Speaker
Description
Metal nanoparticles in reactive atmospheres are not static. Redox cycling reshapes their surface and generates transient oxide species, affecting catalytic performance. Field emission microscopy (FEM) is applied to Co and Cu nanotips to follow this interplay at nm and ms resolution which enables feature-resolved observation of oxidation and reduction during hydrogen oxidation at 10⁻⁵ mbar.
On the annealed Co tip, oscillations were resolved, with an FEM signature of periodic splitting of single bright spots into symmetric triplets. We relate this nano-scale feedback process to the growth and collapse of (111)-truncated edges of Co cubes. Ex-situ scanning electron microscopy (SEM) confirmed that the Co apex had transformed into cubic oxide grains. In previous environmental SEM measurements of the same system dynamics, oxide related volume increase initiated porous networks and regulated pore diameter. However, the mechanistic origin of dynamics expected within pores could not be investigated. Correlating in-situ/operando microscopy insights by shared metrics of dynamic behavior allows to bridge mechanistic understanding over length scales. On Cu tips, for similar experiments, a complex self-organization was resolved. Spatiotemporal analysis identified classes of pattern dynamics and their interplay, including periodic explosion-like events ending bright spots that occurred at high field strengths. Ex-situ SEM imaged a canyon-and-plateau topography consistent with combined oxide growth and H-assisted field evaporation of its sharp edges.
Overall, we demonstrate how cyclic redox dynamics transiently modifies nanoparticle morphologies and local reaction. In this way, employing self-sustained oscillations as a probe, controlled oxidation and reduction processes are accessed at the nanoscale with high temporal resolution within a single experiment.