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Fe3O4 nanoparticle composites with supported Au nanoparticles as recyclable high-performance nanocatalysts
Materials Today Chemistry 5 (2017) 43e51
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Superparamagnetic POT/Fe3O4 nanoparticle composites with supported Au nanoparticles as recyclable high-performance nanocatalysts Jun Sun*, Lin Chen Institute of Applied Technology, Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei 230031, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 March 2017 Received in revised form 13 June 2017 Accepted 13 June 2017
A novel method has been developed to successfully synthesize Fe3O4 nanoparticles with tunable size and morphology supported on shells of poly(o-Toluidine)(POT) hollow microspheres. The as-prepared POT/ Fe3O4 nanoparticle composites can be used as novel and magnetic-responsive catalyst supports to produce highly efficient and recyclable noble metal catalysts. The size of Fe3O4 nanoparticles supported on shells of POT hollow microspheres can be tuned from 4 to 12 nm by changing the concentration of Fe ions. The roles of the doping acid of POT and Zeta potentials of Fe3O4 nanoparticles and POT in the formation of the POT/Fe3O4 nanoparticle composites were discussed. Furthermore, gold nanoparticles that were supported on the as-synthesized POT/Fe3O4 nanoparticle composites have been achieved by utilizing the reactivity of POT towards Au ions. The size of gold nanoparticles can be tuned by altering the concentration of HAuCl4. Finally, the catalytic activity of the obtained POT/Fe3O4/Au composites for 4nitrophenol (4NP) reduction is investigated. The results demonstrate that such magnetic-responsive polymer-supported gold nanoparticles can be easily recovered and reused five times still remains high catalytic performance, which indicate their potential applications in the field of catalysis. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Polyaniline Fe3O4 nanoparticles Au nanoparticles Nanocatalyst
1. Introduction Gold nanoparticles with high surface areas have been extensively studied for their unique optical and electronic properties together with their various applications in fields of electronics, photonics, catalysts, biotechnology [1e4]. To date, various methods have been developed to prepare gold nanoparticles-based catalysts with controllable size. However, gold nanoparticles are unstable and tend to aggregate and therefore lose their initial catalytic activity and selectivity because of their high surface energy [5]. To solve this problem, gold nanoparticles have been supported onto various supporting materials to generate hybrid catalysts to protect against the aggregation of gold nanoparticles. Typically, the supports for gold nanoparticles are base on activated carbon or metal oxides [6e8]. However, in recent years, the polymer based supports have been attracted more and more attentions because of their merits of mild and simplified synthetic conditions and higher catalytic performance [9].
* Corresponding author. E-mail address: [email protected] (J. Sun). http://dx.doi.org/10.1016/j.mtchem.2017.06.001 2468-5194/© 2017 Elsevier Ltd. All rights reserved.
Up to now, many different polymers have been utilized to stabilize and support gold nanoparticles such as poly(vinylpyrrolidone)(PVP) [10e12], polystyrene derivatives [13], and others [14e16]. Among these polymers, conducting polymers of polyaniline(PANI) and its derivatives [17e19], in which benzene rings and amine groups coexist in polymer chains can bind to certain metal atoms on the surface of nanoparticles, have also been developed to act as superior supports for gold nanoparticles. Moreover, it has been reported that dedoped PANI and its derivatives can also be used as reductant for the generation of noble metal nanoparticles from metal salts [20,21]. Thus, PANI and its derivatives can act as both reductant and stabilizer for polymersupported gold nanoparticles, which will facilitate the synthetic procedures for polymer-supported gold nanoparticles within one step. Until now, PANI nanofibers [18], PANI powders and poly(ophenylenediamine)(PoPD) have been reported to use as reactive supports for the preparation of polymer-supported gold nanoparticles [19]. As known to all, the isolation and recovery of gold nanoparticles-based catalysts from the catalytic reaction medium with high efficiency is also worth of consideration due to the scarcity of noble metal resources [22]. However, the isolation and
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recovery efficiency of polymer-supported gold nanoparticles is very low for only centrifugation or filtration can be used. In this regard, magnetic-responsive nanomaterials have emerged as ideal catalyst supports since they can be isolated and recovered by magnetic separation, which is more efficient and rapid than centrifugation or filtration, which prevents mass losses of catalyst or the use of additional solvents [23e25]. However, the synthesis of magneticresponsive polymer-supported gold nanoparticles with controllable size, improved stability and catalytic activity remains a great challenge. Herein, we report that well-defined Fe3O4 nanoparticles with tunable size supported on shells of hollow poly(o-Toluidine)(POT) microspheres, a derivative of PANI, can be successfully synthesized as novel and magnetic-responsive catalyst supports to produce highly efficient and recyclable noble metal catalysts. By altering the concentration of Fe ions, the size of Fe3O4 nanoparticles supported on shells of POT hollow microspheres can be tuned from 4 to 12 nm. It is found that doping acid of POT and Zeta potentials of Fe3O4 nanoparticles and POT show strong affinity toward Fe3O4 nanoparticles into POT hollow microspheres. Formation mechanisms involved have been recommended. Finally, using the redox activity between POT and Au ions, the POT/Fe3O4/Au nanoparticle composites can be fabricated after the addition of Au ions into the POT/Fe3O4 reactive supports, and their catalytic activities were examined by choosing the model catalysis reaction involving the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) in the presence of NaBH4. Superior catalytic performance with high stability and recyclability were revealed, which indicates their potential applications in the field of catalysis.
magnet and washed with distilled water for several times and then dried at 80 C in a vacuum oven. Furthermore, pure Fe3O4 nanoparticles are prepared without POT through similar conditions. 2.4. Synthesis of POT/Fe3O4/Au nanoparticle compositess The POT/Fe3O4/Au nanoparticle composites were synthesized according to the literature [18]. Typically, 3 mg POT/Fe3O4 composite and 20 mg PVP (K-30) were added to 10 mL of deionized water with shaking at room temperature for 30 min. Then, a quantitative amount of HAuCl4 aqueous solution was added (maintain the total volume is 20 mL, the concentration of HAuCl4 is 2 103 M and 8 103 M, respectively.) and the system was shaking at room temperature for 12 h. Finally, the resultant products were separated and collected with a magnet. After washing with deionized water to remove any remaining HAuCl4. The particles were finally dispersed in 10 mL deionized water for catalysis. 2.5. catalytic experiments The reduction of 4-NP by NaBH4 was chosen as a model reaction for testing the efficiency of the POT/Fe3O4/Au nanoparticle composites according to the literature [9,17,18]. Typically, NaBH4 (1.7 mL, 0.015 M) and 4 NP aqueous solution (1.0 mL, 0.2 mM) were mixed in the quartz cell (1 cm path length), Then, 0.3 mL POT/ Fe3O4/Au colloidal solution (0.3 g/L) was added in above solution to initiate the conversion of 4 NP to 4-aminophenol (4 AP) reaction. The whole reaction progress was monitored by the UV-vis spectroscopy at ambient temperature.
2. Experimental section
2.6. Characterization
2.1. Materials
The morphologies of products were examined by field-emission scanning electron microscopy (FESEM) (S-4800, Hitachi Co., Japan) and transmission electron microscopy (TEM) (JEM-2100, JEOL, Japan). The FT-IR spectra (NEXUS-870, Nicolet Instrument Co., USA) were recorded in the range of 400e4000 cm1. The UV-vis spectra (U-4100, Hitachi Co., Japan) of samples were measured in the range between 200 and 900 nm. X-ray diffraction (XRD) patterns were measured on a XD-3 (with CuKa radiation) diffraction meter. The magnetic property of as-prepared POT/Fe3O4 composite was measured by using a vibrating sample magnetometer (VSM, Riken Denshi Co. Ltd., Japan). The Zeta potentials of samples were recorded on Malvern Zetasizer Nano ZS90. The Au contents in the asprepared samples were measured using inductively coupled plasma-atomic emission spectrometry (ICAP, 7400 Duo).
O-Toluidine (Shanghai Chemical Co., Shanghai, China) was distilled under reduced pressure before use. HAuCl4-3H2O (Shanghai Chemical Co.) and other reagents were used as received. 2.2. Synthesis of POT hollow microspheres The POT hollow microspheres were synthesized according to the literature [26]. O-Toluidine (0.257 g) and citric acid (0.084 g) and 50 mL deionized water were mixed together with magnetic stirring at room temperature for 10 min. After that, the mixture was maintained at 0e10 C for 0.5 h before oxidative polymerization. Then 50 mL ammonium persulfate (APS, 0.547 g) aqueous solution was added to the above mixture in one portion. And then the above mixture solution was maintained at 0e10 C without agitation for 12 h. Finally, the product was dedoped with 0.1 M NH4OH (aq) and washed with deionized water for several times, and then dried in a vacuum at 80 C for 24 h before use. H2SO4-doped POT hollow microspheres are prepared by using H2SO4 as doping acid through similar conditions. 2.3. Synthesis of POT/Fe3O4 nanoparticle composites In a typical synthesis, 0.0135 g FeCl2$4H2O, 0.0367 g FeCl3$6H2O and 0.1 g of POT were dispersed in 40 mL of deionized water by using ultrasonication for 15 min. The obtained solution was transferred to a flask and then heated to 80 C in an oil bath under the protection of nitrogen gas atmosphere. As the temperature raised to 80 C, 5 mL of NH4OH (aq) was added to the above solution in one portion. The reaction was allowed to proceed under stirring for 30 min at 80 C. And then the resulting solution was heated to 95 C for another 90 min. Finally, the product was collected with a
3. Results and discussion 3.1. Synthesis and characterization of POT/Fe3O4 nanoparticle composites The reactive template POT hollow microspheres are synthesized by the monomer-droplet strategy without the aid of surfactant according to the literature [26]. Typical scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of POT hollow microspheres are shown in Fig. 1. The POT hollow microstructures are constructed with several POT hollow microspheres and range in size of from 200 nm to several micrometers, with the shell thickness is about 20 nm. As shown in Fig. 1b, there is a hole in the surface of the POT hollow microsphere concluded that the POT microsphere is hollow. As the POT hollow microspheres are in their doping state, dedoping process of POT hollow microspheres with 0.1 M NH4OH (aq) is applied. Then the as-prepared POT hollow microspheres are redispersed in deionized water, and a certain
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Fig. 1. (a, b) SEM and (c, d) TEM images of the POT hollow microspheres.
amount of FeCl2$4H2O and FeCl3$6H2O are added into aqueous suspension of POT. The obtained solution is heated to 80 C under the protection of nitrogen gas atmosphere. And then NH4OH (aq) is added into the above solution to initial precipitation reaction. Finally, Fe3O4 nanoparticles supported on POT hollow microspheres can be formed. As shown in Fig. 2, Fe3O4 nanoparticles with size of ~8 nm are evenly deposited on surfaces of POT hollow microspheres. The selected area electron diffraction (SAED) pattern of POT/Fe3O4 nanoparticle composites is also shown in Fig. 2c, which displays sharp rings indexed to the (440), (511), (220), (311), and (400) planes of a spinel (face centered cubic) Fe3O4 [27]. POT/Fe3O4 nanoparticle composites are further characterized by XRD, FTIR, and UV-vis spectroscopies. As shown in Fig. 3a, eight obvious main peaks indexed to the (111), (220), (311) (222), (400), (422), (511), and (440) planes of a spinel (face centered cubic) Fe3O4, indicating the presence of Fe3O4 nanoparticles in products [28]. As displayed in Fig. 3, the characteristics for CA-doped POT, such as N-H stretching vibrations at 3200-3500 cm1 and C¼C stretching deformation of quinonoid and benzenoid rings at around 1597 and 1498 cm1, respectively, are clearly seen [29]. The ratio of the relative intensity of the two peaks is about 1.0, which indicates that similar nitrogen quinonoid and benzenoid ring structures exist in the POT chains [30]. The band at 1382, 1312, and 1240 cm1, attributed to C-N stretching vibrations in the neighborhood of an
aromatic ring, is characteristic of doped POT. As for POT/Fe3O4 nanoparticle composites, it is revealed that the relative intensity of C¼C stretching vibrations of benzenoid (1498 cm1) to quinonoid (1597 cm1) rings increases significantly after loading Fe3O4 nanoparticles, indicating the dedoped state of POT hollow microspheres [31]. In addition, there is a strong absorption appearing at wavenumbers 574 cm1 can be ascribed to the characteristic Fe-O stretching vibrations in iron oxides [32]. UV-vis spectra of CAdoped POT, dedoped POT, and POT/Fe3O4 nanoparticle composites are shown in Fig. 3. CA-doped POT shows three characteristic absorption bands at about 300e400, 550e650, and ~800 nm wavelengths. The first absorption band arises from p-p* electron transition within benzenoid segments. The second and the third absorption bands are related to doping level and formation of polaron, respectively [29]. After dedoped by using NH4OH (aq), the dedoped POT microspheres show only two adsorption peaks centered at 335 and 606 nm, respectively [33]. As for the UV-vis spectrum of POT/Fe3O4 nanoparticle composites, only two adsorption peaks centered at 365 and 577 nm can be seen, indicating the dedoped state of POT hollow microspheres in the composites. To demonstrate the magnetic property of the prepared POT/ Fe3O4 nanoparticle composites, two vials were loaded with POT hollow microspheres, 150 mg each dispersed in 5.0 mL aqueous
Fig. 2. (a, b) SEM and TEM images of the hollow structured POT/Fe3O4 nanoparticles composites. (c) SAED pattern of the POT/Fe3O4 nanoparticles composites.
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Fig. 4. Magnetization hysteresis at 300 K for the three kinds of hollow structured POT/ Fe3O4 composites synthesized at different Fe ions concentrations. Synthetic conditions: (a) POT ¼ 0.1 g, [FeCl2$4H2O] ¼ 0.85 mM, [FeCl3$6H2O] ¼ 1.7 mM; (b) POT ¼ 0.1 g, [FeCl2$4H2O] ¼ 1.7 mM, [FeCl3$6H2O] ¼ 3.4 mM; (c) POT ¼ 0.1 g, [FeCl2$4H2O] ¼ 8.5 mM, [FeCl3$6H2O] ¼ 17 mM.
after slightly shaking, which proves an effective and reversible response to the magnetic field. Moreover, Fig. 4 further gives the magnetic hysteresis loops of POT/Fe3O4 nanoparticle composites synthesized at different Fe ions concentrations, respectively. The magnetization of POT/Fe3O4 nanoparticle composites are all increased with rise of applied magnetic field. The saturated magnetizations (Ms) of the POT/Fe3O4 nanoparticle composites are 5.7, 15.1, and 29.3 emu/g, respectively. The curves show extremely low remnant magnetization (Mr) and coercivity (Hc), indicating that all of the samples are superparamagnetic at room temperature [34]. Own to the superparamagnetic properties and high saturation magnetization, the as-prepared POT/Fe3O4 nanoparticle composites can be easily separated from the solution by means of an external magnet, which is much simpler than using centrifugation of non-magnetic materials. 3.2. Effect of concentration and doping acid
Fig. 3. (a) XRD pattern of POT/Fe3O4 nanoparticle composites. (b) FTIR spectra of the CA-doped POT, dedoped POT and POT/Fe3O4 nanoparticle composites. (c) UV-vis spectra of the CA-doped POT, dedoped POT and POT/Fe3O4 nanoparticle composites.
solution. The magnetic properties were tested using an external magnet. As illustrated in Fig. 4, in the absence of a magnetic field, POT/Fe3O4 nanoparticle composites were homogeneously dispersed in the aqueous solution. After applying a small Nd-Fe-B magnet to the solution, most POT/Fe3O4 nanoparticle composites moved to one side of the vial closer to the head of the permanent magnet within 3 min. Once removing the magnet, all of the POT/ Fe3O4 nanoparticle composites returned to a dispersed state again
The Fe ions (Fe2þ and Fe3þ) concentration can be used for tuning the size of the Fe3O4 nanoparticles on the POT shells (Fig. 5). When the concentrations of Fe2þ and Fe3þ are maintained at 17 and 34 mM, Fe3O4 nanoparticles are fully supported on surfaces of the shells. The Fe3O4 nanoparticle size is estimated to be 14 nm (Fig. 5a and b and Fig. 6a and b). However, partial aggregation of scattered Fe3O4 nanoparticles (unsupported on POT shells) is obviously seen with size is about 22e35 nm (Fig. 6a and b). These scattered Fe3O4 nanoparticles may attribute to too much Fe ions. If the concentrations of Fe2þ and Fe3þ decrease to 8.5 and 17 mM, Fe3O4 nanoparticles are evenly decorated on POT shells and the mean size of the Fe3O4 nanoparticles is 10 nm (Fig. 5c and d and Fig. 6c and d). Moreover, the aggregation phenomenon of scattered Fe3O4 nanoparticles is seldom seen and the size of scattered Fe3O4 nanoparticles decreases to only about 14e25 nm (Fig. 6c and d). As compared with Fig. 5e and f, if the concentrations of Fe2þ and Fe3þ decrease to 1.7 and 3.4 mM, aggregation-free Fe3O4 nanoparticles with an average size of 6 nm are evenly deposited on POT shells. As the concentrations of Fe2þ and Fe3þ decrease further to 0.85 and 1.7 mM, the average size of the Fe3O4 nanoparticles can reach to 4 nm (Fig. 5g and h). From the above analysis we can conclude that a low concentration of Fe ions is necessary for the formation of
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Fig. 5. TEM images of the POT/Fe3O4 nanoparticle composites synthesized at different Fe ions concentrations. Synthetic conditions: (a, b) POT ¼ 0.1 g, [FeCl2$4H2O] ¼ 17 mM, [FeCl3$6H2O] ¼ 34 mM. (c, d) POT ¼ 0.1 g, [FeCl2$4H2O] ¼ 8.5 mM, [FeCl3$6H2O] ¼ 17 mM. (e, f) POT ¼ 0.1 g, [FeCl2$4H2O] ¼ 1.7 mM, [FeCl3$6H2O] ¼ 3.4 mM (g, h) POT ¼ 0.1 g, [FeCl2$4H2O] ¼ 0.85 mM, [FeCl3$6H2O] ¼ 1.7 mM. Histogram is Fe3O4 particle size distribution.
evenly deposited, aggregation-free Fe3O4 nanoparticles on shells of POT hollow microspheres. Doping acid is another important influencing factor on the morphology of POT/Fe3O4 nanoparticle composites. Morphology of products using H2SO4-doped POT hollow microspheres instead of CA-doped POT hollow microspheres is examined. In comparison with the case of CA-doped POT hollow microspheres (Fig. 7a and b), Fe3O4 nanoparticles apart from POT hollow microspheres can be seen (Fig. 7c and d); however, density of Fe3O4 nanoparticles deposited on surfaces of POT hollow microspheres decreased significantly (Fig. 7c and d and Fig. 8a and b). When the H2SO4doped POT hollow microspheres were dedoped with 0.1 M NH4OH (aq), and then immersed in 0.1 M CA (aq) to obtain CA-redoped POT hollow microspheres for synthesizing POT/Fe3O4 nanoparticle composites, Fe3O4 nanoparticles with size about 10 nm evenly deposited on surfaces of POT hollow microspheres (Fig. 8c). It can be concluded that doping acid of CA that shows strong affinity toward Fe3O4 nanoparticles introduced to POT hollow microspheres will effectively prevent aggregation of Fe3O4 nanoparticles which leads to formation of uniform composites with Fe3O4 nanoparticles evenly deposited on surface of POT hollow microspheres. This can be due to there are three carboxy groups within CA, as CA doped in POT hollow microspheres can improve Fe2þ and Fe3þ ions attached on the surface of the POT hollow microspheres for complexation
[35]. And then amount of Fe3O4 crystal nucleus are formed on the surface of the POT hollow microspheres during early stage of coprecipitation reaction. With the growth of the Fe3O4 crystal nucleus, Fe3O4 nanoparticles in tens of nanometers are formed and supported on surfaces of the POT shells. However, when the concentration of Fe ions is too high, it can see that Fe3O4 nanoparticles aggregations that apart from POT hollow microspheres are mostly seen (Fig. 5aed). 3.3. Formation mechanism of POT/Fe3O4 nanoparticle composites According to the above-mentioned results, a possible formation mechanism is then proposed as shown in Scheme 1. When the precipitant of NH4OH (aq) is added to Fe salts and POT mixture solution, Fe3O4 crystal nucleus are formed. The originally formed Fe3O4 crystal nucleus will locate on the surface of POT hollow microspheres because of their complexation interaction. The morphology of product is evidently related to the concentration of Fe ions. At high concentration of Fe ions, a large amount of Fe3O4 crystal nucleus are formed both on the surface of POT shells and in solution. With the coprecipitation proceeding, the Fe3O4 crystal nucleus will grow and aggregate together. At last, a lots of large size Fe3O4 nanoparticles apart from POT hollow microspheres are formed. When the concentration of Fe ions is relatively low, only a
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Fig. 6. SEM images of the POT/Fe3O4 nanoparticle composite. Synthetic conditions: (a, b) POT-CA ¼ 0.1 g, [FeCl2$4H2O] ¼ 17 mM, [FeCl3$6H2O] ¼ 34 mM. (c, d) POT-CA ¼ 0.1 g, [FeCl2$4H2O] ¼ 8.5 mM, [ FeCl3$6H2O] ¼ 17 mM.
Fig. 7. SEM images of (a, b) CA-doped POT/Fe3O4 and (c, d) H2SO4-doped POT/Fe3O4 composites.
Fig. 8. TEM images of (a) CA-doped POT/Fe3O4, (b) H2SO4-doped POT/Fe3O4 and (c) CA-redoped POT/Fe3O4 composites.
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happened around the surfaces of the POT hollow microspheres where Fe3O4 crystal nucleus mainly deposited on, which will lead to the formation of POT/Fe3O4 nanoparticle composites with Fe3O4 nanoparticles relatively homogeneous supported on the surfaces of the POT hollow microspheres. If the concentration of Fe ions is extremely low, almost all of Fe ions attach on the surface of the CAdoped POT hollow microspheres for complexation and Fe3O4 crystal nucleus are formed on the surface of POT shells. With the coprecipitation proceeding, the Fe3O4 nanoparticles are formed and decorated on the surfaces of POT shells.
Scheme 1. Schematic illustration for the formation of the hollow structured POT/Fe3O4 nanoparticle composites.
small portion of Fe3O4 nanoparticles apart from POT hollow microspheres because of the low concentration of Fe3O4 crystal nucleus. The growth and aggregation processes are more likely
3.4. Synthesis and characterization of POT/Fe3O4/Au nanoparticle composites It is well known that the emeraldine base form of PANI and its derivatives have good reduction properties [36]. Several groups have discovered that the emeraldine base form of conductive polymer can reduce noble metal ions into metallic states, whereas the emeraldine base form of polymer is oxidized into pernigraniline
Fig. 9. TEM images of POT/Fe3O4/Au composites synthesized at different concentration of HAuCl4: (a, b) 2 103 M; (c, d) 8 103 M.
Fig. 10. (a) Successive UV-vis adsorption spectra of the reduction of 4-NP by NaBH4 in the presence of POT/Fe3O4/Au catalysts (gold size: 8 nm). (b) Plot of ln(A0/At) of 4-NP against time.
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Table 1 Comparison of the catalytic activity for the reduction of 4-NP between POT/Fe3O4/Au and some catalysts reported in the literature. Samples
Catalyst used (mg)
NMNPs loading (wt%)
Kapp (s1)
KM (s1g1)
Au/graphene43 Carbon@Au44 This work
0.1 0.1 0.09
24 22.68 8.4
3.17 103 5.42 103 2.28 103
132.08 238.98 301.6
[19,37e40]. Herein, The POT/Fe3O4/Au nanoparticle composites were synthesized according to the literature [18]. The concentration of HAuCl4 was found to have significant influence on the morphology of POT/Fe3O4/Au nanoparticle composites. As compared with Fig. 9, when the concentration of HAuCl4 is 2 mM, Au nanoparticles with an average size of 8 nm supported on surfaces of POT are seen (Fig. 9a and b). If the HAuCl4 concentration increases to 8 mM, gold nanoparticles are evenly decorated on polymer shells and the mean size of the gold nanoparticles is 15 nm (Fig. 9c). A HRTEM micrograph of the edge region of the POT/Fe3O4/ Au nanoparticle composite is presented in Fig. 9d, where the crystal lattice of Au nanoparticles, Fe3O4 nanoparticles, as well as amorphous POT, can be resolved. The measured lattice-fringe spacing of 0.23 nm in these Au nanoparticles corresponds to the (111) crystal facet (Fig. 9d) [41]. The lattice-fringe spacings of the Fe3O4 nanoparticles are measured (0.25, 0.29 nm) which corresponds to crystal facets (311) and (220) of cubic Fe3O4 (Fig. 9d), respectively [32]. In addition, two white arrows are added in Fig. 8d to distinguish the amorphous POT from the composite more clearly.
3.5. Catalytic activities of POT/Fe3O4/Au nanoparticle composites for the NaBH4 reduction of 4NP As show in Fig. 10, the as-prepared POT/Fe3O4/Au was added into 4-NP and NaBH4 aqueous solution to initiate the conversion of 4-NP to 4-AP reaction. The peak at 400 nm (corresponding to the 4NP) is disappeared and accompanied by a gradual development of a new peak at 300 nm (corresponding to the formation of 4-AP) [9,17,18]. The whole catalytic reaction was completely finished within 15 min (Fig. 9a), and the color change from bright yellow to colourless was observed. As shown in Fig. 10a, the tads (a certain period of time about the 4-NP to adsorb onto the catalyst's surfaces before reduction could be initiated [42]) is about 3 min for POT/ Fe3O4/Au catalysts indicating very fast adsorption process. After completion of the first cycle catalytic reaction, the POT/Fe3O4/Au catalysts are recovered by simple magnetic separation and then washed with water and reused in the next cycle. The kinetic reaction rate constant (defined as Kapp) is estimated from the linear relationship, which is 2.28 103 s1 and 0.4 103 s1 for first cycle and fifth cycle (Fig. 10b), respectively [9,18]. The decreased catalysts activity with increasing cycles may result from loss of catalysts during magnetic separation and purification processes of catalysts [18,40]. To further compare the catalytic activity of POT/ Fe3O4/Au with other reported catalysts, the rate constant per gram Kapp active sites was then calculated according to KM ¼ M , where M is the weight of active sites used. As show in Table 1, the KM values for the reported Au/graphene catalyst and Carbon@Au catalyst were 132.08 and 238.98 s1g1, respectively [43,44]. While, the asprepared POT/Fe3O4/Au catalyst with KM of 301.6 s1g1 exhibits better catalytic activity, which is mainly attributed to the low aggregation of the Au nanoparticles on the POT/Fe3O4 support. It is seen that this developed synthesis strategy useful for the design of magnetic catalyst support and other potential applications in field of reusable catalysis.
4. Conclusions POT/Fe3O4 nanoparticle composites have been fabricated as novel and magnetic reactive catalyst supports. The size of the Fe3O4 nanoparticles can be tuned by controlling the concentrations of Fe ions. TEM, SEM, XRD, FTIR, UV-Vis and VSM results revealed the chemical structures, superparamagnetic properties and formation mechanisms of the POT/Fe3O4 nanoparticle composites. Utilizing the redox activity of POT towards Au ions, POT/Fe3O4/Au nanoparticle composites can be fabricated as highly efficient and reusable noble metal catalysts. In addition, the POT/Fe3O4/Au nanoparticle composites showed good catalytic performances and simple recyclability in a model liquid phase reaction. It is believed that as-prepared POT/Fe3O4 nanoparticle composites have potential applications in magnetic catalyst support, biosensors and adsorption/separation fields. Acknowledgements This work was financed by Anhui Provincial Natural Science Foundation (1508085QE110) and Youth Innovation Promotion Association, CAS (2015268). References [1] P.M. Arnal, M. Comotti, F. Schüth, High-temperature-stable catalysts by hollow sphere encapsulation, Angew. Chem. Int. Ed. 45 (2006) 8224e8227. [2] L.C. Kennedy, L.R. Bickford, N.A. Lewinski, A.J. Coughlin, Y. Hu, E.S. Day, J.L. West, R.A. Drezek, A new era for cancer treatment: gold-nanoparticlemediated thermal therapies, Small 7 (2011) 169e183. [3] C. Li, W. Cai, B. Cao, F. Sun, Y. Li, C. Kan, L. Zhang, Mass synthesis of large, single-crystal Au nanosheets based on a polyol process, Adv. Funct. Mater 16 (2006) 83e90. [4] C.-H. Cui, H.-H. Li, S.-H. Yu, A general approach to electrochemical deposition of high quality free-standing noble metal (Pd, Pt, Au, Ag) sub-micron tubes composed of nanoparticles in polar aprotic solvent, Chem. Comm. 46 (2010) 940e942. [5] Q. Yue, Y. Zhang, C. Wang, X. Wang, Z. Sun, X.-F. Hou, D. Zhao, Y. Deng, Magnetic yolkeshell mesoporous silica microspheres with supported Au nanoparticles as recyclable high-performance nanocatalysts, J. Mater. Chem. A 3 (2015) 4586e4594. [6] L. Mandeltort, D.L. Chen, W.A. Saidi, J.K. Johnson, M.W. Cole, J.T. Yates Jr., Experimental and theoretical comparison of gas desorption energies on metallic and semiconducting single-walled carbon nanotubes, J. Am. Chem. Soc. 135 (2013) 7768e7776. [7] R. Liu, A. Sen, Magnetic yolk-shell mesoporous silica controlled synthesis of heterogeneous metal-Titania nanostructures and their applications, J. Am. Chem. Soc. 134 (2012) 17505e17512. [8] X.-K. Kong, Q.-W. Chen, Z.-Y. Sun, Enhanced SERS of the complex substrate using Au supported on graphene with pyridine and R6G as the probe molecules, Chem. Phys. Lett. 564 (2013) 54e59. [9] J. Han, S. Lu, C. Jin, M. Wang, R. Guo, Fe3O4/PANI/m-SiO2 as robust reactive catalyst supports for noble metal nanoparticles with improved stability and recyclability, J. Mater. Chem. A 2 (2014) 13016e13023. [10] I. Washio, Y. Xiong, Y. Yin, Y. Xia, Reduction by the end groups of poly(vinyl pyrrolidone): a new and versatile route to the kinetically controlled synthesis of Ag triangular nanoplates, Adv. Mater 18 (2006) 1745e1749. [11] X. Xia, J. Zeng, L.K. Oetjen, Q. Li, Y. Xia, Quantitative analysis of the role played by poly(vinylpyrrolidone) in seed-mediated growth of Ag nanocrystals, J. Am. Chem. Soc. 134 (2012) 1793e1801. [12] H. Sun, J. He, J. Wang, S.Y. Zhang, C. Liu, T. Sritharan, S. Mhaisalkar, M.Y. Han, D. Wang, H. Chen, Investigating the multiple roles of polyvinylpyrrolidone for a general methodology of oxide encapsulation, J. Am. Chem. Soc. 135 (2013) 9099e9110.
J. Sun, L. Chen / Materials Today Chemistry 5 (2017) 43e51 [13] H. Miyamura, R. Matsubara, Y. Miyazaki, S. Kobayashi, Aerobic oxidation of alcohols at room temperature and atmospheric conditions catalyzed by reusable gold nanoclusters stabilized by the benzene rings of polystyrene derivatives, Angew. Chem. Int. Ed. 46 (2007) 4151e4154. [14] C.J. Murphy, C.J. Orendorff, Alignment of gold nanorods in polymer composites and on polymer surfaces, Adv. Mater 17 (2005) 2173e2177. [15] H. Wu, R. Hong, X. Wang, R. Aroviz, C. You, B. Samarrta, D. Patra, M.T. Tuominen, V.M. Rotello, Controlled formation of patterned gold films via site-selective deposition of nanoparticles onto polymer-templated surfaces, Adv. Mater 19 (2007) 1383e1386. [16] S. Abraham, I. Kim, C.A. Batt, A facile preparative method for aggregation-free gold nanoparticles using poly(styrene-block-cysteine), Angew. Chem. Int. Ed. 46 (2007) 5720e5723. [17] J. Han, R. Chen, M. Wang, S. Lu, R. Guo, Core-shell to yolk-shell nanostructure transformation by a novel sacrificial template-free strategy, Chem. Commun. 49 (2013) 11566e11568. [18] J. Han, L. Li, R. Guo, Novel approach to controllable synthesis of gold nanoparticles supported on polyaniline nanofibers, Macromolecules 43 (2010) 10636e10644. [19] J. Han, Y. Liu, R. Guo, Reactive template method to synthesize gold nanoparticles with controllable size and morphology supported on shells of polymer hollow microspheres and their application for aerobic alcohol oxidation in water, Adv. Funct. Mater 19 (2009) 1112e1117. [20] R.J. Tseng, J. Huang, J. Ouyang, R.B. Kaner, Y. Yang, Polyaniline nanofiber/gold nanoparticle nonvolatile memory, Nano Lett. 5 (2005) 1077e1080. [21] B.J. Gallon, R.W. Kojima, R.B. Kaner, P.L. Diaconescu, Palladium nanoparticles supported on polyaniline nanofibers as a semi-heterogeneous catalyst in water, Angew. Chem. Int. Ed. 46 (2007) 7251e7254. [22] Z. Dong, G. Yu, X. Le, Gold nanoparticle modified magnetic fibrous silica microspheres as a highly efficient and recyclable catalyst for the reduction of 4nitrophenol, New J. Chem. 39 (2015) 8623e8629. [23] M. Rocha, C. Fernandes, C. Pereira, S.L.H. Rebelo, M.F.R. Pereira, C. Freire, Goldsupported magnetically recyclable nanocatalysts: a sustainable solution for the reduction of 4-nitrophenol in water, RSC Adv. 5 (2014) 5131e5141. [24] J. Yoo, N. Park, J.H. Park, J.H. Park, S. Kang, S.M. Lee, H.J. Kim, H. Jo, J.-G. Park, S.U. Son, Magnetically separable microporous Feeporphyrin networks for catalytic carbene insertion into NeH bonds, ACS Catal. 5 (2015) 350e355. [25] B. Karimi, F. Mansouri, H. Vali, A highly water-dispersible/magnetically separable palladium catalyst based on a Fe3O4@SiO2 anchored TEGimidazolium ionic liquid for the SuzukieMiyaura coupling reaction in water, Green Chem. 16 (2014) 2587e2596. [26] J. Han, G. Song, R. Guo, A facile solution route for polymeric hollow spheres with controllable size, Adv. Mater 18 (2006) 3140e3144. [27] X. Zheng, J. Feng, Y. Zong, H. Miao, X. Hu, J. Bai, X. Li, Hydrophobic graphene nanosheets decorated by monodispersed superparamagnetic Fe3O4 nanocrystals as synergistic electromagnetic wave absorbers, J. Mater. Chem. C 3 (2015) 4452e4463. [28] H. Niu, Q. Wang, H. Liang, M. Chen, C. Mao, J. Song, S. Zhang, Y. Gao, C. Chen, Visible-light active and magnetically recyclable nanocomposites for the degradation of organic dye, Materials 7 (2014) 4034e4044.
51
[29] J. Han, Y. Liu, R. Guo, A simple one-step chemical route to gold/polymer core/ shell composites and polymer hollow spheres, J. Appl. Polym. Sci. 112 (2009) 1244e1249. [30] J. Han, Y. Liu, R. Guo, A novel template less method to nanofibers of polyaniline derivatives with size control, J. Polym. Sci. Part A Polym. Chem. 46 (2008) 740e746. [31] H.H. Shih, D. Williams, N.H. Mack, H.L. Wang, Conducting polymer-based electrodeless deposition of Pt nanoparticles and its catalytic properties for regioselective hydrosilylation reactions, Macromolecules 42 (2009) 14e16. [32] L.-L. Yu, H. Bi, Facile synthesis and magnetic property of iron oxide/MCM-41 mesoporous silica nanospheres for targeted drug delivery, J. Appl. Phys. 111 (2012), 07B514e07B514e3. [33] J. Stejskal, P. Kratochvil, N. Radhakrishnan, Polyaniline dispersions-UV-vis absorption spectra, Synth. Met. 61 (1993) 225e231. [34] Y. Deng, D. Qi, C. Deng, X. Zhang, D. Zhao, Superparamagnetic highmagnetization microspheres with an Fe3O4@SiO2 core and perpendicularly aligned mesoporous SiO2 shell for removal of microcystins, J. Am. Chem. Soc. 130 (2008) 28e29. [35] J.-F. Lin, C.-C. Tsai, M.-Z. Lee, Linear birefringence and dichroism in citric acid coated Fe3O4 magnetic nanoparticles, J. Magn. Magn. Mater 372 (2014) 147e158. [36] E.T. Kang, K.G. Neoh, K.L. Tan, Polyaniline: a polymer with many interesting intrinsic redox states, Prog. Polym. Sci. 23 (1998) 277e324. [37] J. Han, L. Wang, R. Guo, Reactive polyaniline-supported sub-10 nm noble metal nanoparticles protected by a mesoporous silica shell: controllable synthesis and application as efficient recyclable catalysts, J. Mater. Chem. 22 (2012) 5932e5935. [38] S. Guo, S. Dong, E. Wang, Polyaniline/Pt hybrid nanofibers: high-efficiency nanoelectrocatalysts for electrochemical devices, Small 5 (2009) 1869e1876. [39] B.J. Gallon, R.W. Kojima, R.B. Kaner, P.L. Diaconescu, Palladium nanoparticles supported on polyaniline nanofibers as a semi-heterogeneous catalyst in water, Angew. Chem. Int. Ed. 46 (2007) 7251e7254. [40] J. Han, Y. Liu, L. Li, R. Guo, Poly(o-phenylenediamine) submicrospheresupported gold nanocatalysts: synthesis, characterization, and application in selective oxidation of benzyl alcohol, Langmuir 25 (2009) 11054e11060. [41] T. Zhang, W. Wang, D. Zhang, X. Zhang, Y. Ma, Y. Zhou, L. Qi, Biotemplated synthesis of gold nanoparticleebacteria cellulose nanofiber nanocomposites and their application in biosensing, Adv. Funct. Mater 20 (2010) 1152e1160. [42] J. Zeng, Q. Zhang, J. Chen, Y. Xia, A comparison study of the catalytic properties of Au-based nanocages, nanoboxes, and nanoparticles, Nano Lett. 10 (2010) 30e35. [43] J. Li, C.Y. Liu, Y. Liu, Au/graphene hydrogel: synthesis, characterization and its use for catalytic reduction of 4-nitropheno, J. Mater. Chem. 22 (2012) 8426e8430. [44] P. Zhang, C.L. Shao, X.H. Li, M.Y. Zhang, X. Zhang, C.Y. Su, N. Lu, K.X. Wang, Y.C. Liu, An electron-rich free-standing carbon@ Au coreeshell nanofiber network as a highly active and recyclable catalyst for the reduction of 4nitrophenol, Phys. Chem. Chem. Phys. 15 (2013) 10453e10458.
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Materials Today Chemistry journal homepage: www.journals.elsevier.com/materials-today-chemistry/
Superparamagnetic POT/Fe3O4 nanoparticle composites with supported Au nanoparticles as recyclable high-performance nanocatalysts Jun Sun*, Lin Chen Institute of Applied Technology, Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei 230031, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 March 2017 Received in revised form 13 June 2017 Accepted 13 June 2017
A novel method has been developed to successfully synthesize Fe3O4 nanoparticles with tunable size and morphology supported on shells of poly(o-Toluidine)(POT) hollow microspheres. The as-prepared POT/ Fe3O4 nanoparticle composites can be used as novel and magnetic-responsive catalyst supports to produce highly efficient and recyclable noble metal catalysts. The size of Fe3O4 nanoparticles supported on shells of POT hollow microspheres can be tuned from 4 to 12 nm by changing the concentration of Fe ions. The roles of the doping acid of POT and Zeta potentials of Fe3O4 nanoparticles and POT in the formation of the POT/Fe3O4 nanoparticle composites were discussed. Furthermore, gold nanoparticles that were supported on the as-synthesized POT/Fe3O4 nanoparticle composites have been achieved by utilizing the reactivity of POT towards Au ions. The size of gold nanoparticles can be tuned by altering the concentration of HAuCl4. Finally, the catalytic activity of the obtained POT/Fe3O4/Au composites for 4nitrophenol (4NP) reduction is investigated. The results demonstrate that such magnetic-responsive polymer-supported gold nanoparticles can be easily recovered and reused five times still remains high catalytic performance, which indicate their potential applications in the field of catalysis. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Polyaniline Fe3O4 nanoparticles Au nanoparticles Nanocatalyst
1. Introduction Gold nanoparticles with high surface areas have been extensively studied for their unique optical and electronic properties together with their various applications in fields of electronics, photonics, catalysts, biotechnology [1e4]. To date, various methods have been developed to prepare gold nanoparticles-based catalysts with controllable size. However, gold nanoparticles are unstable and tend to aggregate and therefore lose their initial catalytic activity and selectivity because of their high surface energy [5]. To solve this problem, gold nanoparticles have been supported onto various supporting materials to generate hybrid catalysts to protect against the aggregation of gold nanoparticles. Typically, the supports for gold nanoparticles are base on activated carbon or metal oxides [6e8]. However, in recent years, the polymer based supports have been attracted more and more attentions because of their merits of mild and simplified synthetic conditions and higher catalytic performance [9].
* Corresponding author. E-mail address: [email protected] (J. Sun). http://dx.doi.org/10.1016/j.mtchem.2017.06.001 2468-5194/© 2017 Elsevier Ltd. All rights reserved.
Up to now, many different polymers have been utilized to stabilize and support gold nanoparticles such as poly(vinylpyrrolidone)(PVP) [10e12], polystyrene derivatives [13], and others [14e16]. Among these polymers, conducting polymers of polyaniline(PANI) and its derivatives [17e19], in which benzene rings and amine groups coexist in polymer chains can bind to certain metal atoms on the surface of nanoparticles, have also been developed to act as superior supports for gold nanoparticles. Moreover, it has been reported that dedoped PANI and its derivatives can also be used as reductant for the generation of noble metal nanoparticles from metal salts [20,21]. Thus, PANI and its derivatives can act as both reductant and stabilizer for polymersupported gold nanoparticles, which will facilitate the synthetic procedures for polymer-supported gold nanoparticles within one step. Until now, PANI nanofibers [18], PANI powders and poly(ophenylenediamine)(PoPD) have been reported to use as reactive supports for the preparation of polymer-supported gold nanoparticles [19]. As known to all, the isolation and recovery of gold nanoparticles-based catalysts from the catalytic reaction medium with high efficiency is also worth of consideration due to the scarcity of noble metal resources [22]. However, the isolation and
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recovery efficiency of polymer-supported gold nanoparticles is very low for only centrifugation or filtration can be used. In this regard, magnetic-responsive nanomaterials have emerged as ideal catalyst supports since they can be isolated and recovered by magnetic separation, which is more efficient and rapid than centrifugation or filtration, which prevents mass losses of catalyst or the use of additional solvents [23e25]. However, the synthesis of magneticresponsive polymer-supported gold nanoparticles with controllable size, improved stability and catalytic activity remains a great challenge. Herein, we report that well-defined Fe3O4 nanoparticles with tunable size supported on shells of hollow poly(o-Toluidine)(POT) microspheres, a derivative of PANI, can be successfully synthesized as novel and magnetic-responsive catalyst supports to produce highly efficient and recyclable noble metal catalysts. By altering the concentration of Fe ions, the size of Fe3O4 nanoparticles supported on shells of POT hollow microspheres can be tuned from 4 to 12 nm. It is found that doping acid of POT and Zeta potentials of Fe3O4 nanoparticles and POT show strong affinity toward Fe3O4 nanoparticles into POT hollow microspheres. Formation mechanisms involved have been recommended. Finally, using the redox activity between POT and Au ions, the POT/Fe3O4/Au nanoparticle composites can be fabricated after the addition of Au ions into the POT/Fe3O4 reactive supports, and their catalytic activities were examined by choosing the model catalysis reaction involving the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) in the presence of NaBH4. Superior catalytic performance with high stability and recyclability were revealed, which indicates their potential applications in the field of catalysis.
magnet and washed with distilled water for several times and then dried at 80 C in a vacuum oven. Furthermore, pure Fe3O4 nanoparticles are prepared without POT through similar conditions. 2.4. Synthesis of POT/Fe3O4/Au nanoparticle compositess The POT/Fe3O4/Au nanoparticle composites were synthesized according to the literature [18]. Typically, 3 mg POT/Fe3O4 composite and 20 mg PVP (K-30) were added to 10 mL of deionized water with shaking at room temperature for 30 min. Then, a quantitative amount of HAuCl4 aqueous solution was added (maintain the total volume is 20 mL, the concentration of HAuCl4 is 2 103 M and 8 103 M, respectively.) and the system was shaking at room temperature for 12 h. Finally, the resultant products were separated and collected with a magnet. After washing with deionized water to remove any remaining HAuCl4. The particles were finally dispersed in 10 mL deionized water for catalysis. 2.5. catalytic experiments The reduction of 4-NP by NaBH4 was chosen as a model reaction for testing the efficiency of the POT/Fe3O4/Au nanoparticle composites according to the literature [9,17,18]. Typically, NaBH4 (1.7 mL, 0.015 M) and 4 NP aqueous solution (1.0 mL, 0.2 mM) were mixed in the quartz cell (1 cm path length), Then, 0.3 mL POT/ Fe3O4/Au colloidal solution (0.3 g/L) was added in above solution to initiate the conversion of 4 NP to 4-aminophenol (4 AP) reaction. The whole reaction progress was monitored by the UV-vis spectroscopy at ambient temperature.
2. Experimental section
2.6. Characterization
2.1. Materials
The morphologies of products were examined by field-emission scanning electron microscopy (FESEM) (S-4800, Hitachi Co., Japan) and transmission electron microscopy (TEM) (JEM-2100, JEOL, Japan). The FT-IR spectra (NEXUS-870, Nicolet Instrument Co., USA) were recorded in the range of 400e4000 cm1. The UV-vis spectra (U-4100, Hitachi Co., Japan) of samples were measured in the range between 200 and 900 nm. X-ray diffraction (XRD) patterns were measured on a XD-3 (with CuKa radiation) diffraction meter. The magnetic property of as-prepared POT/Fe3O4 composite was measured by using a vibrating sample magnetometer (VSM, Riken Denshi Co. Ltd., Japan). The Zeta potentials of samples were recorded on Malvern Zetasizer Nano ZS90. The Au contents in the asprepared samples were measured using inductively coupled plasma-atomic emission spectrometry (ICAP, 7400 Duo).
O-Toluidine (Shanghai Chemical Co., Shanghai, China) was distilled under reduced pressure before use. HAuCl4-3H2O (Shanghai Chemical Co.) and other reagents were used as received. 2.2. Synthesis of POT hollow microspheres The POT hollow microspheres were synthesized according to the literature [26]. O-Toluidine (0.257 g) and citric acid (0.084 g) and 50 mL deionized water were mixed together with magnetic stirring at room temperature for 10 min. After that, the mixture was maintained at 0e10 C for 0.5 h before oxidative polymerization. Then 50 mL ammonium persulfate (APS, 0.547 g) aqueous solution was added to the above mixture in one portion. And then the above mixture solution was maintained at 0e10 C without agitation for 12 h. Finally, the product was dedoped with 0.1 M NH4OH (aq) and washed with deionized water for several times, and then dried in a vacuum at 80 C for 24 h before use. H2SO4-doped POT hollow microspheres are prepared by using H2SO4 as doping acid through similar conditions. 2.3. Synthesis of POT/Fe3O4 nanoparticle composites In a typical synthesis, 0.0135 g FeCl2$4H2O, 0.0367 g FeCl3$6H2O and 0.1 g of POT were dispersed in 40 mL of deionized water by using ultrasonication for 15 min. The obtained solution was transferred to a flask and then heated to 80 C in an oil bath under the protection of nitrogen gas atmosphere. As the temperature raised to 80 C, 5 mL of NH4OH (aq) was added to the above solution in one portion. The reaction was allowed to proceed under stirring for 30 min at 80 C. And then the resulting solution was heated to 95 C for another 90 min. Finally, the product was collected with a
3. Results and discussion 3.1. Synthesis and characterization of POT/Fe3O4 nanoparticle composites The reactive template POT hollow microspheres are synthesized by the monomer-droplet strategy without the aid of surfactant according to the literature [26]. Typical scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of POT hollow microspheres are shown in Fig. 1. The POT hollow microstructures are constructed with several POT hollow microspheres and range in size of from 200 nm to several micrometers, with the shell thickness is about 20 nm. As shown in Fig. 1b, there is a hole in the surface of the POT hollow microsphere concluded that the POT microsphere is hollow. As the POT hollow microspheres are in their doping state, dedoping process of POT hollow microspheres with 0.1 M NH4OH (aq) is applied. Then the as-prepared POT hollow microspheres are redispersed in deionized water, and a certain
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Fig. 1. (a, b) SEM and (c, d) TEM images of the POT hollow microspheres.
amount of FeCl2$4H2O and FeCl3$6H2O are added into aqueous suspension of POT. The obtained solution is heated to 80 C under the protection of nitrogen gas atmosphere. And then NH4OH (aq) is added into the above solution to initial precipitation reaction. Finally, Fe3O4 nanoparticles supported on POT hollow microspheres can be formed. As shown in Fig. 2, Fe3O4 nanoparticles with size of ~8 nm are evenly deposited on surfaces of POT hollow microspheres. The selected area electron diffraction (SAED) pattern of POT/Fe3O4 nanoparticle composites is also shown in Fig. 2c, which displays sharp rings indexed to the (440), (511), (220), (311), and (400) planes of a spinel (face centered cubic) Fe3O4 [27]. POT/Fe3O4 nanoparticle composites are further characterized by XRD, FTIR, and UV-vis spectroscopies. As shown in Fig. 3a, eight obvious main peaks indexed to the (111), (220), (311) (222), (400), (422), (511), and (440) planes of a spinel (face centered cubic) Fe3O4, indicating the presence of Fe3O4 nanoparticles in products [28]. As displayed in Fig. 3, the characteristics for CA-doped POT, such as N-H stretching vibrations at 3200-3500 cm1 and C¼C stretching deformation of quinonoid and benzenoid rings at around 1597 and 1498 cm1, respectively, are clearly seen [29]. The ratio of the relative intensity of the two peaks is about 1.0, which indicates that similar nitrogen quinonoid and benzenoid ring structures exist in the POT chains [30]. The band at 1382, 1312, and 1240 cm1, attributed to C-N stretching vibrations in the neighborhood of an
aromatic ring, is characteristic of doped POT. As for POT/Fe3O4 nanoparticle composites, it is revealed that the relative intensity of C¼C stretching vibrations of benzenoid (1498 cm1) to quinonoid (1597 cm1) rings increases significantly after loading Fe3O4 nanoparticles, indicating the dedoped state of POT hollow microspheres [31]. In addition, there is a strong absorption appearing at wavenumbers 574 cm1 can be ascribed to the characteristic Fe-O stretching vibrations in iron oxides [32]. UV-vis spectra of CAdoped POT, dedoped POT, and POT/Fe3O4 nanoparticle composites are shown in Fig. 3. CA-doped POT shows three characteristic absorption bands at about 300e400, 550e650, and ~800 nm wavelengths. The first absorption band arises from p-p* electron transition within benzenoid segments. The second and the third absorption bands are related to doping level and formation of polaron, respectively [29]. After dedoped by using NH4OH (aq), the dedoped POT microspheres show only two adsorption peaks centered at 335 and 606 nm, respectively [33]. As for the UV-vis spectrum of POT/Fe3O4 nanoparticle composites, only two adsorption peaks centered at 365 and 577 nm can be seen, indicating the dedoped state of POT hollow microspheres in the composites. To demonstrate the magnetic property of the prepared POT/ Fe3O4 nanoparticle composites, two vials were loaded with POT hollow microspheres, 150 mg each dispersed in 5.0 mL aqueous
Fig. 2. (a, b) SEM and TEM images of the hollow structured POT/Fe3O4 nanoparticles composites. (c) SAED pattern of the POT/Fe3O4 nanoparticles composites.
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Fig. 4. Magnetization hysteresis at 300 K for the three kinds of hollow structured POT/ Fe3O4 composites synthesized at different Fe ions concentrations. Synthetic conditions: (a) POT ¼ 0.1 g, [FeCl2$4H2O] ¼ 0.85 mM, [FeCl3$6H2O] ¼ 1.7 mM; (b) POT ¼ 0.1 g, [FeCl2$4H2O] ¼ 1.7 mM, [FeCl3$6H2O] ¼ 3.4 mM; (c) POT ¼ 0.1 g, [FeCl2$4H2O] ¼ 8.5 mM, [FeCl3$6H2O] ¼ 17 mM.
after slightly shaking, which proves an effective and reversible response to the magnetic field. Moreover, Fig. 4 further gives the magnetic hysteresis loops of POT/Fe3O4 nanoparticle composites synthesized at different Fe ions concentrations, respectively. The magnetization of POT/Fe3O4 nanoparticle composites are all increased with rise of applied magnetic field. The saturated magnetizations (Ms) of the POT/Fe3O4 nanoparticle composites are 5.7, 15.1, and 29.3 emu/g, respectively. The curves show extremely low remnant magnetization (Mr) and coercivity (Hc), indicating that all of the samples are superparamagnetic at room temperature [34]. Own to the superparamagnetic properties and high saturation magnetization, the as-prepared POT/Fe3O4 nanoparticle composites can be easily separated from the solution by means of an external magnet, which is much simpler than using centrifugation of non-magnetic materials. 3.2. Effect of concentration and doping acid
Fig. 3. (a) XRD pattern of POT/Fe3O4 nanoparticle composites. (b) FTIR spectra of the CA-doped POT, dedoped POT and POT/Fe3O4 nanoparticle composites. (c) UV-vis spectra of the CA-doped POT, dedoped POT and POT/Fe3O4 nanoparticle composites.
solution. The magnetic properties were tested using an external magnet. As illustrated in Fig. 4, in the absence of a magnetic field, POT/Fe3O4 nanoparticle composites were homogeneously dispersed in the aqueous solution. After applying a small Nd-Fe-B magnet to the solution, most POT/Fe3O4 nanoparticle composites moved to one side of the vial closer to the head of the permanent magnet within 3 min. Once removing the magnet, all of the POT/ Fe3O4 nanoparticle composites returned to a dispersed state again
The Fe ions (Fe2þ and Fe3þ) concentration can be used for tuning the size of the Fe3O4 nanoparticles on the POT shells (Fig. 5). When the concentrations of Fe2þ and Fe3þ are maintained at 17 and 34 mM, Fe3O4 nanoparticles are fully supported on surfaces of the shells. The Fe3O4 nanoparticle size is estimated to be 14 nm (Fig. 5a and b and Fig. 6a and b). However, partial aggregation of scattered Fe3O4 nanoparticles (unsupported on POT shells) is obviously seen with size is about 22e35 nm (Fig. 6a and b). These scattered Fe3O4 nanoparticles may attribute to too much Fe ions. If the concentrations of Fe2þ and Fe3þ decrease to 8.5 and 17 mM, Fe3O4 nanoparticles are evenly decorated on POT shells and the mean size of the Fe3O4 nanoparticles is 10 nm (Fig. 5c and d and Fig. 6c and d). Moreover, the aggregation phenomenon of scattered Fe3O4 nanoparticles is seldom seen and the size of scattered Fe3O4 nanoparticles decreases to only about 14e25 nm (Fig. 6c and d). As compared with Fig. 5e and f, if the concentrations of Fe2þ and Fe3þ decrease to 1.7 and 3.4 mM, aggregation-free Fe3O4 nanoparticles with an average size of 6 nm are evenly deposited on POT shells. As the concentrations of Fe2þ and Fe3þ decrease further to 0.85 and 1.7 mM, the average size of the Fe3O4 nanoparticles can reach to 4 nm (Fig. 5g and h). From the above analysis we can conclude that a low concentration of Fe ions is necessary for the formation of
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Fig. 5. TEM images of the POT/Fe3O4 nanoparticle composites synthesized at different Fe ions concentrations. Synthetic conditions: (a, b) POT ¼ 0.1 g, [FeCl2$4H2O] ¼ 17 mM, [FeCl3$6H2O] ¼ 34 mM. (c, d) POT ¼ 0.1 g, [FeCl2$4H2O] ¼ 8.5 mM, [FeCl3$6H2O] ¼ 17 mM. (e, f) POT ¼ 0.1 g, [FeCl2$4H2O] ¼ 1.7 mM, [FeCl3$6H2O] ¼ 3.4 mM (g, h) POT ¼ 0.1 g, [FeCl2$4H2O] ¼ 0.85 mM, [FeCl3$6H2O] ¼ 1.7 mM. Histogram is Fe3O4 particle size distribution.
evenly deposited, aggregation-free Fe3O4 nanoparticles on shells of POT hollow microspheres. Doping acid is another important influencing factor on the morphology of POT/Fe3O4 nanoparticle composites. Morphology of products using H2SO4-doped POT hollow microspheres instead of CA-doped POT hollow microspheres is examined. In comparison with the case of CA-doped POT hollow microspheres (Fig. 7a and b), Fe3O4 nanoparticles apart from POT hollow microspheres can be seen (Fig. 7c and d); however, density of Fe3O4 nanoparticles deposited on surfaces of POT hollow microspheres decreased significantly (Fig. 7c and d and Fig. 8a and b). When the H2SO4doped POT hollow microspheres were dedoped with 0.1 M NH4OH (aq), and then immersed in 0.1 M CA (aq) to obtain CA-redoped POT hollow microspheres for synthesizing POT/Fe3O4 nanoparticle composites, Fe3O4 nanoparticles with size about 10 nm evenly deposited on surfaces of POT hollow microspheres (Fig. 8c). It can be concluded that doping acid of CA that shows strong affinity toward Fe3O4 nanoparticles introduced to POT hollow microspheres will effectively prevent aggregation of Fe3O4 nanoparticles which leads to formation of uniform composites with Fe3O4 nanoparticles evenly deposited on surface of POT hollow microspheres. This can be due to there are three carboxy groups within CA, as CA doped in POT hollow microspheres can improve Fe2þ and Fe3þ ions attached on the surface of the POT hollow microspheres for complexation
[35]. And then amount of Fe3O4 crystal nucleus are formed on the surface of the POT hollow microspheres during early stage of coprecipitation reaction. With the growth of the Fe3O4 crystal nucleus, Fe3O4 nanoparticles in tens of nanometers are formed and supported on surfaces of the POT shells. However, when the concentration of Fe ions is too high, it can see that Fe3O4 nanoparticles aggregations that apart from POT hollow microspheres are mostly seen (Fig. 5aed). 3.3. Formation mechanism of POT/Fe3O4 nanoparticle composites According to the above-mentioned results, a possible formation mechanism is then proposed as shown in Scheme 1. When the precipitant of NH4OH (aq) is added to Fe salts and POT mixture solution, Fe3O4 crystal nucleus are formed. The originally formed Fe3O4 crystal nucleus will locate on the surface of POT hollow microspheres because of their complexation interaction. The morphology of product is evidently related to the concentration of Fe ions. At high concentration of Fe ions, a large amount of Fe3O4 crystal nucleus are formed both on the surface of POT shells and in solution. With the coprecipitation proceeding, the Fe3O4 crystal nucleus will grow and aggregate together. At last, a lots of large size Fe3O4 nanoparticles apart from POT hollow microspheres are formed. When the concentration of Fe ions is relatively low, only a
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Fig. 6. SEM images of the POT/Fe3O4 nanoparticle composite. Synthetic conditions: (a, b) POT-CA ¼ 0.1 g, [FeCl2$4H2O] ¼ 17 mM, [FeCl3$6H2O] ¼ 34 mM. (c, d) POT-CA ¼ 0.1 g, [FeCl2$4H2O] ¼ 8.5 mM, [ FeCl3$6H2O] ¼ 17 mM.
Fig. 7. SEM images of (a, b) CA-doped POT/Fe3O4 and (c, d) H2SO4-doped POT/Fe3O4 composites.
Fig. 8. TEM images of (a) CA-doped POT/Fe3O4, (b) H2SO4-doped POT/Fe3O4 and (c) CA-redoped POT/Fe3O4 composites.
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happened around the surfaces of the POT hollow microspheres where Fe3O4 crystal nucleus mainly deposited on, which will lead to the formation of POT/Fe3O4 nanoparticle composites with Fe3O4 nanoparticles relatively homogeneous supported on the surfaces of the POT hollow microspheres. If the concentration of Fe ions is extremely low, almost all of Fe ions attach on the surface of the CAdoped POT hollow microspheres for complexation and Fe3O4 crystal nucleus are formed on the surface of POT shells. With the coprecipitation proceeding, the Fe3O4 nanoparticles are formed and decorated on the surfaces of POT shells.
Scheme 1. Schematic illustration for the formation of the hollow structured POT/Fe3O4 nanoparticle composites.
small portion of Fe3O4 nanoparticles apart from POT hollow microspheres because of the low concentration of Fe3O4 crystal nucleus. The growth and aggregation processes are more likely
3.4. Synthesis and characterization of POT/Fe3O4/Au nanoparticle composites It is well known that the emeraldine base form of PANI and its derivatives have good reduction properties [36]. Several groups have discovered that the emeraldine base form of conductive polymer can reduce noble metal ions into metallic states, whereas the emeraldine base form of polymer is oxidized into pernigraniline
Fig. 9. TEM images of POT/Fe3O4/Au composites synthesized at different concentration of HAuCl4: (a, b) 2 103 M; (c, d) 8 103 M.
Fig. 10. (a) Successive UV-vis adsorption spectra of the reduction of 4-NP by NaBH4 in the presence of POT/Fe3O4/Au catalysts (gold size: 8 nm). (b) Plot of ln(A0/At) of 4-NP against time.
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Table 1 Comparison of the catalytic activity for the reduction of 4-NP between POT/Fe3O4/Au and some catalysts reported in the literature. Samples
Catalyst used (mg)
NMNPs loading (wt%)
Kapp (s1)
KM (s1g1)
Au/graphene43 Carbon@Au44 This work
0.1 0.1 0.09
24 22.68 8.4
3.17 103 5.42 103 2.28 103
132.08 238.98 301.6
[19,37e40]. Herein, The POT/Fe3O4/Au nanoparticle composites were synthesized according to the literature [18]. The concentration of HAuCl4 was found to have significant influence on the morphology of POT/Fe3O4/Au nanoparticle composites. As compared with Fig. 9, when the concentration of HAuCl4 is 2 mM, Au nanoparticles with an average size of 8 nm supported on surfaces of POT are seen (Fig. 9a and b). If the HAuCl4 concentration increases to 8 mM, gold nanoparticles are evenly decorated on polymer shells and the mean size of the gold nanoparticles is 15 nm (Fig. 9c). A HRTEM micrograph of the edge region of the POT/Fe3O4/ Au nanoparticle composite is presented in Fig. 9d, where the crystal lattice of Au nanoparticles, Fe3O4 nanoparticles, as well as amorphous POT, can be resolved. The measured lattice-fringe spacing of 0.23 nm in these Au nanoparticles corresponds to the (111) crystal facet (Fig. 9d) [41]. The lattice-fringe spacings of the Fe3O4 nanoparticles are measured (0.25, 0.29 nm) which corresponds to crystal facets (311) and (220) of cubic Fe3O4 (Fig. 9d), respectively [32]. In addition, two white arrows are added in Fig. 8d to distinguish the amorphous POT from the composite more clearly.
3.5. Catalytic activities of POT/Fe3O4/Au nanoparticle composites for the NaBH4 reduction of 4NP As show in Fig. 10, the as-prepared POT/Fe3O4/Au was added into 4-NP and NaBH4 aqueous solution to initiate the conversion of 4-NP to 4-AP reaction. The peak at 400 nm (corresponding to the 4NP) is disappeared and accompanied by a gradual development of a new peak at 300 nm (corresponding to the formation of 4-AP) [9,17,18]. The whole catalytic reaction was completely finished within 15 min (Fig. 9a), and the color change from bright yellow to colourless was observed. As shown in Fig. 10a, the tads (a certain period of time about the 4-NP to adsorb onto the catalyst's surfaces before reduction could be initiated [42]) is about 3 min for POT/ Fe3O4/Au catalysts indicating very fast adsorption process. After completion of the first cycle catalytic reaction, the POT/Fe3O4/Au catalysts are recovered by simple magnetic separation and then washed with water and reused in the next cycle. The kinetic reaction rate constant (defined as Kapp) is estimated from the linear relationship, which is 2.28 103 s1 and 0.4 103 s1 for first cycle and fifth cycle (Fig. 10b), respectively [9,18]. The decreased catalysts activity with increasing cycles may result from loss of catalysts during magnetic separation and purification processes of catalysts [18,40]. To further compare the catalytic activity of POT/ Fe3O4/Au with other reported catalysts, the rate constant per gram Kapp active sites was then calculated according to KM ¼ M , where M is the weight of active sites used. As show in Table 1, the KM values for the reported Au/graphene catalyst and Carbon@Au catalyst were 132.08 and 238.98 s1g1, respectively [43,44]. While, the asprepared POT/Fe3O4/Au catalyst with KM of 301.6 s1g1 exhibits better catalytic activity, which is mainly attributed to the low aggregation of the Au nanoparticles on the POT/Fe3O4 support. It is seen that this developed synthesis strategy useful for the design of magnetic catalyst support and other potential applications in field of reusable catalysis.
4. Conclusions POT/Fe3O4 nanoparticle composites have been fabricated as novel and magnetic reactive catalyst supports. The size of the Fe3O4 nanoparticles can be tuned by controlling the concentrations of Fe ions. TEM, SEM, XRD, FTIR, UV-Vis and VSM results revealed the chemical structures, superparamagnetic properties and formation mechanisms of the POT/Fe3O4 nanoparticle composites. Utilizing the redox activity of POT towards Au ions, POT/Fe3O4/Au nanoparticle composites can be fabricated as highly efficient and reusable noble metal catalysts. In addition, the POT/Fe3O4/Au nanoparticle composites showed good catalytic performances and simple recyclability in a model liquid phase reaction. It is believed that as-prepared POT/Fe3O4 nanoparticle composites have potential applications in magnetic catalyst support, biosensors and adsorption/separation fields. Acknowledgements This work was financed by Anhui Provincial Natural Science Foundation (1508085QE110) and Youth Innovation Promotion Association, CAS (2015268). References [1] P.M. Arnal, M. Comotti, F. Schüth, High-temperature-stable catalysts by hollow sphere encapsulation, Angew. Chem. Int. Ed. 45 (2006) 8224e8227. [2] L.C. Kennedy, L.R. Bickford, N.A. Lewinski, A.J. Coughlin, Y. Hu, E.S. Day, J.L. West, R.A. Drezek, A new era for cancer treatment: gold-nanoparticlemediated thermal therapies, Small 7 (2011) 169e183. [3] C. Li, W. Cai, B. Cao, F. Sun, Y. Li, C. Kan, L. Zhang, Mass synthesis of large, single-crystal Au nanosheets based on a polyol process, Adv. Funct. Mater 16 (2006) 83e90. [4] C.-H. Cui, H.-H. Li, S.-H. Yu, A general approach to electrochemical deposition of high quality free-standing noble metal (Pd, Pt, Au, Ag) sub-micron tubes composed of nanoparticles in polar aprotic solvent, Chem. Comm. 46 (2010) 940e942. [5] Q. Yue, Y. Zhang, C. Wang, X. Wang, Z. Sun, X.-F. Hou, D. Zhao, Y. Deng, Magnetic yolkeshell mesoporous silica microspheres with supported Au nanoparticles as recyclable high-performance nanocatalysts, J. Mater. Chem. A 3 (2015) 4586e4594. [6] L. Mandeltort, D.L. Chen, W.A. Saidi, J.K. Johnson, M.W. Cole, J.T. Yates Jr., Experimental and theoretical comparison of gas desorption energies on metallic and semiconducting single-walled carbon nanotubes, J. Am. Chem. Soc. 135 (2013) 7768e7776. [7] R. Liu, A. Sen, Magnetic yolk-shell mesoporous silica controlled synthesis of heterogeneous metal-Titania nanostructures and their applications, J. Am. Chem. Soc. 134 (2012) 17505e17512. [8] X.-K. Kong, Q.-W. Chen, Z.-Y. Sun, Enhanced SERS of the complex substrate using Au supported on graphene with pyridine and R6G as the probe molecules, Chem. Phys. Lett. 564 (2013) 54e59. [9] J. Han, S. Lu, C. Jin, M. Wang, R. Guo, Fe3O4/PANI/m-SiO2 as robust reactive catalyst supports for noble metal nanoparticles with improved stability and recyclability, J. Mater. Chem. A 2 (2014) 13016e13023. [10] I. Washio, Y. Xiong, Y. Yin, Y. Xia, Reduction by the end groups of poly(vinyl pyrrolidone): a new and versatile route to the kinetically controlled synthesis of Ag triangular nanoplates, Adv. Mater 18 (2006) 1745e1749. [11] X. Xia, J. Zeng, L.K. Oetjen, Q. Li, Y. Xia, Quantitative analysis of the role played by poly(vinylpyrrolidone) in seed-mediated growth of Ag nanocrystals, J. Am. Chem. Soc. 134 (2012) 1793e1801. [12] H. Sun, J. He, J. Wang, S.Y. Zhang, C. Liu, T. Sritharan, S. Mhaisalkar, M.Y. Han, D. Wang, H. Chen, Investigating the multiple roles of polyvinylpyrrolidone for a general methodology of oxide encapsulation, J. Am. Chem. Soc. 135 (2013) 9099e9110.
J. Sun, L. Chen / Materials Today Chemistry 5 (2017) 43e51 [13] H. Miyamura, R. Matsubara, Y. Miyazaki, S. Kobayashi, Aerobic oxidation of alcohols at room temperature and atmospheric conditions catalyzed by reusable gold nanoclusters stabilized by the benzene rings of polystyrene derivatives, Angew. Chem. Int. Ed. 46 (2007) 4151e4154. [14] C.J. Murphy, C.J. Orendorff, Alignment of gold nanorods in polymer composites and on polymer surfaces, Adv. Mater 17 (2005) 2173e2177. [15] H. Wu, R. Hong, X. Wang, R. Aroviz, C. You, B. Samarrta, D. Patra, M.T. Tuominen, V.M. Rotello, Controlled formation of patterned gold films via site-selective deposition of nanoparticles onto polymer-templated surfaces, Adv. Mater 19 (2007) 1383e1386. [16] S. Abraham, I. Kim, C.A. Batt, A facile preparative method for aggregation-free gold nanoparticles using poly(styrene-block-cysteine), Angew. Chem. Int. Ed. 46 (2007) 5720e5723. [17] J. Han, R. Chen, M. Wang, S. Lu, R. Guo, Core-shell to yolk-shell nanostructure transformation by a novel sacrificial template-free strategy, Chem. Commun. 49 (2013) 11566e11568. [18] J. Han, L. Li, R. Guo, Novel approach to controllable synthesis of gold nanoparticles supported on polyaniline nanofibers, Macromolecules 43 (2010) 10636e10644. [19] J. Han, Y. Liu, R. Guo, Reactive template method to synthesize gold nanoparticles with controllable size and morphology supported on shells of polymer hollow microspheres and their application for aerobic alcohol oxidation in water, Adv. Funct. Mater 19 (2009) 1112e1117. [20] R.J. Tseng, J. Huang, J. Ouyang, R.B. Kaner, Y. Yang, Polyaniline nanofiber/gold nanoparticle nonvolatile memory, Nano Lett. 5 (2005) 1077e1080. [21] B.J. Gallon, R.W. Kojima, R.B. Kaner, P.L. Diaconescu, Palladium nanoparticles supported on polyaniline nanofibers as a semi-heterogeneous catalyst in water, Angew. Chem. Int. Ed. 46 (2007) 7251e7254. [22] Z. Dong, G. Yu, X. Le, Gold nanoparticle modified magnetic fibrous silica microspheres as a highly efficient and recyclable catalyst for the reduction of 4nitrophenol, New J. Chem. 39 (2015) 8623e8629. [23] M. Rocha, C. Fernandes, C. Pereira, S.L.H. Rebelo, M.F.R. Pereira, C. Freire, Goldsupported magnetically recyclable nanocatalysts: a sustainable solution for the reduction of 4-nitrophenol in water, RSC Adv. 5 (2014) 5131e5141. [24] J. Yoo, N. Park, J.H. Park, J.H. Park, S. Kang, S.M. Lee, H.J. Kim, H. Jo, J.-G. Park, S.U. Son, Magnetically separable microporous Feeporphyrin networks for catalytic carbene insertion into NeH bonds, ACS Catal. 5 (2015) 350e355. [25] B. Karimi, F. Mansouri, H. Vali, A highly water-dispersible/magnetically separable palladium catalyst based on a Fe3O4@SiO2 anchored TEGimidazolium ionic liquid for the SuzukieMiyaura coupling reaction in water, Green Chem. 16 (2014) 2587e2596. [26] J. Han, G. Song, R. Guo, A facile solution route for polymeric hollow spheres with controllable size, Adv. Mater 18 (2006) 3140e3144. [27] X. Zheng, J. Feng, Y. Zong, H. Miao, X. Hu, J. Bai, X. Li, Hydrophobic graphene nanosheets decorated by monodispersed superparamagnetic Fe3O4 nanocrystals as synergistic electromagnetic wave absorbers, J. Mater. Chem. C 3 (2015) 4452e4463. [28] H. Niu, Q. Wang, H. Liang, M. Chen, C. Mao, J. Song, S. Zhang, Y. Gao, C. Chen, Visible-light active and magnetically recyclable nanocomposites for the degradation of organic dye, Materials 7 (2014) 4034e4044.
51
[29] J. Han, Y. Liu, R. Guo, A simple one-step chemical route to gold/polymer core/ shell composites and polymer hollow spheres, J. Appl. Polym. Sci. 112 (2009) 1244e1249. [30] J. Han, Y. Liu, R. Guo, A novel template less method to nanofibers of polyaniline derivatives with size control, J. Polym. Sci. Part A Polym. Chem. 46 (2008) 740e746. [31] H.H. Shih, D. Williams, N.H. Mack, H.L. Wang, Conducting polymer-based electrodeless deposition of Pt nanoparticles and its catalytic properties for regioselective hydrosilylation reactions, Macromolecules 42 (2009) 14e16. [32] L.-L. Yu, H. Bi, Facile synthesis and magnetic property of iron oxide/MCM-41 mesoporous silica nanospheres for targeted drug delivery, J. Appl. Phys. 111 (2012), 07B514e07B514e3. [33] J. Stejskal, P. Kratochvil, N. Radhakrishnan, Polyaniline dispersions-UV-vis absorption spectra, Synth. Met. 61 (1993) 225e231. [34] Y. Deng, D. Qi, C. Deng, X. Zhang, D. Zhao, Superparamagnetic highmagnetization microspheres with an Fe3O4@SiO2 core and perpendicularly aligned mesoporous SiO2 shell for removal of microcystins, J. Am. Chem. Soc. 130 (2008) 28e29. [35] J.-F. Lin, C.-C. Tsai, M.-Z. Lee, Linear birefringence and dichroism in citric acid coated Fe3O4 magnetic nanoparticles, J. Magn. Magn. Mater 372 (2014) 147e158. [36] E.T. Kang, K.G. Neoh, K.L. Tan, Polyaniline: a polymer with many interesting intrinsic redox states, Prog. Polym. Sci. 23 (1998) 277e324. [37] J. Han, L. Wang, R. Guo, Reactive polyaniline-supported sub-10 nm noble metal nanoparticles protected by a mesoporous silica shell: controllable synthesis and application as efficient recyclable catalysts, J. Mater. Chem. 22 (2012) 5932e5935. [38] S. Guo, S. Dong, E. Wang, Polyaniline/Pt hybrid nanofibers: high-efficiency nanoelectrocatalysts for electrochemical devices, Small 5 (2009) 1869e1876. [39] B.J. Gallon, R.W. Kojima, R.B. Kaner, P.L. Diaconescu, Palladium nanoparticles supported on polyaniline nanofibers as a semi-heterogeneous catalyst in water, Angew. Chem. Int. Ed. 46 (2007) 7251e7254. [40] J. Han, Y. Liu, L. Li, R. Guo, Poly(o-phenylenediamine) submicrospheresupported gold nanocatalysts: synthesis, characterization, and application in selective oxidation of benzyl alcohol, Langmuir 25 (2009) 11054e11060. [41] T. Zhang, W. Wang, D. Zhang, X. Zhang, Y. Ma, Y. Zhou, L. Qi, Biotemplated synthesis of gold nanoparticleebacteria cellulose nanofiber nanocomposites and their application in biosensing, Adv. Funct. Mater 20 (2010) 1152e1160. [42] J. Zeng, Q. Zhang, J. Chen, Y. Xia, A comparison study of the catalytic properties of Au-based nanocages, nanoboxes, and nanoparticles, Nano Lett. 10 (2010) 30e35. [43] J. Li, C.Y. Liu, Y. Liu, Au/graphene hydrogel: synthesis, characterization and its use for catalytic reduction of 4-nitropheno, J. Mater. Chem. 22 (2012) 8426e8430. [44] P. Zhang, C.L. Shao, X.H. Li, M.Y. Zhang, X. Zhang, C.Y. Su, N. Lu, K.X. Wang, Y.C. Liu, An electron-rich free-standing carbon@ Au coreeshell nanofiber network as a highly active and recyclable catalyst for the reduction of 4nitrophenol, Phys. Chem. Chem. Phys. 15 (2013) 10453e10458.