Low temperature methane sensing by electrochemically grown and surface modified ZnO thin films

The functional characteristics of the planar resistive and MIM (metal-insulator-metal) sensors using electrochemically grown nanocrystalline–nanoporous ZnO thin films and surface modified by dipping in an aqueous solution of PdCl2 were investigated for methane sensing. It was found that the operating temperature 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 nitrogen 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.