Shrinking nanowires by kinetically controlled electrooxidation

Article abstract of DOI:10.1021/jp056160k

Nanowires composed of antimony, gold, and bismuth telluride (Bi ₂Te₃) were reduced in diameter by electrooxidation in aqueous solutions. When electrooxidation was carried out using low current densities (J_ox < 150 μAcm^-2), the mean wire diameter decreased in direct proportion to the oxidation time, as expected for a kinetically controlled process. Under these conditions, the diameter uniformity of nanowires remained constant as wires were shrunk from initial diameters of more than 120 nm to less than 40 nm, for Sb and Bi ₂Te₃, and less than 60 nm for Au. Oxidized nanowires remained continuous for more than 100 μm. Electrooxidation at higher current densities rapidly introduced breaks into these nanowires. Electrochemical wire growth and shrinking by electrooxidation were integrated into a single electrochemical experiment that allowed the final mean diameter of nanowires to be specified with a precision of 5-10 nm.

Full text of DOI:10.1021/jp056160k

36
2006, 110, 36-41
Published on Web 12/10/2005
Shrinking Nanowires by Kinetically Controlled Electrooxidation
M. A. Thompson, E. J. Menke, C. C. Martens, and R. M. Penner*
Department of Chemistry and Institute for Surface and Interface Science, UniVersity of California,
IrVine, California 92679-2025
ReceiVed: October 26, 2005; In Final Form: NoVember 21, 2005
Nanowires composed of antimony, gold, and bismuth telluride (Bi2Te3) were reduced in diameter by
electrooxidation in aqueous solutions. When electrooxidation was carried out using low current densities (Jox
< 150 µA cm-2), the mean wire diameter decreased in direct proportion to the oxidation time, as expected
for a kinetically controlled process. Under these conditions, the diameter uniformity of nanowires remained
constant as wires were shrunk from initial diameters of more than 120 nm to less than 40 nm, for Sb and
Bi2Te3, and less than 60 nm for Au. Oxidized nanowires remained continuous for more than 100 µm.
Electrooxidation at higher current densities rapidly introduced breaks into these nanowires. Electrochemical
wire growth and shrinking by electrooxidation were integrated into a single electrochemical experiment that
allowed the final mean diameter of nanowires to be specified with a precision of 5-10 nm.
I. Introduction
were prepared using Nanopure water (F > 18 MΩ cm) and
were purged with N2 prior to each experiment. Nanowires were
electrodeposited on the basal plane surface of ZYB-grade highly
oriented pyrolytic graphite (HOPG, Advanced Ceramics, Inc.,
Cleveland, OH). A 10 × 10 × 1 mm3 HOPG crystal was placed
in a Teflon holder that exposed a circular area of approximately
0.20 cm2 of the HOPG basal plane to the solution using an
O-ring. The depositions were carried out using an EG & G 273A
potentiostat/galvanostat.
The gold plating solution used was 2 mM AuCl3 (Aldrich,
99.999%), 0.1 M KCl (Aldrich, 99.9+%), and 2 mM saccharin.
The deposition was a typical three-pulse potentiostatic ESED
program. Nanowires of antimony and gold were both prepared
by applying three voltage pulses in succession (Scheme 1) as
previously described.28 For example, the three pulses required
for the growth of gold nanowires were as follows: oxidation at
HOPG step edges step at 0.80 V vs SCE for 5 s, nucleation at
Enucl ) -1.00 V for tnucl ) 125 ms, and followed by growth at
Egrowth ) 0.45 VSCE for tgrowth ) 30 s. The corresponding
parameters for the growth of antimony nanowires are sum-
marized in Table 1. The electrodeposition solution for antimony
was aqueous 5 mM SbCl3 (Aldrich, 99.99+%), 0.1 M HNO3,
and 0.1 M tartaric acid (Aldrich, 98%). Bismuth telluride
nanowires were prepared using a cyclic electrodeposition-
stripping method identical to that described previously.12 As
indicated in Scheme 1, nanowire electrooxidation occurred
immediately following electrodeposition and in the same
solution. Electrooxidation potentials for each of the three
materials explored here are listed in Table. 1.
Nanowires are of interest for a diverse range of applications
including chemical sensors,1-6 lasers,7-10 thermoelectric gen-
erators,11-15 and photodetectors.16-20 Nanowire synthesis has
generally involved “bottom-up” assembly from atomic or
molecular precursors.e.g.,21-25 The alternative, “top down”,
approach involves starting with a big wire and reducing its
diameter into the nanometer range. One method for effecting
this size reduction is using ion beam sputtering,26 but it is unclear
whether the dimensional uniformity present in the initial wire
is retained during this size reduction process.
In this letter, we describe a new method for “shrinking”
nanowires that permits the reduction of size to occur with precise
control over the final nanowire diameter and with retention of
the diameter uniformity present in the initial wire or distribution
of wires. Our method involves the electrochemical oxidation
of nanowires in solutions where the products of the oxidation
are soluble. When the oxidation process is carried out slowly,
under conditions of kinetic control, the time rate of change of
the wire radius, dr/dt, becomes independent of the wire radius,
thereby permitting nanowires to be reduced in diameter without
roughening and without the introduction of breaks. In this letter,
we present results for nanowires composed of three disparate
materials: gold, antimony, and bismuth telluride (Bi2Te3) all
synthesized using the electrochemical step edge decoration
(ESED)27 method.
II. Experimental Methods and Materials
The ESED syntheses of gold28 and bismuth telluride,12 but
not antimony nanowires, have been previously described.
Briefly, nanowire plating and electrooxidation experiments were
carried out using a 50-mL glass and Teflon one-compartment,
three-electrode cell using a platinum counter electrode and a
saturated calomel reference electrode (SCE). Aqueous solutions
Scanning electron microscopy (SEM) was carried out using
a Philips FEG 30-XL microscope equipped with an EDAX ele-
mental analyzer operating at an accelerating voltage of 20 keV.
III. Results and Discussion
During ESED nanowire electrodeposition, a constant, convec-
* Address correspondence to: rmpenner@uci.edu.
tion-controlled current, igrowth, is generally observed.29 Under
10.1021/jp056160k CCC: $33.50 © 2006 American Chemical Society

Letters
J. Phys. Chem. B, Vol. 110, No. 1, 2006 37
SCHEME 1: Pulse Program Used to Implement the
Growth by ESED and Thinning of Metal Nanowiresa
dimensionally uniform nanowires of diminutive diameter.
Consider the growth of a “rough” silver (n ) 1, Vm ) 10.26
cm3 mol-1) nanowire consisting of a linear ensemble of r ) 20
nm hemispherical nanoparticles (Figure 1a, left). Within the
context of the ESED experiment, this nanowire is produced by
the nucleation of silver, followed by growth until these particles
are, on average, tangential to one another (at the threshold of
coalescence).6,28 This nanowire has a mean radius, r ) 15.7
nm with σr ) 4.5 nm and is characterized by the distribution of
radii shown in Figure 1b (far left). Now, eq 1 can be used to
predict the profile of this nanowire, as it is grown under
conditions of constant total current. As shown in Figure 1a and
b, growth under conditions of igrowth ) 10-9 A cm-2 for tgrowth
) 600 s produces a smoother nanowire with r ) 65.8 nm and
σr ) 0.9 nm; the wire radius has increased by a factor of 4,
while its roughness has been reduced by 80% (Figure 1d).
Equation 3 can then be used to propagate the wire radii
distribution as this r ) 65.8 nm nanowire is oxidized (n ) 1)
under conditions of kinetic control using Jox ) 10-4 A cm-2
for 500 s. The wire profile (Figure 1d) and the width of the
radius distribution (Figure 1a,c) remain constant as r is reduced
to 10.1 nm. In principle, tox can be extended with further
shrinkage of r to approximately 3 nm before electrooxidation
begins to introduce breaks.
This numerical calculation shows that a relatively large but
“smoothed” nanowire resulting from ESED growth can be
shrunk to 1/20th of its initial radius without inducing roughening
and without introducing breaks. Can a similar result be achieved
experimentally? We address this question now for nanowires
composed of three disparate materials: gold (a noble metal),
antimony (a base metal), and bismuth telluride (Bi2Te3, a
compound). Nanowires of all three materials with diameters in
the 120-150 nm range were prepared using the ESED method
we have previously described.12,28 The conditions of solution
composition, applied potentials, and growth durations are
summarized in Table 1. Cyclic voltammograms for an HOPG
electrode immersed in the solutions employed for electrodepo-
sition and electrooxidation (Figure 2) show a prominent
oxidation peak that is associated with the oxidative removal of
each material from the surface. These oxidation reactions are32,33
a The values of each parameter shown in blue are tabulated in
Table 1.
these conditions, the increase in wire radius, r(t), is ap-
proximately linear with the square root of the deposition time,
tgrowth1/2. For a hemicylindrical wire of initial radius r0, r(t) is
given by25
2igrowth growthVm
t
r(t) ) r0 +
(1)
x
πnFl
where Vm is the molar volume of the deposited material, n is
the number of electrons transferred per mole of deposited
material, F is the Faraday constant (96485 C eq-1), and l is the
total length of nanowires present within 1 cm2 of geometrical
electrode area. The application of eq 1 to a distribution of
nanowires with different radii (or a single nanowire of nonuni-
form radius) leads to narrowing of the diameter distribution as
small nanowires (or constrictions in a particular nanowire) grow
faster than large nanowires (or bulges in a particular nanowire).
For a kinetically controlled oxidation process of the type
Antimony: Sb (s) + H2O f SbO+ (aq) + 2H+ + 3e- (4)
Bi2Te3: Bi2Te3 (s) + 6H2O f 3HTeO2+ (aq) +
2Bi3+ (aq) + 9H+ + 18e- (5)
M(s) f Mn+ (aq) + ne-
(2)
Gold: Au (s) + 3Cl- f AuCl32- (aq) + 1e-
(6)
a time-invariant oxidation current density, jox, is observed.30,31
Note that, in all three cases, the products of the oxidation
reactions are soluble. If oxidation is carried out using potentials
well positive of this peak, a limiting rate for the oxidation
reaction is imposed either by diffusion (e.g., Cl- diffusion in
the case of gold oxidation) or by the solubility of one or more
products of the oxidation reaction.34,35 On the negative edge of
these oxidation peaks, in the limit of small anodic polarizations,
the electrooxidation rate will be limited by the kinetics of the
oxidation reaction, and the current density on the nanowire
surfaces, Jox, becomes constant.31 The red arrows in Figure 2
indicate the potentials we employed for each material (Table
1). These optimum potentials were determined by trial and error,
and in general, they represent the most positive potentials that
satisfied the requirement that oxidation be kinetically controlled.
Multiple samples of nanowires were prepared using identical
For a hemicylindrical wire, it is readily shown that r(t) is given
by
joxtoxVm
r(t) ) ro2
-
(3)
nF
where ro2 is the initial radius of a nanowire subjected to
oxidation, and Jox and tox are the oxidation current density and
duration, respectively. If eq 3 is applied to the oxidation of an
ensemble of nanowires with different radii (or to a single
nanowire with a nonuniform radius), the radius of each nanowire
(or the radius of each section along the axis of a particular
nanowire) decreases in direct proportion to time with the result
that standard deviation of the radius, σr, remains constant.
Coupling nanowire growth according to eq 1 with nanowire
oxidation according to eq 3 provides a means for preparing
nucleation and growth parameters: Enucl, tnucl, Egrowth, and tgrowth
.

38 J. Phys. Chem. B, Vol. 110, No. 1, 2006
Letters
TABLE 1: Parameters Employed for Nanowire Electrodeposition and Electrooxidation
nucl,a tnucl
E
growth, tgrowth
Igrowth
Eox
Jox
c
d
material
E
Sb
Au
Bi2Te3
-1.0 V, 40 ms
-1.0 V, 125 ms
-0.6 V, 5 ms
-0.40 V, 75 s
0.45 V, 30 s
n.a.b
500-1000 µA cm-2
300-800 µA cm-2
n.a.b
-0.060 V
0.810 V
0.370 V
137 µA cm-2
33.4 µA cm-2
109 µA cm-2
a All potentials are referenced to a saturated calomel reference electrode (SCE). See Scheme 1 for a definition of the quantities: Enucl, tnucl
,
E
growth, tgrowth, and Eox. b Bi2Te3 nanowires were prepared using potentiodynamic electrodeposition as previously described.12 c The range of the
pseudo steady-state electrodeposition currents. These current values are normalized by the geometric area of the HOPG electrode. d The oxidation
current density normalized by the area of the nanowires, calculated using eq 3 from the slopes of the plots shown in Figure 4.
Figure 2. Cyclic voltammograms at 20 mV s-1 for an HOPG electrode
immersed in the three solutions employed for electrodeposition and
electrooxidation. For antimony, this solution was aqueous 5 mM SbCl3,
0.1 M tartaric acid, and 0.1 M HNO3. For gold: aqueous 2mM AuCl3,
0.1 KCl, and 2 mM saccharin. For Bi2Te3: aqueous 1.5 mM Bi(NO3)3,
1.0 mM TeO2, and 1 M HNO3. The potentials employed for electrooxi-
dation were -0.060 V vs SCE (Sb), 0.370 V (Bi2Te3), and 0.810 V
Figure 1. (a) Calculated mean nanowire radius, r , and radius
distribution vs time (green). Equations 1 and 3 were used to calculate
r during growth and oxidation, respectively, starting with a nanowire
consisting of hemispherical silver particles with a radius of 20 nm.
(Au).
a 100 µm continuous wire length (Figure 3c,f,i). These SEM
images show that a significant reduction of the wire diameter
is achieved without a significant increase in the diameter
heterogeneity, as compared with the initial state of the wire. It
must be noted here that the oxidation current decreases with
time and in direct proportion to the area of a nanowire and
therefore in proportion to its radius (data not shown). We are
unable to derive quantitative information from the oxidation
current, because it contains contributions from the oxidation of
both nanowires and nanoparticles. These nanoparticles, which
can be seen in the SEMs of Figure 3, are produced in parallel
with the growth of nanowires during the ESED synthesis
process.
Other parameters used in this calculation were the following: igrowth
)
1 × 10-9 A cm-2, Vm ) 10.26 cm3 mol-1, l ) 1 cm, Jox ) 1 × 10-4
A cm-2. Dotted green lines show increase in radius for largest (rmax
)
and smallest (rmin) wire segments. The standard deviation of the radius,
σr, vs time is plotted in blue. (b, c) Histogram of wire radii shown at
several times during the deposition (b) and oxidation (c). (d) Radius
vs axial distance profile of the nanowire as a function of growth time.
The profile seen at 600 s is maintained throughout the oxidation process.
These nanowires were oxidized for various times, tox, using the
same Eox, listed in Table 1. The resulting nanowires were imaged
using scanning electron microscopy, and representative images
are shown in Figure 3. At left (Figure 3a,d,g) are shown
nanowires of each composition before etching. The mean
diameters of these wires were 120 ( 7 nm (Sb), 140 ( 10 nm
(Bi2Te3), and 147 ( 11 nm (Au). The two images at right show
wires subjected to an intermediate oxidation time (Figure 3b,e,h)
and wires subjected to the longest oxidation times that preserved
A statistical analysis of SEM data for more than 50 individual
nanowires at each tox was compiled to prepare the plots shown
in Figure 4. A plot of mean wire diameter versus tox (Figure
4a) shows that the decrease in wire diameter is directly
proportional to tox for nanowires composed of all three materials,

Letters
J. Phys. Chem. B, Vol. 110, No. 1, 2006 39
Figure 3. Representative scanning electron micrographs of antimony (a-c), gold (d-f), and Bi2Te3 (g-i) nanowires. (a) An antimony nanowire
as-prepared using Egrowth ) -0.40 and tgrowth ) 75 s; and after electrooxidation at Eox ) -0.060 V for (b) 250 s and (c) 500 s. (d) A gold nanowire
as-prepared using Egrowth ) 0.45 and tgrowth ) 30 s; and after electrooxidation at Eox ) 0.810 V for (e) 800 s and (f) 1300 s. (g) Bi2Te3 wires
as-prepared using cyclic electrodeposition/stripping as previously described;12 and after electrooxidation at Eox ) 0.370 V for (h) 400 s and (i)
800 s.
Figure 4. (a) Plots of mean nanowire diameter, dia. , vs tox for gold, antimony, and Bi2Te3. Each data point plotted in (a) represents a separate
nanowire growth experiment involving, for a particular material, nucleation and growth under the conditions specified in Table 1, followed by
electrooxidation for various durations, tox. Distribution bars represent (1 σdia for at least 50 diameter measurements for different wires on each
surface. (b) Plot of the σdia vs dia. for each material.
as predicted by eq 3. The vertical error bars attached to the
data points in Figure 4a are (1σdia. We also plot σdia versus tox
separately in Figure 4b. The changes in σdia seen here (a slight
reduction for Sb and Bi2Te3 and a slight increase for Au) are
too small to be statistically significant. Both the linearity of the
diameter versus tox data and the independence of σdia on tox are
consistent with a kinetically controlled electrooxidation process
modeled by eq 3. The data shown in Figure 4 further
demonstrate that electrochemical wire growth (by ESED) and
shrinking by electrooxidation can be integrated into a single
electrochemical experiment that permits the final mean diameter
of nanowires to be specified with a precision of 5-10 nm. The
nanowire length, as well as its diameter, is reduced by
electrooxidation, and this reduction in length is approximately
equal to the absolute change in the wire radius. Since the largest
radius change achieved here is ≈50 nm, and since the nanowires

40 J. Phys. Chem. B, Vol. 110, No. 1, 2006
Letters
Figure 5. Scanning electron micrographs of gold nanowires after the removal, by electrooxidation, of nearly identical quantities of gold (Qox
)
965-1000 µC cm-2) using three different electrooxidation potentials. The initial diameter of all three wires was 147 ( 11 nm. (a) Eox ) 0.810 V
vs SCE, tox ) 1000 s, Qox ) 968.9 µC cm-2. These wires remained continuous for more than 100 µm. (b) Eox ) 0.825, tox ) 600 s, Qox ) 967.4
µC cm-2. These wires had continuous sections 1-5 µm in length. (c) Eox ) 0.837 V, tox ) 300 s, Qox ) 1000.2 µC cm-2. The longest continuous
wire sections were ∼500 nm in length.
employed here are more than 100 µm in length, this reduction
in length of <0.1% is insignificant. It should also be noted that
the Bi2Te3 nanowires investigated here are microcrystalline,12
and it is reasonable to expect variations in the kinetically
controlled oxidation rate with the crystallographic orientation
of the facets for individual microcrystals. In principle, this effect
could cause nanowires to etch anisotropically and to undergo
roughening. However, the SEM data shows that this effect is
insignificant for Bi2Te3 nanowires. It is unlikely that this
isotropic etching behavior would be observed for microcrys-
talline nanowires more generally, and this represents a clear
limitation of this methodology.
Experience (or IM-SURE) program administered by the Un-
dergraduate Research Opportunities Program (UROP) at UCI.
Graphite for this work was supplied by a grant from the EU
Commission FP6 NMP-3 project 505457-1, titled “ULTRA-
1D”.
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the Integrated Micro/Nano Summer Undergraduate Research

Letters
J. Phys. Chem. B, Vol. 110, No. 1, 2006 41
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