Apr 2, 2004
Titania nanotube sensors clean themselves
Researchers at Pennsylvania State University, US, have developed titania nanotube hydrogen sensors that are self-cleaning. The sensors removed coatings of motor oil and stearic acid on exposure to ultraviolet light by a process of photocatalytic oxidation.
Titania nanotubes have photocatalytic properties around 100 times greater than any other form of titania. “Their photocatalytic properties are so large that the material can effectively degrade any contaminate (so long as it does not contain salt, which destroys the photocatalytic properties),” Craig Grimes told nanotechweb.org. “It’s a dirty world out there and one fundamental truth is that sensors in the real world get dirty with use and have to be replaced, which is usually a costly proposition.”
Sensor contaminants can include volatile organic vapours, carbon soot, oil vapours, dust and pollen. In this study the researchers used titania nanotubes that were 200 nm long, had an inner diameter of 22 nm, 13nm thick walls and a 12 nm thick coating of palladium. The palladium layer helps break hydrogen molecules into atomic form and, since it is non-continuous, still enables hydrogen to diffuse into the nanotubes.
Grimes and colleagues exposed the sensors to 1000 ppm of hydrogen at room temperature, which caused a 175,000% change in resistance. Then they coated the devices with a layer of motor oil several tens of microns thick, an action that completely removed the sensors’ sensitivity to hydrogen.
But after exposure to ultraviolet light in air for one hour, the sensors recovered a large proportion of their sensitivity. Following 10 hours in ultraviolet light they were almost back to their pre-contamination levels of performance.
“The recovered sensor has a 1000 part per million hydrogen normalized resistance value of approximately 0.0005%, compared with the 0.0006% value of the sensor prior to contamination,” added Grimes.
That said, the sensors did not recover from contaminants such as the spray-on oil WD-40, which contains salts that degrade the nanotubes’ photocatalytic properties.
As well as their application as high-performance, self-cleaning hydrogen sensors, the devices can also be tailored to detect other chemicals by doping the titania nanotubes with metals such as tin, gold, silver, copper or niobium. The material could also find use in highly efficient self-cleaning photocatalytic surfaces.
“We are making a big push now with respect to the photochemical properties of the material,” said Grimes. “If you could find a way to efficiently split water into hydrogen and oxygen using sunlight, you could make a hydrogen economy a reality rather than a distant dream - with the consequent benefits for the planet.”
The researchers recently reported their work in the Journal of Materials Research and Sensor Letters.
About the author
Liz Kalaugher is editor of nanotechweb.org.