The investigation of hydrogen gas sensing properties of SAW gas sensor based on palladium surface modified SnO2 thin film

The sol-gel method and the magnetron sputtering method were used to deposit the SnO2 thin films on 128° YX LiNbO3 piezoelectric substrate for fabricating different kinds of delay-line surface acoustic wave (SAW) hydrogen gas sensors. To improve hydrogen gas sensing performance of SnO2 sensing films, the bi-layer structure sensitive films was used, which was composed of one pure SnO2 layer and one highly dispersed palladium nanoparticle layer. This bi-layer structure evidently enhanced the hydrogen gas sensing properties of SnO2 thin films. The microstructure, surface morphology and composition of as-prepared SnO2 sensitive films were analyzed by XRD, FESEM and XPS. The oscillator circuit with SAW sensor as resonate was designed to transduce the response of sensitive film into the frequency shift of oscillator. One precise temperature control system was used to ensure the temperature stability of SAW device. The Pd-surfaced-modified SnO2 thin film deposited by magnetron sputtering method has the highest frequency shift of 115.9 kHz to 2000 ppm hydrogen gas at 175 °C. The influence of thickness on the morphology of Pd nanoparticles and hydrogen gas sensing performance was studied. The size of Pd nanoparticle increases with thickness of Pd films and too thick Pd layer will reduce the hydrogen gas sensing performance of SnO2 films.