Fundamental knowledge of metal oxidation processes is an important problem for the understanding of corrosion. In particular, the oxygen reaction with copper surfaces is considered to be a model system and has consequently been extensively studied. However, to our knowledge, most of the studies have been carried out in individual samples of single orientations. In this work, the influence of steps on the oxidation of copper surfaces is studied by the use of a spherically-shaped sample with a (111) plane in its center. This sample allows the possibility to study the orientation dependence continuously up to a deviation of 10^o from the (111) plane.

The copper crystal was cleaned in a UHV preparation chamber with a base pressure < 1X 10^-10 Torr using several cycles of sputter cleaning with Ar⁺ ions of 1 KeV energy and annealing up to 600˚C. The sample, kept at room temperature, was subsequently exposed to oxygen using a leak valve with dosing values in the range 100 to 1000 Langmuir. Two-dimensional surface imaging chemical analysis was carried out at room temperature using a Theta Probe monochromated Al K_α x-ray photoelectron spectroscopy (XPS) system (Thermo Scientific) in snapshot mode with a 100 µm spot size. Data processing was performed using the software Avantage provided with the instrument. Scanning Tunneling Microscopy (STM) was also done at room temperature using a commercially available STM Variable Temperature system (Omicron).

XPS images have shown that the oxygen content in the surface increases in the areas with higher step density. STM images revealed a complex oxidation mechanism. Adsorption of oxygen leads to the formation of a surface oxide by preferential incorporation of Cu atoms from step edges. It was observed at higher dosing that the descending step edge to a large terrace results to be more faceted and jagged than a descending step edge of a smaller terrace. It can be assumed that oxygen landing on the terrace diffuses to the descending step edge and oxidizes it. In this way, larger terraces above a step edge would have more oxygen diffusing to the descending step edge, producing more faceting than in the descending steps of the smaller terraces.