Mapping Shear Bands in Metallic Glasses: From Atomic Structure to Bulk Dynamics.

A deep understanding of the mechanisms controlling shear banding is of fundamental importance for improving the mechanical properties of metallic glasses. Atomistic simulations highlight the importance of nanoscale stresses and strains for shear banding, but corresponding experimental proofs are scarce due to limited characterization techniques. Here, by using precession nanodiffraction mapping in the transmission electron microscope, the atomic density and strain distribution of an individual shear band is quantitatively mapped at 2 nm resolution. We demonstrate that shear bands exhibit density alternation from the atomic scale to the submicron scale and complex strain fields exist, causing shear band segmentation and deflection. The atomic scale density alternation reveals the autocatalytic generation of shear transformation zones, while the density alternation at submicron scale results from the progressive propagation of shear band front and extends to the surrounding matrix, forming oval highly strained regions with density consistently higher (∼0.2%) than the encapsulated shear band segments. Through combination with molecular dynamic simulations, a complete picture for shear band formation and propagation is established.

Surface- and Strain-Mediated Reversible Phase Transformation in Quantum-Confined ZnO Nanowires.

The phase stability of ZnO in a quantum-confinement size regime (sub-2-nm) remains fiercely debated. Applying in situ (scanning) transmission electron microscopy, we present the atomistic view of the phase transitions from the original wurtzite structure to an intermediate body-centered tetragonal and h-MgO structure under tensile strain in quantum-confined ZnO nanowires. Strikingly, such structural transitions are reversible after releasing the stress. Further theoretical calculations mirror the transition pathway and provide basic insight into the overall landscape regarding surface- and strain-dependent phase transition behavior. Our results provide the critical piece to solve the puzzle in phase stability of ZnO, which may prove essential for advancing a variety of nanotechnologies, e.g., quantum-dot light-emitting devices.

Atomistic Interface Dynamics in Sn-Catalyzed Growth of Wurtzite and Zinc-Blende ZnO Nanowires.

Unraveling the phase selection mechanisms of semiconductor nanowires (NWs) is critical for the applications in future advanced nanodevices. In this study, the atomistic vapor-solid-liquid growth processes of Sn-catalyzed wurtzite (WZ) and zinc blende (ZB) ZnO are directly revealed based on the in situ transmission electron microscopy. The growth kinetics of WZ and ZB crystal phases in ZnO appear markedly different in terms of the NW-droplet interface, whereas the nucleation site as determined by the contact angle ϕ between the seed particle and the NW is found to be crucial for tuning the NW structure through combined experimental and theoretical investigations. These results offer an atomic-scale view into the dynamic growth process of ZnO NW, which has implications for the phase-controllable synthesis of II-VI compounds and heterostructures with tunable band structures.

Three-dimensional structures of magnesium nanopores.

The optimization of nanopore-based devices is closely related to the nanopore three-dimensional (3D) structures. In this paper, faceted nanopores were fabricated in magnesium (Mg) by aligning the electron beam (e-beam) along the [0001] direction. Detailed structural characterization by transmission electron microscopy reveals the existence of two 3D structures: hexagonal prism-shaped and hourglass-shaped 3D morphologies. Moreover, the 3D structures of nanopores are also found to depend on the widest nanopore diameter-to-thickness ratio (D/t). A plausible formation mechanism for different 3D structures is discussed. Our results incorporate a critical piece of information regarding the nanopore 3D structures in Mg and may serve as an important design guidance for the size- and shape-controllable fabrication of solid-state nanopores applying the e-beam sculpting technique.

Catalytically Active Oil-Based Lubricant Additives Enabled by Calcining Ni-Al Layered Double Hydroxides.

Layered double hydroxides (LDHs) have lately been hailed as robust lubricant additives for improving tribological properties and as ideal catalysts for synthesizing carbon-based nanomaterials. In this paper, in situ analytical tools are used to track the evolution of the crystal structure and chemical composition of LDHs during calcination. Nickel oxide and elemental nickel can be produced by calcining NiAl-LDH in air (LDH-C-Air) and argon (LDH-C-Ar), respectively. For the base oil with 1 wt % LDH-C-Air, negligible wear can be detected even after a 2 h friction test under a severe contact pressure (∼637 MPa). A relatively thick tribofilm (∼60 nm) with a better mechanical property is formed, which protects the solid surface from severe wear. In addition, the possible formed carbon debris may also prevent the direct collision of asperities and effectively improve the wear resistance. This work provides a unique vision for the application of calcined LDHs with the combination of catalysis and tribology.

Atomistic manipulation of reversible oxidation and reduction in Ag with an electron beam.

Employing electrons for direct control of a nanoscale reaction is highly desirable since it enables fabrication of nanostructures with different properties at atomic resolution and with flexibility of dimensions and location. Here, applying in situ transmission electron microscopy, we show the reversible oxidation and reduction kinetics in Ag, well controlled by changing the dose rate of the electron beam. Aberration-corrected high-resolution transmission electron microscopy observation reveals that O atoms are preferably inserted and extracted along the {111} close-packed planes of Ag, leading to the nucleation and decomposition of nanoscale Ag2O islands on the Ag substrate. By controlling the electron beam size and dose rate, we demonstrated the fabrication of an array of 3 nm Ag2O nanodots in an Ag matrix. Our results open a new pathway to manipulate an atomistic reaction with an electron beam towards the precise fabrication of nanostructures for device applications.

Key Single-Atom Electrocatalysis in Metal-Organic Framework (MOF)-Derived Bifunctional Catalysts.

Metal-organic framework (MOF)-derived materials have attracted increasing interest and show promising catalytic performances in many fields. Intensive efforts have been focused on the structure design and metal-site integration in MOF-derived catalysts. However, the key catalytic processes related with the metal sites in MOF-derived catalysts that dominate the electrocatalytic performance still remain obscure. Herein, we show a neglected but critical issue in the pyrolytic synthesis of MOF-derived catalysts: the coupled evolution of dual sites, that is, metallic sites and single-atom metal sites. The identification of active sites of single-atom sites from the visible particles has been elucidated through the combined X-ray spectroscopic, electron microscopic, and electrochemical studies. Interestingly, after a total removal of metallic cobalt sites, catalysts with purified single-atom metal sites show no faltering activity for either the oxygen reduction reaction (ORR) or hydrogen evolution reaction (HER), while significantly enhanced ORR selectivity is achieved; this reveals the dominant activity and selectivity contribution from single-atom electrocatalysis. The insight of the coupled evolution of dual sites and the as-demonstrated dual-site decoupling strategies open up a new routine for the design and synthesis of MOF-derived catalysts with the optimized single-atom electrocatalysis towards various electrochemical reactions.

Tribological Behavior of NiAl-Layered Double Hydroxide Nanoplatelets as Oil-Based Lubricant Additives.

Layered double hydroxides (LDHs) are a class of naturally occurring inorganic minerals that are composed of divalent and trivalent metal cations. In this study, three different sized NiAl-LDH nanoplatelets were synthesized by varying crystallization time during the microemulsification process. The layered structure and three-dimensional size of nanoplatelets were confirmed by transmission electron microscopy (TEM) and atomic force microscopy (AFM). As lubricant additives, their tribological properties in base oil were evaluated by use of a ball-on-disk reciprocating tribometer under three different loads: 50, 100, and 150 N (which created peak Hertz pressures of 1.74, 2.16, and 2.47 GPa). Under contact pressures of up 2.16 GPa, not only did the coefficient of friction (COF) decrease by about 10% after nano-LDHs were added but also the wear performance improved substantially. These improvements resulted from a protective tribolayer formation on the contact interface, as revealed by detailed surface and structure analytical studies. In particular, cross-sectional TEM images revealed that the larger size nanoplatelets (NiAl-24h), rather than the smaller ones (NiAl-6h) showed the best and most stable tribological performance. This was mainly because of their higher degree of crystallinity, which in turn resulted in the formation of a tribofilm with far superior mechanical properties during sliding. Owing to the simple synthetic method and superior tribological properties as oil-based additives, nano-LDHs hold great potential for use in demanding industrial applications in the future.

Fractal growth of platinum electrodeposits revealed by in situ electron microscopy.

Fractals are commonly observed in nature and elucidating the mechanisms of fractal-related growth is a compelling issue for both fundamental science and technology. Here we report an in situ electron microscopy study of dynamic fractal growth of platinum during electrodeposition in a miniaturized electrochemical cell at varying growth conditions. Highly dendritic growth - either dense branching or ramified islands - are formed at the solid-electrolyte interface. We show how the diffusion length of ions in the electrolyte influences morphology selection and how instability induced by initial surface roughness, combined with local enhancement of electric field, gives rise to non-uniform branched deposition as a result of nucleation/growth at preferred locations. Comparing the growth behavior under these different conditions provides new insight into the fundamental mechanisms of platinum nucleation.

Interactions of titania based nanoparticles with silica and green-tea: Photo-degradation and -luminescence.

An effective way to modify the photocatalytic activity of both anatase and rutile TiO2 nanoparticles by coating the surface with either an inorganic (SiO2/silica) or organic (green-tea) layer using a chemical approach is demonstrated. Tetraethyl orthosilicate (TEOS) was used to cover the surface of TiO2 with silica which facilitates the inhibition of photocatalytic activity, ensuring its application in sunscreens by blocking the reactive oxygen species (ROS). Green-tea extract, rich in epigallocatechin gallate (EGCG), was used to coat/stabilize nano-sized TiO2. The morphology of these coatings was revealed by transmission electron microscopy (TEM) and by energy dispersive spectroscopy (EDS) mapping. These studies showed good coverage for both types of coating, but with somewhat better uniformity of the coating layer on rutile TiO2 compared to anatase due to its more uniform particle geometry. The effectiveness of each coating was evaluated by photodegradation of an organic dye (methyl orange). These studies showed rutile_polyphenol exhibits the highest photocatalytic activity among rutile forms which validates its feasibility to be used in photodegradation.

In situ observation of the sodiation process in CuO nanowires.

In situ transmission electron microscopy was used to observe the dynamic evolution of the morphology and phase transformations in CuO nanowires during the process of sodiation. Our results facilitate a fundamental understanding of the sodiation mechanism in CuO nanostructures used as electrode materials in sodium ion batteries.