The Study In The Exfoliation Of Graphene In Supercritical Carbon Dioxide System

In1981, an important tool for studying nanostructures-scanning tunneling microscope was invented, which marked the advent of the nanotechnology era. Since then, carbon nano-materials began to step into the stage of history. In1985, C60was discovered and its discoverer Walter Harold Kroto won the Nobel Prize in Chemistry in1996; in1991, carbon nanotubes were discovered, then its discoverer Sumio Iijima was awarded with Franklin Medal in2002; in2004, graphene was discovered, thus its discoverer Andre Geim and Konstantin Novoselov won the Nobel Prize in Physics in2010. The three leading roles in the field of the carbon material, have been the focus in various fields since their discovery, but also set off a wave of carbon research boom. Especially, the discovery of graphene materials pushed the study of carbon into another climax.As to its unique two-dimensional crystallographic structure and excellent quality, outstanding electrical, thermal and mechanical properties, graphene attract the attention of researchers in all walks of life. Graphene quickly became one of the hot tops of condensed matter physics and materials science in recent years, covering all aspects of the theory, material preparation and improved applications.In this thesis, exfoliation of graphene material in supercritical carbon dioxide were first studied; followed by the addition of co-intercalation agents to enhance the intercalation-exfoliation of supercritical carbon dioxide, higher yields of graphene were obtained. Exfoliated graphene were characterized By AFM, SEM, TEM and other methods, through which the optimal exfoliation conditions were obtained.The main research work and the results of this paper were as follows:(1) The effects of raw material graphites on the properties of grapheneWe chose two kinds of graphite materials as the reaction precursors and studied its effects on the properties of the sample. Differences in the size of the precursors were reflected on the size of graphene. The size of graphene exfoliated from EG with hundreds of micron size was about few microns, whereas the size of graphene exfoliated from graphite flake with a few microns are hundreds of nanometers, thus the size differ was about an order of magnitude. Except for the size differ, the exfoliating ratio of EG was also very low. As the size of EG were too large, the resolvent molecules so hardly arrived to the inner part of graphite layers that lead to low exfoliating ratio. Therefore, the choose of graphite materials made great influence in graphene.(2) Comparative study of the effect of co-intercalation agents on the yield of grapheneIn pure supercritical carbon dioxide system, exfoliating ratio of graphene was very low. As a solvent commonly used in dispersing graphene, dimethylformamide (DMF) and methyl pyrrolidone (NMP) which were soluble in supercritical carbon dioxide, were introduced into the system in supercritical carbon dioxide as co-intercalation agents to promote the exfoliation of graphene. In addition, sodium dodecyl benzene sulfonate (SDBS) was also added as a comparative study. Settling experiments of Graphene solution showed that the effect of these three solvents on experimental results was:NMP> DMF> SDBS. Under optimal conditions, the concentration of graphene dispersion exfoliated in NMP obtained up to0.13mg/ml, the one of DMF obtained up to0.12mg/ml.(3) The effects of temperature, pressure, time and stirring on the yieldBy control variate method. exfoliating results of graphene in supercritical carbon dioxide system were studied in the range of10-25MPa,40-120℃. It was found that when the temperature is40℃and80℃, exfoliation were better. Maintaining the temperature of80℃, exfoliating under the pressure of15MPa and20MPa reached the best results. We also studied the effect of time on the results, the longer the better, the maximum time of the experiment was24h. In addition, the magnetic stirring was also a great role in promoting the exfoliating. So we got the best exfoliating conditions that was at a temperature of40℃, pressure of20MPa for24h with magnetic stirring.(4) Characterizations were proceeded on the exfoliated grapheneCharacterizations of morphologies, structrue and concentration were taken on exfoliated graphene samples. AFM data displayed that the thickness of the graphene was1.5-3nm, which indicating that the multi-graphene were obtained; XRD data showed that the characteristic peak (002) of graphene was significantly reduced, indicating that the long-range order of graphite was destroyed and the graphite got exfoliated; TEM data suggested some multilayer-transparent structure, and the electron diffraction pattern showed the exfoliated graphene owned good crystallinity, proving that the single-layer or multi-layer graphene sheet were obtained; UV-Vis analysis was used to count concentration of graphene, and the highest concentration was up to0.13mg/ml; particle size analysis showed that the size of the graphene were hundreds of nanometers; Raman data suggested that the exfoliated graphene had few defects,2D peak exhibited multi-graphene.