Mechanistic Transition of Electron Transfer Kinetics from Quantum Electron Tunneling to Trap-Facilitated Hopping through TiO2 Films Grown By Atomic Layer Deposition on SnO2 Electrodes

The understanding of electron transfer (ET) phenomenon across semi-conductor interfaces is crucial for applications in photovoltaic systems and electrochemical heterogeneous catalysis . In order to probe ET mechanisms, we exploit atomic layer deposition (ALD) to conformally coat ultrathin films of TiO2 onto a lower conduction band (CB) SnO2 electrode. Between TiO2 film thicknesses of 1 to 10 Å, quantum tunneling through the insulating TiO2 layer is the kinetically preferred ET pathway.  At films thicker than 10 Å, there is little change to the ET rate as a function of TiO2 thickness.  To our surprise, annealing a 55 Å layer of TiO2 on SnO2, a 10x reduction in ET rate was observed compared to the as deposited TiO2.  At potentials near the CB edge of SnO2, electrochemical impedance spectroscopy of nominally flat amorphous TiO2 indicated the presence of nearly double the density of states (DOS) with respect to their crystalline counterpart; the relationship between ET and DOS was studied in depth. These findings show the first observation of change in ET mechanism across semi-conductor interfaces, and demonstrates the ineffectiveness of thick layers films grown by ALD at completely shutting off electron transfer without annealing post film growth.