This critical review shows the basis of photocatalytic water splitting and experimental points, and surveys heterogeneous photocatalyst materials for water splitting into H2 and O2, and H2 or O2 evolution from an aqueous solution containing a sacrificial reagent. Many oxides consisting of metal cations with d0 and d10 configurations, metal (oxy)sulfide and metal (oxy)nitride photocatalysts have been reported, especially during the latest decade. The fruitful photocatalyst library gives important information on factors affecting photocatalytic performances and design of new materials. Photocatalytic water splitting and H2 evolution using abundant compounds as electron donors are expected to contribute to construction of a clean and simple system for solar hydrogen production, and a solution of global energy and environmental issues in the future (361 references).

Ag cocatalyst-loaded ALa(4)Ti(4)O(15) (A = Ca, Sr, and Ba) photocatalysts with 3.79-3.85 eV of band gaps and layered perovskite structures showed activities for CO(2) reduction to form CO and HCOOH by bubbling CO(2) gas into the aqueous suspension of the photocatalyst powder without any sacrificial reagents. Ag cocatalyst-loaded BaLa(4)Ti(4)O(15) was the most active photocatalyst. A liquid-phase chemical reduction method was better than impregnation and in situ photodeposition methods for the loading of the Ag cocatalyst. The Ag cocatalyst prepared by the liquid-phase chemical reduction method was loaded as fine particles with the size smaller than 10 nm on the edge of the BaLa(4)Ti(4)O(15) photocatalyst powder with a plate shape during the CO(2) reduction. CO was the main reduction product rather than H(2) even in an aqueous medium on the optimized Ag/BaLa(4)Ti(4)O(15) photocatalyst. Evolution of O(2) in a stoichiometric ratio (H(2)+CO:O(2) = 2:1 in a molar ratio) indicated that water was consumed as a reducing reagent (an electron donor) for the CO(2) reduction. Thus, an uphill reaction of CO(2) reduction accompanied with water oxidation was achieved using the Ag/BaLa(4)Ti(4)O(15) photocatalyst.

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The solar energy conversion efficiency considering the energy loss by the external bias for water splitting reached ca. 0.9 or 1.35% using single- or double-stacked photoanodes, respectively, of BiVO(4)/SnO(2)/WO(3) multilayers in a highly concentrated carbonate electrolyte aqueous solution.

Surface modification of conductive oxide glass (F‐SnO2: FTO) anode substrates using 10 oxides was investigated for efficiently producing oxidative hydrogen peroxide (H2O2) from water with hydrogen (H2) production at a Pt cathode. Bismuth vanadate (BiVO4) or titanium oxide modification significantly facilitated oxidative H2O2 production in an aqueous solution of bicarbonate (HCO3−) on the anode substrate in the dark. The BiVO4‐supported FTO anode (BiVO4/FTO) achieved not only approximately twice the H2O2 generation performance compared with a bare FTO substrate but also high H2O2 accumulation, and the maximum selectivity (η(H2O2)) and accumulation reached ca. 35 % and 5 mM, respectively.

The photoelectrochemical production and degradation properties of hydrogen peroxide (H O ) were investigated on a WO /BiVO photoanode in an aqueous electrolyte of hydrogen carbonate (HCO ). High concentrations of HCO species rather than CO species inhibited the oxidative degradation of H O on the WO /BiVO photoanode, resulting in effective oxidative H O generation and accumulation from water (H O). Moreover, the Au cathode facilitated two-electron reduction of oxygen (O ), resulting in reductive H O production with high current efficiency. Combining the WO /BiVO photoanode with a HCO electrolyte and an Au cathode also produced a clean and promising design for a photoelectrode system specializing in H O production (η (H O )≈50 %, η (H O )≈90 %) even without applied voltage between the photoanode and cathode under simulated solar light through a two-photon process; this achieved effective H O production when using an Au-supported porous BiVO photocatalyst sheet.

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.

More confused than ever: Three types of N‐confused corrole isomers (NCCs) were synthesized, and the structures of these isomers were revealed by single‐crystal X‐ray crystallography. The position of the confused pyrrole ring in the NCC has a pronounced effect on optical and anion‐binding properties.