Photoelectrolysis and physical properties of the semiconducting electrode WO2

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.