Light-Induced Electron Transfer in Colloidal Semiconductor Dispersions: Single vs. Dielectronic Reduction of Acceptors by Conduction-Band Electrons

Abstract

Colloidal particles of TiO2 (anatase, 50-Á radius) were produced via hydrolysis of TiCl4 in aqueous solutions and characterized by electron microscopy, light scattering, and absorption and electrophoresis techniques. Laser photolysis and continuous illumination techniques were applied to investigate the reaction of conduction-band electrons (e−CB) with various acceptor molecules. The rate of reduction of methyl viologen is diffusion controlled in alkaline solution (pH <10) but becomes limited by interfacial electron transfer at lower pH. No diffusion limit is observed with the functionalized viologen C14MV2+ which is adsorbed at the surface of the TiO2 particles. Preirradiated samples produce hydrated electrons owing to photoionization of C14MV+. A cofacial dimeric viologen (DV4+) is reduced by e−CB via a simultaneous two-electron-transfer step. In contrast, Rh(bpy)33+ undergoes one-electron reduction to Rh(bpy)32+ by e−CB. Subsequent dark reactions produce Rh(bpy)2+. Heterogeneous rate constants and transfer coefficients for these interfacial redox processes are derived and implications for artificial photosynthetic systems discussed. © 1983, American Chemical Society. All rights reserved.