Dynamic magnetic phenomena in fine-particle goethite

The Role of Defects in Fe(II)-Goethite Electron Transfer

Despite substantial experimental evidence for Fe(II)-Fe(III) oxide electron transfer, computational chemistry calculations suggest that oxidation of sorbed Fe(II) by goethite is kinetically inhibited on structurally perfect surfaces. We used a combination of ⁵⁷Fe Mössbauer spectroscopy, synchrotron X-ray absorption and magnetic circular dichroism (XAS/XMCD) spectroscopies to investigate whether Fe(II)-goethite electron transfer is influenced by defects. Specifically, Fe L-edge and O K-edge XAS indicates that the outermost few Angstroms of goethite synthesized by low temperature Fe(III) hydrolysis is iron deficient relative to oxygen, suggesting the presence of defects from Fe vacancies. This nonstoichiometric goethite undergoes facile Fe(II)-Fe(III) oxide electron transfer, depositing additional goethite consistent with experimental precedent. Hydrothermal treatment of this goethite, however, appears to remove defects, decrease the amount of Fe(II) oxidation, and change the composition of the oxidation product. When hydrothermally treated goethite was ground, surface defect characteristics as well as the extent of electron transfer were largely restored. Our findings suggest that surface defects play a commanding role in Fe(II)-goethite redox interaction, as predicted by computational chemistry. Moreover, it suggests that, in the environment, the extent of this interaction will vary depending on diagenetic history, local redox conditions, as well as being subject to regeneration via seasonal fluctuations.

NIR transmittance tuneability under a magnetic field of colloidal suspensions of goethite (α-FeOOH) nanorods

Goethite (α-FeOOH) nanorods were synthesized and their size and shape were controlled by synthesis parameters. Stable colloidal suspensions were prepared and their transmittance in NIR range was tuned by modifying magnetic field direction and strength.

Magnetic and nuclear structure of goethite (α-FeOOH): a neutron diffraction study

The magnetic structure of two natural samples of goethite (α-FeOOH) with varying crystallinity was analyzed at 15 and 300 K by neutron diffraction. The well crystallized sample has thePb′nmcolor space group and remained antiferromagnetic up to 300 K, with spins aligned parallel to thecaxis. The purely magnetic 100 peak, identifying this color space group, was clearly resolved. The nanocrystalline sample shows a phase transition to the paramagnetic state at a temperature below 300 K. This lowering of the Néel temperature may be explained by the interaction of magnetic clusters within particles. The nuclear structure, refined with the Rietveld and pair distribution function methods, is consistent with reports in the literature.

Pressure-induced hydrogen bond symmetrization in iron oxyhydroxide

Under high pressures the hydrogen bonds were predicted to transform from a highly asymmetric soft O-H···O to a symmetric rigid configuration in which the proton lies midway between the two oxygen atoms. Despite four decades of research on hydroxyl containing compounds, pressure induced hydrogen bond symmetrization remains elusive. Following single crystal x-ray diffraction, Mössbauer and Raman spectroscopy measurements supported by ab initio calculations, we report the H-bonds symmetrization in iron oxyhydroxide, FeOOH, resulting from the Fe(3+) high-to-low spin crossover at above 45 GPa.

Au nanorice assemble electrolytically into mesostars

Star-shaped mesotructures are formed when an aqueous suspension of Au nanorice particles, which consist of prolate hematite cores and a thin Au shell, is subjected to an electric current. The nanorice particles assemble to form hyperbranched micrometer-scale mesostars. To our knowledge, this is the first reported observation of nanoparticle assembly into larger ordered structures under the influence of an electrochemical process (H(2)O electrolysis). The assembly is accompanied by significant modifications in the morphology, dimensions, chemical composition, crystallographic structure, and optical properties of the constituent nanoparticles.

Magnetic fluctuations in nanosized goethite (α-FeOOH) grains

Mössbauer spectra of antiferromagnetic goethite (α-FeOOH) particles usually show an asymmetric line broadening, which increases with increasing temperature, although the magnetic anisotropy is expected to be so large that magnetic relaxation effects should be negligible. By use of high resolution transmission electron microscopy we have studied a sample of goethite particles and have found that the particles contain many defects such as low angle grain boundaries, in accordance with previous studies of other samples of goethite particles. Such defects can result in a magnetic mismatch at the grain boundaries between nanometer-sized grains, leading to a weakened magnetic coupling between the grains. We show that the Mössbauer data of goethite can be explained by fluctuations of the sublattice magnetization directions in such weakly coupled grains. It is likely that the influence of defects such as low angle grain boundaries also plays a role with regards to the magnetic properties in other antiferromagnetic nanograin systems. We discuss the results in relation to Mössbauer studies of α-Fe(2)O(3) and α-Fe(2)O(3)/NiO nanoparticles.

Synthesis and optical characterization of submicrometer gold nanotubes grown on goethite rods

Goethite (FeOOH) rods were used as templates for growing gold nanotubes with a length of a few hundred nanometers and an aspect ratio between 3 and 4. Successful uniform growth required surface modification, followed by the attachment of small Au seeds and one-step seeded growth using formaldehyde as a reducing agent, as previously reported for the growth of Au shells on silica spheres and hematite spindles. The thickness and surface roughness of the obtained shells could be adjusted by simply varying the concentration ratio between seeds (modified goethite rods) and growth reagents (HAuCl 4 and formaldehyde). The morphology of the synthesized gold nanotubes was thoroughly characterized by TEM, SEM, and AFM/MFM. The resulting gold nanotubes display well-defined plasmon resonances, with a strong longitudinal mode centered around ca. 1400 nm and a broad band in the visible resulting from the overlap of a transverse mode and a multipolar mode, as was found from theoretical modeling using the boundary element method, which provides reasonable agreement with the experimental results.

The complex phase behaviour of suspensions of goethite (α-FeOOH) nanorods in a magnetic field

In 1902, Majorana reported the magneto-optical properties of aqueous colloidal suspensions of mixed iron oxides. Oddly enough, the magnetic-field induced birefringence displayed a non-monotonic dependence upon field intensity. This behaviour was later interpreted as due to the existence in these sols of at least two different chemical species. During the course of our studies of mineral liquid crystals, we have revisited this problem by examining aqueous suspensions of pure goethite (alpha-FeOOH) nanorods. Although they are comprised of a single chemical species, these suspensions show the same odd behaviour reported by Majorana. Moreover, we show that, as the volume fraction increases, the suspensions have an isotropic liquid/nematic/rectangular columnar phase sequence, with first-order transitions between these phases. The non-monotonic dependence of the field-induced birefringence can be explained by the existence of a remanent magnetic moment of the nanorods and the negative anisotropy of their magnetic susceptibility. Therefore, the nanorods align parallel to a weak field but realign perpendicular to the field beyond Bc approximately 375 mT. In addition, other interesting phenomena appear upon application of a magnetic field: the disordered (i.e. isotropic in zero-field) phase becomes highly anisotropic and difficult to distinguish from the nematic phase. Both phases then acquire not only quadrupolar order but also dipolar order. The rectangular columnar phase is strongly stabilised versus the nematic one. Our experimental observations raise new theoretical questions about the phase diagram of these suspensions with respect to volume fraction and magnetic field intensity.

Physical properties of aqueous suspensions of goethite (alpha-FeOOH) nanorods. Part I: In the isotropic phase

Depending on volume fraction, aqueous suspensions of goethite (alpha-FeOOH) nanorods form a liquid-crystalline nematic phase (above 8.5%) or an isotropic liquid phase (below 5.5%). In this article, we investigate by small-angle X-ray scattering, magneto-optics, and magnetometry the influence of a magnetic field on the isotropic phase. After a brief description of the synthesis and characterisation of the goethite nanorod suspensions, we show that the disordered phase becomes very anisotropic under a magnetic field that aligns the particles. Moreover, we observe that the nanorods align parallel to a small field (< 350 mT), but realign perpendicular to a large enough field (> 350 mT). This phenomenon is interpreted as due to the competition between the influence of the nanorod permanent magnetic moment and a negative anisotropy of magnetic susceptibility. Our interpretation is supported by the behaviour of the suspensions in an alternating magnetic field. Finally, we propose a model that explains all experimental observations in a consistent way.