Preparation and phase transformation of nanocrystalline copper sulfides (Cu9S8, Cu7S4 and CuS) at low temperature

Nanocrystallites of non-stoichiometric copper sulfides were synthesized via the reaction between [Cu(NH3)(4)](2+) and thiourea in sealed autoclaves at the low temperature of 60 degrees C for 3 h. Thiourea played a key role in the process of Cu9S8 nanocrystal formation. Through adjusting the redox atmosphere, Cu7S4 and CuS were obtained from the freshly formed Cu9S8 nanocrystals. The products were characterized by means of X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), FT-IR spectroscopy and transmission electron microscopy (TEM) techniques. The Cu : S atomic ratios for Cu9S8 and Cu7S4 were 9 : 8.08 and 7 : 4.05, respectively, analyzed by inductively coupled plasma (ICP) spectroscopy. The possible mechanism of phase transformation among copper sulfides (Cu9S8, CuS, and Cu7S4) was discussed. The optical properties of the products were also recorded by means of UV-vis absorption, and photoluminescence spectroscopy.