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Electrodic surfaces of natural chalcopyrite and natural pyrite minerals (El Teniente mine, Chile) have been studied by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy including microanalysis (SEM/EDX). For comparison, fractured and polished mineral surfaces were also studied by XPS. In both electrodes, the formation of Fe(III) species containing oxygen were detected and Cu(II) species containing oxygen were additionally detected for chalcopyrite at advanced oxidation states. The presence of Cu(II) species containing oxygen was not detected by XPS for the initial oxidation states of the chalcopyrite. For pyrite, the present results do not allow confirmation of the presence of polysulfurs such as have been previously proposed. In both minerals, the measurements of SEM and EDX show relevant alterations in the respective surfaces when different potential values were applied. The chalcopyrite surface shows the formation of protrusions with a high concentration of oxygen. The pyrite surface shows a layer of modified material with high oxygen content. The modifications detected by XPS, SEM, and EDX allowed the explanation of the complexity of the equivalent circuit used to simulate the experimental EIS data. At high oxidation states, both minerals showed a pseudoinductive loop in the equivalent circuit, which was due to the active electrodissolution of the minerals which takes place through a surface film previously formed.

Cu x S (x → 2) thin films were obtained by sulfidization of copper thin films previously obtained by spin-coating from a dichloromethane solution of [Cu(II)(2-ethyl hexanoate)2(H2O)2] deposited on ITO substrate, irradiated with UV light and electrochemically reduced. Through cyclic voltammetry experiences performed in a 0.05 M Na2B4O7 buffer solution containing 5 mM Na2S, the electroformation mechanism of Cu x S phase is controlled by a first electron transfer, obtaining an initial formation of Cu(HS)ads that evolves to a Cu x S phase. Potentiostatic current transient recorded in the potential range of −0.8 V ≤ E ≤ −0.7 V showed that the nucleation and growth mechanism of the Cu x S phase obeys a two-dimensional instantaneous process with diffusional and charge-transfer contributions. AFM analysis of the deposits shows that Cu x S phase is preferentially deposited in the valleys left by ITO particles. The average size of Cu x S particles is close to 20 nm. Cyclic voltammetry results, electromotive force determination in the Cu/ /Cu x S galvanic cell, EDAX, and UV analysis demonstrate that the stoichiometric factor x in Cu x S is close to 2. The electro-obtained Cu x S phase was unstable and evolved to other nonstoichiometric compounds at open circuit. The processes responsible for the instability were the own oxidation of Cu x S phase and the water reduction that takes place over Cu x S and bare ITO particles. The last process was studied by electrochemical impedance spectroscopy. Photoelectrochemical measurements in the stability potential range of the Cu x S phase shows that the electro-obtained phase presents a p-type conductivity.

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