Effect of electric field on molecular and electronic structures of graphene with 4N-divacancy defect and application on hydrogen gas storage

Abstract

The effect of external electric fields (EEFs) on geometries and electronic properties of graphene quantum dots (GQDs) and m-4N-divacancy defect GQDs (m-4N-GQDs) was studied using density functional theory (DFT) method with M06-2x functional and 6-31g (d) basis set. An external electric field with strength ranging from -0.035 to 0.035 atomic units (a.u.) was applied normal to the molecular plane. Three different sizes, i.e., C24H12, C54H18, and C96H24, were investigated. The metal doping in m-4N-GQDs consists of Ca, Ca2+, Cr, Cr2+, Fe, and Fe2+. Our results revealed that GQDs and m-4N-GQDs are curved under EEFs in the direction opposite to the applied field. The curvature of the GQDs and m-4N-GQDs is directly proportional to the electric field strength. The curvature of GQDs is in the following order: C96H24 > C54H18 > C24H12. The HOMO-LUMO gap depends on both size and EEFs. HOMO-LUMO gaps of C24H12, C54H18, and C96H24 are 5.87-5.90, 3.98-4.43, and 2.43-3.31 eV, respectively. In addition, the gap varies with the metal doping and the order is as following: Cr ≈ Fe > Ca > Ca2+ > Cr2+ ≈ Fe2+. The HOMO-LUMO gap of m-4N-GQDs are in ranges from 1.38 to 2.98 eV. We also found the HOMO-LUMO gap to be decreased with the increase of the curvature. Therefore, the electronic properties of curved GQDs and m-4N-GQDs could be modified through EEFs. Moreover, the adsorption energies of H2 on Fe2+-4N-GQDs with different degrees of curvature and H2 adsorption positions, inside or outside, of curved Fe2+-4N-GQDs were studied. It was found that the molecular H2 prefers to adsorb outside of curved Fe2+-4N-GQDs, which have a distance between Fe2+ and H2 at 3.0 – 3.5 Å (-2.03 to -0.38 kcal/mol). The adsorption energy at outside H2 adsorption is directly related to the electric field strength, while inside H2 adsorption is similar. The curved Fe2+-4N-GQDs could control molecular H2 adsorption. The curved Fe2+-4N-GQDs can be applied for hydrogen gas storage.