Polypyrrole/silver composite nanotubes for gas sensors

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Abstract

A simple strategy for synthesizing well-defined polypyrrole (PPy) nanotube, which can be used for loading silver nanoparticles without prior PPy nanotube functionalization under mild conditions is demonstrated. It is found that Ag nanoparticles could uniformly decorate onto the PPy nanotube surface in the presence of polyvinylpyrrolidone (PVP) to form PPy/Ag composite nanotubes. Transmission electron microscopy (TEM) images give evidence of the decoration of Ag nanoparticles on the surface of PPy nanotubes. Fourier transform infrared (FTIR) spectra reveal the structure of PPy/Ag composite nanotubes and X-ray diffraction (XRD) directly shows the presence of Ag nanoparticles. The as-prepared PPy/Ag composite nanotubes are applied to the detection of ammonia vapor. Compared with PPy nanotubes, the introduction of metal onto PPy nanotubes is effective in promoting the chemiresistor response to ammonia. Moreover, the response behaviors of PPy/Ag composite nanotubes depend on the distribution of Ag nanoparticles in the nanocomposites.

Introduction

Nanocomposites are new types of materials that are composed of two or more phases of different chemical constituents or structures, and at least one of the chemical and/or structural phases is in nanometric dimension [1], [2], [3], [4]. Conducting polymers have been an attractive class of materials in nanoscience and nanotechnology because of their highly π-conjugated polymeric chains, unique electrical properties, reversible doping/de-doping process, and controllable chemical and electrochemical properties. They can be regarded not only as excellent molecular wires for the fabrication of nanodevices, but also as enhancing materials for constructing sensing platforms with high response [5], [6], [7]. On the other hand, metal nanoparticles have continued to receive considerable interest due to their particular optical, electronic, and catalytic properties and their important applications in many fields such as nanosensors, catalysis, and surface-enhanced Raman scattering. Recently, the composite nanomaterials composed of conducting polymers and noble metal nanoparticles have attracted special attention because of their remarkable properties and their promising applications in many fields of chemical sensors, catalysis, solar cells, medical diagnosis, etc. [8], [9], [10], [11].

Among the studies for different types of nanocomposites, controllable fabrication of the hybrids that consist of highly monodisperse metal nanoparticles and nanometric conducting polymer matrixes, such as nanobelts, nanowires, nanofibers, nanotubes, hollow nanospheres and dendrites, represents a grand challenge in the development of this class of materials. In particular, one-dimensional materials with metal nanoparticles have attracted intensive interest because of their unique properties in fundamental scientific research and practical applications. Such nanostructures possess ultra small size and high surface area features, which offer great promise for gas sensors and biosensors. As a result, considerable efforts have been directed towards attaching certain metal nanoparticles onto different one-dimensional materials and constructing corresponding multifunctional hybrid nanostructures [12], [13], [14].

Although the composites based on conducting polymers and Ag have been reported, the preparation for the composites with controlled nanostructure is still a novel challenge. Thus far, dramatic efforts have been dedicated to develop new methods for fabrication of conducting polymers/Ag composite nanostructures in different systems [15], [16], [17], [18]. Bertino and co-workers described the synthesis of polyaniline/Ag composite nanofibers by γ radiolysis [19]. Li et al. synthesized polyaniline nanotubes decorated with dispersed Ag nanoparticles using nitrocellulose fiber mats as a suitable template through the UV rays irradiation method [20]. Among the family of conducting polymers, polypyrrole (PPy) has shown high performance including environmental stability, electronic conductivity and biocompatibility. Up to now, most reports focused on the preparation of PPy/Ag nanocomposites from the reaction between pyrrole monomers and AgNO3 [16], [17], [21], and there are few report on one-dimensional PPy/Ag composites with nanotubular structure [21].

The reliable, accurate and rapid detection of ammonia vapor is of practical importance in many technological fields, such as industrial processes, clinical diagnosis and environmental monitoring. As is commonly known, PPy is a p-type semiconductor in its conducting state. Recent reports have shown that a redox reaction occurs reversibly in PPy upon exposure to the gas of an electron donor, e.g. NH3. The introduction of NH3 into PPy will result in the decrease the charge-carrier concentration of the polymer and the formation of neutral polymer backbones. After removal of NH3, PPy will return to its original state. Consequently, these phenomena are necessarily accompanied by the decrease and increase in conductivity of PPy reversibly [22]. On the other hand, metal nanoparticles in the organic matrix provide new electron trap site, whose affinity for electrons is strongly affected by binding of a donor molecule such as NH3. Thereby selectively introducing trap sites of choice into the organic matrix may enhance the analytical response [23]. As the results of the high specific surface area and excellent channels for charge transmission in the nanocomposites, it is expected that the PPy/Ag composite nanotubes may show good gas response.

In this work, we report a facile approach to synthesize PPy/Ag composite nanotubes. PPy nanotubes were first prepared by a self-degraded template of a fibrillar complex of Fe(NO3)3 and methyl orange, and then Ag nanoparticles were deposited onto the surface of PPy nanotubes by in situ reduction of AgNO3. The formation mechanism and structural characteristics of the prepared PPy/Ag composite nanotubes were studied. Furthermore, the gas response of PPy/Ag composite nanotubes to ammonia vapor was explored.

Section snippets

Chemicals

Pyrrole monomer was distilled twice and stored in refrigerator before use. Methyl orange (MO), AgNO3, Fe(NO3)3 and polyvinylpyrrolidone (PVP) were purchased from Aldrich and used as received. Other reagents were of analytical grade and used without further purification.

Apparatuses

TEM measurements were conducted on a JEM-200CX transmission electron microscope with an accelerating voltage of 200 kV. SEM images were obtained using a XL-30 ESEM FEG scanning electron microscope. FTIR spectroscopic measurements

Synthesis and characterization of PPy/Ag composite nanotubes

A simple reactive self-degraded template approach was first employed to synthesize PPy nanotubes according to the reported result with some modification [24]. Then, Ag nanoparticles spontaneously adsorbed on the surface of PPy nanofibers when the as-prepared one dimensional nanotubular material was added into the mixture containing AgNO3 and polyvinylpyrrolidone (PVP) for a certain time. Fig. 1 shows the typical TEM images of PPy nanotube and PPy/Ag composite nanotubes prepared without or with

Conclusions

In this study, we have demonstrated the synthesis of well-defined PPy/Ag composite nanotubes by an in situ reduction process under mild conditions. In the presence of PVP, Ag nanoparticles could uniformly absorb onto the surface of PPy nanotube. Moreover, the present PPy/Ag composite nanotubes could be used to construct vapor sensor. The responses of these nanocomposites upon exposure to NH3 vapor were observed by monitoring the change in the resistance of the nanocomposites. The sensors based

Acknowledgements

The work is supported by Key Project in Science & Technology Innovation Cultivation Program of Soochow University, Educational Bureau of Hubei Province (Q20091508), Scientific Research Key Project of MOE (209081), Scientific Research Foundation for Returned Overseas Chinese Scholars of MOE ([2009]1341), State Key Laboratory of Coordination Chemistry (Nanjing University) and NSFC (20904044).

Dr. Xiaoming Yang received her Ph.D. degree in polymer science from Nanjing University in 2006, China. She is currently a lecturer in College of Chemistry, Chemical Engineering and Materials Sciences, Soochow University. Her research interest is in conducting polymers and chemical sensors.

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    Dr. Xiaoming Yang received her Ph.D. degree in polymer science from Nanjing University in 2006, China. She is currently a lecturer in College of Chemistry, Chemical Engineering and Materials Sciences, Soochow University. Her research interest is in conducting polymers and chemical sensors.

    Dr. Liang Li received his M.Sc. (2000) and Ph.D. (2005) degree in polymer science from Nanjing University, China. And he held his postdoctoral position in National University of Singapore and University of Kiel, Germany from 2005 to 2008. He is currently a lecturer in School of Materials Science and Engineering, Wuhan Institute of Technology. His research interest is in composite materials and polymer chemistry.

    Dr. Feng Yan received his Ph.D. (2000) degree in polymer science from Nanjing University, China. Then he did his postdoctoral research in University of Ulm, Rochester University and Eastern Michigan University from 2000 to 2006. He is currently a professor in College of Chemistry, Chemical Engineering and Materials Sciences, Soochow University. His research interest is in polymer chemistry and functional materials.

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