Displacements Of Surface Atoms Research Articles

Assessing the Activity and Stability of Cu-Based Gas Diffusion Electrodes for the CO2 Conversion in Different Electrolytes: Bicarbonate Vs. Ionic Liquid/Acetonitrile

In the context of sustainable strategies proposed to convert CO2 to valuable products, the electrochemical reduction of CO2 (ECR-CO2) represents one of the most promising alternatives1. Herein, the ECR-CO2 has been carried out employing for the first-time synthesised core-shell Cu2O/SnO2-based nanoparticles within a continuous flow cell both in aqueous solution and in the presence of ionic liquids (ILs), which have been known to boost CO2-derived products2. Interestingly, in aqueous potassium bicarbonate-based electrolyte, the synthesised material demonstrated a stable syngas production at -20 mA cm-2, with a CO/H2 ratio of about 10. The electrode stability and selectivity in aqueous environment has been further demonstrated for 24 hours of continuous operations at 10 cm2 scale. However, a CO/H2 ratio of about 3 was reached with 1-Butyl-3-methylimidazolium triflate ([BMIM][TfO]) in acetonitrile (ACN)3. In the aprotic media, stability issues have been faced during operations, visualized in the darkening of the gas diffusion electrodes and in colour changes of the electrolyte. Raman spectroscopy, Field Emission Scanning Electron Microscopy and Electrochemical Impedance Spectroscopy techniques have been employed for the physicochemical characterization of the electrodes and to assess the evolution of the electrochemical interfaces within the system over time. Molecular dynamics simulations have shown that the instability of the GDE could start with the displacement of Cu surface atoms from their equilibrium position by ACN molecules. Furthermore, we observed that the immobilization of ruthenium and rhenium complexes on the catalyst can be crucial to determine its suitability for either the hydroformylation or the carbonylation reaction, making this technology a promising solution for the green transition of the chemical industry. Acknowledgments The financial support of the SUNCOCHEM project (Grant Agreement No 862192) of the European Union’s Horizon 2020 Research and Innovation Action programme and of Fondazione CRT are acknowledged. References IEA. Global Energy Review: CO2 Emissions in 2021. (2021).Kunene, T., Atifi, A. & Rosenthal, J. Selective CO2 Reduction over Rose’s Metal in the Presence of an Imidazolium Ionic Liquid Electrolyte. ACS Appl. Energy Mater. 3, 4193–4200 (2020).Fortunati, A., Hernández, S., et al. Understanding the role of imidazolium-based ionic liquids in the electrochemical CO2 reduction reaction. Communications Chemistry, 6(1), 1–13 (2023).

Static and dynamic atom displacements in nanocrystalline powders from diffraction analysis at finite temperature

We evaluate the accuracy of x-ray diffraction analysis performed by Rietveld refinement in extracting the static and dynamic atomic displacement parameters of spherical gold nanocrystals with sizes of 30 nm and below and within a temperature interval of 10–300 K. A self-consistent computational workflow composed of real- and reciprocal- space analyses is designed for this purpose. We obtain average lattice parameters, static and thermal atomic displacements as well as their temperature and particle size dependence in an ideally monodispersed and realistic nanocrystalline powder generated by molecular dynamics simulations. Then, we compare crystallographic parameters obtained from real- and reciprocal- space analyses to evaluate the accuracy of diffraction analysis. We also extend the comparison to local variations of crystallographic parameters in the nanocrystals and show how Rietveld refinement relates to the average parameters of the core and surface regions separately. We show that 1) average lattice parameters obtained from Rietveld refinement are greater than the mean lattice parameter of small nanocrystals and converge to the core lattice parameter with increasing particle size, 2) Rietveld refinement significantly underestimates static atom displacements of surface atoms in small nanocrystals, 3) dynamic atomic displacements in nanocrystalline gold powders are reliably obtained within 5 % error by Rietveld refinement, provided that thermal diffuse scattering is adequately removed from the diffraction data, 4) the discrepancy between real- and reciprocal-space analysis increases for the smallest gold nanocrystal.

Dynamic mechanical contact behaviors and sintering mechanism of Al nanoparticles subjected to high-speed impact

In this work, dynamic mechanical contact behaviors and sintering mechanism of Al nanoparticles subjected to high-speed impact have been investigated using molecular dynamic simulations by analyzing radial distribution function (RDF), atomic average displacement, mean square displacement (MSD), the sintering neck radius, shrinkage, nanoparticle temperature and the proportion of amorphous atoms. The results shows that the long-ranged ordered structure of Al nanoparticles gradually transforms into short-range ordered structure and finally amorphous structure during the head-on impact. The atomic average displacement, MSD, radius ratio (x/R) and shrinkage at different initial impact speeds first experience a rapid increase to a maximum, followed by a slow decrease and subsequent a recovery to an almost constant. The displacement of surface atoms is always larger than the internal atoms during the head-on impact. There exists one critical impact velocity (e.g., 2800 m/s for given 4 nm in diameter) beyond which the ordered aluminum atoms completely transform into amorphous structure. The contact force, contact radius and contact stress obtained under a given low impact velocity and small strain are quite consistent with the predictions of the continuous Hertz model, but as the impact velocity and normal displacement further increase, the predictions of Hertz model underestimate these contact parameters. The influence of nanoparticle size on the relationship of contact radius and number of contacting atoms is inappreciable. The nanoparticles undergo elastic deformation, and then plastic deformation, followed by mechanical failure and shock-induced melting process into a whole. This work could strengthen our understanding of the industrial processes with applications in energetic nanomaterials, rocket propellant, metal sintering, etc.

Probing the surface structure via the adsorbed hydrogen atoms – The case of Cu(4 1 0)

We show how we could determine (or at least qualitatively deduce) the displacements of surface atoms with respect to an arbitrarily chosen reference, from the trends in the vibrational frequencies of adsorbed hydrogen atoms. The discernible displacements could be as small as 0.1 Å. As a test structure, we consider the case of Cu(4 1 0), which has remained contentious, until recently. Any slight (ca. 0.1 Å) displacement of Cu(4 1 0) surface atoms with respect to an arbitrarily chosen reference structure manifests as either a broadening or narrowing of the corresponding H-Cu(4 1 0) potential energy curve. This suggests the possibility of carrying out adsorbate mediated surface analysis, with hydrogen as the (chosen) adsorbate, in this case.

Nonthermalized Precursor-Mediated Dissociative Chemisorption at High Catalysis Temperatures.

Quasiclassical trajectory calculations and vibrational-state-selected beam-surface measurements of CH4 chemisorption on Ir(111) reveal a nonthermal, hot-molecule mechanism for C-H bond activation. Low-energy vibrationally excited molecules become trapped in the physisorption well and react before vibrational and translational energies accommodate the surface. The reaction probability is strongly surface-temperature-dependent and arises from the pivotal role of Ir atom thermal motion. In reactive trajectories, the mean outward Ir atom displacement largely exceeds that of the transition-state geometry obtained through a full geometry optimization. The study also highlights a new way for (temporary) surface defects to impact high-temperature heterogeneous catalytic reactivity. Instead of reactants diffusing to and competing for geometrically localized lower barrier sites, transient, thermally activated surface atom displacements deliver low-barrier surface reaction geometries to the physisorbed reactants.

Dynamic charge and oxidation state of Pt/CeO2 single-atom catalysts.

The catalytic activity of metals supported on oxides depends on their charge and oxidation state. Yet, the determination of the degree of charge transfer at the interface remains elusive. Here, by combining density functional theory and first-principles molecular dynamics on Pt single atoms deposited on the CeO2 (100) surface, we show that the common representation of a static metal charge is oversimplified. Instead, we identify several well-defined charge states that are dynamically interconnected and thus coexist. The origin of this new class of strong metal-support interactions is the relative position of the Ce(4f) levels with respect to those of the noble metal, allowing electron injection to (or recovery from) the support. This process is phonon-assisted, as the Ce(4f) levels adjust by surface atom displacement, and appears for other metals (Ni) and supports (TiO2). Our dynamic model explains the unique reactivity found for activated single Pt atoms on ceria able to perform CO oxidation, meeting the Department of Energy 150 °C challenge for emissions.

Chemisorption enhancement of single carbon and oxygen atoms near the grain boundary on Fe surface: ab initio study

The chemisorption of single carbon and oxygen atoms on an iron surface with a Σ3(111) grain boundary has been studied by means of first principles modeling. It is shown that adsorption sites close to the grain boundary have higher adsorbability than any site at clean Fe (100), (110), and (111) surfaces. To reveal the mechanism of chemisorption enhancement, several properties are discussed, such as the rearrangement of iron surface atoms, changes in the electronic densities of states, and the spacial charge density differences caused by adsorbates. Due to the higher movability of atoms on the surface near the grain boundary, the chemical bonding of adsorbate to iron atoms in the vicinity of the grain boundary leads to large displacements of surface atoms. The change of the partial density of states of Fe in the vicinity of adsorption sites induced by such rearrangement suggests that the adsorption properties enhancement of the surface is related to the increase of density of states at Fermi level.

Effects of surface motion and electron-hole pair excitations in CO2 dissociation and scattering on Ni(100).

The energy transfer between different channels is an important aspect in chemical reactions at surfaces. We investigate here in detail the energy transfer dynamics in a prototypical system, i.e., reactive and nonreactive scattering of CO2 on Ni(100), which is related to heterogeneous catalytic processes with Ni-based catalysts for CO2 reduction. On the basis of our earlier nine-dimensional potential energy surface for CO2/Ni(100), dynamical calculations have been done using the generalized Langevin oscillator (GLO) model combined with local density friction approximation (LDFA), in which the former accounts for the surface motion and the latter accounts for the low-energy electron-hole pair (EHP) excitation. In spite of its simplicity, it is found that the GLO model yields quite satisfactory results, including the significant energy loss and product energy disposal, trapping, and steering dynamics, all of which agree well with the ab initio molecular dynamics ones where many surface atoms are explicitly involved with high computational cost. However, the GLO model fails to describe the reactivity enhancement due to the lattice motion because it intrinsically does not incorporate the variance of barrier height on the surface atom displacement. On the other hand, in LDFA, the energy transferred to EHPs is found to play a minor role and barely alter the dynamics, except for slightly reducing the dissociation probabilities. In addition, vibrational state-selected dissociative sticking probabilities are calculated and previously observed strong mode specificity is confirmed. Our work suggests that further improvement of the GLO model is needed to consider the lattice-induced barrier lowering.

Liquid Adsorption of Organic Compounds on Hematite α-Fe2O3 Using ReaxFF.

ReaxFF-based molecular dynamics simulations are used in this work to study the effect of the polarity of adsorbed molecules in the liquid phase on the structure and polarization of hematite (α-Fe2O3). We compared the adsorption of organic molecules with different polarities on a rigid hematite surface and on a flexible and polarizable surface. We show that the displacements of surface atoms and surface polarization in a flexible hematite model are proportional to the adsorbed molecule's polarity. The increase in electrostatic interactions resulting from charge transfer in the outermost solid atoms in a flexible hematite model results in better-defined adsorbed layers that are less ordered than those obtained assuming a rigid solid. These results suggest that care must be taken when parametrizing empirical transferable force fields because the calculated charges on a solid slab in vacuum may not be representative of a real system, especially when the solid is in contact with a polar liquid.

Surface electron density models for accurateab initiomolecular dynamics with electronic friction

Ab initio molecular dynamics with electronic friction (AIMDEF) is a valuable methodology to study the interaction of atomic particles with metal surfaces. This method, in which the effect of low-energy electron-hole (e-h) pair excitations is treated within the local density friction approximation (LDFA) [Juaristi et al., Phys. Rev. Lett. 100, 116102 (2008)], can provide an accurate description of both e-h pair and phonon excitations. In practice, its applicability becomes a complicated task in those situations of substantial surface atoms displacements because the LDFA requires the knowledge at each integration step of the bare surface electron density. In this work, we propose three different methods of calculating on-the-fly the electron density of the distorted surface and we discuss their suitability under typical surface distortions. The investigated methods are used in AIMDEF simulations for three illustrative adsorption cases, namely, dissociated ${\mathrm{H}}_{2}$ on Pd(100), N on Ag(111), and ${\mathrm{N}}_{2}$ on Fe(110). Our AIMDEF calculations performed with the three approaches highlight the importance of going beyond the frozen surface density to accurately describe the energy released into e-h pair excitations in case of large surface atom displacements.

Powder diffraction in studies of nanocrystal surfaces: chemisorption on Pt

Atoms at the surface of nanocrystals contribute appreciably to the X-ray diffraction pattern. Phenomena like chemisorption, affecting the displacement of surface atoms with respect to their positions in the perfect crystallographic structure, cause diffraction peak shifts and intensity changes. These effects are easily measurable for small nanocrystals up to 10 nm size. This article reports diffraction effects of chemisorption of adsorbing gases H2, O2, CO and NO for a series ofin situpowder diffraction experiments on nanocrystalline Pt supported on silica. On the basis of previous diffraction observation of Pt surface reconstruction during hydrogen desorption, it was possible to quantify this effectversuscrystallite size and rationalize the observed diffraction peak shift for the other adsorbing species. This enabled the surface reconstruction to be distinguished from the surface relaxation effect, the latter depending monotonically on the adsorption energy. Even if no phase transition occurs, monitoring of a peak's position, intensity, width and gas composition (viamass spectrometry) during a carefully designed physicochemical process (including surface chemical reaction) enables insight into and understanding of the surface structure evolution (e.g.amorphization, relaxation, reconstruction or changes in the overall morphology). The proposed technique can be used as a surface science tool, allowing studies of nanocrystals under high pressure.

A comprehensive DFT investigation of bulk and low-index surfaces of ZrO2 polymorphs.

The bulk structure, the relative stability, and the electronic properties of monoclinic, tetragonal, and cubic ZrO(2) have been studied from a theoretical point of view, through periodic ab initio calculations using different Gaussian basis sets together with Hartree-Fock (HF), pure Density Functional Theory (DFT), and mixed HF/DFT schemes as found in hybrid functionals. The role of a posteriori empirical correction for dispersion, according to the Grimme D2 scheme, has also been investigated. The obtained results show that, among the tested functionals, PBE0 not only provides the best structural description of the three polymorphs, but it also represents the best compromise to accurately describe both the geometric and electronic features of the oxide. The relative stability of the three phases can also be qualitatively reproduced, as long as thermal contributions to the energy are taken into account. Four low-index ZrO(2) surfaces [monoclinic (-111), tetragonal (101 and 111), and cubic (111)] have then been studied at this latter level of theory. Surface energies, atomic relaxations, and electronic properties of these surfaces have been computed. The most stable surface is the cubic one, which is associated to small relaxations confined to the outermost layers. It is followed by the monoclinic (-111) and the tetragonal (101), which have very similar surface energies and atomic displacements. The tetragonal (111) was instead found to be, by far, the less stable with large displacements not only for the outermost but also for deeper layers. Through the comparison of different methods and basis sets, this study allowed us to find a reliable and accurate computational protocol for the investigation of zirconia, both in its bulk and surfaces forms, in view of more complex technological applications, such as ZrO(2) doped with aliovalent oxides as found in solid oxide fuel cells.

Simulating acoustic emission: The noise of collapsing domains

Microstructural changes during mechanical shear of a ferroelastic or martensitic material and their signature in acoustic emission (AE) spectroscopy during strain-induced yield and detwinning are investigated by computer simulation. Complex domain patterns are generated during the main yield event, which leads to large displacements of surface atoms and emission of acoustic waves. Loading beyond the yield point leads, eventually, to a simplification of the domain patterns by local movements of needle domains, the nucleation and movement of kinks in domain walls, and the collapse of domains spanning the entire sample (from surface to surface). These microstructural changes lead to much weaker acoustic emissions than those near the yield point. Nucleation/collapse during a yield event involves an energy drop of some 3.7 meV/atom; the collapse of spanning domains releases 0.56 meV/atom, a kink crashing into the surface changes the energy by 0.017 meV/atom, and the collapsing vertical needle changes the energy by 0.017 meV/atom. All these energy bursts can, in principle, be seen by AE. The large energy spread means that AE spectroscopy measures a mixture of events whereby weak and strong signals may signify smaller and bigger events of the same kind or different microstructural changes with intrinsically different signal strengths. In order to disentangle the various contributions, other observables are needed, such as the time-dependent strain matrix of the deformed sample.

Density functional theory study of water interactions on Mn-doped CeO2(111) surface

Spin-polarized density functional theory (DFT + U) periodic calculations have been performed to study water adsorption and dissociation on the 12.5% Mn-doped CeO2(1 1 1) surface. Our results indicated that Mn cation is the surface active site for water adsorption and dissociation reactions. The H2O molecule preferably adsorbs on a Mn cation, causing some relaxation of the surface O-layer and, thus, facilitating the bonding of one of the HH2O with the nearest oxygen atom. After overcoming an energy barrier of 0.46 eV, the water molecule could dissociate into OH and H species. The latter configuration is about 50% more exothermic than the molecular one, suggesting the Ce0.875Mn0.125O1.9375(1 1 1) surface would be easily hydroxylated under reaction conditions. In addition, the calculations showed that water adsorption on the Mn-doped CeO2(1 1 1) surface did not favor the creation of surface oxygen vacancies as it has been reported for pure CeO2(1 1 1). On the other hand, we created a surface oxygen defect in the slab with structural oxygen vacancies and computed water interactions on the reduced surface. Although, the adsorption of OH species in the O-hole caused many surface and subsurface atomic displacements, no changes in the oxidation state of Mn and Ce cations were detected.

The effect of the exchange-correlation functional on H2 dissociation on Ru(0001)

The specific reaction parameter (SRP) approach to density functional theory (DFT) has enabled a chemically accurate description of reactive scattering experiments for activated H2-metal systems (H2 + Cu(111) and Cu(100)), but its application has not yet resulted in a similarly accurate description of non-activated or weakly activated H2-metal systems. In this study, the effect of the choice of the exchange-correlation functional in DFT on the potential energy surface and dynamics of H2 dissociation on Ru(0001), a weakly activated system, is investigated. In total, full potential energy surfaces were calculated for over 20 different functionals. The functionals investigated include functionals incorporating an approximate description of the van der Waals dispersion in the correlation functional (vdW-DF and vdW-DF2 functionals), as well as the revTPSS meta-GGA. With two of the functionals investigated here, which include vdW-DF and vdW-DF2 correlation, it has been possible to accurately reproduce molecular beam experiments on sticking of H2 and D2, as these functionals yield a reaction probability curve with an appropriate energy width. Diffraction probabilities computed with these two functionals are however too high compared to experimental diffraction probabilities, which are extrapolated from surface temperatures (Ts) ⩾ 500 K to 0 K using a Debye-Waller model. Further research is needed to establish whether this constitutes a failure of the two candidate SRP functionals or a failure of the Debye-Waller model, the use of which can perhaps in future be avoided by performing calculations that include the effect of surface atom displacement or motion, and thereby of the experimental Ts.

Ion-induced effects on grain boundaries and a-Si:H tissue quality in microcrystalline silicon films

Microcrystalline silicon films have been deposited by means of the remote expanding thermal plasma. The effect of ion bombardment on the microcrystalline silicon film properties has been investigated by applying an RF bias to the deposition substrate. The application of the RF substrate bias resulted in the formation of an additional plasma in front of the substrate holder. Neither the SiH4 depletion nor the growth flux was significantly enhanced upon substrate biasing, which suggests that (the composition of) the growth precursor flux is unaffected and that the ion-film interaction mechanisms were responsible for the observed material changes. Moderate bias conditions (i.e., dc bias voltages up to ∼70 V) led to an improved grain boundary passivation and densification of the amorphous silicon tissue, as concluded from the analysis of the infrared Si-Hx stretching modes. These improvements have been ascribed to ion-induced Si surface atom displacement, which enhances the surface diffusion length of the growth precursors. More-energetic ion bombardment (i.e., under applied dc bias voltages of ∼60 V and higher) resulted in enhanced (di)vacancy incorporation via ion-induced Si bulk atom displacement. The film crystallinity was found not to be affected by the ion bombardment, although a reduced crystallite size was observed under ion bombardment conditions where Si bulk displacement had been sufficiently activated. The extent of the ion-film interaction mechanism has been enhanced by increasing the ion-to-Si deposition flux ratio. Under specific ion bombardment conditions, i.e., dc bias voltage in the range of 40–70 V and ion/Si arrival rate ∼0.20, microcrystalline silicon films have been obtained which, on the basis of the Si-Hx stretching modes, are qualified as solar-grade, i.e., the intergranular space is filled with a dense amorphous silicon tissue which completely passivates the crystalline grain boundaries.

Static surface temperature effects on the dissociation of H2 and D2 on Cu(111)

A model for taking into account surface temperature effects in molecule-surface reactions is reported and applied to the dissociation of H(2) and D(2) on Cu(111). In contrast to many models developed before, the model constructed here takes into account the effects of static corrugation of the potential energy surface rather than energy exchange between the impinging hydrogen molecule and the surface. Such an approximation is a vibrational sudden approximation. The quality of the model is assessed by comparison to a recent density functional theory study. It is shown that the model gives a reasonable agreement with recently performed ab initio molecular dynamics calculations, in which the surface atoms were allowed to move. The observed broadening of the reaction probability curve with increasing surface temperature is attributed to the displacement of surface atoms, whereas the effect of thermal expansion is found to be primarily a shift of the curve to lower energies. It is also found that the rotational quadrupole alignment parameter is generally lowered at low energies, whereas it remains approximately constant at high energies. Finally, it is shown that the approximation of an ideal static surface works well for low surface temperatures, in particular for the molecular beams for this system (T(s) = 120 K). Nonetheless, for the state-resolved reaction probability at this surface temperature, some broadening is found.

An analysis on nanovoid growth in body-centered cubic single crystalline vanadium

Molecular dynamics simulations were performed to analyze nanovoid growth in single crystalline vanadium under tension. Radial distribution function at the first nearest neighbor distance was calculated to find out the critical strain rate below which the deformation of specimen was static. Then a tensile stress was exerted on both void contained box and intact box under two constraint conditions. Homogenous dislocations were nucleated in intact box at yield point; while for void contained box with void radius twice the lattice constant, 〈1 1 1〉{1 1 0} shear loops were punched out from void surface. The formation of shear loops was the result of the splitting of purely screw cores on three non-planar planes, as well as their transformations to more stable two-fold non-planar dislocations under tension. The asymmetry of loops was influenced by both strain rate and triaxiality of system. It is also found that, in lower rate cases the yield point and peak stress point coincided; however, the two points separated at higher rate due to the inadequate void growth rate. Mean square displacement of void surface atoms were given out to geometrically depict the void evolution. Moreover, simulations with different initial porosity and box size were performed respectively. It is shown that when void reduced to contain only one vacancy, dislocations can be nucleated independently of the void; when porosity was large enough, the interactions between void and its periodic images were noticeable. Also, when both the void and box were large, triangular prismatic loops on {1 1 0} planes were observed at void surface, which may be contributed to a combined effect of the intersection of shear loops and the ledges along the void surface. Finally, the results of our MD simulations agreed well with that from Lubarda equation.

Orbital-Free DFT Simulations of Elastic Response and Tensile Yielding of Ultrathin [111] Al Nanowires

Elastic properties and mechanisms for tensile yielding of fcc Al nanowires are explored using orbital-free density functional theory. We quasistatically load ultrathin nanowires that have circular cross sections, diameters 1−8 nm, lengths up to ∼20 nm, and axes along the [111] direction. We find that Young’s modulus is roughly consistent for the nanowires and bulk Al but that the equilibrium interlayer spacing, elastic limit, and yield strength are diameter dependent. Plasticity is nucleated by 3-fold symmetric axial displacements of surface atoms for all nanowires examined. However, the nanowire with 4 nm diameter yields via partial slip before achieving the theoretical strength of bulk Al, while the 1 and 2 nm nanowires yield via amorphous mechanisms, above or near the theoretical strength. These results give new insight into the plastic yielding mechanisms of ultrathin fcc nanowires with edge-free cross sections and [111] orientation.

Mechanism of the lattice relaxation in thin epitaxial films of iron oxides: Generalization from the case of ilmenite–hematite solid solution

The overshoot of the in-plane lattice parameter above its expected value was previously observed for iron oxides on different substrates for initial stages of the film growth. The reasons for the overshoot provided in the previously published experimental studies have not satisfactorily explained the effect. In the present article the overshoot is explained in terms of the surface atom displacements at low thicknesses due to the strong lattice relaxation via dislocation formation. The ilmenite–hematite solid solution is used as an example for the experimental and theoretical descriptions of the overshoot. The theoretical model, based on the dislocation density, can be directly applied to describe the lattice relaxation as a function of the film thickness for other iron-based oxide films with a strong film/substrate lattice mismatch.