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1 Solid State Phenomena Vols (2007) pp Online: (2007) Trans Tech Publications, Switzerland doi: / Controlled Synthesis of BaF 2 Nanorods via Hydrothermal Microemulsion Method Y.C. CHEN 1,a, Y.G. ZHANG 1,2, b 1 Department of Chemistry, Anqing Normal College, Anqing , China 2 Department of Chemistry, University of Science & Technology of China, Hefei , China a huaxue@aqtc.edu.cn, b ygz@mail.ustc.edu.cn Keywords: Controlled synthesis, BaF 2, Nanorods, Microemulsion Abstract: BaF 2 nanorods were synthesized by hydrothermal microemulsion method using sodium fluoride (NaF) and barium chloride (BaCl 2 ) as the raw materials. The as-prepared products were characterized by powder X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), and transmission electron microscopy (TEM). The results showed that the products were composed of BaF 2 nanorods with diameters of nm and lengths up to 1µm. A directed aggregation growth process mediated by the microemulsion droplet building blocks is proposed for the formation of BaF 2 nanorods. Further work is in progress to evaluate the possibility of synthesizing other fluoride 1D nanostructures using a similar method. 1. Introduction Nanoscale one-dimensional structures have attracted considerable attention due to their unique electronic, optical, and mechanical properties. The development of synthesis methods, control of morphology, and assembly of desired nanostructure still remain a challenge in this field [1,2,3,4]. Barium fluoride is one of the dielectric fluorides (CaF 2, SrF 2, and BaF 2 ), which have a wide range of potential applications in microelectronic and optoelectronic devices, such as wide-gap insulating overlayers, gate dielectrics, insulators and buffer layers in semiconductor-on-insulator structures, and more advanced three dimensional structure devices [5]. BaF 2 activated with rare- earth ions has also been reported to display unique luminescence properties and can be used as scintillators [6]. With the rapid shrinking in size in electronic devices, nanometer-scale fluorides may play an essential role for their applications in future electronic nanodevices. Recently, Bender et al. have reported the synthesis of Nd: BaF 2 nanoparticles by the reverse microemulsion technique [7], Stouwdam et al. have fabricated lanthanide-doped LaF 3 nanoparticles by the use of capping agents [8], and Li et al. have reported CaF 2 nanocubes by a hydrothermal method with the absence of surfactants [9]. As ideal media for the synthesis of nanoparticles, microemulsion systems have been widely employed to prepare superfine spherical particles [10,11]. Water-in-oil microemulsions contain nanosized water pools which are dispersed in a continuous oil medium and are stabilized by surfactant and cosurfactant molecules localized at the water/oil interface. The nanoscale water pools can provide ideal microreactors for the formation of nanoparticles [12,13]. The hydrothermal method has been utilized in microemulsions as a treatment of the product to improve the crystallinity [14, 15]. There are also a few reports concerning hydrothermal micro- * Project supported by the National Natural Science Foundation of China ( ). 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 Trans Tech Publications, (ID: , Pennsylvania State University, University Park, USA-18/09/15,20:58:22)

2 442 Nanoscience and Technology emulsions as a synthesis method, but only large particles were obtained [16]. When temperature rises, the reverse micelles in microemulsion may be broken, resulting in fast nucleation of clusters in a random orientation and, consequently, a larger particle size [16]. It is this process that limits the application of the hydrothermal method on microemulsions. Here, we report the synthesis of BaF 2 nanorods via hydrothermal microemulsion method. 2. Experimental The BaF 2 nanorods were synthesized as follows: two identical solutions were prepared by dissolving CTAB (1g) in 20 ml of cyclohexane and 1 ml of ethylence glycol (EG). The mixing solution was stirred for 15 min until it became transparent. Next, 1 ml of 0.2 M BaCl 2 aqueous solution and 1 ml of 0.4 M NaF aqueous solution were added to the solutions, respectively. After substantial stirring, the two microemulsion solutions were mixed and stirred for another 10 min. The resulting microemulsion solution was then transferred into a 60 ml stainless Teflon-lined autoclave and heated at 130 ºC for 16 h. After that, the system was then allowed to cool to room temperature naturally. White products were collected by filtration, washed with distilled water and absolute ethanol several times. The final products were dried under a vacuum at 60 ºC for 3 h. 3.Results and discussion The overall crystallinity and purity of the obtained samples were examined by X-ray diffraction (XRD) measurements performed on a Rigaku X-ray diffractometer with Cu-Ka radiation (λ = Å). Fig.1 shows a typical XRD pattern of the as-prepared samples. All of the diffraction peaks in Fig.1 can be readily indexed to a pure cubic phase [space group: Fm3m (225)] with a lattice constant a = Å, which is in good agreement with the standard values for the bulk cubic BaF 2 (JCPDS ). No other impurities have been found in the synthesized products. (222) (400) (331) (420) (422) Intensity (a.u) (200) (220) (311) (111) θ (degree) Fig.1 XRD pattern of the as-prepared samples The size and morphology of as-synthesized BaF 2 products were further examined on a field-emission microscope (JEOL, 6700F) operated at 10 kv and a Hitachi Model H-800 transmission electron microscope operated at 200kV. Fig. 2a shows FE-SEM image of the as-prepared sample. The picture indicates that the products are primarily composed of rodlike structures, which is further confirmed by the TEM image (Fig. 2b). According to the FE-SEM and TEM images, the nanorods are nm in diameter, and up to 1µm in length. More details about the structure of nanorods were investigated by the selected area electron diffraction (SAED) pattern. The SAED image (inset in Fig. 2b) taken from a randomly selected nanoorod from the products, can be indexed as a cubic BaF 2 single crystal, which is in good agreement with the XRD results presented above. Moreover, the SEAD images taken from different positions along the nanorod

3 Solid State Phenomena Vols (without tilting the sample with respect to the electron beam) are found to be almost identical. This indicates that the entire nanorod exhibits single crystalline nature. Fig. 2 (a) FE-SEM image of the as-prepared samples; (b) TEM image and ED pattern (inset) of the as-prepared samples. With the above-mentioned synthetic method, BaF 2 nanorods are obtained instead of nanoparticles. The possible formation mechanism of nanorods is described as follows: It is well known that surfactants can form reverse micelles in nonpolar solvents, and water is readily solubilized in the polar core. These surfactant-covered water pools offer a unique microenvironment for the formation of nanoparticles. The formed nanoparticles can be seen as building blocks for the formation of nanorods at the suitable conditions, which the reverse micelles may array directionally to form rod-like aggregation, and the water channel is able to restrict the growth of the nuclei. In our experiment, CTAB is a cationic surfactant, its hydrophilic tails are directed inward toward the aqueous core in water pools of microemulsions, thus adsorbing sufficient [Ba 2+ ] or [F - ]. When the BaCl 2 and NaF microemulsions containing [Ba 2+ ] or [F - ] were mixed, water droplets would collide and fuse with the exchange of [Ba 2+ ] or [F - ], and simultaneously BaF 2 nuclei form rapidly at the collision interface. With the further progress of the fusion, the self-organization of stacked micelles would lead to rod-like or cylindrical aggregates, which possibly coalesced into each other to a 1-D shape by interaction among the hydrophobic tails of CTAB. The as- formed BaF 2 nuclei which can be seen as building blocks for the formation of BaF 2 nanorods, are gradually assembled into nanorods by a directed aggregation growth process within the water pools of rod-like reverse micelles. In this case, the hydrothermal treatment is used to improve the crystallinity of materials in microemulsions [17]. Certainly, this mechanism proposed needs to be confirmed by more studies. This schematic diagram of the proposed growth mechanism is shown in Fig.3. Ba 2+ + F - collide and fuse coalesce and grow Fig.3 Scheme illustrating the growth mechanism of BaF 2 nanostructures.

4 444 Nanoscience and Technology 4. Conclusions In summary, we have developed a microemulsion-mediated hydrothermal method to prepare BaF 2 nanorods. A directed aggregation growth process mediated by the microemulsion droplet building blocks is proposed for the formation of BaF 2 nanorods. The studies of the properties of as-prepared BaF 2 nanorods are underway. The hydrothermal microemulsion system used in the present work is expected to be extended to the synthesis of other fluoride 1D nanostructures, if appropriate reaction conditions and suitable microemulsions are available. References [1] J. Wang, Z. Deng, Y. Li: Mater. Res. Bull. Vol. 37 (2002), p. 495 [2] M. Chen, Y. Xie, Z. Yao, Y. Qian and G. Zhou: Mater. Res. Bull. Vol. 37 (2002), p. 247 [3] L. Manna, E. C. Scher and A. P. Alivisatos: J. Am. Chem. Soc. Vol. 122(2000), p [4] D.Yu, D. Wang, Z. Meng, J. Lu and Y. Qian: J. Mater. Chem. Vol. 12(2002), p.403 [5] R. Singh, S. Sinha, P. Chou, N.J. Hsu and F.Radpour: J. Appl. Phys. Vol. 66(1989), p.6179 [6] A.J. Wojtowicz: Nucl. Instrum. Methods A. Vol. 486(2002), p.201 [7] C. M. Bender, J. M. Burlitch: Chem. Mater. Vol. 12(2000), p.1969 [8] J. W. Stouwdam, F. C. J. M. van Veggel: Nano Lett. Vol. 2(2002), p.733 [9] X. M. Sun, Y. D. Li: Chem. Commun. 2003, p.1768 [10] M. Schwuger, K. Stickdom and R. Schomacker: Chem. Rev. Vol. 95(1995), p.849 [11] L. M. Gan, B. Liu, C. H. Chew, S. J. Xu, S. J. Chua and G. L. Loy: Langmuir.Vol. 13(1997), p.6427 [12] C. Tojo, M. C. Blanco, F. Rivadulla and M. A. Lopez-Quintela: Langmuir. Vol. 13(1997), p.1970 [13] M. Giustini, G. Palazzo, G. Colafemmina and M. D. Monica: J. Phys. Chem. Vol. 100(1996), p.3190 [14] S.J. Xu, S.J. Chua, B. Liu, L. M. Gan and C. H. Chew: Appl. Phys. Lett. Vol. 73(1998), p.478 [15] W.W. So, J.S. Jang, Y.W. Rhee and K. J. Kim: J.Colloid Interface Sci. Vol. 237(2001), p.136 [16] B. Liu, G.Q. Xu, L.M. Gan, C. H. Chew, W. S. Li and Z. X. Shen: J. Appl. Phys. Vol. 89(2001), p.1059 [17] S. J. Xu, S.J. Chua, B. Liu, L. M. Gan, C. H. Chew and G. Q. Xu: Appl. Phys. Lett. Vol. 73(1998), p.478

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