The large specific surface area of nano-porous materials realizes both high catalytic activity and resource savings. Thin film materials have homogeneity in large areas and can be applied to sensor devices or electrode materials for batteries. The traditional and general method of fabricating a nano-porous noble metallic thin film is the dealloying method, i.e., the selective corrosion of an alloy of a noble metal and a base metal by a strong acid, such as hydrochloric acid or sulfuric acid. In this study, we fabricated a palladium (Pd) thin film with three-dimensional (3D) nano-network structure (3DPdNNF) by the dealloying method without discharging any material with possibly high environmental impact, such as waste solution containing heavy metals or strong acids. The dealloying was conducted in aqueous solutions of organic chelating agents of citric acid and ethylene-diamine-tetraacetic acid (EDTA) and sodium carbonate. All agents are used as a food additive and have environmentally friendly characteristics. Furthermore, the base metal used was aluminum (Al). However, Pd has a hydrogen-storage property with hydrogen gas selective permeation, and it is expected to be useful for developing sensor devices to detect only hydrogen gas. However, pure Pd metal has low repetitive durability for hydrogen gas exposure due to irreversible deformation and destruction caused by its hydrogen-storage property. In this study, we aimed to fabricate a 3DPdNNF by the above method and utilize its stress relaxing action due to the nano-network structure to apply it to a hydrogen gas sensor with repetitive durability.
The thin film preparation was performed using the RF-sputtering method, and Al–Pd alloy (82at% Al–18 at% Pd) films were deposited on substrate (glass, Si wafer, and elastic carbon film) to a thickness of 70 nm. The dealloying process was conducted in mixed aqueous solution of organic chelating acid (100 μmol/L) and sodium carbonate for pH adjustment. The optimized pH value and temperature were 10.0 and 368 K, respectively.
The Al–Pd film was reformed to uniform 3DPdNNF with high Pd purity (> 99 at% Pd). The pore size of the network could be controlled to within the range of 2.90–12.5 nm by three parameters of the gas conditions during the sputtering deposition and the pH values and chelating agents in the dealloying processes. Furthermore, we evaluated the hydrogen gas sensing performance as a 3DPdNNF utilization of a high Pd purity and large specific surface area film. The fabricated 3DPdNNF showed a response to hydrogen gas by changing the electrical resistance in the kΩcm range with repetitive durability for hydrogen gas exposure.