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CdS Crystal Growth of Lamellar Morphology within Templates of Polyelectrolyte/Surfactant Complex Cheng Tao,† Suping Zheng,† Helmuth Mo¨hwald,‡ and Junbai Li*,† International Joint Lab, Key Lab of Colloid and Interface Science, Institute of Chemistry, Chinese Academy of Sciences, Zhong Guan Cun, Beijing 100080, China, and Max-Planck-Institute of Colloids and Interfaces, Am Muehlenberg 2, Golm/Potsdam D-14476, Germany Received March 24, 2003. In Final Form: July 31, 2003 Large lamellar morphological CdS crystals with nanometer-sized thickness were synthesized using the complex templates of poly(styrenesulfonate sodium) and cetyltrimethylammonium bromide at room temperature. The final sample exhibits a high stability after removal of the templates. The synthesized materials were characterized by scanning electron microscopy, atomic force microscopy, transmission electron microscopy, and X-ray diffraction.
1. Introduction Recently, synthesis of inorganic nanocrystals has attracted steadily increasing interest because of their strong size-dependent properties and special optical and electronic features.1-5 As a typical semiconductor material of the II-VI group, cadmium sulfide (CdS) nanocrystals have been widely investigated.6-10 In the past decade, a number of synthetic methods have been developed for the synthesis of CdS nanocrystals.11-24 The controlled synthesis of * To whom correspondence should be addressed. Prof. Dr. Junbai Li. Tel: +86 10 82614087. Fax: +86 10 82612484. E-mail:
[email protected]. † Chinese Academy of Sciences. ‡ Max-Planck-Institute of Colloids and Interfaces. (1) Brus, L. E. J. Chem. Phys. 1984, 80, 4403. (2) Ahmadi, T. S.; Wang, Z. L.; Green, T. C.; Henglein, A.; El-Sayed, M. A. Science 1996, 272, 1924. (3) Alivisatos, A. P. Science 1996, 271, 933. (4) Klimov, V. I.; Mikhailovsky, A. A.; Xu, S.; Malko A.; Hollingsworth, J. A.; Leatherdale, C. A.; Eisler, H.-J.; Bawendi, M. G. Science 2000, 290, 314. (5) Park, J.-I.; Kang, N.-J.; Jun, Y.-W.; Oh, S.-S.; Ri, H.-C.; Cheon, J. ChemPhysChem 2002, 6, 543. (6) Alivisatos, A. P. J. Phys. Chem. 1996, 100, 13226. (7) Peng, Z. A.; Peng, X. J. Am. Chem. Soc. 2001, 123, 183. (8) Korgel, B. A.; Monbouquette, H. G. J. Phys. Chem. 1996, 100, 346. (9) Pinna, N.; Weiss, K.; Sack-Kongehl, H.; Vogel, W.; Urban, J.; Pileni, M. P. Langmuir 2001, 17, 7982. (10) Bekele, H.; Fendler, J. H.; Kelly, J. W. J. Am. Chem. Soc. 1999, 121, 7266. (11) Yu, S. H.; Wu, S. H.; Yang, J.; Han, Z. H.; Xie, Y.; Qian, Y.; Liu, X. Chem. Mater. 1998, 10, 2309. (12) Yang, J.; Zeng, J.; Yu, S.; Yang, L.; Zhou, G.; Qian, Y. Chem. Mater. 2000, 12, 3259. (13) Wu, J.; Jiang, Y.; Li, Q.; Liu, X.; Qian, Y. J. Cryst. Growth 2002, 235, 421. (14) Chen, M.; Xie, Y.; Lu, J.; Xiong, Y.; Zhang, S.; Qian, Y.; Liu, X. J. Mater. Chem. 2002, 12, 748. (15) Yu, W. W.; Peng X. Angew. Chem. 2002, 114, 2474. (16) Jun, Y.-W.; Lee, S.-M.; Kang, N.-M.; Cheon, J. J. Am. Chem. Soc. 2001, 123, 5150. (17) Li, Y.; Wan, J.; Gu, Z. Mater. Sci. Eng. 2000, A286, 106. (18) Mo, X.; Wang, C.; You, M.; Zhu, Y.; Chen, Z.; Hu, Y. Mater. Res. Bull. 2001, 36, 2277. (19) Nikitenko, S. I.; Koltypin, Y.; Mastai, Y.; Koltypin, M.; Gedanken, A. J. Mater. Chem. 2002, 12, 1450. (20) Vossmeyer, T.; Katsikas, L.; Giersig, M.; Popovic, I. G.; Diesner, K.; Chemseddine, A.; Eychmu¨ller, A.; Weller, H. J. Phys. Chem. 1994, 98, 7665. (21) Zhan, J.; Yang, X.; Wang, D.; Li, S.; Xie, Y.; Xia, Y.; Qian, Y. Adv. Mater. 2000, 12, 1348. (22) Xu, D.; Xu, Y.; Chen, D.; Guo, G.; Gui, L.; Tang, Y. Chem. Phys. Lett. 2000, 325, 340.
nanocrystals with a narrow size distribution and uniform shape is very important to produce a well-defined semiconductor. The synthesis employing surfactants as templates has shown a great ability to control the size and shape of CdS nanocrystals.16,24 Polyelectrolyte/surfactant complexes with the combination of polymer backbone and electrostatic interaction exhibited an extremely welldefined three-dimensional structure with special mechanical and dielectric properties in aqueous solution.25-28 Antonietti et al. have demonstrated that the phases of poly(styrenesulfonate) with an oppositely charged cationic surfactant, such as cetyltrimethylammonium derivatives, are essentially layered with various lamellar phases of the repeat periods in a range of 2.5-5.0 nm, such as planar, undulated, and even perforated, which is in the regime of surfactant building blocks.26,27 In the intermediate region having a distance in nanoscale between the ionic and alkane layers, it is considered as an ideal route to induce inorganic crystal growth within the layers in a nanoscale structure. In this study, we prepared CdS crystals with lamellar morphology using the complex templates of polyelectrolyte/ surfactant at room temperature. Our experimental results demonstrate that the facile method is suitable to achieve lamellar CdS crystals within organic complex templates in aqueous solution. The CdS growth along the interspace of template layers leads to an aggregation of many grains in lamellar crystals up to micrometer size and thicknesses of a few nanometers after the removal of the templates. 2. Experimental Section Chemicals. Poly(styrenesulfonate sodium) (PSS) and cetyltrimethylammonium bromide (CTAB) were purchased from Aldrich. Cadmium acetate dihydrate (98%) and sodium sulfide nonahydrate (Na2S‚6H2O) were obtained from ACROS. The chemical reagents were used as received. The water (resistivity (23) Chen, C. C.; Chao, C. Y.; Lang, Z. H. Chem. Mater. 2000, 12, 1516. (24) Simmons, B. A.; Li, S.; John, V. T.; McPherson, G. L.; Bose, A.; Zhou, W.; He, J. Nano Lett. 2002, 2, 263. (25) Antonietti, M.; Conrad, J. Angew. Chem., Int. Ed. Engl. 1994, 33, 1869. (26) Antonietti, M.; Burger, C.; Effing, J. Adv. Mater. 1995, 7, 751. (27) Antonietti, M.; Conrad, J.; Thu¨nemann, A. Macromolecules 1994, 27, 6007. (28) Faul, C. F. J.; Antonietti, M. Adv. Mater. 2003, 15, 673.
10.1021/la034507+ CCC: $25.00 © 2003 American Chemical Society Published on Web 09/04/2003
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higher than 18.2 MΩ) used in all experiments was prepared in a three-stage Millipore Milli-Q plus 185 purification system. Synthesis. Through careful control of experimental conditions, a self-assembled lamellar amphotropic phase can be produced through electrostatic interaction between polyelectrolyte and oppositely charged surfactant molecules in acid aqueous solution. The formation of the lamellar CdS was carried out in aqueous template solution based on the following reaction:
Cd(OAc)2 + Na2S f CdS + 2NaOAc In a typical synthesis, a mixture of 0.005 g of PSS (Mw ) 70 000) and 0.05 g of CTAB was dissolved in 6.0 mL of 0.67 mol/L HCl solution to form a template solution under intensive stirring at room temperature. Then, 1.0 mL of 0.1 M Cd(OAc)2 solution was dispersed in the template solution under ultrasonic treatment.29 After 1.0 mL of 0.1 M Na2S solution was slowly dropped into the solution above, a yellow color change was observed, a positive indication of the presence of CdS. The mixture was treated by ultrasound irradiation generated by a commercial ultrasonic cleaning bath (Bandelin RK 52 H, 35 kHz, 60/120 W) for 0.5 h before aging for 16 h at room temperature. To remove the templates, the sample was filtered and washed with hot water (80 °C) and hot ethanol (70 °C) repeatedly for at least five times, respectively. All the experimental steps above were performed at room temperature. The final product was dried at 60 °C in air. Characterization. Scanning electron microscopy (SEM) images were achieved with a KYKY-2800 with an accelerating voltage of 20 kV. Atomic force microscopy (AFM) images were obtained with a Nanoscope IIIa instrument. AFM measurements were performed in air using the Tapping Mode. Samples for the measurements were prepared by applying a drop of a CdS suspension onto a freshly cleaved mica surface and then drying in air. Transmission electron microscopy (TEM) measurements and selected area electron diffraction (SAED) patterns were performed on a Phillips TECNAI 20 operating at 120 kV. The X-ray diffraction (XRD) pattern of the sample was recorded using a Rigaku X-ray diffractometer D/max-2500 (with Cu KR λ ) 1.5406 Å radiation). Energy-dispersive spectrometry (EDS) was carried out on a Noran Vantage to certify the elemental components of the synthesized sample.
3. Results and Discussion
Figure 1. SEM micrographs of the synthesized samples: (a) CdS aggregates with layered structure; (b) enlarged SEM image of the CdS lamellar morphology.
The SEM images of the synthesized samples are shown in Figure 1a. The large sheets aggregating in a layerby-layer status are obviously illustrated. The average lengths and widths of the aggregates are estimated in the range of 40-200 and 10-50 µm, respectively. A lamellar morphology is achieved strictly according to the PSS-toCTAB molar ratio of 1:1919 in a 0.67 mol/L HCl solution. A SEM image with a higher magnification in Figure 1b shows that each layer is accumulated regularly in a parallel way. To analyze the nanostructure in detail, we performed morphological measurements using AFM to scan a side area of 100 × 100 nm2 (Figure 2). The threedimensional structure of the sheet aggregates is also observed as that from the SEM image in Figure 1b. Through the selected profile analysis presented in Figure 2b, one can distinguish individual layers. The thickness of each layer is measured to be in the range of 5.0-8.0 nm, which matches well with the long period and density calculations.27 The TEM image in Figure 3a also reveals that CdS aggregates possess a lamellar morphology. The different layers can be observed clearly in the fringe of the sample by bright and dark contrast. Those measurements demonstrate that the layered structure of CdS is stable after removal of templates, which is consistent with the prediction of the theoretical model.27 A SAED pattern
(inset of Figure 3a) obtained from the as-synthesized sample shows distinct diffraction rings, which are regarded as the random orientation of the CdS crystallite, confirming the formation of a polycrystalline structure of the lamellar CdS. The larger magnification of TEM image in Figure 3b shows that each individual layer has light and dark lines interlaced in relatively regular way, indicating that each layer consists of nanosized CdS grains aggregated in a two-dimensional way, while the brightness contrast of the grains is due to their different heights. Figure 4 shows the wide-angle XRD pattern of the CdS samples after template removal. The six indexed broad peaks represent the hexagonal CdS structure, which is nearly consistent with the wurtzite structure of CdS in the bulk (JCPDS card no. 41-1049). The broad diffraction peaks indicate that the CdS crystals are in nanoscale. Furthermore, the XRD pattern measurements confirm the formation of CdS polycrystallite and match well with those obtained by SEM and TEM measurements. In fact, it is very interesting to achieve hexagonal CdS crystals at such an ambient temperature since it has been found that as the temperature increases to 160 °C, changes in the CdS nanocrystal structure from amorphous to the hexagonal phase occur with the hydrothermal method.30 Even in the synthesis of 1D CdS nanorods, zinc blende
(29) Wang, Y. Q.; Yin, L. X.; Palchik, O.; Hacohen, Y. R.; Koltypin, Y.; Gedanken, A. Chem. Mater. 2001, 13, 1248.
(30) So, W. W.; Jang, J. S.; Rhee, Y. W.; Kim, K. J.; Moon, S. J. J. Colloid Interface Sci. 2001, 237, 136.
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Figure 2. (a) AFM topographic image of a layered CdS aggregate. (b) The cross-section profile along the line.
nuclei are also preferred at low temperature.16 With EDS, the elemental composition of the lamellar CdS sample is determined. The existence of cadmium LR1 (3.13 keV), Lβ1 (3.31 keV), and Lβ2 (3.53 keV) peaks and sulfur KR1 (2.31 keV) peaks verifies the elements of cadmium and sulfur. The quantitative data show the ratio of Cd/S is 0.98. The composite analysis clearly exhibits that the prepared samples are CdS crystals and the purity of the final product is about 90%. In control experiments, we changed the template environments by varying the pH value of the template solution and the molar ratio of PSS to CTAB in investigating the influence of templates upon the growth of CdS crystals. It is found that the complex could not form ordered-structure templates at neutral or alkaline (pH 10) conditions because of its low solubility. As the template solution remains in a stronger acid environment, the added Na2S solution becomes H2S gas via chemical reaction and there will be no CdS products. It is deduced that HCl plays a key role in the synthesis because it not only helps dissolve the surfactant and polyelectrolyte but also results in the formation of the layered polyelectrolyte/surfactant templates. Increasing the molar ratio of PSS to CTAB to twice or higher than that in the typical synthesis above by adding PSS without changing other experimental conditions, we obtained bundles of “strawlike” CdS particles instead of a lamellar crystal structure as shown in Figure 5. The particles are in a regime of hundreds of nanometers, with a length in the range of 700-900 nm and a width of 250-350 nm. The morphological changes of CdS samples certify that the variation of the molar ratio of the polyelectrolyte and surfactant leads to a new architecture of the template phases.27 The complex formation is a self-assembly process and occurs in a highly cooperative manner following a strict stoichiometry.27 Such geometric confinements dominate the lamellar morphology of complex templates. Every lamella of the templates possesses many polyelectrolyte chains, which elongate the template dimension and stabilize the surfactant phase.27 It can be concluded that
Figure 3. (a) TEM image of the as-synthesized CdS sample, with the inset showing the SAED pattern. (b) TEM image with large magnification.
Figure 4. XRD pattern for the CdS sample.
the complex templates control the growth orientation and size of the CdS aggregates. Of course, the structural change of CdS strongly depends on the actual morphology of the templates, which is determined by the volume fraction and the molecular geometry of both surfactant and polyelectrolyte. Our experimental results show that the crystal formation of CdS with lamellar morphology using complex templates formed by polyelectrolyte and oppositely charged surfactant originates from the layered structure and the stoichiometry of the system.26-28 We are currently investigating the properties of the lamellar CdS crystals by using different complex templates.
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with lengths of micrometer size and thicknesses of a few nanometers at room temperature. The morphology of the inorganic crystallite can be controlled using stoichiometric complex templates. The CdS crystal remains stable after the removal of the templates since the polyelectrolyte/ surfactant complex integrates the self-assembly ability and mechanical properties of the polymer. This approach may be extended to prepare a large size of inorganic materials with ordered nanostructure and desired morphology.
Figure 5. TEM image of the CdS sample synthesized with a molar ratio of PSS-to-CTAB that is twice as much as in the typical synthesis.
4. Conclusions In conclusion, we describe a feasible aqueous synthesis route through polyelectrolyte/surfactant complexes as templates to fabricate lamellar-structured CdS crystals
Acknowledgment. We acknowledge financial support from the National Nature Science Foundation of China (NNSFC29925307 and NNSFC90206035), the Major State Basic Research Development Program (973, Grant. No. G2000078103), and the research contract between the German Max Planck Society and the Chinese Academy of Sciences. LA034507+
