universiti putra malaysia synthesis and …psasir.upm.edu.my/27245/1/fs 2012 11r.pdf · layered...

UNIVERSITI PUTRA MALAYSIA

SYNTHESIS AND CHARACTERIZATION OF ZINC-ALUMINIUM LAYERED DOUBLE HYDROXIDE NANOCOMPOSITE

PREPARED VIA COPRECIPITATION METHOD

ABDULLAH AHMED ALI AHMED

FS 2012 11

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SYNTHESIS AND CHARACTERIZATION OF ZINC-ALUMINIUM

LAYERED DOUBLE HYDROXIDE NANOCOMPOSITE

PREPARED VIA COPRECIPITATION METHOD

By


Thesis Submitted to the School of Graduate Studies, Universiti Putra Malaysia, in

Fulfilment of the Requirements for the Degree of Doctor of Philosophy

September 2012

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DEDICATION

To my wife

with gratitude and love

I dedicate this work

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Abstract of the thesis presented to the senate of Universiti Putra Malaysia in fulfillment

of the requirement for the degree of Doctor of Philosophy

SYNTHESIS AND CHARACTERIZATION OF ZINC-ALUMINIUM LAYERED

DOUBLE HYDROXIDE NANOCOMPOSITE PREPARED VIA

COPRECIPITATION METHOD

By


September 2012

Chairman: Zainal Abidin Talib, PhD

Faculty: Science

Zinc–Aluminium Layered Double Hydroxides (Zn–Al–NO3–LDHs) were prepared

using the coprecipitation method at different Zn2+

/Al3+

molar ratios (2, 3, 4, 5 and 6)

with adjusting pH value at 7.5 and at different pH values (6, 7, 8, 9 and 10) with

constant molar ratio of 4. The second part of the work is the calcination of Zn–Al–NO3–

LDH sample prepared at Zn2+

/Al3+

molar ratio of 4 and pH value of 7 at two temperature

ranges; 50–500 °C and 200–1000 °C. The structural, textural, morphological, thermal

and optical properties were studied for all the samples. In addition, the dielectric

responses and ESR spectra were also investigated.

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Analysis of the XRD spectra of LDH showed that the crystallite size was in the range of

15–36 nm which confirmed that the LDH is a nanocomposite. This is corroborated by

the results of unit cell parameters (a and c) which showed that the construction of LDH

resulted in nanolayers. For the calcined LDH sample at 200 °C and above, the crystallite

size were in the range of 42.7–51.2 nm which is attributed to the formation

nanostructure of ZnO phase.

For the samples prepared at different Zn2+

/Al3+

molar ratios, the crystallinity of LDHs

was found to improve as molar ratio decreased due to the distortion of the hydroxide

sheet networks of the LDH crystal by the larger difference in ionic radii of Zn2+

and

Al3+

. On the other hand, the crystallinity of the samples prepared at different pH-values

was improved as the pH-value increased due to the increasing of the precipitation of

Zn2+

ions into the LDH slurry. For the calcination temperature below 200 °C, LDH

structure was well preserved, but started to collapse and transformed to zinc oxide (ZnO)

phase at 200 °C and above, due to the water loss and decomposition of nitrate anions of

LDH interlayer. At 600 °C and above, ZnAl2O4 spinel was formed and the crystallinity

of ZnO and ZnAl2O4 improved as the calcination temperature increased.

Optical band gap of the samples was calculated using Kubelka-Munk model. The band

gaps arise due to the electronic direct transitions in the brucite-like layers and nitrate

anions (NO3

–

). Band gaps were affected by the molar ratios used due to the formation of

low crystalline phases (ZnO and ZnAl2O4). This affect was also observed by the

variation of pH-value. Band gap decreased as the calcination temperature increased

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within the experimental temperature range of 200–1000 °C. The decreased in band gap

value was related to the improvement of the photocatalytic activity due to the increased

of the ZnO crystal size.

The dielectric response of the LDH is believed to be caused by two charge carriers: the

protons of the LDH water molecules and the NO3

–

interlayer anions, which can be

described by the anomalous low frequency dispersion with the second type of Universal

Power Law. The dominance of ZnO dipoles and charge carriers (NO3

– ions) in the

dielectric relaxation increases with the increasing molar ratio. The dielectric response for

the LDH calcined above 150 °C is similar to the dielectric response of ZnO, because of

the presence of the ZnO phase in the LDH at this temperature range.

The in situ electron spin resonance (ESR) spectra of fresh LDH obtained at room

temperature up to 190 °C were studied. These spectra were due to the presence of nitrate

anions in the LDH interlayer. However, at calcination temperature of 200 °C and above,

ESR spectra (ex situ ESR spectra) were due to the oxygen vacancies of ZnO which was

formed during thermal treatment of LDH. At 600 °C and above, there is an interaction

between the oxygen vacancies of ZnAl2O4 and the aluminium atom. The overlapped

signal can be resolved into two signals from the ZnAl2O4 spinel and the ZnO phase at

1000 °C.

Thermal diffusivity of LDH as a function of in situ temperatures result in a nonlinear

relation which is due to the changing of water content of LDH when the temperature was

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increased. However, thermal diffusivity of LDH as a function of calcined temperatures

showed a linear relation and the slope of these data demonstrated the dependency

between thermal diffusivity and water content of LDH below 200 °C. For temperatures

above 180 °C, the thermal diffusivity behavior was mainly governed by ZnO phase in

the LDH. Thermal diffusivity of LDH as a function of molar ratio also resulted in a

nonlinear relation which is due to the formation other phases namely ZnO and ZnAl2O4

with LDH, when the molar ratio was decreased or pH-value was increased.

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Abstrak tesis yang dikemukakan kepada Senat Universiti Putra Malaysia sebagai

memenuhi keperluan untuk ijazah Doktor Falsafah

SINTESIS DAN PENCIRIAN TERHADAP SEBATIAN NANOKOMPOSIT

ZINK-ALUMINIUM LAPISAN BERGANDA HIROKSIDA DISEDIAKAN

MELALUI KAEDAH SEPEMENDAKAN

Oleh


September 2012

Pengerusi: Zainal Abidin Talib, PhD

Fakulti: Sains

Penyediaan zink-aluminium hiroksida berlapis ganda menggunakan kaedah ko-

pemendakan pada kadar Zn2+

/Al3+

yang berbeza nisbah molar (2, 3, 4, 5 dan 6) dan pada

nilai pH 7.5, dan pada Kedua nilai pH yang berbeza (6, 7, 8, 9 dan 10) dengan

menetapkan nisbah molar Zn2+

/Al3+

pada 4. Bahagian kedua kajian ialah penyediaan

sampel yang terkalsin Zn-Al-NO3-LDH pada nisbah molar 4 dan nilai pH 7 dandua julat

suhu. 50–500 ⁰C dan 200–1000 ⁰C. Kajian berkenaan struktur, tekstur, sifat morfologi,

terma dan optik dilakukan ke atas semua sampel. Analiis berkenaan respons dielektrik

dan spektrum ESR juga telah dilakukan.

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Analisis spektrum XRD LDH menunjukkan bahawa saiz crystallite adalah dalam julat

15-36 nm yang mengesahkan bahawa LDH adalah komposit nano. Ini disokong oleh

keputusan parameter sel unit (a dan c) yang menunjukkan bahawa pembinaan LDH

mengakibatkan lapisan nano. Bagi sampel LDH dikalsin pada suhu 200 °C dan ke atas,

saiz crystallite adalah dalam julat 42.7-51.2 nm yang dikaitkan dengan pembentukan

struktur nano ZnO fasa.

Analisis Spektrum XRD LDH menunjukkan bahawa saiz crystallite adalah dalam julat

15-36 nm yang mengesahkan bahawa LDH adalah Komposit nano. Ini disokong oleh

keputusan parameter sel unit (a dan c) yang menunjukkan bahawa pembinaan LDH

mengakibatkan nanolayers. Bagi sampel LDH calcined pada suhu 200 ° C dan ke atas,

saiz crystallite adalah dalam julat 42.7-51.2 nm yang dikaitkan dengan nanostructure

pembentukan ZnO fasa.

Sampel Zn2+

/Al3+

yang disediakan dengan berbeza nisbah molar menunjukkan

kehabluran LDH meningkat apabila nisbah molar menurun. Perbezaan yang ketara

antara jejari ion Zn2+

dan Al3+

menyebabkan pengherotan kristal rangkaian lapisan

hiroksida kristal LDH. Kehabluran juga didapati meningkat apabila nilai pH meningkat.

Ia disebabkan oleh pemendakan ion Zn2+

meningkat dalam sluri struktur LDH tidak

terjejas apabila pengkalsinan di lakukan di bawah suhu 200 ⁰C. Tetapi, ia mula

mengalami keruntuhan dan berubah menjadi zink oksida pada suhu 200 ⁰C dan ke atas

Ia di sebabkan oleh kehilangan air dan penguraian anion nitrat pada lapisan LDH.

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ZnAl2O4 spinel terbentuk pada suhu 600 ⁰C dan ke atas, dan kehabluran ZnO dan

ZnAl2O4 meningkat apabila suhu pengkalsinan meningkat.

Model Kubelka-Munk digunakan untuk mengira celah jalur tenaga setiap sampel celah

jalur tenaga meningkat disebabkan peralihan elektronik langsung dalam lapisan bersifat

brucite dan anion nitrat (NO3). Perubahan nisbah molar mempengaruhi nilai celah jalur

tenaga. Ia disebabkan oleh pembentukan fasa ZnO dan ZnAl2O4 yang kehablurannya

rendah. Kesan yang sama juga dapat dilihat pada pelbagai nilai pH. Experimen yang di

jalankan dalam julat suhu 200-1000 ⁰C menunjukkan nilai celah jalur tenaga

berkurangan apabila suhu pengkalsinan meningkat. Pengurangan nilai celah jalur tenaga

berkait dengan peningkatan aktiviti fotopemangkin yang di sebabkan oleh peningkatan

saiz hablur ZnO.

Respons dielektrik LDH dipercayai disebabkan oleh dua pembawa cas: iaitu proton dari

molekul air LDH dan lapisan anion, NO3-. Fenomena ini dapat dijelaskan kerana

penyebaran frekuensi rendah yang abnormal (ALFD) berkaitan dengan Hukum Kuasa

Universal ke kedua. Dominan ZnO dwikutub dan pembawa cas (NO3- ion) dalam

pengenduran dielektrik meningkat apabila nisbah molar meningkat. Pengkalsinan LDH

pada suhu lebih dari 150⁰C menunjukkan respons dielektrik yang sama dengan respons

dielektrik ZnO yang disebabkan oleh kehadiran fasa ZnO dalam LDH pada julat suhu

tersebut.

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Elektron putaran resonans in situ (ESR) spectrum dari suhu bilik hingga 190 ⁰C

dijalankan keatas sampel LDH yang baharu (tanpa apa-apa rawatan) . Spektrum yang

diperolehi di sebabkan oleh kehadiran ion nitrat dalam lapisan LDH. Tetapi, pada suhu

pengkalsinan 200 ⁰C dan ke atas, ESR spektrum (ex situ ESR spektrum) di sebabkan

oleh kekurangan oksigen dalam ZnO yang terhasil ketika rawatan haba ke atas LDH

dilakukan. Pada suhu 600 ⁰C dan ke atas, saling tindak balas berlaku antara kekurangan

oxigen dalam ZnAl2O4 dan nukleus aluminium. Isyarat bertindih yang terhasil dapat di

selesaikan menjadi dua isyarat berasingan oleh ZnAl2O4 spinel dan fasa ZnO pada suhu

1000 ⁰C.

Peresapan haba LDH sebagai fungsi terhadap suhu in situ menghasilkan hubungan tidak

linear. Ia disebabkan oleh perubahan kandungan air dalam LDH apabila suhu meningkat.

Sebaliknya, peresapan haba LDH sebagai fungsi terhadap suhu pengkalsinan

menghasilkan hubungan linear dan kecerunan yang di perolehi dari data tersebut

menunjukan penggantungan antara peresapan haba dan kandungan air dalam LDH di

bawah suhu 200 ⁰C. Pada suhu 180 ⁰C ke atas, kelakuan peresapan haba adalah

disebabkan oleh fasa ZnO dalam LDH. Peresapan haba LDH sebagai fungsi terhadap

nisbah molar menghasilkan hubungan tidak linear. Ia di sebabkan oleh pembentukan

fasa-fasa lain (ZnO dan ZnAl2O4) dalam LDH apabila nisbah molar berkurangan atau

nilai pH meningkat.

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ACKNOWLEDGEMENTS

I want to thank Allah who gave me the hope, faith and patience to carry out this work.

I would truly like to express my deepest gratitude and sincere appreciation to my

supervisor, Associate Professor Dr. Zainal Abidin Talib for his guidance, enthusiasm,

leadership, friendship, boundless patience and constant encouragement. Despite his very

busy schedule, he has been a great guiding force and wonderful support extended to me

during my research and the help given to me from beginning of this study until the

completion the thesis.

The whole of the work presented in this thesis was results of a truly collaborative effort.

I would like to thank members of my committee, Professor Dr. Mohd Zobir Hussein and

Professor Dr. Azmi Zakaria for their help and direction in duration of my research.

During the study of my research, numerous people have contributed time and energy to

my knowledge; I would like to thank them all. I would like to thank Ms. Hiazum for her

help during LDH preparation in Nanomaterial Lab. Also I would like to express my

deepest gratitude to Mr. Samer H. Al-Ali for collaboration during the experimental

period of my research. Thank you to Ms. Khor and Mrs. Josephine for their help during

my measurements.

I would like to express my sincere thanks to Thamar University in Yemen for providing

me with financial support through full period time to get PhD degree at Universiti Putra

Malaysia.

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Finally, I especially wish to dedicate this work to my late father. There are no words to

express my appreciation for the love and support my mother, my wife, my son, Ahmed

and my daughter, Sara. This work could not have been possible without their continuous

support encouragement and patience during my studies.

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I certify that a Thesis Examination Committee has met on 18 September 2012 to conduct

the final examination of Abdullah Ahmed Ali Ahmed on his thesis entitled “Synthesis

and Characterization of Zinc-Aluminium Layered Double Hydroxide Nanocomposite

Prepared Via Coprecipitation Method” in accordance with the Universities and

University Colleges Act 1971 and the Constitution of the Universiti Putra Malaysia

[P.U.(A) 106] 15 March 1998. The Committee recommends that the student be awarded

the Doctor of Philosophy.

Members of the Examination Committee are as follows:

Wan Mohamad Daud b Wan Yusoff, PhD

Associate Professor

Faculty of Science

Universiti Putra Malaysia

(Chairman)

Wan Mahmood b Mat Yunus, PhD

Professor

Faculty of Science


(Internal Examiner)

Abdul Halim b Shaari, PhD

Professor

Faculty of Science


(Internal Examiner)

Paduru Venugopal Reddy, PhD Professor

Department of Physics

Osmania University

Hyderabad – India

(External Examiner)

SEOW HENG FONG, PhD

Professor and Deputy Dean

School of Graduate Studies


Date:

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This thesis was submitted to the senate of Universiti Putra Malaysia and has been

acceptance fulfillment of the requirement for the degree of Doctor of Philosophy. The

members of the supervisory Committee were as follow:

Zainal Abidin Talib, PhD

Professor

Faculty of Science


(Chairman)

Mohd Zobir Hussein, PhD

Professor

Faculty of Science


(Member)

Azmi Zakaria, PhD

Professor

Faculty of Science


(Member)

BUJANG BIN KIM HUAT, PhD

Professor and Dean

School of Graduate Studies


Date:

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DECLARATION

I declare that the thesis is my original work except for quotations and citations which

have been duly acknowledged. I also declare that it has not been previously, and is not

concurrently, submitted for any other degree at Universiti Putra Malaysia or at any other

institutions.


Date: 18 September 2012

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TABLE OF CONTENTS

Page

DEDICATION ii

ABSTRACT iii

ABSTRAK vii

ACKNOWLEDGEMENTS xi

APPROVAL xiii

DECLARATION xv

LIST OF TABLES xix

LIST OF FIGURES xxi

LIST OF ABBREVIATIONS xxvii

CHAPTER

1 INTRODUCTION 1

1.1 General description of LDH structure 1

1.2 Historical aspects on LDH 2

1.3 Importance of LDH 4

1.4 Applications of LDH 4

1.5 Problem statements of the study 6

1.6 Scope of the Study 8

1.7 Objectives of the study 8

2 LITERATURE REVIEW 10

2.1 Introduction 10

2.2 Synthesis of LDH 10

2.2.1 Coprecipitation method 11

2.2.1.1 Precipitation at low supersaturation 13

2.2.1.2 Precipitation at high supersaturation 14

2.2.1.3 A method involving separate nucleation and aging

------------------------steps

14

2.2.1.4 Urea hydrolysis 16

2.2.2 Ion-exchange method 17

2.2.3 Hydrothermal methods 18

2.2.4 Rehydration using structural memory effect 19

2.2.5 Other methods 20

2.3 Structural, Textural and Morphological Properties 20

2.4 Optical Properties 24

2.5 Dielectric Properties 28

2.6 Thermal properties 32

2.7 Electron Spin Resonance 37

3 THEORY 42

3.1 Introduction 42

3.2 Review on structural description of LDH 42

3.2.1 General description 42

3.2.2 Metal cations in the LDH layers 44

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3.2.3 Structure of the layers 45

3.2.4 Interlayer anions 48

3.2.5 Stacking and polytype 51

3.3 Optical Properties 53

3.3.1 Band edge absorption in direct gap materials 54

3.3.2 Kubelka-Munk model 59

3.4 Dielectric Properties 62

3.4.1 Dielectric materials 62

3.4.2 Principles of dielectric 63

3.4.3 Frequency-dependent dielectric response 65

3.4.4 Low frequency dispersion 68

3.4.5 Polarization of electrode 71

3.5 Electron Spin Resonance 73

3.5.1 Principles 73

3.5.2 Basics of Electron Spin Resonance 74

3.6 Thermal Diffusivity 76

4 PREPRATION AND CHARACTERISATION TECHNIQUES 80

4.1 Materials 80

4.2 Preparation of Samples 80

4.3 Preparation of Sample pallets 84

4.4 Characterization Techniques 85

4.4.1 Powder X-ray Diffraction 85

4.4.2 Fourier Transform Infrared spectroscopy 88

4.4.3 Thermogravimetric Analysis 88

4.4.4 Scanning Electron Microscopy 89

4.4.5 Inductively coupled plasma 89

4.4.6 Optical properties 90

4.4.7 Dielectric spectroscopy 91

4.4.8 Electron spin resonance 93

4.4.9 Thermal diffusivity 95

5 RESULTS AND DISCUSSION 98

5.1 Introduction 98

5.2 The synthesized samples at different Zn/Al molar ratios and constant

--------pH 7.5

98

5.2.1 Powder X-ray Diffraction study 98

5.2.2 SEM study 99

5.2.3 Fourier Transform Infrared 103

5.2.4 Thermal Analysis 105


5.2.6 Dielectric properties 110

5.2.6.1 Real and imaginary dielectric constants 110

5.2.6.2 The conductivity 114

5.2.7 Electron spin resonance spectra 115


5.3 The synthesized samples at different pH values and constant Zn/Al --

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--------molar ratios = 4 122


5.3.2 SEM study 126





5.3.6.1 Real and imaginary dielectric constants 137

5.3.6.2 The conductivity 139



5.4 The calcined samples at temperature range of 50–500 °C 146


5.4.2 SEM study 150





5.4.6.1 Dielectric studies of LDHs calcined at temperatures --

--- below 200 °C

163

5.4.6.2 Dielectric studies of LDHs calcined at 200, 250 and

--- 300 °C

166

5.4.6.3 In situ dielectric measurements 169



5.5 The calcined samples at temperature range of 200–1000 °C 186


5.5.2 SEM and N2 adsorption–desorption studies 189




6 CONCLUSIONS 204

Future work 207

REFERENCES 208

APPENDIX 222

A Dimensional values of LDH pellets for dielectric measurements 223

B Dimensional values of LDH pellets for thermal diffusivity measurements 225

C Calculation of the optical band gap 227

LIST OF PUBLICATION 230

CONFERENCES 231

BIODATA OF STUDENT 232

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