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Novel One-Step Route for Synthesizing CdS/Polystyrene Nanocomposite Hollow Spheres Dazhen Wu, Xuewu Ge,* Zhicheng Zhang, Mozhen Wang, and Songlin Zhang Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China Received March 8, 2004. In Final Form: May 7, 2004 CdS/polystyrene nanocomposite hollow spheres with diameters between 240 and 500 nm were synthesized under ambient conditions by a novel microemulsion method in which the polymerization of styrene and the formation of CdS nanoparticles were initiated by γ-irradiation. The product was characterized by transmission electron microscopy (TEM), field-emission scanning electron microscopy (FESEM), X-ray powder diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, and thermogravimetric analysis (TGA), which show the walls of the hollow spheres are porous and composed of polystyrene containing homogeneously dispersed CdS nanoparticles. The quantum-confined effect of the CdS/polystyrene nanocomposite hollow spheres is confirmed by the ultraviolet-visible (UV-vis) and photoluminescent (PL) spectra. We propose that the walls of these nanocomposite hollow spheres originate from the simultaneous synthesis of polystyrene and CdS nanoparticles at the interface of microemulsion droplets. This novel method is expected to produce various inorganic/polymer nanocomposite hollow spheres with potential applications in the fields of materials science and biotechnology.
Introduction Nowadays, hollow spheres are drawing intense research interest because their unique structural, optical, and surface properties may lead them to wide applications such as in catalysis, drug delivery, chemical storage, optoelectronics, photonic crystals, and microcavity resonance.1 Various inorganic and polymer hollow spheres have been fabricated by using different templates including polystyrene latex spheres,2 silica sols,3 liquid drops,4 vesicles,5 polymer micelles,6 and emulsion7 and microemulsion8 droplets. More recently, nitrogen (N2) bubbles produced during the reaction have been used as the template for the synthesis of ZnSe semiconductor hollow microspheres,9 and block copolypeptides and homopolymer polypeptides have also been used to assemble inorganic nanoparticles into hollow spheres.10 * To whom correspondence should be addressed. E-mail: xwge@ ustc.edu.cn. (1) (a) Hollow and Solid Spheres and Microspheres: Science and Technology Associated with Their Fabrication and Application, MRS Symposium Proceedings; Wilcox, D. L., Sr., Berg, M., Bernat, T., Kellerman, D., Cochran, J. K., Jr., Eds.; Materials Research Society: Pittsburgh, PA, 1995; Vol. 372. (b) Caruso, F. Top. Curr. Chem. 2003, 227, 145. (2) (a) Breen, M. L.; Dinsmore, A. D.; Pink, R. H.; Qadri, S. B.; Ratna, B. R. Langmuir 2001, 17, 903. (b) Caruso, R. A.; Susha, A.; Caruso, F. Chem. Mater. 2001, 13, 400. (c) Park, M. K.; Onishi, K.; Locklin, J.; Caruso, F.; Advincula, R. C. Langmuir 2003, 19, 8550. (3) (a) Kim, S.-W.; Kim, M.; Lee, W. Y.; Hyeon, T. J. Am. Chem. Soc. 2002, 124, 7642. (b) Mandal, T. K.; Fleming, M. S.; Walt, D. R. Chem. Mater. 2000, 12, 3481. (4) (a) Huang, J. X.; Xie, Y.; Li, B.; Liu, Y.; Qian, Y. T.; Zhang, S. Y. Adv. Mater. 2000, 12, 808. (b) Fowler, C. E.; Khushalani, D.; Mann, S. Chem. Commun. 2001, 2028. (5) (a) Zheng, X. W.; Xie, Y.; Zhu, L. Y.; Jiang, X. C.; Yan, A. H. Ultrason. Sonochem. 2002, 9, 311. (b) Mckelvey, C. A; Kaler, E. W.; Zasadzinski, J. A.; Coldren, B.; Jung, H. T. Langmuir 2000, 16, 8285. (c) Hubert, D. H. W.; Jung, M.; Frederik, P. M.; Bomans, P. H. H.; Meuldijk, J.; German, A. L. Adv. Mater. 2000, 12, 1286. (6) (a) Ma, Y. R.; Ma, J. M.; Cheng, H. M. Langmuir 2003, 19, 4040. (b) Liu, T.; Xie, Y.; Chu, B. Langmuir 2000, 16, 9015. (7) (a) Dinsmore, A. D.; Hsu, M. F.; Nikolaides, M. G.; Marquez, M.; Bausch, A. R.; Weitz, D. A. Science 2002, 298, 1006. (b) Yu, C. Z.; Tian, B. H.; Fan, J.; Stucky, G. D.; Zhao, D. Y. Chem. Lett. 2002, 31, 62. (c) Bao, J. C.; Liang, Y. Y.; Xu, Z.; Si, L. Adv. Mater. 2003, 15, 1832. (8) (a) Walsh, D.; Mann, S. Nature 1995, 377, 320. (b) Jafelicci, M.; Davolos, M. R.; dos Santos, F. J.; de Andrade, S. J. J. Non-Cryst. Solids 1999, 247, 98. (9) Peng, Q.; Dong, Y. J.; Li, Y. D. Angew. Chem., Int. Ed. 2003, 42, 3027.
Inorganic/organic polymer nanocomposites have attracted the interest of a number of researchers, due to their synergistic and hybrid properties derived from several components, and they have successful applications in versatile areas.11 It is unassailable that inorganic/polymer nanocomposite hollow spheres can have more potential applications in the fields of materials science and biotechnology than single inorganic or organic polymer hollow spheres. However, to our knowledge, only two strategies for preparing inorganic/polymer nanocomposite hollow spheres have been reported. First, Caruso and coworkers prepared inorganic/polymer hollow microspheres through the colloid templated electrostatic layer-by-layer self-assembly of oppositely charged inorganic nanoparticles and polymer multilayers, followed by removal of the templated core.12 Second, Yang and co-workers produced SiO2/polyaniline composite hollow spheres using core-gel-shell (polystyrene-sulfonated polystyrene) particle templates.13 However, all of these methods are laborintensive processes. For example, to create hollow centers, dissolution of the templated cores has to be done. Moreover, due to the respective formation of the polymer and inorganic nanoparticles, the inorganic nanoparticles cannot be dispersed homogeneously in the polymer matrix and the aggregation of inorganic nanoparticles cannot be avoided. Thus, for chemists, the development of simple and effective synthetic methods for making inorganic/polymer nanocomposite hollow spheres is a very impending task. γ-Irradiation is one of the most simple and effective methods for the synthesis of inorganic/polymer nanocomposites. Our group has been interested in producing inorganic/polymer nanocomposites by γ-irradiation in microemulsion. Through this method, various inorganic/ polymer nanocomposites such as silver/poly(butyl acrylateco-styrene),14 CdS/polyacrylamide,15 and SiO2/polysty(10) (a) Wong, M. S.; Cha, J. N.; Choi, K.-S.; Demling, T. J.; Stucky, G. D. Nano Lett. 2002, 2, 583. (b) Cha, J. N.; Birkedal, H.; Euliss, L. E.; Bartl, M. H.; Wong, M. S.; Deming, T. J.; Stucky, G. D. J. Am. Chem. Soc. 2003, 125, 8285. (11) (a) Godovsky, D. Y. Adv. Polym. Sci. 2000, 153, 163. (b) Gangopadhyay, R.; De, A. Chem. Mater. 2000, 12, 608. (c) Schmidt, G.; Malwitz, M. M. Curr. Opin. Colloid Interface Sci. 2003, 8, 103. (12) Caruso, F.; Caruso, R. A.; Mo¨hwald, H. Science 1998, 282, 1111. (13) Niu, Z. W.; Yang, Z. Z.; Hu, Z. B.; Lu, Y. F.; Han, C. C. Adv. Funct. Mater. 2003, 13, 949.
10.1021/la049405d CCC: $27.50 © 2004 American Chemical Society Published on Web 05/26/2004
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rene16 have been synthesized. Herein, we present a novel one-step route of microemulsion for synthesizing CdS/ polystyrene nanocomposite hollow spheres. γ-Irradiation was employed to initiate the polymerization of styrene and the formation of CdS nanoparticles at room temperature and ambient pressure. Experimental Section 1. Preparation of the Microemulsion. The nonionic surfactants octyl phenyl poly(ethylene oxide)-4 (OP-4) and octyl phenyl poly(ethylene oxide)-10 (OP-10) were selected as emulsifiers. Kerosene and styrene were used as the continuous oil phase, and an aqueous solution containing CdCl2‚2.5H2O and Na2S2O3‚5H2O, as the dispersed aqueous phase. Typically, the microemulsion was prepared as follows: 10 mL of a prepared CdCl2‚2.5H2O (0.7 M) and Na2S2O3‚5H2O (0.7 M) aqueous solution was added to a mixture of 30 mL of kerosene and 2 mL of styrene; then, OP-4 and OP-10 were titrated alternately into the mixture under stirring until an optically clear microemulsion was suddenly formed. 2. γ-Irradiation and Product Treatment. The microemulsion was deaerated by ultrasonic deaeration for ∼5 min to remove the oxygen solvated in the system and then was irradiated in a field of 2.59 × 1015 Bq by a 60Co γ-ray source at a dose rate of 80 Gy/min for an absorbed dose of 96 kGy. The whole reaction was performed under ambient conditions. After the reaction completed, the microemulsion was destabilized by adding ethanol. The light yellow precipitate was separated by centrifugation and washed repeatedly with distilled water and ethanol. Then, the product was dried in a vacuum at room temperature for 4 h. 3. Characterization. The sample was identified by X-ray powder diffraction (XRD) employing a scanning rate of 0.02 deg/s in the 2θ range from 10 to 70°, using a Japan Rigaku D/max γA X-ray diffractometer equipped with graphite monochromatized Cu KR radiation (λ ) 0.154 178 nm). Fourier transform infrared (FTIR) spectroscopy was carried out on a Bruker Vector-22 FTIR spectrometer. The ultraviolet-visible (UV-vis) absorption spectrum was recorded at room temperature on a UV-2100 Shimadzu spectrophotometer. The photoluminescent (PL) spectrum was taken on a Hitachi 850 fluorescence spectrometer with a Xe lamp at room temperature (λex ) 350 nm). The transmission electron microscopy (TEM) and electron diffraction (ED) images were performed on a Hitachi model H-800 transmission electron microscope with an accelerating voltage of 200 kV. The fieldemission scanning electron microscopy (FESEM) image was obtained on a JEOL JSM-6700 field-emission scanning electron microanalyzer. Thermogravimetric analysis (TGA) was performed with a Shimadzu TGA-50H instrument under a stream of air. The sample was heated at 10 °C/min from 35 to 1000 °C.
Results and Discussion When the microemulsion was irradiated by γ-rays, the initiated reactions can be described as follows:
H2O ∼∼ f eaq-, •H, •OH, H3O+, etc.
(1)
2eaq- + S2O32- f S2-
(2)
S2- + Cd2+ f CdS
(3)
First, owing to the radiolysis of water, active products such as eaq-, •H, and •OH were produced (reaction 1). Then, the reducing species eaq- could reduce S2O32- to S2- ions (reaction 2), which reacted with Cd2+ ions to generate (14) Yin, Y. D.; Xu, X. L.; Xia, C. J.; Ge, X. W.; Zhang, Z. C. Chem. Commun. 1998, 941. (15) Ni, Y. H.; Ge, X. W.; Liu, H. R.; Zhang, Z. C.; Ye, Q. Chem. Lett. 2001, 924. (16) Wu, D. Z.; Ge, X. W.; Chu, S. N.; Zhang, Z. C. Chem. Lett. 2003, 32, 1134.
Figure 1. Schematic illustration of the procedures for preparing CdS/polystyrene nanocomposite hollow spheres.
Figure 2. TEM image of CdS/polystyrene nanocomposite hollow spheres.
CdS nanoparticles (reaction 3). At the same time, the produced radicals such as •H and •OH could initiate the polymerization of styrene (reaction 4). The overall procedure for preparing CdS/polystyrene nanocomposite hollow spheres is shown in Figure 1. Because there was a strong complexation interaction between Cd2+ ions and OP-4 and OP-106a and the polymerization of styrene was mainly initiated by the radicals R• produced from the radiolysis of water drops of the microemulsion,17 the polymerization of styrene and the formation of CdS nanoparticles could take place simultaneously at the interface between water and oil and hence CdS nanoparticles could be dispersed homogeneously in the polystyrene matrix. Furthermore, with γ-irradiation and the generation of the reactions, the component of the microemulsion underwent a change and the microemulsion finally became an emulsion. Thus, submicrometer CdS/ polystyrene nanocomposite hollow spheres could be formed. The TEM image of the as-prepared sample is shown in Figure 2, where we can see the pale center and dark edge (17) Zhang, Z. C.; Ge, X. W.; Zhang, M. W. Irradiation Chemistry of Polymer; Press of the University of Science and Technology of China: Hefei, China, 2000; p 47 (in Chinese).
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Figure 5. XRD pattern of CdS/polystyrene nanocomposite hollow spheres. Figure 3. ED pattern of CdS/polystyrene nanocomposite hollow spheres.
Figure 6. FTIR spectrum of CdS/polystyrene nanocomposite hollow spheres.
Figure 4. FESEM image of CdS/polystyrene nanocomposite hollow spheres.
of the spheres. This suggests the hollow nature of the spheres.18 These hollow spheres exhibit a diameter distribution in the range 240-500 nm and an average diameter of ∼420 nm. The related ED pattern reveals the presence of hexagonal CdS polycrystals in the shell walls (Figure 3). The TEM image also indicates that the walls of the hollow spheres are porous, so that the most surfactant could be removed easily by washing out with distilled water and ethanol. These porous structures of hollow spheres usually were shown by the hollow spheres fabricated by using soft templates in solution.6a,7b,8a The FESEM picture reveals that the mean diameter of the hollow spheres is ∼420 nm and the surface of the hollow spheres is rough (Figure 4). We could not find any break spheres, for which a likely reason is that the effective combination of CdS nanoparticles and polystyrene enhances the mechanical toughness of the hollow spheres and makes the hollow spheres stronger. Figure 5 shows the XRD pattern of the as-prepared CdS/polystyrene nanocomposite hollow spheres. All the diffraction peaks can be indexed as the hexagonal CdS crystal by comparison with the literature (JCPDS card, file no. 41-1049). The average crystallite size was found to be ∼5 nm by rough estimating from Scherrer’s formula.19 Proof of the polymer component of the product was provided by the FTIR spectrum, as presented in Figure 6, where the typical styrene absorption bands at 700, 757, (18) Braun, P. V.; Stupp, S. I. Mater. Res. Bull. 1999, 34, 463. (19) X-ray diffraction procedure; Klug, H. P., Leroy, E. A., Eds.; Wiley: New York, 1974; p 656.
Figure 7. TGA plot of CdS/polystyrene nanocomposite hollow spheres.
1029, 1452, 1493, 1601, 2852, 2924, and 3026 cm-1 can be seen clearly. The 1153-cm-1 weak peak could be contributed by the rudimental surfactants OP-10 and OP-4. However, for the CdS nanoparticles, the vibrational absorption peak of the Cd-S bond, which should be at 405 cm-1,20 cannot be observed, for it is rather weak and is scarcely resolved. Figure 7 shows the result of thermogravimetric analysis of the product. The initial weight loss of 1% (up to 200 °C) is due to the evaporation of physically adsorbed water, and the subsequent loss of 2% (200-274 °C) is due to the decomposition of rudimental surfactant. Between 274 and 540 °C, a weight loss of 21% is attributed to removal of the polystyrene and the residual weight of 76% should be the weight of the CdS nanoparticles. This indicates the weight fraction of CdS in the CdS/polystyrene nanocomposite hollow spheres should be calculated as
WCdS )
76 × 100% ) 78% 21 + 76
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Figure 8. UV-vis absorption spectrum of CdS/polystyrene nanocomposite hollow spheres.
Therefore, the calculated weight fraction of polystyrene should be 22%. Assuming a density of 4.8 g/cm3 for CdS and 1.06 g/cm3 for polystyrene, the volume ratio of CdS to polystyrene in the nanocomposite hollow spheres is
vCdS vpolystyrene
)
0.78/4.8 ) 0.8 0.22/1.06
The CdS/polystyrene nanocomposite hollow spheres are also characterized by room-temperature UV-vis absorption and PL spectra. Figure 8 shows the absorption spectrum of a sample dispersed in ethanol by sonication, which shows a clear maximum at 485 nm, which is assigned to the optical transition of the first excitonic state of the CdS nanoparticles.21 The explicit blue shift of the absorption peak from ∼515 nm for bulk CdS occurred due to the small size of the CdS quantum dots located in the walls of the nanocomposite hollow spheres. Estimation based on the absorption maximum at 485 nm reveals that the mean diameter of the CdS nanoparticles is ∼5 nm,21,22 which is in good agreement with the result obtained from XRD. The PL spectrum of the CdS/polystyrene nanocomposite hollow spheres (as shown in Figure 9) indicates a 400-nm peak, and compared with the emission of bulk (20) Martin, T. P.; Schaber, H. Spectrochim. Acta, Part A 1982, 38, 655. (21) Weller, H. Angew. Chem., Int. Ed. Engl. 1993, 32, 41. (22) Spanhel, L.; Haase, M.; Weller, H.; Henglein, A. J. Am. Chem. Soc. 1987, 109, 5649.
Figure 9. PL spectrum of CdS/polystyrene nanocomposite hollow spheres.
CdS (∼520 nm), 120 nm of blue shift is observed. These features indicate the quantum-confined effect of the CdS/ polystyrene nanocomposite hollow spheres.4a,23 Conclusions In summary, intact CdS/polystyrene nanocomposite hollow spheres with a mean diameter of ∼420 nm have been prepared in microemulsion by simultaneous γ-irradiation synthesis of the CdS nanoparticles and polystyrene at the interface between the water droplets and oil. Compared with previous routes of building hollow spheres, our one-step method is more facile and effective, and it also can be extended to prepare other important inorganic/polymer nanocomposite hollow spheres in largescale at room temperature. The nanocomposite hollow spheres prepared are envisioned to have applications in materials science and biotechnology. Acknowledgment. We thank the Anhui Provincial National Science Foundation (01044804) and the Basic Research Department of Science and Technology Ministry of the People’s Republic of China for support of this work. LA049405D (23) Gurin, V. S.; Artemyev, M. V. J. Cryst. Growth 1994, 138, 993.
