Atomic scale insights into the initial oxidation of Ru(0001) using atomic oxygen

Abstract

With scanning tunneling microscopy (STM) we studied the initial oxidation of Ru(0001) using atomic oxygen (O′) as oxidizing agent at various sample temperatures and exposures. In contrast to the oxidation of Ru(0001) by molecular oxygen, exposure of atomic oxygen is remarkably corrosive even at temperatures as low as 300K. The initial oxidation process is accompanied by the formation of small three-dimensional Ru-oxide domains (clusters) primarily formed at single steps and the formation of larger clusters in the step bunching regions. With increasing O′ exposure both the corrosion at the step edges and the size of the produced clusters progress. The corrosion of the steps leads to the formation of a mobile Ru–O precursor species preceding the actual nucleation and growth of Ru–O clusters. From this study we conclude that the activation of molecular oxygen is the true bottleneck in the initial oxidation of Ru(0001) by O2 exposure. Applying a fixed O′ exposure in the oxidation of Ru(0001), the size of clusters at single steps depends critically on the preparation temperature: For lower temperatures the clusters are small, while the cluster size increases with increasing sample temperature up to 700K. At 700K the Ru-oxide cluster growth accelerated drastically. Anneal of Ru-oxide clusters to 700K for 30min in vacuum leads to thermal decomposition of the Ru-oxide clusters so that this process may modify also the oxidation process at 700K. Flat precursor domains of RuO2 are found to grow at 600K and 700K. At higher O′ exposures (80L of O′) small RuO2(110) domains are observed on a densely with clusters covered Ru(0001) surface. At a sample temperature of 800K corrosion of the Ru(0001) surface is substantial but almost no clusters are observed with STM.