Kinetics and mechanism of electron transfer at nanocrystalline metal-oxide semiconductor/sensitizer interfaces

The kinetics, mechanism, and energetics of interfacial electron transfer (ET) between a metal-oxide semiconductor and a surface-bound sensitizer have been studied through transient absorbance spectroscopy and electrochemical interrogations. The back ET process from colloidal SnO₂ to electrostatically-bound Ru(III) and Os(III) polypyridyl complexes has been investigated through variations in electron concentration, sensitizer formal potential, pH, and separation distance. The kinetics are second order, pH dependent, and occur in both the Marcus normal and inverted regions. Additionally, the distance-dependent studies indicate that the bET kinetics are nonadiabatic and that mixed-rate control may be involved in these interfacial processes. Combining these results, a mechanism where interfacial electron transfer is in competition with electron trap-to-trap hopping has been proposed, although involvement of the SnO ₂ conduction band cannot be excluded.; Kinetic studies of nanocrystalline SnO₂ films covalently sensitized by Ru(II) tris-polypyridyl complexes through phosphonic acid linkages have shown different bET behavior compared to the electrostatic systems. The kinetics are second order but independent of solution pH and occur in the Marcus normal region. From these studies, the trap-to-trap hopping model described for the electrostatic sensitization has been proposed to explain this ET behavior. The kinetic differences between the two interfaces are explained by the method of surface binding, which affects the energetics and pH dependence of the kinetically-relevant redox couple where the bET process originates.; In terms of energetics at the semiconductor surface, the formal potentials of Fe(II), Ru(II) and Os(II) polypyridyl complexes covalently bound through phosphonic acid linkers to a series of metal-oxide substrates (TiO₂, SnO₂, ZrO₂, and Al₂O₃) have been interrogated as a function of solution pH through cyclic voltammetry. Over a range of 13 proton activity units, the potentials are largely independent of solution pH rather than coupled to the Nernstian shifts (−59 mV/pH unit) of the conduction-edge energy. Minor potential changes are caused by pH-induced variations in the metal-oxide zeta (ζ) potential.