The Role of Co-Doping Rare Earth Ion Gd³⁺ in the Photoluminescence Characteristics of CaAl₂o₄: Eu²⁺, Gd³⁺Phosphors

The Role of Co-Doping Rare Earth Ion Gd³⁺ in the Photoluminescence Characteristics of CaAl₂o₄: Eu²⁺, Gd³⁺Phosphors

Nguyen Ngoc Trac1,2, Nguyen Manh Son1, Phan Tien Dung3

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

2 Hue Industrial College, Hue City, Vietnam

3 Vietnam Academy of Science and Technology, Hanoi, Vietnam.

Abstract– CaAl2O4: Eu2+ (1 % mol), Gd3+ (x % mol) phosphors, with x = 0 ÷ 2.5, were prepared by solution combustion method. The structure and luminescent properties were investigated by X- ray diffraction pattern, emission spectra, decay time and glow- curve. The materials had monocline single phase structure. The emission spectra of phosphors had a broad band with maximum at 444 nm due to electron transition from the 4f65d1 to the 4f7 of Eu2+ ion. CaAl2O4: Eu2+, Gd3+ was a long persistence phosphor with high brightness. In these phosphors, Eu2+ ions play the role activators. Whereas, Gd3+ ions generate the hole traps that resulted the long persistent phosphorescence and also act as a sensitizer in the phosphor. The concentration of Gd3+ ion co- doped has strong influence on the luminescence of phosphor.

1. INTRODUCTION

   The alkaline earth aluminate phosphors doped with Eu2+ ions have strong photoluminescence at the visible region. The emission spectra of the phosphors had a broad band that characterized the transition of electronic configuration from

   high-temperature, fast heating rates and short reaction times [8].

2. EXPERIMENTS

   The powder samples with the general formula CaAl2O4: Eu2+ (1 % mol), Gd3+ (x % mol) were prepared by urea-nitrate solution combustion method. Starting materials for synthesized phosphors CaAl2O4 codoped with rare earth ions were the mixture of Ca(NO3)2.4H2O (Merck),Al(NO3)3.9H2O(Merck), Eu2O3(Merck), Gd2O3 (Merck), B2O3(AR) and CO(NH2)2 (AR). Rare earth oxides were nitrified by dissolving into HNO3. A small quantity of B2O3 was added as the flux. Urea (CO(NH2)2) was used to supply fuel and reducing agent. For the combustion synthesis, urea is documented as an ideal fuel [8].

   The reaction for the formation of CaAl2O4: Eu2+, Gd3+ assuming complete combustion, may be written as:

   (1 (0.01+x))Ca(NO3)2+ 0.01Eu(NO3)3+

   4f65d1 to 4f7 of Eu2+ ion. The photoluminescence of material

   xGd(NO ) +2Al(NO )

   + nCH N O Ca

   Al O :

   3 3 3 3 4 2 1-(0.01+x) 2 4

   have strong depend on crystal field [1-3]. In recent years, the influence of Eu2+ on the photoluminescent characteristics of MAl2O4: Eu2+ materials have been extensively studied (M: Sr, Ca, Ba, Mg). Besides, the effect of trivalent rare earth ions codoping on the phosphorescence of alkaline earth aluminate materials were investigated also [4-6]. The persistent lifetime and luminescence intensity of phosphor can be enhanced by co-doping with the second rare earth ions. The trapping mechanism plays an essential role in the persistence ofthese materials [5-7]. The phosphorescence phosphors have important applications with a long afterglow. The CaAl2O4: Eu2+, Gd3+ phosphor is one kind of the long afterglow phosphors. In these phosphors, Eu2+ ions play the role activators. Whereas, Gd3+ ions generated hole traps that resulted the long persistent phosphorescence and also act as a sensitizer in the phosphor [4-5]. The concentration of Gd3+ ion co-doped has strong influence on the luminescence of phosphor.

   In this paper, the influence of Gd3+ ion co-doped with different concentration on luminescent properties of CaAl2O4: Eu2+, Gd3+ phosphor was investigated. The materials were

   Eu2+(1 %), Gd3+(x) + by products.

   With n = 6.69.

   Aqueous solution containing stoichiometric amounts of nitrate metal, urea and B2O3 was mixed and heated by microwavefor 10 minutes to gel form.The mixing and heating mechanism of microwave is different from magnetic heating stirrer. The microwave energy mixes and heats the aqueous solution on a molecular level, which leads to uniform diffusing and rapid water evaporation.Nextly, the gel was dried at 80oC to dehydrate and combusted at temperatures 580oC within 5 minutes.Finally,the white powder product was obtained.Urea concentration was 18 times of product mole (The concentration was calculated by theory: 6.69 times).

   The structureprepared products were characterized by D8- Advance-Bruker X-ray diffractometer. The photoluminescence spectras were measured by FL3-22 fluorescence spectrometer. The glow curves were analysized by Harshaw TLD-3500 equipment.

3. RESULTS AND DISCUSSION

   The phosphors of CaAl O : Eu2+ (1 % mol), Gd3+ (x %

   2 4

   prepared by combustion method. Combustion synthesis has emerged as an important technique for the synthesis and processing of advanced materials that was characterized by

   mol) with x = 0 ÷ 2.5 were successfully prepared by the combustion method. The crystalline structure of CaAl2O4: Eu2+, Gd3+ with different concentration of ion Gd3+were

   confirmed by X-ray diffraction pattern (XRD), the XRD diagrams were showed in Fig. 1. The phosphor had monocline single phase CaAl2O4 structure. There no other phase was observed. The XRD diagrams also indicated that a little amount of rare earth ions did not effect on the structure of the lattice.

   The emission intensity of CaAl2O4: Eu2+, Gd3+ phosphors increases with increasing concentration of Gd3+ ions co-doped, and the luminescence intensity is optimal with 1.5 % mol Gd3+. This suggests that the energy transfer from Gd3+ to Eu2+ ions occurs and Gd3+ ions act as sensitizers. When the concentration of Gd3+ ions increases over 1.5 % mol, the luminescence intensity of phosphor decreases due to the concentration quenching occur.

   600

   8,0×104

   400

   Lin (Cps)

   6,0×104

   (1)

   (3)

   PL Intensity (a.u)

   200

   4,0×104

   30 40 50 60

   2-Theta-Scale

   0 x = 0 % x = 0,5 %

   x = 1,0 % x = 1,5 %

   x = 2,0 % x = 2,5 %

   2,0×104

   0,0

   (2)

   250 300 350 400 450 500 550

   Wavelength (nm)

   Fig. 3. Excitation and emission spectra of

   Fig. 1.XRD of CaAl O : Eu2+(1 % mol), Gd3+ (x % mol)

   CaAl2O4: Eu2+(1 % mol), Gd3+ (1.5 % mol)

   (1) : Excitation spectra of CaAl2O4: Eu2+, Gd3+

   2 4 (2): Excitation spectra of Gd3+was recorded corresponding to maximum emission at wavelength 320 nm.

   (3): Emission spectra ofCaAl2O4: Eu2+, Gd3+when it

   The role of Gd3+ ion co-doping in the photoluminescence characteristics of CaAl2O4: Eu2+ (1 % mol), Gd3+ (x % mol) phosphors was investigated. The emission spectra of the phosphors were and presented in Fig. 2. The samples were excited by radiation with wavelength 285 nm. The results showed that the emission spectra of the phosphors had a same broad band with maximum intensity at 444 nm that characterized the transition of electronic configuration from 4f65d1 to 4f7 of Eu2+ ion. The emissions of Eu3+ and Gd3+ ions were not observed in the spectra, europium ions were reduced into Eu2+ ions in the combustion process and they play the role activator centers in the lattice. The emission spectra of ion

   Gd3+ has a sharp peak at 320 nm when the sample was excited

   wasexcited by radiation with wavelength 285 nm

   Fig. 4 shows the phosphorescent decay time of the CaAl2O4: Eu2+, Gd3+ phosphors with different concentrations of ion Gd3+. The phosphors were excited by radiation with wavelength 365 nm for 2 minutes. The results showed that the phosphor of CaAl2O4: Eu2+, Gd3+ have more long afterglow and high brightness. Whereas, the initial photoluminscence intensity and the lifetime of the phosphor CaAl2O4: Eu2+ is quite low.

   by a radiation with wavelength 285 nm [9]. The both maximum of excitation spectra and emission spectra of ion Gd3+ were covered by excitation spectra of CaAl O : Eu2+,

   1,4

   Photoluminescent Intensity (a.u)

   1,2

   (1) X = 0%

   (2) X = 0.5

   (3) X = 1.0

   (4) X = 1.5

   (5) X = 2.0

   2 4 1,0

   3+

   (6) X = 2.5

   Gd . On the other hand, when the sample was excited by

   radiation with wavelength 285 nm, its emission spectra has a broad band shape with a peak at 444 nm that characterized for emission of Eu2+ ion (fig. 3).

   0,8

   0,6

   0,4

   (4)

   (5)

   (6)

   (3)

   (2)

   (1)

   0,2

   3,0

   2,5

   (4)

   (5)

   (3)

   (2)

   (1)

   (1) x = 0.0

   (2) x = 0.5

   (3) x = 1.0

   (4) x = 1.5

   (5) x = 2.0

   0,0

   1 10 100

   Time (s)

   2+ 3+

   PL Intensity (a.u)

   2,0

   (6)

   (6) x = 2.5

   Fig. 4. Decay time of CaAl2O4: Eu

   (1 % mol), Gd

   (x % mol)

   1,5

   1,0

   0,5

   0,0

   400 450 500 550 60

   Wavelength (nm)

   Fig. 2. Emission spectra of

   The phosphorescent mechanism of material can be suggested as shown in fig. 5.It is indicated that Gd3+ ions generated hole traps near the valence band that resulted the long persistent phosphorescence. These trap levels lie in between the excited state and the ground state of Eu2+ ion. When the sample was excited by UV radiation,the Eu2+ ions

   CaAl2O4: Eu2+(1 % mol), Gd3+ (x % mol); x = 0 ÷ 2.5

   are excited from the ground state (4f7) to the excited state

   (4f65d1): Eu2+ + h Eu2+* and thereby leaving a hole in the valence band, the electron hole pairs are produced in Eu2+ ions. Simultaneously, the Eu2+ ions maybe capture electrons to be reduced to Eu+: Eu2+ + e- Eu+.The Gd3+ ions capture

   9×105

   8×105

   Thermoluminescent Intensity (a.u)

   7×105

   6×105

   (3)

   (4)

   (5)

   (1) x = 0.5%

   (2) x = 1.0%

   (3) x = 1.5%

   (4) x = 2.0%

   (5) x = 2.5%

   some of free holes from valence band to form the Gd4+ cations: Gd3+ + h+ Gd4+. When the excitation was cut off, these captured holes are released slowly at room temperature and recombine with some free electrons, which lead to the persistent afterglow [5-6], [10].

   5×105

   4×105

   3×105

   2×105

   1×105

   (2)

   (1)

   Conduction band

   0

   50 100 150 200 250 300

   Temperature (oC)

   Eu2+ Eu1+

   Fig. 6.Glow curves of CaAl2O4: Eu2+ (1 % mol), Gd3+ (x % mol)

   4f65d1 UV

   4f7

   (Eu2+)*

   444 nm

   Eu2+

   Valence band

   Gd3+Gd4+

   TABLE II. THE VALUES OF THE ACTIVATION ENERGY OF

   CaAl2O4: Eu2+, Gd3+PHOSPHORS

   Sample	E (eV)
   CAO: Eu1%, Gd0.5%	0.63
   CAO: Eu1%, Gd1.0%	0.64
   CAO: Eu1%, Gd1.5%	0.66
   CAO: Eu1%, Gd2.0%	0.65
   CAO: Eu1%, Gd2.5%	0.64

   Fig. 5.Phosphorescent mechanism of CaAl2O4: Eu2+, Gd3+

   The lifetime of phosphors were calculated from fitting experimental decay time with the combination of three exponential functions. The results showed in table 1. It is indicated that the lifetime of phosphors are approximate to each other.

4. CONCLUSION

The phosphors of CaAl2O4: Eu2+, Gd3+ were sucessfully synthesized by combustion method.The emission spectra of phosphors had a broad band with maximum at 444 nm due to electron transition from the 4f65d1 to the 4f7 of ion Eu2+. The

= 0

+ 01

(/1 ) + 02

(/2 ) + 03

(/3 )

materials of CaAl2O4: Eu2+ co-doped with Gd3+ had long persistent phosphorescence. Inside, CaAl2O4: Eu2+ codoped

with Gd3+ had long afterglow with high brightness and their photoluminescent intensity are better than CaAl2O4: Eu2+. In

TABLE I. THE VALUES OF LIFETIME OF

CaAl2O4: Eu2+, Gd3+PHOSPHORS

Sample	1 (s)	2 (s)	3 (s)
CAO: Eu1%, Gd0.5%	6.56	1.27	42.93
CAO: Eu1%, Gd1.0%	6.79	1.38	32.55
CAO: Eu1%, Gd1.5%	8.44	1.59	48.73
CAO: Eu1%, Gd2.0%	7.06	1.54	41.78
CAO: Eu1%, Gd2.5%	6.77	1.34	44.69

And so, their thermoluminescent properties were studied. The glow curves of the phosphors were showed in the fig. 6.

The phosphors were irradiated by UV radiation of D2 lamp for 20second. The results were recorded with heating rates 2oC/s. The glow curves of CaAl O : Eu2+, Gd3+ phosphors

these phosphors, Eu2+ ions play the role activators and Gd3+ ions act as hole traps and sensitizers also.

REFERENCES

[1]. M.Mothudi, Ntwaeborwa, J. R. Botha, H. C. Swart, 2009, Journal of Physica B, Vol. 404, pp. 4440-4444.

[2]. Shin-Hei Choi, Nam-Hoon Kim, Young-Hoon Yun and Sung-Churl Choi, 2006, Journal of Ceramic Processing Research, Vol. 7, No. 1, pp. 62-65.

[3]. Hojin Ryu and K. S. Bartwal, 2007, Research Letters in Materials Science, Vol. 2007, Article ID 23643

[4]. Nguyen Ngoc Trac, Nguyen Manh Son, Phan Tien Dung, Pham Nguyen Thuy Trang, 2013, Journal of Materials Science and Engineering B 3 (6) (2013), pp. 359-363.

[5]. Hojin Ryu and K. S. Bartwal, 2008, Journal of PhysicsD: Applied Physics, Vol. 41, Article ID 235404 (4pp).

[6]. Y. Lin, Z. Tang, Z. Zhang, Cewen Nan, 2003, Jounal of the European

2 4 Ceramic Society23, pp. 175-178.

have a single peak with maximum intensity at around 104oC.

The thermoluminescence intensity of CaAl2O4: Eu2+, Gd3+ phosphor with 1.5 % mol is the strongest. It is indicated that the activator centers and the density of traps produced were more suitable than that of the other concentrations Gd3+ ion. The activation energy was calculated by the Reuven Chen method [11] and the results showed in table 2.

[7]. D. Jia, R. S. Meltzer and W. M. Yen, 2002, Applied Physics Letters,

Vol. 80, No. 9, pp. 1535-1537

[8]. Kashinath C. Patil, S. T. Aruna, Tanu Mimani, 2002, Current Opinion in Solid State and Materials Science, Vol. 6, pp. 507-512

[9]. M. Maqbool and Iftikhar Ahmad, 2007, Journal of Spectroscopy, Vol.

21, Issue 4, pp. 205-210.

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

[11]. S. W. S. McKeever, Thernoluminescence of solids, Cambridge University Press (1985).