Heterogeneous photocatalysis is a process in which the illumination of an oxide semiconductor, usually the anatase form of titanium dioxide, produces photoexcited electrons fe ) and holes (h+). These can migrate to the oxide surface and

The effect of fluoride ions on the photocatalytic degradation of phenol in an aqueous suspension of TiO2 has been investigated. Fluoride ions displace surficial hydroxyl groups and coordinate surface-bound titanium atoms directly. For 0.01 M fluoride concentration and 0.10 g L -1 of TiO2 in the range pH 2-6, the degradation rate of phenol is up to 3 times that in the absence of fluoride ions. This behavior has been correlated with the computed surface speciation. The decrease in the degradation rate of phenol as a function of the substrate concentration observed in naked TiO2 at a high concentration of phenol (over 0.01 M) is largely diminished in the presence of fluoride ions. A photocatalytic model which takes into account the primary events and recombination reactions is able to account for these experimental results. The competition between OH-radical-mediated reaction versus direct electron transfer is discussed. Finally, under a helium atmosphere and in the presence of fluoride ions, phenol is slowly but significantly degraded, although total organic carbon does not decrease, suggesting the occurrence of a photocatalytically induced hydrolysis.

Colloidal T1O2 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 T1O2 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 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 (r > 20 ns) reflect recombination of deep-trapped electrons and holes. It is the latter that dictate the kinetics of the photocatalyzed redox chemistries.

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The photocatalytic transformation of phenol has been investigated on naked TiO2 and on TiO2/F (0.01 M F -) at pH 3.6 in the presence of different alcohols (tert-butyl alcohol, 2-propanol, and furfuryl alcohol). On the basis of a detailed kinetic analysis and the time evolution of the intermediates, it is suggested that on naked TiO2 the oxidation of phenol proceeds for 90% through the reaction with surficial bound hydroxyl radical, the remaining 10% via a direct interaction with the holes. On TiO2/F the reaction proceeds almost entirely via homogeneous hydroxyl radicals because of the unavailability of surface-bound hydroxyl in the presence of fluoride ions. The use of alcohols as a diagnostic tool for the analysis of the photocatalytic mechanism is discussed.