Two-dimensional AlXY (X = S, Se, and Y = Cl, Br, I) monolayers: promising photocatalysts for water splitting with high-anisotropic carrier mobilities

Abstract

We report a new group of two-dimensional (2D) AlXY (X = S, Se, and Y = Cl, Br, I) monolayers by means of first-principles calculations. These AlXY monolayers were found to possess favorable dynamic, thermal, and mechanical stabilities. The calculated Young's modulus and Poisson's ratio reflect significant anisotropy in the mechanical properties of AlXY monolayers. Our electronic calculations indicate that these monolayers exhibit semiconductor character and their bandgaps range from 2.25 eV to 3.31 eV. Among these monolayers, we find that four monolayers, namely AlSI, AlSeCl, AlSeBr, and AlSeI, can be used as efficient photocatalysts. The AlSI monolayer exhibits a favorable photocatalytic activity in an acidic environment, and AlSeBr and AlSeI monolayers can serve as promising photocatalytic materials under acidic and neutral conditions. Notably, the AlSeCl monolayer shows the spontaneous overall photocatalytic water splitting in a wide range of pH conditions (0–14). Furthermore, the bandgaps and band edge positions of AlSCl, AlSBr, AlSI, and AlSeBr monolayers can be tuned to fulfill highly efficient photocatalytic water splitting in an alkaline environment (pH = 14) by the appropriate external biaxial strains. The optical absorption and charge transport properties of the AlXY monolayers are also investigated, and the results reveal that these monolayers exhibit strong absorption of ultraviolet light and high anisotropy in charge carrier transport. Therefore, the AlXY monolayers are expected to act as promising materials for photocatalytic water splitting.