Computational investigation on 2D Janus MSiGeN4 with structural, electronic properties, quantum capacitance, and photocatalytic activity

Inspired by the superior properties of Janus MA2Z4, the electronic properties, quantum capacitance, and photocatalytic activity of Janus MSiGeN4 (M=Cr, Hf, Mo, Nb, Ta, Ti, V, W, and Zr) monolayers are explored. The results indicate that TiSiGeN4, ZrSiGeN4, and HfSiGeN4 are semiconductors with the bandgap larger than 2 eV. NbSiGeN4 is a spin-polarization semiconductor with magnetic moment of 0.61 mu B. VSiGeN4 exhibits the character of half-metal and its spin-up channel hosts a Dirac cone around Fermi level, which is derived from V-d states. The analysis of Bader charge shows that built-in electric field in these systems appears and promotes the charge redistribution across the Janus atomic layer. All systems except NbSiGeN4 and TaSiGeN4 are cathode materials because of |Qp|/|Qn| larger than 1 in aqueous system. Broad voltage keeps the electrode type of all systems unchangeable. MSiGeN4 (M=Hf, Nb, Zr, V, Ti) can perform photocatalytic water splitting, while all MSiGeN4 can reduce N2 to NH3, but are not good CO2 photocatalysts. The Bader charge, work function, and optical properties are further explored.