itethis: New.hem .,2012,36 ,24562459 LETTER...itethis: New.hem .,2012,36 ,24562459 Silver nanowires and nanoparticles from a millifluidic reactor: application to metal assisted silicon

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

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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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