Sensors and Actuators B 135 (2008) 81–88 Contents lists available at ScienceDirect Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb Low temperature methane sensing by electrochemically grown and surface modified ZnO thin films P.K. Basu, S.K. Jana, H. Saha, S. Basu ∗ IC Design & Fabrication Centre, Department of Electronics & Telecommunication Engineering, Jadavpur University, Kolkata 700032, India article info Article history: Received 8 April 2008 Received in revised form 22 July 2008 Accepted 24 July 2008 Available online 31 July 2008 Keywords: Nanocrystalline ZnO thin films Electrochemical anodization Surface modification Resistive and MIM sensors Pd–Ag catalytic metal contact Low operating temperature abstract The functional characteristics of the planar resistive and MIM (metal-insulator-metal) sensors using elec- trochemically grown nanocrystalline–nanoporous ZnO thin films and surface modified by dipping in an aqueous solution of PdCl 2 were investigated for methane sensing. It was found that the operating temper- ature was substantially reduced to 70 ◦ C and 100 ◦ C for the two different configurations, respectively, after this interesting and somewhat novel surface modification step. A high purity Zn was anodized to produce ZnO thin films using a Pt cathode, a calomel reference electrode and a 0.3 M oxalic acid electrolyte. Pd–Ag (26%) was used as the catalytic metal contact to ZnO to fabricate a resistive and an MIM configuration. The response of the order of ∼48, a response time of ∼4.5 s and a recovery time of ∼22.7 s were obtained for the planar resistive structures, while the MIM structures showed a response of the order of ∼32, a response time ∼2.7 s and a recovery time of ∼16 s. The sensors were studied in the presence of 1% methane in nitro- gen and in synthetic air in separate experiments. The performance was somewhat reduced in synthetic air for both the sensor structures while maintaining the optimum operating temperature the same. Both the sensors were stable in 1% methane in nitrogen as well as in 1% methane in synthetic air. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Metal oxide nanostructures can work as sensitive and selective chemical sensors. Nanostructural sensor elements can be config- ured as resistors whose conductance can be modulated by charge transfer across the surface or as a barrier junction device whose properties can be controlled by applying potential across the junc- tion. Functionalizing the surface further offers a possibility to improve their sensing ability. Continuous research and development activities are being pur- sued to explore a gas sensor for detection of low concentrations of methane in the coalmine atmosphere at substantially low tem- perature so that the methane explosion is not further accelerated by the high sensing temperature [1,2]. We have been investigat- ing for the last couple of years on the development of a low temperature methane detector using nanocrystalline ZnO based chemical gas sensors with Pd as the catalytic metal contact on the metal oxide surface. In our previous publications [3–7] we reported the sensing temperature between 210 ◦ C and 250 ◦ C depending upon whether the ZnO sensing film was grown electrochemically or by a sol–gel method, respectively. But these relatively high ∗ Corresponding author. Tel.: +91 3324146217; fax: +91 3324146217. E-mail address: sukumar basu@yahoo.co.uk (S. Basu). temperatures for detection of methane are still not suitable for applications in the coalmines. We report in this communication a novel method of reducing methane sensing temperature con- siderably by the Pd surface modification of the electrochemically deposited nanocrystalline–nanoporous ZnO films using a very low concentration of PdCl 2 solution. We adopted a planar resistive con- figuration and a kind of metal-insulator-metal (MIM) structure. While the resistive sensor showed the temperature of methane detection at 70 ◦ C the MIM sensor structure recorded the maxi- mum response at 100 ◦ C with a minimum response time of ∼2.7 s. Our investigation further demonstrated that the response in syn- thetic air is also substantially high at the same temperatures for both the structures with a little increase in response time. The sta- bility study for more than 1 month in the presence of 1% methane in nitrogen showed a long term operation of both the sensor struc- tures for more than 30 days without any visible deterioration of the response behaviour. 2. Experimental The nanocrystalline–nanoporous ZnO thin films were prepared by electrochemical anodization of high purity Zn (8 mm × 8 mm) (99.9% purity, Aldrich Chemicals, USA) of thickness 0.5 mm using a Pt cathode, a calomel reference electrode, a 0.3 M oxalic acid electrolyte (99%, MERCK, India) and a constant 10V potentio- 0925-4005/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2008.07.021