Fast electrochemistry of surface monolayers on ultramicroelectrodes
Xu, Chuanjing
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https://hdl.handle.net/2142/23394
Description
Title
Fast electrochemistry of surface monolayers on ultramicroelectrodes
Author(s)
Xu, Chuanjing
Issue Date
1992
Doctoral Committee Chair(s)
Faulkner, Larry R.
Department of Study
Chemistry
Discipline
Chemistry
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Chemistry, Analytical
Chemistry, Physical
Language
eng
Abstract
This research concerns the development of electrochemical techniques and the application of these techniques to the quantitative study of kinetic electrode reaction events in the microsecond and nanosecond regimes.
The initial stage of the project focused on methodology, which included the development of an experimental setup that can reliably work on nanosecond timescales and well-behaved ultramicroelectrodes with minimized stray capacitance. The instrumental limit of defining RC circuit time constants is 5 nanoseconds. Experiments involving variations in electrode radius and solution conductivity have been conducted to study the effect of stray capacitance at very short timescales. The results indicate that the stray capacitance arising from the electrode body has been minimized by careful design and construction of small size ultramicroelectrodes. The lower limits of timescales were demonstrated in real chemical environments. A cell time constant of 12 nanoseconds, as well as a surface reaction rate constant of 8.5 $\times$ 10$\sp6$ sec$\sp{-1},$ which corresponds to an average lifetime of 113 nanoseconds, have been obtained. Therefore, the timescale in the measurement of real electrochemical reaction rates has been reduced by 5 orders of magnitude, from several milliseconds at electrodes of conventional size to about one hundred nanoseconds at ultramicroelectrodes.
The separation of charging and faradaic currents for chronoamperometric measurements of surface monolayers has been studied. The kinetic parameters (usually rate constants) can be obtained directly from first order decays of current transients. This strategy is also confirmed by an analysis of the current transients based on the concept of Laplace transformation.
A representative application of high speed measurements is the kinetic study of anthraquinone-2,6-disulfonate (2,6-AQDS) monolayers on mercury surfaces. The kinetics of these surface reactions depend on the proton concentration and the potential applied on the electrode. Isotope effects on the kinetics were also observed. The mechanism of the surface reaction was proposed as an $\rm H\sp+e\sp-H\sp+e\sp-$ process, or as a successive concerted one-electron, one-proton transfer process in both reduction and oxidation directions. The structural effects on the kinetics between 2,6-AQDS and 1,5-AQDS were attributed to steric effects in the adsorbed AQDS monolayers.
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