Sol-Gel Synthesis of CaTiO:Pr Red Phosphors: Tailoring the Synthetic Parameters for Luminescent and Afterglow Applications.

Two sol-gel synthetic routes for the preparation of CaTiO:Pr red emitting phosphors were compared, with the aim of producing nanostructured materials with tailored luminescence/afterglow properties. The effect of the synthetic parameters, such as the addition of a stabilizer and calcination temperature, on the structural, morphological, and optical properties was investigated. The desired perovskite phase was obtained at a calcination temperature of 800 °C or higher. Although the use of acetic acid as the chelating agent leads to micrometric particles with heterogeneous composition, the presence of hydroxypropylcellulose (HPC) results in smaller, less aggregated particles as well as in a high phase purity. At the highest HPC content, surface Ca-rich impurities were detected, although no segregated Ca-rich phases were detectable by X-ray powder diffraction analyses. Luminescence properties were found to be positively related to the phase purity of the oxide, with the highest quantum yields at temperatures equal to or higher than 1000 °C. On the contrary, persistent luminescence properties were highest at intermediate calcination temperatures and for samples synthesized with acetic acid. Overall, a notable role of oxygen vacancies resulting from local Ca excess was observed, acting as trap levels promoting longer relaxation pathways. Thanks to the small-sized particles and best steady-state luminescent properties due to a substantial decrease of lattice defects, the HPC synthesis is a promising strategy for light-emitting diode applications. On the other hand, the acetic acid synthesis promoted a higher defect density, which is required for an efficient yield of light emission in the long time range and is thus more suitable for afterglow applications.