Selective transformation of Cu nanowires to Cu₂S or CuS nanostructures and the roles of the Kirkendall effect and anion exchange reaction

The selective chemical transformation of Cu nanowires to Cu₂S or CuS nanostructures using simple, low-cost and scalable aqueous solution-based sulfurization at low temperatures are demonstrated. The reactive sacrificial Cu nanowire templates with controlled diameters of 270 and 35 nm were synthesized by template-directed electrochemical deposition using anodic alumina templates, and these templates were reacted with a 0.1 M thiourea solution at 90 °C for 10 h for chemical conversion to Cu₂S or CuS nanostructures. Time-series sulfurization experiments indicate that the tubular shape evolved with continuous expansion of Kirkendall voids, which were generated by the differences in outward and inward diffusion rates of Cu and sulfur in Cu sulfide. In addition, a detailed investigation of the effect of the valence states of the sacrificial template surfaces on the stoichiometry of Cu_xS suggest that the anion exchange reaction in the initial stage of the transformation reaction is important in determining the valence states of the final products.