Cubo-octahedral Clusters Research Articles

Fundamental Aspects of Spontaneous Cathodic Deposition of Ru onto Pt/C Electrocatalysts and Membranes under Direct Methanol Fuel Cell Operating Conditions: An in Situ X-ray Absorption Spectroscopy and Electron Spin Resonance Study

In situ X-ray absorption spectroscopy (XAS), using both EXAFS and the Δμ-XANES analysis procedures, is utilized to examine Ru deposition onto Pt/C cathodes at millimolar concentration of Ru in a 1 M HClO4 electrolyte. Also, electron spin resonance (ESR) spectroscopy is utilized to examine the effects of Ru3+ ion exchanged Nafion membranes. The Δμ-XANES analysis of the data allows a determination of the coverage of Ru with time (minutes to hours) and also to identify the binding site of the deposited Ru species (atop/bridged at low coverage and 3-fold at higher coverage) apparently onto the corners and edges initially and at higher coverage onto the faces of the cubooctahedral clusters when exposed to Run+ at OCP. The deposition of Ru appears to be inhibited at potentials where adsorbates (such as H and O(H)) usually adsorb, and Coulomb enhanced at OCP when substantial O exists on the surface. The ESR analysis of Ru3+ in the Nafion membrane indicates significant detrimental changes to the membrane in the p...

Identification of the La6F37 cubooctahedral clusters in mixed crystals (BaF2)1 − x (LaF3) x by the electron paramagnetic resonance method

The electron paramagnetic resonance (EPR) spectra of mixed crystals (BaF2)1 − x (LaF3) x (x = 0, 0.001, 0.002, 0.005, 0.010, 0.020) doped with Ce3+ ions (0.1%) are investigated at a frequency v ≈ 9.5 GHz in magnetic fields up to 1.45 T at temperatures T = 10 and 15 K. The EPR spectrum of “pure” barium fluoride BaF2 (x = 0) is characterized by a single Ce3+-F− center with tetragonal symmetry (i.e., the O center with g ‖ = 2.601 and g ⊥ = 1.555). For a lanthanum trifluoride concentration x ≠ 0, the spectrum exhibits new lines due to the presence of the clusters containing Ce3+ and La3+ ions. The intensity of EPR signals from the O centers decreases rapidly as the lanthanum trifluoride concentration x increases. The lines attributed to a paramagnetic center with tetragonal symmetry and strongly anisotropic g factors (i.e., the K center with g ‖ = 0.725 and g ⊥ = 2.52) are separated in the complex EPR spectrum with the use of the angular dependence of the EPR signal intensity measured for the samples with x ≥ 0.002. This center is identified as a cubooctahedral cluster of the La6F37 type in which one of the La3+ ions is replaced by the Ce3+ ion.

Ab initiotest of the Warren–Averbach analysis on model palladium nanocrystals

Model powder diffraction patterns were calculatedviathe Debye formula from atom positions of a range of energy-relaxed closed-shell cubooctahedral clusters. The energy relaxation employed the Sutton–Chen potential scheme with parameters for palladium. The assumed cluster size distribution followed lognormal distribution of a crystallite volume centred with the diameter of 5 nm, as well as two bimodal lognormal distributions centred around 4 nm and 7 nm. These models allowed an in-depth analysis of the Warren–Averbach method of separating strain and size effects in a peak shape Fourier analysis. The atom-displacement distribution in the relaxed clusters could be directly computed, as well as the strain Fourier coefficients. The results showed that in the case of the unimodal size distribution, the method can still be successfully used for obtaining the column length distribution. However, the strain Fourier coefficients obtained from three reflections (002, 004 and 008) cannot be reliably estimated with the Warren–Averbach method. The primary cause is a non-Gaussian strain distribution and a shift of the diffraction maximum, inherent to the nanoparticles, differing for every constituent cluster in the size distribution. For the bimodal size distributions, the obtained column length distributions tend to be shifted towards the centres of the modes and are less sensitive to the larger size mode.

Electron Paramagnetic Resonance in Mixed Crystals (BaF[sub 2])[sub 1 – x](LaF[sub 3])[sub x] Activated by Ce[sup 3+] Ions

The electron paramagnetic resonance (EPR) spectra of mixed crystals (BaF2)1−x− y (LaF3)x(CeF3)y (y = 0.001 = 0.1%, x = 0–0.02) are investigated in a magnetic field H‖C4 at a frequency of 9.5 GHz. The angular dependence of the EPR spectrum is measured for the sample with x = 0.02. The lines attributed to Ce3+ impurity centers with tetragonal symmetry and g factors (g ‖ = 0.75, g ⊥ = 2.4) close to those measured for the KY3F10: Ce3+ compound are separated in the complex EPR spectrum. The assumption is made that the aforementioned impurity centers are cubooctahedral clusters of the La6F37 type in which one of the La3+ ions is replaced by the Ce3+ ion.

Transferable tight-binding model for palladium and silver

A transferable orthogonal tight-binding (TB) model for Pd and Ag is developed by the simultaneous fit to the energy levels and total energies of a number of clusters of up to 55 atoms in size, as well as the total energies of fcc and bcc structures all obtained from density functional calculations. The effect of the environment is included by incorporating two-center intra-atomic parameters and local environment dependent on-site atomic energy levels. The scheme is tested by comparing its predictions of both bulk and cluster properties with those from ab initio calculations. It is then used to study the behavior of relaxed icosahedral and cubo-octahedral clusters of up to 201 atoms in size.

Hydrothermal Syntheses, Structures, and Magnetic Properties of the U(IV) Fluorides (C5H14N2)2U2F12·5H2O and (NH4)7U6F31

The U(IV) fluorides (C5H14N2)2U2F12·5H2O (AU1-2) and (NH4)7U6F31 have been prepared from the reaction of UO3 with HF and homopiperazine (C5H12N2) at 180°C in aqueous media. AU1-2 contains dodecahedral UF8 units that share two edges to form 1∞[UF4F4/2] one-dimensional chains. The fluoride ligands form hydrogen bonds with the diprotonated homopiperazinium cations to yield channels running down the a axis that contain occluded water molecules. (NH4)7U6F31 is isostructural with Na7Zr6F31, forming a three-dimensional network consisting of edge- and corner-sharing UF8 square antiprisms that create cubooctahedral clusters and small intersecting channels. A disordered fluoride anion resides in the cubooctahedral cavities and the ammonium cations partially fill the channels. AU1-2 and (NH4)7U6F31 display similar magnetic behavior with μeff (300 K)=3.05 and 3.03 μB/U(IV) and θ=−97 and −126 K, respectively. Crystallographic data: AU1-2, monoclinic, space group P21/n, a=6.545(3) Å, b=19.90(1) Å, c=19.07(1) Å, β=98.89(4)°, Z=4, λ=0.71073 Å, R(F)=0.0395 for 293 parameters with 3282 reflections with I>2σ(I); (NH4)7U6F31, rhombohedral, space group R3, a=15.371(3) Å, c=10.791(3) Å, Z=3, λ=0.71073 Å, R(F)=0.0242 for 82 parameters with 841 reflections with I>2σ(I)

Electronic structure and magnetism in 4 -transition metal clusters

We analyze the stability of magnetic states obtained within the tight-binding model for cubooctahedral (Oh) and icosahedral (Ih) clusters of early 4d (Y, Zr, Nb, Mo, and Tc) transition metals. Several metastable magnetic clusters are identified which suggests the existence of multiple magnetic solutions in realistic systems. A bulk-like parabolic behavior is observed for the binding energy of Oh and Ih clusters as a function of the atomic number along the 4 d-series. The charge transfer on the central atom changes sign, while the average magnetic moments present an oscillatory behavior as a function of the number of d electrons in the cluster. Our results are in agreement with other theoretical calculations.

Investigations on supported bimetallic PdPt nanostructures

PdPt bimetallic clusters have been produced by a laser vaporisation source and studied after deposition on a support. We have been able to obtain clusters with a well-defined composition, identical to that of the target rod as deduced from energy dispersive X-ray analysis measurements. An evaporation of Pd atoms upon the impact of the electron beam has been observed, suggesting that the surfaces of the clusters are Pd-enriched. The segregation behaviour using Monte Carlo simulations has been studied for the Pd 15Pt 85 and Pd 50Pt 50 cubo-octahedral clusters composed of 586 and 2406 atoms. The results indicate a large Pd segregation in all the systems, in qualitative agreement with the experimental observations, with preferential occupation of corner and edge sites.

Stability of the ferromagnetic state of Fe atoms at the surface of solids and small clusters

Electronic and magnetic states of Fe atoms of cubo-octahedral clusters of pure iron have been studied in the framework of the coherent locator approximation combined with the continued fraction method and the Hartree-Fock approximation. The calculation shows the stability of the ferromagnetic state of Fe atoms at the cluster and solid surface of pure iron. An analogous investigation of the Fe-Zr system in the ternary alloy approximation resulted in a small number of Fe atoms with magnetic moments oriented anti-parallel with respect to the bulk magnetization, which diminishes with a decrease of the cluster size.

On the influence of topology on the energy profile in metallic Pd clusters

A main ingredient to model the physics of metallic clusters is the knowledge of the spatial and energetic distribution of the atoms. Quenched molecular dynamics simulations with interatomic potentials derived from the electronic structure (tight-binding approximation) show that Pd cubo-octahedral and icosahedral clusters undergo an inhomogeneous contraction, restricted to the surface selvedge and, in the latter case, to the inner core. This wrinkled profile leads to interatomic distances which can be larger than the bulk Pd ones for icosahedral clusters, whereas they all are smaller for cubo-octahedral clusters. Moreover it induces, for the icosahedral shape, the occurrence of a core energy in the energy profile, which leads to a structural transition from icosahedron to cubo-octahedron when the cluster size reaches about 2000 atoms.

Analysis of ac-conductivity data of Ba 2− xBi xF 2+ x solid solutions

Charge carriers concentrations N in the Ba 1− x Bi x F 2+ x (0⩽×⩽0.45) solid solutions were estimated from an analysis of temperature and frequency dependences of the bulk ac-conductivities. They range from 3.10 21 to 1.10 22 (cm) -3 and are temperature independent. In addition, for the (x = 0.05) sample, the parameters of NNN-dipoles are estimated as follows: the reorientation frequency is ν d = 6.10 13 exp.(-0.60 eV./kT) s -1 and the NNN-dipoles concentration is N d = 9.10 19 (cm) -3. The comparison of the N values with the extended defect concentration estimated on the basis of the clustering model which assumes in Ba 1− x Bi x F 2+ x the progressive transformation with increasing x of 4:4:3:0 clusters into 8:12:1:0 cubooctahedral clusters, allows to propose that charge carriers governing the ionic conductivity properties are likely the interstitial fluoride ions belonging to the 4:4:3:0 clusters.

Geometrical characteristics of compact nanoclusters

The geometrical characteristics of icosahedral and cubo-octahedral clusters are studied. The various shells around the central atom that form the cluster, the number of atoms forming each shell, the number of nearest neighbors to each atom, the various surface sites and the total number of bonds are analyzed. General expressions for the total number of atoms, the total number of surface atoms and their distribution and the total number of bonds are given. Some considerations on the cluster shape stability are also discussed.

Chemisorption effects on the spatial atomic distribution of bimetallic nanostructures

The spatial atomic distribution in icosahedral and cubo-octahedral bimetallic clusters with a total number of atoms, N=55 and 147, in the presence of chemisorbed species, is studied. The equilibrium atomic configuration is obtained by calculating the free energy within the regular solution model and by assuming that the surface of the cluster is covered by chemisorbed atoms. Depending on the interaction between chemisorbed species and the cluster components, the atomic distribution in the cluster can be completely modified as compared to the case of clusters with a clean surface. Applications to the chemisorption of H and O on CuNi, CuPd, and NiPt clusters are discussed. We present results for the temperature dependence of the concentration at the different shells around the central atom.

Ionic conduction mechanisms in the Ba 1−xBi xF 2+x solid solution quenched from 700°C : conductivity, NMR and neutron diffraction

An investigation of the Ba 1−xBi xF 2+x (0 ⩽ x ⩽ 0.45) solid solution of CaF 2-type structure has been carried out by conductivity measurements, neutron diffraction and 19F NMR on samples quenched from 700°C . The application of the “clustering processes” model [1] to Ba 1−xBi xF 2+x which involves a conductivity maximum for xmax. # 0.35 has allowed to propose within that solid solution the progressive transformation of 4:4:3 clusters into 8:12:1 cubooctahedral clusters when x increases . The clustering process proposed in Ba 1−xBi xF 2+x is in agreement with the distribution of the fluoride ions in normal and interstitial sites determined by neutron diffraction. The NMR results are interpreted on hand of the existence of different fluoride sublattices and partial exchanges between them at rising temperature . The diffusive character of the mobile F − ions motion is shown above T # 320 K . The NNR study confirms so the validity of clustering process proposed in Ba 1−xBi xF 2+x and shows the unlike electrical behavior of interstitial fluoride ions belonging to 4:4:3 and 8:12:1 clusters.

Bimetallic nanostructures: I. General aspects and the ground state

A systematic study of ordering and segregation in bimetallic nanoclusters is presented. Icosahedral and cubo-octahedral clusters with a total number of particles, N = 55 and 147, are studied. The spatial distribution of the two components, in the cluster, is obtained by calculating the free energy within the regular solution model. Depending on the heat of mixing and on the difference in the cohesive energy of the components, and as a consequence of the large surface to volume ratio, one obtains a variety of phases characteristic only of the nanosystems. By comparing the total energies of the icosahedron and the cubo-octahedron the equilibrium shape is obtained as function of the cluster size and of the interaction energies.

Bimetallic nanostructures: II. Finite temperature and applications

A systematic study of ordering and segregation at finite temperatures in bimetallic nanoclusters is presented. Icosahedral and cubo-octahedral clusters, with a total number of atoms, N = 13, 55 and 147, are studied. The equilibrium configuration is obtained by calculating the free energy within the regular solution model. The theory is applied to CuPd, NiPt and CuNi nanoclusters. We present results for the temperature dependence of the concentrations at the different shells around the central atom. In most of the cases a strong segregation is found.

Corrélations entre propriétés électriques et ordre à courte distance au sein des solutions solides Ca 1− xY xF 2+ x et Ca 1− xLu xF 2+ x

A model has been proposed to correlate the composition dependence of electrical properties and the progressive cluster growth with increasing x in a fluoride anion excess CaF 2-type solid-solution of M 2+ 1− x M′ 2+ α x F 2+ αx ( α = 1,2,3). According to the model, the density of interstitial fluorine ions and the density of vacancies at normal sites are respectively represented by the functions y int. and y □ which depend on three parameters called λ, m, and x e: Y int. = (λ−α)mx 3+λ(m−α)x 2 e (λ−α)x 2+(m−α)x 2 e Y □ = (λ−α)(m−α)(x 3+x 2 e x (λ−α)x 2+(m−α)x 2 e The λ and m parameters respectively define the clustering conditions for the lowest and highest values of x. The x e parameter is the degree of substitution for which a maximum of conductivity in the composition dependence of the electrical properties is encountered. This model, applied to the solid-solutions Ca 1− x Y x F 2+ x (0 ⩽ x ⩽ 0.38) and Ca 1− x Lu x F 2+ x (0 ⩽ x ⩽ 0.40), shows the progressive transformation of 4:4:3 monomer clusters into cubooctahedral 8:12:1 clusters when x increases. The percentage of cubooctahedral clusters in these solid-solutions quenched from 950°C has been determined as a function of composition.

Spatial atomic distribution in small bimetallic clusters

Within the regular solution model the spatial distribution of atoms in cubo-octahedral clusters (55 atoms) is studied. All the sites located at the same distance from the center are considered equivalent and the atoms distributed at random with a characteristic concentration x i ( i = 1, 2, 3, 4). For a system that orders at low temperatures, we calculate the equilibrium values at different temperatures.

Distinguishing icosahedral and cubooctahedral metal clusters by HREM

Distinguishing icosahedral and cubooctahedral metal clusters by HREM

Electronic structure of cubooctahedral clusters of platinum

A study of the electronic structure of small cubooctahedral (fcc) clusters of Pt has been made using the moments method within the framework of the tight binding approximation. We have studied the local electronic density of states (LDS) both by neglecting and by taking into account the effect of spin-orbit (SO) coupling, which is important in Pt. Our results show a marked variation in the LDS on some of the surface atoms when the SO effects are included. The effect is particularly spectacular for the LDS on the corner atoms. In the absence of SO effects, these atoms have a LDS whose central peak becomes important as the cluster size is increased. The inclusion of SO coupling modifies the LDS leading to a deep minima in the centre of the band. Our results, given up to a maximum cluster size of 923 atoms, show that this minimum rests constant with the cluster size. On the other hand, the LDS on the central atoms show a multipeaked structure with some change on the inclusion of SO coupling. For these atoms, the increase in cluster size affects the intensities of the various peaks and the LDS finally converge towards that in the bulk. The opening of a dip in the density of states on the corner atoms is likely to have interesting consequences on the surface magnetism and chemisorptive properties of these aggregates. We are currently studying these effects.