Influence of surface metallic silver deposit and surface fluorination on the photocatalytic activity of rutile TiO2 for the degradation of crystal violet a cationic dye under UV light irradiation

Silver metallization and fluorination on the surface of rutile TiO2 (SRT and FRT) was carried out by photochemical reduction and wet impregnation methods respectively. TiO2, SRT and FRT were characterized by various analytical techniques like PXRD, SEM, EDX, FTIR, PL, UV-vis absorbance and XPS. The photocatalytic degradation of crystal violet (CV) was carried out at three different pH conditions. Acidic pH was found to be more favorable for the degradation, in spite of low adsorption of cationic CV molecules on the catalyst surface. The electrostatic repulsion at this pH drives the CV molecules into the bulk of the solution suggesting the involvement of bulk hydroxyl radicals rather than surface adsorbed hydroxyl radicals. The degradation efficiency can be represented by the following order SRT > FRT > TiO2. The Ti-peroxo (Ti-OO-Ti) complex species formed in presence of H2O2 by the combination of two trapped holes (Ti-O-center dot) in aqueous medium is predicted to enhance the rate of generation of hydroxyl radicals. The exciton mobility is dependent on the polaron effective mass which is higher for rutile TiO2 accounting for its lower activity. The bulk charge carrier transport which is less in bare rutile TiO2 is enhanced in the surface modified TiO2. Effective trapping of photogenerated excitons/electrons by F-/Ag0 can facilitate their migration and increase the activity by tenfolds. The rate of degradation of CV followed two reaction pathways, slower N-demethylation in basic conditions and rapid aromatic cleavage at the central carbon atom in the acidic pH conditions. (C) 2016 Published by Elsevier Ltd.