Search

The transport and optical band gaps for the organic semiconductor tin (II) phthalocyanine (SnPc) and the complete energy band profiles have been determined for organic-inorganic interfaces between SnPc and III-V semiconductors. High throughput measurement of interface energetics over timescales comparable to the growth rates was enabled using in situ and real-time photoelectron spectroscopy combined with Organic Molecular Beam Deposition. Energy band alignment at SnPc interfaces with GaAs, GaP, and InP yields interface dipoles varying from -0.08 (GaP) to -0.83 eV (GaAs). Optical and transport gaps for SnPc and CuPc were determined from photoelectron spectroscopy and from optical absorption using spectroscopic ellipsometry to complete the energy band profiles. For SnPc, the difference in energy between the optical and transport gaps indicates an exciton binding energy of (0.6 ± 0.3) eV. [ABSTRACT FROM AUTHOR]

Abstract: 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). [Copyright &y& Elsevier]

Abstract: The influence of oxygen (O2) and carbon monoxide (CO) on Au nanoparticles supported on TiO2(110) in the size range of 2–3nm has been studied using X-ray photoelectron spectroscopy (XPS) and in situ (high pressure) XPS at 300K for O2 and/or CO pressures of 0.1–1mbar. These experiments were aimed at revisiting Au 4f core level shifts as reported in the literature and most importantly, to establish the dependence of the core-level shifts on the knowledge that there exists a maximum in reactivity for CO oxidation. Two samples were prepared with a coverage corresponding to that maximum (Au coverage 0.14–0.2 ML, particle size estimated to ∼2–2.5nm) while a third sample was expected to be less reactive (Au coverage 0.4 ML, particle size estimated to ∼3.3nm). At elevated O2 pressures, a new Au 4f component at higher binding energy (2.4–2.6eV relative to the Au(0) bulk signal) evolved at all particle sizes. Its appearance was attributed to a radiation-induced activation of oxygen and simultaneous oxidation of gold. The activation was much more efficient on the ∼2–2.5nm particles. The relative intensity of the oxide component depended strongly on O2 pressure and, thus, on the equilibrium coverage of O2. While not present in 0.1mbar O2 regardless of exposure time and particle size, it dominated the Au 4f spectrum of particles ∼2–2.5nm in size at 1mbar oxygen pressure. This pressure-dependent formation reconciles previously conflicting XPS data. Finally, the activated oxygen species were very reactive toward CO as manifested by the rapid disappearance of the new Au 4f component in a 1:1 mixture of CO and O2. The rates of evolution and consumption of this component were found to depend on gold coverage (and thus, particle size) and were highest for the smaller particles. [Copyright &y& Elsevier]

Abstract: Using density functional and STM theory we model the images of TiO2(110), 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. [Copyright &y& Elsevier]

Metal-oxide aqueous interfaces are important in areas as varied as photocatalysis and mineral reforming. Crucial to the chemistry at these interfaces is the structure of the electrical double layer formed when anions or cations compensate for the charge arising from adsorbed H + or OH - . This has proven extremely challenging to determine at the atomic level. In this work, we use a surface science approach, involving atomic level characterization, to determine the structure of pH-dependent model electrified interfaces of TiO 2 (110) with HCl and NaOH using surface X-ray diffraction (SXRD). A comparison with ab initio molecular dynamics calculations reveals the formation of surprisingly compact double layers. These involve inner-sphere bound Cl and Na ions, with respectively H + and O - /OH - in the contact layer. Their exceptionally high electric fields will play a key role in determining the chemical reactivity.

This paper reports on the long-standing puzzle of the atomic structure of the Ag/α-Al 2 O 3 (0001) interface by combining X-ray absorption spectroscopy, to determine Ag local environment [ i.e. average Ag-Ag ( d Ag-Ag ) and Ag-O ( d Ag-O ) interatomic distances and Ag coordination numbers (CN)], and numerical simulations on nanometric-sized particles. The experimental key was the capability of a structural study of clusters involving only a few atoms. The concomitant decrease of d Ag-Ag and CN with decreasing cluster size provides unambiguous fingerprints for the dimensionality of the Ag clusters in the subnanometric regime leading to a series of unexpected results regarding the size-dependent interface structures. At low coverage, Ag atoms sit on surface Al sites to form buckled monolayer-thick islands associated with a Ag-Ag distance (2.75 Å) which fits the alumina lattice. Upon increasing Ag coverage, as 3D clusters appear, the Ag interface atoms tend to leave Al sites to sit atop O atoms as d Ag-Ag increases. The then highlighted size-dependent evolution, is built on structural models which seemed so far contradictory in a static vision of the interface. Theory generalizes the case as it predicts the existence of alumina-supported 2D clusters of Pd and Pt at small coverage and a similar 2D-3D transition upon increasing the size. The structural transformation from 2D Ag clusters to macroscopic 3D islands is accompanied by a noticeable reduction of adhesion energy at the Ag/α-Al 2 O 3 (0001) interface.

The Cr/α-Al 2 O 3 (0001) interface has been explored by X-ray photoemission spectroscopy, X-ray absorption spectroscopy (XAS) and ab initio first-principles calculations of core level shifts including final state effects. After an initial oxidation via a reaction with residual surface OH but no reduction of the alumina substrate, Cr grows in a metallic form without any chemical effect on the initially oxidized Cr. However, Cr metal lacks crystallinity. Long-range (reflection high energy electron diffraction) and short-range (XAS) order are hardly observed. Thus photoemission combined with atomistic simulations becomes a unique tool to explore the chemistry and environment at the Cr/alumina interface. Cr 2p, O 1s and Al 2s shifted components are all explained by the formation of moieties involving Cr 3+ and/or Cr 4+ and of metallic Cr 0 , which supports the previously found Cr buffer mechanism for poorly adhesive metals. Beyond the situation under study, the present data demonstrate the ability of a combined experimental and theoretical approach of core-level shifts to exhaustively describe the general case of disordered metal/oxide interfaces.

The (011) termination of rutile TiO 2 is reported to be particularly effective for photocatalysis. Here, the structure of the interface formed between this substrate and water is revealed using surface X-ray diffraction. While the TiO 2 (011) surface exhibits a (2 × 1) reconstruction in ultra-high vacuum (UHV), this is lifted in the presence of a multilayer of water at room temperature. This change is driven by the formation of Ti-OH at the interface, which has a bond distance of 1.93 ± 0.02 Å. The experimental solution is in good agreement with density functional theory and first-principles molecular dynamics calculations. These results point to the important differences that can arise between the structure of oxide surfaces in UHV and technical environments and will ultimately lead to an atomistic understanding of the photocatalytic process of water splitting on TiO 2 surfaces., Competing Interests: The authors declare no competing financial interest.

Surface X-ray diffraction has been employed to quantitatively determine the geometric structure of an X-ray-induced superhydrophilic rutile-TiO 2 (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., Competing Interests: The authors declare no competing financial interest.

Elucidating the structure of the interface between natural (reduced) anatase TiO 2 (101) and water is an essential step toward understanding the associated photoassisted water splitting mechanism. Here we present surface X-ray diffraction results for the room temperature interface with ultrathin and bulk water, which we explain by reference to density functional theory calculations. We find that both interfaces contain a 25:75 mixture of molecular H 2 O and terminal OH bound to titanium atoms along with bridging OH species in the contact layer. This is in complete contrast to the inert character of room temperature anatase TiO 2 (101) in ultrahigh vacuum. A key difference between the ultrathin and bulk water interfaces is that in the latter water in the second layer is also ordered. These molecules are hydrogen bonded to the contact layer, modifying the bond angles.

Orientation-dependent reactivity and band-bending are evidenced upon Ti deposition (1-10 Å) on polar ZnO(0001)-Zn and ZnO(0001[combining macron])-O surfaces. At the onset of the Ti deposition, a downward band-bending was observed on ZnO(0001[combining macron])-O while no change occurred on ZnO(0001)-Zn. Combining this with the photoemission analysis of the Ti 2p core level and Zn L 3 (L 2 )M 45 M 45 Auger transition, it is established that the Ti/ZnO reaction is of the form Ti + 2ZnO → TiO 2 + 2Zn on ZnO(0001)-Zn and Ti + yZnO → TiZn x O y + (y - x)Zn on ZnO(0001[combining macron])-O. Consistently, upon annealing thicker Ti adlayers, the metallic zinc is removed to leave ZnO(0001)-Zn surfaces covered with a TiO 2 -like phase and ZnO(0001[combining macron])-O surfaces covered with a defined (Ti, Zn, O) compound. Finally, a difference in the activation temperature between the O-terminated (500 K) and Zn-terminated (700 K) surfaces is observed, which is tentatively explained by different electric fields in the space charge layer at ZnO surfaces.

The interaction of water with TiO 2 is crucial to many of its practical applications, including photocatalytic water splitting. Following the first demonstration of this phenomenon 40 years ago there have been numerous studies of the rutile single-crystal TiO 2 (110) interface with water. This has provided an atomic-level understanding of the water-TiO 2 interaction. However, nearly all of the previous studies of water/TiO 2 interfaces involve water in the vapour phase. Here, we explore the interfacial structure between liquid water and a rutile TiO 2 (110) surface pre-characterized at the atomic level. Scanning tunnelling microscopy and surface X-ray diffraction are used to determine the structure, which is comprised of an ordered array of hydroxyl molecules with molecular water in the second layer. Static and dynamic density functional theory calculations suggest that a possible mechanism for formation of the hydroxyl overlayer involves the mixed adsorption of O 2 and H 2 O on a partially defected surface. The quantitative structural properties derived here provide a basis with which to explore the atomistic properties and hence mechanisms involved in TiO 2 photocatalysis.

The wealth of properties of titanium dioxide relies on its various polymorphs and on their mixtures coupled with a sensitivity to crystallographic orientations. It is therefore pivotal to set out methods that allow surface structural identification. We demonstrate herein the ability of photoemission spectroscopy to provide Ti LMV (V = valence) Auger templates to quantitatively analyze TiO2 polymorphs. The Ti LMV decay reflects Ti 4sp-O 2p hybridizations that are intrinsic properties of TiO2 phases and orientations. Ti LMV templates collected on rutile (110), anatase (101), and (100) single crystals allow for the quantitative analysis of mixed nanosized powders, which bridges the gap between surfaces of reference and complex materials. As a test bed, the anatase/rutile P25 is studied both as received and during the anatase-to-rutile transformation upon annealing. The agreement with X-ray diffraction measurements proves the reliability of the Auger analysis and highlights its ability to detect surface orientations.