Controlled synthesis of nickel sulfide polymorphs: studies on the effect of morphology and crystal structure on OER performance

Herein, the Taguchi L-9 orthogonal technique has been employed to design the optimised conditions (sulfur source, temperature, reaction time and surfactant) for the preparation of nickel sulfide polymorphs using hydrothermal method. The experimental results demonstrate the temperature dependant phase transition of NiS → Ni3S4 → NiS2 and the prepared nickel sulfide polymorphs exhibit distinctive morphologies of sugar cubes, spherical apple, rose type, aggregated stones, and tubular bacteria type structures. The effect of morphology and crystalline structure on oxygen evolution reaction (OER) has been investigated for the first time where the OER performance follows the order: NiS > NiS2 > Ni3S4. Among the various morphologies of NiS, and NiS2, the order of OER performance is as follows: NiS(sugar-cubes) > NiS(agglomerated stone particles) > NiS(apple shaped) and NiS2(aggregated stoneparticles) > NiS2(rose type) > NiS2(tubular bacteria) shaped nanostructure.