Basics of Nanoscience 2008 Nanoclusters and nanoparticles II Hannu - PDF document

Basics of Nanoscience 2008 Nanoclusters and nanoparticles II Hannu Häkkinen 22.1.2008 University of Jyväskylä Nanoscience Center Departments of Physics and Chemistry Hannu Häkkinen, Nanoscience Center, University of Jyväskylä Thermodynamic properties 1

Thermodynamic properties - general � Thermodynamics of small systems complicated and not always well defined ! � Experimental concerns: formation of clusters depends on source conditions, sometimes driven by thermodynamics, sometimes kinetics � Nanoclusters exhibit a rich palette of thermodynamic phenomena: size- dependent melting, surface pre-melting, solid-solid structural transitions, freezing transitions, coalescence phenomena… � Sometimes surprises in store: ”non-melting clusters” (melting point appears to be higher that in bulk, e.g. Sn � bonding different in clusters) � Bi-stability of ”phases” Experimentally the best studied cluster melting problem: Na clusters, � work by Haberland group (original exp: Nature 393 , 238 (1998) + many later papers) � Computational challenge: sampling of the phase-space � Good review: Baletto, Ferrando, Rev. Mod. Phys. 77, 371 (2005) Global optimisation and potential energy surfaces • Generally: finding the global optimal geometry for a given cluster size is a highly non-trivial problem • A well-known example: 38-atom Lennard-Jones cluster has a narrow funnel for the global TO ground-state, but a wide funnel for icosahedral local minima • A useful website for global minima: Cambridge Cluster Database www-wales.ch.cam.ac.uk./CCD.html Rev Mod Phys. 77 , 371 (2005) 2

Dynamics of gold clusters from DFT-TB Supercooling to ”wrong” dimensionality (experimental time scale of cooling: MD of Au11- at 750 K: 0.1 to 10 microseconds) co-existence of 2D/3D liquid Koskinen et al, PRL 98, 015701 (2007) Video in EPAPS Electronic, chemical and catalytic properties of gold clusters 3

Chemical and catalytic properties of gold clusters � Bulk gold inert � Finely dispersed gold (as nanoparticles) catalytically active, for review see eg. Haruta, Catal. Today 36 , 153 (1997) � Oxide-supported size-selected clusters catalyze CO oxidation (Yoon, Häkkinen, Landman, Wörz, Antonietti, Abbet, Judai, Heiz, Science 307 , 403 (2005)) � Active site / charge state under debate � Known for long: gold atom chemically active in many oxidation states (rich complex chemistry) � Gas-phase reactivity with O2: anionic gold needed, highly size- dependent reactivity � Reactivity associated with electron transfer to O2 π * orbital Gold: Electron affinity & reactivity with O2 16 Taylor et al JCP 96, 3319 (1992) Anomalously inert Au16- Gantefor group CPL 377, 170 (2003) 4

Electronic structure of free Au clusters: Comparison to bulk band structure I Opahle, PhD Thesis, Dresden ”Band picture” of gold clusters: The Au(5d) derived band ”embedded” in the Au(6s6p) derived conduction electron shells, which Yoon, Koskinen, Huber, Kostko, can be found at E ≈ Emin and E ≈ Ef von Issendorff, Häkkinen, Moseler, Landman, ChemPhysChem 8, 157 (2007) These shells display symmetries expected from the delocalized electron shell model of simple metal clusters (1S-1P-2S/1D-1F…)!! Au16 : electronic structure and reactivity Si@Au16 Si@Au16- Au16 double-anion is a closed-electron-shell + O2 cluster with ”18e” shell closing (jellium-type 1S, 1P, 1D shells in a cage) � high EA � no electron transfer to O2 � no reactivity Dope with Si � ”20e” shell closing (2S now in) � anion now reactive with O2 5

Catalytic oxidation of CO by gas-phase Au 2 - • Low-T activity (low barriers) - (C) • Key intermediates: AuCOO 2 or AuCO 3 - (D) • 2 scenarios I, II • Eley-Rideal mechanism I II J. Am. Chem. Soc. 125, 10437 (2003) Theory: Häkkinen, Landman Experiment: Wöste group (Berlin) � Clusters on a supporting surface 6

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Only indirect observation via IR spectroscopy Yoon et al, Science 307 (2005) 403 Direct observation via STM Au/MgO(100) Sterrer et al, PRL 98 (2007) 96107 Au/NaCl(100) Repp et al, Science 305 (2004) 493 Direct experimental observations of Au clusters with STM Scale: 30 nm minima Sterrer et al, Angew.Chem.In.Ed . 45 (2006) 2630 8

Assumptions Clusters bind to oxygen vacancy (FC) Au atom on plain MgO: 0.8 eV, on FC: 2.8 eV Assume barrier 2 >> barrier 1 Positive bias = negative tip -> STM probes unocc. States Negative bias = positive tip -> STM probes occ. states Tersoff-Hamann approximation: STM probes local DOS Au 8 periodic versus cluster approach I = 0.25 nA 9

Delocalised electrons in gold clusters Janssens et al, NJP 5 (2003) 46 Walter et al, PCCP 8 (2006) 5407 Yoon et al, ChemPhysChem 8 (2007) 157 Flat Au 20 structure motive Same motive like supported Au 8 0eV 0.3eV 0.6eV 10

Flat structure: 2D Harmonic Oscillator model Free electrons of monovalent Gold LDOS of flat Au 20 @MgO 11

STM pictures I = 10 pA HOMO-1 HOMO average LUMO LUMO+1 average Au 13 : open shell cluster positive bias negative bias => STM sees the same picture independent of the bias 12

Conclusions ● Au clusters for 8-20 atoms appear as 1-2 nm particles in STM ● STM shows Gold wfs in the band-gap of MgO ● STM figures show nodes of delocalized wfs and not atoms ● Symmetries of the delocalized states can be understood in a simple monovalent Gold jellium model Ligand-protected gold clusters 13

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Review of optical properties: Wyrwas et al EPJD 2007 15

Science Oct 19, 2007 The first experimental total-structure-determination of a thiolate-protected gold cluster 16

Anatomy of the Stanford cluster I: Au102(p-MBA)44, Two visualizations ( p-MBA = para-mercapto benzoic acid ) Initial coordinates: Jadzinsky et al Science Oct 19, 2007 - necessary H added & CH bonds and COOH groups relaxed Full complex: 762 atoms and 3366 valence electrons (Walter et al, 2008 to be published) Anatomy of the Stanford cluster II: core – shell ! Au102(p-MBA)44 = Au79 + Au23(p-MBA)44 Two views of the (D5h, within 0.4 A) Au79 core 40-atom surface of the core + 21 RSAu-(RSAu)x-SR units (x=0 for 19 units and x=1 for 2 units) 2 Au(core) atoms with 2 SAu bonds each long unit, x=1 17

Anatomy of the Stanford cluster III: The two types of ligands RSAu-(RSAu)x-SR with x=0,1 R=(C6H4)(COOH) Anatomy of the Stanford cluster IV : A metallic, electronically inert Au79 core Radial analysis of charge re-distribution upon ionizing: virtually no changes inside 5Å radius, 10 % of the charge at the surface of Au79, 90% charging inside the Au23(p-MBA)44 protective layer Q(R) Note: 23 Au atoms in the protective layer (cf. ”Divide and Protect” model for Surface layer (40 atoms) of the Au79 core Au38(SR)24 = Au14(Au4SR4)6 Häkkinen, Walter, Grönbeck, JPCB 110, 9927 2006)) 18

(Global) angular momentum analysis of Au(6s6p)-derived ”conduction electron” states in the gold core Evaluate the coefficients c(R0) for each Kohn-Sham state n (done up to ������� I-symmetry) The EDOS of the Stanford cluster region of interest 19

Electronic structure: Angular momentum projected DOS around Fermi level (at E=0) • Au79 core supports the (expected) shell structure (58, 92 e gaps , proper symmetries) • Upon dressing the core with 21 (RSAu-(RSAu)x-SR) units, 21 conduction electrons depleted from the 3S+2D+1H manifold ( � surface- covalent S-Au(core) bonds), thereby revealing the 58 e gap, which becomes the HOMO-LUMO gap of the LPAuNC !! 2P+1G 2P+1G 3S+2D+1H Au79 KS levels and effective radial potential 6s-only calculation (M. Walter) 20