Graphene-templated growth of hollow Ni₃S₂ nanoparticles with enhanced pseudocapacitive performance

Droplet-shape hollow Ni₃S₂ nanoparticles, as well as corresponding partially nickel-filled nanoparticles, of narrow diameter distribution and uniform dispersion were successfully synthesized on two-dimensional graphene templates using a facile process with moderate reaction conditions. The nanoparticle composites were examined as electrochemical supercapacitor materials for energy storage application. We found that the shape of the nanoparticles is dominantly droplet-shape, with shape complementary to graphene support, which ensures good contact between them. The height of the nanoparticles increases linearly with the diameter with a coefficient of 0.44 from the fitting results, and the average height/diameter ratio of those nanoparticles is about 0.6, evidence that the nanoparticles have strong interaction with the graphene template, partially because of graphene-nickel ion interaction which ensures good surface wetting. Such a composite of droplet-shape hollow Ni₃S₂ nanoparticles grown on reduced graphene oxides (rGOs) exhibits a high specific capacitance of 1022.8 F g^-1 at scanning rate of 2 mV s^-1, with a value of 1015.6 F g^-1 obtained at a discharge current density of 1 A g^-1. Improvement of the rate capability can be further obtained by partially filling the hollow core with nickel metal, as 93.6% of the specific capacitance is retained with this structure by increasing the discharge density from 1 A g^-1 to 10 A g^-1. Our method provides a new approach for controlling the structure of graphene-based nanocomposites, with the potential for use in high performance supercapacitor applications.