In this contribution, by employing first-principles calculations and nonadiabatic molecular dynamic simulations, we unequivocally substantiate that the ternary graphene/$\mathrm{ZrA}{\mathrm{s}}_2/\mathrm{HfA}{\mathrm{s}}_2$ van der Walls heterojunction is a highly promising photocatalyst for water splitting. First, through the investigation of stabilities, electronic structures, and optical absorption of two-dimensional $M{X}_2$ series ($M$ = Ti, Zr, Hf; $X$ = N, P, As), $\mathrm{ZrA}{\mathrm{s}}_2/\mathrm{HfA}{\mathrm{s}}_2$ heterojunction was screened out, which features a type-II staggered band arrangement with a Z-type charge transfer mode. Due to the inert activity of intrinsic $\mathrm{ZrA}{\mathrm{s}}_2/\mathrm{HfA}{\mathrm{s}}_2$, the inclusion of external potential still failed to trigger a hydrogen evolution reaction (HER). Second, to boost the photocatalytic activity, graphene was attached to the binary heterostructure. Due to the lower work function of graphene, the energy bands of the ternary graphene/$\mathrm{ZrA}{\mathrm{s}}_2/\mathrm{HfA}{\mathrm{s}}_2$ heterojunction were elevated as a whole, leading to a spontaneous HER. Meanwhile, the built-in electric field was enhanced. The enhanced electric field, on one hand, increases the barrier at the $\mathrm{ZrA}{\mathrm{s}}_2/\mathrm{HfA}{\mathrm{s}}_2$ interface, leading to a longer residence time for the more energetic electrons and holes; and on the other hand, the formation of ohmic contact causes a barrierless transfer for the charges at the graphene/$\mathrm{ZrA}{\mathrm{s}}_2$ interface, and the accumulated electrons rapidly recombine with the holes via the interlayer channel in the effect of enhanced electric field. Moreover, the ternary heterojunction exhibits excellent light absorption characteristics in the visible spectrum, accompanied by a predicted high light conversion efficiency. Our results not only offer a strategy for the photocatalytic performance to be enhanced by attaching graphene to a binary heterostructure but also propose a solid photocatalytic mechanism for the enhanced activity.

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• Electronic structure
• Energy harvesting devices
• Photodissociation
• van der Waals interaction
• Graphene
• Transition metal dichalcogenides
• Ab initio molecular dynamics
• Density functional theory
• First-principles calculations