tsnl Synthesis and Application of Nanosize Semiconductors for Photoxidation of Toxic Organic Chemicals 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 •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 TiO 2 and SnO 2 . •Photocatalysis using visible light and MoS 2 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 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. tsnl Typical Scenario-Dense Non-Aqueous Solvent Pools Rain Organic Pool in Soil Slow 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 time  2) Treatment of large volumes of highly diluted toxins  3) Cost of treatment tsnl 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. tsnl Alternative Approach Use stable, inorganic, semiconductor nanoclusters with tunable bandgaps to oxidize organic chemicals using sunlight MoS 2 nanocrystal + + + + electron/hole transfer reactions CO 2 +HCl sunlight chlorinated aromatic + H 2 O e h +  Clusters can be used in both dispersed and heterogeneous forms (supported) tsnl 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. MoS 2 ). •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. tsnl 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. tsnl MoS 2 layered structure gives chemical stability- Mo, W S, Se 12.3 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 tsnl MoS 2 , Like TiO 2 Has Exceptional Photostability- Energy Valence Band S 3p MoS 2 Mo, 4 d z 1.33 V Conduction Band Mo, 4d xy 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 tsnl MoS 2 synthesis, purification, and characterization- Synthesis in Inverse Micelle System Mo 4+ + 2S 2- = MoS 2 Mo Source: MoCl 4 , S Source: H 2 S, Oil: Octane Typical Surfactant: Tri-ocytlmethylammonium Chloride (TOAC) tsnl Purification by extraction into Acetonitrile (ACN) MoS2 In Oil ACN With hydrophilic cationic Surfactant With Hydrophobic TOAC Surfactant Oil MoS2 In ACN 1) Liquid Chromatography shows the MoS 2 clusters have a net charge. 2) Samples diluted into water are dialized to remove unwanted ions like SO 4 -2 3) Analysis by XRF gives the final [Mo] and [Mo]:[S]~ 1 : 2.4 for D=3 nm. [...]... measured at end of reaction confirms total photooxidation of PCP tsnl Conclusions Photo-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 or absorbance properties of nanosize MoS2 during the photooxidation of this alkyl chloride Both nanosize SnO2 and MoS2 show a strong... medium which separates them and they elute at various times Example - Destruction of an Alkyl Chloride Organic Impurity using dispersed nanosize (D = 3 nm) MoS2 • The amount of chemical in each elution • 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... 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... measured using an absorbance or fluorescence detector and compared to known amounts of the same chemical Organic Impurity Oxidized to CO 2 600 increasing time 400 200 organic impurity t=0 0 3 4 5 6 elution time (min) tsnl 7 8 Optical Absorbance of Nanocluster Catalyst is UnchangedMoS 2 Optical Absorption 800 700 Absorbance 600 before photocatalysis after all organics are oxidized 500 400 300 200 100 0 200... reduces the ability to oxidize a given organic Mixtures of Nanoclusters will likely optimize the photooxidation process tsnl 1400 Photochemical Reactor- 400 W Xe arc lamp with long pass filters Cylindrical reactor with sampling port and overhead illumination tsnl Liquid Chromatography is Used to Follow the Kinetics of Photo-Redox ReactionsBasic Concept • Chemicals (and dispersed nanoclusters) travel through... recombination rate and increases photooxidation rate of organic tsnl 5 Photocatalysis of Phenol Using Nanosize MoS2 Supported on TiO2 Powder 21 Phenol Concentration (mg/l) TiO 2 20 19 18 17 MoS 2 (d=8-10 nm) MoS 2 (d=4.5 nm) 16 λ>450 nm illumination 15 0 100 200 300 400 500 Time (seconds) • Visible (λ>450 nm)Light Absorbance by MoS2 shows exponential photo-oxidation kinetics • A strong size dependence of photo-oxidation... strong size-dependent photocatalytic activity Nanosize MoS2 can be an effective photocatalyst for PCP photo-oxidation even with only visible (λ>400 nm) light tsnl 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...Quantum Size Effects influence the optical and electronic properties of the resulting solutions10 5 10 4 10 3 10 2 10 1 bulk 3.0 nm 0 200 2.5 nm d < 2.5 nm 400 600 bance(a.u.) 10 4.5 nm 800 1000 Wavelength(nm) By adjusting the size alone, the conductance and valence band energy levels can be shifted allowing new types of photocatalytic behavior to occur tsnl Structural/Size Characterization100... 50 two theta (degrees) chemical byproduct impurities 120 Absorbance(a.u.) 60 100 80 Chromatogram of clusters Linewidth(polydispersity) comparable to chemical impurities clusters 60 40 20 0 0 5 10 15 20 Elution time(min) 25 tsnl 60 Light Absorbance and Redox Potentials- µ m) Redox Potentials of Various Semiconductors at pH 7 1000.00 2.0 H 2 O/OH potential 200.00 (bulk) 2 400.00 nm) 600.00 2 (d=8-10... observed tsnl Photocatalysis of Phenol Using Nanosize MoS2 Supported on TiO2 Powder 5 21 20 19 18 17 MoS 2 (d=8-10 nm) Phenol Destruction Rate (%/min) Phenol Concentration (mg/l) TiO 2 4 3 2 1 MoS 2 (d=4.5 nm) 16 λ>450 nm illumination 0 1 15 0 100 200 300 400 500 2 MoS 2 3 4 loading (weight %) Time (seconds) • Visible Light Absorbance by MoS2 • Carrier transfer between MoS2 and TiO2 slurry particles decreases . tsnl Synthesis and Application of Nanosize Semiconductors for Photoxidation of Toxic Organic Chemicals J.P. Wilcoxon, Nanostructures and Advanced. TiO 2 and SnO 2 . •Photocatalysis using visible light and MoS 2 nanoclusters.  Acknowledgement: Div. Of Materials Science and Engineering, Office of Science,