David Ávila-Brande

VO2F: a new transition metal oxyfluoride with high specific capacity for Li ion batteries

Hitherto unreported vanadium oxyfluoride VO2F has been synthesized using a solid state reaction at a pressure of 4 GPa and 800 °C. This long awaited vanadium oxyfluoride fills the existing gap of ReO3-type MO2F compounds of Group 5 elements, from which only NbO2F and TaO2F have been known to exist to date. VO2F crystallizes with the VF3-type structure, space group Rc, with a = 5.1226(1) A and c = 13.0686(3) A as determined by powder X-ray diffraction. Highly structured diffuse streaking observed in electron diffraction patterns evidences local O/F ordering. VO2F exhibits two regions upon discharge in a lithium cell, an upper sloped region in the range of 3.9–2.2 V and a lower plateau at 2.15 V. Discharge of VO2F to 1 V provides a gravimetric capacity of 450 mA h g−1. VO2F can reversibly insert up to 1 Li+ per vanadium above 2.15 V without destruction of the host structure, delivering a gravimetric capacity as high as 250 mA h g−1 and pointing to VO2F as a promising intercalation electrode.

Mechanosensitive Gold Colloidal Membranes Mediated by Supramolecular Interfacial Self-Assembly

The ability to respond toward mechanical stimuli is a fundamental property of biological organisms at both the macroscopic and cellular levels, yet it has been considerably less observed in artificial supramolecular and colloidal homologues. An archetypal example in this regard is cellular mechanosensation, a process by which mechanical forces applied on the cell membrane are converted into biochemical or electrical signals through nanometer-scale changes in molecular conformations. In this article, we report an artificial gold nanoparticle (Au NP)-discrete π-conjugated molecule hybrid system that mimics the mechanical behavior of biological membranes and is able to self-assemble into colloidal gold nanoclusters or membranes in a controlled and reversible fashion by changing the concentration or the mechanical force (pressure) applied. This has been achieved by rational design of a small π-conjugated thiolated molecule that controls, to a great extent, the hierarchy levels involved in Au NP clustering by enabling reversible, cooperative non-covalent (π-π, solvophobic, and hydrogen bonding) interactions. In addition, the Au NP membranes have the ability to entrap and release aromatic guest molecules reversibly (Kb = 5.0 × 105 M-1) for several cycles when subjected to compression-expansion experiments, in close analogy to the behavior of cellular mechanosensitive channels. Not only does our hybrid system represent the first example of a reversible colloidal membrane, but it also can be controlled by a dynamic mechanical stimulus using a new supramolecular surface-pressure-controlled strategy. This approach holds great potential for the development of multiple colloidal assemblies within different research fields.

High pressure synthesis of polar and non-polar cation-ordered polymorphs of Mn2ScSbO6

Two new cation-ordered polymorphs of Mn2ScSbO6 have been synthesised at high-pressure. At 5.5 GPa and 1523 K Mn2ScSbO6 crystallizes in the Ni3TeO6-type structure with the polar R3 space group and cell parameters a = 5.3419 (5) Å and c = 14.0603 (2) Å. Below TC = 42.0 K it exhibits ferrimagnetic order with a net magnetization of 0.6μB arising from unusual site-selective Mn/Sc disorder and is thus a potential multiferroic material. A double perovskite phase obtained at 12 GPa and 1473 K crystallizes in the non-polar P21/n monoclinic space group with cell parameters a = 5.2909 (3) Å, b = 5.4698 (3) Å, c = 7.7349 (5) Å and β = 90.165 (6) °. Magnetization and neutron diffraction experiments reveal antiferromagnetic order below TN = 22.3 K with the spins lying in the ac plane.

Chlorination and solvothermal treatment of Zr(C5H5)2Cl2: a synthetic combination to produce nanometric tetragonal ZrO2.

A novel synthetic strategy based on the combination of the chlorination of an organometallic precursor followed by solvothermal treatment is found to be successful in the synthesis of tetragonal nano-ZrO(2) or nano-ZrO(2), embedded in an amorphous carbon matrix, depending on the solvent employed in the solvothermal step. The chemical and structural features (chemical composition, size and surface defects) of the intermediate and final materials have been determined experimentally mainly by high resolution transmission electron microscopy, electron energy loss spectroscopy, and Z-contrast images. These local techniques reveal that the nanoparticles consist of tetragonal ZrO(2) with an average size of 1.7 ± 0.4 and 6.2 ± 0.9 nm for the embedded in carbon and the free nano-ZrO(2), respectively.

Order, disorder and structural modulations in Bi-Fe-W-O-Br Sillén-Aurivillius intergrowths.

Transmission electron microscopy observations on a new complex oxybromide with nominal composition Bi(4)Fe(1/3)W(2/3)O(8)Br, heated at high temperature, reveal the transformation of its basic structure yielding two types of crystals. The first crystal type shows ordered and disordered extended defects leading to a new family of intergrowths between one Sillén block and n Aurivillius blocks and occasionally between one Aurivillius block and n Sillén blocks. The second type presents a compositionally modulated structure, determined by electron diffraction, with an average composition Bi(4)Fe(1/2)W(1/2)O(8 - delta)Br and unit-cell parameters a = (1/gamma) 3.8, b = 3.8, c = 14.5 A (gamma = 0.10-0.15) in the superspace group Immm[(1 - gamma)00] no. 71.1.

Crystal Structures at Atomic Resolution of the Perovskite-Related GdBaMnFeO5 and Its Oxidized GdBaMnFeO6

Perovskite-related GdBaMnFeO5 and the corresponding oxidized phase GdBaMnFeO6, with long-range layered-type ordering of the Ba and Gd atoms have been synthesized. Oxidation retains the cation ordering but drives a modulation of the crystal structure associated with the incorporation of the oxygen atoms between the Gd layers. Oxidation of GdBaMnFeO5 increases the oxidation state of Mn from 2+ to 4+, while the oxidation state of Fe remains 3+. Determination of the crystal structure of both GdBaMnFeO5 and GdBaMnFeO6 is carried out at atomic resolution by means of a combination of advanced transmission electron microscopy techniques. Crystal structure refinements from synchrotron X-ray diffraction data support the structural models proposed from the TEM data. The oxidation states of the Mn and Fe atoms are evaluated by means of EELS and Mössbauer spectroscopy, which also reveals the different magnetic behavior of these oxides.

Activated nanoporous carbon–gold nanoparticle composite electrode with enhanced volumetric capacitance

A novel and straightforward preparation route of an activated nanoporous carbon, produced from abundant biomass, containing a fine dispersion of gold nanoparticles within the carbon microstructure and its application as a supercapacitor electrode material is presented. A remarkable increase of the volumetric capacitance value, with respect to the activated nanoporous electrode without gold nanoparticles, was observed. Interestingly, the electrochemical series resistances of the symmetric cell decreased up to one order of magnitude in comparison with the unmodified carbon material. The incorporation of gold nanoparticles overcomes the characteristic irreversible charge storage of activated carbon electrodes containing surface oxygenated groups. All these findings pave the way to a novel synthetic route of highly-dense carbon nanocomposites for the design of new electrode architectures with potential application in electrical double-layer capacitor development.

Ordered and Disordered Carbon Structures Detected by TEM in Carbide-derived Carbons Produced from TiC

Nowadays, the selective etching of metal carbides by chlorine gas is one well established method to prepare highly porous and nanostructurated carbon materials with potential applications for hydrogen storage and supercapacitors The obtained products usually named as carbide-derived carbons (CDC) present high purity, high surface area and homogeneous pore size distribution and most of times present a graphitic structure; however, in some cases, disordered carbon nanostructures coexist among the ordered ones [1]. In this study, we report the synthesis and characterisation of CDC materials obtained from TiC at 900 oC. Powder precursor, purity of 99% Aldrich, was heated at 50 °C/min in a tubular furnace during 2 h, in a continuous flow of a mixture containing high purity chlorine gas (25 mL/min) and hydrogen (2.5 mL/min) according the next possible scheme:

Nickelocene as precursor of microporous organometallic-derived carbon and nickel oxide-carbon nanocomposite

Microporous flower-like and spherical carbon particles, made of graphene-like layers, have been obtained via chlorination of nickelocene (Ni(C5H5)2). Their mechanism of formation, in terms of morphology and micro-nanostructure, has been followed from 200 to 900°C. Conventional transmission electron microscopy and high-resolution-TEM observations allow determining that their structure is made of highly disordered graphene-like layers. The Raman spectrum of the high temperature sample exhibits the characteristics D and G bands. The peak positions, the ratio of their intensities (ID/IG) and full width at half maximum suggest a high degree of disorder in the nanostructures. The calculated in-plane correlation length of these graphene-like layers is 1.15nm. In all the carbon particles, electron energy-loss spectroscopy shows sp2 carbon bonding content higher than 95% and mass density in the range of 1.0-1.6g/cm3. Textural studies show Type I adsorption isotherms with surface area of 922m2/g for the sample produced at 900°C. In addition, the basic hydrothermal treatment of the sample chlorinated at 600°C yields a composite material with NiO nanoparticles well dispersed within the carbon matrix.

New Electrode Material Based on Mn3O4 Nanoparticles Embedded in Organometallic-Derived Carbon (ODC)

The electrochemical double layer capacitors (EDLC) exhibit greater power density than batteries and fuel cells, however, the amount of energy they are able to store is much lower [1]. Electric double-layer capacitors EDLCs, the most common devices at present, use carbon-based active materials with high surface area, where the charge is stored on the surface of the electrode and therefore the higher the surface area, the higher the capacitance. On the other hand, pseudo-capacitors such as transition metal oxides or conductive polymers are able to store energy by fast and reversible surface processes originated from the redox reaction of the electrode material with the electrolyte on a Faradaic process where the electrons produced by the redox reaction are transferred across the electrolyte-electrode interface. Here, we propose a new electrode material able to exhibit both mechanisms. In order to achieve pseudo-capacitance compounds with high number of electrons involved on their redox reactions and low molar mass are needed, such as metal oxides of the first transition series. We chose manganese oxides due to their well-known redox behavior and their promising pseudo-capacitance properties.