Controlled Synthesis Of Graphene Oxide By Electrochemical Methods And Its Application On Thermal Conductive Films

Graphene,an atomic thick two-dimensional carbon material,has super-high electron mobility,remarkable mechanical strength and excellent thermal conductivity.Graphene oxide(GO),as the most important derivative of graphene,has the characteristics of high dispersion,good processability and ease of assembly.Therefore,it has many promising applications for ion/molecule separation membrane,optical/electrical functional film,electrical/thermal conductive film,electromagnetic interference shielding film,etc.However,the existing methods of GO preparation and film assembly still face many problems.The chemical oxidation methods of GO preparation suffer from environmental pollution,explosion risk and long reaction time,which seriously limit the scalable production.Alternatively,electrochemical oxidation is a green and safe method for the preparation of GO,but it is difficult to obtain single-layer GO with high oxidation degree,and the reaction mechanism is still lack of in-depth study.In addition,although vacuum filtration,LB membrane assembly,blade coating,continuous centrifugal casting and other methods have been developed for the fabrication of GO films,it still remains a great challenge to fabricate GO film with high efficiency,good alignment and precisely tunable thickness simultaneously.Aiming at these challenges,in this thesis,the electrochemical preparation methods and oxidation mechanism of GO were systematically studied,and efficient and controllable assembly method of highly aligned GO films and their application on thermal conductive films were explored.The main results are as follows:(1)Electrochemically derived nano-sized GO(ENGO)with high oxidation degree was synthesized by one-step electrochemical oxidation,named as and its application as catalysts in two kinds of typical organic reactions was explored.Glassy carbon(GC)was used as the anode in theone-step electrochemical oxidation for GO synthesis,which enables the production of ENGO with lateral size smaller than 20 nm,a thickness of 1 nm～2 nm and a C/O atomic ratio of～1.4.The difficult intercalation structure of GC and the protection of SO42-/HSO4-ensure water electrolytic oxidation,which is the key for the one-step electrochemical oxidation of GO.ENGO with different mean lateral size of 4 nm,8 nm and 13 nm can be selectively synthesized by simply varying the crystallinity of GC.The percentage of carbonyl and carboxyl groups on ENGO increases with the decrease of mean lateral size.As metal-free catalysts,the 13-nm-sized ENGO is found to be beneficial for the oxidative coupling reaction of benzyl amine,while the 4-nm-sized ENGO shows highest conversion rate for the oxidation reaction of benzene.(2)A two-step electrochemical method was invented to realize the synthesis of GO with high efficiency,and the oxidaiton mechanism was explored.Electrochemically derived GO(EGO)was prepared by concentrated sulfuric acid intercalation(the first step)followed by dilute sulfuric acid oxidation and exfoliation(the second step)using flexible graphite paper(FGP)as the anode.The oxidation rate in this method was more than two orders of magnitude higher than that of the traditional chemical oxidation method.The EGO obtained has a lateral size in the range of 1 μm～10μm and a C/O atomic ratio of～1.8.Furthermore,the oxidation degree of EGO can be controlled by varying the H2SO4 concentration in the electrolyte in the second step electrochemical oxidation process.It was found that the oxygen in EGO mainly comes from the water in electrolyte for the second step electrochemical oxidation,while the first step intercalation by concentrated H2SO4 can effectively protect the graphite anode from breaking and inhibit the formation of oxygen gas(O2).The resulting highly reactive oxygen-containing radicals(e.g.·O,·OH,·OOH)enables effecient and green synthesis of EGO by reaction with graphite.(3)A scanning centrifugal casting(SCC)method was developed to realize the fabrication of GO films with highly ordered micro-structure and preciously controlled thickness,which can be used to produce graphene thermal conductive films with excellent heat dissipation performance.To demonstrate the advantages of SCC method,GO films with thickness in the range of 20 μm-330 μm was produced,which show almost the same ordered micro-structure at different film thickness.Such GO films were transformed into graphene films after high-temperature heat treatment and graphitization.Both the electrical and thermal conductivity of the resulting graphene films increase with the rising of heat treatment temperature.Importantly,the thermal conductivity remains almost unchanged for the graphene films with different thickenss.By optimizing the SCC and heat treatment processes,a high thermal diffusivity of 780 mm2 s-1 was achieved for the graphene film with a thickness of 100 μm,which shows the great potential of such SCC-fabricated GO based graphene film for thermal management of electronic devices.