Yellow luminescence of co-doped gadolinium oxyhydroxide

JOURNAL OF RARE EARTHS, Vol. 33, No. 7, July 2015, P. 712

Yellow luminescence of co-doped gadolinium oxyhydroxide Hiroaki Samata1,*, Shungo Imanaka1, Masashi Hanioka1, Tadashi C. Ozawa2 (1. Graduate School of Maritime Sciences, Kobe University, Fukaeminami, Higashinada, Kobe, Hyogo 658-0022, Japan; 2. International Center for Materials Nanoarchitectonics, National Institute for Materials Science, Namiki, Tsukuba, Ibaraki 305-0044, Japan) Received 24 June 2014; revised 15 April 2015

Abstract: Crystals of co-doped gadolinium oxyhydroxide (GdOOH), Gd0.98Eu0.02−xTbxOOH and Gd1−y−zDyyBizOOH, were synthesized by a flux method. The color coordinates in the Commission Internationale de I’Eclairage (CIE) chromaticity diagram of Gd0.98Eu0.02−xTbxOOH, obtained under 254 nm irradiation, shifted along a straight line with the changing values of x to include the yellow region. The CIE coordinates of Dy3+ doped in GdOOH were located in the yellow region, while the emission intensity of Dy3+ under 286 nm irradiation increased by more than 40 times when co-doped with Bi3+. Keywords: gadolinium oxyhydroxide; phosphor; co-doping; yellow luminescence; rare earths

White light-emitting diode (LED) lighting is an evolving technology used for energy conservation purposes[1,2]. Such lighting systems consist of an LED as the light source and fluorescent substances that convert the LED light color to another desired color. A near-ultraviolet LED combined with substances that fluoresce blue and yellow produces squasi-white light. The near-ultraviolet LED does not create white light by itself, which means that its combination with fluorescent substances has good color rendering characteristics[1]. However, to further improve the luminous efficacy and color rendering of this combination, it is important to develop a yellow fluorescent substance excited by near-ultraviolet irradiation[3,4]. There are several ways to obtain yellow fluorescence with trivalent rare-earth ions based on electronic transitions between the 4f orbitals[2,5]. Yellow fluorescence can be obtained using an appropriate mixture of red and green fluorescent ions such as Eu3+ and Tb3+ doped in a suitable host material. The same effect can also be achieved with only one type of ion having an appropriate electron configuration, such as Dy3+ [6]. Generally, the luminescence characteristics of trivalent rare-earth ions are highly affected by the crystal structure and chemical composition of the host material. It is known that the luminescence properties are improved by doping with a sensitizer such as Bi3+, which is efficient for Dy3+ activation[7−10]. Gadolinium oxyhydroxide (GdOOH) is a stable phase obtained by the thermal dehydration of gadolinium hydroxide Gd(OH)3 [11−14] and has a simple layered structure. GdOOH is a possible host material for phosphors, and a few studies on the luminescence properties of Eu3+-

doped GdOOH have been performed[15−17]. We previously reported the synthesis of rare-earth oxyhydroxide crystals that were several millimeters in size; we also reported strong red emission of Eu3+-doped GdOOH with the highest fluorescence quantum yield of 0.27[18−21]. In this study, we synthesized Gd0.98Eu0.02−xTbxOOH and Gd1−y−zDyyBizOOH crystals by a flux method and evaluated their yellow luminescence properties by excitation with near-ultraviolet radiation.

1 Experimental Crystals of co-doped gadolinium oxyhydroxides, Gd0.98Eu0.02−xTbxOOH and Gd1−y−zDyyBizOOH, were synthesized by a flux method. Powders of Gd2O3 (99.9%), Eu2O3 (99.9%), Tb4O7 (99.9%), Dy2O3(99.5%), and Bi2O3 (99.9%) were used as raw materials. Tb4O7 powder was heated in an electric furnace at 800 ºC for 20 h in an Ar/H2 atmosphere to convert it to Tb2O3, which has only Tb3+ ions. Appropriate amounts of raw material powders, with a combined total mass of 0.5 g, were mixed using an agate mortar for 1 h. A mixture of NaOH (15 g) and KOH (5 g) was used as the flux. The powders and flux mixtures were held in a 20 mL zirconium crucible. The crucible was heated in air for 72 h using an electric furnace at a constant temperature. The synthesis temperature was set to be between 310 and 330 ºC depending on the ratio of the raw materials used. The crucible was then removed from the furnace using a pair of forceps and the supernatant liquid of the flux was drained. The crystals were washed thoroughly in distilled water to remove the residual flux and then dried on a hot plate at 50 ºC.

Foundation item: Project supported by JSPS KAKENHI (21560696, 24560827) * Corresponding author: Hiroaki Samata (E-mail: [email protected]; Tel.: +81 78 431 6285) DOI: 10.1016/S1002-0721(14)60475-0

Hiroaki Samata et al., Yellow luminescence of co-doped gadolinium oxyhydroxide

The crystal structures were identified by powder X-ray diffraction (XRD; Rigaku, RINT2000) using Cu Kα radiation generated at 40 kV and 20 mA, in the 2θ range of 10º−80º at room temperature; the data obtained were refined using the Rietveld method[22]. The fluorescence emission spectra of Gd0.98Eu0.02−xTbxOOH were measured at room temperature using a thermoelectrically cooled spectrometer (B&W Tek’s, BTC112E). The fluorescence excitation and emission spectra of Gd1−y−zDyy BizOOH were measured at room temperature using a fluorescence spectrophotometer (Hitachi, F-7000). The excitation spectra were corrected for the spectral distribution of the lamp irradiation by the Rhodamine B method, and the emission spectra were corrected for the spectral response of the instrument by using a substandard light source.

2 Results and discussion 2.1 Crystallographic and luminescence properties of Gd0.98Eu0.02−xTbxOOH crystals Upon synthesis, Gd0.98Eu0.02−xTbxOOH crystals grew at the bottom of the crucible. Fig. 1 shows the powder XRD profiles of Gd0.98Eu0.02−xTbxOOH and the calculated profile of GdOOH using the crystal parameters discussed in a previous study[21]. No second-phase peaks were observed; all peaks can be explained by assuming the crystals have a monoclinic structure belonging to the P21/m space group. The absence of second phases in the rareearth oxyhydroxide crystals synthesized by the flux method used in this study was confirmed by thermogravimetric analysis (TGA), differential thermal analysis (DTA), and powder XRD in our previous studies[19,20].

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Fig. 1 Powder XRD profiles of Gd0.98Eu0.02−xTbxOOH crystals and the calculated profile of GdOOH

Fig. 2(a) shows a photograph of the as-grown crystals of Gd0.98Eu0.02OOH (x=0) taken under irradiation by white fluorescent lamps. The crystals were transparent and had a plate-like shape with flat surfaces. The maximum length of a crystal was about 0.3 mm. Figs. 2 (b)−(f) show photographs of the Gd0.98Eu0.02−xTbxOOH crystals placed on an alumina plate in a light shielding box under 254 nm irradiation of intensity 1.1 mW/cm 2 . Gd0.98Eu0.02OOH (x=0) (Fig. 2(a)) and Gd0.98Tb0.02OOH (x=0.02) (Fig. 2(f)) crystals showed strong red and green emissions, respectively. The emission color of Gd0.98Eu0.02−xTbxOOH crystals changed as a function of the ratios of Eu3+ and Tb3+; each type of crystal, however, showed a uniform color as shown in Fig. 2. The mixed powders of Gd0.98Eu0.02OOH (x=0) and Gd0.98Tb0.02OOH

Fig. 2 Photographs of as-grown crystals of Gd0.98Eu0.02OOH (a) and Gd0.98Eu0.02−xTbxOOH (b−f) crystals under 254 nm irradiation of 1.1 mW/cm2 (a) x=0; (b) x=0;(c) x=0.005; (d) x=0.010; (e) x=0.015; (f) x=0.020

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(x=0.02) crystals that were synthesized separately using an agate mortar for 1 h did not show uniform color. Therefore, in order to obtain crystals exhibiting uniform color luminescence, a proper mixture of raw materials must be used during the synthesis. Fig. 3 shows the emission spectra of Gd0.98Eu0.02−x TbxOOH crystals under 254 nm irradiation measured at room temperature using a thermoelectrically cooled spectrometer. For Gd0.98Eu0.02OOH (x=0) crystals, strong red luminescence from Eu3+ was observed in the visible range. These peaks are attributable to the transition from the exited level of 5D0 to the 7F0−4 levels of the ground term of Eu3+ [23]. The intensity of these peaks decreased by about 10% only, when the temperature increased from room temperature up to 100 ºC. For Gd0.98Tb0.02OOH (x=0.02) crystals, strong green luminescence from Tb3+ was observed. These peaks are attributable to the transition from the exited level of 5D4 to the 7F3−6 levels of the ground term of Tb3+ [24]. On co-doping Y2O3 crystals with Eu3+ and Tb3+, the fluorescence bands of Tb3+ corresponding to the 5D4 to 7F5 transition are reported to be strongly quenched by Eu3+ because of the energy transfer from Tb3+ to Eu3+ [25]. In Gd0.98Eu0.02−xTbxOOH crystals, however, strong quenching of fluorescence from Tb3+ was not observed, and the intensity ratio of Eu3+ and Tb3+ changed depending on the ratio of the raw materials. Fig. 4 shows the CIE chromaticity coordinates of Gd0.98Eu0.02−xTbxOOH crystals. The color coordinates of x=0 and x=0.020 were determined to be (0.64, 0.34) located in the red region and (0.36, 0.61) located in the green region, respectively. Fluorescence quantum yield Φf, which is defined as the ratio of the number of emitted photons to the number of absorbed photons, values for Gd0.98Eu0.02OOH and Gd0.98Tb0.02OOH were 0.24 under excitation at 243 nm[21] and 0.08 at 255 nm, respectively.

Fig. 3 Emission spectra of Gd0.98Eu0.02−xTbxOOH crystals under 254 nm irradiation

JOURNAL OF RARE EARTHS, Vol. 33, No. 7, July 2015

Fig. 4 CIE chromaticity diagram of Gd0.98Eu0.02−xTbxOOH crystals under 254 nm irradiation

In spite of the difference between Φf of Eu3+ and Tb3+ in GdOOH, the color coordinates were shifted along a straight line to include the yellow region, which indicates that different colors including yellow are obtained only by controlling the molar ratio of the starting materials. 2.2 Crystallographic and luminescence properties of Gd1−y−zDyyBizOOH crystals Fig. 5 shows the powder XRD profiles of Gd0.8Dy0.2OOH (y=0.2, z=0), and the results of data refinement by the Rietveld method. For the refinement, the crystal data of NdOOH crystals obtained by the single-crystal XRD analysis were used as the default data[19]. Refinement was done under the following assumptions: atomic coordinates of Dy are exactly the same as those of Gd; and the sum of site occupancy of Gd and Dy is 100%. In Fig. 5, the black squares represent the experi-

Fig. 5 Powder XRD profiles of Gd0.8Dy0.2OOH and the result of data refinement by the Rietveld method (Inset: Dependence of Dy3+ content on unit cell volume of Gd1−yDyyOOH crystals)

Hiroaki Samata et al., Yellow luminescence of co-doped gadolinium oxyhydroxide

mental data, and the red and blue solid lines represent the calculated profile and the difference between the experimental and calculated profiles, respectively. The diffraction data were well refined assuming the crystals had a monoclinic structure belonging to the P21/m space group. In this analysis, the goodness-of-fit indicator S was 1.1692, a value under 1.3 as it should be[26]; the refined crystallographic parameters are listed in Table 1. The inset of Fig. 5 shows the dependence of Dy3+ content on the unit cell volume of Gd1−yDyyOOH (z=0) crystals, which was identified by the Rietveld method to be the same as that of Gd0.8Dy0.2OOH crystals. When synthesizing rare-earth oxyhydroxide crystals by the method we used in this study, the composition of the resulting crystals agrees well with the molar ratio of the starting materials, as was proved in our previous study[21]. When Gd3+ (ionic radius: 0.0938 nm) in GdOOH is replaced by Dy3+ (ionic radius: 0.0912 nm), the unit cell volume decreases in accordance with Vegard’s law[27]. Fig. 6 shows the excitation and emission spectra of Gd1−yDyyOOH (z=0) crystals measured at room temperature using a fluorescence spectrophotometer. The excitation spectra measurements were performed at 579 nm, this system. The sharp peaks in the range of 240−460 nm

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which is the maximum emission wavelength of Dy3+ in are attributable to 4f−4f transitions from the 6H15/2 ground state to the different excited states of Dy3+, while the broad band under 230 nm is attributable to absorption by the host[9]. The emission spectra measurements were performed under 275 nm irradiation, which is close to the peak maxima in the excitation spectra. Fluorescence from 4F9/2 to the 6Hi multiplet was observed (4F9/2→6H15/2: 488 nm, 4F9/2→6H13/2: 579 nm, 4F9/2→6H11/2: 671 nm). When the content of Dy3+ increased from 0.025 to 0.075, the intensity of transitions 4F9/2→6Hi is decreased because of concentration quenching. The CIE coordinates were (0.42, 0.46), (0.41, 0.45), and (0.40, 0.46) corresponding to x=0.025, 0.050, and 0.075, respectively; all coordinates were located in the yellow region. Fig. 7 shows the excitation and emission spectra of Gd0.95−zDy0.05BizOOH. A strong absorption band of Bi3+ at 260−300 nm for the Dy3+ emission at 579 nm was observed. The Dy3+ emission intensity under 286 nm irradiation increased more than 40-fold by co-doping with Bi3+, which proves efficient energy transfer between Bi3+ and Dy3+. Fig. 8 shows the schematic diagram of the possible energy transfer process from the 3P1 band of Bi3+ to the 4F9/2 level of Dy3+ [8,28]. Under specific irradiation,

Table 1 Refined crystallographic parameters obtained by the Rietveld method for Gd0.8Dy0.2OOH Atomic coordinates Atom

g

x

y

B/10–2 nm2

z

Gd

0.800

0.662

0.250

0.305

0.038

Dy

0.200

0.662

0.250

0.305

0.038

O(1)

1.000

0.762

0.750

0.542

3.353

O(2)

1.000

0.242

0.750

0.062

6.420

Atom

β11

β22

β33

β12

β13

β23

Gd

5.670×10−4 6.940×10−4 2.900×10−4

0

1.310×10−4

0

Dy

5.670×10−4 6.940×10−4 2.900×10−4

0

1.310×10−4

0

O(1)

4.994×10−2 6.107×10−2 2.552×10−2

0

1.153×10−2

0

O(2)

9.562×10−2 1.169×10−1 4.887×10−2

0

2.207×10−2

0

Crystal system: monoclinic; Space group: P21/m;

Lattice parameters: a=0.4329 nm, b=0.3705 nm, c=0.6056 nm; α = γ = 90 °,

Fig. 7 Excitation and emission spectra of Gd0.95−zDy0.05BizOOH crystals

β =108.838°; Rwp=2.68%, Re=2.30%, S=1.1692

Fig. 6 Excitation and emission spectra of Gd1−yDyyOOH crystals

Fig. 8 Schematic diagram of energy transfer from Bi3+ to Dy3+

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co-doping with Bi3+ is very efficient for the luminescence of Dy3+ in the Gd0.95−zDy0.05BizOOH system. In addition to Bi3+, co-doping with Ce3+ as a sensitizer introduces the possibility of further improving the luminescence properties of Dy3+. However, we have not yet succeeded in synthesizing Ce3+ co-doped Gd1−yDyyOOH crystals.

3 Conclusions Gd0.98Eu0.02−xTbxOOH and Gd1−y−zDyyBizOOH crystals were synthesized by a flux method. The crystallographic and luminescence properties of these crystals were characterized. All XRD data for the crystals were explained assuming the crystals had a monoclinic structure belonging to the P21/m space group. Under near-ultraviolet irradiation, the color coordinates of Gd0.98Eu0.02−xTbxOOH crystals shifted along a straight line to include the yellow region because of the ratio of raw materials used, and strong quenching of luminescence from Tb3+ was not observed. The CIE coordinates of Dy3+ doped in GdOOH were located in the yellow region, and the Dy3+ emission intensity under a specific irradiation increased by more than 40 times upon co-doping with Bi3+. The luminescence intensity of Dy3+ in GdOOH, however, is not enough for practical applications, and so, research on further increasing the intensity by co-doping with other sensitizers is required. The fact that GdOOH crystals were stable in NaOH/KOH solution between 310 ºC and 330 ºC confirmed that they have high alkali durability. Hence, these materials may be used as high-efficiency hosts for unique applications such as photomedical applications.

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