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New Materials for Photocatalytic Water Splitting Fredrik Skullman MATRL 286G UCSB 5 26 2010 Instructor Ram Seshadri New Materials for Photocatalytic Water Splitting Fredrik Skullman MATRL 286G 05 26 2010 Background Why hydrogen Clean no greenhouse gases Energy security can be produced from abundant sources Economic growth Efficient fuel cells 75 efficiency Portable Car tanks micro fuel cells Honda FCX Clarity New Materials for Photocatalytic Water Splitting Fredrik Skullman MATRL 286G 05 26 2010 Problem Need to build up infrastructure Safety concerns Production today 95 from natural gas which is not renewable and produces CO2 as a byproduct Solution Split water with renewable energy sources New Materials for Photocatalytic Water Splitting Fredrik Skullman MATRL 286G 05 26 2010 Process H2O 2H2 O2 V 1 23V G 238kJ mol Step 1 Photon with energy above 1 23eV 1000 nm is absorbed Step 2 Photoexcited electrons and holes separate and migrate to surface Step 3 Adsorbed species water is reduced and oxidized by the electrons and holes Domen et al New Non Oxide Photocatalysts Designed for Overall Water Splitting under Visible Light J Phys Chem 2007 New Materials for Photocatalytic Water Splitting Fredrik Skullman MATRL 286G 05 26 2010 Photocatalyst material requirements Band gap Band gap 1 23eV and sufficiently small to make efficient use of solar spectrum 3eV Band levels suitable for water splitting High Crystallinity Defects can act as recombination sites Long term stability Charge transfer used for water splitting and not corrosion Domen et al New Non Oxide Photocatalysts Designed for Overall Water Splitting under Visible Light J Phys Chem 2007 New Materials for Photocatalytic Water Splitting Fredrik Skullman MATRL 286G 05 26 2010 d0 and d10 metal oxides Domen et al New Non Oxide Photocatalysts Designed for Overall Water Splitting under Visible Light J Phys Chem 2007 d0 Ti4 TiO2 SrTiO3 K2La2Ti3O10 Zr4 ZrO2 Nb5 K4Nb6O17 Sr2Nb2O7 Ta5 ATaO3 A Li Na K BaTa2O6 W6 AMWO6 A Rb Cs M Nb Ta d10 Ga3 ZnGa2O4 In3 AInO2 A Li Na Ge4 Zn2GeO4 Sn4 Sr2SnO4 Sb5 NaSbO7 New Materials for Photocatalytic Water Splitting Fredrik Skullman MATRL 286G 05 26 2010 d0 and d10 metal oxides d0 Layered perovskites with reaction sites between the layers For example K2La2Ti3O10 K4Nb6O17 ATaO3 A Li Na K Band gap between O2p and d0 usually too big d10 Conduction band with disperse s and p orbitals gives higher mobility Still usually a large band gap Domen et al Recent progress of photocatalysts for overall water splitting Catalysis today 1998 New Materials for Photocatalytic Water Splitting Fredrik Skullman MATRL 286G 05 26 2010 Solution 1 Introduce Nitrogen Domen et al New Non Oxide Photocatalysts Designed for Overall Water Splitting under Visible Light J Phys Chem 2007 N replaces O in certain positions providing a smaller band gap Currently problems with getting the nitrogen there without too many defects Oxygen free options Ta3N5 Ge3N4 New Materials for Photocatalytic Water Splitting Fredrik Skullman MATRL 286G 05 26 2010 Solution 2 Introduce Sulfur Sm2Ti2S2O5 RuddlesdenPopper layered perovskite Introduce higher S3p bands Band gap 2 1 eV 590nm Stable during photocatalysis Still only 1 1 quantum efficiency Domen et al Novel Synthesis and PhotoCatalytic Activity of Oxysulfide Sm2Ti2S2O5 Chem Mater 2007 New Materials for Photocatalytic Water Splitting Fredrik Skullman MATRL 286G 05 26 2010 d10 oxy nitrides GaN ZnO Ga1 xZnx N1 xOx solid solution with RuO2 nanoparticles Wurtzite structure with similar lattice parameters Band interactions give smaller band gap than for the individual semiconductors Bandgap 2 4 2 8 eV Similar material ZnGeN2 ZnO Domen et al Overall Water Splitting on Ga1xZnx N1 xOx Solid Solution Photocatalyst Relationship between Physical Properties and Photocatalytic Activity J Phys Chem 2005 New Materials for Photocatalytic Water Splitting Fredrik Skullman MATRL 286G 05 26 2010 Conclusions Clean cell free hydrogen production possible State of the art A few percent quantum efficiency Plenty of room for improvements Large area and a lot of catalysts needed expensive Other water splitting options currently far more efficient Bruce et al Self organized photosynthetic nanoparticle for cellfree hydrogen production Nature Nanotechnology 2009 New Materials for Photocatalytic Water Splitting Fredrik Skullman MATRL 286G 05 26 2010 Questions


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