Scientific Papers in SCI


Title: Highly Efficient Perovskite Solar Cells with Tunable Structural Color
Author(s): W. Zhang, M. Anaya, G. Lozano, M.E. Calvo, M.B. Johnston, H. Míguez, H.J. Snaith
Source: Nano Letters, (2015)

abstract | fulltext

The performance of perovskite solar cells has been progressing over the past few years and efficiency is likely to continue to increase. However, a negative aspect for the integration of perovskite solar cells in the built environment is that the color gamut available in these materials is very limited and does not cover the green-to-blue region of the visible spectrum, which has been a big selling point for organic photovoltaics. Here, we integrate a porous photonic crystal (PC) scaffold within the photoactive layer of an opaque perovskite solar cell following a bottom-up approach employing inexpensive and scalable liquid processing techniques. The photovoltaic devices presented herein show high efficiency with tunable color across the visible spectrum. This now imbues the perovskite solar cells with highly desirable properties for cladding in the built environment and encourages design of sustainable colorful buildings and iridescent electric vehicles as future power generation sources.

February, 2015 | DOI: 10.1021/nl504349z

Title: Optical Description of Mesostructured Organic-Inorganic Halide Perovskite Solar Cells
Author(s): Anaya, M; Lozano, G; Calvo, ME; Zhang, W; Johnston, MB; Snaith, HJ; Miguez, H
Source: Journal of Physical Chemistry Letters, 6 (2015) 48-53

abstract | fulltext

Herein we describe both theoretically and experimentally the optical response of solution-processed organic–inorganic halide perovskite solar cells based on mesostructured scaffolds. We develop a rigorous theoretical model using a method based on the propagation of waves in layered media, which allows visualizing the way in which light is spatially distributed across the device and serves to quantify the fraction of light absorbed by each medium comprising the cell. The discrimination between productive and parasitic absorption yields an accurate determination of the internal quantum efficiency. State-of-the-art devices integrating mesoporous scaffolds infiltrated with perovskite are manufactured and characterized to support the calculations. This combined experimental and theoretical analysis provides a rational understanding of the optical behavior of perovskite cells and can be beneficial for the judicious design of devices with improved performance. Notably, our model justifies the presence of a solid perovskite capping layer in all of the highest efficiency perovskite solar cells based on thinner mesoporous scaffolds.

January, 2015 | DOI: 10.1021/jz502351s

Title: Synthesis and application of layered titanates in the photocatalytic degradation of phenol
Author(s): Ivanova, S; Penkova, A; Hidalgo, MD; Navio, JA; Romero-Sarria, F; Centeno, MA; Odriozola, JA
Source: Applied Catalysis B: Environmental, 163 (2015) 23-29

abstract | fulltext

This study proposes a direct synthetic route to single titanate sheets through the mild and versatile conditions of the “chimie douce”. The stages of the production include the complexation of the titanium alkoxide precursor by benzoic acid, the formation of titanium oxo-clusters and their controlled transformation into single sheet titanates during the hydrolysis stage. The resulted material appears to be an excellent precursor for self-organized TiO2 nanotubes formation which presents an excellent activity as photocatalyst in the photo-degradation of phenol.

February, 2015 | DOI: 10.1016/j.apcatb.2014.07.048

Title: Evolution of H-2 photoproduction with Cu content on CuOx-TiO2 composite catalysts prepared by a microemulsion method
Author(s): Kubacka, A; Munoz-Batista, MJ; Fernandez-Garcia, M; Obregon, S; Colon, G
Source: Applied Catalysis B: Environmental, 163 (2015) 214-222

abstract | fulltext

Copper oxides in contact with anatase correspond to promising materials with high activity in the photo-production of hydrogen by aqueous reforming of alcohols. By a single pot microemulsion method we obtained a series of Cu-Ti composite systems with controlled copper content in the 0-25 wt.% range. The scanning of such a wide range of composition led to the discovery of two well differentiated maxima in the photo-reaction performance. These maxima present rather high and relatively similar reaction rates and photonic efficiencies but are ascribed to the presence of different copper species. A multi-technique analysis of the materials indicates that the maxima obtained comes from optimizing different steps of the reaction; while the first would be connected with a positive effect on anatase charge handling performance the second seems exclusively related to electron capture by surface copper species.

February, 2015 | DOI: 10.1016/j.apcatb.2014.08.005

Title: Ca-looping for postcombustion CO2 capture: A comparative analysis on the performances of dolomite and limestone
Author(s): Valverde, JM; Sanchez-Jimenez, PE; Perez-Maqueda, LA
Source: Applied Energy, 138 (2015) 202-215

abstract | fulltext

The low cost and wide availability of natural limestone (CaCO3) is at the basis of the industrial competitiveness of the Ca-looping (CaL) technology for postcombustion CO2 capture as already demonstrated by similar to 1 Mw(t) scale pilot projects. A major focus of studies oriented towards further improving the efficiency of the CaL technology is how to prevent the gradual loss of capture capacity of limestone derived CaO as the number of carbonation/calcination cycles is increased. Natural dolomite (MgCa(CO3)(2)) has been proposed as an alternative sorbent precursor to limestone. Yet, carbonation of MgO is not thermodynamically favorable at CaL conditions, which may hinder the capture performance of dolomite. In the work described in this paper we carried out a thermogravimetric analysis on the multicyclic capture performance of natural dolomite under realistic regeneration conditions necessarily implying high calcination temperature, high CO2 concentration and fast transitions between the carbonation and calcination stages. Our study demonstrates that the sorbent derived from dolomite has a greater capture capacity as compared to limestone. SEM analysis shows that MgO grains in the decomposed dolomite are resistant to sintering under severe calcination conditions and segregate from CaO acting as a thermally stable support which mitigates the multicyclic loss of CaO conversion. Moreover, full decomposition of dolomite is achieved at significantly lower calcination temperatures as compared to limestone, which would help improving further the industrial competitiveness of the technology. 

January, 2015 | DOI:

Title: Role of ruthenium on the catalytic properties of CeZr and CeZrCo mixed oxides for glycerol steam reforming reaction toward H2 production
Author(s): Martinez, LM; Araque, M; Centeno, MA; Roger, AC
Source: Catalysis Today, 242 (2015) 80-90

abstract | fulltext

The effect of ruthenium on the physico-chemical properties of CeZr and CeZrCo mixed oxides for H2production by glycerol steam reforming reaction has been studied. The combination of in situ Raman spectroscopy under both reductive and oxidative conditions, H2/O2 pulses and XRD, Raman, BET analysis, H2-TPR and TPD-TPO analyses contributed to the determination of the structural and textural properties, redox behavior, re-oxidation capacity and resistance to carbon deposition of the synthesized catalysts. The results show that the catalytic activity is improved by the (positive) cooperative and complementary effect between cobalt and ruthenium that favors the selectivity toward the steam reforming, selective to H2, with respect to the unselective thermal decomposition of glycerol. Ruthenium stabilizes the cobalt cations inserted in the fluorite structure preventing its rejection as Co3O4; and provides the necessary hydrogen to reduce Ce4+. The combination cobalt–ruthenium modifies positively the redoxproperties of the catalysts, increases the re-oxidation capacity (OSC) and promotes the gasification of the carbon deposits. Under the reaction conditions, the decrease in glycerol conversion came along with a change of selectivity. The formation of H2 and CO2 were strongly decreased, while the formation of CO, C2H4 and condensable products (mainly hydroxyacetone) increase. The differences in the catalytic stability and activity of the catalysts are related to the capability of the catalysts to activate H2O under the reaction conditions, favoring the steam reforming reaction over the thermal decomposition.

March, 2015 | DOI: 10.1016/j.cattod.2014.07.034

Title: Photocatalytic activity of bismuth vanadates under UV-A and visible light irradiation: Inactivation of Escherichia coli vs oxidation of methanol
Author(s): Adan, C; Marugan, J; Obregon, S; Colon, G
Source: Catalysis Today, 240 (2015) 93-99

abstract | fulltext

Four bismuth vanadates have been synthesized by using two different precipitating agents (NH3 and triethylamine) following a hydrothermal treatment at 100 °C for 2 h and at 140 °C for 20 h. Then, solids were characterized by X-ray diffraction, BET surface area, UV–vis spectroscopy and scanning microscopy techniques. The characterization of the synthesized materials showed a well crystallized scheelite monoclinic structure with different morphologies. All materials display optimum light absorption properties for visible light photocatalytic applications. The photocatalytic activity of the catalysts was investigated for the inactivation of Escherichia coli bacteria and the oxidation of methanol under UV–vis and visible light irradiation sources. Main results demonstrate that BiVO4 are photocatalytically active in the oxidation of methanol and are able to inactivate bacteria below the detection level. The activity of the catalyst decreases when using visible light, especially for methanol oxidation, pointing out differences in the reaction mechanism. In contrast with bacteria, whose interaction with the catalyst is limited to the external surface, methanol molecules can access the whole material surface.

February, 2015 | DOI: 10.1016/j.cattod.2014.03.059

Title: Synthesis temperature effect on Na-Mica-4 crystallinity and heteroatom distribution
Author(s): Naranjo, M; Castro, MA; Cota, A; Osuna, FJ; Pavon, E; Alba, MD
Source: Microporous and Mesoporous Materials, 204 (2015) 282-288

abstract | fulltext

The discovery of swelling brittle mica, Na-Mica-4, has been one of the most significant advances in the pursuit for a material with high ion-exchange capacity. For technical applications, the control of the phase evolution during the synthesis is crucial. The main aim of this study was to investigate the effect of Na-Mica-4 synthesis temperature on the crystalline phase evolution, Si–Al distribution in the tetrahedral sheet, the Al occupancy between tetrahedral and octahedral sites and their effects on the interlayer space composition. The synthesis temperature range between 600 °C and 900 °C was explored. At low temperature (600 °C), the precursors were transformed in a low-charged swelling 2:1 phyllosilicate, saponite type, which was progressively aluminum enriched with temperature. The high-charged swelling mica was completely formed at 700 °C, although a minor anhydrous contribution remained up to 850 °C. Up to 800 °C, silicates and fluorides secondary phases were detected as a minor contribution.

March, 2015 | DOI: 10.1016/j.micromeso.2014.11.026

Title: Biotribological behavior of Ag–ZrCxN1−x coatings against UHMWPE for joint prostheses devices
Author(s): Calderon, SV; Sanchez-Lopez, JC; Cavaleiro, A; Carvalho, S
Source: Journal of the Mechanical Behavior of Biomedical Materials, 41 (2015) 83-91

abstract | fulltext

This study aims to evaluate the structural, mechanical and tribological properties of zirconium carbonitrides (ZrCxN1−x) coatings with embedded silver nanoparticles, produced with the intention of achieving a material with enhanced multi-functional properties, including mechanical strength, corrosion resistance, tribological performance and antibacterial behavior suitable for their use in joint prostheses. The coatings were deposited by direct current (DC) reactive magnetron sputtering onto 316 L stainless steel, changing the silver content from 0 to 20 at% by modifying the current density applied to the targets. Different nitrogen and acetylene gas fluxes were used as reactive gases. The coatings revealed different mixtures of crystalline ZrCxN1−x, silver nanoparticles and amorphous carbon phases. The hardness of the films was found to be mainly controlled by the ratio between the hard (ZrCxN1−x) and soft (Ag and amorphous carbon) phases in the films, fluctuating between 7.4 and 20.4 GPa. The coefficient of friction, measured against ultra-high molecular weight polyethylene (UHMWPE) in Hank’s balanced salt solution with 10 g L−1albumin, is governed by the surface roughness and hardness. The UHMWPE wear rates were in the same order of magnitude (between 1.4 and 2.0×10−6 mm3 N−1 m−1), justified by the effect of the protective layer of albumin formed during the tests. The small differences were due to the hydrophobic/hydrophilic character of the surface, as well as to the silver content.

January, 2015 | DOI: 10.1016/j.jmbbm.2014.09.028

Title: Template-free synthesis and luminescent properties of hollow Ln:YOF (Ln = Eu or Er plus Yb) microspheres
Author(s): Martinez-Castro, E; Garcia-Sevillano, J; Cusso, F; Ocana, M
Source: Journal of Alloys and Compounds, 619 (2015) 44-51

abstract | fulltext

A method for the synthesis of hollow lanthanide doped yttrium oxyfluoride (YOF) spheres in the micrometer size range with cubic structure based on the pyrolysis at 600 degrees C of liquid aerosols generated from aqueous solutions containing the corresponding rare earth chlorides and trifluoroacetic acid has been developed. This procedure, which has been used for the first time for the synthesis of YFO based materials, is simpler and advantageous when compared with other methods usually employed for the production of hollow spheres since it does not require the use of sacrificial templates. In addition, it is continuous, which is desirable because of practical reasons. The procedure is also suitable for doping the YOF spheres with europium cations resulting in down converting red phosphors when activated with UV light, or for co-doping with both Er3+ and Yb3+ giving rise to up-converting phosphors, which emit intense red light under near infrared (NIR) irradiation. Because of their optical properties and hollow architecture, the developed materials may find applications in optoelectronic devices and biotechnology. 

January, 2015 | DOI: 10.1016/j.jallcom.2014.09.023

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