Chemistry and microstructure of C-supported Ru catalyst nanoparticles: A correlative study.

Ru@Pt core shell nanoparticles possess optimal catalytic properties that facilitate the anodic oxidation reaction of H2 with decreased Pt loading in hydrogen fuel cells. Moreover, since they preferentially oxidize CO, Pt poisoning is considerably reduced, which significantly improves the stability of the cell. The Ru cores used in this system are usually synthesized by dissolving a RuCl3*H2O precursor in an ethylene glycol-carbon black-NaOH mixture. However, the possibility that remnant Cl and Na from the synthesis process are present in the Ru nanoparticles has not been extensively studied. Therefore, due to the challenges in detecting impurities with traditional characterization methods, here correlative atom probe tomography (APT) with scanning transmission electron microscopy ((S)TEM) techniques were implemented. The capabilities of APT to obtain chemical information with high sensitivity at the nanoscale, in combination with the high spatial resolving power of (S)TEM, provide the necessary resolution to fully characterize the structure and chemical makeup of Ru nanoparticles.

Doubled strength and ductility via maraging effect and dynamic precipitate transformation in ultrastrong medium-entropy alloy.

Demands for ultrahigh strength in structural materials have been steadily increasing in response to environmental issues. Maraging alloys offer a high tensile strength and fracture toughness through a reduction of lattice defects and formation of intermetallic precipitates. The semi-coherent precipitates are crucial for exhibiting ultrahigh strength; however, they still result in limited work hardening and uniform ductility. Here, we demonstrate a strategy involving deformable semi-coherent precipitates and their dynamic phase transformation based on a narrow stability gap between two kinds of ordered phases. In a model medium-entropy alloy, the matrix precipitate acts as a dislocation barrier and also dislocation glide media; the grain-boundary precipitate further contributes to a significant work-hardening via dynamic precipitate transformation into the type of matrix precipitate. This combination results in a twofold enhancement of strength and uniform ductility, thus suggesting a promising alloy design concept for enhanced mechanical properties in developing various ultrastrong metallic materials.

Tracking the Mn Diffusion in the Carbon-Supported Nanoparticles Through the Collaborative Analysis of Atom Probe and Evaporation Simulation.

Carbon-supported nanoparticles have been used widely as efficient catalysts due to their enhanced surface-to-volume ratio. To investigate their structure­property relationships, acquiring 3D elemental distribution is required. Here, carbon-supported Pt, PtMn alloy, and ordered Pt3Mn nanoparticles are synthesized and analyzed with atom probe tomography as model systems. A significant difference of Mn distribution after the heat-treatment was found. Finally, the field evaporation behavior of the carbon support was discussed and each acquired reconstruction was compared with computational results from an evaporation simulation. This paper provides a guideline for studies using atom probe tomography on the heterogeneous carbon-supported nanoparticle system that leads to insights toward a wide variety of applications.

Shear band-driven precipitate dispersion for ultrastrong ductile medium-entropy alloys.

Precipitation strengthening has been the basis of physical metallurgy since more than 100 years owing to its excellent strengthening effects. This approach generally employs coherent and nano-sized precipitates, as incoherent precipitates energetically become coarse due to their incompatibility with matrix and provide a negligible strengthening effect or even cause brittleness. Here we propose a shear band-driven dispersion of nano-sized and semicoherent precipitates, which show significant strengthening effects. We add aluminum to a model CoNiV medium-entropy alloy with a face-centered cubic structure to form the L21 Heusler phase with an ordered body-centered cubic structure, as predicted by ab initio calculations. Micro-shear bands act as heterogeneous nucleation sites and generate finely dispersed intragranular precipitates with a semicoherent interface, which leads to a remarkable strength-ductility balance. This work suggests that the structurally dissimilar precipitates, which are generally avoided in conventional alloys, can be a useful design concept in developing high-strength ductile structural materials.

Three-dimensional atomic mapping of ligands on palladium nanoparticles by atom probe tomography.

Capping ligands are crucial to synthesizing colloidal nanoparticles with functional properties. However, the synergistic effect between different ligands and their distribution on crystallographic surfaces of nanoparticles during colloidal synthesis is still unclear despite powerful spectroscopic techniques, due to a lack of direct imaging techniques. In this study, atom probe tomography is adopted to investigate the three-dimensional atomic-scale distribution of two of the most common types of these ligands, cetrimonium (C19H42N) and halide (Br and Cl) ions, on Pd nanoparticles. The results, validated using density functional theory, demonstrate that the Br anions adsorbed on the nanoparticle surfaces promote the adsorption of the cetrimonium cations through electrostatic interactions, stabilizing the Pd {111} facets. In contrast, the Cl anions are not strongly adsorbed onto the Pd surfaces. The high density of adsorbed cetrimonium cations for Br anion additions results in the formation of multiple-twinned nanoparticles with superior oxidation resistance.

Atom Probe Tomography Investigations of Ag Nanoparticles Embedded in Pulse-Electrodeposited Ni Films.

Atomic mapping of nanomaterials, in particular nanoparticles, using atom probe tomography (APT) is of great interest, as their properties strongly depend on shape, size, and composition. However, APT analyses of nanoparticles are extremely challenging, and there is an urgent need for developing robust and universally applicable sample preparation methods. Herein, we explored a method based on pulse electrodeposition to embed Ag nanoparticles in a Ni matrix and prepare APT specimens from the resulting composite film. By systematically varying the duty cycle during pulse electrodeposition, the dispersion and number density of the nanoparticles within the matrix was significantly enhanced as compared to DC electrodeposition. Several Ag nanoparticles were analyzed with APT from such samples. Shape distortions and biased compositions were observed for the Ag nanoparticles after applying a standard data reconstruction protocol. Numerical simulations of the field evaporation process showed that such artifacts were caused by a difference in the evaporation field of Ni and Ag and a local magnification effect. We expect such detrimental effects to be mitigated by a careful selection of the matrix material, matching the evaporation field of the nanoparticles. Furthermore, we anticipate that the method presented herein can be extended to a wider range of nanomaterials.

Reducing Time to Discovery: Materials and Molecular Modeling, Imaging, Informatics, and Integration.

Multiscale and multimodal imaging of material structures and properties provides solid ground on which materials theory and design can flourish. Recently, KAIST announced 10 flagship research fields, which include KAIST Materials Revolution: Materials and Molecular Modeling, Imaging, Informatics and Integration (M3I3). The M3I3 initiative aims to reduce the time for the discovery, design and development of materials based on elucidating multiscale processing-structure-property relationship and materials hierarchy, which are to be quantified and understood through a combination of machine learning and scientific insights. In this review, we begin by introducing recent progress on related initiatives around the globe, such as the Materials Genome Initiative (U.S.), Materials Informatics (U.S.), the Materials Project (U.S.), the Open Quantum Materials Database (U.S.), Materials Research by Information Integration Initiative (Japan), Novel Materials Discovery (E.U.), the NOMAD repository (E.U.), Materials Scientific Data Sharing Network (China), Vom Materials Zur Innovation (Germany), and Creative Materials Discovery (Korea), and discuss the role of multiscale materials and molecular imaging combined with machine learning in realizing the vision of M3I3. Specifically, microscopies using photons, electrons, and physical probes will be revisited with a focus on the multiscale structural hierarchy, as well as structure-property relationships. Additionally, data mining from the literature combined with machine learning will be shown to be more efficient in finding the future direction of materials structures with improved properties than the classical approach. Examples of materials for applications in energy and information will be reviewed and discussed. A case study on the development of a Ni-Co-Mn cathode materials illustrates M3I3's approach to creating libraries of multiscale structure-property-processing relationships. We end with a future outlook toward recent developments in the field of M3I3.

Involvement of mitogen-activated protein kinases and p21Waf1 in hydroxyurea-induced G1 arrest and senescence of McA-RH7777 rat hepatoma cell line.

Hydroxyurea is commonly used to treat hematologic disorders and some type of solid tumors, but the mechanism for its therapeutic effect is not clearly known. In this study, we examined the effect of hydroxyurea on rat hepatoma McA-RH7777 cells, specifically, on the role of mitogen-activated protein (MAP) kinase signal transduction pathways and p21(Waf1), p27(Kip1) and p53. Rat hepatoma McA-RH7777 cells treated with hydroxyurea for 7 days, caused the inhibition of cell growth in a dose-dependent manner. But, this growth inhibition was not caused by necrosis or apoptosis but instead was associated with cell senescence-like change as evidenced by senescence associated-beta-galactosidase staining, and cells arrest at G1 phase of cell cycle. Phosphorylation of MAP kinases, such as ERK, JNK, and p38, was found to be decreased after treatment of cells with hydroxyurea. But, the expression of p21(Waf1) was increased, while p27(Kip1) and p53 were not detected in hydroxyurea treated rat hepatoma cells. Hydroxyurea treatment induced G1 arrest and a senescence-like changes in rat hepatoma McA-RH7777 cells may be the likely results of signal disruption of MAP kinases (ERK, JNK, and p38 MAP kinase) and p21(Waf1) over-expression.