First principle study of ethanol adsorption and formation of hydrogen bond on Rh(111) surface

Density functional theory (DFT) calculations are performed to study the ethanol adsorption on Rh(111) surfaces. Various adsorption modes, including monomer, dimer, and one-dimensional (1D) chain, are investigated and analyzed in details from energetic, geometrical, vibrational, and electronic points of view, which lead to valuable insights into alcohol molecules adsorption on metal surfaces. It is found that ethanol molecules prefer to adsorb at atop sites and bind to the surfaces through the oxygen atom, independent of the coverage and adsorption modes. The adsorption is exothermic, and the average adsorption energy is -0.5 eV per molecule. Adsorbed ethanol molecules are energetically favorable to agglomerate to dimer and chain by formation of the hydrogen bond. The ethanol adsorption induces significant red shift of the hydroxyl stretching vibration (nu(OH)). It is found that the red shifts are very sensitive to the coverage and adsorption modes. Depending on the nature of the H-bond, be it H-acceptor or H-donor sharing, there is a distinct pair of nu(OH) vibration, which can be seen as the fingerprint of the existence of hydroxyl-contained molecules and the formation of the H-bond. Our results show that the interaction between adsorbate and substrate and the H-bonding between adsorbed ethanol molecules can result in the overall red shift of nu(OH) to the magnitude of 769 cm(-1).