Electrospun ZrO2 nanofibers: precursor controlled mesopore ordering and evolution of garland-like nanocrystal arrays

Abstract

We observed that the hydrolysis–condensation reaction of precursors makes a significant difference in constructing ordered mesopores in electrospun ZrO₂ nanofibers. Transmission-SAXS studies confirm the generation of uniform clusters of size ∼1.44 nm in the ZrOCl₂·8H₂O (inorganic salt) derived sol due to its relatively slow hydrolysis–condensation process. These initial –Zr–O–Zr– clusters acted as building blocks to form uniform 3D ordered cubic (Pmm) mesopores in the presence of Pluronic F127 surfactant. In contrast, the commonly used Zr-alkoxide (zirconium n-propoxide) precursor, which is highly hydrolysable even after the use of a controlling agent, generates larger clusters with broad size distributions due to the uncontrolled hydrolysis–condensation of alkoxy groups. Accordingly, in the presence of F127, the alkoxide derived sol yielded disordered mesopores in the resultant fibers. XRD under dynamic heating conditions (up to 900 °C) and the corresponding TEM studies of the ZrOCl₂·8H₂O derived nanofibers confirmed the retention of mesopores even in the extremely thin nanofibers (diameter ∼15–25 nm) after the amorphous to crystal phase transformation (cubic/tetragonal). An interesting morphological transformation has been observed in the nanofibers at 900 °C where the fibers have been uniformly segmented by distinct single nanocrystals (width ∼15–65 nm) with mesopores. Further heat-treatment at 1100 °C made these segmented nanofibers nonporous, and a garland-like appearance with monoclinic nanocrystal arrays was formed.