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- 2025| ElsevierThe use of surfactants in catalyst synthesis is essential for controlling particle dimensions but often hinders catalytic performance by blocking active sites. To overcome this challenge, this study investigates a solvent-free solid-state pyrolysis approach as an alternative to colloidal synthesis with surfactants. Nickel selenobenzoate was employed as a metal-organic precursor to synthesize NiSe2 nanosheets, aiming to avoid surfactants while preserving active surface sites for catalysis. However, results indicate that merely eliminating surfactants in solvent-free synthesis is insufficient, as the decomposition of the metal-organic precursor leads to an amorphous carbonaceous residue, which negatively impacts electrochemical performance. To understand these effects, NiSe2 nanosheets synthesized via solid-state pyrolysis were compared with those produced using a conventional oleylamine-assisted colloidal approach. Electrochemical tests revealed that oleylamine-capped NiSe2 exhibited superior performance in both supercapacitance and hydrogen/oxygen evolution reactions (HER/OER), highlighting the adverse effects of carbon residue in the solid-state route. Further optimization of solid-state pyrolysis enabled controlled surface and structural modifications, leading to the formation of a NiSe2/Ni0.85Se heterostructure via gradual selenium loss, which improved HER activity. At higher temperatures, further Se loss resulted in a phase transformation from NiSe2 to Ni0.85Se while preserving the nanosheet morphology. Notably, this Ni0.85Se phase exhibited the best HER and OER performance, attributed to enhanced conductivity and the partial conversion of residual carbon into crystalline graphitic carbon at elevated temperatures. These findings underscore the need for careful precursor and method selection in solvent-free synthesis to optimize catalytic material performance, offering valuable insights for the design of next-generation electrocatalysts.
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