Supporting Information

© Wiley-VCH 2007

69451 Weinheim, Germany

High-Yield Synthesis of Single-Crystalline Gold Nanooctahedra

Cuncheng Li, Kevin L. Shuford, Q-Han Park, Weiping Cai, Yue Li, Eun Je Lee, and Sung Oh Cho

24 25 26 27 28 29 30 31 32 33 34 35 36 37 380

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Num

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Edge Length (nm)

A

40 42 44 46 48 50 52 54 56 58 600

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ber

Edge Length (nm)

B

50 52 54 56 58 60 62 64 66 68 700

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Edge Length (nm)

C

Figure S1. The size distribution of Au nanooctahedra obtained at different reaction time. The edge

lengths (average value ± full width at the half maximum) and the reaction time of the Au nanooctahedra

are (A) 30±1 nm at 6 h, (B) 50±2 nm at 24 h, and (C) 60±2 nm at 48 h, respectively. (D) FESEM images

for Au nanooctahedra obtained at 6 h.

1

1000 800 600 400 200 0

Au4d

Inte

nsity

(a.u

.)

Binding enery (eV)

Au4f

C1sN1s

O1s

A

95 90 85 80

83.6 Au4f7/2

Au4f5/2

Inte

nsity

(a.u

.)

Binding enery (eV)

87.3

B

540 535 530 525

O1s

Inte

nsity

(a.u

.)

Binding enery (eV)

532C

405 400 395 390

N1s

Inte

nsity

(a.u

.)

Binding enery (eV)

399.8D

Figure S2. XPS spectra of the repeatedly washed Au nanooctahedra from a survey scan (A), Au 4f (B), O

1s (C), and N 1s (D) energy region. The XPS spectra were referenced to C 1s at 285 eV. The peaks of

both Au 4f7/2 (83.6 eV) and Au 4f5/2 (87.3 eV) shifted by ca. 0.4 eV toward lower binding energies

relative to bulk Au atoms. The peak of O 1s (532 eV) attributed to carboxyl (C=O) oxygen of PVP shifted

by ~0.7 eV to higher binding energy, while the peak of N 1s (399.8 eV) did not shift compared to those of

pure PVP. These verify that PVP molecules strongly interact with Au atoms on the surface of

nanooctahedra.

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Figure S3. The products synthesized at other conditions: in the absence of surfactant PVP (A); the

reaction temperature is 100 oC (B) and 150 oC (C) ; the gold precursor was not preheated at 75 oC (D);

the concentration of NaBH4 is 0 mM (E) and 0.75 mM (F). Scale bars for (A) , (B)-(E), (F) are 2 μm, 500

nm, 100 nm, respectively. The amount of the other reagents and experimental processes were controlled

to be identical to that of the Au nanooctahedra.

In order to demonstrate that Au(0) atoms can be oxidized by Au(III) ions, we performed an experiment

to add Au nanoparticles into AuCl3 PEG 600 solution and investigated the color change of the solution.

Au nanoparticles (2~6 nm, Figure S4A) were synthesized by adding 1.2 mL of 500 mM NaBH4 to 20 mL

AuCl3 PEG 600 solution under stirring at room temperature. The color of Au precursor changed rapidly

from yellow to puce, indicating that Au(0) atoms were directly produced from Au(III) ions due to the

high concentration of NaBH4 (the mole ratio of Au/NaBH4 is 1:6). Before using these Au nanoparticles,

the nanoparticle colloid was kept at 75 oC for more than 24 h to decompose superfluous NaBH4.

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Figure S4. TEM image (A) of the synthesized Au nanoparticles (2 ~ 6 nm in size); FESEM images of the

final products synthesized from sample A (B) and sample B (C), respectively.

When 0.7 mL as-prepared Au colloid was added into 19.6 mL AuCl3 and PEG 600 solution (AuCl3: 0.7

mM, PVP: 100 mM, sample A), the solution color changed from yellow to light puce, which originates

from Au nanoparticles. Subsequently, the solution was heated at 75 oC for 1~2 h and then the solution

became light yellow from light puce, suggesting that the solution mainly consists of Au(III) ions.

Additionally, when 1.4 mL as-prepared Au colloid were added into the solution (sample B), the color of

the solution turned from light puce into colorless after the heating at 75 oC. This indicates that both Au

nanoparticles and Au(III) ions were not the main components in the heated solution. These two results

reflect that Au atoms can be redissolved in the presence of Au(III) ions and that oxidation reaction

between the Au(0) atoms and Au(III) ions can spontaneously occur in our system.

Moreover, high-yield Au nanooctahedra were obtained by further heating sample B at the reaction

temperature of 125 oC (Figure S4C). Whereas, for the sample A, the products had various mixed shapes

(Figure S4B). These results support our argument that almost complete evolution of Au(III) ions into

Au(I) ions before the main reaction is crucial for the selective formation of octahedral Au nanocrystals in

our experiments.

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Numerical Calculations of the Optical Properties

We have computed the optical properties of octahedral gold nanocrystals using the Discrete Dipole

Approximation (DDA). The DDA method solves Maxwell’s equations by partitioning the particle into

small cubic units represented by dipoles.[S1,S2] Each dipole obtains an oscillating polarization from an

incident field, as well as the fields produced from all of the other dipoles in the array. An iterative

procedure is used to calculate the response of the coupled dipole system. The dipole polarizations are then

used to calculate the optical properties of the scattering object. The DDA equations have been reported

numerous times in the literature and therefore not reproduced here. For a more thorough description of the

method including equations, see the work of Draine and coworkers.[S1-S4] The calculations require

frequency dependent optical constants as input. In this work, we have chosen to use experimentally

determined values for gold.[S5]

The optical properties of an octahedron are dependent upon the orientation of the particle with respect

to the incident field. The polarization of the incident field dictates the spectra for the relatively small

particles studied here, while the direction of propagation is less important because the field amplitude

does not vary significantly across the particle. Figure S5 shows the calculated spectrum of a 30 nm

octahedron for two polarizations that are perpendicular to one another. We define the plasmon excitations

as in-plane and out of plane modes, where the aforementioned plane bisects the octahedron into two

equivalent square pyramids. The collective oscillation of electrons parallel to the bisecting plane is

defined as an in-plane mode. The out of plane mode is defined as the collective oscillation of charge

perpendicular to the bisecting plane. This peak red-shifts by ≈ 30 nm and maintains a similar shape when

the octahedron edge length is increased from 30 to 60 nm. Two in-plane modes are present at 567 and 610

nm that both correspond to dipole plasmon excitations. One can imagine two types of in-plane dipole

excitations. They are an edge dipole mode, where the induced polarizations are parallel to the edges of

two neighboring corners, and a cross dipole mode, where the induced polarizations are oriented

diagonally across the square cross section from one corner towards the opposite corner. These two modes

are strongly coupled by mutual interaction, and both are excited simultaneously to some degree when the

incident field is polarized parallel to the bisecting plane. The true induced polarization that occurs is a

combination of the idealized edge and cross dipole states. The degree of mode mixing depends on the

morphology of the particle and polarization of the incident field as will be discussed below.

Panels B-F in Figure S6 display the evolution of the extinction spectrum as the octahedron edge length

is increased from 30 nm to 100 nm. Two incident polarizations that excite in-plane plasmon modes are

shown in each panel. The blue traces have an incident polarization that is parallel to an edge of the

bisecting plane, while the red traces have an incident polarization that is diagonal as depicted in Panel A.

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These orientations have been chosen to preferentially excite the edge dipole mode and the cross dipole

mode, respectively.

Figure S5. Extinction spectrum of a 30 nm octahedron in ethanol. In-plane (out of plane) excitation

occurs when the polarization of the incident field is parallel (perpendicular) to the plane bisecting the

octahedron into square pyramids.

Figure S6. Extinction spectra for Au nanooctahedra with various edge sizes. Panel A displays the

polarization of the incident field. The color of the traces in Panels B-F correspond to the orientation

depicted in Panel A. Note that in some panels the traces are not well resolved.

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Panel B corresponds to a truncated octahedron with a true edge length that is ≈ 24 nm. Octahedra with

truncated tips display only one peak for both incident polarizations as seen in Figure S6 B. It is well

known that large induced polarizations become localized in regions of high curvature, and especially at

tips. Truncating the tip spreads the polarization more evenly around the surface of the particle, which

essentially liminates the interaction of the two modes. Both types of dipole excitations are excited at the

same frequency, yielding a single peak in the spectrum for either incident polarization. Figure S7 shows

the induced polarization and electric field enhancement for the two different orientations at peak

extinction. The induced polarizations are highly uniform, and oriented in a single direction parallel to the

incident field indicating a well-defined dipole excitation. The electric field intensity in Panel C is spread

around the entire surface, and has a maximum enhancement of ≈ 300 occurring at all of the truncated

corners. The field intensity in Panel D shows more localized field enhancements near the corners, with a

maximum intensity of ≈ 600 along the direction of the incident polarization. Note that in this case the

energy carried by the incident field localizes in two regions instead of four leading to greater

enhancements. Figure S7 shows the characteristics of an edge dipole (Figure S7A, C) and a single cross

dipole (Figure S7B, D) in nearly pure states. These vector plots and field patterns can be used to help

analyze the more complex excitations presented below. The calculation results show that no higher-order

multipoles are excited for the nanooctahedra with the sizes from 30 nm to 60 nm.

The mode coupling is substantial in nanoparticles with perfect octahedral shapes, that is, the tips of the

octahedron are not truncated. A perfect 30 nm octahedron produces two peaks in the spectrum as seen in

Figure S6C. The highly polarized regions localized at the tips induce coupling between the two in-plane

modes and thus if one mode is excited, the other mode is also generated. The coupling becomes

maximum when the edge length of octahedron is increased to 60 nm. This can be inferred from the

approximately equal intensities in the spectrum (Figure S6D) and the field patterns that can be seen in

Figure S8. The field enhancements at 582 nm are similar to the edge dipole pattern (Figure S7C), where

the intensity is less localized and distributed more uniformly around the surface. However, a vector plot

of the induced polarization presented in Figure S9A shows that the hot spots on the corners are dipole

moments that are oriented diagonally indicating cross dipole character in addition to edge dipole. These

diagonally oriented polarizations are extremely intense for the resonance at 623 nm as is clearly shown in

the field enhancements in Figure S8C, D. However, again a vector plot of the induced polarization in

Figure S9B shows that some intense dipole moments close to the corners actually do align vertically

indicating edge dipole character in addition to cross dipole character. It is also interesting to note that the

field patterns at interior points of the nanoparticle are consistent for each wavelength regardless of the

incident polarization further supporting the idea that the resulting plasmon excitations are combinations of

the edge and cross dipoles.

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Figure S7. Properties of a nanooctahedron with a 30 nm edge length (truncated) at 572 nm. The black

(white) arrows indicate the polarization of the incident field. Panels A and B are the induced polarizations

and Panels C and D are electric field intensity.

Figure S6 implies that to some extent the polarization of the incident field can be rotated to selectively

excite a particular in-plane dipole mode. However, the presence of two peaks in the spectrum indicates

that a complete decoupling of the modes is not occurring. The incident fields chosen to selectively excite

a particular dipole mode excites both the edge and cross in-plane modes. This strongly suggests mode

mixing and the coupled mode interpretation presented above. It should also be noted that in addition to

incident polarization and truncation, the size of the nanoparticle affects the coupling. Larger octahedron

favors a state more similar to the cross dipole. This is because the induced polarization becomes localized

at the tips almost exclusively, and these dipoles have a propensity to orient diagonally across the square

cross section. As a result, the amount of edge dipole character decreases and the amount of cross dipole

character increases. This trend can be observed in Figure S6 as the intensity of the blue peak decreasing

and the intensity of the red peak increasing, as well as the relative intensities of both spectral features with

regard to incident polarization.

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Figure S8. Electric field intensity of an Au octahedron with a 60 nm edge length. The white arrows

indicate the polarization of the incident field.

Figure S9. Induced polarization of a 60 nm Au octahedron. The black arrows indicate the polarization

of the incident field. Panel A corresponds to the field intensity displayed in Figure S8A, and Panel B

corresponds to the field intensity displayed in Figure S8D.

References [S1] B. T. Draine, Astrophys. J. 1988, 333, 848.

[S2] B. T. Draine, P. J. Flatau, J. Opt. Soc. Am. A 1994, 11, 1491.

[S3] B. T. Draine, J. Goodman, Astrophys. J. 1993, 405, 685.

[S4] J. J. Goodman, B. T. Draine, P. J. Flatau, Opt. Lett. 1991, 16, 1198.

[S5] P. B. Johnson, R. W. Christy, Phys. Rev. B 1972, 6, 4370.

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