Site-specific reactivity of stepped Pt surfaces driven by stress release.

Heterogeneous catalysts are widely used to promote chemical reactions. Although it is known that chemical reactions usually happen on catalyst surfaces, only specific surface sites have high catalytic activity. Thus, identifying active sites and maximizing their presence lies at the heart of catalysis research1-4, in which the classic model is to categorize active sites in terms of distinct surface motifs, such as terraces and steps1,5-10. However, such a simple categorization often leads to orders of magnitude errors in catalyst activity predictions and qualitative uncertainties of active sites7,8,11,12, thus limiting opportunities for catalyst design. Here, using stepped Pt(111) surfaces and the electrochemical oxygen reduction reaction (ORR) as examples, we demonstrate that the root cause of larger errors and uncertainties is a simplified categorization that overlooks atomic site-specific reactivity driven by surface stress release. Specifically, surface stress release at steps introduces inhomogeneous strain fields, with up to 5.5% compression, leading to distinct electronic structures and reactivity for terrace atoms with identical local coordination, and resulting in atomic site-specific enhancement of ORR activity. For the terrace atoms flanking both sides of the step edge, the enhancement is up to 50 times higher than that of the atoms in the middle of the terrace, which permits control of ORR reactivity by either varying terrace widths or controlling external stress. Thus, the discovery of the above synergy provides a new perspective for both fundamental understanding of catalytically active atomic sites and design principles of heterogeneous catalysts.

Preferential segregation of gold at the symmetrical tilt grain boundaries of platinum: an atomic-scale quantitative understanding.

Grain boundary (GB) segregation plays a pivotal role in maintaining and optimizing the remarkable catalytic or mechanical properties of nanocrystalline Pt by reducing the Gibbs free energy and thereby impeding structure degradation. The solute segregation behavior at the Pt GB, however, is not well understood at the atomic level. In this study, we employed first-principles calculations to elucidate the preferential segregation behavior of a single Au atom at the symmetrical tilt GB of Pt. For pure Pt, a linear relationship between the GB energy and excess volume is observed. Therefore, Au exhibits strong segregation tendencies towards GB to release excess energy and volume stored at the strained GB. Although the segregation energy is sensitive to various GB sites, it is interesting to note that the minimum one increases linearly with GB energy. This site-sensitivity of segregation energy can be attributed to mechanical, chemical, and interaction parts, which are quantitatively related to the atomic volume, coordination number, and average bond length, respectively. Finally, the interplay among different structural descriptors is revealed. These insights into the association between GB structures, segregation configuration and energy offers valuable atomic-scale quantitative insights into the segregation behavior of Au in Pt GBs, which holds significant implications for the design of Pt nanomaterials with enhanced thermal stability via GB engineering.

In Situ Visualization of the Pinning Effect of Planar Defects on Li Ion Insertion.

Longevity of Li ion batteries strongly depends on the interaction of transporting Li ions in electrode crystals with defects. However, detailed interactions between the Li ion flux and structural defects in the host crystal remain obscure due to the transient nature of such interactions. Here, by in situ transmission electron microscopy and density function theory calculations, we reveal how the diffusion pathways and transport kinetics of a Li ion can be affected by planar defects in a tungsten trioxide lattice. We uncover that changes in charge distribution and lattice spacing along the planar defects disrupt the continuity of ion conduction channels and dramatically increase the energy barrier of Li diffusion, thus, arresting Li ions at the defect sites and twisting the lithiation front. The atomic-scale understanding holds critical implications for rational interface design in solid-state batteries and solid oxide fuel cells.

Bilayer tetragonal AlN nanosheets as potential cathodes for Li-O2 batteries.

Li-O2 batteries are considered promising electrochemical energy storage devices due to their high specific capacity and low cost. However, this technology currently suffers from two serious problems: low round-trip efficiency and slow reaction dynamics at the cathode. Solving these problems requires designing novel catalysis materials. In this study, a bilayer tetragonal AlN nanosheet as the catalyst is theoretically designed for the Li-O2 electrochemical system, and the discharge/charge process is simulated by a first-principles approach. It is found that the reaction path leading to Li4O2 is energetically more favored than the path to form a Li4O4 cluster on an AlN nanosheet. The theoretical open-circuit voltage for Li4O2 is 2.70 V, which is only 0.14 V lower than the formation of Li4O4. Notably, the discharge overpotential for forming Li4O2 on the AlN nanosheet is only 0.57 V, and the corresponding charge overpotential is as low as 0.21 V. A low charge/discharge overpotential can effectively solve the problems of low round-trip efficiency and slow reaction kinetics. The decomposition pathways of the final discharge product Li4O2 and the intermediate product Li2O2 are also investigated, and the decomposition barriers are 1.41 eV and 1.45 eV, respectively. Our work shows that bilayer tetragonal AlN nanosheets are promising catalysts for Li-O2 batteries.

Synergistic engineering of shell thickness and core ordering to boost the oxygen reduction performance.

When benchmarked against the extended Pt(111), slightly weaker adsorption and stronger cohesion properties of surface Pt are required to improve activity and durability for the oxygen reduction reaction, respectively, making it challenging to meet both requirements on one surface. Here, using Pt(111) over-layers stressed and modified by Pt-TM (TM = Fe, Co, Ni, V, Cu, Ag, and Pd) intermetallics as examples, we theoretically identified ten promising catalysts by synergistically tailoring the skin thickness and substrate chemical ordering to simultaneously achieve weak adsorption and strong cohesion. More specifically, compared with Pt(111), all candidates exhibit 10-fold enhanced activity, half of which show improved durability, such as mono-layer skin on L12-Pt3Co or Pt3Fe, double-layer Pt on L13-Pt3Ni or Pt3Cu, and triple-layer skin on L11-PtCu, while double- or triple-layer skin on L10-PtCo or PtNi and double-layer skin on L12-PtFe3 show slightly poor durability. Although L10 and L12 based nanocrystals have been demonstrated extensively as outstanding catalysts, L11 and L13 ones hold great application potential. The coexistence of high activity and durability on the same surface is because of the different responses of surface adsorption and cohesion properties to the strain effects and ligand effects. When intermetallic-core@Pt-shell nanocrystals are constructed using this slab model, the necessity of protecting or eliminating low-coordinated Pt and the possibility of maximizing Pt(111) facets and core ordering by morphology engineering were highlighted. The current discovery provides a new paradigm toward the rational design of promising cathodic catalysts.

Theoretically evaluating two-dimensional tetragonal Si2Se2 and SiSe2 nanosheets as anode materials for alkali metal-ion batteries.

In this work, based on first-principles calculations, we theoretically predict two kinds of two-dimensional tetragonal Si-Se compounds, Si2Se2 and SiSe2, as the anode materials for alkali metal-ion batteries. The results show that Si2Se2 and SiSe2 are thermally and dynamically stable and have good electronic conductivity. The diffusion barriers of Li, Na and K atoms are 0.07 eV, 0.17 eV and 0.17 eV on the surface of Si2Se2, and 0.45 eV, 0.43 eV and 0.30 eV on the surface of SiSe2, respectively, which indicate excellent rate capability. Most remarkably, Si2Se2 and SiSe2 can deliver high specific capacities. The predicted specific capacities of Si2Se2 are 1252 mA h g-1, 501 mA h g-1 and 250 mA h g-1 for Li, Na and K storage, respectively, and the corresponding specific capacities of SiSe2 are 1441 mA h g-1, 865 mA h g-1 and 180 mA h g-1. In addition, the highest plateaus of open-circuit voltages are 0.50 V vs. Li+/Li, 0.60 V vs. Na+/Na and 1.01 V vs. K+/K for Si2Se2, and 1.13 V vs. Li+/Li, 1.09 V vs. Na+/Na and 1.01 V vs. K+/K for SiSe2, which are beneficial for achieving the high discharge voltage in full cells. Considering these advantages, Si2Se2 and SiSe2 monolayers can be competitive candidates as anode materials for alkali metal-ion batteries.

Unraveling TM Migration Mechanisms in LiNi1/3Mn1/3Co1/3O2 by Modeling and Experimental Studies.

Electrochemical cycling induces transition-metal (TM) ion migration and oxygen vacancy formation in layered transition-metal oxides, thus causing performance decay. Here, a combination of ab initio calculations and atomic level imaging is used to explore the TM migration mechanisms in LiNi1/3Mn1/3Co1/3O2 (NMC333). For the bulk model, TM/Li exchange is an favorable energy pathway for TM migration. For the surface region with the presence of oxygen vacancies, TM condensation via substitution of Li vacancies (TMsub) deciphers the frequently observed TM segregation phenomena in the surface region. Ni migrates much more easily in both the bulk and surface regions, highlighting the critical role of Ni in stabilizing layered cathodes. Moreover, once TM ions migrate to the Li layer, it is easier for TM ions to diffuse and form a TM-enriched surface layer. The present study provides vital insights into the potential paths to tailor layered cathodes with a high structural stability and superior performance.

New Insight into the Confinement Effect of Microporous Carbon in Li/Se Battery Chemistry: A Cathode with Enhanced Conductivity.

Embedding the fragmented selenium into the micropores of carbon host has been regarded as an effective strategy to change the Li-Se chemistry by a solid-solid mechanism, thereby enabling an excellent cycling stability in Li-Se batteries using carbonate electrolyte. However, the effect of spatial confinement by micropores in the electrochemical behavior of carbon/selenium materials remains ambiguous. A comparative study of using both microporous (MiC) and mesoporous carbons (MeC) with narrow pore size distribution as selenium hosts is herein reported. Systematic investigations reveal that the high Se utilization rate and better electrode kinetics of MiC/Se cathode than MeC/Se cathode may originate from both its improved Li+ and electronic conductivities. The small pore size (<1.35 nm) of the carbon matrices not only facilitates the formation of a compact and robust solid-electrolyte interface (SEI) with low interfacial resistance on cathode, but also alters the insulating nature of Li2 Se due to the emergence of itinerant electrons. By comparing the electrochemical behavior of MiC/Se cathode and the matching relationship between the diameter of pores and the dimension of solvent molecules in carbonate, ether, and solvate ionic liquid electrolyte, the key role of SEI film in the operation of C/Se cathode by quasi-solid-solid mechanism is also highlighted.

Molecular dynamics simulations of shock loading of nearly fully dense granular Ni-Al composites.

We used molecular dynamics simulations to study the shock propagation, inhomogeneous deformation, and initiation of the chemical reaction characteristics of nearly fully dense reactive Ni-Al composites. For shocks with piston velocities Up ≤ 2.0 km s-1, particle velocity dispersion was observed at the shock front, which increased on increasing the shock strength. Plastic deformation mainly occurred at the grain boundaries or grain junction during the shock rise and was accompanied by the generation of a potential hot spot in the region where severe plasticity happens. The composite exhibited higher strength and lower reactivity than the mixtures with certain porosity. In addition, the shock-induced premature melting of Al led to the expansion of particle velocity dispersion from the wavefront to the shocked zone and the formation of a heterogeneous velocity field for stronger shocks beyond critical Up (2.5 km s-1). The velocity heterogeneity in the shocked region led to localized shear, strong erosion of Ni, and occurrence of ultrafast chemical reactions. Therefore, the shock-induced premature melting of Al led to the mechanochemical effect and played a role in the shock-induced chemical reaction in the reactive metal system.

Effect of particle packing and density on shock response in ordered arrays of Ni + Al nanoparticles.

We investigate the shock response of Ni + Al reactive nanoparticle systems through molecular dynamics simulations. The powder configurations with varying arrangements and densities are constructed by stacking equal-sized Ni and Al particles based on five typical crystal structures, i.e., zinc-blende, NaCl, CsCl, AuCu and the close-packed. The effects of configuration and shock strength on mechanochemical and diffusion processes in the shock-induced chemical reactions are characterized. A reaction kinetic model is developed to describe these behaviors, assess the extent of mechanochemical effect, and explain the occurrence of ultra-fast reaction. Significant dependence of shock wave velocity, plastic deformation, temperature response, chemistry and microstructure change on particle packing and density is observed under shock loading at the same piston velocity, but we see a relatively weak dependency on the stacking mode with the same density. Our results indicate the important role of particle coordination number and density in shock response of energetic powder materials.

Intrinsic strain-induced segregation in multiply twinned Cu-Pt icosahedra.

We present an atomistic simulation study on the compositional arrangements throughout Cu-Pt icosahedra, with a specific focus on the effects of inherent strain on general segregation trends. The coexistence of radial and site-selective segregation patterns is found in bimetallic nanoparticles for a broad range of sizes and compositions, consistent with prior analytical and atomistic models. Through a thorough comparison between the composition patterns and strain-related patterns, it is suggested that the presence of gradient and site-selective segregation is natural to largely relieve the inherent strain by preferential segregation of big atoms at tensile sites and vice versa, as previously hypothesized in the literature. Analogous to the case of single crystal particles, Cu-rich surface and damped oscillations can also be found in the outer shells of icosahedra, which are dominated by the lowering of both the surface energy and the chemical energy. The thermodynamic stability of segregated icosahedra is similar to segregated cuboctahedra but higher than disordered bulk alloys, validating prior thinking that element segregation driven by strain relief can extend the stability range of multiply-twinned nanoparticles. Our work sheds new light on understanding strain-induced segregation in multiply-twinned nanosystems that have elements with large lattice mismatch and strong alloying ability.

Theoretical prediction of LiScO2 nanosheets as a cathode material for Li-O2 batteries.

Our previous study (J. Mater. Chem. A, 2018, 6, 3171-3180) theoretically predicted that a scandium oxide (ScO2) monolayer can deliver high specific capacity and energy density as the active material of a lithium-ion (Li-ion) battery, but the voltage will drop below 0.5 V when ScO2 is lithiated to LiScO2 during the discharge process. The current study predicts that the discharge product LiScO2 in the Li-ion battery mode can potentially work as the host material of Li-O2 batteries. It is found that the adsorption of O2 on the LiScO2 substrate is energetically favored. The LiScO2 substrate can also provide strong affinities to molecular LiO2 and Li2O2 species. It is interesting to find that the presence of an O2 molecule can oxidize the pre-adsorbed Li2O2 molecule and result in two LiO2 molecules. Hence, the final discharge product of the Li-O2 battery using the LiScO2 cathode is expected to be a crystalline-like LiO2 layer. The discharge voltage related to forming a LiO2 layer on the LiScO2 substrate is 3.50 V vs. Li+/Li according to the present theoretical calculation.

Revealing reaction mechanisms of nanoconfined Li2S: implications for lithium-sulfur batteries.

Using Li2S as an active material and designing nanostructured cathode hosts are considered as promising strategies to improve the performance of lithium-sulfur (Li-S) batteries. In this study, the reaction mechanisms during the delithiation of nanoconfined Li2S as an active material, represented by a Li20S10 cluster, are examined by first-principles based calculations and analysis. Local reduction and disproportionation reactions can be observed although the overall delithiation process is an oxidation reaction. Long-chain polysulfides can form as intermediate products; however they may bind to insoluble S2-via Li atoms as mediators. Activating the charging process only requires an overpotential of 0.37 V if using Li20S10 as the active material. Sulfur allotropes longer than cyclo-S8 are observed at the end of the charge process. Although the discharge voltage of Li20S10 is only 1.27 V, it can still deliver an appreciable theoretical energy density of 1480 W h kg-1. This study also suggests that hole polarons, in Li20S10 and intermediate products, can serve as carriers to facilitate charge transport. This work provides new insights toward revealing the detailed reaction mechanisms of nanoconfined Li2S as an active material in the Li-S battery cathode.

Investigation of the shock-induced chemical reaction (SICR) in Ni + Al nanoparticle mixtures.

Molecular dynamics (MD) simulations are used to investigate the shock-compression response of Ni + Al spherical nanoparticles arranged in a NaCl-like structure. The deformation and reaction characteristics are studied from the particle level to the atomic scale at various piston velocities. Shock-induced chemical reactions (SICRs) occur during non-equilibrium processes, accompanied by a sharp rise in temperature and rapid mixing of atoms. The preferentially deformed Al particles form a high-speed mass flow relative to the Ni at the shock front, which impinges on the Ni particles, and mixing of Ni and Al atoms occurs immediately at the interface. The particle velocity dispersion (PVD) that appears at the shock front has important implications for the initiation of shock-induced chemical reactions. We show that dislocations are mainly generated at the beginning of particle deformation or at the shock front, and do not directly affect the occurrence of SICRs. The intimate contact of the molten Al and the amorphous Ni is found to be critical to the subsequent reactions for the extensive mixing of Ni and Al. We conclude that the mechanisms of SICRs involve mechanochemical processes near the shock front and subsequent interdiffusion processes.

Amorphization and thermal stability of aluminum-based nanoparticles prepared from the rapid cooling of nanodroplets: effect of iron addition.

Despite an intensive investigation on bimetallic nanoparticles, little attention has been paid to their amorphization in the past few decades. The study of amorphization on a nanoscale is of considerable significance for the preparation of amorphous nanoparticles and bulk metallic glass. Herein, we pursue the amorphization process of Al-based nanoparticles with classic molecular dynamics simulations and local structural analysis techniques. By a comparative study of the amorphization of pure Al and Fe-doped Al-based nanodroplets in the course of rapid cooling, we find that Fe addition plays a very important role in the vitrification of Al-based nanodroplets. Owing to the subsurface segregated Fe atoms with their nearest neighbors tending to form relatively stable icosahedral (ICO) clusters, the Fe-centred cluster network near the surface effectively suppresses the crystallization of droplets from surface nucleation and growth as the concentration of Fe attains a certain value. The glass formation ability of nanodroplets is suggested to be enhanced by the high intrinsic inner pressure as a result of small size and surface tension, combined with the dopant-inhibited surface nucleation. In addition, the effect of the size and the added concentration of nanoparticles on amorphization and the thermal stability of the amorphous nanoparticles are discussed. Our findings reveal the amorphization mechanism in Fe-doped Al-based nanoparticles and provide a theoretical guidance for the design of amorphous materials.

Single-layered V2O5 a promising cathode material for rechargeable Li and Mg ion batteries: an ab initio study.

Using first principles calculations based on density functional theory, the adsorption and diffusion properties of Li and Mg atoms on single-layered and bulk V2O5 are investigated. The simulation results show that the diffusion barrier of Li on the single-layered V2O5 is decreased compared with that of the bulk V2O5, which indicates that the Li mobility can be significantly enhanced on the single-layered V2O5. The increased binding energies of Li to single-layered V2O5 make them more attractive for promising cathode materials. Although the diffusion barrier of Mg on the single-layered V2O5 does not decrease, the binding energies of Mg to single-layered V2O5 is increased compared with that of bulk V2O5, thus the single-layered V2O5 is an attractive cathode material for rechargeable ion batteries.

Controlling magnetism of MoS2 sheets by embedding transition-metal atoms and applying strain.

Prompted by recent experimental achievement of transition metal (TM) atoms substituted in MoS2 nanostructures during growth or saturating existing vacancies (Sun et al., ACS Nano, 2013, 7, 3506; Deepak et al., J. Am. Chem. Soc., 2007, 129, 12549), we explored, via density functional theory, the magnetic properties of a series of 3d TM atoms substituted in a MoS2 sheet, and found that Mn, Fe, Co, Ni, Cu and Zn substitutions can induce magnetism in the MoS2 sheet. The localizing unpaired 3d electrons of TM atoms respond to the introduction of a magnetic moment. Depending on the species of TM atoms, the substituted MoS2 sheet can be a metal, semiconductor or half-metal. Remarkably, the applied elastic strain can be used to control the strength of the spin-splitting of TM-3d orbitals, leading to an effective manipulation of the magnetism of the TM-substituted MoS2 sheet. We found that the magnetic moment of the Mn- and Fe-substituted MoS2 sheets can monotonously increase with the increase of tensile strain, while the magnetic moment of Co-, Ni-, Cu- and Zn-substituted MoS2 sheets initially increases and then decreases with the increase of tensile strain. An instructive mechanism was proposed to qualitatively explain the variation of magnetism with elastic strain. The finding of the magnetoelastic effect here is technologically important for the fabrication of strain-driven spin devices on MoS2 nanostructures, which allows us to go beyond the current scope limited to the spin devices within graphene and BN-based nanostructures.

The effect of antisite defects on the mechanical and dynamic stability of Nb3Al.

The A15-type conventional superconductor Nb3Al alloys has been considered as an ideal candidate for next generation high field magnets due to its higher superconducting properties and less sensitivity to stain than that of industrialized Nb3Sn superconductor. First-principles methods are employed to study the potential point defects, vacancy and antisite defects in deviating stoichiometric Nb3Al alloys and their effect on structure and mechanical properties. Our results show that antisite defects are easier to be produced than vacancy defects, and NbAlantisite defects can keep the tetragonal structure of Nb3Al. Furthermore, the influence of antisite defects on dynamic stability of Nb3Al is investigated together with NbAldefects. With the increase of Nb antisite defect content and the formation of orderly arrangement, we found the phonon spectrum yields no more soft phonon modes, which is in contradiction with the dynamical instability of stoichiometric Nb3Al with no defects. Our calculations indicate Nb antisite defects play a crucial role on the dynamic stability of Nb3Al compounds.

Magnesium Mitigation Behavior in P2-Layered Sodium-Ion Battery Cathode.

Heteroatom incorporation can effectively suppress the phase transition of layered sodium-ion battery cathode, but heteroatom behaviors during operating conditions are not completely understood at the atomic scale. Here, density functional theory calculations are combined with experiments to explore the mitigation behavior of Mg dopant and its mechanisms under operating conditions in P2-Na0.67Ni0.33Mn0.67O2. The void formed by Na extraction will pump some Mg dopants into Na layers from TM layers, and the collective diffusion of more than one Mg ion most likely occurs when the Mg content is relatively high in the TM layer, finally aggregating to form Mg-enrich regions (i.e., Mg segregation) apart from Ni vacancies. The void-pump-effect-induced Mg segregation effectively suppresses the P2-O2 phase transition owing to the stronger Mg-O electrostatic attraction that enhances the integrate of two adjacent oxygen layers and prevents the crack growth by mitigating the lattice volume variation under high-voltage cycling. Our work provides a fundamental understanding of heteroatom mitigation behavior in layered cathodes at the atomic level for next-generation energy storage technologies.

Shock-induced plasticity and phase transformation in single crystal magnesium: an interatomic potential and non-equilibrium molecular dynamics simulations.

An effective and reliable Finnis-Sinclair (FS) type potential is developed for large-scale molecular dynamics (MD) simulations of plasticity and phase transition of magnesium (Mg) single crystals under high-pressure shock loading. The shock-wave profiles exhibit a split elastic-inelastic wave in the [0001]HCPshock orientation and a three-wave structure in the [10-10]HCPand [-12-10]HCPdirections, namely, an elastic precursor, a followed plastic front, and a phase-transition front. The shock Hugoniot of the particle velocity (Up) vs the shock velocity (Us) of Mg single crystals in three shock directions under low shock strength reveals apparent anisotropy, which vanishes with increasing shock strength. For the [0001]HCPshock direction, the amorphization caused by strong atomic strain plays an important role in the phase transition and allows for the phase transition from an isotropic stressed state to the product phase. The reorientation in the shock directions [10-10]HCPand [-12-10]HCP, as the primary plasticity deformation, leads to the compressed hexagonal close-packed (HCP) phase and reduces the phase-transition threshold pressure. The phase-transition pathway in the shock direction [0001]HCPincludes a preferential contraction strain along the [0001]HCPdirection, a tension along [-12-10]HCPdirection, an effective contraction and shear along the [10-10]HCPdirection. For the [10-10]HCPand [-12-10]HCPshock directions, the phase-transition pathway consists of two steps: a reorientation and the subsequent transition from the reorientation hexagonal close-packed phase (RHCP) to the body-centered cubic (BCC). The orientation relationships between HCP and BCC are (0001)HCP⟨-12-10⟩HCP// {110}BCC⟨001⟩BCC. Due to different slipping directions during the phase transition, three variants of the product phase are observed in the shocked samples, accompanied by three kinds of typical coherent twin-grain boundaries between the variants. The results indicate that the highly concentrated shear stress leads to the crystal lattice instability in the elastic precursor, and the plasticity or the phase transition relaxed the shear stress.