Development of line-imaging Raman spectroscopy for use with electrochemical systems

Abstract

This work presents the construction and application of a line-imaging Raman spectroscopy instrument for use with electrochemical systems. Line-imaging spectroscopy is able to collect spatially resolved spectra along a contiguous imaging line. Specifically studied was an electrodeposited thin film of CuSCN for possible use in solar cell applications, copper plating as a model electrodeposition system, and nickel hexacyanoferrate derivatized electrodes for possible applications as an electrically controlled cation exchange material. Both solution phase and surface imaging studies were performed. For solution studies, line-imaging Raman spectroscopy was used to simultaneously probe solution and surface chemistry of a thin film of electrodeposited CuSCN, It is shown that the instrument is able to spectrally and spatially identify CuSCN on a wire electrode surface, and Cu(SCN)$\sp+$ species in solution. Images were collected with 5 $\mu$m spatial resolution and 8 cm$\sp{-1}$ spectral slitwidth under the experimental conditions used. The ability to directly image concentration boundary layers during plating is also shown using a copper electrodeposition system. The concentration boundary layer resulting from deposition on a wire electrode was directly imaged in situ at several deposition rates. It is shown that the concentration boundary layer thickness increases, and the surface concentration decreases with deposition rate, as expected. Concentration boundary layer thicknesses of less than 100 $\mu$m were reproducibly resolved.In situ Raman line-images acquired along the surface of nickel hexacyanoferrate derivatized electrodes were used to probe the oxidation and reduction of the thin (ca. 80 nm) surface film. The CW Raman modes of this material are sensitive to oxidation state of the iron-centers in the lattice. A multivariate principle component regression model was developed that predicts the oxidation state in the film lattice based on spectral features in the line-images. The combination of line-imaging Raman spectroscopy and multivariate modeling was used to produce spatial profiles of oxidation state in the film with 5 $\mu$m resolution. Repeated redox cycling of the derivatized surface is shown to cause a loss in the ability of iron centers in the lattice to reversibly switch between Fe$\sp{\rm II}$ and Fe$\sp{\rm III}$, causing the derivatized layer to lose ion exchange capacity. Moreover, the oxidation state of the derivatized electrodes was found to switch nonuniformly across the surface as potential was modulated. As the redox activity of the derivatized electrodes declines, the nickel hexacyanoferrate ultimately reaches a spatially uniform mixed valence state with Fe$\sp{\rm II}$:Fe$\sp{\rm III}$ in a ratio of 4:1. The photoresponse of nickel hexacyanoferrate to 647.1 nm illumination was found to complicate the interpretation of Raman line-images.