Electrocatalysis at Individual Colloidal Nanoparticles: A Quantitative Survey of Four Geometries via Electrochemical Cell Microscopy

Colloidal nanoparticles are inherently heterogeneous, exhibiting variations in size, shape, or composition that will impact their catalytic behavior. Understanding these particle-to-particle variations in catalytic behavior will be critical to realizing more stable, selective, and efficient catalyst systems, but it remains difficult to generate this understanding using conventional characterization techniques. Here, we demonstrate how targeted electrochemical cell microscopy (TECCM) can be utilized to rigorously evaluate the electrocatalytic behavior of hundreds of individual metal nanoparticle catalysts, enabling statistically meaningful insights into these systems to be generated. The electrocatalytic oxidation of hydrazine was studied in a series of Au nanoparticle systems (nanorods, nanospheres, triangular nanoprisms, and nanocubes), directly revealing particle-to-particle variations in key kinetic parameters and catalyst stability. On average, "sharper" nanoparticle geometries were found to exhibit higher initial activities but quickly degraded upon potential cycling. Interestingly, our single particle studies also reveal that while the smoother nanorod and nanosphere geometries exhibit stable behavior in an ensemble sense, this is in fact due to a complicated balance of populations which exhibit increasing or decreasing catalytic behavior over typical experimental time scales. Results from correlated optical spectroscopy and electron microscopy experiments suggest that these observed changes in catalytic behavior are not associated with significant changes in particle structure. Together, these results demonstrate the extensive heterogeneity present in common colloidal nanoparticle systems and the utility of single particle analytical techniques for studying these systems.