Voltammetry and Electrodeposition in Acidic Aqueous Copper(II)–Thiourea Bath. Growth of Complex Copper Sulfide Nanowires

The electrochemical behavior of the CuSO4–thiourea system was investigated by cyclic voltammetry in aqueous hydrochloric and sulfuric acid electrolytes using fluorine-doped tin oxide (FTO) glass electrodes at room temperature. Electrodeposition experiments were conducted under both direct-current (constant-potential) and pulsed-potential conditions. The microstructure of the electrodeposited layers was characterized by X-ray diffraction, high-resolution electron microscopy, and SEM–EDS elemental analysis. Comprehensive resolution of the voltammetric spectra enabled the identification of the redox reactions occurring in the electrolyte and at the substrate. The results indicate that the electrodeposition process depends on complex and adsorbed species involving predominantly the Cu(I) state. Deposition proceeds via the electrochemical reduction of Cu(I)–thiourea and Cu(I)–chloride complexes to metallic copper, followed by the chemical oxidation of the deposited metal to copper sulfides and/or copper chlorides. The deposits exhibited a clustered fibrous morphology that depended strongly on the electrodeposition conditions, and consisted primarily of chalcocite and djurleite CuxS crystallites embedded within a Cu–Cl–S matrix. Well-defined nanowires were obtained under specific pulsed-potential conditions in a hydrochloric acid electrolyte (pH 0.8). Precipitation of Cu–Cl–S nanowires generated in the vicinity of the electrode surface during the anodic pulse period contributed largely to the morphology of the deposited layers.