Synthesis of Li1.20Mn0.432+Nb0.39O2 disordered rock-salt under reducing conditions for Li-ion batteries.

A Mn2+-Li-Nb disordered rock-salt oxide cathode is prepared by a solid-state reaction under 5% H2/N2, and its electrochemical property shows a high voltage plateau at 4.8 V, with irreversible structural changes in the 1st cycle due to O redox processes; this is supported by powder X-ray diffraction and ex situ laboratory Mn K-edge XANES data.

Picoperovskites: The Smallest Conceivable Isolated Halide Perovskite Structures Formed within Carbon Nanotubes.

Halide perovskite structures are revolutionizing the design of optoelectronic materials, including solar cells, light-emitting diodes, and photovoltaics when formed at the quantum scale. Four isolated sub-nanometer, or picoscale, halide perovskite structures formed inside ≈1.2-1.6 nm single-walled carbon nanotubes (SWCNTs) by melt insertion from CsPbBr3 and lead-free CsSnI3 are reported. Three directly relate to the ABX3 perovskite archetype while a fourth is a perovskite-like lamellar structure with alternating Cs4 and polyhedral Sn4 Ix layers. In ≈1.4 nm-diameter SWCNTs, CsPbBr3 forms Cs3 PbII Br5 nanowires, one ABX3 unit cell in cross section with the Pb2+ oxidation state maintained by ordered Cs+ vacancies. Within ≈1.2 nm-diameter SWCNTs, CsPbBr3 and CsSnI3 form inorganic-polymer-like bilayer structures, one-fourth of an ABX3 unit cell in cross section with systematically reproduced ABX3 stoichiometry. Producing these smallest halide perovskite structures at their absolute synthetic cross-sectional limit enables quantum confinement effects with first-principles calculations demonstrating bandgap widening compared to corresponding bulk structural forms.

Zigzag HgTe Nanowires Modify the Electron-Phonon Interaction in Chirality-Refined Single-Walled Carbon Nanotubes.

Atomically thin nanowires (NWs) can be synthesized inside single-walled carbon nanotubes (SWCNTs) and feature unique crystal structures. Here we show that HgTe nanowires formed inside small-diameter (<1 nm) SWCNTs can advantageously alter the optical and electronic properties of the SWCNTs. Metallic purification of the filled SWCNTs was achieved by a gel column chromatography method, leading to an efficient extraction of the semiconducting and metallic portions with known chiralities. Electron microscopic imaging revealed that zigzag HgTe chains were the dominant NW geometry in both the semiconducting and metallic species. Equilibrium-state and ultrafast spectroscopy demonstrated that the coupled electron-phonon system was modified by the encapsulated HgTe NWs, in a way that varied with the chirality. For semiconducting SWCNTs with HgTe NWs, Auger relaxation processes were suppressed, leading to enhanced photoluminescence emission. In contrast, HgTe NWs enhanced the Auger relaxation rate of metallic SWCNTs and created faster phonon relaxation, providing experimental evidence that encapsulated atomic chains can suppress hot carrier effects and therefore boost electronic transport.

Linear and Helical Cesium Iodide Atomic Chains in Ultranarrow Single-Walled Carbon Nanotubes: Impact on Optical Properties.

One-dimensional (1D) atomic chains of CsI were previously reported in double-walled carbon nanotubes with ∼0.8 nm inner diameter. Here, we demonstrate that, while 1D CsI chains form within narrow ∼0.73 nm diameter single-walled carbon nanotubes (SWCNTs), wider SWCNT tubules (∼0.8-1.1 nm) promote the formation of helical chains of CsI 2 × 1 atoms in cross-section. These CsI helices create complementary oval distortions in encapsulating SWCNTs with highly strained helices formed from strained Cs2I2 parallelogram units in narrow tubes to lower strain Cs2I2 units in wider tubes. The observed structural changes and charge distribution were analyzed by density-functional theory and Bader analysis. CsI chains also produce conformation-selective changes to the electronic structure and optical properties of the encapsulating tubules. The observed defects are an interesting variation from defects commonly observed in alkali halides as these are normally associated with the Schottky and Frenkel type. The energetics of CsI 2 × 1 helix formation in SWCNTs suggests how these could be controllably formed.

Hydrothermal Synthesis of Iridium-Substituted NaTaO3 Perovskites.

Iridium-containing NaTaO3 is produced using a one-step hydrothermal crystallisation from Ta2O5 and IrCl3 in an aqueous solution of 10 M NaOH in 40 vol% H2O2 heated at 240 °C. Although a nominal replacement of 50% of Ta by Ir was attempted, the amount of Ir included in the perovskite oxide was only up to 15 mol%. The materials are formed as crystalline powders comprising cube-shaped crystallites around 100 nm in edge length, as seen by scanning transmission electron microscopy. Energy dispersive X-ray mapping shows an even dispersion of Ir through the crystallites. Profile fitting of powder X-ray diffraction (XRD) shows expanded unit cell volumes (orthorhombic space group Pbnm) compared to the parent NaTaO3, while XANES spectroscopy at the Ir LIII-edge reveals that the highest Ir-content materials contain Ir4+. The inclusion of Ir4+ into the perovskite by replacement of Ta5+ implies the presence of charge-balancing defects and upon heat treatment the iridium is extruded from the perovskite at around 600 °C in air, with the presence of metallic iridium seen by in situ powder XRD. The highest Ir-content material was loaded with Pt and examined for photocatalytic evolution of H2 from aqueous methanol. Compared to the parent NaTaO3, the Ir-substituted material shows a more than ten-fold enhancement of hydrogen yield with a significant proportion ascribed to visible light absorption.

Electric Field-Controlled Synthesis and Characterisation of Single Metal-Organic-Framework (MOF) Nanoparticles.

Achieving control over the size distribution of metal-organic-framework (MOF) nanoparticles is key to biomedical applications and seeding techniques. Electrochemical control over the nanoparticle synthesis of the MOF, HKUST-1, is achieved using a nanopipette injection method to locally mix Cu2+ salt precursor and benzene-1,3,5-tricarboxylate (BTC3- ) ligand reagents, to form MOF nanocrystals, and collect and characterise them on a TEM grid. In situ analysis of the size and translocation frequency of HKUST-1 nanoparticles is demonstrated, using the nanopipette to detect resistive pulses as nanoparticles form. Complementary modelling of mass transport in the electric field, enables particle size to be estimated and explains the feasibility of particular reaction conditions, including inhibitory effects of excess BTC3- . These new methods should be applicable to a variety of MOFs, and scaling up synthesis possible via arrays of nanoscale reaction centres, for example using nanopore membranes.

In situ XAFS of acid-resilient iridate pyrochlore oxygen evolution electrocatalysts under operating conditions.

Pyrochlore iridates (Na,Ca)2-xIr2O6·H2O are acid-stable electrocatalysts that are candidates for use in electrolysers and fuel cells. Ir LIII-edge X-ray absorption fine structure spectroscopy in 1 M H2SO4 at oxygen evolution conditions suggests the involvement of the electrons from the conduction band of the metallic particles, rather than just surface iridium reacting.

Ultrafast Optoelectronic Processes in 1D Radial van der Waals Heterostructures: Carbon, Boron Nitride, and MoS2 Nanotubes with Coexisting Excitons and Highly Mobile Charges.

Heterostructures built from 2D, atomically thin crystals are bound by the van der Waals force and exhibit unique optoelectronic properties. Here, we report the structure, composition and optoelectronic properties of 1D van der Waals heterostructures comprising carbon nanotubes wrapped by atomically thin nanotubes of boron nitride and molybdenum disulfide (MoS2). The high quality of the composite was directly made evident on the atomic scale by transmission electron microscopy, and on the macroscopic scale by a study of the heterostructure's equilibrium and ultrafast optoelectronics. Ultrafast pump-probe spectroscopy across the visible and terahertz frequency ranges identified that, in the MoS2 nanotubes, excitons coexisted with a prominent population of free charges. The electron mobility was comparable to that found in high-quality atomically thin crystals. The high mobility of the MoS2 nanotubes highlights the potential of 1D van der Waals heterostructures for nanoscale optoelectronic devices.

Structures of mixed manganese ruthenium oxides (Mn1-xRux)O2 crystallised under acidic hydrothermal conditions.

A synthesis method for the preparation of mixed manganese-ruthenium oxides is presented along with a detailed characterisation of the solids produced. The use of 1 M aqueous sulfuric acid mediates the redox reaction between KRuO4, KMnO4 and Mn2+ to form ternary oxides. At reaction temperature of 100 °C the products are mixtures of α-MnO2 (hollandite-type) and ß-MnO2 (rutile-type), with some evidence of Ru incorporation in each from their expanded unit cell volumes. At reaction temperature of 200 °C solid-solutions ß-Mn1-xRuxO2 are formed and materials with x ≤ 0.6 have been studied. The amount of Ru included in the oxide is greater than expected from the ratio of metals used in the synthesis, as determined by elemental analysis, implying that some Mn remains unreacted in solution. Powder X-ray diffraction (XRD) shows that while the unit cell volume expands in a linear manner, following Vegard's law, the tetragonal lattice parameters, and the a/c ratio, do not follow the extrapolated trends: this anisotropic behaviour is consistent with the different local coordination of the metals in the end members. Powder XRD patterns show increased peak broadening with increasing ruthenium content, which is corroborated by electron microscopy that shows nanocrystalline material. X-ray absorption near-edge spectra show that the average oxidation state of Mn in the solid solutions is reduced below +4 while that of Ru is increased above +4, suggesting some redistribution of charge. Analysis of the extended X-ray absorption fine structure provides complementary local structural information, confirming the formation of a solid solution, while X-ray photoelectron spectroscopy shows that the surface oxidation states of both Ru and Mn are on average lower than +4, suggesting a disordered surface layer may be present in the materials.

Nanocrystalline Transition-Metal Gallium Oxide Spinels from Acetylacetonate Precursors via Solvothermal Synthesis.

The synthesis of mixed-metal spinels based on substituted γ-Ga2O3 is reported using metal acetylacetonate precursors in solvothermal reactions with alcohols as solvents at 240 °C. New oxides of Cr, Mn and Fe have been produced, all of which are formed as nanocrystalline powders, as seen by high-resolution transmission electron microscopy (HR-TEM). The first chromium-gallium mixed oxide is thus formed, with composition 0.33Ga1.87Cr0.8O4 ( = vacant site). X-ray absorption near-edge spectroscopy (XANES) at the chromium K-edge shows the presence of solely octahedral Cr3+, which in turn implies a mixture of tetrahedral and octahedral Ga3+, and the material is stable on annealing to at least 850 °C. An analogous manganese material with average chemical composition close to MnGa2O4 is shown to contain octahedral Mn2+, along with some Mn3+, but a different inversion factor to materials reported by conventional solid-state synthesis in the literature, which are known to have a significant proportion of tetrahedral Mn2+. In the case of iron, higher amounts of the transition metal can be included to give an Fe:Ga ratio of 1:1. Elemental mapping using energy dispersive X-ray spectroscopy on the TEM, however, reveals inhomogeneity in the distribution of the two metals. This is consistent with variable temperature 57Fe Mössbauer spectroscopy that shows the presence of Fe2+ and Fe3+ in more than one phase in the sample. Variable temperature magnetisation and electron paramagnetic resonance (EPR) indicate the presence of superparamagnetism at room temperature in the iron-gallium oxides.

Incorporation of Sb5+ into CeO2: local structural distortion of the fluorite structure from a pentavalent substituent.

Hydrothermal crystallisation of CeO2 from aqueous sodium hydroxide solution at 240 °C using CeCl3·7H2O in the presence of hydrogen peroxide with addition of either SbCl3 or SbCl5 yields polycrystalline samples of antimony-containing ceria directly from solution. Powder X-ray diffraction shows a contraction of the cubic lattice parameter with increasing Sb content, and also a broadening of Bragg peaks, from which Scherrer analysis yields crystallite domain sizes of 5-20 nm. Scanning transmission electron microscopy provides consistent results with observation of highly crystalline particles of a few nm in diameter. X-ray absorption near edge structure spectroscopy at the Ce LIII- and Sb K-edges reveals the presence of Ce4+ and Sb5+ in the solids. To balance charge the presence of co-included Na is proposed, corroborated by elemental analysis. The general chemical formula of the materials can thus be written as (Ce1-xSbx)1-yNayO2-δ (where x < 0.4 and y ≥ x/3). Sb K-edge extended X-ray absorption fine structure spectroscopy of the substituted ceria samples shows that the local structure of Sb resembles that in NaSbO3, where six-coordinate metal sites are found, but with evidence of a longer interatomic correlation due to surrounding Ce/Sb atoms in the fluorite structure; this implies that the Sb is displaced from the ideal eight-coordinate site of the fluorite structure. This structural distortion gives materials that are unstable under reducing conditions, coupled by the ease of reduction to elemental antimony, which is extruded leading to phase separation.

Exploration of the Smallest Diameter Tin Nanowires Achievable with Electrodeposition: Sub 7 nm Sn Nanowires Produced by Electrodeposition from a Supercritical Fluid.

Electrodeposition of Sn from supercritical difluoromethane has been performed into anodic alumina templates with pores down to 3 nm in diameter and into mesoporous silica templates with pores of diameter 1.5 nm. Optimized deposits have been characterized using X-ray diffraction, scanning electron microscopy, and scanning transmission electron microscopy (bright field, high-angle annular dark field, and energy-dispersive X-ray elemental mapping). Crystalline 13 nm diameter Sn nanowires have been electrodeposited in symmetric pore anodic alumina. Direct transmission electron microscopy evidence of sub 7 nm Sn nanowires in asymmetric anodic alumina has been obtained. These same measurements present indirect evidence for electrodeposition through 3 nm constrictions in the same templates. A detailed transmission electron microscopy study of mesoporous silica films after Sn deposition is presented. These indicate that it is possible to deposit Sn through the 1.5 nm pores in the mesoporous films, but that the nanowires formed are not stable. Suggestions of why this is the case and how such extreme nanowires could be stabilized are presented.

Pair Distribution Function Analysis of Structural Disorder by Nb5+ Inclusion in Ceria: Evidence for Enhanced Oxygen Storage Capacity from Under-Coordinated Oxide.

Partial substitution of Ce4+ by Nb5+ is possible in CeO2 by coinclusion of Na+ to balance the charge, via hydrothermal synthesis in sodium hydroxide solution. Pair distribution function analysis using reverse Monte Carlo refinement reveals that the small pentavalent substituent resides in irregular coordination positions in an average fluorite lattice, displaced away from the ideal cubic coordination toward four oxygens. This results in under-coordinated oxygen, which explains significantly enhanced oxygen storage capacity of the materials of relevance to redox catalysis used in energy and environmental applications.

Electrodeposition of tin nanowires from a dichloromethane based electrolyte.

Tin was electrodeposited from a dichloromethane-based electrolyte at ambient temperature into gold coated anodic alumina membranes with nanoscale pores. The tin nanowires are mainly 〈200〉 aligned, together with some 〈101〉 and 〈301〉 wires. Partial filling of the structure and a distribution of wire lengths was found. Grafting of the pores with hydrophobic surface groups was trialled as a means of modifying the deposition, however, it did not increase the proportion of pores in which wires grew. Under potentiostatic conditions the limited rates of nucleation and diffusion down the 1D pores control the growth of the nanowires.

Atomic Defects and Doping of Monolayer NbSe2.

We have investigated the structure of atomic defects within monolayer NbSe2 encapsulated in graphene by combining atomic resolution transmission electron microscope imaging, density functional theory (DFT) calculations, and strain mapping using geometric phase analysis. We demonstrate the presence of stable Nb and Se monovacancies in monolayer material and reveal that Se monovacancies are the most frequently observed defects, consistent with DFT calculations of their formation energy. We reveal that adventitious impurities of C, N, and O can substitute into the NbSe2 lattice stabilizing Se divacancies. We further observe evidence of Pt substitution into both Se and Nb vacancy sites. This knowledge of the character and relative frequency of different atomic defects provides the potential to better understand and control the unusual electronic and magnetic properties of this exciting two-dimensional material.

Local A-Site Layering in Rare-Earth Orthochromite Perovskites by Solution Synthesis.

Cation size effects were examined in the mixed A-site perovskites La0.5 Sm0.5 CrO3 and La0.5 Tb0.5 CrO3 prepared through both hydrothermal and solid-state methods. Atomically resolved electron energy loss spectroscopy (EELS) in the transmission electron microscope shows that while the La and Sm cations are randomly distributed, increased cation-radius variance in La0.5 Tb0.5 CrO3 results in regions of localised La and Tb layers, an atomic arrangement exclusive to the hydrothermally prepared material. Solid-state preparation gives lower homogeneity resulting in separate nanoscale regions rich in La3+ and Tb3+ . The A-site layering in hydrothermal La0.5 Tb0.5 CrO3 is randomised upon annealing at high temperature, resulting in magnetic behaviour that is dependent on synthesis route.

Resonance Raman Spectroscopy of Extreme Nanowires and Other 1D Systems.

This paper briefly describes how nanowires with diameters corresponding to 1 to 5 atoms can be produced by melting a range of inorganic solids in the presence of carbon nanotubes. These nanowires are extreme in the sense that they are the limit of miniaturization of nanowires and their behavior is not always a simple extrapolation of the behavior of larger nanowires as their diameter decreases. The paper then describes the methods required to obtain Raman spectra from extreme nanowires and the fact that due to the van Hove singularities that 1D systems exhibit in their optical density of states, that determining the correct choice of photon excitation energy is critical. It describes the techniques required to determine the photon energy dependence of the resonances observed in Raman spectroscopy of 1D systems and in particular how to obtain measurements of Raman cross-sections with better than 8% noise and measure the variation in the resonance as a function of sample temperature. The paper describes the importance of ensuring that the Raman scattering is linearly proportional to the intensity of the laser excitation intensity. It also describes how to use the polarization dependence of the Raman scattering to separate Raman scattering of the encapsulated 1D systems from those of other extraneous components in any sample.

Ba4Ru3O10.2(OH)1.8: a new member of the layered hexagonal perovskite family crystallised from water.

A new barium ruthenium oxyhydroxide Ba4Ru3O10.2(OH)1.8 crystallises under hydrothermal conditions at 200 °C: powder neutron diffraction data show it adopts an 8H hexagonal perovskite structure with a new stacking sequence, while high resolution electron microscopy reveals regions of ordered layers of vacant Ru sites, and magnetometry shows antiferromagnetism with TN = 200(5) K.

Selective Imaging of Discrete Polyoxometalate Ions on Graphene Oxide under Variable Voltage Conditions.

Monosubstituted lacunary Keggin [CoSiW11O39](6-) ions on graphene oxide (GO) were used in a comparative imaging study using aberration corrected transmission electron microscopy at two different acceleration voltages, 80 and 200 kV. By performing a set of static and dynamical studies, together with image simulations, we show how the use of lower voltages results in better stability and resolution of the underlying GO support while the use of higher voltages permits better resolution of the individual tungsten atoms and leads to less kinetic motion of the cluster, thus leading to a more accurate identification of cluster orientation and better stability under dynamical imaging conditions. Applying different voltages also influences the visibility of both GO and the lighter Co at lower or higher voltages, respectively.

Ordered mesoporous silica films with pores oriented perpendicular to a titanium nitride substrate.

The production of thin mesoporous silica films with small (∼2-3 nm) pores oriented perpendicular to a titanium nitride growth surface is demonstrated using two methods. These are the growth from a Stöber silica solution with surfactant ordering at the surface of the electrode, and electrochemically assisted growth from an acidic sol achieved by polarisation of the electrode surface. The thickness, pore order and pore size that can be achieved with these two methods is contrasted. A number of methods to vary the pore size by using different surfactants and swelling agents are explored. The advantage of applying these growth methods on titanium nitride surfaces is that it provides access to a wider electrochemical window for nanowire growth and sensor applications with non-aqueous electrolytes whilst retaining good film growth and adhesion properties.