(NH4)2S, a highly reactive molecular precursor for low temperature anion exchange reactions in nanoparticles.

Ammonium sulfide ((NH4)2S) exhibits high reactivity as a sulfide reagent in anion exchange reactions that transform CoO to cobalt sulfide nanoparticles (NPs). The faster diffusion of Co(2+) and O(2-) than the incoming S(2-) 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.