National Institute Of Materials Physics - Romania

The composites of transition metal-doped titania and carbon have emerged as promising supports for Pt electrocatalysts in PEM fuel cells. In these multifunctional supports, the oxide component stabilizes the Pt particles, while the dopant provides a co-catalytic function. Among other elements, Sn is a valuable additive. Stong metal-support interaction (SMSI), i.e., the migration of a partially reduced oxide species from the support to the surface of Pt during reductive treatment is a general feature of TiO2-supported Pt catalysts. In order to explore the influence of SMSI on the stability and performance of Pt/Ti0.8Sn0.2O2-C catalysts, the structural and catalytic properties of the as prepared samples measured using XRD, TEM, XPS and electrochemical investigations were compared to those obtained from catalysts reduced in hydrogen at elevated temperatures. According to the observations, the uniform oxide coverage of the carbon backbone facilitated the formation of Pt-oxide-C triple junctions at a high density. The electrocatalytic behavior of the as prepared catalysts was determined by the atomic closeness of Sn to Pt, while even a low temperature reductive treatment resulted in Sn-Pt alloying. The segregation of tin oxide on the surface of the alloy particles, a characteristic material transport process in Sn-Pt alloys after oxygen exposure, contributed to a better stability of the reduced catalysts.

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.

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.

Chiral materials showing Kramers-Weyl fermions represent a suitable platform for quantum technology, i.e., for engineering quantum solenoids, spin-torque devices, polarization-sensitive photodetectors based on quantized circular photogalvanic effect, etc. Accordingly, the stability of this class of materials in oxidative environments, such as the ambient atmosphere, should be carefully investigated to succeed in technology transfer. Here, taking as case-study example the well-recognized topological chiral system cadmium diarsenide (CdAs2), we assess its chemical reactivity towards ambient gases (oxygen and water) and air by density functional theory and experiments. The surface of CdAs2 evolves into an oxide skin, but its thickness remains nanometric even after one year in air, as directly imaged by high-resolution transmission electron microscopy. Accordingly, it is evident that future quantum devices based on Kramers-Weyl fermions could be stable in air, as the oxide layer formed on chiral quantum materials only represents a native oxide, which actually protects bulk features, including Kramers-Weyl fermions (correlated to bulk band structure), from degradation in air.

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.

Unlike the conventional one-dimensional (1D) core-shell nanowires (NWs) composed of p-type shells and n-type cores, in this work, an inverse design is proposed by depositing n-type ZnO (shell) layers on the surface of p-type CuO (core) NWs, to have a comprehensive understanding of their conductometric gas-sensing kinetics. The surface morphologies of bare and core-shell NWs were investigated by field emission scanning electron microscope (FE-SEM). The ZnO shell layer was presented by overlay images taken by electron dispersive X-ray spectroscopy (EDX) and high-resolution transmission electron microscopy (HRTEM). The pronounced crystalline plane peaks of ZnO were recorded in the compared glancing incident X-ray diffraction (GI-XRD) spectra of CuO and CuO-ZnO core-shell NWs. The ZnO shell layers broaden the absorption curve of CuO NWs in the UV-vis absorption spectra. As a result of the heterostructure formation, the intrinsic p-type sensing behavior of CuO NWs towards 250 and 500 ppm of hydrogen (H-2) switched to n-type due to the deposition of ZnO shell layers, at 400 & DEG;C in dry airflow.

This study presents conductometric sensors based on Co3O4 nanowires for hydrogen detection at ppb levels. The nanowires are synthesized through thermal oxidation of a 50 nm cobalt layer, exhibiting diameters between 6-50 nm and lengths of 1-5 & mu;m, primarily growing along the (311) direction of spinal Co3O4. Raman investigation reveals five characteristic peaks at 195, 482, 521, 620, and 692 cm(-1), corresponding to symmetric phonon modes of crystalline Co3O4. Electron paramagnetic resonance measurements confirm the presence of a ferromagnetic phase, attributed to incomplete cobalt oxidation, which disappears after 8 h of thermal aging at 400 & DEG;C. Conductometry measurements are performed in the temperature range of 300-500 & DEG;C. At temperatures above 300 & DEG;C, sensors exhibit abnormal n-type semiconducting behavior due to lattice oxygen's involvement in the hydrogen sensing mechanism. Operating at 450 & DEG;C in dry air, the sensor shows a higher 232% response to 100 ppm H-2 compared to ethanol, acetone, methane, carbon monoxide, and nitrogen dioxide. Remarkably, the sensor maintains a consistent conductance baseline even under high humidity (90%) for 25 d, with three-cycle repeatability. This distinctive gas-sensing capability is attributed to the catalytic activity and elevated operating temperature.

Photocatalytic conversion of H2O, CO2 and N-2 represents one promising approach to harvest and store solar energy, for which efficient visible light responsive semiconductors are inevitable. Often, the presence of a small amount of an additional component called a "cocatalyst", is required to synergistically enhance the performance of the photocatalyst. Tremendous efforts were made in the past to identify inexpensive materials to be used as cocatalysts, for which metal oxides (MOs) are one of the traditional choices. Among alternative categories of materials investigated, the recently discovered MXenes display enormous potential owing to their unique 2D layered structure, tuneable composition, abundant surface functionalities and superior electronic conductivity. Specifically, MOs and MXenes encompass a variety of distinct as well as analogous characteristics that allows them to be tailored to different extents. Unfortunately, a comprehensive overview covering the synthetic, structural and photocatalytic aspects of MOs and MXenes is not available as of now. Herein, we intend to summarize the progress achieved so far in these two families of materials to be used as cocatalysts for the photoconversion of H2O, CO2 and N-2. Followed by a general introduction, we briefly outline the fundamental principles and the role of cocatalysts in photocatalytic reactions. A discussion regarding the use of MOs and MXenes as cocatalysts for the conversion of H2O, CO2 and N-2 is then provided in separate sections. Critical assessment regarding structure and morphology control, surface properties and stability concerns can not only help to recognize the challenges that limit further advancement, but can also highlight the future research directions of these materials for the effective transformation of H2O, CO2 and N-2.

In this work, we demonstrate that solution-processed In2Se3 nanosheets exhibit exceptional selectivity and sensitivity to NO2 gas, making them a promising candidate for gas detection systems. Theoretical simulations and surface-science experiments reveal the unique surface properties of In2Se3 nanosheets, which prevent physisorption of oxygen, carbon monoxide, and carbon dioxide, making them remarkably stable towards oxidation and CO-poisoning. Moreover, we show that NO2 molecules adsorb stably on In2Se3 nanosheets, particularly on Se vacancies, even at high temperatures. The coadsorption of water further enhances NO2 sticking on the In2Se3 surface, making it an ideal material for gas sensing applications in humid and harsh environments. The fabricated In2Se3 gas sensors exhibit excellent and reversible sensing response to NO2 gas, with a limit of detection of 5 ppb at 300 degrees C, and a highly selective response to NO2 compared to other gases and volatile organic compounds. Our sensors outperform other two-dimensional (2D) semiconductors, metal oxides, and their heterostructures, thanks to the unique surface properties of In2Se3 nanosheets. Importantly, the number of layers and termination of the surface almost have no impact on the sensing performance of In2Se3, which is advantageous for practical applications. The high sensitivity, selectivity, and stability of In2Se3 nanosheets make them an exciting platform for the fabrication of high-performance gas sensors, particularly in harsh environments, such as industrial settings or outdoor monitoring. Moreover, our solution processing approach enables scalable production of the sensors. Additionally, their unique surface properties make them an attractive candidate for developing complex composite nanostructures with tailored gas sensing characteristics for various applications.

In this work, we demonstrate that solution-processed In2Se3 nanosheets exhibit exceptional selectivity and sensitivity to NO2 gas, making them a promising candidate for gas detection systems. Theoretical simulations and surface-science experiments reveal the unique surface properties of In2Se3 nanosheets, which prevent physisorption of oxygen, carbon monoxide, and carbon dioxide, making them remarkably stable towards oxidation and CO-poisoning. Moreover, we show that NO2 molecules adsorb stably on In2Se3 nanosheets, particularly on Se vacancies, even at high temperatures. The coadsorption of water further enhances NO2 sticking on the In2Se3 surface, making it an ideal material for gas sensing applications in humid and harsh environments. The fabricated In2Se3 gas sensors exhibit excellent and reversible sensing response to NO2 gas, with a limit of detection of 5 ppb at 300 degrees C, and a highly selective response to NO2 compared to other gases and volatile organic compounds. Our sensors outperform other two-dimensional (2D) semiconductors, metal oxides, and their heterostructures, thanks to the unique surface properties of In2Se3 nanosheets. Importantly, the number of layers and termination of the surface almost have no impact on the sensing performance of In2Se3, which is advantageous for practical applications. The high sensitivity, selectivity, and stability of In2Se3 nanosheets make them an exciting platform for the fabrication of high-performance gas sensors, particularly in harsh environments, such as industrial settings or outdoor monitoring. Moreover, our solution processing approach enables scalable production of the sensors. Additionally, their unique surface properties make them an attractive candidate for developing complex composite nanostructures with tailored gas sensing characteristics for various applications.