gold composite membranes: Preparation, characterization and application

gold composite membranes: Preparation, characterization and application

Colloids and Surfaces A: Physicochem. Eng. Aspects 393 (2012) 153–159 Contents lists available at SciVerse ScienceDirect Colloids and Surfaces A: Ph...

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Colloids and Surfaces A: Physicochem. Eng. Aspects 393 (2012) 153–159

Contents lists available at SciVerse ScienceDirect

Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa

Poly(tetrafluoroethylene)@polyaniline/gold composite membranes: Preparation, characterization and application Zhiquan Shi, Hui Zhou, Yun Lu ∗ Department of Polymer Science and Engineering, State Key Lab Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, People’s Republic of China

a r t i c l e

i n f o

Article history: Received 24 September 2011 Received in revised form 15 November 2011 Accepted 16 November 2011 Available online 25 November 2011 Keywords: Poly(tetrafluoroethylene) Polyaniline Gold Composite membrane Catalytic function

a b s t r a c t The polyaniline/gold (PANI/Au) have been successfully deposited on the surface of the poly(tetrafluoroethylent) (PTFE) membrane by one step process via simple diffusing interfacial reaction. The concentration ratio of aniline monomer to doping agent affected the microstructure and electrical conductivity of the composite membrane. The morphology and structure of the as-prepared composite membrane were characterized by using SEM, XRD, GIXRD, ATR-FTIR, Raman and UV–vis spectroscopy, and a difference on the upside and bottom side surfaces of the membrane was detected. As a candidate of advanced catalytic systems, the catalytic function of the Au on the surface of composite membrane was investigated and showed integrated benefits such as good catalysis, pollution-free, easy separation, convenient operation and regeneration in practical applications. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Poly(tetrafluoroethylene) (PTFE) has many desirable properties, such as high thermal stability, strong resistance to chemicals and good insulating property [1–3], and also present some disadvantages where adhesion-related issues are concerned. A variety of treatments have been practiced to activate PTFE surfaces to enhance their ability of adhesions, for instance chemical etching with sodium naphthalene [4], irradiating with UV lasers, electron and ion beams [5,6], plasma modification [7,8] and coating technology [9]. However, using a simple facile strategy to improve the combinability of PTFE and other polymers or metal materials remains a scientific and technical challenge. Among conducting polymers, polyaniline (PANI) has attracted considerable attention because of its low cost, ease of synthesis, good optical and electrical properties as well as excellent environmental stability [10,11]. However, as pure PANI is usually insoluble in common solvents and infusible at high temperature, its practical applications are limited seriously. Therefore, how to improve the processability of PANI has become the aim of many recent investigations.

∗ Corresponding author. Tel.: +86 25 83686423; fax: +86 25 83686423. E-mail address: [email protected] (Y. Lu). 0927-7757/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2011.11.016

It has been reported that conducting polymer-supported noble metal nanoparticle catalysts show potential applications because the conducting polymers might shuttle the electronic charges to catalyst centres [12,13]. On the other hand, metal nanoparticles incorporated into conducting polymers are also known to enhance the conductivity of the polymers [14,15]. Among these reported metal clusters, there is considerable interest in gold nanoparticles due to their unique catalytic, electronic and optical properties depending on their sizes and shapes [16–19]. Synthesis and immobilization of PANI and Au on a selected matrix have been attractive owing to the great convenience and high efficiency. However, few researches have been reported in this respect except the electrochemical route (ECR) [20,21] which is probably restricted due to the limit of the electrode dimension. In order to meet the requirements for practical applications, it is interesting and significant to introduce a novel method to realize more easily the above assumption. In the present work, the polyaniline/gold (PANI/Au) has been successfully deposited on the surface of the PTFE membrane by one step process via simple diffusing interfacial reaction. The microstructure and morphology of the as-prepared composite membranes surface were characterized by ATR-FTIR, Raman, UV–vis, XRD, GIXRD and SEM measurements. Also, the catalytic activity of the composite membrane embedded with Au nanoparticles was investigated detailedly by taking the reduction of p-nitrophenol as a model reaction.

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membranes was determined by Brucker D8 Advance instrument with Cu K␣ radiation operating at 45 kV and 40 mA and positioned at an incident angle of 0.5◦ . Surface resistances were obtained on ohmmeter at room temperature. Raman spectra were recorded on a Bruker RFS100 Fourier transform Raman spectrometer with the resolution of 1 cm−1 , which is equipped with an air-cooled Nd:YAG laser source(1064 nm) and a Ge detector cooled by liquid nitrogen. Also, the output power is 30–200 mW. Catalytic activity of PTFE@PANI/Au composite membranes was investigated by UV–vis spectra which were recorded on a UV–240 spectrometer (Shimadzu, Japan). Fig. 1. Schematic illustration of reactor to prepare PTFE@PANI/Au composite membranes.

3. Results and discussion 2. Materials and methods 2.1. Materials Aniline (ANI) monomer (Shanghai chemical Co.) was distilled under reduced pressure and refrigerated in the dark. p-Nitrophenol, NaBH4 , p-toluenesulfonic acid (p-TSA) and chloroauric acid (HAuCl4 ) were of AR grade and used without purification. NafionDE 520 (DuPont Co.) was a system with 5 wt% of Nafion diluted in a mixture solvent containing water, propanol, methanol and unspecified ethers. The structure of Nafion is composed mainly of a PTFE backbone with side-chains containing ether groups and a sulfonic acid unit at its end [22]. PTFE membrane (thickness of 20 ± 5 ␮m) was kindly supplied by Sinoma Science & Technology Co., Ltd. (Nanjing, China). 2.2. Fabrication of PTFE@PANI/Au composite membranes In a typical procedure, the pristine PTFE membrane was mounted in the plastic frame and then dipped into the Nafion solution for 30 s. These impregnated membranes were dried at 60 ◦ C for 0.5 h and then immersed in 1 M HCl at room temperature for 24 h. The preparation was carried out in a system which was separated by the PTFE/Nafion composite membranes wetted in 1 M HCl to two compartments (Fig. 1). The upper one consisted of 50 mL of 0.01 M HAuCl4 aqueous solution and the bottom one contained 50 mL of 0.06 M aniline and 0.18 M p-toluenesulfonic acid (p-TSA) aqueous solution. Aniline monomers together with p-TSA and oxidizer solutions (HAuCl4 ) were allowed to counter-diffuse simultaneously to the membrane. The reaction proceeded without agitation for 72 h at room temperature. Finally, the membranes were washed with deionized water and dried in a vacuum at 60 ◦ C. 2.3. Catalytic activity of PTFE@PANI/Au composite membranes A reaction mixture of water (10 mL) and p-nitrophenol aqueous solution (1 mL, 1.5 × 10−3 mol L−1 ) were first put in a beaker where a composite membrane of 2 cm × 2 cm has existed. To this stirring reaction mixture, 4 mL of NaBH4 aqueous solution (1.5 mol L−1 ) was added and the progress of the conversion of p-nitrophenol to paminophenol was monitored via UV–vis spectroscopy by recording the time-dependent absorption spectra of the reaction at a regular time interval of 10 min. 2.4. Characterization Scanning electron microscopy (SEM) images were obtained on a HITACHI S-4800. Attenuated Total Reflectance Fourier transform infrared (ATR-FTIR) spectra of the membranes were recorded on BrukerVECTOR22 spectrometer. X-ray diffraction (XRD) patterns were obtained on a Brucker D8 Advance instrument using Cu K␣ radiation. The grazing incidence X-ray diffraction (GIXRD) of the

PTFE porous membranes were prepared from PTFE fine powder by a series of mechanical operations, such as extrusion, rolling, and bidirectional stretching [23]. Fig. 2a shows the SEM image of pristine PTFE membranes that consist of fibrils and nodes. Between the fibrils and nodes there exist a lot of pores with a mean pore size from 1 ␮m to 3 ␮m in the PTFE membranes. After the treatment of Nafion, the microscale pores seem to be wholly covered by the Nafion and a continuous thin Nafion film appears on the PTFE surface (Fig. 2b). Also, the fibrils can be seen clearly to be immobilized together with the Nafion. Fig. 3 shows the SEM image and optical photographs of PTFE@PANI/Au composite membranes, which reveals that the composite membrane has different morphologies on its upper and bottom surfaces, and three layers in its cross section (see the inset SEM image of Fig. 3a). On the upside (Fig. 3a), the composite membranes display golden color with metallic shine and a surface as smooth as that of PTFE pristine membranes, implying the successful introduction of the gold nanoparticles on the surface of PTFE/Nafion composite membranes. As seen in Fig. 3b, the gold deposited on the upside surface display a special hierarchical nanostructure, and such a nanostructure could give full play to the nano-effect of gold nanoparticles. On the bottom side, the composite membranes show a dark green color (Fig. 3c), indicating that the aniline monomers have also successfully polymerized on the surface of PTFE/Nafion composite membranes. (For interpretation of the references to color in the text, the reader is referred to the web version of the article.) Moreover, it can be found that the spines with an average length of 160 nm and an average diameter of 40 nm bestrew the surface with polyaniline (Fig. 3d). The formation of such morphology may be ascribed to the interaction of aniline and p-TSA [24], that is, the obtained salt of aniline and p-TSA could serves as the “soft template” to induce the formation of PANI nanofibrous structure under certain conditions and then is absorbed on the surface of PTFE/Nafion membrane through electrostatic forces and ionic bonds. Comparing with the pristine PTFE membranes with good insulating property, PTFE@PANI/Au composite membranes have a good conductivity, but the property is different on upside and bottom side. Due to more gold deposited on the upside, the composite membranes have a surface resistance of 5 . On the bottom side, the resistances of composite membranes only reach 50 , which is contributed by the conducting PANI. The surface morphology of PTFE@PANI/Au composite membrane is closely related to the [ANI]/[p-TSA] ratio, as indicated by the results given in Fig. 4. As can be seen, when the ratio changes from 1: 3 to 1:1, the upside morphology of the composite membrane shows an aggregation of Au nanoparticles and the whole membrane displays yellow color with metallic shine (Fig. 4a and b). On the bottom side, the fibrils cover for the spines on the PTFE surface and the whole membrane displays blue color (Fig. 4c and d). The resistances of composite membranes are measured to be 1.5 k on the upside and 3.5 M on the bottom side, which may be

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Fig. 2. SEM images of (a) pristine PTFE membrane, (b) PTFE/Nafion composite membrane. The insets show the higher magnification SEM image.

attributed to both the low doping degree of PANI, resulting from the low concentration of p-TSA, and the different interaction degree of aniline and p-TSA with equal molar concentration. The molecular structure of PTFE@PANI/Au composite membrane on both the upside and the bottom side are characterized. From the ATR-FTIR spectrum of Fig. 5Ab, the presence of polyaniline on the bottom side surface of PTFE/Nafion membranes can be observed by the appearance of additional bands at around 1584 cm−1 and 1493 cm−1 , which belong to the quinoid and benzenoid stretching modes of polyaniline, respectively [25–27]. The bands at 1057 cm−1 and 981 cm−1 , which are associated with –SO3 − symmetric stretching and C–O–C stretching of Nafion, and the bands at 1151 cm−1 and 1205 cm−1 , which are corresponding to the C–F stretching of PTFE [23], indicate the successful combination of Nafion and PTFE. From the ATR-FTIR spectrum of Fig. 5Aa, the bands at 1205 cm−1 and 981 cm−1 are not observable, implying

that there are large amounts of PANI/Au composite fully covering the upside surface of the PTFE/Nafion membrane. The usual XRD patterns of the PTFE@PANI/Au composite membrane on upside and bottom side are shown in Fig. 5B. The composite membranes display the prominent peaks at 2 values of about 38.2◦ , 44.8◦ , 64.5◦ , 77.5◦ and 81.7◦ which are attributed to the 1 1 1, 2 0 0, 2 2 0, 3 1 1 and 2 2 2 Bragg’s diffractions of gold respectively [28], indicating the existence of metallic Au in the PTFE membranes. It is worth noting that, for the composite membrane, the intensity ratios of the 2 0 0 plane diffraction peak to 1 1 1 one (0.27) and the 2 2 0 plane diffraction peak to 1 1 1 (0.17) are all lower than the corresponding values of usual gold powders (0.52 and 0.32) [29,30]. These observations imply that the deposited gold crystal tends to grow with the surface dominated by the lowest energy 1 1 1 facets [31]. Comparing with Fig. 5Bb, the peak at 2 = 18.1◦ and 30◦ ascribed to 1 0 0 and 2 0 1 plane diffractions of

Fig. 3. SEM of PTFE@PANI/Au composite membrane (a, b), upside of membrane, (b) shows the higher magnification SEM image of (a); (c, d), bottom side of membrane, d shows the higher magnification SEM image of (c). The insets showed the optical photographs and a SEM image of the cross section of the membrane (reaction conduction: 0.01 mol L−1 HAuCl4 , [ANI] = 0.06 M, [ANI]/[p-TSA] = 1:3, 72 h).

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Fig. 4. SEM of PTFE@PANI/Au composite membrane (a, b), upside, (b) shows the higher magnification SEM image of (a); (c, d), bottom side, d shows the higher magnification SEM image of (c), The inset shows optical photographs and a SEM image of the cross section of the membrane (reaction conduction: 0.01 mol L−1 HAuCl4 , [ANI] = 0.06 M, [ANI]/[p-TSA] = 1:1, 72 h).

PTFE respectively [32] declines dramatically in intensity or disappears in Fig. 5Ba, suggesting that much more PANI/Au deposited on the upside surface of PTFE/Nafion membranes. Because of the thickness of the composite membrane is only about 20 ␮m, the signal we

got from general X-ray diffraction might include the information of the whole composite membrane instead of that of the surface. In order to get the exact surface information of the membrane, GIXRD was used to give us a better understand of it. The GIXRD

Fig. 5. PTFE@PANI/Au composite membranes (A) ATR-FTIR spectra, (B) usual XRD, (C) GIXRD (a grazing incidence angle of 0.5◦ was used to record the XRD pattern); (a, upside, b, bottom side).

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Fig. 6. PTFE@PANI/Au composite membrane (A) Raman spectra (a, upside, b, bottom side); (B) UV–vis spectrum; (C) cyclic voltammograms, versus Ag/AgCl.

patterns obtained from upside surface of the PTFE@PANI/Au composite membrane (Fig. 5Ca) showed the prominent peaks at 2 values of about 38.2◦ and 44.8◦ related to diffraction from the 1 1 1, 2 0 0 planes of gold crystal, approving the existence of metallic Au on the PTFE membranes. It is notable that no peak assigned to Au was observed in Fig. 5Cb, suggesting that Au was mainly deposited on the upside surface of the composite membrane, which is consistent with the SEM result of cross section of the composite membrane. The PANI on the membranes is in an amorphous nature. Fig. 6Aa shows the Raman spectra of the PTFE@PANI/Au composite membranes. The band at around 1591 cm−1 and 1380 cm−1 are ascribed to C–C stretching vibration, which belong to the quinoid and benzenoid stretching modes of polyaniline, respectively [33,34]. The band at 1512 cm−1 is ascribed to N–H stretching vibration. The band at 1341 cm−1 is a characteristic for the

radical captions stretching vibration. The band close to 1177 cm−1 represents the C–H bending vibrations in the aromatic ring. Also the band at 1235 cm−1 reflects the ring in-plane deformation vibration [35]. The above discussion indicates that the polyaniline have successfully deposited on the PTFE membranes surface. It is thought that the strong SERS response from our substrates can be rationalized by the following: The PTFE@PANI/Au composite membranes presents a uniform nanoscale roughness, and the electromagnetic wave excites localized surface plasmons on the surface, resulting in amplification of the electromagnetic fields near the metal surface [36,37]. There is no peak observed in Fig. 6Ab, because only a little gold deposit on the bottom side of the PTFE membrane. Fig. 6B represents the UV–vis spectrum of composite membrane with air as reference. The peak at 294 nm is assigned to the ␲–␲* electron transition of benzenoid ring segments along the PANI

Fig. 7. Catalytic activity of PTFE@PANI/Au composite membranes for the reduction of p-nitrophenol, (A) UV–vis spectra measured at 10 min intervals ([ANI]/[p-TSA] = 1:3), The inset: [ANI]/[p-TSA] = 1:1; (B) Plot of the absorbance ln(Ct /C0 ) vs time. (a) [ANI]/[p-TSA] = 1:3, (b) [ANI]/[p-TSA] = 1:1.

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chains. The peak at 478 nm, which should be attributed to ␲–␲* quinonoid transition, is characteristic of the protonated PANI. The strong absorption peak at 711 nm is ascribed to ␲-polaron transitions [38]. These data confirm the existence of conducting PANI on the surface of composite membrane. The cyclic voltammograms (CVs) of PTFE@PANI/Au composite membranes (25 mm × 3 mm) as working electrode are carried out with platinum electrode as auxiliary electrode in an aqueous solution of 1.0 mol L−1 LiClO4 at potential scan rate of 50 mV s−1 (Fig. 6C). It can be seen that PTFE@PANI/Au composite membrane exhibits reversible redox reaction with an oxidation wave at approximately −0.39 V and a reduction wave at approximately −0.92 V. As an applied example, the PTFE@PANI/Au composite membranes with a size of 2 cm × 2 cm have been employed for the catalytic reduction of p-nitrophenol by NaBH4 , which is usually simple and fast in the presence of metallic surface [39–42]. Fig. 7A shows a typical UV–vis absorption change of the reaction mixture. Generally, the absorption decrease of p-nitrophenol at 400 nm can be used to evaluate catalyst activity [43]. From these spectra, it can be seen that the absorption of p-nitrophenol at 400 nm decreases obviously within 80 min after the addition of composite membrane, indicating the good catalyst activity. Since the concentration of NaBH4 is excessive in the reaction system, it is suggested that two principal species, p-nitrophenol and p-aminophenol influence the reaction kinetics. In this case, pseudo-first-order kinetics could be applied for the evaluation of rate constants. In Fig. 7B, the ratio of Ct to C0 (Ct and C0 are p-nitrophenol concentrations at time t and 0, respectively) is measured from the relative intensity of respective absorbance, At /A0 . The linear relation of ln(Ct /C0 ) versus time is observed for the composite membrane catalyst, indicating that the reaction follows first-order kinetics. The rate constant (k = 4.12 × 10−2 min−1 ) has been estimated from firstorder reaction kinetics using the slope, and is comparable at the similar catalytic conditions to that (k = 3.65 × 10−2 min−1 ) of the film obtained by casting the poly(acrylonitrile-co-vinyl acetate)graft-poly(3,4-ethylenedioxythiophene) solution on which the gold micropheres were deposited [44]. Moreover, our experiments show that catalytic reaction rate constant of PTFE@PANI/Au composite membranes is almost unchanged in the case that they are washed by water repetitiously. This fact implies that these composite membranes can be used repeatedly. To support this point, we repeated the utilization produces for 8 times and did not find obvious decrease of the catalytic reaction rate constant. The rate constant for the last time of the repeated catalytic reaction is 3.93 × 10−2 min−1 . In our case, the catalytic activity of PTFE@PANI/Au composite membrane is closely related to the [ANI]/[p-TSA] ratio and thus obtained different surface morphologies of the composite membrane. When the [ANI]/[p-TSA] ratio changes from 1:3 to 1:1, the rate constant k decreases to 1.16 × 10−2 min−1 due to the change of morphology of Au particles on the membrane from the special nano-hiberarchy to the aggregated granules. The PTFE@PANI/Au composite membranes have an obvious advantage of easy and convenient operation in the practical catalytic reaction system. For instance, the composite membranes can be processed to different shape according to the practical need, and put into or out from the catalytic reaction system freely. And what is more, because PTFE and PANI are not soluble in organic solvents and resistant to acid and alkali, PTFE@PANI/Au composite membranes can be used in harsh systems and do not pollute the reaction system. Moreover, the tedious regeneration process, as in the case of metal particle catalyst systems, including precipitating, filtering and redispersing could be simplified by washing only with excessive water. This composite membrane is indeed a good material for promoting the practical catalytic applications.

4. Conclusion In summary, the PANI/Au has been successfully deposited on the surface of the PTFE membrane by one step process via simple diffusing interfacial reaction. The content of Au in upside surface of the composite membrane is higher than that in the bottom side, and on the contrary, the content of PANI in bottom side surface of the composite membrane is higher than that in the upside one. The ratio of [ANI]/[p-TSA] affects the micro/nanostructure as well as the electrical conductivity of the composite membrane. The composite membranes can be used as an advanced catalyst material due to their good catalysis, pollution-free, easy separation, convenient operation and regeneration in practical applications. This research provides a new method to synthesize complicated membrane structures via simple diffusing interfacial reaction. Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 20974043), Open Project of State Key Laboratory of Supramolecular Structure and Materials, Jilin University (SKLSSM201105) and the Testing Foundation of Nanjing University. References [1] J.M. Park, L.J. Matienzo, D.F. Spencer, Adhesion and XPS studies on a fluoropolymer–metal interface, J. Adhes. Sci. Technol. 5 (1991) 153–163. [2] E.H.A. Granneman, Thin films in the integrated circuit industry: requirements and deposition methods, Thin Solid Films 228 (1993) 1–11. [3] J.F. Silvain, J.J. Ehrhardt, A. Picco, P. Lutgen, Crystallographic structure and adhesion of aluminum thin films deposited on Mylar, in: ACS Symp. Ser. 440, 1990, pp. 453–466. [4] S.R. Kim, Surface modification of poly(tetrafluoroethylene) film by chemical etching, plasma, and ion beam treatments, J. Appl. Polym. Sci. 77 (2000) 1913–1920. [5] K. Lunkwitz, U. Lappan, D. Lehmann, Modification of fluoropolymers by means of electron beam irradiation, Radiat. Phys. Chem. 57 (2000) 373–376. [6] S.K. Koh, S.C. Park, S.R. Kim, W.K. Choi, H.J. Jung, K.D. Pae, Surface modification of polytetrafluoroethylene by Ar+ irradiation for improved adhesion to other materials, J. Appl. Polym. Sci. 64 (1997) 1913–1921. [7] Y. Yamada, T. Yamada, S. Tasaka, N. Inagaki, Surface modification of poly(tetrafluoroethylene) by remote hydrogen plasma, Macromolecule 29 (1996) 4331–4339. [8] S. Wu, E.T. Kang, K.G. Neoh, Surface modification of poly(tetrafluoroethylene) films by graft copolymerization for adhesion improvement with evaporated copper, Macromolecules 32 (1999) 186–193. [9] H. Gharibi, M. Zhiani, A.A. Enezami, R.A. Mirzaie, M. Kheirmand, K. Kakaei, Study of polyaniline doped with trifluoromethane sulfonic acid in gas-diffusion electrodes for proton-exchange membrane fuel cells, J. Power Sources 155 (2006) 138–144. [10] H. Liu, X. Hu, J. Wang, R. Boughton, Structure, conductivity, and thermopower of crystalline polyaniline synthesized by the ultrasonic irradiation polymerization method, Macromolecules 35 (2002) 9414–9419. [11] X.Y. Zhang, W.J. Goux, S.K. Manohar, Synthesis of polyaniline nanofibers by nanofiber seeding, J. Am. Chem. Soc. 126 (2004) 4502–4503. [12] F. Ficicoglu, F. Kadirgan, Electrooxidation of ethylene glycol on a platinum doped polyaniline electrode, J. Electroanal. Chem. 451 (1998) 95–99. [13] C.S.C. Bose, K. Rajeshwar, Efficient electrocatalyst assemblies for proton and oxygen reduction: the electrosynthesis and characterization of polypyrrole films containing nanodispersed platinum particles, J. Electroanal. Chem. 333 (1992) 235–256. [14] K.G. Neoh, T.T. Young, N.T. Looi, E.T. Kang, K.L. Tan, Oxidation–reduction interactions between electroactive polymer thin films and Au(III) ions in acid solutions, Chem. Mater. 9 (1997) 2906–2912. [15] E.W.H. Jager, E. Smela, O. Inganas, Microfabricating conjugated polymer actuators, Science 290 (2000) 1540–1545. [16] H. Coelfen, S. Mann, Higher-order organization by mesoscale self-assembly and transformation of hybrid nanostructures, Angew. Chem. Int. Ed. 42 (2003) 2350–2365. [17] A.K. Boal, F. Ilhan, J.E. DeRouchey, T.T. Albrecht, T.P. Russell, V.M. Rotello, Selfassembly of nanoparticles into structured spherical and network aggregates, Nature 404 (2000) 746–748. [18] Z.R. Tian, J.A. Voigt, J. Liu, B. Mckenzie, M.J. Mcderott, M.A. Rodriguze, H. Konishi, H.F. Xu, Complex and oriented ZnO nanostructures, Nature 2 (2003) 821–826. [19] M. Muthukumar, C.K. Ober, E.L. Thomas, Competing interactions and levels of ordering in self-organizing polymeric materials, Science 277 (1997) 1225–1232.

Z. Shi et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 393 (2012) 153–159 [20] R. Narayanan, M.A. El-Sayed, Catalysis with transition metal nanoparticles in colloidal solution: nanoparticle shape dependence and stability, J. Phys. Chem. B 109 (2005) 12663–12676. [21] D.I. Enache, D.W. Knight, G.J. Hutchings, Solvent-free oxidation of primary alcohols to aldehydes using supported gold catalysts, Catal. Lett. 103 (2005) 43–52. [22] C.S. Yong, J. Yun, H. Park, W.J. Kim, D.H. Ha, W.S. Yun, Size-controlled synthesis of machinable single crystalline gold nanoplates, Chem. Mater. 17 (2005) 5558–55561. [23] S. Tan, D. Beˇılanger, Characterization and transport properties of Nafion/polyaniline composite membranes, J. Phys. Chem. B 109 (2005) 23480–23490. [24] S. Panigrahi, S. Basu, S. Praharaj, S. Pande, S. Jana, A. Pal, S.K. Ghosh, T. Pal, Synthesis and size-selective catalysis by supported gold nanoparticles: study on heterogeneous and homogeneous catalytic process, J. Phys. Chem. C 111 (2007) 4596–4605. [25] M. Trchova, I. Sedenkova, E. Tobolkova, J. Stejskal, FTIR spectroscopic and conductivity study of the thermal degradation of polyaniline films, Polym. Degrad. Stab. 86 (2004) 179–185. [26] J. Stejskal, M. Trchova, J. Prokes, I. Sapurina, Brominated polyaniline, Chem. Mater. 13 (2001) 4083–4086. [27] H. Varela, R.M. Torresi, D.A. Buttry, Study of charge compensation during the redox process of self-doped polyaniline in aqueous media, J. Braz. Chem. Soc. 11 (2000) 32–38. [28] L.H. Lu, I. Randjelovic, R. Capek, N. Gaponik, J.H. Yang, H.J. Zhang, A. Eychmller, Controlled fabrication of gold-coated 3D ordered colloidal crystal films and their application in surface-enhanced Raman spectroscopy, Chem. Mater. 17 (2005) 5731–5736. [29] C.S. Yong, J. Yun, H. Park, W.J. Kim, D.H. Ha, W.S. Yun, Size-controlled synthesis of machinable single crystalline gold nanoplates, Chem. Mater. 17 (2005) 5558–5561. [30] X.P. Sun, S.J. Dong, E.K. Wang, High-yield synthesis of large single-crystalline gold nanoplates through a polyamine process, Langmuir 21 (2005) 4710–4712. [31] Y.G. Sun, Y.N. Xia, Shape-controlled synthesis of gold and silver nanoparticles, Science 298 (2002) 2176–2179.

159

[32] J. Stejskal, I. Sapurina, M. Trchová, E.N. Konyushenko, Oxidation of aniline: polyaniline granules, nanotubes, and oligoaniline microspheres, Macromolecules 41 (2008) 3530–3536. [33] M.C. Bernard, A. Hugot-Le Goff, Raman spectroscopy for the study of polyaniline, Synth. Met. 85 (1997) 1145–1146. [34] G. Louarn, M. Lapkowski, S. Quillard, A. Pron, J.P. Buisson, S. Lefrant, Vibrational properties of polyaniline-isotope effects, J. Phys. Chem. 100 (1996) 6998–7006. [35] C. Liu, J. Zhang, G. Shi, F.E. Chen, Doping level change of polyaniline film during its electrochemical growth process, J. Appl. Polym. Sci. 92 (2004) 171–177. [36] M. Baibarac, M. Lapkowski, A. Pron, S. Lefrant, I. Baltog, SERS spectra of poly(3hexylthiophene) in oxidized and unoxidized states, J. Raman Spectrosc. 29 (1998) 825–832. [37] E. Hesse, J.A. Creighton, Investigation by surface-enhanced Raman spectroscopy of the effect of oxygen and hydrogen plasmas on adsorbate-covered gold and silver island films, Langmuir 15 (1999) 3545–3550. [38] X. Feng, C. Mao, G. Yang, W. Hou, J. Zhu, Polyaniline/Au composite hollow spheres: synthesis, characterization, and application to the detection of dopamine, Langmuir 22 (2006) 4384–4389. [39] D. Jana, A. Dandapat, G. De, Anisotropic gold nanoparticle doped mesoporous boehmite films and their use as reusable catalysts in electron transfer reactions, Langmuir 26 (2010) 12177–12184. [40] T.K. Sau, A. Pal, T. Pal, Size regime dependent catalysis by gold nanoparticles for the reduction of eosin, J. Phys. Chem. B 105 (2001) 9266–9272. [41] K. Hayakawa, T. Yoshimura, K. Esumi, Preparation of gold–dendrimer nanocomposites by laser irradiation and their catalytic reduction of 4-nitrophenol, Langmuir 19 (2003) 5517–5521. [42] K. Esumi, K. Miyamoto, T. Yoshimura, Comparison of PAMAM–Au and PPI–Au nanocomposites and their catalytic activity for reduction of 4-nitrophenol, J. Colloid Interface Sci. 254 (2002) 402–405. [43] J. Lee, J.C. Park, H. Song, A nanoreactor framework of a Au@SiO2 yolk/shell structure for catalytic reduction of p-nitrophenol, Adv. Mater. 20 (2008) 1523–1528. [44] Y.Y. Xia, Z.Q. Shi, Y. Lu, Gold microspheres with hierarchical structure/conducting polymer composite film: preparation, characterization and application as catalyst, Polymer 51 (2010) 1328–1335.