Investigation of photoluminescent properties of rare-earths doped mixed multicomponents structures of phosphovanadates
Motloung, Selepe Joel
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Multicomponent structures of lanthanide phosphovanadate doped with various rare earth ions were successfully synthesized by solution combustion method. These phosphor powders were prepared at 600±10oC using urea as a fuel. Selected series of samples were annealed at different temperatures ranging from (700 –1000oC) while others were annealed at 900oC for 2 hours, which was found to be the optimum temperature. The crystal structure formation, crystallite sizes, and surface morphologies of the prepared phosphor powders were identified by X-ray diffraction (XRD), high resolution transmission electron microscope (HR–TEM) and field emission scanning electron microscopy (FE–SEM). The elemental composition and the stretching modes of vibration of the samples were investigated by energy dispersive x-ray spectroscopy (EDS) and Fourier transform infrared (FTIR) spectrometer respectively. The diffuse reflectance measurements, which were used to estimate the band gap energies, were determined by ultraviolet/visible spectroscopy (UV–vis). The room temperature photoluminescence (PL) data, excitation and emission, were recorded using a HITACHI F700 fluorescence spectrophotometer. The XRD results revealed that GdVO4 and GdPO4 crystallized in a tetragonal structures. The results further indicated that the XRD peaks of GdV1-xPxO4 slightly shifted towards higher values of 2θ angles when the value of x (P content) was increased. The X-ray diffraction peaks of GdV0.5P0.5O4 were found to be a combination of those of bulk GdVO4 and GdPO4. On the other hand, the lanthanum systems, LaV1-xPxO4 (x = 0, 0.25, 0.5, 0.75, 1), the XRD results confirmed the formation of monoclinic structure of LaVO4 for x = 0 and hexagonal structure of LaPO4 for x = 1. The results also revealed that the crystal structure changed from LaVO4 to LaPO4 when the value of x was increased from 0 to 1. The XRD results for the yttrium system, YV0.5P0.5O4 in particular, showed that the peaks were a combination of those of bulk YVO4 and YPO4. In general, the XRD results showed that all the annealed samples were highly crystalline, free of impurities and have small crystallite sizes. Thus, the annealing temperature played a pivotal role to improve the crystallinity of the prepared powder samples. This was confirmed by the pronouncement of the distinct lattice fringes on the HR–TEM images. FE-SEM results revealed that the particles of the prepared powder samples are agglomerated for un-annealed samples and less agglomerated for the annealed samples. Generally, the FE-SEM micrographs showed that the samples have different shapes and sizes. The incorporation of the dopants did not cause any noticeable change on the morphology of the prepared samples. The presence of all these dopants within the host materials were confirmed by EDS. The room temperature diffuse reflectance spectra revealed that the prepared powder samples mainly absorbed in the UV region. The DRS were mostly dominated by the absorption band in the range between 200 and 350 nm peaking at ~ 275 nm. In some instances, some weak f→f bands were also observed beyond 350 nm. The band gap energies were found to be influenced by the phosphorus content within the samples as well as the dopant concentrations. The room temperature PL data revealed that the prepared powder samples could be excited with UV radiation and emit in the visible range. The strong broad band in the UV range between 200 and 350 nm was observed in almost all the samples, although there were minor f→f bands beyond 350 nm wavelength for other samples. PL data also revealed that there was energy transfer from the host to the dopants and between the dopants for doubly doped samples. Generally, the PL intensity was influenced by the vanadium and phosphorus concentrations, the annealing temperature and the dopant concentrations.