AVS2010 Session SS-FrM: Reactivity of Oxides, Mainly TiO2

Iron oxides are used as catalysts in a number of processes crucial to our standard of living, most notably the water-gas shift reaction and the Haber process for producing ammonia. Recent work[1-3] on ultra-thin iron oxide films suggests that their catalytic efficiency can be improved by the addition of metal nanoparticles. We have performed ARPES and LEED experiments on ultra-thin film FeOx grown on Cu(100). In agreement with our STM results, FeO structure (electronic) is observed at monolayer coverage, but Fe3O4 is observed above 2.5 ML. Moreover, Ag growth, at submonolayer coverages on both oxide phases , there is an increase in binding energy of the Ag 4d band indicating charge transfer form the iron oxide. This partial reduction of Ag may provide sites on the film surface for both oxidation and reduction of adsorbed species. In addition to electronic structure, STM atomic/morphological structure will be presented, as well as induced chemisorption changes , probed by both HREELS and TPD .

[1] L. Giordano et al., Physical Review Letters 101, 1 (2008).

[2] C. Lemire et al., Surface Science 552, 27 (2004).

[3] Y. N. Sun et al., Surface Science 603, 3099 (2009).

In this combined STM and DFT study, we examine the adsorption of Rh(CO)₂(acac) (acac is acetylacetonate) on a TiO₂(110) single crystal surface, which is aimed at developing an understanding of the relationship between the preferred adsorption sites of the supported organometallic species, and the nucleation behavior of this precursor compound on different surface terminations. The STM images of clean TiO₂(110) show large terraces with the characteristic row pattern that arises from alternating lines of bridging oxygen and uncapped titanium ions. After exposing Rh(CO)₂(acac) to the surface at room temperature the formation of small particles is observed, with diameters of 1 nm or less, and a height of 2.5 Å (i.e., monolayer height). After annealing the sample to 630˚C the density and number of the particles is significantly reduced. The particles appear to be monodispersed, with their sizes increased to several nm in lateral directions, and about 5 Å in height (i.e., bilayer height). The findings can be rationalized in terms of deligation of the parent species and agglomeration of the denuded rhodium. The details of the adsorption and deligation Process have been characterized with DFT calculations.

Combination of the statistical analysis by s canning tunneling microscopy and density functional theory calculations has been used to investigate the initial stages of molecular oxygen dissociation on the reduced TiO₂(110) surface at 300 K . Major O₂ dissociation channel results in the bridging O vacancy (V_O) healing and deposition of a single O adatom (O_a), while minor channel results in formation of O_a pair on regular Ti sites. For latter channel, an intermediate, metastable nearest-neighbor O_a-O_a configuration is observed after O₂ dissociation. This initial configuration is destabilized by Coulomb repulsion of charged O_a’s that separate further along the Ti row into energetically more favorable second-nearest neighbor configuration. The potential energy profile calculated for O₂ dissociation on Ti rows and following O_a’s separation strongly supports the experimental observations. Our results also suggest that the itinerant electrons associated with the V_O‘s are being utilized in the O₂ dissociation process at the Ti rows, whereas at least two oxygen vacancies per O₂ molecule are required in order for this process to become viable. Overall, the electrons originating from V_O’s provide a larger fraction of charge required for O₂ dissociation, while a smaller fraction can be attributed to Ti interstitials.

[1] Du, Y.; Dohnalek, Z.; Lyubinetsky, I. J. Phys. Chem. C 2008, 112, 2649.

[2] Du, Y.; Deskins, N. A.; Zhang, Z.; Dohnalek, Z.; Dupuis, M.; Lyubinetsky, I. Phys. Chem. Chem. Phys.

2010, DOI: 10.1039/C000250J.

The first scanning tunneling microscopy (STM) study of the rotational dynamics of organic species on oxides is presented. Variable-temperature STM and dispersion-corrected density functional theory (DFT-D) are used to study the alkyl chain conformational disorder and dynamics of 1-, 2-, 3- and 4-octoxy species on rutile TiO₂(110). Initially, the geminate pairs of the octoxy and bridging hydroxyl species are created via octanol dissociation on bridging-oxygen (O_b) vacancy defects. The STM images provide time averaged snapshots of octoxy species rotating among multiple energetically nearly-degenerate configurations accessible at a given temperature. The calculations show that the underlying corrugated potential energy surface is a result of the interplay between attractive van der Waals dispersion forces leading to weak attractive C^…Ti and repulsive C^…O_b interactions which lead to large barriers of 50-70kJmol^-1 for the rotation of the octoxy alkyl chains across the O_b rows. The relative populations of various conformations as well as the rotational barriers are found to be perturbed as a result of additional C^…hydroxyl repulsions when the geminate hydroxyl groups are present.

The integration of molecular systems with TiO₂ is of interest in a wide range of emerging applications. Previously we developed a photochemical reaction which highly stably and covalently binds terminal alkenes to nanocrystalline and single crystal TiO₂. Here we further develop the reaction to allow the controlled binding of short chain molecules with highly flexible, modular functionalities. Using XPS, FTIR and AFM, we demonstrate that by adding methyl substitutions around the alkene double bond, uniform grafting of the molecules can be controlled from the monolayer to the multilayer regime. Through secondary reactions, click chemistries can be used to attach photo sensitizers and biologically active molecules with the stable linker to the surface. These highly functional molecules grafted to TiO₂ facilitate the study of charge transfer and photo catalysis for wide range of applications involving organic semiconductor oxide interfaces.