Fig. 6. (a) The time response and (b) schematic plot of the surface reaction for the “G300” sensor to 1 ppm NO₂ and UV illumination in the recovery step. The periods of 1 to 4 indicate the initial state, adsorption of NO₂, pre-cleaning process and short term UV illumination, respectively. (c) The time response for the “G” and “G300” sensors exposed to different concentrations of NO₂ with proper UV illumination time for a full recovery. (d) Absolute value of the response change by the effect of RTA at 300 °C and recovery change by the optimized UV illumination time.