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...

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Bibliographic Details
Published in:The Journal of chemical physics 2019-01, Vol.150 (4), p.041719-041719
Main Authors: Troian-Gautier, Ludovic, Sampaio, Renato N., Piechota, Eric J., Brady, Matthew D., Meyer, Gerald J.
Format: Article
Language:English
Subjects:
Adiabatic flow
Barriers
Core-shell structure
Electron transfer
Electrons
Free energy
Nanoparticles
Rate constants
Temperature dependence
Tin dioxide
Titanium dioxide
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Summary: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, Ea = 32 ± 4 kJ/mol, compared to TiO2 alone, Ea = 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., |ΔGoad|
ISSN:0021-9606
1089-7690
DOI:10.1063/1.5054604