Structure of a model TiO2 photocatalytic interface.

The interaction of water with TiO2 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 TiO2(110) interface with water. This has provided an atomic-level understanding of the water-TiO2 interaction. However, nearly all of the previous studies of water/TiO2 interfaces involve water in the vapour phase. Here, we explore the interfacial structure between liquid water and a rutile TiO2(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 O2 and H2O 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 TiO2 photocatalysis.

Atomic structure and crystalline order of graphene-supported ir nanoparticle lattices.

We present the atomic structure of Ir nanoparticles with 1.5 nm diameter at half height and three layers average height grown on graphene/Ir(111). Using surface x-ray diffraction, we demonstrate that Ir nanoparticles on graphene/Ir(111) form a crystallographic superlattice with high perfection. The superlattice arrangement allows us to obtain detailed information on the atomic structure of the nanoparticles themselves, such as size, shape, internal layer stacking and strain. Our experiments disclose that the nanoparticles reside epitaxially on top of the graphene moiré structure on Ir(111), resulting in significant lateral compressive intraparticle strain. Normal incidence x-ray standing wave experiments deliver additional information on the particle formation induced restructuring of the graphene layer.

Tuning the structure of ultrathin BaTiO3 films on Me(001) (Me=Fe, Pd, Pt) surfaces.

Using surface x-ray diffraction in combination with ab initio calculations, we demonstrate that the atomic structure of ultrathin BaTiO3 (BTO) films grown on Me(001) surfaces (Me=Fe, Pd, Pt) depends on subtle modifications of the interface chemical composition. A complete reversal of the surface termination from a BaO- [BTO on Fe(001)] to a TiO2-terminated film [BTO on Pt(001)] is observed which goes in parallel with the adsorption of submonolayer amounts of oxygen at metal hollow sites of the interface. Our results may suggest a new route to an overall control of both the surface and the interface geometry in BaTiO3/metal contacts.

Bulk electronic structure of quasicrystals.

We use hard x-ray photoemission to resolve a controversial issue regarding the mechanism for the formation of quasicrystalline solids, i.e., the existence of a pseudogap at the Fermi level. Our data from icosahedral fivefold Al-Pd-Mn and Al-Cu-Fe quasicrystals demonstrate the presence of a pseudogap, which is not observed in surface sensitive low energy photoemission because the spectrum is affected by a metallic phase near the surface. In contrast to Al-Pd-Mn, we find that in Al-Cu-Fe the pseudogap is fully formed; i.e., the density of states reaches zero at E(F) indicating that it is close to the metal-insulator phase boundary.

Initial corrosion observed on the atomic scale.

Corrosion destroys more than three per cent of the world's GDP. Recently, the electrochemical decomposition of metal alloys has been more productively harnessed to produce porous materials with diverse technological potential. High-resolution insight into structure formation during electrocorrosion is a prerequisite for an atomistic understanding and control of such electrochemical surface processes. Here we report atomic-scale observations of the initial stages of corrosion of a Cu3Au111 single crystal alloy within a sulphuric acid solution. We monitor, by in situ X-ray diffraction with picometre-scale resolution, the structure and chemical composition of the electrolyte/alloy interface as the material decomposes. We reveal the microscopic structural changes associated with a general passivation phenomenon of which the origin has been hitherto unclear. We observe the formation of a gold-enriched single-crystal layer that is two to three monolayers thick, and has an unexpected inverted (CBA-) stacking sequence. At higher potentials, we find that this protective passivation layer dewets and pure gold islands are formed; such structures form the templates for the growth of nanoporous metals. Our experiments are carried out on a model single-crystal system. However, the insights should equally apply within a crystalline grain of an associated polycrystalline electrode fabricated from many other alloys exhibiting a large difference in the standard potential of their constituents, such as stainless steel (see ref. 5 for example) or alloys used for marine applications, such as CuZn or CuAl.

Orientational ordering of nonplanar phthalocyanines on Cu(111): strength and orientation of the electric dipole moment.

In order to investigate the orientational ordering of molecular dipoles and the associated electronic properties, we studied the adsorption of chlorogallium phthalocyanine molecules (GaClPc, Pc=C32N8H16(-2) on Cu(111) by using the x-ray standing wave technique, photoelectron spectroscopy, and quantum mechanical calculations. We find that for submonolayer coverages on Cu(111) the majority of GaClPc molecules adsorb in a Cl-down configuration by forming a covalent bond to the substrate. For bilayer coverages the x-ray standing wave data indicate a coexistence of the Cl-down and Cl-up configurations on the substrate. The structural details established for both cases and supplementary calculations of the adsorbate system allow us to analyze the observed change of the work function.

Site-specific geometric and electronic relaxations at organic-metal interfaces.

The correlation between the geometric and electronic structures of Zn-phthalocyanine (ZnPc) and F16ZnPc on Cu(111) were studied by x-ray standing wave and angle-resolved photoemission spectroscopy. We found evidence for a distortion of the planar molecules upon adsorption, with the central Zn atom in the molecule protruding towards the substrate. This modifies the energy levels of both the molecule and the substrate, which appear as interface states. The site-specific geometric and electronic relaxations are an important effect for organic-metal interface energetics.

Copper-phthalocyanine based metal-organic interfaces: the effect of fluorination, the substrate, and its symmetry.

Metal-organic interfaces based on copper-phthalocyanine monolayers are studied in dependence of the metal substrate (Au versus Cu), of its symmetry [hexagonal (111) surfaces versus fourfold (100) surfaces], as well as of the donor or acceptor semiconducting character associated with the nonfluorinated or perfluorinated molecules, respectively. Comparison of the properties of these systematically varied metal-organic interfaces provides new insight into the effect of each of the previously mentioned parameters on the molecule-substrate interactions.

A Quantitative Structural Investigation of the 0.1 wt % Nb-SrTiO3(001)/H2O Interface.

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.