Density functional prediction of the stabilities, electronic, optical, and photocatalytic properties of 2D Janus XMoNH (X = S and Se) monolayers

The stabilities, electronic, optical, and photocatalytic properties of two-dimensional (2D) Janus SMoNH and SeMoNH are studied by first-principles calculations. Good thermodynamic, lattice dynamic, thermal, and mechanical stabilities are demonstrated for these two 2D Janus materials. The HSE06 hybrid functional calculation reveals direct bandgaps of 2.31 eV and 2.19 eV for 2D Janus SMoNH and SeMoNH, respectively. Their effective masses and optical absorption coefficients are anisotropic. Along the a(xx) and b(yy) directions, their light absorptions cover from visible light to ultraviolet light. The internal electric fields make photogenerated electrons and holes separate at two opposite surfaces during visible-light-driven water splitting. 2D Janus SMoNH enables photocatalytic water splitting over a broad pH range of 0–14, whereas 2D Janus SeMoNH is effective only from pH = 7 to pH = 14. Under biaxial compressive strain, their bandgaps increase firstly and then decrease, while biaxial tensile strain reduces their bandgaps monotonically. Within the visible spectrum, biaxial compressive and tensile strains induce their optical absorptions blue- and red-shifts, respectively. For the photocatalytic water splitting, the 2D Janus SMoNH remains a broad pH range of 0–14 within biaxial strain from -6 % to +6 %, while the pH range of 2D Janus SeMoNH keeps 7–14 only under biaxial tensile strain.