Surface features of TiO2nanoparticles: combination modes of adsorbed CO probe the stepping of (101) facets

Abstract

Integrated studies of CO adsorption on TiO₂ materials of different morphology and surface complexity identify, for the first time, frustrated translational CO modes by detecting their combination with the CO stretching mode (νCO). All the considered materials exhibit IR spectra with low-intensity bands in the 2235–2205 cm⁻¹ range, a region where components due to strong Lewis acid Ti⁴⁺ sites may be present as well. These observations lead to a powerful method for associating high-wavenumber bands to TiO₂ surface features and interpreting IR spectra of drastically complex/defective TiO₂ materials. The proposed band assignment is based on vibrational analyses of CO–TiO₂ theoretical models, indicating that the frustrated translational mode with frequency in the 30–50 cm⁻¹ range involved in the observed combination bands is perpendicular to the Ti⁴⁺ rows. Our results reveal that this low-energy CO mode is much more sensitive than νCO in probing the TiO₂ surface topography, and that its higher-energy components can be specifically associated with the presence of steps on the (101) faces. In a broader perspective, the frustrated translational CO mode surface sensitivity could become a key tool for detecting specific defective sites on TiO₂ surfaces.