Thermal effects on rapid microwave synthesized Co: ZnAl₂O₄ spinel nanoparticles

A rapid (10 min) microwave-assisted synthesis of Co_xZn_(1-x)Al₂O₄ nanoparticles using metal organic precursors is reported for the first time. The nanoparticles formed at 185 °C were phase-pure in the as-prepared form with an average crystallite size of ∼5 nm. Sample with x = 0.0025 showed intense photoluminescence (PL) emission and was used to study the effect of annealing temperature on the structural and optical properties. Annealing in air until 1200 °C gradually (i) increased the crystallite size to as much as 31 nm (ii) change of colour tone from colourless to blue which has been attributed to the diffusion of Co²⁺ in host lattice (iii) increase in intensity of Co²⁺ bands in the diffuse reflectance spectra (DRS). The emission spectra comprised of the broad ⁴T₁(⁴P) → ⁴A₂(⁴F) spin allowed transition pertaining to tetrahedral Co²⁺ at ∼649 nm whose intensity increased with annealing temperature. The crystal field splitting parameter (Dq) and the Racah parameter B, were calculated from the excitation spectra and the Tanabe-Sugano diagrams. The novelty of this work lies in the simplicity and swiftness of the microwave-assisted synthesis that can be extended to other ceramic phosphors suitable for lasing and sensing applications.