pubs.acs.org/Langmuir © 2009 American Chemical Society
Novel Walnut-like Multihollow Polymer Particles: Synthesis and Morphology Control Xueping Ge,† Mozhen Wang,*,† Hua Wang,† Qiang Yuan,† Xuewu Ge,*,† Huarong Liu,† and Tao Tang‡ †
CAS Key Laboratory of Soft Matter Chemistry, Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China, and ‡State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, PR China Received July 10, 2009. Revised Manuscript Received September 9, 2009
Novel walnut-like multihollow polymer particles were first prepared by γ-ray radiation emulsion polymerization using cross-linked and sulfonated polystyrene spheres (CSPs) as the template. The formation process was studied in detail, and the morphology of walnut-like multihollow polystyrene particles could be controlled by the content of crosslinking agent, sulfonation time of CSP particles, and the weight ratio of monomer/CSP. In addition, an application of walnut multihollow polymer particles on bonding Ag nanoparticles onto the surface was achieved, which could be extended to other noble metal nanoparticles and could have a wide range of potential applications, such as catalysts, sensors, solar cells, and photonic crystals.
Introduction The morphology control of latex particles has been a fascinating area in emulsion polymer science because it significantly enhances the application areas of latex. More recently, this research was further extended to nonspherical polymer particles with special surface morphology,1 because the various applications of nonspherical polymer particles in photonic crystals with the light-scattering properties,2,3 catalysts, directed self-assembly,4 molecular recognition,5 and sensors that are responsive to external field such as shear field and electric field6 have attracted great interest among material scientists. However, the preparation of nonspherical particles with special surfaces has been a challenging task for a long time through a multiple phase system, because the thermodynamic trend of minimization of the interfacial free energy between different phases usually results in spherical particles. In most cases, nonspherical particles are fabricated by modification of the prepared spherical particles. *To whom correspondence should be addressed. E-mail:
[email protected]. cn (X.G.);
[email protected] (M.W.). Tel: 86-551-3601586. (1) Van Blaaderen, A. Nature 2006, 439, 545–546. (2) Lu, Y.; Yin, Y. D.; Xia, Y. N. Adv. Mater. 2001, 13, 415–420. (3) (a) Ge, J. P.; Hu, Y. X.; Zhang, T. R.; Yin, Y. D. J. Am. Chem. Soc. 2007, 129, 8974–8975. (b) Ge, J. P.; Hu, Y. X.; Zhang, T. R.; Huynh, T.; Yin, Y. D. Langmuir 2008, 24, 3671–3680. (4) (a) Binks, B. P.; Fletcher, P. D. I. Langmuir 2001, 17, 4708–4710. (b) Murphy, C. J. Science 2002, 298, 2139–2141. (5) (a) Glotzer, S. C. Science 2004, 306, 419–420. (b) Zhang, Z. L.; Horsch, M. A.; Lamm, M. H.; Glotzer, S. C. Nano. Lett. 2003, 3, 1341–1346. (6) (a) Jogun, S. M.; Zukoski, C. F. J. Rheol. 1999, 43, 847–871. (b) Ho, C. C.; Ottewill, R. H.; Yu, L. Langmuir 1997, 13, 1925–1930. (7) (a) Kim, J . W.; Larsen, R. J.; Weitz, D. A. J. Am. Chem. Soc. 2006, 128, 14374–14377. (b) Mock, E. B.; DeBruyn, H.; Hawkett, B. S.; Gilbert, R. G.; Zukoski, C. F. Langmuir 2006, 22, 4037–4043. (c) Lu, Y.; Xiong, H.; Jiang, X. C; Xia, Y. N.; Prentiss, M.; Whitesides, G. M. J. Am. Chem. Soc. 2003, 125, 12724–12725. (8) (a) Fujibayashi, T.; Komatsu, Y.; Konishi, N.; Yamori, H.; Okubo, M. Ind. Eng. Chem. Res. 2008, 47, 6445–6449. (b) Marczewski, D.; Goedel, W. A. Nano Lett. 2005, 5, 295–299. (c) Yan, Q. F.; Liu, F.; Wang, L. K.; Lee, J. Y.; Zhao, X. S. J. Mater. Chem. 2006, 16, 2132–2134. (9) (a) Koo, H. Y.; Yi, D. K.; Yoo, S. J.; Kim, D. Y. Adv. Mater. 2004, 16, 274– 277. (b) Saito, N.; Nakatsuru, R; Kagari, Y.; Okubo, M. Langmuir 2007, 23, 11506– 11512. (c) Wang, D.; Dimonie, V. L.; Sudol, E. D.; El-Aasser, M. S. J. Appl. Polym. Sci. 2002, 84, 2710–2720. (d) Okubo, M.; Fujibayashi, T.; Yamada, M.; Minami, H. Colloid Polym. Sci. 2005, 283, 1041–1045.
Langmuir 2010, 26(3), 1635–1641
For example, dimers and multiplets,7 ellipsoidal particles,2 spheres with holes,8 and snownman-like9 and disk-like10 particles have been fabricated from microspheres by seeded polymerization or self-assembly of complex structures.11 The other methods to prepare nonspherical particles include colloid lithography12 and interference lithography.13 But most of the above-reported methods are only suitable for the preparation of particles with smooth surfaces. Therefore, it still has great significance to develop new methods to fabricate nonspherical polymer particles with special surfaces. On the other hand, multihollow polymer particles have been applied in some advanced industrial field, such as paper coatings, weight-saving thermal insulations, supports for medical assays, enzyme immobilization carriers, chromatographic packing, and sound damping or absorption materials.14 Thus, the integration of nonspherical properties with multihollow polymer particles should be expected to open great potential applications in many emerging fields. But regretfully, there is no related work reported in the open literature. Our group has been focused on the study of radiation emulsion polymerization for a long time.15,16 γ-ray radiation polymerization can be carried out at low temperature so as to keep the stability of emulsion during polymerization. In particular, the products synthesized by γ-radiation are beneficial to their bioapplications because there is no contamination brought in by chemical initiators. In the early work of our group, cage-like (10) (a) Okubo, M.; Fujibayashi, T.; Terada, A. Colloid Polym. Sci. 2005, 283, 793–798. (b) Fujibayashi, T.; Okubo, M. Langmuir 2007, 23, 7958–7962. (11) (a) Chastek, T. T.; Hudson, S. D.; Hackley, V. A. Langmuir 2008, 24, 13897–13903. (b) Kraft, D,J.; Vlug, W. S.; van Kats, C. M.; van Blaaderen, A.; Imhof, A.; Kegel, W. K. J. Am. Chem. Soc. 2009, 131, 1182–1186. (12) Wang, L. K.; Xia, L. H.; Li, G.; Ravaine, S.; Zhao, X. S. Angew. Chem., Int. Ed. 2008, 47, 4725–4728. (13) Jang, J. H.; Ullal, C. K.; Kooi, S. E.; Koh, C. Y.; Thomas, E. L. Nano. Lett. 2007, 7, 647–651. (14) Charles, J. M.; Michael, J. D. Adv. Colloid Interface Sci. 2002, 99, 181–213. (15) He, X. D.; Ge, X. W.; Liu, H. R.; Wang, M. Z.; Zhang, Z. C. Chem. Mater. 2005, 17, 5891–5892. (16) Yuan, Q.; Yang, L. B.; Wang, M. Z.; Wang, H.; Ge, X. P.; Ge, X. W. Langmuir 2009, 25, 2729–2735.
Published on Web 09/24/2009
DOI: 10.1021/la902493r
1635
Article
Ge et al.
Figure 1. SEM images of PS particles prepared by dispersion polymerization (a) and CSP particles (b).
polymer particles with hollow core/porous shell structures were synthesized by γ-ray radiation polymerization in the presence of a sulfonated polystyrene particle (SP) template.15 Recently, the mechanism on the formation of cage-like polymer particles has been further studied in detail.16 In this article, we used crosslinked/sulfonated polystyrene microspheres (CSPs) as the template and provided a simple method to prepare walnut-like multihollow polymer particles by γ-radiation emulsion polymerization. Furthermore, the primary application of walnut-like particles upon bonding with silver nanoparticles was investigated.
deionized water under ultrasonication for 2 min. Then St was added to the solution under stirring. The emulsion was kept for 24 h under continuous stirring in order to let the CSP particles swell by the monomer. The resultant emulsion was irradiated by a 60 Co γ-ray (located in USTC, China) at a dose rate of 50 Gy/min for a total absorbed dose of 45 kGy after removing the dissolved oxygen by bubbling N2 for 10 min. The products were collected by centrifugation (6000 rpm, 5 min) to separate the products from the emulsion, washed with ethanol three times, and then dispersed in ethanol for storage.
Experimental Section
particles (5 mg) were dispersed in AgNO3 solution (0.1 M) with magnetic stirring for 24 h. Then the particles were separated by centrifugation from the unadsorbed Agþ. The collected particles bonded with Agþ were dispersed in the mixture of water (5 mL) and isopropyl alcohol (1 mL), and irradiated by 60Co γ-ray at a dose rate of 20 Gy/min with the total absorbed dose of 30 kGy. 4. Characterization. The morphology of prepared latex particles was studied by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). TEM analysis was taken by a high-resolution transmission electron microscope (H800, JEOL2010); SEM analysis was taken by a field emission scanning electron microscope (FESEM, JEOL JSM6700). Samples for TEM and SEM analysis were prepared at room temperature by dispersing one drop of the ethanol solution of the sample on copper grids and then drying in air. For ultrathin cross section of the particles, the sample was embedded in an epoxy matrix, cured at room temperature, which was then ultramicrotomed to a thickness of 70 nm. The average diameter (Dn), weight average diameter (Dw), and particle distribution index (PDI) of latex particles were calculated by the following equations: P ni Di Dn ¼ P ni P ni D4i Dw ¼ P ni D3i Dw PDI ¼ Dn
1. Materials. Styrene (St) and divinylbenzene (DVB) were purified by passing through a basic alumina column to remove the inhibitor before use. Reagent-grade 2,20 -azobis(isobutyronitrile) (AIBN) was purified by recrystallization in methanol. Reagentgrade concentrated sulfuric acid (98%), sodium dodecyl sulfate (SDS), polyvinylpyrrolidone (PVP, K30), polyvinyl alcohol (PVA), silver nitrate (AgNO3) and ethanol were all purchased from Shanghai Chemical Reagents Co., China, and used without further purification. Deionized water (resistivity >18.2 MΩ 3 cm/ 25 °C) prepared by a Milli-Q 185 system (Millipore, USA) was used for all experiments. 2. Synthesis of Walnut-like Multihollow Polymer Particles. Step 1. Synthesis of Polystyrene (PS) Particles through Dispersion Polymerization. PVP (1.1 g) was dissolved in the mixture of ethanol (75.5 g) and deionized water (3.0 g) in a glass container with a stirrer. St (27.3 g) and AIBN (0.35 g) were added to the above solution. The system was heated to 75 °C and kept for 20 h under stirring at 500 rpm with bubbling N2. The obtained PS particles were collected by centrifugation (6000 rpm, 5 min). After being washed with ethanol three times, the products were dried in a vacuum oven at 40 °C for 24 h.
Step 2. Synthesis of Cross-Linked PS Particles (CP) through Seeded Dispersion Polymerization. The as-prepared PS particles (0.5 g) were first dispersed in SDS solution (60 mL, 0.25%, w/w). Then a monomer solution consisting of St, DVB (0.25-2%, vol%), and AIBN (0.05 g) was added under stirring. The mixture was stirred at 300 rpm for 14 h in order to allow the PS seed particles to be thoroughly swollen by monomer. Then PVA solution (50 mL, 1%, w/w) was added. The polymerization was performed by tumbling at 500 rpm for 10 h at 70 °C. The CPs were collected by centrifugation and washed with ethanol three times, then dried in a vacuum oven at 40 °C for 24 h. Step 3. Synthesis of CSP Particles. A 0.5 g portion of asprepared CP particles was dispersed in 20 mL concentrated sulfuric acid with the aid of ultrasonication. The sulfonation reaction was carried out at 40 °C under magnetic stirring for 2, 5, and 8 h, respectively. After diluted by deionized water, the sample was repeatedly centrifuged and washed with ethanol/ water, then dried in vacuum at 40 °C for 48 h.
Step 4. Synthesis of Walnut-like Multihollow Polymer Particles. A 0.05 g portion of CSP was dispersed into 10 mL of 1636 DOI: 10.1021/la902493r
3. Bonding Ag Nanoparticles onto the Surface of Walnutlike Polymer Particles. The as-prepared walnut-like polymer
where ni is the number of polymer particles with diameter Di. The emulsion droplets formed in step 4 were observed by an OLYMPUS-BX41 optical microscope equipped with a Nikon COOLPIX4500 high-resolution digital charge-coupled device (CCD) camera. The relative content of SO3H groups at particle surface was measured by X-ray photoelectron spectroscopy (XPS).
Results and Discussion 1. Morphology Observation. Figure 1 shows the SEM photographs of seed PS particles (Figure 1a), and CSPs (Figure 1b). Both of them are uniform spherical particles. The diameter of PS spheres increased from 2.9 μm (PDI = 1.01) to 4.2 μm (PDI = 1.01) after cross-linking and sulfonation. Langmuir 2010, 26(3), 1635–1641
Ge et al.
Figure 2. SEM images of CSP particles swollen by St for 24 h (a) before and (b) after polymerization induced by γ-radiation. (c) The TEM image of panel b. (d) Ultrathin cross sections of the sample in b.
Figure 2a shows the SEM image of CSP particles swollen by St for 24 h before polymerization. Obviously, there existed surface patterning on the surface of particle similar to walnut and the particles size was 4.6 μm. Figure 2b,c shows the SEM and TEM photographs of walnut-like multihollow polymer particles after the swollen CSP particles were radiated by γ-ray, which induced the polymerization of St. The surface of walnut-like polymer particles was further patterned by a lot of polymer sheets. Figure 2d shows a TEM image of an ultrathin cross section of these particles. We can see clearly that there were also many hollows in the inner region of these particles. The size of walnutlike particles increased to about 5.4 μm. 2. The Formation Mechanism of the Walnut-like Multihollow Polymer Particles. On the basis of the continuous observation of the process, a formation mechanism on the walnutlike multihollow polymer particles was illustrated in Scheme 1. First, CSP particles were dispersed in water (Scheme 1A). As a result of the strong hydrophilicity of -SO3H on the surface, CSP particles could be dispersed as individual solid particles into water without the help of surfactant. After monomer was added, the individual solid CSP particles adsorbed onto the interface between the oil droplets and water, i.e., a Pickering emulsion was formed (Scheme 1B). However, the adsorption of these sulfonated particles on the surface of oil droplets is a dynamic equilibrium process. Sometimes some particles may fall off into the solution, and some are adsorbed onto the surface of droplets. This process was proved by the observation of optical microscopy as shown in Figure 3. Large monomer droplets were surrounded by a lot of small CSP particles (Figure 3a). After reserving the emulsion for 24 h under continuous stirring, we can see clearly that the large monomer droplets disappeared (Figure 3b). The reason is that the monomer has a good affinity with CSP particles and will penetrate into the CSP particles to swell them. This process leads to the decrease of the size of the monomer droplets since the monomers gradually transferred into the inner region of the CSP particles. When the size decreased to a certain value, Langmuir 2010, 26(3), 1635–1641
Article
the stability of the Pickering emulsion was destroyed. The swollen CSP particles have to disperse again into the water as individual particles (Scheme 1c). At the same time, with the swelling of CSP particles, the mobility of the SP molecular chains was enhanced. The SP chains that were not cross-linked at the surface of CSP were dissolved, and the CP chains remain. Thus, the vacancy (like ditches on the surface of walnut, Figure 2a) can be observed on the exterior of the CSP particles. That is, the surface of CSP particles was first patterned. The patterned CSP was important for further polymerization to form nonspherical polymer particles. On the other hand, because of the limitation of the cross-linked network, some -SO3H groups were be kept in the inner region of the swollen CSP particles, and an osmotic pressure was produced, which forces the water to permeate into the inner region of the CSP particles, as shown in Scheme 1C (inner view). When the system was radiated by γ-ray, the high-energy radiation mainly interacted with the continuous-phase water molecules to form radicals, such as OH• and H•. Then the radicals were captured and anchored preferentially on the sulfonated areas of CSP particles.12,17 Then the polymerization was initiated at the sulfonated areas, and the protruding sheets were formed. As the polymerization proceeded, the monomer transported from the inside of the CSP to the surface because of the higher concentration of monomer in the nucleus of CSP relative to that at the surface until polymerization was ceased. The monomer transport maybe attributed to the elastic stress during the polymerization process because of the presence of the cross-link net.18 Finally, walnut-like multihollow polymer particles were obtained, and some polymer sheets were grown on the protruding sulfonated area (ditches on the surfaces of CSP particles deepened and finally protruding sheets were formed) at the surface of patterned CSP particles (Figure 2b and Scheme 1D). Water inside of CSP particles evaporates after polymerization and hollows formed. Thus walnut-like multihollow polymer particles were prepared. (Figure 2b and Scheme 1D). Figure 4 shows the XPS spectra of PS spheres (Figure 4a), CSP (Figure 4b), and CSP swollen for 24 h before (Figure 4c) and after (Figure 4d) polymerization. As shown in Figure 4, the atomic ratio of C:S on the sample surface of CSP particles was 70.05:4.79 (Figure 4b) after 5 h of sulfonation. After the CSP particles were swollen by monomer for 24 h, the atomic ratio of C:S increased to 70.46:3.29 (Figure 4c). The decrease of S content was attributed to the dissolution of the sulfonated and un-cross-linked PS chains. After polymerization, the atomic ratio of C:S increased to 86.68:2.02 (Figure 4d) because of the further polymerization of PS on the protruding areas. The results as shown in Figure 4 were consistent with the mechanism proposed in Scheme 1. 3. Effects of Cross-Linking and Sulfonation on the Morphology of Polymer Particles. In order to further clarify the significance of cross-linking and sulfonation of PS spheres on the formation of walnut-like polymer particles, two other kinds of seed spheres were prepared for comparison: cross-linked/nonsulfonated PS spheres (CP) and non-cross-linked/sulfonated PS spheres (SP; sulfonation time was 5 h). Especially for the swelling and polymerization of CP, 10 mL of SDS solution (10 mM) was used instead of water because nonsulfonated PS spheres could not be dispersed in water. Figure 5 shows the SEM photographs of (17) (a) Shim, S. E.; Cha, Y. J.; Byun, J. M.; Choe, S. J. Appl. Polym. Sci. 1999, 71, 2259–2269. (b) Lee, C. F. J. Polym. Sci., Part A: Polym. Chem. 2005, 43, 2224– 2236. (18) (a) Sheu, H. R.; El-Aasser, M. S.; Vanderhoff, J. W. J. Polym. Sci., Part A: Polym. Chem. 1990, 28, 629–651. (b) Sheu, H. R.; El-Aasser, M. S.; Vanderhoff, J. W. J. Polym. Sci., Part A: Polym. Chem. 1990, 28, 653–667.
DOI: 10.1021/la902493r
1637
Article
Ge et al.
Figure 3. Optical microscopy images of the emulsion consisting of monomer, CSP particles, and water prepared in Step 4 after being reserved for 1 h (a) and for 24 h (b).
Figure 4. XPS spectra of PS spheres (a), CSP (b), and CSP swollen by St for 24 h before (c) and after (d) polymerization. Scheme 1. The Formation Process of the Walnut-like Multihollow Polymer Particles
polymer particles after swelling and polymerization of CP (Figure 5a) and SP (Figure 5b). As shown in Figure 5a, a lot of secondary nucleation particles were obtained, and the final latex 1638 DOI: 10.1021/la902493r
particles were spherical instead of walnut-like when CP was used. From Figure 5b, we can see clearly that there were also only spherical polymer particles with smooth surfaces when SP was Langmuir 2010, 26(3), 1635–1641
Ge et al.
Article
Figure 5. SEM images of latex particles fabricated from CPs (a) and SPs (b).
Figure 6. SEM images of nonspherical multihollow polymer particles fabricated from CSP particles with different sulfonation times: (a) 2 h; (b) 5 h; (c) 8 h.
Figure 7. SEM images of nonspherical polymer particles obtained from CSP particles with different DVB content (w/w): (a) 0.25%; (b) 0.5%; (c) 2.0%.
used. That is, walnut-like polymer particles could not be obtained when the seed PS spheres were not cross-linked or sulfonated. The results shown in Figure 5 indicated that both the cross-link and sulfonation of PS spheres play important roles in the preparation of walnut-like multihollow polymer particles. The sulfonation could not only help to pattern the surface of CSP and form hollows inside the CSP particles but also favor the CSP particles to capture radicals to initiate polymerization. The cross-linking was important not only for the monomer transport from inside to the surface but also for keeping the skeleton of CSP particles. On the basis of the above results, it can be concluded that the morphology of final walnut-like multihollow particles could be adjusted by changing the extent of the cross-linking and sulfonation of CSP particles. 4. Effect of the Sulfonation Time on the Morphology of Polymer Particles. Commonly, the degree of sulfonation will increase with the sulfonated time. Figure 6 shows the SEM photographs of nonspherical multihollow polymer particles prepared from CSP particles with different sulfonation time. As can be seen from Figure 6a,b, walnut-like polymer particles could be Langmuir 2010, 26(3), 1635–1641
obtained with the CSP particles sulfonated for 2 and 5 h, respectively. With the sulfonation time increasing from 2 to 5 h, the ditches on the surface of walnut-like particles became deeper and wider. Because the content of -SO3H groups increased with the sulfonation time, the higher amount of SP chains in the un-cross-linked area was dissolved during the swelling process. So walnut-like polymer particles with relatively loose surfaces could be prepared after polymerization. When the sulfonation time further increased to 8 h, the amount of dissolved SP chains increased, and more water penetrated into the CSP particles, then flower-like sheets were obtained (Figure 6c). In addition, there were many pores observed on the surface of the flower-like sheets because of the evaporation of water which penetrated into the inside of the swollen CSP particles. 5. Effect of the Cross-Link Agent Content in the Seeded PS Particles on the Morphology of Polymer Particles. Figure 7 shows the SEM photographs of nonspherical polymer particles obtained from CSP particles with different DVB content. With the increase of the DVB content, the morphology of final polymer particles changed from disordered clutter particles DOI: 10.1021/la902493r
1639
Article
Ge et al.
Figure 8. SEM photographs of nonspherical walnut-like multihollow polymer particles prepared under different weight ratios of St to CSP (w/w): (a) 1/1; (b) 2/1; (c) 4/1.
Figure 9. TEM images of silver nanoparticles bonded on the surface of walnut-like multihollow polymer particles after reduction of Agþ by γ-radiation. Panel b is the magnified portion of panel a.
(DVB 0.25%, w/w) (Figure 7a), flower-like particles (DVB 0.5%, w/w) (Figure 7b), and finally to walnut-like particles (DVB 2.0%, w/w) (Figure 7c). This result indicated that, when the crosslinking intensity is quite low (as in the case of DVB 0.25%, w/w), the CSP particles, which were majorly composed of un-crosslinked PS chains, swell thoroughly and even dissolve in the monomer so that the skeleton of CSP lost its strength and collapsed during the swelling process before polymerization. Thus, the disordered clutter particles were obtained after γ-radiation polymerization. When the DVB content increased, the content of cross-linked and SP chains increased. The strength of the CSP skeleton improved correspondingly. Although swollen by monomer, the CSP particles can still keep the former skeleton shape so that flower-like (as the case of DVB 0.5%, w/w) and walnut-like (as the case of DVB, 2.0%, w/w) particles were obtained consequently. That is, when fixing the sulfonation time, the strength of the CSP skeleton increases with the degree of crosslinking. The morphology of walnut-like particles could be easily adjusted by sulfonation time and the content of cross-linking agent in the seed particles. 6. Effect of the Weight Ratio of Monomer to CSP on the Morphology of Polymer Particles. Figure 8 shows the SEM photographs of walnut-like multihollow polymer particles prepared under a different weight ratio of monomer to CSP (1/1, 2/1, and 4/1, w/w). As shown in Figure 8, the only difference in these particles was the compact intensity of the protruding sheets. With the increase of monomer/CSP ratio, the compact density increased. Because there was more monomer presence in the inside of the CPS particles after swelling, the monomer polymerized, which increased the compact intensity of the protruding sheets. 7. Application of Walnut-Like Multihollow Polymer Particles on Bonding with Silver Nanoparticles. Compared 1640 DOI: 10.1021/la902493r
with common spherical multihollow particles, the surface of walnutlike polymer particles was patterned by a lot of polymer sheets; thus this type of walnut-like polymer particles could have light density and high specific surface. It is more favorable for the solute and/or solvent in the environment to disperse or load on the surface, and because the sulfonic acid groups existed on the surface of the walnutlike multihollow polymer particles, metal ions (Mnþ) are be easily bonded on the surface of the particles through the ionic bond between -SO3- and Mnþ. On the other hand, the metal ions are easily reduced in situ by γ-ray radiation. Thus, the metal nanoparticles could be easily bonded on the surface of walnut-like particles. Herein, silver ions were employed in our experiment. It has been admitted that the Agþ ions are reduced by hydrated electron 19 eaq under γ-ray radiation: H2 O f eaq , þ
Ag þeaq f 0 þ
H•,
•OH, etc:
0
Ag Ag þAg f Agþ 2 0 Agþ þe f Ag aq 2 2 3333330 Agþ m þeaq f Agm
Figure 9 and Figure 10 show the TEM images and XPS spectrum of silver nanoparticles bonded on the surface of walnutlike multihollow polymer particles after reduction of Agþ by γ-radiation. It could be seen clearly that silver nanoparticles were dispersed uniformly on the surface, and the size was about 5 nm. This was attributed to the sulfonic groups anchored on the surface of walnut-like particles, which would prevent the aggregation of (19) de Lamaestre, R. E.; Bea, H.; Bernas, H.; Belloni, J.; Marignier, J. L. Phys. Rev. B 2007, 76, 205431.
Langmuir 2010, 26(3), 1635–1641
Ge et al.
Article
Figure 10. XPS spectrum of silver nanoparticles bonded on the surface of walnut-like multihollow polymer particles after reduction of Agþ by γ-radiation.
silver nanoparticles by steric hindrance. The results as shown in Figures 9 and 10 indicated that monodisperse silver nanoparticles could be easily prepared by the Agþ bonding and reduction on the surface of walnut-like multihollow polymer particles.
Conclusion A new and effective process for fabricating novel walnutlike multihollow polymer particles by γ-ray radiation emulsion
Langmuir 2010, 26(3), 1635–1641
polymerization using CSP particles as the template was developed. The formation mechanism of the walnut-like multihollow polymer particles was established on a CSP swelling process by monomer and water penetrating processes. On the basis of the proposed mechanism, the morphology of the final latex particles could be controlled by changing the content of cross-linking agent, sulfonation time of CSP particles, and the weight ratio of monomer/CSP. Combined with the reduction of silver ions under γ-ray radiation, the formation of silver nanoparticles bonding on the surface of walnut multihollow polymer particles could be achieved easily. This method could also be applied to bond other noble metal nanoparticles, such as Au, Pt, Pd, and magnetic Fe3O4, which could have a wide range of potential applications, such as catalysts, sensors, solar cells, and photonic crystals. Further extensive work is underway in our lab. Acknowledgment. The authors gratefully acknowledge the National Natural Science Foundation of China (Nos. 50573070, 50773073, and 50873096), the Foundation of State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry (200904), the Graduate Innovation Fund of USTC, and the Program for Changjiang Scholars and Innovative Research Team in University for support of this work.
DOI: 10.1021/la902493r
1641
