Slide 1Slide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12New Materials for Photocatalytic Water SplittingFredrik SkullmanMATRL 286GUCSB, 5/26/2010Instructor: Ram SeshadriNew Materials for Photocatalytic Water Splitting, Fredrik Skullman, MATRL 286G, 05/26/2010Background – 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 ClarityNew Materials for Photocatalytic Water Splitting, Fredrik Skullman, MATRL 286G, 05/26/2010Problem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 sourcesNew Materials for Photocatalytic Water Splitting, Fredrik Skullman, MATRL 286G, 05/26/2010ProcessStep 1: Photon with energy above 1.23eV (λ<~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. 2007H2O→2H2+O2 ∆V=1.23V, ∆ G=238kJ/molNew Materials for Photocatalytic Water Splitting, Fredrik Skullman, MATRL 286G, 05/26/2010Photocatalyst 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. 2007New Materials for Photocatalytic Water Splitting, Fredrik Skullman, MATRL 286G, 05/26/2010d0 and d10 metal oxidesd0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+: NaSbO7Domen et al. New Non-Oxide Photocatalysts Designed for Overall Water Splitting under Visible Light. J. Phys. Chem. 2007New Materials for Photocatalytic Water Splitting, Fredrik Skullman, MATRL 286G, 05/26/2010d0 and d10 metal oxidesd0+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 gapDomen et al. Recent progress of photocatalysts for overall water splitting. Catalysis today. 1998New Materials for Photocatalytic Water Splitting, Fredrik Skullman, MATRL 286G, 05/26/2010Solution 1: Introduce Nitrogen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, Ge3N4Domen et al. New Non-Oxide Photocatalysts Designed for Overall Water Splitting under Visible Light. J. Phys. Chem. 2007New Materials for Photocatalytic Water Splitting, Fredrik Skullman, MATRL 286G, 05/26/2010Solution 2: Introduce SulfurSm2Ti2S2O5, Ruddlesden-Popper layered perovskiteDomen et al. Novel Synthesis and PhotoCatalytic Activity of Oxysulfide Sm2Ti2S2O5. Chem Mater. 2007Introduce higher S3p bandsBand gap: 2.1 eV (λ= 590nm)Stable during photocatalysisStill only 1.1% quantum efficiencyNew Materials for Photocatalytic Water Splitting, Fredrik Skullman, MATRL 286G, 05/26/2010d10 (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-ZnODomen et al. Overall Water Splitting on (Ga1-xZnx)(N1-xOx) Solid Solution Photocatalyst: Relationship between Physical Properties and Photocatalytic Activity. J. Phys. Chem. 2005New Materials for Photocatalytic Water Splitting, Fredrik Skullman, MATRL 286G, 05/26/2010Conclusions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 efficientBruce et al. Self-organized photosynthetic nanoparticle for cell-free hydrogen production. Nature Nanotechnology. 2009New Materials for Photocatalytic Water Splitting, Fredrik Skullman, MATRL 286G,