Effects of pH Value on Hydrothermal Synthesis of Pyrite CoS2 Micro-Crystals

Pyrite CoS2 micro-crytals were synthesized under hydrothermal conditions employing sodium thiosulfate pentahydrate (Na2S2O3 center dot 5H(2)O) as sulfur source and ethylenediaminetetraacetic acid disodium (EDTA-2Na) as assistant. The effects of pH value of solution on the structures and morphologies of the products were investigated. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to characterize the structural features and the morphology of the samples. The results show that the morphologies of the products could be tuned by the pH values of solutions. The pure-phase pyrite CoS2 micro-crystals with regular hollow sphere-like morphology can be obtained in acid environment (pH=1, 3), while well-faceted octahedron-shaped CoS2 micro-crystals were synthesized in neutral and alkaline environment (pH=7, 11). The mechanisms for the formations of the CoS2 micro-crystals were discussed.