High-resolution Transmission Electron Microscopy Observations Research Articles

High Capacity Sulfurized Alcohol Composite Positive Electrode Materials Applicable for Lithium Sulfur Batteries

New sulfur-carbon composite positive electrode materials were synthesized through sulfurizing primary alcohols, and their electrochemical properties were examined. The sulfurized alcohol composites (SACs) showed relatively higher sulfur contents (more than 60 wt%) and were revealed to be an amorphous structure by high-resolution TEM observations. Raman and XAFS spectra showed the presence of S – S, C – S, and C – C bonds, and the C – C bonds mainly consisted of sp3 components, which makes a clear contrast to the previously reported organosulfur materials where the C – C bonds mainly consisted of sp2 components. The SAC positive electrode cells showed discharge capacities of more than 900 mAh·g−1 with relatively reduced cycle degradation.

Cross-sectional transmission electron microscope observation of Si clathrate thin films grown on Si (111) substrates

By means of high-resolution transmission electron microscopy (HR-TEM) observations, and energy dispersive X-ray analyses performed with a scanning TEM (STEM-EDX), we evaluated the residual Na contents and the induced crystal strains in type-II Si clathrate films grown on a Si substrate and treated with iodine for Na elimination. Cross-sectional TEM and STEM-EDX observations verified the formation of the type-II Si clathrate thin film on the Si substrate. The guest-free (without any Na inclusion) clathrate crystal was obtained by iodine (I2) treatment for more than 3 cycles. The resulting films were polycrystalline. No buffer layer was observed at the boundary of the Si clathrate crystal film and the substrate, suggesting a chemical bonding between them. The crystalline strains defined here by the deviation of the d value were within 3.5%. These findings might allow us to fabricate epitaxial Si clathrate films on Si substrates.

Characterization of Interfacial Charge‐Transfer Photoexcitation of Polychromium‐Oxo‐Electrodeposited TiO2 as an Earth‐Abundant Photoanode for Water Oxidation Driven by Visible Light

Polychromium-oxo-deposited TiO2 (CrIII x Oy /TiO2 ) electrodes were fabricated by a simple electrochemical technique by using different TiO2 basal electrodes (anatase, rutile, and mixed polymorphic phases P25) as earth-abundant photoanodes for visible-light-driven water oxidation. The high-resolution transmission electron microscopy (HR-TEM) observation illustrated that an CrIII x Oy layer with approximately 2-3 nm thickness was formed on the surface of the crystalline TiO2 particles. Upon visible-light irradiation of the electrodes, the photoanodic current based on water oxidation was generated at the CrIII x Oy /TiO2 electrodes. However, the wavelength (below 620 nm) for photocurrent generation at CrIII x Oy /TiO2 -rutile was longer than that (below 560 nm) at CrIII x Oy /TiO2 -P25 by 60 nm, which is in agreement with the difference (0.2 eV) in the conduction band (CB) edge energy between rutile and anatase TiO2 . This gives a quantitative account for the photocurrent generation based on interfacial charge transfer (IFCT) from Cr 3d of the deposited CrIII x Oy layer to the TiO2 CB. The photocurrent generated for CrIII x Oy /TiO2 -rutile was higher than that for CrIII x Oy /TiO2 -anatase, which is ascribed to 1) more effective CrIII x Oy deposition on the rutile particles, 2) a larger electrolyte/CrIII x Oy interface for water oxidation as a result of smaller rutile particles (ca. 30-40 nm) compared with larger P25 particles (ca. 40-80 nm), and 3) more effective use of visible light owing to the low energy IFCT transition of rutile.

Performance enhancement of blue light-emitting diodes with InGaN/GaN multi-quantum wells grown on Si substrates by inserting thin AlGaN interlayers

We have grown blue light-emitting diodes (LEDs) having InGaN/GaN multi-quantum wells (MQWs) with thin AlyGa1−yN (0 < y < 0.3) interlayers on Si(111) substrates. It was found by high-resolution transmission electron microscopy observations and three-dimensional atom probe analysis that 1-nm-thick interlayers with an AlN mole fraction of less than y = 0.3 were continuously formed between GaN barriers and InGaN wells, and that the AlN mole fraction up to y = 0.15 could be consistently controlled. The external quantum efficiency of the blue LED was enhanced in the low-current-density region (≤45 A/cm2) but reduced in the high-current-density region by the insertion of the thin Al0.15Ga0.85N interlayers in the MQWs. We also found that reductions in both forward voltage and wavelength shift with current were achieved by inserting the interlayers even though the inserted AlGaN layers had potential higher than that of the GaN barriers. The obtained peak wall-plug efficiency was 83% at room temperature. We suggest that the enhanced electroluminescence (EL) performance was caused by the introduction of polarization-induced hole carriers in the InGaN wells on the side adjacent to the thin AlGaN/InGaN interface and efficient electron carrier transport through multiple wells. This model is supported by temperature-dependent EL properties and band-diagram simulations. We also found that inserting the interlayers brought about a reduction in the Shockley-Read-Hall nonradiative recombination component, corresponding to the shrinkage of V-defects. This is another conceivable reason for the observed performance enhancement.

Electrochemical hydrogen storage performance of hierarchical Co metal flower-like microspheres

Hierarchical cobalt metal flower-like microsphere (Co-FM) was synthesized by a facile hydrothermal process, and its electrochemical hydrogen storage performance was investigated. The Co-FM was generated by the reduction of beta cobalt hydroxide [β-Co(OH)2] platelets in the presence of a mild reducing agent without the use of any template or surfactant. The obtained sample was characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), high-resolution transmission electron microscopy (HR-TEM), and energy dispersive spectrometry (EDS). The XRD analysis reveals that this sample has a pure hexagonal close-packed (hcp) structure. The FE-SEM and HR-TEM observations indicate that the synthesized particles are flower-like microspheres with an average diameter of ∼1.5μm, composed of individual Co metal platelets. The electrochemical hydrogen storage performance was investigated as the negative electrode for nickel-metal hydride (Ni-MH) battery in aqueous KOH solution. The electrochemical measurements demonstrated that this material showed better reversibility, higher hydrogen storage capacity, and higher rate dischargeability than the commercial Co metal powder with similar particle size (∼2μm) under atmospheric temperature and pressure. Its maximum discharge capacity was ∼360mAhg−1 and remained 300mAhg−1 even after 100 cycles, and the capacity retention rate was ∼83%. This significant electrochemical hydrogen storage performance can be attributed to its hierarchical architecture, which leads to increasing the surface area, reducing the diffusion pathway, and buffering the volume change during cycling.

Behavior of oxidized platinum nanoparticles on an aligned carbon nanotube forest

We observed and analyzed the behavior of platinum nanoparticles (PtNPs) supported on aligned-carbon nanotubes (CNTs) at high temperatures by X-ray photoelectron spectroscopy and high-resolution transmission electron microscope observations. We found that the PtNPs moved toward the inner-side along each CNT on which they were deposited. The mechanism of this behavior is related to the redox reaction of Pt with the carbon atoms in the CNT. We also performed in-situ observation of this process at a high temperature using an environmental transmission electron microscope under an oxygen atmosphere. We found that the PtNPs penetrated down into a high-density aligned CNT forest along the tube axis and that the PtNPs changed their shape to fit the structure of the CNTs during their movement.

Microstructure, Hardness Evolution, and Thermal Stability Mechanism of Mechanical Alloyed Cu-Nb Alloy during Heat Treatment

The microstructure, hardness evolution, and thermal stability of mechanically alloyed (MA-ed) nanocrystalline Cu–10 wt %Nb solid solution during heat treatment were examined using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution TEM (HRTEM) observations, and microhardness measurement. It is found that the pronounced precipitation of Nb from the Cu-Nb supersaturated solid solution occurs at temperatures up to 700 °C, and the annealed alloy shows a bi-nanostructure with Nb nanoparticles dispersed in the nanocrystalline Cu matrix. The bi-nanostructure remains stable with Cu crystalline grain size below 100 nm and Nb particle size around 10 nm even after annealing at 900 °C for 3 h. The microhardness of the annealed sample shows a small increase after annealing at 400 °C, and then it shows a slow decreasing trend with further increasing temperatures. With the help of the kinetics analyses, it is found that the coarsening of the stable Nb nanoparticles is controlled by volume diffusion. The enhanced stability of the nanocrystalline Cu microstructure is mainly attributed to the solute drag and precipitate pinning effects.

The Role of Lattice Misfit on Heterogeneous Nucleation of Pure Aluminum

α-Alumina (Al2O3) single crystals with different termination planes were used as heterogeneous nucleation substrates for liquid aluminum to varying lattice misfits at the interface between substrate and newly nucleated aluminum grain. Undercooling during the nucleation process was measured for interface configurations with varied lattice misfit, while the solidified Al/Al2O3 interfaces were directly observed by high-resolution transmission electron microscopy (HRTEM). Based on experimental results, the effect of lattice misfit on nucleation behavior was systematically investigated following previous misfit-interfacial energy models, with clarification being made by the undercooling measurement and HRTEM observations of the interfaces in the Al/Al2O3 system. When the misfit is smaller than 13 pct, both experimental results and theoretical analysis show that the currently existing models through modification and incorporating energy calculation can be used to fit the detected undercooling of investigated system. Beyond 13 pct, a new hypothesis was developed to accommodate lattice misfit with stacking faults such as microtwins according to the HRTEM analysis. The interfacial energy is then replaced by the stacking fault energy accumulated in the strained area. It is shown that the lattice misfit plays an important role in determining the heterogeneous nucleation of liquid aluminum. The nucleation undercooling is then able to be predicted by the theoretically calculated interfacial energy using the integrated models developed in the work. The prediction results were also verified by the HRTEM analysis on the nucleation interface of the Al/Al2O3 systems and detected undercooling on corresponding systems.

Twin formation in hematite during dehydration of goethite

Twin formation in hematite during dehydration was investigated using X-ray diffraction, electron diffraction, and high-resolution transmission electron microscopy (TEM). When synthetic goethite was heated at different temperatures between 100 and 800 °C, a phase transformation occurred at temperatures above 250 °C. The electron diffraction patterns showed that the single-crystalline goethite with a growth direction of [001]G was transformed into hematite with a growth direction of [100]H. Two non-equivalent structures emerged in hematite after dehydration, with twin boundaries at the interface between the two variants. As the temperature was increased, crystal growth occurred. At 800 °C, the majority of the twin boundaries disappeared; however, some hematite particles remained in the twinned variant. The electron diffraction patterns and high-resolution TEM observations indicated that the twin boundaries consisted of crystallographically equivalent prismatic (100) (010), and (1 $$\bar{1}$$ 0) planes. According to the total energy calculations based on spin-polarized density functional theory, the twin boundary of prismatic (100) screw had small interfacial energy (0.24 J/m2). Owing to this low interfacial energy, the prismatic (100) screw interface remained after higher-temperature treatment at 800 °C.

Probing the Electronic Structures of BaTiO3/SrTiO3 Multilayered Film with Spatially Resolved Electron Energy-Loss Spectroscopy

A multilayered film of BaTiO3/SrTiO3 was grown on a LaAlO3 substrate using dual-target pulsed laser deposition technique. High-resolution scanning transmission electron microscopy observations show that the nine unit-cell BaTiO3 layer and the three unit-cell SrTiO3 layer are alternatively arranged in the epitaxial film where a sharp interface exists between the BaTiO3 and SrTiO3 layers. Electron energy-loss spectroscopy analysis demonstrates that the O-K edge spectra of SrTiO3 and BaTiO3 layers are quite distinct; in particular, the energy-loss peak at 547 eV in the SrTiO3 spectrum splits into two peaks in the BaTiO3 spectrum. The multiple-scattering calculations of O-K edge spectra for BaTiO3 and SrTiO3 agree well with the experimental results. The low-energy region (<542 eV) of the O-K edge spectra for both BaTiO3 and SrTiO3 is mainly caused by the hybridization of the O 2p with Ti 3d orbitals. The splitting peaks between 542 and 552 eV in the O-K edge spectra of BaTiO3 are attributed to its complex cry...

Iron addition induced tunable band gap and tetravalent Fe ion in hydrothermally prepared SnO2 nanocrystals: Application in photocatalysis

Highly iron-doped tin dioxide nanoparticles (Sn1⿿xFexO2 NPs), with x varying from 0 to 0.2, were prepared by simple hydrothermal method. X-ray diffraction (XRD) patterns indicate that Sn1⿿xFexO2 NPs crystallize in the tetragonal rutile-like structure. High-resolution transmission electron microscopy (HRTEM) observations did not show any modification of the SnO2 lattice parameters with Fe addition. Mössbauer spectroscopy indicated Sn4+ substitution by Fe3+ and Fe4+ ions. It was found that iron addition induced high tunable band gap of SnO2 NPs. Photoluminescence (PL) spectra evidenced an improvement of SnO2 crystallinity after Fe introduction. All the results are consistent with the fact that Fe is strongly soluble in SnO2 host. Finally, the photocatalytic efficiency of Sn1-xFexO2 NPs was examined for the degradation of rhodamine B in aqueous solution under visible light irradiation. We show that Fe4+ and Fe3+ ions play a key role in the improvement of the photocatalytic efficiency.

Observations of nanocrystalline cubic boron nitride formed with plasma spraying

Cubic boron nitride (c-BN) has superior hardness and strength as compared to other ceramics that are commonly used as abrasives and in cutting tool applications. Cubic BN powders are generally produced under high pressures, usually greater than 10 GPa. Here, we report on the synthesis of nanocrystalline c-BN from metastable precursors without high external pressures using a novel plasma spray processing technique. Transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) measurements indicate that the initial metastable precursor particles are amorphous and comprised of boron, carbon, nitrogen and oxygen with weak interatomic bonding that may easily undergo phase transformations during plasma spraying. Postmortem X-ray diffraction and TEM analysis of plasma spray deposits reveal the formation of crystalline phases of BN, and high-resolution TEM observations demonstrate that c-BN can be induced during the plasma spray process with the addition of appropriate catalysts. These observations point to a novel pathway for facilitating the transition from a low-density amorphous state to the high-density cubic BN phase.

TiC crystallite formation and the role of interfacial energies on the composition during the deposition process of TiC/a:C thin films

Abstract TiC/amorphous carbon (TiC/a:C) nanocomposite thin films were deposited from two targets by direct current (DC) magnetron sputtering system at room temperature. The film's composition and morphology were studied in detail by High Resolution Transmission Electron Microscopy (HRTEM), Selected Area Electron Diffraction (SAED), X-ray Photoelectron Spectroscopy (XPS) and Raman Spectroscopy. The sputtering power of carbon target (P C ) was kept at the constant 150 W, while the sputtering power of titanium target (P Ti ) was changed between 5 W and 150 W. Additionally, a C/Ti multilayer was deposited and characterized for comparison. The growth mechanism was derived from the XPS, Raman and HRTEM observations on the grown layer structures and it was completed by a semi-empirical equation for the dependence on the average atomic fraction of Ti. The HRTEM investigations confirmed that the first nucleating phase is amorphous carbon due to its lowest surface energy among the possible phases. The second nucleating phase within the amorphous carbon matrix is TiC; its growth is kinetically not limited. The increase of Ti content resulted in larger TiC nanocrystallites and thinner amorphous carbon spacing between the TiC phases shown by HRTEM analysis. The characteristic texture of the crystallite structure was observed in the case of 120 W and 150 W of P Ti . All observation confirmed the two main phases; amorphous carbon + carbide phase. The hcp titanium phase was not formed due to nucleation barrier. The average sputtered composition differs from the average deposited composition due to different nucleation barriers of different phases.

High Capacity Sulfurized Alcohol Composite Positive Electrode Materials Applicable for Li-S Batteries

Introduction Elemental sulfur is one of the promising cathode active materials for high-energy rechargeable lithium batteries because of its high theoretical capacity (ca. 1670 mAh-g-1) and relatively low cost [1]. However, sulfur cathode has some disadvantages, such as low electrical conductivity and dissolution as polysulfides into electrolyte during electrochemical cycling, resulting in shuttling in Li-S batteries. Several attempts have been performed to solve these problems, and the major improvements have been made by forming composites with carbon matrix, whereby sulfur was physically confined in some fine structures [2]. Despite the improved electrical transfer and the suppressed shuttling of polysulfides, these S-C composites cells still showed gradual capacity degradation. This is originated from the low binding energy between sulfur and carbon matrix [3].Another type of S-C composites has been developed, based on the concept of embedding sulfur into a conductive polymer matrix, such as sulfurized poly(acrylonitrile) (S-PAN) [4-6]. In these organosulfur materials, sulfur is thought to be incorporated in cavity of molecular scale, which significantly prevents the dissolution as polysulfides, leading to the excellent cycle stability of the cells. Although many kinds of polymer framework have been investigated, there still remain challenges for exploring the organosulfur materials with higher sulfur contents, using inexpensive and non-toxic reagents [3]. In the present work, we have tried to prepare new type of organosulfur cathode materials using primary alcohol. The structure and the electrochemical properties of the obtained sulfurized alcohol composites (SAC) were examined. Experiments SAC was prepared by refluxing primary alcohol 1-C n H2n+1OH (n = 3 – 10) (1g) and elemental sulfur (5g) in a glass tube equipped in an electric furnace, which was heated at 400oC. After cooling, the resulting powder was ground and then heated again at 300oC under N2 flow in order to eliminate residual elemental sulfur to yield the SAC. We also prepared S-PAN after the method reported previously [7]. The obtained SAC was characterized by XRD, Raman spectroscopy, TEM observation, and elemental analysis, as well as S and C K-edge XAFS measurements (SR Center, Ritsumeikan University). Electrochemical lithium insertion / extraction reactions were carried out at 30oC using lithium coin-type cells with 1M LiPF6 / (EC + DMC) electrolyte at a current density of 30 mA-g-1 between 1.0 and 3.0 V initially with discharging. The electrochemical performances were also examined by assembling all-solid-state cells using the Li2S-P2S5 solid electrolyte and indium anode in a similar manner as described previously [8] at a current density of 30 mA-g-1 between 0.4 and 3.5 V. Results and Discussion The obtained SAC samples were black in color, and the XRD patterns showed no significant peaks, irrespective of the primary alcohol (n-value). High-resolution TEM observations showed no obvious crystalline domains, indicating an amorphous phase. The EDX mapping showed relatively homogeneous distribution of sulfur and carbon in the SAC samples. The elemental analyses indicated that the sulfur content was more than 60 wt%, which was higher than that reported previously for S-PAN (ca. 30 – 53 wt%) [4,5]. The Raman spectra showed some peaks at 480, 1250, 1440cm-1, suggesting the S-S, C-S, and C-C bonds, respectively. Analyses of the D- and G-bands indicated that the C-C bonds mainly consisted of sp3-type configuration, which makes a clear contrast to the S-PAN where the C-C bonds consisted of mainly sp2-configuration.The electrochemical tests for the SAC sample cells showed that the initial discharge capacity was ca. 800 - 1000 mAh-g-1, which was higher than that of S-PAN, due possibly to higher sulfur contents. Also, the all-solid-state cells with the SAC samples showed the discharge capacity of ca. 600 – 800 mAh-g-1. The charge/discharge mechanism was examined using S and C K-edge XAFS measurements, and the results will be presented in the conference. Acknowledgment This work was financially supported partly by “Next-generation storage battery material evaluation technology development” project of NEDO and METI, Japan.

New insights on the formation of supersaturated Cu-Nb solid solution prepared by mechanical alloying

The deformation-driven alloying mechanisms in the immiscible Cu-Nb system during mechanical alloying (MA) process have been studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) observations. It is found that the dominating alloying mechanism changes at the different stages of milling. Initially, the alloying process is mainly governed by interfaces and dislocations, which supply fast diffusion channels between phases. In the intermediate stage, Nb particles with sizes below 8nm undergo a bcc to fcc allotropic transformation, and the interdiffusion between fcc-Nb and fcc-Cu is promoted due to the similarity of lattice structures. Further milling generates disclination defects, which contribute to the formation of fragments or subgrains with sizes below 5nm. The introduction of such defects and fragments benefits the formation of a complete Cu-Nb solid solution. The formation kinetics of solid solution is analyzed by Johnson-Mehl-Avrami equation, which suggests that the speed of the diffusion of Nb atoms into Cu is very slow.

Understanding shape and morphology of unusual tubular starch nanocrystals

Starch nanocrystals (SNC) are aptly described as the insoluble degradation byproducts of starch granules that purportedly display morphologies that are platelet-like, round, square, and oval-like. In this work, we reported the preparation of SNC with unprecedented tubular structures through sulfuric acid hydrolysis of normal maize starch, subsequent exposure to ammonia and relaxation at 4°C. High-resolution transmission electron microscopy observation clearly proved that the SNCs possess tubular nanostructures with polygonal cross-section. After further reviewing the transformations of SNC by acid hydrolysis, ammonia treatment, and curing time at 4°C, a mechanism for T-SNC formation is suggested. It is conjectured that T-SNC gradually self-assembles by combination of smaller platelet-like/square nanocrystals likely loosely aggregated by starch molecular chains from residual amorphous regions. This work paves the way for the pursuit of new approaches for the preparation of starch-based nanomaterials possessing unique morphologies.

Presence of ε-martensite as an intermediate phase during the strain-induced transformation of SUS304 stainless steel

We have investigated the formation process of α′-martensite from the γ-phase induced by external strain using in situ synchrotron diffraction experiments, combined with Lorentz transmission electron microscopy (TEM) and high-resolution TEM observations. It is clearly demonstrated that ε-martensite with hexagonal symmetry appears as an intermediate structure during the plastic deformation of SUS304 stainless steel. In addition to stacking faults and dislocations, interfaces between the twin structures presumably play a key role in the formation of ε-martensite.

Stress-induced twinning of nanocrystalline hexagonal close-packed titanium in Ti-6Al-4V

Twinning behavior of nanocrystalline hexagonal close-packed titanium in Ti-6Al-4V subjected to severe plastic deformation was investigated. High-resolution transmission electron microscopy observations first showed that the deformation twinning of nanocrystalline hexagonal close-packed titanium was induced. The occurrence of deformation twinning was attributed to the extremely high stress due to specific grain orientations and the large strain rate. The twinning mechanism of nanocrystalline hexagonal close-packed titanium in Ti-6Al-4V was the overlapping of two stacking fault ribbons. This study provided a new thinking that it might be possible to design nanocrystalline titanium and its alloys to extensively activate twinning for further improving their properties.

Phase and microstructural evolution in white etching layer of a pearlitic steel during rolling–sliding friction

To understand the microstructural evolution of a pearlitic rail steel during rolling–sliding contact friction, phase structure and mechanical properties of deformed layers, from top surface to inner matrix, were investigated gradually by scanning electron microscopy, electron probe microanalysis (EPMA), X-ray diffraction, transmission electron microscopy (TEM) and nano-hardness indentation. Focused ion beam method was used to prepare site-specific TEM specimens to study the thickness-dependent deformed microstructure. After the deformation, a white etching layer (WEL), was formed on the top surface, which appeared in white color under an optical microscope. Quantitative EPMA analysis revealed slightly lower carbon concentration, inhomogeneously distributed within the WEL. According to high-resolution TEM observation and quantitative electron diffraction analysis, it was found that the WEL was indeed composed of uniform nano-grains of martensite and ferrite, where no cementite and austenite were found. All the results supported that the WEL was formed by severe plastic deformation, and the hardness value of WEL reached 12GPa by nano-hardness indentation.

Emission Characteristics of InGaN/GaN Core-Shell Nanorods Embedded in a 3D Light-Emitting Diode.

We report the selective-area growth of a gallium nitride (GaN)-nanorod-based InGaN/GaN multiple-quantum-well (MQW) core-shell structure embedded in a three-dimensional (3D) light-emitting diode (LED) grown by metalorganic chemical vapor deposition (MOCVD) and its optical analysis. High-resolution transmission electron microscopy (HR-TEM) observation revealed the high quality of the GaN nanorods and the position dependence of the structural properties of the InGaN/GaN MQWs on multiple facets. The excitation and temperature dependences of photoluminescence (PL) revealed the m-plane emission behaviors of the InGaN/GaN core-shell nanorods. The electroluminescence (EL) of the InGaN/GaN core-shell-nanorod-embedded 3D LED changed color from green to blue with increasing injection current. This phenomenon was mainly due to the energy gradient and deep localization of the indium in the selectively grown InGaN/GaN core-shell MQWs on the 3D architecture.