# Presentation: 202516

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

*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 Sandia Corporation, 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.*

- 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.

**J.P. Wilcoxon, **
**Nanostructures and Advanced Materials Chemistry**
**Sandia National Laboratories**
**Albuquerque, N.M**., **87185-1421**
**jpwilco@sandia.gov**
**Colloborators:  T.R. Thurston, P. Provencio, G.A. Samara**

**Talk Outline**

## Typical Scenario-Dense Non-Aqueous Solvent Pools

- 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 time
- 2)  Treatment of large volumes of highly diluted toxins
- 3)  Cost of treatment

- Rain

- Organic Pool in Soil

- Water Table

- Slow Leaching

## Possible Treatment Approaches-

- 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)

- Step 1:  Excavation, Soil Washing

*Cost and large volumes involved are the principal practical concerns.*

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

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

## 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.

## 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.

## MoS2 layered structure gives chemical stability-

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

## MoS2, Like TiO2 Has Exceptional Photostability-

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

## 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)

## Purification by extraction into Acetonitrile (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.

- MoS2
- In Oil

- ACN

- With hydrophilic cationic Surfactant

- With Hydrophobic TOAC Surfactant

- Oil

- MoS2
- In ACN

## Quantum Size Effects influence the optical and electronic properties of the resulting solutions-

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

## Structural/Size Characterization-

- Chromatogram of clusters
- Linewidth(polydispersity) comparable to chemical impurities

## Light Absorbance and Redox Potentials-

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

## Photochemical Reactor-

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

## 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 chosen absorbance wavelength gives the amount of each chemical.
- The stability of the nanosize photocatalyst can 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) MoS****2****.**

## Optical Absorbance of Nanocluster Catalyst is Unchanged-

*No reduction in optical absorbance, nanocluster concentration, or photocatalytic activity were observed*

## Photocatalysis of Phenol Using Nanosize  MoS2 Supported on TiO2 Powder

**Visible Light Absorbance by MoS****2****.**
**Carrier transfer between MoS****2**** and TiO****2**** slurry particles decreases recombination rate and increases photooxidation rate of organic.**

## Pentachlorophenol (PCP) Photocatalysis Studies-

*Intermediate Photooxidation Products Depend on Catalyst Material*

- UV Photooxidaton using D=25 nm, SnO2 Clusters

- PCP UV Photooxidation Results using TiO2 (Degussa)

## PCP Photooxidation Results(Summary)

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

- Visible Light Photooxidation Results

- PCP UV Photooxidation Results

## Conclusions

** ****Photo-oxidation of an alkyl chloride by nanosize MoS****2**** shows a strong size dependence and occurs with weak visible illumination.**
**HPLC analysis demonstrates that no changes occur in the quantity or absorbance properties of nanosize MoS****2**** during the photooxidation of this alkyl chloride.**
**Both nanosize SnO****2**** and MoS****2**** show a strong size-dependent photocatalytic activity.**
** ****Nanosize MoS****2**** can be an effective photocatalyst for PCP photo-oxidation even with only visible (********>400 nm) light.**

## 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, and polycyclic 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.