Sensors and Actuators B 176 (2013) 893–905 Contents lists available at SciVerse ScienceDirect Sensors and Actuators B: Chemical journa l h o mepage: www.elsevier.com/locate/snb Ultra-sensitive H 2 sensors based on flame-spray-made Pd-loaded SnO 2 sensing films C. Liewhiran a,d,∗ , N. Tamaekong b , A. Wisitsoraat c , A. Tuantranont c , S. Phanichphant d a Department of Physics and Materials Science, Faculty of Science, Chiang Mai University, Chiang Mai 50202, Thailand b Program in Materials Science, Faculty of Science, Maejo University, Chiang Mai 50290, Thailand c Nanoelectronics and MEMS Laboratory, National Electronics and Computer Technology Center (NECTEC), Klong Luang, Pathumthani 12120, Thailand d Materials Science Research Center, Faculty of Science, Chiang Mai University, Chiang Mai 50202, Thailand a r t i c l e i n f o Article history: Received 2 June 2012 Received in revised form 29 September 2012 Accepted 19 October 2012 Available online xxx Keywords: SnO2 Pd Flame spray pyrolysis Selectivity H2 sensor a b s t r a c t In this paper, ultra-sensitive hydrogen (H 2 ) gas sensors based on flame-spray-made Pd-catalyzed SnO 2 nanoparticles is presented. Pd-loaded SnO 2 crystalline nanoparticles with high specific surface area and well-controlled size were synthesized by flame spray pyrolysis (FSP) in one step. The particle properties were characterized by XRD, BET, SEM, TEM and EDS analyses. The H 2 -sensing performances in terms of sensor response, response time and selectivity were optimized by varying Pd concentration between 0.2 and 2 wt%. An optimal Pd concentration for H 2 sensing was found to be 0.2 wt%. The optimal sensing film (0.2 wt% Pd/SnO 2 , 10 m in thickness) showed an ultra-high sensor response of ∼10 4 to 1 vol% of H 2 at 200 ◦ C and very short response time within a few seconds. Moreover, the optimum sensing temperature of Pd-loaded SnO 2 films was shifted to a lower value compared with that of unloaded SnO 2 film. The sig- nificant enhancement of H 2 sensing performances was attributed to highly effective spillover mechanism of well-dispersed Pd catalyst in SnO 2 matrix at low Pd-loading concentration. Furthermore, the catalyst selectivity of Pd toward H 2 was found to be significantly higher than those of two other noble metals including Pt and Ru, respectively. Therefore, the flame-made 0.2 wt% Pd/SnO 2 sensors is one of the most promising candidates for highly sensitive and selective detection of H 2 . © 2012 Elsevier B.V. All rights reserved. 1. Introduction Since the first development of commercially available catalytic gas-sensing element, gas sensors have been extensively studied for applications in various fields including urban, domestic life, indus- trial, security, environmental monitoring, medical and agribusiness controls [1–6]. Particularly, increasing attention has been paid on resistive type gas sensors based on semiconducting metal oxides, which may incorporate noble-metal catalysts [4,7–9], because they can be widely applied for the monitoring of explosive, flammable, and toxic gases in industrial communities. Nowadays, the main challenge is to increase the response, selectivity and long-term stability of these sensors [1–50]. These three factors are usually theoretically interrelated and considerably depend on the nature of target gas. Specifically, H 2 is one of the most challenging gases to be detected and controlled because it is col- orless, extremely flammable, explosive and susceptible to leakage ∗ Corresponding author at: Department of Physics and Materials Science, Faculty of Science, Chiang Mai University, Chiang Mai 50202, Thailand. Tel.: +66 81 408 2324; fax: +66 53 892 271. E-mail address: chaikarn l@yahoo.com (C. Liewhiran). from gas-handling equipment [8–15]. The detection of H 2 gas in dif- ferent industrial applications have become especially important for safety reasons since it appears that H 2 will gain significant impor- tance as a clean energy source in the near future. Thus, accurate and fast detection of H 2 is necessary for the extensive adoption of H 2 use in energy production. In addition, the development of a highly sensitive gas sensor for low H 2 concentration of 10–10,000 ppm is also of high interest since H 2 is one of the main gases evolving under pyrolysis in the initial stage of combustion [51] and the detection of small H 2 leakage has become an issue of crucial importance for vigilant control. Recently, the synthesis of H 2 sensors based on wide band gap semiconductors has gained special focus owing to their sensitivity to surface charge and wide temperature stability [1,2,4]. Among semiconductor materials, SnO 2 is the most popular commercial resistive-type n-type semiconducting metal oxide, which is widely used for gas-sensing applications by measuring the change of the resistance upon exposure to reducing gases (H 2 , CO, SO 2 , NH 3 , CH 4 , H 2 S, etc.) [1–50] and oxidizing gases (NO 2 , O 2 , etc.) [19,47,51–54] under an ambient condition. Gas sensors based on SnO 2 films are very attractive because of their distinct advantages such as small size, low cost, high reproducibility and compatibility with micro- fabrication processes and their gas-sensing performances can be 0925-4005/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2012.10.087