Graphene-based multifunctional supramolecular membranes

From a chemistry point of view, graphene is essentially a giant polymeric molecule with unique two-dimensional configuration. As with traditional polymers, graphene and its derivatives can be considered as a new class of molecular building blocks for the formation of a variety of graphene-based macroscopic structures, such as membranes. The properties of graphene-based membranes rely on proper engineering of the structure of graphene sheets and the interactions among them. This project is therefore devoted to investigating the structure and intermolecular interactions of graphene and further exploring applications enabled by the uniqueness of graphene-based membranes. This thesis consists of two main parts. In the first part, the intermolecular interactions of chemically converted graphene (CCG) sheets and their configuration are investigated. By exploiting evaporation-induced deposition methods, self-assembly behavior of CCG sheets at interfaces is revealed. The morphology of the resultant CCG assemblies can be easily tailored by tuning the solution condition or the type of substrate, providing insights into the supramolecular interactions of CCG and its effect on the structure of CCG assemblies. Furthermore, corrugated configuration of CCG sheets in solution is observed by atomic force microscopy tested in liquid state. In contrast to generally impression that the corrugation appears to be static, we find that CCG sheets in solution become more and more corrugated with time. Such a dynamic configuration of CCG sheets leads to modified porous structure of CCG-based membranes when assembling them in a face-to-face manner, and this will also influence future industrial adoption of graphene where storage and transport of graphene and its nanofabrication are required. In the second part, research efforts are made to utilizing the unique features of CCG-based membranes to address key challenges in energy storage and smart membrane areas. Taking polyaniline as the first example, the highly porous, electrically conductive and mechanically stable CCG hydrogel membrane is used as the nanoscaffold to allow in-situ deposition of polyaniline (PANI) and thus to evaluate its electrochemical performance reliably. With scaffolding role and adaptive nature of CCG hydrogel membrane, the effect of coating thickness of PANI, porous structure of the electrode and charging rate are investigated. The results show that CCG-PANI film can exhibit a combination of high capacitance, excellent rate performance and long cycling life that is needed for practical applications, and proper engineering of its nanostructure is the key to realize it. On the other hand, the strong non-covalent interactions between CCG and non-conjugated polymers are revealed by our developed optical characterization methods. Taking advantages of the two-dimensional configuration of CCG, its self-assembly behavior and rich intermolecular interactions with various polymers, we find that CCG sheets can function as an unprecedented membrane- and pore-forming directing agent, as the structural reinforcement nanofiller and as a physical cross-linker to guide the polymers to form a class of ultrathin, uniform, mechanically robust, adaptive and functional hydrogel membranes. The judicious utilization of the unique features of CCG in this work represents a new concept to greatly simplify the membrane formation process and to facilitate the design of functional hydrogel membranes for emerging ‘smart’ membrane applications. The thesis is concluded with personal views on future opportunities of graphene-based membranes where utilization of the rich supramolecular chemistry of CCG may enable the formation of complex membrane structures with multiple functionalities, which however remains very difficult for traditional carbon materials to achieve.