Lusegen A. Bugaev

Atomic structure of nickel phthalocyanine probed by X-ray absorption spectroscopy and density functional simulations

The local atomic structure of Ni in nickel phthalocyanine was studied by K-edge X-ray absorption fine structure spectroscopy. The obtained inter atomic nickel-nitrogen distance differs from the reference X-ray diffraction data so an additional study was performed within density functional theory framework. The justification of the used theoretical approach was provided by a comparison of theoretical free electron densities of states with experimental Ni K-edge X-ray absorption near edge spectra. The refined Ni local environment retain the reference structure of the molecule except for the length of Ni-N bond which increases to 1.90 Å.

Copper-carbon nanocomposites prepared by sold-phase pyrolysis of copper phthalocyanine

By using solid-phase pyrolysis of copper phthalocyanine we have prepared copper nanoparticles in carbon matrices. The elemental composition, structure, and morphology of nanocomposites were investigated by scanning electron microscopy, energy dispersive X-ray microanalysis, and X-ray diffraction. Depending on the temperature and time of pyrolysis, the sizes of copper nanoparticles can be varied from 10 nm to 400 nm. The structure of carbon matrices also strongly depends on the pyrolysis conditions, which allows us to synthesize nanocomposites with given properties.

Atomic structure of PtCu nanoparticles in PtCu/C catalysts from EXAFS spectroscopy data

Deposited electrocatalysts with different distributions of components in PtCu bimetallic nanoparticles involved in their composition were synthesized by simultaneous and sequential reduction of Cu(2+) and Pt(IV) in a carbon suspension. The dependence of the atomic structure of PtCu nanoparticles on the synthesis conditions and the degree of influence of post-treatment was established from analysis of the changes in Fourier transforms of the experimental Pt and Cu EXAFS spectra, as well as the structural parameters obtained by their fitting before and after the treatment of the materials in an acid solution. A technique was proposed for visualizing the atomic structure of synthesized bimetallic nanoparticles. This technique made it possible to determine the character of the distribution of the components over the nanoparticle volume in accordance with the component composition and local atomic structure parameters determined from EXAFS spectroscopy and to obtain the visualization of the distribution of the components in PtCu nanoparticles synthesized by the aforementioned methods.

Effect of Aluminum on the Local Structure of Silicon in Zeolites as Studied by Si K Edge X-ray Absorption Near-Edge Fine Structure: Spectra Simulation with a Non-Muffin Tin Atomic Background

Experimental Si K edge X-ray absorption near-edge fine structure (XANES) of zeolite faujasite, mordenite, and beta are interpreted by means of the FEFF8 code, replacing the theoretical atomic background mu(0) by a background that was extracted from an experimental spectrum. To some extent, this diminished the effect of the inaccuracy introduced by the MT potential and accounted for the intrinsic loss of photoelectrons. The agreement of the theoretical and experimental spectra at energies above the white lines enabled us to identify structural distortion around silicon, which occurs with increasing aluminum content. The Si K edge XANES spectra are very sensitive to slight distortions in the silicon coordination. Placing an aluminum atom on a nearest neighboring T site causes a distortion in the silicon tetrahedron, shortening one of the silicon-oxygen bonds relative to the other three.

Construction of three-dimensional models of bimetallic nanoparticles based on X-ray absorption spectroscopy data

A new method for constructing three-dimensional models of bimetallic nanoparticles is proposed. This method, which is based on X-ray absorption spectroscopy data on the number and type of nearest neighbors, provides information on the distribution of types of atoms over the nanoparticle volume. The application of the method to the study of the structures of platinum–copper and platinum–silver nanoparticles of metal–carbon electrocatalysts has allowed to distinguish the nanoparticles with a core–shell structure from the nanoparticles with structure of disordered alloy or clusterized solid solution.

Determination of the local atomic structure of material from X-ray absorption spectroscopy data without fourier analysis of experimental spectra

A technique for analyzing the extended X-ray absorption fine structure spectra of an atom in different structural states in a material under study is proposed. This technique makes it possible to determine the parameters of the nearest environment of absorbing atoms without applying Fourier filtering and related techniques. The proposed approach is tested by an example of the L3 absorption spectra of platinum, obtained by direct calculation for models of one-component platinum nanoparticles and bimetallic Pt-Ag nanoparticles with different core and shell structures. The error in determining the structural parameters is analyzed, and the range of applicability of the technique proposed is discussed.

The Role of Hydrogen in Formation of Pd XANES in Pd-Nanoparticles

The effects of hydrogen in Pd nanoparticles on the features of Pd K-edge X-ray absorption near edge structure (XANES) are studied. Simulations indicate a linear dependence of the intensities of XANES maxima upon the concentration of hydrogen in the bulk of Pd clusters at low H-concentrations and stepwise increase of the first near edge peak at high concentrations. The effects of the diffusion of H atoms over the interstitial sites are considered. The application of the obtained results to the experimental spectra measured in PdH nanoparticles of sizes from 1.3nm to 10.5nm indicates low H-concentrations and high mobility of H-atoms in the bulk of the nanoparticles.

Pd hydride and carbide studied by means of Pd K-edge X-ray absorption near-edge structure analysis

A Pd K-edge X-ray absorption near edge structure (XANES) analysis for palladium hydride and carbide nanoparticles is presented. It is shown that the presence of H and C atoms changes the Pd unoccupied p-electronic states, affecting the near-edge fine structure. Quantitative XANES analysis is performed using a multidimensional interpolation approach. Theoretical models of PdH and PdC clusters are developed using Monte Carlo methods. The proposed technique allow us to determine hydrogen concentrations in palladium nanoparticles and correctly reproduce all experimental features of XANES spectra for palladium hydride and carbide nanoparticles.

Hydride phase formation in carbon supported palladium hydride nanoparticles by in situ EXAFS and XRD

In the current work we present a detailed analysis of the hydride phase formation in industrial Pd/C nanocatalysts by means of combined in situ X-ray absorption spectroscopy (EXAFS), X-ray diffraction (XRD) and volumetric measurements for the temperatures from - 10 to 50 °C in the hydrogen pressure range from 0 to 1000 mbar. α- and β- hydride phases are clearly distinguished in XRD. For the first time, H/Pd atomic ratio were obtained by theoretical fitting of the near-edge region of the absorption spectra (XANES) and compared with volumetric measurements.

XAFS Study of Local Atomic Structure in InAs at Low Pressures

Local structural distortions in Indium Arsenide at 11 GPa are studied applying the Fourier‐transform analysis to As K‐edge XAFS restricted to a short exploitable energy range of ∼ 250 eV above the edge, due to the Bragg reflections from the diamond anvils. The best model for the distribution of In‐atoms around As is obtained by combining available XRD data with a fitting procedure of the XAFS data, where alternative models for As coordination have been tested. InAs at 0.4 GPa was used as reference compound to minimize the number of variable parameters.