Ammonium sulfide ((NH4)2S) exhibits high reactivity as a sulfide reagent in anion exchange reactions that transform CoO to cobalt sulfidenanoparticles (NPs). The faster diffusion of Co2+ and O2− than the incoming S2− during the anion exchange causes a significant expansion of the NP voids. The low temperature (70 °C) anion exchange reaction produces amorphous cobalt sulfide NPs with Co:S ratio of ca. 3:4, which are converted into crystalline NPs with a major phase of cubic Co3S4 by annealing at high temperature in an organic solution.
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