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Hohai University
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National University of Singapore
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CTAB-Influenced Electrochemical Dissolution of Silver Dendrites
Colm O’Regan,†,‡,§ Xi Zhu,⊥ Jun Zhong,†,‡ Utkarsh Anand,†,‡,§,∥ Jingyu Lu,†,‡,§,∥ Haibin Su,*,⊥
and Utkur Mirsaidov*,†,‡,§,∥
†Centre for BioImaging Sciences, Department of Biological Sciences, National University of Singapore, 14 Science Drive 4, Singapore
117543
‡Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117551
§Centre for Advanced 2D Materials, National University of Singapore, 6 Science Drive 2, Singapore 117546
∥NanoCore, National University of Singapore, 4 Engineering Drive 3, Singapore 117576
⊥Division of Materials Science, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798
*S Supporting Information
ABSTRACT: Dendrite formation on the electrodes of a
rechargeable battery during the charge−discharge cycle limits
its capacity and application due to short-circuits and potential
ignition. However, understanding of the underlying dendrite
growth and dissolution mechanisms is limited. Here, the
electrochemical growth and dissolution of silver dendrites on
platinum electrodes immersed in an aqueous silver nitrate
(AgNO3) electrolyte solution was investigated using in situ
liquid-cell transmission electron microscopy (TEM). The
dissolution of Ag dendrites in an AgNO3 solution with added cetyltrimethylammonium bromide (CTAB) surfactant was
compared to the dissolution of Ag dendrites in a pure aqueous AgNO3 solution. Significantly, when CTAB was added, dendrite
dissolution proceeded in a step-by-step manner, resulting in nanoparticle formation and transient microgrowth stages due to
Ostwald ripening. This resulted in complete dissolution of dendrites and “cleaning” of the cell of any silver metal. This is critical
for practical battery applications because “dead” lithium is known to cause short circuits and high-discharge rates. In contrast to
this, in a pure aqueous AgNO3 solution, without surfactant, dendrites dissolved incompletely back into solution, leaving behind
minute traces of disconnected silver particles. Finally, a mechanism for the CTAB-influenced dissolution of silver dendrites was
proposed based on electrical field dependent binding energy of CTA+ to silver.
■ INTRODUCTION
Metallic dendrite growth on negative electrodes during the
charge−dischargecycle of a rechargeable battery is a major
obstacle to the commercialization of next-generation electro-
chemical storage devices.1−4 These dendritic structures are
responsible for the short-circuiting of batteries in portable
electronics5−7 and their formation can lead to cell ignition or
thermal runaway.8,9 Thus, dendrite formation in electro-
chemical cells presents an immediate safety concern regarding
next-generation battery development.10 Compounding this
problem is the continued miniaturization of device struc-
tures,11,12 which necessitates the development of methods that
enable advanced operando investigations of nanoscale reactions
inside an electrochemical cell.13−15 Despite the advancement of
such techniques, there is still a significant knowledge gap with
regards to dendrite formation and dissolution on electrodes.
The probing of these processes and the underlying mechanisms
is crucial in order to obtain a better understanding of how to
prevent electrochemical dendrite growth.16
Here, we report the use of in situ liquid-cell transmission
electron microscopy (TEM)17,18 to investigate the electro-
chemical dissolution of silver dendrites on platinum electrodes
immersed in an AgNO3 electrolyte solution. CTAB surfactant
was added to an aqueous solution of AgNO3 and the
“dissolution” of dendrites formed was compared to dendrite
dissolution in a pure aqueous AgNO3 electrolyte. The use of
CTAB is a common method for the formation and modulation
of silver (Ag) nanostructure shape.19−21 Previously, surfactants
have also been used to control the electrochemical growth of
Ag dendrites,22 but their application toward the understanding
of growth or dissolution mechanisms is still missing.
In our case, we show that dendrites completely dissolve in
solution when CTAB is added. The dissolution starts at the
dendrite tip and proceeds down to the electrode surface,
completely “cleaning” the solution of silver metal. This prevents
the formation of any “dead” metal in the electrochemical liquid
cell, critical for avoiding short circuits and high-discharge rates
in battery systems.3,23 Without CTAB, the dissolution does not
start at the tip, and in some cases the silver does not fully
Received: January 6, 2016
Revised: March 25, 2016
Published: March 27, 2016
Article
pubs.acs.org/Langmuir
© 2016 American Chemical Society 3601 DOI: 10.1021/acs.langmuir.6b00037
Langmuir 2016, 32, 3601−3607
pubs.acs.org/Langmuir
http://dx.doi.org/10.1021/acs.langmuir.6b00037
dissolve. Instead, the dendrite disconnects from the electrode
surface, leaving silver metal in solution.
■ RESULTS AND DISCUSSION
The electrochemical cell setup used for this study is illustrated
in Figure 1. Our electrochemical liquid cell was assembled in a
Hummingbird Scientific flow holder using two ultrathin (50
nm) electron transparent SiNx membranes (Supporting
Information, Figure S1). The silver nitrate (AgNO3) electrolyte
was injected into the cell at 2 μL min−1 using a Hummingbird
Scientific flow-through pump station. Ag was selected as the
metal of choice because it has sufficient mass−thickness
contrast for imaging inside liquids within a transmission
electron microscope due to its large atomic number (Z =
47). More importantly, it also has a positive standard
thermodynamic potential (+0.80 V) with respect to the
standard hydrogen electrode (SHE),24 so the reduction
reaction at the electrode surface proceeds spontaneously, as
given by the reactions below:
+ → = +
→ + =
+ → + = +
+ −
+ −
+ +
E
E
E
Ag e Ag 0.80V vs SHE
H 2H 2e 0.000V vs SHE
Ag H 2H Ag 0.80V vs SHE
0
2 0
2 0
Consequently, the electroplating of Ag is relatively consistent,
when compared to other metals such as copper (+0.34 V) and
lead (−0.13 V).24 AgNO3 was used specifically as the
electrolyte because it dissolves readily in water, meaning facile
electrolyte preparation. Platinum is a common electrode
material used for electrochemical experiments because it does
not corrode or dissolve in solution. Additionally, excellent
reproducibility of the electrode potential25 and the effective
catalyzing of the proton reduction at the electrode surface made
it a suitable material for our in situ studies.
A DC current−voltage profile was applied and the dynamics
of the interface between the electrode and liquid electrolyte
were monitored in real time inside a JEOL 2010FEG
transmission electron microscope at a frame rate of 10 frames
s−1 and 1024 × 1024 pixels with an Orius SC200 camera
(Gatan Inc. Pleasanton, CA).
Dendrite growth and dissolution from this AgNO3 solution
in our electrochemical cell is shown in Figure 2A. This shows
an in situ TEM image series of Ag dendrites growing and
dissolving on the working electrode when an electrical bias was
applied (Supporting Information, Video 1). The image series of
dendrite growth dynamics in a thin film were acquired after the
liquid receded due to bubble formation26 from electron beam
exposure. For all observations reported, the potential difference
applied between the working and counter electrodes was
between +2 V and −2 V. The working electrode is visible in the
bottom right corner of all frames as a reference (Figure 2A).
Dendrite growth usually proceeded over several voltage cycles
(Figures S2 and S3), where dendrites grew with a negative bias
and dissolved with a positive bias. Electron diffraction analysis
was performed on dendrites in both CTAB and aqueous
AgNO3 solutions, and are indexed to the face-centered-cubic
silver lattice (JCPDS file No. 04-0783) (Figure S4).
The area of two branches of the dendrite shown in Figure
2A, during growth and dissolution (t = 0 s to t = 97.1 s), is
given in Figure 2B. The initial phase of growth of the dendrite
in Figure 2 is fast, from t = 5.5 s to t = 12 s, as seen from the
projected area curves, after which growth slows down and the
area of the dendrite levels off due to a depletion of Ag+ ions
surrounding the electrode. During the growth, most of the
reduced Ag0 in solution was deposited on the electrode and
contributed toward dendrite growth. Hence, as time proceeded,
less and less Ag+ ions were available in solution for reduction.
When the bias was switched from −2 to +2 V at t = 28 s, the
dendrites dissolved back into solution over a time period of
approximately 60 s (panels: t = 66.9 s, t = 77.2 s, t = 97.1 s,
Figure 2A and the brown shaded region of Figure 2B).
Importantly, the dissolution started at the electrode surface and
proceeded upward toward the dendrite tip, as observed by
Schneider et al.27 This disconnected the dendrite from the
electrode surface, leaving “dead” silver in solution. This can be
seen in the last panel of Figure 2A (t = 97.1 s), where even after
90 s, pieces of Ag dendrite are still present in solution. Note
that the effect of the electron beam on dendrite dissolution was
also tested and was found to be negligible (Figure S5). We
avoided in situ imaging at high magnification to keep the
electron flux rate low (≤1 e Å−2 s−1) and to minimize the beam
induced effects (Figure S6).
To investigate the effect of surfactants on Ag dendrite
dissolution, the aforementioned experiment was also performed
in a solution containing 1 mM AgNO3 and 0.5 mM CTAB
surfactant. The dissolution of dendrites with a positive bias of
+2 V in a 1 mM AgNO3 solution with and without CTAB
surfactant is shown in Figure 3A and B, respectively. Figure 3A
represents a time frame of 0 to 13.2 s, and Figure 3B represents
a time frame of 0 to 15.4 s. The CTAB concentration was kept
to 0.5 mM, as CTAB exhibits a critical micelle concentration of
approximately 1 mM.28 As shown in Figure 3A (Video 2),
dendrite dissolution proceeded in a stepwise manner. This
process involved the dendrite completely dissolving in solution,
starting at the tip, with nanoparticle formation observed. In
Figure 1. Schematic of the liquid cell setup used during the in situ
TEM study. The top and bottom SiNx membranes sealed the AgNO3
solution in place. The solution was injected into the cellusing a
syringe and flow station. The bottom membrane contained three
electrodes, spaced 20 μm apart. The cell, after assembly, was then
placed in a flow-holder and inserted into the transmission electron
microscope. The bottom panel shows the magnified electrode region
of the liquid cell.
Langmuir Article
DOI: 10.1021/acs.langmuir.6b00037
Langmuir 2016, 32, 3601−3607
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pure aqueous AgNO3, the dendrites did not dissolve completely
into solution, with small trace amounts of silver particulates
remaining behind, as seen in Figure 3B (Video 3).
For Figure 3A (also see Figure S7 and Video 2), at t = 2 s,
the dendrites began to dissolve, starting at the tip. Dissolution
initially occurred from t = 2 s to t = 3.8 s, after which
approximately one-third of the dendrite has dissolved. This was
followed by nanoparticle formation, and microgrowth of new
branches at t = 3.8 s at the tip of the dissolving dendrite due to
Ostwald ripening - smaller particles giving way to larger
particles, which eventually dissolved back into solution. At t =
5.5 s, dendrite dissolution began again and proceeded to t = 7.2
Figure 2. (A) TEM image series showing one cycle of Ag growth and dissolution on the working electrode upon the application of an electrical bias,
as indicated by the label on the electrode. Importantly, parts of the dendrite, as well as Ag particulates are still visible at t = 97.1 s, approximately 60 s
after dissolution started. (B) 3D schematic of the dendrite in Figure 2A at t = 9 s, along with the area curves for the two branches labeled 1 and 2.
The red curve represents branch 1, and the yellow curve represents branch 2. The blue shaded region represents a negative bias (−2 V) applied to
the electrode shown in (A), resulting in dendrite growth, while the brown shaded region represents a positive bias (+2 V) resulting in dissolution.
Figure 3. Comparison of dendrite dissolution in an aqueous AgNO3 + CTAB versus AgNO3 solution. (A) TEM image series showing dendrite
dissolution in 1 mM AgNO3 + 0.5 mM CTAB aqueous solution. Particle formation and microgrowth was observed during stage-wise dissolution.
The dendrite dissolved from its tip toward the root. No leftover Ag metal was observed in solution after the dendrite dissolved. (B) TEM image
series showing dendrite dissolution in 1 mM AgNO3 aqueous solution in the absence of CTAB surfactant. Dendrite dissolution started at its root.
Remaining Ag nanoparticulates are observed in solution.
Langmuir Article
DOI: 10.1021/acs.langmuir.6b00037
Langmuir 2016, 32, 3601−3607
3603
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http://dx.doi.org/10.1021/acs.langmuir.6b00037
s where more particle formation and microgrowth was observed
at the dendrite tip. At t = 7.2 s, two-thirds of the dendrite had
dissolved into solution. Dissolution began once again at t = 8.5
s, and proceeded until the entire dendrite had dissolved. This
dissolution starts at the dendrite tip and proceeds toward the
electrode surface (Figure S7). Plots showing the change in area
as a function of time for dendrites in both Figure 3A and B are
given in Figure S8.
Significantly, this dissolution process from dendrite tip to
electrode surface may benefit battery technology due to the
prevention of “dead” lithium−metal which has disconnected
from the electrode, and can cause short circuits and high
discharge rates.3,23 Therefore, dissolution starting at the tip and
moving down toward the electrode, which is observed in CTAB
solution, means the entire dendrite is removed. The formation
of “dead” silver during the dendrite dissolution in an aqueous
AgNO3 solution (in the absence of CTAB surfactant) is also
shown in Figure S9.
In order to resolve the observed mechanism of dendrite
dissolution in a AgNO3 solution with added CTAB surfactant,
the binding energies of CTA+ to a Ag (111) surface were
computed by first-principles method, and plotted against the
electric field strength (see Methods and Materials), in Figure 4.
This plot shows how the binding energy of CTA+ decreases as
the field strength increases. When a positive external electrical
bias is applied, the binding energy between CTA+ and the silver
dendrite surface decreases as the electric field strength
increases. This reduction in binding energy leads to the
desorption of CTA+ from the Ag dendrite surfaces, exposing
the Ag to the solution and causing it to oxidize to Ag+ ions. Due
to the concentration of electrical lines near sharp edges,29 the
regions with high curvature in the dendrite, such as the tips of
the dendrite branches, exhibit the highest electric field
strength.30,31 The higher the electric field strength (at sharp
edges and dendrite tips), the less the CTA+ coverage and the
more Ag is exposed to the solution and oxidized to form Ag+
ions. The change in relative CTA+ coverage of silver was
plotted in the inset of Figure 4 at T = 300 K. This plot shows
the CTA+ coverage change as a function of the electric field
strength. The reduction in CTA+ coverage over the dendrite
surface will be greater as the binding energy decreases. This
plot suggests that for the electric field strength of 2.53 × 108 V
m−1 (estimated from a Debye length of 7.9 nm; see Methods
and Materials), an almost 100% reduction in CTA+ coverage
would occur.
This dissolution process of dendrites in an AgNO3 solution
with added CTAB surfactant is shown schematically in Figure
5. We propose that the dissolution process of dendrites in
CTAB solution is due to the detachment of CTA+ ions from
the Ag. Step 1 of the process began when the bias is switched to
positve, which modulates the electric field and influences the
binding energy. The higher the field strength, the lower the
binding energy of CTA+ to the Ag surface, as already seen in
Figure 4. The electric field will be stronger at the sharp points
of the dendrite tips. Therefore, CTA+ ions will desorb first in
the vicinity of these regions to expose silver directly to the
solution in step 2. Step 3 in Figure 5 is the oxidation of silver at
the dendrite branch tips followed by dissolution of these
exposed regions of dendrite. Finally, step 4 is Ostwald ripening
leading to microgrowth at the dendrite tip and particle
formation during the dissolution process. This dissolution
stage with the four above-mentioned steps is then repeated
until the dendrite is fully dissolved back into solution. Figure
3A (Video 2) shows three dissolution stages, which are
highlighted in the bottom left section of Figure 5. The first
major dissolution stage occurred at approximately 4.0 s, the
second at 8.4 s, and the final stage at approximately 13.0 s.
Ostwald ripening of CTAB-capped nanoparticles has been
observed previously,32,33 as well as Ostwald ripening of
nanoparticles stabilized by other molecules.34 Jang et al.33
reported the Ostwald ripening of CTAB-capped Au nano-
particles which originated from the combination of redox
reactions between H2O2 and AuNP’s under weakly acidic
conditions. In particular, they highlighted the role of Br− ions in
the process.33 In our case of electrochemical growth, H2O2 was
not requiredto initiate the oxidation of CTA+-covered Ag
dendrites, as this was accomplished when the electrical bias was
switched to positive. Particle formation was initiated by a
reduction in coverage of the dendrite surface by CTA+ due to
the positive bias. The surfactant stabilized the surface of the Ag
dendrites initially, which explains why they did not dissolve
back into solution from root to tip,32 as observed with the pure
aqueous AgNO3 solution. However, the gradual detachment of
CTA+ due to the strong electric field allowed oxidation of Ag
due to the positive bias and subsequent Ostwald ripening to
form larger particles in solution. We propose that Ostwald
ripening occurred due to the simultaneuous oxidation of the Ag
dendrites due to the positive bias and reduction caused by Br−
in solution, leading to microgrowth and particle formation. This
was likely due to the formation of AgBrx
− complexes in
solution, as seen with Au by Jang et al.33 Also, this Ostwald
ripening process resulted in the complete dissolving of silver
dendrites in CTAB solution, “cleaning” the cell of any
remaining metal.
This formation of particles in solution during the dendrite
dissolution process is an interesting occurrence, considering the
catalytic capability of Ag and other metals. Overall, formation of
electrode-bound nanoparticles in battery systems may prove
Figure 4. Binding energy of CTA+ to the Ag dendrite surface plotted
as a function of the electric field strength. Inset shows a plot of the
change in relative coverage versus the electric field strength, | ⃗|E . This
relative change in CTA+ coverage of the Ag surface at T = 300 K is
g i v e n b y = −ΔC C E k T/ exp( / )0 b B . H e r e ,
Δ = | ⃗| − | ⃗| =E E E E E( ) ( 0)b b b . The higher the electric field strength
(at sharp edges and dendrite tips), the less the CTA+ coverage, and the
more Ag is exposed to the solution and oxidized.
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advantageous, as silver nanoparticles have been shown to
enhance the performance of Li−air batteries by catalyzing the
formation of Li−O bonds.35 Other metals have shown similar
behavior and this suggests Ag particles may hold promise in
future battery technologies.36
■ CONCLUSIONS
In summary, this study revealed, through in situ liquid-cell
TEM, that CTAB surfactant influenced the electrochemical
dendrite dissolution in an AgNO3 solution. When CTAB was
added, Ag dendrites dissolved through the process of
nanoparticle formation and microgrowth due to Ostwald
ripening. The dendrites proceeded to dissolve from tip to
root (i.e., toward the electrode surface) in steps, leading to
complete dendrite dissolution and the absence of remaining
“dead” silver. In contrast, without CTAB, the dendrites did not
dissolve completely into solution. Instead, they dissolved from
root to tip and thus disconnected from the electrode surface at
the root, leaving behind small traces of silver metal. This study
suggests possible control of electrochemical dendrite growth
and dissolution in battery systems using additives. Silver
nanoparticle formation is an important study for Li battery
systems due to their electrocatalytic behavior. Hence, future
work may highlight specific additives that can be utilized to
suppress the growth or enhance the dissolution of different
dendrites.
■ METHODS AND MATERIALS
Materials. A 10 mM AgNO3 electrolyte solution was prepared by
dissolving 0.017 g of AgNO3 powder (Cat# 204390, Sigma-Aldrich
Co., St Louis, MO) in 10 mL of water. This was then diluted down to
1 mM by adding 0.1 to 0.9 mL of ultra pure H2O (Cat# 320072,
Sigma-Aldrich Co., St Louis, MO).
A 10 mM CTAB solution was prepared by dissolving 0.036 g of
CTAB surfactant (Cat# H9151, Sigma-Aldrich Co., St Louis, MO) in
10 mL of water. This was then diluted down to 0.5 mM by adding 0.05
to 0.95 mL of 1 mM AgNO3 solution.
Experimental Methods. Prior to loading, the top and bottom
chips of the cell were oxygen plasma treated to render their membrane
surfaces hydrophilic. This was done with a plasma coater (K100X
Glow Discharge System, Quorum Technologies, Lewes, East Sussex,
U.K.) discharging at 10 mA for 30 s with a base pressure of 2 × 10−1
mbar. The holder tubing system was flushed slowly with ultrapure
water at a rate of 200 μL min−1 for approximately 1 h prior to loading
the AgNO3 solution to ensure all possible contaminants were flushed
from the system. The top and bottom chips were then secured in place
within the liquid flow-through holder (Hummingbird Scientific, Lacey,
WA), and the integrity of the membranes tested using a pumping
station. The solution was loaded at a rate of 2 μL min−1 from a 1 mL
syringe, until approximately 100 μL had passed through the flow-
holder. The low rate of 2 μL min−1 was used to ensure the integrity of
the ultrathin SiNx membranes remained intact during loading. Finally,
the integrity of the cell membranes was tested again using the pumping
station (Hummingbird Scientific, Lacey, WA).
We used a JEOL 2010FEG transmission electron microscope
operating at 200 kV for in situ imaging with electron flux rates of less
than 1 e Å−2 s−1. Imaging and recording was performed using an
ORIUS SC200 CCD camera (Gatan Inc. Pleasanton, CA), at a rate of
10 frames s−1 and 1024 × 1024 pixels. When irradiated with the
electron beam, the liquid in the cell retracted, leaving behind an
ultrathin layer of electrolyte solution on the membrane, which was
ideal for imaging dendrite formation.
Finally, electrical data was recorded using an Agilent B2901A
Precision Source/Measure Unit and a Keithley 2450 SourceMeter that
were switched between +2 and −2 V. DC voltage profiles were used
for our experiments.
Image Processing. We implemented our image processing
algorithm in the Miniconda python37 distribution using the
numpy,38 opencv,39 scikit-image,40 cython,41 mahotas,42 and matplot-
lib43 libraries. The sequence of the raw image frames were inverted so
that dendrites have higher intensity values than the background. In
order to get rid of the illumination gradient the inverted images were
subjected to a Gaussian blur σ = 1 pixels, followed by a gray scale top-
hat transform with a disc shaped structuring element of radius 20
Figure 5. Schematic showing the mechanism of Ag dendrite dissolution in AgNO3 + CTAB aqueous solution. Step 1 involves the switching of the
bias to positive, which influences the binding energy of CTA+ to the Ag surface. The electric field was strongest at the tips and sharp points of the
dendrite, which results in CTA+ detaching from these areas first, as seen in step 2. Step 3 is the oxidation of this exposed Ag to Ag+ ions in solution,
resulting in dendrite dissolution beginning at the dendrite’s upper regions. This is followed by step 4, the formation of larger particles and
microgrowth at the dendrite tips due to Ostwald ripening. This four-step stage of the dissolution process repeats from tip to electrode surface until
the entire dendrite is dissolved. The TEM image on the bottom left represents the dendrite after the first dissolution stage has finished.
Langmuir Article
DOI: 10.1021/acs.langmuir.6b00037
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pixels. The background subtracted images were again subject to
another Gaussian blurring with σ = 2 pixels to smoothen out the high
frequency noise features. Then, the Otsu44 threshold algorithm was
used to segment out the dendrites, where all the pixels above a certain
intensity value were marked as 1, and 0 otherwise. Note that, atthe
end of this step, most of the regions with dendrites were marked as 1.
The dendrites had a very poor contrast, and the above method
failed for some branches in the field of view. In order to get rid of this
fluctuation in segmentation, the observation that dendrites keep
growing for a certain duration of time was used, implying that the area
of dendrites can only increase. Boolean “OR” operations were
performed on the frames where dendrites were growing, wherein the
segmentation result of n+1th frame was replaced by the Boolean OR
result between nth frame and n+1th frame. A similar operation
between the nth and n−1th frames was carried out when the dendrites
were dissolving.
Further refinement of the segmented images was done based on the
area of the segmented regions. A connected region which does not fall
within a certain permissible limit of area was classified as 0, and
henceforth considered to be a part of the background.
Binding Energy Calculations. Here, we chose the Ag (111)
surface to model the dendrite facets since this surface has the lowest
surface energy and so is the most stable. The surface structure
consisted of 384 Ag atoms, with one CTAB molecule adsorbed on it.
First principle density functional theory was implemented in SIESTA
code,45 within the local density approximation.46 A cutoff energy of
4082 eV and a double-ζ polarized basis45 were chosen for all elements,
to investigate the energetic nature of this structure. The computed
binding energy (Eb) was defined as
= − −+ +E E E Eb CTA /Ag(111) Ag(111) CTA
where ECTA+, EAg(111), and ECTA+/Ag(111) are the computed energies of
CTA+, the Ag(111) surface, and CTA
+ adsorbed on the Ag(111) surface,
respectively. The computed binding energy was 2.23 eV, which is in
good agreement with previous results.21 To account for the change of
the binding energy due to the external electric field, we calculated the
binding energy with the presence of the electric field using SIESTA
code. The electric field dependent binding strength was plotted in
Figure 4. Since the variation of the binding energy lead to a change in
the coverage of CTA+ on the Ag dendrites, the relative change in
coverage of CTA+, C/C0 on Ag dendrites is given by
47
= −ΔC
C
E k Texp( / )
0
b B
Here, kB is the Boltzmann constant, while = | ⃗|C C E( ) and
= | ⃗| =C C E( 0)0 0 are the CTA+ coverage of the Ag surface in the
presence of an electric field of strength | ⃗|E and in the absence of an
electric field (| ⃗| =E 0), respectively. We computed the change in the
binding energy, Δ = | ⃗| − | ⃗| =E E E E E( ) ( 0)b b b , at a temperature of T =
300 K using SIESTA codes. The relative coverage of silver surface by
CTA+ ions was plotted in the inset of Figure 4.
The Debye length (δ) was calculated using47
δ
ε ε
=
⎛
⎝⎜
⎞
⎠⎟
k T
e N C2
0 B
2
A
1/2
where C is the ion concentration of CTAB and AgNO3 (0.5 and 1 mM
respectively), e = −1.6 × 10−19 C is the charge of an electron, ε = 78 is
the relative permittivity of water, ε0 = 8.854 × 10
−12 C2 J−1 m−1 is the
permittivity of free space, NA = 6.022 × 10
23 mol−1 is Avogadro’s
constant, and T = 300 K. δ was calculated to be 7.9 nm, which led to
an estimated electrical field strength of δ| ⃗| = ∼ ×E V / 2.5 108 V m−1
for a potential of 2 V.
■ ASSOCIATED CONTENT
*S Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.lang-
muir.6b00037.
Schematic of electrochemical liquid cell, SAED analysis,
detailed image series showing dendrite tip dissolution,
image series showing “dead” metal left behind in solution
in pure aqueous AgNO3 solution, and further image
series of dendrite growth in pure aqueous AgNO3
solution (PDF)
Growth and dissolution of silver dendrite on a platinum
electrode immersed in a pure aqueous 1 mM silver
nitrate solution; video shows 1 cycle of growth and
dissolution at −2 V and +2 V bias voltage, respectively
(AVI)
Dissolution of silver dendrite from a platinum electrode
immersed in a 1 mM silver nitrate and 0.5 mM CTAB
solution; video highlights the step-by-step dissolving of
silver dendrites upon the application of +2 V bias to the
electrode, along with particle formation due to Ostwald
ripening; all silver metal is “cleaned” from solution after
complete dissolution (AVI)
Dissolution of silver dendrite on a platinum electrode
immersed in a pure aqueous 1 mM silver nitrate solution;
video highlights the disconnection of the dendrites from
the electrode and the incomplete dissolving of the
dendrite into solution, upon the application of +2 V bias;
no Ostwald ripening is observed and traces of Ag can be
seen after dissolving (AVI)
■ AUTHOR INFORMATION
Corresponding Authors
*E-mail: hbsu@ntu.edu.sg (H.S., binding energy calculation).
*E-mail: phyumm@nus.edu.sg (U.M., experiments).
Notes
The authors declare no competing financial interest.
■ ACKNOWLEDGMENTS
This work was supported by the Singapore National Research
Foundation’s Competitive Research Program funding (NRF-
CRP9-2011-04).
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