Optimized preparation and characterization of Co-N codoped TiO2 with enhanced visible light activity: An insight into effect of dopants on surface redox reactions of photogenerated charge carriers for hydroxyl radical formation

Abstract

In this study, a statistical experiment design approach using response surface methodology was applied in the optimization of preparing Co-N codoped TiO2 photocatalyst. The prepared Co-N codoped TiO2 significantly enhanced the photocatalytic degradation of dye Congo Red under visible light irradiation, as compared to undoped and N, Co monodoped TiO2. As proved by various characterizations, the enhanced photocatalytic activity of Co-N codoped TiO2 can be attributed to the narrowing of energy band structure and the existence of surface oxygen vacancies (OVs) and Co ions as the charge traps, thus resulting in the improvement of visible light absorption and charge transfer. The electron spin resonance and fluorescence measurements confirmed that the formation of hydroxyl radicals (•OH) can be promoted during Co-N codoped TiO2 photocatalysis, due to the more efficient generation and separation of electron–hole pairs. Moreover, it was found that the oxidation of H2O by holes is the relatively major pathway of •OH formation, and OVs could effectively reduce the electron competition between dissolved O2 and Co dopants to ensure the hole- and electron-induced surface redox reactions to proceed smoothly. Based on the above results, a more comprehensive mechanism of Co-N codoping for enhancing TiO2-assisted photocatalysis was thereby proposed.