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

Energy harvested directly from sunlight offers a desirable approach toward fulfilling, with minimal environmental impact, the need for clean energy. Solar energy is a decentralized and inexhaustible natural resource, with the magnitude of the available solar power striking the earth’s surface at any one instant equal to 130 million 500 MW power plants.1 However, several important goals need to be met to fully utilize solar energy for the global energy demand. First, the means for solar energy conversion, storage, and distribution should be environmentally benign, i.e. protecting ecosystems instead of steadily weakening them. The next important goal is to provide a stable, constant energy flux. Due to the daily and seasonal variability in renewable energy sources such as sunlight, energy harvested from the sun needs to be efficiently converted into chemical fuel that can be stored, transported, and used upon demand. The biggest challenge is whether or not these goals can be met in a costeffective way on the terawatt scale.2

http://pubs.acs.org/doi/pdf/10.1021/cr1002326

Solar Water Splitting Cells

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

As a fascinating conjugated polymer, graphitic carbon nitride (g-C3N4) has become a new research hotspot and drawn broad interdisciplinary attention as a metal-free and visible-light-responsive photocatalyst in the arena of solar energy conversion and environmental remediation. This is due to its appealing electronic band structure, high physicochemical stability, and “earth-abundant” nature. This critical review summarizes a panorama of the latest progress related to the design and construction of pristine g-C3N4 and g-C3N4-based nanocomposites, including (1) nanoarchitecture design of bare g-C3N4, such as hard and soft templating approaches, supramolecular preorganization assembly, exfoliation, and template-free synthesis routes, (2) functionalization of g-C3N4 at an atomic level (elemental doping) and molecular level (copolymerization), and (3) modification of g-C3N4 with well-matched energy levels of another semiconductor or a metal as a cocatalyst to form heterojunction nanostructures. The constructi...

Graphitic Carbon Nitride (g-C3N4)-Based Photocatalysts for Artificial Photosynthesis and Environmental Remediation: Are We a Step Closer To Achieving Sustainability?

Nano-sized TiO 2 photocatalytic water-splitting technology has great potential for low-cost, environmentally friendly solar-hydrogen production to support the future hydrogen economy. Presently, the solar-to-hydrogen energy conversion efficiency is too low for the technology to be economically sound. The main barriers are the rapid recombination of photo-generated electron/hole pairs as well as backward reaction and the poor activation of TiO 2 by visible light. In response to these deficiencies, many investigators have been conducting research with an emphasis on effective remediation methods. Some investigators studied the effects of addition of sacrificial reagents and carbonate salts to prohibit rapid recombination of electron/hole pairs and backward reactions. Other research focused on the enhancement of photocatalysis by modification of TiO 2 by means of metal loading, metal ion doping, dye sensitization, composite semiconductor, anion doping and metal ion-implantation. This paper aims to review the up-to-date development of the above-mentioned technologies applied to TiO 2 photocatalytic hydrogen production. Based on the studies reported in the literature, metal ion-implantation and dye sensitization are very effective methods to extend the activating spectrum to the visible range. Therefore, they play an important role in the development of efficient photocatalytic hydrogen production.

A review and recent developments in photocatalytic water-splitting using TiO2 for hydrogen production

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

Efficient photocatalytic decomposition of organic contaminants over CaBi2O4 under visible-light irradiation.

A photocatalyst, AgInW2O8, with a layered structure was synthesized. The physical and photophysical properties of the photocatalyst were characterized by XRD, BET measurement, UV−visible diffuse reflectance, and photoluminescence, respectively. The photocatalytic properties of the photocatalyst for H2 evolution with Pt cocatalyst from CH3OH/H2O solution and O2 evolution from AgNO3 solution were observed under UV and visible light irradiation. The AgInW2O8 photocatalyst presented not only the activity of O2 evolution but also the activity of H2 evolution. Also the photocatalyst showed the ability to evolve H2 from pure water under full arc irradiation. On the basis of all the results, the band structure of the photocatalyst was discussed.

Photophysical and photocatalytic properties of AgInW2O8

Photoelectrochemical decomposition of water on nanocrystalline BiVO4 film electrodes under visible light.

The research on a new series of solid photocatalysts with different crystal structures was reviewed. The first system is A2B2O7 pyrochlore-crystal type: Bi2MNbO7 (M=Al, Ga, In and Y, rare earth, and Fe), which is cubic system and space group Fd3m. The second system is ABO4 stibotantalite-crystal type: BiMO4 (M=Nb5+, Ta5+), in which both the triclinic system with space group P1 in the case of M=Ta and the orthorhombic system with space group Pnna in the case of M=Nb. The third system is ABO4 wolframite-crystal type: InMO4 (M=Nb5+, Ta5+), which is monoclinic system and space group P2/a. Although these photocatalysts crystallize in the different crystal structure, they contain the same octahedral TaO6 and/or NbO6 in the different photocatalysts. The band structure of the photocatalysts is defined by Ta/Nb d-level for a conduction band and O 2p-level for a valence band. The band gaps of the photocatalysts were estimated to be between 2.7 and 2.4 eV. Metal doped InTaO4 photocatalysts were also investigated. Under visible light (λ>420 nm) or ultra-violet irradiation, the H2 and/or O2 evolutions were observed from pure water as well as aqueous CH3OH/H2O and AgNO3 solutions. The photocatalytic activity increases significantly by loading co-catalysts such as Pt, RuO2 and NiOx on the surface of the photocatalysts. Finally, direct water splitting into H2 and O2 under visible light irradiation was firstly established using newly synthesized NiOx (partly oxidized nickel) promoted In0.9Ni0.1TaO4 photocatalyst.

Zhigang Zou

Papers

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

Efficient photocatalytic decomposition of organic contaminants over CaBi2O4 under visible-light irradiation.

Photophysical and photocatalytic properties of AgInW2O8

Photoelectrochemical decomposition of water on nanocrystalline BiVO4 film electrodes under visible light.

Direct water splitting into H2 and O2 under visible light irradiation with a new series of mixed oxide semiconductor photocatalysts