Alvaro Mayoral

University of Zaragoza, Caesaraugusta, Aragon, Spain

Are you Alvaro Mayoral?

Claim your profile

Publications (84)312.96 Total impact

  • Source
  • Source
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We report a systematic study on the structural and magnetic properties of Co_{x}Fe_{3-x}O_{4} magnetic nanoparticles with sizes between 5 to 25 nm, prepared by thermal decomposition of Fe(acac)_{3} and Co(acac)_{2}. The large magneto-crystalline anisotropy of the synthesized particles resulted in high blocking temperatures (42 K < T_B < 345 K for 5 < d <13 nm ) and large coercive fields (H_C \approxeq 1600 kA/m for T = 5 K). The smallest particles ( d=5 nm) revealed the existence of a magnetically hard, spin-disordered surface. The thermal dependence of static and dynamic magnetic properties of the whole series of samples could be explained within the Néel-Arrhenius relaxation framework without the need of ad-hoc corrections, by including the thermal dependence of the magnetocrystalline anisotropy constant K_1(T) through the empirical Brükhatov-Kirensky relation. This approach provided K_1(0) values very similar to the bulk material from either static or dynamic magnetic measurements, as well as realistic values for the response times (tau_0 \simeq 10^{-10} s). Deviations from the bulk anisotropy values found for the smallest particles could be qualitatively explained based on Zener's relation between K_1(T) and M(T).
  • [Show abstract] [Hide abstract]
    ABSTRACT: In this work we investigate the formation of PtAu monodisperse alloyed nanoparticles by ageing pure metallic Au and Pt small nanoparticles (sNPs), nanoparticle size <5 nm, under certain conditions. We demonstrate that those bimetallic entities can be obtained by controlling the size of the initial metallic sNPs separately prepared and by selecting their appropriate capping agents. The formation of this spontaneous phenomenon was studied using HR-STEM, EDS, ionic conductivity, UV-Vis spectroscopy and cyclic voltammetry. Depending on the type of capping agent used and the size of the initial Au sNPs, three different materials were obtained: (i) AuPt bimetallic sNPs showing a surface rich in Au atoms, (ii) segregated Au and Pt sNPs and (iii) a mixture of bimetallic nanoparticles as well as Pt sNPs and Au NPs. Surface segregation energies and the nature of the reaction environment are the driving forces to direct the distribution of atoms in the bimetallic sNPs. PtAu alloyed nanoparticles were obtained after 150 h of reaction at room temperature if a weak capping agent was used for the stabilization of the nanoparticles. It was also found that Au atoms diffuse towards Pt sNPs, producing a surface enriched in Au atoms. This study shows that even pure nanoparticles are prone to be modified by the surrounding nanoparticles to give rise to new nanomaterials if atomic diffusion is feasible.
    Nanoscale 06/2015; 7(22):10152-10161. DOI:10.1039/c5nr01819f · 6.74 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Nano-hydrotalcite (nano-HT), particles have been supported on silica and tested for chromium(VI) retention through anion exchange mechanism. Ultra high resolution has been achieved in the structural characterization of the nano-hydrotalcite particles allowing identification of the Cr within few layers of the nanoparticles. Compared to a reference hydrotalcite (HT), in which the Cr(VI) retention takes place via memory effect, nano-HT/SiO2 exhibits better adsorption capacity, for 4 mg/L Cr(VI) solutions, even when the adsorption mechanism of the later is anion exchange. This higher adsorption capacity can be attributed to the considerable smaller size of the crystals, which would favor adsorption kinetics and would minimize possible hindrances between the layers of hydrotalcite. The adsorption behavior of nano-HT/SiO2 can be described by a Freundlich isotherm suggesting a non-uniform surface, which is consistent to their arrangement in nanocrystallites observed by TEM.
    Journal of Environmental Chemical Engineering 05/2015; 3(3). DOI:10.1016/j.jece.2015.05.009
  • [Show abstract] [Hide abstract]
    ABSTRACT: Super-paramagnetic iron oxide-based nanoparticles (SPIONS) have attracted an enormous amount of attention for their potential use in biomedical applications, due to their good biocompatibility and low toxicity. The current study considers citric acid-conjugated manganese ferrite and its synergy to be used in MRI and in hyperthermia treatment, thus showing theragnostic applications. High colloidal stability was obtained with this functionalisation. SPIONS with super-paramagnetic behaviour of crystal sizes of approximately 20 nm were obtained via an electrochemical synthesis method. One of the highest specific absorption rate (SAR) values was achieved in this work (1661 W g-1), under a magnetic field of 30 mT at 717 kHz frequency, compared with other magnetic ferrites in the literature. These nanoparticles dissipate heat through Néel relaxation, and together with the high SAR value obtained indicate an excellent material for hyperthermia treatment of cancer. In addition, these nanoparticles exhibit transverse relaxivity behaviour, with an r2 value of 394 mM-1 s-1, i.e., at least two times higher than the value of a commercial magnetic contrast agent based on iron oxides. Finally, no toxicity effects of these nanoparticles are evidenced; as a result, these nanoparticles are appropriate for in vivo application.
    The Journal of Physical Chemistry C 03/2015; 119(12):6828-6834. DOI:10.1021/jp510937r · 4.84 Impact Factor
  • Source
    Alvaro Mayoral, Daniel Llamosa, Yves Huttel
    [Show abstract] [Hide abstract]
    ABSTRACT: Core@shell Co@Au nanoparticles of around 8 nm have been produced by inert gas condensation method, revealing for the first time that most of the nanoparticles present an icosahedral shape in agreement with the theoretical prediction. Additionally, we report the existence of a novel morphology which consists of a Co icosahedron surrounded by fcc Au facets, reported here for the first time.
    Chemical Communications 02/2015; 51(40):8442. DOI:10.1039/C5CC00774G · 6.72 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The synthesis of nano-sized ZIF-11 with an average size of 36±6 nm is reported. This material has been named nano zeolitic imidazolate framework-11 (nZIF-11). It has the same chemical composition and thermal stability properties and analogous H2 and CO2 adsorption properties as the conventional microcrystalline ZIF-11 (i.e. 1.9±0.9 µm). nZIF-11 has been obtained following the centrifugation route, typically used for solid separation, as a fast new technique (pioneering for MOFs) for obtaining nanomaterials where the temperature, time and rotation speed can easily be controlled. Compared with traditional synthesis consisting of stirring + separation, the reaction time was lowered from several hours to a few minutes when using this centrifugation synthesis technique. Employing the same reaction time (2, 5 or 10 min), micro-sized ZIF-11 was obtained in the traditional synthesis while nano-scale ZIF-11 was achieved only by using centrifugation synthesis. The small particle size obtained for nZIF-11 allowed the use of the wet MOF sample as a colloidal suspension stable in chloroform. This helped to prepare mixed matrix membranes (MMMs) by direct addition of the membrane polymer (polyimide Matrimid®) to the colloidal suspension, avoiding particle agglomeration resulting from drying. The MMMs were tested in H2/CO2 separation, improving the pure polymer membrane performance, with permeation values of 95.9 Barrer of H2 and a H2/CO2 separation selectivity of 4.4 at 35 ⁰C. When measured at 200 ⁰C, these values increased to 535 Barrer and 9.1.
    02/2015; 3(12). DOI:10.1039/C4TA06820C
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Metal nanoparticles with precisely controlled size are highly attractive for heterogeneous catalysis. However, their poor thermal stability remains a major concern in their application at realistic operating conditions. This paper demonstrates the possibility of synthesizing gold nanoparticles with exceptional thermal stability. This has been achieved by using a simple conventional deposition-precipitation technique. The material employed as catalyst consists of gold supported on a TiO2-impregnated SiO2 bimodal mesoporous support. The resulting material shows gold nanoparticles with a narrow size distribution around 3.0 nm, homogeneously dispersed over the TiO2/SiO2 material. Most interestingly, the gold nanoparticles show exceptional thermal stability; calcination temperatures as high as 800 °C have been employed, and negligible changes in the gold particle size distribution are apparent. Additionally, the presence of an amorphous titanium silicate phase is partially preserved, and these factors lead to remarkable activity to catalyze a range of oxidation reactions.Keywords: heterogeneous catalysis; gold nanoparticles; stability; mesoporous materials; catalytic oxidation
    ACS Catalysis 02/2015; 5(2):1078-1086. DOI:10.1021/cs501741u · 7.57 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The effect of synthesis pH and H2O/EtOH molar ratio on the textural properties of different aluminium trimesate metal organic frameworks (MOFs) prepared in the presence of the well-known cationic surfactant cetyltrimethylammonium bromide (CTAB) at 120 °C was studied with the purpose of obtaining a MOF with hierarchical pore structure. Depending on the pH and the solvent used, different topologies were obtained (namely, MIL-96, MIL-100 and MIL-110). On the one hand, MIL-110 was obtained at lower temperatures than those commonly reported in the literature and without additives to control the pH; on the other hand, MIL-100 with crystallite sizes as small as 30 ± 10 nm could be easily synthesized in a mixture of H2O and EtOH with a H2O/EtOH molar ratio of 3.4 at pH 2.6 in the presence of CTAB. The resulting material displays a hierarchical porosity that combines the microporosity from the MOF and the non-ordered mesopores defined in between the MOF nanoparticles. Interestingly, the maximum of the pore size distribution could be varied between 3 and 33 nm. Finally, at pH 2.5 and using water as a solvent, platelets of MIL-96, a morphology never observed before for this MOF, were synthesized with a (001) preferential crystal orientation, the (001) plane running parallel to the bipyramidal cages of the MIL-96 topology.
    CrystEngComm 02/2015; 17(7):1693-1700. DOI:10.1039/C4CE02324B · 3.86 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: A ca. one-μm single crystal of MOF MIL-53(Al) was attached to a glass support surface via covalent bonding.•The breathing behavior of MOF MIL-53(Al) was monitored in real-time with the help of environmental SEM.•This allowed the attribution of porosity to the geometry in MIL-53(Al) crystals.•The new characterization and the corresponding crystallographic calculations were in agreement with the previous related literature.
    Microporous and Mesoporous Materials 02/2015; 203. DOI:10.1016/j.micromeso.2014.10.016 · 3.21 Impact Factor
  • Source
  • Source
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Complex Core@Shell and Core@Shell@Shell nanoparticles are systems that combine the functionalities of the inner core and outer shell materials together with new physico-chemical properties originated by their low (nano) dimensionality. Such nanoparticles are of primer importance in the fast growing nanotechnology as building blocks for more sophisticated systems and a plethora of applications. Here it is shown that, although conceptually simple, a modified gas aggregation approach allows the one-step generation of well-controlled complex nanoparticles. In particular it is demonstrated that the atoms of the core and shell of the nanoparticles can be easily inverted, avoiding intrinsic constraints of chemical methods.
    Nanoscale 11/2014; 6(22):13483. DOI:10.1039/c4nr02913e · 6.74 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The functionalization with amine groups was developed on the SBA-15, and its effect in the laccase immobilization was compared with that of a Periodic Mesoporous Aminosilica. A method to encapsulate the laccase in situ has now been developed. In this work, spherical aberration (Cs) corrected scanning transmission electron microscopy combined with high angle annular dark field detector and electron energy loss spectroscopy were applied to identify the exact location of the enzyme in the matrix formed by the ordered mesoporous solids.
    10/2014; 2(11):113304. DOI:10.1063/1.4897281
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Nowadays, there is a great interest in the synthesis of colloidal dispersions of nanoparticles of elements with high atomic weight because of their potential application as contrast agents in X-ray imaging. Therefore, we have attempted to synthesize water-dispersible tungsten and bismuth nanoparticles by scanning-assisted pulsed laser ablation of pure tungsten and bismuth bulk targets immersed in a liquid medium. To this end, a battery of techniques has been used to characterize the products obtained under different experimental conditions. Results showed that laser ablation of tungsten or bismuth targets in distilled water by non-overlapping nanosecond laser pulses yielded large aggregates of amorphous tungsten or bismuth oxide/hydroxide nanoparticles, whereas the use of femtosecond laser pulses always produced well defined tungsten(III) oxide or bismuth(III) oxide nanocrystals. On the other hand, the use of short-chain alcohols (methanol, ethanol and n-propanol) as solvent produced colloidal dispersions of crystalline tungsten or bismuth-based nanoparticles. We have found that the crystalline structure of these nanoparticles depends on the pulse duration and pulse overlap degree. Thus, small tungsten(III) or bismuth(III) oxide nanocrystals were obtained by using non-overlapping nanosecond laser pulses. However, laser ablation by overlapping nanosecond laser pulses yielded tungsten-tungsten(III) oxide or bismuth-bismuth(III) oxide core-shell nanoparticles; the use of femtosecond laser pulses always produced well constructed core-shell nanoparticles independently of the pulse overlap. Pulse irradiance time frequency also played a significant role in controlling the rate production, size and dispersity of the as-synthesized nanoparticles; thus, the increase in the irradiance resulted in an increase in the size and particle rate production together with a significant decrease in the dispersity of the final nanoparticles. In conclusion, we have demonstrated the ability to pulsed laser ablation in liquid media to synthesize water-dispersible tungsten and bismuth based-nanopaticles with an adequate control of their crystalline structure, composition, particle size, dispersity and colloidal stability.
    Materials science and engineering congress 2014, Darmstadt; 09/2014
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: This work investigates the synthesis of water dispersible core-shell iron-iron oxide nanoparticles by using scanning-assisted, pulsed laser ablation of a pure iron bulk target immersed in a liquid medium. We have also explored the dependence of the structure, size, size distribution, aggregation degree and colloidal stability of the as-synthesized nanoparticles on the laser parameters (wavelength, irradiance time frequency pulse, and laser pulse overlap) and the characteristics of the liquid medium (solvent composition and presence of hydrophilic polymers). A battery of techniques has been used to characterize the as-synthesized nanoparticles. Results showed that laser ablation of the iron target in distilled water by nanosecond laser pulses yielded small iron oxide nanoparticles [magnetite or maghemite] embedded in a matrix of amorphous hematite, whereas the use of femtosecond laser pulses produced well isolated crystalline iron oxide nanoparticles. On the other hand, the use of organic solvents yielded colloidal dispersions consist mainly of iron-iron oxide (maghemite/magnetite) core-shell nanoparticles. In this case, the increase in the pulse irradiance time frequency resulted in an increase in the particle size together with a significant decrease in the dispersity of these nanoparticles; moreover, most of the nanoparticles synthesized at high irradiance values showed an amorphous carbon coating on their surface. On the other hand, the decrease in the pulse laser overlap degree produced an important reduction in the size of the obtained nanoparticles. Pulse duration also played a significant role in controlling the particle aggregation, thus the use of femtosecond laser pulses always produced well isolated and homogeneous core-shell nanoparticles. Moreover, we found that the use as solvent of a mixture of hydrophilic polymers (Polyvinylpyrrolidone and Tetraethylene glycol) dissolved in ethanol resulted in a strong decrease of the particle aggregation and improved their colloidal stability in water.
    materials science and engineering congress 2014, Darmstadt; 09/2014
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We describe the preparation of an effective and reusable heterogeneous asymmetric catalyst. A novel chiral periodic mesoporous phenylene-silica containing high density of bis(oxazoline) moieties is prepared by co-condensation method with 1,4-bis(triethoxysilyl)benzene. After copper(II) coordination, the material is extremely efficient on the kinetic resolution of the 1,2-diphenylethane-1,2-diol with persistent high enantioselectivities (91 - > 99%) and yields (46-43% in maximum 50% resolution) at least for five consecutive cycles. Characterization of the material after the catalytic experiments showed that the heterogeneous catalyst was very robust keeping the integrity of the structure.
    Journal of Catalysis 09/2014; 320(1):63-69. DOI:10.1016/j.jcat.2014.09.017 · 6.07 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Gold nanoparticles have received much attention in recent years due to their unique size-dependent properties, as they find useful applications in materials science [Mayoral et al. (Nanoscale 2:335-342, 2010)], catalysis [Schwerdtfeger (Angew Chem Int Ed 42:1892-1895, 2003)] [Hashmi and Hutchings (Angew Chem Int Ed 45:7896-7936, 2006)] and biology [Sperling et al. (Chem Soc Rev 37:1896-1908, 2008)]. The preparation of such nanoparticles benefits from modern chemical knowledge, and a large variety of several procedures have been developed aiming at controlling the size and shape of these metal nanoparticles. Here, we show that two eighteenth-century recipes (Online Resource 1) used at that time to prepare drinkable solutions of gold, used as drugs, actually generate gold nanoparticles, clusters and even monoatomic species of gold. These simple methods involve the dissolution of gold in a solution of ammonium chloride in nitric acid (aqua regia) and the mixing of the resulting solution with rosemary or cinnamon essential oils. The complex mixture of compounds resulting from the fast reaction between aqua regia and the essential oils behave simultaneously as reductants and stabilisers of the nascent gold particles. These results not only prove that historical speculations on the presence of finely divided gold particles floating in these solutions were basically correct but they could also serve as a source of inspiration for new experimental approaches procuring the generation of stable sub-nanometer gold nanoparticles.
    09/2014; 47(3):161-165. DOI:10.1007/s13404-014-0139-8
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The use of hydrogen as an energy vector leads to the development of materials with high hydrogen adsorption capacity. In this work, a new layered stannosilicate, UZAR-S3, is synthesized and delaminated, producing UZAR-S4. UZAR-S3, with the empirical formula Na4SnSi5O14 3.5H2Oand lamellar morphology, is a layered stannosilicate built from SnO6 and SiO4 polyhedra.Thedelamination processused herecomprises three stages: protonation with acetic acid, swelling with nonylamine and the delamination itself with an HCl/H2O/ethanol solution. UZAR-S4 is composed of sheets a few nanometers thick with a high aspect ratio and a surface area of 236 m2/g, twenty times higher than that of UZAR-S3. At �196 �C forUZAR-S4, H2 adsorption reached remarkable values of 3.7 and 4.2 wt% for 10 and 40 bar, respectively, the latter value giving a high volumetric H2 storage capacity of 26.2 g of H2/L.
    International Journal of Hydrogen Energy 08/2014; 39(25):13180–13188. DOI:10.1016/j.ijhydene.2014.06.149 · 2.93 Impact Factor

Publication Stats

362 Citations
312.96 Total Impact Points

Institutions

  • 2010–2015
    • University of Zaragoza
      • Department of Chemical Engineering and Environmental Technology
      Caesaraugusta, Aragon, Spain
  • 2014
    • Klinikum Darmstadt
      Darmstadt, Hesse, Germany
  • 2009–2012
    • University of Texas at San Antonio
      • Department of Physics and Astronomy
      San Antonio, Texas, United States
  • 2007–2010
    • University of Birmingham
      • School of Chemistry
      Birmingham, England, United Kingdom
  • 2007–2008
    • Stockholm University
      • Department of Materials and Environmental Chemistry
      Tukholma, Stockholm, Sweden