Fe 3d Orbitals Research Articles

A Graphene-Supported Single-Atom FeN5 Catalytic Site for Efficient Electrochemical CO2 Reduction.

Electrochemical conversion of CO2 into valued products is one of the most important issues but remains a great challenge in chemistry. Herein, we report a novel synthetic approach involving prolonged thermal pyrolysis of hemin and melamine molecules on graphene for the fabrication of a robust and efficient single-iron-atom electrocatalyst for electrochemical CO2 reduction. The single-atom catalyst exhibits high Faradaic efficiency (ca. 97.0 %) for CO production at a low overpotential of 0.35 V, outperforming all Fe-N-C-based catalysts. The remarkable performance for CO2 -to-CO conversion can be attributed to the presence of highly efficient singly dispersed FeN5 active sites supported on N-doped graphene with an additional axial ligand coordinated to FeN4 . DFT calculations revealed that the axial pyrrolic nitrogen ligand of the FeN5 site further depletes the electron density of Fe 3d orbitals and thus reduces the Fe-CO π back-donation, thus enabling the rapid desorption of CO and high selectivity for CO production.

A Graphene‐Supported Single‐Atom FeN5 Catalytic Site for Efficient Electrochemical CO2 Reduction

AbstractElectrochemical conversion of CO2 into valued products is one of the most important issues but remains a great challenge in chemistry. Herein, we report a novel synthetic approach involving prolonged thermal pyrolysis of hemin and melamine molecules on graphene for the fabrication of a robust and efficient single‐iron‐atom electrocatalyst for electrochemical CO2 reduction. The single‐atom catalyst exhibits high Faradaic efficiency (ca. 97.0 %) for CO production at a low overpotential of 0.35 V, outperforming all Fe‐N‐C‐based catalysts. The remarkable performance for CO2‐to‐CO conversion can be attributed to the presence of highly efficient singly dispersed FeN5 active sites supported on N‐doped graphene with an additional axial ligand coordinated to FeN4. DFT calculations revealed that the axial pyrrolic nitrogen ligand of the FeN5 site further depletes the electron density of Fe 3d orbitals and thus reduces the Fe–CO π back‐donation, thus enabling the rapid desorption of CO and high selectivity for CO production.

Formation and migration of helium pair in bcc Fe from first principle calculations

We investigated the energetics and configurations of two interstitial He atoms in bcc Fe crystal using first-principle method. Two interstitial He atoms will bind together as helium pair if the initial He-He distance is less than 2.82 Å. From a formation energy decomposing analysis we found that, pairing of two He atoms would reduce the interface between He and Fe matrix which mitigates the perturbation of He-1s and Fe-3d orbitals. As a dominant part of the fomation energy, the electron variation energy caused by two close He atoms is less than that of two far He atoms, which accounts for the reason of binding between two close He atoms. Besides, the migration of helium pair along direction [1 0 0] was investigated by a He-He formation energy hypersurface. Through the comparison with Nudged Elastic Band calculation, it’s found that this special energy hypersurface was effective in revealing the migration of helium pair in metals.

Iron-incorporated chalcopyrite of an intermediate band for improving solar wide-spectrum absorption

The high efficiency chalcopyrite tandem cell with two solar absorbers of CuGaS2 and CuInSe2 remains difficult to be fabricated, but intermediate band chalcopyrite is practical to be developed. Herein, the Fe incorporation into the Ga site of chalcopyrite is successful to induce obvious solar multiband absorption. Three intermediate bands are evidenced by the UV–Vis–NIR absorption spectra, which is confirmed by first principle density functional theory (DFT) calculations. These intermediates bands improve solar energy conversion by absorbing more photons in several solar energy ranges including 2.21–1.48 eV, 1.48–0.73 eV, and around 0.57 eV to enhance solar thermal collection and photocatalytic degradation. The consistent nature of the intermediate bands suggested by theoretical analyses mainly are composed of the nondegenerate the eg and t2g states of Fe-3d orbitals.

Formally Ferric Heme Carbon Monoxide Adduct.

Formally ferric carbonyl adducts are reported in a series of thiolate-bound iron porphyrins. Resonance Raman data indicate the presence of both Fe-S and Fe-CO bonds, and EPR data of this S = 1/2 species indicate a ligand-based electron hole, giving this complex an Fe(II)-thiyl radical electronic ground state. The FTIR data show that the C-O vibrations are substantially higher than in the corresponding ferrous-thiolate CO adducts. DFT calculations reproduce the spectroscopic features and indicate that backbonding to the low lying π* orbitals of the bound CO stabilizes the Fe 3d orbitals resulting in a stabilization of the ferrous-thiyl radical ground state compared to the five-coordinate ferric-thiolate precursor complexes. Access to stable thiyl radicals will help understand these elusive species that are mostly encountered as short-lived reactive reaction intermediates.

Magnetic iron doping in Cu2SnS3 ceramics for enhanced thermoelectric transport properties

Thermoelectric properties of Fe-doped Cu2SnS3 (Cu2Sn1−xFexS3, x = 0–0.25) ceramics were studied in relation to magnetoelectric interaction. Phase composition changed from monoclinic-dominating to cubic-and-tetragonal-dominating with increasing x. Electrical conductivity improved effectively with a metallic behavior below 520 K, while it increased unusually at higher temperatures with x over 0.15, caused by an increased carrier concentration as a result of the ferro-to-paramagnetism transition in heavily doped samples. Benefiting from the Fe-induced spin entropy and the enhanced density of states due to the participation of Fe-3d orbitals, the Seebeck coefficient is much higher than that for non-magnetism doping, leading to a large power factor of 1.15 mW m−1 K−2. With a low lattice thermal conductivity owing to cation-disordering and soft bonding, a maximal ZT of ∼0.75 has been achieved at 723 K in the x = 0.15 sample, which stands one among the top of Cu-based ternary chalcogenides.

Detecting the orbital character of the spin fluctuation in the iron-based superconductors with the resonant inelastic X-ray scattering spectroscopy

The orbital distribution of the spin fluctuation in the iron-based superconductors (IBSs) is the key information needed to understand the magnetism, superconductivity and electronic nematicity in these multi-orbital systems. In this work, we propose that the resonant inelastic X-ray scattering (RIXS) technique can be used to probe selectively the spin fluctuation on different Fe 3d orbitals. In particular, the spin fluctuation on the three t2g orbitals, namely, the 3dxz, 3dyz and the 3dxy orbital, can be selectively probed in the scattering geometry by aligning the direction of the outgoing photon in the y-, x- and z-direction. Such orbital-resolved information on the spin fluctuation is invaluable for the study of the orbital-selective physics in the IBSs and can greatly advance our understanding of the relation between orbital ordering and spin nematicity in the IBSs and the orbital-selective pairing mechanism in these multi-orbital systems.

Ab initio study on structural and electronic properties of ReOFeAs (Re: La, Sm, Nd, Ce, Gd) under hydrostatic pressure

This study employs density-functional theory (DFT) to probe the effect of external pressure on the pristine ReOFeAs (Re: La, Sm, Nd, Ce, Gd). As-Fe-As angle depends non-linearly on pressure. In addition, As-Fe-As angle and As-distance from Fe-layer in SmOFeAs and NdOFeAs obtain a configuration met in unconventional superconductors at around 10 GPa. Last, the structural variations coincide with changes in the electronic density of states which stem from the Re-4f and Fe-3d orbitals.

Spin polarization and magnetic properties at the C60/Fe4N(001) spinterface

C60-Induced redistribution of Fe d-orbitals results in magnetic moment inversion, and the inverted spatial spin polarization is obtained.

First-Principles Calculations of the Structure and Magnetic Phases of FeAs2 Compound under Pressure

Understanding of different magnetic configurations for the FeAs2 iron pnictide compound is carried out using first-principles studies based on spin density functional theory (DFT) within the generalized gradient approximation (GGA), including the spin–orbit coupling (SOC). The calculated stable phase is in the marcasite (Pnnm) with nonmagnetic spin-ordering. We find that the FeAs2 compound in the nonmagnetic (NM) marcasite phase undergoes pressure-induced phase transition to the antiferromagnetic (AFM1) marcasite phase at 12[Formula: see text]GPa, then to the AFM CuAl2 ([Formula: see text]4/mcm) phase at 63[Formula: see text]GPa. The phase transition is also accompanied by semiconducting (marcasite phase) to metallic (CuAl2 phase) transition. The calculated electronic density of states profile shows the hybridization of the Fe-3[Formula: see text] and As-4[Formula: see text] orbitals plays an important role in determining the electronic and magnetic characters of this compound. The associated phase transition results in increased Fe-3d orbitals around the Fermi energy level.

Inverted orbital polarization in strained correlated oxide films

Manipulating the orbital occupation of valence electrons via epitaxial strain in an effort to induce new functional properties requires considerations of how changes in the local bonding environment affect the band structure at the Fermi level. Using synchrotron radiation to measure the x-ray linear dichroism of epitaxially strained films of the correlated oxide CaFeO3, we demonstrate that the orbital polarization of the Fe valence electrons is opposite from conventional understanding. Although the energetic ordering of the Fe 3d orbitals is confirmed by multiplet ligand field theory analysis to be consistent with previously reported strain-induced behavior, we find that the nominally higher energy orbital is more populated than the lower. We ascribe this inverted orbital polarization to an anisotropic bandwidth response to strain in a compound with nearly filled bands. These findings provide an important counterexample to the traditional understanding of strain-induced orbital polarization and reveal a new method to engineer otherwise unachievable orbital occupations in correlated oxides.

N2Reduction on Fe‐Based Complexes with Different Supporting Main‐Group Elements: Critical Roles of Anchor and Peripheral Ligands

AbstractDue to the enigmatic existence of the carbon atom in the MoFeS cluster of iron‐molybdenum cofactor (FeMoco), the design of biomimetic model catalysts featuring a dative bond between a transition metal and a main group atom is an important topic for efficient reduction of N2to NH3at ambient conditions. Different anchor atoms (X) for the trigonal bipyramidal (XPiPr3)Fe (X = B, C, N) catalyst scaffold are investigated by theory. The calculations show that from Lewis acidic B anchor to Lewis basic N or P anchor, the molecular orbital energy levels consisting mainly of Fe 3d orbitals are pushed higher, thus causing stronger backdonation bonding with theπ*orbitals of N2, which promotes activation of N2but impedes reduction of N2. The redox‐flexible bonding can act as an electron reservoir and buffer the oxidation state variation of Fe. However, when the FeX bond is less flexible, the peripheral ligands can serve as an extra electron reservoir in the model with Lewis basic anchor. Thus, engineering the peripheral ligand with proper electron‐withdrawing substituent groups can promote reduction of N2. The ability to balance well the electron‐donating ability of anchor and electrophilicity of the peripheral ligand is a promising direction toward novel efficient catalysts for N2fixation.

DFT study of electronic structure properties of SrAFe4As4 (A = Rb and Cs) superconductors

Using the full-potential linearised augmented plane wave method and the PBE generalized gradient approximation, the electronic structure properties of SrAFe4As4 (A = Rb and Cs) superconductors were determined and compared with those of their parents SrFe2As2 (Fmmm), SrFe2As2 (I4/mmm), RbFe2As2 and CsFe2As2. It is found that associated with the Van Hove singularities, densities of states of SrAFe4As4 around Fermi energy level, mostly caused by Fe-3d orbitals, are relatively high but not enough to account for enhancement of Tc in these materials. We suggest rather a relationship between Tc values and interband scattering strength, which depends on the number of electronic bands crossing EF and forming hole-like pockets around the Γ point and electron-like pockets at the corners of Brillouin zone. It is established that Fermi surfaces of SrAFe4As4 are described by 2D and manifest the behaviour of ±s-wave gap in Cs- and nodal gap in Rb-based. Electron Localization Function demonstrates the presence of both valence and metallic bondings. However, there is a stronger covalent bonding in AFe2As2 than in SrAFe4As4, suggesting that the weakness of covalency accompanies higher Tc values.

Origin of Improved Photoelectrochemical Water Splitting in Mixed Perovskite Oxides

AbstractOwing to the versatility in their chemical and physical properties, transition metal perovskite oxides have emerged as a new category of highly efficient photocatalysts for photoelectrochemical (PEC) water splitting. Here, to understand the underlying mechanism for the enhanced PEC water splitting in mixed perovskites, ideal epitaxial thin films of the BiFeO3–SrTiO3 system are explored. The electronic structure and carrier dynamics are determined from both experiment and density‐functional theory calculations. The intrinsic phenomena are measured in this ideal system, contrasting to commonly studied polycrystalline solid solutions where extrinsic structural features obscure the intrinsic phenomena. It is determined that when SrTiO3 is added to BiFeO3 the conduction band minimum position is raised and an exponential tail of trap states from hybridized Ti 3d and Fe 3d orbitals emerges near the conduction band edge. The presence of these trap states strongly suppresses the fast electron–hole recombination and improves the photocurrent density in the visible‐light region, up to 16× at 0 VRHE compared to the pure end member compositions. This work provides a new design approach for optimizing the PEC performance in mixed perovksite oxides.

An investigation of the interrelationship between pressure and correlation in LaFeAsO under pressure

Here, we investigated the interrelationship between pressure and correlation in the LaFeAsO compound by the density functional theory method combined with the dynamical mean field theory (DFT + DMFT) method. The spectral function and the occupation number (ON) of Fe-3d orbitals at different pressures were extracted from the calculations, and the importance of the role of correlation in superconductivity in iron-based compounds was indicated. The measured ON of Fe-3d orbitals demonstrated charge transfer between them and also the crucial role of the Fe-3dxy (xy) orbital in superconductivity in these materials. A switching behavior between high and low TC through decreasing the correlation was observed in our calculations, originating from the orbital switching. This shows the significant role of the orbital degrees of freedom in iron-based superconductors.

Non-covalent control of spin-state in metal-organic complex by positioning on N-doped graphene

Nitrogen doping of graphene significantly affects its chemical properties, which is particularly important in molecular sensing and electrocatalysis applications. However, detailed insight into interaction between N-dopant and molecules at the atomic scale is currently lacking. Here we demonstrate control over the spin state of a single iron(II) phthalocyanine molecule by its positioning on N-doped graphene. The spin transition was driven by weak intermixing between orbitals with z-component of N-dopant (pz of N-dopant) and molecule (dxz, dyz, dz2) with subsequent reordering of the Fe d-orbitals. The transition was accompanied by an electron density redistribution within the molecule, sensed by atomic force microscopy with CO-functionalized tip. This demonstrates the unique capability of the high-resolution imaging technique to discriminate between different spin states of single molecules. Moreover, we present a method for triggering spin state transitions and tuning the electronic properties of molecules through weak non-covalent interaction with suitably functionalized graphene.

Electronic structure and fermiology of LaFe2Ge2

The Density functional theory has been used to investigate the electronic structure properties of layered ternary compound LaFe2Ge2. The main trends in band structure, Fermi level, DOS, and charge density for the Fe2Ge2-layer with nonmagnetic La atom have been analyzed. The electron charge density has shown a hybridization between Fe-3d and Ge-4p causing the splitting of Fe-3d holes and electrons states which increases the Fe magnetic moment. The DOS of Fe-3d and Ge-4p orbitals has indicated that only a small set of 3d and 4p electronic states has been hybridized just under Fermi energy. Additionally, most of Fe-3d and Ge-4p states have been hybridized in the energy range from −5 eV to −2 eV. The magnetic moment of LaFe2Ge2 is 4.96 μB per formula unit where a localized magnetic moment of 2.54 μB is associated with the Fe muffin-tin sphere. The Fe-4s and Ge-4s, 4p bands crossing Fermi level have formed a complicated Fermi surface for majority channel, whereas the spin down Fermi surface consists of simple narrow tubes.

The structural, elastic, electronic and optical properties of orthorhombic FeX2 (X=S, Se, Te)

The structural, elastic, electronic and optical properties of FeX2 (X = S, Se, Te) with orthorhombic (space group Pnnm) structure have been investigated through the first-principles calculations. The calculated lattice constants, internal structure parameters and two kinds of Fe-X bond lengths agree reasonably with the experimental results. The corresponding single crystal elastic constants, polycrystalline elastic moduli, transverse, longitudinal, average elastic wave velocities, Debye temperature and minimum thermal conductivity decrease from FeS2 to FeTe2 while the elastic anisotropy index increases from FeS2 to FeTe2. All the three compounds show the brittle nature and covalent bonding. The orthorhombic FeX2 are non-magnetic semiconductors with indirect band gaps of 1.17eV, 0.64eV and 0.27eV, respectively. There have strong hybridizations between Fe-d orbitals and X-p orbitals. The real part and imaginary part of the dielectric function, refractive index and absorption coefficient are obtained and analyzed, the results show that the three compounds possess excellent absorption properties in ultraviolet and visible light regions. Both elastic properties and optical properties exhibit obvious anisotropy in the three compounds.

Large Charge-Transfer Energy in LiFePO4 Revealed by Full-Multiplet Calculation for the Fe L3 -edge Soft X-ray Emission Spectra.

We analyzed the Fe 3d electronic structure in LiFePO4 /FePO4 (LFP/FP) nanowire with a high cyclability by using soft X-ray emission spectroscopy (XES) combined with configuration-interaction full-multiplet (CIFM) calculation. The ex situ Fe L2,3 -edge resonant XES (RXES) spectra for LFP and FP are ascribed to oxidation states of Fe2+ and Fe3+ , respectively. CIFM calculations for Fe2+ and Fe3+ states reproduced the Fe L3 RXES spectra for LFP and FP, respectively. In the calculations for both states, the charge-transfer energy was considerably larger than those for typical iron oxides, indicating very little electron transfer from the O 2p to Fe 3d orbitals and a weak hybridization on the Fe-O bond during the charge-discharge reactions.

Structural, optical and electronic properties of RFeO3

Effect of R site ionic radius on the structural, optical and electronic properties of rare-earth orthoferrites (RFeO3) (where, R = La, Pr, Nd, Sm, Eu, Tb, Dy, Y, Ho, Er, and Yb) have been investigated experimentally and theoretically. Synchrotron-based X-ray diffraction studies followed by Rietveld refinement confirmed that the lattice constants, R–O bond length and Fe–O–Fe bond angle increase systematically, whereas the Fe–O bond length (i.e. overlap between O-2p and Fe-3d orbitals) associated with FeO6 octahedra almost remains constant throughout RFeO3 series. The energy bandgap (Eg) of all the samples have been measured using optical absorption spectroscopy and the same is found between ~ 1.94 and 1.97 eV, i.e. Eg almost remains constant throughout RFeO3 series. First principle DFT analysis confirmed that the “density of states” near Fermi level in all the studied samples is dominated by O-2p and Fe-3d orbitals. Thus, it appears that the constant value of Fe–O bond length (i.e. Fe-O overlap) associated with FeO6 octahedra might be responsible for the constant value of band gap in these samples.