[Supporting Information] Polyhedral Au Nanocrystals Exclusively Bound by {110} Facets: The Rhombic Dodecahedron

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1 [Supporting Information] Polyhedral Au Nanocrystals Exclusively Bound by {110} Facets: The Rhombic Dodecahedron Gyoung Hwa Jeong, Minjung Kim, Young Wook Lee, Wonjun Choi, Won Taek Oh, Q-Han Park, and Sang Woo Han *, Department of Chemistry, Research Institute of Natural Science, and Environmental Biotechnology National Core Research Center, Gyeongsang National University, Jinju , Korea Department of Physics, Korea University, Seoul, , Korea * Corresponding author. swhan@gnu.ac.kr Experimental Chemicals and Materials. HAuCl 4 (Aldrich, 99.9+%), N,N-dimethylformamide (DMF, DAE JUNG, 99.5%), 4-aminobenzenethiol (4-ABT, TCI, 96%), and benzenethiol (BT, TCI, 96%) were all used as received. Other chemicals, unless specified, were reagent grade, and triply distilled water with a resistivity of greater than 18.0 MΩ cm was used in the preparation of aqueous solutions. Preparation of Nanocrystals. In a typical synthesis of the rhombic dodecahedral Au nanocrystals, an aqueous solution of HAuCl 4 (10 mm, 2.0 ml) was added to 23.0 ml of DMF solution. Next, this solution was heated at 90 ~ 95 C for about 15 h in a conventional forced-convection drying oven. The rhombic dodecahedral Au nanocrystals could not be prepared when the reaction time was shorter or longer than 15 h. The shorter reaction time yielded the agglomerated small irregularly shaped particles, and the longer reaction time resulted in the formation of particles with rather complex polyhedral geometries. The resultant Au sol was purified by centrifugation (11,000 rpm for 10 min and then 10,000 rpm for 15 min) and washing with ethanol to remove excess reagents. Characterization of Nanocrystals. The extinction spectra were recorded with Agilent 8453 and Shimadzu UV-3600 spectrophotometers. Scanning electron microscopy (SEM) images of the samples were taken with a field-emission scanning electron microscope (FESEM, Phillips Model XL30 S FEG). High-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) S1

2 characterizations were performed with a JEOL JEM 2100F field-emission transmission electron microscope operating at 200 kv. X-ray diffraction (XRD) patterns were obtained with a Bruker AXS D8 Advance diffractometer using Cu Kα ( nm) radiation. The XRD samples were prepared by dropping 5 µl of a DMF solution of Au nanocrystals onto a silicon substrate and dried for 30 min. This process was repeated more than 3 times. Cyclic voltammetry (CV) measurements were carried out in a threeelectrode cell using a CH Instrument Model 600C potentiostat. The drop-casting films of nanocrystals on indium tin oxide (ITO) substrates served as working electrodes. Before CV measurements, the nanocrystals-modified substrates were cleaned by washing with ethanol. Pt wire and Ag/AgCl (in saturated KCl) were used as the counter and reference electrodes, respectively. All cyclic voltammograms were obtained at room temperature. The electrolyte solutions were purged with high-purity N 2 gas before use for about 20 min. Raman spectra were obtained using a Jobin Yvon/HORIBA LabRAM spectrometer equipped with an integral microscope (Olympus BX 41). The nm line of an air-cooled He/Ne laser was used as an excitation source. Raman scattering was detected with 180 geometry using a thermoelectrically cooled pixel charge coupled device (CCD) detector. The Raman band of a silicon wafer at 520 cm -1 was used to calibrate the spectrometer. The Raman samples were prepared by dropping 30 µl of a M ethanol solution of 4-ABT or BT onto the drop-casting film of nanocrystals on the Si substrate and dried under ambient condition. Theoretical Simulation of Optical Properties. The optical properties of rhombic dodecahedral Au nanocrystals have been computed using the discrete dipole approximation (DDA). S1 Since the extinction spectrum is dependent upon the orientation of the rhombic dodecahedral particle with respect to the incident light, we have computed the extinction spectrum for a particle with edge length of 140 nm for various orientations and taken an average over the solid angle by varying the polar and the azimuth angles by π/10. Dielectric medium was chosen to be DMF and the interval of the wavelength of incident light was 10 nm. The frequency dependent optical constant of Au has been taken from the literature. S2 To understand the nature of peaks in the calculated extinction spectrum, we compared it with the analytic solution of Mie scattering S3 for a similar spherical structure with radius of 114 nm. S2

3 Figure S1. Cyclic voltammograms of (a) rhombic dodecahedral Au nanocrystals and (b) Au nanoplates on ITO electrode with a scan rate of 20 mvs -1 in 0.1 M HClO 4. The CV trace of rhombic dodecahedral Au nanocrystals is distinctly different from that of the Au nanoplates which are enclosed mainly by {111} planes, and correlates well with that obtained from bulk Au(110) single-crystal electrode. S4 The Au nanoplates were prepared by following the literature. S5 S3

4 Figure S2. SEM image of the Au nanocrystals prepared at 135 C. Figure S3. SEM image of the Au nanocrystals prepared in the presence of poly(vinyl pyrrolidone) (PVP, Alfa Aesar, Mw = 630,000, final concentration = 2 mg/ml). S4

5 Figure S4. Surface-enhanced Raman scattering (SERS) spectra of (a) 4-ABT and (b) BT molecules adsorbed on the rhombic dodecahedral Au nanocrystals. The calculated Raman scattering enhancement factors (EF) are and for 4-ABT and BT, respectively. The EFs were calculated by using the following relationship: EF = (I SERS /I Raman )/(N SERS /N Raman ), S6 where I SERS and I Raman are the measured SERS intensity of the adsorbed species on the nanocrystals films and the measured Raman scattering intensity of the analyte molecule in bulk, respectively, and N SERS and N Raman are the number of adsorbates illuminated by the laser light to obtain the corresponding SERS and Raman spectra, respectively. I SERS and I Raman were measured at ~ 1100 cm -1 and N SERS and N Raman were calculated on the basis of the estimate of the concentration of surface species ( mol/cm 2 for 4-ABT S7 and mol/cm 2 for BT S8 ), density of bulk 4-ABT and BT (19.3 g/cm 3 and 1.07 g/cm 3 for 4-ABT and BT, respectively), and the sampling areas (~1 µm in diameter). The calculated EF values are higher than those for isolated spherical Au nanoparticles ( ) S9 in spite of their large particle sizes. The efficient SERS properties of the rhombic dodecahedral nanocrystals can be ascribed to the fact that the nanocrystals are enclosed by highenergy {110} surfaces that are believed to be more SERS active than the other lowindex facets, i.e., {111} and {100} surfaces. S10 S5

6 References (S1) (a) Draine, B. T. Astrophys. J. 1988, 333, 848. (b) Draine, B. T.; Flatau, P. J. J. Opt. Soc. Am. A 1994, 11, (S2) Johnson, P. B.; Christy, R. W. Phys. Rev. B 1972, 6, (S3) Kreibig, U.; Vollmer, M. Optical Properties of Metal Clusters; Springer: New York, (S4) Hamelin, A. J. Electroanal. Chem. 1996, 407, 1. (S5) Lee, K. Y.; Kim, M.; Lee, Y. W.; Choi, M. Y.; Han, S. W. Bull. Korean Chem. Soc. 2007, 28, (S6) Kwon, K.; Lee, K. Y.; Kim, M.; Lee, Y. W.; Heo, J.; Ahn, S. J.; Han, S. W. Chem. Phys. Lett. 2006, 432, 209. (S7) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications (2 nd ed); John Wiley & Sons: New York, 2001; p 565. (S8) Kim, N. H.; Kim, K. Chem. Phys. Lett. 2004, 393, 478. (S9) Wang, H.; Halas, N. J. Adv. Mater. 2008, 20, 82. (S10) Zhang, J.; Li, X.; Sun, X.; Li, Y. J. Phys. Chem. B 2005, 109, S6