TY - JOUR
T1 - Barrier-Layer-Mediated Electron Transfer from Semiconductor Electrodes to Molecules in Solution
T2 - Sensitivity of Mechanism to Barrier-Layer Thickness
AU - Avila, Jason R.
AU - Katz, Michael J.
AU - Farha, Omar K.
AU - Hupp, Joseph T.
N1 - Funding Information:
This work was supported by the U.S. Dept. of Energy, Office of Science, Office of Basic Energy Sciences, Grant DE-FG02-87ER13808, and by Northwestern University. Ellipsometry was performed in the KECK II facility of the NUANCE Center at Northwestern University.
Publisher Copyright:
© 2016 American Chemical Society.
PY - 2016/9/22
Y1 - 2016/9/22
N2 - 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.
AB - 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.
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U2 - 10.1021/acs.jpcc.6b02651
DO - 10.1021/acs.jpcc.6b02651
M3 - Article
AN - SCOPUS:84988566086
VL - 120
SP - 20922
EP - 20928
JO - Journal of Physical Chemistry C
JF - Journal of Physical Chemistry C
SN - 1932-7447
IS - 37
ER -