1. Introduction
Over a period of three decades, metal oxide semiconductor based sensors have been under extensive investigations due to their applications in industrial and domestic sectors . Amongst the large number of metal oxides, particularly SnO2, ZnO, WO3 and TiO2 have been the aterials of choice in most of these investigations. It has been widely accepted that the gas sensing mechanism consists of ‘receptor’ and ‘transducer’ functions. The receptor function deals with the surface chemical/ catalytic property, whereas the transducer function is due to the surface semiconducting property and grain size of the base oxide used. The receptor function can be tailored by dispersing foreign species on the metal oxide grains. In this regards, attempts have been carried out by ‘doping’ the base metal oxide systems with metallic species, such as Al, Cu, Pt, Pd, etc., in order to obtain better sensor characteristics in terms of sensitivity, selectivity and response time . The recent advances in synthesis of nanostructures having different shapes and sizes have been exploited to design and fabricate smart sensors, thereby tailoring the transducer function responsible for the sensing mechanism .However, the complexity involved in the synthesis process and need of advanced characterization techniques limit the applicability of the nanomaterial based sensor systems. In this context, the simple and cost effective conventional synthesis techniques, such as vacuum evaporation, sputtering, chemical bath deposition, spray pyrolysis, etc., used for making thin film sensors are still attractive. In addition to the sensing of oxidizing and reducing gases, various researchers have attempted detection of volatile organic compounds (VOCs) .In this regard, Patel et al. have reported sensing chracteristics of an indium tin oxide thin film for methanol vapour. Fabrication of a SnO2 based sensor array for recognition of VOCs has been attempted by Lee et al. The authors have studied the effect of various additives such as Pd, Pt, La2O3, CuO, Sc2O3, TiO2,WO3, ZnO and V2O5
on the sensitivity and the selectivity of the SnO2 sensor array. The authors have observed high and selective sensitivity to the VOCs at 400 ◦C. Cobalt oxide Co3O4, owing to its better catalytic activity, can be thought to be an active additive in the base oxide matrix so as to improve its sensing performance. In this regard, a Co3O4 based isobutene sensor operating at low temperature has been reported by Choi and Min .
Acetone vapour sensing characteristics of cobalt-doped SnO2 thin films
