The red phosphor NaEu(MoO4)2 prepared by the combustion method
Introduction
More and more interests are focused on the white-light-emitting diodes (W-LEDs), which have many advantages, such as high efficiency, long lifetime, low power consumption and environmental-friendly characteristics [1]. At present, the most common method to obtain the white light is to combine a yellow-emitting phosphor with blue LED chip, however, there are some problems for such “blue + yellow” white LEDs, such as lower color-rendering index, lower luminous efficiency [2], [3]. In order to improve the luminous efficiency, a new approach using near UV (around 400 nm) InGaN-based LED chip coated with blue/green/red tricolor phosphors was introduced [2]. The current red-emitting phosphor for near UV InGaN-based white LEDs is Y2O2S:Eu3+, which shows some problems: lower efficiency compared with that of blue and green phosphors, shorter working lifetime under UV irradiation, and instability due to releasing of sulfide gas [3], [4], hence it is urgent to find new red phosphors with high efficiency.
The double molybdates AB(MoO4)2 (A = Li+, Na+, K+, Rb+, Cs+; B = trivalent rare earth ions), which share the scheelite-like (CaWO4) iso-structure, show excellent thermal and hydrolytic stability, then they were considered to be efficient luminescent hosts [5], [6], [7], [8]. In this scheelite structure, Mo(VI) is coordinated by four oxygen atoms in a tetrahedral site and the alkali metal ion/rare earth ion site is eight coordinated [9]. In our previous work [10], the photo-luminescent properties of NaLa1 − x(MoO4)2:xEu3+ were systematically investigated, and the results suggested that the phosphor NaEu(MoO4)2 showed the intense red emission with the good CIE (Commission Internationale de l’Eclairage, International Commission on Illumination) chromaticity coordinates under 400 nm excitation.
The phosphors particles prepared by the conventional solid-state reaction were of bigger size with irregular morphology, which were not suitable for the application. Therefore, the wet-chemical methods seem to be an attractive alternative to the classical approach [11]. The combustion process [12], [13], [14] to prepare the phosphor is very facile, and only takes a few minutes, which has been extensively applied to the preparation of various nano-scale inorganic phosphors. So in present paper, the phosphor NaEu(MoO4)2 was prepared by the combustion method, and its photo-luminescence properties were investigated.
Section snippets
Preparation of NaEu(MoO4)2 by the combustion method
The starting materials include 99.99 % purity Eu2O3, analytical reagent grade HNO3, (NH4)6Mo7O24·4H2O, NaHCO3, EDTA and urea. Firstly, 2 mmol (NH4)6Mo7O24·4H2O was dissolved in aqueous solution. Secondly, Eu2O3 (1.2317 g) were dissolved with nitric acid aqueous solution, and EDTA (2.5000 g), NaHCO3 (0.5880 g) were added in the aqueous solution, adjusting the pH to ∼ 5 with NH3·H2O/citric acid. Then the (NH4)6Mo7O24·4H2O aqueous solution was added in the Eu3+-containing solution. The transparent
Results and discussion
The XRD patterns of NaEu(MoO4)2 prepared by the combustion method fired at 600 °C (a), 700 °C (b), 800 °C (c), and prepared by the solid state reaction at 800 °C (d) are shown in Fig. 1. The single crystalline phase of scheelite structure has been formed at 600 °C, and these XRD patterns of the samples are consistent with that given in JCPDS card 25-0828 [Na0.5Gd0.5MoO4], the result shows that these phosphors are of single phase with the scheelite structure.
The SEM micrographs of NaEu(MoO4)2
Conclusions
The phosphor NaEu(MoO4)2 was prepared the combustion method, and its structure, the morphology and the luminescent properties were compared with that of NaEu(MoO4)2 prepared by the solid state reaction. The emission intensity of the phosphor prepared by the combustion method fired at 800 °C is about 1.6 times stronger than that prepared by the solid state reaction fired at 800 °C.
Acknowledgements
This work was financially supported by the National Natural Science Foundation of China (20571088), and by a research grant from the Guangdong province government (ZB2003A07).
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Crystal and electronic structure, thermochemical and photophysical properties of europium-silver sulfate monohydrate AgEu(SO<inf>4</inf>)<inf>2</inf>·H<inf>2</inf>O
2021, Journal of Solid State ChemistryCitation Excerpt :Moreover, double sulfates of rare-earth elements are of interest because of catalytic properties in redox processes [28,29]. The information on double sulfates containing trivalent europium is extremely episodic, while the corresponding double molybdates and europium tungstates with monovalent cations are well studied [30–33]. Employment of silver ions instead of alkaline ones in the structure of rare-earth containing double salts is expected to result in both variation of local field upon rare earth ions and variation of a complex of physical properties of a material that are important for some applications [34].
Local structure of molybdates solid solutions containing europium by results of atomistic simulation
2020, Materials Today CommunicationsSynthesis and spectroscopic investigation of nanostructured europium oxalate: A potential red emitting phosphor
2015, Solid State SciencesCitation Excerpt :In most of the systems emission quenching is observed at higher concentrations of Eu3+ which limits the doping concentration and thereby the luminous intensity [22,23]. However no concentration quenching is observed in some fully concentrated systems like, NaEu(MoO4)2, EuP5O14 and K5Eu(MoO4) etc. and they exhibit remarkable optical properties [24–26]. Among them europium oxalate is an important candidate which can produce strong red emission without exhibiting any concentration quenching, but less investigated other than that on the micro structured europium oxalate [27,28].
Hydrothermal synthesis and properties of a red-emitting phosphor of fully concentrated Eu<sup>3+</sup> based oxalate Eu<inf>2</inf>(C<inf>2</inf>O <inf>4</inf>)<inf>3</inf>·10H<inf>2</inf>O
2014, Materials Research BulletinCitation Excerpt :Luminescent materials based on fully concentrated Eu3+-compounds have attracted many interests because of their remarkable properties [14]. It has been found that some fully concentrated Eu3+-compounds show no concentration quenching at all, for example, Eu2WO6 [15], NaEu(MoO4)2 [16], EuP5O14 and K5Eu(MoO4)4 [17] etc. These red phosphors are very favorable to suffer from high photon flux during its application without many luminescence quenching.
Effects of doped-Li+ and-Eu<sup>3+</sup> ions content on structure and luminescent properties of Li<inf>x</inf>Sr<inf>1-2x</inf>(MoO<inf>4</inf>): Eu<sup>3+</sup><inf>x</inf> red-emitting phosphors for white LEDs
2012, Materials Chemistry and PhysicsCitation Excerpt :At the same time, the host structure is another important factor. It was found molybdates with a scheelite structure, which has an excellent thermal and chemical stability, were considered to be efficient luminescent hosts under near-UV or blue excitation owing to MoO42+ tetrahedron unit [7,8,19]. Generally, phosphors with a low doped-Ln3+ ion content will yield a weak luminescence, while an over-doping level perhaps brings about quenching of the luminescence [20].