Two-dimensional T′-phase MA2N4 (M=Mo/W, A=Si/Ge) nanosheets: First-principles insights into the structural stability, electronic property and catalytic performance for hydrogen evolution reaction

For the emerging MA2Z4 materials, most researches focus on their perfect trigonal-prismatic (H-phase) and octahedral (T-phase) geometries, while the distorted octahedral (T '-phase) one is rarely studied. In this work, we perform a first-principles study on the T '-phase MA2N4 (M=Mo/W, A=Si/Ge) nanosheets to investigate their structural, mechanical, electronic and catalytic properties. As a metastable phase, the T '-MA2N4 nanosheets still possess sufficient stability from the energetic, dynamical, thermal and mechanical points of view. Unlike the semiconducting H-phase counterparts, the T '-MA2N4 nanosheets display a semimetallic or metallic behaviour depending on the lattice constants. Interestingly, these T '-MA2N4 nanosheets exhibit highly efficient catalytic performance for hydrogen evolution reaction (HER). The Gibbs free energy of hydrogen adsorption on the T '- MoGe2N4 nanosheet is close to zero, which is even superior to the well-known Pt catalyst. Furthermore, all the surface N atoms behave as active sites, giving rise to excellent basal HER activity. For these MA2N4 systems, the T '-phase will become more favourable than the H-phase via a combined effect of electron injection and tensile strain. Our study demonstrates that the T '-phase MA2N4 nanosheets possess peculiar electronic properties and promising HER activity, opening up potential applications in nano-devices and renewable energy.