National Institute Of Materials Physics - Romania

Floquet topological insulators (FTIs) are materials which undergo topological phase transitions under a time periodic perturbation causing time reversal symmetry breaking. In this Letter, we propose a spin filter model based on a FTI realized by irradiating a honeycomb lattice with circularly polarized light, in the presence of intrinsic spin-orbit coupling. The main ingredient of our proposed mechanism of Floquet topological spin filter (FTSF) implementation is the presence of an on-site staggered potential which controls independently the topological phases of the two existent spin states. After giving a numerical example of the occurrence of the FTSF phase, we argue that the origin of the FTSF phase resides in the spatial inversion symmetry breaking due to the presence of a staggered potential. The light helicity degree of freedom may be used to select the filtered spin state. Moreover, due to the topological properties of our model, the spin will be purely filtered in a Hall transport experiment. We discuss also the experimental feasibility.

Bimetallic (Ta/Ti, V, Co, Nb) mesoporous MCM-41 nanoparticles were obtained by direct synthesis and hydrothermal treatment. The obtained mesoporous materials were characterized by XRD, XRF, N2 adsorption/desorption, SEM, TEM, XPS, Raman, UV-Vis, and PL spectroscopy. A more significant effect was observed on the mesoporous structure, typically for MCM-41, and on optic properties if the second metal (Ti, Co) did not belong to the same Vb group with Ta as V and Nb. The XPS showed for the TaTi-MCM-41 sample that framework titanium is the major component. The new nanoparticles obtained were used as catalysts for oxidation with hydrogen peroxide of olefinic compounds (1,4 cyclohexadiene, cyclohexene, styrene) and photodegradation of organic pollutants (phenol, methyl orange) from water. The results showed improvementsin activity and selectivity in oxidation reactions by the addition of the second metal to the Ta-MCM-41 catalyst. The slow addition of H2O2 was also beneficial for the selectivity of epoxide products and the stability of the catalysts. The band gap energy values decreased in the presence of the second metal, and the band edge diagram evidenced positive potential for all the conduction bands of the bimetallic samples. The highestlevels of photocatalytic degradation were obtained for the samples with TaTi and TaV.

Carbon dioxide (CO2) is reversibly adsorbed and desorbed from ferroelectric (001) oriented, BaO-terminated barium titanate, as revealed in real time by high resolution and ultrafast photoelectron spectroscopy and certified by low energy electron diffraction. Desorption proceeds when the substrate is heated above its Curie temperature. The amount of CO2 adsorbed is derived to be between one molecule for a surface BaO unit cell (adsorption below room temperature) and one molecule for two unit cells (adsorption above room temperature). The molecule is bound with its carbon to surface oxygen, forming a CO3 structure. The BaTiO3(001) surface is unaffected by repeated cycles of adsorption-desorption. The relatively high amount of CO2 adsorbed and the stability of the substrate after repeated adsorption and desorption processes promotes barium titanate as a promising candidate for decarbonization technologies.

In this study, the Cu2Zn1-xCoxSnS4 (CZn1-xCoxTS) films with partial cation substitution of cobalt are synthetized by sol gel spin coating, followed by sulfurization treatment. The incorporation of cobalt cation in the CZTS crystalline lattice as well as the phase transition from kesterite to stannite were confirmed by the X-ray diffraction (XRD) and Raman spectroscopy data. The XRD pattern shows peak-shifting toward higher 2 theta by increasing the Co concentration, indicating a decrease in lattice parameters. The red shift of Raman peaks by increasing x from 0 to 0.6, confirms the phase transition. The CZn1-xCoxTS morphology was observed by scanning electron microscopy, showing large grain size as x increases and a good distribution of elements for all films. Xray photoelectron spectroscopy was employed to study the valence of cations/anions and to probe the chemical bonds. The optical band gap showed a parabolic behavior versus the molar ratio Co/(Co + Zn), this deviation from Vegard's law being induced by the difference in electronegativity between cobalt and zinc. The pure CZTS has a band gap of 1.47 eV, while for CZn0.6Co0.4TS the gap is 1.17 eV, which indicates that the incorporation of cobalt cation produces a red-shift of the band to band transition energy.

The synthesis of carbon quantum dots (CQDs) from high molecular weight biomass-derived precursors poses a significant challenge due to the complex molecular structures and low conversion efficiency. This work demonstrates a green, rapid, and sustainable continuous hydrothermal flow synthesis (CHFS) approach for nitrogen-doped carbon quantum dots (NCQDs) from various biomass-derived precursors, including high molecular weight polymeric sources like chitosan, lignin, and humic acid. We find that the precursor structure significantly impacts the size of the fabricated NCQDs and their optical properties. Citric acid, a low molecular weight precursor, yields NCQDs with excitation-independent emission, higher quantum yields, and low non-radiative losses, while NCQDs derived from polymeric precursors exhibit excitation-dependent, red-shifted, and lower efficiency emission. Theoretical calculations, performed to understand the configuration and distribution of nitrogen dopants within the NCQD structure, show that pyridinic and graphitic nitrogen atoms exhibit a strong preference to aggregate near the centre of the edge of the NCQD and not in the vertices nor in the graphitic core, thus affecting the HOMO and LUMO, bandgap, and light absorption and emission wavelengths. The life cycle assessment (LCA) analysis highlights the green and scalable advantages of the CHFS process for producing NCQDs compared to batch methods, making it a sustainable and economically viable approach for large-scale NCQD synthesis from high molecular weight biomass-derived precursors. Hence, the combination of experimental data and theoretical calculations provides a comprehensive understanding of the structure-property relationships in these NCQDs.

This research targets the need for eco-friendly strategies in the synthesis of bioactive materials, addressing the importance of valorization of vegetal waste. This study focuses on developing biohybrids containing biomimetic lipid vesicles and phytosynthesized gold-silver chloride nanoparticles (AuAgCl NPs) derived from Achillea millefolium L. extract. By leveraging the natural antioxidant and antimicrobial properties of the plant, the research proposes a sustainable approach to creating materials with potential biomedical applications. The biomimetic membranes were loaded with chlorophyll a, a natural spectral marker. Three types of bioactive materials (biohybrids) were developed by varying the lipid vesicle/AuAgCl NP ratio. Optical (UV-Vis, fluorescence emission, FTIR), structural (XRD), elemental (EDX, XPS), and morphological (TEM) studies were performed to characterize the bio-developed materials. The hydrophobic/hydrophilic characteristics of the samples were investigated by measuring the water contact angle, and their size was estimated by DLS and TEM. Zeta potential measurements were used to evaluate the physical stability of phyto-developed particles. Antioxidant properties of phyto-particles were investigated through the chemiluminescence technique. The obtained biomaterials exhibited high antioxidant activity and antiproliferative activity against HT-29 and B-16 cancer cells. Therapeutic index values were calculated for each biohybrid. Additionally, the bio-prepared hybrids revealed biocidal action against Staphylococcus aureus and Enterococcus faecalis. The phyto-developed biomaterials are promising in biomedical applications, particularly as adjuvants in cancer therapy.

This work examines the effect of incorporating a third cation M (M = Mg, Al, Mn, or Fe) into Nb-promoted NiO catalysts (Ni0.85Nb0.15O) on their physicochemical properties and catalytic performance in the oxidative dehydrogenation (ODH) of ethane into ethylene. Therefore, a series of mixed oxides with the composition Ni0.765Nb0.135M0.1O was synthesized, characterized, and analyzed for catalytic behavior. The addition of the third cation markedly modified the redox and semiconductive properties of the catalysts, thus affecting their ODH performance. The selectivity toward ethylene was strongly dependent on the cation modifier, as it modulated the density of nonselective sites on the catalyst surface. In the low-temperature region, Mn-NiNbO demonstrated the highest conversion rates but with the lowest ethylene selectivity, whereas Fe-NiNbO, despite being less active than the undoped NiNbO, exhibited the highest ethylene selectivity. Overall, undoped NiNbO emerged as the best catalyst in terms of both activity and selectivity at isoconversion. All catalysts underwent partial reduction under reaction conditions, with the degree of reduction inversely correlated with the specific reaction rates. Although none of the catalysts remained stable at 400 degrees C, Al-NiNbO showed the least deactivation over time. The deactivation was linked to a reduction in p-type conductivity and a loss of redox functionality during operation.

Atomically clean SrTiO3(001) is characterized by low energy electron diffraction, core level and valence band photoelectron spectroscopy, the latter also with spin resolution. Samples prepared by a sputtering-annealing procedure exhibited in-gap states in the valence band spectra, Ti3+ components in Ti 2p core level spectra and a noticeable spin asymmetry in the 3-9 eV binding energy range, which corresponds to valence states of mainly O 2p character. Upon annealing in oxygen, the spin asymmetry vanishes, accompanied by the intensity decrease of the contribution of titanium low ionization states and of in-gap states, indicating that these three phenomena are mutually connected. The observed spin asymmetry may be generated by indirect exchange mediated by the in-gap states between O 2p orbitals, or by the partial Ti 3d character of these states, which acquire non-zero spin in case of incomplete oxygen coordination.

A series of M(x)CoCeMgAlO mixed oxides with different transition metals (M = Cu, Fe, Mn, and Ni) with an M content x = 3 at. %, and another series of Fe(x)CoCeMgAlO mixed oxides with Fe contents x ranging from 1 to 9 at. % with respect to cations, while keeping constant in both cases 40 at. % Co, 10 at. % Ce and Mg/Al atomic ratio of 3 were prepared via thermal decomposition at 750 degrees C in air of their corresponding layered double hydroxide (LDH) precursors obtained by coprecipitation. They were tested in a fixed bed reactor for complete methane oxidation with a gas feed of 1 vol.% methane in air to evaluate their catalytic performance. The physico-structural properties of the mixed oxide samples were investigated with several techniques, such as powder X-ray diffraction (XRD), scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDX), elemental mappings, inductively coupled plasma optical emission spectroscopy (ICP-OES), X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction under hydrogen (H2-TPR) and nitrogen adsorption-desorption at -196 degrees C. XRD analysis revealed in all the samples the presence of Co3O4 crystallites together with periclase-like and CeO2 phases, with no separate M-based oxide phase. All the cations were distributed homogeneously, as suggested by EDX measurements and elemental mappings of the samples. The metal contents, determined by EDX and ICP-OES, were in accordance with the theoretical values set for the catalysts' preparation. The redox properties studied by H2-TPR, along with the surface composition determined by XPS, provided information to elucidate the catalytic combustion properties of the studied mixed oxide materials. The methane combustion tests showed that all the M-promoted CoCeMgAlO mixed oxides were more active than the M-free counterpart, the highest promoting effect being observed for Fe as the doping transition metal. The Fe(x)CoCeMgAlO mixed oxide sample, with x = 3 at. % Fe displayed the highest catalytic activity for methane combustion with a temperature corresponding to 50% methane conversion, T50, of 489 degrees C, which is ca. 40 degrees C lower than that of the unpromoted catalyst. This was attributed to its superior redox properties and lowest activation energy among the studied catalysts, likely due to a Fe-Co-Ce synergistic interaction. In addition, long-term tests of Fe(3)CoCeMgAlO mixed oxide were performed, showing good stability over 60 h on-stream. On the other hand, the addition of water vapors in the feed led to textural and structural changes in the Fe(3)CoCeMgAlO system, affecting its catalytic performance in methane complete oxidation. At the same time, the catalyst showed relatively good recovery of its catalytic activity as soon as the water vapors were removed from the feed.

The fractal characterization of supported nanoparticles is a useful tool for obtaining structural and morphological information that strongly impacts catalytic properties. We have synthesized and characterized Pt supported on TiO2 nanostructures. Triblock copolymers with thermosensitive properties were used as templating agents during the synthesis process. In addition to the several techniques used for the characterization of the materials, we carried out fractal analysis. The prepared materials showed a reduction in the band gap of TiO2 from 3.44 to 3.01 eV. The extended absorption in the 500-700 nm regions is mostly attributed to the presence of supported Pt nanoparticles. The ability of the nanostructured Pt/TiO2 catalysts to generate H-2 in an aqueous solution was evaluated. The test reaction was carried out in the presence of methanol, as a hole scavenger, under simulated solar light. Pt/TiO2-3TB shows the highest rate of H-2 (4.17 mmol h(-1) g(cat)(-1)) when compared to Pt/TiO2-0TB (3.65 mmol h(-1) g(cat)(-1)) and Pt/TiO2-6TB (2.29 mmol h(-1) g(cat)(-1)) during simulated solar light irradiation. Pt/TiO2-3TB exhibits a more structured organization (fractal dimensions of 1.65-1.74 nm at short scales, 1.27-1.30 nm at long scales) and a distinct fractal behavior. The generation of hydrogen via photocatalysis can be linked to the fractal characteristics.