STM study of high-coverage structures of atomic oxygen on Pt(1 1 1): p(2 × 1) and Pt oxide chain structures

We used scanning tunneling microscopy (STM) to investigate high-coverage structures of atomic oxygen that develop on Pt(1 1 1) under ultrahigh vacuum (UHV) conditions. We find that oxygen atoms arrange into a p(2 × 1) structure that grows in three rotationally degenerate domains as the oxygen coverage increases above 0.25 ML (monolayers). STM also shows that atomic-scale protrusions and chains, with apparent heights of 1.7 Å, begin to develop within the p(2 × 1) domains once the coverage reaches about 0.40 ML at 450 K. We attribute these features to a Pt oxide chain compound that forms as oxygen atoms adsorb between the close-packed oxygen rows of the surrounding p(2 × 1) structure. As the coverage increases to 0.75 ML, the chains form an interconnected network of Y-shaped structures with regions locally resembling a honeycomb. Each branch of the Y-structure consists of two to three side-by-side Pt oxide chains about 19-24 Å in length. We suggest that uniaxial strain causes the chains to select specific lengths that are commensurate along the close-packed directions of the Pt(1 1 1) substrate, and that stress relief governs the chain branching and formation of the interconnected network. These results demonstrate that Pt oxide chain formation occurs during the early stages of Pt(1 1 1) oxidation, and that long-range effects determine the structural characteristics of the resulting chain network. These findings may have important implications for understanding the oxidation and reactivity of Pt surfaces under oxidizing conditions.