Phase-Engineered Synthesis of Mo _<i>x</i> C (α-MoC _{1– <i>x</i>} /β-Mo ₂ C) via Carbothermal Reduction and Its Application in Hydrogen Evolution Reaction

Nanoscale molybdenum carbide (MoxC) is a prominent catalyst, yet the controlled synthesis of specific crystalline phases remains challenging. Herein, we develop a straightforward electrospinning approach combined with controlled preoxidation and carbonization processes to fabricate MoxC/carbon nanofibers (MoxC/C) composites with tunable crystalline phases. By systematic variation of the carbon-to-molybdenum ratio (C/Mo) in precursor fibers, it has been confirmed that the C/Mo ratio is the critical determinant of the MoxC crystalline phase. Specifically, higher C/Mo ratios favor α-MoC1–x formation, while lower ratios selectively yield β-Mo2C. Moreover, precise regulation of the preoxidation temperature enables dynamic modulation of the C/Mo ratio in precursor fibers, driving a continuous phase transition from α-MoC1–x to β-Mo2C. This process also improves the MoxC content of the composites and significantly enhances their catalytic performance in the hydrogen evolution reaction (HER). Overall, this operationally simple strategy offers a controllable and scalable route for the synthesis of phase-selective transition-metal carbides.