Attractive Electron Delocalization Behavior of FeCoMoPB Amorphous Nanoplates for Highly Efficient Alkaline Water Oxidation.

The rational design of high-performance and cost-effective electrocatalysts to overcome the kinetically sluggish water oxidation reaction is a grand challenge in water electrolysis. Transitional metals with incompletely filled d orbitals are expected to have intrinsic electronic interaction to promote the reaction kinetics, however, the construction of multiple active sites is still a bottleneck problem. Here, inspired by an amorphous alloy design strategy with chemical tunability, a noble-metal-free FeCoMoPB amorphous nanoplate for superior alkaline water oxidation is developed. The achieved overpotentials at current densities of 10, 100, and 500 mA cm-2 are 239, 281, and 331 mV, respectively, while retaining a reliable stability of 48 h, outperforming most currently available electrocatalysts. Experimental and theoretical results reveal that the chemical complexity of the amorphous nanoplate leads to the formation of multiple active sites that is able to greatly lower the free energy of the rate-determining step during the water oxidation reaction. Moreover, the Mo element would result in an electron delocalization behavior to promote electron redistribution at its surrounding regions for readily donating and taking electrons. This amorphous alloy design strategy is expected to stimulate the development of more efficient electrocatalysts that is applicable in energy devices, such as metal-air batteries, fuel cells, and water electrolysis.

Controllable construction of hierarchically porous carbon composite of nanosheet network for advanced dual-carbon potassium-ion capacitors.

Benefitting from the abundance and inexpensive nature of potassium resources, potassium-ion energy storage technology is considered a potential alternative to current lithium-ion systems. Potassium-ion capacitors (PICs) as a burgeoning K-ion electrochemical energy storage device, are capable of delivering high energy at high power without sacrificing lifespan. However, owing to the sluggish kinetics and significant volume change induced by the large K+-diameter, matched electrode materials with good ion accessibility and fast K+ intercalation/deintercalation capability are urgently desired. In this work, pine needles and graphene oxide (GO) are utilized as precursors to fabricate oxygen-doped activated carbon/graphene (OAC/G) porous nanosheet composites. The introduction of GO not only induces the generation of interconnected nanosheet network, but also increases the oxygen-doping content of the composite, thus expanding the graphite interlayer spacing. Experimental analysis combined with first-principle calculations reveal the transport/storage mechanism of K+ in the OAC/G composite anode, demonstrating that the high surface area, sufficient reactive sites, enlarged interlayer distance and open channels in the porous nanosheet network contribute to rapid and effective K+ diffusion and storage. When incorporated with pine needle-activated carbon as cathode, the assembled dual-carbon PICs can function at a high voltage of 5 V, exhibiting a high energy density of 156.7 Wh kg-1 at a power density of 500 W kg-1 along with a satisfied cycle life, which highlights their potential application in economic and advanced PICs.

Atomic-level crystallization in selective laser melting fabricated Zr-based metallic glasses.

As a promising additive manufacturing technique, selective laser melting (SLM) provides the possibility of fabricating metallic glassy components free of the constraints of geometrical complexity and dimensions. However, unexpected crystallization greatly affects the microstructure and degrades the mechanical performance of SLM-fabricated metallic glasses (MGs). To clarify the crystallization mechanism and the effect of laser processing on the crystallization, we investigate the atomic-level crystallization in the SLM Zr90Cu10 MG by using molecular dynamics simulations. The results show that crystallization highly related to scan speed lies in the atomic-level cluster changes. Lower scan speed leads to a dramatically increased fraction of the BCC crystal phase, accompanied by the nucleation of a few HCP and FCC crystal phases. As scan speed increases, more icosahedron-like clusters are formed, leading to the formation of the MG, while the nucleation of the crystal phase is suppressed. The suppression of crystallization is further attributed to a higher average temperature variation rate induced by higher scan speed, which reduces the relaxation time, preventing the nucleation and growth of crystal phases. This work contributes to the understanding of the crystallization in MGs during the SLM process at the atomic level, providing guidance to suppress the crystallization in the SLM process of desired metallic glassy components.

Nitrogen/Oxygen Co-Doped Hierarchically Porous Carbon for High-Performance Potassium Storage.

Although the insertion of potassium ions into graphite has been proven to be realistic, the electrochemical performance of potassium-ion batteries (PIBs) is not yet satisfactory. Therefore, more effort is required to improve the specific capabilities and achieve a long cycling life. The mild carbonization process in molten salt (NaCl-KCl) is used to synthesize nitrogen/oxygen co-doped hierarchically porous carbon (NOPC) for PIBs by using cyanobacteria as the carbon source. This exhibits highly reversible capacities and ultra-long cycling stability, retaining a capacity of 266 mA h g-1 at 50 mA g-1 (100 cycles) and presents a capacity of 104.3 mA h g-1 at 1000 mA g-1 (1000 cycles). Kinetics analysis reveals that the potassium ion (K+ ) storage of NOPC is controlled by a capacitive process, which plays a crucial role in the excellent rate performance and superior reversible ability. The high proportion of capacitive behavior can be ascribed to the hierarchically porous structure and improved conductivity resulting from nitrogen and oxygen doping. Furthermore, density functional theory (DFT) calculations theoretically validate the enhanced potassium storage effect of the as-obtained NOPC. More importantly, the route to NOPC from cyanobacteria in molten salt provides a green approach to the synthesis of porous carbon materials.

Critical Analysis of an FeP Empirical Potential Employed to Study the Fracture of Metallic Glasses.

An empirical potential that has been widely used to perform molecular dynamics studies on the fracture behavior of FeP metallic glasses is shown to exhibit spinodal decomposition in the composition range commonly studied. The phosphorous segregation induces a transition from ductility to brittleness. During brittle fracture the atomically sharp crack tip propagates along a percolating path with higher P concentration. This embrittlement is observed to occur over a wide range of chemical compositions, and toughness decreases linearly with the degree of compositional segregation over the entire regime studied. Stable glass forming alloys that can be quenched at low quench rates do not, as a rule, exhibit such thermodynamically unstable behavior near to or above their glass transition temperatures. Hence, the microstructures exhibited in these simulations are unlikely to reflect the actual microstructures or fracture behaviors of the glassy alloys they seek to elucidate.

In situ formation of spherical MoS2 nanoparticles for ultra-low friction.

The motion resistance and energy dissipation of rolling friction are much lower than those of sliding friction at the macroscale. But at the microscale, the impact of rolling on friction remains an open question. Here, we show that spherical MoS2 nanoparticles can be formed in situ at a friction interface by scrolling and wrapping MoS2 nanosheets under the induction of a reciprocating shear stress, when an MoS2 coating constructed from loosely stacked nanosheets is tested in a vacuum of 3.5 × 10-3 Pa. An ultra-low friction state can be readily realized with friction coefficients of 0.004-0.006, which are one order of magnitude lower than that of a pulse laser deposited MoS2 coating without nanoparticles formed in a friction process. Accordingly, the spherical nanoparticles are highlighted as the key factor in the ultra-low friction. Classical molecular dynamics simulations further reveal that the motion mode of the MoS2 nanoparticle is stress-dependent. This finding confirms access to ultra-low friction by introducing rolling friction based on the microstructural evolution of the coating.

Dandelion-like nickel/cobalt metal-organic framework based electrode materials for high performance supercapacitors.

Metal-organic frameworks (MOFs), serving as a promising electrode material in the supercapacitors, have attracted tremendous interests in recent years. Here, through modifying the molar ratio of the Ni2+ and Co2+, we have successfully fabricated Ni-MOF and Ni/Co-MOF by a facile hydrothermal method. The Ni/Co-MOF with a dandelion-like hollow structure shows an excellent specific capacitance of 758 F g-1 at 1 A g-1 in the three-electrode system. Comparing with Ni-MOF, the obtained Ni/Co-MOF has a better rate capacitance (89% retention at 10 A g-1) and cycling life (75% retention after 5000 circulations). Besides, the assembled asymmetric supercapacitor based on Ni/Co-MOF and active carbon exhibits a high specific energy density of 20.9 W h kg-1 at the power density of 800 W kg-1. All these results demonstrate that the mixed-metal strategy is an effective way to optimize the morphology and improve the electrochemical property of the MOFs.

An Asymmetric Supercapacitor Based on Activated Porous Carbon Derived from Walnut Shells and NiCo2O4 Nanoneedle Arrays Electrodes.

A facile method was utilized to convert a common biomass of walnut shells into activated porous carbon by carbonization and activation with nitricacid treatment. The obtained activated carbon (WSs-2) exhibited excellent electrochemical performance with high specific capacitance of 137 F · g-1 at 1 A · g-1 and super cycling performance of 96% capacitance retention at 5 A · g-1 after 5000 cycles. In addition, NiCo2O4 nanoneedle arrays with good electrochemical properties were successfully prepared by a simple hydrothermal method. An aqueous asymmetric supercapacitor (ASC) device based on WSs-2 and NiCo2O4 was assembled, which delivered 21 Wh · kg-1 at a power density of 424.5 W · kg-1, and maintained 19 Wh · kg-1 at power density of 4254 W · kg-1 as well as excellent cycling stability of 99.3% capacitance retention after 5000 cycles at 4 A · g-1. Through this method, low-cost, environmentally friendly and large-scale carbon materials can be fabricated and applied in supercapacitor electrodes.

Multilayer hexagonal silicon forming in slit nanopore.

The solidification of two-dimensional liquid silicon confined to a slit nanopore has been studied using molecular dynamics simulations. The results clearly show that the system undergoes an obvious transition from liquid to multilayer hexagonal film with the decrease of temperature, accompanied by dramatic change in potential energy, atomic volume, coordination number and lateral radial distribution function. During the cooling process, some hexagonal islands randomly appear in the liquid first, then grow up to grain nuclei, and finally connect together to form a complete polycrystalline film. Moreover, it is found that the quenching rate and slit size are of vital importance to the freezing structure of silicon film. The results also indicate that the slit nanopore induces the layering of liquid silicon, which further induces the slit size dependent solidification behavior of silicon film with different electrical properties.

Wettability and Coalescence of Cu Droplets Subjected to Two-Wall Confinement.

Controlling droplet dynamics via wettability or movement at the nanoscale is a significant goal of nanotechnology. By performing molecular dynamics simulations, we study the wettability and spontaneous coalescence of Cu droplets confined in two carbon walls. We first focus on one drop in the two-wall confinement to reveal confinement effects on wettability and detaching behavior of metallic droplets. Results show that Cu droplets finally display three states: non-detachment, semi-detachment and full detachment, depending on the height of confined space. The contact angle ranges from 125° to 177°, and the contact area radius ranges from 12 to ~80 Å. The moving time of the detached droplet in the full detachment state shows a linear relationship with the height of confined space. Further investigations into two drops subjected to confinement show that the droplets, initially distant from each other, spontaneously coalesce into a larger droplet by detachment. The coalescing time and final position of the merged droplet are precisely controlled by tailoring surface structures of the carbon walls, the height of the confined space or a combination of these approaches. These findings could provide an effective method to control the droplet dynamics by confinement.

Theoretical study of the pathway to heterogeneous nucleation of liquid copper on the groove substrate with different wedge angles.

Heterogeneous nucleation is of significance in controlling the crystal growth, but the key route is still limited. Simulations are performed to explore the microscopic details of how the wedge substrate spreads its structural information to a growing crystal and further affects the solidification process. The simulation results show that, owing to the induced effect from the substrate, the copper atoms become layered at the liquid-solid interface in a "V"-shaped pattern and tend to form a twin crystal. The structural information delivery of the substrate decays with the distance away from the substrate, and the final solidified structure would gradually recover its inherent structure. Interestingly, the wedge angle of 90° seems to be an exception at which the solidified structure exhibits a perfect crystal due to the nearly perfect match with the Cu atoms. Moreover, the cooling rate and the atomic structure of the substrate are also found to have striking correlations with the final structures. These research results are favorable for a better understanding of the inherent relation between the solidified structure and the substrate.

Synergy and pinning effects in a monatomic liquid film in confined conditions.

The freezing behavior of a monatomic liquid film in confined conditions during rapid cooling was studied by molecular dynamics simulations. We illustrated the synergy and pinning effects of the local icosahedral order during freezing. Our results show that the icosahedron contributes to nucleation through the synergy with other short-range ordered structures and participates in crystal growth via assimilation, but the pinning effect should be overcome when crystals grow. Furthermore, a semi-ordered morphology with maze-like nano-patterns emerged due to the cooperation between the synergy effect and the pinning effect. Our findings shed light on the correlation between the local icosahedral order and the crystalline medium-range order, providing a better understanding of the rapid solidification.

Layering transition in confined silicon.

The structure of quasi-2D liquid silicon confined to slit nanopores has been investigated using molecular dynamics (MD) simulations. An obvious structural change from a low-density low-coordinated liquid to a high-density highly coordinated liquid has been found in the confined silicon with the increase of the slit size. This kind of structural transition results from layering in the confined silicon, which disappears with the increase of temperature. In the process of layering transition, the coordination distribution of quasi-2D liquid undergoes an evolutionary process from the initial non-uniform distribution to the final uniform distribution. In addition, our results also indicate that the increase of pressure will also induce a layering transition in the confined silicon.

Theoretical study of core-shell composite structure made of carbon nanoring and aluminum nanowire.

Molecular dynamics (MD) simulations have been performed to investigate the interaction between the carbon nanorings (CNRs) and Al nanowires (NWs). Our results show that the NW can activate, guide and stabilize the self-assembly of the CNR to form a double-deck helix, as a result of the combined action of the van der Waals interaction and the offset face-to-face π-π stacking interaction. The cross section of the NWs has a negligible effect on the helix-forming process, whereas the size (diameter and length) of the CNRs should meet some required conditions to guarantee the helical configuration. It is worth noting that the helical conformation of the hydrogen-terminated CNRs is energetically more stable than scrolling or stacking conformation. Furthermore, we also study how the surface roughness affects the self-assembly of the CNRs.

Dewetting properties of metallic liquid film on nanopillared graphene.

In this work, we report simulation evidence that the graphene surface decorated by carbon nanotube pillars shows strong dewettability, which can give it great advantages in dewetting and detaching metallic nanodroplets on the surfaces. Molecular dynamics (MD) simulations show that the ultrathin liquid film first contracts then detaches from the graphene on a time scale of several nanoseconds, as a result of the inertial effect. The detaching velocity is in the order of 10 m/s for the droplet with radii smaller than 50 nm. Moreover, the contracting and detaching behaviors of the liquid film can be effectively controlled by tuning the geometric parameters of the liquid film or pillar. In addition, the temperature effects on the dewetting and detaching of the metallic liquid film are also discussed. Our results show that one can exploit and effectively control the dewetting properties of metallic nanodroplets by decorating the surfaces with nanotube pillars.

Liquid-liquid phase transition and structure inheritance in carbon films.

Molecular dynamics simulations are performed to study the cooling process of quasi-2D liquid carbon. Our results show an obvious liquid-liquid phase transition (LLPT) from the twofold coordinated liquid to the threefold coordinated liquid with the decrease of temperature, followed by a liquid-solid phase transition (LSPT). The LLPT can be regarded as the preparation stage of LSPT. During the cooling process, the chain structures firstly self-assemble into some ring structures and then aggregate into some stable islands which can further connect together to form a complete polycrystalline film. The threefold coordinated structures play an important role in the formation of atomic rings. The inheritance of the threefold coordinated structures provides essential condition to form rings and islands.

Atomic insight into copper nanostructures nucleation on bending graphene.

Some findings in heterogeneous nucleation that the structural features of a growing crystal are usually inherited from the heterogeneous nucleus, although attracting more and more attention, are not yet well understood. Here we report numerical simulations of copper nucleation on bending graphene (BG) to explore the microscopic details of how the curved surface influences the freezing structure of the liquid metal. The simulation result clearly shows that copper atoms become layered at the solid-liquid interface in a "C"-shaped pattern resembling the BG. This kind of shape control decays with increasing distance from the wall and the outmost layers transform into twin crystal composed of two fcc wedges. It is found that the final structures have striking correlations with the curvature radius, central angle and arc length of the BG. Our study would provide an opportunity for comprehensive and satisfactory understanding of the heterogeneous nucleation on curved surfaces.

Spontaneous encapsulation behavior of ionic liquid into carbon nanotube.

Molecular dynamics simulations and density functional theory have been performed to investigate the spontaneous encapsulation of 1-butyl-3-methylimidazolium chloride ([Bmim][Cl]) into single-walled carbon nanotubes (SWCNTs). This phenomenon can be attributed to the van der Waals attractive force, hydrogen bonds and especially the π-π stacking effect. The [Bmim][Cl] molecules enter SWCNTs with larger diameters more rapidly, showing an interesting dependence on tube size. A high temperature is not beneficial to, and may even disrupt, the encapsulation of the [Bmim][Cl] molecules. It is also worth noting that the graphene nanoribbon entering the SWCNT would have an extremely different effect on this encapsulation process from when they wrap around the outer surface. Furthermore, the [Bmim][Cl] molecules can assist water transport in the SWCNT by expelling water molecules from the SWCNT. The proposed discoveries eventually provide a powerful way to fabricate nanoscale materials and devices and tune their properties.

Wavelike deformation traveling on a carbon nanotube.

The propagation of wavelike ripples on a carbon nanotube (CNT) induced by the radial impact of a C(60) molecule is investigated by molecular dynamics simulations. The ripples start at the impact point and spreads through the tube, accompanied by energy transfer. The ripples would effectively reduce the local energy concentration around the impact point. The propagation of ripples is clearly affected by the diameter but is independent of the chirality of the CNT. Noticeable diffraction occurs when the deformation ripples encounter obstacles or narrow slits, which indicates that the propagation of ripples can be used to detect defects in the CNT. This work provides new and exciting possibilities for CNTs to serve as energy buffers, sensors and new nanoelectromechanical devices.

Helical encapsulation of graphene nanoribbon into carbon nanotube.

Molecular dynamics (MD) simulations were performed to study interaction between the graphene nanoribbon (GNR) and single-wall carbon nanotube (SWCNT). The GNR enters the SWCNT spontaneously to display a helical configuration which is quite similar to the chloroplast in the spirogyra cell. This unique phenomenon results from the combined action of the van der Waals potential well and the π-π stacking interaction. The size of SWCNT and GNR should satisfy some certain conditions in the helical encapsulation process. A DNA-like double helix would be formed inside the SWCNT with the encapsulation of two GNRs. A water cluster enclosed in the SWCNT has great effect on the formation of the GNR helix in the tube. Furthermore, we also studied the possibility that the spontaneous encapsulation of GNR is used for substance delivery. The expected outcome of these properties is to provide novel strategies to design nanoscale carriers and reaction devices.