Uneven Oxidation and Surface Reconstructions on Stepped Cu(100) and Cu(110)

Abstract

How defects such as surface steps affect oxidation, especially initial oxide formation, is critical for nano-oxide applications in catalysis, electronics, and corrosion. We posit that surface reconstruction, a crucial intermediate oxidation step, can highlight initial oxide formation preferences and thus enable bridging the temporal and spatial scale gaps between atomistic simulations and experiments. We investigate the surface-stepinduced uneven surface oxidation on Cu(100) and Cu(110), using atomic-scale in situ environmental transmission electron microscopy experiments with dynamical gas control and advanced data processing. We show that the Cu(100)-O (2√2 × √2)R45°missing row reconstruction strongly favors upper terraces over lower terraces, while Cu(110)-O (2 × 1) "added row" reconstructions indicate slight preferences for upper or lower terraces, depending on oxygen concentration. The observed formation site preference and its variation with surface orientation and oxygen concentration are mechanistically explained by Ehrlich−Schwobel barrier differences for oxygen diffusion on stepped surfaces.

The Role of Grain Boundary Sites for the Oxidation of Copper Catalysts during the CO Oxidation Reaction

“…Upon extended oxygen exposure, the stable overlayer is breached by Cu 2 O islands that nucleate on the surface and grow both horizontally and vertically, which leads to subsurface oxidation. , The transition from MRR is suggested to take place after a critical oxygen exposure . Notably, however, grain boundaries, facet-edges and -corners, and step-edges can decrease the required oxygen exposure and facilitate oxide island nucleation. ,, Apart from the mentioned studies on extended surfaces, we have also reported the oxide island growth by in situ ADF-STEM on similar Cu nanoparticles as investigated here …”

“…37 Notably, however, grain boundaries, facet-edges and -corners, and step-edges can decrease the required oxygen exposure and facilitate oxide island nucleation. 16,29,38 Apart from the mentioned studies on extended surfaces, we have also reported the oxide island growth by in situ ADF-STEM on similar Cu nanoparticles as investigated here. 24 Translating the above to our experimental conditions, it becomes clear that starting from a CO-covered surface and sequentially exposing it to a CO + O 2 reaction gas mixture constitutes significantly different conditions, for which the pathway of the subsurface Cu oxidation has not been as extensively studied.…”

“…We hypothesize that this is because a higher density of surface steps and other low coordination sites accompanied by lattice strain can be found at the intersection between the grain boundaries and the particle surface. Such sites have previously been reported to enhance the oxide formation rate 29 and can act as channels for oxygen diffusion on the surface. 30 The second observation is that the oxidation has progressed deeper into the polycrystalline particles than into the single crystals and is deepest at a position of a grain boundary, where typically an oxide apex has formed (Figure 3k, S4.1).…”

In-situ Environmental TEM Observations of Structural Evolution of Oxygenated Cu(110) in CO

“…1b). 2 This (2×1) restructuring is characterized by Cu-O-Cu chains growing preferentially along the [001] direction in every other [110]-(1×1) spacing, as shown by the side and top views of the density-functional theory (DFT)-relaxed structure model in Fig. 1c.…”

Quantifying Atomic Scale Oxidation Dynamics of Cu Using In situ ETEM and Advanced Data Analysis

“…The studied surface reconstruction was found to be uneven on stepped surfaces, namely due to step induced O diffusion barrier differences. [1] The second example concerns the oxide growth process. As shown in Figure 2, epitaxial Cu 2 O islands on Cu(100) were observed to grow layer-by-layer along Cu 2 O(110) planes, instead of previously expected Cu 2 O(100) planes, using in situ ETEM.…”