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

When used as a photocatalyst, titanium dioxide (TiO(2)) absorbs only ultraviolet light, and several approaches, including the use of dopants such as nitrogen, have been taken to narrow the band gap of TiO(2). We demonstrated a conceptually different approach to enhancing solar absorption by introducing disorder in the surface layers of nanophase TiO(2) through hydrogenation. We showed that disorder-engineered TiO(2) nanocrystals exhibit substantial solar-driven photocatalytic activities, including the photo-oxidation of organic molecules in water and the production of hydrogen with the use of a sacrificial reagent.

https://science.sciencemag.org/content/sci/early/2011/01/19/science.1200448.full.pdf

Increasing Solar Absorption for Photocatalysis with Black Hydrogenated Titanium Dioxide Nanocrystals

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?

Ag cocatalyst-loaded ALa4Ti4O15 (A = Ca, Sr, and Ba) photocatalysts with 3.79–3.85 eV of band gaps and layered perovskite structures showed activities for CO2 reduction to form CO and HCOOH by bubbling CO2 gas into the aqueous suspension of the photocatalyst powder without any sacrificial reagents. Ag cocatalyst-loaded BaLa4Ti4O15 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 BaLa4Ti4O15 photocatalyst powder with a plate shape during the CO2 reduction. CO was the main reduction product rather than H2 even in an aqueous medium on the optimized Ag/BaLa4Ti4O15 photocatalyst. Evolution of O2 in a stoichiometric ratio (H2+CO:O2 = 2:1 in a molar ratio) indicated that water was consumed as a reducing reagent (an electron donor) fo...

Photocatalytic reduction of carbon dioxide over Ag cocatalyst-loaded ALa4Ti4O15 (A = Ca, Sr, and Ba) using water as a reducing reagent.

Photophysical and photocatalytic properties of A5Nb4O15 (A = Sr and Ba), Ba3LaNb3O12, ALa4Ti4O15 (A = Ca, Sr, and Ba), and La4Ti3O12 with layered perovskite structures, in which a plane in parallel with (111) of a simple perovskite structure was exposed at interlayer, were investigated. These oxides were obtained by a polymerizable complex method at 973–1473 K though only A5Nb4O15 (A = Sr and Ba) were prepared by a solid state reaction even at 1673 K. The shapes of these complex metal oxides were plate-like derived from the perovskite layered structure. These band gaps were estimated to be 3.7–4.1 eV from the onsets of diffuse reflection spectra. These oxides showed photoluminescence at 77 K. These oxides loaded with NiO cocatalysts showed activities for water splitting under UV irradiation. NiOx/BaLa4Ti4O15 and NiOx/Ba5Nb4O15 showed the highest activities among the titanates and niobates tested in the present study. NiOx/BaLa4Ti4O15 and NiOx/Ba5Nb4O15 gave 15% and 17% of quantum yields at 270 nm, respectively. Photocatalytic activities of ALa4Ti4O15 (A = Ca, Sr, and Ba) strongly depended on the alkaline earth metal ion. Pt, Au, Ni, and PbO2 were selectively photodeposited on basal or edge plane of the BaLa4Ti4O15 plate-like powder while these were randomly loaded on CaLa4Ti4O15. It was suggested that this difference in the surface property was the one of the important factors affecting photocatalytic ability for ALa4Ti4O15 (A = Ca, Sr, and Ba).

Yugo Miseki

Papers

Heterogeneous photocatalyst materials for water splitting

Photocatalytic reduction of carbon dioxide over Ag cocatalyst-loaded ALa4Ti4O15 (A = Ca, Sr, and Ba) using water as a reducing reagent.

Water splitting into H2 and O2 over niobate and titanate photocatalysts with (111) plane-type layered perovskite structure

Highly efficient photoelectrochemical water splitting using a thin film photoanode of BiVO4/SnO2/WO3 multi-composite in a carbonate electrolyte

Photocatalytic Water Splitting for Solar Hydrogen Production Using the Carbonate Effect and the Z‐Scheme Reaction