(544ai) In Situ observation of Cu2o Island Reductive Shrinking on Cu(100) Facet Under Methanol Using Environmental Transmission Electron Microscopy

Authors: 
Chi, H., University of Pittsburgh
Li, M., University of Pittsburgh
Veser, G., University of Pittsburgh
Curnan, M., Pohang University of Science and Technology (POSTECH)
Andolina, C. M., University of Pittsburgh
Yang, J. C., University of Pittsburgh
In situ environmental transmission electron microscopy (ETEM) has progressed dramatically in recent years, currently allowing for temperature, time, and pressure resolved imaging of oxidation at the atomic scale. Recent in situ studies have provided a significant understanding of atomic scale Cu oxidation, especially during early stage oxidation. In contrast, the reduction of metal oxides, such as Cu2O, has been studied much less extensively. However, such reductions play an important role in many technologies such as catalytic reaction design, environmental stability, and electrochemistry.

In this work, we investigate the structural dynamics of Cu2O islands on Cu(100) under MeOH in an ETEM (Hitachi H9500) equipped with a differential pumping system, as well as a specially designed double-tilt heating holder and gas injection system. Cu(100) thin films with a thickness of 600 Å were produced by e-beam evaporation of 99.999% pure Cu pellets on NaCl(100) substrates. Cu2O nano-islands on Cu facets were created via controlled in situ oxidation of copper films, followed by reduction via MeOH vapor. ETEM images and videos were evaluated using Fiji-ImageJ software (NIH). We observed a uniform two-stage shrinking behavior from anisotropic shrinking, in which island radius shrinks much faster than height, to isotropic shrinking, in which radius and height share similar shrinking rates, for a total of 7 measured Cu2O islands. Moreover, we found that, regardless of initial island radius, isotropic shrinking starts when the radius is equal or below 2 nm. We propose that this behavior is caused by a preferential reaction between MeOH and stepped surfaces on the sides of the Cu2O islands (e.g. Cu(100) with atomic steps), which show substantially higher reactivity than island top terraces (Cu(100)). We are calculating MeOH adsorption energies and diffusion barriers for Cu(100) atomic steps and Cu(100) surfaces using DFT to further corroborate this hypothesis and explain the differences in reactivity between these surfaces. The combination of in situ observations and theoretical calculations in this work will hence serve to reveal insights into methanol reactivity on different crystallographic facets of Cu2O surfaces. More broadly, this work aims to contribute to the understanding and manipulation of supported metal oxide systems, including those that are widely used in heterogeneous catalysis.