Polyaniline Composite Nanotubes - American Chemical Society

Aug 5, 2009 - Silver/Polyaniline Composite Nanotubes: One-Step Synthesis and Electrocatalytic Activity .... mats as a suitable template through the UV...
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J. Phys. Chem. C 2009, 113, 15175–15181

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Silver/Polyaniline Composite Nanotubes: One-Step Synthesis and Electrocatalytic Activity for Neurotransmitter Dopamine Yu Gao, Decai Shan, Fei Cao, Jian Gong,* Xia Li, Hui-yan Ma, Zhong-min Su, and Lun-yu Qu Key Laboratory of Polyoxometalate Science of Ministry of Education, Department of Chemistry, Northeast Normal UniVersity, Changchun, Jilin 130024, P. R. China ReceiVed: May 22, 2009; ReVised Manuscript ReceiVed: July 16, 2009

Silver/polyaniline composite nanotubes have been successfully synthesized via a self-assembly process assisted by excess ammonium persulfate and silver nitrate as oxidant without using any acid molecule reagent or hard template. Scanning electron microscopy (SEM), energy-dispersive X-ray spectra (EDX), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), ultraviolet-visible absorption spectra (UV-vis), and X-ray photoelectron spectroscopy (XPS) were performed to characterize the resulting polyaniline samples. SEM and TEM images indicate that the as-obtained polyaniline composite entirely consists of uniform nanotubes in high yield with a diameter of about 100 nm. The TEM image shows that the average size of the dispersed silver nanoparticles decorated on the surface of the silver/polyaniline composite nanotubes is about 10 nm. A tentative mechanism is proposed in detail to elucidate the formation of the tubular nanostructures in such system. The composite nanotubes are immobilized on the surface of an indium tin oxide and applied to construct a sensor, which exhibits higher electrocatalytic activity toward reduction of dopamine than pure polyaniline. Furthermore, the high surface area, small diameter, and porous nature of the silver/polyaniline composite nanotubes and introduction of the silver nanoparticles give significantly better performance in both gas sensitivity and time response. 1. Introduction In modern chemistry and materials science, especially onedimensional (1D) nanomaterials, nanowires,1 nanorods,2 nanotubes,3 or nanofibers4 have attracted intense interest over the past years because of their importance in fundamental research that depends on their particular sizes and morphologies. It is generally believed that the properties of nanomaterials strongly depend on the size, shape, and dimensionality. Since the discovery of carbon nanotubes by Iijima,5 the investigation of nanotubular materials has been of great interest to many areas of science and technology due to their novel physical properties and potential applications that range from catalysts, electronic devices, sensors, and energy storage media to artificial muscles and intramolecular junctions.6-10 In particular, such nanostructures possess ultrasmall size and high surface area features, which offer great promise for gas sensors and biosensors.11 During the last decades, conducting polymers have been an attractive class of materials for a variety of advanced technologies as they possess electronic, magnetic, and optical properties similar to metals while retaining the flexibility and processability of conventional polymers.12-14 Among the family of conducting polymers, polyaniline (PANI) with various morphologies, such as nanorods, nanofibers, nanotubes, microspheres, hollow octahedral, leaf-like shape, etc., has been widely investigated because of its straightforward polymerization, chemical stability, relatively high conductivity, and potential applications in molecular electronic devices, fuel cells, nonlinear optics, sensors, etc.13,15-19 In particular, controlled synthesis of PANI with tubular nanostructures possessing the advantage of both lowdimensional systems and organic conductors are important * To whom correspondence should be addressed. Phone: +86-43185099765. Fax: + 86-431-85099668. E-mail: [email protected].

subjects in nanoscience and nanotechnology, especially in the area of nanodevices.20 Recently, the multifunctionality of metal/PANI composites is particularly useful, which have attracted considerable attention due to their enhanced gas sensing properties and electrocatalytic activity, memory devices, and others, as compared to those of pure PANI.21-28 Naturally, hybridization of the metal elements and PANI molecule can generate a new family of materials, designated as metals-doped PANI compound, which enrich the property library and widen the application scope offered by each of them individually. Among all kinds of metals-doped PANI compounds, the composites of silver (Ag)/PANI have attracted increased attention due to their interesting properties, such as catalysis, conductive inks, thick film pastes, and adhesives for various electronic components and sensors.29-32 Although the composites based on PANI and Ag have been reported, the preparation for the composites with nanostructure is still a novel challenge. Thus far, dramatic efforts have been dedicated to develop new methods for the fabrication of Ag/PANI composite nanostructures in different systems. For instance, Bertino and co-workers described the synthesis of PANI composite nanofibers decorated with noble-metal (Ag or Au) nanoparticles by γ radiolysis.33 Wang and co-workers reported chemical deposition of metal particles (Ag, Au, and Pt) and clusters with various sizes and morphologies on top of the PANI thin films and porous asymmetry membranes.34 Moreover, our group recently successfully synthesized Ag/PANI composite nanotubes decorated with dispersed Ag nanoparticles using nitrocellulose (NC) fiber mats as a suitable template through the UV rays irradiation method.35 Although great progress has been achieved on the synthesis for Ag/PANI composite nanostructures, they may cause some reduction in the conductivity or yield oligomers.36 One can overcome these difficulties by developing a simple,

10.1021/jp904788d CCC: $40.75  2009 American Chemical Society Published on Web 08/05/2009

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one-step, and effective method for fabricating novel assemblies of the Ag/PANI composite nanotubes without using any acid molecule reagent, hard template, or aid of other techniques. Due to its easily controllable reaction conditions and the relatively abundant reactant sources, the so-called soft chemical route, based on a solution process, might provide an attractive option for large-scale production of nano- and micromaterials with special morphologies.37 In the present work, we developed a simple self-assembly polymerization method for the synthesis of highly uniform and monodisperse Ag/PANI composite nanotubes without using any acid molecule reagent and hard template. The possible formation process and preliminary growth mechanism for the nanotubular composites are proposed. Furthermore, the detailed reaction process, current-voltage characteristics, and gas sensitivity of the Ag/PANI composite nanotubes have been studied. 2. Experimental Section 2.1. Materials. Aniline (Beijing Chemical Co.) was distilled twice under vacuum before use. Other chemicals were analytical-grade reagents and purchased from Beijing Chemical Co. without further purification. Water used in all experiments was distilled twice. 2.2. Preparation of Ag/PANI Composite Nanotubes. In a typical synthesis, APS (0.22 g, 0.965 mmol) was dissolved in 8 mL of deionized water to prepare an oxidant solution at room temperature. The oxidant solution was then added dropwise to the aniline monomer (0.03 mL, 0.322 mmol) solution. Then, AgNO3 (0.02 g, 0.117 mmol) was added to the above mixture. The reaction mixture was continuously stirred for 6 h at room temperature. Finally, the polymerization system was immobilized for 48 h at 0-5 °C. In this process, the color of the mixture changed from originally buff to blue and then to dark green. After reaction, the remaining precipitate was washed several times with deionized water, ethanol, and ethyl ether and then dried under vacuum for 24 h at 50 °C. 2.3. Preparation of Pure PANI. In a typical synthesis, APS (0.22 g, 0.965 mmol) was dissolved in 8 mL of deionized water to prepare an oxidant solution at room temperature. The oxidant solution was then added dropwise to the aniline monomer (0.03 mL, 0.322 mmol) solution. Then the reaction mixture was continuously stirred for 6 h in a water bath at room temperature. Finally, the polymerization system was immobilized for 48 h at 0-5 °C. The remaining precipitate was washed several times with deionized water, ethanol, and ethyl ether and then dried under vacuum for 24 h at 50 °C. 2.4. Characterization. SEM images were obtained using a XL-30 ESEM FEG scanning electron microscope operated at 20 kV with gold sputtered on samples with energy-dispersive X-ray analysis attached to SEM. Low- and high-resolution TEM was performed by using a JEM-2100F instrument with a field emission gun operating at 200 kV. The samples for TEM observations were prepared by dispersing some products in ethanol. This procedure was followed by ultrasonic vibration for 1 min and deposition of a drop of the dispersion onto a carbon-coated copper grid. The excess liquid was wicked away with a filter paper, and the grid was dried at 50 °C. FT-IR spectra were obtained using an Alpha-Centauri 560 Fourier transform infrared spectrophotometer (frequency range from 4000 to 400 cm-1) with a KBr pellet. UV-vis absorption spectra of the samples dispersed in distilled water through ultrasonic irradiation were obtained with Beckman-DU-8B UV spectrophotometer in the range of 350-850 nm. XRD patterns were performed on a D/Max IIIC X-ray diffractometer using a Cu KR radiation

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Figure 1. (A, B) SEM images and (C) TEM image of Ag/PANI composite nanotubes. (D) Corresponding EDX pattern of the Ag/PANI composite nanotubes. Synthetic conditions: [An], 0.322 mM; [APS], 0.965 mM; [AgNO3], 0.117 mM; 15 °C; 48 h.

source. Scans were made from 3° to 90° (2θ) at a speed of 2° min-1. XPS was performed on an ESCALAB-MKII spectrometer (VG Co., U.K.) with Al KR X-ray radiation as the X-ray source for excitation. Electrochemical experiments were all performed with a CHI 800B electrochemical workstation in a conventional three-electrode electrochemical cell using indium tin oxide (ITO)/Ag/PANI composite nanotubes and ITO/pure PANI as the working electrode, platinum wire as the auxiliary electrode, and Ag/AgCl (saturated with KCl) as the reference electrode. All potentials were reported versus the Ag/AgCl reference electrode. ITO substrate (4 cm × 1 cm) as the working electrode was pretreated before it was used. It was sonicated in ethanol for 5 min, followed by rinsing with water and ultrasonic agitation in concentrated NaOH in a 1:1 (v/v) water/ethanol bath for 15 min. The ITO substrate was then rinsed further with water for another 15 min under sonication and dried with a high-purity nitrogen stream. The ITO substrate was covered with the testing samples.38 3. Results and Discussion 3.1. Characterization of Ag/PANI Composite Nanotubes. Figure 1 shows SEM and TEM images of Ag/PANI composite nanotubes obtained through a simple one-step self-assembly polymerization process. As shown in Figure 1A and 1B, the dominant morphology is a 1D nanostructure with tubular shape, which has an opening at the ends. The diameter of the nanotubes ranges from 95 to 120 nm, and the length can reach several thousand nanometers under a typical experimental process. A closer look at the nanotubes reveals that the outer surface of the nanotubes is rough with some nanoparticles (Figure 1B). In addition, the dramatic difference in terms of contrast between the center and fringe of the nanotubes, which confirms the hollow nature of this 1D nanostructure, could be observed by TEM investigation (Figure 1C). Meanwhile, dispersed Ag nanoparticles with an average size of 10 nm decorating the PANI nanotubes are dark spots in the wall in the TEM micrograph. The frequency distributions of the outer diameter of Ag/PANI composite nanotubes and the diameter of the Ag nanoparticles are shown in the Supporting Information (Figure S1a and S1b) based on analysis of a large number of SEM and TEM images. The thickness of the nanotubes from the TEM image is about 40-50 nm and varies with the corresponding nanotubes. Figure 2A shows the SEM image of the PANI synthesized in the same polymerization condition but without using AgNO3. However,

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Figure 2. (A) SEM image and (B) corresponding EDX pattern of pure PANI. Synthetic conditions: [An], 0.322 mM; [APS], 0.965 mM; 15 °C; 48 h.

Figure 5. XRD patterns of (a) Ag/PANI composite nanotubes and (b) pure PANI. (c) Standard data for Ag (JCPDS No. 04-0783) is also presented in the figures for comparison.

Figure 3. FT-IR spectra of (a) Ag/PANI composite nanotubes and (b) pure PANI.

Figure 4. UV-vis spectra of (a) Ag/PANI composite nanotubes and (b) pure PANI.

it can be found that the morphology of PANI obtained shows mainly short nanorods including some nonfibrous particulates.39 Both of the PANIs are confirmed with EDX patterns as shown in Figures 1D and 2B, which reveal the presence of carbon, nitrogen, and oxygen except the minor difference from the Ag element. These results also indicate the successful preparation of Ag/PANI composite nanotubes. FT-IR and UV-vis spectroscopies allow more insight into the information on the structure of the resulting pure PANI and Ag/PANI composite nanotubes, as shown in Figures 3 and 4. For the pure PANI (Figure 3a), the peaks in the frequency range of 2900-3500 cm-1 are due to the stretching vibrations of the leucoemeraldine component. The peak at 1140.43 cm-1 assigned to the characteristic QdNH+-B (where Q and B denote quinoid ring and benzene ring, respectively) is also observed.40,41 The peak at 1299.95 cm-1 relates to the C-N stretching vibration with aromatic conjugation. The well-resolved peaks at 1575.59 and 1495.45 cm-1 correspond to the CdC stretching vibration of benzeniod and quinoid rings, respectively. The FT-IR spectrum of the Ag/PANI composite nanotubes is very similar and also shows all the bands of PANI-emeraldine salt,

confirming polymer formation (Figure 3b). However, it can be found that the integrated peak intensity of benzeniod rings against quinoid rings of the composite is higher than that of pure PANI, showing that PANI in the Ag/PANI nanocomposite has a higher conjugation length than that of pure PANI.42 UV-vis absorption spectra of the pure PANI and Ag/PANI composite nanotubes are shown in Figure 4. The characteristic bands of pure PANI appear at 357, 425, and 812 nm with a free carrier tail extending into the near-infrared region, which are attributed to π-π* transition of benzenoid rings, polaron-π* transition, and π-polaron transition, respectively (Figure 4 a).43 However, for the Ag/PANI composite nanotubes, the characteristic bands at 357 and 425 nm are absent but at 405 nm the band is observed because the absorption of Ag nanoparticles overlaps those of PANI (Figure 4b). The absorption band of the Ag nanoparticles usually appears at around 400 nm, which is caused by surface plasmon resonance.44 It is difficult to differentiate these bands due to the overlap.33 The UV-vis absorption spectra further support formation of Ag/ PANI composite. Figure 5 shows XRD patterns of pure PANI and Ag/PANI composite nanotubes. Pure PANI is amorphous and have two broad bands centered at 2θ ) 20.4° and 24.87°, which are ascribed to the periodicity parallel and perpendicular to PANI chains, respectively (Figure 5a).37b For the Ag/PANI composite nanotubes, the XRD pattern reveals the presence of Ag in the nanocomposites (Figure 5b). Some sharp peaks centered at 2θ ) 38°, 44°, 64°, and 77° are observed corresponding to (111), (200), (220), and (311) silver planes, respectively, which coincide well with the literature values (JCPDS No. 04-0783) (Figure 5c).45 Apart from the sharp peaks of Ag, two broad bands centered at 2θ ) 21.48° and 25.11° are also observed. All these results indicate that Ag/PANI composite nanotubes have been obtained. Figure 6A presents the whole XPS analysis of Ag/PANI composite nanotubes for the binding energy (Eb) of 0-1200 eV. Figure 6B is the part of the same spectrum focused on the region Eb ) 362-382 eV. Two peaks at 367.9 and 374 eV correspond to the Ag 3d5/2 and Ag 3d3/2 energy levels of the Ag atom, respectively.46 These values are in accord with those of Ag(0). The C 1s and N 1s spectra at a binding energy of 284.7 and 398.9 eV (-NH structure) indicate the chemical environment of the C and N elements here due to the effect of PANI matrix (Figure 6C and 6D). Thus, the results allow us to further confirm silver nanoparticles have been effectively assembled on the surface of the PANI nanotubes.

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Figure 6. XPS spectra of (A) Ag/PANI composite nanotubes, (B) Ag 3d, (C) C 1s, and (D) N 1s.

3.2. Possible Formation of Ag/PANI Composite Nanotubes. On the basis of the above experimental results, it is believed that the growth of the tubelike morphology is not assisted by any acid molecule reagent or directed by hard templates. Then what is the possible formation process for the Ag/PANI composite nanotubes? To our knowledge, the standard reduction potential of Ag+ + e- f Ag is E0 ) +0.79 V, which is lower than 1.02 V of aniline. Thus, it is hard for AgNO3 to act as an oxidant in the early stages of aniline polymerization. Aniline monomer is oxidized first by APS, S2O8 2- + 2e- f 2SO42- (+2.01 V), to form reactive aniline cation-radicals, simultaneously producing H2SO4 by the reduction of APS in the early stages. Two initially formed aniline cation-radicals combine into a dimer which is further oxidized by APS to form a dimer cation-radical. These dimer cation-radicals can act as surfactants to template the formation of nanotubes under the condition of excess oxidant.39 Then the Ag anion provided by AgNO3 acts as an electron acceptor and is reduced to Ag0 while the dimer cation-radical oxidizes to as-synthesized resulting PANI.47 Meanwhile, the dimer cation-radical surfactant transforms from spherical micelles into tubular structured micelles for the later formation of the nanotubes. The growth process of the silver nanoparticles and polymerization of the dimer cation-radical surfactant continue simultaneously.39 Finally, the PANI nanotubes with dispersed Ag nanoparticles decorated on the surface are successfully prepared. 3.3. Electrochemical Behavior of PANI Composite Nanotubes. Cyclic voltammetry is one of the most common techniques used in electrochemistry to determine electroactivity. It is well known that PANI bears good electrochemical behavior and has been applied to the field of chemically modified electrodes. Here, CVs of the ITO electrode modified with pure PANI and the Ag/PANI composite nanotubes were measured in 0.1 M N2-saturated H2SO4 solution at different scan rates, as shown in Figure 7. The CVs of the pure PANI show three well-

defined reversible redox processes,38 confirming that the polymer has electroactivity (Figure 7A). The CVs of Ag/PANI composite nanotubes (Figure 7B) display, in addition to the three reversible redox peaks of PANI, one redox process of Ag, which further proves the presence of Ag in the PANI composites.48 When the scan rate is varied from 10 to 100 mV/s (from inside to outside corresponding to V ) 10, 20, 30, 40, 50, 60, 80, and 100 mV/s), the cathodic peak potentials are almost the same as the corresponding anodic peak potentials. The cathodic peak potentials shift to the negative direction and the corresponding anodic peak potentials shift to the positive direction with increasing scan rate. The peak currents of the four redox couples increase linearly with the scan rate from 10 to 100 mV/s. Taking the first redox peak as a representative, the redox peak current has a good linear relationship with increasing scan rate, as indicated by the inset of Figure 7, indicating that the electrode process is a surface-controlled process. Interest in catalysis by metal nanoparticles is increasing dramatically because of the advantages offered by “semiheterogeneous catalysts”.49 Also, as we known, dopamine (DA) is an important neurotransmitter in mammalian central nervous systems, and the loss of DA-containing neurons may lead to serious diseases including Parkinson’s disease.29 Detection of DA has been a subject of considerable interest. In this work, Ag/PANI composite nanotubes were immobilized onto the surface of an ITO, which was taken as an example of developing a possible application as a DA biosensor. The CVs of pure PANI and Ag/PANI composite nanotubes modified electrodes before and after adding DA with various concentrations at a scan rate of 50 mV/s are shown in Figure 8A and 8B, respectively. The electrode modified with Ag/PANI composite nanotubes shows high electrocatalytic current toward the increase of DA compared with that modified with the pure PANI. The anodic current of the Ag/PANI composite nanotubes modified ITO is observed at a more positive potential compared with that of pure PANI-

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Figure 7. CVs of the ITO electrodes modified with (A) Ag/PANI composite nanotubes and (B) pure PANI in 0.1 M N2-saturated H2SO4 with different scan rates (from inner curve to outer curve: 10, 20, 30, 40, 50, 60, 80, and 100 mV/s, respectively). Insets show the relationship of the redox current of peak I and scan rate.

Figure 8. CVs of ITO electrodes modified with (A) Ag/PANI composite nanotubes and (B) pure PANI cross-linking as work electrodes in 0.1 M N2-saturated H2SO4 solution containing DA with various concentrations of 0.0, 0.5, 1.0, 2.0, 3.0, and 4.0 mM (a-f). Scan rate: 50 mV/s.

modified ITO, implying that the composites have a strong catalytic effect on the oxidation of DA. The oxidation peak current increases steadily with increasing DA concentration, showing the catalytic property of the modified electrode in the oxidation of DA to dopamine-quinone. Furthermore, the electrocatalytic anodic current in the Ag/PANI composite nanotubes modified ITO (73.8 µA) is higher than that in the pure PANI-modified ITO (48.6 µA) at a DA concentration of 4 mM, which indicates that the Ag/PANI composite nanotubes modified ITO can act as an electron-transfer mediator between the modified electrode and DA even at a very low concentration of DA. The improved electrocatalytic oxidation of DA for the Ag/PANI-modified electrode may be due to the fact that the charge transport is enhanced in the Ag/PANI composite system, which facilitates the electrical contact of the redox DA with

the electrode. The enhanced electron transfer in the Ag/PANI composite system is attributed to the charge hopping through the metallic conductor Ag nanoparticles that mediate the effective charge migration through the polyaniline. The effective transport of the electrons to the electrode in the Ag/PANI matrix leads to the efficient electrocatalytic oxidation of DA.29 3.4. Sensitivity Behavior of PANI Composite Nanotubes. As a result of the high specific surface area and excellent channels for charge transmission, we deduce that the Ag/PANI composite nanotubes obtained in our experiment may have good gas sensitivity. The characterization results of the sensor’s gas sensitivity to vapor is similar what was found in the literature.50,51 PANI has been used as a sensing material for a variety of toxic gases such as CO, NO2, and NH3.52 Detection of NH3 in air is of interest for environmental monitoring and process control

Figure 9. (A) Resistance changes of (a) Ag/PANI composite nanotubes and (b) pure PANI upon exposure to 100 ppm of NH3. (B) Ag/PANI composite nanotubes exposed to different concentration of NH3. (C) Reversible circulation response change of Ag/PANI composite nanotubes upon exposure to 100 ppm of NH3. The y axis is the normalized resistance (R/R0), where R0 is the initial resistance of the dedoped PANI before exposure to the test gas (t ) 0) and R is the time-dependent resistance of the PANI exposed to the test gas.

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because of its high toxicity. Figure 9A displays real-time change in resistance of Ag/PANI composite nanotubes and pure PANI upon exposure to 100 ppm of NH3 dispersed in an ambient environment. A resistance value (R/R0) of about 15 is obtained for Ag/PANI composite nanotubes, but it is about 8 for pure PANI. It can be seen that Ag/PANI composite nanotubes have higher gas sensitivity and a more rapid respond time to NH3 than the pure PANI. To our knowledge, metal modifiers in the organic films provide a new electron trap site, whose affinity for electrons is strongly affected by binding of a donor molecule such as NH3.53 Because each trap site affects the collection of several charges, significant “amplification” of each analyte chemisorption event is possible. Optimization of response may be achieved through the appropriate modification of the organic thin films with respect to the type and distribution of modifier sites. Therefore, selectively introducing trap sites of choice into the pure organic films can enhance the sensitivity and analytical response.53 Figure 9B shows the response of the Ag/PANI composite nanotubes to different concentrations of NH3 from 5 to 100 ppm. The resistance of the Ag/PANI composite nanotubes increases gradually with the increase of the concentration of the NH3. Even though the NH3 concentration is as low as 5 ppm, the change in resistance of Ag/PANI composite films is still very obvious, which indicates that the Ag/PANI composite nanotubes synthesized by this method have superior gas sensing performance and can act as an “electronic nose” in chemical detection and pattern recognition. The results pave the way for the synthesis, which has the advantage of smaller, less expensive, and more sensitive devices. Another important property of the sensor is its reversibility at room temperature. We find that the sensor can be recovered completely with N2 at room temperature, so it can be used repeatedly. From Figure 9C, the response in the first and third circles almost returns to the original baseline, which illustrates a reasonable reproducibility of the sensor. Therefore, Ag/PANI composite nanotubes not only bear diaphanous morphology but also have better performance in both sensitivity and time response in comparison with the pure PANI. 4. Conclusion We successfully prepared Ag/PANI composite nanotubes by a self-assembly polymerization process using ammonium persulfate and silver nitrate as oxidant. Dispersed Ag nanoparticles decorate the surface of the PANI nanotubes. A possible formation mechanism for Ag/PANI composite nanotubes has been proposed in detail. The Ag/PANI composite nanotubes can be applied to the chemically modified electrode, which show enhanced electrocatalytic activity for oxidation of DA compared with that of the pure PANI-modified electrode. Meanwhile, this composite nanomaterial has super gas sensitivity because of its high surface area, small diameter, and porous nature of the tubular morphology and the introduction of the silver nanoparticles. Our work also paves the way for the synthesis of novel metal/PANI composite nanotubes, and it is expected that this synthetic approach may be applicable for the synthesis of other conducting polymer nanotubes decorated with dispersed metal nanoparticles. Acknowledgment. This work was supported by the Program for Changjiang Scholars and Innovative Research Team in University and the Science Foundation of Jilin Province (20070505).

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