DOI : 10.17577/IJERTV3IS20566
- Open Access
- Total Downloads : 391
- Authors : Pham Nguyen Thuy Trang, Nguyen Manh Son, Nguyen Quang Liem, Ho Van Tuyen
- Paper ID : IJERTV3IS20566
- Volume & Issue : Volume 03, Issue 02 (February 2014)
- Published (First Online): 13-03-2014
- ISSN (Online) : 2278-0181
- Publisher Name : IJERT
- License:
This work is licensed under a Creative Commons Attribution 4.0 International License
Oxidation of Europium-ion in the BaMgAl10O17: Eu2+ Phosphor During the Annealing
Pham Nguyen Thuy Trang
Dept. of Physics, College of Sciences, Hue Uni.
77 Nguyen Hue St., Hue City, Vietnam
Abstract BaMgAl10O17: Eu2+ blue emitting phosphors have been prepared by urea-nitrate solution combustion synthesis at 5900C for 5 minutes. After combustion process, the phosphor was annealed at different temperatures in reducing atmosphere and in air. This material had hexagonal single phase crystal structure that was confirmed by X-ray diffraction (XRD). The experimental results of photoluminescence showed that the emission spectra was a broad band with maximum intensity at the wavelength max = 450 nm due to transitions from the 4f65d1 to the 4f7 electronic configuration of Eu2+. After the annealing process, emission spectra were shown the emission lines corresponding to the 5D0 7FJ electronic transition of Eu2+, which were the typical emission of Eu3+. When annealed temperature
Nguyen Manh Son1, Nguyen Quang Liem2, Ho Van Tuyen3 1 Dept. of Physics, College of Sciences, Hue Uni.
77 Nguyen Hue St., Hue City, Vietnam
2 Vietnam Academy of Science and Technology
18 Hoang Quoc Viet St., Cau Giay Dist., Hanoi, Vietnam.
3 Department of Natural Science, Duy Tan University.
K7/25 Quang Trung Street, Da Nang City.
In this paper, we report luminescent degradation of BaMgAl10O17: Eu2+ phosphors prepared with urea – nitrate solution combustion method in reducing media and in the air.
The problem of luminescent degradation due to oxidation will be clarified in this paper. The oxidation process Eu2+ Eu3+ in the BAM: Eu2+ phosphor and the change of emission intensity of Eu2+ ion in the lattice were investigated to determine how various sites are affected during the oxidation process [6]. The photoluminescent properties of the phosphors will be presented and discussed.
-
EXPERIMENTS
raises, luminescent intensity of Eu2+ decreases and luminescent intensity of Eu3+ increases. Many research have indicated that
BaMgAl
10O
17: Eu phosphors were prepared by urea –
oxidation of Eu2+ Eu3+ occurs in the lattice during thermal treatment process.
KeywordsBaMgAl O , nanoparticle, combustion,
nitrate solution combustion synthesis. This method was
detailed in our other papers [7], [ 8].
The samples were prepared with invariable concentration of Eu2+ (3 %mol). After the combustion process, the
degradation.
10 17
phosphors were annealed at different temperature from 200 0C to 1200 0C for 15 minutes in the reduced atmosphere and in
I. INTRODUCTION
BaMgAl10O17: Eu2+ (BAM: Eu2+) blue emitting phosphor is one of the important phosphors utilized in luminescent devices. BAM: Eu2+ has been widely used in modern lighting, displays and optical communications fields such as manufacturing tricolor fluorescent lamps (FL), field emission displays (FED), plasma display panels (PDPs) and liquid crystal displays (LCD) [1], [2]. Because of its high luminance efficiency and brightness under vacuum ultraviolet light excitation. Emission spectra of BAM: Eu2+ phosphor have a broad band with peak at 450 nm due to transition from the 4f65d excited state to the 4f7 ground state of Eu2+ ion. However, the thermal stability of BAM: Eu2+ is not good. It undergoes determination of luminance intensity and color quality by thermal treatment during the manufacturing process [3]. This low durability is one of the major factors limiting the longevity of optical devices.
BAM has a hexagonal crystal structure and belongs to the P63/mmc space group. Al3+ ions and O2- ions formed a rigid 3D network structure as AlO4 tetrahedron and AlO6 octahedron in spinel layer. Besides, Mg2+ ions occupied one Al site in a unit cell while Ba2+ ions occupy the conduction layer sites, which connected two spinel layers. The BR site is the substitution site of Ba and the other two sites are interstitial sites.In BAM: Eu2+, Eu2+ ion can be located at three sites in the lattice [4], [5].
the air.
The synthesized products were characterized by X-ray diffraction (XRD) using a Bruker D8-Advance X-ray diffractometer. Excitation and emission spectra were measured by FL3-22 fluorescence spectrometer. The morphology were taken on by FESEM-Hitachi S-4800.
-
RESULTS AND DISCUSSION
Relative intensity/arb. units
-
X-ray diffraction of BAM: Eu
8000C
6000C
Pre_annealing
20 30 40 0 50 60 70
2 ( )
Fig.1. XRD diagrams of the BAM: Eu annealed at different temperature in air
The crystalline structure of BaMgAl10O17: Eu phosphor was confirmed by X-ray diffraction diagram (XRD), the result of samples annealed at different temperature in reduced media were presented in our other paper [8]. Figure 1 illustrates the XRD patterns for BAM: Eu phosphors annealed at different temperature in air. These results have hexagonal single phase structure that is classified into -alumina structure with a space group P63/mmc [5]. On the other hand, there were no detectable changes in the structure of BAM: Eu phosphors by annealing at different temperature in both environment.
1.0×106
PL intensity (a.u)
8.0×105
6.0×105
4.0×105
2.0×105
0.0
-
reduced atmosphere
-
air
1
2
0 200 400 600 800 1000 1200
Temperature (0C)
-
-
Influence of annealing on luminescent properties of BAM: Eu
Figure 2 presents emission spectra of BAM: Eu annealed at 800 0C in the reducing atmosphere and in air, excited by radiation 365 nm. The result indicate that the spectra have a same broad band with maximum peak at about 450 nm, corresponding to the 4f65d – 4f7 electronic transition of Eu2+ ion.
Fig.3. Dependency of maximum intensity PL on annealin temperatures in the reducing atmosphere and in air
These oxygen vacancies which are close enough to Eu2+ centers can capture electrons from Eu2+ centers and then the Eu3+ centers are created. Therefore, when annealing in air, the emission intensity of Eu2+ center decreases faster than in the reduced media [9], [10].
6.0×105
PL Intensity (a.u)
4.0×105
-
reduced atmosphere
-
air
1
2
2.0
PL Intensity (a.u)
1.5
1.0
450 nm
448 nm
468 nm
(1)
(2)
-
-
Experiment
-
Fitting
-
Peak 1
-
Peak 2
-
Peak 3
-
-
2.0×105
0.5
501 nm
0.0
400 450 500 550 600
Wavelength (nm)
0.0
350 400 450 500 550 600
Wavelength (nm)
Fig.2. Emission spectra of BAM: Eu annealed at 8000C in the reducing
atmosphere and in air with emission wavelength ex = 365 nm Fig.4. The emission spectra of BAM: Eu2+ was fitted with three
Gaussian peaks
When annealed temperature raises from 200 0C to 800 0C, maximum luminescent intensity of the phosphors decreases insignificantly. But as annealed temperature is above 800 0C, maximum emission intensity decreases fast as showed in figure 3.
On another hand, the results in figure 2 and figure 3 show when phosphors are annealed in air, the degradation speed of luminescent intensity is faster than in reduced media. In addition, oxygen vacancies in BAM: Eu phosphor in the reducing media exist more than these in air. This degradation could due to activator centers Eu2+ were oxidized to Eu3+ in the lattice [7] and his process occurred fast at temperature above 800 0C.
In order to clarify the assignment of the emission centers in the lattice, these emission spectra were fitted with combination of three Gaussian peaks.
Figure 4 gives the results of peak-fitting for the emission spectra of BAM: Eu annealed at 800 0C. The emission band was separated into three peaks which had maximum wavelength at 448 nm (peak 1), 468 nm (peak 2) and 501 nm (peak 3), corresponsively. These 3 peaks are ascribed to 3 sites of Eu2+ (BR, aBR, mO) in the BAM lattice [4],[5].
Corresponding author: Pham Nguyen Thuy Trang.
2.0
PL Intensity (a.u)
1.5
1.0
0.5
0.0
01
I
I
I
02
I
03
0
0 200 400 600 800 1000 1200
Emission spestra of BAM: Eu phosphor annealed at different temperatures with excitation wavelength ex = 394 nm present in the figure 6 and figure 7.
According to the above results, emission spectra of BAM: Eu with excitation wavelength ex = 394 nm are narrow lines at the range of 550-720 nm that is corresponding to 5D0 – 7FJ (J=0,1,2,3,4) transitions of Eu3+ ion. These results indicated, when the annealing temperature increases then the luminescent intensity of Eu3+ ion increases. Thus, Eu ions in the BAM lattice can exist simultaneously divalent and trivalent states.
In addition, the change of maximum emission intensity of
Temperature (0C)
Eu2+
and Eu3+
ions depend on annealed temperatures was
Fig.5. Maximum intensity of Gaussian peaks and total maximum intensity as function of annealed temperture
It is clearly indicated that the three Gaussian peaks do not have the same intensity. Besides, the figure 5 also shows that maximum emission intensity (I0) of the phosphors and these of
shown in figure 8. When annealed temperature of sample increases, maximum luminescent intensity of Eu2+ ion decreases and maximum luminescent intensity of Eu3+ ion increases. It obviously indicates that the oxidation from Eu2+Eu3+ occurred.
3 Gaussian peaks (I01: peak 1, I02: peak 2, I03: peak 3) decrease when the annealed temperature increase and faster degradation of intensity of peak 2 and 3 compare with the peak 1 leads to shift slightly shorter wavelength. These results is similar to that when samples were annealed in air.
3.0
2.5
PL Intensity (a.u)
2.0
1.5
1.0
Eu2+
Eu3+
7×105
6×105
5×105
4×105
3×105
6.0×105
PL Intensity (a.u)
5
5D – 7F
-
Pre_annealing (2) 4000C
0 2
(3) 8000C
5 (4) 10000C
4 (5) 12000C
3
0.5
0.0
0 200 400 600 800 1000 1200
Wavelength (nm)
2×105
0
0
4.0×10
5D – 7F 2
Fig.8. Maximum emission intensity of Eu2+ ion and Eu3+ ion
0
1
2.0×105
5D – 7F
1
0
4
5D – 7F
5D – 7F
as function of different annealed temperatures
Excitation spectra of pre-annealed and annealed BAM: Eu
0 3
0.0
600 650 700 750
Wavelength (nm)
Fig.6. Emission spectra of Eu3+ ion in lattice BAM annealed at different temperatures in reducing atmosphere, ex=394 nm
at different temperature with emission wavelength em = 450 nm present in the figure 9. The spectra consist of some overlapped broad bands from 280 nm to 420 nm. These bands correspond with excitation transitions of Eu2+ ions that located different positions in the lattice.
The oxidation from Eu2+ ion to Eu3+ ion in lattice BAM was demonstrated by luminescence of Eu3+ ion at different annealed temperatures in the reducing atmosphere and in air.
2.5×107
PL Intensity (a.u)
2.0×107
1.5×107
(1)
(1) Pre_annealing (2) 4000C
(3) 8000C
(4) 10000C
(5) 12000C
5 7 (1) pre_annealing
3×105
D0- F2 (2) 4000C (3) 8000C
1.0×107
Intensity PL (a.u)
2×105
5 (4) 10000C
4 (5) 12000C
3 5D – 7F
5.0×106
0.0
(4)
(5)
0 4
2
5D – 7F 1
250 300 350 400
0 1
5
7
1×105
Wavelength (nm)
D – F 5 7
0 3
0 0 D – F
0
600 650 700 750
Wavelength (nm)
Fig.7. Emission spectra of Eu3+ ion in lattice BAM annealed at different temperatures in air, ex=394 nm
Fig.9. Excitation spectra of samples BAM: Eu annealed at different temperatures with emission wavelength em = 450 nm
When annealed temperature of sample increases, maximum positions of excitation bands of ion Eu2+ do not change but maximum intensity decreases significantly.
1.0×107 (1) Pre_annealing
luminescent intensity is faster than in reduced media. This degradation occurred fast at temperature above 800 0C.
7F – 5L 0
0 6
PL Intensity (a.u)
8.0×106
(2) 400 C
(3) 8000C
(4) 10000C
(5) 12000C
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6.0×106
10 17
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360 390 420 450 480 510 540
10 17
Wavelength (nm)
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-
-
CONCLUSION
BaMgAl10O17: Eu phosphors were prepared by urea – nitrate solution combustion method, after samples annealed in reducing atmosphere and in the air. These phosphors have hexagonal single phase structure. The emission spectra was a broad band with maximum intensity at the wavelength max = 450 nm due to transitions from the 4f65d1 to the 4f7 electronic configuration of Eu2+ ions that occupy three different sites (BR, aBR, mO) in the BAM lattice. When annealing temperature increases, emission spectra of BAM : Eu2+ shift to short wavelength. Degradation of luminescent intensity of Eu2+ ion is due to the oxidation of Eu2+Eu3+ in the lattice when annealed temperature raises. At the same time, when phosphors are annealed in air, the degradation speed of
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