Field Scanning Transmission Electron Microscopy Research Articles

BiO 2-x Nanosheets with Surface Electron Localizations for Efficient Electrocatalytic CO 2 Reduction to Formate

BiO _2-x Nanosheets with Surface Electron Localizations for Efficient Electrocatalytic CO ₂ Reduction to Formate

Nanoscale precipitates with quasicrystal domains enhanced strength-ductility synergy in a Mg–6Zn–4Al–1Sn–0.5Mn alloy

Precipitation strengthening is one of the significant strengthening mechanisms for metallic alloys owing to its excellent strengthening effect. Here, we effectively introduce homogenous nanosized precipitates into a Mg–6Zn–4Al–1Sn-0.5Mn (wt.%) alloy via extrusion and double aging to enhance strength-ductility synergy. The yield strength, ultimate tensile strength, and total elongation of the Mg alloy are 303.1 ± 15.8 MPa, 392.5 ± 3.9 MPa, and 21.1 ± 2.4%, respectively. Using transmission electron microscopy and atomic level high angle annual dark field-scanning TEM, in addition to the icosahedral quasicrystalline (IQC) particles at the grain boundary, we find that there are two kinds of nanoprecipitates in the matrix. The majority of nanoprecipitates are cuboid IQC phases with nanodomains parallel to the basal plane of the Mg matrix, while the minority of nanoprecipitates are rod-like β1′ phases containing IQC clusters vertical to the basal plane. These homogenous nanoscale precipitates and grain refinement contribute 50.5% and 30.2% of the yield strength. This work sheds new light on understanding the nanoscale IQC precipitate strengthening mechanisms of magnesium alloys and provides a novel method to fabricate alloys with superior mechanical properties.

Dilute long period stacking/order (LPSO)-variant phases along the composition gradient in a Mg-Ho-Cu alloy

We have systematically investigated the microstructures of as-cast Mg 97.49 Ho 1.99 Cu 0.43 Zr 0.09 alloy by atomic resolution high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM), revealing the coexistence of 18R, 14H and 24R long period stacking/order (LPSO) phases with fully coherent interfaces along step-like composition gradient in a blocky intermetallic compound distributed at grain boundary. The short-range order (SRO) L1 2 -type Cu 6 Ho 8 clusters embedded across AB'C’A-stacking fault layers are directly revealed at atomic scale. Importantly, the order degree of SRO clusters in the present dilute alloy is significant lower than previous 6M and 7M in-plane order reported in ternary Mg-TM (transition metal)-RE (rare earth) alloys, which can be well matched by 9M in-plane order. This directly demonstrates that SRO in-plane L1 2 -type clusters can be expanded into more dilute composition regions bounded along the definite TM/RE ratio of 3/4. In addition, the estimated chemical compositions of solute enriched stacking fault (SESF) in all LPSO variants are almost identical with the ideal SESF composition of 9M in-plane order, regardless of the type of LPSO phases. The results further support the viewpoint that robust L1 2 -type TM 6 RE 8 clusters play an important role in governing LPSO phase formation.

DMPFIT: A Tool for Atomic-Scale Metrology via Nonlinear Least-Squares Fitting of Peaks in Atomic-Resolution TEM Images

Despite the wide availability and usage of Gatan’s DigitalMicrograph software in the electron microscopy community for image recording and analysis, nonlinear least-squares fitting in DigitalMicrograph is less straightforward. This work presents a ready-to-use tool, the DMPFIT software package, written in DigitalMicrograph script and C++ language, for nonlinear least-squares fitting of the intensity distribution of atomic columns in atomic-resolution transmission electron microscopy (TEM) images with a general two-dimensional (2D) Gaussian model. Applications of the DMPFIT software are demonstrated both in atomic-resolution conventional coherent TEM (CTEM) images recorded by the negative spherical aberration imaging technique and in high angle annular dark field (HAADF) scanning TEM (STEM) images. The implemented peak-finding algorithm based on the periodicity of 2D lattices enables reliable and convenient atomic-scale metrology as well as intuitive presentation of the resolved atomic structures.

Synergy of dual single Ni and Co atoms on borate modified g-C3N4 for photocatalytic CO2 reduction

Dual modulation of electron and catalytic reduction as well as hole and catalytic oxidation targeting g-C3N4 is critical for efficient photocatalytic CO2 reduction. Herein, we decorated dual single Ni2+ and Co2+ atoms on borate modified ultrathin porous g-C3N4 nanosheets (NiCo-B-PCN) by a facile ion exchange method. Optimal NiCo-BO-PCN photocatalyst by regulating bimetallic molar ratio realized 43-fold CO2 photoconversion compared to pristine CN under full light irradiation. The single-atom dispersion of Ni2+ and Co2+ sites was evidenced by the high angle angular dark field-scanning transmission electron microscopy. As revealed by the characterizations such as transient absorption spectra and electrochemical curves, the photoactivity enhancement was attributed to the synergetic effects that single Ni2+ atoms could capture electrons and benefit the CO2 reduction while single Co2+ atoms could extract holes and catalyze the water oxidation. This work has provided a novel dual-modulation strategy by constructing dual single-atom photocatalysts for efficient solar fuel production.

Nano-sized alumina supported palladium catalysts for methane combustion with excellent thermal stability

Pd/Al2O3 catalysts supported on Al2O3 of different particle sizes were synthesized and applied in methane combustion. These catalysts were systematically characterized by Brunauer-Emmett-Teller (BET), X-ray diffraction (XRD), high resolution-transmission electron microscopy (HR-TEM), high-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM), H2-temperature-programmed reduction (H2-TPR), O2-temperature-programmed oxidation (O2-TPO), X-ray photoelectron spectroscopy (XPS), and X-ray absorption fine structure (XAFS). The characterization results indicated that nano-sized Al2O3 enabled the uniform dispersion of palladium nanoparticles, thus contributing to the excellent catalytic performance of these nano-sized Pd/Al2O3 catalysts. Among them, Pd/Al2O3-nano-10 (Pd/Al2O3 supported by alumina with an average particle size of 10 nm) showed superior catalytic activity and stability for methane oxidation under harsh practical conditions. It maintained excellent catalytic performance for methane oxidation for 50 hr and remained stable even after harsh hydrothermal aging in 10 vol.% steam at 800°C for 16 hr. Characterization results revealed that the strong metal-support interactions and physical barriers provided by Al2O3-nano-10 suppressed the coalescence ripening of palladium species, and thus contributed to the superior sintering resistance of the Pd/Al2O3-nano-10 catalyst.

Iron Single Atoms Anchored on Carbon Matrix/g-C3N4 Hybrid Supports by Single-Atom Migration-Trapping Based on MOF Pyrolysis.

Numerous efforts have been devoted to realizing the high loading and full utilization of single-atom catalysts (SACs). As one of the representative methods, atom migration-trapping (AMT) is a top-down strategy that converts a certain volume of metal nanoparticles (NPs) or metal-based precursors into mobile metal species at high temperature, which can then be trapped by suitable supports. In this study, high-loading iron single atoms anchored onto carbon matrix/g-C3N4 hybrid supports were obtained through a single-atom migration-trapping method based on metal–organic framework (MOF) pyrolysis. It is confirmed, by high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM), X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS), that the Fe(acac)3 precursor is reduced to Fe single atoms (SAs), which are not only anchored onto the original N-doped carbon (NC), but also onto g-C3N4, with an Fe-N coordination bond. Further electrochemical results reveal that Fe-C3N4-0.075 possesses a better half-wave potential of 0.846 V and onset potential of 0.96 V compared to Fe-N-C, the product obtained after pyrolysis of Fe(acac)3@ZIF-8. As opposed to SAs prepared by the pyrolysis process only, SAs prepared by AMT are commonly anchored onto the surface of the supports, which is a simple and effective way to make full use of the source metal and prepare SACs with higher exposing active sites.

Slow Photon-Enhanced Heterojunction Accelerates Photocatalytic Hydrogen Evolution Reaction to Unprecedented Rates

Slow Photon-Enhanced Heterojunction Accelerates Photocatalytic Hydrogen Evolution Reaction to Unprecedented Rates

Influence of twins found in adiabatic shear bands on dynamic recrystallization of a near β Ti-5.5Mo-7.2Al-4.5Zr-2.6Sn-2.1Cr alloy

Much efforts have been made on the studies of the microstructure evolution near adiabatic shear bands (ASBs) in titanium alloys. However, the deeper evolution mechanism is not very clear. Therefore, in the present, the dynamic compression test utilizing split Hopkinson pressure bar at room temperature and a strain rate of ∼3000/s was carried out on a hot-rolled near β-typed Ti-5.5Mo-7.2Al-4.5Zr-2.6Sn-2.1Cr alloy, and the microstructures of the central region and the transition region of the adiabatic shear bands were carefully investigated by high-angle annular dark field-scanning transmission electron microscope (HAADF-STEM). It was found that the central region of ASBs was composed of the softening regions (SRs) and the hardening regions (HRs). Moreover, detailed observation indicated that in the SRs, the microstructure consisted of dynamic recrystallization (DRX) grains. While in the HRs, the {11–22}<11-2-3> twin in α grains and the {332}<113> twin in β grains were observed, and a great number of dislocations were accumulated at the interface of twins and parent phases (α and β phases) to form dislocation walls, thus promoting the DRX process in the ASB central region. It illustrated a new DRX mechanism in ASBs. Meanwhile, no twins were found in the ASB transition region. Further research on the average compositions of α and β phases in the HRs of ASBs by energy dispersion spectrum method combined with d-electron theory revealed that the twin in α grains was mainly caused by the severe deformation under dynamic loading, while that in β phase was the result of the instability of β phase.

Mapping 1D Confined Electromagnetic Edge States in 2D Monolayer Semiconducting MoS2 Using 4D-STEM.

Four-dimensional (4D) scanning transmission electron microscopy is used to study the electric fields at the edges of 2D semiconducting monolayer MoS2. Sub-nanometer 1D features in the 2D electric field maps are observed at the outermost region along zigzag edges and also along nanowire MoS-terminated MoS2 edges. Atomic-scale oscillations are detected in the magnitude of the 1D electromagnetic edge state, with spatial variations that depend on the specific periodic edge reconstructions. Electric field reconstructions, along with integrated differential phase contrast reconstructions, reveal the presence of low Z number atoms terminating many of the uniform edges, which are difficult to detect by annular dark field scanning transmission electron microscopy due to its limited dynamic range. Density functional theory calculations support the formation of periodic 1D edge states and also show that enhancement of the electric field magnitude can occur for some edge terminations. The experimentally observed electric fields at the edges are attributed to the absence of an opposing electric field from a nearest neighbor atom when the electron beam propagates through the 2D monolayer and interacts. These results show the potential of 4D-STEM to map the atomic scale structure and fluctuations of electric fields around edge atoms with different bonding states than bulk atoms in 2D materials, beyond conventional imaging.

Effects of Electron Microscope Parameters and Sample Thickness on High Angle Annular Dark Field Imaging.

Scanning transmission electron microscopy (STEM) developed into a very important characterization tool for atomic analysis of crystalline specimens. High-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) has become one of the most powerful tools to visualize material structures at atomic resolution. However, the parameter of electron microscope and sample thickness is the important influence factors on HAADF-STEM imaging. The effect of convergence angle, spherical aberration, and defocus to HAADF imaging process has been analyzed through simulation. The applicability of two HAADF simulation software has been compared, and suggestions for their usage have been given.

Interactive transformation mechanisms of multiple metastable precipitates in a Si-rich Al-Mg-Si alloy

ABSTRACT The precipitate evolution mechanism of the Al-Mg-Si alloys with a Si-rich composition have been a long-term challenge due to the complex transformation paths among multiple precipitates. In the present work, the interactive transformation process of multiple metastable precipitates in a Si-rich Al-Mg-Si alloy were thoroughly studied by atomic resolution high angle annular dark field scanning transmission electron microscopy (HAADF-STEM), in-situ heating TEM and first-principles calculation. It was revealed that the β″ phase transforms simultaneously to various types of precipitates in the over-aged stage. Two different evolution paths are proposed. Firstly, the β″ phase transforms directly to the U1, U2 and β′ phases. Secondly, the U2 phase, as an intermediate phase, transforms to the U1, β′ and B′ phases during the prolonging aging. The formation and evolution of substructures, including the QP lattice and Al–Si columns, are the key for structural transformation of precipitates. During the severe over-aging, the evolution of precipitates is governed by the dissolution of small precipitates (β′ and B′ phases) and the growth of the large ones (U1 phase). These results provide scientific basis for both property improvement of the commercial Al-Mg-Si alloys and development of new 6xxx aluminum alloys.

Dendrimer-encapsulated PtSn bimetallic ultrafine nanoparticles supported on graphitic mesoporous carbon as efficient electrocatalysts for methanol oxidation

Catalytic performance of Pt based nanocatalysts significantly depends on their morphologies, compositions and structures. In this work, the generation-4 phenylazomethine-tetraphenylmethane dendrimer (TPMG4)-encapsulated PtSn bimetallic ultrafine nanoparticles with tunable ratio of Pt and Sn (PtnSn60-n@TPMG4/GMC, n = 56, 48, 32) supported on graphitic mesoporous carbon (GMC) were fabricated through co-reduction method by using the phenylazomethine dendrimer as a template. The morphologies, compositions, and structures of resulting PtnSn60-n@TPMG4/GMC were detected by UV–vis absorption spectra (UV–vis), inductively coupled plasma-optical emission spectroscopy (ICP-OES), high angle annular dark field scanning transmission electron microscopy (HAADF-STEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) measurements. These physical characterizations show that dendrimer-encapsulated PtnSn60-n nanoparticles are anchored on the surface of graphitic mesoporous carbon with highly dispersion and uniformly ultrafine size of ∼1 nm, The atomic ratio of Pt and Sn in PtnSn60-n bimetallic nanoparticles can be accurately controlled. The electrochemical characterizations demonstrate that all PtnSn60-n@TPMG4/GMC samples exhibit more excellent electrocatalytic performance for methanol oxidation than single metal Ptm@TPMG4/GMC and commercial carbon supported platinum catalyst (Pt/C). Especially, the Pt56Sn4@TPMG4/GMC displays the highest catalytic activity with 1.6 times higher than Pt60/GMC and 1.9 times higher than Pt/C. Also, the Pt56Sn4@TPMG4/GMC exhibits the high carbon monoxide (CO) tolerance and superior catalytic stability towards methanol oxidation. Consequently, constructing Pt based bimetallic nanoparticles with controllable compositions and structures is an effective way to improve the catalytic performance and reduce the amount of Pt used in the catalysts, which will be widely concerned in the field of direct methanol fuel cells.

Atomic-scale structure clarification of the planar Z phase and its influence on the magnetic properties in Sm(CoFeCuZr)z permanent magnets

2:17-type SmCo-based permanent magnets with special cellular structure composed of 2:17R, 1:5H and planar Z phase have exhibited great potential for high temperature application. However, there are still disputes on the atomic structure of the planar phase and its influence on magnetic properties. Here using advanced aberration corrected high-angle annular dark field scanning transmission electron microscopy we found that most of the planar Z phases possess SmZr4Co13–H (P-6m2) or SmZr2Co9–2H (P-6m2) structure instead of SmZr2Co9–3R (R-3m) structure. This experimental observation is corroborated well with our calculation of the formation enthalpy as well as Gibbs energy that SmZr4Co13–H and SmZr2Co9–2H are more thermodynamically stable. Moreover, we found that the transition from the matrix to the planar phase includes both Sm-Co co-displacement along the basal plane as well as Zr replacement for Sm 1a and Co 2c sites. The calculated magnetocrystalline anisotropy of the planar phase under relaxed state exhibits easy axis of magnetization, while that of the planar phase under strain state inside the magnets transforms to easy plane of magnetization. Micromagnetic simulation indicates that the magnetocrystalline anisotropy of the planar phase determines the initial nucleation process of reversal magnetization inside the 2:17R cells, which influences significantly on the coercivity. Tuning the magnetocrystalline anisotropy of the ultrathin planar phase inside the magnets to easy axis will benefit the achievement of higher coercivity. Our investigation may provide insights for developing SmCo-based permanent magnets with higher performance from atomic scale perspective.

AutomAl 6000: Semi-automatic structural labelling of HAADF-STEM images of precipitates in Al–Mg–Si(–Cu) alloys

When the Al–Mg–Si(–Cu) alloy system is subjected to age hardening, different types of precipitates nucleate depending on the composition and thermomechanical treatment. The main hardening precipitates extend as needles, laths or rods along the <100> directions in the aluminium matrix. It has been found that the structures of all metastable precipitates may be generalized as stacks of <100> columns, where most of these columns are replaced by solute elements. In the precipitates, a column relates to neighbour columns by a set of simple structural principles, which allows identification of species and relative longitudinal displacement over the (100) cross-section.Aberration-corrected high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) is an important tool for studying such precipitates. With the goal of analysing atomic resolution HAADF-STEM images of precipitate cross-sections in the Al–Mg–Si(–Cu) system, we have developed the stand-alone software AutomAl 6000, which features a column characterization algorithm based on the symbiosis of a statistical model and the structural principles formulated in a digraph-like framework. The software can semi-autonomously determine the 3D column positions in the image, as well as column species. In turn, AutomAl 6000 can then display, analyse and/or export the structure data.This paper describes the methodology of AutomAl 6000 and applies it on three different HAADF-STEM images, which demonstrate the methodology. The software, as well as other resources, are available at http://automal.org. The source code is also directly available from https://github.com/Haawk666/AutomAl-6000.

The aging behavior, microstructure and mechanical properties of AlN/AZ91 composite

Microstructure evolution and mechanical properties of the aging treated AlN/AZ91 composites were systematically investigated by optical microscopy (OM), high resolution scanning electron microscopy (HRSEM) with an energy dispersive spectrum (EDS), and high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM). The results show that the higher fracture elongation (14 ± 1%) and ultimate tensile strength (275 ± 6 MPa) were simultaneously obtained in the peak-aged AlN/AZ91 composites. Comparied with AZ91 matrix alloy, the strength was increased by about 44% and the elongation was approximately five times higher, which mainly attributed to the precipitation of nano-sized γ-Mg17Al12 phase and the activation of non-basal slip systems induced by in-situ AlN particles at room temperature. However, the in-situ formation of AlN reinforcements consumed part of Al element in the matrix alloy, which resulted into the volume fraction decreasing of γ-Mg17Al12 precipitates, and then the age hardening and strengthening efficiency were reduced in the AlN/AZ91 composites. On the other hand, the mismatch of thermal expansion coefficient between AlN particles and AZ91 matrix generated high density dislocations around AlN particles, which promoted the precipitation of γ-Mg17Al12 phase, and then the peak aging time and temperature were decreased.

Fabrication and Excellent Performances of Bismuth Telluride-Based Thermoelectric Devices.

The barrier layer between thermoelectric (TE) legs and electrodes has crucial impact on the electrothermal conversion efficiency of the TE device; however, the interfacial reaction of the Ni metal barrier layer with TE legs in traditional Bi2Te3-based devices is harmful to the device performance. Herein, a high-quality barrier layer of a Ni-based alloy has been fabricated on both n-type and p-type Bi2Te3-based TE legs by the electroplating method. The in situ XRD results indicate that the as-prepared Bi2Te3-based TE legs with a Ni-based alloy barrier layer remain stable even at 300 °C. The high-resolution high-angle annular dark field scanning transmission electron microscopy images reveal that the Ni-based alloy barrier layer has more excellent stability than that of the Ni metal barrier layer. The Bi2Te3-based TE devices with excellent structural and performance stabilities were assembled with the as-grown high-performance n-type and p-type Bi2Te3-based leg with a Ni-based alloy barrier layer, which have lower internal resistance and higher cooling and power generation performances. A maximum cooling temperature difference over 65 K and a maximum cooling capacity of 55 W were obtained for the high-performance Bi2Te3-based TE devices. This work provides a new strategy for high-temperature applications of commercial Bi2Te3-based TE devices.

Constructing atomic Co1–N4 sites in 2D polymeric carbon nitride for boosting photocatalytic hydrogen harvesting under visible light

Two‒dimensional polymeric carbon nitride (2DPCN) is a versatile support for constructing single atom catalysts (SACs). Herein, we demonstrated a facile strategy for constructing atomic Co sites on 2DPCN. Atomic dispersion of Co was confirmed by the aberration‒corrected high angle annular dark field scanning transmission electron microscopy, while the extended X‒ray absorption fine structure spectroscopy analysis precisely corroborated the existence of atomic Co1–N4 non square planar sites on 2DPCN support. Remarkably, Co1/2DPCN catalyst yielded H2 at a rate of 28.3mmolH2 h−1 gCo−1, which is approximately 4.2 times higher than that of CoNPs/2DPCN (6.8mmolH2 h−1g Co−1) counterpart. Notably, with 0.75 wt % Pt as a co‒catalyst, H2 evolution activity for Co1/2DPCN and CoNPs/2DPCN reached as high as 138.6 mmolH2 h−1 gmetal−1 and 22.4mmolH2 h−1 gmetal−1, respectively, which could be attributed to the synergistic effect of atomic Co1–N4 sites and Pt. Both DFT calculations and spectroscopic results revealed that as constructed atomic Co1–N4 sites, reduced band gap energy, improved light harvesting ability and decreased energy barrier for H2 evolution.

Unravelling Structure and Formation Mechanisms of Ruddlesden-Popper-Phase-like Nanodomains in Inorganic Lead Halide Perovskites.

Ultrastable CsPbBr3 nanoplates against electron beam irradiations are fabricated and nanodomains with anomalous high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) contrasts are observed within CsPbBr3 nanoplates. Atomic resolution energy dispersive X-ray spectroscopy (EDS) mapping, which requires even higher beam currents and may cause significant damages on electron beam sensitive materials, are obtained without any detectable damages or decomposition. Combining HAADF-STEM images, atomic resolution EDS mapping, and image simulations has revealed detailed structure and chemistry of the nanodomains to be induced by Ruddlesden-Popper faults (RP faults) rather than any chemical intermixing or formation of new phases. A formation mechanism is also proposed on the basis of the atomic structure of the nanodomains. This result promotes an atomic-level understanding of inorganic lead halide perovskites and may help to reveal their structure-property relationship.

Ultrahigh ciprofloxacin accumulation and visible-light photocatalytic degradation: Contribution of metal organic frameworks carrier in magnetic surface molecularly imprinted polymers

Pharmaceuticals pollutions causes inevitable threat for both water environment and human health. However, both the selective and efficient accumulation together with degradation in real wastewater is still one of the major challenges which need further exploration. Here, we designed and synthesized MIL-100 based mesoporous carries supported surface molecularly imprinted polymers. The functional monomer was successfully grafted on the metal organic frameworks (MOFs) carries with magnetic core (Fe3O4). The simultaneous hydrolysis of functional precursor and condensation of the hydrolyzed precursors form a three–dimensional polymer network. Thanks to the high surface area and abundant mesoporous channels formed by the fast microwave irradiation method, it can facilitate mass transfer process and achieve high uptake capacity. This MIL-100 based imprinted polymer showed both ultrahigh selectivity (α(QMIP/QNIP) = 3.54) and highest uptake capacity calculated by the Langmuir equation (273.65 mg/g) for Ciprofloxacin so far. In order to prove the role of MIL-100 insides, pore size distribution, surface area, high angle annular dark field-scanning transmission electron microscopy (HAADF–STEM) images were studied detailly. Adsorption mechanism have been proposed based on X-ray Photoelectron Spectroscopy (XPS) and Infrared Spectroscopy (IR) spectra before and after the treatment. Importantly, the influence of environmental competitors including the inorganic ions and natural organic matters have been evaluated separately. Furthermore, the material can effectively remove targeted compound which was spiked in real secondary effluents from Beijing, substantiating that this novel MOFs–based material is among the rank of excellent CIP adsorbent. Furthermore, the iron-based active sites can be accessible for CIP, resulting the completely degradation under visible − light irradiation with small amount of hydrogen peroxide added. Our study presents a facile approach by introducing MILs to improve the performance of imprinted polymers.