Electrochemical actuation of growing copper dendrimers in water

Article abstract of DOI:10.1021/jp061879t

We report the observation of electrochemical actuation in growing dendritic fibers made of self-assembled copper nanostructures (of 100 nm or less in diameter), on the metallic cathode of an ordinary aqueous electrochemical cell. This could be achieved when a copper anode and another metal cathode is placed in aqueous solution of either dilute HCl or CuSO₄. or CuCl ₂, or H₂O, and upon application of a DC voltage in the range of 1.5-12.0 V. The actuation could be observed with an on-off cycle of the applied voltage in the above range. The phenomenon was also observed with the growth of bimetallic structures using a number of electrolytes such as Ag(NO₃), Pb(NO₃)₂, CoCl₂, NiCl ₂, CdCl₂, ZnCl₂, and HAuCl₄. The fibrous structures remained stretched as they grew longer with time in the presence of an applied voltage. The extent of actuation was dependent on the applied voltage, the concentration of Cu²⁺ ions in the solution, and also on ions such as Na⁺, which did not get deposited at the cathode. The observed phenomena under different conditions have been explained on the basis of electrocapillarity.

Full text of DOI:10.1021/jp061879t

J. Phys. Chem. B 2006, 110, 22917-22922
22917
Electrochemical Actuation of Growing Copper Dendrimers in Water
†
‡
,†
,†,§
Biswa Ranjan Panda, Pinjala Nagaraju Rao, Anumita Paul,* and Arun Chattopadhyay*
Department of Chemistry, Department of Chemical Engineering, and Centre for Nanotechnology,
Indian Institute of Technology Guwahati, Guwahati 781 039, India
ReceiVed: March 27, 2006; In Final Form: August 18, 2006
We report the observation of electrochemical actuation in growing dendritic fibers made of self-assembled
copper nanostructures (of 100 nm or less in diameter), on the metallic cathode of an ordinary aqueous
electrochemical cell. This could be achieved when a copper anode and another metal cathode is placed in
aqueous solution of either dilute HCl or CuSO4, or CuCl2, or H O, and upon application of a DC voltage in
2
the range of 1.5-12.0 V. The actuation could be observed with an on-off cycle of the applied voltage in the
above range. The phenomenon was also observed with the growth of bimetallic structures using a number of
3 , 3 2 2 2 2 2 4
electrolytes such as Ag(NO ) Pb(NO ) , CoCl , NiCl , CdCl , ZnCl , and HAuCl . The fibrous structures
remained stretched as they grew longer with time in the presence of an applied voltage. The extent of actuation
2
+
was dependent on the applied voltage, the concentration of Cu ions in the solution, and also on ions such
+
as Na , which did not get deposited at the cathode. The observed phenomena under different conditions have
been explained on the basis of electrocapillarity.
Introduction
In addition, the fibers generated in the three-dimensional cell
also had similar structures and components. The presence of a
One of the primary focuses of nanotechnology based de-
vices1 is the fabrication and operation of nanoscale actuators,
which could be controlled by internal as well as external handles.
Solution-based actuators assume significance in the wake of
non-copper metal cation in the solution resulted in bimetallic
,2
2
+
fibers consisting of Cu (from Cu released by the anode) and
the other metal from solution. The fibrous structures remained
stretched as they grew longer with time. The extent of actuation
3
-5
observation of actuation of nanoscale structures with electrical
or self-generated forces and of devices such as the single
molecule torsional pendulum made of carbon nanotubes. In
addition, there has been a recent surge in efforts in making
polymer based actuators. Also, there have been reports of
observation of actuation in fluidic devices. Further, autonomous
movements have been induced into self-assembled structures
using gas bubbles generated by decomposition of H2O2.
Although, there have been several reports of the generation of
2
+
was dependent on the applied voltage, the concentration of Cu
6
+
ions in the solution, and also on ions such as Na , which did
not get deposited at the cathode. The observed angle of actuation
with voltage is consistent with the Lippmann equation, based
7
8
1
6
on electrocapillarity.
9
1
0
Experimental
Silver (Ag), Aluminum (Al), Gold (Au), Copper (Cu) wires
and stainless steel (SS) plates used in the experiments were
1
1-14
metal nanofibers, including electrochemical generation,
there is no report of actuation of mono or bimetallic fibrous
structures in aqueous solution, which are important in the
generation of futuristic devices. These structures would be
significant as they can be self-assembled from hybrid materials
and would exhibit actuation in the presence of appropriate
electromagnetic waves, magnetic and electric fields, and due
to specific molecular and ionic interactions in a solution.
We report the unprecedented observation of electrochemical
actuation in growing dendritic fibers15 made of self-assembled
copper nanostructures, on the cathode of an ordinary aqueous
electrochemical cell, with a copper anode and any common
metal cathode, upon application of a DC voltage in the range
of 1.5-12.0 V, and in the presence of one of the species such
as dilute HCl, CuSO4, CuCl2, H2O, or a host of other salts.
Typical scanning electron microscopic images of fibers grown
identically on glass slides, exhibited dendritic fibers consisting
of self-assembled Cu structures of 100 nm or less in diameter.
obtained from commercial sources. HCl (∼ 11.3 M), CuSO ‚
4
5H O, CuCl ‚2H O CdCl CoCl ‚6H O, Pb(NO ) , AgNO ,
2
2
2
,
2,
2
2
3 2
3
ZnCl , NiCl ‚6H O used herein were obtained from Merck
2
2
2
(India) and HAuCl (Hydrogen tetrachloroaurate (III), 17 wt.
4
%,) was procured from Sigma-Aldrich. For cathode we used
either commercially available Ag wire (∼2 mm diameter) or
Al wire (∼2 mm diameter) of typically 5 cm in length or SS
plate (cross section of 1 cm × 3 cm). Aqueous solution of HCl,
CuSO4, CuCl2, CdCl2, CoCl2, Pb (NO3)2, AgNO3, ZnCl2, NiCl2,
HAuCl4, or milli-Q water was used as the electrolyte. While
the cathode metal and the electrolyte were varied, the anode
was always the same in all the experiments, i.e., a 2 mm
diameter and 5 cm length Cu wire. The two electrodes were
placed in the electrolyte; about 2 cm apart in a 100 mL beaker
containing 100 mL of the solution and a potential difference
was applied across the electrodes using an ordinary DC
eliminator. When the concentration of the electrolyte was kept
-
4
at or below 5.0 × 10 M, self-assembled dendritic fibrous
structures grew on the cathode. This concentration is an upper
limit for the observation of dendritic growth, above which an
ordinary deposition of bulk metallic copper on the cathode was
observed. Furthermore, we could observe fibrous growth of the
*
Corresponding author phone: 91-361-258 2304; fax: +91 361 258
2
349; e-mail: anumita@iitg.ernet.in; arun@iitg.ernet.in.
†
Department of Chemistry.
‡
§
Department of Chemical Engineering.
Centre for Nanotechnology.
1
0.1021/jp061879t CCC: $33.50 © 2006 American Chemical Society
Published on Web 10/20/2006

22918 J. Phys. Chem. B, Vol. 110, No. 45, 2006
Panda et al.
structures in as low as 1.0 × 10-7 M concentration of the
electrolyte, although it took typically 1 h to achieve a 10 mm
length fibrous structure when a potential of 12 V was main-
tained. On the other hand, under identical conditions, but with
-
4
higher electrolyte concentration of 1.0 × 10 M, it took about
5
min to grow a fiber of similar length. In practice, there is no
lower limit of concentration as we find that the fibrous growth
of copper could also be obtained with only milli-Q (Millipore)
grade (resistivity 18.2 MΩ/cm) water as the electrolytic medium
and copper as the anode. However, the deposition was slow,
taking about 1 h to grow to a length of about 10 mm at a
deposition voltage of 12 V and when only 25 mL of milli-Q
water was used as the electrolyte. This observation is consistent
with report of similar dendritic silver growth from water using
Ag as the anode.11 It is to be noted that fibers grown at lower
concentrations are thinner and more fragile than those grown
at higher concentrations of the electrolyte. In addition, we find
that small amounts of gaseous product evolve at the cathode
(possibly H2) and the anode (possibly O2 and/or Cl2). We find
that gaseous evolution at the electrodes is greater at lower
concentrations of electrolyte and negligible at higher concentra-
tions of the electrolyte used in our experiments.
Composition of the Dendritic Fibrous Structure Grown
in Different Electrolytes. Scanning electron microscopic (SEM)
and energy-dispersive X-ray (EDX) analysis was pursued for
the composition analysis of various fibers. The dendritic fibers
were grown on a glass slide using a drop of the respective
electrolyte and applying a DC voltage of 6.0 V across the
electrodes immersed in the drop. Depending on the electrolyte
being used, the dendritic fibers with graded composition may
be grown. We have studied a variety of metal salts as electrolytes
and find that only those metal cations whose standard reduction
-
-
potential is less than that of water (2H2O + 2e f H2 + 2OH ,
E° ) -0.83V) yield the bimetallic fiber. For the salts of these
2+
metals (other than Cu ), in each case, we obtained graded
bimetallic dendritic fiber formation (see Table SI-1 in the
Supporting InformationsI). In addition, we have collected fibers
from the three-dimensionally grown fibers in regular cell and
studied them under SEM.
Results and Discussion
When the salt concentration was kept at a value equal to or
-
4
less than 5.0 × 10 M and copper metal anode was used
dendritic fibrous structures continuously grew on the cathode.
Scanning electron micrographs (SEM) of such structures grown
on a glass slide with cathode and anode clamped firmly to the
surface of the glass slide and using a drop of the electrolyte,
such that the “as grown” structure is preserved when recording
SEM, revealed the dendritic growth with assembly of particles
that have typical dimensions in the range of 100 nm or less
(Figures 1a). The close proximity of the glass slide in this setup
Figure 1. (a) Scanning electron micrograph of dendritic Cu grown on
a glass slide with an expanded view in the inset. (b) Scanning electron
micrograph of dendritic Cu grown in a regular electrochemical cell
with an expanded view in the inset. (c) Actuation of fibers at the cathode
with field on at 6.0 V and subsequent relaxation with the field off (d).
interferes with appropriate and controlled actuation, possibly
due to the role played by the additional interfacial forces, even
though we did observed movements of the fibers at the time of
the growth on the slides. Hence the actuation results were all
recorded on a regular electrochemical setup. It is to be noted
that because the dendritic structures grown in a normal
electrochemical cell had to be lifted off to be observed under
SEM, the overall symmetry of the dentritic structure (Figure
is probably similar in the two different electrochemical cells.
The actuation could be observed without any hindrance in the
normal electrochemical cell as the glass walls were well
separated from the growing dentrictic structures. We also
mention here that the structures in the regular cell (as well as
on glass slides) could be grown at any applied voltage between
1.5 and 12.0 V and they remained stretched perpendicular to
the electrode as long as the potential was applied. Although,
the fibers were made up of assemblies of nanoscale structures
1
b) could have been disturbed in the process. However in this
case despite the sample handling, we find that the structures
are composed of assembly of particles with dimensions typically
in the range of 100 nm very similar to those in Figure 1a (inset).
This suggests that the mechanism of growth of dentritic fibers

Electrochemical Actuation of Copper Dendrimers
J. Phys. Chem. B, Vol. 110, No. 45, 2006 22919
the final diameters of the fibers were of micrometer sizes and
hence the actuation of the fibers could easily be observed by
naked eye.
Turning off the voltage turns off the actuation (i.e., the fibers
switch to relaxed position along the electrode). The actuation
process was reversible as long as the time during the voltage
“
off” position did not exceed typically 7-9 s. If the time
exceeded this limit, the fibrous dendritic structure agglomerated,
which prevented further actuation in the next voltage “on” cycle.
A typical on-off cycle of actuation is shown in Figures 1c and
Figure 2. (a) Cone shaped nanofiber suspended at one of its end from
the cathode of an electrochemical cell. (b) Qualitative representation
1
d (refer to the Supporting InformationsII for video).
of the forces acting on the fiber. F
gravitational force, F is the viscous drag force, and F
required for actuation.
b
is the buoyancy force, F
g
the
Electrochemical deposition of dendritic metal fibers is known
d
a
is the force
to occur at rather low electrolytic concentrations, where the
16
reaction is diffusion controlled. This occurs via preferential
growth at the tip in comparison to stunted growth across the
length of the fiber due to kinetic factors. Thus, the fibers grow
longer and assume dendritic structures at the cost of thickness
of the deposited structures. Under these conditions the length
of the growing fiber would act as a polarizable electrode where
changes in interfacial tension, γ, is related to the changes in
0.03 N/m and is the potential when γ for copper electrode is
maximum. Thus
ꢀ
2
∆
γ ) γ - γmax ) -
V
(2b)
(
)
8πd
9,16
applied potential and to changes in solution composition: and
is governed by
Thus the applied electrical potential would lead to changes in
the interfacial tension, which in turn, would produce actuation
of the fibers as explained below.
qM
Actuation of the fiber could possibly be understood to occur
in a manner similar to the wetting and de-wetting characteristics
of a drop of liquid on a flat substrate surface. The drop spreads
or recoils to minimize the surface free energy as governed by
surface tension of the liquid drop, the surface tension of the
substrate surface and the interfacial tension between the liquid
and the substrate surface. In a similar fashion when a micro-
scopic fiber is subjected to different interfacial stresses the result
will be the actuation of the fiber as it cannot move, being bound
to the surface of the electrode. A model calculation shown below
indicates that, when a small fiber attached to an electrode (in
the present case Cu fiber on cathode) is subjected to an applied
voltage, to minimize the interfacial tension across the electrified
interface the fiber will actuate and will tend to remain perpen-
dicular to the electrode at a relatively high voltage. The extent
of deflection would depend on the magnitude of the applied
voltage, bulk solution ionic strength of an electrolyte and con-
centration of depositing species and shape and size of the fiber.
However, this can happen only if the cations (in the present
case) get continuously deposited at the tip of the fiber, while a
kinetic barrier prevents the growth across the length of the fiber
thus creating the situation of a polarizable electrode. For sim-
plicity of calculation and to be in line with the above argument
we consider the fiber to be conical in shape, with the base diam-
eter fixed at 100 nm and the length at 1 cm. The fiber is consid-
ered to be made up of Cu metal deposited at the cathode from
an aqueous solution of CuCl2 and the copper anode. A pictorial
representation of the fiber is shown in Figure 2a. The forces
acting on the fiber are represented qualitatively in Figure 2b.
Assumptions:
dγ ) -q dV - dµ -
∑Γidµi
(1)
M
j
zjF
where, dγ is the differential interfacial tension at the polarizable
electrode;
qM, the charge density on the electrode,
ꢀ
q )
V
(1a)
M
4πd
dV is the differential applied voltage, µj is the chemical potential
of species j which are released at the electrodes, zj is the charge
on the species j, dµi is the chemical potential of species i which
react on the unpolarizable electrode(s), Γi is the surface excess
o
i
o
i
of ith species; Γi ) ci/A - c /A, c is bulk concentration of i, ci
is concentration of species i at the electrode, and A is the cross
sectional area of the electrode.
At constant composition, the second and third terms are zero.
Hence
dγ ) -q dV
(2)
M
is the Lippmann equation.16
The solution leads to
ꢀ
2
γ ) γmax
-
V
(2a)
(
)
8
πd
where, d ) ø-1 is the Debye separation of double layer for a
Length of the conical fiber, l ) 1 cm, and its base radius, r
plane electrode, at 25 °C,
)
100 nm,
-
3
FCu ) 8230 kg m (same as for bulk copper)
-1
ꢀkt
ø
)
The fiber is polarizable along its length with deposition
2
0
2
x
8πe
∑
n
i
z
i
occurring at the tip only.
Gravitational force:
-
4
-1
2
-11
For 10 M CuCl2 solution, ø ) 13.1 nm
Fg ) πr lFCu x 9.81 ) 2.54 × 10
N
-
6
-1
-8
1
0
M CuCl2 solution, ø ) 131 nm, 10 M CuCl2
Buoyancy force:
-
1
2
-12
solution, ø ) 1310 nm, V ) applied potential difference, ꢀ )
Fb ) πr lFH2O x 9.81 ) 3.08 × 10
N
-
10
2
2
permittivity of bulk water ) 6.9 × 10
C /Nm
The viscous drag force acting on fiber during its movement
is (using Stokes equation)
or permittivity of structured water (at the electrode-
electrolyte interface)17 ) 5.31 × 10
-11
C /Nm , γmax for Cu )
2
2
Fd ) 3 πηUdeκ

22920 J. Phys. Chem. B, Vol. 110, No. 45, 2006
Panda et al.
TABLE 1: Actuation Velocity of a Typical Fibera
concentration of
electrolyte (M)
applied
voltage (V)
length of the
fiber (mm)
angular
deflection
time required for
the deflection (s)
velocity (m/s)
-
-
-
4
4
4
-4
-4
-4
1
1
1
.03 × 10
.03 × 10
.03 × 10
6
6
6
5.0
5.0
5.0
45°
45°
45°
7.0
7.5
9.0
5.61 × 10
5.23 × 10
4.36 × 10
mean velocity:
5.07 × 10-4m/s
where, κ ) convection factor for a fiber of uniform cross
section ) 1.25 to 1.5
In the present system assuming growth is slower than
actuation, r, h, ∆F, and g can be set constant, and hence, γ ∝
1/cos θ
In both cases shown in Figure 3, θ2 > θ1, such that average
angle θ of actuation of the filament is as follows:
ηH2O ) 8.905 × 10 N/sec m2
-2
-
1/3
de ) effective diameter ) ((6/π) x volume of fiber)
-9 2
-1/3
For the present fiber, de ) ((6/π) x π(100 × 10 ) x 0.01)
-
6
)
8.4 × 10
m
1
80° + θ - θ
1
2
The measured velocity of the fiber, U ) rdθ/dt ) 5.07 ×
θ )
(
)
-
4
1
0
m/s (Table 1)
Fd ) 3.57 × 10 N (when κ ) 1.25).
2
-
9
∴
-4
In absence of an applied field the net actuation force is zero.
This means the summation of γ1 and γ2 is equal to 0. This is
possible when the fiber is relaxed, i.e., θ1 ) 0° and θ2 ) 180°.
So, γ1 ) (rhg∆F/2) and γ2 ) -(rhg∆F/2) and thus the resultant
force F1 + F2 ) 0, cannot move the fiber away from main
electrode. Thus the fiber remains relaxed.
At a constant composition of 1.0 × 10 M CuCl2, the Debye
length d ) 13.1 nm and at an applied potential of V ) 6.0 V
eq 2b provides
-
2
∆
∆
γ ) 7.54 × 10 N/m for bulk water,
-3
γ ) 5.80 × 10 N/m for structured water.
The force due to interfacial tension at the polarizable electrode
On the other hand under the application of a potential the
net force is nonzero and the actuation of fiber occurs, which is
possible if ∆γ is nonzero when voltage is applied.
When actuation occurs, then θ > 0°, ∆γ is negative and the
resultant force is in the direction of actuation, as explained with
an example below:
If θ1 ) 60° and θ2 ) 110°, then θ ) 65° with the fiber
sloping downward (Figure 3a). Then γ1 ) 2.00 x (rhg∆F/2);
γ2 ) -2.92 x (rhg∆F/2) and R, the resultant force lifts the fiber
during actuation (Figures 3e and f).
Similar arguments can be extended to a fiber which is
originally directed upward (Figure 3b) and it can be shown in
that case also the fiber actuates in the direction of increasing θ.
This is because, in both cases, we are assuming the fiber has
two-half surfaces: lower and upper. Because of the symmetry
of a fiber, the maximum actuation for any fiber occurs when
the surface tension (∴ contact angle) above and below the fiber
)
electrocapillary force ) actuation force ) Fa
∴
Fa ) ∆γ × (half circumference) ) ∆γ x πr
where r is the radius of the fiber base.
∴
and similarly, Fa ) 1.82 × 10 N, for structured water.
-
8
Fa ) 0.0754 × πr ) 2.37 × 10 N, for bulk water,
-9
From the above calculation of forces Fd, Fg, and Fb, acting
on the actuating fiber, it is abundantly clear that buoyancy and
gravity forces are insignificant as compared to the drag force.
Moreover, Coulombic interaction between two fibers, which are
separated in the present case by millimeters, is too small (on
-
17
18
the order of 10
N) to make a meaningful contribution to
actuation and hence discounted in the discussion altogether.
Also, the magnitude of actuation force is greater than the
dominant drag force, and hence the actuation of fiber occurs
due to interfacial tension at the polarizable electrode.
Further, we show here that actuation can be upward or
downward depending on the original direction of orientation
of the fiber during growth. Consider two orientations of the
fiber in Figures 3a (initially pointing downward) and 3b (initially
pointing upward). The fiber shown in Figure 3a would ideally
remain attached to the cathode such that axis of the fiber (dotted
line) is directed down along the cathode, i.e., θ1 ) 0° when the
applied voltage V ) 0. When this fiber actuates it moves upward
under the application of external voltage i.e., θ1 > 0° when V
>
0. On the other hand the fiber shown in Figure 3b would
ideally remain attached to the cathode such that axis of the fiber
dotted line) is directed up along the cathode, i.e., θ1 ) 0° when
(
the applied voltage V ) 0. And the fiber would actuate and
move downward under the application of external voltage i.e.,
θ1 > 0°when V > 0. The corresponding vector (force) diagrams
are schematically shown in Figures 3c, 3d, 3e, and 3f.
The relation between interfacial tension (γ) and contact angle
(θ) for the electrified interface formed by the fiber and the
surrounding fluid layer is
rhg∆F
γ )
(3)
2cosθ
Figure 3. Schematic illustration of the orientation of fibers (a and b)
with angles relative to the cathode. Schematic illustrations of the
direction of actuation of fibers with corresponding vector diagrams (c,
d, e, f).
where, r ) radius, h ) height of fluid, ∆F ) difference of
densities between the copper fiber and the fluid forming the
electrified interface, and g is the gravitational acceleration.

Electrochemical Actuation of Copper Dendrimers
J. Phys. Chem. B, Vol. 110, No. 45, 2006 22921
TABLE 2: Cu2 Concentration Dependent Actuation of the
+
Fiber
2
concentration of CuCl (M) applied voltage (V) angle of deflection
-
-
-
-
-
5
5
5
5
5
5.00 × 10
5.02 × 10
5.04 × 10
5.06 × 10
5.08 × 10
6.0
6.0
6.0
6.0
6.0
14°
17°
56°
63°
90°
TABLE 3: Tabulated Data of Concentration of Cu2 with
+
Deposition Time
time (min)
concentration of Cu2 (M)
+
Figure 4. Plot of angle of actuation verses applied voltage. The applied
voltages and the corresponding θ values are in the table (inset).
-
-
-
4
4
4
0
5
0
5
0
25
30
34
4.21 × 10
4.28 × 10
3.82 × 10
3.91 × 10
3.49 × 10
2.85 × 10
3.12 × 10
8.18 × 10
1
1
2
are equal, i.e., 90°. During actuation, more of the surface is
exposed (similar to spreading of the drop on a flat substrate
due to the favoring of interfacial tension in the system). The
copper metal is not a fluid, but is composed of assemblies of
nanoparticles so it responds to this electrocapillary induced stress
by actuating the fiber at various possible junctions. When the
voltage is turned off, γ ≈ γmax ≈ 0 (for Cu) and hence the
actuation force is zero and the fiber relaxes. However, when
voltage is applied, then γ < γ max and fiber actuate as explained
above.
The above model presents a simple yet realistic account for
the actuation of a fiber by taking into account the driving and
opposing forces of actuation. Additionally, the actuation of the
branched dendritic structure observed in our study could be
explained by the same model, where a fiber is attached in a
branched fashion to a main fiber. The branched fiber actuates
on the main fiber, in just the same way as the main fiber actuates
on the cathode, upon the application of voltage. Further, the
calculation of drag force for a branched fiber would be more
involved. One can also extend the argument to the whole length
of a long and flexible fiber by considering that it is composed
of multiple small fibers of increasingly thinner average cross
section from its base to its tip. The approximations involved
are that the fibers are all of identical shapes and sizes. It is to
be noted that the conical shape used in our model is a
representative shape and is chosen just to demonstrate the
various forces acting on the fiber. Similar conclusions can be
obtained by choosing a differently shaped fiber as long as the
fiber is polarizable along its length. For simplifying the model
formulation, interaction between the adjacent fibers is neglected
and it is assumed that there is no growth of fiber during the
measurement of actuation.
-4
-4
-
-
-
4
4
5
1
centration of CuCl2. We found that the final actuation angle
was higher when the concentration of CuCl2 electrolyte was
higher (Table 2). When 2.0 mm dendritic structures were grown
at a CuCl2 concentration of 5.00 × 10 M then an actuation
of 90° could be observed at an applied voltage of 12.0 V. After
this when the voltage was stepped down to 6 V the angle of
actuation reduced to 14°. Upon gradual addition of a concen-
trated solution of CuCl2 to the actuated system, the angle of
actuation increased gradually and reached a value of 90°at a
-
5
-
5
final concentration of 5.08 × 10 M of CuCl2. The maximum
actuation angle is as always 90°. For example, when a much
-
4
higher concentration of CuCl2 solution was used (1.0 × 10
M) at an applied voltage of 6 V the angular deflection was still
0°. Similar results were obtained when CuSO4 was used as
9
the electrolytic medium instead of CuCl2.
When CuCl2 is added at constant applied voltage, the
1
6
condition of electro-neutrality reduces eq 1 to
dγ ) -Γ dµ
-
(4)
Where Γ- is surface excess of anions at the anode and dµ is
the chemical potential change of the electrolyte. At higher CuCl2
concentrations dµ is more positive as compared to when
concentration of CuCl2 is lower. Hence addition of CuCl2 makes
dγ more negative. As a result, more actuation is observed with
increasing concentration of CuCl2 at constant applied voltage.
Unfortunately, in this experiment, a relation between the angle
of actuation and dγ is not straightforward and depends on the
exact growth of the fiber. There are additional issues, such as
the extent of polarizability of the surface of the fibers and the
rate of growth of the fiber tips, which need to be taken into
account to quantify the effect.
Factors Affecting the Actuation of Fibers. We have studied
in detail the dependence of actuation on various factors such
voltage, concentration of electrolyte, concentration of other ions,
etc. These are reported below.
Extent of Deflection Versus Applied Voltage. It was
observed that the extent of deflection increased with applied
voltage. The results in Figure 4 demonstrate such a trend for a
single dendritic fiber of 2.0 mm length, initially grown at 12 V
2
+
Observed Variation of Concentration of Cu Ions in the
Solution as a Function of Time of Electrodeposition. Con-
2
+
centration of Cu was monitored by measuring the optical
density of the copper ammine complex at 605 nm after addition
of ammonia to a part of the electrolytic solution. The concentra-
tion of copper ion reduced with the time allowed for elec-
trodeposition of the copper dendritic structures (Table 3 and
Figure 5). When milli-Q water and no additional salt was used
as the electrolyte along with copper anode, then the concentra-
-4
in a 1.0 × 10 M CuCl2 electrolytic solution and then subjected
to various applied voltages. Unfortunately, a relation between
the angle of actuation and dγ is not straightforward and depends
on the exact growth conditions due to the reasons explained in
preceding paragraphs. However, our results in Figure 4 are
consistent with the Lippmann equation (eq 2). The plot of 1/cos
θ is found to vary linearly with the square of applied voltage
2
+
-5
tion of Cu reached a steady-state value of ∼6.6 × 10
M
2
(
V ), where θ is the experimentally observed angle of actuation,
(
after 50 min. of deposition at 12 V), which is close to the
consistent with that predicted by Lippmann equation (refer to
part B of the Supporting InformationsI).
Extent of Actuation at a Constant Voltage Versus Con-
-
5
limiting concentration of 8.18 × 10 M achieved after a 134
min of deposition using CuCl2 (Table 3).

22922 J. Phys. Chem. B, Vol. 110, No. 45, 2006
Panda et al.
(Lippmann equation), produced by changes in interfacial tension
at the electrified interfaces. An externally applied potential
induces a gradient in interfacial tension on two opposite sides
of the fiber resulting into its actuation. The actuation of fibers
could also be controlled by controlling the applied potential and
the concentration of electrolytes. This work is expected to propel
further research work in the field of micro and nano electro-
mechanical systems.
Acknowledgment. We thank the Department of Science and
Technology (DST), Government of India for financial support
(
no. SR/S5/NM-01/2005, and DST/TSG/ME/2003/83). A.C. is
Figure 5. Graphical plot showing the approach to a steady-state
concentration of Cu
2
+
a recipient of Swarnajayanti Fellowship Award (2/2/2005-S.F.)
of DST. A.P. thanks Council of Scientific and Industrial
Research (CSIR), New Delhi for a grant (01(1947)/04/EMR-
II).
TABLE 4: Effect of NaCl on the Actuation of the Fiber
concentration of NaCl (M) applied voltage (V) angle of deflection
0
5
1
12 V
12 V
12 V
90°
Note Added after ASAP Publication. Due to a production
error in the version posted ASAP on October 20, 2006, an error
occurred in the corresponding author footnote, eq 2b, and Table
-
-
5
4
0
.0 × 10
.0 × 10
M
M
21
0
0
1
2
. The correct version was published ASAP on October 24,
006.
The Effect of Addition of NaCl on the Extent of Actuation
of Fiber. When a few drops of concentrated NaCl was added
to a growing and actuated (with 90° deflection) copper fiber
electrolytic system (with 1.0 × 10 M CuCl2 electrolytic
solution and grown at 12.0 V till fibers were 1.5 mm long) the
maximum angle of actuation dropped (Table 4). Similar results
were obtained if Na2SO4 was added instead of NaCl.
The decrease in actuation of fibers on addition of NaCl can
be rationalized on the basis of ionic strength effects. Addition
of a nonreactive salt at constant applied voltage implies that eq
Supporting Information Available: Tables, figures, and a
-
4
video showing further details of the analysis. This material is
available free of charge via the Internet at http://pubs.acs.org.
References and Notes
(
1) Drexler, K. E. Engines of Creation: The Coming Era of Nano-
technology. Anchor Books: New York, 1990.
2) Ratner, M.; Ratner, D. Nanotechnology: A Gentle Introduction To
The Next Big Idea; Pearson Education: India, 2003, pp 1-12.
3) Che, G.; Miller, S. A.; Fisher, E. R.; Martin, C. R. Anal. Chem.
999, 71(15), 3187
4) Pei, Q.; Inganas, O. AdV. Mater. 1992, 4(4), 277.
(5) Smela, E.; Inganas, O.; Lundstrom, I. Science 1995, 268, 1735.
6) Paxton, W. F.; Sen, A.; Mallouk, T. E. Chem.- -Eur. J. 2005, 11(22),
462.
(
1
simplifies to
(
1
qM
dγ ) - dµj
zjF
(
(5)
(
6
where, zj ) +2 for Cu2+ ions.
(7) Meyer, J. C.; Paillet, M.; Roth, S. Science 2005, 309, 1539.
Initially the system is at a steady state with constant applied
voltage V, prior to the addition of NaCl. So the composition is
fixed and as per eq 1, dγ is constant and the fiber remains
actuated at a constant angle. The presence of additional
nonreactive ions (NaCl or Na2SO4) increases the ionic strength
(8) Jager, Edwin W. H.; Smela, E.; Inganas, O. Science 2000,
290(5496), 1540.
(9) Le Berre, M.; Chen, Y.; Crozatier, C.; Zhang, Z. L. Microelectro.
Eng. 2005, 78-79, 93.
(
10) Ismagilov, R. F.; Schwartz, A.; Bowden, N.; Whitesides, G. M.
Angew. Chem., Int. Ed. 2002, 41(4), 652.
(11) Smith, P. A.; Nordquist, C. D.; Jackson, T. N.; Mayer, T. S.; Martin,
B. R.; Mbindyo, J.; Mallouk, T. E. Appl. Phys. Lett. 2000, 77(9), 1399.
I) of the solution thus making dµCu2+ more negative in eq 5.
(
This leads to more positive dγ and reduction of the net actuation
force in the presence of NaCl. Higher the concentration of NaCl
lesser the actuation.
(
12) Yuan, Y. J.; Andrews, M. K.; Marlow, B. K.. Appl. Phys. Lett.
2004, 85(1), 130.
13) Huang Y.; Duan X.; Wei Q.; Lieber C. M. Science 2001, 291(5504),
30.
14) Cheng, C.; Gonela, R. K.; Gu, Q.; Haynie, D. T.. Nano Lett. 2005,
5(1), 175.
(15) “Parfenov, A.; Gryczynski, I.; Malicka, J.; Geddes, C. D.; Lakowicz,
J. R.; J. Phys. Chem. B 2003, 107, 8829.
16) O’M Bockris, J.; Reddy, A. K. N. Modern Electrochemistry, 1st
ed.; Plenum Publishing Corporation: New York, 1973; Vol. 2, p. 702.
(17) ꢀ ) K x ꢀo, where K is the dielectric constant of the medium; ꢀ is
the permittivity of the medium; and ꢀo is the permittivity of vacuum where
a) K ) 78 for bulk water and K ) 6 for structured water; O’M Bockris,
J.; Reddy, A. K. N. Modern Electrochemistry, 1st ed.; Plenum Publishing
(
6
Effect of Anode Material on Growth of Dendritic Struc-
ture of Copper Nanofiber. We found that when the anode was
made of metals other than copper, dendritic copper did not grow
on the cathode, even when the electrolyte was a copper salt.
On the other hand, if after growing a copper fiber using copper
anode we replaced the anode with any other metal, such as Ag
or stainless steel; we found that the growth as well as the
actuation stopped. This observation is important and suggests
that the origin of actuation is not due to the applied potential
drop as such across the two electrodes. On the other hand the
continuous growth of the fiber was observed to be a prerequisite
for actuation.
(
(
(
Corporation: New York, 1973; Vol. 2, p. 757, and (b) ꢀ ) 8.85419 ×
o
-
12
2
-1
10 C N m-2; Atkins, P.; Paula, D. Atkins’ Physical Chemistry, 7th ed.;
Oxford University Press: New York, 2006
(
18) The electrolytic cell is considered as a parallel plate capacitor, where
the cathode and anode are separated by a distance (d) of 2.0 cm and the
two consecutive fibers (that actuate) on the cathode are 1.0 mm apart (R).
Under an applied potential (V) of 6.0 V, the charge density on the cathode,
σ ) ꢀV/d, where ꢀ ) permittivity of the medium between the two plates
(electrodes) that is of water here. If one considers the radius of the fiber to
Conclusion
Herein we have been able to demonstrate electrochemical
actuation of growing dendritic mono and bimetallic fibers with
nanoscale structures in aqueous solutions. We have been able
to account for the actuation based on electrocapillary forces
-
9
be 100 nm and length 1 cm, then the charge on a fiber q) σ x 3.14 × 10
C. The electrostatic force between the two consecutive fibers is then F )
q /4πꢀR ) 4.87x10 N, for bulk water and F ) 3.74x10 N for structured
2
2
-17
-18
water.