Complex behavior of hydrogen sensor using nanoporous palladium film prepared by evaporation

Palladium is widely used as a hydrogen-sensing material owing to its high affinity for H-2 adsorption at room temperature. In order to improve the gas-sensing performance, three-dimensional (3D) nanoporous Pd films with interconnected network structure were prepared by thermal evaporation at relatively high process pressures. The morphology, particle size, and crystallinity of the Pd films were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD). Nanoporous Pd films with open pores and porosity greater than 96.4% were successfully fabricated. All Pd films were composed of metallic Pd and Pd oxide. The gas-sensing properties were evaluated by the change in resistance from 0.05 to 2% H-2. When exposed to H-2, the Pd films exhibited complex behavior, with a successive decrease and increase in resistance. This behavior was attributed to three effects - scattering, geometry, and reduction-oxidation. Nanoporous Pd films with a particle size of similar to 20 nm showed high sensitivity and fast reaction. The results demonstrated that 3D nanoporous Pd films with appropriate particle size were a promising candidate for a H-2 sensor.