Comparing Mo2CTx MXene and Ultrathin Mo2C

Abstract

Two-dimensional transition metal carbides have emerged as a material class of broad scientific and technological interest, yet terminological ambiguity conflates two fundamentally distinct systems: MXenes and ultrathin carbide crystals. Here, we provide a systematic, side-by-side comparison of these two material families using molybdenum carbide (Mo2C) as a model system, chosen because it is uniquely accessible in both forms: as termination-stabilized Mo2CTx MXene obtained by selective etching of Mo2Ga2C, and as single-crystalline ultrathin films grown by chemical vapor deposition (CVD) on Cu/Mo substrates. Through parallel analysis of crystal structure, morphology, vibrational properties, and surface chemistry, we demonstrate that the two systems differ in many aspects. CVD-grown Mo2C, with lateral dimensions up to ∼100 µm, exhibits well-faceted crystal morphology, crystallizes in the thermodynamically stable orthorhombic α-phase, and shows sharp x-ray diffraction peaks indicative of a strong preferred orientation, with the (200) planes aligned parallel to the surface, analogous to the (00l) planes in layered MXenes. In contrast, Mo2CTx MXene consists of subnanometer-thin flakes with lateral dimensions of ∼3 µm, exhibits broad (00l) diffraction peaks, and features termination-rich surface chemistry, where electronegative Tx groups (predominantly = O) modulate both Mo 3d and C 1s core-level binding energies. By combining Raman spectroscopy with first-principles DFT calculations, we assign the vibrational modes in both systems and identify a termination-induced out-of-plane A1g mode at 476 cm−1 for Mo2CTx MXene, which is absent in the pristine carbide. Based on these distinctions, we provide a synthesis-rooted comparative reference and terminology guide for distinguishing MAX-derived MXenes from CVD-grown ultrathin transition metal carbides.