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. 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 Random Ordered Linked 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.

Strength of binding to surface ligands (surfactants) 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 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 Cu 1.28 3.49 114 1.9 Ag 1.45 2.95 78 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: 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

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.

“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.

(CuAu3)N Clusters (Cu3Au)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). h PdxPt1x Laser ablation of Pd-Pt target Pt-rich core Pd-rich shell 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)