Two-dimensional Janus square-XAu4Y (X, Y = S, Se, and Te, X ≠ Y) monolayers with built-in electric field: Potential candidates for photocatalytic overall water splitting

• Janus square-XAu 4 Y monolayers possess intrinsic built-in electric fields. • Their band edge alignments are suitable for overall water splitting. • They have high carrier mobility and strong optical absorption . • The largest solar-to-hydrogen efficiency of these monolayers is 28.5%. • Strain engineering can be applied to regulate their photocatalytic performances. The intrinsic built-in electric field (BEF) of the two-dimensional (2D) Janus monolayer effectively suppresses the recombination of photoexcited holes and electrons, thereby improving photocatalytic efficiency. Through first-principles calculations, we have revealed a new family of Janus square-XAu 4 Y (X, Y = S, Se, and Te, X ≠ Y) monolayers, which show great promise as photocatalysts for water splitting. Our calculations indicate that this new family exhibits an intrinsic BEF, a direct bandgap, strong optical absorption across the infrared to visible light range, high electron mobility, and a significant disparity between electron and hole mobilities—key factors that boost photocatalytic performance. In an alkaline environment (pH = 10), the band edge alignments of SAu 4 Te and SeAu 4 Te monolayers fulfill the requirements for overall water splitting (OWS). The solar-to-hydrogen (STH) efficiency of 28.5 % for SAu 4 Te and 23.9 % for SeAu 4 Te surpasses the theoretical limit of 18 %. Moreover, both monolayers retain a high STH efficiency of around 25 % even under 5 % biaxial tensile strain. Present findings extend the scope of photocatalysts for OWS to Janus square-XAu 4 Y monolayers, which is expected to inspire the future experimental fabrications and the discovery of other types of 2D Janus materials for photocatalytic water splitting.