Electrosynthesis and electrochemical characterization of a thin phase of Cu_xS (x ← 2) on ITO electrode

Cu_xS (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_xS phase is controlled by a first electron transfer, obtaining an initial formation of Cu(HS)_ads that evolves to a Cu_xS 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_xS phase obeys a two-dimensional instantaneous process with diffusional and charge-transfer contributions. AFM analysis of the deposits shows that Cu_xS phase is preferentially deposited in the valleys left by ITO particles. The average size of Cu_xS particles is close to 20 nm. Cyclic voltammetry results, electromotive force determination in the Cu/Cu_aq²⁺/Cu_xS galvanic cell, EDAX, and UV analysis demonstrate that the stoichiometric factor x in Cu_x is close to 2. The electro-obtained Cu_xS phase was unstable and evolved to other nonstoichiometric compounds at open circuit. The processes responsible for the instability were the own oxidation of Cu_xS phase and the water reduction that takes place over Cu_xS and bare ITO particles. The last process was studied by electrochemical impedance spectroscopy. Photoelectrochemical measurements in the stability potential range of the Cu_xS phase shows that the electro-obtained phase presents a p-type conductivity.