Poly(acrylonitrile-co-vinyl acetate)/Ag composite microspheres: one-pot fabrication and application as catalyst.
During the past two decades, noble metal nanoparticles have been the subject of a considerable number of publications due to their unique behavior, different from that of single atoms and bulk materials, and their various applications in catalytic and many other fields (1-3). The immobilization of metallic nanoparticles on supports to form composites has proved to be a good way for catalysts to retain high activity on recycling as well as convenient operation (4).
Recently, polymer microspheres have found wide applications in the fields of chromatographies, controlled reservoirs, and carriers, as well as being supports in the catalytic, environmental, biomedical, and other fields (5-8). Polymer microspheres with functional groups such as carbonyl, hydroxyl, and amino groups on their surfaces can be used to immobilize the noble metal nanoparticles. For instance, Akashi and coworkers described in situ formation of platinum (9) and silver nanoparticles (10) on poly(N-isopropylacrylamide)-coated polystyrene microspheres. Yang's groups reported the preparation of gold nanoparticles on polymer microspheres through the coordination of mercapto and carboxylic groups with gold colloids (4), (8). However, as to the fabrication of metal/polymer composite microspheres, most approaches are restricted to prepare polymer microspheres first, followed by in situ reduction metal ions species. To the best of our knowledge, little has been reported on the one-pot fabrication of the aforementioned composites.
In this article, a one-pot route for large-scale preparation of polymer/silver nanoparticles composite microspheres is developed. The nanocomposite has been characterized by various techniques, and we also investigate its catalytic activity in detail by taking the reduction of p-nitrophenol as a model reaction.
The random poly(acrylonitrile-co-vinyl acetate) [P(AN-co-VA)] ([M.sub.n] = 77,000, [M.sub.w]/[M.sub.n] = 1.47), is purchased from Anqing Petrochemical Group, China. The chemical structure is given as
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[AgNO.sub.3], dimethylformamide (DMF), p-nitrophenol and poly(vinylpyrrolidone) ([PVP.sub.k30], [M.sub.n] = 10,000, [M.sub.w]/[M.sub.n] = 4.9) are reagent grade and used as received without further purification.
The One-Pot Fabrication of P(AN-co-VA )/Ag Composite Microspheres
The typical fabrication process for P(AN-co-VA)/Ag composite microspheres is as follows: 0.2 g of random P(AN-co-VA) is dissolved in 15 mL of DMF. Then 5 mL of distilled water is added at a rate of 0.5 vol% per minute with strong stirring (500 r [min.sup.-1]) at room temperature. After the clear solution turns turbid, 5 mL of 5 mmol [L.sup.-1] [PVP.sub.k30] and [AgNO.sub.3] aqueous solution is added successively with continuous stirring for 12 h. P(AN-co VA)/Ag composite microspheres are obtained after being centrifugated, filtrated, washed by distilled water in turn, and finally dried in vacuum at 40[degrees]C.
For comparison, pure P(AN-co-VA) microspheres are also prepared according to the same experimental procedure except no addition of PVP and [AgNO.sub.3].
Catalytic Activity of P(AN-co-VA)/Ag Composite Microspheres
Typically, a reaction mixture of water (3 mL), aqueous p-nitrophenol solution (0.10 mL, 3.0 X [10.sup.-3] mM), purified and dried P(AN-co-VA)/Ag composite microspheres (0.01 g) are first taken in a quartz cuvette. To this stirring reaction mixture, aqueous [NaBH.sub.4] (1 mL, 3.0 X [10.sup.-2] mM) is then added. The progress of the conversion of p-nitrophenol to p-aminophenol is then monitored via measuring the UV-vis spectroscopy of the upper solution of the centrifugated reaction mixture every other 10 min.
The morphologies of the P(AN-co-VA)/Ag composites and P(AN-co-VA) microspheres are observed from transmission electron microscopy (TEM; JEM-200CX) and scanning electron microscopy (SEM; JSM-5610). Dynamic light scattering (DLS) is performed on a Brookhaven instrument, using an argon ion laser with a wavelength of 532 nm and output power of 10 mW at room temperature. X-ray diffraction (XRD) spectra are acquired with a Rigaku D/MAX-RC diffractometer using Cu [K.sub.[alpha]] radiation in the 2[theta] range 5-85[degrees] at 45 kV. The UV-vis spectra are recorded on a UV-240 spectrometer (Shimadzu, Japan).
RESULTS AND DISCUSSION
The Morphology and Structure of P(AN-co-VA)/Ag Composites
An insight into the morphology of the resulting P(AN-co-VA) particles and P(AN-co-VA)/Ag composites is gained from SEM and TEM. The P(AN-co-VA) particle shows spheric morphology and smooth surface (Fig. 1a and b). From DLS experiments, it is found that those microspheres are in the range of 260-920 nm and the average diameter is 487 nm (inset of b). No obvious change is observed when using different P(AN-co-VA) copolymers containing different ratios of AN and VA units (Fig. 1c and d). The obtained micrographs of P(AN-co-VA)/Ag composites clearly display spherically waxberry-like structure with rough surfaces, as shown in Fig. 1e and f. Each composite microsphere is decorated by many Ag nanoparticles with diameter of about 20-110 nm. The selected area electron diffraction (SAED) pattern (inset of f) further confirms the presence of Ag nanoparticles on the surface of P(AN-co-VA)/Ag composite microsphere.
Figure 2 is the XRD diffraction pattern of P(AN-co-VA)/Ag composite microspheres. Obviously, several strongest diffraction peaks are observed. The peak occurs at around 20 = 16.8[degrees], which corresponds to a planar spacing d = 5.240 E and can be indexed to the (100) plane of a hexagonal structure of P(AN-co-VA) (11). The other peaks at 38.1, 44.0, 64.5, and 77.0[degrees] can be assigned to diffraction from the 111, 200, 220, and 311 planes (12) of Ag nanoparticles on the surface of composite microsphere, respectively.
Formation of the P(AN-co-VA)/Ag Composite Microspheres
Under the experimental conditions, why and how the waxberry-like P(AN-co-VA)/Ag composite microspheres worth while investigating. As shown in Fig. 1, it is proposed that the formation of the waxberry-like P(AN-co-VA)/Ag composite microspheres includes two self-assembly processes: (i) the formation process of P(AN-co-VA) microspheres; (ii) the growing process from P(AN-co-VA) microspheres to waxberry-like P(AN-co-VA)/Ag composites. In the past several decades, the self-assembly processes of various copolymers in selective solvent have been widely investigated (13-15) and it is well known that the differences in the solubility between different segments of those copolymers are the basis of their micellization (16), (17). In our case, as to the P(AN-co-VA)/DMF solution, with the water added initially, the mixed solvent becomes progressively less favorable for the hydrophobic PAN segments of P(AN-co-VA). With the water content further increasing, more random polymer chains meet the phase separation condition, and then transfer from the solution into small particles due to the difference of the solubility of hydrophobic PAN-segment and relative hydrophilic ester groups (18). To keep the lowest total interfacial energy, these small particles tend to keep spheric shape, thus the P(AN-co-VA) microspheres form. As for the growing process from P(AN-co-VA) microspheres to waxberry-like P(AN-co-VA)/Ag composites, it is suggested that the electronegative groups (--CN) on the P(AN-co-VA) spheres surface act as anchored sites for the growth of Ag nanoparticles by attracting or chelating positive [Ag.sup.+] (19), (20), followed by the reduction of PVP. For commercially available PVP, its ends are terminated with the hydroxyl (--OH) group because of the involvement of water as a polymerization medium and the presence of hydrogen peroxide (21). According to the reported research (22), it is the hydroxyl (--OH) group that reduces the Ag ion and the reaction can be expressed as follows:
[FIGURE 1 OMITTED]
The schematic illustration of the formation of P(AN-co-VA)/Ag composite microspheres is described in Fig. 3. To further test the above proposal, we subsequently chose different metal compounds such as [HAuCl.sub.4], Pd[([NO.sub.3]).sub.2], [CuSO.sub.4] and [FeSO.sub.4] to replace [AgNO.sub.3], respectively. As expected, we only obtain the P(AN-co-VA)/Pd composite with similar morphology as that of P(AN-co-VA)/Ag composites. Although [AuCl.sub.4.sup.-] can be reduced by the hydroxyl end group of PVP, Au nanoparticles do not attach on the P(AN-co-VA) microspheres due to the weak attraction between the electronegative -CN and negative [AuCl.sub.4]. Contrarily, the reducing power of the hydroxyl end group of PVP is too weak to reduce [Cu.sup.2+] and [Fe.sup.2+] although both [Cu.sup.2+] and [Fe.sup.2+] have strong attraction with electronegative--CN.
[FIGURE 2 OMITTED]
[FIGURE 3 OMITTED]
[FIGURE 4 OMITTED]
Influence of Concentration of [AgNO.sub.3] on the P(AN-co-VA)/Ag Composite Microspheres
To further investigate the self-assembly process of P(AN-co-VA)/Ag composite microspheres, the influence of the concentration of [AgNO.sub.3] on constructing P(AN-co-VA)/Ag composite microspheres has been systematically investigated. For example, as the concentration of [AgNO.sub.3] is lower than 0.1 mmol [L.sup.-1] (Fig. 4a), only several Ag nanoparticles are observed on the surface of microsphere. With the increase of [AgNO.sub.3] concentration, both the density and size of Ag nanoparticles tend to increase (Fig. 4b). Bulk quantities of composite microspheres with high density of Ag nanoparticles on their surfaces are obtained at 1 mmol [L.sup.-1] of [AgNO.sub.3] (Fig. 4c). It is found that continuous increase of concentration of [AgNO.sub.3] does not increase the density of Ag nanoparticles obviously because of finite--CN groups on the surface of P(AN-co-VA) microspheres. Therefore, we conclude that the composite microspheres containing different density of Ag nanoparticles can be obtained easily by controlling the concentration of [AgNO.sub.3] aqueous solution.
[FIGURE 5 OMITTED]
Catalytic Activity of the P(AN-co-VA)/Ag Composite Microspheres
To investigate the catalytic activity of the as-prepared P(AN-co-VA)/Ag composite microspheres, they are employed for the catalytic reduction of p-nitrophenol by [NaBH.sub.4]. It is well known that this reaction is simple and fast in the presence of metallic surface (23-26). We confirm that this reaction does not occur using the P(AN-co-VA) mirospheres, even for lasting 72 h. Figure 5a shows a typical UV--vis absorption change of the reaction mixture by the addition of composite microspheres which prepared from 1 mM of [AgNO.sub.3] aqueous solution. Usually, the absorption decrease of p-nitrophenol at 400 nm can be used to test the catalyst activity (27). From these spectra, it can be seen that the absorption of p-nitrophenol at 400 nm decreases obviously within 50 min after the addition of composite microspheres, indicating the excellent catalyst activity. As the [BH.sub.4.sup.-] concentration remains essentially constant throughout the reaction due to the excessive [NaBH.sub.4], only the two principal species, p-nitrophenol and p-aminophenol could influence the reaction kinetics. Therefore, in this case, pseudo-first-order kinetics could be applied for the evaluation of rate constants. In Fig. 5b, the ratio of [C.sub.t] to [C.sub.0], where [C.sub.t] and [C.sub.0] are p-nitrophenol concentrations at time t and 0, respectively, is measured from the relative intensity of respective absorbance, [A.sub.t]/[A.sub.0]. The linear relation of ln([C.sub.t]/[C.sub.0]) versus time is observed for the composite microspheres catalyst, indicating that the reaction follows first-order kinetics. The rate constant (K = 3.87 X [10.sup.-2] [min.sup.-1]) has been estimated from first-order reaction kinetics using the slope of straight line, indicating a high catalytic reduction rate which can also be comparable to some research results [23, 28]. The composite microsphere catalyst is stable and could be recycled by simply precipitating, filtering, and redispersing process. Our experiment results show that the catalyst reaction rate constant has no obvious change after 10 times repeated utilization of the composite microsphere catalyst, indicating that the composite microsphere is a good candidate for the practical catalytic applications.
In conclusion, P(AN-co-VA)/Ag composite microspheres have been successfully fabricated in one step. The difference in the solubility between different segments of P(AN-co-VA) is the basis of the formation of P(AN-co-VA) microspheres, while the -CN on the surface of P(AN-co-VA) microspheres plays an important role in the formation of P(AN-co-VA)/Ag composite microspheres. The composite microspheres containing high density of Ag nanoparticles, which can be obtained easily by controlling the concentration of [AgNO.sub.3] aqueous solution, exhibit excellent catalytic activity and may have other potential applications such as optics, electronics and so on. This study may shed some light on the self-assembly of other metal/polymer composite microspheres.
The authors thank Prof. Yun Lu (Nanjing University, China) for her help.
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Hongping Xiao, (1) Youyi Xia (2)
(1) School of Chemistry and Materials Engineering, Wenzhou University, Wenzhou, Zhejiang 325035, People's Republic of China
(2) Department of Polymer Science and Engineering, School of Chemistry and Chemical Engineering, Anhui University of Technology, Maanshan 243002, People's Republic of China
Correspondence to: Hongping Xiao; e-mail: firstname.lastname@example.org Contract grant sponsor: Zhejiang Provincial Science and Technology Project of People's Republic of China; contract grant number: 2009C31133; contract grant sponsor: Natural Science Fund of Anhui Provincial Education Committee of People's Republic of China; contract grant number: KJ2009A013Z; contract grant sponsor: Young Teacher Fund and SRTP Fund of Anhui University of Technology; contract grant number: QZ200906.
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[c] 2010 Society of Plastics Engineers
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|Author:||Xiao, Hongping; Xia, Youyi|
|Publication:||Polymer Engineering and Science|
|Date:||Sep 1, 2010|
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