Reactive species, holes, and electrons in photoexcited nanocrystalline TiO 2 films were studied by transient absorption spectroscopy in the wavelength range from 400 to 2500 nm. The electron spectrum was obtained through a hole-scavenging reaction under steady-state light irradiation. The spectrum can be analyzed by a superposition of the free-electron and trapped-electron spectra. By subtracting the electron spectrum from the transient absorption spectrum, the spectrum of trapped holes was obtained. As a result, three reactive speciess trapped holes and free and trapped electronsswere identified in the transient absorption spectrum. The reactivity of these species was evaluated through transient absorption spectroscopy in the presence of hole-and electronscavenger molecules. The spectra indicate that trapped holes and electrons are localized at the surface of the particles and free electrons are distributed in the bulk.

The transient absorption of nanocrystalline TiO(2) films in the visible and IR wavelength regions was measured under the weak-excitation condition, where the second-order electron-hole recombination process can be ignored. The intrinsic dynamics of the electron-hole pairs in the femtosecond to picosecond time range was elucidated. Surface-trapped electrons and surface-trapped holes were generated within approximately 200 fs (time resolution). Surface-trapped electrons, which gave an absorption peak at around 800 nm, and bulk electrons, which absorbed in the IR wavelength region, decayed with a 500-ps time constant due to relaxation into deep bulk trapping sites. It is already known that, after this relaxation, electrons and holes survive for microseconds. We interpreted these long lifetimes in terms of the prompt spatial charge separation of electrons in the bulk and holes at the surface.

To investigate the primary process of photocatalytic oxidation of TiO2, interfacial charge-transfer reaction of trapped holes formed in nanocrystalline TiO2 films by UV irradiation was directly measured by highly sensitive femtosecond and nanosecond transient absorption spectroscopy under low intensity excitation condition to avoid fast electron-hole recombination. Accordingly, the rates and yields of photocatalytic oxidation of several alcohols adsorbed on TiO2 were evaluated successfully.

The transient absorption of nanocrystalline TiO 2 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 TiO 2 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 et al. Phys. Chem. Chem. Phys. 2007, 9, 1453. Electron-hole dynamics in the TiO 2 nanoparticle from 100 fs to 1 ns has been understood in detail, and its relation to the photocatalytic nature of TiO 2 nanoparticles is discussed. † Part of the "Hiroshi Masuhara Festschrift".

We fabricated dye-adsorbed NiO solar cells (pDSCs) with six different metal-free organic dyes having various ground-state oxidation potentials. Under monochromatic light irradiation, all dyes showed cathodic current at wavelengths where the dyes absorb, suggesting hole injection occurred from the adsorbed dyes to the valence band of NiO. Absorbed photon-to-current conversion efficiency (APCE) tends to increase with the increase of energy difference (∆E) between the valence band edge of NiO and the ground state of the dyes. The maximum APCE of 30% was obtained with 0.6 eV of the ∆E. The apparent hole diffusion coefficient in the NiO electrode was nearly independent from light intensity, and the values were estimated to be 4 × 10 -8 cm 2 /s. On the other hand, hole lifetime depends on light intensity, ranging from 3 × 10 -2 to 1 × 10 0 s. Investigation of the anchoring site of the dyes and the results of molecular orbital calculations suggested that electron injection from the dye to the valence band of NiO, occurring just after the hole injection, is the major factor of the relatively low efficiency even with the case of large ∆E.