Electrospinning of flexible and functionalized nano-fiber membranes by means of sol-gel technology (178EA0713)
Traditonal ceramics, based on clay, have been used for over 25 000 years. Specific properties associated with ceramic materials are brittleness, compressive strength and stability in both harsh chemical and thermal environments. Ceramic materials can be produced via sol-gel technology. The sol-gel process is a low temperature method enabling the production of ceramics with an excellent control of product purity and composition. Developments in advanced ceramics have significantly increased over the last decades. More recently, also the interest in nanotechnology expanded rapidly. Nanofibers, typically having a diameter of less than 500 nm, are known for their unique properties including a large specific surface area, a high porosity and small pore sizes. Ceramic nanofibers can be produced by combining electrospinning and sol-gel technology. The combination of the unique properties of nanofibers with the versatility of the sol-gel process gives the opportunity to produce ceramic nanofibrous membranes usable in various advanced applications. This PhD explores the production of novel silica nanofibers. These novel nanofibers are produced by applying a preparation procedure different from what is typically used in literature for the production of ceramic nanofibers. In contrast to literature no organic polymer is added to the electrospinning solution, hereby giving the possibility to avoid the heat treatment step typically used to remove the organic polymer after electrospinning. This heat treatment results most of the time in fragile ceramic nanofibrous membranes, that have the tendency to break into little pieces. Avoiding the heat treatment thus avoids a deleterious effect on the nanofibrous membranes themselves as well as the possible destruction of added functionalities. Both the sol-gel process and the electrospinning process allow for easy functionalization of these silica nanofibers offering applicability in various applications. In this PhD focus is given to two application domains being water treatment and colorimetric sensors. Chapter 1 provides an overview on sol-gel technology, the electrospinning process and the envisioned applications. Subsequently, the materials and methods used throughout this PhD are described in Chapter 2. When electrospinning of silica nanofibers is carried out without organic polymer addition in the electrospinning solution, proper control of the sols rheology is essential to enable stable and scalable electrospinning. One of the characteristics that highly affects the electrospinning process is the viscosity of a solution. Therefore, in Chapter 3 the viscosity range that enables uniform and reproducible nanofibers is determined. Tetraethyl orthosilicate (TEOS) is selected as sol-gel precursor, since this is the most commonly used and most thoroughly studied sol-gel precursor. The importance of the route to obtain this viscosity is discussed as well. Additionally, the resistance of these membranes to high temperatures (1000U+0403‹C) is shown and the interesting changes in hydrophilic properties are denoted. It was found that not only the viscosity of the sol has a major influence on the electrospinning process, but also other parameters including colloidal particle sizes, ethanol concentration and degree of crosslinking are determining the electrospinning process and the resulting nanofibers. Therefore, additional attention is given to the characterization of these parameters in Chapter 4 by using multiple characterization techniques. Moreover, successful upscaling of these silica nanofibers was carried out. Subsequently, the following chapters elaborate on the application of functionalized silica nanofibers for water treatment and colorimetric sensors. The unique properties of nanofibrous membranes makes them ideal for a wide range of filtration applications. Moreover, they show to be an excellent porous support for the immobilization of titanium dioxide (TiO2) nanoparticles. These TiO2 nanoparticles are capable of oxidizing many organic (micro)pollutants. In Chapter 5 both polyamide 6 and silica nanofibers are functionalized with TiO2 nanoparticles. Two techniques for functionalization are compared, namely inline functionalization and postfunctionalization via dip-coating. Their high photocatalytic activity is demonstrated and compared by decoloring of Methylene Blue. The high value of TiO2 functionalized nanofibrous membranes for organic (micro)pollutants removal is shown through complete degradation of isoproturon. Due to the high specific surface area and high porosity of nanofibrous membranes, the sensitivity and response time of sensors can be improved. Color changing polymer nanofibers have already shown to be highly valuable in previous research. Color changing ceramic nanofibers can be applied in an even broader application domain due to their chemical resistance. Chapter 6 and 7 both focus on the production of large area, flexible, reusable colorimetric sensors. In line with Chapter 5, different functionalization techniques are compared, including both physical entrapment (dye-doping) and covalent immombilization of the dye within the nanofibrous matrix. Covalent immobilization of a dye significantly improves dye leaching compared to dye-doped membranes. Therefore, color changing dyes are covalently linked to a selected sol-gel precursor in both chapters. In Chapter 6 color-changing nanofibers are tested in their response toward pH-changes in water, hydrochloric acid and ammonia vapors, and biogenic amines. The importance of the covalent linkage is proven, as only the membranes with a covalently link between dye and sol-gel matrix are suitable to be used as pH-sensor in aqueous acidic environments. All nanofibrous membranes showed an immediate and clear color change upon exposure to hydrogen chloride and ammonia vapors. Their sensitivity towards biogenic amines demonstrates the versatility of these membranes. Chapter 7 focuses on solvatochromic nanofibrous sensor materials. Three functionalization techniques are used, namely dye doping, covalent coupling inside the nanofibers and covalent immobilization on the membranes via coating. A major limitation in Chapter 7 for the inline covalently coupled nanofibrous membranes was, however, their instability in the solvents resulting in insufficient immobilization of the dye giving high dye leaching. Although further studies are still needed it was already clear that coating of pure silica nanofibers overcomes this problem and broadens the applicability on various substrates.
Finally, Chapter 8 summarizes the main conclusions of this PhD and provides an outlook towards possible future work. In this work an in-depth study is thus carried out on the electrospinning process of TEOS sols and the parameters influencing this process. The reproducibility and scalability of the production of silica nanofibers is demonstrated. The implementation of this knowledge for the production of membranes applicable for water treatment and for color-changing sensor materials shows the major potential for advanced applications.