A single Au nanoparticle anchored inside the porous shell of periodic mesoporous organosilica hollow spheres

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1 Nano Research DOI /s Nano Res 1 A single Au nanoparticle anchored inside the porous shell of periodic mesoporous organosilica hollow spheres Ying Yang 1 ( ), Wen Zhang 1, Ying Zhang 1, Anmin Zheng 2, Hui Sun 1, Xinsong Li 1, Suyan Liu 1, Pengfang Zhang 1, and Xin Zhang 1 ( ), Just Accepted Manuscript DOI /s on June 17, 2015 Tsinghua University Press 2015 Just Accepted This is a Just Accepted manuscript, which has been examined by the peer -review process and has been accepted for publication. A Just Accepted manuscript is published online shortly after its acceptance, which is prior to technical editing and formatting and author proofing. Tsinghua University Press (TUP) provides Just Accepted as an optional and free service which allows authors to ma ke their results available to the research community as soon as possible after acceptance. After a manuscript has been technically edited and formatted, it will be removed from the Just Accepted Web site and published as an ASAP article. Please note that technical editing may introduce minor changes to the manuscript text and/or graphics which may affect the content, and all legal disclaimers that apply to the journal pertain. In no event shall TUP be held responsible for errors or consequences arising fr om the use of any information contained in these Just Accepted manuscripts. To cite this manuscript please use its Digital Object Identifier (DOI ), which is identical for all formats of publication.

2 Template for Preparation of Manuscripts for Nano Research This template is to be used for preparing manuscripts for submission to Nano Research. Use of this template will save time in the review and production processes and will expedite publication. However, use of the template is not a requirement of submission. Do not modify the template in any way (delete spaces, modify font size/line height, etc.). If you need more detailed information about the preparation and submission of a manuscript to Nano Research, please see the latest version of the Instructions for Authors at TABLE OF CONTENTS (TOC) Authors are required to submit a graphic entry for the Table of Contents (TOC) in conjunction with the manuscript title. This graphic should capture the readers attention and give readers a visual impression of the essence of the paper. Labels, formulae, or numbers within the graphic must be legible at publication size. Tables or spectra are not acceptable. Color graphics are highly encouraged. The resolution of the figure should be at least 600 dpi. The size should be at least 50 mm 80 mm with a rectangular shape (ideally, the ratio of height to width should be less than 1 and larger than 5/8). One to two sentences should be written below the figure to summarize the paper. To create the TOC, please insert your image in the template box below. Fonts, size, and spaces should not be changed. A Single Au Nanoparticle Anchored inside the Porous Shell of Periodic Mesoporous Organosilica Hollow Spheres Ying Yang*, Wen Zhang, Ying Zhang, Anmin Zheng, Hui Sun, Xinsong Li, Suyan Liu, Pengfang Zhang and Xin Zhang* China University of Petroleum,China Novel Au@void@PMO hollow hybrids having a single Au nanoparticle anchored inside the porous shell of periodic mesoporous organosilica with superior catalytic efficiency and recyclability have been demonstrated.

3 Nano Research DOI (automatically inserted by the publisher) Research Article A Single Au Nanoparticle Anchored inside the Porous Shell of Periodic Mesoporous Organosilica Hollow Spheres Ying Yang 1 ( ), Wen Zhang 1, Ying Zhang 1, Anmin Zheng 2, Hui Sun 1, Xinsong Li 1, Suyan Liu 1, Pengfang Zhang 1 and Xin Zhang 1 ( ) Received: day month year Revised: day month year Accepted: day month year (automatically inserted by the publisher) Tsinghua University Press and Springer-Verlag Berlin Heidelberg 2014 KEYWORDS gold nanoparticles, periodic mesoporous organosilica, yolk-shell nanostructure, core-shell interaction, 4-nitropheol reduction ABSTRACT An ideal metal catalyst requires easier encounter of reaction reagents, more active sites exposed, and higher stability resistant to leaching or particle agglomeration. Anchoring a metal core inside the porous shell, though scarcely reported, may combine these advantages because of its integration of conventional supported metal arrangement into a core@void@shell architecture. However, it is extremely difficult to achieve this owing to the weak core-shell affinity. Herein, we report for the first time to overcome this challenge through increasing the core-shell interaction for the synthesis of a novel Au@void@PMO (Periodic Mesoporous Organosilica) architecture with a single Au core firmly anchored inside the porous shell of hollow PMO sphere. The non-covalent interactions between PVP (poly(vinyl pyrrolidone)) groups from functionalized Au and ethane moieties in PMO facilitate the movement of Au core to the porous shell during the selective alkaline etching of Au@SiO2@PMO. Shell anchored Au cores are superior to those suspended in the conventional Au@void@PMO, in terms of encounter of reagents and exposure of active sites, and thereby show better catalytic efficiency toward 4-nitrophenol reduction. The methodology demonstrated here can provide a new insight for preparing versatile multifunctional nanostructures with cores anchored inside the hollow shells.

4 2 1 Introduction The yolk-shell nanostructures, represent a special class of core-shell structures with a distinctive core@void@shell configuration, have aroused considerable interest recently due to their tailorability and functionality in both cores and hollow shells, which endow them with intriguing properties and thereby potential applications in nanoreactors, drug delivery, lithium-ion batteries, sensors, energy conversion and storage systems. 1-5 So various yolk-shell arrangements, in which the movable core commonly suspended in the hollow space is protected by the outer shell that acts as a blocking layer preventing core from leaching or aggregation, were successfully fabricated as novel catalysts However, the introduction of shells undoubtedly delays the encounter of reaction reagents with catalysts, causing the declined catalytic efficiency as compared to the exposed catalysts. Besides, the escape of active cores and consequent mutual coagulation may occur by the crush of shell structures during reaction or purification process. 7 Encapsulation of metal cores in the porous shells, pioneered by Han's recent research, may be an effective way out of the dilemma mentioned above. They developed a swelling-evaporation strategy to fabricate novel Au-polymer hollow hybrids having a single Au core encapsulated in each porous polymer shell, and demonstrated their improved catalytic efficiency and recyclability compared to the conventional Au-polymer yolk-shell nanoreactor. 11 In spite of this, the embedded core can hardly provide sufficient surface for adsorbing and activating reagents, and easy encounter of reagents cannot be really realized during reaction, and therefore a higher activity is hindered. As a result, there is still opportunity for the development of novel yolk-shell nanoreactors with superior catalytic efficiency. In order to further improve the catalytic efficiency of yolk-shell nanoreactors, it is rational to suppose that anchoring active cores inside the porous shell through a certain core-shell interaction would be the solution. Such a special configuration could integrate the supported metal into a core@void@shell architecture, and it would provide both easy encounter of reagents and more accessible active sites for subtract adsorbing and activating. However, to our knowledge, anchoring a single metal core inside the porous shell has never been achieved in the extensively studied yolk-shell architectures. The absence of effective strategy to control the core location is the main obstacle for its further development. Thus, accurate control of the core location in a yolk-shell nanostructure is crucial but remains challenging. 2 Experimental Herein, we report a novel Au@void@PMO nanosphere consisting of a single PVP-functionalized Au anchored inside ethane-bridged PMO networks, synthesized by a seeded growth based stepwise synthetic strategy. The schematic illustration of formation of such a Au@void@PMO hybrid is shown in Figure 1. Initially, PVP-stabilized Au cores (Au-5) were prepared by seeding growth of tiny Au particles (Au-2) in the presence of ascorbic acid, which was transformed from hydrogen tetrachloroaurate (HAuCl4) by using NaBH4 and PVP. The resulting Au-5 cores were then coated with silica through the stöber method to provide a Au@SiO2 nanostructure, which was further coated with a PMO layer by CATB-directed self-assembly of 1,4-Bis(triethoxysilyl)ethane (BTEE), affording a sandwiched nanostructure, Au@SiO2@as-PMO. The CTAB and spacer silica layer were then selectively removed by subsequent acidic ethanol washing and alkaline dissolution, respectively, yielding the Au@void@PMO hybrid with a single Au core anchored inside the porous shell of PMO hollow sphere. Figure 1 Schematic description of Au@void@PMO synthesis.

5 3 3 Results and discussion Transmission electron microscopy (TEM) shows tiny Au seed particles centered at ca. 2.3 nm (Figure 2a), and seed grown Au particles with a controlled size of 5.8 nm (Figure 2b). The growth of Au particles also can be reflected from the plasma absorption band, 12 which exhibits a red-shift from 520 to 530 (Figure S1). When coated with silica, the formed Au@SiO2 nanosphere shows a slightly increased Au core size of 6.4 nm (Figure S2) and a shell thickness of ca. 106 nm (Figure 2c). The brightand dark-field TEM images (Figure 2d,g) of the Au@void@PMO hybrid show a single Au core encircled in the hollow PMO sphere (d = 165 nm), in accordance with the size of the hollow sphere observed by SEM (Figure S3).From the high magnification TEM image, it is clear that a single Au core is planted onto the inner walls (wall thickness =13.2 nm) of the hollow sphere with the average size of 7.0 nm (Figure 2e), corresponding to the bright spot in the dark-field TEM image (Figure 2h). Here, the hollow void diameter is ca. 144 nm (Figure S4), larger than the size of Au@SiO2 template. High resolution TEM image displays clear lattice fringes throughout the whole Au particle, and the fringes in (111) direction is nm (Figure 2f). The elemental maps of Au@void@PMO show that Si and O are evenly distributed throughout the shell skeleton, in which the Au is inserted, further demonstrating the yolk-shell nanostructure with a single Au anchored inside the PMO shell (Figure 2). Figure 2 TEM images and the corresponding size distributions (inset) of (a) Au-2, (b) Au-5, (c) Au@SiO2, (d) Au@void@PMO and (e) Au@void@PMO in high magnification, high resolution TEM image of (f) Au particles in Au@void@PMO, dark-field TEM images of (g,h) Au@void@PMO, and HADDF-STEM image along with Si, O, Au elemental maps of (i) Au@void@PMO. Nano Research

6 The chemical composition of was characterized by Fourier Transform Infrared (FT-IR) spectroscopy (Figure 3a). The C- H vibrations of CH2-CH2 moieties appearing in the range of 2925~ 2850 cm 1 are clearly observed. 13 Tough these absorptions may overlap with those of remaining C-H stretching from CTA + fragment at 2925 and 2847 cm 1, the clearly decreased υas (CH2) and υs (CH2) stretching intensity as compared to Au@SiO2@as-PMO reveals that the surfactant CTAB has been mostly removed. 14 The FT-IR spectrum also shows the Si-C stretching modes at 701 and 785 cm 1, 15 providing the integrity of the ethane organic groups in Au@void@PMO. Solid-state 29 Si nuclear magnetic resonance (NMR) spectroscopy was performed to further verify the composition. Au@void@PMO shows the existence of both Q n and T n sites as expected (Figure 3b). The characteristic resonances ranged from -92 to -111 ppm can be ascribed to (HO)2Si(OSi)2 (Q 2, δ, -92), (HO)Si(OSi)3 (Q 3, δ, -102), and Si(OSi)4 (Q 4, δ, -111) silica species, 16 originating from the residue pure silica. The signals at -58 and -66 ppm are attributed to Si atoms bridged by the ethane groups, (HO)SiC(OSi)2 (T 2, δ, -58) and SiC(OSi)3 (T 3, δ, -66), respectively. 17 The T/(T+Q) ratio calculated from the normalized peak area is 0.27, suggesting some pure silica still retained after alkaline washing. The combined results of NMR and FT-IR analysis establish that the framework of Au@void@PMO is built of O1.5Si-CH2-CH2-SiO1.5 units. Figure 3 (a) FT-IR spectra, (b) Solid-state 29 Si NMR spectroscopy of Au@void@PMO, and (c) N2 adsorption/desorption isotherm and the corresponding pore size distribution (inset) of Au@void@PMO. N2 adsorption shows a type IV isotherm with the H3/H4 hybrid hysteresis loop (Figure 3c), typical of hollow mesoporous structure with slit and pilled pores. 18 The H4 loop with parallel and almost horizontal branches at a relative pressure close to the saturation vapour pressure may thus be attributed to the hollow voids between the core and shell. 19 The pore size distribution calculated by the BJH method using the desorption branch is shown in Figure 3c (inset), and Au@void@PMO exhibits a bimodal pore size distribution centered at ca. 4.1 and 7.0 nm with surface area of 367 m 2 g 1 and pore volume of 0.71 cm 3 g 1. Thus, the single Au core was successfully retained owing to the similar particle size to the pore size of shell skeleton. The key factor for realizing the transformation of core-double shell Au@SiO2@as-PMO into hollow hybrid with a single Au anchored inside the PMO shell lies in the strong affinity between PVP groups from functionalized Au and ethane moieties in PMO. To illustrate this point, the naked Au was pre-synthesized and used as a core material for the preparation of yolk-shell nanostructures as the method mentioned above, affording PVP-free Au@void@PMO configuration with a single Au centered at ca. 7.7 nm suspended in the void space (Supporting Information and Figure S5a), which shows a surface area of 372 m 2 /g and pore size at ca. 3.9 nm (Figure S6, Table S1). Similarly, during CTAB-directed self-assembly process, TEOS was used instead of BTEE, yielding Au@void@mSiO2 arrangement with a single Au located in the void space with an average core size of 7.4 nm (Supporting Information and Figure S5b). The resulting Au@void@mSiO2 exhibits a surface area of 283 m 2 /g and pore size at ca. 3.7 nm (Figure S7, Table S1). These configurations agree with the previously reported Au@TiO2, Au@C, Au@POMA (poly(o-methyoxyaniline)), Au@HMSM (hollow mesoporous silica microspheres), and Au@SiO2 systems with movable Au cores However, these Au cores can be fastened and their location in the void space also can be controlled, though it is chronically ignored. Our study presents a first example to anchor a single Au core (7.6 nm, Figure S5c) inside the porous shell, by full consideration of the interactions between PVP groups from functionalized Au and ethane moieties in PMO.

7 5 Selective etching of the middle layer not only creates void space, but also provides possibility for core migration into the shell originated from some non-covalent interaction, like hydrogen bonding and σ-σ pilling between PVP and ethane moieties. To evaluate the accessibility and catalytic performance of yolk-shell nanostructure, the liquid-phase reduction of 4-nitrophenol (4-NP) to 4-aminopheol (4-AP) in the presence of sodium borohydride (NaBH4) was chosen as a model reaction. The catalytic reduction of 4-NP can be easily monitored by the reactant 4-nitrophenolate anion (λmax = 400 nm) through UV-vis spectrophotometry. The reaction did not proceed in the absence of catalyst. However, when Au@void@PMO was introduced into the solution, the absorption at 400 nm quickly decreased while the absorption at 298 nm increased accordingly (Figure 4a). The reduction of 4-NP to 4-AP was completely finished in 15 min, and the color change from bright yellow to light pink was observed (Figure 4b). Considering the NaBH4 concentration is much higher than that of 4-NP (CNaBH4/C4-NP = 100), the reaction can be considered as a pseudo-first-order reaction with regard to 4-NP only to evaluate the catalytic rate. A linear relation of ln(ct/co) versus time, where Ct and C0 are 4-NP concentrations at time t and 0, respectively, was observed for Au@void@PMO catalyst, indicating that the reduction reaction can be considered as a pseudo-first order reaction (Figure 4c). The rate constant is estimated to be min 1, which is much higher than those by using Au-5 colloid solution and Au-MCM-41 (SBET = 615 m 2 /g, pore size = 4.0 nm, Au size = 7.1 nm, see Supporting Information, Figure S8 and Figure S5d), suggesting the positive role of shell in preventing leaching and particle conglomeration. However, when using Au@SiO2 with a single Au coated by a dense silica layer, no measurable catalytic activity could be detected, probably because that the mass transportation was highly retarded owing to the absence of void space. In this study, the Au@void@PMO proves to be more efficient than the comparatively prepared Au yolk-shell nanospheres, PVP-free Au@void@PMO and Au@void@mSiO2, which have movable cores in the void space, exhibiting smaller rate constant of and min 1. To compare different Au-containing nanospheres, we calculated the ratio of rate constant K over total weight of Au cores, where K = k/m. Thus the activity factor K was calculated to be 473, 250, 20, and 100 min -1 mg -1 for Au@void@PMO, Au@void@mSiO2, PVP-free Au@void@PMO, and Au-MCM-41, respectively. It is clear that Au@void@PMO shows the largest activity factor, which is about 236 times larger than previously reported ratio for Au-POMA (2 min -1 mg -1 ), times larger than previously reported ratio for Au@ SiO2 (3 min -1 mg -1 ), 24 and ca. 15 times more than that for the reported Au@C (32 min -1 mg -1 ) 11 yolk-shell nanoreactors (Table S2). The best performance of Au@void@PMO arises from the combined advantages of easy encounter of reagents and more active surface exposed offered by the anchored Au core inside the PMO shell. Considering the shielding effect of PVP, Au@void@PMO was irradiated and examined as a catalyst, and the rate constant was reduced by 0.1 min 1 due to the release of Au core into the void space (Figure S9), making the encounter of reagents difficult, further confirming the combined advantages mentioned above. Finally, the recyclability of Au-5, Au- MCM-41 and Au@void@PMO hybrid was revealed (Figure 4d). It is found that Au-5 and Au-MCM-41 can be reused twice, and show an apparent decrease in catalytic efficiency. It is not surprising that pure Au-5 lost its catalytic activity when recycled due to the conglomeration occurred. Because of the insignificant support shelter for loosely adsorbed Au nanoparticles, severe leaching occurred when Au-MCM-41 was reused. With respect to Au@void@PMO, owing to its controlled core size and the shielding role of shell, it can survive for at least 4 cycles, proving good recyclability. A leaching test for Au@void@PMO catalyst was performed to verify the heterogeneity of the catalytic process. To test for leaching, we filtered the catalyst (after reaction for 5 min). At this time, half the volume was filtered and the resulting clear solution was allowed to react. The percentage of leaching was estimated by comparing the absorption-wavelength plot of the twin reactions with (Figure S10a) and without solid (Figure S10b). It was found that 4-nitrophenol cannot be converted in the filtrate. The absence of Au in the filtrate and the inactivity of filtrate indicate that the Au@void@PMO Nano Research

8 6 is stable. These results suggest that the large majority of the catalysis is carried out by truly heterogeneous However, the gradual loss of activity can be ascribed to the shielding effect of reagents still retained after reaction. Moreover, TEM analysis of the spent reveals that the configuration did not show any change after four runs (Figure S11) compared with the fresh one, suggesting the high structural stability of movement, and also gives a new insight for fabrication of such special yolk-shell nanostructures by accurate control of the core location. Acknowledgements The authors gratefully acknowledge financial support from the National Natural Science Foundation of China ( , , ), Beijing Natural Science Foundation ( ), the Science Foundation of China University of Petroleum, Beijing ( YJRC018), Ministry of Science and Technology of China (No. 2011BAK15B05), and Specialized Research Fund for the Doctoral Program of Higher Education ( ). Figure 4 (a) UV-vis spectroscopy of the reaction mixture, (b) optical photos of the color change during the reaction, (c) the relationship between ln(ct/c0) and reaction time (t), and (d) the recyclability test. 4 Conclusions In summary, a novel yolk-shell nanostructure with a single Au nanoparticle anchored inside the porous shell of hollow PMO sphere has been successfully fabricated by a seed grown stepwise synthetic strategy. Selective etching of the middle silica layer facilitates the transformation of core-double shell Au@SiO2@as-PMO to yolk-shell Au@void@PMO, and provides the possibility for anchoring the Au core inside the shell based on the non-covalent interactions between PVP groups from functionalized Au and ethane moieties in PMO. The superior catalytic efficiency has also been established, indicating its potential application as efficient and recyclable catalyst involved in liquid-phase catalysis. The development of Au@void@PMO proves the concept of non-covalent interaction-induced Electronic Supplementary Material: Supplementary material (please give brief details, e.g., further details of the annealing and oxidation procedures, STM measurements, AFM imaging and Raman spectroscopy measurements) is available in the online version of this article at (automatically inserted by the publisher). References [1] Li, G. D.; Tang, Z. Y. Noble metal nanoparticle@metal oxide core/yolk-shell nanostructures as catalysts: recent progress and perspective. Nanoscale 2014, 6, [2] Pérez-Lorenzo, M.; Vaz, B.; Salgueiriño, V.; Correa-Duarte, M. A. Hollow-shelled nanoreactors endowed with high catalytic activity. Chem. Eur. J. 2013, 19, [3] Liu, J.; Qiao, S. Z.; Chen, J. S.; Lou, X. W.; Xing, X. R.; Lu, G. Q. Yolk/shell nanoparticles: new platforms for nanoreactors, drug delivery and lithium-ion batteries. Chem. Commun. 2011, 47, [4] Park, J. C.; Song, H. Metal@silica yolk-shell nanostructures as versatile bifunctional nanocatalysts. 2011, 4, [5] Lou, X. W.; Archer, L. A.; Yang, Z. C. Hollow micro-/nanostructures: synthesis and applications. Adv. Mater. 2008, 20, [6] Ko, Y. N.; Kang, Y. C.; Park, S. B. Continuous one-pot synthesis of sandwich structured core-shell particles and transformation to yolk-shell particles. Chem. Commun. 2013, 49, [7] Arnal, P. M.; Comotti, M.; Schth, F. High-temperature-stable catalysts by hollow sphere encapsulation. Angew. Chem. Int.

9 7 Ed. 2006, 45, [8] Liu, R.; Mahurin, S. M.; Li, C.; Unocic, R. R.; Idrobo, J. C.; Gao, H.; Pennycook, S. J.; Dai, S. Dopamine as a carbon source: the controlled synthesis of hollow carbon spheres and yolk-structured carbon nanocomposites. Angew. Chem. Int. Ed. 2011, 50, [9] Yang, T. Y.; Liu, J.; Zheng, Y.; Monteiro, M. J.; Qiao, S. Z. Facile fabrication of core-shell-structured Ag@carbon and mesoporous yolk-shell-structured Ag@carbon@silica by an extended Stöber method. Chem. Eur. J. 2013, 19, [10] Dong, K.; Liu, Z.; Ren, J. S. A general and eco-friendly self-etching route to prepare highly active and stable Au@metal silicate yolk-shell nanoreactors for catalytic reduction of 4-Nitrophenol. CrystEngComm 2013, 15, [11] Han, J.; Wang, M. G.; Chen, R.; Han, N.; Guo, R. Beyond yolk-shell nanostructure: A single Au nanoparticle encapsulated in the porous shell of polymer hollow spheres with remarkably improved catalytic efficiency and recyclability. Chem. Commun. 2014, 50, [12] Li, B. X.; Gu, T.; Ming, T.; Wang, J. X.; Wang, P.; Wang, J. F.; Yu, J. C. (Gold core)@(ceria shell) nanostructures for plasmon-enhanced catalytic reactions under visible light. ACS Nano 2014, 8, [13] Liu, J.; Yang, H. Q.; Kleitz, F.; Chen, Z. G.; Yang, T.Y.; Strounina, E.; Lu, G. Q.; Qiao, S. Z. Yolk-shell hybrid materials with a periodic mesoporous organosilica shell: ideal nanoreactors for selective alcohol oxidation. Adv. Funct. Mater. 2012, 22, [14] Gallo, J. M. R.; Pastore, H. O.; Schuchardt, U. Silylation of [Nb]-MCM-41 as an efficient tool to improve epoxidation activity and selectivity. J. Catal. 2006, 243, [15] Wahab, M. A.; Kim, II.; Ha, C. -S. Hybrid periodic mesoporous organosilica materials prepared from 1,2-Bis(triethoxysilyl)ethane and (3-Cyanopropyl)triethoxysilane. Micropor. Mesopor. Mater. 2004, 69, [16] Jin, Y.; Wang, P. J.; Yin, D. H.; Liu, J. F.; Qiu, H. Y.; Yu, N. Y. Gold nanoparticles stabilized in a novel periodic mesoporous organosilica of SBA-15 for styrene epoxidation. Micropor. Mesopor. Mater. 2008, 111, [17] Zhuang, T. Y.; Shi, J. Y.; Ma, B. C.; Wang, W. Chiral norbornane-bridged periodic mesoporous organosilicas. J. Mater. Chem. 2010, 20, [18] Corma, A. From microporous to mesoporous molecular sieve materials and their use in catalysis. Chem. Rev. 1997, 97, [19] Robles-Dutenhefner, P. A.; Rocha, K. A. D.; Sousa, E. M. B.; Gusevskaya, E. V. Cobalt-catalyzed oxidation of terpenes: Co-MCM-41 as an efficient shape-selective heterogeneous catalyst for aerobic oxidation of isolongifolene under solvent-free conditions. J. Catal. 2009, 265, [20] Lee, I.; Joo, J. B.; Yin, Y. D.; Zaera, F. A yolk@shell nanoarchitecture for Au/TiO2 catalysts. Angew. Chem. Int. Ed. 2011, 50, [21] Liu, R.; Qu, F. L.; Guo, Y. L.; Yao, N.; Priestley, R. D. Au@carbon yolk-shell nanostructures via one-step core-shell-shell template. Chem. Commun. 2014, 50, [22] Han, J.; Chen, R.; Wang, M. G.; Lu, S.; Guo, R. Core-shell to yolk-shell nanostructure transformation by a novel sacrificial template-free strategy. Chem. Commun. 2013, 49, [23] Wang, S. N.; Zhang, M. C.; Zhang, W. Q. Yolk-shell catalyst of single Au nanoparticle encapsulated within hollow mesoporous silica microspheres. ACS Catal. 2011, 1, [24] Lee, J.; Park, J. C.; Song, H. A nanoreactor framework of a Au@SiO2 yolk/shell structure for catalytic reduction of p-nitrophenol. Adv. Mater. 2008, 20, Nano Research

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11 Electronic Supplementary Material A Single Au Nanoparticle Anchored inside the Porous Shell of Periodic Mesoporous Organosilica Hollow Spheres Ying Yang 1 ( ), Wen Zhang 1, Ying Zhang 1, Anmin Zheng 2, Hui Sun 1, Xinsong Li 1, Suyan Liu 1, Pengfang Zhang 1 and Xin Zhang 1 ( ) Supporting information to DOI /s12274-****-****-* (automatically inserted by the publisher) 1. Experimental Section 1.1. Materials Hydrogen tetrachloroaurate (HAuCl4, Aldrich, > 99.9%), poly(vinylpyrrolidone) (PVP, Ourchem), 1,4-Bis(triethoxysilyl)ethane (BTEE, 92%) and sodium borohydride (NaBH4, 96%) are commercially acquired. Tetraethylorthosilicate (TEOS), sodium citrate, ascorbic acid, cetyltrimethyl ammonium bromide, 4-nitrophenol, sodium hydroxide, ammonia (25 wt%), concentrated hydrochloric acid (37 wt%), and ethanol are of analytical grade and used directly without any further purification Synthesis Preparation of 2.3 ± 0.3 nm gold seed A 50 ml of aqueous solution containing 5 ml HAuCl4 (10 mm) and mg of PVP was prepared in a conical flask, and stirred at 0 o C for 30 min. Then, 5 ml of ice-cold, freshly prepared NaBH4 solution (0.1 M) was added to the solution while stirring. The solution turned pink immediately after adding NaBH4, indicating particle formation. The resulting gold seed colloidal solution was stored in a refrigerator Seeding growth A 35 ml of aqueous solution containing 5 ml HAuCl4 (10 mm) and mg of PVP was prepared in a conical flask, and stirred at 0 o C for 30 min. Then 10 ml of seed solution was added while stirring, and further stirred for 30 min. And then 15 ml of freshly prepared ascorbic acid solution (5 mm) was added slowly, and further stirred for 2 h. The resulting gold seed colloidal solution was stored in a refrigerator Synthesis of Au@SiO2 The coating of silica on PVP-stabilized gold nanoparticles centered at ca. 5.8 nm can be achieved though a slightly modified stöber process. In a typical synthesis, 6 ml of above-mentioned Au colloidal solution was mixed 18.9 ml of ethanol, 0.84 ml of ammonia, and stirred at room temperature for 5 min. Immediately Nano Research

12 afterwards, a solution of TEOS (1.19 ml) in 12.8 ml of ethanol was added. The reaction mixture was then stirred for an additional 12 h at room temperature. And the resulting gel was stored in a refrigerator Synthesis of yolk-shell Au@void@PMO nanosphere First, the core-double shell Au@SiO2@as-PMO nanosphere was prepared via coating PMO layer on Au@SiO2, by using CTAB-directed self-assembly of BTEE under alkaline conditions. In a typical synthesis, 0.15 g of CTAB was dissolved in 30 ml of deionized water and stirred for 0.5 h, followed by addition of 18 ml of Au@SiO2 colloidal solution. After further stirring for 2 h, 0.15 g of CTAB combined with 0.35 ml of NaOH (2 M) was added and the resulting mixture was heated to 80 ºC ml BTEE was added at this temperature and further stirred for 24 h. The resulting gel was then filtered off and dried at 50 o C, yielding Au@SiO2@as-PMO hybrid. The structure-directing agent was removed by extraction of the solid with dilute ethanolic HCl acid solution at 40 o C for 5 h (20 ml of 0.5 M ethanolic HCl for 0.5 g of solid). The resulting Au@SiO2@PMO was mixed with 5 ml NH3 H2O and 10 ml of ethanol, and stirred at 60 o C for 24 h to remove the middle silica layer, affording Au@void@PMO hybrid Comparative synthesis Preparation of MCM-41 supported Au catalyst, Au-MCM-41 Typically, 0.45 g hexadecyltrimethylammonium bromide (CTAB) was dissolved in 18.5 ml of distilled water, and then 7.0 ml of ethanol, 6.8 ml of ammonia, and 2.6 ml of tetraethoxysilane (TEOS) was added sequentially to the homogeneous micelle solution. The resulting mixture was further stirred at room temperature for 1 h, and then transferred into a teflon-lined stainless steel autoclave and aged at 100 o C for 24 h. After cooling down to room temperature, the products were filtered, washed with distilled water repeatedly, and dried overnight at 60 o C in air. The structure-directing agent was removed by extraction of the solid with dilute ethanolic HCl acid solution at 40 o C for 5 h (20 ml of 0.5 M ethanolic HCl for 0.5 g of solid), affording pure siliceous MCM-41. Then the 0.25 g of MCM-41 was added into 2.7 ml of Au-5 colloidal solution and stirred for 24 h. Further filtration, washing with de-ionized water and ethanol, and drying at 60 o C, afforded Au-MCM Preparation of Au@viod@mSiO2 The procedure and the dosage of reagent used for Au@viod@mSiO2 synthesis is identical to that for Au@void@PMO synthesis, except for using BTEE instead of TEOS Preparation of PVP-free Au@void@PMO First, PVP-free Au colloidal solution was prepared by a two-stepwise reduction procedure according to literature method. 1 Typically, 5 ml of HAuCl4 (10 mmol/l) was diluted with 45 ml of de-ionized water and stirred for 5 min. Then 1 ml of aqueous sodium citrate (1 wt%) was added into the solution rapidly and stirred for 5 min. And then 1 ml of NaBH4 (using 1 wt% of aqueous sodium citrate as the solvent) was added and stirred for further 5 min, yielding PVP-free Au colloidal solution cantered at ca. 6 nm. The following precursor for silica coating, supermolecular assembly, as well as template and silica removal is identical to that for Au@void@PMO synthesis, finally affording PVP-free Au@void@PMO Characterization SEM analysis was performed on a FEI-Quanta 200F field-emission scanning microscope operated at 15 kv with an EDX detector to determine the morphology of the prepared samples. Transmission electron microscope (TEM) measurements were carried out on a JEOL JEM-2100 transmission electron microscope with a tungsten filament at an accelerating voltage of 200 kv. HADDF-STEM and elemental mapping were carried out by using a FEI Tecnai G2 F20 transmission electron microscope operated at a voltage of 200 kv. The samples were prepared by placing a drop of the prepared solution on the surface of a copper grid. Powder X-ray diffraction

13 (XRD) was conducted using D8 Advance (Bruker) diffractometer. Diffractogrames were recorded in reflection mode using Ni-filtered CuKα radiation (λ = nm). The samples were scanned in the 2θ range from 5 to 60 in a step of 2 /min. The operation voltage and current were kept at 40 kv and 40 ma. N2 adsorption/desorption isotherms were recorded on a Micromeritics ASAP 2020 automated sorption analyzer. Before measurements, the samples were outgassed at 120 o C for 4 h. The specific surface area was calculated by using the Brunauer-Emmett-Teller (BET) method and the pore size distributions were measured by using Barrett-Joyner-Halenda (BJH) analyse from the desorption branch of the isotherms. UV-vis spectroscopy was carried out with a PC 456 UV-vis spectrometer. All the measurements were performed at room temperature. The infrared spectra of samples were recorded in KBr disks using a Nicolet Nexus 870 FTIR spectrometer. The 29Si solid-state NMR experiment was carried out on a Varian Infinityplus-400 spectrometer at a resonance frequency of 79.4 MHz. The experiment was recorded using a 5 mm MAS probe at a spinning rate of 6 khz. 29 Si MAS NMR spectra with high power proton decoupling w ere recorded using a π/2 pulse length of 7.2 us and a recycle delay of 80 s. The chemical shift of 29 Si was externally referenced to kaolinite (-91.5 ppm). Metal content was estimated by inductively coupled plasma atomic emission spectroscopy (ICP-AES) analysis conducted on a Perkin Elmer emission spectrometer. Certain amount of vacuum-dried material was dissolved in 3 ml of boiling aqua fortis solution. A 1.5 ml of hydrofluoric acid was added to dissolve the silica support and then completely evaporated. The residue was acidified with concentrated HNO3 before dilution to ml with deionized water. Each solution was filtered through a 0.45 μm polyethersulfone filter and then submitted for metal analysis Catalytic reduction of 4-nitrophenol The reduction of 4-nitrophenol (4-NP) was carried out in a quartz cuvette and monitored using a UV-vis spectroscopy at room temperature. For comparison, the aqueous 4-NP solution (0.01 M) was prepared and measured prior to monitoring the change of absorption. Then a total of 25 μl of aqueous 4-NP solution was mixed with 2.5 ml of fresh NaBH4 (0.03 M) solution. Subsequently, a given amount of gold catalyst was added to start the reaction, and the UV spectrometry was employed to in situ monitor the reduction by measuring the absorbance of the solution at 400 nm as a function of time at an interval of 2.5 min. After the reaction was finished, the mixture was centrifuged, washed with ethanol and water three times. The resulting catalyst was re-dispersed in 25 μl of aqueous 4-NP solution and 2.5 ml of fresh NaBH4 solution, and the test was repeated to probe the stability. Reference (1) Quinten, M.; Leiner, A.; Krenn, J. R.; Aussenegg, F. R. Opt. Lett. 1998, 23, Nano Research

14 Table S1 Textual properties of Au-containing catalysts Materials SBET (m 2 /g) Pore width (nm) Au@void@PMO , 7.0 PVP-free Au@void@PMO Au@void@mSiO Au-MCM Table S2 Comparison of Au content and catalytic performance of various Au-containing catalysts Materials Au loading Catalyst k K (wt%) amount (mg) (min -1 ) (min -1 mg -1 ) Au@void@PMO Au-MCM Au@void@mSiO PVP-free Au@void@PMO Au-POMA (mg) 0.3 ml Au@SiO (mg) 1 ml Au@C (mg) 0.3 ml Au@C Au@MgSiO Au@C Au@HMSM umol/l - -

15 0.16 Abasorbance Au-2 Au Wavelength (nm) Figure S1 UV-vis spectra of Au-2 and Au nm Frequence (%) Particle size (nm) Figure S2 Au particle size distribution for Au@SiO Nano Research

16 Figure S3 SEM image of nm Frequence (%) Hollow radius (nm) Figure S4 Hollow radius of Au@void@PMO.

17 Figure S5 TEM images of (a) PVP-free (b) (c) and (d) Au-MCM-41, and the corresponding Au size distributions (inset). Nano Research

18 Quantity adsorbed (cm 3 g -1, STP) DV/dlog(W) (cm 3 g -1 Å -1, STP) Pore width (nm) Relative pressure (P/P 0 ) Figure S6. N2 adsorption/desorption isotherm and the corresponding pore size distribution curve (inset) for PVP-free Au@void@PMO. 10 Quantity adsorbed (cm 3 g -1, STP) DV/dlog(W) (cm 3 g -1 Å -1, STP) Pore width (nm) Relative pressure (P/P 0 ) Figure S7. N2 adsorption/desorption isotherm and the corresponding pore size distribution curve (inset) for Au@void@mSiO2.

19 Quantity adsorbed (cm 3 g -1, STP) DV/dlog(W) (cm 3 g -1 Å -1 ) Pore width (nm) Relative pressure (P/P 0 ) Figure S8. N2 adsorption/desorption isotherm and the corresponding pore size distribution curve (inset) for Au-MCM-41. Figure S9 TEM images of Au@void@PMO after irradiation. Nano Research

20 a Abasorbance Wavelength (nm) 2.5 b 2.0 Absorbance Wavelength (nm) Figure S10 Absorbance-wavelength plot for 4-nitrophenol reduction over Au@void@PMO-containing mixture (a) and filtrate (b). Heterogeneous reaction check for Au@void@PMO by continuing the reaction after removing the Au@void@PMO by filtration after 5min. Reaction conditions: 3 mg Au@void@PMO, 0.01 M 4-nitropheol 2.5 μl, 0.01 M NaBH4 2.5 ml.

21 Figure S11 TEM images of spent Nano Research