Amparo Fuertes

N-doped porous carbon capsules with tunable porosity for high-performance supercapacitors

A procedure for the fabrication of N-doped hollow carbon spheres with a high rate capability for supercapacitors has been developed. The approach is based on a nanocasting method and the use of a nitrogen-rich compound (pyrrole) as a carbon precursor. The carbon particles thus produced combine a large BET surface area (∼1500 m2 g−1) with a porosity made up of mesopores of ∼4 nm, a high nitrogen content (∼6 wt%) and a capsule morphology which entails short ion diffusion paths derived from the shell morphology (thickness ∼60 nm). The porous properties of these hollow particles can be enhanced by means of an additional activation step with KOH. The activation process does not alter the hollow structure or spherical morphology, but strongly modifies the pore structure from a mesoporous network to a microporous one. The N-doped carbon capsules were tested in aqueous and organic electrolytes. In an aqueous medium (1 M H2SO4), the mesoporous carbon capsules offer the best performance due to the pseudocapacitive contribution of the N-groups, exhibiting a specific capacitance of ∼240 F g−1 at 0.1 A g−1 and a capacitance retention as high as 72% at 80 A g−1. In contrast, in an organic electrolyte (1 M TEABF4/AN), where the charge storage mechanism is based on the formation of the electric double-layer, the microporous capsules perform better due to the larger specific surface area. Thus, the microporous carbon capsules display a specific capacitance of up to 141 F g−1 at 0.1 A g−1 and an outstanding capacitance retention of 93% for an ultra-high discharge current density of 100 A g−1.

Chemistry and applications of oxynitride perovskites

Oxynitride perovskites of early transition metals and rare earth or alkaline earth elements have been reported in the last decade as non-toxic pigments, colossal magnetoresistive materials, high permittivity dielectrics and photocatalysts. Nitride and oxide may occupy the same sites in the perovskite structure, forming solid solutions with ratios that are adjusted by synthesis. The valence states of cations, the bond covalency and the energy of the electronic states can be tuned by the introduction of nitrogen as a consequence of its lower electronegativity, higher electronic polarisability and higher formal anion charge relative to oxygen. This feature article will discuss the influence of the above factors on the crystal chemistry and properties of this group of materials that have emerged as a result of the development of new and controlled synthetic methodologies.

Large coupled magnetoresponses in EuNbO2N

We have explored a new strategy to discover materials with large resistive or capacitive responses to magnetic fields by synthesizing EuMO2N (M = Nb, Ta) perovskites that combine ferromagnetic order of S = 7/2 Eu2+ spins with possible off-center distortions of the d0 M5+ cations enhanced by covalent bonding to N. EuNbO2N shows colossal magnetoresistances at low temperatures and a giant magnetocapacitance. However, the latter response originates from a microstructural effect rather than an intrinsic multiferroism.

Electronic tuning of two metals and colossal magnetoresistances in EuWO1+ xN2- x perovskites

A remarkable electronic flexibility and colossal magnetoresistance effects have been discovered in the perovskite oxynitrides EuWO(1+x)N(2-x). Ammonolysis of Eu(2)W(2)O(9) yields scheelite-type intermediates EuWO(4-y)N(y) with a very small degree of nitride substitution (y = 0.04) and then EuWO(1+x)N(2-x) perovskites that show a wide range of compositions -0.16 <or= x <or= 0.46. The cubic lattice parameter varies linearly with x, but electron microscopy reveals a tetragonal superstructure. The previously unobserved x < 0 regime corresponds to oxidation of Eu (hole doping of the Eu:4f band), whereas x > 0 materials have chemical reduction of W (electron doping of the W:5d band). Hence, both the Eu and W oxidation states and the hole/electron doping are tuned by varying the O/N ratio. EuWO(1+x)N(2-x) phases order ferromagnetically at 12 K, and colossal magnetoresistances (CMR) are observed in the least doped (x = -0.04) sample. Distinct mechanisms for the hole and electron magnetotransport regimes are identified.

Oxygen excess and superconductivity at 45 K in La2CaCu2O6+y

Abstract Preparation of single-phase stoichiometric La 2 CaCu 2 O 6+ y from oxide precursors is reported along with high-resolution neutron powder diffraction studies. To date this is the only route that allows the stoichiometric phase to be obtained. An air-heated sample having y =0.0378(8) displays a transition onset at 45 K to diamagnetic susceptibility. Nevertheless, the maximum amount of superconducting phase inferred from these flux exclusion experiments is only 1% in volume. It is also found that the diamagnetic signal is not substantially modified by changing the annealing atmosphere at normal pressures. Neutron diffraction data show a high atomic ordering of La and Ca ions with a strong preference (75%) of Ca ions for sites eight-fold coordinated located between the Cu-O 2 planes. The other 25% is occupied by La ions, around which the excess oxygen is located with partial occupancies, yielding a higher coordination number for some of these La ions. Comparison of this structure with that of the nonsuperconducting oxide La 1.9 Ca 1.1 Cu 2 O 6+ y , suggests that the observed small superconductivity islands are related to clustered oxygen excess intercalated between the two Cu-O 2 planes, along with the La ions. The small overall concentration of defects observed here, and thus the small number of holes, is responsible for the absence of bulk superconductivity in La 2 CaCu 2 O 6+ y .

Enhanced Thermal Oxidation Stability of Reduced Graphene Oxide by Nitrogen Doping

Nitrogen-doped reduced graphene oxide (N-doped RGO) samples with a high level of doping, up to 13 wt. %, have been prepared by annealing graphene oxide under a flow of pure ammonia. The presence of nitrogen within the structure of RGO induces a remarkable increase in the thermal stability against oxidation by air. The thermal stability is closely related with the temperature of synthesis and the nitrogen content. The combustion reaction of nitrogen in different coordination environments (pyridinic, pyrrolic, and graphitic) is analyzed against a graphene fragment (undoped) from a thermodynamic point of view. In agreement with the experimental observations, the combustion of undoped graphene turns out to be more spontaneous than when nitrogen atoms are present.

Metal oxynitrides as emerging materials with photocatalytic and electronic properties

Oxynitrides of transition metals, alkaline earth metals and rare earth metals are intensively investigated as a group of materials to expand and tune the properties of oxides. The differences in polarizability, electronegativity and anion charge of nitrogen and oxygen induce changes in the physical and chemical properties of oxides by nitrogen introduction. The effects on properties arise from the higher covalency of the metal–nitrogen bond and the changes in the energies of electronic levels, and are important in slightly doped nitrogen metal oxides as in stoichiometric oxynitrides. More intense recent progress in oxynitride research has been made in some specific fields such as photocatalysis in water splitting and other processes as the observed small band gaps lead to activity in the visible light range. The stabilization of new perovskite oxynitrides, with the oxidation states of cations tuned by N/O stoichiometry, has led to new magnetic and dielectric materials. The lower electronegativity of nitrogen and larger crystal field splitting induced by N3− shifts the emission wavelengths of phosphors to the red, and oxynitridosilicates have been investigated as components of white LEDs.

The first lithium manganese oxynitride, Li7.9MnN5 −yOy: preparation and use as electrode material in lithium batteries

The first lithium manganese oxynitride, Li7.9MnN5 − yOy, was obtained by solid state reaction of binary nitrides and lithium oxide under nitrogen flow. It is an interesting candidate for negative electrode material in lithium ion batteries yielding a reversible capacity of 310 mAh g−1.

The tubular crystal structure of the new phase Bi4Sr8Cu5O19+x related to the superconducting perovskites

Abstract The single crystal X-ray structure of a new perovskite-related phase with composition Bi4Sr8Cu5O19+x is reported. This new oxide is apparently not a superconductor although it shows close structural relationship with the Bi2Sr2CuO6 superconducting oxide. The main difference between both structures is the presence in Bi4Sr9Cu5O19+x of perovskite layers stacked perpendicularly to the CuO2 planes. The nonobservance of superconductivity in this new phase gives additional support to the idea that two-dimensionality is essential for high Tc superconductivity.

Direct Solid-State Synthesis at High Pressures of New Mixed-Metal Oxynitrides: RZrO2N (R = Pr, Nd, and Sm)

New oxynitrides of RZrO(2)N (R = Pr, Nd, and Sm) have been synthesized via a direct solid-state reaction of R(2)O(3) with Zr(2)ON(2) at 1200-1500 degrees C under 2-3 GPa pressure. Powder X-ray diffraction shows that all three phases adopt an orthorhombic Pnma perovskite superstructure [a = 5.8537(1) A, b = 8.1707(1) A, and c = 5.7093(1) A for NdZrO(2)N] and the structural distortion increases with decreasing R(3+) ionic radius. This method may enable new mixed-metal oxynitrides to be synthesized without the use of nitriding gas atmospheres.