Abstract: described is a room-temperature hydrogen sensor comprised of a TiO₂-nanotube array able to recover substantially from sensor poisoning through ultraviolet (UV) photocatalytic oxidation of the contaminating agent; in this case, various grades of motor oil. The TiO2 nanotubes comprising the sensor are a mixture of both anatase and rutile phases, having nominal dimensions of 22-nm inner diameter, 13.5-nm wall thickness, and 400-nm length, coated with a 10-nm-thick noncontinuous palladium layer. At 24°C, in response to 1000 ppm of hydrogen, the sensors show a fully reversible change in electrical resistance of approximately 175,000%. Cyclic voltammograms using a 1 N KOH electrolyte under 170 mW/cm² UV illumination show, for both a clean and an oil-contaminated sensor, anodic current densities of approximately 28 mA/cm² at 2.5 V. The open circuit oxidation potential shows a shift from 0.5 V to –0.97 V upon UV illumination.

A room-temperature titania-nanotube hydrogen sensor able to self-clean photoactively from environmental contamination, Mor, Gopal K., Carvalho Maria A., Varghese Ooman K., Pishko Michael V., and Grimes Craig A. , Journal of Materials Research, 02/2004, Volume 19, Issue 2, p.628?634, (2004)

The Science Blog reports that researchers at Penn State in the USA are developing self-cleaning titania nanotube hydrogen sensors. The hydrogen sensors are titania nanotubes coated with a discontinuous layer of palladium. Hydrogen sensors are widely used in the chemical, petroleum and semiconductor industries. They are also used as diagnostic tools to monitor certain types of bacterial infections.

"The photocatalytic properties of titania nanotubes are so large - a factor of 100 times greater than any other form of titania - that sensor contaminants are efficiently removed with exposure to ultraviolet light, so that the sensors effectively recover or retain their original hydrogen sensitivity in real world application"

"By doping the titania nanotubes with trace amounts of different metals such as tin, gold, silver, copper, niobium and others, a wide variety of chemical sensors can be made. This doping does not alter the photocatalytic properties of the titania nanotubes" says Dr. Craig A. Grimes, associate professor of Electrical Engineering and Materials Science and Engineering.1

• 1. A room-temperature titania-nanotube hydrogen sensor able to self-clean photoactively from environmental contamination,
  Mor, Gopal K., Carvalho Maria A., Varghese Ooman K., Pishko Michael V., and Grimes Craig A.
  , Journal of Materials Research, 02/2004, Volume 19, Issue 2, p.628?634, (2004)
  

In a letter to Nature E. Kim and M. H. W. Chan (Pennsylvania State University, USA) note that when liquid ⁴He is cooled below 2.176 K, it undergoes a phase transition and becomes a superfluid with zero viscosity. They claim that in addition to superflow in the liquid phase, superflow can also occur under some conditions in the solid phase of one of the helium isotopes (⁴He), and present results to back this up. In other words - evidence for a "supersolid". A supersolid behaves like a superfluid (flows without resistance) although it has crystalline solid characteristics.1

• 1. Probable observation of a supersolid helium phase,
  Kim, E., and Chan M. H. W.
  , Nature, 1/2004, Volume 427, Issue 6971, p.225 - 227, (2004)
  

Abstract: When liquid ⁴He is cooled below 2.176 K, it undergoes a phase transition—Bose–Einstein condensation—and becomes a super- fluid with zero viscosity. Once in such a state, it can flow without dissipation even through pores of atomic dimensions. Although it is intuitive to associate superflow only with the liquid phase, it has been proposed theoretically that superflow can also occur in the solid phase of ⁴He. Owing to quantum mechanical fluctuations, delocalized vacancies and defects are expected to be present in crystalline solid ⁴He, even in the limit of zero temperature. These zero-point vacancies can in principle allow the appearance of superfluidity in the solid. However, in spite of many attempts, such a 'supersolid' phase has yet to be observed in bulk solid ⁴He. Here we report torsional oscillator measurements on solid helium confined in a porous medium, a configuration that is likely to be more heavily populated with vacancies than bulk helium. We find an abrupt drop in the rotational inertia5 of the confined solid below a certain critical temperature. The most likely interpretation of the inertia drop is entry into the supersolid phase. If confirmed, our results show that all three states of matter—gas, liquid and solid—can undergo Bose–Einstein condensation.

Probable observation of a supersolid helium phase, Kim, E., and Chan M. H. W. , Nature, 1/2004, Volume 427, Issue 6971, p.225 - 227, (2004)