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Synthesis and Application of Nanosize Semiconductors for Photoxidation of Toxic Organic Chemicals

J.P. Wilcoxon,Nanostructures and Advanced Materials ChemistrySandia National LaboratoriesAlbuquerque, N.M., 87185-1421jpwilco@sandia.govColloborators: T.R. Thurston, P. Provencio, G.A. Samara

Talk Outline

•Industrial Solvents in the Environment (Impregnated Sediments, Water Table)•Brief History of the problem and possible remediation approaches(Bioremediation, Soil Washing, Adsorption, Photooxidation)•Photocatalysis using UV light and nanosize TiO2 and SnO2.•Photocatalysis using visible light and MoS2 nanoclusters.

Acknowledgement: Div. Of Materials Science and Engineering, Office of Science, US Dept. of Energy under contract DE-AC-04-AL8500. Sandia is a multiprogram laboratory operated by SandiaCorporation, a Lockheed-Martin Company, for the US Dept. of Energy. This work performed under the aupices of the DOE Environmental Science/Environmental Research (ER/ES) Program.

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Typical Scenario-Dense Non-Aqueous Solvent PoolsRain

Organic Pool in SoilSlow Leaching

Water Table

Examples-Cleaning Solvents- Tri-Chloroethylene (TCE)Herbicides/Fungacides/Pesticides (Pentachlorophenol (PCP), DDT)Explosives (e.g. TNT)

Major Remediation Issues-1) Low Solubility (1-10 ppm) in water provides continuous leaching with time2) Treatment of large volumes of highly diluted toxins 3) Cost of treatment

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Possible Treatment Approaches-

Step 1: Excavation, Soil Washing

Conventional Treatment Options:

1)Filtration and/or Adsorption of toxic chemicals in aqueous supernatant from Step 1

2)Chemical Oxidation or Total Mineralization of the the Organics

3)Deep UV Photooxidation of the Organics

4)Photocatalytic oxidation of the Organics (e.g. colloidal titania slurries)

Cost and large volumes involved are the principal practical concerns.

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Alternative Approach Use stable, inorganic, semiconductor nanoclusters with tunable bandgaps to oxidize organic chemicals using sunlight

MoS2 nanocrystal

+ + + +

electron/hole

transfer reactionsCO2 + HCl

sunlight

- - - -chlorinated

aromatic + H2O

e

h+

Clusters can be used in both dispersed and heterogeneous forms (supported)

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Advantanges of this Approach-

•The light absorption and energy levels of the semiconductor valence and conduction bands can be adjusted in a single material by changing the size (quantum confinement effect).

•A covalent semiconductor material with excellent photostability and low toxicity can be selected (e.g. MoS2).

•Our synthesis allows easy chemical modification of the nanocluster surface properties (e.g. deposition of a metal).

•Small size of nanocluster vastly reduces electron-hole recombination rate and undesired light scattering.

•Nanoclusters are easily deposited on bulk support materials from a dispersed liquid phase.

•Both dispersed and supported nanoclusters can be studied, allowing complete characterization of the photocatalyst microstructure.

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Photocatalysts Material Requirements -

1) Efficient conversion of sunlight to electron-hole pairs.

2) Surface trapping of electrons and holes before recombination.

3) Catalyst photostability.

4) Inexpensive, chemically-stable, environmentally benign materials.

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MoS2 layered structure gives chemical stability-

Mo, W

S, Se12.3 Å

weak van-der-Waals forces

:N

:N

bipyridine (bpy)

Binding of substrate organic chemical occurs at metal edge sites.Electron transfer rates allow an estimation of shift of the redox potential with size

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MoS2, Like TiO2 Has Exceptional Photostability-

Energy

Valence Band

S 3p

MoS2

Mo, 4 dz

1.33 V

Conduction Band

Mo, 4dxy

0.1 V

+

-

+

-

light

S 3p

CdS

Cd, 5s

+

-

+

-

2.4 V

Covalent Semi-conductors (Stable)Ionic II-VI Materials Carrier Excitation Weakens Chemical Bonds (Unstable)

Kinetic stability occurs because both valence and conduction bands are localized on the metal, so carrier excitation doesn’t weaken any chemical bonds

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MoS2 synthesis, purification, and characterization-Synthesis in Inverse Micelle System

Mo4+ + 2S2- = MoS2

Mo Source: MoCl4, S Source: H2S, Oil: Octane Typical Surfactant: Tri-ocytlmethylammonium Chloride (TOAC)

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Purification by extraction into Acetonitrile (ACN)

MoS2In Oil

ACN With hydrophilic cationic Surfactant

With Hydrophobic TOAC Surfactant

Oil

MoS2In ACN

1) Liquid Chromatography shows the MoS2 clusters have a net charge.

2) Samples diluted into water are dialized to remove unwanted ions like SO4-2

3) Analysis by XRF gives the final [Mo] and [Mo]:[S]~ 1 : 2.4 for D=3 nm.

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Quantum Size Effects influence the optical and electronicproperties of the resulting solutions-

10 0

10 1

10 2

10 3

10 4

10 5

200 400 600 800 1000

banc

e(a.

u.)

Wavelength(nm)

bulk

4.5 nm3.0 nm

2.5 nm

d < 2.5 nm

By adjusting the size alone, the conductance and valence band energy levels can be shifted allowing new types of photocatalytic behavior to occur

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Structural/Size Characterization-

0

20

40

60

80

100

10 20 30 40 50 60

MoS2(d=4.5 nm)

MoS2 powder std

Inte

nsity

(a.u

.)

two theta (degrees)

<002>

<004>

<100><101>

<102>

<103>

<006><105>

<110><106>

0

20

40

60

80

100

120

140

160

0 5 10 15 20 25

250 nm350 nm

Abs

orba

nce(

a.u.

)

Elution time(min)

clusters

chemical byproductimpurities

Chromatogram of clustersLinewidth(polydispersity) comparable to chemical impurities

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Light Absorbance and Redox Potentials-

-1.0

0.0

1.0

2.0

3.0

Redox Potentials of Various Semiconductors at pH 7

Pote

ntal

vs.

NH

E

H2O/OH. potential

TiO2MoS2(bulk)

MoS2(d=4.5 nm)MoS2

(d=8-10 nm)

0.00

200.00

400.00

600.00

800.00

1000.00

200 400 600 800 1000 1200 1400

Spec

tral

Irra

dian

ce (W

/m2 / µ

m)

Wavelength (nm)

TiO

2,bu

lk2 (

d=4.

5 nm

)

2 (d=

8-10

nm

)

2 (bu

lk)

Greater light absorbance reduces the ability to oxidize a given organic.Mixtures of Nanoclusters will likely optimize the photooxidation process.

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Photochemical Reactor-

400 W Xe arc lamp with long pass filters.Cylindrical reactor with sampling port and overhead illumination.

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Liquid Chromatography is Used to Follow the Kinetics of Photo-Redox Reactions-Basic Concept -

• Chemicals (and dispersed nanoclusters) travel through a porous medium which separates them and they elute at various times.

• The amount of chemical in each elution peak is measured using an absorbance or fluorescence detector and compared to known amounts of the same chemical.

• Intermediate break-down products are also identified.

• The size of the elution peak at a chosenabsorbance wavelength gives the amount of each chemical.

• The stability of the nanosize photocatalystcan be determined from changes in the complete absorbance spectrum at its elution peak.

Example - Destruction of an Alkyl Chloride Organic Impurity using dispersed nanosize (D = 3 nm) MoS2.

0

200

400

600

800

1000

3 4 5 6 7 8

Abs

orba

nce

(250

nm)

elution time (min)

MoS2

Photocatalyst

t = 0organic impurity

increasing time

Organic Impurity Oxidized to CO2

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Optical Absorbance of Nanocluster Catalyst is Unchanged-

0

100

200

300

400

500

600

700

800

200 250 300 350 400 450 500 550 600

MoS 2 Optical Absorption

before photocatalysisafter all organics

are oxidized

Abs

orba

nce

Wavelength (nm)

No reduction in optical absorbance, nanocluster concentration, orphotocatalytic activity were observed

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Photocatalysis of Phenol Using Nanosize MoS2 Supported on TiO2 Powder

• Visible Light Absorbance by MoS2.

• Carrier transfer between MoS2 and TiO2 slurry particles decreases recombination rate and increases photooxidation rate of organic.

0

1

2

3

4

5

1 2 3 4 5Ph

enol

Des

truc

tion

Rat

e (%

/min

)MoS 2 loading (weight %)

15

16

17

18

19

20

21

0 100 200 300 400 500

Phen

ol C

once

ntra

tion

(mg/

l)

Time (seconds)

TiO 2

MoS 2(d=4.5 nm)

MoS 2(d=8-10 nm)

λ>450 nm illumination

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Photocatalysis of Phenol Using Nanosize MoS2 Supported on TiO2 Powder

15

16

17

18

19

20

21

0 100 200 300 400 500

Phen

ol C

once

ntra

tion

(mg/

l)

Time (seconds)

TiO 2

MoS 2(d=4.5 nm)

MoS 2(d=8-10 nm)

λ>450 nm illumination

• Visible (λ>450 nm)Light Absorbance by MoS2 shows exponential photo-oxidation kinetics.

• A strong size dependence of photo-oxidation rate is observed.

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Pentachlorophenol (PCP) Photocatalysis Studies-UV Photooxidaton using D=25 nm,

SnO2 ClustersPCP UV Photooxidation Results

using TiO2 (Degussa)

0

50

100

150

200

250

300

0 5 10 15

t=0t=15t=30t=120t=240 min

A(2

15nm

)

t(min)

pcp

t=7.62t=5.75

0

50

100

150

200

0 2 4 6 8 10 12 14

t=0 mint=30t=120t=480

A(2

15nm

)t(min)

pcp

t=5.90t=3.31

Intermediate Photooxidation Products Depend on Catalyst Material

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PCP Photooxidation Results(Summary)

Visible Light Photooxidation ResultsPCP UV Photooxidation Results

0.01

0.1

1

0 100 200 300 400 500 600 700

Rel

ativ

e PC

P co

ncen

tratio

n

time(min)

No Catalyst

TiO2(d=20 nm)

SnO2(d=25 nm)

SnO2(d=58 nm)TiO2(Degussa)

0.01

0.1

1

0 100 200 300 400 500 600 700 800

No Catalyst or TiO2

CdS(0.1 mg/ml) powderMoS2,d=4.5,0.036 mg/mlMoS2,d=3.0 nm,0.09 mg/mlR

elat

ive

PC

P c

once

ntra

tion

time(min)

CO2 measured at end of reaction confirms total photooxidation of PCP.

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ConclusionsPhoto-oxidation of an alkyl chloride by nanosize MoS2 shows a strong size

dependence and occurs with weak visible illumination.

HPLC analysis demonstrates that no changes occur in the quantity orabsorbance properties of nanosize MoS2 during the photooxidation of this alkyl chloride.

Both nanosize SnO2 and MoS2 show a strong size-dependent photocatalyticactivity.

Nanosize MoS2 can be an effective photocatalyst for PCP photo-oxidation even with only visible (λ>400 nm) light.

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Future Directions•Improve nanocluster/support interactions by heat treatments after deposition of nanoclusters to improve photocatalysis kinetics.

•Examine nanocluster systems with mixed sizes (bandedges and potentials) to optimize solar absorbance while still allowing a sufficient driving force for the photooxidation process.

•Examine the photooxidation of long-lived organics such as pesticides, andpolycyclic aromatics using nanosize MoS2 to determine reaction kinetics and final breakdown products.

•Investigate alternative, highly stable nanocluster catalysts (RuS2, WS2) and compare with MoS2.

•Investigate other small molecule photoredox reactions (e.g. H2O or H2S reduction.