Atomic

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Atomic Molecular Clusters6. Bimetallic
Nanoalloy Clusters

• Nanoalloys are clusters of two or more metallic
  elements.
• A wide range of combinations and compositions are
  possible for nanoalloys.
• Bimetallic nanoalloys (AaBb) can be generated
  with controlled size (ab) and composition (a/b).
• Structures and the degree of A-B
  segregation/mixing may depend on the method of
  generation.

• Nanoalloys can be generated in cluster beams or
  as colloids.
• They can also be generated by decomposing
  bimetallic organometallic complexes.

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Why study nanoalloys?

• Nanoalloys are of interest in catalysis (e.g.
  catalytic converters in automobiles), and for
  electronic and magnetic applications.
• Fabrication of materials with well defined,
  controllable properties combining flexibility
  of intermetallic materials with structure on the
  nanoscale.
• Chemical and physical properties can be tuned by
  varying cluster size, composition and atomic
  ordering (segregation or mixing).
• May display structures and properties distinct
  from pure elemental clusters (e.g. synergism in
  catalysis by bimetallic nanoalloys).
• May display properties distinct from bulk alloys
  (e.g. Ag and Fe are miscible in clusters but not
  in bulk alloys).

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Properties of interest

• Dependence of geometrical structure and atomic
  ordering (mixing vs. segregation) on cluster size
  and composition.
• Comparison with bulk alloys and their surfaces.
• Kinetic vs. thermodynamic growth.
• Dynamical processes (diffusion and melting).
• Electronic, optical and magnetic properties.
• Catalytic activity.

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Isomerism in nanoalloys

• Nanoalloys exhibit geometrical (structural),
  permutational and compositional isomerism.
• Homotops (Jellinek) are Permutational Isomers of
  AaBb having the same number of atoms (ab),
  composition (a/b) and geometrical structures, but
  a different arrangement of A and B atoms.
• Compositional Isomers have the same number of
  atoms and geometrical structures, but different
  compositions (a/b).

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Homotops

• The number of homotops (NH) rises combinatorially
  with cluster size and is maximized for 50/50
  mixtures.
• e.g. for A10B10 there will be 185,000 homotops
  for each geometrical structure though many will
  be symmetry-equivalent.

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Segregation Patterns in Nanoalloys
Layered
Random
Ordered
Linked
Core-Shell
Segregated
Mixed
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Atomic ordering in AaBb nanoalloys depends on

• Relative strengths of A-A, B-B and A-B bonds
• if A-B bonds are strongest, this favours mixing,
  otherwise segregation is favoured, with the
  species forming strongest homonuclear bonds
  tending to be at the centre of the cluster.
• Surface energies of bulk elements A and B
• the element with lowest surface energy tends to
  segregate to the surface.
• Relative atomic sizes
• smaller atoms tend to occupy the core
  especially in compressed icosahedral clusters.

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• Charge transfer
• partial electron transfer from less to more
  electronegative element favours mixing.
• Strength of binding to surface ligands
  (surfactants)
• may draw out the element that binds most strongly
  to the ligands towards the surface.
• Specific electronic/magnetic effects.

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Core-Shell Nanoalloys

• Core of metal A surrounded by a thin shell of
  metal B
• which has the tendency to segregate to the
  surface
• (e.g. B/AAg/Pd, Ag/Cu, Ag/Ni).
• The outer shell is strained, and can present
  unusual catalytic
• properties

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Elemental Properties
Element Ra / Å Ecoh / eV Esurf / meV Å?2 Electroneg.
Ni 1.25 4.44 149 1.8
Pd 1.38 3.89 131 2.2
Pt 1.39 5.84 159 2.2
Cu 1.28 3.49 114 1.9
Ag 1.45 2.95 78 1.9
Au 1.44 3.81 97 2.4
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• Examples Ag combined with Cu, Pd, Ni
• (Theoretical Study by Ferrando)
• Ag has greater size and lower surface energy
• tends to segregate to the surface
• Ag-Cu tendency to phase separation.
• Ag-Pd experimental interest (Henry)
  possibility of forming solid solutions.
• Ag-Ni experimental interest (Broyer) strong
  tendency to phase separation, huge size
  mismatch.
• Different kinds of deposition procedures direct
  deposition and inverse deposition.
• Growth of three-shell onion-like nanoparticles

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Doping of single impurities in a Ag core
When the impurity atom is smaller than the core
atoms, the best place in an icosahedron is in the
central site radial (inter-shell) distances can
expand and intra-shell distances can contract.
In fcc clusters, the Ag atoms accommodate better
around an impurity in a subsurface site, because
they are more free to relax to accommodate the
size mismatch.
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Inverse Deposition
Deposition on icosahedra deposited A atoms
diffuse quickly to the cluster centre, where they
nucleate an inner core ? core-shell A-B structure.
Deposition on TO (fcc) clusters deposited A
atoms stop in subsurface sites where they
nucleate an intermediate layer ? three-shell
onion-like A-B-A structure.
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Normal vs. Inverse Deposition

• Inverse deposition deposition of metal that
  prefers to occupy the core, onto a core of the
  other metal.
• Ag deposited on Cu, Pd or Ni cores ? core-shell
  structures.
• Cu, Pd or Ni deposited on Ag cores (inverse
  deposition), the final result depends on the
  temperature and on the structure of the initial
  core
• starting with Ag icosahedra ? core-shell
  structures
• starting with Ag fcc polyhedra (TO) ? three-shell
  onion-like structures.
• Growth of three-shell structures takes place
  because single impurities are better placed in
  sites which are just one layer below the surface.
  This is true for fcc clusters.

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Cu-Au Nanoalloys

• Cu, Au and all Cu-Au bulk alloys exhibit fcc
  packing.
• Ordered alloys include Cu3Au, CuAu and CuAu3.
• Mixing is weakly exothermic.
• Useful model system (elements from same group).
• Experimental studies of Cu-Au nanoalloys by Mori
  and Lievens.
• Theoretical studies of Cu-Au nanoalloys by Lopez
  and Johnston.

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(Cu3Au)N Clusters
(CuAu3)N Clusters
Cu atoms prefer to occupy bulk sites.

• Au atoms prefer to
• occupy surface sites.

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Ni-Al Nanoalloys

• Ni, Al and most bulk alloys exhibit fcc packing.
• Ordered alloys include Ni3Al, NiAl (bcc) and
  NiAl3.
• Mixing is strongly exothermic.
• Ni-Al nanoalloys useful model system (very
  different metals).
• Application in heterogeneous catalysis
  synergism detected in reductive dehalogenation of
  organic halides by Ni-Al nanoparticles (Massicot
  et al.).
• Experimental studies of Ni-Al nanoalloys by Parks
  and Riley.
• Theoretical studies by Jellinek, Gallego and
  Johnston.

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• The larger Al atom can accommodate more than 12
• neighbouring Ni atoms.

• Different cluster geometries are found as a
  function of cluster size.

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• Clusters with approximate composition Ni3Al,
  show significant Ni-Al mixing.
• There is a slight tendency for surface enrichment
  by Al.

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Pd-Pt Nanoalloys

• Pd, Pt and all Pd-Pt bulk alloys exhibit fcc
  packing.
• In bulk, Pd-Pt forms solid solutions for all
  compositions (no ordered phases!).
• Mixing is weakly exothermic.
• Experimental studies of catalytic hydrogenation
  of aromatic hydrocarbons by Pd-Pt nanoalloys
  (Stanislaus Cooper) indicate a synergistic
  lowering of susceptibility to poisoning by S,
  compared with pure metallic particles.

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• EDX and EXAFS studies of (1-5 nm) Pd-Pt
  nanoalloys (Renouprez Rousset) indicate
  fcc-like structures, with Pt-rich cores and a
  Pd-rich surfaces (i.e. with segregation).

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• Theoretical studies (Johnston) agree with
  experiment.
• Bond strengths Pt-Pt gt Pt-Pd gt Pd-Pd
• (i.e. Ecoh(Pt) gt Ecoh(PdPt) gt Ecoh(Pd))
• favours segregation, with Pt at core.
• Surface energy Esurf(Pd) lt Esurf (Pt)
• favours segregation, with Pd on surface.
• Almost no difference in atomic size and
  electronegativity.

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Ag-Au Nanoalloys

• Ag, Au and all Ag-Au bulk alloys exhibit fcc
  packing.
• In the bulk, Ag-Au forms solid solutions for all
  compositions (no ordered phases!).
• Mixing is weakly exothermic.
• There is experimental interest in how the shape
  and frequency of the plasmon resonance of Ag-Au
  clusters varies with composition and
  segregation/mixing.
• Recent TEM studies of core-shell Ag-Au clusters
  indicate a degree of inter-shell diffusion.

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• Some structural motifs for Ag-Au clusters from
  theoretical studies (Johnston Ferrando).
• Au atoms preferentially occupy core sites and Ag
  atoms occupy surface sites.

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General Results of Theoretical Studies

• Icosahedral and fcc-like (e.g. truncated
  octahedral) structures compete.
• Other structure types (e.g. decahedra) may also
  be found, as well as disordered (amorphous)
  structures.
• The lowest energy structures are size- and
  composition-dependent.
• Doping a single B atom into a pure AN cluster can
  lead to an abrupt change in geometry.

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Specific Results

• Cu-Au the surface is richer in Au (lower surface
  energy), despite Au-Au bonds being strongest.
  The smaller Cu atoms prefer to adopt core sites.
• Ni-Al shows a greater degree of mixing as the
  Ni-Al interaction is strongest (strongly
  exothermic mixing). There is a slight preference
  for Al atoms on the surface (larger atoms,
  smaller surface energy).
• Pd-Pt segregates so that the surface is richer
  in Pd (lower surface energy) and the core is
  richer in Pt (strongest M-M bonds) even though
  the bulk alloy is a solid solution at all
  compositions.
• Ag-Au segregates so that the surface is richer
  in Ag (lower surface energy) and the core is
  richer in Au (strongest M-M bonds) even though
  the bulk alloy is a solid solution at all
  compositions.

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Coated Nanoalloys Ni-Pt-(CO) Clusters(Longoni)
Ni36Pt4(CO)456?
Ni24Pt14(CO)444?
Ni37Pt4(CO)466?