Reduced Graphene Oxide as a Platform for the Immobilization of Amino-Cyclodextrins.

In the present work, we reported on a method to combine amino ß-cyclodextrins (CD1) with reduced graphene oxide (obtained by the electrochemical reduction of graphene oxide, erGO) to produce a glassy carbon electrode (GCE) modified with both CD1 and erGO (CD1-erGO/GCE). This procedure avoids the use of organic solvents such as hydrazine or long reaction times and high temperatures. The material combining both CD1 and erGO (CD1-erGO/GCE) was characterized by SEM, ATR-FTIR, Raman, XPS, and electrochemical techniques. As proof-of-concept, the determination of the pesticide carbendazim was carried out. The spectroscopic measurements, especially XPS, proved that CD1 was covalently attached to the surface of the erGO/GCE electrode. The attachment of cyclodextrin at the reduced graphene oxide produced an increase in the electrochemical behavior of the electrode. The cyclodextrin-functionalized reduced graphene oxide, CD1-erGO/GCE, showed a larger sensitivity (1.01 µA/µM) and a lower limit of detection for carbendazim (LOD = 0.50 µM) compared with the non-functionalized material, erGO/GCE, (sensitivity = 0.63 µA/µM and LOD = 4.32 µM, respectively). Overall, the results of the present work show that this simple method is suitable to attach cyclodextrins to graphene oxide, maintaining their inclusion abilities.

Two Shared Icosahedral Metallacarboranes through Iron: A Joint Experimental and Theoretical Refinement of Mössbauer Spectrum in [Fe(1,2-C2B9H11)2]Cs.

Mössbauer and X-ray photoelectron spectroscopies (XPS) are complemented with high-level quantum-chemical computations in the study of the geometric and electronic structure of the paramagnetic salt of the metallacarborane sandwich complex [Fe(1,2-C2B9H11)2]Cs = FeSanCs. Experimental 57Fe isomer shifts and quadrupole splitting parameters are compared with the theoretical prediction, with good agreement. The appearance of two sets of Cs(3d) doublets in the XPS spectrum, separated by 2 eV, indicates that Cs has two different chemical environments due to ease of the Cs(+) cation moving around the sandwich complex with low-energy barriers, as confirmed by quantum-chemical computations. Several minimum-energy geometries of the FeSanCs structure with the corresponding energies and Mössbauer parameters are discussed, in particular the atomic charges and spin population and the surroundings of the Fe atom in the complex. The Mössbauer spectra were taken at different temperatures showing the presence of a low-spin Fe atom with S = 1/2 and thus confirming a paramagnetic FeIII species.

Evidence of carbon-supported porphyrins pyrolyzed for the oxygen reduction reaction keeping integrity.

Fe(III) 5,10,15,20-(tetraphenyl)porphyrin chloride (FeTPP) and Co(III) 5,10,15,20-(tetraphenyl)porphyrin chloride (CoTPP) were adsorbed on carbon Vulcan and studied as electrocatalysts for the oxygen reduction reaction (ORR) before and after pyrolysis. The pyrolysis process was also simulated through ab initio molecular dynamic simulations and the minimum energy path for the O2 dissociation after the interaction with the metal center of the FeTPP and CoTPP were calculated. After the pyrolysis the FeTPP showed the best performances reducing O2 completely to H2O with increased limiting current and lower overpotential. Tafel slops for the various catalysts did not change after the pyrolytic process suggesting that the mechanism for the ORR is not affected by the heat treatment. TEM images, X-ray diffraction, XPS spectroscopy, 57Fe Mössbauer, and DFT simulations, suggest that there is no breakdown of the macrocyclic complex at elevated temperatures, and that the macro cyclic geometry is preserved. Small variations in the Metal-O2 (M-O2) binding energies and the M-N bond length were observed which is attributed to the dispersive interaction between the macrocycles and the irregular surface of the Vulcan substrate induced by the heat treatment and causing better interaction with the O2 molecule. The theoretical strategy herein applied well simulate and explain the nature of the M-N-C active sites and the performances towards the ORR.

High-Voltage Stabilization of O3-Type Layered Oxide for Sodium-Ion Batteries by Simultaneous Tin Dual Modification.

O3-type layered oxide materials are considered to be a highly suitable cathode for sodium-ion batteries (NIBs) due to their appreciable specific capacity and energy density. However, rapid capacity fading caused by serious structural changes and interfacial degradation hampers their use. A novel Sn-modified O3-type layered NaNi1/3Fe1/3Mn1/3O2 cathode is presented, with improved high-voltage stability through simultaneous bulk Sn doping and surface coating in a scalable one-step process. The bulk substitution of Sn4+ stabilizes the crystal structure by alleviating the irreversible phase transition and lattice structure degradation and increases the observed average voltage. In the meantime, the nanolayer Sn/Na/O composite on the surface effectively inhibits surface parasitic reactions and improves the interfacial stability during cycling. A series of Sn-modified materials are reported. An 8%-Sn-modified NaNi1/3Fe1/3Mn1/3O2 cathode exhibits a doubling in capacity retention increase after 150 cycles in the wide voltage range of 2.0-4.1 V vs Na/Na+ compared to none, and 81% capacity retention is observed after 200 cycles in a full cell vs hard carbon. This work offers a facile process to simultaneously stabilize the bulk structure and interface for the O3-type layered cathodes for sodium-ion batteries and raises the possibility of similar effective strategies to be employed for other energy storage materials.

Obtention and Characterization of Ferrous Chloride FeCl2·4H2O from Water Pickling Liquors.

As a hazardous waste, water pickling liquors must be properly treated. An alternative consists of promoting the formation of ferrous salts from this residue due to their higher ferrous content. Since FeCl2·4H2O is widely used in several applications, obtaining pure crystals of this material appears to be an interesting prospect. However, this compound has scarcely been investigated. In the present work, FeCl2·4H2O crystals were obtained from water pickling liquors. Their structural and morphological characteristics were investigated by X-ray diffraction, scanning electron microscopy as well as Mössbauer spectroscopy. In addition, the photoluminescence study of the obtained samples was also assessed. It was observed that after some aging time, the obtained crystals changed in colour from green to more yellowish. As such, the aged sample was also evaluated, and their structural characteristics were compared with the original crystals. Despite this, the obtained crystals exhibit a FeCl2·4H2O structure, which is not modified with the aging of the sample.

New Highly Charged Iron(III) Metal-Organic Cube Stabilized by a Bulky Amine.

In this work, we report a new octanuclear cluster based on FeIII and the ligand 1H-imidazole-4,5-dicarboxylic acid, [Et3NH]12[Fe8(IDC)12]·10DMF·13H2O (1), with a metal core containing eight FeIII connected by only one type of organic ligand. A peak at 573 m/z in the mass spectra of the compound suggests the adduct species {[Fe8(IDC)12]+8H}4-. By X-ray photoelectron spectroscopy, the oxidation state of the iron cation was confirmed to be 3+, also identifying the presence of a quaternary nitrogen species, which act as a countercation of the anionic metal core [Fe8(IDC)12]12-. Mössbauer spectra recorded at different temperatures show an isomer shift and quadrupole splitting parameters that confirm the existence of only FeIII-HS in the structure of 1. X-ray analysis reveals that compound 1 crystallizes in the orthorhombic system space group Ibam, confirming a molecular cluster structure with an almost regular cube as geometry, with the FeIII atoms located at the corners of the cube and connected by µ-1κ2 N,O:2κ2 N',O‴-IDC3- bridges. Additionally, the magnetic measurements reveal a weak antiferromagnetic coupling in the Fe8 III coordination cluster (J = -3.8 cm-1). To the best of our knowledge, 1 is the first member of the family of cubes assembled with 1H-imidazole-4,5-dicarboxylic acid and FeIII cation, exhibiting high pH stability over a broad pH range, making it an ideal candidate for the design of supramolecular structures and metal-organic frameworks.

Treatment of antibiotic cephalexin by heterogeneous electrochemical Fenton-based processes using chalcopyrite as sustainable catalyst.

The development of heterogeneous Fenton-based electrochemical advanced oxidation processes is important for the removal of organic pollutants at industrial level in the near future. This work reports the application of heterogeneous photoelectro-Fenton (HPEF) with UVA light as an enhanced alternative to the more widespread heterogeneous electro-Fenton (HEF) process. The treatment of the antibiotic cephalexin using chalcopyrite as a sustainable catalyst was studied using an undivided IrO2/air-diffusion cell. XPS analysis showed the presence of Fe(III), Cu(I) and Cu(II) species on the surface. The amount of Fe2+ ions dissolved upon chalcopyrite exposure to continuous stirring and air bubbling was proportional to chalcopyrite content. In all cases, the occurrence of pH self-regulation to an optimum value near 3 was observed. The HEF and HPEF treatments of 100 mL of 50 mg L-1 cephalexin solutions with 0.050 M Na2SO4 have been studied with 1.0 g L-1 chalcopyrite at 50 mA cm-2. Comparative homogeneous EF and PEF with dissolved Fe2+ and Cu2+ catalysts were also performed. HPEF was the most effective process, which can be mainly explained by the larger production of homogeneous and heterogeneous OH and the photodegradation of the complexes formed between iron and organics. The effect of applied current and catalyst concentration on HPEF performance was assessed. Recycling experiments showed a long-term stability of chalcopyrite. Seven initial aromatics and six cyclic by-products of cephalexin were identified, and a plausible degradation route that also includes five final carboxylic acids is proposed.

Oxygen Reduction Reaction at Penta-Coordinated Co Phthalocyanines.

From the early 60s, Co complexes, especially Co phthalocyanines (CoPc) have been extensively studied as electrocatalysts for the oxygen reduction reaction (ORR). Generally, they promote the 2-electron reduction of O2 to give peroxide whereas the 4-electron reduction is preferred for fuel cell applications. Still, Co complexes are of interest because depending on the chemical environment of the Co metal centers either promote the 2-electron transfer process or the 4-electron transfer. In this study, we synthetized 3 different Co catalysts where Co is coordinated to 5 N atoms using CoN4 phthalocyanines with a pyridine axial linker anchored to carbon nanotubes. We tested complexes with electro-withdrawing or electro-donating residues on the N4 phthalocyanine ligand. The catalysts were characterized by EPR and XPS spectroscopy. Ab initio calculations, Koutecky-Levich extrapolation and Tafel plots confirm that the pyridine back ligand increases the Co-O2 binding energy, and therefore promotes the 4-electron reduction of O2. But the presence of electron withdrawing residues, in the plane of the tetra N atoms coordinating the Co, does not further increase the activity of the compounds because of pull-push electronic effects.

Co2TiO4/Reduced Graphene Oxide Nanohybrids for Electrochemical Sensing Applications.

For the first time, the synthesis, characterization, and analytical application for hydrogen peroxide quantification of the hybrid materials of Co2TiO4 (CTO) and reduced graphene oxide (RGO) is reported, using in situ (CTO/RGO) and ex situ (CTO+RGO) preparations. This synthesis for obtaining nanostructured CTO is based on a one-step hydrothermal synthesis, with new precursors and low temperatures. The morphology, structure, and composition of the synthesized materials were examined using scanning electron microscopy, X-ray diffraction (XRD), neutron powder diffraction (NPD), and X-ray photoelectron spectroscopy (XPS). Rietveld refinements using neutron diffraction data were conducted to determine the cation distributions in CTO. Hybrid materials were also characterized by Brunauer-Emmett-Teller adsorption isotherms, Scanning Electron microscopy, and scanning electrochemical microscopy. From an analytical point of view, we evaluated the electrochemical reduction of hydrogen peroxide on glassy carbon electrodes modified with hybrid materials. The analytical detection of hydrogen peroxide using CTO/RGO showed 11 and 5 times greater sensitivity in the detection of hydrogen peroxide compared with that of pristine CTO and RGO, respectively, and a two-fold increase compared with that of the RGO+CTO modified electrode. These results demonstrate that there is a synergistic effect between CTO and RGO that is more significant when the hybrid is synthetized through in situ methodology.

Evidence of anomalous switching of the in-plane magnetic easy axis with temperature in Fe3O4 film on SrTiO3:Nb by v-MOKE and ferromagnetic resonance.

The evolution of the magnetic anisotropy directions has been studied in a magnetite (Fe3O4) thin film grown by infrared pulsed-laser deposition on SrTiO3(100):Nb substrate. The magnetic easy axes at room temperature are found along the in-plane 〈100〉 film directions, which means a rotation of the easy axis by 45° with respect to the directions typically reported for bulk magnetite and films grown on single-crystal substrates. Moreover, when undergoing the Verwey transition temperature, TV, the easy axis orientation evolves to the 〈110〉 film directions. This anomalous behavior has been demonstrated by measuring first the angular dependence of coercivity and remanence well above and below TV by high-resolution vectorial magneto-optical Kerr effect (v-MOKE). Ferromagnetic resonance (FMR) measurements have additionally proven a well-defined fourfold magnetic anisotropy induced during growth with confirmed easy axis directions along 〈100〉 for T > TV and 〈110〉 for T < TV. These results provide a clear proof of the possibility of tuning magnetic anisotropy in Fe3O4 thin films by proper control on the growth parameters and substrate choice.

Topotactic Fluorine Insertion into the Channels of FeSb2O4-Related Materials.

This paper discusses the fluorination characteristics of phases related to FeSb2O4, by reporting the results of a detailed study of Mg0.50Fe0.50Sb2O4 and Co0.50Fe0.50Sb2O4. Reaction with fluorine gas at low temperatures (typically 230 °C) results in topotactic insertion of fluorine into the channels, which are an inherent feature of the structure. Neutron powder diffraction and solid state NMR studies show that the interstitial fluoride ions are bonded to antimony within the channel walls to form Sb-F-Sb bridges. To date, these reactions have been observed only when Fe2+ ions are present within the chains of edge-linked octahedra (FeO6 in FeSb2O4) that form the structural channels. Oxidation of Fe2+ to Fe3+ is primarily responsible for balancing the increased negative charge associated with the presence of the fluoride ions within the channels. For the two phases studied, the creation of Fe3+ ions within the chains of octahedra modify the magnetic exchange interactions to change the ground-state magnetic symmetry to C-type magnetic order in contrast to the A-type order observed for the unfluorinated oxide parents.

Comparison of the catalytic activity for O2 reduction of Fe and Co MN4 adsorbed on graphite electrodes and on carbon nanotubes.

We have compared the electrocatalytic activity of several substituted and unsubstituted Co and Fe N4-macrocyclic complexes (MN4) for the electro-reduction of oxygen with the complexes directly adsorbed on the edge plane of pyrolytic graphite or adsorbed on carbon nanotubes (CNTs). In the presence of CNTs, one order of magnitude higher surface concentrations of MN4 catalysts per geometric area unit could be adsorbed leading to a lower overpotential for the oxygen electro-reduction and activities in the same order of magnitude as the commercially available Pt/C catalysts in basic pH. From Koutecky-Levich regression analysis, the total number of electrons transferred was approximately 2 for all the Co complexes and 4 for all the Fe ones, both in the presence and in the absence of the carbon nanotubes. Furthermore, the Tafel slopes did not vary due to the presence of the CNTs and presented values in the range of -0.06 V decade-1 for the CoN4 compounds and in the range of -0.04 V decade-1 for FeN4. When plotting the log of kinetic current densities (i.e. log jk) at a constant potential for each complex divided by the surface concentration Γ, and the number of electrons transferred n for the ORR for each catalyst, versus the difference between the redox potential of the metal active site of the Co(ii)/(i) or Fe(iii)/(ii) catalyst and the reversible potential of the reaction they promote, the catalytic activity increases when the formal potential of the complex becomes more positive and the data obtained with complexes adsorbed on graphite are in agreement with the data obtained when using CNTs indicating that the increase in jk when CNTs are present is only due to an increase in the number of active sites per geometric area of the electrode.

Reduced Graphene Oxides: Influence of the Reduction Method on the Electrocatalytic Effect towards Nucleic Acid Oxidation.

For the first time a critical analysis of the influence that four different graphene oxide reduction methods have on the electrochemical properties of the resulting reduced graphene oxides (RGOs) is reported. Starting from the same graphene oxide, chemical (CRGO), hydrothermal (hTRGO), electrochemical (ERGO), and thermal (TRGO) reduced graphene oxide were produced. The materials were fully characterized and the topography and electroactivity of the resulting glassy carbon modified electrodes were also evaluated. An oligonucleotide molecule was used as a model of DNA electrochemical biosensing. The results allow for the conclusion that TRGO produced the RGOs with the best electrochemical performance for oligonucleotide electroanalysis. A clear shift in the guanine oxidation peak potential to lower values (~0.100 V) and an almost two-fold increase in the current intensity were observed compared with the other RGOs. The electrocatalytic effect has a multifactorial explanation because the TRGO was the material that presented a higher polydispersity and lower sheet size, thus exposing a larger quantity of defects to the electrode surface, which produces larger physical and electrochemical areas.

Magnetic interactions in cubic-, hexagonal- and trigonal-barium iron oxide fluoride, BaFeO2F.

(57)Fe Mössbauer spectra have been recorded from the hexagonal (6H)- and trigonal (15R)- modifications of BaFeO2F and are compared with those previously recorded from the cubic form of BaFeO2F. The spectra, recorded over a temperature range from 15 to 650 K show that all of the iron in all the compounds is in the Fe(3+) state. Spectra from the 6H- and 15R-modifications were successfully fitted with components that were related to the Fe(1) and Fe(2) structural sites in the 6H variant and to the Fe(1), Fe(2) and Fe(3) structural sites in the 15R form. The magnetic ordering temperatures were determined as 597 ± 3 K for 6H-BaFeO2F and 636 ± 3 K for 15R-BaFeO2F. These values are surprisingly close to the value of 645 ± 5 K determined for the cubic form. The magnetic interactions in the three forms are compared with a view to explaining this similarity of magnetic ordering temperature.

Co on Fe3O4(001): Towards precise control of surface properties.

A novel approach to incorporate cobalt atoms into a magnetite single crystal is demonstrated by a combination of x-ray spectro-microscopy, low-energy electron diffraction, and density-functional theory calculations. Co is deposited at room temperature on the reconstructed magnetite (001) surface filling first the subsurface octahedral vacancies and then occupying adatom sites on the surface. Progressive annealing treatments at temperatures up to 733 K diffuse the Co atoms into deeper crystal positions, mainly into octahedral ones with a marked inversion level. The oxidation state, coordination, and magnetic moments of the cobalt atoms are followed from their adsorption to their final incorporation into the bulk, mostly as octahedral Co(2+). This precise control of the near-surface Co atoms location opens up the way to accurately tune the surface physical and magnetic properties of mixed spinel oxides.

Lanthanide sorbent based on magnetite nanoparticles functionalized with organophosphorus extractants.

In this work, an adsorbent was prepared based on the attachment of organophosphorus acid extractants, namely, D2EHPA, CYANEX 272, and CYANEX 301, to the surface of superparamagnetic magnetite (Fe3O4) nanoparticles. The synthesized nanoparticles were coated with oleic acid, first by a chemisorption mechanism and later by the respective extractant via physical adsorption. The obtained core-shell functionalized magnetite nanoparticle composites were characterized by dynamic light scattering, scanning electron microscopy, transmission electron microscopy, thermogravimetry, infrared absorption and vibrating sample magnetometry. All the prepared nanoparticles exhibited a high saturation magnetization capacity that varied between 72 and 46 emu g-1 and decreased as the magnetite nanoparticle was coated with oleic acid and functionalized. The scope of this study also included adsorption tests for lanthanum, cerium, praseodymium, and neodymium and the corresponding analysis of their results. Sorption tests indicated that the functionalized nanoparticles were able to extract the four studied lanthanide metal ions, although the best extraction performance was observed when the sorbent was functionalized with CYANEX 272, which resulted in a loading capacity of approximately 12-14 mgLa/gMNP. The magnetization of the synthesized nanoparticles was verified during the separation of the lanthanide-loaded sorbent from the raffinate by using a conventional magnet.

Fluorine insertion into the Ruddlesden-Popper phase La2BaFe2O7: the structure and magnetic properties of La2BaFe2O5F4.

Fluorination of the n = 2 Ruddlesden-Popper phase La2BaFe2O7 occurs at ~300 °C in flowing 10% F2 in N2 to form La2BaFe2O5F4. This oxide fluoride contains 2F(-) ions in interstitial sites within the rocksalt regions and 2F(-) ions that have substituted for O(2-) ions in apical sites within the rocksalt layers. The fluorination results in an expansion along c of 7.6% to yield a tetragonal unit cell of dimensions a = 3.96237(7) Å, c = 22.3972(5) Å. The structure and magnetic properties have been examined by Mössbauer spectroscopy, neutron powder diffraction and magnetic susceptibility measurements. La2BaFe2O5F4 becomes antiferromagnetically ordered at temperatures below ~500 K, and the magnetic order shows a striking resemblance to that observed in La2BaFe2O7. The magnetic moments on Fe(3+) are perpendicular to [001] and aligned along ±{100} directions above 300 K, but at temperatures below 200 K, they rotate by 45° to lie along ±{110}. Mössbauer spectroscopy suggests the presence of Fe(3+) within the primary phase, but also indicates that fluorination results in some particle fragmentation to form a paramagnetic component of the fluorinated material.

Potentiodynamic Polarization Studies and Surface Chemical Composition of Bismuth Titanate (BixTiyOz) Films Produced through Radiofrequency Magnetron Sputtering.

The applications of Bismuth Titanate (BixTiyOz) materials have been focused on their electronic and optical properties, but with respect to the use of these compounds in applications like corrosion resistance, have been very few or nonexistent. For this reason, in the present investigation BixTiyOz thin films were deposited using RF magnetron sputtering onto silicon wafers, stainless steel 316L, and titanium alloy (Ti6Al4V) substrates, in order to carry out a study of the corrosion behavior of this compound. The structural properties of the coatings were studied through X-ray diffraction (XRD), the morphology was determined using Scanning Electron Microscopy (SEM), the corrosion resistance behavior of the coated and uncoated substrates was evaluated via the Potentiodynamic Polarization technique, and surface chemical composition was evaluated through X-ray photoelectron spectroscopy (XPS). The XRD results indicated that the films were amorphous. The SEM micrographs showed that the deposited films were homogeneous, but in some cases there were cracks. The potentiodynamic polarization technique showed that the corrosion current in the coated substrates decreased by an order of two magnitudes with respect to the uncoated substrates, but in both cases the corrosion mechanism was pitting due to the pores in the film. The XPS analysis shows that the deposited films contain both Bi3+ and Ti4+.

Ultrasmall iron oxide nanoparticles for biomedical applications: improving the colloidal and magnetic properties.

A considerable increase in the saturation magnetization, M(s) (40%), and initial susceptibility of ultrasmall (<5 nm) iron oxide nanoparticles prepared by laser pyrolysis was obtained through an optimized acid treatment. Moreover, a significant enhancement in the colloidal properties, such as smaller aggregate sizes in aqueous media and increased surface charge densities, was found after this chemical protocol. The results are consistent with a reduction in nanoparticle surface disorder induced by a dissolution-recrystallization mechanism.

Valence band circular dichroism in non-magnetic Ag/Ru(0001) at normal emission.

For the non-magnetic system of Ag films on Ru(0001), we have measured the circular dichroism of photoelectrons emitted along the surface normal, the geometry typically used in photoemission electron microscopy. Photoemission spectra were acquired from micrometer-sized regions having uniformly thick Ag films on a single, atomically flat Ru terrace. For a single Ag layer, we find a circular dichroism that exceeds 6% at the d-derived band region around 4.5 eV binding energy. The dichroism decreases as the Ag film thickness increases to three atomic layers. We discuss the origin of the circular dichroism in terms of the symmetry lowering that can occur even in normal emission.