High-speed Shear And Electrochemical Stripping Of Graphite

In China, there are abundant high-quality graphite mineral resources; however the ability to deeply process the graphite is insufficient. As a result, graphite resources are now facing the embarrassment of "low-priced exports of raw materials, high-priced imports of products". It is urgent to solve the problems in developing new graphite materials and deeply processing the graphite. Graphene has broad application prospects in the electronics, energy storage materials and transparent conductive electrodes due to the excellent carrier mobility, good thermal conductivity,light transmittance and low resistivity. The quality of graphene prepared by Hummers' method is not uniform and the produced waste acid is harmful to the environment.The graphene produced by liquid phase exfoliation has low yield and also has non uniform quality, which is in consequence difficult to achieve the industrial-scale production. Via the chemical vapor deposition (CVD) method, monolayer graphene with high quality can be obtained, however the method is costly and cumbersome. It is easy to exfoliate graphite by high-speed shearing machine. However, the yield of graphene is relatively low and the quality is not equal as well. At present, the electrochemical exfoliation of natural graphite is considered as a feasible and simple way to prepare graphene. Under the force of electrical fielt, the charged ions or ion groups would intercalate into the graphite layers and expand the graphite layer. Thus,graphene is achieved by graphite exfoliation.To date, limited researches on the electrochemical exfoliation were conducted and the method gives relatively low yield as well. Based on the maneuverability of each methods above in industrial production, mechanical shearing and electrochemical exfoliation are adopted in this work in order to improve the yield and quality homogeneity of graphene.In this work, high-speed mechanical shearing and electrochemical stripping on graphite are mainly investigated. This paper is divided into three parts. In the first part, after mechanical shearing on the graphite, the most suitable mechanical shearing speed and shearing time are respectively optimized according to the amount and size of few-layer-graphene which exists in the upper solution. The second part and the third part demonstrate the electrochemical intercalation and exfoliation methods on the graphite in the aqueous solution and organic solution, respectively. The few-layer graphene is obtained by altering the applied voltages and the electrolyte proportions. Afterwards, the obtained graphite characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM)and Raman spectrometry. The optimal concentrations and proportions of the electrolyte as well as the most proper voltage parameters are determined by analyzing the stripping amounts, the concents and yields of few-layer-graphene which are prepared under different conditions. The specific results are as following:Few-layer-graphene can be obtained by high-speed shearing on the natural graphite by mechanical exfoliation. The few-layer-graphene in the upper solution is averagely as thin as around 3 nanometers, and the (002) interlayer spacing of few-layer-graphene is 0.353 nanometers. The experimental results show the few-layer-graphene content in the upper solution is increased under appropriate shear speed and cutting time, meanwhile, the size of graphite particles can be decreased by the mechanical shear. The mechanical shearing under the condition of cooling bath is benefit to lower the size of few-layer-graphene and raise the amount of graphene in the upper solution. Under the room temperature, the yield of few-layer-graphene is ca. 6.5% when the shear rate and cutting time are 15000 rpm and 15 min, respectively.Few-layer-graphene can be obtained by electrochemical exfoliation in the aqueous solution.The few-layer-graphene in the upper solution is averagely as thin as around 2 nanometers and the(002) interlayer spacing of few-layer-graphene is increased to 0.445 nanometers. The Raman peak of 2D shows a blue shift and suggests the protonation between electron and graphite during the electrochemical exfoliation process. The charged ions (O22-?OH-) intercalate and enlarge the graphite layer. The electrochemical exfoliation at different sodium hydroxide, hydrogen peroxide concentrations and different applied voltages is performed on the graphite electrode by the orthogonal test method. The optimal concentrations of sodium hydroxide(3.5 mol L-1),hydrogen peroxide(1.5 mol L-1) are obtained by evaluating the graphite peeling quality, the few-layer-graphene content and size in the upper solution. When the applied voltage is 5 V, the graphene yield is 8.7%.Few-layer-graphene can be obtained by electrochemical exfoliation in the organic solution.The few-layer-graphene in the upper solution is averagely as thin as around 2.1 nanometers and the (002) interlayer spacing of few-layer-graphene is increased to 0.481 nanometers. The Raman peak of 2D shows a blue shift and implies the protonation between electron and graphite. It is confirmed that the charged ion groups ((CH4)4N+) intercalate and enlarge the graphite layer. The graphite exfoliation amount is positively correlated with the electrolyte concentration and the applied voltage. An appropriate increase of tetramethylammonium hydroxide concentration can increase the solution conductivity. The tetramethylammonium cation has easy access to migrate and insert into the graphite layer which is forced by the electric field, so that the graphite layer is peeled off. When electrochemically exfoliating the graphite in the organic solution, the optimal tetramethylammonium hydroxide concentration is 0.2?0.3 mol·L-1. The graphene yield is 7.9%at the applied potential of 30 V.