Design Of Assembled Structure Of Graphene Film And Its Performances Based On Chemistry Modulation Of Basic Units

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.