Special Properties of Au Nanoparticles

Transcription

1 Special Properties of Au Nanoparticles Maryam Ebrahimi Chem 7500/750 March 28 th,

2 Outline Introduction The importance of unexpected electronic, geometric, and chemical properties of nanoparticles in fundamental research and (device) applications Gold Nanoparticles (I) size-dependent catalytic activity (II) size-dependent electronic and optical properties References 2

3 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 3

4 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 supported nano-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 4

5 Gold Nanoparticles Supported Nano-Au catalysts exhibit: an extraordinary high activity for low-temperature catalytic combustion Partial oxidation of hydrocarbon Hydrogenation of unsaturated hydrocarbons Reduction of nitrogen oxides Propylene epoxidation Methanol synthesis Environmental catalysis 5

6 (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 6

7 The Most Active Size: nm D.W. Goodman et al., Science, 281 (1998) 1647 Room temperature 7

8 The Most Active Size: nm D.W. Goodman et al., Catalysis Letters,99 (2005) 1 & Catalysis Today,111 (2006)

9 Gold monolayers & bilayers that completely wet the oxide support, eliminate direct support effects. D.W. Goodman et al., Science, 306 (2004) 252 9

10 Particle thickness and shape (CO Adsorbs strongly on the Au bilayer structure) On the basis of kinetic studies and scanning tunneling microscopy (STM): Au consists of bilayer islands that have distinctive electronic and chemical properties compared to bulk Au. Two well-ordered Au films (monolayer and bilayer) completely wet an ultrathin titania surface. 10

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

12 Strong metal support interaction (SMSI) A key feature of Au grown on TiO x /Mo(112) is the strength of the interaction between the overlayer Au and the support comprised of strong bonding between Au and reduced Ti atoms of the TiO x support, yielding electronrich Au Recent theoretical studies: importance of reduced Ti defect sites at the boundary between Au clusters and a TiO x interface in determining the Au cluster shape and electronic properties via transfer of charge from the support to Au 12

13 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 the support to the Au particles. Au particles don t bind to a perfect TiO 2 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. 13

14 Essential Features of the Interaction of Au with TiO 2 (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 Ti 3+ ) (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 Au D.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 14

15 Au nanoparticles on Highly Ordered Pyrolytic Graphite (HOPG) The O 1s spectra of the Au nanoparticles on HOPG after exposing to atomic oxygen environments (left panel) and subsequently to 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)

16 (II) Au nanoparticles: Optical Properties B. Balamurugan and T. Maruyama, Applied Physics Letters 87(2005) 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 16

17 Au nanoparticles: Optical Properties UV Au on quartz VIS OA spectra of Au nanoparticles deposited on quartz Peak1: plasmon peak related to metals Peak 2: interband transition Sharp drop (Quantum size effect) 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 17

18 Au nanoparticles :Valence-band edge in XPS Contribution of Au 5d electrons No chemical shift 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. 18

19 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 observe the size-dependent 19

20 Unanswered Questions 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 nucleation mechanism? 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? 20

21 References 1. D.W. Goodman et al., Science 281 (1998) D.W. Goodman et al., Science 306 (2004) D.W. Goodman et al., Catalysis Today 111 (2006) D.W. Goodman et al., Catalysis Letters 99 (2005) D.W. Goodman et al., J. Phys. Chem. B 108 (2004) D.W. Goodman et al., Surface Science 600 (2006) L7-L11 7. D.W. Goodman et al., Applied Catalysis A 291 (2005) D.W. Goodman et al., Science 310 (2005) M. Baumer & H-J Freund, Progress in Surface Science 61 (1999) G.A. Somorjai et al., Topics in Catalysis 24 (2003) B. Balamurugan and T. Maruyama, Applied Physics Letters 87 (2005) Y.D. Kim et al., Chemical Physics, 330 (2006)

22 Acknowledgement Professor K.T. Leung Thank You for Your Attention! 22