The influence of light on the gas sensitive properties of microstructured metal oxide thin films

Metal oxide gas sensors are widely used for different applications and operate normally at high temperatures between 300 degrees C and 600 degrees C. Such high temperatures are mainly needed to speed up the desorption of molecules from the gas sensor surface. Goal of the reported investigations is the reduction of the operating temperatures of such devices by the influence of radiation on the gas adsorption/desorption process. Therefore, the influences of radiation on the gas sensing mechanisms at surfaces of different metal oxides (SnO2 ZnO, WO3 and Cr2-xTixO3+z) have been studied for different wavelengths. The experiments were carried out at an operating temperature of 130 degrees C as well as at room temperature. As radiation sources LEDs emitting at different wavelength were used. The sensor response to NO2, CO, NH3 and H-2 has been measured with and without illumination. The investigations have shown that light mainly influences the photo-activation of electron-hole pairs, which results in an increasing of the electrical conductivity of the illuminated metal oxide. The observed influences of photoadsorption and photocatalytic effects are small compared to the photoelectric effect. Only a weak increase of the NO, sensitivity during illumination has been measured.