“Amorphous Nickel Sulfide” Is Hydrated Nanocrystalline NiS with a Core−Shell Structure

The application of a range of experimental techniques shows that "amorphous nickel sulfide" (the material precipitated from aqueous solutions of Ni(II) salts and S(II-) under ambient conditions) is actually a hydrated nanoparticulate material with an approximate formula NiS.1.5H(2)O. The particles comprise a crystalline, anhydrous core (diameter ca. 1-3 nm) with the millerite (NiS) structure, surrounded by a hydrated shell phase. The materials prepared under acidic conditions (pH = 3 and 5) transform with age to form polydymite (Ni(3)S(4)) and heazlewoodite (Ni(3)S(2)), while materials prepared at pH = 7 and 9 do not undergo this transformation. At pH = 12, the preparation procedure yields NiAs-type NiS as a metastable phase.