High sensitivity and fast response hydrogen sensors based on electrochemically etched porous titania thin films

Aluminium doped porous titania (TiO2) thin films were grown by thermal oxidation, followed by electrochemical etching. The thermal oxidation of 0.25 mm thick titanium substrates having a thick gold coating on the back surface and a thin layer of Ti–2 wt% Al on the front surface was performed at 800 °C. Electrochemical etching of the oxide samples were carried out with and without 400 W UV light illumination in a 0.1 M dilute sulphuric acid medium at 10 V potentiostatic bias for 30 min. Palladium (Pd) metal dots (3 mm diameter) were deposited on the oxide surface of the electrochemically-etched samples to fabricate a vertical sensor configuration of Pd/(porous TiO2)/Ti–Au. Similarly planar Pd/(porous TiO2)/Ti sensor structures were fabricated by depositing Pd metal dots at the center and Ti metal at the periphery of the etched oxide surface. The hydrogen sensor study with both sensor structures was performed in 500 and 1000 ppm hydrogen at different temperatures (200 to 400 °C). Varied response and sensitivity were obtained. The sensor structures involving photo electrochemically etched oxide surfaces showed appreciably high sensitivity and fast response. In fact the response time calculated for the vertical sensor configuration of Pd/(porous TiO2)/Ti–Au with UV light etched titania thin films was 5 s in 1000 ppm hydrogen at 300 °C. The effect of Pd dot diameter (2 and 3 mm) on the sensitivity and response was studied. Although Pd dot diameter had little effect on hydrogen sensitivity the response time changed. A detailed analysis of the sensor performance in terms of response, recovery, sensitivity and stability for both the vertical and planar structures has been carried out and a sensing mechanism has been proposed.