Sanjubala Sahoo

Mesoporous Iron Sulfide for Highly Efficient Electrocatalytic Hydrogen Evolution

We report a facile synthetic protocol to prepare mesoporous FeS2 without the aid of hard template as an electrocatalyst for the hydrogen evolution reaction (HER). The mesoporous FeS2 materials with high surface area were successfully prepared by a sol-gel method following a sulfurization treatment in an H2S atmosphere. A remarkable HER catalytic performance was achieved with a low overpotential of 96 mV at a current density of 10 mA·cm-2 and a Tafel slope of 78 mV per decade under alkaline conditions (pH 13). The theoretical calculations indicate that the excellent catalytic activity of mesoporous FeS2 is attributed to the exposed (210) facets. The mesoporous FeS2 material might be a promising alternative to the Pt-based electrocatalysts for water splitting.

First-principles studies on graphene-supported transition metal clusters.

Theoretical studies on the structure, stability, and magnetic properties of icosahedral TM13 (TM = Fe, Co, Ni) clusters, deposited on pristine (defect free) and defective graphene sheet as well as graphene flakes, have been carried out within a gradient corrected density functional framework. The defects considered in our study include a carbon vacancy for the graphene sheet and a five-membered and a seven-membered ring structures for graphene flakes (finite graphene chunks). It is observed that the presence of defect in the substrate has a profound influence on the electronic structure and magnetic properties of graphene-transition metal complexes, thereby increasing the binding strength of the TM cluster on to the graphene substrate. Among TM13 clusters, Co13 is absorbed relatively more strongly on pristine and defective graphene as compared to Fe13 and Ni13 clusters. The adsorbed clusters show reduced magnetic moment compared to the free clusters.

Ni9Te6(PEt3)8C60 Is a Superatomic Superalkali Superparamagnetic Cluster Assembled Material (S3-CAM)

First-principles theoretical studies enable an electronic and magnetic characterization of the recently synthesized Ni9Te6(PEt3)8C60 ionic material consisting of Ni9Te6(PEt3)8 superatoms and C60. The PEt3 ligands are shown to create an internal coulomb well that lifts the quantum states of the Ni9Te6 cluster, lowering its ionization potential to 3.39 eV thus creating a superalkali motif. The metallic core has a spin magnetic moment of 5.3 μB in agreement with experiment. The clusters are marked by low magnetic anisotropy energy (MAE) of 2.72 meV and a larger intra-exchange coupling exceeding 0.2 eV, indicating that the observed paramagnetic behavior around 10K is due to superparamagnetic relaxations. The magnetic motifs separated by C60 experience a weak superexchange that stabilizes a ferromagnetic ground state as observed around 2 K. The calculated MAE is sensitive to the charged state that could account for the observed change in magnetic transition temperature with size of the ligands or anion.

First-principles investigations of multimetallic transition metal clusters

This brief overview summarizes the state-of-the-art of simulations of transition metal nanoclusters based on density functional theory calculations. Besides the monometallic clusters like iron, we focus on alloy nanoclusters like Fe-Pt, Co-Pt and (Ni, Co)-Mn-Ga which are of current interest for recording media and actuators involving the magnetic shape memory effect, respectively. Although catalysis is not the subject of the present paper, trimetallic nanoclusters are of special interest because the third element can be used to achieve higher catalytic and selective properties compared to the corresponding monometallic and bimetallic clusters. For clusters of Fe-Pt and Co-Pt below a critical size, the L12 structure with its technologically relevant high magnetocrystalline anisotropy, is difficult to stabilize. For trimetallic systems like Ni-Mn-Ga, the rather versatile properties of the bulk material can be used to achieve shape changes or magnetocaloric effects (depending on the composition) also in nanoclusters. More importantly, it might be cheaper to manufacture the nanocrystalline materials from the trimetallic nanoclusters than to fabricate corresponding single-crystal bulk systems.

Segregation and ordering in binary transition metal clusters

Quantum mechanical calculations on 13 and 55-atom Fe–Ni clusters with icosahedral geometry have been performed using density functional theory (DFT) and a generalised gradient approximation (GGA) based exchange and correlation functional. It is observed that in the lowest-energy isomers found for both system sizes and all studied compositions, Fe atoms occupy the central position. Ni atoms, however, are scattered to maximise the number of Fe–Ni bonds, and thus segregate to the surface. The mixing energy for icosahedral clusters, with respect to composition, shows a similar trend as in bulk.

First-principles calculation of cluster geometries and magnetization of pure Ni and Fe–Ni clusters

We report the results of ab initio calculations for Ni N clusters with N varying in the range 2 ≤ N ≤ 16 as well as for Fe8Ni5 clusters using density functional theory within the generalized gradient approximation. The cluster geometries are relaxed with no symmetric constraints allowing for non-collinear magnetization density. The lowest-energy structures obtained are used to evaluate the magnetic moment, binding energy and the HOMO–LUMO gap of Ni N clusters. The results are compared with experimental data and previous ab initio calculations. The properties of mixed 13-atom icosahedral clusters for a Fe8Ni5 composition are also investigated. The lowest-energy structure is determined by the Ni atoms occupying the surface positions and forming a ring with a large number of Fe–Ni bonds.

Effect of anisotropy on small magnetic clusters

The effect of dipolar interaction and local uniaxial anisotropy on the magnetic response of small spin clusters where spins are located on the vertices of icosahedron, cuboctahedron, tetrahedron and square geometry have been investigated. We consider the ferromagnetic and antiferromagnetic spin-1/2 and spin-1 Heisenberg model with uniaxial anisotropy and dipolar interaction and apply numerical exact diagonalization technique in order to study the influence of frustration and anisotropy on the ground state properties of the spin-clusters. The ground state magnetization, spin-spin correlation and several thermodynamic quantities such as entropy and specific heat are calculated as a function of temperature and magnetic field.

Conceptual Basis for Understanding C-C Bond Activation in Ethane by Second Row Transition Metal Carbides.

It has been suggested that the addition of carbon to Mo and W may improve their catalytic properties and even grant these metal carbides behaviors similar to those of late transition metals such as Pd and Pt. First-principles studies on the C-C bond activation of ethane by 4d transition metal (TM) atoms and TMC molecules have been carried out to develop a conceptual model underlying the changes. We find that the addition of carbon to TM atoms leads to large variations in the activation barrier depending on the metal, and that MoC indeed reveals a pronounced reduction in the C-C bond activation energy. A critical examination of molecular orbitals shows that the changes in reactivity are not linked to a dramatic increase in the filling of 4d states as implied by the analogy with Pd. The reactivity is governed by the location and filling of the 5s and 4d orbitals, with the different orbitals controlling different facets of reactivity. The 5s state controls the initial binding of ethane, with a strong anticorrelation between the ethane binding energy and the 5s occupation, while the location of the 4dz(2) orbital controls the reaction barrier that controls the activation energy for cleaving the C-C bond.

First-principles investigations of caloric effects in ferroic materials

We study the magnetic interactions in Ni-Mn-based Heusler alloys which are suitable candidates for refrigeration based on magnetocaloric, barocaloric, and elastocaloric effects, where the adiabatic temperature change of the Heusler material is induced by applying a magnetic field, hydrostatic pressure, or compressive strain. The predominantly ferromagnetic interactions of the Heusler alloys with austenite cubic structure at high temperatures are modified by the appearance of antiferromagnetic interactions in the alloys with Mn-excess because of the much shorter distances between the Mn-excess atoms and those on the original Mn-sublattice. This leads to a larger entropy change across the magnetostructural transformation in Ni50Mn25+x(Ga,In,Sn,Sb)25−x alloys and is also responsible for the appearance of the inverse magnetocaloric effect in the martensitic phase. In Ni-excess Ni-Mn-Ga alloys the influence of antiferromagnetic correlations is weaker and the large entropy change across the magnetostructural tr...

Using graphene to control magnetic anisotropy and interaction between supported clusters

Stabilization of magnetic order in clusters/nanoparticles at elevated temperatures is a fundamentally challenging problem. The magnetic anisotropy energy (MAE) that prevents the thermal fluctuations of the magnetization direction can be around 1–10 K in free transition metal clusters of around a dozen atoms. Here we demonstrate that a graphene support can lead to an order of magnitude enhancement in the anisotropy of supported species. Our studies show that the MAE of supported Co5 and Co13 clusters on graphene increase by factors of 2.6 and 25, respectively. The enhancement is linked to the splitting of selected electronic orbitals that leads to the different orbital contributions along the easy and hard axis. The conductive support enables a magnetic interaction between the deposited species and the nature of the magnetic interaction can be controlled by the separation between supported clusters or by vacancies offering an unprecedented ability to tune characteristics of assemblies.