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Supporting Online Material for

Carving at the Nanoscale: Sequential Galvanic Exchange and

Kirkendall Growth at Room Temperature
Edgar González, Jordi Arbiol, Víctor F. Puntes*

*To whom correspondence should be addressed. E-mail: victor.puntes@icn.cat

Published 9 December 2011, Science 334, 1377 (2011)

DOI: 10.1126/science.1212822

This PDF file includes:

Materials and Methods

SOM Text
Figs. S1 to S5
Tables S1 and S2
References
 Supporting Online Material for
Carving at the Nanoscale: Sequential Galvanic Exchange and Kirkendall
Growth at Room Temperature

Edgar González1,2,#, Jordi Arbiol3,4 and Víctor F. Puntes1,2,4,5

1
Institut Català de Nanotecnologia (ICN), Campus UAB, 08193 Bellaterra, Spain.
2
Universitat Autònoma de Barcelona (UAB), Campus UAB, 08193 Bellaterra, Spain.
3
Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), Campus UAB, 08193 Bellaterra, Spain.
4
Institució Catalana de Recerca i Estudis Avançats (ICREA), 08010 Barcelona, Spain.
5
Centre d'Investigacions en Nanociència i Nanotecnologia CIN2 (ICN-CSIC), Campus UAB, 08193
Bellaterra, Spain.

#
Current address: Sección de Nanociencia y Nanotecnología, Instituto Geofísico, Pontificia Universidad
Javeriana, 110231 Bogotá, Colombia.

correspondence to: victor.puntes@icn.cat

This PDF file includes:

Materials and Methods

• Materials: chemicals utilized in the synthetic procedures
• Methods: Synthesis of hollow nanoparticles
• Methods: Production of PdAuAg double-walled nanoboxes
• Methods: Synthesis protocol of complex polymetallic hollow nanoparticles
by sequential Galvanic Replacement and Kinkerdall effect
• Methods: Microscopy Analysis
SOM Text
• Calculation of diffusion times
Figs. S1 to S5
Tables S1 to S2

1
Materials and Methods

Materials: chemicals utilized in the synthetic procedures

Ethylene glycol, anhydrous 99.8%, polyvinylpyrrolidone K 30, hexadecyltrimethyl-

ammonium bromide, benzyldodecyldimethylammonium chloride, L-ascorbic acid 99+%,
silver nitrate ReagentPlus, > 99.8% , sodium sulfide nonahydrate, 99.99+% metal basis
and gold (III) chloride hydrate, 99.999% were purchased from Sigma Aldrich.

Methods: Synthesis of hollow nanoparticles

Silver nanoparticles used as a template for production of hollow nanostructures were

synthesized using the polyol protocol (25). The preparation of bimetallic nanoparticles at
room temperature comprised the steps of: a) adding cetylammonium bromide and
ascorbic acid into an aqueous medium comprising nanoparticles of a first noble metal, b)
adding a salt of a second noble metal in a molar concentration from 30 to 100 µM and
with a flow rate of from 20to 50 µl/min, wherein the second noble metal had a higher
reduction potential than the first noble metal, and wherein the first and the second noble
metals had different diffusion coefficients, and c) isolating the obtained nanoparticles.

For the preparation of trimetallic nanoparticles, the process for the preparation of
bimetallic nanoparticles was carried out and a salt of a third noble metal in a molar
concentration of from 3.5x10-3 M to 4x10-3 M and with a flow rate of from 200 µl/min to
270 µl/min was further added before step c).

When preparing bimetallic nanoparticles in the form of nanocages, the process

ensured the formation of pores with control in the size and localization. This may be an
important condition for its use as carrier and delivery system, and on the other hand
allows a fine modulation of the optical response. For the preparation of bimetallic
nanoparticles in the form of nanocages at room temperature the process comprised the
steps of: a) adding benzyldodecyldimethylammonium chloride in a molar concentration
of from 3 x10-2 to 3.5 x10-2 M in an aqueous medium comprising nanoparticles of a first
noble metal, b) adding a salt of a second noble metal in a molar concentration of from 1.4
x10-4 to 3 x10-4 M and with a flow of from 100 to 200 µl/min, herein the second noble
metal had a higher reduction potential than the first noble metal, and the first and the
second noble metals had different diffusion coefficients, and c) isolating the obtained
nanoparticles

Methods: Production of PdAuAg double-walled nanoboxes

Trimetallic structures of palladium-gold-silver can be produced by the same method

for Au-Ag double-walled nanoboxes. Gold was added and a few seconds after palladium
was also added. The final product is a double-walled nanobox with a core of silver in the
center, which indicates that silver drain was isotropic towards the center. The core of
silver formed in the center of the box can disappear with greater exposure to an oxidizing

2
environment. By studying metal distribution characterization from EDX and Z-contrast
spectra, we find that gold and silver form an alloy with an Ag-rich inner part. Palladium
is located preferentially on the surfaces of the walls, which is advantageous for catalytic
applications. Plasmon resonance presents a redshift to 670 nm during the first 2 minutes,
and then shifts to 590 nm after 40 minutes. This value is close to that obtained for Au-Ag
double-walled nanoboxes.

Methods: Synthesis protocol of complex polymetallic hollow nanoparticles by sequential

Galvanic Replacement and Kinkerdall effect

It is of interest to explore routes that allow better control over the interior
morphology of the nanoparticle, to enable us to increase the levels of complexity that we
have achieved with the production of double-walled structures. To do this, we designed a
synthetic protocol of a similar type, where in the first step a pattern is carved. This
geometric pattern - which is formed by cavities symmetrically distributed - is support for
a second process of carving, although finer (thin enveloping cavities) than the previous.

The Scheme below (Scheme A) shows the synthesis protocol that we developed.
First of all, a metal nanoparticle (template) that can be oxidized using galvanic
replacement is required. In our specific case of silver nanoparticles, to obtain complex
polymetallic hollow structures two stages are required:

Stage I: Control agents (CA(I)) and metal salt precursor (M(I)) with higher reduction
potential of the template material, are necessary to "carve" a pattern in the
nanoparticle. The chemical environment must be suitable for initial oxidation to
occur simultaneously at specific sites, e.g., corners or center of the faces. From these
"starters" cavities can be obtained, where the topology and size are determined by
concentration and reaction time (control parameters).

Stage II: Once formed, the interior hollow pattern structure that serves as support for
a second carving, metal salts (M(II)), and control agents (CA(II)) that smooth and
produce new and finer cavities, are added. The composition of the particle changes
with respect to stage I, and in some cases, changes occur in the original topology.

With this method we have obtained interesting hollow nanoparticles with novel
topologies for noble metals such as silver, gold, palladium, and platinum. Since silver has
the lowest reduction potential, it is used as the sacrificial template, while a palladium salt
that favors the production of symmetrical and segmented cavities (this behavior is also
observed with platinum salt) may be used as an oxidizing agent. For the second stage, a
fine "carve" can be done with gold salt in the presence of CTAB and ascorbic acid to
allow reduction of AuCl4− to AuCl2− and dissolve the silver chloride.

3
Scheme A. Synthesis protocol of complex polymetallic hollow nanoparticles by
sequential Galvanic Replacement and Kinkerdall effect. CA represents controlling agent
and M represents Metal. In this case CA(I) corresponds to CTAB, CA(II) corresponds to
AA, M(I) corresponds to Pd and M(II) corresponds to Gold.

Methods: Microscopy Analysis

Conventional and high-resolution transmission electron microscopy analyses were

performed on a Jeol2010F microscope with a field emission gun and a point to point
resolution of 0.19 nm working at 200 keV. For the HAADF STEM and EDX mapping a
FEI Tecnai F30 field emission gun microscope working at 300 keV was used. The
absorbance spectra were recorded using a Shimadzu UV-2401 PC spectrophotometer.

SOM Text

Calculation of diffusion times

In studies reported in (22) about spontaneous alloying of Au-Ag nanoparticles at

room temperature, the role of defect density (particularly vacancies) at the gold-silver
interface was considered as an explanation for faster than expected diffusion. Such
defects may originate from the curvature of the surfaces and by replacement of the
capping agents at the surface during the formation of the gold layer. The Br− that forms
AgBr and is deposited on the surface of the template can play an important role in this
consideration. The activation barrier for diffusion is given by: ∆H = ∆Hf + ∆Hm, where
∆Hf is the activation barrier for the creation of the defect, and ∆Hm is the activation
barrier for the migration. For Au and Ag (26): ∆Hf = 1.0 ± 0.15 eV and ∆Hm = 0.75 ±

4
0.10 eV therefore ∆H = 1.76 eV. So, the diffusion coefficient at room temperature is
increased to 10-19-10-20 m2/s (instead of 10-24 as in the bulk). Thus, for calculations of
nanoscale systems, it is necessary to be careful in using the values of diffusion
coefficients taken from the bulk. To illustrate this, consider the case of the diffusion
process of the Cu-Ni film couple. At 300 °C the diffusion coefficient of Cu in Ni is
3.8×10-28 m2/s. With this value, the interdiffusion time for a 100 nm thick Ni film would
be ~6×1012 s (2.09×105 years!). This value does not correspond with that obtained by
experiments, where in less than one hour changes of film reflectivity are observed,
indicative of the effective diffusion of Cu in Ni. In those situations the diffusion
coefficient on a nanometric scale should be several orders of magnitude higher than that
used for bulk-type systems.

5
 (A) (B)

Fig. S1. Scalability. (A) Large volumes synthesized of nanoboxes colloid; (B) TEM
image of AuAg nanoboxes synthesized in large volumes. By scalable we mean that it can
be scaled up readily since there are no T gradients involved in these protocols; the mass
gradients versus reaction kinetics allow the mixture of large volumes of reactants if fast
stirring is used and the reaction ends when the nanostructures are completely gold coated
and all Au3+ has been exhausted. By applying these principles, the synthesis has been
scaled from 10 to 1000 ml.

6
Fig. S2. Solubility of AgCl. The photograph shows a sample of crystals of silver chloride
(left), which are completely solubilized in an aqueous solution of CTAB [0.1 M] (right).

7
 (A) (B)

Fig. S3. Evolution of size in the transition from silver nanocubes to AuAg double-
walled nanoboxes. (A) Lengths of the main geometric elements of the Au/Ag
nanoboxes; (B) change of length of edge (exterior wall) in the transition from silver
nanocube to Au/Ag nanobox. The TEM image in Figure S1 shows the lengths of the main
geometric elements of the nanoboxes. The mean length of the edge of the exterior wall is
approximately L~ 59 nm, the width of the inner wall Si ~ 5.5 nm, the width of the outer
wall Se ~ 7 nm, the length of the diagonal D ~ 76 nm, the gap between the walls is e ~
1.5 - 2.0 nm, which is an interesting value for the study of quantum confinement. In the
double-walled nanoboxes, the outer wall thickness is approximately 1/6 (17%) of the
lateral dimension of the silver nanocubes (template).

8
Fig. S4. EDX spectra of Au/Ag double-walled nanoboxes.

9
 (A)

(B)

Figure S5. XRD profiles of AuAg double-walled nanoboxes. (A) XRD profile of silver
nanocubes used as template in the synthesis of gold/silver nanoboxes. All the peaks can
be well indexed to face-centered cubic silver. (B) XRD profile of Au-Ag nanoboxes.

10
Table S1. Abundance of nanostructures in a typical sample. Nanoparticles have been
compared with the model (2nd. column) identifying total number of facets, angles and
distribution of pores, with some degree of tolerance as can be inferred from the images
presented in the TEM Gallery (Table S2). To calculate de abundance (4th. column) of
each nanostructure (1st. column), at least 150 particles have been randomly chosen from
random TEM images. Please note we considered only TEM images of the samples where
the particles ended up oriented and apart enough to be clearly distinguishable one from
another (the 3rd. colum shows an example of such images).

Nanostruture Scheme TEM image Abundance

62±7 %

43±6%

45±5%

53±6%

11
61±7%

58±5%

49± 5%

88±9%

91±8%

52±6%

12
55±6%

67±5%

45±5%

55±6%

30±4%

32±4%

29±4%

13
38±4%

80±7%

20±3 %

14
Table S2. TEM Gallery.

15
16
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