Interior Atoms Research Articles

Edge-pinning effect of graphene nanoflakes sliding atop graphene

Edge effect is one of the detrimental factors preventing superlubricity in laminar solid lubricants. Separating the friction contribution from the edge atom and inner atom is of paramount importance for rational design of ultralow friction across scales in van der Waals heterostructures. To decouple these contributions and provide the underlying microscopic origin at the atomistic level, we considered two contrast models, namely, graphene nanoflakes with dimerized and pristine edges sliding on graphene monolayer based on extensive ab initio calculations. On one hand, we found edge pinning effect by dimerization is obvious for misaligned contact. This case providing local commensuration along edges is reminiscent of Aubry’s pinned phase. The contribution of per dimerized edge carbon atom to the sliding potential energy corrugation is even 1.5 times more than that of an atom in bilayer graphene under commensurate contact. Thus, the dynamic and random edge dimerization during sliding will impact the tribological properties significantly and may be the important sources to account for the marked discrepancies in measured friction parameters [Qu et al., Phys. Rev. Lett. 2020, 125, 126102]. On the other hand, we found the edge contribution to friction is lattice orientation dependent and suppressed in aligned contact. This rationalizes the experimental finding where friction is dominated by interior atoms rather than edge pinning [Liao et al., Nat. Mater. 2022, 21, 47–53]. To eliminate the undesirable edge effects, we adopt strain engineering and edge fluorination to the tribological system constructed here. However, dimerized edges as high frictional pinning sites are robust to both approaches. We hope the detailed atomic information identified here would help for improving superlubric systems.

Homogeneous Vacancies-Enhanced Orbital Hybridization for Selective and Efficient CO2-to-CO Electrocatalysis.

Crafting vacancies offers an efficient route to upgrade the selectivity and productivity of nanomaterials for CO2 electroreduction. However, defective nanoelectrocatalysts bear catalytically active vacancies mostly on their surface, with the rest of the interior atoms adiaphorous for CO2-to-product conversion. Herein, taking nanosilver as a prototype, we arouse the catalytic ability of internal atoms by creating homogeneous vacancies realized via electrochemical reconstruction of silver halides. The homogeneous vacancies-rich nanosilver, compared to the surface vacancies-dominated counterpart, features a more positive d-band center to trigger an intensified hybridization of the Ag_d orbital with the C_P orbital of the *COOH intermediate, leading to an accelerated CO2-to-CO transformation. These structural and electronic merits allow a large-area (9 cm-2) electrode to generate nearly pure CO with a CO/H2 Faradaic efficiency ratio of 6932 at an applied current of 7.5 A. These findings highlight the potential of designing new-type defects in realizing the industrialization of electrocatalytic CO2 reduction.

Comparative study of surface disorder in gold and silver nanoparticles obtained by molecular dynamics in cooling processes

This work presents structural analyses of gold and silver nanoparticles with a surface-to-volume aspect ratio of about 50%. The simulations were performed by molecular dynamics using the Sutton–Chen potential. The atoms were randomly placed in a box with periodic boundary conditions to realize a cooling process under the consideration of a canonical ensemble. The results of the potential energy as a function of temperature are presented, showing the transitions to the solidification phase. At low temperatures, the radial distribution for all nanoparticles follows patterns similar to those reported for metals with FCC structure. Coordination number and interatomic distance were established as parameters to analyze surface particularities. In general, a decrease in the inter-atomic distance was observed with respect to the bulk. In gold nanoparticles, the formation of surface planes is favored and the interatomic distances increase with the coordination number under a well-defined tendency. Interior atoms have an interatomic distances closer to the expected value. In silver nanoparticles, atoms with a coordination number equal 9, located in surface, are at distances closer to the expected value than the internal atoms, at the scale considered. However, the trend with the increments of particle size shows go to a similar gold tendency. Silver led to a more disordered surface with respect to Gold. Therefore, the properties that depend on the surface ordering will exhibit more abrupt transitions with particle size in Silver at the treated scale.

Quantification of the atomic surfaces and volumes of a metal cluster based on the molecular surface model

The atomic volume and surface are important geometric quantities for calculating various macroscopic physical quantities from atomistic models. This paper proposes a new analytical method to calculate the atomic volumes and surfaces of a metal cluster. This method adopts metallic radii to describe atom sizes and constructs the overall volume/surface by the molecular surface (MS) model. It divides cluster atoms into two types: interior atoms and boundary atoms. For an interior atom, the method defines a variational Voronoi cell as its volume. For a boundary atom, the method defines the intersection of the overall cluster volume and its variational Voronoi cell as its volume. The atomic surfaces are calculated along with the volume calculations. Compared with other methods, the new method considers the effect of atom sizes and does not rely on the size and location of the simulation box. Therefore, the new method can accurately calculate the overall volume of a metal cluster of arbitrary shape and the individual atomic volumes. This method provides computational support for multiscale coupled calculations from the microscale to macroscale.

Core-level binding energy shifts between interior, terrace and edge atoms in MnO(001) thin films

Understanding the physical origin of core-level photoemission line shapes can offer valuable information about the chemical and physical properties of surfaces. For instance, in a large number of transition metals oxides, changes in the line shape allow the accurate determination of their oxidation states and cation site symmetry. Yet, despite this importance, experimental investigations on core-level shifts have been much neglected in recent years. In order to provide further evidence of the physical relevance, we have, in this contribution, introduced a new aspect of interior-, terrace- and edge- atom core-level binding energy shifts to describe monolayers of MnO(001) films grown on an Au(111) substrate. By this means we were able to distinguish the line shape contributions related to different types of atomic sites. We show that their relative intensities and energy shifts are able to provide information about the relative amount of under coordinated atoms on the surface and, thus give insights into their catalytic properties. Our findings reveal the importance of a detailed surface science characterization to provide the correct interpretation of distinct photoemission line shapes when considering thin film samples as well as nanostructures in general.

Atomic Simulation of the Melting and Mechanical Behaviors of Silicon Nanowires

Molecular dynamics simulations using a three-body potential show that the melting and mechanical behaviors of silicon nanowires are strongly dependent on their cross-section area. For the wire with a small cross-section area, rearrangements of surface atoms greatly affect thermal stability in a relatively low temperature regime. For these wires with a relatively large area, while some surface atoms adjust their positions, most of the interior atoms hold their tetrahedra packing patterns. At a high temperature, the accumulation of structural disorder can quickly extend into the entire wire, which resembles the melting of the bulk phase. By applying the uniaxial tensile, these silicon nanowires present the typical mechanical behavior of plastic materials. The atomic local stress in the necking region is apparently larger than that outside of the necking region. As the cross-section area becomes large, both the yield strength and tensile strength increase. With the increasing temperature, the elasticity decreases significantly.

First-principles study of the Ti/Al3Ti interfacial properties

In order to theoretically clarify the interface properties of the Ti/Al3Ti laminated composites, based on the interface orientation relationship characterized by electron back scattering diffraction technology, the adhesion work, electronic structure and bonding properties of the Ti/Al3Ti interfaces were calculated by first-principles calculations. Four Ti(0 0 0 1)/Al3Ti(001) models with different terminations (Al-terminated and Al + Ti-terminated) and different stacking sites (top-sites, and center-sites) were investigated. Results showed that the Al3Ti(001) surface energies of Al + Ti-termination (1.96–2.53 J/m2) are higher than that of Al -termination (1.87–1.29 J/m2), which indicates that the Al-terminated surface is more stable; Al-termination-center-sited model has the largest adhesion work (3.202 J/m2) and the strongest charge interaction, which is the best stability among the four Ti/Al3Ti interfaces; The calculated PDOS suggests that the interfacial bond of Al-termination-center-sited interface is the Al - Ti covalent bond, it is the result of the hybridization between the Ti-3d and Al-3p, and the interior atoms in A13Ti (001) slab have more covalent features than the interfacial atoms.

Prediction of beryllium clusters (Ben; n = 3-25) from first principles.

Evolutionary searches using the USPEX method (Universal Structure Predictor: Evolutionary Xtallography) combined with density functional theory (DFT) calculations were performed to obtain the global minimum structures of beryllium (Ben, n = 3-25) clusters. The thermodynamic stability, optoelectronic and photocatalytic properties as well as the nature of bonding are considered for the most stable clusters. It is found that the cluster with n = 15 is the transition point at which the configurations change from 3D hollow cages to filled cage structures (with an interior atom appearing in the structure). All the ground state structures are energetically favorable with negative binding energies, suggesting good synthetic feasibility for these structures. The calculated relative stabilities and electronic structure show that the Be4, Be10 and, Be17 clusters are the most stable structures and can be considered as superatoms. The electron configurations of Be4, Be10 and Be17 clusters with 8, 20 and 34 electrons are identified as 1S2 1P6, 1S2 1P6 1D10 2S2, 1S2 1P6 1D10 2S2 1F14, respectively. Theoretical simulations determined that all the ground state structures exhibit excellent thermal stability, where the upper-limit temperature that the structures can tolerate is 900 K. During AIMD simulation of O2 adsorption onto the Be17 cluster an interesting phenomenon was happening in which the pristine Be17 cluster becomes a new stable Be17O16 cluster. Based on ELF (electron localization function) analysis, it can be concluded that the Be-Be bonds in the small clusters are primarily of van der Waals type, while for the larger clusters, the bonds are of metallic nature. The Ben clusters show very strong absorption in the UV and visible regions with absorption coefficients larger than 105 cm-1, which suggests a wide range of potential advanced optoelectronics applications. The Be17 cluster has a suitable band alignment in the visible-light excitation region which will produce enhanced photocatalytic activities (making it a promising material for water splitting).

Layered double hydroxide supported cobalt nanocluster: size control and the effect in catalytic hydrogen generation

Synthesizing metal nanoclusters with diameters smaller than 5nm is challenging, but desirable because of the high ratio of surface area to interior atom. However, in this report it was achieved by utilizing magnesium-aluminium layered double hydroxide (Mg/Al-LDH) as a host for cobalt citrate anion precursor, which was later reduced into cobalt nanoclusters (Co-NC). Size of the Co-NC was controlled by changing the concentration of cobalt-citrate (Co-citrate) precursor during anion exchange. XRD and FTIR showed that Co-citrate precursor was successfully intercalated on the LDH while nitrogen adsorption/desorption isotherms confirmed that mesopores in the sample were formed after chemical reduction. Furthermore, TEM/STEM observations confirmed the formation of Co-NC. It was also verified that reducing the concentration of Co-citrate from 4mM to 0.5mM resulted in a reduction in the size of Co-NC from 4.4 to 1.3 nm. However, catalytic hydrogen generation from sodium borohydride (NaBH4) hydrolysis experiment indicated that catalytic activity decreased as the size of Co-NC decreases. This is mainly attributed to the limitation in mass transport within the interlamellar space of the smaller cluster LDH compared to the bigger one. Overall, Co-NC-LDH is a promising catalyst for NaBH4 hydrolysis. However, an optimum Co-NC size is critical for enhanced catalytic activity.

Melting mechanism of Pt–Pd–Rh–Co high entropy alloy nanoparticle: An insight from molecular dynamics simulation

The temperature elevation process by the molecular dynamics (MD) simulation was used to investigate the melting mechanism of PtPdRhCo and Pt41Pd18Rh5Co32 high-entropy alloy nanoparticles (HEA-NPs) with diameters of 5, 10, and 15 nm in the FCC arrangement. The second-nearest neighbor modified embedded-atom method (2NN MEAM) potential was used and the cross-element parameters were parametrized by the guaranteed convergence particle swarm optimization (GCPSO) based on the reference data prepared by the density functional theory (DFT) calculation. According to the MD results, three characteristic temperatures during the heating process are found, which include the surface local shear resistance temperature Tslsr, surface pre-melting temperature Tsm, and melting point Tm. By observing the distribution of atomic local shear strain ηiMises with the increasing temperature, the definition of Tslsr is first proposed, below which atoms with high local shear strains are blocked at the surface of HEA-NPs. As the system temperature is above Tslsr, the interior atoms near the surface begin to possess higher local shear strains and this influence continuously spread toward the core of HEA-NPs at higher temperatures. At Tsm, a certain amount of atoms within the thin surface shell are melted, leading to the first discontinuity of binding energy-temperature profile. At Tm, the HEA-NPs structure completely becomes liquid phase and the second discontinuity of binding energy-temperature profile can be found. Although the binding energies of these two HEA-NPs types of the same size are almost identical, the Tslsr, Tsm, and Tm values of PtPdRhCo are relatively higher than those of Pt41Pd18Rh5Co32. Each element type of PtPdRhCo shows a collective behavior during the heating process, resulting in a similar local structural evolution at different temperatures. For Pt41Pd18Rh5Co32, Co and Pd atoms tend to be distorted more easily than Pt and Rh atoms during the heating process, which can be regarded as a kind of defect. This is the main reason for relatively lower Tslsr, Tsm, and Tm values of Pt41Pd18Rh5Co32.

Effects of grain size and temperature on mechanical properties of nano-polycrystalline Nickel-cobalt alloy

Influences of grain size and temperature on the mechanical properties of nano-polycrystalline Ni-Co alloy were investigated with molecular dynamics simulations. It is found that the critical grain size of Hall-Petch relationship is 4.3 nm, at which the maximum flow stress of 4.83 GPa is obtained. In samples with the d > 4.3 nm, the average flow stress increases with the decrease of d, according with the Hall-Petch relationship, caused by the dislocation glide and growth of deformation twin with the breaking of individual grain boundaries. For samples with the d < 4.3 nm, the evolution of flow stress and d is consistent with the inverse Hall-Petch relationship, attributed to the softening caused by the rotation of grain and migration of grain boundary, in which small grain merging can be observed, promoting grain boundary movement, and the extrusion of atoms leads to thinning of grain boundary. In addition, under high temperature loading, the fraction of grain boundary atoms increases, while the fraction of grain interior atoms decreases gradually. Also, the grain boundary will melt with the increase of the atomic disorder degree and the density of partial dislocation decreases. Simultaneously, the resistance to grain growth of samples at high temperature is lower than that of samples at low temperature. These conclusions have positive significance for the design of this fascinating nano-polycrystalline alloy.

Structural, magnetic, and electronic properties of small M-Pt (M = Fe, Co, and Ni) clusters: Insight from density-functional calculations

Structural, magnetic, and electronic properties of M13−nPtn (M = Fe, Co, and Ni) clusters are investigated using the plane wave pseudopotential method within the spin-polarized density-functional theory. Some novel lowest-energy configurations, superior to those reported so far, are identified in this work. There exist markedly structural differences among M13−nPtn clusters with the same M/Pt ratios except for M10Pt3, M9Pt4, and M5Pt8. All these clusters display the same trend of atomic distribution, that is, M atoms are preferentially located in the interior and Pt atoms tend to occupy the sites near surface. The magic compositions with better stability are found to be Fe7Pt6, Co6Pt7, and Ni6Pt7. The magnetic moments of M atoms in M-Pt clusters are sensitive to the charge transfer and geometric structure. In comparison to the M clusters, alloying of Pt with M atoms noticeably enhances the magnetic moments of Fe and Co atoms in Fe-rich and Co-rich clusters, and those of Ni atoms in all Ni-Pt clusters. According to the electronic density of states and Bader charge analysis, it is found that with increasing Pt content in M13−nPtn clusters, the spin-up channel has a shift toward higher energy level, and the average amount of charge transferred from M to Pt atoms gradually increases.

Determining phase transition using potential energy distribution and surface energy of Pd nanoparticles

Molecular dynamics simulation is employed to understand the thermodynamic behavior of cuboctahedron (cub) and icosahedron (ico) nanoparticles with 2–20 number of shells (55–28,741 atoms). The embedded atom method was used to describe the interatomic potential. Conventional melting criteria such as potential energy and specific heat capacity (Cp) caloric curves as well as structure analysis by radial distribution function (G(r)) and common neighbor analysis (CNA) were utilized simultaneously to provide a comprehensive picture of the melting process. It is shown that the potential energy distribution and surface energy (γp) proposed here are holding several advantages over previous criteria. In particular, potential energy distribution can distinguish between interior and surface atoms and even corner, edge and plane atoms at the surface. While G(r) and CNA are not surface sensitive methods and cannot distinguish between surface melting and an allotropic transition. It is also shown that allotropic change appears more clearly in Cp and γp rather than potential energy. However, determining accurate Cp requires enough sampling to be averaged. Finally, a few issues in the current methods for determining γp were discussed and a simple method based on available models was proposed which, independent of estimation of the surface area, predicts the correct temperature and size-dependent trend in agreement with Guggenheim-Katayama and Tolman’s models, respectively.

Nature of interlayer carbon–carbon covalent bonding in graphene-based materials

This study provides a fundamental understanding of the nature of C–C interlayer covalent bonding in graphene-based materials by using the dispersion-corrected density functional theory B3LYP-D3/6-31+G(2df,p)//B3LYP-D3/6-31G(d,p) (single-point energies at the larger 6-31+G(2df,p) basis set using fully optimized geometries at the B3LYP-D3/6-31G(d,p) level). With a bilayer hydrogenated graphene model, a partial covalent bonding was found upon dehydrogenation from opposite layers between interior carbon atoms even at the distance of greater than 4.00 A. To facilitate such bonding carbon atoms must transform from its sp3 to sp2 hybridization upon dehydrogenation so that pz orbitals can extend farther for better overlap at a large distance. The structure containing a single partial covalent bond was found to be less stable compared to its nonbonding triplet state. However, adding normal interlayer covalent bonds at the edge helps to stabilize such partial bond. In addition, forming a partial interlayer covalent bond greatly reduces band gap to 1 eV, whereas the formation of a normal covalent bond causes only a slight change in the band gap. Thus, controlling the population of the two types of interlayer bonds can, therefore, open up a promising way to control the band gap for organic semiconductor materials.

Mechanosensitive Ion Permeation across Subnanoporous MoS2 Monolayers

We use all-atom molecular dynamics simulations informed by density functional theory calculations to investigate aqueous ion transport across sub-nanoporous monolayer molybdenum disulfide (MoS$_2$) membranes subject to varying tensile strains. Driven by a transmembrane electric field, highly mechanosensitive permeation of both anions and cations is demonstrated in membranes featuring certain pore structures. For pores that are permeable when unstrained, we demonstrate ion current modulation by a factor of over 20 in the tensile strain range of 0 - 4%. For unstrained pores that are impermeable, a clear strain-induced onset of permeability is demonstrated within the same range of strains. The underlying mechanism is shown to be a strain-induced reduction of the generally repulsive ion-pore interactions resulting from the ions' short-range interactions with the atoms in the pore interior and desolvation effects. The mechanosensitive pores considered in this work gain their electrostatic properties from the pore geometries and in principle do not require additional chemical functionalization. Here we propose the possibility of a new class of mechanosensitive nanoporous materials with permeation properties determined by the targeted engineering of vacancy defects.

Size and shape dependent melting temperature of metallic nanomaterials

This study aims to characterize the size and shape dependent melting temperature of nanomaterials. Considering that surface atoms and interior atoms affect the melting of materials in different manners, we thus define an equivalent relationship between the contribution of surface atoms and interior atoms. Based on this definition, a criterion of melting is proposed through introducing a critical energy storage density of melting, the sum of the contribution of surface atoms and the interior atoms. According to the proposed criterion, a new theoretical model without any adjustable parameters is developed to characterize the size effect of melting temperatures of nanomaterials. The model predictions are in good agreement with the available experimental data or molecular dynamics simulations. This model uncovers the quantitative relationship between the melting temperature, size, atomic diameter and shape of nanomaterials. In addition, this model is extended to predict the size dependent glass transition temperatures of polymers. This study can help to better understand and characterize the size dependent melting temperatures of nanomaterials, as well as the size dependent glass transition temperatures of polymers.

3D surface condensation of large atomic shear strain in nanoscale metallic glasses under low uniaxial stress

Nanoscale metallic glasses (MGs) are frequently used in experimental and computational studies to probe the deformation mechanisms in amorphous metals. Potential consequences of the significant surface to volume ratio in these extremely small materials, nevertheless, are not well understood. Here, using molecular dynamics simulations and novel selective 3D visualization, we show that significant irreversible atomic shear strain condenses on the 3D surface of these materials under low uniaxial stress, while the interior atoms are bearing much lower, mostly reversible shear strain. This is observed for various sample geometries, dimensions, strain rates and temperatures, and attributable to the correlations of atomic shear strain with atomic potential energy and coordination number. The results reveal the profound influence of the surface on the strain partitioning in nanoscale MGs across the 3D volume, critical to the initiation and continuation of plasticity.

Size-dependent surface energy of Ar and Si nanovoids

A simple model for the size dependence of liquid–vapor surface energy of a liquid drop, free of any adjustable parameter, has been extended for the size dependence of the surface energy of nanovoids in this work. Considering nanovoids, there is a negative curvature on the void surface, which is completely different from nanoparticles or liquid drops, which have positive curvature surfaces. Thus curvature has an important effect on material properties. Moreover, by introducing negative curvature into the model for deduction of liquid–vapor surface energy, a successful prediction for the behaviour of argon (Ar) and silicon (Si) surface energy on the surface of nanovoids are obtained, which further confirms the role of curvature. Furthermore, it is shown that the surface energy of nanovoids increases as void size is reduced, suggesting a greater difference in energy between surface atoms and interior atoms, especially when void size is smaller than 4 nm. This is owing to the fact that the surface energy gradually approaches that of the bulk when the void size is larger than 4 nm. Thus, the effect of size on the surface energy of voids is significant when the voids are of small dimensions.

Toward affordable and sustainable use of precious metals in catalysis and nanomedicine

Abstract

Unraveling Kinetically-Driven Mechanisms of Gold Nanocrystal Shape Transformations Using Graphene Liquid Cell Electron Microscopy.

Mechanisms of kinetically driven nanocrystal shape transformations were elucidated by monitoring single particle etching of gold nanocrystals using in situ graphene liquid cell transmission electron microscopy (TEM). By systematically changing the chemical potential of the oxidative etching and then quantifying the facets of the nanocrystals, nonequilibrium processes of atom removal could be deduced. Etching at sufficiently high oxidation potentials, both cube and rhombic dodecahedra (RDD)-shaped gold nanocrystals transform into kinetically stable tetrahexahedra (THH)-shaped particles. Whereas {100}-faceted cubes adopt an { hk0}-faceted THH intermediate where h/ k depends on chemical potential, {110}-faceted RDD adopt a {210}-faceted THH intermediate regardless of driving force. For cube reactions, Monte Carlo simulations show that removing 6-coordinate edge atoms immediately reveals 7-coordinate interior atoms. The rate at which these 6- and 7-coordinate atoms are etched is sensitive to the chemical potential, resulting in different THH facet structures with varying driving force. Conversely, when RDD are etched to THH, removal of 6-coordinate edge atoms reveals 6-coordinate interior atoms. Thus, changing the driving force for oxidation does not change the probability of edge atom versus interior atom removal, leading to a negligible effect on the kinetically stabilized intermediate shape. These fundamental insights, facilitated by single-particle liquid-phase TEM imaging, provide important atomic-scale mechanistic details regarding the role of kinetics and chemical driving force in dictating shape transformations at the nanometer length scale.