NH_3 sensing property and mechanisms of quartz surface acoustic wave sensors deposited with SiO_2, TiO_2, and SiO_2-TiO_2 composite films

摘要

Pristine SiO_2, TiO_2 and composite SiO_2-TiO_2 films of 200nm 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.