Sensors and Actuators B 119 (2006) 431–434 Complex plane impedance plot as a figure of merit for tin dioxide-based methane sensors S. Chakraborty, A. Sen ∗ , H.S. Maiti Central Glass and Ceramic Research Institute, Kolkata 700032, India Received 12 September 2005; received in revised form 21 December 2005; accepted 22 December 2005 Available online 25 January 2006 Abstract Thick film methane sensors have been fabricated from nanosized tin dioxide powder containing antimony oxide and palladium. The powder has been prepared by sonication-assisted simultaneous precipitation and the sensors made with this powder showed optimum resistance for device applications and good sensitivity towards methane. This contrasts with the thick film sensors prepared with the powder synthesized without sonication, which showed very high resistance at the operating temperature. The complex plane impedance spectroscopy of the sensors (both in air and in the presence of gas) can be a good indicator of the sensor quality. It has been observed that the nature of the complex plane impedance plot of the sensors fabricated by using powders synthesized through sonication-assisted simultaneous precipitation matches well with that of high-quality imported Figaro (Japan) sensors. © 2005 Elsevier B.V. All rights reserved. Keywords: Gas sensor; Impedance spectroscopy; Methane; Tin dioxide 1. Introduction The impedance spectroscopy [1] is a powerful tool for studying various materials like ionically conducting glasses, amorphous semiconductors, electronically conducting poly- mers, ionically conducting polymers and transition metal oxides. Impedance spectroscopy has also been employed to study gas adsorption behavior of semiconductor gas sensors like SnO 2 , In 2 O 3 , Ga 2 O 3 and WO 3 . Among the various materials, SnO 2 has been widely studied for gas sensor [2] applications. SnO 2 is an n-type semiconductor with a direct bandgap of 4 eV and an indi- rect band gap of 2.6 eV [3]. SnO 2 is a nonstoichiometric oxide having oxygen vacancies and electron donor states. Normally, atmospheric oxygen becomes chemisorbed on the surface, con- suming the free electrons as given below: O 2 + 2e − → 2O − ads (1) O 2 + e − → O 2 − ads (2) Any reducing gas like methane, butane and hydrogen, if present in the ambient, produces a counter-reaction, where the reducing ∗ Corresponding author. Tel.: +91 33 24733469 E-mail address: asen@cgcri.res.in (A. Sen). gas reacts with the highly reactive chemisorbed oxygen, frees the bound electrons and increases the conductivity of the semicon- ducting oxide, thus generating a signal. Although semiconductor gas sensors based on SnO 2 have already been in the market for a long time, the modifications [4–8] of the sensing properties, such as the sensitivity and selectivity, are still under way to meet their ever expanding demands in new applications. In the present study, nanosized SnO 2 -based powder contain- ing antimony and palladium has been prepared by a sonication- assisted simultaneous precipitation route. The impedance char- acteristics of the sensors (in air as well as in the presence of methane) in thick film form were compared with those made from powder prepared without sonication. The impedance spectra of Figaro (Japan) sensors were also studied for comparison. 2. Experimental A batch containing tin dioxide, 0.25% (by weight with respect to tin dioxide) antimony oxide and 10 wt% palladium was pre- pared by sonication-assisted simultaneous precipitation tech- nique. The composition of the powder was selected [9] keeping in view the role of Pd as a catalyst to improve the sensitivity and the role of Sb as an n-type dopant to enhance the carrier 0925-4005/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2005.12.045