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A general hydrothermal route to synthesis of nanocrystalline lanthanide stannates: Ln2Sn2O7 (Ln = Y, La–Yb)
Journal of Alloys and Compounds 464 (2008) 508–513
A general hydrothermal route to synthesis of nanocrystalline lanthanide stannates: Ln2Sn2O7 (Ln = Y, La–Yb) Hongliang Zhu ∗ , Dalai Jin, Luming Zhu, Hong Yang, Kuihong Yao, Zhenqiang Xi Center of Materials Engineering, Zhejiang Sci-Tech University, Xiasha University Town, Hangzhou 310018, PR China Received 11 April 2007; accepted 6 October 2007 Available online 12 October 2007
Abstract A general hydrothermal process with use of lanthanide (III) nitrates and Na2 SnO3 as precursors has been proposed for synthesizing nanocrystalline lanthanide stannates with general formula: Ln2 Sn2 O7 (Ln = Y, La–Yb). Stannates of all lanthanides except for the radioactive promethium were successfully synthesized. Characterization by XRD and TEM revealed that all the products were phase-pure nanocrystalline lanthanide stannates with pyrochlore-type structure. Photoluminescent properties of three samples (i.e. Tb2 Sn2 O7 , Dy2 Sn2 O7 and Yb2 Sn2 O7 ) are also presented. The mole ratio of Ln(NO3 )3 :Na2 SnO3 and hydrothermal temperature were two key factors for this general hydrothermal route. © 2007 Elsevier B.V. All rights reserved. PACS: 78.55.−m; 81.16.Be; 81.07.Wx Keywords: Nanostructures; Chemical synthesis; Luminescence
1. Introduction During the past decade, pyrochlore-type oxides (A2 B2 O7 ) have emerged as important functional materials due to their interesting thermal, electrical, optical, magnetic and catalytic properties [1–7]. Among various pyrochlore-type oxides, lanthanide stannates (Ln2 Sn2 O7 (Ln = Y, La–Lu)) are a series of lanthanide–tin oxides, which have wide applications such as ionic/electric conductors [8], catalysts [9], lithium ion batteries [10], phosphors [11,12] and resistance to radiation damage [13]. More interestingly, lanthanide stannates have environment-related high temperature catalytic applications such as automobile exhaust gas and combustion gas control, due to their high stability and melting points (>2000 ◦ C) [14]. Up to now, conventional solid-state reaction is still the most commonly used synthetic method for preparation of rare-earth pyrochlore oxides. This synthesis route employs a solid-state reaction of SnO2 with appropriate rare-earth oxides at high tem-
∗
Corresponding author. Tel.: +86 571 86843266; fax: +86 571 86843266. E-mail address: [email protected] (H. Zhu).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.10.024
perature (>1200 ◦ C) for a long time (several days) [15,16]. In contrast, the hydrothermal method is much simpler, more practical and more cost-effective. In addition, the products prepared by the hydrothermal method are of higher chemical homogeneity, more accurate stoichiometry and smaller size. Therefore, the hydrothermal method is a satisfactory route to the synthesis of lanthanide stannates. Hydrothermal synthesis of La2 Sn2 O7 and Y2 Sn2 O7 :Eu3+ nanocrystals has been presented by several literatures [14,17,18]. In this paper, we propose a general hydrothermal route for the synthesis of nanocrystalline lanthanide stannates. Nanocrystalline lanthanide stannates (Ln2 Sn2 O7 (Ln = Y, La–Yb)), except for that of the radioactive promethium, have been successfully synthesized via this hydrothermal route. Compared to previous literatures [14,17,18], the hydrothermal route presented here is simpler, milder and more general. 2. Experimental procedure All the reagents were of analytical grade and were used without any further purification. In a typical procedure, 4 mmol of lanthanide nitrate hydrate (Ln(NO3 )3 ·6H2 O (Ln = Y, La–Yb)) and 4 mmol of sodium stannate hydrate (Na2 SnO3 ·3H2 O) were added to 100 ml of deionized water under magnetic stirring. The pH of the solution was measured using a pH meter (Model PHS-
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Table 1 The parameters and results for the experiments on the preparation of nanocrystalline Ln2 Sn2 O7 (Ln = Y, La–Yb) via the general hydrothermal route No.
Lanthanide nitrate
pH
Product
JCPDS no.
Grain size by XRD (nm)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Y(NO3 )3 ·6H2 O La(NO3 )3 ·6H2 O Ce(NO3 )3 ·6H2 O Pr(NO3 )3 ·6H2 O Nd(NO3 )3 ·6H2 O Sm(NO3 )3 ·6H2 O Eu(NO3 )3 ·6H2 O Gd(NO3 )3 ·6H2 O Tb(NO3 )3 ·6H2 O Dy(NO3 )3 ·6H2 O Ho(NO3 )3 ·6H2 O Er(NO3 )3 ·6H2 O Tm(NO3 )3 ·6H2 O Yb(NO3 )3 ·6H2 O Lu(NO3 )3 ·6H2 O
7.13 7.46 7.18 7.00 6.67 6.70 6.75 6.74 6.64 6.60 6.56 6.45 6.35 6.30 6.11
Y2 Sn2 O7 La2 Sn2 O7 Ce2 Sn2 O7 Pr2 Sn2 O7 Nd2 Sn2 O7 Sm2 Sn2 O7 Eu2 Sn2 O7 Gd2 Sn2 O7 Tb2 Sn2 O7 Dy2 Sn2 O7 Ho2 Sn2 O7 Er2 Sn2 O7 Tm2 Sn2 O7 Yb2 Sn2 O7 Lu2 Sn2 O7
73-1684 13-0082
8 20 20 25 25 25 20 20 20 15 25 20 20 25 20
13-0184 13-0185 13-0181 13-0182 13-0186 13-0188 88-0450 13-0179 13-0180 13-0172 88-0454 88-0455
Fig. 1. XRD patterns of the Ln2 Sn2 O7 (Ln = Tb, Dy and Yb) nanocrystals: (a) Tb2 Sn2 O7 , (b) Dy2 Sn2 O7 and (c) Yb2 Sn2 O7 .
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Fig. 2. TEM images of the Ln2 Sn2 O7 (Ln = Tb, Dy and Yb) nanocrystals; the insets are their relative SAED patterns: (a) Tb2 Sn2 O7 , (b) Dy2 Sn2 O7 and (c) Yb2 Sn2 O7 .
3C, Shanghai Rex Instruments Factory, China). Then, the above solution was transferred into a Teflon-lined stainless steel autoclave of 120 ml capacity and sealed. The autoclave was heated at 200 ◦ C for 24 h and cooled naturally to room temperature. Finally, the product was collected by centrifugation, washed with distilled water and alcohol several times and dried at 60 ◦ C for 24 h in air. The parameters and results for the experiments are given in Table 1. Phase identification of the lanthanide stannates was carried out by using an X-ray diffractometer (Model ARL X’TRA, Thermo Electron Corporation, USA) over a 2θ range from 20◦ to 90◦ at a scan rate of 5 ◦ /min. Their morphologies were observed by a transmission electron microscope (Model JEM 200 CX, JEOL Corporation, Japan) operated at 160 kV. The PL emission spectra were obtained on a spectrometer (Model spectrapro 2500i, Acton Research Corp., USA), using a He–Cd laser (325 nm) as a source of excitation.
3. Results and discussion All the lanthanide stannates were characterized by XRD, and their data were analyzed by a Thermo ARL WinXRD software package. As shown in Table 1, all the products synthesized by the hydrothermal reaction of lanthanide nitrate hydrates with sodium stannate hydrate can be indexed to their respective lanthanide stannates (Ln2 Sn2 O7 (Ln = Y, La–Lu)) with cubic
pyrochlore-type crystal structure. Average crystalline sizes of the products estimated by the Scherrer’s formula are also given in Table 1. Herein, we take Tb2 Sn2 O7 , Dy2 Sn2 O7 and Yb2 Sn2 O7 as examples to demonstrate their structures, morphologies and photoluminescent properties. XRD patterns of the Tb2 Sn2 O7 , Dy2 Sn2 O7 and Yb2 Sn2 O7 nanocrystals are shown in Fig. 1. Interestingly, they exhibited almost the same XRD patterns, except for a small 2θ difference. All the peaks of Fig. 1(a) can be indexed to cubic pyrochlore˚ which is in type Tb2 Sn2 O7 with the lattice constant a = 10.43 A, good agreement with the values from the standard card (JCPDS no. 13-0188). Likewise, XRD patterns of Fig. 1(b) and (c) can be indexed to cubic pyrochlore-type Dy2 Sn2 O7 and Yb2 Sn2 O7 , respectively. In addition, no impurity peaks such as those of SnO2 or Ln(OH)3 were detected, meaning that the products were phase-pure Ln2 Sn2 O7 . The relatively broad peaks of Fig. 1 indicate the small crystallite diameter of the Ln2 Sn2 O7 products. TEM images and selected area electron diffraction (SAED) patterns of the Ln2 Sn2 O7 (Ln = Tb, Dy and Yb) nanocrystals are shown in Fig. 2. Fig. 2 reveals that all the products are nanocrystals with a diameter of ∼20 nm. The SAED ring pat-
H. Zhu et al. / Journal of Alloys and Compounds 464 (2008) 508–513
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Fig. 3. PL emission spectra of the Ln2 Sn2 O7 (Ln = Tb, Dy and Yb) nanocrystals: (a) Tb2 Sn2 O7 , (b) Dy2 Sn2 O7 and (c) Yb2 Sn2 O7 .
terns taken from several nanocrystals confirm that the products are well crystallized. The d values corresponding to the three brightest rings of Fig. 2(a) (from the inner to the outer) are 2.60, ˚ consistent with the {4 0 0}, {4 4 0} and {4 4 4} 1.82 and 1.52 A, planes of the Tb2 Sn2 O7 (JCPDS no. 13-0188), respectively. The three brightest rings of the SAED patterns of the Dy2 Sn2 O7 nanocrystals (see the inset of Fig. 2(b)) corresponded well to the {4 4 4}, {5 3 1} and {6 6 2} planes of the Dy2 Sn2 O7 (JCPDS no. 88-0450), respectively. Likewise, d values of the rings of Fig. 2(c) were consistent with the JCPDS data (no. 88-0454) of the Yb2 Sn2 O7 . Up to now, a variety of rare-earth phosphates, vanadates, borates, aluminates and silicates have been developed as luminescent materials [19–22]. In contrast, rare-earth stannate phosphors are still not well studied. Due to their high chemical stability, high melting points and efficient photoluminescence, rare-earth stannates may be promising phosphor materials [11,17,18,23]. In this paper, photoluminescent (PL) properties
of the Ln2 Sn2 O7 (Ln = Tb, Dy and Yb) nanocrystals were investigated, and their PL emission spectra are shown in Fig. 3. As shown in Fig. 3(a), the Tb2 Sn2 O7 nanocrystals display obvious photoluminescence in the spectral range of 480–630 nm, corresponding to the 5 D4 → 7 Fj (j = 3–6) transitions [24]. The strongest emission band was located at 543 nm, which is a typical green band caused by the 5 D4 → 7 F5 transition of Tb3+ [24]. Fig. 3(b) shows that the Dy2 Sn2 O7 nanocrystals exhibited the PL emission bands centered at 480 and 573 nm, which were assigned to the 4 F9/2 → 6 H15/2 and 4 F9/2 → 6 H13/2 transitions, respectively. The luminescence of rare-earth ions has been well studied, but one kind of rare-earth luminescence is still relatively unknown: charge transfer (CT) luminescence. This transition is the reverse of the well-known charge transfer absorption [25,26]. Charge transfer luminescence of Yb3+ became of great interest due to possible applications of Yb-containing materials for scintillation detectors in neutrino physics [27]. CT luminescence is characterized by a broad emission band in the UV–vis
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Prior to the hydrothermal stage, H2 SnO3 and OH− were formed via the hydrolysis of Na2 SnO3 (represented in reaction (1)). Then, rare-earth ions (Ln3+ ) reacted with the OH− ions formed in reaction (1) to form Ln(OH)3 precipitate. Reaction (3) represents the formation of Ln2 Sn2 O7 via reaction between Ln(OH)3 and H2 SnO3 under hydrothermal condition. As shown in Fig. 4c and d, SnO2 nanoparticles were formed (reaction (4)) when excess H2 SnO3 existed in the solution. 4. Conclusions
Fig. 4. XRD patterns of the products prepared by four comparative experiments using different mole ratios of Eu(NO3 )3 :Na2 SnO3 (a) 3:2, (b) 1:1, (c) 2:3 and (d) 0:1.
spectral region [26,28]. Interestingly, as shown in Fig. 3(c), a broad CT emission band centered at 525 nm is clearly found in Yb2 Sn2 O7 . This means that the Yb2 Sn2 O7 nanocrystals exhibited strong charge transfer luminescence of Yb3+ at room temperature. Different from the hydrothemal process for synthesizing Y2 Sn2 O7 presented by previous literatures [9,17,18], in this hydrothermal process lanthanide stannates were formed by reaction of lanthanide nitrates with Na2 SnO3 . The mole ratio of Ln(NO3 )3 :Na2 SnO3 and hydrothermal temperature were two key factors for this general hydrothermal route. Herein, we take hydrothermal synthesis of Eu2 Sn2 O7 as an example to show the effects of the two key factors. Four comparative experiments using different mole ratios of Eu(NO3 )3 :Na2 SnO3 (e.g. 3:2, 1:1, 2:3 and 0:1) were carried out under the same reaction conditions. XRD patterns of the four products are shown in Fig. 4. Fig. 4 reveals that phase-pure Eu2 Sn2 O7 was obtained when the mole ratios were higher than 1:1. In addition, the Eu(NO3 )3 + Na2 SnO3 (mole ratio 1:1) solution was hydrothermally processed at 100, 150, 200 and 250 ◦ C. Phase-pure crystalline Eu2 Sn2 O7 was obtained above 150 ◦ C. According to the above results and the related experiments, the chemical mechanism for hydrothermal formation of the Ln2 Sn2 O7 can be expressed as follows: Na2 SnO3 + H2 O ⇔ H2 SnO3 + 2OH−
(1)
Ln3+ + 3OH− → Ln(OH)3 ↓
(2)
2Ln(OH)3 + 2H2 SnO3 H2 SnO3
hydrothermal
−→
hydrothermal
−→
SnO2 + H2 O
Ln2 Sn2 O7 + 5H2 O
(3) (4)
Nanocrystalline stannates of all lanthanides (Ln2 Sn2 O7 (Ln = Y, La–Yb)), except for that of the radioactive promethium, were successfully synthesized via this general hydrothermal route. Different from the hydrothemal process for Y2 Sn2 O7 presented by previous literatures, in this general hydrothermal process lanthanide stannates were formed by reaction of lanthanide nitrates with Na2 SnO3 . Consequently, this hydrothermal process was much simpler and milder. The mole ratio of Ln(NO3 )3 :Na2 SnO3 and hydrothermal temperature were two key factors for this general hydrothermal route. Three samples (i.e. Tb2 Sn2 O7 , Dy2 Sn2 O7 and Yb2 Sn2 O7 ) exhibited obvious photoluminescence in the visible range. References [1] J. Lian, L.M. Wang, S.X. Wang, J. Chen, L.A. Boatner, R.C. Ewing, Phys. Rev. Lett. 87 (2001) 145901. [2] J.H. Zhao, H.P. Kunkel, X.Z. Zhou, G. Williams, M.A. Subramanian, Phys. Rev. Lett. 83 (1999) 219. [3] S. Lutique, R.J.M. Konings, V.V. Rondinella, J. Somers, T. Wiss, J. Alloys Compd. 352 (2003) 1. [4] A. Nag, P. Dasgupta, Y.M. Jana, D. Ghosh, J. Alloys Compd. 384 (2004) 6. [5] J.M. Sohn, M.R. Kim, S. Ihl Woo, Catal. Today 83 (2003) 289. [6] N. Ali, P. Hill, X. Zhang, F. Willis, J. Alloys Compd. 181 (1992) 281. [7] C.P. Grey, C.M. Debson, A.K. Cheethan, R.J.B. Jakeman, J. Am. Chem. Soc. 111 (1989) 505. [8] T. Yu, H.L. Tuller, Solid State Ionics 86–88 (1996) 177. [9] K. Li, H. Wang, H. Yan, J. Mol. Catal. A: Chem. 249 (2006) 65. [10] N. Sharma, G.V. Subba Rao, B.V.R. Chowdari, J. Power Sources 159 (2006) 340. [11] S. Wang, M. Lu, G. Zhou, Y. Zhou, H. Zhang, S. Wang, Z. Yang, Mater. Sci. Eng. B: Solid 133 (2006) 231. [12] K. Li, T. Zhang, H. Wang, H. Yan, J. Solid State Chem. 179 (2006) 1029. [13] K.E. Sickafus, L. Minervini, R.W. Grimes, J.A. Valdez, M. Ishimaru, F. Li, K.J. McClellan, T. Hartmann, Science 289 (2000) 748. [14] J. Moon, J. Am. Ceram. Soc. 84 (2001) 2531. [15] K. Tezuka, Y. Hinasu, J. Solid State Chem. 143 (1999) 140. [16] A.M. Srivastava, Mater. Res. Bull. 37 (2001) 745. [17] Z. Lu, J. Wang, Y. Tang, Y. Li, J. Solid State Chem. 177 (2004) 3075. [18] K. Li, H. Li, H. Zhang, R. Yu, H. Wang, H. Yan, Mater. Res. Bull. 41 (2006) 191. [19] D.W. Cooke, R.E. Muenchausen, K.J. McClellan, B.L. Bennett, Opt. Mater. 27 (2005) 1781. [20] B. Yan, X. Su, Opt. Mater. 29 (2007) 547. [21] H. Lin, D. Yang, G. Liu, T. Ma, B. Zhai, Q. An, J. Yu, X. Wang, X. Liu, E. Yue-Bun Punc, J. Lumin. 113 (2005) 121.
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[25] E. Nakazawa, Chem. Phys. Lett. 56 (1978) 161. [26] L. van Pieterson, M. Heeroma, E. de Heer, A. Meijerink, J. Lumin. 91 (2000) 177. [27] R.S. Raghavan, Phys. Rev. Lett. 78 (1997) 3618. [28] M. Nikl, A. Yoshikawa, T. Fukuda, Opt. Mater. 26 (2004) 545.
A general hydrothermal route to synthesis of nanocrystalline lanthanide stannates: Ln2Sn2O7 (Ln = Y, La–Yb) Hongliang Zhu ∗ , Dalai Jin, Luming Zhu, Hong Yang, Kuihong Yao, Zhenqiang Xi Center of Materials Engineering, Zhejiang Sci-Tech University, Xiasha University Town, Hangzhou 310018, PR China Received 11 April 2007; accepted 6 October 2007 Available online 12 October 2007
Abstract A general hydrothermal process with use of lanthanide (III) nitrates and Na2 SnO3 as precursors has been proposed for synthesizing nanocrystalline lanthanide stannates with general formula: Ln2 Sn2 O7 (Ln = Y, La–Yb). Stannates of all lanthanides except for the radioactive promethium were successfully synthesized. Characterization by XRD and TEM revealed that all the products were phase-pure nanocrystalline lanthanide stannates with pyrochlore-type structure. Photoluminescent properties of three samples (i.e. Tb2 Sn2 O7 , Dy2 Sn2 O7 and Yb2 Sn2 O7 ) are also presented. The mole ratio of Ln(NO3 )3 :Na2 SnO3 and hydrothermal temperature were two key factors for this general hydrothermal route. © 2007 Elsevier B.V. All rights reserved. PACS: 78.55.−m; 81.16.Be; 81.07.Wx Keywords: Nanostructures; Chemical synthesis; Luminescence
1. Introduction During the past decade, pyrochlore-type oxides (A2 B2 O7 ) have emerged as important functional materials due to their interesting thermal, electrical, optical, magnetic and catalytic properties [1–7]. Among various pyrochlore-type oxides, lanthanide stannates (Ln2 Sn2 O7 (Ln = Y, La–Lu)) are a series of lanthanide–tin oxides, which have wide applications such as ionic/electric conductors [8], catalysts [9], lithium ion batteries [10], phosphors [11,12] and resistance to radiation damage [13]. More interestingly, lanthanide stannates have environment-related high temperature catalytic applications such as automobile exhaust gas and combustion gas control, due to their high stability and melting points (>2000 ◦ C) [14]. Up to now, conventional solid-state reaction is still the most commonly used synthetic method for preparation of rare-earth pyrochlore oxides. This synthesis route employs a solid-state reaction of SnO2 with appropriate rare-earth oxides at high tem-
∗
Corresponding author. Tel.: +86 571 86843266; fax: +86 571 86843266. E-mail address: [email protected] (H. Zhu).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.10.024
perature (>1200 ◦ C) for a long time (several days) [15,16]. In contrast, the hydrothermal method is much simpler, more practical and more cost-effective. In addition, the products prepared by the hydrothermal method are of higher chemical homogeneity, more accurate stoichiometry and smaller size. Therefore, the hydrothermal method is a satisfactory route to the synthesis of lanthanide stannates. Hydrothermal synthesis of La2 Sn2 O7 and Y2 Sn2 O7 :Eu3+ nanocrystals has been presented by several literatures [14,17,18]. In this paper, we propose a general hydrothermal route for the synthesis of nanocrystalline lanthanide stannates. Nanocrystalline lanthanide stannates (Ln2 Sn2 O7 (Ln = Y, La–Yb)), except for that of the radioactive promethium, have been successfully synthesized via this hydrothermal route. Compared to previous literatures [14,17,18], the hydrothermal route presented here is simpler, milder and more general. 2. Experimental procedure All the reagents were of analytical grade and were used without any further purification. In a typical procedure, 4 mmol of lanthanide nitrate hydrate (Ln(NO3 )3 ·6H2 O (Ln = Y, La–Yb)) and 4 mmol of sodium stannate hydrate (Na2 SnO3 ·3H2 O) were added to 100 ml of deionized water under magnetic stirring. The pH of the solution was measured using a pH meter (Model PHS-
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Table 1 The parameters and results for the experiments on the preparation of nanocrystalline Ln2 Sn2 O7 (Ln = Y, La–Yb) via the general hydrothermal route No.
Lanthanide nitrate
pH
Product
JCPDS no.
Grain size by XRD (nm)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Y(NO3 )3 ·6H2 O La(NO3 )3 ·6H2 O Ce(NO3 )3 ·6H2 O Pr(NO3 )3 ·6H2 O Nd(NO3 )3 ·6H2 O Sm(NO3 )3 ·6H2 O Eu(NO3 )3 ·6H2 O Gd(NO3 )3 ·6H2 O Tb(NO3 )3 ·6H2 O Dy(NO3 )3 ·6H2 O Ho(NO3 )3 ·6H2 O Er(NO3 )3 ·6H2 O Tm(NO3 )3 ·6H2 O Yb(NO3 )3 ·6H2 O Lu(NO3 )3 ·6H2 O
7.13 7.46 7.18 7.00 6.67 6.70 6.75 6.74 6.64 6.60 6.56 6.45 6.35 6.30 6.11
Y2 Sn2 O7 La2 Sn2 O7 Ce2 Sn2 O7 Pr2 Sn2 O7 Nd2 Sn2 O7 Sm2 Sn2 O7 Eu2 Sn2 O7 Gd2 Sn2 O7 Tb2 Sn2 O7 Dy2 Sn2 O7 Ho2 Sn2 O7 Er2 Sn2 O7 Tm2 Sn2 O7 Yb2 Sn2 O7 Lu2 Sn2 O7
73-1684 13-0082
8 20 20 25 25 25 20 20 20 15 25 20 20 25 20
13-0184 13-0185 13-0181 13-0182 13-0186 13-0188 88-0450 13-0179 13-0180 13-0172 88-0454 88-0455
Fig. 1. XRD patterns of the Ln2 Sn2 O7 (Ln = Tb, Dy and Yb) nanocrystals: (a) Tb2 Sn2 O7 , (b) Dy2 Sn2 O7 and (c) Yb2 Sn2 O7 .
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Fig. 2. TEM images of the Ln2 Sn2 O7 (Ln = Tb, Dy and Yb) nanocrystals; the insets are their relative SAED patterns: (a) Tb2 Sn2 O7 , (b) Dy2 Sn2 O7 and (c) Yb2 Sn2 O7 .
3C, Shanghai Rex Instruments Factory, China). Then, the above solution was transferred into a Teflon-lined stainless steel autoclave of 120 ml capacity and sealed. The autoclave was heated at 200 ◦ C for 24 h and cooled naturally to room temperature. Finally, the product was collected by centrifugation, washed with distilled water and alcohol several times and dried at 60 ◦ C for 24 h in air. The parameters and results for the experiments are given in Table 1. Phase identification of the lanthanide stannates was carried out by using an X-ray diffractometer (Model ARL X’TRA, Thermo Electron Corporation, USA) over a 2θ range from 20◦ to 90◦ at a scan rate of 5 ◦ /min. Their morphologies were observed by a transmission electron microscope (Model JEM 200 CX, JEOL Corporation, Japan) operated at 160 kV. The PL emission spectra were obtained on a spectrometer (Model spectrapro 2500i, Acton Research Corp., USA), using a He–Cd laser (325 nm) as a source of excitation.
3. Results and discussion All the lanthanide stannates were characterized by XRD, and their data were analyzed by a Thermo ARL WinXRD software package. As shown in Table 1, all the products synthesized by the hydrothermal reaction of lanthanide nitrate hydrates with sodium stannate hydrate can be indexed to their respective lanthanide stannates (Ln2 Sn2 O7 (Ln = Y, La–Lu)) with cubic
pyrochlore-type crystal structure. Average crystalline sizes of the products estimated by the Scherrer’s formula are also given in Table 1. Herein, we take Tb2 Sn2 O7 , Dy2 Sn2 O7 and Yb2 Sn2 O7 as examples to demonstrate their structures, morphologies and photoluminescent properties. XRD patterns of the Tb2 Sn2 O7 , Dy2 Sn2 O7 and Yb2 Sn2 O7 nanocrystals are shown in Fig. 1. Interestingly, they exhibited almost the same XRD patterns, except for a small 2θ difference. All the peaks of Fig. 1(a) can be indexed to cubic pyrochlore˚ which is in type Tb2 Sn2 O7 with the lattice constant a = 10.43 A, good agreement with the values from the standard card (JCPDS no. 13-0188). Likewise, XRD patterns of Fig. 1(b) and (c) can be indexed to cubic pyrochlore-type Dy2 Sn2 O7 and Yb2 Sn2 O7 , respectively. In addition, no impurity peaks such as those of SnO2 or Ln(OH)3 were detected, meaning that the products were phase-pure Ln2 Sn2 O7 . The relatively broad peaks of Fig. 1 indicate the small crystallite diameter of the Ln2 Sn2 O7 products. TEM images and selected area electron diffraction (SAED) patterns of the Ln2 Sn2 O7 (Ln = Tb, Dy and Yb) nanocrystals are shown in Fig. 2. Fig. 2 reveals that all the products are nanocrystals with a diameter of ∼20 nm. The SAED ring pat-
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Fig. 3. PL emission spectra of the Ln2 Sn2 O7 (Ln = Tb, Dy and Yb) nanocrystals: (a) Tb2 Sn2 O7 , (b) Dy2 Sn2 O7 and (c) Yb2 Sn2 O7 .
terns taken from several nanocrystals confirm that the products are well crystallized. The d values corresponding to the three brightest rings of Fig. 2(a) (from the inner to the outer) are 2.60, ˚ consistent with the {4 0 0}, {4 4 0} and {4 4 4} 1.82 and 1.52 A, planes of the Tb2 Sn2 O7 (JCPDS no. 13-0188), respectively. The three brightest rings of the SAED patterns of the Dy2 Sn2 O7 nanocrystals (see the inset of Fig. 2(b)) corresponded well to the {4 4 4}, {5 3 1} and {6 6 2} planes of the Dy2 Sn2 O7 (JCPDS no. 88-0450), respectively. Likewise, d values of the rings of Fig. 2(c) were consistent with the JCPDS data (no. 88-0454) of the Yb2 Sn2 O7 . Up to now, a variety of rare-earth phosphates, vanadates, borates, aluminates and silicates have been developed as luminescent materials [19–22]. In contrast, rare-earth stannate phosphors are still not well studied. Due to their high chemical stability, high melting points and efficient photoluminescence, rare-earth stannates may be promising phosphor materials [11,17,18,23]. In this paper, photoluminescent (PL) properties
of the Ln2 Sn2 O7 (Ln = Tb, Dy and Yb) nanocrystals were investigated, and their PL emission spectra are shown in Fig. 3. As shown in Fig. 3(a), the Tb2 Sn2 O7 nanocrystals display obvious photoluminescence in the spectral range of 480–630 nm, corresponding to the 5 D4 → 7 Fj (j = 3–6) transitions [24]. The strongest emission band was located at 543 nm, which is a typical green band caused by the 5 D4 → 7 F5 transition of Tb3+ [24]. Fig. 3(b) shows that the Dy2 Sn2 O7 nanocrystals exhibited the PL emission bands centered at 480 and 573 nm, which were assigned to the 4 F9/2 → 6 H15/2 and 4 F9/2 → 6 H13/2 transitions, respectively. The luminescence of rare-earth ions has been well studied, but one kind of rare-earth luminescence is still relatively unknown: charge transfer (CT) luminescence. This transition is the reverse of the well-known charge transfer absorption [25,26]. Charge transfer luminescence of Yb3+ became of great interest due to possible applications of Yb-containing materials for scintillation detectors in neutrino physics [27]. CT luminescence is characterized by a broad emission band in the UV–vis
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Prior to the hydrothermal stage, H2 SnO3 and OH− were formed via the hydrolysis of Na2 SnO3 (represented in reaction (1)). Then, rare-earth ions (Ln3+ ) reacted with the OH− ions formed in reaction (1) to form Ln(OH)3 precipitate. Reaction (3) represents the formation of Ln2 Sn2 O7 via reaction between Ln(OH)3 and H2 SnO3 under hydrothermal condition. As shown in Fig. 4c and d, SnO2 nanoparticles were formed (reaction (4)) when excess H2 SnO3 existed in the solution. 4. Conclusions
Fig. 4. XRD patterns of the products prepared by four comparative experiments using different mole ratios of Eu(NO3 )3 :Na2 SnO3 (a) 3:2, (b) 1:1, (c) 2:3 and (d) 0:1.
spectral region [26,28]. Interestingly, as shown in Fig. 3(c), a broad CT emission band centered at 525 nm is clearly found in Yb2 Sn2 O7 . This means that the Yb2 Sn2 O7 nanocrystals exhibited strong charge transfer luminescence of Yb3+ at room temperature. Different from the hydrothemal process for synthesizing Y2 Sn2 O7 presented by previous literatures [9,17,18], in this hydrothermal process lanthanide stannates were formed by reaction of lanthanide nitrates with Na2 SnO3 . The mole ratio of Ln(NO3 )3 :Na2 SnO3 and hydrothermal temperature were two key factors for this general hydrothermal route. Herein, we take hydrothermal synthesis of Eu2 Sn2 O7 as an example to show the effects of the two key factors. Four comparative experiments using different mole ratios of Eu(NO3 )3 :Na2 SnO3 (e.g. 3:2, 1:1, 2:3 and 0:1) were carried out under the same reaction conditions. XRD patterns of the four products are shown in Fig. 4. Fig. 4 reveals that phase-pure Eu2 Sn2 O7 was obtained when the mole ratios were higher than 1:1. In addition, the Eu(NO3 )3 + Na2 SnO3 (mole ratio 1:1) solution was hydrothermally processed at 100, 150, 200 and 250 ◦ C. Phase-pure crystalline Eu2 Sn2 O7 was obtained above 150 ◦ C. According to the above results and the related experiments, the chemical mechanism for hydrothermal formation of the Ln2 Sn2 O7 can be expressed as follows: Na2 SnO3 + H2 O ⇔ H2 SnO3 + 2OH−
(1)
Ln3+ + 3OH− → Ln(OH)3 ↓
(2)
2Ln(OH)3 + 2H2 SnO3 H2 SnO3
hydrothermal
−→
hydrothermal
−→
SnO2 + H2 O
Ln2 Sn2 O7 + 5H2 O
(3) (4)
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