Formation of Quasi-2D [Mo_n+1C_n] Layered Structures via Carburizing Metal Films toward Surface Functional Group-Free...

MXenes have a characteristic quasi-two-dimensional structure composed of covalently bonded transition metal (M) and carbon or nitrogen (X) sublayers, [M_n+1X_n], stacked with an interlayer spacing. Conventional MAX-etched MXenes inevitably have inhomogeneous surface functional groups (T_x), while chemical vapor deposition (CVD) has so far yielded only ultrathin α-Mo₂C bulk crystals, failing to form a layered structure. Here, we report a CVD approach that enables the formation of layered [Mo_n+1C_n] structures through controlled carburization of predeposited Mo films without T_x sources. The resulting films are composed of multiple MXene phases, including Mo₂C, Mo₃C₂, and Mo₄C₃ layered motifs. Regulated carbon diffusion results in discrete [Mo_n+1C_n] slabs with a well-defined interlayer spacing. In addition, a signature T_x-free, Mo-exposed surface exhibits enhanced nitrogen reduction reaction (NRR) activity compared with a CVD-grown α-Mo₂C film, achieving an NH₃ yield of 6.52 μg h^-1 cm^-2. Given the broad applicability to other MXenes, we anticipate that this work will stimulate further fundamental research and application on MXenes.