An (hk0)-oriented p-type CaFe2O4 (E(g): 1.9 eV) photocathode was prepared, and hydrogen and oxygen gases were produced from a photocell short-circuited by connecting the CaFe2O4 and n-type TiO2 electrodes under illumination without applying an external voltage. The open-circuited voltage was 0.97 V and the short-circuit current was about 200 μA/cm(2), and the amount of evaluated hydrogen and oxygen gases after 2 days of reaction were about 70 and 4 μmol, respectively.
Rhodium-doped calcium niobate nanosheets were synthesized by exfoliating layered KCa(2)Nb(3-x)Rh(x)O(10-δ) and exhibited high photocatalytic activity for H(2) production from a water/methanol system without cocatalyst loading. The maximum H(2) production rate of the nanosheets was 165 times larger than that of the parent Rh-doped layered oxide. The quantum efficiency at 300 nm was 65%. In this system, the methanol was oxidized to formaldehyde (main product), formic acid, and carbon dioxide by holes, whereas electrons cause the reduction of water to H(2). The conductivity of the parent layered oxide was decreased by doping, which indicates the octahedral RhO(6) unit in the lattice of the nanosheet functions as an electron trap site. The RhO(6) units in the nanosheet probably also act as reaction sites for H(2) evolution.
Photocatalytic
activity of pure TiO2 is limited to ultraviolet
(UV) light due to the wide bandgap of anatase and rutile phases. The
bandgap of high-pressure phases of TiO2 can theoretically
coincide with visible light, but these phases are unstable at ambient
pressure. In this work, the high-pressure TiO2-II (columbite)
phase with large fractions of oxygen vacancies was stabilized by inducing
plastic strain to anatase under 6 GPa. The material could absorb visible
light as a consequence of bandgap narrowing by ∼0.7 eV. Formation
of nanosized TiO2-II enhanced the hydrogen generation efficiency
under visible light, and the efficiency improved after removing the
oxygen vacancies by annealing.
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