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In situ ETEM study of surface reconstruction formation on stepped Cu surfaces during oxidation

Surface reconstruction is the process through which the crystal structure of surface atoms changes from bulk to other structures, when there are absorbates such as gas molecules on the surface. This is an important step in surface chemistry that is critical for applications in catalysis and corrosion. During metal oxidation, surface reconstruction is commonly observed in many metals and alloys, and is considered a key step for the ensuing oxide nucleation and growth process. However, the role of surface reconstruction on oxidation remains debated due to a lack of experimental verification. Surface reconstruction is usually studied using Scanning Tunneling Microscopy. Limited by the time resolution of this technique, the dynamic processes of surface reconstruction formation and evolution remain unclear. On realistic surfaces, defects such as surface steps are omnipresent and have been found to affect interfacial atom diffusion and absorbate distribution. This could impact reconstruction or even later oxidation processes.[1,2] Although multiple surface reconstructions during oxidation have been reported, the dynamic processes forming surface reconstructions, and the effects of surface defects such as surface steps on the reconstruction process, are still unclear. In this work, we show that in situ Environmental TEM could be a powerful tool to investigate surface reconstruction dynamics during oxidation. Using atomic resolution in situ Environmental TEM with precisely controlled low O2 pressure, the dynamic formation process of surface reconstruction on stepped Cu surfaces is investigated. In situ ETEM observation was performed using a Hitachi H-9500 ETEM, with a Hitachi double-tilt heating holder and a homemade gas delivery system with three gas injection lines. The gas injection lines are connected to the pumping system of the ETEM, so that the residual gas in the pipeline can be quickly removed and gas exchange can be finished within minutes. Single crystalline Cu(100) thin films with nominal thicknesses of 60 nm were annealed