Synthesis of gold nano- and microplates in hexagonal liquid crystals

Article abstract of DOI:10.1021/jp0449152

Single-crystalline gold nano- and microplates with triangular or hexagonal shapes are synthesized by reduction of HAuCl₄ in lyotropic liquid crystal (LLC) mainly made of polyethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) block copolymers and water after adding a small amount of capping agents, cetyltrimethylammonium bromide (CTAB) or tetrabutylammonium bromide (TBAB). During the growth of such plates, capping agents play the crucial role. It is found that there is an optimal value of CTAB or TBAB concentration for producing microplates. The selective adsorption of CTAB or TBAB on certain crystallographic facets may be the key point of the supposed mechanism. Although LLC does not really act as a template, it provides an ordered structure confining CTAB as well as the nascent metal nuclei, which enhances the oriented attachment of nuclei and thus the consequent growth of single-crystal plates.

Full text of DOI:10.1021/jp0449152

J. Phys. Chem. B 2005, 109, 3189-3194
3189
Synthesis of Gold Nano- and Microplates in Hexagonal Liquid Crystals
Luyan Wang,† Xiao Chen,*,† Jie Zhan,‡ Yongcun Chai,† Chunjie Yang,† Limei Xu,†
Wenchang Zhuang,† and Bo Jing†
Key Laboratory of Colloid and Interface Chemistry, Shandong UniVersity, Ministry of Education,
Jinan 250100, China, and Institute of Crystal Material, Shandong UniVersity, Jinan 250100, China
ReceiVed: NoVember 6, 2004; In Final Form: December 21, 2004
Single-crystalline gold nano- and microplates with triangular or hexagonal shapes are synthesized by reduction
of HAuCl4 in lyotropic liquid crystal (LLC) mainly made of poly(ethylene oxide)-poly(propylene oxide)-
poly(ethylene oxide) block copolymers and water after adding a small amount of capping agents,
cetyltrimethylammonium bromide (CTAB) or tetrabutylammonium bromide (TBAB). During the growth of
such plates, capping agents play the crucial role. It is found that there is an optimal value of CTAB or TBAB
concentration for producing microplates. The selective adsorption of CTAB or TBAB on certain crystallographic
facets may be the key point of the supposed mechanism. Although LLC does not really act as a template, it
provides an ordered structure confining CTAB as well as the nascent metal nuclei, which enhances the oriented
attachment of nuclei and thus the consequent growth of single-crystal plates.
1. Introduction
The capping agents in the reaction systems, poly(vinyl alcohol)
and dimyristoyl-L-R-phosphatidyl-DL-glycerol, are considered
Inorganic nanocrystals have been widely studied in the past
decades because of their great potential applications in electron-
ics, biology, photonics, and so forth.1 Although the synthesis
of spherical nanocrystals is well developed for many kinds of
materials, fabrication of an anisotropic nanocrystal is still a
challenge for researchers in this field. The shape and size are
key parameters in such preparation because of their close relation
to the physical and chemical properties of final material.2 Many
efforts have been devoted to prepare such anisotropic inorganic
nanocrystals such as rods, wires, cubes, disks, and other special
morphologies.3 Particularly, one- or two-dimensional (1D or 2D)
nanostructures for noble metals attract much attention.4 Nano-
rods or wires of silver and gold, with controllable diameters
and aspect ratios, have been synthesized by electrochemical,
photochemical, templating, and seeding growth methods.5-13
2D silver nanodisks are also intensely investigated by various
solution-phase methods.14-16 However, in contrast to a large
amount of reports on gold nanorod and silver nanodisks, few
concern systems of gold nanodisks or planar nanoparticles
though triangular gold nanoplates had been observed more than
50 years ago.17 The methods having been used are generally
by reduction of AuIII to Au0 through chemical, photochemical,
or biological process.18-23 For example, a chemical synthetic
method within inverse micelles was used by Stoeva et al. to
get platelike gold superlattices from nanoparticles assembling.18
Liz-Marzan’s group obtained flat triangular and hexagonal
nanoplates of ∼50-200 nm in diameter, by reduction of AuIII
with salicylic acid in aqueous solution, accompanied with
smaller close-to-spherical gold nanoparticles.19 The produced
polygons are of relatively low concentration, which leads to
only the longitudinal resonance visible in UV-vis spectrum.
Photoreduction process was employed by Zhou et al. and Ibano
et al. to produce triangular or hexagonal gold nanoplates.20,21
to play important roles in controlling the nanoparticle shape
while their growth mechanisms have not been discussed in
detail. In addition, Tsuji et al. reported a microwave-polyol
method to produce gold nanoplates but with an unsatisfactory
shape control since the product was a mixture of nanoplates
and square or spherical particles.22
Though these different ways have been established to get
anisotropic gold nanoplates, results are not promising because
these methods are often characterized by relative low yields or
small product sizes. Shape control of metal nanoparticles has
proved more difficult, and there is still great interest in
developing new methods for systematic manipulation of platelike
gold nanocrystals and investigating their growth mechanism.
A biological method is recently reported to obtain high-yield
gold nanoprisms with the extract from the lemongrass plant as
the reducer.23a Gold microplates are obtained from a thermal
process at 70 °C as well as from a wet-chemical route in an
aqueous medium.23b,23c We have also tried the preparation of
gold nano- and microplates from lyotropic liquid crystals (LLC)
made of poly(ethylene oxide)-block-poly(propylene oxide)-
block-poly(ethylene oxide) (PEO-PPO-PEO) block copoly-
mers.24 In this paper, we extend this research trying to obtain
products of controllable shape and size. More attention is paid
to different factors affecting on product formation and growth
mechanism. Cetyltrimethylammonium bromide (CTAB) and
tetrabutylammonium bromide (TBAB), as capping agents, are
added to systematically control the morphology of gold products.
2. Experimental Section
Materials. The amphiphilic block copolymer Pluronic P123
(EO20PO70EO20, Mn ) 5800) is purchased from Aldrich.
Hydrogen tetrachloroauric acid (HAuCl4‚4H2O, g99.9%), CTAB
(99.0%), and TBAB (g99.0%) are all purchased from Shanghai
Reagent Co. The water used is highly purified with the resistivity
g18.0 MΩ‚cm. All chemicals are used as received without
further purification.
* To whom correspondence may be addressed. E-mail: xchen@
sdu.edu.cn. Tel: +86-531-8365420. Fax: +86-531-8564750.
† Key Laboratory of Colloid and Interface Chemistry.
‡ Institute of Crystal Material.
10.1021/jp0449152 CCC: $30.25 © 2005 American Chemical Society
Published on Web 02/04/2005

3190 J. Phys. Chem. B, Vol. 109, No. 8, 2005
Wang et al.
Synthesis of Gold Products. On the basis of the phase
diagram25 reported by Alexandridis, Pluronic P123 can form a
hexagonal phase at higher concentration (weight percentage of
40-50%) when mixed with water at room temperature. In our
study, 45% P123 is selected where the PEO-PPO-PEO
molecules self-assemble into cylinders with PEO blocks in water
domains. To prepare gold nanocrystals, tetrachloroauric acid
aqueous solution (0.03 or 0.015 M) instead of water is used to
construct hexagonal phase with PEO-PPO-PEO and CTAB
or TBAB of lower weight percentages. The samples are sealed
in glass tubes and left at room temperature for further study.
Small-Angle X-ray Scattering (SAXS) Measurement.
Phase behaviors of LLCs are investigated by SAXS on a
HMBG-SAX X-ray small angle system (Austria) with Ni-filtered
Cu KR radiation (0.154 nm) operating at 50 kV and 40 mA.
The sample-to-detector distance is 277 cm. The temperature is
kept at 25 ( 0.1 °C. The relative positions of SAXS scattering
peaks along the scattering vector (q) axis are used to determine
the lyotropic phase structure. For hexagonal structure (cylindrical
assemblies crystallized in a hexagonal lattice), the peak positions
should obey the relation 1:x3:2:x7.25
Characterization of Products. Morphologies of gold par-
ticles are observed under a JEM-100CX II (JEOL) transmission
electron microscope operated at 100 kV. To prepare transmission
electron microscopy (TEM) samples, the reacted mixtures are
dispersed in water under sonication and centrifuged at ∼3600
rpm for 10 min. Then the upper solution containing unreduced
ions and unbound surfactant molecules is removed. Such
obtained samples are redispersed in water. A little drop of
resulting dispersion is put onto a Formvar-covered copper grid
(230 mesh) for TEM measurement. The optical properties of
obtained gold products dispersed in water are characterized on
a HP 8453E UV-vis spectrometer with resolution of 1 nm.
Figure 1. SAXS patterns of P123 (45%) hexagonal liquid crystal
phases measured 3 days after preparation. HAuCl4 concentration is
0.03 M.
TABLE 1: Lattice Spacings (d) for LLC Phases with
Different Compositions
systemsa
d (nm)
systemsa
d (nm)
a1
b1
c1
d1
e1
13.6
13.2
12.8
12.6
12.4
a2
b2
c2
d2
e2
14.1
13.7
13.3
12.8
12.6
a System composition corresponds to those denoted in Figure 1.
3. Results and Discussion
3.1. Preparation of Gold Products from CTAB-Doped
Systems. 3.1.1. LLC Phase Structures. The SAXS patterns
of P123 samples are shown in Figure 1. The relative position
of SAXS diffraction peaks along the scattering vector (q) axis
is used to determine the LLC phase structure. The lattice spacing
(d) is the distance between adjacent rows of cylinders in the
hexagonal structure and is determined from the first peak
position (q1).25 All systems here maintain structural character-
istics of the hexagonal phase with their d spacings being listed
in Table 1. It can be noted that all systems accommodating gold
products exhibit enlarged d values, which is easy to understand
when considering the swelling effect of particles formed in LLC
phase. From Figure 1, we can also find differences between
curve shapes for samples with or without gold formation (parts
a-c of Figure 1). First, the intensity of peaks 1 and 2 become
drastically weaker for systems having AuIII reduction, which
may originate from the deterioration of long-range ordering in
the LLC phase by included gold particles and decreased
viscosity due to oxidation of ethylene oxide chains in polymer
molecules. Another change is the elevation of scattering curve
in the low q region for samples a1, b1, and c1, which is an
obvious indication for the production of gold nanoparticles
according to the analysis by Firestone.26 For systems with
∼1.5-2.0% CTAB, however, two SAXS patterns before and
after reduction are similar to each other, suggesting that products
do not much influence the structures of reacting systems,
consistent with the TEM observation as discussed in the
following section.
Figure 2. TEM images of gold nano- and microcrystals obtained from
P123-HAuCl4 (0.03 M) systems measured 3 days after preparation.
CTAB concentrations are (a) 0%, (b) 0.5%, (c) 1.0%, (d) 1.5%, and
(e) 2.0%.
3.1.2. Structural Characterization of Gold Products. In
ordered cylindrical arrays self-assembled from PEO-PPO-
PEO, the reduction occurs in the aqueous domain and PEO
blocks will slowly reduce AuIII ions to Au through oxidation
of their oxyethylene groups.27 TEM images for samples
produced from different systems are shown in Figure 2.
Spherical nanoparticles are main products (ca. 200 nm) coexist-
ing with few triangular nanoplates when no CTAB is added.
Upon an increase of the CTAB concentration to 0.5%, more
platelike gold nanocrystals appear accompanied with spheres
(Figure 2b). As shown in Figure 2c, however, adding 1.0%
CTAB into the LLC results in much more and larger gold plates.
They exhibit equilateral or truncated triangular and hexagonal

Synthesis of Gold Nano- and Microplates
J. Phys. Chem. B, Vol. 109, No. 8, 2005 3191
TEM results. To further study such manipulation effect and
monitor the products formation, the optical absorption properties
of obtained product dispersions are compared as a function of
CTAB concentration. As shown in Figure 4A, the UV-vis
absorption curves for five investigated samples change gradually
with varying CTAB amount. On the basis of the shape of the
curves a1, b1, and c1, we can conclude that the main products
of these three systems are spherical nanoparticles at beginning
of reduction. The symmetrical shape of absoption peak in curves
a1 and b1 reflects a uniform size distribution of spherical
particles. With an increase in CTAB concentration to 1.0%, a
new shoulder band at ∼660-740 nm appears on the spectrum
except for the original absorption peak centered around 560 nm
(Figure 4c1), which may be attributed to the occurrence of
platelike particles and product aggregation.29 For samples with
1.5 and 2.0% CTAB, there is no distinct absorption exhibiting
slower reduction rate of AuIII possibly due to the capping effect
of CTAB molecules.
Figure 3. TEM images of nanocrystals prepared from P123-CTAB
(1.0%)-HAuCl4 (0.03 M) system. a, b, and c represent an equilateral
triangle, a truncated triangle, and a hexagon, respectively. d, e, and f
represent their corresponding images from tilt experiments with the
tilt angle of -50°.
With extended reaction time, spectral changes of the above
systems are depicted in Figure 4B. The surface plasmon
resonance (SPR) band at 547 nm (Figure 4a1) is now red-shifted
to 563 nm for a system without CTAB (Figure 4a2), and its
shape becomes also unsymmetrical, reflecting the formation of
larger particles and their aggregates mixing with few platelike
structures (Figure 2a). For a sample with 0.5% CTAB, the red-
shifted band becomes broader owing to nanoplate formation
(Figure 4b2). At 1.0% CTAB, two peaks begin to occur,
including a broad peak at 600-800 nm relating to the large-
scale formation of platelike products and the band around 569
nm (Figure 4c2). Note that systems with more CTAB exhibit
still a broad and flat feature extending well into the red (curves
d2 and e2 of Figure 4), which is similar to those of other
nanostructured gold materials, such as thin films and nanopar-
ticle aggregates.29 On the basis of TEM images in parts d and
e of Figure 2, it is reasonably considered that such changes are
originated from the aggregation and strong interaction of gold
products.19,29,30
shapes with the edge size even longer than 10 µm. The triangles
and hexagons are characterized by a low and uniform contrast,
indicating a flat feature. When CTAB concentration rises to
1.5%, the product morphology changes greatly. It exhibits both
diversity and decreased size verified also by SAXS patterns
(parts d and e of Figure 1), including nanoplates, particles
aggregates, or monodispersive nanoparticles (with diameters
even smaller than 1 nm, shown in Figure 2d). A similar result
occurs in a system with 2.0% CTAB, except for the smaller
particles and less nanoplates.
To investigate the nature of obtained microplates, tilt experi-
ments are performed, respectively. The electron diffraction (ED)
pattern (inset in Figure 2c) clearly suggests a face-centered cubic
(fcc) single crystal with an atomically flat surface and its [-111]
orientation parallel to the electron beam. All facets of single
crystals parallel to the grid are preferentially along {111} planes.
Such features indicate the faceted morphology of gold micro-
plates and their inherent anisotropy, which is important since
many optical and electronic properties will depend on the
orientation of crystal materials.18
The flatness of these plates is reflected by their low contrast
appearance. They are proved also not pyramids by TEM
observation under tilt angle. It is found that the projection shapes
of triangular and hexagonal particles are similar to original
samples and also in uniform contrast (see Figure 3), indicating
the plate but not pyramid nature.28
3.1.3. Optical Properties. Optical properties of prepared gold
nano- and microcrystals are strongly related to their shape and
size, which seem to be tailored by CTAB molecules as seen in
3.2. Effect of TBAB. To compare the capping effect, TBAB
is used instead of CTAB with a weight percentage of 0.5-
2.0%. Like CTAB, TBAB affects little on the hexagonal phase
without HAuCl4 and the phase repeat spacings are nearly the
same as those without TBAB. When HAuCl4 is introduced as
the precursor, the SAXS curves (not shown here) after reduction
exhibit features just like those in Figure 1, showing deterioration
of the hexagonal phase because of products formation and
oxidation of ethyleneoxide groups.
TEM images of products from these samples are shown in
Figure 5. Although both samples with less TBAB (0.5 and 1.0%)
Figure 4. UV-vis absorption spectra of gold products prepared, respectively, from P123-HAuCl4 (0.03 M) systems with CTAB concentration at
(a) 0%, (b) 0.5%, (c) 1.0%, (d) 1.5%, and (e) 2.0%. They are measured, respectively, at 5 h (A) and 3 days (B) after sample preparation.

3192 J. Phys. Chem. B, Vol. 109, No. 8, 2005
Wang et al.
Figure 7. TEM images of gold products formed in P123-HAuCl4
(0.015 M) systems with adding (a) 0%, (b) CTAB 0.5%, (c) CTAB
1.0%, (d) TBAB 0.5%, and (e) TBAB 1.0% measured 3 days after
preparation.
Figure 5. TEM images of gold products formed in P123-HAuCl4
(0.03 M) systems measured 3 days after preparation with adding TBAB
(a) 0.5%, (b) 1.0%, (c) 1.5%, and (d) 2.0%.
Figure 8. UV-vis absorption spectra of gold products prepared,
respectively, from P123-HAuCl4 (0.015 M) systems with CTAB or
TBAB concentration at (a) 0%, (b) CTAB 0.5%, (c) CTAB 1.0%, (d)
TBAB 0.5%, and (e) TBAB 1.0% measured 3 days after sample
preparation.
when short-chain branched TBAB is added to reaction systems,
products tend to exhibit diversity of morphologies, with a
mechanism to be discussed in section 3.4.
Figure 6. UV-vis absorption spectra of gold products prepared,
respectively, from P123-HAuCl4 (0.03 M) systems with TBAB
concentration at (a) 0.5%, (b) 1.0%, (c) 1.5%, and (d) 2.0% measured
3 days after sample preparation.
3.3. Effect of HAuCl4 Concentration. When the concentra-
tion of HAuCl4 is decreased from 0.03 to 0.015 M, some
changes are also noticed. It can be expected that less HAuCl4
will have little influence on a 45% polymer system without
CTAB or TBAB except for the smaller particle size, as
confirmed by comparing TEM images (Figure 7a) and the
absorption spectrum (Figure 8a) with those of 0.03 M HAuCl4
system. But it can be seen from morphologies (parts b-e of
Figure 7) that both nanoplates and nanoparticles exist in
products. And their corresponding absorption spectra in Figure
8 show that the products tend to be of smaller sizes, present
morphological diversity, aggregate more easily, and interact
strongly with each other.19,23,29,30 Such changes are similar to
those observed when increasing CTAB or TBAB concentration
because of the same tendency by decreasing HAuCl4 amount.
3.4. Growth Mechanism of Gold Nano- and Microplates.
From analysis of SAXS and TEM results, it should be noted
that the products do not replicate the long-range ordered
structure of the hexagonal phase; that is, there is no obviously
templating effect occurred in the reaction systems. Evidently,
addition of capping agents leads to appearance of microplates.
Our results seem to confirm the conclusion from Pileni, i.e.,
anisotropic nanocrystal formation is more related to selective
adsorption of ions during the crystal growth than to the nature
of the soft templates.32 Scheme 1 illustrates a possible mech-
anism for gold nanocrystal growth, which comprises mainly two
exhibit particles of platelike shape (curves a1 and b1 of Figure
5), products with other morphologies are also formed. In a 0.5%
TBAB sample, platelets with quadrangular shape are observed
(Figure 5a2) in a large amount besides gold plates, as indicated
by their optical absorption shape (Figure 6a). It keeps increasing
into the IR region with no indication of leveling off, which may
result from the strong interaction between small particles growth
and aggregation of quadrangular or platelike particles.29,30 The
ED pattern (Figure 5a3) shows their polycrystalline feature. In
a 1.0% TBAB system, such platelets also appear but in a lesser
amount. It should be noted that another type of product emerges
when we enlarge the area marked b2 in Figure 5b1, i.e., smaller
particles and their aggregates with varied shapes (see Figure
5b2). The absorption curve b in Figure 6, showing a broad peak
around 560 nm with a running-up tail after 850 nm, should be
attributed to mixing and aggregation of plates and particles and
to the interactions between them.19,23,29,30 When TBAB con-
centration is increased to 1.5 and 2.0%, the product size is
reduced obviously. Figure 5c presents mainly spherical nano-
particles with relatively uniform size corresponding to typical
SPR adsorption feature (Figure 6c). In curves d1 and d2 of Figure
5 (2.0% TBAB), more spherical nanoparticles appear with a
large size range varying from 1 to 200 nm, accompanied with
few irregular plates, corresponding to a broad shape of the UV-
vis absorption curve (Figure 6d).31 These results suggest that,

Synthesis of Gold Nano- and Microplates
J. Phys. Chem. B, Vol. 109, No. 8, 2005 3193
SCHEME 1: Schematic Growth Mechanism of Gold
Microplates from the P123 (45%)-CTAB
(1.0%)-HAuCl4 (0.03 M) System: (a) Cross Section of
the Hexagonal Phase, (b) Side View of Two Adjacent
Cylinders, and (c) Growth Process of Products
binding6-8,13,32 between CTAB and Au, combining both Br-
and CTA+ adsorption, inhibits particles growth randomly and
favors flat single-crystal growth with {111} facets extending.
It should be noted here that P123, as a poor stabilizer for noble
metal,34 may also play some role of a capping agent, which
can be confirmed from the appearance of few Au plates in the
P123-HAuCl4 system. But it is not the key factor in CTAB-
containing systems. With CTAB concentration increasing from
0.5 to 1.0%, such capping adsorption induces higher nanocrystal
surface coverage and favors a large-scale production of micro-
plates. With plates growing larger, the hexagonal arrays are
deformed, inducing decayed SAXS intensities as exhibited in
curves a2, b2 and c2 of Figure 1.
As for smaller nanoparticles produced when CTAB concen-
tration increases, it can be explained by enhanced adsorption
of cations and halide ions on gold surfaces.37 Adding more
CTAB (>1.0%) will lead to more binding of cations and anions
with Au and thus an increased mobility of gold surface
atoms.37,38 This effect not only prevents crystals from growing
along only certain directions but also makes it easy for unstable
small gold nanoparticles to dissolve, aggregate, or fuse to metal
clusters.37,38 Such a process can be testified by parts d and e of
Figure 2. It is also consistent with results reported in other
systems5d,37d and can be used to explain the effect of decreased
HAuCl4 concentration on products.
By comparison to CTAB-added systems, TBAB molecules
tend to result in diverse product morphologies. This may be
attributed to its short less-hydrophobic butyl groups, which
makes it act more like a cosurfactant and be located at the polar/
apolar interface in LLC but not interdigitated into PPO domains.
Therefore TBAB could move more freely than CTAB, ac-
companying a less oriented attachment of gold nuclei. Such
effects may enhance the gold aggregation in a different direction,
resulting in product morphological diversity.35a,37c
processes: (i) the formation of Au atoms and/or small clusters
in the LLC aqueous domain as the nascent crystal nuclei (I in
Scheme 1); (ii) the subsequent anisotropic growth from these
nuclei (II and III in Scheme 1).
At the initial stage, the dominant process is metal Au
nucleation. During preparation, when light-yellow HAuCl4
aqueous solution is added to samples containing CTAB, the
color is turned to salmon pink within a few minutes, indicating
the CTA+-AuCl4- complexion formation.6,33 With time going,
reduction of AuIII occurs and the color deepens accompanying
small gold nuclei production, which can be verified by absorp-
tion spectra in Figure 4A. Noted that, at this period, there are
possibly three main factors influencing the nucleation, including
-
LLC structure, CTAB adsorption, and CTA+-AuCl4 com-
plexion formation. The LLC structure directs gold nuclei being
distributed in water domains separated by hydrophobic cylinders,
indicating a long-range ordered nuclei arrangement. As for
CTAB adsorption, it has been reported in the literature6,13 to
control the gold nanorod production in a bilayer fashion. In our
case, CTAB molecules cannot move as freely as in the solution
phase because their alkyl chains are interdigitated in the
hydrophobic PPO cores inside the cylinders of LLC. Thus the
gold nuclei adsorbed by CTAB have to be arranged side by
side near the hydrophobic/hydrophilic interface (see Scheme
4. Conclusions
In summary, we have established a novel and simple method
for controllable preparation of gold nano- and microplates.
During plate growth, added CTAB or TBAB molecules may
play the crucial role in controlling the shape and size by
adsorbing selectively on certain crystallographic facets. We find
there is an optimal value of capping agent concentration for
obtaining gold microplates. In excess of this value, diverse
morphologies would be formed because CTAB or TBAB
headgroups and Br- can bind with Au in different direction
besides {111} facets. Although the morphology of gold products
does not replicate the long-range ordered structure of hexagonal
phase, the templating effect indeed occurs at the first stage on
CTAB and gold nuclei arrangement, which is important for flat
product formation. Therefore the LLC phase provides an ideal
reaction environment to control the shape of gold particles.
Obtained results suggest an effective way to produce particles
with controllable shape and size as well as a clue to understand
the mechanism for crystal growth.
-
1). The surfactant-AuCl4 complexion, however, retards the
nucleation rate, which may thus favor the consequent singe
crystal growth.34
During crystal growth, oriented attachment is considered to
be a probable mechanism which gives rise to homogeneous
single crystals,35 though there are some other possibilities for
gold plate formation. It involves spontaneous self-organization
of adjacent particles so that they share a common crystal-
lographic orientation, followed by joining of these particles at
a planar interface.24,35 On the basis of this mechanism, with
more AuCl4- being reduced, nuclei fixed by CTAB molecules
orientationally attach and coalescence with adjacent ones,
decreasing their surface energy and enhancing platelike nano-
crystal production. During this process, CTAB also seems to
play the crucial role. We have seen that large microplates can
be obtained when CTAB concentration is added to a certain
value (1.0% in this system). Exceeding it, more CTAB
molecules will reduce the products’ size and make them
irregular. It has been suggested that atoms on different crystal-
lographic facets might have different interaction strengths with
a polymeric or surfactant capping agent, leading to the aniso-
tropic growth of a solid material.7a,36 In this case, the
Acknowledgment. We thank the supports from the National
Natural Science Foundation of China (20073025, 20373035),
Specialized Research Fund for the Doctoral Program of Higher
Education (20020422060), and Excellent Young Scientists
Awarding Found of Shandong Province (01BS21).
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