Charge Separation and Trapping in N-Doped TiO₂ Photocatalysts: A Time-Resolved Microwave Conductivity Study

We studied charge separation and trapping processes in N-doped TiO2 (N-TiO2) photocatalysts by means of time-resolved microwave conductivity (TRMC). We observed that the trapping rate increased with N-doping due to the increase in the oxygen vacancy. Furthermore, the trapping rate increased markedly when Cu was loaded onto the photocatalysts, and this increase indicated a rapid electron transfer to this Cu cocatalyst. For N-TiO2, the charge separation efficiency under visible light excitation (450 nm, Ti 3d ← N 2p transition) was found to be one-third as high as the efficiency observed under ultraviolet excitation (Ti 3d ← O 2p transition). We discuss possible mechanisms for the charge separation and trapped processes, which is essential for the design of efficient doped-TiO2 photocatalysts.