Titanium dioxide is a promising candidate for photocatalytic H2 fuel production, and understanding water splitting on TiO2 surfaces is vital toward explaining and improving the generation of H2. In ...this work, we electron irradiate anatase TiO2(101) at room temperature to create metastable surface oxygen vacancies in order to investigate their ability to dissociate H2O. Our scanning tunneling microscopy investigations suggest that the surface oxygen vacancies can dissociate H2O by forming bridging OH species. This claim is supported by theoretical calculations from the literature and our previously published spectroscopic measurements.
Combining X-ray photoelectron spectroscopy with the standing wave technique, we investigated adsorption of a monolayer of water on Ti-oxide-terminated SrTiO3(001) in ultra-high vacuum (UHV). At room ...temperature, the surface is water-free but hydroxylated. A quarter monolayer of hydroxyl is tightly bound 1.85 ± 0.06 Å above the TiO2 surface. Deposited at a low temperature, a monolayer of water adsorbs with the oxygen located 2.55 ± 0.2 Å above the surface, apparently close to atop Ti, but H2O is unstable at 200 K. A fraction desorbs, in part under the X-ray beam, but a major fraction of H2O dissociates immediately, with the liberated hydrogen most likely attaching to a surface oxygen. The produced hydroxyls bind only loosely to the surface, are unstable at 200 K, and rapidly desorb once the surface is water-free. Although our study was conducted in UHV, the presented results suggest that Ti-oxide-terminated SrTiO3(001) may possess a high catalytic activity toward hydrolysis under realistic conditions.
We present a detailed experimental and theoretical characterization of the adsorption of the perylene derivative 4,9-diaminoperylenequinone-3,10-diimine (DPDI) on Cu(111) and compare it to its ...threefold dehydrogenated derivative 3deh-DPDI, which forms in a surface reaction upon annealing. While DPDI itself does not give rise to long-range ordered structures due to lack of appropriate functional groups, 3deh-DPDI acts as an exoligand in a Cu-coordinated honeycomb network on Cu(111). The main focus of this work lies on the analysis of intermolecular and molecule-substrate interactions by combining results from scanning tunneling microscopy, x-ray photoelectron spectroscopy, x-ray standing wave measurements, and density functional theory. We show, in particular, that the interactions between metal atoms and organic ligands effectively weaken the molecule-surface interactions for 3deh-DPDI leading to an increase in molecule-substrate distances compared to the DPDI precursor. Our experimental findings also shed light on the applicability of current theories, namely van der Waals corrections to density functional theory.
Surface X-ray scattering (SXS) measurements were carried out to monitor the potential induced structure changes such as surface reconstruction lifting, adsorption of oxygen species, formation of ...surface oxide bilayer, reduction of surface oxide, and surface reconstruction at Au(111)/H2SO4 and Au(100)/H2SO4 interfaces in situ in real time using intense high energy X-ray. The phase transition of the reconstruction/lifting, adsorption of oxygen species, and surface oxide formation/reduction at the Au(100) electrode proceed much slower, slightly slower, and faster, respectively, than at the Au(111) electrode.
•We examine potential induced dynamic structure changes at Au(111) and Au(100) by SXS.•Rates of potential induced structure changes are dependent on faces.•Reconstruction/lifting at Au(100) proceeds much slower than that at Au(111).•Adsorption of oxygen species at Au(100) proceeds slightly slower than that at Au(111).•Surface oxide formation/reduction at Au(100) proceeds faster than at Au(111).
X-ray reflectivity and spectroscopy, voltammetry, and scanning probe microscopy were used to investigate the dealloying of Cu x Au (with x ≈ 4) below the critical potential. We studied ultrathin ...films deposited on oxidized silicon wafers with a thickness of 2.2, 8.5, and 43 nm, which had been determined precisely by X-ray reflectivity. With the help of quantitative X-ray fluorescence (XRF) analysis, the average composition of the films before and after the leaching was determined. In this way, the amount of material going into solution could precisely be analyzed. A compositional depth profile was obtained by angle-dependent hard X-ray photoelectron spectroscopy before and after the dealloying. Two novel findings are reported. (1) Close to the Cu/Cu2+ redox potential, a pronounced increase in current is observed in the voltammogram. While this charge transfer seems to indicate the dissolution of Cu2+, XRF analysis proves that no copper has been lost to the solution. (2) The XRF analysis shows that some gold is going into solution well below the critical potential and far below the Au/Au3+ redox potential. We assign this to stripping of gold atoms from highly unfavorable surface bonding sites.
Atomic-layer 2D crystals have unique properties that can be significantly modified through interaction with an underlying support. For epitaxial graphene on SiC(0001), the interface strongly ...influences the electronic properties of the overlaying graphene. We demonstrate a novel combination of x-ray scattering and spectroscopy for studying the complexities of such a buried interface structure. This approach employs x-ray standing wave-excited photoelectron spectroscopy in conjunction with x-ray reflectivity to produce a highly resolved chemically sensitive atomic profile for the terminal substrate bilayers, interface, and graphene layers along the SiC0001 direction.
An X-ray standing wave (XSW) is created in the overlap region of two coherent X-ray waves, e.g. by diffraction or reflection. The XSW intensity maxima move, when traversing the range of total ...reflection, causing strong modulation of the photo-excitation of a particular element or atomic species, recorded by electron or X-ray fluorescence spectroscopy. The XSW technique is a Fourier technique, particularly useful for identifying and structurally characterizing diluted, active species. In simple cases, a single XSW measurement allows characterization with pm resolution. Otherwise, employing several XSW measurements, an image can be created by Fourier inversion allowing one to identify individual sites. The principle, strength and limitations of the XSW technique are reviewed briefly and we focus on three examples for identifying active sites: catalytically active Al in scolecite, magnetically active and counter-active sites of Mn in GaMnAs and sites on the SrTiO3(001) surface active in the splitting of water.
Interface energetics are of fundamental importance in organic and molecular electronics. By combining complementary experimental techniques and first-principles calculations, we resolve the complex ...interplay among several interfacial phenomena that collectively determine the electronic structure of the strong electron acceptor tetrafluoro-tetracyanoquinodimethane chemisorbed on copper. The combination of adsorption-induced geometric distortion of the molecules, metal-to-molecule charge transfer, and molecule-to-metal back transfer leads to a net increase of the metal work function.
The structural chemistry and reactivity of 1,3,8,10‐tetraazaperopyrene (TAPP) on Cu(111) under ultra‐high‐vacuum (UHV) conditions has been studied by a combination of experimental techniques ...(scanning tunneling microscopy (STM) and X‐ray photoelectron spectroscopy, XPS) and DFT calculations. Depending on the deposition conditions, TAPP forms three main assemblies, which result from initial submonolayer coverages based on different intermolecular interactions: a close‐packed assembly similar to a projection of the bulk structure of TAPP, in which the molecules interact mainly through van der Waals (vDW) forces and weak hydrogen bonds; a porous copper surface coordination network; and covalently linked molecular chains. The Cu substrate is of crucial importance in determining the structures of the aggregates and available reaction channels on the surface, both in the formation of the porous network for which it provides the Cu atoms for surface metal coordination and in the covalent coupling of the TAPP molecules at elevated temperature. Apart from their role in the kinetics of surface transformations, the available metal adatoms may also profoundly influence the thermodynamics of transformations by coordination to the reaction product, as shown in this work for the case of the Cu‐decorated covalent poly(TAPPCu) chains.
TAPP on copper: 1,3,8,10‐Tetraazaperopyrene (TAPP) is chemically activated on a copper (111) surface, with its mobile adatoms, to form a highly ordered surface coordination network. At elevated temperatures it reacts to generate covalently coupled polymers, as illustrated. The copper substrate is crucially important in determining the structures of the aggregates and available reaction channels on the surface.
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