Cobalt sulfide (CoS x ) nanocrystallites capped with mercaptoacetic acid (MAA) were synthesized via a colloidal precipitation method, with reaction temperature and precursor ratios systematically varied to control phase formation and particle size. Single-phase cubic Co 9 S 8 nanocrystallites were obtained at optimized conditions, while the residual supernatant yielded mixed-phase cobalt sulfides upon secondary treatment. Comprehensive characterization using EDX, XRD, TGA, FTIR, UV-Vis, and photoluminescence spectroscopy revealed that first-precipitation nanocrystallites are cobalt-rich, thermally stable up to 1000 °C, and exhibit strong quantum confinement effects with tunable band gaps. Microwave-treated supernatant samples demonstrated increased sulfur content, modified surface coordination, and mixed-phase formation. The nanocrystallites displayed absorption edges in the near-infrared region and energy level splitting indicative of strong confinement. These findings highlight the potential of MAA-capped cobalt sulfide nanocrystallites for optoelectronic, photonic, and energy-related applications, including supercapacitors and photocatalytic hydrogen production.
Malafekh et al. (2026) studied this question.