Takeo Arai

The photoelectrochemical properties of porous BiVO4 thin-film electrodes on conducting glass for H2 production from water under visible light were investigated. BiVO4 films were prepared by the metal-organic decomposition method, and particles were 90-150 nm in diameter. Under visible-light irradiation, H2 and O2 evolved in a stoichiometric ratio (H2/O2 = 2) from an aqueous solution of Na2SO4 with an external bias. The photocurrent increased with addition of methanol. The band structure of BiVO4 was investigated by open-circuit potential, flat-band potential, X-ray photoelectron spectroscopy, and calculations based on density functional theory. The top of the valence-band potential of BiVO4 was shifted negatively compared to the potentials of the conventional oxide semiconductors without Bi. We surmise that hybridization between the O-2p and Bi-6s orbitals might contribute to the negative shift of the BiVO4 valence band. Treatment with an aqueous solution of AgNO3 improved the photocurrent of the BiVO4 electrode significantly. The maximum incident photon-to-current conversion efficiency at 420 nm was 44%. This value was the highest among mixed-oxide semiconductor electrodes under visible light irradiation. AgNO3 treatment also improved the stability of the photocurrent. The Ag+ ion in/on the BiVO4 catalyzed the intrinsic photogeneration of oxygen with the holes.

Photoelectrochemical reduction of CO(2) to HCOO(-) (formate) over p-type InP/Ru complex polymer hybrid photocatalyst was highly enhanced by introducing an anchoring complex into the polymer. By functionally combining the hybrid photocatalyst with TiO(2) for water oxidation, selective photoreduction of CO(2) to HCOO(-) was achieved in aqueous media, in which H(2)O was used as both an electron donor and a proton source. The so-called Z-scheme (or two-step photoexcitation) system operated with no external electrical bias. The selectivity for HCOO(-) production was >70%, and the conversion efficiency of solar energy to chemical energy was 0.03-0.04%.

A highly efficient and visible light responsive photocatalyst was developed by combining the p-type semiconductor CuBi2O4 and the n-type semiconductor WO3. CuBi2O4/WO3 showed higher reactivity than typical TiO2-based photocatalysts for the complete oxidation of acetaldehyde into CO2 under both UV and visible light. From photoelectrochemical measurements, the reaction mechanism is explained based on the model of the p−n photochemical diode for reductivity/oxdizability improvement.

Solar formate production from CO2 and H2O was achieved with no external electrical bias by combining an InP/[RuCP] semiconductor/metal-complex hybrid photocathode with a reduced SrTiO3 (r-STO) photoanode. The conversion efficiency from solar to chemical energy was improved from 0.03 to 0.14% compared to a previous system utilizing a TiO2 photoanode. Stimulated electron transfer from the photoanode to the photocathode is the main cause for the observed improvement, due to an enlarged difference in the band-energy position between r-STO and InP. Since r-STO showed high H2O oxidation selectivity even in the presence of formate, a r-STO/InP/[RuCP] wireless device successfully performed solar CO2 reduction in a one-compartment reactor with no proton exchange membrane, yielding a solar conversion efficiency of 0.08%.

Photoelectrochemical reduction of CO(2) to HCOO(-) was successfully achieved by a p-type InP photocathode modified with an electropolymerized ruthenium complex in water. This technique decreased the required applied potential for CO(2) reduction by utilizing solar energy. The carbon and proton sources of HCOO(-) were identified by a tracer experiment to be CO(2) and H(2)O, respectively.