Comparison of response towards NO2 and H2S of PPy and PPy/TiO2 as SAW sensitive films

We have compared the performance of polypyrrole (PPy) and PPy/TiO2 as surface acoustic wave sensitive films for NO2 detection at room temperature. Our sensors, based on a dual-delay-line pattern, were fabricated on ST-X quartz substrates by the combination of polymerization and self-assembly method. Scanning electron microscopy (SEM) and Fourier transform infrared spectroscopic (FT-IR) techniques were used to characterize the PPy and PPy/TiO2 films. The dynamical frequency shift of sensors was measured as a function of NO2 concentration. When exposed to NO2, sensors with PPy and PPy/TiO2 films shows a negative frequency shift tendency and a positive one, respectively. The latter has a faster response speed and a higher sensitivity. A negative frequency shift of 30 Hz was measured for a PPy film with 15 ppm NO2, while nearly the same amount of positive frequency shift was obtained for a PPy/TiO2 film with only 10 ppm NO2. The selectivity of both films to NO2 and H2S were also assessed. Negative frequency shifts of about 90 and 50 Hz were observed for a sensor with a PPy film to 100 ppm NO2 and 100 ppm H2S, respectively. In contrast, positive frequency shifts of 110 and 5 Hz for a PPy/TiO2 film were measured to NO2 and H2S, respectively. Therefore, the use of a PPy/TiO2 sensitive film can improve the sensor selectivity of NO2 from H2S.