Hydrothermal growth and characterization of zirconia nanostructures on non-stoichiometric zirconium oxide

Powder of nominal composition ZrO, which is a mixture of at least two different phases, was treated hydrothermally to grow zirconia nanostructures. The processes were performed in an autoclave using NaOH as the mineralizer at concentrations between 0 M and 25 M, at a fixed temperature of 200 °C and during 7 days. X-ray diffraction reveals the transformation of raw compound to monoclinic zirconia (ZrO2) phase for all the concentrations of NaOH, which is confirmed by Raman spectroscopy. Transmission electron microscopy (TEM) indicates the formation of nanobars with diameter between 20 and 50 nm and length from 100 nm up to 200 nm. The growth of nanostructures was favored at higher concentrations, whereas the nanostructure length saturates at around 200 nm. Scanning electron microscopy (SEM) suggests that the growth of the nanostructures is perpendicular to the surface of raw powders. An emission peak of around 490 nm is observed in the photoluminescence (PL) spectra of zirconia nanostructures assigned to blue emission, with a red-shift at an increasing NaOH concentration. This is attributed to oxygen vacancies and a quantum size effect originated from the nanostructures produced during the hydrothermal treatment.