2,188 research outputs found
Doped golden fullerene cages
Full text linkA first-principles investigation of the effect of the doping of golden cages of 32 atoms is proposed. It is shown that Ag and Cu doping affects the geometrical stability of the icosahedral fullerene Au32 cage, where Ag-doping leads to a new, low symmetric, and prolate motif while Cu-doping leads to a lump, incomplete decahedral shape. Most significantly, the HOMO-LUMO gap depends strongly on the cluster geometry while its dependence on the cluster chemical composition seems to be weaker
Structural properties of sub-nanometer metallic clusters
No full textAt the nanoscale, the investigation of structural features becomes fundamental as we can establish relationships between cluster geometries and their physicochemical properties. The peculiarity lies in the variety of shapes often unusual and far from any geometrical and crystallographic intuition clusters can assume. In this respect, we should treat and consider nanoparticles as a new form of matter. Nanoparticle structures depend on their size, chemical composition, ordering, as well as external conditions e.g. synthesis method, pressure, temperature, support. On top of that, at finite temperatures nanoparticles can fluctuate among different structures, opening new and exciting horizons for the design of optimal nanoparticles for advanced applications. This article aims to overview geometrical features of transition metal clusters and of their various rearrangements
Hierarchical self-assembly of Au-nanoparticles into filaments: evolution and break
No full textWe compare the assembly of individual Au nanoparticles in a vacuum and between two Au(111) surfaces via classical molecular dynamics on a timescale of 100 ns. In a vacuum, the assembly of three nanoparticles used as seeds, initially showing decahedral, truncated octahedral and icosahedral shapes with a diameter of 1.5-1.7 nm, evolves into a spherical object with about 10-12 layers and a gyration radius similar to 2.5-2.8 nm. In a vacuum, 42% show just one 5-fold symmetry axis, 33% adopt a defected icosahedral arrangement, and 25% lose all 5-fold symmetry and display a face-centred-cubic shape with several parallel stacking faults. We model a constrained version of the same assembly that takes place between two Au(111) surfaces. During the dynamics, the two Au(111) surfaces are kept fixed at distances of 55 & Aring;, 55.5 & Aring;, 56 & Aring;, and 56.5 & Aring;. The latter distance accommodates 24 Au layers with no strain, while the others correspond to nominal strains of 1.5%, 2.4%, and 3.3%, respectively. In the constrained assembly, each individual seed tends to reorganize into a layered configuration, but the filament may break. The probability of breaking the assembled nanofilament depends on the individual morphology of the seeds. It is more likely to break at the decahedron/icosahedron interface, whilst it is more likely to layer with respect to the (111) orientation when a truncated octahedron sits between the decahedron and the icosahedron. We further observe that nanofilaments between surfaces at 56 & Aring; have a >90% probability of breaking, which decreases to 8% when the surfaces are 55 & Aring; apart. We attribute the dramatic change in probability of breaking to the peculiar decahedron/icosahedron interface and the higher average atomic strain in the nanofilaments. This in silico experiment can shed light on the understanding and control of the formation of metallic nanowires and nanoparticle-assembled networks, which find applications in next-generation electronic devices, such as resistive random access memories and neuromorphic devices
The effect of chemical ordering and lattice mismatch on structural transitions in phase segregating nanoalloys
Full text linkWe elucidate the effect of lattice mismatch and chemical ordering on structural transitions in bimetallic nanoalloys.</p
Modelling Janus Nanoparticles
No full textJanus particles are compartmentalised objects with different, usually even opposite, chemical make-ups on their two hemispheres, which make them a unique class of materials at the meso- and nanoscale. The name is reminiscent of Janus the ancient Roman god with two heads looking in opposite directions, simultaneously forward to the future and backward to the past.First made by Casagrande and Veyssie (Compt Rend Ac Sci 306:1423, 1988) of one hydrophilic and one hydrophobic hemisphere, raised to an international attention as “new animals with a polar and an apolar face” by de Gennes in his Nobel lecture, Janus particles are acquiring more and more importance because of their wide range of technological applications. Thanks to their toposelective and functionalised coating, they are utilised as surfactants, drug-delivers and catalysts. The flexibility proper of soft matter has been thought to be applied to bimetallic nanoalloys where the Janus motif represents a layered segregation effect leading to two different chemical regions in the cluster. Janus nanoalloys have been proposed in magneto-optical applications, such as structural colourations on textile, and as flip-flop switchers in the presence/absence of a magnetic field.The numerical modelling of Janus particles is a useful tool to understand and drive their synthesis and preparation. In this chapter, the attention is focused, but not limited, on the modelling of the growth of bi-comparmentalised nanoalloys, where the cases of gold–platinum (AgPt) and silver–cobalt (AgCo) are presented as paradigmatic examples
Chemical order and magnetic properties in small M
No full textA systematic analysis of the chemical order, structure stability and magnetic behaviour
of small transition metal binary nanoalloys is performed employing spin-polarised
ab-initio simulations. The doping of icosahedral geometries at 13 and 19 atoms of magnetic
materials with two impurities both magnetic (Fe, Co, Ni, Pt) and non-magnetic (Ag, Cu) is
considered. In CoFe, the most favourable substitutional sites are those which maximise the
total magnetic moment of the system: Fe dopants tend to occupy surface sites while Co
atoms stay in the inner. For all the other nanoalloys, the doping sites respect a chemical
order that leads to a surface energy minimization often followed by a depression of the
total magnetization. The ferromagnetic arrangement is always the energetically most
favourable order apart from the Ag-doped case where the anti-ferromagnetic alignment is
almost degenerate to the ferromagnetic phase
Pt38 as a promising ethanol catalyst: a first principles study
No full textThis first-principles study predicts Pt38 nanoparticles as a catalyst for ethanol reactions. Starting from the adsorption properties, we shed light on the effectiveness of Pt-based nanoclusters as ethanol catalysts. First, the ethanol adsorption on Pt38 shows that the most stable site positions the molecule with the oxygen anchored on top of an edge, whereas CH3 is oriented towards the facet and the molecule remains in trans-symmetry. The ethanol-oxygen adsorbed on top of a facet Pt-atom offers the least stable configuration and the longer Pt-O distance (2.318 Å), while the shorter Pt-O distance (2.237 Å) is found when ethanol is on top of an edge site and the molecule is vertically oriented with Gauche symmetry. A shorter Pt-O distance correlates with higher radial breathing of the nanoparticle after ethanol adsorption. Atomic charge redistribution is calculated on all the considered systems and cases. In any event, we show that the Pt-anchor receives a charge, whilst oxygen-ethanol donates electrons. Orbital analysis shows that Pt-anchors and ethanol-oxygen atoms primarily exchange p-charge. Energy barriers associated with the ethanol bond cleavage show that the C-C bond break is slightly more favourable on Pt38 than on an extended Pt(111). In addition, we find that the cleavage of the hydroxyl O-H ethanol bond shows a higher energy barrier while the removal of an H-atom from the CH3 group is easier. These three facts indicate that the Pt38 nanoparticle enhances ethanol catalysis and hence is a good candidate for ethanol-based fuel cells
A fluxional anionic water trimer
No full textWe explore the energy landscape of anionic water clusters using Car–Parrinello molecular dynamics with a proper self-interaction correction. While neutral water trimers preserve a ring structure over long time periods, significant changes in the structure of their anionic form, (H2O), occur at temperatures above 100 K. These rearrangements are such that the excess electron attaches and spatially localises close to a new form of double-acceptor water molecule. Such structural rearrangement arises from cooperation among the dipole moments of each single water molecule within the cluster. We discuss the excess electron's attachment to water clusters up to 21 molecules, rationalising the structural binding motifs needed to bind it
A novel structural motif for free CoPt nanoalloys
No full textThe growth of small free platinum-cobalt nanoclusters has been modelled through the one-by-one atom deposition technique based on classical Molecular Dynamics. A novel structural motif, reminiscent of two stacked poly-icosahedral pancakes, has been found in close competition with a Marks-decahedron and a face-centred-cubic geometry at around 75 atoms for a cobalt-rich stoichiometry. The results have been confirmed at the density-functional level, which reveals an unexpected stability of this new morphology over a wide range of chemical compositions. Magnetic properties have been also discussed
A DFT Study on the O2 Adsorption Properties of Supported PtNi Clusters
Full text linkWe present a systematic study on the adsorption properties of molecular oxygen on Pt, Ni and PtNi clusters previously deposited on MgO(100) by means of density functional theory calculations. We map the different adsorption sites for a variety of cluster geometries, including icosahedra, decahedra, truncated octahedra and cuboctahedra, in the size range between 25–58 atoms. The average adsorption energy depends on the chemical composition, varying from 2 eV for pure Ni, 1.07 for pure Pt and 1.09 for a Pt s h e l l Ni c o r e nanoalloy. To correlate the adsorption map to the adsorption properties, we opt for a geometrical descriptor based on the metallic coordination up to the second coordination shell. We find an almost linear relationship between the second coordination shell and adsorption energy, with low coordination sites, such as those located at the (111)/(111) and (111)/(100) cluster edges-displaying adsorption energies above 1 eV, while higher coordination sites such as (111) cluster facets have an interaction of 0.4 eV or lower. The inclusion of van der Waals corrections leads to an overall increase of the O 2 adsorption energy without an alteration of the general adsorption trends
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