Sensitization of a wide-gap oxide semiconductor with a visible-light-absorbing dye has been studied for decades as a means of producing H2 from water. However, efficient overall water splitting using a dye-sensitized oxide photocatalyst has remained an unmet challenge. Here we demonstrate visible-light-driven overall water splitting into H2 and O2 using HCa2Nb3O10 nanosheets sensitized by a Ru­(II) tris-diimine type photosensitizer, in combination with a WO3-based water oxidation photocatalyst and a triiodide/iodide redox couple. With the use of Pt-intercalated HCa2Nb3O10 nanosheets further modified with amorphous Al2O3 clusters as the H2 evolution component, the dye-based turnover number and frequency for H2 evolution reached 4580 and 1960 h–1, respectively. The apparent quantum yield for overall water splitting using 420 nm light was 2.4%, by far the highest among dye-sensitized overall water splitting systems reported to date. The present work clearly shows that a carefully designed dye/oxide hybrid has great potential for photocatalytic H2 production, and represents a significant leap forward in the development of solar-driven water splitting systems.

Tantalum and nitrogen co-doped rutile TiO2 nanorods were developed as a visible-light-active water oxidation photocatalyst for solar-driven Z-scheme water splitting.

Water splitting using a semiconductor photocatalyst with sunlight has long been viewed as a potential means of large-scale H production from renewable resources. Different from anatase TiO , rutile enables preferential water oxidation, which is useful for the construction of a Z-scheme water-splitting system. The combination of rutile TiO with a suitable H -evolution photocatalyst such as a Pt-loaded BaZrO -BaTaO N solid solution enables solar-driven water splitting into H and O . While rutile TiO is a wide-gap semiconductor with a bandgap of 3.0 eV, co-doping of rutile TiO with certain metal ions and/or nitrogen produces visible-light-driven photocatalysts, which are also useful as a component for water oxidation in visible-light-driven Z-scheme water splitting. The key to achieving highly efficient water oxidation is to maintain a charge balance of dopants in the rutile, because single doping typically produces trap states that capture photogenerated electrons and/or holes. Here we provide a concise summary of rutile TiO -based photocatalysts for water-splitting systems.

Mixed-anion compounds (e.g., oxynitrides and oxysulfides) are potential candidates as photoanodes for visible-light water oxidation, but most of them suffer from oxidative degradation by photogenerated holes, leading to low stability. Here we show an exceptional example of a stable, mixed-anion water-oxidation photoanode that consists of an oxyfluoride, Pb 2 Ti 2 O 5.4 F 1.2 , having a band gap of ca. 2.4 eV. Pb 2 Ti 2 O 5.4 F 1.2 particles, which were coated on a transparent conductive glass (FTO) support and were subject to postdeposition of a TiO 2 overlayer, generated an anodic photocurrent upon band gap photoexcitation of Pb 2 Ti 2 O 5.4 F 1.2 (λ <520 nm) with a rather negative photocurrent onset potential of ca. −0.6 V vs NHE, which was independent of the pH of the electrolyte solution. Stable photoanodic current was observed even without loading a water oxidation promoter such as CoO x . Nevertheless, loading CoO x onto the TiO 2 /Pb 2 Ti 2 O 5.4 F 1.2 /FTO electrode further improved the anodic photoresponse by a factor of 2−3. Under AM1.5G simulated sunlight (100 mW cm −2 ), stable water oxidation to form O 2 was achieved using the optimized Pb 2 Ti 2 O 5.4 F 1.2 photoanode in the presence of an applied potential smaller than 1.23 V, giving a Faradaic efficiency of 93% and almost no sign of deactivation during 4 h of operation. This study presents the first example of photoelectrochemical water splitting driven by visible-light excitation of an oxyfluoride that stably works, even without a water oxidation promoter, which is distinct from ordinary mixed-anion photoanodes that usually require a water oxidation promoter.

The Z-scheme CO2 reduction activity of metal complex–semiconductor hybrid photocatalysts was investigated in detail with a focus on the interfacial electron transfer process. Semiconductors of GaN:ZnO solid solutions, TaON, and Ta/N-codoped TiO2 were examined as components of the hybrid photocatalyst in combination with a binuclear Ru­(II) complex. The (photo)­physical properties of the semiconductor part were found to strongly affect the efficiency of interfacial electron transfer from/to the Ru complex photosensitizer unit, which was attached to the semiconductor surface. The photocatalytic activity of the hybrids showed a reasonable relationship with the efficiencies of forward and backward electron transfer. Among the three semiconductors, the highest activity was obtained with GaN:ZnO, which had the most negative conduction band potential among the semiconductors examined. The experimental results clearly demonstrated that analyses of the emission quenching process of the excited photosensitizer moiety of the binuclear Ru­(II) complex allowed visualization of the interfacial electron transfer between the semiconductor and the Ru complex, giving us a rational guideline to improve the efficiency of the hybrid photocatalyst for Z-scheme CO2 reduction.

Nitrogen/fluorine-codoped rutile TiO2 photocatalyst was developed toward solar-driven Z-scheme water splitting.

Water splitting using a semiconductor photocatalyst has been extensively studied as a means of solar-tohydrogen energy conversion. Powder-based semiconductor photocatalysts, in particular, have tremendous potential in cost mitigation due to system simplicity and scalability. The control and implementation of powder-based photocatalysts are, in reality, quite complex. The identification of the semiconductor−photocatalytic activity relationship and its limiting factor has not been fully solved in any powder-based semiconductor photocatalyst. In this work, we present systematic and quantitative evaluation of photocatalytic hydrogen and oxygen evolution using a model strontium titanate powder/aqueous solution interface in a half reaction. The electron density was controlled from 10 16 to 10 20 cm −3 throughout the strontium titanate powder by charge compensation with oxygen nonstoichiometry (the amount of oxygen vacancy) while maintaining its crystallinity, chemical composition, powder morphology, and the crystal and electronic structure of the surface. The photocatalytic activity of hydrogen evolution from aqueous methanol solution was stable and enhanced by 40-fold by the electron doping. The enhancement was correlated well with increased Δabsorbance, an indication of prolonged lifetime of photoexcited electrons, observed by transient absorption spectroscopy. Photocatalytic activity of oxygen evolution from aqueous silver nitrate solution was also enhanced by 3-fold by the electron doping. Linear correlation was found between the photocatalytic activity and the degree of surface band bending, ΔΦ, above 1.38 V. The band bending, potential downhill for electronic holes, enlarges the total flux of photoexcited holes toward the surface, which drives the oxygen evolution reaction.

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