Rapid annealing: A novel processing technique for Cr:ZnAl₂O₄ nanoparticles

Nanocrystalline Cr:ZnAl₂O₄ was synthesized for the first time by rapid thermal annealing the corresponding hydroxide precipitate in just 10 min. The XRD patterns showed that synthesis between 500 °C and 700 °C had smaller crystallites (∼2.4 – 4 nm), while at 800 °C and 900 °C, had ∼10 nm crystallites. HRTEM images of the 900 °C sample showed the well-developed lattice fringes confirming the high crystallinity. The particles were polyhedral having an average size of ∼16 nm. The diffuse reflectance spectra showed characteristic ⁴A_2g → ⁴T_1g and ⁴A_2g → ⁴T_2g transitions, while the photoluminescence emission spectrum comprised of the zero phonon line (R-line) along with other multi-phonon side bands that are characteristic of the spin – parity forbidden transitions of the Cr³⁺ ion. The emission spectra and photoluminescence lifetimes are comparable with reports on sensing and imaging applications. Our work demonstrates the suitability of this technique to quickly synthesise phosphors for a variety of applications. - Highlights: • 10 min synthesis of highly crystalline Cr:ZnAl₂O₄ nanoparticles by Rapid Annealing. • Polyhedral, single crystalline particles obtained at 900 °C with average size 16 nm. • Optical properties were comparable with those obtained by conventional annealing. • Well resolved, characteristic Cr³⁺ emission bands obtained above 800 °C.