Reaction Anisotropy and Size Resolved Oxidation Kinetics of Zinc Nanocrystals

In this work, size-classified substrate-free Zn nanocrystals (NCs) are prepared and investigated for their oxidation kinetics using an in-flight tandem ion-mobility method. The first mobility characterization size selects the NCs, while the second mobility characterization measures changes in mass resulting from a controlled oxidation of the NCs. This method allows for a direct measurement of mass change of individual particles and thus enables us to explore the intrinsic reactivity of NCs while minimizing the sampling error introduced by mass and heat transfer. Two reaction regimes were observed for Zn NC oxidation. A shrinking core model is used to extract the size-dependent oxidation activation energies. We also observed a strong anisotropy effect in the oxidation process as imaged by electron microscopy. An oxidation mechanism is proposed that qualitatively explains the oxidation anisotropy and its relationship to the surface energy of the Zn NCs.