This invention relates to hydrogen sensors, and more particularly, to a high-performance flexible hydrogen sensor.
Hydrogen sensing is an important issue in a wide range of hydrogen related areas, such as industrial processing, fuel cells, hydrogen storage and hydrogen separation, etc. Traditional hydrogen sensors are usually fabricated with pure palladium (Pd) films on inorganic substrates, such as glass, quartz, silicon wafers, etc. The rigidity and/or fragility of these traditional hydrogen sensors associated with the use of rigid substrates might limit their applications in portable devices, transportation vehicles, aeronautic and civil engineering that require flexible, lightweight, and mechanic shock resistant sensing elements.
In contrast, thin polymer sheets with electrical and chemical inertness provide a useful class of substrates for fabricating hydrogen sensors. Typical designs of hydrogen sensors rely on the change in resistance of thin Pd films with continuous or discontinuous morphologies, as well as nanowires or nanotubes made of Pd upon exposure to hydrogen. These Pd micro- and nano-structures are generally not suitable for flexible design because their resistivities tend to change significantly due to the formation of cracks or change in gaps between individual Pd grains under bending deformation.
Another class of sensors depend on Schottky contacts formed between Pd films and semiconductors (e.g., GaAs, InP, Si, etc.), where the reaction between Pd and hydrogen (i.e. formation of palladium hydride) lowers the transport barrier through the contacts, thus increases current flow. These commercial field effect transistors (FET) sensors are usually fabricated on rigid semiconductor wafers.
It has recently been shown that individual single walled carbon nanotubes (SWNTs) as well as nanotube networks grown through chemical vapor deposition (CVD) had enhanced sensing capability for hydrogen when they were decorated or fabricated with palladium (Pd) via electron beam evaporation (EBE). Furthermore, solution-based SWNT films modified with palladium nanoparticles through chemical reactions and physical deposition, e.g. thermal evaporation and sputtering, can also serve as sensing elements for hydrogen detection. However, solution processed carbon nanotubes are limited by their performance in comparison to CVD carbon nanotubes. Therefore, CVD SWNTs modified with Pd nanoparticles provide a promising class of new materials for fabricating high-performance hydrogen sensing elements.
One problem is that the high temperature steps (˜900° C.) involved in the growth of SWNTs, as well as thermal annealing for removing surfactants of solution SWNTs, are generally not compatible with flexible plastic substrates that can only withstand temperatures to ˜300° C.
A dry transfer process has been developed for transferring CVD nanotubes onto plastic substrates where device fabrications can be processed at relatively low temperatures (≦150° C.). This separation of high-temperature steps and low-temperature ones enables the fabrication of flexible thin-film transistors (TFTs) as well as other classes of devices on plastic sheets with the use of CVD SWNTs.
It is, therefore, desirable to provide a strategy for generating flexible hydrogen sensor, which overcomes most, if not all of the preceding disadvantages.