Insights into the thermal decomposition and conversion mechanism of nickel xanthates to nickel sulfides†

Metal sulfides are promising materials for a wide range of applications, from environmental applications to energy conversion and storage. Like many other transition metal sulfides, nickel sulfide exists in different stoichiometries and phases, which influence their chemical and physical properties. While this feature enables the compound's diversified applications, it also makes it necessary to develop simple and reproducible methods to prepare nickel sulfide with defined composition, phase, and morphology. For metal xanthates, the design of the xanthate ligand allows to tune the properties of the metal sulfide obtained by their thermal conversion. To efficiently tailor the precursor to the application, it is imperative to understand the degradation mechanism of the precursors and the formation of the nickel sulfide phases. In this study, we synthesized a series of nickel xanthates bearing alkyl side chains of varying lengths and branching: methyl, ethyl, n-propyl, iso-propyl, iso-butyl, n-pentyl, neo-pentyl, and n-hexyl. Together with two additional nickel xanthates, we systematically investigated their thermal decomposition behavior and the resulting decomposition products using thermogravimetric analysis with coupled gas chromatography/mass spectrometry, pyrolysis gas chromatography/mass spectrometry, single crystal and powder X-ray diffraction, and grazing incidence wide angle X-ray scattering. Based on these findings, we propose a two-step decomposition mechanism that combines alkyl transfer between the ligands with an extended version of the literature-known Chugaev mechanism, which describes alkene formation from xanthates. This refined mechanism can explain the conflicting degradation products reported in literature so far. Additionally, we studied the influence of the ligand on the formed nickel sulfide using temperature dependent X-ray scattering experiments. The decomposition of the xanthates leads to the initial formation of α-NiS at low temperatures for all the precursors, followed by a phase transformation at higher temperatures. Depending on the precursor both pure α- or β-NiS and various mixed phases can be obtained.