Controllable Fabrication of Nanocrystal-Loaded Photonic Crystals

State Key Laboratory of Material-Oriented Chemical Engineering, College of ... is undoubtedly one of the simplest and cheapest techniques for the fabr...
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Controllable Fabrication of Nanocrystal-Loaded Photonic Crystals with a Polymerizable Macromonomer via the CCTP Technique Lili Yan, Ziyi Yu, Li Chen, Caifeng Wang, and Su Chen* State Key Laboratory of Material-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering, Nanjing University of Technology, Nanjing 210009, PR China Received March 4, 2010. Revised Manuscript Received April 8, 2010 We report an alternative strategy for fabricating nanocrystal-loaded photonic crystals with a polymerizable macromonomer via catalytic chain-transfer polymerization (CCTP) in which CoBF acted as a chain-transfer agent (CTAs). First, monodisperse, functionalized PS-co-PMAA microspheres with polystyrene (PS) cores and poly(methacrylic acid) (PMAA) shells were controllably prepared using styrene and the as-prepared PMAA macromonomer by surfactant-free emulsion polymerization. Then Cd0.4Zn0.6S nanocrystals capped with as-prepared PMAA macromonomer were synthesized by an in situ growth method. Finally, Cd0.4Zn0.6S nanocrystal-loaded PS-co-PMAA microspheres hybrids were obtained through simply mixing an aqueous solution of PS-co-PMAA microspheres with an aqueous solution of Cd0.4Zn0.6S nanocrystals in the appropriate proportions; the multianchor -COOH groups on the surface of core-shell PS-co-PMAA microspheres favor incorporation with Cd0.4Zn0.6S nanocrystals. Scanning electron microscopy (SEM) images confirm that PS-co-PMAA microspheres are uniformly surrounded by Cd0.4Zn0.6S nanocrystals. In addition, discrete electronic and photonic states can be combined both with PS-co-PMAA photonic crystals and fluorescent semiconductor Cd0.4Zn0.6S nanocrystals.

Introduction In the past few decades, photonic crystals1-8 (PCs) have received a great deal of attention because of their potential applications in areas such as optical communications, frequency conversion, and sensing. Until now, a variety of elegant techniques have been proposed for constructing photonic crystals. Among them, the synthesis of colloidal crystals from monodisperse microspheres9 is undoubtedly one of the simplest and cheapest techniques for the fabrication of PCs, including microspheres of monodisperse poly(methyl methacrylate) (PMMA),10,11 polystyrene (PS),12 and silica.13 Recently, the construction of functionalized colloidal crystals14,15 has been of special interest in photonic crystals because functionalized colloidal crystals favor the further improvement of the performance of various PC devices, together *To whom correspondence should be addressed. E-mail: [email protected]. cn. Fax: þ86-25-83172258.

(1) John, S. Phys. Rev. Lett. 1987, 58, 2486–2489. (2) Yablonovich, E. Phys. Rev. Lett. 1987, 58, 2059–2062. (3) Norris, D. J.; Arlinghaus, E. G.; Meng, L.; Heiny, R.; Scriven, L. E. Adv. Mater. 2004, 16, 1393–1399. (4) Russell, P. Science 2003, 299, 358–362. (5) Lee, Y. J.; Pruzinsky, S. A.; Braun, P. V. Langmuir 2004, 20, 3096–3106. (6) Ge, J. P.; Hu, Y. X.; Yin, Y. D. Angew. Chem., Int. Ed. 2007, 46, 7428–7431. (7) Yan, Q. F.; Wang, L. K.; Zhao, X. S. Adv. Funct. Mater. 2007, 17, 3695– 3706. (8) Cui, L. Y.; Li, Y. F.; Wang, J. X.; Tian, E. T.; Zhang, X. Y.; Zhang, Y. Z.; Song, Y. L.; Jiang, L. J. Mater. Chem. 2009, 19, 5499–5502. (9) Xia, Y. N.; Gates, B.; Yin, Y. D.; Lu, Y. Adv. Mater. 2000, 12, 693–713. (10) Gu, Z. Z.; Chen, H. H.; Zhang, S.; Sun, L. G.; Xie, Z. Y.; Ge, Y. Y. Colloids Surf., A 2007, 302, 312–319. (11) Egen, M.; Zentel, R. Macromol. Chem. Phys. 2004, 205, 1479–1488. (12) Imura, Y.; Nakazawa, H.; Matsushita, E.; Morita, C.; Kondo, T.; Kawai, T. J. Colloid Interface Sci. 2009, 336, 607–611. (13) Yamada, Y.; Nakamura, T.; Ishi, M.; Yano, K. Langmuir 2006, 22, 2444– 2446. (14) Jeong, U.; Wang, Y. L.; Ibisate, M.; Xia, Y. N. Adv. Funct. Mater. 2005, 15, 1907–1921. (15) Lange, B.; Metz, N.; Tahir, M. N.; Fleischhaker, F.; Theato, P.; Schr€oder, H.; M€uller, W. E. G.; Tremel, W.; Zentel, R. Macromol. Rapid Commun. 2007, 28, 1987–1994. (16) Fleischhaker, F.; Zentel, R. Chem. Mater. 2005, 17, 1346–1351.

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with the synergistic effect of other nanocrystals (NCs).16,17 To this end, a variety of methods have been explored for preparing functionalized polymer microspheres, such as the polymerization of the main monomer with functional monomers,18,19 modification of the final polymer latexes,20 and the incorporation of block copolymer21 during polymerization. In the case of the assembly of colloidal crystals and semiconductor nanocrystals, these functionalized colloidal crystals can provide appropriate anchor sites for assembling semiconductor nanocrystals22-24 onto the surfaces of colloidal crystals, which finally allows functionalized colloidal crystals to permit the combination of the electronic and photonic states within a single structure. For this purpose, a great deal of research has contributed to the further modification of PCs with incorporated semiconductor nanocrystals. Rogach et al.25 reported on the fabrication of 3D colloidal photonic crystals by the selforganization of submicrometer-sized PS latex spheres with semiconductor nanocrystals via electrostatic adsorption. Nakamura and co-workers26,27 obtained titania-nanosheet-coated polystyrene latex or silica spheres by the LBL assembly coating method. (17) Paquet, C.; Yoshino, F.; Levina, L.; Gourevich, I.; Sargent, E. H.; Kumacheva, E. Adv. Funct. Mater. 2006, 16, 1892–1896. (18) Fleischhaker, F.; Lange, B.; Zentel, R. Macromol. Symp. 2007, 254, 210– 216. (19) Yang, S. Y.; Chen, S.; Tian, Y.; Feng, C.; Chen, L. Chem. Mater. 2008, 20, 1233–1235. (20) Covolan, V. L.; Mei, L. H. I; Rossi, C. L. Polym. Adv. Technol. 1997, 8, 44–50. (21) Bousquet, A.; Perrier-Cornet, R.; Ibarboure, E.; Papon, E.; Labrugere, C.; Heoguez, V.; Rodrı´ guez-Hernandez, J. Macromolecules 2007, 40, 9549–9554. (22) Chen, S.; Zhu, J.; Shen, Y. F.; Hu, C. H.; Chen, L. Langmuir 2007, 23, 850– 854. (23) Hou, L. R.; Chen, L.; Chen, S. Langmuir 2009, 25, 2869–2874. (24) Guo, L.; Chen, S.; Chen, L. Colloid Polym. Sci. 2007, 285, 1593–1600. (25) Rogach, A.; Susha, A.; Caruso, F.; Sukhorukov, G.; Kornowski, A.; Kershaw, S.; M€ohwald, H.; Eychm€uller, A.; Weller, H. Adv. Mater. 2000, 12, 333–337. (26) Nakamura, H.; Ishii, M.; Tsukigase, A.; Harada, M.; Nakano, H. Langmuir 2005, 21, 8918–8922. (27) Nakamura, H.; Ishii, M.; Tsukigase, A.; Harada, M.; Nakano, H. Langmuir 2006, 22, 1268–1272.

Published on Web 04/21/2010

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Article Scheme 1. Schematic Illustration of the Fabrication Route to Forming Photonic Crystals Incorporated with Nanocrystals

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there has been no previous report on the application of the CCTP method to the functionalization of photonic crystals.

Experimental Section

Wang et al.28 synthesized nanocomposite SiO2 colloidal particles containing homogeneously dispersed Ag quantum dots by the in situ chemical reduction of Ag ions in a microemulsion. Bawendi et al.29 described a robust procedure for incorporating CdSe/ZnS core-shell NCs into a silica or titania shell grown on preformed submicrometer-diameter silica microspheres. Zhang et al.30 employed an in situ strategy for the production of monodisperse polymer-microsphere-coated semiconductors (CdS) and metal (Ag) nanocrystals. Han et al.31 proposed an approach to incorporating nanoparticles into the natural photonic crystals within peacock feathers. Unfortunately, it still remains a challenge to accumulate many more semiconductor QDs into photonic crystals. This motivated us to explore an alternative and reliable strategy for fabricating functional PCs. In this work, we demonstrate a straightforward procedure for obtaining NC-PCs (Scheme 1). The synthesis started from PMAA macromonomers prepared via catalytic chain-transfer polymerization (CCTP) in which CoBF acted as a chain-transfer agent (CTAs) according to our previously published procedure.32 CCTP is a powerful synthesis route to producing a relatively low-molecular-weight polymer chain with a terminal double bond. In our previous work, we have successfully prepared PMAA macromonomer,32 PMMA macromonomer,33 PMAAb-PBA block copolymer,32,34 and another macromonomer35 via the CCTP technique. Herein, we fabricated core-shell PSco-PMAA microspheres using styrene and an as-prepared PMAA macromonomer via surfactant-free emulsion polymerization, and then we prepared Cd0.4Zn0.6S nanocrystals capped with an asprepared PMAA macromonomer by the in situ growth method. The as-prepared PMAA macromonomers act not only as comonomers for obtaining functional PS microspheres but also as ligands for constructing NCs. Finally, we obtained Cd0.4Zn0.6S nanocrystal-loaded PS-co-PMAA microphere hybrids through simply mixing an aqueous solution of PS-co-PMAA microspheres with an aqueous solution of Cd0.4Zn0.6S nanocrystals in appropriate proportions; multianchor -COOH groups on the surfacse of core-shell PS-co-PMAA microspheres favor incorporation with Cd0.4Zn0.6S nanocrystals. To the best of our knowledge, (28) Wang, W.; Asher, S. A. J. Am. Chem. Soc. 2001, 123, 12528–12535. (29) Chan, Y.; Zimmer, J. P; Stroh, M.; Stechel, J. S.; Jain, R. K.; Bawendi, M. G. Adv. Mater. 2004, 16, 2092–2097. (30) Zhang, J. G.; Coombs, N.; Kumacheva, E.; Lin, Y. K.; Sargent, E. H. Adv. Mater. 2002, 14, 1756–1759. (31) Han, J.; Su, H. L.; Song, F.; Gu, J. J.; Zhang, D.; Jiang, L. M. Langmuir 2009, 25, 3207–3211. (32) Chen, L.; Yan, L. L.; Li, Q.; Wang, C. F.; Chen, S. Langmuir 2010, 26, 1724–1733. (33) Lu, Z.; Wang, J. Y.; Li, Q.; Chen, L.; Chen, S. Eur. Polym. J. 2008, 45, 1072– 1079. (34) Yang, S. Y.; Li, Q.; Chen, L.; Chen, S. J. Mater. Chem. 2008, 18, 5599–5603. (35) Wang, C. F.; Cheng, Y. P.; Wang, J. Y.; Zhang, D.; Hou, L. R.; Chen, L.; Chen, S. Colloid Polym. Sci. 2009, 287, 829–937.

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Materials. Methacrylic acid (MAA) (Aldrich, 99%) and styrene (Aldrich, 99%) were purified by distillation under reduced vacuum to remove inhibitor. Initiators 2,2-azobis(2-(2-imidazolin-2-yl)propane) dihydrochloride (VA-044) and potassium persulfate (KPS) were purified by recrystallization from methanol. Cadmium chloride (CdCl2 3 2.5H2O), zinc acetate (Zn(CH3COO)2 3 2H2O), sodium sulfide (Na2S 3 9H2O), sodium hydrogen carbonate (NaHCO3), and acetone were purchased from Aldrich and used without further purification. Cobalt catalyst bis(aqua)bis((difuoroboryl)dimethylglyoximato)cobalt (CoBF) was prepared according to the literature36 and further confirmed by elemental analysis (C, 22.86%; N, 13.40%; H, 3.89%). Preparation of the PMAA Macromonomer. The PMAA macromonomer was prepared via catalytic chain-transfer polymerization as we described previously.32 Briefly, 0.3 g of azo initiator VA-044, 13 mg of CoBF dissolved in 2 mL of acetone, and 150 g of deionized water were added to a 500 mL flask equipped with a magnetic stirrer under oxygen-free conditions by six freeze-pump-thaw cycles. The flask was consecutively evacuated and purged with nitrogen six times and then heated to about 55 °C with continuous stirring. MAA (74 g) mixed with CoBF (7.5 mg) was added over about 1 h. The reaction was left at 55 °C for another hour and then immediately stopped with icewater. The PMAA macromonomer was precipitated in diethyl ether, separated by centrifugation, and dried under vacuum. Preparation of Cd0.4Zn0.6S Nanocrystals Capped with PMAA Macromonomers. Cd0.4Zn0.6S nanocrystals capped with PMAA macromonomers were prepared by a stepwise procedure. In the first step, 5.81 mmol of PMAA macromonomer was dissolved in 30 mL of deionized water. After the PMAA macromonomers completely dissolved, the solution was mixed with 4 mL of an aqueous solution of cadium chloride (CdCl2 3 2.5H2O, 0.194 mmol) and zinc acetate (Zn(CH3COO)2 3 2H2O, 0.291 mmol) and stirred vigorously for 4 h. In a second step, 4 mL of an aqueous solution of sodium sulfide (Na2S 3 9H2O, 0.097 mmol) was slowly added dropwise to the above solution with stirring. Then, the reaction was carried out for an additional 4 h at room temperature. Finally, Cd0.4Zn0.6S nanocrystals were capped with PMAA macromonomers.

Preparation of Monodisperse PS-co-PMAA Microspheres. Monodisperse core-shell PS-co-PMAA microspheres were prepared using styrene and the as-prepared PMAA macromonomer by surfactant-free emulsion polymerization. The typically procedures were introduced as follows: 6.9 g of styrene, 0.6 g of asprepared PMAA macromonomers, and 1.2 g of NaHCO3 were dissolved in 130 mL of deionized water in a 250 mL four-necked, round-bottomed flask equipped with a condenser, a nitrogen inlet, a thermometer, and a stirrer. Then, the mixture was stirred for 1 h to remove oxygen from the solution. Finally, polymerization was initiated by adding 20 mL of an aqueous solution containing 0.0375 g of K2S2O8 at 93 °C. The reaction was allowed to proceed for 2 h at 93 °C under stirring. After the mixture was quenched with air and filtered afterwards to remove minor traces of agglomerates, the end product was obtained.

Preparation of Cd0.4Zn0.6S Nanocrystal-Loaded PS-coPMAA Microsphere Hybrids. Cd0.4Zn0.6S nanocrystal-loaded PS-co-PMAA microsphere hybrids were prepared by mixing 10 g of an aqueous solution of PS-co-PMAA microspheres and 2 g of an aqueous solution of as-prepared Cd0.4Zn0.6S nanocrystals at room temperature and then stirring for 4 h to obtain a uniform solution. Preparation of Colloidal Crystal Films. The colloidal crystal films were prepared via a vertical deposition method on glass (36) Bakac, A.; Espenson, J. H. J. Am. Chem. Soc. 1984, 106, 5197–5202.

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Figure 1. (a) Raman shift spectra of PMAA macromonomers prepared (1) via free radical polymerization (solvent, H2O; reaction time, 5 h) and (2) via CCTP (solvent, H2O; reaction time, 1 h). (b) GPC profile of the PMAA macromonomer via CCTP.

Figure 2. (a) FT-IR spectra of (1) the PMAA macromonomer and (2) Cd0.4Zn0.6S nanocrystals capped with the PMAA macromonomer. (b) UV-vis absorption and PL spectra of Cd0.4Zn0.6S nanocrystals capped with the PMAA macromonomer. slides, which were treated with an H2SO4/H2O2 (7:3 v/v) mixture for 12 h to create clean hydrophilic surfaces. The glass slides were vertically positioned in a vial containing PS-co-PMAA colloidal suspensions at a concentration of 2 wt % with a relative humidity of 70% for 24 h. Following the evaporation of water, the colloidal crystal films were made under inducement of the capillary force. Characterization. Molecular weight distributions were analyzed by gel permeation chromatography (GPC) performed on a Waters 1525/2414/2487 system. The terminal double bond via CCTP can be demonstrated by Raman spectra collected on an NXR FT-Raman module by sharing an interferometer installed in the FT-IR bench. Transmission electron microscope (TEM) observation was performed with a JEOL JEM-2010 transmission electron microscope. The sample was placed on a copper grid that was left to dry before being transferring into the TEM sample chamber. Scanning electron microscope (SEM) observations were obtained using a Hitachi S-4800 sacnning electron mocroscope. The average diameters of microspheres were measured by using dynamic light scattering (DLS) (Zetasizer 3000HSA, Malvern Instruments). FT-IR spectra were recorded on a Nicolet-Nexus 670 spectrometer. The reflection spectra of colloidal crystals were obtained by a Perkin-Elmer Lambda 950 UV-vis spectrometer. Photographs were taken with an optical microscope (LEICA, DM--4000M). Fluorescence images were captured by an inverted fluorescence microscope (Nikon, TE2000).

Results and Discussion Evidence for the successful preparation of low-molecularweight PMAA macromonomer with unsaturated carbon-carbon double bonds in end groups via CCTP can be demonstrated by Raman spectra and GPC characterization. Figure 1a shows the Raman spectra of the PMAA macromonomer via CCTP and the corresponding PMAA prepared by free radical polymerization for comparison. As seen in Figure 1a, the Raman shift appearing at 1640 cm-1 in curve 2 of Figure 1a can be assigned to the unsaturated carbon-carbon double bonds in the end groups of the PMAA macromonomer, indicating that we have successfully synthesized the PMAA macromonomer with terminal unsaturated Langmuir 2010, 26(13), 10657–10662

carbon-carbon double bonds via CCTP. In general, PMAA prepared by free radical polymerization usually has a high molecular weight and an uncontrollable structure. However, as a result of the presence of CoBF, a PMAA macromonomer with Mn = 1600 and PDI = 1.40 was fabricated successfully (seen in Figure 1b), which demonstrates that CCTP is an effective method of producing controllable polymers with low molecular weights and relatively narrow molecular weight distributions. Cd0.4Zn0.6S nanocrystals capped with PMAA macromonomers were prepared by a stepwise procedure. Figure 2a shows the FT-IR spectra of the PMAA macromonomer and Cd0.4Zn0.6S nanocrystals capped with PMAA macromonomers. As seen in Figure 2a, the PMAA macromonomer in curve 1 shows a strong peak at 1700 cm-1 that is ascribed to the CdO stretching vibration. However, when the PMAA macromonomer acts as a ligand in the construction of Cd0.4Zn0.6S nanocrystals, this CdO stretching vibration peak shifts to the position noticed at 1570 cm-1 during the coordination interaction between nanocrystals and the PMAA macromonomer ligand (seen in curve 2 of Figure 2a). This result also illustrates that Cd0.4Zn0.6S nanocrystals have been successfully prepared using the PMAA macromonomer as the ligand. The UV-vis absorption and fluorescene emission spectra of Cd0.4Zn0.6S nanocrystals capped with PMAA macromonomers are shown in Figure 2b. According to Brus’s formula,37,38 the particle size of Cd0.4Zn0.6S nanocrystals can be calculated to be 3.6 nm on the basis of the wavelength of the maximum absorption peak noticed at 375 nm in the UV-vis spectrum. The PL spectrum of Cd0.4Zn0.6S NCs shows a broad peak centered at 565 nm, mainly originating from the broad emission of the recombination of surface electrons and holes. To obtain desirable properties of PCs, the surface chemical processing of monodisperse microspheres is of major importance because the number of -COOH groups on the surface of (37) Brus, L. E. J. Chem. Phys. 1984, 80, 4403. (38) Brus, L. E. J. Phys. Chem. 1986, 90, 2555.

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Figure 3. (a) TEM and (b) SEM images of monodisperse PS-co-PMAA microspheres with a diameter of 235 nm (m(PMAA)/m(St) = 8/92 g/g).

functionalized monodisperse microspheres can further serve as linkers between the NCs and the appropriate anchor sites, which would favor the further production of NC-loaded functionalized PCs. In this case, we initially synthesized core-shell-structure PSco-PMAA microspheres via emulsion polymerization, producing multianchor -COOH groups on the surfaces of monodisperse microspheres. Then, we fabricated NC-loaded functionalized PCs via the interaction between NCs and multianchor -COOH groups on the surfaces of functionalized PCs. Monodisperse PSco-PMAA microspheres with a hydrophobic core of polystyrene (PS) and a hydrophilic shell of poly(methacrylic acid) were prepared using styrene and as-prepared PMAA macromonomer as the monomers in one step by means of batch surfactant-free emulsion polymerization. The core-shell structure of as-prepared PS-co-PMAA microspheres was demonstrated using transmission electron microscopy (TEM) (Figure 3a). The diameter of a microsphere is about 235 nm, which is a little smaller than the hydrodynamic diameter obtained by dynamic light scattering (Dh = 258.8 nm). The reason can be attributed to aqueous evaporation during the preparation of colloidal crystal films of the emulsion. As seen in Figure 3a, the dark regions represent the PS core, which are surrounded by the bright domains of the PMAA shell. In the literature,39,40 the nucleation mechanism of this core-shell latex particle is suggested to be homogeneous. Accordingly, persulfate radicals produced by potassium persulfate at high temperature induce further polymerization of the PMAA macromonomer bearing terminal carbon-carbon double bonds in end groups in the aqueous phase, resulting in oligomer radicals. As the oligomer radicals reached a critical length, they precipitated and agglomerated to form primary particles. Subsequently, particle growth continues inside the primary particles. Hence, the hydrophilicity of the PMAA macromonomer is the main polymerization parameter that influences the latex sphere morphology (i.e., the outermost layer of the latex is different from the interior, which is enriched with polar groups). Thus, the final morphology of the PS-co-PMAA microspheres is implied to be a core-shell structure. Figure 3b demonstrates the morphology of the colloidal crystal film assembled from the PS-co-PMAA emulsion. The image of the film displays a well-ordered structure and a close-packed array, indicating a narrow size distribution of PS-co-PMAA microspheres. The FT-IR spectra of PS-co-PMAA microspheres and pure PS microspheres are presented in Figure 4. By comparison with the IR spectrum of the pure PS control sample, the IR spectrum of PS-co-PMAA shows two new absorption peaks. The strong peak at 1680 cm-1 is ascribed to the dissymmetric stretching of CdO whereas the broad peak noticed at about 3445 cm-1 is ascribed to (39) Wang, P. H.; Pan, C. Y. Colloid Polym. Sci. 2001, 279, 98–103. (40) Wang, P. H.; Pan, C. Y. Colloid Polym. Sci. 2002, 280, 152–159.

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Figure 4. FT-IR spectra of PS and PS-co-PMAA microspheres (m(PMAA)/m(St) = 8/92 g/g).

Figure 5. Variation of the average hydrodynamic diameter (Dh) of the PS-co-PMAA microspheres (m(PMAA)/m(St) = 8/92 g/g) as a function of pH.

the stretching of the -OH group. In addition, the slight dissymmetric stretching vibration of CdO suggests the existence of strong hydrogen bond interactions. This characterization demonstrates that the PMAA macromonomer has been successfully incorporated into the final polymer microspheres, which is in good agreement with TEM results. On the basis of the above results, the PMAA macromonomer has been successfully copolymerized with styrene via surfactantfree emulsion polymerization and PMAA is mainly located in the outer layer on the surfaces of microspheres. Therefore, we can assume that these PS-co-PMAA microspheres are pH-responsive because of carboxyl groups on the surfaces of the microspheres.41 Figure 5 displays the variation of the average hydrodynamic diameter of PS-co-PMAA microspheres with pH. It can be (41) Gao, Q.; Xu, Y.; Wu, D.; Sun, Y. H.; Li, X. A. J. Phys. Chem. C 2009, 113, 12753–12758.

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Figure 6. Relationship of the particle size of PS-co-PMAA microspheres as a function of the mass concentration of the PMAA macromonomer. Table 1. Effect of the Weight Ratio of the PMAA Macromonomer to the Styrene Monomer on the Average Hydrodynamic Diameters of Microspheres as Measured by DLS sample PS PS-co-PMAA PS-co-PMAA PS-co-PMAA PS-co-PMAA

St (wt %)

PMAA (wt %)

100 95 92 90 88

0 5 8 10 12

Dh (nm) 236 246 258.8 270 303.9

observed that the average hydrodynamic diameter of the PSco-PMAA microspheres increases from 240 to 262 nm by adjusting the pH and maintains a constant diameter of 262 nm when the pH reaches about 6. This behavior can be explained as follows: at lower pH, PS-co-PMAA microspheres are relatively hydrophobic because the majority of carboxyl groups are protonated and the average hydrodynamic diameter is smaller. However, increasing the pH to slightly above the pKa of the MAA gives partially negatively charged PS-co-PMAA particles. Finally, further increasing the pH of the solution (pH 6) leads to PS-co-PMAA microspheres containing the MAA segment that is completely charged, with the average hydrodynamic diameter reaching about 262 nm. This result can be further illustrated by the PMAA macromonomer located in the outer layer on the surface of PS-co-PMAA microspheres and the homogeneous nucleation mechanism. The effect of the weight ratio of the PMAA macromonomer to the styrene monomer on the microspheres’ average hydrodynamic diameter was also investigated, and the results are presented in Table 1 and Figure 6. In this case, the weight ratios of the PMAA macromonomer to styrene were fixed at 0:100, 5:95, 8:92, 10:90, and 12:88. From Table 1 and Figure 6, it can be seen that an increase in PMAA concentration from 0 to 12% causes an increase in the average hydrodynamic diameter of the microspheres from 236 to 303.9 nm. That is, the higher concentration of the PMAA macromonomer leads to larger microspheres, which is similar to previous reports42,43 in which increased particle size coupled with a broadening of the size distribution occurs at a relatively high functional monomer concentration. This may be explained by the formation of a large quantity of polyelectrolytes,44 causing bridging flocculation of the growing particle. (42) Schild, R. L.; El-Aasser, M. S.; Poelhein, G. W.; Vanderhoff, J. W. In Emulsion Latexes and Dispersions; Becher, P., Yudenfreund, M. N., Eds.; Marcel Dekker: New York, 1978; pp 99-128. (43) Liu, L. J.; Kriegger, I. M. J. Polym. Sci., Polym. Chem. Ed. 1981, 19, 3013– 3026. (44) M€uller, H.; Leube, W.; Tauer, K.; F€orster, S.; Antonietti, M. Macromolecules 1997, 30, 2288–2293.

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Figure 7. Representative scanning electron micrographs (SEM) of PS-co-PMAA microspheres with the following m(PMAA)/m(St) g/g values: (a) 0/100, (b) 5/95, (c) 8/92, (d) 10/90, and (e) 12/88.

Figure 8. PS-co-PMAA microsphere reflection spectra. (Inset) Optical micrographs of the corresponding reflection spectra of PS-co-PMAA microspheres.

Figure 7 shows scanning electron microscopy (SEM) micrographs of colloidal crystals assembled from PS-co-PMAA microspheres with different weight ratios of PMAA macromonomer to styrene monomer. SEM observation clearly reveals that all of the microspheres with different weight ratios are monodisperse and hexagonal-type close packed, except that the weight ratio of PMAA macromonomer to styrene was 12:88; namely, when the PMAA weight content reaches 12%, there are a few smaller particles. It also illustrates that the PMAA macromonomer successfully played the role of a hydrophilic comonomer without forming new particles45 and was located in the outer layer of the surface of the PS-co-PMAA microsphere. Figure 8 shows the reflection spectra of colloidal crystals made from PS-co-PMAA monodisperse microspheres with different weight ratios of PMAA macromonomer to styrene monomer. Obvious reflections can be clearly observed, indicating the good quality of the formed microspheres corresponding to the results of SEM. In addtion, the peak position shifts to a longer wavelength as the hydrodynamic diameter of the PS-co-PMAA microspheres increases. Colloidal crystals formed from monodisperse polymer microspheres possess a photonic bandgap feature, which is proven by the dips in the optical transmission spectra or the peaks in the corresponding reflection spectra.46 Therefore, this characterization demonstrates that all of the PS-co-PMAA monodisperse microspheres with different weight ratios of PMAA (45) Kobayashi, H.; Chaiyasat, A.; Oshima, Y.; Suzuki, T.; Okubo, M. Langmuir 2009, 25, 101–106. (46) Wang, J. X.; Wen, Y. Q.; Hu, J. P.; Song, Y. L.; Jiang, L. Adv. Funct. Mater. 2007, 17, 219–225.

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Figure 10. Reflection spectra (a) PS-co-PMAA photonic crystals (m(PMAA)/m(St) = 8/92 g/g) and (b) Cd0.4Zn0.6S nanocrystalloaded photonic crystals.

Figure 9. SEM images of monodisperse PS-co-PMAA microspheres coated with Cd0.4Zn0.6S nanocrystals under different magnification (m(PMAA)/m(St) = 8/92 g/g). (Insets) Optical micrographs and fluorescence microscope images of PS-co-PMAA microspheres coated with Cd0.4Zn0.6S nanocrystals.

macromonomer to styrene monomer can be used for the fabrication of photonic crystals. It also can be seen that all of the films fabricated by PS-co-PMAA microspheres with different hydrodynamic diameters simultaneously exhibit brilliant colors from blue to red (Figure 8 inset). These colors come from the brilliance of the light inside the ordered structure formed by these microspheres, further confirming the well-ordered structure of PS-co-PMAA microspheres. Scanning electron microscope (SEM) images of Cd0.4Zn0.6S nanocrystals and PS-co-PMAA microsphere hybrids under different magnification are presented in Figure 9. Figure 9a shows that Cd0.4Zn0.6S nanocrystals and PS-co-PMAA microspheres hybrids are very regular, and Figure 9b under higher magnification further confirms that the PS-co-PMAA microspheres are uniformly surrounded by Cd0.4Zn0.6S nanocrystals. This can be attributed to the fact that multianchor -COOH groups on the surfaces of PS-co-PMAA microspheres favor incorporation with Cd0.4Zn0.6S nanocrystals capped with PMAA macromonomer. However, we do not observe any nanocrystals existing on the top surface of microspheres from Figure 9. According to the reported literature,47 it could be explained by the fact that Cd0.4Zn0.6S nanocrystals are generally so small that they are easy to draw from the top surfaces of microspheres into the niches between microspheres, accompanied by water evaporation during film formation of the colloidal crystals. The photographic images of the film of Cd0.4Zn0.6S nanocrystals and PS-co-PMAA microsphere hybrids under ultraviolet light and daylight are shown in the insets of Figure 9a,b, respectively. The film is green when examined in daylight; however, under 302 nm ultraviolet light, the film (47) Norris, D. J.; Arlinghaus, E. G.; Meng, L.; Heiny, R.; Scriven, L. E. Adv. Mater. 2004, 16, 1393–1399.

10662 DOI: 10.1021/la1009169

has a light-yellow color. Figure 10 shows the reflection spectra of PS-co-PMAA photonic crystals and Cd0.4Zn0.6S nanocrystalloaded photonic crystals. By comparison, the peak position for Cd0.4Zn0.6S nanocrystal-loaded photonic crystals presents red shifts of about 9 nm. This small difference in the peak position with respect to that of nanocrystal-loaded photonic crystals does not influence the color of the final film. Also, it is in good agreement with the result of the photographic image in Figure 9b. The film composed of nanocrystal-loaded photonic crystals is still green. This finding can also be manifested in the fabrication of nanocrystal-loaded photonic crystals with the polymerizable macromonomer via the CCTP technique as an alternative effective method of permitting the combination of electronic and photonic states within a single structure.

Conclusions We have demonstrated a new strategy for fabricating nanocrystal-loaded photonic crystals with a polymerizable macromonomer via catalytic chain-transfer polymerization (CCTP). The as-prepared PMAA polymerizable macromonomer acts not only as a comonomer for obtaining functionalized PS-co-PMAA microspheres but also as a ligand for constructing Cd0.4Zn0.6S nanocrystals. In particularly, PS-co-PMAA microspheres are uniformly surrounded by Cd0.4Zn0.6S nanocrystals to obtain hybrids of integrating electronic confinement and photonic confinement, hence opening promising avenues in the design and construction of next-generation photoelectric devices based on photonic crystals. A parallel approach to that of this investigation constitutes a promising way to fabricate other nanocrystal-loaded functional photonic crystals with polymerizable macromonomers via the CCTP technique. Hence, systematic research on other high-quality functional photonic crystal will follow. Acknowledgment. This work was supported by the Natural Science Foundation (NSF) of China (grant no. 20606016), the National Natural Science Foundation of China-NSAF (grant nos. 10676013 and 10976012), and the NSF of the Jiangsu Higher Education Institutions of China (grant nos. 07KJA53009 and 09KJB530005).

Langmuir 2010, 26(13), 10657–10662