【作者】 张宇；

【导师】 张好斌；

【作者基本信息】 北京化工大学 ， 材料科学与工程， 2022， 博士

【摘要】 基于氧化石墨烯（GO）组装得到的轻质高强石墨烯薄膜已在电磁屏蔽、电子及储能器件等领域中得到广泛研究。对石墨烯薄膜进行结构设计,同时改善其力学、电学性能可有效提升其应用价值,但这一过程通常受到其组装形态缺陷的限制。将GO组装为致密薄膜时,纳米片的趋肤效应和凝胶化过程会降低成型薄膜的取向程度,并在内部生成应力集中点,最终削弱其力学、电学性能;通过增强层间键合作用可优化薄膜的载荷传递,但此时其取向程度难以得到同步改善,因此薄膜无法兼具高的强度和模量。相比于致密结构,利用GO构筑泡孔结构可为得到的多孔薄膜提供更低密度和丰富活性位点,但未经优化的泡孔形貌将阻碍片间形成足够连接和接触,削弱材料的力学强度和导电性。综上所述,优化石墨烯薄膜的组装结构是提升其性能的关键。为实现这一目的,本工作提出一种基于对纳米片表面化学及其影响认知的新策略,通过调节基元的化学结构或组成,调控石墨烯薄膜的成型过程,因而有效修复其结构缺陷,实现高性能石墨烯薄膜的制备。在此过程中系统性分析了“基元化学-成型影响因素-组装形态”之间的联系。主要成果总结如下:（1）基于化学结构调控策略的高取向导电石墨烯薄膜的制备及其性能研究:为提升液相组装石墨烯薄膜的取向程度,通过调控GO纳米片化学结构,实现其自发规整堆叠。利用改进的氧化方法制备GO（GO-m）,以适度的氧化能力抑制羧基的生成。系统分析GO化学结构与其亲水性、凝胶化能力及组装行为的关系,证明与常规Hummers法的产物（GO-c）相比,GO-m仅具有痕量羧基,利用该化学结构可减弱其分散液的凝胶化能力,有利于纳米片在组装过程中进行构象调整,自发形成取向结构。探究GO薄膜微观结构对其力学性能的影响,在此基础上通过温和化学处理制备还原GO-m（r GO-m）薄膜,使其保持规整致密的堆叠形态,从而具备远优于富含褶皱形貌的r GO-c的断裂强度（545.5 MPa）与模量（47.8 GPa）。对比r GO薄膜的基本性质及形态,证实GO-m的化学结构同时有利于其达到更高的还原程度,这使高取向r GO-m薄膜也具有优异的电导率(1.2×10⁵S m^-1),从而保证其以3μm的厚度在X波段实现41.6 d B的电磁屏蔽效能。经表面化学优化的GO-m纳米片的自发规整组装行为具有普适性,因此利用可规模化的刮涂法,制备了大尺寸长程取向石墨烯薄膜,这为高性能石墨烯薄膜的宏量高效制备提供了新路线。（2）基于化学结构调控策略的氢键增强氧化石墨烯薄膜的制备及其应用研究:为同步改善薄膜的层间键合作用和取向程度,精细调控GO化学结构,在实现其规整堆叠的同时,为其提供更强的层间氢键作用。在GO-m分散液中加入Na OH,使纳米片上的环氧基部分转化为羟基,而得到羟基化GO-m（HyGO-m）。探究Na OH用量对GO的化学结构、对水亲和力及薄膜组装形态的影响,证实液相处理将使HyGO-m羟基含量和氧化程度升高,但同样仅具有痕量羧基,因此表现出接近于GO-m的对水亲和性及规整组装行为;而与GO-m相比,HyGO-m纳米片上分布更多含氧基团,形成层间氢键的能力更强,从而在薄膜内部构建更密集的氢键交联网络。进一步分析液相处理引入的钠离子的影响,以优化Na OH用量,最终使HyGO-m薄膜的力学强度和模量分别被提升至631 MPa和55.9GPa。强层间作用也为HyGO-m薄膜提供在面外方向上的弯曲形变能力,因此可将其作为基底,通过丝网印刷制备柔性电路板。（3）基于化学发泡策略的导电石墨烯-MXene多孔薄膜的制备及其性能研究:为增强石墨烯薄膜在电磁屏蔽领域的应用优势,对致密薄膜进行化学发泡处理,制备轻质导电多孔薄膜,在此过程中利用调节基元化学组分的方法优化其泡孔形貌。将GO和MXene液相共混制备GO-MXene（G-M）致密薄膜,探究基元组分对其化学性质和发泡行为的影响。相对于GO,MXene具有更少的含氧基团,因此通过调节MXene组分的含量,可有效减少G-M薄膜总体的反应活性位点,使其在化学发泡处理中排放更少气体,抑制内部泡孔生长和结构过度膨胀,最终得到完善的连续交联泡孔形貌;与r GO多孔薄膜相比,r G-M在受控发泡过程中保留了更多片间交联位点,这提供高效的载荷传递能力,使其最大断裂强度达到24.5MPa,为轻质多孔组装薄膜的最高值之一。对比致密薄膜和多孔薄膜的电磁屏蔽性能,证明r G-M的多孔形貌为其提供延长的电磁波传播路径,使其损耗电磁能量的能力增强,得到更高的屏蔽效能（33.2 d B）;探究基元组分、材料厚度及热处理因素的影响,在此基础上将r G-M多孔薄膜的屏蔽效能提升至52.6 d B。基于前驱体化学发泡行为调控的策略可有效均衡多孔薄膜的密度和电导率,制备综合性能优异的轻质多功能石墨烯薄膜。更多还原

【Abstract】 Graphene oxide（GO）can be assembled as a basic unit to obtain lightweight,high-strength graphene films,which have been extensively investigated in the fields of electromagnetic shielding,electronics,and energy devices.The structural design of graphene films and the improvement of their mechanical and electrical properties at the same time can effectively enhance their values of application,but this process is often limited by defects in their assembled structure.When assembling GO into dense films,the skin effect and the gelation process of the nanosheets reduce the orientation of the formed film and create stress concentration points inside it,ultimately weakening its mechanical and electrical properties;the load transfer of the films can be optimized by enhancing their interlayer bonding,while the orientation degree cannot be improved simultaneously,so the films can’t combine high strength and modulus.Compared to dense structures,the construction of porous structures using GO can provide lower densities and abundant active sites for the resulting porous films,but unoptimized porous morphology will prevent sufficient connections and contacts between sheets,weakening the mechanical strength and electrical conductivity of the materials.To sum up,the optimization of the microscopic assembly structure of graphene films is the key to improving their performance.To achieve this,this work proposes a new strategy based on the knowledge of the surface chemistry of nanosheets and its effects,to regulate the forming process of graphene films by modulating the chemical structure or composition of the basic units,thus effectively repairing the structural defects,and achieving the preparation of high-performance graphene films.In this process,a systematic analysis of the relationship between“unit chemistry-factors influencing forming-assembled morphology”is also conducted.The main results are summarized below.（1）Preparation of highly aligned and electrically conductive graphene films based on chemical-structure-engineering strategy and study of their performances:To enhance the orientation degree of liquid-phase assembled graphene films,the chemical structure of GO nanosheets is modulated to achieve their spontaneously regular stacking.A modified oxidation method is applied to prepare GO（GO-m）,utilizing its proper oxidation capacity to inhibit the formation of carboxyl groups.Systematic analysis is conducted about the chemical structure of GO in relation to its hygroscopicity,gelation capacity and assembly behavior.The results indicates that,compared to the product of the conventional Hummers’method（GO-c）,GO-m only has trace carboxyl groups,and this chemical structure can be used to reduce the gelation ability of the GO dispersion,and facilitate the conformational adjustment of the nanosheets during assembly process to spontaneously form an oriented structure.The influence of the microstructure of GO films on their mechanical properties is investigated.On this basis,reduced GO-m（r GO-m）films are prepared by mild chemical treatment to maintain the regular and dense stacking morphology,resulting in a much better fracture strength（545.5 MPa）and modulus（47.8 GPa）than those of r GO-c with a wrinkled morphology.A comparison of the basic properties and morphologies of r GO films confirms that the chemical structure of GO-m also favors a higher degree of reduction,which allows the highly oriented r GO-m films for an excellent electrical conductivity(1.2×10⁵S m^-1),thus guaranteeing an electromagnetic shielding effectiveness of 41.6 d B at X-band with a thickness of 3μm for the film.The spontaneously regular stacking behavior of GO-m with optimized surface chemistry is universal,and thus a scalable blade coating method is utilized to prepare a large-size long-range oriented graphene films,which provides a new route for the large-scale and efficient preparation of high-performance graphene films.（2）Preparation of hydrogen-bond-reinforced graphene oxide films based on chemical-structure-engineering strategy and study of their applications:To simultaneously improve the degree of interlayer bonding and orientation of the films,the chemical structure of GO is finely tuned to provide stronger interlayer hydrogen bonding while achieving their regular stacking.The hydroxylated GO-m（HyGO-m）is obtained by adding Na OH to the GO-m dispersion,which results in the partially conversion of the epoxy groups on the nanosheets to hydroxyl groups.The effect of Na OH usage on the chemical structure,affinity to water,and assembled morphology of GO film is investigated,and it is confirmed that HyGO-m obtains the increased hydroxyl content and oxidation degree after liquid phase treatment,while also decorated with only trace carboxyl groups,thus exhibiting a water affinity and regular stacking behavior close to that of GO-m;compared with GO-m,HyGO-m nanosheets are decorated by more oxygen-containing surface terminations,and are thus more capable of forming interlayer hydrogen bonds,resulting in a denser hydrogen bonding crosslinking network inside the films.The effect of the Na⁺ions introduced by the liquid phase treatment is further analyzed to optimize the Na OH dosage,and the mechanical strength and modulus of the HyGO-m films are ultimately elevated to 631 MPa and 55.9GPa,respectively.The strong interlayer interaction also provides HyGO-m films with the ability to perform the bending deformation in the out-of-plane direction,so they can be used as substrates for the preparation of flexible circuit boards by screen printing.（3）Preparation of electrically conductive graphene-MXene porous films based on chemical foaming strategy and study of their performances:To enhance the advantages of graphene films for electromagnetic shielding applications,the dense films are chemically foamed to prepare lightweight conductive porous films,during which the porous morphology is optimized by adjusting the components of basic units.GO-MXene（G-M）dense films are prepared by liquid phase mixing of GO and MXene,and the influence of the components of basic units are investigated on their chemical properties and foaming behaviors.Compared with GO,MXene has fewer oxygen-containing functional groups.As a result,the introduction of MXene can be utilized to effectively inhibit the overgrowth of pores in reduced GO-MXene（r G-M）porous films,allowing them to construct an improved continuous crosslinked cellular porous morphology.Compared to r GO porous film,r G-M retains more interlamellar crosslinking sites during controlled foaming,which provides efficient load transfer capabilities,resulting in a maximum fracture strength of 24.5 MPa,one of the highest values for lightweight porous assembled films.The electromagnetic shielding performance of dense and porous films are compared,demonstrating that the porous morphology of r G-M provides an extended path for electromagnetic wave propagation,resulting in an enhanced ability to attenuate electromagnetic energy and a higher shielding effectiveness（33.2 d B）.The influences of components of basic units,material thickness,and heat treatment are investigated,and,on this basis,the shielding effectiveness of the r G-M porous film is elevated to 52.6d B.Based on the strategy to modulate the chemical foaming behavior of precursor,the density and electrical conductivity of porous films can be effectively balanced to produce lightweight multifunctional graphene films with excellent overall performances.更多还原