Photo-electrochemical and photo-catalytic reactions of pigmentary titanium dioxide

Abstract

The photo-e.m.f. and currents generated by platinum electrodes covered with pigmentary titanium dioxide are dependent on the ions, and their concentration, present in solution. For inert anions, e.g., sulphate and perchlorate, the faster the decay of the photo-voltage, the smaller the peak photo-current. The magnitude of the decay of the photo-voltage and overshoot of the photo-current are interpreted as reflecting the time required for re-equilibration at the semiconductor electrolyte interface on termination and commencement of illumination, respectively. For chloride ions, the higher the concentration of the ion, the greater the time required for re-equilibration. Titanium dioxide photo-catalyses the oxidation of anions such as iodide, thiocyanate or azide. It is possible that the species so formed are reduced at the bare platinum surface of the unhomogeneously covered electrode so causing a drop in e.m.f. current. The fall-off in e.m.f. is as predicted by the Nernst equation.