(190o) Correlation Between Sensitivity and Morphology of Flame-Made SnO2 Thin Film Gas Sensors

Since the discovery of the gas sensing capabilities of metal oxides in early 1950s, metal oxide based gas sensors have been representing a fast growing industry. A wide variety of metal oxides are used as gas sensor materials, such as tin dioxide (SnO₂) and indium oxide (In₂O₃), in which SnO₂ stands out as the most prominent one owing to its superior electronic and chemical properties. This work describes a direct deposition of SnO₂ thin films using a flame aerosol reactor (FLAR) method, focusing on the investigation on the correlation between film morphology and sensing properties. The FLAR offers several advantages over other methods owing to the versatility of the method, which is based on the formation of metal-oxide particles through the combustion of precursors either from gases, liquids, or solid materials. Another feature is that the FLAR can be utilized for in situ direct deposition of sensitive layers onto diverse substrates, which requires no annealing step and entails a good reproducibility in comparison to wet-chemical methods. The SnO₂ thin films were deposited on commercial alumina chips using the FLAR system. The morphology of the thin films was controlled by changing various parameters, such as precursor concentration, burner-substrate distance, and deposition time. The results showed that three typical morphologies of the thin films were achieved, i.e., columnar, column-granular, and granular structures. The as-prepared thin films with columnar structures demonstrated improved performance, in particular to ethanol vapour. It is considered that in the case of columnar structured thin film sensors, there are less grain boundaries compared with those formed from particles. The highly oriented single crystal SnO₂ has well distributed space charge layer and less resistance than polycrystalline SnO₂, which can enhance the charge transfer inside the film and hence improve the chemical and electrical properties of the sensor.