Transcript Document

Atomic & Molecular Clusters
6. 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 (a+b) 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.
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).
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.
Isomerism in nanoalloys
• Nanoalloys exhibit geometrical (structural), permutational and
compositional isomerism.
• Homotops (Jellinek) are Permutational Isomers of AaBb – having
the same number of atoms (a+b), 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).
Homotops
• The number of homotops (NH) rises
combinatorially with cluster size and is
maximized for 50/50 mixtures.
N H  PA,B
N
N!
N!


N A ! N B ! N A ! ( N-N A )!
• e.g. for A10B10 there will be ~ 185,000
homotops for each geometrical structure –
though many will be symmetry-equivalent.
Segregation Patterns in Nanoalloys
Layered
Linked
Random
Ordered
Core-Shell
Segregated
Mixed
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.
• 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.
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/A=Ag/Pd, Ag/Cu, Ag/Ni).
• The outer shell is strained, and can present unusual catalytic
properties
Elemental Properties
Ecoh / eV Esurf / meV Å2 Electroneg.
Element
Ra / Å
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
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:
• Ag-Pd:
• Ag-Ni:
tendency to phase separation.
experimental interest (Henry); possibility
of forming solid solutions.
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
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 intrashell 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.
“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.
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.
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.
(Cu3Au)N Clusters
(CuAu3)N Clusters
Au atoms prefer to
occupy surface sites.
Cu atoms prefer to
occupy bulk sites.
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.
• The larger Al atom can accommodate more than 12
neighbouring Ni atoms.
Ni14Al
Ni15Al
Ni16Al
• Different cluster geometries are found as a function
of cluster size.
Ni28Al10
Ni29Al10
Ni41Al14
• Clusters with
approximate
composition “Ni3Al”,
show significant Ni-Al
mixing.
• There is a slight
tendency for surface
enrichment by Al.
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.
• 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).
PdxPt1x
Pd-rich shell
h
Pt-rich core
Laser ablation of
Pd-Pt target
• Pd-Pt particle has same composition
as target.
• But core-shell segregation is observed.
• Theoretical studies (Johnston) agree with
experiment.
• Bond strengths Pt-Pt > Pt-Pd > Pd-Pd
(i.e. Ecoh(Pt) > Ecoh(PdPt) > Ecoh(Pd))
– favours segregation, with Pt at core.
• Surface energy Esurf(Pd) < Esurf (Pt)
– favours segregation, with Pd on surface.
• Almost no difference in atomic size and
electronegativity.
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.
• Some structural motifs for Ag-Au
clusters from theoretical studies
(Johnston & Ferrando).
• Au atoms preferentially occupy core
sites and Ag atoms occupy surface
sites.
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.
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.
Coated Nanoalloys
Ni-Pt-(CO) Clusters
(Longoni)
[Ni36Pt4(CO)45]6
[Ni37Pt4(CO)46]6
[Ni24Pt14(CO)44]4