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Novel porous CaSnO3:Eu3+ and Ca2SnO4:Eu3+ phosphors by co-precipitation synthesis and postannealing approach: A general route to alkaline-earth stannates
Journal of Alloys and Compounds 497 (2010) 354–359
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Novel porous CaSnO3 :Eu3+ and Ca2 SnO4 :Eu3+ phosphors by co-precipitation synthesis and postannealing approach: A general route to alkaline-earth stannates Xiang Ying Chen ∗ , Chao Ma, Shi Ping Bao, Hai Yan Zhang School of Chemical Engineering, Anhui Key Laboratory of Controllable Chemistry Reaction & Material Chemical Engineering, Hefei University of Technology, Hefei, Anhui 230009, PR China
a r t i c l e
i n f o
Article history: Received 1 December 2009 Received in revised form 2 March 2010 Accepted 3 March 2010 Available online 10 March 2010 Keywords: Phosphors Chemical synthesis Optical properties Scanning electron microscopy
a b s t r a c t Novel porous CaSnO3 :Eu3+ and Ca2 SnO4 :Eu3+ phosphors have been successfully prepared by postannealing the corresponding precursors at elevated temperatures. The precursors were firstly obtained by co-precipitation method at room temperature. The as-prepared phosphors were well characterized by means of X-ray powder diffraction (XRPD) and field emission scanning electron microscopy (FESEM). The photoluminescent excitation and emission spectra illustrate the red-emitting nature of CaSnO3 :Eu3+ and Ca2 SnO4 :Eu3+ phosphors. Moreover, CaSnO3 :Eu3+ phosphors with various shapes were obtained by introducing certain kinds of additives. Importantly, this facile synthetic method can be readily extended to prepare many other alkaline-earth stannates including BaSnO3 :Eu3+ , Ba2 SnO4 :Eu3+ , SrSnO3 :Eu3+ , and Sr2 SnO4 :Eu3+ phosphors. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Alkaline-earth stannates have received considerable attentions over the past few years because of their potential applications in various fields, such as photocatalyst [1], ferromagnet [2], and anode materials for lithium ion batteries [3]. Recently, photoluminescence (PL) properties of alkaline-earth stannates, such as emission color and efficiency, for the phosphor host materials of rare-earth activators have been reported, including green emitting CaSnO3 :Tb3+ [4], reddish orange emitting CaSnO3 :Sm3+ [5], and red-emitting Ca2 SnO4 :Eu3+ [6]. As reported, only two phases of CaSnO3 and Ca2 SnO4 exist in the CaO–SnO2 binary system. CaSnO3 , with orthorhombic perovskite structure, is constructed of octahedral SnO6 which connect to each other by sharing vertexes. Besides, the structure of Ca2 SnO4 consists of SnO6 octahedra which are linked by edges and the Ca2+ which are surrounded by seven oxygen ions is in an arrangement of low symmetry [6]. The SnO4 anions involved in Ca2 SnO4 are optically inert, and it could be a candidate for host materials [7–8]. Ca2 SnO4 :Eu3+ ,Y3+ phosphors were synthesized through solid state reaction between SnO2 and CaCO3 at 1400 ◦ C for 12 h [9]. But this synthetic method needs a higher thermal treatment temperature and produces a lower chemical homogeneity. As an
∗ Corresponding author. Tel.: +86 551 2901450; fax: +86 551 2901450. E-mail address: [email protected] (X.Y. Chen). 0925-8388/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2010.03.065
alternative synthetic strategy, sol–gel route was developed to prepare Ca2 SnO4 :Eu3+ phosphors, whose calcination operation is at 900 ◦ C in air for 6 h [10]. On the other hand, Li and co-workers synthesized MSn(OH)6 (where M = Ca, Sr and Ba) as precursors via a hydrothermal route at 200 ◦ C for 5 h and then converted them into MSnO3 :Eu3+ by calcination at 800 ◦ C for 3 h [11]; Niu and coworkers prepared MSnO3 :Eu3+ [12] and M2 SnO4 :Eu3+ [13] (where M = Ca, Sr and Ba) via a Pechini-type sol–gel route at 90 ◦ C for 24 h followed by calcination at 1000 ◦ C. However, as far as we know, few reports were mentioned on the systematical preparation of MSnO3 :Eu3+ and M2 SnO4 :Eu3+ (where M = Ca, Sr and Ba) phosphors by the facile co-precipitation synthesis especially at room temperature followed by calcination in air. Herein, for the first time, porous CaSnO3 :Eu3+ and Ca2 SnO4 :Eu3+ phosphors were successfully synthesized by calcining the precursors obtained by co-precipitation route at room temperature. Photoluminescence properties of the cubic CaSnO3 :Eu3+ and Ca2 SnO4 :Eu3+ phosphors were characterized. In particular, we could prepare other alkaline-earth stannates including BaSnO3 :Eu3+ , Ba2 SnO4 :Eu3+ , SrSnO3 :Eu3+ , and Sr2 SnO4 :Eu3+ phosphors by the same synthetic method. 2. Experimental All the chemicals are of analytical grade and used as received without further purification. In this study, a series of stannates phosphors containing CaSnO3 :Eu3+ Ca2 SnO4 :Eu3+ , BaSnO3 :Eu3+ , Ba2 SnO4 :Eu3+ , SrSnO3 :Eu3+ , and Sr2 SnO4 :Eu3+ were prepared by the co-precipitation method followed by postannealing treatment at
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elevated temperatures in air. Besides, when preparing Ca2 SnO4 :Eu3+ , Ba2 SnO4 :Eu3+ , and Sr2 SnO4 :Eu3+ phosphors, certain amount of sodium oxalate (Na2 C2 O4 ) was added, acting as complexing agent. 2.1. Preparation of CaSnO3 :Eu3+ phosphors In a typical procedure, CaCl2 ·2H2 O (2 mmol) and Eu(NO3 )3 (0.1 mmol) were dissolved in 15 mL distilled water, and Na2 SnO3 (2 mmol) were also dissolved in 15 mL distilled water. Then the Na2 SnO3 solution was added into CaCl2 and Eu(NO3 )3 solution dropwise under magnetic stirring. Finally, NaOH solution was added until the pH is ca. 10. After 4 h, the resulting white powder was filtered off, washed with distilled water and absolute ethanol for several times, and then dried under vacuum at 80 ◦ C for 4 h. The above powder was postannealed at 800 ◦ C for 3 h in a horizontal furnace in air, resulting in white product. 2.2. Preparation of Ca2 SnO4 :Eu3+ phosphors The typical procedure for preparing Ca2 SnO4 :Eu3+ phosphors is similar to that of CaSnO3 :Eu3+ phosphors, as given above, except that Na2 C2 O4 (2 mmol) was introduced into the reaction system. The resulting powder as precursor was then postannealed at 1100 ◦ C for 3 h in a horizontal furnace in air. Similarly, BaSnO3 :Eu3+ , Ba2 SnO4 :Eu3+ , SrSnO3 :Eu3+ , and Sr2 SnO4 :Eu3+ phosphors were obtained by the co-precipitation method followed by postannealing treatment as shown above. 2.3. Characterization X-ray powder diffraction (XRPD) patterns were obtained on a Rigaku Max-2200 with Cu K␣ radiation. Field emission scanning electron microscopy (FESEM) images were taken with a Hitachi S-4800 scanning electron microscope. Photoluminescent analysis was conducted on a Hitachi F-4500 spectrophotometer with Xe lamp at room temperature.
3. Results and discussion The phase, crystallinity and purity of samples were characterized by means of XRPD technique. When designating the initial molar ratio of CaCl2 ·2H2 O, Eu(NO3 )3 , and Na2 SnO3 as 1:0.05:1 at pH 10, large numbers of white powder occurs, acting as precursor, which takes the form of hydrate, CaSn(OH)6 [11,14]. The precur-
Fig. 1. XRPD patterns of samples: (a) CaSnO3 :Eu3+ phosphors; (b) Ca2 SnO4 :Eu3+ phosphors.
sor was further calcined at 800 ◦ C for 3 h in air to give rise to final product. The corresponding XRPD pattern is shown in Fig. 1a, whose reflection peaks can be readily indexed as perovskite-type orthorhombic CaSnO3 (JCPDS Card No. 31-0312). On the other hand, when designating the initial molar ratio of CaCl2 ·2H2 O, Eu(NO3 )3 , Na2 SnO3 , and Na2 C2 O4 as 2:0.05:1:1 at pH 10, lots of white powder occurs, also acting as precursor, which is actually composed of CaSn(OH)6 and CaC2 O4 . Next, the final product was obtained by calcining the above precursor at 1100 ◦ C for 3 h in air. The resulting XRPD pattern is shown in Fig. 1b, and all the diffraction peaks therein can be readily indexed as perovskite-type orthorhombic Ca2 SnO4 (JCPDS 46-0112). Remarkably, Shi and co-workers once prepared Eu-doped Ca2 SnO4 sample by solid state reaction at 1250 ◦ C for 2 h [6], using CaCO3 , SnO2 and Eu2 O3 as raw materials.
Fig. 2. FESEM images of the samples: (a,b) CaSnO3 :Eu3+ phosphors; (c,d) Ca2 SnO4 :Eu3+ phosphors. The inset in (d) is the corresponding enlarged FESEM image.
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Fig. 3. (a) Excitation spectrum of CaSnO3 :Eu3+ phosphors at em = 613 nm; (b) emission spectra of CaSnO3 :Eu3+ phosphors at ex = 394 nm and ex = 273 nm, respectively; (c) excitation spectrum of Ca2 SnO4 :Eu3+ phosphors at em = 617 nm; (d) emission spectra of Ca2 SnO4 :Eu3+ phosphors at ex = 394 nm and ex = 273 nm, respectively.
Thus, we can clearly see that the present calcination temperature for orthorhombic Ca2 SnO4 (1100 ◦ C) is much lower than that by solid state reaction, indicating the advantage of the synthetic strategy involved herein. In addition, the existence of Eu in the products can hardly be detected from the XRPD patterns due to its low concentration added. Besides, full width at half maximum (FWHM) of the diffraction peaks in Fig. 1 is fairly narrow, revealing high crystallinity of the as-obtained samples. FESEM technique was employed to vividly depict the genuine size and shape of the as-prepared samples. In case of CaSnO3 :Eu3+ phosphor, its representative FESEM images are shown in Fig. 2a and b, which displays many sub-micrometer cubes. Interestingly, further observation on these cubes tells us that they are porous in nature, containing large numbers of holes (Fig. 2b). This kind of porosity mainly arises from the releasing of H2 O in the process of calcining CaSn(OH)6 , and similar result was also reported by Sun and co-workers [14]. Regarding Ca2 SnO4 :Eu3+ phosphor, similar experimental results as those of CaSnO3 :Eu3+ phosphor also take place, as shown in Fig. 2c and d. The Ca2 SnO4 :Eu3+ sample is primarily made up of cubes, which also consist of large numbers of holes, indicating the porous nature as a whole. The formation of holes can be ascribed to the releasing of H2 O and CO2 by calcining the precursor containing CaSn(OH)6 and CaC2 O4 . To our knowledge, this kind of porous Ca2 SnO4 :Eu3+ phosphor is not thus far reported in literature. Besides, considering the formation mechanism for porous CaSnO3 :Eu3+ and Ca2 SnO4 :Eu3+ phosphors, the so-called “Top-down process” should be referred herein, which generally involves starting with a bulk solid and obtaining a nanostructure by structural decomposition. Given the principle raised
by “Top-down process”, we have recently prepared a series of nanoporous aluminates phosphors under certain circumstances such as MgAl2 O4 :Eu3+ [15] and ZnAl2 O4 :Eu3+ [16] phosphors. The photoluminescence emission (PL) and photoluminescence excitation (PLE) spectra of the as-prepared CaSnO3 :Eu3+ phosphors and Ca2 SnO4 :Eu3+ phosphors were determined at room temperature. In the case of the cubic CaSnO3 :Eu3+ phosphors, the typical excitation spectrum ranging from 250 to 550 nm at em = 613 nm is shown in Fig. 3a. As previously reported, the broad band at 273 nm comes from the charge transfer bands (CTB) between Eu3+ ions and the surrounding oxygen anions. Concerning sharp bands centered at 533, 464, 394 and 362 nm, they can be attributed to the characteristic transitions of Eu3+ from 7 F0 → 5 Di (i = 1, 2, 3 and 4). Fig. 3b depicts the typical red photoluminescence from Eu3+ ions in porous CaSnO3 :Eu3+ phosphors when rooting the excitation wavelengths at 393 nm and 273 nm respectively. A series of sharp bands at 589, 613, 652, and 695 nm are assignable to the characteristic transitions of Eu3+ from 5 D0 → 7 Fj (j = 1, 2, 3, and 4) [17], namely the 5 D0 → 7 F1 (589 nm), 5 D0 → 7 F2 (613 nm), 5 D0 → 7 F3 (652 nm), and 5 D0 → 7 F4 (695 nm). Besides, the most intense emission peak at 613 nm occurs through the forced electric dipole transition (5 D0 → 7 F2 ), whose intensity is very sensitive to the site symmetry of Eu3+ ions. Regarding the band located at 589 nm, it derives from the magnetic dipole transition (5 D0 → 7 F1 ), which is independent of the symmetry and the site occupied by Eu3+ ions in the host. As a rule, the intensity ratio of (5 D0 → 7 F2 )/(5 D0 → 7 F1 ), known as the asymmetry ratio, reveals the degree of distortion from the inversion symmetry of the local environment of Eu3+ ions in the host matrix [17]. And the asymmetry ratios for CaSnO3 :Eu3+ phosphors are ca. 2.18 and
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Fig. 4. CaSnO3 :Eu3+ phosphors with various shapes by introducing surfactant into the reaction system: (a,b) SDBS; (c,d) sodium citrate; (e,f) PVP.
2.98 calculated from Fig. 3b1 and 3b2 , respectively. On the other hand, taking into account the ionic radius of Eu3+ (0.95 Å) and Ca2+ (0.99 Å), Eu3+ ions can incorporate into perovskite-type CaSnO3 lattice and form substituted solid solution in the course of calcination at elevated temperature, certainly producing defects such as oxygen ionic vacancies owing to charge compensation mechanism. With respect to the as-prepared Ca2 SnO4 :Eu3+ phosphors, similar PL and PLE spectra occur except for minor difference, as illustrated in Fig. 3c and d. Noticeably, as compared to those of CaSnO3 :Eu3+ phosphors, Einstein shift happens towards 5 D0 → 7 F2 and 5 D0 → 7 F4 in Ca2 SnO4 :Eu3+ phosphors, which might owe to different crystal structures of the host materials [6].
4. Shape manipulation towards CaSnO3 :Eu3+ phosphors As is well known, the properties of nanomaterials strongly depend on the shape and size in nature [18]. It is thus of significance to manipulate these characteristics as we desire. Among the synthetic strategies for achieving this goal, certain kind of surfactant as shape-directing agent is usually introduced into the
Fig. 5. XRPD patterns of samples: (a) BaSnO3 :Eu3+ phosphors; (b) Ba2 SnO4 :Eu3+ phosphors; (c) SrSnO3 :Eu3+ phosphors; (d) Sr2 SnO4 :Eu3+ phosphors. Notes: Two unknown diffraction peaks occur in Figure 5d, as marked by asterisks.
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Fig. 6. FESEM images of the samples: (a,b) BaSnO3 :Eu3+ phosphors; (c,d) Ba2 SnO4 :Eu3+ phosphors; (e,f) SrSnO3 :Eu3+ phosphors; (g,h) Sr2 SnO4 :Eu3+ phosphors. The inset in 6 h is the corresponding enlarged FESEM image.
reaction system [19]. For instance, we recently prepared porous MgAl2 O4 :Eu3+ phosphors having rod-like or spherical shapes by adjusting the amounts of urea and poly(ethylene glycol) involved. It reveals that adding poly(ethylene glycol) 2000, primarily acting as a shape-director, into reaction system is crucial for preparing spherical MgAl2 O4 :Eu3+ phosphors [15]. In present study, in order
to manipulate the shape of CaSnO3 :Eu3+ phosphors, certain kinds of surfactants were introduced into the reaction system. Fig. 4a and b shows the typical FESEM images of CaSnO3 :Eu3+ by adding sodium dodecylbenzenesulfonate (SDBS) into the reaction system. Compared with Fig. 2a and b, it is obvious that these cubes are more uniform and regular, enclosing with six {1 0 0} planes. The possi-
X.Y. Chen et al. / Journal of Alloys and Compounds 497 (2010) 354–359
ble function of SDBS is to kinetically control the growth rates of six {1 0 0} planes by interacting with these faces through adsorption and desorption. This might make the CaSnO3 :Eu3+ phosphors grow up equally along 1 0 0 direction, commonly resulting in uniform cubes in shape. Besides, by introducing sodium citrate into the reaction system, the CaSnO3 :Eu3+ phosphors are composed of lots of hornless cubes, as shown in Fig. 4c and d. This implies that the sodium citrate added can slightly favor the formation of {1 1 1} planes, leading to the hornlessness of cubic CaSnO3 :Eu3+ phosphors. In addition, to our surprise, a great deal of eight-horn-shaped CaSnO3 :Eu3+ phosphors occur when introducing polyvinylpyrrolidone (PVP) into the reaction system, as shown in Fig. 4e and f, which originates from the assumption that the surfactant PVP added is selectively adsorbed on the {1 0 0} planes. This can thus reduce the growth rate of crystals along 1 0 0 direction, and increase the growth rate of crystals along 1 1 1 direction, inducing the formation of eight-horn-shaped CaSnO3 :Eu3+ phosphors. Moreover, as reported by Cheng and Lu [20], this kind of eight-horn-shaped shape is just the intermediate stage for CaSnO3 :Eu3+ phosphors. And with increasing the reaction time, cubic CaSnO3 :Eu3+ phosphors enclosing with six {1 0 0} planes will eventually come into being.
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a whole. As for the Ba2 SnO4 :Eu3+ phosphors, it is made up of lots of micrometer particles, which are in fact constructed of nanoscale rods, as depicted in Fig. 6c and d. Fig. 6e and f shows the FESEM images of the SrSnO3 :Eu3+ phosphors, which consist of a great deal of irregular particles. Regarding the Sr2 SnO4 :Eu3+ phosphors, it is composed of shuttle-like particles with ca. tens of micrometer in size, as shown in Fig. 6g and h. 6. Conclusions In summary, we demonstrate a general synthetic route, i.e., the co-precipitation method followed by postannealing treatment, to successfully prepare a series of alkaline-earth stannates phosphors, including MSnO3 :Eu3+ and M2 SnO4 :Eu3+ (M = Ca, Ba, and Sr) phosphors. The photoluminescent excitation and emission spectra of CaSnO3 :Eu3+ and Ca2 SnO4 :Eu3+ phosphors were studied in brief, illustrating the red-emitting feature. Besides, various kinds of additives were introduced to manipulate the shape and size of the CaSnO3 :Eu3+ phosphors. This facile synthetic method is expected to prepare other kinds of stannates phosphors, especially having porous feature in essence. Acknowledgements
5. The generality of the present synthetic method It is remarkable in this study that the present synthetic method can be extended to prepare other alkaline-earth stannates phosphors including BaSnO3 :Eu3+ , Ba2 SnO4 :Eu3+ , SrSnO3 :Eu3+ , and Sr2 SnO4 :Eu3+ phosphors by the co-precipitation method followed by postannealing treatment. Fig. 5a gives the typical XRPD pattern for the sample obtained by calcining the precursor at 800 ◦ C for 3 h, keeping BaCl2 ·2H2 O, Eu(NO3 )3 , and Na2 SnO3 as 1:0.05:1 at pH 10. Apparently, all the diffraction peaks in Fig. 5a can be assigned to cubic phase BaSnO3 (JCPDS Card No. 74-1300). Next, when calcining the precursor at 1100 ◦ C for 3 h, keeping BaCl2 ·2H2 O, Eu(NO3 )3 , Na2 SnO3 , and Na2 C2 O4 as 2:0.05:1:1 at pH 10, large amount of white powder appears, whose diffraction peaks can be indexed as tetragonal phase Ba2 SnO4 (JCPDS Card No. 74-1349), as shown in Fig. 5b. Similarly, cubic phase SrSnO3 (JCPDS Card No. 22-1442) and tetragonal phase Sr2 SnO4 (JCPDS Card No. 24-1241) were obtained by the co-precipitation method followed by postannealing treatment, which are shown in Fig. 5c and d, respectively. The representative FESEM images with various magnifications of the BaSnO3 :Eu3+ phosphors are shown in Fig. 6a and b. The panoramic view in Fig. 6a tells us that the BaSnO3 :Eu3+ phosphors is composed of irregular particles. Furthermore, the closer observation in Fig. 6b reveals that these large particles actually consist of abundant nanoscale particles, possessing the porous nature as
This project was supported by the Scientific Research Foundation for the Returned Overseas Chinese Scholars, Ministry of Education of China and Anhui Provincial Natural Science Foundation (090414194). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
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Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Novel porous CaSnO3 :Eu3+ and Ca2 SnO4 :Eu3+ phosphors by co-precipitation synthesis and postannealing approach: A general route to alkaline-earth stannates Xiang Ying Chen ∗ , Chao Ma, Shi Ping Bao, Hai Yan Zhang School of Chemical Engineering, Anhui Key Laboratory of Controllable Chemistry Reaction & Material Chemical Engineering, Hefei University of Technology, Hefei, Anhui 230009, PR China
a r t i c l e
i n f o
Article history: Received 1 December 2009 Received in revised form 2 March 2010 Accepted 3 March 2010 Available online 10 March 2010 Keywords: Phosphors Chemical synthesis Optical properties Scanning electron microscopy
a b s t r a c t Novel porous CaSnO3 :Eu3+ and Ca2 SnO4 :Eu3+ phosphors have been successfully prepared by postannealing the corresponding precursors at elevated temperatures. The precursors were firstly obtained by co-precipitation method at room temperature. The as-prepared phosphors were well characterized by means of X-ray powder diffraction (XRPD) and field emission scanning electron microscopy (FESEM). The photoluminescent excitation and emission spectra illustrate the red-emitting nature of CaSnO3 :Eu3+ and Ca2 SnO4 :Eu3+ phosphors. Moreover, CaSnO3 :Eu3+ phosphors with various shapes were obtained by introducing certain kinds of additives. Importantly, this facile synthetic method can be readily extended to prepare many other alkaline-earth stannates including BaSnO3 :Eu3+ , Ba2 SnO4 :Eu3+ , SrSnO3 :Eu3+ , and Sr2 SnO4 :Eu3+ phosphors. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Alkaline-earth stannates have received considerable attentions over the past few years because of their potential applications in various fields, such as photocatalyst [1], ferromagnet [2], and anode materials for lithium ion batteries [3]. Recently, photoluminescence (PL) properties of alkaline-earth stannates, such as emission color and efficiency, for the phosphor host materials of rare-earth activators have been reported, including green emitting CaSnO3 :Tb3+ [4], reddish orange emitting CaSnO3 :Sm3+ [5], and red-emitting Ca2 SnO4 :Eu3+ [6]. As reported, only two phases of CaSnO3 and Ca2 SnO4 exist in the CaO–SnO2 binary system. CaSnO3 , with orthorhombic perovskite structure, is constructed of octahedral SnO6 which connect to each other by sharing vertexes. Besides, the structure of Ca2 SnO4 consists of SnO6 octahedra which are linked by edges and the Ca2+ which are surrounded by seven oxygen ions is in an arrangement of low symmetry [6]. The SnO4 anions involved in Ca2 SnO4 are optically inert, and it could be a candidate for host materials [7–8]. Ca2 SnO4 :Eu3+ ,Y3+ phosphors were synthesized through solid state reaction between SnO2 and CaCO3 at 1400 ◦ C for 12 h [9]. But this synthetic method needs a higher thermal treatment temperature and produces a lower chemical homogeneity. As an
∗ Corresponding author. Tel.: +86 551 2901450; fax: +86 551 2901450. E-mail address: [email protected] (X.Y. Chen). 0925-8388/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2010.03.065
alternative synthetic strategy, sol–gel route was developed to prepare Ca2 SnO4 :Eu3+ phosphors, whose calcination operation is at 900 ◦ C in air for 6 h [10]. On the other hand, Li and co-workers synthesized MSn(OH)6 (where M = Ca, Sr and Ba) as precursors via a hydrothermal route at 200 ◦ C for 5 h and then converted them into MSnO3 :Eu3+ by calcination at 800 ◦ C for 3 h [11]; Niu and coworkers prepared MSnO3 :Eu3+ [12] and M2 SnO4 :Eu3+ [13] (where M = Ca, Sr and Ba) via a Pechini-type sol–gel route at 90 ◦ C for 24 h followed by calcination at 1000 ◦ C. However, as far as we know, few reports were mentioned on the systematical preparation of MSnO3 :Eu3+ and M2 SnO4 :Eu3+ (where M = Ca, Sr and Ba) phosphors by the facile co-precipitation synthesis especially at room temperature followed by calcination in air. Herein, for the first time, porous CaSnO3 :Eu3+ and Ca2 SnO4 :Eu3+ phosphors were successfully synthesized by calcining the precursors obtained by co-precipitation route at room temperature. Photoluminescence properties of the cubic CaSnO3 :Eu3+ and Ca2 SnO4 :Eu3+ phosphors were characterized. In particular, we could prepare other alkaline-earth stannates including BaSnO3 :Eu3+ , Ba2 SnO4 :Eu3+ , SrSnO3 :Eu3+ , and Sr2 SnO4 :Eu3+ phosphors by the same synthetic method. 2. Experimental All the chemicals are of analytical grade and used as received without further purification. In this study, a series of stannates phosphors containing CaSnO3 :Eu3+ Ca2 SnO4 :Eu3+ , BaSnO3 :Eu3+ , Ba2 SnO4 :Eu3+ , SrSnO3 :Eu3+ , and Sr2 SnO4 :Eu3+ were prepared by the co-precipitation method followed by postannealing treatment at
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elevated temperatures in air. Besides, when preparing Ca2 SnO4 :Eu3+ , Ba2 SnO4 :Eu3+ , and Sr2 SnO4 :Eu3+ phosphors, certain amount of sodium oxalate (Na2 C2 O4 ) was added, acting as complexing agent. 2.1. Preparation of CaSnO3 :Eu3+ phosphors In a typical procedure, CaCl2 ·2H2 O (2 mmol) and Eu(NO3 )3 (0.1 mmol) were dissolved in 15 mL distilled water, and Na2 SnO3 (2 mmol) were also dissolved in 15 mL distilled water. Then the Na2 SnO3 solution was added into CaCl2 and Eu(NO3 )3 solution dropwise under magnetic stirring. Finally, NaOH solution was added until the pH is ca. 10. After 4 h, the resulting white powder was filtered off, washed with distilled water and absolute ethanol for several times, and then dried under vacuum at 80 ◦ C for 4 h. The above powder was postannealed at 800 ◦ C for 3 h in a horizontal furnace in air, resulting in white product. 2.2. Preparation of Ca2 SnO4 :Eu3+ phosphors The typical procedure for preparing Ca2 SnO4 :Eu3+ phosphors is similar to that of CaSnO3 :Eu3+ phosphors, as given above, except that Na2 C2 O4 (2 mmol) was introduced into the reaction system. The resulting powder as precursor was then postannealed at 1100 ◦ C for 3 h in a horizontal furnace in air. Similarly, BaSnO3 :Eu3+ , Ba2 SnO4 :Eu3+ , SrSnO3 :Eu3+ , and Sr2 SnO4 :Eu3+ phosphors were obtained by the co-precipitation method followed by postannealing treatment as shown above. 2.3. Characterization X-ray powder diffraction (XRPD) patterns were obtained on a Rigaku Max-2200 with Cu K␣ radiation. Field emission scanning electron microscopy (FESEM) images were taken with a Hitachi S-4800 scanning electron microscope. Photoluminescent analysis was conducted on a Hitachi F-4500 spectrophotometer with Xe lamp at room temperature.
3. Results and discussion The phase, crystallinity and purity of samples were characterized by means of XRPD technique. When designating the initial molar ratio of CaCl2 ·2H2 O, Eu(NO3 )3 , and Na2 SnO3 as 1:0.05:1 at pH 10, large numbers of white powder occurs, acting as precursor, which takes the form of hydrate, CaSn(OH)6 [11,14]. The precur-
Fig. 1. XRPD patterns of samples: (a) CaSnO3 :Eu3+ phosphors; (b) Ca2 SnO4 :Eu3+ phosphors.
sor was further calcined at 800 ◦ C for 3 h in air to give rise to final product. The corresponding XRPD pattern is shown in Fig. 1a, whose reflection peaks can be readily indexed as perovskite-type orthorhombic CaSnO3 (JCPDS Card No. 31-0312). On the other hand, when designating the initial molar ratio of CaCl2 ·2H2 O, Eu(NO3 )3 , Na2 SnO3 , and Na2 C2 O4 as 2:0.05:1:1 at pH 10, lots of white powder occurs, also acting as precursor, which is actually composed of CaSn(OH)6 and CaC2 O4 . Next, the final product was obtained by calcining the above precursor at 1100 ◦ C for 3 h in air. The resulting XRPD pattern is shown in Fig. 1b, and all the diffraction peaks therein can be readily indexed as perovskite-type orthorhombic Ca2 SnO4 (JCPDS 46-0112). Remarkably, Shi and co-workers once prepared Eu-doped Ca2 SnO4 sample by solid state reaction at 1250 ◦ C for 2 h [6], using CaCO3 , SnO2 and Eu2 O3 as raw materials.
Fig. 2. FESEM images of the samples: (a,b) CaSnO3 :Eu3+ phosphors; (c,d) Ca2 SnO4 :Eu3+ phosphors. The inset in (d) is the corresponding enlarged FESEM image.
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Fig. 3. (a) Excitation spectrum of CaSnO3 :Eu3+ phosphors at em = 613 nm; (b) emission spectra of CaSnO3 :Eu3+ phosphors at ex = 394 nm and ex = 273 nm, respectively; (c) excitation spectrum of Ca2 SnO4 :Eu3+ phosphors at em = 617 nm; (d) emission spectra of Ca2 SnO4 :Eu3+ phosphors at ex = 394 nm and ex = 273 nm, respectively.
Thus, we can clearly see that the present calcination temperature for orthorhombic Ca2 SnO4 (1100 ◦ C) is much lower than that by solid state reaction, indicating the advantage of the synthetic strategy involved herein. In addition, the existence of Eu in the products can hardly be detected from the XRPD patterns due to its low concentration added. Besides, full width at half maximum (FWHM) of the diffraction peaks in Fig. 1 is fairly narrow, revealing high crystallinity of the as-obtained samples. FESEM technique was employed to vividly depict the genuine size and shape of the as-prepared samples. In case of CaSnO3 :Eu3+ phosphor, its representative FESEM images are shown in Fig. 2a and b, which displays many sub-micrometer cubes. Interestingly, further observation on these cubes tells us that they are porous in nature, containing large numbers of holes (Fig. 2b). This kind of porosity mainly arises from the releasing of H2 O in the process of calcining CaSn(OH)6 , and similar result was also reported by Sun and co-workers [14]. Regarding Ca2 SnO4 :Eu3+ phosphor, similar experimental results as those of CaSnO3 :Eu3+ phosphor also take place, as shown in Fig. 2c and d. The Ca2 SnO4 :Eu3+ sample is primarily made up of cubes, which also consist of large numbers of holes, indicating the porous nature as a whole. The formation of holes can be ascribed to the releasing of H2 O and CO2 by calcining the precursor containing CaSn(OH)6 and CaC2 O4 . To our knowledge, this kind of porous Ca2 SnO4 :Eu3+ phosphor is not thus far reported in literature. Besides, considering the formation mechanism for porous CaSnO3 :Eu3+ and Ca2 SnO4 :Eu3+ phosphors, the so-called “Top-down process” should be referred herein, which generally involves starting with a bulk solid and obtaining a nanostructure by structural decomposition. Given the principle raised
by “Top-down process”, we have recently prepared a series of nanoporous aluminates phosphors under certain circumstances such as MgAl2 O4 :Eu3+ [15] and ZnAl2 O4 :Eu3+ [16] phosphors. The photoluminescence emission (PL) and photoluminescence excitation (PLE) spectra of the as-prepared CaSnO3 :Eu3+ phosphors and Ca2 SnO4 :Eu3+ phosphors were determined at room temperature. In the case of the cubic CaSnO3 :Eu3+ phosphors, the typical excitation spectrum ranging from 250 to 550 nm at em = 613 nm is shown in Fig. 3a. As previously reported, the broad band at 273 nm comes from the charge transfer bands (CTB) between Eu3+ ions and the surrounding oxygen anions. Concerning sharp bands centered at 533, 464, 394 and 362 nm, they can be attributed to the characteristic transitions of Eu3+ from 7 F0 → 5 Di (i = 1, 2, 3 and 4). Fig. 3b depicts the typical red photoluminescence from Eu3+ ions in porous CaSnO3 :Eu3+ phosphors when rooting the excitation wavelengths at 393 nm and 273 nm respectively. A series of sharp bands at 589, 613, 652, and 695 nm are assignable to the characteristic transitions of Eu3+ from 5 D0 → 7 Fj (j = 1, 2, 3, and 4) [17], namely the 5 D0 → 7 F1 (589 nm), 5 D0 → 7 F2 (613 nm), 5 D0 → 7 F3 (652 nm), and 5 D0 → 7 F4 (695 nm). Besides, the most intense emission peak at 613 nm occurs through the forced electric dipole transition (5 D0 → 7 F2 ), whose intensity is very sensitive to the site symmetry of Eu3+ ions. Regarding the band located at 589 nm, it derives from the magnetic dipole transition (5 D0 → 7 F1 ), which is independent of the symmetry and the site occupied by Eu3+ ions in the host. As a rule, the intensity ratio of (5 D0 → 7 F2 )/(5 D0 → 7 F1 ), known as the asymmetry ratio, reveals the degree of distortion from the inversion symmetry of the local environment of Eu3+ ions in the host matrix [17]. And the asymmetry ratios for CaSnO3 :Eu3+ phosphors are ca. 2.18 and
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Fig. 4. CaSnO3 :Eu3+ phosphors with various shapes by introducing surfactant into the reaction system: (a,b) SDBS; (c,d) sodium citrate; (e,f) PVP.
2.98 calculated from Fig. 3b1 and 3b2 , respectively. On the other hand, taking into account the ionic radius of Eu3+ (0.95 Å) and Ca2+ (0.99 Å), Eu3+ ions can incorporate into perovskite-type CaSnO3 lattice and form substituted solid solution in the course of calcination at elevated temperature, certainly producing defects such as oxygen ionic vacancies owing to charge compensation mechanism. With respect to the as-prepared Ca2 SnO4 :Eu3+ phosphors, similar PL and PLE spectra occur except for minor difference, as illustrated in Fig. 3c and d. Noticeably, as compared to those of CaSnO3 :Eu3+ phosphors, Einstein shift happens towards 5 D0 → 7 F2 and 5 D0 → 7 F4 in Ca2 SnO4 :Eu3+ phosphors, which might owe to different crystal structures of the host materials [6].
4. Shape manipulation towards CaSnO3 :Eu3+ phosphors As is well known, the properties of nanomaterials strongly depend on the shape and size in nature [18]. It is thus of significance to manipulate these characteristics as we desire. Among the synthetic strategies for achieving this goal, certain kind of surfactant as shape-directing agent is usually introduced into the
Fig. 5. XRPD patterns of samples: (a) BaSnO3 :Eu3+ phosphors; (b) Ba2 SnO4 :Eu3+ phosphors; (c) SrSnO3 :Eu3+ phosphors; (d) Sr2 SnO4 :Eu3+ phosphors. Notes: Two unknown diffraction peaks occur in Figure 5d, as marked by asterisks.
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Fig. 6. FESEM images of the samples: (a,b) BaSnO3 :Eu3+ phosphors; (c,d) Ba2 SnO4 :Eu3+ phosphors; (e,f) SrSnO3 :Eu3+ phosphors; (g,h) Sr2 SnO4 :Eu3+ phosphors. The inset in 6 h is the corresponding enlarged FESEM image.
reaction system [19]. For instance, we recently prepared porous MgAl2 O4 :Eu3+ phosphors having rod-like or spherical shapes by adjusting the amounts of urea and poly(ethylene glycol) involved. It reveals that adding poly(ethylene glycol) 2000, primarily acting as a shape-director, into reaction system is crucial for preparing spherical MgAl2 O4 :Eu3+ phosphors [15]. In present study, in order
to manipulate the shape of CaSnO3 :Eu3+ phosphors, certain kinds of surfactants were introduced into the reaction system. Fig. 4a and b shows the typical FESEM images of CaSnO3 :Eu3+ by adding sodium dodecylbenzenesulfonate (SDBS) into the reaction system. Compared with Fig. 2a and b, it is obvious that these cubes are more uniform and regular, enclosing with six {1 0 0} planes. The possi-
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ble function of SDBS is to kinetically control the growth rates of six {1 0 0} planes by interacting with these faces through adsorption and desorption. This might make the CaSnO3 :Eu3+ phosphors grow up equally along 1 0 0 direction, commonly resulting in uniform cubes in shape. Besides, by introducing sodium citrate into the reaction system, the CaSnO3 :Eu3+ phosphors are composed of lots of hornless cubes, as shown in Fig. 4c and d. This implies that the sodium citrate added can slightly favor the formation of {1 1 1} planes, leading to the hornlessness of cubic CaSnO3 :Eu3+ phosphors. In addition, to our surprise, a great deal of eight-horn-shaped CaSnO3 :Eu3+ phosphors occur when introducing polyvinylpyrrolidone (PVP) into the reaction system, as shown in Fig. 4e and f, which originates from the assumption that the surfactant PVP added is selectively adsorbed on the {1 0 0} planes. This can thus reduce the growth rate of crystals along 1 0 0 direction, and increase the growth rate of crystals along 1 1 1 direction, inducing the formation of eight-horn-shaped CaSnO3 :Eu3+ phosphors. Moreover, as reported by Cheng and Lu [20], this kind of eight-horn-shaped shape is just the intermediate stage for CaSnO3 :Eu3+ phosphors. And with increasing the reaction time, cubic CaSnO3 :Eu3+ phosphors enclosing with six {1 0 0} planes will eventually come into being.
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a whole. As for the Ba2 SnO4 :Eu3+ phosphors, it is made up of lots of micrometer particles, which are in fact constructed of nanoscale rods, as depicted in Fig. 6c and d. Fig. 6e and f shows the FESEM images of the SrSnO3 :Eu3+ phosphors, which consist of a great deal of irregular particles. Regarding the Sr2 SnO4 :Eu3+ phosphors, it is composed of shuttle-like particles with ca. tens of micrometer in size, as shown in Fig. 6g and h. 6. Conclusions In summary, we demonstrate a general synthetic route, i.e., the co-precipitation method followed by postannealing treatment, to successfully prepare a series of alkaline-earth stannates phosphors, including MSnO3 :Eu3+ and M2 SnO4 :Eu3+ (M = Ca, Ba, and Sr) phosphors. The photoluminescent excitation and emission spectra of CaSnO3 :Eu3+ and Ca2 SnO4 :Eu3+ phosphors were studied in brief, illustrating the red-emitting feature. Besides, various kinds of additives were introduced to manipulate the shape and size of the CaSnO3 :Eu3+ phosphors. This facile synthetic method is expected to prepare other kinds of stannates phosphors, especially having porous feature in essence. Acknowledgements
5. The generality of the present synthetic method It is remarkable in this study that the present synthetic method can be extended to prepare other alkaline-earth stannates phosphors including BaSnO3 :Eu3+ , Ba2 SnO4 :Eu3+ , SrSnO3 :Eu3+ , and Sr2 SnO4 :Eu3+ phosphors by the co-precipitation method followed by postannealing treatment. Fig. 5a gives the typical XRPD pattern for the sample obtained by calcining the precursor at 800 ◦ C for 3 h, keeping BaCl2 ·2H2 O, Eu(NO3 )3 , and Na2 SnO3 as 1:0.05:1 at pH 10. Apparently, all the diffraction peaks in Fig. 5a can be assigned to cubic phase BaSnO3 (JCPDS Card No. 74-1300). Next, when calcining the precursor at 1100 ◦ C for 3 h, keeping BaCl2 ·2H2 O, Eu(NO3 )3 , Na2 SnO3 , and Na2 C2 O4 as 2:0.05:1:1 at pH 10, large amount of white powder appears, whose diffraction peaks can be indexed as tetragonal phase Ba2 SnO4 (JCPDS Card No. 74-1349), as shown in Fig. 5b. Similarly, cubic phase SrSnO3 (JCPDS Card No. 22-1442) and tetragonal phase Sr2 SnO4 (JCPDS Card No. 24-1241) were obtained by the co-precipitation method followed by postannealing treatment, which are shown in Fig. 5c and d, respectively. The representative FESEM images with various magnifications of the BaSnO3 :Eu3+ phosphors are shown in Fig. 6a and b. The panoramic view in Fig. 6a tells us that the BaSnO3 :Eu3+ phosphors is composed of irregular particles. Furthermore, the closer observation in Fig. 6b reveals that these large particles actually consist of abundant nanoscale particles, possessing the porous nature as
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