Alvaro Mayoral

University of Zaragoza, Caesaraugusta, Aragon, Spain

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Publications (44)179.26 Total impact

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    ABSTRACT: Atomic carbon chains have raised interest for their possible applications as graphene interconnectors as the thinnest nanowires; however, they are hard to synthesize and subsequently to study. We present here a reproducible method to synthesize carbon chains in situ TEM. Moreover, we present a direct observation of the bond length alternation in a pure carbon chain by aberration corrected TEM. Also, cross bonding between two carbon chains, 5 nm long, is observed experimentally and confirmed by DFT calculations. Finally, while free standing carbon chains were observed to be straight due to tensile loading, a carbon chain inside the walls of a carbon nanotube showed high flexibility.
    Carbon 01/2014; 66. · 5.87 Impact Factor
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    ABSTRACT: Few-layer graphene has been achieved in liquid dispersion from graphite by the assistance of titanosilicate JDF-L1, using ultrasound and methanol as dispersive media. After a sedimentation step, both the dispersed and the sedimented phases were collected and then the titanosilicate was removed by alkaline hydrothermal dissolution from the mixed materials to obtain few-layer graphene (FLG) and sedimented material, respectively. The production of smaller particles was confirmed by means of N2 adsorption and zeta-potential measurements, so that the BET specific surface area increased from 20 m2/g of the raw graphite to 333 ± 22 m2/g in FLG. Raman spectroscopy shows a decrease in the ratio of intensities of the peaks G and 2D from 3.8 in the raw material to 2.5 in FLG. Particles as fine as 1.3 nm, corresponding to 4-layered FLG, were observed by AFM, while high-resolution TEM showed defect-free regions of graphene.
    Carbon. 01/2014; 73:99–105.
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    ABSTRACT: FeCo-alloy graphite-coated nanoparticles with mean particle diameter under 8 nm have been synthesized following a CVD carbon-deficient method. The superior magnetic properties of FeCo-alloy nanoparticles makes them good candidates to be used as magnetic filler in magneto-polymer composites. Thanks to the protective effect of the graphite shell, FeCo nanoparticles are stable under oxygen atmosphere up to 200 ° C. The as-prepared nanoparticles presented a highly long range chemically ordered core being ferromagnetic at room temperature with a saturation magnetization at room temperature close to the bulk value. After annealing at 750 K the saturation magnetization and the coercive field increase. To investigate the processes involved in the thermal treatment, the temperature dependence of the magnetization and the particle composition, size and structure have been characterized before and after annealing. Besides powder x-ray diffraction (XRD) and x-ray photoelectron spectroscopy (XPS), a detailed study by means of advanced transmission electron microscopy (TEM) techniques has been carried out. In particular, aberration corrected scanning transmission electron microscopy (STEM), has shown that nanoparticles became faceted after the thermal treatment, as a mechanism to reach the thermodynamic equilibrium within the metastable phase. This outstanding feature, not previously reported, leads to an increase of the shape anisotropy, which in turn might be the origin of the observed increase of the coercive field after annealing.
    Nanotechnology 11/2013; 24(50):505702. · 3.84 Impact Factor
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    ABSTRACT: The catalytic activity of gold depends on particle size, with the reactivity increasing as the particle diameter decreases. However, investigations into behaviour in the subnanometre regime (where gold exists as small clusters of a few atoms) began only recently with advances in synthesis and characterization techniques. Here we report an easy method to prepare isolated gold atoms supported on functionalized carbon nanotubes and their performance in the oxidation of thiophenol with O2. We show that single gold atoms are not active, but they aggregate under reaction conditions into gold clusters of low atomicity that exhibit a catalytic activity comparable to that of sulfhydryl oxidase enzymes. When clusters grow into larger nanoparticles, catalyst activity drops to zero. Theoretical calculations show that gold clusters are able to activate thiophenol and O2 simultaneously, and larger nanoparticles are passivated by strongly adsorbed thiolates. The combination of both reactants activation and facile product desorption makes gold clusters excellent catalysts.
    Nature Chemistry 09/2013; 5(9):775-81. · 21.76 Impact Factor
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    ABSTRACT: The use of tetrakis-(hydroxymethyl)-phosphonium chloride (THPC) as simultaneous reducing agent and stabilizing ligand has been extended to the single-step synthesis at room temperature of a wide variety of monometallic nanoparticles and bi-/tri- metallic nanoalloys containing noble metals with potential application in catalysis. The colloidal suspensions exhibit mean diameters below 4 nm with narrow size distributions and high stability in aqueous solution for long periods of time.
    RSC Advances 06/2013; 3(26):10427-10433. · 2.56 Impact Factor
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    ABSTRACT: The controlled filling of the pores of highly ordered mesoporous antiferromagnetic Co3O4 replicas with ferrimagnetic FexCo3-xO4 nanolayers is presented as a proof-of-concept toward the integration of nanosized units in highly ordered, heterostructured 3D architectures. Antiferromagnetic (AFM) Co3O4 mesostructures are obtained as negative replicas of KIT-6 silica templates, which are subsequently coated with ferrimagnetic (FiM) FexCo3-xO4 nanolayers. The tuneable magnetic properties, with a large exchange bias and coercivity, arising from the FiM/AFM interface coupling, confirm the microstructure of this novel two-phase core-shell mesoporous material. The present work demonstrates that ordered functional mesoporous 3D-materials can be successfully infiltrated with other compounds exhibiting additional functionalities yielding highly tuneable, versatile, non-siliceous based nanocomposites.
    Nanoscale 05/2013; · 6.23 Impact Factor
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    ABSTRACT: Spatially-resolved electron energy loss spectroscopy (SR-EELS) using scanning transmission electron microscope (STEM) allows the identification and determination of the spatial distribution of the components/elements of immuno-functionalized core-shell superparamagnetic magnetite nanoparticles. Here we report that SR-EELS measurements allow the direct identification and study of the biological moieties (protein G and anti-HRP antibody) in complex bio-nanocarriers of relevance for biomedical applications. Our findings show that the bio-macromolecules are located on specific areas on the nanoparticles surface. In addition, efficiency of this functionalization was evaluated by means of biochemical techniques.
    ACS Nano 04/2013; · 12.06 Impact Factor
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    ABSTRACT: Microporous titanosilicate ETS-10 crystals have been analyzed by advanced electron microscopy techniques. With the last generation of spherical aberration corrected electron microscopes, truly atomic resolution images have been recorded. Owing to the extremely high-resolution images that have been obtained, the multiple defects (stacking faults, lack of porosity and “double-pores”) present in this type of material can be analyzed in great detail.
    ChemCatChem 04/2013; 5(9):2595-2598. · 5.18 Impact Factor
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    ABSTRACT: The distribution of bismuth in InAs1-xBix/GaAs quantum dots is analyzed by atomic-column resolution electron microscopy and imaging simulation techniques. A random Bi distribution is measured in the case of <0.03 ML/s Bi flux during the InAs growth with no significant variations in the shape or size of quantum dots, resulting in a low redshift and the degradation of the photoluminescence. However, for a 0.06 ML/s Bi flux the lateral indium segregation into the quantum dots is enhanced and Bi is incorporated inside them. As a result, a strong redshift and an increase of the peak intensity are found in this sample.
    Applied Physics Express 01/2013; 6:042103. · 2.73 Impact Factor
  • Alvaro Mayoral, Rosa M Blanco, Isabel Diaz
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    ABSTRACT: In the current manuscript we report a detailed characterization based on spherical aberration (Cs) corrected scanning transmission electron microscopy (STEM) of enzyme (lipase) loaded ordered mesoporous silica (SBA-12) at an accelerating voltage of 80 kV. The extremely high resolution images combined with electron energy loss spectroscopy (EELS) analysis have allowed a complete and unambiguous determination of the presence of the enzyme inside the pores.
    Journal of Molecular Catalysis B Enzymatic 01/2013; 90:23-35. · 2.82 Impact Factor
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    ABSTRACT: One of the widely accepted uses of ordered mesoporous materials is as supports of enzymes for biotechnological applications. Enzymes have been trapped, anchored, or encapsulated in organized porous networks of the mesoporous range (2– 50 nm). The reactivity of the surface of mesoporous materials has enabled the synthesis of various supports by using different forces for the immobilization process. To design catalysts for specific applications, we have developed functionalized mesoporous materials with tunable hydrophobicity for the immobilization of lipase. More recently, we moved to the immobilization of laccase with amino-functionalized ordered mesoporous materials. In this case, it is required to use pore expanders along with optimized functionalization techniques. Advanced TEM techniques have been applied to locate not only the functional groups but also the macromolecules inside the silica matrix.
    ChemCatChem 01/2013; 5. · 5.18 Impact Factor
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    ABSTRACT: This paper reports the decoration of single wall carbon nanotubes (SWCNTs) with platinum (Pt) nanoparticles using an electrochemical technique, rotating disk slurry electrode (RoDSE). Pt/SWCNTs were electrochemically characterized by cyclic voltammetry technique (CV) and physically characterized through the use of transmission electron microscopy (TEM), energy dispersive spectroscopy - X-ray florescence (EDS-XRF) and X-ray diffraction (XRD). After characterization it was found that electrodeposited nanoparticles had an average particle size of 4.1 ± 0.8 nm. Pt/SWCNTs were used as sensing material for methane (CH4) detection and showed improved sensing properties in a range of concentration from 50 ppm to 200 ppm parts per million (ppm) at room temperature, when compared to other Pt/CNTs-based sensors. The use of this technique for the preparation of Pt/SWCNTs opens a new possibility in the bulk preparation of samples using an electrochemical method and thus their potential use in a wide variety of applications in chemical sensing, fuel cell and others.
    Journal of The Electrochemical Society 12/2012; 160(2):H1-H7. · 2.59 Impact Factor
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    ABSTRACT: The use of GaAsSbN capping layers on InAs/GaAs quantum dots (QDs) has recently been proposed for micro- and optoelectronic applications for their ability to independently tailor electron and hole confinement potentials. However, there is a lack of knowledge about the structural and compositional changes associated with the process of simultaneous Sb and N incorporation. In the present work, we have characterized using transmission electron microscopy techniques the effects of adding N in the GaAsSb/InAs/GaAs QD system. Firstly, strain maps of the regions away from the InAs QDs had revealed a huge reduction of the strain fields with the N incorporation but a higher inhomogeneity, which points to a composition modulation enhancement with the presence of Sb-rich and Sb-poor regions in the range of a few nanometers. On the other hand, the average strain in the QDs and surroundings is also similar in both cases. It could be explained by the accumulation of Sb above the QDs, compensating the tensile strain induced by the N incorporation together with an In-Ga intermixing inhibition. Indeed, compositional maps of column resolution from aberration-corrected Z-contrast images confirmed that the addition of N enhances the preferential deposition of Sb above the InAs QD, giving rise to an undulation of the growth front. As an outcome, the strong redshift in the photoluminescence spectrum of the GaAsSbN sample cannot be attributed only to the N-related reduction of the conduction band offset but also to an enhancement of the effect of Sb on the QD band structure.
    Nanoscale Research Letters 11/2012; 7(1):653. · 2.52 Impact Factor
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    ABSTRACT: An effort to electrochemically develop smaller and well dispersed catalytic materials on high surface area carbon supports is required for improved fuel cell performance. A high surface area carbon material of interest is carbon nano-onions (CNOs), also known as multilayer fullerenes. The most convenient synthetic method for CNOs is by annealing nanodiamond particles, thus keeping the size of the precursors and providing the possibility to prepare very small nanocatalysts using electrochemical techniques. In terms of pure metal catalysts, platinum is the most common catalyst used in fuel cells. The combination of Pt nanoparticles with CNOs could lead to new catalytic nanomaterials. In this work this was accomplished by using a rotating disk-slurry electrode technique. The Pt/CNO catalysts were prepared from slurries that contained functionalized CNOs and K2PtCl6 as the platinum precursor, in aqueous 0.1M H2SO4 solution. X-ray photoelectron spectroscopy results showed that 37.1% of the Pt on the CNOs is metallic Pt while 62.9% had higher binding energies, evidence of higher oxidation states on CNOs. However, aberration-corrected scanning transmission electron microscopy of the Pt/CNOs confirmed the presence of Pt atoms and clusters on CNOs. Thermal gravimetric analysis of Pt/CNOs showed better thermal stability and lower onset potential, for the electrochemical oxidation of methanol, when compared with commercial Pt/Vulcan catalyst material. Computational method confirmed Pt atoms location at CNOs surface sites. Geometric parameters for distances between Pt atoms in the 3Pt/CNOs molecular system from our theoretical calculations are in agreement with the respective parameters obtained experimentally. The combination of CNO with RoDSE presents a new highly dispersed catalyst nanomaterial.
    Langmuir 11/2012; · 4.19 Impact Factor
  • Alvaro Mayoral, Cesar Magen, Miguel Jose-Yacaman
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    ABSTRACT: The optical response of multibranched gold nanoparticles is studied by means of electron energy-loss spectroscopy (EELS) in aberration corrected STEM mode. In every case the plasmon response is constant and variations in the maxima positions were found to be dependent on the branches aspect ratio. The good spatial resolution combined with the high energy resolution (0.18 eV) of the monochromated electron beam allows mapping the different plasmonic modes along the entire nanoparticles ranging from 0.7 eV up to 2.25 eV.
    Chemical Communications 07/2012; 48(69):8667-9. · 6.38 Impact Factor
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    ABSTRACT: Chiral ordered mesoporous silica (COMS) was synthesized in the presence of amino acid proline by combining tetraethyl orthosilicate and quaternized aminosilane silica sources. The as-prepared materials were activated by calcination or microwave chemical extraction to remove the organic templates. The powder X-ray diffraction and N(2) adsorption characterization revealed in COMS the structural and textural features of MCM-41-type silica. The chirality of the material was disclosed by mixed and separate l- and d-proline adsorption on the COMS prepared with l-proline (l-Pro-COMS) and d-proline (d-Pro-COMS). It was found that the maximum l-proline and d-proline adsorption capacities on l-Pro-COMS were ca. 2.3 and 0.6 mmol/g, respectively, while the adsorption of d-proline was higher than that of l-proline on d-Pro-COMS. Finally, both activation routes yielded enantioselective silicas able to separate proline racemate.
    Langmuir 04/2012; · 4.19 Impact Factor
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    ABSTRACT: Ultra-small magnetic nanoparticles consisting of NiCo and FeNi alloys enclosed within graphitic shells (NiCo/G and FeNi/G) have been synthesized. The particles, which retained the face centered cubic (fcc) symmetry of the original bulk metals, together with the graphitic coating were characterized by means of aberration corrected scanning transmission electron microscopy (STEM), obtaining mean particle sizes of 2.6 nm and 6.2 nm for NiCo/G and FeNi/G, respectively. Due to the enhancement of the thermal stability by the graphite shell, the graphite coated FeNi and NiCo were stable under oxygen atmosphere up to 170 °C. The effectiveness of the graphite shell was confirmed when unprotected bimetallic FeNi and NiCo were prepared and chemical characterization revealed that more than 60 at.% of the samples was oxygen due to the massive oxidation of the bimetallic nanoparticles.
    Nanotechnology 02/2012; 23(8):085601. · 3.84 Impact Factor

Publication Stats

41 Citations
179.26 Total Impact Points

Institutions

  • 2010–2014
    • University of Zaragoza
      • • Department of Chemical Engineering and Environmental Technology
      • • Departamento de Química Inorgánica
      Caesaraugusta, Aragon, Spain
    • University of Birmingham
      • School of Chemistry
      Birmingham, England, United Kingdom
  • 2010–2013
    • Spanish National Research Council
      • Institute of Catalysis and Petrochemistry
      Madrid, Madrid, Spain
  • 2009–2011
    • University of Texas at San Antonio
      • Department of Physics and Astronomy
      San Antonio, Texas, United States
  • 2008
    • Stockholm University
      Tukholma, Stockholm, Sweden