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).

BiVO4 photocatalysts for O2 evolution, which work under visible light irradiation, were prepared by an aqueous process. The BiVO4 photocatalysts were obtained by the reaction of layered potassium vanadate powder (KV3O8 and K3V5O14) with Bi(NO3)3 for 3 days in aqueous media at room temperature. Highly crystalline monoclinic and tetragonal BiVO4 were selectively synthesized by changing the ratio of vanadium to bismuth in the starting materials. X-ray diffraction and scanning electron microscopy measurements showed that the monoclinic BiVO4 was formed via a tetragonal BiVO4 intermediate. Tetragonal BiVO4 with a 2.9 eV band gap mainly possessed an ultraviolet absorption band while monoclinic BiVO4 with a 2.4 eV band gap had a characteristic visible light absorption band in addition to the UV band. The UV bands observed in the tetragonal and monoclinic BiVO4 were assigned to the band transition from O2p to V3d whereas the visible light absorption was due to the transition from a valence band formed by Bi6s or a hybrid orbital of Bi6s and O2p to a conduction band of V3d. The photocatalytic activity for O2 evolution from an aqueous silver nitrate solution under UV irradiation (300 < λ < 380 nm) on the tetragonal BiVO4 was comparable to that on the monoclinic BiVO4. The monoclinic BiVO4 also showed the high photocatalytic activity for the O2 evolution under visible light irradiation (λ > 420 nm). When the monoclinic BiVO4 was calcined at 700−800 K the activity was increased. The activity of this monoclinic BiVO4 was much higher than that of BiVO4 prepared by a conventional solid-state reaction. The quantum yield at 450 nm for the O2 evolution on the monoclinic BiVO4 was 9%.

NiO-loaded NaTaO(3) doped with lanthanum showed a high photocatalytic activity for water splitting into H(2) and O(2) in a stoichiometric amount under UV irradiation. The photocatalytic activity of NiO-loaded NaTaO(3) doped with lanthanum was 9 times higher than that of nondoped NiO-loaded NaTaO(3). The maximum apparent quantum yield of the NiO/NaTaO(3):La photocatalyst was 56% at 270 nm. The factors affecting the highly efficient photocatalytic water splitting were examined by using various characterization techniques. Electron microscope observations revealed that the particle sizes of NaTaO(3):La crystals (0.1-0.7 microm) were smaller than that of the nondoped NaTaO(3) crystal (2-3 microm) and that the ordered surface nanostructure with many characteristic steps was created by the lanthanum doping. The small particle size with a high crystallinity was advantageous to an increase in the probability of the reaction of photogenerated electrons and holes with water molecules toward the recombination. Transmission electron microscope observations and extended X-ray absorption fine structure analyses indicated that NiO cocatalysts were loaded on the edge of the nanostep structure of NaTaO(3):La photocatalysts as ultrafine particles. The H(2) evolution proceeded on the ultrafine NiO particles loaded on the edge while the O(2) evolution occurred at the groove of the nanostep structure. Thus, the reaction sites for H(2) evolution were separated from those of O(2) evolution over the ordered nanostep structure. The small particle size and the ordered surface nanostep structure of the NiO/NaTaO(3):La photocatalyst powder contributed to the highly efficient water splitting into H(2) and O(2).

Photocatalytic water splitting using particulate semiconductors is a potentially scalable and economically feasible technology for converting solar energy into hydrogen. Z-scheme systems based on two-step photoexcitation of a hydrogen evolution photocatalyst (HEP) and an oxygen evolution photocatalyst (OEP) are suited to harvesting of sunlight because semiconductors with either water reduction or oxidation activity can be applied to the water splitting reaction. However, it is challenging to achieve efficient transfer of electrons between HEP and OEP particles. Here, we present photocatalyst sheets based on La- and Rh-codoped SrTiO3 (SrTiO3:La, Rh; ref. ) and Mo-doped BiVO4 (BiVO4:Mo) powders embedded into a gold (Au) layer. Enhancement of the electron relay by annealing and suppression of undesirable reactions through surface modification allow pure water (pH 6.8) splitting with a solar-to-hydrogen energy conversion efficiency of 1.1% and an apparent quantum yield of over 30% at 419 nm. The photocatalyst sheet design enables efficient and scalable water splitting using particulate semiconductors.

BiVO 4 powder with scheelite structure was obtained by hydrolyzing a nitric acid solution of Bi(NO 3 ) 3 and Na 3 VO 4 with bases (Na 2 CO 3 and NaHCO 3 ) at room temperature. Tetragonal BiVO 4 of a high-temperature form was obtained after 4.5 h of preparation time while monoclinic BiVO 4 was done after 46 h. Although the structure and the band gap of tetragonal BiVO 4 with scheelite structure were similar to those of monoclinic BiVO 4 , the photocatalytic activity of the tetragonal BiVO 4 for O 2 evolution from an aqueous AgNO 3 solution under visible light irradiation was negligible. In contrast, the monoclinic BiVO 4 showed high photocatalytic activity. Distortion of a Bi-O polyhedron by a 6s 2 lone pair of Bi 3+ plays an important role for high photocatalytic activity of the monoclinic BiVO 4 under visible light irradiation.

The effectiveness of reduced graphene oxide as a solid electron mediator for water splitting in the Z-scheme photocatalysis system is demonstrated. We show that a tailor-made, photoreduced graphene oxide can shuttle photogenerated electrons from an O(2)-evolving photocatalyst (BiVO(4)) to a H(2)-evolving photocatalyst (Ru/SrTiO(3):Rh), tripling the consumption of electron-hole pairs in the water splitting reaction under visible-light irradiation.

Mn-, Ru-, Rh-, and Ir-doped SrTiO 3 possessed intense absorption bands in the visible light region due to excitation from the discontinuous levels formed by the dopants to the conduction band of the SrTiO 3 host. Mn-and Ru-doped SrTiO 3 showed photocatalytic activities for O 2 evolution from an aqueous silver nitrate solution while Ru-, Rh-, and Ir-doped SrTiO 3 loaded with Pt cocatalysts produced H 2 from an aqueous methanol solution under visible light irradiation (λ > 440 nm). The Rh(1%)-doped SrTiO 3 photocatalyst loaded with a Pt cocatalyst (0.1 wt %) gave 5.2% of the quantum yield at 420 nm for the H 2 evolution reaction.

Highly crystalline monoclinic scheelite BiVO4 powders are synthesized from aqueous Bi(NO3)3 and NH4VO3 solutions over a wide range of pH by a hydrothermal process. BiVO4 powders with various morphologies, surface textures, and grain shapes are selectively synthesized by adjusting the pH. The dependence of the Raman peak position and intensity on the synthesis conditions indicates that the symmetry distortions in the local structure of the synthesized BiVO4 are affected by the preparation conditions. These variations in the local structure result in the modification of the electronic structure of BiVO4, which results in a blue‐shift in the UV‐vis absorption spectrum of hydrothermally synthesized BiVO4 in comparison with a well‐crystallized sample prepared by homogeneous coprecipitation. The photocatalytic activities for O2 evolution from an aqueous AgNO3 solution under visible‐light irradiation are strongly dependent on the pH used in the synthesis. The differences in the photocatalytic activities between BiVO4 samples prepared under various conditions is attributed to the degree of structural distortion, leading to differences in the mobility of photogenerated holes formed in the valence band, which consists of Bi 6s and O 2p orbitals.