National Institute Of Materials Physics - Romania

Our study focuses on disclosing the mechanisms standing behind the improved photocatalytic performance of SrTiO3, where Al modification of the perovskite structure boosts the photocatalytic O2 evolution activity of the Al:SrTiO3 system. By adapting the synthesis method that produces well-crystallized materials with low defect density and employing surface modification techniques, we aim to enhance the photocatalytic efficiency of SrTiO3 via Al2O3 nanoceramic oxide doping at concentrations ranging from 0 to 10% and further examining of the relationship between doping process and the changes in the electronic and crystalline structure of SrTiO3. The prepared Al-based SrTiO3 perovskite samples (Al3%:SrTiO3, Al7%:SrTiO3, Al10%:SrTiO3) were thoroughly characterized to understand their structural, electronic, and morphological properties. Complementary X-ray techniques were employed to assess the stoichiometry (X-ray photoelectron spectroscopy - XPS), local environment, and chemical state (X-ray absorption spectroscopy - XAS in both total electron yield (TEY) and fluorescence yield (TFY). The comprehensive characterization enables us to understand the changes in the electronic properties and morphological features of the modified samples elucidating the surface formation mechanism while providing insights into the structural modifications induced by Al doping in the SrTiO3 perovskite lattice. Our findings give new perspectives for the development of Al-modified SrTiO3 perovskite materials with enhanced photocatalytic performance providing rich insights into the optimization of photocatalytic processes for applications in environmental remediation and sustainable energy production.

The process of N-doping is frequently employed to enhance the properties of carbon quantum dots. However, the precise requirements for nitrogen precursors in producing high-quality N-doped carbon quantum dots (NCQDs) remain undefined. This research systematically examines the influence of various nitrogen dopants on the morphology, optical features, and band structure of NCQDs. The dots are synthesized using an efficient, eco- friendly, and rapid continuous hydrothermal flow technique. This method offers unparalleled control over synthesis and doping, while also eliminating convention-related issues. Citric acid is used as the carbon source, and urea, trizma base, beta-alanine, L-arginine, and EDTA are used as nitrogen sources. Notably, urea and trizma produced NCQDs with excitation-independent fluorescence, high quantum yields (up to 40%), and uniform dots with narrow particle size distributions. Density functional theory (DFT) and time-dependent DFT modelling established that defects and substituents within the graphitic structure have a more significant impact on the NCQDs' electronic structure than nitrogen-containing functional groups. Importantly, for the first time, this work demonstrates that the conventional approach of modelling single-layer structures is insufficient, but two layers suffice for replicating experimental data. This study, therefore, provides essential guidance on the selection of nitrogen precursors for NCQD customization for diverse applications. This manuscript highlights continuous hydrothermal flow synthesis (CHFS) that directly obtains luminescent nitrogen doped carbon quantum dots using a simple precursor, citric acid, whilst varying the N-dopants to systematically engineer carbon dots with varied but controllable optical characteristics. It reports that a common approach of modelling single-layer structures is insufficient, but two layers suffice to reproduce the experimental data. image

Visible light-mediated photodegradation offers a sustainable and environmentally friendly solution for removing emerging contaminants from water sources. Herein, we report on the synergy effect between adsorption and visible-light-driven photo-degradation of 17 fl - estradiol using (Au)TiO 2 nanotubes-graphene composites. The graphene structures (graphene oxide - GO or thermally reduced graphene oxide - trGO) provide a platform for strong adsorption of 17 fl -estradiol molecules and facilitate the charge transfer between plasmonic Au nanoparticles and TiO 2 , leading to an improvement of the overall degradation process. To better understand the effect of Au nanoparticles and TiO 2 separately, three different concentrations of Au (1 %, 2.5 %, and 5 %) were prepared and characterized. The estrogenic molecules preferred GO over trGO, suggesting that hydrogen bonds govern the interaction between GO and 17 fl -estradiol. Consequently, a higher photocatalytic efficiency of GOcontaining composites was observed when removing estrogenic molecules. The role of adsorption proved crucial for capturing and retaining more contaminant molecules on the composite surface, thus increasing the odds of surface reactions with the photogenerated carriers. A total removal of 17 fl -estradiol has been obtained for Au (1 %, 2.5 %, and 5 %) TiO 2 nanotubes-GO. The mechanism behind the adsorption and photo-degradation processes has been discussed.

Considering the increasing need for sustainable and economical energy storage solutions, the integration of layered materials such as MoS2 into these systems represents an important step toward enhancing energy sustainability and efficiency. Exploring environmentally responsible fabrication techniques, this study assesses wrinkled MoS2 thin films synthesized from distinct Mo and MoS2 targets, followed by sulfurization conducted in a graphite box. We utilized magnetron sputtering to deposit precursor Mo and MoS2 films on Si substrates, achieving thicknesses below 20 nm. This novel approach decreases sulfur by up to tenfold during sulfurization due to the confined space technique, contributing also to avoiding the formation of toxic gases such as SO2 or the necessity of using H2S, aligning with sustainable materials development. Thinner MoS2 layers were obtained post-sulfurization from the MoS2 precursors, as shown by X-ray reflectometry. Raman spectroscopy and grazing X-ray diffraction analyses confirmed the amorphous nature of the as-deposited films. Post-sulfurization, both types of films exhibited crystalline hexagonal MoS2 phases, with the sulfurized Mo showing a polycrystalline nature with a (100) orientation and sulfurized MoS2 displaying a (00L) preferred orientation. The X-ray photoelectron spectroscopy results supported a Mo:S ratio of 1:2 on the surface of the films obtained using the MoS2 precursor films, confirming the stoichiometry obtained by means of energy dispersive X-ray spectroscopy. Scanning electron microscopy and atomic force microscopy images revealed micrometer-sized clusters potentially formed during rapid cooling post-sulfurization, with an increased average roughness. These results open the way for the further exploration of wrinkled MoS2 thin films in advanced energy storage technologies.

Bulk nanocrystalline La0.67Ba0.33_xKxMnO3 (with x = 0, 0.05, 0.1, and 0.2) manganites have been prepared by the modified sol-gel method (Pechini). The single-phase rhombohedral crystal structure with the R-3c (no. 167) space group was verified by X-ray diffraction (XRD) and sustained by Rietveld refinement. As follows from the results of XRD structural analyses, the increase in K-doping triggers an increase in the distortion of the MnO6 octahedra, which eventually causes the narrowing of the eg bandwidth. Mn is in a mixed valence state of Mn4+/ Mn3+ as inferred by X-ray photoelectron spectroscopy. Magnetic measurements confirm that the Curie tem-perature decreases from 348 K for La0.67Ba0.33MnO3 to 316 K for La0.67Ba0.13K0.20MnO3. The increasing of the Mn4+ ion concentration at the B-site sublattice and A-site ionic disorder (sigma 2) breaks up the double exchange interaction between the Mn3+ and Mn4+ ions. The ferromagnetic to paramagnetic second-order magnetic phase transition at TC is also confirmed by electron paramagnetic resonance. According to magnetic field-dependent magnetization isotherms at different temperatures, La0.67Ba0.13K0.20MnO3 shows a relatively large magneto -caloric effect (1400 mJ cm_ 3 K_ 1 at 316 K under 5 T applied magnetic field), which raises the possibility of using this material for room-temperature micro-scale magnetic cooling.

This work investigates the conduction mechanism of hydrothermally grown Gd2O3-sensitive material in order to explain its electrical resistance behaviour when exposed to increasing concentrations of CO2 under in-field conditions. To achieve this, the experimental investigation began with X-ray photoelectron spectroscopy of the Gd2O3 microstructure to verify the oxidation states of the surface. Subsequently, the impact of constant atmospheric factors such as oxygen and relative humidity on the electrical resistance of the Gd2O3 layer was examined. Finally, a progressive dosing of CO2 concentrations ranging from 400 to 3000 ppm was conducted. The DC electrical resistance measurements were performed using a computer-controlled Gas Mixing System operated under a dynamic gas flow regime. Experimental data was validated using the Boltzmann distribution statistics and the grain-to-grain Schottky barrier model. The results highlight the preservation of the n-type semiconductor behaviour of Gd2O3 irrespective of the background relative humidity and bring the oxidising character of CO2 to the fore.

In the quest for advanced materials suitable for next-generation electronic and optoelectronic applications, tungsten disulfide (WS2) ultrathin films have emerged as promising candidates due to their unique properties. However, obtaining WS2 directly on the desired substrate, eliminating the need for transfer, which produces additional defects, poses many challenges. This paper aims to explore the synthesis of WS2 ultrathin films via physical vapor deposition (PVD) followed by sulfurization in a confined space, addressing the challenge of film formation for practical applications. Precursor layers of tungsten and WS2 were deposited by RF magnetron sputtering. Subsequent sulfurization treatments were conducted in a small, closed, graphite box to produce WS2 films. The physical and chemical properties of these precursor and sulfurized layers were thoroughly characterized using techniques such as X-ray reflectometry (XRR), X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). The findings reveal notable distinctions in film thickness, structural orientation, and chemical composition, attributable to the different precursor used. Particularly, the sulfurized layers from the tungsten precursor exhibited a preferred orientation of WS2 crystallites with their (00L) planes parallel to the substrate surface, along with a deviation from parallelism in a small angular range. This study highlights the necessity of precise control over deposition and sulfurization parameters to tailor the properties of WS2 films for specific technological applications.

In a milestone paper [F. D. M. Haldane, Phys. Rev. Lett. 61, 2015 (1988)], Haldane elaborated a model of graphene within the time -reversal symmetry breaking is achieved by next -nearest -neighbors imaginary counterrotating hopping, hence conferring topological properties. In recent years, the time -reversal symmetry turned out to be broken also by light irradiation in so-called Floquet topological insulators (FTIs). On the other hand, Kane and Mele introduced a spin -orbit coupling (SOC) model [C. L. Kane et al., Phys. Rev. Lett. 95, 226801 (2005)] inspired by the Haldane's mechanism. In this paper, we present the topological properties of a FTI possessing SOC, using graphene as the playground. It was found that the interplay between sublattice subspace and the spin one triggers interesting topological phase transitions. Basically, in a FTI with SOC, two topological phases may be excited: charge quantum Hall effect (CQHE) and, respectively, spin quantum Hall effect (SQHE) phases. Also, it was demonstrated that the CQHE and SQHE coexistence is forbidden by the topology of the system. As well, it was identified a special driving regime of spin filter (SF), in which only one spin state is topological and, consequently, will be filtered in quantum transport.

Ti-aluminosilicate gels were used as supports for the immobilization of Fe, Co, and Ni oxides (5%) by impregnation and synthesis of efficient photocatalysts for the degradation of beta-lactam antibiotics from water. Titanium oxide (1 and 2%) was incorporated into the zeolite network by modifying the gel during the zeolitization process. The formation of the zeolite Y structure and its microporous structure were evidenced by X-ray diffraction and N-2 physisorption. The structure, composition, reduction, and optical properties were studied by X-ray diffraction, H-2-TPR, XPS, Raman, photoluminescence, and UV-Vis spectroscopy. The obtained results indicated a zeolite Y structure for all photocatalysts with tetracoordinated Ti4+ sites. The second transitional metals supported by the post-synthesis method were obtained in various forms, such as oxides and/or in the metallic state. A red shift of the absorption edge was observed in the UV-Vis spectra of photocatalysts upon the addition of Fe, Co, or Ni species. The photocatalytic performances were evaluated for the degradation of cefuroxime in water under visible light irradiation. The best results were obtained for iron-immobilized photocatalysts. Scavenger experiments explained the photocatalytic results and their mechanisms. A different contribution of the active species to the photocatalytic reactions was evidenced.

The primary objective of this research was to develop efficient solid catalysts that can directly convert the lactic acid (LA) obtained from lignocellulosic biomass into alanine (AL) through a reductive amination process. To achieve this, various catalysts based on ruthenium were synthesized using different carriers such as multi-walled carbon nanotubes (MWCNTs), beta-zeolite, and magnetic nanoparticles (MNPs). Among these catalysts, Ru/MNP demonstrated a remarkable yield of 74.0% for alanine at a temperature of 200 degrees C. This yield was found to be superior not only to the Ru/CNT (55.7%) and Ru/BEA (6.6%) catalysts but also to most of the previously reported catalysts. The characterization of the catalysts and their catalytic results revealed that metallic ruthenium nanoparticles, which were highly dispersed on the external surface of the magnetic carrier, significantly enhanced the catalyst's ability for dehydrogenation. Additionally, the -NH2 basic sites on the catalyst further facilitated the formation of alanine by promoting the adsorption of acidic reactants. Furthermore, the catalyst could be easily separated using an external magnetic field and exhibited the potential for multiple reuses without any significant loss in its catalytic performance. These practical advantages further enhance its appeal for applications in the reductive amination of lactic acid to alanine.