Role of primary active species and TiO2 surface characteristic in UV-illuminated photodegradation of Acid Orange 7

Controversy still exists over whether photocatalytic oxidation proceeds via OH radicals, valence band holes, O2− or H2O2. In this paper, the role of these primary oxidants in the photodegradation of an azo dye, Acid Orange 7 (AO7) in UV-illuminated TiO2 suspension was investigated. Little influence of methanol or isopropanol on the degradation indicated that the oxidation was not primarily proceeding by reaction between OH radicals and AO7, because alcohols could effectively scavenge OH radicals. But the presence of I− (hole scavenger) significantly inhibited the degradation, thus suggesting that holes played a major role. Then, experiments carried out in acetonitrile, methanol or isopropanol solvent confirmed the major role of holes, since OH radicals were minimized in dry solvent. In addition, it was suggested that O2− and H2O2 had a negligible effect when Cr(VI) was used as electron scavenger instead of O2. The effects of surface modification by F− or SO42− on the initial steps of photodegradation were also investigated. The hydroxyl groups on the surface of TiO2 were replaced by F− or SO42− and there were little adsorbed AO7 molecules to be trapped by hole, so the initial process could shift progressively from hole-dominated surface reaction to homogeneous radical reaction in bulk solution. Discussion about the relationship between adsorption and photodegradation indicated that the degradation rate was not strongly affected by the actual location amount of organics on TiO2 surface.