National Institute Of Materials Physics - Romania

The commercially ubiquitous liquid electrolytes for lithium-ion batteries have several shortcomings in terms of safety. Therefore, development of solid electrolytes, especially those that are glass-based, has been gaining increasing interest in recent times. However, the fundamental understanding of the changes in the glass structure and the corresponding changes in the properties due to the addition of dopants is necessary for the development of glasses. Therefore, here, we report a study on the role of vanadium on the glass structure, ionic conduction, crystallization behavior, and other properties of lithium silicate-based glasses (23Li(2)O-2.64K(2)O-2.64Al(2)O(3)-71.72SiO(2)) as a solid electrolyte for high-temperature Li-ion battery applications. Furthermore, we proposed a mathematical model to describe/quantify the ion-conducting channels' connectivity in glasses. The experimental glass structures were assessed using Si-29, V-51, Al-27 nuclear magnetic resonance, Fourier transform infrared, and ultraviolet-visible spectroscopy techniques. The ionic conductivity was measured by impedance spectroscopy, and the crystallization behavior was studied by optical microscopy and X-ray diffraction. Furthermore, molecular dynamics simulations were also used to gain structural insights of the glasses. In the designed compositions, the addition of vanadium decreased the overall concentration of Li+ ions. However, the results revealed that the ionic conductivity improved with the addition of vanadium in spite of a decrease in the number of charge carriers. This suggests that vanadium makes the pathways easier for the conducting ions. Thus, we conclude that vanadium modifies the conduction channels to promote better hoping of the ions from one site to another.

In this work, the photodegradation process of atorvastatin calcium (ATC) is reported as depending on: (1) the presence and the absence of excipients in the solid state; (2) the chemical interaction of ATC with phosphate buffer (PB) having pH equal to 7 and 8; and (3) hydrolysis reaction of ATC in the presence of aqueous solution of NaOH. The novelty of this work consists in the monitoring of the ATC photodegradation by photoluminescence (PL). The exposure of ATC in solid state to UV light induces the photo-oxygenation reactions in the presence of water vapors and oxygen from air. According to the X-ray photoelectron spectroscopic studies, we demonstrate that the photo-oxygenation reaction leads to photodegradation compounds having a high share of C=O bonds compared to ATC before exposure to UV light. Both in the presence of PB and NaOH, the photodegradation process of ATC is highlighted by a significant decrease in the intensity of the PL and photoluminescence excitation (PLE) spectra. According to PLE spectra, the exposure of ATC in the presence of NaOH to UV light leads to the appearance of a new band in the spectral range 340-370 nm, this belonging to the photodegradation products. Arguments concerning the chemical compounds, that resulted in this last case, are shown by Raman scattering and FTIR spectroscopy.

Graphene oxide (GO) and N-doped graphene [(N)G] graphenes were submitted to H-2 glow discharge under different discharge regimes, in both the negative glow and positive column plasma regions. The resulted catalysts were fully characterized using several techniques such as Raman, DRIFT and XPS spectroscopy, powder X-ray diffraction, H-2 pulse chemisorption and H-2-, CO2- and NH3-TPD experiments. Density functional theory calculations were performed taking a slab model of graphene sheet with an optimized C-C bond length (1.426 angstrom) and a 16 angstrom vacuum layer between sheets. An overview of these characterizations showed that the O/C atomic ratio of GO is influenced by the plasma regime, indicating the occurrence of O removal, as also predicted by DFT calculations. In the case of (N)G, the plasma treatment also removes pyridinic N with an increase of the C/N ratio. The efficiency of the plasma modification has been checked through catalytic tests in hydroisomerization of 1-octene and hydrogenation of alpha-methyl-styrene. Contrarily to classical thermal activation requiring high temperatures, the generation of the defects by treating with plasma occurs at voltages in the range of 2 5 kV. In consequence, the hydrogenation and isomerization of alkenes resulted with high yields and good selectivities. Graphene prepared from sodium alginate from brown algae was considered as reference in these investigations.

A series of PtCeTi-SBA-15 catalysts, with different concentrations of Pt (0.25 %, 0.5 %, 1.0 %), were synthesized. New Ti-SBA-15 material was obtained by direct synthesis and used as support in successive immobilization of Pt and Ce precursors, changing their order of addition. The characterization of the catalysts has been achieved taking an ensemble of techniques which showed preservation of the support porous structure, high dispersion of the amorphous metal oxides and Pt?, the formation of metal crystals for samples with 1wt% Pt and variation of Pt? content on surface in condition of ceria addition. The best conversion (100 %) was obtained in oxidation of hydrocarbons (CH4, C3H8 and C6H14) for the samples with 1% Pt and a significant diminish of activity was obtained for catalyst in which Ce was added on PtTi-SBA-15 sample. Differently of these, CO oxidation was totally irrespective of the catalyst composition.

The structural role of V in 28Li(2)O-72SiO(2) (in mol%) lithium silicate glass doped with 0.5 mol% V2O5 was assessed using Si-29 and V-51 Nuclear Magnetic Resonance (NMR), Fourier-transform infrared (FTIR), and X-ray photoelectron (XPS) spectroscopy techniques. Despite the low amount of V2O5 used, the structural information obtained or deduced from the statistical analysis of the NMR data could explain the evolution of glass properties after V2O5 addition. The XPS results indicated that all vanadium exists in 5+ oxidation state. Both the Si-29 NMR and FTIR data point toward an increase in the polymerization of the silicate network, caused by the V2O5 acting as network former, capable to form various QVn tetrahedral units (for n = 0, 1, and 2) in the glasses. These QVn units, which are similar to phosphate units, scavenge the Li+ ions and cause the silicate network to polymerize. However, in an overall balance, the entire glass network is depolymerized due to the additional nonbridging oxygens contributed by the vanadium polyhedra. The addition of vanadium causes the network to expand and increases the ionic conductivity.

Ferromagnetism in 3d metals is re-examined in a simple band model. It is shown that the molecular field model cannot account for the low values of coercive field in ferromagnetic pure metals, and that the standard Stoner theory of band ferromagnetism incorrectly evaluates the total electronic energy and cannot predict reasonable Curie temperatures. A simple band model for magnetism is formulated, yielding a criterion for ferromagnetism even in absence of a Hubbard term, involving only the density of states (DOS) at the Fermi level, its derivative and the filling of the 3d band. By introducing a double-peaked DOS, one may explain the occurrence of ferromagnetism in bcc Fe, hcp-fcc Co and fcc Ni, the stabilization of fcc-hcp or bcc structures across all 3d elements, the occurrence of antiferromagnetism in chromium, and derive reasonable Curie temperatures. 'Re-entrant' ferromagnetism is predicted at ultrahigh temperatures, suggesting an alternate origin for the geomagnetic field.

Highly dispersed supported NbOx species were prepared via a deposition precipitation-carbonization (DPC)-like method. As precursors for niobium species and carrier ammonium niobate(V) oxalate hydrate and humins were used. Characterization of the resulted catalysts indicated bi-functional acid-base niobium species anchored onto a highly hydrophobic graphite-like carbon structure. These catalysts were investigated in the one-pot conversion of glucose to 5-hydroxymethylfurfural (HMF) in a biphasic system consisting of a mixture of a 20 wt% NaCl aqueous solution phase and an organic extracting phase (methyl-isobutyl-ketone (MIBK), 2-tert-butylphenol (TBP) or 2sec-butylphenol (SBP)). The optimization conditions afforded the highest yield to HMF (96 %) for the GHNb1.2 catalyst (with 2.5 wt%Nb), the base/acid sites ratio of 1.76, in biphasic TBP/water system, at 180 ?C after 8 h.

Polymer electrolyte membrane fuel cells require cheap and active electrocatalysts to drive the oxygen reduction reaction. Nitrogen-doped carbons have been extensively studied regarding their oxygen reduction reaction. The work at hand looks beyond the nitrogen chemistry and brings to light the role of oxygen. Nitrogen-doped nanocarbons were obtained by a radio-frequency plasma route at 0, 100, 250, and 350 W. The lateral size of the graphitic domain, determined from Raman spectroscopy, showed that the nitrogen plasma treatment decreased the crystallite size. Synchrotron radiation photoelectron spectroscopy showed a similar nitrogen chemistry, albeit the nitrogen concentration increased with the plasma power. Lateral crystallite size and several nitrogen moieties were plotted against the onset potential determined from oxygen reduction reaction curves. There was no correlation between the electrochemical activity and the sample structure, as determine from Raman and synchrotron radiation photoelectron spectroscopy. Near-edge X-ray absorption fine structure (NEXAFS) was performed to unravel the carbon and nitrogen local structure. A difference analysis of the NEXAFS spectra showed that the oxygen surrounding the pyridinic nitrogen was critical in achieving high onset potentials. The work shows that there were more factors at play, other than carbon organization and nitrogen chemistry.

Carbon monoxide is adsorbed at room temperature on graphene formed on atomically clean Pt(001)-hex by chemical vapor deposition, starting with ethylene, in ultrahigh vacuum. The graphene formation is characterized in situ by high resolution photoelectron spectroscopy (HRPES), by low energy electron diffraction (LEED) and by near-edge X-ray absorption fine structure (NEXAFS). The formation of graphene destroys the hex reconstruction of Pt(001) and graphene exhibits totally in-plane sp(2) bonding. CO adsorption is characterized by HRPES and a rigid shift towards higher binding energies by about 96 meV is experienced by Pt 4f core levels, together with a shift towards lower binding energy by 36 meV of the C 1s level corresponding to graphene, while the amplitude analysis of carbon and platinum peaks suggests the intercalation of carbon oxide between graphene and the metal substrate. The presence of oxidized carbon is evidenced by a separate component in the C 1s spectrum (attributed to carbon bond to oxygen) and by the occurrence of the O 1s signal. The coverage expressed in terms of the ratio of the integral amplitudes of the carbon bond to oxygen to the amplitude of the carbon from graphene approaches 3 %, yielding a CO coverage of Pt(001) of about 0.12 monolayer. The derived atomic ratio (O 1s):(C 1s bond to O) is initially close to 1, then evolves in time towards values close to 2, which means that CO is progressively oxidized upon adsorption and irradiation with soft X-rays. The relative amount of oxygen and oxidized carbon decreases in time under irradiation with soft X-rays. Weakly bound graphene on incommensurate metal surfaces may be used as atomic scale nanoreactors for trapping and immediate oxidation of carbon monoxide.