pH-D ependent Aqueous Solution-Grown Highly Nanocrystalline Nickel Cobalt Sulfides (NiCo2S4)

NiCo2S4 is an important bimetallic spinel-type sulfide that is typically synthesized using a two-step method, which commonly involves water as a reaction medium. Here, we show that highly crystalline and high surface area NiCo2S4 could be synthesized into a nanoflake-entangled shape. Control of solution pH under a hydrothermal condition affects the compositions and morphologies of products. It reveals that the pK(a)s of two transition metal ions and l-cysteine (l-Cys) play a crucial role in the formation of NiCo2S4. Various spectroscopic analyses were performed to characterize these products, including X-ray diffraction, energy-dispersive X-ray spectroscopy, gas sorption, scanning electron microscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy.