The production and characterization of teos-based fibers by electrospinning method

In recent years, significant scientific and technological advancements in the fields of nanotechnology and materials engineering have substantially increased interest in the production of fibrous materials, particularly due to their superior thermal and chemical properties. Fibers with high surface area, small diameters, and flexible morphologies are attractive for a wide range of industrial applications. These include energy storage systems, biomedical materials, filtration, sensors, and catalyst supports. In this context, silica-based compounds -especially tetraethyl orthosilicate (TEOS) -have emerged as prominent precursors for fiber production due to their chemical stability, high-temperature resistance, and environmental inertness. Among silica-based compounds, TEOS is widely utilized in sol-gel applications such as thin films, aerogels, and fiber production, owing to its ability to form thermally stable, chemically inert, and highly porous network structures. However, direct fiber production from TEOS without using a carrier polymer poses certain technical challenges, particularly in terms of solution spinnability and structural integrity. Therefore, optimizing electrospinning parameters and understanding the behavior of TEOS-based systems under various physical and chemical modifications are critically important. In this study, fibers were successfully produced from TEOS-based solutions without any polymeric carrier via the electrospinning method, and the resulting fibers were comprehensively characterized in terms of morphological, structural, chemical, thermal, and electrochemical properties. Furthermore, to investigate their potential in energy applications, TEOS solutions were doped with nickel–manganese–cobalt (NMC), a ternary metal oxide widely used in lithium-ion battery technologies. The NMC-doped TEOS fibers were comparatively evaluated against pure TEOS fibers. The sol-gel precursor solutions were prepared using TEOS, ethanol, water, and hydrochloric acid. These solutions were subjected to controlled aging at 80°C to guide viscosity, partial hydrolysis, and condensation reactions. Aging duration significantly affected the fiber-forming capability, particularly in terms of viscosity and gelation behavior. Electrospinning was carried out at voltages ranging from 18 to 28 kV, flow rates between 0.5 and 2.0 mL/h, and a tip-to-collector distance of 10–15 cm. Under these conditions, a total of 12 fiber samples (undoped and NMC-doped TEOS fiber) were successfully fabricated. FTIR analyses of TEOS-based sol-gel solutions aged at room temperature for different durations revealed that the structural transformation proceeded in a limited manner and at a slow rate. The broad ─OH stretching band observed at 3400-3200 cm-1 exhibited only a slight decrease, indicating that hydrolysis and condensation reactions progressed slowly under these conditions. The weak C─H stretching bands in the 2900-2850 cm-1 range gradually diminished over time but did not completely disappear. The strong Si─O─Si stretching peak at around 1100 cm-1, the Si─OH shoulder band near 950 cm-1, and the characteristic vibrations at 800 cm⁻¹ and 450 cm⁻¹ remained stable across all samples. Overall, the findings demonstrated that room-temperature aging up to 24 hours caused only partial reduction of ─OH groups, without significant strengthening of Si─O─Si bonds, confirming that the sol-gel network formation advanced rather slowly. Confocal microscopy was used to examine the three-dimensional morphology of the fibers. Structural characteristics such as surface roughness, fiber alignment, and isotropy were investigated through this technique. Homogeneous fiber samples exhibited smoother and more aligned morphologies, whereas heterogeneous samples showed irregular and branched structures. SEM (Scanning Electron Microscopy) images provided detailed insight into surface morphology and fiber diameter. Fibers produced at voltages between 24-27 kV and flow rates of 1.0-1.5 mL/h displayed bead-free and uniform structures. According to ImageJ-based diameter analysis, the average diameter of pure TEOS fibers was as low as 0.345 μm, while it increased to approximately 1.072 μm in NMC-doped samples. This increase was attributed to the elevated viscosity caused by NMC addition, and possibly changes in surface tension. Homogeneous samples displayed narrow diameter distributions, whereas heterogeneous ones showed wider variation. Elemental analysis showed that pure TEOS fibers predominantly contained silicon and oxygen, while NMC-doped fibers successfully incorporated nickel (Ni), manganese (Mn), and cobalt (Co) into the fiber structure. Structural analysis using X-ray diffraction (XRD) indicated that pure TEOS fibers exhibited amorphous features, whereas the presence of NMC led to the emergence of low-intensity crystalline phases. These crystalline structures are believed to result from the thermal reorganization of NMC components at elevated temperatures, which could positively impact electrochemical performance. Thermal analyses were performed using both Differential Thermal Analysis (DTA) and Thermogravimetric Analysis (TGA), providing valuable insights into the thermal stability and decomposition behavior of the fibers. TGA results indicated a total weight loss of approximately 19% in pure TEOS fibers, attributed to the evaporation of ethanol and water as well as the removal of residual organic compounds. DTA results corroborated this behavior, confirming that the fibers reached thermal stability beyond 600 °C. In NMC-doped fibers, a weight loss of approximately 8% around 200 °C was associated with the removal of volatile components, while an additional 4% loss between 200-650 °C was attributed to organic decomposition and possible oxidative rearrangements. A distinct exothermic peak accompanied by a minor weight reduction observed at around 750 °C was linked to the crystallization or phase transitions of metal oxide phases. Structural rearrangements were detected in the 900-1000 °C range, and the total weight loss was determined to be approximately 13%. This behavior offers significant advantages for high-temperature applications and thermally stable material development. Electrochemical characterization was conducted using Electrochemical Impedance Spectroscopy (EIS) for both pure and NMC-doped TEOS fibers. The results clearly showed that pure TEOS fibers exhibited very high impedance values with no observable electrical conductivity, confirming their suitability for passive roles such as separator layers, insulating interphases, or dielectric support structures rather than as active electrode materials. In contrast, NMC-doped TEOS fibers exhibited a noticeable decrease in impedance and demonstrated the onset of conductivity. These results suggest that the NMC-doped fiber system introduced partial ionic and/or electronic pathways, thereby making them suitable candidates for use in hybrid electrode architectures or as functional interlayers supporting active materials. Additionally, the mechanical integrity and volume-buffering capacity provided by the NMC-doped fibers indicate their potential to contribute to electrode stability. The impedance profiles obtained from both fiber types show that the materials can be designed for different roles depending on the application requirements. In conclusion, this thesis demonstrated that TEOS-based fibers can be produced via electrospinning without the use of a carrier polymer. The influence of process parameters on fiber morphology was systematically evaluated, and it was found that NMC doping significantly modified both physical and electrochemical properties of the fibers. Although NMC-doped fibers have not yet reached the performance level of fully active electrode materials, the observed improvements in conductivity and thermal stability highlight their potential for integration into future energy storage systems. For future studies, the enhancement of functional properties through the incorporation of different metal oxide dopants is recommended. Developing multilayer composite structures and conducting full-cell battery testing of these fibers will also be essential steps forward. This thesis lays a solid foundation for the design and advancement of TEOS-based nanofibers for multifunctional engineering applications.