Available online at www.sciencedirect.com Synthetic Metals 158 (2008) 29–32 A polyaniline/WO 3 nanofiber composite-based ZnO/64 ◦ YX LiNbO 3 SAW hydrogen gas sensor A.Z. Sadek a,∗ , W. Wlodarski a , K. Shin b , R.B. Kaner c , K. Kalantar-zadeh a a RMIT University, School of Electrical and Computer Engineering, Melbourne, VIC 3001, Australia b Department of Applied Chemistry, Sejong University, Seoul 143-747, Republic of Korea c Department of Chemistry and Biochemistry, University of California, Los Angeles, CA 90095, USA Received 4 July 2007; accepted 22 November 2007 Available online 18 January 2008 Abstract Polyaniline/WO 3 nanofiber composite-based surface acoustic wave (SAW) gas sensor has been investigated towards hydrogen (H 2 ). Chemical oxidative polymerization of aniline was employed to synthesize polyaniline nanofibers with WO 3 nanoparticles. The nanocomposite was deposited onto a layered ZnO/64 ◦ YX LiNbO 3 SAW transducer. The sensor was exposed to various concentrations of H 2 gas and operated at room temperature. The sensor response was found to be 7 kHz towards 1% of H 2 in synthetic air. A fast response and recovery with good repeatability in a stable baseline condition were observed at room temperature. © 2007 Elsevier B.V. All rights reserved. Keywords: Polyaniline nanofiber; WO 3 ; Nanocomposite; Gas sensor; H 2 1. Introduction Devices based on semiconductor metal oxide thin films have been extensively used for gas sensing based on film conduc- tivity changes caused by interaction with gas molecules [1–3]. Intense research and development have been conducted to design highly sensitive, selective and stable gas sensors since Seiyama first observed gas sensing effects in metal oxides [4]. The gas sensing mechanism involves chemisorption of oxygen on the oxide surface followed by charge transfer during the reaction of chemisorbed oxygen with target gas molecules [5]. The adsorbed gas atoms inject electrons into or extract electrons from the metal oxide, depending on whether they are reducing or oxidizing agents, respectively [6]. Tungsten trioxide is an n-type semicon- ductor, which has been widely used to detect H 2 [7], NO 2 [8], NH 3 [9],O 3 [10] and H 2 S [11,12] gases. Recently, it has been reported [2,3,11–13] that WO 3 in the nanostructured forms has better performance than polycrystalline forms for gas sensing applications. However, such sensor requires an elevated tem- perature (300–500 ◦ C) for optimum operation which eventually ∗ Corresponding author Tel.: +61 3992 55280; fax: +61 3992 52007. E-mail address: sadek@ieee.org (A.Z. Sadek). reduces the sensor life time. Also, an elevated temperature opera- tion requires extra circuitry with high power consumption which reduces portability. Thus it would be highly desirable if the sen- sor could operate at room temperature with high sensitivity and fast response and recovery. As an alternative to metal oxide materials, conducting poly- mers have received increasing interest for sensor design due to their room temperature operation, low production cost, ease of deposition onto a wide variety of substrates [14]. Among the family of conducting polymers, polyaniline is one of the most highly studied materials because of its simple synthesis, environ- mental stability and straightforward non-redox acid doping/base dedoping process to control conductivity [15]. By changing the doping level and morphology, the conductivity of polyaniline can be tuned for specific applications such as sensors, actuators, rechargeable battery electrodes, anticorrosion coatings, display devices and field effect transistors (FETs) [16]. The conduc- tivity of polyaniline also depends on the oxidation state of the main polymer chain [17]. Redox active chemicals and gases can affect the conductivity of polyaniline by changing its inherent oxidation state. Neutral, volatile organic compounds are able to change the conductivity of doped polyaniline films as a result of polymer swelling, chain alignment, crystallization, solvation or by affecting the doping level [18,19]. Polyaniline gas sensors 0379-6779/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2007.11.008