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 Γ 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 (∼3 × 10⁴/cm) in the visible light region and an even larger absorption coefficient (∼10⁵/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.