Hierarchically structured diamond composite with exceptional toughness.

The well known trade-off between hardness and toughness (resistance to fracture) makes simultaneous improvement of both properties challenging, especially in diamond. The hardness of diamond can be increased through nanostructuring strategies1,2, among which the formation of high-density nanoscale twins - crystalline regions related by symmetry - also toughens diamond2. In materials other than diamond, there are several other promising approaches to enhancing toughness in addition to nanotwinning3, such as bio-inspired laminated composite toughening4-7, transformation toughening8 and dual-phase toughening9, but there has been little research into such approaches in diamond. Here we report the structural characterization of a diamond composite hierarchically assembled with coherently interfaced diamond polytypes (different stacking sequences), interwoven nanotwins and interlocked nanograins. The architecture of the composite enhances toughness more than nanotwinning alone, without sacrificing hardness. Single-edge notched beam tests yield a toughness up to five times that of synthetic diamond10, even greater than that of magnesium alloys. When fracture occurs, a crack propagates through diamond nanotwins of the 3C (cubic) polytype along {111} planes, via a zigzag path. As the crack encounters regions of non-3C polytypes, its propagation is diffused into sinuous fractures, with local transformation into 3C diamond near the fracture surfaces. Both processes dissipate strain energy, thereby enhancing toughness. This work could prove useful in making superhard materials and engineering ceramics. By using structural architecture with synergetic effects of hardening and toughening, the trade-off between hardness and toughness may eventually be surmounted.

Self-healing of fractured diamond.

Materials that possess the ability to self-heal cracks at room temperature, akin to living organisms, are highly sought after. However, achieving crack self-healing in inorganic materials, particularly with covalent bonds, presents a great challenge and often necessitates high temperatures and considerable atomic diffusion. Here we conducted a quantitative evaluation of the room-temperature self-healing behaviour of a fractured nanotwinned diamond composite, revealing that the self-healing properties of the composite stem from both the formation of nanoscale diamond osteoblasts comprising sp2- and sp3-hybridized carbon atoms at the fractured surfaces, and the atomic interaction transition from repulsion to attraction when the two fractured surfaces come into close proximity. The self-healing process resulted in a remarkable recovery of approximately 34% in tensile strength for the nanotwinned diamond composite. This discovery sheds light on the self-healing capability of nanostructured diamond, offering valuable insights for future research endeavours aimed at enhancing the toughness and durability of brittle ceramic materials.

Hierarchical Nanoassembly of MoS2/Co9S8/Ni3S2/Ni as a Highly Efficient Electrocatalyst for Overall Water Splitting in a Wide pH Range.

The design of low-cost yet high-efficiency electrocatalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) over a wide pH range is highly challenging. We now report a hierarchical co-assembly of interacting MoS2 and Co9S8 nanosheets attached on Ni3S2 nanorod arrays which are supported on nickel foam (NF). This tiered structure endows high performance toward HER and OER over a very broad pH range. By adjusting the molar ratio of the Co:Mo precursors, we have created CoMoNiS-NF- xy composites ( x: y means Co:Mo molar ratios ranging from 5:1 to 1:3) with controllable morphology and composition. The three-dimensional composites have an abundance of active sites capable of universal pH catalytic HER and OER activity. The CoMoNiS-NF-31 demonstrates the best electrocatalytic activity, giving ultralow overpotentials (113, 103, and 117 mV for HER and 166, 228, and 405 mV for OER) to achieve a current density of 10 mA cm-2 in alkaline, acidic, and neutral electrolytes, respectively. It also shows a remarkable balance between electrocatalytic activity and stability. Based on the distinguished catalytic performance of CoMoNiS-NF-31 toward HER and OER, we demonstrate a two-electrode electrolyzer performing water electrolysis over a wide pH range, with low cell voltages of 1.54, 1.45, and 1.80 V at 10 mA cm-2 in alkaline, acidic, and neutral media, respectively. First-principles calculations suggest that the high OER activity arises from electron transfer from Co9S8 to MoS2 at the interface, which alters the binding energies of adsorbed species and decreases overpotentials. Our results demonstrate that hierarchical metal sulfides can serve as highly efficient all-pH (pH = 0-14) electrocatalysts for overall water splitting.

Sub-1 nm Nanowire Based Superlattice Showing High Strength and Low Modulus.

Polymers possess special dimension-dependent processing flexibility which is always absent in inorganic materials. Traditional inorganic nanowires own similar dimensions to polymers, but usually lack near-molecular diameters and the related properties. Here we report that inorganic nanowires with sub1 nm diameter and microscale length can be electrospinningly processed into superstructures including smooth fibers and large-area mat by tuning the viscosity and surface tension of the colloidal nanowires solution. These superstructures have shown both flexible texture and excellent mechanical properties (712.5 MPa for tensile strength, 10.3 GPa for elastic modulus) while retaining properties arising from inorganic components.

In Situ Observation of Twin Boundary Sliding in Single Crystalline Cu Nanowires.

Using a homemade, novel, in situ transmission electron microscopy (TEM) double tilt tensile device, plastic behavior of single crystalline Cu nanowires of around 150 nm are studied. Deformation twins occur during the tests as predesigned before the experiments. In situ observation of twin boundary sliding (TBS) caused by full dislocation (extended dislocation) is first revealed at the atomic scale which is confirmed by molecular dynamics (MD) simulation results. Combined with twin boundary migration and multiple dislocations nucleated from surface, TBS causes a superlarge fracture strain which is over 166% and a severe necking which is over 93%, far beyond the typical values for most nanomaterials without twins.

Study of the Mechanical Behavior of Radially Grown Fivefold Twinned Nanowires on the Atomic Scale.

In situ bending tests and dynamic modeling simulations are for the first time revealing the mechanical behavior of copper nanowires (NW) with radially grown fivefold twin structures on the atomic scale. Combining the simulations with the experimental results it is shown that both the twin boundaries (TBs) and the twin center act as dislocation sources. TB migration and L-locks are readily observed in these types of radially grown fivefold-twin structures.

Plastic deformation enabled energy dissipation in a bionanowire structured armor.

It has been challenging to simultaneously achieve high strength and toughness in engineered materials because of the trade-off relation between the two distinct properties. Nature, however, has elegantly solved this problem. Seashells, commonly referred to as nature's armors, exhibit an unusual resilience against predatory attacks. In this letter, we report an unexpected phenomenon in a bionanowire structured armor-conch shell where the shell's basic building blocks, i.e., the third-order lamellae, exhibit an exceptional plasticity with a maximum strain of 0.7% upon mechanical loading. We attribute such a plastic deformation behavior to the lamella's unique nanoparticle-biopolymer architecture, in which the biopolymer mediates the rotation of aragonite nanoparticles in response to external attacks. We also found that electron beam irradiation facilitates the lamella's plasticity. These findings advance our understanding of seashell's energy dissipating strategy and provide new design guidelines for developing high performance bioinspired materials and sensors.

Crystalline liquid and rubber-like behavior in Cu nanowires.

Via in situ TEM tensile tests on single crystalline copper nanowires with an advanced tensile device, we report here a crystalline-liquid-rubber-like (CRYS-LIQUE-R) behavior in fracturing crystalline metallic nanowires. A retractable strain of the fractured crystalline Cu nanowires can approach over 35%. This astonishing CRYS-LIQUE-R behavior of the fracturing highly strained single crystalline Cu nanowires originates from an instant release of the stored ultralarge elastic energy in the crystalline nanowires. The release of the ultralarge elastic energy was estimated to generate a huge reverse stress as high as ~10 GPa. The effective diffusion coefficient (D(eff)) increased sharply due to the consequent pressure gradient. In addition, due to the release of ultrahigh elastic energy, the estimated concomitant temperature increase was estimated as high as 0.6 Tm (Tm is the melting point of nanocrystalline Cu) on the fractured tip of the nanowires. These factors greatly enhanced the atomic diffusion process. Molecular dynamic simulations revealed that the very high reverse stress triggered dislocation nucleation and exhaustion.

Quantitative evidence of crossover toward partial dislocation mediated plasticity in copper single crystalline nanowires.

In situ tensile tests of Cu single crystalline nanowires in a high-resolution transmission electron microscope reveal a novel effect of sample dimensions on plasticity mechanisms. When the single crystalline nanowire size was reduced to <∼150 nm, the normal full dislocation slip was taken over by partial dislocation mediated plasticity (PDMP). For the first time, we demonstrate this transition in a quantitative manner by assessing the relative contributions to plastic strain from PDMP and full dislocations. The crossover sample size is consistent, well within model predictions. This discovery represents yet another "sample size effect", beyond other reported influence of sample dimensions on the mechanical behavior of metals, such as dislocation starvation or source truncation, and the "smaller is stronger" trend.

Stabilizing Single-Atomic Pt by Forming PtFe Bonds for Efficient Diboration of Alkynes.

Precisely tailoring the oxidation state of single-atomic metal in heterogeneous catalysis is an efficient way to stabilize the single-atomic site and promote their activity, but realizing this approach remains a grand challenge to date. Herein, a class of stable single-atomic catalysts with well-tuned oxidation state of Pt by forming PtFe atomic bonds is reported, which are supported by defective Fe2 O3  nanosheets on reduced graphene oxide (PFARFNs). These as-synthesized materials can greatly enhance the catalytic activity, stability, and selectivity for the diboration of alkynes. The PFARFNs exhibit high conversion of 99% at 100 °C with an outstanding turnover frequency (TOF) of 545 h-1 , and a relatively high conversion of 58% at room temperature (25 °C) with a TOF of 310 h-1 , which has been hardly achieved previously. Through both experimental and theoretical investigation, it is demonstrated that the fast electron transfer from Fe to Pt in Fe-Pt-O atomic sites in PFARFNs can not only stabilize the single-atomic Pt, but also significantly improve their catalytic activity.

Approaching the theoretical elastic strain limit in copper nanowires.

Three sets of uniaxial tensile tests have been performed in situ in transmission electron microscopy/high-resolution electron microscopy on Cu nanowires (NWs) to accurately map out the sample size dependence of elastic strain limit. Atomic-resolution evidence was obtained for an exceedingly large recoverable strain (as much as 7.2%) that can be sustained in the lattice of a single-crystalline Cu NW with a diameter of ∼5.8 nm. This ultrahigh elastic strain is consistent with the predictions from molecular dynamics simulations for nanowires and approaches the ideal elastic limit predicted for Cu by ab initio calculations.

Multiscale engineered artificial tooth enamel.

Tooth enamel, renowned for its high stiffness, hardness, and viscoelasticity, is an ideal model for designing biomimetic materials, but accurate replication of complex hierarchical organization of high-performance biomaterials in scalable abiological composites is challenging. We engineered an enamel analog with the essential hierarchical structure at multiple scales through assembly of amorphous intergranular phase (AIP)-coated hydroxyapatite nanowires intertwined with polyvinyl alcohol. The nanocomposite simultaneously exhibited high stiffness, hardness, strength, viscoelasticity, and toughness, exceeding the properties of enamel and previously manufactured bulk enamel-inspired materials. The presence of AIP, polymer confinement, and strong interfacial adhesion are all needed for high mechanical performance. This multiscale design is suitable for scalable production of high-performance materials.

In situ observation of dislocation behavior in nanometer grains.

Using a newly developed nanoscale deformation device, atomic scale and time-resolved dislocation dynamics have been captured in situ under a transmission electron microscope during the deformation of a Pt ultrathin film with truly nanometer grains (diameter d< ~ 10 nm). We demonstrate that dislocations are highly active even in such tiny grains. For the larger grains (d ~ 10 nm), full dislocations dominate and their evolution sometimes leads to the formation, destruction, and reformation of Lomer locks. In smaller grains, partial dislocations generating stacking faults are prevalent.

Atomic mechanisms governing the elastic limit and the incipient plasticity of bending Si nanowires.

Individual single-crystalline Si nanowires (NWs) were bent by forming loops or arcs with different radius. Positional-resolved atomic level strain distribution (PRALSD) along both of the radial and axial directions were calculated and mapped directly from the atomic-resolution strained high-resolution electron microscopy (HREM) images of the bent Si NWs. For the first time, the neutral-strain axis shifted from the compressive zone to the tensile region was directly demonstrated from the PRALSD along the radial direction. Bending-induced ripple-buckling of the bent Si NW was observed and a significant strain variation along the bending axial direction in the compressive region was revealed. The tensile surface atomic steps and the compressive buckling are the physical origin of the asymmetric tensile-compressive properties of postelastic instabilities and the incipient plasticity. Both of the tensile surface atomic-steps and the compressive buckling initiated versatile ductile plastic dislocation events.

Strong structural occupation ratio effect on mechanical properties of silicon carbide nanowires.

Materials' mechanical properties highly depend on their internal structures. Designing novel structure is an effective route to improve materials' performance. One-dimensional disordered (ODD) structure is a kind of particular structure in silicon carbide (SiC), which highly affects its mechanical properties. Herein, we show that SiC nanowires (NWs) containing ODD structure (with an occupation ratio of 32.6%) exhibit ultrahigh tensile strength and elastic strain, which are up to 13.7 GPa and 12% respectively, approaching the ideal theoretical limit. The ODD structural occupation ratio effect on mechanical properties of SiC NWs has been systematically studied and a saddle shaped tendency for the strength versus occupation ratio is firstly revealed. The strength increases with the increase of the ODD occupation ratio but decreases when the occupation ratio exceeds a critical value of ~ 32.6%, micro twins appear in the ODD region when the ODD segment increases and soften the ODD segment, finally results in a decrease of the strength.

Dual-Phase Super-Strong and Elastic Ceramic.

Ceramic materials exhibit very high stiffness and extraordinary strength, but they typically suffer from brittleness. Amorphization and size confinement are commonly used to reinforce materials. However, the inverse Hall-Petch effect and the shear-band softening effect usually limit further improvement of their performance under a critical size. With an optimum structure design, we demonstrate that dual-phase zirconia nanowires (DP-ZrO2 NWs) with nanocrystals embedded in an amorphous matrix as a strengthening phase can overcome these problems simultaneously. As a result of this structure, in situ tensile tests demonstrate that the mechanical properties have been enormously improved in a way that does not follow both the inverse Hall-Petch effect and the shear band softening effect. The elastic strain approaches ∼7%, and the ultimate strength is 3.52 GPa, accompanied by a high toughness of ∼151 MJ m-3, making the DP-ZrO2 NW composite the strongest and toughest ZrO2 ever achieved. The findings provide a way to improve the mechanical properties of ceramics in a controllable manner, which may serve as a pervasive approach to be broadly applied to a variety of materials.

Room-temperature superplasticity in Au nanowires and their atomistic mechanisms.

We report experimental observation of room-temperature superplasticity and the distinct nanosize effect on the deformation mechanisms of Au nanowires. The Au nanowires were subjected to in situ tensile straining in a transmission electron microscope by using a home-made strain actuator, and a super large plastic strain with ∼150% uniform elongation and ∼260% total strain were observed before fracture. The plastic deformation started through full dislocation slip and was followed by the activities of stacking fault ribbons (or dissociated full dislocations) that were generated from surface sources and disappeared at the other end surfaces. With the reduction of the diameter of Au nanowires, the deformation changed to the twinning mode through partial dislocation emissions from sample surfaces. The mechanisms behind the observed phenomena are discussed in detail. These results shed light on the size-controlled plasticity of nano-metals.

Effects of twin orientation and spacing on the mechanical properties of Cu nanowires.

The role of twin orientation in mechanical behaviors of nanomaterials is drawing increasing attention. In this paper, atomistic simulations on the tensile deformation of twinned Cu nanowires (NWs) are implemented to investigate the twin orientation and spacing effects. The results of numerical simulations reveal that the tensile deformation mechanisms can be divided into three types with the twin orientation varying from 0° to 90°: dislocations slip intersecting with twin boundary (TB), stacking faults formed parallel to the TB and TB migration. Detail analysis about dislocation motion is carried out to illustrate the plastic deformation mechanisms. In addition, with the increasing of the TB spacing, there is a transition from yield with strain hardening to yield with nearly constant flow stress. The peak stress decreases with the increase of TB spacing, which can be attributed to surface roughness caused by crystal reorientation. Our findings also suggest a possible approach to tune the mechanical behaviors of low dimensional nanostructures.

A General Bioinspired, Metals-Based Synergic Cross-Linking Strategy toward Mechanically Enhanced Materials.

Creating lightweight engineering materials combining high strength and great toughness remains a significant challenge. Despite possessing-enhanced strength and stiffness, bioinspired/polymeric materials usually suffer from clearly reduced extensibility and toughness when compared to corresponding bulk polymer materials. Herein, inspired by tiny amounts of various inorganic impurities for mechanical improvement in natural materials, we present a versatile and effective metal ion (Mn+)-based synergic cross-linking (MSC) strategy incorporating eight types of metal ions into material bulks that can drastically enhance the tensile strength (∼24.1-70.8%), toughness (∼18.6-110.1%), modulus (∼21.6-66.7%), and hardness (∼6.4-176.5%) of multiple types of pristine materials (from hydrophilic to hydrophobic and from unary to binary). More importantly, we also explore the primarily elastic-plastic deformation mechanism and brittle fracture behavior (indentation strain of >5%) of the synergic cross-linked graphene oxide (Syn-GO) paper by means of in situ nanoindentation SEM. The MSC strategy for mechanically enhanced integration can be readily attributed to the formation of the complicated metals-based cross-linking/complex networks in the interfaces and intermolecules between functional groups of materials and various metal ions that give rise to efficient energy dissipation. This work suggests a promising MSC strategy for designing advanced materials with outstanding mechanical properties by adding low amounts (<1.0 wt %) of synergic metal ions serving as synergic ion-bonding cross-linkers.

Atomic scale observation of oxygen delivery during silver-oxygen nanoparticle catalysed oxidation of carbon nanotubes.

To probe the nature of metal-catalysed processes and to design better metal-based catalysts, atomic scale understanding of catalytic processes is highly desirable. Here we use aberration-corrected environmental transmission electron microscopy to investigate the atomic scale processes of silver-based nanoparticles, which catalyse the oxidation of multi-wall carbon nanotubes. A direct semi-quantitative estimate of the oxidized carbon atoms by silver-based nanoparticles is achieved. A mechanism similar to the Mars-van Krevelen process is invoked to explain the catalytic oxidation process. Theoretical calculations, together with the experimental data, suggest that the oxygen molecules dissociate on the surface of silver nanoparticles and diffuse through the silver nanoparticles to reach the silver/carbon interfaces and subsequently oxidize the carbon. The lattice distortion caused by oxygen concentration gradient within the silver nanoparticles provides the direct evidence for oxygen diffusion. Such direct observation of atomic scale dynamics provides an important general methodology for investigations of catalytic processes.