Microwave-assisted hydrothermal synthesis of nanocrystalline zirconia

A simple, fast and energy efficient microwave-assisted hydrothermal method was developed for the preparation of nanocrystalline zirconia from commercially available ZrOCl₂·8H₂O and KOH. The synthesis was conducted at 180 °C for 20 min by two ways: direct decomposition of ZrOCl₂·8H₂O (sample Z), and precipitation of ZrOCl₂·8H₂O with KOH and dehydration of hydroxides (sample ZK). The as-synthesized powders were calcined at 500 °C, and all the resulting products were characterized by XRD, FE-SEM, HR-TEM and SAED. Both the as-synthesized and calcined nanoparticles were highly crystalline. A single monoclinic phase was obtained for sample Z, while for sample ZK a tetragonal phase was achieved as the main phase with a minor fraction of monoclinic. The particles of the as-synthesized sample Z showed irregular and semi-hexagonal shapes, although they changed to spherical or ellipsoidal shapes during heat treatment. The particles of the sample ZK, both as-synthesized and calcined, exhibited nearly spherical or ellipsoidal shapes. The average crystallite size for the as-synthesized samples Z and ZK were 3.2 ± 0.8 and 5.5 ± 0.9 nm, respectively, while for the calcined ones the values were 8.5 ± 1.2 and 7.6 ± 1.2 nm, respectively.