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)

Heterogeneous photocatalyst materials for water splitting

2.3. Evaluation of Photocatalytic Water Splitting 6507 2.3.1. Photocatalytic Activity 6507 2.3.2. Photocatalytic Stability 6507 3. UV-Active Photocatalysts for Water Splitting 6507 3.1. d0 Metal Oxide Photocatalyts 6507 3.1.1. Ti-, Zr-Based Oxides 6507 3.1.2. Nb-, Ta-Based Oxides 6514 3.1.3. W-, Mo-Based Oxides 6517 3.1.4. Other d0 Metal Oxides 6518 3.2. d10 Metal Oxide Photocatalyts 6518 3.3. f0 Metal Oxide Photocatalysts 6518 3.4. Nonoxide Photocatalysts 6518 4. Approaches to Modifying the Electronic Band Structure for Visible-Light Harvesting 6519

Semiconductor-based Photocatalytic Hydrogen Generation

The photocatalytic splitting of water into hydrogen and oxygen using solar energy is a potentially clean and renewable source for hydrogen fuel. The first photocatalysts suitable for water splitting, or for activating hydrogen production from carbohydrate compounds made by plants from water and carbon dioxide, were developed several decades ago. But these catalysts operate with ultraviolet light, which accounts for only 4% of the incoming solar energy and thus renders the overall process impractical. For this reason, considerable efforts have been invested in developing photocatalysts capable of using the less energetic but more abundant visible light, which accounts for about 43% of the incoming solar energy. However, systems that are sufficiently stable and efficient for practical use have not yet been realized. Here we show that doping of indium-tantalum-oxide with nickel yields a series of photocatalysts, In(1-x)Ni(x)TaO(4) (x = 0-0.2), which induces direct splitting of water into stoichiometric amounts of oxygen and hydrogen under visible light irradiation with a quantum yield of about 0.66%. Our findings suggest that the use of solar energy for photocatalytic water splitting might provide a viable source for 'clean' hydrogen fuel, once the catalytic efficiency of the semiconductor system has been improved by increasing its surface area and suitable modifications of the surface sites.

Direct splitting of water under visible light irradiation with an oxide semiconductor photocatalyst

Photochemical splitting of water into H2 and O2 using solar energy is a process of great economic and environmental interest. Since the discovery of the first water splitting system based on TiO2 and Pt in 1972 by Fujishima and Honda, over 130 inorganic materials have been discovered as catalysts for this reaction. This review discusses the known inorganic catalysts with a focus on structure–activity relationships.

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Inorganic Materials as Catalysts for Photochemical Splitting of Water

Solar driven catalysis on semiconductors to produce clean chemical fuels, such as hydrogen, is widely considered as a promising route to mitigate environmental issues caused by the combustion of fossil fuels and to meet increasing worldwide demands for energy. The major limiting factors affecting the efficiency of solar fuel synthesis include; (i) light absorption, (ii) charge separation and transport and (iii) surface chemical reaction; therefore substantial efforts have been put into solving these problems. In particular, the loading of co-catalysts or secondary semiconductors that can act as either electron or hole acceptors for improved charge separation is a promising strategy, leading to the adaptation of a junction architecture. Research related to semiconductor junction photocatalysts has developed very rapidly and there are a few comprehensive reviews in which the strategy is discussed (A. Kudo and Y. Miseki, Chemical Society Reviews, 2009, 38, 253–278, K. Li, D. Martin, and J. Tang, Chinese Journal of Catalysis, 2011, 32, 879–890, R. Marschall, Advanced Functional Materials, 2014, 24, 2421–2440). This critical review seeks to give an overview of the concept of heterojunction construction and more importantly, the current state-of-the art for the efficient, visible-light driven junction water splitting photo(electro)catalysts reported over the past ten years. For water splitting, these include BiVO4, Fe2O3, Cu2O and C3N4, which have attracted increasing attention. Experimental observations of the proposed charge transfer mechanism across the semiconductor/semiconductor/metal junctions and the resultant activity enhancement are discussed. In parallel, recent successes in the theoretical modelling of semiconductor electronic structures at interfaces and how these explain the functionality of the junction structures is highlighted.

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Visible-light driven heterojunction photocatalysts for water splitting – a critical review

BiVO4 with a 2.3 eV band gap showed an activity for O2 evolution from aqueous solutions containing Ag+ as an electron scavenger under visible light irradiation (λ > 520 nm). The quantum yield was 0.5% at 450 nm.

Photocatalytic O2 evolution under visible light irradiation on BiVO4 in aqueous AgNO3 solution

Photocatalytic activities of Ga2−xInxO3 solid solutions consisting of β-Ga2O3 with a wide band gap and In2O3 with a medium band gap were studied in order to develop new photocatalysts based on oxide solid solutions. The Ga2−xInxO3 solid solutions were characterized by X-ray diffraction, surface area, diffuse reflection spectra, photoluminescent and photoelectrochemical measurements. As the ratio of In was increased from x = 0 to 1 in Ga2−xInxO3, the lattice parameters were increased, indicating the formation of solid solutions as has previously been reported. Corresponding to this, the absorption, excitation and emission spectra were red-shifted, showing a decrease in the band gaps, and the flat band potentials were shifted positively. Among the solid solutions, GaInO3 (or Ga1.14In0.86O3) showed the highest photocatalytic activity for H2 evolution from aqueous methanol solutions and for O2 evolution from aqueous silver nitrate solutions. The dependency of the photocatalytic activities on the composition of the solid solutions was explained by taking the number of available photons and the flat band potentials into consideration.

Photocatalytic activities and photophysical properties of Ga2−xInxO3 solid solution

In2O3(ZnO)m powder with laminal structure like a superlattice showed photocatalytic activities for H2 evolution from an aqueous methanol solution and O2 evolution from an aqueous silver nitrate solution under visible light irradiation (λ > 420 nm). Anodic photocurrent was also observed under visible light irradiation (λ > 480 nm) when In2O3(ZnO)m sintered electrodes were employed. The band gaps of In2O3(ZnO)3 and In2O3(ZnO)9 were 2.6 and 2.7eV, respectively, which were smaller than those of In2O3 and ZnO of starting materials.

New In2O3(ZnO)m Photocatalysts with Laminal Structure for Visible Light-induced H2 or O2 Evolution from Aqueous Solutions Containing Sacrificial Reagents

Lower selectivity for nitrate and nitrite was obtained when Pd was used as a cocatalyst on TiO2 for photocatalytic oxidation of aqueous ammonia in the presence of oxygen. This is thought to be due ...

Photocatalytic Oxidation of Aqueous Ammonia in the Presence of Oxygen over Platinum-loaded TiO2

Chiral separation of catechin and epicatechin by reversed phase high-performance liquid chromatography with β-cyclodextrin stepwise and linear gradient elution modes.

Ikko Mikami

Papers

Photocatalytic O2 evolution under visible light irradiation on BiVO4 in aqueous AgNO3 solution

Photocatalytic activities and photophysical properties of Ga2−xInxO3 solid solution

New In2O3(ZnO)m Photocatalysts with Laminal Structure for Visible Light-induced H2 or O2 Evolution from Aqueous Solutions Containing Sacrificial Reagents

Photocatalytic Oxidation of Aqueous Ammonia in the Presence of Oxygen over Platinum-loaded TiO2

Chiral separation of catechin and epicatechin by reversed phase high-performance liquid chromatography with β-cyclodextrin stepwise and linear gradient elution modes.