2456 New J. Chem., 2012, 36, 2456–2459 This journal is c The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2012
Cite this: New J. Chem., 2012, 36, 2456–2459
Silver nanowires and nanoparticles from a millifluidic reactor: application
to metal assisted silicon etchingw
Ronen Gottesman, Alex Tangy, Ilan Oussadon and David Zitoun*
Received (in Montpellier, France) 28th August 2012, Accepted 26th October 2012
DOI: 10.1039/c2nj40763a
Silver nanowires and nanoparticles are synthesized by a polyol
method in a millifluidic reactor. We have been able to optimize
the flow chemistry reaction conditions to get a high yield of
nanowires in a continuous flow. By changing reaction para-
meters we have demonstrated the synthesis of single crystalline
silver nanoparticles in a rapid reaction time of only 3 minutes.
All results are compared with standard batch and microwave
reactions. An example of application is provided through the
silver nanowire assisted etching of silicon wafers. This colloidal
approach of metal assisted silicon etching allows transferring of
the nanowire shape to silicon.
Owing to its high electrical and/or thermal conductivity, silver has
been widely used in conductive coatings. For this application,
research has been focused on silver nanoparticles and nanowires
in recent years due to their large aspect ratio.1–5 The electrical
percolation threshold of wires is lower than that of silver spheres,
since the probability of junctions between wires is greater than in
the case of spheres. Regarding the synthesis of silver wires, several
methods have been reported including hard-template,6–8 and soft
template synthesis.9 Among these synthesis routes, solution phase
synthesis by polyol reduction is the most intensively studied.10–12
Due to the temperature and alkyl chain dependent reducing power
of polyols, these reducing agents allow sequencing nucleation and
growth processes through careful control of the reagent addition
rate.13 The classical capping agent is a polymer (e.g. polyvinyl
pyrrolidone, PVP), usually foreseen as the directive agent towards
shape control. Indeed, several reports account for different inter-
action strengths with various crystallographic facets of metal
particles, resulting in anisotropic growth.11,14 Ag+ is reduced to
Ag by a polyol at high temperature, Ag atoms form clusters to
decrease the surface free energy, meanwhile, the PVP molecules
adsorb on the surface of silver. Thermodynamics are highly
important to direct the Ag growth since reduction of Ag+ and
adsorption of PVP molecules depend on temperature. Kinetics
plays a significant role in this synthetic process. By finely tuning the
silver ion reduction rates, the formation rates of clusters and seeds,
and the adsorption of PVP molecules, seeds with a decahedral
structure are formed,15 which then lead to the formation and
growth of silver wires as the reaction continues. Ag nanowires
display a fivefold twinned structure bound by five {100} side facets
with the {110} growth direction.16 In an FCC lattice, {111} facets
have the lowest affinity to pyrrolidone. By contrast, {100} facets
have lower atomic density and octahedral interstitial sites of the
lattice are located on these facets, offering more open sites to
coordinate the pyrrolidone. Therefore, when decahedral seeds are
formed during the initial nuclei period, the PVP molecules adsorb
preferentially on the {100} facets and can inhibit the growth along
this direction.
Synthesis of inorganic nanomaterials, such as metallic,
semiconductor and silica nanoparticles, in millifluidic and
microfluidic devices, offers several advantages over macroscale
chemical reactors,17–19 like enhancement of mass and heat
transfer,20,21 reproducibility,22 potential for in situ reaction
monitoring,23 rapid screening of parameters, low reagent
consumption during optimization, safety, and synthesis
parameters independent of the process scale.22 The high surface-
to-volume ratio20,21 of the reactor channels enables precise
temperature control, allowing for the preparation of nano-
particles with narrow size distribution. Noble metals synthesis
using different modulations has been reported in recent years.
Nanostructures of Au and Ag with spherical,24 core–shell25 and
rod26 morphologies were synthesized in continuous flow reactors.
However, studies on the synthesis of noble metal nanowires have
not been reported.
In this communication, we report on the use of a continuous
flow synthesis of Ag nanowires in a conventional polyol
process. The yield of nanowires can be increased by simply
changing the reaction time. We observed that the optimized
concentration ratio between the silver ion precursor (AgNO3)
and the capping agent (PVP) is 1 : 1.5.27 The exploitation of
millifluidics has enabled us to work at temperatures close to or
higher than the solvent boiling point which plays a major role
in the reaction’s kinetics. The Ag nanowires have been used in
metal assisted chemical etching of silicon.
All reactions gave a chemical yield higher than 90% of Ag
nanostructures as deduced from ICP analysis. SEM shows the
formation of nanowires and/or nanocrystals of the samples.
Bar Ilan University, Department of Chemistry and Bar Ilan Instituteof Nanotechnology and Advanced Materials (BINA), Ramat Gan52900, Israel. E-mail: [email protected];Tel: +972(0)37384512w Electronic supplementary information (ESI) available. See DOI:10.1039/c2nj40763a
NJC Dynamic Article Links
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This journal is c The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2012 New J. Chem., 2012, 36, 2456–2459 2457
The nanowires (NWs) yield is defined as the frequency of NWs
compared to all other shapes (faceted, cuboctahedral, icosahedral,
decahedral nanoparticles...), this number is deduced from an
extensive SEM observation. The yield of NWs is first correlated
with reaction time. We have measured yield, length and diameter
of Ag NWs in four different reactions with durations of 3, 15, 30
and 60 min. The NWs display lengths ofB2 mm (minimum value
observed) up to B50 mm (maximum value observed) with an
average value of 10 mm. The NWs that are synthesised in the
30 min reaction have the best yield (92% of nanowires) and are
highly monodisperse with a diameter of 71 nm � 2. When the
reaction time is very short, the NWs yield is poor (11%, Fig. 1A)
but increases with time (Fig. 1B, 31%). The large majority of
nanoparticles and seeds do not grow to form nanowires (3 and 15
minutes reaction time). However, when the reaction time is longer
than the optimum results we achieved (60 min instead of 30 min);
there is clear evidence of non-directional overgrowth (100%
entangled wires). The NWs grow wider, polydisperse and present
kinks and bulges (Fig. 1D). This could be attributed to new seeds
that are formed because of longer reaction time. These seeds
coalesce with the original single crystal nanowire and keep
growing on its surface in directions other than the original
direction of growth (isotropic growth). This mechanism explains
the wider diameter (273 � 63 nm) and the lack of straight and
smooth structure observed at shorter reaction time. When the
reaction time increases more than the estimated optimum time
(from 30 to 60 minutes in our case), the overgrowth we notice is
due to the presence of free Ag ions in the medium, which is
reduced by the EG and added to all the surfaces of the nanowires
as seen in Fig. 1D. This phenomenon also leads to a significant
increase in crystallographic defects which could also explain the
NW bending.
In the following, the discussion will focus on the Ag NWs
grown for 30 minutes (Fig. 1D). The Ag NWs have been
observed by TEM and show a crystalline structure with
a growth direction along the [2, �1, 0] axis (Fig. S2, ESIw).
The optical properties of the NWs dispersion show a
maximum absorption at 460 nm consistent with previous
reported values (Fig. S1, ESIw).14
In an attempt to further explain our findings, we compare
our results to those obtained in previous work where crystal
overgrowth is observed on gold nanorods when increasing
metal cation concentration while a mild reducing agent is
present.28 The unique crystal facet of the starting nanorods
results in anisotropic crystal overgrowth due to preferred Au
overgrowth sites on the original rods. Also mentioned is that
less stable facets are less likely to be coated due to stronger
interaction with the surfactant molecules. Hence, the overgrowth
rate along the [111] direction would be higher than that along
the [110] direction. Due to the fact that Au and Ag have
similar lattice parameters of 0.4078 nm and 0.4086 nm,
respectively, this mechanism applies to epitaxial crystal growth
of Ag on Au nanorod surfaces. One can correlate this report
with our observation of growth on the preferred growth sites
on the original wire in a first step (up to 30 min reaction).
The optimum reaction time in our case means that we are
well within the boundaries of the process window for the
synthesis of nanowires. Two competitive processes explain
the yield of nanowires. One process is the formation of
decahedral seeds (leading eventually to wires), while the
second process is the formation of seeds with different
morphologies (leading eventually to faceted nanoparticles).
In our experiment, when the reaction starts, there is continuous
reduction of silver ions to silver atoms and the formation of
silver colloids which form seeds as time increases. At the
beginning of the reaction, Ag nanoparticles of several shapes
are produced.16 In our case, there is also a mixture of several
shapes of seeds and as time increases, they are being etched to
increase the ratio of decahedral seeds. This occurs at temperatures
higher than 180 1C and the detailed explanation is given
elsewhere.16 When there are still seeds of different types and
the decahedral seeds are not the majority population, there
will also be the formation of faceted nanoparticles in parallel
with nanowires (Fig. 1A and B). If, on the other hand, there
is enough time for the seeds mixture to transform into
decahedral seeds, the concentration of the nanowires will
increase or in other words, their yield will be high (Fig. 1C).
The last scenario is when the reaction time has passed the
optimum time, and the remaining silver content in the reactor
(could be Ag cations or clusters) now forms thicker nanowires
which leads to an overgrowth observed for longer reaction
time (Fig. 1D).
In summary, the reaction temperature is a key parameter in
controlling the type of seeds we receive in the reaction.
Decahedral Ag particles are seeding the unidirectional growth
of the nanowires. If the temperature is sufficiently high, the
majority of seeds will be decahedral. However, it is important
to note and emphasize that the process window has been
found when the temperature and the precursor (AgNO3) to
surfactant (PVP) ratio is constant and the only variable is the
reaction time.
When using solvothermal conditions (i.e. above the solvent’s
boiling point and at elevated pressures) and shortening the
reaction time, we obtain single crystalline nanoparticles (NPs)
(120 � 10 nm) (Fig. 2A and B). The time to complete the
Fig. 1 SEM images of Ag NWs synthesized by flow chemistry
showing typical yield of the NWs as a function of reaction time.
(A–D) 3, 15, 30 and 60 minutes, respectively, reaction times.
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2458 New J. Chem., 2012, 36, 2456–2459 This journal is c The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2012
reaction is decreased to 3 minutes and the temperature and
applied external pressure are 240 1C and 6 bars respectively.
Unlike the reaction at 198 1C (Fig. 1A), there is no trace of
nanowires. In order to find out the ability to synthesize
nanowires at higher yields when increasing the reaction time,
we have increased the reaction time to 15 and 30 minutes, but
the yield of nanowires is still extremely low (Fig. 2C).
When taking into consideration the growth mechanism
discussed above, one can understand the results obtained
under the solvothermal conditions. At elevated temperatures
such as 240 1C, out of the two competitive processes, the
formation of single crystalline FCC lattice Ag nanocrystals is
the dominant process. One can conclude that, under solvothermal
conditions, the pathway to the formation of unidirectionally
grown structures such as rods or nanowires is less favorable than
the pathway to the formation of faceted nanoparticles. In
other words, nucleation is highly favorable under solvothermal
conditions as compared to anisotropic growth. This is supported
by our results showing that the yield of nanowires increases
slightly (but still remains extremely low) when increasing the
reaction time (e.g. from 3 minutes to 15 and 30 minutes).
We compared our typical result of the millifluidic experiments
(30 min reaction time, T = 198 1C) to those obtained using
two other synthetic methods (batch chemical synthesis and
microwave) (ESI,w Fig. S3). Both reactions are done according
to a previously reported experimental protocol.16a The yield of
NWs obtained from batch chemistry reaction is similar to
that from millifluidic reaction with an average NW diameter
of 53 � 7 nm while the yield obtained from microwave reaction
is slightly lower than those from the other two with the
nanoparticles by-product and an average diameter of the nanowires
of 61 � 8 nm. Ag NWs rapid synthesis (3.5 min) using microwave
has been accomplished.15c Our system differs in its extensive
versatility and the unique ability of flow chemistry to upscale the
synthesis.
The principle of metal-assisted etching consists of deposition
of metal particles or films at the Si surface prior to chemical
etching to enhance the Si dissolution and has found many
applications in solar conversion, thermoelectric conversion,
Li-ion storage, and sensing devices.29 This metallization is
performed by various techniques such as sputtering, thermal
evaporation, electrochemical deposition or electroless deposition
in HF solutions. Several batches of Ag NWs were used to
template the metal assisted etching of Si wafers. The etching
follows a two step procedure, namely deposition of Ag NWs
and selective etching of the underneath Si wafer. The etching
conditions are similar to previously reported ones.30,31 Before
etching, Ag NWs need to be annealed to improve the physical
contact with the Si wafer. Fig. 3 shows a SEM image of a wafer
after first (Fig. 3A) and second (Fig. 3B) steps. After etching,
holes corresponding to the Ag NWs morphology are observed.
The average length is kept unchanged between Ag NWs and the
corresponding holes. The only difference comes from the increase
in diameter (31% in average). Ag nanowires can therefore be used
as templates for templating anisotropic holes into Si.
Ag NWs are randomly distributed when the dispersion is
deposited on the Si pattern by drop casting. Therefore, further
improvement can be achieved by aligning them with techniques
like Langmuir–Blodgett32 or local irradiation33 finding applications
toward optical and electronic devices.
We have reported on the synthesis of high yield, high aspect
ratio nanowires using flow chemistry in a millifluidic reactor.
We have optimized the yield of nanowires by controlling the
reaction time at a high temperature close to the solvent’s
boiling point. This enabled the increase in concentration of
decahedral seeds which are proven to be the starting point of
the unidirectional growth of the nanowires. Nucleation is
highly favorable under solvothermal conditions as compared
to anisotropic growth, yielding almost only single crystalline
nanoparticles in a very rapid reaction time (3 minutes). High
yield of metallic nanowires and nanoparticles opens a new
window of opportunities in the use of flow chemistry in
materials science, even for large nanostructures like nanowires.
As an example of application, Ag NWs are used as templates
for a metal assisted etching process.
Experimental
Synthesis of Ag NWs: AgNO3 (99.9%, STREM) and poly-N-
vinylpyrrolidone (K29-32, Mw B 58 000, Aldrich) were stored
in a dessicator and ethylene glycol was dried on molecular
sieves. HF (40% in water, Alfa-Aesar) and H2O2 (30% in
water) were used as received. In a typical reaction, a solution
of [AgNO3] = 0.1 mol L�1 and [PVP] = 0.15 mol L�1 in
ethylene glycol (EG) was injected using an Asia Syringe Pump
(Syrris) through a 1.5 mm I.D. PTFE tube. The PTFE tube
was placed inside a split furnace at a temperature of 198 1C
while the end of the tube was taken out of the furnace and was
connected to a vial to collect Ag NWs (Scheme 1). PTFE
tubing was rolled over an aluminium cylinder to provide an
efficient heat conduction and reproducible geometry. The section of
Fig. 2 (A) SEM image of Ag NPs grown under solvothermal
conditions (reaction time of 3 minutes). (B) TEM images of the
Ag NPs, the inset shows a diffraction pattern of a single crystalline
NP. (C) SEM image of a 30 minutes reaction using solvothermal
conditions showing a mix of NPs and nanowires.Fig. 3 SEM images of (A) Ag nanowires dispersed on a Si wafer; (B)
Si wafer after etching and Ag dissolution.
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This journal is c The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2012 New J. Chem., 2012, 36, 2456–2459 2459
the PTFE tube that was inside the furnace at a temperature of
198 1C was designated as the reaction chamber and all reaction
time calculations were based upon that section’s volume. After the
reaction the precipitate was washed and centrifuged three times in
ethanol at 4000 rpm, then dried at room temperature overnight.
The etching process was achieved by drop casting Ag NWs
dispersion onto the Si wafer (n-type, 4–6 O cm). The Ag NWs
coated Si wafer was then annealed in air at 300 1C for 1 hour. The
Ag NWs coated Si wafer was finally dipped in an aqueous solution
containing 2.9 mol L�1 of HF and 0.5 mol L�1 of H2O2 for 30
minutes. Ag NWs were removed after the etching by dipping the Si
wafer in nitric acid (HNO3). Scanning electron microscopy was
performed on a JEOL-JSM 840. Transmission electron microscope
(TEM) images were obtained using a JEOL-JEM 100SX with
80–100 kV accelerating voltage. Samples for TEM were prepared
by placing a drop of the diluted sample on a 400-mesh carbon-
coated copper grid.
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Scheme 1 Experimental set-up of a millifluidic reactor inserted into a split
furnace.
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