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Synthesis of Atypical Nanoparticles by the Nanostructure in Thin Films of Triblock Copolymers Seung-Min Jeon,† Kyo-Young Jang,† Sung Hwa Lee,† Hae-Woong Park,‡ and Byeong-Hyeok Sohn*,† Department of Chemistry, NANO Systems Institute, Seoul National UniVersity, Seoul 151-747, Korea, and Department of Chemistry, Pohang UniVersity of Science and Technology, Pohang 790-784, Korea ReceiVed May 21, 2008. ReVised Manuscript ReceiVed July 14, 2008 We report the synthesis of atypical nanoparticles in donut shape with or without additional spherical nanoparticles attached on them by using the donut-like nanostructure formed in a thin film of triblock copolymers. In a high-humidity condition, a spin-coated film of triblock copolymer had donut-like holes consisting of the periphery and the center. By selective coordination of precursors of nanoparticles to the periphery of the holes, donut-like oxide nanoparticles were synthesized by oxygen plasma treatment on the film. Moreover, we were able to attach spherical nanoparticles on the donut-like nanoparticles by incorporating the other type of precursors to the center of the holes. Thus, beyond the synthesis of typical spherical nanoparticles, the results here extend potentials of the block copolymer approach to control the shape and complexity of nanoparticles.
Introduction Block copolymers spontaneously assemble into nanometersized domains that have been used for a variety of nanotechnological applications, such as nanolithography and nanopatterning.1,2 As an effective nanostructured template to synthesize and organize nanoparticles in a controlled manner, self-assembled nanodomains of diblock copolymers have been employed. For example, nanoparticles were selectively synthesized or incorporated within spherical, cylindrical, or lamellar nanodomains of diblock copolymers.3-5 The fabrication of ultrahigh density nanoarrays was also realized via deposition of inorganic materials into the nanopores produced by the selective removal of the cylindrical nanodomains perpendicular to the substrate in the thin film of diblock copolymers.6 Selective inclusion of nanoparticles in diblock copolymers also induced the morphological change of nanodomains of copolymers.7,8 In addition, nanoscale micelles of diblock copolymers have been employed as templates to synthesize a variety of nanoparticles,9-20 including noble metal (Au11-13 and Pt9), semi* To whom correspondence should be addressed. Telephone: +82-2883-2154. Fax: +82-2-889-1568. E-mail:
[email protected]. † Seoul National University. ‡ Pohang University of Science and Technology.
(1) Park, C.; Yoon, J.; Thomas, E. L. Polymer 2003, 44, 6725. (2) Hamley, I. W. Nanotechnology 2003, 14, R39. (3) Bockstaller, M. R.; Mickiewicz, R. A.; Thomas, E. L. AdV. Mater. 2005, 17, 1331. (4) Haryono, A.; Binder, W. H. Small 2006, 2, 600. (5) Chai, J.; Wang, D.; Fan, X.; Buriak, J. M. Nat. Nanotechnol. 2007, 2, 500. (6) Thurn-Albrecht, T.; Schotter, J.; Ka¨stle, G. A.; Emley, N.; Shibauchi, T.; Krusin-Elbaum, L.; Guarini, K.; Black, C. T.; Tuominen, M. T.; Russell, T. P. Science 2000, 290, 2126. (7) Thompson, R. B.; Ginzburg, V. V.; Matsen, M. W.; Balazs, A. C. Science 2001, 292, 2469. (8) Sides, S. W.; Kim, B. J.; Kramer, E. J.; Fredrickson, G. H. Phys. ReV. Lett. 2006, 96, 250601. (9) Fo¨rster, S.; Antonietti, M. AdV. Mater. 1998, 10, 195. (10) Yoo, S. I.; Kwon, J. H.; Sohn, B. H. J. Mater. Chem. 2007, 17, 2969. (11) Spatz, J. P.; Mo¨ssmer, S.; Hartmann, C.; Mo¨ller, M. Langmuir 2000, 16, 407. (12) Glass, R.; Mo¨ller, M.; Spatz, J. P. Nanotechnology 2003, 14, 1153. (13) Yun, S. H.; Yoo, S. I.; Jung, J. C.; Zin, W. C.; Sohn, B. H. Chem. Mater. 2006, 18, 5646. (14) Yoo, S. I.; Sohn, B. H.; Zin, W. C.; An, S. J.; Yi, G. C. Chem. Commun. 2004, 2850. (15) Li, X.; Lau, K. H. A.; Kim, D. H.; Knoll, W. Langmuir 2005, 21, 5212.
conductor (ZnO,14 TiO2,15 and GaN16), and magnetic (Co,17 Fe2O3,18,19 and FePt20) nanoparticles. Moreover, by plasma treatment, diblock copolymer templates were effectively removed to produce pure nanoparticles without copolymer templates.11-13 Recently, using triblock copolymers, which show a variety of sophisticated nanostructures,21,22 as well as diblock copolymer micelles, two different types of nanoparticles were synthesized in the specific nanodomain of copolymers.23,24 However, most nanoparticles synthesized with di- or triblock copolymers have a spherical shape.9-20,23,24 In this study, we demonstrate the synthesis of atypical nanoparticles in donut shape with or without additional spherical nanoparticles attached on them with the assistance of the donutlike nanostructure formed in a thin film of triblock copolymers. The copolymer film spin-coated in a high humidity condition had donut-like holes consisting of the periphery and the center. By selective coordination of inorganic salts, which are the precursor of nanoparticles, to the periphery of the holes, donutlike oxide nanoparticles were synthesized by oxygen plasma treatment on the film with the preservation of the nanostructure. Moreover, we were able to synthesize and attach spherical nanoparticles on the donut-like nanoparticles by incorporating the other type of inorganic salts to the center of the holes. Therefore, the nanostructure of triblock copolymers can enable the control over the shape and complexity of nanoparticles beyond the synthesis of typical spherical nanoparticles. (16) Bhaviripudi, S.; Qi, J.; Hu, E. L.; Belcher, A. M. Nano Lett. 2007, 7, 3512. (17) Boyen, H. G.; Ka¨stle, G.; Zu¨rn, K.; Herzog, T.; Weigl, F.; Ziemann, P.; Mayer, O.; Jerome, C.; Mo¨ller, M.; Spatz, J. P.; Garnier, M. G.; Oelhafen, P. AdV. Funct. Mater. 2003, 13, 359. (18) Yun, S. H.; Sohn, B. H.; Jung, J. C.; Zin, W. C.; Lee, J. K.; Song, O. Langmuir 2005, 21, 6548. (19) Bennett, R. D.; Miller, A. C.; Kohen, N. T.; Hammond, P. T.; Irvine, D. J.; Cohen, R. E. Macromolecules 2005, 38, 10728. (20) Ethirajan, A.; Wiedwald, U.; Boyen, H. G.; Kern, B.; Han, L.; Klimmer, A.; Weigl, F.; Ka¨stle, G.; Ziemann, P.; Fauth, K.; Cai, J.; Behm, R. J.; Romanyuk, A.; Oelhafen, P.; Walther, P.; Biskupek, J.; Kaiser, U. AdV. Mater. 2007, 19, 406. (21) Bates, F. S.; Fredrickson, G. H. Phys. Today 1999, 52, 32. (22) Ludwigs, S.; Bo¨ker, A.; Voronov, A.; Rehse, N.; Magerle, R.; Krausch, G. Nat. Mater. 2003, 2, 744. (23) Aizawa, M.; Buriak, J. M. J. Am. Chem. Soc. 2006, 128, 5877. (24) Koh, H. D.; Kang, N. G.; Lee, J. S. Langmuir 2007, 23, 11425.
10.1021/la801568g CCC: $40.75 2008 American Chemical Society Published on Web 08/27/2008
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Figure 1. AFM images of a thin film of PS-P2VP-PEO: (a) height mode and (b) phase mode.
Figure 2. TEM images of a thin film of PS-P2VP-PEO (a) without staining and (b) with I2 staining.
Experimental Section
average diameter of holes was 70 nm. In the phase mode image (Figure 1b), however, each hole was apparently made up of the periphery and the center. The diameter of the central part in the hole was about 30 nm. Considering the molecular weight of each block and the sequence of the blocks in PS-P2VP-PEO copolymers, the majority of the continuous film can be the PS block and the hole can consist of the P2VP block and the PEO block. The structure in the film was further investigated by TEM with or without staining the P2VP block by I2 (Figure 2). In the image without staining shown in Figure 2a, the holes appeared as bright regions because of the thickness difference. However, the dark ring around the hole was observed after the selective staining of the P2VP block, implying that the periphery of the hole corresponds to the P2VP block. Thus, the rest of the hole, i.e., the central part of the hole, should match with the PEO block. In the phase-mode AFM image of Figure 1b, the high phase angle of the central part in the hole could result from the crystalline phase of the PEO block, the melting peak of which was observed in DSC results (Figure 8). On the basis of the AFM and TEM results, we can draw a schematic of the internal morphology in a PS-P2VP-PEO thin film (Figure 3). The continuous matrix is made up of the largest PS block of triblock copolymers. Each hole consists of the P2VP block and the PEO block. Considering the sequence of three blocks in PS-P2VP-PEO copolymers, the hole has to be a donut-like nanostructure as shown in the schematic of Figure 3. The donut-like morphology can also be supported by the shape of nanoparticles synthesized from the nanostructures, which will be discussed along with Figure 6. The donut-like holes could result from the cavity formation of P2VP and PEO blocks by the spin coating in a high-humidity condition. Because both P2VP and PEO blocks can be swollen by water, kinetically frozen holes could be formed by evaporation of water after vitrification of the PS block by fast evaporation of the solvent during the spin coating. A similar cavitation in poly(acrylic acid) (PAA) domains of PS-PAA diblock copolymers because of the swelling by water was reported.25 We incorporated inorganic salts selectively with one of the P2VP and PEO blocks in the donut-like nanostructure formed in a PSP2VP-PEO thin film. To load inorganic salts into the block, the copolymer film on the substrate was immersed into an ethanol solution of inorganic salts. First, as shown in the schematic of Figure 3, FeCl3 was coordinated selectively to the pyridine units of the P2VP block. In a TEM image of Figure 4, the periphery of the holes became dark after loading of FeCl3 as in the case of the P2VP block stained with I2 (Figure 2b), implying that FeCl3 coordinates only to the P2VP block. In FTIR spectra before and after loading of FeCl3 in a PS-P2VP-PEO film shown in Figure
Polystyrene-block-poly(2-vinyl pyridine)-block-poly(ethylene oxide) (PS-P2VP-PEO) was purchased from Polymer Source, Inc. The number average molecular weights of PS, P2VP, and PEO were 75 000, 21 000, and 16 000 g/mol, respectively. The polydispersity index was 1.08. Silicon wafers were cleaned in a piranha solution [70:30 (v/v) of concentrated H2SO4 and 30% H2O2] at 90 °C for 30 min, thoroughly rinsed with deionized water several times, and then blown dry with nitrogen. Substrates after cleaning were immediately used for spin coating of the copolymer. A PS-P2VP-PEO thin film was spin-coated at 2000 rpm onto a clean silicon wafer from a 1.0 wt% tetrahydrofuran (THF) solution at room temperature. The humidity (60-70%) during spin coating was controlled by placing water containers around the spin coater in a closed chamber. To remove residual solvents, a PS-P2VPPEO thin film was dried for longer than 12 h in a vacuum oven. Surface morphologies of PS-P2VP-PEO thin films were investigated by atomic force microscopy (AFM, Nanoscope IIIA, Digital Instrument) in tapping mode with Si cantilevers. To load inorganic salts into PS-P2VP-PEO thin films, a copolymer film on the substrate was immersed into a 1.5 wt % ethanol solution of FeCl3 or LiAuCl4 for 5-10 min and then thoroughly rinsed with deionized water several times. For sequential loading of two inorganic salts, a PS-P2VP-PEO thin film was first immersed into a 1.5 wt % ethanol solution of FeCl3 for 10 min and then into a 1.5 wt % ethanol solution of LiAuCl4 for 5 min. PS-P2VP-PEO thin films containing inorganic salts were treated with oxygen plasma (100 W, 2.0 × 10-2 Torr) for 10 min to synthesize nanoparticles with complete removal of copolymers. PS-P2VP-PEO thin films before and after loading of inorganic salts were examined by transmission electron microscopy (TEM). To prepare samples for TEM, a thin layer carbon was first evaporated onto PS-P2VP-PEO thin films on the substrates and covered with a 25% aqueous solution of poly(acrylic acid) (PAA). After drying in a vacuum oven, the PAA/carbon/PS-P2VP-PEO composite was peeled off from the substrate and floated on deionized water, with the PAA side down. When the PAA layer was completely dissolved, the floating film was collected onto a carbon-coated grid. TEM was performed on a Hitachi H-7600 operating at 100 kV. Nanoparticles synthesized from PS-P2VP-PEO thin films were also visualized by field-emission scanning electron microscopy (FE-SEM, S-4300 Hitachi at 15 kV) after Pt coating on the sample. Differential scanning calorimetry (DSC, Perkin-Elmer, DSC-7) was performed for thermal analysis of PS-P2VP-PEO bulk films with and without inorganic salts. Copolymer films before and after loading of inorganic salts were characterized by Fourier transform infrared (FTIR) spectroscopy (Mattson Infinity Gold). For DSC and FTIR samples, bulk films of PS-P2VP-PEO (50-80 µm thick) with and without inorganic salts were cast from their toluene solutions at room temperature and dried for 24 h in a vacuum oven.
Results and Discussion Figure 1 shows an AFM image of a PS-P2VP-PEO thin film (∼70 nm thick) spin-coated in a high-humidity condition (60-70%), in which dark circular holes were observed. The
(25) Boontongkong, Y.; Cohen, R. E. Macromolecules 2002, 35, 3647.
Synthesis of Atypical Nanoparticles
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Figure 3. Schematic of the nanostructure in a thin film of PS-P2VP-PEO and the fabrication of atypical nanoparticles from the nanostructure.
Figure 4. TEM image of a thin film of PS-P2VP-PEO loaded with FeCl3.
Figure 5. FTIR spectra of PS-P2VP-PEO films containing inorganic salts: (a) without salts and with (b) FeCl3, (c) LiAuCl4, (d) FeCl3, and LiAuCl4.
Figure 6. Images of donut-like iron oxide nanoparticles: (a) FE-SEM and (b) AFM in height mode.
5, the absorption bands at 1435 and 1570 cm-1 (marked by the up arrows) decreased with loading of FeCl3, which correspond to the stretching vibration in noncoordinated pyridine units of
Figure 7. TEM image of a thin film of PS-P2VP-PEO loaded with LiAuCl4.
the P2VP block.26 In addition, the absorption bands at 1540 and 1620 cm-1 (marked by the down arrows) appeared, which match up with the vibration in coordinated pyridine units of the P2VP block.26 Thus, the FTIR result also indicates the selective loading of FeCl3 into the P2VP block. By oxygen plasma treatment on a PS-P2VP-PEO thin film loaded with FeCl3 in the P2VP block, iron oxide nanoparticles were synthesized as shown in the schematic of Figure 3. In our previous report, the oxide nanoparticle synthesized with FeCl3 in diblock copolymers by oxygen plasma treatment was characterized as γ-Fe2O3.18 As shown in Figure 6, nanoparticles were formed over the substrate and some of them were fused together. Apparently, each nanoparticle has a donut-like morphology. The diameter and height of the nanoparticle were about 70 and 20 nm, respectively. We note that the size of nanoparticles could be overestimated because of Pt coating to avoid the charging in FE-SEM. Thus, the morphology of the nanoparticles indicates that atypical nanoparticles in donut shape are synthesized with the preservation of the nanostructure in a PS-P2VP-PEO thin film. We selectively loaded LiAuCl4 into the PEO block in a PSP2VP-PEO thin film by dipping the film into an ethanol solution of LiAuCl4 as in the case of loading of FeCl3. LiAuCl4 can be coordinated to the PEO block by formation of a crown-ether-like coordination between Li+ and ethylene oxide units with AuCl4as a counterion.27 Figure 7 shows a TEM image of the film loaded with LiAuCl4, in which black dots of Au nanoparticles (∼20 nm in diameter) appear in the holes because AuCl4- can be easily reduced to Au nanoparticles by daylight or even by the electron beam in TEM.11-13 Some of the holes did not have Au nanoparticles presumably because Au nanoparticles or their (26) Lyons, A. M.; Vasile, M. J.; Pearce, E. M.; Waszczak, J. V. Macromolecules 1988, 21, 3125. (27) Spatz, J. P.; Roescher, A.; Mo¨ller, M. AdV. Mater. 1996, 8, 337.
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Figure 10. FE-SEM image of donut-like iron oxide nanoparticles with spherical gold nanoparticles. Figure 8. DSC curves of PS-P2VP-PEO films containing inorganic salts: (a) without salts and with (b) FeCl3 and (c) LiAuCl4.
Figure 9. TEM image of a thin film of PS-P2VP-PEO loaded with FeCl3 and LiAuCl4.
precursors could be washed away during the cleaning step after loading of LiAuCl4. The PS block appeared as the continuous gray domain because of the thickness difference between the holes and the continuous film in the TEM image. FTIR and DSC results support the selective loading of LiAuCl4 into the PEO block of triblock copolymers. In the FTIR spectra before and after loading of LiAuCl4 (Figure 5), the characteristic bands of the pyridine unit of the P2VP block were not changed in contrast to the case of FeCl3, indicating that LiAuCl4 is not coordinated to the P2VP block. In the DSC result shown in Figure 8c, the melting peak of the PEO crystalline was not observed in the film loaded with LiAuCl4 because selectively loaded LiAuCl4 in the PEO block could impede the crystallization of the PEO block. In contrast, the melting peak of the PEO crystalline was obtained in the film loaded with FeCl3 (Figure 8b) because FeCl3 was not coordinated to the PEO block but only to the P2VP block. We note that the melting point was somewhat depressed in the film with FeCl3 compared to that in the pristine film presumably because the FeCl3-coordinated P2VP block could harm the formation of PEO crystals, compared to the pure P2VP block without the salt. By sequential loading of FeCl3 and LiAuCl4, as shown in the schematic of Figure 3, we incorporated both FeCl3 and LiAuCl4 in a PS-P2VP-PEO thin film. First, FeCl3 and then LiAuCl4 were loaded by dipping the film into the corresponding solution. In the TEM image of Figure 9, the P2VP block coordinated with FeCl3 appeared as a dark ring around the hole as the case of single loading of FeCl3. In the FTIR spectrum after loading of both salts (Figure 5d), the change of the characteristic bands because of the coordination of FeCl3 to the pyridine unit was also observed. Thus, the TEM image and the FTIR spectrum indicate that the selective loading of FeCl3 to the P2VP block was maintained in the film with both FeCl3 and LiAuCl4. Au nanoparticles in the hole, mostly close to the periphery, were
also observed in the TEM image of Figure 9. Their size (∼10 nm in diameter) is smaller than that in the film with only LiAuCl4, presumably because the coordination of FeCl3 in the P2VP periphery of the hole could affect the loading of LiAuCl4 into the PEO center of the hole. To synthesize iron oxide and gold nanoparticles, the PS-P2VPPEO film with FeCl3 in the P2VP block and LiAuCl4 in the PEO block was treated by oxygen plasma, as shown in the schematic of Figure 3. It was reported that pure gold nanoparticles are formed by oxygen plasma treatment on diblock copolymers containing gold precursors because gold oxide is not stable and turns to pure gold at room temperature.11-13 In the FE-SEM image of Figure 10, donut-like iron oxide nanoparticles were formed, as in the case of the single loading of FeCl3 shown in Figure 6. Additionally, on the donut-like nanoparticle, a spherical gold nanoparticle was clearly visible, although some of donut-like particles did not have a gold nanoparticle because of the absence of gold precursors in some holes as shown in Figure 9. Therefore, we were able to synthesize atypical nanoparticles consisting of donut-like iron oxide nanoparticles and spherical gold nanoparticle with the assistance of the nanostructure in the triblock copolymer thin film. We note that Au nanoparticles appeared bigger than those in the TEM image (Figure 9) because of the Pt coating for FESEM imaging. However, donut-like nanoparticles were somewhat smaller than those synthesized with only FeCl3, even considering the Pt coating, presumably because some of FeCl3 loaded in the P2VP block could be washed away to the ethanol solution of LiAuCl4 during the second loading of LiAuCl4 to the PEO block.
Conclusions We demonstrated the synthesis of atypical nanoparticles with the use of the donut-like nanostructure in thin films of PS-P2VPPEO triblock copolymers. Because of the cavitation, the film had the kinetically frozen donut-like holes consisting of the P2VP periphery and the PEO center. By selective coordination of FeCl3 to the P2VP periphery of the hole and oxygen plasma treatment on the film, donut-like iron oxide nanoparticles were synthesized with the preservation of the nanostructure. Moreover, from the PS-P2VP-PEO film with FeCl3 in the P2VP block and LiAuCl4 in the PEO block, we were able to synthesize nanoparticles consisting of donut-like iron oxide nanoparticles and spherical gold nanoparticle. Thus, beyond the synthesis of typical spherical nanoparticles, the demonstration in this study extends potentials of the block copolymer approach to control the shape and complexity of nanoparticles. Acknowledgment. This work was supported by the Korea Science and Engineering Foundation (Grant R01-2007-00010068-0). LA801568G
