Two-dimensional Janus monolayers Al2XYZ (X/Y/Z = S, Se, Te, X ≠ Y ≠ Z): first-principles insight into the photocatalytic and highly adjustable piezoelectric properties

Abstract

Janus materials possess some unique properties that are different from those of a symmetrical structure, which provides them with broad application prospects. In this work, we investigated the photocatalytic and piezoelectric properties of Janus materials Al₂XYZ (X/Y/Z = S, Se, Te, X ≠ Y ≠ Z) by first-principles calculations. The results show that Janus monolayers Al₂XYZ, with energy, mechanical, thermal, and dynamical stability, have suitable band-edge positions, broad absorption of sunlight (infrared–ultraviolet), high solar-to-hydrogen (STH) efficiency (up to 23.81%) and high carrier transfer in the x-direction (1 × 10⁴ cm² V⁻¹ s⁻¹). In addition, the photocatalytic water decomposition reaction of the Al₂SeTeS monolayer can be carried out spontaneously under an applied potential. The Young's modulus and Poisson's ratio indicated that the Al₂XYZ monolayers possess extraordinary flexibility. The in-plane d₁₁ and the out-of-plane piezoelectric coefficient d₃₁ are up to 41.28 pm V⁻¹ and 0.32 pm V⁻¹. Notably, the in-plane piezoelectric coefficient d₁₁ and the out-of-plane d₃₁ are highly adjustable by applying strain/stacking. For Al₂SeTeS, they increased by approximately one order of magnitude (d₁₁: from 41.28 pm V⁻¹ to 405.63 pm V⁻¹, d₃₁: from 0.07 pm V⁻¹ to 0.44 pm V⁻¹). These outstanding properties allow Al₂XYZ monolayers to be widely used in photocatalysis, flexible nanodevices and piezoelectric sensors.