13. Nanoporous Palladium Films Based Resistive Hydrogen Sensors

Hydrogen sensing technology is significant in many circumstances, such as in the synthesis process of ammonia and methanol, leak detection during shuttle launches and fuel cells. Palladium (Pd) has been widely used in hydrogen sensors for its noteworthy ability to absorb a large quantity of H₂ and its high selective response to H₂. Pd based resistive H₂ sensors have attracted much attention due to their simple device structure and fabrication process. In this chapter, nanoporous Pd films have been demonstrated for hydrogen sensors using anodic aluminum oxide (AAO) template as substrate. Nanoporous Pd films based on AAOs were found to have a quick and reversible response due to their enhanced absorption and desorption of hydrogen compared with dense Pd films that usually have very slow response. The performance of hydrogen sensors depending on different post-deposition annealing temperatures of Pd films has been investigated. A response time as short as 30 s at 1% hydrogen concentration with an anneal temperature of 200°C has been obtained. Then, the sensing performance of hydrogen sensors based on nanoporous Pd supported by AAOs was enhanced by pore-widening treatment of AAO using phosphoric acid (H₃PO₄) as etching solution. It is demonstrated that different concentrations of H₃PO₄ and different pore-widening time lead to different pore-diameters of AAO, resulting in different performance of hydrogen sensors. The optimized hydrogen sensor shows a fast response time of 19 s at hydrogen concentration of 1% and a detection range of H₂ concentration from 0.1% to 2% by pore-widening treatment with a time of 30 min and 5% H₃PO₄ concentration. A novel carbon nanotubes and Pd nanocomposite thin films was introduced for hydrogen sensors, which exhibits very fast response speed with a response time of 8 s at 2% hydrogen gas.