Cs-corrected STEM Research Articles

In Situ Visualization and Mechanistic Understandings on Facet-Dependent Atomic Redispersion of Platinum on CeO2.

Redispersion is an effective method for regeneration of sintered metal-supported catalysts. However, the ambiguous mechanistic understanding hinders the delicate controlling of active metals at the atomic level. Herein, the redispersion mechanism of atomically dispersed Pt on CeO2 is revealed and manipulated by in situ techniques combining well-designed model catalysts. Pt nanoparticles (NPs) sintered on CeO2 nano-octahedra under reduction and oxidation conditions, while redispersed on CeO2 nanocubes above ∼500 °C in an oxidizing atmosphere. The dynamic shrinkage and disappearance of Pt NPs on CeO2 (100) facets was directly visualized by in situ TEM. The generated atomically dispersed Pt with the square-planar [PtO4]2+ structure on CeO2 (100) facets was also confirmed by combining Cs-corrected STEM and spectroscopy techniques. The redispersion and atomic control were ascribed to the high mobility of PtO2 at high temperatures and its strong binding with square-planar O4 sites over CeO2 (100). These understandings are important for the regulation of atomically dispersed platinum catalysts.

Unveiling the Structural Characteristics of Intergrowth Zeolites Synthesized in the Presence of Isopropylimidazolium-Based Cations and Fluoride Anions.

Here, we present the synthesis of RTH/ITE and MEL/MFI intergrowth zeolites using 2-isopropylimidazolium-based cations as organic structure-directing agents (OSDAs) in concentrated fluoride media and their local structural properties. Phase selectivity in the synthesis of zeolite intergrowths was found to differ according to the concentration of OSDA cations and fluoride anions in the synthesis mixture as well as to the type of OSDA employed. Molecular modeling results suggest that the crystallization of intergrowth zeolites in fluoride media may be kinetically rather than thermodynamically controlled, as in ordered zeolites. Cs-corrected STEM analysis of MEL/MFI crystals synthesized at HF/OSDA = 2.0 in the presence of 2-isopropyl-1,3-dipropylimidazolium ions as an OSDA indicates the existence of previously unobserved MEL-MFI intergrowth along the [100] direction, leading to a partial blockage of MEL 10-ring channels.

Quickly Switchable Angular and Spatially Resolved Cs-corrected STEM.

Quickly Switchable Angular and Spatially Resolved Cs-corrected STEM.

Hydrothermal corrosion behavior of Ti3SiC2 MAX phase at atomic scale under simulated PWR conditions

Corrosion-induced Si-layer escape of Ti3SiC2 and transformation from Ti3SiC2 MAX phase to Ti3C2 nanosheets at atomic scale under simulated PWR conditions were directly observed using double CS-corrected STEM, overwriting the conventional opinion that MAX phase corrodes in the form of corresponding parent MX ceramic. Bipyramidal anatase and nanocuboid rutile particles were confirmed to coexist in the corrosion layer by SAED and HRTEM. Long-term hydrothermal corrosion of Ti3SiC2 indicated its operational reliability in PWR conditions simulated by increasing dissolved oxygen content to accelerate corrosion. This work proposes a reference for the application of Ti3SiC2 in PWR fuel cladding.

Most Frequently Asked Questions about the Coercivity of Nd-Fe-B Permanent Magnets

Physically, the coercivity of permanent magnets should scale with the anisotropy field of ferromagnetic compounds, HA; however, the typical coercivity values of commercial polycrystalline sintered magnets are only ~0.2 HA, which is known as Brown’s paradox. Recent advances in multi-scale microstructure characterizations using focused ion beam scanning electron microscope (FIB/SEM), aberration corrected scanning transmission electron microscopy (Cs-corrected STEM), and atom probe tomography (APT) revealed detailed microstructural features of commercial and experimental Nd-Fe-B magnets. These investigations suggest the magnetism of a thin layer formed along grain boundaries (intergranular phase) is a critical factor that influence the coercivity of polycrystalline magnets. To determine the magnetism of the thin intergranular phase, soft X-ray magnetic circular dichroism (XMCD) and electron holography played critical role. Large scale micromagnetic simulations using the models that are close to real microstructure incorporating the recent microstructure characterization results gave insights on how the coercivity and its thermal stability is influenced by the microstructures. Based on these new findings, coercivity of Nd-Fe-B magnets are being improved to its limit. This review replies to most frequently asked questions about the coercivity of Nd-Fe-B permanent magnets based on our recent studies.

Vacancy engineering of two-dimensional W2N3 nanosheets for efficient CO2 hydrogenation.

Peaking carbon emissions and achieving carbon neutrality have become the consensus goal of the international community to solve the environmental problems threatening mankind caused by accumulative greenhouse gases like CO2. Herein we proposed vacancy engineering of two-dimensional (2D) topological W2N3 for efficient CO2 hydrogenation into high value-added chemicals and fuels. Spherical aberration corrected scanning transmission electron microscopy (Cs-corrected STEM) confirmed a large amount of N vacancies on the catalyst surface, which significantly reduced the energy barrier for the formation of the essential intermediates of *CO and *CHO as revealed by density functional theory (DFT) calculations. Consequently, the highly stable catalyst exhibited efficient CO2 hydrogenation superior to many previous reports with a maximum CO2 conversion rate of 24% and a high selectivity of 23% for C2+ hydrocarbons. This work provided not only insight into the vacancy-controlled CO2 hydrogenation mechanism, but also fresh ammunition to bring the remaining potential of 2D topological transition metal nitrides in the field of catalysis.

Structure of an Al64Cu22Co14 decagonal quasicrystal studied by Cs-corrected STEM

During the last several decades, since the discovery of a decagonal quasicrystal, a 2 nm cluster model has been widely accepted as its basic quasi-unit-cell (QUC). Instead of the traditional 2 nm QUC, a 3.2 nm QUC is proposed in this paper. The 3.2 nm QUC can fill all the blank areas. The 3.2 nm QUC consists of 251 atoms. The element type and position of each atom are determined using high-angle annular detector dark-field (HAADF) images taken along three projection directions, i.e., one along the ten-fold symmetry and the other two along the two-fold symmetry with an intersection angle of 18 degrees. The proposed model opens an avenue for further investigation of the aperiodic atomic structure of other quasicrystals.

Phase separation with ordering in aged Fe-Ni-Mn medium entropy alloy

High- and medium-entropy alloys are currently receiving significant attention. Understanding short-range ordering and nano-scaled phase separation concerning elemental interaction is a key to controlling phase stability for designing of high entropy alloys. In this study, we aim to clarify concurrent phase separation and ordering during aging of Fe-35Ni-35Mn (at%) medium-entropy alloy using advanced characterization techniques such as spherical aberration-corrected scanning transmission electron microscopy (Cs-corrected STEM) and three-dimensional atom probe (3DAP). Obvious age hardening occurs after aging at 500 °C for 96 h due to the formation of L10 ordered phase combined with phase separation. By aging at 400 °C or shorter time, obvious precipitation of the ordered phase is not detected but the radial distribution functions obtained by 3DAP present that enrichment of Ni and Mn nearby the same kinds of the element is observed, and this tendency is stronger for Mn. This result suggests that phase separation is induced by Mn clustering in the early stage of aging. A statistical thermodynamic model of phase stability considering L10 ordered structure is developed. The analyses using the model suggest that phase separation is not possible in a disordered state at 500 °C, therefore, nucleation of the L10 phase should precede phase separation at this temperature. On the other hand, phase separation is possible even in a disordered state at 400 °C, and ordering further enhances phase separation.

Zn-Induced Defect Complexity for the High Thermoelectric Performance of n-Type PbTe Compounds.

Although defect engineering is the core strategy to improve thermoelectric properties, there are limited methods to effectively modulate the designed defects. Herein, we demonstrate that a high ZT value of 1.36 at 775 K and a high average ZT value of 0.99 in the temperature range from 300 to 825 K are realized in Zn-containing PbTe by designing complex defects. By combining first-principles calculations and experiments, we show that Zn atoms occupy both Pb sites and interstitial sites in PbTe and couple with each other. The contraction stress induced via substitutional Zn on Pb sites alleviates the swelling stress by Zn atoms occupying the interstitial sites and promotes the solubility of interstitial Zn atoms in the structure of PbTe. The stabilization of Zn impurity as a complex defect extends the region of PbTe phase stability toward Pb0.995Zn0.02Te, while the solid solution region in the other direction of the ternary phase diagram is much smaller. The evolution of defects in PbTe was further explicitly corroborated by aberration-corrected scanning transmission electron microscopy (Cs-corrected STEM) and positron annihilation measurement. The Zn atoms compensate the Pb vacancies (VPb) and Zn interstitials (Zni) significantly improve the electron concentration, producing a high carrier mobility of 1467.7 cm2 V-1 s-1 for the Pb0.995Zn0.02Te sample. A high power factor of 4.11 mW m-1 K-2 is achieved for the Pb0.995Zn0.02Te sample at 306 K. This work provides new insights into understanding the nature and evolution of the defects in n-type PbTe as well as improving the electronic and thermal transport properties toward higher thermoelectric performance.

Topological Defects and Unique Stacking Disorders in Honeycomb Layered Oxide K2Ni2TeO6 Nanomaterials: Implications for Rechargeable Batteries

Endowed with a multitude of exquisite properties such as rich electrochemistry, superb topology, and eccentric electromagnetic phenomena, honeycomb layered oxides have risen to the top echelons of science with applications in diverse fields ranging from condensed matter physics, solid-state chemistry, materials science, and solid-state ionics to electrochemistry. However, these oxides are vastly underutilized as their underlying atomistic mechanisms remain unexplored. Therefore, in this study, atomic-resolution imaging on pristine K2Ni2TeO6 along multiple zone axes was conducted using spherical aberration-corrected scanning transmission electron microscopy (Cs-corrected STEM) to reveal hitherto unreported nanoscale topological defects and curvature which can be associated with various phase transitions. Furthermore, we discover the coexistence of a stacking variant with P3-type sequence alongside the well-reported P2-type stacking sequence in such honeycomb layered oxides. Our findings have the potential to inspire further experimental and theoretical studies into the role of stacking and topology in the functionality of honeycomb layered oxides, for instance, as high-performance electrode materials for rechargeable batteries.

Interstitial carbon induced FCC-Ti exhibiting ultrahigh strength in a Ti37Nb28Mo28-C7 complex concentrated alloy

An interstitial carbon induced-FCC-Ti was observed in a carbon-doped Ti37Nb28Mo28-C7 (atomic percentage) complex concentrated alloys (TNMC alloys). Not only the true crystal structure of this FCC-Ti was directly characterized using Cs-corrected STEM, but also the mechanical properties were tested by nanoindentation and micro-pillar compression for the first time. The FCC-Ti exhibited a high nano hardness of 17.8 GPa and a high elastic modulus of 233.1 GPa. The micropillar compression tests demonstrated the ultrahigh strengths of the interstitial C induced FCC-Ti (4.48 GPa at [1 1 1] direction, 2.67 GPa at [1 0 1] direction, and a CRSS of 1.12 ± 0.07 GPa). The nature of this FCC allotropic transformation was determined to be a rearrangement of lattice structure caused by interstitial C atoms, based on ab-initio studies. Findings in this study provide insights into the characteristics of FCC-Ti as well as the role of interstitial atoms in RCCAs which could open up unlimited possibilities in future material designing.

Effects of rare earth elements on the microstructure and wear properties of TiB2 reinforced aluminum matrix composite coatings: Experiments and first principles calculations

In this work, effects of rare earth elements on microstructure evolution and wear properties of TiB2 reinforced aluminum matrix composite coatings were investigated by the combination of first principles calculations and experimental investigation. The calculated results revealed that Sc exhibited the most effective modification ability due to the most negative adsorption energy and the largest charge transfer compared with La and Y. Based on the calculated results, the rare earth elements modified composite coatings were synthesized by laser processing. The as-obtained composites were characterized by back-scattered electron imaging (BSE) and spherical aberration corrected scanning transmission electron microscopy (Cs-corrected STEM). Results indicated that the addition of Sc can greatly refine the particle size of TiB2, improve the distribution of particles and lead to the morphology transforming from hexagonal disk to sphere. As a result, the microhardness and wear properties of the composite coatings were enhanced significantly. The best properties were achieved in the composite with 0.6 wt% Sc addition, which exhibited highest microhardness of 920 HV, lowest friction coefficient of 0.4 and wear rate of 24.8 mg /h. The preferential adsorption of rare earth elements on TiB2 facets is proposed to be the main modification mechanism which results in the morphology evolution and wear properties improvement.

Atmosphere induced amorphous and permeable carbon layer encapsulating PtGa catalyst for selective cinnamaldehyde hydrogenation

In this work, an alloy catalyst with C@PtGa interface structure was synthesized under H2/CO2 atmosphere by using PtCl62−/MgAlGa-LDHs as precursor. Cs-corrected STEM, EELS and CO chemisorption clearly confirmed the PtGa alloy particles were well covered by amorphous carbon thin layer with the characteristics of porous and permeable. In selective cinnamaldehyde hydrogenation, the C@PtGa catalyst showed a higher selectivity than both bare PtGa catalyst and Ga2O3@PtGa catalyst. The enhanced selectivity was attributed to the geometric decoration of carbon layer as well as the formation of electron-rich Pt sites at C@PtGa interface, which optimized the adsorption mode of cinnamaldehyde. More importantly, the permeable characteristic of carbon layer which maintained the accessibility of reactants to the Pt sites, contributed to the higher conversion compared to Ga2O3@PtGa catalysts with traditional oxide interface layer. Additionally, C@PtGa catalyst exhibited a good reusability with conversion of 88.3% and selectivity of 91.9% after 5 cycles. This work provides an alternative way to realize the boost of activity and selectivity in selective hydrogenation of C=O bonds by intelligently fabricating non-oxides@metal interface.

Synthesis and structural characterization of MoS2 micropyramids

Two-dimensional (2D) materials based on molybdenum sulfide (MoS2) have shown promising applications in semiconductors, optoelectronics, and catalysis. The variety of applications implies a controlled manipulation of purity, shape, and phase of such materials. This work elaborates on the structural characterization of MoS2 micro-assemblies produced in a chemical vapor deposition (CVD) system with emphasis on the pyramidal structures formed at high temperature and low gas rate, on a silicon dioxide (SiO2) substrate. A precise control of temperature and gas rate in the CVD process prompts the growth of pyramidal and other micron-size arrangements of MoS2 layers. An integrative set of high-resolution and analytical electron microscopy techniques, in conjunction with Raman and X-ray photoelectron spectroscopy (XPS), revealed the structural features of the MoS2 microstructures. Raman and XPS confirmed the presence of MoS2 and some residual oxide phases. Ultra-high-resolution scanning electron microscopy provided direct observation of the distinctive stacking of layers forming the pyramidal microstructures. Cross section samples from selected structures were done using focused ion beam. An extent of transmission electron microscopy and Cs-corrected scanning transmission electron microscopy (Cs-corrected STEM) results is discussed. This approach allowed to understand the growth mechanism of the triangular MoS2 microstructures through spiral grow around a screw dislocation, initiated at the center of the assembly.

Imaging the node-linker coordination in the bulk and local structures of metal-organic frameworks

Porous metal-organic frameworks (MOFs) have shown wide applications in catalysis, gas storage and separation due to their highly tunable porosity, connectivity and local structures. However, the electron-beam sensitivity of MOFs makes it difficult to achieve the atomic imaging of their bulk and local structures under (scanning) transmission electron microscopy ((S)TEM) to study their structure-property relations. Here, we report the low-dose imaging of a beam-sensitive MOF, MIL-101, under a Cs-corrected STEM based on the integrated differential phase contrast (iDPC) technique. The images resolve the coordination of Cr nodes and organic linkers inside the frameworks with an information transfer of ~1.8Å. The local structures in MIL-101 are also revealed under iDPC-STEM, including the surfaces, interfaces and defects. These results provide an extensible method to image various beam-sensitive materials with ultrahigh resolution, and unravel the whole framework architectures for further defect and surface engineering of MOFs towards tailored functions.

Observation of Ag Nanoparticles in/on Ag@MIL-100(Fe) Prepared Through Different Procedures.

Loading of active metals, metal clusters, and/or metal nanoparticles in Metal Organic Frameworks (MOFs) is an emergent field with applications in sensors, catalysis, medicine, and even in the polymeric industry. In the present work, MIL-100(Fe) has been synthesized and reacted with AgNO3 through liquid and incipient wetness, and also through solid-state reaction or solid grinding. The aim of this study is to evaluate whether the MIL-100 would uptake metal particles using a similar principle as that of the ion exchange in zeolites, or else, their inherent humidity would favor the “dissolution” of the metal salt, thus yielding very small metal particles. The immobilization of Ag nanoparticles inside the MOF pores was identified by Cs-corrected scanning transmission electron microscopy (Cs-corrected STEM) techniques.

Oxygen vacancies in TiO2/SnOx coatings prepared by ball milling followed by calcination and their influence on the photocatalytic activity

We prepared TiO2/SnOx (x = 1, 2) composite coatings through ball milling followed by calcination. The composite coatings were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), spherical aberration corrected scanning transmission microscopy (Cs-corrected STEM), X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR) and other techniques. Bulk oxygen vacancies in the form of vacancy clusters rather than individual sites were directly observed by Cs-corrected STEM. With an increase in the calcination temperature from 873 K to 1073 K or holding time from five hours to ten hours, the concentration of bulk defects also increased. On the other hand, the photocatalytic activity of the as-prepared samples for degrading MB dye increased with the concentration increase of bulk defects. Sample 973 K-10 h showed the best photocatalytic activity whether under the irradiation of UV light or visible light. With a further increase in the concentration of bulk oxygen vacancies, the photocatalytic activity decreased, although surface Ti3+ defects appeared. In other words, a moderate concentration of bulk defects could benefit the enhancement of the photocatalytic activity of the as-prepared samples. Ball milling followed by calcination is an effective method to introduce defects such as oxygen vacancies.

Extraordinary thermoelectric performance in n-type manganese doped Mg3Sb2 Zintl: High band degeneracy, tuned carrier scattering mechanism and hierarchical microstructure

Zintl phases are ideal candidates for thermoelectric applications due to their rich chemistry and structural complexity. However, the persistent p-type conduction due to intrinsic defects strongly restricts their practical applications. Recently, several typical n-type Zintl materials have been designed, where Te-doped Mg3Sb1.5Bi0.5 as the most promising. To enhance its overall thermoelectric performance, we introduce Mn to synergistically optimize the electrical and thermal transport properties. Both experimental and computational results demonstrate that multiple conduction bands with high band degeneracy are responsible for the enhanced Seebeck coefficient. Mn doping on Mg sites changes the low-temperature carrier scattering mechanism from ionized impurity scattering to mixed scattering with acoustic phonons and ionized impurities, resulting in a significant enhancement of carrier mobility and therefore power factor. Simultaneously, the total thermal conductivity is observably reduced after Mn doping. We employed aberration-corrected scanning transmission electron microscopy (Cs-corrected STEM) to thoroughly investigate its hierarchical microstructure, including sub-micron grains, nanoscale Bi precipitates segregated at grain boundaries, nanoscale endotaxial Bi-rich precipitates within the Mg3Sb2 based matrix, as well as the resulting strain fields around these defects. The synergistic optimization of electrical and thermal transport contributes to extraordinary performance, namely a peak ZT ~ 1.85 at 723 K and an average ZT ~ 1.25 (from 300 K to 723 K), which are the highest ever reported in any n-type thermoelectric material.

High-quality thulium iron garnet films with tunable perpendicular magnetic anisotropy by off-axis sputtering \u2013 correlation between magnetic properties and film strain

Thulium iron garnet (TmIG) films with perpendicular magnetic anisotropy (PMA) were grown on gadolinium gallium garnet (GGG) (111) substrates by off-axis sputtering. High-resolution synchrotron radiation X-ray diffraction studies and spherical aberration-corrected scanning transmission electron microscope (Cs-corrected STEM) images showed the excellent crystallinity of the films and their sharp interface with GGG. Damping constant of TmIG thin film was determined to be 0.0133 by frequency-dependent ferromagnetic resonance (FMR) measurements. The saturation magnetization (Ms) and the coercive field (Hc) were obtained systematically as a function of the longitudinal distance (L) between the sputtering target and the substrate. A 170% enhancement of PMA field (H⊥) was achieved by tuning the film composition to increase the tensile strain. Moreover, current-induced magnetization switching on a Pt/TmIG structure was demonstrated with an ultra-low critical current density (jc) of 2.5 × 106 A/cm2, an order of magnitude smaller than the previously reported value. We were able to tune Ms, Hc and H⊥ to obtain an ultra-low jc of switching the magnetization, showing the great potential of sputtered TmIG films for spintronics.

Selective catalytic cracking of n-hexane to olefins over SSZ-54 fabricated by facile and novel dual templating method

A detailed study of the preparation of SSZ-54, an intergrowth of MTT and TON frameworks using a novel mixture of two templates; isopropyl amine (IPA) and 1,6-hexane diamine (DAH), was carried out. The characterization of the products done using X-ray diffraction (XRD), scanning electron microscopy (SEM) and aberration-corrected scanning transmission electron microscopy (Cs-corrected STEM) unveil assemblies of needles with spans of about 1 µm and 20–30 nm in thickness typical of structures of both MTT and TON frameworks, confirms the formation of SSZ-54. The amount of the structure directing agents, synthesis time and temperature were pivotal in the synthesis of SSZ-54 zeolite. Furthermore, we observed that the higher molecular weighted molecule (DAH) has more influence on the control of the metamorphosis of the SSZ-54 phase. To the best of our knowledge, this is the first application of intergrowth SSZ-54 in the production of light olefins especially propylene from the cracking of n-hexane. Although, initial activity of SSZ-54 is not the best among the three frameworks, its activity, nevertheless, is more stable as compared with its component frameworks (MTT and TON). Also, SSZ-54 zeolite showed high and stable olefin yield than both component frameworks irrespective of SiO2/Al2O3 (Si/Al2) ratio. The shape selective property of SZZ-54 zeolite to mainly olefins owing to its unique innate pore structure was demonstrated as the selectivity to BTX on SSZ-54 was minimized to ca. 1% over 10 h time-on-stream.