SiO2 Particles

Apr 20, 2015 - Double-shelled C/SiO2 hollow microspheres with an outer nanosheet-like silica shell and an inner carbon shell were reported. C/SiO2 aer...
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Activation of the Solid Silica Layer of Aerosol-Based C/SiO2 Particles for Preparation of Various Functional Multishelled Hollow Microspheres Xiangcun Li,* Fan Luo, and Gaohong He* State Key Laboratory of Fine Chemicals, Chemical Engineering Department, Dalian University of Technology, Linggong Road 2#, Dalian 116024, China S Supporting Information *

ABSTRACT: Double-shelled C/SiO2 hollow microspheres with an outer nanosheet-like silica shell and an inner carbon shell were reported. C/SiO2 aerosol particles were synthesized first by a one-step rapid aerosol process. Then the solid silica layer of the aerosol particles was dissolved and regrown on the carbon surface to obtain novel C/SiO2 double-shelled hollow microspheres. The new microspheres prepared by the facile approach possess high surface area and pore volume (226.3 m2 g−1, 0.51 cm3 g−1) compared with the original aerosol particles (64.3 m2 g−1, 0.176 cm3 g−1), providing its enhanced enzyme loading capacity. The nanosheet-like silica shell of the hollow microspheres favors the fixation of Au NPs (C/SiO2/Au) and prevents them from growing and migrating at 500 °C. Novel C/C and C/Au/C (C/Pt/C) hollow microspheres were also prepared based on the hollow nanostructure. C/C microspheres (482.0 m2 g−1, 0.92 cm3 g−1) were ideal electrode materials. In particular, the Au NPs embedded into the two carbon layers (C/Au/C, 431.2 m2 g−1, 0.774 cm3 g−1) show a high catalytic activity and extremely chemical stability even at 850 °C. Moreover, C/SiO2/ Au, C/Au/C microspheres can be easily recycled and reused by an external magnetic field because of the presence of Fe3O4 species in the inner carbon shell. The synthetic route reported here is expected to simplify the fabrication process of doubleshelled or yolk−shell microspheres, which usually entails multiple steps and a previously synthesized hard template. Such a capability can facilitate the preparation of various functional hollow microspheres by interfacial design.

1. INTRODUCTION Hollow spheres with controlled structures and unique physical properties have attracted extensive interests in many fields such as adsorbent, drug delivery system, nanoreactor, and photoelectronics.1−5 Very recently, it was reported that the multishelled hollow spheres with large specific surface area, enhanced structural stability, high loading capacity, and multifunctionality showed significant advantages in many applications over the solid particles or their single-shelled hollow counterparts.1,6,7 Up to now, these hollow microspheres have been typically prepared by multiple coating the surface of colloid particle (e.g., silica or polymer bead) with desirable material layers, followed by selective removal of the colloid template with high-temperature calcination or chemical etching.8−11 The synthesis approach generally involves multistep operations and complex components, leading to difficulty in large-scale production to commercially viable quantities.12 Besides the tedious layer-by-layer assembly procedure, the shells of the nanostructures are commonly made of the same materials. 2,3,10 Several other mechanisms including the Kirkendall effect, Ostwald ripening, dissolution−regrowth, and self-assembly have been reported for the synthesis of various inorganic hollow spheres. However, these methods are limited to yolk−shell or single-shelled hollow particles and always require a previously synthesized hard template.13−17 The © XXXX American Chemical Society

synthesis of multishelled hollow spheres with multicomponent shells by an easy-to-efficient approach is still a challenge. In this study, double-shelled C/SiO2 hollow microspheres with an outer nanosheet-like silica shell and an inner carbon shell were initially reported. C/SiO2 aerosol particles were synthesized first by a one-step rapid aerosol process. Then the solid silica layer of the aerosol particles was dissolved and regrown on the carbon surface to obtain novel C/SiO2 doubleshelled hollow microspheres. The regrowth of silica layer was performed on the carbon shell surface, providing a possibility for rearrangement of other organic/inorganic composites to construct new nanostructures.17−19 Moreover, the dissolution− regrowth process here was facilely completed without any further reagent assistance, differing from the previous reports where the transformation of a solid silica particle or layer to a porous silica shell was generally carried out by a surfaceprotected etching method.18,20,21 Interestingly, the C/SiO2 hollow spheres can be used as ideal templates for the preparation series of SiO2, C, C/C, C/Au/C, and C/Pt/C hollow spheres. The results reported here are expected to simplify the fabrication process of multiple-shelled or yolk− Received: December 30, 2014 Revised: March 29, 2015

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Scheme 1. Formation of Double-Shelled C/SiO2 Hollow Spheres by Exploiting the Dissolution−Regrowth Mechanism and Preparation of Different Hollow Spheres

only results in formation of novel microspheres but also largely induces the increase of the specific surface area and pore volume of the double-shelled hollow spheres (226.3 m2 g−1, 0.51 cm3 g−1, Table S1) compared with the aerosol particles (64.3 m2 g−1, 0.176 cm3 g−1). The new hollow spheres show a high enzyme loading capacity and low enzyme leaching rate because of the high surface area, porous silica shell, and mechanical stability structure. We first report on the synthesis of double-shelled hollow microspheres with the double shells possessing different components by a facile approach. Our route provides an easy access to construct hierarchical nanostructures by interfacial design and has the advantages of being easily scaled up.24 Meanwhile, silica or carbon single-shelled hollow microspheres were obtained by calcining the double-shelled C/SiO2 microspheres in air or etching in HF solution, respectively. The silica shell also shows a high enzyme immobilization capacity due to its large surface area and porous structure. As an ideal template, the C/SiO2 hollow spheres were further coated by a carbon layer to form C/SiO2/C triple hollow spheres; the carbon sources are resorcinol-formaldehyde (RF),25,26 dopamine hydrochloride,27,28 or polypyrrole.29 Subsequently, double-shelled C/C hollow spheres with high specific surface area and pore volume (482.0 m2 g−1, 0.92 cm3 g−1, Table S1) were easily obtained by dissolving the middle silica layer. The novel double-shelled C/C nanostructures show good performance for supercapacitor application and should be excellent electrode materials for other microelectron devices. Considering the flake-like nanostructure of the silica shell of the doubleshelled C/SiO2 hollow spheres, the structure might be a new and excellent support material for catalytic nanoparticles with respect to the favorable fixation of nanoparticles in the flexible shell morphology.30,31 To prove our hypothesis, a soft metal, Au, which is prone to aggregate and loses its activity at high temperature, is selected as a model nanoparticles. It was found that Au nanoparticles were uniformly dispersed on the silica surface, and the nanoparticles maintained dispersion state even after heating treatment at 500 °C. However, the Au NPs grew

shell microspheres, which usually entails multiple steps and a previously synthesized hard template. According to their structure and compositional characters, these hollow spheres show good application performance in different fields such as enzyme loading, electrode materials, and catalytic reaction in solution.

2. RESULTS AND DISCUSSION Formation of Aerosol Particles and the DoubleShelled Hollow Spheres. Our previous work has demonstrated that an aerosol-assisted process is an efficient approach to prepare porous inorganic particles.22,23 The precursor solution is aerosolized, and the resulting droplets pass through a heating furnace where hydrolysis and condensation of the precursor occur to form spherical particles. In this study, a homogeneous precursor solution containing TEOS, sucrose, CTAB, FeCl3·6H2O, and HCl solution with desired proportion was prepared, and then the C/SiO2 aerosol particles were easily obtained by the aerosol-assisted self-assembly approach.12 During the aerosolized process, a silica shell forms extremely rapidly, sealing in the organic species in the particle interior. Subsequent pyrolysis in N2 results in a buildup of internal pressure, forcing carbonaceous species against the silica wall to form an inner carbon shell. As depicted in Scheme 1, C/SiO2 hollow spheres with two thin layers were synthesized by the rapid aerosol process: an outer hydrophilic silica layer and an inner hydrophobic carbon layer. However, the two layers of the C/SiO2 aerosol particles adjoin together tightly, and no porous structure is observed in the shells. We suppose that the silica layer can be rearranged by exploiting the dissolution−regrowth mechanism on the C/SiO2 aerosol particles, leading to formation of double-shelled C/SiO2 hollow spheres with high surface area, porous shell structure, and extensive applications. From Scheme 1, it is apparent that double-shelled C/SiO2 hollow spheres with nanosheet-like silica layer and large interspace between the two layers are obtained, by activating the solid silica layer of the C/SiO2 aerosol particles. Importantly, the simple rearrangement of the silica layer not B

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Figure 1. SEM and TEM images of the C/SiO2 aerosol particles (a, b) and the double-shelled C/SiO2 hollow microspheres with large space between the carbon and silica shells (c, d).

dissolution−regrowth mechanism. The idea is significant here because a double-shelled C/SiO2 hollow sphere is easily obtained, and the novel nanostructure possesses large surface area, porous silica layer, void space between the two layers, and high application performance. We believe that the regrowth of silica layer on a carbon shell surface provides a possibility for rearrangement of different organic−inorganic composite materials. The interfacial design idea gives an easy access to prepare hierarchical nanostructures through a facile and lowcost process. Figure 1c shows the typical SEM image of the double-shelled C/SiO2 hollow spheres prepared by dissolving and regrowing the C/SiO2 aerosol hollow particles. The microspheres are hierarchical and the surface is assembled by a large number of nanosheets. SEM image of a cracked particle clearly reveals a multi-ball-in-ball architecture (the inset in Figure 1c); the relatively smooth surface of the inner shell is characteristic morphology of a carbon layer.9,34 A close look of an individual sphere reveals the existence of considerable nanopores in the shell, which is attributed to the disordered arrangement of the nanosheets. TEM image in Figure 1d shows that all microspheres have two clear rings within one sphere, indicating the formation of hierarchical hollow structures with double walls. The large space of 50−100 nm between the two layers and the mesopores in the silica shell provides the new nanostructure with high surface area and pore volume (226.3 m2 g−1, 0.51 cm3 g−1) and high molecule loading capacity. Formation Mechanism of the Double-Shelled Hollow Spheres. Time-dependent TEM observation of the dissolution−regrowth process was conducted to understand the generalization process of the double-shelled nanostructure. First, the outer silica layer of the C/SiO2 aerosol particles begin to dissolve in alkaline solution. Compared to the precursor C/

into large particles when the C/SiO2/Au spheres was heated at 700 or 900 °C because the flake-shell morphology of silica layer became more smooth at high temperature and could not suppress the growth and aggregation of the small Au NPs. To further control the growth, the C/SiO2/Au spheres were coated with a carbon layer, and C/Au/C double-shelled hollow spheres were obtained by the removal of silica. The C/Au/C hollow spheres, which were first reported here,32,33 showed a high catalytic activity. Moreover, the Au NPs embedded into the two carbon layers showed an extremely chemical stability even at 850 °C. The high surface area, pore volume (431.2 m2 g−1, 0.774 cm3 g−1, Table S1), and the mesoporous carbon layer of the novel C/Au/C nanostructure were responsible for the high catalytic activity. Figure 1a,b shows scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of the C/SiO2 particles synthesized by a rapid aerosol method. The aerosol particles have a relative smooth surface. The SEM image gives some of the broken spheres and the exposed core of the aerosol particles, providing evidence of a double-layer structure. The TEM image reveals an evidence of double layer at progressively increasing resolution of the shell from the surface to interior. The formation mechanism of the aerosol particles involves two steps: the generation of the silica layer due to the preferred silica condensation reaction along the gas−liquid interface of an aerosol droplet and the formation of carbon layer by the dehydration and carbonization of the encapsulated sucrose during the subsequent pyrolysis.12 However, the low surface area (64.3 m2 g−1, 0.176 cm3 g−1) and the absence of porous structure could limit the application of the hollow aerosol particles. In this study, we propose that the solid silica layer of the C/SiO2 aerosol particles can be rearranged by exploiting the C

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Figure 2. SEM and TEM images of double-shelled C/C hollow spheres (a, b), C/SiO2/Au hollow spheres (c, d), and C/Au/C hollow spheres (e, f).

Formation of Different Multishelled Nanostructures. Moreover, silica or carbon shell was easily obtained by calcining the double-shelled C/SiO2 hollow spheres in air or etching in HF (Figure S2a,b). The surface zeta potentials of the single layer silica and carbon hollow spheres were tested to be −42.8 and −1.26 mV, respectively, differing from those of the C/SiO2 aerosol particles (−22.0 mV) and the double-shelled C/SiO2 hollow spheres (−37.1 mV). As an ideal hard template,35,36 the double-shelled spheres were further encapsulated by a carbon layer to form triple C/SiO2/C hollow particles (Figure S2c). Subsequently, double-shelled C/C hollow spheres were obtained by removing the middle silica layer in HF solution. As shown in Figure 2a,b, the novel C/C nanostructure with high surface area and pore volume (482.0 m2 g−1, 0.92 cm3 g−1, Table S1) has promising applications in adsorption and electrode materials. It is known that one key issue in gold catalyst is to stabilize Au NPs and prevent them from aggregating. Several methods such as loading Au NPs on a surface-modified support and embedding Au NPs in mesopores have been developed to keep Au NPs from growing and aggregating.31,32 However, thermal stability has been a problem with these composite Au catalysts. The interaction between the support and the Au NPs is usually a week; Au NPs can move during high-temperature reactions. The flake-like shell morphology should favor the fixation of nanoparticles, and the open shell and interlayer space should allow the substrates more easy access to the catalyst active sites and lead to high catalytic efficiency. Here, Au NPs with controllable weight ratio were bound in the flake shell by the reduction of HAuCl4 on the amino group postgrafted surface. Figure 2c shows a representative TEM image of the C/SiO2/Au composites; there are many black dots of ∼15 nm decorated into the doubleshelled hollow spheres, which can be indexed to the Au NPs. It is clear that all Au NPs are homogeneously located on the silica

SiO2 aerosol particles, the surface of the etched microspheres is rather rough (Figure S1a,b), indicating in situ dissolution of the SiO2 and formation of silicate anions around the microspheres. With the increase of the reaction time and temperature (the etching solution was once heated to boiling point and then cooled down naturally), more silica is dissolved, and lots of hollow particles with single shell and smooth surface are obtained (Figure S1c,d). These particles are ascribed to the carbon spheres, which can be well preserved due to the high chemical stability. When the reaction time is 3 h, however, double-shelled microspheres with a outer silica shell are obtained (Figure S1e), revealing the regrowth of the dissolved silicate anions around the carbon shell with decreasing temperature of the reaction solution. The disordered rearrangement of the nanosheets and unstable dissolution−regrowth condition (magnetic stirring in the whole etching process) could lead to the voidage between the silica shell and carbon core and the mesopores among the silica nanosheets. The ICP detection shows that concentrations of the Si species in etching solution at 80 and 120 min are higher than other stages (Figure S1f), further confirming the dissolution of silica from the carbon shell. At 180 min, the dissolved Si species begin to rearrange on the carbon shell with the decrease of their concentrations. The Si species analysis in the solution agrees well with the morphology evolution of the etched particles. The study presents the first report on the synthesis of doubleshelled hollow spheres with the double shells made of different components by a facile route, in which no additional protecting reagent is required. This method differs from the previously reported dissolution−regrowth process where the transition of solid silica particle or shell to a porous silica shell was generally performed by surface-protected etching. Our study offers a possibility for rearrangement of other organic−inorganic materials to construct various hierarchical nanostructures. D

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Figure 3. Nitrogen adsorption−desorption curves (a) and pore size distributions (b) of different hollow spheres.

Figure 4. Adsorption−desorption properties of the double-shelled C/SiO2 hollow spheres: (a) time profile for GOx immobilization, (b) GOx adsorption isotherm after 3 h, and (c) activity of the GOx loaded. Samples 1 and 2: enzyme loaded with initial concentrations of 0.1456 and 0.0728 mg mL−1, respectively (Figure S7a,b). (d) Leaching rate of GOx loaded.

coagulating. From TEM image in Figure 2e, the Au NPs with uniform size are homogeneously dispersed in the shells of the double-shelled C/C hollow spheres. The nanoparticles are embedded into the outer carbon layer or the C−C interspace (inset image in Figure 2f). Such embedment is critical in this study to suppress the growth of the Au NPs in the shell. The dispersion state of Au NPs remained unchanged even thermal treatment at 850 °C for 3 h (in N2) or etching in HF overnight of the hollow spheres. The C/Au/C nanostructure shows a high catalytic activity for 4-NP reduction reaction due to its high surface and pore volume (431.2 m2 g−1, 0.774 cm3 g−1, Table S1) as well as the porous structure of the outer carbon layer (Figure 2e). Interestingly, the HAuCl4 can be replaced by H2PtCl6 to prepare double-shelled C/Pt/C hollow spheres (Figure S4) using the same procedures, showing the flexibility of the double-shelled C/SiO2 hollow spheres as a support network or hard template.

shell (white dots in Figure 2d), and the Au NPs are uniform in size and well-dispersed in the hierarchical shell without excess aggregation (2.89 wt %).37,38 The loading efficiency of the Au NPs in the shell is tuned by simply altering the concentration of HAuCl4 (Figure 2Sd,e, 0.51 and 0.34 wt %). When the composites were heated at 500 °C, the Au NPs in the shell were observed to maintain their dispersed state without obvious aggregation on the silica surface (Figure 3Sa,b). Under 700 or 900 °C, however, the sizes of Au NPs increased to ∼50 and ∼100 nm, respectively (Figure 3Sc−f). The results were due to the deformation and flatness of the nanosheets under such harsh conditions, which could not prevent the aggregation of the Au NPs. To overcome the problem, C/SiO2/Au composite was subsequently coated using a carbon layer to prepare the C/ Au/C spheres with the removal of silica layer in HF solution. Our hypothesis is that the Au NPs can be fixed between the two layers (sandwich structure) or embedded into the outer carbon layer, protecting the Au NPs from moving together and E

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(Figure S7c,d).43 The single-shelled silica spheres show a loading capacity of ∼28 mg g−1 to the enzyme, lower than that of the C/SiO2 spheres, which is probably ascribed to its week mechanical stability because it has only one wall. This was further confirmed by their morphology change of the spheres with enzyme loaded (Figure S8). With enzyme immobilization, most originally intact silica spheres were deformed and broken. In contrast, the morphology and dispersion of the doubleshelled C/SiO2 spheres have no obvious damage compared to the original spheres, demonstrating high mechanical stability of the hierarchical nanostructure, which is vital for the highly efficient enzyme loading.7,44 Figure 4c shows that the GOx loaded has a lower activity than the free enzyme, which can be ascribed to that the strongly encapsulation inside the inner cavity decreases the activity of the enzyme loaded. However, the activity of the free enzyme decreases rapidly with reaction going on, while the enzyme loaded keeps a stable activity after 10 min. We speculate that the free enzyme is homogeneously dispersed in solution, and the dissolved glucose participates in the catalytic reaction. The GOx is surrounded by the reaction products of gluconic acid and H2O2, leading to the repaid decrease of its activity. For the loaded GOx, however, which first reacts with the glucose around the spheres, and then the products diffuse into the solution quickly from the sphere surface under the driving force of concentration gradient. Thus, the dynamic equilibrium is established, and the GOx loaded can provide a stable catalytic activity in a microenvironment. For the enzyme immobilization via the adsorption or encapsulation approach, the leakage is usually a big problem and disadvantage. Figure 4d shows that the leaching of GOx increases with time. The leaching rate decreases slightly after 8 h, which is probably due to the readsorption of the enzyme onto the spheres. Importantly, only ∼3% of the GOx loaded is leached after 10 h, which can be ascribed to the strong encapsulation and protection of the hierarchical silica shell. Therefore, the double-shelled C/SiO2 hollow spheres are efficient to suppress the enzyme leakage and provide a stable enzyme activity for a long time. The double-shelled C/C hollow spheres have been used as supercapacitor electrode materials. Even at a high rate of 500 mV s−1, the CV curve shows a nearly rectangular shape (Figure S9a), indicating good capacitive performance of the C/C material at high current density.27,45,46 The charge−discharge curves of the electrode at current density of 0.5−20 A g−1 present isosceles triangular shape, demonstrating a close to ideal capacitor behavior (Figure S9b).47 At a low current density of 0.1 A g−1, the specific capacitance is 251 F g−1, which still remain 74 and 64 F g−1 respectively even at 10 and 20 A g−1 (Figure S9c). The specific capacitances of the C/C hollow spheres are higher than those values of typical carbon electrodes (50−100 F g−1);46,48−50 this can be due to its high surface area and porous outer carbon layer which facilitates ion transport during the charging/discharging process.51,52 The performance of the C/C spheres is also superior than that of the single C shell (105 F g−1 at 0.1 A g−1), proving the crucial roles of the outer porous carbon layer and the voidage for improving the energy storage property of the C/C hollow spheres. The capacitance decreases by 5.4% after 5000 cycles, indicating good cycling stability of the C/C nanostructure as electrode materials (Figure S9d).27 The reduction of 4-nitrophenol (4-NP) by NaBH4 was chosen as a model reaction for evaluating the catalytic performance of the Au NPs loaded hollow spheres. Light

The distribution of iron species added in the initial experiments was also studied by an energy dispersive X-ray spectroscopy (EDX) analysis. The iron oxides are homogeneously distributed in the carbon and silica shells (Figure S5A− C and Table S2). For the C/SiO2/Au and C/Au/C hollow spheres, the Au NPs are well distributed in the whole particles (Figure S5D,E), and the detection agrees with the TEM and SEM results. The XRD patterns prove that the iron species in the C/SiO2, C/SiO2/Au, and C/Au/C hollow spheres are mainly Fe3O4 (Figure S6), showing magnetic properties and their recycling possibility from solution by an external magnetic field. Compared to the XRD pattern of the C/SiO2 spheres, the additional reflection peaks at (111), (200), and (220) further indicate the presence of Au in the C/SiO2/Au and C/Au/C materials. Importantly, the well encapsulation of Fe3O4 in the inner carbon shell entails the C/SiO2/Au and C/Au/C strong magnetic properties, leading to formation of recyclable catalysts. N2 adsorption−desorption isotherms and pore size distributions of these hollow spheres are presented in Figure 3a,b. All of the isotherms are type IV with an H1 hysteresis loop, indicating the presence of mesopores.39,40 The corresponding pore size distributions, which were calculated using the BJH method from the desorption branch of the isotherms, further indicated the presence of mesopores in these hollow spheres. The broad pore size distributions can be due to the nanosheetstructured silica shell. For the double-shelled C/SiO2 spheres with hierarchical silica shell, the pore size increases to 10−40 nm compared to that of 2−4 nm in the C/SiO2 aerosol particles, which facilitates the diffusion of the reactants and product molecules. The BET surface area of the hollow spheres is in the range of 177−482 m2 g−1, obviously higher than that of the precursor C/SiO2 aerosol particles (64.3 m2 g−1). The pore volume is in the range of 0.31−0.92 cm3 g−1 for these hollow spheres. Application of Multishelled Hollow Nanostructures. Figure 4a shows that there is an initially fast increase and then a slow increase for the GOx adsorption on the hollow spheres. The loading capacity has no obvious increase after 150 min, indicating the GOx immobilization approaches equilibrium at this time. At 180 min, the loading capacity of GOx on the double-shelled C/SiO2 hollow spheres reaches 27 mg g−1, which is higher than that of the reported results (0.08 mg g−1) at the same adsorption time, and the immobilization equilibrium time in this study is obviously shorter than the previous result (∼8 h).7 The high adsorption rate and capacity of the C/SiO2 hollow sphere can be attributed to its high surface area and hierarchical porous silica shell as well as the mesopores of 10−40 nm in the silica shell which facilitate the diffusion of the enzyme molecules.41,42 However, the C/SiO2 aerosol particles have no loading capacity to the enzyme at all. In practice, the adsorption process is complicated, comprising adsorption of GOx onto the external surface of the silica shell followed by further encapsulation of the enzyme inside the silica inner surface or the interspace. Therefore, the loading capacity increases gradually with the initial concentration of GOx, and a maximum immobilization capacity of 36 mg g−1 is obtained for the double-shelled hollow spheres (Figure 4b). Two classical models, Langmuir and Freundlich, were used to evaluate the kinetics of GOx immobilization in this work. The correlation coefficient of the curve from the Langmuir isotherm fitting is very well, indicating the immobilization of GOx onto the external and inner surface of the silica layer as a monolayer F

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Figure 5. (A) UV−vis spectra of 4-NP before and after the addition of NaBH4 solution, (B) the reduction of 4-NP at different time intervals using C/SiO2/Au (0.51 wt %) as catalyst, (C) the reduction of 4-NP using C/SiO2/Au as catalyst, (a) 0.34 wt %, (b) 0.51 wt %, and (c) 2.89 wt %, (D) the reduction of 4-NP using different Au-based catalysts.

evaluation of catalytic activity, reusability, and chemical stability, the C/Au/C is an excellent catalyst for solution reaction compared with the reported Au-based catalysts.55−57

yellow aqueous 4-NP solution shows absorption at 320 nm, while the absorption maximum shifts to 400 nm due to the formation of 4-nitrophenolate by adding of NaBH4.26,53,54 There was only a little change in the absorption after standing for a long time (Figure 5A), indicating that the reduction almost did not proceed without catalyst. After addition of a small amount (0.5 mg) of C/SiO2/Au (0.51 wt %) in the mixture solution of 4-NP and NaBH4, the adsorption peak at 400 nm significantly decreases as the reaction proceeds. Meanwhile, a new peaks appears at 295 nm and gradually increases, revealing the reduction of 4-NP into 4-aminophenol (Figure 5B).54 Understandably, the high Au NPs loading provides more active sites on the sphere surface, resulting in a high reaction rate, as shown in Figure 5C. However, considering the same Au content in the reaction solution, C/ Au/C (2.08 wt %) double-shelled hollow spheres show a comparable activity to C/SiO 2/Au spheres in catalytic reduction of 4-NP, and over 98% conversion can be reached in 6 min at room temperature (Figure 5D). The high catalytic performance of the C/Au/C nanostructure is attributed to its high surface area. The mesopores (8−30 nm) in the outer carbon shell as well as the voidage between two carbon layers facilitate the diffusion process of phenol molecules and their access to the Au NPs. To investigate reusability, the samples C/ Au/C and C/SiO2/Au were collected by an external magnetic field (the C/SiO2 hollow spheres can also be recycled magnetically) after each reduction reaction and washed with water and ethanol before being used again (Figure S10). Catalytic performance of the C/Au/C spheres was similar without a noticeable reduction in the conversion efficiency of 4NP over the same reaction time even after performing five cycles (Figure S11). Accordingly, in a comprehensive

3. CONCLUSIONS We first report on the synthesis of double-shelled C/SiO2 hollow spheres with the C/SiO2 aerosol particles as a precursor by exploiting the dissolution−regrowth mechanism. The high specific surface area, pore volume, and mechanical stability of the novel spheres benefit the achievement of a high enzyme loading, stable activity, and lower enzyme leaching. Moreover, the hollow spheres can act as ideal support or template for generalized synthesis of a family of new materials such as single carbon and silica shells, C/SiO2/C triple spheres, and C/SiO2/ Au, C/C, and C/Au/C double-shelled hollow spheres. These nanostructures are versatile materials for a broad range of applications such as electrode materials, catalytic reaction, separation, drug delivery, and controlled release. 4. EXPERIMENTAL SECTION Tetraethyl orthosilicate (TEOS), FeCl3·6H2O, and cetyltrimethylammonium bromide (CTAB) were purchased from Sigma-Aldrich. Sucrose and hydrochloric acid (∼37%) were obtained from Damao Chemical Company (China). The precursor solution was prepared by mixing 1.0 g of FeCl3·6H2O and 1.1 g of CTAB in 15 mL of ethanol solution, followed by sonication for 5 min to form a clear solution. Then, 4.5 mL of TEOS, 2.0 mL of a 0.1 M HCl solution, and the dissolution of 1 g of sucrose were added.12 The resulting mixture was sonicated for 10 min to form a transparent aerosol solution. The aerosol solution was atomized using a commercial atomizer (model HD-130, HOLDER) to form aerosol droplets, which passed through a quartz tube with a heating zone. The temperature of the heating zone was held at 200 °C, and the carrying gas pressure of N2 was adjusted to G

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Langmuir yield a droplet residence time of about 20 s through the furnace. The aerosol particles were collected by a microfiltration membrane which was maintained at 100 °C to avoid solvent condensation. The assynthesized particles were calcined in N2 at 500 °C for 3 h to prepare C/SiO2 aerosol hollow particles. To obtain double-shelled C/SiO2 hollow spheres, the aerosol hollow particles were treated in a 5 wt % NaOH for ∼5 h, and the etching solution was once heated to boiling point and then cooled to room temperature naturally. The single silica and carbon hollow spheres were easily obtained respectively by calcining the double-shelled spheres in air (500 °C, 3 h) or treating it in HF solution (10 wt %, 5 h). Loading Au NPs on the C/SiO2 hollow spheres was performed according to the previous report by direct reduction of HAuCl4 on the 3-aminopropyltriethylsilane modified hollow spheres.31 The outer carbon shell of the C/SiO2/C, C/C, C/SiO2/Au/C, and C/Au/C hollow spheres was prepared by coating a carbon precursor on the surface of the corresponding hollow spheres, then the porous carbon layer was formed by dehydration and carbonization treatment of the precursor at 850 °C in N2 for 3 h, and the middle silica layer was selectively removed by etching the carbon based spheres in HF solution. The carbon precursors can be resorcinol formaldehyde (RF),26 dopamine,28 or polypyrrole.29 The concentrations of Au in the hollow spheres were determined by an inductively coupled plasma emission (ICP, Optima 2000DV). Enzyme Assay. Two kinds of solutions ware prepared first. Solution A: 3.5 mg of horseradish peroxidase (HRP) and 3.5 mg of 4aminoantipyrine were dissolved in 20 mL of phosphate buffer solution (pH = 7.0), followed by adding 1 mL of 3.0% phenol solution. Solution B: 10% of glucose solution. Then 1.5 mL of A and 1.5 mL of B were mixed under shaking, followed by adding of 100 μL of HRP solution. Glucose can be oxidized to glucono-δ-lactone (C6H10O6) and H2O2 under catalysis of glucose oxidase (GOx) (eq 1), and the product of H2O2 can react with 4-aminoantipyrine and phenol to form stable quinonimine (C6H5NO) with HRP as catalyst (eq 2). The quinonimine shows a red color having a maximum at 506 nm. A calibration curve of glucose oxidase in phosphate buffer solution was constructed (Figure S5a,b). The free and immobilized GOx activities were determined spectraphotometrically at 506 nm by UV−vis spectroscopy (Persee, Beijing, China) by monitoring H2O2 using 4aminoantipyrine and phenol as substrates in the secondary reaction with HRP. Unit (U) of enzyme: the absorbance intensity for 1 mg of GOx in 1 min, based on 100 μL of solution. O2 + C6H12O6 → C6H10O6 + H 2O2

A platinum foil and Ag/AgCl electrode were used as the counter electrode and reference electrode, respectively. Catalytic Reduction of 4-Nitrophenol. The reduction of 4nitrophenol was carried out in a quartz cuvette and monitored by UV−vis spectroscopy at room temperature. A total of 0.4 mL of aqueous 4-nitrophenol solution (1 mM) was mixed with 2 mL of fresh NaBH4 solution (0.02 M). Then, 2 mL of sphere dispersion was added (0.25 mg mL−1), and the mixture solution was quickly subjected to UV−vis measurements after different reaction times. The catalyst can be recycled by an external magnetic field for reuse.

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ASSOCIATED CONTENT

S Supporting Information *

Formation mechanism of the double-shelled C/SiO2 hollow spheres (Figure S1), TEM images of different hollow spheres, (Figure S2), C/SiO2/Au hollow spheres calcined at different temperatures (Figure S3), C/Pt/C hollow spheres with RF as carbon precursors (Figure S4), element analysis of different hollow spheres (Figure S5), XRD patterns of C/SiO2, C/SiO2/ Au, and C/Au/C hollow spheres (Figure S6), UV−vis scanning curve of GOx (Figure S7), SEM images C/SiO2 hollow spheres and single SiO2 hollow spheres with GOx loaded (Figure S8a), CVs curves of the C/C hollow sphere supercapacitors (Figure S9), recycling of the hollow spheres by an external magnetic field (Figure S10), reusability of C/Au/C and C/SiO2/Au as catalysts (Figure S11), BET surface area and pore volume of different hollow spheres (Table S1), and element content analysis of single SiO2 and carbon hollow spheres (Table S2). The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/la505032a.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (X.L.). *E-mail [email protected] (G.H.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The National Natural Science Foundation of China (21476044, 21006008), National Science Fund for Distinguished Young Scholars of China (21125628), and the Fundamental Research Funds for the Central Universities (DUT15QY08) are greatly appreciated.

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H 2O2 + C6H5OH + C11H13N3O + CO2 → 3C6H5NO + 3H 2O (2) Enzyme Immobilization. 5 mg of double-shelled C/SiO2 hollow spheres was added into 4 mL of GOx phosphate solution (0.0009− 0.1456 mg mL−1). The sample solutions were shook at 25 °C for 180 min to allow the immobilization equilibrium of enzyme for each samples, and they were then separated from solution by centrifugation, decantation, and washed with fresh buffer solution for several times to remove the free enzyme completely. The amount of loaded GOx was calculated by measuring the concentration of enzyme in supernatant and then subtracting from the free enzyme amount added in the experiment. The GOx concentration in the supernatant was determined based on the enzyme assay. Enzyme Leaching. The GOx immobilized hollow spheres (5 mg) were redispersed in a 4 mL of fresh phosphate solution and then equilibrated for a desirable time. The concentration of GOx desorbed in the supernatant buffer solution was determined by the GOx assay. Electrochemical Measurements. The electrochemical measurements (Ivium Technologies BV, Holland) were conducted using a three-electrode mode in a 6 M KOH aqueous solution. The working electrode was prepared as follows. The active materials (80 wt %), acetylene black (15 wt %), and polytetrafluoroethylene (5 wt %) binder were mixed in ethanol. A slurry of the above mixture was subsequently pressed onto nickel foam under a pressure of 10 MPa. The prepared electrode was dried in vacuum oven at 120 °C overnight.

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REFERENCES

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DOI: 10.1021/la505032a Langmuir XXXX, XXX, XXX−XXX