Hierarchically ordered networks comprising crystalline ZrO2 tubes through sol–gel mineralization of eggshell membranes

A facile sol–gel coating procedure has been successfully applied for the ZrO2 coating of eggshell membranes (ESM), resulting in hierarchically ordered thin films with a macroporous network structure comprising crystalline ZrO2 tubes. This hierarchical material, which was obtained through sol–gel mineralization of the ESM template and subsequent calcination at 600 °C, was characterized by XRD, SEM, TEM, HRTEM, and nitrogen sorption measurements. It has been shown that this ZrO2 material exhibits a macroscopic morphology of a film with a thickness about 15 µm; the film has a microstructure of macroporous networks composed of interwoven ZrO2 microtubes with diameters less than 1.0 µm; the tube walls consist of tetragonal ZrO2 nanocrystals with an average crystallite size about 6 nm. It shows a specific surface area of 55 m2 g−1 and a BET average pore size of 7.0 nm, which is mostly due to the ZrO2 nanocrystals constituting the tube walls. It has also been shown that calcination of the initial ESM/zirconium precursor hybrid at 700 °C resulted in significant fusion between neighboring ZrO2 tubes accompanying a tetragonal-to-monoclinic phase transformation of zirconia.