Nanotechnology
Nanotechnology 26 (2015) 381002 (6pp)
doi:10.1088/0957-4484/26/38/381002
Fast Track Communication
A simple method of growing silver chloride
nanocubes on silver nanowires
Hadi Hosseinzadeh Khaligh1,2 and Irene A Goldthorpe1,2
1
Department of Electrical & Computer Engineering, University of Waterloo, Waterloo, ON N2L 3G1,
Canada
2
Waterloo Institute for Nanotechnology, University of Waterloo, Waterloo, ON N2L 3G1, Canada
E-mail: [email protected]
Received 16 July 2015
Accepted for publication 10 August 2015
Published 3 September 2015
Abstract
The growth of AgCl nanocubes directly on the sidewalls of Ag nanowires is demonstrated. The
nanocubes can be simply obtained through extended low temperature annealing of polyolsynthesized silver nanowires in a vacuum. The length of time and temperature of the anneal and
the diameter of the nanowire affect the size and density of the nanocubes obtained. It is
hypothesized that the AgCl material is supplied from reactants leftover from the silver nanowire
synthesis. This novel hybrid nanostructure may have applications in areas such as photovoltaics,
surface enhanced Raman spectroscopy, and photocatalysis.
Keywords: hybrid nanostructure, silver nanowires, silver chloride nanocubes
(Some figures may appear in colour only in the online journal)
1. Introduction
sidewalls, the advantages of nanocubes, such as their larger
surface-area-to-volume ratio and surface plasmon resonance
at different wavelengths, can be combined with nanowires.
Furthermore, if the nanocubes and nanowires are different
materials, the properties of each material can be exploited and
combined in a useful way. Nanowires with nanoparticles on
their surface have been used in applications such as sensors,
catalysts, and photovoltaics [18–20].
There exist several methods of decorating the surface of
nanowires with nanoparticles or nanocubes. The nanowire is
first synthesized and nanoparticles are then grown or attached
on their sidewalls through methods such as chemical vapour
deposition or solution synthesis [24, 25]. For example, for
surface enhanced Raman spectroscopy (SERS) applications,
silver nanocubes were attached onto silver nanowires by
mixing pre-synthesized individual nanocubes and nanowires
together in solution [26]. However, the spatial orientation of
the nanocubes with respect to the nanowire sidewalls was not
controllable. This is not ideal since the maximum surface
plasmon resonance happens when the face of a nanocube is flat
against the sidewall of the nanowire [26]. In another report,
silver phosphate cubes were grown around silver nanowires in
The crux of nanotechnology is exploiting the unique properties that arise when material feature sizes are on the order of
or smaller than ∼100 nm. The size, shape, and composition of
nanostructures can all significantly affect their characteristics
[1–10]. For example, the size of a nanostructure can lead to
changes in surface energy, electronic band structure, and
thermal conductivity, among many other properties [11–16].
Because of this, a multitude of nanostructures such as spheres,
wires, cubes, plates, and polyhedra of varying sizes have been
synthesized and studied over the past couple of decades [1–3].
Integrating more than one shape, feature size, and/or
material into one hybrid nanostructure can impart increased
functionality [17–23]. In this work, nanocubes are grown on
the surface of nanowires. Nanowires, unlike nanocubes and
nanoparticles, have one long dimension. This long axis can be
used to transport electrical current and heat, it makes nanowires easier to contact for use in devices, and it allows for a
connected network to be achieved at a far lower particle
density, the latter being useful for applications such as
transparent electrodes. By adding nanocubes on nanowire
0957-4484/15/381002+06$33.00
1
© 2015 IOP Publishing Ltd Printed in the UK
Nanotechnology 26 (2015) 381002
Figure 1. 90 nm diameter silver nanowires before (a) and after annealing at 100 °C for 3 days (b) and 7 days (c). (d) 200 nm diameter
nanowires after annealing at 100 °C for 7 days.
visible-light driven photocatalyst useful for water splitting and
the decomposition of organic pollutants [36, 37]. Their
attachment to a silver nanowire can enhance their photocatalytic performance since the silver can promote charge
separation and transport generated electrons [29, 38, 39]. In
addition to these potential applications, this communication
helps to explain why many groups observe nanoparticles on
silver nanowires in transparent electrodes [40, 41].
solution. The average edge length of the cubes however was
500 nm, 10 times larger than the diameter of the nanowire and
too big to be used in nanoscale applications [27].
In this work we show for the first time that nanocubes
can be synthesized directly on the sidewalls of a metallic
nanowire through a simple anneal in a vacuum. This is also
the first report of AgCl nanocube/Ag nanowire hybrid
structures. The size and density of the nanocubes are controlled through the annealing temperature and time. The cubes
lie flat against the nanowire facets and are likely epitaxial.
These structures have many potential applications. Firstly, the
nanocubes increase the surface area of the nanowire which is
beneficial for catalytic, sensing, and antibacterial applications
[28, 29]. Secondly, films of silver nanowires have been used
as substrates for SERS applications to increase signal intensity [30]. The addition of nanocubes on their surfaces can
further enhance the SERS signal [31], which improves sensitivity of detection. Thirdly, films of silver nanowires are a
promising transparent electrode for use in photovoltaic devices
[32]. Having nanocubes on the nanowire surfaces may increase
light absorption in a thin-film solar cell due to increased light
scattering, as has been shown to be the case when nanoparticles are added to the transparent electrode of solar cells
[33–35]. Fourthly, the nanocubes are silver chloride, which is a
2. Experiment
Poly(vinylpyrrollidone) (PVP) directed polyol synthesis is a
solution-based method that can be used to synthesize crystalline silver nanostructures of various well-controlled shapes.
In this method, a mixture of AgNO3 and ethylene glycol (EG)
is gradually added to a solution containing EG, PVP and
NaCl, and is heated to 170 °C. Depending on the concentration of AgNO3 and the molar ratio of AgNO3 and PVP,
various shapes such as nanocubes, nanowires, spheres, and
triangular plates can be obtained. For silver nanowire synthesis, the resulting nanowires have a pentagonal cross-section,
with 5 {100} sidewalls [42, 43].
In this study, two different sizes of polyol-synthesized
silver nanowire dispersed in ethanol were commercially
2
Nanotechnology 26 (2015) 381002
Table 1. Density and average size of the nanocubes on nanowires
after annealing at 100 °C.
90 nm
Density of NCs
Average size of NCs
200 nm
Density of NCs
Average size of NCs
3 days
1 week
2 weeks
1.1 NC/μm
60 nm
0.7 NC/μm
80 nm
0.3 NC/μm
100 nm
1 NC/μm
80 nm
0.3 NC/μm
300 nm
0.2 NC/μm
300 nm
obtained; ones with an average diameter and length of 90 nm
and 15 μm, respectively (Blue Nano Inc., Charlotte, NC), and
200 nm and 25 μm, respectively (ACS Materials Inc., Medford, MA). Silicon wafer chips were cleaned in a sonication
bath with ethanol, acetone and distilled water, each for one
minute. The as-received nanowires were diluted with ethanol
and drop cast onto the Si. They were then annealed in low
vacuum for various lengths of time, ranging for 3 to 14 days,
and at temperatures ranging between 100 °C to 150 °C.
Scanning electron microscopy (SEM) images were taken of
the samples before and after annealing. The average density
and size of the nanocubes were calculated from these SEM
images by measuring a minimum of 100 nanocubes for each
experimental condition. Transmission electron microscopy
(TEM) samples were prepared by mechanically rubbing the
silver nanowires from the silicon wafer onto TEM grids.
Energy dispersive x-ray spectroscopy (EDS) spectra were
collected to determine the chemical composition of the
nanocubes and nanowires.
Figure 2. Energy-dispersed x-ray spectroscopy (EDS) spectrum from
(a) a portion of a silver nanowire away from a nanocube and (b) a
nanocube on a nanowire.
nanowire diameter, higher temperatures also led to larger and
less dense nanocubes. However, even at 125 °C the nanocubes had average edge lengths of more than 500 nm. This
large size is not as attractive for nanoscale applications and
thus 100 °C is a more preferable annealing temperature.
EDS spectra of 90 nm nanowires annealed for 7 days at
100 °C in a vacuum are displayed in figure 2. In figure 2(a),
the electron probe was focused on a section of a nanowire
away from a nanocube. Other than carbon, only silver was
detected. When the probe was focused on a nanocube
(figure 2(b)), the spectrum contained an additional peak corresponding to chlorine. This suggests that the nanocubes are
AgCl, since AgCl is the only compound occurring in the
silver-chlorine system. Moreover, it is known that AgCl
nanocubes form in the presence of Ag+, Cl−, and PVP (more
discussion on this later). Attempts at TEM imaging and the
collection of diffraction patterns to characterize the morphology, lattice structure, and the epitaxial relationship of the
nanocubes to the nanowires were made. However, the nanocubes quickly decomposed upon exposure to the electron
beam preventing such characterization. This decomposition
and the inability to image AgCl nanocubes in the TEM has
been reported by others [44, 45].
In the polyol synthesis of silver nanowires, silver ions
(Ag+) bond with chlorine ions (provided by NaCl) and form
AgCl nanocrystals to control the growth rate of the silver
3. Results and discussion
An SEM image of the 90 nm silver nanowires before
annealing is shown in figure 1(a). The sidewalls of the
nanowires are smooth. After annealing for 3 days at 100 °C
(figure 1(b)), nanocubes of various sizes are observed on the
surface of the nanowires. All cubes appear to have smooth
facets and sharp corners. Almost all are oriented such that a
cube face lays flat against the nanowire sidewall, and with
cube edges aligned parallel to the nanowire axis. Their edge
lengths range from 20–120 nm. After annealing for 7 days
(figure 1(c)), the nanocubes are larger, with edge lengths
ranging from 40–200 nm, but are less dense than with the 3
day anneal. In general, as shown in table 1, as the nanowires
are annealed for longer times the obtained nanocubes are
larger and less dense, likely because of the increased time for
Oswald ripening to occur.
The nanowire diameter also affects the size and density
of the nanocubes. An image of 200 nm diameter nanowires
annealed for 7 days at 100 °C is shown in figure 1(d). In
general (table 1), nanocubes obtained on the larger diameter
nanowires are larger and less dense compared to those on
thinner nanowires.
The nanowires were also annealed at temperatures of
125 °C and 150 °C. Similar to increasing annealing time or
3
Nanotechnology 26 (2015) 381002
Figure 3. Nanocubes with sharp edges lie flat on the surface of silver nanowires, with sidewalls parallel to the nanowire axis. These
nanocubes were obtained by annealing 200 nm diameter nanowires at 100 °C for 3 days.
nanostructures [46]. Some AgCl nanocrystals remain in the
solution after the synthesis of the nanowires is terminated.
When the nanowires are deposited on the Si substrate, the
solvent evaporates and the AgCl nanocrystals remain on the
surface of the nanowires and the substrate. Elevated temperatures facilitate the diffusion of these nanoparticles. We
hypothesize that the nanocubes form because of Oswald
ripening of these particles, and the cubic shape is due to the
presence of PVP which selectively binds and stabilizes the
{100} surfaces of AgCl [29]. It has been shown that when
AgCl particles are synthesized in solution by mixing AgNO3,
NaCl, and PVP, and heating the solution to 80 °C, AgCl
nanocubes with {100} sidewalls are obtained [29]. As mentioned above and seen more clearly in figure 3, the nanocubes
have sharp edges and their spatial orientation is not random,
but rather lies flat on the {100} nanowire sidewalls with the
cube walls parallel to the nanowire axes. Thus it is likely that
the nanocubes are epitaxially related to the nanowire. If the
cube faces are {100}, as is the case for all AgCl nanocubes in
the literature [29, 47, 48], the cube edges are thus along the
〈100〉 directions. The axes of polyol synthesized Ag nanowires
are 〈110〉 [49]. The {100} planes of the AgCl and Ag are thus
rotated in-plane 45 degrees with respect to one another and in
this orientation, there is only a 4% lattice mismatch.
Further investigation is required to determine the
mechanism of nanocube formation. Nanocubes were only
observed on the nanowires and not on the Si substrate. This
may be because the Ag nanowires promote nanocube growth
in some manner, or merely because silver has a higher surface
energy than the SiO2 surface of the silicon substrate and
growth and diffusion are easier on higher energy surfaces
[50]. The nanowire sidewalls of {100} may be particularly
favorable because not only are they not the lowest energy
plane of an FCC material like Ag, but also because AgCl(100)
has a much larger lattice mismatch with Ag(110) and Ag(111)
planes making epitaxy on these planes unlikely.
To investigate the effect of leftover components from the
nanowire synthesis solution, some nanowire samples were
washed with acetone before annealing. Annealing the washed
nanowires at 100 °C for 7 days did not result in any nanocube
formation which is likely due to the absence of any AgCl
nanocrystals. Unwashed silver nanowires were also annealed
in air rather than in vacuum. Annealing at 100 °C in air for 3
days resulted in the formation of irregularly shaped nanoparticles on the nanowire sidewalls as well as a few long
rectangular prisms with edge lengths of up to 1 μm. The
irregular shapes may be due to the absence of PVP for shape
control, as PVP decomposes in air below 200 °C in less than
one hour, and would be expected to decompose at lower
temperatures at longer annealing times [51]. The irregular
shapes may also, or instead, be due to the reaction of silver
with oxygen and sulfur atoms in the air [52].
4. Conclusions
This study shows that AgCl nanocubes can be synthesized on
the sidewalls of silver nanowires with a simple annealing
process in a vacuum. The size and density of the nanocubes
are dependent on the temperature and length of the anneal, as
well as the diameter of the nanowires. The presence of
nanocubes on the nanowires could add extra functionality and
future work will involve testing them for applications.
Acknowledgments
This work was supported by the Natural Science and Engineering Research Council (NSERC) of Canada.
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