Surface Tuned Electrospun Nanofibers for Enhanced Mechanical and Functional Performance
Electrospun 1-D nanofibers are extensively investigated for diverse applications owing to their unique morphology, size, high aspect ratio, low density, better pore connectivity and ease to fabricate them at large scales. However, inert nature, low density of active sites, poor electrical and thermal conductivities requires their surface modification either by introducing different chemical functionalities or by constructing heterostructures with other suitable materials where nanofibers act as an active support. The choice of surface modification depends on the desired characteristics and application. This thesis is focused on developing chemical and physical surface modification strategies for carbon and ceramic nanofibers and understanding the underlying mechanisms of enhanced performance when used in polymer nanocomposites as reinforcements and in energy generation and environmental remediation as catalysts.
Polymer nanocomposites are advanced materials in which properties of both the reinforcement, usually in nanoscale length regime, and the polymer matrix phase are combined. The impact of interfacial shear strength, tuned by chemical functionalization, on multifunctional properties of carbon nanofibers (CNFs) reinforced epoxy nanocomposites is investigated. A facile approach was adopted to chemically functionalize the CNF through acid oxidation and amidation treatments. The surface functionalized and pristine CNFs were then incorporated as reinforcements in the epoxy matrix. The surface functionalities on CNF surfaces formed covalent interactions with epoxy monomer through an in-situ reaction. A significant enhancement in the tensile strength and storage modulus was observed which is attributed to the better dispersion and strong interfacial interaction. The thermal stability, dielectric properties and AC/DC conductivity were also investigated and discussed in terms of the interfacial characteristics.
In the next step, the fabrication of these carbon nanofibers was slightly modified to induce in-situ growth of nanostructured carbons over carbon nanofibers aided by FeNi bimetallic nanoparticles. These novel hierarchical carbon-based nanostructures were investigated as electrocatalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) reactions. Interestingly, different morphologies of carbon nanostructures, i.e. carbon nanotubes (CNT), graphene nanotubes (GNT) and graphene sheets (GS), were grown over carbon nanofibers by controlling mixing ratios of the precursors. Specifically, a mixture of carbon nanotubes and graphene nanotubes supported over CNF (G/CNTs@NCNF) showed significant enhancement in ORR with superior stability and methanol tolerance to Pt/C in the alkaline media. Whereas, graphene sheets wrapped carbon nanofibers (GS@NCNF) exhibited superior performance with overpotential 230 mV for OER. However, G/CNT@NCNF showed better stability and improved OER reaction kinetics over GS/NCNF, which indicated that CNTs and GNTs promote charge and mass transfer resulting in better overall performance. These results are of great significance for the design and development of advanced electrocatalysts based on nonprecious metals and metal free heteroatom doped carbon-based nanostructures.
Last part of the dissertation focused on exploring synergistic combination of energy generation with simultaneous environmental remediation. A heterojunction photocatalyst was fabricated by in-situ growth using Co zeolitic imidazolate framework nanoplates decorated BaTi2O5 nanofibers (Co-ZIFL@BaTi2O5) with high exposed active sites and improved solar energy harvesting capability. The suitable band matching resulted in simultaneous organic pollutant degradation through oxidation and H2 production by reduction. Moreover, the organic pollutant degradation synergistically improves the kinetics of hydrogen production. This work not only provides a feasible strategy for fabricating dual functional heterojunction catalysts for pollution degradation and clean energy production, but also sheds light on the underlying mechanism for enhanced H2 production by corresponding wastewater treatment.
 Aziz, I.; Lee, J. G.; Duran, H.; Kirchhoff, K.; Baker, R. T.; Irvine, J. T. S.; Arshad, S. N. Nanostructured Carbons Containing FeNi/NiFe2O4 Supported over N-Doped Carbon Nanofibers for Oxygen Reduction and Evolution Reactions. RSC Advances 2019, 9 (63), 36586–36599.
 Aziz, I.; Duran, H.; Saleem, M.; Yameen, B.; Arshad, S. N. The Role of Interface on Dynamic Mechanical Properties, Dielectric Performance, Conductivity, and Thermal Stability of Electrospun Carbon Nanofibers Reinforced Epoxy. Polymer Composites 2021, 9 (42), 4366-4379.