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

Jason R. Avila, Michael J. Katz, Omar K. Farha, Joseph T Hupp

Research output: Contribution to journalArticle

6 Citations (Scopus)

Abstract

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.

Original languageEnglish
Pages (from-to)20922-20928
Number of pages7
JournalJournal of Physical Chemistry C
Volume120
Issue number37
DOIs
Publication statusPublished - Sep 22 2016

Fingerprint

barrier layers
electron transfer
Semiconductor materials
Electrodes
Molecules
electrodes
Electrons
sensitivity
molecules
delivery
electrons
atomic layer epitaxy
catalysis
thick films
conduction bands
deposits
traps
impedance
Electronic density of states
Atomic layer deposition

ASJC Scopus subject areas

  • Electronic, Optical and Magnetic Materials
  • Energy(all)
  • Surfaces, Coatings and Films
  • Physical and Theoretical Chemistry

Cite this

Barrier-Layer-Mediated Electron Transfer from Semiconductor Electrodes to Molecules in Solution : Sensitivity of Mechanism to Barrier-Layer Thickness. / Avila, Jason R.; Katz, Michael J.; Farha, Omar K.; Hupp, Joseph T.

In: Journal of Physical Chemistry C, Vol. 120, No. 37, 22.09.2016, p. 20922-20928.

Research output: Contribution to journalArticle

@article{041f7c7cbd2e4cf89dc4e82988521c62,
title = "Barrier-Layer-Mediated Electron Transfer from Semiconductor Electrodes to Molecules in Solution: Sensitivity of Mechanism to Barrier-Layer Thickness",
abstract = "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 {\AA} 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 {\AA}, there is little change in ET rate as a function of TiO2 thickness. To our surprise, thermally annealing a 55 {\AA} 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.",
author = "Avila, {Jason R.} and Katz, {Michael J.} and Farha, {Omar K.} and Hupp, {Joseph T}",
year = "2016",
month = "9",
day = "22",
doi = "10.1021/acs.jpcc.6b02651",
language = "English",
volume = "120",
pages = "20922--20928",
journal = "Journal of Physical Chemistry C",
issn = "1932-7447",
publisher = "American Chemical Society",
number = "37",

}

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

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.

UR - http://www.scopus.com/inward/record.url?scp=84988566086&partnerID=8YFLogxK

UR - http://www.scopus.com/inward/citedby.url?scp=84988566086&partnerID=8YFLogxK

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 -