Femtosecond Visible-to-IR Spectroscopy of TiO2 Nanocrystalline Films: Elucidation of the Electron Mobility before Deep Trapping

The transient absorption of nanocrystalline TiO2 films in the visible-to-IR wavelength region was measured under UV excitation at 266 nm in order to purposely generate plural electron−hole pairs in single nanoparticles. Trapped holes, trapped electrons, and bulk electrons were observed as in our previous transient absorption studies, where the electron−hole density reduction to be as low as second-order recombination did not occur, namely, the number of the electron hole pairs was less than unity per TiO2 nanoparticle. The kinetics of the second-order electron−hole recombination, induced in a controlled manner, was analyzed to estimate the mobility of the electrons before their deep trapping, which was found to occur with a time constant of 500 ps in our previous report (Tamaki, Y.; et al. Phys. Chem. Chem. Phys. 2007, 9, 1453−1460). Electron−hole dynamics in the TiO2 nanoparticle from 100 fs to 1 ns has been understood in detail, and its relation to the photocatalytic nature of TiO2 nanoparticles is discussed.