Barriers for interfacial back-electron transfer: A comparison between TiO₂ and SnO₂/TiO₂ core/shell structures

Temperature dependent kinetics for back-electron transfer (BET) from electrons in TiO₂ or SnO₂/TiO₂ core/shell nanoparticles to oxidized donor-bridge-acceptor (D-B-A) sensitizers is reported over a 110° range. Two D-B-A sensitizers (CF₃-p and CF₃-x) were utilized that differed only by the nature of the bridging ligand: a xylyl spacer that largely insulated the two redox active centers and a phenyl bridge that promoted strong electronic coupling and an adiabatic electron transfer mechanism. An Arrhenius analysis revealed that the activation energies were significantly larger for the core/shell oxides, E_a = 32 ± 4 kJ/mol, compared to TiO₂ alone, E_a = 22 ± 6 kJ/mol. The barriers for BET on sensitized TiO₂ were within the same range as previous literature reports, while this study represents the first quantification for SnO₂/TiO₂ core/shell materials. Two different models were proposed to rationalize the larger barrier for the core/shell materials: (1) a band edge offset model and (2) a low energy trap state model with recombination from the TiO₂ rutile polymorph shell. The latter model was preferred and is in better agreement with the experimental data. The kinetic analysis also afforded the forward and reverse rate constants for the intramolecular equilibrium. In accordance with theoretical predictions and previous research, the absolute value of the free energy change was smaller for the adiabatic equilibrium provided by the phenyl bridge, i.e., |ΔG^o _ad| <|ΔG^o|.