Harnessing excess photon energy in photoinduced surface electron transfer between salicylate and illuminated titanium dioxide nanoparticles

Photons absorbed by nanocrystalline TiO2 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-0(S-/S-.) redox potential and interfacial reorganization energy lambda(R) values, which make electron-transfer rates peak at energies inside the valence band of TiO2. Master equation kinetic modeling shows that electron transfer from S into hyperthermal valence band holes takes place at rates consistent with k(sc) similar to 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.