ELECTRON INJECTION BY PHOTOEXCITED RU(BPY)(3)(2- ANALYSES OF THE RECOMBINATION KINETICS BASED ON ELECTROCHEMICAL AND AUGER-CAPTURE MODELS()INTO COLLOIDAL SNO2 )

The photosensitization of colloidal particles of antimony-doped SnO2 b y electrostatically adsorbed Ru(bpy)(3)(2+) (bpy = 2,2'-bipyridine) pr oduced 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 be tween 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 t he range 0.03-7. The rate of recombination (k(rec)) increased nonlinea rly by a factor of 250 over this range in N-e. The nonlinear dependenc e of k(rec) on N-e was analyzed quantitatively using two different mod els: (1) an electrochemical model based on the Butler-Volmer equation and (2) a model based on the Auger recombination mechanism. Both model s gave satisfactory fits to the data with parameters that are physical ly reasonable. The Butler-Volmer equation was used to relate the rate of recombination to the overpotential driving the reaction by making t he assumption that the overpotential is linearly proportional to N-e. That analysis gave values of (2.1 +/- 0.7) x 10(6) s(-1) for the intri nsic rate constant and 0.59 +/- 0.11 for the transference number, with each excess electron contributing 22 +/- 6 mV to the overpotential. I n the Auger-capture model, it was assumed that an equilibrium exists b etween Ru(bpy)(3)(3+) and the stannic hydroxyl radical, >SnO., and tha t e(cb)(-) recombines with >SnO. and not directly with Ru(bpy)(3)(3+). The values of n(0) and B-1 exp(Delta E degrees/k(B)T) were found to b e (1.3 +/- 0.1) x 10(19) cm(-3) and (9 +/- 1) x 10(-34) cm(6) s(-1), r espectively, where n(0) is the equilibrium concentration of free elect rons, B-1 is the coefficient for the conduction band-to-trap Auger tra nsition, and Delta E degrees is the difference in reduction potentials of Ru(bpy)(3)(3+) and >SnO.. Comparison of these data to those for co lloidal particles of undoped SnO2 showed that the doping of SnO2 with antimony does not significantly affect the dynamics of electron recapt ure by Ru(bpy)(3)(3+).