Photoelectrochemical reduction rate of ferricyanide at different TiO<inf>2</inf> forms: comparison of SECM and cyclic voltammetric results

Abstract

The heterogeneous rate constant (k) for the reduction of Fe(CN)63- at different UV-illuminated TiO2 polymorphs was determined by (1) cyclic voltammetry (CV) where a TiO2-modified fluorine-doped tin oxide (FTO)-coated glass was the working electrode, and (2) SECM approach curves at the same electrodes at open circuit. The combination of Fe(CN)63- with photogenerated electrons was found to be the rate determining step. The magnitude of the SECM values of k were in the order: anatase-rutile > anatase > rutile > amorphous, which agrees with the generally accepted photocatalytic efficiencies of the TiO2 polymorphs. In contrast, the CV-determined values of k followed the sequence: amorphous > rutile > anatase-rutile > anatase and were approximately one order of magnitude lower than the SECM-determined values. We suggest this indicates the CV determination of rate constants at photoactive films is problematic, due to factors such as reaction at the underlying electrode and the uncompensated resistance of the film.