Red-emitting enhancement of Bi4Si3O12:Sm3+ phosphor by Pr3+ co-doping for White LEDs application

Red-emitting enhancement of Bi4Si3O12:Sm3+ phosphor by Pr3+ co-doping for White LEDs application

Author’s Accepted Manuscript Red-emitting enhancement of Bi4Si3O12:Sm3+ phosphor by Pr3+ co-doping for White LEDs application Yuqiao Shen, Kehui Qiu, ...

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Author’s Accepted Manuscript Red-emitting enhancement of Bi4Si3O12:Sm3+ phosphor by Pr3+ co-doping for White LEDs application Yuqiao Shen, Kehui Qiu, Wentao Zhang, Yu Zeng www.elsevier.com/locate/ceri

PII: DOI: Reference:

S0272-8842(17)30668-5 http://dx.doi.org/10.1016/j.ceramint.2017.04.066 CERI15046

To appear in: Ceramics International Received date: 23 February 2017 Revised date: 7 April 2017 Accepted date: 10 April 2017 Cite this article as: Yuqiao Shen, Kehui Qiu, Wentao Zhang and Yu Zeng, Redemitting enhancement of Bi4Si3O12:Sm3+ phosphor by Pr3+ co-doping for White LEDs application, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2017.04.066 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Red-emitting enhancement of Bi4Si3O12:Sm3+ phosphor by Pr3+ co-doping for White LEDs application

Yuqiao Shena, Kehui Qiub*, Wentao Zhanga,c, Yu Zenga

a

College of Materials and Chemistry & Chemical Engineering, Chengdu University of

Technology, Chengdu 610059, China b

Institute of Materials Science and Technology, Chengdu University of Technology,

Chengdu 610059, China c

Mineral Resources Chemistry Key Laboratory of Sichuan Higher Education

Institutions, Chengdu 610059, China

*Corresponding author. Institute of Materials Science and Technology, Chengdu University of Technology, Chengdu, Sichuan, 610059, China, Tel.: +86 2884079014; [email protected]com(K. Qiu)

Abstract In this account, Bi4Si3O12:Sm3+ and (Bi4Si3O12:Sm3+, Pr3+) red phosphors were prepared by solution combustion method fueled by citric acid at 900 oC for 1 hour. The effects of co-doping Pr3+ ions on red emission properties of Bi4Si3O12:Sm3+ phosphors, as well as the mechanism of interaction between Sm3+ and Pr3+ ions were investigated by various methods. X-ray diffraction (XRD) and Scanning electron

microscopy (SEM) revealed that smaller amounts of doped rare earth ions did not change the crystal structure and particle morphology of the phosphors. The photoluminescence spectroscopy (PL) indicated that shape and position of the emission peaks of (Bi4Si3O12:Sm3+, Pr3+) phosphors excited at λex=403 nm were similar to those of Bi4Si3O12:Sm3+ phosphors. The strongest emission peak was recorded at 607 nm, which was attributed to the 4G5/2→6H7/2 transition of the Sm3+ ion. The photoluminescence intensities of Bi4Si3O12:Sm3+ phosphors were significantly improved by co-doping with Pr3+ ions and were maximized at Sm3+ and Pr3+ ions doping concentrations of 4mol% and 0.1mol%, respectively. The characteristic peaks of Sm3+ ions were displayed in the emission spectra of (Bi4Si3O12:Sm3+, Pr3+) phosphors excited at respectively λex=443 nm and λex=481 nm (Pr:3H4→3P2, 3

H4→3P0). This indicated the existence of Pr3+→Sm3+ energy transfer in

(Bi4Si3O12:Sm3+, Pr3+) phosphors.

Keywords: Luminescence properties; Red emitting enhancement; Phosphors; Energy transfer

1. Introduction LEDs have widely been used as solid state lighting sources, including in LED display and LED lighting [1,2]. Currently, the applications of white light from combined blue emission of blue chip and yellow light emitted by YAG:Ce3+ have been limited in many fields due to the lack of red light harmonic [3,4]. Thus, the lack

of red phosphors for long term constitutes a serious obstacle to the development of advanced LED-based technologies. Therefore, one way for developing practical white LEDs is to search for both high efficiency and quality red phosphors to improve color rendering [4]. A series of red phosphor have been investigated, such as nitrides, silicates, sulfides, and molybdates et al [5-8]. Silicates have been extensively studied due to their excellent chemical and thermal stability, low-cost, available raw materials, and stable crystal structure [9,10]. Among silicates, Bi4Si3O12 has attracted increasing attention due to its crystal structure. As shown in Fig. 1, the Bi4Si3O12 crystal belongs to the cubic crystal system with the space group of I 43d (a=b=c=10.291Å). When rare earth ions replace Bi3+ in the crystal lattice, a rigid coordination lattice environment with surrounding 12 oxygen coordination sites is formed. This yields a strong crystal field with more conducive ability to light [11,12]. Therefore, bismuth silicates were selected as the matrix of phosphor. Meanwhile, our previous studies dealing with Bi4Si3O12:Sm3+ phosphor revealed that the photoluminescence intensity of Bi4Si3O12:Sm3+ phosphor was slightly stronger than that of Bi4Si3O12:Eu3+ phosphor. Furthermore, The excitation wavelength range of Sm3+ was located at 300-500nm, which was very suitable for emitting white light working LED chip excited by near ultraviolet or blue light, while, the emission wavelength of Sm3+ spanned the orange-red wavelength range [13]. Therefore, Sm3+ ions were selected as activators for the synthesis of red phosphor because not only they have better red fluorescence performance but also lower manufacturing cost.

In 2011, Zhang et al. [14] synthesized Bi4Si3O12:Eu3+ red phosphor with bismuth silicate as a matrix using high-temperature solid phase method [14]. Other studies have then followed by Bi4Si3O12:Dy3+ yellow phosphor and (Bi4Si3O12:Dy3+, Tb3+) white phosphor both published in 2014 using the sol-gel method [15,16]. In addition, our team also synthesized Bi4Si3O12:Sm3+ red phosphor by sol-gel method in 2016 [17]. So far, the preparation of Bi4Si3O12 phosphor by solution combustion method fueled by citric acid has not yet been reported. The numerous advantages of rare earth Sm3+ and Bi4Si3O12 crystal could lead to Bi4Si3O12:Sm3+ phosphor red emission with better performance. In this paper, Bi4Si3O12:Sm3+ red phosphor samples were prepared by solution combustion route fueled by citric acid. The red-light emission properties of Bi4Si3O12:Sm3+ phosphor were further improved by co-doping with Pr3+ ions. The relationship of interaction between Sm3+ and Pr3+ ions in (Bi4Si3O12:Sm3+, Pr3+) phosphor was systematically examined. 2. Experimental 2.1 Materials preparation Bi4Si3O12 red phosphors were prepared by the solution combustion method fueled by citric acid using Bi(NO3)3·5H2O(AR), C6H8O7, Si (OC2H5) (99.99%) and 4 Pr2O(99.99%) as raw materials. The first step consisted of weighing all raw materials 3 according to the stoichiometric ratios, and then Si(OC2H5)4 with distilled water and absolute ethanol were mixed. The pH of the solution was adjusted to pH 2 using concentrated nitric acid. In the second step, a solution of Sm(NO3)3 and Pr(NO3)3 was

obtained by dissolving Sm2O3 and Pr2O3 in concentrated HNO3, and then Bi(NO3)3·5H2O was dissolved in a suitable amount of water to obtain a nitrate solution. Finally, C6H8O7 and all other mentioned mixtures were evenly mixed, and then heated and stirred until a colorless and transparent solution was formed. The resulting transparent solution was rapidly transferred into a crucible, placed in a muffle preheated furnace then ignited at 900 oC for 1 hour to obtain products with uniform dispersion and regular morphology. 2.2 Characterization The crystal structures of the phosphors were examined by a DX-2007 X-Ray diffraction instrument from Dandong (XRD) with Cu-Kɑ radiation set to 40kV and 30 mA at room temperature. The experimental parameters were collected in the 2θ range from 10° to 80 °. The morphology of the phosphors was characterized by Scanning electron microscopy (SEM, Inspect F150). The fluorescence intensity of the phosphor was qualitatively analyzed on a Hitachi F-4600 fluorescence spectrometer using xenon lamp and ordinary photomultiplier. The scanning wavelength range was set between 200 ~ 800nm, the voltage was 400V, and the scanning speed was 1200nm/min.

3. Results and discussion Figure 2 shows the XRD patterns of Bi4Si3O12, Bi4Si3O12:0.04Sm3+ and (Bi4Si3O12:0.04Sm3+, 0.001Pr3+) phosphors prepared by the solution combustion method fueled by citric acid at 900 oC for 1 h. It can be seen that un-doped,

single-doped Sm3+ and co-doped ions induced no obvious miscellaneous peaks when compared to JCPDS cards 35-0215. This indicated that small amounts of doping rare earth ions did not induce changes to the basic crystal structure of Bi4Si3O12. The obtained peaks with sharp diffraction features and high intensities confirmed the good crystallinity of the prepared samples. The radius of Sm3+(0.0964Å) and Pr3+(0.099Å) are very close to that of Bi3+(0.096Å), and all the three valences had positive trivalent while the ionic radius of Si4+ was 0.026Å. Therefore, the trivalent bismuth ion sites could be the most likely to be occupied by the same trivalent samarium and praseodymium ions in Bi4Si3O12. The XRD patterns of Bi4Si3O12:0.04Sm3+ and (Bi4Si3O12:0.04Sm3+, 0.001Pr3+) phosphors were refined by the software Jade, and the lattice constants were estimated as 10.3320Å and 10.3321Å, respectively. This showed that lattice constants of the doped samples were slightly larger, which may be due to the slightly bigger radii of Sm3+ and Pr3+ ions with respect to the slightly smaller radii of Bi3+ ions. Fig. 3 depicts the SEM images of Bi4Si3O12, Bi4Si3O12:0.04Sm3+ and (Bi4Si3O12:0.04Sm3+, 0.001Pr3+) phosphors prepared by the combustion method. All the phosphor samples revealed large numbers of agglomerated lumpy particles and small amounts of porous particles. Both these particles were, in turn, attached to other smaller particles. The porous particles shown in some samples were probably created by gas molecules escaping from the interior of the samples when the wet gel was ignited at 900℃.

The polyhedron morphology of the lumpy samples caused a relatively low diffuse reflectance and led to enhanced absorption and excitation. The latter was beneficial to improve the fluorescence intensity of the phosphors. Moreover, phosphor layers with high packing density and low surface scattering were easily formed for manufacturing LEDs, because all phosphors were in the micrometer grade. The photoluminescence excitation (PLE) and PL spectra of Bi4Si3O12:xSm3+(1≤ x ≤6mol%, λem=607 nm, λex= 403 nm) phosphors prepared by the solution combustion route fueled by citric acid are shown in Fig. 4. The broad excitation spectra ranged from 320-500 nm. The strongest excitation peak was recorded at 403 nm and the other excitation peaks appeared at 344 nm, 373 nm, and 477 nm. These four peaks were respectively assigned to 6H5/2→4F7/2, 6H5/2→4H9/2, 6H5/2→4D7/2 and 6

H5/2→4I13/2 transitions created by the f-f transition of Sm3+ ion in the host lattice

[18]. The intensity of the excitation peaks gradually increased as the concentration of Sm3+ ions rose, and strongest peaks appeared at Sm3+ ions concentration of 4mol%. As shown in figure 4, samples with different Sm3+ doping concentrations basically generated similar peak shapes and peak positions. The main emission peaks were located at 566 nm, 607 nm and 656 nm, assigned to the 4G5/2→6HJ (J=5/2, 7/2, 9/2) transitions of Sm3+ [18]. The strongest emission peak was recorded at 607 nm, which was stronger than that at 654 nm. This indicated that the emission of Sm3+ ion was dominated by 4G5/2→6H7/2 magnetic dipole transition located strictly in the inversion center of symmetry. As Sm3+ concentration rose, the luminescence intensity of the samples increased and reached strongest values at the Sm3+ ions concentration

of 4mol%. However, the luminescence intensity of Bi4Si3O12:Sm3+ phosphor gradually decreased when the concentration exceeded 4mol% of Sm3+ due to concentration quenching [19]. At Sm3+ concentrations below 4mol%, the average distance between adjacent Sm3+ ions in the matrix lattice became longer, and most energies of Sm3+ absorption was released by radiation transition. At concentrations exceeding 4mol%, the average distance between adjacent Sm3+ ions became shorter and the probability of non-radiative transition increased. This led to cross-relaxation between ions, which weakened the luminescence intensities. The critical distance Rc for the quenching of phosphor concentration can be calculated by Eq. (1) [20]: 1/ 3

 3V   Rc  2    4X c N 

(1)

where V represents the unit cell volume, Xc indicates the critical concentration of Sm3+ ions, and N is the number of cations in a Bi4Si3O12 unit cell. The other values are: V=1102.94Å3, Xc=0.04, and N=4. Eq. (1) estimated the critical distance of concentration quenching to 23.61Å. Therefore, the concentration quenching of Sm3+ ions in Bi4Si3O12:Sm3+ phosphor was dominated by multipolar interactions. It is well known that the combustion method has the characteristics of simple process and high rate of synthetic product. The preparation of Bi4Si3O12:Sm3+ phosphor by combustion method has a much shorter process cycle. According to the related research made by our team, the luminescence intensities of the phosphors

prepared by sol-gel method and combustion method both reach maximum when x=0.04 [17]. In addition, the fluorescence intensity of Bi4Si3O12:0.04Sm3+ phosphor prepared by combustion method is similar to that of Bi4Si3O12:0.04Sm3+ phosphor prepared by sol-gel method in Fig. 5. Fig. 6 illustrates the PLE and PL of Bi4Si3O12:Pr3+ phosphors at λem=609 nm and λex=443 nm. The excitation peak of Pr3+ ions (443 nm:3H4→3P2, 468 nm:3H4→1I6, 481nm:3H4→3P0)can clearly be observed from 400-500 nm [21]. The strongest excitation peak was located at 443 nm, indicating that samples in the blue region can effectively be excited to match the blue LED chip. In the range of 600-700nm, the observed emission peaks were recorded at 609 nm and 644 nm, assigned to 3P0→3H6 and 3P0→3F2 of Pr3+ ion [21], respectively. The strongest emission peak appeared at 609 nm. In order to improve the fluorescence properties of phosphors, one common used method is by co-doping with other ions. This method allows the interaction between the ions to yield better luminescence properties of the activator. The emission peaks of Bi4Si3O12:Pr3+ phosphor were excited in the range of 550 nm-700 nm at λex = 443 nm, which was overlapped with the emission peak of Bi4Si3O12:Sm3+ phosphor in the range of 500 nm-750 nm. This strengthened each other’s red light emission intensity, as shown in Fig. 7. In addition, Fig. 11 indicated that excited levels of Pr3+ and Sm3+ ions were close to each other, as 3P0 and 4I9/2. Therefore, it could theoretically be speculated that samples co-doped with Pr3+ and Sm3+ ions would not only produce a fluorescent emission superposition due to some similar energy levels but also induce

energy transfer between Pr3+ and Sm3+. In order to verify the above theoretical suggestion, the effect of different concentrations of Pr3+ on the red light emission properties of Bi4Si3O12:0.04Sm3+ phosphors was investigated. The PLE values of (Bi4Si3O12:0.04Sm3+, xPr3+) (0≤x≤0.3mol%) phosphors doped with different concentrations of Pr3+ ions at λem = 607 nm are gathered in Fig. 8. The excitation peaks did not obviously change when compared to Fig. 4, and the strongest excitation peak was recorded at 403 nm. Figure 9 depicts the PL of (Bi4Si3O12:0.04Sm3+, xPr3+) phosphors excited under λex = 403 nm, where the emission peaks pattern and peaks positions also did not undergo obvious changes when compared to Fig. 4. This indicated that the Sm3+ ion was still the luminescence center. In addition, Fig. 9 revealed that the luminescence intensity of (Bi4Si3O12:Sm3+, Pr3+) phosphors first increased and then declined as the Pr3+ ion concentration gradually rose. This suggested that there may be energy transfer between Sm3+ and Pr3+ ions. The luminescence intensity reached a maximum at x=0.1mol%. On the one hand, the decrease in luminescence intensity could be caused by the excessive concentration of doping ions, and then the transfer rate of energy between the centers that became faster. This increased the probability of the luminescence center closer to the quenching center, thus concentration quenching was reached much easily. Many similar energy levels between Pr3+ and Sm3+ ions also exist, thus energy inversion (Sm3+→Pr3+) may occur when the Pr3+ concentration was very high in (Bi4Si3O12:Sm3+, Pr3+) phosphor. The latter might be a reason for the decrease in the

luminescent intensity of (Bi4Si3O12:Sm3+, Pr3+) phosphors. To clarify the interaction relationship between both Sm3+ and Pr3+ ions, further experiments were carried out on (Bi4Si3O12:Sm3+, Pr3+) phosphor. Fig. 10 illustrates the PL of Bi4Si3O12:Sm3+ and (Bi4Si3O12:Sm3+, Pr3+) phosphors excited at λex = 443 nm (3P0→3H6) and λex = 481 nm (3P0→3F2), respectively. All phosphors depicted the characteristic peaks of Sm3+ ions. However, the emission intensities of (Bi4Si3O12:Sm3+, Pr3+) phosphors were significantly stronger than that of Bi4Si3O12:Sm3+ phosphors under the same excitation conditions. This indicated that part of the energy of Pr3+ ions was transferred to Sm3+ ions (Pr3+→Sm3+), confirming the previous theoretical speculations. On the other hand, the characteristic emission peaks of Pr3+ ions were not observed, which was probably due to the overlapping in emission peaks of Sm3+ ions. Fig. 10 revealed that the intensity of the emission peak of (Bi4Si3O12:Sm3+, Pr3+) phosphor excited at λex = 481 nm was much greater than that excited at λex = 443 nm. Fig. 11 suggested that at λex = 443 nm, the excited state 3P2 level first relaxed to the lower energy level 3P0 by radiation, and then the energy was transferred to the 4I9/2 level of Sm3+. During this process, energy loss was induced from the 3P2 level to the low energy level 3P0. At excitation λex = 481nm, the energy was directly transferred from 3P0 level to 4I9/2 level of Sm3+. The energy transfer efficiency was much higher than that of the former. Therefore, the emission intensities of the peaks in (Bi4Si3O12:Sm3+, Pr3+) phosphor excited at λex = 481nm were obviously greater than those excited at λex = 443nm.

The efficiency of energy transfer from Pr3+ ions to Sm3+ ions could be calculated by Eq. (2) [22]:

T  1 

Is I so

(2)

where ηT represents the energy transfer efficiency, and Is and Iso are the single-doped and co-doped luminescence intensities of the phosphors. Eq. (2) revealed that at doping Pr3+ ion levels of 0.1mol%, the energy transfer efficiency reached a maximum of 59.3%, indicating that the energy transfer from Pr3+ to Sm3+ ions in (Bi4Si3O12:0.04Sm3+, 0.001Pr3+) phosphors was still effective. The possible mechanism of the energy transfer process between Pr3+ and Sm3+ ions is depicted in Fig. 11. The electrons of Pr3+ in the ground state 3H4 were first excited by near UV or blue light, and then the stimulated electrons from the excited state 3P2 level were relaxed to the lower energy level 3P0 by nonradiative relaxation. Part of the energy released by radiation to the ground state of Pr3+ ions formed the characteristic emission peaks (609 nm:3H6 and 644 nm:3F2). Meanwhile, the other part of Pr3+ ions energy was transmitted to the 4I9/2 level of Sm3+ without radiation and then was released to the 4G5/2 level to finally return to the ground state 6HJ (J =5/2, 7/2, 9/2) by radiation. This process may enhance the characteristic emission of Sm3+ and reduce the fluorescence emission intensity of Pr3+ ions. For photoluminescence materials, a chromaticity diagram is usually introduced to qualitatively describe and analyze the luminescent color as all colors can be characterized by the three primary basic colors. Therefore, the luminescent color of rare earth luminescent materials can be expressed by chromaticity coordinates in the

chromaticity diagram. The calculated color coordinates of Bi4Si3O12:0.04Sm3+ and (Bi4Si3O12:0.04Sm3+, 0.001Pr3+) using a CIE software were recorded as (0.578,0.420) and (0.585,0.414), respectively. After co-doping with Pr3+ ions, the color coordinates were closer to the red region (Fig. 12). From the practical point of view, the co-doping Pr3+ ions made Bi4Si3O12:Sm3+ phosphors more suitable for white LEDs.

4. Conclusions Bi4Si3O12:Sm3+ and (Bi4Si3O12:Sm3+, Pr3+) phosphors were prepared by the solution combustion method fueled by citric acid at 900 oC for 1h. Not only the combustion method can shorten the preparation period of the phosphor, but also the fluorescence intensity of the phosphors prepared by this method is similar to that prepared by sol-gel method. In addition, the data showed that Bi4Si3O12:Sm3+ phosphors still exhibited the characteristic emission of Sm3+ ion after co-doping with Pr3+ ions under λex = 403 nm. The strongest emission peak was recorded at 607 nm, which was attributed to the 4G5/2→6H7/2 transition of Sm3+ ion. Co-doping with Pr3+ ions led to energy transfer from Pr3+ ions to the Sm3+ ions and promoted their red emission properties when irradiated by near UV or blue light. The luminescence intensities and energy transfer efficiencies of the phosphors reached their maximum values at Sm3+ and Pr3+ ion concentrations of 4mol% and 0.1mol%, respectively. In addition, the color coordinates (0.585, 0.414) of (Bi4Si3O12:Sm3+, Pr3+) phosphors shifted slightly toward the red region relative to Bi4Si3O12:Sm3+ phosphor color coordinates (0.578, 0.420). The latter would be better when applied to white LEDs.

Acknowledgments This work is supported by the Key Scientific and Technological Research and Development Program (Grant no. 2017GZ0400) in Sichuan Province's Peoples Republic of China.

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Fig. 1: The basic structure of the Bi4Si3O12 crystal.

Fig. 2: XRD patterns of the phosphor samples (a), (b) and (c) annealed at 900 ◦C for 1 h.

Fig. 3 : SEM images of (a)Bi4Si3O12, (b) Bi4Si3O12:0.04Sm3+, and (c) (Bi4Si3O12:0.04Sm3+, 0.001Pr3+) phosphors.

Fig. 4: PLE and PL of Bi4Si3O12:xSm3+(1≤ x ≤6mol%) phosphors under λem=607 nm and λex=403nm.

Fig. 5 PL of Bi4Si3O12:0.04Sm3+ phosphors prepared by the sol-gel method and the combustion method

Fig. 6: PLE spectrum of Bi4Si3O12:Pr3+ phosphor under λem=609 nm and PL spectrum of Bi4Si3O12:Pr3+ phosphor under λex=443 nm.

Fig. 7: Emission spectrum of Bi4Si3O12:Sm3+ phosphor at λem=403 nm and Bi4Si3O12:Pr3+ phosphor at λex=443 nm.

Fig. 8: PLE spectra of (Bi4Si3O12:0.04Sm3+, xPr3+) (0≤ x ≤ 0.3mol%) phosphors under λem=607 nm.

Fig. 9: PL spectra of (Bi4Si3O12:0.04Sm3+, xPr3+) (0≤ x ≤ 0.3mol%) phosphors under λex=403nm. The inset shows the relationship between the emission intensity at 607 nm and the doping concentration of Pr3+ ions.

Fig. 10: PL spectra of Bi4Si3O12:0.04Sm3+ and (Bi4Si3O12:0.04Sm3+, 0.001Pr3+) phosphors under λex=443 nm and λex=481nm.

Fig. 11: Energy level diagrams of Sm3+ and Pr3+ ions, and the energy transfer process from Pr3+ to Sm3+ in (Bi4Si3O12:0.04Sm3+, 0.001Pr3+) phosphor.

Fig.12: CIE 1931 chromaticity diagram for (a) Bi4Si3O12:0.04Sm3+ phosphor and (b) (Bi4Si3O12:0.04Sm3+, 0.001Pr3+ ) phosphor under λex=403 nm.