Author: Mária Čaplovičová

Catalytic graphitization of single-crystal diamond

Catalytic graphitization of single-crystal diamond

Posted on by Mária Čaplovičová

SemirTulić, ThomasWaitz, MáriaČaplovičová, GerlindeHabler, ViliamVretenár, TomaSusi, VieraSkákalová

Carbon, Volume 185, 2021, 300-313

https://doi.org/10.1016/j.carbon.2021.08.082

Abstract

Diamond and graphene are carbon allotropes with starkly different physical characteristics. Their combination into graphene-on-diamond heterostructures could benefit from the complementary properties of both components. Graphitization of single-crystalline diamond surfaces is a promising synthesis route, but a clear understanding of the growth of graphene or graphite from solid carbon sources is so far missing. Using aberration-corrected transmission electron microscopy, Raman spectroscopy, and electrical transport measurements, we provide detailed insight in the mechanisms of structural changes of nickel-catalyzed graphitization of diamond. We propose competing atomistic processes occurring at contact sites of diamond and Ni, depending on diamond surface terminations. One-dimensional etching process dominates on (111) diamond surfaces that remain almost atomically flat during graphitization. Two-dimensional etching of (110) and (100) diamond surfaces results in Ni drilling into the diamond substrate. Our findings also provide evidence on the reaction rates of the catalysis. The most reactive diamond surface in the (100) orientation is covered with the largest amount of well-crystallized graphite, whereas the (111) surface shows the highest stability against catalytic etching. In the latter case, only a thin disordered graphite layer is formed, yielding the lowest electric conductance. By clarifying these etching mechanisms, our results can improve the synthesis of graphene-on-diamond heterostructures.

Effect of Sub-Zero Treatments and Tempering on Corrosion Behaviour of Vanadis 6 Tool Steel

Posted on by Mária Čaplovičová

Peter Jurči, Aneta Bartkowska, Mária Hudáková, Mária Dománková, Mária Čaplovičová, Dariusz Bartkowski

Materials (Basel). 2021 Jul; 14(13): 3759

doi: 10.3390/ma14133759

Abstract

Sub-zero treatment of Vanadis 6 steel resulted in a considerable reduction of retained austenite amount, refinement of martensite, enhancement of population density of carbides, and modification of precipitation behaviour. Tempering of sub-zero-treated steel led to a decrease in population density of carbides, to a further reduction of retained austenite, and to precipitation of M3C carbides, while M7C3 carbides precipitated only in the case of conventionally quenched steel. Complementary effects of these microstructural variations resulted in more noble behaviour of sub-zero-treated steel compared to the conventionally room-quenched one, and to clear inhibition of the corrosion rate at the same time.

Substrate dependent epitaxy of superconducting niobium nitride thin films grown by pulsed laser deposition

Posted on by Mária Čaplovičová

T. Roch, M. Gregor, S. Volkov, M. Čaplovičová, L. Satrapinskyy, A. Plecenik

Applied Surface Science, Volume 551, 2021, 149333

https://doi.org/10.1016/j.apsusc.2021.149333

Abstract

Niobium nitride (NbN) has suitable mechanical properties for application as protective coatings in mechanical engineering, and also its superconductivity can be utilized in thin film devices for sensorics or in combination with ferromagnet in spintronics. Long-range superconducting proximity effect at the heterostructures with a weak ferromagnet can be used for generation of spin-polarized current. For any operational heterostructure application the high quality NbN thin films need to be prepared. In this work we have investigated impact of the substrates on the structure and preferential orientation of niobium nitride thin films fabricated by pulsed laser deposition at 600 °C on Si (0 0 1), MgO (0 0 1), C-plane and R-plane Al2O3 substrates. Growth parameters have been tuned in order to obtain single superconducting δ-NbN phase. Microstructure was analyzed by X-ray diffraction and transmission electron microscopy. Si substrate does not induce the film preferential orientation. Films on MgO are epitaxial (0 0 1) oriented. Films on R-Al2O3 show (1 3 5) orientation with twinned crystallites on the lower symmetry substrate surface. The (1 1 1) epitaxial growth with the largest crystallites and their smallest tilting was achieved on C-Al2O3 substrate leading to the best superconducting properties.

Incorporation mechanism of tungsten in W-Fe-Cr-V-bearing rutile

Posted on by Mária Čaplovičová

MAJZLAN, Juraj – BOLANZ, Ralph – GÖTTLICHER, Jörg – MIKUŠ, Tomáš – MILOVSKÁ, Stanislava – ČAPLOVIČOVÁ, Mária – ŠTEVKO, Martin – RÖSSLER, Christiane – MATTHES, Christian

In American Mineralogist. Vol. 106, (2021)

https://doi.org/10.2138/am-2021-7653

Abstract

Rutile is a common mineral in many types of ore deposits and can carry chemical or isotopic information about the ore formation. For closer understanding of this information, the mechanisms of incorporation of minor elements should be known. In this work, we have investigated natural rutile crystals with elevated concentrations of WO3 (up to 17.7 wt%), Cr2O3,tot (7.5), V2O3,tot (4.1), FeOtot (7.3), and other metals. X-ray absorption spectroscopy (XAS) of rutile at the Fe K, Cr K, V K, and W L1 and L3 edges shows that all cations are coordinated octahedrally. The average oxidation state of V is +3.8, and that of Cr is near +4. Shell-by-shell fitting of the W L3 EXAFS data shows that W resides in the rutile structure. Raman spectroscopy excludes the possibility of hydrogen as a charge-compensating species. High-resolution TEM and electron diffraction confirm this conclusion as the entire inspected area consists of rutile single crystal with variable amounts of metals other than Ti. Our results show that rutile or its precursors can be efficient vehicles for tungsten in sedimentary rocks, leading to their enrichment in W and possibly later fertility with respect to igneous ore deposits. Leucoxene, a nanocrystalline mixture of Ti and Fe oxides, is an especially suitable candidate for such a vehicle.

Ce ion surface-modified TiO2 aerogel powders: a comprehensive study of their excellent photocatalytic efficiency in organic pollutant removal

Posted on by Mária Čaplovičová

Thirunavukkarasu Guru Karthikeyan, Monfort Olivier, Motola Martin, Motlochová Monika, Gregor Maroš, Roch Tomáš, Čaplovicová Maria, Lavrikova Aleksandra Y., Hensel Karol, Brezová Vlasta, Jerigová Monika, Šubrt Ján, Plesch, Gustáv

New J. Chem., 2021, 45, 4174-4184

https://doi.org/10.1039/D0NJ05976E

Abstract

Titanium dioxide aerogel (TiAP) powders were prepared by lyophilization of peroxo-polytitanic gels followed by annealing at 800 °C to obtain an anatase structure. The surface modification of TiAP was performed for the first time by low amounts of Ce ions (in the range of 0.0025 to 0.025 wt%) using a wet impregnation method. The photocatalytic activity of the aerogel samples was investigated for the removal of different organic pollutants (i.e., Rhodamine B, phenol and caffeine) and the results were compared with the reference P25. Both TiAP and Ce ion surface-modified TiAP (Ce/TiAP) have exhibited better degradation efficiencies for the removal of pollutants than P25, especially for Ce/TiAP with an enhancement in the degradation efficiencies of +18% and +37% for the removal of caffeine and Rhodamine B, respectively. These results have been partly explained by the high active surface area of Ce/TiAP compared to TiAP as well as its better photoelectrochemical properties which have shown, for instance, ∼10% increased incident photon-to-electron conversion efficiency at 360 nm. Interestingly, the energetic position of the valence band maximum of Ce/TiAP is shifted from 3.2 eV to 2.8 eV (compared to TiAP), thus improving the generation of reactive oxygen species (ROS), especially hydroxyl radicals. Indeed, the presence of HO˙ is confirmed by electron paramagnetic resonance, and fluorescence spectroscopy and their photoinduced generation are enhanced in the case of Ce/TiAP. Finally, the surface modification of TiAP by cerium ions led not only to better photoinduced properties, thus limiting the electron–hole pair recombination, but also to the improvement of ROS generation via different plausible mechanisms.

Thermally induced structural evolution and age-hardening of polycrystalline V1–xMoxN (x ≈ 0.4) thin films

Posted on by Mária Čaplovičová

Marián Mikula, Stela Uzon, Tomáš Hudec, Branislav Grančič, Martin Truchlý, Tomáš Roch, Peter Švec, Leonid Satrapinskyy, Mária Čaplovičová, Grzegorz Greczynski, Ivan Petrov, Magnus Odén, Peter Kúš, Davide G. Sangiovanni

Surface and Coatings Technology, Volume 405, 2021, 126723

https://doi.org/10.1016/j.surfcoat.2020.126723

Abstract

Rocksalt-structure (B1) (V,Mo)N alloys are inherently hard and tough ceramics. However, the mechanical properties and thermal stability of (V,Mo)N solid solutions at temperatures ⪆ 700 °C of relevance for practical applications have not been previously investigated. In this work, we synthesize single-phase B1 polycrystalline V0.57Mo0.43N0.95 coatings to investigate the effects induced by temperature on the nanostructural evolution and hardness (H) of the material. Nanoindentation measurements show that the as-deposited film (H = 23 ± 3 GPa) becomes ≈30% harder (up to 31 ± 2 GPa) upon annealing at 730 °C. Experimental characterization and analyses, based on dispersive X-ray spectroscopy, X-ray diffraction (XRD), and transmission electron microscopy (TEM), reveal that the age-hardening effect originates from decomposition of the solid solution into coherent strained cubic VN-rich/MoN-rich domains. The experimental results are complemented by the composition/temperature (V,Mo)N phase diagram – constructed upon ab initio molecular dynamics free-energies – which indicates that the separation observed in the solid solutions is of spinodal nature. Films annealed at temperatures exceeding 850 °C undergo structural coarsening, with formation of hexagonal MoxNy and cubic VN phases, which cause a decrease in hardness to ≈22 GPa. Our present findings indicate that (V,Mo)N coatings may offer outstanding mechanical performances during operation at elevated temperatures.

ZnO nanoparticles as photodegradation agent controlled by morphology and boron doping

ZnO nanoparticles as photodegradation agent controlled by morphology and boron doping

Posted on by Mária Čaplovičová

Daniel Furka, Samuel Furka, Mira Naftaly, Erik Rakovský, Mária Čaplovičová, Marián Janek

Catal. Sci. Technol., 2021,11, 2167-2185

https://doi.org/10.1039/D0CY01802C

Abstract

Photolytic degradation of model dyes on pure and boron-doped zinc oxide (ZnO) nanoparticles with pseudohexagonal or elongated spindle-like morphology was investigated. ZnO nanoparticles were prepared by spray-assisted co-precipitation solvothermal synthesis. The bandgap (Eg) of pure ZnO was decreased by boron doping. The prepared nanoparticles were tested for their photocatalytic activity by decomposition of different organic dyes, using phloxine, oxazine and rhodamine as test substances simulating environmental pollutants. The photolysis experiments were done in a 3D printed photoreactor using a photodiode with a wavelength of 365 ± 5 nm as a narrow-line radiation source. The observed first-order reaction kinetics revealed that higher reaction rates were achieved on pseudohexagonal nanoparticles. Increasing the boron content led to accelerated photolysis rates. A significant linear correlation was observed between the optical bandgap energy Eg and the residual dye concentration remaining after 6 hours from the start of the reaction. It was found that ZnO particles with pseudohexagonal morphology decompose organic dyes faster than elongated spindle-like particles, indicating dependence on the surface area as determined by BET analysis. Interestingly, the residual dye concentration varied more strongly with doping when elongated spindle-like nanoparticles were used. It was shown that the efficiency of a photolysis reaction occurring on a solid oxide/solution interface is affected by the crystallographic plane on which it takes place.

Dehydroaromatization of methane over Mo/ZSM-5 zeolites: influence of aluminum distribution in the crystals

Posted on by Mária Čaplovičová

HORŇÁČEK, Michal – HUDEC, Pavol – JORÍK, Vladimír – ČAPLOVIČOVÁ, Mária – ČAPLOVIČ, Ľubomír – KALIŇÁK, Michal – SMIEŠKOVÁ, Agáta

https://doi.org/10.1007/s11144-020-01887-5

Abstract

Mo/ZSM-5 type catalysts based on two ZSM-5 zeolites with comparable crystal morphology, shape and size, Si/Al in bulk (22–23), acidity and amount of Mo (~ 4.7 wt.%) but having a different (homogeneous and non-homogeneous) Al distribution in particles and different acid sites type distribution were investigated in methane dehydroaromatization. From the results follows that Mo is associated with both BAS and LAS in the samples with almost homogenous Al distribution (MoZSM-5/AH). In the case of both samples with non-homogenous Al distribution (MoZSM-5/BH and MoZSM-5/BNH4) Mo preferably associates with BAS. Evidently lower activity and selectivity of Mo/ZSM-5/AH catalyst in methane transformation compared with Mo/ZSM-5/BH we attribute to the fact that from Mo species which interact with Lewis acid sites in sample ZSM-5/AH less active carburized species are formed as from the Mo associated with Brønsted sites in sample ZSM-5/BH.

Green synthesis of stable nanocolloids of monodisperse silver and gold nanoparticles using natural polyphenols from fruits of Sambucus nigra L

Posted on by Mária Čaplovičová

MARIYCHUK, Ruslan – PORUBSKÁ, Jana – OSTAFIN, Marek – ČAPLOVIČOVÁ, Mária – ELIÁŠOVÁ, Adriana

In Applied Nanoscience. Vol. 10, iss. 12 (2020)

https://doi.org/10.1007/s13204-020-01324-y

Abstract

In the present work, we report, for the first time, the production of stable nanocolloids of highly monodisperse silver and gold nanoparticles (AgNPs and AuNPs) using phytosynthesis approach. With the aim to eliminate misinterpretations associated with the use of usually applied crude plant extracts, the presented study was focused on the use of polyphenolic fraction isolated from elderberry fruits (Sambucus nigra L.) for the synthesis. Obtained AgNPs and AuNP were characterized by UV–Vis and FTIR spectroscopy, DLS and TEM. We observed unexpectedly high capability of the natural polyphenols not only to form monodisperse nanoparticles but also to stabilise the concentrated nanocolloids (539 mg/L AgNPs and 739 mg/L AuNPs) for at least two years. Moreover, AgNPs showed strong antibacterial properties against pathogenic bacterial strains Staphylococcus aureus and Escherichia coli (inhibited after 15 min and totally stopped growth after 3 and 5 h at 3.125 mg/L concentration).

Ni-mediated reactions in nanocrystalline diamond on Si substrates: the role of the oxide barrier

Posted on by Mária Čaplovičová

TULIČ, Semir – WAITZ, Thomas – ROMANYUK, Oleksandr – VARGA, Marián – ČAPLOVIČOVÁ, Mária – HABLER, Gerlinde – VRETENÁR, Viliam – KOTLÁR, Mário – KROMKA, Alexander – REZEK, Bohuslav – SKÁKALOVÁ, Viera

In RSC Advances. Vol. 10, iss. 14 (2020)

https://doi.org/10.1039/D0RA00809E

Abstract

Nanocrystalline diamond (NCD) films grown on Si substrates by microwave plasma enhanced chemical vapor deposition (MWPECVD) were subjected to Ni-mediated graphitization to cover them with a conductive layer. Results of transmission electron microscopy including electron energy-loss spectroscopy of cross-sectional samples demonstrate that the oxide layer on Si substrates (∼5 nm native SiO2) has been damaged by microwave plasma during the early stage of NCD growth. During the heat treatment for graphitizing the NCD layer, the permeability or absence of the oxide barrier allow Ni nanoparticles to diffuse into the Si substrate and cause additional solid-state reactions producing pyramidal crystals of NiSi2 and SiC nanocrystals. The latter are found impinged into the NiSi2 pyramids but only when the interfacial oxide layer is absent, replaced by amorphous SiC. The complex phase morphology of the samples is also reflected in the temperature dependence of electrical conductivity, where multiple pathways of the electronic transport dominate in different temperature regions. We present models explaining the observed cascade of solid-state reactions and resulting electronic transport properties of such heterostructures.