Sulfur Extraction for Stoichiometry Control of Cobalt Sulfides by Reducing Gas Annealing

In this study, we propose an anion vacancy engineering process for CoS₂-based nanomaterials using carbon monoxide (CO) thermal treatment. This process enables precise control over sulfur vacancy formation and phase transitions while preserving structural integrity through the tuning of variables such as temperature, annealing time, and gas composition.

Notably, changes in the oxidation state of cobalt and the chemical state of sulfur were systematically observed as a function of the reaction conditions, confirming that the controlled structural evolution of cobalt sulfides was mediated by selective sulfur extraction. Moreover, the values from thermodynamic calculations were in good agreement with the experimental results, demonstrating that the sulfur extraction process follows a thermodynamically favorable pathway.

The versatility of this approach extends beyond specific materials or processing conditions and can be applied to a wide range of transition metal compounds. Additionally, the process demonstrates excellent scalability, as it is not constrained by sample quantity. The ability to finely control structural and chemical properties highlights the applicability of this method to various fields where defect engineering is critical, such as electrocatalysis and energy storage devices.

In particular, sulfur-deficient CoS₂ demonstrated superior electrocatalytic performance, with an overpotential of only 327 mV at 10 mA cm^− 2 for the oxygen evolution reaction, outperforming conventional noble metal catalysts by approximately 60 mV. This result underscores the practical value and broad applicability of the proposed process.