Characterization and Properties of Metallic Iron Nanoparticles:  Spectroscopy, Electrochemistry, and Kinetics (original) (raw)

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.

Comparison of a calculated and measured XANES spectrum of α-Fe2O3

Physical Chemistry Chemical Physics, 2011

Comparison and prediction of the experimental XANES spectrum is a good measurement of the quality of the electronic structure calculations employed, and their ability to predict electronic transitions in solids. Here we present a comparison between BLYP + U and hybrid-BLYP calculations regarding the geometric, magnetic and electronic structures of a-Fe 2 O 3 (hematite). Several values of U and different percentages of Fock-exchange have been screened to see how their contributions affect different properties of hematite, paying particular attention to the electronic structure. To estimate the quality of the various methods the calculated density-ofstates were compared to the experimentally collected XANES spectrum of the iron K-edge, providing information about the orbitals describing the conduction band. We find that in agreement with previous studies DFT + U and hybrid-functional simulations can correctly predict the character of the valence band, but only Fock-exchange higher than 30% or U-values equal or larger than 6 eV properly reproduce the order between the t g and e orbitals in the conduction band, and can, therefore, be used to study and predict XANES spectra and electronic transitions in hematite.

X-Ray Diffraction of Iron Containing Samples: The Importance of a Suitable Configuration

Solid State Phenomena, 2017

X-ray diffraction (XRD) is a commonly used technology to identify crystalline phases. However, care must be taken with the combination of XRD configuration and sample. Copper (most commonly used radiation source) is a poor match with iron containing materials due to induced fluorescence. Magnetite and maghemite are analysed in different configurations using copper or cobalt radiation. Results show the effects of fluorescence repressing measures and the superiority of diffractograms obtained with cobalt radiation. Diffractograms obtained with copper radiation make incontestable phase identification often impossible. Cobalt radiation on the other hand yields high quality diffractograms, making phase identification straightforward.

Diffraction anomalous fine structure study of iron/iron oxide nanoparticles

Journal of Applied Crystallography, 2009

Diffraction anomalous fine structure is a recently developed technique which can provide a measurement of the local structure of a given element in a particular phase or crystallographic site. Most previous investigations have applied the technique to bulk solids, thin films, and nano-dots or wires on crystalline substrates. In this paper, the technique is applied to highly disordered nanometre-sized Fe/Fe oxide core-shell nanocrystalline powders, the diffraction patterns of which exhibit weak and greatly broadened diffraction peaks. Focusing on the oxide shell diffraction peaks, a qualitative analysis of the nearedge spectral region and a quantitative analysis of the extended energy region are provided; in particular, good quality fittings of the extended range spectra are obtained. The local structure is selectively probed around the tetrahedral and octahedral sites of the oxide shell, finding the presence of a high degree of structural disorder. This study demonstrates that diffraction anomalous fine structure can now be fruitfully applied to nanocrystalline powders. research papers J. Appl. Cryst. (2009). 42, 642-648 Carlo Meneghini et al. Diffraction anomalous fine structure of Fe/Fe oxide 643

Studies of iron and iron oxide layers by electron spectroscopes

Applied Surface Science, 2005

Thin iron oxide layers prepared ''in situ'' in the ultra high vacuum on polycrystalline iron substrate were investigated by electron spectroscopy methods-X-ray photoelectron spectroscopy (XPS) and elastic peak electron spectroscopy (EPES), using spectrometer ADES-400. The texture and the average grain size of the iron substrate foil have been examined by glancing angle X-ray diffraction (XRD). Qualitative and quantitative estimation of investigated oxide layers was made using (i) the relative sensitivity factor XPS method, (ii) comparison of binding energy shifts of Fe 2p photoelectron line and (iii) non-linear fitting procedure of Fe 2p photoelectron lines.

PIXE & XRD analysis of nanocrystals of Fe, Ni and Fe 2O 3

Materials Letters, 2007

Nanocrystalline materials have become a subject of both scientific and industrial importance in the past decade. The present work deals with the preparation of α-Fe and Ni powders by high-energy ball mill method and chemically prepared α-Fe2O3 powders of nano crystalline type respectively. There is a need to characterize the trace elements in order to check the purity of these samples. The results of trace element analysis of these nanocrystals by using PIXE, characterization and size determination by XRD using Debye–Scherrer formula with full-width at half-maximum(FWHM) have been discussed.Nanocrystallinity is examined already by (TEM,FTIR and MICRO-RAMAN) experiments done previously.