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ABSTRACT: Passivated gold nanoparticles were synthesized through a microwave-assisted process in a two-phase system, in the presence of 1-dodecanethiol. An average particle size of 1.8 nm of the gold nanoparticles obtained and 0.35 S.D. was determined through HRTEM and STEM analysis. It was observed that these nanoparticles spontaneously self-assemble into self-supported superstructures of 1 μm in diameter avg and 400 nm thickness, yielding an off-white powder which can be handled as a simple powder. XRD analysis indicates that n-alkanethiol molecules used as a passivating compound, besides protecting against crystal growth, interact to form cubic ordered arrays between the nanoparticles. This interaction leads to the superstructure formation, with an average distance between nanoparticles in the array, of 3.56 nm. Theoretical calculations and molecular dynamics simulations were performed to analyze the resulting structure.
Nanoscale 03/2012; 4(7):2281-7. · 5.91 Impact Factor
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ABSTRACT: Bimetallic nanoparticles constitute a promising type of catalysts, mainly because their physical and chemical properties may be tuned by varying their chemical composition, atomic ordering, and size. Today, the design of novel nanocatalysts is possible through a combination of virtual lab simulations on massive parallel computing and modern electron microscopy with picometre resolution on one hand, and the capability of chemical analysis at the atomic scale on the other. In this work we show how the combination of theoretical calculations and characterization can solve some of the paradoxes reported about nanocatalysts: Au-Pd bimetallic nanoparticles. In particular, we demonstrate the key role played by adsorbates, such as carbon monoxide (CO), on the structure of nanoalloys. Our results imply that surface condition of nanoparticles during synthesis is a parameter of paramount importance.
Nanoscale 12/2011; 3(12):5013-9. · 5.91 Impact Factor
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J.Nanopart.Res. 01/2009; submitted.
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ABSTRACT: When the size of a particle decreases around 100 nm or less, there is a change in properties from those shown in the bulk material. In this work approximately 3 nm nanoparticles of Ni, Ti and TiNi bimetallic are produced using physical vapor deposition (PVD). Nanoparticles are characterized by High Resolution Transmission Electron Microscopy (HRTEM), High Angle Annular Dark Field (HAADF), Electron Diffraction (ED). The results show that all nanoparticles maintain the same crystal structure of bulk material but a change in their lattice parameter is produced.
Journal of Nanoscience and Nanotechnology 01/2009; 8(12):6618-22. · 1.56 Impact Factor
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ABSTRACT: Three polycrystalline Bi2Sr2CaCu2O8, Bi2Sr2CaCu2O8 with carbon nanotubes, Bi2Sr2CaCu2O8 with NbSe2 nanotubes were synthesized by solid state reaction method and studied by scanning electron microscopy, X-ray diffraction,
magnetization measurements, and high resolution transmission electron microscopy.
The critical temperature T
C for the three compounds was about 85 K. There is an enhancement in the critical current density, J
C for samples with carbon and NbSe2 nanotubes as compared with pure Bi2Sr2CaCu2O8. The enhancement provides evidence that wetting exists for the two doped samples investigated.
Journal of Superconductivity and Novel Magnetism 06/2008; 21(5):271-277. · 0.65 Impact Factor
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ABSTRACT: In many cases nanostructures present forbidden spots in their electron diffraction patterns when they are observed by transmission electron microscopy (TEM). To interpret their TEM and high resolution transmission electron microscopy (HRTEM) images properly, an understanding of the origin of these spots is necessary. In this work we comment on the origin of the forbidden spots observed in the [111] and [112] electron diffraction patterns of flat gold triangular nanoparticles. The forbidden spots were successfully indexed as corresponding to the first laue Zone (FOLZ) and the HRTEM images presented a contrast produced by the interference of the zero-order Laue zone (ZOLZ) and FOLZ spots. We discuss the use of the forbidden spots in the study of the structure of nanoparticles and show that they are related to the shape and incompleteness of layers in the very thin particles.
Ultramicroscopy 05/2008; 108(9):929-36. · 2.47 Impact Factor
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ABSTRACT: Gold/Palladium nanoparticles were fabricated by inert-gas condensation on a sputtering reactor. With this method, by controlling both the atmosphere on the condensation chamber and the magnetron power, it was possible to produce nanoparticles with a high degree of monodispersity in size. The structure and size of the Au/Pd nanoparticles were determined by mass spectroscopy, and confirmed by atomic force microscopy and electron transmission microscopy measurements. The chemical composition was analyzed by X-ray microanalysis. From these measurements we confirmed that with the sputtering technique we are able to produce particles of 1, 3, and 5 nm on size, depending on the choice of the synthesis conditions. From TEM measurements made both in the regular HREM, as well as in STEM-HAADF mode, we found that the particles are icosahedral in shape, and the micrographs show no evidence of a core-shell structure, in contrast to what is observed in the case of nanoparticles prepared by chemical synthesis.
Faraday Discussions 02/2008; 138:353-62; discussion 421-34. · 5.00 Impact Factor
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ABSTRACT: Modern research in the field of small metallic systems has confirmed that many nanoparticles take Platonic and Archimedean
solids related shapes. A Platonic solid looks the same from any vertex, and intuitively they appear as good candidates for
atomic equilibrium shapes. A good example is the icosahedral (Ih) particle that only shows {111} faces that produce a more rounded structure. Indeed, many studies report the Ih as the most stable particle at the size range r≤20 Å for noble gases and for some metals. In this chapter, we discuss the structure and shape of mono- and bimetallic nanoparticles
in the size range from 1–300 nm. First, AuPd nanoparticles (1–2 nm) that show dodecahedral atomic growth packing. Next, in
the range of 2–5 nm, we discuss a surface reconstruction phenomenon observed also on AuPd and AuCu nanoparticles. These binary
alloy nanoparticles show the fivefold edges truncated, resulting in {100} faces on decahedral structures, an effect largely
envisioned and reported theoretically, with no experimental evidence in the literature before. Next, we review a monometallic
system (≈5 nm) that we termed the decmon. Finally, we present icosahedrally derived star gold nanocrystals (100–300 nm) which resemble the great stellated dodechaedron,
a Kepler-Poisont solid. We conclude that the shape or morphology of some mono- and bimetallic particles evolves with size
following the sequence from atoms to the Platonic solids. As the size increases, they tend to adopt Archimedean related shapes
and then beyond the Archimedean (Kepler-Poisont) solids, up to the bulk structure of solids.
12/2007: pages 55-84;
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Microscopy and Microanalysis 07/2007; 13:84 - 85. · 3.01 Impact Factor
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ABSTRACT: This paper describes the internal structure of Au-Pd nanoparticles exhibiting newly discovered three-layer core/shell morphology, which is composed of an evenly alloyed inner core, an Au-rich intermediate layer, and a Pd-rich outer shell. By exploitation of spatially resolved imaging and spectroscopic and diffraction modes of transmission electron microscopy (TEM), insights were gained on the composition of each one of the observed three layers, indicating a significant extent of intimate alloy among the monometallic elements.
Nano Letters 07/2007; 7(6):1701-5. · 13.20 Impact Factor
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Microscopy and Microanalysis 07/2006; 12:772 - 773. · 3.01 Impact Factor
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ABSTRACT: We report on energetic surface reconstruction phenomena observed on bimetallic nanoparticle systems of AuPd and AuCu, similar to a resolidification effect observed during the cooling process in lead clusters. These binary alloy nanoparticles show the fivefold edges truncated, resulting in [100] facets on decahedral structures, an effect largely envisioned and reported theoretically, with no experimental evidence so far. We demonstrate experimentally as well as by computational simulations that this new eutectic structure is favored in such nanoalloy systems.
Physical Review Letters 06/2004; 92(19):196102. · 7.37 Impact Factor
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ABSTRACT: In the present work we report the synthesis of MoS2 nanotubes with diameters greater than 10nm using a template method. The length and properties of these nanotubes are a direct result of the preparation method. High-resolution transmission electron microscopy is used to study the structure of these highly curved entities. Molecular dynamics simulations of MoS2 nanotubes reveal that one of the stable forms of the nanotubes is a twisted one. The twisting of the nanotubes produces a characteristic contrast in the images, which is also studied using simulation methods. The analysis of the local contrast close to the perpendicular orientation shows geometrical arrays of dots in domain-like structures, which are demonstrated to be a product of the atomic overlapping of irregular curvatures in the nanotubes. The configuration of some of the experimentally obtained nanotubes is demonstrated to be twisted with a behavior suggesting partial plasticity.
Applied Physics A 02/2004; 78(4):513-518. · 1.63 Impact Factor
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ABSTRACT: Gold nanoparticles have been synthesized using n-alkylthiol molecules as a passivating agent. By fixing the length of the thiol chain, it is possible to produce nanocrystal arrays, such as 1D chains, 2D arrays of chains and 3D crystals.
Microscopy and Microanalysis 03/2002; 8(1):64-9. · 3.01 Impact Factor
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ABSTRACT: The active sites of ultra-dispersed Pt/-Al2O3 catalysts are studied using high-resolution electron microscopy, Z-contrast and dark field. In addition we have calculated using a method based in density functional theory the electrostatic potential and charge distribution of the active sites. It is conclude that the most likely Pt clusters that are formed correspond to Pt13 and Pt with icosahedral and decahedral structure. It is shown that this is consistent with the electron microscopy data.
Topics in Catalysis 01/2002; 18(3):167-173. · 2.62 Impact Factor
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ABSTRACT: In this work we present a new method to produce gold nanorods based on the bio-reduction process. The nanorods produced tend to show twins appearing either as bands or concentric forms, generating a new type of nanorods based on a decahedral structure. Examples of these two types of twins are presented here. The main conclusion is that the bio-reduction method to produce nanorods is a good alternative to the electrochemical methods.
Journal of Nanoparticle Research 11/2001; 3(5):475-481. · 3.29 Impact Factor
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ABSTRACT: The application of modern electron microscopy and X-ray diffraction techniques to the study of ancient pigments has proved to be very useful. In the present work we report the study of pigments from Mexica (Aztec) culture which developed in Central Mexico from 1325–1521 AD; we study the blue, ochre, red and black pigments. We found in the most cases the paints were made of a clay substrate that contained metal particles such as Fe, Ti, Mn and Zn, in different phases. This technology was found similar to that of earlier mesoamerican cultures.
Journal of Materials Science 01/2001; 36(3):751-756. · 2.02 Impact Factor
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ABSTRACT: In this work we report experimental and theoretical studies of structural instabilities of gold nanoparticles supported on
a carbon substrate using high-resolution transmission microscopy and molecular dynamics simulations. It is shown that particles
undergo structural fluctuations in which a particle exhibits a change in orientation whilst maintaining the overall structure.
These fluctuations are of a different kind than the ones reported in previous work, in which transitions between different
structures or the appearance, movement or vanishing of twin boundaries are observed. Computer calculations were performed
using a model that included both the particle and the substrate. It is found that during the fluctuations there is a collective
displacement of the atoms at the interface between the gold and the carbon substrate that results in a larger contact area.
Calculations of particle stability that include the interaction with the carbon substrate show that different orientations
of the particle have similar energies. As a result, during the fluctuations the particle becomes trapped between different
orientations of the same structure. This kind of phenomenon can be considered as a new kind of quasimelting.
Applied Physics A 01/2001; 73(3):295-300. · 1.63 Impact Factor
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ABSTRACT: In the present work, we report the growth of ordered arrays of gold nanoparticles passivated with n-alkylthiol molecules using
crystallization in toluene vapor. We kept constant the average particle size and the length of the passivating molecule (1-dodecanethiol).
The temperature and time of growth were varied. We show that in the initial stages of the growth, nano-chains of particles
are produced. These nano-chains aggregate to form two-dimensional arrays. In the initial stages the nano-chains form a two-dimensional
square lattice which then relaxes to a close-packed structure. In latter stages of growth a three-dimensional supercrystal
is produced. It is found that the packing of nanoparticles corresponds to an average FCC lattice. However, large variations
on the local parameters of the lattice are observed. Near the edges of the supercrystal the anomalous packing reported by
Zanchet et al. [20] and Fink et al. [21] was observed. The energy of the observed structures is analyzed using molecular dynamics
simulation. It is concluded that using the vapor growth method, it is possible to produce controlled ordered structures from
one to three dimensions.
Applied Physics A 08/2000; 71(3):237-243. · 1.63 Impact Factor
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ABSTRACT: We have studied the structure and properties of palygorskite clays. A structure analysis was performed using two different models to reproduce the monoclinic and orthorhombic lattices, using the atomic positions and cell parameters of palygorskite structure suggested by several authors, we simulated structures using Molecular Dynamics and Quantum Mechanics. Modifications on the structure and elemental atom changes were made to obtain more stable configurations. X-ray diffraction patterns and high resolution electron microscopy images from simulated structures were compared with experimental results. It could be observed that orthorhombic model shows a better fitting than monoclinic models. These results might help to understand many of the properties of archaeological pigments, such as, Maya Blue, in which palygorskite clay was the main component.
Journal of Materials Science 10/1999; 34(21):5243-5255. · 2.02 Impact Factor
