Sensors and Actuators B 177 (2013) 981–988 Contents lists available at SciVerse ScienceDirect Sensors and Actuators B: Chemical journa l h o mepage: www.elsevier.com/locate/snb Optical response of WO 3 nanostructured thin films sputtered on different transparent substrates towards hydrogen of low concentration Mohd Hanif Yaacob a,b,∗ , Muhammad Zamharir Ahmad b,d , Abu Zafar Sadek c , Jian Zhen Ou b , Jos Campbell c , Kourosh Kalantar-zadeh b , Wojtek Wlodarski b a Faculty of Engineering, Universiti Putra Malaysia, 43400 Serdang, Selangor, Malaysia b School of Electrical and Computer Engineering, RMIT University, GPO Box 2476V, Melbourne 3001, Australia c School of Applied Sciences, RMIT University, GPO Box 2476V, Melbourne 3001, Australia d Mechanisation & Automation Research Center, MARDI HQ, 43400 Serdang, Selangor, Malaysia a r t i c l e i n f o Article history: Received 6 June 2012 Received in revised form 30 October 2012 Accepted 26 November 2012 Available online 7 December 2012 Keywords: Tungsten trioxide Hydrogen Optical sensing Absorbance Transparent substrates RF sputtering a b s t r a c t The gasochromic response of WO 3 nanostructured films coated with a catalytic Pt or Pd layer on different transparent substrates upon exposure to H 2 gas was investigated. WO 3 nanostructured films with 500 nm thickness were coated with a 25 ˚ A thick Pt or Pd layer. The films were prepared on quartz, glass, indium- doped tin oxide (ITO) and fluorine-doped tin oxide (FTO) conductive glass. The nanostructured WO 3 was deposited by RF magnetron sputtering, and the Pt or Pd layer was deposited by DC sputtering. Characterization of the film revealed that the WO 3 was deposited as nanoscale grains of varied size depending on the substrate. WO 3 grains on quartz and glass were 30–40 nm in size. WO 3 grain sizes on ITO and FTO were 40–60 nm and 300–500 nm, respectively. The WO 3 films were observed to show strong gasochromic response, with absorbance changes measured in the Vis-NIR (500–1100 nm) range. The cumulative absorbance response towards H 2 is the highest and more stable for Pd/WO 3 films on quartz, glass and ITO, compared to the FTO substrate. The gasochromic effect was also stronger in Pd/WO 3 films compared to Pt/WO 3 films. © 2012 Elsevier B.V. All rights reserved. 1. Introduction The negative impacts from global warming due to the exten- sive consumption of fossil fuels require the adoption of alternative energy sources that are inexhaustible and clean. One such alter- native centres on the use of hydrogen (H 2 ). H 2 is an ideal future fuel because it significantly reduces greenhouse gas emissions, reduces the global dependence on the fossil fuels and increases the efficiency of the energy conversion process for both internal com- bustion engines and proton exchange membrane fuel cells [1]. The fact that the by-product of the H 2 based fuel cell is only water (H 2 O) [2] emphasizes the huge potential of H 2 as a clean energy source. H 2 is also one of the most commonly used process gases in many industries, such as petrochemical, electronics or semicon- ductor, metallurgy, food, glass, and laboratory analysis [3]. On the other hand, there are major safety concerns related to the prop- erties of H 2 . Even though H 2 has higher auto-ignition temperature (585 ◦ C) compared to the other types of fuels, its low mass and high ∗ Corresponding author at: Faculty of Engineering, Universiti Putra Malaysia, 43400 Serdang, Selangor, Malaysia. Tel.: +60 389464345; fax: +60 386567127. E-mail addresses: hanif.yaacob@gmail.com, imhanif@ieee.org (M.H. Yaacob). diffusion coefficient (0.61 cm 2 s -1 ) makes it difficult to store [3]. The odourless and colourless gas also easily leaks due to its small- est molecular size [4]. H 2 has a wide flammability range (4–75%) and, hence, is easily combusted when mixed with air. The risk of explosion due to H 2 leakage is further increased by its small ignition energy (0.02 mJ) and large flame propagation velocity [5]. Minor leaks from H 2 storage tanks in enclosed environments can quickly reach potentially explosive concentrations. Previous studies have identified WO 3 as a material that is highly sensitive towards H 2 [6,7,8]. When a WO 3 film is combined with a catalytic metal such as palladium (Pd) or platinum (Pt), changes in the film colour from transparent to dark blue are observed upon exposure to H 2 [6]. Pd and Pt are effective catalysts for breaking H 2 molecules into hydrogen ions [8]. The common model used to explain the gasochromic effect of WO 3 is the reduction of the W 6+ (transparent) centre in the WO 3 crystal lattice to W 5+ (blue colour) with the transfer of an H ion and electrons disassociated from H 2 molecules by the catalytic metal [4,9]. The change in the optical properties of the film makes WO 3 a suitable sensing layer in the development of an optical H 2 gas sensor. Compared to the electrical sensor, research in the optical sensing is relatively new and less explored. Nevertheless, the reduced cost in optical components which has been driven by the large 0925-4005/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2012.11.098