Luminescent Characteristics of the CaAl2O4: Eu²⁺, Nd³⁺, Gd³⁺ Phosphors

Luminescent Characteristics of the CaAl2O4: Eu²⁺, Nd³⁺, Gd³⁺ Phosphors

2+

Nguyen Ngoc Trac1,2 , Nguyen Manh Son1, Phan Tien Dung3, Tran Thi Hai Tu1

1 College of Sciences, Hue University, Hue City, Vietnam.

2 Hue Industrial College, Hue City, Vietnam

3 Vietnam Academy of Science and Technology, Hanoi, Vietnam

Abstract CaAl2O4: Eu2+, Nd3+, Gd3+ phosphorescent powder was prepared by the combustion method at 580 oC for 5 minutes. The powder was investigated by X-ray diffractometer, SEM, excitation and emission spectra, decay time and glow curve. The material had monocline single phase structure. The emission spectra were a broad band with peak at 444 nm that due to electron transition from the 4f65d1 to the 4f7 configuration of the Eu2+ ion. CaAl2O4: Eu2+, Nd3+, Gd3+ phosphor was a long persistence phosphor with high brightness that was better than CaAl2O4: Eu2+, Nd3+ phosphor. The results have been presented and discussed.

I. INTRODUCTION

Until recent decade, strontium aluminate phosphors codoped with europium and dysprosium ions have attracted much attention since they show excellent properties [1-3]. Compared with classical sulfide phosphorescent phosphor, aluminates have several valuable properties: high radiation intensity, color purity, longer afterglow, chemical stabilization, safe and no radioactivity, etc. Alkaline earth aluminate doped with rare earth ions (Eu2+, RE3+) have well known as a long afterglow phosphor. In these phosphors, Eu2+

ions play role luminescent centers, RE3+ ions act as holes that

have been nitrified by nitric acid. The reaction for the formation of CaAl2O4: Eu2+, Nd3+, Gd3+ assuming complete combustion, may be written as:

(1 (x+y+z))Ca(NO3)2 + xEu(NO3)3 + yNd(NO3)2 + zGd(NO3)2 + 10Al(NO3)3 + nCH4N2O Ca1-(x+y+z)Al2O4:

Eu2+(x), Nd3+(y), Gd3+(z) + by products.

With x = 1 % mol, y = 0,5 % mol, z = 1 % mol then n = 6.69.

Aqueous solution containing stoichiometric amounts of nitrate metal and urea was mixed by magnetic stirrer and heated at 70 oC for 2 hours to gel. Next, the gel was dried at 80 oC to dehydrate and combusted at temperatures 580 oC within 5 minutes. Urea concentration was 18 times of product mole (theory 6.69). The product was CaAl2O4: Eu2+, Nd3+, Gd3+ with white powder.

X-ray diffraction, SEM, excitation and emission spectra, decay time and glow curve of the prepared phosphors were measured and discussed.

III. RESULT AND DISCUSSION

The crystalline structure of CaAl O : Eu2+, Nd3+,

2 4

had been discussed by Matsuzawa [1]. The emission color of the phosphor is depended on the alkaline earth ion in the alkaline earth aluminate lattice. The SrAl2O4: Eu2+, Dy3+ green emitting phosphor is a long persistent phosphorescence with high brightness, the CaAl2O4: Eu2+, Nd3+ blue emitting phosphor is too [4-6]. Therefore, the doping an other trivalent rare earth ion in the CaAl2O4: Eu2+, Nd3+ phosphor expect to form a better phosphorescence.

Gd3+ phosphor synthesized by solution combustion was confirmed by X-ray diffraction (XRD), the results showed in figure 1. The phosphor powder has monocline single phase structure. The phase structure of starting materials and other compositions had been observed in the XRD. It showed that a little amount of rare earth ions did not effect on the structure of the lattice.

CAO20

Faculty of Chemistry, VNU, D8 ADVANCE-Bruker – CAO18

2 4

In this paper, the CaAl O : Eu2+, Nd3+, Gd3+ phosphors

300

290

280

270

260

with changed concentration of Gd3+ ions were prepared by the combustion process. The results of photoluminescence and thermoluminescence were presented and discussed.

II. EXPERIMENT

Starting materials for the preparation of phosphors CaAl2O4 doped with different rare earth ions by urea – nitrate solution combustion synthesis are the mixture of Ca(NO3)2.4H2O, Al(NO3)3.9H2O, Eu2O3, Nd2O3, Gd2O3, B2O3,

CO(NH2)2. Small quantities of B2O3 were used as a flux. Urea was used to supply fuel and reducing agent. Rare earth oxides

250

240

230

d=2.968

220

210

200

190

180

Lin (Cps)

170

160

150

140

130

120

d=2.600

d=2.518

110

100

d=2.405

90

80

d=1.922

d=1.528

d=1.455

70

d=4.670

d=2.864

d=2.333

d=2.023

60

50

40

30

20

10

0

15 20 30 40 50 60

2-Theta – Scale

File: Hung(ManhSon) CAO20.raw – Type: Locked Coupled – Start: 15.000 ° – End: 65.010 ° – Step: 0.030 ° – Step time: 0.3 s – Temp.: 25 °C (Room) – Time Started: 10 s – 2-Theta: 15.000 ° – Theta: 7.500 ° – C 01-070-0134 (C) – Calcium Aluminum Oxide – CaAl2O4 – Y: 73.28 % – d x by: 1. – WL: 1.5406 – Monoclinic – a 8.70000 – b 8.09200 – c 15.19100 – alpha 90.000 – beta 90.170 – gamma 90.000 – Primitive – P21/

Fig.1: XRD pattern of the CaAl2O4: Eu2+, Nd3+, Gd3+

In order to determine the morphology and particle size of the sample, a scanning electron microscope (SEM) were carried out. Figure 2 displays a typical scanning electron microscope image of the CaAl2O4: Eu2+, Nd3+, Gd3+ phosphor. SEM image showed that the sample had loose, foamy, irregular and crowded particles. The surfaces of the particles had many cracked traces and pores that formed by the escaping gases during the combustion reaction. The size particle is about 1-2 µm.

Emission spectra of phosphors CaAl2O4: Eu2+, Nd3+ codoped with Gd3+ ion have different concentration of Gd3+ ion, presented in the figure 4. Maximum luminescent intensity of the spectra change when concentration of Gd3+ ion change and optimal emission intensity corresponding with concentration of Gd3+ ion 1 % mol. CaAl2O4: Eu2+ (1 % mol), Nd3+ (0,5 % mol), Gd3+ (1 % mol) phosphor not has only high brightness, but also long afterglow is more long than CaAl2O4: Eu2+ (1 % mol), Nd3+ (0,5 % mol) phosphor.

1,5 (2)

(3)

(1)

PL intensity (a.u)

(4)

(1) z = 0.5 % mol

(2) z = 1 % mol

(3) z = 1.5 % mol

(4) z = 2 % mol

1,0

0,5

Fig. 2: SEM photograph of CaAl2O4: Eu2+, Nd3+, Gd3+

Figure 3 shows luminescent spectra of the phosphors CaAl2O4: Eu2+ and CaAl2O4: Eu2+ codoped with Nd3+, Nd3+ and Gd3+, excited by radiation with wavelength 365 nm. It showed that the emission spectra of the phosphors are a same

0,0

400 450 500 550

Wavelength (nm)

Fig. 4: Emission spectra of

broad band with maximum intensity at 444 nm that characterize the electronic transition from 4f65d1 to 4f7 configuration of Eu2+ ion. Emission of Eu3+, Nd3+ and Gd3+

CaAl2O4: Eu2+, Nd3+, Gd3+ (z % mol) with z = 0 ÷ 2

Excitation spectra of phosphors CaAl O

doped and

was not observed in the spectra. It could indicate that 2 4

europium ions were reduced into Eu2+ ions in the combustion reaction and they play role activator centers in the lattice. Whereas, Nd3+ and Gd3+ ions play role hole trap. A part of Eu2+ ions exchanged electric charge in the trap-release process

codped with different rare earth ions at emission wavelength em = 444 nm show in the figure 5. Excitation spectrum of CaAl2O4: Eu2+ phosphor has two broad band with maximum peak at 270 nm and 325 nm. While Excitation spectra of the

CaAl O : Eu2+ codoped with Nd3+ and Nd3+, Gd3+ phosphors

to form the phosphorescence of material. And so, emission 2 4

intenity of the phosphors CaAl O : Eu2+ codoped with Nd3+,

have only a broad band with maximum at wavelength 325 nm.

2 4

Nd3+ and Gd3+

2+ Disappearance of the peak at 270 nm in these phosphors could

phosphor.

2,5

2,0

1,5

1,0

were more strong than CaAl2O4: Eu

(1)

(2)

(3)

due to the peak at 270 nm corresponds with absorption of the O2–RE3+ charge transfer band in the oxide host that contains trivalent rare earth ions [7]. Therefore, the absorption of Eu2+ ions in this band disappeared in the phosphors codoped with trivalent rare earth ions.

1,0×108

8,0×107

PL intensity (a.u)

(1)

6,0×107

PL intensity (a.u)

0,5

0,0

400 440 480 520 560

Wavelength (nm)

4,0×107

2,0×107

0,0

(3) (2)

Fig. 3: Emissiom spectra of CaAl2O4: Eu2+ (1), CaAl2O4: Eu2+, Nd3+, Gd3+ (2)

and CaAl2O4: Eu2+, Nd3+(3)

240 280 320 360 400 440

Wavelength (nm)

Fig. 5: Excitation spectra of CaAl2O4: Eu2+(1), CaAl2O4: Eu2+, Nd3+(2)

and CaAl2O4: Eu2+, Nd3+, Gd3+(3)

Fig. 6 shows the decay times of CaAl2O4: Eu2+, Nd3+, Gd3+ phosphors with different codoped concentration of Gd3+ ion after excitation by UV lamp with radiation wavelength 365 nm. The optimal long afterglow is corresponding to the concentration of Gd3+ ion 1 %mol. In order to compare the long afterglow of phosphors CaAl2O4 doped with different rare earth ions, The decay times of those present in the figure

1. It indicated that CaAl2O4: Eu2+ phosphors codoped with

   Nd3+ and Gd3+ ions formed hole traps in the lattice. The trap density of CaAl2O4: Eu2+, Nd3+, Gd3+ phosphor is more high than these of CaAl2O4: Eu2+, Nd3+ phosphor. Activation energy of CaAl2O4: Eu2+, Nd3+ and CaAl2O4: Eu2+, Nd3+, Gd3+ phosphors was analyzed by R. Chen method that was 0.68 eV and 0.73 eV, respectively.

   The experimental results of the photoluminescence and

   2+ 3+

   Nd3+ or Nd3+, Gd3+ ions have more long than persistence the

   thermoluminescence showed that the CaAl2O4: Eu , Nd ,

   phosphor did not dope, among them CaAl O : Eu2+, Nd3+,

   Gd3+ phosphor is long persistent phosphorescence with high

   3+ 2 4 brightness that is beter than CaAl2O4: Eu2+, Nd3+ phosphor.

   Gd phosphor was optimal long persistence. It indicated that

   all of Nd3+ ad Gd3+ ions formed hole trap in the lattice that trap depth will be determined by thermoluminescent method.

   2,0×108

   1E-4

   PL intensity (a.u)

   1E-5

   1E-6

   (3)

   (2)

   (4)

   (1)

   (1) z = 0.5 % mol

   (2) z = 1 % mol

   (3) z = 1.5 % mol

   (4) z = 2 % mol

   10 100 1000

   Time (s)

   1,5×108

   TL intensity (a.u)

   1,0×108

   5,0×107

   0,0

   (2)

   (1)

   50 100 150 200 250 300

   Temperature (0C)

   Fig. 6: The decay time of CaAl2O4: Eu2+

   (1 % mol), Nd3+

   (0,5 %

   Fig. 8: Glow curve of CaAl2O4: Eu2+, Nd3+ (1) and CaAl2O4: Eu2+, Nd3+, Gd3+(2)

   1E-4

   PL intensity (a.u)

   1E-5

   1E-6

   1E-7

   mol), Gd3+ (z % mol) with z = 0 ÷ 2

   (3)

   (2)

   (1)

   10 100 100

   Time (s)

   Fig. 7: The decay time of CaAl2O4: Eu2+ (1) , CaAl2O4:Eu2+, Nd3+ (2) and CaAl2O4:, Eu2+, Nd3+, Gd3+ (3)

   IV. CONCLUSIONS

   The phosphors CaAl2O4 codoped with different rare earth ions had been successfully synthesized by the urea-nitrate solution combustion method. The materials had monocline single phase structure with average size of particles about 1- 2m. The Nd3+, Gd3+ ions formed hole traps with high trap density and suitable trap depth, Eu2+ ions was the activator centers in the CaAl2O4: Eu2+, Nd3+, Gd3+ lattice. This phosphor was a long persistent phosphorescence with high brightness that was better than CaAl2O4: Eu2+, Nd3+ phosphor.

   REFERENCE

   1. T. Matsuzawa, Y. Aoki, N. Takeuchi, Y. Murayama, 1996, J. Electrochem. Soc, Vol. 143, pp. 2670-2673.

   2. H. Song and D. Chen, 2007, J. Luminescence, Vol. 22, pp. 554-558.

   3. Nguyen Manh Son, Le Thi Thao Vien, Nguyen Ngoc Trac, 2009. Journal of Physics: Conference Series 187, 012017.

   4. Changliang Zhao, Donghua Chen, 2007, Materials Letters 61, pp. 36733675.

      The glow-curve of phosphors codoped with rare earth ions

      have been irradiated by UV radiation of D2 lamp for 1 minute, showed in Fig. 8. The glow curves of CaAl2O4: Eu2+, Nd3+ and

      CaAl O : Eu2+, Nd3+, Gd3+ phosphors have a symmetric and

   5. Yang Zhiping, Yang Yong, Li Xingmin, Li Xu, Liu Chong, Feng Jianwei, 2005. Proc. of SPIE, Vol. 5632, pp. 308-311.

   6. S.Vijay, J. Z. Jun, M. K. Bhide, V. Natarajan, 2007. Optical Materials 30, 446-450.

2 4 o o 7. K.C. Mishra, M. S. Raukas, G. Marking, P. Chen and P. Boolchand,

single peak with maximum temperature at 129

C and 118 C,

2005. Joural of the Electrochemical Society, pp. H183-H190.

respectively. Maximum thermoluminescent intensity of CaAl2O4: Eu2+, Nd3+, Gd3+ phosphor is more strong than these of CaAl2O4: Eu2+, Nd3+ phosphor. It could indicate that the