Preparation And Performance Of Hybrid Materials Based On Two-dimensional Nanosheets

Two-dimensional (2D) nanomaterials such as graphene and MXenes have been widely applied in various areas including energy storage,environment purification, and polymer nancomposites due to their excellent physical, chemical and mechanical properties. However, 2D nanomaterials themselves alone still have some shortages and limitations which seriously prevent their potential in wide applications. For example,graphene is very easy to aggregate due to the strong interactions between nanosheets; graphene as filler is difficult to be uniformly dispersed in polymer matrices because of its chemically inert surfaces. MXenes have high electrical conductivity and abundant functional groups; however,large contact resistance of MXenes will prevent MXenes from forming a conductive network. Therefore, to prepare 2D nanomaterial based hybrids is an important way to broaden the application of 2D materials. In this work, by using supercritical CO2 (scC02), we synthesized many hierarchical graphene based hybrid and conquer the difficulty for hybrid component to be dispersed on graphene surfaces, broadening the application of 2D graphene; moreover, by constructing continuous conductive MXene networks, composites with high electrical conductivity and electromagnetic shielding effectiveness (EMI SE) are prepared. The main work is as follows:(1) In situ reduction of iron oxide with graphene for convenient synthesis of various graphene hybrids: The synthesis of uniformly dispersed catalyst nanoparticles on graphene surfaces is critical for further structure construction of 3D CNT@Graphene (CNT@G). In this work,zero-valent Fe nanoparticles are homogeneously distributed on the chemically inert surfaces of graphene by scCO2-assisted deposition of Fe2O3 and subsequent in-situ carbothermal reaction with graphene as reductant. The reduction mechanism of Fe2O3 with graphene is proposed and verified on account of the XRD and TGA-MS characterization.CNT@G is prepared by CVD method of the in situ synthesized Fe nanoparticles which are coated on graphene and utilized as the catalyst for CNT growth. In addition, y-Fe2O3@G and a-Fe2O3@G hybrids also are conveniently obtained by oxidizing Fe particles at different oxidation conditions.(2) scCO2 assisted synthesis of hierarchical AIOOH@reduced graphene oxide hybrid for efficient removal of fluoride ions: in this work, we report a facile and green method to synthesize hierarchically structured AlOOH@RGO hybrids by scC02-assisted deposition. SEM images shows AlOOH nanolayers are homogeneously and perpendicularly assembled on the RGO substrate, forming a 3D structure with large surface area (513 m2·g-1).The special structures with abundant pores can provide more active sites available for the removal alien substance. Therefore, excellent fluoride removal performance is achieved especially at low concentrations for A1OOH@RGO adsorbents. The maximum fluoride adsorption capacity of A1OOH@RGO is as high as 118.7 mg·g-1, which is one of the best values for Al based adsorbents. The stable and robust hierarchical structures of hybrids endow excellent separability the promising adsorbent for fluoride ions. Moreover, the adsorption mechanism for of A1OOH@RGO and the effects of pH value and co-existing ions on adsorption performance is also analyzed.(3) Decoration of defect-free graphene nanoplatelets with alumina for thermally conductive and electrically insulating epoxy composites:thermallly conductive materials with excellent electrical insulation performance are highly demanded for the thermal management of electronics and systems. High quality graphene with the highest thermal conductivity is a promising candidate, however, it is still a huge challenge to suppress its high electrical conductivity while retain its excellent thermal conductivity. Herein, defect-free graphene nanoplatelets (GNPs)anchored with Al2O3 nanolayers were prepared separately by supercritical dioxide assisted methods. Thus high thermal conductivity and electrical insulation is realized by utilizing the superb thermal conductivity of GNP and suppressing its high electrical conductivity by coating insulating Al2O3 nanolayers. Once incorporated into the epoxy matrix, the Al2O3@GNP/epoxy composites with high thermal conductivity and electrical insulation are also prepared. Al2O3@GNP/epoxy composite possesses a high thermal conductivity of 0.96 W ml·K-1 while maintaining good electrical insulation feature, 436% higher than that of neat epoxy (0.22 W·m-1·K-1), indicating its promising potential as thermally conductive and electrically insulating fillers for polymer composites.(4) Electrostatically assembled 3D Ti3C2Tx@polystyrene nanocomposites for electromagnetic interference shielding: high filler content is usually required for polymer nanocomposites with excellent electrical conductivity due to the large contact resistance. To solve this problem, we first synthesized positively charged polystyrene (PS)microspheres using a cationic organic quaternary ammonium salt as stabilizer and co-monomer; then the PS particles are assembled with intrinsically negatively charged Ti3C2T, by electrostatic interaction. The prepared core-shell Ti3C2T,@PS hybrids are further compression molded to successfully form Ti3C2T,@PS composites with 3D interconnected Ti3C2Tx network, giving high electrical conductivity and EMI SE of the composites. The percolation threshold of PS composites is as low as 0.26 vol% and the composite with only 1.90 vol% Ti3C2Tx exhibits an electrical conductivity of 1081 S·m-1 and EMI SE of 54-62 dB at X-band.To the best of our knowledge, these values are among the highest reported results for polymer based composites. Besides, the composite (1.90 vol%)exhibits an storage modulus of 2226 MPa, 54 % and 56 % higher than those of neat PS and the reference sample prepared by the conventional-pressure molding route, respectively.