310 results on '"Diebold, U."'
Search Results
102. Polarons in materials
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Cesare Franchini, Michele Reticcioli, Martin Setvin, Ulrike Diebold, Franchini C., Reticcioli M., Setvin M., and Diebold U.
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Condensed Matter::Quantum Gases ,Physics ,Condensed matter physics ,Polarons ,Ionic bonding ,Lattice vibration ,02 engineering and technology ,Electron ,Electronic structure ,010402 general chemistry ,021001 nanoscience & nanotechnology ,Polaron ,01 natural sciences ,0104 chemical sciences ,Surfaces, Coatings and Films ,Electronic, Optical and Magnetic Materials ,Biomaterials ,Coupling (physics) ,Polarizability ,Materials Chemistry ,Quasiparticle ,Condensed Matter::Strongly Correlated Electrons ,0210 nano-technology ,Energy (miscellaneous) - Abstract
Polarons are quasiparticles that easily form in polarizable materials due to the coupling of excess electrons or holes with ionic vibrations. These quasiparticles manifest themselves in many different ways and have a profound impact on materials properties and functionalities. Polarons have been the testing ground for the development of numerous theories, and their manifestations have been studied by many different experimental probes. This Review provides a map of the enormous amount of data and knowledge accumulated on polaron effects in materials, ranging from early studies and standard treatments to emerging experimental techniques and novel theoretical and computational approaches. Polarons — quasiparticles arising from the interaction of electrons with lattice vibrations — strongly influence materials properties. This Review provides a map of the theoretical models and experimental techniques used to study polarons in materials, presenting paradigmatic examples of different types of polarons and polaron-driven phenomena.
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- 2021
103. Unraveling CO adsorption on model single-atom catalysts
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Cesare Franchini, Matthias Meier, Ulrike Diebold, Florian Kraushofer, Gareth S. Parkinson, Michael Schmid, Roland Bliem, Jan Hulva, Zdenek Jakub, Hulva J., Meier M., Bliem R., Jakub Z., Kraushofer F., Schmid M., Diebold U., Franchini C., Parkinson G.S., ARCNL (WZI, IoP, FNWI), and IoP (FNWI)
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chemistry.chemical_classification ,Condensed Matter - Materials Science ,Multidisciplinary ,Materials science ,Materials Science (cond-mat.mtrl-sci) ,FOS: Physical sciences ,02 engineering and technology ,Electronic structure ,010402 general chemistry ,021001 nanoscience & nanotechnology ,01 natural sciences ,0104 chemical sciences ,Coordination complex ,Catalysis ,Metal ,Adsorption ,chemistry ,Chemical physics ,Single atom catalysis, DFT, CO adsorption ,visual_art ,Atom ,visual_art.visual_art_medium ,Reactivity (chemistry) ,0210 nano-technology ,Deposition (chemistry) - Abstract
Modeling single-atom reactivity Noble metals often perform best for demanding reactions such as oxygen reduction, an effect often explained by the position of their d-band. One way to minimize the cost of noble metals is to disperse them as single atoms. To model the reactivity of supported single atoms, Hulva et al. evaporated different transition metals such as nickel, silver, and iridium on an Fe 3 O 4 (001) support. Single atoms adsorbed in the same twofold site between underlying rows of surface iron atoms. In studies of CO adsorption as a proxy for reactivity, the d-band was strongly affected by the charge transfer to the support and CO-induced structural changes. These effects can weaken the adsorption energy compared with the expected values based on electronic structure alone. Science , this issue p. 375
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- 2021
104. Resolving the adsorption of molecular O 2 on the rutile TiO 2 (110) surface by noncontact atomic force microscopy
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Ulrike Diebold, Igor Sokolović, Martin Calkovský, Margareta Wagner, Michael Schmid, Cesare Franchini, Michele Reticcioli, Martin Setvin, Sokolovic I., Reticcioli M., Calkovsky M., Wagner M., Schmid M., Franchini C., Diebold U., and Setvin M.
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Multidisciplinary ,Materials science ,Oxide ,chemistry.chemical_element ,Electron ,Substrate (electronics) ,Oxygen ,O2, oxide ,TiO2, nc-AFM ,chemistry.chemical_compound ,Adsorption ,chemistry ,Chemical physics ,Rutile ,Molecule ,Density functional theory ,Tip functionalization - Abstract
Significance Molecular O 2 on semiconducting metal oxides, its adsorption, response to thermal treatment, UV irradiation, and electron/hole injection is at the heart of a wide range of technologies. These processes were studied molecule by molecule on the prototypical model oxide T i O 2 (110), using noncontact AFM. We show that the nonintrusive nature of the nc-AFM and its exceptional spatial resolution due to the functionalization of the tip with a terminal O atom allow identifying oxygen adsorption geometries which were not previously determined by other techniques. In combination with density functional theory calculations, we explain the charge states of the various adsorbed oxygen species, their dynamics upon annealing and UV irradiation, and the role of electron exchange with the T i O 2 substrate.
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- 2020
105. Rapid oxygen exchange between hematite and water vapor
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Gareth S. Parkinson, Cesare Franchini, Michael Schmid, Matthias Meier, Ulrike Diebold, Jiri Pavelec, Florian Kraushofer, Jan Balajka, Zdenek Jakub, Jakub Z., Meier M., Kraushofer F., Balajka J., Pavelec J., Schmid M., Franchini C., Diebold U., and Parkinson G.S.
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Materials science ,Science ,Oxide ,General Physics and Astronomy ,chemistry.chemical_element ,dissolution ,FOS: Physical sciences ,Crystal structure ,dissociation ,Oxygen ,General Biochemistry, Genetics and Molecular Biology ,Article ,chemistry.chemical_compound ,Desorption ,Monolayer ,surface ,Condensed Matter - Materials Science ,Multidisciplinary ,atomic-scale view ,low-temperature ,Materials Science (cond-mat.mtrl-sci) ,General Chemistry ,Hematite ,Surface chemistry ,stabilization ,CO ,Geochemistry ,chemistry ,Physical chemistry ,Chemical physics ,adsorption ,visual_art ,visual_art.visual_art_medium ,isotope-exchange ,Oxygen exchange, DFT, surface ,Density functional theory ,oxide ,Water vapor - Abstract
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.
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- 2022
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106. Reducing the In2O3(111) Surface Results in Ordered Indium Adatoms
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Diebold, U. [Technische Universitat Wien, Wiedner Hauptstrasse]
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- 2014
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107. Local Structure and Coordination Define Adsorption in a Model Ir1/Fe3O4 Single‐Atom Catalyst
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Jakub, Zdenek, Hulva, Jan, Meier, Matthias, Bliem, Roland, Kraushofer, Florian, Setvin, Martin, Schmid, Michael, Diebold, Ulrike, Franchini, Cesare, Parkinson, Gareth S., Jakub Z., Hulva J., Meier M., Bliem R., Kraushofer F., Setvin M., Schmid M., Diebold U., Franchini C., Parkinson G.S., and IoP (FNWI)
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inorganic chemicals ,Heterogeneous Catalysis ,adsorption ,single-atom catalysis ,heterogeneous catalysi ,scanning probe microscopy ,Research Articles ,Research Article - Abstract
Single‐atom catalysts (SACs) bridge homo‐ and heterogeneous catalysis because the active site is a metal atom coordinated to surface ligands. The local binding environment of the atom should thus strongly influence how reactants adsorb. Now, atomically resolved scanning‐probe microscopy, X‐ray photoelectron spectroscopy, temperature‐programmed desorption, and DFT are used to study how CO binds at different Ir1 sites on a precisely defined Fe3O4(001) support. The two‐ and five‐fold‐coordinated Ir adatoms bind CO more strongly than metallic Ir, and adopt structures consistent with square‐planar IrI and octahedral IrIII complexes, respectively. Ir incorporates into the subsurface already at 450 K, becoming inactive for adsorption. Above 900 K, the Ir adatoms agglomerate to form nanoparticles encapsulated by iron oxide. These results demonstrate the link between SAC systems and coordination complexes, and that incorporation into the support is an important deactivation mechanism., The coordination of a single atom to the oxide support has dramatic consequences on its ability to adsorb carbon monoxide. The observed motifs can be rationalized using simple arguments from coordination chemistry, confirming the hypothesis that single‐atom catalysts have much in common with organometallic complexes used in homogeneous catalysis.
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- 2019
108. Bulk and surface characterization of In2O3(001) single crystal
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Diebold, U. [Technische Universitat Wien, Wiedner Hauptstrasse]
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- 2012
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109. Sulfur on TiO{sub 2}(110) studied with resonant photoemission
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Diebold, U
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- 2001
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110. Formaldehyde Adsorption on the Anatase TiO2(101) Surface: Experimental and Theoretical Investigation
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Annabella Selloni, Gareth S. Parkinson, Ulrike Diebold, Honghong Wang, Martin Setvin, Cristiana Di Valentin, Thomas Simschitz, Michael Schmid, Jan Hulva, Setvin, M, Hulva, J, Wang, H, Simschitz, T, Schmid, M, Parkinson, G, Di Valentin, C, Selloni, A, and Diebold, U
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Anatase ,Formaldehyde ,02 engineering and technology ,010402 general chemistry ,Photochemistry ,01 natural sciences ,law.invention ,chemistry.chemical_compound ,Adsorption ,X-ray photoelectron spectroscopy ,law ,Desorption ,Monolayer ,Physical and Theoretical Chemistry ,Electronic, Optical and Magnetic Material ,021001 nanoscience & nanotechnology ,0104 chemical sciences ,Surfaces, Coatings and Films ,Electronic, Optical and Magnetic Materials ,Energy (all) ,General Energy ,chemistry ,Physical chemistry ,Density functional theory ,Scanning tunneling microscope ,0210 nano-technology - Abstract
Formaldehyde (CH2O) adsorption on the anatase TiO2(101) surface was studied with a combination of experimental and theoretical methods. Scanning tunneling microscopy, noncontact atomic force microscopy, temperature-programmed desorption, and X-ray photoelectron spectroscopy were employed on the experimental side. Density functional theory was used to calculate formaldehyde adsorption configurations and energy barriers for transitions between them. At low coverages (
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- 2017
111. Interaction of molecular oxygen with the vacuum-annealed TiO{sub 2}(110) surface: Molecular and dissociative channels
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Diebold, U [Tulane Univ., New Orleans, LA (United States). Dept. of Physics]
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- 1999
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112. The adsorption of chlorine on TiO2(<f>1 1 0</f>) studied with scanning tunneling microscopy and photoemission spectroscopy
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Hebenstreit, E.L.D., Hebenstreit, W., Geisler, H., Ventrice Jr., C.A., Hite, D.A., Sprunger, P.T., and Diebold, U.
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SYNCHROTRON radiation , *PHOTOELECTRON spectroscopy , *CHLORINE , *ADSORPTION (Chemistry) - Abstract
The adsorption of chlorine, dosed from an electrochemical cell, on the (1 1 0)-surface of a bulk-reduced
TiO2 crystal was studied with scanning tunneling microscopy, X-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy (UPS). At room temperature chlorine adsorbs on the fivefold coordinated Ti atoms and on the vacancies in the bridging oxygen rows. Chlorine exposure at 200 and 300° C leads to adsorption at the position of bridging oxygen atoms. In addition,∼ 15 A˚ wide protrusions are formed that contain several chlorine atoms. No long-range ordering was found in the adsorbed layer. UPS shows that adsorption of Cl on the hot surface leads to an increase in the Ti 3d-related defect state. This indicates a replacement of bridging oxygen atoms by chlorine. Interestingly, the rate of the site exchange depends on the bulk reduction state of theTiO2 crystal. After dosing Cl at 200° C, negligible adsorption was found on a slightly reduced, light blue crystal. The data are compared and contrasted with similar experimental results on the S/TiO2 (1 1 0) system. [Copyright &y& Elsevier]- Published
- 2002
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113. Defect chemistry of Eu dopants in NaI scintillators studied by atomically resolved force microscopy
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Martin Setvin, Michael Schmid, Igor Sokolović, Manuel Ulreich, Cesare Franchini, Lynn A. Boatner, Ulrike Diebold, Berthold Stoeger, Michele Reticcioli, Flora Poelzleitner, Ulreich M., Boatner L.A., Sokolovic I., Reticcioli M., Stoeger B., Poelzleitner F., Franchini C., Schmid M., Diebold U., and Setvin M.
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Condensed Matter - Materials Science ,Materials science ,Physics and Astronomy (miscellaneous) ,Dopant ,business.industry ,Physics::Instrumentation and Detectors ,Materials Science (cond-mat.mtrl-sci) ,FOS: Physical sciences ,02 engineering and technology ,Scintillator ,021001 nanoscience & nanotechnology ,01 natural sciences ,0103 physical sciences ,Microscopy ,DFT, STM, scintillator, NaI ,Optoelectronics ,General Materials Science ,010306 general physics ,0210 nano-technology ,business - Abstract
Activator impurities and their distribution in the host lattice play a key role in scintillation phenomena. Here a combination of cross-sectional noncontact atomic force microscopy, x-ray photoelectron spectroscopy, and density-functional theory were used to study the distribution of Eu2+ dopants in a NaI scintillator activated by 3% EuI2. A variety of Eu-based structures were identified in crystals subjected to different postgrowth treatments. Transparent crystals with good scintillation properties contained mainly small precipitates with a cubic crystal structure and a size below 4 nm. Upon annealing, Eu segregated toward the surface, resulting in the formation of an ordered hexagonal overlayer with a EuI2 composition and a pronounced, unidirectional moiré pattern. Crystals with poor optical transparency showed a significant degree of mosaicity and the presence of precipitates. All investigated crystals contained a very low concentration of Eu dopants present as isolated point defects; most of the europium was incorporated in larger structures.
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- 2018
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114. Imaging complex model catalysts in action: From surface science towards industrial practice using high-pressure scanning tunneling microscopy
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Mom, R.V., Frenken, J.W.M., Groot, I.M.N., Diebold, U., Lauritsen, J.V., Koper, M.T.M., Eliel, E.R., and Leiden University
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Surface science ,High-pressure scanning tunneling microscopy ,Operando ,Catalysis - Abstract
Finding a new catalyst is no easy task, especially since our understanding of catalysts at the atomic level is still lacking. In this thesis, a step is made to combine model catalysts that we do understand with realistic industrial conditions. This methodology comprises both the preparation of complex model catalysts and the development of new instrumentation. The model catalysts under study were MoO3 on Al2O3/NiAl(110), MoS2 on Au(111) and AuOx/WO3/ReO3 on Au(111). For MoO3, it is shown that the O2 pressure during physical vapor deposition preparation affects the particle dispersion, allowing for tuning of the structural properties of the model catalyst. For MoS2, the aim was to image the atomic structure of the active sites during the hydrodesulfurization reaction. To achieve this, an in-house developped high-pressure scanning tunneling microscope was modified to increase its corrosion resistance. Thus, it was possible to show that hydrocarbons can play a key role in determining the dominant active site structure of the MoS2 catalyst. Using the same microscope, gold oxide particles were imaged on Au(111). From our images and simple thermodynamic considerations, we determined that these particles are suprisingly stable. Finally, new methodology was developped to provide chemical contrast to high-pressure scanning tunneling microscopy.
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- 2017
115. Coexistence of trapped and free excess electrons inSrTiO3
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Xianfeng Hao, Cesare Franchini, Ulrike Diebold, Zhiming Wang, Michael Schmid, Hao XF, Wang ZM, Schmid M, Diebold U, and Franchini C
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Physics ,Condensed Matter - Materials Science ,Strongly Correlated Electrons (cond-mat.str-el) ,Condensed matter physics ,Materials Science (cond-mat.mtrl-sci) ,FOS: Physical sciences ,Electron ,Type (model theory) ,Condensed Matter Physics ,Polaron ,Electronic, Optical and Magnetic Materials ,Condensed Matter - Strongly Correlated Electrons ,Delocalized electron ,physics, polaron, oxides ,Coulomb ,Density functional theory ,Atomic physics ,Electronic properties - Abstract
The question whether excess electrons in SrTiO3 form free or trapped carriers is a crucial aspect for the electronic properties of this important material. This fundamental ambiguity prevents a consistent interpretation of the puzzling experimental situation, where results support one or the other scenario depending on the type of experiment that is conducted. Using density functional theory corrected with an on-site Coulomb interaction U, it is established that excess electrons form small polarons if the electron density is higher than ~ 1%. For densities below this critical value, the electrons stay delocalized or become large polarons. Our modeling of oxygen deficient SrTiO3 provides an alternative picture with respect to previous theoretical studies endowing firm evidence for the observed, temperature-driven metal-insulator transition in samples with homogeneously distributed oxygen vacancies. Small polarons confined to Ti(3+) sites are immobile at low temperature but can be thermally activated into a conductive state., Comment: 5 pages, 3 figures
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- 2015
116. Anisotropic two-dimensional electron gas at SrTiO3(110)
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Z. Wang, Z. Zhong, X. Hao, S. Gerhold, B. Stxf6ger, M. Schmid, J. Sxe1nchez-Barriga, A. Varykhalov, C. Franchini, K. Held, U. Diebold, Wang ZM, Zhong ZC, Hao XF, Gerhold S, Stoger B, Schmid M, Sanchez-Barriga J, Varykhalov A, Franchini C, Held K, and Diebold U
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2DEG, oxides, surface, electronic properties, physics, chemistry - Abstract
Two-dimensional electron gases (2DEGs) at oxide heterostructures are attracting considerable attention, as these might one day substitute conventional semiconductors at least for some functionalities. Here we present a minimal setup for such a 2DEG-the SrTiO3(110)-(4 x 1) surface, natively terminated with one monolayer of tetrahedrally coordinated titania. Oxygen vacancies induced by synchrotron radiation migrate underneath this overlayer; this leads to a confining potential and electron doping such that a 2DEG develops. Our angle-resolved photoemission spectroscopy and theoretical results show that confinement along (110) is strikingly different from the (001) crystal orientation. In particular, the quantized subbands show a surprising "semiheavy" band, in contrast with the analog in the bulk, and a high electronic anisotropy. This anisotropy and even the effective mass of the (110) 2DEG is tunable by doping, offering a high flexibility to engineer the properties of this system.
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- 2014
117. Charge trapping at the step edges of TiO(2) anatase (101)
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Martin Setvin, Georg Kresse, Xianfeng Hao, Michael Schmid, Gareth S. Parkinson, Jiri Pavelec, Cesare Franchini, Zbynek Novotny, Ulrike Diebold, Benjamin Daniel, Setvin M, Hao XF, Daniel B, Pavelec J, Novotny Z, Parkinson GS, Schmid M, Kresse G, Franchini C, and Diebold U
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Condensed Matter - Materials Science ,Anatase ,Materials science ,Materials Science (cond-mat.mtrl-sci) ,FOS: Physical sciences ,Charge (physics) ,General Chemistry ,Trapping ,Electron ,Catalysis ,law.invention ,Adsorption ,surface, polaron, semiconductor, physics ,law ,Chemical physics ,Density functional theory ,Scanning tunneling microscope ,Spectroscopy - Abstract
A combination of photoemission, atomic force, and scanning tunneling microscopy/spectroscopy measurements shows that excess electrons in the TiO2 anatase (101) surface are trapped at step edges. Consequently, steps act as preferred adsorption sites for O-2. In density functional theory calculations electrons localize at clean step edges, this tendency is enhanced by O vacancies and hydroxylation. The results show the importance of defects for the wide-ranging applications of titania.
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- 2013
118. Hybrid exchange density functional study of vicinal anatase TiO2 surfaces.
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Sanches, F. F., Mallia, G., Liborio, L., Diebold, U., and Harrison, N. M.
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DENSITY functional theory , *TITANIUM dioxide , *METALLIC surfaces , *PHOTOCATALYSIS , *WATER electrolysis , *MOLECULAR structure - Abstract
The observation of photocatalytic water splitting on the surface of anatase TiO2 crystals has stimulated many investigations of the underlying processes. Nevertheless, a molecular-level understanding of the reaction is not available. This requires knowledge of the crystal facets present, the atomistic structure of the surfaces, and thus the reaction sites involved. In this paper we establish the atomistic structure of two surfaces, vicinal to the low-energy (101) surface. We compute the relative stability and electronic properties of the (514) and (516) surfaces and compare these to the low-index (101), (001), and (100) surfaces. The (516) surface is remarkably stable, and is predicted to contribute significantly to the surface area of a crystallite in equilibrium. We simulate constant current scanning tunneling microscopy images and, by comparing with those measured, we conclude that a surface previously observed in a miscut single crystal is the (516) surface described here. The computed stability of this surface indicates that it will be present in TiO2 nanostructures and the relative positions of its band edges suggests that it will play a significant role in the water-TiO2 reactions in solar water splitting. [ABSTRACT FROM AUTHOR]
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- 2014
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119. Digging Its Own Site: Linear Coordination Stabilizes a Pt 1 /Fe 2 O 3 Single-Atom Catalyst.
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Rafsanjani-Abbasi A, Buchner F, Lewis FJ, Puntscher L, Kraushofer F, Sombut P, Eder M, Pavelec J, Rheinfrank E, Franceschi G, Birschitzky V, Riva M, Franchini C, Schmid M, Diebold U, Meier M, Madsen GKH, and Parkinson GS
- Abstract
Determining the local coordination of the active site is a prerequisite for the reliable modeling of single-atom catalysts (SACs). Obtaining such information is difficult on powder-based systems and much emphasis is placed on density functional theory computations based on idealized low-index surfaces of the support. In this work, we investigate how Pt atoms bind to the (11̅02) facet of α-Fe
2 O3 ; a common support material in SACs. Using a combination of scanning tunneling microscopy, X-ray photoelectron spectroscopy, and an extensive computational evolutionary search, we find that Pt atoms significantly reconfigure the support lattice to facilitate a pseudolinear coordination to surface oxygen atoms. Despite breaking three surface Fe-O bonds, this geometry is favored by 0.84 eV over the best configuration involving an unperturbed support. We suggest that the linear O-Pt-O configuration is common in reactive Pt-based SAC systems because it balances thermal stability with the ability to adsorb reactants from the gas phase. Moreover, we conclude that extensive structural searches are necessary to determine realistic active site geometries in single-atom catalysis.- Published
- 2024
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120. Quantitative Measurement of Cooperative Binding in Partially Dissociated Water Dimers at the Hematite "R-Cut" Surface.
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Ryan PTP, Sombut P, Rafsanjani-Abbasi A, Wang C, Eratam F, Goto F, Franchini C, Diebold U, Meier M, Duncan DA, and Parkinson GS
- Abstract
Water-solid interfaces pervade the natural environment and modern technology. On some surfaces, water-water interactions induce the formation of partially dissociated interfacial layers; understanding why is important to model processes in catalysis or mineralogy. The complexity of the partially dissociated structures often makes it difficult to probe them quantitatively. Here, we utilize normal incidence X-ray standing waves (NIXSW) to study the structure of partially dissociated water dimers (H
2 O-OH) at the α-Fe2 O3 (012) surface (also called the (11̅02) or "R-cut" surface): a system simple enough to be tractable yet complex enough to capture the essential physics. We find the H2 O and terminal OH groups to be the same height above the surface within experimental error (1.45 ± 0.04 and 1.47 ± 0.02 Å, respectively), in line with DFT-based calculations that predict comparable Fe-O bond lengths for both water and OH species. This result is understood in the context of cooperative binding, where the formation of the H-bond between adsorbed H2 O and OH induces the H2 O to bind more strongly and the OH to bind more weakly compared to when these species are isolated on the surface. The surface OH formed by the liberated proton is found to be in plane with a bulk truncated (012) surface (-0.01 ± 0.02 Å). DFT calculations based on various functionals correctly model the cooperative effect but overestimate the water-surface interaction., Competing Interests: The authors declare no competing financial interest., (© 2024 The Authors. Published by American Chemical Society.)- Published
- 2024
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121. Exploring inhomogeneous surfaces: Ti-rich SrTiO 3 (110) reconstructions via active learning.
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Wanzenböck R, Heid E, Riva M, Franceschi G, Imre AM, Carrete J, Diebold U, and Madsen GKH
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The investigation of inhomogeneous surfaces, where various local structures coexist, is crucial for understanding interfaces of technological interest, yet it presents significant challenges. Here, we study the atomic configurations of the (2 × m ) Ti-rich surfaces at (110)-oriented SrTiO
3 by bringing together scanning tunneling microscopy and transferable neural-network force fields combined with evolutionary exploration. We leverage an active learning methodology to iteratively extend the training data as needed for different configurations. Training on only small well-known reconstructions, we are able to extrapolate to the complicated and diverse overlayers encountered in different regions of the inhomogeneous SrTiO3 (110)-(2 × m ) surface. Our machine-learning-backed approach generates several new candidate structures, in good agreement with experiment and verified using density functional theory. The approach could be extended to other complex metal oxides featuring large coexisting surface reconstructions., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)- Published
- 2024
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122. Stoichiometric reconstruction of the Al 2 O 3 (0001) surface.
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Hütner JI, Conti A, Kugler D, Mittendorfer F, Kresse G, Schmid M, Diebold U, and Balajka J
- Abstract
Macroscopic properties of materials stem from fundamental atomic-scale details, yet for insulators, resolving surface structures remains a challenge. We imaged the basal (0001) plane of α-aluminum oxide (α-Al
2 O3 ) using noncontact atomic force microscopy with an atomically defined tip apex. The surface formed a complex ([Formula: see text] × [Formula: see text]) R ±9° reconstruction. The lateral positions of the individual oxygen and aluminum surface atoms come directly from experiment; we determined with computational modeling how these connect to the underlying crystal bulk. Before the restructuring, the surface Al atoms assume an unfavorable, threefold planar coordination; the reconstruction allows a rehybridization with subsurface O that leads to a substantial energy gain. The reconstructed surface remains stoichiometric, Al2 O3 .- Published
- 2024
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123. A Multitechnique Study of C 2 H 4 Adsorption on a Model Single-Atom Rh 1 Catalyst.
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Wang C, Sombut P, Puntscher L, Ulreich M, Pavelec J, Rath D, Balajka J, Meier M, Schmid M, Diebold U, Franchini C, and Parkinson GS
- Abstract
Single-atom catalysts are potentially ideal model systems to investigate structure-function relationships in catalysis if the active sites can be uniquely determined. In this work, we study the interaction of C
2 H4 with a model Rh/Fe3 O4 (001) catalyst that features 2-, 5-, and 6-fold coordinated Rh adatoms, as well as Rh clusters. Using multiple surface-sensitive techniques in combination with calculations of density functional theory (DFT), we follow the thermal evolution of the system and disentangle the behavior of the different species. C2 H4 adsorption is strongest at the 2-fold coordinated Rh1 with a DFT-determined adsorption energy of -2.26 eV. However, desorption occurs at lower temperatures than expected because the Rh migrates into substitutional sites within the support, where the molecule is more weakly bound. The adsorption energy at the 5-fold coordinated Rh sites is predicated to be -1.49 eV, but the superposition of this signal with that from small Rh clusters and additional heterogeneity leads to a broad C2 H4 desorption shoulder in TPD above room temperature., Competing Interests: The authors declare no competing financial interest., (© 2024 The Authors. Published by American Chemical Society.)- Published
- 2024
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124. Infrared reflection absorption spectroscopy setup with incidence angle selection for surfaces of non-metals.
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Rath D, Mikerásek V, Wang C, Eder M, Schmid M, Diebold U, Parkinson GS, and Pavelec J
- Abstract
Infrared Reflection Absorption Spectroscopy (IRAS) on dielectric single crystals is challenging because the optimal incidence angles for light-adsorbate interaction coincide with regions of low IR reflectivity. Here, we introduce an optimized IRAS setup that maximizes the signal-to-noise ratio for non-metals. This is achieved by maximizing light throughput and by selecting optimal incidence angles that directly impact the peak heights in the spectra. The setup uses a commercial Fourier transform infrared spectrometer and is usable in ultra-high vacuum (UHV). Specifically, the optical design features sample illumination and collection mirrors with a high numerical aperture inside the UHV system and adjustable apertures to select the incidence angle range on the sample. This is important for p-polarized measurements on dielectrics because the peaks in the spectra reverse the direction at the Brewster angle (band inversion). The system components are connected precisely via a single flange, ensuring long-term stability. We studied the signal-to-noise ratio (SNR) variation in p-polarized IRAS spectra for one monolayer of CO on TiO2(110) as a function of incidence angle range, where a maximum SNR of 70 was achieved at 4 cm-1 resolution in a measurement time of 5 min. The capabilities for s polarization are demonstrated by measuring one monolayer D2O adsorbed on a TiO2(110) surface, where a SNR of 65 was achieved at a peak height ΔR/R0 of 1.4 × 10-4 in 20 min., (© 2024 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).)
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- 2024
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125. NH3 adsorption and competition with H2O on a hydroxylated aluminosilicate surface.
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Franceschi G, Conti A, Lezuo L, Abart R, Mittendorfer F, Schmid M, and Diebold U
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The interaction between ammonia (NH3) and (alumino)silicates is of fundamental and applied importance, yet the specifics of NH3 adsorption on silicate surfaces remain largely unexplored, mainly because of experimental challenges related to their electrically insulating nature. An example of this knowledge gap is evident in the context of ice nucleation on silicate dust, wherein the role of NH3 for ice nucleation remains debated. This study explores the fundamentals of the interaction between NH3 and microcline feldspar (KAlSi3O8), a common aluminosilicate with outstanding ice nucleation abilities. Atomically resolved non-contact atomic force microscopy, x-ray photoelectron spectroscopy, and density functional theory-based calculations elucidate the adsorption geometry of NH3 on the lowest-energy surface of microcline, the (001) facet, and its interplay with surface hydroxyls and molecular water. NH3 and H2O are found to adsorb molecularly in the same adsorption sites, creating H-bonds with the proximate surface silanol (Si-OH) and aluminol (Al-OH) groups. Despite the closely matched adsorption energies of the two molecules, NH3 readily yields to replacement by H2O, challenging the notion that ice nucleation on microcline proceeds via the creation of an ordered H2O layer atop pre-adsorbed NH3 molecules., (© 2024 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).)
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- 2024
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126. High-κ Wide-Gap Layered Dielectric for Two-Dimensional van der Waals Heterostructures.
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Söll A, Lopriore E, Ottesen A, Luxa J, Pasquale G, Sturala J, Hájek F, Jarý V, Sedmidubský D, Mosina K, Sokolović I, Rasouli S, Grasser T, Diebold U, Kis A, and Sofer Z
- Abstract
van der Waals heterostructures of two-dimensional materials have unveiled frontiers in condensed matter physics, unlocking unexplored possibilities in electronic and photonic device applications. However, the investigation of wide-gap, high-κ layered dielectrics for devices based on van der Waals structures has been relatively limited. In this work, we demonstrate an easily reproducible synthesis method for the rare-earth oxyhalide LaOBr, and we exfoliate it as a 2D layered material with a measured static dielectric constant of 9 and a wide bandgap of 5.3 eV. Furthermore, our research demonstrates that LaOBr can be used as a high-κ dielectric in van der Waals field-effect transistors with high performance and low interface defect concentrations. Additionally, it proves to be an attractive choice for electrical gating in excitonic devices based on 2D materials. Our work demonstrates the versatile realization and functionality of 2D systems with wide-gap and high-κ van der Waals dielectric environments.
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- 2024
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127. CO-Induced Dimer Decay Responsible for Gem-Dicarbonyl Formation on a Model Single-Atom Catalyst.
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Wang C, Sombut P, Puntscher L, Jakub Z, Meier M, Pavelec J, Bliem R, Schmid M, Diebold U, Franchini C, and Parkinson GS
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The ability to coordinate multiple reactants at the same active site is important for the wide-spread applicability of single-atom catalysis. Model catalysts are ideal to investigate the link between active site geometry and reactant binding, because the structure of single-crystal surfaces can be precisely determined, the adsorbates imaged by scanning tunneling microscopy (STM), and direct comparisons made to density functional theory. In this study, we follow the evolution of Rh
1 adatoms and minority Rh2 dimers on Fe3 O4 (001) during exposure to CO using time-lapse STM at room temperature. CO adsorption at Rh1 sites results exclusively in stable Rh1 CO monocarbonyls, because the Rh atom adapts its coordination to create a stable pseudo-square planar environment. Rh1 (CO)2 gem-dicarbonyl species are also observed, but these form exclusively through the breakup of Rh2 dimers via an unstable Rh2 (CO)3 intermediate. Overall, our results illustrate how minority species invisible to area-averaging spectra can play an important role in catalytic systems, and show that the decomposition of dimers or small clusters can be an avenue to produce reactive, metastable configurations in single-atom catalysis., (© 2024 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)- Published
- 2024
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128. Dynamics and nano-rheology of interfacial water: general discussion.
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Advincula XR, Blow KE, Bonn M, Bui AT, Cheng Y, Cox SJ, Della Pia F, Diebold U, Fumagalli L, Goel G, Hayton JA, Jiang Y, Kapil V, Kavokine N, Koga K, Laage D, Lahav M, Miao S, Michaelides A, Montero de Hijes P, Morgenstern K, Mukherjee T, O'Neill N, Pan D, Piaggi PM, Rempe SLB, Salvalaglio M, Salzmann CG, Sayer T, Shepelenko M, Sosso GC, Wang S, Webber B, Willard AP, and Yao Y
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- 2024
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129. Ice interfaces: general discussion.
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Advincula XR, Backus EHG, Bartels-Rausch T, Benaglia S, Ben Ari G, Blow KE, Bonn M, Bui AT, Cox SJ, Della Pia F, Diebold U, Finney AR, Franceschi G, Fumagalli L, Goel G, Hayton JA, Holdship C, Jiang Y, Jin D, Kapil V, Kavokine N, Koga K, Laage D, Lahav M, Miao S, Michaelides A, Mohandas N, Morgenstern K, Mukherjee T, Nagata Y, Olvera de la Cruz M, Pan D, Piaggi PM, Rempe SLB, Ryan P, Salzmann CG, Sayer T, Saykally RJ, Shepelenko M, Sosso GC, Whale TF, White JJ, Willard AP, and Zhang P
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- 2024
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130. Interaction of surface cations of cleaved mica with water in vapor and liquid forms.
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Franceschi G, Brandstetter S, Balajka J, Sokolović I, Pavelec J, Setvín M, Schmid M, and Diebold U
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Natural minerals contain ions that become hydrated when they come into contact with water in vapor and liquid forms. Muscovite mica - a common phyllosilicate with perfect cleavage planes - is an ideal system to investigate the details of ion hydration. The cleaved mica surface is decorated by an array of K
+ ions that can be easily exchanged with other ions or protons when immersed in an aqueous solution. Despite the vast interest in the atomic-scale hydration processes of these K+ ions, experimental data under controlled conditions have remained elusive. Here, atomically resolved non-contact atomic force microscopy (nc-AFM) is combined with X-ray photoelectron spectroscopy (XPS) to investigate the cation hydration upon dosing water vapor at 100 K in ultra-high vacuum (UHV). The cleaved surface is further exposed to ultra-clean liquid water at room temperature, which promotes ion mobility and partial ion-to-proton substitution. The results offer the first direct experimental views of the interaction of water with muscovite mica under UHV. The findings are in line with previous theoretical predictions.- Published
- 2024
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131. Electrified/charged aqueous interfaces: general discussion.
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Advincula XR, Backus EHG, Bonn M, Cox SJ, Diebold U, Fellows A, Finney AR, Goel G, Hedley J, Jiang Y, Jin D, Kapil V, Kavokine N, Klein J, Laage D, Mohandas N, Morgenstern K, Mukherjee T, Olvera de la Cruz M, Orlikowska-Rzeznik H, Perkin S, Piaggi PM, Rodellar CG, Ryan P, Sayer T, Seyffertitz M, Shepelenko M, Sosso GC, Thämer M, Vilangottunjalil A, Walker-Gibbons R, Wang Y, Willard AP, and Zhang P
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- 2024
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132. How Water Binds to Microcline Feldspar (001).
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Franceschi G, Conti A, Lezuo L, Abart R, Mittendorfer F, Schmid M, and Diebold U
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Microcline feldspar (KAlSi
3 O8 ) is a common mineral with important roles in Earth's ecological balance. It participates in carbon, potassium, and water cycles, contributing to CO2 sequestration, soil formation, and atmospheric ice nucleation. To understand the fundamentals of these processes, it is essential to establish microcline's surface atomic structure and its interaction with the omnipresent water molecules. This work presents atomic-scale results on microcline's lowest-energy surface and its interaction with water, combining ultrahigh vacuum investigations by noncontact atomic force microscopy and X-ray photoelectron spectroscopy with density functional theory calculations. An ordered array of hydroxyls bonded to silicon or aluminum readily forms on the cleaved surface at room temperature. The distinct proton affinities of these hydroxyls influence the arrangement and orientation of the first water molecules binding to the surface, holding potential implications for the subsequent condensation of water.- Published
- 2024
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133. Evolution of the surface atomic structure of multielement oxide films: curse or blessing?
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Franceschi G, Heller R, Schmid M, Diebold U, and Riva M
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Atomically resolved scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) are used to gain atomic-scale insights into the heteroepitaxy of lanthanum-strontium manganite (LSMO, La
1- x Srx MnO3- δ , x ≈ 0.2) on SrTiO3 (110). LSMO is a perovskite oxide characterized by several composition-dependent surface reconstructions. The flexibility of the surface allows it to incorporate nonstoichiometries during growth, which causes the structure of the surface to evolve accordingly. This happens up to a critical point, where phase separation occurs, clusters rich in the excess cations form at the surface, and films show a rough morphology. To limit the nonstoichiometry introduced by non-optimal growth conditions, it proves useful to monitor the changes in surface atomic structures as a function of the PLD parameters and tune the latter accordingly., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)- Published
- 2023
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134. Effect of Different In 2 O 3 (111) Surface Terminations on CO 2 Adsorption.
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Gericke SM, Kauppinen MM, Wagner M, Riva M, Franceschi G, Posada-Borbón A, Rämisch L, Pfaff S, Rheinfrank E, Imre AM, Preobrajenski AB, Appelfeller S, Blomberg S, Merte LR, Zetterberg J, Diebold U, Grönbeck H, and Lundgren E
- Abstract
In
2 O3 -based catalysts have shown high activity and selectivity for CO2 hydrogenation to methanol; however, the origin of the high performance of In2 O3 is still unclear. To elucidate the initial steps of CO2 hydrogenation over In2 O3 , we have combined X-ray photoelectron spectroscopy and density functional theory calculations to study the adsorption of CO2 on the In2 O3 (111) crystalline surface with different terminations, namely, the stoichiometric, reduced, and hydroxylated surface. The combined approach confirms that the reduction of the surface results in the formation of In adatoms and that water dissociates on the surface at room temperature. A comparison of the experimental spectra and the computed core-level shifts (using methanol and formic acid as benchmark molecules) suggests that CO2 adsorbs as a carbonate on all three surface terminations. We find that the adsorption of CO2 is hindered by hydroxyl groups on the hydroxylated surface.- Published
- 2023
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135. A Multitechnique Study of C 2 H 4 Adsorption on Fe 3 O 4 (001).
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Puntscher L, Sombut P, Wang C, Ulreich M, Pavelec J, Rafsanjani-Abbasi A, Meier M, Lagin A, Setvin M, Diebold U, Franchini C, Schmid M, and Parkinson GS
- Abstract
The adsorption/desorption of ethene (C
2 H4 ), also commonly known as ethylene, on Fe3 O4 (001) was studied under ultrahigh vacuum conditions using temperature-programmed desorption (TPD), scanning tunneling microscopy, X-ray photoelectron spectroscopy, and density functional theory (DFT)-based computations. To interpret the TPD data, we have employed a new analysis method based on equilibrium thermodynamics. C2 H4 adsorbs intact at all coverages and interacts most strongly with surface defects such as antiphase domain boundaries and Fe adatoms. On the regular surface, C2 H4 binds atop surface Fe sites up to a coverage of 2 molecules per (√2 × √2)R45° unit cell, with every second Fe occupied. A desorption energy of 0.36 eV is determined by analysis of the TPD spectra at this coverage, which is approximately 0.1-0.2 eV lower than the value calculated by DFT + U with van der Waals corrections. Additional molecules are accommodated in between the Fe rows. These are stabilized by attractive interactions with the molecules adsorbed at Fe sites. The total capacity of the surface for C2 H4 adsorption is found to be close to 4 molecules per (√2 × √2)R45° unit cell., Competing Interests: The authors declare no competing financial interest., (© 2023 The Authors. Published by American Chemical Society.)- Published
- 2023
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136. Correction to "Why and How Savitzky-Golay Filters Should Be Replaced".
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Schmid M, Rath D, and Diebold U
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[This corrects the article DOI: 10.1021/acsmeasuresciau.1c00054.]., (© 2023 The Authors. Published by American Chemical Society.)
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- 2023
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137. Oxygen-Terminated (1 × 1) Reconstruction of Reduced Magnetite Fe 3 O 4 (111).
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Kraushofer F, Meier M, Jakub Z, Hütner J, Balajka J, Hulva J, Schmid M, Franchini C, Diebold U, and Parkinson GS
- Abstract
The (111) facet of magnetite (Fe
3 O4 ) has been studied extensively by experimental and theoretical methods, but controversy remains regarding the structure of its low-energy surface terminations. Using density functional theory (DFT) computations, we demonstrate three reconstructions that are more favorable than the accepted Feoct2 termination under reducing conditions. All three structures change the coordination of iron in the kagome Feoct1 layer to be tetrahedral. With atomically resolved microscopy techniques, we show that the termination that coexists with the Fetet1 termination consists of tetrahedral iron capped by 3-fold coordinated oxygen atoms. This structure explains the inert nature of the reduced patches.- Published
- 2023
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138. Resolving the intrinsic short-range ordering of K + ions on cleaved muscovite mica.
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Franceschi G, Kocán P, Conti A, Brandstetter S, Balajka J, Sokolović I, Valtiner M, Mittendorfer F, Schmid M, Setvín M, and Diebold U
- Abstract
Muscovite mica, KAl
2 (Si3 Al)O10 (OH)2 , is a common layered phyllosilicate with perfect cleavage planes. The atomically flat surfaces obtained through cleaving lend themselves to scanning probe techniques with atomic resolution and are ideal to model minerals and clays. Despite the importance of the cleaved mica surfaces, several questions remain unresolved. It is established that K+ ions decorate the cleaved surface, but their intrinsic ordering - unaffected by the interaction with the environment - is not known. This work presents clear images of the K+ distribution of cleaved mica obtained with low-temperature non-contact atomic force microscopy (AFM) under ultra-high vacuum (UHV) conditions. The data unveil the presence of short-range ordering, contrasting previous assumptions of random or fully ordered distributions. Density functional theory (DFT) calculations and Monte Carlo simulations show that the substitutional subsurface Al3+ ions have an important role for the surface K+ ion arrangement., (© 2023. The Author(s).)- Published
- 2023
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139. Water Structures Reveal Local Hydrophobicity on the In 2 O 3 (111) Surface.
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Chen H, Blatnik MA, Ritterhoff CL, Sokolović I, Mirabella F, Franceschi G, Riva M, Schmid M, Čechal J, Meyer B, Diebold U, and Wagner M
- Abstract
Clean oxide surfaces are generally hydrophilic. Water molecules anchor at undercoordinated surface metal atoms that act as Lewis acid sites, and they are stabilized by H bonds to undercoordinated surface oxygens. The large unit cell of In
2 O3 (111) provides surface atoms in various configurations, which leads to chemical heterogeneity and a local deviation from this general rule. Experiments (TPD, XPS, nc-AFM) agree quantitatively with DFT calculations and show a series of distinct phases. The first three water molecules dissociate at one specific area of the unit cell and desorb above room temperature. The next three adsorb as molecules in the adjacent region. Three more water molecules rearrange this structure and an additional nine pile up above the OH groups. Despite offering undercoordinated In and O sites, the rest of the unit cell is unfavorable for adsorption and remains water-free. The first water layer thus shows ordering into nanoscopic 3D water clusters separated by hydrophobic pockets.- Published
- 2022
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140. Analysis of Temperature-Programmed Desorption via Equilibrium Thermodynamics.
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Schmid M, Parkinson GS, and Diebold U
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Temperature-programmed desorption (TPD) experiments in surface science are usually analyzed using the Polanyi-Wigner equation and/or transition-state theory. These methods are far from straightforward, and the determination of the pre-exponential factor is often problematic. We present a different method based on equilibrium thermodynamics, which builds on an approach previously used for TPD by Kreuzer et al. ( Surf. Sci. 1988 ). Equations for the desorption rate are presented for three different types of surface-adsorbate interactions: (i) a 2D ideal hard-sphere gas with a negligible diffusion barrier, (ii) an ideal lattice gas, that is, fixed adsorption sites without interaction between the adsorbates, and (iii) a lattice gas with a distribution of (site-dependent) adsorption energies. We show that the coverage dependence of the sticking coefficient for adsorption at the desorption temperature determines whether the desorption process can be described by first- or second-order kinetics. The sticking coefficient at the desorption temperature must also be known for a quantitative determination of the adsorption energy, but it has a rather weak influence (like the pre-exponential factor in a traditional TPD analysis). Quantitative analysis is also influenced by the vibrational contributions to the energy and entropy. For the case of a single adsorption energy, we provide equations to directly convert peak temperatures into adsorption energies. These equations also provide an approximation of the desorption energy in cases that cannot be described by a fixed pre-exponential factor. For the case of a distribution of adsorption energies, the desorption spectra cannot be considered a superposition of desorption spectra corresponding to the different energies. Nevertheless, we present a method to extract the distribution of adsorption energies from TPD spectra, and we rationalize the energy resolution of TPD experiments. The analytical results are complemented by a program for simulation and analysis of TPD data., Competing Interests: The authors declare no competing financial interest., (© 2022 The Authors. Published by American Chemical Society.)
- Published
- 2022
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141. Surface chemistry on a polarizable surface: Coupling of CO with KTaO 3 (001).
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Wang Z, Reticcioli M, Jakub Z, Sokolović I, Meier M, Boatner LA, Schmid M, Parkinson GS, Diebold U, Franchini C, and Setvin M
- Abstract
Polarizable materials attract attention in catalysis because they have a free parameter for tuning chemical reactivity. Their surfaces entangle the dielectric polarization with surface polarity, excess charge, and orbital hybridization. How this affects individual adsorbed molecules is shown for the incipient ferroelectric perovskite KTaO
3 . This intrinsically polar material cleaves along (001) into KO- and TaO2 -terminated surface domains. At TaO2 terraces, the polarity-compensating excess electrons form a two-dimensional electron gas and can also localize by coupling to ferroelectric distortions. TaO2 terraces host two distinct types of CO molecules, adsorbed at equivalent lattice sites but charged differently as seen in atomic force microscopy/scanning tunneling microscopy. Temperature-programmed desorption shows substantially stronger binding of the charged CO; in density functional theory calculations, the excess charge favors a bipolaronic configuration coupled to the CO. These results pinpoint how adsorption states couple to ferroelectric polarization.- Published
- 2022
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142. Competing electronic states emerging on polar surfaces.
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Reticcioli M, Wang Z, Schmid M, Wrana D, Boatner LA, Diebold U, Setvin M, and Franchini C
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Excess charge on polar surfaces of ionic compounds is commonly described by the two-dimensional electron gas (2DEG) model, a homogeneous distribution of charge, spatially-confined in a few atomic layers. Here, by combining scanning probe microscopy with density functional theory calculations, we show that excess charge on the polar TaO
2 termination of KTaO3 (001) forms more complex electronic states with different degrees of spatial and electronic localization: charge density waves (CDW) coexist with strongly-localized electron polarons and bipolarons. These surface electronic reconstructions, originating from the combined action of electron-lattice interaction and electronic correlation, are energetically more favorable than the 2DEG solution. They exhibit distinct spectroscopy signals and impact on the surface properties, as manifested by a local suppression of ferroelectric distortions., (© 2022. The Author(s).)- Published
- 2022
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143. Structure of an Ultrathin Oxide on Pt 3 Sn(111) Solved by Machine Learning Enhanced Global Optimization.
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Merte LR, Bisbo MK, Sokolović I, Setvín M, Hagman B, Shipilin M, Schmid M, Diebold U, Lundgren E, and Hammer B
- Abstract
Determination of the atomic structure of solid surfaces typically depends on comparison of measured properties with simulations based on hypothesized structural models. For simple structures, the models may be guessed, but for more complex structures there is a need for reliable theory-based search algorithms. So far, such methods have been limited by the combinatorial complexity and computational expense of sufficiently accurate energy estimation for surfaces. However, the introduction of machine learning methods has the potential to change this radically. Here, we demonstrate how an evolutionary algorithm, utilizing machine learning for accelerated energy estimation and diverse population generation, can be used to solve an unknown surface structure-the (4×4) surface oxide on Pt
3 Sn(111)-based on limited experimental input. The algorithm is efficient and robust, and should be broadly applicable in surface studies, where it can replace manual, intuition based model generation., Competing Interests: Authors declare that they have no competing interests., (© 2022 The Authors. Angewandte Chemie published by Wiley-VCH GmbH.)- Published
- 2022
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144. Why and How Savitzky-Golay Filters Should Be Replaced.
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Schmid M, Rath D, and Diebold U
- Abstract
Savitzky-Golay (SG) filtering, based on local least-squares fitting of the data by polynomials, is a popular method for smoothing data and calculations of derivatives of noisy data. At frequencies above the cutoff, SG filters have poor noise suppression; this unnecessarily reduces the signal-to-noise ratio, especially when calculating derivatives of the data. In addition, SG filtering near the boundaries of the data range is prone to artifacts, which are especially strong when using SG filters for calculating derivatives of the data. We show how these disadvantages can be avoided while keeping the advantageous properties of SG filters. We present two classes of finite impulse response (FIR) filters with substantially improved frequency response: (i) SG filters with fitting weights in the shape of a window function and (ii) convolution kernels based on the sinc function with a Gaussian-like window function and additional corrections for improving the frequency response in the passband (modified sinc kernel). Compared with standard SG filters, the only price to pay for the improvement is a moderate increase in the kernel size. Smoothing at the boundaries of the data can be improved with a non-FIR method, the Whittaker-Henderson smoother, or by linear extrapolation of the data, followed by convolution with a modified sinc kernel, and we show that the latter is preferable in most cases. We provide computer programs and equations for the smoothing parameters of these smoothers when used as plug-in replacements for SG filters and describe how to choose smoothing parameters to preserve peak heights in spectra., Competing Interests: The authors declare no competing financial interest., (© 2022 The Authors. Published by American Chemical Society.)
- Published
- 2022
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145. CO oxidation by Pt 2 /Fe 3 O 4 : Metastable dimer and support configurations facilitate lattice oxygen extraction.
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Meier M, Hulva J, Jakub Z, Kraushofer F, Bobić M, Bliem R, Setvin M, Schmid M, Diebold U, Franchini C, and Parkinson GS
- Abstract
Heterogeneous catalysts based on subnanometer metal clusters often exhibit strongly size-dependent properties, and the addition or removal of a single atom can make all the difference. Identifying the most active species and deciphering the reaction mechanism is extremely difficult, however, because it is often not clear how the catalyst evolves in operando. Here, we use a combination of atomically resolved scanning probe microscopies, spectroscopic techniques, and density functional theory (DFT)-based calculations to study CO oxidation by a model Pt/Fe
3 O4 (001) "single-atom" catalyst. We demonstrate that (PtCO)2 dimers, formed dynamically through the agglomeration of mobile Pt-carbonyl species, catalyze a reaction involving the oxide support to form CO2 . Pt2 dimers produce one CO2 molecule before falling apart into two adatoms, releasing the second CO. Olattice extraction only becomes facile when both the Pt-dimer and the Fe3 O4 support can access metastable configurations, suggesting that substantial, concerted rearrangements of both cluster and support must be considered for reactions occurring at elevated temperature.- Published
- 2022
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146. Modeling polarons in density functional theory: lessons learned from TiO 2 .
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Reticcioli M, Diebold U, and Franchini C
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Density functional theory (DFT) is nowadays one of the most broadly used and successful techniques to study the properties of polarons and their effects in materials. Here, we systematically analyze the aspects of the theoretical calculations that are crucial to obtain reliable predictions in agreement with the experimental observations. We focus on rutile TiO
2 , a prototypical polaronic compound, and compare the formation of polarons on the (110) surface and subsurface atomic layers. As expected, the parameter U used to correct the electronic correlation in the DFT + U formalism affects the resulting charge localization, local structural distortions and electronic properties of polarons. Moreover, the polaron localization can be driven to different sites by strain: due to different local environments, surface and subsurface polarons show different responses to the applied strain, with impact on the relative energy stability. An accurate description of the properties of polarons is key to understand their impact on complex phenomena and applications: as an example, we show the effects of lattice strain on the interaction between polarons and CO adsorbates., (Creative Commons Attribution license.)- Published
- 2022
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147. Single Rh Adatoms Stabilized on α-Fe 2 O 3 (11̅02) by Coadsorbed Water.
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Kraushofer F, Haager L, Eder M, Rafsanjani-Abbasi A, Jakub Z, Franceschi G, Riva M, Meier M, Schmid M, Diebold U, and Parkinson GS
- Abstract
Oxide-supported single-atom catalysts are commonly modeled as a metal atom substituting surface cation sites in a low-index surface. Adatoms with dangling bonds will inevitably coordinate molecules from the gas phase, and adsorbates such as water can affect both stability and catalytic activity. Herein, we use scanning tunneling microscopy (STM), noncontact atomic force microscopy (ncAFM), and X-ray photoelectron spectroscopy (XPS) to show that high densities of single Rh adatoms are stabilized on α-Fe
2 O3 (11̅02) in the presence of 2 × 10-8 mbar of water at room temperature, in marked contrast to the rapid sintering observed under UHV conditions. Annealing to 50 °C in UHV desorbs all water from the substrate leaving only the OH groups coordinated to Rh, and high-resolution ncAFM images provide a direct view into the internal structure. We provide direct evidence of the importance of OH ligands in the stability of single atoms and argue that their presence should be assumed when modeling single-atom catalysis systems., Competing Interests: The authors declare no competing financial interest., (© 2021 The Authors. Published by American Chemical Society.)- Published
- 2022
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148. Rapid oxygen exchange between hematite and water vapor.
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Jakub Z, Meier M, Kraushofer F, Balajka J, Pavelec J, Schmid M, Franchini C, Diebold U, and Parkinson GS
- Abstract
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" ([Formula: see text]) surface of hematite (α-Fe
2 O3 ) 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., (© 2021. The Author(s).)- Published
- 2021
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149. Direct assessment of the acidity of individual surface hydroxyls.
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Wagner M, Meyer B, Setvin M, Schmid M, and Diebold U
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The state of deprotonation/protonation of surfaces has far-ranging implications in chemistry, from acid-base catalysis
1 and the electrocatalytic and photocatalytic splitting of water2 , to the behaviour of minerals3 and biochemistry4 . An entity's acidity is described by its proton affinity and its acid dissociation constant pKa (the negative logarithm of the equilibrium constant of the proton transfer reaction in solution). The acidity of individual sites is difficult to assess for solids, compared with molecules. For mineral surfaces, the acidity is estimated by semi-empirical concepts, such as bond-order valence sums5 , and increasingly modelled with first-principles molecular dynamics simulations6,7 . At present, such predictions cannot be tested-experimental measures, such as the point of zero charge8 , integrate over the whole surface or, in some cases, individual crystal facets9 . Here we assess the acidity of individual hydroxyl groups on In2 O3 (111)-a model oxide with four different types of surface oxygen atom. We probe the strength of their hydrogen bonds with the tip of a non-contact atomic force microscope and find quantitative agreement with density functional theory calculations. By relating the results to known proton affinities of gas-phase molecules, we determine the proton affinity of the different surface sites of In2 O3 with atomic precision. Measurements on hydroxylated titanium dioxide and zirconium oxide extend our method to other oxides.- Published
- 2021
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150. Unraveling CO adsorption on model single-atom catalysts.
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Hulva J, Meier M, Bliem R, Jakub Z, Kraushofer F, Schmid M, Diebold U, Franchini C, and Parkinson GS
- Abstract
Understanding how the local environment of a "single-atom" catalyst affects stability and reactivity remains a challenge. We present an in-depth study of copper
1 , silver1 , gold1 , nickel1 , palladium1 , platinum1 , rhodium1 , and iridium1 species on Fe3 O4 (001), a model support in which all metals occupy the same twofold-coordinated adsorption site upon deposition at room temperature. Surface science techniques revealed that CO adsorption strength at single metal sites differs from the respective metal surfaces and supported clusters. Charge transfer into the support modifies the d-states of the metal atom and the strength of the metal-CO bond. These effects could strengthen the bond (as for Ag1 -CO) or weaken it (as for Ni1 -CO), but CO-induced structural distortions reduce adsorption energies from those expected on the basis of electronic structure alone. The extent of the relaxations depends on the local geometry and could be predicted by analogy to coordination chemistry., (Copyright © 2021, American Association for the Advancement of Science.)- Published
- 2021
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