National Institute Of Materials Physics - Romania

The development of new thin-film cathodes triggered a recent research interest in energy storage applications. Over the past years, vanadium oxides have been extensively explored as promising electrodes for batteries owing to their rich valence states and remarkable electrochemical properties. Herein, we report on the synthe-sis of undoped and Sn doped V2O3 thin-films on graphene (G)/Al foil by pulsed laser deposition followed by rapid thermal annealing in N2 at low temperature (similar to 430 degrees C). The obtaining V2O3 phase on graphene/Al foil (G/Al) has been confirmed by X-ray diffraction and Raman and X-ray photoelectron spectroscopy analyses. The synthesized vanadium oxide films were tested as cathodes in coin cells. The electrochemical properties have been systematically investigated by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and galvanostatic charge discharge (GCD) measurements. A superior electrochemical performance was observed for the V2O3 on G/Al structures, with an initial capacity of around 300 mAh g-1, with respect to the bare G/Al electrode. The use of the Sn-doped (5 mol%) V2O3 thin-films improved slightly the initial capac-ity up to a value of ca. 311 mAh g-1. Both V2O3/G/Al and Sn-doped V2O3/G/Al exhibited excellent cycling performances after 40 cycles with a capacity maintenance at a C-rate C/20 of 317 mAh g-1. Long-term cycling test (up to 200 cycles) showed that the Sn doping could be an excellent strategy to improve the stability of the electrodes, which yielded a capacity loss of only 0.128% per cycle. Possible mechanisms are presented and dis-cussed. This work could serve as point of reference for future developments in the field of batteries employing vanadium oxide-based thin-films deposited by physical vapor deposition techniques.

In this paper, we present a theoretical perspective regarding the interaction of graphene with circularly polarized light and magnetic field, from the topological insulators point of view. We analyze how these two external fields affect the spectral, topological and transport properties of graphene and correlate the findings in order to explain in a fundamental and unified way the emerging topological phase transitions. In this respect, in the first step we introduce a model for interaction and charge transport. Then, based on the derived theory, we present numerical results aimed to explain the underlying processes which give graphene topological properties. The central point is represented by the time-reversal symmetry breaking which generates chiral edge states, namely electronic states localized at the edges of the system, having opposite velocity directions. We find that the light frequency, intensity and polarization state drastically influence the formation of the chiral edge states and their number. We correlate this effect with quantum Hall transport, analyzing the resulting transversal (Hall) resistance plateaus and their values. Moreover, if a supplementary magnetic field driving is applied, there emerge intricate topological phase transitions, characterized by introducing or removing specific Hall resistance plateaus.

In 1946 and 1949, Charles Kittel proposed a simple model for the size of ferromagnetic domains that is still widely used nowadays [C. Kittel, Phys. Rev. 70 (1946) 965-971 and Rev. Mod. Phys. 21 (1949) 541-583], including for other ferroic systems, such as ferroelectrics and multiferroics. Kittel's theory is revisited in this work, with a more detailed demonstration and emphasizing the main assumptions utilized, by using SI units instead of CGS units, as in the original Kittel's works. The validity limits of the Kittel's scaling law where the domain width varies with the square root of the sample thickness towards low thicknesses is derived, with the possibility of evolution towards large domains for ultralow thicknesses. Further, Kittel's model is extended to the case where the sample has a non-vanishing net magnetization and it is shown how magnetization curves at zero temperature can be obtained. This is discussed by supposing constant width of a pair of neighboring domains with opposed magnetization, or by allowing this width to vary as function on the net magnetization of the sample. Though this latter assumption seems to be more reasonable from the point of view of the evolution towards a single domain state at saturation, it seems that the model able to yield most accurate vales of the coercive field is the domain with fixed width of the pair of domains, which justifies the assumption of domain wall pinning". The introduction of the demagnetization factor associated with the finite size of the film yields a maximum thickness up to which the films present hysteresis curves. The validity of this theory for ferrelectric domains is also briefly discussed.

Hybrid layers consisting of Fe oxide, Fe hydroxide, and nitrogen doped graphene-like platelets have been synthesized by an eco-friendly laser-based method for photocatalytic applications. The complex composite layers show high photodecomposition efficiency towards degradation of antibiotic molecules under visible light irradiation. The photodecomposition efficiency was investigated as a function of relative concentrations of base materials, Fe oxide nanoparticles and graphene oxide platelets used for the preparation of target dispersions submitted to laser irradiation. Although reference pure Fe oxide/Fe hydroxide layers have high absorption in the visible spectral region, their photodecomposition efficiency is negligible under the same irradiation conditions. The high photocatalytic decomposition efficiency of the nanohybrid layer, up to 80% of the initial antibiotic molecules was assigned to synergistic effects between the constituent materials, efficient separation of the electron-hole pairs generated by visible light irradiation on the surface of Fe oxide and Fe hydroxide nanoparticles, in the presence of conducting graphene-like platelets. Nitrogen doped graphene-like platelets contribute also to the generation of electron-hole pairs under visible light irradiation, as demonstrated by the photocatalytic activity of pure, reference nitrogen doped graphene-like layers. The results also showed that adsorption processes do not contribute significantly to the removal of antibiotic molecules from the test solutions. The decrease of the antibiotic concentration under visible light irradiation was assigned primarily to photocatalytic decomposition mechanisms.

Pb(Zr,Ti)O-3 (PZT) is the most common ferroelectric (FE) material widely used in solid-state technology. Despite intense studies of PZT over decades, its intrinsic band structure, electron energy depending on 3D momentum k, is still unknown. Here, Pb(Zr0.2Ti0.8)O-3 using soft-X-ray angle-resolved photoelectron spectroscopy (ARPES) is explored. The enhanced photoelectron escape depth in this photon energy range allows sharp intrinsic definition of the out-of-plane momentum k and thereby of the full 3D band structure. Furthermore, the problem of sample charging due to the inherently insulating nature of PZT is solved by using thin-film PZT samples, where a thickness-induced self-doping results in their heavy doping. For the first time, the soft-X-ray ARPES experiments deliver the intrinsic 3D band structure of PZT as well as the FE-polarization dependent electrostatic potential profile across the PZT film deposited on SrTiO3 and LaxSrMn1-xO3 substrates. The negative charges near the surface, required to stabilize the FE state pointing away from the sample (P+), are identified as oxygen vacancies creating localized in-gap states below the Fermi energy. For the opposite polarization state (P-), the positive charges near the surface are identified as cation vacancies resulting from non-ideal stoichiometry of the PZT film as deduced from quantitative XPS measurements.

Zeolitic imidazole frameworks (ZIFs) and TiO2 based composites have recently attracted interest to control pathogenic microorganism growth. In this context, two novel TiO2/ZIF-67 nanocomposites were synthesized by the microwave-assisted solvothermal method using two different routes: one pot composite (OPC) and two step composite (TSC) syntheses. X-ray powder diffraction and vibrational spectroscopy data revealed the formation of crystalline ZIF-67 and disordered TiO2 in the OPC composite, while both phases were crystalline in the TSC sample. Sphere-like morphologies were obtained for both materials, as indicated by SEM and TEM images. XPS measurements showed mixed-valence Ti (Ti3+/Ti4+) and Co (Co2+/Co3+) on the surface of both materials, while EPR analysis confirmed the presence of Co2+ ions and oxygen-related defects. The results showed that TiO2 has no antimicrobial activity against Staphylococcus aureus, while the TSC has higher and the OPC has lower activity than pure ZIF-67. Highly efficient biocidal activity at a concentration of 2.5 mg mL(-1) was observed for the TSC. The difference in the antimicrobial performance of the nanocomposites can be correlated with the balance between structural order/disorder, which depends on the synthesis route used, and the material solubility. The results also indicate the release of Co ions and the ligand as a possible mechanism together with redox reactions to inactivate bacteria. Thus, this work provides simple and promising synthesis routes to obtain TiO2/ZIF-67 nanocomposites for potential use as new antimicrobial agents.

Several Co(x)CeMgAlO mixed oxides with different cobalt contents x between 7 and 50 at% with respect to cations, but with fixed 10 at% Ce and Mg/Al atomic ratio of 3, were obtained by the calcination of layered double hydroxide (LDH) precursors at 750 degrees C. The mixed oxide catalysts were characterized by XRD, nitrogen adsorption, DRIFT, DR-UV-Vis, SEM-EDX, including elemental mapping, XPS, H2-TPR and O2-TPO techniques. XRD analysis indicated the coexistence of Co3O4 crystallites with periclase-like and ceria phases, distributed homogeneously as shown by EDX measurements. The surface composition determined by XPS and the redox properties studied by H2-TPR and O2-TPO were shown to be key factors controlling the catalytic behavior of the investigated materials. Their catalytic performance was evaluated in the total oxidation of methane, used as a volatile organic compound (VOC) model molecule. Co(40)CeMgAlO mixed oxide showed the highest catalytic activity for methane combustion with a T50 of 528 degrees C, associated with its greatest Co/Ce surface atomic ratio and the best redox properties in the Co(x)CeMgAlO series. In addition, the influence of the contact time on the catalytic activity of the Co(40)CeMgAlO mixed oxide has been examined and its good stability was evidenced from combustion tests with prolonged time on stream. Nevertheless, the addition of water vapors to the reaction mixture induces textural and structural changes of the mixed oxide with negative consequences on its performance.

A multi-step ion-exchange methodology was developed for the fabrication of Cu(LaTa2O7)(2) lamellar architectures capable of wastewater depollution. The (001) diffraction line of RbLaTa2O7 depended on the guest species hosted by the starting material. SEM and TEM images confirmed the well-preserved lamellar structure for all intercalated layered perovskites. The UV-Vis, XPS, and photocurrent spectroscopies proved that Cu intercalation induces a red-shift band gap compared to the perovskite host. Moreover, the UV-Vis spectroscopy elucidated the copper ions environment in the Cu-modified layered perovskites. H-2-TPR results confirmed that Cu species located on the surface are reduced at a lower temperature while those from the interlayer occur at higher temperature ranges. The photocatalytic degradation of phenol under simulated solar irradiation was used as a model reaction to assess the performances of the studied catalysts. Increased photocatalytic activity was observed for Cu-modified layered perovskites compared to RbLaTa2O7 pristine. This behavior resulted from the efficient separation of photogenerated charge carriers and light absorption induced by copper spacer insertion.

In this work, new information concerning the optical properties of black phosphorus (BP) sheets chemically/electrochemically functionalized with diphenyl amine (DPA) and its macromolecular compound (poly(diphenylamine) (PDPA)) in the absence/presence of phosphotungstic acid (PTA) is reported. Raman scattering and FTIR spectroscopy studies indicate that the interaction of BP with PTA leads to the elimination of the PxOy layer onto the surface of the BP sheets. In the case of the chemical interaction of BP with DPA, the reaction product corresponds to DPA chemically functionalized BP sheets having an imino-phosphorane (IP) structure. The electrochemical oxidation of BP sheets chemically functionalized with DPA in the presence of PTA leads to an increase in the weight of P-N bonds as a consequence of the generation of PDPA doped with the PTA heteropolyanions, as shown by FTIR spectroscopy and Raman scattering. This process is evidenced by a shift of the Raman line from 362 cm(-1) to 378 cm(-1), assigned to the A(1g) mode. This change was explained by taking into account the compression of the layers containing P atoms, which is induced by PDPA macromolecular chains. The decrease in the intensity of the PL spectra of DPA as well as PDPA, in the presence of BP, indicates that BP acts as a PL quenching agent for these compounds. A preferential orientation of the PDPA doped with the PTA heteropolyanions on the surface of BP sheets is highlighted by the variation of the binding angle of the PDPA on the surface of BP sheets from 44.7 degrees to 39.9 degrees.

Understanding the phonon anharmonicity and temperature-dependent behavior of phonons that affect the thermal transport properties in 2D materials is crucial for developing efficient thermoelectric and memristor devices. SnSe has attracted significant interest because of its potential applications for developing such novel devices. Here, orthorhombic SnSe nanoflakes with a thickness of less than 100 nm and oriented along the [100] crystal axis were obtained using physical vapor transport at atmospheric pressure. Polarization-resolved Raman spectroscopy of SnSe nanoflakes was performed at a temperature of 5 K. Temperature-dependent frequencies and linewidths of Raman modes in tin selenide were fitted according to the anharmonic phonon coupling theory. The results indicate that both two and three order processes are responsible for the phonon decay in tin selenide. The memristive property was confirmed by electrical measurements of SnSe devices. SnSe memristors have an operating current of 10-4 A, similar to other transition-metal dichalcogenide memristors, but are more energy efficient than memristors based on defect migration, with a threshold voltage of 3 V.