National Institute Of Materials Physics - Romania

Recently, Kittel's theory of ferromagnetic domains in thin films in one dimension, i. e. with the domains extended infinitely over one in-plane direction and with the anisotropy axis oriented perpendicular to the film was revisited by considering the film thickness and unbalanced domains with up and down magnetization, yielding to the computation of magnetic hysteresis [C. M. Teodorescu, Res. Phys. 46 (2023) 106287]. In this work, the above study is extended to samples featuring two-dimensional domain landscapes, for materials with strong magnetic anisotropy, typically characterized by a superunitary ratio between the anisotropy energy and the stray field energy densities. This allows one to compute the most stable structures for vanishing average magnetization < M > together with hysteresis curves for thin films with perpendicular magnetic anisotropy and two-dimensional rectangular domains and also for thin films with in-plane magnetic anisotropy. For two-dimensional films with perpendicular magnetic anisotropy, the most stable structure for < M > = 0 is found to be that with domains infinitely elongated along one in-plane direction, i. e. the one-dimensional case treated in the preceding work. For thin stripes with in-plane magnetization, the domain size l is approximately linear with the stripe lateral size d for low film thickness, while for large film thicknesses it follows a Kittel-like law, but as function of the stripe size l similar to d(1/2). For in-plane magnetized thin films of infinite lateral extent, the most stable structure is the single domain. As for hysteresis curves, the two-dimensional case with perpendicular magnetic anisotropy is shown to evolve from a 2D landscape derived from the checkerboard structure, but with unbalanced domains for magnetization near saturation, towards one-dimensional domain structures for lower magnetization. In some cases and depending also on the demagnetization factor, the one-dimensional case is not reached, and the film exhibit 2D structures on the whole range of the magnetization curve. The hysteresis obtained for thin magnetic stripes with in-plane magnetization also can exhibit a rich structure, with minor cycles on the wings of the magnetization curves evolving towards ,,normal" hysteresis (again, depending on the film thickness, stripe lateral size and demagnetization factor). An infinite thin film with in-plane anisotropy features a steplike magnetization dependence on the applied field.

Thick tungsten oxide layers were prepared electrophoretically in order to be used as photoanodes in photoelectrochemical water oxidation applications. The highest photocurrent density was obtained for a WO3 layer with thickness of similar to 900 nm. Additionally, WO3/BiVO4 and WO3/BiVO4/MIL-101(Fe) heterojunctions have been fabricated using WO3 layer as substrate. WO3/BiVO4 shows an increased value of the electrochemical active surface area indicating that more sites of this photoanode are activated through the formation of the heterojunction. The small V5+ signals observed in the V 2p XPS spectra of these heterojunctions were attributed to the substitution of V5+ atoms with W6+ atoms on the surface of BiVO4. Excessive W doping of the BiVO4 film determined the decrease of the photoelectrochemical performance of WO3/BiVO4 photoanode. The significant improvement of the photoconversion efficiency of the sample decorated with MIL-101(Fe) indicated that this cocatalyst provides sites more efficiently in the photoelectrochemical process. This performance was correlated with the reduced value of the charge transfer resistance at electrode/electrolyte interface obtained from electrochemical impedance spectroscopy investigation.

Motivated by the goal of developing ultralow power, smart and multifunctional nano (micro) electronic devices, research has shown unwavering interest in synthesis methods, architectures and interphase connectivity of composites containing magnetostrictive and piezoelectric phases, known as magnetoelectric (ME) materials. Herein we report on CoFe2O4/0.92Bi0.5Na0.5TiO3-0.08BaTiO3 biphasic ME composites, obtained by sol-gel chemistry, by mixing the precursor sols of the two phases into one precursor sol and further transforming it into gel, with the goal of obtaining homogeneous nanocomposites with magnetoelectric properties. The structural properties, the temperature dependance of dielectric properties, the magnetic and magnetoelectric properties of these biphasic mixtures, with various molar ratios CoFe2O4/ BNT-BT0.08 = 0.5:1, 1:1 and 1.5:1, are investigated. It is observed that the amount of CoFe2O4 and the synthesis in situ of these composites influences their macroscopic properties showing a high difficulty of carrying out an efficient poling resulting in small piezoelectric and magnetoelectric response. It was concluded that the synthesis procedure, the type of architecture and the interphase connectivity are of outmost importance for magnetoelectric properties based on lead-free materials. (c) 2023 Published by Elsevier B.V.

Probing of the free surface ferroelectric properties of thin polar films can be achieved either by estimating the band bending variance under the top-most layer or by studying the extent of the extrinsic charge accumulated outside the surface. Photoemitted or incoming low-energy electrons can be used to characterize locally both properties in a spectromicroscopic approach. Thin ferroelectric lead zirco-titanate (PZT) is investigated by combining low energy/mirror electron microscopy (LEEM/MEM) with photoemission electron microscopy (PEEM) and high-resolution photoelectron spectroscopy (XPS). Significant extrinsic negative compensation charge is proven to accumulate on the surface of the outward polarized thin film, indicated by high MEM-LEEM transition values, up to 15.3 eV, and is correlated with the surface electrostatic potential, which can be partially screened either by electrons interacting with the sample or by soft X-rays through the ejection of secondary electrons and generation of positive charge under the surface. A radiation-induced surface charge compensation effect is observed. The study indicates that air-exposed high quality ferroelectric thin films show large negative surface potentials, determined locally on the surface, which are nevertheless sensitive to beam damage and molecular desorption. These values represent a confirmation of previously estimated surface potential energy values determined from the LEED data on clean surfaces.

[Cu(acac)(phen)(H2O)](ClO4) and [Cu(acac)(bipy)(H2O)](ClO4) catalysts were prepared by the immobilization of mixed-ligand complexes on graphene oxide derivatized with monochloroacetic acid. These catalysts were characterized through an ensemble of techniques including XRD, FTIR, Raman, and XPS while the catalytic behavior has been investigated in the oxidation of cyclohexene, 1-octene and glycerol in the presence of molecular oxygen. Cyclohexanone was the dominant product in the oxidation of cyclohexene resulting in selectivities higher than 50% for a conversion of 14.5%. The turnover frequencies were pretty high, namely, 4.5 h(-1) for [Cu(acac)(phen)(H2O)](ClO4) and 2.3 h(-1) for Cu(acac)(bipy)(H2O)](ClO4). The oxidation of 1-octene also occurred with a pretty high selectivity in 2-octanol as the main product. The role of copper in the oxidative dehydrogenation of glyceric acid towards tartronic acid was as well confirmed. Noteworthy, these catalysts were stable and their recycling occurred with no change in the conversion or selectivity.

Zn1-xFexO nanoparticles with the iron concentrations level in the dilute regime (x = 0.001---0.01) were produced by a sol-gel route from acetate precursors along with an un-doped and 3 at.% Fe-doped reference. The X-ray diffraction of the un-doped and 0.1-1 at.% Fe-doped samples reveal the reflections for only the ZnO wurtzite structure. Fe doping enhances the a-axis lattice constant, the unit cell volume and the microstrain. Iron doping reduces the average crystallite/particle size (confirmed by Scanning Electron Microscopy), improving the surface-to-volume ratio or the concentration of defective surface sites. XPS identifies the iron in both Fe3+ and Fe2+ states. XPS and Fe-57 Mossbauer spectroscopy indicate a broad distribution (distortion) of Fe3+ sites on the surface of ZnO nanoparticles. The blue shift and broadening of the UV emission, and quenching of defect-related photoluminescence in the Fe-doped samples verify the presence of iron in the ZnO lattice and surface intrinsic defects. 0.1-1 at.% Fe-doped ZnO show room temperature ferromagnetism, RTFM, characteristic of dilute magnetic semiconductors, DMS. The magnetization measurements with temperature evidence an antiferromagnetic alignment and an increase of ferromagnetic contribution with Fe doping up to 1 at.%. Zn0.97Fe0.3O reference is a superparamagnetic ZnO/ZnFe2O4 nanocomposite with a blocking temperature of 20 K; HRTEM shows (ultra)fine ZnFe2O4 particles at the surface of ZnO nanoparticles. The analysis of experimental data of 0.1-1 at.% Fe-doped ZnO was done in terms of iron coupling with intrinsic defects, which can generate surface Fe3+ states with geometries similar to the Fe3+ in inverse spinel ZnFe2O4. The superexchange interaction (resembling that in the inverse spinel ZnFe2O4) between the Fe3+ sites with distorted configuration resulting in ferrimagnetism was hypothesised as a possible mechanism of RTFM. Experimental (structural, local chemical, magnetic, optical) and interpretation results can be used to optimize the processing conditions for Fe-doped ZnO to serve as an effective DMS, e.g. for spintronic applications.

Personal and industrial use of internal combustion engines (ICEs) is projected to continue until 2050 and beyond. Yet demands to reduce global dependence on petrochemicals and fossil fuel-derived lubricants are increasing and environmentally necessary. New strategies for maintaining and enhancing ICE performance by reducing friction, wear, fuel consumption, and exhaust emissions will reduce the depletion of mineral and fossil fuel reserves and environmental pollution. This paper reports the tribological enhancement of nano-bio lubricants formulated using 2D nanocomposites of Al2O3/graphene as novel additives in coconut oil, whose performance as a lubricant compares favorably with the mineral-based engine oil 15W40. Structural, compositional, and morphological characterization of the Al2O3/graphene nanocomposite revealed an ultra-fine particle size (< 10 nm) with spherical/laminar morphology and a rich sp2 domain, exhibiting a consistent colloidal stability when formulated as nanofluid. Through the use of various characterization techniques, including friction and wear analysis we gained valuable insight into the tribological mechanism. Our optimization of this 2D tribological system using coconut oil formulation resulted significant reductions in the coefficient of friction (28 %), specific fuel con-sumption (8 %), and exhaust pollutant emissions (CO, SO2, and NOx). This work demonstrates the benefits of using nano-bio lubricant formulated using coconut oil and 2D-based hybrids as base stock and additives, delivering solutions to global challenges such as improving fuel consumption while reducing environmental pollution; solutions that can be transferred to other areas where lubricants are a necessity.

Zeolite Y samples with microporous and hierarchical structures containing Ti-Ni and Ti-Co oxides were obtained as active photocatalysts. Different Ti amounts (5, 10% TiO2) were supported, followed by the loading of Ni or Co oxides (5%). X-ray diffraction evidenced the presence of TiO2 as an anatase. N-2 adsorption-desorption results showed type IV isotherms for hierarchical zeolite Y samples, and a combination of type IV and I isotherms for zeolite Y samples. UV-Vis diffuse reflectance spectra showed a shift in the absorption band to visible with increasing Ti loading and especially after Co and Ni addition. A significant effect of the support was evidenced for Ti and its interaction with Co/Ni species. The zeolite Y support stabilized Ti in the 4+ oxidation state while hierarchical zeolite Y support favored the formation of Ti3+ species, Ni-0 and Ni2+ and the oxidation of Co to 3+ oxidation state. Photocatalytic activity, under UV and visible light irradiation, was evaluated by the degradation of amoxicillin, used as a model test. The photocatalytic mechanism was investigated using ethanol, p-benzoquinone and KI as & BULL;OH and & BULL;O-2(-) radicals and hole (h(+)) scavengers. The best results were obtained for the immobilized Ni-Ti species on the hierarchical zeolite Y support.

Nanostructured oxides (SiO2, TiO2) were synthesized using the sol-gel method and mod-ified with noble metal nanoparticles (Pt, Au) and ruthenium dye to enhance light harvesting and promote the photogeneration of reactive oxygen species, namely singlet oxygen (O-1(2)) and hydroxyl radical (center dot OH). The resulting nanostructures were embedded in a transparent polyvinyl alcohol (PVA) hydrogel. Morphological and structural characterization of the bare and modified oxides was performed using scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), UV-Vis spectroscopy, and X-ray photoelectron spectroscopy (XPS). Additionally, electrokinetic potential measurements were conducted. Crystallinity data and elemental analysis of the investigated systems were obtained through X-ray diffraction and X-ray fluorescence analyses, while the chemical state of the elements was determined using XPS. The engineered ma-terials, both as simple powders and embedded in the hydrogel, were evaluated for their ability to generate reactive oxygen species (ROS) under visible and simulated solar light irradiation to establish a correlation with their antibacterial activity against Staphylococcus aureus. The generation of singlet oxygen (O-1(2)) by the samples under visible light exposure can be of significant importance for their potential use in biomedical applications.

Nanostructured surfaces based on silver nanoparticles decorated ZnO-CuO core-shell nanowire arrays, which can assure protection against various environmental factors such as water and bacteria were developed by combining dry preparation techniques namely thermal oxidation in air, radio frequency (RF) magnetron sputtering and thermal vacuum evaporation. Thus, high-aspect-ratio ZnO nanowire arrays were grown directly on zinc foils by thermal oxidation in air. Further ZnO nanowires were coated with a CuO layer by RF magnetron sputtering, the obtained ZnO-CuO core-shell nanowires being decorated with Ag nanoparticles by thermal vacuum evaporation. The prepared samples were comprehensively assessed from morphological, compositional, structural, optical, surface chemistry, wetting and antibacterial activity point of view. The wettability studies show that native Zn foil and ZnO nanowire arrays grown on it are featured by a high water droplet adhesion while ZnO-CuO core-shell nanowire arrays (before and after decoration with Ag nanoparticles) reveal a low water droplet adhesion. The antibacterial tests carried on Escherichia coli (a Gram-negative bacterium) and Staphylococcus aureus (a Gram-positive bacterium) emphasize that the nanostructured surfaces based on nanowire arrays present excellent antibacterial activity against both type of bacteria. This study proves that functional surfaces obtained by relatively simple and highly reproducible preparation techniques that can be easily scaled to large area are very attractive in the field of water repellent coatings with enhanced antibacterial function.