Selective Synthesis of Hexagonal and Tetragonal Dysprosium Orthophosphate Nanorods by a Hydrothermal Method Yue-Ping Fang,†,‡ An-Wu Xu,*,† Ai-Miao Qin,† and Rui-Jin Yu†
CRYSTAL GROWTH & DESIGN 2005 VOL. 5, NO. 3 1221-1225
School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275, China, and School of Chemistry and Chemical Engineering, Guangxi Normal University, Guilin, Guangxi, 541004, China Received December 14, 2004;
Revised Manuscript Received January 23, 2005
ABSTRACT: A simple hydrothermal method has been designed for the selective synthesis of hexagonal DyPO4‚ 1.5H2O and tetragonal DyPO4 nanorods in solution. This study added a new example for selectively controlling different crystal polymorphs of dysprosium orthophosphate nanocrystals through adjusting the temperature and the pH value of the solution. The phase transition and shape evolution were fully investigated and were found to be strongly dependent on the temperature and pH value of the reaction system. The nanostructure and shape of hexagonal DyPO4‚1.5H2O and tetragonal DyPO4 were characterized by X-ray diffraction, transmission electron microscopy (TEM), and high-resolution TEM. The metastable hexagonal DyPO4‚1.5H2O can be trapped even at 120 °C and pH 1-2 by the hydrothermal method. And the stable tetragonal DyPO4 nanorods can be obtained at 200 °C and pH 6-7 by the same method. The evolution process of stable tetragonal and metastable hexagonal phases under different temperatures and pH values via the hydrothermal process was investigated for the first time. 1. Introduction Lanthanide compounds have been widely used as high-performance luminescent devices, magnets, catalysts, time-resolved fluorescence labels for biological detection, and other functional materials based on the optoelectronic and chemical characteristics resulting from the 4f shell of their ions.1-6 The majority of their properties are dependent on the compositions, crystal types, shapes, and sizes. Recently, much attention has focused on the preparation and characterization of the 1D nanostructures of lanthanide compounds, such as lanthanide hydroxides,7-12 oxides,6 and orthophosphates.13-15 In our previous study,15 it has been shown that pure LnPO4 compounds change structures with decreasing Ln ionic radius: i.e., the orthophosphates from Ho to Lu exist only in the tetragonal zircon (xenotime) structure and consist of nanoparticles, while the orthophosphates from La to Dy exist in the hexagonal structure and have wirelike morphology under the same hydrothermal treatment. It has been demonstrated that the obtained hexagonal LnPO4‚1.5H2O (Ln ) La f Tb) can convert to the monoclinic monazite structured products, and their morphologies remained the same after calcination at 900 °C in air (hexagonal DyPO4‚1.5H2O is an exceptional case; it transformed to tetragonal DyPO4 by calcination), while the tetragonal structure for (Ho f Lu)PO4 remains unchanged by calcination. Among lanthanide orthophosphates, dysprosium orthophosphate exists in the boundary positions between the hexagonal and the tetragonal. According to previous studies, it can crystallize in the hexagonal, monoclinic, orthorhombic, or tetragonal polymorph under certain conditions.12-20 This implies that * To whom correspondence should be addressed. E-mail: cedc17@ zsu.edu.cn. Fax: 86-20-84111088. † Sun Yat-Sen University. ‡ Guangxi Normal University.
dysprosium orthophosphate may crystallize in the tetragonal phase besides in the hexagonal under certain hydrothermal conditions. The effects of the temperature and pH value on the shape, size, and phase formation for various nanocrystals under hydrothermal conditions have been reported previously.21-23 Only a few examples about the phase transitions of nanocrystals have been reported.24,25 In which polymorph can DyPO4 crystallize by hydrothermal treatment under different temperatures and pH values? Herein, we report selected-control synthesis of single crystalline hexagonal DyPO4‚1.5H2O and tetragonal DyPO4 nanorods, which were prepared at 120 °C and pH 1-2 and at 200 °C and pH 6-7, respectively, by a simple hydrothermal method; the shape and phase evolution process of dysprosium orthophosphate from nanorods and the hexagonal to nanoparticles or nanorods and the tetragonal under hydrothermal conditions was investigated for the first time; and the X-ray diffraction (XRD), transmission electron microscopy (TEM), and electronic diffraction (ED) patterns were used for the identification of different phases in the obtained DyPO4 products. 2. Preparation and Characterization As-prepared dysprosium orthophosphate samples were prepared by a simple hydrothermal method. The chemical reaction we employed can be formulated as shown in eq 1:
Dy(NO3)3 + NH4H2PO4 ) DyPO4 (colloid) + NH4NO3 + 2HNO3 (1) 2.1. Preparation of the Samples. Analytical grade Dy(NO3)3 solution (10 mL, 0.5 M) was mixed with 10 mL of NH4H2PO4 solution (0.5 M) to form a suspension solution. The resulting suspension having pH 1-2 was poured into a Teflonlined stainless steel autoclave (capacity, 33 mL), and 8 mL of water was added until about 85% of the autoclave capacity
10.1021/cg0495781 CCC: $30.25 © 2005 American Chemical Society Published on Web 03/03/2005
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Fang et al. Table 1. Summary of the Main Results on the Products Obtained under Different pH Values and Temperature Conditions with Other Conditions Kept Constant as-prepared samples
pH value
temp (°C)
products detected by XRD
sample 1 sample 2
1-2 5-6
120 120
sample 3
5-6
180
sample 4
6-7
200
hexagonal DyPO4‚1.5H2O hexagonal DyPO4‚1.5H2O,a tetragonal DyPO4b hexagonal DyPO4‚1.5H2O,b tetragonal DyPO4a tetragonal DyPO4
a
Figure 1. The XRD patterns of the obtained samples under hydrothermal treatment for 12 h: (A) pure hexagonal DyPO4‚ 1.5H2O (sample 1, 120 °C and pH 1-2); (B) a mixture of hexagonal DyPO4‚1.5H2O and tetragonal DyPO4 (sample 2, 120 °C and pH 5-6); (C) a mixture of hexagonal DyPO4‚1.5H2O and tetragonal DyPO4 (sample 3, 180 °C and pH 5-6); (D) pure tetragonal DyPO4 (sample 4, 200 °C and pH 6-7). H: hexagonal DyPO4‚1.5H2O (JCPDS 21-0316); T: tetragonal DyPO4 (JCPDS 76-1532). was filled. The autoclave was then sealed and maintained at 120 °C for 12 h. After the reaction was completed, the resulting solid product was filtered, washed with distilled water and absolute alcohol to remove ions possibly remaining in the final products, and finally dried at 60 °C in air. The obtained sample is hexagonal DyPO4‚1.5H2O. Similarly, other samples were prepared by the same method only altering the reaction temperature and the pH value in solution, such as the sample of tetragonal DyPO4 prepared at 200 °C and pH 6-7. The pH value was adjusted using aqueous ammonia (10%). 2.2. Characterization. The phase purity of the products was examined by XRD using a Rigaku/Max-3A X-ray diffractometer with Cu KR radiation (λ ) 1.5418 Å); the operation voltage and current were maintained at 40 kV and 40 mA, respectively. The morphology and structure of the samples were examined with scanning electron microscopy (SEM, JEOL JSM-6330F) and transmission electron microscopy (JEOL-2010). Thermogravimetric analysis (TGA) was carried out under air, at a heating rate of 10 °C/min, using a NETZSH TG-209 instrument.
3. Results and Discussion 3.1. Selective Synthesis of the Hexagonal DyPO4‚ 1.5H2O and Tetragonal DyPO4. The reaction at 120 °C and pH 1-2 led to the formation of the pure hexagonal phase [15] [space group: P3121 [152]] of DyPO4‚ 1.5H2O with lattice constants a ) 6.86 Å and c ) 6.32 Å (JCPDS 21-0316), as detected by the XRD pattern shown in Figure 1A. However, the reaction at 200 °C and pH 6-7 led to the formation of the pure tetragonal phase [space group: I41/amd [141]] of DyPO4 with lattice constants a ) 6.91 Å and c ) 6.05 Å (JCPDS 761532), as demonstrated by the XRD pattern shown in Figure 1D. The results obtained by hydrothermal treatment under different pH values and temperatures are summarized in Table 1 and Figure 1. From the XRD patterns (Figure 1) and Table 1, it can be clearly seen
The predominant phase. b A minor phase in the final product.
that when the pH value and/or temperature was increased, the tetragonal phase DyPO4 appeared in the final products, indicating that this phase could be formed due to the change of hydrothermal conditions such as the pH value and temperature. The present results indicate that the pH value and temperature have significant effects on the crystal growth and phase formation. The appearance of the tetragonal phase DyPO4 with the increase in the reaction temperature could be understood from the viewpoint of thermodynamics/kinetics: there is the free energy and the internal energy difference between the hexagonal and tetragonal phase. The metastable hexagonal phase can be readily transformed into stable tetragonal form with reaction time increased according to thermodynamic expectations. In addition, the lower pH value will favor the formation of the metastable hexagonal phase. This phenomenon is quite similar to the more favorable formation of vaterite than calcite under more acidic conditions.26 3.2. Shape and Phase Evolution with Hydrothermal Conditions Changing. The shape and phase evolution with changes of the pH value and temperature of the local solution were investigated in detail by XRD patterns, SEM, and TEM. XRD data show (Figure 1B,C) that the (101), (200), (112), (220), and (301) diffraction peaks for the tetragonal DyPO4 phase appear in the XRD pattern of sample 2 and the (101), (200), (211), (112), (220), (202), and (301) diffraction peaks for the tetragonal DyPO4 phase appear in the XRD pattern of sample 3, respectively, indicating that samples 2 and 3 contain the tetragonal DyPO4 phase. In addition, the intensity of any diffraction peak for the tetragonal DyPO4 phase in the XRD pattern of sample 3 is much greater compared to that in the XRD pattern of sample 2, indicating that the content of tetragonal DyPO4 in sample 3 is higher than that in sample 2. The above-mentioned results can be further confirmed by SEM and TEM measurements. As shown in Figure 2A, a typical SEM image of sample 1 obtained at 120 °C and pH 1-2 displays the morphology of nanorods with uniform size, and nanorods aggregate into large bundles, with diameters of ca. 20-40 nm and lengths ranging from 2.5 to 4 µm. The formation of the dumbbell-shaped bundle structure made of nanorods can be observed for several systems, such as the growth of fluoroapatite in gelatin gels.27 The exact growth mechanism is unknown, although some explanation was given in the literature based on the role of intrinsic electric fields which direct the growth of dipole crystals.28 When the pH value was increased from 1-2 to 5-6 while other conditions were kept identical, the
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Figure 3. TEM, HRTEM, and ED images of sample 4 obtained at 200 °C and pH 6-7: (A) TEM image. (B) HRTEM image taken from a single nanorod; (inset) the corresponding electron diffraction pattern shows a single crystal recorded from the [010] zone axis.
Figure 2. SEM and TEM images of the products obtained under hydrothermal treatment at different temperatures and pH values: (A) pure hexagonal DyPO4‚1.5H2O (sample 1); (B) a mixture of hexagonal DyPO4‚1.5H2O and tetragonal DyPO4 (sample 2); (C) a mixture of hexagonal DyPO4‚1.5H2O and tetragonal DyPO4 (sample 3); (D) pure tetragonal DyPO4 (sample 4).
nanorods with diameters of 20-80 nm and lengths ranging between 100 nm and 2 µm were still found to be the major phase (Figure 2B), coexisting with a minor amount of nanoparticles (white square box in Figure 2B; the inset in the upper right is an enlarged image of the white square box). On the other hand, when the temperature was increased from 120 to 180 °C with other conditions kept constant, the nanoparticles with a size of about 50-100 nm were the major phase and the nanorods with diameters of 60-100 nm and lengths of about 2 µm were the minor phase (Figure 2C). When the product was obtained at pH 6-7 and 200 °C, a typical TEM image in Figure 2D shows that pure tetragonal DyPO4 crystals (sample 4) display a typical elongated morphology of nanorods with diameters of 10-40 nm and lengths ranging between 60 and 250 nm. These results are good agreement with XRD data (Figure 1). The morphologies of tetragonal DyPO4 observed at 120 and 180 °C were nanoparticles, but it turned to nanorods at 200 °C (Figure 2). Peng et al. have demonstrated that the shape evolution of nanocrystals is strongly dependent on the relative chemical potential,29 and in the case of one-dimensional nanostructure growth it would be advantageous to have a higher chemical potential,30 which is mainly determined by the pH value and solute concentrations of the solutions in the reaction system. Here, the strong temperature- and/or pHdependent relation with the shape of nanocrystals is due to the sensitive influence of the temperature and/or pH on the solute concentrations ([Dy3+] and [PO43-]). At the very beginning, the direct mixing of the two solutions led to the formation of a large number of amorphous DyPO4 particles. Later on, changing the reaction temperature and/or pH significantly affected the monomer concentrations. With the increase of temperature in the reaction system, the solubility of amorphous DyPO4 increases, implying it has a higher chemical potential as compared with that at lower temperatures. A high
chemical potential is preferable for the growth of 1D nanostructures. This could explain why nanorods can be observed at higher temperature. The obvious 1D growth stage observed here is also in agreement with the previous model proposed by Peng et al.29,30 Generally speaking, in the hydrothermal reaction system, the promotion of anisotropic growth of nanorods/nanowires in a ligand-free system could be related to several parameters, including the intrinsic structural features of specific faces, the local solution details, the foreign energy activation, and the autogenous pressure. The shape evolution of nanocrystals that we observed here is similar to the formation of tungstate nanorods,22 lanthanide hydroxide nanotubes,12 and LiMn2O4 nanobelts.31 The detailed formation mechanism of such shape evolution under hydrothermal conditions needs more clarification in the future. 3.3. The Composition and Micro-/Nanostructure of the Products. The morphology and micro-/nanostructure of hexagonal DyPO4‚1.5H2O (sample 1) has been reported previously.15 The micro-/nanostructure of sample 4 was further characterized by TEM and highresolution TEM (HRTEM). TEM and HRTEM images and the selected area electron diffraction (SAED) patterns of tetragonal DyPO4 (sample 4) are presented in Figure 3A,B, respectively. The HRTEM image (Figure 3B) taken from a single nanorod shows the clearly resolved planes of (002) and (200). The (002) planes are oriented parallel to the nanorod growth axis, indicating that the growth direction of nanorods is along the c axis. The SAED pattern (inset in Figure 3B) taken from a single nanorod can be indexed as a tetragonal DyPO4 single crystal recorded from the [010] zone axis. The composition of samples 2 and 3 was further characterized by TEM. As shown in Figure 4, a TEM image (Figure 4A) shows that sample 3 is made up of fewer nanowires, with diameters of 10-100 nm and lengths ranging between 2 and 3 µm, and more nanoparticles. The ED patterns (Figure 4B), taken from a single nanorod of sample 3, can be indexed as a hexagonal DyPO4‚1.5H2O single crystal recorded from the [010] zone axis. This indicates that the nanorods in sample 3 are hexagonal DyPO4‚1.5H2O. The polycrystalline ED ring (Figure 4C), taken from the nanoparticles, can only be indexed as the tetragonal DyPO4 phase. This indicates that the nanoparticles in sample 3 are tetragonal DyPO4. Thus sample 3 is a mixture of
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the hexagonal and the tetragonal, as shown in Figure 5. This suggests that DyPO4 can crystallize in the tetragonal phase besides in the hexagonal under certain hydrothermal conditions, which has been demonstrated by our present experiments. It is obvious that the present work is more integrated and systematic than our previously reported study for DyPO4. 4. Conclusions
Figure 4. TEM and ED images of sample 3 obtained at 180 °C and pH 5-6: (A) TEM image; (B) ED pattern taken from a longer nanorod in sample 3; (C) ED ring taken from the nanoparticles in the white square box indicated in image A.
In summary, we have succeeded in selective synthesis of hexagonal DyPO4‚1.5H2O and tetragonal DyPO4 nanorods through adjusting the temperature and pH value of the solution by a simple hydrothermal method. This study added a new example for selectively controlling different phases of dysprosium orthophosphate through manipulating the balance between kinetics and thermodynamics in a solution system. Phase transition and shape evolution were fully investigated and were found to be strongly dependent on the temperature and pH value of the reaction media. The nanostructure and shape of hexagonal DyPO4‚1.5H2O and tetragonal DyPO4 were characterized by XRD, TEM, and HRTEM. The evolution process of metastable hexagonal and stable tetragonal phases under different temperatures and pH values via the hydrothermal process has been discussed. Our study demonstrated that it is possible to selectively synthesize other inorganic nanocrystals with controllable phases and structural specialty. Further extension of this approach and investigation of the properties of these new kinds of 1D nanostructures are ongoing. Acknowledgment. Support from the National Natural Science Fundation of China (20371053) and the Guangdong Province NNSF (031574) is gratefully acknowledged. References
Figure 5. Plots of all the measured lattice constants a (top) and c (bottom) of the obtained hexagonal LnPO4‚1.5H2O and tetragonal LnPO4 against lanthanide ionic radius.
the hexagonal DyPO4‚1.5H2O and tetragonal DyPO4, and the tetragonal is the predominant phase. The plots of the measured lattice constants a (top) and c (bottom) of the obtained hexagonal and tetragonal LnPO4 (including tetragonal DyPO4) against lanthanide ionic radius are presented in Figure 5. The two lines are almost parallel to each other in each part (top or bottom) in contrast to that in Figure 1 of our previous paper.15 DyPO4 exists in the boundary positions between
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CG0495781
