Evaluation of the flat-band potentials by measurements of anodic/cathodic photocurrent transitions

Abstract

The photoprocesses on semiconducting n-TiO₂ and n-SrTiO₃ 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 pre-illumination can be related to the negative charge required to reduce them. This charge correlates proportionally with V_tr, the voltage at which transition from anodic to cathodic photocurrent occurs. The influence on V_tr 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 i_ph against V. Experimental evidence is provided that the most reliable values of V_fb 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.