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General synthesis and characterization of monocrystalline 1D-nanomaterials of hexagonal and orthorhombic lanthanide orthophosphate hydrate
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Journal of Crystal Growth 262 (2004) 408–414
General synthesis and characterization of monocrystalline 1D-nanomaterials of hexagonal and orthorhombic lanthanide orthophosphate hydrate 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 & 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 7 September 2003; accepted 22 October 2003 Communicated by M. Schieber
Abstract Hexagonal LnPO4 nH2O (Ln=La, Ce, Pr, Nd, Sm, Eu, Gd) and orthorhombic LnPO4 nH2O (Ln=Tb, Dy) onedimensional nanomaterials were prepared through hydrothermal reaction under fine control of the acidity in the mother liquors and with a heating temperature in the range 140–200 C. The products were characterized by X-ray diffraction, transmission electron microscopy, high-resolution transmission electron microscopy, and infrared spectroscopy. The length and width of the nanowires/nanorods were in the ranges 0.2–5 mm and 10–300 nm, respectively. The phasic and morphological changes of the products due to the acidity of the stock solution and the heating temperature used are also discussed. r 2003 Elsevier B.V. All rights reserved. PACS: 61.46.+w; 81.07.b; 81.10.h Keywords: A1. Crystal morphology; A1. Low dimensional structures; A1. Nanostructures; A2. Hydrothermal crystal growth; B1. Nanomaterials; B1. Phosphates
1. Introduction One-dimensional (1D) nanostructures, especially nanowires, nanobelts, nanotubes and nanorods, have recently gained interest and importance based on their novel properties associated with the reduced dimensionality and their potential appli*Corresponding author. Tel.: +8610-6275-4179; fax: +86106275-4179. E-mail address: [email protected] (C.-H. Yan).
cations in nanotechnology [1–4]. Various preparation means towards diverse 1D-nanomaterials (elemental and compound semiconductors, metal oxides, metals, inorganic salts and so on), including templating direction, catalytic growth, electrochemistry, chemical vapor deposition, and solution-based solvothermal or hydrothermal treatment have been extensively developed [1–8]. Among these methods, hydrothermal route was demonstrated to be robust, straightforward and promising for the large-scale fabrication of
0022-0248/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2003.10.058
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1D-nanomaterials with well-controlled shape, size, phase purity, crystallinity and chemical composition. More recently, a growing number of works have dealt with 1D-nanostructured materials of various compounds of lanthanide elements, which are well known for their unique 4f electron configuration, abundant energy levels and wide applications in magnets, phosphors, catalysts, biochemical probes and medical diagnostics [9–12]. As an important category of lanthanide inorganic salts, lanthanide phosphates are broadly used in the production of luminescent or laser materials, moisture sensors, heat-resistant materials, nuclear waste disposal, versatile biological labels and photon up-conversion materials [13– 18]. In addition to their wide applications, lanthanide orthophosphates with the chemical formula LnPO4 nH2O ðn ¼ 023Þ (Ln3+=lanthanide ion) are also rich in polymorphs. There are five kinds of polymorphs, monazite (monoclinic, naturally abundant), xenotime (tetragonal, naturally abundant), rhabdophane (hexagonal), weinschenkite (monoclinic), and orthorhombic [19]. In our previous work, seven monazite-type lanthanide orthophosphate nanowires were obtained [20]. Zhang et al. also reported the preparation of CePO4 nanowires [21]. However, due to the phase-diversity of lanthanide orthophosphates and their potential applications, it is necessary to screen the optimal synthetic condition and prepare their 1D-nanomaterials across the lanthanide series. In this paper, we will report the general synthesis and characterization of monocrystalline 1D-nanomaterials of lanthanide orthophosphate hydrate in rhabdophane and orthorhombic phases by a solution-based hydrothermal route under fine control of both acidity in the mother liquors and heat-treatment temperatures in the range 140–200 C.
2. Experimental procedure 2.1. Preparation A typical synthesis started with quantitative solutions of Ln(NO3)3 (La, Ce, Pr, Nd, Sm, Eu,
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Gd, Tb, Dy: AR, >99.9%, Liyang-RhodiaFounder Rare Earths New Materials Corp. of China) and diluted H3PO4, NH4H2PO4 or (NH4)2HPO4 (AR, Beijing Chem. Corp. of China) in a given molar ratio of Ln:H3PO4=1:1.1 to prepare the stock solution (40 ml, pH 0.8) with the lanthanide cation concentration of 0.05 mol l1 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 120– 200 C for 24 h under autogenous pressure in an electric oven. As the autoclave cooled down to room temperature, products in the color of Ln3+ ions were obtained and collected. The precipitates were washed by deionized water, centrifugally filtered off and dried at 60 C overnight. 2.2. Characterization methods Water content in the dried products was determined with a thermo-gravimetry analyzer (Dupont 1090B, USA) in air at a heating rate of 10 C/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 ( (l ¼ 1:5418 A). The sizes, morphologies and growth orientation of the products were characterized by transmission electronic spectroscopy (200CX, JEOL, Japan) at 160 kV and highresolution transmission electronic spectroscopy 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 cm1 with a Nic-Plan IR Microscope.
3. Results and discussion In Figs. 1a and b, we show the typical XRD pattern of the rhabdophane-type NdPO4 H2O nanowires and GdPO4 H2O nanorods, respectively. All the reflections can be distinctly indexed to a pure hexagonal phase with lattice constants ( and c ¼ 6:3688ð19Þ A ( (space a ¼ 6:9683ð7Þ A group: P6222) for NdPO4 H2O (JCPDS No.50( and c ¼ 6:3150ð5Þ A ( 620), and a ¼ 6:8781ð2Þ A (space group: P31 21) for GdPO4 H2O (JCPDS
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No.39-232). For TbPO4 0.5H2O, we noted that its XRD pattern (Fig. 1c) agreed well with that of orthorhombic HoPO4 2H2O (JCPDS No. 20-475) and DyPO4 1.5H2O (JCPDS No. 20-385). There-
Fig. 1. XRD patterns of LnPO4 nH2O products: (a) Nd, (b) Gd, and (c) Tb.
fore, its crystal structure was ascribed to orthorhombic (space group: P222) with lattice ( b ¼ 13:6927ð10Þ A ( and constants a ¼ 6:9192ð5Þ A, ( c ¼ 9:0929ð5Þ A. However, the orthorhombic structural data given in the JCPDS cards were of poor quality, in particular, the peak at 2y ¼ 15 was not indexed. Figs. 2a–d display the typical TEM images of hexagonal LnPO4 nH2O (Ln=La, Ce, Pr, Nd) nanowires, respectively. Fig. 2f displays the typical TEM image of hexagonal GdPO4 H2O nanorods. Fig. 2e shows the typical TEM image of hexagonal EuPO4 H2O nanomaterials, a mixture of ultrathin nanowires and nanorods. The mixed morphologies always existed when the pH value changed within 0.5–3. The case of Sm is similar to that of Eu. Therefore, a morphological transition from nanowires to nanorods across the series from light to middle rare earths is obvious and interesting. As displayed in Fig. 2g, the TbPO4 0.5H2O nanorods were pure in morphology. Fig. 2h depicts the TEM image of orthorhombic DyPO4 0.5H2O bundled
Fig. 2. TEM images of LnPO4 nH2O products: (a) La, (b) Ce, (c) Pr, (d) Nd, (e) Eu, (f) Gd, (g) Tb, and (h) Dy.
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Fig. 4. FTIR spectra of LnPO4 nH2O 1D-nanomaterials: (a) Nd, (b) Gd, and (c) Tb.
Fig. 3. HRTEM images and SAED patterns of LnPO4 nH2O products: (a) Nd, (b) Gd, and (c) Tb.
nanowires. From Fig. 2, it was found that the asprepared 1D-nanomaterials have a length up to 5 mm and a width within 15–300 nm. Figs. 3a and b show the HRTEM images of a single NdPO4 H2O nanowire and a single GdPO4 H2O nanorod, respectively. Both of them exhibited well-defined lattice fringes and clear spotty selected area electron diffraction (SAED) patterns, revealing the monocrystalline nature of the products. Both the growth directions were determined to be along [0 0 1] (c-axis). The HRTEM image and SAED pattern shown in Fig. 3c indicate that the TbPO4 0.5H2O nanorods are also single crystals. The growth direction was not deduced due to the above-mentioned confused structural data about the orthorhombic phase in the literature. We suggest structural study on orthorhombic phase to be important to clarify the
problems. However, an X-ray single crystal analysis of orthorhombic structure is currently beyond the scope of this work. IR analysis was performed on the LnPO4 nH2O products. The site symmetry of tetrahedral PO3 4 anions in the hexagonal structure is C2 [22]. Under such symmetry, selection rules allow three out of four fundamental vibration modes to be IR active. As illustrated in Fig. 4, three active vibration modes ascribed to tetrahedral PO3 anions were 4 present in the IR spectra of LnPO4, and the patterns are basically consistent with the IR spectra of bulk rhabdophane type LnPO4 [22]. No extra peaks possibly related to HPO2 were 4 observed [22]. Typically for NdPO4 H2O nanowires, we noted the n1 band around 972 cm1 (weak), the n4 band composed of three peaks at 619 cm1 (middle strong), 577 cm1 (very weak), and 546 cm1 (weak) and the n3 band around 1063 cm1 (strong). GdPO4 H2O nanorods showed a similar picture with slightly purple shifted peaks (Fig. 4b). Shown in Fig. 4c, IR spectrum of orthorhombic TbPO4 0.5H2O nanorods is in clear accordance with the IR spectrum of orthorhombic DyPO4 nH2O [22]. Controlled experiments were carried out to screen the optimal conditions for preparing phase pure monocrystalline LnPO4 nH2O 1D-nanomaterials. The crystallinity, phase purity and morphological uniformity of the products were found to be highly correlative with the heat treatment
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Table 1 Optimal preparative conditions, crystal structures and morphologies of LnPO4 nH2O 1D-nanomaterials Ln
Preparative conditions
Structure
pH
Temp. ( C)
La Ce Pr Nd Sm
0.8 0.8 0.8 0.8 0.8
140 140 140 140 140
Hexagonal Hexagonal Hexagonal Hexagonal Hexagonal
Eu
1.3
180
Hexagonal
Gd Tb Dy
1.3 0.8 0.8
180 180 140
Hexagonal Orthorhombic Orthorhombic
temperature, the starting acidity in the mother liquors, and the intrinsic characteristics (ionic radius, solubility product, ionic mobility, etc.) of individual lanthanide element [23,24]. The optimal preparative conditions, crystal structures and morphologies of the LnPO4 nH2O nanowires/ nanorods are summarized in Table 1. Temperature-dependent experiments revealed that pure hydrated monocrystalline nanowires/ nanorods could only be prepared at the temperatures within 140–200 C under a pH value within 0.5–1.5. For instance, when pH=0.8, dehydrated monazite phase would emerge for La, Ce and Pr at the temperatures above 140 C, for Nd and Sm above 160 C, and for Eu and Gd above 220 C. Also when pH=0.8, orthorhombic phase would transform to tetragonal phase above 200 C for Tb and Dy. At a given heat-treatment temperature, it was observed that the morphologies of the products strongly depend upon the starting acidity and the formation of colloidal precipitates in the mother liquors. LnPO4 nH2O 1D-nanomaterials with a high aspect ratio could only be obtained in a fairly narrow pH range within 0.5–1.5 in the presence of the colloidal precipitates. The hexagonal LnPO4 nH2O colloidal precipitates were aggregates of nearly spherical nanoparticles with sizes around 80–100 nm (see Fig. 5). Those nanoparticles could serve as anisotropic seeds for the growth
Morphology Length (mm)
Width (nm)
1–3 0.8–3 0.3–2 0.5–2.5 0.2–0.9(rod) 0.2–1(wire) 0.8–3(rod) 1–5(wire) 1–4 0.25–0.7 3–5
20–90 20–100 20–50 15–60 70–150 (rod) 10–30 (wire) 100–300 (rod) 10–30 (wire) 150–300 30–70 B70 (single) 300–500 (bundle)
Nanowires Nanowires Nanowires Nanowires Nano-rods/wires Nano-rods/wires Nanorods Nanorods Bundled Nanowires
Fig. 5. XRD pattern and SEM image of NdPO4 H2O colloidal precipitate prepared at pH 0.8 before hydrothermal treatment (inset was taken from the highlighted section).
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of highly anisotropic nanostructures in the solution solid process via the dissolution and crystallization mechanism in present case [25–27]. The colloidal precipitates collected by centrifugally filtering and drying is far less in quantity than the final products after hydrothermal treatment, and the solubility of lanthanide orthophosphates decreases with increasing temperature [28]. Therefore, it is clear that a large portion of lanthanide orthophosphates still existed in the mother liquor and would be attached to the surfaces of the colloidal precipitates and then grow with a preferred crystal facet when heated. Under this condition, well-defined LnPO4 nH2O 1D-nanomaterials were obtained in a fairly narrow pH range within 0.5–1.5, which might favor high chemical potential and fast ionic motion for the LnPO4 crystals [9,27]. Either under too weak (pH>2) or too strong acidic conditions (pHo0.25), the aspect ratio of the LnPO4 nH2O 1D-nanomaterials would be dramatically reduced. Another main driving force for forming LnPO4 nH2O 1D-nanomaterials in this work is supposed to be their highly anisotropic structures. Although we have no exact structural data in describing rhabdophane and orthorhombic polymorphs, their close relation to the monoclinic monazite and tetragonal xenotime phases hints that the hexagonal and orthorhombic phases might share some basic factors with monoclinic and tetragonal phases, such as the [PO4]–Ln–[PO4] chains along the preferred crystal growth direction [20,29–31].
4. Conclusion Hexagonal LnPO4 nH2O (Ln=La, Ce, Pr, Nd, Sm, Eu, Gd) and orthorhombic LnPO4 nH2O (Ln=Tb, Dy) monocrystalline 1D-nanomaterials have been successfully synthesized in bulk quantities by a simple, facile and clean solution-based hydrothermal method, in which the selective control of the acidity in the stock solutions and heat treatment temperatures was very important. The length and width of the nanowires/nanorods were in the range 0.2–5 mm and 10–300 nm, respectively. We expect the hydrothermal route
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related to acidity control in a relatively narrow range employed in this work can be extended to the preparation of both more lanthanide phosphate compounds and other pure and doped 1D-nanomaterials with highly anisotropic structures. Furthermore, the as-obtained LnPO4 nH2O 1D-nanomaterials are believed to have both academic and practical interests in the crystallography of low-dimensional nanomaterials, lanthanide chemistry and nanotechnology.
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.
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[16] P. Schuetz, F. Caruso, Chem. Mater. 14 (2002) 4509. [17] S. Nishihama, T. Hirai, I. Komasawa, J. Mater. Chem. 12 (2002) 1053. [18] S. Heer, O. Lehmann, M. Haase, H.U. Gudel, . Angew. Chem. Int. Ed. 42 (2003) 3179. [19] H. Ito, Y. Fujishiro, T. Sato, A. Okuwaki, Br. Ceram. Trans. 94 (1995) 146. [20] Y.W. Zhang, Z.G. Yan, L.P. You, R. Si, C.H. Yan, Eur. J. Inorg. Chem. (2003) 4099. [21] Y.J. Zhang, H.M. Guan, J. Crystal Growth 256 (2003) 156. [22] R. Kijkowska, J. Mater. Sci. 38 (2003) 223. [23] X.W. Liu, R.H. Byrne, Geochim. Cosmochim. Acta 61 (1997) 1625.
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General synthesis and characterization of monocrystalline 1D-nanomaterials of hexagonal and orthorhombic lanthanide orthophosphate hydrate 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 & 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 7 September 2003; accepted 22 October 2003 Communicated by M. Schieber
Abstract Hexagonal LnPO4 nH2O (Ln=La, Ce, Pr, Nd, Sm, Eu, Gd) and orthorhombic LnPO4 nH2O (Ln=Tb, Dy) onedimensional nanomaterials were prepared through hydrothermal reaction under fine control of the acidity in the mother liquors and with a heating temperature in the range 140–200 C. The products were characterized by X-ray diffraction, transmission electron microscopy, high-resolution transmission electron microscopy, and infrared spectroscopy. The length and width of the nanowires/nanorods were in the ranges 0.2–5 mm and 10–300 nm, respectively. The phasic and morphological changes of the products due to the acidity of the stock solution and the heating temperature used are also discussed. r 2003 Elsevier B.V. All rights reserved. PACS: 61.46.+w; 81.07.b; 81.10.h Keywords: A1. Crystal morphology; A1. Low dimensional structures; A1. Nanostructures; A2. Hydrothermal crystal growth; B1. Nanomaterials; B1. Phosphates
1. Introduction One-dimensional (1D) nanostructures, especially nanowires, nanobelts, nanotubes and nanorods, have recently gained interest and importance based on their novel properties associated with the reduced dimensionality and their potential appli*Corresponding author. Tel.: +8610-6275-4179; fax: +86106275-4179. E-mail address: [email protected] (C.-H. Yan).
cations in nanotechnology [1–4]. Various preparation means towards diverse 1D-nanomaterials (elemental and compound semiconductors, metal oxides, metals, inorganic salts and so on), including templating direction, catalytic growth, electrochemistry, chemical vapor deposition, and solution-based solvothermal or hydrothermal treatment have been extensively developed [1–8]. Among these methods, hydrothermal route was demonstrated to be robust, straightforward and promising for the large-scale fabrication of
0022-0248/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2003.10.058
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1D-nanomaterials with well-controlled shape, size, phase purity, crystallinity and chemical composition. More recently, a growing number of works have dealt with 1D-nanostructured materials of various compounds of lanthanide elements, which are well known for their unique 4f electron configuration, abundant energy levels and wide applications in magnets, phosphors, catalysts, biochemical probes and medical diagnostics [9–12]. As an important category of lanthanide inorganic salts, lanthanide phosphates are broadly used in the production of luminescent or laser materials, moisture sensors, heat-resistant materials, nuclear waste disposal, versatile biological labels and photon up-conversion materials [13– 18]. In addition to their wide applications, lanthanide orthophosphates with the chemical formula LnPO4 nH2O ðn ¼ 023Þ (Ln3+=lanthanide ion) are also rich in polymorphs. There are five kinds of polymorphs, monazite (monoclinic, naturally abundant), xenotime (tetragonal, naturally abundant), rhabdophane (hexagonal), weinschenkite (monoclinic), and orthorhombic [19]. In our previous work, seven monazite-type lanthanide orthophosphate nanowires were obtained [20]. Zhang et al. also reported the preparation of CePO4 nanowires [21]. However, due to the phase-diversity of lanthanide orthophosphates and their potential applications, it is necessary to screen the optimal synthetic condition and prepare their 1D-nanomaterials across the lanthanide series. In this paper, we will report the general synthesis and characterization of monocrystalline 1D-nanomaterials of lanthanide orthophosphate hydrate in rhabdophane and orthorhombic phases by a solution-based hydrothermal route under fine control of both acidity in the mother liquors and heat-treatment temperatures in the range 140–200 C.
2. Experimental procedure 2.1. Preparation A typical synthesis started with quantitative solutions of Ln(NO3)3 (La, Ce, Pr, Nd, Sm, Eu,
409
Gd, Tb, Dy: AR, >99.9%, Liyang-RhodiaFounder Rare Earths New Materials Corp. of China) and diluted H3PO4, NH4H2PO4 or (NH4)2HPO4 (AR, Beijing Chem. Corp. of China) in a given molar ratio of Ln:H3PO4=1:1.1 to prepare the stock solution (40 ml, pH 0.8) with the lanthanide cation concentration of 0.05 mol l1 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 120– 200 C for 24 h under autogenous pressure in an electric oven. As the autoclave cooled down to room temperature, products in the color of Ln3+ ions were obtained and collected. The precipitates were washed by deionized water, centrifugally filtered off and dried at 60 C overnight. 2.2. Characterization methods Water content in the dried products was determined with a thermo-gravimetry analyzer (Dupont 1090B, USA) in air at a heating rate of 10 C/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 ( (l ¼ 1:5418 A). The sizes, morphologies and growth orientation of the products were characterized by transmission electronic spectroscopy (200CX, JEOL, Japan) at 160 kV and highresolution transmission electronic spectroscopy 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 cm1 with a Nic-Plan IR Microscope.
3. Results and discussion In Figs. 1a and b, we show the typical XRD pattern of the rhabdophane-type NdPO4 H2O nanowires and GdPO4 H2O nanorods, respectively. All the reflections can be distinctly indexed to a pure hexagonal phase with lattice constants ( and c ¼ 6:3688ð19Þ A ( (space a ¼ 6:9683ð7Þ A group: P6222) for NdPO4 H2O (JCPDS No.50( and c ¼ 6:3150ð5Þ A ( 620), and a ¼ 6:8781ð2Þ A (space group: P31 21) for GdPO4 H2O (JCPDS
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No.39-232). For TbPO4 0.5H2O, we noted that its XRD pattern (Fig. 1c) agreed well with that of orthorhombic HoPO4 2H2O (JCPDS No. 20-475) and DyPO4 1.5H2O (JCPDS No. 20-385). There-
Fig. 1. XRD patterns of LnPO4 nH2O products: (a) Nd, (b) Gd, and (c) Tb.
fore, its crystal structure was ascribed to orthorhombic (space group: P222) with lattice ( b ¼ 13:6927ð10Þ A ( and constants a ¼ 6:9192ð5Þ A, ( c ¼ 9:0929ð5Þ A. However, the orthorhombic structural data given in the JCPDS cards were of poor quality, in particular, the peak at 2y ¼ 15 was not indexed. Figs. 2a–d display the typical TEM images of hexagonal LnPO4 nH2O (Ln=La, Ce, Pr, Nd) nanowires, respectively. Fig. 2f displays the typical TEM image of hexagonal GdPO4 H2O nanorods. Fig. 2e shows the typical TEM image of hexagonal EuPO4 H2O nanomaterials, a mixture of ultrathin nanowires and nanorods. The mixed morphologies always existed when the pH value changed within 0.5–3. The case of Sm is similar to that of Eu. Therefore, a morphological transition from nanowires to nanorods across the series from light to middle rare earths is obvious and interesting. As displayed in Fig. 2g, the TbPO4 0.5H2O nanorods were pure in morphology. Fig. 2h depicts the TEM image of orthorhombic DyPO4 0.5H2O bundled
Fig. 2. TEM images of LnPO4 nH2O products: (a) La, (b) Ce, (c) Pr, (d) Nd, (e) Eu, (f) Gd, (g) Tb, and (h) Dy.
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Fig. 4. FTIR spectra of LnPO4 nH2O 1D-nanomaterials: (a) Nd, (b) Gd, and (c) Tb.
Fig. 3. HRTEM images and SAED patterns of LnPO4 nH2O products: (a) Nd, (b) Gd, and (c) Tb.
nanowires. From Fig. 2, it was found that the asprepared 1D-nanomaterials have a length up to 5 mm and a width within 15–300 nm. Figs. 3a and b show the HRTEM images of a single NdPO4 H2O nanowire and a single GdPO4 H2O nanorod, respectively. Both of them exhibited well-defined lattice fringes and clear spotty selected area electron diffraction (SAED) patterns, revealing the monocrystalline nature of the products. Both the growth directions were determined to be along [0 0 1] (c-axis). The HRTEM image and SAED pattern shown in Fig. 3c indicate that the TbPO4 0.5H2O nanorods are also single crystals. The growth direction was not deduced due to the above-mentioned confused structural data about the orthorhombic phase in the literature. We suggest structural study on orthorhombic phase to be important to clarify the
problems. However, an X-ray single crystal analysis of orthorhombic structure is currently beyond the scope of this work. IR analysis was performed on the LnPO4 nH2O products. The site symmetry of tetrahedral PO3 4 anions in the hexagonal structure is C2 [22]. Under such symmetry, selection rules allow three out of four fundamental vibration modes to be IR active. As illustrated in Fig. 4, three active vibration modes ascribed to tetrahedral PO3 anions were 4 present in the IR spectra of LnPO4, and the patterns are basically consistent with the IR spectra of bulk rhabdophane type LnPO4 [22]. No extra peaks possibly related to HPO2 were 4 observed [22]. Typically for NdPO4 H2O nanowires, we noted the n1 band around 972 cm1 (weak), the n4 band composed of three peaks at 619 cm1 (middle strong), 577 cm1 (very weak), and 546 cm1 (weak) and the n3 band around 1063 cm1 (strong). GdPO4 H2O nanorods showed a similar picture with slightly purple shifted peaks (Fig. 4b). Shown in Fig. 4c, IR spectrum of orthorhombic TbPO4 0.5H2O nanorods is in clear accordance with the IR spectrum of orthorhombic DyPO4 nH2O [22]. Controlled experiments were carried out to screen the optimal conditions for preparing phase pure monocrystalline LnPO4 nH2O 1D-nanomaterials. The crystallinity, phase purity and morphological uniformity of the products were found to be highly correlative with the heat treatment
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Table 1 Optimal preparative conditions, crystal structures and morphologies of LnPO4 nH2O 1D-nanomaterials Ln
Preparative conditions
Structure
pH
Temp. ( C)
La Ce Pr Nd Sm
0.8 0.8 0.8 0.8 0.8
140 140 140 140 140
Hexagonal Hexagonal Hexagonal Hexagonal Hexagonal
Eu
1.3
180
Hexagonal
Gd Tb Dy
1.3 0.8 0.8
180 180 140
Hexagonal Orthorhombic Orthorhombic
temperature, the starting acidity in the mother liquors, and the intrinsic characteristics (ionic radius, solubility product, ionic mobility, etc.) of individual lanthanide element [23,24]. The optimal preparative conditions, crystal structures and morphologies of the LnPO4 nH2O nanowires/ nanorods are summarized in Table 1. Temperature-dependent experiments revealed that pure hydrated monocrystalline nanowires/ nanorods could only be prepared at the temperatures within 140–200 C under a pH value within 0.5–1.5. For instance, when pH=0.8, dehydrated monazite phase would emerge for La, Ce and Pr at the temperatures above 140 C, for Nd and Sm above 160 C, and for Eu and Gd above 220 C. Also when pH=0.8, orthorhombic phase would transform to tetragonal phase above 200 C for Tb and Dy. At a given heat-treatment temperature, it was observed that the morphologies of the products strongly depend upon the starting acidity and the formation of colloidal precipitates in the mother liquors. LnPO4 nH2O 1D-nanomaterials with a high aspect ratio could only be obtained in a fairly narrow pH range within 0.5–1.5 in the presence of the colloidal precipitates. The hexagonal LnPO4 nH2O colloidal precipitates were aggregates of nearly spherical nanoparticles with sizes around 80–100 nm (see Fig. 5). Those nanoparticles could serve as anisotropic seeds for the growth
Morphology Length (mm)
Width (nm)
1–3 0.8–3 0.3–2 0.5–2.5 0.2–0.9(rod) 0.2–1(wire) 0.8–3(rod) 1–5(wire) 1–4 0.25–0.7 3–5
20–90 20–100 20–50 15–60 70–150 (rod) 10–30 (wire) 100–300 (rod) 10–30 (wire) 150–300 30–70 B70 (single) 300–500 (bundle)
Nanowires Nanowires Nanowires Nanowires Nano-rods/wires Nano-rods/wires Nanorods Nanorods Bundled Nanowires
Fig. 5. XRD pattern and SEM image of NdPO4 H2O colloidal precipitate prepared at pH 0.8 before hydrothermal treatment (inset was taken from the highlighted section).
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of highly anisotropic nanostructures in the solution solid process via the dissolution and crystallization mechanism in present case [25–27]. The colloidal precipitates collected by centrifugally filtering and drying is far less in quantity than the final products after hydrothermal treatment, and the solubility of lanthanide orthophosphates decreases with increasing temperature [28]. Therefore, it is clear that a large portion of lanthanide orthophosphates still existed in the mother liquor and would be attached to the surfaces of the colloidal precipitates and then grow with a preferred crystal facet when heated. Under this condition, well-defined LnPO4 nH2O 1D-nanomaterials were obtained in a fairly narrow pH range within 0.5–1.5, which might favor high chemical potential and fast ionic motion for the LnPO4 crystals [9,27]. Either under too weak (pH>2) or too strong acidic conditions (pHo0.25), the aspect ratio of the LnPO4 nH2O 1D-nanomaterials would be dramatically reduced. Another main driving force for forming LnPO4 nH2O 1D-nanomaterials in this work is supposed to be their highly anisotropic structures. Although we have no exact structural data in describing rhabdophane and orthorhombic polymorphs, their close relation to the monoclinic monazite and tetragonal xenotime phases hints that the hexagonal and orthorhombic phases might share some basic factors with monoclinic and tetragonal phases, such as the [PO4]–Ln–[PO4] chains along the preferred crystal growth direction [20,29–31].
4. Conclusion Hexagonal LnPO4 nH2O (Ln=La, Ce, Pr, Nd, Sm, Eu, Gd) and orthorhombic LnPO4 nH2O (Ln=Tb, Dy) monocrystalline 1D-nanomaterials have been successfully synthesized in bulk quantities by a simple, facile and clean solution-based hydrothermal method, in which the selective control of the acidity in the stock solutions and heat treatment temperatures was very important. The length and width of the nanowires/nanorods were in the range 0.2–5 mm and 10–300 nm, respectively. We expect the hydrothermal route
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related to acidity control in a relatively narrow range employed in this work can be extended to the preparation of both more lanthanide phosphate compounds and other pure and doped 1D-nanomaterials with highly anisotropic structures. Furthermore, the as-obtained LnPO4 nH2O 1D-nanomaterials are believed to have both academic and practical interests in the crystallography of low-dimensional nanomaterials, lanthanide chemistry and nanotechnology.
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.
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