Artículos SCI

Abstract

In this article we investigate the structure–activity link of anatase-type Ti–M (M = V, Mo, Nb, and W) mixed oxides used for toluene photo-oxidation under sunlight-type excitation. An analysis of the local and long-range structural and electronic characteristics of the mixed oxides show that only structurally highly homogeneous anatase-type oxides with electronic properties exclusively leading to a band gap decrease drive to efficient visible light-driven photocatalysts. Within our microemulsion preparation method, this only occurs for Ti–V and Ti–W series of samples. The isoelectronic (V4+) substitution of Ti4+ ions at the anatase lattice is characterized by a low solubility limit (ca. 2.5 at. %), and drives to a limited modification of the band gap and to a moderate enhancement of the photo-activity with respect to bare titania reference systems. W presence at anatase cation positions occurs with concomitant presence of cation vacancies derived by the charge imbalance between the W6+ and Ti4+ species. A unique W-vacancy local arrangement is detected by the structural characterization, leading to both an important band gap decrease and enhancement of the photo-activity upon sunlight excitation.

Abstract

TiO2 thin films have been prepared by physical vapour deposition (PVD) and plasma enhanced chemical vapour deposition (PECVD) to study the UV-induced photo-activity of this material. Wetting angle variations and photo-catalytic activity for the degradation of dyes upon UV illumination have been compared for thin films with different crystalline structure (amorphous, rutile and anatase), microstructure (columnar, compact, etc.) and porosities as estimated from the values of their refraction indices and their direct assessment with a quartz crystal monitor. The surface of the thin films became superhydrophilic upon UV light irradiation and then it recovered its original state by keeping the samples in the dark. Wetting angle decays follow very similar kinetics for amorphous and crystalline films, independently of their actual porosities. By contrast the photo-catalytic activity was very dependent on the crystalline structure of the films (anatase > rutile > amorphous) and on their porosities. The different behaviour depicted by the films with regard to these two properties suggests that they respond to different though related mechanisms and that they cannot be considered as equivalent when trying to prove the photo-activity of TiO2.

Abstract

Nanocomposite films composed by small crystallites of hard phases embedded in an amorphous lubricant matrix have been extensively studied as protective coatings. These kinds of coatings have often to work in extreme environments, exposed to high temperatures (above 800–900 °C), and/or oxidizing/corrosive atmospheres, which may resist. As a result, it is important to study the behavior of such coatings at high temperatures (thermal stability) and in the presence of oxygen (oxidation resistance). In this sense, we have selected a TiC/a-C nanocomposite coating with good mechanical and tribological properties in order to do several thermal tests under three different environments: high vacuum (10−6 mbar), low vacuum (10−1 mbar), and air. Our observations allow us to establish that the film microstructure is stable at least up to 1 000 °C in high vacuum. When oxygen is present, the practical temperature of use is reduced at 700 °C (low partial pressure) and 300 °C (air) by formation of Ti oxides and C removal.

Abstract

We have carried out a study of adsorption, on the (1 0 1) surface of anatase TiO2, of two industrially relevant rhodamine molecules [rhodamine 6G (R6G) and rhodamine 800 (R800)] employing molecular dynamics. These theoretical studies have shown that R6G must adsorb on surfaces under basic conditions. Moreover, the adsorption of this molecule shows a strong dependence upon the pH of the system, i.e. under basic conditions the adsorption energy is quite high, under neutral conditions the adsorption energy is lower and under acidic conditions an even lower adsorption energy indicates that there must be very little adsorption under such conditions. By contrast, for R800, there is little dependence of the adsorption energy upon the pH, suggesting that the amount of adsorption of these molecules is little affected by this parameter. These theoretical results are in qualitative agreement with the experimental results consisting of the incorporation of these dye molecules into porous thin films.

Abstract

Sonication in a 1:1 mixture (volume ratio) of water and concentrated H2O2 (30%) is a soft method for particle size reduction of phyllosilicate minerals like vermiculites. Repeated sonication causes a particle size reduction to about 70 nm for the Santa Olalla and to 45 nm for the Ojén-vermiculite. In this context the question arises whether the strong oxidising effect of the hydrogen peroxide affects the oxidation state of the iron in the vermiculites. Therefore, the Fe3+/Fetotal ratio before and after sonication was determined by means of Mössbauer spectroscopy. Whereas this ratio was found to remain almost constant in the Santa Olalla vermiculite, it increased from 0.79 to 0.85 in case of the Ojén sample. In the latter case, the oxidation is accompanied by a decrease of the layer charge.

Surprisingly, sonication in pure water leads to a decrease of the Fe3+/Fetotal ratio in the case of the Ojén-vermiculite, i.e., to an increase of the Fe2+ fraction to roughly twice the value before sonication. Again the Fe3+/Fetotal ratio of the Santa Olalla vermiculite remains unchanged. The surface area SBET of the reduced Ojén-vermiculite amounts to 50 m2/g, which is close to the value obtained in the presence of hydrogen peroxide. The results presented should be taken as a warning that particle size reduction by sonication may be accompanied by a change of the redox state and the layer charge of the material.

Abstract

Two-component Co–Ce samples deposited onto SiO2 have been prepared, characterized and tested in the reaction of complete n-hexane and CO oxidation. It was established that cerium enhanced the catalytic activity of cobalt in the reaction of n-hexane oxidation, although this depended on the sequence of cobalt and cerium introduction. Co-impregnation of Co and Ce resulted in a close interaction between Co3O4 and CeO2 leading to more surface oxygen species available and, therefore, a better reactivity.

Abstract

This work reports the synthesis, characterization and catalytic activity for CO oxidation of gold catalysts supported on calcium hydroxyapatite. On both, the hydroxyapatite support and the gold-supported hydroxyapatite catalyst, the CO conversion shows a peak near 100% of conversion at room temperature. The generation of structural vacancies by interaction of CO with the solid provokes the formation of peroxide species in the presence of gaseous oxygen, which seems to be responsible of this high conversion of CO at room temperature. Moreover, the influence of the pre-treatment temperature on the activity has been observed and related with the elimination of carbonate species and the generation of structural defects in the apatite structure, which are able to modify the gold oxidation state.

Abstract

A series of Ce1−xEuxO2−x/2 mixed oxides was synthesized by coprecipitation. The solids were characterized by means of XRF, SBET, XRD, UV−vis, and Raman techniques, and their catalytic activities toward CO oxidation were tested. A solid solution, with CeO2 F-type structure, is formed for europium contents (measured as Eu2O3 by XRF) ≤20% wt. For higher contents, the solid solution is not formed, but a physical mixture is detected. The existence of oxygen vacancies in the solids with Eu2O3 contents between 3 and 17% wt was demonstrated by the presence of bands at 532 and 1275 cm−1 in their Raman spectra. The catalytic performances of the solids correlate with the amount of these punctual defects in the solid solution.

Abstract

Characterization of four amalgam surfaces, with different alteration degrees from Andalusia historical mirrors, has been carried out by grazing-incidence X-ray diffraction (GIXRD), and other spectroscopic techniques (SEM/EDX, XPS, and REELS). The combination of all these techniques allows determining the corrosion state of the amalgams. The results show that the amalgams are composed in all cases of a binary alloy of tin and mercury. As mercury has high vapour pressure at RT, it slowly segregates and eventually evaporates, it leaves finely divided particles of tin that easily can be oxidize, forming tin monoxide (SnO) and tin dioxide (SnO2). In one of the samples, most of the amalgam remains unoxidized, since Hg0.1Sn0.9 and metallic Sn phases are the major components; in two other samples, Hg0.1Sn0.9 and Sn phases are not detected while SnO2 and SnO phases appear. Finally, in the last studied sample, only SnO2 phase is detected. The surface analyses of these samples by XPS show that, for most of them an unique chemical species (Sn4+) is found.

Abstract

This work investigates the possibility of using plasmas to treat high boiling point and viscous liquids (HBPVL) and cokes resulting as secondary streams from the refining of oil. For their revalorisation, the use of microwave (MW) induced plasmas of water is proposed, as an alternative to more conventional processes (i.e., catalysis, pyrolysis, combustion, etc.). As a main result, this type of energetic cold plasma facilitates the conversion at room temperature of the heavy aromatic oils and cokes into linear hydrocarbons and synthesis gas, commonly defined as syngas (CO + H2 gas mixture). The exposure of the coke to this plasma also facilitates the removal of the sulfur present in the samples and leads to the formation on their surface of a sort of carbon fibers and rods network and new porous structures. Besides, optical emission measurements have provided direct evidence of the intermediates resulting from the fragmentation of the heavy oils and cokes during their exposure to the water plasma. Furthermore, the analysis of the mass spectra patterns suggests a major easiness to break the aromatic bonds mainly contained in the heavy oils. Therefore, an innovative method for the conversion of low value residues from oil-refining processes is addressed.

Abstract

Ta2O5 thin films with different nanostructure and surface roughness have been prepared by electron evaporation at different angles between the evaporation source and the substrates. Large variation of refraction indexes (n) from 1.40 to 1.80 were obtained by changing the geometry of evaporation and/or by annealing the evaporated films at increasing temperatures up to 1000 °C to make them crystalline. Very flat and compact thin films (n = 2.02) were also obtained by assisting the growth by bombardment with O2+ ions of 800 eV kinetic energy. A similar correlation has been found between the wetting contact angle of water and the roughness of the films for the evaporated and evaporated + annealed samples, irrespective of their procedure of preparation and other microstructural characteristics. When the films were illuminated with UV light of h > Eg = 4.2 eV (Eg, band gap energy of Ta2O5), their surface became superhydrophilic (contact angle < 10°) in a way quite similar to those reported for illuminated TiO2 thin films. The rate of transformation into the superhydrophilic state was smaller for the crystalline than for the amorphous films, suggesting that in Ta2O5 the size of crystal domains at the surface is an important parameter for the control of this kinetics. Changes in the water contact angle on films illuminated with visible light were also found when they were subjected to implantation with N2+ ions of 800 eV kinetic energy. The origin of this photoactivity is discussed in terms of the electronic band gap states associated with the nitrogen-implanted atoms. The possibility of preparing antireflective and self-cleaning coatings of Ta2O5 is discussed.

Abstract

Amorphous hydrogenated silicon carbide (a-SiC:H) films are produced by remote microwave hydrogen plasma (RHP)CVD using triethylsilane (TrES) as the single-source precursor. The reactivity of particular bonds of the precursor in the activation step is examined using tetraethylsilane as a model compound for the RHP-CVD experiments. The susceptibility of a TrES precursor towards film formation is characterized by determining the yield of RHP-CVD and comparing it with that of the trimethylsilane precursor. The effect of substrate temperature (Ts) on the rate of the RHP-CVD process, chemical composition, and chemical structure of the resulting a-SiC:H films is reported. The substrate temperature dependence of the film growth rate implies that film growth is independent of the temperature and RHP-CVD is a mass transport-limited process. The examination of the a-SiC:H films, performed by means of X-ray photoelectron spectroscopy (XPS), elastic recoil detection analysis (ERDA), and Fourier transform infrared absorption spectroscopy (FTIR), reveals that the increase in the substrate temperature from 30 °C to 400 °C causes the elimination of organic moieties from the film and the formation of a Si-carbidic network structure. On the basis of the results of the structural study, the chemistry involved in film formation is proposed.

Abstract

Despite its biological importance, the mechanism of construction of cutin, the polymer matrix of plant cuticles, has not yet been elucidated. Recently, progress on lipid barrier formation of polymers such as cutin and suberin has been recently reviewed by Pollard et al. In their review the authors state that the ubiquitous cutin is the least understood of the plant extracellular polymers and that major questions about cutin structure and its macromolecular assembly remain to be resolved. At the time this paper was being published our research group has developed a new hypothesis on plant cutin synthesis.

Abstract

Compounds containing rare earths are of increasing technological interest especially because of their unique mechanical, magnetic, electrical, and optical properties. Among them, rare earth oxyorthosilicates are attractive scintillators for γ- and X-ray spectroscopy and detection. However, there are many structural aspects of those compounds that are not clear. In this research, the structure parameters for Sc2Si2O5, X2-polymorph, have been refined from powder X-ray diffraction (XRD) data and the 29Si MAS NMR spectrum is reported for the first time. X2-Sc2SiO5 polymorph was synthesized by the sol–gel method and characterized by XRD and 29Si MAS NMR. The XRD pattern was indexed in a monoclinic unit cell with space group I2/c; the resulting unit cell parameters were a=9.9674(2) Å, b=6.4264(9) Å, c=12.0636(2) Å, and β=103.938(1)°. The 29Si MAS NMR spectrum showed a unique signal at −79.5 ppm, compatible with the unique Si crystallographic site in the unit cell. Finally, the band valence method has been applied to the calculation of a “shift parameter,” which is correlated with the NMR chemical shift.

Abstract

Mixing behaviour of solid crystalline monolayers adsorbed onto graphite from different mixtures of undecanoic and dodecanoic acids at submonolayer coverage has been investigated. X-ray diffraction measurements have been collected from a variety of compositions as a function of temperature. An extensive phase separation is found for all the compositions – the scattering patterns characteristic of the pure material crystalline structures being preserved across the entire composition range. The temperature dependence of the monolayer melting points and their depression is also clearly indicative of separation of the two surface components, in clear contrast to that expected if the two carboxylic acids mixed ideally in the monolayer.

Abstract

MCM-22 zeolites with Si/Al in the 9–46 range were synthesised in rotating autoclave and characterised by X-ray diffraction, 1H, 29Si and 27Al magic angle spinning nuclear magnetic resonance, scanning electron microscopy and nitrogen physisorption. For the Si/Al = 21, 30 and 46 samples both X-ray diffraction and scanning electron microscopy revealed the crystallisation of pure MCM-22. Besides the latter, crystals of ferrierite also formed during the synthesis of the Si/Al = 9 sample. Based on the 1H MAS NMR spectra of dehydrated samples, the different proton species present on the MCM-22 samples were determined and quantified. Information about the incorporation of Al ions into the zeolite framework, as well as on the preferential crystallographic sites occupied in dependence on the Si/Al ratio of the sample, was obtained by 27Al MAS NMR spectroscopy. From 29Si MAS NMR spectra, differences in the degree of crystallinity of the samples were assessed, the results being in agreement with the diffraction data. Nitrogen physisorption runs revealed the microporous nature of the adsorbents, with a supermicropore to ultramicropore volume ratio in good agreement, for the best crystallised samples, with the porous structure with supercages and sinusoidal channels of the ideal MCM-22 crystal.

Abstract

The liquid-phase adsorption of thiophene from thiophene/iso-octane solutions has been investigated in batch conditions at room temperature and atmospheric pressure on MCM-22 zeolites with Si/Al in the 9–46 range. Thiophene adsorption was found to occur in two steps whatever the Si/Al ratio of the adsorbent. The presence of ferrierite besides the MCM-22 phase caused a significant loss of the adsorption performance. For pure MCM-22 samples, the Si/Al ratio influenced the adsorption performance. Based on the acid properties of the samples, investigated by adsorption microcalorimetry of ammonia, the adsorption features were interpreted by assuming that positively charged species were originated during the first step; these species underwent successive reaction with weakly adsorbed species formed in the second step, leading to heavy molecular weight organosulphur compounds. Direct evidence for the occurrence of reactive adsorption of thiophene involving its transformation into heavy molecular weight organosulphur compounds was obtained by GC/MS investigation of the nature of the adsorbed material recovered after the adsorption experiments. The peculiar structure of MCM-22 zeolites made possible the formation of long-sized organosulphur compounds. Due to the mechanism by which thiophene is transformed (i.e. progressive addition of other thiophene molecules), the size of the resulting products was found to depend also on the concentration of the weakly adsorbed thiophene molecules able to interact with those already activated through protonation.

Abstract

A device for solar-energy conversion was introduced in which a porous and highly reflecting 1D photonic crystal (1D PC) was coupled to a dye-sensitized nanocrystals anatase (NC-TiO₂) electrode. The results show that the transparency of the PC-based dye-sensitized solar cells (DSSC) in the visible range of the electromagnetic spectrum is very similar to that of the reference cell. The multilayer whose photonic bandgap has a larger overlap with the absorption band of the ruthenium dye, gives rise to a larger enhancement of the photocurrent. It is also seen that the porous 0.5μm thick PC, whose deleterious effect is compensated by the large increment in photocurrent. The spectral photoelectric response of the cell clearly shows the effect that coupling to a PC has on the current photogenerated in the dye-sensitized electrode.

Abstract

The microstructure of TiO2 thin films prepared by plasma enhanced chemical vapour deposition has been assessed by using water adsorption–desorption isotherms measured by means of a quartz crystal monitor (QCM). Thin films have been deposited by using titanium tetraisopropoxide as a precursor and by changing different experimental parameters of the deposition procedure such as temperature of the substrate, pressure, and gas composition in the plasma. The films were characteristic of different microstructures that, according to their scanning electron micrographs, have been categorized as columnar, homogeneous and crystalline. They also have different refraction indices with values between 1.95 and 2.41. Water and toluene adsorption isotherms have been measured by means of a QCM monitor for the films heated in vacuum to remove the water previously adsorbed in their pores. The analysis of the adsorption-desorption isotherms by means of the so called “t-plots” and the determination of the pore size distribution curves rendered that the three kinds of microstructures presented different kinds of isotherms and water adsorption behaviours. Columnar films consisted of micro- and meso-pores had a very high adsorption hystheresis at low pressures. Homogeneous films only had micropores and presented no adsorption hystheresis. Crystalline films consisted of both micro- and meso-pores but had no adsorption hystheresis at low pressures. A zone scheme has been proposed to account for the microstructure of the films depending on the plasma conditions utilized. The implications of the different water adsorption behaviours of the films for the determination of their refraction indices are discussed.

Abstract

This work reports about the preparation of plasma polymerized thin films of perylene with thicknesses 30−150 nm and their characterization by different methods and the analysis of their optical properties. Highly absorbent and fluorescent films have been obtained by this method that combines the sublimation of the perylene molecules and their controlled polymerization by the interaction with remote Ar plasma. The polymeric films are very flat with a root mean square (rms) roughness in the range 0.3−0.4 nm. In contrast with the sublimated layers of perylene that present a high scattering of light, the polymerized films depict the well-defined absorption bands in the region 400−450 nm and fluorescence spectra of the perylene molecule at 475 nm. The films are formed by a matrix formed by cross-linked fragments of perylene and intact molecules that confer the observed optical properties to this material. The optical and microstructural characteristics of this type of thin films and the possibility to perform their deposition by using lithographic procedures make them suitable for their integration into photonic components for various applications. A preliminary study of the use of these films as an optical sensor of NO2 is also presented.

Abstract

Silica monospheres with a diameter of 330 nm modified with aminosilane compounds of three different basicities have been prepared. Surface coverage of the silica with an organic compound leads to an increase of the point of zero charge (PZC) of the silica surface from 2.1 to 5.1, 6.5 and 7.2 values, depending on the amine used. From these silicas, gold-containing catalysts have been prepared by a deposition–precipitation method at the same pH as the PZC of the support. The best results have been obtained using 3-(Diethoxymethylsilyl) propylamine as a modifying agent, which has allowed obtaining a good dispersion of the gold particles with an average size of 3.8 nm.

Abstract

ZnO/ZnS bilayer antireflection coatings have been prepared by spray pyrolysis using aqueous solutions of zinc acetate and thiourea or zinc chloride and thiourea. The structure, surface morphology, chemical composition and optical transmittance of the bilayer have been examined as a function of the composition of the initial solution. X-ray photoelectron spectroscopy analysis and Ar ion-beam sputter etching was carried out to obtain a depth profile of bilayer. Neither carbon nor other by-products, which could alter the optical transmittance of the bilayer were found in either the interface or bulk. The differences between the bilayers arise from the annealing of the ZnS underlayer, as well as the precursor used to prepare it.

Abstract

The characterization of iron in microsamples by conventional X-ray diffraction is difficult due to its low concentration in thin layers and its low reflecting power relative to other phases. Synchrotron radiation can provide unique information because of high intensity, sample penetration, small beam diameter and fast data collection. In this study, micro X-ray diffraction (mu-XRD) data were obtained of two samples taken from wall paintings at San Agustin's Church in Cordoba. Crystalline iron phases such as goethite, lepidocrocite and hematite in the cross-section of the painting thin layers were identified. with a good spatial resolution. Conventional XRD only detected hydrocerussite and cerussite rather than the full range of iron phases found in the mu-XRD experiments. 

Abstract

Photocatalytic oxidation of 2,4-dichlorophenoxyacetic acid was performed over ZrO2, Cu/ZrO2 and Fe/ZrO2 catalysts prepared by the sol-gel method. The samples were annealed at 400 degrees C. Textural and electronic characterization was carried out using BET and UV-Vis in order to establish the relationship between surface, pore volume and E. with the photoactivity of the materials. The degradation of the acid was followed by UV-Vis spectroscopy. The disappearance of the herbicide in solution follows approximately pseudo-first order kinetics. The apparent rate constants were calculated for the three catalysts. The results reveal that Fe/ZrO2 exhibits the best photoactivity for the degradation of 2,4-dichlorophenoxyacetic acid.

Abstract

Carbon nanotubes (CNTs) have been grown both on a sample of stainless steel (317-2R) and on the same steel coated with cobalt colloid nanoparticles. Both materials are suitable supports for the growth of vertically aligned carbon nanotubes, although a more sparse growth of significantly thicker carbon nanotubes is observed in the case of the bare steel. We find that carbon nanotubes grown directly on the stainless steel support show very poor tribological behaviour whereas the support using nanoparticles for carbon nanotube growth displayed interesting tribological properties with friction coefficients of approximately 0.1-0.2. The modified CNT material (studied by Raman spectroscopy) adheres to both mating surfaces avoiding direct contact between asperities and plough so the friction and wear processes decrease greatly.

Abstract

In the present work we synthesized inorganic oxide nanoparticle carriers of platinum compounds and tested their therapeutic effect on animal models in which C6 glioma cells have been inoculated. TiO2-containing Pt(NH3)(4)Cl-2 complexes were synthesized using sol-gel methods. The platinum species are chemically bonded to the TiO2 carrier, as shown by Fourier transform infrared spectroscopy of probe molecules. Treatment with TiO2-Pt nanoparticles reduces tumour growth rate by up to 56%, showing that a synergistic effect exists between the TiO2 carrier and the platinum drug.

Abstract

Gold nanoparticles with an average particle size below 3 nm have been synthesized and stabilized with different thiol-derivatized molecules for studying the influence of the capping molecule on the magnetic behavior. Thiolated-alkane chains with different lengths as well as a thiol-containing biomolecule (tiopronin) have been selected as protecting shells for the synthesized NPs. Magnetic characterization indicates that the appearance of a ferromagnetic-like behavior is related not only with the formation of Au-S bonds linking the protective molecules to the nanoparticle surface but also with the formation of the nanoparticle itself as well as with the geometry of the capping molecule. The later seems to determine whether the protective monolayer shell is ordered or not. The simultaneous presence of Au-Au and Au-S bonds together with a reduced particle diameter, and the formation of an ordered monolayer protective shell, have been proved to be key parameters for the ferromagnetic-like behavior exhibited by thiol-functionalized gold NPs.

Abstract

The adsorption and self-assembly of hydroxylated long chain fatty acids on mica has been studied by AFM. The presence of secondary midchain hydroxyl groups is found to strongly affect the self-assembly process via the reinforcement of lateral interactions between molecules by -OH center dot center dot center dot HO- hydrogen bonding. On the other side, the availability of an exposed terminal hydroxyl group in the acid molecule triggers the formation of a second layer due to the strength of tile -OH center dot center dot center dot HOOC- tail-to-head interaction. Surface coverage (theta) data follow a Langmuir-type relationship vs solution concentration (c). The analysis of the slope of 1/theta vs 1/c plots provides a relative estimation of the adsorption enthalpy for the formation of the first and second monolayers. Based on that, both the contribution of molecule-substrate and lateral molecule-molecule energy terms are found to be relevant in the self-assembly of this type of molecule oil mica.