Eu3+- and Ho3+-doped Ba3Al2O6- and MgAl2O4-type phosphors and their photoluminescent properties

Eu

- and Ho

-doped Ba

Al

O

- and MgAl

O

-type phosphors

and their photoluminescent properties

Esra O¨ ztu¨rk1•_{Erkul Karacaoglu}1

Received: 9 March 2017 / Accepted: 9 September 2017 / Published online: 9 November 2017 Ó Akade´miai Kiado´, Budapest, Hungary 2017

Abstract The aluminate hosts were basically activated with the Eu3?- and Ho3?-ions which were prepared by solid-state reaction in this study. The DTA/TG results were used as a reference for the heat treatments of the samples. The Ba3Al2O6:Eu3?, Ho3?phosphor was well indexed to the expected phase, and MgAl2O4:Eu3?, Ho3?could not be indexed as single phase by powder X-ray diffraction analysis to the any major phase in spite of repeated and increased heat treatments for this system. Thus, MgAl2O4 and MgO impurities were obtained. Noteworthy photolu-minescent results were obtained since the transitions of Eu3?-ions occurred for both of the samples.

Keywords Aluminate phosphor Solid-state reaction method Photoluminescence  Eu3?_{ Ho}3?

Introduction

Rare-earth ion-activated aluminate-type hosts have been used as inorganic photoluminescence materials for a long time and exhibit high stability, chemical resistance, brightness and long afterglow. Also, they are easy and convenient for laboratory and industrial processing pur-poses as well as being environmentally friendly and suit-able for the manufacture of lighting and display devices. These phosphors have found major applications in fluo-rescence lamps and display devices such as

electroluminescent panels (ELP), field emission displays (FED) and plasma display panels (PDP). Their high quantum efficiency with europium ion activation in the visible region is well understood. The photoluminescence properties of these phosphors depend very strongly on the host’s structure as well as on the type of activator ion. The performance of new aluminate-based photoluminescent phosphors is much superior to that of conventional ZnS-type materials [1–3].

Because barium aluminates when doped with rare earths have important applications and many studies, for example, on the effect of specific dopants on phosphorescence, have been conducted considering them as a phosphor material, e.g. BaAl2O4:Eu2?, Dy3? [4] and BaAl2O4:Ce3? [5], in this research, we aimed to develop aluminate-type hosts and to use a popular activator ion, namely the Eu3?- and Ho3?-ions, to obtain inorganic phosphor systems with high efficiency and chemical stability under room temperature for next generation applications.

Materials and methods

The solid-state reaction method was chosen to produce the phosphors. Therefore, high-purity raw materials in stoi-chiometric proportions were used to prepare the samples, i.e. BaCO3 (99.9%), 4MgCO3Mg(OH)25H2O (A.R.), Al2O3(99.9%), Eu2O3(99.99%) and Ho2O3(99.99%). As for the method, the raw materials, which were weighed in stoichiometric rates, were ground as required in an agate mortar to get a homogeneous mixed powder. After the dry-milling process, each batch from sample was then analysed with thermal analysis (DTA/TG) to obtain the thermal properties at a specific temperature range. The preheating stage was applied at 800°C for 2 h according to the DTA/ & Esra O¨ztu¨rk

esracircir@gmail.com

1 _{Department of Metallurgical and Materials Engineering,}

Faculty of Engineering, Karamanog˘lu Mehmetbey University, Karaman, Turkey

TG results. The major heat treatments for the Ba3Al2O6 :-Eu3?, Ho3? and MgAl2O4:Eu3?, Ho3? phosphor systems were at 1200°C for 24 h and 1200 °C for 24 h ? 1300 °C for 6 h, respectively.

All sintering conditions were under open atmosphere, and the samples were cooled down to the room temperature slowly, and then the end products were dry-milled to powder form for characterisations.

The decomposition or oxidation process of the samples was determined by differential thermal analysis (DTA) and thermogravimetric (TG) analysis (Seiko Instruments Inc./ Exstar TG/DTA 6200) at a heating rate of 10°C min-1 from room temperature to 1300°C. The X-ray diffractions, i.e. phases after the sintering processes of the powder form samples, were defined with a Bruker axs D8 Advance model X-ray diffractometer (XRD), which was run at 40 kV and 30 mA (Cu-Ka radiation) in a step-scan mode (0.02°/2h). The surface morphology and elemental analysis of samples were achieved by a scanning electron micro-scope (SEM). Finally, the photoluminescence (PL) prop-erties including the excitation and emission spectra of the products were analysed by a fluorometer (Photon Tech-nology International (PTI), QuantaMasterTM30).

Results and discussion

Thermal analysis

The thermal analysis result of initial mixtures for Ba3Al 2-O6:Eu3?, Ho3? system includes only BaCO3, which decomposes to BaO. The DTA/TG/DTG curves were related only to this reactant which can be clearly seen in Fig.1.

Figure1 shows that the major mass loss within the temperature range 900–1250°C is related to the decom-position of BaCO3to BaO and CO2. The thermal behaviour of BaCO3under heating can be given as follows:

BaCO3! D

BaOþ CO2 ð1Þ

BaCO3has three crystallographic structures depending on the temperature: rhombohedral (natural witherite), b-hexagonal and a-cubic forms. In this study, it was proved that the phase transformation temperatures of BaCO3 comply with the literature. There were three endothermic reactions which were an orthorhombic to rhombohedral transition at 820°C, a rhombohedral to cubic phase tran-sition at 985°C and the last one was for the decomposition of BaCO3to BaO at 1160°C in accordance with the TG curve [6].

The mass loss within the temperature range 50–1300°C was about 18.3%. It was proved that the experimental result was close to the theoretical mass change which was calculated (18.8%).

There is only one raw material that can be decomposed that is 4MgCO3Mg(OH)25H2O, for the other phosphor, namely MgAl2O4:Eu

3?

, Ho3?. The DTA/TG/DTG curves which were related only for this reactant are presented in Fig.2.

Figure2shows the mass losses in two stages which start from 200 to 345 °C and a higher decrease from 345 to 640 °C which are related to the decomposition of 4MgCO3Mg(OH)25H2O. The thermal behaviour of this material under heating can be summarised as follows:

50 83 100 TG/% 300 550 800 DTG/mg/min Endo 820 °C 985 °C DTA/μV 1160 °C 1050 1300 –200 –150 –100 –50 0 50 100 Temperature/°C

Fig. 1 DTA/TG/DTG curves of the starting materials for Ba3Al

2-O6:Eu3?, Ho3? 50 57 100 87 TG/% 300 550 800 DTG/mg/min Endo 260 °C 442 °C DTA/μV 1050 1300 –10 0 10 20 Temperature/°C

Fig. 2 DTA/TG/DTG curves of the starting materials for MgAl

4MgCO₃ Mg OHð Þ25H2O! D 4MgCO₃  Mg OHð Þ2þ5H2O ð2Þ 4MgCO3 Mg OHð Þ2! D 4MgCO3þ MgO þ H2O ð3Þ

4MgCO₃!D 4MgOþ 4CO2 ð4Þ

The first endothermic peak (at 260°C) is associated with the departure of the hydroxyl group from Mg(OH)2(2 and 3). The second endothermic peak (at 442°C) shows the decomposition of MgCO3which changes into MgO [7, 8]. At the end of the decompositions, the TG curve exhibits a total mass loss equal to *42.1%, which is very similar to the calculated mass loss (*44.0%).

X-ray diffraction (XRD) analysis

The first heat treatments were applied according to the thermal analysis results to decompose the hydrates or carbonates forms of raw materials; this was done to facil-itate the formation temperatures of the phosphor systems. Thus, first of all, the pre-sintering stage was applied to samples at 800°C for 2 h for the decomposition process. The major sintering processes were determined and/or repeated according to the XRD results.

The XRD pattern of the Ba3Al2O6:Eu3?, Ho3?phosphor which was sintered in a single step at 1200°C for 24 h is given in Fig.3.

The XRD pattern of Ba3Al2O6:Eu3?, Ho3? was well indexed considering the patterns of standard Ba3Al2O6 (PDF 01-078-6164) and complied with the previous limited researches about this structure [9]. Moreover, it was proved that the trace amounts of Eu3?-dopant and Ho3?co-dopant rare-earth ions did not significantly influence the host, as

well as can be seen in the XRD patterns. The indexed Ba3Al2O6has a cubic structure with the P213 space group which includes the lattice parameters that can be expressed as follows: a = 16.4984 A˚ , b = 16.4984 A˚ , c = 16.4984 A˚ , a = 90°, b = 90° and c = 90°.

Although the heat treatments for MgAl2O4:Eu3?, Ho3? were in two stages, the desired single MgAl2O4phase and MgO impurities phase were obtained. Thus, the compara-tive XRD patterns of MgAl2O4:Eu3?, Ho3? sintered at different temperatures are given in Fig.4.

The two XRD patterns of the MgAl2O4:Eu3?, Ho3? phosphor belong to the first sample which was sintered at 1200°C for 24 h and the second one that was subjected to two sintering stages: 1200°C for 24 h ? 1300 °C for 6 h. Even though two-stage heat treatments were applied, both XRD patterns were in agreement with the MgAl2O4 dom-inant phase (PDF 01-077-11193) and the other prominent MgO phase. Actually, SEM images of MgAl2O4 system show irregular distribution. Otherwise, Ba3Al2O6:Eu3?, Ho3? system has an excellent grain size and boundary (Fig.5).

It can be seen from the SEM images that the grains of Ba3Al2O6:Eu3?, Ho3? are more well grained in terms of the shape of particles than MgAl2O4:Eu3?, Ho3? because Ba3Al2O6:Eu3?, Ho3?have a single phase. In addition, the particle size distribution of Ba3Al2O6:Eu3?, Ho3? is homogenous.

Photoluminescence properties

The PL spectra of the two samples including excitation and emission bands are shown in Figs.6and7. Both of the host lattices, namely Ba3Al2O6 and MgAl2O4, were activated with Eu3? and co-activated with Ho3?-ions. Thus, it was

15 0 2000 4000 6000 20 25 30 35 40 45 50 55 60 2 /° Intensity/cps (4 0 0) (3 3 1) (5 1 1) (0 2 6) (7 1 1) (4 4 4) (4 4 0) (8 0 0) (8 4 4) θ

Fig. 3 XRD pattern of Ba3Al2O6:Eu3?, Ho3?which was sintered at

1200°C for 24 h 6000 3000 0 10 20 30 40 50 60 70 80 90 1200 °C – 24 h 1200 °C – 24 h + 1300 °C – 6 h + : MgO

_*

*

*

*

*

_{* *}

*

* *

*

Fig. 4 Comparative XRD patterns of the MgAl2O4:Eu3?, Ho3?

system which was sintered at 1200°C for 24 h (below) and 1200 °C for 24 h ? 1300°C for 6 h (above)

expected to obtain similar PL results except for the inten-sities, and then it occurred as follows:

The PL spectra of the phosphor samples were both excited between 260 and 280 nm, which provided effective emissions. Therefore, the comments as well as results were written for both of the samples. All the excitation bands originated from the intra-4f forbidden transitions of the Eu3?activator ion. The most intense excitations are related to the charge-transfer state (CTS) band. It is well known that typical Eu3?-activated phosphors mostly show strong CTS transition band excitation around 200–300 nm. Fur-thermore, the CTS is related to an electron transferred from the oxygen 2p orbital to the empty 4f orbital of europium, which may be ascribed as ligand-to-Eu3? charge-transfer transitions (LMCT) [3,10,11]. The excitation bands at 389 and 391 nm are both attributed to the7F0-5L6transition of Eu3?. Those at 465 and 463 nm are both related to the 7_F

0?5D2transition of Eu3?[3,12,13]. All the emission bands in the range of 500–750 nm were related to the 5_D

1?7F0 (541 nm), 5D0?7F0 (582 nm), 5D0?7F1 (590 nm), 5D0?7F2 (608 and 614 nm), 5D0?7F3 (649 nm) and 5D0?7F4(700 and 702 nm) transitions of Eu3? [13–15]. Additionally, although the MgAl2O4:Eu3?, Ho3? sample could not be synthesised in the expected single phase, remarkable PL results were observed with the excitation and emission bands which were related to the activator rare-earth ion, i.e. Eu3?. Another important point to be considered is that the Ba3Al2O6:Eu3?, Ho3? phos-phor, which was well indexed as a single phase, had more intense PL results which were more than double the PL results of the MgAl2O4:Eu3?, Ho3? phosphor. This indi-cated that the single phase formation helped to obtain more effective PL spectra.

Fig. 5 SEM images of aBa3Al2O6:Eu3?, Ho3?and bMgAl2O4:Eu3?, Ho3? 0 200 250 300 350 400 450 500 389 nm f–f transitions of Eu3+ 465 nm 265 nm 590 nm 541 nm 5D 1 7F 0 7F 3 5D 0 7_F 4 5_D 0 7_F 2 5_D 0 7_F 1 5_D 0 649 nm 608 nm 702 nm Emission Excitation Eu3+–CT 550 600 650 700 750 200 400 600 800 1000 1200 Intensity/a.u. Wavelength/nm

Fig. 6 PL spectra of Ba3Al2O6:Eu3?, Ho3?

0 200 300 400 500 f–f transitions of Eu3+ 278 nm 582 nm 5D 0 7F 0 7_F 4 5_D 0 7_F 2 5_D 0 700 nm 614 nm 391 nm 463 nm Emission Excitation Eu3+–CT 600 700 800 900 100 200 300 400 500 600 Intensity/a.u. Wavelength/nm

Conclusions

The phosphors for this research were prepared by the solid-state technique which is known to be an effective and simple method. Heat treatments were applied considering the DTA/TG results of the batches of the phosphor sys-tems. Although Ba3Al2O6:Eu3?, Ho3? was formed as a single phase and indexed to major desired phase, MgAl 2-O4:Eu

3?

, Ho3?phosphor could not be indexed to expected single phase while increasing temperature being applied in addition to the first heat treatment. It was well indexed, namely MgAl2O4and MgO, in XRD. Despite this result, the PL analysis indicated that the MgAl2O4:Eu3?, Ho3? phosphor exhibited prominent excitation and emission bands. The Ba3Al2O6:Eu

3?

, Ho3? phosphor had remark-able PL results thanks to the dopant Eu3?-ions, as expected.

Both of the Eu3?-activated phosphors presented emis-sions in the red region that are attributed to the transitions of the Eu3?-ion. Ho3? co-doping gave no PL results, although it was thought to enhance the PL properties. Thus, further research is required for these phosphors to fully understand the effect of the Ho3?-ion as co-dopant. Acknowledgements The authors kindly thank Karamanoglu Meh-metbey University’s Scientific Research Projects Commission (BAP, project number: 17-M-14), in the Republic of Turkey for its financial support.

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