Direct comparison of PdAu alloy thin films and nanoparticles upon hydrogen exposure.

Nanostructured metal hydrides are able to efficiently detect hydrogen in optical sensors. In the literature, two nanostructured systems based on metal hydrides have been proposed for this purpose, each with its own detection principle: continuous sub-100 nm thin films read out via optical reflectance/transmittance changes, and nanoparticles for which the detection relies on localized surface plasmon resonance. Despite their apparent similarities, their optical and structural response to hydrogen has never been directly compared. In response, for the case of PdAu (y = 0.15-0.30) alloys, we directly compare these two systems and establish that they are distinctively different. We show that the dissimilar optical response is not caused by the different optical readout principles, but results from a fundamentally different structural response to hydrogen due to the different nanostructuring. The measurements empirically suggest that these differences cannot be fully accounted by surface effects, but that the nature of the film-substrate interaction plays an important role and affects both the hydrogen solubility and the metal-to-metal hydride transition. In a broader perspective, our results establish that the specifics of nanoconfinement dictate the structural properties of metal hydrides, which in turn control the properties of nanostructured devices including the sensing characteristics of optical hydrogen sensors and hydride-based active plasmonic systems.