Surface X-ray diffraction has been employed to quantitatively determine the geometric structure of an X-ray-induced superhydrophilic rutile-TiO2(110)(1 × 1) surface. A scatterer, assumed to be ...oxygen, is found at a distance of 1.90 ± 0.02 Å above the five-fold-coordinated surface Ti atom, indicating surface hydroxylation. Two more oxygen atoms, situated further from the substrate, are also included to achieve the optimal agreement between experimental and simulated diffraction data. It is concluded that these latter scatterers are from water molecules, surface-localized through hydrogen bonding. Comparing this interfacial structure with previous studies suggests that the superhydophilicity of titania is most likely to be a result of the depletion of surface carbon contamination coupled to extensive surface hydroxylation.
The origin of the catalytic activity of gold nanoparticles remains debated despite extensive studies. This in operando work investigates the relationship between catalytic activity and size/shape of ...gold nanoparticles supported on TiO2(110) during CO oxidation. The nanoparticles were synthesized by vapor deposition in ultrahigh vacuum. Their geometry was monitored in the presence of O2, Ar, or a mixture of O2 + CO and of Ar + CO by grazing incidence small-angle X-ray scattering simultaneously with the catalytic activity. The occurrence of CO oxidation induces a sintering directly correlated to the reaction rate. The catalytic activity is optimum for a nanoparticle’s diameter of 2.1 ± 0.3 nm and a height of about six atomic layers. Below this size, the activity drop corresponds to a height decrease. Rescaling of activities obtained in different experimental conditions shows consistency of these results with published data using both “model” and “real” catalysts.
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).
•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.
Surface X-ray diffraction has been employed to quantitatively determine the geometric structure of an X-ray-induced superhydrophilic rutile-TiO
(110)(1 × 1) surface. A scatterer, assumed to be ...oxygen, is found at a distance of 1.90 ± 0.02 Å above the five-fold-coordinated surface Ti atom, indicating surface hydroxylation. Two more oxygen atoms, situated further from the substrate, are also included to achieve the optimal agreement between experimental and simulated diffraction data. It is concluded that these latter scatterers are from water molecules, surface-localized through hydrogen bonding. Comparing this interfacial structure with previous studies suggests that the superhydophilicity of titania is most likely to be a result of the depletion of surface carbon contamination coupled to extensive surface hydroxylation.
Tartaric acid (TA) is one of the most known chiral modifier used in heterogeneous enantioselective catalysis and understanding how it interacts with oxide supports is of significant importance for ...the design of efficient supported metal catalysts. This work presents a detailed surface science characterisation of (R,R)-TA adsorption on a rutile TiO2(110) surface from a chemical and structural point of view. X-ray Photoelectron Spectroscopy (XPS), High Resolution Electron Energy Loss Spectroscopy (HREELS), Low Energy Electron Diffraction (LEED) and Scanning Tunnelling Microscopy (STM) were used to decipher the adsorption mode and geometrical arrangement of TA on TiO2(110). For XPS analysis, spectra were compared to references recorded on model metal surfaces, namely Au(111) and Cu(110), on which a better understanding of the TA chemical nature is available. On TiO2, both XPS and HREELS converged to show the monotartrate chemical nature (only one acidic group is deprotonated) of the adsorbed TA molecules, while STM and LEED evidenced a (2 × 1) 2D network arrangement. TA molecules are anchored via both oxygen atoms of one carboxylate group binding directly to two adjacent Ti5c of the underlying oxide surface. No chiral ordered domains were observed whatever the coverage, suggesting that surface chirality is induced at the molecular level.
Display omitted
•R,R-Tartaric acid was adsorbed on Rutile TiO2 singles crystals.•XPS and HREELS data show that RRTA is adsorbed as monotartrate species.•STM at low coverage show a random adsorption site along the Ti5c rows.•STM at high coverage show a (2 × 1) adlayer of TA on TiO2.
Geometric structure of TiO2(011)(2 x 1) Torrelles, X; Cabailh, G; Lindsay, R ...
Physical review letters,
10/2008, Letnik:
101, Številka:
18
Journal Article
Recenzirano
Surface x-ray diffraction has been employed to elucidate the surface structure of the (011)-(2 x 1) termination of rutile TiO2. The data are inconsistent with previously proposed structures. Instead, ...an entirely unanticipated geometry emerges from the structure determination, which is terminated by zigzag rows of twofold coordinated oxygen atoms asymmetrically bonded to fivefold titanium atoms. The energetic stability of this structure is demonstrated by ab initio total energy calculations.
Here, we show that the stoichiometry and, consequently, the chemical activity toward hydroxylation of MgO(100) films grown by reactive deposition on Ag(100) strongly depend on the O2 partial pressure ...during film growth. Oxygen-deficient films undergo dramatic relative oxygen uptake either by exposure to a partial pressure of water vapor or by aging in vacuum for a sufficiently long time. Conversely, on stoichiometric monolayer MgO islands, photoemission analysis of the O 1s level and scanning tunneling microscopy images are consistent with the prediction that dissociative adsorption of water occurs only at the borders of the islands.
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.
Local defects present in CeO
2
−
x
films result in a mixture of Ce
3+ and Ce
4+ oxidation states. Previous studies of the Ce 3
d region with XPS have shown that depositing metal nanoparticles on ...ceria films causes further reduction, with an increase in Ce
3+ concentration. Here, we compare the use of XPS and resonant photoemission spectroscopy (RESPES) to estimate the concentration of Ce
3+ and Ce
4+ in CeO
2
−
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 CeO
2
−
x
, resonant peaks are observed for the 4
d–4
f transitions when the photon energy matches the resonant energy; (
hν
=
121.0
eV) for Ce
3+ and (
hν
=
124.5
eV) for Ce
4+. 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 Ce
3+. Results from RESPES indicate reduction of CeO
2
−
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.
PDF
