Nanocrystalline monocalcium aluminate "Krotite": band gap engineering and near IR emission by selective excitation of native defects owing to annealing

This research deals with the synthesis of "Krotite", i.e., monocalcium aluminate "CaAl2O4", using the citrate sol-gel method. Various complementary techniques have explored the consequences of annealing temperature on the crystallinity, morphology, optical band gap, and emission mechanism. XRD and FTIR certified the formation of impurity-free CaAl2O4 powder within the size range of 5-10 nm. The granular surface morphology of the "Krotite" powder and its particle size distribution was examined by SEM and TEM, respectively.The direct band gap of the prepared samples was calculated (similar to 4-5 eV) using Tauc's plots. The red shift in absorbance spectra was observed with annealing due to modulation in the undoped "Krotite" band gap. Photoluminescence (PL) results demonstrate blue emission at 420 nm after providing energy corresponding to direct excitation. More interestingly, near-infrared (IR) emission has also been found after exciting with indirect excitation energy. The citrate sol-gel method employed in the work is very economical, environment-friendly, and compatible with the long-term goal of continuous industrial-scale manufacturing of monocalcium aluminate.