Investigation of phase formation dependency of photoluminescence properties of Eu3+ in Mg4Al2O7:Eu3+,Dy3+ and Ca4Al2O7:Eu3+,Dy3+ red-emitting phosphors

Investigation of phase formation dependency

of photoluminescence properties of Eu

in Mg

Al

O

:Eu

,Dy

and Ca

Al

O

:Eu

,Dy

red-emitting phosphors

Esra O¨ ztu¨rk•_{Erkul Karacaoglu}

Received: 20 August 2014 / Accepted: 12 January 2015 / Published online: 31 January 2015 Ó Akade´miai Kiado´, Budapest, Hungary 2015

Abstract Rare earth-doped Mg4Al2O7:Eu3?,Dy3? and

Ca4Al2O7:Eu3?,Dy3? were synthesized by the solid-state

reaction method at 1,400°C. The phosphors were charac-terized by X-ray powder diffraction, photoluminescence, thermogravimetry and differential thermal analysis and scanning electron microscopy. X-ray powder diffraction studies show that the Mg4Al2O7:Eu3?,Dy3?phosphor was

crystallized in the triclinic crystal system but that Ca4Al2O7:

Eu3?,Dy3?was not. The phosphors show the characteristic broad band phosphorescence of Eu3?. This broad band phosphorescence has red emission bands in the range of 550–700 nm corresponding to 5D0?7Fj (j: 1, 2, 3,)

transitions of Eu3?.

Keywords Photoluminescence
Aluminates
Thermal analysis
XRD

Introduction

Different aluminate ceramics are good candidate materials for optical and electronic applications [1–3]. At the same time, aluminate materials could be successfully used as a solid oxide fuel cell functional anode material [4]. Calcium aluminates have low density and hardness; therefore, they are used in the steel and cement industry [5]. Magnesium aluminates have interesting mechanical and refractory properties [6].

For example, Eu3? and Dy3?rare earth ions are widely used as the activator and co-activator to improve materials performance, such as photoluminescence properties [7,8]. Eu3? is one of the most used rare earth ions due to its f– f transitions in red wavelength region [9–11]. Red light-emitting Eu3?phosphors are used in display panels and lamps. Many Eu3?-doped materials are being examined for use in new flat panel display technologies [12]. It is well known that a proper host material doped with Eu3?phosphors can generate red emission from the 5D0level as well as blue and green

emissions from higher5D levels, including5D1,5D2and5D3

of Eu3?, which lays the basis for producing single-phased white light-emitting phosphors [13–15].

In this paper, we focused on aluminates doped with Eu3? and Dy3? ions and their photoluminescence proper-ties and phase formation. Mg4Al2O7:Eu3?,Dy3?- and

Ca4Al2O7:Eu3?,Dy3? -based phosphors were applied heat

treatment by solid-state reaction at 1,400 °C. Their crystal structure, morphological characterization, photolumines-cence properties and excitation mechanism were investi-gated. The dependency of the luminescence properties of Eu3?ions on phase formation was discussed.

Experimental Starting materials

4MgCO3.Mg(OH)2.5H2O (Merck, A.R.), Al2O3 (Acros,

99.0 %), CaCO3 (Sigma-Aldrich, 99.0 %) and the rare

earth oxides Eu2O3(Alfa Aesar, 99.99 %) and Dy2O3(Alfa

Aesar, 99.99 %) were used without further purification. Eu2O3was used as activator ions’ source, and Dy2O3was

used as co-activator ions source for helping excitation energy capturing.

E. O¨ ztu¨rk (&)
E. Karacaoglu

Department of Materials Science and Engineering, Faculty of Engineering, Karamanog˘lu Mehmetbey University,

Karaman 70200, Turkey e-mail: esracircir@gmail.com

Preparation

Mg4-x-yAl2O7: xEu3?, yDy3?and Ca4-x-yAl2O7: xEu3?,

yDy3? (x: 10 % mole and y: 1 % mole) samples were prepared by a conventional solid-state reaction method. The starting materials were weighed according to the nominal composition of (Mg3.89Eu0.10Dy0.01)Al2O7 and

(Ca3.89Eu0.10Dy0.01)Al2O7 and mixed homogeneously

using a ball mill for 3 h. The samples were taken from each mixed powder for thermogravimetric and differential thermal analysis (TG/DTA). The milled and mixed powder samples were annealed at 1,400°C in an open atmosphere in alumina crucibles for 2 h.

Characterization

Firstly, the thermal behavior of the powder samples was investigated by using an SII6000 Exstar 6300 TG/DTA system. The samples were heated at a rate of 10°C min-1

from room temperature to 1,000°C in nitrogen atmo-sphere. After the thermal treatment, their crystal systems were checked by an X-ray powder diffractometer (XRD). XRD data were collected with a Bruker AXS D8 Advance diffractometer which was run at 40 kV and 40 mA, 2h = 10°–90° and a step size of 0.002° using CuKa radi-ation. All XRD data were collected at room temperature in air. After the samples were pelletized and coated with a silver layer, the morphological characterization and ele-mental analysis of the pelletized and coated samples were monitored on a LEO 440 model scanning electron micro-scope using an accelerating voltage of 20 kV. The emission and excitation spectra of the powder samples were obtained out in the ranges of 550–700 and 200–400 nm, respec-tively, by photoluminescence spectrometer (PL) with a Photon Technology International Quanta Master 30 model phosphorescence/fluorescence spectrofluorometer equipped with pulsed xenon lamp.

Results and discussion

Figure1 shows the DTA/TG curves of Mg4Al2O7:Eu ?3

, Dy?3. The curves below 200°C include the dehydration of 4MgCO3.Mg(OH)2.5H2O. The first endothermic peak is (at

250°C, point A) attributed to the deviation of the hydroxyl group from Mg(OH)2. The second endothermic peak (at

437°C, point B) shows the decomposition of MgCO3

which changes into MgO. The third endothermic peak (at 817°C) without any loss of mass is attributed to the phase forming of Mg4Al2O7:Eu?3,Dy?3. Indeed, according to the

XRD data in Fig.2 and Table 1, Mg4Al2O7:Eu3?,Dy3?

had a triclinic crystal system with unit cell parameters of a = 390 pm, b = 450 pm, c = 705 pm; a = 125.74°,

b = 59,01°, c = 93,97° and V = 123 9 106_pm3_.

How-ever, Ca4Al2O7:Eu3?,Dy3? did not have a crystal system.

Figure3 illustrates the DTA/TG curves of Ca4Al2O7:

Eu?3,Dy?3. There is only a single endothermic peak, and this peak (at 792°C) is attributed to the decomposition of CaCO3which changes into CaO. As shown in Fig. 3, there

is no peak attributed to any phase-forming process after 792 °C. Actually, the XRD pattern of Ca4Al2O7:Eu3?,

Dy3? did not index after heating up to at 1,400°C. Fig-ure4 shows the XRD patterns of Ca4Al2O7:Eu3?,Dy3?.

This result shows that the nominal composition of Ca4Al2O7:Eu3?,Dy3? was not good or the reaction

tem-perature was insufficient for the phase-forming process. Ca4Al2O7:Eu3?,Dy3? powder samples melted at

tempera-tures over 1,400 °C. Therefore, the annealed Ca4Al2O7:

Eu3?,Dy3? powder samples at 1,400°C were character-ized. Surprisingly, although Ca4Al2O7:Eu3?,Dy3? did not

have a crystal system, its emission spectra contain three bands. Figure 5 gives the PL spectra of Ca4Al2O7:Eu3?,

Dy3?. The three emission bands at 590, 617 and 656 nm belong to Eu3?ions. The emission bands at 590, 617 and

200 50.0 0.0 50.0 100.0 150.0 200.0 250.0 300.0 60.0 70.0 80.0 90.0 100.0 B' A' B A 400 600 DTG TG DTA Endo Mass/% _DTA/μV Temperature/°C DTG/ μ g min –1 800 1000 0.00 5.00 10.00 15.00

Fig. 1 TG/DTG/DTA curves of Mg4Al2O7:Eu?3,Dy?3

10 20 (101) (001) (110) (2–11) (–111) (–1–23) (–1–11) (–101) 1,400 °C (–110) (–2–2–2) 30 40 I/cps 2θ/° 50 60 70 80 90

656 nm are due to the transitions of Eu3?, namely5D0?7F1, 5_D

0?7F2and5D0?7F3, respectively [16,17]. The three

bands at 362, 383 and 395 nm in the excitation spectra

belong to the transitions of Eu3?ions, namely7F0,1,2?5D0, 7_F

0?5L7and7F0?5L6, respectively [18,19]. The

exci-tation band at 270 nm can be seen as a charge transfer tran-sition between the emission level of the Ca4Al2O7host crystal

and Eu3?ions [18].

Figure6 shows the excitation and emission spectra of the Mg4Al2O7:Eu?3,Dy?3 phosphor. The four bands at

299, 362, 383 and 395 nm in the excitation spectra belong to the transitions of Eu3? ions, namely 7F0?

5

F4, 7_F

0,1,2?5D0, 7F0?5L7 and 7F0?5L6, respectively

[18–20]. Similarly to Ca4Al2O7:Eu?3,Dy?3which does not

have a crystal system, there are three emission peaks at 591, 615 and 654 nm in the emission spectra of Mg4Al2

O7:Eu?3,Dy?3. These peaks are due to the transitions of

Eu3?, namely 5D0?7F1, 5D0?7F2, 5D0?7F3,

respectively [16]. This kind of red luminescence is a typ-ical luminescence for Eu?3 ions. It is known that the Table 1 XRD data of Mg4Al2O7:Eu?3,Dy?3

h k l dobs dcalc 2hobs 2hcalc I/Io

0 0 1 4.6427 4.6433 19.101 19.103 20.2 1 0 1 3.8666 3.8692 22.983 22.967 9.4 1 1 0 2.8462 2.8456 31.404 31.411 23.4 -1 -1 1 2.4289 2.4283 36.980 36.989 70.7 -1 0 1 2.1003 2.1003 43.032 43.031 100 -1 1 0 2.0160 2.0165 44.926 44.916 29.6 2 -1 1 1.5535 1.5534 59.453 59.457 27.9 -1 1 1 1.4866 1.4865 62.419 62.421 50.2 -1 -2 3 1.4267 1.4267 63.358 63.358 36.1 -2 -2 2 1.2141 1.2142 78.758 78.756 11.1 200 400 600 Temperature/°C 800 1000 65.00 70.00 75.00 80.00 85.00 90.00 95.00 200.0 400.0 600.0 800.0 0.0 Mass/% DTG TG DTA Endo A' A –10.00 –5.00 0.00 5.00 10.00 DTG/ μ g min –1 DTA/μV

Fig. 3 TG/DTG/DTA curves of Ca4Al2O7:Eu?3,Dy?3

10 20 30 40

2θ/°

50 60 70 80 90 1,400 °C

I/cps

Fig. 4 XRD patternof Ca4Al2O7:Eu?3,Dy?3

200 0 10 20 30 40 300 362 383 270 395 590 617 656 Excitation Emission 400 Wavelength/nm Intensity/a.u. 500 600 700

Fig. 5 The excitation and emission spectra of Ca4Al2O7:Eu?3,Dy?3

phosphor 200 0 10 20 30 40 300 362 383 299 395 591 615 654 Excitation Emission 400 Wavelength/nm Intensity/a.u. 500 600 700

Fig. 6 The excitation and emission spectra of Mg4Al2O7:Eu?3,Dy?3

excitation and emission spectra of Eu?3 ions consist of bands in the ranges of 200–500 and 500–700 nm, respec-tively [21]. In both phosphors, Eu?3 ions have similar luminescence intensity. There were no typical emission peaks of Dy3? in the spectra of the Mg4Al2O7:Eu

?3

,Dy?3 and Ca4Al2O7:Eu?3,Dy?3phosphors.

Figure7a, b show the SEM images of Mg4Al2O7:

Eu?3,Dy?3and Ca4Al2O7:Eu?3,Dy?3powders annealed at

1,400°C. The microstructure of the phosphors consisted of fine irregular grains. Elemental analysis results show that there were no impurity atoms and the experimental and calculated nominal compositions were in good agreement.

Conclusions

In this paper, Mg4Al2O7:Eu?3,Dy?3 and Ca4Al2O7:Eu?3,

Dy?3luminescent materials were obtained at 1,400°C by conventional solid-state reaction method. The Mg4Al2

O7:Eu?3,Dy?3 phosphor has a triclinic crystal system but

Ca4Al2O7:Eu?3,Dy?3 does not. Despite this, the

charac-teristic emission peaks of Eu?3 in the both host systems were detected. According to the results, we can state that the luminescence properties of Eu?3 in the M4Al2O7(M:

Mg, Ca) host system are independent of the structural properties of M4Al2O7(M: Mg, Ca).

Acknowledgements This work was supported by Karamanog˘lu Mehmetbey University BAP under Project Number 48-M-12.

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