Imaging the ordering of a weakly adsorbed two-dimensional condensate: ambient-pressure microscopy and spectroscopy of CO2 molecules on rutile TiO2(110)

Abstract

Disorder–Order transitions in a weakly adsorbed two-dimensional film have been identified for the first time using ambient-pressure scanning tunneling microscopy (AP-STM) and X-ray photoelectron spectroscopy (AP-XPS). As of late, great effort has been devoted to the capture, activation and conversion of carbon dioxide (CO₂), a ubiquitous greenhouse gas and by-product of many chemical processes. The high stability and non-polar nature of CO₂ leads to weak bonding with well-defined surfaces of metals and oxides. CO₂ adsorbs molecularly on the rutile TiO₂(110) surface with a low adsorption energy of ∼10 kcal mol⁻¹. In spite of this weak binding, images of AP-STM show that a substantial amount of CO₂ can reside on a TiO₂(110) surface at room temperature forming two-dimensionally ordered films. We have employed microscopic imaging under in situ conditions, soft X-ray spectroscopy and theory to decipher the unique ordering behavior seen for CO₂ on TiO₂(110).