Study On The Structural Transformation And Electrical Properties Of Products Formed By The Oxidation-reduction Of Graphite

The preparation of graphene has been achieved based on theoxidation-reduction method, the structural and el ectrical properties theproducts of various stages obtained in the process of preparinggraphene have been studied based on the experimental characterizationand theoretical study. The graphite oxide samples with differentoxidation levels were prepared by changing the process factorparameters based on the Hummers me thod. The stable graphene oxidesuspensions were prepared by ultrasonic stripping of gr aphite oxide andthen followed by high-speed centrifugation. The reduction of graphite-and graphene-oxide have been carri ed out by using hydrazine hydrateas reductant, obtaining different reduced graphite oxide sa mples andchemically converted graphene with different reduction levels.Moreover, the preparation of graphene oxide me mbrane andgraphene-based me mbrane were achieved by vacuum filtration ofgraphene oxide and graphene suspensions, respectively. The bandstructure of graphite, graphite oxide and graphene have been alsocalculated by using the CASTEP code with density functional theory(DFT) method. Then a series of testing and analysis of differentsa mples were carried out, such as XRD、 FT IR、 Ra man、 UV-vis、TG-DTA、 LPSA、 Zeta potential analysis、 SEM、 AFM and Four-pointprobe measurement.The results show that, by increasing the dosage of potassiu mpermanganate or extending the reaction ti me of35℃-stage, theintercalation and oxidation of graphite can be i mproved based on theHummers method, forming graphite oxide sa mples with higheroxidation levels. After oxidation, the structural layers of graphite bonda large number of oxygen-bearing functional groups, leading to theincrease of the layer spacing, the destruction of π-bond conjugate dsyste m, and the loss of electrical property. With the increase ofoxidation levels, the degree of the intercalation and oxidation of obtained graphite oxide samp les increases, resulting in the increase ofthe content of oxygen-bearing functional groups in structure, theweakening of the thermal stability, and the decrease of the electricalproperty. Graphite oxide with higher oxidation levels can be welldispersed in water or in the alkaline solution, forming a stable grapheneoxide colloidal suspension by the aid of ultrasonication andcentrifugation. The generating single graphene oxide sheets havethickness of~1.3-1.5nm and lateral dimensions of~1-2μm, and canform a paper-like macroscopic membrane with orderly structuralfeature like in graphite oxide by self-assembly resulted from vacuumfiltration. Due to the incompletely removal of oxygen-containingfunctional groups bonded on the structural layers, and the destructionof carbon layers caused during the oxidation and reduction process, thereduced graphite oxide can form graphite-like crystal phase in structure.After reduction, the π-bond conjugated system in the structure ofgraphene oxide recoveri es, the conductive property can be partiallyrestored, and the thickness of single sheets decreased to~1nm. Theobtained chemically converted graphene nanosheets can for mgraphene-based ma croscopic me mbrane with amorphous feature byself-assembly. The DFT calculation results suggest that the valenceband and conduction band of graphene and graphite overlap at Fermilevel, showing half-metallic feature. The O2p orbital electrons inoxygen-containing functional groups can bond to the C2p orbita lelectrons in the valence band nearby the Fermi level in graphene,leading to electronic states of the valence band nearby the Fermi levelweaken, making C8O2(OH)2model to generate a wide band gap. By there moval of oxygen-containing functional groups, partial electronicstates contributed by the C2p orbital electrons bonded to the O2porbital electrons in valence band, move towards the Fer mi level,revealing the recovery of delocalized π electrons in structure, givingrise to the C8OH mo del generating a relatively narrower band gap.The research of this thesis reveals the inherent relationshipbetween the process parameter used in the preparation of graphene, the structural transformation and electrical property of products, and hasimportant theoretical and practical significance in the selection ofappropriate preparation method for graphite oxie and graphene, thecontrol of preparation process and the design of controlled property forproducts.