Phase Transitions of Cobalt Oxide Bilayers on Au(111) and Pt(111): The Role of Edge Sites and Substrate Interactions

Well-characterized metal oxides supported on single crystal surfaces serve as valuable model systems to study fundamental chemical properties and reaction mechanisms in heterogeneous catalysis or as new thin film metal oxide catalysts in their own right. Here, we present scanning tunneling microscopy and X-ray photoelectron spectroscopy results for cobalt oxide nanoislands that reveal the detailed atomistic mechanisms leading to transitions between Co-O bilayer and O-Co-O trilayer, induced by oxidation in O2 and reductive vacuum annealing treatments, respectively. By comparing between two different noble metal substrates, Au(111) and Pt(111), we further address the influence of the substrate. Overall, nanoisland edges act to initiate both the oxidation and reduction processes on both substrates. However, important influences of the choice of substrate were found, as the progress of oxidation includes intermediate steps on Au(111) not observed on Pt(111), where the oxidation on the other hand takes place at a significantly higher rate. During reductive treatment of trilayer, the bilayer structure gradually reappears on Pt(111), but not on Au(111) where the reduction rather results in the appearance of a stacked cobalt oxide morphology. These observations point to strong differences in the catalytic behavior between Au and Pt supported cobalt oxides, despite the otherwise strong structural similarities.