Electron Injection by Photoexcited Ru(bpy)32+ into Colloidal SnO2: Analyses of the Recombination Kinetics Based on Electrochemical and Auger-Capture Models

The photosensitization of colloidal particles of antimony-doped SnO 2 by electrostatically adsorbed Ru(bpy) 3 2+ (bpy = 2,2‘-bipyridine) produced Ru(bpy) 3 3+ and a conduction band electron (e cb - ) with a quantum efficiency close to 1. The influence of the injected electron population density on the dynamics of the subsequent recombination between Ru(bpy) 3 3+ and e cb - was investigated by nanosecond laser flash photolysis. The initial average number of Ru(bpy) 3 3+ /e cb - pairs per particle ( N e ) created by the laser pulse was varied in the range 0.03−7. The rate of recombination ( k rec ) increased nonlinearly by a factor of 250 over this range in N e . The nonlinear dependence of k rec on N e was analyzed quantitatively using two different models: (1) an electrochemical model based on the Butler−Volmer equation and (2) a model based on the Auger recombination mechanism. Both models gave satisfactory fits to the data with parameters that are physically reasonable. The Butler−Volmer equation was used to relate the rate of recombination to the overpotential driving the reaction by making the assumption that the overpotential is linearly proportional to N e . That analysis gave values of (2.1 ± 0.7) × 10 6 s -1 for the intrinsic rate constant and 0.59 ± 0.11 for the transference number, with each excess electron contributing 22 ± 6 mV to the overpotential. In the Auger-capture model, it was assumed that an equilibrium exists between Ru(bpy) 3 3+ and the stannic hydroxyl radical, >SnO •, and that e cb - recombines with >SnO • and not directly with Ru(bpy) 3 3+ . The values of n 0 and B 1 exp(Δ E °/ k B T ) were found to be (1.3 ± 0.1) × 10 19 cm -3 and (9 ± 1) × 10 -34 cm 6 s -1, respectively, where n 0 is the equilibrium concentration of free electrons, B 1 is the coefficient for the conduction band-to-trap Auger transition, and Δ E ° is the difference in reduction potentials of Ru(bpy) 3 3+ and >SnO • . Comparison of these data to those for colloidal particles of undoped SnO 2 showed that the doping of SnO 2 with antimony does not significantly affect the dynamics of electron recapture by Ru(bpy) 3 3+ .