Revealing the Relationship between Semiconductor Electronic Structure and Electron Transfer Dynamics at Metal Oxide–Chromophore Interfaces

Interfacial charge recombination dynamics in nanocrystalline SnO 2 and TiO 2 thin films sensitized with phosphonate-derivatized ruthenium chromophores (Ru(bpy) 2 (4,4′-(PO 3 H 2 ) 2 bpy)] 2+, RuP) have been investigated in aqueous media by nanosecond transient absorption spectroscopy . Back electron transfer (BET) rates for RuP–SnO 2 were observed to be 2–3 times greater than for RuP–TiO 2 . Additionally, rates of charge recombination for RuP–TiO 2 show a significant pH dependence, while only a subtle influence of pH is observed for BET in RuP–SnO 2 . Cyclic voltammetry measurements indicate the exponential distribution of intra-band-gap trap states varies with pH for both SnO 2 and TiO 2 nanocrystalline thin films. BET rates for RuP–SnO 2 and RuP–TiO 2 are correlated with the distribution, identity, and occupation of localized trap states within the nanocrystalline metal oxide films, which are pH specific. Recombination between injected electrons and oxidized chromophores is influenced by the identity of metal oxide localized trap states populated and the specific pathways by which BET can proceed.