ARTICLE pubs.acs.org/Langmuir
Formation of a “Hard Microemulsion” and Its Role in Controllable Synthesis of Nanoparticles within a Functional Polymer Matrix Qingli Qian, Zhijun Huang, Xiaohui Zhang, and Guoqing Yuan* Beijing National Laboratory for Molecular Sciences (BNLMS), Laboratory of New Materials, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
bS Supporting Information ABSTRACT: Microemulsions are often used in the synthesis of nanoparticles in solution. In this work, we put forward the concept of a “hard microemulsion”, which is based on the differential partitioning of water and ethanol solvent molecules inside functional polymer matrices. When the mixture of water and organic solvent enters the functional polymer, the liquid molecules should partition to different regions. Water should concentrate in the microdomains rich in hydrophilic functional groups, forming water-enriched cores, whereas organic solvents should localize near the alkyl polymer skeleton, forming organic liquid enriched outer layers. From a macroscopic view, the swollen polymer matrix is divided into numerous “microdroplets”, resembling frozen waterin-oil microemulsions. We define such a structure as a “hard microemulsion”. The water-enriched microdroplets may act as templates for synthesizing inorganic nanoparticles. We demonstrate the utility of hard microemulsions for the controllable synthesis of silver and platinum nanoparticles inside different macroreticular functional polymers.
’ INTRODUCTION New strategies for materials preparation are of fundamental importance in materials science and technology.1 Synthesis of nanoparticles can be accomplished in various ways,1 6 and the major advances over the recent decades are based on the selfassembly process of amphiphiles,1,7 in which microemulsion systems played a predominant role.2 5,7,8 Nanoparticles are unstable and prone to agglomeration, and they are difficult to separate from solutions. Thus, in recent years immobilization of nanoparticles has become a promising research field, and functional polymer supported nanoparticles are among the most important directions.9 However, random migration of reactant atoms during liquid-phase synthesis may cause maldistribution and agglomeration of nanoparticles inside functional polymers, especially inside macroporous polymers.10,11 Currently, the strategies for controllable synthesis of nanoparticles inside functional polymers are mostly based on the size-restricting effect of polymer chain frameworks. In this case, the size of the microrooms divided by polymer chain frameworks should be uniform; hence, dendrimers or hyperbranched polymers,12 polymer films,13 and gel-type resins14,15 have been used as supports for nanoparticles syntheses. Nevertheless, the microdistribution of nanoparticles inside functional polymers cannot be controlled. As for macroporous polymers, the reactant atoms can migrate into their mesopores and macropores, making both size and distribution control of the nanoparticles impossible.9e,15,16 In this paper, we put forward a new strategy, based on the formation of a “hard microemulsion” consisting of a polymer matrix into which water and ethanol are partitioned, for controllable synthesis of nanoparticles inside porous functional polymers. r 2011 American Chemical Society
’ EXPERIMENTAL SECTION Materials. The solvents and chemicals bought from Alfa Aesar were of reagent grade and were used as received. The carboxylated macroreticular resin (D113), made from copolymerization of acrylic acid and divinylbenzene, is bead shaped (20 40 mesh) and light yellow in water (Figure S1). The sulfonated macroreticular resin (D001), i.e., sulfonated copolymer of styrene and divinylbenzene, is bead shaped (25 40 mesh) and light brown in water (Figure S2). The resins were kindly offered by Jiangsu Suqing Water Treatment Engineering Group Co., Ltd. The detailed information on the resins is presented in the Supporting Information (Table S1). Preparation of Resin-Supported Silver Nanoparticles. Carboxylated macroreticular resin (16 mL), silver nitrate (9.0 g), and deionized water (180 mL) were put into a 250 mL flask. After mechanically stirred for 24 h, the resin was rinsed with 200 mL of water for two times, and the resin containing silver ion (0.28 mmol/mL) was obtained. In a typical synthesis, carboxylated macroreticular resin beads (1 mL) containing silver ions were put into the solution of water and ethanol (40 mL, 70 vol % in ethanol), after mechanically stirred for 1 h, sodium hydroxide (0.30 g) dissolved in an additional 10 mL of solution was slowly added to start the reaction. The resin beads turned gray rapidly and then gradually turned black. After 5 h reaction, the resinsupported silver nanoparticles were prepared. All above steps were conducted at room temperature. When the reaction was conducted in other solvents, conditions were identical to the typical synthesis described above, except when pure water was used, in which case sodium Received: June 5, 2011 Revised: November 7, 2011 Published: November 10, 2011 736
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Scheme 1. Formation of Hard Microemulsion in Macroreticular Resin Matrix
Scheme 2. Synthesis of Silver Nanoparticles inside Microdroplets Formed in Carboxylated Macroreticular Resin
borohydride (0.30 g) was used as the reducing agent instead of sodium hydroxide.
three-dimensions meshes available in the bulk of a given polymer particle.10 However, when macro- and mesopores exist in a polymer matrix, the metal atoms and small nanoparticles may move into them, causing metal maldistribution and agglomeration, an issue that is not addressed by Corain’s theory.16b It must be borne in mind that in functional polymers the functional groups are mostly located inside the swollen polymer matrix, so that even when permanent pores are present only a small fraction of the functional groups are truly positioned on the pore walls.15 Accordingly, the metal ions immobilized by the functional groups are mostly distributed inside the swollen polymer matrix. If the migration of reactant atoms inside the polymer matrix is restricted during the nanoparticle synthesis, the maldistribution and agglomeration of nanoparticles can be effectively prevented. On the basis of this point, we put forward our strategy for controllable synthesis of nanoparticles inside functional polymers. The strategy is based on the creation of liquid nanoreactors inside polymer matrix, as is depicted in Scheme 1. When the mixture of water and organic solvent enters a functional polymer, the liquid molecules will partition differently. Water tends to concentrate in the microdomains rich in hydrophilic functional groups, forming water-enriched cores, whereas organic solvent tends to enrich near the alkyl polymer skeleton, forming organic liquid enriched outer layers. From a macroscopic view, the swollen polymer matrix is divided into numerous “microdroplets”, resembling a frozen water-in-oil microemulsion. We define such a structure as a “hard microemulsion”. The water-enriched microdroplets may act as templates for synthesizing inorganic nanoparticles, and we define this synthetic approach as the “hard microemulsion method”. Controllable Synthesis of Resin-Supported Silver Nanoparticles. To test the above strategy, we performed a self-assembly process based on carboxylated macroreticular resin (D113). Silver ions were dispersed into the resin matrix by ion exchange reaction. The resin containing silver ions was put into the solution of ethanol and water. Because of different affinity, water concentrates in carboxyl focused sites, forming water (carboxyl) cores, whereas ethanol enriches near the alkyl polymer skeleton, forming ethanol (alkyl) outer layers. When the partitioning process is complete, the hard microemulsion is formed in the resin matrix. Scheme 2 shows the synthesis of silver nanoparticles in the microdroplets. When sodium hydroxide is added to initiate the reaction (I), the silver ions are quickly converted into silver oxide and separate from the carboxyl groups; then the silver oxide is gradually reduced to silver atoms by ethanol,17 and the silver atoms self-assemble into silver nanoparticles in the water (carboxyl) cores (II). The silver nanoparticles generated in situ are immobilized by the surrounding carboxyl groups and alkyl skeleton.
Preparation of Resin-Supported Platinum Nanoparticles. Sulfonated macroreticular resin (5 mL), platinum tetrachloride (1.0 g), and water (40 mL) were put into a 50 mL flask. After mechanically stirred for 72 h, the resin was rinsed with 40 mL of water for two times, and the resin containing platinum ion (0.06 mmol/mL) was obtained. In a typical synthesis, macroreticular resin (1 mL) containing platinum ions was put into the solution of water and ethanol (40 mL, 60 vol % in ethanol), after being mechanically stirred for 15 h, sodium borohydride (0.30 g) dissolved in an additional 10 mL of solution was slowly added to start the reaction. After 5 h reaction, the resin-supported platinum nanoparticles were prepared. When the reaction was carried out in water, water was used as solvent instead of the mixed solution; other conditions were identical to the typical synthesis. All above steps were conducted at room temperature. Characterization. Images of metal nanoparticles were taken on a JEM-2011 transmission electron microscope (TEM). The size and morphology of the resin beads were characterized by a Hitachi S-4300 scanning electron microscope (SEM). The global distribution of silver nanoparticles in the resin beads was analyzed by energy dispersive X-ray spectroscopy (EDS) using a Hitachi S-4300 SEM. The metal content in resins was detected via IRIS Intrepid II inductively coupled plasma atomic emission spectrometry (ICP-AES).
’ RESULTS AND DISCUSSION Formation of “Hard Microemulsion”. Functional polymers are available as two major categories: gel-type and macroreticular ones.9a Gel-type resins (usually lightly cross-linked, 2 8%) do not possess any porosity in the dry state, but they develop an extensive nanoporosity in the swollen state. Macroreticular polymers (higher cross-linking degrees, 8 20%) do possess a permanent porosity even in the dry state. Besides the nanoporosity in swollen state, macroreticular polymers possess plentiful macro- and mesopores as well as good mechanical strength, offering them better mass/heat transfer properties and practicability. Corain’s group, with an endeavor lasting more than 15 years, made fundamental contributions to preparation and application of gel-type polymer supported metal nanoparticles.16b In Corain’s theory, swollen gel can be viewed as a three-dimensional polymer chain framework filled with a liquid;16a metal atoms are generated isotropically in every domain of the polymer framework, and they tend to freely aggregate to growing metal nanoparticles that can move through the spheroidal volume until the nanoparticle reaches the largest mesh available in that volume. At this point the size control inside the spheroidal space volume is achieved. Finally, the size of the observed metal nanoparticles will be dictated by the size of the largest and most abundant 737
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Figure 3. TEM images of carboxylated macroreticular resin supported silver nanoparticles prepared before (a) and after (b) hard microemulsion formation.
maldistribution and agglomeration of silver nanoparticles were observed. Many particles of medium (20 50 nm) and large (near 100 nm) size are distributed in the polymer support (Figure 2a,c). The silver atoms evidently migrated during the reduction process in solution and finally redistributed according to the size of the compartments in the polymer. For gel-type resins, these compartments are of uniform size; thus, the metal aggregates in geltype resins are controllable in size.10,11 However, macroreticular resins possess both small compartments in the polymer matrix and larger compartments (i.e., mesopores and macropores). During the reduction process, metal atoms are free to migrate into large compartments, resulting in lower occupancy of the smaller compartments. We can imagine such migration process from Figure 2b,d. Silver atoms are generated in every domain of the polymer framework, and they aggregate freely to form small nanoparticles. The small nanoparticles continue to move through the polymer chain meshes and further aggregate to form larger nanoparticles, until the size of the nanoparticle exceeds the largest mesh size of the microdomains in the polymer matrix. As a result, some of the nanoparticles reside in the polymer matrix and their sizes are controlled by the polymer chain framework, whereas a considerable part of the nanoparticles move into macro- and mesopores, causing maldistribution and agglomeration. The formation of a hard microemulsion effectively prevents the maldistribution and agglomeration. Hence, we can conclude that the hard microemulsion plays a key role in controllable synthesis of nanoparticles inside the functional polymer. To further consolidate the above conclusion, we conducted two additional experiments. The chemicals in the experiments were identical and used in equal amounts, and the operations in both experiments were similar to the typical synthesis. The only difference between the two experiments was the sequence of solvent addition (Figure 3), where the hard microemulsion can form prior to nanoparticle formation only in the second case. Silver nanoparticles generated in the two experiments differ greatly in distribution and monodispersity. Such distinction originates from the formation process of the hard microemulsion. In the former experiment, the formation process had not begun when sodium hydroxide was added to start the reaction; thus, silver atoms can move randomly, causing maldistribution
Figure 1. TEM images (a, b) and size distribution (c) of carboxylated macroreticular resin supported silver nanoparticles prepared by the hard microemulsion method.
Figure 2. TEM images of carboxylated macroreticular resin supported silver nanoparticles prepared in ethanol (a, b) and water (c, d).
Figure 1 shows typical TEM images of the resin-supported silver nanoparticles prepared by the hard microemulsion method. The silver nanoparticles are uniformly distributed in the resin support (Figure 1a). They are round shaped (Figure 1b) and 2 8 nm in size (Figure 1c). Furthermore, EDS mapping data indicate good distribution of silver nanoparticles throughout the resin beads (Figure S3). The Role of the Hard Microemulsion. To verify the role of hard microemulsion, we removed the water cores of the microdroplets and conducted above experiments in ethanol. Separately, we also eliminated the ethanol outer layers of the microdroplets by conducting the experiment in water (sodium borohydride as reducing agent). The results of both experiments are shown in Figure 2. Without the hard microemulsion, severe 738
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the experiment was conducted without the formation of a hard microemulsion. Consequently, the agglomeration of metal nanoparticles in macroporous polymers is spontaneous during the synthesis, and the hard microemulsion is essential for controlling their size and distribution.
Figure 4. TEM images of carboxylated macroreticular resin supported silver nanoparticles prepared in the water ethanol solvents of different compositions: (a) 40 vol % in ethanol and (b) 90 vol % in ethanol.
’ CONCLUSIONS In summary, we have put forward the concept of a hard microemulsion, which can be achieved by differential partitioning of water and organic solvent molecules inside functional polymers. The water-enriched microdroplets may act as templates for synthesizing inorganic nanoparticles inside functional polymers. We verified the role of the hard microemulsion by fabricating silver and platinum nanoparticles supported by carboxylated and sulfonated macroreticular resin, respectively. The good designability and tunability of polymer chain structure, porosity, and functional groups offer numerous opportunities for this strategy. ’ ASSOCIATED CONTENT
bS
Supporting Information. Detailed information on polymer support, photos, SEM images, and EDS mapping data included. This material is available free of charge via the Internet at http:// pubs.acs.org.
Figure 5. TEM images of sulfonated macroreticular resin supported platinum nanoparticles prepared by hard microemulsion method (inset: size distribution histogram) (a) and prepared in water (b).
and agglomeration (Figure 3a). Whereas in the latter case, formation of the hard microemulsion occurred before the reaction, thus silver atoms were confined in the preformed microdroplets and self-assembled into uniformly distributed and monodisperse nanoparticles (Figure 3b). Influence of Solvent Composition. The formation of a hard microemulsion is based on the differential partitioning of solvent molecules inside functional polymers; thus, the solvent composition is expected to be an important factor that influences the final structure and its ability to control nanoparticle formation. To test this hypothesis, we synthesized silver nanoparticles supported by the carboxylated macroreticular resin with various solvent compositions. The results show that the solvent composition affects the size distribution of the silver nanoparticles formed inside the resin, with the suitable solvent composition being between 50 and 80 vol % in ethanol. When the synthesis was conducted in solvents beyond this composition range, the monodispersity of the silver nanoparticles inside the resin deteriorated (Figure 4). There is a strong possibility that the hard microemulsion cannot form completely at those conditions; thus, the silver maldistribution and agglomeration cannot be effectively restricted. Controllable Synthesis of Resin-Supported Platinum Nanoparticles. To demonstrate the broad feasibility of the hard microemulsion method, we also prepared well-distributed platinum nanoparticles in sulfonated macroreticular resin (D001). In addition, the platinum content in the sulfonated resin was much lower than the silver content in the former carboxylated resin. The platinum nanoparticles (about 2 5 nm) prepared by the hard microemulsion method are uniformly scattered in the resin support (Figure 5a), whereas they form aggregates when the synthesis was conducted in water (Figure 5b). Although the platinum nanoparticles are sparse in resin support, they are clearly smaller than the agglomerated particles that formed when
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]; Tel: +86-10-62560247; Fax: +8610-62559373.
’ ACKNOWLEDGMENT We thank the financial support from Center for Molecular Science, Chinese Academy of Sciences (CMS-Y200725). The resins used in the experiments were kindly offered by Jiangsu Suqing Water Treatment Engineering Group Co., Ltd. We are grateful for the help of Cheng Hu from that corporation. ’ REFERENCES (1) Wang, X.; Zhuang, J.; Peng, Q.; Li, Y. Nature 2005, 437, 121–124. (2) Cushing, B. M.; Kolesnichenko, V. L.; O’Connor, C. J. Chem. Rev. 2004, 104, 3893–3946. (3) Masala, O.; Seshadri, R. Annu. Rev. Mater. Res. 2004, 34, 41–81. (4) Daniel, M. C.; Astruc, D. Chem. Rev. 2004, 104, 293–346. (5) Laurent, S.; Forge, D.; Port, M.; Roch, A.; Robic, C.; Elst, L. V.; Muller, R. N. Chem. Rev. 2008, 108, 2064–2110. (6) Pecher, J.; Mecking, S. Chem. Rev. 2010, 110, 6260–6279. (7) Lee, Y. S. Self-Assembly and Nanotechnology: A Force Balance Approach; John Wiley & Sons, Inc.: Hoboken, NJ, 2008; Chapter 3. (8) Taleb, A.; Petit, C.; Pileni, M. P. Chem. Mater. 1997, 9, 950–959. (9) (a) Corain, B.; Centomo, P.; Lora, S.; Kralik, M. J. Mol. Catal. A: Chem 2003, 204 205, 755–762. (b) Suzuki, D.; Kawaguchi, H. Langmuir 2005, 21, 8175–8179. (c) Lu, Y.; Mei, Y.; Drechsler, M.; Ballauff, M. Angew. Chem., Int. Ed. 2006, 45, 813–816. (d) Xing, R.; Liu, Y.; Wu, H.; Li, X.; He, M.; Wu, P. Chem. Commun. 2008, 47, 6297–6299. (e) Corain, B.; Zecca, M.; Canton, P.; Centomo, P. Philos. Trans. R. Soc. London, A 2010, 368, 1495–1507. (10) Artuso, F.; D’Archivio, A. A.; Lora, S.; Jerabek, K.; Kralik, M.; Corain, B. Chem.—Eur. J. 2003, 9, 5292–5296. (11) Corain, B.; Jerabek, K.; Centomo, P.; Canton, P. Angew. Chem., Int. Ed. 2004, 43, 959–962. 739
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