Ag-Nanoparticle-Loaded Mesoporous Silica: Spontaneous Formation

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Ag-Nanoparticle-Loaded Mesoporous Silica: Spontaneous Formation of Ag Nanoparticles and Mesoporous Silica SBA-15 by a One-Pot Strategy and Their Catalytic Applications Jie Han, Ping Fang, Wenjuan Jiang, Liya Li, and Rong Guo* School of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou, 225002, Jiangsu, P. R. China ABSTRACT: A facile one-step method was proposed for the successful synthesis of Ag-nanoparticle-loaded mesoporous silica SBA-15 composites, where silver ions and their corresponding reductant aniline were added in the traditional synthetic system of mesoporous silica SBA-15 containing P123 as the surfactant and TEOS as the silica source. Mesoporous silica SBA-15 and Ag nanoparticles were spontaneously formed with Ag nanoparticles embedded in channels and even implanted in frameworks of mesoporous silica SBA-15. A tentative formation process was then proposed according to experimental observations. Furthermore, catalytic activities of Agnanoparticle-loaded silica SBA-15 composites toward the reduction of 4-nitrophenol in the presence of NaBH4 and the reduction of H2O2 were also investigated.

1. INTRODUCTION Since the discovery of the cooperative assembly of surfactants with silicates by Mobil scientists in 1992, which led to the synthesis of MCM-41 and the M41S family of mesoporous materials,1 a series of ordered mesoporous materials have been prepared for their extensive usage in field of controlled drug release.2 Due to robust structures and well-ordered arrays of uniform nanometer-sized channel pores, mesoporous silica materials have been extensively investigated as hard templates for the synthesis of nanoparticles, nanowires, and nanowire networks.3 Noble metal nanoparticles have been extensively studied because of their unique optical and electronic properties, together with their various applications in fields such as electronics, photonics, catalysts, and nano- and biotechnology. Recently, numerous methods have been developed to prepare stable noble metal nanoparticles with controllable shape and size. Use of mesoporous silica materials is established to be one of the most effective ways, with an efficient template effect. Mostly, as-formed mesoporous silica materials are first treated with noble metal ions followed by reduction of noble metal ions to noble metal nanoparticles that are confined in mesoporous channels. Noble metal ions can be incorporated into mesoporous channels through chemical adsorption between noble metal ions and functionalized mesoporous channels. For example, N-trimethoxysilylpropylN,N,N-trimethylammonium chloride (TPTAC)3a and 3-aminopropyltriethoxysilane3b functionalized SBA-15 have been used as a template for deposition of Au nanowires and Ag nanoparticles, respectively. Besides, physical adsorption driven by capillary force also can be utilized to incorporate noble metal ions into mesoporous silica channels.3c−h,4 Nevertheless, synthesis of composites of mesoporous silica loaded with noble metal nanoparticles by a much more straightforward strategy, namely, spontaneous formation of noble metal © 2012 American Chemical Society

nanoparticles and mesoporous silica for simplifying the synthetic procedures, has been seldom seen. Herein, a facile one-step method has been proposed for the successful synthesis of Ag-nanoparticle-loaded mesoporous silica SBA-15 composites. In the traditional synthetic system of mesoporous silica SBA-15 containing P123 and TEOS, aniline was first added for solubilization in P123 micelles, followed by addition of silver nitrate. After hydrolysis and condensation processes of silica precursor together with redox reaction between aniline and silver ions, Ag nanoparticles loaded mesoporous silica SBA-15 (Ag−SBA-15) composites could be formed through a coassemble process. Effects of AgNO3 concentration on the structure of SBA-15 and size of Ag nanoparticles were investigated. Fourier-transform infrared (FT-IR), X-ray diffraction (XRD), and N2 adsorption− desorption were used to characterize the products. Formation mechanisms involved were then proposed according to experimental observations. In addition, catalytic performances of Ag−SBA-15 composites toward the reduction of 4nitrophenol (4NP) in the presence of NaBH4 and the reduction of H2O2 were also investigated. It is believed that such composites will find promising applications in the fields of catalysis and sensors.

2. EXPERIMENTAL SECTION Chemicals. Aniline monomer (Shanghai Chemical Co.) was distilled under reduced pressure before use. Tetraethoxysilane [(C2H5O)4Si, TEOS, SCRC], Pluronic P123 (Sigma), AgNO 3 (Shanghai Chemical Co.), and other reagents were used as received. Received: July 10, 2011 Revised: January 22, 2012 Published: February 16, 2012 4768

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Figure 1. TEM images of (a, b) pure SBA-15 and (c, d) Ag(0.7)−SBA-15 composites. The inset shows corresponding defects of large cavities remained in Ag(0.7)−SBA-15 composites. certain amount of AgNO3 aqueous solution (0.1 mol L−1, keeping the molar ratio of AgNO3 to aniline at 1:1) was added all at once. The total volume of the reaction system was controlled at 15.0 mL. The resulting mixture was left to stir for 24 h at 35 °C and then transferred into a Teflon bottle and heated at 100 °C for 24 h without stirring. After cooling to room temperature, the solid product was recovered by filtration, washed with distilled water, and calcined at 550 °C for 6 h under air flow to remove the surfactant and polymer. The filtration solutions were examined by atomic absorption spectroscopy (AAS) analysis, where the remaining silver was neglectable as compared with the initial charged silver. As a result, it is believed that silver ions are totally converted to silver nanoparticles that exclusively loaded in channels of SBA-15. For clarity, the composites synthesized at AgNO3 concentration of 0.3, 0.7, and 1.2 mmol L−1 are denoted as Ag(0.3)− SBA-15, Ag(0.7)−SBA-15, and Ag(1.2)−SBA-15, respectively. NaBH 4 Reduction of 4NP Catalyzed by Ag−SBA-15 Composites. Typically, an aqueous solution of NaBH4 (1.0 mL, 1.5 × 10−2 mol L−1) was mixed with aqueous 4NP solution (1.7 mL, 2.0 × 10−4 mol L−1) in the quartz cell (1 cm path length), leading to a color

The water used in this study was deionized by Milli-Q Plus system (Millipore, France), having 18.2 MΩ electrical resistivity. Preparation of Mesoporous Silica SBA-15. A typical synthesis of SBA-15 was according to ref 2e but with some modifications as follows: to 10 mL of aqueous solution held at 35 °C containing 1.75 g of P123 (0.30 mmol) was added 2.25 mL of HCl (35%), and the mixture was stirred for 30 min, followed by the addition of 1.0 g (4.7 mmol) of TEOS. The resulting mixture was left to stir for 24 h at 35 °C and then transferred into a Teflon bottle and heated at 100 °C for 24 h without stirring. After cooling to room temperature, the solid product was recovered by filtration, washed with distilled water, and calcined at 550 °C for 6 h under air flow to remove the surfactant. Preparation of Ag−SBA-15 Composites. A typical synthesis of Ag−SBA-15 composites was as follows: to 10 mL of aqueous solution held at 35 °C containing 1.75 g of P123 (0.30 mmol) was added 2.25 mL of HCl (35%), and the mixture was stirred for 30 min, followed by addition of a certain amount of aniline and 1.0 g (4.7 mmol) of TEOS. The resulting mixture was left to stir for 1 h at 35 °C, and then a 4769

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Figure 2. TEM images of (a) Ag(0.3)−SBA-15 and (b) Ag(1.2)−SBA-15 composites. change from light yellow to yellow-green. Then, Ag(0.7)−SBA-15 catalysts (0.3 mL, 2.0 × 10−4 mol L−1 relative to Ag nanoparticles) were added to the mixture and quickly placed in the cell holder of the spectrophotometer. The progress of the conversion of 4NP to 4aminophenol (4AP) was then monitored via UV−vis spectroscopy by recording the time-dependent absorbance spectra of the reaction mixture in a scanning range of 200−600 nm at ambient temperature. Electrochemical Detection of H2O2. Electrochemical measurements were performed with a CHI 660C electrochemical analyzer (Chenhua Instruments, Shanghai, China). Prior to the fabrication, a glass carbon electrode (GCE) was polished with 1.0, 0.3, and 0.05 μm alumina slurries followed by rinsing with doubly distilled water, and then dried in air. GCE/Ag(0.7)−SBA-15 was fabricated by casting 3 μL of concentrated Ag(0.7)−SBA-15 aqueous dispersions on GCE. Then, the solvent was allowed to evaporate at room temperature in air for 1 h. A conventional three-electrode cell was used, including a pretreated GCE as the working electrode, an Hg/HgO electrode as the reference electrode, and platinum foil as the counter electrode. The potentials were measured with an Hg/HgO electrode as the reference electrode. All the experiments were carried out at ambient temperature. Cyclic voltammetry (CV) measurements were performed in a 10 mL electrochemical cell at 25 ± 0.2 °C. Prior to the electrochemical measurements, 0.2 mol L−1 sodium phosphate buffer solutions (PBS, pH 6.8) were purged with highly pure N2 for 15 min, and a N2 atmosphere was kept over the solutions during the electrochemical measurements. Aliquots of standard solutions of H2O2 were successively added to the cell. Characterization. The morphologies of mesoporous silica SBA-15 and Ag−SBA-15 composites were examined by transmission electron microscopy (TEM, Tecnai-12 Philip Apparatus Co.). Fourier-transform infrared (FTIR) spectra were recorded in the range of 400−4000 cm−1 using FTIR spectroscopy (Nicolet-740, United States). The samples were prepared in a pellet form with spectroscopic-grade KBr. The X-ray diffraction (XRD) patterns were recorded on a German Bruker AXS D8 ADVANCE X-ray diffractometer. The products were recorded in the 2θ range from 0.5° to 85.0° in steps of 0.04° with a count time of 1 s each time. N2 adsorption−desorption measurements were conducted by N2 physisorption (Thermo Sorptomatic 1990) at 77 K. The as-calcined samples were outgassed for 4 h at 250 °C under vacuum (p < 10−2 Pa) in the degas port of the sorption analyzer. The BET specific surface areas of samples were evaluated using adsorption data in a relative pressure range from 0.05 to 0.25. The pore size distributions were calculated from the adsorption branch of the

isotherm using the thermodynamic-based Barrett−Joyner−Halenda (BJH) method.

3. RESULTS AND DISCUSSION Morphology, Characterization, and Formation Mechanism of Ag−SBA-15 Composites. Mesoporous silica SBA15 with a 2D-hexagonal symmetry is normally synthesized using nonionic surfactant P123 under acidic conditions according to the nanocasting concept. Take mesoporous silica SBA-15 synthesized with nonionic surfactant P123, for example. The network grows through equilibrium with a phase of diluted spheroidal micelles, which transform with time into the viscous phase; meanwhile, the silicate precursors adsorb and polymerize at the hydrophilic micellar interface, followed by the template removal processes to obtain mesoporous silica SBA-15 materials.1b,3f,5 If hydrophobic noble metal nanoparticles are formed simultaneously in this system, they are inclined to locate in the hydrophobic environment of the inner part of micelles. As a result, the template removal process of calcination will result in noble metal nanoparticles locating in channels of mesoporous silica SBA-15 materials. Aniline is a special molecule that has aroused considerable attention in the past decades because the conducting polymer polyaniline (PANI) can be formed when ammonium persulfate (APS) is added as oxidant to initiate the polymerization of aniline in acidic aqueous solution.6 Recent reports have shown that when noble metal ions and aniline are mixed together, noble metal ions can be reduced to metallic state in the form of nanoparticles, aniline monomers will be oxidized to PANI,7 and noble metal nanoparticles are always capped by PANI, showing the hydrophobic feature of noble metal nanoparticles.8 More importantly, aniline molecules will be solubilized in micelles due to their hydrophobic feature,9 and then the redox reactions between silver ions and aniline monomers will happen in micelles that favors the location of Ag nanoparticles in micelles. Therefore, silver nitrate and aniline are chosen as raw materials of Ag nanoparticles. In the traditional synthetic system of mesoporous silica SBA-15 containing P123 and TEOS, aniline monomers are first added for solubilization in P123 micelles, followed by addition 4770

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of silver nitrate. After hydrolysis and condensation processes of silica precursor together with redox reaction between aniline and silver ions, Ag−SBA-15 composites can be formed through a coassemble process. Figure 1 shows the typical TEM images of mesoporous silica SBA-15 and Ag(0.7)−SBA-15 composites. As shown in Figure 1a, well-defined 2D-hexagonal symmetry is clearly seen, and the pore diameter is estimated to be 9 nm (Figure 1b). When silver nitrate and aniline are added in the reaction system, 2D-hexagonal symmetry also can be identified; however, distortion of the long-range ordering also can be seen (Figure 1c). Clearly, observation reveals the black dots that should come from Ag nanoparticles embedded in channels and even implanted in the framework of mesoporous silica. Magnified TEM image in Figure 1d shows the pore diameter of about 10 nm, which is slighter higher that that of pure SBA15. Interestingly, Ag nanoparticles embedded in mesoporous silica are estimated to be 15 nm, which is larger than the pore size. After carefully checking the products, we also find the defects of large cavities remained (inset in Figure 1d). Effects of AgNO3 concentration on the morphology of SBA15 and the size of Ag nanoparticles were then considered. At relatively low AgNO3 concentration of 0.3 mmol L−1, the majority of the long-range ordering of SBA-15 channels is maintained, and less Ag nanoparticles are found located in channels of SBA-15 (Figure 2a). The size of most Ag nanoparticles is comparable to the pore size (10 nm), whereas some larger ones embedded in channels are also observed. As for 0.7 mmol L−1 AgNO3 concentration, a distortion of the long-range ordering of SBA-15 and an increase in density and size (∼15 nm) of Ag nanoparticles can be evidenced (Figure 1c,d). If AgNO3 concentration is further increased to 1.2 mmol L−1, severe distortion of the long-range ordering of SBA-15 is clearly seen in Figure 2b, and the size of Ag nanoparticles is estimated to be 20 nm. Therefore, it is concluded that the size of Ag nanoparticles will be increased, and the distortion of the long-range ordering of SBA-15 will be enhanced for Ag−SBA15 composites with increasing AgNO3 concentration. It should be noted that the destruction of mesoporous silica structures and aggregation in Ag nanoparticles can be evidenced if the AgNO3 concentration is higher than 1.5 mmol L−1, which is unfavorable for the formation of Ag-nanoparticle-loaded SBA15 composites. FTIR spectra of pure SBA-15 and Ag(0.7)−SBA-15 composites before and after calcination are given in Figure 3. As for pure SBA-15 in Figure 3a, the broad band around 3420 cm−1 can be attributed to surface silanols and adsorbed water molecules, whose deformational vibrations cause the absorption band near 1637 cm−1; the absorption bands at 1087, 808, and 464 cm−1 are assigned to Si−O−Si groups.10 With regard to the spectrum of Ag(0.7)−SBA-15 before calcination (Figure 3b), the stretching vibration of the CH2 groups around 2975 cm−1 indicated the presence of surfactant P123. Besides, the broad band around 3420 cm−1 in pure SBA-15 is found to shift to 3380 cm−1 in Ag(0.7)−SBA-15 composites, which may be a result of the N−H stretching vibration of PANI at 3320 cm−1; the characteristic bands at 1460 and 1378 cm−1, which come from the CC stretching deformation of the benzenoid rings of PANI and the C−N stretching in the neighborhood of a quinonoid ring, respectively, indicate the presence of PANI in products.6c The spectrum of Ag(0.7)−SBA-15 after calcination (Figure 3c) is almost identical to that of pure SBA-15, indicating the successful removal of surfactant and PANI polymer.

Figure 3. FTIR spectra of (a) pure SBA-15 and Ag(0.7)−SBA-15 composites (b) before and (c) after calcination.

Figure 4a shows small-angle XRD patterns of pure SBA-15 and Ag−SBA-15 composites after calcination. As for pure SBA-

Figure 4. (a) Small-angle and (b) wide-angle XRD patterns of pure SBA-15 and Ag−SBA-15 composites.

15, there are three peaks assignable to (100), (110), and (200) in a highly ordered 2D-hexagonal symmetry3a with d100 spacing of 9.02 nm. In the case of Ag−SBA-15 after calcination, the intensity of the peaks is obviously lower than that of the original SBA-15, which may be ascribed to a pore-filling effect resulting in weakening in the scattering contrast. A similar 4771

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Table 1. Structural Parameters for Pure SBA-15 and Ag−SBA-15 Composites

a

sample

d-spacing, d100 (nm)

unit cell parameter, a0 (nm)a

SBA-15 Ag(0.3)−SBA-15 Ag(0.7)−SBA-15 Ag(1.2)−SBA-15

9.02 9.63 9.85 10.20

10.42 11.12 11.37 11.78

wall thickness, dw (nm)b surface area (m2/g) 1.40 1.49 1.52 1.58

456 522 654 718

pore volume (cm3/g)

pore size (nm)

0.36 0.49 1.07 1.36

8.9 9.5 9.9 10.2

a0 = 2/31/2d100. bdw = a0 − d100.

phenomena was noted in previous reports.11 The wide-angle XRD patterns of pure SBA-15 and Ag−SBA-15 composites after calcination are given in Figure 4b. The broad band between 2θ = 15° and 35° is the characteristic band of amorphous silica walls of the pristine material.12 In addition to the broad band, four distinct diffractions at about 38.0°, 44.3°, 64.3°, and 77.3° that correspond to the (111), (200), (220), and (311) planes of Ag,13 respectively, indicate the presence of Ag nanoparticles. The physicochemical properties of SBA-15 and Ag−SBA-15 composites as determined by the XRD results are given in Table 1. N2 adsorption−desorption isotherms were further conducted to evaluate the surface and structure properties of SBA-15 and Ag−SBA-15 composites. Figure 5a shows the typical N2

surfaces areas and pore volumes as calculated according to the BET method are also compiled in Table 1. As displayed in Table 1, it is clearly seen that the pore size increases with loading amount of Ag nanoparticles in SBA-15, which is consistent with TEM results. Although the formation mechanism of Ag−SBA-15 composites is not fully clear at this stage, the above-mentioned results allow us to suggest a tentative formation process responsible for the formation of Ag−SBA-15 composites, as shown in Scheme 1. When aniline monomers are added in solution of P123 and Scheme 1. Schematic Illustration for the Formation of Ag− SBA-15 Composites

TEOS, aniline molecules are inclined to locate in the inner part of micelles due to their obvious hydrophobic feature.9 When silver ions are added, redox reactions between aniline and silver ions lead to the formation of Ag nuclei and PANI polymer. Considering that amine groups can bind to noble metal atoms on the surface of nanoparticles through lone-pair electrons on nitrogen atoms,15 Ag nanoparticles will be wrapped by PANI polymer showing hydrophobic feature of Ag nanoparticles. As a result, as-formed polymer-stabilized Ag nuclei will be located in inner part of micelles. Simultaneously, silicate precursors will adsorb and polymerize at the hydrophilic micellar interface. Growth of Ag nuclei in three dimensions will make rodlike micelles distort with prominences. Thereafter, Ag−SBA-15 composites will be formed after calcination for removal of micelle template and PANI polymer. Calcination process for removal of PANI polymer will lead to the cavities remaining on the wall of channels (inset in Figure 1d). In comparison with the formation process of pure mesoporous silica SBA-15 materials, addition of aniline monomers that solubilized in micelles will lead to increased size of micelles and finally result in larger pore size of SBA-15. Catalytic Activities of Ag−SBA-15 Composites in the NaBH4 Reduction of 4NP. Catalytic applications of assynthesized Ag−SBA-15 composites were then considered. Considering the short-range ordering of channels, large pore size, and implanted Ag nanoparticles in frameworks, it is believed that Ag−SBA-15 composites will show pronounced catalytic efficiency and high stability with excellent reusability. The contents of Ag nanoparticles in Ag−SBA-15 composites were roughly calculated to be 0.2, 0.4, and 0.7 wt % for Ag(0.3)−SBA-15, Ag(0.7)−SBA-15, and Ag(0.7)−SBA-15, respectively, according to results from energy dispersive spectra. In addition, it is found that silver ions are almost completely

Figure 5. (a) N2 adsorption−desorption isotherms and (b) BJH pore size distributions of pure SBA-15 and Ag−SBA-15 composites.

adsorption−desorption isotherms of pure SBA-15, which feature hysteresis loops with sharp adsorption and desorption branches.14 The N2 adsorption−desorption isotherms of Ag− SBA-15 composites show similar trends for pure SBA-15. The pore size distributions for pure SBA-15 and Ag−SBA-15 composites are shown in Figure 5b, and the pore sizes corresponding to their maxima are listed in Table 1. The 4772

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transferred into metallic states as Ag nanoparticles. As a result, amounts of catalysts in Ag−SBA-15 composites are better measured according to the AgNO3 precursor concentration. Catalytic activities of Ag(0.7)−SBA-15 composites were examined by choosing the model catalysis reaction involving reduction of 4NP to 4AP by NaBH4. Such a reaction catalyzed by noble metal catalysts has been reported intensively because this reaction can be rapidly and easily characterized.16 In the absence of Ag catalysts, the mixture of 4NP and NaBH4 shows an absorbance band at λmax = 400 nm corresponding to the adsorption of 4NP ions in alkaline conditions. This peak remains unaltered with time, indicating that the reduction does not take place in the absence of catalysts.8a,17 However, the addition of Ag(0.7)−SBA-15 composites to the above reaction mixture causes a fading and ultimate bleaching of the yellow color of 4NP in aqueous solution within 3 min. It is commonly accepted that when noble metal nanoparticles are used for catalytic reduction, BH4− and 4NP first diffuse from aqueous solution to the surfaces of noble metal nanoparticles, and then the bare noble metal nanoparticles serve as catalysts to transfer electrons from BH4− to 4NP, leading to the production of 4AP.16a,d Time-dependent absorbance spectra of this reaction mixture show the disappearance of the peak at 400 nm accompanied by a gradual development of a new peak at 300 nm corresponding to the formation of 4AP (Figure 6a). The adsorption spectra after 5 min are shifted to less than 400 nm

and almost unchanged with increasing reaction time, indicating that such adsorption should come from catalysts.17a Since excess NaBH4 is present in the reaction solution and the reduction of 4NP by NaBH4 is negligible in the absence of Ag nanoparticles, the reaction can be considered as pseudofirst-order with respect to the concentration of 4NP. As the absorbance of 4NP is proportional to its concentration in the medium, the ratio of absorbance at time t (At) to that at t = 0 (A0), i.e., At/A0, could be used as the ratio of concentration of 4NP at time t to that at t0, i.e., At/A0 = Ct/C0. As shown in Figure 6b, ln At versus t is obtained. Upon the addition of Ag nanocatalysts, a certain period of time (defined as tads) is normally required for the 4NP to adsorb onto the catalyst’s surfaces before reduction can be initiated.16b As shown in Figure 6b, the tads is almost undetectable, indicating very fast adsorption time. It is believed that the large pore size of SBA-15 resulted from solubilization of aniline monomers in micelles, and decomposition of PANI makes it favorable for BH4− and 4NP to diffuse from aqueous solution to surfaces of Ag nanoparticles, resulting in low tads. The kinetic reaction rate constant (defined as kapp) is estimated to be 9.0 × 10−3 s−1 from the linear relationship given in Figure 6b, which is superior to that of calcium alginate-supported Ag nanoparticles with smaller size as catalysts18 and even comparable to that of unsupported polygonal gold nanoparticles with similar size17b and polymer-stabilized gold nanoparticles with smaller size.17a In addition, the reusability of Ag−SBA-15 composites as heterogeneous catalysts involved in the reduction of 4NP is also considered. It is found that no obvious catalytic loss is evidenced within six cycles. Unlike other noble-metal-nanoparticle-loaded mesoporous silica materials synthesized by a two-step strategy, where noble nanoparticles are just located in channels of mesoporous silica,3,4 Ag nanoparticles in Ag(0.7)− SBA-15 composites are loaded and even implanted in frameworks of mesoporous silica SBA-15 by our proposed novel one-step method. Therefore, they will show good recyclability, which evidences their potential in catalysis. Electrochemical Detection of H2O2. It has been reported that Ag nanoparticles show good catalytic activity toward the reduction of H2O2, opening the door for designing of enzymeless H2O2 sensors.19 To demonstrate the sensing application of Ag(0.7)−SBA-15 composites, we constructed an enzymeless H2O2 sensor by immobilization of the composites on a bare GCE surface. Figure 7a shows cyclic voltammetry (CV) of a bare GCE in N2-saturated 0.2 mol L−1 PBS at pH 6.8 in the presence of 1.0 mmol L−1 H2O2 with scan rate of 50 mV s−1. The response toward the reduction of H2O2 is pretty weak (about 4.6 μA in intensity at −0.60 V). Figure 7b shows CV of the Ag(0.7)−SBA-15-composites-modified GCE in N2-saturated 0.2 mol L−1 PBS at pH 6.8 in the absence of H2O2 with scan rate of 50 mV s−1. It is found that the response is also pretty weak (about 4.8 μA in intensity at −0.60 V). However, a remarkable current peak about 18.6 μA in intensity is observed at −0.48 V for the Ag(0.7)−SBA-15-compositesmodified GCE in N2-saturated 0.2 mol L−1 PBS at pH 6.8 in the presence of 0.1 mol L−1 H2O2 with a scan rate of 50 mV s−1 (Figure 7c). Above observations indicate that Ag nanoparticles contained in the composites exhibit good catalytic activity toward the reduction of H2O2. In addition, it is found that the oxidation peak current of Ag−SBA-15-composites-modified GCE is increased with the gradual addition of H2O2 (Figure 8), showing the catalytic property of the modified electrode in the reduction of H2O2. However, it is observed that there is a

Figure 6. (a) Successive UV−vis absorbance spectra of the reduction of 4NP by NaBH4 in the presence of Ag(0.7)−SBA-15 composites. (b) Normalized absorbance at the peak position for 4NP (400 nm) as a function of time in the presence of Ag(0.7)−SBA-15 composites. 4773

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Figure 9. Amperometric response of Ag(0.7)−SBA-15-compositesmodified GCE for consecutive addition of 10 μmol L−1 of H2O2 to 0.2 mol L−1 PBS at pH 6.8 under stirring. Work potential 0.60 V. The inset shows the linear plot of the current versus H2O2 concentration.

Figure 7. CVs of (a) bare GCE and (c) Ag(0.7)−SBA-15-compositesmodified GCE in N2-saturated 0.2 mol L−1 PBS at pH 6.8 in the presence of 1.0 mmol L−1 H2O2. (b) CV curve of Ag(0.7)−SBA-15composites-modified GCE in N2-saturated 0.2 mol L−1 PBS at pH 6.8 in the absence of H2O2. Potential scan rate: 50 mV s−1.

4. CONCLUSIONS In summary, by adding aniline and silver nitrate, which are chosen as raw materials of Ag nanoparticles in the typical reaction system of mesoporous silica SBA-15 containing P123 and TEOS, Ag−SBA-15 composites with Ag nanoparticles loaded in channels and even implanted in frameworks of mesoporous silica SBA-15 have been successful fabricated. Mechanistic studies reveal that solubilization of aniline monomers in micelles and thereafter redox reactions between aniline monomers and silver ions led to the formation of Ag nanoparticles located in inner part of micelles; meanwhile, silicate precursors are adsorbed and polymerized on surfaces of micelles, leading to the formation of mesoporous silica SBA-15. Finally, Ag−SBA-15 composites are obtained after calcination for removal of micelle template and PANI polymer. Our proposed method may open an efficient and straightforward route to mesoporous silica SBA-15-supported Ag nanoparticles and is also anticipated to construct other mesoporous-silicasupported noble metal nanoparticles by simply changing surfactants or/and noble metal ions, which will be our continuing interest. Catalytic performances revealed that Ag− SBA-15 composites showed excellent catalytic activity toward the reduction of 4NP in the presence of NaBH4 and the reduction of H2O2. Therefore, it is believed that Ag−SBA-15 composites will find promising applications in fields of catalysis and sensors.

Figure 8. CVs of Ag(0.7)−SBA-15-composites-modified GCE in N2saturated 0.2 mol L−1 PBS at pH 6.8 in the presence of different concentration of H2O2 at a scan rate of 50 mV s−1.

negative shift in the potentials. This is because only a low level of loading of the composites is possible by the adsorption process, which leads to ratio mismatch between the concentration of the composites and H2O2 at higher concentrations of H2O2.20 Figure 9 shows the typical current−time plot of Ag(0.7)− SBA-15-composites-modified GCE in 0.2 mol L−1 PBS buffer at pH 6.8 on successive step change of H2O2 concentrations under optimized conditions. When an aliquot of H2O2 is dropped into the stirring PBS solution, the reduction current rises steeply to reach a stable value. The sensor can achieve 95% of the steady state current within 10 s, indicating a fast amperometric response behavior. It is clearly seen that the steps are more horizontal in the region of lower concentration of H 2 O 2 and the noises become higher with increased concentration of H2O2. The inset in Figure 9 shows the calibration curve of the sensor. The linear detection range is estimated to be from 10 to 6.0 mmol L−1 (r = 0.992), and the detection limit is estimated to be 6.2 μmol L−1 at a signal-tonoise ratio of 3. These values are comparable with the reported results, where graphene or polymer-supported Ag nanoparticles were used.19

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Funding is acknowledged from the National Natural Scientific Foundation of China (No. 20903079 and 21073156) and the Priority Academic Program Development of Jiangsu Higher Education Institutions.

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REFERENCES

(1) (a) Lu, Y. Angew. Chem., Int. Ed. 2006, 45, 7664−7667. (b) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J.

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dx.doi.org/10.1021/la204503b | Langmuir 2012, 28, 4768−4775