Synthesis and characterization of nanoporous gold nanowires

Article abstract of DOI:10.1021/jp0222200

We report on a two-step process for the fabrication of nanoporous nanowires. The fabrication process involves electrochemical deposition of a single-phase binary alloy into a nanoporous template, followed by subsequent etching of the less noble component. These particles are characterized by diameters of 20 nm or larger, lengths of up to tens of microns, and a porous structure with controlled pore dimensions. In contrast to conventional solid nanoparticles, nonporous nanowires have a high surface area and a well-defined pore morphology. Furthermore, nanoporous segments can be incorporated into multisegment nanorods or nanowires. Here, we demonstrate the fabrication of nanoporous gold (NPG) nanowires and multisegment nanowires incorporating Au/NPG layers and Pt/NPG layers.

Full text of DOI:10.1021/jp0222200

4494
J. Phys. Chem. B 2003, 107, 4494-4499
Synthesis and Characterization of Nanoporous Gold Nanowires
Chunxin Ji and Peter C. Searson*
Department of Materials Science and Engineering, Johns Hopkins UniVersity,
Baltimore, Maryland 21218
ReceiVed: October 14, 2002
We report on a two-step process for the fabrication of nanoporous nanowires. The fabrication process involves
electrochemical deposition of a single-phase binary alloy into a nanoporous template, followed by subsequent
etching of the less noble component. These particles are characterized by diameters of 20 nm or larger,
lengths of up to tens of microns, and a porous structure with controlled pore dimensions. In contrast to
conventional solid nanoparticles, nonporous nanowires have a high surface area and a well-defined pore
morphology. Furthermore, nanoporous segments can be incorporated into multisegment nanorods or nanowires.
Here, we demonstrate the fabrication of nanoporous gold (NPG) nanowires and multisegment nanowires
incorporating Au/NPG layers and Pt/NPG layers.
Introduction
Solid metallic nanoparticles have been exploited for a wide
range of applications that take advantage of their high surface-
1
-4
to-volume ratio. Recently, there has been increasing interest
5
-9
in the anisotropic properties of nanorods and nanowires.
Here, we report on the fabrication of nanoporous nanowires.
These particles are characterized by diameters of 20 nm or
larger, lengths of up to tens of microns, and a porous structure
with controlled pore dimensions. In contrast to conventional
solid nanoparticles, nanoporous nanowires have a high surface
area and a well-defined pore morphology. Furthermore, non-
porous segments can be incorporated into multisegment nano-
wires.
Nanoporous structures are formed when a single-phase binary
alloy containing a noble metal is etched in a suitable solution.
Preferential dissolution of the less noble component of the alloy
results in the formation of a porous structure of the more noble
element. This effect was first reported in the 1920s and has
Figure 1. Cyclic voltammograms for 10 mM KAg(CN) , 10 mM KAu-
2
2 2 3
(CN) , and 0.25 M Na CO (pH 13) on gold at a scan rate of 10 mV
-
1
s
.
subsequently been observed in Cu-Zn, Au-Cu (ref 10 and
references therein), and Au-Ag1
1-14
alloys. The evolution of
the porous structure during the dealloying process involves
etching of the less noble metal coupled with coarsening of the
more noble metal by surface diffusion.1
0,11,14-19
The Au-Ag system is of interest because Au and Ag both
have face-centered-cubic structures with similar lattice param-
eters and exhibit solid solubility across the entire composition
range. Porous Au films have been prepared by dealloying
AuxAg1-x alloys with Au concentrations in the range of 15-35
1
1-14
atom %.
AuxAg1-x alloys have been prepared by cast-
ing,1
3,14
thermal evaporation, or sputter deposition. To our
15
knowledge, there have been no reports of the dealloying of
electrodeposited AuxAg1-x alloys. Various solutions have been
developed for the electrodeposition of silver2
0-22
and gold,
23-29
Figure 2. Steady-state deposition currents for gold (O) and silver (4)
and it is known that both gold and silver can be deposited from
-
-
at -1.2 V versus Au(CN)
deposited from a solution containing KAu(CN)
(pH 13), and silver was deposited from a solution containing KAg-
(CN) and 0.25 M Na CO (pH 13).
2
or Ag(CN)
2
concentration. Gold was
basic cyanide solutions.22-26
2
and 0.25 M Na CO
2
3
In this paper, we describe a two-step process for the
fabrication of nanoporous nanowires. In the first step, a single-
2
2
3
phase, binary alloy is electrochemically deposited into a
*
To whom correspondence should be addressed. E-mail:
3
0-32
searson@jhu.edu.
nanoporous template.
In the second step, the solid nano-
1
0.1021/jp0222200 CCC: $25.00 © 2003 American Chemical Society
Published on Web 04/19/2003

Synthesis and Characterization of NPG Nanowires
J. Phys. Chem. B, Vol. 107, No. 19, 2003 4495
Experimental Section
AuxAg1-x alloy thin films were electrodeposited from solu-
tions containing KAg(CN)2 (99.9%, Alfa Aesar), KAu(CN)2
(99.99%, Alfa Aesar), and 0.25 M Na2CO3 (99.5%, ACS grade)
at pH 13. Thin films were deposited onto gold or sliver
evaporated onto Si(100) wafers. Prior to deposition, the
substrates were sequentially cleaned with acetone, methanol,
and deionized distilled H2O. Experiments were performed in a
three-electrode cell with a platinum mesh counter electrode and
a Ag/AgCl (3 M NaCl) reference electrode (Ueq ) 0.2 V vs
SHE) positioned close to the substrate surface using a Luggin
capillary.
X-ray photoelectron spectra were recorded with a Perkin-
Elmer X-ray photoelectron spectrometer using Mg KR as the
7
5
X-ray source. The Au 4f and Ag 3d peaks were used to
determine the relative concentrations of Au and Ag in the films.
Scanning electron microscopy (SEM) images were obtained with
a JEOL JSM-6700 field emission SEM instrument.
Figure 3. Mole fraction of gold in Au
1.2 V versus the mole fraction of Au(CN)
the thickness of the films was about 3 µm. The composition of the
films was determined from the XPS Au 4f and Ag 3d peaks after
sputter cleaning of the surface.
x
Ag1-x alloy films deposited at
-
2
in solution. In all cases
-
7
5
AuxAg1-x alloy nanowires were fabricated by electrodeposi-
tion3
0-32
into Al2O3 membranes (Anodisc, Whatman) with a
specified pore diameter of 100 nm. We note that the pores are
00 nm in diameter only for a distance of about 1 µm from the
wires are removed from the template, and one component is
chemically etched from the alloy. For example, an AxB1-x alloy
is deposited into a porous template, and component B is then
chemically etched from the solid nanowires. As long as the
concentration of B in the alloy is in the range, e.g., 65-85 atom
1
back side of the membrane; for the rest of the thickness, the
33
diameter is 200-350 nm. A copper film, evaporated onto the
back side of the Al2O3 membrane, served as the working
electrode.
%
Ag for AuxAg1-x alloys (0.15 e x e 0.35), nanoporous
The AuxAg1-x alloy nanowires were deposited from a solution
containing 100 mM KAg(CN) , 20 mM KAu(CN) , and 0.25
nanowires of component A are formed. The pore structure and
surface area are dependent on the concentration of component
B and any subsequent annealing treatment.13
2
2
M Na CO (pH 13) or from a solution containing 50 mM
2
3
KAg(CN)2, 20 mM KAu(CN)2, and 0.25 M Na2CO3 (pH 13)
Figure 4. Plan-view SEM images of Au
Etched Au0.40Ag0.60 film deposited from a solution containing 60 mM KAg(CN)
Au0.32Ag0.68 film deposited from a solution containing 50 mM KAg(CN) , 20 mM KAu(CN)
film deposited from a solution containing 100 mM KAg(CN) , 30 mM KAu(CN) , and 0.25 M Na
, 20 mM KAu(CN) , and 0.25 M Na CO (pH 13). The thickness of the as-deposited films
x
Ag1-x alloy films after etching in concentrated nitric acid. The films were deposited at -1.2 V on Au. (a)
, 35 mM KAu(CN) , and 0.25 M Na CO (pH 13), (b) etched
, and 0.25 M Na CO (pH 13), (c) etched Au0.26Ag0.74
CO (pH 13), and (d) etched Au0.18Ag0.82 film
2
2
2
3
2
2
2
3
2
2
2
3
deposited from a solution containing 100 mM KAg(CN)
2
2
2
3
was about 3 µm.

4496 J. Phys. Chem. B, Vol. 107, No. 19, 2003
Ji and Searson
Figure 5. SEM images of NPG nanowires prepared by the etching of
Au0.18Ag0.82 alloy nanowires deposited in an Al template at -1.2 V
from a solution containing 100 mM KAg(CN) , 20 mM KAu(CN)
and 0.25 M Na CO (pH 13). The nanowires were removed from the
Figure 6. SEM images of an NPG nanowire prepared by the etching
of Au0.32Ag0.68 alloy nanowires deposited at -1.2 V from a solution
2
O
3
2
2
,
containing 50 mM KAg(CN)
CO
etched in concentrated nitric acid.
2
, 20 mM KAu(CN)
2 2
, and 0.25 M Na -
(pH 13). The nanowires were removed from the template and
2
3
3
template and etched in concentrated nitric acid.
at -1.2 V (vs Ag/AgCl). Pure gold was deposited from a
solution containing 20 mM KAu(CN)2 and 0.25 M Na2CO3 (pH
1
3) at -1.2 V (vs Ag/AgCl). Pure platinum was deposited from
a solution containing 0.0167 M ammonium hexachloroplatinate
(NH4)2PtCl6; 44% Pt, Alfa Aesar] and 0.409 M sodium
[
hydrogen phosphate (Na2HPO4; 98%, Alfa Aesar) (pH 8) at
-
1.0 V (vs Ag/AgCl).34
The nanowires were stripped from the templates in the
following way: The evaporated copper film was dissolved using
.5 M CuCl2 and 1 M HCl, and the alumina template was then
0
dissolved in 2 M KOH. The nanowires were cleaned by
sequential ultrasonication, centrifugation, and rinsing with
deionized distilled water. The nanowires were then transferred
and stored in ethanol. Nanoporous gold (NPG) nanowires were
prepared by etching of the AuxAg1-x alloy nanowires in
concentrated nitric acid.
Figure 7. Specific surface area of NPG nanowires versus annealing
temperature. The NPG nanowires were obtained by the etching of (O)
Au0.18Ag0.82 alloy nanowires; (4) Au0.26Ag0.74 alloy nanowires; and (c)
Au0.32Ag0.68 alloy nanowires. The solid symbols correspond to the
unannealed NPG nanowires.
The surface area of the nanoporous gold nanowires was
determined from BET measurements (Coulter SA3100) using
nitrogen adsorption. The AuxAg1-x alloy nanowires were etched
with concentrated nitric acid, and samples of at least 100 mg
of the NPG nanowires were used for each BET measurement.
Before measurement of the surface area, the samples were
annealed at temperatures between 50 and 300 °C for 1 h at a
observed up to about -1.0 V. At more positive potentials, a
peak is seen at about -0.45 V due to the stripping of silver.
Figure 2 shows the steady-state deposition current for pure
-
-
gold from Au(CN)2 solution and for pure silver from Ag(CN)2
solution at -1.2 V. At this potential, the deposition currents
for gold and silver both increase linearly with concentration,
indicating that the deposition process is diffusion-limited for
-
5
pressure of about 2 × 10 Torr.
Results and Discussion
35-39
both elements.
The slopes of the two curves are essentially
Figure 1 shows a current-voltage curve for the deposition
of AuxAg1-x from a solution containing 10 mM KAg(CN)2, 10
mM KAu(CN)2, and 0.25 M Na2CO3 (pH 13). The deposition
peaks at about -0.6 and -1.05 V correspond to the deposition
the same, consistent with the fact that the diffusion coefficients
-
-
-5
2
for Ag(CN)2 and Au(CN)2 are both about 1.6 × 10 cm
-
1 39-41
s .
The composition of AuxAg1-x alloy films deposited at -1.2
35
peaks for silver and gold, respectively. On the reverse scan, a
V, where the deposition currents for both elements are diffusion-
-
2
-
diffusion-limited deposition current of about 0.8 mA cm is
limited, is dependent on the concentrations of Ag(CN)2 and

Synthesis and Characterization of NPG Nanowires
J. Phys. Chem. B, Vol. 107, No. 19, 2003 4497
Figure 8. SEM images of NPG nanowires annealed for 1 h at (a) 50, (b) 100, (c) 150, (d) 200, (e) 250, and (f) 300 °C. The nanowires were
synthesized by the etching of Au0.18Ag0.82 alloy nanowires deposited at -1.2 V from a solution containing 100 mM KAg(CN) , 20 mM KAu(CN)
and 0.25 M Na CO (pH 13).
2
2
,
2
3
Au(CN)2- in solution. Figure 3 shows the mole fraction of Au
nm for x ) 0.26 and to 30-40 nm for x ) 0.18. Similar results
-
in the alloy films versus the fraction of Au(CN)2 in solu-
have been reported for the dealloying of AuxAg1-x films
-
-
11-14
tion determined from the ratio [Au(CN)2 ]/([Ag(CN)2 ] +
deposited by vapor deposition.
For films with x > 0.40,
-
[
Au(CN)2 ]). The linear relationship with a slope close to 1.0
there is no significant change in surface morphology before and
after etching. Cross-sectional SEM images revealed a continuous
porous network across the entire thickness of the film for porous
Au layers formed by the etching of alloys with x e 0.32.
NPG nanowires were fabricated by removing Au Ag alloy
illustrates that AuxAg1-x is an ideal binary alloy system where
the ratio of the diffusion-limited deposition currents at any
potential and concentration is equal to the composition ratio in
the film. The small deviation from ideal behavior is due to the
difference in deposition efficiencies: the deposition efficiency
for gold films at this potential is 0.78, whereas the deposition
efficiency for silver films is 0.75.
x
1-x
nanowires from the alumina template and etching the silver in
nitric acid. Figure 5 shows SEM images of NPG nanowires
fabricated by etching Au0.18Ag0.82 alloy nanowires. The NPG
nanowires exhibit a porous structure with a ligament size of
about 20-30 nm and a pore size of about 20-30 nm.
Figure 4 shows plan-view SEM images of a series of NPG
films prepared by the etching of electrodeposited AuxAg1-x films
with 0.18 e x e 0.40. The AuxAg1-x films were deposited at
The morphology of the nanowires is dependent on the alloy
composition. Figure 6 shows SEM images of NPG nanowires
fabricated by the etching of Au0.32Ag0.68 alloy nanowires. In
this case, the ligament size is about 35-50 nm, and the pore
size about 10-40 nm. The increase in ligament size with
increasing gold concentration in the AuxAg1-x alloy is opposite
-
-
-
1.2 V from solutions with different Au(CN)2 /Ag(CN)2
ratios. For x ) 0.40 (Figure 4a), the dealloyed film exhibits
sparsely distributed nanopores that are 10-30 nm in diameter.
For x ) 0.32, a continuous porous network is formed with a
characteristic ligament size of about 10-20 nm. With decreasing
mole fraction of gold, the ligament size increased to 20-30
1
1-14
to the dependence seen in thin films
(see Figure 4) and is

4498 J. Phys. Chem. B, Vol. 107, No. 19, 2003
Ji and Searson
Figure 10. SEM images of multicomponent nanowires. (a) Nanowires
with NPG and solid Au and segments fabricated by the etching of
Figure 9. Diameter of the NPG nanowires obtained from SEM images
in Figure 8 versus (a) annealing temperature and (b) surface area.
n
(Au0.18Ag0.82/Au) nanowires. (b) Nanowires with NPG and solid Pt
segments fabricated by the etching of Pt/Au0.18Ag0.82/Pt nanowires.
related to coarsening in the cylindrical geometry. These images
illustrate that the porosity and morphology of the NPG nanow-
ires can be controlled by varying the composition of the
AuxAg1-x alloy.
from Au0.18Ag0.82 alloys than for those fabricated from Au0.32
Ag0.68 alloys.
-
Figure 9 displays a plot of the diameter of the NPG nano-
wires, obtained from the SEM images shown in Figure 8, as a
function of annealing temperature and surface area. From Figure
9a, it can be seen that for annealing temperatures below about
150 °C, the diameter remains constant with increasing annealing
temperatures whereas the specific surface area decreases from
The surface area of the NPG nanowires was determined by
gas adsorption measurements. Figure 7 shows the specific
surface area versus annealing temperature for NPG nanowires.
For the unannealed nanowires, the surface area increased with
decreasing atomic fraction of gold in the AuxAg1-x alloy. The
2
-1
2
-1
surface area was 6.9 m g for NPG nanowires synthesized
6.9 to 4 m g , as shown in Figure 7. In this temperature range,
the decrease in surface area is due to coarsening. For annealing
temperatures above 150 °C, the diameter decreases dramatically
from 275 to 125 nm, as can be seen in Figure 9b, and the surface
area decreases accordingly. The decrease in wire diameter with
annealing above 150 °C is associated with the onset of sintering.
2
-1
by dealloying Au0.18Ag0.82 and 3.7 m g for NPG nanowires
synthesized by dealloying Au0.32Ag0.68. Annealing the nanowires
at temperatures above 50 °C results in a decrease in the specific
surface area due to coarsening.13 With increasing annealing
temperature, the specific surface area decreases rapidly and
2
-1
2
reaches a constant value of 2.0 m g for x ) 0.18 and 1.3 m
Nanoporous segments can be incorporated into multisegment
nanorods or nanowires. Figure 10a shows an SEM image of a
bundle of multisegment nanowires with alternating NPG and
solid Au segments formed by the etching of (Au0.18Ag0.82/Au)2
nanowires in nitric acid. The Au0.18Ag0.82 segments were
-
1
g
for x ) 0.32.
The influence of annealing on the morphology of the NPG
nanowires can be seen in the SEM images shown in Figure 8
for NPG nanowires synthesized by the etching of Au0.18Ag0.82.
The images show that the NPG nanowires coarsen with
increasing annealing temperature, resulting in a decrease in the
porosity. Sintering between nanowires can be seen when the
annealing temperature is higher than 175 °C. After annealing
at 300 °C, the nanowires are almost solid cylinders, consistent
with surface area measurements shown in Figure 8. Note that
the diameter of the NPG nanowires after annealing at 300 °C
is larger for nanowires fabricated from AuxAg1-x alloys with
higher gold contents (larger x). Thus, the surface area after
annealing at 300 °C is higher for NPG nanowires fabricated
-
deposited from a solution containing 100 mM Ag(CN)2 and
-
20 mM Au(CN)2 at -1.2 V, and the Au segments were
-
deposited at -1.2 V from a 20 mM Au(CN)2 solution. The
multisegment nanowires are reasonably robust, although a small
number of the gold end segments have become detached during
ultrasonication and centrifugation. Figure 10b shows multiseg-
ment nanowires with a central NPG segment and solid platinum
segments at each end. The nanowires were prepared by the
etching of Pt/Au0.18Ag0.82/Pt nanowires. The Au0.18Ag0.82 seg-
ment was deposited from a solution containing 100 mM

Synthesis and Characterization of NPG Nanowires
J. Phys. Chem. B, Vol. 107, No. 19, 2003 4499
Ag(CN)2- and 20 mM Au(CN)2- at -1.2 V. Before etching,
the diameter of the Pt/Au0.18Ag0.82/Pt nanowire was about 320
nm. After etching, the diameter of the NPG segment decreased
to about 200 nm, and from the original composition, we obtained
a porosity of 53% for NPG nanowires fabricated by the etching
of Au0.18Ag0.82 alloys. The Pt/NPG/Pt nanowires were very
robust, with no evidence of segments becoming detached.
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Nanoporous gold nanowires have been fabricated by deal-
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in Al2O3 membranes from a solution containing KAg(CN)2,
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1
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Acknowledgment. This work was supported by the JHU
MRSEC (DMR-0080031) and the David and Lucile Packard
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(
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