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Icosahedral gold–platinum alloy nanocrystals in hollow silica: a highly active and stable catalyst for Ullmann reactions†
Received 20th August 2013, Accepted 28th October 2013
Xiaoli Wu,ab Longfei Tan,*a Dong Chen,c Xianwei Menga and Fangqiong Tang*a
DOI: 10.1039/c3cc46383d www.rsc.org/chemcomm
Icosahedral Au–Pt alloy nanocrystals are prepared in porous hollow silica nanospheres via a hydrothermal method without using capping agents. These nanoparticles with unique shape and structure exhibit excellent catalytic activity and stability in Ullmann reactions.
Metal nanocrystals have attracted great attention because of their excellent catalytic performance in synthetic organic chemistry, green chemistry and energy processes.1 The catalytic properties of metal nanocrystals are closely related to their size and shape.2 The crystallographic planes on the surface, as well as the number of atoms on edges or corners of the nanocrystals shall strongly affect their catalytic properties.3 Recently, icosahedral (Ih) nanocrystals have shown excellent catalytic activity due to their unique exposed surface, bounded by {111} facets and built by twin boundary defects.4 For example, Yang and co-workers reported that an icosahedral Pt–Ni nanocrystal was more active than other low-indexed surfaces, such as its {100} surface-bound cube nanocrystal towards the oxygen reduction reaction.4b Several icosahedral metal nanocrystals have been reported, such as Au, Ag, Pd and their alloys.5 Generally, capping agents, such as poly(vinyl pyrrolidone) (PVP) or triblock copolymers were utilized to stabilize and regulate the shape of metal icosahedral nanocrystals in synthetic processes.6 However, the usage of capping agents would bring some disadvantages. For one thing, the capping molecule may block the active sites of surfaces of nanocrystals and reduce their catalytic activity.7 For another, the stabilizing effect of capping agents is not strong enough to keep metal nanocrystals stable in harsh catalytic reactions.8 Therefore, an efficient strategy should be developed to obtain active, stable and shape controlled nano-catalysts.
Hollow nanospheres with porous shells were ideal nanoreactors to fabricate metal nanocrystals and form a core–void–shell (so called yolk–shell) structure. Up to now, various metal nanocatalysts have been prepared in hollow structures for enhanced activity and stability.9 However, the reported metal cores are almost nearspherical nanoparticles. To tune nanocrystal’s shape inside of hollow spheres is of importance and still a great challenge. In this work, we successfully prepared icosahedral Au–Pt alloy nanocrystals into hollow silica nanospheres (Ih Au–Pt@SiO2) for the first time. Pt(IV) ions instead of capping agents were adopted to assist the formation of icosahedral Au cores. This nanocatalyst may lead to unique catalytic properties as the morphology of metal nanocrystals is critically important. The Ullmann reaction, homocoupling of arylhalides, has provided versatile routes for constructing biaryl compounds in industrial organic synthesis.10 So far only a few reports have covered the catalytic application of gold nanocatalysts for Ullmann reactions, such as small gold nanoparticles and nanoclusters,11 which usually endure poor stability during catalysis reaction for their high surface energy. In this work, Ih Au–Pt@SiO2 showed both excellent catalytic activity and cycle stability in homocoupling reaction of iodobenzene due to the presence of the icosahedral Au–Pt core and the protection of the mesoporous shell. Ih nanoparticles inside of hollow silica (Ih Au–Pt@SiO2) were more active than spherical nanoparticles (Sr Au@SiO2), showing a shapedependent catalytic performance. The synthesis route of Ih Au–Pt@SiO2 nanoparticles is shown in Scheme 1. In brief, hybrid silica nanospheres with an etchable and reductive middle layer were designed firstly.12 Then the middle layer was selectively etched under hydrothermal conditions with metal precursors, simultaneously providing a cavity for the
a
Laboratory of Controllable Preparation and Application of Nanomaterials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China. E-mail: [email protected], [email protected]; Tel: +86-10-82543521 b University of Chinese Academy of Sciences, Beijing 100049, China c Beijing Creative Nanophase Hi-Tech Company Limited, 100086, China † Electronic supplementary information (ESI) available: Experimental details and relevant figures. See DOI: 10.1039/c3cc46383d
This journal is © The Royal Society of Chemistry 2014
Scheme 1
The synthesis route of Ih Au–Pt@SiO2.
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Fig. 1 (a and b) TEM images and (c) compositional backscattered SEM image of Ih Au–Pt@SiO2. (d) TEM image of unsupported Ih Au–Pt nanoparticles, and the inset shows a 3D model of a Ih nanocrystal. (e) HRTEM image of Ih Au–Pt nanoparticles. (f) Enlarged image of the square region in (e), the white line is a twin boundary.
in situ growth of metal cores. In this work, the nearly monodisperse hybrid silica nanospheres with an average diameter of 110 3 nm were mixed with HAuCl4 and a small amount of H2PtCl6, and then heated at 200 1C for 3 h. The SEM image (Fig. S1, ESI†) shows uniform morphology of silica shells. The TEM images (Fig. 1a and b) and backscattered SEM image (Fig. 1c) establish that the nanoparticles are located exclusively inside of hollow silica spheres. The thickness of the silica shell is about 10 nm. The BET surface area of Ih Au–Pt@SiO2 nanoparticles is 175.2 m2 g 1 and the average pore size of the shell is determined to be 1.9 nm (Fig. S2, ESI†), which ensures the smooth passage of reactants and products through the silica shell while prevents Au–Pt cores from leakage. For the better observation of Ih Au cores, the silica shell of Ih Au–Pt@SiO2 was carefully etched by hydrofluoric acid, and meanwhile PVP was added to keep the Au cores dispersed. Fig. 1d shows the PVP-protected isolated Ih Au–Pt nanoparticles, which are mainly icosahedra with sizes from B14 to B22 nm. The Au–Pt icosahedron is a multiply twinned structure having 12 corners, 30 edges and 20 planes with the {111} surface, which shows a projected hexagonal shape under TEM. The ratio of icosahedral shape was above 90% as observed from TEM image. The inset of Fig. 1d shows a schematic illustration of the three-dimensional (3D) model of the Ih Au–Pt nanocrystal, which matches very well with the high-resolution TEM (HRTEM) image (Fig. 1e). Lattice planes and twin boundaries of nanocrystals can be seen from Fig. 1f, an enlarged image of Fig. 1e. The d-spacing of adjacent lattice planes is 0.235 nm, corresponding to the {111} planes of facecentered cubic (fcc) gold. The two lattice planes are separated by a twin boundary as shown in Fig. 1f. The results of HRTEM
540 | Chem. Commun., 2014, 50, 539--541
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observation indicate that the Au–Pt nanoparticle contains primarily {111} planes on the surface which are separated by multiple twin boundaries, confirming the icosahedral structure of cores. Compared with UV-vis spectra of Sr Au@SiO2, 5 nm blue-shift of the absorption peaks was observed for Ih Au–Pt@SiO2 (Fig. S3, ESI†). The slight change may be ascribed to the presence of a small amount of Pt. The powder X-ray diffraction (XRD) patterns of Ih Au–Pt@SiO2 nanoparticles (Fig. S4, ESI†) could be indexed to {111}, {200}, and {220} diffraction peaks of fcc gold, which were not affected by Pt. The intensity ratios of {200} and {220} to {111} are 0.22 and 0.14, respectively, lower than the conventional intensity ratios (0.53 and 0.33) of fcc gold, indicating that the {111} planes are dominant in Ih cores.4a PVP or triblock copolymers are common surface capping agents in solution-phase synthesis of Ih Au nanocrystals.6 However, in this work, Pt ions instead of capping agents were adopted to regulate the shape of Au nanocrystals. After adding Pt ions, the cores with icosahedral shape were obtained via a simultaneous etching and growth route. We speculate the Pt ion-assisted formation mechanism of icosahedral Au–Pt cores as follows. Under hydrothermal treatment, H2PtCl6 was reduced together with HAuCl4 by alkylamino groups in hybrid silica nanospheres. Due to the weak reducing ability of alkylamino groups, the nucleation and growth of metal atoms are slow, which promote the formation of twinned seeds.4b Pt atoms might grow preferentially at the corners and edges of nanocrystals to reduce the formation energy of the twin boundaries of twinned Au nanocrystals, which was beneficial to produce icosahedral Au–Pt alloy nanoparticles as a result.4a,b The distribution of the Pt element in an Ih nanocrystal was detected by high-angle annular dark-field scanning TEM (HAADF-STEM)-energy dispersive X-ray (EDX) spectroscopy. The STEM image and elemental mappings indicate that Au and Pt are distributed in the nanocrystal (Fig. 2). The atom ratio of Au to Pt is determined to be 13.5 by SEM-EDX spectroscopy (Fig. S5, ESI†), which is close to the theoretical molar ratio (13.3) of added HAuCl4 and H2PtCl6. The Ih Au–Pt@SiO2 catalyst was next applied to catalyze the Ullmann homocoupling reaction of iodobenzene (Fig. S6, ESI†). Before catalytic reaction, the nanoparticles were calcined in air at 500 1C for 1 h to remove the residual organic groups of hollow silica. The Ih Au–Pt@SiO2 catalyst without calcination only gave a yield of 61.6% using iodobenzene as the substrate. After calcination, the Ih Au–Pt cores retain their original size, shape and dispersity (Fig. S7, ESI†). The catalytic results of different catalysts are shown in Table 1. The green solvent, a mixture of ethanol and water (v/v = 4/1), was used. We found that the existence of water in ethanol was favorable to improve the activity of catalysts, probably because of the suitable solvent polarity and enhanced alkalinity of K2CO3 in aqueous
Fig. 2 (a) STEM image of Ih Au–Pt@SiO2. (b and c) STEM-EDX spectral element mappings for Au (red) and Pt (green).
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Table 1
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Ullmann reactions of iodobenzene catalyzed by different catalystsa
Entry
Catalyst
Solvent
1 2 3 4 5b 6c
lh Au@SiO2 lh Au@SiO2 Sr Au@SiO2 Unsupported Ih Au Pt/C Pd/C
EtOH EtOH–H2O EtOH–H2O EtOH–H2O EtOH–H2O EtOH–H2O
Yield (%) (4/1) (4/1) (4/1) (4/1) (4/1)
84.2 99.2 53.6 47.9 0 86.5
a Reaction conditions: 0.5 mmol substrate, 1 mmol K2C03, 5 mg catalyst with equal Au loading (10 wt%), 10 mL solvent, 80 1C, reaction time 4 h. b 5 mg Pt/C (10 wt% Pt loading). c 10 mg Pd/C (5 wt% Pd loading), reductant 0.8 mmol HCO2Na.
solution than in pure ethanol (entry 1). The Ih Au–Pt@SiO2 nanoparticles exhibited excellent catalytic activity towards the homocoupling of iodobenzene. A 99.2% yield of biphenyl was obtained with Ih Au–Pt@SiO2 at relatively low temperature (80 1C) (entry 2), which was even higher than the commercially available Pd/C nanocatalysts (entry 6). As a control, Au@SiO2 nanoparticles with spherical Au cores (9.8 1.9 nm) were prepared by a similar method and then applied as catalysts (Fig. S8, ESI†). The yield of biphenyl of Sr Au@SiO2 was nearly half of Ih Au–Pt@SiO2 (entry 3), with by-products of benzene from the hydro-deiodination of iodobenzene. Besides, the commercial Pt/C nanocatalysts showed no catalytic activity in this reaction (entry 5). It is worth noting that the surfaces of Ih Au–Pt cores are bound by {111} facets and twin boundary defects.4a,b The high catalytic activity of Ih Au–Pt@SiO2 may attribute to their unique surfaces. These results indicate that the high catalytic activities of Ullmann reactions can be achieved by tuning the shape of Au nanoparticles. The catalytic activities of substrates with substituent groups were also measured. The yield of electron-rich 4-iodoanisole and electron-poor 1-iodo-4-nitrobenzene is 78.3% and 81.7%, respectively. To evaluate the protection effect of hollow silica spheres towards Ih Au–Pt cores, we carefully removed the silica shell of Ih Au–Pt@SiO2 by HF solution and added PVP molecules to stabilize Ih Au–Pt cores (Fig. 1d). The unsupported Ih Au–Pt nanoparticles were further utilized to catalyze homocoupling reaction of iodobenzene, but only a low yield (47.9%) of biphenyl was achieved (entry 4). The unsupported Ih Au–Pt nanoparticles showed severe aggregation after the first cycle of reaction, and no appreciable activity was observed in the succedent cycles. This indicates that the protection of PVP molecules was not strong enough to stabilize Ih nanoparticles during catalytic reactions, so that their activity would dramatically decrease because of aggregation. In contrast, Ih Au–Pt@SiO2 could maintain catalytic activity even after 5 cycles of reactions with high yields (>90%) (Fig. S9, ESI†). Accordingly, the hollow silica spheres can efficiently prevent Ih cores from aggregation, and significantly improve their activity and stability. In conclusion, icosahedral Au–Pt alloy nanoparticles were prepared in hollow silica spheres for the first time. The usage of Pt ions in the synthetic process was critical to acquire Ih nanoparticles. The Ih Au–Pt@SiO2 nanoparticles have shown outstanding catalytic performance in Ullmann homocoupling reactions.
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The Ih Au–Pt@SiO2 nanoparticles were more active than Sr Au@SiO2, probably because Ih nanocrystals possess a higher proportion of {111} planes and twin boundary defects than Sr nanocrystals. Besides, Ih Au–Pt @SiO2 showed higher activity and stability than PVP-stabilizing Ih Au–Pt nanoparticles, confirming the efficient protection of hollow silica for cores. Tuning the shape of metal nanoparticles and encapsulating them in hollow spheres provide new insights into the construction of high performance noble metal nanocatalysts with both excellent activity and stability. This work was supported by the National Natural Science Foundation of China (no. 51202260, 81171454 and 61171049).
Notes and references 1 (a) B. Yuan, Y. Pan, Y. Li, B. Yin and H. Jiang, Angew. Chem., Int. Ed., 2010, 49, 4054; (b) M. Subhramannia and V. K. Pillai, J. Mater. Chem., 2008, 18, 5858; (c) M. Stratakis and H. Garcia, Chem. Rev., 2012, 112, 4469; (d) J. Lee, J. C. Park and H. Song, Adv. Mater., 2008, 20, 1523; (e) A. Corma and H. Garcia, Chem. Soc. Rev., 2008, 37, 2096; ( f ) Z. Peng and H. Yang, Nano Today, 2009, 4, 143. 2 (a) H. Lee, S. E. Habas, S. Kweskin, D. Butcher, G. A. Somorjai and P. Yang, Angew. Chem., 2006, 118, 7988; (b) G. R. Zhang, D. Zhao, Y. Y. Feng, B. Zhang, D. S. Su, G. Liu and B. Q. Xu, ACS Nano, 2012, 6, 2226; (c) S. Kundu, S. Lau and H. Liang, J. Phys. Chem. C, 2009, 113, 5150; (d) M. A. Mahmoud, C. E. Tabor, M. A. El-Sayed, Y. Ding and Z. L. Wang, J. Am. Chem. Soc., 2008, 130, 4590; (e) C. Y. Chiu, P. J. Chung, K. U. Lao, C. W. Liao and M. H. Huang, J. Phys. Chem. C, 2012, 116, 23757. 3 H. Zhang, M. S. Jin, Y. J. Xiong, B. Lim and Y. N. Xia, Acc. Chem. Res., 2013, 46, 1783. 4 (a) F. Kim, S. Connor, H. Song, T. Kuykendall and P. Yang, Angew. Chem., Int. Ed., 2004, 43, 3673; (b) J. Wu, L. Qi, H. You, A. Gross, J. Li and H. Yang, J. Am. Chem. Soc., 2012, 134, 11880; (c) L. F. Chen, G. J. Leong, M. Schulze, H. N. Dinh, B. Pivovar, J. C. Hu, Z. W. Qi, Y. J. Fang, S. Prikhodko, M. Pozuelo, S. Kodambaka and R. M. Richards, ChemCatChem, 2012, 4, 1662; (d) W. Zhou, J. B. Wu and H. Yang, Nano Lett., 2013, 13, 2870. 5 (a) Q. B. Zhang, J. P. Xie, J. H. Yang and J. Y. Lee, ACS Nano, 2009, 3, 139; (b) J. H. Wu, Z. P. Guan, S. K. Yang, P. Y. Yuan, Q. H. Xu and G. Q. Xu, Nanoscale, 2013, 5, 2983; (c) J. Xu, S. Y. Li, J. Weng, X. F. Wang, Z. M. Zhou, K. Yang, M. Litt, X. Chen, Q. Cui, M. Y. Cao and Q. Q. Zhang, Adv. Funct. Mater., 2008, 18, 277; (d) C. C. Huang, W. C. Lai, C. Y. Tsai, C. H. Yang and C. S. Yeh, Chem.–Eur. J., 2012, 18, 4107; (e) M. Tsuji, M. Ogino, R. Matsuo, H. Kumagae, S. Hikino, T. Kim and S. H. Yoon, Cryst. Growth Des., 2010, 10, 296; ( f ) B. Lim, M. J. Jiang, J. Tao, P. H. C. Camargo, Y. M. Zhu and Y. N. Xia, Adv. Funct. Mater., 2009, 19, 189; (g) C. C. Li, R. Sato, M. Kanehara, H. B. Zeng, Y. Bando and T. Teranishi, Angew. Chem., Int. Ed., 2009, 48, 6883; (h) N. Goubet, Y. Ding, M. Brust, Z. L. Wang and M. P. Pileni, ACS Nano, 2009, 3, 3622. 6 (a) B. Lim, Y. Xiong and Y. Xia, Angew. Chem., Int. Ed., 2007, 46, 9279; (b) K. Kwon, K. Y. Lee, Y. W. Lee, M. Kim, J. Heo, S. J. Ahn and S. W. Han, J. Phys. Chem. C, 2007, 111, 1161; (c) J. Xu, S. Li, J. Weng, X. Wang, Z. Zhou, K. Yang, M. Liu, X. Chen, Q. Cui, M. Cao and Q. Zhang, Adv. Funct. Mater., 2008, 18, 277; (d) J. Xiao and L. Qi, Nanoscale, 2011, 3, 1383; (e) S. W. Lu, V. Frain and M. Arbab, J. Phys. Chem. C, 2010, 114, 12850; ( f ) M. Zhou, S. H. Chen and S. Y. Zhao, J. Phys. Chem. B, 2006, 110, 4510. 7 H. Lee, C. Kim, S. Yang, J. W. Han and J. Kim, Catal. Surv. Asia, 2012, 16, 14. 8 R. Narayanan and M. A. El-Sayed, J. Phys. Chem. B, 2004, 108, 8572. 9 (a) J. C. Park, E. Heo, A. Kim, M. Kim, K. H. Park and H. Song, J. Phys. Chem. C, 2011, 115, 15772; (b) Z. Chen, Z. M. Cui, F. Niu, L. Jiang and W. G. Song, Chem. Commun., 2010, 46, 6524; (c) L. Tan, D. Chen, H. Liu and F. Tang, Adv. Mater., 2010, 22, 4885; (d) Y. H. Ng, S. Ikeda, T. Harada, S. Higashida, T. Sakata, H. Mori and M. Matsumura, Adv. Mater., 2007, 19, 597; (e) J. Li and H. C. Zeng, Angew. Chem., Int. Ed., 2005, 44, 4342. 10 (a) J. P. Corbet and G. Mignani, Chem. Rev., 2006, 106, 2651; (b) L. Yin and J. Liebscher, Chem. Rev., 2007, 107, 133. 11 (a) G. Li, C. Liu, Y. Lei and R. Jin, Chem. Commun., 2012, 48, 12005; (b) B. Karimi and F. K. Esfahani, Chem. Commun., 2011, 47, 10452. 12 D. Chen, L. Li, F. Tang and S. Qi, Adv. Mater., 2009, 21, 3804.
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Cite this: Chem. Commun., 2014, 50, 539
Icosahedral gold–platinum alloy nanocrystals in hollow silica: a highly active and stable catalyst for Ullmann reactions†
Received 20th August 2013, Accepted 28th October 2013
Xiaoli Wu,ab Longfei Tan,*a Dong Chen,c Xianwei Menga and Fangqiong Tang*a
DOI: 10.1039/c3cc46383d www.rsc.org/chemcomm
Icosahedral Au–Pt alloy nanocrystals are prepared in porous hollow silica nanospheres via a hydrothermal method without using capping agents. These nanoparticles with unique shape and structure exhibit excellent catalytic activity and stability in Ullmann reactions.
Metal nanocrystals have attracted great attention because of their excellent catalytic performance in synthetic organic chemistry, green chemistry and energy processes.1 The catalytic properties of metal nanocrystals are closely related to their size and shape.2 The crystallographic planes on the surface, as well as the number of atoms on edges or corners of the nanocrystals shall strongly affect their catalytic properties.3 Recently, icosahedral (Ih) nanocrystals have shown excellent catalytic activity due to their unique exposed surface, bounded by {111} facets and built by twin boundary defects.4 For example, Yang and co-workers reported that an icosahedral Pt–Ni nanocrystal was more active than other low-indexed surfaces, such as its {100} surface-bound cube nanocrystal towards the oxygen reduction reaction.4b Several icosahedral metal nanocrystals have been reported, such as Au, Ag, Pd and their alloys.5 Generally, capping agents, such as poly(vinyl pyrrolidone) (PVP) or triblock copolymers were utilized to stabilize and regulate the shape of metal icosahedral nanocrystals in synthetic processes.6 However, the usage of capping agents would bring some disadvantages. For one thing, the capping molecule may block the active sites of surfaces of nanocrystals and reduce their catalytic activity.7 For another, the stabilizing effect of capping agents is not strong enough to keep metal nanocrystals stable in harsh catalytic reactions.8 Therefore, an efficient strategy should be developed to obtain active, stable and shape controlled nano-catalysts.
Hollow nanospheres with porous shells were ideal nanoreactors to fabricate metal nanocrystals and form a core–void–shell (so called yolk–shell) structure. Up to now, various metal nanocatalysts have been prepared in hollow structures for enhanced activity and stability.9 However, the reported metal cores are almost nearspherical nanoparticles. To tune nanocrystal’s shape inside of hollow spheres is of importance and still a great challenge. In this work, we successfully prepared icosahedral Au–Pt alloy nanocrystals into hollow silica nanospheres (Ih Au–Pt@SiO2) for the first time. Pt(IV) ions instead of capping agents were adopted to assist the formation of icosahedral Au cores. This nanocatalyst may lead to unique catalytic properties as the morphology of metal nanocrystals is critically important. The Ullmann reaction, homocoupling of arylhalides, has provided versatile routes for constructing biaryl compounds in industrial organic synthesis.10 So far only a few reports have covered the catalytic application of gold nanocatalysts for Ullmann reactions, such as small gold nanoparticles and nanoclusters,11 which usually endure poor stability during catalysis reaction for their high surface energy. In this work, Ih Au–Pt@SiO2 showed both excellent catalytic activity and cycle stability in homocoupling reaction of iodobenzene due to the presence of the icosahedral Au–Pt core and the protection of the mesoporous shell. Ih nanoparticles inside of hollow silica (Ih Au–Pt@SiO2) were more active than spherical nanoparticles (Sr Au@SiO2), showing a shapedependent catalytic performance. The synthesis route of Ih Au–Pt@SiO2 nanoparticles is shown in Scheme 1. In brief, hybrid silica nanospheres with an etchable and reductive middle layer were designed firstly.12 Then the middle layer was selectively etched under hydrothermal conditions with metal precursors, simultaneously providing a cavity for the
a
Laboratory of Controllable Preparation and Application of Nanomaterials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China. E-mail: [email protected], [email protected]; Tel: +86-10-82543521 b University of Chinese Academy of Sciences, Beijing 100049, China c Beijing Creative Nanophase Hi-Tech Company Limited, 100086, China † Electronic supplementary information (ESI) available: Experimental details and relevant figures. See DOI: 10.1039/c3cc46383d
This journal is © The Royal Society of Chemistry 2014
Scheme 1
The synthesis route of Ih Au–Pt@SiO2.
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Fig. 1 (a and b) TEM images and (c) compositional backscattered SEM image of Ih Au–Pt@SiO2. (d) TEM image of unsupported Ih Au–Pt nanoparticles, and the inset shows a 3D model of a Ih nanocrystal. (e) HRTEM image of Ih Au–Pt nanoparticles. (f) Enlarged image of the square region in (e), the white line is a twin boundary.
in situ growth of metal cores. In this work, the nearly monodisperse hybrid silica nanospheres with an average diameter of 110 3 nm were mixed with HAuCl4 and a small amount of H2PtCl6, and then heated at 200 1C for 3 h. The SEM image (Fig. S1, ESI†) shows uniform morphology of silica shells. The TEM images (Fig. 1a and b) and backscattered SEM image (Fig. 1c) establish that the nanoparticles are located exclusively inside of hollow silica spheres. The thickness of the silica shell is about 10 nm. The BET surface area of Ih Au–Pt@SiO2 nanoparticles is 175.2 m2 g 1 and the average pore size of the shell is determined to be 1.9 nm (Fig. S2, ESI†), which ensures the smooth passage of reactants and products through the silica shell while prevents Au–Pt cores from leakage. For the better observation of Ih Au cores, the silica shell of Ih Au–Pt@SiO2 was carefully etched by hydrofluoric acid, and meanwhile PVP was added to keep the Au cores dispersed. Fig. 1d shows the PVP-protected isolated Ih Au–Pt nanoparticles, which are mainly icosahedra with sizes from B14 to B22 nm. The Au–Pt icosahedron is a multiply twinned structure having 12 corners, 30 edges and 20 planes with the {111} surface, which shows a projected hexagonal shape under TEM. The ratio of icosahedral shape was above 90% as observed from TEM image. The inset of Fig. 1d shows a schematic illustration of the three-dimensional (3D) model of the Ih Au–Pt nanocrystal, which matches very well with the high-resolution TEM (HRTEM) image (Fig. 1e). Lattice planes and twin boundaries of nanocrystals can be seen from Fig. 1f, an enlarged image of Fig. 1e. The d-spacing of adjacent lattice planes is 0.235 nm, corresponding to the {111} planes of facecentered cubic (fcc) gold. The two lattice planes are separated by a twin boundary as shown in Fig. 1f. The results of HRTEM
540 | Chem. Commun., 2014, 50, 539--541
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observation indicate that the Au–Pt nanoparticle contains primarily {111} planes on the surface which are separated by multiple twin boundaries, confirming the icosahedral structure of cores. Compared with UV-vis spectra of Sr Au@SiO2, 5 nm blue-shift of the absorption peaks was observed for Ih Au–Pt@SiO2 (Fig. S3, ESI†). The slight change may be ascribed to the presence of a small amount of Pt. The powder X-ray diffraction (XRD) patterns of Ih Au–Pt@SiO2 nanoparticles (Fig. S4, ESI†) could be indexed to {111}, {200}, and {220} diffraction peaks of fcc gold, which were not affected by Pt. The intensity ratios of {200} and {220} to {111} are 0.22 and 0.14, respectively, lower than the conventional intensity ratios (0.53 and 0.33) of fcc gold, indicating that the {111} planes are dominant in Ih cores.4a PVP or triblock copolymers are common surface capping agents in solution-phase synthesis of Ih Au nanocrystals.6 However, in this work, Pt ions instead of capping agents were adopted to regulate the shape of Au nanocrystals. After adding Pt ions, the cores with icosahedral shape were obtained via a simultaneous etching and growth route. We speculate the Pt ion-assisted formation mechanism of icosahedral Au–Pt cores as follows. Under hydrothermal treatment, H2PtCl6 was reduced together with HAuCl4 by alkylamino groups in hybrid silica nanospheres. Due to the weak reducing ability of alkylamino groups, the nucleation and growth of metal atoms are slow, which promote the formation of twinned seeds.4b Pt atoms might grow preferentially at the corners and edges of nanocrystals to reduce the formation energy of the twin boundaries of twinned Au nanocrystals, which was beneficial to produce icosahedral Au–Pt alloy nanoparticles as a result.4a,b The distribution of the Pt element in an Ih nanocrystal was detected by high-angle annular dark-field scanning TEM (HAADF-STEM)-energy dispersive X-ray (EDX) spectroscopy. The STEM image and elemental mappings indicate that Au and Pt are distributed in the nanocrystal (Fig. 2). The atom ratio of Au to Pt is determined to be 13.5 by SEM-EDX spectroscopy (Fig. S5, ESI†), which is close to the theoretical molar ratio (13.3) of added HAuCl4 and H2PtCl6. The Ih Au–Pt@SiO2 catalyst was next applied to catalyze the Ullmann homocoupling reaction of iodobenzene (Fig. S6, ESI†). Before catalytic reaction, the nanoparticles were calcined in air at 500 1C for 1 h to remove the residual organic groups of hollow silica. The Ih Au–Pt@SiO2 catalyst without calcination only gave a yield of 61.6% using iodobenzene as the substrate. After calcination, the Ih Au–Pt cores retain their original size, shape and dispersity (Fig. S7, ESI†). The catalytic results of different catalysts are shown in Table 1. The green solvent, a mixture of ethanol and water (v/v = 4/1), was used. We found that the existence of water in ethanol was favorable to improve the activity of catalysts, probably because of the suitable solvent polarity and enhanced alkalinity of K2CO3 in aqueous
Fig. 2 (a) STEM image of Ih Au–Pt@SiO2. (b and c) STEM-EDX spectral element mappings for Au (red) and Pt (green).
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Table 1
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Ullmann reactions of iodobenzene catalyzed by different catalystsa
Entry
Catalyst
Solvent
1 2 3 4 5b 6c
lh Au@SiO2 lh Au@SiO2 Sr Au@SiO2 Unsupported Ih Au Pt/C Pd/C
EtOH EtOH–H2O EtOH–H2O EtOH–H2O EtOH–H2O EtOH–H2O
Yield (%) (4/1) (4/1) (4/1) (4/1) (4/1)
84.2 99.2 53.6 47.9 0 86.5
a Reaction conditions: 0.5 mmol substrate, 1 mmol K2C03, 5 mg catalyst with equal Au loading (10 wt%), 10 mL solvent, 80 1C, reaction time 4 h. b 5 mg Pt/C (10 wt% Pt loading). c 10 mg Pd/C (5 wt% Pd loading), reductant 0.8 mmol HCO2Na.
solution than in pure ethanol (entry 1). The Ih Au–Pt@SiO2 nanoparticles exhibited excellent catalytic activity towards the homocoupling of iodobenzene. A 99.2% yield of biphenyl was obtained with Ih Au–Pt@SiO2 at relatively low temperature (80 1C) (entry 2), which was even higher than the commercially available Pd/C nanocatalysts (entry 6). As a control, Au@SiO2 nanoparticles with spherical Au cores (9.8 1.9 nm) were prepared by a similar method and then applied as catalysts (Fig. S8, ESI†). The yield of biphenyl of Sr Au@SiO2 was nearly half of Ih Au–Pt@SiO2 (entry 3), with by-products of benzene from the hydro-deiodination of iodobenzene. Besides, the commercial Pt/C nanocatalysts showed no catalytic activity in this reaction (entry 5). It is worth noting that the surfaces of Ih Au–Pt cores are bound by {111} facets and twin boundary defects.4a,b The high catalytic activity of Ih Au–Pt@SiO2 may attribute to their unique surfaces. These results indicate that the high catalytic activities of Ullmann reactions can be achieved by tuning the shape of Au nanoparticles. The catalytic activities of substrates with substituent groups were also measured. The yield of electron-rich 4-iodoanisole and electron-poor 1-iodo-4-nitrobenzene is 78.3% and 81.7%, respectively. To evaluate the protection effect of hollow silica spheres towards Ih Au–Pt cores, we carefully removed the silica shell of Ih Au–Pt@SiO2 by HF solution and added PVP molecules to stabilize Ih Au–Pt cores (Fig. 1d). The unsupported Ih Au–Pt nanoparticles were further utilized to catalyze homocoupling reaction of iodobenzene, but only a low yield (47.9%) of biphenyl was achieved (entry 4). The unsupported Ih Au–Pt nanoparticles showed severe aggregation after the first cycle of reaction, and no appreciable activity was observed in the succedent cycles. This indicates that the protection of PVP molecules was not strong enough to stabilize Ih nanoparticles during catalytic reactions, so that their activity would dramatically decrease because of aggregation. In contrast, Ih Au–Pt@SiO2 could maintain catalytic activity even after 5 cycles of reactions with high yields (>90%) (Fig. S9, ESI†). Accordingly, the hollow silica spheres can efficiently prevent Ih cores from aggregation, and significantly improve their activity and stability. In conclusion, icosahedral Au–Pt alloy nanoparticles were prepared in hollow silica spheres for the first time. The usage of Pt ions in the synthetic process was critical to acquire Ih nanoparticles. The Ih Au–Pt@SiO2 nanoparticles have shown outstanding catalytic performance in Ullmann homocoupling reactions.
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The Ih Au–Pt@SiO2 nanoparticles were more active than Sr Au@SiO2, probably because Ih nanocrystals possess a higher proportion of {111} planes and twin boundary defects than Sr nanocrystals. Besides, Ih Au–Pt @SiO2 showed higher activity and stability than PVP-stabilizing Ih Au–Pt nanoparticles, confirming the efficient protection of hollow silica for cores. Tuning the shape of metal nanoparticles and encapsulating them in hollow spheres provide new insights into the construction of high performance noble metal nanocatalysts with both excellent activity and stability. This work was supported by the National Natural Science Foundation of China (no. 51202260, 81171454 and 61171049).
Notes and references 1 (a) B. Yuan, Y. Pan, Y. Li, B. Yin and H. Jiang, Angew. Chem., Int. Ed., 2010, 49, 4054; (b) M. Subhramannia and V. K. Pillai, J. Mater. Chem., 2008, 18, 5858; (c) M. Stratakis and H. Garcia, Chem. Rev., 2012, 112, 4469; (d) J. Lee, J. C. Park and H. Song, Adv. Mater., 2008, 20, 1523; (e) A. Corma and H. Garcia, Chem. Soc. Rev., 2008, 37, 2096; ( f ) Z. Peng and H. Yang, Nano Today, 2009, 4, 143. 2 (a) H. Lee, S. E. Habas, S. Kweskin, D. Butcher, G. A. Somorjai and P. Yang, Angew. Chem., 2006, 118, 7988; (b) G. R. Zhang, D. Zhao, Y. Y. Feng, B. Zhang, D. S. Su, G. Liu and B. Q. Xu, ACS Nano, 2012, 6, 2226; (c) S. Kundu, S. Lau and H. Liang, J. Phys. Chem. C, 2009, 113, 5150; (d) M. A. Mahmoud, C. E. Tabor, M. A. El-Sayed, Y. Ding and Z. L. Wang, J. Am. Chem. Soc., 2008, 130, 4590; (e) C. Y. Chiu, P. J. Chung, K. U. Lao, C. W. Liao and M. H. Huang, J. Phys. Chem. C, 2012, 116, 23757. 3 H. Zhang, M. S. Jin, Y. J. Xiong, B. Lim and Y. N. Xia, Acc. Chem. Res., 2013, 46, 1783. 4 (a) F. Kim, S. Connor, H. Song, T. Kuykendall and P. Yang, Angew. Chem., Int. Ed., 2004, 43, 3673; (b) J. Wu, L. Qi, H. You, A. Gross, J. Li and H. Yang, J. Am. Chem. Soc., 2012, 134, 11880; (c) L. F. Chen, G. J. Leong, M. Schulze, H. N. Dinh, B. Pivovar, J. C. Hu, Z. W. Qi, Y. J. Fang, S. Prikhodko, M. Pozuelo, S. Kodambaka and R. M. Richards, ChemCatChem, 2012, 4, 1662; (d) W. Zhou, J. B. Wu and H. Yang, Nano Lett., 2013, 13, 2870. 5 (a) Q. B. Zhang, J. P. Xie, J. H. Yang and J. Y. Lee, ACS Nano, 2009, 3, 139; (b) J. H. Wu, Z. P. Guan, S. K. Yang, P. Y. Yuan, Q. H. Xu and G. Q. Xu, Nanoscale, 2013, 5, 2983; (c) J. Xu, S. Y. Li, J. Weng, X. F. Wang, Z. M. Zhou, K. Yang, M. Litt, X. Chen, Q. Cui, M. Y. Cao and Q. Q. Zhang, Adv. Funct. Mater., 2008, 18, 277; (d) C. C. Huang, W. C. Lai, C. Y. Tsai, C. H. Yang and C. S. Yeh, Chem.–Eur. J., 2012, 18, 4107; (e) M. Tsuji, M. Ogino, R. Matsuo, H. Kumagae, S. Hikino, T. Kim and S. H. Yoon, Cryst. Growth Des., 2010, 10, 296; ( f ) B. Lim, M. J. Jiang, J. Tao, P. H. C. Camargo, Y. M. Zhu and Y. N. Xia, Adv. Funct. Mater., 2009, 19, 189; (g) C. C. Li, R. Sato, M. Kanehara, H. B. Zeng, Y. Bando and T. Teranishi, Angew. Chem., Int. Ed., 2009, 48, 6883; (h) N. Goubet, Y. Ding, M. Brust, Z. L. Wang and M. P. Pileni, ACS Nano, 2009, 3, 3622. 6 (a) B. Lim, Y. Xiong and Y. Xia, Angew. Chem., Int. Ed., 2007, 46, 9279; (b) K. Kwon, K. Y. Lee, Y. W. Lee, M. Kim, J. Heo, S. J. Ahn and S. W. Han, J. Phys. Chem. C, 2007, 111, 1161; (c) J. Xu, S. Li, J. Weng, X. Wang, Z. Zhou, K. Yang, M. Liu, X. Chen, Q. Cui, M. Cao and Q. Zhang, Adv. Funct. Mater., 2008, 18, 277; (d) J. Xiao and L. Qi, Nanoscale, 2011, 3, 1383; (e) S. W. Lu, V. Frain and M. Arbab, J. Phys. Chem. C, 2010, 114, 12850; ( f ) M. Zhou, S. H. Chen and S. Y. Zhao, J. Phys. Chem. B, 2006, 110, 4510. 7 H. Lee, C. Kim, S. Yang, J. W. Han and J. Kim, Catal. Surv. Asia, 2012, 16, 14. 8 R. Narayanan and M. A. El-Sayed, J. Phys. Chem. B, 2004, 108, 8572. 9 (a) J. C. Park, E. Heo, A. Kim, M. Kim, K. H. Park and H. Song, J. Phys. Chem. C, 2011, 115, 15772; (b) Z. Chen, Z. M. Cui, F. Niu, L. Jiang and W. G. Song, Chem. Commun., 2010, 46, 6524; (c) L. Tan, D. Chen, H. Liu and F. Tang, Adv. Mater., 2010, 22, 4885; (d) Y. H. Ng, S. Ikeda, T. Harada, S. Higashida, T. Sakata, H. Mori and M. Matsumura, Adv. Mater., 2007, 19, 597; (e) J. Li and H. C. Zeng, Angew. Chem., Int. Ed., 2005, 44, 4342. 10 (a) J. P. Corbet and G. Mignani, Chem. Rev., 2006, 106, 2651; (b) L. Yin and J. Liebscher, Chem. Rev., 2007, 107, 133. 11 (a) G. Li, C. Liu, Y. Lei and R. Jin, Chem. Commun., 2012, 48, 12005; (b) B. Karimi and F. K. Esfahani, Chem. Commun., 2011, 47, 10452. 12 D. Chen, L. Li, F. Tang and S. Qi, Adv. Mater., 2009, 21, 3804.
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