Abstract: In-situ plasma processing serves as a cost-effective strategy for modulating surface structure while simultaneously functionalizing material surface, owing to the low-contamination environment and its capability to induce strong surface-substrate interaction. However, current research primarily focuses on correlating plasma discharge parameters with the resultant surface morphology and facet orientation, often overlooking the dynamic evolution of critical parameters during plasma processing, particularly the surface thermal field. This leads to suboptimal surface structure modulation, thereby limiting the practical applicability of plasma-based surface engineering. Herein, we introduce a facile cooling-mediated N₂-plasma processing strategy to directly engineer iron nitride nano-framework on Fe surface, while concurrently modulating the surface facets. Operando plasma diagnostics, combined with numerical simulations, are employed to unravel the role of the surface-thermal field in governing the formation of catalytically favorable facets. Given the strong dependence of hydrogen evolution reaction (HER) behavior on surface structure, the resultant iron nitride frameworks via cooling-mediated plasma processing (cFeNC) exhibit improved catalytic performance compared to those fabricated through conventional thermally preserved plasma (hFeN). Density functional theory (DFT) calculations further confirm that the enhanced catalytic behaviors of cFeNC arise from the preferential exposure of highly reactive facets. Our strategy presents a cost-effective pathway for facet engineering of nitride surfaces, providing a promising route towards advanced electrocatalytic materials.

Key words: Surface structural modulation, Nitride formation mechanism, Selective facet exposure, Electrocatalytic H₂ evolution