Titania nanotube arrays promise self-cleaning, hydrogen-sensing ability


By Betsy Ziobron

UNIVERSITY PARK, Pa.—Scientists at Penn State University have demonstrated the hydrogen-sensing and self-cleaning capabilities of cost-effective titania nanotube arrays—a technology that could provide a two-fold benefit for cleanrooms and have a profound affect on both hydrogen sensing and contamination control.

According to researchers, titania nanotubes, which are made by an anodization technique, are 1,500 times better than the next best material for sensing hydrogen.

Hydrogen sensors are utilized in several industries, from quality control of food and identifying biochemical warfare to monitoring pollution and bacterial infections. In the semiconductor and lighting industries, where the production of gases is commonplace, these sensors have helped ensure safe levels of hydrogen ranging from 0.1 parts per million (ppm) to 100 ppm.

Images of the titania nanotube array prepared using an anodization potential of 10 volts (cross-sectional view).
Click here to enlarge image

"The sensitivity [of titania nanotubes] comes from the nano-architecture, not the surface area," says Dr. Craig Grimes, Penn State associate professor of electrical engineering, and materials science and engineering. "Hydrogen molecules get dissociated at the titania surface, diffusing into the titania lattice and acting as electron donors." Grimes says this means sensors are able to detect hydrogen levels in parts per billion.

When Penn State researchers investigated the photocatalytic oxidation of contaminants on the titania nanotubes, they found contaminants to be self-cleanable through exposure to ambient light.

"We're already seeing self-cleaning windows, predominantly in Japan, where the photocatalytic properties of titania keep the windows clean," says Grimes.

According to the researchers, the transparent titania nanotubes have photocatalytic activities 100 times greater than any other form of titania. Contaminants that contain salts, however, degrade the nanotubes' photocatalytic properties.

"The opportunity exists to create wall coatings that, with exposure to ambient light, highly efficiently destroy any contaminate that lands on the wall," says Grimes. "That's great news for any environment where contamination is of concern."

For HVAC and hoods, too?

There is no reason, Grimes says, why titanium nanotubes could not be applied to all mechanically-rigid surfaces in a cleanroom environment to create an air-scrubbing surface: "It would be difficult to apply them to bunny suits or other flexible surfaces; however, they could be applied to glass, metal, Teflon, etc." The titania nanotubes could even be applied to the surfaces of HVAC systems and hoods to help decrease harmful contaminants.

The research team says the titania nanotubes' self-cleaning properties let hydrogen sensors recover their hydrogen sensitivity when coated with contaminants. In tests, researchers coated sensors with a layer of motor oil several tens of microns thick that extinguished the hydrogen sensitivity. After just one hour of exposure to ultraviolet light, the sensors had recovered a large portion of their sensitivity, and after 10 hours, they almost fully regained their hydrogen sensitivity.

"In many locations, hydrogen sensors can become contaminated by a variety of substances, and the self-cleaning function of titania nanotubes extends sensor lifetime and minimizes sensor errors to increase safety," says Grimes.

Titania nanotubes can be made by the mile rather inexpensively, and the material is not used up when sensing hydrogen. Once the gas clears, the nanotubes can be used again.

Over the next couple of years, as titania nanotube technology becomes available for real-world technology, improved hydrogen sensing and contamination control in many cleanrooms across the nation may be seen.

In addition to Grimes, the Penn State researchers include: Gopal K. Mor and Oomman K. Varghese, postdoctoral fellows; Michael V. Pishko, associate professor of chemical engineering, and Maria A. Carvalho, graduate student in chemical engineering. They reported their results in recent issues of the Journal of Materials Research and Sensor Letters.

BETSY ZIOBRON is a Westbrook, Conn.-based freelance technical writer covering a variety of high-tech industries.