Synthesis of n-hydrated nickel sulfates from mechanically alloyed nanocrystalline nickel sulfides†
In this work, hydrated nickel sulfates (NSHs) were spontaneously obtained by storing nanocrystalline nickel sulfide composites (NiS2–NiS) under ambient conditions over several months. The samples containing different proportions of NiS2–NiS nanophases were produced by mechanochemistry by simply ball-milling Ni and S elements in Ni34S66 composition in the absence of solvent. X-ray powder diffraction (XRPD) was used to follow the evolution of phase transitions of the milled samples over about 5 years. A few weeks after the milling process, the monoclinic NiSO4·6(H2O) phase is detected. Rietveld analysis showed that the NiS2 and NiS phase fractions decrease over time, and more than 90% of nickel sulfate hexahydrate is present in the end product of the sample with a milling time of 24 h. The small crystallite size phases (∼20 nm) and high micro-strain of the nickel sulfide nanophases favor this process, suggesting that sulfate nucleation occurs at disordered regions, also called the interfacial component, and that the growth rates are determined by the exposure time of the sample to the ambient atmosphere. DSC analysis shows at least one phase transition, dehydration and the presence of sulfur in some aged samples. Magnetic measurements at room temperature indicate mixed magnetism in the fresh samples and paramagnetic behavior in the aged ones. We have also observed that the growth of the NSH phases can be accelerated by keeping the samples under high humidity conditions, reaching more than 80% of NSH phases after 35 days. The chemical reactions involved in the milling and aging, as well as a tentative model to explain this spontaneous composition transition from nickel sulfides to nickel sulfate hydrates, are presented.