Cr-doped ZnAl₂O₄: Microwave solution route for ceramic nanoparticles from metalorganic complexes in minutes

A combination of microwave irradiation and metalorganic precursors in solution was used for the synthesis of Cr:ZnAl₂O₄ nanoparticles with high yields (~92%). Though the spinel phase is formed after ~40 minutes, 60 minutes of microwave irradiation was required for the complete transformation of precursors into the spinel. The as-prepared material is nanocrystalline and phase-pure and was subjected to annealing in air at different temperatures. Annealing improved the crystallinity, and the material turned pink at 1200°C. The structural and optical properties were investigated by XRD, HR-TEM, FE-SEM, FT-IR, diffuse reflectance spectroscopy, and photoluminescence spectroscopy. The two bands in the excitation spectra (400 and 540 nm) became more intense as the annealing temperature was raised. The 400 nm band was asymmetric and consisted of two peaks, suggesting a trigonal distortion. The emission spectra consisted of a zero phonon line (ZPL), along with its associated multi-phonon side bands. The high Dq/B (3.21) suggested the presence of Cr³⁺ in a strong crystal field. With annealing, the emission lifetimes increased from ~7 to ~34 ms. This work demonstrates rapid, low-temperature synthesis of a red-emitting phosphor, with potential applications in bio-imaging, sensors, and lighting.