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.

The stoichiometric splitting of water into H2 and O2 (H2/O2 = 2) under visible light irradiation (lambda > 420 nm) took place for the first time using a mixture of Pt-WO3 and Pt-SrTiO3 (Cr-Ta-doped) photocatalysts and an IO3-/I- shuttle redox mediator.

The nanocrystalline BiVO4 film electrode on conducting glass showed an excellent efficiency (IPCE = 29% at 420 nm) for the decomposition of water under visible light.

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We studied the photoelectrochemical properties of nanoporous TiO2 electrodes sensitized by a series of benzothiazole merocyanine dyes, 3-carboxyalkyl-5-[2-(3-alkyl-2-benzothiazolinyldene) ethylidene]-2-thioxo-4-thiazolidinone, having a different alkyl chain length. Broadening of absorption spectra and a large red shift of the absorption threshold up to 680 nm were observed by adsorbing dyes on a porous TiO2 electrode, suggesting the formation of various kinds of J aggregates of the dye on the TiO2 surface. The dye was fixed by a chelate-like linkage of the carboxylate anchoring group on the TiO2 surface, rather than by an ester linkage. The solar light-to-power conversion efficiency (ηsun) and the incident photon-to-current conversion efficiency (IPCE) increased with increasing the length of alkyl chain attached to the benzothiazole ring. The maximum ηsun value was 4.5%, which is the second highest value among organic dye systems reported so far (AM-1.5, 100 mW/cm2). The J aggregates of the dyes having long alkyl chains showed an excellent ability for sensitization on the TiO2 electrode, whereas those having short alkyl chains hardly showed the sensitization especially at a light wavelength (λ) of >600 nm. The IPCE decreased significantly with increasing methylene length between the carboxyl anchoring group and the dye chromophore, that is, the conjugated skeleton, suggesting that the distance between the chromophore and the TiO2 surface affected the IPCE. Moreover, a formation of fine packing of the dye chromophores in the J aggregate has proved to be important to obtain a high IPCE. We tested also the stability of the dye in a sealed solar cell under simulated solar light with an UV cut filter. The ηsun and the color of the dye remained unchanged for more than 750 h, and the total turnover number of the dye for the photon-to-electron conversion exceeded 1.6 × 106 cycles, suggesting that the merocyanine dye itself is fundamentally stable during the photoreaction in this system without UV irradiation.