Effects of porosity and particle size on the gas sensing properties of SnO2 films

Metal oxide semiconductors are widely used as gas sensing materials; thus, improving their gas sensing properties is of some interest. The microstructure of a SnO2 film was controlled using the thermal evaporation technique at a relatively high process pressure. Scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM), and Brunauer-Emmett-Teller (BET) analysis were used to characterize microstructures, crystallinity, particle size, and the surface area that was dramatically altered as a function of the process pressure. In all cases, SnO2 films had interconnected network structures with open pores; continuous grain growth was observed through the neck between the SnO2 nanoparticles. The responses of sensors fabricated at different depositional pressure were evaluated by monitoring changes in the electrical resistance of CO gas. The gas sensor deposited at 0.2 Torr showed a high response and short response time owing to its high porosity (97%) and nano-sized particles (8.4 nm). The results confirm that porosity and particle size play key roles in determining the gas response.