Two-dimensional MX 2 Y 4 systems: ultrahigh carrier transport and excellent hydrogen evolution reaction performances.

Very recently, two-dimensional MoSi 2 N 4 has been synthetized (Y.-L. Hong, Z. Liu, L. Wang, T. Zhou, W. Ma, C. Xu, S. Feng, L. Chen, M.-L. Chen and D.-M. Sun, Chemical vapor deposition of layered two-dimensional MoSi 2 N 4 materials, Science , 2020, 369 , 670-674.). In this work, we systematically explore the mechanical, electronic, and catalytic properties of the MX 2 Y 4 (M = Cr, Hf, Mo, Ti, W, Zr; X = Si, Ge; Y = N, P, As) monolayers by first-principles calculations. These observed monolayers exhibit an isotropic Young's moduli of 165-514 N m -1 and a Poisson's ratio of 0.26-0.33. The calculated band structures indicate that their bandgaps are in the range of 0.49-2.05 eV at the HSE06 level. In particular, a high electron mobility of about 1.04 × 10 4 cm 2 V -1 s -1 is observed in TiSi 2 N 4 monolayers, which shows potential for high-speed electronic devices. MX 2 Y 4 monolayers also reveal decent performances in the hydrogen evolution reaction. More importantly, the Gibbs free energy change of the TiSi 2 N 4 (ZrSi 2 N 4 ) monolayer is as small as 0.078 eV (-0.035 eV), even being comparable with that of Pt (-0.09 eV). This investigation suggests that the MoSi 2 N 4 family monolayers have potential advanced applications such as photocatalytic, electrocatalytic, and photovoltaic devices.