Structural stability and optical properties of nanocrystalline zirconia

Nanoparticles of the cubic phase of zirconia (ZrO₂) of size range 4.5-8.7 nm have been synthesized by alkaline hydrolysis of a zirconium salt followed by solvothermal reaction. Subsequently, the room-temperature stability of cubic ZrO₂ nanoparticles has been explored with the goal of understanding how crystal structure tends to transform into a structure of higher symmetry with decreasing crystallite size. The room-temperature-stable cubic phase in nanosized ZrO₂ has been observed to transform into the monoclinic phase at 873 K. The crystalline phases of ZrO₂ nanocrystals have been determined quantitatively by Rietveld refinement. Lattice constant and internal strain increase with decreasing particle size. Growth kinetics studies have established that cubic nanocrystals are more stable at smaller crystallite sizes. UV-visible absorption spectra show an absorption peak at 275 nm which indicates a lowering of the band gap energy. Photoluminescence spectra of zirconia nanoparticles show an emission peak at 305 nm at room temperature. The modification of the optical properties is explained on the basis of oxygen vacancies present within the samples.