Showing papers on "Transmission electron microscopy published in 2021"

One-pot prepared mesoporous silica SBA-15-like monoliths with embedded Ni particles as selective and stable catalysts for methane dry reforming

Abstract: Ni@SBA-15 monoliths with up to 5 wt.% of Ni were successfully synthetized by means of an original and easy one-pot sol-gel method. Transmission Electron Microscopy (TEM), X-ray Photoelectron Spectroscopy (XPS), Temperature-Programmed Reduction (TPR), Pair Distribution Function (PDF) and X-Ray Diffraction (XRD) were used for the structural characterization of the samples. After H2-reduction, those solids exhibited small Ni° particles (between 1–3 nm) highly dispersed (one of the highest dispersion reported in the literature to date for 5 wt.% Ni/Silica materials) in strong interaction with the silica support. Scanning Transmission Electron Microscopy in the High Angle Annular Dark Field (STEM/HAADF) mode, chemical mapping by Energy Dispersive X-Ray (EDX) spectroscopy and electron tomography in STEM-HAADF mode highlighted the presence of Ni particles homogeneously distributed, especially in the mesopores. Such confined Ni nanoparticles were shown to be very selective and stable in the dry reforming of methane.

Enhanced NO2 gas sensing properties based on Rb-doped hierarchical flower-like In2O3 microspheres at low temperature

Abstract: In this work, Rb-doped hierarchical flower-like In2O3 microspheres were synthesized via a facile one-step solvothermal method along with the subsequent thermal treatment. The morphology and structure of the as-synthesized samples were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). Gas sensors based on the pure and Rb-doped In2O3 were fabricated and their gas sensing properties were systematically investigated. The sensors based on 1 mol% Rb-doped In2O3 exhibited ultrahigh response (Rg/Ra = 1502) to 5 ppm NO2 at 75 °C, which was over 2 times higher than that of pure In2O3. Moreover, the sensors showed good response towards NO2 down to ppb level (Rg/Ra = 10.2–100 ppb), a very low theoretical detection limit of 3.5 ppb, high selectivity and reliable repeatability. The excellent and enhanced NO2 gas sensing properties were mainly owing to its high specific surface area, novel hierarchical structure and increased surface chemisorbed oxygen species induced by Rb doping.

Ammonia gas sensing properties and density functional theory investigation of coral-like Au-SnSe2 Schottky junction

Abstract: This paper introduces a coral-like Au-modified SnSe2 nanocomposite prepared by a simple hydrothermal method. The pure- and Au-modified SnSe2 film sensors were prepared on the substrate with interdigitated electrodes by drop coating. The compositions, morphologies and microstructures of the as-synthesized nanomaterials were observed using transmission electron microscopy (TEM), X-ray diffraction (XRD), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The Au-modified SnSe2 nanocomposite has a coral-like morphology assembled from many irregular and very thin nanosheets. The gas performances of the film sensors at room temperature were tested through a series of experiments. The experimental results demonstrated that the Au-modified SnSe2 nanocomposite film sensor had excellent response characteristics, extremely short response/recovery time, outstanding gas selectivity and stability towards ammonia gas compared with the pure one. In addition, the first-principle density functional theory (DFT) was employed to simulate the effect of Au modification on gas adsorption behavior, clarifying the internal mechanism of gas sensing by combining with Au-SnSe2 Schottky junction. These results verified that the Au-modified SnSe2 film sensor is a high-performance candidate for enhanced ammonia gas sensing properties at room temperature.

Magnetic Field-Assisted Laser Ablation of Titanium Dioxide Nanoparticles in Water for Anti-Bacterial Applications

Abstract: Titanium oxide nanoparticles (TiO2) were produced by pulsed Nd:YAG laser ablation in water under the effect of an external magnetic field. Various techniques such as X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), energy Dispersive X-ray (EDX), transmission electron microscopy (TEM), UV–Vis spectroscopy, and Raman spectroscopy were used to characterize the TiO2 nanoparticles. The XRD analysis of titanium oxide nanoparticles revealed that the synthesized nanoparticles were polycrystalline with mixed of tetragonal anatase and rutile TiO2. Scanning electron microscope shows the formation of spherical nanoparticles and the particles agglomeration decreases and the particle size from increases from 25 to 35 nm when the magnetic field applied. The optical energy gap of TiO2 nanoparticles decreased from 4.6 to 3.4 eV after using the magnetic field during the ablation. Raman studies show the existence of five vibration modes belong to TiO2. The antibacterial effect assay revealed a largest inhibition zone in S. aureus and E. coli, with a more potent effect for TiO2 NPs prepared by magnetic field when compared with that prepared without presence of magnetic field.

Microstructure evolution and mechanical properties of the electron-beam welded joints of cast Al–Cu–Mg–Ag alloy

Abstract: In this work, the microstructures of the electron-beam welded joints of cast Al–Cu–Mg–Ag alloy plate were investigated in terms of the element segregation, grain structure and precipitation behavior by scanning electron microscopy, transmission electron microscopy and atom probe tomography observation as well as differential scanning calorimetry and electron probe micro analysis. The hardness distribution of the weld cross-section was also described using microhardness mapping. A model of strengthening mechanisms was proposed to analysis the microstructure-strength relationship of the welded joint. It is found that the occurrence of softening behavior in fusion zone after welding is closely related to the dissolution of Ω and θ′ strengthening phases. The precipitation of a large number of fine Ω, θ′ plates and some small dispersed precipitates (Ag-rich particles, GP zones and Mg–Ag phases) may be the main reason for the increase in the strength of the welded joint after post-welding heat treatment. It is because the supersaturated solid solution produced after welding due to the rapid cooling rate as well as the strong binding energy between Ag atoms and vacancies/Mg atoms promotes the precipitation of small dispersoids.

Anomalous nanoparticle surface diffusion in LCTEM is revealed by deep learning-assisted analysis.

Abstract: The motion of nanoparticles near surfaces is of fundamental importance in physics, biology, and chemistry. Liquid cell transmission electron microscopy (LCTEM) is a promising technique for studying motion of nanoparticles with high spatial resolution. Yet, the lack of understanding of how the electron beam of the microscope affects the particle motion has held back advancement in using LCTEM for in situ single nanoparticle and macromolecule tracking at interfaces. Here, we experimentally studied the motion of a model system of gold nanoparticles dispersed in water and moving adjacent to the silicon nitride membrane of a commercial LC in a broad range of electron beam dose rates. We find that the nanoparticles exhibit anomalous diffusive behavior modulated by the electron beam dose rate. We characterized the anomalous diffusion of nanoparticles in LCTEM using a convolutional deep neural-network model and canonical statistical tests. The results demonstrate that the nanoparticle motion is governed by fractional Brownian motion at low dose rates, resembling diffusion in a viscoelastic medium, and continuous-time random walk at high dose rates, resembling diffusion on an energy landscape with pinning sites. Both behaviors can be explained by the presence of silanol molecular species on the surface of the silicon nitride membrane and the ionic species in solution formed by radiolysis of water in presence of the electron beam.

Optoelectronic properties of highly porous silver oxide thin film

Abstract: In this paper, we report oxidation time effect on highly porous silver oxide nanowires thin films fabricated using ultrasonic spray pyrolysis and oxygen plasma etching method. The NW’s morphological, electrical, and optical properties were investigated under different plasma etching periods and the number of deposition cycles. The increase of plasma etching and oxidation time increases the surface roughness of the Ag NWs until it fused to form a porous thin film of silver oxide. AgNWs based thin films were characterized using X-ray diffraction, scanning electron microscope, transmission electron microscope, X-ray photoemission spectroscopy, and UV–Vis spectroscopy techniques. The obtained results indicate the formation of mixed mesoporous Ag2O and AgO NW thin films. The Ag2O phase of silver oxide appears after 300 s of oxidation under the same conditions, while the optical transparency of the thin film decreases as plasma etching time increases. The sheet resistance of the final film is influenced by the oxidation time and the plasma application periodicity.

Microstructure and properties of CoCrFeNiMo0.2 high-entropy alloy enhanced by high-current pulsed electron beam

Abstract: Recently, high-entropy alloys have drawn lots of attention due to their outstanding properties In this paper, a promising new surface modification technique was acted on CoCrFeNiMo02 high-entropy alloy to improve its mechanical and corrosion properties CoCrFeNiMo02 high-entropy alloys were synthesized via vacuum arc melting then, their surfaces were processed by high-current pulsed electron beam (HCPEB) Microstructure, microhardness, wear and corrosion resistance were studied systematically by means of X-ray diffraction (XRD), optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) Before and after HCPEB irradiation, CoCrFeNiMo02 high-entropy alloy had a face-centered cubic (FCC) structure, and the surface of the irradiated samples revealed preferential orientation on the (111) and (200) planes In addition, a compact and homogenization surface layer formed after irradiation Also, high-amplitude stress caused high-density dislocations and stacking faults on the surface After HCPEB irradiation, the properties of the samples were significantly improved, the maximum microhardness (3929 HV) and lowest wear rate (092 × 10−4 mm3·N−1·m−1) was obtained after 35-pulsed irradiation Furthermore, corrosion resistance was obviously enhanced after 25-pulsed irradiation

Carbon coated cobalt oxide (CC-CO3O4) as electrode material for supercapacitor applications

Abstract: Carbon coated cobalt oxide (CC-CO3O4) was prepared by colloidal processing using cobalt oxide and sucrose. Sucrose was used as a soluble carbon precursor in the preparation of CC-CO3O4. CC-CO3O4 was characterized by Brunauer–Emmett–Teller (BET) analysis, X-ray diffraction (XRD), Raman spectroscopy, Scanning electron microscopy (SEM), Transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS) respectively. Electrochemical properties of CC-CO3O4 were measured in 1 M KOH electrolyte. CC-CO3O4 exhibited the maximum specific capacitance of 395 F g−1 at a scan rate of 5 mV s−1. The enhanced electrochemical performance of CC-CO3O4 may be due to the increased conductivity of the composite electrode by carbon coating over the cobalt oxide nanoparticles.

4D-STEM of Beam-Sensitive Materials.

Abstract: ConspectusScanning electron nanobeam diffraction, or 4D-STEM (four-dimensional scanning transmission electron microscopy), is a flexible and powerful approach to elucidate structure from "soft" materials that are challenging to image in the transmission electron microscope because their structure is easily damaged by the electron beam. In a 4D-STEM experiment, a converged electron beam is scanned across the sample, and a pixelated camera records a diffraction pattern at each scan position. This four-dimensional data set can be mined for various analyses, producing maps of local crystal orientation, structural distortions, crystallinity, or different structural classes. Holding the sample at cryogenic temperatures minimizes diffusion of radicals and the resulting damage and disorder caused by the electron beam. The total fluence of incident electrons can easily be controlled during 4D-STEM experiments by careful use of the beam blanker, steering of the localized electron dose, and by minimizing the fluence in the convergent beam thus minimizing beam damage. This technique can be applied to both organic and inorganic materials that are known to be beam-sensitive; they can be highly crystalline, semicrystalline, mixed phase, or amorphous.One common example is the case for many organic materials that have a π-π stacking of polymer chains or rings on the order of 3.4-4.2 A separation. If these chains or rings are aligned in some regions, they will produce distinct diffraction spots (as would other crystalline spacings in this range), though they may be weak or diffuse for disordered or weakly scattering materials. We can reconstruct the orientation of the π-π stacking, the degree of π-π stacking in the sample, and the domain size of the aligned regions. This Account summarizes illumination conditions and experimental parameters for 4D-STEM experiments with the goal of producing images of structural features for materials that are beam-sensitive. We will discuss experimental parameters including sample cooling, probe size and shape, fluence, and cameras. 4D-STEM has been applied to a variety of materials, not only as an advanced technique for model systems, but as a technique for the beginning microscopist to answer materials science questions. It is noteworthy that the experimental data acquisition does not require an aberration-corrected TEM but can be produced on a variety of instruments with the right attention to experimental parameters.

Enhancement of Corrosive-Resistant Behavior of Zn and Mg Metal Plates Using Biosynthesized Nickel Oxide Nanoparticles

Abstract: In this work, nickel oxide nanoparticles (NiO NPs) were synthesized using ultrasonic wave-assisted green synthesis route with Delonix elata leaf extract as a reducing and capping agent. The phase structure, crystallinity, thermal and physical stability, surface morphology, and surface area of the produced NiO NPs were investigated using X-ray diffraction, field-emission scanning electron microscopy high-resolution transmission electron microscopy, thermogravimetric/differential thermal analysis, and Brunauer–Emmett–Teller analysis. The surface properties such as roughness and hardness of NiO NP-coated plates were determined using atomic force microscopy and nanoindentation techniques. The electrochemical corrosion behavior of NiO NPs was studied in the presence of an aqueous electrolyte medium, that is, 3.5% NaCl, 6 M KOH, 1 M HCl, and 1 M H2SO4. The Tafel plot showed that the corrosive nature of Zn and Mg plates significantly decreases when the plates were coated with the prepared high surface area and mesoporous NiO NPs under all electrolytes, especially in acidic medium, that is, 1 M H2SO4.

Electrical conductivity and dielectric properties of Sr doped M-type barium hexaferrite BaFe12O19

Abstract: The hexaferrite Ba1−xSrxFe12O19 compounds with x = 0, 0.5 and 1 were synthesized by the autocombustion method. X-ray diffraction (XRD), Raman spectroscopy and transmission electron microscopy (TEM) were used for structural and morphological studies.

Peering into buried interfaces with X-rays and electrons to unveil MgCO3 formation during CO2 capture in molten salt-promoted MgO.

Abstract: The addition of molten alkali metal salts drastically accelerates the kinetics of CO2 capture by MgO through the formation of MgCO3. However, the growth mechanism, the nature of MgCO3 formation, and the exact role of the molten alkali metal salts on the CO2 capture process remain elusive, holding back the development of more-effective MgO-based CO2 sorbents. Here, we unveil the growth mechanism of MgCO3 under practically relevant conditions using a well-defined, yet representative, model system that is a MgO(100) single crystal coated with NaNO3. The model system is interrogated by in situ X-ray reflectometry coupled with grazing incidence X-ray diffraction, scanning electron microscopy, and high-resolution transmission electron microscopy. When bare MgO(100) is exposed to a flow of CO2, a noncrystalline surface carbonate layer of ca. 7-A thickness forms. In contrast, when MgO(100) is coated with NaNO3, MgCO3 crystals nucleate and grow. These crystals have a preferential orientation with respect to the MgO(100) substrate, and form at the interface between MgO(100) and the molten NaNO3. MgCO3 grows epitaxially with respect to MgO(100), and the lattice mismatch between MgCO3 and MgO is relaxed through lattice misfit dislocations. Pyramid-shaped pits on the surface of MgO, in proximity to and below the MgCO3 crystals, point to the etching of surface MgO, providing dissolved [Mg2+…O2–] ionic pairs for MgCO3 growth. Our studies highlight the importance of combining X-rays and electron microscopy techniques to provide atomic to micrometer scale insight into the changes occurring at complex interfaces under reactive conditions.

Nanoscale Chemical and Structural Analysis during In Situ Scanning/Transmission Electron Microscopy in Liquids.

Abstract: Liquid-cell scanning/transmission electron microscopy (S/TEM) has impacted our understanding of multiple areas of science, most notably nanostructure nucleation and growth and electrochemistry and corrosion. In the case of electrochemistry, the incorporation of electrodes requires the use of silicon nitride membranes to confine the liquid. The combined thickness of the liquid layer and the confining membranes prevents routine atomic-resolution characterization. Here, we show that by performing electrochemical water splitting in situ to generate a gas bubble, we can reduce the thickness of the liquid to a film approximately 30 nm thick that remains covering the sample. The reduced thickness of the liquid allows the acquisition of atomic-scale S/TEM images with chemical and valence analysis through electron energy loss spectroscopy (EELS) and structural analysis through selected area electron diffraction (SAED). This contrasts with a specimen cell entirely filled with liquid, where the broad plasmon peak from the liquid obscures the EELS signal from the sample and induces beam incoherence that impedes SAED analysis. The gas bubble generation is fully reversible, which allows alternating between a full cell and thin-film condition to obtain optimal experimental and analytical conditions, respectively. The methodology developed here can be applied to other scientific techniques, such as X-ray scattering, Raman spectroscopy, and X-ray photoelectron spectroscopy, allowing for a multi-modal, nanoscale understanding of solid-state samples in liquid media.

Single Femtosecond Laser-Pulse-Induced Superficial Amorphization and Re-Crystallization of Silicon.

Abstract: Superficial amorphization and re-crystallization of silicon in and orientation after irradiation by femtosecond laser pulses (790 nm, 30 fs) are studied using optical imaging and transmission electron microscopy. Spectroscopic imaging ellipsometry (SIE) allows fast data acquisition at multiple wavelengths and provides experimental data for calculating nanometric amorphous layer thickness profiles with micrometric lateral resolution based on a thin-film layer model. For a radially Gaussian laser beam and at moderate peak fluences above the melting and below the ablation thresholds, laterally parabolic amorphous layer profiles with maximum thicknesses of several tens of nanometers were quantitatively attained. The accuracy of the calculations is verified experimentally by high-resolution transmission electron microscopy (HRTEM) and energy dispersive X-ray spectroscopy (STEM-EDX). Along with topographic information obtained by atomic force microscopy (AFM), a comprehensive picture of the superficial re-solidification of silicon after local melting by femtosecond laser pulses is drawn.

Strengthening mechanisms in selective laser melted 316L stainless steel

Abstract: The microstructure and chemical composition of a 316L stainless steel prepared by selective laser melting have been characterized using electron backscatter diffraction, transmission electron microscopy and atom probe tomography (APT). A multi-scale microstructure in the 316L stainless steel was observed in the as-built samples, consisting of equiaxed and columnar grains, dislocation cell blocks, dislocation cells, individual dislocations and nano-sized particles. The misorientations across the dislocation cells were determined based on local crystallographic orientation measurements using a Kikuchi pattern method. The dislocation cells have very small misorientation angles, with an average angle of 0.9°, and are arranged to form dislocation cell-blocks, with cell-block boundary misorientation angles generally larger than 2°. The APT data reveal that alloying elements are evenly distributed in the matrix as well as a high nitrogen content in the as-built material. Based on quantification of the microstructural parameters, good agreement is found between the yield strength as calculated from a linear sum of different strengthening contributions, and the experimentally measured value, with significant contributions both from dislocation strengthening and solid solution strengthening effects.