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.