Square-shaped Nd2Sn2O7 Nanocrystals Prepared by ...

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Applied Mechanics and Materials Vol. 238 (2012) pp 79-82 © (2012) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/AMM.238.79

Square-shaped Nd2Sn2O7 Nanocrystals Prepared by Salt-assistant Combustion Method Yuping Tong1,a, Xi Chen1,b, Shunbo Zhao1,c and Lude Lu2,d 1

School of Civil Engineering and Communication, North China University of Water Resources and Electric Power, Zhengzhou 450011, China

2

Key Laboratory Soft Chemistry and Functional Materials of Ministry Education, Nanjing University of Science and Technology, Nanjing 210094, China a

[email protected]; [email protected]; [email protected]; d [email protected]

Keywords: nanomaterials; ceramics; salt-assistant combustion method; X-ray.

Abstract. Ultrafine square-shaped pyrochlore-type Nd2Sn2O7 nanocrystals were synthesized by a convenient salt-assisted combustion process using glycine as fuel. The products were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), Raman spectroscopy, Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). The results indicate that the products are phase-pure nanocrystals with pyrochlore-type structure. TEM and HRTEM images reveal that the products are composed of well-dispersed square-shaped Nd2Sn2O7 nanocrystals with the average size of 30 nm and the crystallite is structurally uniform and crystalline. The presented method provides a convenient and low-cost route for the synthesis of oxide materials nanostructures. Introduction The pyrochlore structure-type has a wide variety of compositions with a wide range of technologically important properties, such as the interesting thermal, optical, electrical, magnetic and cataltytic properties [1-4]. The pyrochlore oxides have the chemical composition A2B2O7, and their basic framework is a three-dimensional corner-sharing network of BO6 octahedra [5]. Among various pyrochlore oxides, lanthanide stannates (Ln2Sn2O7 (Ln=Y, La-Lu), form a group of isostructural compounds. There has been an initial interest in stannate pyrochlores stemmed from their high efficiency for the oxidative coupling of methane and their potential use as high-temperature gas sensors or fast ion conductors [6], due to their high stability and melting points. Great efforts have been made to devise versatile techniques to synthesize Ln2Sn2O7, such as conventional solid state reaction [7], co-precipitation [8], sol-gel [9], hydrothermal technique [10] and so forth. Conventional solid state synthesis employs a solid-state reaction of SnO2 with appropriate rare-earth oxides at high temperature (>1200oC) for a long time (several days). The method is simple, nevertheless it inherits several drawbacks. As is well known, the high temperature in the preparation process can easily lead to nonuniform in part in chemical component and large sized particles. The present paper has shown that nanosized perovskite oxide powder can be prepared by glycine combustion synthetic method [11], and this may provide another possible method to synthesize Nd2Sn2O7 nanoparticles. In many previous reports, it was shown that inert salt played an important role in the reaction process [12-14]. The emphasis of this work is to prepare and characterize Nd2Sn2O7 nanocrystals via salt-assisted combustion method (SACM) using glycine as fuel. Experimental Preparation of Nd2Sn2O7 Nanocrystals. All reagents were of analytical grade and used without further purification. The fabrication procedure of Nd2Sn2O7 is illustrated as followed. Nd(NO3)3·nH2O obtained from Nd2O3 dissolved in HNO3 and SnCl4·5H2O were used as the All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, www.ttp.net. (ID: 218.206.243.165-28/09/12,09:05:08)

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precursors of Nd and Sn, respectively. Firstly, appropriate amount of glycine was dissolved in 60 ml deionized water. Then proper amounts of Nd(NO3)3·nH2O, SnCl4·5H2O and KCl were added to above glycine aqueous solution in turn. The mixed solution was vigorously stirred for 2h at 60oC and evaporated at 110oC. At this stage, the viscous liquids swelled, followed by the evolution of gases, and self-propagating solution combustion slowly occurred, yielding loose powders. The obtained powders were calcined at 700oC for 5h in air. In order to remove salt, the as-calcined powders were filtered and washed with hot deionized water and ethanol. Finally, the product was dried in an oven at 80oC. For comparison, a parallel experiment was done in the absence of KCl. Instrumentation. The crystalline phase structure was determined by Bruker D8 Advance X-ray diffractometer (XRD) using Cu Kα radiation. Raman spectra were run on a Renishaw in Raman microscope. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images were recorded on a JEOL JEM-2100 transmission electron microscope. Results and Discussion The XRD measurement of Nd2Sn2O7 sample prepared by salt-assisted glycine combustion process is shown in Fig. 1, and all the diffraction data can be indexed to the cubic phase of Nd2Sn2O7 crystals (JCPDS No: 13-0185). No peaks of impurity are detected in the XRD patterns, indicating that pure crystalline Nd2Sn2O7 was fabricated by the presented procedures. (222)

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2q/degree XRD pattern of the Nd2Sn2O7 nanocrystals

Fig.1

The pyrochlore structure was also identified by Raman spectra. And the Raman spectrum of Nd2Sn2O7 nanocrystals is shown in Fig. 2. The Raman bands at 302, 403, 497 and 671 cm-1 are all clearly visible, which is similar to that reported [13]. The band at 671 cm-1 is attributed to the stretching vibration of O-Sn. 7000

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Raman spectrum of Nd2Sn2O7 nanocrystals

The direct information about the size, shape and agglomeration of the produced Nd2Sn2O7 nanocrystals are shown in Fig. 3. As shown in Fig.3a, the samples obtained are inseparable three-dimensional network agglomerates with their particle size in the submicrometer range in the

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combustion process without KCl. Fig.3b reveals that the products are composed of well-dispersed square-shaped Nd2Sn2O7 nanocrystals with the average size of 30 nm. Compared with conventional combustion synthesis, the introduction of KCl in the solution combustion reaction process can effectively prevent nanocrystallites from sintering and forming inseparable three-dimensional network, which could result in the formation of well-dispersed nanoparticles. Also, a better dispersibility was shown than that reported similar series by hydrothermal method [11]. The corresponding HRTEM image (Fig.3c) shows the clear lattice orientation and the regular succession of the atomic planes, indicating that the crystallite is structurally uniform and crystalline. The lattice spacing of 0.301 nm is consistent with that of the (222) plane of Nd2Sn2O7 nanocrystals.

Fig. 3 Representative TEM images of Nd2Sn2O7 nanocrystals: (a) TEM image in the absence of KCl; (b) TEM image and (c) HRTEM image in the presence of KCl. The role of KCl in the formation process of well-dispersed Nd2Sn2O7 nanocrysals was assumed as follows. As is well known, the self-propagating combustion reaction can release large amount of heat in a very short time and result in the instant high temperature of the reaction system, and the particles can be formed. Meanwhile, the salt precipitation in situ is completed in an instant to form a thin layer of salt crust on the surface of the newly formed nanoparticles. After the rapid cooling, the salt-coated Nd2Sn2O7 particles are trapped in the salt matrix, which prevents the reagglomeration. And well-dispersed Nd2Sn2O7 nanocrystals can be obtained eventually. Conclusions Nd2Sn2O7 nanocrystals were successfully synthesized via this simple salt-assisted combustion method. Compared with conventional solid state reaction, the preparation temperature was lowered (from 1200oC to 700oC) and the procedure was simpler and milder. The control route was easily manipulative and well repeatable. It was expected to create other oxides nanocrystals using this method. The TEM results indicated Nd2Sn2O7 nanocrystals were square-shaped with good dispersibility and its average size was 30nm. And Nd2Sn2O7 nanocrystals obtained by this procedure showed the better dispersibility and uniformity than that reported similar series by other method. It was also expected to create other oxides nanocrystals using this method.

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Acknowledgments The work was supported by the Technological Leading Talent plan of Zhengzhou City (096SYJH23105), Natural Science Research Projects of Education Department of Henan Province (2010B430018) and Key Programs for Science and Technology Development of Henan Province (122102210239). References [1] K. J. Moreno, A. F. Fuetes, J. García-Barriocanal, et al, Mechanochemical synthesis and ionic conductivity in the Gd2(Sn1 yZry)2O7 (0≤y≤1) solid solution, Journal of Solid State Chemistry, 179 (2006) 323-330. [2] D. W. Hwang, H. G. Kim, J. S. Lee, et al, Photocatalytic hydrogen production from water over M-doped La2Ti2O7 (M=Cr, Fe) under visible light irradiation, Journal of Physics and Chemistry B, 109 (2005) 2093-2102. [3] J. M. Sohn, M. R. Kim, S. Ihl Woo, The catalytic activity and surface characterization of Ln2B2O7 (Ln=Sm, Eu, Gd and Tb; B=Ti or Zr) with pyrochlore structure as novel CH4 combustion catalyst, Catalysis Today, 83 (2003) 289-297. [4] H. G. Kim, D. W. Hwang, S. W. Bae, et al, Photocatalytic water splitting over La2Ti2O7 synthesized by the polymerizable complex method, Catalysis Letters, 91 (2003) 193-198. [5] K.W. Li, H. L. Li, H. M. Zhang, et al, Hydrothermal synthesis of Eu3+-doped Y2Sn2O7 nanocrystals, Materials Research Bulletin, 41 (2006) 191-197. [6] B. J. Kennedy, B. A. Hunter, C. J. Howard, Structural and bonding trends in tin pyrochlore oxides, Journal of Solid State Chemistry, 130 (1997) 58-65. [7] Ismunandar, B. J. Kennedy, B. A. Huner, et al, Bonding and structural variation in doped Bi2Sn2O7, Journal of Solid State Chemistry, 131 (1997) 317-325. [8] V. Ravi, S. Adyanthaya, M. Aslam, et al, Synthesis of tin bismuth pyrochlore, Materials Letters, 40 (1999) 11-13. [9] Z. Lu, J. Wang, Y. Tang, et al, Synthesis and photoluminescence of Eu3+-doped Y2Sn2O7 nanocrystals, Journal of Solid State Chemistry, 177 (2004) 3075-3079. [10] H. Zhu, D. Jin, L. Zhu, et al, A general hydrothermal route to synthesis of nanocrystalline lanthanide stannates: Ln2Sn2O7 (Ln: Y, La-Yb), Journal of Alloys and Compounds, 464 (2008) 508-513. [11] Y. Wang, J. Zhu, X. Yang, et al, Preparation and characterization of LaNiO3 nanocrystals, Materials Research, 41 (2006) 1565-1570. [12] Y. Tong, J. Zhu, L. Lu, et al, Preparation and characterization of Ln2Zr2O7 (Ln = La and Nd) nanocrystals and their photocatalytic properties, Journal of Alloys and Compounds, 465 (2008) 280-284. [13] Y. Tong, Y. Wang, Salt-assistant combustion synthesis of nanocrystalline Nd2(Zr1-xSnx)2O7 (0≤x≤1), Materials Characterization, 60 (2009) 1382-1386. [14] W. Chen, F. Li, J. Yu, Salt-assisted combustion synthesis of highly dispersed pervoskite NdCoO3 nanoparticles, Materials Letters, 61 (2007) 397-400.