Tunable structural phases and electronic properties of group V MSi2N4 (M = V, Nb, Ta) nanosheets via surface hydrogenation: a first-principles study

In recent experiments, researchers have discovered a new family of layered MA(2)Z(4) materials, which exhibit fascinating phase-dependent electronic properties. Here, we have performed a comprehensive first-principles calculation to investigate the hydrogenation effects on the structural phases and electronic properties of group V MSi2N4 (M = V, Nb, Ta) nanosheets. It is found that the hydrogenation will induce structural transitions through layer gliding in both the SiN surface layers and the central MN2 part. The tetrahedral coordination of Si atoms will be transformed into the octahedral one, which gives rise to an H alpha-to-H beta transition in the semihydrogenated system. When the system is fully hydrogenated, the central MN2 layer will further experience an H-to-T phase transition due to the change of d electron count. Accompanied with these phase transitions, the electronic properties of the MSi2N4 nanosheets are affected remarkably, and they will be converted into small-gap semiconductors/metals by the semi-/full- hydrogenation. In particular, due to the Janus structure of semi-hydrogenated MSi2N4H1 nanosheets, their band gaps can be substantially modulated by the electric field and high solar-to-hydrogen efficiencies of 18-20% are present in these systems. Our study demonstrates that surface hydrogenation is an effective way to tune the geometrical and electronic structures of group V MSi2N4 nanosheets, which allows them to have fascinating electronic and photocatalytic applications.