A. C. Oliveira

The influence of cobalt population on the structural properties of CoxFe3−xO4

CoxFe3−xO4-based (x=0.05–1.6) nanoparticles prepared by combustion reaction were investigated using x-ray diffraction, Raman spectroscopy, and Mossbauer spectroscopy. The Mossbauer data revealed Co2+ in both tetrahedral (site A) and octahedral (site B) sites of the cubic ferrite structure. For x⩽0.4 the experimental data indicated the synthesis of a core-shell structure, with hematite as the shell and cobalt ferrite as the core of the nanoparticle. Differences in crystalline structure between the two phases support the evidences we found of a highly stressed core-shell interface, leading to symmetry reduction of the tetrahedral and octahedral sites.

Spectroscopic Study of Maghemite Nanoparticles Surface-Grafted with DMSA

Nanosized maghemite (below 10 nm average diameter), surface-functionalized with meso-2,3-dimercaptosuccinic acid (DMSA), was investigated with respect to the content of DMSA molecules attached onto its surface and the onset of S-S bridges due to oxidation of neighboring S-H groups. To support our investigation, we introduced the use of photoacoustic spectroscopy to monitor thiol groups (S-H) conjugated with Raman spectroscopy to monitor the disulfide bridges (S-S). The normalized intensity (N(R)) of the Raman feature peaking at 500 cm(-1) was used to probe the S-S bridge whereas the normalized intensity (N(P)) of the photoacoustic band-S (0.42-0.65 μm) was used to probe the S-H moiety. The perfect linearity observed in the N(R) versus (1 - N(P)) plot strongly supports the oxidation process involving neighboring S-H groups as the DMSA surface grafting coefficient increases whereas the approach used in this report allows the evaluation of the [S-H]/[S-S] ratio. The observation of the reduction of the hydrodynamic diameter as the nominal DMSA-grafting increases supports the proposed model picture, in which the intraparticle (interparticle) S-S bridging takes place at higher (lower) DMSA-grafting values.

Effects of fatty acid surfactants on the magnetic and magnetohydrodynamic properties of ferrofluids

We prepared Fe3O4 magnetic nanoparticles having diameters of approximately 12 nm by chemical coprecipitation, which were coated with three different fatty acid surfactants: oleic acid, lauric acid, and myristic acid. From x-ray diffraction, transmission electron microscopy, and Mossbauer spectroscopy measurements we confirmed that Fe3O4 is the only phase present in the samples. The zero field cooled magnetization curves for the nanoparticles exhibit broad peaks, consistent with superparamagnetic blocking for the polydisperse samples, and a saturation magnetization smaller than that for bulk Fe3O4. Although there are minimal differences in the magnetic properties of the nanoparticles having different surfactants, we find significant changes in the hydrodynamic response depending on chain length. Hyperthermia measurements show considerably larger response for oleic acid-coated samples, while magneto-optical studies indicate that these samples have slower dynamics of aggregation under the influence of a dc f...

Effect of the Zn content in the structural and magnetic properties of ZnxMg1−xFe2O4 mixed ferrites monitored by Raman and Mössbauer spectroscopies

Samples of ZnxMg1−xFe2O4 (0≤x≤1) synthesized by the combustion reaction method were investigated by x-ray diffraction, Mossbauer spectroscopy, and Raman spectroscopy. All the samples are found to have a cubic spinel structure and the lattice parameter increases linearly with increasing Zn-content (x). The Mossbauer data showed that the replacement of Mg2+ ions for Zn2+ ions changes substantially the hyperfine parameter. Moreover, it was verified the presence of Fe3+ ions both in A and B sites. The Raman spectra showed five predicted Raman bands for the spinel structure and it was observed the splitting of the A1g Raman mode into tree branches, where each one have been attributed to peaks belonging to each ion (Zn, Mg, and Fe) in the tetrahedral positions.

Photoacoustic spectroscopy: a promising technique to investigate magnetic fluids

In the present study, we investigate biocompatible magnetic fluids (BMFs) by photoacoustic spectroscopy and the investigation are based on magnetic nanoparticles surface-coated with citric acid and dispersed in physiological media (pH8 and 0.9 salinity).

Characterization of FeMCM-41 and FeZSM-5 catalysts to styrene production

FeMCM-41 and FeZSM-5 catalysts have been prepared and tested in the dehydrogenation of ethylbenzene to produce styrene. These new catalytic systems have high specific areas and can stabilize the trivalent state of iron. It was found that the FeZSM-5 catalysts are more active and selective than the FeMCM-41 ones and this behavior is explained in terms of the higher amount of the active oxidation state (Fe 3+ ) on the catalyst surface. These catalysts also result more active and selective than α-Fe 2 O 3 (hematite) and are less toxic than the chromium-containing commercial catalysts used for this purpose.

Study of magnetic susceptibility of magnetite nanoparticles

In this study dynamic susceptibility (DS) was used to investigate magnetite nanoparticles dispersed in styrene-divinylbenzene copolymer. Transmission electron microscopy (TEM) data have been used to support the DS data analysis. The field dependence of the peak position of the imaginary component of DS was analyzed using a picture of an asymmetric double well potential for the relaxation of the magnetic moment associated with the magnetite nanoparticle. The size dependence of the magnetic susceptibility was included in the data analysis. Nanoparticle-size parameters obtained from the analysis of the DS data (19.1 and 18.2 nm) are in excellent agreement with the values obtained from the fitting of the TEM data (19.2 and 20.1 nm).

Facile approach to suppress γ-Fe2O3 to α-Fe2O3 phase transition beyond 600 °C in Fe3O4 nanoparticles

Magnetic iron oxide nanoparticles on a zeolite template have been synthesized using wet chemical approach. The average particle size initially decreases from 8.5 to 6 nm (increasing zeolite concentration from 0 to 75 mg) but increases to 11 nm for higher zeolite concentration (100 mg). Room temperature magnetization curves show an initial decrease in saturation magnetization from 62 to 42 emu per gram due to decrease in particle size as well as increase in contribution from nonmagnetic zeolite template. Further increase in zeolite concentration to 100 mg results in a significant increase in saturation magnetization from 42 to 51 emu per gram. Calorimetric studies show a continuous enhancement in γ-Fe2O3 to α-Fe2O3 phase transition temperature from 590 to 715 °C by increasing the zeolite concentration from 0 to 75 gm. The exothermic peak corresponding to the γ-Fe2O3 to α-Fe2O3 phase transition has been completely suppressed for nanoparticles prepared in presence of 100 mg of zeolite. Mossbauer spectra of as-synthesized nanoparticles show an increase of superparamagnetic components from 7 to 36% corresponding to increase in zeolite concentration from 0 to 100 mg. Mossbauer spectra of pure Fe3O4 nanoparticles annealed at 500 °C shows formation of pure α-Fe2O3 phase and Mossbauer spectra of particles prepared in presence of 25 mg shows only 18% of α-Fe2O3 phase after annealing at 550 °C. Further increase in zeolite concentration to 50 and 75 mg (annealed at 550 °C) leads to pure γ-Fe2O3. Annealing of Fe3O4 nanoparticles prepared in the presence of 100 mg of zeolite at 650 °C shows formation of only 8% α-Fe2O3 phase. Our results show an easy and effective method to enhance the thermal stability of magnetic iron oxide nanoparticles making it suitable for high temperature applications.

Evaluation of Fe/MCM-41 catalysts in the water gas shift reaction

Abstract The demand for high purity hydrogen is largely met by the water gas shift reaction. It is favored by low temperature and excess of steam due to its reversibility and exothermicity, which requires two steps in industrial processes. In the range of 320-450°C (high temperature shift, HTS), the reaction is carried out over hematite-based catalysts, which show high stability in performance but loose specific surface area with time, demanding for new catalysts. To fit this requirement, Fe/MCM-41 catalysts with different iron contents were studied in this work. Hematite and magnetite nanoparticles on MCM-41 were found in the fresh and in the spent catalysts, respectively. It was noted that the specific surface areas decreased with the increase of iron in solids, due to the presence of the hematite nanoparticles in the pores. Supported-hematite nanoparticles were more easily reduced than hematite and thus produced more easily the active phase (magnetite). The most active catalyst was the sample with the highest amount of iron, which was the most resistant against sintering. Also, it was more active than a commercial catalyst and thus is promising for industrial applications with the advantage of not being toxic.

Optical and magnetic properties of Co-doped ZnO nanoparticles and the onset of ferromagnetic order

In this study, we report on the optical and magnetic properties of Co-doped ZnO nanoparticles with increasing Co-content (CoxZn1−xO; x = 0.000, 0.005, 0.010, 0.030, 0.050, 0.070, and 0.100) synthesized by the combustion reaction method. The X-ray diffraction patterns and the Raman spectra of all samples indicated the formation of the ZnO hexagonal wurtzite phase (space group C46V). The Raman data also show the formation of a secondary Co3O4 phase, which is barely seen in the X-ray spectra. Photoacoustic spectroscopy and electron paramagnetic resonance confirm the presence of the two phases (CoxZn1−xO and Co3O4). Vibrating sample magnetometer measurements performed at room temperature exhibited hysteresis loops, indicating the presence of long-range magnetic ordering in the samples. Analysis of the magnetization as a function of magnetic field and temperature shows that the ferromagnetism in the as-synthesized samples comes from small Co-metallic inclusions, with an estimated radius of about 4.8 nm and blo...