NH3 sensing property and mechanisms of quartz surface acoustic wave sensors deposited with SiO2, TiO2, and SiO2-TiO2 composite films.

Pristine SiO 2 , TiO 2 and composite SiO 2 -TiO 2 films of 200 nm thick were coated on surface of quartz acoustic wave (SAW) sensors with sol-gel and spin coating technique. Their performance and mechanisms for sensing NH 3 were systematically investigated. Sensors made with the TiO 2 and SiO 2 -TiO 2 films showed positive frequency shifts, whereas SiO 2 film exhibits a negative frequency shift to NH 3 gas. it is believed that the negative frequency shift was mainly caused by the increase of NH 3 mass loading on the sensitive film while the positive frequency shift was associated to the condensation of the hydroxyl groups (-OH) on the film making the film stiffer and lighter, when exposed to NH 3 gas. It demonstrated that humidity played a significant factor on the sensing performance. Comparative studies exhibited that the sensor based on the composite SiO 2 -TiO 2 film had a much better sensitivity to NH 3 at a low concentration level (1 ppm) with a response of 2 KHz, and also showed fast response and recovery, excellent selectivity, stability and reproducibility. [ABSTRACT FROM AUTHOR]