Abstract
Europium ion doped BaMgAl10O17 blue phosphor nanopowder has been fabricated by urea–nitrate solution combustion synthesis at 590 °C for 5 min. These phosphors were codoped with different europium ion concentrations (1–8 mol%). The experimental results of x-ray diffraction (XRD), scanning electron microscopy (SEM) and photoluminescence showed that the phosphors have a hexagonal single phase structure, the average particle size of the powders was about 50 nm and the emission spectra have a broad band with maximum intensity at wavelength λmax=455 nm due to transitions from the 4f65d1 to the 4f7 electronic configuration of Eu2+ ion. The maximum emission of phosphor corresponds to the europium concentration 7 mol%.
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1. Introduction
BaMgAl 10 O 17:Eu 2+ blue phosphor has been used extensively in manufacturing tricolor fluorescent lights (FL), field emission displays (FED), plasma display panels (PDPs) and liquid crystal displays (LCD) [1, 2]. Emission spectra of BaMgAl 10 O 17:Eu 2+ phosphor have a broad band with peak at 455 nm due to transition from the 4f 65d excited state to the 4f 7 ground state of ion Eu 2+. There are many synthesis technologies of this phosphor [3–6]. Every technology has some advantages. Among them, combustion synthesis has the following remarkable advantages: low heating temperature and short reaction time. However, luminescent properties of materials depend strongly on the technology conditions [2, 7]. For BaMgAl 10 O 17:Eu 2+ phosphors prepared by urea–nitrate solution combustion synthesis, urea plays the role of fuel as well as reducing agent. Besides, the initiating combustion temperature influences the product. In the present experimental work, we study the influence of urea concentration and the initiating combustion temperature on luminescent properties of BaMgAl 10 O 17:Eu 2+ phosphors prepared by urea–nitrate solution combustion synthesis, and also the influence of concentration on emission intensity.
2. Experimental
Starting materials for the preparation of BaMgAl 10 O 17:Eu 2+ phosphors by urea–nitrate solution combustion synthesis are a mixture of Ba(NO 3)2, Mg(NO 3)2·6H 2 O, Al(NO 3)3·9H 2 O and Eu 2 O 3 oxide. Urea was used to supply fuel and reducing agent. Eu 2 O 3 oxide has been nitrified by nitric acid. The reaction for the formation of BaMgAl 10 O 17:Eu 2+, assuming complete combustion, may be written as (1−x)Ba(NO 3)2+xEu(NO 3)3+Mg(NO 3)2+10Al(NO 3)3+28.34CH 4 N 2 O→Ba (1−x) Eu x MgAl 10 O 17+ by products [8].
Aqueous solution containing stoichiometric amounts of nitrate metal and urea was mixed by a magnetic stirrer and heated at 60 °C for 2 h to gel. Next, the gel was dried at 80 °C to dehydrate and combusted at different temperatures within 5 min. The product was BaMgAl 10 O 17:Eu 2+ (1 mol%) with white powder. The influence of heating temperature and urea concentration on luminescent properties was investigated. The samples were prepared with combustion temperature changed from 570 to 630 °C, concentration of Eu 2+ ions changed from 0 to 8 mol% and changing the urea mole (n urea ) from 30 to 80 times the product mole (n BAM ). For convenience, we set
in this case 30 ⩽n⩽ 80.
3. Results and discussions
3.1. The effects of combustion technology on the structure and luminescence of BaMgAl 10 O 17:Eu 2+ blue phosphor
The crystallographic phase of phosphor with different urea concentrations at a constant combustion temperature of 590 °C was confirmed by x-ray diffraction (XRD) and the results are shown in figure 1. The XRD pattern indicated that product did not appear at BaMgAl 10 O 17 phase with n=30. With n=40, 50 and 70, products occurred at a low amount of undesirable phase beside the BaMgAl 10 O 17 phase. The material had a hexagonal single phase structure with n=60.
Figure 1 XRD diagram of the samples with different concentrations of urea.
Luminescent spectra of BaMgAl 10 O 17:Eu 2+ phosphors prepared with different concentrations of urea are shown in figure 2. Emissions of phosphors with concentrations n=40, 50, 60 and 70 have a broad band with peak at 455 nm that characterized the transition of electronic configuration from the 4f 65 d excited state to the 4f 7 ground state of Eu 2+ ions.
Figure 2 Emission spectra of phosphors prepared with different concentrations of urea.
The emission of the sample with n=30 has weak luminescent intensity, the emission maximum shifts to a longer wavelength and emission also exists at 617 nm of Eu 3+ ions. It showed that the low concentration of urea did not suffice for the complete reduction. Besides, with n=80, the luminescent intensity is very low and the position of maximum radiation intensity shifts to a longer wavelength region.
Figure 3 shows the change of maximum luminescent intensity of the phosphors as a function of urea concentration. The phosphor with n=60 was not only a single-phase structure but also has a better intensity of luminescence than the other samples.
Figure 3 The dependence of maximum emission intensity of Eu 2+ ions as a function of urea concentration.
From the investigated results of the XRD patterns, the invariable concentration of urea were chosen as n=60 to synthesize the phosphor at different combustion temperatures. Their XRD diagrams are presented in figure 4. It shows that samples had a hexagonal single-phase structure when the combustion temperature was at 590 °C. At other temperatures, the structure of the materials appeared not only in BaMgAl 10 O 17 phase but also in another sub-phase.
Figure 4 XRD diagram of the samples at different combustion temperatures.
Luminescent spectra of the phosphors prepared with variable combustion temperature and constant urea concentration are presented in figure 5. Broadband luminescent spectra of the samples characterized the transition of Eu 2+ ions with maximum luminescent intensity at 455 nm wavelength. However, the luminescent spectra also show a low broadband emission with maximum wavelength at 520 nm when the sample was heated at a temperature of 570 °C. This suggests that the structure of this phosphor also exists in some unwanted phase, when heating temperature is not appropriate. Auxiliary emission band could be the radiation of ion Eu 2+ in this lattice. The change of luminescent intensity of the phosphors BaMgAl 10 O 17:Eu 2+ on the heating temperature is described in figure 6. The results show that the heated sample at 590 °C had the highest luminescent intensity.
Figure 5 Emission spectra of phosphor with different heating temperatures.
Figure 6 The dependence of maximum intensity as a function of combustion temperature.
A SEM image of the samples is shown in figure 7. The average particle size of the powder is about 50 nm. However, the particle distribution is not uniform.
Figure 7 SEM image of BaMgAl 10 O 17.
3.2. The effect of concentration of Eu2+ ions on luminescent characteristics
Phosphors BaMgAl 10 O 17:Eu 2+ with activator concentration ranging from 0 to 8 mol% were prepared by the combustion of corresponding metal nitrates and urea solution with urea concentration 60 n BAM at 590 °C. The prepared phosphors had a single phase structure. Luminescent spectra of the phosphors were recorded by exciting at 365 nm and are presented in figure 8. It shows that relative emission intensity increased with increasing activator concentration Eu 2+ but the emission maximum did not change. Above 7 mol% Eu 2+ ion, a sudden drop of relative intensity was observed, probably due to concentration quenching. In figure 9, the optimum activator concentration was found to be 7 mol% for maximum emission intensity.
Figure 8 Emission spectra of BaMgAl 10 O 17:Eu 2+ with variable Eu 2+ ion concentration.
Figure 9 The plot of emission intensity as a function of Eu 2+ ion concentration.
4. Conclusion
The urea concentration and combustion temperature in the combustion technology influenced the crystalline structure and optical properties of the products. BaMgAl 10 O 17:Eu 2+ phosphor nanopowder was prepared by a urea–nitrate solution combustion method. Nanosized blue phosphor BaMgAl 10 O 17:Eu 2+ had a single hexagonal structure phase that was synthesized with n=60 and combustion temperature 590 °C. Note that the value n=28.33 was derived from the theoretical calculation in [8]. With the increase of Eu 2+ concentration, the emission intensity increased but the maximum of the spectra did not change. The optimum concentration of Eu 2+ ions was 7 mol% in order to achieve the highest emission.