Interfacial cohesive interaction and band modulation of two-dimensional MoS2/graphene heterostructure

Abstract

To improve the efficiency of water-splitting, a key way is to select suitable semiconductor or design semiconductor based heterostructure to enhance charge separation of photogenerated h+-e- pairs. It is possible for a two-dimensional (2D) heterostructure to show more efficient charge separation and transfer in a short transport time and distance. Among numerous heteromaterials, the 2D layered MoS2 has become a very valuable material in photocatalysis-driven field due to the appropriate electronic structure, peculiar thermal and chemical stability, and low-cost preparation. To couple with MoS2, layered graphene will be an ideal candidate due to extremely high carrier mobility, large surface area, and good lattice match with MoS2. At present, a lot of researches focus on the synthesis and modification of MoS2/graphene heterostructure. However, it is hard to detect directly the weak interaction between MoS2 and graphene through the experiment. Here, an effective structural coupling approach is described to modify the photoelectrochemical properties of MoS2 sheet by using the stacking interaction with graphene, and the corresponding effects of interface cohesive interaction on the charge redistribution and the band edge of MoS2/graphene heterostructure are investigated by using the planewave ultrasoft pseudopotentials in detail. Three dispersion corrections take into account the weak interactions between MoS2 and graphene, resulting in an equilibrium layer distance d of about 0.34 nm for the MoS2/graphene heterostructure. The results indicate that the lattice mismatch between monolayer MoS2 and graphene is low in contact and a van der Waals interaction forms in interface. Further, it is identified by analyzing the energy band structures and the threedimensional charge density difference that in the MoS2 layer in interface there appears an obvious electron accumulation, which presents a new n-type semiconductor for MoS2 and a p-type graphene with a small band gap ( 0.1 eV). In addition, Mo 4d electrons in the upper valence band can be excited to the conduction band under irradiation. And the orbital hybridization between Mo 4d and S 3p will cause photogenerated electrons to transfer easily from the internal Mo atoms to the external S atoms. The build-in internal electric field from graphene to MoS2 will facilitate the transfer and separation of photogenerated charge carriers after equilibrium of the MoS2/graphene interface. It is identified that the hybridization between the two components induces a decrease of band gap and then an increase of optical absorption of MoS2 in visible-light region. It is noted that their energy levels are adjusted with the shift of their Fermi levels based on our calculated work function. The results show that the Fermi level of monolayer MoS2 is located under the conduction band and more positive than that of graphene. After the equilibrium of the MoS2/graphene interface, the Fermi level shifts toward the negative direction for MoS2 and the positive direction for graphene, respectively, until they are equal. At this time, the conduction band and valence band of MoS2 are pulled to the negative direction a little, and then form a slightly upward band bending close to the interface between MoS2 and graphene. Combining the decrease of the band gap of MoS2 in heterostructure, the potential of the conduction band minimum of MoS2 in heterostructure will increase to -0.31 eV, which enhances its reduction capacity. A detailed understanding of the microcosmic mechanisms of interface interaction and charge transfer in this system can be helpful in fabricating 2D heterostructure photocatalysts.