Hydrogen-sensing properties of anodically oxidized TiO2 film sensors: Effects of preparation and pretreatment conditions

Porous oxide thin films were prepared by anodic oxidation of a Ti plate in H2SO4 and HF solutions. The film (s-TiO2) prepared in the H2SO4 solution was a mixture of anatase and rutile and contained micron-size pores. In contrast, no crystal phase appeared in the film (f-TiO2) prepared in the HF solution, while the film showed submicron-size pores. A Pd electrode was deposited by evaporation on the surface of the film to make a sensor configuration of Pd (top electrode)/oxide film/Ti (bottom electrode). Both the thin film sensors exhibited a fast response to H2 and fast recovery at 250 °C in air as well as in N2 atmosphere. The magnitude of the response of s-TiO2 was superior to that of f-TiO2, especially in N2 atmosphere. The H2 response of s-TiO2 at 250 °C in air was found to vary significantly with the pretreatment conditions: additional pretreatment at 600 °C for 1 h in N2 resulted in a deteriorated H2 response especially in air, in comparison with that just after the firing at 600 °C for 1 h in air for ensuring the electrical contact. Thus, the surface oxidation state and morphology of the Pd electrode was suggested to affect the H2 response of the sensors. It was revealed that the s-TiO2 sensor without additional treatment in N2 showed superior H2-sensing performance to the N2-treated s-TiO2 sensor when operated in N2 atmosphere. In addition, the response to N2-diluted H2 was larger than that to air-diluted H2, and the interference from water vapor was negligible in N2 atmosphere.