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

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Hannu Häkkinen, Nanoscience Center, University of Jyväskylä

Basics of Nanoscience 2008 Nanoclusters and nanoparticles II

Hannu Häkkinen 22.1.2008

University of Jyväskylä Nanoscience Center Departments of Physics and Chemistry

Thermodynamic properties

2 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)

3 Dynamics of gold clusters from DFT-TB

MD of Au11- at 750 K: co-existence of 2D/3D liquid Supercooling to ”wrong” dimensionality (experimental time scale of cooling: 0.1 to 10 microseconds) Koskinen et al, PRL 98, 015701 (2007) Video in EPAPS

Electronic, chemical and catalytic properties of gold clusters

4 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

Taylor et al JCP 96, 3319 (1992) Gantefor group CPL 377, 170 (2003) 16

Anomalously inert Au16-

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Electronic structure of free Au clusters: Comparison to bulk band structure

Yoon, Koskinen, Huber, Kostko, von Issendorff, Häkkinen, Moseler, Landman, ChemPhysChem 8, 157 (2007)

”Band picture” of gold clusters: The Au(5d) derived band ”embedded” in the Au(6s6p) derived conduction electron shells, which can be found at E ≈ Emin and E ≈ Ef These shells display symmetries expected from the delocalized electron shell model of simple metal clusters (1S-1P-2S/1D-1F…)!!

I Opahle, PhD Thesis, Dresden

Au16 : electronic structure and reactivity

Si@Au16 Si@Au16- + O2 Au16 double-anion is a closed-electron-shell 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

6 Catalytic oxidation of CO by gas-phase Au2

• J. Am. Chem. Soc. 125, 10437 (2003)

Theory: Häkkinen, Landman Experiment: Wöste group (Berlin)

• Low-T activity (low barriers)
• Key intermediates: AuCOO2
• (C)
• r AuCO3
• (D)
• 2 scenarios I, II
• Eley-Rideal mechanism

I II

Clusters on a supporting surface

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, ).

.

• .

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Au/NaCl(100) Repp et al, Science 305 (2004) 493 Au/MgO(100) Sterrer et al, PRL 98 (2007) 96107 Yoon et al, Science 307 (2005) 403

Direct

• bservation

via STM Only indirect observation via IR spectroscopy

Sterrer et al, Angew.Chem.In.Ed. 45 (2006) 2630

Direct experimental observations of Au clusters with STM minima Scale: 30 nm

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Assumptions Assume barrier 2 >> barrier 1 Positive bias = negative tip

• > STM probes unocc. States

Negative bias = positive tip

• > STM probes occ. states

Clusters bind to oxygen vacancy (FC)

Au atom on plain MgO: 0.8 eV, on FC: 2.8 eV

Tersoff-Hamann approximation: STM probes local DOS Au8 periodic versus cluster approach I = 0.25 nA

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Delocalised electrons in gold clusters

Walter et al, PCCP 8 (2006) 5407 Yoon et al, ChemPhysChem 8 (2007) 157 Janssens et al, NJP 5 (2003) 46

Flat Au20 structure motive 0eV 0.3eV 0.6eV Same motive like supported Au8

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Flat structure: 2D Harmonic Oscillator model Free electrons of monovalent Gold LDOS of flat Au20@MgO

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LUMO LUMO+1 average HOMO-1 HOMO average STM pictures I = 10 pA

Au13: open shell cluster

positive bias negative bias => STM sees the same picture independent of the bias

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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

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,"-+12**3 ./ 4 2 53 '6 !,7"82**13 . . / , 9' 2**"3 .,. . "# :;2"""3

%.;) % .

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Review of optical properties: Wyrwas et al EPJD 2007

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Science Oct 19, 2007

The first experimental total-structure-determination

• f a thiolate-protected gold cluster

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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

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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

Surface layer (40 atoms) of the 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

Note: 23 Au atoms in the protective layer (cf. ”Divide and Protect” model for Au38(SR)24 = Au14(Au4SR4)6 Häkkinen, Walter, Grönbeck, JPCB 110, 9927 2006))

Q(R)

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(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

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Electronic structure: Angular momentum projected DOS around Fermi level (at E=0) 2P+1G 3S+2D+1H

• 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

Au79 KS levels and effective radial potential 6s-only calculation (M. Walter)

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AuNC Phosphine-chemistry

• various low-nuclearity Phosphine / halide stabilized gold

compounds known & structure-resolved since late 1970’s !

• Au39 compound crystallized & characterized by Teo et al 1992 (JACS):

Any similarites to thiolate-chemistry? Traditional answer : NO! Au39(PH3)14Cl6- Au39 core Au11(PH3)7(SCH3)3 Au11(PH3)7Cl3 Compounds studied here: D3 C3v

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Surviving gaps: 8, 18, 34, 58 (cf: model ->) 8 e HL gap in undecagold 34e shell closing in Au39 compound: below HL gap : 2S/1F character (as expected) above HL gap: 1G/2P character (as expected) deHeer RMP1993 Undecagold (Au11) and Au39 superatoms

0.5 eV Au102(p-MBA)44 0.6 eV Au58- (a,c) 58e (34e + 2P61G18) 0.8 eV Au39Cl6(PH3)14

• 1.0 eV

Au34- (a,b) 34e (8e + 1D102S21F14) 2.1 eV Au11(PH3)7Cl3 8e 1.5 eV Au11(PH3)7(SMe)3 N/A (*) 8e (1S21P6) Gap Cluster compound Gap Cluster Theory (this work) Experiment Shell closing

Electron shell closings & HOMO-LUMO gaps of LPAuNC ”superatoms” (this work) compared to measured gaps of known gas-phase clusters

(*) anionic gas-phase clusters planar, 3D shell model not applicable (would work for Au9+ though) (a) Taylor et al, J Chem Phys 96, 3319 (1992) (b) Lechtken et al, Angew. Chemie Int Ed 46, (2007) (c) Häkkinen et al, Phys. Rev. Lett. 93, 093401 (2004)

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Summary: the unifiying superatom concept for LPAuNCs

• co-existence of

(i) globular conduction electron shell structure in a (fairly) symmetric, close-packed Au core (ii) the ”right” number of (pseudo)halide ligands (thiolate, halide) for chemical passivation of the core surface & stabilizing the core e-shells

• thiolate- , phosphine/halide- and phosphine/thiolate protection

can be understood with the same footing

• works from small to medium to large sizes
• closings of 8, 34, 58 conduction electron shells disclosed in this work
• for the first time, gives a solid theoretical concept to predict the

electronic structure (including the symmetries of the photo-active levels around Ef in the gold core!) for a host of mass/composition-resolved, but not yet structure-resolved thiolate-protected compounds (4 kDa, 6 kDa, 8 kDa, 14 kDa, 29 kDa…)

• application to LPAuNC novel
• 58 Au102(SR)44

92 …. (40) 34 Au39(PR3)Cl6- Au75(SR)40(+) (20) 18 Au44(SR)28(2-) 8 Au11(PR3)X3 Au25(SR)18(-1) 2

The Periodic Table of LPAuNC Superatoms

Magic conduction e numbers in the metal core; already known compounds (atomic structure resolved)

Self-organized growth of LPAuNCs in solution tries to take all the forming compounds to ”rare-gas superatom” configuration most reactive - - - less reactive - - - - INERT, THERMODYNAMIC GROUND STATES !! [LB]s • ANXM(z),

24 Summary: Cluster = a system with some ions and electrons !