Coprecipitation synthesis, characterization and density functional theory study of CaX2O4 (X: Al, Cr) spinel nanocrystallites

Spinel oxides are a type of material that can be used in a wide range of applications, such as photocatalysis, hydrogen production and environmental protection. In this respect, calcium aluminate (CaAl₂O₄) and chromate (CaCr₂O₄) spinels were synthesized in this study by the coprecipitation method using potash solution as a precipitant. CaAl₂O₄ and CaCr₂O₄ spinels were annealed at 900 and 1100°C, respectively, for different periods. The obtained spinels were first characterized by thermal analysis, and the phase composition of the oxides was analysed using X-ray diffraction. Hydroxyl groups and absorbed water in the obtained precipitates disappeared after calcination and were observed via Fourier transform infrared spectroscopy. BET and SEM/EDS analyses were also used to determine the total surface area of the powder particles, the size of the grains and the morphology of the powders of the synthesized nanoparticles, respectively. The structural and morphological analyses revealed the formation of single-phase CaAl₂O₄ and dual-phase CaCr₂O₄, with specific surfaces for each spinel of 44.2165 and 5.7190 m² g⁻¹, respectively. Moreover, DFT calculations were performed on the materials, and the direct bandgaps of these spinels were found to be 4.365 eV for CaAl₂O₄ and 2.256 eV for CaCr₂O₄. The results indicated that different compositions led to different optical bandgaps. Finally, the results indicate that due to the suitable characteristics and properties of the produced spinel oxides, they are among the promising materials that may be employed as semiconductors for various applications.