Atomic & Molecular Clusters 6. Bimetallic “Nanoalloy” Clusters

1. 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 (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.

2. 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 clustersize, 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).

3. 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.

4. 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).

5. 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.

6. Segregation Patterns in Nanoalloys Layered Random Ordered Linked Core-Shell Segregated Mixed

7. 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.

8. 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.

9. 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

10. Elemental Properties

11. Examples: Ag combined with Cu, Pd, Ni (Theoretical Study by Ferrando) • Ag has greater size and lower surface energy • tends tosegregate 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

12. 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.

13. “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.

14. 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 depositedon Ag cores (inverse deposition), the finalresult 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.

15. 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.

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

17. 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.

18. Ni14Al Ni15Al Ni16Al Ni28Al10 Ni29Al10 Ni41Al14 • The larger Al atom can accommodate more than 12 • neighbouring Ni atoms. • Different cluster geometries are found as a function of cluster size.

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

20. 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.

21. 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. • 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).

22. 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.

23. 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.

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

25. 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.

26. 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.

27. Coated Nanoalloys Ni-Pt-(CO) Clusters(Longoni) [Ni36Pt4(CO)45]6 [Ni37Pt4(CO)46]6 [Ni24Pt14(CO)44]4