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Probing gas adsorption on individual facets of a metal nanoparticle

Metal nanoparticle surfaces comprise of multiple planes with various atomic arrangements that interact with gases differently1,2. Identification of gas adsorption properties on all facets is an essential prerequisite for rational design of metal nanoparticles for catalysis, energy storage and gas sensing. Adsorbed gas molecules alter the electron density at metal surfaces3, changing the energy of the surface plasmon resonance4,5. All-optical methods using light as the excitation source can identify, in situ, gas-metal interactions in an ensemble of metal nanoparticles by measuring energy shifts of either stationary surface plasmon resonances over an entire particle, or delocalized symmetric modes on multiple regions in a particle6-8. Such methods preclude the characterization of facet-dependent gas adsorption for individual nanoparticles. Here, by using in situ electron-energy-loss spectroscopy in an environmental scanning-transmission electron microscope, we show that localized, stationary surface plasmons on individual facets of triangular crystalline Au nanoparticles in vacuum and in gaseous environments can be excited using a nanometer size electron probe. We then show that, by exploiting this localized spatial resolution, selective gas adsorption on specific sets of facets can be characterized. We anticipate that this method can be extended to quantify the concentration of adsorbed molecules, derive the binding energy for specific facets for certain gas species, and design facet-controlled nanoparticles to achieve specific gas adsorption properties.