A Self-Cleaning, Room-Temperature Titania-Nanotube Hydrogen Gas Sensor

Chemical sensors are of critical importance for industrial process control, medical diagnosis, and helping to ensure a safe environment. Although rarely considered in the literature (D. E. Williams et al., J. Chem. Soc., Faraday Trans. 91, 3307 (1995); D. E. Williams and K. F. E. Pratt, J. Chem. Soc., Faraday Trans. 91, 1961 (1995)), a fundamental problem with chemical sensor use is that the sensors become contaminated, or poisoned, limiting their useful lifetime; typically the more sensitive the sensor, the more susceptible it is to contamination. In addition to the gas mixtures of interest, real-world operation results in sensors being exposed to various organic vapors, carbon smoke and soot, and volatile organic compounds that will contaminate a sensor surface, degrading its sensitivity, result in spurious measurements, and limit the useful sensor lifetime. Our dual motivation for the present work is to demonstrate, to our knowledge for the first time, a sensor able to self-clean and recover from environmental insult. The titania-nanotube room-temperature hydrogen gas sensor is able to self-clean with exposure to UV light, fully recovering its initial properties after being contaminated by either motor oil and/or stearic acid. The sensor response to hydrogen is fully reversible, with a change in electrical resistance of 3 orders of magnitude upon exposure to 1000 ppm hydrogen at 25 °C. The self-cleaning properties are demonstrated by observing the sensor base resistance, and the resistance change upon exposure to a 1000 ppm hydrogen atmosphere, before and after contamination with exposure of the sensor to a 270 mW/cm² UV light of mixed (365 nm, 254 nm) wavelength. The sensor comprises an array of 22-nm inner-diameter titania nanotubes 200 nm in length, fabricated by anodizing a Ti foil, subsequently annealed at 500 °C for 6 h in an oxygen atmosphere, and then coated with a 12-nm palladium layer.