Ag Composite Nanotubes Prepared through UV Rays

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Polyaniline/Ag Composite Nanotubes Prepared through UV Rays Irradiation via Fiber Template Approach and their NH3 Gas Sensitivity Xia Li, Yu Gao, Jian Gong,* Li Zhang, and Lunyu Qu Key Laboratory of Polyoxometalates Science of Ministry of Education, Northeast Normal UniVersity, Changchun, Jilin, 130024, Peoples Republic of China ReceiVed: August 22, 2008; ReVised Manuscript ReceiVed: October 26, 2008

Polyaniline(PANI)/Ag composite nanotubes have been successfully prepared using a suitable fiber as template in the presence of H4SiW12O40 (SiW12) and silver nitrate (AgNO3) as a dopant and an oxidant, respectively. The Ag nanoparticles dispersed in the PANI nanotubes are very uniform. The structure and morphology of the product are characterized by Fourier transform infrared (FT-IR) spectrum, X-ray diffraction (XRD) pattern, scanning electron microscope (SEM), and transmission electron microscopy (TEM) images. UV rays play an important role in the polymerization. A potential formation mechanism of the PANI/Ag composite nanotubes is suggested. The average inner diameter of PANI/Ag composite nanotubes is 170 nm, which is consistent with the diameter of the template. The high surface areas, small diameter, and porous nature of the PANI/Ag composite nanotubes give significantly better performance in both gas sensitivity and time response. Moreover, the introduction of metal into organic films is effective in promoting the chemiresistor sensitivity to NH3. The gas-response to NH3 gas is examined at room temperature. Meanwhile, the reversible circulation response change of PANI/Ag composite nanotubes has a reasonable reproducibility and has more potential applications in the area of sensor development. 1. Introduction There has been tremendous interest in the synthesis of conducting polymers due to their excellent electronic properties, with conductivities covering the whole range from insulator to metal while retaining lightweight, mechanical properties, and promising applications in nanodevices.1-3 Among the family of conducting polymers, polyaniline (PANI) has been one of the most extensively studied because of its stability and reversible acid-base doping/dedoping chemistry.4 In recent years, one-dimensional (1D) conducting polymer nanotubes have received great attention due to the potential advantages of having an organic conductor exhibit low-dimensionality. Such materials are potentially useful for applications that depend on ultrasmall, high surface area features such as chemical sensors.5 However, 1D nanostructures (namely, 1D nanotubes) have been difficult to fabricate because of their diaphanous morphology.6 As one knows, PANI nanotubes are frequently fabricated by employing “hard” template, such as the channel of microporous membranes,7 zeolites,8 particle track-etched membrane,9 and “soft” template such as micelles, surfactants, or seeding.10,11 However, template fabrication and removal are tedious when hard templates are employed in the polymerization and soft templates limit the range of chemicals that can be used. For PANI, only a few acids have been reported to function as dopants and soft “internal” templates until date. In addition, forming and removing the templates will usually lead to poor reproducibility and low yields. Furthermore, it is rather difficult to retain the ordered nanostructure of PANI nanotubes after removing the template.12 Therefore, selecting core polymer as the template to control over the morphology of PANI nanotubes still remains a challenge. Recently, researches on the synthesis of conducting PANI/ metal composites have attracted considerable attention due to their enhanced sensing and catalytic capabilities, as compared * Corresponding author. E-mail: [email protected].

with those of pure PANI.13,14 Naturally, hybridization of the metal elements and PANI molecule can introduce a new family of materials, designated as PANI compound-doped metals, which enrich the property library and widen the application scope offered by each of them individually. Moreover, it is reported that the introduction of metal into organic films is effective in promoting the chemiresistor sensitivity to NH3, through the apparent creation of new chemisorption sites.15 The methodology of preparation of those PANI/metal composites involves metal synthesis by chemical reduction of a metal cation in the presence of the aniline molecule. The reduction can be carried out either with water-soluble reducing agents16,17 or with a powder of metal.18 However, most of the distribution of the metal nanoparticles in the PANI matrix is not very uniform.19 In order to enlarge the family of the PANI/metal composites decorated with uniform dispersed metal nanoparticles, a new synthesis process is required. Encouraged by our success in the preparation of PANI nanotubes using template method,6,20,21 in this paper, we report a facile approach to synthesize conducting PANI nanotubes decorated with uniform dispersed Ag nanoparticles using nitrocellulose (NC) fiber mats as suitable template in the presence of SiW12 as a dopant through UV rays irradiation method for the first time. SiW12 is usually used as the dopant, which offers protons to obtain SiW12 doped PANI (emeraldine salt form of PANI).6,20,21 In the end, we obtain the SiW12 doped PANI in the PANI/Ag composite nanotubes. AgNO3 is used as a single oxidant in this work. Compared with other methods, the NC fiber mats template is easy to be removed while the PANI/Ag tubular morphology remains. Moreover, the Ag nanoparticles dispersed in the PANI nanotubes are very uniform. The possible formation mechanism of the PANI/Ag composite nanotubes prepared by this method is discussed. The gas sensitivity and response to NH3 are investigated.

10.1021/jp807535v CCC: $40.75  2009 American Chemical Society Published on Web 12/08/2008

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Figure 1. (A) SEM image and (B) frequency diameter distribution of the NC fiber mats (86 fibers were measured to get the size distribution).

Figure 2. SEM (A,B) and TEM (C,D) images of PANI/Ag composite nanotubes obtained using NC as a suitable fiber template through UV rays irradiation method at low magnification (A,C), and at higher magnifications (B,D). Inset of A: EDAX spectrum of PANI/Ag composite nanotubes. Inset of D: ED pattern of Ag nanoparticles in the PANI nanotubes.

2. Experimental Section 2.1. Materials. Aniline (Beijing Chemical Co.) was distilled twice under vacuum before use. SiW12 used here was selfsynthesized according to a literature procedure.22 AgNO3 was purchased from Beijing Chemical as a single oxidant without further purification. Water used in all the experiments was distilled twice. 2.2. Polymerization. The NC fiber mats were prepared according to ref 20; 0.5 g of SiW12 was dissolved in 20 mL of distilled water. Then, the NC fiber mats (0.1 g) were soaked in the solution. The solution was stirred under supersonic for 30 min in an ice-water bath. A 0.3 g sample of AgNO3 and 0.08 mL of aniline were added respectively under vigorous agitation. The mixture was kept in the ice bath for 48 h and meanwhile irradiated by UV rays with 254 nm wavelength, providing a power density of 0.75 mW cm-2. Then the NC fibers coated PANI/Ag composites were washed with distilled water until pH ) 6-7. The PANI/Ag/NC fiber mats were soaked with acetone under supersonic stirring for 10 min. Finally, the precipitated powder was washed with distilled water and ethanol, respectively, and dried in vacuum for 12 h.

2.3. Characterization. The morphology of the product was observed with an XL-30 ESEM FEG SEM operated at 20 KV. The TEM images were obtained using a Philips JEM-2010 TEM at an acceleration voltage of 200 KV. FT-IR spectrum was measured on an Alpha-Centauri 650 spectrometer with a KBr pellet. The frequency range was 4000-400 cm-1. The XRD was measured with a D/max 2200 PC spectrometer with a Cu KR source. Scans were made from 3 to 90° (2θ) at the speed of 2° min-1. 3. Results and Discussions Figure 1 shows the morphology and the diameter distribution of the NC fiber mats. As one can see that, the average diameter of the NC fiber mats is 172 nm. Figure 2 shows SEM and TEM images of PANI/Ag composite nanotubes obtained using NC fiber mats as the template. As shown in Figure 2A, the SEM image shows that most of the resulting products are tubular. A closer look at the nanotubes reveals that the average outer and inner diameters are 200 and 170 nm, respectively (Figure 2B). The length of the nanotubes is several thousand nanometers. Some broken nanotubes are

Polyaniline/Ag Composite Nanotubes

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Figure 3. (A) FT-IR spectrum and (B) XRD pattern of PANI/Ag composite nanotubes prepared using NC as a suitable fiber template through UV rays irradiation method (filled triangles indicate the peaks ascribed to SiW12, and filled squares indicate the peaks ascribed to PANI). The inset of A is the structure of SiW12.

observed in the SEM images which indicate they are hollow. This is further proven by the sharp contrast between the dark edge and the pale center in the TEM images (Figure 2C,D). Uniform dispersed Ag nanoparticles decorating the PANI nanotubes are dark spots in the TEM image (Figure 2D). The composition of the PANI/Ag composite nanotubes are confirmed by energy-dispersive X-ray analysis (EDAX), which reveals that the presence of carbon, nitrogen, oxygen, Ag, Si, and W (inset of the Figure 2A). Gold element comes from the thin film of gold sputtered before SEM and EDAX measurements. These results indicate the successful preparation of PANI/Ag composite nanotubes, in which PANI is doped with SiW12. Furthermore, the inner diameter of the PANI/Ag nanotubes is consistent with the diameter of the NC fiber mats. We deduce that the result is due to the polymerization of aniline mainly occurs on the surface of the NC fiber mats because of the big BET surface area of the NC fibers.20 The conclusion can be confirmed by the fact that there is scarcely precipitation in the solution. With soaking the PANI/Ag/NC fiber mats in acetone under supersonic stirring for 10 min, PANI/Ag composite nanotubes are obtained because of the very good solubility of the NC core in acetone solution. The electron diffraction (ED) pattern shows that Ag nanoparticles in the PANI nanotubes are polycrystalline. The above results show, when AgNO3 is employed as a single oxidant to form PANI/Ag composite, the UV rays play an important role in the polymerization. As we know, the potential of Ag+/Ag is only 0.79 V (vs normal hydrogen electrode, NHE), which is lower than 1.02 V (vs NHE) of aniline.23 So it is hard for Ag+ to accept electrons and be reduced without the photoinduced process. The most important idea to be noted here is that the PANI/Ag composite nanotubes can not be obtained when UV rays are absent from the polymerization. The possible mechanism for the tubular formation of PANI/Ag composite nanotubes is that the photons in the UV rays excite the aniline monomer to the excited state, forming solvated electrons.23 The electrons can be transferred from the excited aniline monomers to the silver ions, which leave aniline radical cations and Ag.24 Aniline radical cations interacts each other and polymerize to PANI on the surface of NC fiber mats, with Ag nanoparticles decorated on. By removing the NC fiber mats in acetone solution, PANI/Ag composite nanotubes can be obtained. Moreover, the Ag nanoparticles are very uniform dispersed in PANI nanotubes. The molecular structure of the resulting PANI/Ag composite nanotubes is characterized by FT-IR spectroscopy as shown in Figure 3A. The characteristic peaks at 1583 and 1487 cm-1 are

due to the CdC stretching vibration of quinoid and benzenoid rings, respectively. Moreover, the peak at 1305 cm-1 corresponds to the CsN stretching vibration with aromatic conjugation. The peak at 1143 cm-1 relates to the CsH in-plane bending mode of permigraniline which is also observed. The peak of the out-of-plane deformation of CsH in the 1, 4-disubstituted benzene ring can all be observed at 828 cm-1.25,26 Four characteristic peaks of SiW12 (788.13 cm-1 ascribed to WsOcsW, 878.39 cm-1 ascribed to WsObsW, 917.51 cm-1 ascribed to SisOa, and 968.65 cm-1 ascribed to WdOd) attest to the presence of SiW12 in the PANI.27 These results demonstrate the successful polymerization of PANI doped with SiW12. Figure 3B shows XRD of PANI/Ag composite nanotubes. As one can see, one sharp peak at 2θ ) 7.5° (d ) 11.777 Å), which is close to the PANI repeat distance.21 This result suggests that PANI doped with SiW12 leads to a more ordered structure with relatively distinct Bragg reflection.28 The broad peak centered at 2θ ) 27° may be ascribed to the periodicity parallel to the PANI chain, while the weak broad peaks at higher angles may be caused by the periodicity perpendicular to the PANI chain.29 Meanwhile, the XRD of the PANI/Ag composite nanotubes also exhibit sharp peaks corresponding to 111, 200, 220, and 311 Bragg reflections of Ag.30 As a result of the high specific surface area and excellent channels for charge transmission, we deduce that the PANI/Ag composite nanotubes obtained in our experiment may have good gas sensitivity. The characterization of the sensor′s gas sensitivity to NH3 gas is similar to that in refs 12 and 31. The structure of sensor in our experiments is according to the previous literature.6 The PANI/Ag composite films were fabricated using a drop-coating technique. The films were deposited onto porcelain tube that consisted of 1 × 1 mm2 Pt electrode. The substrates were equipped with integrated electrodes to the sensitive film. The films were prepared by starting from pastes obtained by adding to the above powders an organic vehicle together with a small percentage of glass frit for improving the adhesion of the layers to the substrates. Then the films were dried for 1 h under vacuum at 40 °C. The PANI/ Ag composite device was placed in an airproof test box (about 27 L). Six volt DC current was applied. The box and the device were flushed with high-purity N2 continuously until the electrical resistance (DC) reaches a steady value. Then, a certain amount of volatile solvent was injected into the test box with a syringe. The change in electrical resistance were monitored and recorded automatically with a computer. The gas sensitivity of the PANI/ Ag composite nanotubes was defined as the ratio of R/R0,6,21,32

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Figure 4. (A) Resistance change of the PANI/Ag composite nanotubes exposed to different concentrations of NH3. (B) Response of PANI nanotubes with (a) and without (b) Ag upon exposure to 100 ppm of NH3. (C) The reversible circulation response change of PANI/Ag composite nanotubes upon exposure to NH3 (100 ppm). The y axis is the normalized resistance (R/R0), where R0 is the initial resistance before exposure to the test gas (t ) 0) and R is the time dependent resistance of the PANI/Ag composite nanotubes exposed to the NH3 gas.

in which R0 is the initial resistance of the PANI/Ag composite nanotubes before being exposed to the test gas and R is the time-dependent resistance of the PANI/Ag composite nanotubes exposed to the test gas. Figure 4A shows the response of the PANI/Ag composite nanotubes to different concentrations of NH3 from 5 to 100 ppm. The resistance changes (R/R0) reflect the effect of the gas concentration on the sensitivity of the PANI/Ag composite nanotubes as the sensor. The resistance of the PANI/Ag composite nanotubes increases gradually with the increase of the concentration of the NH3. We find that, PANI/Ag composite films have a small increase in resistance when exposed to 5 ppm of NH3, which indicates that the PANI/Ag composite nanotubes synthesized by this method have superior gas sensing performance even to very low concentration of NH3 gas, and can be 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. 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 NH3.15 Because each trap site affects the collection of several charges, significant “amplification” of each analyte chemisorption event is possible. The 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. Thereby selectively introducing trap sites of choice into the pure organic films can enhance the sensitivity and analytical response.15 In contrast, the response of PANI nanotubes without Ag is also discussed. Ag can be easily removed by immersion of the PANI/Ag composite nanotubes in 1 M HNO3. Figure 4B shows the responses of PANI nanotubes with and without Ag upon exposure to 100 ppm of NH3, respectively. There is, however, a significant difference in the resistance value for the PANI nanotubes with and without Ag. A resistance value (R/ R0) of about 3.0 is obtained for PANI nanotubes with Ag and 2.0 is obtained for PANI nanotubes without Ag. On the basis of the above experimental results, we can conclude that Ag nanoparticles play an important role in improving the sensitivity and response rate of the PANI nanotubes. Another important property of the PANI/Ag composite device is its reversibility. We find that it can be completely recovered with high purity N2 at room temperature, so the sensor can be used repeatedly. It can also be seen from Figure 4C, which shows that the response in the first and third circles almost

returns to the original baseline. Therefore, PANI/Ag composite nanotubes not only bear diaphanous morphology but also appear to perform better in both sensitivity and time response in comparison with conventional PANI. 4. Conclusions In summary, we have presented a novel PANI/Ag composite nanotube using NC as a suitable fiber template through UV rays irradiation method. The Ag nanoparticles are uniformly dispersed. The average outer and inner diameters of PANI/Ag composite nanotubes are 200 and 170 nm, which indicate their superior performance as chemical sensors. Furthermore, the high surface areas, small diameter, and porous nature of the PANI/ Ag composite nanotubes appear to have better performance in both sensitivity and time response. Our work also paves the way for the synthesis of novel PANI/metal composite nanotubes, and it is expected that this synthetic approach may be applicable for the synthesis of other conducting polymer nanotubes decorated with uniform 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). References and Notes (1) Martin, C. R. Science 1994, 266, 1961–1966. (2) Heeger, A. J. Angew. Chem., Int. Ed. 2001, 40, 2591–2611. (3) Hohnholz, D.; Okuzaki, H.; MacDiarmid, A. G. AdV. Funct. Mater. 2005, 15, 51–56. (4) Tran, H. D.; Kaner, R. B. Chem. Commun. 2006, 3915–3917. (5) Virji, S.; Huang, J.; Kaner, R. B.; Weiller, B. H. Nano. Lett. 2004, 4, 491–496. (6) Gao, Y.; Yao, S.; Gong, J.; Qu, L. Y. Macromol. Rapid Commun. 2007, 28, 286–291. (7) Martin, C. R. Chem. Mater. 1996, 8, 1739–1746. (8) Wu, C. G.; Bein, T. Science 1994, 264, 1757–1759. (9) Martin, C. R. Acc. Chem. Res. 1995, 28, 61–68. (10) Zhang, Z. M.; Wei, Z. X.; Wan, M. X. Macromolecules 2002, 35, 5937–5942. (11) Huang, K.; Wan, M. X. Chem. Mater. 2002, 14, 3486–3492. (12) Ma, X. F.; Li, G.; Wang, M.; Cheng, Y. N.; Bai, R.; Chen, H. Z. Chem.-Eur. J. 2006, 12, 3254–3260. (13) Kitani, A.; Akashi, T.; Sugimoto, K.; Ito, S. Synth. Met. 2001, 121, 1301–1302. (14) Granot, E.; Katz, E.; Basnar, B.; Willner, I. Chem. Mater. 2005, 17, 4600–4609. (15) Brina, R.; Collins, G. E.; Lee, P. A.; Armstrong, N. R. Anal. Chem. 1990, 62, 2357–2365. (16) Behar-Levy, H.; Avnir, D. AdV. Funct. Mater. 2005, 15, 1141–1146.

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