The production of hydrogen from water using a catalyst and solar energy is an ideal future energy source, independent of fossil reserves. For an economical use of water and solar energy, catalysts that are sufficiently efficient, stable, inexpensive and capable of harvesting light are required. Here, we show that an abundant material, polymeric carbon nitride, can produce hydrogen from water under visible-light irradiation in the presence of a sacrificial donor. Contrary to other conducting polymer semiconductors, carbon nitride is chemically and thermally stable and does not rely on complicated device manufacturing. The results represent an important first step towards photosynthesis in general where artificial conjugated polymer semiconductors can be used as energy transducers.

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A metal-free polymeric photocatalyst for hydrogen production from water under visible light

Titanium Dioxide Nanomaterials: Synthesis, Properties, Modifications, and Applications

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

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 Novel Aqueous Process for Preparation of Crystal Form-Controlled and Highly Crystalline BiVO4 Powder from Layered Vanadates at Room Temperature and Its Photocatalytic and Photophysical Properties

BiVO4 powder with scheelite structure was obtained by hydrolyzing a nitric acid solution of Bi(NO3)3 and Na3VO4 with bases (Na2CO3 and NaHCO3) at room temperature. Tetragonal BiVO4 of a high-temperature form was obtained after 4.5 h of preparation time while monoclinic BiVO4 was done after 46 h. Although the structure and the band gap of tetragonal BiVO4 with scheelite structure were similar to those of monoclinic BiVO4, the photocatalytic activity of the tetragonal BiVO4 for O2 evolution from an aqueous AgNO3 solution under visible light irradiation was negligible. In contrast, the monoclinic BiVO4 showed high photocatalytic activity. Distortion of a Bi−O polyhedron by a 6s2 lone pair of Bi3+ plays an important role for high photocatalytic activity of the monoclinic BiVO4 under visible light irradiation.

Selective Preparation of Monoclinic and Tetragonal BiVO4 with Scheelite Structure and Their Photocatalytic Properties

Mn-, Ru-, Rh-, and Ir-doped SrTiO3 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 SrTiO3 host. Mn- and Ru-doped SrTiO3 showed photocatalytic activities for O2 evolution from an aqueous silver nitrate solution while Ru-, Rh-, and Ir-doped SrTiO3 loaded with Pt cocatalysts produced H2 from an aqueous methanol solution under visible light irradiation (λ > 440 nm). The Rh(1%)-doped SrTiO3 photocatalyst loaded with a Pt cocatalyst (0.1 wt %) gave 5.2% of the quantum yield at 420 nm for the H2 evolution reaction.

Photocatalytic Activities of Noble Metal Ion Doped SrTiO3 under Visible Light Irradiation

TiO2 and SrTiO3 codoped with antimony and chromium showed intense absorption bands in the visible light region and possessed 2.2 and 2.4 eV of energy gaps, respectively. TiO2 codoped with antimony and chromium evolved O2 from an aqueous silver nitrate solution under visible light irradiation, while SrTiO3 codoped with antimony and chromium evolved H2 from an aqueous methanol solution. The activity of TiO2 photocatalyst codoped with antimony and chromium was remarkably higher than that of TiO2 doped with only chromium. It was due to that the charge balance was kept by codoping of Sb5+ and Cr3+ ions, resulting in the suppression of formation of Cr6+ ions and oxygen defects in the lattice which should work as effectively nonradiative recombination centers between photogenerated electrons and holes.

Visible-light-response and photocatalytic activities of TiO2 and SrTiO3 photocatalysts codoped with antimony and chromium

Photophysical and photocatalytic properties of perovskite-type materials AgMO3 (M:  Ta and Nb) consisting of Ag+ and d0 ions were investigated. The band gaps of AgTaO3 and AgNbO3 were 3.4 and 2.8 eV, respectively, being 0.6 eV smaller than the band gaps of NaTaO3 (4.0 eV) and NaNbO3, even if the crystal structures of AgMO3 were similar to those of NaMO3. It was found from the electronic band structure study, using the plane-wave-based density functional method, that a hybrid orbital of Ag 4d and O 2p formed a valence band at a more negative level than O 2p orbitals. AgTaO3 showed photocatalytic activity for pure water splitting into H2 and O2 under UV-light irradiation. AgNbO3 has arisen as a new visible-light-driven photocatalyst possessing the ability to evolve H2 or O2 from water in the presence of sacrificial reagents.

Hideki Kato

Papers

A Novel Aqueous Process for Preparation of Crystal Form-Controlled and Highly Crystalline BiVO4 Powder from Layered Vanadates at Room Temperature and Its Photocatalytic and Photophysical Properties

Selective Preparation of Monoclinic and Tetragonal BiVO4 with Scheelite Structure and Their Photocatalytic Properties

Photocatalytic Activities of Noble Metal Ion Doped SrTiO3 under Visible Light Irradiation

Visible-light-response and photocatalytic activities of TiO2 and SrTiO3 photocatalysts codoped with antimony and chromium

Role of Ag+ in the Band Structures and Photocatalytic Properties of AgMO3 (M: Ta and Nb) with the Perovskite Structure