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The synthesis and photoluminescent properties of calcium tungstate nanocrystals
ARTICLE IN PRESS
Journal of Crystal Growth 289 (2006) 231–235 www.elsevier.com/locate/jcrysgro
The synthesis and photoluminescent properties of calcium tungstate nanocrystals Lingna Suna,b, Minhua Caoc, Yonghui Wangc, Genban Suna,b, Changwen Hua,b, a
Institute for Chemical Physics, Beijing Institute of Technology, Beijing 100081, People’s Republic of China b Department of Chemistry, Beijing Institute of Technology, Beijing 100081, People’s Republic of China c Institute of Polyoxometalate Chemistry, Northeast Normal University, Changchun 130024, People’s Republic of China Received 15 July 2005; received in revised form 6 October 2005; accepted 18 October 2005 Available online 27 December 2005 Communicated by T.F. Kuech
Abstract CaWO4 nanocrystals with average diameters of 20–30 nm and nanorods with mean lengths of 600–1000 nm were controllably synthesized through a facile microemulsion-mediated hydrothermal procedure. And the factors affecting the morphologies of the products, such as the molar ratio (w) between water and CTAB, the reaction temperature and the concentration of the reactants, were studied and discussed. With the increase of the w value, the diameter of the as-synthesized products increased gradually, except in the case of w ¼ 10 where nanorods were formed. With the temperature increasing, the morphologies of the products changed from nanocrystals to nanorods, and finally to nanocrystals again. However, the concentration of the reactants exhibited no obvious influence on the shape and size of the CaWO4 product. A possible mechanism is proposed for the selective formation of the different morphologies. The photoluminescent properties of the CaWO4 nanocrystals are also reported. All the products of CaWO4 with different morphologies exhibited the only green peak at 435 nm with an excitation wavelength at 238 nm. r 2005 Elsevier B.V. All rights reserved. PACS: 78.55.Hx; 81.07.Bc; 81.10.Dn Keywords: A1. Characterization; B1. Nanomaterials; B1. Tungstates
1. Introduction During the past decade, low-dimensional nanomaterials have attracted much attention because they may havepotential applications, such as photonics, microelectronics and catalysis [1–5]. In particular, metal tungstate (MWO4, M ¼ Ca2+, Ba2+, Pb2+, Sr2+, etc.) nanostructures attract great interest due to their structural properties and promising applications [6]. For example, calcium tungstate (CaWO4) has scheelite structure and is interesting because of its luminescence, thermoluminescence, and stimulated Raman scattering behavior [7]. Because of its superior Corresponding author. Department of Chemistry, Beijing Institute of Technology, Beijing 100081, People’s Republic of China. Tel.: +86 10 86665743; fax: +86 10 68913293. E-mail address: [email protected] (C. Hu).
0022-0248/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2005.10.125
luminescence properties CaWO4 has been extensively used as blue phosphor (433 nm) in oscilloscopes and fluorescent lightening and as a scintillator for detecting X-rays and grays in medical applications [8,9]. This material doped with rare-earth ions has optical and laser properties [10,11]. Nanometer-scale calcium tungstate may play an essential role for its applications in future luminescent nanodevices. At present, rod-, ellipsoid- and sphere-like PbWO4 nanostructures have been prepared in a simple anionic microemulsion system [12]. And PbWO4 nano- and microcrystals were also prepared via a simple, seedless, and high-yield wet chemical route [13]. Further, Shi et al. [14] prepared BaWO4 nanowires in cationic reverse micelles. The synthesis of penniform superstructures of BaWO4 nanowires was also directed by the block copolymer in reverse micelles [15]. In addition, Yu et al. [7] synthesized 1D and 2D CdWO4 nanoparticles by a
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facile aqueous-solution route. The synthesis of CdWO4 nanorods was also achieved by a hydrothermal method [16]. They also synthesized a family of single-crystalline tungstate nanorods/nanowires such as ZnWO4, FeWO4, MnWO4, Bi2WO6, Ag2WO4, and Ag2W2O7 [17]. Although CaWO4 nanoparticles with the sizes of 20–45 nm [18] and nanocrystals with the sizes of 40 nm [19] were solvothermally synthesized, control of the crystalline structure with a fine monomorphology has not yet been achieved. Microemulsion systems, which are certain combinations of water, oil, surfactant and cosurfactant, have already been used as media to synthesize nanoparticles [20,21]. In a water-in-oil (w/o) microemulsion, nanosized water pools are dispersed in a continuous phase and stabilized by the surfactant (S) and co-surfactant at the water/oil interface. The size of the water pool can be carefully controlled by adjusting the [H2O]/[S] ratio, provided wX10 [22]. In the past 2 years, our group has reported the synthesis of BaF2 whiskers, hydroxyapatite nanofibers, Co–Cu complex nanorods/nanorings, Co3[Co(CN)6]2 truncated nanocubes and LaPO4/CePO4 nanorods/nanowires via a microemulsion-mediated hydrothermal method [23–27]. Herein, we present a facile microemulsion-mediated hydrothermal procedure for the synthesis of extremely small CaWO4 nanocrystals and nanorods with controlled sizes. 2. Experiment Cetyltrimethylammonium bromide (CTAB) and absolute ethanol were purchased from Beijing Beihua Fine Chemicals Co. Ltd. Cyclohexane and n-pentanol were bought from The First Tianjin Chemical Reagent Co. Ltd. Sodium tungstate (Na2WO4) and calcium chloride (CaCl2) were analytical grade reagents. All the chemicals were used without further purification. Water used in this study was distilled and deionized. In a typical synthesis, two identical solutions were separately prepared by dissolving CTAB (2 g) in 50 mL of cyclohexane and 2 mL of n-pentanol and were mechanically agitated for about 20 min until they became transparent. Then, 2 mL of CaCl2 aqueous solution and 2 mL of Na2WO4 aqueous solution were added to the above solution respectively and stirred for 15 min. Then the two optically transparent microemulsion solutions were mixed and stirred for another 15 min. The resulting solution was transferred into a stainless Teflon-lined autoclave and heated at 140 1C for 24 h. The suspension thus obtained was naturally cooled to room temperature and samples were collected and washed five times with absolute ethanol and distilled water, followed by drying in vacuum at 60 1C for 5 h. The overall crystallinity and purity of the as-synthesized samples were analyzed by X-ray power diffraction with monochromatized Cuka incident radiation by SHIMADZU XRD-6000 operated at 40 kV voltage and 50 mA current. X-ray diffraction (XRD) patterns were recorded
from 101 to 801 (2y) with a scanning step of 0.021. The size distribution, morphology and selected-area electron diffraction (SAED) patterns of the CaWO4 products were characterized by transmission electron microscopy (TEM, JEOL, Model JEM-2000, 160 kV and HRTEM, JEOL, JEM-2010F, 200 kV). Photoluminescence spectra were examined using a fluorescence spectrophotometer (SHIMADZU, RF-5301PC) with Xe lamp at room temperature.
3. Results and discussions 3.1. Characterization of CaWO4 nanocrystals The scheelite possesses a tetragonal structure with W in tetrahedral coordination and Ca in eight-fold coordination to oxygen [8]. The phase composition and structure of the product, which was obtained when the temperature was 140 1C and the molar ratio was 20, were examined by powder XRD (see Fig. 1). All the strong peaks can be assigned to the tetragonal phase CaWO4 (a ¼ 5:216 and c ¼ 11:313 A˚) reported in the literature (JCPDS 77-2234). No other impurities were detected in the product. The morphology and structure of the product were further characterized by transmission electron microscopy (TEM), high-resolution TEM (HRTEM) and SAED. Fig. 2a shows a typical TEM image of CaWO4 nanocrystals with mean diameters of 20–30 nm. These nanocrystals are well dispersed with good monodispersity. Fig. 2a (inset) displays the SAED image taken from the same product. The homocentric circles in this diffraction pattern indicate the polycrystalline nature, and confirm that the structures are sheelite. The reason for this phenomenon could be that the size of polyhedron is so small that we could not get a diffraction pattern of a single polyhedron. The statistical information of the nanocrystal size is shown in Fig. 2b. The
Fig. 1. XRD pattern of the product, confirming the formation of pure CaWO4 phase.
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Fig. 2. (a) TEM image of CaWO4 nanocrystals synthesized with the concentration of the reactants of 0.7 M and w ¼ 20 at 140 1C; inset shows the corresponding electron diffraction patterns. (b) Size distribution of CaWO4 nanocrystals synthesized with the concentration of the reactants of 0.7 M and w ¼ 20 at 140 1C. (c, d) TEM images of CaWO4 nanocrystals obtained at T ¼ 140 1C with the reactant concentration of 0.7 M but the w value increasing from 10, 30. (e) High-resolution TEM image of the CaWO4 product, recorded from one nanocrystal. (f) EDXA of the same product, further confirming the element composition of the product.
information was obtained by measuring every nanocrystal on the same TEM photograph, on which there are more than 140 nanocrystals. In Fig. 2b, the column of 20 nm is the highest and the column of 25 nm is the second highest. The percentages of the nanocrystals with diameters of 20, 25, 29 nm are 40%, 28.6%, 16.4%, respectively. It means that there are about 85% nanocrystals with the diameter of
20–30 nm. The HRTEM image (Fig. 2e) suggests that the nanocrystals are structurally uniform with an interplanar spacing of 0.31 nm, corresponding to the (1 1 2) lattice spacing of CaWO4 and in agreement with the observations from the SAED image. EDX measurement made on the assynthesized CaWO4 nanocrystals indicates that the nanocrystals are composed only of Ca, W and O (Fig. 2f). It is
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obvious that the weak copper peaks in the figure are raised from the TEM grid. EDX analysis also demonstrates that the composition within the experimental error is consistent with the stoichiometry of CaWO4. 3.2. Effect of the reaction conditions on the morphology and size of CaWO4 nanostructures It was found in our experiments that the molar ratio (w) between water and CTAB and the reaction temperature both have significant effects on the morphology of CaWO4. The products obtained at an invariable temperature (T ¼ 140 1C) but with w ¼ 5, 10, 20, 30, 40, respectively, are discussed as follows. When the w value was as low as 5, the product contained only nanocrystals with average diameters of 10–20 nm. However, nanorods with lengths of 600–1000 nm were obtained with w ¼ 10 (Fig. 2c). When the w value was increased to 20, there were only nanocrystals in the product again with diameters increasing to 20–30 nm, but the nanocrystals became more uniform and were similar to nanospheres (Fig. 2a). With w further increasing to 30 and 40, the diameters of the nanocrystals increased to about 40–80 nm (Fig. 2d) and 60–100 nm. But the diameters were not uniform any more in the latter case. Thus we can conclude that with the increase of w (except w ¼ 10), the diameters of the as-synthesized products increased gradually (Table 1). Fig. 3 shows the TEM images of the products obtained when the w value was kept constant (w ¼ 20) but the temperature adjusted to 140, 150, and 180 1C. With T ¼ 140 1C, only nanocrystals with average diameters of 20–30 nm (Fig. 3a) were formed. When the temperature Table 1 Different morphologies of the products obtained with various molar ratios (w) between water and CTAB Products CaWO4 CaWO4 CaWO4 CaWO4 CaWO4
(S1) (S2) (S3) (S4) (S5)
w
Morphology and size
5 10 20 30 40
Nanocrystals (10–20 nm) Nanorods (600–1000 nm) Nanocrystals (20–30 nm) Nanocrystals (40–80 nm) Nanocrystals (60–100 nm)
was increased to 150 1C, however, the product contained both nanorods (about 500 nm in length) and nanocrystals (10–20 nm in mean diameters) (Fig. 3b). The diameters of the nanorods were similar to those of the nanocrystals and there were nanocrystals on the top of the nanorods. With T ¼ 180 1C, the product was composed of only nanocrystals with average diameters of 20–30 nm, (Fig. 3c). It can be seen that the temperature has significant effect on the morphologies and sizes of the final products (Table 2). But the mechanism remains unclear and has to be further investigated. Furthermore, we also studied the effect of the concentration of the reactants with a constant w value (w ¼ 20) and temperature (T ¼ 140 1C). When the concentration of CaCl2 was adjusted to 0.05, 0.5 or 0.7 M, the morphologies of the resulting CaWO4 product showed no obvious changes, i.e. they were all nanocrystals with diameters of 20–30 nm. Therefore, the conclusion was that the concentration of the reactants was not an important influence factor of the shape and size of the CaWO4 product. 3.3. Formation mechanism and photoluminescent properties of the CaWO4 nanostructures As stated above, with the increase of the w value, the diameters of the as-synthesized products increased gradually. When two microemulsion solutions containing CaCl2 and NaWO4 respectively were mixed together, CaWO4 nucleation and irreversible micellar fusion may be concomitant. When the w value is very low, the low water content in the microemulsion leads to tiny droplets [24]. Accordingly, the fused droplets containing CaWO4 nucleus would be small and thus nanocrystals with small diameters
Table 2 Different morphologies of products obtained at different reaction temperatures Products
T (1C)
Morphology (size)
CaWO4 (S6) CaWO4 (S7) CaWO4 (S8)
140 150 180
Nanocrystals (20–30 nm) Nanocrystals (10–20 nm) and nanorods Nanocrystals (20–30 nm)
Fig. 3. Changes of the morphologies with different reaction conditions. (a–c) TEM images of CaWO4 nanocrystals obtained with w ¼ 20 and the reactant concentration of 0.7 M but at temperatures of 140 1C, 150 1C and 180 1C.
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at 435 nm when excited at 238 nm. These interesting properties of CaWO4 nanocrystals have potential applications. Acknowledgments This work was supported by the Natural Science Foundation of China (NSFC) (No. 20271007, 20331010 and 90406002), and by Specialized Research Fund for the Doctoral Program of Higher Education (SRFDP) (No. 20030007014). References
Fig. 4. PL spectra of the CaWO4 nanocrystals. Curves A–C were obtained from samples with average diameters of 10–20, 40–80 and 60–100 nm, which were synthesized with the w value of 5, 30, 40, respectively.
can be produced [22,26]. When the w value is a little higher, the low water content may make the rates slow at which two spherical droplets fuse, which would result in a short cylinder droplet. In such a microemulsion droplet, a rodlike nanocrystalline of CaWO4 would grow [23]. When the w value is high enough, the water droplets become bigger, and the diameters of the forming nanocrystals would increase. The surfactant, CTAB, stabilizes the water droplets at the water/oil interface and the absorption between CTAB and the crystal seeds controls the growth of the crystals. The room temperature PL spectra of the products are shown in Fig. 4. The curves A–C were obtained from samples with average diameters of 10–20, 40–80, and 60–100 nm, which were synthesized with the w value of 5, 30, and 40, respectively. When excited at 238 nm, all the CaWO4 products with different morphologies exhibited the only green peak at 435 nm. The peak positions are the same. The luminescence properties of CaWO4 can be utilized for applications in lasers, and fluorescent lamps in the future. 4. Conclusion In summary, we have demonstrated a facile hydrothermal microemulsion method to synthesize uniform CaWO4 nanocrystals and nanorods with controlled diameters. We also found the molar ratio (w) between water and CTAB, and the reaction temperature could affect the morphology, but the concentration of the reactants had no important influence on the shape and size of the CaWO4 product. The possible mechanism is proposed for the selective formation of the different morphologies. All the CaWO4 products with different morphologies exhibited the only green peak
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Journal of Crystal Growth 289 (2006) 231–235 www.elsevier.com/locate/jcrysgro
The synthesis and photoluminescent properties of calcium tungstate nanocrystals Lingna Suna,b, Minhua Caoc, Yonghui Wangc, Genban Suna,b, Changwen Hua,b, a
Institute for Chemical Physics, Beijing Institute of Technology, Beijing 100081, People’s Republic of China b Department of Chemistry, Beijing Institute of Technology, Beijing 100081, People’s Republic of China c Institute of Polyoxometalate Chemistry, Northeast Normal University, Changchun 130024, People’s Republic of China Received 15 July 2005; received in revised form 6 October 2005; accepted 18 October 2005 Available online 27 December 2005 Communicated by T.F. Kuech
Abstract CaWO4 nanocrystals with average diameters of 20–30 nm and nanorods with mean lengths of 600–1000 nm were controllably synthesized through a facile microemulsion-mediated hydrothermal procedure. And the factors affecting the morphologies of the products, such as the molar ratio (w) between water and CTAB, the reaction temperature and the concentration of the reactants, were studied and discussed. With the increase of the w value, the diameter of the as-synthesized products increased gradually, except in the case of w ¼ 10 where nanorods were formed. With the temperature increasing, the morphologies of the products changed from nanocrystals to nanorods, and finally to nanocrystals again. However, the concentration of the reactants exhibited no obvious influence on the shape and size of the CaWO4 product. A possible mechanism is proposed for the selective formation of the different morphologies. The photoluminescent properties of the CaWO4 nanocrystals are also reported. All the products of CaWO4 with different morphologies exhibited the only green peak at 435 nm with an excitation wavelength at 238 nm. r 2005 Elsevier B.V. All rights reserved. PACS: 78.55.Hx; 81.07.Bc; 81.10.Dn Keywords: A1. Characterization; B1. Nanomaterials; B1. Tungstates
1. Introduction During the past decade, low-dimensional nanomaterials have attracted much attention because they may havepotential applications, such as photonics, microelectronics and catalysis [1–5]. In particular, metal tungstate (MWO4, M ¼ Ca2+, Ba2+, Pb2+, Sr2+, etc.) nanostructures attract great interest due to their structural properties and promising applications [6]. For example, calcium tungstate (CaWO4) has scheelite structure and is interesting because of its luminescence, thermoluminescence, and stimulated Raman scattering behavior [7]. Because of its superior Corresponding author. Department of Chemistry, Beijing Institute of Technology, Beijing 100081, People’s Republic of China. Tel.: +86 10 86665743; fax: +86 10 68913293. E-mail address: [email protected] (C. Hu).
0022-0248/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2005.10.125
luminescence properties CaWO4 has been extensively used as blue phosphor (433 nm) in oscilloscopes and fluorescent lightening and as a scintillator for detecting X-rays and grays in medical applications [8,9]. This material doped with rare-earth ions has optical and laser properties [10,11]. Nanometer-scale calcium tungstate may play an essential role for its applications in future luminescent nanodevices. At present, rod-, ellipsoid- and sphere-like PbWO4 nanostructures have been prepared in a simple anionic microemulsion system [12]. And PbWO4 nano- and microcrystals were also prepared via a simple, seedless, and high-yield wet chemical route [13]. Further, Shi et al. [14] prepared BaWO4 nanowires in cationic reverse micelles. The synthesis of penniform superstructures of BaWO4 nanowires was also directed by the block copolymer in reverse micelles [15]. In addition, Yu et al. [7] synthesized 1D and 2D CdWO4 nanoparticles by a
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facile aqueous-solution route. The synthesis of CdWO4 nanorods was also achieved by a hydrothermal method [16]. They also synthesized a family of single-crystalline tungstate nanorods/nanowires such as ZnWO4, FeWO4, MnWO4, Bi2WO6, Ag2WO4, and Ag2W2O7 [17]. Although CaWO4 nanoparticles with the sizes of 20–45 nm [18] and nanocrystals with the sizes of 40 nm [19] were solvothermally synthesized, control of the crystalline structure with a fine monomorphology has not yet been achieved. Microemulsion systems, which are certain combinations of water, oil, surfactant and cosurfactant, have already been used as media to synthesize nanoparticles [20,21]. In a water-in-oil (w/o) microemulsion, nanosized water pools are dispersed in a continuous phase and stabilized by the surfactant (S) and co-surfactant at the water/oil interface. The size of the water pool can be carefully controlled by adjusting the [H2O]/[S] ratio, provided wX10 [22]. In the past 2 years, our group has reported the synthesis of BaF2 whiskers, hydroxyapatite nanofibers, Co–Cu complex nanorods/nanorings, Co3[Co(CN)6]2 truncated nanocubes and LaPO4/CePO4 nanorods/nanowires via a microemulsion-mediated hydrothermal method [23–27]. Herein, we present a facile microemulsion-mediated hydrothermal procedure for the synthesis of extremely small CaWO4 nanocrystals and nanorods with controlled sizes. 2. Experiment Cetyltrimethylammonium bromide (CTAB) and absolute ethanol were purchased from Beijing Beihua Fine Chemicals Co. Ltd. Cyclohexane and n-pentanol were bought from The First Tianjin Chemical Reagent Co. Ltd. Sodium tungstate (Na2WO4) and calcium chloride (CaCl2) were analytical grade reagents. All the chemicals were used without further purification. Water used in this study was distilled and deionized. In a typical synthesis, two identical solutions were separately prepared by dissolving CTAB (2 g) in 50 mL of cyclohexane and 2 mL of n-pentanol and were mechanically agitated for about 20 min until they became transparent. Then, 2 mL of CaCl2 aqueous solution and 2 mL of Na2WO4 aqueous solution were added to the above solution respectively and stirred for 15 min. Then the two optically transparent microemulsion solutions were mixed and stirred for another 15 min. The resulting solution was transferred into a stainless Teflon-lined autoclave and heated at 140 1C for 24 h. The suspension thus obtained was naturally cooled to room temperature and samples were collected and washed five times with absolute ethanol and distilled water, followed by drying in vacuum at 60 1C for 5 h. The overall crystallinity and purity of the as-synthesized samples were analyzed by X-ray power diffraction with monochromatized Cuka incident radiation by SHIMADZU XRD-6000 operated at 40 kV voltage and 50 mA current. X-ray diffraction (XRD) patterns were recorded
from 101 to 801 (2y) with a scanning step of 0.021. The size distribution, morphology and selected-area electron diffraction (SAED) patterns of the CaWO4 products were characterized by transmission electron microscopy (TEM, JEOL, Model JEM-2000, 160 kV and HRTEM, JEOL, JEM-2010F, 200 kV). Photoluminescence spectra were examined using a fluorescence spectrophotometer (SHIMADZU, RF-5301PC) with Xe lamp at room temperature.
3. Results and discussions 3.1. Characterization of CaWO4 nanocrystals The scheelite possesses a tetragonal structure with W in tetrahedral coordination and Ca in eight-fold coordination to oxygen [8]. The phase composition and structure of the product, which was obtained when the temperature was 140 1C and the molar ratio was 20, were examined by powder XRD (see Fig. 1). All the strong peaks can be assigned to the tetragonal phase CaWO4 (a ¼ 5:216 and c ¼ 11:313 A˚) reported in the literature (JCPDS 77-2234). No other impurities were detected in the product. The morphology and structure of the product were further characterized by transmission electron microscopy (TEM), high-resolution TEM (HRTEM) and SAED. Fig. 2a shows a typical TEM image of CaWO4 nanocrystals with mean diameters of 20–30 nm. These nanocrystals are well dispersed with good monodispersity. Fig. 2a (inset) displays the SAED image taken from the same product. The homocentric circles in this diffraction pattern indicate the polycrystalline nature, and confirm that the structures are sheelite. The reason for this phenomenon could be that the size of polyhedron is so small that we could not get a diffraction pattern of a single polyhedron. The statistical information of the nanocrystal size is shown in Fig. 2b. The
Fig. 1. XRD pattern of the product, confirming the formation of pure CaWO4 phase.
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Fig. 2. (a) TEM image of CaWO4 nanocrystals synthesized with the concentration of the reactants of 0.7 M and w ¼ 20 at 140 1C; inset shows the corresponding electron diffraction patterns. (b) Size distribution of CaWO4 nanocrystals synthesized with the concentration of the reactants of 0.7 M and w ¼ 20 at 140 1C. (c, d) TEM images of CaWO4 nanocrystals obtained at T ¼ 140 1C with the reactant concentration of 0.7 M but the w value increasing from 10, 30. (e) High-resolution TEM image of the CaWO4 product, recorded from one nanocrystal. (f) EDXA of the same product, further confirming the element composition of the product.
information was obtained by measuring every nanocrystal on the same TEM photograph, on which there are more than 140 nanocrystals. In Fig. 2b, the column of 20 nm is the highest and the column of 25 nm is the second highest. The percentages of the nanocrystals with diameters of 20, 25, 29 nm are 40%, 28.6%, 16.4%, respectively. It means that there are about 85% nanocrystals with the diameter of
20–30 nm. The HRTEM image (Fig. 2e) suggests that the nanocrystals are structurally uniform with an interplanar spacing of 0.31 nm, corresponding to the (1 1 2) lattice spacing of CaWO4 and in agreement with the observations from the SAED image. EDX measurement made on the assynthesized CaWO4 nanocrystals indicates that the nanocrystals are composed only of Ca, W and O (Fig. 2f). It is
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obvious that the weak copper peaks in the figure are raised from the TEM grid. EDX analysis also demonstrates that the composition within the experimental error is consistent with the stoichiometry of CaWO4. 3.2. Effect of the reaction conditions on the morphology and size of CaWO4 nanostructures It was found in our experiments that the molar ratio (w) between water and CTAB and the reaction temperature both have significant effects on the morphology of CaWO4. The products obtained at an invariable temperature (T ¼ 140 1C) but with w ¼ 5, 10, 20, 30, 40, respectively, are discussed as follows. When the w value was as low as 5, the product contained only nanocrystals with average diameters of 10–20 nm. However, nanorods with lengths of 600–1000 nm were obtained with w ¼ 10 (Fig. 2c). When the w value was increased to 20, there were only nanocrystals in the product again with diameters increasing to 20–30 nm, but the nanocrystals became more uniform and were similar to nanospheres (Fig. 2a). With w further increasing to 30 and 40, the diameters of the nanocrystals increased to about 40–80 nm (Fig. 2d) and 60–100 nm. But the diameters were not uniform any more in the latter case. Thus we can conclude that with the increase of w (except w ¼ 10), the diameters of the as-synthesized products increased gradually (Table 1). Fig. 3 shows the TEM images of the products obtained when the w value was kept constant (w ¼ 20) but the temperature adjusted to 140, 150, and 180 1C. With T ¼ 140 1C, only nanocrystals with average diameters of 20–30 nm (Fig. 3a) were formed. When the temperature Table 1 Different morphologies of the products obtained with various molar ratios (w) between water and CTAB Products CaWO4 CaWO4 CaWO4 CaWO4 CaWO4
(S1) (S2) (S3) (S4) (S5)
w
Morphology and size
5 10 20 30 40
Nanocrystals (10–20 nm) Nanorods (600–1000 nm) Nanocrystals (20–30 nm) Nanocrystals (40–80 nm) Nanocrystals (60–100 nm)
was increased to 150 1C, however, the product contained both nanorods (about 500 nm in length) and nanocrystals (10–20 nm in mean diameters) (Fig. 3b). The diameters of the nanorods were similar to those of the nanocrystals and there were nanocrystals on the top of the nanorods. With T ¼ 180 1C, the product was composed of only nanocrystals with average diameters of 20–30 nm, (Fig. 3c). It can be seen that the temperature has significant effect on the morphologies and sizes of the final products (Table 2). But the mechanism remains unclear and has to be further investigated. Furthermore, we also studied the effect of the concentration of the reactants with a constant w value (w ¼ 20) and temperature (T ¼ 140 1C). When the concentration of CaCl2 was adjusted to 0.05, 0.5 or 0.7 M, the morphologies of the resulting CaWO4 product showed no obvious changes, i.e. they were all nanocrystals with diameters of 20–30 nm. Therefore, the conclusion was that the concentration of the reactants was not an important influence factor of the shape and size of the CaWO4 product. 3.3. Formation mechanism and photoluminescent properties of the CaWO4 nanostructures As stated above, with the increase of the w value, the diameters of the as-synthesized products increased gradually. When two microemulsion solutions containing CaCl2 and NaWO4 respectively were mixed together, CaWO4 nucleation and irreversible micellar fusion may be concomitant. When the w value is very low, the low water content in the microemulsion leads to tiny droplets [24]. Accordingly, the fused droplets containing CaWO4 nucleus would be small and thus nanocrystals with small diameters
Table 2 Different morphologies of products obtained at different reaction temperatures Products
T (1C)
Morphology (size)
CaWO4 (S6) CaWO4 (S7) CaWO4 (S8)
140 150 180
Nanocrystals (20–30 nm) Nanocrystals (10–20 nm) and nanorods Nanocrystals (20–30 nm)
Fig. 3. Changes of the morphologies with different reaction conditions. (a–c) TEM images of CaWO4 nanocrystals obtained with w ¼ 20 and the reactant concentration of 0.7 M but at temperatures of 140 1C, 150 1C and 180 1C.
ARTICLE IN PRESS L. Sun et al. / Journal of Crystal Growth 289 (2006) 231–235
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at 435 nm when excited at 238 nm. These interesting properties of CaWO4 nanocrystals have potential applications. Acknowledgments This work was supported by the Natural Science Foundation of China (NSFC) (No. 20271007, 20331010 and 90406002), and by Specialized Research Fund for the Doctoral Program of Higher Education (SRFDP) (No. 20030007014). References
Fig. 4. PL spectra of the CaWO4 nanocrystals. Curves A–C were obtained from samples with average diameters of 10–20, 40–80 and 60–100 nm, which were synthesized with the w value of 5, 30, 40, respectively.
can be produced [22,26]. When the w value is a little higher, the low water content may make the rates slow at which two spherical droplets fuse, which would result in a short cylinder droplet. In such a microemulsion droplet, a rodlike nanocrystalline of CaWO4 would grow [23]. When the w value is high enough, the water droplets become bigger, and the diameters of the forming nanocrystals would increase. The surfactant, CTAB, stabilizes the water droplets at the water/oil interface and the absorption between CTAB and the crystal seeds controls the growth of the crystals. The room temperature PL spectra of the products are shown in Fig. 4. The curves A–C were obtained from samples with average diameters of 10–20, 40–80, and 60–100 nm, which were synthesized with the w value of 5, 30, and 40, respectively. When excited at 238 nm, all the CaWO4 products with different morphologies exhibited the only green peak at 435 nm. The peak positions are the same. The luminescence properties of CaWO4 can be utilized for applications in lasers, and fluorescent lamps in the future. 4. Conclusion In summary, we have demonstrated a facile hydrothermal microemulsion method to synthesize uniform CaWO4 nanocrystals and nanorods with controlled diameters. We also found the molar ratio (w) between water and CTAB, and the reaction temperature could affect the morphology, but the concentration of the reactants had no important influence on the shape and size of the CaWO4 product. The possible mechanism is proposed for the selective formation of the different morphologies. All the CaWO4 products with different morphologies exhibited the only green peak
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