上课Solar_Cell.ppt

* * Principle o f photocatalytic hydrogen generation 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. H2O→2H2+O2 ?V=1.23V, ? G=238kJ/mol Photocatalyst material requirements Band gap: Band gap1.23eV 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. d0 and d10 metal oxides 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 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 disperses and p orbitals gives higher mobility. - Still usually a large band gap Solution 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, Ge3N4 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 Solution 2: Introduce Sulfur Sm2Ti2S2O5, Ruddlesden-Popper layered perovskite Introduce higher S3p bands Band gap: 2.1 eV (λ= 590nm) Stable during photocatalysis Still only 1.1% quantum efficiency High Temperature Heat Chemical Cycle Low Temperature Heat Water Oxygen Hydrogen Hydrogen THERMOCHEMICAL HYDROGEN ? Utilize a series of ch