Improvement of the Stability of Colloidal Gold Superparticles by

Jan 22, 2010 - ... microscopy (TEM) was conducted using a Hitachi H-800 electron microscope at an acceleration voltage of 200 kV with a CCD cinema...
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Improvement of the Stability of Colloidal Gold Superparticles by Polypyrrole Modification Jie Wu, Xue Zhang, Tongjie Yao,† Jing Li, Hao Zhang,* and Bai Yang State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012, P. R. China. †Current address: Nature Science Research Center, Harbin Institute of Technology, Harbin 150001, P. R. China. Received November 29, 2009. Revised Manuscript Received January 6, 2010 The colloidal gold superparticle (SP)/polypyrrole (PPy) core/shell composites were successfully prepared by oxidative polymerization of pyrrole monomer on the surface of poly(N-vinylpyrrolidone) (PVP)-grafted colloidal gold SPs. These core/shell composites showed strong catalytic activity and excellent stability. Control experiments indicated that the morphology and the thickness of PPy shell were controllable by adjusting the dosage of pyrrole monomer. Meaningfully, the resulting SP/PPy core/shell composites were quite stable in water, which could be stored for more than half a year without damaging their structures. As an example, we demonstrated the use of these composites as catalyst for the reduction of methylene blue (MB) dye with a reducing agent of sodium borohydride. The composites exhibited highly catalytic activity and long-term stability, implying promising applications of SP/PPy composites in catalysis.

Introduction Directing the self-assembly of colloidal nanoparticles (NPs) into one-, two-, and three-dimensional (1D, 2D, and 3D) arrays is the current means for enhancing and integrating the performance of NPs, which promotes both the academic studies and the technical applications of NPs ranging from biological labeling to optoelectronic devices.1-4 Typically, such self-assembly is the hallmark of supramolecular chemistry; namely, the assembly of NPs into higher-order structures is through specific and noncovalent interactions, such as electrostatic interactions, hydrogen bonding, van der Waals interactions, and so forth.5-8 By flexible control of these weak interactions, omnifarious nanostructures from NPs have been fabricated. For example, Bawendi and co-workers originally discovered that monodispersed NPs capped by hydrophobic ligands with long alkyl chains spontaneously tended to form highly ordered superlattices.6 The formation of this structure was mainly driven by the hydrophobic-hydrophobic interaction between the long alkyl chains during the slow evaporation of organic solvents. Meaningfully, the operation of the aforementioned process using oil droplets in microemulsions as templates led to novel colloidal superparticles (SPs), defined as *Corresponding author: Fax þ86-431-85193423; e-mail hao_zhang@ jlu.edu.cn. (1) Yin, X. B.; Qi, B.; Sun, X. P.; Yang, X. R.; Wang, E. K. Anal. Chem. 2005, 77, 3525–3530. (2) Seker, F.; Malenfant, P. R. L.; Larsen, M.; Alizadeh, A.; Conway, K.; Kulkarni, A. M.; Goddard, G.; Garaas, R. Adv. Mater. 2005, 17, 1941–1945. (3) Tain, E. T.; Wang, J. X.; Zheng, Y. M.; Song, Y. L.; Jiang, L.; Zhu, D. B. J. Mater. Chem. 2008, 18, 1116–1122. (4) Tang, Z. Y.; Kotov, N. A.; Giersig, M. Science 2002, 297, 237–240. (5) Wu, J.; Zhang, H.; Zhang, J. H.; Yao, T. J.; Sun, H. Z.; Yang, B. Colloids Surf., A 2009, 348, 240–247. (6) Jin, J.; Iyoda, T.; Cao, C. S.; Song, Y. L.; Jiang, L.; Li, T. J.; Zhu, D. B. Angew. Chem., Int. Ed. 2001, 40, 2135–2138. (7) Hong, X.; Li, J.; Wang, M. J.; Xu, J. J.; Guo, W.; Li, J. H.; Bai, Y. B.; Li, T. J. Chem. Mater. 2004, 16, 4022–4027. (8) Zhang, Q.; Zhang, T. R.; Ge, J. P.; Yin, Y. D. Nano Lett. 2008, 8, 2867–2871. (9) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Science 1995, 270, 1335–1338. (10) Zhuang, J. Q.; Wu, H. M.; Yang, Y. G.; Cao, Y. C. Angew. Chem., Int. Ed. 2008, 47, 2208–2212. (11) Gur, I.; Fromer, N. A.; Geier, M. L.; Alivisatos, A. P. Science 2005, 301, 462.

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the water-dispersible colloidal particles made of hundreds of NPs through self-assembly.10-13 To date, SPs with various NP components can be fabricated through this protocol. The components include the NPs of Au, Fe3O4, CdSe, NaYF4, and so forth,10,14-16 thus greatly extending the applications of NPs in nanoscience and nanotechnology. Unfortunately, the resulting SPs are usually fragile because in comparison to covalent interactions, the hydrophobic-hydrophobic interaction between each NP is easier to be broken, which limits the practical applications of SPs. Typically, SPs are foremost fabricated using various surfactants, such as dodecyltrimethylammonium bromide (DTAB), sodium dodecyl sulfate, and so forth.10 Although the resulting SPs possesses uniform size and ordered NP arrays, the disassembly of SPs occurs in long time storage or in some special conditions, such as vigorous stirring or in the presence of organic solvent. Accordingly, polymers are expected to improve the stability of SPs by forming core/shell composites, namely SP core with polymer shell. In this scenario, SPs must be dispersed in water because they will fast disassemble in other solvents. Consequently, the polymers used in this strategy must soluble in water. For example, hydrophilic polymer of poly(N-vinylpyrrolidone) (PVP) was applied to form SP/PVP composites.15 The resulting composites indicated improved chemical and physical stability beyond virgin SPs.10,15 Because of the water solubility of PVP, however, the disassembly of the composite SPs still occurred slowly. The SPs disassembled after weeks of storage or even faster in alkaline solution. For practical applications, new protocol is required to further improve the stability of SPs. It is known that pyrrole is slightly soluble in water, making it possible to prepare polypyrrole (PPy) in aqueous media. Moreover, (12) Rosi, N. L.; Mirkin, C. A. Chem. Rev. 2005, 105, 1547. (13) Jin, R. C.; Jureller, J. E.; Kim, H. Y.; Scherer, N. F. J. Am. Chem. Soc. 2005, 127, 12482. (14) Bai, F.; Wang, D. S.; Huo, Z. Y.; Chen, W.; Liu, L. P.; Liang, X.; Chen, C.; Wang, X.; Peng, Q.; Li, Y. D. Angew. Chem., Int. Ed. 2007, 46, 6650–6653. (15) Zhuang, J. Q.; Wu, H. M.; Yang, Y. G.; Cao, Y. C. J. Am. Chem. Soc. 2007, 129, 14166–14167. (16) Zhuang, J. Q.; Shaller, A. D.; Lynch, J.; Wu, H. M.; Chen, O.; Li, A. D. Q.; Cao, Y. C. J. Am. Chem. Soc. 2009, 131, 6084–6085.

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PPy possesses excellent stability in various conditions, such as water, organic solvent, air, and so forth, implying that the stability of SPs could be improved through PPy coating.17-20 Furthermore, PPy can be easily synthesized by various approaches, such as oxidative polymerization and electrochemical polymerization.21,22 Consequently, in this paper, we report the synthesis of gold SP/PPy composites with defined core/shell structures, which possessed both high stability and strong catalytic activity.

Experimental Section Materials. Pyrrole monomer, poly(N-vinylpyrrolidone) (PVP,

Mw = 5.5  104), and dodecyltrimethylammonium bromide (DTAB) were purchased from Sigma-Aldrich. Pyrrole monomer was distilled under reduced pressure and stored at -4 °C prior to use. The aqueous chloroauric acid (HAuCl4) with a specific AuCl4concentration was prepared using analytically pure HAuCl4 reagent which was purchased from Alfa Aesar. Gelatin, sodium borohydride (NaBH4), ethylene glycol, toluene, chloroform, methylene blue (MB) dye, and FeCl3 3 6H2O were analytical grade and used as received. Octadecyl-p-vinylbenzyldimethylammonium chloride (OVDAC) was synthesized according to ref 23. In all preparations, absolute ethanol and deionized water were used. Synthesis of OVDAC-Stabilized Gold NPs. OVDACstabilized gold NPs were prepared through NaBH4 reduction of HAuCl4.24 Typically, a 10 mL HAuCl4 aqueous solution (30 mM) was mixed with 30 mL of OVDAC chloroform solution (7 mg/ mL) under vigorous stirring. After Au3þ was completely transferred to the chloroform phase, 8.3 mL of NaBH4 aqueous solution (15.3 mg/mL) was added dropwise into the aforementioned mixtures under vigorous stirring. The mixtures were stirred at room temperature for 30 min, and then, the chloroform solution was separated to obtain OVDAC-stabilized Au NPs. Synthesis of DTAB-Capped SPs. The SPs were synthesized according to the modified literature method.10,14 First, 1 mL of chloroform solution of gold NPs was evaporated to remove chloroform and followed by adding 1 mL of toluene to form the toluene solution of gold NPs. Subsequently, the toluene solution of gold NPs was mixed with 5 mL of aqueous solution of 2.8 mg/mL DTAB under vigorous stirring. After removing toluene by evaporation at 55 °C, the DTAB-capped SPs were obtained. Synthesis of PVP-Capped SPs. The solution of DTABcapped SPs was mixed with an ethylene glycol solution of PVP (2.0 mM, 5 mL); after stirring for several minutes, the gelatin aqueous solution (1.0 wt %, 0.5 mL) was added. Then, the temperature of the mixture was kept at 70 °C for 1 h. After cooling to the room temperature, the products were separated by centrifugation and the PVP-capped SPs were synthesized. Preparation of SP/PPy Core/Shell Composites. Pyrrole monomer was injected into PVP-capped SPs solution by using syringe under magnetic stirring at ambient temperature for half an hour. Then, 1 mL of 10 mg/mL aqueous FeCl3 3 6H2O solution was added into the system, and the polymerization took place. Keep stirring for 12 h, SP/PPy core/shell composites were obtained. The products were collected and washed by water and (17) Yao, T. J.; Lin, Q.; Zhang, K.; Zhao, D. F.; Lv, H.; Zhang, J. H.; Yang, B. J. Colloid Interface Sci. 2007, 315, 434–438. (18) Yoon, C. O.; Sung, H. K.; Kim, J. H.; Barsoukov, E.; Kim, J. H.; Lee, H. Synth. Met. 1999, 99, 201–212. (19) Cascalheira, A. C.; Aeiyach, S.; Lacaze, P. C.; Abrantes, L. M. Electrochim. Acta 2003, 48, 2523–2529. (20) Yuan, Y. J.; Adeloju, S. B.; Wallace, G. G. Eur. Polym. J. 1999, 35, 1761– 1772. (21) Yao, T. J.; Wang, C. X.; Wu, J.; Lin, Q.; Lv, H.; Zhang, K.; Yu, K.; Yang, B. J. Colloid Interface Sci. 2009, 338, 573–577. (22) Yao, T. J.; Li, X.; Lin, Q.; Wu, J.; Ren, Z. Y.; Wang, C. X.; Zhang, J. H.; Yu, K.; Yang, B. Polymer 2009, 50, 3938–3942. (23) Zhang, H.; Wang, C. L.; Li, M. J.; Ji, X. L.; Zhang, J. H.; Yang, B. Chem. Mater. 2005, 17, 4783–4788. (24) Gittins, D. I.; Caruso, F. Angew. Chem., Int. Ed. 2001, 40, 3001–3004.

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Figure 1. TEM images of (a) OVDAC-stabilized gold NPs and (b) PVP-capped SPs. Inset of (b) shows the highly magnified image, and the arrows indicate the individual gold NPs. ethanol for several times. The thickness of the PPy shell was controlled by adjusting the concentration of pyrrole monomer. Catalysis of the Reduction of MB Dye. The catalytic property of pure PPy, isolated gold NPs, DTAB-capped SPs, PVP-capped SPs, and SP/PPy composites were investigated by studying the change of the absorbance intensity at the maximum absorbance wavelength (λmax) of the MB dye. In a typical procedure, a certain amount of the samples was homogeneously dispersed into the aqueous solution of MB dye (67.0 mg/L) and followed by a rapid injection of 0.5 mL of aqueous solution containing NaBH4 (56.0 mg) under stirring. The blue color of the mixture gradually vanished, indicating the samples catalyzed the reduction of the MB dye. Characterization. Transmission electron microscopy (TEM) was conducted using a Hitachi H-800 electron microscope at an acceleration voltage of 200 kV with a CCD cinema. A JEOL JSM6700F scanning electron microscope (SEM) with primary electron energy of 3 kV was employed to examine the surface morphologies of products. UV-vis absorption spectra were obtained using a Shimadzu 3100 UV-vis spectrophotometer. Fourier-transform infrared (FTIR) spectra were measured in wavenumber ranging from 400 to 4000 cm-1 using a Nicolet Avatar 360 FTIR spectrophotometer. Dynamic light scattering (DLS) and zeta potential measurements were performed using a Zetasizer Nano-ZS (Malvern Instruments).

Results and Discussion Gold NPs used in our studies were synthesized by using OVDAC as stabilizer. Figure 1a displays the TEM image of the as-prepared OVDAC-stabilized gold NPs. The gold NPs with a spherical morphology were uniform, and their diameter was about 6 nm. These NPs were used as the building blocks to fabricate SPs. PVP-capped SPs were prepared through a modified reaction.15 First, a toluene solution of OVDAC-stabilized gold NPs was mixed with an aqueous DTAB solution. By means of vigorous stirring, a stable oil-in-water microemulsion was obtained. After evaporating the low-boiling-point solvent of toluene, gold NPs assembled into nanoparticle-micelle through hydrophobic van der Waals interactions. Afterward, the nanoparticle-micelle aqueous solution was mixed with PVP-ethylene glycol solution under vigorous stirring and kept at 70 °C for 1 h. Finally, PVP-capped SPs were separated from the reaction solution by centrifugation. Figure 1b was the TEM image of the PVP-capped SPs. From the inset, it could be seen clearly that the SPs were constructed from small gold NPs, and the diameter was around 200 nm. Although the stability of PVP-capped SPs was greatly improved due to the protection of PVP, the resultant SPs still disassembled after weeks of storage in water. To further improve the stability of gold SPs, PPy was applied to cap SPs through oxidative polymerization.17 Experimentally, the oxidant of FeCl3 3 6H2O was added into the aqueous solution of PVP-capped SPs in the presence of pyrrole monomer to initiate polymerization. The suspension color gradually turned black, Langmuir 2010, 26(11), 8751–8755

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Figure 2. SEM images of SP/PPy core/shell composites with different concentrations of pyrrole monomer: (a) 0.57, (b) 1.14, (c) 2.28, (d) 2.85, (e) 8.55, and (f) 14.25 mmol/L.

Figure 3. TEM images of SP/PPy core/shell composites with different concentrations of pyrrole monomer: (a) 0.57, (b) 1.14, (c) 2.28, (d) 2.85, (e) 8.55, and (f) 14.25 mmol/L.

indicating the polymerization of pyrrole monomer. Figure 2 displays the morphologies of SP/PPy core/shell composites with different concentrations of pyrrole monomer. In Figure 2a, the concentration of pyrrole monomer was 0.57 mmol/L. PPy antennas with length of tens of nanometers have formed and randomly grown on the surface of SPs. When the concentration of pyrrole monomer increased from 1.14 mmol/L (Figure 2b) to 2.28 mmol/L (Figure 2c), the PPy antennas disappeared. Compared with the pure SPs in Figure 1b, the surface of composites in Figure 2b,c became much rougher, and this was the feature of PPy homopolymer, suggesting the PPy shell had coated on the SPs surface. When the concentration of pyrrole monomer reached 2.85 mmol/L (Figure 2d), besides SP/PPy composites, some pure PPy NPs appeared around these composites. With further increasing the concentrations of pyrrole monomer to 8.55 mmol/L (Figure 2e) and 14.25 mmol/L (Figure 2f), the number of pure PPy NPs gradually increased obviously. However, it had no influence on the size of pure PPy NPs, the diameters of them were all around 40 nm. In order to confirm the core/shell structure of composites and investigate the influence of pyrrole monomer concentration on the thickness of PPy shell, TEM images are shown in Figure 3. In Figure 3a, the concentration of pyrrole monomer was 0.57 mmol/L; only PPy antennas randomly grew on SPs surface, and no complete shell formed. When the concentration of pyrrole monomer increased to 1.14 mmol/L, the PPy shell was quite uniform and ultrathin; the average thickness was about 10 nm (Figure 3b). In Figure 3c, the concentration of pyrrole monomer reached 2.28 mmol/L, and the thickness of PPy shell increased to 16 nm. When the concentration of pyrrole monomer further increased to Langmuir 2010, 26(11), 8751–8755

Figure 4. FTIR spectra of (a) DTAB-capped SPs, (b) PVP-capped SPs, and (c) SP/PPy core/shell composites.

2.85 mmol/L, the average thickness of PPy shell increased to 20 nm (Figure 3d). In parts e and f of Figure 3, the concentration of pyrrole monomer was 8.55 and 14.25 mmol/L, respectively. Compared with the thickness of PPy shell in Figure 3d, the thickness of these two samples did not change obviously and were both around 20 nm. However, more and more pure PPy NPs appeared around the core/shell composites. This result was consistent with the SEM observations. Besides, the formation of pure PPy NPs with increasing the concentration of pyrrole monomer was attributed to the limited protection from PVP; namely, PVP could not effectively prevent the PPy from aggregating when the hydrodynamic diameter of PVP was shorter than the thickness of the PPy shell.25,27 Consequently, the experimental results showed when the concentration of pyrrole monomer exceeded 2.28 mmol/L, the PPy shell was too thick to form a hydrogen bond between the carbonyl group of PVP backbone and the amino group on pyrrole ring. It led to the formation of the pure PPy NPs originating from the polymerization of pyrrole monomer in the solution. The formation of SP/PPy core/shell composites were further confirmed by FTIR spectra (Figure 4). As shown in curve (a), the (25) Lascelles, S. F.; Armes, S. P. J. Mater. Chem. 1997, 7, 1339–1347.

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Figure 5. TEM image of SP/PPy composites without assistance of PVP.

characteristic peak at 3421 cm-1 corresponded to stretching mode of N-H of DTAB. Compared curve (b) with curve (a), a strong peak at 1662 cm-1 was observed due to the pyrrolidone carbonyl group on PVP chains, which suggested PVP had successfully grafted on the surface of SPs. Curve (c) was the FTIR spectrum of SP/PPy composites, from which the PPy peaks were distinguished clearly. In this context, the characteristic bands, located at 1561 and 1460 cm-1, were assigned to the stretching mode of the pyrrole ring. The peaks at 1315 and 1038 cm-1 were related to the in-plane vibrations of =C-H. The band at 1212 cm-1 was assigned to the C-N stretching mode, and the peak at 793 cm-1 was attributed to C-H wagging vibration.26 Thus, by combining the FTIR spectra and SEM and TEM images, we firmly concluded that the SP/PPy core/shell composites have been successfully prepared. In our system, PVP was particularly important for the synthesis of SP/PPy core/shell composites. Because of the hydrophobic interaction between DTAB alkane and PVP backbone, PVP strongly adsorbed on the SPs surface.27 Most importantly, PVP could also provide active sites for pyrrole monomer loading on.28-30 Pyrrole monomer could polymerize both on the surface of SPs and in solution. As PVP adsorbed on the surface of SPs, pyrrole monomer preferred to polymerize on the surface through the formation of hydrogen bond between the amino group on pyrrole ring and the carbonyl group on PVP backbone. Without PVP, however, the PPy shell could not effectively coat on the surface of DTAB-capped SPs. In a control experiment, it failed to form a complete PPy shell around DTAB-capped SPs through pyrrole polymerization in the absence of PVP (Figure 5). As we known, DTAB is a cationic surfactant and the PPy backbone displays the positive charge.31 Therefore, during the polymerization, PPy chains could not deposit onto the surface of DTABcapped SPs due to the electrostatic repulsion. Note that the stability of SPs was significantly improved through PPy modification, thus extending the practical applications of SPs, for instance catalysis use. It has been experimentally demonstrated that metal NPs had high catalytic activity in the (26) Li, X.; Wan, M. X.; Wei, Y.; Shen, J. Y.; Chen, Z. J. J. Phys. Chem. B 2006, 110, 14623–14626. (27) Smith, J. N.; Meadows, J.; Wiliams, P. A. Langmuir 1996, 12, 3773–3778. (28) Hao, L.; Zhu, C.; Chen, C.; Kang, P.; Hu, Y.; Fan, W.; Chen, Z. Synth. Met. 2003, 139, 391–396. (29) Hao, L.; Zhu, C.; Jiang, W.; Chen, C.; Hu, Y.; Chen, Z. J. Mater. Chem. 2004, 14, 2929–2934. (30) Fujii, S.; Armes, S. P.; Jeans, R.; Devonshire, R. Chem. Mater. 2006, 18, 2758. (31) Li, C.; Bai, H.; Shi, G. Q. Chem. Soc. Rev. 2009, 38, 2397–2409. (32) Ahmadi, T.; Wang, Z. L.; Green, T. C.; Henglein, A.; EI-Sayed, M. A. Science 1996, 272, 1924–1926.

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Figure 6. UV-vis spectra of (a) the initial mixture with MB dye and NaBH4, (b) the mixture reacting for 180 min without adding core/shell composites, and (c) the mixture reacting for 2 min after the addition of 20 mg core/shell composites. The inset shows optical photos of solution (a), (b), and (c).

reduction reactions of nitrophenols, hydrogenation, and various dyes.32-36 Reduction of MB dye is usually used as a standard to evaluate the catalytic activity of gold NPs or gold composites. However, to our best knowledge, no study has reported the catalytic property of SPs due to their poor stability. Here, we explored the catalytic property of the SP/PPy core/shell composites by studying the evolution of the absorbance intensity at λmax. The preliminary catalytic testing was carried out by reducing MB dye in water with NaBH4 as the reducing agent. Figure 6 illustrates the UV-vis spectra of the MB dye during the reaction in the presence or absence of the SP/PPy composites. Curve (a) is the UV-vis spectrum of the initial mixture with the color of blue (inset of Figure 6), and the λmax appeared at 665 nm. Curve (b) corresponds to the UV-vis spectrum of the mixture without the SP/PPy composites after reacting for 180 min at room temperature. It could be seen that the absorbance intensity at the λmax of MB decreased, but did not completely disappear, and the color of the mixture became light blue (inset of Figure 6). Through the addition of 20 mg of SP/PPy core/shell composites as catalyst to the mixture, moreover, the absorbance at λmax completely vanished within 2 min (inset of Figure 6). The experimental results confirmed that the core/shell composites had very good catalytic performance. High catalytic activity is regarded as the advantages of NPbased heterogeneous catalysts. However, in most cases, these merits suffer from the challenge of low efficiency of separation and the decrease of catalytic activity due to the NP aggregation.37,38 These problems were solved by forming SP/PPy composites. In a control experiment, DTAB- and PVP-capped SPs were dispersed in water under stirring for 5 days. Under TEM (Figure 7a,b), DTAB-capped SPs and PVP-capped SPs were found collapsed, and gold NPs aggregated into the macroprecipitate and deposited on the bottom of the beaker. The poor stability of DTAB-capped SPs and PVP-capped SPs was attributed to the solubility of DTAB and PVP in water. Under vigorous (33) Jiang, Z. J.; Liu, C. Y.; Sun, L. W. J. Phys. Chem. B 2005, 109, 1730–1735. (34) Patel, A. C.; Li, S.; Wang, C.; Zhang, W. J.; Wei, Y. Chem. Mater. 2007, 19, 1231–1238. (35) Bhattacharjee, S.; Dotzauer, D. M.; Bruening, M. L. J. Am. Chem. Soc. 2009, 131, 3601–3610. (36) Liang, C. H.; Xia, W.; van den Berg, M.; Wang, Y. M.; Soltani-Ahmadi, H.; Schluter, O.; Fischer, R. A.; Muhler, M. Chem. Mater. 2009, 21, 2360–2366. (37) Medasani, B.; Park, Y. H.; Vasiliev, I. Phy. Rev. B 2007, 75, 235436–235441. (38) Lee, J.; Park, J. C.; Song, H. Adv. Mater. 2008, 20, 1523–1528.

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Figure 7. TEM images of (a) DTAB-capped SPs after stirring in water for 5 days, (b) PVP-capped SPs after stirring in water for 5 days, and (c) SP/PPy stored in water for half a year.

stirring, DTAB and PVP detached from the surface of SPs and dissolved in water, which dramatically reduced the stability of SPs and resulted in SPs coagulation. In comparison, PPy exhibited excellent stability in water, making SP/PPy composites dispersed in water but maintained the original core/shell structures for longer time. In our experiment, the SP/PPy composites could be stored in water for more than half a year without structural collapse (Figure 7c). DLS measurements further indicated that the hydrodynamic diameter of these SP/PPy composites was 314 nm and the zeta potential was -31.0 mV, which are close to freshly prepared SP/PPy composites. These results firmly proved that PPy shell significantly enhanced the stability of gold SPs. In the previous studies, the NP/polymer composites used as the catalysts in liquid-phase reactions were usually composed of single or several metal NPs core and a polymer shell. Their size was very small, only about tens of nanometers due to the small diameter of metal NPs.39-42 One of the limits in the application of these composites was the difficulty to separate them from the reaction solution by common centrifugation or filtration. Therefore, assistant supporters were usually required in these systems.43-45 However, in our study, we first used SP as a core to replace the traditional small size NPs. After catalytic reaction, the SP/PPy composites could be easily separated from the reaction solution by centrifugation at 5000 rpm because of the large size of SP/PPy composites, thus avoiding the pollution of reaction solution. Since the SP/PPy composites could be easily separated from the reaction solution by centrifugation, they were reusable after multiple cycles of reactions. As shown in Figure 8, eight successive catalytic reactions were carried out using pure PPy, isolated gold NPs, DTAB-capped SPs, PVP-capped SPs, and SP/PPy composites. In each cycle, after the solution turned colorless, the samples were quickly separated from the solution by centrifugation and rinsed with deionized water for the next cycle of catalysis. Even after eight successive reactions, the catalytic property of SP/PPy composites had no change, represented by the fast color change of MB solution from blue to colorless within 2 min. In comparison, pure PPy had no catalytic activity. The catalytic ability of isolated gold NPs vanished rapidly after only 1 cycle, and the catalytic (39) Ye, S. J.; Lu, Y. J. Phys. Chem. C 2008, 112, 8767–8772. (40) Selvan, S. T.; Spatz, J. P.; Klok, H. A.; Moller, M. Adv. Mater. 1998, 10, 132–134. (41) Pillalamarri, S. K.; Blum, F. D.; Bertino, M. F. Chem. Commun. 2005, 4584–4585. (42) Feng, X. M.; Huang, H. P.; Ye, Q. Q.; Zhu, J. J.; Hou, W. H. J. Phys. Chem. C 2007, 111, 8463–8468. (43) Miyazaki, K.; Nishida, Y.; Matsuoka, K.; Iriyama, Y.; Abe, T.; Matsuoka, M.; Kikuchi, K.; Ogumi, Z. Electrochemistry 2007, 75, 217–220. (44) Yan, W. F.; Mahurin, S. M.; Chen, B.; Overbury, S. H.; Dai, S. J. Phys. Chem. B 2005, 109, 15489–15496. (45) Kovalenko, G. A.; Rudina, N. A.; Chuenko, T. V.; Ermakov, D. Y.; Perminova, L. V. Carbon 2009, 47, 428–435.

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Figure 8. Conversion of MB dye in eight successive cycles of reduction with pure PPy, isolated gold NPs, DTAB-capped SPs, PVP-capped SPs, and SP/PPy composites.

activity of DTAB-capped SPs and PVP-capped SPs lost after 3 cycles and 4 cycles, respectively. We also studied the influence of PPy shell thickness on catalytic performance through the addition of SP/PPy composites with the shell thickness of 10, 16, and 20 nm into MB-NaBH4 solution (Figure 3b-d). Within 2 min, the color of all solutions vanished at the same time, indicating that the thickness of PPy shell had no influence on the catalytic activity. It was attributed to the incompact structure of PPy shell in aqueous media, which permitted the penetration of ions and small molecules freely.28

Conclusion In summary, we have successfully synthesized gold SP/PPy composites with defined core/shell structure and high stability. In the preparation process, PVP played a key role; without it, PPy could not cover on the surface of SPs and form a complete shell. The influence of pyrrole monomer concentrations on the morphology and shell thickness of resulting composites was carefully investigated. Experimental results indicated the optimized concentrations of pyrrole monomer ranged between 1.14 and 2.28 mmol/L. If the concentration was too high, pure PPy NPs would appear in the system. If the concentration was too low, pyrrole monomer would polymerize into PPy antennas and randomly grow on the surface of SPs. The resultant SP/PPy composites exhibited high catalytic activity and excellent stability in the catalysis applications, for instance, the reduction of MB dye with NaBH4. Most importantly, the method presented here demonstrates a new protocol to fabricate SP/polymer core/shell composites with enhanced performance, which is potentially applicable to various functional SP with optical, electrical, and magnetic properties. Acknowledgment. This work was supported by NSFC (20704014, 20974038, 20921003), the 973 Program of China (2007CB936402, 2009CB939701), the FANEDD of China (200734), the Program of Technological Progress of Jilin Province (20080101), the Special Project from MOST of China, and the Program for New Century Excellent Talents in University.

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