Barrier-Layer-Mediated Electron Transfer from Semiconductor Electrodes to Molecules in Solution: Sensitivity of Mechanism to Barrier-Layer Thickness

Electron transfer (ET) phenomena at and near semiconductor/molecule interfaces are of fundamental significance for applications involving liquid-junction photovoltaics, organic photovoltaics, and electrochemical heterogeneous catalysis. To probe mechanisms of electron delivery through barrier layers at such interfaces, we make use of atomic layer deposition to deposit ultrathin films of TiO₂ conformally onto SnO₂ electrodes. In the presence of TiO₂ films (i.e., barrier layers) up to 10 Å thick, electrons are delivered from the electrode to molecules in solution by tunneling through the layers, as evidenced, in part, by an exponential decrease in ET rate with layer thickness. For films thicker than 10 Å, there is little change in ET rate as a function of TiO₂ thickness. To our surprise, thermally annealing a 55 Å layer of TiO₂ on SnO₂ yielded a 10-fold decrease in ET rate compared to that imposed by the as-deposited layer. At applied potentials near the conduction-band edge of SnO₂, and significantly below the band edge of TiO₂, electrochemical impedance spectroscopy with nominally flat, as-deposited TiO₂ indicates the presence of nearly twice the density of electronic states as found with air-annealed samples. These and related observations point to a barrier-layer-thickness-dependent change in the mechanism of electron delivery, from the underlying electrode to solution species, from one based on tunneling to one entailing trap-facilitated hopping. The findings have design implications for the application of interfacial barrier layers to electrochemical and photoelectrochemical problems.