Observing Early-Stage Zn Oxidation Using Environmental Transmission Electron Microscopy

Zn oxidation is a fundamental process in fuel cells, aqueous Zn batteries, and catalytic applications such as methanol synthesis. Understanding the oxidation of zinc is of critical importance in developing the broad applications of zinc and zinc oxide in energy, catalysis and electronics. Although Zn oxidation has been studied for over a century, the atomic mechanisms behind early-stage oxidation remain elusive. For instance, the native Zn oxide layer on Zn (0001) has been interpreted through the Volmer–Weber (VW) mechanism. However, the early electron spectroscopy studies indicated that the initial zinc oxide film growth (&lt; 2 nm) was controlled by a different growth mechanism.<br/><br/>Environmental transmission electron microscopy (ETEM) is well known as a powerful technique for elucidating metal oxidation mechanism at the atomic scale. The capabilities of ETEM can be enhanced through the addition of a secondary electron (SE) detector which enables surface information to be obtained simultaneously with the STEM signal during gas reactions. To date, the application of SE-STEM in metal oxidation studies has been limited. Despite comprehensive conventional ETEM studies on the oxidation of metals such as Cu, Ni, and Al, the Zn system has not been studied, possibly due to the challenges in preparing an oxide-free starting Zn surface and the potential for Zn contamination inside the TEM column.<br/><br/>In this work, we investigate the initial stages of zinc oxidation using an aberration-corrected ETEM equipped with an SE detector. We first fabricate pristine, oxide-free Zn surfaces by electron irradiation in the TEM. We show movies of the decomposition of zinc oxide and the formation of the oxide-free Zn surface at elevated temperatures. These movies show sublimation-induced faceting and surface reconstruction of the Zn surface, and we discuss the mechanisms at work. Subsequently, by exposing the newly formed Zn facets to different O₂ concentrations, we observe the initial stages of Zn oxidation at various temperatures. Simultaneous SE-STEM imaging enables the evolution of sample surface morphology to be compared with the changes in the bulk during the processes of Zn surface formation, sublimation and oxidation. On higher oxygen exposure, an epitaxial oxide forms with a moiré structure arising from mismatch. We conclude that ETEM provides new insights into the Zn oxidation mechanism, and we anticipate broader applications of SE-STEM in studying the oxidation processes of other metals.