CuO Barrier Limited Corrosion of Solid Cu2O Leading to Preferential Transport of Cu(I) Ion for Hollow Cu7S4 Cube Formation

Abstract

Highly ordered, uniform Cu7S4 hollow cubes have been successfully synthesized in a mild, low-temperature condition from freshly prepared solid Cu2O cubes. Cu2O cubes have been synthesized at ∼80 °C, exploiting the water-soluble Cu(II)–EDTA complex (λmax = 730 nm) as precursor and glucose as reducing agent under alkaline conditions. In the synthetic pathway, Cu2O solid cubes act as corrosion-prone, sacrificial templates. Kinetic parameters describe the corrosion of Cu2O solid cubes in the presence of sulfide ions, which is the product of hydrolysis of thioacetamide. Corrosion results in a nonstoichiometric hollow Cu7S4 structure like a solid cubic template. Strong affinity of Cu(I) toward sulfide (“soft”–“soft” interaction) fetches Cu(I) from the central region of the solid Cu2O template, making hollow cubes of Cu7S4. Mechanistically, the thin film of the oxidized surface layer on Cu2O cubes protects the template. Then the oxidized layer offers resistance to the passage of sulfide ions for its inward transportation. Conversely, soft–soft affinity fetches Cu(I) ions from inside. Finally, hollow Cu7S4 cubes are formed at the solid–liquid interface. The transformation process has been further examined and confirmed from UV–visible spectroscopy, scanning electron microscopy, transmission electron microscopy, X-ray diffraction, Fourier transform infrared spectra, and impedance measurement. Hollow Cu7S4 cubes with increased surface area are generated from solid Cu2O cubes via Kirkendall diffusion.