In‐Situ ESTEM Observations of Asymmetric Oxidation and Reduction in Copper Nanoparticles

Abstract

A fundamental understanding of the oxidation and corrosion mechanisms of metals is of critical importance to improving their performance in catalysis, and other industrial applications. 1 For applications in nanocatalysis a metals oxidation pathway and subsequent reduction can lead to the rearrangement of catalytically active surface facets 2 as well as deactivation through sintering and Ostwald ripening. 3 In particular we are studying copper which can readily oxidize at room temperature and has two native oxides, cuprous oxide (Cu 2 O) and cupric oxide (CuO). The oxidation of copper has been previously reported to be dependent on its crystallography 4 as well as the interaction between the copper and the substrate. 5 In this talk we will discuss the use of environmental scanning transmission electron microscopy (ESTEM) 6 to study the in‐situ oxidation of copper. Environmental STEM was carried out in a modified JEOL 2200 which allowed for the introduction of gases into the microscope and using a DENSsolutions holder to control the reaction temperature. The copper is studied in the form of nanoparticles of 2 – 50 nm in size. With high angle annular dark field (HAADF) STEM we use conditions that are ideal to track the oxidation front as it progresses across the copper nanoparticles by following the changes in Z‐contrast with time. In the case of copper, the oxidation occurred via the heterogeneous nucleation of the oxide phase (Cu 2 O) from the smallest point on the nanoparticle (Figure 1a and 1b). When the process is reversed, via reducing the particles with hydrogen, it was also observed that the reduction was initially nucleated from the smallest part of the nanoparticle and then spread across the particle. Preliminary analysis of the data suggests that once the oxidized or reduced phase is nucleated the reaction is mediated by the Cu/Cu 2 O interface.