PHOTOCALORIMETRY AT SEMICONDUCTOR ELECTRODES - THEORY, TECHNIQUE AND APPLICATIONS

A review is given on the theory and technique of phoocalorimetry using photo-acoustic or pyroelectric detection of the temperature change at the back surface of a thermally thin semiconductor electrode under ch opped monochromatic illumination. Relations are given for determining the internal quantum efficiency (eta(a)) of the photocurrent, the uppe r limit of the internal energy conversion efficiency (L(G)) attainable with the electrode in a photoelectrolytic or regenerative cell, and t he Peltier heat (Q(PE)) for the reaction at an n-type photoanode. The theory includes competitive processes such as photocorrosion of the se miconductor and oxidation of a solute. Photoanodic oxidation of water has been studied on polycrystalline n-TiO2 thin-film electrodes (rutil e and Be-doped anatase) at pH 0.3-13.8. The lower values of eta(a) and L(G) at the anatase film are attributed to a shorter hole diffusion l ength than in the rutile film. The pH dependence of Q(PE) is similar a t both modifications of TiO2. It is controlled mainly by the pH-depend ent entropy change of the leading net reactions in acid and strongly b asic solution. The faradaic efficiency for photoanodic oxidation of Cl - competing with that of water at the rutile electrode has been determ ined. The values for formation of Cl-2 in acid solution decrease with increasing pH. Results for pH 9 are consistent with formation of ClO-. Photocorrosion of n-GaAs (100) in 0.5 M H2SO4 has been studied as a f unction of concentration of I-. On increasing [I-], the faradaic effic iency for photoanodic oxidation of I- approaches unity at [I-] greater than or equal to 4 M, indicating complete stabilization of GaAs. For n-GaAs/I- (7 M), L(G) was 5% at 633 nm. At a photoanode made from natu ral pyrite, L(G) was circa 0.5% for I- (1 M)/I-2 (1 mM) in 0.5 M H2SO4 at 633 nm.