Studies on a resistive gas sensor based on sol–gel grown nanocrystalline p-TiO2 thin film for fast hydrogen detection

A thin layer (~1 μm) of sol–gel grown nanocrystalline p-type TiO2 was deposited on a thermally oxidized p-Si (2–5 Ω cm resistivity and (1 0 0) orientation) substrate. The surface was characterized by X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM), which also confirmed the nanocrystallinity of the material. Optical absorption spectroscopy was carried out to calculate the band gap of the material. Two lateral Pd contacts were used as the catalytic metal electrodes on TiO2 to fabricate the resistive gas sensor for hydrogen sensing. Detail gas response characteristics, selectivity and the stability of the sensor structure were studied. The sensors showed high response (~55%) to hydrogen with an appreciable short response time of 2 s at the optimized temperature, 175 °C and biasing voltage, 0.1 V in a steady dynamic atmosphere of 1% H2 with N2 as carrier gas. For practical applications, similar set of sensor experiments was also performed in air ambient. At 100 °C and 1.0 V bias the response magnitude was reduced to 49% but the response time came down to 1.3 s. The recovery time was lowest (~34 s) at 150 °C. The reduction in the recovery time in air is possibly due to quick removal of residual hydrogen from the surface of the sensor by interaction with oxygen present in air. The sensors showed selectivity to hydrogen and good stability. There was no degradation after working for 42 h in a discrete mode (6 h/day) in nitrogen and also in air. A possible gas sensing mechanism was suggested with a qualitative energy band diagram.