Maria do Carmo Martins Alves

Nanosized powders of NiZn ferrite: Synthesis, structure, and magnetism

The structure and magnetic behavior of nanostructured powders of stoichiometric NiZn ferrite, Ni0.5Zn0.5Fe2O4, synthesized by coprecipitation, are investigated by extended x-ray-absorption fine structure spectroscopy (EXAFS), x-ray diffraction, Mossbauer spectroscopy, and vibrating sample magnetometry. Samples of high purity and high homogeneity were obtained by annealing at relatively low temperatures (300–800 °C) resulting in nanoparticles with average diameter between 9 and 90 nm, as determined by x-ray diffraction. EXAFS was applied to follow Ni, Zn, and Fe cations distribution and the evolution of the short range order of the samples with increasing annealing temperature. Our results show ferrimagnetic NiZn ferrite nanosized powders with high purity, 1:1 Ni to Zn stoichiometric ratio and superparamagnetic behavior. Moreover, the samples exhibit good structural ordering already after heat treatment at 400 °C. Analysis by vibrating sample magnetometry indicated a critical particle diameter for the tran...

Incorporation of Ni into natural goethite: An investigation by X-ray absorption spectroscopy

Abstract Goethite (α-FeOOH) is abundant at the Earth’s surface and has the unusual capacity to adsorb and fix ions from migrating solutions. Understanding the mechanisms by which foreign elements are incorporated into natural goethite has implications for environmental and mining problems. X-ray absorption spectroscopy (XAS) was used to obtain structural information on the local environment around Ni in natural Ni-containing goethite (1.8-4.1 mol% Ni) from Vermelho lateritic deposit of Serra dos Carajás (Brazil) and in synthetic analogues. The data were collected at the LNLS XAS beam line at the Ni and Fe K-edges, at room temperature, and at the Ni K-edge at 8 K. Nickel was found in essentially the same environment in all natural and synthetic samples, with negligible thermal disorder. The coordination polyhedron is a tetragonal dipyramid of oxygen atoms showing that Ni preserves its usual local symmetry. This finding is compatible with a model in which substitution of Ni for Fe is accompanied by a proton capture resulting in NiO2(OH)4 octahedra. The polyhedral linkages are similar to that of pure a-FeOOH, consisting of four shared edges at about the same metal-metal distances, as in the pure mineral. The third and longest metal-metal distance is about 6% larger than the expected corner-sharing distance in the α-FeOOH structure, showing that incorporation of Ni locally distorts and opens the structure.

Structural characterization of the Co2FeZ (Z=Al, Si, Ga, and Ge) Heusler compounds by x-ray diffraction and extended x-ray absorption fine structure spectroscopy

This work reports on the structure of Fe containing, Co2-based Heusler compounds that are suitable for magnetoelectronic applications. The compounds Co2FeZ (where Z=Al, Si, Ga, and Ge) were investigated using the x-ray diffraction (XRD) and extended x-ray absorption fine structure (EXAFS) techniques. Using XRD, it was shown conclusively that Co2FeAl crystallizes in the B2 structure whereas Co2FeSi crystallizes in the L21 structure. For compounds containing Ga or Ge, the XRD technique cannot be used to easily distinguish between the two structures. For this reason, the EXAFS technique was used to elucidate the structure of these two compounds. Analysis of the EXAFS data indicated that both compounds crystallize in the L21 structure.

Environment of hafnium and silicon in Hf-based dielectric films: An atomistic study by x-ray absorption spectroscopy and x-ray diffraction

The atomic structure of HfSiO and HfSiON was investigated before and after thermal annealing using x-ray diffraction and x-ray absorption spectroscopy. In HfSiO, the Hf atoms are arranged in a monoclinic HfO2 structure with Hf as second nearest neighbors, while Si is in a SiO2 environment. Thermal annealing induces crystallization of HfSiO with subtle changes in Hf–Hf distances. In the case of HfSiON, a stable structure is observed around the Hf atoms, which remains unaffected after annealing. Nitrogen is present in the first coordination shell of the Hf atoms, with Si in a SiON environment.

Structural properties of the quaternary Heusler alloy Co2Cr1-xFexAl

The quarternary substitutional series Co2Cr1?xFexAl was investigated by means of surface and bulk sensitive techniques in order to exploit its structural and compositional properties. Both bulk and powder samples of the alloy series were investigated to obtain specific information about this material.The long range order was determined by means of x-ray diffraction and neutron diffraction, while the site specific (short range) order was proved by extended x-ray absorption fine structure spectroscopy. The magnetic structure was investigated by M?ssbauer spectroscopy in transmission and scattering modes in order to compare and separate powder and bulk properties. The chemical composition was analysed by means of x-ray photo emission spectroscopy combined with Auger electron spectroscopy depth profiling. The results from these methods are compared to get an insight into the differences between surface and bulk properties and the appearance of disorder in such alloys.The material shows an extremely high sensitivity to oxygen. In particular, powder materials show a high amount of oxygen contamination. Therefore, an additional oxide-mediated tunnel magneto-resistance may always contribute to measurements of magneto-resistive effects because the oxide layers will provide natural tunnelling barriers. In addition, the results suggest that thin films have to be produced under ultra-high vacuum conditions.

X-ray absorption and XPS study of titanium mixed oxides synthesized by the sol–gel method

Abstract The mixed oxides SiO 2 /TiO 2 and SiO 2 /TiO 2 /Sb 2 O 5 were prepared with different Ti and Sb amounts and investigated by X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) at the Ti K edge. There is no evidence for structural modification of titanium environment in the samples having between 5.6 and 20.3 wt% of TiO 2 . All the experimental evidence indicates that titanium has a local environment close to that of TiO 2 anatase. XAS show that this similarity is extends up to the next neighbouring Ti shells. XPS analysis shows that Ti and Sb are present as TiO 2 and Sb 2 O 5 .

Synthesis and characterization of SiO2–CrO3, SiO2–MoO3, and SiO2–WO3 mixed oxides produced using the non-hydrolytic sol–gel process

Silica-based mixed oxide xerogels, namely SiO2–CrO3, SiO2–MoO3, and SiO2–WO3, were prepared using the non-hydrolytic sol–gel process. The materials were synthesized using metal chloride:tetraethoxysilane (TEOS) molar ratios of 0.1:2; 0.2:2 and 0.4:2 for each metal chloride and 1:2 SiCl4:TEOS molar ratio. All of the xerogels containing Cr, Mo or W had considerably greater surface areas than that of SiO2. The small angle X-ray scattering experiments suggest that the surface roughness of the aggregates in SiO2–CrO3 is less than that of SiO2–MoO3 and SiO2–WO3. The morphological characteristics of the silica-based mixed oxide xerogels were not affected by the nature and amount of metal chloride employed in the synthesis. An irregular morphology was observed for SiO2–CrO3, SiO2–MoO3 and SiO2–WO3, but a lamellar structure was observed for SiO2. X-ray photoelectron spectroscopy analysis suggests that tungsten species were preferentially distributed on the outmost part of the grain. The resulting particle diameter was shown to be lower for the mixed oxides compared to that of bare silica. Furthermore, the presence of metals (Cr, Mo and W) on silica caused a decrease in the size of the particles as the atomic radii of these metals increased. According to the Fourier transform infrared spectroscopy and Raman, Cr, Mo and W were incorporated within the silica framework.

Pd–M/C (M = Pd, Cu, Pt) Electrocatalysts for Oxygen Reduction Reaction in Alkaline Medium: Correlating the Electronic Structure with Activity

The increasing global needs for clean and renewable energy have fostered the design of new and highly efficient materials for fuel cells applications. In this work, Pd-M (M = Pd, Cu, Pt) and Pt nanoparticles were prepared by a green synthesis method. The carbon-supported nanoparticles were evaluated as electrocatalysts for the oxygen reduction reaction (ORR) in alkaline medium. A comprehensive electronic and structural characterization of these materials was achieved using X-ray diffraction, high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy. Their electrochemical properties were investigated by cyclic voltammetry, while their activities for the ORR were characterized using steady-state polarization experiments. The results revealed that the bimetallic nanoparticles consist of highly crystalline nanoalloys with size around 5 nm, in which the charge transfer involving Pd and M atoms affects the activity of the electrocatalysts. Additionally, the samples with higher ORR activity are those whose d-band center is closer to the Fermi level.

Structural characterization of ZrN implanted with high Co fluences

Abstract Films of ZrN were prepared using Zr sputtering assisted by N bombardment on heated substrates. They were submitted to Co implantation at 300 K with various fluences to reach concentrations as high as 10%–30% in the doped zone. The long and short range orders were studied via X-ray absorption spectroscopy performed at the Zr and Co K edges and X-ray diffraction. ZrN appears stable under the amount of energy deposited in the lattice, no amorphization is observed. Co ions precipitate to form clusters. This is interpreted as due to the great stability of the matrix together with the fact that Co–N bonds are not chemically favoured. Such a precipitation was already seen in the case of Cu and Ni implantation in the AlN matrix.

Interacting Superparamagnetic Iron(II) Oxide Nanoparticles: Synthesis and Characterization in Ionic Liquids

Interacting superparamagnetic iron(II) oxide nanoparticles (NPs) with sizes of 5.3 ± 1.6 nm were prepared by simple decomposition of [Fe(COT)2] (COT = 1,3,5,7-cyclooctatetraene) with 5 bar of H2 in 1-n-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (BMI·NTf2) ionic liquid (IL). The static and dynamic magnetic characterization revealed a superparamagnetic behavior with weak dipolar interactions of these NPs. In situ structural studies by X-ray absorption spectroscopy demonstrated that they consist of nanostructured FeO. This approach is an appropriate method to prepare and stabilize nanostructured FeO particles, where the presence of an IL proved to be fundamental to suppress the aggregation and usual overoxidation of the FeO NPs.