Electrospinning of Ceria and Nickel Oxide Nanofibers

Electrospinning uses an electrical charge to draw very fine fibers from a liquid. It has very high potential for industrial processing. Electrospinning is cost effective, repeatable and it can produce long, continuous nanofibers. Polymers such as polyalcohol, polyamides, and PLLA can be easily electrospun. The increase in demand for clean energy combined with the research work in progress and the potential advantages of electrospun electrodes over conventionally fabricated SOFCs makes electrospinning a strong candidate. In this thesis, ceramic nanofibers (ceria and nickel oxide) that can potentially be used in SOFCs are fabricated. A three-phase approach is implemented in the fabrication of ceria and nickel oxide nanofibers. The first phase involves the preparation of the composite ceramic-polymer solution to be electrospun. The second phase gives the processing conditions such as voltage applied, feed rate, and gauge of syringe tip used for successfully electrospinning composite ceramic-polymer fibers. The final stage demonstrates the temperature cycles used to burn out the polymer and calcine the ceramic particles in the ceramic-polymer nanofibers leaving behind ceria and nickel oxide nanofibers. Techniques such as scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray Diffraction (XRD) were used to measure the average diameter of the fibers formed and to understand the chemical composition and crystallanity of the nanofibers after calcination. This thesis also discusses the advantages and possibility of fabricating side-by-side nanofibers and oriented nanofiber mats

Laser-Induced Functional Carbon Nanofibers for Electrochemical Sensing

The development of electrochemical sensors utilizing non-enzymatic detection strategies is a topic of high interest for many researchers in order to replace classic expensive and less stable enzymatic approaches. In this thesis, recent developments in non-enzymatic sensing were reviewed. The general principles of (nano)catalysis and preparation of nanomaterials were discussed focusing on carbon materials and metal-based catalysts. Carbon nanomaterials stand out for their great electron transfer properties and, especially for nanofibers, high surface-to-volume ratio with multiple analyte interaction sites. Doping carbon nanofibers with heteroatoms or metal nanoparticles further introduces nanocatalytic functionalities. Hereby, the type of atom and metal, respectively, determines the selectivity as well as sensitivity of the generated composite. The fabrication of laser-induced carbon materials was quite recently found to be a simple and effective way to receive such hybrids. In this regard, several substrates are suitable for being carbonized e.g. polymer films containing metal salts. Already constructed non-enzymatic platforms, based on laser-induced graphene, for sensing in aqueous solution but also with gaseous analytes were presented. The current achievements in wearables were emphasized which guide the prospective trends. Concluding, the key aspects were summarized and thoughts on improvements and suggestion for future evolvements were shared. In this thesis the strategy of one-step laser-carbonization of electrospun nanofibers to obtain carbon nanofibers was developed as a superior process over traditional chemical vapor deposition and thermal carbonization of electrospun nanofibers which are laborious, time-consuming and inflexible. Polyimide, more precisely Matrimid® 5218, served as carbon precursor and polymer solutions of it were electrospun into nanofiber mats. Afterwards, carbon nanofibers were prepared in a facile manner via direct lasing on as-spun mats at ambient conditions with a CO2-laser. This method allows the generation of electrodes with any design and shape controlled by PC software. Compared to both mentioned conventional procedures, large-scale production of carbon nanofibers at affordable costs is possible in a short time. The morphology of laser-induced carbon nanofibers (LCNFs) can not only be controlled by lasing parameters such as laser power and speed. During electrospinning, metal nanoparticles can be incorporated into nanofibers by simply doping the spinning solutions with metal salt e.g. iron(III) acetylacetonate before. This metal, as studied for iron, additionally contributes to the homogeneous carbonization during lasing process by a kind of heat transfer ability. Therefore, the respective metal content relative to the polymer also enables tuning of the obtained LCNF morphology. Continuously scribing of several electrodes in a row vs. discontinuously i.e. one electrode prior to the next also has a huge impact on the heat input because of different durations of scribing one line. It was demonstrated that the electrochemical properties of LCNF electrodes can be optimized due to the direct link to the morphology or rather electrochemical surface area. The latter was found to be much greater than the geometric area. The 3D porous network structure of LCNFs with an average pore size in low micrometer range facilitates interaction with molecules in aqueous solution and hence allows high electron transfer rates, which was displayed by very low peak-to-peak separations, as studied with [Fe(CN)6]4-/3- redox marker. The type of metal salt incorporated in LCNFs defines its catalytic properties. Nickel salt can be evenly embedded into carbon matrix with low nanometer nanoparticle size. The electrospun nanofiber diameter does not change with varying nickel content. However, it was shown that the expansion of nanofibers during carbonization is influenced assumedly by the mentioned heat transfer ability. A significant smaller increase of LCNF diameter compared to electrospun nanofibers was achieved with increasing the nickel content, which resulted in better fiberosity. In contrast to electrodeposited nickel particles, it was evinced that nickel in LCNFs is stably adhered to the carbon and does not leach out during several hours of shaking incubation in phosphate buffer at body temperature. Prevented desorption of potential toxic metals brings the application of LCNFs in vivo one step closer. With its catalytic behavior towards glucose, Ni-LCNF was utilized for amperometric glucose sensing. The electroanalytical performance with a high sensitivity and a low limit of detection turned out to be excellent and the linear range covers the real glucose levels in blood. Further, ascorbic acid and uric acid did not produce interference at their relevant levels. Those great electrochemical characteristics are attributed to derive from nickel nanocatalyst on the one hand and 3D fiberosity on the other hand. Electron microscopic images gave a hint that LNCFs could be hollow which additionally could increase the interaction of catalyst nanoparticles with analytes in solution but also gaseous molecule samples. With focus on catalysis, LCNFs with several different metals can be created to enable a variety of application possibilities. Palladium containing LCNFs were furthermore prepared in order to electrochemically detect hydrogen peroxide. In this experiments, the amperometric sensitivity towards H2O2 was enhanced by improving the electrochemical properties of Pd-LCNF either electrochemically by cyclic voltammetry cycling, application of a constant negative potential or chemically by reduction during incubating Pd-LCNF electrodes in NaBH4 solutions for some hours. By fabrication of bimetallic Pd/Fe-LCNF hybrids, the detection potential of H2O2 could be significantly reduced which gives a first hint on successful achievement of a synergistic effect. As these investigations were only preliminary ones, further optimizations to reach low micromolar limit of detection prior to final sensor development have to be carried out

Development of a Wireless MEMS Multifunction Sensor System and Field Demonstration of Embedded Sensors for Monitoring Concrete Pavements, Volume II

This two-pronged study evaluated the performance of commercial off-the-shelf (COTS) micro-electromechanical sensors and systems (MEMS) embedded in concrete pavement (Final Report Volume I) and developed a wireless MEMS multifunctional sensor system for health monitoring of pavement systems (Final Report Volume II). The Volume I report focused on the evaluation of COTS MEMS sensors embedded in concrete pavement sections. The Volume II report covers the set of MEMS sensors that were developed as single-sensing units for measuring moisture, temperature, strain, and pressure. These included the following sensors: (1) nanofiber-based moisture sensors, (2) graphene oxide (GO)–based moisture sensors, (3) flexible graphene strain sensors with liquid metal, (4) graphene strain and pressure sensors, (5) three-dimensional (3D) planar and helical structured graphene strain sensors, (6) temperature sensors, and (7) water content sensors. In addition, the MEMS temperature sensors and the MEMS water content sensors were integrated into one sensing unit as a multifunctional sensor. A wireless signal transmission system was built for MEMS sensor signal readings. Characterization of the sensors was conducted and sensor responses were analyzed using different applications. The sensors developed were installed and tested inside concrete. The results demonstrated the capability to detect sensor response changes at the installed locations

The Electrospun Ceramic Hollow Nanofibers

Hollow nanofibers are largely gaining interest from the scientific community for diverse applications in the fields of sensing, energy, health, and environment. The main reasons are: their extensive surface area that increases the possibilities of engineering, their larger accessible active area, their porosity, and their sensitivity. In particular, semiconductor ceramic hollow nanofibers show greater space charge modulation depth, higher electronic transport properties, and shorter ion or electron diffusion length (e.g., for an enhanced charging–discharging rate). In this review, we discuss and introduce the latest developments of ceramic hollow nanofiber materials in terms of synthesis approaches. Particularly, electrospinning derivatives will be highlighted. The electrospun ceramic hollow nanofibers will be reviewed with respect to their most widely studied components, i.e., metal oxides. These nanostructures have been mainly suggested for energy and environmental remediation. Despite the various advantages of such one dimensional (1D) nanostructures, their fabrication strategies need to be improved to increase their practical use. The domain of nanofabrication is still advancing, and its predictable shortcomings and bottlenecks must be identified and addressed. Inconsistency of the hollow nanostructure with regard to their composition and dimensions could be one of such challenges. Moreover, their poor scalability hinders their wide applicability for commercialization and industrial use