Scalable Route to Colloidal NiCo_3-S₄ Nanoparticles with Low Dispersity Using Amino Acids

The thiospinel group of nickel cobalt sulfides (NiCo_3-S₄) are promising materials for energy applications such as supercapacitors, fuel cells, and solar cells. Solution-processible nanoparticles of NiCo_3-S₄ have advantages of low cost and fabrication of high-performance energy devices due to their high surface-to-volume ratio, which increases the electrochemically active surface area and shortens the ionic diffusion path. The current approaches to synthesize NiCo_3-S₄ nanoparticles are often based on hydrothermal or solvothermal methods that are difficult to scale up safely and efficiently and that preclude monitoring the reaction through aliquots, making optimization of size and dispersity challenging, typically resulting in aggregated nanoparticles with polydisperse sizes. In this work, we report a scalable "heat-up" method to colloidally synthesize NiCo_3-S₄ nanoparticles that are smaller than 15 nm in diameter with less than 15% in size dispersion, using two inexpensive, earth-abundant sulfur sources. Our method provides a reliable synthetic pathway to produce phase-pure, low-dispersity, gram-scale nanoparticles of ternary metal sulfides. This method enhances the current capabilities of NiCo_3-S₄ nanoparticles to meet the performance demands to improve renewable energy technologies.