Z-schematic water splitting was successfully demonstrated using metal sulfide photocatalysts that were usually unsuitable for water splitting as single particulate photocatalysts due to photocorrosion. When metal sulfide photocatalysts with a p-type semiconductor character as a H2-evolving photocatalyst were combined with reduced graphene oxide-TiO2 composite as an O2-evolving photocatalyst, water splitting into H2 and O2 in a stoichiometric amount proceeded. In this system, photogenerated electrons in the TiO2 with an n-type semiconductor character transferred to the metal sulfide through a reduced graphene oxide to achieve water splitting. Moreover, this system was active for solar water splitting.

A Rh-doped SrTiO(3) (SrTiO(3):Rh) photocatalyst electrode that was readily prepared by pasting SrTiO(3):Rh powder onto a transparent indium tin oxide electrode gave a cathodic photocurrent under visible-light irradiation (λ > 420 nm), indicating that the SrTiO(3):Rh photocatalyst electrode possessed p-type semiconductor character. The cathodic photocurrent increased with an increase in the amount of doped Rh up to 7 atom %. The incident-photon-to-current efficiency at 420 nm was 0.18% under an applied potential of -0.7 V vs Ag/AgCl for the SrTiO(3):Rh(7 atom %) photocatalyst electrode. The photocurrent was confirmed to be due to water splitting by analyzing the evolved H(2) and O(2). The water splitting proceeded with the application of an external bias smaller than 1.23 V versus a Pt counter electrode under visible-light irradiation and also using a solar simulator, suggesting that solar energy conversion should be possible with the present photoelectrochemical water splitting.

An efficient BiVO 4 thin film electrode for overall water splitting was prepared by dipping an F-doped SnO 2 (FTO) substrate electrode in an aqueous nitric acid solution of BiðNO 3 Þ 3 and NH 4 VO 3 , and subsequently calcining it. X-ray diffraction of the BiVO 4 thin film revealed that a photocatalytically active phase of scheelite-monoclinic BiVO 4 was obtained. Scanning electron microscopy images showed that the surface of an FTO substrate was uniformly coated with the BiVO 4 film with 300-400 nm of the thickness. The BiVO 4 thin film electrode gave an excellent anodic photocurrent with 73% of an IPCE at 420 nm at 1.0 V vs. Ag∕AgCl. Modification with CoO on the BiVO 4 electrode improved the photoelectrochemical property. A photoelectrochemical cell consisting of the BiVO 4 thin film electrode with and without CoO, and a Pt counter electrode was constructed for water splitting under visible light irradiation and simulated sunlight irradiation. Photocurrent due to water splitting to form H 2 and O 2 was confirmed with applying an external bias smaller than 1.23 V that is a theoretical voltage for electrolysis of water. Water splitting without applying external bias under visible light irradiation was demonstrated using a SrTiO 3 ∶Rh photocathode and the BiVO 4 photoanode.hydrogen production | solar energy conversion | visible light response T he development of powdered photocatalysts and semiconductor photoelectrodes for water splitting have been studied extensively in view of utilization of solar energy, since the report of the Honda-Fujishima effect (1). There are advantageous and disadvantageous points for water splitting using the powdered photocatalysts and photoelectrochemical cells. Although a powdered photocatalyst system is simple, H 2 and O 2 are produced as a mixture. In contrast to it, photoelectrochemical cells give H 2 separately from O 2 gas. Moreover, even if powdered photocatalysts do not possess band potentials suitable for water splitting (the bottom of the conduction band <0 V and the top of the valence band >1.23 V vs. NHE at pH 0), water splitting may be achieved by application of these powdered photocatalysts to photoelectrodes with applying some external bias. However, the electrical conductivity of semiconductor electrode is indispensable, resulting in that the number of the photoelectrode materials is limited.It has been reported that many metal oxides are active photocatalysts for water splitting into H 2 and O 2 stoichiometrically under UV light irradiation (2). Photoelectrodes, such as TiO 2 (1, 3, 4), SrTiO 3 (5, 6), BaTiO 3 (7), and KTaO 3 (8) have been reported for photoelectrochemical water splitting under UV light irradiation. Development of a photoelectrode material with visible light response has been sought for efficient utilization of solar energy. It has been reported that Fe 2 O 3 (9-11), WO 3 (12-14), BiVO 4 (15-24), and SrTiO 3 ∶Rh (25) of metal oxide electrodes respond to visible light. Recently, some (oxy) nitride materials such as TaON (26,27), Ta 3 N 5 (27, 28), SrN...

The occupied and unoccupied in-gap electronic states of a Rh-doped SrTiO3 photocatalyst were investigated by X-ray emission spectroscopy and X-ray absorption spectroscopy for different Rh impurity valence states and doping levels. An unoccupied midgap Rh4+ acceptor state was found 1.5 eV below the SrTiO3 conduction band minimum. Both Rh4+ and Rh3+ dopants were found to have an occupied donor level close to the valence band maximum of SrTiO3. The density of states obtained from first-principles calculations show that all observed spectral features can be assigned to electronic states of substitutional Rh at the Ti site and that Rh:SrTiO3 is an unusual titanate compound with a characteristic p-type electronic structure. The Rh doping results in a large decrease of the bandgap energy, making Rh:SrTiO3 an attractive material for use as a visible-light-driven H2-evolving photocatalyst in a solar water splitting reaction.

The effect of iridium valence in Ir:SrTiO3 on the electronic structure and the photocatalytic activity in a water splitting reaction was studied. Epitaxial thin film photoelectrodes were grown with controlled Ir valence and used to measure the electrochemical efficiency of Ir:SrTiO3. The positions of the in-gap Ir impurity levels were determined by optical and X-ray photoelectron spectroscopies. Comparison with ab initio calculations was used to assign the observed electronic states to either Ir4+ or Ir3+ dopants in SrTiO3. The measurements show that Ir3+:SrTiO3 forms a single midgap impurity state that is strongly localized, completely quenching the photoelectrochemical response. An anodic photoresponse was seen in Ir4+:SrTiO3 under visible-light illumination up to a wavelength of 600 nm (hν = 2.0 eV). Ir4+:SrTiO3 contains an in-gap state close to the top of the SrTiO3 valence band. The performance of Ir4+:SrTiO3 in electrochemical reactions is compared with cathodic Rh3+:SrTiO3, clearly illustrating the importance of strong dopant d-electron hybridization with the oxygen 2p valence band of SrTiO3 for improving the energy conversion efficiency in SrTiO3-based photocatalysts.