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−.
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