Influence of humidity on the gas sensing characteristics of SnO2

Today there is a high demand for devices, which can detect small concentrations of various gases for various applications e. g. leakage detection of industrial processes, mobile phone, automated car and house ventilation etc. Sensors based on SMOX (Semiconducting Metal OXides) can be an inexpensive and robust solution. Despite their simple measuring principle the sensing process of the most commonly used oxide, SnO2, is still not fully understood. This understanding is necessary to exploit its full potential in ambient conditions. The influence humidity has on the sensing process is one of the major aspects of operation in application relevant conditions. It is the focus of the presented study. Four different SnO2 materials were chosen as representatives for a wide range of SnO2 materials. Their sensing performance towards CO, EtOH, CH4, and H2 in different humidity backgrounds has been investigated with DC resistance, catalytic conversion and DRIFTS (Diffuse Reflectance Infrared Fourier Spectroscopy) measurements to gain information about their general sensing properties and surface reaction mechanisms. It was shown that water does not on all SnO2 materials react as a reducing gas - as so far reported - but can react also in an electroneutral way. Furthermore, also the influence of Cobalt and Palladium doping on the selectivity towards the above mentioned gases and the sensor signal was thoroughly investigated. The attained knowledge about the general sensing characteristics and mechanisms of SnO2 serves as a toolbox for future straight forward sensor developments.