National Institute Of Materials Physics - Romania

MAX phases and MXenes are important materials that have recently gained great popularity due to their special properties, which render them particularly useful in many applications, including catalytic ones. This can be seen in the large number of publications that appear annually on these materials and their applications. This review aims to evaluate MAX phases and MXenes as materials for heterogeneous, non-electrocatalytic, catalytic applications. The review begins with a brief introduction to the MAX phase and MXene properties that recommend them as potential materials for heterogeneous catalytic applications, followed by four sections grouped according to the processes in which they have already proven effective. These include supports to activate the C-H or C-O bonds in applications such as dehydrogenation of light or aromatic alkanes, methanol formation from CH4, dry reforming, and CO oxidation or the water gas shift reaction (Section 2), and their use in fine chemical reactions (Section 3) and in chemical degradation (Section 4). The last section deals with photocatalytic applications (Section 5). The review ends by highlighting the huge potential of these materials for a wide range of heterogeneous catalytic applications as well as the challenges ahead.

Glasses from lithium-aluminum-zinc-boron-phosphorous oxide system co-doped with terbium (Tb3+) and dysprosium (Dy3+) oxides were studied for magneto-optical applications in lasers. The Fourier Transform Infrared and Raman Spectroscopy complementary analysis suggested the depolymerization of the borophosphate glass network by adding and increasing the rare-earth (RE) oxide content. Main UV-vis absorption maxima of Tb and Dy ions were identified at 348 and 1266 nm. Spectroscopic ellipsometry indicated a maximum refractive index of 1.56, at 400 nm, for the highest RE content. The Verdet constant amplified by increasing the RE content, reaching for the 9 mol% RE co-doped sample a value of -0.075 min/Oe/cm at 630 nm. The Faraday rotation angle was additionally confirmed by using a Faraday Cell Device, being also related to the specific paramagnetic behavior evidenced by Superconducting Quantum Interference Device magnetometry. The magneto-optical properties recommend such vitreous co-doped materials for magneto-optical devices.

Ni0.5Zn0.5Fe2O4 nanoparticles (NPs) with the spinel structure were synthesized by the cryo-chemical method with further heat treatment in the temperature range of 200-800 degrees C. Crystalline NPs began to form in one-stage and the degree of crystallinity grew with the increase of the heating temperature. Particles sizes, their size distributions and magnetization also tended to growth directly with the increasing the heating temperature of NPs. Magnetic fluids based on the obtained Ni0.5Zn0.5Fe2O4 NPs demonstrated effective and self-controlled heating up to the certain temperatures under the effect of an alternating magnetic field in contrast to known in literature Fe3O4 NPs with the spinel structure, which heated up uncontrolled to the extremely high phase transition temperature.

Polycrystalline NiO thick film-based gas sensors have been exposed to different test gas atmospheres at 250 degrees C and measured via simultaneous electrical resistance and work function investigations. Accordingly, we decoupled different features manifested toward the potential changes, i.e., work function, band-bending, and electron affinity. The experimental results have shown that the presence of moisture induces an unusual behavior toward carbon monoxide (CO) detection by considering different surface adsorption sites. On this basis, we derived an appropriate detection mechanism capable of explaining the lack of moisture influence over the CO detection with NiO-sensitive materials. As such, CO might have both chemical and dipolar interactions with pre-adsorbed or lattice oxygen species, thus canceling out the effect of moisture. Additionally, morphology, structure, and surface chemistry were addressed, and the results have been linked to the sensing properties envisaging the role played by the porous quasispherical-hollow structures and surface hydration.

Preventing and controlling the spread of multidrug-resistant (MDR) bacteria implicated in healthcare-associated infections is the greatest challenge of the health systems. In recent decades, research has shown the need for passive antibacterial protection of surfaces in order to reduce the microbial load and microbial biofilm development, frequently associated with transmission of infections. The aim of the present study is to analyze the efficiency of photocatalytic antimicrobial protection methods of surfaces using the new photocatalytic paint activated by light in the visible spectrum. The new composition is characterized by a wide range of analytical methods, such as UV-VIS spectroscopy, electron microscopy (SEM), X-ray powder diffraction (PXRD) or X-ray photoelectron spectroscopy (XPS). The photocatalytic activity in the UV-A was compared with the one in the visible light spectrum using an internal method developed on the basis of DIN 52980: 2008-10 standard and ISO 10678-2010 standard. Migration of metal ions in the composition was tested based on SR EN1186-3: 2003 standard. The new photocatalytic antimicrobial method uses a type of photocatalytic paint that is active in the visible spectral range and generates reactive oxygen species with inhibitory effect against all tested microbial strains.

In this work, a multi-analytical approach involving nitrogen porosimetry, small angle neutron and X-ray scattering, Fourier transform infrared (FTIR) and nuclear magnetic resonance (NMR) spectroscopies, X-ray diffraction, thermal analysis and electron microscopy was applied to organically modified silica-based xerogels obtained through the sol-gel process. Starting from a tetraethoxysilane (TEOS) precursor, methyltriethoxysilane (MTES) was added to the reaction mixture at two different pH values (2.0 and 4.5) producing hybrid xerogels with different TEOS/MTES molar ratios. Significant differences in the structure were revealed in terms of the chemical composition of the silica network, hydrophilic/hydrophobic profile, particle dimension, pore shape/size and surface characteristics. The combined use of structural characterization methods allowed us to reveal a relation between the cavity dimensions, the synthesis pH value and the grade of methyl substitution. The effect of the structural properties on the controlled Captopril release efficiency has also been tested. This knowledge facilitates tailoring the pore network for specific usage in biological/medical applications. Knowledge on structural aspects, as reported in this work, represents a key starting point for the production of high-performance silica-based hybrid materials showing enhanced efficacy compared to bare silica prepared using only TEOS.

The advancement of nanotechnology in the last decade has developed an abundance of novel and intriguing TiO2-based nanomaterials that are widely used in many sectors, including industry (as a food additive and colorant in cosmetics, paints, plastics, and toothpaste) and biomedicine (photoelectrochemical biosensing, implant coatings, drug delivery, and new emerging antimicrobial agents). Therefore, the increased use of engineered nanomaterials in the industry has raised serious concern about human exposure and their unexpected cytotoxic effects. Since inhalation is considered the most relevant way of absorbing nanomaterials, different cell death mechanisms induced in MRC-5 lung fibroblasts, following the exposure to functionalized TiO2 NPs, were investigated. Long-term exposure to TiO2 nanoparticles co-doped with 1% of iron and nitrogen led to the alteration of p53 protein activity and the gene expression controlled by this suppressor (NF-kB and mdm2), DNA damage, cell cycle disruptions at the G2/M and S phases, and lysosomal membrane permeabilization and the subsequent release of cathepsin B, triggering the intrinsic pathway of apoptosis in a Bax- and p53-independent manner. Our results are of major significance, contributing to the understanding of the mechanisms underlying the interaction of these nanoparticles with in vitro biological systems, and also providing useful information for the development of new photocatalytic nanoparticles that are active in the visible spectrum, but with increased biocompatibility.

Cu2ZnSnS4 (CZTS) is a complex quaternary material, and obtaining a single-phase CZTS with no secondary phases is known to be challenging and dependent on the production technique. This work involves the synthesis and characterization of CZTS absorber layers for solar cells. Thin films were deposited on Si and glass substrates by a combined magnetron sputtering (MS) and pulsed laser deposition (PLD) hybrid system, followed by annealing without and with sulfur powder at 500 degrees C under argon (Ar) flow. Three different Cu2S, SnS2, and ZnS targets were used each time, employing a different target for PLD and the two others for MS. The effect of the different target arrangements and the role of annealing and/or sulfurization treatment were investigated. The characterization of the absorber films was performed by grazing incidence X-ray diffraction (GIXRD), X-ray reflectometry (XRR), Raman spectroscopy, scanning electron microscopy, and regular transmission spectroscopy. The film with ZnS deposited by PLD and SnS2 and Cu2S by MS was found to be the best for obtaining a single CZTS phase, with uniform surface morphology, a nearly stoichiometric composition, and an optimal band gap of 1.40 eV. These results show that a new method that combines the advantages of both MS and PLD techniques was successfully used to obtain single-phase Cu2ZnSnS4 films for solar cell applications.

The feasibility of mixed-cation mixed-halogen perovskites of formula A(x)A'1-xPbXyXz'X3-y-z '' is analyzed from the perspective of structural stability, opto-electronic properties and possible degradation mechanisms. Using density functional theory (DFT) calculations aided by machine-learning (ML) methods, the structurally stable compositions are further evaluated for the highest absorption and optimal stability. Here, the role of the halogen mixtures is demonstrated in tuning the contrasting trends of optical absorption and stability. Similarly, binary organic cation mixtures are found to significantly influence the degradation, while they have a lesser, but still visible effect on the opto-electronic properties. The combined framework of high-throughput calculations and ML techniques such as the linear regression methods, random forests and artificial neural networks offers the necessary grounds for an efficient exploration of multi-dimensional compositional spaces.

Surface plasmon resonance containing amorphous As2S3 film is proposed as a chemical sensor to highlight several liquid hydrocarbons. Method is label-free and is based on detection of small changes in the refractive index. As2S3 operates as plasmonic waveguide which confines the probing beam to the interface with liquid hydrocarbons. The method can easily distinguish hydrocarbons with very close refractive indices. The film thicknesses were optimized to obtain the best sensitivity and resolving power Minimum reflectance of SPR less than 1 % was found for optimal calculated film thicknesses, the sensitivity to the refractive index changes being 2.10(-5).