Substrate effects on photoelectrochemical kinetics in hydrogen production

The surface of p-Si has been treated in a number of ways, and the rate of the photoelectrochemical reduction of water to hydrogen measured on each type of surface. Correspondingly, in-situ ellipsometric determinations of thickness and refractive index of the surface films were made; corresponding XPS, ISS and SIMS studies were also carried out. The photoelectrochemical activities of the surfaces, as measured by the positive shift on the potential axis of the mid-current point of the photocurrent/potential curve, differed greatly. In treatments with HF, the chemical structure of the surface remains that of SiO2; in treatments with aquaregia and HF, the surface becomes SiO. After hydrogen evolution, SiOH bonds appear. The degree of dependence of the photoelectrochemical activity on the surface characteristics indicates that a reaction at the semiconductor/solution interface controls the overall (consecutive) photoelectrochemical reaction. The increase in rate with change of surface structure depends on the following factors in increasing order of importance: the presence of band gap surface states, the (established) jump in the order of magnitude of conductance of SiOx at x = 1.8, and the (argued) increasing availability of Si bonds during increasing reduction of the surface, thus causing an increase in the rate constant of a rate-determining proton transfer. © 1984.