Migration Of Atoms Research Articles

Molecular dynamics simulation of the effect of nickel transition layer on the deposition of carbon atoms and graphene growth on cemented carbide surfaces

WC-Co cemented carbide has excellent cutting performance, which is a potential tool material. But when used for cutting ultra-high strength and high hardness materials, the machining accuracy and service life of the tool are significantly reduced. It was used to cut still the challenges to be solved, which led to the development of coated tools. Graphene is a potential coating material for cemented carbide cutting tools due to its excellent mechanical properties. In this paper, molecular dynamics (MD) was applied to simulate the deposition of nickel transition layer and high-temperature catalytic growth of graphene in cemented carbide. The physical vapor deposition method was used to simulate the Ni and C atomic deposition and high temperature annealing processes, and a combination of potential functions was applied to simulate the two deposition processes without interruption. The effects of deposition temperature and incident energy on the growth of graphene were analyzed. The healing mechanism of nickel-based catalytic defective graphene under high-temperature annealing was explored in detail.<br>Simulation Discovery：At the deposition temperature of 1100 K, the coverage of graphene is higher and the microstructure is flat. The higher temperature helps to provide enough kinetic energy of carbon atoms to overcome the potential energy barrier of nucleation, promote the migration and rearrangement of carbon atoms, and reduce graphene growth defects. Too high temperature leads to the formation of multilayered reticulation and disordered structure by carbon atoms continuously stacking up at the deposited carbon rings, which causes a low coverage rate of graphene. The increase of incident energy helps to reduce the vacancy defects in the film, but too high energy leads to poor continuity of the film, agglomeration, the stacking effect of carbon atoms and the tendency of epitaxial growth are more obvious. When the incident energy is 1 eV, the surface roughness of the film is lower, and more monolayer graphene can be grown. During annealing at 1100 K, the carbon film simultaneously underwent dissolution and nucleation in the Ni transition layer, and the nickel transition layer catalytic defect graphene healed. The graphene film became more uniform, and the number of hexagonal carbon rings increased. Appropriate high-temperature annealing facilitates the repair and reconstruction of defective carbon rings and the rearrangement of carbon chains into rings. Therefore, a high quality graphene coating was prepared after graphene deposition/ annealing at 1100 K and incident energy of 1 eV. The simulation results provide reference for the preparation of cemented carbide graphene coated tools.

Structure evolution and specific effects for the catalysis of atomically ordered intermetallic compounds.

Atomically ordered intermetallic compounds (IMCs) have been extensively studied for exploring catalysts with high activity, selectivity, and longevity. Compared to random alloys, IMCs present a more pronounced geometric and electronic effect with desirable catalytic performance. Their well-defined structure makes IMCs ideal model catalysts for studying the catalytic mechanism. This review focuses especially on elemental composition, electron transfer, and structure/phase evolution under high temperature treatment conditions, providing direct evidence for the migration and rearrangement of metal atoms through electron microscopy. We then present the outstanding applications of IMCs in growing single-walled nanotubes, hydrogenation/dehydrogenation reactions, and electrocatalysis from the perspective of electronic, geometric, strain, and bifunctional effects of ordered IMCs. Finally, the current obstacles associated with the use of in situ techniques are proposed, as well as future research possibilities.

Study on the relationship between structure and magnetic properties of τ-phase MnAl prepared by cryo-milling

The unique characteristics, such as high magnetic moment, high Curie temperature, large magnetic crystalline anisotropy, and low cost, make the τ-phase MnAl a promising candidate as the market alternatives which could fill the gap between the rare earth magnets and ferrite magnets. Because at low temperatures the metal will become more brittle, the cryomilling technology may allow the τ-phase MnAl to be broken quickly and introduce few crystal defects, and then obtain better permanent magnetic properties. In this study, the cryomilling technology was used to grind the τ-phase MnAl, and the structural and magnetic properties of the obtained powder sample were investigated. It was found that compared to room-temperature ball milling, the morphology of cryomilled powders is granular and the powder agglomeration is dramatically suppressed at a low temperature, and as a result the larger particles tend to be broken down into smaller ones rather than being agglomerated to larger thin layers. The grain size D reduced continuously from 145 nm to 36 nm when the sample was milled for 120 minutes. The coercivity increased with increasing the cryo-milling time, while the saturation magnetization decreased. The maximum coercivity of up to 4.9 kOe was obtained by cryo-milling for 100 minutes. By the neutron diffraction analysis, it was confirmed that the decrease of the saturation magnetization with increasing milling time is mainly due to the migration of Mn atoms from 1a site to the 1d site and the decrease of Mn atomic magnetic moment.

A novel core-shell nanostructure of Ti-Au nanocrystal with PtNi alloy skin: Enhancing the durability for oxygen reduction reaction

The sluggish oxygen reduction reaction (ORR) at the cathode largely hinders the practical application of the proton-exchange membrane fuel cells (PEMFCs). Pt-based nanocrystals are the most effective electrocatalysts for the oxygen reduction reaction, however, suffer high cost, scarcity and unsatisfied durability. Herein, we report a new category of annealed multimetallic core-shell catalyst of gold core doped with titanium and platinum-nickel (PtNi) shell with Pt-rich surface as a highly active and robust electrocatalyst for ORR. The introduction of titanium can effectively stabilize the gold core, avoiding the outward migration of Au atoms and ensuring the exposure of Pt-Ni active sites. The experimental findings revealed that the annealed Ti-Au@PtNi NPs catalyst provides 19.26 and 9.84 times higher mass and specific activities than commercial Pt/C catalysts for the oxygen reduction reaction. Moreover, durability tests of annealed Ti-Au@PtNi NPs show no noticeable activity loss even after 20000 potential cycles between 0.6 and 1.0 V vs. RHE, demonstrating a promising catalyst for the oxygen reduction reaction.

Theoretical study of two-dimensional BeB<sub>2</sub> monolayer as anode material for magnesium ion batteries

<sec>Rechargeable lithium-ion batteries as the main energy storage equipment should possess high power density, excellent reversible capacity, and long cycle life. However, due to the high cost and dendrite growth of Li, searching for non-Li-ion batteries is urgent. Compared with lithium, magnesium has abundant resources, small ionic radius, and high energy density. Therefore, magnesium-ion batteries (MIBs) can serve as the next generation metal-ion batteries. Two-dimensional materials based on Be or B element acting as the anode of metal-ion batteries always exhibit high theoretical storage capacity. Using first-principles calculations, we systematically explore the potential of BeB<sub>2</sub> as MIBs anode. The optimized BeB<sub>2</sub> monolayer structure shown in Fig. (a) consists of two atomic layers, where each Be atom is coordinated with six B atoms, and each B atom is coordinated with three Be atoms.</sec><sec>The lattice constants are <i>a</i> = <i>b</i> = 3.037 Å with a thickness of 0.554 Å. From the phonon spectrum calculations, the absence of imaginary modes indicates the dynamic stability of BeB<sub>2</sub> monolayer. The presence of a Dirac cone further suggests the excellent conductivity (Fig.(b)). Three stable adsorption sites (Be<sub>1</sub>: top of Be atoms; Be<sub>2</sub> and B<sub>2</sub>: bottom of Be and B atoms) are labeled in Fig. (a). Taking symmetry into account, we consider three pathways to evaluate the migration of Mg atom on BeB<sub>2</sub> monolayer (Fig.(c)). The corresponding lowest diffusion energy barrier is 0.04 eV along Path III. The stable configuration with the maximum adsorption Mg concentration is shown in Fig.(d), which generates a theoretical capacity of 5250 mA·h·g<sup>–1</sup>. The calculated average open-circuit voltage is 0.33 V. Based on <i>ab initio</i> molecular dynamics simulations, the total energy of BeB<sub>2,</sub> with Mg adsorbed, fluctuates within a narrow range, suggesting that BeB<sub>2</sub> can sustain structural stability after storing Mg at room temperature (Fig.(e)). Finally, for practical application, we investigate the adsorption and diffusion behavior of Mg on bilayer BeB<sub>2</sub>. Three configurations are considered: <i>AA</i> stacking (overlapping of Be atoms in upper layer with Be atoms in lower layer), <i>AB</i> stacking (overlapping of Be atoms in upper layer with B atoms in lower layer), and <i>AC</i> stacking (overlapping of Be atoms in upper layer with B—B bonds in lower layer). The most stable configuration is <i>AB</i> stacking (shown in Fig.(f)) with the interlayer spacing of 3.12 Å and the binding energy of –120.97 meV/atom. Comparing with the BeB<sub>2</sub> monolayer structure, the adsorption energy of Mg is –2.24 eV for Be<sub>1</sub>, –1.38 eV for B<sub>5</sub> site, and –1.90 eV for B<sub>4</sub> site, while the lowest diffusion energy barrier is 0.13 eV along the path of B<sub>5</sub>-Be<sub>3</sub>-B<sub>5</sub>. Therefore, according to the above-mentioned properties, we believe that BeB<sub>2</sub> monolayer can serve as an excellent MIBs anode material.</sec>

Coalescence behavior of size-selected gold and tantalum nanoclusters under electron beam irradiation: insights into nano-welding mechanisms.

The emerging technique of nano-welding (NW) via precisely regulating the fusion of nanoclusters (NCs) in nanotechnology has attracted significant attention for its innovative approach. Employing the gas-phase condensation cluster source with a lateral time-of-flight (TOF) mass-selector, size-selected gold (Au), and tantalum (Ta) NCs were prepared. This study explores the coalescence behavior of size-selected Au and Ta NCs under electron beam irradiation, aiming to investigate the related mechanism governing the welding process. Intrinsically driven by the reduction of excess surface energy, electron beam induces atomic thermal migration, fostering sintering neck growth at cluster interfaces. During this process, atomic diffusion and recrystallization enable NCs to alter shape while retaining stable facet planes. Aberration-corrected scanning transmission electron microscopy (AC-STEM) showcases the formation of single or polycrystalline sintered clusters, during which some lattice distortions can be eliminated. Interestingly, oxidized Ta clusters experience knock-on damage caused by elastic scattering of electron beams, partially deoxidizing them. Additionally, electron-phonon inelastic scattering transforms oxidized Ta clusters from amorphous to crystalline structures. Moreover, the quantum size effect and surface effect of NCs facilitate the surpassing of miscibility limits during Au-Ta heterogeneous welding processes. This investigation bridges the gap between fundamental research on cluster materials and their practical applications.

Hydrogen diffusion in zirconium hydrides from on-the-fly machine learning molecular dynamics

Reactor pressure vessels and fuel cladding tubes have repeatedly failed due to zirconium hydrides. Zirconium hydride precipitation and growth are directly affected by hydrogen atom transport properties, which would make nuclear fuel storage less safe over long periods of time. Herein, we employ first-principles calculations to investigate the hydrogen diffusion mechanism in zirconium hydrides, utilizing on-the-fly machine learning force field molecular dynamics. It is verified that the machine learning force field can accurately describe the hydrogen atomic diffusion properties in zirconium hydrides at several temperatures and compositions. The atomic migration paths of hydrogen in zirconium hydrides as well as their barriers and pre-factors are also calculated. According to our results, the temperature and composition of zirconium hydrides affect the microscopic dynamical behavior of hydrogen.

Intermetallic Pd-Y nanoparticles/N-doped carbon nanotubes as multi-active catalysts for oxygen reduction reaction, ethanol oxidation reaction, and zinc-air batteries.

Intermetallic nanomaterials are unique in terms of their band gap, atomic-level arrangement, and well-defined stoichiometry, which allows them to exhibit significantly enhanced catalytic performance in electrochemical applications. However, the preparation of durable intermetallic catalysts with a lower content of platinum group metals is challenging, while the lack of control over the loss of active components limits their long-term application due to weak interaction between the support and the nanostructure. Here, we have designed the intermetallic alloyed nanoparticles (NPs) of PdY on N-doped carbon nanotubes (PdY/NCNTs) as a multifunctional catalyst for the oxygen reduction reaction (ORR), the ethanol oxidation reaction (EOR), and zinc-air batteries (ZABs). The strong adhesion through nitrogen ensures the anchoring of alloyed PdY NPs on the NCNTs, which restrains atomic migration and sintering during their conversion to intermetallic phases. This study confirms that there is negligible active site leaching owing to the strong and multiple dative bonds between the NCNTs and PdY NPs. Therefore, this catalyst exhibits remarkable catalytic activity, resulting in a mass activity of 1317 and 2902 mA mgPd-1 at jk and jf for the ORR and the EOR, respectively, and remains stable for a longer period. In addition, the PdY/NCNT-containing air cathode-fabricated ZAB achieved a higher power density (0.236 W cm-2) compared to the benchmark Pt/C.

Sintering behavior and activation energy calculation of nanoparticulate Fe3O4: a molecular dynamics simulation

This study employs molecular dynamic simulations to investigate the sintering mechanisms of Fe3O4 nanoparticles and determine the sintering activation energy. The melting temperature has been calculated using atomic volume to verify the accuracy of this EAM function and its parameters. Neck width has been defined to demonstrate the sintering degree under different temperatures, from 1373 K to 1773 K. Following sections divide the whole evolution into three mechanisms: surface diffusion, grain boundary diffusion and viscous flow according to the atomic migration vector. Finally, the sintering activation energy has been figured out based on the neck diameter ratio which later has been employed to predict the neck growth curve at 1523 K. Therefore, this investigation reveals the sintering mechanisms and introduces a method to predict the growth rate using activation energy.

Inhibition of roof-type Cu6Sn5 grains on migration of Cu atoms under temperature gradient

Inhibition of roof-type Cu6Sn5 grains on migration of Cu atoms under temperature gradient

Solid and Liquid State Dewetting of Thin Silicon Films Sandwiched between Two Silicon Carbide Substrates

Silicon carbide (SiC) is a very promising material for power electronics thanks to its wide band gap and high thermal conductivity. Stacking of SiC substrates of different crystalline quality and/or doping levels through wafer bonding is of particular interest for applications. Although a direct SiC/SiC bonding may result in an unstable interface, a layer of intermediate material playing a role of a “glue” can be sandwiched between the SiC substrates to ensure their good attachment. However, the processing of the power electronics devices at a high temperature may degrade the properties of this material which evolution under high temperature annealing must be studied in advance.In this work, we exploited the possibility to use silicon (Si) as a bonding material. We studied the structure evolution of a nanometer-thick polycrystalline Si layer sandwiched between two monocrystalline SiC substrates with a high temperature annealing. We focused our attention on understanding the impact of as-bonded Si layer thickness and the temperature of annealing on the eventual Si layer morphology. Tests were conducted for Si films of as-bonded thickness of 8 nm, 20 nm and 40 nm at temperatures ranging from 950°C to 1400°C, i.e. below Si melting temperature of 1414°C, and above it up to 1700°C. The experiments were realized using transmission electron microscopy (TEM), scanning electron microscopy (SEM) and high-resolution X-ray diffraction techniques. Cross-sectional TEM images were used to describe the structure and morphology of the interface. SEM was used to observe the inner surface of one of the SiC substrates once the other was removed by a cleavage. X-ray measurements were carried out to determine Si film thickness and texture as well as the strain in Si and SiC.We show that annealing at temperatures ranging from 950°C to 1400°C results in pitting, i.e. creation of voids, at the Si/SiC interfaces which are observed for any studied as-bonded thickness of a Si layer. We attribute this effect to a solid state dewetting of Si from SiC. This dewetting effect evolves into complete piercing of the silicon film and formation of cylindrical voids within the Si layer which grow in diameter with annealing temperature. We also observe a thickening of the silicon layer. The driving force for a solid state dewetting will be discussed in terms of SiC/Si interface energy, vacancies precipitation and Si atoms diffusion.When annealed to relatively higher temperatures allowing for Si melting followed by its solidification, we put into evidence the formation of step bunching on the internal surfaces of both SiC substrates. Regarding Si, it is concentrated along the SiC steps. The mechanism of the reconstruction of SiC internal surfaces resulting in the creation of step bunching will be discussed in relation to Si liquid state dewetting, carbon atoms dissolution, recondensation and migration within liquid Si.The effect of as-bonded silicon film thickness on the Si dewetting process is not pronounced after annealing at 950°C: all samples contain pits of a similar size and density. When the annealing temperature is slightly increased above 1000°C, the pits in the Si films of a lower initial thickness transform into cylindrical voids piercing the layers. The thicker layers contain both pits and cylindrical voids. At a high enough temperature of 1400°C only cylindrical voids were observed in all samples. The thicker layers present the lowest densities of voids.For the annealing at temperatures higher than 1400°C, the as-bonded thickness of a Si layer had a direct impact on the height of the steps formed on the internal surfaces of the SiC substrates. The eventual steps were higher the thicker the Si layer was used for bonding.The role of the diffusion and interaction of Si and C atoms and vacancies during annealing will be discussed in relation to the dynamics of solid and liquid state Si dewetting, size and density of pits and cylindrical voids and eventual Si layer morphology.Figure 1, on the left, SEM images showing the evolution of Si solid state dewetting with annealing temperature, from pitting to the formation of cylindrical voids. On the right, SEM and cross-sectional TEM images showing the step bunching on the SiC internal surfaces and the Si accumulation at the steps obtained after Si liquid state dewetting. Figure 1

Transforming microstructures and mechanical properties of (CoCrNi)93-xAl7Ndx medium entropy alloy films by annealing

In this context, we aim to investigate the effects of Nd content and annealing on the microstructures and mechanical properties of a series of (CoCrNi)93-xAl7Ndx (x = 0, 0.22, 0.56, 1.05, 1.73, 2.55, 3.53) medium entropy alloy films. The results showed that doping with Nd resulted in grain refinement and triggered a transition from a single FCC solid solution to nanocrystalline and amorphous structures. The mechanical properties, evaluated using nanoindentation and micropillar compression tests, showed that 0.56 at.% Nd doping yielded the optimum outcome due to the formation of a solid solution and grain refinement. However, the mechanical properties dropped sharply with further increases in Nd content due to the inverse Hall-Petch effect and the transition from an FCC to an amorphous matrix. The annealing treatment (550 °C for 10 min) was found to enhance atomic diffusion and migration, facilitating lattice rearrangement and the formation of a large number of precipitates, including BCC Cr-rich, B2 NiAl, L12 Ni3Al, and HCP NdNi5 phases. In addition, we also found that annealing resulted in an increase in strength and a decrease in ductility, which was contrary to the behavior of conventional metal materials.

Effect of transition metals co-dopant on eliminating boron and phosphorous impurities from silicon

In this work, we report the results of the systematic studies of the interactions between boron and phosphorous impurities in silicon matrix and transitional metals (TM) co-dopants: iron, cobalt, and nickel. Calculations results demonstrate the favorability of the stable pairs of substitutional boron and phosphorous defects. Calculated energies of the formation of substitutional and interstitial TM defects explain the high solubility and mobility of this kind of impurity. The simulations also reveal reciprocal affinity between B, P, and TM impurities that leads to the migration of metal atoms toward boron and phosphorous defects. Redistribution of the charge density caused by the presence of TM in the neighboring position leads to a significant decrease in the energy cost for the migration of B and P substitutional impurities to the interstitial void.

Strongly-Interacted NiSe2/NiFe2O4 Architectures Built Through Selective Atomic Migration as Catalysts for the Oxygen Evolution Reaction.

The interactions between the catalyst and support are widely used in many important catalytic reactions but the construction of strong interaction with definite microenvironments to understand the structure-activity relationship is still challenging. Here, strongly-interacted composites are prepared via selective exsolution of active NiSe2 from the host matrix of NiFe2O4 (S-NiSe2/NiFe2O4) taking advantage of the differences of migration energy, in which the NiSe2 possessed both high dispersion and small size. The characteristics of spatially resolved scanning transmission X-ray microscopy (STXM) coupled with analytical Mössbauer spectra for the surface and bulk electronic structures unveiled that this strongly interacted composite triggered more charge transfers from the NiSe2 to the host of NiFe2O4 while stabilizing the inherent atomic coordination of NiFe2O4. The obtained S-NiSe2/NiFe2O4 exhibits overpotentials of 290mV at 10mAcm-2 for oxygen evolution reaction (OER). This strategy is general and can be extended to other supported catalysts, providing a powerful tool for modulating the catalytic performance of strongly-interacted composites.

MBE growth of SnTe films on GaAs substrates with ZnTe buffer layers

SnTe (100) single-domain thin films have been previously grown on GaAs substrates by molecular beam epitaxy but obtaining excellent crystallinity can be difficult. In this paper, ZnTe buffer layers are introduced to improve the crystallinity of SnTe films. The effects of the molecular beam flux ratio (JTe/JSn) and growth temperature (Tg) on the crystallinity are investigated to achieve high-quality layers. The surface morphology and electrical properties of the SnTe thin films are also evaluated in addition to the conventional X-ray diffraction (XRD)measurements. The crystallographic properties of the grown samples are shown to be significantly improved by introducing a ZnTe buffer layer. XRD (θ–2θ) full width at half maximum of the lowest value obtained so far is about 0.14°. It is also found that lowering JTe/JSn to 0.6 results in a reduced growth rate, and an increased growth temperature (Tg = 220 °C) promotes the migration of atoms at the growth front. Both effects contribute to improvement in the crystallographic quality, including the layer surface smoothness. The cross-sectional transmission electron microscopy observation of the sample indicates the abrupt interface formation with different lattice structures. It is also found that low growth temperatures (below 200 °C) are not favorable for the SnTe layer growth, which is confirmed by the surface morphology.

Study on Phase Electromigration and Segregation Behavior of Cu-Cored Sn-58Bi Solder Interconnects under Electric Current Stressing

Cu-cored solder interconnects have been demonstrated to increase the performance of interconnect structures, while the quantitative understanding of the effect of the Cu-cored structure on microstructure evolution and atomic migration in solder interconnects is still limited. In this work, the effect of the Cu-cored structure on phase migration and segregation behavior of Sn-58Bi solder interconnects under electric current stressing is quantitatively studied using a developed phase field model. Severe phase segregation and redistribution of Bi-rich phase are observed in the Cu-cored Sn-58Bi interconnects due to the more pronounced current crowding effect near the Cu core periphery. The average current density and temperature gradient in Sn-rich phase and Bi-rich phase decrease with an increase in the diameter of the Cu core. The temperature gradient caused by Joule heating is significantly reduced owing to the presence of the Cu core. Embedding of the Cu core in the solder matrix could weaken the directional diffusion flux of Bi atoms, so that the enrichment and segregation of the Bi phase towards the anode side are significantly reduced. Furthermore, the voltage across the solder interconnects is correspondingly changed due to the phase migration and redistribution.

The occurrence of oxygen in lignite derivatives and residue from catalytic ethanolysis of Shengli lignite under an inert atmosphere

Lignite is rich in aromatic structures and oxygen atoms, which have the potential to produce organic oxygen-containing compounds. Investigating the cleavage of the >CH–O– bond and oxygen transfer behavior in the catalytic ethanolysis of lignite is indispensable. Herein, A difunctional catalyst composed of HZSM-5 and Ni nanoparticles was fabricated for splitting the >CH–O– bond in the absence of H2. It is found that the >CH–O– bond in Benzyloxy benzene (BOB) could be completely broken with ethanol in the absence of H2. Ethanol can be activated and release H+ over Ni/HZSM-5, which transfers and attacks the >CH–O– bond for obtaining toluene and phenols. The soluble portions from ethanolysis of Shengli lignite were analyzed with fourier transform infrared spectrometer and gas chromatograph/mass spectrometer for tracking the migration of oxygen atoms. It is apparent that esters with higher relative content (32.6 %) were detected in soluble portion 1 (SP1), while alcohols (35.3 %) dominate catalytically soluble portion 1 (CSP1). Moreover, a large number of alkyl-substituted phenols were identified in both SP1 and CSP1. Additionally, related analysis of SL and residual coal manifested that >CH–O– and >C=O bonds were the main forms of existence in residue from catalytic ethanolysis of SL. In a word, exploring the occurrence of oxygen in catalytic ethanolysis of lignite has the potential to produce lignite-derived organic oxygenated chemicals.

Migration and aggregation of Pt atoms on metal oxide-supported ceria nanodomes control reverse water gas shift reaction activity

Single-atom catalysts (SACs) are particularly sensitive to external conditions, complicating the identification of catalytically active species and active sites under in situ or operando conditions. We developed a methodology for tracing the structural evolution of SACs to nanoparticles, identifying the active species and their link to the catalytic activity for the reverse water gas shift (RWGS) reaction. The new method is illustrated by studying structure-activity relationships in two materials containing Pt SACs on ceria nanodomes, supported on either ceria or titania. These materials exhibited distinctly different activities for CO production. Multimodal operando characterization attributed the enhanced activity of the titania-supported catalysts at temperatures below 320 ˚C to the formation of unique Pt sites at the ceria-titania interface capable of forming Pt nanoparticles, the active species for the RWGS reaction. Migration of Pt nanoparticles to titania support was found to be responsible for the deactivation of titania-supported catalysts at elevated temperatures. Tracking the migration of Pt atoms provides a new opportunity to investigate the activation and deactivation of Pt SACs for the RWGS reaction.

Reinforcing or weakening? Determined by the interfacial nanostructures in graphene reinforced copper matrix composites

Graphene reinforced copper matrix composites (Gr/Cu) has been intensively studied, however, precisely controlling interfacial structure, understanding the reinforcing or weakening mechanism at nanoscale are still scientific problems by experiment method. Using a molecular dynamics simulation method, we examined the intrinsic relationship between nanostructures and evolutions mechanisms of various Gr/Cu composites. It was found that reinforcement of the Gr/Cu composites interface was dependent on its nanostructures. Under compression loading, interface with Gr/Cu{100} was more likely to experience a plastic deformation comparing to Gr/Cu{111} and Gr/Cu{110} interfaces. Graphene could lubricate the atoms migration at the Gr/Cu{100} interface but hinder these process at the Gr/Cu{111} and Cu{110} interfaces. Dislocation analysis confirmed that Shockly (1/6 〈112〉) dislocation was a main factor affecting the interface evolutions and dislocation propagation was limited by graphene sheets at the interface. Besides, the face-centred cubic (FCC) to hexagonal close-packed (HCP) transition induced by formation of stacking faults were observed to dominate in plastic deformation under higher compression strains to some extent, and the graphene sheet in grain boundaries could reverse the transition direction, which kept the plastic deformations under high compression strain more stable. Our findings may provide more insight to understand the interface characteristics-deformation relation of the graphene reinforced copper matrix composites.

The Unimolecular Chemistry of Methyl Chloroformate Ions and Neutrals: A Story of Near-Threshold Decomposition.

The near-threshold dissociation of ionized and neutral methyl chloroformate (CH3COOCl, MCF) was explored with imaging photoelectron photoion coincidence spectroscopy. The threshold photoelectron spectrum (TPES) for MCF was acquired for the first time; the large geometry changes upon ionization of MCF result in a broad, poorly defined TPES. Franck-Condon simulations are consistent with an adiabatic ionization energy (IE) of 10.90 ± 0.05 eV. Ionized MCF dissociates by chlorine atom loss at a measured 0 K appearance energy (AE) of 11.30 ± 0.01 eV. Together with the above IE, this AE suggests a reaction barrier of 0.40 ± 0.05 eV, consistent with the SVECV-f12 computational result of 0.41 eV. At higher internal energies, the loss of CH3O• becomes competitive due to its lower entropy of activation. Pyrolysis of neutral MCF formed the anticipated major products CH3Cl + CO2 (R1) and the minor products HCl + CO + CH2O (R2). The thermal decomposition products were identified by their photoion mass-selected threshold photoelectron spectrum (ms-TPES). Possible reaction pathways were explored computationally to confirm the dominant ones: R1 proceeds by a concerted Cl atom migration via a four-membered transition state in agreement with the mechanism proposed in the literature. R2 is a two-step reaction first yielding 2-oxiranone by HCl loss, which then decomposes to CH2O and CO. Kinetic modeling of the neutral decomposition could simulate the observed reactions only if the vibrational temperature of the MCF was assumed not to cool in the expansion.