Energy Level Alignment Modulation of ZnO Surfaces by Graphene Coupling: A Density Functional Theory Investigation

The graphene/ZnO (G/ZnO) interface is promising for photocatalysis due to its potential to enhance charge separation. Using density functional theory, we investigate the structural and electronic properties of G/ZnO interfaces in both parallel (basal-plane) and perpendicular (edge-contact) configurations on the nonpolar ZnO(101̅0) surface. In the case of the pristine ZnO(101̅0) surface, the Fermi level is located in the band gap, suggesting that the band bending from the bulk to the surface is small. In the case of the parallel G/ZnO(101̅0) interface, although the graphene has semimetallic density of states, the interaction is mainly due to van der Waals-like weak interaction and the Fermi level is located just below the conduction band minimum, indicating the induced band bending is still small. In contrast, in the case of perpendicular G/ZnO(101̅0) interfaces, the chemical bonds between the graphene edges and the ZnO surface are formed and electron transfer takes place from ZnO(101̅0) to graphene. This results in the shift of the Fermi level toward the valence band maximum and induces a large upward band bending from the n-type bulk ZnO to the interface. These findings highlight the importance of interface orientation and local energy level shifts in elucidating underlying charge transfer mechanisms for electronic and photocatalytic applications.