In Situ Hydrogenation of Single Bimetallic Nanoparticles Visualized by Environmental Transmission Electron Microscopy

Pd-based alloys find applications in a multitude of technologies including hydrogen sensors, gas separation membranes, fuel cells, and hydrogenation catalysts. The high activity of Pd towards H₂ splitting and reversible hydrogen absorption dynamics are key to each application, and the atomic structure of the alloy material can dictate the behavior for the desired application. For example, adding small amounts of Ag to Pd decreases the hydrogen induced lattice strain built up in gas separation membranes, and higher Ag doping greatly increases the selectivity of Pd-based hydrogenation catalysts by hindering PdH formation. At the nanoscale, properties such as particle morphology, faceting, and composition can have profound impacts on the behavior of nanoparticles (NPs) but are hidden from view by traditional ensemble characterization methods.<br/>In this work, <i>in situ</i>, high resolution, environmental transmission electron microscopy was used to characterize the structural changes undergone by such AgPd NPs in gaseous hydrogen environments. Using selected area electron diffraction, we studied the thermodynamics of hydride formation in bimetallic AgPd NPs with 0.5-30% Ag composition. Interestingly, we found that the system changes from a first-order phase transition, as seen in pure Pd NPs, to a second-order phase transition with increasing Ag content. Using a direct electron detection camera, we tracked the dynamics of hydrogen induced lattice expansion within individual NPs by imaging and analyzing the lattice fringes in real time during a transition. By using Fourier transforms (FT) of the real space in-situ movies, we observed the lattice parameter change in the particle as it transitioned from a H-poor alpha phase to a H-rich beta phase. The FT can be filtered for the distinct lattice parameter to reveal the real space distribution of H within the lattice with up to 13 millisecond temporal resolution. By using HRTEM and defect contrast imaging, we developed a statistical perspective on the dynamics of this phase transformation. We discovered that phase nucleation always occurs in the low coordination site tips of these triangular plate NPs, and the progression of the phase front is in part limited by a strain relaxation process, in addition to the surface limiting processes identified in previous studies. Using molecular dynamics simulations, we supported the hypothesis that strain effects dictate the motion of phase front propagation in these systems. Additionally, we did not observe a preference for nucleation to occur at the smallest tips of these NPs, as may be expected from classical nucleation theory. By coupling a fiber optic cable through a cathodoluminescence holder, we were able to optically excite our NPs, <i>in situ</i>, and study the role of light on phase transformations. Using boundary element method optical simulations, we showed that nucleation is correlated with electric field enhancement from the local surface plasmon resonance focused on the sharpest tips of the particle. In other words, we observe a change in the nucleation behavior in dark and illuminated conditions which can be attributed to plasmon-driven effects.<br/>Such precise analysis allows us to explore the impacts of nanostructure, faceting, and composition on Pd hydride formation, giving insight into the design of optimal nanomaterials for industrially important processes. Our results also explore interesting fundamental questions about how phase transformations occur in nanoscale materials, such as the role of strain in a confined system. Finally, the exploration of light-driven processes is a significant and exciting topic in modern research. Our initial experiments demonstrate that plasmon resonances can play a role in phase transformation processes, thereby allowing for a well-controlled model system to be studies with nanometer scale resolution.