Scientific Papers in SCI

Abstract

The enhanced thermal stability of polymer-clay nanocomposites over the original polymers is one of their most interesting features, and it has been profusely studied within the last decades. Here, a thorough kinetic analysis of polystyrene and a montmorillonite-polystyrene nanocomposite has been performed making use of state-of-the-art kinetic procedures. It has been found that the degradation mechanism changes from a chain scission process for the polymer to a complex two-step nucleation-driven reaction for the nanocomposite. This mechanism change can explain the delayed onset of degradation found in the nanocomposite. Moreover, observation by transmission electron microscopy (TEM) has shown that the clay platelets within the composite could act as nucleation centers for the decomposition.

Abstract

Gold supported on cryptomelane-type OMS-2 catalysts deposited on AISI 304 stainless steels monoliths have been prepared for the first time, characterised and tested in the CO oxidation reaction. An easy and non-conventional method of incorporation of gold to the cryptomelane solid is used. This method allows the preparation of the monolithic catalysts without altering the structural and textural characteristics of the parent OMS-2 material. Although these catalysts do not show an optimal performance for the oxidation of CO, the presence of small gold particles enhances the catalytic performances of the cryptomelane producing promissory CO oxidation catalysts. The non-conventional gold deposition favours a partial loss of K ⁺ into the channels, resulting in an increment of the average oxidation state of manganese which favours the catalytic behaviour of these kinds of materials. This study can be taken as a starting point to obtain very active gold catalysts supported on OMS-2 materials through the optimisation of the gold-support interaction and the decrease in the gold particle size.

Abstract

Thermally stable nanoarchitectures are realized on the Ag(100) surface by self-assembly of asymmetrically substituted arenes. The process is instigated by adsorption-induced molecule → surface charge transfer that gives rise to in-plane dipole moments. Observation and calculation indicate that cooperative interactions further enhance the stability of these polarizable systems.

Abstract

Monophase nanocrystalline powders belonging to the La_1−xSr_xMnO_3±δ system (0≤x≤1) with a perovskite structure have been obtained by mechanochemistry synthesis using a planetary ball milling equipment from La₂O₃, SrO, and Mn₂O₃ mixtures. The solid state reaction was complete after one hour of milling treatment. For all the compositional range, the diffraction domain was very small and the structure appeared as a pseudo cubic perovskite. After annealing at 1100 °C under static air, the symmetry evolution due to the La substitution by Sr was analyzed by X-ray and electron diffraction. Samples with x=0, 0.25, 0.5, and 0.75 were assigned to R-3c space group (1 6 7) in the rhombohedral system and perovskite structure. However, the symmetry of the last term of the system (x=1), SrMnO_3±δ sample, changed to P6₃/mmc space group (1 9 4) in the hexagonal system. The terms with x=0.8, 0.85, and 0.9 presented mainly rhombohedral symmetry.

Abstract

The total oxidation of toluene is studied over catalytic systems based on perovskite with general formula AA′CoO _3-δ (A = La, A′ = Sr). The systematic and progressive substitution of La ³⁺ by Sr ²⁺ cations in the series (La _1-xSr _xCoO _3-δ system) of the perovskites have been studied to determine their influence in the final properties of these mixed oxides and their corresponding reactivity performance for the total oxidation of toluene as a model volatile organic compound with detrimental effects for health and environment. The structure and morphology of the samples before and after reaction have been characterized by XRD, BET and FE-SEM techniques. Additional experiments of temperature programmed desorption of O ₂ in vacuum and reduction in H ₂ were also performed to identify the main surface oxygen species and the reducibility of the different perovskites. It is remarkable that the La _1-xSr _xCoO _3-δ series presents better catalytic performance for the oxidation of toluene, with lower values for the T ₅₀ (temperature of 50 % toluene conversion) than the previously studied LaNi _1-yCoyO ₃ series.

Abstract

Advanced nanostructured materials that demonstrate useful activity under solar excitation in fields concerned with the elimination of pollutants, partial oxidation and the valorization of chemical compounds, water splitting and CO ₂ reduction processes, are discussed. Point defects present in nanoparticulated anatase present both 5-fold- and 6-fold-coordinated titanium atoms, as well as 2-fold- and 3-fold-coordinated oxygens. The requirement of using sunlight as the excitation source for the degradation reaction demands, as a principal requirement, the modification of the electronic characteristics of a UV absorber system such as anatase-TiO ₂. Some reports also indicate the need for large doping concentrations for N-doping in specific cases where notable changes in the valence band onset are subsequently observed. The effect of cetyltrimethylammonium bromide (CTAB) on the crystallization is reported by Yin et al. They showed that the presence of CTAB induces the appearance of BiOBr during the synthesis at 80°C using an aqueous method.

Abstract

Self-assembled monolayers of n-octadecylamine on mica (ODA/mica SAMs) have been investigated by atomic force microscopy (AFM) and by attenuated total reflectance infrared (ATR-FTIR) and X-ray photoelectron (XPS) spectroscopies. Topographic data characterizes a stable configuration with the alkyl skeleton tilted approximate to 46 degrees from the surface normal that is rationalized according to a well established structural alkyl chain packing model. Extended contact with air increases molecular tilting up to approximate to 58 degrees. ATR-FTIR and XPS reveal the presence of protonated amino groups within the monolayer and its increment upon exposure to air. The transition between both tilted states is explained assuming the protonation reaction as the driving force and introducing a model to evaluate an electrostatic repulsions term in the overall cohesive energy balance of the system. ODA molecules in the self-assembled monolayer respond to their spontaneous protonation by atmospheric water by tilting as a mechanism to relax the repulsions between -NH3+ heads.

Abstract

Growth of amorphous SiO₂ thin films deposited by reactive magnetron sputtering at low temperatures has been studied under different oxygen partial pressure conditions. Film microstructures varied from coalescent vertical column-like to homogeneous compact microstructures, possessing all similar refractive indexes. A discussion on the process responsible for the different microstructures is carried out focusing on the influence of (i) the surface shadowing mechanism, (ii) the positive ion impingement on the film, and (iii) the negative ion impingement. We conclude that only the trend followed by the latter and, in particular, the impingement of O^- ions with kinetic energies between 20 and 200 eV, agrees with the resulting microstructural changes. Overall, it is also demonstrated that there are two main microstructuring regimes in the growth of amorphous SiO₂ thin films by magnetron sputtering at low temperatures, controlled by the amount of O₂ in the deposition reactor, which stem from the competition between surface shadowing and ion-induced adatom surface mobility.

Abstract

A series of ZnO and Fe ₂O ₃ modified ceria/alumina supports and their corresponding gold catalyst were prepared and studied in the CO oxidation reaction. ZnO-doped solids show a superior catalytic activity compared to the bare CeO ₂-Al ₂O ₃, which is attributed to the intimate contact of the ZnO and CeO ₂ phases, since an exchange of the lattice oxygen occurs at the interface. In a similar way, Fe ₂O ₃-modified supports increase the ability of the CeO ₂-Al ₂O ₃ solids to eliminate CO caused by both the existence of Ce-Fe contact surface and the Fe ₂O ₃ intrinsic activity. All of the gold catalysts were very efficient in oxidising CO irrespective of the doping metal oxide or loading, with the ZnO containing systems better than the others. The majority of the systems reached total CO conversion below room temperature with the ZnO and Fe ₂O ₃ monolayer loaded systems the most efficient within the series.

Abstract

A FTIR study of the CO adsorption on a Zn-modified ceria is presented. The results indicate that at lower activation temperatures only Ce ⁴⁺ carbonyls were detected, which were reduced with the increase of the activation temperature. At higher activation temperatures, up to three Zn ²⁺ carbonyls were also identified according to the arrangement of the Zn ²⁺ cations. The consecutive CO adsorptions demonstrated an irreversible modification of the surface, resulting in an agglomeration of the zinc cations. A stepwise conversion of the isolated Zn ²⁺ carbonyls into carbonyls of the closely situated zinc cations followed by formation of bigger zinc oxide clusters was observed. The carbon monoxide coordinated on the isolated Zn ²⁺ cations at the interface with ceria reacts with the lattice oxygen leading to formation of oxygen vacancies. An insight into the origin of the activation during the CO oxidation process is proposed.

Abstract

Herein we demonstrate improved mass transport through nano-particle one-dimensional photonic crystals of enhanced porosity. Analysis is made by impedance spectroscopy using iodine and ionic liquid based electrolytes and shows that newly created large pores and increased porosity improve the diffusion of species through the photonic crystal. This achievement is based on the use of a polymeric porogen (polyethylene glycol), which is mixed with the precursor suspensions used for the deposition of nanoparticle TiO2 and SiO2 layers and then eliminated to generate a more open interconnected void network, as confirmed by specular reflectance porosimetry. A compromise between pore size and optical quality of these periodic structures is found.

Abstract

Catalysts of 1 wt% copper oxide supported on cerium oxide or cerium–terbium mixed oxides are comparatively examined with respect to their redox and catalytic properties for CO oxidation. Characterization of the catalysts had shown that they contain highly dispersed CuO-type entities on the corresponding nanostructured fluorite supports with copper dispersion increasing with increasing amounts of terbium in the support. In contrast, the CO oxidation catalytic activity decreases with increasing amounts of terbium in the support. On the basis of operando-DRIFTS experiments, one of the factors that could explain such behaviour is related to the greater difficulty in generating reduced copper sites active for the reaction in the presence of terbium, which in turn is evidenced to constitute an induction stage. Analysis of the redox properties is complemented by XPS which confirms the greater resistance to copper reduction in the presence of terbium.

Abstract

Engineered nanoparticles (ENPs) are increasingly being incorporated into commercial products. A better understanding is required of their environmental impacts in aquatic ecosystems.

This review deals with the ecotoxicity effects of silver and gold ENPs (AgNPs and AuNPs) in aquatic organisms, and considers the means by which these ENPs enter aquatic environments, their aggregation status and their toxicity. Since ENPs are transported horizontally and vertically in the water column, we discuss certain factors (e.g., salinity and the presence of natural organic materials), as they cause variations in the degree of aggregation, size range and ENP toxicity. We pay special attention to oxidative stress induced in organisms by ENPs.

We describe some of the main analytical methods used to determine reactive oxygen species, antioxidant enzyme activity, DNA damage, protein modifications, lipid peroxidation and relevant metabolic activities. We offer an overview of the mechanisms of action of AgNPs and AuNPs and the ways that relevant environmental factors can affect their speciation, agglomeration or aggregation, and ultimately their bio-availability to aquatic organisms.

Finally, we discuss similarities and differences in the adverse effects of ENPs in freshwater and salt-water systems.

Abstract

This work reports the synthesis at room temperature of transparent and colored W_xSi_yO_z thin films by magnetron sputtering (MS) from a single cathode. The films were characterized by a large set of techniques including X-ray photoelectron spectroscopy (XPS), Rutherford backscattering spectrometry (RBS), Fourier transform infrared (FT-IR), and Raman spectroscopies. Their optical properties were determined by the analysis of the transmission and reflection spectra. It was found that both the relative amount of tungsten in the W–Si MS target and the ratio O₂/Ar in the plasma gas were critical parameters to control the blue coloration of the films. The long-term stability of the color, attributed to the formation of a high concentration of W⁵⁺ and W⁴⁺ species, has been related with the formation of W–O–Si bond linkages in an amorphous network. At normal geometry (i.e., substrate surface parallel to the target) the films were rather compact, whereas they were very porous and had less tungsten content when deposited in a glancing angle configuration. In this case, they presented outstanding electrochromic properties characterized by a fast response, a high coloration, a complete reversibility after more than one thousand cycles and a relatively very low refractive index in the bleached state.

Abstract

Amorphous silicon oxynitride coatings with similar composition and different closed porosity were prepared by magnetron sputtering. Pores size, shape and distribution were evaluated by scanning electron microscopy and transmission electron microscopy. Raman and EELS analysis proved that the pores are filled with molecular nitrogen trapped during deposition. The mechanical properties evaluated by nanoindentation shows that the presence of closed nano-porosity does not compromise the mechanical integrity of these coatings. The introduction of closed porosity is shown as a good strategy for obtaining lower dielectric constant silicon oxynitride coatings with similar composition while keeping the good mechanical properties (∼13 GPa) characteristic of this type of coatings. The presence of close porosity gives also a good stability of coatings properties as compared to open porosity microstructures where gas phase in contact with the coatings can affect coatings properties.

Abstract

Four different mono and bimetallic Ni–Co/ZrO₂ catalysts have been studied by means of in situ XAS, X-ray diffraction, TPR, and measurements of the catalytic activity in the dry reforming reaction of methane (DRM). Even though the cobalt monometallic system has no activity for the methane reforming reaction, both bimetallic catalysts (with 1:1 and 1:2 Ni/Co ratio, respectively), showed a better activity and stability than the nickel monometallic system. The XRD data indicate that a mixed cobalt–nickel spinel is formed by calcination of the precursor solids, leading to the formation of an alloy of both metals after reduction in hydrogen. In situ XAS experiments showed a much better resistance of metals in the bimetallic systems to be oxidized under reaction conditions at temperatures until 750 °C. After these results, we proposed the formation in the bimetallic systems of a more reducible nickel–cobalt alloy phase, which remains completely metallic in contact with the CO₂/CH₄ reaction mixture at any temperature. The presence of adjacent nickel and cobalt sites seems to avoid the deactivation of cobalt in the DRM reaction. In the case of cobalt sites, the presence of adjacent nickel atoms seems to prevent the deposition of carbon over the cobalt sites, now showing its higher activity in the dry reforming reaction. Simultaneously, this higher activity of the cobalt sites in the bimetallic system produces more hydrogen as a product, maintaining the nickel atoms completely reduced under reaction conditions. This synergic effect accounts for the better performance of the bimetallic systems and points at both, the oxidation state of nickel particles under reaction conditions and the carbon deposition processes, as important factors responsible for differences in catalytic activities and stabilities in this hydrocarbon reaction.

Abstract

In the present manuscript the authors have systematically investigated the composition and microstructure of a series of ternary nitrides (TixV1-xNy) (0.0 <= x <= 1.0) prepared by mechanosynthesis, using XRD, SEM, EELS, XAS and TGA. The ternary titanium-vanadium nitride (TixV1-xNy) has been obtained in all range of compositions by the mechanical treatment of the two metals under nitrogen pressure in a planetary mill with a maximum milling time of 3 h and without any post-heating treatment. The materials' microhardnesses were measured after sinterisation and compared to those reported in the literature for these types of materials. When compared with the previously reported data for bulk samples, these values are similar or higher for compositions within the range x = 0.5 to x = 0.77 (TixV1-xN).

Abstract

Aluminum incorporation into the high pressure polymorph of TiO2 with the structure of alpha-PbO2 has been studied from 10 to 20 GPa and 1300 degrees C by XRD, high-resolution Al-27 MAS-NMR and TEM. Al-doped alpha-PbO2 type TiO2 can be recovered at atmospheric pressure. Al2O3 solubility in alpha-PbO2 type TiO2 increases with increasing the synthesis pressure. The alpha-PbO2 type TiO2 polymorph is able to incorporate up to 35 wt.% Al2O3 at 13.6 GPa and 1300 degrees C, being the substitution of Ti4+ by Al3+ on normal octahedral sites and the formation of oxygen vacancies the mechanism of solubility. The transition to the higher pressure TiO2 polymorph with the ZrO2 baddeleyite structure, akaogiite, has not been observed in the quenched samples at room pressure. The microstructure of the recovered sample synthesized at 16 GPa and 1300 degrees C points to the existence of an intermediate non-quenchable aluminum titanium oxide phase at these conditions.

Abstract

We demonstrate that the microstructure and electrical properties of Ge₂Sb₂Te₅ films can be changed by a nanoscale mechanical process. Nanoscratching is used to define modified areas onto an as-deposited crystalline Ge₂Sb₂Te₅ film. Scanning tunneling microscopy measurements show that the modified areas have a very low electrical conductivity. Micro-Raman measurements indicate that the mechanically induced microstructural changes are consistent with a phase transformation from crystalline to amorphous, which can be reversed by laser irradiation.

Abstract

Reactive magnetron sputtering was used to deposit titanium oxycarbide thin films. The overall set of results showed that the oxygen flow rate, and thus the composition of the atmosphere in the deposition chamber, controls the composition of the titanium oxycarbide thin films obtained by reactive sputtering. Rutherford Backscattering Spectroscopy analysis revealed the existence of three major types of films, indexed to their particular composition ratios. A detailed study by X-ray photoelectron spectroscopy was carried out in order to characterize the evolution of the TiC, CO and CC bonds induced by the increase of the oxygen partial pressure, which was found to be closely related with the different zones of composition that were suggested. Micro-Raman spectroscopy and X-ray diffraction measurements allowed describing the complex nature of the film structure, namely in what concerns different phases and their evolution, texture phenomena and grain size evolution as a function of the particular composition and film types (different zones). Electrical conductivity revealed a transition from a metallic to a semi-conducting behavior as a function of the oxygen concentration in the films, in good agreement with the different zones that were suggested. Similarly, optical properties supported this gradual change and for oxygen contents higher than 67 at.%, the films exhibited typical reflectance of insulator materials (interferences) in the UV, visible and near IR regions.

Abstract

The restoration of the Cavaille-Coll Romantic organ housed in La Merced Church of Burgos, Spain is described in this paper. The organ was affected by a fire that took place in the church. The effect of the fire on the pipes differed depending on their location within the instrument. A combination of analytical techniques (X-ray diffraction, scanning electron microscopy with energy-dispersive X-ray analysis, particle-induced X-ray emission, metallography, and specific density) allowed the accurate determination of the microstructures and compositions of the alloys used to make the different pipes of the organ, some of which had a high tin content and others which had a high lead content. The most damaged pipes were replaced by reconstructed pipes made out of metallic sheets of the same composition as the originals, to ensure a historically accurate sound.

Abstract

The chemical composition and microstructure of rutile grains in a ultra-high pressure metamorphic gneiss of the Saxonian Erzgebirge, Germany have been studied by Raman spectroscopy, SEM, EMPA and TEM. Rutile inclusions in garnet contain free dislocations, iron-enriched dislocations and exsolved ilmenite lamellae, while subgrain boundaries are observed in rutile grains of the rock matrix. The previously reported alpha-PbO2 type TiO2 phase could not be confirmed by our TEM observations. On the basis of Zr solubility in the rutile and the presence of microdiamonds, minimum metamorphic peak conditions of 3.95 GPa and 915 degrees C are estimated.

Abstract

The thermal aging of both a quenched and a cold rolled homogeneous supersaturated Cu-9 % wt Ni-5.5 wt % Sn alloy has been studied from differential scanning calorimetry (DSC) and microhardness measurements. An increase of the hardness during the aging of the quenched sample, because of the precipitation of a γ' phase, takes place. On the contrary, no hardness increase was observed during the aging of the cold rolled sample. A theoretical analysis of the enthalpy determined from the first DSC exothermic peak suggests that a segregation of the solute towards the dislocations occurs during the aging of the cold rolled alloy. The values of the n Avrami-Erofeev coefficients estimated from the kinetic analysis supports the above interpretations.

Abstract

Osmotic shock in a vesicle or cell is the stress build-up and subsequent rupture of the phospholipid membrane that occurs when a relatively high concentration of salt is unable to cross the membrane and instead an inflow of water alleviates the salt concentration gradient. This is a well-known failure mechanism for cells and vesicles (for example, hypotonic shock) and metal alloys (for example, hydrogen embrittlement). We propose the concept of collective osmotic shock, whereby a coordinated explosive fracture resulting from multiplexing the singular effects of osmotic shock at discrete sites within an ordered material results in regular bicontinuous structures. The concept is demonstrated here using self-assembled block copolymer micelles, yet it is applicable to organized heterogeneous materials where a minority component can be selectively degraded and solvated whilst ensconced in a matrix capable of plastic deformation. We discuss the application of these self-supported, perforated multilayer materials in photonics, nanofiltration and optoelectronics.

Abstract

Ethanol oxidative dehydrogenation over Pt/TiO2 photocatalyst, in the presence and absence of blue phosphors, was performed. The catalyst was prepared by photodeposition of Pt on sulphated TiO2. This material was tested in a gas-solid photocatalytic fluidized bed reactor at high illumination efficiency. The effect of the addition of blue phosphors into the fluidized bed has been evaluated. The synthesized catalysts were extensively characterized by different techniques. Pt/TiO2 with a loading of 0.5 wt% of Pt appeared to be an active photocatalyst in the selective partial oxidation of ethanol to acetaldehyde improving its activity and selectivity compared to pure TiO2. In the same way, a notable enhancement of ethanol conversion in the presence of the blue phosphors has been obtained. The blue phosphors produced an increase in the level of ethanol conversion over the Pt/TiO2 catalyst, keeping at the same time the high selectivity to acetaldehyde.

Abstract

In spite of the large amount of work performed by many investigators during last decade, the actual understanding of the kinetics of thermal degradation of cellulose is still largely unexplained. In this paper, recent findings suggesting a nucleation and growth of nuclei mechanism as the main step of cellulose degradation have been reassessed and a more appropriate model involving chain scission and volatilization of fragments has been proposed instead. The kinetics of cellulose pyrolysis have been revisited by making use of a novel kinetic method that, without any previous assumptions regarding the kinetic model, allows performing the kinetic analysis of a set of experimental curves recorded under different heating schedules. The kinetic parameters and kinetic model obtained allows for the reconstruction of the whole set of experimental TG curves.