Phase Evolution, Polymorphism, and Catalytic Activity of Nickel Dichalcogenide Nanocrystals

The nickel chalcogenide family contains multiple phases, each with varying properties that can be applied to an expansive range of industrially relevant processes. Specifically, pyrite-type NiS2 and NiSe2 have been used as electrocatalysts for oxygen or hydrogen evolution reactions. These pyrites have also been used in batteries and solar cells due to their optoelectronic and transport properties. The phase evolution of pyrite NiS2 and polymorphism of NiSe2 have briefly been studied in the literature, but there has been limited work focusing on the phase transformations within each of these two systems. Using experiments and calculations, we detail how pyrite NiS2 nano-crystals decompose into hexagonal alpha-NiS, and how the synthesis of pyrite NiSe2 nanocrystals is affected by the presence of two polymorphs, a metastable orthorhombic marcasite phase and a more stable cubic pyrite phase. Each reaction can be controlled by fine-tuning the reaction parameters, including temperature, time, and the precursor identity and concentration. Interestingly, both NiS2 and NiSe2 nanopyrites are active catalysts in the selective reduction of nitrobenzene to aniline, in agreement with other catalysts containing an fcc (sub) lattice. Our results demonstrate a feasible, logical process for synthesizing nanocrystalline pyrites without common byproducts or impurities. This work can help in solving a major problem suspected in preventing pyrite FeS2 and similar materials from large-scale use: the presence of small amounts of secondary phases and impurities.