Au nanoparticles supported on reducible metal oxide surfaces are known to be active catalysts for a number of reactions including CO oxidation and hydrogen production. The exact choice of a metal oxide support has been shown to have a marked impact on activity, suggesting that interactions between Au and the support play a key role in catalysis. For TiO2, a model substrate for Au catalysis, it had been thought that bridging oxygen vacancies are involved in binding Au atoms to the (110) surface based on indirect evidence. However, a recent scanning transmission electron microscopy study of single Pt atoms on TiO2(110) suggests that subsurface vacancies are more important. To clarify the role of bridging or subsurface vacancies we employ scanning tunneling microscopy to determine the bonding site of single Au atoms on TiO2(110). Using in situ deposition as well as a manipulation method, we provide definitive evidence that the bonding site is atop surface oxygen vacancies.

Despite the proven properties of the anatase phase of TiO2 related to photocatalysis, detailed mechanistic information regarding a photooxidation reaction has not yet been derived from single-crystal studies. In this work, we have studied the photooxidation of ethanol (as a prototype hole-scavenger organic molecule) adsorbed on the anatase TiO2(101) surface by STM and online mass spectrometry to determine the adsorbate species in the dark and under UV illumination in the presence of O2 and to extract kinetic reaction parameters under photoexcitation. The reaction rate for the photooxidation of ethanol to acetaldehyde was found to depend on the O2 partial pressure and surface coverage, where the order of the reaction with respect to O2 is close to 0.15. Carbon–carbon bond dissociation leading to the formation of CH3 radicals in the gas phase was found to be a minor pathway, which is contrary to the case of the rutile TiO2(110) single crystal. Our STM images distinguished two types of surface adsorbates upon ethanol exposure that can be attributed to its molecular and dissociative modes. A mixed adsorption is also supported by our DFT calculations, in which we determined similar energies of adsorption (Eads) for the molecular (1.11 eV) and dissociative (0.93 eV) modes. Upon UV exposure at (and above) 3 × 10–8 mbar O2, a third species was identified on the surface as a reaction product that can be tentatively attributed to acetate/formate species on the basis of C 1s XPS results. The kinetics of the initial oxidation steps were evaluated using the STM and mass spectrometry data.

Excess electrons facilitate redox reactions at the technologically relevant anatase TiO2(101) surface. The availability of these electrons is related to the defect concentration at the surface. We present two-photon (2PPE, 3.10–3.54 eV) and ultraviolet (UPS, 21.2 & 40.8 eV) photoemission spectroscopy measurements evidencing an increased concentration of excess electrons following electron bombardment at room temperature. Irradiation-induced surface oxygen vacancies are known to migrate into the sub-surface at this temperature, quickly equilibrating the surface defect concentration. Hence, we propose that the irradiated surface is hydroxylated. Peaks in UPS difference spectra are observed centred 8.45, 6.50 and 0.73 eV below the Fermi level, which are associated with the 3σ and 1π hydroxyl molecular orbitals and Ti 3d band gap states, respectively. The higher concentration of excess electrons at the hydroxylated anatase (101) surface may increase the potential for redox reactions.

•Hydroxylated TiO2 shows water coverage-dependent two-photon photoexcitation.•The resonance signal reaches a maximum at a nominal water coverage of a monolayer.•This enhancement likely arises from a partial dissociation of the water monolayer.•Monolayer water does not alter the Ti 3d band gap states in photoemission data.Excited electrons and holes are crucial for redox reactions on metal oxide surfaces. However, precise details of this charge transfer process are not known. We report two-photon photoemission (hν = 3.23 eV) measurements of rutile TiO2(110) as a function of exposure to water below room temperature. The two-photon resonance associated with bridging hydroxyls is enhanced following water exposure, reaching a maximum at a nominal coverage of one monolayer. Higher coverages attenuate the observed resonance. Ultraviolet photoemission spectroscopy (hν = 21.22 eV) of the initial, band gap states shows little change up to one monolayer water coverage. It is likely that the enhancement arises from dissociation within the adsorbed water monolayer, although other mechanisms cannot be excluded.Figure optionsDownload full-size imageDownload high-quality image (162 K)Download as PowerPoint slide

Near edge X-ray absorption fine structure (NEXAFS) measurements of CO on Pd nanoparticles have been simulated. This was achieved by calculating the CO π* resonance signal of CO on a nanoparticle both as a function of the angle of incidence (I vs θ) and the direction of the electric field vector E of the incident photon beam (I vs β), with the nanoparticle defined as a \(\left( {111} \right)\) top facet with \(\left\{ {111} \right\}\) and \(\left\{ {100} \right\}\) side facets. The dependence of the π* resonance intensity signal of CO covered nanoparticles on the particle geometry and orientation as well as the bond orientation of CO is examined. In addition, we compare our simulations to a set of C K-edge NEXAFS experimental data obtained from a single Pd nanoparticle decorated with CO. Our simulation predicts that the nanoparticle has a high lateral aspect ratio of 37.7 ± 4.1.

Interest in resolving the mechanisms behind ceria’s activity has been intense due to the numerous industrial applications including those in heterogeneous catalysis. In this work, we study the reduction and reoxidation of ultrathin CeO2(111) nanoislands on Rh(111) and Pt(111) substrates, so-called inverse model catalysts, with a combination of real and reciprocal space techniques based on X-ray photoemission electron microscopy (XPEEM) and low energy electron microscopy. Soft X-ray microfocused illumination was employed to reduce the ceria islands, which we are able to control by varying the oxygen partial pressure within the measurement chamber. Low energy electron diffraction measurements of the irradiated ceria films demonstrate the formation of an ordered array of oxygen vacancies leading to a (√7 × √7)R19.1° superstructure attributed to the ι-phase (Ce7O12)(111). Resonant photoelectron spectroscopy provides the required high sensitivity to detect small changes in Ce3+ concentration. The high spatial resolution of the XPEEM allows us to determine that the reduction of the ceria occurs initially at the interface of the islands with the Rh support. Reoxidation of the CeO2–x(111) to CeO2(111) proceeds via spillover of activated oxygen adsorbed on the Rh(111) surface as a (2 × 2) overlayer. Our results highlight the important role that the noble metal plays in the regeneration of the stoichiometric ceria surface, a vital step in many reactions on ceria. This differs from the commonly proposed Mars–van Krevelen model in which reoxidation involves direct reaction of the ceria with O2.

The positions of atoms in and around acetate molecules at the rutile TiO2(110) interface with 0.1 M acetic acid have been determined with a precision of ±0.05 Å. Acetate is used as a surrogate for the carboxylate groups typically employed to anchor monocarboxylate dye molecules to TiO2 in dye-sensitized solar cells (DSSC). Structural analysis reveals small domains of ordered (2 × 1) acetate molecules, with substrate atoms closer to their bulk terminated positions compared to the clean UHV surface. Acetate is found in a bidentate bridge position, binding through both oxygen atoms to two 5-fold titanium atoms such that the molecular plane is along the [001] azimuth. Density functional theory calculations provide adsorption geometries in excellent agreement with experiment. The availability of these structural data will improve the accuracy of charge transport models for DSSC.

The photochemistry of TiO2 has been studied intensively since it was discovered that TiO2 can act as a photocatalyst. Nevertheless, it has proven difficult to establish the detailed charge-transfer processes involved, partly because the excited states involved are difficult to study. Here we present evidence of the existence of hydroxyl-induced excited states in the conduction band region. Using two-photon photoemission, we show that stepwise photoexcitation from filled band gap states lying 0.8 eV below the Fermi level of rutile TiO2(110) excites hydroxyl-induced states 2.73 eV above the Fermi level that has an onset energy of ∼3.1 eV. The onset is shifted to lower energy by the coadsorption of molecular water, which suggests a means of tuning the energy of the excited state.

The interaction of carbon monoxide (CO) molecules with the facets of noble metal nanoparticles forms the basis of many important catalytic reactions. Using scanning tunneling microscopy (STM), we have studied the adsorption of CO molecules on the (111) facets of Pd nanocrystals supported on a rutile TiO2(110) substrate. We observed four compact CO overlayers with coverages ranging between 0.5 and 0.6 monolayers. Examination of the positions of the CO molecules in each of the unit cells reveals that one of the overlayers has a rhombic (√7 × √7) R19.1°-4CO structure. The other three form rectangular structures, namely, (7 × √3) rect-8CO, c(5 × √3) rect-3CO, and c(9 × √3) rect-5CO. These are closely related via a soliton model previously proposed on the basis of infrared absorption spectroscopy and low-energy electron diffraction. By imaging the CO molecules, we provide direct evidence for the soliton model.

Supported metal nanoparticles form the basis of heterogeneous catalysts. Above a certain nanoparticle size, it is generally assumed that adsorbates bond in an identical fashion as on a semiinfinite crystal. This assumption has allowed the database on metal single crystals accumulated over the past 40 years to be used to model heterogeneous catalysts. Using a surface science approach to CO adsorption on supported Pd nanoparticles, we show that this assumption may be flawed. Near-edge X-ray absorption fine structure measurements, isolated to one nanoparticle, show that CO bonds upright on the nanoparticle top facets as expected from single-crystal data. However, the CO lateral registry differs from the single crystal. Our calculations indicate that this is caused by the strain on the nanoparticle, induced by carpet growth across the substrate step edges. This strain also weakens the CO–metal bond, which will reduce the energy barrier for catalytic reactions, including CO oxidation.

•Ultrathin CeO2(1 1 1) films were prepared on Pt(1 1 1) as a model catalyst.•A combined microscopic and spectroscopic approach was used to probe the ceria films.•STM and LEEM monitor the morphology.•Secondary electron XPEEM permits measurement of the local oxidation state.•The ceria nanostructures display a uniform stoichiometry (CeO1.75).CeO2−x(1 1 1) ultrathin films consisting of small, discrete islands decorating a Pt(1 1 1) substrate have been studied using a combination of Scanning Tunnelling Microscopy, Low-Energy Electron Microscopy, and Low-Energy Electron Diffraction. Significantly, the chemical nature of the ceria film has also been probed using X-ray Absorption Spectroscopy (XAS) combined with X-ray PhotoEmission Electron Microscopy (XPEEM) in the same ultrahigh vacuum system. XAS spectra over the Ce M5 absorption edge demonstrated that the ceria islands contained ∼50% Ce4+and ∼50% Ce3+, leading to an overall stoichiometry of CeO1.75, which was uniform across the film. The unique advantage of this experimental setup is the application of multiple techniques on the same sample: high-resolution STM to monitor the morphology, XPEEM to probe the stoichiometry, and LEEM to act as a bridge between the two.

Surface X-ray diffraction has been employed to elucidate the structure of the interface between a well-characterized (001) surface of 0.1 wt % Nb–SrTiO3 and liquid H2O. Results are reported for the clean surface, the surface in contact with a drop of liquid water, and the surface after the water droplet has been removed with a flow of nitrogen. The investigation revealed that the clean surface, prepared via annealing in 1 × 10–2 mbar O2 partial pressure, is unreconstructed and rough on a short length scale. The surface is covered with large terraces, the topmost layer of which is either TiO2 or SrO with an area ratio of about 7/3. For the surface in contact with water, our results reveal that associative H2O adsorption is favored for the TiO2-terminated terrace whereas adsorption is dissociative for the SrO-terminated terrace, which validates recent first-principles calculations. After removal of the water droplet, the surface largely resembles the water-covered surface but now with a disordered overlayer of water present on the surface.

Nanometer-sized gold particles supported on ceria are an important catalyst for the low-temperature water–gas shift reaction. In this work, we prepared a model system of epitaxial, ultrathin (1–2 nm thick) CeO2–x(111) crystallites on a Rh(111) substrate. Low-energy electron microscopy (LEEM) and X-ray photoemission electron microscopy (XPEEM) were employed to characterize the in situ growth and morphology of these films, employing Ce 4f resonant photoemission to probe the oxidation state of the ceria. The deposition of submonolayer amounts of gold at room temperature was studied with scanning tunneling microscopy (STM) and XPEEM. Spatially resolved, energy-selected XPEEM at the Au 4f core level after gold adsorption indicated small shifts to higher binding energy for the nanoparticles, with the magnitude of the shift inversely related to the particle size. Slight reduction of the ceria support was also observed upon increasing Au coverage. The initial oxidation state of the ceria film was shown to influence the Au 4f binding energy; more heavily reduced ceria promoted a larger shift to higher binding energy. Understanding the redox behavior of the gold/ceria system is an important step in elucidating the mechanisms behind its catalytic activity.

As models for probing the interactions between TiO2 surfaces and the dye molecules employed in dye-sensitized solar cells, carboxylic acids are an important class of molecules. In this work, we present a scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED) study of three small carboxylic acids (formic, acetic, and benzoic) that were reacted with the TiO2(110) surface via a dipping procedure. The three molecules display quite different adsorption behavior, illustrating the different interadsorbate interactions that can occur. After exposure to a 10 mM solution, formic acid forms a rather disordered formate overlayer with two distinct binding geometries. Acetic acid forms a well-ordered (2 × 1) acetate overlayer similar to that observed following deposition from vapor. Benzoic acid forms a (2 × 2) overlayer, which is stabilized by intermolecular interactions between the phenyl groups.Keywords: acetic acid; benzoic acid; dipping; formic acid; STM;

•Three methods for synthesis of ultra-thin rutile TiO2(110) films•Resulting films have been found to be equivalent.•Increase in adsorbed hydroxyls accompanies an increase in Ti3 + states.We present a study of the growth and reactivity of ultra-thin films of TiO2 grown on W(100). Three approaches to film growth are investigated, each resulting in films that show order in low-energy diffraction (LEED) and a low level of non-stoichiometry in X-ray photoelectron spectroscopy (XPS). H2O is used as a probe of the reactivity of the films, with changes in the Ti 2p and O 1s core levels being monitored by XPS. Evidence for the dissociation of H2O on the TiO2(110) ultra-thin film surface is adduced. These results are discussed with reference to related studies on native TiO2(110).

Electron beam-induced surface activation (EBISA) has been used to grow wires of iron on rutile TiO2(110)-(1 × 1) in ultrahigh vacuum. The wires have a width down to ∼20 nm and hence have potential utility as interconnects on this dielectric substrate. Wire formation was achieved using an electron beam from a scanning electron microscope to activate the surface, which was subsequently exposed to Fe(CO)5. On the basis of scanning tunneling microscopy and Auger electron spectroscopy measurements, the activation mechanism involves electron beam-induced surface reduction and restructuring.

Ultrathin films of rutile TiO2(110) have been grown on a W(100)-O(2 × 1) surface and characterized with a combination of scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED). LEED shows the presence of two orthogonal rotational domains of rutile TiO2(110). In line with this, STM images reveal that the rutile TiO2 grows as discrete islands and can be aligned in either of the principal directions of the underlying substrate W(100)-O(2 × 1). High-resolution STM images reveal atomic-scale rows and the presence of point defects on the rutile islands that are characteristic of the native rutile TiO2(110)-(1 × 1).

We have investigated the (001) surface structure of lithium titanate (Li2TiO3) using auger electron spectroscopy (AES), low-energy electron diffraction (LEED), and scanning tunneling microscopy (STM). Li2TiO3 is a potential fusion reactor blanket material. After annealing at 1200 K, LEED demonstrated that the Li2TiO3(001) surface was well ordered and not reconstructed. STM imaging showed that terraces are separated in height by about 0.3 nm suggesting a single termination layer. Moreover, hexagonal patterns with a periodicity of ∼0.4 nm are observed. On the basis of molecular dynamics (MD) simulations, these are interpreted as a dynamic arrangement of Li atoms.

High resolution X-ray photoemission electron microscopy (XPEEM) and low energy electron microscopy (LEEM) have been used to investigate the growth of ultrathin CeOx(111) on Re(0001), a model catalyst system. Rotational domains of CeOx(111) are identified with microprobe low energy electron diffraction (LEED) and dark-field LEEM. In the regions not covered by the ceria islands, a surface rhenium-oxide layer has been observed using energy-filtered XPEEM imaging and spectroscopy. The oxidation state of the ceria is key to its catalytic activity. For this reason we have employed resonant photoelectron spectroscopy of the Ce 4f contributions to the valence band to monitor the relative Ce3+ and Ce4+ concentrations. The overall stoichiometry of the moderately reduced film was CeO1.63. Resonant energy-filtered XPEEM imaging of the Ce oxidation state allowed us to confirm the uniformity of this stoichiometry across the ceria islands that constituted the film.

The adsorption and reactivity of acetic acid on anatase TiO2(101) has been investigated with scanning tunneling microscopy (STM). At low coverage, acetic acid is observed to have a characteristic appearance in STM consistent with a dissociative bidentate binding geometry. At room temperature acetic acid has a relatively strong interaction with the anatase (101) surface and a near-unity sticking probability. When deposited at elevated temperatures (420 K), a saturated coverage displays a partially ordered superstructure with two domains across small regions of the anatase surface. The periodicity of these domains was found to be (2 × 1), again consistent with a bidentate binding geometry of the acetate to two neighboring Ti5c sites along the [010] direction. Heating the acetate-covered surface to 570 K in ultrahigh vacuum resulted in clean desorption of ∼90% of the molecules, leaving only a small fraction undesorbed that were mainly situated at the step edges of the anatase. STM tip pulsing of +6 V was also found to desorb acetate molecules from the surface.

Commercial TiO2 (Hombikat, UV-100) was impregnated with different loadings of zinc nitrate solution and subsequently calcined at different temperatures in order to obtain a stable homogeneous solid composite of ZnO/TiO2. The prepared samples were characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), high resolution transmission electron microscopy (HR-TEM), UV–vis and Raman spectroscopy, inductively coupled plasma mass spectroscopy (ICP), X-ray photoelectron spectroscopy (XPS) as well as N2 adsorption and desorption measurements. Results show that ZnO was incorporated within the TiO2 crystals and did not form a separate bulky phase or metallic zinc. Moreover, the calcination temperature dramatically modifies the texture properties of the prepared samples compared with original Hombikat TiO2. The photocatalytic performance of the prepared samples was evaluated by monitoring the degradation of methyl orange dye under black light illumination. Three main parameters were studied; ZnO loading, surface area and initial pH of the methyl orange solution. The variation in ZnO loading appears to have less influence on the catalytic activity than either the surface area or the pH.Highlights► Hombikat TiO2 was impregnated with different ZnO loadings and calcined at various temperatures to obtain ZnO/TiO2 composites. ► Increasing the calcination temperature lead to a reduction of the surface area, and lowered the nanocomposites photocatalytic activity. ► At low calcination temperatures zinc salt residues (used during loading) lead to lower pH in the test solution. ► This pH reduction increased the photocatalytic activity of the nanocomposites. ► The ZnO loading appears to have less influence on the catalytic activity than the surface area or the pH.

Scanning tunneling microscopy (STM) has been used to investigate the adsorption of benzoic acid on the rutile TiO2(110)(1 × 1) and the reconstructed TiO2(110)(1 × 2) surfaces. Benzoic acid binds to both surfaces dissociatively via a bridging geometry to two Ti5c sites. At a slightly elevated sample temperature during deposition onto the (110) (1 × 1) surface, a well-ordered (2 × 1) overlayer was formed at saturated benzoate coverage. On the reconstructed (110) (1 × 2) surface, benzoate was observed to adsorb between the (1 × 2) strands leading to a (2 × 2) superstructure at higher coverage. Elongation along the [11̅0] direction in the STM images indicates a rotation of the benzene ring of 90° relative to the carboxylate group, which is reasonably explained by hydrogen bond interactions between terminating O-atoms on the surface and H-atoms of the ring.

Local defects present in CeO2 − x films result in a mixture of Ce3+ and Ce4+ oxidation states. Previous studies of the Ce 3d region with XPS have shown that depositing metal nanoparticles on ceria films causes further reduction, with an increase in Ce3+ concentration. Here, we compare the use of XPS and resonant photoemission spectroscopy (RESPES) to estimate the concentration of Ce3+ and Ce4+ in CeO2 − x films grown on Pt (111), and the variation of this concentration as a function of Pd deposition. Due to the nature of the electronic structure of CeO2 − x, resonant peaks are observed for the 4d–4f transitions when the photon energy matches the resonant energy; (hν = 121.0 eV) for Ce3+ and (hν = 124.5 eV) for Ce4+. This results in two discrete resonant photoemission peaks in valence band spectra. The ratio of the difference of these peaks with off-resonance scans gives an indication of the relative contribution of Ce3+. Results from RESPES indicate reduction of CeO2 − x on deposition of Pd, confirming earlier findings from XPS studies.► We use resonant photoemission to show that Pd nanoparticles reduce an ultrathin CeO2(111) film. ► The resonant photoemission data are in qualitative agreement with XPS results on the same surface. ► Earlier calculations suggest that the reduction of the film arises from charge transfer from Pd to the film.

Atomically resolved scanning tunnelling microscopy (STM) images have been obtained on ultrathin films of CeO2(111) supported on Pt(111). The ultrathin films were grown in two ways, by reactive deposition in an oxygen atmosphere and by postoxidation of Ce/Pt surface alloys. STM results are compared with previously reported high-temperature STM and noncontact atomic force microscopy (NC-AFM) images of the native CeO2(111) surface. The similarity between these images is striking and allows a number of defects and adsorbates in our ultrathin film to be assigned. Moreover, the similarity in structure between the native oxide and the ceria ultrathin film indicates that it is an excellent topographic mimic of the native oxide.

Oxygen vacancies on metal oxide surfaces have long been thought to play a key role in the surface chemistry. Such processes have been directly visualized in the case of the model photocatalyst surface TiO2(110) in reactions with water and molecular oxygen. These vacancies have been assumed to be neutral in calculations of the surface properties. However, by comparing experimental and simulated scanning tunneling microscopy images and spectra, we show that oxygen vacancies act as trapping centers and are negatively charged. We demonstrate that charging the defect significantly affects the reactivity by following the reaction of molecular oxygen with surface hydroxyl formed by water dissociation at the vacancies. Calculations with electronically charged hydroxyl favor a condensation reaction forming water and surface oxygen adatoms, in line with experimental observations. This contrasts with simulations using neutral hydroxyl where hydrogen peroxide is found to be the most stable product.

We have used scanning tunneling microscopy (STM), noncontact atomic force microscopy (NC-AFM), low energy electron diffraction (LEED), and ab initio calculations to study adsorbates resulting from exposure of rutile TiO2(110)1 × 1 to methyl phosphonic acid (CH3P═O(OH)2). At low exposures, adsorbates appear on the 5-fold coordinated Ti (Ti5c) rows. As the coverage of adsorbates approaches 0.5 ML, STM images show an ordered 2 × 1 overlayer consistent with LEED. We propose that the phosphonic acid is deprotonated with the resulting phosphonate bridging across two adjacent Ti5c atoms in the [001] direction. This bridging conformation would lead to the observed 2 × 1 overlayer and is analogous to that found for a range of carboxylates adsorbed on TiO2(110).

Palladium nanoparticles supported on rutile TiO2(110)-1 × 1 have been studied using the complementary techniques of scanning tunneling microscopy and X-ray photoemission electron microscopy. Two distinct types of palladium nanoparticles are observed, namely long nanowires up to 1000 nm long, and smaller dotlike features with diameters ranging from 80−160 nm. X-ray photoemission electron microscopy reveals that the nanoparticles are composed of metallic palladium, separated by the bare TiO2(110) surface.

Metal oxides have considerable potential as insulating supports for nanoscale electronic devices. One of the key attributes of metal oxide surfaces is their capacity to be modified by electron beams and scanning probe tips. Such modifications can involve the creation of O vacancies or an area of a different reconstruction, which in principle can act as anchoring points or templates for molecules or metal interconnects. In this Prospective we describe previous attempts at well-defined modification in order to illustrate this potential.

Surface X-ray diffraction has been used to investigate the structure of TiO2(1 1 0)(3 × 1)–S. In concert with existing STM and photoemission data it is shown that on formation of a (3 × 1)–S overlayer, sulphur adsorbs in a position bridging 6-fold titanium atoms, and all bridging oxygens are lost. Sulphur adsorption gives rise to significant restructuring of the substrate, detected as deep as the fourth layer of the selvedge. The replacement of a bridging oxygen atom with sulphur gives rise to a significant motion of 6-fold co-ordinated titanium atoms away from the adsorbate, along with a concomitant rumpling of the second substrate layer.

The reaction of Fe3O4(1 1 1) with water vapour has been studied with scanning tunnelling microscopy (STM) and with X-ray and UV-photoemission as a function of water partial pressure and temperature. The photoemission results point to dissociation to form surface hydroxyls at a partial pressure of 10−6 mbar H2O and a substrate temperature of about 200 K. At 298 K it is known that dissociation occurs at around 10−3 mbar [Kendelewicz et al., Surf. Sci. 453 (2000) 32]. This difference suggests that an intermolecular mechanism of dissociation is involved. It also suggests that the pressure dependence arises from a coverage term rather than differences in the Gibbs Free Energies of the oxide and hydroxide, as previously proposed. The STM results indicate that dissociation takes place on a termination of Fe3O4(1 1 1) thought to contain a 1/4 monolayer (ML) of Fe3+ ions on top of a close-packed oxygen monolayer.

Photoelectron spectroscopy and scanning tunneling microscopy have been used to investigate how the oxidation state of Ce in CeO2−x(111) ultrathin films is influenced by the presence of Pd nanoparticles. Pd induces an increase in the concentration of Ce3+ cations, which is interpreted as charge transfer from Pd to CeO2−x(111) on the basis of DFT+U calculations. Charge transfer from Pd to Ce4+ is found to be energetically favorable even for individual Pd adatoms. These results have implications for our understanding of the redox behavior of ceria-based model catalyst systems.

Using density functional and STM theory we model the images of TiO2(1 1 0), its defects, molecularly adsorbed water, its recently suggested pseudo-dissociated precursor and final dissociation product. The simulated STM images and the corresponding line scans agree with the available experimental data: oxygen vacancies are imaged as smaller protrusions than unpaired or paired hydroxyl groups. Finally, we obtain simulated images of undissociated and pseudo-dissociated water molecules. These simulations are discussed in view of their appearance in the experiments.Simulated Tersoff–Hamann STM images (1.5 V) for optimized rutile TiO2(1 1 0) in the presence of one oxygen vacancy, one unpaired hydroxyl, one molecularly adsorbed water molecule, its pseudo-dissociated state and final dissociation product. In agreement with the experiment oxygen vacancies appear as smaller protrusions than unpaired and paired hydroxyl groups, respectively.

Reduced phases of ultrathin rutile TiO2(110) grown on Ni(110) have been characterized with scanning tunneling microscopy and low-energy electron diffraction. Areas of 1×2 reconstruction are observed as well as {132} and {121} families of crystallographic shear planes. These phases are assigned by comparison with analogous phases on native rutile TiO2(110).Keywords: crystallographic shear plane; facet; rutile; self-assembly; STM; TiO2; ultrathin film

The adsorption site of K on TiO2(1 1 0) 1 × 1 has been investigated at low coverage (<0.15 ML, where 1 monolayer corresponds to the number of surface unit cells). A combination of surface extended X-ray absorption fine structure (SEXAFS), scanning tunneling microscopy (STM), and non-contact atomic force microscopy (NC-AFM) was used for this purpose. SEXAFS identifies a threefold O hollow site in which K bonds to two bridging O atoms and one in-plane O atom at K–O bond distances of 2.49 ± 0.12 Å and 2.73 ± 0.07 Å. STM and NC-AFM images evidence a condensation of K atoms on either side of the bridging O row.