The behavior of semiconducting electrodes for photoelectrolysis of water is examined in terms of the physical properties of the semiconductor. The semiconductor-electrolyte junction is treated as a simple Schottky barrier, and the photocurrent is described using this model. The approach is appropriate since large-band-gap semiconductors have an intrinsic oxygen overpotential which removes the electrode reaction kinetics as the rate-limiting step. The model is successful in describing the wavelength and potential dependence of the photocurrent in WO3 and allows a determination of the band gap, optical absorption depth, minority-carrier diffusion length, flat-band potential, and the nature of the fundamental optical transition (direct or indirect). It is shown for WO3 that minority-carrier diffusion plays a limited role in determining the photoresponse of the semiconductor-electrolyte junction. There are indications that the diffusion length in this low carrier mobility material is determined by diffusion-controlled bulk recombination processes rather than the more common trap-limited recombination. It is also shown that the fundamental optical transition is indirect and that the band-gap energy depends relatively strongly on applied potential and electrolyte. This effect seems to be the result of field-induced crystallographic distortions in antiferroelectric WO3.

The electron affinities of several metal oxide semiconductors that have been used as anodes in photoelectrochemical cells are calculated using the atomic electronegativities of the constituent atoms. These electron affinities are quantitatively related to the measured flatband potentials by considering the effects of specific adsorption of potential-determining ions (for metal oxides used in photoelectrolysis, these are usually OH-and H+). Methods are discussed for determining the pH at which net adsorbed surface charge and thus potential across the Helmholtz layer is zero (point of zero zeta potential, pzzp). This pH value is shown to correlate with the electronegativity of the metal oxides. The application of these ideas to other semiconductor-electrolyte systems is discussed.

Expansions around the Gaussian and Lorentzian limits for randomly perturbed spectral lines and their shift, width, and asymmetry

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