Microwave solution route to ceramic ZnAl₂O₄ nanoparticles in 10 minutes: Inversion and photophysical changes with thermal history

Microwave-assisted synthesis of ZnAl₂O₄ nanoparticles in minutes using metalorganic precursors is reported. Phase-pure ZnAl₂O₄ with an average crystallite size of ∼5 nm is formed in the solution medium at 185 °C. Annealing in air at temperatures between 500 and 1200 °C increases the crystallite size to ∼32 nm. The as-prepared particles are largely shapeless, whereas polyhedral crystallites with well-defined grain boundaries can be seen in the HR-TEM image of the annealed samples. Diffuse reflectance spectroscopy provides insight into the structural development of the oxide spinel. Rapid synthesis leads to significant crystallographic inversion (∼33%), as observed by X-ray photoelectron spectroscopy. Photoluminescence spectroscopy shows that the different emission bands are due both to anti-site defects in the form of zinc interstitials caused by cationic inversion and to oxygen and zinc vacancies. Optical measurements suggest that inhomogeneity in cationic distribution, probably caused by the rapidity of synthesis, is prevalent even after annealing at temperatures up to 1200 °C, and plays a significant role in controlling the emission properties of the spinel. The microwave-assisted technique using metalorganic precursors is an easy path to the rapid synthesis of doped ZnAl₂O₄ phosphors.