High-resolution Transmission Electron Microscopy Characterization Research Articles

Electron Density Optimization and the Anisotropic Thermoelectric Properties of Ti Self-Intercalated Ti1+ xS2 Compounds.

Polycrystalline Ti1+ xS2 (0.111 ≤ x ≤ 0.161) with high density and controllable composition were successfully prepared using solid-state reaction combined with plasma-activated sintering. Ti1+ xS2 showed strong (00 l) preferred orientation with Lotgering factor of 0.32-0.60 perpendicular to the pressing direction (⊥), whereas the preferred orientation was not obvious along the pressing direction (∥). This structural anisotropy resulted in distinct anisotropic thermoelectric transport properties in Ti1+ xS2. At 300 K, while the Seebeck coefficient was weak anisotropic, the power factor and lattice thermal conductivity of Ti1+ xS2 was much larger in the perpendicular direction as compared to that of the parallel direction, with an anisotropic ratio of 1.8-2.7 and 1.3-1.7, respectively. Theoretical calculations of formation energy of defects suggested that the excess Ti was most probably intercalated into the van der Waals gaps in metal-rich Ti1+ xS2, consistent with X-ray diffraction, high-resolution transmission electron microscopy characterization and transport measurements. With increasing x, the carrier concentration and power factor of Ti1+ xS2 dramatically increased because of the donor behavior of Ti interstitials, which was accompanied by a significant decrease in the lattice thermal conductivity owing to the strengthened phonon scattering from structural disorder. Because of its strongest (00 l) preferred orientation and largest carrier mobility among all samples, Ti1.112S2 had the highest power factor of 22 μW cm-1 K-2 at 350 K perpendicular to the pressing direction, close to the value (37.1 μW cm-1 K-2) achieved in single-crystal TiS2. We found out that the maximum power factor and dimensionless figure of merit ZT could be achieved at an optimum carrier concentration of about 5.0 × 1020 cm-3. Finally, Ti1.142S2 acquired the highest ZT value of 0.40 at 725 K perpendicular to the pressing direction because of the beneficial preferred orientation, improved power factor, and reduced lattice thermal conductivity.

Porous Amorphous Electrocatalyst Development for Active and Durable Oxygen Evolution

Material novelty is momentous for electrocatalyst in striding into a renewable energy era. Oxygen evolution reaction (OER) is of great significance for hydrogen production via water electrolysis, but has a high energy barrier that limits the energy conversion efficiency. Herein we report a highly-efficient and long-term durable NiFePB OER electrocatalyst, which composes of unique porous amorphous conductive solids with finely tuned nonmetal elements. The NiFePB phase formation is confirmed with X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS). X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) characterizations determine a porous amorphous structure. The outstanding metallic conductivity and corrosion resistance properties are simulated with density functional theory (DFT) calculations. Benefiting from these unique features, the NiFePB exhibits an extraordinarily low overpotential of 197 mV to reach an OER current density of 10 mA/cm2 and 233 mV to reach 100 mA/cm2 under chronopotentiometry condition, with the Tafel slope harmoniously conforming to 34 mV/dec. The catalyst also has an impressive long-term stability, evidenced by a limited activity decay for more than 50-h in a wide current density range from 10 to 200 mA/cm2. This work strategically directs a way for heading up a promising energy conversion alternative.

Deformation mechanisms of Al0.1CoCrFeNi high entropy alloy at ambient and cryogenic temperatures

The deformation mechanisms of an Al0.1CoCrFeNi high entropy alloy (HEA) produced by the vacuum levitation melting (VLM) method were studied in the compression test at temperatures range from 298 K to 77 K. The nominal yield strength was 120, 168, 275 MPa for the alloy tested at 298, 200, and 77 K, respectively. Note that all the compression tests were done only up to the ~ 50% plastic strains. In addition, the remarkable strain-hardening ability and the serration phenomena were observed during the deformation of the alloy at 77 K. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) characterizations revealed that at 298 and 200 K, the deformation in the alloy occurs on the normal FCC {111}< 110 > slip systems by planar dislocation glide. In contrast, at 77 K, nano-twinning becomes an additional deformation mechanism. Overall, the significant increase in the nominal yield strength, extensive strain-hardening ability, and the serration phenomena are attributed to the capability of extensive nano-twinnings at 77 K.

Direct structure imaging of partially collapsed omega domains in phase-separated V–Ti alloy through atom column contrast interpretation

The metastable ω phase stabilizes either in hexagonal (P6/mmm) or trigonal ( $$ P\overline{3} m1 $$ ) structures depending upon the extent of collapse of the atomic planes. The stability between these two structures depends on the atomic scale shear mechanism of the ω formation and also on the relative composition of the bcc stabilizer. Detailed interpretation of the bcc → ω phase transformation demands understanding of structure, chemistry and interface of ω phase along with the bcc matrix down to the atomic level. Present paper deals with the structural imaging of partially collapsed ω structure and its lattice correspondence with the bcc phase in a phase-separated V–Ti alloy. High-resolution TEM characterization with the aid of phase-contrast image simulation and atomic structure modeling has been systematically carried out to study the structural aspects of the nanostructured ω. The contrast of the collapsed atoms and the corner atoms are understood through systematic studies of the experimental images and their corresponding column intensities, as seen along simulated exit waves and their phase and amplitude parts along different zone axis. Attempts have also been made to understand the qualitative nature of the electron channeling behavior along the different atomic positions of the ω structure.

Characterization of thermal-induced distorted face-centered cubic structure in pure titanium

After 900 °C high-vacuum annealing and air cooling, a special band was observed in pure titanium. Energy-dispersive spectrum showed that the composition of the band and the matrix is the same. By tilting the band to different zone axes and taking selected-area electron diffraction (SAED) patterns, we found that the structure of the band is distorted face-centered cubic (fcc) with the lattice constants a, b, and c deviating slightly from each other on the basis of the analyses of the SAED patterns together with the tilting angles. The lattice constants a, b, and c are calculated based on the measurement of interplanar spacing according to high-resolution transmission electron microscopy (HRTEM) images. The orientation relation between the hexagonal-close-packed (hcp) matrix and the distorted fcc band is [11\(\overline{2}\)0]hcp//[110]fcc and (0001)hcp//(1\(\overline{1}\)1)fcc according to SAED analysis. The phase boundary between the matrix and the band was observed to demonstrate serrated configuration composed of two kinds of micro-steps: (1\(\overline{1}\)1)//(0002) and (\(\overline{1}\)11)//(10\(\overline{1}\)1) on the basis of Cs-corrected HRTEM characterization. The distorted fcc band is deduced to be a thermal-induced metastable phase transformed from the high-temperature body-centered cubic (bcc) structure during the allotropic transformation from bcc to hcp based on orientation relation analysis.

Band alignment of 2D WS2/HfO2 interfaces from x-ray photoelectron spectroscopy and first-principles calculations

We report on the growth of two-dimensional (2D) WS2 on high-k HfO2/Si substrates by reactive sputtering deposition. Raman, x-ray photoelectron spectroscopy (XPS), and high-resolution transmission electron microscopy characterizations indicate that the 2D WS2 layers exhibit high-quality crystallinity and exact stoichiometry. Through high-resolution XPS valence spectra, we find a type I alignment at the interface of monolayer WS2/HfO2 with a valence band offset (VBO) of 1.95 eV and a conduction band offset (CBO) of 1.57 eV. The VBO and CBO are also found to increase up to 2.24 eV and 2.09 eV, respectively, with increasing WS2 layers. This is consistent with the results obtained from our first-principles calculations. Our theoretical calculations reveal that the remarkable splitting and shift of the W 5dz2 orbital originating from interlayer orbital coupling in thicker WS2 films induce a reduction of its bandgap, leading to an increase in both the VBO and CBO. This observation can be attributed to the asymmetric splitting at different high symmetric k-points caused by the interlayer orbital coupling.

Substrate Ligand Effects on Atomically Thin 2D Platinum on Graphenated 3D Structures

This work explores the novel concept of electrochemically deposited wetted monolayer to few multilayers of Pt on a graphenated 3D Ni foam (Pt-ML/GR/Ni). Commercially available, highly porous, Ni foam capped with graphene was utilized as a substrate for layer-by-layer Pt growth by surface-limited electrochemical deposition. A survey of literature reveals that our resulting Pt-ML/GR/Ni architecture is the first instance of 3D rendered graphene being used as a template for 2D Pt growth via room-temperature synthesis. XPS, CV, SEM/EDS and high-resolution TEM characterization reveals that graphene covers much of the Ni foam and that the Pt grows epitaxially on the graphene. This scheme allows us to approach theoretical limits of Pt utilization for electrocatalytic applications, such as the canonical oxygen reduction reaction (ORR). Analysis of the ORR overpotential in Pt-ML/GR/Ni in comparison to that of the Ni-free Pt-ML/GR shows that the Dirac cone of the Pt/GR is intact, making the graphene essentially ‘electron transparent’ to the Ni-to-Pt charge transfer. Such Pt-ML/GR architectures can thus allow the surface Pt atoms to be electronically tuned by an underlying ligand of choice. This synthesis approach may prove extremely valuable for high-utilization, low-loading, electronically tunable catalysts. We present here a thorough investigation of the atomic/electronic structure and of the related evolution of electrocatalytic properties as a function of the number of graphene-templated 2D Pt layers.

White LED based on CsPbBr3 nanocrystal phosphors via a facile two-step solution synthesis route

All-inorganic CsPbBr3 perovskite nanocrystals with mean size of ∼300 nm were synthesized via a facile two-step solution synthesis route. High-resolution transmission electron microscopy and X-ray diffraction characterizations confirm the as-prepared CsPbBr3 nanoparticles are single-crystals with good crystallinity, although a CsPb2Br5 impure phase also exists in the final product. Power-/temperature-dependent and time-resolved photoluminescence measurements reveal the notable and stable excitonic emission feature of these CsPbBr3 nanocrystals. As a proof-of-concept demonstration, a LED with p-GaN/n-ZnO heterojunction as blue-violet excitation chip and CsPbBr3 nanocrystals as green phosphors, was fabricated and a white electroluminescence was achieved from this perovskite nanocrystals decorated LED.

Insights into the Electrochemical Reaction Mechanism of a Novel Cathode Material CuNi2(PO4)2/C for Li-Ion Batteries.

In this work, we first report the composite of CuNi2(PO4)2/C (CNP/C) can be employed as the high-capacity conversion-type cathode material for rechargeable Li-ion batteries (LIBs), delivering a reversible capacity as high as 306 mA h g-1 at a current density of 20 mA g-1. Furthermore, CNP/C also presents good rate performance and reasonable cycling stability based on a nontraditional conversion reaction mode. X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) characterizations show that CNP is reduced to form Cu/Ni and Li3PO4 during the discharging process, which is reversed in the following charging process, demonstrating that a reversible conversion reaction mechanism occurs. X-ray absorption spectroscopy (XAS) discloses that Ni2+/Ni0 exhibits a better reversibility compared to Cu2+/Cu during the conversion reaction process, while Cu0 is more difficult to be reoxidized during the recharge process, leading to capacity loss as a consequence. The fundamental understanding obtained in this work provides some important clues to explore the high-capacity conversion-type cathode materials for rechargeable LIBs.

Reversible conversion of MoS2 upon sodium extraction

Intercalation and conversion are two of the most popular forms of lithium (Li) or sodium (Na) storage. Conversion reactions usually involve transference of more than one electron. Therefore, the electrode materials that store Li- and Na-ions by way of such reactions theoretically deliver specific capacities a few times higher than those of the intercalation materials. Intercalation and conversion reactions can occur in sequence upon Na storage in MoS2, but the reversibility of the latter upon Na extraction is still controversial. In this article, we clarify that the conversion reaction of nano-MoS2 can be reversible in a carbon matrix and the extent of reversibility is highly dependent on the dynamic properties of the composite. In addition, hexagonal NaMoS2 is recognized as a new phase in the early stage of Na extraction from the reduction products of MoS2, Mo+Na2S, on the basis of high-resolution transmission electron microscopy (HRTEM) characterization and first-principles calculations. These findings enrich the understanding of reaction mechanism of MoS2 upon Na storage and removal and are helpful to the design and applications of the transition metal sulfides.

In-situ formation of supramolecular aggregates between chitin nanofibers and silver nanoparticles

Chitin and chitin derivatives have gained significant research interest over the years due to a number of beneficial properties that can be exploited in various application fields. Particularly, interactions between their nanostructures and other nanomaterials are of great interest. In situ photo-reduction of AgCl in chitin nanofiber aqueous dispersions resulted in significant loss of colloidal stability of both chitin nanofibers (CNF) and silver nanoparticles. UV–vis spectroscopy was used to characterize the extinction profiles of in-situ prepared CNF and several silver nanoparticle mixtures over the reaction steps. High resolution TEM characterization of the resulting structures indicated the presence of the aggregated form of nanofiber and nanoparticles. Energy filtered TEM analysis confirmed the existence of both CNF and silver nano particles in the aggregate, with silver in its chemically reduced state (Ag(0)). FT-IR, and 13C solid state NMR revealed the presence of strong interactions between Ag and CNF through hydroxyl and carbonyl moieties of the CNF structure. It was concluded that these interactions led to the formation of a supramolecular aggregate in the in-situ mixture as a result of wrapping of CNF around photo-reduced silver nanoparticles which resulted in the colloidal instability.

Alignment control and atomically-scaled heteroepitaxial interface study of GaN nanowires.

Well-aligned GaN nanowires are promising candidates for building high-performance optoelectronic nanodevices. In this work, we demonstrate the epitaxial growth of well-aligned GaN nanowires on a [0001]-oriented sapphire substrate in a simple catalyst-assisted chemical vapor deposition process and their alignment control. It is found that the ammonia flux plays a key role in dominating the initial nucleation of GaN nanocrystals and their orientation. Typically, significant improvement of the GaN nanowire alignment can be realized at a low NH3 flow rate. X-ray diffraction and cross-sectional scanning electron microscopy studies further verified the preferential orientation of GaN nanowires along the [0001] direction. The growth mechanism of GaN nanowire arrays is also well studied based on cross-sectional high-resolution transmission electron microscopy (HRTEM) characterization and it is observed that GaN nanowires have good epitaxial growth on the sapphire substrate following the crystallographic relationship between (0001)GaN∥(0001)sapphire and (101[combining macron]0)GaN∥(112[combining macron]0)sapphire. Most importantly, periodic misfit dislocations are also experimentally observed in the interface region due to the large lattice mismatch between the GaN nanowire and the sapphire substrate, and the formation of such dislocations will favor the release of structural strain in GaN nanowires. HRTEM analysis also finds the existence of "type I" stacking faults and voids inside the GaN nanowires. Optical investigation suggests that the GaN nanowire arrays have strong emission in the UV range, suggesting their crystalline nature and chemical purity. The achievement of aligned GaN nanowires will further promote the wide applications of GaN nanostructures toward diverse high-performance optoelectronic nanodevices including nano-LEDs, photovoltaic cells, photodetectors etc.

A new efficient visible-light-driven composite photocatalyst comprising ZnFe2O4 nanoparticles and conjugated polymer from the dehydrochlorination of polyvinyl chloride

This communication reports the development of a new efficient visible-light-driven composite photocatalyst comprising ZnFe2O4 nanoparticles and conjugated polymer (CPVC) from the dehydrochlorination of polyvinyl chloride (PVC). ZnFe2O4/CPVC nanocomposites were synthesized via three main steps: (i) the synthesis of ZnFe2O4 nanoparticles by a sol-gel method; (ii) the adsorption of PVC in tetrahydrofuran solution by ZnFe2O4 nanoparticles to form ZnFe2O4/PVC nanocomposites; and (iii) the dehydrochlorination of PVC in the ZnFe2O4/PVC nanocomposites by heating at 150°C for 2h. X-ray diffraction, Fourier transform infrared spectroscopy and high-resolution transmission electron microscopy characterization revealed that the as-synthesized product was core/shell structured ZnFe2O4/CPVC nanocomposites. The photocatalytic results demonstrated that the ZnFe2O4/CPVC nanocomposites had much higher photocatalytic activity than ZnFe2O4 nanoparticles in the reduction of aqueous Cr(VI) under visible-light (λ>420nm) irradiation. Thus, ZnFe2O4/CPVC nanocomposites are promising for use as a new efficient visible-light-driven photocatalyst.

High thermal stability and sluggish crystallization kinetics of high-entropy bulk metallic glasses

Metallic glasses are metastable and their thermal stability is critical for practical applications, particularly at elevated temperatures. The conventional bulk metallic glasses (BMGs), though exhibiting high glass-forming ability (GFA), crystallize quickly when being heated to a temperature higher than their glass transition temperature. This problem may potentially be alleviated due to the recent developments of high-entropy (or multi-principle-element) bulk metallic glasses (HE-BMGs). In this work, we demonstrate that typical HE-BMGs, i.e., ZrTiHfCuNiBe and ZrTiCuNiBe, have higher kinetic stability, as compared with the benchmark glass Vitreoy1 (Zr41.2Ti13.8Cu12.5Ni10Be22.5) with a similar chemical composition. The measured activation energy for glass transition and crystallization of the HE-BMGs is nearly twice that of Vitreloy 1. Moreover, the sluggish crystallization region ΔTpl-pf, defined as the temperature span between the last exothermic crystallization peak temperature Tpl and the first crystallization exothermic peak temperature Tpf, of all the HE-BMGs is much wider than that of Vitreloy 1. In addition, high-resolution transmission electron microscopy characterization of the crystallized products at different temperatures and the continuous heating transformation diagram which is proposed to estimate the lifetime at any temperature below the melting point further confirm high thermal stability of the HE-BMGs. Surprisingly, all the HE-BMGs show a small fragility value, which contradicts with their low GFA, suggesting that the underlying diffusion mechanism in the liquid and the solid of HE-BMGs is different.

Magnesia interface nanolayer modification of Pt/Ta3N5 for promoted photocatalytic hydrogen production under visible light irradiation

Deposition of a co-catalyst is a general strategy for promoting the water splitting performance of semiconductor-based photocatalysts, but the interface barrier of the co-catalyst/semiconductor system often leads to unfavorable interfacial charge transfer and separation. In this work, the interface issue of the Pt/Ta3N5 proton reduction system was addressed via a magnesia interface nanolayer (MIN) modification strategy, and its effect on the structure and properties of both the Ta3N5 semiconductor and the Pt co-catalyst was investigated. UV–visible diffuse reflectance spectroscopy, field emission scanning electron microscopy, and high-resolution transmission electron microscopy characterizations indicate that the MIN can not only effectively passivate the Ta3N5 semiconductor, but also favor the deposition of Pt co-catalyst with small particle size and uniform dispersion, which can increase the catalytic active sites and enlarge the interfacial contact area between Ta3N5 and Pt. Time-resolved infrared spectroscopy further evidences that the promoted charge separation process is achieved by this magnesia interface engineering strategy. Based on our modification, the optimal H2 evolution rate on the Pt/MgO(in)–Ta3N5 photocatalyst reaches 22.4μmolh−1, which is ca. 17 times that of pristine Pt/Ta3N5 photocatalyst.

Preparation of SnO&lt;sub&gt;2&lt;/sub&gt;-TiO&lt;sub&gt;2&lt;/sub&gt;/Zeolite Y Composites with Enhanced Photocatalytic Activity

SnO2-TiO2/zeolite Y composites were prepared by the impregnation of tin chloride and tetrabutyl titanate solution with zeolite Y and subsequent calcination at 500. SnO2-TiO2 heterostructures coated on zeolite Y is ascertained by X-ray diffraction, high-resolution transmission electron microscopy and UV-vis diffuse reflectance spectra characterization. The photocatalytic studies suggested that the SnO2-TiO2/zeolite Y showed enhanced photocatalytic efficiency of photodegradation of methyl orange compared with TiO2/ zeolite Y and SnO2/zeolite Y under UV light irradiation.

Infrared photoluminescence from GeSi nanocrystals embedded in a germanium–silicate matrix

We investigate the structural and optical properties of GeO/SiO2 multilayers obtained by evaporation of GeO2 and SiO2 powders under ultrahigh vacuum conditions on Si(001) substrates. Both Raman and infrared absorption spectroscopy measurements indicate the formation of GeSi nanocrystals after postgrowth annealing at 800°C. High-resolution transmission electron microscopy characterizations show that the average size of the nanocrystals is about 5 nm. For samples containing GeSi nanocrystals, photoluminescence is observed at 14 K in the spectral range 1500–1600 nm. The temperature dependence of the photoluminescence is studied.

Structural and chemical characterization of novel NixZn1−xGa2O4 nanocatalysts at atomic resolution

NixZn1−xGa2O4 has already been demonstrated as a noteworthy example of potentially useful catalytic properties such as NOx reduction. In our previous work, it was interesting to find out that the operating temperature of NiGa2O4 catalyst in NOx reduction can be tuned by simple chemical substitution of Ni2+ by Zn2+. It is believed that the mechanism behind such stoichiometry-dependence on operating temperature should be strongly correlated with microstructure, surface morphology as well as the local composition of the nanocatalysts. In the present investigation, NixZn1−xGa2O4 solid solution was synthesized via a hydrothermal ion-exchange reaction, using NaGaO2 and the corresponding acetic salts as the starting materials. By means of a state-of-the-art aberration corrected STEM and high resolution TEM, the structural and chemical characterization at the atomic scale on the NixZn1−xGa2O4 nanocatalyst was carried out, including the crystal structure, size, morphology, surface structure and local composition. It is found that the catalyst was solid solution and most possible exposed facets may be (111).

Improvement of Air/Fuel Ratio Operating Window and Hydrothermal Stability for Pd-Only Three-Way Catalysts through a Pd-Ce2Zr2O8 Superstructure Interaction.

The extremely severe and persistent haze problems in some parts of the world including China have prompted the implementation of increasingly stringent tailpipe regulations. This places increasingly higher performance requirements for three-way catalysts, and in particular a widening of the air/fuel (λ) ratio operating window to facilitate operation of the on-board diagnostic system. A new pathway is presented here by tuning the nanostructure of TWCs to improve their λ activities and hydrothermal stability. High-temperature reduction and a mild-temperature reoxidation treatment for alumina-modified ceria-zirconia brought about the formation of a cubic, fully oxidized, pyrochlore-like superstructure, Ce2Zr2O8. The combination of Pd and the Ce2Zr2O8 superstructure greatly improved the λ window for Pd-only three-way catalysts. X-ray powder diffraction (XRD), temperature-programmed reduction with H2 (H2-TPR) and high-resolution transmission electron microscopy (HRTEM) characterization confirmed the interaction between Pd and the Ce2Zr2O8 superstructure, which modifies the dynamic oxygen storage capacity in comparison to the conventional Pd-Ce(Zr)O2 interaction, due to higher low-temperature reducibility for the Ce2Zr2O8 superstructure than for Ce(Zr)O2. Furthermore, the retention of the Ce2Zr2O8 superstructure derived from the interaction with Pd results in superior λ and light-off performances after hydrothermal aging treatment at 1000 °C for 12 h in air containing 10% H2O.

21.4: Deposition Conditions and HRTEM Characterization of CAAC IGZO

Crystallinity and texture of c‐axis aligned crystal indium gallium zinc oxide (CAAC IGZO) films deposited by RF reactive sputtering were studied and characterized over a range of deposition conditions. The characteristic CAAC (009) peak at 2θ=30° was observed by X‐ray diffraction, and nanocrystalline domain texture was determined using a general area detector diffraction system (GADDS). Highly ordered CAAC films were obtained near the InGaZnO4 composition at a substrate temperature of 310°C and in 10% O2. High‐resolution transmission electron microscopy (HRTEM) confirmed the formation of CAAC showing 2‐3 nm domains aligned over 15 nm x 15 nm fields of view. Cross‐section HRTEM of the CAAC/substrate interface shows formation of an initially disordered IGZO layer prior to CAAC formation, suggesting a nucleation mechanism similar to ZnO thin films.