National Institute Of Materials Physics - Romania

The presence of a metallic layer can influence the properties of high-temperature superconductors underneath. We investigate the influence of metallic structures deposited in form of nanoparticles or continuous layers by electron beam evaporation or ion beam sputtering on the properties of Y1Ba2Cu3O7-x (YBCO) thin films. To generally avoid diffusion of metal atoms an additional barrier layer is introduced. Detailed measurements of the magnetic moment of the superconductor as a function of temperature and magnetic field have been performed using SQUID magnetometry. It is found that the modification of the superconducting properties of coated YBCO strongly depends on the deposition method of the metal on top rather than the type of metal (Ni or Ag), its magnetic properties (ferromagnetic or paramagnetic) or its morphology (nanoparticles or thin film). The main result is a transient increase of the critical temperature T-c and critical current density J(c) that was observed for samples prepared by electron beam evaporation.

Niobia-based magnetic nanocomposites were prepared by covering magnetite nanoparticle cores (Fe3O4, MNP) with either Nb2O5 or Nb2O5-SiO2 shells using a two-step procedure. In the first step magnetite nanoparticles were prepared by the coprecipitation method. The second step involved their coverage with either Nb2O5 shells, through a precipitation method, or with Nb2O5-SiO2 shells, through a sol-gel protocol followed by precipitation in the presence of the CTAB surfactant. The obtained materials were exhaustively characterised by X-ray diffraction, Mossbauer spectroscopy, magnetic measurements, ICP-OES, DRIFT and Raman spectroscopy, and CO2 - and NH3-TPD measurements, and investigated for glucose dehydration to HMF. The catalytic performances were directly correlated to the nature of the supported niobia phases, which, in turn, has been dictated by the niobia content and the preparation route. The high selectivity to HMF was correlated with to the large pseudohexagonal niobium oxide (TT-Nb2O5) phase while the catalytic activity was directly correlated to the small nanoparticles size. A proper combination of these features led to an optimum catalytic system for the selective production of HMF through glucose dehydration. A third important feature making the developed catalyst promising is its magnetic property, ensured by the magnetite nanoparticles core. This allowed its easy separation from the reaction products.

In the present paper is presented an efficient strategy for synthesis of mesoporous TiO2 modified with different Ce concentrations through a sol-gel process in the presence of triblock Pluronic P123 as structure directing agent, integrated with evaporation-induced self- assembly (EISA) approach. The nanocomposite consisted mostly of small crystallite of anatase. The presence of a new crystal phase corresponding to cerium titanate was evidenced in Ce-modified powder samples. All materials were characterized by SEM and TEM microscopies, UV-vis, XPS and N-2 adsorption-desorption isotherms in order to examine the textural and structural characteristics and the chemical nature of their surface. Furthermore, the photoelectrochemical characterization evidenced the effect of the TiO2 mesoporous structure, the amount of cerium and the oxidation state of Ti and Ce in the new photocatalysts on the enhanced photovoltaic performance. The photocatalytic activity of the as-synthesized materials was evaluated for phenol photodegradation in aqueous media and the reactive species involved in photocatalytic process were established by using some radical scavengers in the photodegradation experiments. Based on the obtained results, these new Ce-modified mesoporous TiO2 photocatalysts could be considered for degradation of organic compounds from wastewaters by advanced oxidation processes or for photoanodes construction for efficient photoelectrochemical fuel cell (PFC) systems.

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.

Gold is deposited on atomically clean, inwards polarized, ferroelectric lead zirco-titanate deposited by pulsed laser deposition on strontium titanate (001) single crystal, then carbon monoxide adsorption and desorption experiments are investigated by in situ fast photoelectron spectroscopy using synchrotron radiation. Atomic force microscopy and high resolution photoelectron spectroscopy are consistent with the formation of 50?100 nm nanoparticles, and their Au 4f core levels point to a negative charge state of gold. As compared with a similar experiment performed on ferroelectric lead zirco-titanate with similar polarization state and without gold, the saturation coverage after exposure to carbon monoxide increases by about 68 %, and also most of the additional carbon is found in oxidized state. Desorption experiments with in situ follow-up by photoelectron spectroscopy are performed as function of temperature, and the neutral carbon intensity decreases when the ferroelectric polarization decreases, while the components corresponding to oxidized carbon remain unchanged. It looks that neutral carbon adsorption is strictly related to the polarization of the ferroelectric film, while carbon still found in molecular form is related to its carbonyl bonding on metal nanoparticles, independent of the polarization state of the substrate. Desorbed carbon at higher temperature uptakes oxygen from the substrate.

Environmentally-friendly bio-organic materials have become the centre of recent developments in organic electronics, while a suitable interfacial modification is a prerequisite for future applications. In the context of researches on low cost and biodegradable resource for optoelectronics applications, the influence of a 2D nanostructured transparent conductive electrode on the morphological, structural, optical and electrical properties of nucleobases (adenine, guanine, cytosine, thymine and uracil) thin films obtained by thermal evaporation was analysed. The 2D array of nanostructures has been developed in a polymeric layer on glass substrate using a high throughput and low cost technique, UV-Nanoimprint Lithography. The indium tin oxide electrode was grown on both nanostructured and flat substrate and the properties of the heterostructures built on these two types of electrodes were analysed by comparison. We report that the organic-electrode interface modification by nano-patterning affects both the optical (transmission and emission) properties by multiple reflections on the walls of nanostructures and the electrical properties by the effect on the organic/electrode contact area and charge carrier pathway through electrodes. These results encourage the potential application of the nucleobases thin films deposited on nanostructured conductive electrode in green optoelectronic devices.

We report the obtaining of a new composite starting from pyrene thiazole, a compound certified by nuclear magnetic resonance and its covalent grafting on the surface of graphene oxide. Novel material was synthesized in two stages: the first involving transformation of carboxyl groups of graphene oxide into acid chlorides and the second the amide reaction between acid chloride and amine group of pyrene thiazole (PTC). Numerous characterization methods have been used to certify this material, such as: Raman spectroscopy, fluorescence, infrared spectroscopy and X-ray photoelectron spectroscopy, scanning electron microscopy and transmission electron microscopy. Their results show the successful covalent functionalization of graphene oxide with pyrene thiazole through the formation of amide bonds. The electrochemical investigation consisted of evaluating the redox behavior of the carbon screen printed electrodes modified with the new composite (GO-PTC) using caffeic acid, as analyte. From analytical point of view, it is relevant to be able to quantify the presence of caffeic acid and for such reason we used as analytical method the square wave voltammetry. The results showed that the GO-PTC modified carbon screen printed electrodes were able to detect the caffeic acid over more than one order of magnitude (linear working range: 0.005-0.1 mM) and GO-PTC modified electrodes can be considered promising for other analytical investigations.

We present hybrid nanomaterial architectures, consisting of carbon nanowalls (CNW) templates decorated with tungsten oxide nanoparticles, synthesized using a mechanism based on tungsten oxide sublimation, vapor transport, followed by vapor condensation, in the absence or presence of plasma. The key steps in the decoration mechanism are the sublimation of tungsten oxides, when are exposed in vacuum at high temperature (800 degrees C), and their redeposition on colder surfaces (400-600 degrees C). The morphology and chemical composition of the hybrid architectures, as obtained from Scanning Electron Microscopy and X-ray Photoelectron Spectroscopy, are discussed with respect to substrate nature and the physical conditions of synthesis. We pointed out that the decoration process is strongly dependent on the temperature of the CNW templates and plasma presence. Thus, the decoration process performed with plasma was effective for a wider range of template temperatures, in contrast with the decoration process performed without plasma. The results are useful for applications using the sensing and photochemical properties of tungsten oxides, and have also relevance for fusion technology, tungsten walls erosion and material redeposition being widely observed in fusion machines.

The specific roles played by both support and noble metals in light absorption, charge separation, and the formation of center dot OH and O-2(-) (ROS) are analyzed for light-triggered oxidation of phenol (Ph) over pristine and over noble metal (Ag, Au, Pt) -loaded TiO2. Experiments show that the supported noble metals act as a light visible absorber, assist the separation of photo-charges and reduction of O-2 to O-2(-). The O-2(-) oxidizes mildly Ph to oxygenated products (hydroquinone, benzoquinone, and 1,2-dihydroxibenzene). In a parallel process, center dot OH radicals, yielded by TiO2, mineralize Ph to CO2 by fast reaction sequences. Radical quenching and photo electrochemical measurements (surface photovoltage) confirm independently that the production of center dot OH and O-2(-) scale with oxidative conversion of Ph. The selectivity to CO2 and mild oxidation products is the result of the interplay between catalyst activity for center dot OH and for O-2(-) production.

The aim of this paper is to prepare multi-layered ITO thin films by a low cost and environmental-friendly method for different applications (optoelectronics, sensors, etc.). ITO films with 15 layers were obtained by successive depositions using the sol-gel & dip-coating method on three different substrates: glass, SiO2/glass and SiO2/Si. Comparative structural, morphological and optical characterization were performed by X-ray Diffraction (XRD), Atomic Force Microscopy (AFM), Cross Section Transmission Electron Microscopy (XTEM) coupled with Selected Area Electron Diffraction (SAED), Infrared Spectroscopic Ellipsometry (IRSE) and Raman spectroscopy analyses. The optical constants (refractive index n and extinction coefficient k) were determined in a large spectral range (300-27500 cm-1) by spectroscopic ellipsometry (SE). The thicknesses determined by SE were confirmed by HRTEM (High Resolution TEM) measurements which also presents in detail the textural properties of the ITO films at nanometric level. A comparison between IRSE and Raman analysis in the infrared active region was presented.