Two unexplored two-dimensional MSe2 (M = Cd, Zn) structures as the photocatalysts of water splitting and the enhancement of their performances by strain

Exploring suitable photocatalysts is vital for the generation of hydrogen fuel from the solar energy via the water splitting. Penta two-dimensional (2D) materials owning to the versatile properties received much attention recently. In this work, we defined two new 2D penta structures, namely MSe2 (M = Cd, Zn), and evaluated their potentials as the photocatalysts by density functional theory (DFT) calculations. We found that both structures are mechanically, kinetically and thermally stable according to their elastic constant calculations, vibrational phonon calculations and ab initio molecular dynamics (AIMD) simulations, respectively. CdSe2 and ZnSe2 sheets are semiconductors with band gaps of 2.13 and 2.26 eV, respectively, obtained on the basis of Herd-Scuseria-Ernzerh (HSE06) functional. Their optical absorptions cover the range from the visible light region to the near-ultraviolet region. Interestingly, the band edge positions of MSe2 (M = Cd, Zn) sheets can perfectly engulf the redox potentials of the water-splitting reaction. Besides, we found that, by applying suitable strain, their band gaps and optical absorptions can be notablely improved while their suitable band edge alignments for the water splitting are still maintained. This work discloses two new 2D penta structures which are expectable to be the promising photocatalysts for water splitting.