Mechanism of photocatalytic degradation of dye MG by TiO2-film electrode with cathodic bias potential

Abstract

Photoelectrocatalytic degradation of malachite green (MG) under visible light irradiation with TiO2-film electrode has been investigated to reveal the mechanism for TiO2 photocatalytic degradation of dyes. The supported TiO2 electrode was prepared in laboratory and detected by scanning electron micros-copy and X-ray diffractometry. We have examined the degradation kinetics, change in degradation rate of MG and photocurrent change with the bias potential, voltage-induced adsorption of dyes, accumu-lation of electrons in the TiO2 electrode, effect of various additives such as benzoquinone (BQ), and formation of active oxygen species by UV-visible spectroscopy, TOC and spin-trap ESR spectrometry. It was found that the dyes could controllably interact with TiO2 surface by external bias and charge of dyes. The cationic dye MG underwent efficient mineralization at negative bias under visible light irradiation, but N-dealkylation process predominated at positive bias under visible light irradiation. At negative bias of −0.4 V vs SCE, the O2·− and dye·+ were formed simultaneously at the electrode/electrolyte interface during degradation of cationic MG. Experimental results imply both the superoxide radical and the dye cationic radical are essential to the mineralization of the dyes under visible-light-induced photocatalytic conditions.