Journal
ELECTROCHIMICA ACTA
Volume 114, Issue -, Pages 105-115Publisher
PERGAMON-ELSEVIER SCIENCE LTD
DOI: 10.1016/j.electacta.2013.09.168
Keywords
Viologen; HOPG; Phase transition; Anion effect; Electroreflectance
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Funding
- MEXT, Japan [24550158]
- Tokyo Ohka Foundation
- Kato Foundation for Promotion of Science
- Japan Society for the Promotion of Science
- Grants-in-Aid for Scientific Research [13J09394, 24550158] Funding Source: KAKEN
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We found that dibenzyl viologen (dBV) on an HOPG electrode undergoes a two-step first order faradaic phase transition at high concentrations of bromide ion (Br-). Results of voltammetric and electroreflectance measurements were used to describe the mechanism of the two-step transition processes. When [Br-] > 180 mM, the transition step at less negative potential was ascribed to a phase transition between a gas-like adsorption layer of dBV dication (dBV(2+)) and a mesophase of dBV radical cation (dBV(center dot+)). Most likely, the mesophase is a two-dimensional (2D) ordered phase composed of co-adsorbed dBV(center dot+) and Br- where both are in direct contact with the HOPG surface. The transition step at more negative potential was ascribed to a phase transition between the dBV Br- mesophase and a 20 condensed phase of dBV(center dot+). In the condensed phase being denser than the mesophase, dBV(center dot+) molecules are pi-stacked due to face-to-face interaction between bipyridinium radical cations. This transition step involves also a reduction process of dBV(2+) to dBV(center dot+) followed by its incorporation into the condensed phase. The two-step transition was not observed in KCl solution of any concentration, either in KBr solution of [Br-] < 75 mM. Other viologens examined, including benzyl-heptyl viologen, did not exhibit such a two-step transition but single-step one. The nature of the transition, especially in the [Br-] range from 75 to 180 mM, was closely analyzed. (C) 2013 Elsevier Ltd. All rights reserved.
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