二硫化钼/石墨烯异质结的界面结合作用及其对带边电位影响的理论研究

采用基于色散修正的平面波超软赝势方法研究了二硫化钼/石墨烯异质结的界面结合作用及其对电荷分布和带边电位的影响.研究表明二硫化钼与石墨烯之间可以形成范德瓦耳斯力结合的稳定堆叠结构.通过能带结构计算,发现二硫化钼与石墨烯的耦合导致二硫化钼成为n型半导体,石墨烯转变成小带隙的p型体系.并通过电子密度差分图证实了界面内二硫化钼附近聚集负电荷,石墨烯附近聚集正电荷,界面内形成的内建电场可以抑制光生电子-空穴对的复合.石墨烯的引入可以调制二硫化钼的能带,使其导带底上移至-0.31 eV,提高了光生电子还原能力,有利于光催化还原反应.

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

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.