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 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

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

Electron Microdiffraction [011] [112] [310] 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).

X-Ray Absorption Spectroscopy Absorption coefficient (  ) 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, Absorption Photon Energy

Extended X-ray Absorption Fine Structure oscillation of the X-ray absorption coefficient near and edge local (<10 Å) structure surrounding the absorbing atom EXAFS Pt foil I0I0 ITIT x Pt L 3 edge (11564 eV)

Excitation of a photoelectron with wavenumber k = 2  / h initial final e-e- E0E0 PE = h - E 0 RiRi Oscillations,  i (k): final state interference between outgoing and backscattered photoelectron R i - distance to shell-i A i (k) - backscattering amp. Basics of EXAFS

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

|  (r)|(Å -3 ) r (Å) R 2 R 3 R 4 Pt L 3 edge, Pt foil R 1 Fourier Transform Resolve the scattering from each distance (R i ) into r-space

Multiple-Shell Fit Calculate F i (k) and  i (k) for each shell-i (i = 1 to 6) using the FEFF computer code Non-linear least-square refinement: vary N i, R i,  2 i using the EXAFS equation

Multiple Scattering Paths In-plane atom Above-plane atom Absorbing atom

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.

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.

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

Temperature Dependence Temperature dependence on the extended x-ray absorption spectra for 10% Pt/C. As the temperature increases, the dynamic disorder (  D 2 ) increases, causing the amplitude to decrease.

First Shell Fitting: 10% Pt/C 200 K300 K 473 K 673 K

Size Dependent Scaling of Bond Length and Disorder The EXAFS Disorder,  2, is the sum of the static,  s 2, and dynamic,  d 2, disorder as follows: The dynamic disorder,  d 2, can be separated by using the following relationship:

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

Theoretical vs. Experimental Spherical Hemispherical

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.

Surface Relaxation Theoretical: = 2.74 Å  2 = Å 2 Experimental: = 2.753(4) Å  2 = (2) Å 2 Theoretical: = Å  2 = Å 2 Experimental: = 2.753(4) Å  2 = (2) Å 2 Bulk Relaxation Bond Length Distributions: 10% Pt/C BULK = 2.77 Å FOIL = 2.761(2) Å

Bond Length Distributions: 40% Pt/C Theoretical: = Å  2 = Å 2 Experimental: = 2.761(7) Å  2 = (2) Å 2 Bulk Relaxation Surface Relaxation Theoretical: = 2.76 Å  2 = Å 2 Experimental: = 2.761(7) Å  2 = (2) Å 2 BULK = 2.77 Å FOIL = 2.761(2) Å

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

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