SYNTHESIS AND BANDGAP ENGINEERING

OF VANADOSILICATE AM-6 FOR PHOTOCATALYTIC APPLICATIONS

A Dissertation Presented

by

Mariam N. Ismail

to

The Department of Chemical Engineering

In partial fulfillment of the requirements For the degree of

Doctor of Philosophy

In the field of

Chemical Engineering

March 22, 2011

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ACKNOWLEDGEMENTS

I would like to begin by expressing my deepest and most sincere gratitude to all

of those who contributed to my growth throughout my graduate studies here at

Northeastern University. I would like to especially thank Dr. Albert Sacco Jr. for all his

continuous support and guidance throughout the years. Dr. Sacco, you have shaped me

into the researcher that I am today, and I will forever be grateful for all the assistance

you’ve provided. You’re continuous support has motivated me to be the best that I can

be. I would like to also thank Dr. Juliusz Warzywoda, Dr. Katherine S. Ziemer, and Dr.

Teiichi Ando for serving on my committee and for their insightful suggestions

throughout. Dr. Michael Manning, you have been such a great support to have in the lab

throughout the years. Thank you for all your insightful discussions and aid that you have

provided. Dr. Juliusz Warzywoda, you have played such a crucial role in my growth

throughout these years. You have been a great mentor and friend. I most certainly would

not be here without you. Thank you for everything you have done for me and your

continuous support as I close out this chapter in my life. I look forward to what the future

holds.

I would also like to acknowledge the support of my past and present colleagues. I

would like to express my sincerest gratitude to Dr. Zhaoxia (Ivy) Ji for being a great

senior labmate at CAMMP and teaching me lessons that I will forever carry with me.

Onnaz Ozkanat, you have been a true friend and colleague. Jessy Elhajj, I am so thankful

for meeting a colleague and most importantly a friend like you. Brian McMahon, thank

you for all your support throughout these years. I would also like to thank all my former

CAMMP labmates, Jon Leong, Dennis M. Callahan, Eko A. Pandowo, Shihara

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Shafeque, Samantha Rosenberg, Nina Bordeaux, Stephanie Fernandez, and Frank

Marealle. It was such great pleasure to have worked with you all. Natalia Maximova and

Kathleen McCarthy, it was a pleasure sharing an office with you two. I would like to also

thank all the undergraduate students who have contributed to this work; Naftali Fraiman,

Daniel Shea, Devyesh Rana, and Ugonna Ibe. I would like to express my deepest

gratitude to all of our collaborators who contributed significantly to this work. I thank

Dr. Trevor L. Goodrich, Bing Sun, and Dr. Katherine S. Ziemer for their XPS analysis,

Edward Viveiros, Dr. Ronald Willey and Dr. Zhaoxia Ji for their BET surface area

analysis, Dr. Tatyana Chernenko and Dr. Max Diem for their Raman analysis, and Rob

Eagan for all his technical support.

Finally, I would like to thank my family and friends for all their support and

encouragement. I would like to express my deepest gratitude to my mother, Doha Onsi,

and all my siblings, Ahmed, Abdul, Ebraheim, Sarah, Omar, and Norah. You have all

kept me grounded and motivated me to be the best that I can be. This dissertation is

dedicated to all my family and friends for all their continuous support and guidance. I

would not be the woman that I am today without any of you. I thank you all.

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ABSTRACT

Vanadosilicate AM-6 is a large-pore microporous material, isostructural with

titanosilicate ETS-10. This material contains in its structure semiconducting monatomic

…V-O-V-O-V… chains that are embedded in a silica matrix. Microporosity, pore

regularity, and the presence of stoichiometric amounts of vanadium in the silicate

framework make AM-6 a promising material in both traditional and advanced zeolite-

type materials applications. AM-6 has recently been demonstrated to exhibit

photocatalytic activity in the visible light range. Diverse applications require AM-6

crystals with controlled characteristics such as purity, size, morphology, topography, and

defect concentration. However, to date, AM-6 has only been synthesized using seeds of

ETS-10, and no control of the crystal characteristics has been reported. The use of

titanosilicate ETS-10 crystals in the synthesis of vanadosilicate AM-6 limits the ability to

control product characteristics without also influencing the AM-6 product titanium

content.

In this investigation, a novel method for synthesizing AM-6 crystals without the

use of seeds was developed. The use of a structure-directing agent facilitated the

synthesis of pure (i.e., Ti-free) AM-6 products with various sizes, morphologies,

topographies, and defect concentrations. AM-6 crystals appeared to grow via a two-

dimensional nucleation crystal growth mechanism. UV-vis, FTIR, and Raman

spectroscopic analyses suggested the presence of two oxidation states of vanadium (V4+

and V5+) in AM-6. The optical properties of AM-6 were modified by varying the

supersaturation levels in the synthesis mixtures and consequently the rate of surface

nucleation relative to the rate of layer lateral spreading. Structural spectroscopic analyses

v

were carried out to elucidate the effect of varying these supersaturation levels on the

…V-O-V-O-V… chains. “High quality” crystals with less interrupted (i.e., more

continuous) chains in their bulk as determined from XRD and UV-vis analyses were

found to be more effective towards the photodegradation of methylene blue under visible

light irradiation than “low quality” crystals with more discontinuous chains in their bulk.

These findings were attributed to the presence of more V5+ species in the “outer surface

region” of “high quality” crystals than the “low quality” crystals as determined from

Raman analysis using different laser excitation lines.

In situ isomorphous framework substitutions of transition metals were carried out

in efforts to enhance the photocatalytic activity of AM-6 by introducing new ligand-to-

metal charge-transfer transitions and separating the V4+ and V5+ oxidation states along

the …V-O-V-O-V… chains. The proximity of these vanadium oxidation states has been

reported to promote electron-hole recombination. Successful incorporation of Cr3+, Fe3+,

and Co2+ in the AM-6 framework was established by EDX, XRD, UV-vis, Raman and

FTIR spectroscopic analyses. All transition metal ions-incorporated AM-6 products

showed a red shift of their bandgap energies (3.62-3.78 eV) compared to unmodified

AM-6 (3.82 eV), and new low energy charge-transfer transitions. Chromium-substituted

AM-6 showed a substantial improvement in the photodegradation of methylene blue

under UV and visible light irradiation compared to unmodified AM-6.

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TABLE OF CONTENTS LIST OF FIGURES ......................................................................................................... viii LIST OF TABLES ........................................................................................................... xiv 1.0 INTRODUCTION .................................................................................................. 1

1.1 Semiconductor Photocatalysis ............................................................................ 2 1.1.1 Semiconductors ............................................................................................. 3 1.1.2 Photocatalysis Mechanism ............................................................................ 4

1.2 Improving Photocatalytic Activity ...................................................................... 7 1.2.1 Surface Modifications ................................................................................... 7 1.2.2 Framework/Lattice Modifications ................................................................ 9

1.3 Vanadosilicate AM-6 Isostructural with Titanosilicate ETS-10 ....................... 10 1.4 Effect of Presence of Vanadium on Structure and Electronic Properties ......... 12

2.0 CRITICAL LITERATURE REVIEW .................................................................. 19 2.1 Vanadosilicate AM-6 ........................................................................................ 20

2.1.1 Seeded Synthesis of AM-6 ........................................................................... 20 2.1.2 Photocatalytic Activity of Seeded AM-6 ..................................................... 26

2.2 Unseeded Synthesis Techniques – Structure Directing Agents ........................ 28 2.2.1 Synthesis using SDAs .................................................................................. 28 2.2.2 Effect of SDA on Crystallization Kinetics ................................................... 33

2.3 Methods of Improving Photocatalytic Activity ................................................ 35 2.3.1 Surface Modifications ................................................................................. 36

2.3.1.1 Inducing Defects via Post-Synthesis Modifications ............................ 36 2.3.1.2 Inducing Defects via Crystallization Parameters ................................. 42 2.3.1.3 Integration with Noble Metal Nanoparticles ....................................... 46

2.3.2 Bandgap Engineering via Framework/Lattice Modifications .................... 51 2.3.2.1 Transition Metal Isomorphous Framework Substitutions ................... 51 2.3.2.2 Doping with Nonmetals ....................................................................... 58

2.4 Photocatalysis using Microporous Materials .................................................... 61 3.0 EXPERIMENTAL ................................................................................................ 67

3.1 Unseeded AM-6 Synthesis ................................................................................ 67 3.2 Transition Metal Isomorphous Framework Substitutions ................................. 68 3.3 Photocatalytic Investigations ............................................................................ 69

3.3.1 Model Organic Compound ......................................................................... 70 3.3.2 Photocatalysis in a Slurry Reactor ............................................................. 71

3.4 Characterization Techniques ............................................................................. 75 3.4.1 X-ray Powder Diffraction ........................................................................... 75 3.4.2 Field Emission Scanning Electron Microscopy .......................................... 76 3.4.3 Energy Dispersive X-ray Spectroscopy ...................................................... 77 3.4.4 Thermogravimetric Analysis ....................................................................... 77 3.4.5 Particle Size Distribution ............................................................................ 77 3.4.6 X-ray Photoelectron Spectroscopy ............................................................. 78 3.4.7 Raman Spectroscopy ................................................................................... 78 3.4.8 Fourier Transform Infrared Spectroscopy.................................................. 79 3.4.9 Diffuse Reflectance UV-vis Spectroscopy ................................................... 79

4.0 RESULTS AND DISCUSSION ........................................................................... 82

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4.1 Unseeded AM-6 Crystal Growth ...................................................................... 83 4.1.1 Investigation of Crystallization Parameters ............................................... 86

4.1.1.1 Effect of Templating Agent ................................................................. 87 4.1.1.2 Effect of TMA Molar Content ............................................................. 89 4.1.1.3 Effect of Si/V Molar Ratio .................................................................. 91 4.1.1.4 Effect of Synthesis pH ......................................................................... 94 4.1.1.5 Effect of Crystallization Temperature ................................................. 95

4.1.2 Crystal Growth Mechanism ...................................................................... 100 4.1.3 Crystallization Kinetics ............................................................................. 103 4.1.4 Effect of Crystal Quality on the Optical Properties of AM-6 ................... 104 4.1.5 Removal of TMA from the Micropores of AM-6 ....................................... 108

4.1.5.1 Calcination in Air .............................................................................. 108 4.1.5.2 Gaseous Ammonia Treatment at Elevated Temperatures ................. 111

4.1.6 Spectroscopic Characterization of Unseeded AM-6 ................................. 112 4.2 Isomorphous Framework Substitution ............................................................ 117

4.2.1 X-ray Powder Diffraction and Unit Cell Analysis .................................... 119 4.2.2 Effect of Substitution on the Development of the Crystal Surfaces .......... 121 4.2.3 Optical and Electronic Properties of TM-AM-6 ....................................... 124 4.2.4 Effect of TM-Substitution on the Local Environment of the VO6 Chains .. 128 4.2.5 Effect of TM-Substitution on the Framework Vibrations of AM-6 ........... 130

4.3 Photocatalytic Degradation of Methylene Blue .............................................. 131 4.3.1 Photolysis of Methylene Blue .................................................................... 132 4.3.2 Effect of Isomorphous Framework Substitution on the Photocatalytic Activity 133

4.3.2.1 Photocatalytic Activity under UV Light Irradiation .......................... 134 4.3.2.2 Photocatalytic Activity under Visible Light Irradiation .................... 138

4.3.3 Photocatalyst Re-use ................................................................................. 141 4.3.4 Effect of Crystal Quality on the Photocatalytic Activity ........................... 143

5.0 CONCLUSIONS................................................................................................. 147 6.0 RECOMMENDATIONS .................................................................................... 151 7.0 NOMENCLATURE ........................................................................................... 155 8.0 REFERENCES ................................................................................................... 157 9.0 APPENDICES .................................................................................................... 165

9.1 Location of Substituted TM Ion Relative to the AM-6 Crystal Surface ......... 167 9.2 Particle Size Distribution Analysis ................................................................. 167 9.3 FTIR Spectroscopic Analysis ......................................................................... 168 9.4 Photocatalyst Re-Use – SEM Analysis ........................................................... 170 9.5 Preparation of Silver Nanoparticles ................................................................ 172

9.5.1 Synthesis of ETS-10 Crystals .................................................................... 172 9.5.2 Preparation of Ag0-ETS-10 ....................................................................... 173

9.6 Photocatalytic Degradation of MB in a Free-Floating Ag System ................. 173 9.6.1 Photocatalytic Degradation of Methylene Blue on Ag+-ETS-10 .............. 176 9.6.2 Photocatalytic Degradation of Methylene Blue on Ag0-ETS-10 ............... 178

References for Appendix 9.0 .......................................................................................... 181

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LIST OF FIGURES

Figure 1: Schematic of the energy diagram for an irradiated semiconductor; the Fermi level splits into two "quasi-Fermi levels". ........................................................ 4

Figure 2: Schematic of semiconductor photocatalysis. ....................................................... 5Figure 3: Simplified energy level diagram in semiconductors (a), and metals (b) [10]. .... 8Figure 4: Potential roles of metal nanoparticles in photocatalysis: (a) Schematic of

charge separation in a semiconductor-nanoparticle system; (b) Oscillation of electrons in a metal nanoparticle upon exposure to light causing injections of e-’s in the CB of a SC. ....................................................................................... 9

Figure 5: Framework of AM-6 illustrating the linkage of the VO6 octahedral chains (light grey) with SiO4 tetrahedra (dark grey) [19]. .................................................. 11

Figure 6: Two chemical environments for the Si atoms in vanadosilicate AM-6 [adapted from 22]. ........................................................................................................... 12

Figure 7: Density functional theory cluster region for ONIOM models; M – teal; Si – light gray; O – red; Na – blue/purple. The numbering scheme was used to identify different V structures [23]. ................................................................ 13

Figure 8: Band structure of ETS-10, 2-V4+, 2-V5+, and 1,2,3-V5+ models illustrating the addition of a mid-gap state upon V (V4+ or V5+) substitution [24]. ................ 16

Figure 9: (a) UV-vis spectra for ETS-10 samples substituted with 13, 33, 43, and 100% V, and (b) magnification of the low energy region of the spectra shown in (a) [24]. ................................................................................................................. 17

Figure 10: XRD patterns of titanosilicate ETS-10 and vanadosilicate AM-6 [17]. .......... 21Figure 11: Raman spectra of ETS-10 and AM-6 measured at room temperature [17]. .... 22Figure 12: SEM images of (a) ETS-10, (b) AM-6, and (c) 0.43ETVS-10 crystals [31]. . 24Figure 13: UV-vis spectra of ETS-10, P25, and AM-6 products [33]. ............................. 25Figure 14: IR peak area of polyethylene versus reaction time for conversion of ethylene

on ETS-10, 0.43ETVS-10, 0.33ETVS-10, AM-6, and P25 TiO2 irradiated under UV light [31]. ........................................................................................ 27

Figure 15: SEM images of ETS-10 crystals synthesized in the absence of TMACl (a), and in the presence of TMACl (b) [38]. ......................................................... 31

Figure 16: SEM images of ETS-10 products obtained from mixtures with molar composition 4.2Na2O : 1.5KF : 5.5SiO2 : 1.0TiO2 : 300H2O : xOrganic at 473 K: (a) x = 0, 24 hr; (b) x = 0.2 TPA, 15 hr; (c) x = 0.2 TEA, 15 hr; (d) x = 0.2 TMA, 18 hr; (e) x = 0.4 EA, 24 hr [39]. ......................................................... 32

Figure 17: Scheme for determining nucleation, transition, and crystallization stages on the crystallization curve of ETS-10 [39]. ........................................................ 34

Figure 18: HR-TEM micrographs of as-synthesized ETS-10 (a), and 8 wt.% HF treated ETS-10 (b) samples [50]. ................................................................................ 37

Figure 19: UV-vis (a) and Raman (b) spectra of AM-6 before NH4+ exchange (1), after

NH4+ exchange (2), and after exchange and dehydration (3) [33]. ................. 41

Figure 20: SEM images of ETS-10 crystals with varying defect concentrations: (a) T = 200 oC, 2 d, Si/Ti = 4.97, cation/Ti = 1.95; (b) T = 200 oC, 6 d, Si/Ti = 5.02, cation/Ti = 1.90; (c) T = 200 oC, 4 d, Si/Ti = 4.94, cation/Ti = 1.78; (d) T = 200 oC, 1 d, Si/Ti = 4.98, cation/Ti = 1.84; (e) T = 200 oC, 2 d, Si/Ti = 4.97,

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cation/Ti = 1.75; (f) T = 200 oC, 2.6 d, Si/Ti = 4.87, cation/Ti= 1.70; (g) T = 170 oC, 11 d, Si/Ti = 4.71, cation/Ti = 1.73 [49]. ........................................... 43

Figure 21: Raman spectra of ETS-10 samples with varying defect concentration: (a) T = 200 oC, 2 d, Si/Ti = 4.97, cation/Ti = 1.95; (b) T = 200 oC, 6 d,Si/Ti = 5.02, cation/Ti = 1.90; (c) T = 200 oC, 4 d, Si/Ti = 4.94, cation/Ti = 1.78; (d) T = 200 oC, 1 d, Si/Ti = 4.98, cation/Ti = 1.84; (e) T = 200 oC, 2 d, Si/Ti = 4.97, cation/Ti = 1.75; (f) T = 200 oC, 2.6 d, Si/Ti = 4.87, cation/Ti = 1.70; (g) T = 170 oC, 11 d, Si/Ti = 4.71, cation/Ti = 1.73 [49]. ........................................... 44

Figure 22: UV-vis absorption spectra of ETS-10 samples synthesized with varying defect concentrations converted using the Kubelka-Munk function and fitted with two Gaussian peaks and a mixed Gaussian-Lorentzian peak. The solid line passing through the data points is the sum of the fitted peaks [49]. ............... 45

Figure 23: UV-vis absorption spectra of ZnO (a) and 1-5 mol% Ag-ZnO (b-d) [12]. ..... 47Figure 24: Room temperature PL spectra of (a) ZnO and (b-d) 1, 3, and 5 mol% Ag-

modified ZnO, respectively, in the presence of rhodamine 6G [12]. ............... 48Figure 25: Uv-vis spectra of (a) ETS-10 and (b) ETCoS-10 [60]. ................................... 53Figure 26: UV-vis spectra of Cr- and Co- incorporated ETS-10 samples [63]. ............... 54Figure 27: Raman spectra of ETS-10 and ETNbS-10 [65]. .............................................. 56Figure 28: Comparison of electronic structures for ETS-10 and all 33% substituted

transition metal models. The red lines illustrate the h-O(2p) state, the black lines illustrate the unoccupied midgap states, and the gray lines illustrate the occupied midgap state. The light gray rectangles illustrate unoccupied conduction band states, and the dark gray rectangles illustrate occupied valence band states [66]. ................................................................................. 58

Figure 29: UV-vis spectra showing a red shift in the band edge due to nitrogen doping of TiO2 nanoparticles [68]. .................................................................................. 60

Figure 30: UV-vis spectra of various modified and unmodified photocatalysts [70]. ...... 61Figure 31: Maximum length, molecular width, and minimum width of (a) symmetric

1,3,5-trihydroxybenzene and (b) asymmetric 2,4-dichlorophenol as determined by molecular orbital calculations [75]. ........................................ 63

Figure 32: UV-vis absorption spectrum of MB; insert: MB (thiazine) molecular structure [79]. ................................................................................................................. 70

Figure 33: Absorbance of MB as a function of concentration. ......................................... 71Figure 34: Temperature change throughout a photocatalytic process. ............................. 73Figure 35: Temporal change for adsorption of MB on AM-6 crystals under dark

conditions while stirring at 1000 rpm. ............................................................ 74Figure 36: XRD pattern of as-synthesized pure AM-6 product [120]. ............................. 76Figure 37: Plot of [F(R∞)hν]n vs. hν with n values of: (a) 2, (b) 1/2, (c) 3, and (d) 3/2.

Inserts are magnifications of the lower absorption regions of each plot. Red dotted lines indicate forbidden transitions, whereas black solid lines indicate allowed transitions. ......................................................................................... 81

Figure 38: Proposed mechanism for the formation of zeolites utilizing a structure directing agent [86]. ........................................................................................ 83

Figure 39: FE-SEM images of crystals synthesized at 430 K from mixture with molar composition 5.6Na2O : 1.3K2O : 4.85SiO2 : 0.5V2O5 : wTMABr : 169H2O: (a) w = 0; unidentified crystalline product, and (b) w = 2.0; AM-6 product. ....... 84

x

Figure 40: XRD patterns of ETS-10 product (a), and products synthesized at 503 K from mixture with molar composition 5.6Na2O : 1.3K2O : 4.85SiO2 : 0.5V2O5 : wTMABr : 169H2O: (b) w = 2; AM-6, and (c) w = 0; unidentified crystalline material. Tick marks indicate where the AM-6 reflections should appear. .... 86

Figure 41: XRD patterns of products synthesized at 503 K from mixture with molar composition 5.6Na2O : 1.3K2O : 4.85SiO2 : 0.5V2O5 : 2.0SDA : 169H2O: (a) SDA = TMABr; AM-6 product, (b) SDA = TEABr; unidentified crystalline product, and (c) SDA = TPABr; unidentified crystalline product. ................. 88

Figure 42: XRD patterns of AM-6 products synthesized at 503 K from mixture with molar composition 5.6Na2O : 1.3K2O : 4.85SiO2 : 0.5V2O5 : wTMA : 209H2O with: (a) w = 0.0, (b) w = 0.25, (c) w = 0.5, (d) w = 1.0, (e) w = 2.0, and (f) w = 3.0. ................................................................................................ 90

Figure 43: FE-SEM images showing the effect of mixture TMA content on the product purity and size of AM-6 crystals obtained from mixture with molar composition 5.6Na2O : 1.3K2O : 4.85SiO2 : 0.5V2O5 : wTMA : 209H2O with: (a) w = 0.0, (b) w = 0.25, (c) w = 0.5, (d) w = 1.0, (e) w = 2.0, and (f) w = 3.0.

......................................................................................................................... 91Figure 44: FE-SEM images of AM-6 crystals synthesized at 503 K from mixtures with

molar compositions 5.6Na2O : 1.3K2O : ySiO2 : 0.5V2O5 : 2.0TMA : zH2O with: (a) y = 3.4, z = 199, (b) y = 3.8, z = 202, (c) y = 4.3, z = 206, (d) y = 4.85, z = 209, (e) y = 5.5, z = 214, and (f) y = 6.2, z = 216. ..................................... 92

Figure 45: Particle size distributions (PSDs) of AM-6 products synthesized at 503 K from mixtures with molar compositions 5.6Na2O : 1.3K2O : ySiO2 : 0.5V2O5 : 2.0TMA : zH2O with: (a) y = 3.4, z = 199, (b) y = 3.8, z = 202, (c) y = 4.3, z = 206, (d) y = 4.85, z = 209, and (e) y = 5.5, z = 214. ........................................ 93

Figure 46: XRD patterns and average full-width-at-half-maximum (FWHM) of AM-6 products synthesized at 503 K from mixtures with molar compositions 5.6Na2O : 1.3K2O : ySiO2 : 0.5V2O5 : 2.0TMA : zH2O with: (a) y = 3.4, z = 199, (b) y = 3.8, z = 202, (c) y = 4.3, z = 206, (d) y = 4.85, z = 209, and (e) y = 5.5, z = 214. ..................................................................................................... 94

Figure 47: FE-SEM images of AM-6 crystals synthesized at 503 K from mixtures with molar compositions 5.6Na2O : 1.3K2O : 4.85SiO2 : 0.5V2O5 : 2.0TMA : 209H2O with: (a) pH = 10.5; (b) pH = 10.8; (c) pH = 11.0. ........................... 95

Figure 48: FE-SEM images of AM-6 crystals synthesized from mixtures with molar compositions 5.6Na2O : 1.3K2O : 4.3SiO2 : 0.5V2O5 : 2.0TMA : 206H2O at: (a) 503 K, 4 d; (b) 473 K, 6 d; (c) 453 K, 6 d; (d) 438 K, 8 d; (e) 430 K, 14 d.

......................................................................................................................... 96Figure 49: XRD patterns of AM-6 products synthesized from mixtures with molar

compositions 5.6Na2O : 1.3K2O : 4.3SiO2 : 0.5V2O5 : 2.0TMA : 206H2O at: (a) 430 K, 14 d; (b) 438 K, 8 d; (c) 453 K, 6 d; (d) 473 K, 6 d; (e) 503 K, 4 d.

......................................................................................................................... 98Figure 50: XRD patterns of AM-6 products synthesized from mixtures with molar

compositions 4.5Na2O : 1.3K2O : 4.85SiO2 : 0.5V2O5 : 2.2TMAOH : 206H2O at: (a) 430 K, 14 d; (b) 503 K, 3 d. .................................................................. 99

xi

Figure 51: FE-SEM images of AM-6 crystals synthesized from mixtures with molar compositions 4.5Na2O : 1.3K2O : 4.85SiO2 : 0.5V2O5 : 2.2TMAOH : 206H2O at: (a) 430 K, 14 d; (b) 503 K, 3 d. .................................................................. 99

Figure 52: Kubelka-Munk function UV-vis spectra and calculated bandgap energies of AM-6 products synthesized from mixtures with molar compositions 5.6Na2O : 1.3K2O : 4.3SiO2 : 0.5V2O5 : 2.0TMA : 209H2O at: (a) 503 K, 4d; (b) 473 K, 6 d; (c) 453 K, 6d; (d) 438 K, 8 d; (e) 430 K, 14 d. ...................................... 100

Figure 53: Low-voltage, high-resolution FE-SEM image of the nano-sized square shaped island-like structures and terraces on a square face of uncoated AM-6 crystal synthesized at 503 K from mixture with molar composition 4.5Na2O : 1.3K2O : 4.85SiO2 : 0.5V2O5 : 2.2TMAOH : 209H2O. ................................ 101

Figure 54: Crystallization curves of AM-6 synthesized from mixture with molar composition 5.6Na2O : 1.3K2O : 4.3SiO2 : 0.5V2O5 : 2.0TMABr : 206H2O at: (a) 503 K, (b) 473 K, and (c) 443 K. Symbols depict the experimental data points. Estimated error for each data point is ± 5 %. .................................... 104

Figure 55: UV-vis spectra AM-6 products synthesized at 503 K from mixtures with molar compositions 5.6Na2O : 1.3K2O : ySiO2 : 0.5V2O5 : 2.0TMA : zH2O with: (a) y = 3.4, z = 199, (b) y = 3.8, z = 202, (c) y = 4.3, z = 206, (d) y = 4.85, z = 209, and (e) y = 5.5, z = 214. ................................................................... 105

Figure 56: Kubelka-Munk function UV-vis spectra and calculated bandgap energies of AM-6 products synthesized at 503 K from mixtures with molar compositions 5.6Na2O : 1.3K2O : ySiO2 : 0.5V2O5 : 2.0TMA : zH2O with: (a) y = 3.4, z = 199, (b) y = 3.8, z = 202, (c) y = 4.3, z = 206, (d) y = 4.85, z = 209, and (e) y = 5.5, z = 214. ................................................................................................... 106

Figure 57: XRD patterns and % crystallinity retained (% C) of AM-6 products synthesized at 503 K from mixture with molar composition 5.6Na2O : 1.3K2O : 4.85SiO2 : 0.5V2O5 : 2TMABr : 209H2O calcined in air for 30 min at: (a) 698 K, (b) 673 K, (c) 648 K, (d) 623 K, and (e) un-calcined. ............ 109

Figure 58: XRD patterns of seeded AM-6 product calcined in air for 30 min (a), unseeded AM-6 product calcined in air for 30 min (b), and un-calcined unseeded AM-6 product (c). ......................................................................... 110

Figure 59: TGA analysis of AM-6 products synthesized at 503 K from mixture with molar composition 4.5Na2O : 1.3K2O : 4.85SiO2 : 0.5V2O5 : 2.2TMAOH : 209H2O: (a) as-synthesized product, and (b) product treated with gaseous ammonia at 673 K for 50 min [120]. ............................................................ 112

Figure 60: FTIR absorption spectra of AM-6 products synthesized at 503 K from mixture with molar composition 5.6Na2O : 1.3K2O : 4.85SiO2 : 0.5V2O5 : 2TMABr : 209H2O: (a) as-synthesized AM-6, (b) product treated with gaseous ammonia at 648 K for 1 h, (c) AM-6 synthesized in the absence of TMA using ETS-10 seeds, and (d) ETS-10 product [120]. ........................................................... 114

Figure 61: Raman spectra recorded with 785 nm laser (I) and 532 nm laser (II) of AM-6 products synthesized at 503 K from mixtures with molar compositions 5.6Na2O : 1.3K2O : ySiO2 : 0.5V2O5 : 2.0TMA : zH2O with: (a) y = 4.85 z = 209, and (b) y = 4.3, z = 206. ........................................................................ 117

Figure 62: XRD patterns of modified and unmodified AM-6 products synthesized at 503 K from mixture with molar composition 2.25Na2O : 1.3K2O : 4.85SiO2 :

xii

0.5V2O5 : 2.25TMAOH : 209H2O : 0.025TM: (a) Fe-AM-6, (b) Cr-AM-6, (c) Co-AM-6, and (d) unmodified AM-6. Star symbols mark peaks due to unidentified impurities. ................................................................................. 120

Figure 63: FE-SEM images of modified and unmodified AM-6 crystals synthesized at 503 K from mixture with molar composition 2.25Na2O : 1.3K2O : 4.85SiO2 : 0.5V2O5 : 2.25TMAOH : 209H2O : 0.025TM: (a) Fe-AM-6, (b) Cr-AM-6, (c) Co-AM-6, and (d) unmodified AM-6. .......................................................... 122

Figure 64: FE-SEM images of modified AM-6 crystals synthesized at 503 K from mixture with molar composition 2.25Na2O : 1.3K2O : 4.85SiO2 : 0.5V2O5 : 2.25TMAOH : 209H2O : sTM: (a) s = 0.0375Fe/V, (b) s = 0.10Cr/V, and (c) s = 0.20Co/V. ................................................................................................ 123

Figure 65: (a) UV-vis spectra of unmodified AM-6 and TM-AM-6 products grown at 503 K from mixture with molar composition 2.25Na2O : 1.3K2O : 4.85SiO2 : 0.5V2O5 : 2.25TMAOH : 209H2O : 0.0375TM; (b) magnification of the lower energy region of the spectra in (a) used to highlight the features specific to the substitution of various TM ions. ................................................................... 125

Figure 66: UV-vis spectra for TM ion modified AM-6 products synthesized at 503 K from mixture with molar composition 2.25Na2O : 1.3K2O : 4.85SiO2 : 0.5V2O5 : 2.25TMAOH : 209H2O : xTM substituted with: (a) Fe-AM-6, x = 0.025-0.05; (b) Cr-AM-6, x = 0.025-0.10; (c) Co-AM-6, x = 0.025-0.20. ... 127

Figure 67: Raman spectra recorded with 785 nm laser of modified and unmodified AM-6 products synthesized at 503 K from mixture with molar composition 2.25Na2O : 1.3K2O : 4.85SiO2 : 0.5V2O5 : 2.25TMAOH : 209H2O : 0.0375TM : (a) Fe-AM-6, (b) Cr-AM-6, (c) Co-AM-6, and (d) unmodified AM-6. Star symbols mark bands due to unidentified impurities. ................. 130

Figure 68: Diffuse reflectance FTIR absorption spectra of modified and unmodified AM-6 products synthesized at 503 K from mixture with molar composition 2.25Na2O : 1.3K2O : 4.85SiO2 : 0.5V2O5 : 2.25TMAOH : 209H2O : 0.025TM : (a) Fe-AM-6, (b) Cr-AM-6, (c) Co-AM-6, and (d) unmodified AM-6. ............................................................................................................ 131

Figure 69: Direct photolysis of methylene blue at room temperature under UV light irradiation (310-400 nm). .............................................................................. 132

Figure 70: Direct photolysis of methylene blue at room temperature under visible light irradiation (420-630 nm). .............................................................................. 133

Figure 71: Temporal spectral changes of MB in aqueous 0.025Cr-AM-6 suspension under UV light (310-400 nm) irradiation. ..................................................... 134

Figure 72: Photocatalytic degradation kinetics of MB under UV light (310-400 nm) irradiation on unmodified AM-6 and 0.025TM-AM-6: TM = Fe; Cr; Co. MB photolysis pseudo-first-order reaction rate constant was determined to be 0.0003 min-1 ± 0.0001 min-1. ........................................................................ 135

Figure 73: Photocatalytic degradation kinetics of MB under UV light (310-400 nm) irradiation on transition metal substituted AM-6 (TM-AM-6): (a) Fe-AM-6; (b) Cr-AM-6; (c) Co-AM-6. ......................................................................... 138

Figure 74: Photocatalytic degradation kinetics of MB under visible light (420-630 nm) irradiation on unmodified AM-6 and 0.025TM-AM-6: TM = Fe; Cr; Co. MB

xiii

photolysis pseudo-first-order reaction rate constant was determined to be 0.0028 min-1 ± 0.0004 min-1. ........................................................................ 140

Figure 75: Photocatalytic degradation kinetics of MB under visible light (420-630 nm) irradiation on transition metal substituted AM-6 (TM-AM-6): (a) Fe-AM-6; (b) Cr-AM-6; (c) Co-AM-6. ......................................................................... 141

Figure 76: XRD patterns and average (avg.) full-width-at-half-maximum (FWHM) of 0.025Cr-AM-6 before photocatalysis (a), after the 1st reaction cycle (b), and after the 2nd reaction cycle (c) under UV light irradiation. ........................... 142

Figure 77: Photocatalytic degradation kinetics of MB on fresh and previously used 0.025Cr-AM-6 under UV light (310-400 nm) irradiation. ........................... 143

Figure 78: Absorbance from the supernatants of equilibrated MB solutions illustrating the adsorption of MB on “fresh” vs. “used” photocatalyst. .......................... 143

Figure 79: Photocatalytic degradation kinetics of MB under UV light (310-400 nm) irradiation on unmodified AM-6 crystals with varying surface defect concentrations: "low quality” AM-6; "high quality” AM-6. ........................ 145

Figure 80: Photocatalytic degradation kinetics of MB under visible light (420-630 nm) irradiation on unmodified AM-6 crystals with varying surface defect concentrations: "low quality” AM-6; "high quality” AM-6. ........................ 146

Figure 81: High-resolution FE-SEM images of the square faces of ETS-10 crystals synthesized at 473 K from mixtures with molar composition 5.5SiO2 : xNa2O : yK2O : 1.0TiO2 : 300H2O with x/y ratio of: (a) 2.0, (b) 1.5, and (c) 1.0 [92].

....................................................................................................................... 151Figure 82: ESR spectra of ·OH spin adduct signals obtained under photoirradiation of: (a)

PBN, (b) NB, and (c) DBNBS, in the presence of (i) ETS-10 or (ii) TiO2 for comparison [75]. ........................................................................................... 153

xiv

LIST OF TABLES Table 1: Photocatalysis Reaction Scheme in an Aqueous System [adapted from 6,7]. ...... 6Table 2: Size, polarity, and photocatalytic conversion of 22 aromatic substrates on ETS-

10 [75]. ............................................................................................................... 64Table 3: Chemical compositions as determined by EDX analysis of seeded [31,33] and

unseeded AM-6 crystals. .................................................................................... 85Table 4: Full-width-at-half-maximum (FWHM) of all hkl reflections in the 5-37.5 º2θ

range for AM-6 products synthesized from mixtures with molar compositions 5.6Na2O : 1.3K2O : 4.3SiO2 : 0.5V2O5 : 2.0TMA : 206H2O at: 503 K; 473 K; 453 K; 438 K; 430 K. ......................................................................................... 97

Table 5: BET surface area and micropore volume of AM-6 and ammonia-treated AM-6 (i.e., TMA-free AM-6) falling within the range of reported values for ETS-10 [110] and AM-6 synthesized with seeds [31]. ................................................. 111

Table 6: Chemical compositionsa of various transition metal (TM) ions isomorphously substituted AM-6 products. .............................................................................. 119

Table 7: Unit cell parameters of various TM-AM-6 products. ....................................... 121Table 8: The average full-width-at-half-maximum (FWHM) of AM-6 reflections in the

5-37.5 º2θ range for different TM -AM-6 samples (TM = Fe, Cr, Co; mixture TM/V = 0.025). ................................................................................................ 124

Table 9: Pseudo-first-order reaction rate constants (k) for the photodegradation of MB under UV light (310-400 nm) irradiation, BET surface areas, and bandgap energies (Eg) of various 0.025TM-substituted AM-6 products (TM = Fe, Cr, Co). ................................................................................................................... 136

Table 10: Pseudo-first-order reaction rate constants (k) for the photodegradation of MB on various 0.025TM-substituted AM-6 products (TM = Fe, Cr, Co) under visible light (420-630 nm) irradiation. ............................................................. 140

Table 11: Pseudo-first-order reaction rate constants (k) for the photodegradation of MB under UV and visible light irradiation, BET surface areas, and bandgap energies (Eg) of different unmodified AM-6 products. .................................... 144

1

1.0 INTRODUCTION

The 21st century is being referred to as the environment century, with

photocatalysis technologies leading the way at an estimated annual market of 1.2 billion

US dollars [1]. Semiconductor photocatalysis has attracted increasing attention in the

recent years as an alternative means for environmental cleaning using solar energy.

Typically, photocatalysts are activated by ultraviolet (UV) irradiation because of their

wide bandgaps. Since UV light accounts for a small fraction (~4-5%) of solar energy

compared to visible light (~45%), any shift in the optical response of these

semiconductors will increase energy utilization

Titanium dioxide, TiO2 P25, has been widely used for photocatalysis over the

past few decades due to its stability and nontoxicity which make it useful for the

photodegradation of organic and inorganic pollutants [2]. However, one of the major

drawbacks in using TiO2 as a photocatalyst is its relatively wide bandgap (~3.2 eV),

which requires photons in the near UV light region for excitation. This makes TiO2

inefficient for use as a natural sunlight photocatalyst. In addition, modifications using

various transition metal ions and/or main group metals, which may red shift the bandgap

energy into the visible light region, are not easily incorporated into the TiO2 lattice.

Therefore, other photocatalysts with more structural flexibility have been of recent

interest.

Titanosilicate ETS-10 is a large-pore microporous zeolite-type material

containing semiconducting monatomic …Ti-O-Ti-O-Ti… chains that are embedded in a

silica matrix. These monatomic chains have a reported bandgap energy of ~4.03 eV [3],

which is still well within the UV light irradiation range. However, ETS-10 has a more

2

flexible structure than TiO2, which readily allows for transition metal and main group

metal modifications.

Large-pore microporous vanadosilicate AM-6, recently invented by Rocha et al.

[17], is isostructural with titanosilicate ETS-10. The framework of AM-6 contains

monatomic semiconductor …V-O-V-O-V… chains that have a reported bandgap energy

of 3.80 eV, which is narrower than bandgap energy of ETS-10. In addition, AM-6 has

exhibited photocatalytic activity in the visible light range due to its visible light range

absorption features [31]. However, due to the synthesis method of Rocha et al. [17]

which utilizes seeds of titanosilicate ETS-10, the modification of the optical properties of

AM-6 is limited. Therefore, the (unseeded) synthesis and bandgap engineering of

vanadosilicate AM-6 photocatalyst has been investigated towards the photocatalytic

degradation of organic contaminants under both UV and visible light irradiation.

However, in order to synthesize a photocatalyst efficient under both the UV and visible

light range, knowledge of the mechanism of semiconductor photocatalysis, electronic

and optical properties of semiconductor photocatalysts, and ways to improve the

photocatalytic activity is imperative. This section provides the background necessary to

facilitate the understanding of the investigated topic.

1.1

The term photocatalysis is defined as “the acceleration of a photoreaction by the

presence of a catalyst” [4,5], where “photoreaction” may be defined as a “photoinduced”

or “photoactivated” reaction. This definition also includes the process of

“photosensitization”, a process by which a photochemical or photophysical alteration

occurs in a chemical entity as a result of the initial absorption of radiation by another

Semiconductor Photocatalysis

3

chemical species (called the photosensitizer) [4,5]. There are two types of photocatalytic

reactions: (1) homogeneous photocatalysis, and (2) heterogeneous photocatalysis.

Homogeneous photocatalysis is defined as a photocatalytic reaction taking place in a

homogeneous phase, whereas heterogeneous photocatalysis is defined as a photocatalytic

reaction taking place in a heterogeneous system (e.g., two phases: solid-liquid or solid-

gas). Heterogeneous photocatalysis is often referred to as semiconductor photocatalysis.

1.1.1 Semiconductors

Semiconductors are materials comprised of two non-overlapping energy bands: a

low energy electron-occupied valence band (VB), and a high energy non-occupied

conduction band (CB). The innate gap between these two bands is referred to as the

bandgap energy (Eg) of a material (Figure 1). The bandgap energy is defined as the

energy difference between the valence band maximum (VBM) and the conduction band

minimum (CBM). Located between these two bands is the Fermi level: i.e., the free

energy that pertains to the electrons in a semiconductor (Ef). If the Fermi level is shifted

towards the CB, the semiconductor is considered to be n-type (i.e., extra electrons). If the

Fermi level is shifted towards the VB, the semiconductor is considered to be p-type (i.e.,

extra holes) [6]. When a semiconductor absorbs light with energy greater than or equal to

its bandgap energy, the promotion of electrons (e-) from the VB to the CB occurs,

leaving behind positive holes (h+) in the VB. This photoexcitation results in the splitting

of the Fermi level into two “quasi-Fermi levels”: nEf* for the electrons and pEf* for the

holes [6]. The nEf* and pEf* values represent the electrochemical potentials for the

electrons and holes, respectively. The maximum nEf* and pEf* values have potential

4

energies corresponding to their thermodynamic levels: Ecb for electrons (e-, i.e., nEf*),

and Evb for holes (h+, i.e., pEf*).

Figure 1: Schematic of the energy diagram for an irradiated semiconductor; the

Fermi level splits into two "quasi-Fermi levels".

A redox (i.e., reduction-oxidation process) reaction is hypothesized to take place

if the irradiated semiconductor is in contact with (or in the presence of) a suitable

electron acceptor and electron donor species, and if the redox potentials for this reaction

lies between the Ecb and Evb. From a thermodynamic viewpoint, the hypothesized

oxidation processes will only occur if the potential, i.e., Eox, is higher than the valance

band energy, Evb. Similarly, the reduction processes will only occur if the potential, i.e.,

Ered, is lower than the conduction band energy, Ecb.

1.1.2 Photocatalysis Mechanism

When a semiconductor (photo)catalyst (SC) is irradiated with photons of energy

greater than or equal to its bandgap energy, the electrons in the valence band are excited

into the non-occupied conduction band, thereby generating electron-hole pairs [7,8]

(Figure 2).

5

Figure 2: Schematic of semiconductor photocatalysis.

The absorption of the energy followed by the generation of the electron-hole

pairs is the initiation step in a photocatalytic process (Table 1, reaction T1). In an

aqueous environment, following the initiation step are the adsorption and trapping steps,

in which the hole traps (positive holes, h+) are believed to react with hydroxide ions

(OH-) (Table 1, reaction T3-a) or water molecules (H2O) (Table 1, reaction T3-b), since

OH- and H2O groups are abundant adsorbents, or adsorbed substrates (Ri,ads) (Table 1,

reaction T4). Photooxidation occurs through the trapping of the holes (h+) that form

hydroxyl radicals (OH·) which are associated with the Mn+ (Mn+ = transition metal ion in

the crystalline lattice) site due to their proximity (Table 1, reaction T3). Once the

hydroxyl radicals are produced, there are four possible cases for the hydroxyl attack: (1)

the hydroxyl radical associated with the Mn+ site (Mn+-OH·) may attack an adsorbed

molecule (R1,ads) (Table 1, reaction T7) or (2) a solution phase molecule (R1) (Table 1,

reaction T9); or (3) the hydroxyl radical may diffuse away from the surface and react

with an adsorbed molecule (R1,ads) (Table 1, reaction T8) or (4) a solution phase

6

molecule (R1) (Table 1, reaction T10). For systems without a reducible adsorbate present

(i.e., Mn+ (aq)), photoreduction occurs through the trapping of the photo-excited electrons

which can readily reduce molecular oxygen (O2) to form superoxide radicals (O2·-, Table

1, reaction T6). These superoxide radicals may be further reduced to hydrogen peroxide

(H2O2, Table 1, reaction T11). In addition, under acidic conditions, H+ may protonate the

superoxide radical to form peroxide radicals, HO2·, (Table 1, reaction T12). In systems

where a reducible adsorbate, Mn+ (aq), is present, the adsorbate will capture the

photogenerated electrons, and reduce Mn+ (aq) to an M(n-1)+ state (Table 1, reaction T5).

Excitation SC → e- + h+ (T1) Recombination e- + h+ → heat (T2) Adsorption/Trapping Mn+-OH- + h+ ↔ Mn+-OH· (T3-a)*

M n+-H2O + h+ ↔ M n+-OH· + H+ (T3-b) Ri,ads + h+ ↔ Ri,ads

+ (T4)1 Mn+ + e- ↔ M(n-1)+ (T5) M(n-1)+ + O2 ↔ Mn+-O2·- (T6)

Hydroxyl Attack Case I Case II Case III Case IV

Mn+-OH· + R1,ads → Mn+ + R2,ads (T7)2 OH· + R1,ads → R2,ads (T8)2 Mn+-OH· + R1 → R2

(T9)3 OH· + R1 → R2 (T10)3

Reactions of other radicals

e- + Mn+-O2·- + 2(H+) ↔ Mn+(H2O2) (T11) Mn+-O2·- + (H+) ↔ Mn+(HO2·) (T12)

Table 1: Photocatalysis Reaction Scheme in an Aqueous System [adapted from 6,7]. * Mn+ = transition metal, e.g., Ti, V, W, Zn. 1 Direct hole and adsorbed organic Ri,ads reaction. 2 Hydroxyl attack of one adsorbed organic R1,ads to form another adsorbed product R2,ads. 3 Hydroxyl attack of one organic R1 to form another organic product R2 in solution phase.

According to laser flash photolysis measurements performed on TiO2 by

Hoffman et al. [9], there appears to be competition between the trapping reactions (Table

1, T3-T6) and interfacial charge transfer (Table 1, T7-T12), and the electron-hole

recombination (Table 1, T2) in a photocatalytic process. The reported proposed

7

characteristic times suggest that the overall efficiency for the interfacial charge transfer is

determined by two critical processes, i.e., the competition between the electron-hole

recombination and trapping (picoseconds to nanoseconds) followed by the competition

between trapped carrier recombination and interfacial charge transfer (microseconds to

milliseconds) [9]. Therefore, an increase in the adsorption/trapping and interfacial charge

transfer rates, or a decrease in the recombination rate for a given photocatalyst should

enhance its photocatalytic activity. Framework modifications and/or integration of noble

metal nanoparticles with photocatalysts have been typically employed to increase the

trapping and interfacial charge transfer rates as well as potentially decrease the electron-

hole recombination rates. Increase in the external/accessible M sites (i.e., M-OH groups)

may also increase the adsorption/trapping rates of a photocatalyst. Section 1.2 will

discuss the feasibility of using these methods for enhancing the photocatalytic activity of

vanadosilicate AM-6.

1.2

As discussed in Section

Improving Photocatalytic Activity

1.1.2, the efficiency of a photocatalyst can be enhanced

by increasing the adsorption/trapping and interfacial charge transfer rates, and/or by

decreasing the rate of recombination of the photoexcited electrons and holes. Surface and

framework modifications of photocatalysts have been shown to increase the

adsorption/trapping rates and reduce the electron-hole recombination rate, respectively.

1.2.1 Surface Modifications

Integration of metal nanoparticles with semiconductors (SC) is one means of

improving the photocatalytic activity via surface modifications. Like semiconductors,

8

metals are characterized by two energy bands: a low energy valence band (VB), and a

high energy conduction band (CB). Whereas the conduction and valence bands of

semiconductors are separated by a well-defined bandgap (Figure 3a), metals have

overlapping bands in which electrons can move freely (Figure 3b). Metal nanoparticles,

such as silver, have a distinct well-defined surface plasmon absorption bands in the

visible light range. The surface plasmon absorption bands of the metal nanoparticles

originate from the collective oscillations of the free conduction band electrons (i.e.,

localized surface plasmons) excited by light at an incident wavelength where resonance

occurs. These resonances arise when the wavelength of the incident light exceeds the

diameter of the metal nanoparticles.

Figure 3: Simplified energy level diagram in semiconductors (a), and metals (b) [10]. The integration of noble metal nanoparticles has been shown to be one effective

way for enhancing the photocatalytic activity of various semiconductors [11,12,13,14].

In a photocatalytic system irradiated under UV light, metal nanoparticles have the

capability of trapping the photoexcited electrons from the semiconductor photocatalyst

(Figure 4a) (i.e., act as a sink for the photoexcited electrons), which then leaves the holes

9

for photoxidation of the organic compounds [12,14]. Metal nanoparticles can also extend

the light absorption into the visible light range and enhance the surface electron

excitation by surface plasmons excited by visible light [11,12,13] (Figure 4b).

Figure 4: Potential roles of metal nanoparticles in photocatalysis: (a) Schematic of

charge separation in a semiconductor-nanoparticle system; (b) Oscillation of electrons in a metal nanoparticle upon exposure to light causing injections of e-’s in the CB of a SC.

1.2.2 Framework/Lattice Modifications

The photocatalytic performance of semiconductors, such as the widely used

crystalline TiO2 and titanosilicate ETS-10 (and hypothesized here, vanadosilicate AM-6),

can be improved through doping with various transition and main group metals, as well

as nonmetals. Transition metals incorporated into substitutional and/or interstitial sites in

the crystal lattice may act as electron and/or hole traps, which may result in a decrease in

the electron-hole recombination rates. The electron-hole recombination rate may be

altered through the following process when the irradiated semiconductor generates

electron-hole pairs [15]:

Mn+ + ecb- → M(n-1)+ electron trap (Equation 1)

Mn+ + hvb+ → M(n+1)+ hole trap (Equation 2)

10

The incorporation of transition metals into the crystal lattice or framework of

these semiconductors produces new low energy ligand-to-metal charge-transfer (LMCT)

transitions which occasionally produce absorption features in the visible light range, and

can potentially red shift the bandgap energy of the photocatalyst. The new low energy

LMCT transitions may also result in midgap states (i.e., states that are positioned above

the valence band and below the conduction band) which can behave as electron or hole

trapping sites, depending on whether these states are unoccupied or occupied,

respectively. Ions such as Fe3+ and Cr3+ can act as both electron and hole traps [23];

whereas V4+ [23] and Mn2+ can only act as hole traps, and V5+ [23] can only behave as

electron traps. Ions with closed-shell electronic configurations (such as V5+, electronic

configuration: 1s22s22p63s23p6, i.e., [Ar]) will have little effect on the observed

photocatalytic activity. Choi et al. [15] and Zhang et al. [16] hypothesized that trapping

either only electrons or only holes is not effective because the immobilized charge

species can easily recombine with their mobile counterparts.

1.3

Vanadosilicate AM-6 (Aveiro/Manchester no. 6), first reported by Rocha et al.

[17], is a large-pore, microporous crystalline material with unit cell composition of

(Na,K)2VSi5O13·3H2O, isostructural with titanosilicate ETS-10 [17]. The AM-6

framework is comprised of corner-sharing SiO4 tetrahedra and VO6 octahedra linked

through the bridging of oxygen atoms forming three-dimensional large-pore channel

system, whose minimal pore dimensions are defined by the 12-membered ring channels,

pore openings of approximately 4.9 × 7.6 Å (

Vanadosilicate AM-6 Isostructural with Titanosilicate ETS-10

Figure 5) [19]. Great interest arises in the

well-defined, monatomic …V-O-V-O-V… chains, which run in two orthogonal

11

directions, and are surrounded by the tetrahedral silicate units. These chains have been

reported to be similar to the monatomic …Ti-O-Ti-O-Ti… chains in ETS-4 and ETS-10

[18]. These chains, also known as rods, impart some of the interesting optical properties

of these materials (vide infra, Section 1.4) [21].

Figure 5: Framework of AM-6 illustrating the linkage of the VO6 octahedral chains

(light grey) with SiO4 tetrahedra (dark grey) [19].

As with titanium (Ti) atoms in ETS-10, each vanadium (V) atom in the chains is

surrounded by two types of oxygen (O) atoms – type I, and type II (Figure 6). Type I,

also referred to as axial (Oa) oxygen atoms, are those that bridge the V atoms along the

…V-OI-V-OI-V… chains. Type II, also referred to as equatorial (Oeq) oxygen atoms, are

those that bridge each V atom with the silicon (Si) atom of the surrounding SiO4 groups

(V-OII-Si). Four resonance lines were observed in 29Si solid-state NMR spectrum for

ETS-10 which were assigned to Si(3Si, 1Ti) (i.e., Si linked via OII bridges to three Si

atoms and one Ti atom), and Si(4Si, 0Ti) (i.e., Si linked via O bridges to four Si atoms)

[20,22]. The intensity ratio of these two groups of signals (i.e., Si(3Si, 1Ti) and Si(4Si,

0Ti)) was 4:1, respectively [22]. Since AM-6 is isostructural with ETS-10, two types of

12

chemical environments for silicon atoms can also be inferred: Si linked via O bridges to

four Si atoms, (4Si, 0V); and Si linked via OII bridges to three Si atoms and one V atom,

(3Si, 1V) (Figure 6). The intensity ratio of the Si(4Si,0V) sites to Si(3Si, 1V) sites is 1:4.

Figure 6: Two chemical environments for the Si atoms in vanadosilicate AM-6

[adapted from 22].

The rods formed by the bridging of Si with V are orthogonally stacked with each

other, with a displacement of 1/4 unit cell in the [100] direction (a-direction) or the [010]

direction (b-direction), giving rise to four possible connections [20]. The disorder in the

ETS-10 framework, and consequently the disorder in the AM-6 framework [17], is

caused by the random stacking of these four possible connections [20,21,22].

1.4

In order to utilize what is known regarding modification of the ETS-10 structure

for enhancing AM-6 photocatalytic activity, it is necessary to first understand the effect

of vanadium (V) substitution on the ETS-10 structure. Shough et al. [23,24] developed

computational models using hybrid density functional theory/molecular mechanism

(DFT/MM) methods to investigate the geometric and energy changes upon varying the

concentrations of V4+ and V5+ in the …M-O-M-O-M… (M = Ti and/or V) chains that

exist in vanadium-substituted ETS-10. Three main systems were investigated: (i) V4+, (ii)

Effect of Presence of Vanadium on Structure and Electronic Properties

13

V5+, and (iii) V4+/V5+. For these systems, models included one, two, or three vanadium

atoms (the three vanadium atom model represented 100% AM-6) to investigate the effect

of the vanadium concentration on the …M-O-M-O-M… (M = Ti and/or V) chains. The

numbering system (i.e., M1, M2, M3) shown in Figure 7 was used to identify the

location of the different vanadium-substituted structures as well as the geometric

parameters.

Figure 7: Density functional theory cluster region for ONIOM models; M – teal; Si

– light gray; O – red; Na – blue/purple. The numbering scheme was used to identify different V structures [23].

The following structures were modeled: 2-V4+, 1,3-V4+, 1,2,3-V4+, 2-V5+, 1,3-V5+,

1,2,3-V5+, and 2-V4+/1,3-V5+. Upon substitution of V4+ for Ti4+, an extra electron is

added to the ETS-10 system without altering the net charge. Since the ionic radii of T4+

(0.745 Å [25]) and V4+ (0.72 Å [25]) are very similar, only a slight compression in the

M–OI–M bonds and angles was observed in the V4+ substitution systems (i.e., 2-V4+, 1,3-

V4+, 1,2,3-V4+). However, upon substitution of V5+ the net nuclear charge of the system

is increased; therefore, requiring the removal of one Na+ for each V5+ addition. To

address this issue the authors constructed three additional (positively charged) models:

14

[2-V5+]+, [1,3-V5+]2+, [1,2,3-V5+]+. Regardless of the model studied (positive or neutral

V5+ model), the authors observed a shortening of some of the M–O–M axial and

equatorial bond lengths with respect to the ETS-10 structure, which was due to the

smaller ionic radius of V5+ (0.68 Å [25]) as compared to the ionic radii of V4+ (0.72 Å

[25]) and T4+ (0.745 Å [25]). Upon high substitutions of V5+ throughout the chains (e.g.:

1,2,3-V5+ model), the removal of Na+ ions resulted in a large asymmetry surrounding the

central atom (one short and one long V5+–O bond length); i.e., formation of V5+=O (i.e.,

short bond), which is in agreement with the experimental observations of Rocha et al.

[17]. The removal of the Na+ ions surrounding the O–M–O chains for the V5+ substituted

models had a large impact on the local chain geometry. Upon increasing the presence of

V5+ in the structure, the asymmetry throughout the O–M–O chains became more

prominent resulting in a decrease in the stability of the structure. Therefore, the

conclusion of this theoretical study was that the incorporation of V4+ into the …M-O-M-

O-M… (M = Ti and/or V) chains are necessary in order to stabilize the structure,

especially in the case of fully vanadium-substituted (100% V) ETS-10, i.e., AM-6.

Electron paramagnetic resonance (EPR), 51V NMR and DFT calculations of the

NMR parameters were carried out to further confirm the structural location and

coordination environment of V in ETS-10 samples experimentally substituted with 11,

33, 43, and 100% V [26]. For low V-substitutions, a clear well-resolved hyperfine signal

was observed in the EPR spectra. Upon increasing the V content, the distance between

the vanadium atoms decreased, causing a loss in the hyperfine structure, which resulted

in one broad signal for 100% V-substituted ETS-10 (i.e., AM-6). This has been attributed

to the electron-electron coupling as a result of multiple V4+ atoms in close proximity to

15

one another. These results suggested octahedral coordination of V4+ along the …M-O-M-

O-M… chains [26], which are also in agreement with the modeling performed by

Shough and coworkers [23]. 51V NMR results suggested the presence of V5+, which were

in close proximity (i.e., a single oxygen bridge apart) to V4+ as a result of the electron-

nuclear interaction between the V5+ nuclei and the unpaired electron on V4+. DFT

calculations of the NMR parameters for models in which V5+ is in the octahedral

coordination at the ends of the chains were in good agreements with the experimental

observations. These results clearly suggest the presence of octahedrally coordinated V4+

and V5+, with V5+ located at the terminal sites of the …V-O-V-O-V… chains in AM-6.

The changes of electronic structure of ETS-10 upon varying degrees of V

substitution were also investigated using DFT/MM methods [23,24]. For ETS-10 and all

V-substituted ETS-10 models, the top of the valence band is dominated by the O(2p)

orbitals of the axial oxygens (type I oxygens, OI), which remain unchanged regardless of

the V (V4+ and/or V5+) concentration throughout the …M-O-M-O-M… (M = Ti or V)

chains. On the other hand, the bottom of the conduction band, dominated by the metal-d

orbital, is significantly changed upon V substitution and oxidation state of the V atom

(V4+ and/or V5+) throughout the chains. As previously mentioned, the addition of V4+

results in the addition of an extra electron to the system, but does not alter the net charge.

This extra electron occupies the localized V4+(dxy) mid-gap state, which as a result

introduces a “metal-to-metal” transition from the V4+(dxy) mid-gap state to the metal

(dxz/dyz) state at the bottom of the conduction band (Figure 8). Since V5+ is isoelectronic

(electronic configuration: 1s22s22p63s23p6) with Ti4+, the mid-gap energy state remains

unoccupied. Further studies [23] were performed to investigate the excess electron and

16

hole states by probing the highest occupied molecular orbital (HOMO) of the negative

ion (i.e., O(2p)) and the lowest unoccupied molecular orbital (LUMO) of the positive ion

(i.e., Ti(3d) or V(3d)). For mixed V4+/V5+ substituted models (i.e., 2-V4+/1,3-V5+), an

excess electron is delocalized along the V5+(3d) states thereby becoming an electron trap,

whereas hole traps occur along the metal V4+(3d) states. In addition, the lowering of the

conduction band energy occurs due to the high oxidation state and the small ionic radius

of V5+ (see Section 2.3.2 for further discussion).

Figure 8: Band structure of ETS-10, 2-V4+, 2-V5+, and 1,2,3-V5+ models illustrating

the addition of a mid-gap state upon V (V4+ or V5+) substitution [24]. To investigate the effect of V substitution on the optical and electronic properties

of ETS-10, the authors [24] preformed UV-vis spectroscopic analysis on ETS-10

samples experimentally substituted with 11, 33, 43, and 100% V (Figure 9). All V-

substituted samples exhibited two low-energy optical transitions at 2.1 eV and between

2.5 and 4 eV attributed to the localized V(dxy) mid-gap states, which were not observed

in the unmodified ETS-10 spectra. The authors assigned the optical transition at 2.1 eV

17

to the metal-to-metal transitions of octahedral V4+. The optical transitions between 2.5

and 4 eV have been assigned to a range of band gap energies (O(2p) → metal(d xy/dyz)) in

all V-substituted samples. As illustrated in Figure 9a, the low-energy absorption edge for

all partially substituted V samples shifts to lower eV values with increasing V

substitution. The authors attributed these shifts to the lowering of the conduction band

energy. This is in agreement with the theoretical calculations performed by Shough et al.

[23,24], as discussed above. However, upon complete V-substitution (100% V, AM-6),

the low-energy absorption edge shifts to higher eV values with respect to the partially

substituted samples (33% and 44%), yet is still lower compared to that of unmodified

ETS-10. In addition, the authors observed a new absorption feature in the 2.5 – 3 eV

range (Figure 9b), which were attributed to the OI(2p) → V5+(dxy) transition of octahedral

V5+ [31].

Figure 9: (a) UV-vis spectra for ETS-10 samples substituted with 13, 33, 43, and 100%

V, and (b) magnification of the low energy region of the spectra shown in (a) [24].

The experimental UV-vis spectroscopic results suggest that the incorporation of

V5+ and V4+ species into the …M-O-M-O-M… chains (partial and complete V-

substitution of ETS-10) results in the lowering of the optical bandgap energy with the

appearance of new low-energy optical transitions in the visible light range. These

18

findings provide a new window of opportunity in exploring fully V-substituted ETS-10

(AM-6) as a potential visible light photocatalyst.

19

2.0 CRITICAL LITERATURE REVIEW

Vanadosilicate AM-6 is a large-pore microporous material, isostructural with

titanosilicate ETS-10 [17]. Microporosity, pore regularity, and the presence of

stoichiometric amounts of vanadium in the silicate framework make AM-6 a promising

material in both traditional [27,28] and advanced zeolite-type materials applications

[19,30,31]. Recent results have shown one dimensional ferromagnetic ordering along the

monatomic …V-O-V-O-V… chains in AM-6 at temperatures below 50 K. Therefore,

AM-6 has the potential to be used as a porous magnet with properties that can be

modulated by incorporation of magnetic guest molecules [19]. The monatomic …V-O-

V-O-V… chains in AM-6 are similar to the …Ti-O-Ti-O-Ti… chains in ETS-4 and ETS-

10, which have shown potential for use in quantum wires applications [18,29,30]. As its

titanosilicate ETS-10 analogue, AM-6 retains the photocatalytic properties of crystalline

TiO2, but has a more flexible chemical structure than the latter. Unlike TiO2 and ETS-10,

AM-6 has exhibited absorption features in the visible light range, due to the presence of

V4+ and V5+ along the …V-O-V-O-V… chains, making this material a promising visible

light photocatalyst [31]. The diverse potential applications require AM-6 crystals with

controlled characteristics (such as purity, size, morphology, topography, defect

concentration, etc.). However, to date, AM-6 has only been synthesized using ETS-10

seeds [17,31], and no control of the crystal characteristics was reported. The use of

titanosilicate ETS-10 crystals in the synthesis of vanadosilicate AM-6 limits the ability to

control product characteristics without also influencing the product titanium content.

Therefore, an unseeded method for crystallizing AM-6 is essential.

20

The proximity of the V4+ and V5+ along the monatomic …V-O-V-O-V… chains

in AM-6 was shown to promote the electron-hole recombination in AM-6 products

synthesized in the presence of titanosilicate ETS-10 seeds, which decreased the

photocatalytic activity [31]. Due to the more flexible structure of AM-6 relative to the

conventional TiO2 photocatalyst, the separation of these V atoms is hypothesized to be

possible through surface or framework modifications. These modifications may also

modify the optical and electronic properties, and thus photocatalytic performance, of

AM-6.

2.1

Large-pore microporous vanadosilicate AM-6 has recently attracted interest in

UV and visible light photocatalysis [31]. However, to date AM-6 has only been

synthesized using seeds of titanosilicate ETS-10 [17,31]. For given applications, crystal

characteristics such as size, morphology, topography, and defect concentration, as well

as product purity may be of interest. Therefore, it is necessary to establish an unseeded

crystallization method in order to better control the desired crystal characteristics without

affecting the purity of the product (i.e., titanium-free product) and to elucidate the

mechanism of crystal formation/growth.

Vanadosilicate AM-6

2.1.1 Seeded Synthesis of AM-6

To date, AM-6 has only been synthesized using seeds of its analogue,

titanosilicate ETS-10 [17,19,31,32,33,34]. Rocha et al. [17] first synthesized AM-6 in

1997. A typical synthesis procedure was described as follows: first, an alkaline solution

was prepared by mixing 6.26 g sodium silicate (8 wt.% Na2O, 27 wt.% SiO2), 8.03 g

21

deionized (DI) water, 0.20 g sodium hydroxide (NaOH), 0.99 g potassium chloride (KCl),

and 3.08 g sodium chloride (NaCl). A second solution was prepared by mixing 7.60 g DI

water and 1.50 g vanadyl sulfate (VOSO4·5H2O). The two solutions were combined and

0.10 g of ETS-10 seeds were added to the resulting gel, with a molar composition of

Na2O : 0.23K2O : 0.97SiO2 : 0.10V2O5 : 30H2O. Crystallization was carried out for 3

days at 230 oC under autogenous pressure. The X-ray powder diffraction (XRD) patterns

of AM-6 obtained using this procedure and ETS-10 are very similar (Figure 10).

However, the AM-6 reflection at ~ 24.7o 2θ appears to have a slightly smaller d-spacing

value (3.599 Å) than that of ETS-10 (3.614 Å). Similar results have been reported

[19,31,34]. This is consistent with the fact that V4+ (ionic radius: 0.72 Å [25]) and V5+

(ionic radius: 0.68 Å [25]) ions are slightly smaller than Ti4+ (ionic radius: 0.745 Å [25])

ions. In addition, the AM-6 product appeared to contain a small amount of quartz

impurity as indicated by the presence of the XRD reflection at ~ 26.6o 2θ. However, the

authors only reported the presence of quartz impurity through SEM analysis.

Figure 10: XRD patterns of titanosilicate ETS-10 and vanadosilicate AM-6 [17].

The authors also performed Raman spectroscopic analysis of their AM-6

products (Figure 11) and ETS-10 for reference. The Raman spectrum of ETS-10

22

exhibited a characteristic strong band at 730 cm-1, typically associated with TiO6

octahedra; whereas the Raman spectrum of AM-6 gave a strong sharp signal at 870 cm-1.

The authors attributed this signal to VO6 octahedra. In addition, a weak band at 946 cm-1,

attributed to the presence of terminal V=O bonds in defect sites or on the crystal surface,

was observed. Even though the authors showed a magnified AM-6 Raman spectrum (x14

magnification, Figure 11), they did not address the appearance of the weak band at ~730

cm-1, typically attributed to TiO6 octahedra, which may be due to the ETS-10 seeds used

in the synthesis preparation.

Figure 11: Raman spectra of ETS-10 and AM-6 measured at room temperature

[17]. The presence of V4+ was confirmed through EPR, which displayed a single broad

resonance centered at about g = 1.9545. When the AM-6 product was heated at 400 oC,

the structure partially collapsed. Upon heating at 450 oC, the product was completely

amorphous as suggested by XRD analysis. These results suggest low thermal stability of

AM-6 products synthesized in the presence of ETS-10, as compared to its analogue ETS-

10. Therefore, methods that produce products with higher thermal stability should be

pursued.

23

To eliminate the quartz impurity phase accompanying the AM-6 product obtained

from the synthesis procedure utilized by Rocha et al. [17], Nash et al. [31] and Yeates et

al. [19] carried out the same AM-6 synthesis procedure but at a lower temperatures (185

oC and 195 oC, respectively, vs. 230 oC). Nash et al. [31] also explored the effect of

various amounts of V substituted for Ti in the ETS-10 framework (V/(V+Ti) = 0.11 ±

0.01, 0.30 ± 0.02, 0.41 ± 0.02, and 0.99 ± 0.01). A typical synthesis procedure [31]

(V/(V+Ti) = 0.33) was described as follows: two solutions were prepared and then mixed

to form the synthesis gel. One solution contained 11.13 g sodium silicate (14% NaOH,

27% SiO2), 2.24 g KCl, 1.70 g NaCl, and 7.42 g DI water. The second solution contained

14.85 g DI water, 0.80 g P25 TiO2, and 1.27 g VOSO4·5H2O. The pH of the gel was

adjusted to 10.4 using either NaOH or HCl. Synthesis of AM-6 crystals was carried as

reported by Rocha et al. [17] using ETS-10 seeds.

The XRD patterns of the product showed only AM-6 reflections, with no

evidence for the presence of quartz impurity phase. However, the vanadium-incorporated

ETS-10 (ETVS-10) samples contained anatase (from the unreacted Ti source used in

ETS-10 synthesis) and quartz impurities, with the amount of quartz impurity decreasing

with increasing V loading level (no quartz present for the V/(V+Ti) = 0.43). The average

size of the AM-6 crystals (~1.3 µm), as inferred from SEM analysis (Figure 12), was

larger than the average size of the ETS-10 crystals (~0.5 µm) which were used as seeds

in the synthesis procedure. Since the crystal growth mechanism of AM-6 was not

reported [17,19,31,32,33,34], the possible growth of the ETS-10 seeds, resulting in the

final crystals with an ETS-10 core and AM-6 shell (i.e., core-shell morphology), cannot

be ruled out. Another possibility involves ETS-10 seeds providing the “initial breed”

24

nuclei from which AM-6 crystals develop with the seed crystals either growing

heteroepitaxially or not growing at all. Similar to the ETS-10 crystal morphology (Figure

12a), AM-6 crystals exhibited truncated square bipyramidal habit (Figure 12b). However,

unlike ETS-10, the ETVS-10 crystals (Figure 12c) exhibited a plate-like morphology

with a different degree of development of the square and trapezoidal facets than ETS-10

and AM-6 crystals.

Figure 12: SEM images of (a) ETS-10, (b) AM-6, and (c) 0.43ETVS-10 crystals [31]. The UV-vis spectrum of the AM-6 product showed a significant difference

compared to the spectra of ETS-10 and P25 TiO2 products (Figure 13). In addition to the

red shift of the AM-6 spectrum relative to that of the ETS-10 spectrum, three new

absorbance features in the visible light region were observed at 450 nm, 594 nm, and

above 800 nm. The authors attributed the absorbance at 450 nm to a charge-transfer

transition that occurs in V5+, and the 594 nm absorbance to either metal-to-metal

transitions associated with V4+ or to a charge-transfer transition, suggesting the presence

of two oxidation states in the partially (ETVS-10) and completely (AM-6) vanadium-

substituted ETS-10 samples. The photocatalytic properties of these products will be

discussed in Section 2.1.2.

25

Figure 13: UV-vis spectra of ETS-10, P25, and AM-6 products [33].

The broad and diversified range of traditional and potential advanced applications

(such as using AM-6 as a potential visible light photocatalyst) require AM-6

crystals/products with controlled and varied characteristics, such as size, morphology,

topography, defect concentration, and purity. As mentioned above, the AM-6 crystal

growth mechanism was not reported, and only one synthesis composition was explored

[17,19,31,32,33,34]. Additionally, control of the product characteristics cannot be

achieved via the reported synthesis procedure without directly influencing the product

(and possibly crystal) purity (i.e., Ti/P25 TiO2 content as a result of using ETS-10 seeds).

Therefore, it is necessary to establish an unseeded crystallization method in order to

obtain “pure” (i.e., titanium-free) AM-6 for fundamental (structural) studies as well as

for applications (such as photocatalysis) that may require crystals/products with

controlled characteristics/purity.

26

2.1.2 Photocatalytic Activity of Seeded AM-6

Nash et al. [31] explored the photocatalytic activity of AM-6, ETS-10, and

ETVS-10 in the polymerization of ethylene by recording changes to the resulting

polyethylene product IR peak areas (Figure 14) in the C-H stretching region. The

0.43ETVS-10 (V/(V+Ti) = 0.43) sample showed the highest activity in the

polymerization of ethylene when compared to ETS-10, 0.33ETVS-10, and AM-6 under

UV light irradiation. However, the size and morphology of these samples were not

comparable. As previously discussed (Section 3.1.1), ETS-10 crystals were ~0.5 µm in

size (Figure 12a), whereas the AM-6 crystals were ~1.5 µm in size (Figure 12b). As with

all photocatalytic investigations, the activity is dependent on the ability of the material to

produce electron-hole pairs, the adsorption effects of a given material, and the amount of

defects present in the material. The authors did not report any surface area measurements

on their materials. Taking into consideration the variation in the size and degree of

development of the square and trapezoidal facets of ETS-10, ETVS-10, and AM-6

crystals, and the fact that the concentration of structural defects is related to the

topography/morphology [49], the adsorption of ethylene may have varied from sample to

sample, thereby affecting the photocatalytic activity. In addition, the 0.43ETVS-10

product contained anatase impurities, which may have enhanced the photocatalytic

activity of the material under UV light irradiation. Therefore, it is necessary to synthesize

materials of high purity and comparable size, topography and morphology in order to

effectively investigate and compare their photocatalytic activities. In addition, no

photolysis studies were investigated. Therefore, the reported activity results appear to be

inconclusive.

27

Figure 14: IR peak area of polyethylene versus reaction time for conversion of

ethylene on ETS-10, 0.43ETVS-10, 0.33ETVS-10, AM-6, and P25 TiO2 irradiated under UV light [31].

In order to investigate the photocatalytic activity of the 2.5-3 eV (~450 nm) peak,

attributed to the V5+ along the …V-O-V-O-V… chains, the authors [31] applied a 400

nm filter (400-800 nm irradiation range). Both the ETVS-10 (V/(V+Ti) = 0.43) and AM-

6 samples showed visible light photocatalytic activity towards the polymerization of

ethylene. However, the 0.43ETVS-10 sample showed greater activity than AM-6 sample.

This may be attributed to the presence of the V5+ sites (electron traps) in close proximity

to the V4+ sites (hole traps) along the …V-O-V-O-V… chains in AM-6, which may

promote electron-hole recombination thereby lowering the photocatalytic activity

[23,24,31]. At low concentrations of V, the two different oxidation states of vanadium

are hypothesized to be far enough from one another, which increases the charge

separation in the semiconductor. However, the 0.33ETVS-10 sample, which should have

the two vanadium oxidation states far enough from each other, showed poor

photocatalytic activity when compared to AM-6 and 0.43ETVS-10 under UV light

irradiation (Figure 14). Thus, the hypothesis that the presence of the V5+ sites (electron

28

traps) in close proximity to the V4+ sites (hole traps) along the …V-O-V-O-V… chains in

AM-6 results in a decrease of the photocatalytic activity of AM-6 has not been proven

definitively. To determine the photocatalytic activity of the 2.1 eV (~590 nm) peak,

attributed to the V4+ along the …V-O-V-O-V… chains, a 515 nm filter (515-800 nm

irradiation range) was employed. The authors reported no photocatalytic activity for

either of the samples, which suggested that no charge separation occurs at this transition

(OI(2p) → V 4+(dxy) transition) alone [31]. Therefore, in order to improve the

photocatalytic activity of pure vanadosilicate AM-6, it is probably necessary to separate

the different oxidation states and/or control the ratio of V4+/V5+ throughout the

monatomic semiconductor chains.

2.2

Organic structure directing agents (SDAs) have been used in the syntheses of

various zeolite and zeolite-type materials [35,36,37,38,39,40,84,85]. These SDAs are

said to affect the gel chemistry of synthesis mixtures resulting in structures that

otherwise would not be synthesized in the absence of SDAs. Zeolite nucleation and

crystal growth that occur in the presence of SDAs are hypothesized [86] to be affected by

the inorganic-organic composites that are produced from the ordering of water and silica

and/or aluminosilicate species present in the synthesis mixtures [84].

Unseeded Synthesis Techniques – Structure Directing Agents

2.2.1 Synthesis using SDAs

Valtchev [36] investigated the “structure directing” role of various organic bases,

such as pyrrolidine (Py), tetramethylammonium chloride (TMACl), tetraethylammonium

chloride (TEACl), tetrepropylammonium bromide (TPABr), tetrabutylammonium

29

chloride (TBACl), 1,2-diaminoethane (En), and 1,6-diaminohexane (Dh) towards the

crystallization of ETS-10. At 473 K and in the absence of organic base, ETS-10

crystallized in 7 days. An increase in crystallization time was observed for products

synthesized in the presence of En, Py, TEACl, and TPACl. Additional impurity phases,

such as, quartz, ETS-4, crystobalite, and a molecular sieve with the MFI-type structure,

were also observed. No quantitative chemical analysis of these products or pH of the

parent gel/reaction mixture were given; therefore, these impurities may be due to high

mixture Si/Ti ratio and/or pH of the synthesis mixtures. The use of TBABr and Dh did

not produce ETS-10 product [36]. Only TMACl resulted in a pure, fully crystalline ETS-

10 product (as determined by XRD and SEM) with a decrease in the crystallization time

from 7 days to 2 days at 200 °C. Therefore, these results suggested the “structure

directing” role of TMACl and the “structure breaking” role of the other organics

investigated (i.e., En, Py, TEACl, TPACl, TBABr, and Dh). Similar observations were

reported by Valtchev and Mintova in the synthesis of ETS-10 using TMACl [37].

Typically, the decomposition temperature of TMA is in the rage of 240-400 °C [36,37].

However, the increase in decomposition temperature to 290-570 °C, as determined by

both differential thermal analysis (DTA) and thermogravimetric analysis (TGA),

suggested that TMA is present in the pores of ETS-10 [36,37]. In addition, Valtchev and

Mintova [37] reported a substantial decrease in the Na+ ions and water content in ETS-10

products synthesized in the presence of TMACl when compared to ETS-10 products

synthesized without TMACl. These results suggest that the TMA+ ions replace the

hydrated Na+ ions, which is in agreement with the similar size of hydrated Na+ (2.51 Å

[87]) and TMA+ ions (~6.0-7.4 Å [37]).

30

Das et al. [35] investigated the effect of SDAs that were significantly different in

structure from TMA (such as choline chloride and hexaethyl diquat-5) in the synthesis of

ETS-10. Both choline chloride and hexaethyl diquat-5 resulted in ETS-10 products as

determined by XRD. However, the crystal Si/Ti ratio from the chemical composition

analysis was higher in these products (i.e., 6.54 and 6.37 for ETS-10 synthesized using

choline chloride and hexaethyl diquat-5, respectively) than the theoretical value of ETS-

10 (i.e., Si/Ti = 5.0) [20,22]. The authors [35] concluded from the chemical composition

analysis that choline chloride and hexaethyl diquat-5 replace the hydrated K+ ions in

ETS-10 since the K/Ti ratio for ETS-10 synthesized using choline chloride and

hexaethyl diquat-5 (0.36 and 0.26, respectively) were much lower than that of ETS-10

synthesized without organic (0.76) [35]. However, these ETS-10 crystals were

synthesized using ETS-4 seeds which could result in an increase in the K/Ti ratio,

thereby making this analysis inadequate. The morphology of ETS-10 crystals varied

significantly when choline chloride (cuboidal crystals) was used in synthesis compared

to hexaethyl diquat-5 (“wheat-shaped” polycrystallites).

Yang et al. [38] investigated the influence of tetramethylammonium chloride

(TMACl) on the size and the degree of development of the square and trapezoidal faces

of titanosilicate ETS-10 crystals. Upon addition of TMACl, the crystal size decreased

from ~4 µm (Figure 15a) to ~1 µm (Figure 15b), and an increase in the crystal aspect

ratio (i.e., decrease of the size of the square face relative to the size of the trapezoidal

face) was observed. The authors [38] however did not correlate the variation in size and

degree of development of the square and trapezoidal faces with the well-known ETS-10

crystal growth mechanism [89]. Since ETS-10 grows via a two-dimensional nucleation

31

crystal growth mechanism [89], the increase in the crystal aspect ratio (and decrease in

crystal size) upon addition of TMACl (Figure 15b) implies an increase in the two-

dimensional nucleation rate relative to the layer lateral spreading rate. Pure (i.e.,

amorphous/unreacted material-free) ETS-10 products were obtained upon addition of

TMACl [38].

Figure 15: SEM images of ETS-10 crystals synthesized in the absence of TMACl (a),

and in the presence of TMACl (b) [38].

Kim et al. [39] investigated the effect of various tetraalkylammonium salts (TAA:

TMA, TEA, and TPA) and ethanolamine (EA) on the morphology and crystallization

times of titanosilicate ETS-10 crystals. At 473 K and 24 hours, fully crystalline ETS-10

was synthesized from mixtures with molar composition 4.2Na2O : 1.5KF : 5.5SiO2 :

1.0TiO2 : 300H2O. The resulting crystals exhibited well-defined square truncated

bipyramidal morphology with sharp edges (Figure 16a). A slight change in the crystal

morphology (relatively thick square plate-like crystals with truncated bipyramidal

morphology with faulting on the sides) was observed upon the addition of 0.2TPA (TPA

molar content = 0.2) (Figure 16b). The addition of 0.2TMA, 0.2TEA, and 0.4EA resulted

in bipyramidal morphologies with round edges (Figure 16c-e). The presence of organic

appeared to accelerate the crystallization of ETS-10. Similar observations were reported

32

by Pavel et al. [40]. Therefore, it is of interest to investigate various types of SDAs in

efforts to obtain (without the use of ETS-10 seeds) pure AM-6 products with various

morphologies, and decrease the crystallization times.

Figure 16: SEM images of ETS-10 products obtained from mixtures with molar

composition 4.2Na2O : 1.5KF : 5.5SiO2 : 1.0TiO2 : 300H2O : xOrganic at 473 K: (a) x = 0, 24 hr; (b) x = 0.2 TPA, 15 hr; (c) x = 0.2 TEA, 15 hr; (d) x = 0.2 TMA, 18 hr; (e) x = 0.4 EA, 24 hr [39].

Calcination in air at 450 °C for 5 h of ETS-10 products obtained using TMACl

was sufficient to successfully remove the organic from the ETS-10 pores (while

preserving the framework structure) as determined by DTA and TG analysis [38]. Two

weight loss steps were observed: the first weight loss step (9.3 wt.%) was below 250 °C,

attributed to dehydration; whereas the second weight loss step (5.2 wt.%) was above

250 °C, attributed to the burning-off of the organic species. Similar results were

previously reported for ETS-10 synthesized using TMA [36,37,39,40]. Therefore,

calcination in air will be carried out in efforts to remove the SDA from the micropores of

AM-6.

33

2.2.2 Effect of SDA on Crystallization Kinetics

The relative rates of nucleation and crystallization in zeolite synthesis can be

described based on crystallization curves obtained at different temperatures [41]. The

influence of the SDAs during the different stages of ETS-10 crystallization was

described in several reports [35,37,39,40] through the kinetic analysis of the

crystallization process using the modified Avrami-Erofeev equation (Equation 3):

α = 1- exp [-k(t – t0)n] Equation 3

Warzywoda et al. [42] suggested that factors other than the crystallization of

zeolites (e.g., initial rearrangement of species in the reaction mixture) may affect the

induction time, i.e., the nucleation time before the appreciable amounts of crystalline

phase are observed in the crystallizing system by e.g., X-ray powder diffraction, and

therefore the Avrami model may be misleading during the transition from the nucleation

to the growth stages [43]. Therefore, Valtchev and Mintova [37] introduced a transition

period to separate and better define the nucleation and crystallization. A point that

corresponded to 15% crystallinity of the product was used as the upper limit of the

transition period for all three crystallization curves obtained at the different temperatures

(200, 180, and 160 °C). The synthesis of ETS-10 in the presence of TMA was shown to

be energetically more favorable (i.e., TMA accelerates the nucleation and crystal growth

for ETS-10) and decreased the nucleation time and accelerated the crystal growth rate

when compared to ETS-10 synthesized without TMA [37]. However, the use of an

arbitrary point (i.e., 15% crystallinity) in different crystal growth rates could lead to an

under- or over-estimation in the apparent activation energy for the transition period. To

compensate for this, Kim et al. [39] used a different approach in which the nucleation

34

region was subdivided into two regions: the induction period during which no crystals

were detected, and the transition period where “slow” crystal growth was observed. The

crystallinity in the transition period (where “slow” crystal growth occurs) was

determined by differentiating Equation 3 twice and setting the resulting expression to

zero. A tangent line was then drawn from the inflection point to the time axis which

provided the intercept (ttr) on the time axis. A vertical line was then drawn leading to the

corresponding slope (dα/dt )tr and crystallinity (αtr). Figure 17 summarizes the procedure

for determining the nucleation, transition, and crystallization stages on the crystallization

curve of ETS-10 [39].

Figure 17: Scheme for determining nucleation, transition, and crystallization stages

on the crystallization curve of ETS-10 [39].

Similar to what was reported by Valtchev and Mintova [37], Kim et al. [39]

indicated the crystallization of ETS-10 was accelerated upon addition of organic. The

reported activation energies increased in the following order: TMA < TEA < TPA < EA

< no organic [39]. Pavel et al. [40] reported similar observations with the rate of

35

crystallization for ETS-10 increasing in the following order: TMABr < TEABr < TPABr

< TBABr < no organic. This may be due to the hydrophilic character of the TMA+ cation

compared with the hydrophobic character of the TBA+ cation [40].

Crystallization kinetics at 473 K for ETS-10 synthesized in the presence of

choline chloride suggested that the induction period was longer than for ETS-10

synthesized in the presence of hexaethyl diquat-5 [35]. These results indicated that the

formation of nuclei is slower when choline chloride is used as an SDA than when

hexaethyl diquat-5 is used. When compared to ETS-10 synthesized in the absence of any

SDA, increase in SDA concentration resulted in an increase in the crystallization time

regardless of the SDA used, and a decrease in the % crystallinity for products obtained

using choline chloride. These results are in disagreement with previous observations

using templates such as TMA [36,37,39,40], En [36], Py [36], TEA [36,39,40], and TPA

[36,39,40]. Therefore, the effect of type of SDA and temperature on AM-6 crystallization

kinetics will be investigated.

2.3

As previously discussed, for AM-6 to be effectively utilized as a photocatalyst, it

is necessary to separate the two oxidation states of vanadium present in the monatomic

semiconductor chains and increase the number of surface defects (i.e, active sites). The

separation of the two oxidation states and the increase in the number of active sites can

be achieved by in situ isomorphous framework substitution using other transition metals

and/or via post-synthesis modifications, respectively. Since AM-6 is a relatively new

material, literature regarding modifications of this material is limited. The sections below

Methods of Improving Photocatalytic Activity

36

discuss various techniques for surface and framework modifications of conventional

photocatalytic materials, such as TiO2 and ETS-10 (i.e., analogue of AM-6).

2.3.1 Surface Modifications

The idealized framework of AM-6 consists of corner-sharing SiO4 tetrahedra and

VO6 octahedra linked through the bridging of oxygen atoms [17]. The well-defined

monatomic semiconductor …V-O-V-O-V… chains separated by the silica matrix [17]

makes this material viable for advanced applications such as photocatalysis. Considering

the photocatalytic mechanism (Table 1), in order to increase the photocatalytic activity of

a semiconductor, the number of surface hydroxyl groups, i.e., surface defects, (M-OH

groups, where M = V in AM-6 or Ti in ETS-10, see reaction T3-a in Table 1) should be

increased. Increase of the number of external accessible defect sites (i.e., M-OH sites)

may be achieved through post-synthesis modifications of as-synthesized materials and/or

by changing crystallization parameters to affect the nucleation and crystal growth rates in

the material. Integration of noble metal nanoparticles with semiconductors is another

means for improving the photocatalytic activity of these photocatalysts. These metal

nanoparticles can reduce the electron-hole recombination rate by acting as electron sinks

and/or extending the light absorption into the visible region and enhance the surface

electron excitation as a result of surface plasmon resonance (SPR).

2.3.1.1 Inducing Defects via Post-Synthesis Modifications

One means of increasing the number of external accessible defect sites is through

post-synthesis acid treatments [44,45,46,4750]. Llabrés i Xamena et al. [50] reported

more than an order of magnitude increase in the photocatalytic degradation rate towards

37

2,4-dichlorophenol (DCP) and 2,4,5-trichlorophenol (TCP) using HF (1.13 M) -modified

ETS-10 samples. These samples were prepared by contacting 1.5 g of as-synthesized

ETS-10 crystals with 5 mL of HF at various concentrations (0.56 M to 4.52 M) for a

“few seconds”, then thoroughly rinsing with DI water. Upon HF treatment, the Si/Ti

molar ratio progressively decreased from 4.92 (as-synthesized ETS-10) to 4.39 (ETS-10

contacted with 4.52 M HF), which resulted in a 5% loss in crystallinity of ETS-10 as

determined by XRD. In addition, the authors reported a minimal increase in the specific

external surface area from 21 m2 g-1 (as-synthesized ETS-10) to 27-30 m2 g-1 using the t-

plot method (HF-treated ETS-10). High-resolution transmission electron microscopy

(HRTEM) analysis of the HF-treated ETS-10 (Figure 18) showed mild erosion of the

crystallite borders/edges as compared to the as-synthesized material (well-defined and

sharp borders), which suggests that the HF attack affects only the external surface of the

crystals. This mild attack causes disruption of the bridging Ti-O-Si bonds and/or Ti-O-Ti

chains, which may have potentially increased the number of accessible Ti sites (i.e., Ti-

OH groups).

Figure 18: HR-TEM micrographs of as-synthesized ETS-10 (a), and 8 wt.% HF

treated ETS-10 (b) samples [50].

38

Hydrogen peroxide (H2O2) was used as a probe molecule for qualitative

determination of exposed Ti centers in the ETS-10 framework after acid treatment. The

exposed Ti centers can readily interact with H2O2 forming a yellow-orange colored

peroxo-titanite species absorbing at approximately 26000-28000 cm-1 (~360-385 nm).

The intensity of this band increased with higher concentrations of HF, indicating an

increase in accessible Ti-OH sites. However, since H2O2 has been shown to possibly

increase the number of accessible Ti surface sites (i.e., Ti-OH groups) [46] conclusions

must be drawn with caution. Even though the authors [50] have shown an increase in the

accessible Ti centers which resulted in the enhanced photocatalytic activity towards the

degradation of DCP and TCP, the comparable photocatalytic activity towards the

degradation of chlorophenol (CP) of unmodified ETS-10 and HF-modified ETS-10 was

explained by the fact that CP diffused into the pores of ETS-10 and therefore is protected

from photodegradation. The authors hypothesized that the number of UV photons

reaching the internal Ti sites may be lower than at the surface of the crystal. However, no

photon penetration depth calculations were shown to prove this hypothesis.

Goa et al. [44] reported similar findings for ETS-10 samples contacted with

aqueous hydrochloric (HCl) acid, ammonium chloride (NH4Cl), and citric acid. The

liquid to solid ratio was 100 mL to 1 g. After contact for 4 h at room temperature, the

crystals were thoroughly washed with DI water, dried at 373 K for 3 h, and calcined at

723 K. The citric acid treatment involved heating 0.55 g ETS-10 crystals in 0.25 M citric

acid in a 20 mL round bottom flask at 353 K for 4 h. The crystals were then washed

thoroughly with DI water, dried at 373 K for 3 h, and calcined at 723 K for 6 h. XRD

analysis of all treated samples suggested that upon calcination the ETS-10 structure

39

remained intact for HCl concentrations up to 0.02 M, NH4Cl concentrations up to 3.7 M,

and citric acid concentrations up to 0.25 M. Through XRD and elemental EDX analyses

of all the treated samples the authors suggested that the threshold of the amount of Ti

removed from the ETS-10 structure was ~5 mol%. UV-vis spectroscopic analysis (not

shown here) of all the acid-treated ETS-10 samples suggested a change in the local

structure around Ti. In the HCl-treated samples the 215 nm absorption peak, typically

attributed to the OII(2p) → Ti3d charge transfer transition in the Si-OII-Ti groups , and

the 252 and 280 nm peaks attributed to the OI(2p) → Ti3d charge transfer transition in

the Ti-OI-Ti chains disappeared, while the 242 nm peak which was attributed to

pentacoordinated Ti species appeared. The NH4Cl- and citric acid-treated samples also

resulted in a disappearance of the 215, 252, and 280 nm peaks. Therefore, the extent of

disruption throughout the …V-O-V-O-V… chains in modified AM-6 products may be

evaluated using UV-vis spectroscopy.

Lv et al. [45] reported an increase in the amount of accessible Ti centers (i.e., Ti-

OH sites) as determined by the gradual increase of the 360-385 nm absorption shoulder,

attributed to the peroxo-titanate formation, upon treatment of ETS-10 crystals with 1 M

nitric acid (HNO3) with pH ranging from 4-7. Pavel et al. [46] reported similar findings

for ETS-10 samples treated with H2O2 with concentrations ranging from 5 to 30 wt.%.

XRD analysis indicated that all HNO3-treated samples maintained at least 93%

crystallinity under all pH conditions [45]. The gradual disappearance of the 724 cm-1

Raman band, attributed to the Ti-O-Ti chains, with decreasing pH (band disappearing at

pH = 4) suggested partial or complete disruption of the chains [45,47]. Fourier transform

infrared (FTIR) analysis also suggested partial or complete disruption of the chains as

40

indicated by the gradual decrease of the 749 cm-1 band belonging to Ti-O-Ti chains

[45,46]. The partial or complete disruption may lead to a change in the coordination state

of the Ti atoms, which has been shown to improve the photocatalytic properties of the

material [45,46,47]. In addition to the disruption of the Ti-O-Ti chains, Pavel et al. [46]

observed an inversion of the 3732 cm-1 and 3697 cm-1 FTIR bands, typically attributed to

Si-OH, and Ti-OH groups respectively, with the 3732 cm-1 band being more intense

upon treatment of ETS-10 crystals with 30 wt.% H2O2. These results may suggest an

increase in the number of Si-OH groups due to the removal of Si species from the silica

matrix. Increase in the number of Si-OH groups was also inferred from the appearance of

two broad low intensity signals (~ -90 ppm and -102 ppm, attributed to Si(3Si, 1 Ti)) in

the 29Si CP-MAS NMR spectra [45,47], and the disappearance of the -103.8 ppm signal,

typically attributed to Si(4Si, 0Ti), in 29Si MAS NMR spectra suggesting the breaking of

Si-O-Si bonds [45]. Therefore, even though it appears that post-synthesis acid treatments

may have increased the surface defects (i.e., M-OH groups) [46], it appears that these

surface defects may in fact be attributed to photocatalytically inactive Si-OH groups and

not photocatalytically active Ti-OH groups.

Nash et al. [33] performed NH4+ ion exchange on seeded AM-6 products in

efforts to change the coordination environment and increase the defect concentration. 1 g

of AM-6 crystals, synthesized using ETS-10 seeds [31], was added to 500 ml of 0.1 M

NH4NO3 and allowed to exchange for 19 h at 70 °C. EDX analysis showed a substantial

decrease in the Na+ and K+ ion content upon ion exchange. It was suggested that the

NH4+ ions replace the Na+ and K+ cations inside the pores. The slight increase in the

micropore volume upon ion exchange further confirmed the replacement of Na+ and K+

41

cations with the NH4+ ions, which was expected since the molar volume of the

ammonium ion is larger than that of a proton. Change in the coordination environment

and/or oxidation state of V only occurred upon dehydration of the ion-exchanged

samples as suggested by the red shift of absorption edge in the AM-6 spectra and the

increase in intensity of the AM-6 absorption features (Figure 19a,3). Similar results

were obtained from Raman analysis (Figure 19b,3). As-synthesized AM-6 exhibited the

typical 867 cm-1 peak, attributed to V-O-V stretch along the VO6 chains, and a weak

peak around 939 cm-1, attributed to terminal V=O (Figure 19b,1). Upon ion exchange,

the 867 cm-1 peak nearly disappeared, suggesting that only some of the V in the chains

remained unaffected (Figure 19b, 2). However, after dehydration of the NH4+-exchanged

sample, a substantial increase of the 939 cm-1 peak intensity was observed (Figure 19b,3),

which may imply that the V atoms in AM-6 changed coordination from octahedral to

tetrahedral upon dehydration. FTIR analysis suggested an increase in the tetrahedral V-

OH groups upon ion exchange.

Figure 19: UV-vis (a) and Raman (b) spectra of AM-6 before NH4

+ exchange (1), after NH4

+ exchange (2), and after exchange and dehydration (3) [33].

42

The authors concluded from XRD data that the AM-6 structure remained intact

after ion exchange and dehydration. However, a substantial decrease in XRD peak

intensity and broadening of the peak was observed. Therefore, it appears that dehydration

causes some structural collapse. The authors [33] reported complete oxidation of

ethylene to CO2 on ion-exchanged and dehydrated AM-6 under UV light irradiation.

Similar results were reported for NH4+-exchanged ETS-10 (heated to form H+-ETS-10)

in the gas-phase photooxidation of ethylene under UV light irradiation [48]. The authors

[48] attributed this enhanced activity to the generation of the photocatalytic sites (i.e., Ti-

OH groups) on the external surface of NH4+-exchanged ETS-10. However, since ion

exchange followed by dehydration [33,48] and post-synthesis acid treatments

[44,45,46,47,50] caused partial structural collapse, and since AM-6 is much less stable

than ETS-10 [17], other means will be investigated in efforts to change the coordination

and/or oxidation state of the V atoms in unseeded AM-6 while maintaining structural

integrity.

2.3.1.2 Inducing Defects via Crystallization Parameters

Another means of increasing the number of active sites (i.e., surface hydroxyl

groups), without affecting the crystallinity of the material, is by controlling the relative

nucleation and growth rates [49]. By controlling synthesis parameters such as

crystallization temperature (200 oC vs. 170 oC) and time (2-11 days), Southon and Howe

[49] prepared seven batches (A-G) of fully crystalline (as indicated by XRD analysis)

titanosilicate ETS-10 samples with varying defect concentrations, all with near identical

Si/Ti ratio and moderate variance in the cation/Ti ratio (1.95 to 1.73). According to the

two-dimensional nucleation crystal growth mechanism [20], the defects in ETS-10

43

crystals occur where the spreading surface nuclei (islands) meet during crystal growth

and the concentration of structural defects in ETS-10 crystals is related to their

morphology/topography (i.e., crystals with more structural defects have more faulted

topography) [49]. This was observed for all ETS-10 samples (Figure 20) which

suggested crystallization parameters affect the faulting on the crystals surfaces.

Figure 20: SEM images of ETS-10 crystals with varying defect concentrations: (a) T

= 200 oC, 2 d, Si/Ti = 4.97, cation/Ti = 1.95; (b) T = 200 oC, 6 d, Si/Ti = 5.02, cation/Ti = 1.90; (c) T = 200 oC, 4 d, Si/Ti = 4.94, cation/Ti = 1.78; (d) T = 200 oC, 1 d, Si/Ti = 4.98, cation/Ti = 1.84; (e) T = 200 oC, 2 d, Si/Ti = 4.97, cation/Ti = 1.75; (f) T = 200 oC, 2.6 d, Si/Ti = 4.87, cation/Ti= 1.70; (g) T = 170 oC, 11 d, Si/Ti = 4.71, cation/Ti = 1.73 [49].

Typically, defects in the ETS-10 framework occur from the termination of the

…Ti-O-Ti-O-Ti… chains and the Ti-O-Si-O-Si linkages, resulting in an increase in the

Ti-OH and Ti-O-Si-OH sites. The broadening and shift to higher frequency of the 724

cm-1 Raman peak (Figure 21a), typically attributed to the Ti-O stretch of the Ti-O-Ti

44

chains, with increasing faulting on the crystal surfaces (Figure 20) suggested disruption

throughout the …Ti-O-Ti-O-Ti… chains [49]. Introduction of stacking defects will

shorten the average Ti-O-Ti chain length.

Figure 21: Raman spectra of ETS-10 samples with varying defect concentration: (a)

T = 200 oC, 2 d, Si/Ti = 4.97, cation/Ti = 1.95; (b) T = 200 oC, 6 d,Si/Ti = 5.02, cation/Ti = 1.90; (c) T = 200 oC, 4 d, Si/Ti = 4.94, cation/Ti = 1.78; (d) T = 200 oC, 1 d, Si/Ti = 4.98, cation/Ti = 1.84; (e) T = 200 oC, 2 d, Si/Ti = 4.97, cation/Ti = 1.75; (f) T = 200 oC, 2.6 d, Si/Ti = 4.87, cation/Ti = 1.70; (g) T = 170 oC, 11 d, Si/Ti = 4.71, cation/Ti = 1.73 [49].

UV-vis spectroscopic analysis was used to determine the effect of stacking

defects on the optical properties of ETS-10 (Figure 22). The asymmetric band centered at

~280 nm, typically attributed to the delocalized transitions along the …Ti-O-Ti-O-Ti…

chains from the top of the valence band (filled O(2p) orbitals) to the bottom of the

conduction band (empty Ti4+(3d) orbitals), was deconvoluted into two Gaussian peaks,

~295 nm (Figure 22, red vertical line) and ~270 nm (Figure 22, blue vertical line). A

third peak ~216 nm (Figure 22, green vertical line) was also observed. Upon increase in

defect concentrations (Figure 20), the 270 nm peak broadened and increased in intensity,

45

whereas the 216 nm peak became much less intense throughout the series. These changes

were partly attributed to the introduction of stacking defects into the …Ti-O-Ti-O-Ti…

chains which shortened the average length of the chains [49]. Similar spectroscopic

characterizations were performed on variously-sized ETS-10 crystals [71,93].

Figure 22: UV-vis absorption spectra of ETS-10 samples synthesized with varying

defect concentrations converted using the Kubelka-Munk function and fitted with two Gaussian peaks and a mixed Gaussian-Lorentzian peak. The solid line passing through the data points is the sum of the fitted peaks [49].

The authors [49] hypothesized that the level of defects in ETS-10 may potentially

enhance the photocatalytic activity through the disruption of the …Ti-O-Ti-O-Ti…

chains thereby increasing the number of accessible sites (i.e., Ti-OH groups). However,

46

no photocatalytic results were presented. Furthermore, Llabrés i Xamena [50] showed a

substantial increase in the photodegradation of DCP and TCP using ETS-10 crystals with

increased specific external surface areas (i.e., increased number of accessible sites).

Therefore, it is of interest to structurally and photocatalytically investigate the

concentration/number of defects in unseeded AM-6 crystals.

2.3.1.3 Integration with Noble Metal Nanoparticles

The integration of noble metal nanoparticles, such as silver (Ag), has been shown

to be an effective way for enhancing the photocatalytic activity of various

semiconductors [11,12,13,14]. Georgekutty et al. [12] reported a substantial increase in

the photodegradation rate of rhodamine 6G under UV light irradiation (four-fold increase)

and visible light (five-fold increase) using Ag-modified ZnO samples as compared to

unmodified ZnO samples under irradiation using a Q-Sun solar light source (0.68 W/m2

at 340 nm). The Ag-modified ZnO photocatalysts were prepared by dissolving silver

nitrate (AgNO3) with various concentrations (1, 3, and 5 mol%) in ethanol and zinc

acetate-oxalic acid solution. The solution was stirred for 2 h at 60 oC, then dried

overnight at 80 oC. The dried gel was then calcined at 400 oC for 2 h to form the Ag-

modified ZnO powder. The XRD patterns of the powders confirmed the presence of

metallic Ag (Ag0) through the appearance of reflections at ~ 38.2, 44.2, 64.4, and 77.4

º2θ, with the peaks increasing in intensity for AgNO3 concentrations increasing from 1 to

5 mol%. These results are in agreement with data for ETS-10 samples modified with Ag0

loading levels greater than or equal to 6 mM AgNO3 (i.e., Ag0-ETS-10-n (n≥ 6)) [51].

Further confirmation of metallic Ag in the samples was obtained from the DSC analysis

which revealed the intense exothermic peak at 390 oC, which the authors reported to be

47

due to the thermal decomposition of Ag2O to metallic Ag0. However, UV-vis

spectroscopic analysis did not show any SPR absorption features that are typically

associated with Ag0. An increase in the absorption spectra of ZnO was observed upon

increasing the Ag mol% concentration in the powders (Figure 23). Typically, the

increase in absorption is attributed to Ag+-exchanged materials [13,63]. The absence of

the surface plasmon resonance absorption bands, even at high concentrations (5 mol%),

was in disagreement with recent findings which reported the appearance of SPR

absorption bands even at very low concentrations (i.e., Ag0-ETS-10-n (n = 3), loading

levels of 3 mM) [51].

Figure 23: UV-vis absorption spectra of ZnO (a) and 1-5 mol% Ag-ZnO (b-d) [12].

The authors [12] performed room temperature photoluminescence (PL) studies

(excitation of 325 nm) to provide information regarding the optical and photochemical

properties of the Ag-modified ZnO powders. The authors attributed the decrease in the

UV emission (390 nm) with increasing silver content to a decrease in the electron-hole

recombination, which was hypothesized to be due to an electron trapping effect of Ag

[12,52]. The transfer of electrons from ZnO to Ag is favored since the Fermi level of Ag

48

lies below the conduction band of ZnO. Therefore, when ZnO is illuminated with energy

of an appropriate wavelength, the conduction band electrons are captured by Ag, which

as a result reduces the electron-hole recombination rate, thus decreasing the emission

intensity (390 nm). However, in the photodegradation of rhodamine 6G an increase in

the intensity of the UV emission (390 nm, ZnO) with increasing amount of Ag was

observed, while the emission at ~550 nm (rhodamine 6G) decreased (Figure 24). The

authors attributed these findings to the degradation of rhodamine 6G via a simultaneous

two-stage charge transfer process: the electrons from the dye dominating the conduction

band of ZnO, while also injecting electrons into Ag.

Figure 24: Room temperature PL spectra of (a) ZnO and (b-d) 1, 3, and 5 mol%

Ag-modified ZnO, respectively, in the presence of rhodamine 6G [12]. The injection of electrons will continue until the overall Fermi level of Ag-

modified ZnO (Ag, 0.15 V as reported by the authors) shifts towards a more negative

potential, then ultimately equilibrating with that of ZnO (-0.8 V vs. NHE). Once this is

achieved, Ag discharges electrons into solution where they react with dissolved oxygen

to form superoxide ions and OH radicals which degrade the dye. Therefore, based on the

49

proposed mechanism, the degradation of rhodamine 6G can be attributed to both the

sensitizing properties of the dye molecule and the electron scavenging role of Ag [12,13].

Seery et al. [13] also reported a four-fold increase in the photodegradation rate of

rhodamine 6G using Q-Sun solar light (0.68 W/m2 at 340 nm) in the presence of Ag-

modified TiO2 (degradation rate of 0.24 min-1) vs. unmodified TiO2 (degradation rate of

0.06 min-1). The Ag-modified TiO2 samples were prepared via addition of silver nitrate

(1, 2, 3 and 5 mol%) to a titanium isopropoxide and acetic acid mixture. A transparent

sol was prepared upon stirring for 8 h. The sol then was irradiated using a 250 W bulb

for 60 min, after which the samples were dried at 100 oC and calcined at 600 oC for 2 h.

XRD analysis of the Ag-modified TiO2 (anatase phase) samples did not show any peaks

corresponding to metallic silver (38.2, 44.2, 64.4, and 77.4 º2θ) for material calcined at

600 oC. This could be due to the incomplete transformation of AgNO3 → Ag2O → Ag.

Calcination at higher temperatures (700 oC) resulted in the appearance of metallic silver,

at the expense of formation of a rutile phase as indicated by the XRD patterns (not

shown here). UV-vis analysis of the prepared Ag-modified TiO2 materials calcined at

600 oC did not show any SPR absorption features typically attributed to the formation of

nanometer-sized metal clusters, which further confirmed the incomplete transformation

into metallic Ag. Therefore, since it is unclear whether the authors used the 700 oC-

calcined material that resulted in Ag0 and a rutile phase or the 600 oC-calcined material

that did not result in Ag0 but remained in anatase phase, the enhanced photocatalytic

activity may be due to either the electron scavenging role of Ag+ ions (i.e., 600 oC-

calcined material) or Ag0 particles (i.e., 700 oC-calcined material). Nonetheless, addition

50

of Ag resulted in enhanced photodegradation of rhodamine 6G under visible light

irradiation.

In photocatalytic systems irradiated under UV light, metal nanoparticles have the

capability of trapping the photoexcited electrons from the semiconductor photocatalyst

(i.e., acts as a sink for the photoexcited electrons), which then leaves the holes for

photooxidation of the organic compounds. Dawson and Kamat [53] reported a 40%

enhancement in the oxidation of thiocyanate under UV light irradiation in the presence of

low concentrations of gold-(Au)-modified TiO2 nanoparticles (quantum yield of 0.09 for

unmodified TiO2 versus 0.13 for Au-modified TiO2 with ratio of 0.17 Au:TiO2). Upon

increasing the Au:TiO2 concentration the photooxidation of thiocyanate decreased,

which was attributed to the inability of the photogenerated holes to reach thiocyanate

[53]. The decrease in photooxidation of thiocyanate may be due to a saturation of Au

nanoparticles on the TiO2 surface, which would result in blocking the accessible sites

(i.e., Ti-OH). Therefore, it is hypothesized that the integration of noble metal

nanoparticles with AM-6 may improve the photocatalytic efficiency, regardless of the

light energy used (UV or visible), by trapping the photoexcited electrons and/or

enhancing surface electron excitation by plasmon resonances excited by visible light.

However, recent reports [54,55,56,57,58,59] have suggested that these nanoparticles

integrated on TiO2 surfaces are unstable during a photocatalytic process. Core-shell type

morphologies have been pursued in efforts to stabilize these nanoparticles

[54,55,56,57,58,59]. Therefore, it is of interest to investigate the stability of these

nanoparticles integrated with semiconductor photocatalysts, such as ETS-10 and AM-6

(See Appendix 9.6 for results).

51

2.3.2 Bandgap Engineering via Framework/Lattice Modifications

The unique and flexible framework of vanadosilicate AM-6 allows the

incorporation of a wide range of dopants, which may improve the photocatalytic

performance of the material. ETS-10, titanosilicate analogue to AM-6, has been modified

with various transition metals [60,63,64,65] and main group metals [61,62,70] in efforts

to enhance its photocatalytic performance. Therefore, it is hypothesized that the

electronic properties of the VO6 chains may be modified through incorporation of

transition metal ions into the chains (i.e., substitution of V in the…V-OI-V-OI-V…

chains) or close to the chains (i.e., substitution of Si in the V-OII-Si group).

2.3.2.1 Transition Metal Isomorphous Framework Substitutions

Isomorphous framework substitutions improve the photocatalytic activity by

introducing new charge transfer transition states without affecting the crystal structure.

Isomorphous framework substitutions can occur either in the …Ti-O-Ti-O-Ti… chains

or the silicate matrix in the case of ETS-10. Eldewik and Howe [60] prepared cobalt-

substituted ETS-10 (ETCoS-10) samples via in situ synthesis through the addition of

cobalt(II) nitrate (Co(NO3)2) to the Ti precursor solution. Inductively coupled plasma

atomic emission spectroscopy (ICP-AES) gave a unit cell composition of

Na1.8K0.33Co0.06TiSi4.93 for ETCoS-10 sample which provided indirect evidence for the

substitution of Co2+ in the framework since the Ti/(Co+Si) ratio was similar to the Ti/Si

ratio of the as-synthesized material (0.20). The (Na+K)/Ti ratio of ETCoS-10 was found

to be slightly higher (2.13) than that of as-synthesized ETS-10 (2.0), which may be due

to Co2+ in the extra framework sites. In addition, XRD patterns of ETCoS-10 showed a

slight shift to lower 2θ angles (a shift of 0.25o for the 24.6o 2θ peak) when compared to

52

as-synthesized ETS-10. The unit cell parameters of both materials were calculated: ETS-

10, a = 14.85 Å, c = 27.08 Å; Co-ETS-10, a = 15.11 Å, c = 27.79 Å. The expansion of

the unit cell coincides with the smaller ionic radius of Si4+ (40 pm) as compared to that

of Co2+ (72 pm). Anderson et al. [61,62] reported similar findings, although smaller unit

cell expansions due to the smaller ionic radii when Al3+ (53 pm) or Ga3+ (61 pm) were

substituted for Si4+ (40 pm) in the ETS-10 framework. The exact location of the Co2+

substitution was difficult to determine by 29Si NMR analysis since Co2+ is paramagnetic

and can affect the surroundings of the nuclei. Therefore, the authors conducted EXAFS

analysis and found that the Si substitution by Co occurs adjacent to the TiO6 chains (i.e.,

substitution occurs in the Ti-OII-Si groups). UV-vis spectroscopic analysis was

performed to investigate the effect of Co substitution on ETS-10 (Figure 25). The triplet

structure band between 14,000 cm-1 and 19,000 cm-1 was attributed to d-d transitions of

tetrahedral Co2+. Upon substitution with Co2+, the band edge red shifted with respect to

the band edge of unmodified ETS-10. The authors attributed this shift to the close

proximity of the Co2+ to the TiO6 chains since the band gap of ETS-10 is associated with

the Ti-OI charge transfer transitions in the Ti-OI bonds perpendicular to the chain axis

(i.e., the bridging of the Si in the in the Ti-OII-Si groups).

53

Figure 25: Uv-vis spectra of (a) ETS-10 and (b) ETCoS-10 [60].

Uma et al. [63] prepared Cr- and Co- isomorphously substituted ETS-10 via in

situ synthesis by the addition of the transition metal salt solution (chromium(III) nitrate

or cobalt(II) nitrate) to the Si precursor solution such that the M/Ti molar ratio is 0.05 (M

= Cr or Co). Crystallization was carried out at 230 oC for 24-72 h. Upon complete

crystallization, the products were washed with DI water, and dried at 110 oC. Selected

ETCrS-10 and ETCoS-10 samples were calcined at 500 oC for 1 h. UV-vis spectroscopic

analysis showed a red shift in the band edge for ETCrS-10 when compared to ETS-10. A

strong absorption around 600-700 nm was observed, which is characteristic of Cr3+ in

octahedral coordination. Similar findings were reported by Brandão et al. [64]. Upon

calcination at 500 oC, the intensity of the 600-700 nm band decreased and an absorption

at around 450 nm appeared, which suggested the oxidation of Cr3+ to Cr4+ (Figure 26).

Cobalt substitution in ETS-10 did not result in a shift of the band edge of ETS-10,

contrary to what has been reported by Eldewik and Howe [60]. This could be due to the

different synthesis techniques and Ti sources used by each group – addition of Co(NO3)2

to the alkaline solution with a Ti source of titanium(III) chloride (TiCl3) [60] versus

addition of Co(NO3)2 to the Si precursor solution with a Ti source of TiO2 (anatase) [63].

54

However, a 500-600 nm absorption was observed which suggested the presence of Co2+

and Co3+, and this absorption feature increased in intensity upon calcination (Figure 26).

Figure 26: UV-vis spectra of Cr- and Co- incorporated ETS-10 samples [63].

Even though the Co- and Cr- substituted ETS-10 samples exhibited absorption in

the visible light range, only the samples that were calcined at 500 oC showed activity in

the decomposition of acetaldehyde under visible light irradiation. However, the authors

[63] did not report activity for the as-synthesized material under visible light irradiation,

therefore making it difficult to attribute the increase in activity to the substitution of Co

and Cr. In addition, calcination at 500 oC typically results in destruction of the

framework. Therefore, if this is the case, which was not reported by the authors, the

increase in the rate of formation of CO2 may be due to defects in the framework and not

the substitution of Co and Cr.

Rocha et al. [65] prepared a titanoniobosilicate ETS-10 (ETNbS-10) with Nb/Ti

molar ratios of 0.12, 0.38, and 0.47 via in situ crystallization technique. A typical

synthesis preparation required the addition of 1.50 g of niobium oxalate (Nb(HC2O4)5 to

the alkaline solution prepared by mixing 10.0 g sodium silicate (8% Na2O, 27% SiO2),

55

15.03 g DI water, 0.80 g NaOH, 0.96 g KF, 0.38 g KCl, 1.02 g NaCl, and 4.63 g TiCl3.

ETS-10 seeds (0.10 g) were added to the final gel. Crystallization was carried out for 3

days at 230 oC. The products were then filtered, washed with DI water, and dried at

ambient temperature. XRD analysis of the products suggested pure highly crystalline

materials for Nb/Ti molar ratios of 0.12 and 0.38. Upon increasing the molar ratio to 0.47

some niobosilicate and quartz impurities were detected by XRD. The XRD patterns of

ETS-10 and all ETNbS-10 samples were identical, and no significant shift of the

reflections was observed. This was anticipated since the ionic radius of Ti4+ (0.745 Å

[25]) is very similar to the ionic radius of Nb5+ (0.78 Å [25]). All the resonances in the

29Si NMR spectra broadened considerably (except for the resonance at – 103.7 pm,

which shifted slightly to lower frequencies in addition to broadening) upon incorporation

of Nb into the ETS-10 framework. Appearance of a broad asymmetric resonance at 100

ppm in the 93Nb NMR spectrum of ETNbS-10 confirmed the presence of distorted

octahedrally coordinated Nb in the framework. Raman spectroscopic analysis (Figure 27)

further confirmed the presence of octahedrally coordinated Nb by revealing the gradual

disappearance of the 735 cm-1 peak, typically attributed to Ti-O-Ti chains, and the

gradual appearance of a band at 664 cm-1, typically attributed to octahedrally coordinated

NbO6, with increasing Nb content. However, the distinct 664 cm-1 peak for sample

containing Nb/Ti=0.47 may also be due to the presence of some niobosilicate impurities

that were reported by the authors. In conclusion, the above results show the feasibility of

substituting transition metals (i.e., Co2+, Cr3+, and Nb5+) into the ETS-10 framework.

56

Figure 27: Raman spectra of ETS-10 and ETNbS-10 [65].

Shough et al. [66] constructed a variety of models to understand and predict the

electronic structure upon the incorporation of transition metals such as V5+, Nb5+, Mo5+,

V4+, Cr3+, Fe3+, and Cu2+ into the octahedral Ti sites of ETS-10. The models constructed

included one (~33% substitution), two (~66% substitution), and/or three (~100%

substitution) substituted Ti atoms in the modeled cluster in order to investigate the

effects of varying the concentration throughout the …M-O-M-O-M… chains (where M =

transition metal). The effects of V4+ and V5+ substitution (partial and complete

substitution) were previously discussed in section 1.4. Therefore, this section will focus

more on the substitution effects using the other transition metals considered (Nb5+, Mo5+,

Cr3+, Fe3+, and Cu2+). The authors reported only 33% and 66% substituted models for

Cr3+ and Fe3+, and 33% substituted model for Cu2+ substitution. The inability to model a

full substitution (100%) was found to be due to the additional counter-ions in the large

channels of ETS-10 that were needed to achieve electroneutrality. The authors suggested

that the ETS-10 structure is unstable at a high concentration of counter-ions present in

the large channels, therefore limiting the full substitution (100%) of the lower oxidation

57

state metals (Cr3+, Fe3+, and Cu2+). The bottom of the conduction band for all the models

studied in this investigation was dominated by the metal d states, thereby making these

energies dependent on the relative orbital energies of the metals coordinated along the

…M-O-M-O-M… chain. The effect of transition metal substitution on the conduction

band energy is essential in predicting the ligand-to-metal charge transfer (LMCT)

transition. Factors affecting the d-orbital energies are: (i) effective charge (i.e., oxidation

state), (ii) ionic radius, and (iii) the number of d electrons. The oxidation state is directly

related to the amount of octahedral splitting, ΔO (i.e., the energy difference between the

splitting of the two sets of d-orbitals). As the oxidation state of a transition metal

increases, the splitting increases, and the t2g orbital energies decrease. Since the t2g orbital

energies are positioned at the bottom of the conduction band, the increase in oxidation

state results in the lowering of the conduction band energy. On the other hand, the ionic

radius is inversely related to the amount of octahedral splitting, thereby making it

directly related to the conduction band energy. For example, smaller ionic radius results

in lowering the conduction band energy. Lastly, increasing the number of d electrons

results in the lowering of the conduction band energy. This is clearly seen in Figure 28 in

the case of substitution of V5+, which has the smallest ionic radius and highest oxidation

state. Therefore, the incorporation of transition metals into the vanadosilicate AM-6

framework will be investigated in efforts to introduce new LMCT transitions and red

shift the bandgap energy.

58

Figure 28: Comparison of electronic structures for ETS-10 and all 33% substituted

transition metal models. The red lines illustrate the h-O(2p) state, the black lines illustrate the unoccupied midgap states, and the gray lines illustrate the occupied midgap state. The light gray rectangles illustrate unoccupied conduction band states, and the dark gray rectangles illustrate occupied valence band states [66].

2.3.2.2 Doping with Nonmetals

Doping with nonmetals, such as carbon (C), nitrogen (N), and sulfur (S) has been

shown as another means to enhance the photocatalytic activity of a given material

without destroying the crystal structure. Ohno [67] prepared S-doped TiO2 photocatalysts

by mixing titanium isopropoxide with thiourea at a molar ratio of 1 to 4 in ethanol. The

resulting product was stirred for 1 h at room temperature and concentrated under reduced

pressure. The white powder was then calcined at various temperatures to obtain the

yellow powder (taken as indication of S-doped material). The observation of the S(2p)

peaks around 168 eV and 167.6 eV, attributed to S6+ and S4+, respectively, indicated

successful incorporation S in the TiO2 lattice. However, considering the ionic radius of

S6+ and S4+, 43 pm and 51 pm, respectively, the incorporation of such oxidation states

59

into the lattice (in place of O2-) seem very questionable. In general, the substitution of S

into the lattice has been reported to be very difficult considering the vast differences in

the ionic radius of S2- (1.84 Å [25]) and O2- (1.40 Å [25]) [69]. Nonetheless, the authors

reported a significant red shift in the UV-vis spectra of S-doped TiO2 calcined at 400 and

500 oC when compared to pure TiO2 powder.

The S-doped TiO2 sample calcined at 500 oC showed the highest activity in the

photocatalytic decomposition of 2-propanol under visible light irradiation. On the other

hand, the S-doped TiO2 sample calcined at 400 oC showed lower degradation rate as

compared to the S-doped TiO2 sample calcined at 500 oC. This difference may be due to

the improved crystallinity of the S-doped samples. Even though the authors showed a

substantial enhancement in the degradation rates under visible light irradiation upon

doping TiO2 with S, no detailed characterization results were reported regarding the

photoactive sites responsible for the degradation of 2-propanol using the prepared

materials.

Burda et al. [68] reported an increase in the photodegradation rate of methylene

blue under UV and visible light irradiation upon doping TiO2 with nitrogen. UV-vis

spectroscopic analysis showed a red shift in the band edge of TiO2 (380 nm for pure

sample) upon incorporation of N (600 nm) (Figure 29). However, it remains unclear

where the N is located with respect to the TiO2 lattice, and what causes the enhanced

photocatalytic activity of the nonmetal-doped materials. A recent review by Kuznetsov

and Serpone [69] suggested that for materials doped with nonmetals, such as N, three

types of doping can occur: substitutional doping, interstitial doping, and a combination

of the two. The authors also suggested that bandgap narrowing may be due to the inter-

60

mixing of the N(2p) with the O(2p) states of the valence band [69], which coincide with

the observed bandgap narrowing of TiO2-xNx (red shift with respect to undoped TiO2).

Figure 29: UV-vis spectra showing a red shift in the band edge due to nitrogen

doping of TiO2 nanoparticles [68]. Recently, Shankar and Ye [70] prepared a C, S, N co-doped ETS-10

photocatalyst (hereafter referred to as vis/ETS-10) by finely milling a 1:1 ratio of as-

synthesized ETS-10 with thiourea followed by calcination at 400 oC for 4 h. The

resulting product was orange-red in color. XPS analysis confirmed the presence of C, S,

and N. UV-vis spectroscopic analysis showed a significant red shift upon doping ETS-10

with thiourea. The bandgap energies of the synthesized products were calculated to be

3.80 eV for as-synthesized ETS-10 and 2.25 eV for vis/ETS-10. Previous reports

attributed bandgap shifting to be dependent upon the length of the …Ti-O-Ti-O-Ti…

chains in ETS-10, i.e., longer chains/higher quality crystals (larger crystals) resulted in a

red shift in the bandgap energy [71,93]. Therefore, any size effects due to finely milling

thiourea with ETS-10 can be neglected. The authors attributed the shift of the bandgap

energy to atomic orbital mixing as a result of partial substitution of oxygen by the

anionic species near the …Ti-O-Ti-O-Ti… chains. However, a recent report claimed the

61

red shift observed after anion doping in the case of TiO2 is due to the formation of color

centers (as a result of the loss of an O atom upon doping) [72]. In addition, bandgap

engineering via substitution of the O atom in the …Ti-O-Ti-O-Ti… chains, and

consequently the …V-O-V-O-V… chains in AM-6, is likely unfavorable since the

oxidation potential of the hole would decrease [73]. Therefore, the more favorable

approach to bandgap engineering will be via transition metal isomorphous framework

substitutions in place of the V (and/or Si) atoms in the AM-6 framework.

Figure 30: UV-vis spectra of various modified and unmodified photocatalysts [70].

2.4

Titanosilicate ETS-10 has been shown to selectively degrade organic

contaminants based on size [50,74,75] and polarity [75]. Calza et al. [74] first reported

the shape selectivity of ETS-10 towards the photodegradation of three molecules with

different steric hindrances: (1) phenol (P), (2) 1,3,5-trihydroxybenzene (3HP), and (3)

2,3-dihydroxynaphthalene (3HPP). All photocatalysis experiments were performed under

identical conditions: 5ml of aqueous suspension containing 1 x 10-4 M organic

compound and 1 g/l ETS-10 catalyst were irradiated using a 1500 W Xenon lamp with a

Photocatalysis using Microporous Materials

62

310 nm cut-off filter (photon flux = 7.9 x 10-6 Einstein min-1). It was unclear from the

experimental section whether these reactions were mass transfer limited or kinetically

controlled, and whether these reactions were taking place in a slurry reactor or an

immobilized reactor. Nonetheless, the authors reported the rate of 3HPP degradation

(4.46 min-1) to be 56 times higher than that of P (0.08 min-1), which was attributed to the

size variation of 2HPP and P (only a schematic illustrated the varying size; no values

were given). The authors concluded based on these results that the “internal cavities” of

ETS-10 behave as a protective environment against the degradation of species that can

easily diffuse into the pores due to their small size. The lower degradation rate of 3HP

(0.38 min-1) however was not addressed. While it was of benefit that the authors

performed these investigations using three molecules of varying steric hindrances, the

polarities of the molecules may have played a role in the degradation rate. According to

the polarity parameter, log S, calculated on the basis of the ALOGPS 2.1 program [76],

the polarity of the studied molecules was as follows (from less polar to more polar):

2HPP, 3HP, and P. According to Shiraishi et al. [75] the more polar the molecule, the

easier it will diffuse into the polar pores of ETS-10 (the presence of Na+ and K+ make the

pore environment of ETS-10 highly polarized [75]). Therefore, the low degradation rate

of 3HP, as well as P, may be attributed to its polarity, in addition to their smaller size

relative to 2HPP.

Shiraishi et al. [75] performed an extensive study in which 22 different aromatic

substrates with varying size and polarity were photocatalytically investigated in an ETS-

10 system. A typical photocatalysis experiment was as follows: In a Pyrex glass tube

(capacity of 20 ml) 10 mg of ETS-10 catalyst was suspended by means of sonication for

63

5 min in 10 ml of pH 7.0 buffer solution along with 20 μmol of dissolved substrate. The

sample was irradiated for 0.5 h (while stirring and purging under an N2 atmosphere)

using a high-pressure 300 W Hg lamp with the light intensity at 320-400 nm of 905 mW

m-2. The temperature was kept constant (at 313 K) throughout the reaction. After each

photocatalysis reaction, the catalyst was recovered by centrifugation, and the supernatant

was analyzed using high-performance liquid chromatograph (HPLC). With the bandgap

energy of ETS-10 being 4.03 eV, it is questionable whether or not the irradiation range

(λ > 320 nm) used was sufficient to activate the photocatalyst. Nonetheless, the extent of

the conversion of the aromatic substrates was shown to be dependent on size and polarity

of the substrates. The size of the substrates was determined by molecular orbital

calculations in which an effective molecular width (EMW) was used as the size

parameter. For symmetric substrates, such as 1,3,5-trihydroxybenze (Figure 31a), the

EMW was defined to be the perpendicular length of the substrate (EMW for 1,3,5-

trihydroxybenzene = 0.6132 nm). For asymmetric substrates, such as 2,4-dichlorophenol

(Figure 31b), the EMW was defined to be the average of the minimum width and the

molecular width of the substrate (EMW for 2,4-dichlorophenol = 0.605 nm).

Figure 31: Maximum length, molecular width, and minimum width of (a)

symmetric 1,3,5-trihydroxybenzene and (b) asymmetric 2,4-dichlorophenol as determined by molecular orbital calculations [75].

64

The polarity of each substrate was determined by a polarity parameter, log S,

calculated on the basis of the ALOGPS 2.1 program [76]. The more negative the log S

value the less polar the substrate; the less negative the log S values, the more polar the

substrate. As indicated above, due to the presence of Na+ and K+ ions in the pores of

ETS-10, the inner pore environment is highly polarized [75]. Therefore, more polar

substrates (i.e., highly polarized substrates) will diffuse more easily into the pore of ETS-

10 if size permits, whereas less polar (and large) substrates will not diffuse easily into the

pores. Table 2 summarizes the conversion of all 22 aromatic substrates along with their

EMWs and log S values.

No. Substrate EMW (nm) log S Conversion (%) 1 p-xyleneglycol 0.511 – 0.57 0 2 benzylalcohol 0.511 – 0.61 0 3 phenol 0.513 – 0.30 0 4 benzene 0.513 – 1.84 6.9 5 chlorobenzene 0.513 – 2.28 7.2 6 ethylbenzene 0.519 – 2.89 10.2 7 4-ethylphenol 0.522 – 1.31 0 8 4-chlorophenol 0.524 – 0.97 0 9 p-chlorotoluene 0.526 – 3.04 18.2 10 Toluene 0.529 – 2.26 6.9 11 p-cresol 0.533 – 0.66 0 12 4-chlororesorcinol 0.540 – 0.68 0 13 2-chlorophenol 0.541 – 0.94 0 14 2-hydroxybenzylalcohol 0.551 – 0.24 0 15 Naphthalene 0.563 – 3.32 4.2 16 3-chlorophenol 0.578 – 0.98 8.8 17 2,5-dichlorophenol 0.581 – 1.77 7.7 18 2,3-dihydroxynapthalene 0.583 – 2.42 7.6 19 2,6-dichlorophenol 0.603 – 1.77 4.0 20 2,4-dichlorophenol 0.605 – 1.81 11.4 21 1,3,5-trihydroxybenzene 0.613 – 0.53 5.6 22 2,4,5-trichlorophenol 0.627 – 2.60 6.7

Table 2: Size, polarity, and photocatalytic conversion of 22 aromatic substrates on ETS-10 [75].

65

As shown in Table 2, the conversions of the substrates depended on two

parameters: size and polarity. The extent of conversion for substrates with EMW > 0.551

nm (no.15-22) was dependent on size, whereas the extent of conversion for substrates

with EMW < 0.551 nm (no. 1-14) became more dependent on the polarity of the

substrate rather than the size. The relatively high conversion rates of substrates with

EMW > 0.551 nm supported the hypothesis of size-selectivity of the photocatalytic

activity for ETS-10. The hindering effect due to size allowed the larger (i.e., EMW >

0.551 nm) substrates to efficiently react with the external surface of ETS-10. Substrates

with EMW < 0.551 nm (no.1-14) showed dispersed conversions (from as low as 0 % to

as high as 18.2 %). These differences in conversions may be attributed to the varying

polarity of the substrates. As illustrated in Table 2, the conversion of substrates with log

S values larger than – 1.50 log S (i.e., less polar substrates) was nearly zero, whereas the

conversion of substrates with log S values smaller than – 1.50 log S (i.e., highly polarized

substrates) was high and comparable to that of some of the larger substrates. These

results agree with the hypothesis that the highly polarized ETS-10 pores block the less

polar small (EMW < 0.551 nm) substrates from entering the pores, thereby allowing

small less polar substrates to react efficiently on the external surface of ETS-10.

In addition to their findings, the authors [75] briefly discussed the selectivity of

the photocatalytic transformations of some of the aforementioned aromatic substrates.

Two types of selective organic transformations were observed on ETS-10. Type (1) is

the reaction of large substrates to form small polar substrates; example being the

transformation of 2,5-dichlorophenol (no.17, EMW = 0.581 nm, log S = – 1.77) to 4-

chlororesorcinol (no.12, EMW = 0.540 nm, log S = – 0.68). No further degradation of 4-

66

chlororesorcinol will occur since it is small enough and highly polar to diffuse into the

pores of ETS-10. Type (2) is the reaction of less polar substrates to small polar substrates;

example being the transformation of toluene (no.10, EMW = 0.529 nm, log S = – 2.26) to

benzylalcohol (no.2, EMW = 0.511 nm, log S = – 0.61). No further degradation of

benzylalcohol will occur since it is highly polar and will diffuse easily into the pores of

ETS-10. The findings of Shiraishi et al. [75] are of great interest in terms of choosing the

right molecule to evaluate the effectiveness of AM-6 and modified-AM-6 samples. Since

AM-6 is isostructural to ETS-10 and contains Na+ and K+ ions in the pores, it is

presumed that the pores of AM-6 are also highly polarized. Therefore, substrates of size

larger than the pore size of AM-6 and/or substrates of low polarity should be selected for

photocatalysis experiments.

67

3.0 EXPERIMENTAL

There were three main objectives in this investigation. The first objective was to

develop an unseeded AM-6 synthesis method in order to control the desired crystal

characteristics such as purity, size, morphology, topography, and defect concentration

without affecting the product and crystal purity (i.e., TiO2 content as a result of using

ETS-10 seeds during synthesis); the second objective was to perform a series of in situ

and post synthesis modifications of the unseeded (i.e., titanium-free) AM-6 products in

efforts to enhance the photocatalytic activity of AM-6 in both the UV and visible light

range and red shift its bandgap energy; the third objective was to evaluate the

effectiveness of these modifications in a slurry reactor system under UV and visible light

irradiation.

3.1

Hydrothermal synthesis of vanadosilicate AM-6 was carried out in 10 ml Teflon-

lined stainless steel autoclaves for 1-20 days at 430-503 K using mixtures with molar

compositions xNa2O : 1.3K2O : ySiO2 : 0.5V2O5 : wTMA : zH2O, where x=4.5 or 5.6,

y=3.4-5.5, w=0-3.0, and z=199-214. These mixtures were obtained by mixing two

aqueous precursor solutions: solution A containing sodium chloride (NaCl), potassium

chloride (KCl), sodium hydroxide (when/if necessary) (NaOH), sodium silicate, and a

structure directing agent such as TMA (tetramethylammonium hydroxide (TMAOH) or

tetramethylammonium bromide (TMABr)), TEA (tetraethylammonium bromide), or

TPA (tetrapropylammonium bromide); and solution B containing vanadyl sulfate

(VOSO4·5H2O). A typical mixture with x=4.5, y=4.85, w=2.2 TMAOH, and z=209

required 6.42 g NaCl (Aldrich), 3.43 g KCl (Fisher), and 14.57 g TMAOH (25 wt.%

Unseeded AM-6 Synthesis

68

solution in water, Aldrich) for solution A (NaOH was not required). After dissolving

these components in 20.02 g DI water, 18.05 g N-brand sodium silicate (28.89% SiO2,

8.89% Na2O, PQ Corp.) was added and hand-shaken until solution A appeared

transparent. Then, solution B, prepared by dissolving 4.49 g VOSO4·5H2O (Aldrich) in

23.0 g DI water, was added to solution A and the resulting mixture (olive green gel) was

hand-shaken for 5 minutes. When required, the initial pH was adjusted by adding

aqueous sodium hydroxide solution (10.0 g NaOH pellets (Aldrich) in 100.0 g DI water)

to the solution A or final gel. The pH was measured using an Orion pH meter model

720A. After measuring pH (pH≈11.0 for the gel preparation described above) the

mixture was transferred to the Teflon-lined autoclaves. After crystallization, the products

were cooled down to room temperature by quench cooling in a cold water bath, filtered,

washed with 1 L of DI water, and dried in air at ~343 K [120]. All products were

analyzed by XRD, SEM, EDX, UV-vis, Raman, and FTIR spectroscopy. Treatment of

fully crystalline AM-6 products with gaseous ammonia was carried out for 1 h at 623-

673 K in a stainless steel reactor using a 2 vol.% mixture of NH3 in He flowing at 2-10

cm3 s-1 to remove the TMA from its pores [120].

3.2

Transition metal (TM) ion-substituted AM-6 products with crystal TM/V molar

ratios of 0.04-0.23 (

Transition Metal Isomorphous Framework Substitutions

Table 6) were hydrothermally synthesized using a modification of

the AM-6 synthesis procedure utilizing tetramethylammonium cations (TMA+) [120] and

mixture molar composition of 2.25Na2O : 1.3K2O : 4.85SiO2 : 0.5V2O5 : 2.25TMAOH :

209H2O : xTM, x = 0.025-0.20. Typically (for x = 0.0375), solution A was prepared by

mixing 0.445 g NaCl (Aldrich), 1.977 g KCl (Fisher), TM salt (0.153 g Cr(NO3)3 • 9H2O

69

(99%, Aldrich), 0.155 g Fe(NO3)3 • 9H2O (98+%, Aldrich), or 0.111 g Co(NO3)2 • 6H2O

(98+%, Aldrich)), 12.471 g DI water, 8.367 g TMAOH (25 wt.% solution in water,

Aldrich), and 11.218 g sodium silicate (26.5% SiO2, 10.6% Na2O, Aldrich). Solution B

was prepared by dissolving 2.581 g VOSO4·5H2O (Aldrich) in 10.0 g DI water. Solution

B was added to solution A and the resulting mixture (olive green gel) was hand-shaken

for 5 minutes. When required, 0.5 g H2SO4 was added to solution B to adjust the initial

pH of the mixture (pH ≈ 11.1). After crystallization at 503 K for 1 -6 days, the products

were cooled to room temperature, filtered and washed with 1 L of DI water, and air-dried

overnight at 343 K [77].

3.3

All unmodified and modified AM-6 products were evaluated photocatalytically

under both UV and visible light irradiation. All photocatalytic experiments were carried

out in a slurry reactor. Optimum pH, adsorption-desorption equilibrium, and mass

transfer limitations were investigated in order to achieve optimum photocatalytic

conditions.

Photocatalytic Investigations

As discussed in Section 2.4, microporous materials selectively degrade organic

compounds based on size [50,74,75] and polarity [75]. Small and highly polar organic

compounds can diffuse into the pores of these (microporous) materials, and be protected

from photodegradation. Therefore, a large organic compound with low polarity was

chosen to evaluate the photocatalytic activity of unmodified and modified AM-6

products.

70

3.3.1 Model Organic Compound

Based on the size and polarity roles of organic compounds in photodegradation

using microporous materials, such as AM-6, methylene blue (MB), a brightly colored

cationic thiazine dye with the absorbance maximum at a wavelength of 664 nm (Figure

32), was chosen. The size (14.3 Å × 6.1 Å × 4.0 Å [78]) and polarity (log S = -4.45

[76]) of MB allows for photodegradation to occur only on the external surfaces of AM-

6 (pore sizes of AM-6: 4.9 Å × 7.6 Å [17]). In addition, the photodegradation of MB

can easily be monitored using UV-vis spectrophotometer.

Figure 32: UV-vis absorption spectrum of MB; insert: MB (thiazine) molecular

structure [79]. The absorption maximum at ~664 nm was used to determine the MB

concentration in all the photocatalysis experiments investigated. Beer-Lambert’s law was

used to relate the absorbance of MB to the concentration of MB. A linear correlation was

observed (Figure 33).

71

Figure 33: Absorbance of MB as a function of concentration.

3.3.2 Photocatalysis in a Slurry Reactor

The photocatalytic activities of all modified and unmodified AM-6 products (and

Ag0-ETS-10, See Appendix 9.6) were evaluated in a continuously stirred slurry reactor.

All photocatalytic experiments were conducted in a 200 mL Pyrex beaker batch system.

A homogenous catalyst suspension of 500 mg L-1 in a 100 mL aqueous methylene blue

(MB, Sigma-Aldrich) solution, which had a concentration of 10 mg L-1, was prepared by

sonication for 5 min. Effect of pH on the % crystallinity of the photocatalyst retained

after photocatalysis was investigated; % crystallinity was estimated by normalizing the

integrated intensity of the ~24.6 °2θ XRD AM-6 peak after photocatalysis to that

measured before photocatalysis. At pH ≤ 6.5, ~80% crystallinity was retained. Under

“natural” pH conditions (i.e., pH = 7.5-8.0), 100% crystallinity was retained. Therefore,

all photocatalysis investigations were conducted under “natural” pH conditions in order

to maintain the crystallinity of the photocatalyst throughout the photocatalytic process.

The light was supplied by a 500 W Xe arc lamp (Oriel Model 66924). To deliver only

72

UV light (310-400 nm), a dichroic mirror (280-400 nm) followed by a UVB/C blocking

filter was used. To deliver only visible light, a cold mirror (420-630 nm) was used

instead. 4 mL aliquots were taken at various time intervals and centrifuged to remove the

crystals from the supernatant prior to analysis. Under both irradiation ranges, the

temporal MB concentration change during each experiment was monitored using a Cary

5000 UV-vis spectrometer. The MB concentration in each sample was determined from

the absorbance maximum at ~664 nm in the UV-vis spectrum using the correlation

established in Figure 33. For comparison, direct photolysis experiments were performed

under both UV (310-400 nm) and visible (420-630 nm) light irradiation.

The temperature change throughout the photocatalytic experiments was

monitored. The temperature change was less than 5 °C up to 9 min (Figure 34) under

both UV and visible light irradiation. A significant increase in temperature was observed

for irradiation time ≥ 15 min ( Figure 34). Therefore, only data collected up to 9 min of

irradiation were used in determining the pseudo-first-order reaction rate constant [81] of

each experiment. All experiments were considered to be performed at room temperature

(~19-20 °C).

73

Figure 34: Temperature change throughout a photocatalytic process.

For measured conversion to reflect the real kinetics of degradation (and

consequently the photocatalytic reaction rate), mass transfer limitations must be

eliminated [6]. In heterogeneous photocatalysis systems, mass transfer includes both

internal mass transfer (i.e., the transfer of the organic into the pores of AM-6) and

external mass transfer (i.e., transfer of the organic to the active sites on the photocatalyst

surface) [6]. Since AM-6 was synthesized using TMA, a pore-blocking structure

directing agent (Section 4.1.1.1), and since the low polarity (i.e., log S = -4.45 [76]) of

the MB molecule will not allow for MB to diffuse into the highly polar environment of

the AM-6 pores, internal mass transfer limitations can be eliminated in this investigation.

External mass transfer can be eliminated by using a high stirring rate under which the

MB photodegradation rate no longer changes. Therefore, the stirring rate of the slurry

reactor was increased using the most active photocatalytic sample tested. The rate of MB

conversion did not change above 750 rpm. Therefore, a stirring rate of 1000 rpm was

utilized to eliminate external mass transfer as well as to uniformly disperse the AM-6

74

crystals, and to aerate the mixture during the light irradiation to avoid photobleaching of

MB that occurs at insufficient dissolved oxygen concentrations [80,81].

To investigate the adsorption kinetics of MB on AM-6, a 4 mL aliquot was taken

every 30 min from the vigorously stirred (~1000 rpm) reactor containing 500 mg L-1

AM-6 suspension and 10 mg L-1 MB (under dark conditions), centrifuged to remove the

crystals, then analyzed using the UV-vis spectrophotometer (Figure 35). This was

repeated until no change in the MB concentration was observed. It was determined that

120 min of magnetic stirring of the suspensions in the dark was sufficient to reach

adsorption-desorption equilibrium. Therefore, all the photocatalytic experiments were

conducted by leaving the mixture (of AM-6 and MB) under continuous vigorous stirring

in the dark for 120 min before irradiation.

Figure 35: Temporal change for adsorption of MB on AM-6 crystals under dark

conditions while stirring at 1000 rpm.

All crystals were collected post-photocatalysis for XRD analysis to estimate the

extent of crystallinity retained.

75

3.4

In order to quantitatively and qualitatively (i.e., structural analysis) analyze both

the unmodified and modified vanadosilicate AM-6 products, several characterization

techniques were utilized. Identification of the product purity and % crystallinity, and

crystal phases was determined using X-ray powder diffraction (XRD). Crystal size and

morphology of these products were determined using a field emission scanning electron

microscopy (FE-SEM). Particle size distribution (PSD) analysis was performed to

illustrate the crystal size distribution in the products. Quantitative chemical analysis of

the products was performed using two techniques – Energy Dispersive X-ray (EDX)

spectroscopy and X-ray Photoelectron Spectroscopy (XPS). Structural analysis of the

unmodified and modified products was performed using Fourier transform infrared

(FTIR) spectroscopy, and Raman spectroscopy. Thermogravimetric analysis (TGA) was

carried out to confirm the removal of organic from the pores of the AM-6 products.

Diffuse reflectance UV-vis spectroscopy was used to determine the optical properties

and bandgap energies of the unmodified and modified AM-6 products.

Characterization Techniques

3.4.1 X-ray Powder Diffraction

All unmodified and modified vanadosilicate AM-6 products were analyzed using

X-ray powder diffraction (XRD) on a Bruker D5005 θ: 2θ Bragg-Brentano

diffractometer equipped with a curved graphite crystal diffracted beam monochromator

and a NaI scintillation detector using Cu Kα radiation (40 kV, 30 mA). Each sample was

ground prior to the measurements in order to minimize the effect of preferred orientation

on the patterns. The Bruker software package EVA was used to obtain relative peak

intensities.

76

Figure 36 shows a typical XRD pattern of AM-6 product obtained from the

unseeded synthesis procedure [120]. The % crystallinity of all products was estimated by

normalizing the integrated intensity of the 24.6 °2θ peak of a given ammonia -

treated/modified sample to that of a standard ammonia-treated/as-synthesized sample.

Fully crystalline samples were obtained from each synthesis temperature/mixture and

were used as standards.

Figure 36: XRD pattern of as-synthesized pure AM-6 product [120].

3.4.2 Field Emission Scanning Electron Microscopy

A Hitachi S-4700 FE-SEM was used to characterize the morphology and estimate

the crystal size of the as-synthesized and modified AM-6 products. Aluminum sample

holders coated with carbon paint were used as supports for the uncoated crystals. Crystal

suspensions were prepared by dispersing a small amount of product in DI water and

sonicating for several seconds. Evaporation of water from the crystal suspension resulted

in a film of crystals on the carbon-coated aluminum holder. Secondary electron imaging

77

mode at 3 or 6 mm working distance with an accelerating voltage of 2kV and a beam

current of 10 μA was used to obtain the SEM images. The low accelerating voltage (2

kV) minimized/eliminated the charging effects of the uncoated crystals.

3.4.3 Energy Dispersive X-ray Spectroscopy

Quantitative chemical analysis for the unmodified and modified AM-6 products

was performed using energy dispersive X-ray (EDX) spectroscopy. A Phoenix EDAX X-

ray analyzer equipped with a Sapphire super ultra thin window detector attached to the

Hitachi S-4700 FE-SEM was used. All samples were analyzed using an accelerating

voltage of 15-20 kV, beam current of 10 µA, and counting time of 100 seconds. Due to

the relatively large penetration depth of EDX beam (micrometer scale penetration),

“thick” films of AM-6 crystals were prepared in order to obtain an accurate quantitative

analysis.

3.4.4 Thermogravimetric Analysis

Thermogravimetric analysis (TGA) was carried out to determine to the extent of

organic (TMA) removal from the unseeded AM-6 products. A Mettler-Toledo

TGA/SDTA851e module was used with a heating rate of 5 °K min-1 and 50 mL min-1 air

flow. Both the as-synthesized and ammonia-treated samples were analyzed.

3.4.5 Particle Size Distribution

Particle size distributions of the products were determined using an API

Aerosizer LD equipped with an API Aero-Disperser dry powder dispersion system (TSI,

Inc., Particle Instruments/Amherst). Dry powder samples were dispersed in an air stream

prior to sizing. The density of AM-6 was assumed to be that of the density of ETS-10,

78

1750 kg/m3. The geometric diameter measured by the API Aerosizer, defined as the

equivalent spherical diameter of a particle with the same density as the measured particle,

was determined by comparing the time-of-flight of the analyzed particle to that of the

spherical particles, which were used to calibrate the instrument. Therefore, the AM-6

crystals/particles, whose shape deviates from spherical, were expected to have the

geometric diameters smaller than their largest dimensions.

3.4.6 X-ray Photoelectron Spectroscopy

Quantitative chemical analysis of the as-synthesized and modified AM-6

products was performed using X-ray photoelectron spectroscopy (XPS) (Appendix 9.1).

Sample rotation capability enabled angle resolved XPS. A PHI model 04-548 Mg/Al

dual anode non-monochromatic Mg Kα (hυ = 1253.6 eV) X-ray source and a PHI model

10-360 hemispherical analyzer was used for analysis. The XPS spectra were fitted using

80% Gaussian and 20% Lorenzian functions, as determined from a clean Au 4f7/2

photoelectron peak. The integrated Shirley method was used for background subtraction.

The full-width-at-half-maximum (FWHM) values were determined for the V 2p3/2 and

O 1s peaks in the V2O5 spectra. These FWHM values were then used for the V 2p3/2 and

O 1s peaks in all AM-6 spectra. Sample charging was corrected by referencing all the

binding energies (BE) to the adventitious carbon C 1s BE at 284.4 eV.

3.4.7 Raman Spectroscopy

The Raman spectra of all unmodified and TM-modified AM-6 products were

obtained using a CRM200 Confocal Raman Microscope (WITec GmbH, Ulm, Germany)

employing a 532 nm, 633 nm, or 785 nm excitation wavelength and an air objective

79

(100x / NA = 0.9, WD = 0.2 mm). Excitation was provided by a HeNe laser (Melles

Griot). The exciting laser radiation was coupled into a Zeiss microscope through a

wavelength-specific single mode optical fiber. The incident laser beam was collimated

via an achromatic lens and passes a holographic band-pass filter before it was focused

onto the sample through the microscope objective. The sample is located on a piezo-

electrically driven microscope scanning stage with an x,y resolution of ca. 3 nm and a

repeatability of ±5 nm, and z resolution of ca. 0.3 nm and ±2 nm repeatability. The

Raman back-scattered radiation was detected by a back-illuminated deep-depletion,

1024×128 pixel charge-coupled device camera operating at -82°C.

3.4.8 Fourier Transform Infrared Spectroscopy

Diffuse reflectance Fourier transform infrared (FTIR) spectra of all unmodified

and modified AM-6 products in the framework vibrations region were recorded at ~293

K on a Magna-IR 560 spectrometer with 2 cm-1 resolution by averaging 128 scans.

Powders were mixed with KBr having a KBr to AM-6 weight ratio of 1:200. KBr (99+%,

infrared grade, Acros) was used as the background.

3.4.9 Diffuse Reflectance UV-vis Spectroscopy

Diffuse reflectance UV-vis (DR-UV-vis) spectroscopy was performed using a

Cary 5000 UV-vis-NIR spectrometer equipped with a Praying Mantis accessory. Powder

SpectralonTM standard was used as the reference. The measurements were taken in

ambient air in the appropriate wavelength range with a bandwidth of 1.0 nm.

Since the Eg transitions (lower energy absorption region) are not typically sharp,

precise measurements of the bandgap energy (Eg) value cannot be determined by the

80

optical spectrum. Therefore, to determine the bandgap energies of all unmodified and

modified AM-6 products, the collected DR-UV-vis spectra (optical spectra) were

converted into the Kubelka-Munk function (F(R∞)) using the Cary Win UV software.

The bandgap energies of all unmodified and modified AM-6 products were determined

using the method derived from the general power law of Davis and Mott (Equation 4)

[82]:

αħω (ħω – Eg)n ( Equation 4)82

Where: α = absorption coefficienct (m-1);

ħ = Dirac’s constant, 1.05 x 10-34 J·s ;

ω = angular frequency of the incident radiation (s-1);

Eg = bandgap energy (eV);

n = 2, 3, 1/2, and 3/2 for indirect allowed, indirect forbidden, direct allowed, and

direct forbidden transitions, respectively.

Equation 4 can also be written as:

[F(R∞)hν]n (hν – Eg) (Equation 5)83

Where: F(R∞) = Kubelka-Munk function;

h = Plank’s constant, 6.626 x 10-34 J·s = 4.136 x 10-15 eV·s ;

ν = frequency of light (s-1) = c/λ, where c = speed of light, 3.0 x 108 m·s-1,

and λ = wavelength (nm);

Eg = bandgap energy (eV);

n = 2, 3, 1/2, and 3/2 for indirect allowed, indirect forbidden, direct allowed, and

direct forbidden transitions, respectively.

81

The value of n was determined by examining the plots of [F(R∞)hν]n vs. hν using

n values of 2, 3, 1/2, and 3/2 (Figure 37). The exponent which resulted in the best linear

fit of the lower energy absorption region was used (i.e, n = ½, Figure 37b). The bandgap

energy (Eg) of all unmodified and modified AM-6 products was determined from the

straight line intercept in the lower energy absorption region of the plot of [F(R∞)hν]1/2 vs.

hν (Figure 37b).

Figure 37: Plot of [F(R∞)hν]n vs. hν with n values of: (a) 2, (b) 1/2, (c) 3, and (d) 3/2.

Inserts are magnifications of the lower absorption regions of each plot. Red dotted lines indicate forbidden transitions, whereas black solid lines indicate allowed transitions.

82

4.0 RESULTS AND DISCUSSION

Vanadosilicate AM-6 is a large-pore microporous material, isostructural with

titanosilicate ETS-10. Microporosity, pore regularity, and the presence of stoichiometric

amounts of vanadium in the silicate framework make AM-6 a promising material in both

traditional and advanced zeolite-type materials applications. However, to date AM-6 has

only been synthesized using seeds of ETS-10, and no control of the crystal

characteristics (such as purity, size, morphology, topography, and defect concentration)

were reported. The use of titanosilicate ETS-10 crystals in the synthesis of vanadosilicate

AM-6 limits the ability to control product characteristics without also influencing the

product titanium content. Therefore, a novel AM-6 crystallization method was developed

which did not require the use of ETS-10 seeds. A structure directing agent (SDA),

tetramethylammonium (TMA) cation, was used in the unseeded syntheses. Effects of

various synthesis parameters, such as TMA molar content, Si/V molar ratio, and

temperature on AM-6 product purity, crystal size, morphology, and crystal quality were

investigated.

Recent reports suggested visible light photocatalytic activity of AM-6

synthesized using titanosilicate ETS-10 seeds. However, the close proximity of V4+ to

V5+ along the …V–O–V–O–V… chains in AM-6 promoted electron-hole recombination,

thereby decreasing the photocatalytic activity. The new unseeded synthesis method was

used to crystallize AM-6 products with varying distributions of V4+/V5+ throughout the

monatomic …V–O–V–O–V… chains, and perform modifications, such as isomorphous

framework substitutions, in efforts to enhance the photocatalytic activity of AM-6 and

red shift its bandgap energy.

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4.1

The main objective of this investigation was to develop an unseeded synthesis

method for crystallizing AM-6 in order to control the desired crystal characteristics such

as purity, size, morphology, topography and defect concentration without affecting the

product and crystal purity (i.e., TiO2 content as a result of using ETS-10 seeds during

synthesis), for photocatalytic applications.

Unseeded AM-6 Crystal Growth

Quaternary ammonium ions, such as tetramethylammonium (TMA) cations, have

been used as organic structure-directing agents (SDAs) in synthesis of microporous

aluminosilicate zeolite molecular sieves [84], and other related zeolite-type open-

framework materials [35,36,37,38,39,40,85]. The role of these SDAs is thought to

involve ordering water molecules and silica, and/or aluminosilicate species present in the

synthesis mixture [84], where these locally-ordered inorganic–organic composite

structures are hypothesized to participate in zeolite nucleation and crystal growth [86]

(Figure 38). Based on this discussion, the hydrothermal synthesis of AM-6 was carried

out using SDAs.

Figure 38: Proposed mechanism for the formation of zeolites utilizing a structure

directing agent [86].

84

Large-pore vanadosilicate AM-6 was hydrothermally synthesized (Figure 39b,

Figure 40b) in the absence of titanosilicate ETS-10 seeds by utilizing a SDA, TMA [120],

using mixtures with molar compositions xNa2O : 1.3K2O : ySiO2 : 0.5V2O5 : wTMA :

zH2O, where x=4.5 or 5.6, y=3.4-6.2, w=0.25-3.0, and z=199-216. In the absence of

TMA, AM-6 did not crystallize (Figure 40c); other unidentified crystalline material(s)

were formed instead (Figure 39a and Figure 40c). When synthesized from mixtures with

x=5.6, y=4.3-5.5, w=0, and z=206-214 the unidentified crystalline products typically

contained thin (thickness <<1 μm), hexagonal plates with a lateral dimension smaller

than ~1 µm (Figure 39a). AM-6 crystals formed in the absence of seeds typically had

truncated square bipyramidal habit with various irregularities and faulting on their

surfaces (Figure 39b). The effect of TMA molar content on the morphology, size, and

purity of AM-6 products will be discussed in Section 4.1.1.2.

Figure 39: FE-SEM images of crystals synthesized at 430 K from mixture with

molar composition 5.6Na2O : 1.3K2O : 4.85SiO2 : 0.5V2O5 : wTMABr : 169H2O: (a) w = 0; unidentified crystalline product, and (b) w = 2.0; AM-6 product.

EDX analysis of individual AM-6 crystals synthesized from mixtures with

different molar compositions showed identical Si/V ratios with a value of 5.39 ± 0.15.

This value is similar to the theoretical (Si/Ti = 5.0 [20]) and experimental (Si/Ti = 5.0 ±

85

0.5) [20,22]) values for the Si/Ti ratio in isostructural ETS-10 crystals, and experimental

values for Si/V ratios in AM-6 crystals synthesized using seeds (Si/V = 5.23 ± 0.47)

[17,31] (Table 3). A substantial decrease in the (Na+K)/V ratio was observed for AM-6

crystals synthesized using TMA (Table 3, (Na+K)/V = 1.40 ± 0.10) when compared to

the (Na+K)/V ratio of AM-6 crystals synthesized using seeds (Table 3, (Na+K)/V = 2.47

± 0.13) [33]. Since the TMA+ ions (~6.0-7.4 Å [85]) are similar in size to the hydrated

Na+ ions (2.51 Å [87]), these results suggest that the TMA+ ions likely replace the Na+

ions that balance the negative charge [37] in AM-6 framework, which would result in a

decrease in the (Na+K)/V ratio, as observed.

Unseeded AM-6 Seeded AM-6 Si/V in crystals (mol mol-1) 5.39 ± 0.15 5.23 ± 0.47 [31] (Na+K)/V in crystals (mol mol-1) 1.40 ± 0.10 2.47 ± 0.13 [33] Table 3: Chemical compositions as determined by EDX analysis of seeded [31,33]

and unseeded AM-6 crystals.

The XRD patterns of ETS-10 (Figure 40a) and AM-6 (Figure 40b) products were

nearly identical; however, the AM-6 reflections showed slightly smaller d-spacing values

compared to the corresponding ETS-10 reflections. This indicates a smaller unit cell

volume for AM-6 relative to ETS-10 (e.g., 1516.8 Å3 ± 2.4 Å3 vs. 1528.6 Å3 ± 1.6 Å3 for

a tetragonal body centered unit cell [88] in samples shown in Figure 40b, and Figure 40a,

respectively). This is in agreement with the smaller ionic radii of both hexacoordinate

V4+ (0.72 Å [25]) and hexacoordinate V5+ (0.68 Å [25]) compared to hexacoordinate Ti4+

(0.745 Å [25]). Similar results were obtained for AM-6 synthesized with ETS-10 seeds

[17,31].

86

Figure 40: XRD patterns of ETS-10 product (a), and products synthesized at 503 K

from mixture with molar composition 5.6Na2O : 1.3K2O : 4.85SiO2 : 0.5V2O5 : wTMABr : 169H2O: (b) w = 2; AM-6, and (c) w = 0; unidentified crystalline material. Tick marks indicate where the AM-6 reflections should appear.

4.1.1 Investigation of Crystallization Parameters

Southon and Howe [49] reported through a series of spectroscopic investigations

on various ETS-10 samples that the level of defects in titanosilicate ETS-10 may modify

the optical and electronic properties of this material, which in return may enhance its

photocatalytic performance. These levels of defects may be controlled through the

variation of two-dimensional nucleation rate (layer growth rate) during ETS-10 synthesis

[49]. It was hypothesized that similar control can be accomplished for defect

concentration in AM-6 (i.e., AM-6 crystal growth occurs via the same mechanism as

ETS-10 crystal growth). Anderson et al. [89] reported that the rate of surface nucleation

relative to the rate of lateral layer spreading may be influenced by parameters such as

addition of SDAs, modification of mixture molar ratios, synthesis pH, and temperature,

which would vary the supersaturation levels in the synthesis mixtures. Therefore, it was

87

necessary to investigate the effects of these crystallization parameters in order to

discover/understand the AM-6 crystal growth mechanism, and synthesize products with

various defect levels for photocatalytic investigations.

4.1.1.1 Effect of Templating Agent

A compositional study was performed to investigate the “structure directing” role

of various “templating agents” (SDAs), such as tetramethylammonium (TMA) cation,

tetraethylammonium (TEA) cation, and tetrapropylammonium (TPA) cation, towards the

crystallization of AM-6. At 503 K, mixture Si/V molar ratio of 4.85 (i.e., y = 4.85), and

mixture SDA/V molar content of 2.0 (i.e., w = 2.0), AM-6 products only crystallized in

the presence of TMA (Figure 41a). Using TEA and TPA as “templating agents” did not

result in AM-6 products (Figure 41b and c). Similar results were observed for mixture

Si/V molar ratios of 3.4-6.2 (i.e., y =3.4-6.2). Therefore, these “templates” (i.e., TEA and

TPA) are said to have “structure breaking” roles rather than “structure directing” roles

[36]. Davis and Lobo [84] stated that “structure direction implies that a specific structure

is synthesized via a single organic species”, therefore TMA, to the limited extent of three

different organic “templates” investigated here, may be provisionally concluded to act as

a structure directing agent in the synthesis of AM-6.

88

Figure 41: XRD patterns of products synthesized at 503 K from mixture with molar

composition 5.6Na2O : 1.3K2O : 4.85SiO2 : 0.5V2O5 : 2.0SDA : 169H2O: (a) SDA = TMABr; AM-6 product, (b) SDA = TEABr; unidentified crystalline product, and (c) SDA = TPABr; unidentified crystalline product.

Nitrogen sorption at 77 K showed the BET surface area of 100 m2 g-1 for TMA-

containing AM-6 product and 326 m2 g-1 for TMA-free AM-6 products obtained via

treatment with gaseous ammonia at 673 K for 50 min [120]. The micropore volume of

TMA-containing AM-6 product (0.056 cm3 g-1) was significantly smaller than that of

TMA-free AM-6 product (0.130 cm3 g-1). These values suggest that TMA has a pore-

filling role stabilizing the formation of AM-6 structure in a similar fashion to that

hypothesized for ETS-10 crystallization in the presence of TMA [36]. The pore-filling

role of TMA in the crystallization of AM-6 is also supported by the similarity between

the size of the TMA molecule (~6.0-7.4 Å [36,85]) and the size of AM-6 pores (4.9 Å

×7.6 Å [17]).

89

4.1.1.2 Effect of TMA Molar Content

The addition of SDAs has been reported to influence the supersaturation levels in

the synthesis mixtures which may vary the rate of surface nucleation relative to the rate

of layer growth [89]. Therefore, the effect of the TMA content on the crystallization of

AM-6 was investigated. The product purity, size of the AM-6 crystals, surface

topography, and degree of development of their square and trapezoidal faces, which may

be quantified by calculating the “aspect ratio” R (i.e., the combined height of two

truncated pyramids in a crystal divided by the width of the shared base [92]), were

affected by varying the mixture TMA content. With increasing amount of TMA in the

synthesis mixture, the fraction of AM-6 in the products increased (Figure 42 and Figure

43) while the size of AM-6 crystals decreased (Figure 43). This shows that the presence

of TMA in synthesis mixtures influenced the type of phase being crystallized, and

suggests that the concentration of TMA in synthesis mixture affected (three-dimensional)

nucleation of AM-6. Thus, TMA appears to act as a structure-directing agent [84,85] in

unseeded AM-6 synthesis, as hypothesized. Typically at 503 K with w≥2.0, these

mixtures resulted in nearly pure (≥95% of AM-6 by mass) AM-6 products (Figure 43e,f).

The trace impurities observed in these products by FE-SEM were not detected by XRD

(Figure 42d-f). Morphology of these impurities varied (needles, hexagonal plates) and

depended on the starting composition.

90

Figure 42: XRD patterns of AM-6 products synthesized at 503 K from mixture with

molar composition 5.6Na2O : 1.3K2O : 4.85SiO2 : 0.5V2O5 : wTMA : 209H2O with: (a) w = 0.0, (b) w = 0.25, (c) w = 0.5, (d) w = 1.0, (e) w = 2.0, and (f) w = 3.0.

For products synthesized with Si/V molar ratio = 4.85 at 503 K and w<2.0

TMABr (i.e., lower purity AM-6 products) (Figure 42a-d and Figure 43a-d), the size of

AM-6 crystals decreased while the fraction of AM-6 in the product increased with

increasing w. This can be attributed to the structure directing role of TMA in AM-6

synthesis and its effect on AM-6 nucleation as indicated earlier.

91

Figure 43: FE-SEM images showing the effect of mixture TMA content on the

product purity and size of AM-6 crystals obtained from mixture with molar composition 5.6Na2O : 1.3K2O : 4.85SiO2 : 0.5V2O5 : wTMA : 209H2O with: (a) w = 0.0, (b) w = 0.25, (c) w = 0.5, (d) w = 1.0, (e) w = 2.0, and (f) w = 3.0.

4.1.1.3 Effect of Si/V Molar Ratio

The AM-6 products grown from mixtures with x = 5.6, y = 4.3-5.5, w = 2.0

TMABr, and z = 206-214 at 503 K were nearly pure (≥95% AM -6 by mass) AM-6

products (Figure 44c-e and Figure 46c-e). The trace impurities observed in these

products by FE-SEM were not detected by XRD. Morphology of these impurities varied

(needles, hexagonal plates), and depended on the starting composition. The AM-6

products grown from mixtures with x = 5.6, y = 3.4-3.8, w = 2.0 TMABr, and z = 109-

202 at 503 K (Figure 44a-b) were somewhat less pure (i.e., <95% of AM-6 by mass).

Thus, at constant TMA content the increasing mixture Si/V ratio improved the purity of

AM-6 product.

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Figure 44: FE-SEM images of AM-6 crystals synthesized at 503 K from mixtures

with molar compositions 5.6Na2O : 1.3K2O : ySiO2 : 0.5V2O5 : 2.0TMA : zH2O with: (a) y = 3.4, z = 199, (b) y = 3.8, z = 202, (c) y = 4.3, z = 206, (d) y = 4.85, z = 209, (e) y = 5.5, z = 214, and (f) y = 6.2, z = 216.

Increase of y from 3.4 to 6.2 resulted in the decrease of the maximum dimension

of the square crystal base from ~9.0 to ~1.5 µm (R ≈ 0.25 – 0.50, Figure 44) as well as in

the decreased of the average product size from ~5.0 to ~1.9 µm (Figure 45). The average

crystal size and broader particle size distribution in products obtained at lower mixture

Si/V ratios are self-consistent and suggest lower AM-6 nucleation rates. Thus, at

constant TMA content the decreasing mixture Si/V ratio resulted in lower

supersaturation levels in the crystallizing system with respect to AM-6 formation.

93

Figure 45: Particle size distributions (PSDs) of AM-6 products synthesized at 503 K

from mixtures with molar compositions 5.6Na2O : 1.3K2O : ySiO2 : 0.5V2O5 : 2.0TMA : zH2O with: (a) y = 3.4, z = 199, (b) y = 3.8, z = 202, (c) y = 4.3, z = 206, (d) y = 4.85, z = 209, and (e) y = 5.5, z = 214.

The quality of the AM-6 crystals grown from mixtures with x = 5.6, y = 3.4-5.5,

w = 2.0 TMABr, and z = 206-214 at 503 K were assessed through a detailed X-ray

analysis (Figure 46). All reflections (indicated by avg. FWHM, Figure 46) for the AM-6

products synthesized at 503 K y = 4.85-5.5 were broader compared to the corresponding

reflections in the AM-6 products synthesized from mixtures with y = 3.4-4.3. Crystals in

the latter products (Figure 44a-c) show less faulting and fewer irregularities on the

surfaces than crystals in the former products (Figure 44d-e). Since the increasing faulting

in the crystal morphology of ETS-10 (structural analogue of AM-6 which grows via the

same mechanism, vide infra) correlates well with the increasing crystallographic disorder

[49], line broadening in the products grown at 503 K may be attributed in part to lattice

disorder. Therefore, AM-6 products grown at higher mixture Si/V ratios (i.e., y = 4.85-

5.5) result in crystals with greater degree of faulting and irregularities. Since crystals

94

with more faulted topographies imply more structural defects [49], which may enhance

the photocatalytic activity of AM-6 (Section 2.3.1.2), it will be of interest to investigate

the photocatalytic activity of AM-6 products with various degrees of surface defects (i.e.,

topographical faulting, Figure 44c,d).

Figure 46: XRD patterns and average full-width-at-half-maximum (FWHM) of

AM-6 products synthesized at 503 K from mixtures with molar compositions 5.6Na2O : 1.3K2O : ySiO2 : 0.5V2O5 : 2.0TMA : zH2O with: (a) y = 3.4, z = 199, (b) y = 3.8, z = 202, (c) y = 4.3, z = 206, (d) y = 4.85, z = 209, and (e) y = 5.5, z = 214.

4.1.1.4 Effect of Synthesis pH

The synthesis pH has been reported to affect the solubility of silica species in the

synthesis mixture [85], which may vary the supersaturation levels in the synthesis

mixtures and thereby the rate of surface nucleation relative to the rate of layer growth

[89]. Therefore, the effect of initial pH of the synthesis mixture was investigated. At 503

K and constant mixture Si/V ratio and TMABr content, at low pH (e.g., pH = 10.5), AM-

6 did not crystallize (Figure 47a, also confirmed by XRD analysis). Increasing the initial

pH in the synthesis mixtures from ~10.8 to ~11.0 resulted in pure AM-6 products (Figure

95

47b,c). Therefore, only a relatively narrow “optimal” pH range synthesized pure AM-6

products. Increasing the initial pH in the synthesis mixtures from ~10.8 to ~11.0 also

resulted in the increase of the size of the square crystal faces relative to the size of the

trapezoidal faces in these AM-6 products (Figure 47b and c). Thus, the initial pH affects

the supersaturation levels in the synthesis mixtures.

Figure 47: FE-SEM images of AM-6 crystals synthesized at 503 K from mixtures

with molar compositions 5.6Na2O : 1.3K2O : 4.85SiO2 : 0.5V2O5 : 2.0TMA : 209H2O with: (a) pH = 10.5; (b) pH = 10.8; (c) pH = 11.0.

4.1.1.5 Effect of Crystallization Temperature

The AM-6 products crystallized from mixture with x =5.6, y = 4.3, w = 2.0

TMABr, and z = 206 at lower temperatures (430-453 K) were free of the trace impurities

(Figure 48c-e and Figure 49c-e), which were present in the products synthesized at

higher temperatures (473-503 K) (Figure 48a,b and Figure 49a,b). Thus, the lower

crystallization temperature had a beneficial effect on the AM-6 product purity. Crystals

grown at lower temperatures (i.e., 430-453 K) showed a decrease in the crystal aspect

ratio R (i.e., more platy AM-6 crystals with R < 0.1 and thickness smaller than ~100-200

nm, Figure 48c-e).

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Figure 48: FE-SEM images of AM-6 crystals synthesized from mixtures with molar

compositions 5.6Na2O : 1.3K2O : 4.3SiO2 : 0.5V2O5 : 2.0TMA : 206H2O at: (a) 503 K, 4 d; (b) 473 K, 6 d; (c) 453 K, 6 d; (d) 438 K, 8 d; (e) 430 K, 14 d.

Detailed X-ray analyses (Table 4 and Figure 49) of AM-6 products grown at 430-

503 K were carried out to investigate the effect of crystallization temperature on the

growth mechanism of AM-6. The indexing of the AM-6 reflections is based on the

tetragonal body centered unit cell [90]. The FWHM of all 0kl and h0l reflections (except

that of the 200 reflection), typically sharp in the tetragonal system polymorph A in

isostructural ETS-10 crystals [22], increased considerably (at least 1.5 times) for the

products grown at 430-453 K compared to those grown at 473-503 K, whereas the

FWHM of the 200 reflection did not change within measurement error. All hkl

reflections with non-zero indices also broadened for the former products (Table 4).

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hkl 503 K 473 K 453 K 438 K 430 K 101 0.0546 ±

0.0032 0.0805 ± 0.0052

0.1164 ± 0.0157

0.1234 ± 0.0145

0.1609 ± 0.0221

004 0.0690 ± 0.0084

0.0847 ± 0.0080

0.1125 ± 0.0088

0.1210 ± 0.0145

0.1330 ± 0.0079

105 0.0671 ± 0.0024

0.0956 ± 0.0028

0.1658 ± 0.0062

0.1893 ± 0.0099

0.2806 ± 0.0148

200 0.0767 ± 0.0146

0.0646 ± 0.0094

0.0863 ± 0.0231

0.0739 ± 0.0188

0.0994 ± 0.0297

202 0.0671 ± 0.0021

0.0784 ± 0.0029

0.0996 ± 0.0029

0.1103 ± 0.0031

0.1100 ± 0.0027

116 0.0726 ± 0.0026

0.1197 ± 0.0048

0.1897 ± 0.0146

0.2172 ± 0.0305

0.2614 ± 0.0465

008 0.0541 ± 0.0099

0.0905 ± 0.0136

0.1814 ± 0.0172

0.2029 ± 0.0384

0.1972 ± 0.0237

204 0.0668 ± 0.0016

0.0785 ± 0.0024

0.0941 ± 0.0031

0.1045 ± 0.0033

0.0980 ± 0.0035

109 0.0644 ± 0.0048

0.1031 ± 0.0178

0.2300 ± 0.0461

---- ----

217 0.0926 ± 0.0060

0.1323 ± 0.0109

0.1300 ± 0.0218

0.1372 ± 0.0289

0.1959 ± 0.0498

208 0.0762 ± 0.0019

0.0944 ± 0.0024

0.1318 ± 0.0040

0.1488 ± 0.0045

0.1721 ± 0.0045

224 0.0753 ± 0.0026

0.0862 ± 0.0031

0.1160 ± 0.0075

0.1214 ± 0.0066

0.1288 ± 0.0065

1110 0.0720 ± 0.0046

0.0883 ± 0.0168

0.1463 ± 0.0704

---- ----

Table 4: Full-width-at-half-maximum (FWHM) of all hkl reflections in the 5-37.5 º2θ range for AM-6 products synthesized from mixtures with molar compositions 5.6Na2O : 1.3K2O : 4.3SiO2 : 0.5V2O5 : 2.0TMA : 206H2O at: 503 K; 473 K; 453 K; 438 K; 430 K.

XRD line broadening due to fine crystal/particle size in the c dimension was

observed for the platey crystals grown at lower temperatures which have a small (~100-

200 nm) thickness (Figure 48c-e). The line broadening also had a strong visual effect on

the XRD patterns of platey crystals, i.e., the broad peaks (e.g., 105, 116, 109, and 1110

98

reflections) nearly disappeared in the patterns of crystals grown at lower temperatures

(430-453 K) (Figure 49a-c).

Figure 49: XRD patterns of AM-6 products synthesized from mixtures with molar

compositions 5.6Na2O : 1.3K2O : 4.3SiO2 : 0.5V2O5 : 2.0TMA : 206H2O at: (a) 430 K, 14 d; (b) 438 K, 8 d; (c) 453 K, 6 d; (d) 473 K, 6 d; (e) 503 K, 4 d.

A similar effect was observed in the XRD patterns of AM-6 synthesized at 430 K

using TMAOH (Figure 50a), where the 105, 116, 109, and 1110 reflections practically

disappeared compared to the corresponding reflections of AM-6 grown at 503 K using

TMAOH (Figure 50b).

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Figure 50: XRD patterns of AM-6 products synthesized from mixtures with molar

compositions 4.5Na2O : 1.3K2O : 4.85SiO2 : 0.5V2O5 : 2.2TMAOH : 206H2O at: (a) 430 K, 14 d; (b) 503 K, 3 d.

The crystals grown at 430 K using TMAOH had all dimensions considerably

smaller than 1 µm with “thickness” (i.e., dimension along the crystal c axis) smaller than

~100-200 nm (Figure 51a). The small crystal c dimension illustrated in Figure 48c-e and

Figure 51a can be expected to contribute to the observed XRD line broadening [91].

These results imply that similarly to ETS-10 [92] the square crystal facets in AM-6 are

the growth surfaces.

Figure 51: FE-SEM images of AM-6 crystals synthesized from mixtures with molar

compositions 4.5Na2O : 1.3K2O : 4.85SiO2 : 0.5V2O5 : 2.2TMAOH : 206H2O at: (a) 430 K, 14 d; (b) 503 K, 3 d.

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The effect of synthesis temperature on the bandgap energy of AM-6 was also

investigated (Figure 52). The absorption edge of AM-6 products synthesized at lower

temperatures (i.e., T < 503 K, Figure 52b-e) was blue shifted with respect to the

absorption edge of AM-6 products synthesized at higher temperatures (i.e., T = 503 K,

Figure 52a). The calculated bandgap energies (using the method described in Section

3.4.9) for these products are as follows: for T = 503 K, Eg = 3.58 eV ± 0.02 eV; T = 473

K, Eg = 3.57 eV ± 0.03 eV; T = 453 K, Eg = 3.75 eV ± 0.01 eV; T = 438 K, Eg = 3.80 eV

± 0.02 eV; T = 430 K, Eg = 3.80 eV ± 0.01 eV. A detailed discussion regarding the

quality of the crystals with respect to their absorption spectra is given in Section 4.1.4.

Figure 52: Kubelka-Munk function UV-vis spectra and calculated bandgap energies

of AM-6 products synthesized from mixtures with molar compositions 5.6Na2O : 1.3K2O : 4.3SiO2 : 0.5V2O5 : 2.0TMA : 209H2O at: (a) 503 K, 4d; (b) 473 K, 6 d; (c) 453 K, 6d; (d) 438 K, 8 d; (e) 430 K, 14 d.

4.1.2 Crystal Growth Mechanism

High-resolution FE-SEM (Figure 53) revealed multiple nano-sized (size ~20-50

nm), square-shaped island-like structures and terraces on the square faces of AM-6

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crystals. The edges of these surface structures are oriented parallel to the edges of the

square crystal faces. These morphological and microtopographic features of AM-6

crystals are similar to those of ETS-10 crystals [20,22,95], and suggest, in agreement

with our hypothesis, a two-dimensional nucleation crystal growth mechanism with a

multi-nucleation, multilayer growth pattern for AM-6 similar to that established before

for ETS-10 [49,92]. The defects in ETS-10 form where the spreading surface nuclei

(islands) meet during crystal growth [49], and the concentration of structural defects in

ETS-10 crystals is related to their morphology/topography [49]. Therefore, these results

suggest, in agreement with our hypothesis, that defect concentration in AM-6 crystals

were controlled using the unseeded synthesis in a similar manner to that which has been

demonstrated for ETS-10 [49,92,93] (vide supra, Section 2.3.1.2).

Figure 53: Low-voltage, high-resolution FE-SEM image of the nano-sized square

shaped island-like structures and terraces on a square face of uncoated AM-6 crystal synthesized at 503 K from mixture with molar composition 4.5Na2O : 1.3K2O : 4.85SiO2 : 0.5V2O5 : 2.2TMAOH : 209H2O.

Crystals obtained at different temperatures (Figure 48), and from mixtures with

different Si/V ratio (Figure 44) and TMA content (Figure 43) exhibited the same general

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truncated square bipyramidal morphology, and the same type of surface features on their

square faces as the features illustrated in Figure 53. This suggests that the hypothesized

layer-by-layer AM-6 crystal growth mechanism, where multiple surface nuclei form and

propagate on the square crystal facets, operates over the entire range of the synthesis

parameters investigated. No evidence of a layer-by-layer crystal growth mechanism was

observed on the trapezoidal faces in AM-6, in agreement with the observations made for

isostructural ETS-10 [92]. Thus, these trapezoidal faces are likely a result of piling up of

the growth layers advancing on the neighboring square faces [22,92]. Consequently, the

morphological variation (i.e., change in the degree of development of the square and

trapezoidal crystal faces) of AM-6 crystals shown in Figure 44 (effect of the mixture

Si/V ratio) and Figure 48 (effect of the crystallization temperature) is likely a result of

variation of the normal growth rates of their square faces. Growth rates normal to the

crystal faces grown by a layer-by-layer mechanism depend on the relative rates of supply

of layers and their lateral spreading [94]. Thus, the increasing size of the square faces

relative to the size of the trapezoidal faces observed for AM-6 crystals synthesized from

mixtures with decreasing Si/V ratio (Figure 44) as well as at lower temperatures (Figure

48) suggests that these changes of synthesis parameters decreased the two-dimensional

nucleation rate faster than the lateral spreading rate. This implies [95] that the decreasing

mixture Si/V ratio (at constant TMA content) and crystallization temperature decreased

supersaturation levels in the crystallizing system. Similar effect of the mixture Si/V ratio

on supersaturation levels was concluded earlier from the analysis of the particle size

distributions of AM-6 products synthesized from mixtures with the constant TMA

content and variable Si/V ratio (Figure 45). Self-consistency of the morphological and

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size changes observed for crystals obtained upon changing the mixture Si/V ratio

strongly supports the hypothesized mechanism of AM-6 crystal growth.

4.1.3 Crystallization Kinetics

Figure 54 shows the “crystallization curves” for AM-6 synthesized at 503, 473,

and 443 K using TMABr (x=5.6, y=4.3, w=2.0, and z=206). These curves were

constructed by fitting the empirical Kholmogorov (Avrami) equation [96,97], z(t)=1-

exp(-ktn), to the percent crystallinity (C; C=z×100) data obtained from the XRD analysis.

In the Kholmogorov equation, z(t) is the fraction of crystals in the solid (i.e., crystalline

and amorphous) product at time t, and k and n are constants. These curves exhibit

sigmoid shape that characterizes typical zeolite crystallization process [98], which can be

divided into: (1) an induction stage, which likely involves rearrangements in the

synthesis mixture [99] and nucleation of crystals, and (2) a crystal growth stage when

crystal growth predominates and occurs initially at a higher supersaturation, which

decreases near the end of crystallization as the species consumed from solution cannot be

replenished because of the insufficient amount of amorphous material left in the system.

Based on data shown in Figure 54, the induction time, defined as the time necessary to

reach a few percent crystallinity and determined as a point where the tangent to the

“crystallization curve” at its steepest slope crosses the time axis, decreased with the

increasing synthesis temperature. The crystal growth rate, assumed to be proportional to

the rate of crystallization [98], dz/dt, defined as the steepest slope of the “crystallization

curve” (d2z/dt2=0), increased with the increasing temperature. Similar observations were

made for the crystallization of ETS-10 (the AM-6 homologue) in the presence of TMA

[37,39,40].

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Figure 54: Crystallization curves of AM-6 synthesized from mixture with molar

composition 5.6Na2O : 1.3K2O : 4.3SiO2 : 0.5V2O5 : 2.0TMABr : 206H2O at: (a) 503 K, (b) 473 K, and (c) 443 K. Symbols depict the experimental data points. Estimated error for each data point is ± 5 %.

4.1.4 Effect of Crystal Quality on the Optical Properties of AM-6

UV-vis spectroscopy has been used to determine the “crystal quality” by probing

the average chain length of the …Ti–O–Ti–O–Ti… chains in isostructural ETS-10, and

examine the effect of crystal size and quality on the optical properties of ETS-10

[18,49,93]. Therefore, UV-vis spectroscopic analysis was undertaken to further

investigate defects in a series of AM-6 crystals shown in Figure 44.

The UV-vis absorption spectra of AM-6 products synthesized at 503 K from

mixtures with molar compositions 5.6Na2O : 1.3K2O : ySiO2 : 0.5V2O5 : 2.0TMA : zH2O

(where y = 3.4-5.5 and z = 199-214) are shown in Figure 55 and Figure 56. In addition to

the broad absorption in the 200-350 nm region attributed to the O(2p) → V(3d) charge -

transfer transition [24,31], these samples showed an absorption shoulder between ~400-

500 nm, absorption band centered at ~594 nm, and a slight increase in absorbance at

105

~800 nm. The absorption shoulder in the range of ~400-500 nm has been attributed to the

OI2- → V5+ charge-transfer in distorted octahedral V5+ [31]. The absorption bands

centered at ~594 nm and above 800 nm correspond to metal-to-metal transitions of

distorted octahedral V4+ [100,101,102]. Therefore, these results suggest two different

oxidation states for vanadium in AM-6 synthesized in the absence of seeds. Similar

conclusions were reached for AM-6 prepared with seeds [31].

Figure 55: UV-vis spectra AM-6 products synthesized at 503 K from mixtures with

molar compositions 5.6Na2O : 1.3K2O : ySiO2 : 0.5V2O5 : 2.0TMA : zH2O with: (a) y = 3.4, z = 199, (b) y = 3.8, z = 202, (c) y = 4.3, z = 206, (d) y = 4.85, z = 209, and (e) y = 5.5, z = 214.

The intensity of the absorption band at ~594 nm increased significantly in

crystals obtained using decreasing mixture Si/V ratios (Figure 55). Detailed X-ray

analysis (Section 4.1.1.3, Figure 46) suggested “higher quality” AM-6 crystals for

products synthesized from mixtures with low Si/V ratios (i.e., y = 3.4-4.3). Therefore,

based on the increase of the ~594 nm band intensity, it is suggested that “higher quality”

AM-6 crystals (i.e., products synthesized from mixtures with y = 3.4-4.3) contain more

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V4+ sites (i.e., less V5+ sites) in their framework as probed by UV-Vis spectroscopy than

“lower quality” AM-6 crystals (i.e., crystals with more topographical faulting obtained

from mixtures with y = 4.85-5.5). In addition, the absorption edge of “higher quality”

AM-6 crystals (for which UV-vis absorption spectra are shown in Figure 55a-c) was red

shifted with respect to the absorption edge of “lower quality” AM-6 crystals (Figure 55d-

e). In agreement with this observation, the values for the bandgap energies (Eg) (Figure

56) of these “high quality” crystals (crystal morphologies shown in Figure 44a-c) were

lower when compared to crystals with more topographical faulting (i.e., “lower quality”

crystals, Figure 44d-e). The calculated bandgap energies of these products synthesized

from mixture Si/V ratios from 3.4 to 5.5 are as follows: 3.41 eV ± 0.03 eV; 3.49 eV ±

0.01 eV; 3.58 eV ± 0.02 eV; 3.70 eV ± 0.01eV; 3.82 eV ± 0.02 eV. These bandgap

energies are the lowest reported to date for vanadosilicate AM-6.

Figure 56: Kubelka-Munk function UV-vis spectra and calculated bandgap energies

of AM-6 products synthesized at 503 K from mixtures with molar compositions 5.6Na2O : 1.3K2O : ySiO2 : 0.5V2O5 : 2.0TMA : zH2O with: (a) y = 3.4, z = 199, (b) y = 3.8, z = 202, (c) y = 4.3, z = 206, (d) y = 4.85, z = 209, and (e) y = 5.5, z = 214.

107

FE-SEM images of AM-6 crystals grown at 503 K from mixtures with x = 5.6, y

= 3.4-5.5, w = 2.0 TMABr, and z = 206-214 (Figure 44) show an increase in the degree

of faulting and irregularities on the crystal surfaces with increasing mixture Si/V ratio.

As deduced in Section 4.1.2, AM-6 grows via layer-by-layer crystal growth mechanism

where multiple surface nuclei form and propagate on the square crystal facets. Since

stacking defects occur where these surface nuclei meet during crystal growth, the smaller

crystals with more topographical faulting may have greater defect concentration in the

AM-6 structure, and thus more discontinuities in the …V–O–V–O–V… chains. These

discontinuities in the AM-6 chains can be surmised from the effective mass model

[93,103,104,105,106,107,108], which compares the bandgap shift (∆E g) of a

semiconductor quantum wire (i.e., here the VO6 chains in AM-6) with diameter d and

length l to the bandgap of the corresponding bulk semiconductor (Equation 6):

ΔEg = h 2 + h 2

4μxyd2 8μzl2 Equation 6

where h is Plank’s constant, and µxy and µz are the reduced effective masses of the

excitons in the transverse and axial directions of the quantum wire, respectively. The first

term on the right-hand side in Equation 6 represents the radial quantum confinement

effect and the second term represents the axial quantum confinement effect. Large Eg

values for the semiconductor quantum wire imply larger ∆Eg. If a constant wire diameter

d for all AM-6 products synthesized from varying mixture Si/V ratios is assumed (i.e., all

chains in these products are monatomic; no missing insulating silica matrix between the

individual chains), the first term on the right-hand side in Equation 6 becomes constant.

As ∆E g decreases with decreasing mixture Si/V ratio (Figure 56), the value for the

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second term becomes smaller. This implies the longer wire length l in “high quality”

crystals (Figure 44a-c and Figure 46a-c). Since the V5+ species in AM-6 form where the

chains terminate [26], these results complement the UV-vis data for the ~594 nm band

shown in Figure 55 according to which the “high quality” crystals (with more continuous

chains) have more V4+ species. According to the photocatalytic mechanism (Table 1,

Section 1.1.2) and the fact that discontinuity in the …V–O–V–O–V… chains in AM-6

likely produces V–OH groups based on the analogous situation observed for …Ti–O–Ti–

O–Ti… chains/Ti–OH groups in ETS-10 [49], the “low quality” crystals (i.e., y = 4.85-5.5)

would result in more V-OH which may enhance the photocatalytic activity of AM-6.

Therefore, it is of interest to investigate the photocatalytic activity of “high quality” and

“low quality” AM-6 crystals.

4.1.5 Removal of TMA from the Micropores of AM-6

As-synthesized AM-6 crystals contain TMA in the micropores as shown by the

high-temperature (>573 K) weight loss in the TGA pattern (Figure 59a) corresponding to

the combustion of the SDA. Since the size of TMA cations (7.4 Å [36]) is comparable to

the size of AM-6 pores (4.9 Å ×7.6 Å [17]), TMA may have acted as a pore-filler

stabilizing the formation of AM-6 structure (vide supra, Section 4.1.1.1). Calcination in

air and ammonia treatments at elevated temperatures were carried out in efforts to

remove the TMA from the micropores of AM-6 while retaining its framework integrity.

4.1.5.1 Calcination in Air

According to literature findings [36,37,38,39,40], TMA can be effectively

removed from the ETS-10 structure, while retaining high crystallinity, by heating in air

109

at elevated temperatures (~ 723 K [38]). Therefore, efforts in removing TMA from the

micropores of AM-6 were carried out by calcination in air at various temperatures (623-

698 K). The percent crystallinity (% C) of each calcined AM-6 product, estimated by

normalizing the integrated intensity of the ∼24.6 °2θ peak of the sample to that of an un-

calcined fully crystalline (i.e., % C = 100 %) product (Figure 57e), was monitored at the

various calcination temperatures. High % C (i.e., % C ≥ 80%) was retained at calcination

temperatures up to 673 K and calcinations times of 30 min (Figure 57b-d). The AM-6

products calcined at 673 K for more than 30 minutes showed nearly complete framework

destruction (i.e., very low % C)

Figure 57: XRD patterns and % crystallinity retained (% C) of AM-6 products

synthesized at 503 K from mixture with molar composition 5.6Na2O : 1.3K2O : 4.85SiO2 : 0.5V2O5 : 2TMABr : 209H2O calcined in air for 30 min at: (a) 698 K, (b) 673 K, (c) 648 K, (d) 623 K, and (e) un-calcined.

In agreement with literature on the low (thermal) stability of AM-6 synthesized

with seeds [31], thermal treatment of TMA-AM-6 (i.e., AM-6 synthesized using TMA)

in air at temperatures higher than 673 K resulted in the extensive/complete destruction of

the crystalline structure (Figure 57a). However, unseeded AM-6 was found to be more

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thermally stable than seeded AM-6 when calcined in air at 673 K for 30 min (i.e., 80%

crystallinity retained for unseeded product versus approximately 0% crystallinity for

seeded product, Figure 58). Therefore, this unseeded synthesis method resulted in

products with higher thermal stability than the seeded synthesis method.

Figure 58: XRD patterns of seeded AM-6 product calcined in air for 30 min (a),

unseeded AM-6 product calcined in air for 30 min (b), and un-calcined unseeded AM-6 product (c).

Nitrogen sorption at 77 K of AM-6 calcined in air for 30 min at 673 K showed a

micropore volume of 0.059 cm3 g-1 which was very similar to un-calcined AM-6 (i.e.,

TMA-AM-6; 0.056 cm3 g-1,Table 5). These results suggest that calcination in air at 673

K resulting in the products with a relatively high % C was not sufficient to effectively

remove TMA from the micropore of AM-6. Therefore, a less severe method for

removing TMA was pursued.

111

Sample Surface Area (m2 g-1) Micropore Volume (cm3 g-1) As-synthesized AM-6 100 0.056

AM-6 treated with gaseous ammonia at 673 K (TMA-free AM-6)

326 0.130

AM-6 calcined in air at 673 K for 30 min

150 0.059

Seeded AM-6 384 0.152 Seeded AM-6 [31] Not reported 0.129 [31]

ETS-10 [110] 402 [110] 0.128 [110] Table 5: BET surface area and micropore volume of AM-6 and ammonia-treated

AM-6 (i.e., TMA-free AM-6) falling within the range of reported values for ETS-10 [110] and AM-6 synthesized with seeds [31].

4.1.5.2 Gaseous Ammonia Treatment at Elevated Temperatures

TMA was effectively removed from the pores by treatment with ammonia at

elevated temperatures (673 K) [109]. This was surmised from the absence of the SDA’s

weight loss step in the TGA pattern of AM-6 crystals previously treated with gaseous

ammonia (Figure 59b). Concomitantly, the water content in AM-6 increased after the

ammonia treatment (e.g., from 5.9 wt.% to 10.3 wt.%, Figure 59). This suggests that

porosity increased/developed so that additional water can enter into the channels. XRD

analysis indicated a minimal (~5%) loss of crystallinity of AM-6 products due to the

ammonia treatment at 673 K. Nitrogen sorption at 77 K showed the BET surface area of

326 m2 g-1 and micropore volume of 0.13 cm3 g-1 for AM-6 after the ammonia treatment

(i.e., TMA-free AM-6) (Table 5). These values match very well literature data for AM-6

synthesized with seeds [31] and ETS-10 [110] (Table 5), and indicate fully developed

microporosity with minimum pore blocking. The weight gain steps at temperatures

higher than 700 K likely correspond to the oxidation of V4+ to V5+.

112

Figure 59: TGA analysis of AM-6 products synthesized at 503 K from mixture with

molar composition 4.5Na2O : 1.3K2O : 4.85SiO2 : 0.5V2O5 : 2.2TMAOH : 209H2O: (a) as-synthesized product, and (b) product treated with gaseous ammonia at 673 K for 50 min [120].

4.1.6 Spectroscopic Characterization of Unseeded AM-6

The FTIR spectra of various AM-6 and ETS-10 samples in the framework

vibrations region are shown in Figure 60. These spectra are in good agreement with the

earlier literature data [38,64]. Each major band in ETS-10 (Figure 60d) at approximately

1040, 745, 660, 570, and 450 cm-1 appears to have a counterpart in AM-6 (Figure 60a-c).

However, in contrast to ETS-10 (Figure 60d), the intense and sharp band at 870 cm-1 is

observed in AM-6 samples (Figure 60a-c). Since the V–O stretching frequencies in the

six-coordinate oxovanadium (IV) compounds with V–O–V bridging appear somewhat

below 900 cm-1 [111,112,113], this band can be attributed to the V–O stretching

vibrations that involve octahedrally coordinated V4+ in AM-6. This is in agreement with

the assignment of this band in AM-6 synthesized using ETS-10 seeds [64]. The spectrum

of as-synthesized AM-6 obtained with TMA contains a weak band at 948 cm-1 (Figure

60a), which may be attributed to the TMA cations [38]. However, upon complete

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removal of TMA from the pores by ammonia treatment (as illustrated by the TGA results

in Figure 59, by the nitrogen sorption results in Table 5, as well as by disappearance of

the IR bands in the 1400-3100 cm-1 range (not shown), attributed [114] to N–C–H, H–C–

H, and C–H vibrations) the intensity of this band decreases only to some extent, resulting

in the appearance of a shoulder at 948 cm-1 (Figure 60b). AM-6 synthesized in the

absence of TMA using ETS-10 seeds [17] also exhibits a shoulder at 948 cm-1 (Figure

60c). Since ETS-10 does not show any IR absorption at this frequency (Figure 60d), this

suggests that the band at 948 cm-1 in the organic-free AM-6 may be due to the

framework vibrations, and that in AM-6 synthesized with TMA both the organic and the

framework contribute to the IR band observed at 948 cm-1. This band in the organic-free

AM-6 can be tentatively attributed to a V=O stretching since the IR bands typically

found in the 910-1035 cm-1 range in oxovanadium (IV) complexes are assigned to this

vibrational mode [111,112]. Thus, the band at 948 cm-1 may indicate the presence of

V=O bonds. These may be terminal V=O bonds in defect sites or on the external AM-6

crystal surfaces, postulated by Rocha et al. [17] based on the 946 cm-1 band observed in

the Raman spectrum. Recent theoretical results of Shough et al. [23,24] suggest that the

formation of V=O bonds (i.e., a five-fold coordination around the V5+ centers) along the

…V–O–V–O–V… chain may be caused by the presence of V5+ in the adjacent positions

within the chain. The weak IR band at 936 cm-1 in vanadosilicate AM-15 was also

attributed to the V=O bonds [64]. A distinct IR band at 930-960 cm-1 in vanadosilicate

microporous zeolites and mesoporous molecular sieves, and vanadia-silica aero-gels is

typically assigned to the Si–O–V stretching mode [115,116,117] (this implies that the

Si–O and V–O bonds are equivalent), but it is likely that the Si–O and V–O bonds are

114

nonequivalent [118], and thus this IR band may indicate a localized Si–O vibration in the

Si–O–V linkages.

Figure 60: FTIR absorption spectra of AM-6 products synthesized at 503 K from

mixture with molar composition 5.6Na2O : 1.3K2O : 4.85SiO2 : 0.5V2O5 : 2TMABr : 209H2O: (a) as-synthesized AM-6, (b) product treated with gaseous ammonia at 648 K for 1 h, (c) AM-6 synthesized in the absence of TMA using ETS-10 seeds, and (d) ETS-10 product [120].

Raman spectroscopy has been used as a means of determining the crystal quality

and disorder along the monatomic chains in titanosilicate ETS-10 [18,45,48,49,71,119].

Therefore, Raman spectroscopic analysis was undertaken to further investigate the

disorder in “high quality” AM-6 crystals (i.e., y = 4.3, Figure 44c) and “low quality”

AM-6 crystals (i.e., y = 4.85, Figure 44d). The Raman spectra of “high quality” and “low

quality” AM-6 products obtained using two different excitation lines (785 nm and 532

nm) are shown in Figure 61. In both cases, the spectra were dominated by a narrow band

at ~860 cm-1, characteristic of V–O–V stretch along the …V–O–V–O–V… chains

[17,19,31,33], and a band at ~940 cm-1 which has been attributed to the short V=O bonds

at defect or terminating surface sites in AM-6 [17,31,33]. However, the ~940 cm-1 band

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was more prominent for the “high quality” crystals (Figure 61, Ib and IIb), which may

indicate more V5+ terminating surface sites. In the case of the “high quality” crystals, the

Raman beam probes predominantly the more disrupted chains at the outer surface of the

crystal. This in turn implies that “high quality” crystals have more layers on the outer

surface that are not completed. (i.e., chains in the near outer surface region for the “high

quality” crystals are more disrupted). This (i.e., larger number of incomplete layers on

the outer surface of the crystals) was also reported for the “high quality” large ETS-10

crystals using atomic force microscopy (AFM) analysis [92] (AM-6 crystals synthesized

in this investigation were not easily amenable to AFM analysis due to their small

size/intricate shape and topography). This was manifested by the higher surface

roughness measured by AFM for large “high quality” ETS-10 crystals compared to that

measured for smaller crystals [92]. Also, broader ~860 cm-1 bands (Figure 61, II)

observed upon using the 532 nm (i.e., more surface sensitive excitation line) is consistent

with the notion that Raman excitation lines employed here probed predominantly the

outer surface crystal features, and that the outer crystal surface has a larger number of

disrupted chains than the crystal interior due to the fact that crystal exterior has layers

that are not yet completed due to the mechanism AM-6 crystal growth (vide supra,

Section 4.1.2). The broader ~860 cm-1 Raman band observed for the “high quality”

crystals (FWHM = 26 cm-1) than for the “low quality” crystals (FWHM = 20 cm-1)

(Figure 61, II), and the band increased FWHM observed upon using the 532 nm

excitation laser (i.e., more surface sensitive, Figure 61, II) are thus consistent with the

higher intensity of the ~940 cm-1 Raman band for these “high quality” crystals (and the

increased intensity of the ~940 cm-1 band observed upon probing crystals with the 532

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nm excitation line, Figure 61, IIb). This is again likely due to the limited Raman laser

penetration depth. As deduced from Section 4.1.4, “high quality” crystals have less

disrupted chains on average (i.e., throughout the entire crystal volume). Based on the

Raman observations of disorder along the monatomic chains in titanosilicate ETS-10 [49]

analogue of AM-6, the FWHM of the Raman ~860 cm-1 band is proportional to the

length distribution of the disrupted chains, i.e., the more disrupted the chains in AM-6

are, the broader the chain length distribution is, and thus the higher the FWHM of the

~860 cm-1 band is observed. Based on this, it follows that the more disrupted chains in

“low quality” crystals would have a broader distribution of chain lengths, and thus the

“lower quality” crystals would have broader ~860 cm-1 band. Even though this was not

observed from the Raman analysis, presumably due to the hypothesized limited

penetration depth of the Raman lines employed, the higher intensity of the ~940 cm-1

band and the broader ~860 cm-1 band for the “high quality” crystals as well as the

increased FWHM of the ~860 cm-1 band observed using more surface sensitive

excitation line are self consistent.

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Figure 61: Raman spectra recorded with 785 nm laser (I) and 532 nm laser (II) of

AM-6 products synthesized at 503 K from mixtures with molar compositions 5.6Na2O : 1.3K2O : ySiO2 : 0.5V2O5 : 2.0TMA : zH2O with: (a) y = 4.85 z = 209, and (b) y = 4.3, z = 206.

4.2

The introduction of heteroatoms into the framework of microporous materials is

an important process since it allows the fine-tuning of their properties [21], specifically,

their optical properties. In addition, since the close proximity of V4+ with V5 + along the

…V–O–V–O–V… chains in AM-6 promotes electron-hole recombination and decreases

the photocatalytic activity [31], isomorphous substitution was carried out in efforts to

Isomorphous Framework Substitution

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decrease the electron-hole recombination rate by separating these two oxidation states,

and consequently enhance the photocatalytic activity of AM-6.

The chemical analyses of the transition metal (TM)-substituted products (Table 6)

provided indirect evidence for TM isomorphous substitution in the vanadosilicate AM-6

framework. The elemental composition of unmodified AM-6 was found to be in good

agreement with the ideal unit cell composition for this material [17,120]. The crystal

TM/V ratios differed slightly from those used in the synthesis mixture. However, the

TM/V ratios in all crystalline products increased with increasing TM/V ratios in the

synthesis mixture. The crystal Si/(V+TM) ratio for all TM-substituted products was

identical within measurement error with that of unmodified AM-6. For low TM-

substituted products (i.e., mixture TM/V ≤ 0.05), the (Na+K)/(V+TM) ratio was identical

with that of unmodified AM-6, whereas for high TM-substituted products (i.e., mixture

TM/V ≥ 0.10), the (Na+K)/(V+TM) ratio decreased significantly when compared to that

of unmodified AM-6. This may be due to the presence of extra-framework sites in which

the TM ions would balance some of the framework charges and result in an overall

decrease in the (Na + K) content. Similar observations were reported for Cr-substituted

ETS-10 [64].

119

TM/V in synthesis

mixture (mol mol-1)

TM/V in crystals

(mol mol-1)

Si/(V+TM) in crystals (mol mol-1)

(Na+K)/(V+TM) in crystals

(mol mol-1)

Fe-AM-6 0.025 0.058 ± 0.005 5.71 ± 0.24 1.36 ± 0.08 0.0375 ----b ----b ----b 0.05 ----b ----b ----b 0.10 ----b ----b ----b 0.20 ----b ----b ----b

Cr-AM-6 0.025 0.052 ± 0.002 5.74 ± 0.18 1.23 ± 0.05 0.0375 0.060 ± 0.004 5.93 ± 0.26 1.56 ± 0.05 0.05 0.083 ± 0.005 5.46 ± 0.43 1.20 ± 0.42 0.10 0.229 ± 0.008 5.09 ± 0.20 0.64 ± 0.05 0.20 ----b ----b ----b

Co-AM-6 0.025 0.037 ± 0.003 5.36 ± 0.20 1.37 ± 0.07 0.0375 0.049 ± 0.005 5.79 ± 0.07 1.54 ± 0.06 0.05 0.061 ± 0.002 5.30 ± 0.16 1.36 ± 0.08 0.10 0.165 ± 0.001 5.57 ± 0.06 1.0 ± 0.07 0.20 ----b ----b ----b

AM-6 0 0 5.39 ± 0.15 1.40 ± 0.10 Table 6: Chemical compositionsa of various transition metal (TM) ions

isomorphously substituted AM-6 products. a calculated from EDX data b EDX data uncertain due to large amount of impurity in the AM-6 product

4.2.1 X-ray Powder Diffraction and Unit Cell Analysis

Figure 62 shows the XRD patterns of all transition metal (TM) substituted AM-6

products synthesized from mixture with TM/V = 0.025. Within measurement error, all

these samples showed identical peak positions and relative intensities to those of

unmodified AM-6 (Figure 62d). An additional peak at ~21.7o 2θ was observed for

samples synthesized from mixtures with Co/V ≥ 0.05, Cr/V ≥ 0.025, and Fe/V ≥ 0.025.

This peak increased in intensity with increasing mixture TM/V content. Additional

impurity peaks (not shown here) were observed for products obtained from mixtures

with Co/V ≥ 0.10, Cr/V ≥ 0.20, and Fe/V ≥ 0.0375. These additional peaks were located

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at the same angles in all TM-AM-6 samples synthesized. This suggests the same

impurity phase but with different relative content formed in all samples investigated.

However, no additional peaks due to oxide phases (cobalt oxide, chromium oxide or iron

oxide for Co-AM-6, Cr-AM-6 and Fe-AM-6, respectively) were observed.

Figure 62: XRD patterns of modified and unmodified AM-6 products synthesized at

503 K from mixture with molar composition 2.25Na2O : 1.3K2O : 4.85SiO2 : 0.5V2O5 : 2.25TMAOH : 209H2O : 0.025TM: (a) Fe-AM-6, (b) Cr-AM-6, (c) Co-AM-6, and (d) unmodified AM-6. Star symbols mark peaks due to unidentified impurities.

The unit cell volumes of TM-AM-6 crystals calculated from the XRD patterns

(space group 141) showed little expansion/contraction regardless of the mixture TM/V

content (and the resulting crystal TM/V content). The unit cell volumes for AM-6 and

TM-AM-6 grown from mixture with TM/V = 0.0375 are as follows: AM-6, 1519.22 Å3

± 3.785 Å3; Co-AM-6, 1519.03 Å3 ± 3.785 Å3; Cr-AM-6, 1519.77 Å3 ± 3.566 Å3; Fe-

AM-6, 1517.52 Å3 ± 3.718 Å3. These XRD observations and calculated unit cell volumes

(Table 7) are consistent with the similar ionic radii of Co2+ (0.72 Å for tetrahedral

coordination, and 0.78 Å for octahedral coordination [25]), Co3+ (0.685 Å for octahedral

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coordination [25]), Cr3+ (0.755 Å for octahedral coordination [25]), and Fe3+ (0.785 Å for

octahedral coordination [25]) when compared to that of V4+ (0.72 Å for octahedral

coordination [25]) and V5+ (0.68 Å for octahedral coordination [25]) in unmodified AM-

6. Eldewik et al. [121] and Brandao et al. [64] reported similar observations for Fe-

substituted [121] and Cr-substituted [64] titanosilicate ETS-10, where the ionic radius of

Ti4+ is 0.745 Å [25].

Sample Mixture TM/V content

a(b) (Å) c (Å) Vol (Å3)

Fe-AM-6 0.025 7.4446 ± 0.00473 27.364 ± 0.02964 1516.57 ± 3.569 0.0375 7.43836 ± 0.00520 27.4272 ± 0.02881 1517.52 ± 3.718 0.05 7.44436 ± 0.00591 27.3739 ± 0.02706 1517.02 ± 3.778

Cr-AM-6 0.0375 7.45343 ± 0.00549 27.3459 ± 0.02393 1519.77 ± 3.566 0.05 7.45099 ± 0.00463 27.3474 ± 0.02542 1518.26 ± 3.298 0.10 7.43827 ± 0.00501 27.3929 ± 0.02332 1515.59 ± 3.331

Co-AM-6 0.025 7.45501 ± 0.00637 27.3384 ± 0.02895 1519.40 ± 4.207 0.0375 7.45343 ± 0.00561 27.3434 ± 0.02697 1519.03 ± 3.785 0.05 7.44963 ± 0.0056 27.3321 ± 0.02695 1516.85 ± 3.777 0.10 7.44855 ± 0.00474 27.349 ± 0.0296 1517.35 ± 3.572 0.20 7.44596 ± 0.00477 27.3957 ± 0.03801 1518.88 ± 4.054

AM-6 0 7.45482 ± 0.0056 27.3367 ± 0.02695 1519.22 ± 3.785 Table 7: Unit cell parameters of various TM-AM-6 products.

4.2.2 Effect of Substitution on the Development of the Crystal Surfaces

The degree of development of the crystal square and trapezoidal faces in the

truncated square bipyramidal TM-AM-6 crystals, which may be quantified by calculating

the crystal “aspect ratio” R, and size of the TM-AM-6 products varied depending on the

substituted TM ion (Figure 63) and TM/V content. Unmodified AM-6 crystals exhibited

the typical [120] truncated square bipyramidal morphology (Figure 63d) with R = 0.9 ±

0.2 and an average product particle size of 0.80 µm as determined by PSD analysis

(Appendix A, 9.2). Substitution using mixture Co/V = 0.025 had very little effect on the

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relative crystal faces development and size of the Co-AM-6 crystals as determined by

FE-SEM (Figure 63c) and PSD analyses (Appendix A, 9.2), respectively. However, an

increase in the mixture Co/V content resulted in an increase in the crystal aspect ratio

from R = 1.0 ± 0.1 for 0.025Co-AM-6 (i.e., mixture Co/V = 0.025) to R = 4.2 ± 0.4 for

0.20Co-AM-6 (i.e., mixture Co/V = 0.20), and a decrease in the fraction of Co-AM-6 in

the products as observed by FE-SEM (Figure 64c), which was in agreement with the

XRD observations. The relative development of crystal faces for Fe-AM-6 (Figure 63a)

and Cr-AM-6 (Figure 63b) differed significantly from that of unmodified AM-6 upon

even the lowest TM-substitution investigated, i.e., R = 2.4 ± 0.4 for 0.025Fe-AM-6

(mixture Fe/V = 0.025) and R = 3.0 ± 0.9 for 0.025Cr-AM-6 (mixture Cr/V = 0.025).

Similar to Co substitutions, an increase of the mixture TM/V content for Fe and Cr

substitutions also resulted in an increase of the crystal aspect ratio.

Figure 63: FE-SEM images of modified and unmodified AM-6 crystals synthesized

at 503 K from mixture with molar composition 2.25Na2O : 1.3K2O : 4.85SiO2 : 0.5V2O5 : 2.25TMAOH : 209H2O : 0.025TM: (a) Fe-AM-6, (b) Cr-AM-6, (c) Co-AM-6, and (d) unmodified AM-6.

123

The fraction of Fe-AM-6 and Cr-AM-6 in the products estimated by FE-SEM

decreased upon increasing mixture Fe/V content (Figure 64a) and Cr/V content (Figure

64b), respectively, which is consistent with the XRD observations. Since AM-6 grows

via a two-dimensional nucleation crystal growth mechanism [120], the morphological

variation of crystals observed in Figure 63 upon TM substitutions is a result of change in

the degree of development of the square and trapezoidal crystal faces, which are

inversely proportional to the growth rates normal to the crystal faces. Growth rates

normal to the crystal faces developed by a two-dimensional nucleation crystal growth

mechanism depend on the relative rates of supply of layers and layer lateral spreading

[122]. Therefore, the increase in the crystal aspect ratio (i.e., decrease of the size of the

square face relative to the size of the trapezoidal face) upon incorporation of TM

(mixture Fe/V ≥ 0.025, Cr/V ≥ 0.025, and Co/V ≥ 0.10) suggests that the presence of TM

increased the two-dimensional nucleation rate faster than the layer lateral spreading rate

[123].

Figure 64: FE-SEM images of modified AM-6 crystals synthesized at 503 K from

mixture with molar composition 2.25Na2O : 1.3K2O : 4.85SiO2 : 0.5V2O5 : 2.25TMAOH : 209H2O : sTM: (a) s = 0.0375Fe/V, (b) s = 0.10Cr/V, and (c) s = 0.20Co/V.

Faulting/irregularities on the crystal surfaces imaged by FE-SEM appeared to be

similar for all synthesized products (Figure 63). These observations coincided with the

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identical (within error of analysis) average FWHM of AM-6 reflections obtained from

the XRD data (Table 8). This suggests TM substitutions did not introduce additional

crystalline defects/disorder in the AM-6 framework to those that may have been caused

by the AM-6 crystal growth mechanism.

Sample Average FWHM (o) Fe-AM-6 0.2145 ± 0.0491 Cr-AM-6 0.1574 ± 0.0085 Co-AM-6 0.1997 ± 0.0499

AM-6 0.1889 ± 0.0295 Table 8: The average full-width-at-half-maximum (FWHM) of AM-6 reflections in

the 5-37.5 º2θ range for different TM-AM-6 samples (TM = Fe, Cr, Co; mixture TM/V = 0.025).

4.2.3 Optical and Electronic Properties of TM-AM-6

UV-vis spectroscopic analysis (Figure 65 and Figure 66) was performed to

determine the oxidation states of the TM ions substituted in AM-6 as well as determine

bandgap energies of the TM-AM-6 products. Spectrum of unmodified AM-6 (Figure 65a)

exhibited the typical intense broad absorption in the 200-350 nm region attributed to the

O(2p) → V(3d) charge-transfer transition [24,31]. In addition, the characteristic AM-6

absorption features in the visible light range, i.e., a shoulder at ~450 nm and an

absorption band centered at ~590 nm, were observed. Upon incorporation of TM ions

into the framework, the absorption edge of AM-6 red shifted (Figure 65a) and new

absorption features (Figure 65), which increased in intensity with increasing mixture

(and crystal) TM concentration (Figure 66), were observed in the low energy region of

the UV-vis spectra.

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Figure 65: (a) UV-vis spectra of unmodified AM-6 and TM-AM-6 products grown

at 503 K from mixture with molar composition 2.25Na2O : 1.3K2O : 4.85SiO2 : 0.5V2O5 : 2.25TMAOH : 209H2O : 0.0375TM; (b) magnification of the lower energy region of the spectra in (a) used to highlight the features specific to the substitution of various TM ions.

Upon incorporation of Fe into the AM-6 framework (Fe-AM-6, Figure 65 and

Figure 66a), three new/increased absorption bands at ~380-450, ~590, and >725 nm were

observed. The absorption shoulder in the range of ~380-450 nm has been reported to be

convoluted into two peaks [124] with the lower region (~380 nm) being attributed to the

126

direct O2- → Fe3+ charge-transfer transition in octahedral Fe3+ [125], and the higher

absorption region (~450 nm) attributed to Fe3+ → Fe3+ intercationic charge-transfer

transition [126]. The absorption features at ~590 nm and > 725 nm have both been

attributed to the d → d transition of octahedral Fe 3+ ions [124]. Similar observations

were reported for Fe3+ substituted in the Ti4+ octahedral sites of ETS-10 [121]. Cr-AM-6

gave two new/increased absorption bands at ~430 nm and a prominent feature at 610 nm

(Figure 65and Figure 66b) which have been attributed to Cr3+ ions surrounded by six

oxygen ligands in octahedral coordination [127,128]. Co-AM-6 gave an absorption

feature in the range of ~400-500 nm and a feature centered at ~600 nm (Figure 65 and

Figure 66c), which have been attributed to charge-transfer transitions in tetrahedrally

coordinated Co2+ [129,130]. However, it has also been reported that the distinction

between the different Con+ (where n = 2 or 3) species (i.e., tetrahedral and/or octahedral)

is difficult due to the overlapping octahedral and tetrahedral transitions in this range

[130]. Uma et al. [63] assigned the absorption feature at ~500-600 nm in Co-substituted

ETS-10 to the presence of Co2+ and Co3+. EDX analysis suggested Co2+ substitution for

Si4+ in the ETS-10 framework [63]. However, the authors did not report unit cell

dimension calculations to confirm this. Eldewik and Howe [60] assigned the absorption

feature between 14000 and 19000 cm-1 (i.e., ~526-714 nm) to the d → d transitions in

tetrahedral Co2+ in Co-substituted ETS-10, which coincided with the expansion in the

unit cell volume of ETS-10 upon incorporation of Co2+ for the tetrahedral Si4+ sites [60].

However, the identical unit cell volume of Co-AM-6 and unmodified AM-6 obtained

here (vide supra, Section 4.2.1) suggests that the Co2+/3+ species do not substitute for the

tetrahedral Si4+ sites in the vanadosilicate framework (ionic radius of Si4+ = 0.40 Å [25]),

127

and thus it is concluded that the Co2+/3+ species most likely substitute for the V

octahedral sites.

Figure 66: UV-vis spectra for TM ion modified AM-6 products synthesized at 503 K

from mixture with molar composition 2.25Na2O : 1.3K2O : 4.85SiO2 : 0.5V2O5 : 2.25TMAOH : 209H2O : xTM substituted with: (a) Fe-AM-6, x = 0.025-0.05; (b) Cr-AM-6, x = 0.025-0.10; (c) Co-AM-6, x = 0.025-0.20.

The bandgap energies of all TM-AM-6 samples were determined using the

method of Davis and Mott [82], where an n value of ½ was used since it resulted in the

best linear fit in the lower absorption region of AM-6 (Section 3.4.9). Upon

incorporation of TM ions into the framework, the bandgap energy of AM-6 shifted to

lower values. The calculated bandgap energy values are as follows: 3.62 eV ± 0.01 eV

for 0.0375Cr-AM-6; 3.68 eV ± 0.02 eV for 0.0375Fe-AM-6; 3.78 eV ± 0.01 eV for

0.0375Co-AM-6; and 3.82 eV ± 0.03 eV for unmodified AM-6. These values did not

128

change with increasing crystal TM/V content. The red shift in the absorption edge (i.e.,

bandgap energy) observed for TM-AM-6 can be attributed to the charge-transfer

transitions between the metal ion d electrons and the AM-6 conduction band or valence

band [15]. In addition, this shift is an indication that the isomorphous TM ion

substitutions are occurring in or near the VO6 chains.

4.2.4 Effect of TM-Substitution on the Local Environment of the VO6 Chains

Raman spectroscopic analysis (excitation line of 785 nm) was used to investigate

the effect of isomorphous substitutions of TM ions into the AM-6 framework by probing

the local environment of the …V–O–V–O–V… chains. Unmodified AM-6 showed the

characteristic asymmetric band at 860 cm-1 (Figure 67d) assigned to VO6 octahedra [17,31].

Upon incorporation of TM ions, this band shifted slightly to higher frequency (Figure 67a-

c). Similar observations of the high frequency shift for the 724 cm-1 band, attributed to the

…Ti–O–Ti–O–Ti… chains in ETS-10, were reported for Nb-substituted ETS-10 [65] and

Co-substituted ETS-10 [60]. These results were attributed to successful incorporation of

the TM ions into the ETS-10 framework. However, shifts in the Raman frequencies have

also been reported to be due to disorder in the semiconductor chains in ETS-10. Southon

and Howe [49] reported a shift and broadening of the 724 cm-1 band due to the stretching

mode of the …Ti–O–Ti–O–Ti… chains upon increasing disorder in various ETS-10

samples. These observations were in agreement with the SEM observations indicating an

increase in the faulting/irregularities on the ETS-10 crystal surfaces [49]. In the case of

TM-substituted AM-6, the FWHM (20 cm-1) of the 860 cm-1 band did not change when

compared to unmodified AM-6 (20 cm-1), and therefore suggests the near identical

structural order in AM-6 framework of the TM-modified and unmodified crystals (note

129

that upon increased “quality” of AM-6 crystals (i.e., less interrupted …V–O–V–O–V…

chains) grown from mixtures with decreasing Si/V ratio, the FWHM of 860 cm-1 Raman

band, acquired using the same excitation line of 785 nm, increased from 20 cm-1 to 26 cm-

1, vide supra, Section 4.1.6). These observations were also consistent with the identical

FWHM of the AM-6 reflections and topographical features determined by XRD and FE-

SEM, respectively (Table 8 and Figure 63). This suggests the shift of the 860 cm-1

Raman band to higher frequencies is likely due to the incorporation of TM in the AM-6

framework, and is not caused by increased structural disorder caused by isomorphous

substitution. No additional bands due to TM oxide phases (i.e., cobalt oxide, chromium

oxide or iron oxide for Co-AM-6, Cr-AM-6 and Fe-AM-6, respectively) were observed,

which further confirmed the presence of the TM ions in the substitutional sites of the

AM-6 framework. However, upon Fe and Cr substitution, additional bands were

observed (Figure 67a and b, respectively) which may be due to the unidentified

impurities in the product. These observations are in agreement with the appearance of the

~ 21.7o 2θ reflection for samples with Cr/V ≥ 0.025 and Fe/V ≥ 0.025 (Figure 62).

130

Figure 67: Raman spectra recorded with 785 nm laser of modified and unmodified

AM-6 products synthesized at 503 K from mixture with molar composition 2.25Na2O : 1.3K2O : 4.85SiO2 : 0.5V2O5 : 2.25TMAOH : 209H2O : 0.0375TM : (a) Fe-AM-6, (b) Cr-AM-6, (c) Co-AM-6, and (d) unmodified AM-6. Star symbols mark bands due to unidentified impurities.

4.2.5 Effect of TM-Substitution on the Framework Vibrations of AM-6

The FTIR spectra of the various TM-substituted AM-6 products in the framework

vibrations region are shown in Figure 68. Unmodified AM-6 (Figure 68d) exhibited an

intense sharp band at ~870 cm-1 typically attributed to V–O stretching vibrations that

involve octahedrally coordinated V4+ [120], and a weak band at ~948 cm-1 characteristic of

the presence of V5+ in AM-6. Results in Section 4.1.6 (Figure 60) confirmed the

assignment of the weak band at ~948 cm-1 to framework vibrations and not only the TMA+

cations used in synthesis since this band (~948 cm-1) was still present after ammonia-

treatment (i.e., removal of TMA from the pores of AM-6). No TM oxide phases were

detected upon incorporation of TM ions in AM-6 framework (Figure 68a-c). However, it

was observed that the band at ~570 cm-1, typically attributed to Si–O rocking [131] and O–

131

V–O bond stretching vibrations, shifted to lower wavenumbers upon substitution of AM-6

with TM ions (Figure 68). In addition, as the mixture (and crystal) TM concentration

increased, the intensity of these bands (e.g., ~870 cm-1 and ~948 cm-1) decreased

(Appendix 9.3), and the bands at ~870 cm-1 and ~570 cm-1 shifted to higher and lower

wavenumbers, respectively (Appendix 9.3). Similar findings were reported for Nb-

substituted ETS-10 [65]. These observations further confirm successful isomorphous

substitution of transition metal ions into the AM-6 framework.

Figure 68: Diffuse reflectance FTIR absorption spectra of modified and unmodified

AM-6 products synthesized at 503 K from mixture with molar composition 2.25Na2O : 1.3K2O : 4.85SiO2 : 0.5V2O5 : 2.25TMAOH : 209H2O : 0.025TM : (a) Fe-AM-6, (b) Cr-AM-6, (c) Co-AM-6, and (d) unmodified AM-6.

4.3

In this investigation, methylene blue (MB) was chosen as a model dye to evaluate

the photocatalytic activity of unmodified and modified AM-6 products. The MB

concentration during each photocatalytic experiment was determined from the

Photocatalytic Degradation of Methylene Blue

132

absorbance maximum at ~664 nm in the UV-vis spectrum using the correlation

established in Figure 33.

4.3.1 Photolysis of Methylene Blue

Photolysis of methylene blue (MB) was carried out in order to effectively

evaluate the photocatalytic activity of unmodified and modified AM-6 products towards

the photodegradation of MB under both UV and visible light irradiation. Figure 69

shows a very slow photolysis process under UV light irradiation. The UV photolysis data

were fitted using the pseudo-first-order reaction mechanism [81] and the rate constant

was determined to be 0.0003 min-1 ± 0.0001 min-1.

Figure 69: Direct photolysis of methylene blue at room temperature under UV light

irradiation (310-400 nm).

The direct photolysis of MB under visible light irradiation (Figure 70) was

significantly different compared to photolysis under UV light irradiation. The pseudo-

first-order reaction rate constant was determined to be 0.0028 min-1 ± 0.0004 min-1,

which suggests that the photodegradation route involving visible light irradiation of MB

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is much faster compared to the photodegradation route involving UV light irradiation.

These results are consistent with the photodegradation mechanism of MB under visible

light irradiation involving a singlet oxygen pathway, produced by the energy transfer

from the dye (i.e., dye-sensitization), which reacts with the ground state dye to give N-

methylated products [132]. Furthermore, a hypsochromic effect (i.e., blue shift in the

spectral band at ~664 nm), arising from N-demethylation of MB, was more prominent

under visible light irradiation (Figure 70).

Figure 70: Direct photolysis of methylene blue at room temperature under visible

light irradiation (420-630 nm).

4.3.2 Effect of Isomorphous Framework Substitution on the Photocatalytic Activity

Photodegradation of MB occurs via two routes: (1) the photoreduction of MB to

leuco-methylene blue (LMB) by a scavenging electron donor under anaerobic conditions;

and (2) photooxidation of MB by O2 [80]. The first mechanism (1) is reversible upon

exposure to oxygen. A hypsochromic effect and a decrease in the MB absorbance

maximum at ~664 nm were observed for all TM-modified AM-6 samples under both UV

and visible light irradiation. An example of the temporal changes and shift of the ~664

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nm MB peak during MB photodegradation under UV light irradiation on 0.025Cr-AM-6

are shown in Figure 71. These observations (i.e., hypsochromic effect and decrease in

MB absorbance) suggested oxidative N-demethylation of the dimethylamino groups in

MB occurring together with the oxidative degradation of the MB chromophore [81].

Formation of N-demethylated intermediates implied [80] that the AM-6-sensitized

photoreduction of MB possible during the UV (and visible) light activation of a

photocatalyst [80] did not occur in this investigation. Therefore, the concentration of

dissolved oxygen in the reaction mixtures and the rate of trapping the photogenerated

conduction band electrons by oxygen adsorbed onto the AM-6 surface (under UV light

irradiation) appeared to be sufficient [80] to result in the photodegradation of MB.

Figure 71: Temporal spectral changes of MB in aqueous 0.025Cr-AM-6 suspension

under UV light (310-400 nm) irradiation.

4.3.2.1 Photocatalytic Activity under UV Light Irradiation

The pseudo-first-order reaction rate constants (k) were determined based on the

photodegradation kinetics data shown in Figure 72, and the k values are shown in Table

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9. All these k values were larger than the direct UV light photolysis rate constant

determined for MB (0.0003 min-1 ± 0.0001 min-1) under the same reaction conditions.

Therefore, all TM-modified and unmodified AM-6 samples were active in the

photodegradation of MB under UV light irradiation. These activities are in the following

order: unmodified AM-6 ≈ Fe-AM-6 < Co-AM-6 << Cr-AM-6.

Figure 72: Photocatalytic degradation kinetics of MB under UV light (310-400 nm)

irradiation on unmodified AM-6 and 0.025TM-AM-6: TM = Fe; Cr; Co. MB photolysis pseudo-first-order reaction rate constant was determined to be 0.0003 min-1 ± 0.0001 min-1.

The BET surface area measurements of these products (Table 9) increased in the

following order: Fe-AM-6 < unmodified AM-6 < Cr-AM-6 ~ Co-AM-6. The activities of

these products (k values, Table 9) did not correlate well with the BET surface area values

of the different TM-substituted AM-6 products (e.g., 0.025Fe-AM-6 and 0.025Co-AM-6

showed similar rate constants, however their surface areas were significantly different,

Table 9). Therefore, other factors were evidently affecting the photocatalytic activity of

these products.

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Sample k (min-1) Surface Area (m2 g-1) Eg (eV) Unmodified AM-6 0.0017 ± 0.0007 99.8 ± 10.0 3.82 ± 0.01

0.025Fe-AM-6 0.0024 ± 0.0005 58.7 ± 6.0 3.68 ± 0.02 0.025Cr-AM-6 0.0197 ± 0.0042 127.4 ± 12.7 3.62 ± 0.01 0.025Co-AM-6 0.0039 ± 0.0006 134.4 ± 13.4 3.78 ± 0.01

UV Direct Photolysis 0.0003 ± 0.0001 --- --- Table 9: Pseudo-first-order reaction rate constants (k) for the photodegradation of

MB under UV light (310-400 nm) irradiation, BET surface areas, and bandgap energies (Eg) of various 0.025TM-substituted AM-6 products (TM = Fe, Cr, Co).

In the UV irradiation range investigated here, the photodegradation of MB can be

mainly ascribed to the bandgap photoexcitation of (unmodified and modified) AM-6

(Figure 2). MB photodegradation mechanism occurs via a photocatalytic pathway (Table

1) where the photoexcited electrons in the conduction band and the positive holes in the

valence band, which arise from the photoexcitation of AM-6, participate in the

degradation of MB via the formation of reactive oxygen radicals (e.g., OH•, HO2•, Table

1, reactions T7-T10) [80,81]. Therefore, the higher photocatalytic activities of TM-AM-6

samples compared to that of unmodified AM-6 may be attributed to the somewhat

broader UV irradiation range utilization by the TM-substituted AM-6 products, as

indicated by the lower bandgap energies of all TM-AM-6 products compared to that of

unmodified AM-6 (Table 9). However, the broader UV irradiation range utilization by a

photocatalyst cannot explain the absence of correlation between k and Eg values for TM-

AM-6 products (Table 9). Thus, it appears that the photocatalytic activities of these TM-

AM-6 products are dependent on the type of TM ions isomorphously substituted in the

AM-6 framework.

The increase in photocatalytic activity (Table 9) of TM-AM-6 products under UV

light irradiation may be attributed to the electron and/or hole trapping effect as a result of

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TM-substitution (Equation 1 and Equation 2, Section 1.2.2). Since both 0.025Co-AM-6

and 0.025Cr-AM-6 showed increased activity compared to unmodified AM-6 products,

it can be hypothesized that the energy levels of these substituted TM ions (i.e., Cr and Co)

lie within the AM-6 bandgap (i.e., TM substitutions result in midgap states). These

midgap states can serve as electron and/or hole trapping sites along the …V–O–V–O–

V… chains in AM-6, depending on whether these states are unoccupied or occupied,

respectively. The variations in the photocatalytic activities of the TM-AM-6 products

may be attributed to the different energy levels (i.e., redox potential), oxidation states,

and d electronic configurations of the substituted TM ion. Even though Fe3+ and Cr3+

have identical oxidation states, and behave as both electron and hole traps [15,16], their

effects on AM-6 photocatalytic activity are substantially different (Figure 72 and Table

9). The trapped holes in Cr4+ (Cr3+ + hvb+ → Cr4+) and Fe4+ (Fe3+ + hvb

+ → Fe4+) can

either migrate to the surface or recombine [15]. According to the DFT calculations of

various transition metal substituted ETS-10 [66] and since photogenerated holes prefer

states of higher energy, a trapped hole in the localized Cr3+ (dxy) can easily transition into

the more delocalized dxz/dyz state (Section 2.3.2.1, Figure 28), which can be more

effective in transporting the photogenerated holes to the active sites of AM-6. This

would make 0.025Cr-AM-6 more active than 0.025Fe-AM-6, as observed (Figure 72).

The low activity of 0.025Co-AM-6 may be attributed to the slightly higher Eg value

(Table 9).

The photocatalytic activities of the TM-AM-6 products substantially decreased

with increasing TM concentrations (Figure 73). This may be attributed to the decrease in

the fraction of TM-AM-6 in the product (Figure 64, Section 4.2.1 and Section 4.2.2),

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and/or an increase in the recombination rates as a result of a decrease in the average

distance between the trapping sites, which would promote recombination. Similar

observations were reported for high levels of TM-doped TiO2 [15,16].

Figure 73: Photocatalytic degradation kinetics of MB under UV light (310-400 nm)

irradiation on transition metal substituted AM-6 (TM-AM-6): (a) Fe-AM-6; (b) Cr-AM-6; (c) Co-AM-6.

4.3.2.2 Photocatalytic Activity under Visible Light Irradiation

Previous reports [31] have shown visible light photocatalytic activity of AM-6

towards the polymerization of ethylene. The authors [31] attributed the visible

photocatalytic activity to the O2- → V5+ charge-transfer transition at ~450 nm. Therefore,

visible light irradiation (420-630 nm) was used to investigate the visible light

photocatalytic activity of unmodified and TM-modified AM-6 products towards the

photodegradation of MB. The pseudo-first-order reaction rate constants (k) were

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determined based on the photodegradation kinetics data shown in Figure 74, and the k

values are shown in Table 10. These k values for all materials investigated except for

0.025Cr-AM-6 were identical within experimental error to that of direct visible light

photolysis for MB (0.0028 min-1 ± 0.0004 min-1) under the same reaction conditions.

Therefore, 0.025Fe-AM-6, 0.025Co-AM-6, and unmodified AM-6 samples were

considered inactive towards the photodegradation of MB under visible light irradiation.

The absence of MB photodegradation on these AM-6 products (i.e., Fe-AM-6, Co-AM-6,

and unmodified AM-6) under visible light irradiation may be due to the ineffectiveness

of the electron excitation for the interfacial charge-transfer. These results are consistent

with the UV-vis spectroscopic analysis (Section 4.2.3), i.e., both spectra of Fe-AM-6 and

Co-AM-6 showed visible light absorption features which were due to Fe3+ → Fe3+

intercationic charge-transfer transitions and d → d transitions, respectively. The higher

photocatalytic activity of 0.025Cr-AM-6 (i.e., k = 0.0046 min-1 ± 0.0004 min-1)

compared to the other TM-modified and unmodified AM-6 products may be attributed to

the OI2- → Cr3+ charge-transfer transitions occurring in the visible light range (Figure 65,

Section 4.2.3), which likely resulted in an electron excitation for the interfacial charge-

transfer.

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Figure 74: Photocatalytic degradation kinetics of MB under visible light (420-630

nm) irradiation on unmodified AM-6 and 0.025TM-AM-6: TM = Fe; Cr; Co. MB photolysis pseudo-first-order reaction rate constant was determined to be 0.0028 min-1 ± 0.0004 min-1.

Sample k (min-1)

Unmodified AM-6 0.0029 ± 0.0007 0.025Fe-AM-6 0.0031 ± 0.0004 0.025Cr-AM-6 0.0046 ± 0.0004 0.025Co-AM-6 0.0034 ± 0.0006

Vis Direct Photolysis 0.0028 ± 0.0004 Table 10: Pseudo-first-order reaction rate constants (k) for the photodegradation of

MB on various 0.025TM-substituted AM-6 products (TM = Fe, Cr, Co) under visible light (420-630 nm) irradiation.

The photocatalytic activities of the TM-AM-6 products remained near identical

with within experimental error with increasing TM concentrations (Figure 75). These

results are consistent with the UV-vis spectroscopic analyses of Fe-AM-6 and Co-AM-6

in that the visible light absorption features observed were due to Fe3+ → Fe3+

intercationic charge-transfer transitions for Fe-AM-6, and d → d transitions for Co -AM-

6. The slight decrease in the photocatalytic activity of Cr-AM-6 may be due to the

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decrease in the fraction of Cr-AM-6 product with increasing Cr/V content (Figure 64,

Section 4.2.1 and Section 4.2.2).

Figure 75: Photocatalytic degradation kinetics of MB under visible light (420-630

nm) irradiation on transition metal substituted AM-6 (TM-AM-6): (a) Fe-AM-6; (b) Cr-AM-6; (c) Co-AM-6.

4.3.3 Photocatalyst Re-use

The repetitive use of a photocatalyst is very important for practical applications.

Therefore, to investigate the lifetime of the photocatalyst, consecutive experiments were

conducted under identical conditions using the most active photocatalyst (i.e., 0.025Cr-

AM-6). In these experiments, after the completion of the 1st run of photodegradation of

MB under UV light irradiation, the photocatalyst at the end of the 1st reaction cycle was

collected, washed thoroughly with 1 L DI water, dried and analyzed using XRD and

SEM for evaluation of crystallinity. XRD analysis (Figure 76) indicated a slight loss (i.e.,

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~5% loss) in crystallinity upon completion of the 1st reaction cycle. However, no

discernable loss was shown by SEM analysis (Appendix 9.4). The 2nd reaction cycle

utilized the same products collected from the 1st reaction cycle.

Figure 76: XRD patterns and average (avg.) full-width-at-half-maximum (FWHM)

of 0.025Cr-AM-6 before photocatalysis (a), after the 1st reaction cycle (b), and after the 2nd reaction cycle (c) under UV light irradiation.

The pseudo-first-order reaction rate constants (k) were determined based on the

photodegradation kinetics data shown in Figure 77. Even though 0.025Cr-AM-6 retained

high % crystallinity after the 1st reaction cycle, it appears that the photocatalytic activity

significantly decreased: k1st rxn cycle = 0.0197 min-1 versus k2nd rxn cycle = 0.0023 min-1. This

drop-off in the reaction rate constant for the “used” catalyst (i.e., catalyst used in the 2nd

reaction cycle) may be attributed largely to the deactivation of the active sites on Cr-AM-

6 post-photocatalysis. Deactivation of these active sites may result from MB adsorbed

onto the photocatalyst after the 1st reaction cycle. This is shown in Figure 78 with the

“used” catalyst adsorbing less MB than the “fresh” catalyst (i.e., catalyst used in the 1st

reaction cycle).

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Figure 77: Photocatalytic degradation kinetics of MB on fresh and previously used

0.025Cr-AM-6 under UV light (310-400 nm) irradiation.

Figure 78: Absorbance from the supernatants of equilibrated MB solutions

illustrating the adsorption of MB on “fresh” vs. “used” photocatalyst.

4.3.4 Effect of Crystal Quality on the Photocatalytic Activity

The pseudo-first-order reaction rate constants (k) of unmodified AM-6 crystals

with varying degrees of crystal quality (Sections 4.1.1.3 and 4.1.4) were determined

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based on the photodegradation kinetics data shown in Figure 79 (UV light irradiation)

and Figure 80 (visible light irradiation), and the resulting k values are shown in Table 11.

Sample kUV (min-1) kvis (min-1) Surface Area (m2/g)

Eg (eV)

“High quality” AM-6 crystals*

0.0022 ± 0.0003

0.0049 ± 0.0002

18.6 ± 1.9 3.58 ± 0.02

“Low quality” AM-6 crystals**

0.0017 ± 0.0007

0.0029 ± 0.0007

99.8 ± 10.0 3.70 ± 0.01

Table 11: Pseudo-first-order reaction rate constants (k) for the photodegradation of MB under UV and visible light irradiation, BET surface areas, and bandgap energies (Eg) of different unmodified AM-6 products.

* “High quality” crystals: AM-6 crystals synthesized with mixture Si/V ratio of 4.3 ** “Low quality” crystals: AM-6 crystals synthesized with mixture Si/V ratio of 4.85

Under UV light irradiation (310-400 nm), the k values of the “high quality” and

“low quality” AM-6 crystals were identical within error, yet still significantly larger than

the direct UV light photolysis rate constant (0.0003 min-1 ± 0.0001 min-1). These results

however did not correlate with the BET surface area measurements and the bandgap

energies of these products; e.g., BET surface area of “low quality” crystals was ~5 times

greater than that of “high quality” crystals (Table 11). Therefore, these results suggest

that the effect of “quality” (including BET surface area, and bandgap energy) of the

product is negligible with regard to the photodegradation of MB under UV light

irradiation.

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Figure 79: Photocatalytic degradation kinetics of MB under UV light (310-400 nm)

irradiation on unmodified AM-6 crystals with varying surface defect concentrations: "low quality” AM-6; "high quality” AM-6.

“High quality” crystals showed increased photocatalytic activity compared to

“low quality” crystals (Table 11) which were inactive towards the photodegradation of

MB under visible light irradiation (direct visible light MB photolysis, k = 0.0028 min-1 ±

0.0004 min-1). Similar to the k values obtained using UV light irradiation, these (i.e.,

recorded using visible light irradiation) k values did not correlate well with the BET

surface areas of the crystals (Table 11). However, Raman spectroscopic results obtained

using a surface sensitive excitation line at 532 nm (Section 4.1.6, Figure 61, II)

suggested the presence of more V5+ species (~940 cm-1 band) in the “outer surface region”

of “high quality” crystals than “low quality” crystals. Nash et al. [31] reported AM-6

visible light photocatalytic activity due to the OI2- → V5+ charge-transfer transitions at

~450 nm. Therefore, the increase in the photocatalytic activity of “high quality” crystals

under visible light irradiation may be due to the presence of more V5+ in the outer surface

146

region of AM-6 crystals and thus the OI2- → V5+ charge-transfer transitions which likely

resulted in an electron excitation for the interfacial charge-transfer.

Figure 80: Photocatalytic degradation kinetics of MB under visible light (420-630

nm) irradiation on unmodified AM-6 crystals with varying surface defect concentrations: "low quality” AM-6; "high quality” AM-6.

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5.0 CONCLUSIONS

The use of titanosilicate ETS-10 seeds in the synthesis of vanadosilicate AM-6

limits the ability to control product characteristics without also influencing the product

titanium content. To circumvent this, a novel method for synthesizing pure (i.e., Ti free)

vanadosilicate AM-6, which does not require the use of titanosilicate ETS-10 seeds has

been developed. Products were synthesized at 430-503 K for 1-20 days (“optimal” pH =

10.8-11.0) using mixtures with molar compositions xNa2O : 1.3K2O : ySiO2 : 0.5V2O5 :

wSDA : zH2O, where x=4.5 or 5.6, y=3.4-5.5, w=0-3.0, and z=199-214. The XRD

patterns of AM-6 products were nearly identical to its analogue ETS-10; however, the

AM-6 reflections showed slightly smaller d-spacing values (i.e., smaller unit cell

dimensions) compared to the corresponding ETS-10 reflections. The smaller unit cell

volume for AM-6 relative to ETS-10 is in agreement with the smaller ionic radii of both

hexacoordinate V4+ (0.72 Å) and hexacoordinate V5+ (0.68 Å) compared to

hexacoordinate Ti4+ (0.745 Å).

Various templating agents, such as tetramethylammonium (TMA) cation,

tetraethylammonium (TEA) cation, and tetrapropylammonium (TPA) cation, were

investigated in crystallization of AM-6. Only TMA resulted in AM-6 products; AM-6

did not crystallize in the presence of TEA and TPA. Therefore these templates (i.e., TEA

and TPA) had a “structure breaking” role rather than “structure directing” role. By

varying the mixture TMA content, AM-6 products with varying product purity, size of

crystals, surface topography, and degree of development of the crystals’ square and

trapezoidal faces were obtained. The fraction of the AM-6 in the product increased with

increasing mixture TMA content, which further confirmed the “structure directing” role

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of TMA. Pure (≥ 95% of AM -6 by mass) AM-6 products were obtained with mixture

TMA/V content ≥ 2.0 (from mixtures with y = 4.85-5.5). In addition, the fraction of AM-

6 in the products increased with increasing mixture Si/V ratio for constant TMA content

(i.e., w = 2.0). The increase in mixture Si/V ratio also affected the size of the crystals;

smaller crystals were obtained using higher mixture Si/V ratios. XRD line broadening for

AM-6 products synthesized at higher mixture Si/V ratios (i.e., y = 4.85-5.5) suggested

that these crystals have greater degree of faulting and irregularities. These results were in

agreement with the detailed UV-vis spectroscopic analysis which suggested that the

…V–O–V–O–V… chains in AM-6 crystals grown at lower mixture Si/V ratios (i.e., y =

3.4-4.3) are less interrupted (i.e., more continuous chains) when compared to the …V–O–

V–O–V… chains in crystals grown at higher mixture Si/V ratios (i.e., y = 4.85-5.5).

The lower crystallization temperature (430-453 K) had a beneficial effect on the

AM-6 product purity; AM-6 products crystallized from mixtures with constant TMA

content (i.e., w = 2.0) and mixture Si/V ratio (i.e., y = 4.3) were free of trace impurities.

In addition, the degree of development of the crystals’ square and trapezoidal faces were

also affected by decreasing the crystallization temperature; plate-like AM-6 products

were grown at lower temperatures. Therefore, these results suggest that the

supersaturation levels in the crystallizing system are affected by varying parameters such

as TMA content, mixture Si/V ratios, pH, and crystallization temperature, which are in

agreement with the hypothesized layer-by-layer AM-6 crystal growth mechanism, where

multiple surface nuclei form and propagate on the square crystal facets. In addition, this

new synthesis method allowed crystallization of AM-6 at lower temperatures than those

reported to date, and offers greater flexibility to tailor AM-6 for a variety of applications

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that may require the controlled crystal characteristics such as size, morphology,

microtopography, and defect concentration.

TMA was effectively removed from the pores of AM-6 by treatment with

gaseous ammonia at elevated temperatures with little or no adverse effects on product

crystallinity. This was surmised from the increase in micropore volume (0.056 cm3 g-1 for

TMA-containing AM-6 vs. 0.130 cm3 g-1 for TMA-free AM-6), and the absence of the

TMA weight loss step in the TGA patterns of AM-6 crystals previously treated with

gaseous ammonia.

UV-vis, FTIR and Raman spectroscopic analyses suggested the presence of two

oxidation states of vanadium (V4+, V5+) in the AM-6 framework. Raman spectroscopic

analysis using two different excitation lines (785 nm and 532 nm) suggested the presence

of more V5+ than V4+ in AM-6 crystals grown from mixtures with low mixture Si/V

ratios (i.e., y = 4.3) compared to crystals grown using mixtures with y = 4.85. These

results are consistent with the enhanced photocatalytic activity (under both UV and

visible light irradiation) of AM-6 products synthesized from low mixture Si/V ratios.

Partial isomorphous framework substitutions of various transition metal (TM)

ions (Fe3+, Cr3+, and Co2+) have been investigated for AM-6. UV-vis spectroscopic

analysis suggested the presence of Fe3+ in the octahedral coordination for Fe-substituted

AM-6, Cr3+ in the octahedral coordination for Cr-substituted AM-6, and Co2+ in the

octahedral coordination for Co-substituted AM-6. These findings were consistent with

the calculated nearly identical unit cell volumes of the TM-substituted AM-6 products

compared to unit cell volume of unmodified AM-6. The bandgap energy of AM-6

decreased from 3.82 eV to 3.62-3.78 eV upon TM substitution. New absorption features

150

in the low energy region of the UV-vis spectra were observed upon TM substitutions; the

intensity of these features increased with increasing mixture TM/V ratio. The increasing

mixture TM/V ratios resulted in increasing crystal TM/V ratios in the TM-AM-6

products. Successful TM substitutions were confirmed by the shift to higher frequency of

the 860 cm-1 Raman band, and the intensity decrease of the FTIR bands upon increasing

TM (mixture and crystal) concentration. All TM-substituted AM-6 products showed

enhanced photocatalytic activity in the photodegradation of methylene blue under UV

light irradiation. However, these activities substantially decreased for products with

increasing TM concentrations. This was attributed to the decrease in the fraction of TM-

AM-6 in the products having increasing TM concentrations. Similar observations were

reported for photodegradation of MB under visible light irradiation. Only Cr-substituted

AM-6 obtained from mixture Cr/V ratio = 0.025 showed enhanced visible light

photocatalytic activity under visible light irradiation. This was attributed to the O2- →

Cr3+ charge-transfer transitions occurring in the visible light range which resulted in an

electron excitation for the interfacial charger transfer. The photocatalytic activity of the

“used” 0.025Cr-AM-6 photocatalyst significantly decreased after the 1st reaction cycle.

This was mainly attributed to the deactivation of the active sites on Cr-AM-6.

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6.0 RECOMMENDATIONS

Throughout the course of the investigations discussed in this dissertation, there

were certain aspects that require further attention. The following section will discuss

recommendations towards the continued understanding of AM-6 crystal growth

mechanism and enhancing its photocatalytic activity.

The first recommendation is in regard to further assessing the variations of the

topographical features upon the varied defect concentration of various quality (i.e.,

“high” and “low” quality) AM-6 crystals. High-resolution FE-SEM analysis (Figure 81)

of titanosilicate ETS-10 crystals grown using various mixture Na2O/K2O ratios showed

island-like structures of various sizes; the island-like structures increased from ~50 nm to

over 200 nm with decreasing mixture Na2O/K2O ratio (i.e., increasing crystals size) [92].

Since the junction between the spreading surface nuclei is likely the place where defects

(i.e., discontinuity in the …Ti–O–Ti–O–Ti… chains) in ETS-10 occur [49], therefore

crystals with larger island-like sizes likely contain fewer defects along the …Ti–O–Ti–

O–Ti… chains in ETS-10. These results were consistent with UV-vis and Raman

spectroscopic analyses which suggested more continuous chains in ETS-10 products

grown from lower mixture Na2O/K2O ratios [71].

Figure 81: High-resolution FE-SEM images of the square faces of ETS-10 crystals

synthesized at 473 K from mixtures with molar composition 5.5SiO2 : xNa2O : yK2O : 1.0TiO2 : 300H2O with x/y ratio of: (a) 2.0, (b) 1.5, and (c) 1.0 [92].

152

Since UV-vis spectroscopic analysis suggested more continuous …V–O–V–O–

V… chains in “high quality” AM-6 and the detailed XRD analysis suggested fewer

defects in these crystals, while Raman spectroscopic analysis suggest more terminating

V5+ on the outer suface of these crystals, high-resolution FE-SEM analysis of the square

faces of vanadosilicate AM-6 crystals grown at varying mixture Si/V ratios (i.e., “low

quality” and “high quality”) should be conducted and correlated with the results

presented in Sections 4.1.1.3, 4.1.4, and 4.1.6.

The second recommendation is in regard to the photocatalysis investigations of

this work. Since methylene blue (MB) was shown to be active (i.e., dye-sensitized) under

the visible light irradiation range used (420-630 nm) due to photoexcitation in the visible

light range (note the MB absorption peaks in the visible light range in Figure 32), it was

difficult to assess the photocatalytic activities of the modified and unmodified AM-6

products under this irradiation range. Therefore, these photocatalysis investigations

should be performed using either a molecule that does not absorb light in the visible light

range (i.e., is inactive in this irradiation range), or using monochromatic light sources

with wavelengths that do not overlap with the MB absorption spectrum. Using

monochromatic light sources can also be beneficial in determining the photocatalytic

activity of specific absorption features in unmodified and modified AM-6 products.

The third and last recommendation of this work is to further confirm the

degradation mechanism proposed in Sections 4.3.2 and 4.3.4 using electron spin

resonance (ESR) spectroscopy. ESR spectroscopy has been used to further confirm the

selective photodegradation mechanism of organic substrates with various sizes and

polarities on ETS-10 [75]. Three different spin-trapping aromatic substrates were

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investigated: α-phenyl-N-tert-butylnitrone (PBN) with an EMW of 0.544 nm and log S

value of – 1.30; nitrosobenzene (NB) with an EMW of 0.513 nm and log S value of –

2.08; and sodium 3,5-dibromo-4-nitrosobenzenesulfonate (DBNBS) with an EMW of

0.882 nm (log S value could not be determined for this structure). In all cases where TiO2

was used as the photocatalyst, distinct spin adduct signals were present (Figure 82ii).

Conversely, ETS-10 showed no spin adduct signals for the case of PBN (Figure 82i(a)),

which suggested that PBN does not react with the ·OH group present on the external

surface of ETS-10. These results coincide with the size (EMW < 0.551 nm) and high

polarization of PBN, which allowed for PBN to diffuse into the protective pore

environment of ETS-10. Both NB (Figure 82i(b)) and DBNBS (Figure 82i(c)) showed

spin adduct signals which suggested that both substrates reacted with the ·OH group

present on the external surface of ETS-10. These ESR data are consistent with the low

degree of polarization of NB (log S > – 1.50) and the large size of DBNBS (EMW >

0.551 nm).

Figure 82: ESR spectra of ·OH spin adduct signals obtained under photoirradiation

of: (a) PBN, (b) NB, and (c) DBNBS, in the presence of (i) ETS-10 or (ii) TiO2 for comparison [75].

154

ESR studies have also shown that various transition metal ions (such as, Fe, V,

and Mo) doped in the TiO2 lattice can act as electron and/or hole traps under UV light

irradiation [133]. Therefore, in addition to using ESR spectroscopy to further confirm the

proposed MB photodegradation mechanism, ESR can also be used to further elucidate

the roles (i.e., electron and/or hole traps) of transition metal ions isomorphously

substituted in the AM-6 framework.

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7.0 NOMENCLATURE

AM-6 – Aveiro/Manchester no. 6 Materials

ETS-10 – Engelhard titanosilicate no. 10 MB – Methylene blue Mn+ – Transition metal ion SDA(s) – Structure directing agent(s) TiO2 – Titanium dioxide TEA –Tetraethylammonium cation TMA – Tetramethylammonium cation TMAOH – Tetramethylammonium hydroxide TPA – Tetrapropylammonium cation TM – Transition metal (isomorphous substitutions)

29Si CP MAS NMR – Charge-polarized magic angle spinning nuclear magnetic resonance

Characterization Techniques

BET – Brunauer, Emmett, and Teller method DTA – Differential thermal analysis EDX – Energy dispersive X-ray spectroscopy EPR – Electron Paramagnetic Resonance EXAFS – Extended X-ray absorption fine structure FE-SEM – Field emission scanning electron microscopy FTIR – Fourier transform infrared spectroscopy HPLC – High-performance liquid chromatography HR-TEM – High-resolution transmission electron microscopy ICP-AES – Inductively couple plasma atomic emission spectroscopy PSD – Particle size distribution TGA – Thermogravimetric analysis XPS – X-ray photoelectron spectroscopy XRD – X-ray powder diffraction UV-vis – Ultraviolet-visible light spectroscopy

a(b) – Crystal dimension identified by the width of the shared base of the crystal Miscellaneous Abbreviations

c – Crystal dimension identified by the combined height of the truncated pyramids of a crystal % C – Percent crystallinity CB – Conduction band CBM – Conduction band maximum e- – Electrons Eg – Bandgap energy Ef – Fermi level Ecb – Conduction band potential energy Evb – Valence bance potential energy

156

Eox – Oxidation potential of a given reaction Ered – Reduction potential of a given reaction EMW – Effective molecular width FWHM – Full-width-at-half-maximum h+– Holes HOMO – Highest occupied molecular orbital k – Pseudo-first-order reaction rate constant LTMC – Ligand-to-metal charge-transfer LUMO – Lowest occupied molecular orbital nEf

* – Electrochemical potential for electrons NHE – Normal hydrogen electrode pEf

* – Electrochemical potential for holes R – Aspect ratio (R = c/a) SC – Semiconductor SPR – Surface Plasmon resonance VB – Valence band VBM – Valence band maximum

157

8.0 REFERENCES 1. http://sangakukan.jp/journal/journal_contents/2010/06/articles/1006-03-2/1006-03-

2_earticle.html 2. Kavita, K., R. Chaudhary, and R.L. Sawhney, Treatment of hazardous organic and

inorganic compounds through aqueous-phase photocatalysis: a review, Ind. Eng. Res., 43, pp. 7683-7696, 2004.

3. Borello, E., C. Lamberti, S. Bordiga, A. Zecchina, and C.O. Arean, Quantum-size effects in the titanosilicate molecular sieve, Appl. Phys. Lett., 71, pp. 2319-2321, 1997.

4. Serpone, N., and A. Salinaro, Terminology, relative photonic efficiencies and quantum yields in heterogeneous photocatalysis. Part I; Suggested protocol (Technical Report), Pure Appl. Chem., 71, pp.303-320, 1999.

5. Mills, Andrew, and S.L. Hunte, An overview of semiconductor photocatalysis, J. Photochem. Photobiol., A, 108, pp.1-35, 1997.

6. Serpone, N., and E. Pelizzetti, Photocatalysis – Fundamentals and Applications

7. Turchi, C.S., and D.F. Ollis, Photocatalytic degradation of organic water contaminants: mechanisms involving hydroxyl radical attack, J. Catal., 122, pp.178-192, 1990.

, Canada: Wiley, pp. 159-166, 1989.

8. Lasa, H. D., B. Serrano, and M. Salaices, Photocatalytic reaction engineering

9. Hoffman, M.R., S.T. Martin, W. Choi, and D.W. Bahnemann, Environmental applications of semiconductor photocatalysis, Chem. Rev., 95, pp.69-96, 1995.

, New York: Springer, pp. 1-47, 2005.

10. Kamat, P.V., Photophysical, photochemical, and photocatalytic aspects of metal nanoparticles, J. Phys. Chem. B, 106, pp. 7729-7744, 2002.

11. Sobana, N., M. Muruganadham, and M. Swaminathan, Nano-Ag particles doped TiO2 for efficient photodegradation of direct azo dyes, J. Mol. Catal. A: Chem., 258, pp.124-132, 2006.

12. Georgekutty, R., M.K. Seery, and S.C. Pillai, A highly efficient Ag-ZnO photocatalyst: synthesis, properties, and mechanism, J. Phys. Chem. C, 112, pp. 13563-13570, 2008.

13. Seery, M.K., R. George, P. Floris, and S.C. Pillai, Silver doped titanium dioxide nanomaterials for enhanced visible light photocatalysis, J. Photochem. Photobiol., A, 189, pp. 258-263, 2007.

14. Dawson, A., and P.V. Kamat, Semiconductor-metal nanocomposites. Photoinduced fusion and photocatalysis of gold-capped TiO2 (TiO2/gold) nanoparticles, J. Phys. Hem. B, 105, pp. 960-966, 2001.

15. Choi, W., A. Termin, and M.R. Hoffmann, The role of metal ion dopants in quantum-sized TiO2: Correlation between photoreactivity and charge carrier recombination dynamics, J. Phys. Chem., 98, pp.13669-13679, 1994.

16. Zhang, J., L. Xiao, Y. Cong, and M. Anpo, Preparation and characterization of multi-functional titanium dioxide photocatalysts, Topics in Catalysis, 47, pp.122-130, 2008.

17. Rocha, J., P. Brandão, Z. Lin, M.W. Anderson, V. Alfredsson, and O. Terasaki, The first large-pore vanadosilicate framework containing hexacoordinated vanadium, Angew. Chem. Int. Ed. Engl., 36, pp. 100-102, 1997.

158

18. Yilmaz, B., J. Warzywoda, and A. Sacco Jr., Spectroscopic characterization of the quantum wires in titanosilicate ETS-4 and ETS-10, Nanotechnology, 17, pp. 4092-4099, 2006.

19. Yeates, R.M., M.J. Murdoch, P.D. Southon, A.C. Mclaughlin, R.F. Howe, F. Bonino, S. Bordiga, and A. Damin, AM-6: a microporous one-dimensional ferromagnet, Dalton Trans., pp. 8025-5032, 2009.

20. Anderson, M.W., O. Terasaki, T. Ohsuna, A. Philippou, S.P. MacKay, A. Ferreira, J. Rocha, and S. Lidin, Structure of microporous titanosilicate ETS-10, Nature, 367, pp. 347-351, 1994.

21. Rocha, J., and M.W. Anderson, Microporous titanosilicates and other novel mixed octahedral-tetrahedral framework oxides, Eur. J. Inorg. Chem., pp. 801-818, 2000.

22. Anderson, M.W., O. Terasaki, T. Ohsuna, P.J.O Malley, A. Philippou, S.P. MacKay, A. Ferreira, J. Rocha, and S. Lidin, Microporous titanosilicate ETS-10: a structural survery, Philos. Mag. B, 71, pp. 813-841, 1995.

23. Shough, A.M., D.J. Doren, M. Nash, and R.F. Lobo, Effect of vanadium substitution on the structure and photocatalytic behavior of ETS-10, J. Phys. Chem. C, 111, pp. 1776-1782, 2007.

24. Shough, A.M., R.F. Lobo, and D.J. Doren, A visible light Photocatalyst: effects of vanadium substitution on ETS-10, Phys. Chem. Chem. Phys., 9, pp. 5096-5104, 2007.

25. M. Winter, WebElements, http://www.webelements.com/, 1993. 26. Ooms, K., T. Polenova, A-M. Shough, D.J. Doren, M.J. Nash, and R.F. Lobo,

Identification of mixed valence vanadium in ETS-10 using electron paramagnetic resonance, 51V solid-state nuclear magnetic resonance, and density functional theory studies, J. Phys. Chem. C, 113, pp.10477-10484, 2009.

27. Seeralha, F.N., J.M. Lopes, F. Lemos, D. Prazares, M. Aires-Barros, J. Rocha, J. Cabral, and F.R. Ribeiro, Titanosilicates as supports for an enzymatic alcoholysis reaction, React. Kinet. Catal. Lett., 69, pp. 217-222, 2000.

28. Rocha, J., and Z. Lin, Microporous mixed octahedral-pentahedral-tetrahedral framework silicates, Mineral. Geochem., 57, pp. 173-201, 2005.

29. Yilmaz, B., A. Sacco Jr., and J. Deng, Electrical transport through monatomic titania chains, Appl. Phys. Lett., 90, pp. 152101-152103, 2007.

30. Datta, S.J., and K.B. Yoon, Synthesis of ideal AM-6 and elucidation of V4+-to-O charge transfer in vanadate quantum wires, Angew. Chem. Int. Ed., 49, pp. 4971-4975, 2010.

31. Nash, M. J., S. Rykov, R. F. Lobo, D. J. Doren, and I. Wachs, Photocatalytic activity of vanadium-substituted ETS-10, J. Phys. Chem. C, 111, pp. 7029-7037, 2007.

32. Philippou, A., Naderi, M., Rocha, J., and M.W. Anderson, Dehydration of t-butanol over basic ETS-10, ETAS-10 and AM-6, Catal. Lett. 53, pp. 221-224, 1998.

33. Nash, M. J., R. F. Lobo, and D. J. Doren, Photocatalytic oxidation of ethyelene by ammonium exchanged ETS-10 and AM-6, Appl. Catal. B: Environ., 88, pp. 232-239, 2009.

34. Brandao, P., A.A. Valente, J. Rocha, and M.W. Anderson, Synthesis, characterization and catalytic activity of vanadium-containing ETS-10, Stud. Surf. Sci. Catal., 142, pp. 327-334, 2002.

159

35. Das, T.K., A.J. Chandwadkar, A.P. Budhkar, and S. Sivasanker, Studies on the synthesis of ETS-10 II. Use of organic templates, Micropor. Mater. 5, pp. 401-410, 1996.

36. Valtchev, V.P., Influence of different organic bases on the crystallization of titanium silicate ETS-10, J. Chem. Soc., Chem. Commun., pp. 261-262, 1994.

37. Valtchev, V.P., and S. Mintova, Synthesis of titanium silicate ETS-10: The effect of tetramethylammonium on the crystallization kinetics, Zeolites, 14, pp. 697-700, 1994.

38. Yang, X., J-L. Paillaud, H.F.W.J. van Breukelen, H. Kessler, and E. Duprey, Synthesis of microporous titanosilicate ETS-10 with TiF4 or TiO2, Micropor. Mesopor. Mater., 46, pp. 1-11, 2001.

39. Kim, W.J., S.D. Kim, H.S. Jung, and D.T. Hayhurst, Compositional and kinetic studies on the crystallization of ETS-10 in the presence of various organics, Micropor. Mesopor. Mater., 56, pp. 89-100, 2002.

40. Pavel, C.C., J.B. Nagy, N. Bilba, A. Nastro, C. Perri, D. Vuono, P. De Luca, and I.V. Asaftei, Influence of the TAABr salts on the crystallization of ETS-10, Micropor. Mesopor. Mater., 71, pp. 77-85, 2004.

41. Culfaz, A., and L.B. Sand, in: W.M. Meir, J.B. Utterhoeven (Eds.), ACS Symp. Series no. 121, Washington DC, 1973, p.140.

42. Warzywoda, J., R.D. Edelman, and R.W. Thompson, Thoughts on the induction time in zeolite crystallization, Zeolites, 9, pp. 187-192, 1989.

43. Gonthier, S.,L. Gora, I. Guray, and R.W. Thompson, Further comments on the role of autocatalytic nucleation in hydrothermal zeolite synthesis, Zeolites, 13, pp. 414-418, 1993.

44. Goa, Y., H. Yoshitake, P. Wu, and T. Tatsumi, Controlled detitanation of ETS-10 materials through the post-synthetic treatment and their applications to the liquid-phase epoxidation of alkenes, Micropor. Mesopor. Mater., 70, pp. 93-101, 2004.

45. Lv, L., J.K. Zhou, F. Su, and X.S. Zhao, Local structure changes of microporous titanosilicate ETS-10 upon acid treatment, J. Phys. Chem. C, 111, pp. 773-778, 2007.

46. Pavel, C.C., S-H. Park, A. Dreier, B. Tesche, and W. Schmidt, Structural defects induced in ETS-10 by postsynthesis treatment with H2O2 solution, Chem. Mater., 18, pp. 3813-3820, 2006.

47. Lv, L., F.Y. Lee, J. Zhou, F. Su, and X.S. Zhao, XPS study on microporous titanosilicate ETS-10 upon acid treatment, Micropor. Mesopor. Mater., 96, pp. 270-275, 2006.

48. Krisnandi, Y.K., and R.F. Howe, Effects of ion-exchange on the photoreactivity of ETS-10, Appl. Catal. A-Gen., 307, pp. 62-69, 2006.

49. Southon, P.D., and R.F. Howe, Spectroscopic studies of disorder in the microporous titanosilicate ETS-10, Chem. Mater., 14, pp. 4209-4218, 2002.

50. Llabrés i Xamena, F.X., P. Calza, C. Lamberti, C. Prestipino, A. Damin, S. Bordiga, E. Pelizzetti, and A. Zecchina, Enhancement of the ETS-10 titanosilicate activity in the shape-selective photocatalytic degradation of large aromatic molecules by controlled, J. Am. Chem. Soc., 125, pp. 2264-2271, 2003.

51. Ismail, M.N., Z. Ji, D.M. Callahhan Jr., E. Pandowo, Z. Cai, T.L. Goodrich, K.S. Ziemer, J. Warzywoda, and A. Sacco Jr., The role of silver nanoparticles on silver modified titanosilicate ETS-10 in visible light photocatalysis, Appl. Catal. B, 102, pp. 323-333, 2011.

160

52. Tada, H., K. Teranishi, Y-I. Inubushi, and S. Ito, TiO2 photocatalytic reduction of bis(2-dipyridyl)disulfide to 2-mercaptopyridine by H2O: incorporation effect of nanometer-sized Ag particles, Chem. Commun., pp. 2345-2346, 1998.

53. Dawson, A., and P.V. Kamat, Semiconductor-metal nanocomposites. Photoinduced fusion and photocatalysis of gold-capped TiO2 (TiO2/gold) nanoparticles, J. Phys. Chem. B, 105, pp. 960-966, 2001.

54. Pastoriza-Santos, I., D.S. Koktysh, A.A. Mamedov, M. Giersig, N.A. Kotov, and L.M. Liz-Marzan, One-pot synthesis of Ag@TiO2 core-shell nanoparticles and their layer-by-layer assembly, Langmuir, 16, pp. 2731-2735, 2000.

55. Tom, R.T., A.S. Nair, N. Singh, M. Aslam, C.L. Nagendra, R. Philip, K. Vijayamohanan, and T. Pradeep, Freely dispersible Au@TiO2, Au@ZrO2, Ag@TiO2, and Ag@ZrO2 core-shell nanoparticles: one-step synthesis, characterization, spectroscopy, and optical limiting properties, Langmuir, 19, pp. 3439-3445, 2003.

56. Hirakawa, T., and P.V. Kamat, Charge-separation and catalytic activity of Ag@TiO2 core-shell composite clusters under UV-irradiation, J. Am. Chem. Soc., 127, pp. 3928-3934, 2005.

57. Wang, W., J. Zhang, F. Chen, D. He, and M. Anpo, Preparation and photocatalytic properties of Fe3+-doped Ag@TiO2 core-shell nanoparticles, J. Colloid Interface Sci., 323, pp. 182-186, 2008.

58. Li, Y., H. Zhang, Z. Guo, J. Han, X. Zhao, Q. Zhao, and S-J. Kim, Highly efficient visible-light-induced photocatalytic activity of nanostructured AgI/TiO2 photocatalyst, Langmuir, 24, pp. 8351-8357, 2008.

59. Chuang, H-Y., and D-H. Chen, Fabrication and photocatalytic activities in visible and UV light regions of Ag@TiO2 and NiAg@TiO2 nanoparticles, Nanotechnology, 20, pp. 105704/1-105704/10, 2009.

60. Eldewik, A., and R.F. Howe, Cobalt substitution in ETS-10, Micropor. Mesopor. Mater., 48, pp. 65-71, 2001.

61. Anderson, M.W., A. Philippou, Z. Lin, A. Ferreira, and J. Rocha, Al,Ti Avoidance in the microporous titanoaluminosilicate ETAS-10, Angew. Chem. Int. Ed. Engl., 34, pp. 1003-1005, 1995.

62. Anderson, M.W., J. Rocha, Z. Lin, A. Philippou, I. Orion, and A. Ferreira, Isomorphous substitution in the microporous titanosilicate ETS-10, Microporous Mater., 6, pp. 195-204, 1996.

63. Uma, S., S. Rodrigues, I.N. Martyanov, and K.J. Klabunde, Exploration of photocatalytic activities of titanosilicate ETS-10 and transition metal incorporation ETS-10, Micropor. Mesopor. Mater., 67, pp.181-187, 2004.

64. Brandão, P., A. Philippou, A. Valente, J. Rocha, and M. Anderson, Synthesis and characterization of chromium-substituted ETS-10, Phys. Chem. Chem. Phys., 3, pp. 1773-1777, 2001.

65. Rocha, J., P. Brandão, J.D. Pedrosa de Jesus, A. Philippou, and M.W. Anderson, Synthesis and charactertization of microporous titanoniobosilicate ETNbS-10, Chem. Commun., pp. 471-472, 1999.

66. Shough, A.M., B. Ogunnaike, and D.J. Doren, Transition metal substitution in ETS-10: DFT calculations and a simple model for electronic structure prediction, Chem. Mater., 21, pp. 1232-1241, 2009.

161

67. Ohno, T. Preparation of visible light active S-doped TiO2 photocatalysts and their photocatalytic activities, Water Sci. Technol., 49, pp. 159-163, 2004.

68. Burda, C., Y. Lou, X. Chen, A.C.S. Samia, J. Stout, and J.L Gole, Enhanced nitrogen doping in TiO2 nanoparticles, Nano Lett., 3, pp.1049-1051, 2003.

69. Kuznetsov, V.N., and N. Serpone, On the origin of the spectral bands in the visible absorption spectra of visible-light-active TiO2 specimens analysis and assignments, J. Phys. Chem. C, 113, pp. 15110-15123, 2009.

70. Shankar, M.V., and J. Ye, Inorganic alkaline-sols as precursors for rapid synthesis of ETS-10 microporous titanosilicates and their photocatalytic reforming of methanol under visible-light irradiation, Catal. Commun., 11, pp. 261-265, 2009.

71. Z. Ji, “Synthesis and engineering of titanosilicate ETS-10 for enhanced photocatalytic activity in an optical fiber reactor”, in Chemical Engineering. vol. Ph.D. Boston: Northeastern University, 2009, pp. 231-259.

72. Serpone, N. Is the band gap of pristine TiO2 narrowed by anion- and cation-doping of titanium dioxide in second generation photocatalysts?, J. Phys. Chem. B, 110, pp. 24287-24293, 2006.

73. Linsebigler, A.L., G. Lu, and J.T. Yates, Jr., Photocatalysis on TiO2 surfaces: principles, mechanisms, and selected results, Chem. Rev., 95, pp. 735-758, 1995.

74. Calza, P., C. Paze, E. Pelizzetti, and A. Zecchina, Shape-selective photocatalytic transformation of phenols in an aqueous medium, Chem. Commun., pp. 2130.2131, 2001.

75. Shiraishi, Y., D. Tsukamoto, and T. Hirai, Selective photocatalytic transformations on microporous titanosilicate ETS-10 driven by size and polarity of molecules, Langmuir, 24, pp. 12658-12663, 2008.

76. http://www.vcclab.org/lab/alogps 77. Ismail, M.N., U.K. Ibe, T. Chernenko, M. Diem, J. Warzywoda, A. Sacco Jr.,

Synthesis and characterization of vanadosilicate AM-6 with transition metal ions isomorphously substituted in the framework, Micropor. Mesopor. Mater., (accepted with minor revisions).

78. Pelekani, C., and V.L. Snoeyink, Competitive adsorption between atrazine and methylene blue on activated carbon: the importance of pore size distribution, Carbon, 38, pp. 1423-1436, 2000.

79. http://pubchem.ncbi.nlm.nih.gov/summary/summary.cgi?cid=6099 80. Mills, A., and J. Wang, Photobleaching of methylene blue sensitized by TiO2: an

ambiguous system?, J. Photochem. Photobiol. A, 127, pp. 123-134, 1999. 81. Zhang, T., T. Oyama, A. Aoshima, H. Hidaka, J. Zhao, and N. Serpone,

Photooxidation N-demethylation of methylene blue in aqueous TiO2 dispersions under UV irradiation, J. Photochem. Photobiol. A, 140, pp. 163-172, 2001.

82. Davis, E.A, and N.F. Mott, Conduction in noncrystalline systems. V. Conductivity, optical absorption, and photoconductivity in amorphous semiconductors, Philos. Mag., 22, pp. 903-922, 1970.

83. Delgass, W.N, G.L. Haller, R. Kellerman, and J.H. Lunsford, Diffuse reflectance and photoacoustic spectroscopies, Spectroscopy in Heterogeneous Catalysis

84. Davis, M.E., and R.F. Lobo, Zeolite and molecular sieve synthesis, Chem. Mater., 4, pp. 756-768, 1992.

, Academic Press, New York, p. 128, 1979.

162

85. Lok, B.M., T.R. Cannon, and C.A. Messina, The role of organic molecules in molecular sieve synthesis, Zeolites, 3, pp. 282-291, 1983.

86. Burkett, S.L., and M.E. Davis, Mechanism of structure direction in the synthesis of Si-ZSM-5: An investigation by intermolecular 1H-29Si CP MAS NMR, J. Phys. Chem., 98, pp. 4647-4653, 1994.

87. Wang, D., B. Tejerina, I. Lagzi, B. Kowalczyk, and B.A. Grzybowski, Bridging interactions and selective nanoparticle aggregation mediated by monovalent cations, ACS Nano, 5, pp. 530-536, 2011.

88. Wang, X., and A.J. Jacobson, Crystal structure of the microporous titanosilicate ETS-10 refined from single crystal X-ray diffraction data, Chem. Commun., pp. 973-974, 1999.

89. Anderson, M.W., J.R. Agger, N. Hanif, and O. Terasaki, Growth models in microporous materials, Micropor. Mesopor. Mater., 48, pp. 1-9, 2001.

90. Wang, X., and A.J. Jacobson, Crystal structure of the microporous titanosilicate ETS-10 refined from single crystal X-ray diffraction data, Chem. Commun., pp. 973-974, 1999.

91. Cullity, B.D., Elements of X-ray Diffraction

92. Ji, Z., J. Warzywoda, and A. Sacco Jr., Synthesis and morphological control of large titanosilicate ETS-10 crystals, Micropor. Mesopor. Mater., 109, pp.1-11, 2008.

, second ed., Addison-Wesley, Reading, MA, 1978.

93. Jeong, N.C., M.H. Lee, and K.B. Yoon, Length-dependent band-gap shift of TiO32-

molecular wires embedded in zeolite ETS-10, Angew. Chem. Int. Ed., 46, pp. 5868-5872, 2007.

94. A. Tiller, The Science of Crystallization: Macroscopic Phenomena and Defect Generation, Cambridge University Press, Cambridge, 1991.

95. J.W. Mullin, Crystallization, third ed., Butterworth-Heinemann, Oxford, 1993. 96. Zhdanov, S.P., and N.N. Samulevich, Nucleation and crystal growth of zeolites in

crystallizing aluminosilicate gels, Proc. Int. Conf. Zeolites, 5th, pp. 75-84, 1980. 97. Thompson, R.W., Analysis of zeolite crystallizations using Avrami transformation

methods, Zeolites, 12, pp. 680-684, 1992. 98. Thompson, R.W., and A. Dyer, Mathematical analyses of zeolite crystallization,

Zeolites, 5, pp. 202-210, 1985. 99. Den Ouden, C.J.J, and R.W. Thompson, Analysis of zeolite crystallization using the

“crystallization curve”, Ind. Eng. Chem. Res., 31, pp. 369-373, 1992. 100. Luan, Z., J. Xu, H. He, J. Klinowski, and L. Kevan, Synthesis and spectroscopic

characterization of vanadosilicate mesoporous MCM-41 molecular sieves, J. Phys. Chem., 100, pp. 19595-19602, 1996.

101. Catana, G., R.R. Rao, B.M. Weckhuysen, P. Van Der Voort, E. Vansant, and R.A. Schoonheydt, Supported vanadium oxide catalysts: quantitative spectroscopy, preferential adsorption of V4+/V5+, and Al2O3 coating of zeolite Y, J. Phys. Chem. B, 102, pp. 8005-8012, 1998.

102. Morey, M., A. Davidson, H. Eckert, and G. Stucky, Pseudotetrahedral O3/2V:O centers immobilized on the walls of a mesoporous, cubic MCM-48 support: preparation, characterization, and reactivity toward water as investigated by 51V NMR and UV-Vis spectroscopies, Chem. Mater., 8, pp. 486-492, 1996.

163

103. Luca, V., M. Osborne, D. Sizgek, C. Griffith, and P.Z. Araujo, Photodegradation of methylene blue using crystalline titanosilicate quantum-confined semiconductor, Chem. Mater. 18, pp. 6132-6138, 2006.

104. Gudiksen, M.S., J. Wang, and C.M. Lieber, Size-dependent photoluminescence from single indium phosphide nanowires, J. Phys. Chem. B, 106, pp. 4036-4039, 2002.

105. Buhro, W.E., and V.L. Colvin, Semiconductor nanocrystals. Shape matters, Nature Mater., 2, pp. 138-139, 2003.

106. Kan, S., T. Mokari, E. Rothenberg, and U. Banin, Synthesis and size-dependent properties of zinc-blende semiconductor quantum rods, Nature Mater., 2, pp. 155-158, 2003.

107. Lamberti, C. Electron-hole reduced effective mass in monoatomic …–O–Ti–O–Ti–O–… quantum wires embedded in the siliceous crystalline matrix of ETS-10, Micropor. Mesopor. Mater., 30, pp. 155-163, 1999.

108. Borello, E. C. Lamberti, S. Bordiga, C.O. Arean, A. Zecchina, Quantum-size effects in the titanosilicate molecular sieve, Appl. Phys. Lett., 71, pp. 2319-2321, 1997.

109. Kresnawahjuesa, O., D.H. Olson, R.J. Gorte, and G.H. Kuhl, Removal of tetramethylammonium cations from zeolites, Micropor. Mesopor. Mater., 51, pp. 175-188, 2002.

110. Tiscornia, I., S. Irusta, P. Pradanos, C. Tellez, and J. Coronas, Preparation and characterization of titanosilicate Ag-ETS-10 for propylene and propane adsorption, J. Phys. Chem. C, 111, pp. 4702-4709, 2007.

111. Hamilton, D.E., Reinvestigation of the vanadium-oxygen stretch in the IR spectrum of bis[N-(4-chlorophenyl)salicylideneiminato]oxovanadium(IV), Inorg. Chem., 30, pp. 1670-1671, 1991.

112. Mathew, M, A.J. Carty, and G.J. Palenik, Unusual complex containing bridging vanadyl groups. Crystal structure of N,N'-propylenebis(salicylaldiminato) oxovanadium(IV), J. Am. Chem. Soc., 92, pp. 3197-3198, 1970.

113. McCormick, B.J., and R.A. Bozis, Vanadyl(IV)-monothio-β-diketone complexes. II. Bridged compounds, Inorg. Chem., 10, pp. 2806-2808, 1971.

114. Sawicka, M., P. Storoniak, P. Skurski, J. Błażejowski, and J. Rak, TG-FTIR, DSC and quantum chemical studies of the thermal decomposition of quaternary methylammonium halides, Chem. Phys., 324, pp. 425-437, 2006.

115. Baltes, M., K. Cassiers, P. Van Der Voort, B.M. Weckhuysen, R.A. Schoonheydt, and E.F. Vansant, MCM-48-supported Vanadium Oxide Catalysts, Prepared by the Molecular Designed Dispersion of VO(acac)2: A Detailed Study of the Highly Reactive MCM-48 Surface and the Structure and Activity of the Deposited VOx, J. Catal., 197, pp. 160-171, 2001.

116. Rao, P.R.H.P., R. Kumar, A.V. Ramaswamy, and P. Ratnasamy, Synthesis and characterization of a crystalline vanadium silicate with MEL structure, Zeolites, 13, pp. 663-670, 1993.

117. Dutoit, D.C.M., M. Schneider, P. Fabrizioli, and A. Baiker, Vanadia-Silica low-temperature aerogels: Influence of aging and vanadia loading on structural and chemical properties, Chem. Mater., 8, pp. 734-743, 1996.

118. Smirnov, K.S., and B. van de Graaf, On the origin of the band at 960 cm-1 in the vibrational spectra of Ti-substituted zeolites, Micropor. Mater., 7, pp. 133-138, 1996.

164

119. Llabrés i Xamena, F.X., A. Damin, S. Bordiga, and A. Zecchina, Healing of defects in ETS-10 by selective UV irradiation: a Raman study, Chem. Commun. pp. 1514-1515, 2003.

120. Ismail, M.N., N.D. Fraiman, D.M. Callahan Jr., G. Gursoy, E. Viveiros, O. Ozkanat, Z. Ji, R.J. Willey, J. Warzywoda, and A. Sacco Jr., First unseeded hydrothermal synthesis of microporous vanadosilicate AM-6, Micropor. Mesopor. Mater., 120, pp. 454-459, 2009.

121. Eldewik, A., V. Luca, N.K. Singh, and R.F. Howe, Iron substitution in the microporous titanosilicate ETS10, Proceedings of the 12th International Zeolite Conference, 3, pp. 1507-1514, 1999.

122. I. Sunagawa, Crystals: Growth, Morphology, and Perfection, Cambridge University Press, Cambridge 2005.

123. J.W. Mullin, Crystallization, third ed., Butterworth-Heinemann, Oxford, 1993. 124. Escribano, V.S., J.M.G. Amores, E. Finocchio, M. Daturi, and G. Busca,

Characterization of α-(Fe,Al)2O3 solid-solution powders, J. Mater. Chem., 5, pp. 1943-1951, 1995.

125. Schumacher, L.C., S. Mamiche-Afara, M.F. Weber, and M.J. Dignam, Photoelectrochemical properties of sputtered and plasma-oxidized iron oxide films, J. Electrochem. Soc., 132, pp. 2945-2950, 1985.

126. De Haart, L.G.J, and G. Blasse, Photoelectrochemical properties of ferrites with the spinel structure, J. Electrochem. Soc., 132, pp. 2933-2938, 1985.

127. Zhu, Z., Z. Chang, and L. Kevan, Synthesis and characterization of mesoporous chromium-containing silica tube molecular sieves CrMCM-41, J. Phys. Chem. B, 103, pp. 2680-2688, 1999.

128. Weckhuysen, B.M., L.M. De Ridder, P.J. Grobet, and R.A. Schoonheydt, Redox behavior and dispersion of supported chromium catalysts, J. Phys. Chem., 99, pp. 320-326, 1995.

129. Capek, L., J. Dedecek, P. Sazama, and B. Wichterlova, The decisive role of the distribution of Al in the framework of beta zeolites on the structure and activity of Co ion species in propane-SCR-NOx in the presence of water vapour, J. Catal., 272, pp. 44-54, 2010.

130. Birk, Y., M. Kacimi, M. Ziyad, and F. Bozon-Verduraz, Titania-supported cobalt and cobalt-phosphorus catalysts: characterization and performances in ethane oxidative dehydrogenation, J. Catal., 202, pp. 118-128, 2001.

131. Mihailova, B., V. Valtchev, S. Mintova, and L. Konstantinov, Vibrational spectra of ETS-4 and ETS-10, Zeolites, 16, pp. 22-24, 1996.

132. Takizawa, T., T. Watanabe, and K. Honda, Photocatalysis through excitation of adsorbates. 2. A comparative study of rhodamine B and methylene blue on cadmium sulfide, J. Phys. Chem., 82, pp. 1391-1396, 1978.

133. Gratzel, M, and R.F. Howe, Electron paramagnetic resonance studies of doped TiO2 colloids, J. Phys. Chem., 94, pp. 2566-2572, 1990.

165

9.0 APPENDICES

166

Appendix A:

Further Analysis of TM-Substituted AM-6

167

9.1

It has been reported [9.

Location of Substituted TM Ion Relative to the AM-6 Crystal Surface

1] that effective electron trapping and de-trapping is only

valid for transition metals (TM) substituted close to the surface site at which interfacial

charge-transfer occurs [9.2, 9.3]. Therefore, angle-resolved XPS (ARXPS) was utilized

to determine the location of the substituted TM ion relative to the surface of the AM-6

crystal. A decrease in the take-off angle (TOA) from 90° (i.e., normal to the surface;

approximate penetration depth of 10 nm) to 60° (i.e., increased surface sensitivity;

approximate penetration depth of 4-6 nm) [9.4

9.2

] resulted in an increase in the Cr/V ratio

for the 0.05Cr-AM-6 product (Section 4.2): at 90° TOA, Cr/V = 0.27, whereas at 60°

TOA, Cr/V = 0.47. These results suggest the presence of Cr3+ sites near the AM-6

crystal surface.

As discussed in Section 4.2.2, unmodified AM-6 crystals had an average product

particle size of 0.80 µm as determined by PSD analysis (Figure 9.2.1). Substitution using

mixture Co/V = 0.025 had very little effect on the average particle size of the resulting

(i.e., Co-AM-6) crystals as determined by FE-SEM (Figure 63c) and PSD analyses

(Figure 9.2.1).

Particle Size Distribution Analysis

168

Figure 9.2.1: Particle size distributions (PSDs) of Co-modified and unmodified AM-

6 products synthesized at 503 K from mixture with molar composition 2.25Na2O : 1.3K2O : 4.85SiO2 : 0.5V2O5 : 2.25TMAOH : 209H2O : 0.025TM.

9.3

In Section 4.2.5, it was suggested that as the mixture (and crystal) TM

concentration increased, the intensity of bands at ~870 cm-1 and ~948 cm-1 decreased,

and the bands at ~870 cm-1 and ~570 cm-1 shifted to higher and lower wavenumbers,

respectively. These observations are clearly illustrated in the Figure 9.3.1, Figure 9.3.2,

and Figure 9.3.3 for all TM-substituted AM-6 products. The decreased intensity of FTIR

bands observed for products with higher TM ion contents is likely caused by the

decreasing amount of AM-6 in these products.

FTIR Spectroscopic Analysis

169

Figure 9.3.1: Diffuse reflectance FTIR absorption spectra of Cr-AM-6 products

synthesized at 503 K from mixture with molar composition 2.25Na2O : 1.3K2O : 4.85SiO2 : 0.5V2O5 : 2.25TMAOH : 209H2O : xCr : (a) x = 0.10, (b) x = 0.05, (c) x = 0.025, and (d) unmodified AM-6.

Figure 9.3.2: Diffuse reflectance FTIR absorption spectra of Fe-AM-6 products

synthesized at 503 K from mixture with molar composition 2.25Na2O : 1.3K2O : 4.85SiO2 : 0.5V2O5 : 2.25TMAOH : 209H2O : xFe : (a) x = 0.05, (b) x = 0.0375, (c) x = 0.025, and (d) unmodified AM-6.

170

Figure 9.3.3: Diffuse reflectance FTIR absorption spectra of Co-AM-6 products

synthesized at 503 K from mixture with molar composition 2.25Na2O : 1.3K2O : 4.85SiO2 : 0.5V2O5 : 2.25TMAOH : 209H2O : xCo : (a) x = 0.10, (b) x = 0.05, (c) x = 0.025, and (d) unmodified AM-6.

9.4

In section 4.3.3, the repetitive use of a photocatalyst was investigated. XRD and

SEM were used to evaluate the crystallinity of the photocatalyst after each reaction cycle.

XRD analysis (Figure 76) indicated a slight loss (i.e., ~5% loss) in crystallinity upon

completion of the 1st reaction cycle. However, no discernable loss was shown by SEM

analysis (Figure 9.4.1).

Photocatalyst Re-Use – SEM Analysis

Figure 9.4.1: FE-SEM images of of 0.025Cr-AM-6 before photocatalysis (a), after

the 1st reaction cycle (b), and after the 2nd reaction cycle (c) under UV light irradiation.

171

Appendix B:

The Role of Silver Nanoparticles Deposited on Titanosilicate ETS-10 on the Photocatalytic Activity of Ag-modified ETS-10 under Visible Light Irradiation

172

As discussed in Section 2.3.1.3, modification of semiconductors with noble metal

particles enhances the photocatalytic activity by noble metal particles either acting as

electron sinks or extending the light absorption into the visible range and enhance the

surface electron excitation by surface plasmons excited by visible light. However, the

stability of these nanoparticles is questionable [9.5,9.6,9.7,9.8,9.9,9.10

9.5

]. Therefore, the

incorporation of silver nanoparticles with the readily studied ETS-10 (Ag0-ETS-10) was

initially investigated in efforts to elucidate the role of silver nanoparticles on the

photocatalytic activity of Ag-modified ETS-10.

Ag0 nanoparticles were prepared by irradiating aqueous silver nitrate solutions

(AgNO3, Ultrapure grade, Acros) of predetermined concentrations under UV light (500

W Xe arc lamp; 280-400 nm) while purging with N2 for 30 min. Appearance of the XRD

peak at approximately 44.2 o2θ for the filtered solids confirmed the formation of silver

metal nanoparticles with the average crystallite size of ~25 nm [9.

Preparation of Silver Nanoparticles

11

9.5.1 Synthesis of ETS-10 Crystals

].

ETS-10 crystals were hydrothermally synthesized for 3 days at 503K from

mixtures with the composition 5.2Na2O : 0.5K2O : TiO2 : 5.5SiO2 : 113H2O. The

synthesis mixtures were prepared using anatase (99.0%, Aldrich), N-Brand sodium

silicate (28.59% SiO2, 8.88% Na2O, PQ), sodium chloride (99.0%, Sigma), potassium

chloride (99.0%, Fisher), and deionized water (resistivity > 18MΩ cm) [9.12]. After

crystallization, the ETS-10 products were filtered with 1 L of deionized water and dried

overnight at 373 K in ambient air.

173

9.5.2 Preparation of Ag0-ETS-10

To obtain Ag0-ETS-10, 1 g of as-synthesized ETS-10 crystals was added to a 200

mL aqueous AgNO3 (Ultrapure grade, Acros) solution with concentrations varying from

3 to 30 mM. The pH of the crystal suspension was adjusted to be ~6.8, and the

suspension was stirred at ~353 K for 2 h. After ion exchange (Ag+-ETS-10) [9.11], the

samples were filtered and washed with 1 L of deionized water, followed by overnight

drying at 353 K in ambient air. The Ag+-ETS-10 products were reduced to Ag0-ETS-10

by purging with H2 at 20 cm3 s-1 (STP) for 2 h at 423 K. The Na+ ion-exchanged ETS-10

samples are referred to as Ag+-ETS-10-n, and the corresponding Ag0 nanoparticle-

modified ETS-10 samples are referred to as Ag0-ETS-10-n, where n = 3, 6, 10, 15, and

30 mM is the concentration of AgNO3 solution [51,71].

9.6

Figure 9.6.1 illustrates the photocatalytic degradation kinetics of MB on the free-

floating silver introduced into the system as nanoparticles (Ag0) or formed from Ag+ ions

during MB photodegradation. The data in Figure 9.6.1 show that the rate constants for

free-floating silver initially present in the system as either Ag+ or Ag0 appeared to be

identical under the conditions of this investigation. Figure 9.6.1 suggests that regardless

of the starting oxidation state of Ag (i.e., cationic or metallic) the reaction rate constants

are virtually identical for a given amount of free-floating silver. In addition, since

starting concentrations of MB are identical, and the rate constants measured for the

higher silver content reaction mixtures (i.e., silver loadings equivalent to that for Ag-

ETS-10-15 sample) are similar to the rate constants measured for the lower silver content

reaction mixtures (i.e., silver loadings equivalent to that for Ag-ETS-10-6 sample), it

Photocatalytic Degradation of MB in a Free-Floating Ag System

174

appears that the photodegradation of MB is independent on the amount of silver present;

however, it is related to the loss in surface area of the silver particles per unit volume of

solution.

Figure 9.6.1: Photocatalytic degradation of methylene blue utilizing free-floating

silver (Ag+ and Ag0) with the silver loading equivalent to that in Ag+–ETS-10-15 and Ag+–ETS-10-6 under visible light (420–630 nm) irradiation. The data points labeled Ag+–ETS-10-15 and Ag+–ETS-10-6 depict results obtained from experiments run in the presence of aqueous AgNO3 solution. The data points labeled Ag0–ETS-10-15 and Ag0–ETS-10-6 depict results obtained from experiments run in the presence of preformed Ag0 nanoparticles.

This particle surface area loss was deduced from the temporal changes of the UV-

vis spectra of MB solution photodegraded on free-floating silver. An example of this is

shown in Figure 9.6.2 for the free-floating silver system formed from Ag+ ions. Similar

temporal changes were observed for the free-floating silver introduced into the system as

nanoparticles. The gradual increase in the absorption maxima assigned to the quadrupole

and dipole plasmon resonance bands and the slight shift of their position to longer

wavelengths seen in Figure 9.6.2 have been attributed to the increase of silver particle

size in a suspension of constant particle concentration [9.13]. Thus, the presence of

175

plasmon resonance bands as well as their temporal evolution shown in Figure 9.6.2

suggests that Ag+ ions were reduced by trapping the electrons from the photoexcited MB

and the resulting silver nanoparticles subsequently grew in size during MB

photodegradation. This surface area loss (i.e., loss in dispersion of the free-floating silver

particles) is also hypothesized to be responsible for the decrease of the apparent reaction

rate constant determined at longer (i.e., between 2-9 minutes, k = 0.093-0.139 min-1)

reaction times versus those measured initially (i.e., after 2 minutes, k = 0.394-0.449 min-1)

(Figure 9.6.1). This loss was also observed for Ag-ETS-10 samples in which the silver

particles grew faster for Ag-ETS-10-15 than for Ag-ETS-10-6 (Section 9.6.1, Figure

9.6.4b).

Figure 9.6.2: Absorption spectra of silver nanoparticle suspensions (silver loading

equivalent to that in Ag+–ETS-10-15) taken at different times during the photocatalytic degradation of methylene blue utilizing free-floating silver system formed from Ag+ ions. The spectrum collected at t = 0min depicts methylene blue.

0.0

0.5

1.0

1.5

2.0

250 350 450 550 650 750

Abs

orba

nce

(a.u

.)

Wavelength (nm)

0 min 2 min4 min 6 min9 min

DipoleQuadrupole

176

9.6.1 Photocatalytic Degradation of Methylene Blue on Ag+-ETS-10

All Ag+-ETS-10 samples showed photocatalytic activity for degradation of MB

under visible light irradiation (Figure 9.6.3), and this activity increased with increasing

degree of sample Ag+ exchange. Assuming a pseudo-first-order reaction mechanism

[9.14

The change of color of the reaction suspension from bright blue to dark gray over

the course of the photocatalytic experiments in the presence of Ag+-exchanged ETS-10

samples is indicative of the formation of metallic silver. Vamathevan et al. [9.

], the reaction rate constant increased from 0.041 min-1 for Ag+-ETS-10-3 to 0.263

min-1 for Ag+-ETS-10-15, Figure 9.6.3. The MB degradation rate constant of 0.007 min-1

determined for the as-synthesized ETS-10 sample (no silver) indicates the unmodified

ETS-10 has a very low activity under visible light irradiation (MB photolysis rate =

0.005 min-1), and acts mainly as a substrate for MB adsorption in these studies.

15

] also

reported a color change of Ag+-modified TiO2 from white to brown owing to the

reduction of Ag+ ions by the photogenerated electrons during photocatalytic oxidation of

sucrose under UV light irradiation.

Figure 9.6.3: Photocatalytic degradation kinetics of methylene blue on Ag+-exchanged ETS-10 samples with different degree of Ag+ exchange under visible light (420–630 nm) irradiation.

177

The FE-SEM images (Figure 9.6.4) of Ag+-ETS-10-15 crystals which before use

had “featureless” surfaces with only occasional steps/striations, after photocatalysis

showed small particles on the crystal surfaces. Some of these particles (≤ 15 nm)

appeared to be attached to ETS-10 crystal surfaces (Figure 9.6.4b); however, many

relatively larger particles (~100-200 nm) appeared to be present on their own, i.e.,

suspended free in the reaction mixture as suggested by the aggregated particles located

between ETS-10 crystals (Figure 9.6.4a) and separated from ETS-10 crystals (Figure

9.6.4b). The XRD analysis of the Ag+-ETS-10-15 sample performed post-photocatalysis

revealed additional peaks at approximately 38.2, 44.2, 64.4, and 77.4 º2that are due to

metallic silver (PDF No. 4-0783, JCPDS). This confirmed formation of Ag0 particles

during the photocatalytic degradation of MB using the Ag+-ETS-10 samples.

Figure 9.6.4: FE-SEM images of Ag+-exchanged ETS-10 sample (Ag+–ETS-10-15)

after photocatalytic experiment showing the formation of metallic Ag particles (a) in solution, and (b) on the ETS-10 crystal surfaces.

To estimate the relative importance of the photoreduction of Ag+ ions and any

subsequent action of metallic Ag0 nanoparticles in the photodegradation activity of MB

over Ag+-ETS-10 samples, consecutive experiments were conducted under identical

reaction conditions. In these experiments the first run used an Ag+-exchanged ETS-10

178

sample, whereas the second run utilized the same sample recovered from the first run. As

shown in Figure 9.6.5, the photocatalytic activities of the “used” Ag+-ETS-10-6 and Ag+-

ETS-10-15 samples were lower than those of the corresponding “fresh” samples.

Interestingly, the calculated rate constants for the “used” crystals are within a factor of 2-

3 of those for Ag0-ETS-10 samples for similar silver loadings (Section 9.6.2, Figure

9.6.6).

Figure 9.6.5: Photocatalytic degradation kinetics of methylene blue on fresh and

previously used Ag+-exchanged Ag+–ETS-10-6 and Ag+–ETS-10-15 samples under visible light (420–630 nm) irradiation.

9.6.2 Photocatalytic Degradation of Methylene Blue on Ag0-ETS-10

The release of Ag0 particles into the solution through photoreduction of Ag+ ions,

suggested by FE-SEM observations (Figure 9.6.4), presents a significant drawback of

using Ag+-ETS-10 samples in liquid systems. Therefore, the Ag0 nanoparticle-modified

ETS-10 samples prepared via H2 reduction of Ag+-ETS-10 samples prior to reaction

were investigated in an effort to achieve a more stable catalysis system. All Ag0-ETS-10

samples were active in the photodegradation of MB under visible light irradiation

179

(Figure 9.6.6). However, unlike the Ag+-ETS-10 samples which showed increased

photoactivity with increasing Ag+ ion loading (Figure 9.6.3), the highest photocatalytic

activity for Ag0 nanoparticle-modified ETS-10 was obtained using the intermediate Ag0-

ETS-10-6 sample (Figure 9.6.6). Further increase of Ag0 loading levels resulted in a

lower photocatalytic activity of the Ag0-ETS-10 samples. A maximum reaction rate for

an intermediate silver loading has been observed for the Ag0 nanoparticle-modified TiO2

and other photocatalysts [9.16,9.17,9.18,9.19

].

Figure 9.6.6: Photocatalytic degradation kinetics of methylene blue on Ag0 nanoparticle modified ETS-10 samples with different Ag0 loading levels under visible light (420–630 nm) irradiation.

As with Ag+-ETS-10 samples, it appears that Ag0 particles leave the surfaces of

these Ag0-ETS-10 products (Figure 9.6.7), with more silver leaving the surfaces at higher

loading levels (i.e., n ≥ 10) and larger particle sizes, than for the lower silver loadings

(i.e., n ≤ 6) and smaller particle sizes. The lower activity of Ag0-ETS-10 samples with

higher Ag0 contents (Figure 9.6.6) appears to be related to a large number of large Ag0

nanoparticles in these samples, and is likely the result of silver particles leaving the

surface (Figure 9.6.7). These results are in agreement with the photodegradation results

180

in the free-floating silver systems where the apparent activity at longer times (larger

silver nanoparticles) was lower than that measured at early times (smaller silver

nanoparticles) (Figure 9.6.1 and Figure 9.6.2).

Figure 9.6.7: FE-SEM images of Ag0 nanoparticle-modified ETS-10 sample (Ag0–

ETS-10-15) before (a) and after (b) photocatalytic experiment under visible light (420–630 nm) irradiation showing that silver nanoparticles leave the ETS-10 surface.

Therefore, these findings clearly suggest that silver nanoparticles act as the

catalyst in the photodegradation of MB on Ag-modified ETS-10, and that the presence of

these nanoparticles on the semiconductor surfaces is unstable (i.e., Ag0 nanoparticles

leave the ETS-10 surfaces).

181

References for Appendix 9.0

9.1. Choi, W., A. Termin, and M.R. Hoffmann, The role of metal ion dopants in

quantum-sized TiO2: Correlation between photoreactivity and charge carrier recombination dynamics, J. Phys. Chem., 98, pp. 13669-13679, 1994.

9.2. Lasa, H. D., B. Serrano, and M. Salaices, Photocatalytic reaction engineering, New York: Springer, pp. 1-47, 2005.

9.3. Palmisano, G., V. Augugliaro, M. Pagliaro, and L. Palmisano, Photocatalysis: a promising route for 21st century organic chemistry, Chem. Commun., pp. 3425-3437, 2007.

9.4. Barr, T.L., Modern ESCA: The Principles and Practice of X-Ray Photoelectron Spectroscopy, CRC Press, Boca Raton, 1994.

9.5. Pastoriza-Santos, I., D.S. Koktysh, A.A. Mamedov, M. Giersig, N.A. Kotov, and L.M. Liz-Marzan, One-pot synthesis of Ag@TiO2 core-shell nanoparticles and their layer-by-layer assembly, Langmuir, 16, pp. 2731-2735, 2000.

9.6. Tom, R.T., A.S. nair, N. Singh, M. Aslam, C.L. Nagendra, R. Philip, K. Vijayamohanan, and T. Pradeep, Freely dispersible Au@TiO2, Au@ZrO2, Ag@TiO2, and Ag@ZrO2 core-shell nanoparticles: one-step synthesis, characterization, spectroscopy, and optical limiting properties, Langmuir, 19, pp. 3439-3445, 2003.

9.7. Hirakawa, T., and P.V. Kamat, Charge-separation and catalytic activity of Ag@TiO2 core-shell composite clusters under UV-irradiation, J. Am. Chem. Soc., 127, pp. 3928-3934, 2005.

9.8. Wang, W., J. Zhang, F. Chen, D. He, and M. Anpo, Preparation and photocatalytic properties of Fe3+-doped Ag@TiO2 core-shell nanoparticles, J. Colloid Interface Sci., 323, pp. 182-186, 2008.

9.9. Li, Y., H. Zhang, Z. Guo, J. Han, X. Zhao, Q. Zhao, and S-J. Kim, Highly efficient visible-light-induced photocatalytic activity of nanostructured AgI/TiO2 photocatalyst, Langmuir, 24, pp. 8351-8357, 2008.

9.10. Chuang, H-Y., and D-H. Chen, Fabrication and photocatalytic activities in visible and UV light regions of Ag@TiO2 and NiAg@TiO2 nanoparticles, Nanotechnology, 20, pp. 105704/1-105704/10, 2009.

9.11. Ismail, M.N., Z. Ji, D.M. Callahhan Jr., E. Pandowo, Z. Cai, T.L. Goodrich, K.S. Ziemer, J. Warzywoda, and A. Sacco Jr., The role of silver nanoparticles on silver modified titanosilicate ETS-10 in visible light photocatalysis, Appl. Catal. B, 102, pp. 323-333, 2011.

9.12. Rocha, J., A. Ferreira, Z. Lin, and M.W. Anderson, Synthesis of microporous titanosilicate ETS-10 from TiCl3 and TiO2: a comprehensive study, Micropor. Mesopor. Mater., 23, pp. 256-263, 1998.

9.13. Evanoff, D.D. Jr., and G. Chumanov, Size-controlled synthesis of nanoparticles. 1. Silver-only aqueous suspensions via hydrogen reduction, J. Phys. Chem. B., 108, pp. 13948-13956, 2004.

9.14. Zhang, T., T. Oyama, A. Aoshima, H. Hidaka, J. Zhao, and N. Serpone, Photooxidation N-demethylation of methylene blue in aqueous TiO2 dispersions under UV irradiation, J. Photochem. Photobiol. A, 140, pp. 163-172, 2001.

182

9.15. Vamathevan, V., H. Tse, R. Amal, G. Low, and S. McEvoy, Effects of Fe3+ and

Ag+ ions on the photocatalytic degradation of sucrose in water, Catal. Today, 68, pp. 201-208, 2001.

9.16. V. Vamathevan, R. Amal, D. Beydoun, G. Low, and S. McEvoy, Photocatalytic oxidation of organics in water using pure and silver-modified titanium dioxide particles, J. Photochem. Photobiol. A., 148, pp. 233-245, 2002.

9.17. Zhang, L., J. Yu, h.Y. Yip, Q. Li, K.W. Kwong, A-W. Xu, and K. Po, Ambient light reduction strategy to synthesize silver nanoparticles and silver-coated TiO2 with enhanced photocatalytic and bactericidal activities, Langmuir, 19, pp. 10372-10380, 2003.

9.18. H.M. Sung-Suh, J.R. Choi, H.J. Hah, S.M. Sang, and Y.C. Bae, Comparison of Ag deposition effects on the photocatalytic activity of nanoparticulate TiO2 under visible and UV light irradiation, J. Photochem. Photobiol. A, 163, pp. 37-44, 2004.

9.19. Georgekutty, R., M.K. Seery, and S.C. Pillai, A highly efficient Ag-ZnO photocatalyst: synthesis, properties, and mechanism, J. Phys. Chem. C, 112, pp. 13563-13570, 2008.