Mechanistic Study On Graphene Oxidation By KMnO₄ And Resultant Carbon-carbon Unzipping

With the rapid development of electronic devices,people have increasingly higher demands on the dimension,edge shape and device performance of the core material.Graphene,a typical type of two-dimensional nanomaterial has excellent electrical,thermal and mechanical properties,whose preparation and shape control has long been a hot topic.Oxidation of graphite followed by chemical reduction has been widely used in the mass production of graphene.It has been pointed that single-layer graphite,i.e.graphene would break up into fragments with irregular shapes during oxidation and sonication,causing inconsistent performances of graphenes from the same batch.At present,the mechanism of graphene oxidation and fragmentation is not clear.Understanding the mechanism and preference of the unzipping process is not only essential to the pursuit of large-size graphene,but also conducive to tailor graphene with controllable shapes and sizes.In this thesis,we used density functional theory(DFT)method to study the oxidation process of graphene and the resultant C-C bond unzipping.In Chapter 3,we investigated the oxidation process of perfect graphene by potassium permanganate in aqueous solution and the mechanism of C-C bond unzipping driven by the hydroxyl-epoxy pair through theoretical calculations.We revealed the origin of the oxidation groups on the graphene surface,and pointed out that the bifacial hydroxy-epoxy pair is an important precursor for the C-C bond fracture.DFT calculations show that the hydroxyl group introduced by the hydrolysis of permanganate ester plays an important role in the oxidative cutting of graphene.Oxidative unzipping of the defect-free graphene will occur from the edge and the inner plane simultaneously in terms of comparable maximum barrier heights along these two reaction routes.Unlike the previous unzipping mechanism that only produces graphene flakes with zigzag edges,our mechanism involves both sides of the graphene sheet,and the hydroxyl group,which is often overlooked in the current studies.This new mechanism allows richer edge states after oxidative cutting,which are consistent with the experimental results.In Chapter 4,we studied the oxidative cutting mechanism of the common defective graphene(with single and double vacancy,SV and DV).Our results manifest that the permanganate tends to bind with the carbon atoms at the defective area.Three dangling carbon atoms at the SV bind strongly with three oxygen atoms of permanganate.As a result,the edge hydroxyl group can not be easily formed via the hydrolysis reaction in situ.Therefore,oxidative cutting of graphene might not be initiated from the SV.While for the DV,permanganate tends to bind one carbon atom at the defect with one oxygen atom.Although the hydroxyl group at the defect is still difficult to form via hydrolysis,the edge hydroxyl group can be instead created by in-plain hydroxyl hopping and induce C-C breaking.We calculated the energy barrier of C-C bond cracking in the linear and nonlinear directions of graphene with DV,and found that there was no obvious tendency between these two.Although DV has long been considered to be a chemically stable defect in graphene,when the inner hydroxyl group hops to the DV,it will accelerate the oxidative etching process.