Preparation, Assembly And Application Of Graphene

Graphene is a one atom thick and closely packed two-dimensional lattice, which is viewed as a basic building unit for well-known carbonaceous materials including fullerene, carbon nanotubes and graphite. Since the first report for the free-standing graphene published by Geim, it is considered as a promising candidate for various applications and becomes a hot research topic in the field of chemistry, materials science and physics due to its unusual and intriguing mechanical, electric, thermal and optical properties. However, graphene is difficult to produce in large scale and intends to exist in aggregation state. In order to make use of its high tensile strength, Young's modulus, thermal conductivity, specific surface area, thermostability and chemical resistance, it is necessary to investigate the morphology and structure deeply. As a basic building unit for construction of carbon materials, graphene can be thought as "soft" two-dimensional macromolecules for the preparation of assembled structure under the assistance of interfacial effect and metal catalyst. Moreover, the high-quality sp2 carbon lattice, quasi-two-dimensional crystal structure and high aspect ratio of graphene provide the basis for the applications in anode material and conducting agent for lithium-ion batteries. So the lithium storage mechanism is to be further studied and developed. Due to the prior mechanical and other physical properties, graphene can be utilized as the reinforced filler in polymer-based composites.In this paper, the preparation, assembly and applications of graphene and graphene nanosheets (GNSs) were investigated. First, GNSs with different lateral size and surface chemistry were prepared by the oxidation/exfoliation process of graphite. The morphology and structure of GNSs as well as electrochemical properties of GNSs as anode material and conductive agent were studied by SEM,TEM,HRTEM,AFM,XRD,BET and a variety of electrochemical testing techniques. On the basis of above investigation, the relationship between the morphology, structure and electrochemical properties at high rate was analyzed. The high aspect ratio and electric conductivity of GNSs contributed to high reversibly capacity, rate and cycle performance. The assembly ability of GNSs as a basic building block for novel carbon materials was investigated deeply through the preparation of hollow graphene oxide spheres and graphene-encapsulated metal microspheres under the assistance of water-in-oil interface and metal catalyst. Moreover, the graphene as the reinforced filler in the epoxy composites was studied. The well-dispersed GNS/epoxy composites were obtained by the chemical modification and ultrasonic treatment. The mechanical properties and thermal stability of GNS/epoxy composites were investigated in comparison with MWCNTs/epoxy composites. These researches had great academic and practical significances for the broadening of graphene and the promotion of the development of its application in the field of energy storage, self-assembly and polymer-based composites.The results indicated that the length of oxidation time, rate of oxidant addition, thermal expansion time, ultrasonic treatment power and time exert the significant influences on the expansion rate, morphology, structure, specific surface area and electric conductivity of exfoliated graphite and graphene nanosheets. GNS electrode exhibits a 672 and 554 mAh g-1 reversible capacities at the higher current density of 0.2 and 1 mA cm-2 respectively. It displays excellent cycle and rate performance. The AC impedance spectra represent high rate performance of graphene nanosheets which is influenced by diffusion path and velocity of lithium ions. The high aspect ratio and electric conductivity of graphene endow an internal resistance of GNS electrodes. The high aspect ratio and sp2-hybridized carbon organization of GNSs ensure the formation of stable conductive network and efficient electronic transport throughout the anode, which endows higher reversible capacity, better cycling stability and excellent high-rate performance of electrode when GNSs are used as the additive for lithium-ion batteries. When the addition content is same, compare to the traditional conducting agent-acetylene black, the reversible capacity and coulomb efficiency of graphite electrode with GNSs increased by 25-40% and 10-15%, respectively. With the increase of GNS addition from 2 to 10%, the reversible capacity, cycle and rate performance of graphite electrode enhance gradually, which is from 180 to 422 mAh g-1.Hollow graphene oxide spheres (HGOSs) were fabricated from graphene oxide nanosheets (GONs) utilizing water-in-oil (W/O) emulsion technique without surfactant. The fine morphology of HGOSs is obtained when the emulsion time and blending rate are proper. The oxidation time for preparing GONs is a crucial factor for the formation and morphology of HGOSs. With the increase of oxidation time, the morphology and surface topography of HGOSs vary from the irregular and rough to uniform and smooth shape with a decreasing diameter. The reaction between ammonia and functional groups on the surface of graphene nanosheets is important for the formation of HGOSs. The heat treated HGOSs exhibit 485 and 310 mAh g-1 reversible capacities when the current density is 0.2 and 1 mA cm-2, respectively. The enhanced electrochemical properties are attributed to the hollow structure, thin and porous shells consisting of graphene.Graphene encapsulated iron microspheres (GEIMs) were fabricated by heat treatment (1500℃) of the mixture of graphene oxide nanosheets (GONs) and metal catalyst for 2 hours. The morphology of GEIMs is influenced by amount of oxidant addition, heat treatment temperature and time, catalyst addition, catalyst species and mixture method. The surface morphology of GEIMs consisted of hexagonal/pentagonal graphene and most crease angles are 60°, suggests a graphitic structure. GONs provided a convenient environment and source for the formation of graphene shells via the precipitation of dissolved carbon from micron-size drops of molten iron. The modified hollow graphene encapsulated iron microspheres (M-GEIMs) anchoring on the graphene nanosheets were obtained after the removal of ferric in the GEIMs. When used as the anode materials for lithium-ion batteries, the M-GEIMs anchored on the GNSs exhibit excellent cycle capability and a higher reversible capacity of about 420 mA h g-1 and possess great potential application in lithium-ion batteries. It is attributed to the relatively high graphitic degree, aspect ratio and electric conductivity.Graphene nanosheets (GNSs)/epoxy nanocomposites were prepared via ultrasonic dispersion and cast moulding method. GNSs were well dispersed and highly loaded in the epoxy matrix through an ultrasonic process. The mechanical performances and fracture morphologies of GNSs/epoxy composites were compared with that of multi-walled carbon nanotubes (MWCNTs)/epoxy composites. The tensile strength and Young's modulus improved with the increase of GNSs addition, which are higher than those of the MWCNTs/epoxy composites with the same addition percentage. When loading is 6 wt%, GNS/epoxy composites exhibits higher tensile strength (72 MPa), elastic modulus (1279 MPa) and fracture elongation (11.5%), higher than those of MWCNTs/epoxy composites (55 MPa,979 MPa and 11.3%). The high mechanical properties of MWCNTs/epoxy composites are ascribed to the reinforced skeleton function and stress transfer of graphene nanosheets by the well interface interaction, which is similar to short fiber reinforced mechanism. High mechanical properties of GNS/epoxy composites are ascribed that the stress transfer is obtained by interface interaction formed by rich nanochannels and functional groups. With the enhancement of loading, the initial and terminal decomposition temperature of two composites increased. When the addition is 6 wt%, the initial and terminal decomposition temperature of GNS/epoxy composites increased by 36 and 194℃compared with pure epoxy, which are high than those of MWCNTs/epoxy composites (15 and 165℃) The thermal stability of GNSs/epoxy composites is enhanced due to high heat resistance of graphene and carbonized skeleton.