No abstract

Photons absorbed by nanocrystalline TiO 2 particles at 254 nm are found to be 7.7 times more efficient than those at 366 nm for driving the photocatalytic oxidation of salicylate S in aerated aqueous sols. The occurrence of this phenomenon is ascribed to the conjunction of (1) short diffusion times of photogenerated carriers to the surface of nanoparticles, a fact that allows chemical reaction to compete with energy relaxation, and (2) favorable donor E o (S -/S • ) redox potential and interfacial reorganization energy λ R values, which make electrontransfer rates peak at energies inside the valence band of TiO 2 . Master equation kinetic modeling shows that electron transfer from S into hyperthermal valence band holes takes place at rates consistent with k sc ∼ 10 4 cm s -1 at optimal exoergicity, if the excess energy is dissipated into the crystal lattice within a few picoseconds. Hydroxyl ions as donors would require much slower thermalization rates.

Electron accumulation in TiO 2 ethanolic sols prepared by the HCl hydrolysis of a titanium alkoxide has been scrutinized by UV−vis and EPR spectroscopy. Unexpectedly, Ti(III) centers, g = 1.9551, formed af ter controlled monochromatic irradiation of the sols could be detected at room temperature by EPR spectroscopy. The yield of the paramagnetic signal and the number of accumulated reducing species, detected in dark titration experiments, increase as the water to titanium molar ratio, h, used in the synthesis diminishes. A 3.8% Ti(III) production efficiency was estimated for h = 6.5. Bidentate ethoxide coordination to the titanium dioxide surface and the replacement of surface hydroxylic groups by chloride ions is directly inferred by FTIR and EPR spectroscopies. Both findings are proposed to account for the room temperature detection of the Ti(III) species, and the higher electron storage capacity of the colloids prepared with lower h values.

The action spectra of the colloidal TiO2-photosensitized oxidations of bifunctional aromatics in 1 mM phosphate colloidal media provide firm evidence that electron transfer from outer-sphere donors can compete with excited hole relaxation at nanoparticle interfaces. The possibility and extent of the competition are largely determined by the dependence of Marcus nuclear factors on the donors' reversible redox potentials E°D/D +• relative to the valence band edge. Good electron donors are degraded by OH radicals produced in the oxidation of water by thermal holes, whereas direct electron transfer into excited holes (the pathway favored by less oxidizable substrates) leads to enhanced quantum efficiencies at short wavelengths. The ultimate decline of the quantum efficiency for the oxidation of phthalate (the most endoergic donor of the set) by λ ≤ 320 nm photons indicates that the relaxation of highly excited carriers takes place in discrete steps commensurate with electron transfer reorganization energies. The latter observation is consistent with the opening of low order multiphonon channels for the disposal of kinetic energy quanta larger than the depth of surface ν̄O - H ∼ 3700 cm-1 vibrational sinks.