Precise Stoichiometry‐Controlled Phase Engineering of Cobalt Sulfides

ABSTRACT Phase transformation represents a key strategy for designing two‐dimensional (2D) materials with tailored properties. Among various approaches, stoichiometry modulation is particularly effective, as 2D systems often host multiple thermodynamically stable phases with distinct compositions. However, precise stoichiometry control remains challenging due to comparable formation energies and intertwined kinetic pathways of competing phases. Here, we devise a distance‐gradient chemical vapor deposition (DG‐CVD) method that spatially tunes precursor concentrations, enabling controlled synthesis of cobalt sulfides with distinct compositions, including CoS 2 , CoS, and Co 9 S 8 . Through this approach, we realize two complementary routes to Co 9 S 8 —direct vapor‐phase growth and solid‐state conversion from hexagonal CoS—demonstrating that precise control of the reaction environment (sulfur chemical potential and temperature) enables phase selection. Ex situ scanning transmission electron microscopy (STEM) resolves the kinetic phase transition from CoS to Co 9 S 8 through a transient tetragonal intermediate . On‐chip electrochemical measurements reveal that Co 9 S 8 exhibits a low hydrogen evolution reaction (HER) overpotential of ∼173 mV at 10 mA cm −2 with improved kinetics. Density functional theory (DFT) calculations attribute the enhanced activity to charge redistribution and an upshift of the Co d ‐band center. These findings underscore that stoichiometry control and kinetic insight drive scalable phase engineering and rational design of 2D materials.