Investigation On The Functional Applications Of Graphene Oxide

Graphene-based materials have attracted extensive attention around the world since Andre K. Geim and Konstantin Novoselov peeled a few graphene sheets from highly oriented pyrolytic graphite by a ’scotch tape’ method in 2004. As an almost perfect two-dimensional crystal material, graphene presents lots of the excellent physical properties including electronic, optical, thermal, mechanical, magnetic, chemical properties and so on, and it provides broad prospects in practical applications. Great progress in graphene-based materials has been made, but it still faces huge challenges, especially in the large-scale production, green technology, self-assembly technology,3D functional materials and new composite materials. Every specific application requires the corresponding structure and properties of graphene-based materials.Recently, graphene oxide (GO) synthesized from graphite powder by a modified Hummers’ method, has traditionally served as a precursor for graphene, but now has become a promising basic building block to make graphene-based derivates. It can make up some shortcomings in the preparation and application of graphene-base materials, because of its significant advantages such as low cost, easy preparation, scalable production, good solubility, chemical stability, low toxicity and biological compatibility. Abundant oxygen-containing functional groups in the basal plane and at the edges, such as epoxide, hydroxyl, carbonyl and carboxyl, allow the interactions between GO and a wide range of organic and inorganic materials in non-covalent, covalent and/or ionic manner, to create graphene-based derivates with unusual properties. In addition, it also can directly blend or in-situ composite with other functional nanomaterials to obtain composite functional nanomaterials. These applications mentioned above are based on the understanding of the structure and properties of graphene oxide, however, there are still many mysteries of the properties and applications of graphene oxide, which restrict its further use, in particular, the functional applications of graphene oxide needs to be further explored.In this work, GO was prepared by a modified Hummers’ method, and its functional applications were further explored before and after functionalized modifications. These research results are as follows:1. Without using any surfactants or stabilizing agents, a facile and eco-friendly ultrasonication-assisted exfoliation strategy was developed to fabricate GO sheets with different sizes. GO sheets were self-assembled into various functional structures, including films, microfibers, submicron rods, and nanorods at the liquid/air interface. The relevant assembly mechanisms of various functional structures were proposed on the basis of the repulsive electrostatic forces, attractive van der Waals forces, and π-π stacking.2. Borate cross-linked reduced graphene oxide (B-RGO) sheets were synthesized by using GO sheets, H3BO3 and NaOH as precursors through a mild hydrothermal process in one-pot. Interestingly, the resulting B-RGO sheets with good optical and electrical properties were assembled into highly conductive B-RGO paper by a simple filtration through an Anodisc membrane filter. Based on the analyses of EDX, XPS, XRD, Raman and FTIR, it was reasoned that borate ions act as a cross-linking agent to patch up B-RGO sheets in the basal plane, and NaOH serves as an effective reducing agent to restore a part of of graphitic framework in the hydrothermal process. The synergistic effects of borate ions and NaOH enhance the electrical conductivities of the resulting B-RGO sheets and B-RGO paper.3.3D porous N-RGO hydrogels were easily synthesized by using GO sheets and Hexamethylenetetramine ((CH2)6N4, HMT) as precursors through a facile hydrothermal process in one pot. HMT acts as not only a nitrogen dopant and reducing agent in the hydrothermal reaction process, but also a modifier in the formation process of the 3D porous hydrogels. Interestingly, the 3D porous N-RGO hydrogels with high specific surface area and abundant functional groups exhibit excellent adsorption capability towards various organic dyes, such as MB, pure blue ink and rhodamine 6G4. A facile Mn(II)-assisted assembly was designed to fabricate microbowls by using GO nanosheets as basic building blocks in aqueous solution. The dispersive GO nanosheets with a size of ～50 nm were exfoliated ultrasonically from the oxidized soot powder in deionized water, and then assembled into microbowls assisted by Mn(II) coordination based on a coordinating-tiling-collapsing manner. The as-synthesized microbowls have a caliber size of 3-6μm and a wall thickness of nanometer scale. The morphological evolution process and the formation mechanism were analyzed based on the observations of the GO’s microstructures. A room temperature superparamagnetism of the as-prepared microbowls was observed interestingly.5. A convenient method was developed to synthesize ZnO QDs/RGO nanocomposites by using ZnO QDs solution and GO solution as precursors through a mild hydrothermal process. Through electrostatic interaction and subsequent chemical interactions, the positively charged ZnO QDs were anchored onto the negatively charged RGO sheet surface to form ZnO QDs/RGO nanocomposites. The as-synthesized nanocomposites exhibited enhanced photocatalytic activity on methylene blue compared to that of ZnO powder under UV illumination. This strategy was also applied to anchor various nanostructures (Fe3O4, Au and Ag NPs) onto the RGO sheet surface to form different functional nanocomposites.