Abstract
Calcia-alumina binary compounds doped with rare earths and some transition metals cations show persistent luminescence from the visible to the infrared range. Specifically, the blue light can be obtained through the Eu2+ activator center in a potential host, such as dodecacalcium hepta-aluminate (Ca12Al14O33) and monocalcium aluminate (CaAl2O4). By doping with Nd3+, the persistent luminescence can be substantially prolonged; for this reason, the Eu/Nd pair is a potential choice for developing long-lasting blue luminescence. Herein, the phase evolution of the calcia-alumina system via molten salt synthesis is reported as a function of the synthesis temperature and the atmospheric environment. The fraction of CaAl2O4 phase increases when the temperature is higher. Synthesized microparticles of platelet-type morphology represent isolated nanostructured ceramic pieces. Under visible light, the particles are white. This indicates that the followed process solves the dark-gray coloring of phosphor when is synthesized in a reduced atmosphere at high temperature. As regards the synthesis mechanism, which is assisted by the molten flux, the dissolution−diffusion transport process is promoted at the surface of the alumina microparticles. In fact, the emission intensity can be modulated through the phase of the Eu-doped calcium-aluminate discrete platelets synthesized. Consequently, the photoluminescence intensity depends also on the oxidation state of the Eu ion. X-ray absorption near-edge structure and photoluminescence measurements corroborate the Eu reduction and the grain coarsening with the enhancement of the blue emission. The doped phosphors with Eu/Nd show a broad and strong absorption in the region of 320–400 nm and a broad emission band at around 440 nm when they are excited in this absorption range. From a broader perspective, our findings prove that the Ca12Al14O33 and CaAl2O4 phases open new opportunities for research into the design of blue long-lasting emitters for a wide range of fields from ceramic to optoelectronic materials.
Highlights
Research on rare earth luminescent materials has been stimulated over the last decade by several important industrial applications, such as bioimaging and energy harvesting
We evaluated the vast synthesis conditions to facilitate the formation of the desired crystalline structure
We found that the ratio 3:1 offers the most suitable synthesis conditions, analogously to synthesis for the SrAl2O4 phosphor
Summary
Research on rare earth luminescent materials has been stimulated over the last decade by several important industrial applications, such as bioimaging and energy harvesting. There is a great demand for rare earth-doped oxide materials with excellent luminous efficiency and high thermal stability. The emission of light in a fixed range (Ultraviolet, (UV), to Near Infrared Region, (NIR)) is required, and the capability of materials to act as an optical storage is needed. These materials should emit after the removal of the activation light source. The research is focused mainly upon green emission due to the large sensitivity in this region of the human eye (luminous efficacy) in light and dark conditions, the persistent emission other wavelength ranges (blue or red light emission) is desirable
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