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Special Properties of Au Nanoparticles

Maryam EbrahimiChem 7500/750March 28th, 2007

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Outline● Introduction

The importance of unexpected electronic, geometric, and chemicalproperties of nanoparticles in fundamental research and (device)applications

● Gold Nanoparticles (I) size-dependent catalytic activity

(II) size-dependent electronic and optical properties

● References

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Introduction● Metal oxide interface, metal coatings or dispersed metals on oxide supports play

an important role in many technological areas.

● One of the areas where deposited metal particles are technically employed to a large extent is heterogeneous catalysis (room temperature).

● There is still a lack of fundamental knowledge about the essential properties of thin metal films and small metal particles on oxide supports. So, an increasing number of model studies like “model catalysis” have been introduced. One approach comes from ultrahigh vacuum (UHV) surface science aiming at an understanding of the elementary steps involved on a microscopic level.

● Particle-size effects and the role of metal-support interactions

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Gold Nanoparticles• Au has long been known as being catalytically far less active than other transition

metals.

• Because of its inertness, Au was formerly considered as an ineffective catalyst.

• The ineffective assumption was based on studies where Au was present as relatively large particles (diameter > 10 nm) or in bulk form such as single crystal.

• A decrease of particle size causes a positive core level shift in XPS studies.

• Haruta et al. have shown exceptionally high CO oxidation activity on supportednano-Au catalysts even at sub-ambient temperatures (200 K).

• The effect of not only the particle size but also the support on the catalytic activity of a catalyst at room temperature

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Gold NanoparticlesSupported Nano-Au catalysts exhibit:

● an extraordinary high activity for low-temperaturecatalytic combustion

● Partial oxidation of hydrocarbon● Hydrogenation of unsaturated hydrocarbons● Reduction of nitrogen oxides● Propylene epoxidation● Methanol synthesis● Environmental catalysis

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(I) The structure of Catalytically Active Gold on Titania

● Cluster size and morphology, particle thickness and shape

● Support effects:Nature of the support material, Surface defects, Metal-Support charge transfer, Au- support interface

● Metal oxidation state

● Au-oxide contact area

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The Most Active Size: 3-3.5 nmD.W. Goodman et al., Science, 281 (1998) 1647

Room temperature

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The Most Active Size: 3-3.5 nmD.W. Goodman et al., Catalysis Letters,99 (2005) 1 & Catalysis Today,111 (2006) 22-33

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Gold monolayers & bilayers that completely wet the oxide support,eliminate direct support effects.

D.W. Goodman et al., Science, 306 (2004) 252

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Particle thickness and shape(CO Adsorbs strongly on the Au bilayer structure)

• On the basis of kinetic studiesand scanning tunnelingmicroscopy (STM): Au consistsof bilayer islands that havedistinctive electronic andchemical properties comparedto bulk Au.

• Two well-ordered Au films(monolayer and bilayer)completely wet an ultrathintitania surface.

11D.W. Goodman et al., Science, 306 (2004) 252 & Catalysis Today,111 (2006) 22-33

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Strong metal support interaction (SMSI)• A key feature of Au grown on TiOx/Mo(112)

is the strength of the interaction between theoverlayer Au and the support comprised of strong bonding between Au and reduced Tiatoms of the TiOx support, yielding electron-rich Au

• Recent theoretical studies: importance of reduced Ti defect sites at the boundary between Au clusters and a TiOxinterface in determining the Au cluster shape and electronic properties via transfer of charge from the support to Au

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Surface Defects• The introduction of defects into a crystal can dramatically change its

electronic properties• Defects can affect the chemistry of bare metal-oxide surfaces• Au particles bind more strongly to a defective surface than to a defect

deficient surface. There is significant charge transfer from thesupport to the Au particles. Au particles don’t bind to a perfect TiO2 surface.

• Defect sites on the oxide support play an important role in the wetting of Au particles yielding electron-rich Au. But the support itself need not be directly involved in the CO oxidation reaction sequence.

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Essential Features of the Interaction ofAu with TiO2

(1) wetting of the support by the cluster(2) strong bonding between the Au atoms at the interface

with surface defects (reduced Ti sites like Ti3+)(3) electron-rich Au(4) annealing at temperatures in excess of 750 K, sufficient

to create and mobilize surface and bulk defects, is crucial in preparing an active catalyst

(5) oxidation leads to deactivation via sintering of AuD.W. Goodman: Au particle size is related to activity,bilayer Au structure and the strong interaction between Au and defect sites on the TiO2 surface and critical for CO oxidation activity

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Au nanoparticles on Highly Ordered Pyrolytic Graphite (HOPG)

● The O 1s spectra of the Au nanoparticles on HOPG after exposing to atomic oxygenenvironments (left panel) and subsequentlyto CO (right panel).

● Each spectrum was fitted using 2 Gaussian functions, one representing the oxidic species, and the other one, non-oxidic (subsurface oxygen) species.One can see that the oxidic species exclusively react with CO, loosing their intensities after CO exposure.

Y.D. Kim et al., Chemical Physics, 330 (2006) 441-448

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(II) Au nanoparticles: Optical Properties

Au on carbon-coated copper grid

zoom

TEM micrographs of Au nanoparticles prepared by radio-frequency (rf) magnetron sputtering of a high-purity Au target a, b, c prepared in 5, 10, 15 s deposition time, respectively

B. Balamurugan and T. Maruyama, Applied Physics Letters 87(2005) 143105

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Au nanoparticles: Optical Properties

Au nanoparticles with 2.4 nm size, undergo a transition from metal to insulator:1) An absence of the surface plasmon peak (oscillation of the conduction electrons in a metal)

2) A sharp interband transition (from occupied d -level to an empty state in the conduction band above the Fermi level) in optical absorption (OA) spectra

VIS

UVAu on quartz

Sharp drop(Quantum size effect)

OA spectra of Au nanoparticles deposited on quartzPeak1: plasmon peak related to metalsPeak 2: interband transition

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Valence-band XPS spectra of Au nanoparticle samples having different average particle sizes: a) 2.5 nm, b) 3.1 nm, and c) 6.4 nm.

XPS spectra of Au 4f7/2 and Au 4f5/2 core electrons of Au nanoparticle samples having different average particle sizes: a) 2.5 nm, b) 3.1 nm, and c) 6.4 nm.

Au nanoparticles :Valence-band edge in XPSContribution of Au 5d electrons

No chemical shift

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● The metallic nature of Au nanoparticles

● The interband absorption dominates the plasmon absorption on decreasing particle size.

● Thiol-capped Au nanoparticles undergo a metal to insulator transition at a particle size of 2.4 nm.

● Au nanoparticles of size 1.6 nm, stabilized by chemical ligands, show an insulating nature due to the strong intact ligand shells with the nanoparticles and becomes metallic on the removal of the chemical ligands by x-ray exposure

● The contamination-free nanoparticle surface is essential to observethe size-dependent

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● Electronic properties of deposited metal clusters and thin films:How does the electronic structure develop with increasing size/thickness?

● Metal-oxide interface: what is the nature and strength of the bonding?● Adsorption and adhesion energies.● Diffusion of metal atoms on oxide supports.● Nucleation and growth: what are the activation energies for the

elementary steps involved? What is the prevailing nucleationmechanism? Under which conditions are ordered/disordered particles

formed? Is the growth process influenced by an ambient of certain gases?● Interaction with gases: in which way does the interaction strength/adsorption

energy change with size? Is the particle shape altered by gas adsorption?● Catalytic activity: how does the activity/selectivity change with dispersion. ● Are metal-support interactions of relevance?

Unanswered Questions

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References1. D.W. Goodman et al., Science 281 (1998) 1647-16502. D.W. Goodman et al., Science 306 (2004) 252-2553. D.W. Goodman et al., Catalysis Today 111 (2006) 22-334. D.W. Goodman et al., Catalysis Letters 99 (2005) 1-45. D.W. Goodman et al., J. Phys. Chem. B 108 (2004) 16339-163436. D.W. Goodman et al., Surface Science 600 (2006) L7-L117. D.W. Goodman et al., Applied Catalysis A 291 (2005) 32-368. D.W. Goodman et al., Science 310 (2005) 291-2939. M. Baumer & H-J Freund, Progress in Surface Science 61 (1999)

127-19810. G.A. Somorjai et al., Topics in Catalysis 24 (2003) 61-7211. B. Balamurugan and T. Maruyama, Applied Physics Letters 87

(2005) 14310512. Y.D. Kim et al., Chemical Physics, 330 (2006) 441-448

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Acknowledgement● Professor K.T. Leung

Thank You for Your Attention!