Tuning laminar structures of graphene oxide-based membranes for water treatment

Membrane separation technologies are gaining attention for sustainable water supply since they are energy efficient, environmentally benign and economical. However, development of state-of-the-art membrane materials with well-defined nanostructures and excellent selectivity, high permeate flux and antifouling properties, remains challenging. Due to their excellent physical and chemical properties, two-dimensional (2D) graphene oxide (GO) is promising for use as building blocks for membrane construction. This thesis describes innovative research which constructs and tunes laminar structures of GO-based membranes for water treatment. In this thesis, membrane separation technologies and membrane materials were introduced. Bioinspired conception for fabrication of polymeric, ceramic, metallic and nanomaterial-based free-standing membranes was then reviewed. GO could be combined with traditional membrane materials, forming composite membranes for enhanced separation performance. GO nanosheets (NS) could also be assembled into laminar structures solely for molecular and ion sieving. Laminar structures are attractive for designing GO-based membranes with desirable properties, which was a focus of this thesis. Precisely tuning the crumpled laminar structure of GO-based membranes is crucial. I attempted to reduce GO and intercalate nano-fillers into the laminar structure. I developed a novel 1D graphitic carbon nitride nanotube (g-C3N4 NT) intercalated reduced GO (rGO) nanofiltration (NF) membrane with enhanced photo-induced self-cleaning performance. The g-C3N4 NT photocatalyst enlarged the rGO interlayer spacing for improved water permeability and endowed the composite membrane with visible-light photocatalytic activity for efficient removal of organic dyes from water. My g-C3N4 NT/rGO membrane exhibits superior water permeability (4.87 L·m−2·h−1·bar−1) and synergetic Rhodamine B (RhB) removal rate (> 98%) for long-term operation. The RhB removal mechanism on the as-prepared composite membranes under integrated photocatalytic filtration was also established. Apart from reducing GO and increasing laminar interlayer space to optimize the laminar structure of GO, I explored the potential for preparation of "nanoporous GO membranes" by in-situ ion beam modification. Ultra-thin (150-200 nm) GO films were modified by low energy carbon ion beams with ion fluences of 1 × 1015 ions·cm-2 - 1 × 1017 ions·cm-2. Low energy carbon ion beam irradiation can simultaneously reduce and drill nanoscale pores on GO surfaces in a controllable manner, which could be used for engineering GO-based separation membranes. Ion beam modification was then used to prepare real GO ion-sieving membranes. The thickness of the GO layer and the ion fluence applied control the ion beam irradiated GO structure and the consequent mono-/di-valent metal ion separation performance. All ion beam irradiated GO membranes exhibited enhanced K+ selectivity compared with the ion separation performance of the corresponding GO membranes without ion beam treatment. Pure GO membrane with 0.5 mg GO loading amount exhibited a K+ ion permeation rate up to 1.4 × 10-3 mol·m-2·h-1 and an infinite separation factor to di-valent ions, after ion beam irradiation with fluence of 1 × 1016 ions·cm-2. Ion beam modification was also successfully applied to EDA-modified GO membranes for enhanced selectivity, demonstrating the potential for applicability to other chemically modified GO membranes. This work is a step towards the development of high-performance GO-based membranes for use in water desalination, gas separation and biomedical applications.