Hollow Hafnium Oxide (HfO₂) Fibers: Using an Effective Combination of Sol-Gel, Electrospinning, and Thermal Degradation Pathway

In recent years, hafnium oxide (HfO₂) has gained increasing interest because of its high dielectric constant, excellent thermal stability, and high band gap. Although HfO₂ bulk and film materials have been prepared and well-studied, HfO₂ fibers, especially hollow fibers, have been less investigated. In this study, we present a facile preparation method for HfO₂ hollow fibers through a unique integration of the sol-gel process and electrospinning technique. Initially, polystyrene (PS) fibers are fabricated by using electrospinning, followed by dipping in a HfO₂ precursor solution, resulting in HfO₂-coated PS fibers. Subsequent thermal treatment at 800 °C ensures the selective pyrolysis of the PS fibers and complete condensation of the HfO₂ precursors, forming HfO₂ hollow fibers. Scanning electron microscopy (SEM) characterizations reveal HfO₂ hollow fibers with rough surfaces and diminished diameters, a transformation attributed to the removal of the PS fibers and the condensation of the HfO₂ precursors. Our study also delves into the influence of precursor solution molar ratios, showcasing the ability to achieve smaller HfO₂ fiber diameters with reduced precursor quantities. Validation of the material composition is achieved through thermogravimetric analysis (TGA) and energy-dispersive spectroscopy (EDS) mapping. Additionally, X-ray diffraction (XRD) analysis provides insights into the crystallinity of the HfO₂ hollow fibers, highlighting a higher crystallinity in fibers annealed at 800 °C compared with those treated at 400 °C. Notably, the HfO₂ hollow fibers demonstrate a water contact angle (WCA) of 38.70 ± 5.24°, underscoring the transformation from hydrophobic to hydrophilic properties after the removal of the PS fibers. Looking forward, this work paves the way for extensive research on the surface properties and potential applications of HfO₂ hollow fibers in areas such as filtration, energy storage, and memory devices.