The impact of heterogeneous surface oxide formation on the electrochemical performance of single silver nanoparticles is explored using in situ superlocalization optical microscopy. Silver nanoparticles are well-known to form a natural oxide layer on their surface, but the effect of this oxide layer on electrochemical reactions is not well understood. Here we track the temporal and spatial dependence of electrodissolution of single silver nanoparticles in order to study the role of surface oxide layers on electrochemical reactions. Heterogeneity in electrodissolution kinetics is observed by following the time-dependent loss in scattering intensity from individual silver nanoparticles using dark-field scattering. Both fast and slow dissolution kinetics are observed, with the dominant pathway changing as a function of applied potential. To understand this, superlocalization imaging is employed to follow the spatial variance of the electrodissolution process and reveals that the silver nanoparticles undergo electrodissolution in either a spatially symmetric or asymmetric manner. We hypothesize that asymmetric dissolution events are due to heterogeneity in the exposed silver sites on the nanoparticle surface because of incomplete surface oxide formation. Polarization-resolved measurements support this hypothesis by revealing anisotropic dissolution of the nanoparticles over time. By tracking the electrodissolution behavior of silver nanoparticles in both the temporal and spatial domains, we provide an improved understanding of how heterogeneity in electrochemical reactions is impacted by nanoparticle surface properties.

Studying multiple simultaneous electrochemical reactions using typical electrochemical methods is challenging, because the measured current is a convolution of concurrent electrochemical reactions. Thus, to monitor multiple simultaneous electrochemical reactions, secondary techniques, such as imaging or spectroscopy are increasingly useful. Herein we use dark-field optical microscopy to visualize the electrodeposition of silver oxide (Ag x O y ) particles using the Ag + ions generated by the concurrent electrodissolution of individual Ag nanoparticles at high anodic potential. We propose that the formation of Ag x O y particles is based on an aggregative growth mechanism, where electrodeposited Ag x O y nanoclusters aggregate over time to form a larger Ag x O y particle. The electrodeposited Ag x O y particles catalyze water oxidation and decrease the local pH, which alters the reaction equilibrium by hindering continued growth of the Ag x O y particles at 1.2 V and consuming the Ag x O y particles and producing Ag + ions at open circuit. Overall the understanding obtained by imaging these reactions is not possible to decode using the measured ensemble current.

Localization-based super-resolution imaging techniques have improved the spatial resolution of optical microscopy well below the diffraction limit, yet encoding additional information into super-resolved images, such as anisotropy and orientation, remains a challenge. Here we introduce calcite-assisted localization and kinetics (CLocK) microscopy, a multiparameter super-resolution imaging technique easily integrated into any existing optical microscope setup at low cost and with straightforward analysis. By placing a rotating calcite crystal in the infinity space of an optical microscope, CLocK microscopy provides immediate polarization and orientation information while maintaining the ability to localize an emitter/scatterer with <10 nm resolution. Further, kinetic information an order of magnitude shorter than the integration time of the camera is encoded in the unique point spread function of a CLocK image, allowing for new mechanistic insight into dynamic processes such as single-nanoparticle dissolution and single-molecule surfaceenhanced Raman scattering.