Visible Light Photodegradation of Phenol Using Nanoscale TiO2 and ZnO Impregnated with Merbromin Dye: A Mechanistic Investigation

ZnO and TiO2 nanoparticles were impregnated with merbromin dye and used as modified photocatalysts for degradation of phenol. Dye-modified ZnO and TiO2 showed significantly higher photocatalytic activity than neat ZnO and TiO2 under visible light illumination. Moreover, the prepared dye-modified ZnO showed superior photocatalytic efficiency in degradation of phenol compared to the dye-modified TiO2. In a period of 4 hours, dye-modified ZnO removed phenol almost completely while dye-modified TiO2 degraded it only to 47%. A cooperative working mechanism involving the possible photoactivation of both surface bound dye and semiconductor is proposed for the dye-modified systems. The suggested pathway is based on charge-transfer formalism. Furthermore, the study proposes some reasons for difference in reactivity of the dyemodified ZnO and TiO2 catalysts. Less aggregation of dye molecules on the surface of ZnO compared to TiO2 causes more prolonged lifetime of excited state of the dye on the surface of ZnO. Also, energy gap between the conduction band of semiconductor and LUMO level of merbromin in dye-modified ZnO is larger than dye-modified TiO2. Both above lead to more effective electron injection from the dye to ZnO which is hypothesized to be mainly responsible for the enhancement of the reaction rate of phenol degradation.