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Au@carbon yolk–shell nanostructures via one-step core–shell–shell template† Rui Liu,a Fengli Qu,ab Yunlong Guo,a Nan Yaoc and Rodney D. Priestley*ac
Received 14th September 2013, Accepted 13th November 2013 DOI: 10.1039/c3cc47050d www.rsc.org/chemcomm
A facile one-step Sto ¨ ber route to synthesize high-quality core– shell–shell templates is reported for the fabrication of Au@carbon yolk–shell nanostructures. The converted Au@carbon yolk–shell nanostructures exhibited high catalytic performance as illustrated by the reduction reaction of o-nitrophenol.
Uniform core–shell or core–shell–shell (CSS) nanoparticles with different functional compositions are being widely investigated because of their potential applications in drug delivery, catalysis, photonic crystals, biodiagnostics and energy storage.1–5 In general, the synthesis of core–shell or core–multiple shell nanoparticles requires multiple coating processes.1–5 One application of the core–shell–shell nanoparticle is their use as a template to develop various new nanostructures. Among them, the rattle-type or yolk– shell nanostructure is a novel and promising nanostructure, in which a movable core is encapsulated inside a polymeric or inorganic shell.6–8 Yolk–carbon shell nanostructures have received significant attention. For example, Pt@carbon and Sn@carbon yolk–shell structures have shown excellent performance in catalytic hydrogenation reactions and lithium batteries, respectively.9,10 In both cases, the carbon shell functioned as a barrier to prevent the encapsulated nanoparticle from coalescence. Furthermore, the inherent electrical conductivity as well as excellent chemical and thermal stability of carbon coatings are especially beneficial for catalytic and electrochemical applications. As alluded to above, templating is the most widely used method to synthesize yolk–carbon shell nanostructures. Typically, spherical core–shell metal–silica nanoparticles are coated with a carbon precursor to form CSS nanoparticles, followed by carbonization and subsequently, the selective removal of the silica interlayer to obtain yolk–carbon shell nanostructures.11,12 a
Department of Chemical and Biological Engineering, Princeton University, Princeton, New Jersey, 08544, USA. E-mail: [email protected] b College of Chemistry and Chemical Engineering, Qufu Normal University, Qufu, 273165, China c Princeton Institute for the Science and Technology of Materials, Princeton University, Princeton, NJ 08544, USA † Electronic supplementary information (ESI) available. See DOI: 10.1039/c3cc47050d
478 | Chem. Commun., 2014, 50, 478--480
It is well known that the type of carbon precursor plays a vital role in both the preparation and final physical and chemical properties of the resulting carbon framework.13,14 Among the various carbon precursors, resorcinol-formaldehyde (RF) resin, a threedimensional network structured polymer, is particularly interesting due to the attractive properties of RF resin derived carbon such as low cost, high surface area, high porosity, controllable pore structure, low electrical resistivity, good electrical conductivity, and outstanding thermal and mechanical properties.15,16 In terms of ¨ber method has been successfully applied for preparation, the Sto the preparation of monodisperse RF polymer spheres due to its ability to form a coordinated covalently bonded framework during ¨ber polymerization similar to silica.17 The extension of the Sto method for polymer synthesis opens the pathway for synthetic strategies in the preparation of RF based polymer spheres, carbon spheres and other nanostructures. For example, a one-step method has been reported to produce uniform silica@polymer spheres with a core–shell structure by combining the synthesis processes of silica ¨ber conditions, which could and resorcinol-formaldehyde under Sto further be converted to hollow polymer or carbon spheres.18 In addition, the preparation of polymer/silica/surfactant nanoparticles by the co-sol–gel process with tetraethoxysilane (TEOS) and resorcinolformaldehyde precursor in the presence of cetyltrimethylammonium bromide (CTAB) resulted in the formation of various hierarchical structures, including hollow mesoporous silica tubes/ spheres, radially porous silica spheres, rattle-type mesoporous carbon tubes/spheres and hollow mesoporous carbon spheres.19 We report herein for the first time a one-step strategy for the preparation of monodisperse Au–silica–RF polymer CSS nanostructures and their transformation to Au@carbon yolk–shell nanostructures. As illustrated in Scheme 1, the CSS template is simply obtained by feeding Au colloid,20 TEOS, resorcinol, and formaldehyde in a mixture of alcohol and aqueous ammonia, a process similar to the synthesis ¨ber method.21 Then, Au@C yolk–shell of silica spheres by the Sto nanostructures are obtained by carbonization and selective removal of the silica. The synthetic route reported here is expected to simplify the fabrication process of yolk–shell nanostructures, which usually entails multiple steps and a previously synthesized hard template.
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Scheme 1 One-step synthesis of Au–SiO2–RF polymer CSS nanocomposite and its conversion to Au@C yolk–shell nanostructure.
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precursor in a dilute HF solution. During the etching process, the silica layer was removed and the Au@RF polymer yolk–shell nanostructures (Fig. 1D) were obtained. Both results from the above experiments clearly confirm the CSS Au–SiO2–RF polymer ¨ber Si spheres can be tuned by nanostructure. As the diameter of Sto varying the concentration of NH4OH and TEOS or the ethanol/ water ratio;17,18 and the RF thickness can be tuned by varying the initial monomer feed concentration, it is expected that the yolk– shell diameter and thickness can be finely controlled. These studies are currently underway and will be published elsewhere. It has been suggested that the polymerization reactions of silica and resorcinol-formaldehyde occurring at different time scales is key to the formation of silica@RF polymer core–shell nanomaterials, although both occur under similar reaction conditions (i.e., reaction ¨ber medium, temperature and catalyst).18 The synthesis time of Sto silica particles is B1 hour at 30 1C while the formation of RF particles occurs slower and requires a reaction time greater than 24 h at 30 1C.17 The electrostatic interaction between the RF oligomer and the early staged formed silica particles would direct the condensation of RF around the silica particles.18 At the same time, previous reports have shown that silica coating of Au nanoparticles or Ag nanowires is a fast process, achieving maximum coating thickness within B1 hour.22,23 Combining the fast coating of metal nano¨ber silica with the slow polymerization of Sto ¨ber RF particles by Sto polymer, as shown in Scheme 1, provides a route to form CSS nanostructures. A series of TEM images at different reaction time intervals were obtained to observe the growth process. As shown in Fig. S1 (ESI†), Au@SiO2 core–shell nanoparticles with an B150 nm diameter were formed within 1 h while no obvious RF polymer layer was observed after 18 h of reaction time. Only after 24 h of reaction time was a 5 nm thick outer RF layer observed. After hydrothermal treatment at 100 1C for 24 h the RF layer grew to a thickness of 18 nm, a value consistent with that determined from Fig. 1B. As demonstrated, the different reaction kinetics is key to the successful formation of the CSS nanostructures by this approach. Monodispersed Au@carbon yolk–shell nanostructures were obtained after carbonization of the CSS template in N2 and after
Fig. 1 (A) SEM and (B) TEM images of Au–SiO2–RF CSS. (C) Au@silica core–shell nanoparticle produced by the calcination of CSS in air and (D) Au@RF yolk–shell nanoparticle after etching silica in HF.
A scanning electron microscopic (SEM) image (Fig. 1A) shows ¨ber that the CSS nanoparticles obtained using the one-step Sto approach have a spherical morphology. A transmission electron microscopic (TEM) image (Fig. 1B) confirms that the CSS precursor nanoparticles are monodisperse and uniformly spherical, with a diameter of B180 nm. Furthermore, the characteristics of the individual nanoparticles are clearly visualized by TEM. The obtained CSS structure consists of an Au core, a SiO2 interlayer and a polymeric outer shell. In more detail, an 18 nm thick RF shell encircles a 150 nm thick silica shell while a 15 nm diameter Au nanoparticle serves as the core. We conducted two verification experiments to further confirm the CSS nanostructure. Au@silica core–shell nanoparticles (Fig. 1C) were obtained after calcination at 600 1C. The outer polymer shell degraded in the air atmosphere indicating its organic nature. In the second experiment, we stirred the CSS
This journal is © The Royal Society of Chemistry 2014
Fig. 2
(A), (B) SEM and (C), (D) TEM images of Au@C yolk–shell nanostructure.
Chem. Commun., 2014, 50, 478--480 | 479
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Fig. 3 (A) Reaction scheme and associated color, and (B) UV/Vis spectra of o-nitrophenol reduction at the different reaction time after the addition of Au@C yolk–shell catalyst. Insets: graphic illustration of the conversion of o-nitrophenol in 10 min versus the number of catalyst recycles.
etching in HF solution. The Energy-dispersive X-ray (EDX) spectrum of the nanostructures (Fig. S2, ESI†) clearly identifies the peaks of C and Au. The SEM image in Fig. 2A indicates that the Au@C preserves a spherical morphology with a particle size of approximately 170 nm. The TEM image in Fig. 2C confirms the yolk–shell nanostructure of a carbon shell encircling one metal nanoparticle. On occasion two nanoparticles are present within a single carbon shell, which is typically observed in other processes known to create yolk–shell structures.6–8 From the SEM image of a broken particle (Fig. 2B) and the TEM image of an individual yolk–shell nanostructure (Fig. 2D), it is clear that the gold nanoparticle is studded onto the inner surface of the hollow shell. Here, the hollow void diameter is B150 nm, which is consistent with the size of the silica particle in the CSS template. The carbon shell is B13 nm thick. The 25% shrinkage of the RF shell during the carbonization process is comparable with the reported 19% shrinkage of solid RF polymer particles7 and 25% shrinkage of a polydopamine shell layer.24 The gold-catalyzed reduction of o-nitrophenol by NaBH4 to o-aminophenol (Fig. 3A) was chosen as a model reaction to evaluate the catalytic capability of the Au@C yolk–shell nanostructures. The reduction reaction did not proceed without the presence of the catalyst. However, when the Au@C yolk–shell catalyst was introduced into the solution, the absorption at 400 nm quickly decreased while the absorption at 295 nm increased (see Fig. 3B). The reduction of o-nitrophenol into o-aminophenol was completed in 10 min. The complete conversion of o-nitrophenol could also be visually appreciated by the color change of the solution from yellow to clear (see Fig. 3A). The apparent rate constant k calculated from the ln(Ct/C0) vs. time plot (Fig. S3, ESI†) was 0.48 min 1. The fast reaction kinetics of the nitrophenol reduction is believed to result from the nanorattle structure characteristics, such as the large free reaction voids inside the carbon sphere, highly dispersed gold nanoparticle in each capsule12 and high porosity of the carbon shell derivative from RF.11,18 The reaction kinetics are similar to that reported for Au@C yolk–shell nanostructures prepared using multiple coating steps.11 The reaction time observed here is significantly faster than that reported for the same reaction using Au@SiO2 yolk–shell nanoparticle catalyst with an B30–50 nm thick silica shell layer.25 This difference is likely due to the different thicknesses of the shell layers, which control the rate of diffusion. Stability against coalescence is an important issue for nanocrystal-based catalysts.26,27 The stability of the Au@C yolk–shell nanostructure was investigated by repeating the reduction reaction with the same catalyst five times (see inset Fig. 3B). After each reaction, the catalyst was recycled by centrifugation,
480 | Chem. Commun., 2014, 50, 478--480
Communication
followed by washing with distilled water and drying in vacuum overnight. The catalyst showed high activity after 5 successive reaction cycles, with conversion near 100% within B10 min of reaction time. The yolk–shell nanostructures were still clearly visualized after 5 reaction cycles (see TEM images in Fig. S4, ESI†). Clearly, the presence of the carbon shell was efficient in working as a capping agent to stabilize the catalytic nanoparticles by preventing their aggregation, making the catalyst reusable after multiple cycles of reactions. In summary, we have developed a facile one-step approach to produce uniform Au–silica–polymer spheres with a core–shell–shell ¨ber silica and structure by combining the synthesis processes of Sto resorcinol-formaldehyde polymerization. The Au@carbon yolk–shell nanostructures were obtained by carbonization and selective removal of the silica. The strategy presented in this work provides a simple and versatile synthetic approach towards designing core–multiple shell hybrid nanoparticles and therefore other various novel nanostructures for biological, energy, and environmental applications. R.D.P. acknowledges support of the National Science Foundation (NSF) Materials Research Science and Engineering Center program through the Princeton Center for Complex Materials (DMR-0819860). F.Q. acknowledges support of the National Natural Science Foundation of China (21005047, 21375076).
Notes and references 1 2 3 4 5
6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
Y. J. Wang, A. S. Angelatos and F. Caruso, Chem. Mater., 2008, 20, 848. A. H. Lu, E. L. Salabas and F. Schuth, Angew. Chem., Int. Ed., 2007, 46, 1222. Y. Zhao and L. Jiang, Adv. Mater., 2009, 21, 3621. J. W. Chung, Y. Guo, S.-Y. Kwak and R. D. Priestley, J. Mater. Chem., 2012, 22, 6017. G. Lu, S. Z. Li, Z. Guo, O. K. Farha, B. G. Hauser, X. Y. Qi, Y. Wang, X. Wang, S. Y. Han, X. G. Liu, J. S. DuChene, H. Zhang, Q. C. Zhang, X. D. Chen, J. Ma, S. C. J. Loo, W. D. Wei, Y. H. Yang, J. T. Hupp and F. W. Huo, Nat. Chem., 2012, 4, 310. M. Kim, K. Sohn, H. B. Na and T. Hyeon, Nano Lett., 2002, 2, 1383. A. B. Fuertes, M. Sevilla, T. Valdes-Solis and P. Tartaj, Chem. Mater., 2007, 19, 5418. J. X. Zhu, T. Sun, H. H. Hng, J. Ma, F. Y. C. Boey, X. W. Lou, H. Zhang, C. Xue, H. Y. Chen and Q. Y. Yan, Chem. Mater., 2009, 21, 3848. S. Ikeda, S. Ishino, T. Harada, N. Okamoto, T. Sakata, H. Mori, S. Kuwabata, T. Torimoto and M. Matsumura, Angew. Chem., Int. Ed., 2006, 45, 7063. W. M. Zhang, J. S. Hu, Y. G. Guo, S. F. Zheng, L. S. Zhong, W. G. Song and L. J. Wan, Adv. Mater., 2008, 20, 1160. X. Fang, S. Liu, J. Zang, C. Xu, M. S. Zheng, Q. F. Dong, D. Sun and N. Zheng, Nanoscale, 2013, 5, 6908. R. Liu, S. M. Mahurin, C. Li, R. R. Unocic, J. C. Idrobo, H. Gao, S. J. Pennycook and S. Dai, Angew. Chem., Int. Ed., 2011, 50, 6799. S. Y. Li, Y. R. Liang, D. C. Wu and R. W. Fu, Carbon, 2010, 48, 839. X. Wang, R. Liu, M. M. Waje, Z. Chen, Y. Yan, K. N. Bozhilov and P. Feng, Chem. Mater., 2007, 19, 2395. S. A. Al-Muhtaseb and J. A. Ritter, Adv. Mater., 2003, 15, 101. A. M. Elkhatat and S. A. Al-Muhtaseb, Adv. Mater., 2011, 23, 2887. J. Liu, S. Z. Qiao, H. Liu, J. Chen, A. Orpe, D. Zhao and G. Q. Lu, Angew. Chem., Int. Ed., 2011, 50, 5947. A. B. Fuertes, P. Valle-Vigon and M. Sevilla, Chem. Commun., 2012, 48, 6124. X. H. Zhang, Y. N. Li and C. B. Cao, J. Mater. Chem., 2012, 22, 13918. ¨th, Angew. Chem., Int. Ed., 2006, 45, 8224. P. M. Arnal, M. Comotti and F. Schu ¨ber, A. Fink and E. Bohn, J. Colloid Interface Sci., 1968, 26, 62. W. Sto Y. Yin, Y. Lu, Y. Sun and Y. Xia, Nano Lett., 2002, 2, 427. Y. Lu, Y. Yin, Z. Y. Li and Y. Xia, Nano Lett., 2002, 2, 785. R. Liu, Y. Guo, G. Odusote, F. Qu and R. D. Priestley, ACS Appl. Mater. Interfaces, 2013, 5, 9167. J. Lee, J. C. Park and H. Song, Adv. Mater., 2008, 20, 1523. M. Comotti, C. D. Pina, R. Matarrese and M. Rossi, Angew. Chem., Int. Ed., 2004, 43, 5812. ´s-Solı´s, P. Valle-Vigo ´n, M. Sevilla and A. B. Fuertes, J. Catal., T. Valde 2007, 251, 239.
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Au@carbon yolk–shell nanostructures via one-step core–shell–shell template† Rui Liu,a Fengli Qu,ab Yunlong Guo,a Nan Yaoc and Rodney D. Priestley*ac
Received 14th September 2013, Accepted 13th November 2013 DOI: 10.1039/c3cc47050d www.rsc.org/chemcomm
A facile one-step Sto ¨ ber route to synthesize high-quality core– shell–shell templates is reported for the fabrication of Au@carbon yolk–shell nanostructures. The converted Au@carbon yolk–shell nanostructures exhibited high catalytic performance as illustrated by the reduction reaction of o-nitrophenol.
Uniform core–shell or core–shell–shell (CSS) nanoparticles with different functional compositions are being widely investigated because of their potential applications in drug delivery, catalysis, photonic crystals, biodiagnostics and energy storage.1–5 In general, the synthesis of core–shell or core–multiple shell nanoparticles requires multiple coating processes.1–5 One application of the core–shell–shell nanoparticle is their use as a template to develop various new nanostructures. Among them, the rattle-type or yolk– shell nanostructure is a novel and promising nanostructure, in which a movable core is encapsulated inside a polymeric or inorganic shell.6–8 Yolk–carbon shell nanostructures have received significant attention. For example, Pt@carbon and Sn@carbon yolk–shell structures have shown excellent performance in catalytic hydrogenation reactions and lithium batteries, respectively.9,10 In both cases, the carbon shell functioned as a barrier to prevent the encapsulated nanoparticle from coalescence. Furthermore, the inherent electrical conductivity as well as excellent chemical and thermal stability of carbon coatings are especially beneficial for catalytic and electrochemical applications. As alluded to above, templating is the most widely used method to synthesize yolk–carbon shell nanostructures. Typically, spherical core–shell metal–silica nanoparticles are coated with a carbon precursor to form CSS nanoparticles, followed by carbonization and subsequently, the selective removal of the silica interlayer to obtain yolk–carbon shell nanostructures.11,12 a
Department of Chemical and Biological Engineering, Princeton University, Princeton, New Jersey, 08544, USA. E-mail: [email protected] b College of Chemistry and Chemical Engineering, Qufu Normal University, Qufu, 273165, China c Princeton Institute for the Science and Technology of Materials, Princeton University, Princeton, NJ 08544, USA † Electronic supplementary information (ESI) available. See DOI: 10.1039/c3cc47050d
478 | Chem. Commun., 2014, 50, 478--480
It is well known that the type of carbon precursor plays a vital role in both the preparation and final physical and chemical properties of the resulting carbon framework.13,14 Among the various carbon precursors, resorcinol-formaldehyde (RF) resin, a threedimensional network structured polymer, is particularly interesting due to the attractive properties of RF resin derived carbon such as low cost, high surface area, high porosity, controllable pore structure, low electrical resistivity, good electrical conductivity, and outstanding thermal and mechanical properties.15,16 In terms of ¨ber method has been successfully applied for preparation, the Sto the preparation of monodisperse RF polymer spheres due to its ability to form a coordinated covalently bonded framework during ¨ber polymerization similar to silica.17 The extension of the Sto method for polymer synthesis opens the pathway for synthetic strategies in the preparation of RF based polymer spheres, carbon spheres and other nanostructures. For example, a one-step method has been reported to produce uniform silica@polymer spheres with a core–shell structure by combining the synthesis processes of silica ¨ber conditions, which could and resorcinol-formaldehyde under Sto further be converted to hollow polymer or carbon spheres.18 In addition, the preparation of polymer/silica/surfactant nanoparticles by the co-sol–gel process with tetraethoxysilane (TEOS) and resorcinolformaldehyde precursor in the presence of cetyltrimethylammonium bromide (CTAB) resulted in the formation of various hierarchical structures, including hollow mesoporous silica tubes/ spheres, radially porous silica spheres, rattle-type mesoporous carbon tubes/spheres and hollow mesoporous carbon spheres.19 We report herein for the first time a one-step strategy for the preparation of monodisperse Au–silica–RF polymer CSS nanostructures and their transformation to Au@carbon yolk–shell nanostructures. As illustrated in Scheme 1, the CSS template is simply obtained by feeding Au colloid,20 TEOS, resorcinol, and formaldehyde in a mixture of alcohol and aqueous ammonia, a process similar to the synthesis ¨ber method.21 Then, Au@C yolk–shell of silica spheres by the Sto nanostructures are obtained by carbonization and selective removal of the silica. The synthetic route reported here is expected to simplify the fabrication process of yolk–shell nanostructures, which usually entails multiple steps and a previously synthesized hard template.
This journal is © The Royal Society of Chemistry 2014
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Scheme 1 One-step synthesis of Au–SiO2–RF polymer CSS nanocomposite and its conversion to Au@C yolk–shell nanostructure.
ChemComm
precursor in a dilute HF solution. During the etching process, the silica layer was removed and the Au@RF polymer yolk–shell nanostructures (Fig. 1D) were obtained. Both results from the above experiments clearly confirm the CSS Au–SiO2–RF polymer ¨ber Si spheres can be tuned by nanostructure. As the diameter of Sto varying the concentration of NH4OH and TEOS or the ethanol/ water ratio;17,18 and the RF thickness can be tuned by varying the initial monomer feed concentration, it is expected that the yolk– shell diameter and thickness can be finely controlled. These studies are currently underway and will be published elsewhere. It has been suggested that the polymerization reactions of silica and resorcinol-formaldehyde occurring at different time scales is key to the formation of silica@RF polymer core–shell nanomaterials, although both occur under similar reaction conditions (i.e., reaction ¨ber medium, temperature and catalyst).18 The synthesis time of Sto silica particles is B1 hour at 30 1C while the formation of RF particles occurs slower and requires a reaction time greater than 24 h at 30 1C.17 The electrostatic interaction between the RF oligomer and the early staged formed silica particles would direct the condensation of RF around the silica particles.18 At the same time, previous reports have shown that silica coating of Au nanoparticles or Ag nanowires is a fast process, achieving maximum coating thickness within B1 hour.22,23 Combining the fast coating of metal nano¨ber silica with the slow polymerization of Sto ¨ber RF particles by Sto polymer, as shown in Scheme 1, provides a route to form CSS nanostructures. A series of TEM images at different reaction time intervals were obtained to observe the growth process. As shown in Fig. S1 (ESI†), Au@SiO2 core–shell nanoparticles with an B150 nm diameter were formed within 1 h while no obvious RF polymer layer was observed after 18 h of reaction time. Only after 24 h of reaction time was a 5 nm thick outer RF layer observed. After hydrothermal treatment at 100 1C for 24 h the RF layer grew to a thickness of 18 nm, a value consistent with that determined from Fig. 1B. As demonstrated, the different reaction kinetics is key to the successful formation of the CSS nanostructures by this approach. Monodispersed Au@carbon yolk–shell nanostructures were obtained after carbonization of the CSS template in N2 and after
Fig. 1 (A) SEM and (B) TEM images of Au–SiO2–RF CSS. (C) Au@silica core–shell nanoparticle produced by the calcination of CSS in air and (D) Au@RF yolk–shell nanoparticle after etching silica in HF.
A scanning electron microscopic (SEM) image (Fig. 1A) shows ¨ber that the CSS nanoparticles obtained using the one-step Sto approach have a spherical morphology. A transmission electron microscopic (TEM) image (Fig. 1B) confirms that the CSS precursor nanoparticles are monodisperse and uniformly spherical, with a diameter of B180 nm. Furthermore, the characteristics of the individual nanoparticles are clearly visualized by TEM. The obtained CSS structure consists of an Au core, a SiO2 interlayer and a polymeric outer shell. In more detail, an 18 nm thick RF shell encircles a 150 nm thick silica shell while a 15 nm diameter Au nanoparticle serves as the core. We conducted two verification experiments to further confirm the CSS nanostructure. Au@silica core–shell nanoparticles (Fig. 1C) were obtained after calcination at 600 1C. The outer polymer shell degraded in the air atmosphere indicating its organic nature. In the second experiment, we stirred the CSS
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Fig. 2
(A), (B) SEM and (C), (D) TEM images of Au@C yolk–shell nanostructure.
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Fig. 3 (A) Reaction scheme and associated color, and (B) UV/Vis spectra of o-nitrophenol reduction at the different reaction time after the addition of Au@C yolk–shell catalyst. Insets: graphic illustration of the conversion of o-nitrophenol in 10 min versus the number of catalyst recycles.
etching in HF solution. The Energy-dispersive X-ray (EDX) spectrum of the nanostructures (Fig. S2, ESI†) clearly identifies the peaks of C and Au. The SEM image in Fig. 2A indicates that the Au@C preserves a spherical morphology with a particle size of approximately 170 nm. The TEM image in Fig. 2C confirms the yolk–shell nanostructure of a carbon shell encircling one metal nanoparticle. On occasion two nanoparticles are present within a single carbon shell, which is typically observed in other processes known to create yolk–shell structures.6–8 From the SEM image of a broken particle (Fig. 2B) and the TEM image of an individual yolk–shell nanostructure (Fig. 2D), it is clear that the gold nanoparticle is studded onto the inner surface of the hollow shell. Here, the hollow void diameter is B150 nm, which is consistent with the size of the silica particle in the CSS template. The carbon shell is B13 nm thick. The 25% shrinkage of the RF shell during the carbonization process is comparable with the reported 19% shrinkage of solid RF polymer particles7 and 25% shrinkage of a polydopamine shell layer.24 The gold-catalyzed reduction of o-nitrophenol by NaBH4 to o-aminophenol (Fig. 3A) was chosen as a model reaction to evaluate the catalytic capability of the Au@C yolk–shell nanostructures. The reduction reaction did not proceed without the presence of the catalyst. However, when the Au@C yolk–shell catalyst was introduced into the solution, the absorption at 400 nm quickly decreased while the absorption at 295 nm increased (see Fig. 3B). The reduction of o-nitrophenol into o-aminophenol was completed in 10 min. The complete conversion of o-nitrophenol could also be visually appreciated by the color change of the solution from yellow to clear (see Fig. 3A). The apparent rate constant k calculated from the ln(Ct/C0) vs. time plot (Fig. S3, ESI†) was 0.48 min 1. The fast reaction kinetics of the nitrophenol reduction is believed to result from the nanorattle structure characteristics, such as the large free reaction voids inside the carbon sphere, highly dispersed gold nanoparticle in each capsule12 and high porosity of the carbon shell derivative from RF.11,18 The reaction kinetics are similar to that reported for Au@C yolk–shell nanostructures prepared using multiple coating steps.11 The reaction time observed here is significantly faster than that reported for the same reaction using Au@SiO2 yolk–shell nanoparticle catalyst with an B30–50 nm thick silica shell layer.25 This difference is likely due to the different thicknesses of the shell layers, which control the rate of diffusion. Stability against coalescence is an important issue for nanocrystal-based catalysts.26,27 The stability of the Au@C yolk–shell nanostructure was investigated by repeating the reduction reaction with the same catalyst five times (see inset Fig. 3B). After each reaction, the catalyst was recycled by centrifugation,
480 | Chem. Commun., 2014, 50, 478--480
Communication
followed by washing with distilled water and drying in vacuum overnight. The catalyst showed high activity after 5 successive reaction cycles, with conversion near 100% within B10 min of reaction time. The yolk–shell nanostructures were still clearly visualized after 5 reaction cycles (see TEM images in Fig. S4, ESI†). Clearly, the presence of the carbon shell was efficient in working as a capping agent to stabilize the catalytic nanoparticles by preventing their aggregation, making the catalyst reusable after multiple cycles of reactions. In summary, we have developed a facile one-step approach to produce uniform Au–silica–polymer spheres with a core–shell–shell ¨ber silica and structure by combining the synthesis processes of Sto resorcinol-formaldehyde polymerization. The Au@carbon yolk–shell nanostructures were obtained by carbonization and selective removal of the silica. The strategy presented in this work provides a simple and versatile synthetic approach towards designing core–multiple shell hybrid nanoparticles and therefore other various novel nanostructures for biological, energy, and environmental applications. R.D.P. acknowledges support of the National Science Foundation (NSF) Materials Research Science and Engineering Center program through the Princeton Center for Complex Materials (DMR-0819860). F.Q. acknowledges support of the National Natural Science Foundation of China (21005047, 21375076).
Notes and references 1 2 3 4 5
6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
Y. J. Wang, A. S. Angelatos and F. Caruso, Chem. Mater., 2008, 20, 848. A. H. Lu, E. L. Salabas and F. Schuth, Angew. Chem., Int. Ed., 2007, 46, 1222. Y. Zhao and L. Jiang, Adv. Mater., 2009, 21, 3621. J. W. Chung, Y. Guo, S.-Y. Kwak and R. D. Priestley, J. Mater. Chem., 2012, 22, 6017. G. Lu, S. Z. Li, Z. Guo, O. K. Farha, B. G. Hauser, X. Y. Qi, Y. Wang, X. Wang, S. Y. Han, X. G. Liu, J. S. DuChene, H. Zhang, Q. C. Zhang, X. D. Chen, J. Ma, S. C. J. Loo, W. D. Wei, Y. H. Yang, J. T. Hupp and F. W. Huo, Nat. Chem., 2012, 4, 310. M. Kim, K. Sohn, H. B. Na and T. Hyeon, Nano Lett., 2002, 2, 1383. A. B. Fuertes, M. Sevilla, T. Valdes-Solis and P. Tartaj, Chem. Mater., 2007, 19, 5418. J. X. Zhu, T. Sun, H. H. Hng, J. Ma, F. Y. C. Boey, X. W. Lou, H. Zhang, C. Xue, H. Y. Chen and Q. Y. Yan, Chem. Mater., 2009, 21, 3848. S. Ikeda, S. Ishino, T. Harada, N. Okamoto, T. Sakata, H. Mori, S. Kuwabata, T. Torimoto and M. Matsumura, Angew. Chem., Int. Ed., 2006, 45, 7063. W. M. Zhang, J. S. Hu, Y. G. Guo, S. F. Zheng, L. S. Zhong, W. G. Song and L. J. Wan, Adv. Mater., 2008, 20, 1160. X. Fang, S. Liu, J. Zang, C. Xu, M. S. Zheng, Q. F. Dong, D. Sun and N. Zheng, Nanoscale, 2013, 5, 6908. R. Liu, S. M. Mahurin, C. Li, R. R. Unocic, J. C. Idrobo, H. Gao, S. J. Pennycook and S. Dai, Angew. Chem., Int. Ed., 2011, 50, 6799. S. Y. Li, Y. R. Liang, D. C. Wu and R. W. Fu, Carbon, 2010, 48, 839. X. Wang, R. Liu, M. M. Waje, Z. Chen, Y. Yan, K. N. Bozhilov and P. Feng, Chem. Mater., 2007, 19, 2395. S. A. Al-Muhtaseb and J. A. Ritter, Adv. Mater., 2003, 15, 101. A. M. Elkhatat and S. A. Al-Muhtaseb, Adv. Mater., 2011, 23, 2887. J. Liu, S. Z. Qiao, H. Liu, J. Chen, A. Orpe, D. Zhao and G. Q. Lu, Angew. Chem., Int. Ed., 2011, 50, 5947. A. B. Fuertes, P. Valle-Vigon and M. Sevilla, Chem. Commun., 2012, 48, 6124. X. H. Zhang, Y. N. Li and C. B. Cao, J. Mater. Chem., 2012, 22, 13918. ¨th, Angew. Chem., Int. Ed., 2006, 45, 8224. P. M. Arnal, M. Comotti and F. Schu ¨ber, A. Fink and E. Bohn, J. Colloid Interface Sci., 1968, 26, 62. W. Sto Y. Yin, Y. Lu, Y. Sun and Y. Xia, Nano Lett., 2002, 2, 427. Y. Lu, Y. Yin, Z. Y. Li and Y. Xia, Nano Lett., 2002, 2, 785. R. Liu, Y. Guo, G. Odusote, F. Qu and R. D. Priestley, ACS Appl. Mater. Interfaces, 2013, 5, 9167. J. Lee, J. C. Park and H. Song, Adv. Mater., 2008, 20, 1523. M. Comotti, C. D. Pina, R. Matarrese and M. Rossi, Angew. Chem., Int. Ed., 2004, 43, 5812. ´s-Solı´s, P. Valle-Vigo ´n, M. Sevilla and A. B. Fuertes, J. Catal., T. Valde 2007, 251, 239.
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