Semiconductor-type SnO2-based NO2 sensors operated at room temperature under UV-light irradiation

- The SnO2 sensor showed larger response to 5 ppm NO2 especially under weak UV-light irradiation than In2O3 and WO3 sensors.
- The UV-light irradiation reduced resistances of all sensors and accelerated their response and recovery speeds.
- The appropriate amount of Pd loading enhanced the NO2 response of the SnO2 sensor under weak UV-light irradiation.
- The 0.05 wt% Pd loading onto the SnO2 sensor reduced the dependence of the NO2 response on humidity.

NO2-sensing properties of typical oxide (SnO2, In2O3, or WO3)-based semiconductor gas sensors were measured at 30 °C with and without UV-light irradiation (main wavelength: 365 nm), and effects of noble-metal (Pd or Pt) loading, UV-light intensity (0-134 mW cm−2) and relative humidity in target gas (0-80%RH) on their NO2-sensing properties were investigated in this study. The UV-light irradiation effectively reduced the resistances of all sensors, enhanced their NO2 responses in some cases, and tended to accelerate their response and recovery speeds in dry air, because the UV-light irradiation promoted the adsorption and desorption of NO2-species on the surface. The SnO2 sensor showed the largest NO2 response in dry air, among all the pristine oxide sensors, especially under weak UV-light irradiation (≤35 mW cm−2), together with relatively fast response and recovery speeds. The Pd or Pt loading onto SnO2 enhanced the NO2 response of the SnO2 sensor and accelerated their response and recovery speeds in dry air, while XPS analysis indicated that most of the Pd and Pt nanoparticles loaded on the surface were oxidized after heat treatment at 500 °C. Among all the sensors, the 0.05 wt% Pd-loaded SnO2 sensor showed the largest NO2 response under weak UV-light irradiation (≤35 mW cm−2), together with relatively fast response and recovery speeds. The addition of moisture to the target gas had adverse effects on the NO2 responses and the response speeds of the SnO2 and 0.05 wt% Pd-loaded SnO2 sensors, but the weak UV-light irradiation (7 mW cm−2) largely reduced the dependence of the NO2 response of the 0.05Pd/SnO2 sensor on relative humidity while maintaining the large NO2 response, probably because the weak UV-light irradiation promotes the desorption of physisorbed water molecules and then the effective adsorption of NO2 on the 0.05Pd/SnO2 surface.