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Atomic Scale Ordering in Metallic Nanoparticles

Atomic Scale Ordering in Metallic Nanoparticles. Structure: Atomic packing: microstructure? Cluster shape? Surface structure? Disorder?. Characterization. Electron Microscopy Scanning Transmission Electron Microscopy (STEM) Electron Diffraction

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Atomic Scale Ordering in Metallic Nanoparticles

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  1. Atomic Scale Ordering in Metallic Nanoparticles • Structure: • Atomic packing: microstructure? • Cluster shape? • Surface structure? • Disorder?

  2. Characterization • Electron Microscopy • Scanning Transmission Electron Microscopy (STEM) • Electron Diffraction • X-ray Absorption Spectroscopy • X-ray Absorption Near Edge Spectroscopy (XANES) • Provides information on chemical states • Oxidation state • Density of states • Extended X-ray Absorption Fine Structure (EXAFS) • Provides local (~10 Å) structural parameters • Nearest Neighbors (coordination numbers) • Bond distances • Disorder

  3. (111) (001) (110) Face Centered Cubic Structure

  4. [011] [112] [310] Electron Microdiffraction Electron diffraction probes the ordered microstructure of the nanoparticles. Above are 3 sample diffraction patterns for ~ 20 Å Pt nanoparticles. All are indexed as face-centered cubic (fcc).

  5. Absorption Photon Energy X-Ray Absorption Spectroscopy • Absorption coefficient (m) vs. incident photon energy • The photoelectric absorption decreases with increasing energy • “Jumps” correspond to excitation of core electrons Adapted from Teo, B. K. EXAFS: Basic Principles and Data Analysis; Springer-Verlag: New York, 1986.

  6. Pt L3 edge (11564 eV) Pt foil I0 IT EXAFS x Extended X-ray Absorption Fine Structure • oscillation of the X-ray absorption coefficient near and edge • local (<10 Å) structure surrounding the absorbing atom

  7. hn e- Ri PE = hn - E0 E0 initial final • Excitation of a photoelectron with wavenumber k = 2p/l • Oscillations, ci(k): final state interference between outgoing and backscattered photoelectron Ri - distance to shell-i Ai(k) - backscattering amp. Basics of EXAFS

  8. Data Analysis m Convert to wave number m0 m0(0) Subtract background and normalize Resulting data is the sum of scattering from all shells

  9. R 1 Pt L3 edge, Pt foil ) -3 (r)|(Å R c 3 | R R 4 2 r (Å) Fourier Transform Resolve the scattering from each distance (Ri) into r-space

  10. Multiple-Shell Fit Calculate Fi(k) and di(k) for each shell-i (i = 1 to 6) using the FEFF computer code Non-linear least-square refinement: vary Ni, Ri, s2i using the EXAFS equation

  11. SS1 TR2 DS SS4 TS SS2 SS3 TR1 SS5 TR3 In-plane atom Above-plane atom Absorbing atom Multiple Scattering Paths

  12. X-Ray Absorption Near Edge Spectroscopy (XANES) XANES measurements for reduced 10%, 40% Pt/C, 60% Pt/C Pt/C, and Pt foil at 200, 300, 473 and 673 K. A total of 16 measurements are shown. All overlay well with bulk Pt (Pt foil); therefore, the samples are reduced to their metallic state.

  13. Size Dependence Size dependence on the extended x-ray absorption spectra. The amplitude of the EXAFS signal is directly proportional to the coordination numbers for each shell; therefore, as the cluster size increases, the amplitude also will increase.

  14. Multiple Shell Fitting Analysis 40% Pt/C 10% Pt/C

  15. Temperature Dependence Temperature dependence on the extended x-ray absorption spectra for 10% Pt/C. As the temperature increases, the dynamic disorder (D2) increases, causing the amplitude to decrease.

  16. First Shell Fitting: 10% Pt/C 200 K 300 K 673 K 473 K

  17. The EXAFS Disorder, s2, is the sum of the static, ss2, and dynamic, sd2, disorder as follows: The dynamic disorder, sd2, can be separated by using the following relationship: Size Dependent Scaling of Bond Length and Disorder

  18. Structure and Morphology Hemispherical cuboctahedron, (111) basal plane • Determining shape and texture • Electron microscopy • X-Ray absorption spectroscopy • Molecular modeling Hemispherical cuboctahedron, (001) basal plane Spherical cuboctahedron

  19. Theoretical vs. Experimental Spherical Hemispherical

  20. Molecular Modeling: Understanding Disorder • Probe bulk vs. surface relaxation. • Bulk: • Allow relaxation of entire structure. • Surface: • Allow relaxation of atoms bound in surface sites only.

  21. Surface Relaxation Bulk Relaxation • Theoretical: • <d1NN> = 2.74 Å • 2 = 0.0022 Å2 • Experimental: • <d1NN> = 2.753(4) Å • 2 = 0.0017(2) Å2 • Theoretical: • <d1NN> = 2.706 Å • 2 = 0.0003 Å2 • Experimental: • <d1NN> = 2.753(4) Å • 2 = 0.0017(2) Å2 Bond Length Distributions: 10% Pt/C <d1NN>BULK= 2.77 Å <d1NN>FOIL= 2.761(2) Å

  22. Bulk Relaxation Surface Relaxation • Theoretical: • <d1NN> = 2.689 Å • 2 = 0.0002 Å2 • Experimental: • <d1NN> = 2.761(7) Å • 2 = 0.0010(2) Å2 • Theoretical: • <d1NN> = 2.76 Å • 2 = 0.0013 Å2 • Experimental: • <d1NN> = 2.761(7) Å • 2 = 0.0010(2) Å2 Bond Length Distributions: 40% Pt/C <d1NN>BULK= 2.77 Å <d1NN>FOIL= 2.761(2) Å

  23. Future Directions • In-depth modeling of relaxation phenomena. • Further understanding the “nano-phase” behavior of bimetallic • particles. • Polymer matrices as supports and stabilizers for nanoparticles. • Silanes • Hydrogels

  24. Acknowledgments Dr. Ralph Nuzzo Dr. Andy Gewirth Dr. Tom Rauchfuss Dr. John Shapley Dr. Anatoly Frenkel Dr. Michael Nashner Dr. Ray Twesten Dr. Rick Haasch Nuzzo Research Group Funding: Department of Energy Office of Naval Research

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