Separation And Evaluation Of Chemically Modified Graphene

Graphene is a new two-dimensional planar structure carbon materials closely packed by single layer carbon atoms. Since the first report for the free-standing graphene at room temperature discovered by scotch tape method, it has been demonstrated its unique potential applications in various fields such as nanoelectronics, sensors, nanocomposites, batteries, supercapacitors biomedical, hydrogen storage, mechanical, and thermal and optical. It has been suggested as a potential alternative to single walled carbon nanotubes and monocrystalline silicon for future technological applications and becomes a hot research topic in the field of the chemistry, materials science and physics, due to its low cost and novel properties. The prerequisite for exploiting these potential applications is the availability of bulk quantities of high-quality graphene sheets with appropriate sizes and specific functionalization. A variety of methods have been developed to prepare high-quality graphene (always are pristine graphene), such as micromechanical exfoliation of graphite, chemical vapor deposition (CVD), epitaxial growth on electrically insulating surface and exfoliation of graphite in liquid-phase. However, these methods are difficult to meet the need of industrial production, due to its bulk production, layer selective, size and cost. How to get bulk high-quality graphene and graphene with specific size are the two main problems in research on graphene. The most promising route for the bulk production of graphene sheet is the chemical oxidation and exfoliation of graphite in the liquid phase and followed by reduction, due to its simplicity, reliability, ability for large-scale production, relatively low material cost and versatile in terms of being well-suited to chemical functionalization. This thesis focused on how to get monodisperse chemically modified graphene in size and surface chemistry and evaluation criteria for reduced graphene oxide, to facilitate the realization of its large-scale application. The main results are listed as following.1. Separation of chemically modified grapheneGraphite oxide was prepared by modified Hummers method, and then was subjected to ultrasonic treatment to prepare graphene oxide. The chemically reduced graphene oxide sheets were prepared by reduction of GO with hydrazine. Graphene obtained by this method always has a wide size distribution, while the application of graphene always need appropriate sizes and specific functionalization, so we need to o sort graphene according to their differences in sheet size and surface chemistry. The density gradient ultracentrifugation rate separation, which always used in purification of biological macromolecules, has been developed for sorting chemically modified graphenes by taking advantage of their differences in sedimentation rate. This rapid, versatile, scalable, efficient, non-destructive, and reproducible method separates single-layer and monodisperse graphene oxide and chemically reduced graphene oxide sheets (50-700 nm) according to differences in size and chemical properties (UV-vis,XPS and fluorescence) in a time as short as 5 min. Specific sheet size (<20 nm) and extent of graphene functionalization can be targeted by adjusting the separation parameters, including centrifugation time and density gradient profile. Unwanted aggregates and even impurities (e.g., silicates in graphene) were removed without interfering with the separation. The separation method provides a way of obtaining graphene sheets with suitable sizes in bulk quantities and might pave the way for realization of technological applications of graphene. It also provides a potential analytical method to assess the size distribution and stability of suspensions of chemically modified graphene by comparison against standards.In addition, we discussed the possible separation mechanism by theoretical model analysis and comparative experiment (Isopycnic separation and one uniform layer density gradient separation). The results show that the density difference between particles and gradient medium, particle size and surface chemistry plays a major role in the density gradient ultracentrifugation rate separation. The wide size range and high spatial resolution obtained with the multilayer gradient separation are especially important when the method is employed for analysis of an unknown sample without any information available. With the analysis of separation mechanism, we can more easily separate the sample with specific features.2. Evaluation criteria for reduced graphene oxideDifferent reduction methods result in reduced graphene oxide with different properties and reduction degree in final products. How to evaluate the reduction methods and choose or develop an appropriate one for a specific application is a troublesome problem. In this thesis, we present a simple classification for the reported reduction methods as followed:using reducing agents, performing under various conditions and the others by the summary and analysis of the literature on reduction methods. And then we have demonstrated a systemic comparison for six selected reduction methods (hydrazine, alkali, sodium borohydride, solvothermal, high temperature and two-step method). The systemic comparison includes four parts:dispersibility, reduction degree, defect repair degree, and electrical conductivity. Based on the systemic comparison for the reduction methods, we provide an evaluation criterion for half quanlitively judging the reduction method. The dispersion behavior can be obtained from AFM. Combined XPS spectra with UV-vis measurement, we can simply and clearly gain important information about reduction degree. Using Raman to test the defect on the reduced graphene oxide would be a good choice. According to the electrical conductivity test, we can make an appropriate adjustment for the degree of reduction or defect repair. Under this evaluation criterion, two-step method is the best one for better reduced graphene oxide synthesis in the six reduction methods which we chose. RGO reduced by two-step method has better reduction degree, defect repair degree and electrical conductivity, but the relatively weak dispersibility and tedious preparation process still need to improve.An ideal reduction method is not only to remove the oxygen-containing functional groups, but also to repair the defect to obtain high quality reduced graphene oxide. In practice, the reduction degree and the defect repair degree are the two key factors for effective reduction of graphene oxide. The combination of these two factors will thus give the criterion to choose the best reduction method. In addition, we can rationally design the reaction process for transforming the functional groups (the main oxygen-containing functional groups are difficult to remove or reduce) of graphene oxide before reduction or repair, and also helped to develop one-pot synthesis of reduced graphene oxide according to XPS and Raman data. The evaluation criteria would also be help to understand the mechanism of reduction further and then help to select or develop more effective reduction method.