Invited Paper NS+EN+SS-TuA7
In Situ Visualization of Intercalation-Driven Nanoparticle Phase Transitions using Plasmon-EELS

Tuesday, October 20, 2015, 4:20 pm, Room 212B

A number of energy-relevant processes rely on nanomaterial phase transitions induced by solute intercalation. However, many of these phase transitions are poorly understood, since observing them in nanomaterials – and in particular in individual nanoparticles – can be extremely challenging. This presentation will describe a novel technique to investigate intercalation-driven phase transitions in individual nanoparticles, based on in-situ environmental transmission electron microscopy (TEM) and plasmon electron energy loss spectroscopy (EELS). As a model system, this presentation will focus on the hydrogenation of palladium nanoparticles. We use the plasmon-EEL signal at varying hydrogen pressures as a proxy for hydrogen concentration in the particle. First, we investigate the hydriding properties of single-crystalline particles, free from defects and grain boundaries, and free from elastic interactions with the substrate. We obtain single particle loading and unloading isotherms for particles ranging from approximately 10 nm to 100 nm, allowing us to address outstanding questions about the nature of phase transitions and surface energy effects in zero-dimensional nanomaterials. We find that hydrogen loading and unloading isotherms of single crystals are characterized by abrupt phase transitions and macroscopic hysteresis gaps. These results suggest that thermodynamic phases do not coexist in single-crystalline nanoparticles, in striking contrast with ensemble measurements of Pd nanoparticles. Then, we extend our single-particle techniques to explore the hydriding properties of polycrystalline and multiply-twinned nanoparticles, including Pd nanorods and icosahedra. In contrast to single crystalline nanoparticles, these particles exhibit sloped isotherms and narrowed hysteretic gaps. Based on these results, we develop a model to deconvolve the effects of disorder and strain on the phase transitions in nanoscale systems. Lastly, we describe techniques to generate high-resolution plasmon-EELS (and hence phase) maps of nanoparticles. These mapping studies promise unprecedented insight into the internal phase of nanomaterials, and can be complemented with diffraction and dark-field imaging studies. We will discuss how these results could be used to interpret the thermodynamics of Li-ion insertion in battery electrodes, hydrogen absorption in state-of the-art metal hydride catalysts, or ion exchange reactions in quantum dot syntheses.