High-density electrosorbed carbon monoxide monolayers on Pt(111) under atmospheric pressure

Abstract

We report structure studies of high-density CO monolayers on Pt(111) surfaces in equilibrium with gaseous CO near atmospheric pressure, using surface x-ray scattering (SXS) and scanning tunneling microscopy (STM). We were able to stabilize extremely well-ordered CO monolayers by emersion transfer from an electrochemical cell. We found the hexagonal close-packed $(2\ifmmode\times\else\texttimes\fi{}2)\text{\ensuremath{-}}3\mathrm{C}\mathrm{O}$ structure at room temperature in $\ensuremath{\sim}1\phantom{\rule{0.3em}{0ex}}\mathrm{atm}$ CO gas pressure. We also confirm the $(\sqrt{19}\ifmmode\times\else\texttimes\fi{}\sqrt{19})\text{\ensuremath{-}}13\mathrm{C}\mathrm{O}$ structure previously proposed in STM studies [Vestergaard et al., Phys. Rev. Lett. 88, 259601 (2002)]. Based on SXS and STM measurements, a tentative phase diagram is proposed. Detailed SXS structure studies of the $(2\ifmmode\times\else\texttimes\fi{}2)\text{\ensuremath{-}}3\mathrm{C}\mathrm{O}$ structure suggest a surface structure and substrate reconstruction less symmetric than previously studies suggested. We also find that $(\sqrt{19}\ifmmode\times\else\texttimes\fi{}\sqrt{19})\text{\ensuremath{-}}13\mathrm{C}\mathrm{O}$ structure induces substrate reconstructions including the dimerization of Pt bonded to bridging CO and the rotation of the Pt cluster with atop CO.