Alexios P. Douvalis

Ferromagnetism in Fe-doped SnO2 thin films

Thin films grown by pulsed-laser deposition from targets of Sn0.95Fe0.05O2 are transparent ferromagnets with Curie temperature and spontaneous magnetization of 610 K and 2.2 A m2 kg−1, respectively. The 57Fe Mossbauer spectra show the iron is all high-spin Fe3+ but the films are magnetically inhomogeneous on an atomic scale, with only 23% of the iron ordering magnetically. The net ferromagnetic moment per ordered iron ion, 1.8 μB, is greater than for any simple iron oxide with superexchange interactions. Ferromagnetic coupling of ferric ions via an electron trapped in a bridging oxygen vacancy (F center) is proposed to explain the high Curie temperature.

Nanoscale zero-valent iron supported on mesoporous silica: Characterization and reactivity for Cr(VI) removal from aqueous solution

MCM-41-supported nanoscale zero-valent iron (nZVI) was sytnhesized by impregnating the mesoporous silica martix with ferric chloride, followed by chemical reduction with NaHB4. The samples were studied with a combination of characterization techniques such as powder X-ray diffraction (XRD), Fourier-transform infrared (FT-IR) and Mössbauer spectroscopy, N2 adsorption measurements, transmission electron microscopy (TEM), magnetization measurements, and thermal analysis methods. The experimental data revealed development of nanoscale zero-valent iron particles with an elliptical shape and a maximum size of ∼80 nm, which were randomly distributed and immobilized on the mesoporous silica surface. Surface area measurements showed that the porous MCM-41 host matrix maintains its hexagonal mesoporous order structure and exhibits a considerable high surface area (609 m(2)/g). Mössbauer and magnetization measurements confirmed the presence of core-shell iron nanoparticles composed of a ferromagnetic metallic core and an oxide/hydroxide shell. The kinetic studies demonstrated a rapid removal of Cr(VI) ions from the aqueous solutions in the presence of these stabilized nZVI particles on MCM-41, and a considerably increased reduction capacity per unit mass of material in comparison to that of unsupported nZVI. The results also indicate a highly pH-dependent reduction efficiency of the material, whereas their kinetics was described by a pseudo-first order kinetic model.

SnO2 doped with Mn, Fe or Co: Room temperature dilute magnetic semiconductors

Room temperature ferromagnetism is found in (Sn1−xMx)O2 (M=Mn, Fe, Co, x=0.05) ceramics where x-ray diffraction confirms the formation of a rutile-structure phase. Room temperature saturation magnetization of 0.2 and 1.8 Am2 kg−1 for (Sn0.95Mn0.05)O2 and (Sn0.95Fe0.05)O2, respectively, corresponds to a moment of 0.11 or 0.95 μB per Mn or Fe atom. The Curie temperatures are 340 and 360 K, respectively. The magnetization cannot be attributed to any identified impurity phase. 57Fe Mossbauer spectra of the Fe-doped SnO2 samples, recorded at room temperature and 16 K, show that about 85% of the iron is in a magnetically ordered high spin Fe3+ state, the remainder being paramagnetic.

Ferromagnetism of a graphite nodule from the Canyon Diablo meteorite.

There are recent reports of weak ferromagnetism in graphite and synthetic carbon materials such as rhombohedral C60 (ref. 4), as well as a theoretical prediction of a ferromagnetic instability in graphene sheets. With very small ferromagnetic signals, it is difficult to be certain that the origin is intrinsic, rather than due to minute concentrations of iron-rich impurities. Here we take a different experimental approach to study ferromagnetism in graphitic materials, by making use of meteoritic graphite, which is strongly ferromagnetic at room temperature. We examined ten samples of extraterrestrial graphite from a nodule in the Canyon Diablo meteorite. Graphite is the major phase in every sample, but there are minor amounts of magnetite, kamacite, akaganéite, and other phases. By analysing the phase composition of a series of samples, we find that these iron-rich minerals can only account for about two-thirds of the observed magnetization. The remainder is somehow associated with graphite, corresponding to an average magnetization of 0.05 Bohr magnetons per carbon atom. The magnetic ordering temperature is near 570 K. We suggest that the ferromagnetism is a magnetic proximity effect induced at the interface with magnetite or kamacite inclusions.

Synthesis and Characterization of γ-Fe2O3/Carbon Hybrids and Their Application in Removal of Hexavalent Chromium Ions from Aqueous Solutions

Magnetic Fe(2)O(3)/carbon hybrids were prepared in a two-step process. First, acetic acid vapor interacted with iron cations dispersed on the surface of a nanocasted ordered mesoporous carbon (CMK-3). In the second step, the primarily created iron acetate species underwent pyrolysis and transformed to magnetic iron oxide nanoparticles. X-ray diffraction, Fourier-transform infrared, and Raman spectroscopies were used for the chemical and structural characterization of the hybrids, while surface area measurements, thermal analysis, and transmission electron microscopy were employed to determine their physical, surface, and textural properties. These results revealed the preservation of the host carbon structure, which was homogenously and controllably loaded (up to 27 wt %) with nanosized (ca. 20 nm) iron oxides inside the mesoporous system. Mössbauer spectroscopy and magnetic measurements at low temperatures confirmed the formation of γ-Fe(2)O(3) nanoparticles exhibiting superparamagnetic behavior. The kinetic studies showed a rapid removal of Cr(VI) ions from the aqueous solutions in the presence of these magnetic mesoporous hybrids and a considerably increased adsorption capacity per unit mass of sorbent in comparison to that of pristine CMK-3 carbon. The results also indicate highly pH-dependent sorption efficiency of the hybrids, whereas their kinetics was described by a pseudo-second-order kinetic model. Taking into account the simplicity of the synthetic procedure and possibility of magnetic separation of hybrids with immobilized pollutant, the developed mesoporous nanomaterials have quite real potential for applications in water treatment technologies.

Zero-valent iron/iron oxide-oxyhydroxide/graphene as a magnetic sorbent for the enrichment of polychlorinated biphenyls, polyaromatic hydrocarbons and phthalates prior to gas chromatography–mass spectrometry

A composite magnetic material consisting of zero-valent iron, iron oxide-oxyhydroxide and graphene was synthesized and used successfully as a sorbent for the micro solid-phase extraction of PAHs, PCBs and phthalic acid esters. The components endow the composite with multiple characteristics such as adsorption capability and facile removal due to its magnetic properties. Due to the π-π electrostatic stacking property of graphene, the high specific surface area and the adsorption capability of both components, the resulting black flaky Fe(0)/iron oxide-oxyhydroxide/graphene composite showed high extraction efficiency for the target analytes from water samples. Compared with the neat graphene, the composite material has improved properties in terms of microextraction capabilities as both the hydrophobic graphene and zero-valent iron participate in the adsorption of the hydrophobic molecules. The precision from the extraction of all three groups of compounds was lower than 7% and the recoveries were from 90 to 93% from a spiked lake water sample. The high recoveries in relation to the low final volume of the desorption solvent ensure high preconcentration efficiency and a promising sorbent for analytical applications.

Biomimetic Multifunctional Porous Chalcogels as Solar Fuel Catalysts

Biological systems that can capture and store solar energy are rich in a variety of chemical functionalities, incorporating light-harvesting components, electron-transfer cofactors, and redox-active catalysts into one supramolecule. Any artificial mimic of such systems designed for solar fuels production will require the integration of complex subunits into a larger architecture. We present porous chalcogenide frameworks that can contain both immobilized redox-active Fe(4)S(4) clusters and light-harvesting photoredox dye molecules in close proximity. These multifunctional gels are shown to electrocatalytically reduce protons and carbon disulfide. In addition, incorporation of a photoredox agent into the chalcogels is shown to photochemically produce hydrogen. The gels have a high degree of synthetic flexibility, which should allow for a wide range of light-driven processes relevant to the production of solar fuels.

Photocatalytic Hydrogen Evolution from FeMoS-Based Biomimetic Chalcogels

The naturally abundant elements used to catalyze photochemical processes in biology have inspired many research efforts into artificial analogues capable of proton reduction or water oxidation under solar illumination. Most biomimetic systems are isolated molecular units, lacking the protective encapsulation afforded by a proteins tertiary structure. As such, advances in biomimetic catalysis must also be driven by the controlled integration of molecular catalysts into larger superstructures. Here, we present porous chalcogenide framework materials that contain biomimetic catalyst groups immobilized in a chalcogenide network. The chalcogels are formed via metathesis reaction between the clusters [Mo(2)Fe(6)S(8)(SPh)(3)Cl(6)](3-) and [Sn(2)S(6)](4-) in solution, yielding an extended, porous framework structure with strong optical absorption, high surface area (up to 150 m(2)/g), and excellent aqueous stability. Using [Ru(bpy)(3)](2+) as the light-harvesting antenna, the chalcogels are capable of photocatalytically producing hydrogen from mixed aqueous solutions and are stable under constant illumination over a period of at least 3 weeks. We also present improved hydrogen yields in the context of the energy landscape of the chalcogels.

Nitrogenase-mimic iron-containing chalcogels for photochemical reduction of dinitrogen to ammonia

Significance In nature, nitrogenase fixes nitrogen into biologically usable forms under ambient conditions. Today, half of the world’s nitrogen fixation is achieved through the industrial Haber–Bosch process, which operates at elevated temperature and pressure. Here, we present a synthetic nitrogenase mimic in the form of chalcogel composed of molybdenum and iron-containing biomimetic clusters that can accomplish photocatalytic N2 fixation and conversion to NH3 at ambient temperature and pressure. Surprisingly, the iron–sulfur chalcogels without molybdenum are observed to have a higher activity toward N2 reduction. The results reported here will greatly expand the scope of materials design and engineering for the creation of highly active iron-based N2 reduction catalysts operating in mild conditions. A nitrogenase-inspired biomimetic chalcogel system comprising double-cubane [Mo2Fe6S8(SPh)3] and single-cubane (Fe4S4) biomimetic clusters demonstrates photocatalytic N2 fixation and conversion to NH3 in ambient temperature and pressure conditions. Replacing the Fe4S4 clusters in this system with other inert ions such as Sb3+, Sn4+, Zn2+ also gave chalcogels that were photocatalytically active. Finally, molybdenum-free chalcogels containing only Fe4S4 clusters are also capable of accomplishing the N2 fixation reaction with even higher efficiency than their Mo2Fe6S8(SPh)3-containing counterparts. Our results suggest that redox-active iron-sulfide–containing materials can activate the N2 molecule upon visible light excitation, which can be reduced all of the way to NH3 using protons and sacrificial electrons in aqueous solution. Evidently, whereas the Mo2Fe6S8(SPh)3 is capable of N2 fixation, Mo itself is not necessary to carry out this process. The initial binding of N2 with chalcogels under illumination was observed with in situ diffuse-reflectance Fourier transform infrared spectroscopy (DRIFTS). 15N2 isotope experiments confirm that the generated NH3 derives from N2. Density functional theory (DFT) electronic structure calculations suggest that the N2 binding is thermodynamically favorable only with the highly reduced active clusters. The results reported herein contribute to ongoing efforts of mimicking nitrogenase in fixing nitrogen and point to a promising path in developing catalysts for the reduction of N2 under ambient conditions.

Structural and magnetic properties of (Sr2−xCax)FeReO6

The compounds (Sr2−xCax)FeReO6 (x=0, 0.2, 0.5, 1.0, 1.5, and 2.0) were synthesized by solid state reaction. The resistivity exhibits a metallic behavior for Sr2FeReO6 and insulating behavior for Ca2FeReO6. Structural transformation is observed from tetragonal (x⩽1) to monoclinic (x⩾1.5). The Curie temperature increases from 405 K for x=0 to 539 K for x=2. The saturation magnetic moment is always less than the 3 μB anticipated for a ferrimagnetic configuration of Fe3+ and Re5+, suggesting some antisite disorder. All compounds exhibit significant coercivity, which increases with increasing Ca content, from 0.2 T for x=0 to 1.1 T for x=2. The unexpectedly large coercivity in these compounds is attributed to intrinsic magnetic anisotropy of the Re5+ ions. Mossbauer spectra indicate a small admixture of 0.2–0.3 electrons in 3d↓(Fe)t2g orbitals.