Shape- and Size-Controlled Synthesis of Calcium Molybdate Doughnut-Shaped Microstructures

We report a solution-phase rapid-injection-based route for the synthesis of calcium molybdate (CaMoO4) microstructures with well-defined shape and size. This tertiary metal oxide material has been widely used as phosphors, scintillator, and laser materials for a long time because of its attractive luminescence property. Complex three-dimensional Structures, mainly doughnut-shaped microparticles with highly controllable size and aspect ratio, can be obtained at 100 degrees C by controlling the reaction conditions including the concentration of reactants, molar ratio between reactants, molybdenum source, pH, and the volume ratio of the mixed solvents. Other shapes such as spindles, nanosheets, and microspheres can also be produced by pushing the reaction conditions away from those optimal for doughnuts. X-ray diffraction, scanning electron microscopy, transmission electronic microscopy, and X-ray energy dispersive spectrometer were used to characterize the obtained samples. The photoluminescence spectra of the CaMoO4 doughnut microstructures reveal a strong and broad emission with a maximum at 512 nm, which is blue-shifted by similar to 20 nm compared to the value for the bulk samples, The advantages of this synthetic route over the previously reported ones include the precise control over particle size and shape, simple synthetic procedure, relatively low reaction temperature, and high reproducibility of the process.