Two distinct branch-stem interfacial structures of silver dendrites with vertical and slanted branchings

Article abstract of DOI:10.1016/j.cplett.2009.06.087

Silver dendrites synthesized by sonoelectrochemical deposition are investigated via transmission electron microscopy (TEM) in order to clarify the formation of specific branching angles. Two kinds of dendrites are found to form simultaneously; one has sla

Full text of DOI:10.1016/j.cplett.2009.06.087

Chemical Physics Letters 477 (2009) 179–183
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Chemical Physics Letters
journal homepage: www.elsevier.com/locate/cplett
Two distinct branch–stem interfacial structures of silver dendrites with vertical
and slanted branchings
*
Shaochun Tang, Sascha Vongehr, Xiangkang Meng
National Laboratory of Solid State Microstructures, Department of Materials Science and Engineering, Nanjing University, Nanjing 210093, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
Silver dendrites synthesized by sonoelectrochemical deposition are investigated via transmission elec-
tron microscopy (TEM) in order to clarify the formation of specific branching angles. Two kinds of den-
drites are found to form simultaneously; one has slanted angles while the other shows orthogonal
branching. Investigation of the branch–stem interfacial structures reveals that the vertical branches
attach at a characteristic transition layer around the main stem, while the slanted growth involves twin-
ning induced dislocations. It is suggested that oriented attachment of nanoparticles followed by epitaxial
growth leads to obliqueness, while crystallization of amorphous phase involving grain rotation and
realignment results in vertical branching.
Received 12 May 2009
In final form 26 June 2009
Available online 1 July 2009
Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction
dendrites by various sonoelectrochemical methods with the assis-
tance of organic capping agents such as nitrilotriacetate [14], gela-
Much effort has been devoted to understand non-equilibrium
growth phenomena theoretically and experimentally [1,2]. Among
such phenomena, dendrites are one of the most intensively inves-
tigated topics due to their attractive structures and large surface
area [3], both leading to many applications, for example in cataly-
sis [4], chemical/biochemical sensors [5] and surface-enhanced
Raman scattering [6]. Detailed description of the structure is
important to understand the underlying growth mechanisms,
which in turn is crucial to achieve designable structures (e.g. with
well defined branch–stem angles) resulting in desired properties.
Many studies have focused on the mechanisms of dendritic growth
[1,2,7–10]. Some novel growth scenarios have been recently pro-
posed. Ding et al. [9] reported a double-interface growth mode of
silver in a replacement reaction where dendritic growth was con-
trolled by the formation of an amorphous phase and its crystalliza-
tion. Cheng et al. [8] demonstrated that dendritic structure can
emerge from oriented attachment of nanoparticles. Ostwald ripen-
ing forms the initial particles and also smoothes the morphology
after attachment. Dislocations are known to originate at the bond-
ing interface in the oriented attachment of nanoparticles [7,8].
As for growth under non-equilibrium conditions, the distance
from equilibrium is important [11,12] but generally not easily con-
trolled [13]. Electrochemistry is in this context especially conve-
nient, because the ‘distance’ can be tuned continuously and
reversibly by the electrode potential. Particular attention has been
focused on the study of the formation and growth of silver
tin [15] and poly (vinyl pyrrolidone) [16]. Generally, branching
angles vary in a range from 15° to 90° [5]. Many factors affecting
structure have been discussed [2,13,16], but the detailed growth
mechanisms are still unknown. It remains unclear how the
branches emerge from the stems; how the interplay of nucleation
and growth at the microscopic scale results in specific densities,
branching angles and so on.
In this work, we describe the discovery and investigation of two
branch–stem interfacial structures occurring simultaneously in a
sonoelectrochemical process involving silver: the surfactant-free,
constant-current method leads to both, vertical branching and also
a slanted growth, sometimes even along the same stem. High res-
olution transmission electron microscopy (HRTEM) reveals two
distinct microstructures underlying the different interfaces. The
origin of such dissimilar interfacial structures will be discussed
in light of the many new proposals concerning non-equilibrium
growth.
2. Experiment
Silver dendrites were fabricated by a sonoelectrochemical reac-
tion with silver perchloride solutions as electrolytes. The simple
two-electrode setup was described previously [16–19]. Two iden-
tical silver sheets (2 cm ꢀ 2 cm) were cleaned with acetone, hydro-
chloric acid and distilled water to remove surface contamination.
The sheets were fixed on the two opposing sides of an electro-
chemical cell 4 cm apart in order to serve as the working cathode
and counter anode. The electrolyte concentration (C), current den-
sity (J), and the reaction time (t) were about 1.0 ꢀ 10ꢁ4 to 5 ꢀ 10ꢁ3
* Corresponding author. Fax: +86 25 8359 5535.
E-mail address: mengxk@nju.edu.cn (X. Meng).
0009-2614/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2009.06.087

180
S. Tang et al. / Chemical Physics Letters 477 (2009) 179–183
mol/L, 1.25 to 2.5 mA/cm2 and 10 to 20 min, respectively. Electrol-
ysis was conducted at room temperature under continuous 40 kHz
ultrasonic vibration at 50 W power. Products were precipitated
and purified by five centrifugation/rinsing/redispersion steps with
deionized water and ethanol.
and exhibit the branching angles more clearly, i.e. a vertical (inset
of Fig. 2a) and a slanted growth (inset of Fig. 2b) clearly distin-
guishes the dendrites. Sometimes, both types of growth even
appear on different sections of the same individual dendrite
(Fig. 2c). The areas representing vertical and slanted growth are
denoted as area ‘V’ and ‘S’, respectively.
The products were collected, dried at 40 °C for 12 h in an
oven, and characterized by X-ray diffraction (XRD) with a D/Max-
To reveal the origin of such dissimilar branchings, the branch–
stem interfacial structure was analyzed with much higher resolu-
tion. Fig. 3a shows a magnification of the area V, and the inset
further magnifies the squared area (black dashed). The strong con-
trast between the innermost and subsequent outer parts of the
stem in area V suggests a core–shell structure. We henceforth call
this subsequent shell ‘transition layer’, because there is yet another
outermost coating. That outermost surface of the dendrite is less
obvious (has less contrast) and is therefore specifically labeled in
the inset of Fig. 3a. A HRTEM image (Fig. 3b) magnifying the area
inside the larger dashed square in the inset of Fig. 3a reveals that
the outermost layer is amorphous, which indicates that an amor-
phous phase or growth front may have been forming first in this
sonoelectrochemical growth process. A few very small crystallites
with different crystallographic orientations are present in the
amorphous layer (arrows in the inset of Fig. 3a and white dashed
circles in 3b). Such an amorphous layer has been observed in the
growth of silver dendrites fabricated by a replacement reaction
[9]. Similar amorphous regions involving InAs dendrites were re-
ported by May et al. [23]. It is clearly seen (Fig. 3b) that the transi-
tion layer is single-crystalline. The fringe spacing of 0.23 nm
implies that these are interplanar spacings of (1 1 1) planes. More-
over, these fringes run continuously through both the transition
layer and the branch. Both belong to a single crystal and the con-
nection between branch and stem is not simply a physical contact
but rather like an epitaxial growth. The branches are not perpen-
dicular to the stem without the presence of the transition layer,
as shown in the upper section of the dendrite in Fig. 2c. The vertical
growth of branches is therefore attributed to this transition layer.
The core–shell interface between the transition layer and the inner
dark region (labeled ‘interface 2’ in Fig. 3b) is further investigated
with the images 3c and 3d. Fig. 3c shows the fast Fourier trans-
forms (FFT) of the areas A, B and C. The similarity between all
the hexagonal FFT patterns indicates that the transition layer and
the inner stem all belong to fcc silver. The electron beam transmits
through the whole core–shell structure, and the two types of spots
in the FFT corresponding to areas B and C demonstrate the core–
shell structure of stem and surrounding transition layer. Fig. 3d
displays the HRTEM of the smaller dashed square in the inset of
Fig. 3a. From the FFT pattern one can determine that the direction
orthogonal to h1 1 –1i in Fig. 3d is h0 1 1i. Also the branching direc-
tion is indicated in Fig. 3d, and it is parallel to the arrow in the inset
of Fig. 3a; there is no rotation between the images at different
magnifications. The branching direction happens to lie at about
30° away from h0 1 1i, i.e. it is not related to any specific crystallo-
graphic direction (e.g. a well known fast direction or suchlike). This
implies that the branching angle is under the control of global
diffusion while the crystal directions had an influence only locally
(grain rotation and realignment).
RA X-ray diffractometer (using Cu
Ka = 1.5418 Å radiation).
Transmission electron microscopy (TEM) samples were prepared
by dripping suspensions of the dendrites onto a carbon film
covering a copper grid and subsequent drying. TEM measurement
was performed with an FEI TECNAI F20 microscope operating at
200 kV.
3. Results and discussion
Fig. 1 shows an XRD pattern of the as-synthesized product from
a typical experiment (C, V and t were 1.1 ꢀ 10ꢁ3 mol/L, 1.25 mA/
cm2 and 20 min). The XRD pattern reveals that these Ag dendrites
are cubic Ag (a = 4.09 Å, JCPDS 04-0783). The diffraction peaks ob-
served can be indexed as (1 1 1), (2 0 0), (2 2 0) and (3 1 1) planes
of the face-centered cubic (fcc) structure, respectively. An absence
of other peaks indicates a high purity of the silver.
Over the whole range of investigated conditions, the sonoelect-
rochemical reaction always produces two types of dendrites. In the
TEM images they are easily distinguished by their specific angles
between branches and the main stem. TEM images of two typical
dendritic structures in the product (Fig. 1) are shown in Fig. 2a
and b. In Fig. 2a, the branches are approximately parallel to each
other and attach basically perpendicular to the stem (branching
angle is 90°). Also, most branches point towards one side of the
stem. The structure looks two dimensional although it grew in
solution. It is known that the growth happened in solution, because
without ultrasound irradiation, all the products adhere to the cath-
ode while the electrolyte remains clear, and our TEM observations
(not shown) evidenced that the products adhering to the cathode
are large conglomerates totally different from the dendrites shown
in Fig. 2a. The branches of the dendrite in Fig. 2b show angles of
53–61° towards the main stem; coinciding with many previous
reports [5,20,21]. Such angles indicate that the growth is globally
diffusion-controlled. This slanted branching is more commonly
observed under non-equilibrium conditions [5,16,21,22] and also
happens to be the more prominent in our products. The insets in
Fig. 2a and b magnify the areas inside the white dashed squares
(111)
The slanted branching from the circled area ‘S’ of Fig. 2c, is
further investigated with Fig. 4. It can be seen that the branches
have round cross sections (Fig. 4a). The dendrite has a bilaterally
symmetric structure with branches distributed roughly evenly be-
tween the sides of the stem. Fig. 4b magnifies the squared area in
Fig. 4a; no transition layer is visible. The HRTEM image in Fig. 4b
shows a connection region between a stem and one of the branches
(the square in Fig. 4a). It shows edge dislocations at the connecting
region between the crystal structure of the stem and that of the
branch (twinning), which both turn out to be single-crystalline
domains. The fringe spacings perpendicular to twin plane are
(200)
(311)
(220)
65
30
35
40
45
50
55
60
70
75
80
θ (degrees)
Fig. 1. XRD pattern of as-synthesized silver dendrites.

S. Tang et al. / Chemical Physics Letters 477 (2009) 179–183
181
Fig. 2. TEM images of typical silver dendrites (a) the vertical growth and (b) the slanted branching. The insets in the corners are magnifications of the dashed squares and (c)
an individual silver dendrite including both kinds of branchings.
Fig. 3. TEM and HRTEM images of a typical silver dendrite (a) local-magnification of area V in Fig. 2c; the inset further magnifies the black dashed square, (b) HRTEM image
magnifying the area inside the larger dashed square in the inset of (3a), (c) HRTEM image of the area of ‘interface 2’ indicated in image (b). The right-hand insets are the FFT
patterns corresponding to the squared areas labeled A, B, and C, and (d) HRTEM with even higher resolution showing the smaller dashed square in the inset of (3a).
Fig. 4. (a) The area S shown in Fig. 2c, and (b) HRTEM image of the squared area in (a).

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S. Tang et al. / Chemical Physics Letters 477 (2009) 179–183
measured to be 0.23 nm, which indicates (1 1 1) interplanar spac-
ing. Another set of fringes in the branch displays the (1 0 0) inter-
planar spacing of 0.20 nm, but the thereby inferred h1 0 0i
direction is not quite parallel to the branch. This is expected, be-
cause the branching angles are 53–61°, so they are not exactly
equal but quite close to the angle of 54.7° between the h1 0 0i
and the h1 1 1i direction perpendicular to the (1 1 1) twin plane
(Fig. 4b). Such angles indicate that the growth is globally diffu-
sion-controlled, but locally it results from oriented attachment,
which has been found to be responsible for the formation disloca-
tions during early crystal growth [24]. Nano-particle aggregation
by oriented attachment of nano-crystallites, has been recognized
as an important crystal growth mechanism for dentritic morphol-
ogy in solution [7,8,25–28]. The formation of dislocations is often a
direct consequence of oriented attachment of a nano-particle that
attaches with a small misorientation or atomic nonflatness at the
bonding interface, and so marks the occurrence of the initial parti-
cle–particle bonding [8,24] that initiates a new branch. The inset
again magnifies the marked square just above in order to point
out another edge dislocation (defined as having a Burgers vector
normal to the dislocation line) found within the connecting region.
Dislocations formed at interfaces due to oriented attachments in
other systems have also been reported by Penn and Banfield
[8,24]. The formation of defects coincides with the direct (no tran-
sition layer), slanted branching.
Our TEM observations reveal that two types of branch–stem
interfacial structures can appear even on the same silver dendrite.
This results from different nucleation and growth modes of the
branches on the stem. Generally, branch formation involves several
growth mechanisms: atom-by-atom crystal growth, amorphous
growth with later crystallization, oriented attachment of nanopar-
ticles and grain rotation and realignment, etc. Also in the case of
our sonoelectrochemical deposition system one can expect that
some of these mechanisms are responsible for the specific dendrite
branching angles. The proposed growth mechanisms for the here
observed silver dendrites are as follows:
A hypothesis consistent with the appearance of both growth
modes during the same experiment is a strong influence of the
ultrasound. Compared to the micro scale, ultrasound has a long
wavelength and is thus always focused differently at different loca-
tions in the reaction vessel (standing wave patterns). Thus, it pro-
vides different growth environments resulting in different growth
modes during the same experiment. Ultrasonic vibration may play
an important role especially for the vertical growth of the branches
because it is very difficult to find vertical branches for electro-
chemically grown silver dendrites without ultrasonic vibration
[31]. Such strong influence on the growth mechanism has been
suggested previously for the case of Ag sonoelectrochemical depo-
sition on silica spheres from AgNO3 solution [32].
4. Conclusion
In summary, our study shows an interesting phenomenon in
sonoelectrochemical growth processes where two branching an-
gles are found to coincide with distinct branch–stem interfacial
micro structures. It is proposed that during slanted branching,
the attachment of nanoparticle seeds to the stem’s surface results
in the observed twinning and its characteristic small edge disloca-
tions. The epitaxial crystal growth via atom-by-atom addition
develops a single-crystalline branch. During the vertical growth
of branches, silver may first deposit to form an initially amorphous
phase (growth front) departing orthogonally from the stem. Silver
nucleation in the amorphous layer matures the growing branch via
grain rotations into a single crystal. This work advances the under-
standing of the underlying growth mechanisms in dendrite
branching, which may be useful to achieve designable structures
(e.g. with well defined branch–stem angles) resulting in desired
properties.
Acknowledgments
This work was supported by a grant from the State Key Program
for Basic Research of China (2004CB619305), and National Natural
Science Foundation of China (50571044 and 50831004).
(1) In the slanted growth, the attachment of a nanoparticle seed
to the stem’s surface forms a nucleus for the growth of a side
branch. The observed twins and twinning induced disloca-
tions (Fig. 4b) are indicative of this. Regularities in dendrite
side branches (frequency and angle) can be attributed to
twinning [29]. Twinning is a common result of two fcc struc-
tures joining with their {1 1 1} facets to share a crystallo-
graphic plane [30]. Such oriented attachment has been
found to be responsible for the dislocations during early
crystal growth [24]. In the slanted growth mode observed
here, the branching angles are not exactly 55°, and therefore
they are likely affected by the diffusion of Ag precursors [5].
The epitaxial crystal growth via atom-by-atom addition
develops a single-crystalline branch.
(2) As for the vertical growth, this orthogonality is well known
to arise from growth within an amorphous layer, as
observed during vapor deposition experiments using cata-
lyst [23]. Indeed, we found the transition layer around the
main stem and a small amorphous layer around the whole
structure to be always present in vertically grown dendrites.
In our sonoelectrochemical deposition process, silver may
first deposit to form an initially amorphous phase (as simi-
larly observed by Ding et al. [9]) departing orthogonally
from the stem. In the area behind the growth front, amor-
phous silver spontaneously crystallizes and matures by
grain rotation and realignment. This is also indicated by
the presence of very small grains with different orientations.
The later observable transition layer is the matured part of
this deposition process from amorphous Ag.
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