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Difficulties associated with the integration of liquids into a UHV environment make surface-science style studies of mineral dissolution particularly challenging. Recently, we developed a novel experimental setup for the UHV-compatible dosing of ultrapure liquid water, and studied its interaction with TiO2 and Fe3O4 surfaces. Here, we describe a simple approach to vary the pH through the partial pressure of CO2 (pCO2) in the surrounding vacuum chamber, and use this to study how these surfaces react to an acidic solution. The TiO2(110) surface is unaffected by the acidic solution, except for a small amount of carbonaceous contamination. The Fe3O4(001)-(rt2 x rt2)R45 surface begins to dissolve at a pH 4.0-3.9 (pCO2 = 0.8-1 bar) and, although it is significantly roughened, the atomic-scale structure of the Fe3O4(001) surface layer remains visible in scanning tunneling microscopy (STM) images. X-ray photoelectron spectroscopy (XPS) reveals that the surface is chemically reduced, and contains a significant accumulation of bicarbonate (HCO3-) species. These observations are consistent with Fe(II) being extracted by bicarbonate ions, leading to dissolved iron bicarbonate complexes (Fe(HCO3)2), which precipitate onto the surface when the water evaporates.

Oxygen exchange at oxide/liquid and oxide/gas interfaces is important in technology and environmental studies, as it is closely linked to both catalytic activity and material degradation. The atomic-scale details are mostly unknown, however, and are often ascribed to poorly defined defects in the crystal lattice. Here we show that even thermodynamically stable, well-ordered surfaces can be surprisingly reactive. Specifically, we show that all the 3-fold coordinated lattice oxygen atoms on a defect-free single-crystalline “r-cut” (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$1\bar{1}02$$\end{document}11¯02) surface of hematite (α-Fe2O3) are exchanged with oxygen from surrounding water vapor within minutes at temperatures below 70 °C, while the atomic-scale surface structure is unperturbed by the process. A similar behavior is observed after liquid-water exposure, but the experimental data clearly show most of the exchange happens during desorption of the final monolayer, not during immersion. Density functional theory computations show that the exchange can happen during on-surface diffusion, where the cost of the lattice oxygen extraction is compensated by the stability of an HO-HOH-OH complex. Such insights into lattice oxygen stability are highly relevant for many research fields ranging from catalysis and hydrogen production to geochemistry and paleoclimatology., Knowing how individual water molecules interact with surfaces is crucial for understanding surface and interface phenomena. Here, the authors show how local water-water interactions enable an unforeseen and surprisingly rapid mechanism of atom exchange between a common mineral and its surroundings.

The oxygen evolution reaction (OER) is thought to occur via a four-step mechanism with *O, *OH, and *OOH as adsorbed intermediates. Linear scaling of the *OH and **OOH adsorption energies is proposed to limit the oxides’ efficiency as OER catalysts, but the use of simple descriptors to screen candidate materials neglects potentially important water–water interactions. Here, we use a combination of temperature-programmed desorption (TPD), X-ray photoemission spectroscopy (XPS), noncontact atomic force microscopy (nc-AFM), and density functional theory (DFT)-based computations to show that highly stable HO–H2O dimer species form at the (1102) facet of hematite; a promising anode material for photoelectrochemical water splitting. The UHV-based results are complemented by measurements following exposure to liquid water and are consistent with prior X-ray scattering results. The presence of strongly bound water agglomerates is generally not taken into account in OER reaction schemes but may play a role in det...

HAL Id: hal-03021641 https://hal.archives-ouvertes.fr/hal-03021641, Establishing the atomic‐scale structure of metal‐oxide surfaces during electrochemical reactions is a key step to modeling this important class of electrocatalysts. Here, we demonstrate that the characteristic (√2×√2)R45° surface reconstruction formed on (001)‐oriented magnetite single crystals is maintained after immersion in 0.1 M NaOH at 0.20 V vs. Ag/AgCl and we investigate its dependence on the electrode potential. We follow the evolution of the surface using in situ and operando surface X‐ray diffraction from the onset of hydrogen evolution, to potentials deep in the oxygen evolution reaction (OER) regime. The reconstruction remains stable for hours between −0.20 and 0.60 V and, surprisingly, is still present at anodic current densities of up to 10 mA cm−2 and strongly affects the OER kinetics. We attribute this to a stabilization of the Fe3O4 bulk by the reconstructed surface. At more negative potentials, a gradual and largely irreversible lifting of the reconstruction is observed due to the onset of oxide reduction., We gratefully acknowledge financial support by AGENCIA PICT 20141415 and 2016069, EC‐MEC (ANR‐15‐CE30‐0024‐01 and DFG‐Ma1618/2020), European Research Council—European Union's Horizon 2020 (864628), Austrian Science Fund FWF (Project Z‐250 Wittgenstein Prize), and RyC program RYC‐2017‐21931 and Basque Government Project (IT‐1255‐19). D.G. acknowledges Federico López for assistance with the TOC. We thank the ESRF for the PhD fellowship for T.W. and the ID03 beamline staff—in particular H. Isern.

Establishing the atomic-scale structure of metal-oxide surfaces during electrochemical reactions is a key step to modeling this important class of electrocatalysts. Here, we demonstrate that the characteristic (√2×√2)R45° surface reconstruction formed on (001)-oriented magnetite single crystals is maintained after immersion in 0.1 M NaOH at 0.20 V vs. Ag/AgCl and we investigate its dependence on the electrode potential. We follow the evolution of the surface using in situ and operando surface X-ray diffraction from the onset of hydrogen evolution, to potentials deep in the oxygen evolution reaction (OER) regime. The reconstruction remains stable for hours between -0.20 and 0.60 V and, surprisingly, is still present at anodic current densities of up to 10 mA cm

Difficulties associated with the integration of liquids into a UHV environment make surface-science style studies of mineral dissolution particularly challenging. Recently, we developed a novel experimental setup for the UHV-compatible dosing of ultrapure liquid water and studied its interaction with TiO

In this work, the adsorption height of Ag adatoms on the Fe3O4(001) surface after exposure to CO was determined using normal incidence x-ray standing waves. The Ag adatoms bound to CO (Ag1CO) are found to be pulled out of the surface to an adsorption height of 1.15 A ± 0.08 A, compared to the previously measured height of 0.96 A ± 0.03 A for bare Ag adatoms and clusters. Utilizing DFT+vdW+U calculations with the substrate unit cell dimension fixed to the experimental value, the predicted adsorption height for Ag1CO was 1.16 A, in remarkably good agreement with the experimental results.

A preference for acids When titanium dioxide surfaces are exposed to water under ambient conditions, an ordered overlayer forms. Balajka et al. studied this process with scanning tunneling microscopy and x-ray photoelectron spectroscopy for water adsorption under vacuum conditions and in air (see the Perspective by Park). The ordered overlayer was only formed in air, the result of the adsorption of organic acids (formic and acetic acids). Although other species such as alcohols were present in much higher concentrations in air, the bidentate adsorption and entropic effects favored acid adsorption. Science , this issue p. 786 ; see also p. 753

Accurately modelling the structure of a catalyst is a fundamental prerequisite for correctly predicting reaction pathways, but a lack of clear experimental benchmarks makes it difficult to determine the optimal theoretical approach. Here, we utilize the normal incidence X-ray standing wave (NIXSW) technique to precisely determine the three dimensional geometry of Ag1 and Cu1 adatoms on Fe3O4(001). Both adatoms occupy bulk-continuation cation sites, but with a markedly different height above the surface (0.43 ± 0.03 A (Cu1) and 0.96 ± 0.03 A (Ag1)). HSE-based calculations accurately predict the experimental geometry, but the more common PBE + U and PBEsol + U approaches perform poorly.

Alkali antimonide photocathodes are capable of generating high brightness electron beams given their high quantum efficiency and low mean transverse energy (MTE). Increasing the brightness of the photoemitted electron beams beyond the current state of the art requires surface control of the photocathode at the atomic scale, since the beam brightness at the cathode is degraded by a rough, inhomogeneous surface. In this work, we grow cesium antimonide photocathodes on single crystal substrates (Al2O3, TiO2, 3C–SiC, and a control sample on Si) and study the resulting surface morphology with scanning tunneling microscopy (STM). We observe dramatic changes in surface morphology across substrates. In particular, we demonstrate 10 times larger island size and significantly reduced roughness on two samples grown on 3C–SiC(100) substrates as compared to samples on other substrates. By computing the local electric fields which these surfaces would generate in an electron accelerator source, we estimate the roughness-induced contribution to MTE. Across samples, the calculated contribution to MTE varies by a factor of 17, and the smallest value computed is 12 meV at an extraction field of 50 MV/m, which is smaller than typical values for alkali antimonides. Additionally, we show that oxidation, commonly encountered in vacuum transfer and in accelerator operation, does not affect the measured surface morphology. Our findings suggest that even in high field environments, the brightness of the photocathodes grown on 3C–SiC will be primarily determined by the material's electronic properties rather than by spurious fields generated by photocathode roughness.

Atomic-scale investigations of metal oxide surfaces exposed to aqueous environments are vital to understand degradation phenomena (e.g., dissolution and corrosion) as well as the performance of these materials in applications. Here, we utilize a new experimental setup for the ultrahigh vacuum-compatible dosing of liquids to explore the stability of the Fe

Atomic-scale investigations of metal oxide surfaces exposed to aqueous environments are vital to understand degradation phenomena (e.g., dissolution and corrosion) as well as the performance of these materials in applications. Here, we utilize a new experimental setup for the ultrahigh vacuum-compatible dosing of liquids to explore the stability of the Fe3O4(001)-(√2 × √2)R45° surface following exposure to liquid and ambient pressure water. X-ray photoelectron spectroscopy and low-energy electron diffraction data show that extensive hydroxylation causes the surface to revert to a bulklike (1 × 1) termination. However, scanning tunneling microscopy (STM) images reveal a more complex situation, with the slow growth of an oxyhydroxide phase, which ultimately saturates at approximately 40% coverage. We conclude that the new material contains OH groups from dissociated water coordinated to Fe cations extracted from subsurface layers and that the surface passivates once the surface oxygen lattice is saturated with H because no further dissociation can take place. The resemblance of the STM images to those acquired in previous electrochemical STM studies leads us to believe that a similar structure exists at the solid-electrolyte interface during immersion at pH 7.

High-performance photocathodes for many prominent particle accelerator applications, such as x-ray free-electron lasers, cannot be grown in situ. These highly reactive materials must be grown and then transported to the electron gun in an ultrahigh-vacuum (UHV) suitcase, during which time monolayer-level oxidation is unavoidable. Thin film Cs3Sb photocathodes were grown on a variety of substrates. Their performance and chemical state were measured by x-ray photoelectron spectroscopy after transport in a UHV suitcase as well as after O2-induced oxidation. The unusual chemistry of cesium oxides enabled trace amounts of oxygen to drive structural reorganization at the photocathode surface. This reorganization pulled cesium from the bulk photocathode, leading to the development of a structurally complex and O2-exposure-dependent cesium oxide layer. This oxidation-induced phase segregation led to downward band bending of at least 0.36 eV as measured from shifts in the Cs 3d5/2 binding energy. At low O2 exposures, the surface developed a low work function cesium suboxide overlayer that had little effect on quantum efficiency (QE). At somewhat higher O2 exposures, the overlayer transformed to Cs2O; no antimony or antimony oxides were observed in the near-surface region. The development of this overlayer was accompanied by a 1000-fold decrease in QE, which effectively destroyed the photocathode via the formation of a tunnel barrier. The O2 exposures necessary for degradation were quantified. As little as 100 L of O2 irreversibly damaged the photocathode. These observations are discussed in the context of the rich chemistry of alkali oxides, along with potential material strategies for photocathode improvement.

Researchers around the world have observed the formation of molecularly ordered structures of unknown origin on the surface of titanium dioxide (TiO

The normal incidence X-ray standing wave (NIXSW) technique has been used to follow the evolution of the adsorption geometry of Ni adatoms on the Fe3O4(001)-(√2 × √2)R45° surface as a function of temperature. Two primary surface region sites are identified: a bulk-continuation tetrahedral site and a sub-surface octahedral site, the latter site being preferred at higher annealing temperatures. The ease of incorporation is linked to the presence of subsurface cation vacancies in the (√2 × √2)R45° reconstruction and is consistent with the preference for octahedral coordination observed in the spinel compound NiFe2O4.\ud \ud

The rutile TiO2(011) surface exhibits a (2 × 1) reconstruction when prepared by standard techniques in ultrahigh vacuum (UHV). Here we report that a restructuring occurs upon exposing the surface to liquid water at room temperature. The experiment was performed in a dedicated UHV system, equipped for direct and clean transfer of samples between UHV and liquid environment. After exposure to liquid water, an overlayer with a (2 × 1) symmetry was observed containing two dissociated water molecules per unit cell. The two OH groups yield an apparent “c(2 × 1)” symmetry in scanning tunneling microscopy (STM) images. On the basis of STM analysis and density functional theory (DFT) calculations, this overlayer is attributed to dissociated water on top of the unreconstructed (1 × 1) surface. Investigation of possible adsorption structures and analysis of the domain boundaries in this structure provide strong evidence that the original (2 × 1) reconstruction is lifted. Unlike the (2 × 1) reconstruction, the (1 × 1) surface has an appropriate density and symmetry of adsorption sites. The possibility of contaminant-induced restructuring was excluded based on X-ray photoelectron spectroscopy (XPS) and low-energy He+ ion scattering (LEIS) measurements.

Electrochemical surface science of oxides is an emerging field with expected high impact in developing, for instance, rationally designed catalysts. The aim in such catalysts is to replace noble metals by earth-abundant elements, yet without sacrificing activity. Gaining an atomic-level understanding of such systems hinges on the use of experimental surface characterization techniques such as scanning tunneling microscopy (STM), in which tungsten tips have been the most widely used probes, both in vacuum and under electrochemical conditions. Here, we present an in situ STM study with atomic resolution that shows how tungsten(VI) oxide, spontaneously generated at a W STM tip, forms 1D adsorbates on oxide substrates. By comparing the behavior of rutile TiO2(110) and magnetite Fe3O4(001) in aqueous solution, we hypothesize that, below the point of zero charge of the oxide substrate, electrostatics causes water-soluble WO3 to efficiently adsorb and form linear chains in a self-limiting manner up to submonolayer coverage. The 1D oligomers can be manipulated and nanopatterned in situ with a scanning probe tip. As WO3 spontaneously forms under all conditions of potential and pH at the tungsten–aqueous solution interface, this phenomenon also identifies an important caveat regarding the usability of tungsten tips in electrochemical surface science of oxides and other highly adsorptive materials.

The structure of the solid-liquid interface often defines function and performance of materials in applications. To study the interface at the atomic scale, we extended an ultrahigh vacuum (UHV) surface-science chamber with an apparatus that allows to bring a sample in contact with ultrapure liquid water without exposure to air. In this process, a sample, typically a single crystal prepared and characterized in UHV, is transferred into a separate, small chamber. This chamber already contains a volume of ultrapure water ice, whose vapor pressure is reduced to UHV range by cooling it to cryogenic temperatures. Upon warming, the ice melts and forms a liquid droplet, which is deposited on the sample. First experiments carried out on rutile TiO2(110) single crystals using this apparatus exhibit unprecedented purity, as tested by X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM). These results enabled to separate the effect of pure water from the effect of low-level impurities present in the air. Other possible uses of the setup are discussed., 12 pages, 4 figures