Correlation between Deposition Parameters and Hydrogen Production in CuO Nanostructured Thin Films.

In this article, we report a systematic investigation of the role of (i) substrate temperature, (ii) oxygen partial pressure, and (iii) radio frequency (rf) power on the crystal structure and morphology of CuO nanostructured thin films prepared by means of rf-magnetron sputtering starting from a Cu metal target. On selected films, photocatalytic tests have been carried out in order to correlate the structural and morphological properties of the thin films prepared under different conditions with the photocatalytic properties and to find out the key parameters to optimize the CuO nanostructured films. All of the synthesized films were single-phase CuO nanorods of variable diameter between 80 and 200 nm. Better-aligned rods were obtained at relatively low substrate temperatures and from low to intermediate oxygen partial pressures, resulting in more efficient photocatalytic activities. Our investigation suggests a relevant role of the crystallographic orientation of the CuO tenorite film on the photocatalytic activity, as demonstrated by the significant improvement in H2 evolution for highly oriented films.

Microfluorescence analysis of nanostructuring inhomogeneity in optical fibers with embedded gallium oxide nanocrystals.

A spectroscopic protocol is proposed to implement confocal microfluorescence imaging to the analysis of microinhomogeneity in the nanocrystallization of the core of fibers belonging to a new kind of broadband fiber amplifier based on glass with embedded nanocrystals. Nanocrystallization, crucial for achieving an adequate light emission efficiency of transition metal ions in these materials, has to be as homogeneous as possible in the fiber to assure optical amplification. This requirement calls for a sensitive method for monitoring nanostructuring in oxide glasses. Here we show that mapping microfluorescence excited at 633 nm by a He-Ne laser may give a useful tool in this regard, thanks to quasi-resonant excitation of coordination defects typical of germanosilicate materials, such as nonbridging oxygens and charged Ge-O-Ge sites, whose fluorescence are shown to undergo spectral modifications when nanocrystals form into the glass. The method has been positively checked on prototypes of optical fibers--preventively characterized by means of scanning electron microscopy and energy dispersive spectroscopy--fabricated from preforms of Ni-doped Li2O-Na2O-Sb2O3-Ga2O3-GeO2-SiO2 glass in silica cladding and subjected to heat treatment to activate gallium oxide nanocrystal growth. The method indeed enables not only the mapping of the crystallization degree but also the identification of drawing-induced defects in the fiber cladding.

Deepening the shear structure FeNb11O29: influence of polymorphism and doping on structural, spectroscopic and magnetic properties.

FeNb11O29 is an intriguing and promising material that has been emerging in the last few years. It is isostructural with Nb12O29, one of the rare compounds in which Nb displays a local magnetic moment and shows both antiferromagnetic ordering and metallic conductivity at low temperatures. Both the two polymorphic monoclinic and orthorhombic forms have a mono-dimensional magnetic arrangement, but the different disposition of the structural building blocks leads to a strong frustration phenomenon of the magnetic order in the high-temperature orthorhombic form. Whereas Nb12O29 has been widely studied, barely few magnetic data can be found on its analogous FeNb11O29, for which a role of the Fe3+ localized d electrons in affecting the original magnetic behaviour can be foreseen. In this paper, we report how we synthesized undoped and, for the first time, Mn- and V-doped FeNb11O29. Both the monoclinic and orthorhombic polymorphs, stable in different temperature ranges, were then thoroughly structurally characterized. With the help of micro-Raman spectroscopy, we investigated the differences introduced into the vibrational levels by doping, while EPR data allowed us to obtain information on the transition metal ions and to point out the related peculiar structural features. Static magnetization measurements evidenced the paramagnetic character of the compounds and the high-spin configuration of Fe3+ ions.

Absence of long-range magnetic order in the La1.4Sr0.8Ca0.8Mn2O7 bilayered manganite.

This paper presents the results of a high-resolution neutron diffraction and magnetometry investigation on the optimally doped (x = 0.3) La(1.4)Sr(0.8)Ca(0.8)Mn(2)O(7) bilayered manganite. In particular, two samples with different oxygen contents have been studied to put in prominence the role of the Mn average valence states at fixed cation concentration. The results show, for the first time, the absence of long-range magnetic order in this optimally doped manganite when the A-site of the structure is doped with equal proportions of isovalent Ca and Sr. This holds for both samples, which present different lattice effects with T, thus suggesting the primary role of cation disorder as the source of the lack of long-range order. The presence, for both samples, of defined insulating- to metallic-like transitions suggests that the transport properties are not linked to the evolution of long-range order and that two-dimensional spin ordering in the layers of the perovskite blocks may be sufficient to "assist" the hole hopping. A possible reason for the suppression of magnetic order induced by the Ca doping is a size effect coupled to the cation size mismatch between the Sr and Ca ions.

Doping effects in single-layered La0.5Sr1.5MnO4 manganite.

In this paper we report the results of the synthesis and structural, transport, and magnetic characterization of pure La(0.5)Sr(1.5)MnO(4) and B-site lightly doped samples, i.e. La(0.5)Sr(1.5)Mn(0.95)B(0.05)O(4), where B = Ru, Co, and Ni. The choice was made in order to probe the charge ordering/orbital ordering ground state of the monolayered La(0.5)Sr(1.5)MnO(4) manganite as a consequence of the cation doping. It is shown that even a light doping is successful in suppressing the charge and orbital order found in pure La(0.5)Sr(1.5)MnO(4). No long-range magnetic order has been detected in any of the doped samples but the setup of a spin-glass state with a common freezing temperature ( approximately 22 K). Structural parameters show an anisotropy in the lattice constant variation, with the tetragonal distortion increasing as the cell volume reduces, which may suggest a variation in the orbital character of the e(g) electrons along with the overall cation size.

New materials for Li-ion batteries: synthesis and spectroscopic characterization of Li2(FeMnCo)SiO4 cathode materials.

Improving cathode materials is mandatory for next-generation Li-ion batteries. Exploring polyanion compounds with high theoretical capacity such as the lithium metal orthosilicates, Li2MSiO4 is of great importance. In particular, mixed silicates represent an advancement with practical applications. Here we present results on a rapid solid state synthesis of mixed Li2(FeMnCo)SiO4 samples in a wide compositional range. The solid solution in the P21/n space group was found to be stable for high iron concentration or for a cobalt content up to about 0.3 atom per formula unit. Other compositions led to a mixture of polymorphs, namely Pmn21 and Pbn21. All the samples contained a variable amount of Fe(3+) ions that was quantified by Mössbauer spectroscopy and confirmed by the TN values of the paramagnetic to antiferromagnetic transition. Preliminary characterization by cyclic voltammetry revealed the effect of Fe(3+) on the electrochemical response. Further work is required to determine the impact of these electrode materials on lithium batteries.

Influence of Ru doping on the structure, defect chemistry, magnetic interaction, and carrier motion of the La(1-x)Na(x)MnO(3+delta) manganite.

In this work, we report a structural, electrical, and magnetic characterization of the La(1-x)Na(x)Mn(1-y)Ru(y)O(3+delta) (LNMRO) system with x = 0.05 and 0.15 and y = 0, 0.05, and 0.15, also comprising an investigation of the role of the oxygen content on the related redox properties. The experimental investigation has been realized with the aid of X-ray powder diffraction, electron microprobe analysis, thermogravimetry, electrical resistivity and magnetization measurements, and electron paramagnetic resonance. We demonstrate that the effect of ruthenium doping on the studied LNMRO compounds is not only directly related to the Ru/Mn substitution and to the Ru oxidation state but also indirectly connected to the oxygen content in the sample. Our data show that ruthenium addition can "improve" electrical and magnetic properties of nonoptimally (low) cation-doped manganites, causing an increase of the T(C) value and the insurgence of magnetoresistance effect, as observed for the x = 0.05 and y = 0.05 sample (MR congruent with 60% at 7 T and at approximately 260 K).

Superconductivity in Sm-doped [n]phenacenes (n = 3, 4, 5).

We report here the discovery of a new aromatic hydrocarbon superconductor, Sm-doped chrysene, with Tc ∼ 5 K, and compare its behavior with those measured in the full series of Sm-doped [n]phenacene superconductors, with n = 3, 4, 5, thus determining the trend of Tc as a function of the number of fused benzene rings and for an odd or even number of units.

Polymorphism and magnetic properties of Li2MSiO4 (M = Fe, Mn) cathode materials.

Transition metal-based lithium orthosilicates (Li2MSiO4, M = Fe, Ni, Co, Mn) are gaining a wide interest as cathode materials for lithium-ion batteries. These materials present a very complex polymorphism that could affect their physical properties. In this work, we synthesized the Li2FeSiO4 and Li2MnSiO4 compounds by a sol-gel method at different temperatures. The samples were investigated by XRPD, TEM, (7)Li MAS NMR, and magnetization measurements, in order to characterize the relationships between crystal structure and magnetic properties. High-quality (7)Li MAS NMR spectra were used to determine the silicate structure, which can otherwise be hard to study due to possible mixtures of different polymorphs. The magnetization study revealed that the Néel temperature does not depend on the polymorph structure for both iron and manganese lithium orthosilicates.