Optical Properties and Photocatalytic Applications of Two-Dimensional Janus Group-III Monochalcogenides

Photocatalytic water splitting has received much attention for the production of renewable hydrogen from water, and two-dimensional (2D) materials show great potential for use as efficient photocatalysts. In this paper, the stabilities and electronic and optical properties of Janus group-III monochalcogenide M2XY (M = Ga and In and X/Y = S, Se, and Te) monolayers were investigated using first principles calculations. The band gaps of the Janus M2XY monolayers are in the range of 1.54-2.98 eV, which satisfies the minimum band gap requirement of photocatalysts for overall water splitting. Indirect-to-direct band gap transitions occur in the M2XTe (M = Ga and In and X = S and Se) monolayers. These transitions were induced by the valence band maximum at the Gamma point, being composed of the p(x) and p(y) orbitals of the M and Y atoms in M2XTe instead of the p(z) orbitals of the M and X atoms in the MX and other M2XY monolayers. The Janus M2XY monolayers have a considerable optical absorption coefficient (similar to 3 x 10(4)/cm) in the visible light region and an even larger absorption coefficient(similar to 10(5)/cm) in the near ultraviolet region. This study not only highlights the efficient photocatalytic performance of the 2D MX and M2XY monolayers but also provides an approach for tuning the band structures of 2D photocatalysts by forming Janus structures.