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Synthesis of Fe3O4@SiO2–Ag magnetic nanocomposite based on small-sized and highly dispersed silver nanoparticles for catalytic reduction of 4-nitrophenol
Journal of Colloid and Interface Science 383 (2012) 96–102
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Synthesis of Fe3O4@SiO2–Ag magnetic nanocomposite based on small-sized and highly dispersed silver nanoparticles for catalytic reduction of 4-nitrophenol Yue Chi a, Qing Yuan b, Yanjuan Li a, Jinchun Tu c, Liang Zhao a, Nan Li a,⇑, Xiaotian Li a,⇑ a
School of Material Science and Engineering, Key Laboratory of Automobile Materials of Ministry of Education, Jilin University, Changchun 130012, China School of Physics and Materials Engineering, Liaoning Key Lab of Optoelectronic Films & Materials, Dalian Nationalities University, Dalian 116600, China c College of Material and Chemical Engineering, Ministry of Education Key Laboratory of Application Technology of Hainan Superior Resources Chemical Materials, Hainan University, Haikou 570228, China b
a r t i c l e
i n f o
Article history: Received 6 April 2012 Accepted 12 June 2012 Available online 19 June 2012 Keywords: Core–shell nanostructured composite Silver nanoparticles Magnetic property Catalytic activity
a b s t r a c t In this work, we report a facile method to generate core–shell structured Fe3O4@SiO2–Ag magnetic nanocomposite by an in situ wet chemistry route with the aid of polyvinylpyrrolidone as both reductant and stabilizer. This method can effectively prevent Ag nanoparticles from aggregating on the silica surface, thus resulting highly dispersed and small-sized Ag nanoparticles. The as-prepared nanocomposite is composed of a central magnetite core with a strong response to external fields, an interlayer of SiO2, and numerous highly dispersed Ag nanoparticles with a narrow size distribution. Furthermore, the Fe3O4@SiO2–Ag nanocomposite showed high performance in the catalytic reduction of 4-nitrophenol and could be easily recycled by applying an external magnetic field while maintaining the catalytic activity without significant decrease even after running 15 times. Ó 2012 Elsevier Inc. All rights reserved.
1. Introduction Noble metal nanoparticles have shown remarkable potential for numerous applications in electronic, chemical, biological, and catalytic fields due to their distinctive properties, when compared to their bulk counterparts [1–3]. As a relatively inexpensive noble metal, silver nanoparticles (AgNPs) have been applied in a variety of catalytic reactions, such as selective butadiene epoxidation, ethanol oxidation, and selective NOx reduction [4–7]. In the field of catalysis, dispersion and size of active metals play an important role in overall performance [8–11]. However, it is noteworthy that metal nanoparticles are active and tend to coalesce during preparation and catalytic process due to van der Waals forces and high surface energy. Therefore, when working with nanoparticles, the ability to stabilize active metals in nanoscale is paramount to achieve desired performance markers [12,13]. As we know, numerous processes have been developed to synthesize small-sized and highly dispersed AgNPs, and the AgNPs can be produced in high yield using solution based methods, but the subsequent collection and assembly of the individual AgNPs from solutions are major challenges [14,15]. Partially in order to address these problems, increasing effort has been aimed toward the introduction of AgNPs on/into solid supports (such as polymer, carbon, and metal oxides) with different nanostructures (including spheres, fibers, mesoporous silica, and so on) to form composite catalysts, which seems to ⇑ Corresponding authors. Fax: +86 431 85168444. E-mail address: [email protected] (X. Li). 0021-9797/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2012.06.027
be the best method to avoid the aggregation of small-sized AgNPs without the decrease in their catalytic activity [16–22]. Recently, magnetic particles have been extensively employed as alternative catalyst supports, in view of their convenient catalyst recycling, high surface area resulting in high catalyst loading capacity, high dispersion, and outstanding stability [23,24]. The incorporation of magnetic components into AgNPs-based catalysts may, therefore, enhance the separation and recovery of nanosized Ag. However, compared with silver, the magnetic particles are much more sensitive and unstable, especially under acidic conditions. It has been reported that silica as a protecting shell can be utilized to coat the Fe3O4 particles to form a core–shell (Fe3O4@SiO2) structure [25]. Meanwhile, the silica shell can prevent the aggregation of the Fe3O4 particles and provide numerous surface Si–OH groups for further modification [26]. Recently, Hu et al. have developed a simple solution-phase method to synthesize Agcoated Fe3O4@SiO2 composite microspheres through the Ag-mirror reaction [27]. Despite of the progresses, there is still a critical need to develop facile and feasible methods to get small and highly dispersed AgNPs with a narrow size distribution on the Fe3O4@SiO2 support. Polyvinylpyrrolidone (PVP) was often employed as a stabilizer in the preparation of composite spheres. Recently, PVP was also used to reduce noble metal ions (Au, Ag, Pd, and Pt) to prepare nanostructured metals [28–31]. In this paper, we combine the reducing function of PVP with its stabilizing function to prepare Fe3O4@SiO2–Ag composite spheres. The synthesis offers good controllability of size and dispersion of AgNPs with high-yield
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and short reaction time. We also demonstrate the high catalytic efficiency of Fe3O4@SiO2–Ag nanocomposite toward 4-nitrophenol (4-NP) by sodium borohydride. In addition, the obtained Fe3O4@SiO2–Ag catalyst can be efficiently recovered from the reaction solution by using external magnetic fields for many cycles without significant loss of either material or catalytic activity. 2. Experimental 2.1. Materials and reagents Silver nitrate (analytical grade) and PVP (K30, analytical grade) were purchased from Sinopharm Chemical Reagent Co. Ltd. Other chemicals were purchased from Beijing Chemical Corp. All chemicals were used as received without any further purification.
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(0.1 wt%) was then added. The reaction was carried out at 298 K with continuous stirring. At various time intervals, parts of the mixture were taken and centrifuged for the determination with UV–vis absorption spectra at the wavelength of the absorbance maximum in the range of 250–500 nm. To study the reusability of Fe3O4@SiO2–Ag nanocomposite, the used sample was separated from the solution by a magnet after monitoring the whole reduction process. The recycled sample was washed with ethanol and water several times for the next cycling. Similar to the above reduction process, the obtained magnetic nanocomposite was redispersed in 1.0 g of deionized water, then mixed with 30 mL of aqueous 4-NP solution (0.12 mM) and 29 mL of NaBH4 (12 mM) solution. With a stiff limit of 8 min kept for completion of the reaction, the solution was centrifuged and measured using UV–vis spectroscopy. The procedure was repeated 15 times.
2.2. Synthesis of Fe3O4 microspheres 2.6. Characterization The magnetic particles were synthesized through a solvothermal reaction according to the reported method [32] with some modification. Typically, 1.08 g of FeCl36H2O was dissolved in 20 mL of ethylene glycol under magnetic stirring. A clear yellow solution was obtained after stirring for 0.5 h. Subsequently, 1.8 g of sodium acetate was added to this solution and being stirred for another 1 h. Afterward, 0.25 g of trisodium citrate was added. After forming a homogeneous dispersion, the mixture was transferred into a Teflon-lined stainless-steel and heated at 200 °C for 10 h. The black magnetic particles were collected with the help of a magnet filed, followed by washing with a recycle of ethanol and deionized water six times. The product was then dried under vacuum at 50 °C for 12 h. 2.3. Synthesis of Fe3O4@SiO2 microspheres The interlayers of SiO2 were prepared through a modified Stöber method [33]. In a typical process, as-prepared Fe3O4 particles (0.1 g) were dispersed in a mixture of ethanol (40 mL), deionized water (10 mL), and concentrated ammonia aqueous solution (28 wt%, 1.2 mL) by ultrasonication for 1 h. Subsequently, 0.35 mL of tetraethyl orthosilicate (TEOS) was added dropwise. After being stirring for 2 h, the products were collected and washed with deionized water and then dried under vacuum at 60 °C for further use. 2.4. Constructing Fe3O4@SiO2–Ag nanocomposite A simple in situ wet chemistry method was employed for the immobilization of AgNPs onto the core–shell structured Fe3O4@SiO2 microspheres to form Fe3O4@SiO2–Ag nanocomposite. First, 0.06 g of Fe3O4@SiO2 was dispersed into 10 mL of 3 10 2 M Ag(NH3)2NO3 solution under mechanical stirring at room temperature. After the [Ag(NH3)2]+ ions were adsorbed onto the surfaces of silica spheres for around 30 min via electrostatic attraction between [Ag(NH3)2]+ ions and the negatively charged Si–OH groups, the dispersion was added into 15 mL of ethanol containing PVP (0.1 g), followed by heating the solution by reflux at 70 °C for 4 h. The final products were magnetically separated, washed several times with a recycle of ethanol and deionized water, and dried at 50 °C for 12 h. 2.5. Catalyzed reduction of 4-NP To study the catalytic activity, 30 mL of 4-NP aqueous solution (0.12 mM) was mixed with 29 mL of fresh NaBH4 solution (12 mM). 1 g of aqueous dispersed Fe3O4@SiO2–Ag nanocomposite
The morphologies of the as-prepared nanocomposite were observed by the scanning electron microscope (SEM, JEOL JSM6700F), energy dispersive X-ray (EDX), and transmission electron microscopy (TEM; JEM 3010). X-ray diffraction (XRD) patterns were collected using a Bruker D8 Advance X-ray diffractometer to identify the crystal structures with a Cu Ka X-ray source operating at 40 kV and 100 mA. Fourier transform infrared (FTIR) spectra were recorded with a PerkinElmer spectrometer using KBr pellets, and the thickness of the pellet being about 1.3 mm. Each spectrum was collected at room temperature under atmospheric pressure with a resolution of 4 cm 1. X-ray photoelectron spectroscopy (XPS) was accomplished using a PHI-5702 multi-functional X-ray photoelectron spectrometer with pass energy of 29.35 eV and an Mg Ka line excitation source. Magnetization measurements were performed on a MPM5-XL-5 superconducting quantum interference device (SQUID) magnetometer at 300 K. UV–vis absorption spectra were recorded using a UV–vis spectrophotometer (UV2550). The samples were placed in a 1 cm 1 cm 3 cm quartz cuvette, and the spectra were recorded at room temperature. 3. Results and discussion 3.1. The characterization of Fe3O4@SiO2–Ag core–shell nanocomposite The synthetic procedure for Fe3O4@SiO2–Ag core–shell nanocomposite is illustrated in Fig. 1e. Firstly, the Fe3O4 particles were prepared via a robust solvothermal reaction based on a high temperature reduction of Fe(III) salts with ethylene glycol in the presence of trisodium citrate. The magnetite particles capped with citrate groups exhibit excellent dispersibility in polar solvents such as water and ethanol, which favors the subsequent coating. SEM images of the Fe3O4 particles confirm the uniform size of about 200 nm and nearly spherical shape (Fig. 1a). Subsequently, silica was coated on the Fe3O4 to form Fe3O4@SiO2 core–shell microspheres through hydrolysis of TEOS according to the Stöber method. This coating process is indispensable for two reasons: first, the SiO2 shell can effectively prevent the aggregation and chemical degradation of the Fe3O4 particles in a harsh liquid environment; second, [Ag(NH3)2]+ ions could be adsorbed on Fe3O4@SiO2 microspheres by the electrostatic attraction between negatively charged surface Si–OH groups and positively charged [Ag(NH3)2]+ ions. The SEM image (Fig. 1b) shows that the Fe3O4@SiO2 core–shell microspheres still keep the morphological properties of pure Fe3O4 except for a slightly larger particle size about 270 nm. Finally, these [Ag(NH3)2]+ ions adsorbed on Fe3O4@SiO2 microspheres were reduced and protected by PVP through an in situ wet chemistry
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Fig. 1. SEM images of Fe3O4 microspheres (a), Fe3O4@SiO2 microspheres (b), Fe3O4@SiO2–Ag nanocomposite (c), EDX spectrum of Fe3O4@SiO2–Ag nanocomposite (d), and schematic illustration of the formation process of Fe3O4@SiO2–Ag nanocomposite (e).
method to obtain Fe3O4@SiO2–Ag nanocomposite. It can be seen that Fe3O4@SiO2–Ag nanocomposite is well dispersed without the occurrence of aggregation after deposition of high-density AgNPs (Fig. 1c), indicating the uniform property of our simple. The small size of Ag nanoparticles results in low contrast in SEM images, so they could not be clearly distinguished from the Fe3O4@SiO2 matrix. The EDX characterization demonstrates that the nanocomposite contains three necessary and diagnostic elements of the precursors, Fe, Si, and Ag (Fig. 1d, Cu arising from the sample carrier and C from the adhesive used for sample preparation are discounted). In order to further study the morphological and structural features of the Fe3O4@SiO2–Ag nanocomposite, TEM observations were carried out. It can be clearly seen from Fig. 2a that the Fe3O4@SiO2 microspheres have a core–shell structure. The magnetic cores are black spheres with an average size of around 200 nm, and the silica shell shows a gray color with an average thickness of about 35 nm. TEM images of Fe3O4@SiO2–Ag nanocomposite directly show that many small and highly dispersed
AgNPs are immobilized on the surface of Fe3O4@SiO2 microspheres (Fig. 2b). The size distribution of AgNPs is narrow with an average size of 3.65 nm (Fig. 2c). No large Ag particles were observed. Instead, there are nearly monodispersed nanoparticles, indicating that the in situ wet chemistry method has a good control over the formation of AgNPs. Specifically, the crystal lattice of AgNPs can be well observed in the insert of Fig. 2c, which clearly shows the formation of Ag nanocrystal. The phase and purity of the as-prepared samples were examined by XRD. A typical XRD pattern of the obtained Fe3O4 sample was shown in Fig. 3a, all of the diffraction peaks could be readily indexed to a face-centered cubic structure (Fd3m space group) of magnetite according to JCPDS card No. 19-0629. The XRD pattern of Fe3O4@SiO2 microspheres exhibited almost the same feature as pure Fe3O4 (Fig. 3b), except that a broad peak centered at 22° of 2h corresponding to SiO2 was observed, indicating that the coated SiO2 shell is amorphous. In the case of Fe3O4@SiO2–Ag (Fig. 3c), besides the characteristic diffraction of Fe3O4 microspheres, the
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Fig. 2. TEM images of Fe3O4@SiO2 microspheres (a) and Fe3O4@SiO2–Ag nanocomposite (b). Size distribution histogram of AgNPs calculated from the TEM images (c). Inset shows the corresponding HRTEM image of the AgNPs.
Fig. 3. Powder XRD patterns of Fe3O4 microspheres (a), Fe3O4@SiO2 microspheres (b), and Fe3O4@SiO2–Ag nanocomposite (c).
Fig. 4. FTIR spectra of Fe3O4 microspheres (a), Fe3O4@SiO2 microspheres (b), and Fe3O4@SiO2–Ag nanocomposite (c).
obvious diffraction peaks at 2h = 37.9°, 44.1°, 64.3°, and 77.2° can be readily indexed to face-centered-cubic structure of Ag (JCPDS card No. 4-783), indicating that the crystallized AgNPs could be obtained by PVP reduction. In order to further confirm the composition and structure of samples, FTIR spectra were measured. As shown in Fig. 4a, Fe3O4 microspheres show typical bands centered at 585 and 635 cm 1, which are related to the Fe–O stretches [34]. The absorption bands at 1635 and 1406 cm 1 assigned to the carboxylate groups are clearly visible, indicating the large amount of carboxylate groups, which is crucial to the subsequent coating. After the coating of silica, Fe3O4@SiO2 microspheres show some new bands centered around 795 cm 1 and 1080–1100 cm 1 (Fig. 4b). The new absorption at 795 cm 1 can be ascribed to the symmetric vibration of Si–O–Si, whereas the band at 1080–1100 cm 1 assigned to the asymmetric stretching vibration of Si–O–Si. These results indicate that SiO2 is immobilized on the surfaces of Fe3O4 microspheres. Fig. 4c shows the FTIR spectrum of Fe3O4@SiO2–Ag nanocomposite. Because AgNPs do not have absorption in the infrared region [26], the FTIR spectrum of Fe3O4@SiO2–Ag nanocomposite is almost the same as that of Fe3O4@SiO2 microspheres. Then, XPS has been utilized as a useful tool for qualitatively determining the surface component and composition of the samples. Fig 5A shows the fully scanned spectra in the range of 0–1200 eV. The peaks on the curve of Fe3O4@SiO2–Ag nanocom-
posite are assigned to Si, O, Ag, and C elements. The C element was attributed to the adventitious carbon-based contaminant. Therefore, it is concluded that Si, O, and Ag exist in Fe3O4@SiO2– Ag nanocomposite, while Si and O exist in Fe3O4@SiO2 microspheres. Fig. 5B shows the XPS spectrum in the O 1s region of Fe3O4@SiO2–Ag nanocomposite, which could be fitted into three peaks. The binding energy at 533.2 eV is attributed to surface hydroxyl groups of silica (Si–O–H), the 532.3 eV peak represents the lattice oxygen of silica (Si–O–Si), and the peak at 531.0 eV suggests the formation of Si–O–Ag unit. The XPS spectrum of Ag 3d of Fe3O4@SiO2–Ag nanocomposite as shown in Fig. 5C can be assigned to two components. The major component is Ag0 indicating the formation of AgNPs, and the lesser component is Ag–O– due to the surface of AgNPs being bound to the anionic oxygen of SiO2 shell [35–37]. The peaks at Ag0 3d5/2 and Ag0 3d3/2 (368.0 eV and 374.0 eV) of Fe3O4@SiO2–Ag nanocomposite shift to lower binging energy in compared with pure silver (368.2 eV for Ag 3d5/2, 374.2 eV for Ag 3d3/2), further confirming that there is an interaction between the SiO2 shell and AgNPs. The magnetic properties of the samples were characterized using a SQUID magnetometer measured at 300 K as shown in Fig. 6. Pure Fe3O4 microspheres, Fe3O4@SiO2 microspheres, and Fe3O4@SiO2–Ag nanocomposite, have magnetization saturation (Ms) values of 78.5, 66.4, and 63.8 emu/g, respectively. Because of the presence of SiO2 shell, the Ms value of Fe3O4 microspheres
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Fig. 5. (A) XPS fully scanned spectra of Fe3O4@SiO2 microspheres (a) and Fe3O4@SiO2–Ag nanocomposite (b). XPS spectra of the Fe3O4@SiO2–Ag nanocomposite: (B) O 1s, and (C) Ag 3d.
Fig. 6. Magnetization curves of Fe3O4 microspheres, Fe3O4@SiO2 microspheres, and Fe3O4@SiO2–Ag nanocomposite.
is higher than that of Fe3O4@SiO2 microspheres and Fe3O4@SiO2– Ag nanocomposite. Compared with Fe3O4@SiO2 microspheres, Fe3O4@SiO2–Ag nanocomposite exhibits a slightly smaller Ms value, which is attributed to the slight increase in the mass and size caused by the depositing AgNPs on the surfaces of Fe3O4@SiO2 microspheres. It should be noted that the core–shell structured Fe3O4@SiO2–Ag nanocomposite still shows strong magnetization, suggesting its suitability for magnetic separation and recovery. 3.2. Application of Fe3O4@SiO2–Ag nanocomposite for catalytic reduction of 4-NP As it is known, 4-aminophenol (4-AP) is very useful in a wealth of applications that include analgesic and antipyretic drugs, photographic developers, corrosion inhibitors, and so on [38–40]. The reduction of 4-NP to 4-AP by NaBH4 was employed as a model system to evaluate the catalytic activity of Fe3O4@SiO2–Ag nanocomposite. Although the reaction was a thermodynamically feasible process involving E0 for 4-NP/4-AP = 0.76 V and H3BO3/ BH4 = 1.33 V versus normal hydrogen electrode, it was kinetically restricted in the absence of a catalyst (no change in the absorption even after 2 days). In agreement with previous results, the absorption peak of 4-NP changed from 317 to 400 nm immediately when treated with an aqueous solution of NaBH4 (Fig. 7a), which corresponds to a color change of light yellow to yellow-green due to the formation of 4-nitrophenolate ion [41]. After addition of a small
amount (1.0 mg) of the Fe3O4@SiO2–Ag nanocomposite, the reduction commences and the time-dependent absorption spectra show a decrease in intensity of the absorption peak at 400 nm and a concomitant increase of a new peak at 295 nm corresponding to 4-AP (Fig. 7b). After the completion of reduction reaction, the peak due to the nitro compound was no longer observed, which indicated that the catalytic reduction of 4-NP had proceeded successfully. For comparison, a blank test was conducted with a mixture of 4-NP, NaBH4, and Fe3O4@SiO2 microspheres containing no AgNPs. There was little decrease in the absorbance of nitro compound at 400 nm monitored by UV, meanwhile, no absorption peak assigned to 4-AP was appeared at around 295 nm, indicating that the catalytic reduction of 4-NP did not occur. The rate of reduction reaction catalyzed by Fe3O4@SiO2–Ag was assumed to be independent of the concentration of NaBH4 because this reagent was used in large excess compared to 4-NP. Therefore, the kinetic data were fitted by a first-order rate law. Linear relationship between ln(C/C0) and reaction time is obtained in the reduction catalyzed by Fe3O4@SiO2–Ag nanocomposite (Fig. 7c), and the rate constant k is calculated to be 7.67 10 3 s 1. Reported ratios of rate constants k over the weight of catalyst employed are 1.30, 0.41, and 0.09 s 1 g 1 for coral-like dendrite, banana leave-like dendrite, and spherical Ag nanostructures, respectively [42]. These are all lower than the 7.67 s 1 g 1 of Fe3O4@SiO2–Ag nanocomposite synthesized in this work, although a substantial part of the weight is actually the Fe3O4 and SiO2 in this case. Compared with other matrix-supported AgNPs, the rate constant k of Fe3O4@SiO2–Ag is much higher than previous reported ratios for Ag supported halloysite nanotubes (0.087 s 1 g 1) and Ag doped carbon spheres (1.69 s 1 g 1) [18,43]. The good catalytic activity of Fe3O4@SiO2–Ag nanocomposite is derived from its highly dispersed and small-sized AgNPs. Highly dispersed AgNPs can make 4-nitrophenolate ion easily access to the AgNPs that serve as an electron relay in the system for an oxidant and a reductant. The small size of AgNPs makes a large potential difference, thus leading to a high rate of reduction. In addition, a smaller particle size can result in a larger surface-to-volume ratio and more exposed atoms on the surface, which act as the potential catalytic sites [44–47]. Furthermore, we have also monitored the cyclic stability of Fe3O4@SiO2–Ag nanocomposite by monitoring the catalytic activity during successive cycles of the reduction reaction. The Fe3O4@SiO2–Ag nanocomposite can be recovered from the catalytic reaction solution by using a magnet. With a stiff limit of 8 min kept for completion of the reaction, the catalyst exhibited similar catalytic performance without significant reduction in the conversion even after running 15 cycles (>99% in eight successive reactions, Fig. 7d). The conversion starts to decrease slightly after eight cycles, implying that the reaction rate is only slightly reduced after multiple
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Fig. 7. UV–vis spectra of (a) 4-NP before and after adding NaBH4 solution and (b) the reduction of 4-NP in aqueous solution recorded every 2 min using Fe3O4@SiO2–Ag nanocomposite (1 mg) as a catalyst, (c) ln(C/C0) versus reaction time for the reduction of 4-NP, (d) the reusability of Fe3O4@SiO2–Ag nanocomposite as a catalyst for the reduction of 4-NP with NaBH4.
reuses. These results indicate that the Fe3O4@SiO2–Ag nanocomposite might possess a profound application in the fields of catalytic reduction of 4-NP to 4-AP. 4. Conclusions In summary, we demonstrated an effective route to prepare Fe3O4@SiO2–Ag core–shell structured nanocomposite by combining the sol–gel process and a facile in situ wet chemistry method with the aid of the dual function of PVP as both reductant and stabilizer. The as-prepared nanocomposite shows a number of important features as a recyclable catalyst for the reduction of 4NP: it contains a highly field-responsive magnetic Fe3O4 core for efficient magnetic separation, a SiO2 interlayer for protecting the Fe3O4 core from chemical- and/or photo-dissolution, and smallsized and highly dispersed AgNPs for enhanced catalytic activity. This method is efficient to get small and highly dispersed AgNPs with a narrow size distribution on the Fe3O4@SiO2 support and presents a new paradigm for the fabrication of magnetically separable core–shell structured catalysts based on noble metal nanoparticles, which could be very useful in catalytic applications. Acknowledgment This work was supported by the National Natural Science Foundation of China for Project Nos. 21076094 and 20743005.
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Synthesis of Fe3O4@SiO2–Ag magnetic nanocomposite based on small-sized and highly dispersed silver nanoparticles for catalytic reduction of 4-nitrophenol Yue Chi a, Qing Yuan b, Yanjuan Li a, Jinchun Tu c, Liang Zhao a, Nan Li a,⇑, Xiaotian Li a,⇑ a
School of Material Science and Engineering, Key Laboratory of Automobile Materials of Ministry of Education, Jilin University, Changchun 130012, China School of Physics and Materials Engineering, Liaoning Key Lab of Optoelectronic Films & Materials, Dalian Nationalities University, Dalian 116600, China c College of Material and Chemical Engineering, Ministry of Education Key Laboratory of Application Technology of Hainan Superior Resources Chemical Materials, Hainan University, Haikou 570228, China b
a r t i c l e
i n f o
Article history: Received 6 April 2012 Accepted 12 June 2012 Available online 19 June 2012 Keywords: Core–shell nanostructured composite Silver nanoparticles Magnetic property Catalytic activity
a b s t r a c t In this work, we report a facile method to generate core–shell structured Fe3O4@SiO2–Ag magnetic nanocomposite by an in situ wet chemistry route with the aid of polyvinylpyrrolidone as both reductant and stabilizer. This method can effectively prevent Ag nanoparticles from aggregating on the silica surface, thus resulting highly dispersed and small-sized Ag nanoparticles. The as-prepared nanocomposite is composed of a central magnetite core with a strong response to external fields, an interlayer of SiO2, and numerous highly dispersed Ag nanoparticles with a narrow size distribution. Furthermore, the Fe3O4@SiO2–Ag nanocomposite showed high performance in the catalytic reduction of 4-nitrophenol and could be easily recycled by applying an external magnetic field while maintaining the catalytic activity without significant decrease even after running 15 times. Ó 2012 Elsevier Inc. All rights reserved.
1. Introduction Noble metal nanoparticles have shown remarkable potential for numerous applications in electronic, chemical, biological, and catalytic fields due to their distinctive properties, when compared to their bulk counterparts [1–3]. As a relatively inexpensive noble metal, silver nanoparticles (AgNPs) have been applied in a variety of catalytic reactions, such as selective butadiene epoxidation, ethanol oxidation, and selective NOx reduction [4–7]. In the field of catalysis, dispersion and size of active metals play an important role in overall performance [8–11]. However, it is noteworthy that metal nanoparticles are active and tend to coalesce during preparation and catalytic process due to van der Waals forces and high surface energy. Therefore, when working with nanoparticles, the ability to stabilize active metals in nanoscale is paramount to achieve desired performance markers [12,13]. As we know, numerous processes have been developed to synthesize small-sized and highly dispersed AgNPs, and the AgNPs can be produced in high yield using solution based methods, but the subsequent collection and assembly of the individual AgNPs from solutions are major challenges [14,15]. Partially in order to address these problems, increasing effort has been aimed toward the introduction of AgNPs on/into solid supports (such as polymer, carbon, and metal oxides) with different nanostructures (including spheres, fibers, mesoporous silica, and so on) to form composite catalysts, which seems to ⇑ Corresponding authors. Fax: +86 431 85168444. E-mail address: [email protected] (X. Li). 0021-9797/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2012.06.027
be the best method to avoid the aggregation of small-sized AgNPs without the decrease in their catalytic activity [16–22]. Recently, magnetic particles have been extensively employed as alternative catalyst supports, in view of their convenient catalyst recycling, high surface area resulting in high catalyst loading capacity, high dispersion, and outstanding stability [23,24]. The incorporation of magnetic components into AgNPs-based catalysts may, therefore, enhance the separation and recovery of nanosized Ag. However, compared with silver, the magnetic particles are much more sensitive and unstable, especially under acidic conditions. It has been reported that silica as a protecting shell can be utilized to coat the Fe3O4 particles to form a core–shell (Fe3O4@SiO2) structure [25]. Meanwhile, the silica shell can prevent the aggregation of the Fe3O4 particles and provide numerous surface Si–OH groups for further modification [26]. Recently, Hu et al. have developed a simple solution-phase method to synthesize Agcoated Fe3O4@SiO2 composite microspheres through the Ag-mirror reaction [27]. Despite of the progresses, there is still a critical need to develop facile and feasible methods to get small and highly dispersed AgNPs with a narrow size distribution on the Fe3O4@SiO2 support. Polyvinylpyrrolidone (PVP) was often employed as a stabilizer in the preparation of composite spheres. Recently, PVP was also used to reduce noble metal ions (Au, Ag, Pd, and Pt) to prepare nanostructured metals [28–31]. In this paper, we combine the reducing function of PVP with its stabilizing function to prepare Fe3O4@SiO2–Ag composite spheres. The synthesis offers good controllability of size and dispersion of AgNPs with high-yield
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and short reaction time. We also demonstrate the high catalytic efficiency of Fe3O4@SiO2–Ag nanocomposite toward 4-nitrophenol (4-NP) by sodium borohydride. In addition, the obtained Fe3O4@SiO2–Ag catalyst can be efficiently recovered from the reaction solution by using external magnetic fields for many cycles without significant loss of either material or catalytic activity. 2. Experimental 2.1. Materials and reagents Silver nitrate (analytical grade) and PVP (K30, analytical grade) were purchased from Sinopharm Chemical Reagent Co. Ltd. Other chemicals were purchased from Beijing Chemical Corp. All chemicals were used as received without any further purification.
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(0.1 wt%) was then added. The reaction was carried out at 298 K with continuous stirring. At various time intervals, parts of the mixture were taken and centrifuged for the determination with UV–vis absorption spectra at the wavelength of the absorbance maximum in the range of 250–500 nm. To study the reusability of Fe3O4@SiO2–Ag nanocomposite, the used sample was separated from the solution by a magnet after monitoring the whole reduction process. The recycled sample was washed with ethanol and water several times for the next cycling. Similar to the above reduction process, the obtained magnetic nanocomposite was redispersed in 1.0 g of deionized water, then mixed with 30 mL of aqueous 4-NP solution (0.12 mM) and 29 mL of NaBH4 (12 mM) solution. With a stiff limit of 8 min kept for completion of the reaction, the solution was centrifuged and measured using UV–vis spectroscopy. The procedure was repeated 15 times.
2.2. Synthesis of Fe3O4 microspheres 2.6. Characterization The magnetic particles were synthesized through a solvothermal reaction according to the reported method [32] with some modification. Typically, 1.08 g of FeCl36H2O was dissolved in 20 mL of ethylene glycol under magnetic stirring. A clear yellow solution was obtained after stirring for 0.5 h. Subsequently, 1.8 g of sodium acetate was added to this solution and being stirred for another 1 h. Afterward, 0.25 g of trisodium citrate was added. After forming a homogeneous dispersion, the mixture was transferred into a Teflon-lined stainless-steel and heated at 200 °C for 10 h. The black magnetic particles were collected with the help of a magnet filed, followed by washing with a recycle of ethanol and deionized water six times. The product was then dried under vacuum at 50 °C for 12 h. 2.3. Synthesis of Fe3O4@SiO2 microspheres The interlayers of SiO2 were prepared through a modified Stöber method [33]. In a typical process, as-prepared Fe3O4 particles (0.1 g) were dispersed in a mixture of ethanol (40 mL), deionized water (10 mL), and concentrated ammonia aqueous solution (28 wt%, 1.2 mL) by ultrasonication for 1 h. Subsequently, 0.35 mL of tetraethyl orthosilicate (TEOS) was added dropwise. After being stirring for 2 h, the products were collected and washed with deionized water and then dried under vacuum at 60 °C for further use. 2.4. Constructing Fe3O4@SiO2–Ag nanocomposite A simple in situ wet chemistry method was employed for the immobilization of AgNPs onto the core–shell structured Fe3O4@SiO2 microspheres to form Fe3O4@SiO2–Ag nanocomposite. First, 0.06 g of Fe3O4@SiO2 was dispersed into 10 mL of 3 10 2 M Ag(NH3)2NO3 solution under mechanical stirring at room temperature. After the [Ag(NH3)2]+ ions were adsorbed onto the surfaces of silica spheres for around 30 min via electrostatic attraction between [Ag(NH3)2]+ ions and the negatively charged Si–OH groups, the dispersion was added into 15 mL of ethanol containing PVP (0.1 g), followed by heating the solution by reflux at 70 °C for 4 h. The final products were magnetically separated, washed several times with a recycle of ethanol and deionized water, and dried at 50 °C for 12 h. 2.5. Catalyzed reduction of 4-NP To study the catalytic activity, 30 mL of 4-NP aqueous solution (0.12 mM) was mixed with 29 mL of fresh NaBH4 solution (12 mM). 1 g of aqueous dispersed Fe3O4@SiO2–Ag nanocomposite
The morphologies of the as-prepared nanocomposite were observed by the scanning electron microscope (SEM, JEOL JSM6700F), energy dispersive X-ray (EDX), and transmission electron microscopy (TEM; JEM 3010). X-ray diffraction (XRD) patterns were collected using a Bruker D8 Advance X-ray diffractometer to identify the crystal structures with a Cu Ka X-ray source operating at 40 kV and 100 mA. Fourier transform infrared (FTIR) spectra were recorded with a PerkinElmer spectrometer using KBr pellets, and the thickness of the pellet being about 1.3 mm. Each spectrum was collected at room temperature under atmospheric pressure with a resolution of 4 cm 1. X-ray photoelectron spectroscopy (XPS) was accomplished using a PHI-5702 multi-functional X-ray photoelectron spectrometer with pass energy of 29.35 eV and an Mg Ka line excitation source. Magnetization measurements were performed on a MPM5-XL-5 superconducting quantum interference device (SQUID) magnetometer at 300 K. UV–vis absorption spectra were recorded using a UV–vis spectrophotometer (UV2550). The samples were placed in a 1 cm 1 cm 3 cm quartz cuvette, and the spectra were recorded at room temperature. 3. Results and discussion 3.1. The characterization of Fe3O4@SiO2–Ag core–shell nanocomposite The synthetic procedure for Fe3O4@SiO2–Ag core–shell nanocomposite is illustrated in Fig. 1e. Firstly, the Fe3O4 particles were prepared via a robust solvothermal reaction based on a high temperature reduction of Fe(III) salts with ethylene glycol in the presence of trisodium citrate. The magnetite particles capped with citrate groups exhibit excellent dispersibility in polar solvents such as water and ethanol, which favors the subsequent coating. SEM images of the Fe3O4 particles confirm the uniform size of about 200 nm and nearly spherical shape (Fig. 1a). Subsequently, silica was coated on the Fe3O4 to form Fe3O4@SiO2 core–shell microspheres through hydrolysis of TEOS according to the Stöber method. This coating process is indispensable for two reasons: first, the SiO2 shell can effectively prevent the aggregation and chemical degradation of the Fe3O4 particles in a harsh liquid environment; second, [Ag(NH3)2]+ ions could be adsorbed on Fe3O4@SiO2 microspheres by the electrostatic attraction between negatively charged surface Si–OH groups and positively charged [Ag(NH3)2]+ ions. The SEM image (Fig. 1b) shows that the Fe3O4@SiO2 core–shell microspheres still keep the morphological properties of pure Fe3O4 except for a slightly larger particle size about 270 nm. Finally, these [Ag(NH3)2]+ ions adsorbed on Fe3O4@SiO2 microspheres were reduced and protected by PVP through an in situ wet chemistry
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Fig. 1. SEM images of Fe3O4 microspheres (a), Fe3O4@SiO2 microspheres (b), Fe3O4@SiO2–Ag nanocomposite (c), EDX spectrum of Fe3O4@SiO2–Ag nanocomposite (d), and schematic illustration of the formation process of Fe3O4@SiO2–Ag nanocomposite (e).
method to obtain Fe3O4@SiO2–Ag nanocomposite. It can be seen that Fe3O4@SiO2–Ag nanocomposite is well dispersed without the occurrence of aggregation after deposition of high-density AgNPs (Fig. 1c), indicating the uniform property of our simple. The small size of Ag nanoparticles results in low contrast in SEM images, so they could not be clearly distinguished from the Fe3O4@SiO2 matrix. The EDX characterization demonstrates that the nanocomposite contains three necessary and diagnostic elements of the precursors, Fe, Si, and Ag (Fig. 1d, Cu arising from the sample carrier and C from the adhesive used for sample preparation are discounted). In order to further study the morphological and structural features of the Fe3O4@SiO2–Ag nanocomposite, TEM observations were carried out. It can be clearly seen from Fig. 2a that the Fe3O4@SiO2 microspheres have a core–shell structure. The magnetic cores are black spheres with an average size of around 200 nm, and the silica shell shows a gray color with an average thickness of about 35 nm. TEM images of Fe3O4@SiO2–Ag nanocomposite directly show that many small and highly dispersed
AgNPs are immobilized on the surface of Fe3O4@SiO2 microspheres (Fig. 2b). The size distribution of AgNPs is narrow with an average size of 3.65 nm (Fig. 2c). No large Ag particles were observed. Instead, there are nearly monodispersed nanoparticles, indicating that the in situ wet chemistry method has a good control over the formation of AgNPs. Specifically, the crystal lattice of AgNPs can be well observed in the insert of Fig. 2c, which clearly shows the formation of Ag nanocrystal. The phase and purity of the as-prepared samples were examined by XRD. A typical XRD pattern of the obtained Fe3O4 sample was shown in Fig. 3a, all of the diffraction peaks could be readily indexed to a face-centered cubic structure (Fd3m space group) of magnetite according to JCPDS card No. 19-0629. The XRD pattern of Fe3O4@SiO2 microspheres exhibited almost the same feature as pure Fe3O4 (Fig. 3b), except that a broad peak centered at 22° of 2h corresponding to SiO2 was observed, indicating that the coated SiO2 shell is amorphous. In the case of Fe3O4@SiO2–Ag (Fig. 3c), besides the characteristic diffraction of Fe3O4 microspheres, the
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Fig. 2. TEM images of Fe3O4@SiO2 microspheres (a) and Fe3O4@SiO2–Ag nanocomposite (b). Size distribution histogram of AgNPs calculated from the TEM images (c). Inset shows the corresponding HRTEM image of the AgNPs.
Fig. 3. Powder XRD patterns of Fe3O4 microspheres (a), Fe3O4@SiO2 microspheres (b), and Fe3O4@SiO2–Ag nanocomposite (c).
Fig. 4. FTIR spectra of Fe3O4 microspheres (a), Fe3O4@SiO2 microspheres (b), and Fe3O4@SiO2–Ag nanocomposite (c).
obvious diffraction peaks at 2h = 37.9°, 44.1°, 64.3°, and 77.2° can be readily indexed to face-centered-cubic structure of Ag (JCPDS card No. 4-783), indicating that the crystallized AgNPs could be obtained by PVP reduction. In order to further confirm the composition and structure of samples, FTIR spectra were measured. As shown in Fig. 4a, Fe3O4 microspheres show typical bands centered at 585 and 635 cm 1, which are related to the Fe–O stretches [34]. The absorption bands at 1635 and 1406 cm 1 assigned to the carboxylate groups are clearly visible, indicating the large amount of carboxylate groups, which is crucial to the subsequent coating. After the coating of silica, Fe3O4@SiO2 microspheres show some new bands centered around 795 cm 1 and 1080–1100 cm 1 (Fig. 4b). The new absorption at 795 cm 1 can be ascribed to the symmetric vibration of Si–O–Si, whereas the band at 1080–1100 cm 1 assigned to the asymmetric stretching vibration of Si–O–Si. These results indicate that SiO2 is immobilized on the surfaces of Fe3O4 microspheres. Fig. 4c shows the FTIR spectrum of Fe3O4@SiO2–Ag nanocomposite. Because AgNPs do not have absorption in the infrared region [26], the FTIR spectrum of Fe3O4@SiO2–Ag nanocomposite is almost the same as that of Fe3O4@SiO2 microspheres. Then, XPS has been utilized as a useful tool for qualitatively determining the surface component and composition of the samples. Fig 5A shows the fully scanned spectra in the range of 0–1200 eV. The peaks on the curve of Fe3O4@SiO2–Ag nanocom-
posite are assigned to Si, O, Ag, and C elements. The C element was attributed to the adventitious carbon-based contaminant. Therefore, it is concluded that Si, O, and Ag exist in Fe3O4@SiO2– Ag nanocomposite, while Si and O exist in Fe3O4@SiO2 microspheres. Fig. 5B shows the XPS spectrum in the O 1s region of Fe3O4@SiO2–Ag nanocomposite, which could be fitted into three peaks. The binding energy at 533.2 eV is attributed to surface hydroxyl groups of silica (Si–O–H), the 532.3 eV peak represents the lattice oxygen of silica (Si–O–Si), and the peak at 531.0 eV suggests the formation of Si–O–Ag unit. The XPS spectrum of Ag 3d of Fe3O4@SiO2–Ag nanocomposite as shown in Fig. 5C can be assigned to two components. The major component is Ag0 indicating the formation of AgNPs, and the lesser component is Ag–O– due to the surface of AgNPs being bound to the anionic oxygen of SiO2 shell [35–37]. The peaks at Ag0 3d5/2 and Ag0 3d3/2 (368.0 eV and 374.0 eV) of Fe3O4@SiO2–Ag nanocomposite shift to lower binging energy in compared with pure silver (368.2 eV for Ag 3d5/2, 374.2 eV for Ag 3d3/2), further confirming that there is an interaction between the SiO2 shell and AgNPs. The magnetic properties of the samples were characterized using a SQUID magnetometer measured at 300 K as shown in Fig. 6. Pure Fe3O4 microspheres, Fe3O4@SiO2 microspheres, and Fe3O4@SiO2–Ag nanocomposite, have magnetization saturation (Ms) values of 78.5, 66.4, and 63.8 emu/g, respectively. Because of the presence of SiO2 shell, the Ms value of Fe3O4 microspheres
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Fig. 5. (A) XPS fully scanned spectra of Fe3O4@SiO2 microspheres (a) and Fe3O4@SiO2–Ag nanocomposite (b). XPS spectra of the Fe3O4@SiO2–Ag nanocomposite: (B) O 1s, and (C) Ag 3d.
Fig. 6. Magnetization curves of Fe3O4 microspheres, Fe3O4@SiO2 microspheres, and Fe3O4@SiO2–Ag nanocomposite.
is higher than that of Fe3O4@SiO2 microspheres and Fe3O4@SiO2– Ag nanocomposite. Compared with Fe3O4@SiO2 microspheres, Fe3O4@SiO2–Ag nanocomposite exhibits a slightly smaller Ms value, which is attributed to the slight increase in the mass and size caused by the depositing AgNPs on the surfaces of Fe3O4@SiO2 microspheres. It should be noted that the core–shell structured Fe3O4@SiO2–Ag nanocomposite still shows strong magnetization, suggesting its suitability for magnetic separation and recovery. 3.2. Application of Fe3O4@SiO2–Ag nanocomposite for catalytic reduction of 4-NP As it is known, 4-aminophenol (4-AP) is very useful in a wealth of applications that include analgesic and antipyretic drugs, photographic developers, corrosion inhibitors, and so on [38–40]. The reduction of 4-NP to 4-AP by NaBH4 was employed as a model system to evaluate the catalytic activity of Fe3O4@SiO2–Ag nanocomposite. Although the reaction was a thermodynamically feasible process involving E0 for 4-NP/4-AP = 0.76 V and H3BO3/ BH4 = 1.33 V versus normal hydrogen electrode, it was kinetically restricted in the absence of a catalyst (no change in the absorption even after 2 days). In agreement with previous results, the absorption peak of 4-NP changed from 317 to 400 nm immediately when treated with an aqueous solution of NaBH4 (Fig. 7a), which corresponds to a color change of light yellow to yellow-green due to the formation of 4-nitrophenolate ion [41]. After addition of a small
amount (1.0 mg) of the Fe3O4@SiO2–Ag nanocomposite, the reduction commences and the time-dependent absorption spectra show a decrease in intensity of the absorption peak at 400 nm and a concomitant increase of a new peak at 295 nm corresponding to 4-AP (Fig. 7b). After the completion of reduction reaction, the peak due to the nitro compound was no longer observed, which indicated that the catalytic reduction of 4-NP had proceeded successfully. For comparison, a blank test was conducted with a mixture of 4-NP, NaBH4, and Fe3O4@SiO2 microspheres containing no AgNPs. There was little decrease in the absorbance of nitro compound at 400 nm monitored by UV, meanwhile, no absorption peak assigned to 4-AP was appeared at around 295 nm, indicating that the catalytic reduction of 4-NP did not occur. The rate of reduction reaction catalyzed by Fe3O4@SiO2–Ag was assumed to be independent of the concentration of NaBH4 because this reagent was used in large excess compared to 4-NP. Therefore, the kinetic data were fitted by a first-order rate law. Linear relationship between ln(C/C0) and reaction time is obtained in the reduction catalyzed by Fe3O4@SiO2–Ag nanocomposite (Fig. 7c), and the rate constant k is calculated to be 7.67 10 3 s 1. Reported ratios of rate constants k over the weight of catalyst employed are 1.30, 0.41, and 0.09 s 1 g 1 for coral-like dendrite, banana leave-like dendrite, and spherical Ag nanostructures, respectively [42]. These are all lower than the 7.67 s 1 g 1 of Fe3O4@SiO2–Ag nanocomposite synthesized in this work, although a substantial part of the weight is actually the Fe3O4 and SiO2 in this case. Compared with other matrix-supported AgNPs, the rate constant k of Fe3O4@SiO2–Ag is much higher than previous reported ratios for Ag supported halloysite nanotubes (0.087 s 1 g 1) and Ag doped carbon spheres (1.69 s 1 g 1) [18,43]. The good catalytic activity of Fe3O4@SiO2–Ag nanocomposite is derived from its highly dispersed and small-sized AgNPs. Highly dispersed AgNPs can make 4-nitrophenolate ion easily access to the AgNPs that serve as an electron relay in the system for an oxidant and a reductant. The small size of AgNPs makes a large potential difference, thus leading to a high rate of reduction. In addition, a smaller particle size can result in a larger surface-to-volume ratio and more exposed atoms on the surface, which act as the potential catalytic sites [44–47]. Furthermore, we have also monitored the cyclic stability of Fe3O4@SiO2–Ag nanocomposite by monitoring the catalytic activity during successive cycles of the reduction reaction. The Fe3O4@SiO2–Ag nanocomposite can be recovered from the catalytic reaction solution by using a magnet. With a stiff limit of 8 min kept for completion of the reaction, the catalyst exhibited similar catalytic performance without significant reduction in the conversion even after running 15 cycles (>99% in eight successive reactions, Fig. 7d). The conversion starts to decrease slightly after eight cycles, implying that the reaction rate is only slightly reduced after multiple
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Fig. 7. UV–vis spectra of (a) 4-NP before and after adding NaBH4 solution and (b) the reduction of 4-NP in aqueous solution recorded every 2 min using Fe3O4@SiO2–Ag nanocomposite (1 mg) as a catalyst, (c) ln(C/C0) versus reaction time for the reduction of 4-NP, (d) the reusability of Fe3O4@SiO2–Ag nanocomposite as a catalyst for the reduction of 4-NP with NaBH4.
reuses. These results indicate that the Fe3O4@SiO2–Ag nanocomposite might possess a profound application in the fields of catalytic reduction of 4-NP to 4-AP. 4. Conclusions In summary, we demonstrated an effective route to prepare Fe3O4@SiO2–Ag core–shell structured nanocomposite by combining the sol–gel process and a facile in situ wet chemistry method with the aid of the dual function of PVP as both reductant and stabilizer. The as-prepared nanocomposite shows a number of important features as a recyclable catalyst for the reduction of 4NP: it contains a highly field-responsive magnetic Fe3O4 core for efficient magnetic separation, a SiO2 interlayer for protecting the Fe3O4 core from chemical- and/or photo-dissolution, and smallsized and highly dispersed AgNPs for enhanced catalytic activity. This method is efficient to get small and highly dispersed AgNPs with a narrow size distribution on the Fe3O4@SiO2 support and presents a new paradigm for the fabrication of magnetically separable core–shell structured catalysts based on noble metal nanoparticles, which could be very useful in catalytic applications. Acknowledgment This work was supported by the National Natural Science Foundation of China for Project Nos. 21076094 and 20743005.
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