Size-dependent oxidation in iron/iron oxide core-shell nanoparticles (original) (raw)
Related papers
2010
The crystal structure of magnetite nanoparticles may be transformed to maghemite by complete oxidation, but under many relevant conditions the oxidation is partial, creating a mixed-valence material with structural and electronic properties that are poorly characterized. We used X-ray diffraction, Fe K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy, and soft X-ray absorption and emission spectroscopy to characterize the products of oxidizing uncoated and oleic acid-coated magnetite nanoparticles in air. The oxidization of uncoated magnetite nanoparticles creates a material that is structurally and electronically indistinguishable from maghemite. By contrast, while oxidized oleic acid-coated nanoparticles are also structurally indistinguishable from maghemite, Fe L-edge spectroscopy revealed the presence of interior reduced iron sites even after a 2-year period. We used X-ray emission spectroscopy at the O K-edge to study the valence bands (VB) of the iron oxide nanoparticles, using resonant excitation to remove the contributions from oxygen atoms in the ligands and from low-energy excitations that obscured the VB edge. The bonding in all nanoparticles was typical of maghemite, with no detectable VB states introduced by the long-lived, reduced-iron sites in the oleic acid-coated sample. However, O K-edge absorption spectroscopy observed a ∼0.2 eV shift in the position of the lowest unoccupied states in the coated sample, indicating an increase in the semiconductor band gap relative to bulk stoichiometric maghemite that was also observed by optical absorption spectroscopy. The results show that the ferrous iron sites within ferric iron oxide nanoparticles coated by an organic ligand can persist under ambient conditions with no evidence of a distinct interior phase and can exert an effect on the global electronic and optical properties of the material. This phenomenon resembles the band gap enlargement caused by electron accumulation in the conduction band of TiO 2 .
Morphology and Electronic Structure of the Oxide Shell on the Surface of Iron Nanoparticles
Journal of the American Chemical Society, 2009
An iron (Fe) nanoparticle exposed to air at room temperature will be instantly covered by an oxide shell that is typically ∼3 nm thick. The nature of this native oxide shell, in combination with the underlying Fe 0 core, determines the physical and chemical behavior of the core-shell nanoparticle. One of the challenges of characterizing core-shell nanoparticles is determining the structure of the oxide shell, that is, whether it is FeO, Fe 3 O 4 , γ-Fe 2 O 3 , R-Fe 2 O 3 , or something else. The results of prior characterization efforts, which have mostly used X-ray diffraction and spectroscopy, electron diffraction, and transmission electron microscopic imaging, have been framed in terms of one of the known Fe-oxide structures, although it is not necessarily true that the thin layer of Fe oxide is a known Fe oxide. In this Article, we probe the structure of the oxide shell on Fe nanoparticles using electron energy loss spectroscopy (EELS) at the oxygen (O) K-edge with a spatial resolution of several nanometers (i.e., less than that of an individual particle). We studied two types of representative particles: small particles that are fully oxidized (no Fe 0 core) and larger core-shell particles that possess an Fe core. We found that O K-edge spectra collected for the oxide shell in nanoparticles show distinct differences from those of known Fe oxides. Typically, the prepeak of the spectra collected on both the core-shell and the fully oxidized particles is weaker than that collected on standard Fe 3 O 4 . Given the fact that the origin of this prepeak corresponds to the transition of the O 1s electron to the unoccupied state of O 2p hybridized with Fe 3d, a weak pre-edge peak indicates a combination of the following four factors: a higher degree of occupancy of the Fe 3d orbital; a longer Fe-O bond length; a decreased covalency of the Fe-O bond; and a measure of cation vacancies. These results suggest that the coordination configuration in the oxide shell on Fe nanoparticles is defective as compared to that of their bulk counterparts. Implications of these defective structural characteristics on the properties of core-shell structured iron nanoparticles are discussed.
Physical Review B, 2012
Fe 3 O 4 nanoparticles have been investigated as they are biocompatible and their surface can be functionalized. We synthesized iron oxide nanoparticles using a water-in-oil microemulsion method. Bare and silica-coated iron oxide nanoparticles of a core size of 6 nm dispersed in ethanol have been investigated by means of x-ray absorption spectroscopy (XAS). Due to a dedicated experimental setup the particles can be measured directly in dispersion. XAS allows us to disentangle the contributions of the Fe 2+ and Fe 3+ ions and therefore to estimate the amount of Fe 3 O 4 in the particles. In case of the silica coated particles a high amount of magnetite was obtained. In contrast, the bare nanoparticles showed indications of a further oxidation into γ -Fe 2 O 3 even in dispersion.
Journal of Applied Crystallography, 2014
The size-driven expansion and oxidation-driven contraction phenomena of nonstoichiometric magnetite–maghemite core–shell nanoparticles have been investigated by the total scattering Debye function approach. Results from a large set of samples are discussed in terms of significant effects on the sample average lattice parameter and on the possibility of deriving the sample average oxidation level from accurate, diffraction-based, cell values. Controlling subtle experimental effects affecting the measurement of diffraction angles and correcting for extra-sample scattering contributions to the pattern intensity are crucial issues for accurately estimating lattice parameters and cation vacancies. The average nanoparticle stoichiometry appears to be controlled mainly by iron depletion of octahedral sites. A simple law with a single adjustable parameter, well correlating lattice parameter, stoichiometry and size effects of all the nanoparticles present in the whole set of samples used in ...
Applied Surface Science, 2005
Iron-based core-shell nanostructures were synthesized by laser pyrolysis in a two-steps procedure. In a first step, using a cross-flow configuration, the laser radiation was heating a gas phase mixture containing iron pentacarbonyl (vapors) entrained by an ethylene flow, which plays also the role of an energy transfer agent. Secondly, a carefully controlled in situ passivation of the freshly formed pyrophoric iron nanoparticles created a protective iron oxide shell. The produced nanoparticles (22 nm size diameters) with core-shell features were analyzed by TEM, XRD, SAED and Raman spectroscopy. Majoritary iron and gamma iron oxide/magnetite and minoritary carbon phases were identified. In laser pyrolysis experiments in which the reaction temperature was increased, the catalyzed homogeneous nucleation and growth of carbon nanotubes in the gas phase was observed and is presented here for the first time. #
Journal of Inorganic and Organometallic Polymers and Materials, 2019
The goal of this paper was to synthesize and characterize core-shell iron-carbon nanoparticles. For this purpose, nanoparticles were synthetized via a hydrothermal co-precipitation route, applying a 2 2 factorial experimental design with a central point, and varying both the concentration of the iron precursor (iron nitrate) and the reaction temperature. The nanoparticles were characterized via the following analysis: vibrating sample magnometry (VSM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), energy dispersive X-ray spectroscopy (EDX), high resolution field emission gun scanning electron microscopy (SEM/FEG) and transmission electron microscopy (TEM) analysis. The results showed that the hydrothermal co-precipitation synthesis route enabled the production of Fe 3 O 4-Fe 2 O 3 @C core-shell nanoparticles with dimensions between 4 and 8 nm. An increase in iron nitrate concentration and temperature during synthesis entailed a decrease in the remnant field and the magnetization of the nanoparticles.
Journal of Magnetism and Magnetic Materials, 2014
The synthesis of iron oxide nanoparticles, i.e., magnetite was attempted by using only ferrous ion (Fe 2 þ ) as a magnetite precursor, under an ambient atmosphere. The room temperature reverse co-precipitation method was used, by applying two synthesis protocols. The freshly prepared iron oxide was also immediately coated with Stöber silica (SiO 2 ) layer, forming the coreshell structure. The phase, stoichiometry, crystallite and the particle size of the synthesized powders were determined by using X-ray diffraction (XRD) and transmission electron microscope (TEM), while the magnetic and oxidation behaviors were studied by using the vibrating sample magnetometer (VSM) and Mössbauer spectroscopy. Based on the results, the bare iron oxide nanoparticles are in the stoichiometry between the magnetite and the maghemite stoichiometry, i.e., oxidation occurs. This oxidation is depending on the synthesis protocols used. With the silica coating, the oxidation can be prevented, as suggested by the fits of Mössbauer spectra and low temperature magnetic measurement.
The Journal of Physical Chemistry B, 2002
The structures of Fe 2 O 3 nanoparticles with different sizes were investigated using Fe K-edge X-ray absorption near-edge structure (XANES) and the FEFF calculations, as well as surface modification with enediol ligands. The studies not only revealed the existence of under-coordinated Fe sites in the nanoparticles but also confirmed that these under-coordinated sites were located on the surface. Upon binding of enediol ligands, surface sites were restructured to octahedral sites. In particular, the nature of the surface defects and their correlation with the unique properties of the nanoparticles were discussed. Model calculations were conducted for Fe m O n (m g 1, n g 4) clusters of various sizes centered at Fe sites with octahedral (O h), distorted octahedral (C 3V) and tetrahedral (T d) coordination geometry using FEFF8.10 programs. The main features of the calculated spectra agree with the experimental results and were correlated to the density of states, the Fe coordination geometry, and the long-range order of the lattice.
Oriental Journal of Chemistry, 2019
Synchrotron X-ray absorption near edge spectroscopy (XANES) and extended X-ray fine structure (EXAFS) are used to study the effect of temperature in the co-precipitation synthesis of iron oxides. Nanoparticles are rod-like and agglomerate into microscale aggregates. XANES spectra complement X-ray diffractometry in the phase identification of inverse spinel structured maghemite (γ-Fe 2 O 3). In addition to the smallest crystallite size, EXAFS analysis revealed the highly distorted structure in the sample synthesized at 90 O C. The samples synthesized at lower temperatures (25 O C, 60 O C) are ferrimagnetic with much larger magnetizations. The variation in magnetic properties with the synthetic temperature is related to the differences in crystallite size and distortion in the structure of γ-Fe 2 O 3 nanoparticles.