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Characterization of bitumen oils
Andraž Mavrič, Matjaž Valant, final research report

Found in: osebi
Keywords: izolacija, bitumen, termična analiza
Published: 14.09.2016; Views: 3651; Downloads: 0
.pdf Fulltext (1,34 MB)

Amorphous nanocomposite of polycarbosilanes and aluminum oxide
Andraž Mavrič, 2018, doctoral dissertation

Abstract: This work presents a paradigm for high temperature stabilization of bulk amorphous aluminium oxide. The thermodynamic stabilization is achieved by preparing a nanocomposite, where polymethylsilane dendritic molecules are dispersed in an aluminium hydroxide gel. Upon heat-treatment the gel transforms to the amorphous aluminium oxide that is stable up to 900°C. The dispersion of the macromolecules and their covalent bonding to the alumina matrix induce homogeneously distributed strain fields that keep the alumina amorphous. The first part of the thesis focuses on the synthesis, characterization and solubility properties of the dendritic polymethylsilane. The polymethylsilane is synthetized by electrochemical polymerization from trichloromethylsilane monomer. The polymerization mechanism, involving a single polymerization pathway, is identified. The polymer growth proceeds through reduction of the monomers to the silyl anions and their addition to the growing polymer. The solubility of three chemically related but topologically different polysilanes (linear, dendritic and network) were studied by dynamic light scattering. At room temperature the agglomerates in a range from 500 to 1300 nm are present. They undergo de-agglomeration at slightly elevated temperatures of around 40°C. The de-agglomeration results in formation of stable solutions, where a hydrodynamic diameter of the individual polymer molecules was measured to be in a range from 20 to 40 nm. The obtained diameters of two dendritic polymethylsilane macromolecules, synthesized under different electrolysis conditions, are much larger than the theoretical size estimated for an ideal dendrimer. We determined by 29Si NMR that the reason for this is in a large number of branching irregularities (defects) contained in the molecular structure. Combining the experimental values obtained by DLS and density measurements with a structural model that considers the branching irregularities, it is shown that the inclusion of the defects allows the dendritic polymer to exceed the sterical limitations and form the hyperbranched dendritic structure. The final size depends on a relative amount of the branching defects. In the second part, the synthetized polymethylsilane molecules were successfully used for the nanocomposite formation. The aluminium hydroxide gel with the dispersed polymethylsilane molecules was prepared as a precursor. Upon heat-treatment it gives the amorphous aluminium oxide stable up to 900°C. The dispersed macromolecules induce homogeneously distributed strain fields that keep the aluminium oxide amorphous during the thermal treatment the dispersed macromolecules covalently bind to the matrix, inducing the interface strain. The amorphous state was confirmed by the presence of penta-coordinated aluminium detected by 27Al NMR and a low bandgap measured by UV-vis absorption spectroscopy.
Found in: osebi
Keywords: amorphous aluminium oxide, polymethylsilane, nanocomposite, electropolymerization, solubility, agglomeration, de-agglomeration, dendrimer, hyperbranched dendritic structure, dynamic light scattering, thermal analysis, transmission electron microscopy, scanning electron microscopy, X-ray diffraction, infrared spectroscopy, UV-Vis spectroscopy
Published: 19.07.2018; Views: 4958; Downloads: 205
.pdf Fulltext (5,07 MB)

Deposition of porphyrin thin films by electrophoresis
Matjaž Valant, Changzhu Wu, Mads Nybo Sørensen, Tina Škorjanc, Andraž Mavrič, 2022, published scientific conference contribution abstract

Abstract: Porphyrins are heterocyclic macrocycles consisting of interconnected pyrrole subunits acting as ligands for metal ions. Metallised metalloporphyrins naturally occur as cofactors in a series of enzymes, acting as active sites for biochemical transformations. Mimicking nature, a variety of functionalized porphyrins have been prepared for different catalytic purposes [1]. These organometallic complexes have isolated metal centers in tailored coordination environments to drive catalytic reactions in homogeneous solutions. The activity and selectivity of isolated metal ions acting as single-atom catalysts are defined by the coordination environment. Depending on the porphyrin structure, the solubility of these macrocycles and their processability in solutions can be altered. While such adjustments to the structure might ease the processability, the catalytic properties might also be altered. Because homogenous catalysis presents challenges with the separation and recyclability of the catalyst, it is common to fix metalloporphyrins into molecular organic frameworks or deposit them onto a substrate. Commonly used deposition techniques face several challenges. For instance, thermal evaporation can cause partial or complete degradation of some thermally-labile functional groups attached to the porphyrins. Similarly, spin coating commonly results in an uneven thickness and uneven morphology of the deposited films. To overcome these difficulties, we present an alternative method for the deposition of porphyrin thin films that is suitable for a wide range of functionalized porphyrins. The electrophoresis can force the molecules to deposit on a conductive substrate such as a metal foil or transparent conductive oxide by applying the electric field generated by a DC power supply. The film thickness can be precisely controlled by changing the voltage value, deposition time, or solution concentration using even a small amount of material [2, 3]. Six different functionalized porphyrin molecules have been successfully deposited onto the copper foil substrate by optimizing key parameters, including applied electric field, the duration of electrophoresis, the size of the copper electrodes, and solvent polarity. To demonstrate the generality of our approach, we have selected a broad range of porphyrins that incorporate the following functional groups: phenyl rings, carboxylic acids, pyridyl rings, methyl benzyl ethers, methyl benzoyl esters, and cobalt (II) metalized macrocycle. The impact of this study extends above catalysis to various applications of porphyrins thin films on conductive substrates such as optoelectronics and sensors.
Found in: osebi
Keywords: porphyrin, thin film, electrophoresis
Published: 26.09.2022; Views: 435; Downloads: 0
.pdf Fulltext (27,64 MB)

Metal hydroxides for energy conversion and energy storage
Andraž Mavrič, invited lecture at foreign university

Abstract: Electrocatalysts, electrochromic devices, and pseudo-capacitors based on transition metal (oxy)hydroxides depend on the reversibility of the reduction-oxidation process of metal cations. Rapid switching between different redox states is often involved, particularly in electrocatalysis where redox metal sites act as active centers for electron transfer to the reactant. To ensure long-term durability, the reversibility of the redox metal sites should be robust. Nickel hydroxide is a model catalyst for the oxygen evolution reaction (OER) and the basic representative of the layered double hydroxides. It is frequently combined with other transition metals (e.g. Fe, Co, Mn), forming some of the most active OER electrocatalysts in alkaline media. [1] I will present the use of in-situ spectroscopy to track the reversibility of redox states of the Ni(OH)2 during its lifetime. During the operation at 200 mA cm-2 in 1 M KOH electrolyte, the catalytic activity of Ni(OH)2 gradually degrades until lastly, the catalyst breaks down. During the catalyst lifetime, the reduction-oxidation reversibility of the Ni2+/3+ redox couple is lost and the catalyst converts into an inactive phase. The reversibility of the redox couple is monitored by the in-situ UV/Vis spectroscopy. During the catalyst lifetime, the reversibility of the redox peak is lost. The activity collapse is attributed to the structural amorphization/disordering of the layered Ni(OH)2 catalyst, as confirmed by TEM investigations and in-situ Raman spectroscopy. [2] Similarly, the redox reversibility of metal sites is also important for long cycle life in supercapacitors, based on the pseudo-capacitance mechanism. Contrary to catalysts, for supercapacitors, the water oxidation needs to be suppressed to increase the working voltage range. I will discuss the mechanisms for the deactivation of transition metal hydroxides to serve as capacitors and approaches to increase power density. Finally, I will discuss the use of mixed metal hydroxides to serve as precursors for a copper oxide-based catalytic system for CO2 hydrogenation to methanol. Thermal decomposition of hydrotalcite-based hydroxide precursor is followed by in-situ x-ray diffraction. The conditions to prepare disordered oxide in contact with catalytical active Cu metal are identified and the catalytic performance of catalysts with crystalline and disordered oxide phases are compared. [1] A. Mavrič, C. Cui, (2021), Advances and Challenges in Industrial-Scale Water Oxidation on Layered Double Hydroxides, ACS Appl. Energy Mater., 4, 12032-12055. [2] A. Mavrič, M. Fanetti, Y. Lin, M. Valant, C. Cui, (2020), Spectroelectrochemical Tracking of Nickel Hydroxide Reveals Its Irreversible Redox States upon Operation at High Current Density, ACS Catal., 10, 9451-9457.
Found in: osebi
Summary of found: ...and disordered oxide phases are compared. [1] A. Mavrič, C. Cui, (2021), Advances and Challenges in... ...ACS Appl. Energy Mater., 4, 12032-12055. [2] A. Mavrič, M. Fanetti, Y. Lin, M. Valant, C....
Keywords: electrochemistry, energy storage, CO2 hydrogenation, methnaol
Published: 13.10.2022; Views: 424; Downloads: 0
.pdf Fulltext (469,20 KB)

Plasma-enhanced atomic layer deposition of amorphous Ga2O3 for solar-blind photodetection
Qiang Zhou, Guang-Wei Deng, Dong-Liang Liu, Kuang Feng, Bo-Lin Li, Matjaž Valant, Andraž Mavrič, Nadiia Pastukhova, Bo-Yu Fan, Min-Ji Yang, Ze-Yu Fan, Yan-Bo Li, 2022, original scientific article

Abstract: Wide-bandgap gallium oxide (Ga2O3) is one of the most promising semiconductor materials for solar-blind (200 nm–280 nm) photodetection. In its amorphous form, a-Ga2O3 maintains its intrinsic optoelectronic properties while can be prepared at a low growth temperature, thus is compatible with Si integrated circuits (ICs) technology. Herein, the a-Ga2O3 film is directly deposited on pre-fabricated Au interdigital electrodes by plasma enhanced atomic layer deposition (PE-ALD) at a growth temperature of 250 °C. The stoichiometric a-Ga2O3 thin film with a low defect density is achieved owing to the mild PE-ALD condition. As a result, the fabricated Au/a-Ga2O3/Au photodetector shows a fast time response, high responsivity, and excellent wavelength selectivity for solar-blind photodetection. Furthermore, an ultra-thin MgO layer is deposited by PE-ALD to passivate the Au/a-Ga2O3/Au interface, resulting in the responsivity of 788 A/W (under 254 nm at 10 V), a 250-nm-to-400-nm rejection ratio of 9.2×103, and the rise time and the decay time of 32 ms and 6 ms, respectively. These results demonstrate that the a-Ga2O3 film grown by PE-ALD is a promising candidate for high-performance solar-blind photodetection and potentially can be integrated with Si ICs for commercial production.
Found in: osebi
Keywords: Amorphous gallium oxide, Passivation layer, Plasma enhanced atomic layer deposition, Responsivity, Solar-blind photodetector
Published: 25.10.2022; Views: 414; Downloads: 0
.pdf Fulltext (1,65 MB)

A comparative study of nanolaminate CrN/Mo2N and CrN/W2N as hard and corrosion resistant coatings
Orfeo Sbaizero, Marco Lazzarino, Gregor Kapun, Mattia Fanetti, Simone Dal Zilio, Andraž Mavrič, Marco Beltrami, Miha Čekada, 2013, original scientific article

Abstract: Nanolaminate coatings (NLC) consisting of alternated CrN coupled with either cubic tungsten nitride (β-W2N) or molybdenum nitride (γ-Mo2N) were deposited on cold worked tool steel substrates using reactive DC reactive magnetron sputtering for improved mechanical and corrosion resistance. The CrN/γ-Mo2N and CrN/β-W2N nanolaminate systems were found to perform better than the corresponding single-layer systems, with both mechanical and electrochemical properties improving by decreasing the individual layer thickness from 100 to 5 nm. The CrN/β-W2N NLC combined the high hardness value of W2N with the low corrosion current of CrN. The CrN/γ-Mo2N NLC showed synergistic improvements consisting of both higher hardness and lower corrosion currents with respect to the constituent materials alone. The dependence of mechanical and corrosion properties on the bilayer period is discussed in terms of the grain size, residual stresses and texture of the constituent materials and the nanostructured character of the multilayer architecture.
Found in: osebi
Keywords: Nanolayered coatings, Transition metal nitrides, Sputtering, Nanoindentation, Corrosion resistance, Mechanical properties
Published: 12.01.2023; Views: 277; Downloads: 0
.pdf Fulltext (12,52 MB)

Non-covalent ligand-oxide interaction promotes oxygen evolution
Qianbao Wu, Junwu Liang, Mengjun Xiao, Chang Long, Lei Li, Zhenhua Zheng, Andraž Mavrič, Xia Zheng, Hai-Wei Liang, Hongfei Liu, Matjaž Valant, Wei Wang, Zhengxing Lv, Jiong Li, Chunhua Cui, 2023, original scientific article

Abstract: Strategies to generate high-valence metal species capable of oxidizing water often employ composition and coordination tuning of oxide-based catalysts, where strong covalent interactions with metal sites are crucial. However, it remains unexplored whether a relatively weak “non-bonding” interaction between ligands and oxides can mediate the electronic states of metal sites in oxides. Here we present an unusual non-covalent phenanthroline-CoO2 interaction that substantially elevates the population of Co4+ sites for improved water oxidation. We find that phenanthroline only coordinates with Co2+ forming soluble Co(phenanthroline)2(OH)2 complex in alkaline electrolytes, which can be deposited as amorphous CoOxHy film containing non-bonding phenanthroline upon oxidation of Co2+ to Co3+/4+. This in situ deposited catalyst demonstrates a low overpotential of 216 mV at 10 mA cm−2 and sustainable activity over 1600 h with Faradaic efficiency above 97%. Density functional theory calculations reveal that the presence of phenanthroline can stabilize CoO2 through the non-covalent interaction and generate polaron-like electronic states at the Co-Co center.
Found in: osebi
Keywords: water oxidation, cobalt hydroxide, ligand-metal interactions
Published: 23.02.2023; Views: 280; Downloads: 9
.pdf Fulltext (1,77 MB)

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