Photocalorimetry at semiconductor electrodes: theory, technique and applications

Abstract

A review is given on the theory and technique of photocalorimetry using photo-acoustic or pyroelectric detection of the temperature change at the back surface of a thermally thin semiconductor electrode under chopped monochromatic illumination. Relations are given for determining the internal quantum efficiency ( η a) of the photocurrent, the upper limit of the internal energy conversion efficiency ( L G) attainable with the electrode in a photoelectrolytic or regenerative cell, and the Peltier heat ( Q PE) for the reaction at an n-type photoanode. The theory includes competitive processes such as photocorrosion of the semiconductor and oxidation of a solute. Photoanodic oxidation of water has been studied on polycrystalline n-TiO 2 thin-film electrodes (rutile and Be-doped anatase) at pH 0.3–13.8. The lower values of η a and L G at the anatase film are attributed to a shorter hole diffusion length than in the rutile film. The pH dependence of Q PE is similar at both modifications of TiO 2. It is controlled mainly by the pH-dependent entropy change of the leading net reactions in acid and strongly basic solution. The faradaic efficiency for photoanodic oxidation of Cl − competing with that of water at the rutile electrode has been determined. 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 H 2SO 4 has been studied as a function of concentration of I −. On increasing [I −], the faradaic efficiency for photoanodic oxidation of I − approaches unity at [I −] ≥ 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 natural pyrite, L G was circa 0.5% for I − (1 M)/I 2 (1 mM) in 0.5 M H 2SO 4 at 633 nm.