Characterization of Redox States of Nickel Hydroxide Film Electrodes by In Situ Surface Raman Spectroscopy

Abstract

Thin films of nickel hydroxide deposited on gold electrodes have been characterized in detail by in situ surface Raman spectroscopy in conjunction with electrochemical techniques. Raman spectra were obtained for film thicknesses varying from less than one equivalent monolayer to several hundred monolayers, as determined from the faradaic charge for the cyclic voltammetric oxidation of Ni ( OH ) 2 . For the thinnest films, Raman bands at 455 cm −1 and at 480 and 560 cm −1 were obtained for the reduced and oxidized films, respectively, using 647.1 nm excitation at roughened gold. These signals, identified with Ni‐OH and Ni‐O vibrations from deuterium isotope data, were diagnosed as arising from surface‐enhanced Raman scattering (SERS) in view of their absence for the reduced film when using smooth gold surfaces and/or green/blue laser excitation. Raman spectra were obtained using the latter conditions for thicker oxidized films, which were consistent with resonance Raman scattering (RRS). Analysis of the dependence of the 480 / 560 cm − 1 band intensities as a function of film thickness under conditions where SERS or RRS predominates enables the relative contributions of these mechanisms as well as the influence of film absorbance to be assessed quantitatively. Raman spectra were also obtained in air and after “film aging” by potential cycling or heating. The spectral changes following the latter treatments support the evolution of a less hydrogen‐bound Ni ( OH ) 2 film structure, i.e. , the transformation of α ‐ to β ‐ Ni ( OH ) 2 . By combining cyclic voltammetry with analysis of the dissolved film using atomic absorption spectroscopy, the effective oxidation state of nickel in the oxidized films is determined to be + 3.7 ( ± 0.1 ) . On the basis of the spectroscopic and electrochemical measurements, the most likely structure for the oxidized film is a hydrated form of M ( NiO 2 ) 3 , where M is the supporting electrolyte cation.