Heterostructured Cobalt Silicide Nanocrystals: Synthesis in Molten Salts, Ferromagnetism, and Electrocatalysis.

Nanoscale heterostructures of covalent intermetallics should give birth to a wide range of interface-driven physical and chemical properties. Such a level of design however remains unattainable for most of these compounds, due to the difficulty to reach a crystalline order of covalent bonds at the moderate temperatures required for colloidal chemistry. Herein, we design heterostructured cobalt silicide nanoparticles to trigger magnetic and catalytic properties in silicon-based materials. Our strategy consists in controlling the diffusion of cobalt atoms into silicon nanoparticles, by reacting these particles in molten salts. By adjusting the temperature, we tune the conversion of the initial silicon particles toward homogeneous CoSi nanoparticles and core-shell nanoparticles made of a CoSi shell and a silicon-rich core. The increased interface-to-volume ratio of the CoSi component in the core-shell particles yields distinct properties compared to the bulk and homogeneous nanoparticles. First, the core-shell particles exhibit increased ferromagnetism, despite the bulk diamagnetic properties of cobalt monosilicide. Second, the core-shell nanoparticles act as efficient precatalysts for alkaline water oxidation, where the nanostructure is converted in situ into a layered cobalt silicon oxide/(oxy)hydroxide with high and stable oxygen evolution reaction (OER) electrocatalytic activity. This work demonstrates a route to design heterostructured nanocrystals of covalent intermetallic compounds and shows that these new structures exhibit very rich, yet poorly explored, interface-based physical properties and reactivity.

Tuning Magnetoconductivity in LaMnO3 NPs through Cationic Vacancy Control.

The inclusion of La-Mn vacancies in LaMnO3 nanoparticles leads to a noticeable change in conductivity behavior. The sample retains its overall insulator characteristic, with a typical thermal activation mechanism at high temperatures, but it presents high magnetoconductivity below 200 K. The activation energy decreases linearly with the square of the reduced magnetization and vanishes when the sample is magnetized at saturation. Therefore, it turns out that electron hopping between Mn3+ and Mn4+ largely contributes to the conductivity below the Curie temperature. The influence of the applied magnetic field on conductivity also supports the hypothesis of hopping contribution, and the electric behavior can be explained as being due to an increase in the hopping probability via spin alignment.

Atomic Resolution Electron Microscopy: A Key Tool for Understanding the Activity of Nano-Oxides for Biomedical Applications.

Transition metal oxides constitute one of the most fruitful sources of materials with continuously increasing potential applications prompted by the expectations derived from the reduction of the particle size. The recent advances in transmission electron microscopy, because of the development of lenses, have made it possible to reach atomic resolution, which can provide answers regarding the performance of the transition metal nano-oxides. This critical information is related not only to the ability to study their microstructural characteristics but also their local composition and the oxidation state of the transition metal. Exploring these features is a well-known task in nano-oxides for energy and electronic technologies, but they are not so commonly used for elucidating the activity of these oxides for biomedical applications. Nevertheless, the identification at the atomic level of a certain dopant or the unambiguous determination of the oxidation state of a transition metal in a nano-oxide can be important questions to be answered in a certain biomedical application. In this work, we provide several examples in transition metal nano-oxides to show how atomic-resolution electron microscopy can be a key tool for its understanding.

Unambiguous localization of titanium and iron cations in doped manganese hollandite nanowires.

New insights into the chemical and structural features of iron or titanium-doped KxMnO2 hollandites are reported. Neutron diffraction and atomically resolved transmission electron microscopy elucidate the localization of the dopant cations that could be one of the key factors governing the functional activity of these nanomaterials.

Biomimetic Synthesis of Sub-20 nm Covalent Organic Frameworks in Water.

Covalent organic frameworks (COFs) are commonly synthesized under harsh conditions yielding unprocessable powders. Control in their crystallization process and growth has been limited to studies conducted in hazardous organic solvents. Herein, we report a one-pot synthetic method that yields stable aqueous colloidal solutions of sub-20 nm crystalline imine-based COF particles at room temperature and ambient pressure. Additionally, through the combination of experimental and computational studies, we investigated the mechanisms and forces underlying the formation of such imine-based COF colloids in water. Further, we show that our method can be used to process the colloidal solution into 2D and 3D COF shapes as well as to generate a COF ink that can be directly printed onto surfaces. These findings should open new vistas in COF chemistry, enabling new application areas.

Complex structural ordering of the oxygen deficiency in La0.5Ca2.5Mn2O7-δ Ruddlesden-Popper phases.

Ruddlesden-Popper oxides, (AO)(ABO3)n, occupy a prominent place in the landscape of materials research because of their intriguing potential applications. Compositional modifications to the cation sublattices, A or B, have been explored in order to achieve enhanced functionalities. However, changes to the anionic sublattice have been much less explored. In this work, new oxygen-deficient manganese Ruddlesden-Popper-related phases, La0.5Ca2.5Mn2O6.5 and La0.5Ca2.5Mn2O6.25, have been synthesized by controlled reduction of the fully oxidized n = 2 term La0.5Ca2.5Mn2O7. A complete structural and compositional characterization, by means of neutron diffraction, electron diffraction and atomically resolved scanning transmission electron microscopy and electron energy-loss spectroscopy techniques, allows the proposition of a topotactic reduction pathway through preferential oxygen removal in the [MnO2] layers along [031] and [0{\bar 1}3] directions. The gradual decrease of the Mn oxidation state, accommodated by short-range ordering of anionic vacancies, reasonably explains the breaking of ferromagnetic interactions reinforcing the emergence of antiferromagnetic ones. Additional short-range order-disorder phenomena of La and Ca cations have been detected in the reduced La0.5Ca2.5Mn2O7-δ, as previously reported in the parent compound.

Experimental Evidence of the Origin of Nanophase Separation in Low Hole-Doped Colossal Magnetoresistant Manganites.

While being key to understanding their intriguing physical properties, the origin of nanophase separation in manganites and other strongly correlated materials is still unclear. Here, experimental evidence is offered for the origin of the controverted phase separation mechanism in the representative La1-xCaxMnO3 system. For low hole densities, direct evidence of Mn(4+) holes localization around Ca(2+) ions is experimentally provided by means of aberration-corrected scanning transmission electron microscopy combined with electron energy loss spectroscopy. These localized holes give rise to the segregated nanoclusters, within which double exchange hopping between Mn(3+) and Mn(4+) remains restricted, accounting for the insulating character of perovskites with low hole density. This localization is explained in terms of a simple model in which Mn(4+) holes are bound to substitutional divalent Ca(2+) ions.

Chemical analysis at atomic resolution of isolated extended defects in an oxygen-deficient, complex manganese perovskite.

A general approach to the structural and analytical characterization of complex bulk oxides that exploits the advantage of the atomic spatial resolution and the analytical capability of aberration-corrected microscopy is described. The combined use of imaging and spectroscopic techniques becomes necessary to the complete characterization of the oxygen-deficient colossal magnetoresistant La(0.56)Sr(0.44)MnO(2.5)-related perovskite. In this compound, the formation of isolated (La/Sr)O and MnO rock-salt-type planar defects are identified from atomically resolved High Angle Annular Dark Field (HAADF) images. The location of the oxygen atomic columns from Annular Bright Field (ABF) images indicates edge-sharing MnO6 octahedra in the MnO planes and the study performed by Electron Energy Loss Spectroscopy (EELS) reveals different Mn oxidation states derived from the corner- or edge-sharing MnO6 octahedra environment.

A novel zwitterionic bioceramic with dual antibacterial capability.

A novel zwitterionic SBA-15 type bioceramic with dual antibacterial capability has been synthesized. The co-condensation route has been employed to functionalize SBA-15 with primary and secondary amine groups. The resulting material exhibits textural and nanostructural properties comparable to those of pure silica SBA-15, as confirmed by XRD, HR-TEM and N2 adsorption porosimetry. The presence of -NH3 ⊕/-SiO⊖ and >NH2 ⊕/-SiO⊖zwitterionic pairs on the material surface is evidenced by FTIR and 1H →13C CP/MAS solid state NMR. The homogeneous distribution of this zwitterionic pairs agrees with the results derived from STEM-EDS studies. ζ-Potential measurements indicate that the zwitterionic nature of this material is preserved at the physiological pH of 7.4. In vitro bacterial assays using S. aureus demonstrate that the zwitterionic material is capable of inhibiting 99.9% of the bacterial adhesion compared to pure silica SBA-15. Moreover, cephalexin loading and delivery assays indicate that the zwitterionic sample is capable of releasing antibiotic molecules over long time periods. This dual antibacterial capability, i.e. antibiofouling and bactericidal, opens up promising expectations for the treatment of bone implant infections.

Magnetoresistance in La0.5Sr0.5MnO2.5.

Resistance measurements indicate the presence of magnetoresistance in the La(0.5)Sr(0.5)MnO(2.5) brownmillerite related compound. An 80 % of magnetoresistance is found at 75 K. In spite of the partial break-up occurring at the 3D network of octahedra sharing corners, characteristic of the full oxygen content perovskite phase, the oxygen deficient compound exhibits complex magnetic and electric properties. Such behavior can be explained on the basis of ferromagnetic and metallic clusters randomly distributed at the octahedral layers separated from each other by an insulating antiferromagnetic matrix. AC susceptibility measurements suggest spin glass behavior at low temperature as a consequence of the competition between different magnetic interactions.

Magnetic structure and electronic study of complex oxygen-deficient manganites.

Neutron diffraction and X-ray absorption near-edge structure (XANES) studies have been performed in La0.5Ca0.5MnO2.5, La0.5Sr0.5MnO2.5 and Nd0.5Sr0.5MnO2.5 oxygen-deficient perovskite compounds obtained by topotactic reduction. They all exhibit a brownmillerite structure with G-type antiferromagnetic ordering. Mn2+, Mn3+ and Mn4+ coexist at the octahedral sites, whereas only Mn2+ is placed in the tetrahedral positions. A magnetic moment of 1.6 microB has been detected at the tetrahedral layers, which can be explained by assuming Mn2+ is in a low-spin configuration.

Ferromagnetism in a new manganese-related Brownmillerite: La0.5Sr0.5MnO2.5.

The topotactic reduction of La0.5Sr0.5MnO3 leads to ordering of the anionic vacancies in the La0.5Sr0.5MnO2.5 composition. The isolated material, which is isostructural with Sr2Fe2O5, crystallises in the brownmillerite structural type with unit cell parameters a=0.54117(3), b=1.67608(12), c=0.54004(3) nm and space group Ibm2. Its microstructural characterisation by means of electron diffraction and high-resolution electron microscopy suggests a complex microstructure arising from the coherent intergrowth of different brownmillerite-type domains that show short-range ordering at the A sub-lattice. The layer structure of La0.5Sr0.5MnO2.5 leads to a double magnetic behaviour where a ferromagnetic two-dimensional component is present.