Showing papers on "Transmission electron microscopy published in 2020"

The effect of shape and size of ZnO nanoparticles on their antimicrobial and photocatalytic activities: a green approach

Abstract: In this paper, ZnO nanoparticles (NPs) having potent photocatalytic and antimicrobial activities have been synthesized by using the aloe vera plant extract. The ZnO NPs have been synthesized using $$(\hbox {Zn}(\hbox {CH}_3\hbox {COO})_{2}{\cdot } 2\hbox {H}_{2}\hbox {O})$$ (5, 10 and $$50\,\hbox {mmol}\, \hbox {kg}^{-1}$$ ), at temperature $$70{^{\circ }}\hbox {C}$$ and pH 11.5. The synthesized NPs were examined using UV–Visible, X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectroscopy and transmission electron microscopy (TEM) characterization techniques. TEM analysis confirms the synthesis of ZnO NPs with hexagonal, spherical, cylindrical and cuboidal shapes decorated under different concentrations of precursor metal salt. UV–Visible studies revealed that ZnO NPs have a wide energy band gap varying from 3.36 to 3.43 eV. The synthesized ZnO NPs were examined for the photocatalytic degradation of methyl orange dye which resulted in up to 95% degradation. ZnO NPs are also inspected for the antibacterial activity against Bacillus subtilis (MTCC 441), Staphylococcus aureus (MTCC 737) and Escherichia coli (MTCC 739) pathogenic bacteria.

ZIF-67 MOF-derived unique double-shelled Co3O4/NiCo2O4 nanocages for superior Gas-sensing performances

Abstract: Unique nanostructures of bimetallic organic materials assisted by new organic frameworks are promising candidates for gas sensor applications. For the first time, we successfully report the design and synthesis of Co3O4/NiCo2O4 double-shelled nanocages (DSNCs) via a facile template-assisted method. The DSNCs were characterized using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Brunauer–Emmett–Teller (BET) analysis, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), thermogravimetric (TG) analysis, and transmission electron microscopy (TEM). The nanocages had a high surface-to-volume ratio and a high surface area of 103 m2 g−1. The as-prepared nanostructures were exposed to various reducing gases at different controlled gas concentrations and working temperatures. The fabricated sensors exhibited high selectivity and gas-sensing responses toward H2S at an optimal temperature of 250 °C for 100 ppm. The outstanding gas-sensing performance revealed the potential application of the unique double-shelled hollow nanostructures to the gas sensor industry.

Formation of Defects in Two-Dimensional MoS2 in the Transmission Electron Microscope at Electron Energies below the Knock-on Threshold: The Role of Electronic Excitations

Abstract: Production of defects under electron irradiation in a transmission electron microscope (TEM) due to inelastic effects has been reported for various materials, but the microscopic mechanism of damage development in periodic solids through this channel is not fully understood. We employ non-adiabatic Ehrenfest, along with constrained density functional theory molecular dynamics, and simulate defect production in two-dimensional MoS2 under electron beam. We show that when excitations are present in the electronic system, formation of vacancies through ballistic energy transfer is possible at electron energies which are much lower than the knock-on threshold for the ground state. We further carry out TEM experiments on single layers of MoS2 at electron voltages in the range of 20-80 kV and demonstrate that indeed there is an additional channel for defect production. The mechanism involving a combination of the knock-on damage and electronic excitations we propose is relevant to other bulk and nanostructured semiconducting materials.

Gradient transition zone structure in “steel–copper” sample produced by double wire-feed electron beam additive manufacturing

Abstract: This paper describes the results of an investigation into a microstructure formation on a wire-feed electron beam additive manufactured “steel–copper” bimetallic sample. The peculiarities of a gradient zone structure with a smooth change of components’ concentration are revealed. The heterogeneity of copper and steel distribution in the gradient zone is provided by copper solidification and precipitation mechanisms. Both solidification of coarse copper inclusions in the interdendrite areas or along the dendrite boundaries and precipitation of fine Cu-based particles at the cooling stage from the solid solution of Cu in γ-Fe are the main factors of structure formation during the double wire gradient zone deposition. The presence of such fine copper precipitates from the supersaturated solid solution was revealed by means of transmission electron microscopy. The shape of copper particles in the gradient zone varies from spherical to oblong and irregular. The shape of steel particles and/or grains is mainly determined by the peculiarities of the crystallization zone and is characterized by the primary crystallization of γ-iron dendrites from the liquid melt. A physical scheme describing a variation in phase composition and microstructure in gradient zone of the bimetallic specimen was proposed.

Application of Chemically Exfoliated Boron Nitride Nanosheets Doped with Co to Remove Organic Pollutants Rapidly from Textile Water

Abstract: Two-dimensional layered materials doped with transition metals exhibit enhanced magnetization and improved catalytic stability during water treatment leading to potential environmental applications across several industrial sectors. In the present study, cobalt (Co)-doped boron nitride nanosheets (BN-NS) were explored for such an application. Chemical exfoliation process was used to exfoliate BN-NS and the hydrothermal route was adopted to incorporate Co dopant in various concentrations (e.g., 2.5, 5, 7.5, and 10 wt%). X-ray diffraction (XRD) study indicated that crystallinity improved upon doping with the formation of a hexagonal phase of the synthesized material. Selected area electron diffraction (SAED) confirmed enhanced crystallinity, which corroborates XRD results. Interlayer spacing was evaluated through a high-resolution transmission electron microscope (HR-TEM) equipped with Gatan digital micrograph software. Compositional and functional group analysis was undertaken with energy dispersive X-ray (EDS) and Fourier transform infrared (FTIR) spectroscopy, respectively. Field emission scanning electron microscope (FE-SEM) and HR-TEM were utilized to probe surface morphologies of prepared samples. Bonding modes in the sample were identified through Raman analysis. Optical properties were examined using UV-vis spectroscopy. Photoluminescence spectra were acquired to estimate the separation and recombination of excitons. Magnetic properties were studied by means of hysteresis loop acquired using VSM measurements. Methylene blue dye was degraded with as-prepared host and doped nanosheets used as catalysts and investigated through absorption spectra ranging from 250 to 800 nm. The experimental results of this study indicate that Co-doped BN-NS showed enhanced magnetic properties and can be used to degrade dyes present as an effluent in industrial wastewater.

Phase Transitions in Metal–Organic Frameworks Directly Monitored through In Situ Variable Temperature Liquid-Cell Transmission Electron Microscopy and In Situ X-ray Diffraction

Abstract: Zr6-based metal-organic frameworks (MOFs) with tetratopic organic linkers have been extensively investigated owing to their versatile structural tunability. While diverse topologies and polymorphism in the resulting MOFs are often encountered with tetratopic linkers and Zr6 nodes, reports on phase transitions within these systems are rare. Thus, we have a limited understanding of polymorph transformations, hindering the rational development of pure phase materials. In this study, a phase transition from a microporous MOF, scu-NU-906, to a mesoporous MOF, csq-NU-1008, was discovered and monitored through in situ variable temperature liquid-cell transmission electron microscopy (VT-LCTEM), high-resolution transmission electron microscopy (HRTEM), and in situ variable temperature powder X-ray diffraction (VT-PXRD). It was found that the microporous- to-mesoporous transformation in the presence of formic acid occurs via a concomitant dissolution-reprecipitation process.

Atomic mechanism of metal crystal nucleus formation in a single-walled carbon nanotube

Abstract: Knowing how crystals nucleate at the atomic scale is crucial for understanding, and in turn controlling, the structure and properties of a wide variety of materials. However, because of the scale and highly dynamic nature of nuclei, the formation and early growth of nuclei are very difficult to observe. Here, we have employed single-walled carbon nanotubes as test tubes, and an ‘atomic injector’ coupled with aberration-corrected transmission electron microscopy, to enable in situ imaging of the initial steps of nucleation at the atomic scale. With three different metals we observed three main processes prior to heterogeneous nucleation: formation of crystal nuclei directly from an atomic seed (Fe), from a pre-existing amorphous nanocluster (Au) or by coalescence of two separate amorphous sub-nanometre clusters (Re). We demonstrate the roles of the amorphous precursors and the existence of an energy barrier before nuclei formation. In all three cases, crystal nucleus formation occurred through a two-step nucleation mechanism. Crystal nucleation processes are difficult to probe experimentally because of the spatial and temporal scales involved. Now, the heterogeneous nucleation of three different metals has been observed by electron microscopy with atomic resolution—using single-walled carbon nanotube as test tubes—and, in each case, shown to adopt a two-step nucleation mechanism involving a metastable amorphous precursor.

Microstructures and properties of SLM-manufactured Cu-15Ni-8Sn alloy

Abstract: Easily segregated Cu-15Ni-8Sn alloy bulk material was fabricated using a selective laser melting (SLM) process. The microstructure of SLM-manufactured Cu-15Ni-8Sn alloy was investigated using optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD), and transmission electron microscopy (TEM). Differences in the microstructures and elemental segregation of gas-atomized alloy powder, cast ingots, and SLM-manufactured samples were analyzed. The statistical average grain size of the SLM-manufactured Cu-15Ni-8Sn alloy was 4.03 μm. Microstructures of the SLM-manufactured sample were mainly composed of epitaxially grown slender cellular structures with submicron widths. Microsegregation was detected by TEM, and 80- to 200-nm Sn-enriched precipitates were dispersed between cellullar structures. Many dislocations and dislocation tangles appeared around the precipitates. An EBSD test revealed that most local misorientations within 3 degrees were concentrated in fusion line regions. Compared with cast ingots, the yield strength Rp0.2, ultimate tensile strength Rm, elongation A, and elastic modulus E of the SLM-manufactured sample increased by 67%, 24.6%, 360%, and 7%, respectively. Moreover, the SLM-manufactured Cu-15Ni-8Sn alloy could be directly aged at 350℃ for 12 h, reaching Rm = 991.1 MPa and A =3 %, with no need for solid solution treatment or cold working.

In‐situ Transmission Electron Microscope Techniques for Heterogeneous Catalysis

Abstract: The physicochemical properties of heterogeneous catalysts in static or inert environments often deviate greatly from the properties under in‐situ or working conditions. To gain an insightful understanding of realistic catalyst properties and the corresponding catalytic mechanisms, it is essential to identify and rationalize changes in catalysts under reaction conditions. In recent years, in‐situ transmission electron microscope (in‐situ TEM) techniques have been increasingly developed, offering a unique approach to visualize the evolution of heterogeneous catalysis with ultra‐high spatial resolution, good energy resolution and reasonable temporal resolution under controllable or even realistic catalytic conditions. In this review, we have summarized recent advances in the in‐situ TEM analysis of heterogeneous catalysis, which suggests the great potential of this technique in this important field. Furthermore, technical challenges and possible solutions are discussed.

Structural and Electrical Comparison of Si and Zr Doped Hafnium Oxide Thin Films and Integrated FeFETs Utilizing Transmission Kikuchi Diffraction

Abstract: The microstructure of ferroelectric hafnium oxide plays a vital role for its application, e.g., non-volatile memories. In this study, transmission Kikuchi diffraction and scanning transmission electron microscopy STEM techniques are used to compare the crystallographic phase and orientation of Si and Zr doped HfO2 thin films as well as integrated in a 22 nm fully-depleted silicon-on-insulator (FDSOI) ferroelectric field effect transistor (FeFET). Both HfO2 films showed a predominately orthorhombic phase in accordance with electrical measurements and X-ray diffraction XRD data. Furthermore, a stronger texture is found for the microstructure of the Si doped HfO2 (HSO) thin film, which is attributed to stress conditions inside the film stack during crystallization. For the HSO thin film fabricated in a metal-oxide-semiconductor (MOS) like structure, a different microstructure, with no apparent texture as well as a different fraction of orthorhombic phase is observed. The 22 nm FDSOI FeFET showed an orthorhombic phase for the HSO layer, as well as an out-of-plane texture of the [111]-axis, which is preferable for the application as non-volatile memory.

Synthesis and Characterization of Nitrogen-doped Carbon Nanotubes Derived from g-C3N4.

Abstract: Here, nitrogen-doped carbon nanotubes (CNT-N) were synthesized using exfoliated graphitic carbon nitride functionalized with nickel oxides (ex-g-C3N4-NixOy). CNT-N were produced at 900 °C in two steps: (1) ex-g-C3N4-NixOy reduction with hydrogen and (2) ethylene assisted chemical vapor deposition (CVD). The detailed characterization of the produced materials was performed via atomic force microscopy (AFM), transmission electron microscopy (TEM), Raman spectroscopy, X-ray diffraction (XRD) and thermogravimetric analysis (TGA). The possible mechanism of nanotubes formation is also proposed.