Observations of Shape-Dependent Hydrogen Uptake Trajectories from Single Nanocrystals

In this work, H-2 absorption and desorption in faceted, crystalline Au/Pd core/shell nanocrystals and their interaction with a SiOx/Si support were studied at the single-particle level. Dark-field microscopy was used to monitor the changing optical properties of these Au/Pd nanoparticles (NPs) upon exposure to H-2 as reversible H-2 uptake from the Pd shell proceeded. Analysis of the heterogeneous ensemble of NPs revealed the H-2 uptake trajectory of each nanocrystal to be shape-dependent. Differences in particle uptake trajectories were observed for individual particles with different shapes, faceting, and Pd shell thickness. In addition to palladium hydride formation, the single-particle trajectories were able to decipher specific instances where palladium silicide formation and Au/Pd interdiffusion occurred and helped us determine that this was more frequently seen in those particles within an ensemble having thicker Pd shells. This noninvasive, plasmonic-based direct sensing technique shows the importance of single-particle experiments in catalytically active systems and provides a foundation for studying more complex catalytic processes in inhomogeneous NP systems.