M. P. Dare-Edwards

Electrochemistry and photoelectrochemistry of iron(III) oxide

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 d.c. photocurrent onset to very anodic potentials but also reduce the observed efficiency to remarkably low values.

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

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.

The Transport and Kinetics of Minority Carriers in Illuminated Semiconductor Electrodes

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.

The efficiency of photogeneration of hydrogen at p-type III/V semiconductors

Abstract Studies have been carried out on p-GaP and other III/V p-type semiconductors in an attempt to understand the reasons for the very low efficiency of photogeneration of hydrogen at potentials just positive of flatband. The efficiency for this process only rises to significant levels at potentials >0.6 V from flatband, which prevents the effective use of these photoelectrodes in solar photoelectrolysis cells. The poor response has been found to be caused by surface hydrogen atoms formed in the first step of the photo-assisted hydrogen evolution reaction. In addition to their subsequent conversion to H 2 , these atoms may be reoxidised by holes tunnelling to the surface from the valence band; this process is only suppressed at very negative potentials. Evidence for this model is provided by ac and dc experiments and a detailed impedance analysis. It has been found that the response of p-GaP may be improved dramatically by the adsorption of a layer of Ru(III) chloride. This also appears to effect a reduction in the tunnelling reaction.

Sensitisation of semiconducting electrodes with ruthenium-based dyes

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.

Inorganic materials for photoelectrolysis

Abstract The photoelectrolysis of water by sunlight represents a significant technical target that illustrates well the materials problems facing the designer of suitable catalytic electrodes for any photoelectrolytic process. Two design approaches are discussed: stable materials with suitable band parameters and large-bandgap semiconductors sensitized by an inorganic dye.

Photoelectrochemistry of nickel(II) oxide

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.

New anode materials for photoelectrolysis

The design strategy for a successful oxide anode for use in photoelectrolysis cells is reviewed. Conduction bands based on Ti4+ and Nb5+ can be made to possess sufficiently small electron affinities to allow the cell to operate without external bias. For example, data on SrTi0.75Zr0.25O3 are presented to show that the Ti4+ conduction band can be fine-tuned to the required electron affinity using Zr4+ as dopant. The Zr4+ dopant also enhances remarkably oxygen evolution in the dark, and a mechanism for this observation is advanced. In order to reduce the bandgap in oxide titanates, incorporation of a second metal-ion sublattice appears to be essential. Three types of substituent cation that can, in principle, give rise to a valence band at the right energy are discussed: (i) transition metals giving rise to ndm bands, (ii) lanthanides giving rise to 4fm bands and (iii) reduced B-metals giving rise to ns2 bands. Optical absorption studies of MnTiO3 and NaCeTi2O6 indicate that the bulk Mn2+ 3d5 and Ce3+ 4f1 levels are at about the right energy, but holes photogenerated in these levels become trapped in surface–polaron sites, as they only oxidise water slowly. Analysis of the transient photocurrents for MnTiO3 shows that holes arrive at the surface in both the O2–:2p6 band and the Mn2+:3d5 levels and the former oxidise water rapidly.

Evaluation of p-type PdO as a photocathode in water photoelectrolysis

Abstract The electronic and electrochemical properties of vapour-grown single-crystal PdO are reported; this PdO is a p-type semiconductor with a bandgap of about 0.8 eV, corresponding to a strongly forbidden d-d transition. A higher-energy transition, with a threshold near 2.2eV, is assigned to 0 2p — Pd 4d charge transfer. The flat-band potential PdO appears to be 0.7 ± 0.1V (NHE) in 0.5M H 2 SO 4 , but the photoresponse of the crystals is poor owing both to unfavourable bulk properties and to feeble faradaic kinetics for hydrogen evolution. The cathodic decomposition of PdO to palladium metal is a strongly competing reaction under potential biases that invert the surface region.

Alternating-current techniques in semiconductor electrochemistry

A new theory of alternating-current effects in semiconductors is presented. The equivalent circuit used by a large number of workers to describe the effects of electroactive surface states is shown to have a firm physical basis at the molecular level, and the connection between the empirically derived circuit elements and such microscopic parameters as rate constants and surface-state densities is given. The implementation of programs designed to obtain such circuit elements from the raw experimental data is described and some examples of the theory are given.