Accepted Manuscript Synthesis of mesoporous silica hollow nanospheres with multiple gold cores and catalytic activity Junchen Chen, Zhaoteng Xue, Shanshan Feng, Bo Tu, Dongyuan Zhao PII: DOI: Reference:
S0021-9797(14)00296-3 http://dx.doi.org/10.1016/j.jcis.2014.05.005 YJCIS 19557
To appear in:
Journal of Colloid and Interface Science
Received Date: Accepted Date:
26 March 2014 9 May 2014
Please cite this article as: J. Chen, Z. Xue, S. Feng, B. Tu, D. Zhao, Synthesis of mesoporous silica hollow nanospheres with multiple gold cores and catalytic activity, Journal of Colloid and Interface Science (2014), doi: http://dx.doi.org/10.1016/j.jcis.2014.05.005
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Synthesis of mesoporous silica hollow nanospheres with multiple gold cores and catalytic activity
Junchen Chen, Zhaoteng Xue, Shanshan Feng, Bo Tu*, and Dongyuan Zhao*
Department of Chemistry and Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Laboratory of Advanced Materials, Fudan University, Shanghai 200433, P. R. China. E-mail: [email protected], [email protected] Tel: +86-21-5163-0205; Fax: +86-21-5163-0307.
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Abstract: The core–shell Au@resorcinol-formaldehyde (RF) nanospheres with multiple cores have been successfully synthesized by a modified Stöber method. After coating mesoporous silica and the calcination, the Au@meso-SiO2 hollow nanospheres with multiple gold cores can be obtained, which have a high surface area (~ 537 m2/g) and uniform pore size (~ 2.5 nm). The Au@meso-SiO2 hollow nanospheres can be used as a catalyst for the reduction of 4-nitrophenol by NaBH4 into 4-aminophenol, and exhibit excellent catalytic performance.
Keywords: Mesoporous silica, hollow nanospheres, gold, multiple cores, synthesis, catalysis
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1. Introduction In recent years, a special class of core–shell structures with core@void@shell configuration (the hollow nanospheres with cores) has attracted considerable attention because of the following advantages. (I) The moveable cores can afford much more exposed active sites to interact with the guest molecules more effectively. (II) The outer shells can prevent the aggregation of neighboring nanoparticles. (III) The hollow spaces between the cores and the shells can provide large space for loading functional molecules. Due to the functionalities of the freely movable cores, the protective shells and the spaces between them, hollow nanospheres have been widely applied in the fields of catalysis, biomedicine, lithium-ion batteries and so on [1-13]. Up to now, a variety of synthetic strategies have been developed to fabricate hollow nanospheres with metal cores. According to the sequence of preparing cores and
shells,
the
methods
can
be
classified
into
pre-shell/post-core
and
pre-core/post-shell approaches. In the pre-shell/post-core approaches, the metal ions are reduced to grow metal cores in the pre-formed hollow shells [14-17]. Most of the hollow nanospheres are synthesized by pre-core/post-shell approaches, firstly, a sandwich structure (core/template/shell) is fabricated by using hard or soft templates, and then, the voids can be formed via selectively removing the intermediate sacrificial template layer, using a solvent or calcination [18-24]. Generally, the inner sacrificial shells are composed of silica or carbon precursors for the hard-templating method. The silica shells can be selectively removed by NaOH [21], NH3·H2O [10], Na2CO3 [25], NaBH4 [19, 26], H2O [11, 27] and
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organosilane [16, 28]. However, compared with etching silica in solution, calcination in air to remove the carbonaceous shells is much more simple. Many carbon precursors can be used to fabricate carbon-based nanospheres, such as resorcinol-formaldehyde (RF) resin [29-32], phenol-formaldehyde resin [33, 34], saccharide [35, 36], polydopamine [37] and so on. Among them, RF resin, a three-dimensional (3-D) network structured polymer, has been paid much attention due to the attractive properties such as low cost, controllable morphology and outstanding stability. Until now, various porous shells have been fabricated to encapsulate metal nanoparticles, such as SiO2, ZrO2, TiO2 and C [3, 5, 38-44]. The porous outer shells can provide convenient channels for the diffusion and transport of reactants to reach the surface of the active cores, and protect the active metal cores from leakage. In particular, mesoporous silicas have been paid much attention because of their high surface area, large pore volume, tunable uniform pore size, high stability, controllable morphology, facile surface functionalization and high biocompatibility [45]. The Stöber method has attracted many interests for the fabrication of silica shells on nanoparticles through the facile hydrolysis and condensation of tetraethyl orthosilicate (TEOS) in basic aqueous solutions [46]. Functional nanoparticles have been encapsulated in silica to form the core–shell structures, including noble-metal nanoparticles, metal oxides, and quantum dots [25, 47-49], because the encapsulated nanoparticles can maintain their specific catalytic, magnetic, electronic or optical properties. Among them, gold@silica nanospheres have been applied in many fields
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such as catalysis, drug delivery and bioimaging [15-17, 22, 50]. In the case of nanocatalysts, the strategy of encapsulating gold nanoparticles in hollow mesoporous silica nanospheres is a good way to obtain great catalytic performance, because the gold cores can move freely inside the cavity of the shells to sufficiently expose the surface of the active cores. Herein, we demonstrate the one-pot synthesis of the core–shell Au@RF nanospheres with multiple gold cores by a modified Stöber method, using resorcinol and formaldehyde as the precursors, NH3·H2O as the catalyst, surfactant CTAB and F127 as the costabilizers. After the core–shell Au@RF@meso-SiO2 nanospheres were calcined, Au@meso-SiO2 hollow nanospheres with multiple gold cores could be achieved. The obtained Au@meso-SiO2 hollow nanospheres have a high surface area (~ 537 m2/g) and uniform pore size (~ 2.5 nm). Furthermore, the Au@meso-SiO2 hollow nanospheres were successfully employed in the catalytic reduction of 4-nitrophenol as a model reaction.
2. Experimental 2.1 Chemicals Cetyltrimethylammonium bromide (CTAB, 99.0 wt%), formaldehyde solution (37.0 wt%), resorcinol (99.0 wt%), aqueous ammonia (NH3·H2O, 25.0 wt%) and chloroauric acid tetrahydrate (≥ 47.8 wt% Au) were purchased from Sinopharm Chemical Reagent Company. Tetraethyl orthosilicate (TEOS, 98.0 wt%), NaBH4 (99.0 wt%), 4-nitrophenol (4-NP, 99.0 wt%) and absolute anhydrous ethanol (99.7 wt%)
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were obtained from Shanghai Chemical Company. Tri-block copolymer Pluronic F127 (EO106PO70EO106, EO = ethylene oxide, PO = propylene oxide, 99.0 wt%) was purchased from Sigma-Aldrich. All chemicals were used without additional purification. Deionized water was used for all experiments.
2.2 Synthesis Synthesis
of
core–shell
Au@resorcinol-formaldehyde
polymer (Au@RF)
nanospheres: In a typical synthesis, 0.125 g of F127 and 0.05 g of CTAB were dissolved in the solution containing 10 mL of deionized water and 4 mL of ethanol. After stirring at 30 °C for 15 min, 0.05 mL of NH3·H2O was added. After further stirring for 50 min, 0.1 g of resorcinol was added. Stirring for additional 30 min, 0.14 mL of formaldehyde solution was added. The mixture was stirred for 24 h at 30 °C, then, 0.5 mL of 0.1 M HAuCl4 aqueous solution was added. After stirring for 30 min, the mixture was transferred to a Teflon-lined auto-clave and heated for 24 h at 100 °C under static conditions. The products were centrifuged, washed three times with water and once with absolute ethanol, and then dried at 60 °C overnight, after which the core–shell Au@RF nanospheres were obtained. Synthesis of core–shell Au@RF@meso-SiO2 nanospheres: The core–shell nanospheres with double shells were prepared through a surfactant-templating sol–gel approach by using CTAB as a template. In a typical synthesis, 0.075 g of CTAB was dissolved in the solution containing 25 mL of deionized water and 13 mL of ethanol. After stirring at 30 °C for 15 min, 1 mL of an ethanol dispersion solution containing
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0.05 g of Au@RF nanospheres was added. The mixture was homogeneously dispersed by ultrasonication for 15 min, followed by the addition of 0.25 mL of NH3·H2O. Stirring for additional 10 min, a solution containing 0.11 g of TEOS and 1.0 mL of ethanol was added dropwise with continuous stirring for 1 min. After reaction for 6 h, the products were centrifuged, washed for three times with water and once with absolute ethanol, and then dried at 60 °C overnight, after which the core–shell Au@RF@meso-SiO2 nanospheres were obtained. Synthesis of Au@meso-SiO2 hollow nanospheres: Removal of the polymer shells and
surfactants
was
achieved
by
the
calcination
of
the
core–shell
Au@RF@meso-SiO2 nanospheres in air from room temperature to 550 °C for 6 h at a rate of 1 °C·min-1, after which the Au@meso-SiO2 hollow nanospheres were obtained.
2.3 Catalytic Reduction The catalytic reduction process of 4-nitrophenol (4-NP) was carried out in a quartz cuvette and monitored by in-situ measuring the UV-vis absorption spectra. At first, 0.125 mL of 0.2 M freshly prepared NaBH4 solution was added in the solution containing 0.0125 mL of 0.005 M 4-NP and 1.0 mL of deionized water. Subsequently, 0.05 mL of aqueous dispersion of 0.0125 wt% Au@meso-SiO2 hollow nanospheres was added, and the reaction started immediately. The gradual change of the solution color from light bright yellow to colorless was observed during the reaction. To study the recyclability of the catalysts, the used Au@meso-SiO2 hollow nanospheres were separated from the reaction mixture by centrifugation at the end of each run, washed
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once with absolute ethanol and once with deionized water, then re-dispersed in water by ultrasonication and added in a fresh reaction solution. After reaction for 20 min, the solution was measured using UV-vis spectroscopy.
2.4 Characterization Scanning electron microscopy (SEM) images were obtained on a FE-SEM S-4800 microscope (Hitachi, Japan) operated at 1 kV. Transmission electron microscopy (TEM) images were taken by a JEM-2011F microscope (JEOL, Japan) operated at 200 kV. The samples for TEM measurements were dispersed in ethanol, and then dipped and dried on holey carbon films supported on a Cu grid. Energy dispersive X-ray spectroscopy (EDX) was recorded on an IET-200 EDX instrument (Oxford, England). Thermogravimetric analysis (TGA) was monitored by using a Mettler Toledo TGA/DSC 1 analyzer (Switzerland) from 30 to 600 °C under air (40 ml·min-1) with a heating rate of 5 °C·min-1. X-ray diffraction (XRD) patterns were recorded on a D8 advance X-ray diffractometer (Bruker, Germany) with Ni-filtered Cu Kα radiation (40 kV, 40 mA). Nitrogen adsorption/desorption isotherms were measured with a Tristar II 3020 analyzer (Micromeritics, USA) at 77 K. Before the measurements, the samples were degassed in vacuum at 180 °C for at least 6 h. The surface area and pore size were obtained by using Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) methods, respectively. The carbon contents of the samples were measured on a Vario EL Ⅲ elemental analyzer (Germany). The UV-vis spectra were collected using a UV-3600 spectrophotometer (Shimadzu, Japan).
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3. Results and discussion SEM images (Fig. 1a) show that the Au@RF nanospheres have well spherical morphology, the size of the core-shell Au@RF nanospheres is quite uniform with the average diameter of about 850 nm. The monodispersed spherical morphology of the core–shell Au@RF nanospheres can be further confirmed from the TEM images (Fig. 1b). TEM images display that many gold nanoparticles as cores with a size of about 15 nm are located randomly in one Au@RF nanosphere. After further coating a layer of mesoporous silica on the core–shell Au@RF nanospheres by using CTAB as the template, the core–shell Au@RF@meso-SiO2 nanospheres with double shells can be obtained. TEM images (Fig. 2a) show that the Au@RF@meso-SiO2 nanospheres are composed of many small gold cores, inner shell of crosslinked-RF and outer shell of mesoporous silica. After calcination in air, the Au@meso-SiO2 hollow nanospheres can be obtained, due to the removal of the polymer shells and surfactants (Fig. 2b). Many larger gold cores with the diameters from 20 to 80 nm are dispersed in the interior void, due to the aggregation of the smaller gold nanoparticles (~ 15 nm) during the calcination. The high-resolution TEM (HRTEM) image (Fig. 2c) displays the radial mesoporous silica as a shell with unobstructed mesopores in one Au@meso-SiO2 hollow nanosphere. The HRTEM image of the gold core (Fig. 2d) demonstrates the polycrystalline structure with fringes associated with a d-spacing of ~ 0.235 nm, which can be assigned to the (111) planes of the cubic gold lattice. The TGA curve (Fig. 2e) of the core–shell
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Au@RF@meso-SiO2 nanospheres demonstrates a weight loss of ~ 70 % in the temperature range from 200 to 520 °C in air, which is attributed to the decomposition of crosslinked-RF polymer, surfactants CTAB and F127. However, no obvious weight loss of the Au@meso-SiO2 hollow nanospheres is observed, suggesting the entire removal of the RF polymer and surfactants. In addition, elemental analysis shows that the mass percentage of carbon in the core–shell Au@RF@meso-SiO2 nanospheres is about 50.80 wt%, while for the Au@meso-SiO2 hollow nanospheres, the carbon content is only 0.20 wt%, also indicating the almost complete removal of the RF polymer and surfactants. As demonstrated by the EDX spectra (Fig. 2f), the loading amount of Au in Au@meso-SiO2 hollow nanospheres is about 37.10 wt%. The wide-angle XRD patterns of the Au@meso-SiO2 hollow nanospheres (Fig. 3a) show five well-resolved diffraction peaks in the range of 20 – 90 °, which can be indexed to the 111, 200, 220, 311 and 222 reflections of cubic metal gold (JCPDS 04-0784). It further indicates the good crystallinity of the gold nanoparticle cores. The nitrogen adsorption-desorption isotherms of the Au@meso-SiO2 hollow nanospheres (Fig. 3b) display type-IV curves with a capillary condensation step at a low relative pressure (P/P0 = 0.2 – 0.4), suggesting the existence of uniform mesopores. Moreover, the hysteresis loop at a higher pressure (P/P0 = 0.75 – 0.97) may reflect the interparticle packed pores. The BET surface area and total pore volume are calculated to be as large as 537 m2/g and 0.31 cm3/g, respectively. According to the BJH model, the Au@meso-SiO2 hollow nanospheres have a pore
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size distribution of approximately 2.5 nm (Fig. 3b, inset), the sharp peaks also indicate the uniform mesopores of the silica shells. The formation process of the Au@meso-SiO2 hollow nanospheres with multiple gold cores can be speculated as follows (Fig. 4). At first, in the mixed solution of Pluronic F127, CTAB, NH3·H2O, ethanol and H2O (Fig. 4a), the RF resin colloidal nanospheres were synthesized by a modified Stöber method (Fig. 4b), using resorcinol and formaldehyde as the precursor, NH3·H2O as the catalyst, surfactant CTAB and F127 as costabilizers. After adding HAuCl4 aqueous solution, the core–shell Au@RF nanospheres with multiple gold cores can be achieved (Fig. 4c). We suppose that the crosslinked-RF polymer nanospheres have many micropores [51, 52], which can provide access for the diffusion of HAuCl4, so the gold nanoparticles can be formed in the RF nanospheres by hydrothermal treatment. After coating a layer of mesoporous silica (meso-SiO2) on the core–shell Au@RF nanospheres, core–shell Au@RF@meso-SiO2 nanospheres with double shells can be obtained (Fig. 4d), because of the self-assembly of CTAB and oligomers of the hydrolyzed TEOS under a basic condition. After removal of the polymer shells and surfactants by calcination in air, the Au@meso-SiO2 hollow nanospheres with multiple gold cores can be achieved (Fig. 4e). The diameters of gold cores in Au@meso-SiO2 hollow nanospheres are larger than that of core–shell Au@RF@meso-SiO2 nanospheres, because the neighboring gold nanoparticles aggregate together during calcination. The catalytic performance of the Au@meso-SiO2 hollow nanospheres was investigated by using the liquid-phase reduction of 4-nitrophenol (4-NP) by NaBH4 to
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4-aminophenol (4-AP) as a model reaction (Fig. 5a). After adding NaBH4 into the aqueous solution of 4-NP, the color of the solution changed from light yellow to light bright yellow. Moreover, the UV-vis absorption peak shifted from 317 to 400 nm (Fig. 5b) because of the deprotonation of 4-NP. No obvious change of the UV-vis absorption spectra was measured after 24 h, suggesting that the reduction reaction did not start without the Au@meso-SiO2 hollow nanosphere catalysts. After the addition of Au@meso-SiO2 hollow nanospheres as the catalyst, the reduction reaction started immediately, and the color of the reaction solution turned lighter and lighter. The absorption intensity at 400 nm became weaker and weaker along with the increase of reaction time, at the same time, an absorption peak around 300 nm appeared due to the formation of 4-AP (Fig. 5c). The linear relationship between ln(Ct/C0) and reaction time of the reduction reaction is demonstrated in Fig. 5d, where Ct and C0 are the concentration of 4-NP at time t and 0, which can be measured from the relative intensity of the absorbance At and A0, respectively. The reduction reaction matched first-order kinetics, and the rate constant k of the reaction with the Au@meso-SiO2 hollow nanosphere catalysts was calculated to be 0.08 min-1, indicating that the obtained Au@meso-SiO2 hollow nanospheres have relatively high catalytic activity [15, 17, 39, 53]. We suggest that mesopores of the silica shells can provide convenient channels for the reactant molecules to diffuse and subsequently interact with the gold cores with catalytic activity. The recyclability of the Au@meso-SiO2 hollow nanospheres displayed in Fig. 5e reveals that the conversion percentage of 4-NP slightly dropped along with the increase of cycles, however, it is still as high as 94 %
12
after five cycles, indicating the good stability of the Au@meso-SiO2 hollow nanospheres.
4. Conclusions The core–shell Au@RF nanospheres with multiple gold cores have been successfully fabricated through the polymerization of resorcinol and formaldehyde. After coating a layer of mesoporous silica by using CTAB as the template, and then calcination in air, the Au@meso-SiO2 hollow nanospheres with multiple gold cores can be obtained, with high surface area (~ 537 m2/g) and uniform pore size (~ 2.5 nm). In the model catalytic reaction of reducing 4-nitrophenol by NaBH4 into 4-aminophenol, the Au@meso-SiO2 hollow nanospheres exhibit excellent catalytic performance and stability.
Acknowledgements This work was supported by the China National Key Basic Research Program (973 Project) (No. 2013CB934104, 2012CB224805), NSF of China (21210004), the Shanghai Leading Academic Discipline Project (B108), and the Science and Technology Commission of Shanghai Municipality (08DZ2270500).
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Fig. 1 (a) SEM and (b) TEM images of the core–shell Au@RF nanospheres synthesized by a modified Stöber method.
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Fig. 2 TEM images of (a) the core–shell Au@RF@meso-SiO2 nanospheres and (b) the Au@meso-SiO2 hollow nanospheres, HRTEM images of (c) the mesoporous silica shell and (d) the gold core, (e) TGA curves of (1) Au@meso-SiO2 and (2) Au@RF@meso-SiO2, and (f) EDX spectra of the Au@meso-SiO2 hollow nanospheres. The inset in (a) displays the HRTEM images of the core–shell Au@RF@meso-SiO2 nanospheres.
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Fig. 3 (a) The wide-angle XRD pattern and (b) N2 sorption isotherms of the Au@meso-SiO2 hollow nanospheres. The inset (b) displays the pore size distribution of the Au@meso-SiO2 hollow nanospheres.
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Fig. 4 Synthesis procedures of the Au@meso-SiO2 hollow nanospheres obtained from core–shell Au@RF nanospheres.
22
Fig. 5 (a) Catalytic reduction equation of 4-NP to 4-AP, (b) UV-vis spectra of 4-NP (1) before and (2) after the addition of NaBH4, (c) Time-dependent UV-vis spectra of the reaction solution in the presence of the Au@meso-SiO2 hollow nanospheres, (d) Plot of ln(Ct/C0) against the reaction time, the R2 is the coefficient of determination obtained from the linear fitting and (e) The recyclability of the Au@meso-SiO2 hollow nanospheres as the catalyst for the reduction of 4-NP with NaBH4.
23
Figure captions: Fig. 1 (a) SEM and (b) TEM images of the core–shell Au@RF nanospheres synthesized by a modified Stöber method. Fig. 2
TEM images of (a) the core–shell Au@RF@meso-SiO2 nanospheres and (b)
the Au@meso-SiO2 hollow nanospheres, HRTEM images of (c) the mesoporous silica shell and (d) the gold core, (e) TGA curves of (1) Au@meso-SiO2 and (2) Au@RF@meso-SiO2, and (f) EDX spectra of the Au@meso-SiO2 hollow nanospheres. The inset in (a) displays the HRTEM images of the core–shell Au@RF@meso-SiO2 nanospheres. Fig. 3 (a) The wide-angle XRD pattern and (b) N2 sorption isotherms of the Au@meso-SiO2 hollow nanospheres. The inset (b) displays the pore size distribution of the Au@meso-SiO2 hollow nanospheres. Fig. 4 Synthesis procedures of the Au@meso-SiO2 hollow nanospheres obtained from core–shell Au@RF nanospheres. Fig. 5 (a) Catalytic reduction equation of 4-NP to 4-AP, (b) UV-vis spectra of 4-NP (1) before and (2) after the addition of NaBH4, (c) Time-dependent UV-vis spectra of the reaction solution in the presence of the Au@meso-SiO2 hollow nanospheres, (d) Plot of ln(Ct/C0) against the reaction time, the R2 is the coefficient of determination obtained from the linear fitting and (e) The recyclability of the Au@meso-SiO2 hollow nanospheres as the catalyst for the reduction of 4-NP with NaBH4.
24
25
Highlights: 1. Core–shell Au@RF nanospheres with multiple cores were obtained by a Stöber method. 2. Au@meso-SiO2 hollow nanospheres can be obtained by silica coating and calcination. 3. The Au@meso-SiO2 hollow nanospheres have high surface area and uniform pore size. 4. The Au@meso-SiO2 nanospheres exhibit great catalytic performance and stability.
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S0021-9797(14)00296-3 http://dx.doi.org/10.1016/j.jcis.2014.05.005 YJCIS 19557
To appear in:
Journal of Colloid and Interface Science
Received Date: Accepted Date:
26 March 2014 9 May 2014
Please cite this article as: J. Chen, Z. Xue, S. Feng, B. Tu, D. Zhao, Synthesis of mesoporous silica hollow nanospheres with multiple gold cores and catalytic activity, Journal of Colloid and Interface Science (2014), doi: http://dx.doi.org/10.1016/j.jcis.2014.05.005
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Synthesis of mesoporous silica hollow nanospheres with multiple gold cores and catalytic activity
Junchen Chen, Zhaoteng Xue, Shanshan Feng, Bo Tu*, and Dongyuan Zhao*
Department of Chemistry and Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Laboratory of Advanced Materials, Fudan University, Shanghai 200433, P. R. China. E-mail: [email protected], [email protected] Tel: +86-21-5163-0205; Fax: +86-21-5163-0307.
1
Abstract: The core–shell Au@resorcinol-formaldehyde (RF) nanospheres with multiple cores have been successfully synthesized by a modified Stöber method. After coating mesoporous silica and the calcination, the Au@meso-SiO2 hollow nanospheres with multiple gold cores can be obtained, which have a high surface area (~ 537 m2/g) and uniform pore size (~ 2.5 nm). The Au@meso-SiO2 hollow nanospheres can be used as a catalyst for the reduction of 4-nitrophenol by NaBH4 into 4-aminophenol, and exhibit excellent catalytic performance.
Keywords: Mesoporous silica, hollow nanospheres, gold, multiple cores, synthesis, catalysis
2
1. Introduction In recent years, a special class of core–shell structures with core@void@shell configuration (the hollow nanospheres with cores) has attracted considerable attention because of the following advantages. (I) The moveable cores can afford much more exposed active sites to interact with the guest molecules more effectively. (II) The outer shells can prevent the aggregation of neighboring nanoparticles. (III) The hollow spaces between the cores and the shells can provide large space for loading functional molecules. Due to the functionalities of the freely movable cores, the protective shells and the spaces between them, hollow nanospheres have been widely applied in the fields of catalysis, biomedicine, lithium-ion batteries and so on [1-13]. Up to now, a variety of synthetic strategies have been developed to fabricate hollow nanospheres with metal cores. According to the sequence of preparing cores and
shells,
the
methods
can
be
classified
into
pre-shell/post-core
and
pre-core/post-shell approaches. In the pre-shell/post-core approaches, the metal ions are reduced to grow metal cores in the pre-formed hollow shells [14-17]. Most of the hollow nanospheres are synthesized by pre-core/post-shell approaches, firstly, a sandwich structure (core/template/shell) is fabricated by using hard or soft templates, and then, the voids can be formed via selectively removing the intermediate sacrificial template layer, using a solvent or calcination [18-24]. Generally, the inner sacrificial shells are composed of silica or carbon precursors for the hard-templating method. The silica shells can be selectively removed by NaOH [21], NH3·H2O [10], Na2CO3 [25], NaBH4 [19, 26], H2O [11, 27] and
3
organosilane [16, 28]. However, compared with etching silica in solution, calcination in air to remove the carbonaceous shells is much more simple. Many carbon precursors can be used to fabricate carbon-based nanospheres, such as resorcinol-formaldehyde (RF) resin [29-32], phenol-formaldehyde resin [33, 34], saccharide [35, 36], polydopamine [37] and so on. Among them, RF resin, a three-dimensional (3-D) network structured polymer, has been paid much attention due to the attractive properties such as low cost, controllable morphology and outstanding stability. Until now, various porous shells have been fabricated to encapsulate metal nanoparticles, such as SiO2, ZrO2, TiO2 and C [3, 5, 38-44]. The porous outer shells can provide convenient channels for the diffusion and transport of reactants to reach the surface of the active cores, and protect the active metal cores from leakage. In particular, mesoporous silicas have been paid much attention because of their high surface area, large pore volume, tunable uniform pore size, high stability, controllable morphology, facile surface functionalization and high biocompatibility [45]. The Stöber method has attracted many interests for the fabrication of silica shells on nanoparticles through the facile hydrolysis and condensation of tetraethyl orthosilicate (TEOS) in basic aqueous solutions [46]. Functional nanoparticles have been encapsulated in silica to form the core–shell structures, including noble-metal nanoparticles, metal oxides, and quantum dots [25, 47-49], because the encapsulated nanoparticles can maintain their specific catalytic, magnetic, electronic or optical properties. Among them, gold@silica nanospheres have been applied in many fields
4
such as catalysis, drug delivery and bioimaging [15-17, 22, 50]. In the case of nanocatalysts, the strategy of encapsulating gold nanoparticles in hollow mesoporous silica nanospheres is a good way to obtain great catalytic performance, because the gold cores can move freely inside the cavity of the shells to sufficiently expose the surface of the active cores. Herein, we demonstrate the one-pot synthesis of the core–shell Au@RF nanospheres with multiple gold cores by a modified Stöber method, using resorcinol and formaldehyde as the precursors, NH3·H2O as the catalyst, surfactant CTAB and F127 as the costabilizers. After the core–shell Au@RF@meso-SiO2 nanospheres were calcined, Au@meso-SiO2 hollow nanospheres with multiple gold cores could be achieved. The obtained Au@meso-SiO2 hollow nanospheres have a high surface area (~ 537 m2/g) and uniform pore size (~ 2.5 nm). Furthermore, the Au@meso-SiO2 hollow nanospheres were successfully employed in the catalytic reduction of 4-nitrophenol as a model reaction.
2. Experimental 2.1 Chemicals Cetyltrimethylammonium bromide (CTAB, 99.0 wt%), formaldehyde solution (37.0 wt%), resorcinol (99.0 wt%), aqueous ammonia (NH3·H2O, 25.0 wt%) and chloroauric acid tetrahydrate (≥ 47.8 wt% Au) were purchased from Sinopharm Chemical Reagent Company. Tetraethyl orthosilicate (TEOS, 98.0 wt%), NaBH4 (99.0 wt%), 4-nitrophenol (4-NP, 99.0 wt%) and absolute anhydrous ethanol (99.7 wt%)
5
were obtained from Shanghai Chemical Company. Tri-block copolymer Pluronic F127 (EO106PO70EO106, EO = ethylene oxide, PO = propylene oxide, 99.0 wt%) was purchased from Sigma-Aldrich. All chemicals were used without additional purification. Deionized water was used for all experiments.
2.2 Synthesis Synthesis
of
core–shell
Au@resorcinol-formaldehyde
polymer (Au@RF)
nanospheres: In a typical synthesis, 0.125 g of F127 and 0.05 g of CTAB were dissolved in the solution containing 10 mL of deionized water and 4 mL of ethanol. After stirring at 30 °C for 15 min, 0.05 mL of NH3·H2O was added. After further stirring for 50 min, 0.1 g of resorcinol was added. Stirring for additional 30 min, 0.14 mL of formaldehyde solution was added. The mixture was stirred for 24 h at 30 °C, then, 0.5 mL of 0.1 M HAuCl4 aqueous solution was added. After stirring for 30 min, the mixture was transferred to a Teflon-lined auto-clave and heated for 24 h at 100 °C under static conditions. The products were centrifuged, washed three times with water and once with absolute ethanol, and then dried at 60 °C overnight, after which the core–shell Au@RF nanospheres were obtained. Synthesis of core–shell Au@RF@meso-SiO2 nanospheres: The core–shell nanospheres with double shells were prepared through a surfactant-templating sol–gel approach by using CTAB as a template. In a typical synthesis, 0.075 g of CTAB was dissolved in the solution containing 25 mL of deionized water and 13 mL of ethanol. After stirring at 30 °C for 15 min, 1 mL of an ethanol dispersion solution containing
6
0.05 g of Au@RF nanospheres was added. The mixture was homogeneously dispersed by ultrasonication for 15 min, followed by the addition of 0.25 mL of NH3·H2O. Stirring for additional 10 min, a solution containing 0.11 g of TEOS and 1.0 mL of ethanol was added dropwise with continuous stirring for 1 min. After reaction for 6 h, the products were centrifuged, washed for three times with water and once with absolute ethanol, and then dried at 60 °C overnight, after which the core–shell Au@RF@meso-SiO2 nanospheres were obtained. Synthesis of Au@meso-SiO2 hollow nanospheres: Removal of the polymer shells and
surfactants
was
achieved
by
the
calcination
of
the
core–shell
Au@RF@meso-SiO2 nanospheres in air from room temperature to 550 °C for 6 h at a rate of 1 °C·min-1, after which the Au@meso-SiO2 hollow nanospheres were obtained.
2.3 Catalytic Reduction The catalytic reduction process of 4-nitrophenol (4-NP) was carried out in a quartz cuvette and monitored by in-situ measuring the UV-vis absorption spectra. At first, 0.125 mL of 0.2 M freshly prepared NaBH4 solution was added in the solution containing 0.0125 mL of 0.005 M 4-NP and 1.0 mL of deionized water. Subsequently, 0.05 mL of aqueous dispersion of 0.0125 wt% Au@meso-SiO2 hollow nanospheres was added, and the reaction started immediately. The gradual change of the solution color from light bright yellow to colorless was observed during the reaction. To study the recyclability of the catalysts, the used Au@meso-SiO2 hollow nanospheres were separated from the reaction mixture by centrifugation at the end of each run, washed
7
once with absolute ethanol and once with deionized water, then re-dispersed in water by ultrasonication and added in a fresh reaction solution. After reaction for 20 min, the solution was measured using UV-vis spectroscopy.
2.4 Characterization Scanning electron microscopy (SEM) images were obtained on a FE-SEM S-4800 microscope (Hitachi, Japan) operated at 1 kV. Transmission electron microscopy (TEM) images were taken by a JEM-2011F microscope (JEOL, Japan) operated at 200 kV. The samples for TEM measurements were dispersed in ethanol, and then dipped and dried on holey carbon films supported on a Cu grid. Energy dispersive X-ray spectroscopy (EDX) was recorded on an IET-200 EDX instrument (Oxford, England). Thermogravimetric analysis (TGA) was monitored by using a Mettler Toledo TGA/DSC 1 analyzer (Switzerland) from 30 to 600 °C under air (40 ml·min-1) with a heating rate of 5 °C·min-1. X-ray diffraction (XRD) patterns were recorded on a D8 advance X-ray diffractometer (Bruker, Germany) with Ni-filtered Cu Kα radiation (40 kV, 40 mA). Nitrogen adsorption/desorption isotherms were measured with a Tristar II 3020 analyzer (Micromeritics, USA) at 77 K. Before the measurements, the samples were degassed in vacuum at 180 °C for at least 6 h. The surface area and pore size were obtained by using Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) methods, respectively. The carbon contents of the samples were measured on a Vario EL Ⅲ elemental analyzer (Germany). The UV-vis spectra were collected using a UV-3600 spectrophotometer (Shimadzu, Japan).
8
3. Results and discussion SEM images (Fig. 1a) show that the Au@RF nanospheres have well spherical morphology, the size of the core-shell Au@RF nanospheres is quite uniform with the average diameter of about 850 nm. The monodispersed spherical morphology of the core–shell Au@RF nanospheres can be further confirmed from the TEM images (Fig. 1b). TEM images display that many gold nanoparticles as cores with a size of about 15 nm are located randomly in one Au@RF nanosphere. After further coating a layer of mesoporous silica on the core–shell Au@RF nanospheres by using CTAB as the template, the core–shell Au@RF@meso-SiO2 nanospheres with double shells can be obtained. TEM images (Fig. 2a) show that the Au@RF@meso-SiO2 nanospheres are composed of many small gold cores, inner shell of crosslinked-RF and outer shell of mesoporous silica. After calcination in air, the Au@meso-SiO2 hollow nanospheres can be obtained, due to the removal of the polymer shells and surfactants (Fig. 2b). Many larger gold cores with the diameters from 20 to 80 nm are dispersed in the interior void, due to the aggregation of the smaller gold nanoparticles (~ 15 nm) during the calcination. The high-resolution TEM (HRTEM) image (Fig. 2c) displays the radial mesoporous silica as a shell with unobstructed mesopores in one Au@meso-SiO2 hollow nanosphere. The HRTEM image of the gold core (Fig. 2d) demonstrates the polycrystalline structure with fringes associated with a d-spacing of ~ 0.235 nm, which can be assigned to the (111) planes of the cubic gold lattice. The TGA curve (Fig. 2e) of the core–shell
9
Au@RF@meso-SiO2 nanospheres demonstrates a weight loss of ~ 70 % in the temperature range from 200 to 520 °C in air, which is attributed to the decomposition of crosslinked-RF polymer, surfactants CTAB and F127. However, no obvious weight loss of the Au@meso-SiO2 hollow nanospheres is observed, suggesting the entire removal of the RF polymer and surfactants. In addition, elemental analysis shows that the mass percentage of carbon in the core–shell Au@RF@meso-SiO2 nanospheres is about 50.80 wt%, while for the Au@meso-SiO2 hollow nanospheres, the carbon content is only 0.20 wt%, also indicating the almost complete removal of the RF polymer and surfactants. As demonstrated by the EDX spectra (Fig. 2f), the loading amount of Au in Au@meso-SiO2 hollow nanospheres is about 37.10 wt%. The wide-angle XRD patterns of the Au@meso-SiO2 hollow nanospheres (Fig. 3a) show five well-resolved diffraction peaks in the range of 20 – 90 °, which can be indexed to the 111, 200, 220, 311 and 222 reflections of cubic metal gold (JCPDS 04-0784). It further indicates the good crystallinity of the gold nanoparticle cores. The nitrogen adsorption-desorption isotherms of the Au@meso-SiO2 hollow nanospheres (Fig. 3b) display type-IV curves with a capillary condensation step at a low relative pressure (P/P0 = 0.2 – 0.4), suggesting the existence of uniform mesopores. Moreover, the hysteresis loop at a higher pressure (P/P0 = 0.75 – 0.97) may reflect the interparticle packed pores. The BET surface area and total pore volume are calculated to be as large as 537 m2/g and 0.31 cm3/g, respectively. According to the BJH model, the Au@meso-SiO2 hollow nanospheres have a pore
10
size distribution of approximately 2.5 nm (Fig. 3b, inset), the sharp peaks also indicate the uniform mesopores of the silica shells. The formation process of the Au@meso-SiO2 hollow nanospheres with multiple gold cores can be speculated as follows (Fig. 4). At first, in the mixed solution of Pluronic F127, CTAB, NH3·H2O, ethanol and H2O (Fig. 4a), the RF resin colloidal nanospheres were synthesized by a modified Stöber method (Fig. 4b), using resorcinol and formaldehyde as the precursor, NH3·H2O as the catalyst, surfactant CTAB and F127 as costabilizers. After adding HAuCl4 aqueous solution, the core–shell Au@RF nanospheres with multiple gold cores can be achieved (Fig. 4c). We suppose that the crosslinked-RF polymer nanospheres have many micropores [51, 52], which can provide access for the diffusion of HAuCl4, so the gold nanoparticles can be formed in the RF nanospheres by hydrothermal treatment. After coating a layer of mesoporous silica (meso-SiO2) on the core–shell Au@RF nanospheres, core–shell Au@RF@meso-SiO2 nanospheres with double shells can be obtained (Fig. 4d), because of the self-assembly of CTAB and oligomers of the hydrolyzed TEOS under a basic condition. After removal of the polymer shells and surfactants by calcination in air, the Au@meso-SiO2 hollow nanospheres with multiple gold cores can be achieved (Fig. 4e). The diameters of gold cores in Au@meso-SiO2 hollow nanospheres are larger than that of core–shell Au@RF@meso-SiO2 nanospheres, because the neighboring gold nanoparticles aggregate together during calcination. The catalytic performance of the Au@meso-SiO2 hollow nanospheres was investigated by using the liquid-phase reduction of 4-nitrophenol (4-NP) by NaBH4 to
11
4-aminophenol (4-AP) as a model reaction (Fig. 5a). After adding NaBH4 into the aqueous solution of 4-NP, the color of the solution changed from light yellow to light bright yellow. Moreover, the UV-vis absorption peak shifted from 317 to 400 nm (Fig. 5b) because of the deprotonation of 4-NP. No obvious change of the UV-vis absorption spectra was measured after 24 h, suggesting that the reduction reaction did not start without the Au@meso-SiO2 hollow nanosphere catalysts. After the addition of Au@meso-SiO2 hollow nanospheres as the catalyst, the reduction reaction started immediately, and the color of the reaction solution turned lighter and lighter. The absorption intensity at 400 nm became weaker and weaker along with the increase of reaction time, at the same time, an absorption peak around 300 nm appeared due to the formation of 4-AP (Fig. 5c). The linear relationship between ln(Ct/C0) and reaction time of the reduction reaction is demonstrated in Fig. 5d, where Ct and C0 are the concentration of 4-NP at time t and 0, which can be measured from the relative intensity of the absorbance At and A0, respectively. The reduction reaction matched first-order kinetics, and the rate constant k of the reaction with the Au@meso-SiO2 hollow nanosphere catalysts was calculated to be 0.08 min-1, indicating that the obtained Au@meso-SiO2 hollow nanospheres have relatively high catalytic activity [15, 17, 39, 53]. We suggest that mesopores of the silica shells can provide convenient channels for the reactant molecules to diffuse and subsequently interact with the gold cores with catalytic activity. The recyclability of the Au@meso-SiO2 hollow nanospheres displayed in Fig. 5e reveals that the conversion percentage of 4-NP slightly dropped along with the increase of cycles, however, it is still as high as 94 %
12
after five cycles, indicating the good stability of the Au@meso-SiO2 hollow nanospheres.
4. Conclusions The core–shell Au@RF nanospheres with multiple gold cores have been successfully fabricated through the polymerization of resorcinol and formaldehyde. After coating a layer of mesoporous silica by using CTAB as the template, and then calcination in air, the Au@meso-SiO2 hollow nanospheres with multiple gold cores can be obtained, with high surface area (~ 537 m2/g) and uniform pore size (~ 2.5 nm). In the model catalytic reaction of reducing 4-nitrophenol by NaBH4 into 4-aminophenol, the Au@meso-SiO2 hollow nanospheres exhibit excellent catalytic performance and stability.
Acknowledgements This work was supported by the China National Key Basic Research Program (973 Project) (No. 2013CB934104, 2012CB224805), NSF of China (21210004), the Shanghai Leading Academic Discipline Project (B108), and the Science and Technology Commission of Shanghai Municipality (08DZ2270500).
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Fig. 1 (a) SEM and (b) TEM images of the core–shell Au@RF nanospheres synthesized by a modified Stöber method.
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Fig. 2 TEM images of (a) the core–shell Au@RF@meso-SiO2 nanospheres and (b) the Au@meso-SiO2 hollow nanospheres, HRTEM images of (c) the mesoporous silica shell and (d) the gold core, (e) TGA curves of (1) Au@meso-SiO2 and (2) Au@RF@meso-SiO2, and (f) EDX spectra of the Au@meso-SiO2 hollow nanospheres. The inset in (a) displays the HRTEM images of the core–shell Au@RF@meso-SiO2 nanospheres.
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Fig. 3 (a) The wide-angle XRD pattern and (b) N2 sorption isotherms of the Au@meso-SiO2 hollow nanospheres. The inset (b) displays the pore size distribution of the Au@meso-SiO2 hollow nanospheres.
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Fig. 4 Synthesis procedures of the Au@meso-SiO2 hollow nanospheres obtained from core–shell Au@RF nanospheres.
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Fig. 5 (a) Catalytic reduction equation of 4-NP to 4-AP, (b) UV-vis spectra of 4-NP (1) before and (2) after the addition of NaBH4, (c) Time-dependent UV-vis spectra of the reaction solution in the presence of the Au@meso-SiO2 hollow nanospheres, (d) Plot of ln(Ct/C0) against the reaction time, the R2 is the coefficient of determination obtained from the linear fitting and (e) The recyclability of the Au@meso-SiO2 hollow nanospheres as the catalyst for the reduction of 4-NP with NaBH4.
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Figure captions: Fig. 1 (a) SEM and (b) TEM images of the core–shell Au@RF nanospheres synthesized by a modified Stöber method. Fig. 2
TEM images of (a) the core–shell Au@RF@meso-SiO2 nanospheres and (b)
the Au@meso-SiO2 hollow nanospheres, HRTEM images of (c) the mesoporous silica shell and (d) the gold core, (e) TGA curves of (1) Au@meso-SiO2 and (2) Au@RF@meso-SiO2, and (f) EDX spectra of the Au@meso-SiO2 hollow nanospheres. The inset in (a) displays the HRTEM images of the core–shell Au@RF@meso-SiO2 nanospheres. Fig. 3 (a) The wide-angle XRD pattern and (b) N2 sorption isotherms of the Au@meso-SiO2 hollow nanospheres. The inset (b) displays the pore size distribution of the Au@meso-SiO2 hollow nanospheres. Fig. 4 Synthesis procedures of the Au@meso-SiO2 hollow nanospheres obtained from core–shell Au@RF nanospheres. Fig. 5 (a) Catalytic reduction equation of 4-NP to 4-AP, (b) UV-vis spectra of 4-NP (1) before and (2) after the addition of NaBH4, (c) Time-dependent UV-vis spectra of the reaction solution in the presence of the Au@meso-SiO2 hollow nanospheres, (d) Plot of ln(Ct/C0) against the reaction time, the R2 is the coefficient of determination obtained from the linear fitting and (e) The recyclability of the Au@meso-SiO2 hollow nanospheres as the catalyst for the reduction of 4-NP with NaBH4.
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Highlights: 1. Core–shell Au@RF nanospheres with multiple cores were obtained by a Stöber method. 2. Au@meso-SiO2 hollow nanospheres can be obtained by silica coating and calcination. 3. The Au@meso-SiO2 hollow nanospheres have high surface area and uniform pore size. 4. The Au@meso-SiO2 nanospheres exhibit great catalytic performance and stability.
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