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

•Optimized preparation of Co-N codoped TiO2 was achieved by RSM application.•Co-N codoped TiO2 exhibited enhanced visible light activity for CR degradation.•Co-N codoping synergistically improved visible light absorption and charge transfer.•OVs could effectively reduce electron competition between dissolved O2 and Co ions.•Enhanced mechanism of •OH formation over Co-N codoped TiO2 was further clarified.