Measurement of the free-energy dependence of interfacial charge-transfer rate constants using ZnO/H2O semiconductor/liquid contacts

Thomas W. Hamann, Florian Gstrein, Bruce S. Brunschwig, Nathan S Lewis

Research output: Contribution to journalArticle

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Abstract

The dependence of electron-transfer rate constants on the driving force for interfacial charge transfer has been investigated using n-type ZnO electrodes in aqueous solutions. Differential capacitance versus potential and current density versus potential measurements were used to determine the energetics and kinetics, respectively, of the interfacial electron-transfer processes. A series of nonadsorbing, one-electron, outer-sphere redox couples with formal reduction potentials that spanned approximately 900 mV allowed evaluation of both the normal and Marcus inverted regions of interfacial electron-transfer processes. All rate processes were observed to be kinetically first-order in the concentration of surface electrons and first-order in the concentration of dissolved redox acceptors. The band-edge positions of the ZnO were essentially independent of the Nernstian potential of the solution over the range 0.106-1.001 V vs SCE. The rate constant at optimal exoergicity was observed to be approximately 10-16 cm4 s-1. The rate constant versus driving force dependence at n-type ZnO electrodes exhibited both normal and inverted regions, and the data were well-fit by a parabola generated using classical Marcus theory with a reorganization energy of 0.67 eV. NMR line broadening measurements of the self-exchange rate constants indicated that the redox couples had reorganization energies of 0.64-0.69 eV. The agreement between the reorganization energy of the ions in solution and the reorganization energy for the interfacial electron-transfer processes indicated that the reorganization energy was dominated by the redox species in the electrolyte, as expected from an application of Marcus theory to semiconductor electrodes.

Original languageEnglish
Pages (from-to)7815-7824
Number of pages10
JournalJournal of the American Chemical Society
Volume127
Issue number21
DOIs
Publication statusPublished - Jun 1 2005

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Semiconductors
Contacts (fluid mechanics)
Free energy
Charge transfer
Rate constants
Electrons
Semiconductor materials
Oxidation-Reduction
Liquids
Electrodes
Surface Tension
Electrolytes
Capacitance
Current density
Nuclear magnetic resonance
Ions
Kinetics

ASJC Scopus subject areas

  • Chemistry(all)

Cite this

Measurement of the free-energy dependence of interfacial charge-transfer rate constants using ZnO/H2O semiconductor/liquid contacts. / Hamann, Thomas W.; Gstrein, Florian; Brunschwig, Bruce S.; Lewis, Nathan S.

In: Journal of the American Chemical Society, Vol. 127, No. 21, 01.06.2005, p. 7815-7824.

Research output: Contribution to journalArticle

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