A computational study of the interaction of oxygenates with the surface of rutile TiO2(110). Structural and electronic trends

A variety of OH containing molecules in their different modes of adsorption onto the rutile TiO₂(110) are studied by means of density functional theory. A special focus is given to ethanol, ethylene glycol and glycerol. The different species were analyzed with respect to the adsorption energy, work function, and atomic Bader charges. Our results show that dissociated adsorption is favored in all cases. Within these modes, the strongest binding is observed in the case of bidentate fully dissociated adsorption, followed by bidentate partially dissociated then the monodentate dissociated modes. The dependence is also noted upon charge transfer analysis. Species adsorbing with two dissociated OH groups show a negative charge which is roughly twice as large compared to those exhibiting only one dissociated group. In the case of molecular adsorption, we find a small positive charge on the adsorbate. The change in work functions obtained is found to be negative in all studied cases. We observe a trend of the work function change being more negative for glycerol (3 OH groups) followed by ethylene glycol (2 OH groups) and the remaining alcohols (1 OH group), thus indicating that the number of OH groups present is an important factor in regards to work function changes. For the complete series of adsorbates studied (methanol, ethanol, isopropanol, ethylene glycol, glycerol, hydrogen peroxide and formic acid) there is a linear relationship between the change in the work function and the adsorption energy for the molecular adsorption mode. The relationship is less pronounced for the dissociated adsorption mode for the same series.

CO adsorption on the calcite(10.4) surface: a combined experimental and theoretical study

Detailed information on structural, chemical, and physical properties of natural cleaved (10.4) calcite surfaces was obtained by a combined atomic force microscopy (AFM) and infrared (IR) study using CO as a probe molecule under ultrahigh vacuum (UHV) conditions. The structural quality of the surfaces was determined using non-contact AFM (NC-AFM), which also allowed assigning the adsorption site of CO molecules. Vibrational frequencies of adsorbed CO species were determined by polarization-resolved infrared reflection absorption spectroscopy (IRRAS). At low exposures, adsorption of CO on the freshly cleaved (10.4) calcite surface at a temperature of 62 K led to the occurrence of a single C-O vibrational band located at 2175.8 cm^-1, blue-shifted with respect to the gas phase value. For larger exposures, a slight, coverage-induced redshift was observed, leading to a frequency of 2173.4 cm^-1 for a full monolayer. The width of the vibrational bands is extremely small, providing strong evidence that the cleaved calcite surface is well-defined with only one CO adsorption site. A quantitative analysis of the IRRA spectra recorded at different surface temperatures revealed a CO binding energy of -0.31 eV. NC-AFM data acquired at 5 K for sub-monolayer CO coverage reveal single molecules imaged as depressions at the position of the protruding surface features, in agreement with the IRRAS results. Since there are no previous experimental data of this type, the interpretation of the results was aided by employing density functional theory calculations to determine adsorption geometries, binding energies, and vibrational frequencies of carbon monoxide on the (10.4) calcite surface. It was found that the preferred geometry of CO on this surface is adsorption on top of calcium in a slightly tilted orientation. With increased coverage, the binding energy shows a small decrease, revealing the presence of repulsive adsorbate-adsorbate interactions.

Unraveling the effect of Al doping on CO adsorption at ZnO(101̄0)

Al doping enhances the adsorption of CO on ZnO(101̄0) by facilitating π-back donation from the surface to CO.

Interplay between Adsorbates and Polarons: CO on Rutile TiO_{2}(110)

Polaron formation plays a major role in determining the structural, electrical, and chemical properties of ionic crystals. Using a combination of first-principles calculations, scanning tunneling microscopy, and atomic force microscopy, we analyze the interaction of polarons with CO molecules adsorbed on the reduced rutile TiO_{2}(110) surface. Adsorbed CO shows attractive coupling with polarons in the surface layer, and repulsive interaction with polarons in the subsurface layer. As a result, CO adsorption depends on the reduction state of the sample. For slightly reduced surfaces, many adsorption configurations with comparable adsorption energies exist and polarons reside in the subsurface layer. At strongly reduced surfaces, two adsorption configurations dominate: either inside an oxygen vacancy, or at surface Ti_{5c} sites, coupled with a surface polaron. Similar conclusions are predicted for TiO_{2}(110) surfaces containing near-surface Ti interstitials. These results show that polarons are of primary importance for understanding the performance of polar semiconductors and transition metal oxides in catalysis and energy-related applications.