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Controlled synthesis and characterization of monazite type monocrystalline nanowires of mixed lanthanide orthophosphates
Solid State Communications 130 (2004) 125–129 www.elsevier.com/locate/ssc
Controlled synthesis and characterization of monazite type monocrystalline nanowires of mixed lanthanide orthophosphates Zheng-Guang Yana, Ya-Wen Zhanga,*, Li-Ping Youb, Rui Sia, Chun-Hua Yana,* a
State Key Laboratory of Rare Earth Materials Chemistry and Applications and PKU-HKU Joint Laboratory on Rare Earth Materials and Bioinorganic Chemistry, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China b Electron Microscopy Laboratory, Peking University, Beijing 100871, China Received 8 August 2003; accepted 28 December 2003 by Z.Z. Gan
Abstract Monazite type monocrystalline nanowires of mixed lanthanide orthophosphates LnTb(PO4)2 (Ln ¼ Ce, Nd, Sm) were synthesized via a simple hydrothermal process under optimal heat-treatment temperature (240 8C) and the starting acidity of the stock solution (pH 0.8). The as-synthesized nanowires were characterized by X-ray diffraction, transmission electron microscopy (TEM) and high-resolution TEM coupled with energy dispersive X-ray analysis, and infrared spectroscopy. The length and width of the nanowires were in the range of 2 – 10 mm and 20 – 350 nm, respectively. q 2004 Elsevier Ltd. All rights reserved. PACS: 61.46. þ w; 81.05.Ys; 81.10.Dn Keywords: A. Nanostructures; B. Chemical Synthesis; C. Transmission electron microscopy
1. Introduction One-dimensional (1D) nanomaterials including nanowires, nanobelts, nanotubes and nanorods have been a focal area of intensive researches for their unique electronic, optical and catalytic properties associated with the reduced dimensionality, and their great importance both in mesoscopic physics and nanoscale device fabrication [1 – 7]. In order to synthesize 1D nanomaterials based on a variety of chemicals such as elemental semiconductors, II – VI and III – V semiconductors, metal oxides, metal and inorganic salts, a lot of methods including templating direction, catalytic growth, electrochemistry, chemical vapor deposition, and solution based solvothermal or hydrothermal treatment have been developed [1 – 4]. Among these techniques, hydrothermal route is a robust and promising one for the fabrication of 1D nanomaterials with wellcontrolled shape, size, phase-purity, crystallinity and chemical composition on large scale [2,4– 7]. * Corresponding authors. Tel./fax: þ 86-10-62754179. E-mail address: [email protected] (C.H. Yan). 0038-1098/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2003.12.044
Lanthanide compounds form a large family of versatile materials in electronic, optic, catalytic and more areas, and several studies have been conducted on 1D nanomaterials of lanthanide compounds [8– 11]. Lanthanide orthophosphates have been extensively applied as phosphors, laser hosts, heat resistant materials and moisture sensors based on their electronic and luminescent properties. Several mixed orthophosphates composed of two lanthanide elements were also suggested to be used as disposals of nuclear waste or as host lattices for spectroscopic investigations [12– 18]. Lanthanide orthophosphates with the chemical formula LnPO4·n H2O ðn ¼ 0 – 3Þ (Ln3þ ¼ lanthanide ion) have five polymorphs: monazite (monoclinic), xenotime (tetragonal), rhabdophane (hexagonal), weinschenkite (monoclinic), and orthorhombic [18]. Very recently, we have managed to synthesize nanowires of light and medium lanthanide orthophosphates exhibiting monazite structure by a hydrothermal method, in which we finely tuned the acidity and selectively controlled the heat treatment temperature [19]. Since structural study has revealed that three lanthanide orthophosphate composites, LnTb(PO4)2 (Ln ¼ Ce, Nd,
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Sm), also exhibit monazite structure [17], it is of considerable interest to employ the hydrothermal method to synthesize monocrystalline nanowires of these compounds. In this communication, the controlled hydrothermal synthesis and characterization of monocrystalline LnTb(PO4)2 (Ln ¼ Ce, Nd, Sm) nanowires are presented.
2. Experimental 2.1. Preparation A typical synthesis started with quantitative mixed solutions of Ln(NO3)3 (Ce, Nd, or Sm; AR) and Tb(NO3)3 (AR, . 99.9%, Liyang-Rhodia-Founder Rare Earths New Materials Corp. of China) and diluted H3PO4 (AR, Beijing Chem. Corp. of China) in a given molar ratio of Ln:Tb:H3PO4 ¼ 0.5:0.5:1.1 to prepare the stock solution (40 mL, pH 0.8) with the total lanthanide cation concentration of 0.05 mol L21 in a Teflon cup (50 mL). After 15 min of stirring, the cup was transferred into a stainless steel autoclave, and subjected to hydrothermal treatment at 240 8C for 24 h under autogenous pressure in an electric oven. As the autoclave cooled down to room temperature, products in the color of Ln3þ ions were obtained and collected. The precipitates were washed by deionized water, centrifugally filtered off, and dried at 60 8C overnight. The yields of the products were about 80 – 90%. 2.2. Characterization methods Water content in the products was determined to be negligible with a thermo-gravimetry and differential thermal analyzer (Du Pont 2100) in air at a heating rate of 10 8C/min, using a-Al2O3 as a reference. Crystal structures of the products were identified by a powder X-ray diffractometer (Rigaku Dmax-2000, Japan), employing Cu Ka radiation ˚ ). The sizes, morphologies and stoichiometry (l ¼ 1:5418 A of the products were characterized by transmission electronic spectroscopy (200CX, JEOL, Japan) at 160 kV and high-resolution transmission electronic spectroscopy coupled with energy dispersive X-ray analysis (EDAX) at a resolution of 159 eV (H-9000, Hitachi, Japan) at 300 kV. FT-IR spectra were obtained on a Nicolet Magna 750 FTIR spectrometer at a resolution of 4 cm21 with a Nic-Plan IR Microscope.
Fig. 1. XRD patterns of NdTb(PO4)2. (a) nanowires; (b) colloidal precipitates prepared at pH 0.8 before hydrothermal treatment.
The monazite structure was also identified for CeTb(PO4)2 ˚, and SmTb(PO4)2. The lattice constants are a ¼ 6:670 A ˚ , c ¼ 6:373 A ˚ and b ¼ 104:88 for CeTb(PO4)2, b ¼ 6:878 A ˚ , b ¼ 6:820 A ˚ , c ¼ 6:333 A ˚ and b ¼ and a ¼ 6:630 A 104:18 for SmTb(PO4)2. No peaks associated with rhabdophane or xenotime by-products were found in the XRD patterns and no explicitly widening or splitting of the peaks was observed, showing that the as-synthesized nanowires are of pure monazite phase. The EDAX spectrum typically shown in Fig. 2 was performed in a selected area, and informed us that Ln (Ce, Nd, or Sm) and Tb coexisted in the product with the atomic ratios of nearly 1:1, the same as that added during the preparation. From Fig. 1(b), we note that the NdTb(PO4)2 colloidal precipitates before hydrothermal treatment exhibited a hexagonal structure, consistent with that of NdPO4·H2O (JCPDS No. 50-0620). The weak and widened peaks in Fig. 1(b) also indicate both poor crystallinity and nanoparticle formation for the colloidal precipitates. The as-formed nanoparticles could serve as anisotropic seeds for the growth of highly anisotropic nanostructures in the solution solid process via the dissolution and crystallization mechanism in the present case [19].
3. Results and discussion The typical XRD pattern of NdTb(PO4)2 nanowires (Fig. 1(a)) agreed well with that of monazite NdPO4 (JCPDS No. 46-1328), and thus its crystal structure was confirmed to be monazite type with lattice constants calculated to be ˚ , b ¼ 6:870 A ˚ , c ¼ 6:365 A ˚ and b ¼ 104:08: a ¼ 6:670 A
Fig. 2. EDAX spectrum of NdTb(PO4)2 nanowires.
Z.-G. Yan et al. / Solid State Communications 130 (2004) 125–129
127
The TEM images of LnTb(PO4)2 (Ln ¼ Ce, Nd, Sm) nanowires displayed in Fig. 3 indicate the nanowires to be in a length of 2 –10 mm and a width within a range varied according to the different Ln element added, which was 20 – 90, 30 – 200 and 30 – 350 nm for Ce-, Nd- and Smcontaining nanowires, respectively. The differences in width and morphological uniformity are obviously related to the differences in their intrinsic characteristics (solubility of according phosphate, ion radius and ionic mobility and so on) of the different Ln3þ ions added [19 – 23], which is worth of further investigation by kinetically controlled
Fig. 4. HRTEM images and SAED patterns of a single LnTb(PO4)2 nanowire. (a) CeTb; (b) NdTb.
Fig. 3. TEM images of LnTb(PO4)2 nanowires. (a) CeTb; (b) NdTb; (c) SmTb.
experiments and theoretical analysis. Fig. 4(a) and (b) show the HRTEM images and selected area electronic diffraction (SAED) patterns of CeTb(PO4)2 and NdTb(PO4)2 nanowires, respectively. The well-defined lattice fringes and clear spotty SAED patterns prove the nanowires to be of monocrystalline nature. The growth direction of LnTb(PO4)2 nanowires is determined to be all along [001] (c axis). The smooth morphology and perfect monocrystalline nature of the as-synthesized nanowires indicated that a reversible pathway between the solution phase and the solid phase which allows building blocks in LnPO4 crystals to adopt correct positions was established during the hydrothermal treatment employed. IR analysis was performed upon the LnTb(PO4)2 nanowires. The site symmetry of tetrahedral PO32 anions 4 in the monazite structure is C1 [20]. Under such symmetry, no selection rule works and all possible combinations of fundamental vibrations would be IR active. As shown in Fig. 5, all four active vibration modes ascribed to tetrahedral PO32 4 anions were present in the IR spectra of LnTb(PO4)2, and the patterns are basically consistent with the IR spectra of bulk monazite type LnPO4 [20]. No extra peaks related to were observed. Typically for NdTb(PO4)2 nanoP2O42 7 wires, we noted that the spectrum exhibited increased resolution in the n3 region, and the presence of band n2
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4. Conclusion
Fig. 5. FTIR spectra of LnTb(PO4)2 nanowires. (a) CeTb; (b) NdTb; (c) SmTb.
around 491 cm21 (very weak), and the band n1 around 963 cm21 (middle strong). The n3 band is composed of four peaks at 1003 cm21 (middle strong), 1034 and 1068 cm21 (shoulder, very strong), and 1102 cm21 (strong). The n4 band consists of four peaks at 547 cm21 (middle strong), 586 and 575 cm21 (shoulder, middle strong), and 619 cm21 (very strong). Our recent researches upon the preparation of individual LnPO4 nanowires in monazite structure have proved that the heat treatment temperature and the starting acidity of the stock solution are both major factors governing the crystallinity, phase purity and the aspect ratio of the nanomaterials. Therefore, an optimal synthesis temperature (.200 8C) and starting acidity (pH 0.5– 1.5) promising to achieve high crystallinity and phase purity was chosen to prepare nanowires of the mixed lanthanide orthophosphates. When heat-treated below 220 8C at a given acidity, hexagonal lanthanide orthophosphate hydrates will readily form. At a given temperature of 240 8C, either under too weak (pH . 2) or under too strong acidic conditions (pH , 0.25), the aspect ratio of the 1D LnPO4·n H2O nanomaterials would be considerably reduced [19]. The highly anisotropic structure of monazite type LnPO4 is demonstrated to be also important for the growth of nanowires, since the growth along c-axis forms double O atoms connected Ln-[PO32 4 ]-Ln chains while other five O atoms coordinated to Ln3þ ions would make the packing of the building blocks along the other directions not easily. The successful synthesis of LnTbPO4 instead of a mixture of LnPO4·n H2O and TbPO4·n H2O is confirmed by its high morphological purity. In our recent work about TbPO4·n H2O, only hexagonal nanorods and tetragonal nanocubes were obtained under altered acidities at 240 8C [19], both of which were not observed in the as-obtained LnTbPO4 products (see Fig. 3).
We have successfully synthesized monocrystalline LnTbPO4 (Ln ¼ Ce, Nd, Sm) nanowires in bulk quantities by a simple, facile and clean solution based hydrothermal method under optimal heating temperature (240 8C) and the starting acidity of the stock solution (pH 0.8). The length and width of the nanowires were in the range of 2 – 10 mm and 20 –350 nm, respectively. This successful synthesis also demonstrated the hydrothermal method to be capable of synthesizing nanowires of monazite type lanthanide orthophosphates no mater individual, lightly-doped or mixed. It is expected that the hydrothermal route employed in this work can be also extended to the preparation of other pure and doped 1D nanomaterials with highly anisotropic structures. Currently, our researches on the luminescent behaviors of doped LnPO4·n H2O 1D nanomaterials associated with the reduced dimensionality are under progress.
Acknowledgements Grants-in-aids from NSFC (Nos. 20171003, 20221101, 50272006 and 20023005), MOST of China (G19980613), and Founder Foundation of Peking University are gratefully acknowledged.
References [1] J.T. Hu, T.W. Odom, C.M. Lieber, Acc. Chem. Res. 435 (1999) 32. [2] Y.N. Xia, P.D. Yang, Y.G. Sun, Y.Y. Wu, B. Mayers, B. Gates, Y.D. Yin, F. Kim, H.Q. Yan, Adv. Mater. 15 (2003) 353. [3] Y.Y. Wu, H.Q. Yan, M. Huang, B. Messer, J.H. Song, P.D. Yang, Chem. Eur. J. 8 (2002) 1260. [4] G.R. Patzke, F. Krumeich, R. Nesper, Angew. Chem., Int. Ed. 41 (2002) 2446. [5] J. Zhang, L.D. Sun, C. Liao, C.H. Yan, Chem. Commun. 3 (2002) 262. [6] J. Zhang, L.D. Sun, H.L. Su, C.S. Liao, J.L. Yin, C.H. Yan, Chem. Mater. 14 (2002) 4172. [7] D.F. Zhang, L.D. Sun, J.L. Yin, C.H. Yan, Adv. Mater. 15 (2003) 1022. [8] X. Wang, Y.D. Li, Angew. Chem., Int. Ed. 41 (2002) 4790. [9] A.W. Xu, Y.P. Fang, L.P. You, H.Q. Lin, J. Am. Chem. Soc. 125 (2003) 1494. [10] C.F. Wu, W.P. Qin, G.S. Qin, D. Zhao, J.S. Zhang, S.H. Huang, S.Z. Lu, H.Q. Liu, H.Y. Lin, Appl. Phys. Lett. 82 (2003) 520. [11] M. Yada, M. Mihara, S. Mouri, M. Kuroki, T. Kijima, Adv. Mater. 14 (2000) 309. [12] K. Riwotzki, H. Meyssamy, H. Schnablegger, A. Kornowski, M. Haase, Angew. Chem., Int. Ed. 40 (2001) 573. [13] K. Riwotzki, H. Meyssamy, A. Kornowski, M. Haase, J. Phys. Chem. B 104 (2000) 2824. [14] H. Meyssamy, K. Riwotzki, Adv. Mater. 11 (1999) 840.
Z.-G. Yan et al. / Solid State Communications 130 (2004) 125–129 [15] S. Nishihama, T. Hirai, I. Komasawa, J. Mater. Chem. 12 (2002) 1053. [16] P. Schuetz, F. Caruso, Chem. Mater. 14 (2002) 4509. [17] D.F. Mullica, E.L. Sappenfield, L.A. Boatner, Inorg. Chim. Acta 244 (1996) 247. [18] H. Ito, Y. Fujishiro, T. Sato, A. Okuwaki, Br. Ceram. Trans. 94 (1995) 146. [19] Y.W. Zhang, Z.G. Yan, L.P. You, R. Si, C.H. Yan, Eur. J. Inorg. Chem. (2003) 4099.
129
[20] R. Kijkowska, E. Cholewka, B. Duszak, J. Mater. Sci. 38 (2003) 223. [21] X.W. Liu, R.H. Byrne, Geochim. Cosmochim. Acta 61 (1997) 1625. [22] N.N. Greenwood, A. Earnshaw, Chemistry of the Elements, second ed., Butterworth-Heinemann, Oxford, 1998, p. 1236. [23] R. Kijkowska, J. Mater. Sci. 38 (2003) 229.
Controlled synthesis and characterization of monazite type monocrystalline nanowires of mixed lanthanide orthophosphates Zheng-Guang Yana, Ya-Wen Zhanga,*, Li-Ping Youb, Rui Sia, Chun-Hua Yana,* a
State Key Laboratory of Rare Earth Materials Chemistry and Applications and PKU-HKU Joint Laboratory on Rare Earth Materials and Bioinorganic Chemistry, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China b Electron Microscopy Laboratory, Peking University, Beijing 100871, China Received 8 August 2003; accepted 28 December 2003 by Z.Z. Gan
Abstract Monazite type monocrystalline nanowires of mixed lanthanide orthophosphates LnTb(PO4)2 (Ln ¼ Ce, Nd, Sm) were synthesized via a simple hydrothermal process under optimal heat-treatment temperature (240 8C) and the starting acidity of the stock solution (pH 0.8). The as-synthesized nanowires were characterized by X-ray diffraction, transmission electron microscopy (TEM) and high-resolution TEM coupled with energy dispersive X-ray analysis, and infrared spectroscopy. The length and width of the nanowires were in the range of 2 – 10 mm and 20 – 350 nm, respectively. q 2004 Elsevier Ltd. All rights reserved. PACS: 61.46. þ w; 81.05.Ys; 81.10.Dn Keywords: A. Nanostructures; B. Chemical Synthesis; C. Transmission electron microscopy
1. Introduction One-dimensional (1D) nanomaterials including nanowires, nanobelts, nanotubes and nanorods have been a focal area of intensive researches for their unique electronic, optical and catalytic properties associated with the reduced dimensionality, and their great importance both in mesoscopic physics and nanoscale device fabrication [1 – 7]. In order to synthesize 1D nanomaterials based on a variety of chemicals such as elemental semiconductors, II – VI and III – V semiconductors, metal oxides, metal and inorganic salts, a lot of methods including templating direction, catalytic growth, electrochemistry, chemical vapor deposition, and solution based solvothermal or hydrothermal treatment have been developed [1 – 4]. Among these techniques, hydrothermal route is a robust and promising one for the fabrication of 1D nanomaterials with wellcontrolled shape, size, phase-purity, crystallinity and chemical composition on large scale [2,4– 7]. * Corresponding authors. Tel./fax: þ 86-10-62754179. E-mail address: [email protected] (C.H. Yan). 0038-1098/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2003.12.044
Lanthanide compounds form a large family of versatile materials in electronic, optic, catalytic and more areas, and several studies have been conducted on 1D nanomaterials of lanthanide compounds [8– 11]. Lanthanide orthophosphates have been extensively applied as phosphors, laser hosts, heat resistant materials and moisture sensors based on their electronic and luminescent properties. Several mixed orthophosphates composed of two lanthanide elements were also suggested to be used as disposals of nuclear waste or as host lattices for spectroscopic investigations [12– 18]. Lanthanide orthophosphates with the chemical formula LnPO4·n H2O ðn ¼ 0 – 3Þ (Ln3þ ¼ lanthanide ion) have five polymorphs: monazite (monoclinic), xenotime (tetragonal), rhabdophane (hexagonal), weinschenkite (monoclinic), and orthorhombic [18]. Very recently, we have managed to synthesize nanowires of light and medium lanthanide orthophosphates exhibiting monazite structure by a hydrothermal method, in which we finely tuned the acidity and selectively controlled the heat treatment temperature [19]. Since structural study has revealed that three lanthanide orthophosphate composites, LnTb(PO4)2 (Ln ¼ Ce, Nd,
126
Z.-G. Yan et al. / Solid State Communications 130 (2004) 125–129
Sm), also exhibit monazite structure [17], it is of considerable interest to employ the hydrothermal method to synthesize monocrystalline nanowires of these compounds. In this communication, the controlled hydrothermal synthesis and characterization of monocrystalline LnTb(PO4)2 (Ln ¼ Ce, Nd, Sm) nanowires are presented.
2. Experimental 2.1. Preparation A typical synthesis started with quantitative mixed solutions of Ln(NO3)3 (Ce, Nd, or Sm; AR) and Tb(NO3)3 (AR, . 99.9%, Liyang-Rhodia-Founder Rare Earths New Materials Corp. of China) and diluted H3PO4 (AR, Beijing Chem. Corp. of China) in a given molar ratio of Ln:Tb:H3PO4 ¼ 0.5:0.5:1.1 to prepare the stock solution (40 mL, pH 0.8) with the total lanthanide cation concentration of 0.05 mol L21 in a Teflon cup (50 mL). After 15 min of stirring, the cup was transferred into a stainless steel autoclave, and subjected to hydrothermal treatment at 240 8C for 24 h under autogenous pressure in an electric oven. As the autoclave cooled down to room temperature, products in the color of Ln3þ ions were obtained and collected. The precipitates were washed by deionized water, centrifugally filtered off, and dried at 60 8C overnight. The yields of the products were about 80 – 90%. 2.2. Characterization methods Water content in the products was determined to be negligible with a thermo-gravimetry and differential thermal analyzer (Du Pont 2100) in air at a heating rate of 10 8C/min, using a-Al2O3 as a reference. Crystal structures of the products were identified by a powder X-ray diffractometer (Rigaku Dmax-2000, Japan), employing Cu Ka radiation ˚ ). The sizes, morphologies and stoichiometry (l ¼ 1:5418 A of the products were characterized by transmission electronic spectroscopy (200CX, JEOL, Japan) at 160 kV and high-resolution transmission electronic spectroscopy coupled with energy dispersive X-ray analysis (EDAX) at a resolution of 159 eV (H-9000, Hitachi, Japan) at 300 kV. FT-IR spectra were obtained on a Nicolet Magna 750 FTIR spectrometer at a resolution of 4 cm21 with a Nic-Plan IR Microscope.
Fig. 1. XRD patterns of NdTb(PO4)2. (a) nanowires; (b) colloidal precipitates prepared at pH 0.8 before hydrothermal treatment.
The monazite structure was also identified for CeTb(PO4)2 ˚, and SmTb(PO4)2. The lattice constants are a ¼ 6:670 A ˚ , c ¼ 6:373 A ˚ and b ¼ 104:88 for CeTb(PO4)2, b ¼ 6:878 A ˚ , b ¼ 6:820 A ˚ , c ¼ 6:333 A ˚ and b ¼ and a ¼ 6:630 A 104:18 for SmTb(PO4)2. No peaks associated with rhabdophane or xenotime by-products were found in the XRD patterns and no explicitly widening or splitting of the peaks was observed, showing that the as-synthesized nanowires are of pure monazite phase. The EDAX spectrum typically shown in Fig. 2 was performed in a selected area, and informed us that Ln (Ce, Nd, or Sm) and Tb coexisted in the product with the atomic ratios of nearly 1:1, the same as that added during the preparation. From Fig. 1(b), we note that the NdTb(PO4)2 colloidal precipitates before hydrothermal treatment exhibited a hexagonal structure, consistent with that of NdPO4·H2O (JCPDS No. 50-0620). The weak and widened peaks in Fig. 1(b) also indicate both poor crystallinity and nanoparticle formation for the colloidal precipitates. The as-formed nanoparticles could serve as anisotropic seeds for the growth of highly anisotropic nanostructures in the solution solid process via the dissolution and crystallization mechanism in the present case [19].
3. Results and discussion The typical XRD pattern of NdTb(PO4)2 nanowires (Fig. 1(a)) agreed well with that of monazite NdPO4 (JCPDS No. 46-1328), and thus its crystal structure was confirmed to be monazite type with lattice constants calculated to be ˚ , b ¼ 6:870 A ˚ , c ¼ 6:365 A ˚ and b ¼ 104:08: a ¼ 6:670 A
Fig. 2. EDAX spectrum of NdTb(PO4)2 nanowires.
Z.-G. Yan et al. / Solid State Communications 130 (2004) 125–129
127
The TEM images of LnTb(PO4)2 (Ln ¼ Ce, Nd, Sm) nanowires displayed in Fig. 3 indicate the nanowires to be in a length of 2 –10 mm and a width within a range varied according to the different Ln element added, which was 20 – 90, 30 – 200 and 30 – 350 nm for Ce-, Nd- and Smcontaining nanowires, respectively. The differences in width and morphological uniformity are obviously related to the differences in their intrinsic characteristics (solubility of according phosphate, ion radius and ionic mobility and so on) of the different Ln3þ ions added [19 – 23], which is worth of further investigation by kinetically controlled
Fig. 4. HRTEM images and SAED patterns of a single LnTb(PO4)2 nanowire. (a) CeTb; (b) NdTb.
Fig. 3. TEM images of LnTb(PO4)2 nanowires. (a) CeTb; (b) NdTb; (c) SmTb.
experiments and theoretical analysis. Fig. 4(a) and (b) show the HRTEM images and selected area electronic diffraction (SAED) patterns of CeTb(PO4)2 and NdTb(PO4)2 nanowires, respectively. The well-defined lattice fringes and clear spotty SAED patterns prove the nanowires to be of monocrystalline nature. The growth direction of LnTb(PO4)2 nanowires is determined to be all along [001] (c axis). The smooth morphology and perfect monocrystalline nature of the as-synthesized nanowires indicated that a reversible pathway between the solution phase and the solid phase which allows building blocks in LnPO4 crystals to adopt correct positions was established during the hydrothermal treatment employed. IR analysis was performed upon the LnTb(PO4)2 nanowires. The site symmetry of tetrahedral PO32 anions 4 in the monazite structure is C1 [20]. Under such symmetry, no selection rule works and all possible combinations of fundamental vibrations would be IR active. As shown in Fig. 5, all four active vibration modes ascribed to tetrahedral PO32 4 anions were present in the IR spectra of LnTb(PO4)2, and the patterns are basically consistent with the IR spectra of bulk monazite type LnPO4 [20]. No extra peaks related to were observed. Typically for NdTb(PO4)2 nanoP2O42 7 wires, we noted that the spectrum exhibited increased resolution in the n3 region, and the presence of band n2
128
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4. Conclusion
Fig. 5. FTIR spectra of LnTb(PO4)2 nanowires. (a) CeTb; (b) NdTb; (c) SmTb.
around 491 cm21 (very weak), and the band n1 around 963 cm21 (middle strong). The n3 band is composed of four peaks at 1003 cm21 (middle strong), 1034 and 1068 cm21 (shoulder, very strong), and 1102 cm21 (strong). The n4 band consists of four peaks at 547 cm21 (middle strong), 586 and 575 cm21 (shoulder, middle strong), and 619 cm21 (very strong). Our recent researches upon the preparation of individual LnPO4 nanowires in monazite structure have proved that the heat treatment temperature and the starting acidity of the stock solution are both major factors governing the crystallinity, phase purity and the aspect ratio of the nanomaterials. Therefore, an optimal synthesis temperature (.200 8C) and starting acidity (pH 0.5– 1.5) promising to achieve high crystallinity and phase purity was chosen to prepare nanowires of the mixed lanthanide orthophosphates. When heat-treated below 220 8C at a given acidity, hexagonal lanthanide orthophosphate hydrates will readily form. At a given temperature of 240 8C, either under too weak (pH . 2) or under too strong acidic conditions (pH , 0.25), the aspect ratio of the 1D LnPO4·n H2O nanomaterials would be considerably reduced [19]. The highly anisotropic structure of monazite type LnPO4 is demonstrated to be also important for the growth of nanowires, since the growth along c-axis forms double O atoms connected Ln-[PO32 4 ]-Ln chains while other five O atoms coordinated to Ln3þ ions would make the packing of the building blocks along the other directions not easily. The successful synthesis of LnTbPO4 instead of a mixture of LnPO4·n H2O and TbPO4·n H2O is confirmed by its high morphological purity. In our recent work about TbPO4·n H2O, only hexagonal nanorods and tetragonal nanocubes were obtained under altered acidities at 240 8C [19], both of which were not observed in the as-obtained LnTbPO4 products (see Fig. 3).
We have successfully synthesized monocrystalline LnTbPO4 (Ln ¼ Ce, Nd, Sm) nanowires in bulk quantities by a simple, facile and clean solution based hydrothermal method under optimal heating temperature (240 8C) and the starting acidity of the stock solution (pH 0.8). The length and width of the nanowires were in the range of 2 – 10 mm and 20 –350 nm, respectively. This successful synthesis also demonstrated the hydrothermal method to be capable of synthesizing nanowires of monazite type lanthanide orthophosphates no mater individual, lightly-doped or mixed. It is expected that the hydrothermal route employed in this work can be also extended to the preparation of other pure and doped 1D nanomaterials with highly anisotropic structures. Currently, our researches on the luminescent behaviors of doped LnPO4·n H2O 1D nanomaterials associated with the reduced dimensionality are under progress.
Acknowledgements Grants-in-aids from NSFC (Nos. 20171003, 20221101, 50272006 and 20023005), MOST of China (G19980613), and Founder Foundation of Peking University are gratefully acknowledged.
References [1] J.T. Hu, T.W. Odom, C.M. Lieber, Acc. Chem. Res. 435 (1999) 32. [2] Y.N. Xia, P.D. Yang, Y.G. Sun, Y.Y. Wu, B. Mayers, B. Gates, Y.D. Yin, F. Kim, H.Q. Yan, Adv. Mater. 15 (2003) 353. [3] Y.Y. Wu, H.Q. Yan, M. Huang, B. Messer, J.H. Song, P.D. Yang, Chem. Eur. J. 8 (2002) 1260. [4] G.R. Patzke, F. Krumeich, R. Nesper, Angew. Chem., Int. Ed. 41 (2002) 2446. [5] J. Zhang, L.D. Sun, C. Liao, C.H. Yan, Chem. Commun. 3 (2002) 262. [6] J. Zhang, L.D. Sun, H.L. Su, C.S. Liao, J.L. Yin, C.H. Yan, Chem. Mater. 14 (2002) 4172. [7] D.F. Zhang, L.D. Sun, J.L. Yin, C.H. Yan, Adv. Mater. 15 (2003) 1022. [8] X. Wang, Y.D. Li, Angew. Chem., Int. Ed. 41 (2002) 4790. [9] A.W. Xu, Y.P. Fang, L.P. You, H.Q. Lin, J. Am. Chem. Soc. 125 (2003) 1494. [10] C.F. Wu, W.P. Qin, G.S. Qin, D. Zhao, J.S. Zhang, S.H. Huang, S.Z. Lu, H.Q. Liu, H.Y. Lin, Appl. Phys. Lett. 82 (2003) 520. [11] M. Yada, M. Mihara, S. Mouri, M. Kuroki, T. Kijima, Adv. Mater. 14 (2000) 309. [12] K. Riwotzki, H. Meyssamy, H. Schnablegger, A. Kornowski, M. Haase, Angew. Chem., Int. Ed. 40 (2001) 573. [13] K. Riwotzki, H. Meyssamy, A. Kornowski, M. Haase, J. Phys. Chem. B 104 (2000) 2824. [14] H. Meyssamy, K. Riwotzki, Adv. Mater. 11 (1999) 840.
Z.-G. Yan et al. / Solid State Communications 130 (2004) 125–129 [15] S. Nishihama, T. Hirai, I. Komasawa, J. Mater. Chem. 12 (2002) 1053. [16] P. Schuetz, F. Caruso, Chem. Mater. 14 (2002) 4509. [17] D.F. Mullica, E.L. Sappenfield, L.A. Boatner, Inorg. Chim. Acta 244 (1996) 247. [18] H. Ito, Y. Fujishiro, T. Sato, A. Okuwaki, Br. Ceram. Trans. 94 (1995) 146. [19] Y.W. Zhang, Z.G. Yan, L.P. You, R. Si, C.H. Yan, Eur. J. Inorg. Chem. (2003) 4099.
129
[20] R. Kijkowska, E. Cholewka, B. Duszak, J. Mater. Sci. 38 (2003) 223. [21] X.W. Liu, R.H. Byrne, Geochim. Cosmochim. Acta 61 (1997) 1625. [22] N.N. Greenwood, A. Earnshaw, Chemistry of the Elements, second ed., Butterworth-Heinemann, Oxford, 1998, p. 1236. [23] R. Kijkowska, J. Mater. Sci. 38 (2003) 229.