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Structure-performance correlations of engineered graphene based materials for water treatment: experimental and theoretical investigations
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posted on 23.02.2017by Sheath, Phillip Andrew
Water purification technologies would benefit from the rich variety of chemical and physical amenability which graphene based materials are now associated with. For instance, adsorbent materials rely on both physical and chemical characteristics; membranes require microstructural control for physical separations and an optimised photocatalyst would require finely tuned electronic properties. It is becoming increasingly important to understand and control the material processing of graphene based materials towards engineered, high performance, industrially scalable and cost-effective products.
This dissertation aims to investigate graphene oxide (GO) and reduced GO (RGO) material processing techniques and their application in water-treatment technologies towards advancing an understanding of their structure-performance relationship using both experimental and theoretical density functional theory (DFT) techniques.
DFT results showed that on an RGO model, the hydroxyl group enhances the adsorption of methylene blue (MB) dye. The DFT results informed an experimental study in which the MB adsorption capacity of various RGO derivatives is investigated. GO was chemically reduced by sodium borohydride, strategically chosen to produce a surface rich with hydroxyl groups, and its adsorption capacity is compared to the reduction products of hydrazine and thermally reduced GO, all coated onto sand for use within technologically attractive fixed-bed columns. The sodium borohydride derived products adsorb significantly more MB than any of those reduced by hydrazine or by thermal treatment. Detailed chemical and physical characterisation was used to determine that the hydroxyl group is indeed the primary adsorption instigator, although not necessarily the primary adsorption site. In accordance with the theoretical results, the enhanced adsorption is attributed to dispersion interactions which, by the presence of the hydroxyl group, are enhanced by more π-electrons on the graphene basal plane.
The importance of structural order in the rejection and flux capabilities of RGO membranes is determined by comparing those produced by capillary-force assisted self-assembly (CAS, a novel meniscus driven self-assembly process) and traditional vacuum filtration membranes. The CAS membranes displayed inherent microstructural ordering of the RGO platelets, that was manifest in the rejection and permeanace of CAS membranes, and exhibited ~4.5 times improved permeanace and a sharp size cut-off estimated to be 2 nm based on its rejection of organic species. CAS membranes have a larger permeance, despite its smaller nano-channels, as consequence of slip-flow between the hydrophobic nano-channels which constitute RGO membranes and is accentuated due to the ordered structure in CAS.
The cluster expansion, which greatly simplifies the prediction of material physical properties from a handful of DFT calculations, was used to obtain the concentration dependant bandgap and valence/conduction band character of two graphene functionalisations (hydroxylated and fluorinated). It is predicted that hydroxylated graphene could generate hydroxyl radicals under visible light over a narrow range of concentration however fluorinated graphene is predicted to have visible light band gap over a wide range of concentration and has optimal valence band and conduction band for direct and indirect degradation of organic pollutants for the purification of water. Fluorinated graphene is a particularly strong candidate for further investigation by experiment.