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

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 Al(2)XYZ (X/Y/Z = S, Se, Te, X not equal Y not equal Z) by first-principles calculations. The results show that Janus monolayers Al(2)XYZ, 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 x 10(4) cm(2) 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 Al(2)XYZ monolayers possess extraordinary flexibility. The in-plane d(11) and the out-of-plane piezoelectric coefficient d(31) are up to 41.28 pm V-1 and 0.32 pm V-1. Notably, the in-plane piezoelectric coefficient d(11) and the out-of-plane d(31) are highly adjustable by applying strain/stacking. For Al2SeTeS, they increased by approximately one order of magnitude (d(11): from 41.28 pm V-1 to 405.63 pm V-1, d(31): from 0.07 pm V-1 to 0.44 pm V-1). These outstanding properties allow Al(2)XYZ monolayers to be widely used in photocatalysis, flexible nanodevices and piezoelectric sensors.