Ten-minute synthesis of ZnAl2O4nanoparticles by rapid annealing: dopant-inducedacceleration in the crystal growth

Abstract

Rapid annealing (RA) technique was used for the swift synthesis of doped and undoped ZnAl₂O₄ nanoparticles. For a set temperature of 800 °C and heating rates of 150–200 °C/min, the corresponding hydroxide precursors transformed into crystalline and phase-pure ZnAl₂O₄ within 10 min. The influence of Fe³⁺, Cr³⁺, and Co²⁺ doping in ZnAl₂O₄ was studied to determine the time taken to form phase pure materials by rapid annealing. Crystallites obtained were larger for Co²⁺ and Cr³⁺-doping compared to undoped and Fe³⁺-doped ZnAl₂O₄. With higher Cr³⁺ doping, larger crystallites were obtained, probably due to higher thermal radiation absorption by the Cr³⁺. Structural investigations showed a direct relationship between the dopant ion and the extent of material crystallization. Comparatively, larger crystallites for Cr³⁺- and Co²⁺-doped samples indicate that beyond conventional thermal effects, the absorption cross section of the doped samples significantly influenced the crystallization process. To test the growth dependence on these ions, Cr₂O₃ was synthesized under similar conditions. Surprisingly, the crystal growth was ten times higher (Cr₂O₃ —47 nm; ZnCr_xAl_2−xO₄ — ~ 5 nm) within the same 10-min synthesis. The individual nanoparticles were single-crystalline, as seen from HRTEM proving that the nature of dopant has a crucial role in the accelerated crystal growth using RA. Compared to the usual high temperature and longer durations required for the synthesis of ceramic materials, our approach is novel being swift, energy efficient, and reliable. This opens up the possibility of implementing RA as an effective technique for synthesizing various nanoparticles in quick time.