Femtosecond Diffuse Reflectance Spectroscopic Study on Nitrogen-Doped Titanium Oxide Photocatalysts

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Femtosecond time-resolved diffuse reflectance measurements under weak excitation conditions successfully detected the intrinsic charge separation and trapping dynamics in nitrogen-doped titanium oxide(N-TiO_2) powders without second-order electron-hole recombination processes. The decay rate of the population of the surface-trapped electrons decreased nonlinearly with increasing N atom concentration after 360 nm light excitation, which can be explained by the electron trapping process being diffusion-limited. The deep trapping of electrons within 1 ps was evident after 450 nm light excitation, which can probably be explained by quick and deep trapping of the electron in the Ti3d level by an adjacent oxygen vacancy induced near N atoms. For metal loaded N-TiO_2 system, it has been revealed that the photoinduced electron transfer occurred from N-TiO_2 to the loaded Cu or Fe species under UV light excitation whereas no reduction of the loaded Cu or Fe species was detected under visible light excitation.