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Preparation of raspberry-like polypyrrole composites with applications in catalysis
Journal of Colloid and Interface Science 338 (2009) 573–577
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Preparation of raspberry-like polypyrrole composites with applications in catalysis Tongjie Yao a, Chuanxi Wang a, Jie Wu a, Quan Lin a, Hui Lv a, Kai Zhang a, Kui Yu b, Bai Yang a,* a b
State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012, People’s Republic of China Steacie Institute for Molecular Sciences, National Research Council Canada, Ottawa K1A OR6, Canada
a r t i c l e
i n f o
Article history: Received 7 February 2009 Available online 6 May 2009 Keywords: Polypyrrole (PPy) Silver (Ag) SiO2 Composites Catalysis
a b s t r a c t Raspberry-like composites were prepared by coating the silver/polypyrrole core/shell composites onto the surface of silica spheres via oxidation polymerization of pyrrole monomer with [Ag(NH3)2]+ ions as oxidants. The whole process allowed the absence of stabilizers, which greatly improved the quality of the conducting polymer composites. The morphology of the resulting composites was investigated, which can be described as raspberry-like; also, the structure and composition of the composites were characterized in detail. A possible formation mechanism was proposed. The present synthetic strategy substantially extended the scope of metal/conducting polymer composite synthesis. The raspberry-like composites exhibited excellent catalytic properties in the reduction of methylene blue dye with the reducing agent of sodium borohydride. Ó 2009 Elsevier Inc. All rights reserved.
1. Introduction Metal nanoparticles, such as silver (Ag) and gold (Au), have been extensively studied due to their unique catalytic properties [1–3]. The catalytic properties of metal nanoparticles are usually size dependent; once they aggregate to the macro-sized particles, their catalytic properties will be dramatically decreased. Unfortunately, nano-sized metal particles in solution tend to coalesce readily due to high surface energy together with Van der Waals forces [4,5]. Recently, severe particle aggregation during reaction poses the main obstacle to the commercialization of metal nanoparticle catalysts in terms of reusability. Accordingly, various techniques have been developed to prevent metal nanoparticles from aggregation, such as stabilizing nanoparticles by ligands and dispersing nanoparticles on supporters [2,6]. In these techniques, embedding metal nanoparticles into the core of functional polymers has been proven to be an effective approach to preserve their catalytic property [7,8]. Among various functional polymers, conducting polymer is the most attractive candidate. Compared with other polymers, one distinct advantage of embedding metal nanoparticles into the conducting polymer is that oxidation polymerization of monomer and reduction of metal salt can be finished in one step. Many research groups have focused their attention on the fabrication of metal/conducting polymer core/shell composites. For example, Selvan and co-workers have successfully synthesized Au/polypyrrole (PPy) composites using HAuCl4 and pyrrole monomer with a special diblock copolymer as a stabilizer [9]. Lu and co-workers have synthesized Ag/PPy composites by ultraviolet * Corresponding author. Fax: +86 431 85193423. E-mail addresses: [email protected], [email protected] (B. Yang). 0021-9797/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2009.05.001
(UV)-induced polymerization [10]. However, the size of these core/ shell composites is very small and usually around tens of nanometers [9,10]. Once they are used as catalysts, it is difficult to remove them completely from their reaction solution by common techniques such as filtration and centrifugation. Such difficulty leads to the pollution of the reaction solution and hence restricts their wide applications. It has been proven that silanol groups on the silica spheres can help to immobilize ions through electrostatic interactions and hydrogen bonds [11–13]. In this paper, we report a novel way to prepare PPy composites by using silica spheres loaded with [Ag(NH3)2]+ ions as the supporter. Oxidation polymerization of pyrrole monomer and reduction of [Ag(NH3)2]+ ions took place on the surface of silica spheres simultaneously and raspberry-like composites were formed in one step. No stabilizers were used in the whole process, which significantly improved the quality of the composites. The catalytic property of raspberry-like composites was investigated by reducing the methylene blue (MB) dye with sodium borohydride (NaBH4) as the reducing agent. The raspberry-like composites could be easily removed from the reaction solution by centrifugation due to the large size of silica sphere supporters and then can be reused. 2. Materials and methods 2.1. Materials The pyrrole monomer and sodium borohydride (NaBH4) were purchased from Sigma–Aldrich. Pyrrole monomer was distilled under reduced pressure and stored at 4 °C prior to use. Silver nitrate (AgNO3), ammonium hydroxide (NH3H2O, 25 wt% in water),
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T. Yao et al. / Journal of Colloid and Interface Science 338 (2009) 573–577
tetraethyl orthosilicate (TEOS), isopropanol, methylene blue (MB) dye, and hydrofluoric (HF) acid were analytical grade and used as received. In all preparations, absolute ethanol and deionized water were used. 2.2. Preparation of raspberry-like composites Fig. 1 outlines the procedure of the preparation of raspberrylike composites, which begins with soaking 0.1 g monodisperse silica spheres with an average diameter of 470 nm in [Ag(NH3)2]+ solution at room temperature for 6 h. Then the spheres were separated from the suspension by centrifugation and dried in an oven at 60 °C for 3 h (Fig. 1b). Subsequently, the spheres were redispered in water by sonication. Afterward, pyrrole monomer was added into the solution drop by drop under stirring at ambient temperature; the raspberry-like composites were prepared after 12 h stirring (Fig. 1c). 2.3. Preparation of Ag/PPy core/shell composites The silica spheres were completely etched by soaking the raspberry-like composites in 10 wt% HF acid under stirring for 24 h. After being washed by water several times, Ag/PPy core/shell composites were obtained (Fig. 1d). 2.4. Catalysis of the reduction of MB dye The catalytic property of raspberry-like composites was explored by studying the change of the absorbance intensity at the maximum absorbance wavelength (kmax) of the MB dye. In a typical procedure, a certain amount of the raspberry-like composites was homogeneously dispersed into the MB dye (0.9 mg) solution in water (4.5 mL), followed by a rapid injection of 0.5 mL of aqueous solution containing NaBH4 (14 mg) under stirring. The blue color of the mixture gradually vanished, indicating that the Ag NPs catalyzed the reduction of the MB dye. 2.5. Characterization A JEOL JSM-6700F scanning electron microscope (SEM) with primary electron energy of 3 kV was employed to examine the surface morphologies of products. The structure and shell thickness of the composites were determined by a JEOL-2010 transmission electron microscope (TEM) operating at 200 kV. Fourier-transform infrared (FTIR) spectra were measured in wavenumbers ranging from 400 to 4000 cm1 using a Nicolet Avatar 360 FTIR spectrophotometer. Ultraviolet–Visible (UV–Vis) spectra were acquired using a Shimadzu 3100 UV–Vis spectrophotometer. X-ray diffraction (XRD) data were collected on a Siemens D-5005 X-ray diffractometer with Cu Ka radiation (k = 1.5418 Å). 3. Results and discussion In recent years, many researches have used Ag+ ions as the oxidant to initiate the polymerization of pyrrole monomer accompa-
nied by the reduction of the Ag+ ions to Ag nanoparticles (Ag NPs) simultaneously [10,14–16]. The standard reduction potential of Ag+/Ag0 (0.799 V) is very close to that of Fe3+/Fe2+ (0.771 V) which is a commonly used oxidant in pyrrole monomer polymerization. In this study, after the addition of pyrrole monomer into the suspension of silica spheres loaded with [Ag(NH3)2]+ ions, the color of the suspension gradually turned from white to khaki, suggesting that [Ag(NH3)2]+ ions were reduced to Ag NPs. The final color of the solution was black, indicating the polymerization of the pyrrole monomer. The SEM image in Fig. 2 shows that the nano-sized particles randomly decorate the surface of silica spheres, which makes the morphology of composites raspberry-like. The XRD pattern of the as-prepared composites in Fig. 3 shows four diffraction peaks at 2h = 38.1, 44.3, 64.6, and 77.3°, which are corresponding to (1 1 1), (2 0 0), (2 2 0), and (3 1 1) Bragg reflections of silver, respectively. The positions of all the sharp peaks are in good agreement with those reported for Ag NPs [17]. In order to confirm the core/shell structure of the Ag/PPy nanocomposite, HF acid was selected to etch silica spheres. Fig. 4 shows the SEM image of the nano-sized Ag/PPy composites with the diameter ranging from 30 to 80 nm. As commonly known, it was difficult to prevent the aggregation of conducting polymers as the polymer chains were prone to forming deposits of sediments due to intermolecular hydrogen bond and strong p–p* interaction [15,16]. However, in our study, the composites show good dispersity. A typical TEM image is shown in the inset of Fig. 4, and it confirms that the composites possess a core/shell structure. Ag NPs, which scatter electrons more than organic polymers, formed the core, and PPy formed the shell. The PPy shell is quite uniform and ultrathin; the average thickness is ca. 5 nm. The weight ratios of silica sphere and Ag/PPy core/shell composites were measured: the content of silica spheres was 89.2 wt%, while the content of Ag/ PPy core/shell composites was 10.8 wt%. The molecular structures of the raspberry-like composites and Ag/PPy core/shell composites were characterized by FTIR spectra as shown in Fig. 5. In curve (a), the characteristic peak of silica spheres at 1106 cm1 due to the Si–O–Si stretching vibration is so strong that many weak bands of PPy are disguised. After soaking raspberry-like composites in HF acid for 24 h, the band of silica sphere at 1106 cm1 disappears, which indicates that the silica spheres were completely etched away by HF acid. The PPy peaks can be distinguished clearly in curve (b). The characteristic bands are located at 1561 and 1460 cm1 due to the stretching mode of the pyrrole ring. The peaks at 1315 and 1038 cm1 are related to the in-plane vibrations of @C–H. The band at 1212 cm1 is assigned to the C–N stretching mode and the peak at 793 cm1 is attributable to C–H wagging vibration. Furthermore, a band assigncounterions is observed at able to N–O stretching of NO 3 1380 cm1, which suggests that NO 3 ions are doped ions in PPy molecular chains [18,19]. We have successfully prepared raspberry-like composites and Ag/PPy core/shell composites without any stabilizers. It has been reported that only PPy homopolymer and macroscopic silver were obtained, when no stabilizers were used [11,17]. In previous
Fig. 1. Fabrication process of raspberry-like composites and Ag/PPy core/shell composites.
T. Yao et al. / Journal of Colloid and Interface Science 338 (2009) 573–577
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Fig. 2. SEM image of raspberry-like composites. The concentrations of pyrrole monomer and [Ag(NH3)2]+ ions are 0.3 M. Inset shows the high magnified image.
Fig. 5. FTIR spectra of (a) raspberry-like composites and (b) Ag/PPy core/shell composites.
Fig. 3. XRD pattern of raspberry-like composites.
loading of pyrrole monomer [20–22]. However, it is well known that the stabilizers can hardly be removed in the final products, which decreases the purity of the conducting polymer composites [23]. To our best knowledge, there was little study on the preparation of metal/conducting polymer core/shell composites without stabilizers. Compared with traditional preparation processes, there are two major differences in our system: first, in traditional works, the pyrrole monomer and the oxidant were mixed together in solution, which easily induced the homopolymerization of pyrrole monomer and the coalescence of Ag NPs in the absence of stabilizers. In our work, the silica spheres loaded with [Ag(NH3)2]+ ions played a role as the source of oxidant. The [Ag(NH3)2]+ ions existed on the surface of the silica spheres due to the electrostatic interaction and hydrogen bond [11–14]. After the addition of pyrrole monomer, the [Ag(NH3)2]+ ions could be reduced and form Ag NPs while the pyrrole monomer polymerized around the silver to hinder the coalescence of Ag NPs. The reaction only occurred on the surfaces of silica spheres and the reaction rate in our study was much slower than that in traditional work. Two compared experiments were carried out: The 0.3 M pyrrole monomer was added into the silica sphere suspension which was soaked in 0.3 M [Ag(NH3)2]+ solution, and the color of suspension did not turn black until 30 min later. However, if 0.3 M pyrrole monomer was directly added into 0.3 M [Ag(NH3)2]+ solution, the color became black within 10 min. Second, in previous reports, AgNO3 was usually used as the oxidant in pyrrole monomer polymerization [11,15–17]. The potential of AgNO3 reducing to Ag NPs can be calculated by the Nernst equation [24]
E1 ¼ E0 þ Fig. 4. SEM image of Ag/PPy core/shell composites with diameter ranging from 30 to 80 nm. Inset shows the typical TEM image of the Ag/PPy composites. The PPy shell is ultrathin with a thickness of ca. 5 nm.
preparation systems, the conducting polymer monomer, stabilizer, and oxidant were mixed together. Stabilizers such as poly(vinyl pyrrolidone) and poly(vinyl alcohol) played a vital role in those synthesis processes: first, they could effectively prevent the PPy from aggregation; second, they could provide active sites for the
2:303RT lg½Agþ ; F
ð1Þ
where E0 is the standard potential and the value is 0.799 V for Ag+/ Ag0; F is Faraday constant and the value is 9.65 104 C mol1; R is the idea gas constant and the value is 8.314 J K1 mol1; T is the temperature (K). From Eq. (1) we can see that the E1 is a function of temperature and the concentration of AgNO3 solution. In our study, [Ag(NH3)2]+ ions were selected to replace AgNO3. In [Ag(NH3)2]+ solution, the following reaction occurred:
Agþ þ 2NH3 AgðNH3 Þþ2 :
ð2Þ
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According to the general principle of the chemical equilibrium, Eq. (2) can be expressed as [24]
Kx ¼
½AgðNH3Þ2 þ ½Agþ ½NH3 2
ð3Þ
;
where Kx is the stability constant and its value is 1.6 107. Combining Eq. (3) with Eq. (1), Eq. (4) can be obtained:
E2 ¼ E0 þ
þ 2:303RT ½AgðNH3Þ2 : lg F K x ½NH3 2
ð4Þ þ
3 Þ2 Comparing Eq. (1) with Eq. (4), the value of lg ½AgðNH is less K ½NH 2
x
3
than that of lg[Ag+] due to the large value of Kx, which results in E2 lower than E1 while other parameters are the same. That means the [Ag(NH3)2]+ ion is a weaker oxidant compared with AgNO3. Taking our system into account, the polymerization rate of pyrrole monomer is slower by using [Ag(NH3)2]+ ions as the oxidant than that by using AgNO3, which was favorable to hindering the coalescence of Ag NPs and homopolymerization of pyrrole monomer. Being a weak oxidant, [Ag(NH3)2]+ ion played a key role in the fabrication of raspberry-like composites and core/shell composites without any stabilizers. It has been experimentally demonstrated that Ag NPs have high catalytic activity in reduction reactions of nitrophenols, hydrogenation, and various dyes [25–27]. Reduction of MB dye is usually used as a standard for determining the catalytic activity of Ag NPs or silver composites. Here, we also explored the catalytic property of the raspberry-like composites by studying the evolution of the absorbance intensity at kmax. The preliminary catalytic testing was carried
out by reducing MB dye in water with NaBH4 as the reducing agent. Fig. 6 illustrates the UV–Vis spectra of the MB dye during the reaction in the presence or absence of the raspberry-like composites. Curve (a) is the UV–Vis spectrum of the initial mixture with the color of blue (inset of Fig. 6) and the kmax appears at 665 nm. The Curve (b) is corresponding to the UV–Vis spectrum of the mixture without the raspberry-like composites after reacting for 72 h at room temperature. We can see that the absorbance intensity at kmax of MB decreases, but does not completely disappear. The color of the mixture becomes light blue (inset of Fig. 6). After 10 mg of raspberry-like composites as catalyst was added into the mixture, the absorbance at kmax completely vanished within 2 min, and the color turned colorless as seen in the inset of Fig. 6. The experimental results confirmed that the raspberry-like composites had very good catalytic performance and the Ag NPs were effectively protected by PPy shell from aggregation. After reaction, the raspberry-like composites can be easily removed from the reaction solution by centrifugation at 4000 rpm due to the large size of the silica sphere supporters. The concentration of [Ag(NH3)2]+ ions had a direct influence on the morphologies and the catalytic properties of raspberry-like composites. Fig. 7a–c demonstrate the morphologies of the raspberrylike composites as a function of the [Ag(NH3)2]+ concentration. The [Ag(NH3)2]+ concentrations of samples a–c are 0.03, 0.2, and 0.5 M, respectively, while the other parameters are the same. When the concentration of [Ag(NH3)2]+ ions is low, for example, 0.03 M, the silica spheres are coated by a small amount of Ag/PPy core/shell composites (Fig. 7a). The amount of the Ag/PPy composites deposited on the surface of silica spheres increases with the increase of the [Ag(NH3)2]+ concentration. However, at high [Ag(NH3)2]+ concentration, for example, 0.5 M, some core/shell composites aggregate on the surface of silica spheres (Fig. 7c), probably because of the fast polymerization of the pyrrole monomer at high [Ag(NH3)2]+ concentrations. To investigate the relationship between the concentration of [Ag(NH3)2]+ ions and the catalytic property of the raspberry-like composites, samples a–c with the same mass were added into three identical MB dye solutions. After the injection of aqueous NaBH4 solution and being stirred for 90 s, the mixture was immediately measured by a UV–Vis spectrometer. The rate of color disappearance was used to compare the catalytic activity of the raspberry-like composites. Fig. 8 shows that the greater the concentration of [Ag(NH3)2]+ ions used, the weaker the absorbance intensity at kmax. The UV–Vis spectra and SEM images suggest that the more Ag/PPy core/shell composites deposited on the surface of silica spheres, the better the catalytic property of the raspberry-like composites. 4. Summary
Fig. 6. UV–Vis spectra of (a) the initial mixture with MB dye and NaBH4, (b) the mixture reacting for 72 h without adding raspberry-like composites and (c) the mixture reacting for 2 min after the addition of 10 mg raspberry-like composites. The inset shows optical photos of solutions (a–c).
In summary, we have successfully synthesized raspberry-like composites in a one-step reaction without any stabilizers. After etching silica spheres by HF acid, Ag/PPy core/shell composites were obtained; the PPy shell was uniform with the thickness of ca. 5 nm. A possible mechanism of the fabrication of PPy composites without stabilizers was proposed: silica spheres used as the source of oxidant
Fig. 7. SEM images of raspberry-like composites synthesized under different concentrations of [Ag(NH3)2]+ ions while other parameters equal: (a) 0.03 M, (b) 0.2 M and (c) 0.5 M.
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B06009), and National Basic Research 2007CB936402 and 2007CB936403).
Program
(Nos.
References
Fig. 8. UV–Vis spectra of MB dye solution reduced by NaBH4 as a function of [Ag(NH3)2]+ concentrations.
and the selection of [Ag(NH3)2]+ ions as oxidant played key roles. The raspberry-like composites exhibited very good catalytic performance based on the reduction results of MB dye with NaBH4 as the reducing agent. The catalytic property of the raspberry-like composites was improved with an increase of [Ag(NH3)2]+ concentration. The method presented here may open up new opportunities to fabricate other raspberry-like core/shell conducing polymer composites with potential in various applications. Acknowledgments This work was supported by the National Nature Science Foundation of China (Nos. 20534040 and 20674026), ‘‘111” project (No.
[1] X.H. Peng, Q.M. Pan, G.L. Rempel, Chem. Soc. Rev. 37 (2008) 1619. [2] X. Roucoux, J. Schulz, H. Patin, Chem. Rev. 102 (2002) 3757. [3] R.M. Crooks, M.Q. Zhao, L. Sun, V. Chechik, L.K. Yeung, Acc. Chem. Res. 34 (2001) 181. [4] B. Medasani, Y.H. Park, I. Vasiliev, Phys. Rev. B 75 (2007) 235436. [5] J. Lee, J.C. Park, H. Song, Adv. Mater. 20 (2008) 1523. [6] M. Boutros, A. Denicourt-Nowicki, A. Roucoux, L. Gengembre, P. Beaunier, A. Gédéon, F. Launay, Chem. Commun. (2008) 2920. [7] M. Graeser, E. Pippel, A. Greiner, J.H. Wendorff, Macromolecules 40 (2007) 6032. [8] F. Wen, W.Q. Zhang, G.W. Wei, Y. Wang, J.Z. Zhang, M.C. Zhang, L.Q. Shi, Chem. Mater. 20 (2008) 2144. [9] S.T. Selvan, J.P. Spatz, H.A. Klok, M. Möller, Adv. Mater. 10 (1998) 132. [10] X.M. Yang, Y. Lu, Mater. Lett. 59 (2005) 2484. [11] Y. Kobayashi, V. Salgueirio-Maceira, L.M. Liz-Maren, Chem. Mater. 13 (2001) 1630. [12] Z.M. Chen, X. Chen, L.L. Zheng, T. Gang, T.Y. Cui, K. Zhang, B. Yang, J. Colloid Interface Sci. 285 (2005) 146. [13] C.A.R. Costa, C.A.P. Leite, F. Galembeck, J. Phys. Chem. B 107 (2003) 4747. [14] S. Fujii, A. Aichi, K. Akamatsu, H. Nawafune, Y. Nakamura, J. Mater. Chem. 17 (2007) 3777. [15] P. Dallas, D. Niarchos, D. Vrbanic, N. Boukos, S. Pejovnik, C. Trapalis, D. Petridis, Polymer 48 (2007) 2007. [16] S.Y. Jing, S.X. Xing, L.X. Yu, C. Zhao, Mater. Lett. 61 (2007) 4528. [17] M.Y. Mohan, T. Premkumar, K. Lee, K.E. Geckeler, Macromol. Rapid Commun. 27 (2006) 1346. [18] X.M. Feng, Z.Z. Sun, W.H. Hou, J.J. Zhu, Nanotechnology 18 (2007) 195603. [19] X. Li, M.X. Wan, Y. Wei, J.Y. Shen, Z.J. Chen, J. Phys. Chem. B 110 (2006) 14623. [20] T.J. Yao, Q. Lin, K. Zhang, D.F. Zhao, H. Lv, J.H. Zhang, B. Yang, J. Colloid Interface Sci. 515 (2007) 434. [21] S.F. Lascelles, S.P. Armes, J. Mater. Chem. 7 (1997) 1339. [22] S. Fujii, S.P. Armes, R. Jeans, R. Devonshire, S. Warren, S.L. McArthur, M.J. Burchell, F. Postberg, R. Srama, Chem. Mater. 18 (2006) 2758. [23] F. Epron, F. Henry, O. Sagnes, Makromol. Chem. Macromol. Symp. 35/36 (1990) 527. [24] M.L. Thompson, L.J. Kateley, J. Chem. Educ. 76 (1999) 95. [25] T. Ahmadi, Z.L. Wang, T.C. Green, A. Henglein, M.A. EI-Sayed, Science 272 (1996) 1924. [26] Z.J. Jiang, C.Y. Liu, L.W. Sun, J. Phys. Chem. B 109 (2005) 1730. [27] A.C. Patel, S. Li, C. Wang, W.J. Zhang, Y. Wei, Chem. Mater. 19 (2007) 1231.
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Preparation of raspberry-like polypyrrole composites with applications in catalysis Tongjie Yao a, Chuanxi Wang a, Jie Wu a, Quan Lin a, Hui Lv a, Kai Zhang a, Kui Yu b, Bai Yang a,* a b
State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012, People’s Republic of China Steacie Institute for Molecular Sciences, National Research Council Canada, Ottawa K1A OR6, Canada
a r t i c l e
i n f o
Article history: Received 7 February 2009 Available online 6 May 2009 Keywords: Polypyrrole (PPy) Silver (Ag) SiO2 Composites Catalysis
a b s t r a c t Raspberry-like composites were prepared by coating the silver/polypyrrole core/shell composites onto the surface of silica spheres via oxidation polymerization of pyrrole monomer with [Ag(NH3)2]+ ions as oxidants. The whole process allowed the absence of stabilizers, which greatly improved the quality of the conducting polymer composites. The morphology of the resulting composites was investigated, which can be described as raspberry-like; also, the structure and composition of the composites were characterized in detail. A possible formation mechanism was proposed. The present synthetic strategy substantially extended the scope of metal/conducting polymer composite synthesis. The raspberry-like composites exhibited excellent catalytic properties in the reduction of methylene blue dye with the reducing agent of sodium borohydride. Ó 2009 Elsevier Inc. All rights reserved.
1. Introduction Metal nanoparticles, such as silver (Ag) and gold (Au), have been extensively studied due to their unique catalytic properties [1–3]. The catalytic properties of metal nanoparticles are usually size dependent; once they aggregate to the macro-sized particles, their catalytic properties will be dramatically decreased. Unfortunately, nano-sized metal particles in solution tend to coalesce readily due to high surface energy together with Van der Waals forces [4,5]. Recently, severe particle aggregation during reaction poses the main obstacle to the commercialization of metal nanoparticle catalysts in terms of reusability. Accordingly, various techniques have been developed to prevent metal nanoparticles from aggregation, such as stabilizing nanoparticles by ligands and dispersing nanoparticles on supporters [2,6]. In these techniques, embedding metal nanoparticles into the core of functional polymers has been proven to be an effective approach to preserve their catalytic property [7,8]. Among various functional polymers, conducting polymer is the most attractive candidate. Compared with other polymers, one distinct advantage of embedding metal nanoparticles into the conducting polymer is that oxidation polymerization of monomer and reduction of metal salt can be finished in one step. Many research groups have focused their attention on the fabrication of metal/conducting polymer core/shell composites. For example, Selvan and co-workers have successfully synthesized Au/polypyrrole (PPy) composites using HAuCl4 and pyrrole monomer with a special diblock copolymer as a stabilizer [9]. Lu and co-workers have synthesized Ag/PPy composites by ultraviolet * Corresponding author. Fax: +86 431 85193423. E-mail addresses: [email protected], [email protected] (B. Yang). 0021-9797/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2009.05.001
(UV)-induced polymerization [10]. However, the size of these core/ shell composites is very small and usually around tens of nanometers [9,10]. Once they are used as catalysts, it is difficult to remove them completely from their reaction solution by common techniques such as filtration and centrifugation. Such difficulty leads to the pollution of the reaction solution and hence restricts their wide applications. It has been proven that silanol groups on the silica spheres can help to immobilize ions through electrostatic interactions and hydrogen bonds [11–13]. In this paper, we report a novel way to prepare PPy composites by using silica spheres loaded with [Ag(NH3)2]+ ions as the supporter. Oxidation polymerization of pyrrole monomer and reduction of [Ag(NH3)2]+ ions took place on the surface of silica spheres simultaneously and raspberry-like composites were formed in one step. No stabilizers were used in the whole process, which significantly improved the quality of the composites. The catalytic property of raspberry-like composites was investigated by reducing the methylene blue (MB) dye with sodium borohydride (NaBH4) as the reducing agent. The raspberry-like composites could be easily removed from the reaction solution by centrifugation due to the large size of silica sphere supporters and then can be reused. 2. Materials and methods 2.1. Materials The pyrrole monomer and sodium borohydride (NaBH4) were purchased from Sigma–Aldrich. Pyrrole monomer was distilled under reduced pressure and stored at 4 °C prior to use. Silver nitrate (AgNO3), ammonium hydroxide (NH3H2O, 25 wt% in water),
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tetraethyl orthosilicate (TEOS), isopropanol, methylene blue (MB) dye, and hydrofluoric (HF) acid were analytical grade and used as received. In all preparations, absolute ethanol and deionized water were used. 2.2. Preparation of raspberry-like composites Fig. 1 outlines the procedure of the preparation of raspberrylike composites, which begins with soaking 0.1 g monodisperse silica spheres with an average diameter of 470 nm in [Ag(NH3)2]+ solution at room temperature for 6 h. Then the spheres were separated from the suspension by centrifugation and dried in an oven at 60 °C for 3 h (Fig. 1b). Subsequently, the spheres were redispered in water by sonication. Afterward, pyrrole monomer was added into the solution drop by drop under stirring at ambient temperature; the raspberry-like composites were prepared after 12 h stirring (Fig. 1c). 2.3. Preparation of Ag/PPy core/shell composites The silica spheres were completely etched by soaking the raspberry-like composites in 10 wt% HF acid under stirring for 24 h. After being washed by water several times, Ag/PPy core/shell composites were obtained (Fig. 1d). 2.4. Catalysis of the reduction of MB dye The catalytic property of raspberry-like composites was explored by studying the change of the absorbance intensity at the maximum absorbance wavelength (kmax) of the MB dye. In a typical procedure, a certain amount of the raspberry-like composites was homogeneously dispersed into the MB dye (0.9 mg) solution in water (4.5 mL), followed by a rapid injection of 0.5 mL of aqueous solution containing NaBH4 (14 mg) under stirring. The blue color of the mixture gradually vanished, indicating that the Ag NPs catalyzed the reduction of the MB dye. 2.5. Characterization A JEOL JSM-6700F scanning electron microscope (SEM) with primary electron energy of 3 kV was employed to examine the surface morphologies of products. The structure and shell thickness of the composites were determined by a JEOL-2010 transmission electron microscope (TEM) operating at 200 kV. Fourier-transform infrared (FTIR) spectra were measured in wavenumbers ranging from 400 to 4000 cm1 using a Nicolet Avatar 360 FTIR spectrophotometer. Ultraviolet–Visible (UV–Vis) spectra were acquired using a Shimadzu 3100 UV–Vis spectrophotometer. X-ray diffraction (XRD) data were collected on a Siemens D-5005 X-ray diffractometer with Cu Ka radiation (k = 1.5418 Å). 3. Results and discussion In recent years, many researches have used Ag+ ions as the oxidant to initiate the polymerization of pyrrole monomer accompa-
nied by the reduction of the Ag+ ions to Ag nanoparticles (Ag NPs) simultaneously [10,14–16]. The standard reduction potential of Ag+/Ag0 (0.799 V) is very close to that of Fe3+/Fe2+ (0.771 V) which is a commonly used oxidant in pyrrole monomer polymerization. In this study, after the addition of pyrrole monomer into the suspension of silica spheres loaded with [Ag(NH3)2]+ ions, the color of the suspension gradually turned from white to khaki, suggesting that [Ag(NH3)2]+ ions were reduced to Ag NPs. The final color of the solution was black, indicating the polymerization of the pyrrole monomer. The SEM image in Fig. 2 shows that the nano-sized particles randomly decorate the surface of silica spheres, which makes the morphology of composites raspberry-like. The XRD pattern of the as-prepared composites in Fig. 3 shows four diffraction peaks at 2h = 38.1, 44.3, 64.6, and 77.3°, which are corresponding to (1 1 1), (2 0 0), (2 2 0), and (3 1 1) Bragg reflections of silver, respectively. The positions of all the sharp peaks are in good agreement with those reported for Ag NPs [17]. In order to confirm the core/shell structure of the Ag/PPy nanocomposite, HF acid was selected to etch silica spheres. Fig. 4 shows the SEM image of the nano-sized Ag/PPy composites with the diameter ranging from 30 to 80 nm. As commonly known, it was difficult to prevent the aggregation of conducting polymers as the polymer chains were prone to forming deposits of sediments due to intermolecular hydrogen bond and strong p–p* interaction [15,16]. However, in our study, the composites show good dispersity. A typical TEM image is shown in the inset of Fig. 4, and it confirms that the composites possess a core/shell structure. Ag NPs, which scatter electrons more than organic polymers, formed the core, and PPy formed the shell. The PPy shell is quite uniform and ultrathin; the average thickness is ca. 5 nm. The weight ratios of silica sphere and Ag/PPy core/shell composites were measured: the content of silica spheres was 89.2 wt%, while the content of Ag/ PPy core/shell composites was 10.8 wt%. The molecular structures of the raspberry-like composites and Ag/PPy core/shell composites were characterized by FTIR spectra as shown in Fig. 5. In curve (a), the characteristic peak of silica spheres at 1106 cm1 due to the Si–O–Si stretching vibration is so strong that many weak bands of PPy are disguised. After soaking raspberry-like composites in HF acid for 24 h, the band of silica sphere at 1106 cm1 disappears, which indicates that the silica spheres were completely etched away by HF acid. The PPy peaks can be distinguished clearly in curve (b). The characteristic bands are located at 1561 and 1460 cm1 due to the stretching mode of the pyrrole ring. The peaks at 1315 and 1038 cm1 are related to the in-plane vibrations of @C–H. The band at 1212 cm1 is assigned to the C–N stretching mode and the peak at 793 cm1 is attributable to C–H wagging vibration. Furthermore, a band assigncounterions is observed at able to N–O stretching of NO 3 1380 cm1, which suggests that NO 3 ions are doped ions in PPy molecular chains [18,19]. We have successfully prepared raspberry-like composites and Ag/PPy core/shell composites without any stabilizers. It has been reported that only PPy homopolymer and macroscopic silver were obtained, when no stabilizers were used [11,17]. In previous
Fig. 1. Fabrication process of raspberry-like composites and Ag/PPy core/shell composites.
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Fig. 2. SEM image of raspberry-like composites. The concentrations of pyrrole monomer and [Ag(NH3)2]+ ions are 0.3 M. Inset shows the high magnified image.
Fig. 5. FTIR spectra of (a) raspberry-like composites and (b) Ag/PPy core/shell composites.
Fig. 3. XRD pattern of raspberry-like composites.
loading of pyrrole monomer [20–22]. However, it is well known that the stabilizers can hardly be removed in the final products, which decreases the purity of the conducting polymer composites [23]. To our best knowledge, there was little study on the preparation of metal/conducting polymer core/shell composites without stabilizers. Compared with traditional preparation processes, there are two major differences in our system: first, in traditional works, the pyrrole monomer and the oxidant were mixed together in solution, which easily induced the homopolymerization of pyrrole monomer and the coalescence of Ag NPs in the absence of stabilizers. In our work, the silica spheres loaded with [Ag(NH3)2]+ ions played a role as the source of oxidant. The [Ag(NH3)2]+ ions existed on the surface of the silica spheres due to the electrostatic interaction and hydrogen bond [11–14]. After the addition of pyrrole monomer, the [Ag(NH3)2]+ ions could be reduced and form Ag NPs while the pyrrole monomer polymerized around the silver to hinder the coalescence of Ag NPs. The reaction only occurred on the surfaces of silica spheres and the reaction rate in our study was much slower than that in traditional work. Two compared experiments were carried out: The 0.3 M pyrrole monomer was added into the silica sphere suspension which was soaked in 0.3 M [Ag(NH3)2]+ solution, and the color of suspension did not turn black until 30 min later. However, if 0.3 M pyrrole monomer was directly added into 0.3 M [Ag(NH3)2]+ solution, the color became black within 10 min. Second, in previous reports, AgNO3 was usually used as the oxidant in pyrrole monomer polymerization [11,15–17]. The potential of AgNO3 reducing to Ag NPs can be calculated by the Nernst equation [24]
E1 ¼ E0 þ Fig. 4. SEM image of Ag/PPy core/shell composites with diameter ranging from 30 to 80 nm. Inset shows the typical TEM image of the Ag/PPy composites. The PPy shell is ultrathin with a thickness of ca. 5 nm.
preparation systems, the conducting polymer monomer, stabilizer, and oxidant were mixed together. Stabilizers such as poly(vinyl pyrrolidone) and poly(vinyl alcohol) played a vital role in those synthesis processes: first, they could effectively prevent the PPy from aggregation; second, they could provide active sites for the
2:303RT lg½Agþ ; F
ð1Þ
where E0 is the standard potential and the value is 0.799 V for Ag+/ Ag0; F is Faraday constant and the value is 9.65 104 C mol1; R is the idea gas constant and the value is 8.314 J K1 mol1; T is the temperature (K). From Eq. (1) we can see that the E1 is a function of temperature and the concentration of AgNO3 solution. In our study, [Ag(NH3)2]+ ions were selected to replace AgNO3. In [Ag(NH3)2]+ solution, the following reaction occurred:
Agþ þ 2NH3 AgðNH3 Þþ2 :
ð2Þ
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According to the general principle of the chemical equilibrium, Eq. (2) can be expressed as [24]
Kx ¼
½AgðNH3Þ2 þ ½Agþ ½NH3 2
ð3Þ
;
where Kx is the stability constant and its value is 1.6 107. Combining Eq. (3) with Eq. (1), Eq. (4) can be obtained:
E2 ¼ E0 þ
þ 2:303RT ½AgðNH3Þ2 : lg F K x ½NH3 2
ð4Þ þ
3 Þ2 Comparing Eq. (1) with Eq. (4), the value of lg ½AgðNH is less K ½NH 2
x
3
than that of lg[Ag+] due to the large value of Kx, which results in E2 lower than E1 while other parameters are the same. That means the [Ag(NH3)2]+ ion is a weaker oxidant compared with AgNO3. Taking our system into account, the polymerization rate of pyrrole monomer is slower by using [Ag(NH3)2]+ ions as the oxidant than that by using AgNO3, which was favorable to hindering the coalescence of Ag NPs and homopolymerization of pyrrole monomer. Being a weak oxidant, [Ag(NH3)2]+ ion played a key role in the fabrication of raspberry-like composites and core/shell composites without any stabilizers. It has been experimentally demonstrated that Ag NPs have high catalytic activity in reduction reactions of nitrophenols, hydrogenation, and various dyes [25–27]. Reduction of MB dye is usually used as a standard for determining the catalytic activity of Ag NPs or silver composites. Here, we also explored the catalytic property of the raspberry-like composites by studying the evolution of the absorbance intensity at kmax. The preliminary catalytic testing was carried
out by reducing MB dye in water with NaBH4 as the reducing agent. Fig. 6 illustrates the UV–Vis spectra of the MB dye during the reaction in the presence or absence of the raspberry-like composites. Curve (a) is the UV–Vis spectrum of the initial mixture with the color of blue (inset of Fig. 6) and the kmax appears at 665 nm. The Curve (b) is corresponding to the UV–Vis spectrum of the mixture without the raspberry-like composites after reacting for 72 h at room temperature. We can see that the absorbance intensity at kmax of MB decreases, but does not completely disappear. The color of the mixture becomes light blue (inset of Fig. 6). After 10 mg of raspberry-like composites as catalyst was added into the mixture, the absorbance at kmax completely vanished within 2 min, and the color turned colorless as seen in the inset of Fig. 6. The experimental results confirmed that the raspberry-like composites had very good catalytic performance and the Ag NPs were effectively protected by PPy shell from aggregation. After reaction, the raspberry-like composites can be easily removed from the reaction solution by centrifugation at 4000 rpm due to the large size of the silica sphere supporters. The concentration of [Ag(NH3)2]+ ions had a direct influence on the morphologies and the catalytic properties of raspberry-like composites. Fig. 7a–c demonstrate the morphologies of the raspberrylike composites as a function of the [Ag(NH3)2]+ concentration. The [Ag(NH3)2]+ concentrations of samples a–c are 0.03, 0.2, and 0.5 M, respectively, while the other parameters are the same. When the concentration of [Ag(NH3)2]+ ions is low, for example, 0.03 M, the silica spheres are coated by a small amount of Ag/PPy core/shell composites (Fig. 7a). The amount of the Ag/PPy composites deposited on the surface of silica spheres increases with the increase of the [Ag(NH3)2]+ concentration. However, at high [Ag(NH3)2]+ concentration, for example, 0.5 M, some core/shell composites aggregate on the surface of silica spheres (Fig. 7c), probably because of the fast polymerization of the pyrrole monomer at high [Ag(NH3)2]+ concentrations. To investigate the relationship between the concentration of [Ag(NH3)2]+ ions and the catalytic property of the raspberry-like composites, samples a–c with the same mass were added into three identical MB dye solutions. After the injection of aqueous NaBH4 solution and being stirred for 90 s, the mixture was immediately measured by a UV–Vis spectrometer. The rate of color disappearance was used to compare the catalytic activity of the raspberry-like composites. Fig. 8 shows that the greater the concentration of [Ag(NH3)2]+ ions used, the weaker the absorbance intensity at kmax. The UV–Vis spectra and SEM images suggest that the more Ag/PPy core/shell composites deposited on the surface of silica spheres, the better the catalytic property of the raspberry-like composites. 4. Summary
Fig. 6. UV–Vis spectra of (a) the initial mixture with MB dye and NaBH4, (b) the mixture reacting for 72 h without adding raspberry-like composites and (c) the mixture reacting for 2 min after the addition of 10 mg raspberry-like composites. The inset shows optical photos of solutions (a–c).
In summary, we have successfully synthesized raspberry-like composites in a one-step reaction without any stabilizers. After etching silica spheres by HF acid, Ag/PPy core/shell composites were obtained; the PPy shell was uniform with the thickness of ca. 5 nm. A possible mechanism of the fabrication of PPy composites without stabilizers was proposed: silica spheres used as the source of oxidant
Fig. 7. SEM images of raspberry-like composites synthesized under different concentrations of [Ag(NH3)2]+ ions while other parameters equal: (a) 0.03 M, (b) 0.2 M and (c) 0.5 M.
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B06009), and National Basic Research 2007CB936402 and 2007CB936403).
Program
(Nos.
References
Fig. 8. UV–Vis spectra of MB dye solution reduced by NaBH4 as a function of [Ag(NH3)2]+ concentrations.
and the selection of [Ag(NH3)2]+ ions as oxidant played key roles. The raspberry-like composites exhibited very good catalytic performance based on the reduction results of MB dye with NaBH4 as the reducing agent. The catalytic property of the raspberry-like composites was improved with an increase of [Ag(NH3)2]+ concentration. The method presented here may open up new opportunities to fabricate other raspberry-like core/shell conducing polymer composites with potential in various applications. Acknowledgments This work was supported by the National Nature Science Foundation of China (Nos. 20534040 and 20674026), ‘‘111” project (No.
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