【作者】 郭鹏；

【导师】 宋怀河；

【作者基本信息】 北京化工大学 ， 材料科学与工程， 2010， 博士

【摘要】 石墨烯是一种由碳原子紧密堆积构成的二维晶体,是包括富勒烯、碳纳米管、石墨在内的碳的同素异形体的基本组成单元。自2004年首次报道独立存在的石墨烯以来,它在力学、热学、电学、光学等方面的优异性能,使之成为近年来化学,材料科学及物理学领域的研究热点。但是石墨烯具有不易大量制备,宏观以聚集态形式存在的缺点,为了充分利用其高强度,高模量,高导电性,良好的耐化学耐热性,高比表面积等特点,需要对其结构和形貌进行深入考察。作为炭材料最小的构筑单元,二维大分子石墨烯在水油界面和催化剂作用下具有自组装潜力。此外,由于其特殊的二维晶体结构,高的纵横比及高的电子迁移率使其在储能领域具有广阔的应用前景,但其在储能领域的应用范围及储能机理还有待进一步拓展与考察。通过物理或者化学改性的方法将石墨烯应用于聚合物基复合材料的力学增强方面也具有重要的学术价值。本论文旨在针对石墨烯纳米片的制备,组装及应用开展前瞻性研究。以石墨为原料,利用化学氧化法,设计制备具有不同片层厚度和表面化学特征的石墨烯,采用SEM、TEM、HRTEM、AFM、XRD、BET和一系列电化学手段系统考察石墨烯的形貌、结构和作为锂离子电池负极材料及导电添加剂的电化学性能,包括可逆容量、库仑效率、循环性能、储锂机理及动力学性能等；并在此基础上,分析石墨烯的形貌、结构与其电化学性能的相关性,得出高纵横比,高导电率对石墨烯高容量,高倍率及循环性能的影响因素。利用水油乳液界面或催化剂分别在常温和高温下自组装氧化石墨烯,设计制备了石墨烯中空微球和石墨烯包覆金属微球,探明了石墨烯作为其他维度炭材料的基本构筑单元的自组装特性。此外,考察了石墨烯作为环氧树脂复合材料添加剂的性能,通过化学修饰,超声分散等手段制备了分散及界面结合良好的石墨烯／环氧树脂复合材料,并与碳纳米管／环氧树脂复合材料进行对比性考察,探讨了添加量对复合材料力学性能,热学稳定性的影响。这些研究拓宽了石墨烯的研究领域并促进了石墨烯在储能,自组装,复合材料领域的发展,具有重要的理论和现实意义。研究结果表明,氧化处理时间、氧化剂添加量、热剥离温度、热剥离时间、超声剥离功率,超声剥离时间等工艺参数显著影响着可剥离石墨及其膨胀倍率,形貌,比表面积,导电率等。石墨烯作为锂离子电池负极材料,在0.2 mA cm-2电流密度下,可逆容量达672mAh g-1,为同电流密度下人造石墨的2倍。在较大电流密度下(1 mA cm-2)充放电时,可逆容量可达到554 mAh g-1,表现出良好的循环性能和倍率性能。交流阻抗谱图分析得出石墨烯的高倍率性能主要受锂离子固相扩散速率的影响,特殊的高纵横比和导电性能可获得较低的内阻。具有高导电率及纵横比的石墨烯作为导电材料,与活性材料石墨以特殊的面接触方式构建锂离子电池负极,可有效提高电极的电化学活性,并降低电荷转移电阻。在相同用量下,与常用的乙炔黑导电剂相比,负极材料的比容量提高25-40%,库仑效率提高10-15%；随着石墨烯添加量的提高(2-10%),复合电极的比容量,循环性能,倍率性能也逐渐提高。石墨烯添加量从2%上升到10%,可逆容量由180提高到422 mAh g-1。以人造石墨为原料,使用水油乳液法制备石墨烯中空微球。适中的乳化时间,乳化后排水速度有利于形成粒径均匀,形貌规整的石墨烯中空微球。氨分子与石墨烯表面官能团的反应及氢键结合,对于乳液环境石墨烯中空微球形成作用较大。与石墨烯电极相比,当电流密度为0.2及1 mA cm-2时,石墨烯中空微球可逆容量分别为485及310mAh g-1,均高于人造石墨的可逆容量,但低于石墨烯的可逆容量,主要是由石墨烯中空微球表面丰富的含氧官能团所造成的。良好的循环与倍率性能,主要是由于石墨烯壳层结构便于锂离子在其两面嵌入,且球形中空的石墨烯堆垛碳骨架在充放电过程中起了缓解应力的作用。以氧化石墨烯为碳源,通过与金属盐等混合得到复合物,利用惰性气体保护的高温管式炉在1500℃处理2小时得到直径在2-10μm的石墨烯包覆金属微球,该包覆结构具有规则的六元和五元环的表面形貌特征。氧化剂添加量,热处理温度,热处理时间,催化剂比例,催化剂类型,催化剂与氧化石墨烯的混合方式等工艺参数显著影响石墨烯包覆金属微球的形貌。通过分析推断石墨烯包覆金属微球是利用碳的固相溶解析出得到,微球表面的六元环结构证实了石墨烯包覆结构的高度石墨化特征。利用盐酸处理产物得到石墨烯纳米片层与石墨烯中空微球杂化材料。该材料具有高比容量(420 mA g-1),良好的循环性能,高倍率性能优良等电化学特征,这与其特殊的高纵横比,高导电性有关,更与高温催化法得到产物的石墨化特点有关。以石墨烯纳米片和多壁碳纳米管为增强剂,利用超声波分散,浇注法制备了石墨烯／环氧树脂(GNS/epoxy)和多壁碳纳米管／环氧树脂(MWCNTs/epoxy)复合材料。GNS/epoxy复合材料的力学性能好于同比例添加量下的MWCNTs/epoxy复合材料。当添加量同为6 wt%时,GNS/epoxy复合材料的拉伸强度(72 MPa)、弹性模量(1279 MPa)和断裂伸长率(11.5%)均高于MWCNTs/epoxy复合材料的55 MPa,979 MPa及11.3%。MWCNTs/epoxy复合材料界面增韧机制与短切碳纤维类似,通过界面将环氧树脂基体的应力传递到碳纳米管上。而GNS/epoxy复合材料则利用石墨烯丰富的纳米孔道、边缘极性官能团与聚合物基体进行界面键合,通过互锁机制,将应力传导到平行于石墨烯的方向。随添加量的提高,两种复合材料的起始及终止分解温度均有提高。当添加量同为6 wt%时,GNS/epoxy复合材料的起始及终止分解温度与纯环氧树脂相比,分别提高36及194℃,高于MWCNTs/epoxy复合材料的15及165℃。这主要是归功于石墨烯自身良好的热稳定性以及碳化骨架保护作用。更多还原

【Abstract】 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.更多还原