Synthesis And Luminescence Properties Of Rare Earth Doped BiPO₄ Luminescent Materials

White light emitting diodes (LEDs) are new generation green lighting devices due to advantages such as energy conservation, environmental friendliness, high efficiency and long lifetime. Nowadays, the combination of blue diode chip with Y3Al5O12:Ce3+ yellow phosphor is the most common method for the realization of white LEDs. Nevertheless, these devices suffer from low color rendering index (CRI) because of the deficiency of red light component. Consequently, more attention has been paid to the development of novel red phosphors. Phosphates have been used as hosts for many phosphhors due to their extremely high physical and chemical stability. Among the various phosphates, BiPO4 is regarded as an efficient host, comparable to LnPO4 in structure and ionic radius.In this article, Bi1-xPO4:xLn3+(Ln=Sm, Eu) phosphors were successfully synthesized via hydrothermal method. The phase structure, morphology and luminescence properties of as-prepared samples were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM) and fluorescence spectroscopy. In order to synthesis low temperature monoclinic phase (LTMP) Bi1-xPO4:xLn3+(Ln=Sm, Eu) and improve their luminescence properties, the influences of pH value, Ln3+doping, molar ratio etc on the structure, morphology and luminescence properties of the samples were investigated. The main results are as follows:(1) In order to synthesis low temperature monoclinic phase Bi1-xPO4:xSm3+, un-doped BiPO4 and Bi1-xPO4:xSm3+ were prepared at different pH values. The influences of pH and Sm3+ doping on the structure, morphology and luminescence properties of Bi1-xPO4:xSm3+ were investigated. With the pH in the range of 0.5-2.0, all diffraction peaks of the octahedron-like BiPO4 can be indexed to LTMP. At pH=1.0-1.5, XRD results reveal that a phase transition from low temperature monoclinic phase to hexagonal phase (HP) occurs with increasing Sm3+ doping. At pH=0.5, Bi1-xPO4:xSm3+ with LTMP could be obtained within a very wide x range (x=0-0.15). The results suggest that crystalline structure and morphology of the samples can be readily tuned by adjusting the pH and Sm3+ doping. Furthermore, the structural transition from LTMP to HP, induced by Sm3+ doping can be suppressed at pH 0.5. The photoluminescence (PL) spectra suggest that the optimal emission intensity of Bi0.89PO4:0.11Sm3+ appears at pH 0.5.(2) In order to improve the red light component of phosphors, Bi1-xPO4:xEu3+ phosphors were prepared by a simple hydrothermal method. The influences of pH and Eu3+ doping on the structure, morphology and luminescence properties of Bi1-xPO4:xEu3+ were investigated. XRD and FE-SEM results reveal that octahedron-like Bi0.95PO4:0.05Eu3+ with LTMP could be obtained within a very wide pH range (pH=0.5-1.5). At pH 2.0, the microrod-like with LTMP and nanoparticles with HP appear. With the pH increasing to 3.0, the sample is composed of agglomerated nanoparticles with HP. By comparing the CIE chromaticity coordinates of Bi0.89PO4:0.11Eu3+ (0.6249,0.3747) and Bi0.89PO4:0.11Sm3+(0.5599,0.4368), we identify that the Eu3+ions is a better activator for red phosphors.(3) In order to enhance the emission intensity of Bi1-xPO4:xEu3+ phosphors, various types of surfactants and chelating agent were used to prepare a series of samples by a hydrothermal method. The PL results reveal that the emission intensity is significantly affected by the surfactants and that the highest luminescence intensity was observed for the cetyltrimethyl ammonium bromide (CTAB)-assisted Bi1-xPO4:xEu3+ phosphors. The influences of CTAB concentration, pH and Eu3+ doping on the structure, morphology and luminescence properties of Bi1-xPO4:xEu3+ were investigated. XRD and FE-SEM results reveal that octahedron-like (LTMP) sample gradually tend to disappear and be replaced with irregular polyhedron (LTMP) with an increase in CTAB concentration. The optimal CTAB concentration required for maximum PL emission intensity of Bi0.87PO4:0.11Eu3+ is 2 mmol.(4) In order to enhance the emission intensity of Bi1-xPO4:xEu3+ phosphors, microrod-like Bi1-xPO4:xEu3+ were selectively prepared by a simple hydrothermal method. The influences of molar ratio PO43-/[Bi3++Eu3+], pH and Eu3+ doping on the structure, morphology and luminescence properties of Bi1-xPO4:xEu3+ were investigated. At molar ratio<1.0, a phase transition from low temperature monoclinic phase (LTMP) to hexagonal phase (HP) occurs with decreasing NH4H2PO4, accompanied by a morphology transition from octahedron-like to agglomerated nanoparticles. At molar ratio>1.0, the morphology varies from octahedron-like (LTMP) to microrod-like (LTMP) with increasing NH4H2PO4. The emission intensity is initially enhanced with the increase in molar ratio, then reaches a maximum with the molar ratio of PO43-/[Bi3++Eu3+] at 3.0 and subsequently stays almost the same even with further increase in molar ratio from 3.0 to 5.0. The results suggest that molar ratio play a key role in controlling the structure, morphology and luminescence properties of BiPO4:Eu3+ phosphors.