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Synthesis of Novel Metal Sulfide-Polymer Composite Microspheres Exhibiting Patterned Surface Structures Chaoliang Bai, Yu Fang,* Ying Zhang, and Beibei Chen School of Chemistry and Materials Science, Shaanxi Normal University, Xi’an 710062, People’s Republic of China Received August 24, 2003. In Final Form: October 27, 2003
In recent years, extensive studies have been carried out on the preparation and characterization of composite microspheres because of their importance in potential wide-ranging applications.1-7 Moreover, combination of different components in the nanosize range can yield new materials that may combine the advantages of each component and of nanomaterials.8 Among these composites, many recent efforts have been focused on the integration of inorganic nanoparticles into the interior of polymer microspheres,9 envelope of inorganic cores with shells of either inorganic or organic materials,10-14 and synthesis of soft core-hard shell composite microspheres.15-17 These efforts would offer opportunities to explore their novel collective mechanical, thermal, optical, magnetic, and electronic properties.18,19 Various approaches have been designed to prepare the composite materials to obtain required properties and structures. Among them, the template approach is particularly useful because of its ability to construct highly ordered materials in a controllable manner and obtain interesting structures, such as hollow spheres, hollow fibers, and three-dimensional hollow frames.20-26 A range of natural and artificial matrixes have been investigated * To whom correspondence should be addressed. E-mail: yfang@ snnu.edu.cn. Tel.: 0086 29 5307534. Fax: 0086 29 5307025. (1) Siegel, R. W. Chem. Eng. News 1999, 77 (23), 25. (2) Bigi, A.; Boanini, E.; Walsh, D.; Mann, S. Angew. Chem., Int. Ed. 2002, 41, 2163. (3) Yang, D.; Qi, L. M.; Ma, J. M. Adv. Mater. 2002, 14, 1543. (4) Lizama, B.; Lopez-Castanares, R.; Vilchis, V.; Vazquez, F. Mater. Res. Innovat. 2001, 5, 63. (5) Gao, Y.; Choudhury, N. R.; Dutta, N.; Matisons, J.; Reading, M.; Delmotte, L. Chem. Mater. 2001, 13, 3644. (6) Antonietti, M.; Goltner, C. Angew. Chem., Int. Ed. 1997, 36, 910. (7) Winiarz, J. G.; Zhang, L. M.; Lai, M.; Friend, C. S.; Prasad, P. N. J. Am. Chem. Soc. 1999, 121, 5287. (8) Ozin, G. A. Chem. Commun. 2000, 419. (9) Du, H.; Xu, G. Q.; Chin, W. S.; Huang, L.; Ji, W. Chem. Mater. 2002, 14, 4473. (10) Monteiro, O. C.; Esteves, A. C. C.; Trindade, T. Chem. Mater. 2002, 14, 2900. (11) Azad Malik, M.; O’Brien, P.; Revaprasadu, N. Chem. Mater. 2002, 14, 2004. (12) Guan, S. Y.; Inagaki, S.; Ohsuna, T.; Terasaki, O. J. Am. Chem. Soc. 2000, 122, 5660. (13) Kapoor, M. P.; Inagaki, S. Chem. Mater. 2002, 14, 3509. (14) Carg, A.; Matijevic, E. J. Colloid Interface Sci. 1988, 126, 243. (15) Okubo, M.; Lu, Y.; Wang, Z. Colloid Polym. Sci. 1998, 276, 833; 1999, 277, 77. (16) Vazquez, F.; Schneider, M.; Pith, T.; Lambla, M. Polym. Int. 1996, 41, 1. (17) Nemirovski, N.; Silverstein, M. S.; Naikis, M. Polym. Adv. Technol. 1996, 7, 247. (18) Chan, Y. N.; Craig, G. S. W.; Schrock, R. R.; Cohen, R. E. Chem. Mater. 1992, 4, 885. (19) Forster, S.; Antonietti, M. Adv. Mater. 1998, 10, 195. (20) Mann, S. Angew. Chem., Int. Ed. 2000, 39, 3392. (21) Murphy, W. L.; Mooney, D. J. J. Am. Chem. Soc. 2002, 124, 1910. (22) Caruso, R. A.; Antonietti, M. Chem. Mater. 2001, 13, 3272. (23) Kawahshi, N.; Matijevic, E. J. Colloid Interface Sci. 1991, 143, 1.
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as templates. Compared with other templates, the size, composition, charge nature, and even cross-linking density of microgels, which are cross-linked spongelike polymeric microparticles, are controllable.27,28 Their narrow size distribution combined with the inherent stability makes them ideal templates for preparing spherical inorganicorganic composite materials. Antonietti and his colleagues29 prepared various noble-metal colloids of special shapes by using polymeric microgel as microreactors. Snowden, Silver and co-workers30 used microgels of poly(N-isopropylacrylamide-co-acrylic acid) and poly(N-isopropylacrylamide-co-acrylamido-2-methylpropane sulfonic acid) to stimulate the formation of spherical Y2O3/ Eu phosphors. Shen et al.31,32 have reported the synthesis of PbS and ZnS nanocomposites via the microgels of Pb or Zn methacrylate, which were then copolymerized into the polystyrene matrix. Pan and Wang33 have focused on the studies of polymeric latex for a number of years, and a variety of polymer-inorganic composites in the nanometer range have been prepared. Kim et al.34 embedded ZnO nanoparticles into poly(methyl methacrylate) microspheres by in situ suspension polymerization. Ge and co-workers35 prepared Ag-polystyrene and CdS-polystyrene microspheres by using a γ-radiation technique. Recently, Lyon and his colleagues36 studied the phenomenon of microlens formation in microgel/gold colloid composite materials via a photothermal patterning technique. Actually, microgels provide a particularly versatile method for controlling interfacial/surface structures and facilitate manipulation of surface morphologies at a nanometer- or micrometer-size range. This is because the network structure of a microgel might control and direct the precipitation of inorganic compounds and thereby control the final size and morphology of the composite microspheres. New materials or materials with new surface morphologies may be constructed by incorporating particular functional groups into the microgels and controlling their distribution within them. On the basis of these considerations, novel composite microspheres of ZnS-poly(N-isopropylacrylamide-co-methacrylic acid) (ZnS-PNIPAM-MAA) and CdS-PNIPAM-MAA exhibiting patterned surface structures have been prepared. It is believed that the low specific gravity, high specific area, great toughness, and good stability of this kind of (24) Aizenberg, J.; Black, A. J.; Whitesides, G. Nature 1999, 398, 495. (25) Storhoff, J. J.; Mirkin, C. A. Chem. Rev. 1999, 99, 1849. (26) Woodward, N. C.; Snowden, M. J.; Chowdhry, B. Z. Langmuir 2002, 18, 2089. (27) Saunders, B. R.; Vincent, B. Adv. Colloid Interface Sci. 1999, 80, 1. (28) Murray, M. J.; Snowden, M. J. Adv. Colloid Interface Sci. 1995, 54, 73. (29) Antonietti, M.; Grohn, F.; Hartmann, J.; Bronstein, L. Angew. Chem., Int. Ed. 1997, 36, 2080. (30) Martinez-Rubio, I.; Ireland, T. G.; Fern, G. R.; Silver, J.; Snowden, M. J. Langmuir 2001, 17, 7145. (31) Gao, M. Y.; Yang, Y.; Yang, B.; Bian, F.; Shen, J. J. Chem. Soc., Chem. Commun. 1994, 2779. (32) Yang, Y.; Huang, J.; Liu, S.; Shen, J. J. Mater. Chem. 1997, 7, 131. (33) Wang, P. H.; Pan, C. Y. Colloid Polym. Sci. 2000, 278, 245; 2002, 280, 152; Eur. Polym. J. 2000, 36, 2279. (34) Shim, J. W.; Kim, J. W.; Han, S. H.; Chang, I. S.; Kim, H. K.; Kang, H. H.; Lee, O. S.; Suh, K. D. Colloid Surf., A 2002, 207, 105. (35) Wu, D. Z.; Ge, X. W.; Huang, Y. H.; Zhang, Z. C.; Ye, Q. Mater. Lett. 2003, 57, 3549. (36) Jones, C. D.; Serpe, M. J.; Schroeder, L.; Lyon, L. A. J. Am. Chem. Soc. 2003, 125, 460; 2003, 125, 5292.
10.1021/la035561t CCC: $27.50 © 2004 American Chemical Society Published on Web 12/03/2003
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composite microspheres may make them valuable in such applications as separation, catalysis, radiation absorbance, and shock absorption. In this contribution, we describe the syntheses of the composite microspheres mentioned above. The template used was prepared by reverse suspension polymerization of the aqueous solution of NIPAM and MAA initiated by ammonium persulfate (APS) in n-heptane. In a typical synthesis, 0.6 g of a neutral surfactant mixture formed by Span-80 and Tween-80 (5:1 volume ratio) was added to 75 mL of n-heptane, and the mixture was stirred vigorously under a nitrogen atmosphere, giving a fully emulsified surfactant solution. Subsequently, 1.1 g (10 mmol) of NIPAM, 0.086 g (1 mmol) of MAA, 1.5 mL of aqueous Zn(Ac)2 solution (0.3 mol/L), 0.02 g of N,N′methylenebisacrylamide, and 0.3 mL of APS (216 mg/mL in the aqueous phase) were dissolved in 6 mL of doubledistilled water. This solution was added to the organic phase. Polymerization was initiated by addition of 0.5 mL of a promoter, N,N,N′,N′-tetramethylethylenediamine, solution (50 mg/mL). The reaction was conducted for 3 h at 26 °C under stirring (360 rpm), resulting in PNIPAMMAA microgels containing Zn2+. For the preparation of the composite microspheres, H2S was introduced slowly for 25 min with constant stirring. The time was chosen because the formation of ZnS in the system is found to be practically complete within this period. The system was further stirred and purged with N2 for 3 h before separation by filtration. The microspheres obtained were washed thoroughly with double-distilled water and acetone and then dried at room temperature. Other composite microspheres were prepared in a similar way, but different compositions were used. The general morphology of the microspheres was examined by scanning electron microscopy (SEM; Philip XL-20), using an accelerating voltage of 20 kV. Figure 1 shows typical SEM images of the as-fabricated ZnS-PNIPAM-MAA microspheres (the SEM images of the template and that of an additional composite of microspheres are given in Supporting Information). The products range from 60 to 70 µm in diameter (Figure 1a,c,e), and exhibit complex but periodically patterned surface structures (Figure 1b,d,f). Upon further examination of the images, it can be observed that patterns exhibited by the surfaces of the ZnS-PNIPAM-MAA composite microspheres depend on the composition of the template and amount of ZnS precipitated. The patterns exhibited by the microspheres prepared with PNIPAMMAA (20%) as a template (Figure 1d,f) are more coarse as compared to the one with PNIPAM-MAA (10%) as a template. The surface structure of the microspheres containing more ZnS is characterized by a rise and fall morphology and looks like a pile of “noodle pieces” (a kind of special Chinese noodles, Figure 1f). Nevertheless, the internal relations between the structures shown in Figure 1b,d,g are obvious, even though they are very different. The dependence of the surface morphologies of the composite microspheres on the composition of the template and on the ratio of the metal sulfide to the template may be understood by considering the nonhomogeneous distribution of the metal ions within the microgels and the specific precipitation method employed. The interactions, mainly association and coordination, between MAA and Zn2+ should be stronger than those between NIPAM and Zn2+, and thereby the distribution and diffusion of Zn2+ within the two microgels with different compositions should be different, resulting in different precipitation behaviors of the sulfide. As for the difference between Figure 1d and Figure 1g, the “noodle piece”-like structures
Notes
Figure 1. SEM images of the composite microspheres of ZnSPNIPAM-MAA (a, c, e) and images of their enlarged surface structures (b, d, f). The formulations for the preparation of the three ZnS-PNIPAM-MAA composite microspheres are different. (a, b) MAA/NIPAM 10% (w/w), 1.5 mL of aqueous Zn(Ac)2 solution (0.3 mol/L); (c, d) MAA/NIPAM 20% (w/w), 1.5 mL of aqueous Zn(Ac)2 solution (0.3 mol/L); and (e, f) MAA/NIPAM 20% (w/w), 1.5 mL of aqueous Zn(OAc)2 solution (1.0 mol/L).
of the surface shown in Figure 1g might be formed gradually by further precipitation of ZnS on the basis of the microspheres shown in Figure 1c,d. It is to be noted that the precipitation of metal sulfide in the present method is a controlled process. This is because the precipitation might proceed gradually from the surface of the microgel into the inner part of it. It is the Zn2+ ions existing at the outer part of the microgels that met H2S first and thereby precipitated locally. As the precipitation decreased the local concentration of the ion, free Zn2+ in the microgels would diffuse to the place to make up the deficiency. These processes continued until the ions were consumed completely, provided the precipitating agent was introduced adequately. Clearly, the precipitation might stop at positions other than the center of the microgels because of the limited amount of metal ions available. This implies that the composite microspheres prepared in this way might adopt a core-shelllike structure. This speculation has been confirmed by the studies of the systems of PbS-PMAA37 and CdSPNIPAM-MAA (cf. Figure 2d). As for the formation of the (37) Zhang, Y.; Fang, Y.; Wang, S.; Lin, S. Y. J. Colloid Interface Sci., accepted for publication.
Notes
Figure 2. SEM images of the composite microspheres of CdSPNIPAM-MAA (a, b, d, e) and images of their enlarged surface structures (c, f). The compositions of the template used for the preparation of the composite microspheres are different. (a, b, c, d) MAA/NIPAM 10% (w/w) and (e, f) MAA/NIPAM 20% (w/ w), 1.5 mL of aqueous Zn(Ac)2 solution (0.3 mol/L).
patterned surface structures of the composite microspheres, the chains and joints of the template network may play crucial roles. Considering the fact that the network structures of the microgels may occupy some space at a molecular level and the interactions between the carboxyl groups of the microgels and the ions may affect the distribution of Zn2+ within the microgels, it is speculated that the structure of the interface between the organic n-heptane phase and the aqueous microgel phase and the distribution of Zn2+ within the microgels might be heterogeneous, resulting in nonhomogeneous precipitation of the sulfide. It is this heterogeneity that produces the patterned surface structures. Clearly, different microgels may have different network structures and dif-
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ferent functional groups and functional group distributions and thereby have a different effect upon the precipitation of the metal sulfide, resulting in different surface patterns (the crystal structures of the sulfides are a reflection of the effect, see Supporting Information). The surface structures of metal sulfide-polymer composite microspheres depend not only on the nature of the template but also on the nature of the sulfide. Figure 2 presents the SEM images of CdS-PNIPAM-MAA composite microspheres and their enlarged surface structures. The preparation procedures for these composites are the same as those used for the preparation of ZnS-PNIPAMMAA composites. It can be seen that the surface structures of the CdS-PNIPAM-MAA composite microspheres are basically different from those of ZnS-PNIPAM-MAA no matter which template was used. Upon further examination of the images shown in Figure 2, it can be observed that the surface structures of the CdS-PNIPAM-MAA composite microspheres are unique and full of wrinkles (which look like they have been folded artificially). Variation in the content of MAA in the microgels only results in a difference in the density of the wrinkles. As expected, the inner density of the composite microspheres is lower than that of the outer part of the microspheres because of the precipitation starting from the surface of the template (cf. the image of the cross section of the microspheres shown in Figure 2d). It is to be noted, however, that there is no distinctive interface between the “core” and the “shell”, that is to say the composite microspheres prepared in this method may adopt “coreshell”-like structures. In conclusion, for the first time, ZnS-PNIPAM-MAA and CdS-PNIPAM-MAA composite microspheres exhibiting patterned surface structures have been prepared by employing the microgel template method. It has been revealed that the surface structures of these composite microspheres depend not only upon the nature of the template and that of the metal sulfide but also upon the ratio of the metal sulfide to the template. It is speculated that the microgels mainly play roles of confinement and guidance of the precipitation of the sulfides. On the basis of the proposed microgel template method, a variety of composite microspheres with similar patterned surface structures may be prepared via simple precipitation conversion. Acknowledgment. The authors would like to thank the NSF of China (20173035), the Key Project Fund of MOE, China (03148), and the NSF of Shaanxi Province (2002B11) for financial support. Supporting Information Available: SEM imagines of the PNIPAM-MAA template and additional metal sulfidePNIPAM-MAA composite microspheres and X-ray Diffraction patterns of the template and metal sulfide-PNIPAM-MAA composite microspheres. This material is available free of charge via the Internet at http://pubs.acs.org. LA035561T
