Substitution Of Fe Atoms Research Articles

Electrocatalytic oxygen evolution with pure and substituted M6(SR)12 (M = Pd, Fe, Rh) complexes

The development of new technologies to generate fuels from water splitting requires highly active and cost-effective catalysts. Organometallic complexes have attracted considerable interests due to their scientific significance and their ability to efficiently photo- and electro-catalyze H2 and O2 evolution reactions. Here, we combine density functional theory (DFT) and computational electrochemical analysis to predict the oxygen evolution reaction (OER) activity of a series of organometallic complexes containing methyl thiol ligands and compare it to that of Ni complex. We find that the OER activity versus the diameter of the hexagonal ring shows a maximum activity which corresponds to that of Ni complex. We also find an existence of linear correlations of the adsorption energies with the ring diameter of the oxidized complexes. In an effort to improve the OER activity of these complexes, we substitute a metal cation with Ni, and we show that Ni-doped complexes give rise to smaller overpotentials. The enhanced overpotential can be explained in terms of the overall affinity of active sites for reactive species resulting in a shift of their binding energies. Finally, we find that substituting Fe atoms into small organometallic Ni complex does not appear to have the same influence that has been observed in bulk materials. We believe these predictions will help guide catalyst design by identifying active and robust OER catalysts, capable to reduce the energy required for the anodic water oxidation. Reducing anodic overpotentials will help reduce the overall energy requirements of both H2 evolution and CO2 reduction systems.

DFT calculations of graphene monolayer in presence of Fe dopant and vacancy

In the present work, the effects of Fe doping and vacancies on the electronic, magnetic and optical properties of graphene are studied by density functional theory based calculations. The conductive behavior is revealed for the various defected graphene by means of electronic density of states. However, defected structures show different magnetic and optical properties compared to those of pure one. The ferromagnetic phase is the most probable phase by substituting Fe atoms and vacancies at AA sublattice of graphene. The optical properties of impure graphene differ from pure graphene under illumination with parallel polarization of electric field, whereas for perpendicular polarization it remains unchanged. In presence of defect and under parallel polarization of light, the static dielectric constant rises strongly and the maximum peak of Im ε(ω) shows red shift relative to pure graphene. Moreover, the maximum absorption peak gets broaden in the visible to infrared region at the same condition and the magnitude and related energy of peaks shift to higher value in the EELS spectra. Furthermore, the results show that the maximum values of refractive index and reflectivity spectra increase rapidly and represent the red and blue shifts; respectively. Generally; substituting the C atom with Fe has more effect on magnetic and optical properties relative to the C vacancies.

Influence of impurity elements on the corrosion of α-uranium surface: a density functional theory study

Influence of substitutional Fe, Al and Mg atoms on the corrosion of α-U(110) surface was studied by DFT + U method. The calculations results indicate that impurity atoms inhibit the dissociation of H2O molecular. Although the impurity atoms do not impede the dissociation of O2 molecular, they lower the binding energy of the dissociated O atoms with the substitutional surfaces. Through the investigation on the surface diffusion and into-bulk penetration, it could be confirmed that Fe, Al and Mg atoms accelerate the hydrogenation of α-uranium surface, while improve the anti-oxidizing ability of α-uranium.

Accelerated Hydrogen Evolution Kinetics on NiFe-Layered Double Hydroxide Electrocatalysts by Tailoring Water Dissociation Active Sites.

Owing to its earth abundance, low kinetic overpotential, and superior stability, NiFe-layered double hydroxide (NiFe-LDH) has emerged as a promising electrocatalyst for catalyzing water splitting, especially oxygen evolution reaction (OER), in alkaline solutions. Unfortunately, as a result of extremely sluggish water dissociation kinetics (Volmer step), hydrogen evolution reaction (HER) activity of the NiFe-LDH is rather poor in alkaline environment. Here a novel strategy is demonstrated for substantially accelerating the hydrogen evolution kinetics of the NiFe-LDH by partially substituting Fe atoms with Ru. In a 1 m KOH solution, the as-synthesized Ru-doped NiFe-LDH nanosheets (NiFeRu-LDH) exhibit excellent HER performance with an overpotential of 29 mV at 10 mA cm-2 , which is much lower than those of noble metal Pt/C and reported electrocatalysts. Both experimental and theoretical results reveal that the introduction of Ru atoms into NiFe-LDH can efficiently reduce energy barrier of the Volmer step, eventually accelerating its HER kinetics. Benefitting from its outstanding HER activity and remained excellent OER activity, the NiFeRu-LDH steadily drives an alkaline electrolyzer with a current density of 10 mA cm-2 at a cell voltage of 1.52 V, which is much lower than the values for Pt/C-Ir/C couple and state-of-the-art overall water-splitting electrocatalysts.

Self-consistent mapping: Effect of local environment on formation of magnetic moment in α−FeSi2

The Hohenberg-Kohn theorem establishes a basis for mapping the exact energy functional to a model one provided that their charge densities coincide. We suggest to use a mapping in a similar spirit, but here the parameters of the formulated multiorbital model should minimize the difference between the self-consistent charge and spin densities. The analysis of the model allows for detailed understanding of the role played by different parameters of the model in the physics of interest. After finding the areas of interest in the phase diagram of the model, we return to the ab initio calculations and check if the effects discovered are confirmed or not. Because of the last controlling step, we call this approach hybrid self-consistent mapping approach (HSCMA). As an example of the approach we present a study of the effect of silicon atoms substitution by the iron atoms and vice versa on the magnetic properties in the iron silicide $\ensuremath{\alpha}\ensuremath{-}{\mathrm{FeSi}}_{2}$. We find that while the stoichiometric $\ensuremath{\alpha}\ensuremath{-}{\mathrm{FeSi}}_{2}$ is nonmagnetic, the substitutions generate different magnetic structures depending on the type of local environment of the substitutional Fe atoms. Besides, contrary to the commonly accepted statement that the destruction of the magnetic moment is controlled only by the number of Fe-Si nearest neighbors, we find that actually it is controlled by the Fe-Fe next-nearest-neighbor hopping parameter. This finding led us to the counterintuitive conclusion: an increase of Si concentration in ${\mathrm{Fe}}_{1\ensuremath{-}x}{\text{Si}}_{2+x}$ ordered alloys may lead to ferromagnetism. The calculation within GGA-to-DFT confirms this conclusion.

Spin filter properties of armchair graphene nanoribbons with substitutional Fe atoms

ABSTRACTThe spin filter capability of a (0,8) armchair graphene nanoribbon with Fe atoms at substitutional sites is investigated by density functional theory in combination with the non-equilibrium Green's function technique. For specific arrangements, a high degree of spin polarisation is achieved. These include a single substitution at an edge position or double substitution in the central sector of the transmission element. The possibility of switching between majority and minority spin polarisation by changing the double substitution geometry is predicted. Including the bias dependence of the transmission function proves to be essential for correct representation of the spin-resolved current-voltage profiles.

Graphene as a flexible template for controlling magnetic interactions between metal atoms

Metal-doped graphene produces magnetic moments that have potential application in spintronics. Here we use density function theory computational methods to show how the magnetic interaction between metal atoms doped in graphene can be controlled by the degree of flexure in a graphene membrane. Bending graphene by flexing causes the distance between two substitutional Fe atoms covalently bonded in graphene to gradually increase and these results in the magnetic moment disappearing at a critical strain value. At the critical strain, a carbon atom can enter between the two Fe atoms and blocks the interaction between relevant orbitals of Fe atoms to quench the magnetic moment. The control of interactions between doped atoms by exploiting the mechanical flexibility of graphene is a unique approach to manipulating the magnetic properties and opens up new opportunities for mechanical-magnetic 2D device systems.

Thermodynamic Properties of Heusler [formula omitted

We investigated the thermodynamic properties of Heusler compounds Fe2−xCoxmnSi (0.00≤x≤2.00). The specific heats CP(T) for compounds with x≤0.1 exhibit a λ-type anomaly arising from spin rearrangements at TR. With increasing x, TR decreases linearly and vanishes at x∼0.169. The magnetic entropy, STR, derived from the magnetic specific heat, Cm(T), released at TR decreases by increasing x. This means the canting angle of spins from the [111] direction decreases by the substitution of Fe atoms with Co atoms, based on the magnetic structure model of Fe2MnSi proposed by Miles et al. For compounds with 0.5≤x, CP(T) in the low-T range can be reproduced by Debye T3 law. The electronic specific heat coefficient decreases monotonically with x.

Structural and magnetic properties of (Zn,Fe)Te thin films grown by MBE under Zn-rich flux condition

We have performed the structural and magnetic characterization of Zn1-xFexTe thin films grown by molecular beam epitaxy (MBE). Thin films of Zn1-xFexTe with Fe composition up to x~0.06 were grown by MBE with the supply of excess Zn flux over Te flux. From the structural analyses by x-ray absorption fine structure (XAFS), it is revealed that the grown film with a low Fe content x=0.015 is composed of pure diluted phase with substitutional Fe atoms while the precipitation of an extrinsic phase is suggested at higher Fe contents x=0.03~0.06. The magnetization measurement reveals a clear distinction in the magnetic-field (M-H) and temperature (M-T) dependences between the low and high Fe contents; a hysteretic M-H dependence and the blocking phenomenon in the M-T dependence at the higher Fe content. These behaviors are attributed to the superparamagnetism, originating from the formation of an extrinsic precipitate.

Effect of Nb on Magnetic Properties, Microstructures, and Site Preference in Nd-Fe-B Nanograin Single-Phase Alloy

The effect of Nb addition on the magnetic properties, microstructure, and site preference has been investigated in single-phase Nd. 0.94 T, Hci = 919.0 kA/m, and (BH)max = 143.1 kJ/m 3 were achieved by annealing the melt-spun ribbons with x = 1.5 at 650 ∘C for 10 min. Additions of Nb were effective in refining the grain size and improving the magnetic properties. Transmission electron microscopy (TEM) showed that there was no second phase or precipitate found in grain boundaries. The results of three-dimensional atom probe (3DAP) indicated that Nb may enter into the amorphous structure and Nd2Fe14B. The content of Nb in Nd2Fe14B was very small. Theoretical calculation showed that Nb preferentially substituted Fe atoms at the 8j2 sites and the volume of a cell would slightly enlarge.

Ti doped hematite thin film photoanode with enhanced photoelectrochemical properties

Ti-doped Fe2O3 thin films were prepared on fluorine-doped SnO2 substrate as visible light active photoelectrochemical anodes. The fabricated films were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), atomic force microscope (AFM), X-ray energy dispersive spectroscopy and X-ray photoelectron spectroscopy (XPS). XRD data showed all films exhibited rhombohedral hematite phase, and the cell parameters showed that Titanium atoms substituted Fe atoms in the hematite lattice. AFM demonstrated that Ti doping could decrease the particle size on the surface compared with pure hematite. XPS results presented that Ti atom concentration was about 2.23 % in the doped film surface. The incident photon to electron conversion efficiency of Ti doped α-Fe2O3 film reached 23 % at 400 nm under 0.30 V bias versus AgCl in 1 M NaOH, which was nearly four times than that of undoped film. Titanium atoms in α-Fe2O3 lattice could increase the conductivity of hematite film. And excited electrons and holes in the bulk film could be separated more efficiently, rather than recombining with each other rapidly as that in pure hematite, which ultimately prolonged the life of electrons and holes and obtained the high efficiency Fe2O3 photo anode.

Mössbauer Spectroscopy on Fe Impurities in Si Materials

Based on a series of Mössbauer spectroscopic investigations on Fe impurities in p-type and n-type Si materials, we propose a new model for Fe impurities in Si matrix, consisting not only of interstitial Fe, but also substitutional Fe atoms with different charge states. Mössbauer spectroscopy enables us to observe directly these components which transform each other by changing external conditions such as under light illumination, under external voltage, and also under external stress. This means that not only interstitial Fe impurities, but also substitutional Fe impurities appear to be a source for producing “electrically active Fe impurities” in Si materials.

Dielectric properties of iron-containing bismuth titanate solid solutions with a layered perovskite structure

The complex oxides based on bismuth titanate with substituting iron atoms Bi4Ti2.98Fe0.02O11.99, Bi4Ti2.5Fe0.5O11.75, and Bi4Ti2Fe1O11.5 have been studied using dielectric spectroscopy in the frequency range from 30 to 106 Hz at a temperature of 296 K. The frequency dependences of permittivity ɛ′, dielectric loss ɛ″, and complex part of conductivity σ′ have been studied, and the dependences ɛ″(ɛ′) have been constructed. The microstructure images of the studied ceramic materials have been obtained using atomic force microscopy. It has been shown that, as Fe3+ ions are introduced as substituting atoms, there is a critical concentration x (0.5 < x < 1) above which the dielectric properties substantially change with variations in the number of perovskite-like layers, and the correlation is observed between the concentration of substituting Fe atoms and the grain size on the surface of the samples.

Local structure and optical absorption characteristic investigation on Fe doped TiO2 nanoparticles**Supported by National Basic Research Program of China (2012CB825801), Science Fund for Creative Research Groups of NSFC (11321503), National Natural Science Foundation of China (11321503, 11179004) and Guangdong Natural Science Foundation (S2011040003985)

The local structures and optical absorption characteristics of Fe doped TiO2 nanoparticles synthesized by the sol-gel method were characterized by X-ray diffraction (XRD), X-ray absorption fine structure spectroscopy (XAFS) and ultraviolet-visible absorption spectroscopy (UV-Vis). XRD patterns show that all Fe-doped TiO2 samples have the characteristic anatase structure. Accurate Fe and Ti K-edge EXAFS analysis further reveal that all Fe atoms replace Ti atoms in the anatase lattice. The analysis of UV-Vis data shows a red shift to the visible range. According to the above results, we claim that substitutional Fe atoms lead to the formation of structural defects and new intermediate energy levels appear, narrowing the band gap and extending the optical absorption edge towards the visible region.

Formation of different magnetic phases and high Curie temperature ferromagnetism in Fe57-implanted ZnO film

We investigated magnetic properties of ZnO thin film implanted with Fe 57 ions to the fluence of 1.00×10 17 ions/cm 2 . Both vibrating sample magnetometry and magneto-optical Kerr effect measurements revealed strong room temperature ferromagnetism with similar hysteresis loops. Temperature dependent measurements showed a very high Curie temperature around 850 K. Conversion electron Mössbauer spectroscopy experiments proved the existence of paramagnetic Fe +3 and Fe +2 ions and also the presence of substitutional Fe atoms in the hexagonal ZnO crystal resulting in intrinsic ferromagnetic order. • The implanted iron atoms form different magnetic phases in the host ZnO film. • Mössbauer data proved the existence of paramagnetic Fe +3 and Fe +2 ions and substitutional Fe atoms. • The observed RT ferromagnetism is attributed to the iron atoms occupying ZnO crystal lattice. • Temperature dependent measurements showed a very high T C around 850 K.

Effect of alloying elements on the properties of Zr and the Zr–H system

The effect of the alloying elements Sn, Fe, Cr, Ni, Nb, and O on hydrogen-containing alpha-zirconium and zirconium hydrides is investigated using ab initio quantum mechanical calculations and classical simulations. Cr, Fe, and Ni atoms attract interstitially dissolved H atoms whereas interstitial oxygen atoms show no pronounced interaction with H atoms. The alloying elements destabilize the hydride phases in the order Sn>Fe>Cr>Ni>Nb. Hence, substitutional Sn (if atomically dispersed), Cr and Fe atoms are likely to delay hydride precipitation, effectively increasing the hydrogen solubility. Nb and Sn influence the mobility of Zr self-interstitial atoms (SIA’s), which diffuse rapidly and preferentially parallel to the basal planes forming interstitial dislocations loops perpendicular to the basal planes (a-loops). Nb suppresses this diffusion of SIA’s, thereby reducing the rate of formation of interstitial a-loops. Sn atoms, if present on substitutional sites, have a similar, but smaller effect. If SIA’s approach substitutional Fe, Cr, and Ni atoms, the simulations indicate a spontaneous swap promoting the smaller transition metal atoms into interstitial atoms, which diffuse very rapidly with a preference in the c-direction, thereby facilitating their segregation to energetically more favorable sites such as vacancies, vacancy c-loops, grain boundaries, surfaces, and intermetallic precipitates.

Magnetic structures of Mn3−xFexSn2: An experimental and theoretical study

We investigate the magnetic structure of Mn${}_{3\ensuremath{-}x}$Fe${}_{x}$Sn${}_{2}$ using neutron powder diffraction experiments and electronic structure calculations. These alloys crystallize in the orthorhombic Ni${}_{3}$Sn${}_{2}$ type of structure ($Pnma$) and comprise two inequivalent sites for the transition metal atoms (4$c$ and 8$d$) and two Sn sites (4$c$ and 4$c$). The neutron data show that the substituting Fe atoms predominantly occupy the 4$c$ transition metal site and carry a lower magnetic moment than Mn atoms. Four kinds of magnetic structures are encountered as a function of temperature and composition: two simple ferromagnetic structures (with the magnetic moments pointing along the $b$ or $c$ axis) and two canted ferromagnetic arrangements (with the ferromagnetic component pointing along the $b$ or $c$ axis). Electronic structure calculations results agree well with the low-temperature experimental magnetic moments and canting angles throughout the series. Comparisons between collinear and noncollinear computations show that the canted state is stabilized by a band mechanism through the opening of a hybridization gap. Synchrotron powder diffraction experiments on Mn${}_{3}$Sn${}_{2}$ reveal a weak monoclinic distortion at low temperature ($\ensuremath{\beta}\ensuremath{\sim}90.08$${}^{\ensuremath{\circ}}$ at 175 K). This lowering of symmetry could explain the stabilization of the $c$-axis canted ferromagnetic structure, which mixes two orthorhombic magnetic space groups, a circumstance that would otherwise require unusually large high-order terms in the spin Hamiltonian.

Half-metallic antiferromagnetic behavior of double perovskite Sr2OsMoO6: First principle calculations

Electronic structure calculations based on density functional theory within the generalized gradient approximation for double perovskite Sr2FeMoO6 and Sr2OsMoO6 have been performed using the accurate full potential augmented spherical wave method. By substituting Fe atoms by Os in the double perovskite structure oxides we have shown that it is possible to realize half-metallic antiferromagnets with 100% spin polarization of the conduction electrons crossing the Fermi level, without showing a net magnetization. To support our results, GGA+U electronic structure calculations have been performed showing that the half-metallic antiferromagnetic state still persists. We conclude that the origin of the antiferromagnetism in Sr2OsMoO6 may be attributed to both superexchange and generalized double exchange mechanisms via the B(3d,5d)–O(2p)–B′(4d) coupling.

Effects of oxygen vacancy on the electrical and magnetic properties of anatase Fe0.05Ti0.95O2−δ films

Epitaxial anatase Fe0.05Ti0.95O2−δ (δ: oxygen vacancy) thin films were grown on LaAlO3 substrates by pulsed laser deposition. Structural characterizations by X-ray diffraction confirmed the incorporation of substituting Fe atoms into anatase TiO2 lattice, no impurity phases or clusters formation were detected. The influence of growth oxygen pressure on the crystal structure and properties of Fe0.05Ti0.95O2−δ semiconductor films has been investigated systematically. As the oxygen pressure decreases, more oxygen vacancies are generated in the thin film. At the same time, the optical band gap narrows down and the electrical properties improve correspondingly. Room temperature ferromagnetism was observed in all samples and saturation magnetization increased from 0.8emu/cm3 to 8.3emu/cm3 with the increase of oxygen vacancy concentration. Strong correlation between ferromagnetism and oxygen vacancy density was established. This is explained by a modified bounded magnetic polaron model.

Magnetic frustration effect in Mn-rich Sr2Mn1−xFexMoO6 system

Variation in lattice parameters and magnetic properties of Sr2Mn1−xFexMoO6 (0≤x≤0.4) has been studied in the present work. Our findings suggest that the crystal structures at room temperature for all the compositions studied here are pseudocubic in nature. Substitution of as small as 10% Fe in place of Mn destroys the long range antiferromagnetic ordering observed in Sr2MnMoO6 and introduces local magnetic frustration, i.e., short range ferromagnetic interactions centered around the substituted Fe atoms, although the shape of the magnetic susceptibility creates an illusion of antiferromagnetic ordering. Different local structural environments around Mn and Fe atoms are thought to be responsible for the presence of such magnetic frustration. The results have been compared with those observed previously in SrLaMnMoO6. Our detailed measurements and interpretations are clearly at variance with the magnetic phase diagrams of Mn-rich phases of Sr2Mn1−xFexMoO6 compounds reported earlier, although the experimental results appear to be similar in both cases.