Reversible Diameter Modulation of Single-Walled Carbon Nanotubes by Acetonitrile-Containing Feedstock

Changing the carbon feedstock from pure ethanol to a 5 vol % mixture of acetonitrile in ethanol during the growth of vertically aligned single-walled carbon nanotubes (SWNTs) reduces the mean diameter of the emerging SWNTs from approximately 2 to 1 nm. We show this feedstock-dependent change is reversible and repeatable, as demonstrated by multilayered vertically aligned SWNT structures. The reversibility of this process and lack of necessity for catalyst modification provides insight into the role of nitrogen in reducing the SWNT diameter

Atomic-Scale Tracking of a Phase Transition from Spinel to Rocksalt in Lithium Manganese Oxide

For the intercalation type cathode in lithium-ion batteries, the structural framework of electrode is expected to remain unchanged during lithium insertion and extraction. Unfavorable phase transition in electrode materials, which has been frequently observed, modifies the structural framework, which leads to capacity loss and voltage decay. Here, we track atoms motion/shift in lithium manganese oxide during a phase transition from spinel to rocksalt by using atomically resolved aberration corrected scanning transmission electron microscopy and spectroscopy. We find that when given energy, the transition metal cation can readily hop between oxygen tetrahedral and octahedral sites in oxygen deficient lithium manganese oxide similar to lithium diffusion behavior, which leaves the anion structure framework almost unchanged. During this phase transition, the intermediate state, migration length, and atomic structure of phase boundaries are revealed, and the mechanism is discussed. Our observations help us to understand the past experimental phenomena and provide useful information to stabilize the structure of electrode materials and thus improve the cycling life of lithium-ion batteries

Electron Microscopic Observation of Selective Excitation of Conformational Change of a Single Organic Molecule

Atomic resolution transmission electron microscopic observations at different electron acceleration voltages enabled us to observe visually the energy relaxation process of one conformer into another via rotation of various parts of the molecule. Cross-correlation analysis of sequential transmission electron microscopy (TEM) images or of the difference between experimental and simulated TEM images has been utilized for investigation of the conformational mobility and for structure identification of conformers

Electron Microscopic Observation of Selective Excitation of Conformational Change of a Single Organic Molecule

Atomic resolution transmission electron microscopic observations at different electron acceleration voltages enabled us to observe visually the energy relaxation process of one conformer into another via rotation of various parts of the molecule. Cross-correlation analysis of sequential transmission electron microscopy (TEM) images or of the difference between experimental and simulated TEM images has been utilized for investigation of the conformational mobility and for structure identification of conformers

Morphology-Controlled Synthesis of Cubic Cesium Hydrogen Silicododecatungstate Crystals

Cubic particles of cesium hydrogen silicododecatungstate crystals are obtained for the first time by the use of spherical seed crystals and control of the Cs⁺ to SiW₁₂O₄₀^4– (Cs/POM) ratio in the synthetic solution. The morphology of the particles is controlled between rhombic dodecahedra faceted with {110} planes (thermodynamically stable morphology) and cubes faceted with {100} planes. Scanning transmission electron microscopy–energy-dispersive X-ray spectroscopy analysis of the cross-section shows that the cubes possess a core–shell structure, and the Cs/POM ratio of the shell (ave. 3.24) is larger than that of the core (ave. 2.76), suggesting the existence of anion (POM) vacancies in the shell. Solid state magic angle spinning NMR spectroscopy, nitrogen adsorption, and water vapor sorption measurements of the cubes show that the porous core is covered by the dense shell, and only water molecules can diffuse through the dense shell via the anion vacancies. Despite the small amounts of acidic protons, the cubes exhibit moderate proton conductivity (2.5 × 10^–4 S cm^–1) at room temperature under water vapor (298 K, <i>P</i>/<i>P</i>₀ = 0.95), suggesting that mobile water molecules in the anion vacancies contribute to the proton conduction

Self-Limiting Chemical Vapor Deposition Growth of Monolayer Graphene from Ethanol

Using low-pressure chemical vapor deposition (LPCVD), we, for the first time, realize the self-limiting growth behavior of monolayer graphene on commercially available electroplated copper foils from a carbon precursor other than methane, and systematically investigate the growth of graphene from ethanol and compare its self-limiting behavior over copper facets with different identities. Results show that the growth of graphene from ethanol in the LPCVD process is a substrate-mediated process, in which the domains of graphene are determined by the lattice axes on the copper facets. Moreover, during the evolution of the domains, low-index copper facets of Cu(111) and Cu(100) play a critical role in the following self-limiting process of a continuous graphene sheet, whereas the Cu(110) and high-index facets favor nucleation and formation of secondary layers. In addition, a high degree of similarity exists between graphene grown from ethanol and methane, showing that, when the carbon flux is sufficiently low, the self-limiting growth of graphene on copper surfaces using LPCVD is independent of the precursor structure of ethanol and methane

Amine/Hydrido Bifunctional Nanoporous Silica with Small Metal Nanoparticles Made Onsite: Efficient Dehydrogenation Catalyst

Multifunctional catalysts are of great interest in catalysis because their multiple types of catalytic or functional groups can cooperatively promote catalytic transformations better than their constituents do individually. Herein we report a new synthetic route involving the surface functionalization of nanoporous silica with a rationally designed and synthesized dihydrosilane (3-aminopropylmethylsilane) that leads to the introduction of catalytically active grafted organoamine as well as single metal atoms and ultrasmall Pd or Ag-doped Pd nanoparticles via on-site reduction of metal ions. The resulting nanomaterials serve as highly effective bifunctional dehydrogenative catalysts for generation of H₂ from formic acid

Hierarchically Structured Thermoelectric Materials in Quaternary System Cu–Zn–Sn–S Featuring a Mosaic-type Nanostructure

Multinary chalcogenide semiconductors in the Cu–Zn–Sn–S system have numerous potential applications in the fields of energy production, photocatalysis and nonlinear optics, but characterization and control of their microstructures remains a challenge because of the complexity resulting from the many mutually soluble metallic elements. Here, using state-of-the-art scanning transmission electron microscopy, energy dispersive spectroscopy, first-principles calculations and classical molecular dynamics simulations, we characterize the structures of promising thermoelectric materials Cu₂(Zn,Sn)­S₃ at different length scales to gain a better understanding of how the various components influence the thermoelectric behavior. We report the discovery of a mosaic-type domain nanostructure in the matrix grains comprising well-defined cation-disordered domains (the “tesserae”) coherently bonded to a surrounding network phase with semiordered cations. The network phase is found to have composition Cu_4+<i>x</i>Zn_<i>x</i>Sn₂S₇, a previously unknown phase in the Cu–Zn–Sn–S system, while the tesserae have compositions closer to that of the nominal composition. This nanostructure represents a new kind of phonon-glass electron-crystal, the cation-disordered tesserae and the abrupt domain walls damping the thermal conductivity while the cation-(semi)­ordered network phase supports a high electronic conductivity. Optimization of the hierarchical architecture of these materials represents a new strategy for designing environmentally benign, low-cost thermoelectrics with high figures of merit