How fast is interfacial hole transfer? In situ monitoring of carrier dynamics in anatase TiO2 nanoparticles by femtosecond laser spectroscopy

Abstract

By comparing the transient absorption spectra of nanosized anatase TiO₂ colloidal systems with and without SCN⁻, the broad absorption band around 520 nm observed immediately after band-gap excitation for the system without SCN⁻ has been assigned to shallowly trapped holes. In the presence of SCN⁻, the absorption from the trapped holes at 520 nm cannot be observed because of the ultrafast interfacial hole transfer between TiO₂ nanoparticles and SCN⁻. The hole and electron trapping times were estimated to be <50 and 260 fs, respectively, by the analysis of rise and decay dynamics of transient absorption spectra. The rate of the hole transfer from nanosized TiO₂ colloid to SCN⁻ is comparable to that of the hole trapping and the time of formation of a weakly coupled (SCN···SCN)^•− is estimated to be ∽2.3 ps with 0.3 M KSCN. A further structural change to form a stable (SCN)₂^•− is observed in a timescale of 100∽150 ps, which is almost independent of the concentration of SCN⁻.