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Enhanced luminescence of CaSb2O6:Bi3+ blue phosphors by efficient charge compensation
Materials Science in Semiconductor Processing 41 (2016) 265–269
Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Enhanced luminescence of CaSb2O6:Bi3 þ blue phosphors by efficient charge compensation Shiyue Yao, Liumin Chen, Yanlin Huang, Weifeng Li n College of Chemistry, Chemical engineering and Materials Science, Soochow University, Suzhou 215123, China
art ic l e i nf o
a b s t r a c t
Article history: Received 17 June 2015 Received in revised form 30 August 2015 Accepted 20 September 2015
This paper reported an enhanced photoluminescence of CaSb2O6:Bi3 þ by efficient charge compensation. Charge compensated CaSb2O6:Bi3 þ ,M þ (M¼ Li, Na and K) phosphors were prepared using a co-precipitation technique followed by heat-treatment. The structure and morphology of the as-prepared CaSb2O6:Bi3 þ ,M þ samples were characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM). The results revealed that the obtained CaSb2O6:Bi3 þ ,M þ samples are hexagonal crystal structure and this structure was retained regardless of co-doping by Li þ , Na þ or K þ . All samples showed sphere-like shape with particle size of 40–80 nm. The optical properties of products were studied by UV– vis diffuse reflectivity, photoluminescence spectra and luminescence decay measurements. Under the excitation of 336 nm light, all of the samples exhibited a strong blue emission peaking around 437 nm, which is attributed to the 3P1–1S0 transition of the Bi3 þ ion. It was found that the charge compensation has significant effect on the photoluminescence properties of CaSb2O6:Bi3 þ and the best luminescence properties have been achieved for CaSb2O6:0.75Bi3 þ ,0.75 Na þ . The mechanism for the enhancement of the blue emission has also been studied in detail. Our results suggested that the optical properties of oxide nanostructures can be tailored through co-doping with aliovalent ions and the favorable luminescence properties of CaSb2O6:Bi3 þ ,Na þ make it potential for lighting and display applications. & 2015 Elsevier Ltd. All rights reserved.
Keywords: CaSb2O6 Bi3 þ Luminescence Nanoparticles Charge compensation
1. Introduction Recently, phosphors applied in lighting and display systems, are required to be improved in terms of optical spectrum, luminous efficiency, thermal stability, long life and fine particle size powders. Therefore, considerable attention has been paid on searching for new materials and synthesis techniques to improve the properties of phosphors. Generally speaking, the luminescence properties of phosphors are related not only to optical active ions but also to host lattice. Trivalent bismuth cation with outer 6s2 electronic configuration is evidenced to be an interesting luminescence activator and the spectroscopic properties of Bi3 þ in a variety of host lattices have been extensively studied [1–5]. The ground state of the free Bi3 þ ion is 1S0 while the 6s16p1 excited states are 1P0, 3P1, 3P2 and 1P1 in order of increasing energy. Theoretically, the 1S0-3P1 transition is an allowed electric dipole transition and the 1S0-3P0,1,2 transitions are forbidden. However, 1 S0-3P1,2 transitions become more allowed due to spin–orbit coupling. The observed Bi3 þ emission band at room temperature is assigned to the 3P1-1S0 transition owing to the lower energy of n
Corresponding author. E-mail address: [email protected] (W. Li).
http://dx.doi.org/10.1016/j.mssp.2015.09.015 1369-8001/& 2015 Elsevier Ltd. All rights reserved.
3
P1 than that of 1P1 [6,7]. Furthermore, the emission performances of Bi3 þ show a strong dependence on the environmental conditions because its outer s2 electron orbit is not shielded. As a matter of fact, the emission peaks of Bi3 þ can appear in the ultraviolet, blue, green, or even red wavelength regions with different matrices. CaSb2O6, one of ternary antimonites, has a hexagonal crystal structure, which can be viewed as two-dimensional infinite sheets of edge-sharing SbO6 octahedra alternating with layers of Ca2 þ ions. The CaSb2O6 compound is optical transparent (band gap: 3.59 eV) [8] and chemically stable. Therefore, the CaSb2O6 could be a desirable luminescence host material and the luminescence properties of Bi3 þ -doped CaSb2O6 have been reported [9–11]. However, the visible emission MSb2O6:Bi (M¼ Ca, Sr) always quenched at room temperature due to the volatility of Bi2O3 and the tendency of Bi3 þ to be oxidized to Bi5 þ in the conventional sintering process [12]. It is believed that lattice modification is one of the most effective strategies for improving luminescent properties of already known compounds [13,14]. Such a lattice modification can be easily realized by co-doping, which can modify the local site symmetry of activator ion or induce defect, and then change radiation relaxation process. Many literatures has reported that alkali metal ions like Li þ , Na þ K þ are favorable lattice modifiers to enhance
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S. Yao et al. / Materials Science in Semiconductor Processing 41 (2016) 265–269
luminescence because of their low oxidation states and distinct ionic radii [15–18]. In previous work, we developed CaSb2O6:Bi3 þ blue phosphors using a co-precipitation method [9]. These phosphors are stable and show good luminescent properties at room temperature. The purpose of this work is to improve the luminescent properties of Bi3 þ -activated CaSb2O6 phosphors via co-doping with monovalent ions, such as Li þ , Na þ and K þ . The effect of alkali metal ions codoping on the structure, morphology and optical properties in CaSb2O6: Bi3 þ system has been studied and possible mechanism has been proposed.
2. Experimental 2.1. Preparation of CaSb2O6 nanocrystals All chemicals, including SbCl3, CaCl22H2O, Bi2O3, NaOH, Na2C2O4, Li2CO3, Na2CO3, and K2CO3 (Sinopharm Chemical Reagent Co. Ltd, China) were analytical grade and used without further purification. The deionized water was used throughout the experiment. CaSb2O6:xBi3 þ (0.25 rx r2 mol%) and CaSb2O6:0.75Bi3 þ , 0.75M þ (M ¼Li, Na and K) precursors were obtained by a simple chemical co-precipitation method. Typically Bi2O3 and SbCl3 were dissolved in dilute hydrochloric acid with stirring and moderate heating to obtain BiCl3 and SbCl3 stock solutions, respectively. Meanwhile, CaCl2 solution was prepared by dissolving CaCl22H2O in deionized water. After that, stoichiometric BiCl3, SbCl3 and CaCl2 were mixed together to form a clear and homogeneous solution. The mixed solution was dropped in a precipitator (NaOH–Na2C2O4 solution) and then white precipitation was obtained. After aging, filtration, washing and drying, the white precursor was obtained. 1) The precursor was calcined at 1000 °C for 4 h to obtain CaSb2O6: xBi3 þ products. 2) In a synthesis procedure of CaSb2O6:0.75Bi3 þ , 0.75M þ , M2CO3 (atom ratio: Bi:M ¼1:1) was introduced in the above precursor, and blended and milled thoroughly. Finally, the mixture was calcined in muffle at 1000 °C for 4 h. 2.2. Characterization The structure of the samples was characterized by XRD on a X’Pert Pro MPD diffractometer (Panalytical, Holland) using CuKα radiation (λ ¼1.5406 Å). The SEM images were taken on Hitachi s-4600 (Japan). The photoluminescence (PL) spectra were obtained on an F-2500 fluorescence spectrometer (Hitachi, Japan) at room temperature. The diffuse reflectance spectra (DRS) of samples were obtained on a UV-3150 spectrophotometer (Shimadzu, Japan) using BaSO4 as a reference standard in the wavelength range of 200–800 nm. The fourth harmonic (266 nm) of a pulsed Nd:YAG laser (Spectron Laser Sys. SL802G) was employed to examine the luminescence decay and decay profiles were recorded with a LeCloy 9301 digital storage oscilloscope in which the signal was fed from PMT.
3. Results and discussion The XRD patterns of the as-obtained undoped and doped CaSb2O6 are shown in Fig. 1. It can be clearly seen that all the samples have the similar XRD patterns and their diffraction peaks can be perfectly indentified as pure hexagonal structure of CaSb2O6 with the space group of P-31m (JCPDS card No. 46-1496). There is no diffraction peak corresponding to any starting materials or other impure phases, indicating that the samples obtained are single phase and doping at the investigated concentrations do
Fig. 1. XRD patterns of CaSb2O6, CaSb2O6:0.75Bi and CaSb2O6:0.75Bi,0.75M (M¼ Li, Na and K).
not change the crystal structure. Moreover, the good crystalline of the samples, confirmed by the strong and sharp diffraction peaks in Fig. 1, is advantageous to luminescence. Considering the similarity of the effective ionic radii between Ca2 þ ions (1.00 Å) and Bi3 þ ions (1.03 Å), it is rational that the dopants are preferably substituted into the Ca2 þ sub-crystal sites. According to the Scherrer equation, the average crystallite sizes of the nanocrytals were determined to be in the range of 54–66 nm. Fig. 2 displays the SEM images of undoped and doped CaSb2O6 samples. From the SEM micrograph in Fig. 2a, it is obvious that the particles of pure CaSb2O6 are roughly spherical with an average diameter of about 70 nm. The SEM images (Fig. 2b) of CaSb2O6:0.75Bi give the similar morphological properties to those of pure CaSb2O6. With the co-doping by Bi3 þ and M þ , the particle size is slightly decreased (Fig. 2c–e), and their morphology is regular and uniform. The result is consistent with those of XRD measurements (Fig. 1). The diffuse reflectance spectra of pure and doped CaSb2O6 samples are given in Fig. 3. All the samples show a remarkable drop in reflection around 300 nm, corresponding to the valenceto-conduction band transitions of the CaSb2O6 host. The intense reflection in the visible spectral range is in agreement with the observed white daylight color for undoped CaSb2O6. It is thereby expected that CaSb2O6:Bi,M could achieve high luminescent efficiency owing to the high reflectance in emission region. On the other hand, the doped CaSb2O6 samples show different optical absorption behavior. A broad absorption band extending from 265 nm to 355 nm can be clearly observed from the absorption spectra of the CaSb2O6:0.075Bi and CaSb2O6:0.75Bi,0.75M samples. Apparently, the absorption band should be attributed to the Bi3 þ -related absorption due to the absence of them in the pure CaSb2O6 sample. Fig. 4 shows typical excitation and emission spectra of CaSb2O6:0.75Bi phosphor. The sample displays a broad emission band (right of Fig. 4) in the wavelength range of 370-550 nm with a peak center at about 437 nm, corresponding to 3P1–1S0 transition of Bi3 þ . It is believed that Bi3 þ will replace Ca2 þ on the single crystallographic site available in CaSb2O6 with some kind of charge compensation, indicating that mainly one emission Bi3 þ center will present in CaSb2O6:0.75Bi. However, from Fig. 4 one can see that the emission curve of CaSb2O6:0.75Bi is asymmetric, which is ascribed to the disturbance of defect, such as cation vacancies (VCa )‵‵ [9]. On the basis of our previous work, the doping concentration of activator is an important factor for luminescence materials and the concentration dependent emission intensity at 437 nm of CaSb2O6:xBi was given in the inset of Fig. 4. It can be
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Fig. 2. SEM images of (a) pure CaSb2O6, (b) CaSb2O6:0.75Bi, (c) CaSb2O6:0.75Bi,0.75Li, (d) CaSb2O6:0.75Bi,0.75Na and (e) CaSb2O6:0.75Bi,0.75K.
seen that the optimal Bi3 þ content is 0.75 mol% [9]. The excitation spectrum of CaSb2O6:0.75Bi (left of Fig. 4) consists of three bands, which are attributed to the transition from the ground state of 1S0 to the various excited states of Bi3 þ ion. Among them, the excitation band peaking at 336 nm corresponding to 1S0–3P1 transition is most prominent. The effect of charging compensators, including Li þ , Na þ and K þ ions on the blue emission of CaSb2O6:0.75Bi was investigated and the result is given in Fig. 5. It can be seen from Fig. 5 that the shape and positions are similar in the PL spectra for all the samples. On the other hand, emission intensity has been improved by the introduction of alkali metal ions and the emission intensities of CaSb2O6:0.75Bi,0.75Li, CaSb2O6:0.75Bi,0.75Na and CaSb2O6:0.75Bi,0.75K are about 1.24, 1.67 and 1.42 times higher than that of CaSb2O6:0.75Bi, respectively. It is believed that in the CaSb2O6:0.75Bi sample, the substitution of Bi3 þ for Ca2 þ is
accompanied by the formation of Ca vacancies for charge balance. Therefore, the defect chemical reaction could be described as following equation:
Bi2O3 + CaSb2O6 → 2Bi•Ca + V "Ca + 3O×O
(1)
However, the charge compensation can be realized by (MCa )‵ for Bi,M-doped CaSb2O6, as shown in Eq. (2).
Bi2O3 + M2O + CaSb2O6 → 2Bi•Ca + 2(MCa )‵ + 4O×O
(2)
Generally speaking, vacancy in lattice is the luminescent quenching center [19]. The occurrence of (MCa )‵ will decrease the (VCa)‵‵ content in the CaSb2O6 and thus improve their emission properties. Since Na þ has the similar ionic radius (1.02 Å) to that of Ca2 þ (1.00 Å), it is expected that the incorporation of Na þ will cause the less distortion of crystal lattice compared to Li þ (0.76 Å)
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Fig. 6. Emission decay curves of CaSb2O6:0.75Bi and CaSb2O6:0.75Bi,0.75M (M¼ Li, Na and K) excited by 355 nm. Fig. 3. UV–vis DRS of CaSb2O6, CaSb2O6:0.75Bi and CaSb2O6:0.75Bi,0.75M (M¼ Li, Na and K).
emission upon 355 nm pulsed excitation were examined. Fig. 6 shows the normalized time decay emission at 437 nm for CaSb2O6:0.75Bi and CaSb2O6:0.75Bi,0.75M. All the samples exhibit non-exponential feature, suggesting that there exist more than one relaxation process. Moreover, the non-exponential character becomes more evident with the co-doping of M þ . The non-exponential profiles of luminescence decay should be associated with the cross-relaxation or the energy transfer process between the activator ions or the induced defects, which make the decay change and lead to non-exponential decay curve for CaSb2O6:Bi system. In addition, the decay curves can be well-reproduced by a double-exponential function as
I = A1exp( − t /τ1) + A2 exp( − t /τ2)
Fig. 4. Excitation (left) and emission (right) spectra of CaSb2O6:0.75Bi. Inset: plot of the emission intensity vs. Bi-doping content.
(3)
where I is the emission intensity, A1 and A2 are constants, and τ1 and τ2 are luminescent lifetimes. To determine the magnitudes of decay times, the average lifetimes were estimated using the following equation:
τaver =
A1τ12 + A2 τ22 A1τ1 + A2 τ2
(4)
Therefore, the τaver values were obtained and listed in Table 1. In the CaSb2O6 system, the decay time of 291 ns is close to that of other Bi3 þ -doped phosphor [20]. However, the average lifetime gradually decreases with the co-addition of M þ ions and the shortest decay time occurs for CaSb2O6:0.75Bi,0.75Li sample. As discussed above, the emission behavior of CaSb2O6:0.75Bi,0.75M (Fig. 5) indicated that there is energy transfer in the phosphor and the lattice defects such as (BiCa) and (MCa)’ may play an important role on this energy transfer process. It should be noted that the average distance between nearest neighbor doping ions (Bi3 þ and/ or M þ ) in the CaSb2O6:0.75Bi,0.75M samples is closer than that in the CaSb2O6:0.75Bi due to the higher doping concentration. This causes higher probabilities of the energy transfer and further the shorter lifetime (Fig. 6 and Table 1). On the other hand, the doping of M þ in CaSb2O6 will induce lattice relaxation of crystal because of the different ionic radii between Ca2 þ and M þ . For all the Fig. 5. Emission spectra of CaSb2O6:0.75Bi and CaSb2O6:0.75Bi,0.75M (M¼ Li, Na and K).
and K þ (1.38 Å) in the Bi,M-codoped CaSb2O6. This could be the fact that the CaSb2O6:0.75Bi,0.75 Na exhibits the strongest emission, as shown in Fig. 5. Therefore, Na þ could be the optimal charge compensator in the CaSb2O6:0.75Bi system. In order to elucidate the role that alkali metal ions play on the luminescence properties of Bi3 þ , the decay times of the 437 nm
Table 1 The luminescence decay lifetimes and CaSb2O6:0.75Bi and CaSb2O6:0.75Bi,0.75M.
CIE
chromaticity
coordinates
Samples
τaver (ns)
CIE color coordinates
CaSb2O6:0.75Bi CaSb2O6:0.75Bi,0.75Li CaSb2O6:0.75Bi,0.75Na CaSb2O6:0.75Bi,0.75K
291 183 256 226
(0.1505, 0.0490) (0.1495, 0.0529) (0.1497, 0.0509) (0.1496, 0.0518)
of
S. Yao et al. / Materials Science in Semiconductor Processing 41 (2016) 265–269
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corresponding to 3P1–1S0 transition of Bi3 þ ions and the optimal Bi-doping concentration is determined to be 0.75 mol%. Interestedly, the emission can be notably enhanced by the introduction of alkali ions due to the effective charge compensation. Among them, Na þ ion has the best effect on enhancing luminescence properties and improve the emission intensity to 1.67 times that of CaSb2O6:Bi, which is attributed to the similar ionic radii between Na þ and Ca2 þ . In addition, the decay curves of samples exhibit nonlinear feature and the decay time decreases with the co-doping by alkali ions, indicating multiple relaxation process.
Acknowledgements This work was supported by the Priority Academic Program Development of Jiangsu Higher Education Institutions. Fig. 7. The CIE chromaticity CaSb2O6:0.75Bi,0.75Na. 2þ
diagram
for
CaSb2O6:0.75Bi
and
References 3þ
þ
þ
þ
þ
investigated cations (Ca , Bi , Li , Na and K ), Li has the smallest radii. Therefore, the substitution of Li þ on Ca2 þ crystal site will lead to large lattice relaxation and make crystal lattice smaller. Thereby, one can imagine that the average distance between nearest neighbor doping ions (Bi3 þ and/or Li þ ) in the CaSb2O6:0.75Bi,0.75Li sample becomes closer than those in the other phosphors and contribute to a short decay time. However, further work is necessary. Table 1 gives the Commission Internationale de I’Eclairage (CIE) color coordinates of all samples. With the co-addition of M þ , the color tone changes from (0.1505, 0.0490) of CaSb2O6:0.75Bi to (0.1497, 0.0509) of CaSb2O6:0.75Bi,0.75Na, as shown in Fig. 7. On the other hand, the CIE color coordinates of CaSb2O6:0.75Bi,0.75M change little, which could be attributed to the low concentration of the charge compensator.
4. Conclusions In this study, Bi3 þ -doped and Bi3 þ ,M þ -codoped (M ¼Li, Na, K) CaSb2O6 nanoparticles were successfully prepared by a co-precipitation method and their luminescence properties were investigated in detail. Under the excitation of UV light, CaSb2O6:Bi nano-phosphors exhibit intense blue emission peaking at 437 nm,
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Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Enhanced luminescence of CaSb2O6:Bi3 þ blue phosphors by efficient charge compensation Shiyue Yao, Liumin Chen, Yanlin Huang, Weifeng Li n College of Chemistry, Chemical engineering and Materials Science, Soochow University, Suzhou 215123, China
art ic l e i nf o
a b s t r a c t
Article history: Received 17 June 2015 Received in revised form 30 August 2015 Accepted 20 September 2015
This paper reported an enhanced photoluminescence of CaSb2O6:Bi3 þ by efficient charge compensation. Charge compensated CaSb2O6:Bi3 þ ,M þ (M¼ Li, Na and K) phosphors were prepared using a co-precipitation technique followed by heat-treatment. The structure and morphology of the as-prepared CaSb2O6:Bi3 þ ,M þ samples were characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM). The results revealed that the obtained CaSb2O6:Bi3 þ ,M þ samples are hexagonal crystal structure and this structure was retained regardless of co-doping by Li þ , Na þ or K þ . All samples showed sphere-like shape with particle size of 40–80 nm. The optical properties of products were studied by UV– vis diffuse reflectivity, photoluminescence spectra and luminescence decay measurements. Under the excitation of 336 nm light, all of the samples exhibited a strong blue emission peaking around 437 nm, which is attributed to the 3P1–1S0 transition of the Bi3 þ ion. It was found that the charge compensation has significant effect on the photoluminescence properties of CaSb2O6:Bi3 þ and the best luminescence properties have been achieved for CaSb2O6:0.75Bi3 þ ,0.75 Na þ . The mechanism for the enhancement of the blue emission has also been studied in detail. Our results suggested that the optical properties of oxide nanostructures can be tailored through co-doping with aliovalent ions and the favorable luminescence properties of CaSb2O6:Bi3 þ ,Na þ make it potential for lighting and display applications. & 2015 Elsevier Ltd. All rights reserved.
Keywords: CaSb2O6 Bi3 þ Luminescence Nanoparticles Charge compensation
1. Introduction Recently, phosphors applied in lighting and display systems, are required to be improved in terms of optical spectrum, luminous efficiency, thermal stability, long life and fine particle size powders. Therefore, considerable attention has been paid on searching for new materials and synthesis techniques to improve the properties of phosphors. Generally speaking, the luminescence properties of phosphors are related not only to optical active ions but also to host lattice. Trivalent bismuth cation with outer 6s2 electronic configuration is evidenced to be an interesting luminescence activator and the spectroscopic properties of Bi3 þ in a variety of host lattices have been extensively studied [1–5]. The ground state of the free Bi3 þ ion is 1S0 while the 6s16p1 excited states are 1P0, 3P1, 3P2 and 1P1 in order of increasing energy. Theoretically, the 1S0-3P1 transition is an allowed electric dipole transition and the 1S0-3P0,1,2 transitions are forbidden. However, 1 S0-3P1,2 transitions become more allowed due to spin–orbit coupling. The observed Bi3 þ emission band at room temperature is assigned to the 3P1-1S0 transition owing to the lower energy of n
Corresponding author. E-mail address: [email protected] (W. Li).
http://dx.doi.org/10.1016/j.mssp.2015.09.015 1369-8001/& 2015 Elsevier Ltd. All rights reserved.
3
P1 than that of 1P1 [6,7]. Furthermore, the emission performances of Bi3 þ show a strong dependence on the environmental conditions because its outer s2 electron orbit is not shielded. As a matter of fact, the emission peaks of Bi3 þ can appear in the ultraviolet, blue, green, or even red wavelength regions with different matrices. CaSb2O6, one of ternary antimonites, has a hexagonal crystal structure, which can be viewed as two-dimensional infinite sheets of edge-sharing SbO6 octahedra alternating with layers of Ca2 þ ions. The CaSb2O6 compound is optical transparent (band gap: 3.59 eV) [8] and chemically stable. Therefore, the CaSb2O6 could be a desirable luminescence host material and the luminescence properties of Bi3 þ -doped CaSb2O6 have been reported [9–11]. However, the visible emission MSb2O6:Bi (M¼ Ca, Sr) always quenched at room temperature due to the volatility of Bi2O3 and the tendency of Bi3 þ to be oxidized to Bi5 þ in the conventional sintering process [12]. It is believed that lattice modification is one of the most effective strategies for improving luminescent properties of already known compounds [13,14]. Such a lattice modification can be easily realized by co-doping, which can modify the local site symmetry of activator ion or induce defect, and then change radiation relaxation process. Many literatures has reported that alkali metal ions like Li þ , Na þ K þ are favorable lattice modifiers to enhance
266
S. Yao et al. / Materials Science in Semiconductor Processing 41 (2016) 265–269
luminescence because of their low oxidation states and distinct ionic radii [15–18]. In previous work, we developed CaSb2O6:Bi3 þ blue phosphors using a co-precipitation method [9]. These phosphors are stable and show good luminescent properties at room temperature. The purpose of this work is to improve the luminescent properties of Bi3 þ -activated CaSb2O6 phosphors via co-doping with monovalent ions, such as Li þ , Na þ and K þ . The effect of alkali metal ions codoping on the structure, morphology and optical properties in CaSb2O6: Bi3 þ system has been studied and possible mechanism has been proposed.
2. Experimental 2.1. Preparation of CaSb2O6 nanocrystals All chemicals, including SbCl3, CaCl22H2O, Bi2O3, NaOH, Na2C2O4, Li2CO3, Na2CO3, and K2CO3 (Sinopharm Chemical Reagent Co. Ltd, China) were analytical grade and used without further purification. The deionized water was used throughout the experiment. CaSb2O6:xBi3 þ (0.25 rx r2 mol%) and CaSb2O6:0.75Bi3 þ , 0.75M þ (M ¼Li, Na and K) precursors were obtained by a simple chemical co-precipitation method. Typically Bi2O3 and SbCl3 were dissolved in dilute hydrochloric acid with stirring and moderate heating to obtain BiCl3 and SbCl3 stock solutions, respectively. Meanwhile, CaCl2 solution was prepared by dissolving CaCl22H2O in deionized water. After that, stoichiometric BiCl3, SbCl3 and CaCl2 were mixed together to form a clear and homogeneous solution. The mixed solution was dropped in a precipitator (NaOH–Na2C2O4 solution) and then white precipitation was obtained. After aging, filtration, washing and drying, the white precursor was obtained. 1) The precursor was calcined at 1000 °C for 4 h to obtain CaSb2O6: xBi3 þ products. 2) In a synthesis procedure of CaSb2O6:0.75Bi3 þ , 0.75M þ , M2CO3 (atom ratio: Bi:M ¼1:1) was introduced in the above precursor, and blended and milled thoroughly. Finally, the mixture was calcined in muffle at 1000 °C for 4 h. 2.2. Characterization The structure of the samples was characterized by XRD on a X’Pert Pro MPD diffractometer (Panalytical, Holland) using CuKα radiation (λ ¼1.5406 Å). The SEM images were taken on Hitachi s-4600 (Japan). The photoluminescence (PL) spectra were obtained on an F-2500 fluorescence spectrometer (Hitachi, Japan) at room temperature. The diffuse reflectance spectra (DRS) of samples were obtained on a UV-3150 spectrophotometer (Shimadzu, Japan) using BaSO4 as a reference standard in the wavelength range of 200–800 nm. The fourth harmonic (266 nm) of a pulsed Nd:YAG laser (Spectron Laser Sys. SL802G) was employed to examine the luminescence decay and decay profiles were recorded with a LeCloy 9301 digital storage oscilloscope in which the signal was fed from PMT.
3. Results and discussion The XRD patterns of the as-obtained undoped and doped CaSb2O6 are shown in Fig. 1. It can be clearly seen that all the samples have the similar XRD patterns and their diffraction peaks can be perfectly indentified as pure hexagonal structure of CaSb2O6 with the space group of P-31m (JCPDS card No. 46-1496). There is no diffraction peak corresponding to any starting materials or other impure phases, indicating that the samples obtained are single phase and doping at the investigated concentrations do
Fig. 1. XRD patterns of CaSb2O6, CaSb2O6:0.75Bi and CaSb2O6:0.75Bi,0.75M (M¼ Li, Na and K).
not change the crystal structure. Moreover, the good crystalline of the samples, confirmed by the strong and sharp diffraction peaks in Fig. 1, is advantageous to luminescence. Considering the similarity of the effective ionic radii between Ca2 þ ions (1.00 Å) and Bi3 þ ions (1.03 Å), it is rational that the dopants are preferably substituted into the Ca2 þ sub-crystal sites. According to the Scherrer equation, the average crystallite sizes of the nanocrytals were determined to be in the range of 54–66 nm. Fig. 2 displays the SEM images of undoped and doped CaSb2O6 samples. From the SEM micrograph in Fig. 2a, it is obvious that the particles of pure CaSb2O6 are roughly spherical with an average diameter of about 70 nm. The SEM images (Fig. 2b) of CaSb2O6:0.75Bi give the similar morphological properties to those of pure CaSb2O6. With the co-doping by Bi3 þ and M þ , the particle size is slightly decreased (Fig. 2c–e), and their morphology is regular and uniform. The result is consistent with those of XRD measurements (Fig. 1). The diffuse reflectance spectra of pure and doped CaSb2O6 samples are given in Fig. 3. All the samples show a remarkable drop in reflection around 300 nm, corresponding to the valenceto-conduction band transitions of the CaSb2O6 host. The intense reflection in the visible spectral range is in agreement with the observed white daylight color for undoped CaSb2O6. It is thereby expected that CaSb2O6:Bi,M could achieve high luminescent efficiency owing to the high reflectance in emission region. On the other hand, the doped CaSb2O6 samples show different optical absorption behavior. A broad absorption band extending from 265 nm to 355 nm can be clearly observed from the absorption spectra of the CaSb2O6:0.075Bi and CaSb2O6:0.75Bi,0.75M samples. Apparently, the absorption band should be attributed to the Bi3 þ -related absorption due to the absence of them in the pure CaSb2O6 sample. Fig. 4 shows typical excitation and emission spectra of CaSb2O6:0.75Bi phosphor. The sample displays a broad emission band (right of Fig. 4) in the wavelength range of 370-550 nm with a peak center at about 437 nm, corresponding to 3P1–1S0 transition of Bi3 þ . It is believed that Bi3 þ will replace Ca2 þ on the single crystallographic site available in CaSb2O6 with some kind of charge compensation, indicating that mainly one emission Bi3 þ center will present in CaSb2O6:0.75Bi. However, from Fig. 4 one can see that the emission curve of CaSb2O6:0.75Bi is asymmetric, which is ascribed to the disturbance of defect, such as cation vacancies (VCa )‵‵ [9]. On the basis of our previous work, the doping concentration of activator is an important factor for luminescence materials and the concentration dependent emission intensity at 437 nm of CaSb2O6:xBi was given in the inset of Fig. 4. It can be
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Fig. 2. SEM images of (a) pure CaSb2O6, (b) CaSb2O6:0.75Bi, (c) CaSb2O6:0.75Bi,0.75Li, (d) CaSb2O6:0.75Bi,0.75Na and (e) CaSb2O6:0.75Bi,0.75K.
seen that the optimal Bi3 þ content is 0.75 mol% [9]. The excitation spectrum of CaSb2O6:0.75Bi (left of Fig. 4) consists of three bands, which are attributed to the transition from the ground state of 1S0 to the various excited states of Bi3 þ ion. Among them, the excitation band peaking at 336 nm corresponding to 1S0–3P1 transition is most prominent. The effect of charging compensators, including Li þ , Na þ and K þ ions on the blue emission of CaSb2O6:0.75Bi was investigated and the result is given in Fig. 5. It can be seen from Fig. 5 that the shape and positions are similar in the PL spectra for all the samples. On the other hand, emission intensity has been improved by the introduction of alkali metal ions and the emission intensities of CaSb2O6:0.75Bi,0.75Li, CaSb2O6:0.75Bi,0.75Na and CaSb2O6:0.75Bi,0.75K are about 1.24, 1.67 and 1.42 times higher than that of CaSb2O6:0.75Bi, respectively. It is believed that in the CaSb2O6:0.75Bi sample, the substitution of Bi3 þ for Ca2 þ is
accompanied by the formation of Ca vacancies for charge balance. Therefore, the defect chemical reaction could be described as following equation:
Bi2O3 + CaSb2O6 → 2Bi•Ca + V "Ca + 3O×O
(1)
However, the charge compensation can be realized by (MCa )‵ for Bi,M-doped CaSb2O6, as shown in Eq. (2).
Bi2O3 + M2O + CaSb2O6 → 2Bi•Ca + 2(MCa )‵ + 4O×O
(2)
Generally speaking, vacancy in lattice is the luminescent quenching center [19]. The occurrence of (MCa )‵ will decrease the (VCa)‵‵ content in the CaSb2O6 and thus improve their emission properties. Since Na þ has the similar ionic radius (1.02 Å) to that of Ca2 þ (1.00 Å), it is expected that the incorporation of Na þ will cause the less distortion of crystal lattice compared to Li þ (0.76 Å)
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Fig. 6. Emission decay curves of CaSb2O6:0.75Bi and CaSb2O6:0.75Bi,0.75M (M¼ Li, Na and K) excited by 355 nm. Fig. 3. UV–vis DRS of CaSb2O6, CaSb2O6:0.75Bi and CaSb2O6:0.75Bi,0.75M (M¼ Li, Na and K).
emission upon 355 nm pulsed excitation were examined. Fig. 6 shows the normalized time decay emission at 437 nm for CaSb2O6:0.75Bi and CaSb2O6:0.75Bi,0.75M. All the samples exhibit non-exponential feature, suggesting that there exist more than one relaxation process. Moreover, the non-exponential character becomes more evident with the co-doping of M þ . The non-exponential profiles of luminescence decay should be associated with the cross-relaxation or the energy transfer process between the activator ions or the induced defects, which make the decay change and lead to non-exponential decay curve for CaSb2O6:Bi system. In addition, the decay curves can be well-reproduced by a double-exponential function as
I = A1exp( − t /τ1) + A2 exp( − t /τ2)
Fig. 4. Excitation (left) and emission (right) spectra of CaSb2O6:0.75Bi. Inset: plot of the emission intensity vs. Bi-doping content.
(3)
where I is the emission intensity, A1 and A2 are constants, and τ1 and τ2 are luminescent lifetimes. To determine the magnitudes of decay times, the average lifetimes were estimated using the following equation:
τaver =
A1τ12 + A2 τ22 A1τ1 + A2 τ2
(4)
Therefore, the τaver values were obtained and listed in Table 1. In the CaSb2O6 system, the decay time of 291 ns is close to that of other Bi3 þ -doped phosphor [20]. However, the average lifetime gradually decreases with the co-addition of M þ ions and the shortest decay time occurs for CaSb2O6:0.75Bi,0.75Li sample. As discussed above, the emission behavior of CaSb2O6:0.75Bi,0.75M (Fig. 5) indicated that there is energy transfer in the phosphor and the lattice defects such as (BiCa) and (MCa)’ may play an important role on this energy transfer process. It should be noted that the average distance between nearest neighbor doping ions (Bi3 þ and/ or M þ ) in the CaSb2O6:0.75Bi,0.75M samples is closer than that in the CaSb2O6:0.75Bi due to the higher doping concentration. This causes higher probabilities of the energy transfer and further the shorter lifetime (Fig. 6 and Table 1). On the other hand, the doping of M þ in CaSb2O6 will induce lattice relaxation of crystal because of the different ionic radii between Ca2 þ and M þ . For all the Fig. 5. Emission spectra of CaSb2O6:0.75Bi and CaSb2O6:0.75Bi,0.75M (M¼ Li, Na and K).
and K þ (1.38 Å) in the Bi,M-codoped CaSb2O6. This could be the fact that the CaSb2O6:0.75Bi,0.75 Na exhibits the strongest emission, as shown in Fig. 5. Therefore, Na þ could be the optimal charge compensator in the CaSb2O6:0.75Bi system. In order to elucidate the role that alkali metal ions play on the luminescence properties of Bi3 þ , the decay times of the 437 nm
Table 1 The luminescence decay lifetimes and CaSb2O6:0.75Bi and CaSb2O6:0.75Bi,0.75M.
CIE
chromaticity
coordinates
Samples
τaver (ns)
CIE color coordinates
CaSb2O6:0.75Bi CaSb2O6:0.75Bi,0.75Li CaSb2O6:0.75Bi,0.75Na CaSb2O6:0.75Bi,0.75K
291 183 256 226
(0.1505, 0.0490) (0.1495, 0.0529) (0.1497, 0.0509) (0.1496, 0.0518)
of
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corresponding to 3P1–1S0 transition of Bi3 þ ions and the optimal Bi-doping concentration is determined to be 0.75 mol%. Interestedly, the emission can be notably enhanced by the introduction of alkali ions due to the effective charge compensation. Among them, Na þ ion has the best effect on enhancing luminescence properties and improve the emission intensity to 1.67 times that of CaSb2O6:Bi, which is attributed to the similar ionic radii between Na þ and Ca2 þ . In addition, the decay curves of samples exhibit nonlinear feature and the decay time decreases with the co-doping by alkali ions, indicating multiple relaxation process.
Acknowledgements This work was supported by the Priority Academic Program Development of Jiangsu Higher Education Institutions. Fig. 7. The CIE chromaticity CaSb2O6:0.75Bi,0.75Na. 2þ
diagram
for
CaSb2O6:0.75Bi
and
References 3þ
þ
þ
þ
þ
investigated cations (Ca , Bi , Li , Na and K ), Li has the smallest radii. Therefore, the substitution of Li þ on Ca2 þ crystal site will lead to large lattice relaxation and make crystal lattice smaller. Thereby, one can imagine that the average distance between nearest neighbor doping ions (Bi3 þ and/or Li þ ) in the CaSb2O6:0.75Bi,0.75Li sample becomes closer than those in the other phosphors and contribute to a short decay time. However, further work is necessary. Table 1 gives the Commission Internationale de I’Eclairage (CIE) color coordinates of all samples. With the co-addition of M þ , the color tone changes from (0.1505, 0.0490) of CaSb2O6:0.75Bi to (0.1497, 0.0509) of CaSb2O6:0.75Bi,0.75Na, as shown in Fig. 7. On the other hand, the CIE color coordinates of CaSb2O6:0.75Bi,0.75M change little, which could be attributed to the low concentration of the charge compensator.
4. Conclusions In this study, Bi3 þ -doped and Bi3 þ ,M þ -codoped (M ¼Li, Na, K) CaSb2O6 nanoparticles were successfully prepared by a co-precipitation method and their luminescence properties were investigated in detail. Under the excitation of UV light, CaSb2O6:Bi nano-phosphors exhibit intense blue emission peaking at 437 nm,
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