Preparation of Two-dimensional Electrically Conductive Nanocomposites and Their Performances

Author:Liu Zuo

Supervisor:yu zhong zhen


Degree Year:2018





Two-dimensional(2D)nanomaterials,such as graphene and MXenes,have shown huge potential for applications in various fields due to their excellent physical and chemical properties.To harness the intriguing attributes of graphene and MXenes for practical applications,it is feasible to assemble these nanoscale sheets into macroscopic assemblies,such as fibers,films,and aerogels.Tuning the morphologies and structures of the assemblies during the assembling process can not only utilizes the properties of these nanomaterials as needed but also extend their applications.Therefore,to study the assembly strategy of two-dimensional nanomaterials is of great significance for the preparation of macroscopic functional materials.However,there are still technical obstacles to construct novel macroscopic structures and the assemblies of graphene and MXene also show some dificiencies when they are used in a real environment.For example,the unavoidable degradation of MXene materials in humid air or water and the poor mechanical properties of MXene film may affect their stability and reliability when applied in real environment.In this work,by proposing some novel assembling strategy,we synthesized magnetic porous graphene films for broadband electromagnetic interference(EMI)shielding,high-performance MXene films with excellent mechanical properties and high conductivities,and hydrophobic MXene foams with high electrical conductivity.Moreover,we also report a facile approach of assembling the Ti3C2Tx MXene sheets into superflexible and conductive three-dimensional architectures reinforced by polyimide.The main work includes:(1)To enrich the shielding mechanisms and decrease the densities of pristine graphene films,magnetic,conductive,and lightweight porous graphene films are fabricated in this work by a highly efficient hydrazine-induced reducing and foaming approach,which shows an excellent broadband EMI shielding performance.The cellular structure of pristine porous graphene film is optimized by adding chitosan to enhance the interlayer interactions between the reduced graphene oxide(RGO)sheets.Moreover,considering the synergetic effect of electrical and magnetic constituents on.EMI attenuation,iron pentacarbonyl(IP)flakes are integrated into the conductive porous structure to further enhance the EMI shielding performance.The magnetic and conductive porous film with a density of 0.12 g/cm3 and a small thickness of 0.3 mm exhibits a high electrical conductivity of more than 2000 S/m and an excellent broadband EMI shielding performance of>38 dB in the full frequency range 8.2-59.6 GHz.The positive effects of chitosan on the cellular structure and the favorable contribution of magnetic IP particles on shielding performance are also addressed.(2)In this work,we demonstrate the fabrication of freestanding,hydrophobic,lightweight,and flexible MXene foams by assembling MXene sheets into films followed by a hydrazine-induced foaming process.In contrast to conventional hydrophilic MXene materials,our MXene foam is the first reported hydrophobic and porous MXene material.The foaming mechanism and the conversion of the surface wettability were elucidated.The hydrophobic and porous MXene foam exhibited excellent water resistance and strong absorption capacity.More importantly,a greatly enhanced EMI shielding performance was achieved for the MXene foam compared to its unfoamed film counterpart because of its favorable porous structure.(3)Up to now,MXene sheets have been assembled into thin films and layered paper-like materials for producing macroscopic architectures for practical applications.However,construction of three-dimensional MXene macrostructures with outstanding mechanical properties has never been realized.In this work,we report a facile approach of assembling the Ti3C2Tx MXene sheets into superflexible and conductive three-dimensional architectures reinforced by polyimide.The aerogel is lightweight,superelastic,and mechanically flexible.Furthermore,the aerogel with tunable electrical conductivity and internal cellular structures suggests a great potential in applications of flexible strain sensor,shock/vibration absorbers,thermal shock barriers,thermal insulation/flame-retardant skins,and porous microwave-absorbing materials.The maximum reflection loss(RL)value of the aerogel is-45.4 dB at 9.59 GHz with a thickness of only 3 mm and the effective absorption bandwidth with RL values lower than-10 dB(reflection loss more than 90%)reaches up to 3.7 GHz(8.3-12.0 GHz),which covers almost the whole X-band.For the aerogel with a thickness of 2 mm,the maximum RL value is-25.3 dB at 15.28 GHz and the effective absorption bandwidth reaches up to 5.1 GHz,which is better than most of the literatures about MXene-based microwave-absorbing materials report(4)The highly conductive pristine MXene film usually shows poor mechanical properties with an ultimate tensile strength of~23.2 MPa.However,the electrical conductivity of the MXene film is inevitably decresed by orders of magnitude when the insulating polymers are introduced as cross-linking agents.In this work,MXene based films with excellent mechanical performances and high conductivities are prepared by introducing oxygen-containing graphene oxide(GO)sheets to enhance the interactions between nanosheets.Tensile strength is 2.7 times and 9.0 times higher than that of pristine MXene films when the GO content is 10 wt%and 50 wt%for the MXene-GO films,respectively.Meanwhile,the electrical conductivity of the MXene based film containing 50 wt%GO is measured as high as 46165 S/m,which is promising for applications in high-performance EMI shielding materials and flexible supercapacitor electrodes.This strategy also offers a new avenue for the development of high-performance macroscopic assemblies of MXenes.