We have performed a resonant photoemission study on both stoichiometric and reduced ${\mathrm{TiO}}_2$ (110) surfaces using synchrotron radiation. A resonant enhancement of the O 2p valence-band photoemission intensity is observed for both stoichiometric and reduced ${\mathrm{TiO}}_2$ surfaces as the photon energy is swept through the Ti 3p→3d optical transition energy, which indicates a strong hybridization between the oxygen and titanium ions. Compared to the stoichiometric surface, the amplitude of the resonance is smaller for the reduced surface. In addition, the reduced surface has higher emission intensity from the O 2p bonding orbitals and lower emission intensity from the nonbonding orbitals. For both stoichiometric and reduced surfaces, the intensity of emission from the O 2p bonding orbitals resonates more than that from the O 2p nonbonding orbitals, and the resonance profile for the nonbonding orbitals maximizes at a higher photon energy than that for the bonding orbitals. The resonant enhancement of the O 2p bonding orbitals is consistent with theoretical calculations, which predict that the O 2p–Ti 3d hybridization involves primarily those O 2p orbitals. The energy dependence of the resonant enhancement for the O 2p nonbonding orbitals suggests that it results predominantly from resonant processes involving Ti 3p→4s excitation at the surface.

We have studied model catalysts comprised of vanadium or vanadia deposited onto both stoichiometric and reduced single-crystal TiO 2 (110) surfaces by using XPS, UPS, Auger, LEED and work-function measurements. Changes in the electronic and geometric structure of TiO 2 supports are monitored during overlayer formation in the submonolayer and monolayer regimes. Deposition of vanadium in an O 2 ambient onto the stoichiometric TiO 2 surface results in the formation of lower oxides of vanadium that interact only weakly with the support. However, the stoichiometric TiO 2 surface is attacked violently by vanadium metal atoms deposited in UHV; in the submonolayer range, V-O bonding results in electron transfer to the substrate due to the strong affinity between V and O. This substrate reduction is confirmed by the appearance of Ti 3+ features in XPS spectra. For greater than monolayer coverage, a metallic overlayer is formed. In both the vanadia/TiO 2 and V/TiO 2 systems, the initially sharp (1 × 1) LEED patterns disappear for small coverages, indicating disordering of the surfaces. Although defects on TiO 2 are generally active chemisorption sites, the reduced TiO 2 surface interacts only weakly with metallic vanadium.

We demonstrate that water is almost universally present on apparently dry self-assembled monolayers, even on those considered almost hydrophobic by conventional methods such as water contact goniometry. The structure and kinetics of nanoscale water adsorption onto these surfaces were investigated using X-ray and neutron reflectometry, as well as atomic force microscopy. Condensation of water on hydrophilic surfaces under ambient conditions formed a dense sub-nanometre surface layer; the thickness of which increased with exponentially limiting kinetics. Tapping mode AFM measurements show the presence of nanosized droplets that covered a small percentage (∼2%) of the total surface area, and which became fewer in number and larger in size with time. While low vacuum pressures (∼10⁻⁸ bar) at room temperature did nothing to remove the adsorbed water from these monolayers, heating to temperatures above 65 °C under atmospheric conditions did lead to evaporation from the surface. We demonstrate that water contact angle measurements are not necessarily sensitive to the presence of nanoscale adsorbed water and do not vary with time. For the most part they are a poor indicator of the kinetics and the amount of water condensation onto these surfaces at the molecular level. In summary, this study reveals the need to exclude air containing even trace amounts of water vapor from such surfaces when characterizing using techniques such as X-ray reflectometry.