Supporting Information Wiley-VCH 2007 69451 Weinheim, Germany
Chemical Sharpening of Gold Nanorods: The Rod-to-Octahedron Transition Enrique Carbó-Argibay, Benito Rodríguez-González, Jessica Pacifico, Isabel Pastoriza-Santos*, Jorge Pérez-Juste, and Luis M. Liz-Marzán* Departamento de Química Física and Unidad Asociada CISC Universidade de Vigo, 36310, Vigo, Spain, pastoriza@uvigo.es; lmarzan@uvigo.es
Heating Experiments. Since Au nanorods are sensitive to thermal heating, that is, they can easily reshape into spherical particles by simply heating up to moderate temperatures, 1 gold nanorod dispersions in DMF were heated at different temperatures to identify an appropriate reaction temperature, at which no significant reshaping was observed. The temperature was tuned by variation of the sonicating power of the ultrasonic homogenizer. Shown below is the time evolution of the UV-visible absorption spectra of a nanorod dispersion in DMF ([PVP]=2.5 mm) at 75, 85, 95 and 105 ºC. From analysis of the spectra, it is clear that thermal heating produces a blue shift of the LSP band due to reshaping. Additionally, we observe that at lower temperatures, the process is notably slower. Whereas at 105 ºC the LSP band shifted around 200 nm in 200 min, at 75 ºC the shift in the same time period amounts to just 17 nm (see summary plot in Figure S2). 3 Temperature=75ºC Temperature=85ºC Absorbance 2 0' 20' 40' 70' 110' 200' 1 0 3 Absorbance 2 Temperature=95ºC Temperature=105ºC 1 0 400 600 800 Wavelength (nm) 400 600 800 1000 Wavelength (nm) Figure S1. Time evolution of the UV-visible absorption spectra of a nanorod dispersion in DMF ([PVP]=2.5 mm) at 75, 85, 95 and 105 ºC, as indicated. 1 H. Petrova, J. Perez-Juste, I. Pastoriza-Santos, G. V. Hartland, L. M. Liz-Marzán, P. Mulvaney, Phys. Chem. Chem. Phys. 2006, 8, 814.
Shift of the LSPR (nm) 200 175 150 125 100 75 50 25 0 Time=200 minutes 75 80 85 90 95 100 105 Temperature (C) Figure S2. Blue Shift of the LSP band as a function of the heating temperature.
Table 1. Dimensions (length and width), aspect ratio and morphology (TEM image) of the final products obtained after growing Au nanorods using different R ([HAuCl 4 ]/[Au seed]) values. The HAuCl 4 concentration is indicated. * The length and the width were determined from the side length of the octahedrons. *
Structural Analysis of Sharpened Rods. TEM Tilting experiments carried out on sharpened rods prepared with R=8.2 revealed a nearly square cross-section (Figure 4 in the manuscript). This morphology can in principle give rise to two possible interpretations of the electron diffraction pattern shown in Figure 3: a) the particle presents four lateral {110} faces and two tips enclosed by four {111} faces (Figure S3, right) and is lying on the substrate with one of the {110} faces; or b) the particle presents two tips enclosed by {111} faces, but it is enclosed by four {100} lateral faces (Figure S3, left) and it is lying on a edge parallel to [001] direction. Although both models agree with the measured tip aperture angle (71º), the latter is obviously less probable, since it assumes particles lying on an edge. Additionally, based on the following comments we can absolutely discard this second option: In TEM, most of the particles display fringes of equal-thickness contours at the tips (see Figure S4), indicating that the thickness of the tip increases from the apex to the particle body, where the contrast is homogeneous, revealing uniform thickness in the central part, which can only correspond to a particle lying on a face (not an edge). A particle lying on an edge would present concentric fringes as shown in the particle at the right-hand side of Figure S4. A final evidence against interpretation b is presented on Figure S5, where rotation of the particle along its long axis in ± 45º is shown to give rise to an increase of both the observed thickness and the tip aperture angle (from ca. 73º to 90º). Although in both models the ± 45º rotation of the particle should lead to a similar change in tip aperture angle, only in model a (see Figure S5, middle panel) an increase in the particle width is expected. In model b, the same tilting would produce a decrease of the particle width as sketched in the bottom panel of Figure S5. Figure S3. Cartoon of the only two possible structural models for gold nanorods with sharp tips, which are in accordance with the electron diffraction pattern (SAED) obtained from the nanoparticles.
Figure S4. High-resolution TEM image of two sharpened rods. One of them (left) is resting on a face (a tip aperture of ca 71º and fringes of equal-thickness contours parallel to the particle body are observed at the tips), while the other (right) is resting on an edge, as indicated by a tip aperture of ca 90º and fringes of equal-thickness contours concentric to the particle body. Scale bar is 20 nm.
Figure S5. (a-c) TEM images of a rod grown with R=8.2, tilted around the [001] zone axis, with tilt angles as indicated in the images. (d-f) Model a tilting around [001] zone axis, showing an increase in the particle thickness and tip aperture angle (from ca. 70º up to 90º). (g-i) Model b tilting around [001] zone axis, resulting in a decrease of particle thickness.
Structural Analysis of the Octahedrons. Although the octahedral morphology has been clearly evidenced by SEM and AFM images (see Figure 5 in the manuscript), TEM tilting and SAED experiments were carried out to corroborate those results. Figure S6 (top panel) shows an apparently hexagonal nanoparticle, which is actually an octahedron oriented in the [111] zone axis as revealed by the corresponding SAED pattern. Additionally, the SAED analysis also shows that the octahedron is single crystal with all the facets of {111} type. When the same particle is tilted by 19.5º around the ( 220) direction, that is, maintaining the reciprocal lattice common tilt axis, a new particle profile is observed (Figure S6, bottom panel) which corresponds to an octahedron orientated in the [112] zone axis (as evidenced by SAED). Therefore, 19.5º is the experimental angle observed between [111] and [112] zone axes, which is in agreement with the theoretical tilt angle (19.47º) predicted for an octahedron. Figure S6. Cartoon (left), TEM image (centre) and corresponding diffraction pattern (right) for an octahedron oriented in the [111] (top panel) or [112] (bottom panel) zone axis.