A Surface Science Approach to Unveiling the TiO₂ Photocatalytic Mechanism: Correlation between Photocatalytic Activity and Carrier Lifetime

Abstract

Establishing an accurate view of the photocatalytic mechanism of titanium dioxide (TiO2) has been a challenging task since the discovery of the Honda-Fujishima effect. Despite the great success of catalytic studies in elucidating the chemical and physical aspects of photocatalysis, many questions remain. A surface science approach, which is characterized by the use of atomically well-defined surfaces in precisely controlled environments, is a powerful tool to shed light on the fundamental mechanism, especially the dynamics of photoexcited carriers. In the present contribution, recent progress in photocatalytic research that correlates photocatalytic activity and carrier dynamics on rutile and anatase TiO2 is reviewed. A special focus is placed on the lifetime of photoexcited carriers. We present a method to determine the carrier lifetime; pump-probe time-resolved soft X-ray photoelectron spectroscopy, utilizing an ultraviolet laser as a pump light and a synchrotron radiation as a probe light. The carrier lifetime is found to be linearly correlated with the photocatalytic decomposition/desorption rate of acetic acid adsorbed on single-crystal TiO2 surfaces. The important role of a potential barrier on the TiO2 surface, which influences the carrier lifetime and the photocatalytic activity, is discussed.

Effects of oxygen vacancies on the photoexcited carrier lifetime in rutile TiO2

“…Previous results suggested that a bulk O V and surface O V play different roles. [68][69][70][71][72] It was proposed that the surface O V can trap the photoexcited electrons. Furthermore, the electron and hole can be separated due to the surface band bending and the e-h recombination is suppressed.…”

“…Furthermore, the electron and hole can be separated due to the surface band bending and the e-h recombination is suppressed. 69,70 Moreover, the surface O V is also known as a chemically reactive site, which can improve the photocatalytic properties. 17,18 However, on the other hand, there are many reports showing that the bulk O V acts as an e-h recombination center, 68,[72][73][74][75] which is in agreement with our results.…”

Photocatalytic Activity Enhancement of Anatase/Rutile‐Mixed Phase TiO2 Nanoparticles Annealed with Low‐Temperature O2 Plasma

“…[46,47] This difference is based on the fact that the energy band structures of anatase and rutile phase crystals exhibit an indirect bandgap and a direct bandgap, respectively. [48,49] The direct bandgap structure of rutile phase crystal allows the photoexcited charge carriers to easily recombine each other with the emission of photons. In contrast, the indirect bandgap structure of anatase phase crystal allows the photoexcited charge carriers to recombine each other with the assistance of phonons.…”

“…This assistance of phonons suggests that the photogenerated electrons in the anatase phase crystal do not directly recombine the photogenerated holes, extending the photoexcited carrier lifetime and increasing the photoexcited carrier concentration. [48,49] Thus, the phase transformation to the anatase phase would contribute to the enhanced photoexcited carrier concentration under UV irradiation.…”

Erratum: A Surface Science Approach to Understanding TiO₂ Photocatalyst: Correlation between Carrier Lifetime and Photocatalytic Activity

“…Surface indices of rutile TiO 2 were wrongly displayed in Table 3 in the original article [1]. The correct table is shown below.…”