G. Atanasova

Electrochemical growth of thin La2O3 films on oxide and metal surfaces

This work presents a new electrochemical method (i.e. new electrolyte composition and electrolysis regime) for direct formation of La2O3 on ZrO2 and other oxide and metal substrates from nonaqueous electrolytes, data on the kinetics of the electrochemical processes and scanning electron microscopy (SEM) and XPS studies of the morphology, structure, dispersion and chemical composition of La2O3.

Silica supported copper and cerium oxide catalysts for ethyl acetate oxidation

The formation of active sites in the silica supported copper and cerium oxide bi-component catalysts for total oxidation of ethyl acetate was studied by Nitrogen physisorption, XRD, XPS, UV-Vis, Raman, FTIR of adsorbed CO spectroscopies and TPR. It was found that the interaction between the copper oxide nanoparticles and the supported on the silica ceria ones is realized with the formation of interface layer of penetrated into ceria lattice copper ions in different oxidative state. This type of interaction improves the dispersion of copper oxide particles and provides higher accessibility of the reactants to the copper active sites even at low copper amount.

Effect of porous structure on the formation of active sites in manganese hosted in ordered mesoporous silica catalysts for environmental protection

Recently ordered mesoporous silicas have been considered as suitable catalyst supports due to their high surface area, well developed porous volume and tuned size, shape and topology of mesopores. Among them SBA-15 and KIT-6 are most promising and studied materials as host matrix of metal/metal oxide nanoparticles. Both structures are characterized with cylindrical mesopores which are 2D- and 3D-packed in the SBA-15 and KIT-6 silicas, respectively. The flexibility of the oxidation state of manganese ensures high oxygen storage capacity of its oxides and provokes their wide application as catalysts in various redox processes. The aim of the current investigation is to clear the effect of pore topology in SBA-15 and KIT-6 mesoporous silicas on the state of the hosted in them manganese oxide nanoparticles. The samples were obtained by incipient wetness impregnation of silicas with manganese nitrate and conventional SiO2 was also used as a reference support. A complex of physicochemical techniques, such as nitrogen physisorption, X-ray diffraction, UV–Vis, XPS, FTIR and temperature-programmed reduction with hydrogen was used for samples characterization. The obtained modifications were tested as potential catalysts for environmental protection via total oxidation of VOCs (ethyl acetate) or hydrogen production from methanol as clean and effective alternative fuel. It was established that the porous structure of mesoporous silica supports influences in a complex way the catalytic behaviour of their manganese modifications, which is determined by the specificity of the reaction medium.

Preparation and Characterization of Al2O3 Thin Films for Catalytic Activity Studies

In the present work, the surface properties of various Al oxide films were investigated. The oxide films were produced on a stainless steel by spray pyrolysis and cathodic deposition methods. The films obtained represent typical layers that can be used as a support in model systems to investigate alumina-based catalysts. Information about the chemical environment of the Al and O ions in the oxide films depending on the preparation conditions has been deduced from the binding energies of the Al 2p and O 1s electron core levels and corresponding Auger parameters.

Fabrication of ZnO nanostructures by PLD

Different types of ZnO nanostructures were fabricated on metal (Au or Ag) coated silicon substrates by applying the pulsed laser deposition (PLD) method. The samples were prepared at substrate temperatures in the range of 300 – 650 °C, oxygen pressure of 5 Pa, and laser fluence ≤ 1 J.cm-2– process parameters usually used for thin-film deposition. The metal layer is essential for the preparation of nanostructures. The nanostructures grown at different substrate temperatures showed obvious morphological differences. The substrate temperature increase led to changes in the morphology of the nanostructures from nanowhiskers to nanowalls when a thin Au layer was used. It was also observed that the type and thickness of the metal layer affect the morphology of the nanostructure.

PLD fabrication of ZnO nanostructures on metal-coated substrates

In this work, ZnO nanostructures were fabricated on metal (a metal alloy containing Fe, Cr, Mn and Ni) coated silicon substrates by applying pulsed laser deposition. The samples were prepared at substrate temperatures in the range of 550 – 650 °C, oxygen pressure of 5 Pa, and laser fluence ≤ 1 J cm−2 i.e., process parameters usually used for thin-film deposition. We found that the metal layers role is substantial in the preparation of nanostructures, the morphology of the catalyst layer determines the growth of the ZnO nanowalls and the increase of the process temperature leads to nanorods formation on the nanowalls.

Ag/ZnO nanocomposites prepared by laser methods

The composite nanostructures of double layer, multilayer and mosaic Ag/ZnO containing nano-sized particles have been produced by laser deposition and annealing procedures. The samples are investigated and the results indicated that laser annealing of Ag or Ag/ZnO layer is a feasible method to tune the optical properties of the nanostructures. Their morphology and optical absorption were investigated as a function of annealing regime and preparation procedure, leading to different silver arrangement. All annealed samples showed surface plasmon resonance (SPR) properties. The red shifted SPR was observed with the increase of the number of layers and is associated with the reduction of the interparticle distance. The absorption minimum of a mosaic samples broadens towards the longer wavelengths with reducing the thickness, due to the broader NPs size distribution. A ZnO excitonic feature visible in the spectra of double layer and multilayer structures disappear in the spectrum of a mosaic sample. The X-ray photoelectron spectroscopy was used to study the composition and chemical state of the double layer Ag/ZnO nanocomposites on the surface and in depth. The binding energy of Ag3d5/2 shifts to the lower values compared to the pure metallic Ag due to the interaction between Ag and ZnO.

XPS study of ZnO nanostructures prepared by laser ablation

The subject of the present work is different types of nanostructured ZnO films produced by pulsed laser deposition. The influence was investigated of the surface morphology on the physicochemical and optical properties of the nanostructured films. X-ray photoelectron spectroscopy was used to study the composition and chemical state of the nanostructured ZnO. The changes occurring on the surface of the films after prolonged exposure in air are considered in the light of practical applications of ZnO films. The change in the number of adsorbed species on the surface with the time affects the surface conductivity, the latter in turn modifying the optical properties of the samples.

Optical properties of Ag/ZnO nanocomposite coatings

In this work, ZnO nanostructures were fabricated on Ag-coated substrates by pulsed laser deposition. Ag was incorporated into ZnO nanostructures during the growth by using a composite, or mosaic, target. The effect was studied of Ag doping on the morphology, structure, composition, and photoluminescence of the ZnO nanostructures. It was found that the increase of the dopant concentration leads to a deterioration of the crystalline structure, as well as to a change of the sample morphology. It was also found that the manner of incorporating Ag into the ZnO matrix affects the photoluminescent performance of the nanocomposite coatings.

Influence of the preparation method of sulfated zirconia nanoparticles for levulinic acid esterification

Zirconia nanomaterials were prepared by hydrothermal synthesis with or without template and were modified by post synthesis method with sulfate groups. The materials were thoroughly characterized by X-ray powder diffraction, TEM, N2 physisorption, FTIR spectroscopy of adsorbed pyridine TG analysis and XPS spectroscopy. The catalytic performance of nanosized ZrO2 catalysts and their sulfated modifications was studied in levulinic acid esterification with ethanol. The sulfate group’s dispersion was predetermined by the use of template during the mesoporous zirconia synthesis. A relation between sulfate groups leaching and the applied synthesis conditions (with or without template) of the zirconia nanoparticles was found. Sulfated materials showed significantly higher activity compared to non-sulfated ones. Furthermore, it has been found that the presence of template during the mesoporous ZrO2 nanoparticles preparation influences significantly the zirconia phase and catalytic performance in levulinic acid esterification.