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

Dye-sensitized solar cells (DSCs) offer the possibilities to design solar cells with a large flexibility in shape, color, and transparency. DSC research groups have been established around the worl ...

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Dye-Sensitized Solar Cells

The challenges for further development of Li rechargeable batteries for electric vehicles are reviewed. Most important is safety, which requires development of a nonflammable electrolyte with either a larger window between its lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) or a constituent (or additive) that can develop rapidly a solid/ electrolyte-interface (SEI) layer to prevent plating of Li on a carbon anode during a fast charge of the battery. A high Li-ion conductivity (σ Li > 10 ―4 S/cm) in the electrolyte and across the electrode/ electrolyte interface is needed for a power battery. Important also is an increase in the density of the stored energy, which is the product of the voltage and capacity of reversible Li insertion/extraction into/from the electrodes. It will be difficult to design a better anode than carbon, but carbon requires formation of an SEI layer, which involves an irreversible capacity loss. The design of a cathode composed of environmentally benign, low-cost materials that has its electrochemical potential μ C well-matched to the HOMO of the electrolyte and allows access to two Li atoms per transition-metal cation would increase the energy density, but it is a daunting challenge. Two redox couples can be accessed where the cation redox couples are "pinned" at the top of the O 2p bands, but to take advantage of this possibility, it must be realized in a framework structure that can accept more than one Li atom per transition-metal cation. Moreover, such a situation represents an intrinsic voltage limit of the cathode, and matching this limit to the HOMO of the electrolyte requires the ability to tune the intrinsic voltage limit. Finally, the chemical compatibility in the battery must allow a long service life.

Challenges for Rechargeable Li Batteries

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 dye-sensitized solar cells (DSC) provides a technically and economically credible alternative concept to present day p–n junction photovoltaic devices. In contrast to the conventional systems where the semiconductor assume both the task of light absorption and charge carrier transport the two functions are separated here. Light is absorbed by a sensitizer, which is anchored to the surface of a wide band semiconductor. Charge separation takes place at the interface via photo-induced electron injection from the dye into the conduction band of the solid. Carriers are transported in the conduction band of the semiconductor to the charge collector. The use of sensitizers having a broad absorption band in conjunction with oxide films of nanocrstalline morphology permits to harvest a large fraction of sunlight. Nearly quantitative conversion of incident photon into electric current is achieved over a large spectral range extending from the UV to the near IR region. Overall solar (standard AM 1.5) to current conversion efficiencies (IPCE) over 10% have been reached. There are good prospects to produce these cells at lower cost than conventional devices. Here we present the current state of the field, discuss new concepts of the dye-sensitized nanocrystalline solar cell (DSC) including heterojunction variants and analyze the perspectives for the future development of the technology.

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Dye-sensitized Solar Cells

Iron(III) oxide has been extensively studied as a possible n-type semiconductor for use in solar photoelectrolysis cells However, its properties have remained curiously elusive; even such fundamental properties as bandgap and flat-band potential are still controversial, and this uncertainty has hindered any rational evaluation of the use of the material in solar cells In this paper, an extensive study of the surface and bulk properties of both single-crystal and polycrystalline Fe2O3 is reported Surface pretreatment is found to have a major effect on the photoelectrochemical properties Even in properly treated samples, however, the photocurrent onset is found to be delayed due to the small value of the faradaic rate constant for the oxidation of water, and a semi-quantitative treatment of this case is provided Improper surface treatment is shown to lead to a substantial conversion of the surface of Fe2O3 to Fe3O4; this introduces a large fraction of recombination sites at the surface that not only delay dc photocurrent onset to very anodic potentials but also reduce the observed efficiency to remarkably low values

Electrochemistry and photoelectrochemistry of iron(III) oxide

SEMICONDUCTING electrodes stable in aqueous solution during the photoelectrochemical oxidation of water have generally been large-bandgap oxides, and their photosensitisation to visible light is of continuing interest1. In particular, n-type TiO2 (rutile, with a bandgap of 3.0 eV) is a stable photoanode for the photoelectrolysis of water by uv light2, and considerable effort has been devoted to its photosensitisation to sunlight1. One approach to photosensitisation is the use of a dye that, on excitation by visible light, transfers an electron to the conduction band of the solid and subsequently returns to its reduced ground state by the oxidation of water. The three major problems to be avoided are (1) irreversible degradative oxidation of the dye3, (2) failure of the oxidised form of the dye to oxidise water, reduction being carried out by a supersensitiser, such as hydro-quinone, that is consumed by the reaction4,5, and (3) inefficient electron transfer from the dye to the solid6. To circumvent the first two of these problems, we have selected an inorganic complex as the dye, a derivative of [Ru(BIPY)3]2+ (BIPY = 2,2′-bipyridine). [Ru(BIPY)3]3+ is known to oxidise water in suitable pH conditions evolving about 80% of the theoretical yield of oxygen7. The last problem was avoided by chemically attaching the complex to the electrode surface, a procedure recently demonstrated for organic dyes (such as rhodamine-B) that require the use of a supersensitiser4,5. We report here that a single-crystal n-type TiO2 electrode, chemically modified by the attachment of a monolayer of a derivative of [Ru(BIPY)3]2+, will produce significant anodic photocurrents when irradiated with visible light in the absence of a supersensitiser.

Chemical modification of a titanium (IV) oxide electrode to give stable dye sensitisation without a supersensitiser

The electrochemistry and photoelectrochemistry of a (110) face of single-crystal lithiated NiO has been studied. The Mott–Schottky and photocurrent appearance potential suggest a flat-band potential of +0.8 V (NHE), and earlier capacitance data are used to fix a second acceptor level at +0.4 eV above the Li acceptor states. A simple geometrical interpretation of the anodic transients at 0.95 and 1.30 V (NHE) is provided, and the Tafel slope observed at anodic potentials is shown to be consistent with an electrochemical process having charge transfer as a rate-determining step. The optical data are interpreted with an energy-level scheme using a value of ca. 3 eV for the Hubbard parameter U defining the correlation splitting of the Ni 3d8 and Ni 3d9 configurations, and the data are consistent with a direct allowed transition at the band edge that is assigned to O 2p6→ Ni 3d9. Comparison with the X.p.e.s. spectrum and the known position of the O 2p6 band edge in other transition-metal oxides leads to a suggested Ni 3d8–O 2p6 band-edge separation of ca. 1.4 eV. The very low efficiencies found for NiO can be understood only if the minority-carrier diffusion length becomes comparable with the semiconductor Debye length. The very low ratio of free carriers to trapped holes in NiO is shown to make this material impractical for solar-energy devices.

Photoelectrochemistry of nickel(II) oxide

Practical photoelectrolysis of water by sunlight requires the development of suitable semiconductor electrodes. One approach is the photosensitisation of wide-bandgap oxide semiconductors by the chemical attachment of a suitable dye to the surface. We report on the photosensitisation of n-TiO2, n-SrTiO3 and n-SnO2 with chemically attached Ru(bipy)2(bpca) in which the 2,2′-bipyridine-4,4′ dicarboxylic acid (bpca) is chemically attached to the semiconductor surface by two ester linkages. Kinetic analysis reveals a surprisingly low quantum efficiency (ηe≈ 0.25%) for electron injection from the photoexcited dye into the semiconductor and a low rate constant for reoxidation of the dye. A suggestion is made how the dye molecule might be designed to improve the situation radically. Some deterioration in performance was observed after illumination of the sensitised electrode for many hours.

Sensitisation of semiconducting electrodes with ruthenium-based dyes

The differential equation describing the transport and kinetics of photogenerated minority carriers in a semiconductor electrode is solved analytically to obtain a solution in terms of confluent hypergeometric functions. Much simpler approximate solutions are also obtained. Comparison of these solutions with results from the full expression show that the approximate solutions hold to within 5%. A simple physical significance can be attached to the different terms in the approximate solutions. The form of the photocurrent voltage curves that arise from the different solutions is discussed.

M. P. Dare-Edwards

Papers

Electrochemistry and photoelectrochemistry of iron(III) oxide

Chemical modification of a titanium (IV) oxide electrode to give stable dye sensitisation without a supersensitiser

Photoelectrochemistry of nickel(II) oxide

Sensitisation of semiconducting electrodes with ruthenium-based dyes

The Transport and Kinetics of Minority Carriers in Illuminated Semiconductor Electrodes