Promoting 2D Growth in Colloidal Transition Metal Sulfide Semiconductor Nanostructures via Halide Ions

Wet-chemically synthesized 2D transition metal sulfides (TMS) are promising materials for catalysis, batteries and optoelectronics, however a firm understanding on the chemical conditions which result in selective lateral growth has been lacking. In this work we demonstrate that Ni₉S₈, which is a less common nonstoichiometric form of nickel sulfide, can exhibit two-dimensional growth when halide ions are present in the reaction. We show that the introduction of halide ions reduced the rate of formation of the nickel thiolate precursor, thereby inhibiting nucleation events and slowing growth kinetics such that plate-like formation was favored. Structural characterization of the Ni₉S₈ nanoplates produced revealed that they were single-crystal with lateral dimensions in the range of ∼100–1000 nm and thicknesses as low as ∼4 nm (about 3 unit cells). Varying the concentration of halide ions present in the reaction allowed for the shape of the nanostructures to be continuously tuned from particle- to plate-like, thus offering a facile route to controlling their morphology. The synthetic methodology introduced was successfully extended to Cu₂S despite its different growth mechanism into ultrathin plates. These findings collectively suggest the importance of halide mediated slow growth kinetics in the formation of nanoplates and may be relevant to a wide variety of TMS