Morphology control and luminescence properties of YAG:Eu phosphors prepared by spray pyrolysis

Starting from nitrate aqueous solutions with citric acid and polyethylene glycol (PEG) as additives, Y₃Al₅O₁₂:Eu (YAG:Eu) phosphors were prepared by a two-step spray pyrolysis (SP) method. The obtained YAG:Eu phosphor particles have spherical shape, submicron size and smooth surface. The effects of process conditions of the spray pyrolysis on the crystallinity, morphology and luminescence properties of phosphor particles were investigated. The emission intensity of the phosphors increased with increasing of sintering temperature and solution concentration due to the increase of the crystallinity and particles size, respectively. Adequate amount of PEG was necessary for obtaining spherical particles, and the optimum emission intensity could be obtained when the concentration of PEG was 0.10 g/ml in the precursor solution. Compared with the YAG:Eu phosphor prepared by citrate-gel (CG) method with non-spherical morphology, spherical YAG:Eu phosphor particles showed a higher emission intensity.     

Y3Al5O12:Tb phosphor particles prepared by spray pyrolysis from spray solution with polymeric precursors and ammonium fluoride flux

High brightness Y₃Al₅O₁₂:Tb phosphor particles with spherical shape and fine size were prepared by spray pyrolysis from spray solution with polymeric precursors and ammonium fluoride flux. The effect of ammonium fluoride flux on the characteristics of the Y₃Al₅O₁₂:Tb phosphor particles prepared by spray pyrolysis was investigated. Ammonium fluoride flux dissolved into spray solution improved the photoluminescence intensity of the Y₃Al₅O₁₂:Tb phosphor particles prepared by spray pyrolysis. The optimum addition amount of ammonium fluoride flux to produce the maximum photoluminescence intensity was 1 wt.% of the Y₃Al₅O₁₂:Tb phosphor particles. The spherical shape of the as-prepared particles obtained by spray pyrolysis from spray solution with polymeric precursors and small amount of ammonium fluoride flux had maintained after post-treatment below 1300 °C. The optimum photoluminescence intensity of the Y₃Al₅O₁₂:Tb phosphor particles prepared from polymeric precursors and ammonium fluoride flux was 156% of that of the phosphor particles prepared from pure aqueous solution.     

UV and VUV characteristics of (YGd)2O3:Eu phosphor particles prepared by spray pyrolysis from polymeric precursors

Red-emitting (YGd)₂O₃:Eu phosphor particles, with high luminescence efficiency under vacuum ultraviolet (VUV) and ultraviolet (UV) excitation, were prepared by a large-scale spray pyrolysis process. To control the morphology of phosphor particles under severe preparation conditions, spray solution with polymeric precursors were introduced in spray pyrolysis. The prepared (YGd)₂O₃:Eu phosphor particles had spherical shape and filled morphology even after post-treatment irrespective of Gd/Y ratio. In the case of solution with polymeric precursors, long polymeric chains formed by esterification reaction in a hot tubular reactor; the droplets turned into viscous gel, which retarded the precipitation of nitrate salts and promoted the volume precipitation of droplets. The brightness of (YGd)₂O₃:Eu phosphor particles increased with increasing gadolinium content, and the Gd₂O₃:Eu phosphor had the highest luminescence intensity under UV and VUV excitation. The maximum peak intensity of Gd₂O₃:Eu phosphor particles under UV and VUV were 118 and 110% of the commercial Y₂O₃:Eu phosphor particles, respectively.     

Europium-doped yttrium silicate nanophosphors prepared by flame synthesis

Europium-doped yttrium silicate (Y₂SiO₅:Eu³⁺) nanophosphors were successfully synthesized by flame spray pyrolysis method. The effect of silicon concentration on the crystal structure and morphology of the Y₂SiO₅:Eu³⁺ phosphors were investigated. As-prepared phosphor consists of spherical nanoparticles with filled morphology, high crystallinity, narrow size distribution, and intense photoluminescence. The crystal structure and photoluminescence intensity of Y₂SiO₅:Eu³⁺ nanophosphors are strongly affected by the ratio of silicon to yttrium in the precursor solution, and the maximum photoluminescence intensity is obtained from particles prepared from the silicon to yttrium ratio of 1.25. A concentration quenching limit is observed at 30 mol% Eu of yttrium. The photoluminescence intensity also increases with the increase of the concentration of precursor solution. This work demonstrates the advantages of flame spray pyrolysis method for the preparation of multi-component nanophosphor, which can be found potential application in lamp and display industries.     

Gd2O3:Eu phosphor powders prepared using a size-controllable droplet generator

Gd₂O₃:Eu phosphor powders were prepared by a filter expansion aerosol generator (FEAG) process capable of changing the mean size of droplets. The change in the mean size of the Gd₂O₃:Eu phosphor powders according to the concentrations of polyethylene glycol added to spray solutions was caused by the difference in the mean size of the droplets produced via the FEAG process. The mean sizes of droplets produced by the FEAG process were affected by the surface tension and viscosity of the spray solutions. The mean sizes of the Gd₂O₃:Eu phosphor powders obtained from the spray solutions with the same concentration of metal salts changed from 1.5 to 4.2 μm according to the concentrations of polyethylene glycol and citric acid added to the spray solutions. The maximum photoluminescent intensity of the phosphor powders obtained from the spray solutions with polymeric precursors and boric acid flux was 144% of that of the phosphor powders obtained from the aqueous spray solutions without flux.     

Metal composition of Y2O3:Eu powder evaluated using particle analyzer

Y₂O₃:Eu red phosphor powder was synthesized with powders of metal–ethylenediaminetetraacetic acid (EDTA) complexes as a starting material. The compositional analysis of each metal–EDTA complexes particle and Y₂O₃:Eu particle was performed using a particle analyzer. In this study, first, the particles of a mixture of Y– and Eu–EDTA complexes were obtained by a spray drying method from solution consisting of Y– and Eu–EDTA·NH₄. Then, the metal–EDTA complex powder was fired in obtaining the Y₂O₃:Eu red phosphor. The metal composition of each particle was scattered for the powder of the metal–EDTA mixture, while it became narrow for the Y₂O₃:Eu powder. The intensity of cathodoluminescence obtained from the Y₂O₃:Eu powder increased with increasing the fired temperature. In addition, the maximum intensity was obtained from the sample with x=0.12 for Y_2−xO₃:Eu_x.     

Metal composition of Y2O3:Eu powder evaluated using particle analyzer

Y₂O₃:Eu red phosphor powder was synthesized with powders of metal–ethylenediaminetetraacetic acid (EDTA) complexes as a starting material. The compositional analysis of each metal–EDTA complexes particle and Y₂O₃:Eu particle was performed using a particle analyzer. In this study, first, the particles of a mixture of Y– and Eu–EDTA complexes were obtained by a spray drying method from solution consisting of Y– and Eu–EDTA$NH₄. Then, the metal–EDTA complex powder was fired in obtaining the Y₂O₃:Eu red phosphor. The metal composition of each particle was scattered for the powder of the metal–EDTA mixture, while it became narrow for the Y₂O₃:Eu powder. The intensity of cathodoluminescence obtained from the Y₂O₃:Eu powder increased with increasing the fired temperature. In addition, the maximum intensity was obtained from the sample with xZ0.12 for Y_2KxO₃:Eu_x.     

Effect of preparation temperature on the characteristics of Eu-doped borate phosphor particles in the spray pyrolysis

Eu-doped gadolinium borate (GdBO₃:Eu) phosphor particles with fine size and uniform morphology were prepared by spray pyrolysis. The effects of the preparation temperatures on the characteristics of the GdBO₃:Eu phosphor particles prepared by spray pyrolysis were analyzed. The precursor particles obtained at preparation temperatures below 1400 °C had hollow and porous inner structures. On the other hand, the precursor particles obtained at preparation temperature of 1600 °C had dense structure and uniform morphologies. The precursor particles with glass phases were formed at high-preparation temperatures above 1400 °C. The precursor particles prepared at preparation temperature of 1400 °C turned into GdBO₃:Eu phosphor particles with fine size and regular morphology after post-treatment. The GdBO₃:Eu phosphor particles prepared from the precursor particles obtained at a preparation temperature of 1400 °C had the maximum photoluminescence intensity, which was similar to that of the commercial (Y,Gd)BO₃:Eu phosphor particles.     

Study of the effect of Li<Superscript>+</Superscript> concentration on the photoluminescence properties of Y<Subscript>2</Subscript>O<Subscript>3</Subscript>:Eu<Superscript>3+</Superscript> phosphors

Y₂O₃:Eu³⁺ phosphors were prepared by hydrothermal method. Effect of the doping concentration of Eu³⁺ on the photoluminescence properties of Y₂O₃:Eu³⁺ phosphor was studied in details. It was found that the strongest emission intensity is achieved as atomic ratio of Y³⁺ to Eu³⁺ is 8. As concentration of Eu³⁺ exceeds the critical concentration, the emission intensity decreases dramatically due to the concentration quenching of Eu³⁺. Also, the effect of Li⁺ on the photoluminescence performance of the Y₂O₃:Eu³⁺ phosphor is studied in this work. According to the results, the doping of Li⁺ may greatly improve the PL performance of the Y₂O₃:Eu³⁺ phosphors due to the flux effect and improved crystallinity caused by the doping of Li⁺.     

Effect of boron concentration in Y2O3:(Eu,B) phosphor on luminescence property

Y₂O₃:(Eu,B) red phosphor was obtained by a thermal decomposition technique of a mixture of yttrium–ethylenediaminetetraacetic acid (Y–EDTA), Eu–EDTA and boric acid. The doping ratio of boron, [B]/([Y]C[Eu]), in the starting material was varied from 0 to 1. The properties of morphology, crystallization behavior, metal composition and photoluminescence of resulting Y₂O₃:(Eu,B) powder were examined. The Y₂O₃:(Eu,B) powder had spherical shape with a diameter of 1–5 mm with some hollows on the surface. No secondary cohesive particles were seen. X-ray diffractometry revealed that although the crystallinity of powder was improved with addition of small amounts of boron, it decreased with excess amounts of boron. Several peaks indexed as the impurity phases of Y₃BO₆ and YBO₃ were observed on the samples obtained under condition of excess amounts of boron. The photoluminescence intensity observed at 611 nm was dependent upon crystallinity of the sample.     

Luminescence and crystallinity of flame-made Y2O3:Eu3+ nanoparticles

Cubic and/or monoclinic Y₂O₃:Eu³⁺ nanoparticles (10–50 nm) were made continuously without post-processing by single-step, flame spray pyrolysis (FSP). These particles were characterized by X-ray diffraction, nitrogen adsorption and transmission electron microscopy. Photoluminescence (PL) emission and time-resolved PL intensity decay were measured from these powders. The influence of particle size on PL was examined by annealing (at 700–1300°C for 10 h) as-prepared, initially monoclinic Y₂O₃:Eu³⁺ nanoparticles resulting in larger 0.025–1 μm, cubic Y₂O₃:Eu³⁺. The influence of europium (Eu³⁺) content (1–10 wt%) on sintering dynamics as well as optical properties of the resulting powders was investigated. Longer high-temperature particle residence time during FSP resulted in cubic nanoparticles with lower maximum PL intensity than measured by commercial micron-sized bulk Y₂O₃:Eu³⁺ phosphor powder. After annealing as-prepared 5 wt% Eu-doped Y₂O₃ particles at 900, 1100 and 1300°C for 10 h, the PL intensity increased as particle size increased and finally (at 1300°C) showed similar PL intensity as that of commercially available, bulk Y₂O₃:Eu³⁺ (5 μm particle size). Eu doping stabilized the monoclinic Y₂O₃ and shifted the monoclinic to cubic transition towards higher temperatures.     

Synthesis and photoluminescence properties of (Y,Gd)BO3 : Eu phosphor prepared by ultrasonic spray

The mixed solution of acetate of Y, Gd, Eu and boric acid diluted in water was used as the precursor for the ultrasonic spray for the synthesis of (Y,Gd)BO₃ : Eu phosphor. It was found that (Y,Gd)BO₃ : Eu phosphor made by ultrasonic spray pyrolysis had a spherical shape and a narrow size distribution having a mean size of 1.3 m, while it had irregular, coarse and non-uniform size distribution for the phosphor formed by solid-state reaction. The as-sprayed particles was amorphous but they converted into the same polycrystalline phase of solid state reaction after post heat treatment at 1100 °C for 2 hr. The emitting intensity under 147 nm VUV excitations for the spray-formed (Y,Gd)BO₃ : Eu phosphor, however, was inferior to the later one. It was found that the optimum concentrations of Gd and Eu were 30% and 5%, respectively in (Y_1–x Gd _x )_1–y BO₃ : Eu _y phosphors prepared both by spray and solid state reaction.     

Direct synthesis of strontium titanate phosphor particles with high luminescence by flame spray pyrolysis

SrTiO₃:Pr, Al phosphor particles with high luminescence intensities were directly prepared by flame spray pyrolysis without post-treatment. They had better crystallinity than those prepared by general spray pyrolysis with post-treatment and solid-state reaction methods. In addition, they had complete spherical shape and narrow size distribution. On the other hand, the particles prepared by general spray pyrolysis had irregular shape, and poorer brightness than those prepared by solid-state reaction method, while the particles prepared by flame spray pyrolysis had comparable photoluminescence and cathodoluminescence intensities with those of particles prepared by solid-state reaction method. The photoluminescence intensity of SrTiO₃:Pr, Al particles prepared by flame spray pyrolysis was as much as 4.7 times higher than that of particles prepared by general spray pyrolysis.     

Estimation of yttrium oxide microstructural parameters using the Williamson–Hall analysis

ABSTRACT

Yttrium oxide (Y₂O₃) particles were synthesised from a yttrium nitrate solution via ultrasonic spray pyrolysis (USP) method. The effects of temperature and precursor concentration on morphology and microstructural parameters were investigated. Y₂O₃ particles were characterised by scanning-electron-microscope energy-dispersive spectroscopy, Raman spectroscopy and X-ray diffraction analysis. Based on X-ray peak broadening, the crystallite size was calculated using the modified Debye–Scherrer (MDS) method. Furthermore, the crystallite size, crystal strain and the energy density of the crystal were evaluated using the Williamson–Hall (W–H) analysis integrated with the uniform deformation model, the uniform stress deformation model and the uniform deformation energy density model. A comparative evaluation of Y₂O₃ crystallite size using the MDS and W–H methods was carried out.

This is part of a thematic issue on Nanoscale Materials Characterisation and Modeling by Advances Microscopy Methods - EUROMAT.     

Preparation and photoluminescence properties of Y2O3:Eu, Bi phosphors by molten salt synthesis for white light-emitting diodes

Y₂O₃:Eu, Bi red phosphors were first prepared by molten salt synthesis (MSS) method at a low temperature. X-ray diffraction (XRD), scanning electron microscopy (SEM), and fluorescence spectrophotometer were used to characterize the as-synthesized phosphors. The results show that as-obtained Y₂O₃:Eu, Bi phosphors have good cubic crystallinity, presenting octahedral morphology with smooth surface and relatively uniform particle size. Bi³⁺ ion as a sensitizer plays a significant effect on the emission intensity of Y₂O₃:Eu, Bi by energy transfer from Bi³⁺ to Eu³⁺ and the optimal concentration of Bi³⁺ is 1.5 mol%. Y₂O₃:Eu, Bi emits excellent red light as the excitation wavelength is between 330 and 420 nm or excited by 254, 466 nm. Meanwhile, its emission intensity is as strong as that of sample prepared by solid-state reaction. So the as-fabricated red phosphors by MSS method would have a promising application in the area of white light-emitting diodes.     

Morphology and luminescent properties of Al<Superscript>3+</Superscript>/Mg<Superscript>2+</Superscript>-doped Y<Subscript>2</Subscript>O<Subscript>3</Subscript>:Eu<Superscript>3+</Superscript> phosphor

Al³⁺/Mg²⁺ doped Y₂O₃:Eu phosphor was synthesized by the glycine-nitrate solution combustion method. In contrast to Y₂O₃:Eu which showed an irregular shape of agglomerated particles (the mean particle size >10 μm), the morphology of Al³⁺/Mg²⁺ doped Y₂O₃:Eu crystals was quite regular. Al³⁺/Mg²⁺ substituting Y³⁺ in Y₂O₃:Eu resulted in an obvious decrease of the particle size. Meanwhile, higher the Al³⁺/Mg²⁺ concentration, smaller the particle size. In particular, the introduction of Al³⁺ ion into Y₂O₃ lattice induced a remarkable increase of PL and CL intensity. While, for Mg²⁺ doped Y₂O₃:Eu samples, their PL and CL intensities decreased. The reason that causes the variation of PL and CL properties for Al³⁺ and Mg²⁺ doped Y₂O₃:Eu crystals was concluded to be related to sites of Al³⁺ and Mg²⁺ ions inclined to take and the difference of ion charge.     

Directed growth of nanoarchitected hybrid phosphor particles synthesized at low temperature

Sub-micronic particles are of considerable interest for a wide variety of applications, such as catalyst or optical ceramics, due to their unique properties determined by size, composition and structure. In this work, we have reported a simple, rapid, single-step aerosol processing for the continuous synthesis of nanostructured particles having homogeneous composition and narrow size distributions, good crystallinity and fluorescence response. This paper presents the synthesis, optimization and characterization of hybrid Ag@Y₂O₃:Eu (9 at.% Eu³⁺) phosphor particles by means of spray pyrolysis method from water solutions of common nitrates precursors. The effect of silver concentration on particle structure, morphology and functional properties was specially evaluated. The as-prepared samples were additionally heated from 800 to 1200 °C/12 h under constant argon flow to avoid the silver oxidation. It was evident the cubic phase with Ia-3 symmetry as the principal one in all as-prepared and thermally treated samples. For the case high silver nitrate concentrations in precursor solutions a minority crystalline phase having Fm3m symmetry was identified. The luminescence emission spectra have been taken after excitation at 235 nm wavelength. It is evident the increase in the emission caused by the presence of metallic silver nanoparticles onto Y₂O₃:Eu³⁺ particle surface. It was also determined the silver concentration influence on the fluorescence response.     

The Effect of flux types on the formation of green light emitting phosphor particles with spherical shape and filled morphology

(CeTb)MgAl₁₁O₁₉ (CTMA) phosphor particles were prepared by high-temperature spray pyrolysis from spray solutions with various types of flux materials. The particles prepared from spray solutions with ammonium dihydrogen phosphate and lithium carbonate fluxes had spherical shape and filled morphology at temperatures of 900^∘C and 1650^∘C. On the other hand, the particles prepared from spray solutions without flux material had hollow and fractured morphology at temperatures of 900^∘C and 1650^∘C. The melting of flux material formed the spherical intermediate particles with filled morphology. These spherical intermediate particles were formed from spray solutions with flux material that transformed into spherical CTMA phosphor particles with filled morphology at a high-preparation temperature. The phosphor particles prepared by spray pyrolysis from the spray solution with appropriate flux materials at 1650^∘C had high photoluminescence intensities, spherical shape, and filled morphology.     

Homogeneous coatings of nanosized Fe2O3 particles on Y2O2S:Eu

The red emitting phosphor Y₂O₂S:Eu commonly applied in colour television tubes was pigmented with a thin homogeneous layer of nanosized Fe₂O₃ particles based on a novel coating strategy. In a first step the phosphor was covered with nanosized Fe₃O₄ particles which themselves were prepared by reduction of an Fe(III)-salt with hydrazine. Thereafter, Fe₃O₄ was converted to Fe₂O₃ by heating to 450°C in air. Surface composition and body colour of the corresponding phosphor samples were investigated applying ESCA and measuring diffuse reflectance. The size of the Fe₂O₃ particles as well as the homogeneity of the resulting coating were studied by SEM. Furthermore, the adhesion of Fe₂O₃ particles on the phosphor surface was examined.     

Synthesis and photoluminescence of Y,Eu co-doped ZnO red phosphor

A red emitting ZnO·Y₂O₃:Eu phosphor has been prepared using pyrolysis technique at temperatures ≤1000 °C. When NH₄Cl was used as an ingredient, its luminescence efficiency was quite high indicating that Cl^? ions act as charge compensators since the introduction of Y³⁺ and Eu³⁺ cations in ZnO lattice demands the introduction of equal amount of excess anions. However, Na⁺ or Li⁺ quenches the luminescence efficiency of ZnO·Y₂O₃:Eu. Due to ZnO host absorption, the excitation peaks of ZnO·Y₂O₃:Eu phosphor near 260 nm and 394 nm are suppressed while the one at 468 nm is intense. This red emitting phosphor may find applications when monochromatic excitation such as lasers are involved. XRD data of (Zn_0.93Y_0.07)O_z:Eu³⁺,Cl^? shows the presence of the ZnO phase as well as the Y₂O₃ phase. It shows that Y₂O₃ forms a sublattice within ZnO host. This is supported by the PL data of (Zn_0.93Y_0.07)O_z:Eu³⁺,Cl^? which showed no significant change in the PL efficiency with increase in ZnO molar concentration.