Characteristics And Applications For Defects Of Graphene

Graphene is a kind of new materials which maybe used in many fields. With the development of study on graphene, characteristics and applications for the defects of graphene have drawn the attention of many researchers.The defects of graphene include intrinsic defects and extrinsic defects. The intrinsic defects are caused by non-sp~2 carbon atoms and the extrinsic defects are caused by foreign atoms(such as O and N) covalently bonded with C atoms in graphene. Besides, after stacking or merging, multilayer graphene can form defects. These defects have different effects on the electronic, optical and mechanical properties of graphene sheets; therefore studying, using and controlling them are very important.Extrinsic defects such as O atoms in graphene can form some kinds of oxygen-containing functional groups(such as C-O-C, C-OH and C(O)O), the graphene which has a number of O atoms may have good dispersability in water, this kind of graphene is usually named as graphene oxide(GO). In this thesis, molecular dynamic simulation and experiments were used to study the differences between the behaviors of GO in water and salt solutions, results showed that GO disperse uniformly in water but agglomerate in salt solutions. Simulation showed that GO can disperse in water due to the hydration of its oxygen-containing functional groups, the hydration of GO is weakened when salt is present because salt ions are preferentially hydrated over GO. Thus the movement ability of GO in the salt solution decreases(for example, the ability of GO in water is 4 times than GO in NaCl solution). The decrease of movement ability of GO makes it in salt solution tend to agglomerate. Further studies showed that the agglomeration behavior is related to salt concentration and ion species.Due to weak movement ability of GO in salt solutions, GO membrane is very stable in salt solutions such as Na2SO4 solution even with ultrasonic treatment(30 kHz, 100 W). Further study showed that reduced GO(r-GO) membrane could be prepared in salt solutions by hydrothermal method, the resistivity of the r-GO membrane was only 0.08 ?cm, which was much lower than that for the original GO membrane(>20 k?cm). The reason for the change of resistivity is elimination of some oxygen-containing functional groups in graphene.Based on good dispersability in water of GO, graphene oxide was coated on the surface of high strength carbon spheres by assembly method in water. After elimination of some oxygen-containing functional groups by high temperature treatment, conductive high strength carbon spheres(40 % resistance of original carbon spheres) were prepared. Besides, high strength carbon spheres can not be produced industrially in China, thus a novel method was used for preparing these spheres by ourselves. The average strength for the obtained carbon sphere(800 μm) is 50 N, which achieved the international advanced level.Some oxygen-containing functional groups in GO can be eliminated by high temperature treatment in inert gas, however, new intrinsic defects appear during treatment because some C atoms leave graphene together with O atoms at high temperature. Simulations using Gaussian software showed that charge distributions of atoms in the graphene sheets are relatively uniform due to large conjugated л bond, but atomic defects in the graphene sheets change the uniform charge distributions. The change of polarity makes these defect-regions have higher chemical activities. Based on the different chemical activities, air at high temperature was used to selectively oxidize defect-regions of graphene, which makes the number of them decrease.Formation and merging defects of graphene during the mesophase process in which mesocarbon microbeads(MCMBs) form in isotropic pitch were studied. A novel method was used and found that ordered regions of graphene are almost less than 5 μm in an interface between MCMBs and their pitch matrix, which is quite different from long-range ordered regions inside MCMBs. Further studies showed that the number of sp~2 and non-sp~2 carbon atoms of the graphene in the interface is probably equal to that inside MCMBs, the differences of defects in both regions are just the scale of long-range ordered regions which form in merging process of MCMBs.