This content is excerpted from Sensor Technology Alert and Newsletter, a sensor intelligence service published by the Technical Insights unit of Frost & Sullivan.
Hydrogen (H2) has numerous applications--petroleum distillation and chemical production are some examples of its use in the industrial sector. Its wide range of applications has instigated researchers to develop highly sensitive, stable, specific, robust, selective and affordable hydrogen sensors that enable its safe and accurate use. Various types of sensor technologies, such as catalytic, electrochemical, resistive semiconductor, Schottky junction, fiberoptic, and combinations of these have been developed and utilized. Metal oxide semiconductor technology can offer lower cost. However, the historical need for operating these materials at elevated temperatures to improve sensitivity can be a limiting factor with respect to their application spectrum.
Researchers at the Pennsylvania State University, department of electrical engineering, and materials science and engineering in collaboration with researchers at SentechBiomed Corporation have developed highly ordered titania nanotube array hydrogen sensors on a titanium (Ti) substrate. Craig Grimes and his research team suggest the use of a highly ordered, nanotubular structure that has both size-dependent and surface area-related properties. This could further improve gas sensing, photocatalytic, and photoelectrochemical properties. Additionally the research team has developed and refined anodization processes for the fabrication of highly ordered nanotube arrays from Ti metal foils. Despite its remarkable hydrogen sensitivities, the fabrication of the nanotube arrays from the Ti foil may have limited use at high temperatures because the metal electrode on top of the nanotube array (for electrical contact) may cause a short circuit on diffusion into the array. In addition, these devices may also be exposed to mechanical shock or vibrations. To overcome these difficulties, the researchers have worked on the fabrication of the highly ordered titania nanotube arrays from Ti thin films.
For this the team has developed an anodization-based process to grow the nanotube arrays from Ti thin films on a variety of substrates, including glass, silicon, and alumina. The nanotube array showed remarkable hydrogen sensitivity. At room temperature in response to 1000 parts per million (ppm) hydrogen, the researchers found a change in electrical resistance of 100 billion%. The sensors exhibited a four-order magnitude drop in resistance and negligible sensitivity to other reducing gases such as methane, carbon monoxide, and ammonia. The modification of the fabrication technique helped to make the sensors transparent. In addition, the sensors were found to be mechanically and electrically stable over a tested temperature range of 25 degrees C to 250 degrees C.
The sensor comprising of a 22 nm inner-diameter titanium oxide (TiO2) nanotube array was observed to provide a resistance change in the order of 104; in response to 1000 ppm H2 at 290 degrees C. It has now been observed that these nanotube arrays, having an inner-diameter of approximately 22 nm, tube length ranging from 200 nm to 6 micrometers, when coated with a thin layer (approximately 10 nm) of palladium exhibit ultrahigh sensitivity to hydrogen at room temperature (typically approximately 23 degrees C). "Realizing the potential of these nanostructured materials, we have concentrated our efforts toward the development of {a}room, or low, temperature hydrogen sensor based on titania nanotube arrays. The sensitivity demonstrated by the highly ordered nanotube array of hydrogen sensors is unprecedented. It is a very important step for gas sensors in general. Specifically, the hydrogen sensitivity we show would enable part per trillion monitoring. Such sensitivity would be quite useful in biomedical applications, enabling hydrogen economy, and in oil refining where hydrogen is used frequently as a diagnostic tool," Grimes told Sensor Technology.
All the research work as of now has been done within the group, and the team has a patent pending with regard to this work. In addition, the National Science Foundation and the National Institutes of Health have provided partial support for this work.