Showing papers on "Transmission electron microscopy published in 2018"

Direct observation of noble metal nanoparticles transforming to thermally stable single atoms.

Abstract: Single noble metal atoms and ultrafine metal clusters catalysts tend to sinter into aggregated particles at elevated temperatures, driven by the decrease of metal surface free energy. Herein, we report an unexpected phenomenon that noble metal nanoparticles (Pd, Pt, Au-NPs) can be transformed to thermally stable single atoms (Pd, Pt, Au-SAs) above 900 °C in an inert atmosphere. The atomic dispersion of metal single atoms was confirmed by aberration-corrected scanning transmission electron microscopy and X-ray absorption fine structures. The dynamic process was recorded by in situ environmental transmission electron microscopy, which showed competing sintering and atomization processes during NP-to-SA conversion. Further, density functional theory calculations revealed that high-temperature NP-to-SA conversion was driven by the formation of the more thermodynamically stable Pd-N4 structure when mobile Pd atoms were captured on the defects of nitrogen-doped carbon. The thermally stable single atoms (Pd-SAs) exhibited even better activity and selectivity than nanoparticles (Pd-NPs) for semi-hydrogenation of acetylene.

Atomic-resolution transmission electron microscopy of electron beam-sensitive crystalline materials.

Abstract: High-resolution imaging of electron beam–sensitive materials is one of the most difficult applications of transmission electron microscopy (TEM). The challenges are manifold, including the acquisition of images with extremely low beam doses, the time-constrained search for crystal zone axes, the precise image alignment, and the accurate determination of the defocus value. We develop a suite of methods to fulfill these requirements and acquire atomic-resolution TEM images of several metal organic frameworks that are generally recognized as highly sensitive to electron beams. The high image resolution allows us to identify individual metal atomic columns, various types of surface termination, and benzene rings in the organic linkers. We also apply our methods to other electron beam–sensitive materials, including the organic-inorganic hybrid perovskite CH 3 NH 3 PbBr 3 .

A Polymer Encapsulation Strategy to Synthesize Porous Nitrogen-Doped Carbon-Nanosphere-Supported Metal Isolated-Single-Atomic-Site Catalysts.

Abstract: A novel polymer encapsulation strategy to synthesize metal isolated-single-atomic-site (ISAS) catalysts supported by porous nitrogen-doped carbon nanospheres is reported. First, metal precursors are encapsulated in situ by polymers through polymerization; then, metal ISASs are created within the polymer-derived p-CN nanospheres by controlled pyrolysis at high temperature (200-900 °C). Transmission electron microscopy and N2 sorption results reveal this material to exhibit a nanospheric morphology, a high surface area (≈380 m2 g-1 ), and a porous structure (with micropores and mesopores). Characterization by aberration-corrected high-angle annular dark-field scanning transmission electron microscopy and X-ray absorption fine structure confirms the metal to be present as metal ISASs. This methodology is applicable to both noble and nonprecious metals (M-ISAS/p-CN, M = Co, Ni, Cu, Mn, Pd, etc.). In particular, the Co-ISAS/p-CN nanospheres obtained using this method show comparable (E1/2 = 0.838 V) electrochemical oxygen reduction activity to commercial Pt/C with 20 wt% Pt loading (E1/2 = 0.834 V) in alkaline media, superior methanol tolerance, and outstanding stability, even after 5000 cycles.

Superconductivity in a Copper(II)-Based Coordination Polymer with Perfect Kagome Structure.

Abstract: A highly crystalline copper(II) benzenehexathiolate coordination polymer (Cu-BHT) has been prepared. The two-dimensional kagome structure has been confirmed by powder X-ray diffraction, high-resolution transmission electron microscopy, and high-resolution scanning transmission electron microscopy. The as-prepared sample exhibits bulk superconductivity at about 0.25 K, which is confirmed by the zero resistivity, AC magnetic susceptibility, and specific heat measurements. Another diamagnetic transition at about 3 K suggests that there is a second superconducting phase that may be associated with a single layer or few layers of Cu-BHT. It is the first time that superconductivity has been observed in a coordination polymer.

Reversible superdense ordering of lithium between two graphene sheets

Abstract: Many carbon allotropes can act as host materials for reversible lithium uptake1,2, thereby laying the foundations for existing and future electrochemical energy storage. However, insight into how lithium is arranged within these hosts is difficult to obtain from a working system. For example, the use of in situ transmission electron microscopy3-5 to probe light elements (especially lithium)6,7 is severely hampered by their low scattering cross-section for impinging electrons and their susceptibility to knock-on damage8. Here we study the reversible intercalation of lithium into bilayer graphene by in situ low-voltage transmission electron microscopy, using both spherical and chromatic aberration correction9 to enhance contrast and resolution to the required levels. The microscopy is supported by electron energy-loss spectroscopy and density functional theory calculations. On their remote insertion from an electrochemical cell covering one end of the long but narrow bilayer, we observe lithium atoms to assume multi-layered close-packed order between the two carbon sheets. The lithium storage capacity associated with this superdense phase far exceeds that expected from formation of LiC6, which is the densest configuration known under normal conditions for lithium intercalation within bulk graphitic carbon10. Our findings thus point to the possible existence of distinct storage arrangements of ions in two-dimensional layered materials as compared to their bulk parent compounds.

In-situ liquid cell transmission electron microscopy investigation on oriented attachment of gold nanoparticles.

Abstract: Inside a liquid solution, oriented attachment (OA) is now recognized to be as important a pathway to crystal growth as other, more conventional growth mechanisms. However, the driving force that controls the occurrence of OA is still poorly understood. Here, using in-situ liquid cell transmission electron microscopy, we demonstrate the ligand-controlled OA of citrate-stabilized gold nanoparticles at atomic resolution. Our data reveal that particle pairs rotate randomly at a separation distance greater than twice the layer thickness of adsorbed ligands. In contrast, when the particles get closer, their ligands overlap and guide the rotation into a directional mode until they share a common {111} orientation, when a sudden contact occurs accompanied by the simultaneous expulsion of the ligands on this surface. First-principle calculations confirm that the lower ligand binding energy on {111} surfaces is the intrinsic reason for the preferential attachment at this facet, rather than on other low-index facets.

Antibacterial, magnetic, optical and humidity sensor studies of β-CoMoO4 - Co3O4 nanocomposites and its synthesis and characterization

Abstract: Cobalt Molybdate (β-CoMoO4) and Cobalt Oxide (Co3O4) nanocomposite was prepared via co-precipitation and solid-state methods. Various techniques like powder XRD, FESEM, HRTEM, FTIR, VSM, UV-Vis and PL spectroscopy were used to investigate the structure and morphology of as prepared samples. Powder X-ray diffraction (XRD) reveals monoclinic and cubic structure for β-CoMoO4 and Co3O4 respectively. The surface morphology was observed using field emission electron microscopy (FESEM) and high-resolution transmission electron microscopy (HRTEM), which shows the formation of nanocomposites at nanoscale range, the presence of elements were determined by energy dispersive x-ray spectroscopy (EDX). FTIR analysis confirms the formation and bonding nature of the samples. The anti-ferromagnetic behavior of CMCO64 composite was determined by vibrating sample magnetometer (VSM). The bandgap values were calculated by extrapolating the straight line on the energy axis (hν), and the values of β-CoMoO4, CO3O4 and β-CoMoO4 - CO3O4 composites were determined to be 2.20, 2.09 eV and 1.54-2.44 eV respectively. The weak blue emission peak observed at 489 nm is corresponds to crystal defects only observed in CMCO01 and CMCO64 composite, for CMCO10 the peak shifted to green region. Antibacterial studies illustrate good result for the CMCO64 composite against both Gram-negative and Gram-positive bacteria. The sensor studies were measured at different humidity environment (RH5% to RH98%). It was found that the increase in relative humidity leads to increase in the sensitivity factor of the samples. Among the samples CMCO64 composite possess highest sensitivity factor of (Sf = 4851) with response time of 60 s and recovery time of 230 s respectively.

Highly sensitive NO2 gas sensor of ppb-level detection based on In2O3 nanobricks at low temperature

Abstract: The brick-like In2O3 nanomaterials were synthesized by oil bath precipitation and subsequent calcination method without any templates or surfactants. Technologies of X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) were carried out to characterize the morphological and structural of the In2O3 samples. Photoluminescence spectra (PL) and X-ray photoelectron spectroscopy (XPS) were used to illustrate the influence of material surface states and internal defects on gas-sensing properties. The In2O3 nanomaterials annealed at 400 °C exhibited a high response (402) toward 500 ppb NO2 with the fast response/recovery time (114 s/49 s) at rather low operating temperature of 50 °C. Besides, it had a good linear relationship in the range of 100–500 ppb. Such excellent gas sensing properties could be on account of the relative large surface area, abundant adsorbed oxygen and good electrical properties.

Structural and Chemical Changes to CH3NH3PbI3 Induced by Electron and Gallium Ion Beams

Abstract: Organic-inorganic hybrid perovskites, such as CH3 NH3 PbI3, have shown highly promising photovoltaic performance. Electron microscopy (EM) is a powerful tool for studying the crystallography, morphology, interfaces, lattice defects, composition, and charge carrier collection and recombination properties at the nanoscale. Here, the sensitivity of CH3 NH3 PbI3 to electron beam irradiation is examined. CH3 NH3 PbI3 undergoes continuous structural and compositional changes with increasing electron dose, with the total dose, rather than dose rate, being the key operative parameter. Importantly, the first structural change is subtle and easily missed and occurs after an electron dose significantly smaller than that typically applied in conventional EM techniques. The electron dose conditions under which these structural changes occur are identified. With appropriate dose-minimization techniques, electron diffraction patterns can be obtained from pristine material consistent with the tetragonal CH3 NH3 PbI3 phases determined by X-ray diffraction. Radiation damage incurred at liquid nitrogen temperatures and using Ga+ irradiation in a focused ion beam instrument are also examined. Finally, some simple guidelines for how to minimize electron-beam-induced artifacts when using EM to study hybrid perovskite materials are provided.

Atomic Layer Deposition of Rhenium Disulfide.

Abstract: 2D materials research is advancing rapidly as various new "beyond graphene" materials are fabricated, their properties studied, and materials tested in various applications. Rhenium disulfide is one of the 2D transition metal dichalcogenides that has recently shown to possess extraordinary properties such as that it is not limited by the strict monolayer thickness requirements. The unique inherent decoupling of monolayers in ReS2 combined with a direct bandgap and highly anisotropic properties makes ReS2 one of the most interesting 2D materials for a plethora of applications. Here, a highly controllable and precise atomic layer deposition (ALD) technique is applied to deposit ReS2 thin films. Film growth is demonstrated on large area (5 cm × 5 cm) substrates at moderate deposition temperatures between 120 and 500 °C, and the films are extensively characterized using field emission scanning electron microscopy/energy-dispersive X-ray spectroscopy, X-ray diffractometry using grazing incidence, atomic force microscopy, focused ion beam/transmission electron microscopy, X-ray photoelectron spectroscopy, and time-of-flight elastic recoil detection analysis techniques. The developed ReS2 ALD process highlights the potential of the material for applications beyond planar structure architectures. The ALD process also offers a route to an upgrade to an industrial scale.

Synthetic Control of Two-Dimensional NiTe2 Single Crystals with Highly Uniform Thickness Distributions.

Abstract: Two-dimensional (2D) layered materials have stimulated extensive research interest for their unique thickness-dependent electronic and optical properties. However, the layer-number-dependent studies on 2D materials to date are largely limited to exfoliated flakes with relatively small lateral size and poor yield. The direct synthesis of 2D materials with a precise control of the number of atomic layers remains a substantial synthetic challenge. Here we report a systematic study of chemical vapor deposition synthesis of large-area atomically thin 2D nickel telluride (NiTe2) single crystals and investigate the thickness dependent electronic properties. By controlling the growth temperature, we show that the highly uniform NiTe2 single crystals can be synthesized with precisely tunable thickness varying from 1, 2, 3, . . . to multilayers with a standard deviation (∼0.3 nm) of less than the thickness of a monolayer layer NiTe2. Our studies further reveal a systematic evolution of single crystal domain size and nucleation density with the largest lateral domain size up to ∼440 μm. X-ray diffraction, transmission electron microscopy, and high resolution scanning transmission electron microscope studies demonstrate that the resulting 2D crystals are high quality single crystals and adopt hexagonal 1T phase. Electrical transport studies reveal that the 2D NiTe2 single crystals show a strong thickness-tunable electrical properties, with an excellent conductivity up to 7.8 × 105 S m-1 and extraordinary breakdown current density up to 4.7 × 107 A/cm2. The systematic study and robust synthesis of NiTe2 nanosheets defines a reliable chemical route to 2D single crystals with precisely tailored thickness and could enable the design of new device architectures based on thickness-tunable electrical properties.

A Layered–Tunnel Intergrowth Structure for High-Performance Sodium-Ion Oxide Cathode

Abstract: Delivery of high-energy density with long cycle life is facing a severe challenge in developing cathode materials for rechargeable sodium-ion batteries (SIBs). Here a composite Na0.6MnO2 with layered-tunnel structure combining intergrowth morphology of nanoplates and nanorods for SIBs, which is clearly confirmed by micro scanning electron microscopy, high-resolution transmission electron microscopy as well as scanning transmission electron microscopy with atomic resolution is presented. Owing to the integrated advantages of P2 layered structure with high capacity and that of the tunnel structure with excellent cycling stability and superior rate performance, the composite electrode delivers a reversible discharge capacity of 198.2 mAh g(-1) at 0.2C rate, leading to a high-energy density of 520.4 Wh kg(-1). This intergrowth integration engineering strategy may modulate the physical and chemical properties in oxide cathodes and provide new perspectives on the optimal design of high-energy density and high-stable materials for SIBs.

Fundamental Insight into Zr Modification of Li- and Mn-Rich Cathodes: Combined Transmission Electron Microscopy and Electrochemical Impedance Spectroscopy Study

Abstract: While zirconium-based coatings are known to improve the cycling stability of a number of lithium ion battery cathodes, the microstructural origin of this enhancement remains uncertain. Here we combine advanced transmission electron microscopy (high-resolution transmission electron microscopy, high-angle annular dark field, electron energy loss spectroscopy, and energy-dispersive X-ray spectroscopy) with electrochemical impedance analysis to provide new insight into the dramatic role of Zr surface modification on the electrochemical performance of Li- and Mn-rich (LMR) cathodes (Li[Li0.2Ni0.13Co0.13Mn0.54]O2). It is demonstrated that a Zr-based rock-salt structure layer with a thickness of 1–2 nm is formed on the surface of the LMR. This layer is effective in suppressing the deleterious phase transformation of LMR from initial layered composite combining Li2MO3 and LiMO2 to the disordered rock-salt phase, leading to an enhanced long-term cycling performance and rate capability. Electrochemical impedance sp...

Nanometer Resolution Elemental Mapping in Graphene-Based TEM Liquid Cells.

Abstract: We demonstrate a new design of graphene liquid cell consisting of a thin lithographically patterned hexagonal boron nitride crystal encapsulated on both sides with graphene windows. The ultrathin window liquid cells produced have precisely controlled volumes and thicknesses and are robust to repeated vacuum cycling. This technology enables exciting new opportunities for liquid cell studies, providing a reliable platform for high resolution transmission electron microscope imaging and spectral mapping. The presence of water was confirmed using electron energy loss spectroscopy (EELS) via the detection of the oxygen K-edge and measuring the thickness of full and empty cells. We demonstrate the imaging capabilities of these liquid cells by tracking the dynamic motion and interactions of small metal nanoparticles with diameters of 0.5-5 nm. We further present an order of magnitude improvement in the analytical capabilities compared to previous liquid cell data with 1 nm spatial resolution elemental mapping achievable for liquid encapsulated bimetallic nanoparticles using energy dispersive X-ray spectroscopy (EDXS).

Exposing the {010} Planes by Oriented Self-Assembly with Nanosheets To Improve the Electrochemical Performances of Ni-Rich Li[Ni0.8Co0.1Mn0.1]O2 Microspheres.

Abstract: A modified Ni-rich Li[Ni08Co01Mn01]O2 cathode material with exposed {010} planes is successfully synthesized for lithium-ion batteries The scanning electron microscopy images have demonstrated that by tuning the ammonia concentration during the synthesis of precursors, the primary nanosheets could be successfully stacked along the [001] crystal axis predominantly, self-assembling like multilayers According to the high-resolution transmission electron microscopy results, such a morphology benefits the growth of the {010} active planes of final layered cathodes during calcination treatment, resulting in the increased area of the exposed {010} active planes, a well-ordered layer structure, and a lower cation mixing disorder The Li-ion diffusion coefficient has also been improved after the modification based on the results of potentiostatic intermittent titration technique As a consequence, the modified Li[Ni08Co01Mn01]O2 material exhibits superior initial discharges of 2016 mA h g–1 at 02 C and 1

Ultra-sonication-assisted silver nanoparticles using Panax ginseng root extract and their anti-cancer and antiviral activities.

Abstract: A simple, inexpensive ultra-sonication method was used to synthesize quasi spherical silver nanoparticles (AgNPs) with an aqueous extract from Panax ginseng roots. This method has the advantages of being completely eco-friendly and allows increased reaction rates, uniform dispersal of the nanoparticles in liquids, and effective breaking of aggregates. Biomolecules present in plant extracts are often used to reduce metal ions to nanoparticles in a single-step green synthesis route. The formation of the AgNPs was characterized using UV–visible (UV–vis) spectroscopy, X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), energy- dispersive X-ray analysis (EDX), Fast Fourier Transform (FFT), and high-resolution transmission electron microscopy (HR-TEM). The formation of AgNPs (456 nm) was confirmed by UV–vis spectroscopy. HR-TEM analysis revealed that most of the AgNPs were quasi spherical with sizes ranging from approximately 5 to 15 nm. The crystalline nature of the AgNPs was confirmed by XRD, and the presence of elemental silver was confirmed by energy-dispersive X -ray analysis. The AgNPs showed dose-dependent cytotoxicity towards HeLa cells in vitro (3.88% at 0.005 M, 5.11% at 0.01 M, 7.52% at 0.015 M, 11.19% at 0.02 M, and 19.45% at 0.025 M) as revealed by sulforhodamine B assay. They were also shown to be virucidal against the influenza A virus (strain A/PR/8). Hence, the present facile, eco-friendly, and efficient method results in the synthesis of AgNPs that can act as an alternative biomaterial for future biomedical applications.

Simple Synthesis of Large Graphene Oxide Sheets via Electrochemical Method Coupled with Oxidation Process.

Abstract: In this paper, we report a simple two-step approach for the synthesis of large graphene oxide (GO) sheets with lateral dimensions of ≈10 μm or greater. The first step is a pretreatment step involving electrochemical exfoliation of graphite electrode to produce graphene in a mixture of H2SO4 and H3PO4. The second step is the oxidation step, where oxidation of exfoliated graphene sheets was performed using KMnO4 as the oxidizing agent. The oxidation was carried out for different times ranging from 1 to 12 h at ∼60 °C. Prepared GO batches were characterized using a number of spectroscopy and microscopy techniques such as X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), Fourier-transform infrared spectroscopy (FTIR), and UV–visible spectroscopy. Raman and thermogravimetric analysis techniques were used to study the degree of oxidation in the as-synthesized GO batches. The UV–visible absorption spectrum showed an intense peak at 230 nm and a...

Hydrothermal synthesis of Ce-doped hierarchical flower-like In2O3 microspheres and their excellent gas-sensing properties

Abstract: Pure and 1–5 mol% Ce-doped hierarchical flower-like In 2 O 3 microspheres with uniform sizes were synthesized via environment friendly one-step hydrothermal method. Their phase structures, morphologies properties and element composition were investigated by different kinds of techniques, including X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and X-ray photoelectric spectroscopy (XPS). Their gas-sensing properties were tests for several kinds of Volatile Organic Compounds (VOCs). The results indicated that Ce doping could greatly improve sensing performances of In 2 O 3 gas sensors. Among all the samples (0, 1, 3 and 5 mol% Ce-doped In 2 O 3 ), 3 mol% Ce-doped In 2 O 3 exhibited the highest response toward 200 ppm acetone at 250 °C, having a response of 41.8, which was about 4 times higher than pure In 2 O 3 . Furthermore, good repeatability and long term stability were achieved, showing its prospect for excellent acetone gas sensor with high performance.

Preparation and Characterization of Nickel ferrite Nanoparticles via Co-precipitation Method

Abstract: Nickel ferrite (NiFe2O4) nanoparticles were synthesized using co-precipitation method. The X-ray diffraction (XRD) pattern was used to determine the structure of NiFe2O4 nanoparticles. The presence of NiFe2O4 nanoparticles was confirmed by the FT-IR spectrum. The details of the surface morphology of NiFe2O4 nanoparticles were obtained by Scanning Electron Microscopic analysis. The particle size of the NiFe2O4 nanoparticles could be determined by means of Transmission Electron Microscopy. This work aimed at the investigation of the dielectric properties such as the dielectric loss and the dielectric constant of NiFe2O4 nanoparticles at varied frequencies and temperatures. In addition, the magnetic properties of the NiFe2O4 nanoparticles were studied.

Atomic-Scale Insights into the Oxidation of Aluminum

Abstract: The surface oxidation of aluminum is still poorly understood despite its vital role as an insulator in electronics, in aluminum–air batteries, and in protecting the metal against corrosion. Here we use atomic resolution imaging in an environmental transmission electron microscope (TEM) to investigate the mechanism of aluminum oxide formation. Harnessing electron beam sputtering we prepare a pristine, oxide-free metal surface in the TEM. This allows us to study, as a function of crystallographic orientation and oxygen gas pressure, the full oxide growth regime from the first oxide nucleation to a complete saturated, few-nanometers-thick surface film.

Large-scale synthesis of free-standing N-doped graphene using microwave plasma.

Abstract: Direct assembling of N-graphene, i.e. nitrogen doped graphene, in a controllable manner was achieved using microwave plasmas at atmospheric pressure conditions. The synthesis is accomplished via a single step using ethanol and ammonia as carbon and nitrogen precursors. Tailoring of the high-energy density plasma environment results in a selective synthesis of N-graphene (~0.4% doping level) in a narrow range of externally controlled operational conditions, i.e. precursor and background gas fluxes, plasma reactor design and microwave power. Applying infrared (IR) and ultraviolet (UV) irradiation to the flow of free-standing sheets in the post-plasma zone carries out changes in the percentage of sp2, the N doping type and the oxygen functionalities. X-ray photoelectron spectroscopy (XPS) revealed the relative extension of the graphene sheets π-system and the type of nitrogen chemical functions present in the lattice structure. Scanning Electron microscopy (SEM), Transmission Electron microscopy (TEM) and Raman spectroscopy were applied to determine morphological and structural characteristics of the sheets. Optical emission and FT-IR spectroscopy were applied for characterization of the high-energy density plasma environment and outlet gas stream. Electrochemical measurements were also performed to elucidate the electrochemical behavior of NG for supercapacitor applications.