XPS, SEM, EDX and EIS study of an electrochemically modified electrode surface of natural chalcocite (Cu2S)

An electrode surface of a natural chalcocite mineral obtained from the ‘Chuquicamata’ mine in Chile has been studied by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy including microanalysis (SEM/EDX). Different potentials were applied via CV to the electrode surface in a specially designed preparation chamber of inert (Ar gas) atmosphere. This chamber was coupled, in the case of XPS analysis, to the X-ray photoelectron spectrometer and, in the case of SEM/EDX analysis, to the electron microscope in order to study quasi-in-situ changes in the chemical composition and morphology at the electrode surface induced by the electrochemical treatment and to relate them to the EIS results obtained for the same applied potential. CV was performed in an aqueous 0.05 M borax electrolyte solution of pH 9.2 at 300 K (cycles: 0→−850 mV (cathodic sense: reduction)→+200 mV (anodic sense: oxidation)→−400 mV, all vs. SCE). Along the cyclic voltammogram, which shows two anodic peaks, ten points of applied potential (for 1 or 400 s) have been studied by XPS. Partial oxidation from Cu(I) to Cu(II) is observed at +100 mV versus SCE applied for 400 s which is assigned to the formation of CuO, Cu(OH)2 and possibly Cu3(SO4)(OH)4. For comparison, the fractured mineral surface was also studied by XPS. The measurements of SEM and EDX did not show any relevant alterations, except for the potential of +100 mV versus SCE in the positive-going potential, in which the formation of protrusions with a high concentration of oxygen (44%) was detected. The modifications detected by XPS, SEM and EDX reflect large changes in the electrochemical parameters obtained using EIS and are characteristic of a partially covered electrode; the charge-transfer resistance across the irregularly formed layer on the surface increases for the point +100 mV versus SCE in the positive-going potential and its capacitance is reduced.