Photooxidation mechanism of dye alizarin red in TiO2 dispersions under visible illumination:: an experimental and theoretical examination

Visible light-induced photocatalytic oxidation of the dye alizarin red (AR) has been examined in TiO2 aqueous dispersions. The ESR spin-trapping technique was used to detect active oxygen radicals formed during in situ visible irradiation of AR/TiO2 dispersions. Evidence for the production of superoxide (O-2(.-) or HOO.) (formed in the reduction of O-2) and hydroxyl radicals ((OH)-O-.) (formed by a multistep reduction) in the initial photoexcitation stage is presented. Meanwhile, the pathway of the photooxidation of AR is theoretically predicted on the basis of molecular orbital (MO) calculations by frontier electron densities and point charges on all the individual atoms of the dye. The relatively high negative point charges on the sulfonic oxygens lead to a strong adsorption of the dye onto the TiO2 particle surface through the sulfonate function. The position of the dye molecule attacked by the active oxygen species (e.g. O2(.-) or HOO. radicals) and/or O-2 is correlated with frontier electron densities, there is a perfect agreement between MO calculations and the results of experiments. A plausible mechanism of photooxidation under visible irradiation is discussed. (C) 2000 Elsevier Science B.V. All rights reserved.