SUBNANOSECOND RELAXATION DYNAMICS IN TIO2 COLLOIDAL SOLS (PARTICLE SIZES R(P)=1.0-13.4 NM) - RELEVANCE TO HETEROGENEOUS PHOTOCATALYSIS

Colloidal TiO2 sols with mean particle diameter 2.1, 13.3, and 26.7 nm were examined by picosecond transient absorption and emission spectroscopies. Transient emission decays followed excellent single-exponential kinetics in all cases, with decay times 67 ps (2.1 nm), 405 ps (13.3 nm), and 66 ps (26.7 nm). Transient absorption spectra show that localization (trapping) of the electron as a Ti3+ species is significant in the 2.1 nm TiO2 particles. At the end of the 30 ps laser pulse, the transient spectra are-fully developed and comprise spectra of trapped holes and trapped electrons. This has important consequences in heterogeneous photocatalysis: photooxidations are initiated by surface-trapped holes, h(TR)(+) (presumably as . OH radicals) and not by valence band holes, h(VB)(+). Absorption decay for the 2.1 nm sols is a simple first-order process, and electron/hole recombination is 100% complete by 10 ns. For the 13.3 and 26.7 nm specimens transient absorption decay follows distinct second-order biphasic kinetics; the decay times of the fast components decrease with increase in particle size. By 10 ns, about 90% or more of the photogenerated electron/hole pairs have recombined such that the quantum yield of photooxidations must be 10% or less. The faster components are due to the recombination of shallow-trapped charge carriers, whereas the slower components (tau > 20 ns) reflect recombination of deep-trapped electrons and holes. It is the latter that dictate the kinetics of the photocatalyzed redox chemistries.