Doped and dedoped polyaniline nanofiber based conductometric hydrogen gas sensors

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Abstract

Template-free, rapid polymerisation was employed to synthesize polyaniline nanofibers using chemical oxidative polymerisation of aniline, with HCl as a dopant. The doped and dedoped nanofibers were deposited onto conductometric sapphire transducers for gas sensing applications. The sensors were exposed to various concentrations of hydrogen (H2) gas at room temperature. The sensitivity was measured to be 1.11 for doped and 1.07 for dedoped polyaniline nanofiber sensors upon exposure to 1% H2. Fast response times of 28 and 32 s were observed for dedoped and doped sensors, respectively. The dedoped nanofiber sensor outperforms the doped sensor in terms of baseline stability and repeatability. Due to its room temperature operation, the gas sensor is promising for environmental applications.

Introduction

Conducting polymers are of increasing importance in the development of smart sensors due to their room temperature operation, low fabrication cost, ease of deposition onto a wide variety of substrates [1], [2] and their rich structural modification chemistry [3]. Polyaniline is unique among the family of conducting polymers since its doping level can be controlled through a non-redox acid doping/base dedoping process [4]. By changing the doping level, conductivity of polyaniline can be modified to suit specific applications. Polyaniline in the emeraldine oxidation state can be reversibly switched between electrically insulating emeraldine base and conducting emeraldine salt forms. Polyemeraldine (Fig. 1, top) consists of amine (–NH–) and imine (double bondN–) sites in equal proportions. The imine sites can be protonated to achieve the intermediate bipolaron form (Fig. 1, middle) and finally by dissociation the polaron lattice form (Fig. 1, bottom), resulting in high conductivity [5]. It is widely agreed that polarons are the charge carriers responsible for the high conductivity of polyaniline. By controlling the pH of the dopant acid solution any desired quantity of dopant can be added until all imine nitrogen atoms are doped. The dopant can be removed by a reversible reaction with any strong base such as ammonium hydroxide (NH4OH).

The conductivity of polyaniline depends on both the oxidation state of the main polymer chain and the degree of protonation on imine sites [6]. Any interaction with polyaniline that alters either of these processes will affect its conductivity. Unlike acids and bases, redox active chemicals and gases can change the conductivity of polyaniline by changing its inherent oxidation state.

Depending on the extent of the redox reaction, polyaniline can exist in a range of oxidation states: fully reduced leucoemeraldine, half oxidized emeraldine and fully oxidized pernigraniline. The emeraldine form of polyaniline shows the highest electrical conductivity after it had been doped with protonic acid [8]. Neutral, non-redox organic compounds, such as chloroform or toluene, are able to change the conductance of doped polyaniline films through swelling effects [9]. Functional additives incorporated into the polyaniline structural matrix, such as metals, metal oxides and enzymes can change the electrical characteristics of polyaniline. This versatility has made polyaniline attractive for a broad scope of design and development of smart sensors [10].

Recently, nanostructured materials, in the form of nanowires, nanotubes, nanofibers or nanobelts have received much attention. These one-dimentional materials process extremely high surface area without increasing the device dimension. Therefore, they should have improved performance in applications where a high surface contact area is needed between the device and its environment, such as in sensors [11], [12]. Polyaniline nanofibers which have a cylindrical morphology form porous structures when deposited as thin films. Polyaniline nanofibers, with diameters in the nanometer range, possess larger surface areas per unit mass and permit easier addition of surface functionalities compared with traditional polyaniline which is highly agglomerated [13]. The three-dimensional porous structure of a polyaniline nanofiber film allows easy diffusion of gas molecules into and out of the film and the nano-scale fiber diameters lead to rapid diffusion of gas molecules into the polyaniline [14]. As a result, polyaniline nanofiber based sensors outperform conventional polyaniline thin film sensors in terms of sensitivity and dynamic performances upon exposure to a number of gases [11], [12].

The sensitivity of conventional polyaniline thin film gas sensors depends on the film thickness [15]. Generally, sensor sensitivity increases with a reduction of film thickness as entire thickness of the film is affected by the reactions with gas species in a short period of time. On the other hand, sensitivity of a polyaniline nanofiber sensor is independent of film thickness, due to the porous structure of the film which leads to the predominance of surface phenomena over the bulk material phenomena. This advantage allows the fabrication of sensors with reproducible response that have a large tolerance in thickness variation [11].

Although nanostructured conducting polymers are very promising for sensors, there are few reports of them in the literature. Recently, we have reported [7] a surface acoustic wave based polyaniline nanofiber hydrogen sensor. To the best of our knowledge, polyaniline nanofibers have not yet been used as a conductometric sensor for hydrogen gas sensing. In this paper, we will present and compare the responses of doped and dedoped polyaniline nanofiber based conductometric H2 gas sensors.

Section snippets

Experimental

The classical methods of synthesizing polyaniline nanostructures usually require structure-directing templates, such as zeolite channels [16], nanoporous membranes [17], or surfactants [18]. Complex synthesis conditions require removal of these templates at the end of the reaction, resulting in very low production rate. Recently, template-free, interfacial polymerisation was employed to synthesize polyaniline nanofibers using chemical oxidative polymerisation of aniline [19], [20]. The

Results and discussion

A scanning electron microscope (SEM) image of the polyaniline nanostructures on the sapphire substrate is shown in Fig. 3. The SEM result indicates that the polyaniline layer deposited on the sapphire substrate consists of nanofibers. The average diameter of the both doped and dedoped polyaniline nanofibers is about 30 nm with a length of several microns. The average thickness measured for both of the thin films on sapphire substrate using a profilometer is 0.3 μm, and the deviation in thickness

Conclusion

Conductometric H2 gas sensors based on doped and dedoped polyaniline nanofibers were developed. The sensors were investigated for concentrations of H2 gas in synthetic air. Both of the sensors showed high sensitivity and good repeatability. For dedoped polyaniline nanofiber sensor, a 17 kΩ resistance shift from 260 kΩ baseline, which is equivalent to a sensitivity of 1.07, was observed when exposed to 1% H2 at room temperature. For similar conditions, the doped polyaniline nanofiber sensor

Abu Z. Sadek received the B.Sc. degree in electrical and electronics engineering from the Bangladesh University of Engineering & Technology (BUET), Dhaka, Bangladesh, in 1998, and the M.E. degree in Telecommunications Engineering from the University of Melbourne, Australia, in 2002. He is currently pursuing the Ph.D. degree at the Sensor Technology Lab, School of Electrical and Computer Engineering, RMIT University, Melbourne, Australia. His research interests include chemical and biochemical

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    Abu Z. Sadek received the B.Sc. degree in electrical and electronics engineering from the Bangladesh University of Engineering & Technology (BUET), Dhaka, Bangladesh, in 1998, and the M.E. degree in Telecommunications Engineering from the University of Melbourne, Australia, in 2002. He is currently pursuing the Ph.D. degree at the Sensor Technology Lab, School of Electrical and Computer Engineering, RMIT University, Melbourne, Australia. His research interests include chemical and biochemical sensors, micro and nanotechnology, acoustic propagation and conducting polymers.

    Wojtek Wlodarski has worked in the areas of sensor technology and instrumentation for over 30 years. He has published 4 books and monographs, over 400 papers and holds 29 patents. He is a professor at RMIT University, Melbourne, Australia, and heads the Sensor Technology Laboratory at the School of Electrical and Computer Engineering.

    Dr. Kourosh Kalantar-Zadeh is a tenured academic at the School of Electrical and Computer Engineering, RMIT University, Melbourne, Australia. His research interests include: chemical and biochemical sensors, nanotechnology, MEMS, thermoelectric materials, electronic circuits, and microfluidics. He has published more than 80 scientific papers in the refereed journals and in the proceedings of international conferences. He holds three patents. He is currently authoring a book entitled “Nanotechnology Enabled Sensors”.

    Christina O. Baker received her B.S. in chemistry from the Georgia Institute of Technology, Atlanta, in May 2003. She is a graduate student pursuing a Ph.D. in inorganic chemistry at the University of California, Los Angeles (UCLA) working with Professor Kaner on conducting polymers. Her current research involves the development of polyaniline nanofibers and metal nanoparticle polyaniline nanofiber composites for applications in non-volatile memory, chemical sensors and actuators.

    Richard B. Kaner received a Ph.D. in Inorganic Chemistry from the University of Pennsylvania in 1984 followed by two and a half years of postdoctoral research at UC Berkeley. He is Professor of Chemistry and Professor of Materials Science & Engineering at the University of California, Los Angeles (UCLA). He joined UCLA in 1987 as an assistant professor, earned tenure in 1991 and became a full professor in 1993. He has received awards from the Dreyfus, Fulbright, Guggenheim, Packard and Sloan Foundations for his work on new routes to refractory materials including high-temperature ceramics, intercalation compounds, fulleride superconductors, super hard materials and conducting polymers.

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