Redox Activity and Structural Transition of Heptyl Viologen Adlayers on Cu(100)

Abstract

The redox behaviour and potential-dependent adsorption structure of heptyl viologen (1,1'-diheptyl-4,4'-bipyridinium dichloride, DHV(2+)) on a Cu(100) electrode was investigated in a chloride-containing electrolyte solution by cyclic voltammetry (CV) and in situ electrochemical scanning tunneling microscopy (EC-STM). The dicationic DHV molecules generate a few pairs of current waves in CV measurements which are ascribed to two typical one-electron transfer steps. STM images obtained in a KCl-containing electrolyte solution disclose a well-ordered c(2x2) chloride adlayer on a Cu(100) electrode surface. After injecting DHV(2+) molecules into the KCl electrolyte solution, a highly ordered 2D "dot-array" structure in STM images emerges on the c(2x2)-Cl modified Cu(100) electrode surface. DHV(2+) molecules spontaneously arrange themselves with their molecular planes facing the electrode surface and their long molecular axis parallel to the step edge. Such adsorption structure can be described by mirror domains and rotational domains which stably exist between 200 mV and -100 mV. One-electron reduction of the dications DHV(2+) around -150 mV causes a phase transition from a 'dot-array' assembly to a stripe pattern formed by DHV(*+) radical monocations in STM images which has a bilayer structure. With a further decrease of the applied electrode potential, the structure of the DHV(*+) adlayer undergoes a change from a loose stripe phase to a more compact stripe phase, a subsequent decay of the compact structure, and finally the formation of a new dimer phase. A further electron transfer reaction at -400 mV causes the formation of an amorphous phase on the chloride free electrode surface. In a reverse anodic sweep, the reproduction of the ordered DHV(*+) stacking phase occurs again on top of the chloride lattice.

Structure Determination of Au on Pt(111) Surface: LEED, STM and DFT Study

“…Ultra-thin epitaxial film systems exhibit a variety of interesting properties due to the strong correlation between the electronic structure of the film and its morphology, strain, and defect structure [1,2,3,4,5,6,7,8,9,10]. Structural studies of fcc/fcc systems provide a great deal of information on the connection between the geometrical properties of the adsorbed atomic layers and the atomic arrangements of the substrates.…”

Faradaic Phase Transition of Dibenzyl Viologen on an HOPG Electrode Surface Studied by In Situ Electrochemical STM and Electroreflectance Spectroscopy

“…Apart from HOPG, Jiang and coworkers have recently observed a stripe pattern of adsorbed HV •+ on a chloride modified Cu(100) electrode surface by in situ STM. 35 In their model, the main molecular axis of HV •+ is parallel to the surface in a side-on configuration, exhibiting the formation of π-stacked polymer chains.…”

“…Adsorption layers of organic molecules on electrode surfaces frequently exhibit sharp phase transitions triggered by the potential change. − A better understanding of the potential-driven phase transitions together with the molecular level adsorption structures would lead us to the development of devices that show drastic, sharp, and chemically reversible responses to the potential change at electrified interfaces. Viologens bearing long alkyl chains are among the molecules that exhibit faradaic phase transition, − in many cases of the first-order, on several electrode surfaces. The phase transition is represented by sharp spike peaks in cyclic voltammograms (CVs).…”

Formation of benzyl viologen-containing films at copper and their protective properties

“…There have also been some reports on the adsorption of viologens at copper and copper modified surfaces. Jiang et al [10] have reported the potential-dependent adsorption of heptyl viologen on a Cu(100) surface in a chloride-containing electrolyte. In the absence of the viologens, a highly ordered chloride adlayer was observed on the Cu(100).…”