All-optical hydrogen-sensing materials based on tailored palladium alloy thin films

Optical reflectance measurements were performed to determine the hydrogen response characteristics of 20-nm-thick Pd-Au (Ag) films. The response characteristics displayed a strong dependence on α, mixed α/β, and β Pd-hydride phases formed in the films. The response time peaks in the α → β phase transition region (1625 s at 0.4% H₂ for Pd_0.94Ag_0.06 and 405 s at 1% H₂ for Pd _0.94-Au_0.06), consistent with critical slowing down phenomena. The α → β phase transition region was shifted and inhibited by changing the alloy element to Au and increasing its corresponding content to 40 atom %, respectively. Initial hydrogen uptake rate measurements determined that, due to the adsorption of ambient background gases, the rate-limiting step for α or β phase PdH formation is dissociative chemisorption of hydrogen for each palladium alloy film. By tuning the alloy content and composition of the palladium films, the surface properties of the film become more receptive toward the rapid detection of hydrogen and a novel hydrogen-sensing material using Pd alloyed with 40 atom % Au is presented.