Harnessing Excess Photon Energy in Photoinduced Surface Electron Transfer between Salicylate and Illuminated Titanium Dioxide Nanoparticles

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 Eo(S^-/S^•) redox potential and interfacial reorganization energy λ_R values, which make electron-transfer 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.

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