National Institute Of Materials Physics - Romania

Recently, perovskites have become a hotspot for researchers attempting to exploit metal and oxygen vacancies in structures of the form MTiO3, facilitating the convenient electron/hole migration, thus displaying interesting properties. Magnesium Titanate (MgTiO3) is a prominent part of the perovskite class, exhibiting remarkable electrical, thermal, and chemical properties. Undoped and Mn-doped MgTiO3 samples were obtained using a solid-state reaction starting from previously synthesized MgO and TiO2 powders, which were separately doped with different Mn ion concentrations. The resulting multiphase materials with a major MgTiO3 phase were thoroughly morpho-structurally analyzed employing XRD, STEM, Raman, PL, XPS, and EPR spectroscopy. The electrochemical results indicate that they show superior performance when used as electrode materials for supercapacitor application due to the high defect concentration as shown in EPR and PL spectroscopy and the ferroelectric behavior observed in XPS and XRD. When used in symmetric and asymmetric supercapacitor devices, they show promising results, with specific capacity values reaching up to 109 F/g for the symmetric and 609 F/g for the asymmetric devices, while energy and power density values reached 84.7 Wh/kg and 90.8 kW/kg respectively, proving a great potential in the energy storage field.

In this work, three methods for the synthesis of composites based on poly(ortho-toluidine) (POT) and WS2 are reported: (a) the solid-state interaction (SSI) of POT with WS2 nanoparticles (NPs); (b) the in situ chemical polymerization (ICP) of ortho-toluidine (OT); and (c) the electrochemical polymerization (ECP) of OT. The preparation of WS2 sheets was performed by the ball milling of the WS2 NPs followed by ultrasonication in the solvent N,N'-dimethyl formamide. During the synthesis of the POT/WS2 composites by SSI and ICP, an additional exfoliation of the WS2 NPs was reported. In this work, we demonstrated the following: (a) the ICP method leads to POT/WS2 composites, which contain repeating units of POT in the leucoemeraldine salt (LS) state, while (b) the ECP method leads to POT/WS2 composites, which contain repeating units of POT in the emeraldine salt (ES) state. Capacitances equal to 123.5, 465.76, and 751.6 mF cm-2 in the cases of POT-ES/WS2 composites, synthesized by SSI, ICP, and ECP, respectively, were reported.

Ruthenium-based catalysts were prepared through a deposition-precipitation approach, taking beta zeolites with Si/Al ratios of 12.5, 18.5, and 150, respectively, as supports, and 1-3 wt% loadings of metal. Their activation was performed in the presence of either H2 or NaBH4. The dispersion of the Ru species and the acid-base properties were influenced by both the preparation method and the activation protocol. The catalysts reduced under H2 flow presented well-dispersed Ru(0) and RuOx nanoparticles, while the reduction with NaBH4 led to larger RuOx crystallites and highly dispersed Ru(0). These characteristics exerted an important role in the hydrogenation of levulinic acid (LA) to gamma-valerolactone (GVL). The H2 dissociation occurred via a heterolytic mechanism involving Lewis acid-base pairs associated with RuOx and the framework oxygen (Si-O-Al) located near the zeolite pore edge. The Ru(0) nanoparticles activated the -C=O bond of the LA substrate, while the presence of the carrier zeolite Br & oslash;nsted acid sites promoted the ring-closure esterification of the 4-hydroxyvaleric acid (4-HVA) intermediate to GVL. An optimal combination of these features was achieved for the catalyst with 3 wt% Ru and a Si/Al ratio of 150, which selectively converted LA (XLA = 96.5%) to GVL (SGVL = 97.8%) at 130 degrees C and 10 bars of H2.

Featured Application Optoelectronic devices in space missions.Abstract Transparent conductive oxides are essential materials for many optoelectronic applications. For new devices for aerospace and space applications, it is crucial to know how they respond to the space environment. The most important issue in commonly used low-Earth orbits is proton radiation. This study examines the effects of high-energy proton irradiation (226.5 MeV) on thin films of aluminium-doped zinc oxide (AZO) and indium tin oxide (ITO). We use X-ray diffraction and electron microscopy observations to see the changes in the structure and microstructure of the films. The optical properties and homogeneity of the materials are determined by spectrophotometry and spectroscopic ellipsometry (SE). Analysis of the chemical states of the elements with X-ray photoelectron spectroscopy (XPS) gives insight into what proton irradiation changes at the surface of the oxides. All measurements show that ITO is less influenced than AZO. The proton energy and fluence used in this study simulate about a hundred years in low Earth orbit. This research demonstrates that both transparent conductive oxide thin films can function under simulated space conditions, with ITO showing superior resilience. The ITO film was more homogenous in terms of the total thickness measured with SE, had fewer defects and adsorbates present on the surface, as XPS analysis proved, and did not show a difference after irradiation regarding its optical properties, transmission, refractive index, or extinction coefficient.

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 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.

The development of two-dimensional (2D) materials has gained significant attention due to their unique properties and potential applications in advanced electronics. This study investigates the fabrication and characterization of Fe-doped SnSe semiconductors using an optimized chemical vapor deposition (CVD) method. Fe doping was achieved by dissolving FeCl3 in deionized water, applying it to SnSe powder, and conducting vacuum drying followed by high-temperature CVD at 820 degrees C. Structural and morphological properties were characterized using optical microscopy, scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDX). Results revealed differently shaped flakes, including rectangles, discs and wires, influenced by Fe content. Micro-Raman spectroscopy showed significant vibrational mode shifts, indicating structural changes. X-ray photoelectron spectroscopy (XPS) confirmed the presence of Sn-Se and Fe-Se bonds. Electrical characterization of the memristive devices showed stable switching between high- and low-resistance states, with a threshold voltage of 1.6 V. These findings suggest that Fe-doped SnSe is a promising material for non-volatile memory and neuromorphic computing applications.