Sensors and Actuators B 183 (2013) 364–371 Contents lists available at SciVerse ScienceDirect Sensors and Actuators B: Chemical journal h om epage: www.elsevier.com/ locate/snb Investigation of RF sputtered tungsten trioxide nanorod thin film gas sensors prepared with a glancing angle deposition method toward reductive and oxidative analytes Muhammad Z. Ahmad a,c,∗ , Anurat Wisitsoraat b , Ahmad Sabirin Zoolfakar a,d , Rosmalini Ab Kadir a,d , Wojtek Wlodarski a a School of Electrical & Computer Engineering, RMIT University, GPO Box 2476V, Melbourne 3001, Victoria, Australia b National Electronics and Computer Technology Center, 112 Pahol Yothin Rd., Pathumthani 12120, Thailand c Mechanization & Automation Research Center, MARDI HQ, 43400 Serdang, Selangor, Malaysia d Fakulti Kejuruteraan Elektrik, Universiti Teknologi MARA, 40450 Shah Alam, Selangor, Malaysia a r t i c l e i n f o Article history: Received 9 December 2012 Received in revised form 14 March 2013 Accepted 5 April 2013 Available online 17 April 2013 Keywords: Tungsten trioxide Ethanol sensing NO2 sensing Highly sensitive a b s t r a c t The gas-sensing performances of tungsten trioxide nanorod thin-films toward reducing and oxidiz- ing analytes are reported. The nanostructured thin-films were RF sputtered, via a glancing angle deposition (GLAD) technique, with different thickness onto conductometric transducers. Thin-film micro- characterization of the surface morphology, chemical compositions and crystal structure by SEM, TEM, XRD and XPS showed that the thin layers contained highly porous, nanocrystalline, stoichiometric WO 3 nanorod features with average height varying from 140 to 420 nm and diameter ranging from 40 to 60 nm. Initial vapor sensing test results showed that the ethanol response of WO 3 nanorod thin-film tended to increase with decreasing film thickness. The optimized sensor operating temperature for ethanol detec- tion was 300 ◦ C with a maximum response of 10 to C 2 H 5 OH of 200 ppm for the thinnest film. In contrast, the NO 2 response was improved by increasing the WO 3 nanorod film thickness, and the thickest films exhibited a very large response of 1075 (10 ppm of NO 2 ) at a relatively low optimized operating temper- ature of 150 ◦ C. Moreover, the developed WO 3 based sensors showed good repeatability characteristics when exposed to C 2 H 5 OH, but exhibited some poisoning effects when tested in an NO 2 environment. The interesting surface structure with very high surface-to-volume ratio of the thin films was reasoned to be responsible for the high and fast response when exposed toward both analytes. © 2013 Elsevier B.V. All rights reserved. 1. Introduction Nanostructured based metal oxide thin films have been of great interest in the applications of gas sensing due to their huge surface-to-volume ratio and high density of surface sites [1,2]. These thin-film sensors operate based on a change of the sur- face conductivity of the active material due to surface chemical adsorption of gas species and related space charge effects [3,4]. High quality nanostructures are key to achieving highly responsive sensing devices. Features that improve the quality of nanostruc- tured thin-films include, but are not limited to, size and spacing of nanostructures and their crystallinity. Nanostructures with high porosity are known to possess high surface-to-volume ratios, lead- ing to the enhanced reaction rates between the surface and the ∗ Corresponding author at: School of Electrical & Computer Engineering, RMIT University, GPO Box 2476V, Melbourne 3001, Victoria, Australia. Tel.: +61 03 9925 3690; fax: +61 03 9925 2007. E-mail addresses: zamharir@gmail.com, zamharir@mardi.gov.my (M.Z. Ahmad). molecules [5]. As a result, their gas detection capability increases dramatically with a decrease in structural dimensions and grain size [6]. A number of deposition methods have been employed to obtain various novel metal oxide nanostructures for a myriad for applications ranging from land to space [7]. Single nanowires and its array based devices have been demonstrated to offer great potential in achieving high sensitivity due to their ultrahigh surface-to- volume ratio [5,6]. However, the control of single nanostructure formation has been a significant challenge, and this novelty is still in the early stages of exploration. Presently, a single nanostructure device still requires expensive micro- or nano-scale fabrication pro- cesses, such as e-beam lithography and nano-manipulation, which are impractical for commercial applications. Thin-films compris- ing of nanostructures are practical alternatives that also offer high sensing performances. Nanostructures such as nanorods, nanowires, and nanofibers based on tungsten trioxide (WO 3 ) thin-films have been widely investigated due to its various unique properties [8–10]. Numerous experiments have been reported in regards to its applications, such as in gas sensing and solar cells [11–13]. This n-type semiconductor 0925-4005/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2013.04.027