Enhanced NO2-Sensing Properties of Au-Loaded Porous In2O3 Gas Sensors at Low Operating Temperatures

NO2-sensing properties of semiconductor gas sensors using porous In(2)O(3)powders loaded with and without 0.5 wt% Au (Au/In(2)O(3)and In(2)O(3)sensors, respectively) were examined in wet air (70% relative humidity at 25 degrees C). In addition, the effects of Au loading on the increased NO(2)response were discussed on the basis of NO(2)adsorption/desorption properties on the oxide surface. The NO(2)response of the Au/In(2)O(3)sensor monotonically increased with a decrease in the operating temperature, and the Au/In(2)O(3)sensor showed higher NO(2)responses than those of the In(2)O(3)sensor at a temperature of 100 degrees C or lower. In addition, the response time of the Au/In(2)O(3)sensor was much shorter than that of the In(2)O(3)sensor at 30 degrees C. The analysis based on the Freundlich adsorption mechanism suggested that the Au loading increased the adsorption strength of NO(2)on the In(2)O(3)surface. Moreover, the Au loading was also quite effective in decreasing the baseline resistance of the In(2)O(3)sensor in wet air (i.e., increasing the number of free electrons in the In2O3), which resulted in an increase in the number of negatively charged NO(2)species on the In(2)O(3)surface. The Au/In(2)O(3)sensor showed high response to the low concentration of NO2(ratio of resistance in target gas to that in air: ca. 133 to 0.1 ppm) and excellent NO(2)selectivity against CO and ethanol, especially at 100 degrees C.