Surface Functional Modification Of Graphene Oxide And Its Applications

Graphene, which consists of atom-thick sheets of carbon packed in atwo-dimensional honeycomb lattice, has been used in sensors,nanoelectronics, composite reinforcement and capacitors because of itshigh electrical conductivity, high thermal conductivity and superiormechanical properties. Several methods, such as the mechanicalexfoliation, chemical vapor deposition, epitaxial growth on electricallyinsulating surface have been employed in preparing graphene nanosheets.However, scaling up is a major hurdle with these methods. It has beenwell demonstrated that graphene oxide (GO) is an excellent precursor toprepare graphene by ultrasonic exfoliation and chemical reduction. In thiswork, GO was reduced and functionalized with glucose, ethylenediamineand p-phenylene diamine, respectively. And the main work of this thesisincludes:(1) Poly(vinyl alcohol)(PVA) nanocomposite films with enhancedmechanical properties were prepared by solution blending ofglucose-reduced graphene oxide (G-graphene) with aqueous solution of PVA. Poly(N-vinyl-2-pyrrolidone)(PVP) was selected as the surfactant toimprove the stability of the aqueous suspension of G-graphene. With0.7wt%of PVP-stabilized G-graphene (G-P-graphene), the tensile strengthof PVA increases from105MPa to154MPa and the Young’s modulusincreases from3.3GPa to4.9GPa. These substantial improvements wereattributed to the good dispersion of G-P-graphene nanosheets in PVAmatrix and the strong hydrogen-bonding interaction betweenG-P-graphene nanosheets and PVA macromolecular chains.(2) Ethylenediamine-reduced graphene oxide (ED-RGO) sheets wereprepared by simple refluxing of GO solution with ethylenediamine (ED).This was possible by the nucleophilic substitution reaction of epoxidegroups of GO with amine groups of ED. Compared with otherconventional adsorbents, ED-RGO exhibits relatively a high removal rateand can be easily separated from the solution after adsorption. Moreimportantly, Cr(VI) can be effectively reduced to low toxic Cr(III) speciesat low pH, which follows an indirect reduction mechanism with the aid ofπ electrons on the carbocyclic six-membered ring of ED-RGO. It isexpected that ED-RGO can be applied as a novel adsorbent for Cr(VI)removal from wastewater.(3) A facile and efficient approach was developed to simultaneouslyfunctionalize and reduce GO with p-phenylene diamine (PPD) by simplerefluxing. This was also possible by the nucleophilic substitution reaction of epoxide groups of GO with amine groups of PPD. As a consequence,electrical conductivity of GO-PPD increased to2.1×10~2S/m, which wasnearly9orders of magnitude higher than that of GO. Additionally, afterthe incorporation of GO-PPD in polystyrene (PS), the compositesexhibited a sharp transition from electrically insulating to conductingbehavior with a low percolation threshold of～0.34vol%, which wasattributed to the improved dispersion and the reduction of GO-PPD.Thermal stability of the PS/GO-PPD composite was also～8°C higherthan that of PS.(4) Electrically conductive polycarbonate (PC) nanocomposites wereprepared by solution blending of PC matrix with thermal expansiongraphene or GO-PPD, respectively. PC microcellular electricallyconductive foams were obtained by two-step foaming with subcriticalCO2as an environmentally benign foaming agent. The influences offoaming time, saturated pressure and saturated temperature on the cellsize and cell density were investigated. The foaming conditions were alsooptimized. The average cell diameter and cell density of pure PC matrixwere28.2μm and7.85×10~7cells cm~3, respectively. Compared to thermalexpansion graphene, GO-PPD exhibited an improved dispersion in PCmatrix, which was confirmed by SEM, TEM, XRD, TGA and rheologicalmeasurement. The good dispersion of GO-PPD in PC matrix could inhibitthe cell expansion and had high nucleation efficiency during foaming. The average cell diameter and cell density of PC/GO-PPD were0.76μmand5.69×1011cells cm~3, respectively.Compared to that of the bulknanocomposites, the insulator to semiconductor transition of the foamsshifted to lower GO-PPD content because the conductive network wasnot destroyed by the micron-sized cells.