Phase control during the synthesis of nickel sulfide nanoparticles from dithiocarbamate precursors

Abstract

Square-planar nickel bis(dithiocarbamate) complexes, [Ni(S₂CNR₂)₂], have been prepared and utilised as single source precursors to nanoparticulate nickel sulfides. While they are stable in the solid-state to around 300 °C, heating in oleylamine at 230 °C, 5 mM solutions afford pure α-NiS, where the outcome is independent of the substituents. DFT calculations show an electronic effect rather than steric hindrance influences the resulting particle size. Decomposition of the iso-butyl derivative, [Ni(S₂CNⁱBu₂)₂], has been studied in detail. There is a temperature-dependence of the phase of the nickel sulfide formed. At low temperatures (150 °C), pure α-NiS is formed. Upon raising the temperature, increasing amounts of β-NiS are produced and at 280 °C this is formed in pure form. A range of concentrations (from 5–50 mM) was also investigated at 180 °C and while in all cases pure α-NiS was formed, particle sizes varied significantly. Thus at low concentrations average particle sizes were ca. 100 nm, but at higher concentrations they increased to ca. 150 nm. The addition of two equivalents of tetra-iso-butyl thiuram disulfide, (ⁱBu₂NCS₂)₂, to the decomposition mixture was found to influence the material formed. At 230 °C and above, α-NiS was generated, in contrast to the results found without added thiuram disulfide, suggesting that addition of (ⁱBu₂NCS₂)₂ stabilises the metastable α-NiS phase. At low temperatures (150–180 °C) and concentrations (5 mM), mixtures of α-NiS and Ni₃S₄, result. A growing proportion of Ni₃S₄ is noted upon increasing precursor concentration to 10 mM. At 20 mM a metastable phase of nickel sulfide, NiS₂ is formed and as the concentration is increased, α-NiS appears alongside NiS₂. Reasons for these variations are discussed.