Investigation on Initial Process of Photoexcitation at Semiconductor/Solution Interface by Ultrafast Spectroscopy

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This study aimed at clarification of decisive factor of recombination between photoexcited electron and positive hole in the semiconducting materials by analysis of initial process in the photoirradiated titania particles and electrodes with pump-probe time-resolved diffuse reflectance spectroscopy. By using ultrafast femtosecond laser system with 310 nm pump (excitation) and 620 nm probe pulses, we have proved that the bandgap excitation of titania particles induces very fast rise followed by relatively rapid and slow decay of absorption. The rapid decay obeyed second order kinetics suggesting that the absorption is due to trapped electrons and their recombination with positive holes give the rapid decay. Similar behavior was also observed in the electrode of polycrystalline titania showing that each titania particle in the electrode behaves independently. Dependence of these decays on the pump pulse wavelength was investigated for anatase and rutile titania particles. It was clarified that when the particles were photoexcited by the pump pulse of energy close to the bandgaps of crystallites very fast decay component appeared. We concluded that this is due to the formation of excitons.