Mass-production, Controlled Assembly And Electrochemical Properties Of Graphenes

Graphene, as a novel carbon material which is characterized by uniquetwo-dimensional structure and fascinating electronic, thermal, mechanical and opticalproperties, shows great potential in many fields, such as nanoelectronics, energystorage and conversion, catalyst, ultrafast laser system, high sensitivite sensors,composites and so on. However, in-depth research and large scale application ofgraphene are still hindered by some key issues, such as large scale preparation,uniform dispersion of graphene nanosheets and precise structure control ofgraphene-based materials. With a purpose to promote the basic research and realapplication of graphene, this thesis focuses on mass-production and electrochemicalapplications, and the main work includes vacuum-promoted exfoliation from graphiteoxide, efficient dispersion in aqueous solution, controlled construction ofgraphene-based nanostructure and their applications in electrochemical sensors andenergy storage devices.For the large scale preparation of powdered graphene, a vacuum-promotedexfoliation method at low temperature was for the first time proposed and theobtained graphenes are single-layer dominated and show high quality. Starting withthe investigation of the thermal properties of graphite oxide, we found that most offunctional groups attached to graphite oxide planes were decomposed in a verynarrow range of150²50℃. However, the graphite oxide cannot be fully exfoliatedat such low temperature since the internal force generated by the functional groupsdecomposition is not enough to push the layered strcuture exfoliated. Therefore, weintroduced an outward drawing force at such low temperature and realized fullexfoliation and reduction of graphite oxide layers by creating high vacuumenvironment. Furthermore, the low-temperature exfoliated graphenes show muchhigher capacitance than high-temperature exfoliated ones due to the different surfacechemistry.We proposed a facile method to realize the single-layer dispersion of graphenepowder in aqueous solution. By using single-stranded DNA as a high efficientdispersing agent, graphene was well dispersed in aqueous solution in highconcentration while graphene/DNA hybrid was obtained by self-assembly simultaneously. This hybrid promotes the electron transfer in the electrocatalysisprocess and shows good response to H2O2. Graphene/NiO/DNA hybrid, which can beused as a nonenzymatic sensor, was also prepared by self-assembly and showssensitive response to glucose. The above two hybrids demonstrate good sensingability, such as large detection ranges, short response periods, low detection limit andhigh sensitivity and stability.For the two-dimensional graphene-based macrostructure fabrication, we designedand fabricated a membrane-like hybrid with layered sandwich structure conbiningtwo-dimensional graphene layers with zero-dimensional nanoparticles. A muti-stepstrategy that was conducted under ‘‘homogenous’’ and ‘‘mild’’ conditions wasdeveloped to fabricate this hybrid membrane. This structure possesses goodconductivity and open ion transportation channel and is an ideal structure forelectrochemical energy storage with excellent cycling and power performance.Besides, we found that graphene oxide can be used as a sticking component to linkgraphene sheets together. By introducing graphene oxide in the water dispersedgraphene aided by the surfactant, a macroscopic graphene-based membrane wasfabricated at the liquid-air interface upon heating. The conductivity of suchgraphene-based membrane can be finely tuned by changing the graphene oxidefraction.For the construction of three-dimensional graphene-based structure, we proposed aone-pot self-assembly process under mild condition and obtained a novelthree-dimensional graphene-based macroassembly with core–shell hybrid structure.Such a hybrid structure is characterized by a porous core and layered membrane shell,and its macro-morphology and infrastructure can be easily tuned by changing theKMnO₄fraction. The prepared assembly shows good electrochemical performanceand improved Coulombic efficiency than powdered graphene. Furthermore, byintroducing the external turbulent force in the assembly process, the macroassemblywith smaller size is obtained.