Efficiency of Hot Carrier Trapping by Outer-Sphere Redox Probes at Quantum Dot Interfaces

Abstract

The action spectra of the colloidal TiO_2-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 ν̄_(O-H) ∼ 3700 cm^(-1) vibrational sinks.

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