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Transition metal nitrides are a fascinating class of catalyst materials due to their superior catalytic activity, low electrical resistance, good corrosion resistance and earth abundance; however, their conventional synthesis relies on high-temperature nitridation processes in hazardous environments. Here, we report a direct synthesis of Ni3N/Ni enriched with N-vacancies using one-step magnetron sputtering. The surface state of Ni3N(001) with 75% N-vacancies is hydrogen-terminated and exhibits four inequivalent Ni3-hollow sites. This leads to stronger H* binding compared to Ni(111), and is affirmed as the most stable surface termination under the electrochemical working conditions (pH ≈ 13.8 and E = −0.1 V) from the Pourbaix diagram. The Ni3N/Ni catalyst shows low crystallinity and good wettability and exhibits a low overpotential of 89 mV vs. RHE at 10 mA cm−2 in 1.0 M KOH with excellent stability over 3 days. This performance closely matches that of the Pt catalyst synthesized under the same conditions and surpasses that of other reported earth-abundant catalysts on planar substrates. The application of Ni3N/Ni as a cocatalyst on Si photocathodes produces an excellent ABPE of 9.3% and over 50 h stability. Moreover, its feasibility for practical application was confirmed with excellent performance on porous substrates and robustness at high operating currents in zero-gap alkaline electrolysis cells. Our work demonstrates a general approach for the feasible synthesis of other transition metal nitride catalysts for electrochemical and photoelectrochemical energy conversion applications.

Detrimental to the performance of solid oxide cells (SOCs) is the interface between electrolyte and each electrode: for purely electron-conducting electrodes, it is part of the triple phase boundary where the reactions take place, and for mixed ionic and electronic conductor (MIEC) electrodes, the oxygen ions coming from or moving into the electrolyte are transported through this phase boundary. As such, the performance of SOFCs and SOECs is strongly dependent on the chemical nature of this interface. By combining state-of-the-art electron microscopy, synchrotron-based X-ray spectroscopy and theoretical calculations, we were able to obtain an atomically resolved picture of the interface between yttria-stabilized zirconia (YSZ, with 8 mol% Y2O3) and lanthanum strontium manganite (LSM, (La0.8Sr0.2)0.95MnO3-δ).[1] Energy-dispersive X-ray (EDX) spectroscopy measurements in the transmission electron microscope (TEM) reveal the presence of a significant amount of inter-diffusion between YSZ and LSM. Only strontium does not show any diffusion into YSZ, resulting in a shift of its intensity distribution profile of approximately 0.8 nm with respect to all other elements. All this is in agreement with Monte-Carlo-based simulations that were used to theoretically model the YSZ/LSM boundary including the ion- as well as site-specific swapping probabilities for the diffusion process.[1] According to these simulations, this diffusion region is slightly amorphous. By means of atomically resolved scanning TEM (STEM), this reduction of long-range order was observed experimentally for an approximately 1.5 nm wide slab on the YSZ side of the boundary, clearly distinguishing it from the bulk fluorite structure. In materials science, distinct ‘2D-like’ layers at grain boundaries and interfaces are often referred to as ’complexions’. While these can in general occur in different types, depending on thickness and retained order,[2] they are all characterized by a self-limited (finite) width and thermodynamic stability obtained only by confinement in between two bulk phases (i.e. they cannot exist on their own without neighboring phases).[2] These complexions have recently also been discovered in other energy-related systems such as battery materials.[3] The presence of such a stable complexion might be the reason why the YSZ/LSM interface is not as prone to the formation of lanthanum or strontium zirconates as other perovskites such as lanthanum strontium cobalt ferrite (LSCF).[4] Consequently, it might have significant influence on the chemical stability of such SOCs and a thorough understanding of the complexions between electrolyte and electrode may in the future allow fine tuning of SOC performance and stability. This requires the determination of electrochemical and thermodynamic properties of these interface complexions for which we will, in a next step, employ theoretical simulations and experimental techniques in order to find out how different transport phenomena, widths and stabilities of the layers behave with temperature, time and chemical environment. [1] H. Tuerk, F.-P. Schmidt, T. Götsch et al. in preparation [2] S. J. Dillon et al. Acta Mater. 2007, 55, 6208–6218 [3] J. Timmermann et al. Phys. Rev. Lett. 2020, 125, 206101 [4] W. Wang et al. J. Electrochem. Soc. 2006, 153, A2066

Compound materials, such as transition-metal (TM) carbides, are anticipated to be effective electrocatalysts for the carbon dioxide reduction reaction (CO2RR) to useful chemicals. This expectation is nurtured by density functional theory (DFT) predictions of a break of key adsorption energy scaling relations that limit CO2RR at parent TMs. Here, we evaluate these prospects for hexagonal Mo2C in aqueous electrolytes in a multimethod experiment and theory approach. We find that surface oxide formation completely suppresses the CO2 activation. The oxides are stable down to potentials as low as −1.9 V versus the standard hydrogen electrode, and solely the hydrogen evolution reaction (HER) is found to be active. This generally points to the absolute imperative of recognizing the true interface establishing under operando conditions in computational screening of catalyst materials. When protected from ambient air and used in nonaqueous electrolyte, Mo2C indeed shows CO2RR activity.

Cyclic voltammograms (CVs) are a central experimental tool for assessing the structure and activity of electrochemical interfaces. Based on a mean-field ansatz for the interface energetics under applied potential conditions, we here derive an ab initio thermodynamics approach to efficiently simulate thermodynamic CVs. All unknown parameters are determined from density functional theory (DFT) calculations coupled to an implicit solvent model. For the showcased CVs of Ag(111) electrodes in halide-anion-containing solutions, these simulations demonstrate the relevance of double-layer contributions to explain experimentally observed differences in peak shapes over the halide series. Only the appropriate account of interfacial charging allows us to capture the differences in equilibrium coverage and total electronic surface charge that cause the varying peak shapes. As a case in point, this analysis demonstrates that prominent features in CVs do not only derive from changes in adsorbate structure or coverage but can also be related to variations of the electrosorption valency. Such double-layer effects are proportional to adsorbate-induced changes in the work function and/or interfacial capacitance. They are thus especially pronounced for electronegative halides and other adsorbates that affect these interface properties. In addition, the analysis allows us to draw conclusions on how the possible inaccuracy of implicit solvation models can indirectly affect the accuracy of other predicted quantities such as CVs.

Computational screening of metal oxide catalysts is challenging due to their more localized and intricate electronic structure as compared to metal catalysts and the resulting lack of suitable acti...

The surfaces of compound catalyst materials generally exhibit a wide range of geometric and compositional motives that could act as active sites. We address this inherent complexity by extending co...

Theoretical calculations of core electron binding energies are required for the interpretation of experimental X-ray photoelectron spectra, but achieving accurate results for solids has proven difficult. In this work, we demonstrate that accurate absolute core electron binding energies in both metallic and insulating solids can be obtained from periodic all-electron Δ-self-consistent-field (ΔSCF) calculations. In particular, we show that core electron binding energies referenced to the valence band maximum can be obtained as total energy differences between two (N – 1)-electron systems: one with a core hole and one with an electron removed from the highest occupied valence state. To achieve convergence with respect to the supercell size, the analogy between localized core holes and charged defects is exploited. Excellent agreement between calculated and experimental core electron binding energies is found for both metals and insulators, with a mean absolute error of 0.24 eV for the systems considered.

The deeper reduction of CO to multi-carbon-based fuels with higher energy densities and wider applicability has recently triggered extensive experimental and theoretical research. However, present-day catalysts for the generation of multi-carbon products (C2+) suffer from ultra-high energy barrier for C-C bond formation and poor selectivity, which poses great challenges for practical application. Herein, we propose and evidence with first-principles calculations a novel catalyst for the conversion of CO into more value-added ethylene and ethanol under visible light, consisting of active Cu-B atomic pair decorated graphitic carbon nitride (Cu-B@g-C3N4). Our results show that the low coordinated Cu-B atomic pair anchored on g-C3N4 could effectively reduce the CO dimerization free energy barrier to a record value of 0.63 eV. This high catalytic performance stems from the moderate binding strength of the intermediates modulated by the asymmetric synergy of the atomically dispersed metal Cu and non-metal B that can break the linear scaling relationship of traditional transitional metal catalysts. Moreover, the low-coordinated Cu-B active site with synergistic d-p coupling can significantly suppress the parasitic hydrogen evolution reaction. Compared to pure g-C3N4, the Cu-B@g-C3N4 catalyst also becomes more optically active under visible and even infrared light. Ab initio molecular dynamics simulations suggest that the Cu-B@g-C3N4 catalyst possesses a high thermal stability. Considering that efficient catalysts for C2+ production are currently essentially limited to Cu-bimetallic systems, our work highlights a fully new concept towards the development of novel CO reduction catalysts based on synergistic coupling between metal Cu and non-metal atoms.

At the cell voltages required to reach technologically viable current densities in proton-exchange membrane (PEM) electrolyzers, IrO2 catalysts are suspected to undergo a transformation to an amorphous hydrous form. Here, we present a systematic ab initio thermodynamics study analyzing the shape and stability of IrO2 nanoparticles in this potential range. Our results confirm a thermodynamic instability of the rutile crystal structure induced by the stabilization of highly oxidized O species at the surface already at onset potentials for the oxygen evolution reaction (OER). Intriguingly, this is preceded by a transformation of the equilibrium shape at even lower potentials. Instead of the well-studied IrO2(110) facets, this shape is dominated by IrO2(111) facets that have hitherto barely received attention. Our findings highlight the need to extend detailed characterization studies to this high-potential range, not the least to establish more suitable active-site models for the OER that may then serve as t...

Antiperovskite glasses such as Li3OCl and doped analogues have been proposed as excellent electrolytes for all-solid-state Li ion batteries (ASSB). Incorporating these electrolytes in ASSBs results in puzzling properties. This Letter describes a theoretical Li3OCl glass created by conventional melt-quench procedures. The ion conductivities are calculated using molecular dynamics based on a polarizable force field that is fitted to an extensive set of density functional theory-based energies, forces, and stresses for a wide range of nonequilibrium structures encompassing crystal, glass, and melt. We find high Li+ ion conductivity in good agreement with experiments. However, we also find that the Cl- ion is mobile as well so that the Li3OCl glass is not a single-ion conductor, with a transference number t + ≈ 0.84. This has important implications for its use as an electrolyte for all-solid-state batteries because the Cl could react irreversibly with the electrodes and/or produce glass decomposition during discharge-charge.

Advanced energy materials 11(26), 2100707 (2021). doi:10.1002/aenm.202100707, Published by Wiley-VCH, Weinheim

Based on a mean-field description of thermodynamic cyclic voltammograms (CVs), we analyze here in full generality, how CV peak positions and shapes are related to the underlying interface energetics, in particular when also including electrostatic double layer (DL) effects. We show in particular, how non-Nernstian behaviour is related to capacitive DL charging, and how this relates to common adsorbate-centered interpretations such as a changed adsorption energetics due to dipole-field interactions and the electrosorption valency – the number of exchanged electrons upon electrosorption per adsorbate. Using Ag(111) in halide-containing solutions as test case, we demonstrate that DL effects can introduce peak shifts that are already explained by rationalizing the interaction of isolated adsorbates with the interfacial fields, while alterations of the peak shape are mainly driven by the coverage-dependence of the adsorbate dipoles. In addition, we analyze in detail how changing the experimental conditions such as the ion concentrations in the solvent but also of the background electrolyte can affect the CV peaks via their impact on the potential drop in the DL and the DL capacitance, respectively. These results suggest new routes to analyze experimental CVs and use of those for a detailed assessment of the accuracy of atomistic models of electrified interfaces e.g. with and without explicitly treated interfacial solvent and/or approximate implicit solvent models.

ACS nano 15(6), 9638 - 9648 (2021). doi:10.1021/acsnano.0c10377, The synthesis of large, defect-free two-dimensional materials (2DMs) such as graphene is a major challenge toward industrial applications. Chemical vapor deposition (CVD) on liquid metal catalysts (LMCats) is a recently developed process for the fast synthesis of high-quality single crystals of 2DMs. However, up to now, the lack of in situ techniques enabling direct feedback on the growth has limited our understanding of the process dynamics and primarily led to empirical growth recipes. Thus, an in situ multiscale monitoring of the 2DMs structure, coupled with a real-time control of the growth parameters, is necessary for efficient synthesis. Here we report real-time monitoring of graphene growth on liquid copper (at 1370 K under atmospheric pressure CVD conditions) via four complementary in situ methods: synchrotron X-ray diffraction and reflectivity, Raman spectroscopy, and radiation-mode optical microscopy. This has allowed us to control graphene growth parameters such as shape, dispersion, and the hexagonal supra-organization with very high accuracy. Furthermore, the switch from continuous polycrystalline film to the growth of millimeter-sized defect-free single crystals could also be accomplished. The presented results have far-reaching consequences for studying and tailoring 2D material formation processes on LMCats under CVD growth conditions. Finally, the experimental observations are supported by multiscale modeling that has thrown light into the underlying mechanisms of graphene growth., Published by Soc., Washington, DC

Controllable large-scale synthesis of two-dimensional materials (2DMs) such as graphene is a prerequisite for industrial applications. Chemical vapor deposition (CVD) is currently the most widespread synthesis method as it is efficient and easy to automatize. The process itself is quite complex and poorly understood, but it is generally believed to involve a number of distinct steps such as hydrocarbon decomposition into surface-bound intermediates, diffusion on the catalytic substrate, generation of nucleation points and, finally, graphene growth. In situ monitoring and tailoring of such a complex procedure is beneficial for understanding the growth kinetics and, eventually, for controlling the graphene growth. Herein, we report on a novel metrology system based on in situ reflectance spectroscopy that has been developed for real-time monitoring of surface changes during graphene growth on Cu foils at high operating temperatures. The implementation of this technique for extracting kinetic parameters of the growth process is presented. Furthermore, a microkinetic model of graphene growth based on density-functional theory (DFT) and the hindered translator / rotator model for enthalpy and entropy corrections is constructed and used to obtain a microscopic understanding of the apparent activation energy and related rate-determining steps in graphene growth.

A Gaussian approximation potential was trained using density-functional theory data to enable a global geometry optimization of low-index rutile IrO_{2} facets through simulated annealing. Ab initio thermodynamics identifies (101) and (111) (1×1) terminations competitive with (110) in reducing environments. Experiments on single crystals find that (101) facets dominate and exhibit the theoretically predicted (1×1) periodicity and x-ray photoelectron spectroscopy core-level shifts. The obtained structures are analogous to the complexions discussed in the context of ceramic battery materials.

We describe the formation of lanthanide-organic coordination networks and complexes under ultra-high-vacuum conditions on a clean Ag(100) surface. The structures comprise single Ho atoms as coordination centers and 1,4-benzenedicarboxylate (from terephtalic acid, TPA) as molecular linkers. Using low temperature scanning tunneling microscopy, we find two different chiral phases of surface-supported metal-organic structures incorporating Ho atoms. Density functional theory calculations can explain the structure of both binding motifs and give possible reasons for their varying formation under the respective Ho/TPA ratios, as well as deposition and annealing temperatures. Metal-ligand interactions drive the formation of cloverleaf-shaped mononuclear Ho-TPA(4) complexes establishing supramolecular arrays stabilized through hydrogen bonding. A 2D lanthanide-organic reticulation is observed when changing the stoichiometry between the two building blocks. The combined insights from scanning tunneling microscopy and density functional theory reveal the relative stability, charge transfer, and bonding environment of both motifs.

We present a novel approach for calculating the static dielectric permittivity profile of a liquid–liquid interface (LLI) from molecular dynamics simulations. To obtain well-defined features, comparable to those observed at solid–liquid interfaces, we find it essential to reference to the instantaneous liquid–liquid interface rather than the more commonly used average Gibbs interface. We provide a coarse-grained approach for the practical definition of the instantaneous interface and present numerical results for the prototypical water/1,2-dichloroethane system. These results show that the parallel components of the dielectric permittivity tensor can be accurately extracted. In contrast, the perpendicular component does not converge to the correct bulk value at large distances from the LLI, highlighting a flaw in the regularly applied coarse-graining procedure.

An accurate atomistic treatment of aqueous solid–liquid interfaces necessitates the explicit description of interfacial water ideally via ab initio molecular dynamics simulations. Many applications, however, still rely on static interfacial water models, e.g., for the computation of (electro)chemical reaction barriers and focus on a single, prototypical structure. In this work, we systematically study the relation between density functional theory-derived static and dynamic interfacial water models with specific focus on the water–Pt(111) interface. We first introduce a general construction protocol for static 2D water layers on any substrate, which we apply to the low index surfaces of Pt. Subsequently, we compare these with structures from a broad selection of reference works based on the Smooth Overlap of Atomic Positions descriptor. The analysis reveals some structural overlap between static and dynamic water ensembles; however, static structures tend to overemphasize the in-plane hydrogen bonding network. This feature is especially pronounced for the widely used low-temperature hexagonal ice-like structure. In addition, a complex relation between structure, work function, and adsorption energy is observed, which suggests that the concentration on single, static water models might introduce systematic biases that are likely reduced by averaging over consistently created structural ensembles, as introduced here.

Even though the surface-assisted dehalogenative coupling constitutes the most abundant protocol in on-surface synthesis, its full potential will only become visible if selectivity issues with polybrominated precursors are comprehensively understood, opening new venues for both organometallic self-assembly and on-surface polymerization. Using the 3,3',5,5'-tetrabromo-2,2',4,4',6,6'-hexafluorobiphenyl (Br4F6BP) at Ag(111), we demonstrate a remote site-selective functionalization at room temperature and a marked temperature difference in double- vs. quadruple activation, both phenomena caused by conformational mechanical effects of the precursor-surface ensemble. The submolecularly resolved structural characterization was achieved by Scanning Tunneling Microscopy, the chemical state was quantitatively assessed by X-ray Photoelectron Spectroscopy, and the analysis of the experimental signatures was supported through first-principles Density-Functional Theory calculations. The non-planarity of the various structures at the surface was specifically probed by additional Near Edge X-ray Absorption Fine Structure experiments. Upon progressive heating, Br4F6BP on Ag(111) shows the following unprecedented phenomena: (1) formation of regular organometallic 1D chains via remote site-selective 3,5'-didebromination; (2) a marked temperature difference in double- vs. quadruple activation; (3) an organometallic self-assembly based on reversibility of C-Ag-C linkages with a thus far unknown polymorphism affording both hexagonal and rectangular 2D networks; (4) extraordinary thermal stability of the organometallic networks. Controlled covalent coupling at the previously Br-functionalized sites was not achieved for the Br4F6BP precursor, in contrast to the comparatively studied non-fluorinated analogue.

Rapid deactivation presently limits a wide spread use of high-temperature solid oxide cells (SOCs) as otherwise highly efficient chemical energy converters. With deactivation triggered by the ongoing conversion reactions, an atomic-scale understanding of the active triple-phase boundary between electrolyte, electrode, and gas phase is essential to increase cell performance. Here, a multi-method approach is used comprising transmission electron microscopy and first-principles calculations and molecular simulations to untangle the atomic arrangement of the prototypical SOC interface between a lanthanum strontium manganite (LSM) anode and a yttria-stabilized zirconia (YSZ) electrolyte in the as-prepared state after sintering. An interlayer of self-limited width with partial amorphization and strong compositional gradient is identified, thus exhibiting the characteristics of a complexion that is stabilized by the confinement between two bulk phases. This offers a new perspective to understand the function of SOCs at the atomic scale. Moreover, it opens up a hitherto unrealized design space to tune the conversion efficiency.

Deliberate doping of oxides is a promising route to further optimize their catalytic performance. To guide corresponding experimental endeavours we perform a density-functional theory based computational screening study for a wide range of metal dopant atoms in rutile RuO2. With a focus on the Deacon process, i.e. the catalytic oxidation of HCl to chlorine and water, we use the rate-controlling Cl desorption energy as a reactivity descriptor. As stability descriptors we employ the dopant surface segregation energy, as well as the dopant thermodynamic stability against precipitation into metal or bulk oxide grains. In the oxygen-rich conditions of the Deacon process, particularly the instability against oxide precipitation represents a strong limitation. In this respect, doping with Cu appears as an optimum compromise between stability and catalytic activity enhancement.

Lithium–titanium-oxide (Li4Ti5O12, LTO) is unique among battery materials due to its exceptional cyclability and high rate capability. This performance is believed to derive at least partly from the occupational disorder introduced via mixed Li/Ti occupancy in the LTO spinel-like structure. We explore the vast configuration space accessible during high-temperature LTO synthesis by Monte Carlo sampling and indeed find lowest-energy structures to be characterized by a high degree of microscopic inhomogeneity. Dynamical simulations in corresponding configurations reveal the dominant fraction of Li ions to be immobile on nanosecond time scales. However, Ti antisite-like defects stabilized by the configurational disorder give rise to a novel correlated ion diffusion mechanism. The resulting fast but localized diffusion could be a key element in the sudden rise in conductivity found in LTO in the early stages of charging and questions the validity of ion mobility measurements for this and other configurationall...

We present a generic analysis of the implications of energetic scaling relations on the possibilities for bifunctional gains at homogeneous bimetallic alloy catalysts. Such catalysts exhibit a large number of interface sites, where second-order reaction steps can involve intermediates adsorbed at different active sites. Using different types of model reaction schemes, we show that such site-coupling reaction steps can provide bifunctional gains that allow for a bimetallic catalyst composed of two individually poor catalyst materials to approach the activity of the optimal monomaterial catalyst. However, bifunctional gains cannot result in activities higher than the activity peak of the monomaterial volcano curve as long as both sites obey similar scaling relations, as is generally the case for bimetallic catalysts. These scaling-relation-imposed limitations could be overcome by combining different classes of materials such as metals and oxides.

Controllable synthesis of defect-free graphene is crucial for applications since the properties of graphene are highly sensitive to any deviations from the crystalline lattice. We focus here on the emerging use of liquid Cu catalysts, which has high potential for fast and efficient industrial-scale production of high-quality graphene. The interface between graphene and liquid Cu is studied using force field and ab initio molecular dynamics, revealing a complete or partial embedding of finite-sized flakes. By analyzing flakes of different sizes we find that the size-dependence of the embedding can be rationalized based on the energy cost of embedding versus bending the graphene flake. The embedding itself is driven by the formation of covalent bonds between the under-coordinated edge C atoms and the liquid Cu surface, which is accompanied by a significant charge transfer. In contrast, the central flake atoms are located around or slightly above 3 {\AA} from the liquid Cu surface and exhibit weak vdW-bonding and much lower charge transfer. The structural and electronic properties of the embedded state revealed in our work provides the atomic-scale information needed to develop effective models to explain the special growth observed in experiments where various interesting phenomena such as flake self-assembly and rotational alignment, high growth speeds and low defect densities in the final graphene product have been observed., Comment: This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in J. Chem. Phys. 153, 074702 (2020) and may be found at https://doi.org/10.1063/5.0020126

Interfacial electrochemistry and photo(electro)catalysis are key processes that convert the energy of photons or electrons to chemical bonds in many energy conversion and storage technologies. Achieving a molecular level understanding of the funda- mental interfacial structure, energetics, dynamics, and reaction mechanisms that govern these processes represents a broad frontier for chemical physics and physical chemistry. This Special Topic contains a collection of articles that range from the development of new experimental and computational techniques to the novel application of those techniques for mechanistic studies, as the principal investigators seek a fundamental molecular understanding of both electrode/electrolyte interfaces and the relevant electrocatalytic, photocatalytic, and photoelectrochemical reactions taking place thereabout. Altogether, this col- lection of articles captures the current state of this very active, frontier research field and highlights the current and remaining key scientific challenges and opportunities.

Computational screening for new and improved catalyst materials relies on accurate and low-cost predictions of key parameters such as adsorption energies. Here, we use recently developed compressed sensing methods to identify descriptors whose predictive power extends over a wide range of adsorbates, multi-metallic transition metal surfaces and facets. The descriptors are expressed as non-linear functions of intrinsic properties of the clean catalyst surface, e.g. coordination numbers, d-band moments and density of states at the Fermi level. From a single density-functional theory calculation of these properties, we predict adsorption energies at all potential surface sites, and thereby also the most stable geometry. Compared to previous approaches such as scaling relations, we find our approach to be both more general and more accurate for the prediction of adsorption energies on alloys with mixed-metal surfaces, already when based on training data including only pure metals. This accuracy can be systematically improved by adding also alloy adsorption energies to the training data., Comment: This document is the unedited Author's version of a Submitted Work that was subsequently accepted for publication in ACS Catalysis, copyright American Chemical Society after peer review. To access the final edited and published work see https://pubs.acs.org/doi/10.1021/acscatal.8b04478

The local structure of the nonplanar phthalocyanine, vanadyl phthalocyanine (VOPc), adsorbed on Cu(111) at a coverage of approximately one-half of a saturated molecular layer, has been investigated by a combination of normal-incidence X-ray standing waves (NIXSW), scanned-energy mode photoelectron diffraction (PhD), and density-functional theory (DFT), complemented by scanning tunnelling microscopy (STM). Qualitative assessment of the NIXSW data clearly shows that both “up” and “down” orientations of the molecule (with V═O pointing out of, and into, the surface) must coexist on the surface. O 1s PhD proves to be inconclusive regarding the molecular orientation. DFT calculations, using two different dispersion correction schemes, show good quantitative agreement with the NIXSW structural results for equal co-occupation of the two different molecular orientations and clearly favor the many body dispersion (MBD) method to deal with long-range dispersion forces. The calculated relative adsorption energies of the differently oriented molecules at the lowest coverage show a strong preference for the “up” orientation, but at higher local coverages, this energetic difference decreases, and mixed orientation phases are almost energetically equivalent to pure “up”-oriented phases. DFT-based Tersoff–Hamann simulations of STM topographs for the two orientations cast some light on the extent to which such images provide a reliable guide to molecular orientation.

In molecular nanotechnology, a single molecule is envisioned to act as the basic building block of electronic devices. Such devices may be of special interest for organic photovoltaics, data storage, and smart materials. However, more often than not the molecular function is quenched upon contact with a conducting support. Trial-and-error-based decoupling strategies via molecular functionalisation and change of substrate have in many instances proven to yield unpredictable results. The adsorbate-substrate interactions that govern the function can be understood with the help of first-principles simulation. Employing dispersion-corrected Density-Functional Theory (DFT) and linear expansion Delta-Self-Consistent-Field DFT, the electronic structure of a prototypical surface-adsorbed functional molecule, namely azobenzene adsorbed to (111) single crystal facets of copper, silver and gold, is investigated and the main reasons for the loss or survival of the switching function upon adsorption are identified. The light-induced switching ability of a functionalised derivative of azobenzene on Au(111) and azobenzene on Ag(111) and Au(111) is assessed based on the excited-state potential energy landscapes of their transient molecular ions, which are believed to be the main intermediates of the experimentally observed isomerisation reaction. We provide a rationalisation of the experimentally observed function or lack thereof that connects to the underlying chemistry of the metal-surface interaction and provides insights into general design strategies for complex light-driven reactions at metal surfaces., 14 pages, 5 figures, submitted to J. Phys. Condens. Matter

Atomic scale switches working at room temperature represent the ultimate level of device miniaturization. Using scanning tunneling microscopy, we find a bistable switching between two mirror-symmetric configurations of self-assembled magic rare earth silicide clusters on the Si(111) 7 X 7 surface. Density functional theory reveals an energy barrier of 1.3 eV between the two cluster configurations, suppressing the switching even at room temperature. However, intentional switch- ing between the two states is possible in the presence of a close tunneling tip due to a tip-induced lowering of the energy barrier.

Developing new methods, other than size and shape, for controlling the optoelectronic properties of semiconductor nanocrystals is a highly desired target. Here we demonstrate that the photoluminescence (PL) of silicon nanocrystals (SiNCs) can be tuned in the range 685-800 nm solely via surface functionalization with alkynyl(aryl) (phenylacetylene, 2-ethynylnaphthalene, 2-ethynyl-5-hexylthiophene) surface groups. Scanning tunneling microscopy/spectroscopy on single nanocrystals revealed the formation of new in-gap states adjacent to the conduction band edge of the functionalized SiNCs. PL red-shifts were attributed to emission through these in-gap states, which reduce the effective band gap for the electron-hole recombination process. The observed in-gap states can be associated with new interface states formed via (-Si-C≡C-) bonds in combination with conjugated molecules as indicated by ab initio calculations. In contrast to alkynyl(aryl)s, the formation of in-gap states and shifts in PL maximum of the SiNCs were not observed with aryl (phenyl, naphthalene, 2-hexylthiophene) and alkynyl (1-dodecyne) surface groups. These outcomes show that surface functionalization with alkynyl(aryl) molecules is a valuable tool to control the electronic structure and optical properties of SiNCs via tuneable interface states, which may enhance the performance of SiNCs in semiconductor devices.

Reactivity at metal-oxide interfaces is of fundamental importance in heterogeneous catalysis. Herein, we report a thorough surface-science study on the growth and chemical activity of ultrathin ZnO...

For studying electrode and electrolyte materials for lithium ion batteries, solid-state (SS) nuclear magnetic resonance (NMR) of lithium moves into focus of current research. Theoretical simulations of magnetic resonance parameters facilitate the analysis and interpretation of experimental Li SS-NMR spectra and provide unique insight into physical and chemical processes that are determining the spectral profile. In the present paper, the accuracy and reliability of the theoretical simulation methods of Li chemical shielding values is benchmarked by establishing a reference scale for Li SS-NMR of diamagnetic compounds. The impact of geometry, ionic mobility and relativity are discussed. Eventually, the simulation methods are applied to the more complex lithium titanate spinel (Li4Ti5O12, LTO), which is a widely discussed battery anode material. Simulation of the Li SS-NMR spectrum shows that the commonly adopted approach of assigning the resonances to individual crystallographic sites is not unambiguous.

Small Cu particles supported on and most likely activated by a ZnO substrate are the active component in the industrial catalyst used to convert syngas (H2, CO, CO2) into methanol, the third most important chemical product worldwide. Although a topic of intense research, the nature of the active site is still under debate. Recently, it has been pointed out that Zn atoms present at the surfaces of the Cu particles exhibit pronounced chemical activity and could explain some of the experimental findings. Another interesting suggestion is the presence of a thin layer of ZnOx species which forms on the surface of the Cu particles under reaction conditions. The importance of such thin oxide layers on the surface of metals under reaction conditions has already been pointed out in other contexts, where it was found that their chemical properties may differ substantially from those of the corresponding bulk oxides. In the case of ZnO this question is particularly interesting, since strong interactions between ZnO and the supporting metal have been reported for ZnO/ Cu and in recent work thin layers of ZnO have been shown to adopt a depolarized, graphitic structure, ZnO(gr), different from the wurtzite-type bulk. The properties of oxide thin films supported on metal substrates have been successfully studied in a number of cases, for example, for thin aluminum oxide films grown by oxidation of Ni/Al alloys. In contrast, the chemical activity of ZnO thin films supported on Cu single crystals has been investigated in a few cases only. Maroie et al. have investigated the adsorption and oxidation processes for single-crystal brass(110), brass(100), and brass(111) surfaces by X-ray photoelectron spectroscopy (XPS). Brass(110) and brass(111) show the same behavior with regard to the interaction with oxygen: the dissociative adsorption of oxygen on the surface is followed by the growth of thin ZnO layers. Wiame et al. reported that after oxidation of (111)-oriented Cu0.7Zn0.3 samples at room temperature the surface is covered by ZnO islands; it was suggested that these islands have (0001) and (0001) surface terminations. A more detailed characterization of the chemical properties of the thin ZnO layers was not carried out in this early work. In the present paper we report a detailed multitechnique investigation of a brass(111) single-crystal substrate (Cu/Zn ratio 9:1) subjected to different oxidation procedures using scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy (FTIR) under ultrahigh vacuum (UHV) conditions. The experimental findings are then interpreted by comparison with the results of a rather extensive set of density functional theory (DFT) calculations. Our results reveal the growth of thin ZnO adlayers with chemical properties that are markedly different from those of normal, wurtzite-type ZnO substrates. XPS data recorded for the brass(111) surface before and after different oxidation procedures for two different exit angles of the photoelectrons are shown in Figure 1. Since it is difficult to discriminate between Cu and Cu on the basis of XPS data, also the corresponding results from Auger electron spectroscopy (AES) are shown. For the clean brass(111) substrate the data indicate a Zn atom concentration of 5%, clearly lower than the 10% expected based on the bulk Cu/Zn ratio. Upon oxidation, the XPS data reveal an increase of the surface Zn concentration. Since it is a crucial question whether, in addition to Zn, also Cu or Cu is present, we have carefully analyzed the XPS and AES data. Neither in the Cu2p XPS data nor in the Cu L3M45M45 Auger data were the characteristic signatures of Cu or Cu species resulting from an oxidation of copper atoms detected. Cu exhibits a L3M45M45 peak at electron kinetic energies of 915 eV– 917 eV, which is clearly absent in the present data (see Figure 1). This observation, which agrees with the conclusions presented in a previous study by Rameshan et al., is expected, since in the presence of the less noble Zn one would expect the formation of ZnO to precede that of CuxO. Oxidation at elevated temperatures results in a substantial increase of the Zn signal, revealing the formation of thicker ZnO adlayers. The thickness of these thin ZnO layers was determined from the intensity of the Zn2p3/2 and Cu2p3/2 XPS signals. Exposure of the samples to 500 L of O2 at room temperature yields a ZnO adlayer with an average thickness of about 1.7 , consistent with the presence of a monolayer. More extended exposures to oxygen at room temperature did not result in a significant further increase of the thickness of the ZnO layer. Even the oxidation of the brass substrate at [*] Dr. V. Schott, Dr. A. Birkner, Dr. M. Xu, Dr. Y. Wang Chair of Physical Chemistry Ruhr-University Bochum (Germany)

X-ray absorption spectroscopy yields direct access to the electronic and geometric structure of hybrid inorganic-organic interfaces formed upon adsorption of complex molecules at metal surfaces. The unambiguous interpretation of corresponding spectra is challenged by the intrinsic geometric flexibility of the adsorbates and the chemical interactions with the interface. Density-functional theory (DFT) calculations of the extended adsorbate-substrate system are an established tool to guide peak assignment in X-ray photoelectron spectroscopy (XPS) of complex interfaces. We extend this to the simulation and interpretation of X-ray absorption spectroscopy (XAS) data in the context of functional organic molecules on metal surfaces using dispersion-corrected DFT calculations within the transition potential approach. On the example of X-ray absorption signatures for the prototypical case of 2H-porphine adsorbed on Ag(111) and Cu(111) substrates, we follow the two main effects of the molecule/surface interaction on XAS: (1) the substrate-induced chemical shift of the 1s core levels that dominates in physisorbed systems and (2) the hybridization-induced broadening and loss of distinct resonances that dominates in more chemisorbed systems., 13 pages, 4 figures

The fabrication of highly active and robust hexagonal ruthenium oxide nanosheets for the electrocatalytic oxygen evolution reaction (OER) in an acidic environment is reported. The ruthenate nanosheets exhibit the best OER activity of all solution-processed acid medium electrocatalysts reported to date, reaching 10 mA cm(-2) at an overpotential of only approximate to 255 mV. The nanosheets also demonstrate robustness under harsh oxidizing conditions. Theoretical calculations give insights into the OER mechanism and reveal that the edges are the origin of the high OER activity of the nanosheets. Moreover, the post OER analyses indicate, apart from coarsening, no observable change in the morphology of the nanosheets or oxidation states of ruthenium during the electrocatalytic process. Therefore, the present investigation suggests that ruthenate nanosheets are a promising acid medium OER catalyst with application potential in proton exchange membrane electrolyzers and beyond.

Carbon Capture and Sequestration (CCS) is one of the promising ways to significantly reduce the CO2 emission from power plants. In particular, amongst several separation strategies, adsorption by nano-porous materials is regarded as a potential means to efficiently capture CO2 at the place of its origin in a post-combustion process. The search for promising materials in such a process not only requires the screening of a multitude of materials but also the development of an adequate evaluation metric. Several evaluation criteria have been introduced in the literature concentrating on a single adsorption or material property at a time. Parasitic energy is a new approach for material evaluation to address the energy load imposed on a power plant while applying CCS. In this work, we evaluate over 60 different materials with respect to their parasitic energy, including experimentally realized and hypothetical materials such as metal–organic frameworks (MOFs), zeolitic imidazolate frameworks (ZIFs), porous polymer networks (PPNs), and zeolites. The results are compared to other proposed evaluation criteria and performance differences are studied regarding the regeneration modes, (i.e. Pressure-Swing (PSA) and Temperature-Swing Adsorption (TSA)) as well as the flue gas composition.

We present first-principles kinetic Monte Carlo (1p-kMC) simulations addressing the CO oxidation reaction at Pd(100) for gas-phase conditions ranging from ultra-high vacuum to ambient pressures and elevated temperatures. For the latter technologically relevant regime there is a long-standing debate regarding the nature of the active surface. The pristine metallic surface, an ultra-thin $$(\sqrt{5}\times \sqrt{5})R27^{\circ}$$ PdO(101) surface oxide, and thicker oxide layers have each been suggested as the active state. We investigate these hypotheses with 1p-kMC simulations focusing on either the Pd(100) surface or the PdO(101) surface oxide and intriguingly obtain a range of (T, p)-conditions where both terminations appear metastable. The predicted bistability regime nicely ties in with oscillatory behavior reported experimentally by Hendriksen et al. (Catal Today 105:234, 2005). Within this regime we find that both surface terminations exhibit very similar intrinsic reactivity, which puts doubts on attempts to assign the catalytic function to just one active state.

Density-functional theory based global geometry optimization is employed to systematically scrutinize the possibility of multidoping of hydrogenated Si clusters to achieve high spin states beyond the septet limit of a single-atom dopant. While our unbiased configurational search reveals that the previously suggested Si18H12 double hexagonal prism structure is generally too small to accommodate two dopants in magnetized state, the larger Si24H24 cage turns out to be suitable for such applications. For dimer dopants M2+ = Cr2+, Mn2+, and CrMn+, the structural integrity of the host cage is conserved in the ground-state structure of corresponding M2+@Si24H24 aggregates, as is the unusually high spin state of the guest dopant, which in the case of Cr2+ already exceeds the single-atom dopant septet limit by almost a factor of 2. Moreover, the possibility of further increasing the cluster spin moment by encapsulating an even larger number of dopants into a suitably sized hydrogenated Si cage is illustrated for the example of a (CrMn+)2@Si28H28 aggregate with a total number of 18 unpaired electrons. These results strongly suggest multidoping of Si clusters as a viable route to novel cluster-based materials for magneto-optic applications.

Blue-color stimulated emission with low threshold power is observed from In- and Zn-MOFs, which feature a highly fluorescent chromophore densely packed and rigidly linked to the metal-ion centers in the solid state. The density-of-states and transition dipole moments are calculated and the stimulated emission phenomenon is correlated with these properties. MOE (Min. of Education, S’pore) Accepted version

Realistic oxide materials are often semiconductors, in particular at elevated temperatures, and their surfaces contain undercoordiated atoms at structural defects such as steps and corners. Using hybrid density-functional theory and ab initio atomistic thermodynamics, we investigate the interplay of bond-making, bond-breaking, and charge-carrier trapping at the corner defects at the (100) surface of a p-doped MgO in thermodynamic equilibrium with an O2 atmosphere. We show that by manipulating the coordination of surface atoms one can drastically change and even reverse the order of stability of reduced versus oxidized surface sites., Comment: 5 papges, 4 figures

Multi‐photon absorption (MPA) is among the most prominent nonlinear optical (NLO) effects and has applications, for example in telecommunications, defense, photonics, and bio‐medicines. Established MPA materials include dyes, quantum dots, organometallics and conjugated polymers, most often dispersed in solution. We demonstrate how metal–organic frameworks (MOFs), a novel NLO solid‐state materials class, can be designed for exceptionally strong MPA behavior. MOFs consisting of zirconium‐ and hafnium‐oxo‐clusters and featuring a chromophore linker based on the tetraphenylethene (TPE) molecule exhibit record high two‐photon absorption (2PA) cross‐section values, up to 3600 GM. The unique modular building‐block principle of MOFs allows enhancing and optimizing their MPA properties in a theory‐guided approach by combining tailored charge polarization, conformational strain, three‐dimensional arrangement, and alignment of the chromophore linkers in the crystal. MOE (Min. of Education, S’pore) Accepted version

We explicitly calculate the free-energy barrier for the initial proton abstraction in the water splitting reaction at rutile ${\mathrm{TiO}}_{2}(110)$ through ab initio molecular dynamics. Combining solid-state embedding, an energy based reaction coordinate and state-of-the-art free-energy reconstruction techniques renders the calculation tractable at the hybrid density-functional theory level. The obtained free-energy barrier of approximately 0.2 eV, depending slightly on the orientation of the first acceptor water molecule, suggests a hindered reaction on the pristine rutile surface.

Efficient structure search is a major challenge in computational materials science. We present a modification of the basin hopping global geometry optimization approach that uses a curvilinear coordinate system to describe global trial moves. This approach has recently been shown to be efficient in structure determination of clusters [Nano Letters 15, 8044-8048 (2015)] and is here extended for its application to covalent, complex molecules and large adsorbates on surfaces. The employed, automatically constructed delocalized internal coordinates are similar to molecular vibrations, which enhances the generation of chemically meaningful trial structures. By introducing flexible constraints and local translation and rotation of independent geometrical subunits we enable the use of this method for molecules adsorbed on surfaces and interfaces. For two test systems, trans-$\beta$-ionylideneacetic acid adsorbed on a Au(111) surface and methane adsorbed on a Ag(111) surface, we obtain superior performance of the method compared to standard optimization moves based on Cartesian coordinates., Comment: 14 pages, 9 figures