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 (M X: 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 Sc CO , Sc CO H , and Ti CO . Besides, the differences between MXenes and MoS 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.