【作者】 李琦；

【导师】 熊传溪； 董丽杰；

【作者基本信息】 武汉理工大学 ， 材料学， 2012， 博士

【摘要】 石墨烯具有独特的载流子输运性质、超高的比表面积、电导率、热导率以及机械强度,在自然科学界引起了广泛关注。然而,这种柔软的超薄二维碳质纳米材料极易自发形成褶皱和折叠结构,导致基于石墨烯的材料和器件性能大幅下降。因此,开发一种简单可行的方法消除石墨烯的褶皱和折叠结构、提高石墨烯材料和器件的性能具有重要的理论意义和应用价值。本博士论文通过采用三种不同的化学方法对石墨烯进行表面修饰制备了具有无溶剂纳米液体特性的石墨烯类流体,使石墨烯片层的褶皱、折叠结构被消除；系统研究了石墨烯类流体的结构、性能及潜在应用。主要研究内容和结果如下：1、通过苯胺单体在氧化石墨烯表面的原位氧化聚合制备了聚苯胺包覆石墨烯；利用聚苯胺的质子酸掺杂机制将柔性长链离子引入到体系中,得到聚苯胺包覆法石墨烯类流体(PANI-GF)。红外光谱(FT-IR)、紫外可见光谱(UV-vis)、元素分析、扫描电镜(SEM)以及透射电镜(TEM)结果表明PANI-GF是以石墨烯为主体、以柔性长链离子掺杂聚苯胺微纤为包覆层的片状杂化纳米材料；流变性能测试数据显示PANI-GF具有类液体特性,室温下粘度为2800Pa.s。2、通过对聚苯胺包覆氧化石墨烯进行化学还原制备了水溶性石墨烯。元素分析数据表明,这种水溶性石墨烯中聚苯胺的质量分数仅为3%。SEM和TEM图片显示,在这种水溶性石墨烯表面不存在聚苯胺微纤结构。如此低含量的修饰分子就能够保证石墨烯材料具有优异的水分散性,这为制备水溶性石墨烯材料提供了一种简单可行的新方法。3、通过3-(三甲氧基硅丙基)二甲基十八烷基氯化铵(DC5700)与氧化石墨烯表面含氧官能团的脱水缩合反应制备了氧化石墨烯有机离子盐；用柔性长链离子与经过化学还原的石墨烯有机离子盐(RGO-DC5700)进行离子交换制备了偶联接枝法石墨烯类流体(CG-GF)。FT-IR和能量弥散X射线(EDX)能谱表明DC5700以及柔性长链离子已接枝到石墨烯表面；TEM图片显示CG-GF中的石墨烯片层十分平整,没有褶皱和折叠结构。流变性能数据表明CG-GF具有类液体性质,粘度比PANI-GF低,室温下为120Pa.s。4、合成了对氨基苯磺酸重氮盐,利用石墨烯表面电子向重氮盐的自发转移机制将重氮盐接枝到石墨烯表面,得到磺化石墨烯(SG)；通过柔性长链离子与SG离子交换制得重氮化合物接枝法石墨烯类流体(Dia-GF)。X射线光电子能谱(XPS)、FT-IR以及EDX能谱表明重氮化合物和柔性长链离子已接枝到石墨烯表面；TEM图片显示Dia-GF也具有平整的微观形貌,在片层表面观察不到任何褶皱与折叠结构；流变性能数据表明Dia-GF具有类液体性质,室温下粘度为170Pa.s；电性能测试结果表明Dia-GF的室温电导率高达233S/m。5、将石墨烯类流体的微观形貌与零维及一维类流体建立了科学的联系,构建了石墨烯类流体的展平模型,提出了石墨烯类流体结构微元相互排斥、流动解缠以及流动取向的自展平机制。通过研究NPE-SG、CTAB-SG以及NPEQ-LG-SG的形貌结构及流变特性,揭示了石墨烯自展平行为的三个关键影响因素,即：有机配体间的离子键作用形式、有机配体的柔性链结构以及有机配体的接枝密度。6、将石墨烯类流体悬浮液滴延在普通铝箔基板上进行SEM观察,发现类流体中的石墨烯片层在铝箔表面完全展平开来；采用涂膜器将石墨烯类流体涂覆在平整的PET基板上并测试宏观薄膜的导电性能,结果表明薄膜电导率高达260S/m,比具有同样有机含量的普通石墨烯薄膜材料的电导率高15倍。这可以归结为不褶皱石墨烯中相邻π电子云的“肩并肩”交叠程度增大,使石墨烯中的π电子离域程度提高。7、分别将石墨烯类流体(Dia-GF)和溶剂热法石墨烯(ST-RGO)与聚偏氟乙烯(PVDF)进行溶液共混；干燥并热压处理后得到石墨烯类流体/聚偏氟乙烯(GF/PVDF)和溶剂热法石墨烯/聚偏氟乙烯(Gr/PVDF)两种复合薄膜。介电性能测试结果显示,两种复合薄膜的渗流阈值比较接近,且均较低,约为0.32vo1.%。纯PVDF薄膜的介电常数仅在10左右,而GF/PVDF和Gr/PVDF在接近渗流阈值处(0.24vol.%)的介电常数分别高达376和139,并且GF/PVDF的介电常数明显更高,这是因为石墨烯类流体在聚合物基体中排列更加有序、片层更加伸展、能够形成更多的“纳电容器”结构。更多还原

【Abstract】 Graphene possesses unique carrier transport properties, ultra-high specific surface area, remarkable electrical and thermal conductivity, as well as outstanding mechanical strength, and thus has attracted extensive attention from natural science communities. However, such soft and ultra-thin carbonaceous membrane tends to form wrinkled and folded structures spontaneously, resulting in substantial decrease of material and device performance. Therefore, developing a simple yet feasible route to eliminate those unfavorable structures and promote material and device performance is of great significance for both fundamental research and application. In this thesis, solvent-free graphene fluids with liquid-like feature were prepared via surface-modification of graphene by employing three different chemical methods, and in these graphene materials the wrinkled and folded structures were removed. We also systematically studied the structures, properties as well as potential applications of the solvent-free graphene fluids. The main content and result of this thesis were listed as follow:1. Polyaniline (PANI) coated graphene was prepared by employing in situ oxidizing polymerization of aniline monomers over the surface of graphene oxide, and then flexible long chain ionic liquid was doped to the backbone of PANI molecule by making use of the protonic acid doping mechanism of PAIN to produce solvent-free graphene fluid (PANI-GF). Fourier transform infrared (FT-IR) spectroscopy, UV-visual (UV-vis) spectroscopy, elemental analysis, scanning electron microscopy (SEM) as well as transmission electron microscopy (TEM) revealed that such hybrid material consisted of graphene sheets coated with flexible long chain doped PANI nanofibers. The rheological measurement indicated that the resultant material behaved in a liquid-like manner and the viscosity was2800Pa-s at room temperature.2. By conducting a chemical reduction on GO@PANI, the water-soluble graphene was obtained. The result of elemental analysis indicated that the PANI fraction in water-soluble graphene was only as low as3%. The results of SEM and TEM observation demonstrated that there was no PANI nanofibers remained on the surface of graphene sheets. Remarkably, the product was very stable in aqueous medium, which offered a simple and feasible route to produce water-soluble graphene.3. By utilizing the dehydration and condensation reaction between organic silane (DC5700) and the oxygen-containing groups on graphene oxide, the graphene oxide organic salt was obtained, which could be chemically reduced to produce graphene organic salt (RGO-DC5700). By further implementing ion exchange on RGO-DC5700with flexible long chain ionic liquid, we obtained the second type of solvent-free graphene fluid (CG-GF). We verified the chemical composition of the product by using FT-IR spectroscopy and energy-dispersive X-ray (EDX) analysis. The TEM images showed that the sheets were completely flat with no detected wrinkled or folded structure. The CG-GF was also characteristic of liquid-like, and its viscosity was remarkably lower over that of the PANI-GF, presenting as120Pa-s at room temperature.4. We also prepared a third-type of solvent-free graphene fluid (Dia-GF) by utilizing the spontaneous electron transfer from graphene to diazonium species followed by a typical ion exchange procedure. X-ray photoelectron spectroscopy, FT-IR spectroscopy, as well as EDX analysis were employed to monitor the whole synthetic procedure and confirmed that the diazonium salt and ionic liquid were grafted to graphene. The TEM images also presented the flat morphology of the resultant sheets, and no any wrinkled or folded structures could be found. The result of rheological measurement implied that the product was virtually liquid-like, and the viscosity was as low as170Pa-s at room temperature. The conductivity measurement indicated that the electrical conductivity of the product reached as high as233S/m.5. Scientific relationship between microscopic morphology of solvent-free graphene fluids and that of solvent-free fluids based on0-dimensional and1-dimensional nanostructures was established. The model for the self-unfolding behavior was constructed. The self-unfolding mechanism of mutual repulsion between micro units as well as flow-induced untangling and flow-induced orienting effects was proposed. By investigating the microscopic morphology and rheological properties of NPE-SG, CTAB-SG and NPEQ-LG-SG, we revealed that the self-unfolding behavior was dominated by three key factors, namely, the ionic interaction between organic species, the soft chain segment and the appropriate grafting density.6. The suspension of a solvent-free graphene fluid was drop-casted on an aluminum foil and observed via SEM. It was found that the sheets with completely unfolded configuration could tie to the flat substrate. We also fabricated a bulk film of the solvent-free graphene fluid on PET substrate by using a wet film applicator. The fact that its conductivity was more than15folds higher over that of ordinary graphene film with the same organic fraction could be attributed to the highly overlapped π orbits of adjacent carbon atoms in a flat graphene sheet, which could promote the delocalization of π electrons.7. The Dia-GF and ST-RGO were fully mixed with PVDF, respectively, using DMF as the solvent. After drying and hot pressing, two kinds of composite films were obtained, which were GF/PVDF and Gr/PVDF. The results of dielectric performance measurement suggested that the two kinds of composite films had very similar percolation threshold which was relatively low, at around0.32vol.%. The permittivity of a pure PVDF film was approximately10, while those of the GF/PVDF and Gr/PVDF composite films near their percolation threshold (0.24vol.%) were as high as376and139, respectively. Besides, the permittivity of GF/PVDF was remarkably higher than that of Gr/PVDF, which was because that the graphene sheets in solvent-free graphene fluid were more orderly arranged and extended, and thus forming more "nano-capacitor" structures.更多还原