Author: Mária Čaplovičová

Structural Breakdown of Natural Epidote and Clinozoisite in High-T and Low-P Conditions and Characterization of Its Products

Kozáková, P.; Miglierini, M.; Čaplovičová, M.; Škoda, R.; Bačík, P.

Minerals 202212, 238

https://doi.org/10.3390/min12020238

Abstract

A heat treatment was performed on selected epidote and clinozoisite crystals to establish the nature of any changes in the optical and crystal-chemical properties and to identify a breakdown product using a wide spectrum of analytical methods. Natural samples were heated from 900 to 1200 °C under atmospheric pressure in ambient oxidation conditions for 12 h. Epidote and clinozoisite were stable at 900 °C; those heated at 1000 °C, 1100 °C, and 1200 °C exhibited signs of breakdown, with the development of cracks and fissures. The average chemical composition of epidote is Ca2.000Al2.211Fe0.742Si2.994O12(OH), while that of clinozoisite is Ca2.017A12.626Fe0.319Si3.002O12(OH). The breakdown products identified by electron microanalysis, powder X-ray diffraction, Raman spectroscopy, and high-resolution transmission electron microscopy were anorthite, pyroxene compositionally close to esseneite, and wollastonite. The decomposition of the epidote-clinozoisite solid solution is controlled by the following reaction: 4 epidote/clinozoisite → 2 pyroxene + 2 wollastonite + 4 anorthite + 2 H2O. Pyroxene likely contains a significant proportion of tetrahedral Fe3+ as documented by the Mössbauer spectroscopy. Moreover, the presence of hematite in the Mössbauer spectrum of the clinozoisite sample heated at 1200 °C can result from the following reaction: 4 epidote → pyroxene + 3 wollastonite + 4 anorthite + hematite + 2 H2O. 

Catalytic graphitization of single-crystal diamond

Catalytic graphitization of single-crystal diamond

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 microscopyRaman 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

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

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.

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

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

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

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.