High‐Performance, Flexible Hydrogen Sensors That Use Carbon Nanotubes Decorated with Palladium Nanoparticles

Hydrogen sensing represents an important issue in a wide range of hydrogen related areas such as industrial processing, fuel cells, hydrogen storage and separation, etc. In the era to exploit “hydrogen economy”, advances in hydrogen sensors become critical to safety for economy, environment and society as well. Traditional hydrogen sensors are fabricated, in general, on inorganic substrates (e.g., glass, quartz, silicon wafers, etc.). The rigidity (and/or fragility) of these sensors associated with the use of rigid substrates might limit their application in various new areas (e.g., portable devices, aerospace science and civil engineering) that require flexible, lightweight, and mechanical shock-resistive sensing elements. In contrast, thin polymer sheets with electrical and chemical inertness provide an ideal class of substrates for fabricating hydrogen sensors, which can be used in applications complementary to those of the conventional sensors. For example, hydrogen-fueled space shuttles can be conformally covered with large-area flexible hydrogen sensor arrays on plastic sheets to reduce their overall weights and to detect any leakage of hydrogen prior to diffusion. Typical designs of hydrogen sensors rely on the change in resistance of thin Pd films with continuous or discontinuous morphologies upon exposure to hydrogen. Nanowires and nanotubes of Pd have also been demonstrated the capability to probe hydrogen molecules. These Pd microand nano-structures are not suitable for flexible design because their resistivities tend to change significantly due to the formation of cracks (and/or change of 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. Also, the commercial field effect transistor (FET) sensors are usually fabricated on rigid semiconductor wafers. Recently, Dai et al. demonstrated individual single-walled carbon nanotube (SWNT) as well as nanotube networks grown through chemical vapor deposition (CVD) had enhanced sensing capability for hydrogen when the SWNTs were decorated with Pd nanoparticles via electron beam evaporation. In addition, solution-based SWNT films modified with Pd nanoparticles through chemical reactions and physical depositions (e.g., thermal evaporation and sputtering) can also serve as sensing elements for hydrogen detection. However, the solution processed carbon nanotubes are limited by their performance comparing with the CVD carbon nanotubes. Therefore, CVD SWNTs modified with Pd nanoparticles provide a promising class of new material for fabricating highperformance hydrogen sensing devices. One more hurdle to overcome is the high temperature step (∼ 900 °C) involved in the growth of SWNTs, which is not compatible with plastic substrates that can only withstand modest temperatures (< 300 °C). Most recently, Rogers and co-workers develop a “dry transfer printing” process for transferring CVD nanotubes onto plastic substrates where device fabrications can be processed at relatively low temperatures (≤ 100 °C). The separation of high-temperature steps and low-temperature ones enables the fabrication of flexible thin-film transistors (TFTs) on plastic sheets with the use of CVD SWNTs. Herein, we combined the “dry transfer printing” technique and modification of SWNTs with Pd nanoparticles to prepare high-performance hydrogen sensors with excellent mechanical flexibility on plastic substrates. To our knowledge, there was no report on flexible hydrogen sensing devices. The asfabricated sensors work efficiently at room temperature and exhibit sensing performance comparable to (or even better than) the traditional sensors on rigid substrates. For example, the sensitivity of the typical flexible sensors was in the range of 50 ∼ 100 % for 0.1 % hydrogen in air at room temperature. The response times were typically around 1.5 s for 1 % C O M M U N IC A TI O N