Detection of hydrogen fluoride using SnO2-based gas sensors: Understanding of the reactional mechanism

Tin dioxide-based gas sensors are very efficient devices for the detection of hydrogen fluoride in trace levels since amounts lower than 50 ppb can be detected. Considering the working temperatures of tin dioxide-based gas sensors which are included between 25 and 500 °C, the best sensitivity was obtained when the sensorʹs temperature was maintained at 380 °C. In order to explain why high sensitivity is obtained for this temperature and then to understand the reactional mechanism between HF molecules and tin dioxide surfaces, X-ray photoelectron spectroscopy investigations were performed on tin dioxide samples treated with HF vapors at temperatures ranging from 200 to 500 °C. For this temperature range, the comparison between the electrical response curves and the XPS characterization, led to the consideration of two separate temperature ranges, where the interaction mechanism between HF and SnO2 can be explained. For temperature lower than 380 °C, the adsorption of HF induces the formation of surface hydroxyl groups and SnF4 species. In that case, the electrical conductance of the sensitive material gradually increases. Beyond this temperature, water vapors desorb from tin dioxide surfaces and then the electrical conductance is lowered. Finally, for both ranges of temperatures, the interaction mechanism occurring at the gas/detector interface is proposed.