Many energy and information storage processes, such as battery charging, hydrogen storage and memory switching, rely on phase changes of nanostructured materials in reactive environments. Compared to their bulk counterparts, nanostructured materials appear to exhibit faster charging and discharging kinetics, an extended life cycle, and size-tunable thermodynamics. However, in ensemble studies of these materials, it is often difficult to discriminate between intrinsic size-dependent properties and effects due to sample size and shape dispersity. Here, we present the first direct measurement of hydrogen absorption and desorption in individual palladium nanocrystals, with sizes ranging from 29 nm down to 13 nm. Our approach is based on in-situ electron energy-loss spectroscopy (EELS) in an environmental transmission electron microscope. By constructing single-particle pressure - energy-loss isotherms, we find that hydrogen intercalation in Pd nanocrystals is regulated by the coherent elastic coupling between the alpha and beta phases of PdHx. Such coupling results in an energy barrier that cannot be overcome by thermal fluctuations, suppressing coexistence of alpha and beta phases. Consequently, hydrogen loading and unloading isotherms are characterized by abrupt phase transitions and macroscopic hysteresis gaps. These findings are in striking contrast with conventional first-order phase transitions, and also deviate from ensemble measurements of Pd nanoparticles, which have suggested a thermally-driven nucleation of the hydride phase. Furthermore, we find that surface energy plays a critical role in determining the energetics of the hydrogen absorption process. Our results provide a general framework for monitoring phase transitions in individual nanocrystals in a reactive environment and highlight the importance of single-particle approaches to the characterization of nanostructured materials.

Visualizing metal-hydride nanoparticle phase transitions with nanometer-scale resolution

248th National Meeting (2014)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.