Fabrication Of Macroscopic Assemblies Of Nanofiber And Their Properties

With the fast development of nanotechnology during the past two decades, high-quality one-dimentional (1D) nanomaterials with various compositions and sizes have been fabricated successfully through different physical and chemical strategies, among which, template-directed synthesis has been one of the most popular strategies for the fabrication of 1D nanostructures. Compared with the templateless methods, the template-directed synthetic method has some obvious advantages, e.g., easy fabrication, low cost, high-through-put, and various compositions of materials. On the other hand, although the properties of nanomaterials are frequently superior to those of their bulk counterparts, translating the unique characteristics of individual nanoscale components into macroscopic materials, such as 2D membrane and 3D monolith, still remains a challenge.The present dissertation will focus on template-directed synthesis, assembly, and application of 1D nanomaterials. A series of high-quality 1D nanomaterials were synthesized through the template-directed strategy, showing the unique superiority and broad applicability of the templating process. The prepared 1D nanomaterials were then successfully assembled into macroscopic functional membrane and aerogel materials. These assembled macroscopic materials exhibited wide application potential in filtration and separation, water purification, anti-biofouling filtration, electrocatalysis, continuous-flow catalysis, and magnetic actuation. The main results can be summarized as follows:1. A novel synthetic route was developed for preparing a family of high-quality noble metal and telluride 1D nanomaterials by using highly active ultrathin nanowire as templates. The keys for successful synthesis of high-quality PtNTs with very high aspect ratio are to use ultrathin Te nanowires as sacrificial templates and ethylene glycol as solvent to control the reaction kinetics. The valence of chosen metal precursors plays a crucial role in selective syntheses of PtNTs, Pt NWs and PdNWs. Highly uniform CdTe nanowires with an average diameter of 12 nm were synthesized using ultrathin Te nanowires as templates via a low temperature hydrothermal process. The templating method can also be extended to synthesize other one dimensional telluride nanostructures such as Ag₂Te and PbTe nanowires with uniform diameter and high aspect ratio. 2. A simple assembling process was developed for fabricating free-standing nanofibrous membranes, which exhibited excellent ability for size-selective separation and water purification. Highly-uniform carbon nanofibers (CNFs) with controllable diameters were fabricated through the template-directed hydrothermal carbonization (HTC) process by using ultrathin Te nanowires as templates and glucose as carbon source. These CNFs with high aspect ratio were then assembled into a new kind of free-standing fibrous membrane by a simple casting-evaporating process. The fabricated CNFs membranes are very flexible and mechanically robust enough for filtration operation under a high applied pressure without any damage. More importantly, these CNFs membranes have very narrow pore size distributions and show excellent size-selective rejection properties. The cutoff sizes of these membranes could be controlled precisely by carefully regulating the diameters of the CNFs. The CNFs membranes exhibited a strong ability for selective separation of nanoparticles with different sizes form solution.In addition, a simple filtration process was demonstrated to decontaminate water by employing the CNFs membranes. This process combines the excellent adsorption behavior of CNFs and the advantages of membrane filtration. The filtration experiments proved that the CNFs membranes could remove methylene blue (MB) efficiently with a very high flux of 1580 L/m²h, which is 10–100 times higher than that of commercial nano- or ultrafiltration membranes with similar rejection properties.Moreover, basing on the unique adsorption and easy-assembling properties of CNFs, a novel free-standing PGCNFs membrane with high graphitization degree and mesoporous nature was fabricated through a catalytic graphitization process. The PGCNFs membranes could be of special interest as electrode materials for supercapacitors or supported materials for catalysts.3. The CNFs were further demonstrated as a versatile nanoscale scaffold for constructing macroscopic multifunctional membranes. A variety of CNFs-based composite nanofibers, including CNFs-Fe₃O₄, CNFs-TiO₂, CNFs-Ag, and CNFs-Au were fabricated through various chemical routes. Importantly, all of them inherit the unique dimensional (high aspect ratio) and mechanical properties (flexibility) of the original CNFs scaffolds thus can be assembled into macroscopic free-standing membranes. The wide application potentials of these multifunctional composite membranes were also demonstrated, such as magnetic actuation, anti-biofouling filtration, and continuous-flow catalysis.4. A smart multi-step templating route was designed for preparing free-standing Pt nanowires membranes, which could be used as an electrocatalyst for oxygen reduction reaction (ORR). This membrane catalyst system consists of long crystalline PtNWs that can improve catalytic activity owing to the preferential exposure of certain crystal facets and less surface defect sites in 1D nanostructures. In addition, the unique nanowire network structure facilitates the electron transport and gas diffusion on the PtNWs electrode. Therefore, the PtNWs membrane has 2.1 and 1.8 times higher specific activity than that of Pt/C and Pt black for ORR, respectively. More importantly, compared with the commercial catalysts, this unique free-standing Pt nanostructural catalyst exhibits remarkably high stability, which is crucial for proton exchange membrane fuel cells (PEMFCs) applications.5. The template-directed HTC process was further developed for fabricating CNFs hydrogels and aerogels in large-scale. Different from the fragility of traditional silica-based aerogels, the CNFs aerogels exhibited high mechanical stability and excellent compressibility. The outstanding mechanical properties of CNFs gels were resulted from their unique structures, where CNFs connected with each other to form abundant junctions and finally leading to a highly stable 3D nanowire network structures. Compared with other processes, the present template-directed HTC route for synthesis of CNFs hydrogels and aerogels possesses two significant advantages, namely, i) easily scale-up for massive synthesis and ii) allowing for controlling the CNFs size, the density and porosity of aerogels, and their mechanical properties. In addition, Pt nanowires aerogels were synthesized for the first time by a multi-step templating process.