Two-Dimensional Janus van der Waals Heterojunctions SnX₂@SnSSe (X = S, Se) as Promising Solar-Driven Water-Splitting Photocatalysts

Abstract

Constructing stable two-dimensional (2D) van der Waals heterojunctions (vdWHs) as potential high-efficiency photocatalysts with an intrinsic electric field and tunable properties is of vital importance to explore direct solar-driven water splitting without sacrificial agents and cocatalysts. Herein, 16 designed 2D MX 2 @MSSe vdWHs (M = Ge, Sn; X = S, Se) by assembling MX 2 and Janus MSSe monolayers with classic T-and H-phases are comprehensively studied by density functional theory. Eight of the T-MX 2 @MSSe vdWHs are considered to be more stable than the corresponding H-phase vdWHs according to ab initio molecular dynamics calculations. The T-GeX 2 @GeSSe and T-SnX 2 @SnSSe vdWHs exhibit semiconductor properties, with tunable band gaps of 0.14−0.49 and 1.20−1.53 eV, respectively. Remarkably, the band alignments of T-SSnS@SSnSe, T-SSnS@SeSnS, T-SeSnSe@SSnSe, and T-SeSnSe@SeSnS vdWHs under an intrinsic electric field straddle the reaction potential of O 2 /H 2 O and H + /H 2 when participating in photocatalytic processes. Additionally, such four T-Sn-based vdWHs exhibit ∼10 4 cm 2 V −1 s −1 carrier mobilities, and the appreciable difference between electron and hole mobilities could render the rapid migration and weaken the recombination of photogenerated carriers. Meanwhile, the T-SnX 2 @SnSSe vdWHs possess broader visible light absorptions than those of their monolayer counterparts. In view of the outstanding characteristics for overall water splitting, the photocatalytic mechanisms of T-SnX 2 @SnSSe vdWHs have been clarified. Moreover, the chemical driving force of the photogenerated carriers in T-SnX 2 @SnSSe vdWHs can apparently facilitate redox reactions. The hydrogen reduction of T-SnX 2 @SnSSe vdWHs along with the water oxidation of T-SSnS@SSnSe and T-SeSnSe@SSnSe vdWHs can occur spontaneously under light irradiation. This work regarding 2D group IV metal chalcogenides could pave the way for competent photocatalyst system construction.