Low-temperature synthesis of single crystalline Ag2S nanowires on silver substrates

Article abstract of DOI:10.1021/jp050126o

We report on the successful synthesis of silver sulfide (Ag₂S) nanowires by a simple and mild gas - solid reaction approach. For the nanowire synthesis, a preoxidized silver substrate is exposed to an atmosphere of an O₂/H₂S mixture at room temperature or slightly above. The resulting Ag₂S nanowires are phase pure with a monoclinic crystal structure and have diameters of a few tens of nanometers and lengths up to 100 ??m. The influence of reaction conditions on the diameter, length, and morphology of the Ag₂S nanowires has been studied by a number of structural and spectroscopic techniques. The nanowire growth mechanism on the Ag substrate has been discussed, which is likely characterized by continuous deposition at the tip. Additionally, we demonstrate thinning and cutting of individual Ag₂S nanowires with electron beams and laser beams, which are potentially useful for nanowire manipulation and engineering. ? 2005 American Chemical Society.

Full text of DOI:10.1021/jp050126o

10100
J. Phys. Chem. B 2005, 109, 10100-10106
Low-Temperature Synthesis of Single Crystalline Ag2S Nanowires on Silver Substrates
Xiaogang Wen, Shuhua Wang, Yutao Xie, Xiao-Yuan Li, and Shihe Yang*
Department of Chemistry, Institute of Nano Science and Technology, The Hong Kong UniVersity of Science and
Technology, Clear Water Bay, Kowloon, Hong Kong, China
ReceiVed: January 8, 2005; In Final Form: March 26, 2005
We report on the successful synthesis of silver sulfide (Ag2S) nanowires by a simple and mild gas-solid
reaction approach. For the nanowire synthesis, a preoxidized silver substrate is exposed to an atmosphere of
an O2/H2S mixture at room temperature or slightly above. The resulting Ag2S nanowires are phase pure with
a monoclinic crystal structure and have diameters of a few tens of nanometers and lengths up to 100 µm. The
influence of reaction conditions on the diameter, length, and morphology of the Ag2S nanowires has been
studied by a number of structural and spectroscopic techniques. The nanowire growth mechanism on the Ag
substrate has been discussed, which is likely characterized by continuous deposition at the tip. Additionally,
we demonstrate thinning and cutting of individual Ag2S nanowires with electron beams and laser beams,
which are potentially useful for nanowire manipulation and engineering.
Introduction
The one-dimensional (1D) nature endows nanowires unique
electrical, optoelectronic, and mechanical properties with fun-
damental significance and practical ramifications.1-11 Although
several nanowire fabrication methods have been developed,12-22
conventional nanowire synthesis often requires high temperature
and/or various templates. We have recently found that Cu2S
nanowire arrays can be grown from a copper surface by reacting
with O2 and H2S at low temperatures, thus avoiding the usage
of templates.23,24 Such a structure of nanowire array on electrode
has permitted convenient measurement of the promising field
electron emission properties of the Cu2S nanowire arrays.25
Gold- and polypyrrole-coated Cu2S nanowire arrays have also
been prepared aiming at electronic applications.26,27 The facile
synthesis of Cu2S nanowire arrays on Cu by the gas-solid
reaction method has raised a number of important questions
about the growth mechanism. For example, why does Cu2S
prefer 1D growth under those conditions and how do the Cu2S
nanowires grow at such low temperatures? Furthermore, can
Figure 1. Typical SEM image of Ag2S nanowires grown at 40 °C for
5 days. Inset: The EDX pattern in an area including many Ag2S
we extend this method to the synthesis of nanowires of other
semiconductor compounds? It is important to address these
questions so as to facilitate the generalization of the gas-solid
reaction method for the synthesis of nanowire materials and
the device applications of these interesting materials.
nanowires.
Silver sulfide is known to exist in several different allotropic
forms including, e.g., R-Ag2S (achantite, monoclinic, stable up
to 178 °C), â-Ag2S (argentite, bcc, 178-600 °C), and γ-Ag2S
(fcc, above 600 °C). Bulk R-Ag2S is an important semiconductor
with an band gap of ∼1 eV at room temperature and a relatively
high absorption coefficient.28,29 It can be used for the manu-
facture of optical and electrical devices such as photoconductors,
photovoltaic cells, IR detectors,30,31 solar-selective coating,32 and
photosensitive films in optoelectronical devices. Ag2S is an
unusual solid ionic conductor, which operates at room temper-
ature and conducts both electrons and ions.33 Ag2S nanoparticles
have been synthesized and studied extensively,34-39 and their
application potential in photography37,38 and luminescent de-
vices39 has been recognized. However, Ag2S nanowires seem
to have been much less explored due to the lack of appropriate
samples. There is a sign that this is starting to change. Very
recently, Peng et al. prepared Ag2S nanowire arrays electro-
chemically by using anodic aluminum oxide (AAO) membrane
The outstanding questions mentioned above have prompted
us to attempt the synthesis of Ag2S, a close fellow of Cu2S,
nanowires on Ag substrates. The synthesis of Ag2S nanowires
turns out to be much more difficult because of the difficulty in
forming the precursor oxide phase. By varying experimental
conditions, we have finally succeeded in the synthesis of Ag2S
nanowires using the gas-solid reaction approach. In this paper,
we present the results of the Ag2S nanowire synthesis and
discuss the growth mechanism based on the experimental results.
The influence of the reaction conditions on the Ag2S nanowire
growth has been systematically studied. We also describe
methods to tailor, reshape, and section the Ag2S nanowires by
using electron beams and laser beams.
* Address correspondence to this author. E-mail: chsyang@ust.hk.
10.1021/jp050126o CCC: $30.25 © 2005 American Chemical Society
Published on Web 04/23/2005

Synthesis of Single Crystalline Ag2S Nanowires
J. Phys. Chem. B, Vol. 109, No. 20, 2005 10101
Figure 2. Low-magnification TEM image (A), SAED pattern (B), and HRTEM image (C) of a single Ag2S nanowire (T ) 40 °C).
as the template.40 As another example, Lu et al. synthesized
Ag2S nanorod arrays by a solution-growth method.41 Our
fabrication of Ag2S nanowires is characterized by the ambient
conditions of the gas-solid reactions, the removal of the need
for templates, and the high aspect ratio and good monocrystal-
linity of the synthesized nanowires. Moreover, there is a
potential to grow uniform Ag2S nanowire arrays on silver
substrates.
Experimental Section
A typical synthesis is as follows. A Ag foil (Aldrich, 99%)
was cleaned with a 4 M HCl aqueous solution and then
deionized water in an ultrasonic bath. The cleaned Ag foil was
then subjected to preoxidation. Two methods were used to
oxidize the Ag foil surface. The first is simply to expose the
Ag foil surface in the ambient air. The color of the Ag surface
changed from shining gray to pale purple, then to yellow, and
to brown black, indicating the formation of Ag2O. Because the
oxidation step above is too time-consuming (up to a few
months), another method was tried, which involves the elec-
trochemical oxidation of the Ag foil. Here the Ag foil was used
as a working electrode, another Ag foil was used a counter
electrode, and an Ag/AgCl electrode was used as a reference
electrode. The preoxidation was accomplished with a Model
600 electrochemical analyzer (CH instruments Inc. U.S.A.) by
keeping the Ag foil at a constant potential (∼0.3 V) for 5-10
min in a 0.1 M NaOH aqueous solution. Afterward, the surface-
oxidized Ag foil was taken out of the electrochemical cell,
washed with deionized water, and dried in air. For the synthesis
of Ag2S nanowires, the oxidized Ag foil was held on the outer
wall of a test tube or a glass rod, which was then loaded into a
sealed glass reactor (600 mL). The reactor was pumped to a
vacuum with a rotary pump for 5 min, followed by the injection
of 280 mL of H2S and 280 mL of O2 (1:1) to the reactor. The
reactor temperature was varied from room temperature to 40
Figure 3. XPS spectra taken from the surface with different extents
of oxidation of Ag foils: (A) Ag 3d5/2 spectra and (B) Ag MNN Auger
spectra.
°C in a water bath (the reactor pressure is ∼1 atm). As the
reaction progressed, a black layer was formed on the Ag foil
surface. After a given reaction time, the Ag foil with the Ag2S
nanowires grown on it was collected for characterization.
The structural phase of the Ag2S nanowires was identified
by powder X-ray diffraction (XRD, Philips PW-1830 X-ray
diffractometer) with a 1.5405 Å Cu KR rotating anode point
source operated at 40 kV and 40 mA. SEM images were taken
with a JEOL 6300 scanning electron microscope equipped with
an energy-dispersive X-ray spectroscopy (EDX) at an accelerat-
ing voltage of 15 kV. TEM, HRTEM, and ED analyses were
conducted on Philips CM 20 and JEOL 2010 transmission
electron microscopes operating at an accelerating voltage of 200
kV. For TEM characterization, the Ag2S nanowires were
transferred onto carbon-coated copper grids by gently sliding
the Cu grids over the surfaces of the Ag2S nanowire films. X-ray
photoelectron spectroscopy (XPS) was performed in a Physical
Electronics PHI 5600 multitechnique system with a monochro-
matic Al KR X-ray source.

10102 J. Phys. Chem. B, Vol. 109, No. 20, 2005
Wen et al.
Figure 4. SEM images of Ag2S nanostructures grown from the silver substrates (30 days) under different surface oxidation conditions (T ) 25
°C). The Ag foils are oxidized in air for 2 (A), 4 (B), and 6 months (C), or oxidized by EC for ∼5 s at 0.3 V (D), 5 min at 0.3 V (E), and 10 min
at 0.3 V (F), respectively.
Results and Discussion
the diffraction peaks can be indexed to the monoclinic
Ag2S: Achantite, a ) 4.221 Å (4.229 Å); b ) 6.928 Å (6.931
Å); c ) 7.840 Å (7.862 Å); â ) 99.61° (99.61°) [the numbers
in parentheses are taken from JCPDS 14-0072]. Control
measurements on the Ag surface after the oxidation step but
before the Ag2S nanowire growth showed a relatively smooth
surface (see Figure S2 in the Supporting Information).
A typical SEM image of the as-synthesized Ag2S nanowires
(40 °C/5 days) is shown in Figure 1. It can be seen that Ag2S
nanowires are abundantly produced with uniform diameters
(40-150 nm) and high aspect ratios (∼1000). The lengths of
the nanowires are up to 100 µm. The elemental analysis of the
nanowires by EDX (inset of Figure 1) shows the significant
presence of only Ag and S with an atomic ratio (Ag/S) of 2.1,
which matches well with the chemical formula of Ag2S. The
powder XRD pattern of the as-synthesized Ag2S nanowires
recorded directly on the Ag (see Figure S1 in the Supporting
Information). Except for the Ag peaks from the substrate, all
Figure 2A shows a low-magnification TEM image of an
individual Ag2S nanowire, which is ∼75 nm in diameter with
the top end slightly thinner than the bottom end. Selected area
electron diffraction (SAED) from the nanowire displays a
spotted pattern (Figure 2B), indicating the monocrystallinity of

Synthesis of Single Crystalline Ag2S Nanowires
J. Phys. Chem. B, Vol. 109, No. 20, 2005 10103
the nanowires. The SAED pattern tallies closely with the [010]
zone axis of Ag2S and suggests the nanowire growth direction
along the normal of (1h01). The HRTEM image in Figure 2C
reveals clear lattice fringes, which are perpendicular to the
nanowire and run through it without interruption. The fringe
spacing measures 0.40 nm, which is very close to the interplanar
spacing of (1h01) and therefore taken to corroborate the above
inference of the Ag2S nanowire growth direction from the SEAD
pattern. This growth direction is quite different from that of
Cu2S nanowires,24 which extend along the [001] direction
instead of the normal of (1h01). Further studies are needed to
clarify the different nanowire crystal growth kinetics of the two
systems.
We have studied the effect of the extent of surface oxidization
on the growth of Ag2S nanowires. The extent of surface
oxidation was monitored by XPS. The energy was calibrated
by fixing the binding energy of C 1s as 285.0 eV. The Ag 3d5/2
peak of a clean Ag foil (Figure 3A(a)) is located at a binding
energy of 368.3 eV, which is characteristic of elemental Ag.
The Ag 3d3/2 peak (3d3/2:3d5/2 ≈ 2:3) was also monitored but
not shown here. After being oxidized in air for 6 months, the
Ag 3d5/2 peak shifted down to 368.0 eV (Figure 3A(b)),
indicating the formation of Ag2O on the silver surface.42 XPS
spectra of the Ag foils oxidized by the electrochemical method
display similar binding energy downshifts (Figure 3A(c) and
3A(d)). More detailed chemical information can be extracted
from the corresponding Ag MNN Auger spectra in Figure 3B.
For the clean Ag foil, the Auger spectrum shows two distinctive
kinetic energy peaks at 357.2 and 351.2 eV (Figure 3B(a)). Upon
oxidization in air or in the electrochemical cell for 5 min, two
new peaks started to develop at kinetic energies of 355.1 and
349.1 eV (Figure 3B(b) and 3B(c)), hinting at the formation of
Ag2O. After being oxidized by the electrochemical method for
a longer time (15 min), the two old peaks are almost completely
replaced with the two new peaks, which indicates the nearly
complete transformation of Ag to Ag2O on the substrate surface
(Figure 3B(d)).43
Figure 5. SEM images of Ag2S nanowires synthesized by reactions
at 25 °C for 30 days (A) and 40 °C for 5 days (B). Inset: TEM images
of a single nanowire grown at the corresponding reaction conditions
(O2/H2S ) 1).
Shown in Figure 4 are SEM images of Ag2S nanostructures
grown on the Ag foils with different extents of oxidation. When
the Ag foil is oxidized insufficiently, e.g., with oxidation in air
for 2 months, only irregularly shaped but faceted Ag2S particles
with an average size of submicron are formed on the surface
(Figure 4A). Some of the Ag2S particles have a cone structure
(about 1 µm long, 300 nm thick at the root, and several nm at
the tip, as seen in the dashed white cycle), which is perhaps the
preferred site for the nanowire growth. For the Ag foil that has
been oxidized in air for 4 months, sharp-tipped Ag2S nanorods
2-3 µm in length emerge (Figure 4B). After the Ag foil is
oxidized in air for an even longer time (6 months), we have
produced copious Ag2S nanowires, which are uniform in
diameter (50-300 nm) and several tens of micrometers in length
(Figure 4C). Electrochemical oxidation at ∼0.16-0.30 V is a
convenient method to shorten the time for the surface oxidization
of the Ag foil. For example, with only ∼5 s of electrochemical
preoxidation at 0.30 V, the Ag2S nanowires already grow faster
than those from the Ag foil preoxidized in air for 4 months
(see Figure 4D; the nanowires are longer than those in Figure
4B). For even longer electrochemical oxidation times (Figure
4E, 0.3 V, 5 min; Figure 4F, 0.3 V, 10 min), the Ag2S nanowires
grow much longer (10-20 µm) after a similar reaction period
with the only difference being that the nanowires in the latter
case are much less straight. It can be concluded that the
preoxidization of the silver surface is necessary for the growth
of the Ag2S nanowires. However, overoxidation may somehow
impede the formation of the Ag2S nanowires; the nanowires
are less straight (Figure 4F). It follows that the Ag2S nanowires
are produced by the following reactions:44
Ag(s) + O2(g) ) Ag2O(s)
∆G ) -11.2 kJ/mol
Ag2O(s) + H2S(g) ) Ag2S(s) + H2O(l)
∆G ) -233.2 kJ/mol
Clearly, the oxidation step is critical to the Ag2S nanowire
growth given that the formation of Ag2O is not very thermo-
dynamically favorable. This is in marked contrast to the
reactions involved in the Cu2S nanowire growth process:44
Cu(s) + O2(g) ) Cu2O(s)
∆G ) -146.0 kJ/mol
Cu2O(s) + H2S(g) ) Cu2S(s) + H2O(l)
∆G ) -143.9 kJ/mol
Here the oxidation step is much more facile simply on
thermodynamic grounds. This explains why Cu2S nanowires are
more easily synthesized than the Ag2S nanowires by the gas-
solid reactions, and in fact, the Cu2S nanowires can be grown
into uniform arrays. It is believed that by varying the reaction
conditions favoring the oxidation step, uniform Ag2S nanowire

10104 J. Phys. Chem. B, Vol. 109, No. 20, 2005
Wen et al.
Figure 6. Low-magnification TEM image of a single Ag2S nanowire before (A) and after (B) electron beam irradiation (200 kV) for 2 h. (C and
D) The corresponding ED pattern and HRTEM image of the nanowire in part B (etched part enclosed by the ellipsoid).
arrays of Ag2S can also be prepared. This work is underway in
our laboratory.
reaction, the solid metal serve sas both a feeding material and
a supporting substrate. The gas phase reagent diffuses to the
oxidized metal surface and reacts at appropriate conditions. After
the formation of a nanoparticle seed, a preferential growth
direction may develop as an entry to the formation of 1D
nanostructures. The active site for the nanowire growth is likely
to be at the tip. As such, the diameter and morphology of the
nanowires are mainly controlled by two factors. One is the
growth rate along the wire direction relative to those of other
directions. The other factor is perhaps the diffusion rate of Ag
atoms from the substrate to the nanowire growth site, from
which the nanowire propagates. Because the atomic diffusion
of Ag in the growing nanowire is expected to be slow at low
temperature, the Ag atom supply becomes shorter and shorter
to sustain the uniform nanowire growth as the nanowire gets
longer and longer and this would result in the formation of a
nanocone structure with a sharp tip as observed in Figure 4A.
As the temperature is increased, however, the atomic diffusion
rate of Ag increases, which provides a sufficient amount of Ag
at the nanowire tip for the growth. As a result, the individual
nanowire (Figure 5B) diameters are more uniform with relatively
blunt tips. When the atomic diffusion of Ag is fast enough
The SEM images in Figure 5 show the effect of reaction
temperature on the growth of Ag2S nanowires. The silver foils
used for the Ag2S nanowire growth were preoxidized under the
same electrochemical conditions (at 0.3 V for 5 min; the
standard potential of Ag2O/Ag is 0.342 V44). It is apparent that
the Ag2S nanowire growth is more efficient (both in abundance
and lengths of the nanowires) at 40 °C (Figure 5B) than at room
temperature (Figure 5A). Notice that the reaction time in Figure
5A is 30 days whereas the reaction in Figure 5B has lasted for
only 5 days. Moreover, the nanowire morphology is also
different at the two different growth temperatures. At room
temperature, the nanowires are grown slowly (only ∼10 µm
long after reaction for 30 days) with a sharp tip (inset of Figure
5A). At 40 °C, however, the nanowire growth is much faster
(several tens of micrometers long after reaction for only 5 days)
and the nanowires are more like a cable with uniform diameters
along the growth direction (see inset of Figure 5B).
Our method for the growth of Ag2S nanowires is characterized
by the gas-solid reaction at low temperatures. The nanowires
synthesized by gas-solid reaction are often accompanied by a
sharp tip.23,24,45,46 In the nanowire growth process of gas-solid

Synthesis of Single Crystalline Ag2S Nanowires
J. Phys. Chem. B, Vol. 109, No. 20, 2005 10105
images were recorded by a CCD through a 50× ultra-long
working distance objective. Presumably due to the light scat-
tering effect when excited with white light, we could observe
the Ag2S nanowires clearly (in a slightly red color) under an
optical microscope though the nanowire is only several tens of
a nanometer in diameter. Figure 7A shows a typical OM image
of the Ag2S nanowires on the substrate. For the facility of
observation, we select a single nanowire to process the cutting
experiment. Figure 7B,C and Figure 7D,E show optical images
of two single Ag2S nanowires before and after being cut by the
laser. For the first nanowire, both broken parts can be seen
(Figure 7C), whereas only the left part can be seen for the second
after cutting (Figure 7E); the other piece may have fallen out
of focus and disappeared from view. It is clear that the nanowires
could be cut very easily when we focus the laser spot on the
nanowires with the help of the crosshairs shown in the images.
The whole laser radiation time was only about 1 s. The success
rate of the cutting was 100% and ∼15 nanowires were cut in
this way in our experiment. The facile thinning and cutting of
Ag2S nanowires by electron beam and laser suggest that the
methods may find use in the fabrication of future nanoscale
devices. Clearly, understanding the atomistic details of these
processes under more controlled conditions is necessary to
facilitate the applications of the radiation-sensitive nanowires.
Conclusion
We have succeeded in synthesizing Ag2S nanowires 40-150
nm in diameter and several tens of micrometers in length by a
gas-solid reaction method on silver substrates. Reaction condi-
tions including surface oxidation of the Ag substrate and reaction
temperature have been investigated. The Ag2S nanowires are
found to grow along the normal of (1h01) instead of [001] as
with Cu2S nanowires. The normal of (1h01) is believed to be
the fastest growing crystal direction for Ag2S under our
experimental conditions. The morphology and size of the
nanowires grown by the gas-solid reaction method are deter-
mined mainly by the competition of the growth rate and the
atomic diffusion rate. On the basis of these studies, a possible
nanowire tip growth mechanism has been discussed. In addition,
we have demonstrated methods for the nanoscale tailoring of
individual Ag2S nanowires. Specifically, by using a high-energy
electron beam and a laser beam, individual Ag2S nanowires
could be thinned out and cut, respectively. In conclusion, the
current work extends the gas-solid reaction method to the
synthesis of Ag2S nanowires under mild conditions and dem-
onstrates simple nanoengineering methods for the thinning and
cutting of the individual nanowires. This is an important step
toward the development of fabrication methods for nanoscale
devices.
Figure 7. Typical optical microscope images of Ag2S nanowires. (A)
Overview of the Ag2S nanowires. (B and D) A single Ag2S nanowire
before the laser irradiation (782 nm, 0.10 mW). (C and E) A single
Ag2S nanowire after the laser irradiation.
compared to the nanowire growth rate, the nanowire diameters
are expected to be uniform from the root to the top (Figure 1).
An interesting phenomenon we observed is that the Ag2S
nanowires are more unstable than nanowires of other semicon-
ductor materials, such as ZnO, under high energy electron beam
radiation. We irradiated a single Ag2S nanowire with an electron
beam in the TEM machine at relatively high magnification
conditions. Figure 6B shows the TEM image from the same
nanowire shown in Figure 6A after being irradiated with the
electron beam (200 kV) for 2 h. Clearly, at the position irradiated
by the electron beam (in the white circle), the diameter of the
nanonanowire decreases from 75 to 50 nm. This suggests that
the Ag2S nanowires is easily etched away by high energy
electrons. Moreover, the crystal structure of the nanowire is
preserved. This is shown in the corresponding ED (Figure 6C)
and HRTEM (Figure 6D): the Ag2S nanowire after electron
beam induced thinning keeps the same monoclinic crystal
structure and the length direction normal to the (1h01) plane.
We also tried to cut the Ag2S nanowires using a laser beam,
which is easier to operate than the high-energy electron beam
because there is no need for a vacuum. For this, a diode laser
(782 nm, 0.10 mW) with a beam spot of 3 µm was used. The
Acknowledgment. We are grateful to the Research Grant
Council of Hong Kong and the Chemistry Department of the
Hong Kong University of Science and Technology for support-
ing the research. S.Y. wishes to thank the Hong Kong Young
Scholar Cooperation Research Foundation of NSFC.
Supporting Information Available: Figures showing the
XRD pattern of the as-prepared Ag2S nanowires on an Ag
substrate and SEM image and XRD pattern of the oxidized Ag
foils before the growth of Ag2S nanowires. This material is
available free of charge via the Internet at http://pubs.acs.org.
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