Charge-Carrier Dynamics in Nitrogen-Doped TiO2 Powder Studied by Femtosecond Time-Resolved Diffuse Reflectance Spectroscopy

Nitrogen-doped TiO 2 (N-TiO 2 ) is one of the most promising and widely investigated visible-light-responsive semiconductor photocatalysts. Femtosecond time-resolved diffuse reflectance (TDR) spectroscopy was employed under weak excitation conditions to clarify the charge-separation and trapping dynamics in N-TiO 2 powder, which exhibits high activity for photocatalytic oxidation under visible light. TDR spectra of N-TiO 2 after 360 nm light excitation (Ti 3d ← O 2p transition) revealed that the surface-trapped electrons and holes were generated immediately after excitation, similar to that for TiO 2, and the population of surface-trapped electrons decreased more than that for TiO 2 due to deep trapping by additionally induced oxygen vacancies. The dependence of trapping dynamics on the N concentration is discussed with respect to the diffusion coefficient of electrons. TDR spectra of N-TiO 2 after 450 nm light excitation (Ti 3d ← N 2p transition) clearly indicated the generation of charge carriers. Compared with the 360 nm excitation, two differences of time evolution were detected: the significant decrease just after excitation and the deep trapping of electrons within 1 ps (time resolution). Possible mechanisms for the dependence of the charge-carrier dynamics in N-TiO 2 on the excitation wavelength are discussed.