Phase-controlled solvothermal synthesis and morphology evolution of nickel sulfide and its pseudocapacitance performance

Phase-controlled solvothermal synthesis has been proposed for the synthesis of nickel sulfide of single phase including α-NiS, Ni3S4 and NiS2 by tuning the reaction time and the addition of surfactant. The phase evolution of nickel sulfide proceeds with the increase of sulfur stoichiometry with longer reaction time in the presence of surfactant. With the addition of hexadecyl-trimethyl-ammonium bromide (CTAB), a higher sulfur stoichiometry NiS2 phase with hollow sphere geometry was synthesized at 9 h, a much shorter time due to the enrichment of S2− on CTAB micelle surface, followed by the transformation to single phase Ni3S4 finally due to dissipation of enriched sulfur to the bulk solution. The application of these three single phase materials in supercapacitors was investigated. The α-NiS electrode material outperformed the Ni3S4 and NiS2 electrodes, exhibiting a much higher specific capacitance of 800 F g−1 at 0.5 A g−1, attributable to the small particle size, high electrical conductivity and the unique hexagonal crystal structure.