Selective synthesis of single-crystalline rhombic dodecahedral, octahedral, and cubic gold nanocrystals

Article abstract of DOI:10.1021/ja804115r

This paper reports a versatile seed-mediated growth method for selectively synthesizing singlecrystalline rhombic dodecahedral, octahedral, and cubic gold nanocrystals. In the seed-mediated growth method, cetylpyridinium chloride (CPC) and CPC-capped sing

Full text of DOI:10.1021/ja804115r

Published on Web 12/22/2008
Selective Synthesis of Single-Crystalline Rhombic
Dodecahedral, Octahedral, and Cubic Gold Nanocrystals
Wenxin Niu,†,‡ Shanliang Zheng,§ Dawei Wang,†,‡ Xiaoqing Liu,†,‡ Haijuan Li,†,‡
Shuang Han,†,‡ Jiuan Chen,†,‡ Zhiyong Tang,*,§ and Guobao Xu*,†
State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry,
and Graduate UniVersity of the Chinese Academy of Sciences, Chinese Academy of Sciences,
Changchun 130022, China, and National Center for Nanoscience and Technology,
Beijing 100190, China
Received June 6, 2008; E-mail: zytang@nanoctr.cn.; guobaoxu@ciac.jl.cn
Abstract: This paper reports a versatile seed-mediated growth method for selectively synthesizing single-
crystalline rhombic dodecahedral, octahedral, and cubic gold nanocrystals. In the seed-mediated growth
method, cetylpyridinium chloride (CPC) and CPC-capped single-crystalline gold nanocrystals 41.3 nm in
size are used as the surfactant and seeds, respectively. The CPC-capped gold seeds can avoid twinning
during the growth process, which enables us to study the correlations between the growth conditions and
the shapes of the gold nanocrystals. Surface-energy and kinetic considerations are taken into account to
understand the formation mechanisms of the single-crystalline gold nanocrystals with varying shapes. CPC
surfactants are found to alter the surface energies of gold facets in the order {100} > {110} > {111} under
the growth conditions in this study, whereas the growth kinetics leads to the formation of thermodynamically
-
less favored shapes that are not bounded by the most stable facets. The competition between AuCl4
reduction and the CPC capping process on the {111} and {110} facets of gold nanocrystals plays an important
role in the formation of the rhombic dodecahedral (RD) and octahedral gold nanocrystals. Octahedral
nanocrystals are formed when the capping of CPC on {111} facets dominates, while RD nanocrystals are
-
formed when the reduction of AuCl4 on {111} facets dominates. Cubic gold nanocrystals are formed by
the introduction of bromide ions in the presence of CPC. The cooperative work of cetylpyridinium and
bromide ions can stabilize the gold {100} facet under the growth condition in this study, thereby leading to
the formation of cubic gold nanocrystals.
1. Introduction
Synthesis of gold nanocrystals bounded by {110} facets has
seldom been reported because the surface energy of the bare
{110} facets is higher than those of bare {111} and {100}
facets.11 In principle, the shape of single-crystalline gold crystals
bounded completely by equivalent {110} facets is supposed to
be rhombic dodecahedral (RD),12,13 i.e., such a crystal would
be a convex polyhedron with 12 rhombic faces that has eight
vertices where three faces meet at their obtuse angles and six
vertices where four faces meet at their acute angles (Figure 1).14
RD gold crystals have been observed in minerals,12 and RD
gold nano- and microcrystals have also been observed in
chemical syntheses.15,16 However, the rational synthesis of
uniform RD gold nanocrystals in high yield has not been
reported to date and is still a challenge that requires exquisite
crystal-growth control.
The properties of noble-metal nanocrystals are dictated by
their sizes and shapes.1-3 Therefore, size- and shape-controlled
synthesis of noble-metal nanocrystals is essential for uncovering
and understanding their intrinsic properties, which will conse-
quently enable us to exploit them for catalytic, plasmonic,
sensing, and spectroscopic applications.1-6 Over the past decade,
great progress has been made in crystallographic control over
the nucleation and growth of gold nanocrystals. Most efforts
thus far have been focused on the synthesis of octahedral and
cubic gold nanocrystals bounded by {111} and {100} facets.7-10
† Changchun Institute of Applied Chemistry.
‡ Graduate University of the Chinese Academy of Sciences.
§ National Center for Nanoscience and Technology.
Although several parameters have been revealed to greatly
affect the growth mode of metal nanocrystals, the exact
mechanisms for the shape-controlled synthesis of metal nanoc-
(1) Murphy, C. J.; Sau, T. K.; Gole, A. M.; Orendorff, C. J.; Gao, J.;
Gou, L.; Hunyadi, S. E.; Li, T. J. Phys. Chem. B 2005, 109, 13857.
(2) Wiley, B.; Sun, Y.; Xia, Y. Acc. Chem. Res. 2007, 40, 1067.
(3) Tao, A. R.; Habas, S.; Yang, P. Small 2008, 4, 310.
(4) Narayanan, R.; El-Sayed, M. A. J. Am. Chem. Soc. 2004, 126, 7194.
(5) Liz-Marzan, L. M. Langmuir 2006, 22, 32.
(11) Wang, Z. L. J. Phys. Chem. B 2000, 104, 1153.
(12) Taber, S. Am. Mineral. 1948, 33, 482.
(6) Rosi, N. L.; Mirkin, C. A. Chem. ReV. 2005, 105, 1547.
(7) Kim, F.; Connor, S.; Song, H.; Kuykendall, T.; Yang, P. Angew. Chem.,
Int. Ed. 2004, 43, 3673.
(13) Francis, C. A. Rocks Miner. 2004, 79, 24.
(14) Weisstein, E. W. CRC Concise Encyclopedia of Mathematics, 2nd
ed.; CRC Press: Boca Raton, FL, 2003; p 2546.
(8) Sau, T. K.; Murphy, C. J. J. Am. Chem. Soc. 2004, 126, 8648.
(9) Seo, D.; Park, J. C.; Song, H. J. Am. Chem. Soc. 2006, 128, 14863.
(10) Li, C.; Shuford, K. L.; Park, Q.-H.; Cai, W.; Li, Y.; Lee, E. J.; Cho,
O. C. Angew. Chem., Int. Ed. 2007, 46, 3264.
(15) Liu, X.; Wu, N.; Wunsch, B. H., Jr.; Stellacci, F. Small 2006, 2, 1046.
(16) Chen, Y.; Gu, X.; Nie, C.-G.; Jiang, Z.-Y.; Xie, Z.-X.; Lin, C.-J. Chem.
Commun. 2005, 4181.
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A R T I C L E S
Niu et al.
that affect the formation of the gold nanocrystals with varying
shapes. Several important parameters that affect the shape
evolutions of the gold nanocrystals are revealed, and the growth
mechanisms are explained in terms of surface energy and growth
kinetics.
2. Experimental Section
2.1. Materials. Chloroauric acid tetrahydrate (HAuCl4 ·4H2O),
sodium borohydride (NaBH4), and cetyltrimethylammonium bro-
mide (CTAB) were obtained from Sinopharm Chemical Reagent
Co., Ltd. L-Ascorbic acid, silver nitrate (AgNO3), and potassium
bromide (KBr) were obtained from Beijing Chemical Reagent
Company. CPC was obtained from Shanghai Sangon Company.
All of the chemicals were of analytical grade and used without
further purification. Doubly distilled water was used throughout
the experiments.
Figure 1. Geometrical models of rhombic dodecahedra: (A) overall view;
(B) view perpendicular to one of the rhombic faces; (C) view centered on
one of the vertices where three faces meet at their obtuse angles; (D) view
centered on one of the vertices where four faces meet at their acute angles.
2.2. Synthesis of Gold Nanorods. 2.2.1. Preparation of
∼1.5 nm CTAB-Capped Gold Seeds. A 125 µL aliquot of 10
mM HAuCl4 solution was added to 5 mL of 100 mM CTAB
solution at 30 °C. After this combination was gently mixed, 0.3
mL of 10 mM ice-cold NaBH4 solution was added all at once,
followed by rapid inversion mixing for 2 min.22,23 The CTAB-
capped gold seed solution was stored at 30 °C for future use.
2.2.2. Synthesis of Gold Nanorods. In sequence, 2 mL of 10
mM HAuCl4 solution, 240 µL of 10 mM AgNO3 solution, 320 µL
of freshly prepared 100 mM ascorbic acid solution, and 48 µL of
the ∼1.5 nm CTAB-capped gold seed solution were added to 40
mL of 100 mM CTAB solution at 30 °C. The solution was
thoroughly mixed after each addition.22,23 Finally, the gold nanorod
solution was left undisturbed and aged for 2 h for further use.
2.3. Synthesis of the CPC-Capped Gold Seeds from Gold
Nanorods. 2.3.1. Secondary Overgrowth of Gold Nanorods. A
30 mL aliquot of the as-synthesized gold nanorod solution was
centrifuged (12 000 rpm, 10 min) and redispersed in water.
Subsequently, the solution was centrifuged (12 000 rpm, 10 min)
again and redispersed in 30 mL of 10 mM CTAB solution at 40
°C. Lastly, 1.5 mL of 10 mM HAuCl4 solution and 0.3 mL of 100
mM ascorbic acid solution were added in sequence and mixed
thoroughly. The mixture was allowed to react at 40 °C for 1 h.
2.3.2. Synthesis of the CPC-Capped Gold Seeds. The CPC-
capped gold seeds were prepared by transformation of the over-
grown gold nanorods to near-spherical nanoparticles.24 Briefly, the
overgrown gold nanorod solution was centrifuged (12 000 rpm, 10
min) and redispersed in 30 mL of 10 mM CTAB solution. Next, at
40 °C, 0.6 mL of 10 mM HAuCl4 solution was added. After the
solution was gently mixed, it was left undisturbed and aged for
12 h. The solution was then washed three times with 100 mM CPC
solution by centrifugation (12 000 rpm, 10 min) and dissolution
and finally dispersed in 30 mL of 100 mM CPC solution. We
designated these nanoparticles as the CPC-capped seeds.
rystals are still not well-understood.3 Most of the synthetic
methods remain empirical, and understanding their growth
mechanisms is still a challenging task. The seed-mediated
growth method separates the nucleation and growth stages of
nanocrystals, which provides better control over the size, size
distribution, and shape evolution of the nanoparticles.1,17
A
typical seed-mediated growth process involves the preparation
of small metal nanoparticles and their subsequent growth in
reaction solutions containing metal salts, reducing agents, and
surfactants. During the growth procedure, the crystal structures
of the metal nanocrystals grown from small seeds are fluctuant.18
The small seeds may grow into single-crystalline, twinned, or
multitwinned structures,19 leading to the formation of polydis-
perse nanostructures. The shape evolution of metal nanocrystals
from seeds with different crystal structures is quite different,20,21
which blurs our understanding of the mechanisms of the growth
process. To preserve the single-crystalline nature of the seeds
during the seed-mediated growth process, one has to judiciously
select appropriate adsorbents20,22 or manipulate the growth
kinetics.8 Therefore, development of a general strategy that can
selectively produce single-crystalline metal nanocrystals without
the need for elaborate selection of appropriate adsorbents or
manipulation of the growth kinetics could enable us to study
the detailed correlations between growth conditions and the
shapes of the single-crystalline nanocrystals.
In this study, we developed a versatile seed-mediated growth
method to selectively synthesize single-crystalline RD, octahe-
dral, and cubic gold nanocrystals through manipulation of the
growth kinetics and selection of appropriate adsorbates. Single-
crystalline gold nanocrystals with diameters of 41.3 nm and
capped by cetylpyridinium chloride (CPC) were prepared and
explored as the seeds in the seed-mediated growth method. The
single-crystalline nature and relatively large sizes of the CPC-
capped seeds can fix the structure of the final nanocrystals as
single-crystalline, enabling us to carefully study the parameters
2.4. Seed-Mediated Growth of Three Types of Gold
Nanocrystals. In a typical synthesis of the RD gold nanocrystals,
100 µL of 10 mM HAuCl4 solution, 200 µL of freshly prepared
100 mM ascorbic acid solution, and 200 µL of the CPC-capped
seed solution were added to 5 mL of 10 mM CPC solution at 30
°C in sequence. The solution was thoroughly mixed after each
addition. The reaction was stopped after 2 h by centrifugation
(12 000 rpm, 10 min). The gold nanocrystal solution was washed
twice with water and concentrated for characterization by electron
microscopy.
In a typical synthesis of the octahedral gold nanocrystals, 100
µL of 10 mM HAuCl4 solution, 13 µL of freshly prepared 100
mM ascorbic acid solution, and 200 µL of the CPC-capped seed
solution were added to 5 mL of 100 mM CPC solution at 30 °C in
(17) Murphy, C. J.; Gole, A. M.; Hunyadi, S. E.; Orendorff, C. J. Inorg.
Chem. 2006, 45, 7544.
(18) Elechiguerra, J. L.; Reyes-Gasga, J.; Yacaman, J. M. J. Mater. Chem.
2006, 16, 3906.
(19) Wu, H.-Y.; Huang, W.-L.; Huang, M. H. Cryst. Growth Des. 2007,
7, 831.
(20) Liu, M.; Guyot-Sionnest, G. J. Phys. Chem. B 2005, 109, 22192.
(21) Lim, B.; Xiong, Y.; Xia, Y. Angew. Chem., Int. Ed. 2007, 46, 9279.
(22) Nikoobakht, B.; El-Sayed, M. A. Chem. Mater. 2003, 15, 1957.
(23) Sau, T. K.; Murphy, C. J. Langmuir 2004, 20, 6414.
(24) Rodriguez-Fernandez, J.; Perez-Juste, J.; Mulvaney, P.; Liz-Marzan,
L. M. J. Phys. Chem. B 2005, 109, 14257.
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Dodecahedral, Octahedral, and Cubic Gold Nanocrystals
A R T I C L E S
Figure 3. (A) TEM image of the CPC-capped gold seeds. (B) TEM image
and corresponding SAED pattern of a single CPC-capped seed. (C) HRTEM
image of the CPC-capped seed in (B).
Figure 2. SEM images of (A) the gold nanorods, (B) the overgrown gold
nanorods, and (C) the CPC-capped gold seeds (scale bars: 200 nm). (D)
UV-vis extinction spectra of (a) the gold nanorods, (b) the overgrown gold
nanorods, and (c) the CPC-capped gold seeds (each spectrum is normalized
against the intensity of its strongest peak).
capped gold seeds have an average diameter of 41.3 nm and a
size distribution of 5.4%. The detailed morphology and struc-
tures of the CPC-capped seeds were studied through TEM.
Figure 3A shows that the shapes of the CPC-capped gold seeds
are slightly ellipsoidal rather than perfectly spherical. Therefore,
the average diameter of the seeds is bigger than the average
width of the overgrown nanorods from which they originate.
The SAED pattern and high-resolution TEM (HRTEM) image
of a single CPC-capped seed demonstrate that the CPC-capped
seeds are single-crystalline (Figure 3B,C). The process of
preparing the CPC-capped seeds was also studied by UV-vis
spectroscopy. The UV-vis extinction spectra of the gold
nanorods, overgrown gold nanorods, and CPC-capped seeds are
shown in Figure 2D. The two peaks in the UV-vis extinction
spectra of the gold nanorods and overgrown gold nanorods can
be attributed to the transverse and longitudinal plasmon modes
of gold nanorods.22 The blue shift in the longitudinal plasmon
mode of the overgrown nanorods confirms a decrease in their
aspect ratios.25 The UV-vis extinction spectrum of the CPC-
capped seeds has only one peak [curve (c) in Figure 2D], which
confirms their near-spherical morphology.
3.2. Synthesis of Single-Crystalline RD, Octahedral, and
Cubic Gold Nanocrystals. 3.2.1. Synthesis and Characterization
of RD Gold Nanocrystals. Synthesis of RD gold nanocrystals
was performed at 30 °C. Typically, 100 µL of 10 mM HAuCl4
solution, 200 µL of 100 mM ascorbic acid solution, and 200
µL of the CPC-capped seed solution were added to 5 mL of 10
mM CPC solution in sequence. The reaction was stopped after
2 h by centrifugation. Figure 4A,B shows SEM images of
orderly assembled and randomly distributed RD gold nanoc-
rystals, respectively. In the ordered assembly of RD gold
nanocrystals, the RD gold nanocrystals are in the orientation
shown in Figure 1C. From the randomly distributed RD gold
nanocrystals, the geometrical shapes of these gold nanocrystals
can be viewed from different directions and identified as RD,
i.e., bounded by 12 equivalent rhombic faces (Figure S1 in the
Supporting Information).14 Figure 4C shows the TEM image
of the RD gold nanocrystals. Most of the RD nanocrystals tend
to lie flat and exhibit elongated hexagonal shapes. For a flat-
sequence. The solution was thoroughly mixed after each addition.
The reaction was stopped after 2 h by centrifugation (12 000 rpm,
10 min).
In a typical synthesis of the cubic gold nanocrystals, 500 µL of
100 mM KBr solution, 100 µL of 10 mM HAuCl4 solution, 13 µL
of freshly prepared 100 mM ascorbic acid solution, and 200 µL of
the CPC-capped seed solution were added to 5 mL of 100 mM
CPC solution at 30 °C in sequence. The solution was thoroughly
mixed after each addition. The reaction was stopped after 2 h by
centrifugation (12 000 rpm, 10 min).
2.5. Instrumentation. Scanning electron microscopy (SEM)
images were taken using an FEI XL30 ESEM FEG scanning
electron microscope operating at 25 kV. Transmission electron
microscopy (TEM) and selected-area electron diffraction (SAED)
studies were performed on a FEI Tecnai G2 20 S-TWIN transmis-
sion electron microscope operated at 200 kV. A drop of the
concentrated nanocrystal solution was deposited on ITO glass or a
TEM grid for SEM or TEM measurements, respectively. During
natural evaporation of water, part of the nanocrystals migrated to
the edge of the drop and formed a solid ring. SEM observations
revealed that ordered assemblies mainly existed at the coffee-ring
region, while random assemblies mainly existed at the center.
UV-vis extinction spectra were taken at room temperature on a
CARY 500 Scan UV-vis-near-IR spectrophotometer using a
quartz cuvette with an optical path of 1 cm. X-ray diffraction (XRD)
measurements were obtained with a Rigaku D/MAX-2500 instru-
ment (Cu KR1 radiation) operated at 50 kV and 250 mA over a
range of 30-90° by step scanning with a step size of 0.02°.
3. Results and Discussion
3.1. Preparation and Characterization of the CPC-Capped
Gold Seeds. Gold nanorods were overgrown and then oxidized
by Au(III)-CTAB complexes to prepare the CPC-capped gold
seeds. The oxidation occurred preferentially at the ends of the
overgrown gold nanorods and eventually led to the formation
of near-spherical nanoparticles.24 These near-spherical nano-
particles were finally dispersed in 100 mM CPC solution and
designated as the CPC-capped gold seeds. As shown in Figure
2, the average width, length, and aspect ratio of the gold
nanorods are 27.2 nm, 58.8 nm, and 2.2, respectively, and the
average width, length, and aspect ratio of the overgrown gold
nanorods are 32.4 nm, 63.7 nm, and 2.0, respectively; the CPC-
(25) Tsung, C.-K.; Kou, X.; Shi, Q.; Zhang, J.; Yeung, M. H.; Wang, J.;
Stucky, G. D. J. Am. Chem. Soc. 2006, 128, 5352.
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Figure 4. SEM images of (A) orderly assembled and (B) randomly
distributed RD gold nanocrystals (scale bars: 200 nm). (C) TEM image of
the RD gold nanocrystals (scale bar: 200 nm). (D) TEM image and
corresponding SAED pattern of a single flat-lying RD gold nanocrystal
recorded along the [011] zone axis (scale bar: 50 nm).
Figure 5. SEM images of larger RD gold nanocrystals at different
magnifications [scale bars: (A) 1 µm, (B) 500 nm, (C) 200 nm]. (D) RD
geometrical models with typical orientations corresponding to RD gold
nanocrystals at the same positions in (C).
lying RD gold nanocrystal, the top and bottom {110} faces of
the RD gold nanocrystal are parallel to the substrate. Directing
the electron beam perpendicular to the upper face of the flat-
lying RD nanocrystal produces the typical SAED pattern of a
gold crystal along the [011] zone axis,12,13,26 as shown in the
inset of Figure 4D. The HRTEM and corresponding SAED
images of gold nanocrystals viewed from different directions
further confirm their RD shapes (Figure S2 in the Supporting
Information). It should be pointed out that the XRD pattern of
the RD gold nanocrystals (Figure S3 in the Supporting Informa-
tion) does not have an abnormally intense (220) peak because
SEM observations (Figure 4A,B) reveal that not all of the RD
nanocrystals lie flat on the substrate.
The TEM image in Figure 4D also reveals that the corners
of the RD gold nanocrystal are slightly stretched-out, which
indicates that the faces of the RD nanocrystals are not perfectly
flat. In order to clearly distinguish the surface morphologies of
the RD nanocrystals, larger RD nanocrystals were also synthe-
sized by reducing the added volume of the seed solution to 25
µL. As shown in Figure 5, the faces of the RD nanocrystals are
concave with relatively flat center parts parallel to the ideal
{110} facets. Therfore, although the gold nanocrystals in Figures
4 and 5 take an RD overall shape, their faces are not perfect
{110} facets, and the center part of each facet is supposed to
be a {110} facet.
3.2.2. Synthesis and Characterization of Octahedral Gold
Nanocrystals. Octahedral gold nanocrystals were synthesized
under conditions similar to those for the synthesis of the RD
gold nanocrystals in Figure 4, except that the concentration of
CPC was increased from 10 to 100 mM and the volume of 100
mM ascorbic acid solution was decreased from 200 to 13 µL.
SEM and TEM images of the octahedral gold nanocrystals are
presented in Figure 6A,B, respectively. The SAED study
indicates that the octahedral gold nanocrystal is a piece of single
crystal bounded by {111} facets (Figure 6C,D).9,10
Figure 6. (A) SEM image of octahedral gold nanocrystals (scale bar: 200
nm). (B) TEM image of octahedral gold nanocrystals (scale bar: 100 nm).
(C) TEM image and (D) corresponding SAED pattern of a single flat-lying
octahedral gold nanocrystal recorded along the [111] zone axis (scale bar:
20 nm).
j
of 100 mM KBr solution was introduced while the other
conditions were kept the same as those in the synthesis of the
octahedral gold nanocrystals. Figure 7A,B shows the SEM and
TEM images of the cubic gold nanocrystals, respectively, and
Figure 7C,D shows the TEM image and corresponding SAED
pattern of a single cubic gold nanocrystal. The square spot array
of the SAED pattern indicates that the cubic gold nanocrystal
is a single crystal bounded by {100} facets.7,9 Cubic gold
nanocrystals are also produced when only 0.05 mL of 100 mM
KBr solution is introduced, as shown in Figure S4 in the
Supporting Information.
3.2.4. Average Edge Lengths, Size Distributions, Yields,
and Corresponding Optical Properties of the Gold Nano-
crystals. Table 1 summarizes the average sizes, size distributions,
and yields of the gold nanocrystals presented in Figures 4, 6,
and 7. All three types of gold nanocrystals are prepared in high
yields with good monodispersity. Compared with the CPC-
capped seeds, the final nanocrystals have improved monodis-
3.2.3. Synthesis and Characterization of Cubic Gold Nano-
crystals. Cubic gold nanocrystals were produced when 0.5 mL
(26) Wang, Z. L. Reflection Electron Microscopy and Spectroscopy for
Surface Analysis; Cambridge University Press: Cambridge, U.K., 1996;
Appendix D.
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Dodecahedral, Octahedral, and Cubic Gold Nanocrystals
A R T I C L E S
Figure 8. UV-vis extinction spectra of (A) the RD gold nanocrystals in
Figure 4, (B) the octahedral gold nanocrystals in Figure 6, and (C) the cubic
gold nanocrystals in Figure 7 (each spectrum is normalized against the
intensity of its strongest peak).
Figure 7. (A) SEM image of cubic gold nanocrystals (scale bar: 500 nm).
(B) TEM image of cubic gold nanocrystals (scale bar: 100 nm). (C) TEM
image and (D) corresponding SAED pattern of a single flat-lying cubic
gold nanocrystal recorded along the [001] zone axis (scale bar: 20 nm).
3.3.1. The Importance of the Size and Crystal Structure
of the Seeds. To investigate how the size and crystal structure
of the seeds affect the growth of gold nanocrystals, another two
types of seeds commonly used in the seed-mediated growth
method were adopted here (under the same growth conditions)
in addition to the CPC-capped gold seeds. These two types of
seeds were ∼3 nm citrate-capped twinned gold seeds and ∼1.5
nm CTAB-capped single-crystalline gold seeds.20 Among the
three types of seeds, the ∼1.5 nm CTAB-capped single-
crystalline seed solution contains bromide ions, and bromide
ions from this seed solution may affect the growth of gold
nanocrystals.33 Therefore, we intentionally adopted the growth
conditions with the addition of KBr to counteract the influence
of the presence or absence of bromide ions in different seed
solutions.
SEM images of the gold nanocrystals synthesized with the
different types of seeds are shown in Figure 9, and their
corresponding shape distributions are summarized in Table S1
in the Supporting Information. When the CPC-capped single-
crystalline gold seeds were used, cubic gold nanocrystals were
produced in high yield (95.2%). In contrast, only 4.0% of the
products were cubic nanocrystals when the ∼3 nm twinned
nanoparticles were used, and 26.1% of the products were cubic
nanocrystals when ∼1.5 nm single-crystalline nanoparticles were
used. These results provide evidence that both the single-
crystalline nature and the relatively large sizes of the CPC-
capped seeds play important roles in the growth of single-
crystalline gold nanocrystals with high yields. It is believed that
the crystal structure of the seeds fluctuates at very small sizes,
whereas their structure will be fixed as single-crystalline or
multitwinned as the size of the crystals increases.18,34 Because
of their relatively large sizes, the CPC-capped single-crystalline
seeds can avoid twinning during the growth process, enabling
us to study the parameters that affect the growth of the gold
nanocrystals. It should also be noted that the present procedure
for preparing the CPC-capped seeds is somewhat tedious.
Ongoing efforts in our group are being directed toward finding
simpler methods for synthesis of high-quality single-crystalline
gold seeds with relatively large sizes.
Table 1. Average Edge Lengths (Lav), Size Distribution Standard
Deviations (σ), and Yields of the RD, Octahedral, and Cubic Gold
Nanocrystals
shape
Lav (nm)
σ (%)
yield (%)
figure no.
RD
octahedral
cubic
53.4
59.8
55.7
5.3
4.1
4.4
∼100
97.2
4
6
7
96.1
persity. In seed-mediated growth procedures, the improvement
in the monodispersity is attributed to the “self-focusing”
tendency that small particles grow faster than larger ones,
causing the final sizes of the nanocrystals to become uniform.27,28
Such observations are consistent with previous reports that the
monodispersity of the grown nanoparticles is better than that
of the seeds.29-31 It is well-known that noble metal nanoparticles
exhibit size- and shape-dependent optical properties arising from
surface plasmon resonances.32 As shown in Figure 8, the
UV-vis extinction spectra of the RD, octahedral, and cubic
gold nanocrystals show peaks at 582, 568, and 551 nm,
respectively. The differences in the UV-vis extinction spectra
may result from the differences in the sizes, shapes, and
outstretched or truncated corners of the gold nanocrystals.
3.3. Growth Mechanisms of the Gold Nanocrystals. In a seed-
mediated growth procedure, several parameters are simulta-
neously responsible for the final shapes of the gold nanocrys-
tals.17 Hence several control experiments are conducted to
elucidate the growth mechanisms of the gold nanocrystals. In
the nucleation stage, the size and crystal structure of seeds are
proved to be important for the formation of single-crystalline
structures of the gold nanocrystals. In the growth stage,
surfactants, growth kinetics, and adsorbates are found to
determine the final shapes of the resultant single-crystalline gold
nanocrystals.
(27) Reiss, H. J. Chem. Phys. 1951, 19, 482.
(28) Peng, X. G.; Wickham, J.; Alivisatos, A. P. J. Am. Chem. Soc. 1998,
120, 5343.
(29) Brown, K. R.; Walter, D. G.; Natan, M. J. Chem. Mater. 2000, 12,
306.
(33) Ha, T. H.; Koo, H. J.; Chung, B. H. J. Phys. Chem. C 2007, 111,
(30) Henglein, A. Langmuir 1999, 15, 6738.
(31) Jana, N. R.; Gearheart, L.; Murphy, C. J. Langmuir 2001, 17, 6782.
(32) Xia, Y.; Halas, N. J. MRS Bull. 2005, 30, 338.
1123.
(34) Im, S. H.; Lee, Y. T.; Wiley, B.; Xia, Y. Angew. Chem., Int. Ed. 2005,
44, 2154.
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A R T I C L E S
Niu et al.
Figure 9. SEM images of gold nanocrystals synthesized with different types of seeds: (A) 41.3 nm CPC-capped single-crystalline gold seeds; (B) ∼3 nm
citrate-capped twinned gold seeds; (C) ∼1.5 nm CTAB-capped single-crystalline gold seeds (all scale bars: 500 nm). The gold nanocrystals were synthesized
by adding 50 µL of 100 mM KBr solution, 100 µL of 10 mM HAuCl4 solution, 15 µL of 100 mM ascorbic acid solution, and the corresponding amount of
seed solution (200 µL for the CPC-capped seed solution, 1 µL for the ∼3 nm citrate-capped twinned seed solution, and 0.02 µL for the ∼1.5 nm CTAB-
capped single-crystalline seed solution) to 5 mL of 100 mM CPC solution at 30 °C.
facets relatively poorly. Moreover, octahedral gold nanocrystals
bounded by {111} facets are obtained at relatively high CPC
concentrations, suggesting that the enhanced capping of CPC
promotes the formation of {111} facets of gold. These observa-
tions suggest that the gold {111} facets should be more stable
than gold {110} facets in the presence of CPC under the growth
conditions in this study. Since gold nanocrystals bounded by
{100} facets are not observed in the presence of CPC, we
suppose that gold {100} facets tend to disappear during the
growth procedure, and it should be the most unstable low-index
facet of gold in the presence of CPC. Therefore, it is reasonable
to assume that CPC alters the surface energies of gold facets,
ordering them as γ{111} < γ{110} < γ{100} under the growth
conditions in this study. The selective interactions of CPC with
different gold facets are responsible for the alteration.
3.3.3. Growth Kinetics of the RD and Octahedral Gold
Nanocrystals. The above surface-energy considerations suggest
that the RD gold nanocrystal is not the thermodynamically most
Figure 10. SEM image of larger octahedral gold nanocrystals synthesized
with 25 µL of the CPC-capped seed solution added (scale bar: 200 nm).
favored shape. The formation of thermodynamically less favored
shapes of nanocrystals are generally governed by growth
kinetics.39-41 In the case of the selective synthesis of RD and
3.3.2. Surface Energies of Different Gold Crystallographic
Facets in the Presence of CPC. Surface energies associated with
octahedral gold nanocrystals, growth kinetics and the capping
effect of CPC must be taken into account simultaneously in
different gold crystallographic facets usually increase in the
order to understand their growth mechanisms.
order γ{111} < γ{100} < γ{110}.
11,35 In a solution-phase synthesis,
Several control experiments were conducted to investigate
the formation mechanisms of RD and octahedral gold nanoc-
rystals. RD gold nanocrystals could still be prepared when the
concentration of CPC was increased to 100 mM and the volume
of 100 mM ascorbic acid solution was between 200 and 25 µL,
while other conditions were the same as those in the synthesis
of the RD gold nanocrystals in Figure 4. The corresponding
SEM images of the RD nanocrystals are shown in Figure 11A,B.
When the volume of ascorbic acid solution was further reduced
to 13 µL, octahedral gold nanocrystals were produced, as shown
in Figure 6. These results indicate that the competition between
adsorbates (including surfactants, polymers, small molecules,
and atomic adsorbates) can interact selectively with different
metal crystal facets and alter their surface energies.3 It has been
reported that gold nanorods have {110} side facets.20,36 These
{110} facets disappear during overgrowth in the presence of
CTAB or poly(vinyl pyrrolidone).37,38 In contrast, CPC can
selectively stabilize the {110} facets of gold, and thus, RD gold
nanocrystals can exist as final products. The preparation of
octahedral gold nanocrystals indicates that CPC can also stabilize
the {111} facets of gold. Through a comparison between the
surface structures of the larger RD and octahedral gold nanoc-
rystals shown in Figures 5 and 10, respectively, we found that
the faces of the octahedral gold nanocrystals are flat and smooth
but that the faces of the RD gold nanocrystals are not well-
defined {110} facets, which suggests that CPC stabilizes {110}
-
AuCl4 reduction and the CPC capping process on the {111}
and {110} surfaces plays an important role. Octahedral gold
nanocrystals bounded by {111} facets were obtained at relatively
high CPC concentrations, suggesting that the capping of CPC
promotes the formation of {111} facets of gold. At relatively
high ascorbic acid concentrations, RD nanocrystals were
(35) Xiong, Y.; Wiley, B.; Xia, Y. Angew. Chem., Int. Ed. 2007, 46, 7157.
(36) Wang, Z. L.; Mohamed, M. B.; Link, S.; El-Sayed, M. A. Surf. Sci.
1999, 440, L809.
(39) Petroski, J. M.; Wang, Z. L.; Green, T. C.; El-Sayed, M. A. J. Phys.
Chem. B 1998, 102, 3316.
(37) Xiang, Y.; Wu, X.; Liu, D.; Feng, L.; Zhang, K.; Chu, W.; Zhou, W.;
Xie, S. J. Phys. Chem. C 2008, 112, 3203.
(40) Xiong, Y.; Cai, H.; Wiley, B. J.; Wang, J.; Kim, M. J.; Xia, Y. J. Am.
Chem. Soc. 2007, 129, 3665.
(38) Carbo-Argibay, E.; Rodriguez-Gonzalez, B.; Pacifico, J.; Pastoriza-
Santos, I.; Perez-Juste, J.; Liz-Marzan, L. M. Angew. Chem., Int. Ed.
2007, 46, 8983.
(41) Tao, A.; Sinsermsuksakul, P.; Yang, P. Angew. Chem., Int. Ed. 2006,
45, 4597.
9
702 J. AM. CHEM. SOC. VOL. 131, NO. 2, 2009

Dodecahedral, Octahedral, and Cubic Gold Nanocrystals
A R T I C L E S
bromide ion has a higher affinity for gold than chloride ion
does.45 Several reports suggest that cationic surfactants with
bromide counterions can stabilize the {100} facets of noble
metals by reducing their surface energies,46-48 and the coopera-
tive work of cetylpyridinium and bromide ions plays a similar
role herein. By using the CPC-capped single-crystalline seeds,
twinning was avoided during the growth process, and therefore,
cubic gold nanocrystals with single-crystalline structures bounded
by {100} facets were formed.
CTAB is also a cationic surfactant with bromide counterions
and is widely used in seed-mediated growth methods.1 It has
been reported that CTAB can stabilize the {100} facets of
gold.46 To investigate whether CTAB plays a similar role in
the growth method presented here, CPC was replaced with
CTAB as the surfactant. CTAB-capped single-crystalline gold
seeds 41.3 nm in size were prepared by a process similar to
that for the CPC-capped seeds, except that the seeds were finally
dispersed in 100 mM CTAB solution. Figure 11D shows that
cubic gold nanocrystals were also obtained as final products
when the CTAB-capped 41.3 nm seeds were used. Therefore,
CTAB plays a role similar to that of the cooperative work of
cetylpyridinium and bromide ions under the growth conditions
in this study.
Figure 11. SEM images of gold nanocrystals: (A) RD nanocrystals
synthesized with 200 µL of 100 mM ascorbic acid in 100 mM CPC solution
at 30 °C (scale bar: 200 nm); (B) RD nanocrystals synthesized with 25 µL
of 100 mM ascorbic acid in 100 mM CPC solution at 30 °C (scale bar: 200
nm); (C) octahedral nanocrystals synthesized with 25 µL of 100 mM
ascorbic acid in 100 mM CPC solution at 20 °C (scale bar: 200 nm); (D)
cubic nanocrystals synthesized in 10 mM CTAB solutions with 13 µL of
100 mM ascorbic acid at 30 °C by using the CTAB-capped 41.3 nm single-
crystalline seeds (scale bar: 500 nm).
4. Conclusion
In conclusion, we report a versatile method to selectively
synthesize single-crystalline rhombic dodecahedral, octahedral,
and cubic gold nanocrystals. This is also the first report regarding
the rational synthesis of RD gold nanocrystals in high yield.
Parameters that are responsible for the final shapes of the gold
nanocrystals have been studied in detail. The adoption of the
CPC-capped gold seeds enabled us to temporarily ignore the
elusive nucleation process in typical growth processes of metal
nanocrystals, and thus, it was possible to study the correlations
between the growth conditions and the shapes of the gold
nanocrystals in the growth process. During the growth process,
important parameters, including surfactants, growth kinetics, and
adsorbates, were found to affect the shape formation of the gold
nanocrystals, and the growth mechanisms are explained in terms
of surface energy and growth kinetics. These results may provide
a foundation for gaining mechanistic insights into the growth
of shape- and structure-controlled noble-metal nanocrystals.
-
obtained, which suggests that the reduction of AuCl4 by
ascorbic acid tends to occur on the {111} facets of gold, leading
to the disappearance of {111} facets and the formation of {110}
facets. Therefore, octahedral gold nanocrystals are formed when
the capping of CPC on {111} facets dominates, while RD gold
nanocrystals are formed when the reduction of AuCl4- on {111}
facets dominates.
The reaction temperature can also affect the growth kinetics
of the gold nanocrystals. As shown in Figure 11C, octahedral
gold nanocrystals were produced when the reaction temperature
was decreased from 30 to 20 °C while the other conditions were
the same as those in the synthesis of the RD gold nanocrystals
shown in Figure 11B. A decrease in reaction temperature can
slow down the reduction of AuCl4- ions by ascorbic acid. The
reaction rate in our system was estimated to increase ∼3.3 times
as the reaction temperature was increased from 20 to 30 °C,
according to the results of Zijlstra et al.42 and the Arrhenius
equation. Low temperature can also favor the adsorption of
Acknowledgment. We gratefully acknowledge support from the
National Natural Science Foundation of China (20875086 for G.X.;
20773033 for Z.T.), the Department of Sciences & Technology of
Jilin Province (20070108), the Ministry of Science and Technology
of the People’s Republic of China (2006BAE03B08), the National
Research Fund for Fundamental Key Projects (2009CB930401),
and the Hundred Talents Program of the Chinese Academy of
Sciences (G.X. and Z.T.). We thank M. Y. Li and Y. D. Fan for
technical assistance and helpful discussions.
CPC.43 Both the retarded reduction of AuCl4 on the {111}
-
facets of gold and the increased capping of CPC on {111} facets
of gold promote the production of octahedral gold nanocrystals
bounded by {111} facets, which is in accordance to the growth
mechanisms we proposed above.
3.3.4. Effect of Bromide Ions. The adsorption of cationic
surfactants based on quaternary ammonium cations onto a gold
surface is mediated by the precedent-specific adsorption of
halide counterions.33 In the case of CPC, chloride counterions
adsorb onto the gold surface and create a negatively charged
layer, after which cetylpyridinium cations adsorb onto the
surface through electrostatic forces.44 When KBr is introduced
into the growth solution, the bromide ions replace the chloride
ions and mediate the adsorption of cetylpyridinium cations, since
Supporting Information Available: Figures S1-S4 and Table
S1. This material is available free of charge via the Internet at
http://pubs.acs.org.
JA804115R
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