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Room temperature synthesis of PbS nanocrystals with different morphologies in PEO–PPO–PEO triblock copolymers
Materials Science and Engineering B100 (2003) 314 /317 www.elsevier.com/locate/mseb
Short communication
Room temperature synthesis of PbS nanocrystals with different morphologies in PEO PPO PEO triblock copolymers /
/
S.H. Liu, X.F. Qian *, J. Yin, H.W. Lu, Z.K. Zhu Research Institute of Polymer Materials, School of Chemistry and Chemical Technology, Shanghai Jiao Tong University, Shanghai 200240, PR China Received 23 August 2002; received in revised form 7 March 2003; accepted 31 March 2003
Abstract PbS nanocrystals with different morphologies were successfully synthesized in poly(ethylene oxide) /poly(propylene oxide) / poly(ethylene oxide) (PEO /PPO /PEO) triblock copolymers at room temperature with thioacetamide as the sulfur source. From TEM analysis, we found that both the solvents and the hydrophilic PEO fraction of the copolymers played a decisive role in controlling the morphology of the final products. # 2003 Elsevier Science B.V. All rights reserved. Keywords: PbS; Nanocrystals; Morphology; PEO /PPO /PEO
1. Introduction In the nanoscale regime, the size and morphology of materials strongly influence their physical properties [1 / 3]. The fabrication of nanocrystals with controlled morphologies is always potentially important in the preparation of materials suitable for optoelectronic and luminescent applications [4]. In the past few years, a number of strategies on nanostructure fabrication have been developed to control not only the size but also the dispersion and morphology of the products. Among these strategies, the template self-assembly process has been demonstrated to be effective since the pre-organized template regulates the nucleation, growth, morphology and orientation of nanocrystals [5 /7]. Till now, many kinds of templates, such as liquid crystals, self-assembled monolayers, porous aluminum oxide, or carbon nanotubes, have been used. For example, Fan and coworkers reported the synthesis of GaN nanowires in carbon nanotubules [8]. Braum et al. synthesized CdS nanoparticles within a liquid-crystalline hexagonal mesophase formed by nonionic organic amphiphiles and water [9].
* Corresponding author. Tel.: /86-21-5474-3268; fax: /86-215474-1297. E-mail address: [email protected] (X.F. Qian).
Recently, block copolymers, which self-assemble in selective solvents to form micelles or micelle-like aggregates, have been applied to the synthesis of nanocrystals with a desired morphology. The use of block copolymers has also attracted considerable interest because of their potential for processability and hence ease of application [10,11]. PbS is a narrow band gap IV /VI semiconductor with a cubic rock salt structure, and is an important semiconductor owing to its unique optical and electronic properties, and its widespread applications in optical and electronic devices such as IR detectors and Pb2 ion-selective sensors [12]. The synthesis of PbS nanocrystals has been tried by many methods such as girradiation method [7], liquid phase synthesis [13], gas phase synthesis [14], and polymer films route [15]. Herein, we reported the synthesis of PbS nanocrystals in poly(ethylene oxide) /poly(propylene oxide) / poly(ethylene oxide) (PEO /PPO /PEO) triblock copolymers with thioacetamide as the sulfur source at room temperature. It was found that both the solvents and the hydrophilic PEO fraction of the copolymers played a key role in controlling the morphology of the final products. Besides, it was worth noting that, thioacetamide was usually used as sulfur source in ultraviolet irradiation [16] or hydrothermal process [17], and to our knowledge, we first used thioacetamide as sulfur source
0921-5107/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0921-5107(03)00117-X
S.H. Liu et al. / Materials Science and Engineering B100 (2003) 314 /317
315
to prepare sulfides at room temperature without adjusting the pH value.
2. Experimental All reactants and solvents were analytical grade. The triblock copolymers L64 [(EO)13 /(PO)70 /(EO)13], P123 [(EO)20 /(PO)70 /(EO)20], F127 [(EO)106 /(PO)70 / (EO)106] were obtained from BASF. In a typical preparation process, 0.06 g Pb(NO3)2 was dissolved in the solution of 0.1 g copolymers dissolved in 20 ml solvents under vigorous stirring. Distilled water, ethanol, as well as their mixtures in different volume ratios (v/v) were served as the solvents. Thioacetamide (50 mol% excessive based on Pb(NO3)2) was then introduced and the solution was stirred in dark for sulfuration reaction for 3 h. The obtained black viscous solution was stable and did not precipitate within 3 months. The resulting solution was cast on a glass substrate and dried at room temperature for 12 h, then at 30 8C in vacuum for 12 h. The obtained film was then washed with deionized water to remove unreacted thioacetamide. Powder samples were prepared by grinding the above film for X-ray diffraction (XRD) experiments. The X-ray powder diffraction (XRD) patterns were recorded at a scanning rate of 48 min 1 in the 2u range of 20/708 using a Rigaku D/max gA X-ray diffract˚ ). Transometer with Cu-Ka radiation (l /1.54178 A mission electron microscopic (TEM) photographs were taken on a Hitachi S-530 TEM. The samples were prepared by mounting a drop of the resulting solution on carbon-coated Cu grids and allowed to dry in air.
3. Results and discussion The formation process of PbS nanocrystallites in copolymers could be described as follows. When thioacetamide was added to the Pb2/copolymer solution, it gradually released sulfide ions upon hydrolysis. Then the released sulfide ions combined with lead ions to form PbS nanocrystals as described in Eqs. (1) and (2). CH3 CSNH2 2H2 O 0 CH3 COOHNH3 H2 S Pb2 H2 S 0 PbS2H
(1) (2)
Fig. 1 showed the XRD pattern of the PbS nanocrystals obtained in P123 with pure distilled water as the solvent. All the diffraction peaks could be indexed to ˚ , which was cubic PbS with the cell constant a /5.931 A very close to that in the JCPDS card (card No. 5-592). No impurities were detected in the XRD pattern. When the mixtures of distilled water and ethanol were used as the solvents, similar XRD patterns were obtained.
Fig. 1. XRD pattern of the P123-PbS nanocrystals obtained in pure distilled water.
Fig. 2 showed the typical TEM micrographs of PbS nanocrystals obtained in P123 in different solvents. As could be seen from Fig. 2, the obtained nanocrystals showed different morphologies when the mixtures of distilled water and ethanol in different volume ratios (v/ v) were used as the solvents. It was interesting to find that the PbS nanocrystals obtained in pure distilled water showed a rice-like morphology, and the morphology of the obtained nanocrystals turned from ‘rice-like’ to ‘spherical’ gradually with the increase of the ratio of ethanol. It was well known that the amphiphilic PEO / PPO /PEO triblock copolymers self-assembled in aqueous solution to form micelles or micelle-like aggregates, offering a template for further synthesis of nanocrystals [18]. The morphology of the template probably played a key role in determining the morphology of the final products. By comparison, in an ethanol solution, the PEO and PPO blocks had similar solubility, so the supermolecular structure could not form. As a result, reactants mixed at the molecular level could only produced small PbS nanocrystals with slight aggregation [19]. Therefore, it was conceivable that the solvent had a significant impact on the morphology of the copolymer micelles, which further determined the morphology of the final products. Besides P123, other copolymers with different hydrophilic PEO fractions were also used. Fig. 3a/b showed the TEM images of PbS nanocrystals synthesized in L64 and F127, respectively. Compared with the ‘rice-like’ PbS crystals synthesized in P123, the products obtained in L64 and F127 showed quite different morphologies. It has been reported that the hydrophilic fraction of copolymers played a decisive role in controlling the morphology of the final products [11]. It was probably because block copolymers with different hydrophilic fractions may form micelles with different morphologies, offering different templates for further synthesis of nanocrystals.
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S.H. Liu et al. / Materials Science and Engineering B100 (2003) 314 /317
Fig. 2. TEM images of the P123-PbS sample obtained in different solvents: (a) distilled water; (b) distilled water/ethanol, 2:1; (c) distilled water/ ethanol, 1:2; (d) ethanol.
4. Conclusions This work showed that it was possible to synthesize PbS nanocrystals in PEO /PPO /PEO triblock copolymers with thioacetamide as the sulfur source at room temperature. It was found that both the solvents and the hydrophilic PEO fraction of the copolymers played a decisive role in controlling the morphology of the final products. When the mixtures of distilled water and ethanol were served as the solvents, morphological
evolution from ‘rice-like’ to ‘spherical’ was observed. This method can be extended to other metal chalcogenides/amphiphilic copolymer systems.
Acknowledgements This work was financially supported by the National Natural Science Foundation of China (50103006), the
S.H. Liu et al. / Materials Science and Engineering B100 (2003) 314 /317
317
Fig. 3. TEM images of the L64-P123 (a) and F127-P123 (b) samples obtained in pure distilled water.
Shanghai Nanomaterials Project and the Shanghai Shu Guang Project.
[8] [9] [10] [11]
References
[12]
[1] A.P. Alivisatos, Science 271 (1996) 933. [2] C.M. Lieber, Solid State Commun. 107 (1998) 607. [3] L. Manna, E.C. Scher, A.P. Alivisatos, J. Am. Chem. Soc. 122 (2000) 12 700. [4] Z.X. Deng, C. Wang, X.M. Sun, Y.D. Li, Inorg. Chem. 41 (2002) 869. [5] A. Blaaderen Van, R. Ruel, P. Wiltzius, Nature 385 (1997) 321. [6] A. Sellinger, P.M. Weiss, A. Nguyen, Y. Lu, R.A. Assink, W. Gong, C.J. Brinker, Nature 394 (1998) 256. [7] Z.P. Qiao, Y. Xie, M. Chen, J.G. Xu, Y.J. Zhu, Y.T. Qian, Chem. Phys. Lett. 321 (2000) 504.
[13] [14] [15] [16] [17] [18] [19]
W.Q. Han, S.S. Fan, Q.Q. Li, Y.D. Hu, Science 277 (1997) 1287. P.V. Braum, P. Osenar, S.I. Stupp, Nature 380 (1996) 325. L. Qi, H. Colfen, M. Antonietti, Nano Lett. 1 (2001) 61. C.S. Yang, D.D. Awschalom, G.D. Stucky, Chem. Mater. 14 (2002) 1277. W.Z. Wang, Y.K. Liu, Y.J. Zhan, C.L. Zheng, G.H. Wang, Mater. Res. Bull. 36 (2001) 1977. C. Wang, W.X. Zhang, X.F. Qian, X.M. Zhang, Y. Xie, Y.T. Qian, Mater. Lett. 40 (1999) 255. C. Kaito, Y. Saito, K. Fujita, Jpn. J. Appl. Phys. 26 (1987) 1973. S.H. Wang, S.H. Yang, Langmuir 16 (2000) 389. C.Y. Wang, X. Mo, Y. Zhou, Y.R. Zhu, H.Y. Liu, Z.Y. Chen, J. Mater. Chem. 10 (2000) 607. H. Zeng, J. Yang, Y. Zhu, Y.F. Liu, Y.T. Qian, H.G. Zheng, Chem. Commun. (2001) 1332. Y.L. Su, H.Z. Liu, J. Wang, J.Y. Chen, Langmuir 18 (2002) 865. Y. Zhu, Y.T. Qian, X. Li, M. Zhang, Chem. Commun. (1997) 1081.
Short communication
Room temperature synthesis of PbS nanocrystals with different morphologies in PEO PPO PEO triblock copolymers /
/
S.H. Liu, X.F. Qian *, J. Yin, H.W. Lu, Z.K. Zhu Research Institute of Polymer Materials, School of Chemistry and Chemical Technology, Shanghai Jiao Tong University, Shanghai 200240, PR China Received 23 August 2002; received in revised form 7 March 2003; accepted 31 March 2003
Abstract PbS nanocrystals with different morphologies were successfully synthesized in poly(ethylene oxide) /poly(propylene oxide) / poly(ethylene oxide) (PEO /PPO /PEO) triblock copolymers at room temperature with thioacetamide as the sulfur source. From TEM analysis, we found that both the solvents and the hydrophilic PEO fraction of the copolymers played a decisive role in controlling the morphology of the final products. # 2003 Elsevier Science B.V. All rights reserved. Keywords: PbS; Nanocrystals; Morphology; PEO /PPO /PEO
1. Introduction In the nanoscale regime, the size and morphology of materials strongly influence their physical properties [1 / 3]. The fabrication of nanocrystals with controlled morphologies is always potentially important in the preparation of materials suitable for optoelectronic and luminescent applications [4]. In the past few years, a number of strategies on nanostructure fabrication have been developed to control not only the size but also the dispersion and morphology of the products. Among these strategies, the template self-assembly process has been demonstrated to be effective since the pre-organized template regulates the nucleation, growth, morphology and orientation of nanocrystals [5 /7]. Till now, many kinds of templates, such as liquid crystals, self-assembled monolayers, porous aluminum oxide, or carbon nanotubes, have been used. For example, Fan and coworkers reported the synthesis of GaN nanowires in carbon nanotubules [8]. Braum et al. synthesized CdS nanoparticles within a liquid-crystalline hexagonal mesophase formed by nonionic organic amphiphiles and water [9].
* Corresponding author. Tel.: /86-21-5474-3268; fax: /86-215474-1297. E-mail address: [email protected] (X.F. Qian).
Recently, block copolymers, which self-assemble in selective solvents to form micelles or micelle-like aggregates, have been applied to the synthesis of nanocrystals with a desired morphology. The use of block copolymers has also attracted considerable interest because of their potential for processability and hence ease of application [10,11]. PbS is a narrow band gap IV /VI semiconductor with a cubic rock salt structure, and is an important semiconductor owing to its unique optical and electronic properties, and its widespread applications in optical and electronic devices such as IR detectors and Pb2 ion-selective sensors [12]. The synthesis of PbS nanocrystals has been tried by many methods such as girradiation method [7], liquid phase synthesis [13], gas phase synthesis [14], and polymer films route [15]. Herein, we reported the synthesis of PbS nanocrystals in poly(ethylene oxide) /poly(propylene oxide) / poly(ethylene oxide) (PEO /PPO /PEO) triblock copolymers with thioacetamide as the sulfur source at room temperature. It was found that both the solvents and the hydrophilic PEO fraction of the copolymers played a key role in controlling the morphology of the final products. Besides, it was worth noting that, thioacetamide was usually used as sulfur source in ultraviolet irradiation [16] or hydrothermal process [17], and to our knowledge, we first used thioacetamide as sulfur source
0921-5107/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0921-5107(03)00117-X
S.H. Liu et al. / Materials Science and Engineering B100 (2003) 314 /317
315
to prepare sulfides at room temperature without adjusting the pH value.
2. Experimental All reactants and solvents were analytical grade. The triblock copolymers L64 [(EO)13 /(PO)70 /(EO)13], P123 [(EO)20 /(PO)70 /(EO)20], F127 [(EO)106 /(PO)70 / (EO)106] were obtained from BASF. In a typical preparation process, 0.06 g Pb(NO3)2 was dissolved in the solution of 0.1 g copolymers dissolved in 20 ml solvents under vigorous stirring. Distilled water, ethanol, as well as their mixtures in different volume ratios (v/v) were served as the solvents. Thioacetamide (50 mol% excessive based on Pb(NO3)2) was then introduced and the solution was stirred in dark for sulfuration reaction for 3 h. The obtained black viscous solution was stable and did not precipitate within 3 months. The resulting solution was cast on a glass substrate and dried at room temperature for 12 h, then at 30 8C in vacuum for 12 h. The obtained film was then washed with deionized water to remove unreacted thioacetamide. Powder samples were prepared by grinding the above film for X-ray diffraction (XRD) experiments. The X-ray powder diffraction (XRD) patterns were recorded at a scanning rate of 48 min 1 in the 2u range of 20/708 using a Rigaku D/max gA X-ray diffract˚ ). Transometer with Cu-Ka radiation (l /1.54178 A mission electron microscopic (TEM) photographs were taken on a Hitachi S-530 TEM. The samples were prepared by mounting a drop of the resulting solution on carbon-coated Cu grids and allowed to dry in air.
3. Results and discussion The formation process of PbS nanocrystallites in copolymers could be described as follows. When thioacetamide was added to the Pb2/copolymer solution, it gradually released sulfide ions upon hydrolysis. Then the released sulfide ions combined with lead ions to form PbS nanocrystals as described in Eqs. (1) and (2). CH3 CSNH2 2H2 O 0 CH3 COOHNH3 H2 S Pb2 H2 S 0 PbS2H
(1) (2)
Fig. 1 showed the XRD pattern of the PbS nanocrystals obtained in P123 with pure distilled water as the solvent. All the diffraction peaks could be indexed to ˚ , which was cubic PbS with the cell constant a /5.931 A very close to that in the JCPDS card (card No. 5-592). No impurities were detected in the XRD pattern. When the mixtures of distilled water and ethanol were used as the solvents, similar XRD patterns were obtained.
Fig. 1. XRD pattern of the P123-PbS nanocrystals obtained in pure distilled water.
Fig. 2 showed the typical TEM micrographs of PbS nanocrystals obtained in P123 in different solvents. As could be seen from Fig. 2, the obtained nanocrystals showed different morphologies when the mixtures of distilled water and ethanol in different volume ratios (v/ v) were used as the solvents. It was interesting to find that the PbS nanocrystals obtained in pure distilled water showed a rice-like morphology, and the morphology of the obtained nanocrystals turned from ‘rice-like’ to ‘spherical’ gradually with the increase of the ratio of ethanol. It was well known that the amphiphilic PEO / PPO /PEO triblock copolymers self-assembled in aqueous solution to form micelles or micelle-like aggregates, offering a template for further synthesis of nanocrystals [18]. The morphology of the template probably played a key role in determining the morphology of the final products. By comparison, in an ethanol solution, the PEO and PPO blocks had similar solubility, so the supermolecular structure could not form. As a result, reactants mixed at the molecular level could only produced small PbS nanocrystals with slight aggregation [19]. Therefore, it was conceivable that the solvent had a significant impact on the morphology of the copolymer micelles, which further determined the morphology of the final products. Besides P123, other copolymers with different hydrophilic PEO fractions were also used. Fig. 3a/b showed the TEM images of PbS nanocrystals synthesized in L64 and F127, respectively. Compared with the ‘rice-like’ PbS crystals synthesized in P123, the products obtained in L64 and F127 showed quite different morphologies. It has been reported that the hydrophilic fraction of copolymers played a decisive role in controlling the morphology of the final products [11]. It was probably because block copolymers with different hydrophilic fractions may form micelles with different morphologies, offering different templates for further synthesis of nanocrystals.
316
S.H. Liu et al. / Materials Science and Engineering B100 (2003) 314 /317
Fig. 2. TEM images of the P123-PbS sample obtained in different solvents: (a) distilled water; (b) distilled water/ethanol, 2:1; (c) distilled water/ ethanol, 1:2; (d) ethanol.
4. Conclusions This work showed that it was possible to synthesize PbS nanocrystals in PEO /PPO /PEO triblock copolymers with thioacetamide as the sulfur source at room temperature. It was found that both the solvents and the hydrophilic PEO fraction of the copolymers played a decisive role in controlling the morphology of the final products. When the mixtures of distilled water and ethanol were served as the solvents, morphological
evolution from ‘rice-like’ to ‘spherical’ was observed. This method can be extended to other metal chalcogenides/amphiphilic copolymer systems.
Acknowledgements This work was financially supported by the National Natural Science Foundation of China (50103006), the
S.H. Liu et al. / Materials Science and Engineering B100 (2003) 314 /317
317
Fig. 3. TEM images of the L64-P123 (a) and F127-P123 (b) samples obtained in pure distilled water.
Shanghai Nanomaterials Project and the Shanghai Shu Guang Project.
[8] [9] [10] [11]
References
[12]
[1] A.P. Alivisatos, Science 271 (1996) 933. [2] C.M. Lieber, Solid State Commun. 107 (1998) 607. [3] L. Manna, E.C. Scher, A.P. Alivisatos, J. Am. Chem. Soc. 122 (2000) 12 700. [4] Z.X. Deng, C. Wang, X.M. Sun, Y.D. Li, Inorg. Chem. 41 (2002) 869. [5] A. Blaaderen Van, R. Ruel, P. Wiltzius, Nature 385 (1997) 321. [6] A. Sellinger, P.M. Weiss, A. Nguyen, Y. Lu, R.A. Assink, W. Gong, C.J. Brinker, Nature 394 (1998) 256. [7] Z.P. Qiao, Y. Xie, M. Chen, J.G. Xu, Y.J. Zhu, Y.T. Qian, Chem. Phys. Lett. 321 (2000) 504.
[13] [14] [15] [16] [17] [18] [19]
W.Q. Han, S.S. Fan, Q.Q. Li, Y.D. Hu, Science 277 (1997) 1287. P.V. Braum, P. Osenar, S.I. Stupp, Nature 380 (1996) 325. L. Qi, H. Colfen, M. Antonietti, Nano Lett. 1 (2001) 61. C.S. Yang, D.D. Awschalom, G.D. Stucky, Chem. Mater. 14 (2002) 1277. W.Z. Wang, Y.K. Liu, Y.J. Zhan, C.L. Zheng, G.H. Wang, Mater. Res. Bull. 36 (2001) 1977. C. Wang, W.X. Zhang, X.F. Qian, X.M. Zhang, Y. Xie, Y.T. Qian, Mater. Lett. 40 (1999) 255. C. Kaito, Y. Saito, K. Fujita, Jpn. J. Appl. Phys. 26 (1987) 1973. S.H. Wang, S.H. Yang, Langmuir 16 (2000) 389. C.Y. Wang, X. Mo, Y. Zhou, Y.R. Zhu, H.Y. Liu, Z.Y. Chen, J. Mater. Chem. 10 (2000) 607. H. Zeng, J. Yang, Y. Zhu, Y.F. Liu, Y.T. Qian, H.G. Zheng, Chem. Commun. (2001) 1332. Y.L. Su, H.Z. Liu, J. Wang, J.Y. Chen, Langmuir 18 (2002) 865. Y. Zhu, Y.T. Qian, X. Li, M. Zhang, Chem. Commun. (1997) 1081.