Low-temperature synthesis of tetragonal phase of hafnium oxide using polymer-blended nanofiber precursor

Hafnium oxide (HfO2), which is known as hafnia, is considered one of the best materials for various future applications such as neuromorphic computing systems and optical coatings. Although tetragonal phase (t-phase) in the multiple crystallographic structures of HfO2 outperforms monoclinic phase (m-phase) most common phase that can be formed at low temperature in physicochemical properties such as dielectric constant, bandgap, high resistance to corrosion and hardness, it is extremely hard to stabilize t-phase below 1670 degrees C. Herein, we synthesized oxygen-deficient tetragonal hafnium oxide nanofibers (t-HfO2-x NFs) using the electrospinning technique, which included the polymer-blended precursors and a magnesium-thermic reaction at a low processing temperature (< 600 degrees C). The small grain size (< 10 nm) of the t-HfO2-x NFs and oxygen vacancies (V-oxygen) synergistically stabilized t-phase in the nanofibrous structure. Oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) performances of the t-HfO2-x NFs in alkaline and acid media were evaluated as a potential application, which were higher than those of the m-HfO2 NFs. Our straightforward approach to make t-phase in HfO2-based materials can be broadly applied for many future applications.