Metastable precursors during the oxidation of the Ru(0001) surface

The interaction of metals with our oxygen-rich atmosphere leads to the oxidation of the metal surfaces. Although this is common knowledge, little is known about the microscopic processes that actuate this oxide formation. Roughly speaking, the reaction sequence may be divided into the initial dissociation of O2 and O chemisorption, followed by oxide nucleation, and finally the growth of the formed oxide film. In this scheme, particularly the transition from a twodimensional on-surface O adlayer to a three-dimensional surface-oxide nucleus has hitherto barely been addressed. Based on a host of density-functional-theory ~DFT! calculations, we present an atomistic pathway for the oxide formation on the Ru~0001! surface. 1 The situation for this surface is in fact unique, as both the initial O chemisorption on the metal and the finally resulting RuO 2(110) oxide patches were already characterized experimentally on an atomic level. 2,3 Bridging this detailed knowledge of the initial and final state of the oxidation, we predict that after the completion of a full monolayer of chemisorbed O on Ru~0001!, the incorporation of O into the lattice leads to the formation of two-dimensional subsurface O islands between the first and second substrate layer. This implies that domains are formed that have a local (1 31) periodicity, and that can be described as a trilayered Oad-Ru-Osub film on top of Ru~0001!. Further O incorporation also occurs between the first and second substrate layer, saturating the underlying metal and almost completely decoupling the O-Ru-O trilayer. The ongoing oxidation results in the successive formation of more of these O-Ru-O trilayers, which at first remain in a loosely coupled stacking sequence. Once a critical film thickness is exceeded, this trilayer stack unfolds into the experimentally reported RuO2(110) rutile structure. 3