RESUMEN

High entropy alloys (HEAs) with a tunable alloy composition and fascinating synergetic effects between various metals have attracted significant attention in the field of electrocatalysis, but their potential is limited by inefficient and unscalable fabrication methodologies. This work proposes a novel solid-state thermal reaction method to synthesise HEA nanoparticles encapsulated in an N-doped graphitised hollow carbon tube. This facile method is simple and efficient and involves no use of organic solvents during the fabrication process. The synthesized HEA nanoparticles are confined by the graphitised hollow carbon tube, which is possibly beneficial for preventing the aggregation of alloy particles during the oxygen reduction reaction (ORR). In a 0.1 M KOH solution, the HEA catalyst FeCoNiMnCu-1000(1 : 1) exhibits an onset and half-wave potential of 0.92 V and 0.78 V (vs. RHE), respectively. We assembled a Zn-Air battery with FeCoNiMnCu-1000 as a catalyst for the air electrode, and a power density of 81 mW cm-2 and a long-term durability of >200 h were achieved, which is comparable to the performance of the state-of-the-art catalyst Pt/C-RuO2. This work herein offers a scalable and green method for synthesising multinary transition metal-based HEAs and highlights the potential of HEA nanoparticles as electrocatalysts for energy storage and conversion.

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V3 O7 ·H2 O nanobelts/reduced graphene oxide (rGO) composites (weight ratio: 86%/14%) are synthesized by a microwave approach with a high yield (85%) through controlling pH with acids. The growth mechanisms of the highly crystalline nanobelts (average diameter: 25 nm; length: ≈20 µm; oriented along the [101] direction) have been thoroughly investigated, with the governing role of the acid upon the morphology and oxidation state of vanadium disclosed. When used as the ZIB cathode, the composite can deliver a high specific capacity of 410.7 and 385.7 mAh g-1 at the current density of 0.5 and 4 A g-1 , respectively, with a high retention of the capacity of 93%. The capacity of the composite is greater than those of V3 O7 · H2 O, V2 O5 nanobelts, and V5 O12 · 6H2 O film. Zinc ion storage in V3 O7 ·H2 O/rGO is mainly a pseudocapacitive behavior rather than ion diffusion. The presence of rGO enables outstanding cycling stability of up to 1000 cycles with a capacity retention of 99.6%. Extended cycling shows a gradual phase transition, that is, from the original orthorhombic V3 O7 · H2 O to a stable hexagonal Zn3 (VO4 )2 (H2 O)2.93 phase, which is a new electrochemical route found in V3 O7 materials. This phase transition process provides new insight into the reactions of aqueous ZIBs.

RESUMEN

A modulated coherent (La,Sr)CoO3-δ/(Ce,Gd)O2-δ heterostructure is characterized for the first time for its electronic and chemical properties. 2D-multilayer architectures are deposited on NdGaO3 (110) single crystal substrate by pulsed laser deposition, resulting in epitaxial structures with in-plane lattice rotation that, via the metal oxides' interfaces, induces mutual structural rearrangements. Our results show that (La,Sr)CoO3-d thin films of 10-100 nm are chemically unstable when exposed to air at 600 °C during electrical cyclic stress-tests. Conversely, improved stability is achieved confining LSC in the nanometric heterostructure. Remarkably, the chemical stabilization occurs without compromising substantially the electrical properties of the LSC component: the heterostructures show unexpected electrical behaviour with dominant electronic contributions, fast conductivity and mixed ionic-electronic properties, depending on the number of interfaces and the nano-scaled layers.

RESUMEN

Zinc-air batteries offer the potential of low-cost energy storage with high specific energy, but at present secondary Zn-air batteries suffer from poor cyclability. To develop economically viable secondary Zn-air batteries, several properties need to be improved: choking of the cathode, catalyzing the oxygen evolution and reduction reactions, limiting dendrite formation and suppressing the hydrogen evolution reaction (HER). Understanding and alleviating HER at the negative electrode in a secondary Zn-air battery is a substantial challenge, for which it is necessary to combine computational and experimental research. Here, we combine differential electrochemical mass spectrometry (DEMS) and density functional theory (DFT) calculations to investigate the fundamental role and stability when cycling in the presence of selected beneficial additives, that is, In and Bi, and Ag as a potentially unfavorable additive. We show that both In and Bi have the desired property for a secondary battery, that is, upon recharging they will remain on the surface, thereby retaining the beneficial effects on Zn dissolution and suppression of HER. This is confirmed by DEMS, where it is observed that In reduces HER and Bi affects the discharge potential beneficially compared to a battery without additives. Using a simple procedure based on adsorption energies calculated with DFT, it is found that Ag suppresses OH adsorption, but, unlike In and Bi, it does not hinder HER. Finally, it is shown that mixing In and Bi is beneficial compared to the additives by themselves as it improves the electrochemical performance and cyclic stability of the secondary Zn-air battery.

RESUMEN

Possible changes in the oxidation state of the oxygen ion in the lithium iron phosphate Li3Fe2(PO4)3 at high voltages in lithium-ion (Li-ion) batteries are studied using experimental and computational analysis. Results obtained from synchrotron-based hard X-ray photoelectron spectroscopy and density functional theory (DFT) show that the oxidation state of O(2-) ions is altered to higher oxidation states (O(δ-), δ<2) upon charging Li3Fe2(PO4)3 to 4.7 V.

Asunto(s)

RESUMEN

Intensive investigations have been conducted to develop epitaxial oxide thin films with superior electromagnetic performance by low-cost chemical solution deposition routes. In this paper, a novel propionate-based precursor solution without involving any other additive was proposed and employed to grow superconducting YBa2Cu3O(7-δ) (YBCO) films on LaAlO3 (LAO) single crystals. The precursor solutions are stable with a long shelf life of up to several months. Since the primary compositions are propionates after evaporating the solvent, the toxic reagents and evolved gases during solution synthesis and heat treatment can be eliminated completely. In this process, rapid pyrolysis and high conversation rate can also be achieved during growth of YBCO films in comparison with the conventional trifluoroacetate metal organic deposition routes. Remarkably, a 210 nm YBCO film exhibits high superconducting performance with a Jc value of 3.7 MA/cm(2) at 77 K, self-field. Nucleation and growth behaviors in the chemical solution process have also been studied. It is revealed that the amount of liquid phase (Ba-Cu-O) is sufficient through the entire thickness within a very short time at high growth temperatures, which results in pronounced densification and fast conversion of the YBCO phase.

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The discovery of two-dimensional electron gases (2DEGs) in SrTiO3-based heterostructures provides new opportunities for nanoelectronics. Herein, we create a new type of oxide 2DEG by the epitaxial-strain-induced polarization at an otherwise nonpolar perovskite-type interface of CaZrO3/SrTiO3. Remarkably, this heterointerface is atomically sharp and exhibits a high electron mobility exceeding 60,000 cm(2) V(-1) s(-1) at low temperatures. The 2DEG carrier density exhibits a critical dependence on the film thickness, in good agreement with the polarization induced 2DEG scheme.

RESUMEN

Lithium-O2 (Li-O2) batteries are currently limited by a large charge overpotential at practically relevant current densities, and the origin of this overpotential has been heavily debated in the literature. This paper presents a series of electrochemical impedance measurements suggesting that the increase in charge potential is not caused by an increase in the internal resistance. It is proposed that the potential shift is instead dictated by a mixed potential of parasitic reactions and Li2O2 oxidation. The measurements also confirm that the rapid potential loss near the end of discharge ("sudden death") is explained by an increase in the charge transport resistance. The findings confirm that our theory and conclusions in ref 1, based on experiments on smooth small-area glassy carbon cathodes, are equally valid in real Li-O2 batteries with porous cathodes. The parameter variations performed in this paper are used to develop the understanding of the electrochemical impedance, which will be important for further improvement of the Li-air battery.

RESUMEN

Crystal structures of Sr4(Sr2Ta2)O11 and Sr4(Sr1.92Ta2.08)O11.12, synthesized by solid state reaction technique in dry and hydrated state have been studied mainly using Transmission Electron Microscopy. Due to the lesser ability of X-rays to probe details in oxygen sublattice, the change in crystal symmetry due to ordering of oxygen vacancies could be detected better using Transmission Electron Microscopy. After detailed analysis through TEM, it was observed that no major change occurs in the cation sublattice. The TEM observations are compared with XRD data and discussed. The crystal symmetries and corresponding unit cells of all the perovskites based on the ordering of oxygen vacancies is deduced. Crystal unit cells based on the observations are proposed with ideal atomic coordinates. Finally an attempt is made to explain the water uptake behaviour of these perovskites based on the proposed crystal structure.

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The effects of Li2CO3 like species originating from reactions between CO2 and Li2O2 at the cathode of non-aqueous Li-air batteries were studied by density functional theory (DFT) and galvanostatic charge-discharge measurements. Adsorption energies of CO2 at various nucleation sites on a stepped (11̅00) Li2O2 surface were determined and even a low concentration of CO2 effectively blocks the step nucleation site and alters the Li2O2 shape due to Li2CO3 formation. Nudged elastic band calculations show that once CO2 is adsorbed on a step valley site, it is effectively unable to diffuse and impacts the Li2O2 growth mechanism, capacity, and overvoltages. The charging processes are strongly influenced by CO2 contamination, and exhibit increased overvoltages and increased capacity, as a result of poisoning of nucleation sites: this effect is predicted from DFT calculations and observed experimentally already at 1% CO2. Large capacity losses and overvoltages are seen at higher CO2 concentrations.

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The inorganic-organic compound Ca(6)(1,3-adamantanedicarboxylate)(4)(CO(3))(OH)(2)(H(2)O)(x) with 0 < x < 15.2 was synthesized by hydrothermal methods. The crystal structure was determined on the basis of high resolution synchrotron powder diffraction data and poly-crystal measurements. The crystal structure of Ca(6)(C(12)H(14)O(4))(4)(CO(3))(OH)(2)(H(2)O)(14) is tetragonal, space group I4(1)/amd (141) with a = 29.12 Å, c = 15.85 Å, V = 13,440 Å(3) and Z = 8. The compound is classified as a 3D inorganic hybrid material with a 3-dimensional inorganic framework consisting of Ca and O, connected to 1,3-adamantanedicarboxylate anions. The structure shows hydrophilic channels in a diamond-like network. In between the channels there exist hydrophobic pores with surfaces defined by adamantane cages. The shortest distance between hydrogen atoms from different molecules in these pores is 3.6 Å. The largest hydrophilic cavity has a diameter of 10 Å and the pores connecting the channels have a diameter of 5 Å. In the as-synthesised state these channels are filled with water molecules. Reversible dehydration-rehydration occurs. The dehydrated compound easily takes up water from ambient air.

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Photocatalytic degradation of acetaldehyde and its photocatalytic mechanisms over Au/TiO2 core-shell nano catalyst were, for the first time, investigated under UV and visible light irradiation. The results indicate that Au/TiO2 core-shell catalyst shows higher activity for the oxidation of acetaldehyde into CO2 under both UV and visible light irradiation comparing with P-25 and metal-deposited TiO2 photocatalysts. When Au/TiO2 core-shell catalyst is excited by UV light, the Au-core acts as the sink to restore the separated electrons, thus to improve the photoinduced charge separation; while under visible light irradiation, the mechanism can be understood as the coordinate effect of the plasmon resonance of Au-core particles and the formation of an impurity energy level induced by TiO(2-x)F(x).

RESUMEN

Ni-doped titanate Cs(x)Ti(2-x/2)Ni(x/2)O(4) and its protonic derivative H(x)Ti(2-x/2)Ni(x/2)O(4) x xH(2)O (x = 0.7) were synthesized and characterized by means of synchrotron X-ray diffraction, Raman scattering, X-ray photoelectron spectroscopy (XPS), and magnetic measurements. Cs(x)Ti(2-x/2)Ni(x/2)O(4) crystallizes in an orthorhombic structure (space group Immm), consisting of infinite two-dimensional (2D) host layers of the lepidocrocite (gamma-FeOOH) type. The substitution of Ni atoms for Ti in the 2D octahedral layers results in negative charges that are compensated by interlayer Cs(+) ions. Raman scattering and XPS indicate that local structural perturbations are induced upon exchange of interlayer Cs ions with protons H(3)O(+). Magnetic measurements reveal typical paramagnetism induced by Ni substitution; the effective paramagnetic moment mu(eff) = 1.57(1) mu(B) and Curie-Weiss temperature -2.51(1) K are obtained for H(x)Ti(2-x/2)Ni(x/2)O(4) x xH(2)O. Ni- and Mg-codoped titanates Cs(x)Ti(2-x/2)(Ni(y)Mg(1-y))(x/2)O(4) (x = 0.7, 0 < or = y < or = 1) were also reported. The crystal structure, interlayer chemistry, and magnetic properties of the titanates depend on the Ni substitution levels, indicating opportunities for tuning of the properties by controlling the nature and level of lattice substitutions.

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In this comment to a recent paper [Anal. Chim. Acta 585 (2007) 241-245], we report a comparison study on Mn oxide-related compounds with different crystallographic forms, which distinguish between beta-MnO(2) and alpha-MnO(2) type materials via Raman scattering (RS) spectroscopy. The tetragonal rutile-type beta-MnO(2) is characterized by a RS band at approximately 667cm(-1) of symmetry A(1g), whereas the alpha-MnO(2) type materials feature two main RS contributions at about 574 and 634cm(-1), belonging to A(g) spectroscopic species of a tetragonal hollandite-type framework. These data represent a clear signature for identifying beta-MnO(2) and alpha-MnO(2) type materials via RS spectroscopy.

RESUMEN

Beta-MnO(2) nanorods were synthesized via a redox reaction of (NH(4))(2)S(2)O(8) and MnSO(4) under hydrothermal conditions. In situ and ex situ x-ray diffraction and scanning electron microscopy were employed to follow the structural and morphological evolution during growth. It was found that the crystallization of beta-MnO(2) nanorods proceeds through two steps: gamma-MnO(2) nanorods form first via a dissolution-recrystallization process and then transform topologically into beta-MnO(2) with increasing temperature. The phase transformation was associated with a short-range rearrangement of MnO(6) octahedra. Vibrational spectroscopic studies showed that the beta-MnO(2) nanorods had four infrared absorptions at 726, 552, 462 and 418 cm(-1) and four Raman scattering bands at 759 (B(2g)), 662 (A(1g)), 576 (Ramsdellite impurity) and 537 (E(g)) cm(-1), which are in agreement with Mn-O lattice vibrations within a rutile-type MnO(6) octahedral matrix.

Asunto(s)

RESUMEN

Manganite (gamma-MnOOH) nanorods with typical diameters of 20-500 nm and lengths of several micrometers were prepared by reacting KMnO(4) and ethanol under hydrothermal conditions. Synchrotron X-ray diffraction (XRD) reveal that the gamma-MnOOH nanorods crystallize in the monoclinic space group P2(1)/c with unit cell dimensions a = 5.2983(3) A, b = 5.2782(2) A, c = 5.3067(3) A, and beta = 114.401(2) degrees . Transmission electron microscopy shows that the gamma-MnOOH nanorods are single crystalline and that lateral attachment occurs for primary rods elongated along 101. X-ray photoelectron spectroscopy studies indicate that the surfaces of the gamma-MnOOH nanorods are hydrogen deficient and compensated by surface complexation. The Raman scattering spectrum features five main contributions at 360, 389, 530, 558, and 623 cm(-1) along with four weak ones at 266, 453, 492, and 734 cm(-1), attributed to Mn-O vibrations within MnO(6) octahedral frameworks. The structural stability of the gamma-MnOOH nanorods was discussed by means of in situ time-resolved synchrotron XRD. The monoclinic gamma-MnOOH nanorods transform into tetragonal beta-MnO(2) upon heating in air at about 200 degrees C. The reaction is topotactic and shows distinctive differences from those seen for bulk counterparts. A metastable, intermediate phase is observed, possibly connected with hydrogen release via the interstitial (1 x 1) tunnels of the gamma-MnOOH nanorods.

RESUMEN

Crystal structures of titanate nanotubes prepared from a NaOH treatment of TiO(2) with subsequent acid washing were discussed from a viewpoint of vibrational spectroscopy. The correlation between the vibrational feature and the polymerization nature of the TiO(6) octahedron was established by analyzing Raman scattering data of crystalline TiO(2) (anatase and rutile) and layered protonic titanates. Then, the polymerization nature of TiO(6) octahedra in the titanate nanotubes was identified by comparing their Raman scattering spectra with those of the crystalline TiO(2) and layered protonic titanates. It demonstrated that the titanate nanotubes consist of two-dimensional TiO(6) octahedral host layers with a lepidocrocite (gamma-FeOOH)-type layered structure. This conclusion was confirmed further by considering the Raman scattering properties of a restacked titanate prepared by assembling TiO(6) octahedral layers derived from the original scroll-like titanate nanotubes. Our findings offered a convenient approach to validate the crystal structures of the products from an alkaline treatment of TiO(2) under different experimental conditions.

RESUMEN

Single-phase layered Nb-substituted titanates, Na(2)Ti(3-x)Nb(x)O(7) (x = 0-0.06) and Cs(0.7)Ti(1.8-x)Nb(x)O(4) (x = 0-0.03), were for the first time synthesized by a novel sol-gel assisted solid state reaction (SASSR) route. Conventional solid state reactions as well as sol-gel synthesis did not succeed in producing phase pure Nb-substituted titanates. In the SASSR synthesis route we combine the advantages of traditional sol-gel technique (i.e., homogeneous products formed at low temperatures) and solid state reaction (i.e., formation of stable, crystalline phases) for preparing single-phase niobium-substituted layered titanates. The obtained products were characterized by X-ray powder diffraction, scanning electron microscopy, inductively coupled plasma-atomic emission spectrometry, Raman spectroscopy, and thermogravimetric analysis. Results indicate that the Ti(IV) in the host layer of the samples could be partially replaced by Nb(V) without structural deterioration. After proton-exchange, more water molecules were intercalated into the interlayer of H(0.7)Ti(1.8-x)Nb(x)O(4) x nH(2)O with increasing niobium content, whereas the interlayer distance of H(2)Ti(3-x)Nb(x)O(7) (x = 0-0.06) was unchanged.

RESUMEN

Raman scattering spectroscopy is employed to characterize a layered titanate HxTi2-x/4[symbol: see text]x/4O4.H2O ([symbol: see text]: vacancy; x=0.7) with lepidocrocite (gamma-FeOOH)-type layered structure. Nine Raman lines corresponding to (3Ag+3B1g+3B3g) Raman-active modes expected from this orthorhombic structure (space group D2h25-Immm) are recorded at 183, 270, 387, 449, 558, 658, 704, 803, and 908 cm(-1), which are assigned to Ti-O lattice vibrations within the two-dimensional (2D) lepidocrocite-type TiO6 octahedral host layers. These intrinsic Raman bands present a clear signature that can be used for probing the protonic titanate HxTi2-x/4[symbol: see text]x/4O4.H2O and the 2D titanate nanosheets, as well as their corresponding derivates.