Internal and external electric field tunable electronic structures for photocatalytic water splitting: Janus transition-metal chalcogenides/C3N4 van der Waals heterojunctions

Structural symmetry-breaking plays a crucial role in determining the electronic structures of two-dimensional (2D) materials. Janus transition-metal dichalcogenides (TMDs) break the out-of-plane mirror symmetry of TMD and induce a built-in electric field in the 2D Janus TMD to present rather exotic properties with applications. There forms a coupled electric field by the intra-layer and inter-layer of Janus TMDs with the stacking van der Waals (vdW) heterojunctions, which can effectively change the intrinsic property and reconstruct the band structure. Here, vdW heterojunction composed of Janus WXY (X/Y = S, Se and Te; X ≠ Y) and C3N4 is explored by using the first-principles calculation method. The results show that all the vdW heterojunctions, except WSTe/C3N4, can meet the band gap and band-edge position requirements of photocatalytic water splitting when they are modulated by a limited external electric field. The detailed electronic analysis shows that the external electric field will affect the band-edge position and further change the band gap of the type-II vdW heterojunction. Finally, only WSeTe/C3N4 vdW heterojunction possessing high optical absorption strength and broad spectrum width is the best candidate for photocatalytic water splitting. This work provides a new understanding and stimulates more studies for photocatalytic water splitting in Janus based vdW heterojunctions.