Nanostructured polyaniline-based composites for ppb range ammonia sensing
Introduction
The existing chemical sensors are typically composed of a sensitive material capable of selective interaction (recognition) with a gas of interest (analyte) and a transducer. The latter converts multiple events of chemical interactions of the sensitive material with an analyte into an electrically measurable response signal. Ideally, such sensors are expected to have a high sensitivity to the analyte, a short response time, and small geometrical dimensions. Among sensitive materials used in these sensors, inorganic metal or semiconductor oxides are probably most frequently employed as a sensitive layer. This happens because they are easy to integrate as variable resistive elements in classical electronic circuits [1]. However, the preparation of these crystalline materials often requires expensive high vacuum and/or high temperature processing operations. Moreover, in most cases the sensor materials themselves operate at high temperatures (above 200 °C), which often limit their applications [2].
Extensive studies have been carried out on nanostructured materials in order to reduce the working temperature and decrease energy consumption. One of the main advantages of these materials is their high surface to volume ratio. Since the developed surface of the active layer is very high, this increases the sensitivity of the sensor device. In turn, this improved sensitivity motivated application of the materials in a form of their nanoparticles or nanowires. Such nanostructures dramatically change their electrical or optical properties while interacting with the analyte [3], [4].
Among the mentioned materials intrinsically conducting polymers (ICP) such as polypyrrole, polyaniline (PANI), polythiophene as well as their derivatives are considered as very promising due to their quite high sensitivity, relative simplicity of their synthesis, light weight, and ability to detect gases at ambient temperature [5], [6], [7], [8], [9], [10], [11]. PANI stands out compared to other ICP through its higher stability and much lower cost. Conductivity of this polymer can be reversibly controlled either by electrochemical or chemical oxidation/reduction processes or by protonation/deprotonation in a simple acid–base chemical interaction. PANI can be blended with thermoplastic polymers to prepare composite materials with a very low percolation threshold of conductivity. The resulting composites effectively combine mechanical properties of the thermoplastic polymer matrix and the electronic properties of PANI and can be used as organic gas sensors [12], [13]. Specifically, nanostructured PANI fabricated as nanofibers, core–shell particles, or nanotubes has recently received a lot of attention due to its ability to form thin films with porous structure [14], [15]. This morphology facilitates penetration of the analyte molecules inside of the sensor film bulk. Therefore, better metrological performances of these materials namely the detection limit, the sensitivity and the response time are expected [16], [17]. Indeed, several recent publications reported that nanostructured PANI based composites can be used as sensing materials with the detection limit below 1 ppm for ammonia gas [18], [19]. In particular, when hybrid nanomaterials with a p–n heterojunction between TiO2 nanowires and a PANI component were applied [20], extremely low detection limits have been demonstrated. However, it is still very complicated to fabricate such materials. Their reproducibility, the rate and the amplitude of the response, as well as sensitivity, strongly depend on several parameters: the active layer thickness and morphology, its porosity, the nature of the dopant and the presence of other components in the material should be taken into account simultaneously. As discussed in the literature, nanomorphology of the sensor materials (in a sense of accessibility of the recognition centers to the analyte molecules) should be considered among the main reasons determining the strength and rate of the sensor response [21], [22], [23]. Based on this understanding, we focused our work on the development of new all-polymer nanostructured composite films suitable for ppb range ammonia sensing. As a starting point, we introduce two different PANI-based systems with artificially created nanoscale architecture: (i) core–shell particles tightly bound in a thin film, and (ii) nanofibers incorporated into the inert polymer matrix. We demonstrate ppb range sensor abilities in both PANI-based nanocomposites. Such a high sensitivity to the analyte has been achieved even without accurate optimization of the dopant content and the polymer film thickness. We compare materials according to their gas sensing behavior and relate the observed detection limit to their morphology. We also address the question of the influence of doping level of PANI on the detection limit for the composites with nanofiber-based structures.
Section snippets
PANI-based core–shell particles
PANI-based core–shell materials were prepared via two routes of chemical oxidative polymerization in water dispersions of submicron polymer particles with anilinium chloride or aniline in the presence of acid dopants being used as initial monomers. The synthesis details have been described in previous publications [24], [25], and will be briefly shown below.
Results and discussion
The dynamic behavior of the developed sensor materials, their reversibility, and detection limits under ammonia gas exposure were studied in terms of their morphology. In order to display their specificities and originality, the obtained results are grouped, discussed, and compared to previously reported findings. Our results are presented below according to the peculiarities of the core–shell or fibrillar nanomorphology of the composite material.
Conclusions
Nanostructured PANI-based composites were prepared via three different techniques and studied as a sensing layer for an electronic organic ammonia gas sensor operating at a room temperature. Two of them had a core–shell structure (PVDF–PANI(DBSA) or PBuA–PANI(HCl)) and one had PANI nanofibers embedded in amorphous dielectric PU matrix (PU–PANI(CSA)). Simple ammonia gas sensor devices constituted by interdigitated gold electrodes patterned on surface oxidized silicon substrate and coated with
Acknowledgements
The authors are grateful to Dr. Larry Scipioni and Mr. Chuong Huynh, Carl Zeiss SMT, Inc., USA for helium ion microscopy study of PVDF–PANI core–shell sample, and to Dr. N. Lavrik, Center for Nanophase Materials Sciences (CNMS) at Oak Ridge National Laboratory, for fabrication of some interdigitated electrodes (CNMS2008-084 grant).
The authors acknowledge Mr. Damien Betrancourt, Département Génie Civil et Environnemental of’ Ecole des Mines de Douai, for SEM Image of PU–PANI films and Mr. Andriy
JL Wojkiewicz received Ph.D. in Material Science from Université Lille Nord de France in 1984 on the Mott Anderson transition in III V semiconductors. He is working as Assistant professor at the department of Chemical and Environment of the Ecole des Mines de Douai, France. He is currently working on nanostructured polyaniline based composites from synthesis, electronic properties to applications. Particularly, his interests are on functionalized polyaniline for electromagnetic shielding and
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JL Wojkiewicz received Ph.D. in Material Science from Université Lille Nord de France in 1984 on the Mott Anderson transition in III V semiconductors. He is working as Assistant professor at the department of Chemical and Environment of the Ecole des Mines de Douai, France. He is currently working on nanostructured polyaniline based composites from synthesis, electronic properties to applications. Particularly, his interests are on functionalized polyaniline for electromagnetic shielding and gas sensing applications.
V.N. Blynyuk is Doctor in Science in Polymer Chemistry (2004) and Ph.D. in Physics and Mechanics in Polymers(1985). He is working as assistant Professor in the department of Physical Science and Engineering, College of Engineering and Applied Science, Western Michigan University since 2001. His interests are in the physical and chemical properties of nanostructured materials, particularly in carbon nanotubes and polyaniline composites for molecular electronics, electro-optics and photovoltaics and gas sensing.
S. Carquigny received a master degree in chemistry and physics at Université de Franche-Comté - UFR Sciences et Techniques – Besançon, Institut UTINAM UMR CNRS 6213 et LMN AC UMR CEA_E4. She is a Ph.D. student at the department of Chemical and Environment of the Ecole des Mines, Douai, France. Her work concerns gas sensing (ammonia, formaldehyde) with polyaniline composites.
N.Elkamchi titular of a master degree in microelectronic and actually Ph.D. student at Ecole des Mines de Douai since 2008. Specialized on conducting polymers, he worked during three years on the study of these materials, especially Polyaniline that was the subject of different applications in the fields of EMI shielding and gas sensing.
N.Redon graduated in 1996 of a Master Degree in engineering in industrial systems, She hold a doctorate in electronics obtained in 2000 at the Université du Littoral Côte d’Opale of Calais (France). Since 2001, she works as researcher at the Chemical and environment department of the Ecole des Mines de Douai. Her research interests are on the development of electronic gas sensors for the analysis of indoor air and/or outside, using intrinsically conductive polymer materials (ICP); the development of composite materials (TiO2/ICP) for the air-treatment by photocatalysis; The development of networks of sensors (TGS type semiconductor) for measurement of odor or signatures of air pollution (data processing by PCA, LDA, neural networks) and the characterization and improvement of physical properties of intrinsic conductive polymers for shielding applications and/or electromagnetic absorption.
T. Lasri received the Ph.D. degree in electronic from the University of Lille in 1992, he is presently Professor of Electronics and Electrical Engineering in the University of Lille. His main research interests, in the Institut d’Electronique, de Microélectronique et de Nanotechnologie (IEMN), encompass the development of measurement techniques, and the conception and realization of systems for microwave and millimeter wave Non Destructive Evaluation (NDE) purposes including the characterization of nano-devices. Another interest is in the area of energy with the development of microgenerators based on thermoelectric transduction. He served as reviewer for many international journals and conferences, he is a TPC member for international conferences and is currently on the editorial board of Sensing and Imaging – An International Journal.
Alexander A. Pud since 2009 is the Head of the Department of Chemistry of Functional Materials of the Institute of Bioorganic Chemistry and Petrochemistry of National Academy of Sciences of Ukraine. He graduated from Kiev Polytechnic Institute, Department of Electrochemical Productions Technology (1979). In 1985 and 2004 he received his Ph.D. degree and Dr. Sci. (HDR) in Polymer Science, at Chemistry Department of Kiev State University, Ukraine respectively. In 1999 he was awarded with Kiprianov Prize of National Academy of Sciences of Ukraine for the cycle of works performed jointly with Prof. Dr. G.S. Shapoval “Electrochemically initiated transformations of macromolecules”. In 2011 he became the Professor in Physical Chemistry. His research interests currently are in fields of chemical and electrochemical formation, properties and functioning of intrinsically conducting polymer (ICP) structures in dispersion and solid-phase media; synthesis, properties and applications of multifunctional host-guest (core-shell) hybrid (nano)composites of ICP (e.g. polyaniline, polythiophene and their derivatives) with both polymers of other nature and inorganic nanoparticles (semiconductor, dielectric, magnetic etc.).
Stephanie Reynaud, she obtained her Ph.D. in 1999 on Chemistry and Physic-Chemistry of Molecular and Macromolecular Materials. After a post-doctoral position both at the Carnegie Mellon Institute in Pittsburgh (Prof. Matyjaszewski group) and within ATOFINA in King of Prussia (USA), she became a permanent researcher at the CNRS within the “Institut Pluridisciplinaire de Recherche sur l’Environnement et les Matériaux”, UMR 5254 (Pau, France). Her domain of expertise is the synthesis and the characterization of polymers. She works on solution and aqueous dispersed phase using either conventional heating or microwave irradiation. Particularly she developed the synthesis of intrinsically conducting polymers for heating, gas sensing (VOC, ammonia) or smart hydrogels for tactile sensors.