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

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 (Pm3[combining macron]m) 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.