Hydrothermal synthesis and multicolor luminescence

Ceramics International 42 (2016) 7781–7786

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Hydrothermal synthesis and multicolor luminescence properties of Dy3 þ /Eu3 þ co-doped KLa(MoO4)2 phosphors Yun Liu a,n, Haoqiang Zuo a, Jinyang Li b, Xiaolei Shi a, Suiyan Ma a, Minzhu Zhao c, Kun Zhang a, Chenyu Wang a a

College of Science, Shaanxi University of Science and Technology, Xi'an 710021, China College of Electrical and Information Engineering, Shaanxi University of Science and Technology, Xi'an 710021, China c School of Materials Science and Engineering, Shaanxi University of Science and Technology, Xi'an 710021, China b

art ic l e i nf o

a b s t r a c t

Article history: Received 28 December 2015 Received in revised form 25 January 2016 Accepted 31 January 2016 Available online 10 February 2016

A series of Dy3 þ /Eu3 þ co-doped KLa(MoO4)2 phosphors were synthesized by a hydrothermal method. The as-prepared samples were characterized by an X-ray diffraction (XRD), a field emission scanning electron microscope (FE-SEM), a photoluminescence (PL) excitation, emission spectra and decay curves. The samples have a single-phase monoclinic structure of KLa(MoO4)2 and consist of microspheres with the mean size of 1.5 μm. Upon excitation at 389 nm, KLa(MoO4)2:Dy3 þ exhibits characteristic peaks at 485 nm and 576 nm. The optimal doping concentration of Dy3 þ is about 7 mol%. KLa0.93 y(MoO4)2:0.07Dy3 þ , yEu3 þ samples show emission peaks of both Dy3 þ and Eu3 þ centered at 485, 576, 595 and 618 nm under 389 nm excitation. It can be found that the integrated emission intensity of Dy3 þ is decreased and the lifetime of Dy3 þ is declined with increasing Eu3 þ concentration, which is due to the energy transfer from Dy3 þ to Eu3 þ . Furthermore, according to Dexter's theory and Reisfeld's approximation, the quadrupole–quadrupole (q–q) interaction is the mechanism for energy transfer between Dy3 þ and Eu3 þ . Moreover, the chromaticity color coordinates of KLa0.93 y(MoO4)2:0.07Dy3 þ , yEu3 þ samples are tuned from the yellowish-white region to white light and eventually to the reddishwhite region with increasing Eu3 þ content. This kind of multicolor tunable phosphors exhibits the great potential applications in the fields of white light diodes, full-color displays and optoelectronic devices. & 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: A. Hydrothermal method C. Multicolor luminescence D. Molybdate

1. Introduction Recently, there has been a growing interest in the preparation of multicolor luminescent materials due to their wide applications in full-color displays, white light emitting diodes (W-LEDs), medical imaging and optoelectronic devices [1–4]. The whiteemitting phosphors applied in white light emitting diodes have been investigated by many researchers. One method for obtaining white-emitting phosphors is via the energy transfer from the sensitizer to the activator. For example, Rivera et al. [5] reported that the chromaticity coordinates of Ce3 þ –Dy3 þ co-doped Al2O3 films were close to the ideal white light coordinates (0.333, 0.333). Som et al. [6] discussed the photoluminescence properties and energy transfer behavior of Dy3 þ –Tb3 þ co-doped Y2O3 nanophosphors. Rare earth ion, which emits white light itself, doped phosphors is another way to achieve white light. It is well known that single Dy3 þ ions doped luminescent materials exhibit two n

Corresponding author. E-mail address: [email protected] (Y. Liu).

http://dx.doi.org/10.1016/j.ceramint.2016.01.210 0272-8842/& 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

characteristic emissions in the blue (4F9/2-6H15/2 transition) and yellow (4F9/2-6H13/2 transition) regions [7,8]. However, due to the lack of red components, the color render index (CRI) of such luminescent materials is very low [9,10]. To compensate the red component, a red luminescence center is introduced into Dy3 þ ions doped luminescent materials. The Eu3 þ ion, which exhibits the orange and red emissions due to its 5D0-7FJ (J ¼1, 2, 3, 4) transitions, is an excellent activator [11]. Therefore, white light with good quality can be achieved by co-doping Dy3 þ and Eu3 þ ions into a single-phase host. Double molybdates ALn(MoO4)2 (A ¼Li, Na, K; Ln ¼La, Gd, Y, Lu), which possess a scheelite-type structure, are important inorganic compounds and have been considered to be excellent luminescent hosts due to their attractive luminescence properties, thermal and chemical stability [12–15]. As one compound of the family, KLa(MoO4)2 has a monoclinic symmetry and Mo6 þ is coordinated with four O2 ions at a tetrahedral site, and La3 þ /K þ site is eight coordinated. But to our best knowledge, little attention has been paid to the luminescence properties of Dy3 þ –Eu3 þ codoped KLa(MoO4)2 phosphors. It is all known that the luminescence properties of phosphors depend not only on the

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Y. Liu et al. / Ceramics International 42 (2016) 7781–7786

composition, but also on the morphology and size. Hydrothermal method is an excellent synthetic route, which can prepare nanomaterials with the controllable morphology and size [16–18]. In the present work, Dy3 þ single-doped and Dy3 þ /Eu3 þ co-doped KLa(MoO4)2 samples were prepared by a hydrothermal method successfully. Their multicolor photoluminescence and the energy transfer mechanism were investigated in detail.

solution. Finally, the precursor solution was transferred into a 60 mL Teflon-lined stainless steel autoclave and maintained at 180 °C for 12 h. After the autoclave was naturally cooled down to room temperature, the precipitates were collected by centrifugation and washed with de-ionized water, and then dried at 80 °C. Eventually, the KLa(MoO4)2:0.07Dy3 þ , 0.01Eu3 þ samples were

2. Experimental 2.1. Sample preparation KLa(MoO4)2:Dy3 þ and KLa(MoO4)2:Dy3 þ , Eu3 þ phosphors were prepared by a hydrothermal method. All chemicals used in the experiments were of analytical grade. In the typical synthesis of KLa0.92(MoO4)2:0.07Dy3 þ , 0.01Eu3 þ samples, firstly, Eu(NO3)3 and Dy(NO3)3 solutions were prepared by dissolving Eu2O3 and Dy2O3 in dilute HNO3, respectively. Then, 1.359 g of La (NO3)3 nH2O, 0.297 g of KOH, 0.45 mL of 0.2 mol L 1 Eu(NO3)3 solution and 3.15 mL of 0.2 mol L 1 Dy(NO3)3 solution were mixed and stirred to obtain a homogeneous solution. Next, 1.605 g of (NH4)6Mo7O24 4H2O was added into the above solution. Subsequently, the pH value of the mixed solution was adjusted to 7 by aqueous ammonia and then stirred for 1 h to form the precursor Fig. 3. The excitation and emission spectra of KLa(MoO4)2:Dy3 þ samples.

Fig. 1. XRD patterns of KLa0.93 y(MoO4)2:0.07Dy3 þ , yEu3 þ samples.

Fig. 4. The emission spectra of KLa(MoO4)2:Dy3 þ with various Dy3 þ doping concentrations under excitation at 389 nm. (Inset: the emission intensity of 4F9/ 6 3þ doping concentration.). 2- H13/2 transition at 576 nm as a function of Dy

Fig. 2. FE-SEM images of KLa(MoO4)2:Dy3 þ , Eu3 þ samples with different magnifications.


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Fig. 7. Decay curves of Dy3 þ in KLa0.93 y(MoO4)2:0.07Dy3 þ , yEu3 þ samples. 3þ

Fig. 5. The excitation spectra of KLa0.92(MoO4)2:0.07Dy monitoring different wavelengths.

, 0.01Eu

3þ

samples by

3. Results and discussion 3.1. Phase identification and morphology

Fig. 6. The emission spectra of KLa0.93 y(MoO4)2:0.07Dy3 þ , yEu3 þ with various Eu3 þ doping concentrations (Inset: the integrated emission intensity of the 4F9/ 6 5 7 3þ doping concentration.) 2- H13/2 and D0- F2 transition as a function of Eu

3þ

obtained. The synthetic process of KLa0.9 y(MoO4)2:0.07Dy , yEu3 þ samples was the same as that of synthesizing KLa0.92(MoO4)2:0.07Dy3 þ , 0.01Eu3 þ samples.

2.2. Characterization X-ray powder diffraction (XRD) was performed by a Rigaku D/ Max-2200 diffractometer with Cu Kα radiation (λ ¼1.5405 Å). The scanning rate was set of 4°/min from 10° to 70° for 2θ. The size and morphology of the samples were observed using an S-4800 field emission scanning electron microscope (FE-SEM) with the accelerating voltage of 10 kV. The excitation, emission spectra and fluorescent decays were recorded by an F-4600 spectrophotometer equipped with a 150 W Xenon lamp as an excitation source. The CIE chromaticity coordinates were calculated by GoCIE software. All measurements were performed at room temperature.

Fig. 1 shows the X-ray diffraction patterns of KLa0.93 y(MoO4)2:0.07Dy3 þ , yEu3 þ phosphors with different doping concentrations of Eu3 þ . As can be seen, all the diffraction peaks of the as-prepared samples are almost matched well with those of the pure monoclinic phase of KLa(MoO4)2 (JCPDS 40-0466) with the lattice constants a ¼5.437 Å, b¼ 12.205 Å, and c ¼5.417 Å, and no diffraction peaks of impurities are observed, indicating that the incorporation of Eu3 þ and Dy3 þ into KLa(MoO4)2 host has little influence on the structure of the host. Moreover, the main peak positions are shifted slightly to the higher degree with the increase of Eu3 þ concentrations, which is due to the ionic radius of the La3 þ ions (0.116 nm) is larger than that of Eu3 þ ions (0.107 nm) [19]. The morphology of KLa(MoO4)2:Eu3 þ , Dy3 þ phosphors was characterized by an FE-SEM. As shown in Fig. 2(a), the as-prepared samples consist of homogeneous microspheres with an average diameter of about 1.5 μm. The high-magnification FE-SEM image in Fig. 2(b) reveals that the surface of microspheres is smooth. 3.2. Luminescence properties 3.2.1. Luminescence properties of KLa1 x(MoO4)2:xDy3 þ Fig. 3 shows the excitation and emission spectra of KLa (MoO4)2:Dy3 þ phosphors. The excitation spectrum is obtained by monitoring the emission of Dy3 þ at 576 nm corresponding to the 4 F9/2-6H13/2 transition. It can be seen that the excitation spectrum consists of a weak band centered at 293 nm, which is due to the charge transfer of O2 –Dy3 þ and O2 –Mo6 þ . Some excitation peaks located at 351, 366, 389, 429 and 453 nm are assigned to the f–f transitions of Dy3 þ from the ground state 6H15/2 to the excited states of 6P7/2, 6P5/2, 4I13/2, 4G11/2 and 4I15/2, respectively [20,21]. Among these transitions, 6H15/2-4I13/2 (389 nm) and 6H15/2-4I15/2 (453 nm) are the stronger peaks. This demonstrates that KLa (MoO4)2:Dy3 þ phosphors can be excited both by the near-UV and blue LED chips. Upon excitation at 389 nm, sharp emission peaks at 485, 576, and 665 nm are obtained, which is originated from the 4F9/2-6H15/2, 4F9/2-6H13/2, and 4F9/2-6H11/2 transition of Dy3 þ , respectively [22]. However, due to the emission intensity of 4F9/2-6H11/2 transition is very weak, the white light cannot be achieved by Dy3 þ singly doped KLa(MoO4)2.

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Table 1 Data of the decay curves of Dy3 þ in KLa(MoO4)2:0.07Dy3 þ , yEu3 þ phosphors. Sample

y

KLa(MoO4)2:0.07Dy3 þ , yEu3 þ

0 0.01 0.05 0.09 0.13 0.17

Fitting parameter

Decay time (ms)

A1

A2

τ1

τ2

τ

22.21337 30.04855 26.04086 34.31754 39.63847 29.77076

18.92158 18.1754 7.9914 3.06139 1.76375 23.11624

0.6244 0.51697 0.36541 0.30303 0.2428 0.1453

7.13313 7.12373 5.6743 5.52336 2.2371 0.30794

6.526 6.416 4.753 3.535 0.823 0.347

Fig. 8. The dependence of the I0/I of Dy3 þ on (a) C6/3 103 (b) C8/3 104 (c) C10/3 105 in KLa0.93 y(MoO4)2:0.07Dy3 þ , yEu3 þ samples.

Fig. 4 gives the emission spectra of KLa1 x(MoO4)2:xDy3 þ under 389 nm excitation. It is clear that the spectra are similar in shape except the emission intensity. The inset curve shows the trend of the relative intensity of 4F9/2-6H13/2 transition as a function of Dy3 þ concentration. The emission intensity is increased with increasing Dy3 þ doping concentration, and reaches the maximum when doping concentration is 7 mol%, then it is decreased due to concentration quenching. The optimal Dy3 þ doping concentration is 7 mol% in KLa(MoO4)2 host. Therefore, the doped Dy3 þ concentration is fixed to 7 mol% in our following research.

3.2.2. Luminescence properties of KLa0.93 y(MoO4)2:0.07Dy3 þ , yEu3 þ Fig. 5 presents the excitation spectra of KLa0.92(MoO4)2:0.07Dy3 þ , 0.01Eu3þ phosphors. When monitoring the 4F9/2-6H13/2 transition of Dy3þ (576 nm) or the 5D0-7F2 transition of Eu3þ (618 nm), the excitation spectra consist of a broad charge transfer band (CTB) and some sharp lines owing to the 4f–4f configuration of Dy3 þ and Eu3 þ , respectively. However, a weak peak centered at 450 nm is observed when monitoring the emission of Eu3þ , which is due to the 6H15/ 4 3þ . This fact illustrates that the Dy3þ ions can 2- I15/2 transition of Dy 3þ transfer the energy to Eu ions in KLa(MoO4)2 host.


Fig. 6 depicts the emission spectra of KLa0.93 y(MoO4)2:0.07Dy3þ , yEu3 þ phosphors upon excitation at 389 nm. It can be seen that the samples exhibit four emission peaks centered at 485 nm, 576 nm, 595 nm and 618 nm corresponding to 4F9/2-6H15/2, 4F9/2-6H13/2, 5 D0-7F1, and 5D0-7F2 transitions, respectively. In other words, the introduction of Eu3þ into the KLa(MoO4)2:Dy3þ samples can obtain blue, yellow, and red emissions. Thus, white light can be obtained by properly adjusting the relative ratios of the Eu3 þ to Dy3þ ions. It can be found that the integrated emission intensity of Eu3 þ is increased while that of Dy3þ is monotonically decreased with the increase of Eu3 þ concentration. These phenomena indicate that the Eu3þ emission is enhanced by the insertion of Dy3 þ , which is attributed to the energy transfer from Dy3 þ to Eu3 þ ions [23,24]. In order to further prove the existence of the energy transfer

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from Dy3 þ to Eu3 þ in KLa(MoO4)2 host, the decay curves of Dy3 þ in KLa0.93 y(MoO4)2:0.07Dy3 þ , yEu3 þ excited at 389 nm and monitored at 576 nm are measured and shown in Fig. 7. It can be found that all of the curves can be well fitted into the second-order exponential function as;

I = A1exp(−t /τ1) + A2 exp(−t /τ2)

(1)

where I is the emission intensity at any time; A1 and A2 are the fitting parameters; τ1 and τ2 are the attenuation times. The average lifetime of Dy3 þ was calculated using the following equation [25].

τ=

A1τ12 + A2 τ22 A1τ1 + A2 τ2

(2)

As shown in Table 1, the average lifetime is determined to be 6.526, 6.416, 4.753, 3.535, 0.823 and 0.347 ms for y ¼0, 1, 5, 9, 13 and 17 mol% in KLa0.93 y(MoO4)2:0.07Dy3 þ , yEu3 þ , respectively. The lifetime of Dy3 þ is declined with the increasing Eu3 þ content. According to Dexter's energy transfer formula of multipolar interaction and Reisfeld's approximation, the following relation can be obtained:

I0 ∝ C n /3 I

Fig. 9. The emission spectra of KLa0.88(MoO4)2:0.07Dy3 þ , 0.05Eu3 þ under different wavelengths excitation.

(3)

where I0 and I are the luminescence intensity of sensitizer without and with the presence of activator; C is the sum of the sensitizer and activator ions concentrations; n ¼6, 8, and 10, corresponding to the dipole–dipole (d–d), dipole–quadrupole (d–q), and quadrupole–quadrupole (q–q) interactions, respectively [26,27]. The relationship between I0/I and Cn/3 is plotted and shown in Fig. 8. It can be seen that a liner relation is observed when n ¼10, indicating that the energy transfer mechanism from Dy3 þ to Eu3 þ in KLa(MoO4)2 host is a quadrupole–quadrupole interaction.

Fig. 10. (a) CIE chromaticity diagram of KLa0.88(MoO4)2:0.07Dy3 þ , 0.05Eu3 þ samples under 389 nm (1), 394 nm (2), 454 nm (3), 465 nm (4) excitation; (b) CIE chromaticity diagram of KLa0.93 y(MoO4)2:0.07Dy3 þ , yEu3 þ samples under 389 nm excitation y¼ 0 (a), y¼0.01 (b), y¼0.05 (c), y¼ 0.09 (d), y¼ 0.13 (e), y ¼0.17 (f), ideal white light point (g).

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3.3. Chromaticity coordination Fig. 9 shows the emission spectra of KLa0.88(MoO4)2:0.07Dy3 þ , 0.05Eu3 þ under different wavelengths excitation. It can be seen that the samples merely show the characteristic emissions of Eu3 þ under 394 and 465 nm excitation. Upon excitation at 454 nm, the intense emissions of Dy3 þ and weak emissions of Eu3 þ are observed. However, it can be seen that the samples exhibit the characteristic emissions of both Eu3 þ and Dy3 þ under 389 nm excitation. The corresponding CIE chromaticity diagram is shown in Fig. 10(a). The CIE chromaticity coordinates are found to be (0.341, 0.356), (0.534, 0.350), (0.446, 0.532) and (0.598, 0.393) when excited by 389, 394, 454 and 465 nm, respectively. Fig. 10(b) shows the CIE chromaticity diagram of KLa0.93 y(MoO4)2:0.07Dy3 þ , yEu3 þ samples excited at 389 nm. The CIE chromaticity coordinates are calculated to be (0.362, 0.405), (0.353, 0.390), (0.341, 0.356), (0.373, 0.370), (0.386, 0.363) and (0.394, 0.353) for y ¼0, 1, 5, 9, 13 and 17 mol%, respectively. It is clear that the chromaticity color coordinates can be tuned from the yellowish-white region to white light region and eventually to the reddish-white region with increasing Eu3 þ concentration. When y¼5 mol%, the chromaticity coordinates are found to be (0.341, 0.356), which is close to the ideal white light (point g). All results suggest that multicolor luminescence can be obtained by adopting different excitation wavelengths or properly adjusting the relative contents of the Eu3 þ to Dy3 þ ions and KLa(MoO4)2: Eu3 þ , Dy3 þ has potential applications in the fields of white light diodes, full-color displays and optoelectronic devices.

[2]

[3]

[4]

[5]

[6]

[7]

[8]

[9]

[10] [11]

[12]

[13] [14]

4. Conclusions In summary, a series of sphere-like KLa(MoO4)2:Dy3 þ , Eu3 þ microarchitectures with an average diameter of 1.5 μm are successfully prepared by a hydrothermal method at 180 °C for 12 h. The Dy3 þ single-doped samples can be excited by 389 nm and 453 nm, and show emission bands centered at 485, 576 and 665 nm. The optimal doping content of Dy3 þ is 7 mol%. The Dy3 þ / Eu3 þ co-doped samples show the characteristic emissions of Dy3 þ and Eu3 þ under 389 nm excitation. The excitation, emission spectra and decay curves data indicate that there exists an energy transfer from Dy3 þ to Eu3 þ in KLa(MoO4)2 host. The energy transfer mechanism is calculated to be quadrupole–quadrupole (q–q) interaction according to Dexter's theory. The emission color of KLa0.93 y(MoO4)2:0.07Dy3 þ , yEu3 þ samples can be tunable from yellowish-white, white to reddish-white by simply adjusting the content of Eu3 þ . Moreover, the tunable photoluminescence is also realized by employing different excitation wavelengths. All results suggest that the as-prepared samples can be potentially applied in the fields of white light diodes, full-color displays and optoelectronic devices.

Acknowledgments This work is financially supported by the National Natural Science Foundation of China (No. 51272148) and the Natural Science Foundation of Shaanxi Province (2014JM6237).

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