A DFT+U study of acetylene selective hydrogenation on oxygen defective anatase (101) and rutile (110) TiO2 supported Pd4 clusterWe have studied the stoichiometric (annealed in oxygen), the slightly oxygen-deficient (annealed in vacuum), and the highly defective (sputtered with Ar + ) Ti0 2 ( Ito) surfaces and their reactivities to molecular oxygen, molecular water, and 50-eV hydrogen ions using x-ray photoelectron spectroscopy (XPS) and low-energy ion scattering spectroscopy (LEIS). The use of isotopically labeled I KO enables us to distinguish adsorbed oxygen from lattice oxygen, and the concentration of surface oxygen vacancies is titrated by 18 0 2 adsorption. LEIS (l-keV He' ) is used to analyze the chemical composition of the outermost surface layer before and after 18 0 2 and H 2 1K O exposure. Water adsorbs on both stoichiometric and slightly O-deficient surfaces [with oxygeil vacancies -0 and 0.08 monolayer (ML), respectively] at room temperature. There is little or no dependence of saturation water coverage (lower limit of -0.07 ML for both surfaces) on the concentration of surface oxygen vacancies. On the highly defective surfaces, the saturation water coverage increases to a lower limit of 0.1 5 ML and the saturation 18 0 coverage increases to 0.4 ML. The interaction of hydrogen with the stoichiometric surface creates defect states that can be observed by XPS and by subsequent adsorption of 18 0.

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Copper overlayers with thicknesses up to several tens of angstroms have been vapor deposited at various substrate temperatures onto rutile TiO2(110) surfaces that have different defect concentrations. The metal films have been studied by means of He+ low-energy ion scattering, x-ray photoelectron spectroscopy, and LEED (low-energy electron diffraction). Our measurements clearly show that Volmer-Weber growth (formation of three-dimensional crystallites) occurs even at sample temperatures as low as 160 K.Defects created by sputtering the substrate prior to Cu deposition do not influence the subsequent growth of the Cu films. The clusters are oriented with their (111)orientation parallel to the surface as confirmed by LEED. During low-temperature annealing, a coarsening of the crystallites takes place. The size of the clusters and the coverage have been modeled using simple assumptions about their shape. 0 An average thickness of approximately 10 A is needed to cover half of the sample with Cu. The growth mode can be attributed to a very weak interaction between Cu and the substrate.

We present a surface force apparatus (SFA) study of the effects of time and loading−unloading rates on the adhesion of solid polymeric surfaces of poly(butylmethacrylate). We used the equilibrium JKR theory of adhesion or contact mechanics as a framework for analyzing the “adhesion dynamics” of two surfaces during nonequilibrium (viscoelastic, plastic) adhesion and separation. PBMA films of thickness ∼2 μm were prepared on curved mica surfaces by casting from a solution of methyl ethyl ketone. Pull-off forces from adhesive contact were measured at different temperatures around the glass−rubber transition temperature (T g ≈ 25 °C) at different loads and contact times, and hysteretic loading−unloading cycles were measured at different rates. On entering the rubber regime, the effective surface energies deduced from the pull-off forces increase dramatically, by up to 3 orders of magnitude above the “equilibrium” value, with increasing contact time and load. Strong entanglements across the interface, probably through reptation, increase the effective area of contact with time, giving rise to the high pull-off forces observed. Bulk viscoelastic deformations of the surface profiles accompany the time-dependent adhesion processes. The existence of at least two different relaxation (energy dissipating) processes, one at the molecular level and the other at the microscopic to macroscopic level, can be inferred from these experiments. The implications of the results for understanding the adhesion, fracture strength, and crack−propagation of elastic versus viscoelastic materials are discussed.