Pattern formation of zinc nanoparticles in silica film by electrodeposition

Article abstract of DOI:10.1021/jp0643505

Zinc nanoparticles were grown within gel-derived silica films by applying a direct current voltage. Pattern formation of metallic Zn was studied as a function of applied voltage and metal-silica ratio. Average particle size varied from 5.2 to 20.2 nm by changing the applied voltage and silica concentration. It was found that the transition from fractal to dendritic morphology was possible due to crystalline anisotropy. From high-resolution transmission electron microscopy images and X-ray diffraction pattern a possible model is proposed to explain this observation.

Full text of DOI:10.1021/jp0643505

J. Phys. Chem. B 2006, 110, 20917-20921
20917
Pattern Formation of Zinc Nanoparticles in Silica Film by Electrodeposition
B. N. Pal and D. Chakravorty*
Unit on Nano Science and Technology, Indian Association for the CultiVation of Science,
JadaVpur, Kolkata-700032, India
ReceiVed: July 11, 2006; In Final Form: August 11, 2006
Zinc nanoparticles were grown within gel-derived silica films by applying a direct current voltage. Pattern
formation of metallic Zn was studied as a function of applied voltage and metal-silica ratio. Average particle
size varied from 5.2 to 20.2 nm by changing the applied voltage and silica concentration. It was found that
the transition from fractal to dendritic morphology was possible due to crystalline anisotropy. From high-
resolution transmission electron microscopy images and X-ray diffraction pattern a possible model is proposed
to explain this observation.
TABLE 1: Sol-Gel Films with Different Ratios of Zn(NO3)2
and SiO2
Introduction
Growth of metal nanoparticles by electrodeposition has
received considerable attention in recent years because of their
fractal geometry and dendritic growth.1-4 The latter has been
studied extensively as a model for understanding diffusion-
limited growth phenomena.5 In most of these electrodeposition
experiments an aqueous solution of electrolyte with a ringlike
anode and a sharp-tip cathode was used. The interesting aspect
of these experiments is the reason behind some patterns to be
dendritic while the others are fractal. In general, the transition
from fractal to dendritic structure is possible when anisotropy
dominates over random noise in the growth process.2 In electro-
chemical deposition, this type of transition is observed when
growth speed is increased.2,3 In the present work, we have grown
both fractal and dendritic structures of zinc nanoparticles inside
a silica film by the electrochemical deposition process. We have
also studied the crossover from fractal to dendritic growth in
two dimensions on the microscopic scale. Dendritic to stringy
transition has also been observed at very high growth speed.
Detailed microstructural analyses through high-resolution trans-
mission electron microscopy (HRTEM) and X-ray diffraction
(XRD) patterns of these films show that the crystalline aniso-
tropy in the nanometer range growth is the reason behind phase
transition in the micrometer range pattern formation. This
mechanism has also been established through a crystallographic
growth model. From XRD pattern and HRTEM images a model
is proposed to show how anisotropy in nanometer scale gener-
ates this transition in the micrometer scale. The details are
reported in this paper.
specimen no.
composition
1
2
3
4
20Zn(NO3)2, 80SiO2
30Zn(NO3)2, 70SiO2
40Zn(NO3)2, 60SiO2
50Zn(NO3)2, 50SiO2
was mixed dropwise when the solution was stirred. The volume
ratio of water, alcohol, and TEOS was kept at 5:5:1. This
mixture was stirred at room temperature for 1 h to obtain a
clear sol. Chemically cleaned and dried microscope glass slides
of dimension 2 cm × 2 cm × 2 cm were dipped into this sol
and drawn at a speed of 1 mm/s. This resulted in the formation
of transparent films with thickness of ∼5 µm. Two metallic
electrodes (aluminum as cathode and silver as anode) with sharp
edges were placed on the surface at a separation of 3 mm. A
direct current (DC) voltage (ranging from 5 to 60 V) was applied
between the electrodes for approximately 20 s at room temper-
ature. Current through the circuit increased abruptly to a value
of a few milliamps. This indicated formation of metallic
channels due to the electrodeposition process.
Optical micrographs of the structures generated by the above
treatments were taken in a Leitz Laborlux 18 Pol S optical
microscope in the transmission mode at magnifications ranging
from 500 to 1000. An atomic force microscope (AFM) of Park
Scientific Instruments, USA, was used to delineate the structure.
The detailed surface morphology was also studied by FESEM
(JEOL, JSM-6700F). The microstructure of the electrodeposited
pattern was studied by a JEOL 2010 transmission electron
microscope.
Experimental Section
Results and Discussion
Parts a, b, and c of Figure 1 are the optical micrographs of
specimen no. 2 in which fractal, dendritic, and stringy growth
of metal were induced by applying a DC voltage of 5, 25, and
40 V, respectively. From these three images it is clear that the
transition of morphology is possible by changing the growth
voltage, i.e., the deposition rate. For this particular sample
(specimen no. 2) fractal to dendritic transition is possible when
deposition voltage crosses 15 V. These dendritic structures can
also be changed to one-dimensional stringy structure when the
voltage increased above 35 V, i.e., at a very high deposition
rate.
The target composition chosen for the gel was xZn(NO3)2,
(100 - x)SiO2 (where x was varied from 20 to 50). The sol-
gel films with different ratios of Zn(NO3)2 and SiO2 prepared
are summarized in Table 1. The precursors used were Zn(NO3)2
and Si(OC2H5)4. Zinc nitrate salt was dissolved in distilled water
and ethyl alcohol solution, which was stirred by a magnetic
stirrer. A few drops of dilute HNO3 was mixed to keep the pH
level ∼4. Weighed amount of tetraethyl orthosilicate (TEOS)
* To whom correspondence should be addressed. Fax: (+91) 33 2473-
2805. E-mail: mlsdc@mahendra.iacs.res.in.
10.1021/jp0643505 CCC: $33.50 © 2006 American Chemical Society
Published on Web 09/19/2006

20918 J. Phys. Chem. B, Vol. 110, No. 42, 2006
Pal and Chakravorty
Figure 2. AFM micrograph of the structure shown in Figure 1a.
Figure 3. Transmission electron micrograph for the sample corre-
sponding to Figure 1a.
micrograph of the fractal structure, which was shown in Figure
1a. From this image it is evident that nanoparticles form the
fractal structure. Figure 3 gives the transmission electron micro-
graph for the sample. The diameters observed are in conformity
with the AFM picture. In high-resolution observation (Fig-
ure 4), it is found that there is no correlation of crystal planes
among the neighboring particles, which indicates there is no
preferred growth direction. This means there is no long-range
ordering among the nanoparticles. Figure 5 gives the histogram
of particle sizes. The data were fitted by a log-normal distribu-
tion function, and the extracted values of mean diameter and
standard deviation are 10.5 nm and 1.1, respectively. Average
particle size depends on the amount of silica present in the film.
Table 2 summarizes the average particle diameter and the
geometrical standard deviation. It is seen that the mean diameter
of Zn nanoparticles increases as the concentration of Zn2+ in
the precursor sol is increased.
Figure 1. Optical micrographs with metal growth in gel specimen of
composition 30Zn(NO3)2, 70SiO2 under different applied voltages. (a)
5 V, (b) 25 V, (c) 40 V.
The fractal dimension of the ramified structure (Figure 1a)
was calculated by a method described earlier.6 The value
obtained is 1.82. This is a consequence of the generalized
diffusion-limited growth mechanism. Figure 2 shows the AFM
In the case of dendritic growth TEM images show that there
is a variation of particle size and particle distribution with respect
to growth voltage. Table 3 summarizes the variation of the
average particle size and their distribution with voltage for speci-

Pattern Formation of Zinc Nanoparticles
J. Phys. Chem. B, Vol. 110, No. 42, 2006 20919
Figure 6. HRTEM for specimen no. 2 with a growth a growth voltage
of 25 V.
Figure 4. HRTEM for sample corresponding to Figure 3.
Figure 7. Selected area electron diffraction pattern obtained from
Figure 6.
Figure 5. Histogram of particle sizes as obtained from Figure 3.
TABLE 2: Average Particle Diameter and the Geometrical
Standard Deviation for Different Specimen
specimen
no.
growth voltage
(V)
mean diameter
(nm)
standard
deviation
1
2
3
4
5
5
5
5
5.2
10.5
16.5
20.2
1.0
1.1
1.1
1.0
Figure 8. Scanning electron micrograph of specimen no. 2 with metal
growth carried out at 25 V.
TABLE 3: Average Particle Diameter and the Geometrical
Standard Deviation for Different Growth Voltage
in a direction perpendicular to the c axis of hexagonal closed
packed (hcp) zinc crystal. The latter implies that the stacked-
plane structure is the microscopic source of the anisotropy,
which forms the micrometer-ranged dendritic structure of Zn.
The selected-area electron diffraction (SAED) pattern (Figure
7) taken from the image position of Figure 6 shows a single-
crystalline structure. The diffraction pattern indicates that the
dendrite is oriented perpendicular to the (0002) plane with the
three branches along the [101h0] (or [100]), [11h00] (or [11h0]),
and [011h0] (or [01h0]) directions.7-8 Figure 8 is the FESEM
image of these dendritic structures. From this image it is seen
that the growth is confined in a particular direction and mini
specimen
no.
growth voltage
(V)
mean diameter
(nm)
standard
deviation
2
2
2
2
20
30
40
50
10.0
9.2
8.5
7.8
1.0
1.0
1.1
1.1
men number 2. Figure 6 shows the HRTEM images for
specimen 2 with growth voltage 25 V. This lattice image shows
that the orientation of neighboring particles is the same, which
is a (101h0) plane. This indicates that the particles had grown

20920 J. Phys. Chem. B, Vol. 110, No. 42, 2006
Pal and Chakravorty
Figure 11. Schematic representation of proposed formation of the
micropine dendritic structure of zinc. (a) Fast growth along [101h0]
direction. (b) Subsequent side-branch growth alone [01h10] and [11h00]
directions. (c) Further growth of minibranches.
directions, for example, [101h0], initiates the growth along the
electric field direction and forms a needle in the direction of
[101h0]. Because of the electric field, growth of the dendrite in
this direction is much faster than along the other directions
(Figure 11a). Subsequent growth along the other two crystal-
lographically equivalent directions [011h0] and [11h00], leads to
the formation of symmetric branches on both sides (Figure 11b).
With further growth, all of the branches become thicker and
finally interconnected (Figure 11c) and form the micropine
dendrite structure as observed in Figure 1b.
Figure 9. X-ray diffractogram of specimen no. 2 with metal phase
grown at different voltages (a) 25 V, (b) 30 V, (c) 40 V.
Now the question is why the six 101h0 directions are the
directions of fastest growth? For electrochemical deposition, new
grains will grow if the size of the initial grain exceeds a critical
grain size (Nc). The critical dimension Nc for a 2D-like growth
is expressed as12
Figure 10. Schematic representation of the six crystallographically
equivalent directions of hcp Zinc.
bsꢀ2
(zeη)2
Nc )
(1)
branches are at 60° angles on both sides of the former. But in
the case of the stringy structure, the side branching totally
disappears. This implies that it does not get sufficient time for
the side branching due to very high growth rate and the growth
is confined only in the [101h0] direction.
where s, ꢀ, z, and b are the area occupied by one metallic atom
on the surface of the nucleus, the edge energy, the effective
electron number, and a constant, respectively.
η is the overpotential and is defined as
η ) E(I) - E0
(2)
This phenomenon was also observed during XRD investiga-
tion. The XRD patterns (Figure 9) show the phase structure
and crystal orientation of the Zn nanparticles. Parts a, b, and c
of Figure 9 are the XRD patterns of specimen no. 2 with growth
voltages 25, 30, and 40 V, respectively. Parts a and b of Figure
9 show that there are two Zn peaks in the XRD spectra, with
the highest at the (0002) crystal plane and another lower peak
at the (101h1). In Figure 9c there is only one peak, which
corresponds to the (0002) plane. By comparison of all these
figures, it is clear that the second peak is reduced with the
increase of growth voltage and completely disappears when the
growth rate is very high. This indicates that the (0002) planes
of zinc nanoparticles are highly oriented and parallel to the glass
substrate.10 From these results it can be concluded that there
exists a preferred growth direction,9 which is normal to the
(0002) crystal plane of the Zn nanoparticle arrays. By normal-
izing the peak areas to the peak intensities of a polycrystalline
bulk Zn standard,11 it is found that more than 95% of the Zn
nanoparticles in the arrays are oriented along a direction
perpendicular to the (0002) crystal plane.
where E(I) and E0 are the external current-induced potential
and the equilibrium potential of the electrode (potential in the
absence of the external current), respectively. From the equation
it is clear that in the present case for a particular growth voltage,
ꢀ is the only parameter, which can control the growth of a new
nucleus. This means the low-surface-energy grains grow faster
than do the high-energy grains. The rapid growth of the low-
surface-energy grains at the expense of the high-energy grains
results in an increase in grain size. So the low-surface-energy
facet appears more easily during the deposition. For Zn metal,
the {101h0} surfaces are the most densely packed, so the (101h0)
planes are the surfaces of lower energy than the (0002) planes.
Hence the (101h0) planes are the energetically most favorable
surfaces for hcp Zn and the formation of the {101h0} facets on
the Zn particle surface is expected more than the {0002} facets.
It is therefore believed that this anisotropy is the reason growth
takes place along the [101h0] direction. At the same time, the
formation of side branches dominated by the [011h0] and [11h00]
directions takes place. In case of a stringy pattern, the side
branching disappears due to very high growth in [101h0]
direction.
The formation of the dendrites could be explained by the
following representation. Metallic Zn has an hcp crystal
structure, which indicates that the (101h10) planes have six
equivalent directions (Figure 10). These directions are ([101h0],
( [11h00], and ( [011h0]. For spatial confinement, one of the
The above discussion indicates that the growth of zinc phase
is taking place in a two-dimensional space. To confirm this,

Pattern Formation of Zinc Nanoparticles
J. Phys. Chem. B, Vol. 110, No. 42, 2006 20921
n ) 1 - â (3)
We therefore obtain a â value around 0.5. This suggests that
the charge motion occurs in a two-dimensional structure. This
result is consistent with the mechanism of zinc pattern formation
discussed earlier.
Conclusions
In summary, we have described an experiment in which a
stable pattern formation of zinc nanoparticles is possible inside
silica film. This pattern can be changed from fractal to dendritic
and dendrtic to stringy only by changing the growth voltage.
For dendritic and stringy structure anisotropy in nanoscale causes
the structural transformation.
Acknowledgment. B.N.P. acknowledges the award of a
Senior Research Fellowship by CSIR New Delhi. D.C. thanks
Indian National Science Academy, New Delhi, for the award
of a Senior Scientist position. The work has been carried out
under the Nano Science and Technology Initiative of Depart-
ment of Science and Technology, Government of India, New
Delhi.
Figure 12. Variation of log(conductance) as a function of log(angular
frequency) in precursor gel film of composition 30 Zn(NO3)2, 70 SiO2.
References and Notes
TABLE 4: High-Frequency Slopes of These Curves Were
Estimated from Figure 12
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we have carried out alternating current conductance measure-
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