Prediction of T- and H-Phase Two-Dimensional Transition-Metal Carbides/Nitrides and Their Semiconducting-Metallic Phase Transition

The discovery of various two-dimensional (2D) nanomaterials with different phases is fundamentally and technologically intriguing, leading to diverse physical and chemical properties for expanded applications. Herein, we performed systematic first-principles calculations to explore a series of single-layer MXenes (M2X: M=Sc, Ti, V, Cr, Mn, Zr, Nb, Mo, Hf, Ta; X=C, N). The new H-phase MXenes are firstly predicted, which are compared with the usually synthesized MXenes named as T-phase MXenes on their structure and electronic properties. Semiconducting-metallic transitions could be realized by T-/H-phase transition for Sc2CO2, Sc2CO2H2, and Ti2CO2. Besides, the differences between MXenes and MoS2 on energy storage are discussed. Our work will provide valuable directions for the discovery of new MXene phases and give a systematic understanding of 2D MXenes.