The highest oxidation state observed in graphene-supported sub-nanometer iron oxide clusters

Size-selected iron oxide nanoclusters are outstanding candidates for technological-oriented applications due to their high efficiency-to-cost ratio. However, despite many theoretical studies, experimental works on their oxidation mechanism are still limited to gas-phase clusters. Herein we investigate the oxidation of graphene-supported size-selected Fe_n clusters by means of high-resolution X-ray Photoelectron Spectroscopy. We show a dependency of the core electron Fe 2p_3/2 binding energy of metallic and oxidized clusters on the cluster size. Binding energies are also linked to chemical reactivity through the asymmetry parameter which is related to electron density of states at the Fermi energy. Upon oxidation, iron atoms in clusters reach the oxidation state Fe(II) and the absence of other oxidation states indicates a Fe-to-O ratio close to 1:1, in agreement with previous theoretical calculations and gas-phase experiments. Such knowledge can provide a basis for a better understanding of the behavior of iron oxide nanoclusters as supported catalysts.

Evolution of Ferromagnetic and Antiferromagnetic States in Iron Nitride Clusters FenN and FenN2 (n = 1-10)

First-principles density functional theory calculations on neutral and singly negatively and positively charged iron clusters Fe_n and iron nitride clusters Fe_nN and Fe_nN₂ (n = 1-10) in the range of 1 ≤ n ≤ 10 revealed that there is a strong competition between ferromagnetic and antiferromagnetic states especially in the Fe_nN₂^0,±1 cluster series. This phenomenon was related to superexchange via a bridging N atom between two iron atoms in the Fe_nN₂^0,±1 cluster series and to a double superexchange effect via a Fe atom shared by two N atoms in the Fe_nN₂^0,±1 series. A thorough examination of the structure-energy-spin state relationships in these clusters is conducted, leading to new insights and confirmation of available experimental results on structural parameters and dissociation energetics. The bond energies of both nitrogen atoms in the Fe_nN₂ series are approximately the same. They weakly depend on the charge of the host cluster and fluctuate around 5.5 eV when moving along the series. The energy of N₂ desorption is relatively small; it varies by about 1.0 eV and depends on the charge of the cluster. The experimental finding that N₂ dissociates on the Fe_n⁺ clusters beginning with n = 4 was supported by the results of our computations. Our computed values of the Fe_n⁺-N bonding energies agree with the experimental data within the experimental uncertainty bars. It was found that the attachment of one or two N atoms does not seriously affect the polarizability, electron affinity, or ionization energy of the host iron clusters independent of the charge.

Anisotropic thermoelectric properties of Weyl semimetal NbX (X = P and As): a potential thermoelectric material

Weyl semimetal, a newly developed thermoelectric material, has aroused much interest due to its extraordinary transport properties. In this work, the thermoelectric transport properties of NbX (X = P and As), a prototypical Weyl semimetal, are investigated using the first-principles calculations together with Boltzmann transport theory. The calculated room-temperature lattice thermal conductivities along the a and c directions are 2.0 W mK-1 and 0.6 W mK-1 for NbP and 1.4 W mK-1 and 0.4 W mK-1 for NbAs, respectively. The low thermal conductivities may be useful in the thermoelectric applications. It is found that the acoustic branches have obvious contribution to the total lattice thermal conductivity, and the size dependence of the thermal conductivities can provide guidance for designing thermoelectric nanostructures. Our results show that the anisotropic structures of these compounds bring about the anisotropy of transport coefficients along the a and c directions, and the preferred direction is the c direction in thermoelectric applications. Moreover, NbP and NbAs show high ZT values of 0.82 and 0.50 along the c direction for p-type at an optimal carrier concentration, indicating that they are potential thermoelectric materials.

Hydrogenation of 3d-metal oxide clusters: Effects on the structure and magnetic properties

The geometrical structures and properties of the M₈ O₁₂ , M₈ O₁₂ H₈ , and M₈ O₁₂ H₁₂ clusters are explored using density functional theory with the generalized gradient approximation for all 3d-metals M from Sc to Zn. It is found that the geometries and total spin magnetic moments of the clusters depended strongly on the 3d-atom type and the hydrogenation extent. More than the half of all of the 30 clusters had singlet lowest total energy states, which could be described as either nonmagnetic or antiferromagnetic. Hydrogenation increases the total spin magnetic moments of the M₈ O₁₂ H₁₂ clusters when MMnNi, which become larger by four Bohr magneton than those of the corresponding unary clusters M₈ . Hydrogenation substantially affects such properties as polarizability, forbidden band gaps, and dipole moments. Collective superexchange where the local total spin magnetic moments of two atom squads are coupled antiparallel was observed in antiferromagnetic singlet states of Fe₈ O₁₂ H₈ and Co₈ O₁₂ H₈ , whereas the lowest total energy states of their neighbors Mn₈ O₁₂ H₈ and Ni₈ O₁₂ H₈ are ferrimagnetic and ferromagnetic, respectively. Hydrogenation leads to a decrease in the average binding energy per atom when moving across the 3d-metal atom series. © 2018 Wiley Periodicals, Inc.

FeOCl/POM Heterojunctions with Excellent Fenton Catalytic Performance via Different Mechanisms

To enhance the Fenton catalytic performance in a neutral solution under indoor sunlight, a novel FeOCl/polyoxometalate (POM) (FeOCl/POM-W and FeOCl/POM-Mo) composite was successfully synthesized for the first time, which shows significantly improved Fenton catalytic activity and stability for phenol degradation compared with FeOCl. Furthermore, the degradation constants ( k) of FeOCl/POM-Mo (0.08 min^-1) and FeOCl/POM-W (0.06 min^-1) are a factor of 4 and 3 times greater than that of FeOCl (0.02 min^-1), respectively. The enhanced catalytic activity is attributed to the formation of FeOCl/POM heterojunctions, which results in efficient separation of photoinduced electron-hole pairs and electron transfer from POM to FeOCl. Density functional theory calculations indicate a strong interface interaction of Fe-O-Mo and Fe-O-W in the FeOCl/POM heterojunctions. A Z-scheme mechanism for FeOCl/POM-Mo and a double-transfer mechanism for FeOCl/POM-W are proposed for the enhanced catalytic performance. This study sheds new light on the design and fabrication of high-performance photo-Fenton catalysts to overcome the environmental crisis.

Collective Superexchange and Exchange Coupling Constants in the Hydrogenated Iron Oxide Particle Fe8O12H8

Motivated by the fact that Fe₂O₃ nanoparticles are used in the treatment of cancer, we have examined the role of ligands on the magnetic properties of these particles by focusing on (Fe₂O₃)₄ as a prototype system with H as ligands. Using the Broken-Symmetry Density Functional Theory, we observed a strong collective superexchange in the hydrogenated Fe₈O₁₂H₈ cluster. The average antiferromagnetic exchange coupling constant between the four iron-iron oxo-bridged pairs was found to be -178 cm^-1, whereas coupling constants between hydroxo-bridged pairs were much smaller. We found that despite the apparent symmetry of the iron atom framework, it is not reasonable to assume this symmetry when fitting the exchange coupling constants. We also analyzed the geometrical and magnetic properties of Fe₈O₁₂H _n for n = 0-12 and found that hydrogenating oxo-bridges would generally inhibit the Fe-O-Fe antiferromagnetic superexchange interactions. Antiferromagnetic lowest total energy states become favorable only when specific distributions of hydrogen atoms are realized. The (HO)₄-Fe₄(all spin-up)-O₄-Fe₄(all spin-down)-(OH)₄ configuration in Fe₈O₁₂H₈ presents such an example. This symmetric configuration can be considered a superdiatomic system.