Two Sharp Phase Change Processes of Diphenyl Viologen at a Au(111) Electrode Surface: Non-Faradaic Transition with Interplay of Ionic Adsorption of Chloride and Bromide and Faradaic One

Abstract

Two phase change processes of diphenyl viologen (dPhV) on a Au(111) electrode in KCl and KBr aqueous solutions were described using the results of voltammetric, electroreflectance (ER), and electrochemical scanning tunneling microscopic (EC-STM) measurements. Both processes exhibited sharp spikelike voltammetric responses. In KCl solution, the phase change at 0.30 V versus Ag/AgCl/saturated KCl was found to be a nonfaradaic order–disorder phase transition, from an ordered adlayer of dPhV dication (dPhV2+) with coadsorbed Cl– at more positive potentials than 0.30 V to a gaslike phase at less positive potentials. The faradaic reaction at −0.09 V was found to be the transition from the gaslike phase to a condensed monolayer of dPhV•+. The EC-STM images of the condensed monolayer showed stripe patterns of rows of π–π stacked dPhV•+. Almost the same set of two processes was observed in KBr solution but not in KF solution. In KF solution, although two voltammetric responses were observed, the peaks were small and broad, indicative of sluggish adsorption state changes of individual dPhV cations. Taken together, specific adsorption of coexistent anions is of critical importance for the occurrence of the sharp nonfaradaic phase transition.