Tin oxide films for nitrogen dioxide gas detection at low temperatures

Porous tin oxide films were prepared via an electrophoretic deposition process and their sensing response toward nitrogen gas (4.1–100 ppm) at 25–120 °C were investigated. The TEM and SEM images indicated a porous microstructure of the SnO2 film composed of nanocrystals. It is found that the sensing response increased monotonically with increasing operating temperature and gas concentration. Above 80 °C, repeatable response and recovery characteristics toward NO2 gas injection and release cycles could be obtained. The response and recovery time at different temperatures were also studied. Typically, the SnO2 sensor had a response of 61 toward 13.7 ppm of NO2 with good selectivity against NH3 and SO2 gases when operated at 105 °C. The response time (T90) and recovery time (T10) were 83 s and 45 s, respectively. Discussion based on surface chemistry theory suggests that at 105 °C, NO2 may interact with SnO2 both by trapping the free electrons directly and indirectly through the surface adsorbed oxygen ions, increasing the potential barrier across grain boundaries of SnO2 and hence making its resistance to increase. In summary, the simplicity and versatility of the electrophoretic deposition make it very attractive and promising in preparation of high-quality oxide gas sensors.