783675.fig.003
Figure 3: Resistance variation at room temperature for two similar samples of nanostructured WO3 exposed to a sequence of UV light and NO2 (1 ppm). The sample thickness (300  m) enlarges the response time. In dark conditions, no response was observed. The extremely high resistance cause spurious noise as resistance values approach to the limits of our equipment. After switching off the light, the PPC effect appears.