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A Chemical Reactor for Hierarchical Nanomaterials with Tunable Structures: A Metal-Triggered Reaction in the Confined Heat Chamber Hyeah Goh,†,‡ Ha-Jin Lee,† Bora Nam,†,‡ Young Boo Lee,† and Won San Choi*,§ †
Jeonju Center, Korea Basic Science Institute (KBSI), 664-14 Dukjin-dong 1-ga, Dukjin-gu, Jeonju 561756, Republic of Korea Graduate School of Analytical Science and Technology, Chungnam National University, 79 Daehangno, Yuseong-gu, Daejeon 305764, Republic of Korea § Department of Applied Chemistry, Hanbat National University, San 16-1, Dukmyung-dong, Yuseong-gu, Deajeon 305719, Republic of Korea ‡
bS Supporting Information ABSTRACT: A novel protocol for the synthesis of core@shell particles with transformable core structures has been introduced, and its use as a chemical reactor for hierarchical nanomaterials with tunable properties has been demonstrated. A metal, metal/polymer, or bimetallic hollow structure was incorporated into the interior of silica capsules via controlled heat treatment of the inorganic-coated polymer particles. A micrometer-sized capsule was used as a reaction chamber to contain and sustain heat, and metal NPs were used as a controller for the transformation of polymer cores. By varying the thermal conductivities of the metal NPs, it was also possible to confer synthesis of hatlike, UFO-like, or bimetallic hollow structures within the capsule. This novel method for core@shell particles is useful for synthesizing nanomaterials with controlled structures and properties. Furthermore, Pt/Au capsule@SiO2 and Au/ polymer@SiO2 showed excellent catalytic properties for the transformation of nitrophenol to aminophenol. This approach could be used to tune the structures and properties of various kinds of polymer matrices. KEYWORDS: core@shell, hierarchical nanostructure, catalytic reactor, hollow capsule, metal nanoparticles
’ INTRODUCTION Core-in-shell capsule structures (denoted as core@shell structures) equipped with a separate core in their cavity have attracted considerable attention as a new class of hierarchically structured materials due to their unique structure, which is characterized by an interstitial space between the core and shell. These structures have great potential for application in various fields including chemical reactors, sensors, catalysts, batteries, drug carriers, and heavy metal ion removers.1 11 Several approaches have been developed for the synthesis of core@shell structures. One typical method is a multiple coating of desired materials onto a preformed core and following removal of a middle layer.12 15 An alternative strategy is a preshell/postcore approach, starting from hollow capsules with further synthesis of cores inside the capsules.3,6,16,17 Unfortunately, most research in this area has been directed toward the development of new synthetic approaches for core@shell structures. Efforts for the control of both the core and the shell structure are quite limited. Taking into account the unique characteristics due to the presence of the distinctive cores, efforts for the control of core structure would be of interest for many applications. The use of hollow capsules as chemical reactors provides an opportunity to explore advanced materials.18 20 Materials synthesized within the spatially confined microcontainers often r 2011 American Chemical Society
show unique properties that cannot be obtained through synthesis in a bulk space.21,22 Polyelectrolyte multilayers (PEMs) are considered as excellent building components for nanoengineered materials.23 32 With PEMs, it is very feasible to prepare chemical reactors with confined space as a result of their ease of successive adsorption over charged particles. In addition, PEM-based hollow capsules could be useful for the synthesis of nanomaterials with novel functionalities. Owing to optimistic expectations on reactions within a confined space, the core@shell structures have been used as nanoreactors as well.1,2,10 However, only a limited number of works demonstrated the possibility of tuning of the properties of the products within a confined space. Thus, study on the reactions within a confined space is highly desirable, and control of the reactions is a great challenge for synthesis of novel materials with desired properties. The goal of the present study is to control the transformation of a polymer core by heat flux within a confined shell space. To do so, a complex structure of a container maintaining heat flux without leakage and a messenger transferring heat flux to the polymer core was designed. Herein, we describe a novel protocol for synthesis of core@shell particles Received: August 2, 2011 Revised: October 4, 2011 Published: October 18, 2011 4832
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with transformable core and its use as a chemical reactor for hierarchical nanomaterials with tunable properties.
’ EXPERIMENTAL SECTION Materials. Weakly cross-linked melamine-formaldehyde (MF) particles (10 wt %, 1.87 μm) were purchased from Microparticle (GmbH). Poly(allylamine hydrochloride) (PAH, Mw 56 000), poly(styrene sulfonate) (PSS, Mw 70 000), silver nitrate (AgNO3, 99.99%), chloroauric acid (HAuCl4), chloroplatinic acid (H2PtCl6), palladium nitrate (N2O6Pd), and sodium borohydride (NaBH4) were purchased from Aldrich. All commercial materials were used without further purification. The water used in all experiments was prepared in a three-stage Milli-Q plus 185 purification system and had a resistivity higher than 18 MΩ cm. Preparation of PEMPs. 1.4 mL of a PSS solution (2 mg/mL) was added to 0.13 mL of an aqueous suspension of positively charged MF particles (10 wt %, 1.87 μm). The dispersion was vigorously agitated by shaking for 15 min to allow the PEs to adsorb onto the MF particles. The resulting dispersion was centrifuged at 10 000g for 3 min; subsequently, the supernatant was removed and 1.4 mL of water was added. The centrifugation washing dispersion cycle was repeated three times. After the formation of a PSS layer on the MF cores, PAH (1.4 mL of a 2 mg/mL solution) layers were deposited using an LbL (layer-by-layer) assembly technique. The adsorption and rinsing steps were repeated until the desired number of layers was obtained. In total, nine PE multilayers were deposited on the MF particles. Preparation of Hatlike Au/Polymer@SiO2. 1.4 mL of a HAuCl4 solution (5.6 mg/1.5 mL) was added to 0.12 mL (2 wt %) of an aqueous suspension of PEMPs. The dispersion was vigorously agitated on a shaking apparatus for 4 h to allow Au ions to adsorb to the amine groups of the PEs. The dispersion was centrifuged at 8000g for 1 min, the supernatant was removed, and 1.4 mL of water was added. This rinsing step was repeated three times. The Au ions within the PEMs were reduced by treatment with a 1 mM NaBH4 solution for 30 min. After rinsing three times, AuNP-embedded PEMPs were redispersed in a solution containing 1 mL isopropyl alcohol, 0.18 mL water, and 0.03 mL NH4OH. The resultant solution, after the addition of 0.03 mL tetraethylorthosilicate (TEOS), was vigorously stirred for 30 min. The rinsing step was repeated three times. To form hatlike Au/polymer@SiO2 structures, a drop of sample solution was applied to a silicon wafer, followed by calcination for 15 min at 500 °C. Preparation of Pt@SiO2. 1.4 mL of a H2PtCl6 solution (5.6 mg/ 1.5 mL) was used to prepare PtNP-embedded PEMPs. Details for the synthesis of Pt@SiO2 structures are the same as the above-described procedures. Preparation of Pt/Au Capsule@SiO2. 0.12 mL (2 wt %) of an aqueous suspension of PtNP-embedded PEMPs was redispersed in 1.4 mL of an HAuCl4 solution (5.6 mg/1.5 mL). The dispersion was vigorously agitated on a shaking apparatus for 4 h to adsorb Au ions onto the PtNP-embedded PEMPs. The dispersion was centrifuged at 8000g for 1 min, the supernatant was removed, and 1.4 mL of water was added. This rinsing step was repeated three times. The Au ions within the PEMs were reduced by treatment with a 1 mM NaBH4 solution for 30 min. After rinsing three times, Pt-AuNP-embedded PEMPs were redispersed in a solution containing 1 mL isopropyl alcohol, 0.18 mL water, and 0.03 mL NH4OH. The resulting solution, after the addition of 0.03 mL tetraethylorthosilicate (TEOS), was vigorously stirred for 30 min. The rinsing step was repeated three times. To form Pt Au capsule@SiO2 structures, a drop of sample solution was applied to a silicon wafer, followed by calcination for 15 min at 500 °C. The rate of increasing temperature was 25 °C/min. It was maintained for 15 min after reaching calcination temperature (500 °C) and cooled to room temperature. Catalytic Activity of Core@Shell Structures for Reduction of 4-Nitrophenol. After calcination, core@shell particles deposited
Figure 1. Schematic illustration for the formation of Au/polymer@ SiO2, Pt/Au capsule@SiO2, and Pt@SiO2 structures produced through reaction in the confined space of a heat chamber. on silicon wafer were separated by mild sonication and redispersed in an aqueous solution for catalytic reaction. To investigate the catalytic reduction of 4-nitropheol to 4-AP, 1 mL of water was placed in a quartz cell. 1.5 mL of freshly prepared 4-NP aqueous solution (0.08 mM) and 700 μL of NaBH4 aqueous solution (100 mM) were added to the quartz cell. The mixed 4-NP solution was measured by UV vis spectrometer. 3 μL (3.8 wt %) of core@shell structures was added with gentle shaking. After the addition of the core@shell structures, UV vis spectra of the mixture were monitored at regular intervals to observe the reaction progress. Characterization. FE-TEM/STEM/EDX (field emission-transmission electron microscopy/scanning transmission electron microscopy/ energy-dispersive X-ray) analyses were carried out using a JEOL (JEM2200 FS) microscope operated at 200 kV. FE-SEM micrographs were obtained using Hitachi S-4700 microscopes. UV vis spectra were obtained on a Shimadzu UV vis-near infrared (NIR) spectrophotometer.
’ RESULTS AND DISCUSSION Figure 1 shows a schematic for the synthesis of the core@shell particles with variable cores. PEM-coated particles (PEMPs) were prepared by successive coating of poly(styrene sulfonate) (PSS) and poly(allylamine hydrochloride) (PAH) onto a melamine-formaldehyde (MF) particles. In total, nine PEMs were deposited on the MF particles (MF/(PSS/PAH)4PSS). Other polymer particles such as polystyrene or polymethylmethacrylate (PMMA) could also be used as a sacrificial templates. The PEMs were used to bind metal salt precursors for synthesis of metal nanoparticles such as gold, silver, platinum, and palladium. Metal nanoparticles (NPs) and silica were synthesized onto the PEMPs in sequence by introducing inorganic precursors into the PEMPs and subsequent reduction and hydrolysis of the inorganic species. Subsequently, the resulting PEMPs equipped with inorganic multiple shells were calcined at 500 °C for 15 min. After the heat treatment, the polymer cores were transformed to various kinds of nanostructures such as Au/polymer cores, Pt/ Au bimetallic capsules, and Pt cores inside the SiO2 capsules. Here, the presence of the silica outer shell helps to maintain heat flux without leakage, controlling the transformation of the polymer core within a confined space. Also, the aspect of transformation depends on the abilities of the metal NPs to absorb and transfer the heat flux within a confined reaction chamber. 4833
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Figure 2. Electron microscopy images showing the formation process of Au/polymer@SiO2 structures. SEM images (A) before and (B) after calcination of PEMP/AuNP/SiO2 for 15 min at 500 °C. (C) SEM image of Au/polymer cores obtained after removal of the SiO2 shell. (D) STEM image of part B. STEM images showing the transformation process of Au/polymer cores during calcination at (E) 1, (F) 15, (G) 30, and (H) 60 min. TEM images after calcination of PEMP/SiO2 for (I) 1 h, (J) 2 h, and (K) 3 h at 500 °C.
Figure 3. (A) SEM image after calcination of PEMP/AuNPs without an SiO2 shell (inset: STEM image). (B) STEM image after calcination of PEMP/SiO2 without AuNPs. STEM images after calcination of PEMP/(C) PtNP or (D) AgNP/SiO2. (Calcination conditions: 500 °C for 15 min.)
Electron microscopy images of the resulting silica capsules with Au/polymer core are shown in Figure 2. Figure 2A is an image of the PEMPs after synthesis of AuNPs and SiO2, showing relatively smooth surfaces. After calcination at 500 °C for 15 min, the overall size of the resulting structures was reduced to 90% of the original size and the surface was a bit crushed as a result of shrinkage upon heat treatment. Energy-dispersive X-ray (EDX) measurement demonstrated that the core@shell structures were composed of Au, SiO2, and carbons from polymers (Figure S1 in the Supporting Information). Scanning transmission electron microscopy (STEM) images reveal the presence of hat-shaped cores of Au/polymer inside the SiO2 capsules (Figure 2D). One core per SiO2 capsule was observed for all cases. The hat-shaped cores can be collected by selective dissolution of the SiO2 outer shell (Figure 2C). Considering that an atom with a high atomic number looks brighter in a STEM image, it seems that the AuNPs are particularly localized on top of the hatlike core. To trace the morphological change of the core upon heat treatment, the samples were exposed to calcination with different time durations. At the beginning of the heat treatment, the size of the polymer core looks comparable with that of the shell (Figure 2E), but eventually, the polymer cores disappear upon prolonged heat treatment, and only the Au cores remained (Figure 2F H). As the calcination proceeded, it was observed that the AuNPs are gathered to a one side of the polymer cores, resulting in a morphological transition from a sphere shape to a crushed hatlike shape. This suggests that the migration of the AuNPs on the polymer cores upon exposure to a heat plays a crucial role in the transformation of the cores. One thing to note is that the silica outer shell in this structure helps to maintain the heat within the structure, promoting the decomposition of the PEMPs. Parts I, J, and K of Figure 2 show the calcined PEMP/SiO2 structures exposed to calcination for 1, 2, and 3 h, respectively, without loading of metal NPs. Three hours are enough to completely remove the PEMPs within the SiO2 shells. However, if there is no
SiO2 outer shell, more than 4 h are needed for the complete removal of the PEMPs (data not shown). When a porous structured material such as hematite was used as a shell, no big difference on the decomposition time between cases with or without a shell was observed. Thus, we can conclude that the dense SiO2 shell, without heat leakage acts, as a reaction chamber for containing and maintaining heat effectively. To elucidate the role of the AuNPs, the same calcination process was carried out on the AuNP-coated PEMPs, without a SiO2 outer shell. After the calcination, walnut-shaped particles of ca. 550 nm in size covered with AuNPs were formed (Figure 3A and inset). Compared to the calcinated PEMPs without AuNPs, these were much more shrunken in size, suggesting that the AuNPs on their surface would promote the transformation and decomposition of the polymer cores (Figure S2 in the Supporting Information). In this case, however, the hat-shaped cores were not formed. Thus, it is concluded that the hatlike core structure can be formed by synergetic effect of the AuNPs and the SiO2 outer shells. Existence of these two components is indispensible for successful formation of the hatlike core structures. When the same calcination process was carried out on SiO2-coated PEMPs for 15 min without the AuNPs, indeed, there was not much progress toward the decomposition of the polymer core (Figure 3B). The conventional wisdom is that metals easily absorb heat compared to polymers and then transfer heat flux to the polymer matrix, and a heat flux that would provide enough thermal energy to decompose or deform polymers is maintained through convection within the dense SiO2 shell without dissipation. The deformation of polymers can be either accelerated or slowed by varying the thermal conductivity of the metal NPs used. To confirm this hypothesis, two types of experiments using metals with high thermal conductivity (Au and Ag for 284 W/m k and 396 W/m k, respectively, at 500 °C) and low thermal conductivity (Pt and Pd for 75.6 W/m k and 75.8 W/m k, respectively, at 500 °C) are conducted. Shaped 4834
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Figure 4. STEM images showing the shape of the change processes of Au or Pt NP-embedded PEMPs during calcination. Microtoming STEM images of PEMP/(A) AuNP or (D) PtNP. STEM images of PEMP/(B) AuNP or (E) PtNP/SiO2 after calcination for 13 min. STEM images of PEMP/(C) AuNP or (F) PtNP/SiO2 after calcination for 15 min.
cores similar to those formed by Au were formed by Ag, while Pt or Pd showed agglomerated cores composed of Pt or Pd, which shows a quite different aspect (Figure 3C and D, and Figure S3 in the Supporting Information). Contrary to our expectations, Pt or Pd, of low thermal conductivity, decomposed a polymer matrix faster than Au or Ag, of high thermal conductivity. To further investigate this phenomenon, the decomposition process of the polymer cores was monitored using metals of high and low thermal conductivities. Because the loading amount of the metal NPs would have and effect on the decomposition of the polymer cores, the PEMPs were loaded with the same amount of the metals. Parts A and D of Figure 4 show the cross-sectional STEM images of the PEMPs loaded with AuNPs and PtNPs, respectively. In the beginning of the heat treatment, the AuNPs decomposed the polymer cores faster than the PtNPs, as expected (Figures 4B and E). For AuNP-loaded PEMPs, a hollow space was formed as a result of partial decomposition of the core, but no hole was observed in case of the PtNP-loaded PEMPs. Toward end of the reaction, however, PtNPs decomposed the polymer core remarkably well, as compared to the AuNPs (Figure 4C and F). A good thermal conductor like Au may quickly absorb heat, rapidly transfer heat, and easily lose heat. For this reason, the polymer core is decomposed well in the beginning of the reaction, but the speed of decomposition slows toward the end. For Pt, just the opposite is true; heat is absorbed with difficulty because of low thermal conductivity. Thus, polymers could not be easily decomposed in the beginning of the reaction. However, once Pt absorbs heat, it decomposes polymers remarkably well because the heat is retained for long periods of time. Because decomposition of the polymer cores could be controlled by use of various metal NPs, a variety of core structures were formed within the shell. By varying metal species and synthesis orders, many interesting core structures could be designed by this approach. Parts A and D of Figure 5 show the Pt/Au capsule@SiO2 shell structures obtained by the controlled calcination of Pt/Au/SiO2-coated PEMPs. The removal of the SiO2 shell by hydrogen fluoride confirmed the formation of a bimetallic capsule with an uneven surface and macropores (Figure 5B). The Pt almost decomposed the inside of the polymer core, leaving a hollow space. However,
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Figure 5. (A) STEM image after calcination of PEMP/PtNP/AuNP/ SiO2. (B) SEM image after the removal of the SiO2 shell of part A. (C) STEM image after calcination of PEMP/AuNP/PtNP/SiO2. (D) EDX data of part A. (Calcination conditions: 500 °C for 15 min.)
Figure 6. Time-dependent UV vis absorption spectral changes of 4-NP reduction in the presence of (A) Au/polymer@SiO2, (B) Au@SiO2, and (C) Pt/Au capsule@SiO2. (D) Their conversion rate during the reaction. The Au@SiO2 structure was obtained by further calcination of Au/polymer@SiO2.
the Au partially decomposed the outside of the core, creating a shell. In case of the calcinated PEMPs with Au/Pt/SiO2 coating (reversed order of metal NPs compared to the above case), a UFO-shaped particle was formed instead of a capsule. Because the Au partially decomposed the inside of the polymer core, on the contrary, the Pt almost decomposed the outside of the core (Figure 5C). These results demonstrate that this approach can be directly used for chemical reaction to prepare nanomaterials with a desired structure for use in further applications. As examples of the utility of the resulting core@shell structures, Au/polymer@SiO2 and Pt/Au capsule@SiO2 were used as nanocatalysts for the conversion of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP). Nitrophenol is among the most common organic pollutants in industrial wastewaters, while aminophenol 4835
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Chemistry of Materials is an important intermediate for the manufacture of antipyretic drugs.33 The cost for production of aminophenol is normally high. Generally, the catalytic reduction of 4-NP by sodium borohydride (NaBH4) has been widely studied using metal catalysts as a result of its low cost and ease of operation.33,34 Gradual dispersion of the AuNPs on the cores, as in case of the Au/polymer@SiO2 structure, could be a favorable structure for effective catalysts. Figure 6A shows the UV visible spectra of the reduced 4-NP by NaBH4 in the presence of the Au/polymer@SiO2. After adding catalysts, a decrease in the absorbance of the 4-NP at 400 nm, along with an increase in the peaks of 4-AP at 230 and 300 nm, was observed with the passage of time. Within 15 min, the reduction of 4-NP was completely terminated and the yellow-colored solution turned colorless. It is worthy of note that this reaction did not proceed without catalysts, even in the presence of an excess amount of NaBH4. The Au@SiO2 structure was obtained by prolonged calcination, and we tested its catalytic activity under the same conditions. It showed relatively low catalytic ability owing to the agglomerated AuNPs upon excessive heat treatment (Figure 6B and D). In case of the Pt/Au capsule@SiO2, surprisingly, the reduction of 4-NP was completely terminated within 5 min, which is the best performance in our results (Figures 6C and D). The Pt@SiO2 structure presented very low catalytic ability as a result of the agglomerated PtNPs upon excessive heat treatment (Figure S4 in the Supporting Information). We think that gradually dispersed AuNPs on a hat-shaped core played a crucial role in the increase of specific surface area of the NPs, and the bimetallic porous capsule that was exposed in multidirections contributed to an increase in surface areas and enhanced the synergetic effect of the catalytic reaction.
’ CONCLUSION In summary, a novel protocol for the synthesis of core@shell particles with transformable core structures has been introduced, and its use as a chemical reactor for hierarchical nanomaterials with tunable properties has been demonstrated. A metal, metal/ polymer, or bimetallic hollow structure was incorporated into the interior of silica capsules via controlled heat treatment of the inorganic-coated polymer particles. Upon controlled heat treatment, metals absorbed heat more easily than polymers; thus, they transferred heat flux to a polymer matrix to deform, and such heat flux was maintained through convection within the dense SiO2 shell without dissipation. Metal NPs with high mobility migrated to a certain direction on polymer cores, which led to a heated metal-induced transformation of the polymer core within the capsules. The transformation could be accelerated or delayed by varying species of the metal NPs capable of absorption and following transfer heat flux to the polymer core. A micrometersized capsule was used as a reaction chamber to contain and sustain heat, and metal NPs were used as a controller for transformation of the polymer cores. By varying the thermal conductivities of the metal NPs, it was also possible to confer synthesis of hat-shaped, UFO-shaped, or bimetallic hollow structures within the capsule. This novel method for the core@shell particles would be useful for synthesizing nanomaterials with controlled structures and properties. Furthermore, Pt/Au capsule@SiO2 and Au/polymer@SiO2 showed excellent catalytic properties for the transformation of 4-nitrophenol to 4-aminophenol. This approach could be used to tune the structures and properties of various kinds of polymer matrices.
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’ ASSOCIATED CONTENT
bS
Supporting Information. Size distribution and EDX data before and after calcination of PEMP/AuNP/SiO2 (Figure S1); SEM image after calcination of PEMPs (Figure S2); STEM/EDX data of Ag/polymer@SiO2, Pt@SiO2, and Pd@SiO2 produced under the same calcination conditions (Figure S3); UV vis data for catalytic ability of Pt@SiO2 structure (Figure S4). This material is available free of charge via the Internet at http:// pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT This work was supported by the Korea Basic Science Institute (KBSI), the National Research Foundation of Korea (NRF), the Ministry of Education, Science and Technology (20090081966), and intra-research fund of Hanbat National University (2011). ’ REFERENCES (1) Kamata, K.; Lu, Y.; Xia, Y. N. J. Am. Chem. Soc. 2003, 125, 2384–2385. (2) Van Gough, D.; Wolosiuk, A.; Braun, P. V. Nano Lett. 2009, 9, 1994–1998. (3) Gao, J. H.; Liang, G.; Zhang, B.; Kuang, Y.; Zhang, X.; Xu, B. J. Am. Chem. Soc. 2007, 129, 1428–1433. (4) Zhao, W. R.; Chen, H. R.; Li, Y. S.; Li, L.; Lang, M. D.; Shi, J. L. Adv. Funct. Mater. 2008, 18, 2780–2788. (5) Zhu, Y. F.; Ikoma, T.; Hanagata, N.; Kaskel, S. Small 2010, 6, 471–478. (6) Liu, J.; Xia, H.; Xue, D. F.; Lu, L. J. Am. Chem. Soc. 2009, 131, 12086–12087. (7) Lou, X. W.; Li, C. M.; Archer, L. A. Adv. Mater. 2009, 21, 2536–2539. (8) Zhang, W. M.; Hu, J. S.; Guo, Y. G.; Zheng, S. F.; Zhong, L. S.; Song, W. G.; Wan, L. J. Adv. Mater. 2008, 20, 1160–1165. (9) Li, H. X.; Bian, Z. F.; Zhu, J.; Huo, Y. N.; Li, H. X.; Lu, Y. F. J. Am. Chem. Soc. 2007, 129, 8406–8407. (10) Choi, W. S.; Koo, H. Y.; Kim, D.-Y. Adv. Mater. 2007, 19, 451–455. (11) Choi, W. S.; Yang, H. M.; Koo, H. Y.; Lee, H.-J.; Lee, Y. B.; Bae, T. S.; Jeon, I. C. Adv. Funct. Mater. 2010, 20, 820–825. (12) Zhang, T. R.; Ge, J. P.; Hu, Y. X.; Zhang, Q.; Aloni, S.; Yin, Y. D. Angew. Chem., Int. Ed. 2008, 47, 5806–5811. (13) Chen, D.; Li, L. L.; Tang, F. Q.; Qi, S. Adv. Mater. 2009, 21, 3804–3807. (14) Zhang, Q.; Ge, J. P.; Goebl, J.; Hu, Y. X.; Lu, Z. D.; Yin, Y. D. Nano Res. 2009, 2, 583–591. (15) Chen, Y.; Chen, H. R.; Guo, L. M.; He, Q. J.; Chen, F.; Zhou, J.; Feng, J. W.; Shi, J. L. ACS Nano 2010, 4, 529–539. (16) Yin, Y. D.; Rioux, R. M.; Erdonmez, C. K.; Hughes, S.; Somorijai, G. A.; Alivisatos, A. P. Science 2004, 304, 711–714. (17) Ren, N.; Yang, Y.-H.; Zhang, Y.-H.; Wang, Q.-R.; Tang, Y. J. Catal. 2007, 246, 215–222. (18) Vriezema, D. M.; Aragons, M. C.; Elemans, J. A. A.W.; Cornelissen, J. J. L. M.; Rowan, A. E.; Nolte, R. J. M. Chem. Rev. 2005, 105, 1445–1489. (19) Arnal, P. M.; Comotti, M.; Schuth, F. Angew. Chem., Int. Ed. 2006, 45, 8224–8227. 4836
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’ NOTE ADDED AFTER ASAP PUBLICATION There was an error in the Acknowledgment in the version published ASAP October 18, 2011; the corrected version published ASAP November 1, 2011.
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