Room-temperature hydrogen sensor based on palladium nanowires

Palladium (Pd) nanowires, synthesized by template-nanomanufacturing techniques, has been studied for hydrogen gas-sensing applications at room temperature. In this study, parallel arrays of Pd nanowires were fabricated by electrodeposition from an aqueous plating solution onto the surface of highly oriented pyrolytic graphite (HOPG). The nanowires were then transferred onto a polystyrene film and silver electrical contact pads were fabricated by shadow masking. The morphology of the nanowires was analyzed using atomic force microscope (AFM) in noncontact mode and the diameter of the observed nanowires was measured to be approximately 250 nm. Scanning electron microscope (SEM) images revealed that the nanowires fabricated by this procedure were parallel and continuous. Electrodes were patterned by shadow masking and the I-V characteristics of the nanowires were studied. Experimental results indicated that the sensors are highly sensitive to hydrogen, showing a two-order change in conductance. The morphology of the nanowires was analyzed using SEM and AFM in order to understand the properties responsible for the high sensitivity of the nanowires. SEM images showed that the nanowires contain nanogaps in absence of H₂. Upon exposure to H₂, the Pd absorbed hydrogen, resulting in the expansion of Pd grains. This expansion results in the closing of the nanogaps. The expansion occurred due to the phase transition from α to β and the Pd lattice expansion.