Photocatalytic activity is determined by the transport property of photoexcited carriers from the interior to the surface of photocatalysts. Because the carrier dynamics is influenced by a space charge layer (SCL) in the subsurface region, an understanding of the effect of the potential barrier of the SCL on the carrier behavior is essential. Here we have investigated the relaxation time of the photoexcited carriers on single-crystal anatase and rutile TiO2 surfaces by time-resolved photoelectron spectroscopy and found that carrier recombination, taking a nanosecond time scale at room temperature, is strongly influenced by the barrier height of the SCL. Under the flat-band condition, which is realized in nanometer-sized photocatalysts, the carriers have a longer lifetime on the anatase surface than the rutile one, naturally explaining the higher photocatalytic activity for anatase than rutile.

Pump−probe time-resolved X-ray photoelectron spectroscopy measurements have been carried out to comparatively assess the relaxation process of the photoexcited states on pristine and Ar + -sputtered TiO 2 (110) surfaces and a TiO 2 (011)-2 × 1 surface, on which the accumulation-type space charge layers are developed. Ultraviolet laser irradiation induces a surface photovoltage (SPV) of around 0.1 eV. The SPV relaxation time on pristine TiO 2 ( 110) is determined to be approximately 100 ns and is doubled on the sputtered surface. In contrast, a much shorter time of 1 ns is observed on TiO 2 (011)-2 × 1. The difference in the relaxation time on the two TiO 2 (110) surfaces is explained by differences in the O vacancy density on the surface as well as the barrier height of the surface potential for the photoexcited holes. A large hole capture cross section of a state characteristic of TiO 2 (011)-2 × 1 is, on the other hand, responsible for the fast SPV relaxation on this surface.

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Time-resolved soft X-ray photoelectron spectroscopy (PES) experiments were performed with time scales from picoseconds to nanoseconds to trace relaxation of surface photovoltage on the ZnO(0001) single crystal surface in real time. The band diagram of the surface has been obtained numerically using PES data, showing a depletion layer which extends to 1 lm. Temporal evolution of the photovoltage effect is well explained by a recombination process of a thermionic model, giving the photoexcited carrier lifetime of about 1 ps at the surface under the flat band condition. This lifetime agrees with a temporal range reported by the previous time-resolved optical experiments.

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