The photoprocesses on semiconducting n-Ti02 and n-SrTiO, electrodes in the presence of reducible surface species are described. These species are generated by pre-illumination with the band-gap light of the electrodes being operated under reverse bias. The amount formed in the course of this preillumination can be re!ated to the negative charge required to reduce them. This charge correlates proportionally with V,,, the voltage at which transition from anodic to cathodic photocurrent occurs. The influence on Vt, of the parameters surface coverage as function of pre-illumination time, wavelength, photon flux and pH of the electrolyte is investigated. The occurrence of the cathodic photoprocess is discussed in terms of an electron-tunnelling mechanism through the space-charge barrier to the solution and a joint model proposed by Bard and Gerischer.The influences of the cathodic photoeffect on the onset potential of the usual anodic photoprocess may affect flat-band potential determinations from curves of iph against V. Experimental evidence is provided that the most reliable values of Vfb from this method can be obtained by scanning from cathodic to anodic potentials. Also irradiation with long-wavelength light, close to the band-gap energy, and high light intensity are recommended.

Photoelectrochemical processes are repeatedly suggested to be utilized for the conversion of light into electrical and/or chemical energy. This study reports on photoreduction processes which can be obtained on cathodically polarized n‐TiO2 electrodes when reducible species such as O2, Fe3+, Ce4+, Fe(CN)3‐6, and H2O2 are present in the electrolyte. The changes of photoreduction are investigated as function of the amount of reducible species transported to the surface as well as the wavelength and intensity of the incident light. It is shown that under specific experimental conditions the cathodic quantum efficiency ηc can reach values up to 1. An energy scheme is suggested to explain the mechanism which is responsible for the photoreduction process. In this scheme a broad distribution of surface states is introduced which covers the forbidden gap of the semiconductor from a mid‐gap position up to levels close to the tower edge of the conduction band ECB. It is assumed that these surface states mediate the transfer of electrons and holes from the illuminated electrode to the solution. Possible reaction channels are discussed.

Photoclectrochemical investigations are described with solid solution HgS1‐xSex electrodes of the compositions x = 0, 0.01, 0.03, and 0.06. The electrodes were prepared by a sublimation and a vapor phase technique, respectively. The semiconductor electrodes were characterized by their current‐potential dependence and their spectral response as function of illumination time.—No corrosion currents were observed in an aqueous electrolyte of pH 2.5 in the potential vs. SCE region −0.45 to +0.9 V in the dark. Upon illumination only the electrode with the highest selenium content corrodes noticeably. Bandgaps of Eg = 2.1 eV with x = 0 and Eg = 1.7 eV with x = 0.06 are determined from wavelength dependent photocurrent spectra. The results indicate that the transition of the hexagonal α‐ to the cubic β‐structure is more hindered in the solid solution electrodes than in the pure α‐HgS system.