Electronic properties of defective graphene functionalized with oxygen and hydroxyl groups for molecular sieve

Abstract

Sub nanometer-defective-graphene, such as graphene with vacancies and graphene oxide, is promising candidates to overcome the water permeability of membrane materials for reverse osmosis technology, which is the leading technology for desalination and water treatment. The advantage of graphene-based materials in this area is attributed to their ability to adsorb contaminants by oxygen-containing functional groups at defects on graphene sheets. In this study, the interaction mechanism between water molecules and defective graphene, including pyridine defective graphene and pyridine graphene oxide, is described through electronic properties by a DFT calculation. The calculation was performed using the pseudopotential plane-wave method by Quantum Espresso with cut-off energy of 40 Rdy and supercell of 4x4 with in-plane periodicity containing 32 atoms. The orbital hybridization of C atom changed from sp2 to sp3 when it bonded to the O atom instead of neighboring C atoms in the graphene structure, led to the formation of a negative charge on the graphene sheet. In particular, hybrid sp2/sp3 orbitals was created on C atoms, which formed the epoxy group with the adsorption energy of -5.80 eV. In the models of monovacancy of graphene (defective graphene) and various positions of oxygen on defective graphene (graphene oxide), charge re-arrangement due to vacancy or presence of oxygen atom and weak binding energy between graphene and water molecules are responsible for attracting cations and allowing water molecules to pass through. The negative charge areas were thought to help increase the spacing of the functionalized defective graphene sheets, creating a path for water molecules to pass through. In addition, the computational model also showed the (non)-magnetic moment of the functionalized defective graphene depending on the functional group attached at the defect position.