Structural Control And Applications Of Crosslinking Graphene Oxide Membranes

Graphene has special 2D monolayer structure and many outstanding properties. Hence, graphene and its derivatives can used as building blocks to construct novel carbon-based materials is an important route to fulfill its final applications. As a significant derivative of graphene, graphene oxide has excellent membrane-formation property, which is potential for application in desalination, pervaporation, supercapacitor, transparent electrode and so on. However, pure graphene oxide membrane becomes very unstable and tends to disintegrate when it is immersed in water. Hence, it is very important to improve its stability. In addition, graphene oxide membranes functions depend on its microstructures and physicochemical properties. In order to achieve desired function, it is vital to design and prepare graphene oxide membranes with suitable microstructures and physicochemical properties. In this study, we focus on the relationship among microstructures, physicochemical properties and function of graphene oxide membranes. By selecting suitable crosslinkers and membrane-preparation methods, we have designed and fabricated graphene oxide membranes with various microstructures and physicochemical properties and investigated their applications.(1) Polyethyleneimine crosslinking graphene oxide laminated film:Graphene oxide nanosheets, with superhigh specific area, can load many substance through hydrogen bond, π-π stacking and van der Waals interaction. Polyethyleneimine, as a kind of branched polymers, has many reactive groups, and its internal cavities can be used as loading sites for many small molecules. Based on such characteristics, we have prepared a polyethyleneimine crosslinking graphene oxide laminated film via filtration method. Because of crosslinking, the mechanic stability of film in water was improved greatly. We have investigated the film’s drug release behavior using ciprofloxacin as model drug. The film showed near-zero order behavior without initial burst effect. Compared to pure graphene oxide flim and polyethyleneimine crosslinking graphene oxide laminated flim, the ciprofloxacin-loaded polyethyleneimine crosslinking graphene oxide laminated flim exhibited excellent antibacterial property.(2) Diamine crosslinking graphene oxide laminated membranes:The 2D nanochannel between adjacent graphene oxide nanosheets has excellent sieve and water-permeation properties, which can be controlled by crosslinking between graphene oxide nanosheets. Based on this fact, we have prepared diamine crosslinking graphene oxide laminated membranes with different d-spacing by seclecting various diamine molecules (ethylenediamine, butanediamine and hexamethylenediamine) as crosslinker. We have investigated the pervaporation performance of these membranes for ethanol/water mixture. Uncrosslinking membrane showed the highest flux but lowest separation factor, while crosslinking membranes showed relatively low flux and high separation factor, and among them, ethylenediamine crosslinking membrane exhibited best performance.(3) Polyethyleneimine crosslinking graphene oxide nonlaminated membrane: Through sol-gel inverse process under high temperature, we have prepared a non-laminated graphene oxide membrane. In non-laminated graphene oxide membrane, graphene oxide nanosheets remained random orientation. The membrane surface is characterized by possessing micro/nanoscale hierarchical structure, which endow the membrane with superhydrophilic and underwater superoleophobic properties. The angle of oil-droplet underwater was as high as 160°. We employed this membrane for separation of a series of oil/water emulsions. High flux and separation efficiency (<10ppm) were achieved under gravity as the only pushing force. Moreover, the membrane showed an outstanding antifouling performance and can be recycled easily for long-term use.(4) Polydopamine crosslinking graphene oxide nonlaminated membrane modified with cyclodextrins:Polydopamine has special autopolymerization, adhesion and adsorption abilities. Cyclodextrins, as a kind of host molecule, has specific inclusion capacity for many organic molecules and heavy metal ions. Through sol-gel inverse process, we have prepared a polydopamine crosslinking graphene oxide nonlaminated membrane modified with cyclodextrins. We has investigated the memebrane for contaminant removal in static and dynamic conditions. The membrane can effectively removel dyes and heavy metal ion of low concentration from water by filtration. Under 0.01 MPa, its flux maintained 12 L/m2h. Compared to orange G and Pb(II), methylene blue can be effectively removed by this membrane with rejection of more than 99%. The results showed that taking advantage of polydopamine, graphene oxide and cyclodextrins, the membrane has synergistic adsorption performance for contaminants.In sum, by selecting suitable crosslinkers and preparation methods, we have designed and fabricated graphene oxide membranes with various microstructures and physicochemical properties and investigate their applications in membrane separation and drug delivery. In addition, we deeply studied the synergistic function mechanism of graphene oxide and crosslinker in specific application. This study would be helpful for designing and fabricating high performance graphene oxide membrane in future.