Fe-oxide Shell Research Articles

Millisecond self-heating and quenching synthesis of Fe/carbon nanocomposite for superior reductive remediation

Fe0-based nanomaterials are extensively applied in environmental remediation, but their passivated oxide shell restricts deep application. However, efforts aimed at revitalizing Fe-oxide shells have shown limited success. Here, we report a “faster win fast” approach by preferential carbon layer deposition in milliseconds to block Fe-oxide shell growth via carbon-assisted flash Joule heating (C-FJH) reaction. C-FJH induced ultra-high temperature and electric shock promoted reductive Fe formation and subsequently melted to a phase-fusional heterostructure (Fe0/FeCl2). Therefore, theoretical calculation confirmed that electron delocalization effect of derived heterostructure promoted electron transfer. Synchronously, rapid self-heating/quenching rate (∼102 K/ms) realized a thin aromatic-carbon layer deposition to sustain both high stability and activity of reductive Fe. The channels of thin aromatic-carbon layer favored inward diffusion of pollutants, which facilitated the subsequent reduction. Accordingly, derived heterostructure and carbon layer jointly contributed to the boosted removal of multiple pollutants (including metal oxyanions, perfluorinated compounds, and disinfection by-products).

Effect of structure of Pd@Fe core–shell cubes on the enhancement of H2 conversion in direct reaction of H2 and O2

Structure engineering is an integrated strategy to regulate the performance of nanoparticles for various catalytic reactions, since the nanoparticle composition, shape, and interface affect the reaction activity and selectivity. Herein, we report a simple synthesis of clear core–shell structured Pd@Fe nanocubes with different shell thicknesses, which exhibit higher H2 conversion efficiency in direct hydrogen peroxide generation compared with ordinary Pd cubes. This result can be attributed to two main reasons: (i) the conformal growth of the Fe oxide shell consisting of loose structured Fe3O4 favors H2 penetration, and (ii) the capping agent (Br−) on the Pd–Fe3O4 interface that blocks the Pd active sites is removed during the shell formation. Therefore, the Fe oxide shell of the Pd@Fe nanoparticles not only helps maintaining the morphology of the nanoparticles but also increases the number of active sites on the inner core surface, confirming the advantage of the core–shell structure.

Synthesis of highly stable Fe/FeOx@citrate colloids with strong magnetic response by mechanochemistry and coprecipitation for biomedical and environmental applications

We present a simple, scalable, and low-cost route for producing aqueous colloids of nanometer size Fe cores coated with iron oxide and stabilized with sodium citrate. The Fe cores were obtained by mechanosynthesis by means of a highly exothermic solid-state reaction between FeCl3 and Mg, in a dispersive inert NaCl medium. The optimal experimental conditions for achieving a full reaction were determined with a Retsch 2000 oscillatory mill. Then the production yield was successfully 16-fold scaled using a rotatory Fritsch Pulverissete 7 mill. To provide biocompatibility and facilitate colloid stabilization, the Fe cores were coated with a Fe-oxide shell produced by chemical coprecipitation, and finally, the system Fe/FeOx was functionalized with sodium citrate in water. The so-obtained Fe/FeOx@Cit (4/3 < x < 3/2) nanoparticles have a mean size of about 11–12 nm, specific saturation magnetization around 130 Am2/kgFe and ζ -potential of −40 mV. The time dependence of the reaction progress could be described by a simple function of the mass ratio of milling balls to processed material and time. The resultant colloids are excellent candidates for biomedical and environmental remediation applications.

Advances in the Synthesis and Long‐Term Protection of Zero‐Valent Iron Nanoparticles

AbstractCore@shell Fe@Fe3O4 nanoparticles (NPs) are synthesized via the thermal decomposition of iron pentacarbonyl (Fe(CO)5) in the presence either of oleylamine (OAm) or a mixture of OAm and oleic acid (OA). The heterostructured nanocomposites formed do so by a postsynthetic modification of isolated Fe seeds. This proves the versatility of the coating procedure and represents a significant advantage over previous work with Co seeds owing to the higher magnetic susceptibility, reduced toxicity, and excellent biocompatibility of Fe. Furthermore, the latter system allows the synthetic methodology to be developed from a two‐pot scenario where seeds are isolated then coated, to an easier and more efficient direct one‐pot scenario. The two‐pot method yields proportionately larger cores. However, in both cases, the monodisperse product reveals a carbonaceous interface between the Fe core and oxide shell. Meanwhile for the one‐pot synthesis, the OA:OAm ratio influences both the morphology and dispersity of the product. This is interpreted in terms of competing interactions of the ligands with the iron precursor. Superparamagnetism (SPM) is observed, and microscopic studies reveal oxidative stability of the Fe(0) cores achieved by either method for &gt;6 months. It is proposed that the carbonaceous interface is critical to this sustained oxidative stability.

High molecular weight components of natural organic matter preferentially adsorb onto nanoscale zero valent iron and magnetite

Nanoscale zero valent iron particles (nano-Fe0) are attractive for in-situ groundwater remediation due to their high reactivity and ability to degrade many different classes of environmental contaminants. It is expected that adsorbed natural organic matter (NOM), which is heterogeneous and typically has a wide molecular weight (MW) distribution, will affect the reactivity and performance of nano-Fe0 as a remediation agent. However, the interaction of NOM with nano-Fe0 has not been well-studied. In this study, we used high performance size exclusion chromatography (HPSEC) to determine if there was preferential sorption of the high MW fraction of NOM onto nano-Fe0 that have a Fe0 core and a Fe-oxide shell (predominantly magnetite). Adsorption of two types of NOM, Suwannee River Humic Acid (SRHA) and Fulvic Acid (SRFA), to nano-Fe0 was compared to magnetite of similar size (nano-Fe3O4) to also assess the effect of the Fe0 core on adsorption of NOM. The results showed that the surface area normalized adsorbed mass (mg/m2) of both SRHA and SRFA onto nano-Fe0 is almost three times than that of nano-Fe3O4. This is attributed to a greater number of reactive sites on nano-Fe0 compared to nano-Fe3O4, and indicates that the surface properties of nano-Fe0 are different that nano-Fe3O4 despite the shell of magnetite on nano-Fe0. The sorption capacity of both SRHA and SRFA onto nano-Fe0 were similar. However, the intermediate sized MW fractions (2–6 kDa) of SRHA were preferentially adsorbed onto the nano-Fe0 surface, whereas the large MW fractions (>3.5 kDa) of SRFA were preferentially adsorbed. These results suggest that NOM interaction with nano-Fe0 are a function of the MW distribution of the NOM in the system studied and indicate that the MW distributions of NOM should be taken into consideration when predicting the fate and performance of nano-Fe0 in environmental remediation.

Synthesis, characterization, and evaluation of iron nanoparticles as hydrogenation catalysts in alcohols and tetraalkylphosphonium ionic liquids: do solvents matter?

Best recyclability seen for Fe nanoparticles in alcohol solvents in which Fe-core, Fe oxide shell nanoparticles are formed.

X-ray Absorption Spectroscopic Studies of the Penetrability of Hollow Iron Oxide Nanoparticles by Galvanic Exchange Reactions

Hollow Fe oxide nanoparticles have many applications in catalysis, drug delivery, and energy storage. Hollow Fe oxide shells can also be used for preventing the sintering of catalytically active cores and for magnetic recovery of bimetallic nanoparticles. However, more studies are required under real reaction conditions on the availability of the interior surface or active cores in hollow nanoparticles. Herein, we introduce a simple approach to study the penetrability of hollow Fe oxide shells by attempting galvanic exchange reactions between the remaining Fe(0) core within the hollow Fe oxide shell and Pd(II) salts. First, in situ high-temperature Fe K-edge XANES was used to monitor the formation of hollow Fe oxide nanoparticles from Fe nanoparticles. Core-void-shell Fe-Fe oxide nanoparticle intermediates were captured at different time intervals and then reacted with Pd(II). The reduction of Pd(II) was characterized by in situ Pd L3-edge XANES spectra. The results show that the core-void-shell nanoparti...

Tetragonal-Like Phase in Core–Shell Iron Iron-Oxide Nanoclusters

Two sizes of iron/iron-oxide (Fe/Fe-oxide) nanoclusters (NCs) of 10 and 35 nm diameters were prepared using a cluster deposition technique. Both these NCs displayed X-ray diffraction (XRD) peaks due to body-centered cubic (BCC) Fe0 and magnetite-like phase. 57Fe Mössbauer spectroscopy measurements confirmed (a) the core–shell nature of the NCs, (b) the Fe-oxide shell to be nanocrystalline and partially oxidized beyond magnetite, and (c) the Fe-oxide spins are significantly canted. In addition to the BCC Fe and magnetite-like phases, a phase similar to tetragonal σ-Fe-Cr (8% Cr) was clearly evident in the larger NC, based on XRD. The origin of the tetragonal-like phase in the larger NC could be due to significant distortion of the Fe0 core lattice planes; subtle peaks due to this phase were also apparent in the smaller NC. Unambiguous evidence for the presence of such a phase was not clear from X-ray photoelectron spectroscopy, vibrating sample magnetometry, X-ray magnetic circular dichroism, or transmission electron microscopy. 57Fe Mössbauer spectroscopy, however, provided support for a non-BCC Fe0 phase in the strongest sixth and the first resonance lines of the Fe0 sextet. To our knowledge, this is the first report of a tetragonal-like phase in the Fe/Fe-oxide core–shell systems.

Zero-valent iron nanoparticles with sustained high reductive activity for carbon tetrachloride dechlorination

Fe oxide shell breakdown and new oxyhydroxide formation during CT dechlorination contribute to pH self-buffering and sustained nZVI reactivity.

Magnetic interaction reversal in watermelon nanostructured Cr-doped Fe nanoclusters

Cr-doped core-shell Fe/Fe-oxide nanoclusters (NCs) were synthesized at varied atomic percentages of Cr from 0 at. % to 8 at. %. The low concentrations of Cr (&amp;lt;10 at. %) were selected in order to inhibit the complete conversion of the Fe-oxide shell to Cr2O3 and the Fe core to FeCr alloy. The magnetic interaction in Fe/Fe-oxide NCs (∼25 nm) can be controlled by antiferromagnetic Cr-dopant. We report the origin of σ-FeCr phase at very low Cr concentration (2 at. %) unlike in previous studies, and the interaction reversal from dipolar to exchange interaction in watermelon-like Cr-doped core-shell NCs.

Watermelon-like iron nanoparticles: Cr doping effect on magnetism and magnetization interaction reversal

Cr-doped core-shell iron/iron-oxide nanoparticles (NPs) containing 0, 2, 5, and 8 at.% of Cr dopant were synthesized via a nanocluster deposition system and their structural and magnetic properties were investigated. We observed the formation of a σ-FeCr phase in 2 at.% of Cr doping in core-shell NPs. This is unique since it was reported in the past that the σ-phase forms above 20 at.% of Cr. The large coercive field and exchange bias are ascribed to the antiferromagnetic Cr2O3 layer formed with the Fe-oxide shell, which also acts as a passivation layer to decrease the Fe-oxide shell thickness. The additional σ-phase in the core and/or Cr2O3 in the shell cause the hysteresis loop to appear tight waisted near the zero-field axis. The exchange interaction competes with the dipolar interaction with the increase of σ-FeCr grains in the Fe-core. The interaction reversal has been observed in 8 at.% of Cr. The observed reversal mechanism is confirmed from the Henkel plot and delta M value, and is supported by a theoretical watermelon model based on the core-shell nanostructure system.

Structural and Mössbauer studies of aerosol FeCu nanoparticles in a wide composition range

Structure and magnetic state of aerosol FeCu nanoparticles of 10–30 nm size with Cu content of 0.6–92.1 at.% have been examined by X-ray diffraction and Mossbauer spectroscopy. The FeCu particles have been shown to consist of an iron core surrounded by a copper and Fe oxide shell. With increasing Cu content the iron core having a bcc structure is reduced down to its complete disappearance followed by vanishing ferromagnetism of the particles. Within the copper content from 4.9 to 74.3 at.% the bcc and fcc phases coexist, with the fcc phase having a lattice constant close to that of pure copper and the bcc lattice constant being slightly higher than that for pure Fe due to embedding Cu atoms into the Fe lattice. At Fe-rich FeCu samples a presence of two-spin (ferromagnetic and paramagnetic) components of the fcc Fe is also observed. In the case of a thin copper shell there is only the ferromagnetic fcc Fe, whereas with further thickening of the shell both spin states of the fcc Fe appear existing up to a 20% Cu content. For FeCu samples with a higher Cu content they disappear due to oxidation of the copper grains. The Cu-rich samples with Cu content higher 80 at.% have a fcc structure, with the lattice constant being slightly higher than that of copper and they are paramagnetic. A slight increase of the lattice constant is due to the penetration of small iron aggregations into the Cu grains. In contact with air, the FeCu particles become covered with Fe3O4 and Cu2O. Their long-term exposure to ambient conditions leads to further oxidation process of Cu2O to CuO.

Size controlled Fe nanoparticles through polyol process and their magnetic properties

The first report on the synthesis of submicron sized pure Fe particles solely by polyol process was published by the authors a couple of years ago. Recent applications in biomedical fields demand stable and high saturation magnetization particles of sizes below 100 nm. In this paper, we report the successful synthesis of size controlled Fe nanoparticles ranging between 90 and 10 nm by polyol process using H 2PtCl 6 as the nucleating agent. The size of cubic Fe particles synthesized without the nucleating agent was 150 nm. The gradual decrease in size was observed with the increase in Pt ion concentration and the minimum size of 10–15 nm was achieved under a Pt ion concentration of 2 × 10 −7 M. The Fe particles retained their cubic morphology for sizes above 25 nm and became spherical and agglomerated with further reduction in size. Saturation magnetizations of Fe particles were size dependent and varied between 210 and 90 Am 2 kg −1. The as-prepared particles with diameters up to 60 nm were highly stable in air due to the formation of a thin passive layer of Fe-oxide shell. Consequently, this particle is considered a better candidate for biomedical application than magnetite due to higher saturation magnetization and biocompatible nature of the oxide layer formed on the surface.

Microstructure and magnetism of nanoparticles withγ-Fecore surrounded byα-Feand iron oxide shells

Iron-carbon nanocomposites have been elaborated by means of a simple chemical procedure based on in situ synthesis of iron nanoparticles within the nanopores of an activated carbon. The Fe nanoparticles present a broad particle-size distribution (5--40 nm). A combined structural and magnetic study seems to suggest that most of the nanoparticles of mean size $\ensuremath{\sim}15\text{ }\text{nm}$ have exotic ``onionlike'' core-shell morphology of $\ensuremath{\gamma}\text{-Fe}$ nucleus surrounded by a concentric double shell of $\ensuremath{\alpha}\text{-Fe}$ and maghemitelike oxide. The true nature of Fe-oxide was successfully evidenced through room temperature x-ray absorption spectroscopy. The whole system does not reach a fully superparamagnetic regime even at 750 K, probably due to higher blocking temperatures for the largest nanoparticles. M\"ossbauer spectrometry indicates that low temperature para-to-antiferromagnetic transition for the $\ensuremath{\gamma}\text{-Fe}$ phase cannot be discarded. In addition, the external Fe-oxide shell exhibits spin-glass behavior giving rise to the freezing of its magnetic moments at low temperatures. Hence, we propose a competing double magnetic coupling: (i) the oxide shell/$\ensuremath{\alpha}\text{-Fe}$ interaction and (ii) the possible antiferromagnetic coupling between $\ensuremath{\gamma}\text{-Fe}$ nucleus and $\ensuremath{\alpha}\text{-Fe}$ layer; as being both responsible for the observed exchange bias effect at $T=5\text{ }\text{K}$ $({H}_{ex}\ensuremath{\approx}150\text{ }\text{Oe})$.

ＦｅＰｔ／酸化鉄コアシェル・ナノ粒子の作製

The present article describes core/shell nanoparticles composed of disordered alloy FePt core and iron-oxide shell with spinel structure for biomedical applications such as magnetic hyperthermia, which requires high ac magnetic susceptibility χ at around room temperature. The nanoparticles are chemically synthesized by a polyol method in octylether as a solvent using Fe(CO)5 and Pt(acac)2 as precursors, and oleic acid and oleylamine as surfactants. The core/shell nanoparticles are formed under larger amount of Fe(CO)5 and surfactants compared with that for a synthesis of equiatomic FePt nanoparticles. For typical core/shell nanoparticles obtained in this study, the size of FePt core and the thickness of Fe-oxide shell are approximately 9 nm and 1.5 nm, respectively. The shell thickness increases with the increase of a concentration of Fe(CO)5, leading to the increase of the peak value of χ and the peak temperature in the temperature dependence of χ.

Aging of Iron Nanoparticles in Aqueous Solution: Effects on Structure and Reactivity

Aging (or longevity) is one of the most important and potentially limiting factors in the use of nano-Fe0 to reduce groundwater contaminants. We investigated the aging of FeH2 (Toda RNIP-10DS) in water with a focus on changes in (i) the composition and structure of the particles (by XRD, TEM, XPS, and bulk Fe0 content) and (ii) the reactivity of the particles (by carbon tetrachloride reaction kinetics, electrochemical corrosion potentials, and H2 production rates). Our results show that FeH2 becomes more reactive between 0 and ∼2 days exposure to water and then gradually loses reactivity over the next few hundred days. These changes in reactivity correlate with evidence for rapid destruction of the original Fe(III) oxide film on FeH2 during immersion and the subsequent formation of a new passivating mixed-valence Fe(II)−Fe(III) oxide shell. The effect of aging on the rate of carbon tetrachloride reduction was best described by the corrosion potential of FeH2, whereas the yield of chloroform from this reaction correlated best with the rate of H2 production. The behavior of unaged nano-Fe0 in the laboratory may be similar to that in field-scale applications for source-zone treatment due to the short reaction times involved. Long-term aged FeH2 acquires properties that are relatively stable over weeks or even months.

Effect of particle size on the magnetic properties of core-shell structured nanoparticles

Effect of particle size on exchange bias observed in Fe∕Fe oxide core/shell structured nanoparticles was investigated. Inert gas condensation was utilized for the synthesis of samples. Two sets of different particle size samples were prepared and the structural and magnetic properties were probed. It was found that the small particles show superparamagnetic behavior and exhibit high exchange bias field, 1574±25Oe at 5K, when field cooled in the presence of 2T magnetic field. Structural analyses of the particles in correlation with the magnetic measurements show that the smaller particle size (6nm Fe core, 1.5nm Fe oxide shell) favors amorphous oxide shell structure, and this in turn causes high magnetocrystalline anisotropy and enhanced exchange bias. Furthermore, we have also observed a vertical shift of the hysteresis loop related to the pinned spins at the ferromagnetic/antiferromagnetic (AFM) interface of the small particles. Decreased core size, high AFM anisotropy, and pinned spins observed from the small size particles support the domain wall model of the exchange bias.

Micromagnetic simulation of magnetization reversal in small particles with surface anisotropy

In order to study the effect of surface anisotropy, we have developed a three-dimensional finite element micromagnetic model. The finite element mesh of the particles is split into a core region with bulk properties and a surface region (shell) of variable thickness with modified material parameters. We have studied the magnetization reversal in FePt nanoparticles. The effect of surface anisotropy strongly reduces their coercivity. An Fe–oxide shell with low anisotropy in the plane of the surface has been considered as well as surface anisotropy with high anisotropy constants and axis orientation perpendicular to the surface.

Electrical Resistivity and Magnetoresistance in Monodispersed Oxide-Coated Fe Cluster Assemblies

We systematically studied electrical resistivity and magnetoresistance (MR) of size-monodispersed oxide-coated Fe cluster assemblies with the mean cluster sizes of d = 9–17 nm prepared by a plasma-gas-condensation-type cluster beam deposition system. The electrical resistivity and magnetoresistance strongly depend on the temperature, surface oxidization degree of the clusters (namely O2 gas flow ratio RO2), Fe cluster size d, and magnetic field. The oxide-coated Fe cluster assemblies exhibit a large negative MR effect which is further enhanced at low temperatures due to the dominant contribution of the spin-dependent tunneling process between the Fe cores through the oxide shell layers. It has been found that the magnetic field dependence of the MR ratio at all temperatures shows no saturation tendency up to a maximum field H = 50 kOe and completely disagrees with the magnetization curves which indicate a saturation tendency. These results have been interpreted by consideration of the magnetic state of the Fe-oxide shell layers, spin-dependent tunneling mechanism, and intercluster magnetic correlation. The high-field nonsaturation behavior in the magnetoresistance effect is attributed to the spin-disordered structure, which is frozen in a spin-glass-like state at low temperatures, in the surface of the Fe-oxide shell crystallites or the whole thinner Fe-oxide shell layers.