Fe₃O₄ Nanocrystals Tune the Magnetic Regime of the Fe/Ni Molecular Magnet: A New Class of Magnetic Superstructures

A new class of organometallic–inorganic magnetic material was engineered by a sonochemically assisted self-assembly process between magnetite nanoparticles (biogenic Fe₃O₄, hard constituent) functionalized with isonicotinic acid and a metamagnetic organometallic complex ([Ni­(en)₂]₃[Fe­(CN)₆]₂·3H₂O, soft constituent). In such bottom-up methodology, hard and soft counterparts form well-organized microdimensional clusters that showed morphological fingerprints and magnetic behavior clearly distinct from those of the initial building units. In the engineered soft–hard material, the magnetite nanocrystals induced ferromagnetic ordering at room temperature of closer contact layers of [Ni­(en)₂]₃[Fe­(CN)₆]₂·3H₂O, thus demonstrating the ability to sensibly modify the [Ni­(en)₂]₃[Fe­(CN)₆]₂·3H₂O paramagnetic regime. The magnetic ordering of [Ni­(en)₂]₃[Fe­(CN)₆]₂·3H₂O was triggered by the intrinsic local field of the hard magnetic nanocrystals, which resembled, to some extent, the effects promoted by large, external magnetic fields

Noncovalent Grafting of a Dy^III₂ Single-Molecule Magnet onto Chemically Modified Multiwalled Carbon Nanotubes

While synthetic methods for the grafting of nanoparticles or photoactive molecules onto carbon nanotubes (CNTs) have been developed in the last years, a very limited number of reports have appeared on the grafting of single-molecule magnets (SMMs) onto CNTs. There are many potential causes, mainly focused on the fact that the attachment of molecules on surfaces remains not trivial and their magnetic properties are significantly affected upon attachment. Nevertheless, implementation of this particular type of hybrid material in demanding fields such as spintronic devices makes of utmost importance the investigation of new synthetic protocols for effective grafting. In this paper, we demonstrate a new experimental protocol for the noncovalent grafting of Dy^III₂ SMM, [Dy₂(NO₃)₂(saph)₂(DMF)₄], where H₂saph = <i>N</i>-salicylidene-<i>o</i>-aminophenol and DMF = <i>N</i>,<i>N</i>-dimethylformamide, onto the surface of functionalized multiwalled CNTs (MWCNTs). We present a simple wet chemical method, followed by an extensive washing protocol, where the cross-referencing of data from high-resolution transmission electron microscopy combined with electron energy loss spectroscopy, conventional magnetic measurements (direct and alternating current), X-ray photoelectron spectroscopy, and Raman spectroscopy was used to investigate the physical properties, chemical nature, and overall magnetic behavior of the resulting hybrids. A key point to the whole synthesis involves the functionalization of MWCNTs with carboxylic groups, which proved to be a powerful strategy for enhancing the ability to process MWCNTs and facilitating the preparation of hybrid composites. While in the majority of analogous hybrid materials the raw carbon material (multiwalled or single-walled nanotubes) is heavily treated to minimize the contribution of contaminant traces of magnetic nanoparticles with important effects on their electronic properties, this method can lead easily to elimination of the largest part of the impurities and provide an effective way to investigate/discriminate the magnetic contribution of the SMM molecules

Iron and Iron Oxide Nanoparticles Synthesized with Green Tea Extract: Differences in Ecotoxicological Profile and Ability To Degrade Malachite Green

Iron-based nanoparticles (FeNPs) have been used successfully in water treatment and environmental cleanup efforts. This study examined ecotoxicity of two FeNPs produced with extract from green tea (smGT, GTFe) and their ability to degrade malachite green (MG). Their physicochemical properties were assessed by transmission electron microscopy, X-ray powder diffraction, dynamic light scattering, and transmission Mössbauer spectroscopy. Using a battery of ecotoxicological bioassays, we determined the toxicity for nine different organisms, including bacteria, cyanobacteria, algae, plants, and crustaceans. Iron and iron oxide nanoparticles synthesized with green tea extract displayed low capacity to degrade MG and were toxic to all tested organisms. Superparamagnetic iron oxide nanoparticles (smGT) derived from GTFe showed no toxic effect on most of the tested organisms up to a concentration of 1 g/L, except for algae and cyanobacteria, and removed 93% MG at a concentration of 125 mg of Fe/L after 60 min. The procedure described in this paper generates new nontoxic superparamagnetic iron oxide NPs from existing and toxic GTFe that are endowed with degradative potential for organic compounds. These findings suggest low ecotoxicological risks and the suitability of these green-synthesized FeNPs for environmental remediation purposes

Triggering Two-Step Spin Bistability and Large Hysteresis in Spin Crossover Nanoparticles via Molecular Nanoengineering

The local entrapment of the spin crossover complex Fe­(II)-tris­[2-(2′-pyridyl)­benzimidazole] into the pluronic polymeric matrix (P123, PEG20–PPG70–PEG20, MW ∼ 5800) yielded the formation of magnetic nanoparticles of ∼26 nm (SCO-Np). Formation of SCO-Np was driven by the emergence of noncovalent interactions between the aromatic −NH group of the benzimidazole moieties present in Fe­(II)-tris­[2-(2′-pyridyl)­benzimidazole] with the aliphatic ether (−O−) groups of the pluronic polymeric matrix. The nanoparticles show spin crossover behavior, two-step spin bistability, and wide magnetic hysteresis, expressed in the temperature range of 170–280 K (Δ<i>T</i>_max = 38 K). The neat SCO molecules, Fe­(II)-tris­[2-(2′-pyridyl)­benzimidazole], on the contrary show only first-order spin transition and negligible hysteresis. The developed matrix-confinement approach of SCO molecules shown in this work yielded an unprecedented and significant improvement of the magnetic cooperativity compared to the neat spin crossover system, despite the decreased dimension of the magnetic domain in the nanosized architecture

Ferrate(VI)-Induced Arsenite and Arsenate Removal by In Situ Structural Incorporation into Magnetic Iron(III) Oxide Nanoparticles

We report the first example of arsenite and arsenate removal from water by incorporation of arsenic into the structure of nanocrystalline iron­(III) oxide. Specifically, we show the capability to trap arsenic into the crystal structure of γ-Fe₂O₃ nanoparticles that are in situ formed during treatment of arsenic-bearing water with ferrate­(VI). In water, decomposition of potassium ferrate­(VI) yields nanoparticles having core–shell nanoarchitecture with a γ-Fe₂O₃ core and a γ-FeOOH shell. High-resolution X-ray photoelectron spectroscopy and in-field ⁵⁷Fe Mössbauer spectroscopy give unambiguous evidence that a significant portion of arsenic is embedded in the tetrahedral sites of the γ-Fe₂O₃ spinel structure. Microscopic observations also demonstrate the principal effect of As doping on crystal growth as reflected by considerably reduced average particle size and narrower size distribution of the “in-situ” sample with the embedded arsenic compared to the “ex-situ” sample with arsenic exclusively sorbed on the iron oxide nanoparticle surface. Generally, presented results highlight ferrate­(VI) as one of the most promising candidates for advanced technologies of arsenic treatment mainly due to its environmentally friendly character, in situ applicability for treatment of both arsenites and arsenates, and contrary to all known competitive technologies, firmly bound part of arsenic preventing its leaching back to the environment. Moreover, As-containing γ-Fe₂O₃ nanoparticles are strongly magnetic allowing their separation from the environment by application of an external magnet

Doping with Graphitic Nitrogen Triggers Ferromagnetism in Graphene

Nitrogen doping opens possibilities for tailoring the electronic properties and band gap of graphene toward its applications, e.g., in spintronics and optoelectronics. One major obstacle is development of magnetically active N-doped graphene with spin-polarized conductive behavior. However, the effect of nitrogen on the magnetic properties of graphene has so far only been addressed theoretically, and triggering of magnetism through N-doping has not yet been proved experimentally, except for systems containing a high amount of oxygen and thus decreased conductivity. Here, we report the first example of ferromagnetic graphene achieved by controlled doping with graphitic, pyridinic, and chemisorbed nitrogen. The magnetic properties were found to depend strongly on both the nitrogen concentration and type of structural N-motifs generated in the host lattice. Graphenes doped below 5 at. % of nitrogen were nonmagnetic; however, once doped at 5.1 at. % of nitrogen, N-doped graphene exhibited transition to a ferromagnetic state at ∼69 K and displayed a saturation magnetization reaching 1.09 emu/g. Theoretical calculations were used to elucidate the effects of individual chemical forms of nitrogen on magnetic properties. Results showed that magnetic effects were triggered by graphitic nitrogen, whereas pyridinic and chemisorbed nitrogen contributed much less to the overall ferromagnetic ground state. Calculations further proved the existence of exchange coupling among the paramagnetic centers mediated by the conduction electrons

Ferrate(VI)-Prompted Removal of Metals in Aqueous Media: Mechanistic Delineation of Enhanced Efficiency via Metal Entrenchment in Magnetic Oxides

The removal efficiency of heavy metal ions (cadmium­(II), Cd­(II); cobalt­(II), Co­(II); nickel­(II), Ni­(II); copper­(II), Cu­(II)) by potassium ferrate­(VI) (K₂FeO₄, Fe­(VI)) was studied as a function of added amount of Fe­(VI) (or Fe) and varying pH. At pH = 6.6, the effective removal of Co­(II), Ni­(II), and Cu­(II) from water was observed at a low Fe-to-heavy metal ion ratio (Fe/M­(II) = 2:1) while a removal efficiency of 70% was seen for Cd­(II) ions at a high Fe/Cd­(II) weight ratio of 15:1. The role of ionic radius and metal valence state was explored by conducting similar removal experiments using Al­(III) ions. The unique combination of X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), in-field Mössbauer spectroscopy, and magnetization measurements enabled the delineation of several distinct mechanisms for the Fe­(VI)-prompted removal of metal ions. Under a Fe/M weight ratio of 5:1, Co­(II), Ni­(II), and Cu­(II) were removed by the formation of MFe₂O₄ spinel phase and partially through their structural incorporation into octahedral positions of γ-Fe₂O₃ (maghemite) nanoparticles. In comparison, smaller sized Al­(III) ions got incorporated easily into the tetrahedral positions of γ-Fe₂O₃ nanoparticles. In contrast, Cd­(II) ions either did not form the spinel ferrite structure or were not incorporated into the lattic of iron­(III) oxide phase due to the distinct electronic structure and ionic radius. Environmentally friendly removal of heavy metal ions at a much smaller dosage of Fe than those of commonly applied iron-containing coagulants and the formation of ferrimagnetic species preventing metal ions leaching back into the environment and allowing their magnetic separation are highlighted

Zero-Valent Iron Nanoparticles with Unique Spherical 3D Architectures Encode Superior Efficiency in Copper Entrapment

The large-scale preparation of spherical condensed-type superstructures of zero-valent iron (nZVI), obtained by controlled solid-state reaction through a morphologically conserved transformation of a magnetite precursor, is herein reported. The formed 3D nanoarchitectures (S-nZVI) exhibit enhanced entrapment efficiency of heavy metal pollutants, such as copper, compared to all previously tested materials reported in the literature, thus unveiling the relevance in the material’s design of the morphological variable. The superior removal efficiency of these mesoporous S-nZVI superstructures is linked to their extraordinary ability to couple effectively processes such as reduction and sorption of the metal pollutant