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Morphology-controlled synthesis, characterization and growth mechanism of PbWO4 nano and macrocrystals
ARTICLE IN PRESS
Journal of Crystal Growth 276 (2005) 577–582 www.elsevier.com/locate/jcrysgro
Morphology-controlled synthesis, characterization and growth mechanism of PbWO4 nano and macrocrystals Guangjun Zhou, Mengkai Lu¨, Feng Gu, Dong Xu, Duorong Yuan State Key Laboratory of Crystal Materials, Shandong University, Shanda Nan Road, Jinan 250100, PR China Received 14 July 2004; received in revised form 30 November 2004; accepted 30 November 2004 Communicated by J. Sherwood Available online 8 January 2005
Abstract PbWO4 nano and macrocrystals with various morphologies have been synthesized via a wet chemical method in the presence of cetyltrimethylammonium bromine (CTAB). The morphological growth processes were investigated and a growth mechanism was proposed. Two different crystal growth processes, the Ostwald ripening process and the oriented attachment process, were held responsible for the formation of the spindle-shaped macrocrystals. The luminescence properties were investigated and the prepared structures displayed a very strong blue/green luminescence at the room temperature. Based on these properties, both the nano and macrostructure materials show promise for applications in the fabrication of photoelectric materials. r 2004 Elsevier B.V. All rights reserved. PACS: 81.05.y; 81.10Aj; 78.66.j Keywords: A1. Characterization; A1. Nanostructures; A2. Growth from solution; B1. Tungstates
1. Introduction Controlling the architecture and morphology of materials at all dimensions from the nanoscale to macroscale is a challenging issue to material scientists [1–7]. Specifically, research on onedimensional (1D) inorganic nano and macrocrysCorresponding author. Tel.: +86 5318364591;
fax: +86 5318565403. E-mail address: [email protected] (M. Lu¨).
tals has attracted extensive attention during the past years due to their fundamental importance in understanding the size- and shape-dependent properties as well as their novel properties and potential applications in fabricating photoelectric sensing device [8–12]. The size- and shape-controlled synthesis of inorganic nano and microcrystallines have been achieved in the solution phase by using appropriate organic additives, such as surfactants, polymers and so on, as capping agents, stabilizing agents, modifying
0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.11.428
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agents, directing agents or soft templates. For example, Yu and Antonietti have presented a facile aqueous solution route for the synthesis of extremely thin 1D and 2D CdWO4 nanoparticles by using copolymers as crystal-growth modifiers [13]. Gou and Murphy have reported the solutionphase synthesis of highly monodisperse Cu2O cubes by using CTAB as the protecting agent [14]. Penniform BaWO4 nanowires have been synthesized by using reverse micelles as soft templates [15]. A highly monodisperse wurtizite ZnO nanoparticles have been synthesized in the presence of PVP [16]. Recently, tungstate materials have attracted much interest because of their luminescence behavior and potential application [13,17–19]. Lead tungstate with a tetragonal scheelite-type structure shows promise for device application due to its interesting excitonic luminescence, thermoluminescence and stimulated Raman scatting behavior [20]. In this report, we demonstrate a facile surfactant-assisted method for the controlled synthesis of PbWO4 nano and macrocrystals of various morphologies (nanoparticles, rhombs, spindles). The morphological growth process were investigated and a mechanism of the formation of the PbWO4 nano and macrocrystals is proposed.
2. Experimental procedure Pb(NO3)2 (Shanghai Chemistry Co.) and Na2WO4 2H2O (Shanghai Chemistry Co.) of analytical grade purity were used as starting materials without further purification. All solutions were made up in distilled water. In a typical experiment, 50 ml of 0.2 M Na2WO4 solutions and 100 ml of 0.1 M CTAB solutions were mixed in a beaker. Two hour of vigorous stirring was necessary to ensure that all the reagents were dispersed homogeneously in the solution at room temperature. Fifty milliliter of 0.2 M Pb(NO3)2 was then added drop wise to the solution under continuous stirring, until the concentrations of Pb2+ and WO2 were equal. A white precipitates formed 4 during the addition of Pb(NO3)2. After the Pb(NO3)2 addition was completed, the mixture was stirred at room temperature for up to 1 h to
encourage the formation of spindle-shaped PbWO4 macrocrystals. Prior to characterization, the precipitate was separated and carefully washed repeatedly with distilled water and absolute ethanol to remove the remaining CTAB and then dried in an oven at 150 1C for 2 h. In order to investigate the morphological growth process, the reaction time was varied in the range 5, 20 and 30 min up to 1 h. X-ray diffraction (XRD) patterns were recorded using a Japan Rigaku D/Max-gA X-ray diffractometer with graphite monochromatized Cu Ka ( Transmission electron irradiation (l ¼ 1:5418 A). microscopy (TEM) studies were carried out using a Japan JEM-100CX transmission electron microscope. Excitation and emission spectra were measured on a Hitachi 850 fluorescence spectrophotometer. All the measurements were carried out at room temperature.
3. Results and discussion 3.1. Morphological development The XRD patterns of the PbWO4 nanoparticles and the spindle-shaped PbWO4 macrocrystals are shown in Figs. 1a and b, respectively. All the reflections of the XRD pattern can be indexed to a pure tetragonal PbWO4 with space group I41/a (JCPDS file number19-0708), the cell parameters ( ¼ 5:45 and cðAÞ ( ¼ 12:05; respectively. were: aðAÞ These XRD patterns indicate that well-crystallized PbWO4 macrocrystals can be obtained under the given synthetic conditions. The morphologies and macrostructures of the as-prepared samples were further investigated by TEM. As shown in Fig. 2, samples prepared under different reaction times in the presence of CTAB displayed an interesting range of morphologies from nanoparticles to rhombic and spindle-shaped macroparticles. Fig. 2a shows the morphology of the samples prepared under continuous stirring for 5 min after the Pb(NO3)2 addition was completed. These conditions yielded PbWO4 nanoparticles of mean diameter 4–5 nm. No other morphologies were present in the precipitates at this stage. The morphologies of the samples prepared under
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Fig. 1. XRD patterns of (a) the PbWO4 nanoparticles prepared for 5 min in the presence of 0.05 M CTAB (b) the spindleshaped PbWO4 macrocrystals prepared for 1 h in the presence of 0.05 M CTAB.
continuous stirring for 20 min are shown in Fig. 2b. The TEM indicated that the obtained samples were a mixture of PbWO4 nano and macrocrystals, in which a few PbWO4 nanoparticles were mixed with rhombic- and spindle-shaped macrocrystals. This suggested that the larger PbWO4 macrocrystals must have formed during the course of reaction by the conversion of the PbWO4 nanoparticles. The morphologies of the samples prepared under continuous stirring for 30 min are shown in Fig. 2c. Here the PbWO4 nanoparticles have vanished with the formation of rhombic- and spindle-shaped macrocrystals. Fig. 2d shows the morphology of the samples prepared under continuous stirring for 1 h. It was clear that the precipitates were well-defined spindle-shaped macrocrystals of about 3.0–3.5 mm in length and 500–800 nm in width. No PbWO4 nanoparticles and rhomb-shaped particles were present in the product. When no surfactants were introduced into the reaction solution, only irregular nanoparticles were formed. No rhombic- and spindle-shaped macrocrystals were found (Fig. 3a). When the CTAB concentration in the reaction solution was
Fig. 2. TEM images of the PbWO4 particles prepared in the present of 0.05 M CTAB for different periods of time (a) 5 min. (b) 20 min. (c) 30 min. (d) 1 h.
changed from 0.05 to 0.005 M, only rhombicshaped particles were present in the products. No spindle-shaped PbWO4 macrocrystals were found as shown in Fig. 3b. These results indicated that the surfactant CTAB played a crucial role in
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Fig. 3. TEM images of (a) the PbWO4 nanoparticles prepared in the absence of CTAB (b) the rhomb-shaped PbWO4 particles prepared in the presence of 0.005 M CTAB.
the formation of spindle-shaped PbWO4 macrocrystals. The CTAB concentration was another important factor. When the concentration of CTAB was reduced to 0.005 M in the solution rhombicshaped particles could be formed, but the spindle-shaped macrocrystals cannot be produced (see Fig. 3b). This indicated that a CTAB concentration, up to [CTAB]/[WO4] ¼ 1, was necessary for the formation of the spindle-shaped PbWO4 macrocrystals. One basic crystal-growth mechanism in solution systems is the well known ‘‘Ostwald ripening process’’ [21]. In the Ostwald ripening process, the initial formation of tiny crystalline nuclei in a supersaturated medium is followed by crystal growth. In this process the larger particles grow at the cost of the small ones due to the energy difference between large particles and small particles. This difference is reflected in the higher solubility of the smaller particles as defined by the Gibbs–Thompson law [22]. Another growth mechanism involving mostly oriented particle aggregation [23–25], which were termed conceptually as ‘‘oriented attachment’’ by Penn and Banfield et al. [26–29], has emerged recently as highlighted by Alivisatos [30]. In this mechanism, the bigger particles grow from small primary particles through an oriented attachment process, in which the adjacent particles are self assembled by sharing a common crystallographic orientation and docking of these particles at a planar interface. Based on the XRD and TEM measurement, the following growth mechanism of PbWO4 crystals was proposed. We propose that the formation
Scheme 1. Formation process of the rhomb-shaped PbWO4 particles by the Ostwald ripening process: (1) primary amorphous nanoparticles, (2) formation of rhomb-shaped particles by surfactant-assisted method at the cost of small nanoparticles and (3) ripened rhomb-shaped particles.
process of the spindle-shaped PbWO4 macrocrystals be divided into two stages. In the first stage, the PbWO4 nanoparticles are produced via a crystallization process in the supersaturated solution. The rhombic-shaped particles are then formed via a transformation process using the nanoparticles as precursors. This stage was a typical ‘‘Ostwald ripening process’’. Fig. 2a confirms that the nanoparticles formed at the early stage. Fig. 2b shows the coexistence of the nanoparticles and rhombic-shaped particles. Fig. 2c shows that the nanoparticles vanish and the larger rhombic-shaped particles form as the reaction proceeds. The formation process of rhombic-shaped particles is schematically illustrated in Scheme 1. In the second stage, the spindle-shaped PbWO4 macrocrystals form from rhombic-shaped particles by an oriented particle aggregation process. This stage was a typical ‘‘oriented attachment process’’. Fig. 2c shows the coexistence of the rhombicand spindle-shaped macrocrystals. Fig. 2d shows that the rhombic-shaped particles vanish and the
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Scheme 2. Formation process of the spindle-shaped PbWO4 macrocrystals by the oriented attachment process: (1) primary rhomb-shaped particles, (2) oriented attachment of rhombshaped particles on the two top of the spindle-shaped macrocrystals and (3) well crystallized spindle-shaped macrocrystals.
longer spindle-shaped macrocrystals form. The formation process of spindle-shaped macrocrystals is schematically illustrated in Scheme 2. 3.2. Spectral properties The PbWO4 emission spectrum consists of at least two components, a blue component peaking around 420 nm and a green one peaking around 500 nm [31]. The blue component is ascribed to the regular lattice of which the emitting level comprises both lead and tungstate contributions, while the green one originates from defect centers associated with oxygen [31,32]. The luminescence spectrum when excited at 300.6 nm, of the PbWO4 nanoparticles obtained in the absence of CTAB is shown in Fig. 4a. This exhibits a small blue peak around 400 nm and a strong green broad emission band around 483 nm. These emissions were blue shifted about 20 and 17 nm, respectively, compared with results reported previously in the above literature. The luminescence spectrum of the spindle-shaped PbWO4 macrocrystals obtained in the present of 0.05 M CTAB was shown in Fig. 4b when excited at 300.6 nm. This exhibited a strong blue peak around 390.2 nm and a weak green broad emission band at around 483.8 nm that were blue shifted about 29.8 and 16.2 nm, respectively, compared to the previously reported results. We believe that the blue shift of the emission components could be related to the lattice defects. The shape of the spectrum of the spindle-shaped PbWO4 differs considerably from that of the PbWO4 nanoparticles. The blue emission peak of
Fig. 4. Emission spectra (lex ¼ 300:6 nm) of (a) the PbWO4 nanoparticles prepared in the absence of CTAB (b) the spindleshaped PbWO4 macrocrystals prepared for 1 h in the present of 0.05 M CTAB.
the spindle-shaped macrocrystals is much stronger than that of the nanoparticles. A probable reason for this difference is that the spindle-shaped macrocrystals have less defective lattices than the nanoparticles and that this greater regularity yields the stronger blue emission. In contrast, because the PbWO4 nanoparticles have more defect centers relative to oxygen, the green emission band of the nanoparticles was stronger than that of spindle-shaped macrocrystals. Based on these properties, the PbWO4 nano and macrostructure materials may have promising for applications in photoelectric materials. The other properties of PbWO4 nano and macrostructure need further investigation; such a study could lead to the
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discovery of novel optical, magnetic, and catalytic properties.
4. Conclusion In summary, PbWO4 nanoparticles and rhombic- and spindle-shaped macrocrystals have been synthesized via a wet chemical method in the present of CTAB. A growth mechanism of PbWO4 crystallines with various morphologies was proposed. Their fluorescence properties were investigated and the prepared structures displayed a very strong blue/green luminescence at the room temperature. A blue shift of the blue and green emission components were observed. Based on these properties, the nano and macrostructure materials may be promising for applications in photoelectric materials.
Acknowledgments The authors thank the Natural Science Foundation of Shandong Province for support. References [1] S. Mann, G.A. Ozin, Nature 382 (1996) 313. [2] T.S. Ahmadi, Z.L. Wang, T.C. Green, A. Henglein, M.L. Ei-Sayed, Science 272 (1996) 1924. [3] H. Yang, N. Coombs, G.A. Ozin, Nature 386 (1997) 692. [4] S. Mann, Angew. Chem. Int. Ed. 39 (2000) 3392. [5] L.A. Estroff, A.D. Hanilton, Chem. Mater. 13 (2001) 3227. [6] Y.G. Sun, Y.N. Xia, Science 298 (2002) 2176. [7] J.Y. Lao, J.Y. Huang, Z.D. Wang, Z.F. Ren, Nano Lett. 3 (2003) 235.
[8] S. Frank, P. Poncharal, Z.L. Wang, W.A. De Heer, Science 280 (1998) 1744. [9] J. Hu, T.W. Odom, C.M. Lieber, Accounts Chem. Res. 32 (1999) 435. [10] Y. Cui, C.M. Lieber, Science 291 (2001) 851. [11] G.R. Patzke, F.K. Krumeich, R. Nesper, Angew. Chem. Int. Ed. 41 (2002) 2446. [12] Y.N. Xia, P.D. Yang, Y.G. Sun, Y.Y. Wu, B. Mayers, B. Gates, Y.D. Yin, F. Kim, Y.Q. Yan, Adv. Mater. 15 (2003) 353. [13] S.H. Yu, M. Antonietti, H. Co¨lfen, M. Giersig, Angew. Chem. Int. Ed. 41 (2002) 2356. [14] L. Gou, C.J. Murphy, Nano Lett. 3 (2003) 231. [15] H. Shi, L. Qi, J. Ma, H. Cheng, J. Am. Chem. Soc. 125 (2003) 3450. [16] L. Guo, S. Yang, Chem. Mater. 12 (2000) 2268. [17] N. Saito, N. Sonoyama, T. Sakata, Bull. Chem. Soc. Jpn. 69 (1996) 2191. [18] S.J. Chen, J.H. Zhou, X.T. Chen, J. Li, L.H. Li, J.M. Hong, Z.L. Xue, X.Z. You, Chem. Phys. Lett. 375 (2003) 185. [19] X.L. Hu, Y.J. Zhou, Langmuir 20 (2004) 1521. [20] M. Kobayashi, M. Ishii, Y. Usuki, Nucl. Instrum. Methods Phys. Res. A 406 (1998) 442. [21] T. Sugimoto, Adv. Colloid Interface Sci. 28 (1987) 65. [22] J.W. Mullin, Crystallization, third ed, Butterworth-Heinemann, Oxford, 1997. [23] J.K. Bailey, C.J. Brinker, M.L. Mecartney, J. Colloid Interface Sci. 157 (1993) 1. [24] V. Privman, D.V. Goia, J. Park, E. Matijevic, J. Colloid Interface Sci. 213 (1999) 36. [25] A. Chemseddine, T. Moritz, Eur. J. Inorg. Chem. (1999) 235. [26] R.L. Penn, J.F. Banfield, Science 281 (1998) 969. [27] R.L. Penn, J.F. Banfield, Geochim. Cosmochim. Acta 63 (1999) 1549. [28] J.F. Banfield, S.A. Welch, H. Zhang, T.T. Ebert, R.L. Penn, Science 289 (2000) 751. [29] R.L. Penn, A.T. Stone, D.R. Veblen, J. Phys. Chem. B 105 (2001) 4690. [30] A.P. Alivisatos, Science 289 (2000) 736. [31] G. Blasse, J. Lumin. 72 (1997) 129. [32] M. Nikl, Phys. Status Solidi (a) 178 (2000) 595.
Journal of Crystal Growth 276 (2005) 577–582 www.elsevier.com/locate/jcrysgro
Morphology-controlled synthesis, characterization and growth mechanism of PbWO4 nano and macrocrystals Guangjun Zhou, Mengkai Lu¨, Feng Gu, Dong Xu, Duorong Yuan State Key Laboratory of Crystal Materials, Shandong University, Shanda Nan Road, Jinan 250100, PR China Received 14 July 2004; received in revised form 30 November 2004; accepted 30 November 2004 Communicated by J. Sherwood Available online 8 January 2005
Abstract PbWO4 nano and macrocrystals with various morphologies have been synthesized via a wet chemical method in the presence of cetyltrimethylammonium bromine (CTAB). The morphological growth processes were investigated and a growth mechanism was proposed. Two different crystal growth processes, the Ostwald ripening process and the oriented attachment process, were held responsible for the formation of the spindle-shaped macrocrystals. The luminescence properties were investigated and the prepared structures displayed a very strong blue/green luminescence at the room temperature. Based on these properties, both the nano and macrostructure materials show promise for applications in the fabrication of photoelectric materials. r 2004 Elsevier B.V. All rights reserved. PACS: 81.05.y; 81.10Aj; 78.66.j Keywords: A1. Characterization; A1. Nanostructures; A2. Growth from solution; B1. Tungstates
1. Introduction Controlling the architecture and morphology of materials at all dimensions from the nanoscale to macroscale is a challenging issue to material scientists [1–7]. Specifically, research on onedimensional (1D) inorganic nano and macrocrysCorresponding author. Tel.: +86 5318364591;
fax: +86 5318565403. E-mail address: [email protected] (M. Lu¨).
tals has attracted extensive attention during the past years due to their fundamental importance in understanding the size- and shape-dependent properties as well as their novel properties and potential applications in fabricating photoelectric sensing device [8–12]. The size- and shape-controlled synthesis of inorganic nano and microcrystallines have been achieved in the solution phase by using appropriate organic additives, such as surfactants, polymers and so on, as capping agents, stabilizing agents, modifying
0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.11.428
ARTICLE IN PRESS 578
G. Zhou et al. / Journal of Crystal Growth 276 (2005) 577–582
agents, directing agents or soft templates. For example, Yu and Antonietti have presented a facile aqueous solution route for the synthesis of extremely thin 1D and 2D CdWO4 nanoparticles by using copolymers as crystal-growth modifiers [13]. Gou and Murphy have reported the solutionphase synthesis of highly monodisperse Cu2O cubes by using CTAB as the protecting agent [14]. Penniform BaWO4 nanowires have been synthesized by using reverse micelles as soft templates [15]. A highly monodisperse wurtizite ZnO nanoparticles have been synthesized in the presence of PVP [16]. Recently, tungstate materials have attracted much interest because of their luminescence behavior and potential application [13,17–19]. Lead tungstate with a tetragonal scheelite-type structure shows promise for device application due to its interesting excitonic luminescence, thermoluminescence and stimulated Raman scatting behavior [20]. In this report, we demonstrate a facile surfactant-assisted method for the controlled synthesis of PbWO4 nano and macrocrystals of various morphologies (nanoparticles, rhombs, spindles). The morphological growth process were investigated and a mechanism of the formation of the PbWO4 nano and macrocrystals is proposed.
2. Experimental procedure Pb(NO3)2 (Shanghai Chemistry Co.) and Na2WO4 2H2O (Shanghai Chemistry Co.) of analytical grade purity were used as starting materials without further purification. All solutions were made up in distilled water. In a typical experiment, 50 ml of 0.2 M Na2WO4 solutions and 100 ml of 0.1 M CTAB solutions were mixed in a beaker. Two hour of vigorous stirring was necessary to ensure that all the reagents were dispersed homogeneously in the solution at room temperature. Fifty milliliter of 0.2 M Pb(NO3)2 was then added drop wise to the solution under continuous stirring, until the concentrations of Pb2+ and WO2 were equal. A white precipitates formed 4 during the addition of Pb(NO3)2. After the Pb(NO3)2 addition was completed, the mixture was stirred at room temperature for up to 1 h to
encourage the formation of spindle-shaped PbWO4 macrocrystals. Prior to characterization, the precipitate was separated and carefully washed repeatedly with distilled water and absolute ethanol to remove the remaining CTAB and then dried in an oven at 150 1C for 2 h. In order to investigate the morphological growth process, the reaction time was varied in the range 5, 20 and 30 min up to 1 h. X-ray diffraction (XRD) patterns were recorded using a Japan Rigaku D/Max-gA X-ray diffractometer with graphite monochromatized Cu Ka ( Transmission electron irradiation (l ¼ 1:5418 A). microscopy (TEM) studies were carried out using a Japan JEM-100CX transmission electron microscope. Excitation and emission spectra were measured on a Hitachi 850 fluorescence spectrophotometer. All the measurements were carried out at room temperature.
3. Results and discussion 3.1. Morphological development The XRD patterns of the PbWO4 nanoparticles and the spindle-shaped PbWO4 macrocrystals are shown in Figs. 1a and b, respectively. All the reflections of the XRD pattern can be indexed to a pure tetragonal PbWO4 with space group I41/a (JCPDS file number19-0708), the cell parameters ( ¼ 5:45 and cðAÞ ( ¼ 12:05; respectively. were: aðAÞ These XRD patterns indicate that well-crystallized PbWO4 macrocrystals can be obtained under the given synthetic conditions. The morphologies and macrostructures of the as-prepared samples were further investigated by TEM. As shown in Fig. 2, samples prepared under different reaction times in the presence of CTAB displayed an interesting range of morphologies from nanoparticles to rhombic and spindle-shaped macroparticles. Fig. 2a shows the morphology of the samples prepared under continuous stirring for 5 min after the Pb(NO3)2 addition was completed. These conditions yielded PbWO4 nanoparticles of mean diameter 4–5 nm. No other morphologies were present in the precipitates at this stage. The morphologies of the samples prepared under
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Fig. 1. XRD patterns of (a) the PbWO4 nanoparticles prepared for 5 min in the presence of 0.05 M CTAB (b) the spindleshaped PbWO4 macrocrystals prepared for 1 h in the presence of 0.05 M CTAB.
continuous stirring for 20 min are shown in Fig. 2b. The TEM indicated that the obtained samples were a mixture of PbWO4 nano and macrocrystals, in which a few PbWO4 nanoparticles were mixed with rhombic- and spindle-shaped macrocrystals. This suggested that the larger PbWO4 macrocrystals must have formed during the course of reaction by the conversion of the PbWO4 nanoparticles. The morphologies of the samples prepared under continuous stirring for 30 min are shown in Fig. 2c. Here the PbWO4 nanoparticles have vanished with the formation of rhombic- and spindle-shaped macrocrystals. Fig. 2d shows the morphology of the samples prepared under continuous stirring for 1 h. It was clear that the precipitates were well-defined spindle-shaped macrocrystals of about 3.0–3.5 mm in length and 500–800 nm in width. No PbWO4 nanoparticles and rhomb-shaped particles were present in the product. When no surfactants were introduced into the reaction solution, only irregular nanoparticles were formed. No rhombic- and spindle-shaped macrocrystals were found (Fig. 3a). When the CTAB concentration in the reaction solution was
Fig. 2. TEM images of the PbWO4 particles prepared in the present of 0.05 M CTAB for different periods of time (a) 5 min. (b) 20 min. (c) 30 min. (d) 1 h.
changed from 0.05 to 0.005 M, only rhombicshaped particles were present in the products. No spindle-shaped PbWO4 macrocrystals were found as shown in Fig. 3b. These results indicated that the surfactant CTAB played a crucial role in
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Fig. 3. TEM images of (a) the PbWO4 nanoparticles prepared in the absence of CTAB (b) the rhomb-shaped PbWO4 particles prepared in the presence of 0.005 M CTAB.
the formation of spindle-shaped PbWO4 macrocrystals. The CTAB concentration was another important factor. When the concentration of CTAB was reduced to 0.005 M in the solution rhombicshaped particles could be formed, but the spindle-shaped macrocrystals cannot be produced (see Fig. 3b). This indicated that a CTAB concentration, up to [CTAB]/[WO4] ¼ 1, was necessary for the formation of the spindle-shaped PbWO4 macrocrystals. One basic crystal-growth mechanism in solution systems is the well known ‘‘Ostwald ripening process’’ [21]. In the Ostwald ripening process, the initial formation of tiny crystalline nuclei in a supersaturated medium is followed by crystal growth. In this process the larger particles grow at the cost of the small ones due to the energy difference between large particles and small particles. This difference is reflected in the higher solubility of the smaller particles as defined by the Gibbs–Thompson law [22]. Another growth mechanism involving mostly oriented particle aggregation [23–25], which were termed conceptually as ‘‘oriented attachment’’ by Penn and Banfield et al. [26–29], has emerged recently as highlighted by Alivisatos [30]. In this mechanism, the bigger particles grow from small primary particles through an oriented attachment process, in which the adjacent particles are self assembled by sharing a common crystallographic orientation and docking of these particles at a planar interface. Based on the XRD and TEM measurement, the following growth mechanism of PbWO4 crystals was proposed. We propose that the formation
Scheme 1. Formation process of the rhomb-shaped PbWO4 particles by the Ostwald ripening process: (1) primary amorphous nanoparticles, (2) formation of rhomb-shaped particles by surfactant-assisted method at the cost of small nanoparticles and (3) ripened rhomb-shaped particles.
process of the spindle-shaped PbWO4 macrocrystals be divided into two stages. In the first stage, the PbWO4 nanoparticles are produced via a crystallization process in the supersaturated solution. The rhombic-shaped particles are then formed via a transformation process using the nanoparticles as precursors. This stage was a typical ‘‘Ostwald ripening process’’. Fig. 2a confirms that the nanoparticles formed at the early stage. Fig. 2b shows the coexistence of the nanoparticles and rhombic-shaped particles. Fig. 2c shows that the nanoparticles vanish and the larger rhombic-shaped particles form as the reaction proceeds. The formation process of rhombic-shaped particles is schematically illustrated in Scheme 1. In the second stage, the spindle-shaped PbWO4 macrocrystals form from rhombic-shaped particles by an oriented particle aggregation process. This stage was a typical ‘‘oriented attachment process’’. Fig. 2c shows the coexistence of the rhombicand spindle-shaped macrocrystals. Fig. 2d shows that the rhombic-shaped particles vanish and the
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Scheme 2. Formation process of the spindle-shaped PbWO4 macrocrystals by the oriented attachment process: (1) primary rhomb-shaped particles, (2) oriented attachment of rhombshaped particles on the two top of the spindle-shaped macrocrystals and (3) well crystallized spindle-shaped macrocrystals.
longer spindle-shaped macrocrystals form. The formation process of spindle-shaped macrocrystals is schematically illustrated in Scheme 2. 3.2. Spectral properties The PbWO4 emission spectrum consists of at least two components, a blue component peaking around 420 nm and a green one peaking around 500 nm [31]. The blue component is ascribed to the regular lattice of which the emitting level comprises both lead and tungstate contributions, while the green one originates from defect centers associated with oxygen [31,32]. The luminescence spectrum when excited at 300.6 nm, of the PbWO4 nanoparticles obtained in the absence of CTAB is shown in Fig. 4a. This exhibits a small blue peak around 400 nm and a strong green broad emission band around 483 nm. These emissions were blue shifted about 20 and 17 nm, respectively, compared with results reported previously in the above literature. The luminescence spectrum of the spindle-shaped PbWO4 macrocrystals obtained in the present of 0.05 M CTAB was shown in Fig. 4b when excited at 300.6 nm. This exhibited a strong blue peak around 390.2 nm and a weak green broad emission band at around 483.8 nm that were blue shifted about 29.8 and 16.2 nm, respectively, compared to the previously reported results. We believe that the blue shift of the emission components could be related to the lattice defects. The shape of the spectrum of the spindle-shaped PbWO4 differs considerably from that of the PbWO4 nanoparticles. The blue emission peak of
Fig. 4. Emission spectra (lex ¼ 300:6 nm) of (a) the PbWO4 nanoparticles prepared in the absence of CTAB (b) the spindleshaped PbWO4 macrocrystals prepared for 1 h in the present of 0.05 M CTAB.
the spindle-shaped macrocrystals is much stronger than that of the nanoparticles. A probable reason for this difference is that the spindle-shaped macrocrystals have less defective lattices than the nanoparticles and that this greater regularity yields the stronger blue emission. In contrast, because the PbWO4 nanoparticles have more defect centers relative to oxygen, the green emission band of the nanoparticles was stronger than that of spindle-shaped macrocrystals. Based on these properties, the PbWO4 nano and macrostructure materials may have promising for applications in photoelectric materials. The other properties of PbWO4 nano and macrostructure need further investigation; such a study could lead to the
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G. Zhou et al. / Journal of Crystal Growth 276 (2005) 577–582
discovery of novel optical, magnetic, and catalytic properties.
4. Conclusion In summary, PbWO4 nanoparticles and rhombic- and spindle-shaped macrocrystals have been synthesized via a wet chemical method in the present of CTAB. A growth mechanism of PbWO4 crystallines with various morphologies was proposed. Their fluorescence properties were investigated and the prepared structures displayed a very strong blue/green luminescence at the room temperature. A blue shift of the blue and green emission components were observed. Based on these properties, the nano and macrostructure materials may be promising for applications in photoelectric materials.
Acknowledgments The authors thank the Natural Science Foundation of Shandong Province for support. References [1] S. Mann, G.A. Ozin, Nature 382 (1996) 313. [2] T.S. Ahmadi, Z.L. Wang, T.C. Green, A. Henglein, M.L. Ei-Sayed, Science 272 (1996) 1924. [3] H. Yang, N. Coombs, G.A. Ozin, Nature 386 (1997) 692. [4] S. Mann, Angew. Chem. Int. Ed. 39 (2000) 3392. [5] L.A. Estroff, A.D. Hanilton, Chem. Mater. 13 (2001) 3227. [6] Y.G. Sun, Y.N. Xia, Science 298 (2002) 2176. [7] J.Y. Lao, J.Y. Huang, Z.D. Wang, Z.F. Ren, Nano Lett. 3 (2003) 235.
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