One-dimensional fibrous materials fabricated at nanoscale such as nanofibers and nanotubes are now fast becoming potential candidates for making composite structures because of their unique morphologies and superior properties. Polymer fibers with diameters of few hundred nanometers up to a few micrometers are commonly synthesized by low-cost and scalable electrospinning technique. These polymer nanofibers can be used as a precursors for making carbon and oxide nanofibers which can be incorporated in composites or further modified by making hybrids with various inorganic and organic constituents. However, the synthetic routes are often complex involving multiple steps with less control over their morphologies and properties. This research work mainly focuses on the fabrication of nanofiber based novel composite structures by developing facile and well-controlled fabrication methods and their potential is demonstrated by using them in energy production, storage and water remediation applications.

In the first part, carbon nanofibers (CNFs) with FeCo nanoparticles (NPs) at their surfaces were fabricated by one-pot electrospinning which resulted in an in-situ growth of carbon nanotubes (CNTs) during heat treatment in an inert atmosphere with FeCo NPs situated at the tips of CNTs@CNFs composites. The growth of CNTs was driven by the absorption of carbon in the FeCo NPs and dewetting of the NPs from the surface of CNF after reaching supersaturation. These CNTs not only prevent FeCo NPs from agglomeration by encapsulating them at the tip but also provide an efficient electron pathway. These materials exhibited excellent activity in overall electrocatalytic water splitting with an over-potential of 283 mV at 10 mA cm-2 and Tafel slope of 38 mV dec-1 for oxygen evolution reaction (OER) with long-term durability of up to 48 hours. In the second part, the synthesis approach was modified where polymer and Fe precursors were blended before electrospinning and CNTs were grown by chemical vapor deposition (CVD) using Fe catalysts. The resulting structure consisted of Fe/Fe3O4 NPs located at the tips of CNTs distributed over CNFs. It was observed that the time allowed for the growth of CNTs had direct effect on the morphology, defects in nanocarbon and the electrochemical properties of the composite structures. The high surface area, mesoporosity and presence of large number of defective sites in the optimized composite structure provided more accessible sites for reversible electrolyte ion absorption. This was evidenced by the excellent performance of this electrode material in electrochemical supercapacitor with a specific capacitance of 195 Fg-1 at 0.5 Ag-1, areal capacitance of 398 mFcm-2 in 1 M KOH and long term durability with 95% capacitance retention even after 5000 charge-discharge cycles. The CNFs also provided the support for the homogeneous growth of evenly distributed and well-separated CNTs that prevent the material from structural deformation during repetitive charge-discharge cycles. In the last part, TiO2 nanofibers (TNFs) with phosphorus doping were fabricated by one-pot electrospinning followed by silver deposition on the surfaces by chemical reduction method. The Ag NPs of ∼8 nm size on 2 % P-doped TNFs showed excellent photocatalytic activity, under simulated solar light, for the photoreduction of Cr(VI) and photodegradation of methylene blue dye, with 96.5 % and 82.7 % increase in pseudo-first order rate constants, respectively, in comparison to the pristine TNFs. The enhanced photocatalytic performance of Ag-PTNFs is attributed to the reduced band-gap, increased charge separation and reduced recombination rates. A proposed photocatalytic mechanism based on the synergistic effect of both Ag and P-doping will be presented which highlights the potential of these composite materials for water remediation.

This dissertation discusses in detail the mechanisms involved in synthesizing the unique morphologies of nanofiber based composite structures and the structure-property relationships for enhanced performances. Overall, the strategies presented in this dissertation are simple yet effective for making composite structures which can be further explored for the fabrication of other metal/metal oxides over carbon substrates with potential applications in energy generation, energy storage and environmental remediation.