Effects of pH on the Structural and Optical Properties of CaAl2O4: Eu2+, Dy3+ Nanoparticles

Abstract

Calcium aluminate phosphor nanomaterials co-doped with europium and dysprosium, (CaAl₂O₄: Eu²⁺, Dy³⁺) were prepared using a facile solution combustion technique. The structural and optical properties were investigated. The X-ray diffraction (XRD) results confirmed the presence of the monoclinic phase in all the samples. The Fourier-transform infrared analysis gave the expected chemical combustion results of the final product with few traces of Ca₃Al₂O₆ impurities at low and very high Potential of Hydrogen (pH). The XRD patterns showed the presence of an impurity phase at low pH. This impurity phase was caused by preferential precipitation in the starting mixture due to a lack of homogeneity in the precursor mixture between the urea and metallic ions hence poor combustion. The diffraction angles of the major peaks shifted to lower 2θ for all the samples except for the sample synthesized at pH = 3.4 which shifted to higher 2θ. The crystallite sizes of the as-prepared samples were determined using the Debye-Scherrer equation. It was noted that there was variation in the crystallite sizes with a change in pH. The effect of pH was also observed in the Ultraviolet-Visible (UV-Vis) studies. It was also noted that the band gap increased with an increase in pH from 2.9 to 4.5. Scanning electron microscope (SEM) micrographs showed that all samples were agglomerated and had irregular shapes with pores and cracks. The study provides a simple route to synthesize CaAl₂O₄: Eu²⁺, Dy³⁺ phosphors with the optimum synthesis pH producing the most crystalline sample for application in lighting devices.

HIGHLIGHTS

• Phosphors are solid-state materials that exhibit the phenomenon of luminescence, whereby they emit light upon excitation. These materials can exist as finely divided powders or as thin films.
• In recent years, researchers have focused on phosphors because of their suitability in many industrial applications including emergency guidance during intentional blackouts, in light emitting devices, fluorescence lamps, watches, demarcation of loads and radiation dosimeter
• Compared to other methods, the combustion method helps to control the amount of each component, gives a uniform mixture, needs low synthesis temperature and takes a short time.
• pH affects the preferential precipitation of the phosphor at low pH for example, there is no complete combustion, and hence the Ca and Al will not be completely precipitated giving rise to poorly developed crystal impurity alongside the pure crystal. At very high pH, there was preferential precipitation of aluminum oxohydroxide in the precursors solution owing to the formation of impurities.

GRAPHICAL ABSTRACT