Massage ball-like, hollow porous Au/SiO₂ microspheres templated by a Pickering emulsion derived from polymer–metal hybrid emulsifier micelles

Abstract

In this study, we reported a novel, facile, Pickering emulsion-templating method to prepare massage ball-like, hollow-structured Au/SiO₂ microspheres. Firstly, oil-in-water Pickering emulsions stabilized by Au@poly(ethylene oxide)-b-poly(4-vinylpyridine) (Au@PEO-b-P4VP) hybrid emulsifier micelles, which were formed by a P4VP/Au complex induced self-assembly process, were generated. Then hollow Au/SiO₂ hybrid microspheres with nano-/submicro-sized protrusions on their shells, termed as massage ball-like microspheres, were successfully synthesized using the generated Pickering emulsion as template, in which the P4VP catalyzed hydrolysis and condensation of tetraethoxysilane (TEOS) in the TEOS/n-decanol mixed oil phase occurred at the oil/water interface. As a result, a continuous SiO₂ shell was formed. The uneven adsorption of polydisperse hybrid micelles at the oil/water interface as well as the volume shrinkage of the oil phase during the early hydrolysis and condensation of TEOS facilitated the formation of protrusions on the shell surface. After further removal of the polymer components embedded in the shell by calcination, hollow Au/SiO₂ hybrid microspheres with micropore/mesopore bimodal porous shells were produced. The as-prepared Au/SiO₂ hybrid microspheres were applied as catalysts for the reduction of p-nitrophenol with NaBH₄, showing a high catalytic activity with a good recyclability owing to the large specific areas, the easily accessible Au active centres, and the enhanced mass transportation.

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1. Introduction

In the past decades, nanometer- or micrometer-sized hollow spheres with porous shells (HSPS) have attracted considerable attention due to their low density and large specific surface areas. These unique properties make them attractive for applications in the fields of catalysis, encapsulation, and energy storage.¹ Among these HSPS materials, hollow porous SiO₂ spheres have been a hot spot in the material synthesis field because of their low toxicity, high biocompatibility, and mechanical stability. A large number of hollow SiO₂ spheres with various shell structures have been successfully synthesized using different strategies involving hard template method,^2–5 soft template method,^6–11 and self-templating method.^12–14

In general, highly uniform HSPS with controlled shape and cavity size can be produced by hard templates strategy, in which cross-linked latex, carbon sphere, metal oxide, and so on^2–5 were employed in combination with surfactant to create hollow cavity and porous shell, respectively. However, the hard template method has several limitations such as the sacrificial use of hard templates and the difficulties in synthesis of different types of templates. In addition, the removal process of the inorganic template usually employs hazardous chemicals such as acids and bases, which is a drawback in industrial applications.¹³ In order to overcome these limitations, efforts have been directed toward soft template method by virtue of its convenience, simpleness and efficiency. More importantly, functional nanoparticles and/or drugs can be facilely and effectively encapsulated in the hollow interior by this strategy.¹ Up to now, emulsion droplets, polymer aggregates, and gas bubbles have been employed as soft templates.^6–11 Although there still remains challenge in the synthesis of monodisperse HSPS, soft template has yet proved to be effective in creating SiO₂ hollow spheres with ordered mesopore structure within their shell layer. HSPS with different pore structures, including cylindrical mesopores perforating their shells,⁷ radially oriented pores,^11,15 hexagonal pores,⁸ and periodic mesopores¹⁶ have been successfully synthesized. Besides, budded, mesoporous SiO₂ hollow spheres have also been synthesized using emulsion droplets as templates, in which oil phase acted as a reservoir of surfactants, which could diffuse through the mesoporous shell to induce a kinetic co-assembly with SiO₂ precursors thus facilitated the formation of budlike structures.¹⁷ The shells of the obtained hollow spheres possess wormhole-like mesopores, while the buds on the shell had an intriguing vesicular mesostructure. Evidently, there is no doubt in technology to synthesize HSPS with different shell structures. From the application point of view, however, there is a great demand to develop an facile method that not only produces porous SiO₂ hollow spheres with controlled pore size but also can implant in situ the metal catalysts in the produced mesopores during the synthesis. And this would largely improve the loading efficiency and dispersivity of metal catalysts against refilling operation.

Pickering emulsion, stabilized by solid particles at the water/oil interface in place of surfactants, was recognized by Pickering in 1907,¹⁸ which has drawn considerable research interest in recent years due to its widespread applications in many areas including cosmetics, food products, and pharmaceutics.^19–21 The highly stable and size-tuneable droplets make Pickering emulsion as an ideal soft template for hollow structure materials.²² Moreover, the functionalities and/or properties of the particle emulsifiers acquired by inherence or post-functionalization can be directly transferred to the synthesized hollow materials.²³

Herein, we presented the synthesis of massage ball-like, hollow porous Au/SiO₂ spheres using Au@PEO-b-P4VP hybrid micelles stabilized Pickering emulsion as the template. The stabilized oil (TEOS + n-decanol) in water Pickering emulsion was generated by homogenizing process using Au@PEO-b-P4VP micelles as emulsifier. The hydrolysis and condensation of the encapsulated TEOS at the oil/water interface led to the formation of hollow SiO₂ microspheres with many nano-/submicro-sized protrusions. Removing the polymer components embedded in the shell by calcination left behind hollow SiO₂ spheres with bimodal micro/mesopore sized shells. The as-prepared Au/SiO₂ hollow porous spheres showed an excellent catalytic activity toward the reduction of p-nitrophenol by NaBH₄.

2. Experimental

2.1 Materials

Poly(ethylene oxide)-b-poly(4-vinylpyridine) (PEO₁₁₃-b-P4VP₁₁₉, M_n = 17 600 g mol⁻¹, PDI = 1.14) was synthesized in our laboratory as described elsewhere.²⁴ The subscript numbers represented the polymerization degree of the respective blocks. The detailed description of the polymer synthesis and characterization was shown in S1, ESI.† Chloroauric acid (HAuCl₄, 47% based on Au), n-decanol and TEOS were purchased from Aladdin Reagent Co. Ltd (China). All other reagents were purchased from the commercial suppliers, which were used as received without further purification.

2.2 Preparation of Au@PEO-b-P4VP hybrid micelles

PEO-b-P4VP diblock copolymer (250 mg) was dissolved in 10 mL of methanol under stirring. And then 0.6 mL of HAuCl₄ aqueous solution (0.01%) was pumped into the solution at a rate of 0.03 mL min⁻¹, and the mixture was continuously stirred for 24 h at room temperature. Subsequently, 25 mL of deionized water was added into the dispersion with a pump at a rate of 0.1 mL min⁻¹. After 24 h of continuous stirring, 5 mL of fresh NaBH₄ aqueous solution (5 mmol L⁻¹) was rapidly added into the above dispersion. After 2 h of reduction reaction, the mixture was dialyzed against deionized water to remove the methanol and the remaining NaBH₄. The final hybrid micelles aqueous solution was marked as Au@PEO-b-P4VP with concentration of 5 mg mL⁻¹.

2.3 Generation of o/w emulsion stabilized by Au@PEO-b-P4VP hybrid micelles

3.0 mL of n-decanol oil and 3.0 mL of Au@PEO-b-P4VP hybrid micelles (5 mg mL⁻¹) aqueous dispersion were charged in 10 mL of glass vessel at room temperature. Emulsions were formed using a XHF-D high speed disperser (Ningbo Scientz Biotechnology Co., Ltd, China) at a stirring rate of 14 000 rpm for 2 min. The content of hybrid micelles was about 0.27 wt%. Unless mentioned otherwise, the particle concentration indicated throughout this study referred to the solids content percent versus the total weight of water and oil for emulsion formation. All emulsions were stored in closed vials at room temperature.

2.4 Preparation of massage ball-like, hollow porous Au/SiO₂ hybrid microspheres

In a typical experiment, the total volume of oil and water phase was fixed to be 6 mL. The volume ratio of oil/water and TEOS/n-decanol was set to be 1 : 3 and 1 : 1, respectively. The content of Au@PEO-b-P4VP hybrid micelles was 0.27 wt%. The mixture was homogenized at 14 000 rpm for 2 min by a XHF-D high speed disperser at room temperature. The resultant emulsion was then transferred to 30 °C thermal constant bath for 3 days under continuously gentle stirring. The obtained white solid products were suspended in the upper layer, which were separated by centrifugation. The collected samples were then dried under reduced press at 60 °C, and then programmed calcined in muffle furnace at 200 °C for 1 h and then 500 °C for 2 h, affording massage ball-like hollow porous SiO₂ hybrid microspheres (Au/SiO₂).

2.5 Reduction of p-nitrophenol by NaBH₄ catalyzed by hollow porous Au/SiO₂ microspheres

In a typical experiment, 2.5 mL of freshly prepared aqueous solution of NaBH₄ (0.3 mol L⁻¹) and 0.5 mL of p-nitrophenol (1.21 × 10⁻⁴ mol L⁻¹) solution were introduced into a cuvette. Next, 1.0 mg of the hollow porous Au/SiO₂ hybrid microspheres was added to the above solution, and time-dependent UV-vis absorption spectra were recorded from changes in the absorption intensity at 400 nm as a function of time. After reaction, the solid catalyst was separated by simple filtration, and the collected catalyst was washed thoroughly with water and ethanol. The dried catalyst was redispersed in a new reaction system for the subsequent catalytic experiments under the same reaction conditions. The experiment was carried out at 25 °C.

2.6 Characterization

The molecular weights and polydispersity indexes (PDI) of polymers were determined by gel permeation chromatography (GPC) measurements, which were performed on a Waters 1515 GPC setup equipped with a Waters 2414 differential refractive index detector in DMF at 80 °C with a flow rate of 1.0 mL min⁻¹. ¹H NMR spectra were recorded with a 400 MHz Bruker AV-400 NMR spectrometer. The average hydrodynamic diameter (〈D_h〉) of the hybrid micelles was measured by dynamic light scattering (DLS) on a Laser Light Scattering Spectrometer (BI-200SM). The sample was filtered through 0.45 μm Millipore filters to remove dust prior to DLS measurements. DLS measurements were performed at a fixed scattering angle of 90°. The results were analyzed in CONTIN mode. Zeta potential of the hybrid micelles in water was measured using a MALVERN Zetasizer Nano ZS instrument. The type of emulsion was conducted by placing one drop of the emulsion phase into neat water or neat oil. An o/w emulsion droplet disperses readily in water but not in oil and vice versa. Optical micrographs (OM) were collected with an optical microscope (Leica, DM 4500P). A drop of the diluted emulsion was placed on a microscope slide and five pictures were taken randomly from different spots of the same sample. The droplets size was analyzed by DLS measurements. The average diameter of the droplets was calculated from all the droplets in a typical image of optical micrograph. The morphologies of the as-prepared products were observed by scanning electron microscope (SEM) and transmission electron microscope (TEM) measurements. SEM measurements were carried out with a microscope (JEOL, JSM-6610LV) operated at an acceleration voltage of 30 kV, and TEM images were taken on a JEOL-2010, operated at an acceleration voltage of 200 kV. The textural properties were characterized by N₂ sorption measurements at 77 K (Micromeritics TriStar II 3020), and the specific surface area was obtained by Brunauer–Emmett–Teller (BET) method. The conversion of catalytic reduction was measured on a Perkin-Elmer Lamda-25 UV-vis spectrometer at room temperature. Power X-ray radiation diffraction (XRD) measurement was performed on a Bruker D8 Discover diffractometer. The Au contents in hybrid micelles and in Au/SiO₂ microspheres were measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES).

3. Results and discussion

3.1 Generation of Pickering emulsion stabilized by Au@PEO-b-P4VP hybrid micelles

Au@PEO-b-P4VP hybrid micelles were prepared according to the procedure described elsewhere,²⁵ and the detailed process was shown in Scheme 1.

Scheme 1 Schematic illustration of preparation of Au@PEO-b-P4VP micelles and massage-ball like, hollow porous Au/SiO₂ hybrid microsphere using Au@PEO-b-P4VP hybrid micelles stabilized Pickering emulsion as the template. The shells of the hybrid micelles were omitted for simplicity.

As indicated in Scheme 1, P4VP/HAuCl₄ complexes were formed immediately upon the addition of HAuCl₄ into the PEO-b-P4VP solution in methanol, resulting in the decrease in the solubility of P4VP block, which further drove the micellization of PEO-b-P4VP copolymer, producing polymeric micelles with PEO being the shells and P4VP/HAuCl₄ complexes being the cores as proved by Chen et al.²⁵ After further reduction with NaBH₄, the HAuCl₄ was then transformed in situ to Au nanoparticles, which were embedded in the cores of polymer micelles. The advantage of this strategy is that well-defined micelles can be obtained at a high polymer concentration. Additionally, the content and the size of Au particles embedded in the cores are adjustable. The as-produced Au@PEO-b-P4VP hybrid micelles were characterized by DLS and TEM. According to DLS measurement, 〈D_h〉 was about 25 nm with PDI of 0.218 (Fig. 1A). Zeta potential measurement indicated that the surfaces of the hybrid micelles were positive charged (about 5 mV). TEM observation revealed that these spherical aggregates had an average diameter of about 15 nm as shown in Fig. 1B. The distinctively high contrast observed in the TEM image was attributed to the embedded Au nanoparticles. The Au content in the hybrid polymer micelle was 0.78% according to ICP-AES analysis.

Fig. 1 (A) DLS curve of the Au@PEO-b-P4VP hybrid micelles in water; (B) TEM image of the Au@PEO-b-P4VP hybrid micelles.

Early reports indicated that core cross-linked star polymers possess excellent emulsifying performances owing to their compact cores and flexible arms.^26,27 The flexible arms permit these micelles to adopt favourable configurations at the oil/water interface.²⁷ As the common star polymer architecture, spherical polymer micelles formed by self-assembly strategy exhibit many advantages, such as their adjustable compositions and/or sizes. However, reports on Pickering emulsion stabilized by the polymer micelles are rather limited.^28–31 To evaluate the emulsifying performances of the as-fabricated Au@PEO-b-P4VP hybrid micelles, a series of n-decanol in water emulsions were prepared with a fixed oil/water volume ratio (1 : 1) but varied micelle contents. Stable emulsions could all be generated with the content of Au@PEO-b-P4VP hybrid micelles in the range of 0.27–0.11 wt%. However, a discernible thin oil layer separated from the emulsion was observed when the hybrid micelles content was 0.11 wt%, revealing that 0.11 wt% of hybrid micelles was not enough to completely emulsify the n-decanol oil (S2, ESI†). Optical microscopy observations displayed that the emulsion droplets presented a broad size distribution, and the mean size of the droplets also showed a slight increase with the decrease of the polymer hybrid micelles content (S2, ESI†). The broad size distribution of the emulsion droplets was resulted from the poor uniformity of the Au@PEO-b-P4VP hybrid micelles. Typical digital photographs and optical microscopy images of the generated n-decanol in water emulsion stabilized by 0.27 wt% of Au@PEO-b-P4VP hybrid micelles were shown in Fig. 2. As shown in Fig. 2A, the creamy emulsion phase reached about 70% of the total mixture volume. DLS measurement revealed a broad droplets size distribution, as could be seen from the droplets size distribution histogram (inset of Fig. 2C). The mean diameter of the oil droplets was 6.3 μm. The stability of the obtained n-decanol in water emulsion was investigated by monitoring the size variation of the emulsion droplets. A slight increase in the droplets size was observed for the emulsion stabilized by 0.27 wt% of the hybrid polymer micelles for a period of 6 months at room temperature (S2, ESI†), which was an indication of ultra-stabilized emulsion. The Pickering emulsions with varied oil/water volume ratios stabilized by a fixed hybrid micelles content of 0.27 wt% were also studied. It was found that the emulsion layer thickness and the mean size of the emulsion droplets decreased with the decrease of the volume ratio of oil to water (data not shown). Fig. 2B was the digital photograph of the generated n-decanol in water emulsion with 1 : 3 of oil/water volume ratio at the hybrid micelles content of 0.27 wt%, and the corresponding optical micrograph of the emulsion droplets was shown in Fig. 2D. As shown inset of Fig. 2D, a broad droplets size distribution was observed. The mean size of the droplets was 5.1 μm, and the emulsion layer thickness was about 42% of the total mixture volume. In fact, stabilized o/w emulsion also could be formed using the PEO-b-P4VP polymer micelles without Au nanoparticles in cores as emulsifier (S3, ESI†). The incorporation of Au nanoparticles in the cores gave the polymer micelles more “solid particle” characteristic. More importantly, Au nanoparticles could thus be implanted into the final nanocomposities obtained from the Pickering emulsion template (vide infra).

Fig. 2 (A and B) Digital photographs and (C and D) optical microscope images of the n-decanol in water Pickering emulsion stabilized by 0.27 wt% of Au@PEO-b-P4VP hybrid micelles with different oil/water volume ratios (A and C: 1 : 1; B and D: 1 : 3). The photographs were took after 24 h standing at room temperature. Scale bar: 10 μm. Insets of (C and D): droplets size distribution histograms.

3.2 Preparation of massage ball-like, hollow porous Au/SiO₂ microspheres

Highly stable Pickering emulsion was an ideal soft template for the preparation of the hollow sphere materials. In order to synthesize hollow SiO₂ microspheres employing the generated o/w emulsion as template, the pure n-decanol oil was partly replaced by TEOS. In this study, the total oil/water volume ratio of the emulsion was fixed to be 1 : 3, while the volume ratio of TEOS to n-decanol in mixed oil phase was changed from 2 : 1 to 1 : 2. The corresponding hybrid micelles content was about 0.27 wt% versus the total weight of oil and water. Pickering emulsions were thus prepared by homogenization method, which was then used as the template to synthesize the SiO₂ microspheres. The digital photographs of the generated o/w emulsion as well as the optical microscopy photographs for the varied TEOS/n-decanol volume ratio were shown in S4, ESI.† It was found that the emulsion layer thickness was nearly the same for different TEOS/n-decanol volume ratios, i.e., about 36% of the total liquid volume. The average diameter of these droplets with a broad size distribution was about 6.5, 9.6, and 12.9 μm, corresponding to the 2 : 1, 1 : 1, and 1 : 2 of TEOS/n-decanol volume ratio, respectively (S4, ESI†).

For the purpose of preparing hollow Au/SiO₂ microspheres, the as-generated Pickering emulsions with different TEOS/n-decanol volume ratios were immediately placed in a 30 °C oil bath and aged for 72 h. In general, acids or bases are required to catalyze TEOS hydrolysis. However, the hydrolysis reaction of TEOS in the present system was found to proceed smoothly without the addition of any base or acid, which was probably attributed to the catalytic effect of the weak basic P4VP core of the hybrid micelle adsorbed at the oil/water interface.³² Therefore, it is reasonable to assume that the TEOS encapsulated in the oil phase firstly diffuse to the oil/water interface and then hydrolyze at the interface. In the current case, the hybrid micelles acted as both emulsifier to stabilize the as-formed Pickering emulsion and catalyst for the TEOS hydrolysis. The morphologies of the obtained spheres were characterized by SEM. Typical SEM image of the microspheres obtained from 1/1 of TEOS/n-decanol volume ratio was depicted in Fig. 3A. As could be seen, the microspheres had a rough surface with many nano-/submicro-sized protrusions. The average diameter of the microsphere was around 4.5 μm, whose size was much smaller than that of the emulsion droplets before solidification because of the volume shrinkage during the TEOS hydrolysis, condensation, and SiO₂ formation process. Increasing the volume ratio of TEOS/n-decanol to 2 : 1 resulted in deflated, wrinkles surfaced SiO₂ microspheres with an average diameter of 3 μm. On the other hand, almost totally broken SiO₂ microspheres were observed when the volume ratio of TEOS/n-decanol was 1 : 2. This was caused by the formation of very thin and weak shells with such a small quantity of TEOS. The corresponding SEM images of hybrid microspheres were shown in S5, ESI.† Moreover, TEM observation further proved that the produced hybrid microspheres at 1/1 of TEOS/n-decanol volume ratio were hollow structures as evidenced by the broken and intact single hollow sphere shown in (Fig. 3B–D). The thickness of the shell was about 28 nm (Fig. 3E). In addition, the nano-/submicro-sized protrusions on the spherical surface were clearly observed, as indicated by arrows in Fig. 3A, C–F. The as-synthesized microspheres were further characterized by powder XRD diffraction. A broad peak centred at 23° was observed, which is the characteristic diffraction of amorphous SiO₂. Furthermore, the diffraction peaks at 2θ = 38°, 44°, 65°, and 78° can be indexed to (111), (200), (220) and (311) planes of Au (inset of Fig. 3E),^32,33 implying the successful preparation of Au/SiO₂ hybrid microspheres.

Fig. 3 (A) SEM image of the SiO₂ hybrid microspheres; (B–F) TEM images of the hollow SiO₂ hybrid microspheres with different magnifications; insets of (A and E): the SEM image at high magnification (A); XRD pattern of the hollow SiO₂ hybrid microspheres (E).

The as-obtained Au/SiO₂ hybrid microspheres were further heat-treated at 500 °C in muffle furnace under air atmosphere. After calcination, the morphologies of these microspheres were again characterized by TEM. Evidently, the spherical architecture was still preserved, and some broken microspheres were observed, as shown in Fig. 4A. Careful observation indicated that the nano-/submicro-sized protrusions on the shell have been converted to hollow structures (Fig. 4B–D), and the average pore size was about 15 nm, being close to the 〈D_h〉 value of the hybrid micelles as mentioned previously. Therefore, the as-produced pores were the result of the pyrolysis of these embedded hybrid micelles. In addition, the Au nanoparticles embedded in the mesopores were clearly seen, as revealed by arrows in Fig. 4D, which were also supported by energy dispersive X-ray spectroscopy (EDX) analysis (inset in Fig. 4D). The Au content in the Au/SiO₂ was 0.28% based on ICP-AES analysis.

Fig. 4 (A) TEM images of the hollow porous Au/SiO₂ hybrid microspheres; (B) TEM image of the protrusions on the shell; (C) TEM image of the porous shell; (D) TEM image of the porous shell at the higher magnification; (E) N₂ adsorption–desorption isotherm for the hollow SiO₂ hybrid microspheres before (a) and after (b) calcinations; insets in (D and E): EDX of the hollow porous Au/SiO₂ hybrid microspheres (D) and pore size distribution of the hollow porous Au/SiO₂ hybrid microspheres (E), respectively.

The nitrogen sorption isotherms of these SiO₂ microspheres were also shown in Fig. 4. As could be seen, the specific surface areas of the SiO₂ microspheres before and after calcination were 174 and 290 m² g⁻¹, respectively (Fig. 4E). The remarkable increase in specific surface area after calcination can be attributed to the formed pore structures. Furthermore, the obtained hollow porous Au/SiO₂ material exhibited a bimodal pore size distribution (inset in Fig. 4E), in which the average pore size was centred at around 1.7 and 11 nm, respectively, based on BJH method. Considering that the hybrid micelles contain Au cross-linked P4VP cores and PEO shells that were embedded in the SiO₂ shell, it is reasonable to assume that the pyrolysis of P4VP cores resulted in the formation of the mesopores, while the pyrolysis of PEO shells contributed to the formation of the micropores, giving a total pore volume of 0.38 cm³ g⁻¹, and a micropore volume of 0.033 cm³ g⁻¹.

The as-produced hollow hybrid microspheres could be explained by the plausible formation mechanism as depicted in Scheme 1. For simplicity, the shells of the hybrid micelles were omitted in Scheme 1. The SiO₂ shell was formed through a self-catalyzed process. As mentioned previously, the Au@PEO-b-P4VP micelles stabilized Pickering emulsion contained a TEOS/n-decanol mixed oil phase. The P4VP cores could act as catalyst for the hydrolysis and condensation of TEOS, while the oil phase acted as reservoir for the TEOS precursors. Therefore, the hybrid micelles at the water/oil interface were the main location of the hydrolysis reaction and some SiO₂ fragments were initially formed around the P4VP cores (Scheme 1a). With the increase of the reaction time, these SiO₂ fragments around the P4VP cores merged together and the location of the reaction changed to the oil/water interface. With the further increase of the reaction time, the thickness of the shell increased continually and SiO₂ microspheres with hollow structure were finally formed, in which these micelles were embedded in the SiO₂ shell (Scheme 1b). The n-decanol oil did not participate in the hydrolysis and condensation reaction of TEOS, which only acted as a soft template to create a larger hollow cavity. During the sol–gel reaction, n-decanol was encapsulated by the formed SiO₂ shell. The encapsulated n-decanol was removed during the following drying and calcinations treatment. On the other hand, the relatively broad size distribution (PDI = 0.218) of Au@PEO-b-P4VP hybrid micelles led to the uneven adsorption of these hybrid micelles at the oil/water interface, where hybrid micelles with large size (single or aggregated) facilitated the formation of protrusions on the shell surface. At the same time, the volume shrinkage of the oil droplets during the early hydrolysis and condensation of TEOS led to the increase in their curvature, which further prompted the aggregation and the expulsion of micelles at the oil/water interface, leading to the formation of protrusions (Scheme 1b and c). After calcination, the polymer components embedded in the SiO₂ shells as well in the SiO₂ protrusions were pyrolysized, leaving behind pore structures (including mesopores and micropores) together with Au nanoparticles being embedded in the interior surfaces of the produced mesopores (Scheme 1d).

3.3 Reduction of p-nitrophenol by NaBH₄ catalyzed by Au/SiO₂

As discussed above, the prepared Au/SiO₂ hybrid microspheres exhibited hollow cavities, and bimodal porous shells as well as functional Au nanoparticles encapsulated in the mesopores. Therefore, Au/SiO₂ hybrid microspheres should be ideal catalytic microreactors, and each mesopore on the microspheres could be considered as an individual reactor. At the same time, the coexisted hollow cavities and micropores can shorten the diffusion path of reactants,¹ which thus leads to an enhanced catalytic efficiency. In order to demonstrate the potential advantage of the as-prepared Au/SiO₂ microspheres, the reduction reaction of p-nitrophenol to p-aminophenol by NaBH₄ was chosen to evaluate the catalytic activity of Au/SiO₂ microspheres. The reduction of p-nitrophenol to p-aminophenol by NaBH₄ has been widely studied,^33–36 and the reaction mechanism is very well recognized. The reaction was monitored spectrophotometrically by measuring the absorption intensity change at 400 nm upon the addition of NaBH₄. For comparison, two sets of control experiments were carried out. In the first experiment, the reaction was carried out in the absence of Au/SiO₂ hybrid microspheres. The experiment result indicated there was almost no change in UV absorbance curves. In the second control experiment, the reaction was carried out in the absence of NaBH₄. The phenomenon was the same to that obtained from the first control experiment. These suggested that the reduction reaction was unable to occur by itself under the experimental conditions without the addition of catalysts or reduction agent, and these were consistent with the early reports.^33,35 Conversely, after the addition of a small amount of Au/SiO₂ microspheres (1.0 mg) to this solution at room temperature (25 °C), the intensity of absorbance peak at 400 nm gradually dropped, accompanied by a concomitant development of a new peak at about 310 nm corresponding to the formation of p-aminophenol (Fig. 5A). These results indicated that Au/SiO₂ microsphere was an effective catalyst in this reaction. Since the concentration of NaBH₄ greatly exceeded that of p-nitrophenol, and the reaction could be considered as a pseudo first-order, and the corresponding pseudo first-order rate constant, K_app, was calculated to be 3.15 × 10⁻³ s⁻¹ from the fit linear of ln A (A = absorbance at 400 nm) vs. time, which was comparable with or higher than those recently reported Au based catalyst.^34,35,37,38 In addition, the recovered catalyst also exhibited a high activity with just slightly decreasing in K_app even after running for 3 successive cycles. The K_app was 3.15 × 10⁻³ s⁻¹, 2.14 × 10⁻³ s⁻¹ and 1.87 × 10⁻³ s⁻¹ for the first, the second and the third running, respectively. The higher catalytic activity of Au/SiO₂ to the reduction of p-nitrophenol by NaBH₄ was as the result of the large specific area of hollow porous SiO₂ spherical support and the easily accessible Au nanoparticles catalytic centres to reaction substrates. At the same time, the coexisting hollow cavity and micropores facilitated the mass transportation.

Fig. 5 (A) Time-evolution UV-vis absorption spectra for the successive reduction of p-nitrophenol with NaBH₄ in presence of hollow Au/SiO₂ microspheres with porous wall; (B) ln A versus reaction time for the reduction of p-nitrophenol by NaBH₄ in the presence of Au/SiO₂ hybrid microspheres for the first (a) the second (b) and the third running (c). A represented the absorption intensity (400 nm) at time t.

4. Conclusions

In summary, massage ball-like, hollow porous Au/SiO₂ microspheres with bimodal porosity were successfully synthesized using Pickering emulsion stabilized by the Au@PEO-b-P4VP hybrid micelles as template. The mesopores and the micropores in the shell layer were produced by the P4VP/Au cores and PEO shells of the hybrid micelles embedded in the SiO₂ layer, respectively. Furthermore, Au nanoparticles could be readily implanted in the produced mesopores by this method. The as-prepared Au/SiO₂ hybrid microspheres catalyst possessed a high activity and a good recyclability. When used to catalyze the reduction reaction of p-nitrophenol by NaBH₄, the corresponding pseudo first-order rate constant, K_app, showed a slight decrease after 3 successive cycles. The high catalytic efficiency was as the result of the lager specific areas and the easily accessible Au nanoparticles as well as the fast mass transportation. In addition, the present method is also suitable for loading other metals (e.g., Pt, Pd, Ru etc.) and their alloys in the produced mesopores for the required catalysis reaction.

Acknowledgements

The authors thank the financial supports from the National Natural Science Foundation of China (21174118, 20974090), the key project of Education Department of Hunan Province (12A134).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Synthesis and characterization of PEO–Br macromolecular ATRP initiator and PEO-b-P4VP block polymer; the digital photographs of n-decanol in water Pickering emulsion and the optical microscopy photographs of emulsions droplets stabilized by different content of Au@PEO-b-P4VP hybrid micelles; the digital photographs of (TEOS + n-decanol) in water Pickering emulsion stabilized by 0.27 wt% of PEO-b-P4VP micelles and the corresponding optical microscopy photographs of emulsions droplets; the digital photographs of (TEOS + n-decanol) in water Pickering emulsion with fixed oil/water volume ratio and different TEOS/n-decanol volume ratio stabilized by 0.27 wt% of Au@PEO-b-P4VP hybrid micelles and the corresponding optical microscopy photographs of emulsions droplets; SEM images of SiO₂ microspheres obtained from Pickering emulsion templates with different TEOS/n-decanol volume ratios. See DOI: 10.1039/c4ra09545f

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