Dynamics of efficient electron–hole separation in TiO₂nanoparticles revealed by femtosecond transient absorption spectroscopy under the weak-excitation condition

Abstract

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.

Carrier dynamics and the role of surface defects: Designing a photocatalyst for gas-phase CO 2 reduction

“…3 and 4, charge scavenging by interacting gases, which typically occurs on the nanosecond-microsecond time scale, is not expected to dominate. Instead, the majority of the transient signal decay in this time regime (<500 ps) is likely due to recombination and relaxation of charge carriers into deeper trap states (45).…”

Photocatalytic Water-Splitting Reaction from Catalytic and Kinetic Perspectives

“…In the case of TiO 2 , surface-trapped electrons and holes are generated within 200 fs after photoexcitation [10]. Surface-trapped and bulk electrons equilibrate and relax into deep trap sites with a time constant of a few hundred picoseconds [10].…”

“…In the case of TiO 2 , surface-trapped electrons and holes are generated within 200 fs after photoexcitation [10]. Surface-trapped and bulk electrons equilibrate and relax into deep trap sites with a time constant of a few hundred picoseconds [10]. Photoexcited electrons react with gaseous O 2 within 10-100 ls [11], whereas surface-trapped holes react with methanol, ethanol, and 2-propanol within 300, 1000, and 3000 ps, respectively [12].…”

“…The dynamics of photoexcited carriers in nanoparticulate TiO 2 [10][11][12][13] and CdS [14,15] have been studied in detail using transient absorption spectroscopy. In the case of TiO 2 , surface-trapped electrons and holes are generated within 200 fs after photoexcitation [10].…”

Injection Kinetics and Electronic Structure at the N719/TiO2 Interface Studied by Means of Ultrafast XUV Photoemission Spectroscopy

“…To date, results of studies on the carrier relaxation dynamics in TiO 2 are sparsely presented in literature. By using the method of transient absorption spectroscopy, Tamaki et al 44 reported on the capture of free electrons into trap states at the semiconductor surface, occurring on a time scale of ∼ 200 fs.…”