Enhanced visible-light photocatalytic activity of direct Z-scheme g-C₃N/MX₂ (M = Mo, W and X = S, Se) nanosheets from theoretical insights

Since the discovery of graphene, two-dimensional (2D) layered materials have attracted extensive research interest. The synergistic effects of two different 2D semiconductors forming a heterostructure may enhance the intrinsic properties. Here, the photoelectric properties of g-C₃N/MX₂ (M = Mo, W and X = S, Se) heterostructures and the effect of their interface interaction on photocatalytic activity have been systematically studied. Different stacking structures of g-C₃N/MX₂ exhibit similar stability and electronic properties. g-C₃N/MoS₂ is metallic, while the other systems retain semiconductor properties. The interface dipole moment and work function confirm the intrinsic electric field direction from g-C₃N to MX₂, which reduces the coulomb interaction of carriers and promotes carrier separation. Moreover, the synergistic effect of the band offset and the intrinsic electric field induces the charge transfer to follow a direct Z-scheme in g-C₃N/MoSe₂, g-C₃N/WS₂, and g-C₃N/WSe₂, such that the conduction band (CB) of g-C₃N becomes a reduction center and the valence band (VB) of MX₂ becomes an oxidation center. Meanwhile, the hydrogen evolution reaction (HER) can occur in either acidic, alkaline, or neutral solutions, and g-C₃N/WS₂ has the smallest potential-determining step. The oxygen evolution reaction (OER) can occur in solutions at most pH levels, and g-C₃N/WSe₂ has the smallest potential-determining step. Electrons mainly migrating in the g-C₃N layer have higher mobility, which facilitates the HER process. All three g-C₃N/MX₂ have similar light-absorption strengths, with the absorption edge being red-shifted compared to g-C₃N or MX₂. Therefore, heterostructure construction does indeed effectively improve the photoelectric performance and photocatalytic activity.