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

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 mn cannot be observed because of the ultrafast interfacial hole transfer between Tio(2) 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 similar to 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 similar to 150 ps, which is almost independent of the concentration of SCN-.