Electrospun metallic nanowires: Synthesis, characterization, andapplicationsAbdullah Khalil, Boor Singh Lalia, Raed Hashaikeh, and Marwan Khraisheh Citation: J. Appl. Phys. 114, 171301 (2013); doi: 10.1063/1.4822482 View online: http://dx.doi.org/10.1063/1.4822482 View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v114/i17 Published by the AIP Publishing LLC. Additional information on J. Appl. Phys.Journal Homepage: http://jap.aip.org/ Journal Information: http://jap.aip.org/about/about_the_journal Top downloads: http://jap.aip.org/features/most_downloaded Information for Authors: http://jap.aip.org/authors

APPLIED PHYSICS REVIEWS

Electrospun metallic nanowires: Synthesis, characterization, andapplications

Abdullah Khalil, Boor Singh Lalia, Raed Hashaikeh,a) and Marwan KhraishehMaterials Science and Engineering Program Masdar Institute of Science and Technology, Abu Dhabi,United Arab Emirates

(Received 10 March 2013; accepted 26 June 2013; published online 1 November 2013)

Metals are known to have unique thermal, mechanical, electrical, and catalytic properties. On the

other hand, metallic nanowires are promising materials for variety of applications such as

transparent conductive film for photovoltaic devices, electrodes for batteries, as well as

nano-reinforcement for composite materials. Whereas varieties of methods have been explored

to synthesize metal nanowires with different characteristics, electrospinning has also been found

to be successful for that purpose. Even though electrospinning of polymeric nanofibers is

a well-established field, there are several challenges that need to be overcome to use the

electrospinning technique for the fabrication of metallic nanowires. These challenges are mainly

related to the multi-steps fabrication process and its relation to the structure evolution of the

nanowires. In addition to reviewing the literature, this article identifies promising avenues for

further research in this area with particular emphasis on the applications that nonwoven metal

wires confined in a nano-scale can open. VC 2013 AIP Publishing LLC.

[http://dx.doi.org/10.1063/1.4822482]

TABLE OF CONTENTS

I. INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

A. Scope of the review . . . . . . . . . . . . . . . . . . . . . . 1

B. Article organization . . . . . . . . . . . . . . . . . . . . . . 2

II. ELECTROSPINNING OF METALLIC

NANOWIRES (MNWS) . . . . . . . . . . . . . . . . . . . . . . 2

III. CHALLENGES IN ELECTROSPINNING OF

MNWS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

A. Precursor optimization . . . . . . . . . . . . . . . . . . 5

B. Jet instabilities . . . . . . . . . . . . . . . . . . . . . . . . . 6

C. Length . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

D. Alignment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

E. Reduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

F. Control of microstructure . . . . . . . . . . . . . . . . 7

IV. PROPERTIES AND APPLICATIONS . . . . . . . . . 8

A. Compositional and morphological analysis . 8

B. Conductivity . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

C. Magnetic properties . . . . . . . . . . . . . . . . . . . . . 11

D. Optical properties. . . . . . . . . . . . . . . . . . . . . . . 11

E. Mechanical properties . . . . . . . . . . . . . . . . . . . 11

F. Sensing characteristics . . . . . . . . . . . . . . . . . . . 13

G. Catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

V. OUTLOOK . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

I. INTRODUCTION

A. Scope of the review

Nanotechnology, the technology which has the highest

potential to shape the future of mankind,1 involves manipu-

lating the matter at the nano (or even atomic) scale. Whereas

nanotechnology has promised to address almost all the scien-

tific domains, the field of materials science and engineering

was found to be one among the most highly influenced by

this rapidly emerging technology. The term “nanomaterials”

evolved as a consequence of the marriage between nanotech-

nology and materials science. Among various classes of 1D

nanomaterials, the “nanofiber” is one where the fiber diame-

ter is 100 nm or less. Different types of materials like poly-

mers, ceramics, as well as metals have been successfully

synthesized in the form of nanofibers using different fabrica-

tion techniques. The term “nanowire (NW)” is commonly

used in case of metals. Due to their 1D-anisotropic structure,

size dependent properties and great potential to act as key

performers in future, microelectromechanical systems

(MEMS) and nanoelectromechanical systems (NEMS), me-

tallic NWs have gained tremendous attention during last dec-

ade. However, very limited real time applications have been

proposed so far due to difficulties encountered in their

reliable characterization and integration with other materials

and systems. Moreover, the cost associated with the current

sophisticated methods for synthesizing metal NWs is a key

hurdle in their commercial use. These include electrochemi-

cal approaches,2–8 nanoimprint lithography,9–14 synthetic

chemical routes,15–21 template assisted techniques,22,23

a)Author to whom correspondence should be addressed. Electronic mail:

rhashaikeh@masdar.ac.ae

0021-8979/2013/114(17)/171301/16/$30.00 VC 2013 AIP Publishing LLC114, 171301-1

JOURNAL OF APPLIED PHYSICS 114, 171301 (2013)

chemical vapor deposition,24,25 and electric arc discharge26

methods. Other sophisticated and unconventional methods

for producing metallic NWs have also been proposed in

several studies. Adelung et al.27–29 have demonstrated the

deposition of Rb NW networks on different semiconducting

substrates using the concept of 1D self-assembly of metal

atoms along the narrow strain zones created across the sur-

face. Similarly, electron beam irradiation under ultra-high

vacuum conditions to produce extremely thin Au NWs (four

atomic rows thick) has also been demonstrated by Kondo

and Takayanagi.30

It can be seen that there are variety of routes which can

be followed for synthesizing metallic NWs keeping in mind

that each method has its own benefits and limitations. The

complexities and cost involved in these methods, however,

make them unsuitable from the view point of economy,

efficiency, and mass production. That is why the large scale

production of metal NWs for commercial usage has not been

made possible so far. Recently, electrospinning, which is a

low cost and scalable technique31 for producing nanofibers,

has been employed for synthesizing metallic NWs. This pro-

cess is not only relatively low cost but also provides a better

control over various process parameters which directly affect

the physical and morphological properties of the nanofiber.

So far, we found nine experimental studies32–40 which are

related to the synthesis of metallic NWs via electrospinning.

The reported results are very encouraging in terms of the

NW quality and properties. However, there are different

aspects which need to be addressed and several challenges

which need to be overcome in electrospinning of metal

NWs and this is the subject of current article. The scope of

this article is to address these issues comprehensively and

highlight various potential applications of these low cost

electrospun metallic NWs in near future.

B. Article organization

The article is composed of three major sections: In

Sec. II, we describe how the electrospinning process is

employed to obtain metal NWs; in Sec. III, we addresses

several issues and problems associated with fabrication of

electrospun metal NWs; and in Sec. IV, we discuss their

reported properties and possible future applications.

II. ELECTROSPINNING OF METALLIC NANOWIRES(MNWs)

Electrospinning is a process of fabricating very thin

fibers through an electrically charged jet of a liquid precur-

sor. The concept was demonstrated by Formhals41 to produce

extremely thin polymeric fibers via electrically charged

liquid. A similar approach was shown earlier by Morton42

for separating the volatile liquid from a solution under the

influence of an electric field resulting in the solidification

of the less volatile phase in the form of filaments. In electro-

spinning, the precursor is usually a solution of different

chemicals, in a mixture of solvent and non-solvent, depend-

ing upon the desired fiber material. This process is not only

relatively cost-effective but also provides a better control

over various process parameters, viz., solution viscosity,

flow rate, electric potential, etc., which directly affect the

properties of the nanofiber. Figure 1 shows the schematics of

the electrospinning process. The process involves the appli-

cation of strong electric field to generate electrically charged

jet from the solution through a tiny nozzle. When the electri-

cal potential overcomes the surface tension of the solution

droplet coming out of the nozzle, the jet emerges from the

droplet end and continues to thin as it approaches the collec-

tor. The solvents evaporate from the jet to form fibers before

approaching the grounded collector. The collector can have

any suitable geometry. During electrospinning, the fibers

are subjected to a complex system of forces comprising ten-

sile, gravitational, inertial, and aerodynamic components.

Moreover, depending upon the liquid precursor, the solution

viscosity effect could be significant. However, the primary

force responsible for electrospinning is the tensile force act-

ing in the axial direction with respect to the fiber flow.43

This tensile component originates due to the induced charges

in the presence of strong electric field. In general, the solu-

tion droplet at the tip of the nozzle is conical in shape, and

the jet is ejected from the vertex of the cone once the applied

electric field crosses the critical value to overcome the sur-

face tension of the solution. This conical region, with a half

angle of 49.3�, was later referred to as the “Taylor Cone” as

it was discovered by Taylor.44 The magnified view of the

Taylor cone is shown in Fig. 2. However, other stable droplet

shapes with different half angles determined by the equilib-

rium of electric and surface tension forces are also possible

for various Newtonian and viscoelastic fluids.45 If the viscos-

ity of the solution is very high, it travels to the collector as a

single fiber jet. However, under most practical circumstan-

ces, the jet usually undergoes instabilities causing further

stretching and elongation of fiber resulting in much thinner

fibers. In general, for sufficiently low viscosity solutions,

the solution breaks up into droplets46,47 rather than fibers

and hence electrospinning becomes impossible. Due to the

viscoelastic nature of a polymer solution, the jet maintains

continuity and resists the droplet formation during electro-

spinning. It can be understood that there are several parame-

ters which will strongly influence the structure and the

properties of the electrospun fibers. These include the

applied electric field (or voltage), nozzle-collector distance,

FIG. 1. Schematics of electrospinning.

171301-2 Khalil et al. J. Appl. Phys. 114, 171301 (2013)

and the solution flow rate. In addition, the spinning environ-

ment and the solution properties could have significant

impact on the electrospun fibers formation, alignment, and

diameter. The electrospinning process has been successfully

employed to synthesize various polymeric, ceramic, and

metallic nanofibers.

One of the earliest successes in electrospinning polymer

fibers was reported by Baumgarten48 who was able to elec-

trospun acrylic fibers having diameter in the sub-micron

range. Although the fiber diameter was in 500 to 1100 nm

range, which is too large to fall within the nanometer range.

Baumgarten concluded many important aspects which served

as a foundation for future researchers. One important experi-

mental finding was the relationship between jet diameter and

applied field. It was also found that the proper spinning envi-

ronment is necessary. The relative humidity should be less

than 60% so that the fibers can dry sufficiently before reach-

ing the collector. Comprehensive studies were carried out

later by Larrondo and Manley.49,50 They electrospun poly-

ethylene and polypropylene fibers and observed that the fiber

diameter decreases with increasing melt temperature and the

applied voltage. Fiber diameter observed to be decreased

with increasing melt temperature and the applied voltage.

Hayati et al.51 showed that optimum solution conductivity is

required for formation of smooth and continuous jets during

electrospinning. The high conductivity of solution results in

the formation of unstable jets resulting in nonuniform fibers.

On the other hand, the solution with lower conductivity,

such as those having paraffinic oil as solvent, forms rela-

tively stable jets and consequently formed uniform fibers. A

major breakthrough in the electrospinning was achieved by

Doshi and Reneker52 who were able to produce electrospun

polyethylene oxide nanofibers with a smallest diameter of

50 nm. They found the nozzle-collector distance as a key

parameter affecting the jet diameter. Inverse relation was

observed between the nozzle-collector distance and the jet

diameter. Another important finding was that an optimum so-

lution viscosity is required to form a stable jet. Porous fibers

can also be formed while electrospinning by controlling the

evaporation rate of the solvent which depends upon the flow

rate,53 polymer’s molecular weight, and the relative humidity

of the environment in which the electrospinning is carried

out.54 A number of reports have been published afterwards

revealing successful electrospinning of different polymeric

nanofibers. Interested readers may follow the Refs. 43 and

55–57 where detailed reviews were carried out on the elec-

trospinning of wide variety of polymeric nanofibers. The

potential applications of electrospun polymeric nanofibers

have been found to be in the areas of optical fiber technol-

ogy,58 advanced textiles,46,59 membrane filtration technol-

ogy,60,61 and various bio-medical applications such as drug

delivery62 and scaffolds for tissue engineering.63

Because of better flow ability, viscoelasticity, and

solubility in different chemicals, the polymers were the first

to be electrospun. However, the electrospinning technique

has been successfully employed to synthesize various

ceramic and metallic nanofibers. Larsen et al.64 have demon-

strated successfully the electrospinning of titania, silica, and

alumina nanofibers using viscous inorganic sols. Others65–67

have also demonstrated the synthesis of PZT and silica nano-

fibers using same approach. In order to improve the viscoe-

lasticity required for electrospinning, a solution consisting

of a polymer as a host for carrying ceramic precursor has

been employed by Li and Xia.68 The polymeric phase is

selectively removed via calcination at high temperature in

air leaving behind the titania nanofibers. This method turned

out to be much better in terms of control of final fiber diame-

ter which can be controlled from tens to hundreds of nano-

meters. The fiber diameter was found to increase with the

increasing polymer/ceramic concentration in the solution.

Direct relation was also observed between the final fiber

diameter and the solution flow rate, whereas the opposite

trend was observed for applied field. Nanofibers of several

ceramics have been synthesized following the same

approach.69–74 Moreover, nanofibers of a non-oxide ceramic,

such as silicon carbide, have been synthesized using the

same technique.64 Several potential applications of ceramic

nanofibers have been proposed. These include structural

reinforcement, sensors/actuators, membranes, catalysis, and

electrodes in energy management devices.43 Interested read-

ers may follow the Ref. 75 where the processing, characteri-

zation and applications of electrospun ceramic nanofibers

have been discussed comprehensively.

As mentioned previously, limited attempts have been

made to utilize this method for producing pristine metallic

NWs. The reason behind this may be the difficulty associated

with the formation of suitable precursor and obtaining rea-

sonable microstructure of the NW. However, the primary

benefits offered by electrospinning are its simplicity, eco-

nomic feasibility, and mass production capability,31,35 which

justifies an extensive research for obtaining high quality

metallic NWs through this process. In all the reported work

related to the electrospun metallic NWs, similar approach

has been followed which comprise of three steps: (1) A metal

salt is selected as precursor, e.g., copper nitrate, copper ace-

tate, etc. The appropriate amount of salt is mixed with a suit-

able polymer such as polyvinyl acetate (PVA) in an aqueous

or other solvent. The purpose of polymer is to provide the

structural support to hold the metal ions during electrospin-

ning and to provide a viscoelastic behavior to the solution

FIG. 2. Image showing formation of Taylor cone. Reproduced by permis-

sion from Han et al., Polymer 49, 1651 (2008). Copyright 2008 by Elsevier

Ltd.

171301-3 Khalil et al. J. Appl. Phys. 114, 171301 (2013)

which is mandatory for electrospinning. (2) The electrospun

composite fibers are then calcined at high temperature under

suitable inert environment to selectively remove the polymer

component. This results in the obvious decrease in fiber

diameter. (3) The calcined fibers are usually metallic oxide

and further reduction to metallic nanofibers is needed. The

reduction of metal oxide nanofibers was achieved using a

reducing agent such as hydrogen gas at 300 �C. The overall

process is schematically illustrated in Figure 3.The third step

was found to be inapplicable in some cases because the cal-

cination step turned out to be enough for oxygen removal.

This aspect will be discussed in detail in later sections. The

first successful attempt to synthesize metallic NW via elec-

trospinning was made by Bognitzki et al.33 Copper NWs

with average diameter of 270 nm were fabricated using the

above mentioned procedure. Wu et al.34,36 synthesized Cu,

Fe, Ni, and Co NWs having diameters of 40 to 100 nm range.

Graeser et al.32 synthesized Fe and Co electrospun NWs of

100 to 200 nm fiber diameters. Barakat et al.35 also synthe-

sized nearly 200 nm Ni NWs. Recently, Hansen et al.37 have

carried out a much comprehensive work on the fabrication of

Cu, Fe, Ni, and Co NWs having diameters 100 to 200 nm.

Kim et al.38,39 have obtained very thin Pt nanowires, 30 nm

diameter, via electrospinning. Barakat et al.35 also synthe-

sized about 200 nm diameter Ni NWs. In another study, Kim

et al.40 obtained Pt nanowires in the diameter range of

100–150 nm via electrospinning. Recently, Hansen et al.37

have carried out a much comprehensive work on the fabrica-

tion of Cu, Fe, Ni, and Co NWs having diameters 100 to

200 nm.

Table I summarizes the important aspects of the work

done so far related to electrospinning of metallic NWs. It can

be seen that the work done so far is encouraging in terms of

the results. However, there are plenty of aspects which need

to be addressed regarding the electrospinning, characteriza-

tion, as well as the applications of electrospun metallic NWs.

As already mentioned, electrospinning has plenty of

controllable parameters which directly affect the properties

of the produced NWs. However, all the studies related to

electrospun metallic NWs, used a single set of parameters

(applied voltage, solution flow rate, collector distance, envi-

ronment), i.e., the optimization of these parameters for

obtaining the metallic NWs with finest microstructure and

highly tuned properties is still an open question. The most

important parameter of interest is the final diameter of the

obtained metallic NW. Theoretical models have been formu-

lated to correlate the final fiber diameter with the processing

parameters during electrospinning.76 The major factors influ-

encing the final fiber diameter were found to be solution flow

rate, current carried by the solution, and the surface tension

of the solution being electrospun. Sigmund et al.31 have

modified this theory for the electrospinning of ceramic NWs

and the experimental findings were found to be in good

agreement with the modified model. Since the procedure for

electrospinning ceramic and metallic NWs is similar, the

modified model of Sigmund et al.31 is expected to be equally

applicable for metal NWs. It is however important to realize

that the complexities posed by the variety of competing

factors during electrospinning makes it nearly impossible to

develop a versatile model which can predict the final fiber

diameter for variety of precursors. For example, the fiber

diameter is supposed to decrease with increasing applied

voltage (or current) due to greater stretching of jet. However,

Deitzel et al.46 showed that high voltage causes an alteration

in the Taylor cone configuration and the produced nanofibers

are composed of high density of beads causing thicker and

nonuniform nanofiber geometry. The studies realized that

electrospinning is not a single stage process. It is rather a

combination of multiple stages involving formation of

Taylor cone, jet emission, and then the whipping region. In

case where the precursor is composed of different compo-

nents or solutes having different electrical properties (for

example, the polymer molecules and salt ions), the localized

attraction/repulsion due to inhomogeneous charge distribu-

tion can cause localized instabilities which can further com-

plicate the electrospinning process. All these aspects make it

very difficult to develop a single versatile model which can

be applicable irrespective of the solution properties. In case

of metallic NWs, the final wire diameter is the one which is

obtained after the removal of polymer as well reduction

of metal oxide NWs. Therefore, besides the processing

variables, the final diameter of a metallic NW is largely

influenced by the solution chemistry which defines the com-

positional properties of electrospun composite wires contain-

ing polymeric as well as metallic phase. This aspect is

evident from the literature32–37 that in spite of using almost

similar electrospinning parameters, the final metal NW diam-

eter was found to be very different in these studies. This is

most likely due to vast difference in the precursor chemistry

and composition. Hence, it is logical to carry out the system-

atic study which can give an indication that how the diameter

of electrospun metal NW is affected by solution composi-

tion. In a similar fashion, the morphology and crystalline

structure of electrospun metal NWs can be correlated to solu-

tion properties and processing variables. For example, highly

imperfect crystalline structure and surface of the NWs have

been reported to be the major reasons for their lower magnet-

ization and conductivity.33–35,37 Therefore, it is important to

develop the NWs with fine and highly oriented crystalline

structure to overcome these issues. This will be dependent

upon not only the electrospinning parameters but also

the precursor composition and the post heat treatment ofFIG. 3. Steps involved in obtaining metal NWs via electrospinning.

171301-4 Khalil et al. J. Appl. Phys. 114, 171301 (2013)

composite wires which is done for selective removal of poly-

mer and oxygen.

III. CHALLENGES IN ELECTROSPINNING OF MNWS

Although electrospinning process is quite simple in terms

of its setup and theory, yet there are several challenges which

need to be overcome for obtaining smooth and continuous

metal NWs. These are discussed separately in Secs. III A–III F.

A. Precursor optimization

As mentioned previously, the precursor which is used

for producing electrospun metallic NWs primarily composed

of three components: A soluble metal salt, a solvent, and a

soluble polymer. The salt acts as a source of metal ions

which later integrate up to yield metal NW and the sole pur-

pose of a polymer is to provide viscosity to the solution as

well as fiber-like connectivity of the electrospun salt. Hence

the interaction of salt and polymer inside the solution is not

chemical. However, depending upon the type of polymer and

salt used, there is a possibility of polymer hydrolysis by the

salt ions. For example, if PVA is used as a polymer and

copper acetate as salt, the PVA will get hydrolyzed due to

the action of acetate ions. To avoid this, acetic acid is usually

added in certain proportion to maximize the solution stabil-

ity, as in case of copper acetate-PVA combination for pro-

ducing copper NWs.37 This causes an increase in the acetate

ion concentration shifting the equilibrium in backward direc-

tion. Acetic acid has also been found very beneficial for

precursor stability in several other electrospinning attempts

for producing niobium oxide,77 titania,78 and barium tita-

nate79 nanofibers.

The interaction of salt and polymer in the precursor can

depend upon the polymer-salt combination used. For

example, in case of copper acetate-PVA combination, the

“chelation” of PVA chain is most likely to be caused by cop-

per acetate via Cu atoms.80,81 The Cu atoms are held across

the adjacent PVA chains via oxygen atoms through dative

bond. This will also increase the cross-linking across the

PVA chains shown below.80

As the polymer is selectively removed, the Cu atoms

combine to yield pure Cu NWs. This chelating interaction is

the most probable type of interaction which could happen in

the precursor used for obtaining metallic NWs.

The type of salt selected for obtaining metal NW seems

to largely influence the final product as evident from litera-

ture. If metal nitrate is selected, the obtained product after

polymer burning is the metal oxide.33,34,36 However, if metal

acetate is used as a precursor salt, the likelihood of oxidation

reduces and the final product is pure metal NW without any

further need of reduction.35,37 Similarly, it is also interesting

to note that using PVA as a polymer also results in pure

metal NWs after polymer burning.35,37 Whereas the use of

other polymers such as polyvinyl butyral (PVB)32,33 and

PVA34,36 leads to the production of metal oxide NWs. This

gives an indication that selecting a proper polymer-salt com-

bination and optimizing the precursor chemistry can have

significant effect on the product composition which is

obtained after the polymer removal step. Moreover, the role

of stabilizers, such as acetic acid, as described before, and

organic solvents such as Isopropanol,33 Isopropyl alcohol,32

and ethanol,79 which are added to improve the volatility of

liquid, can also be significant in this regard but all these

aspects can be clarified only after carrying out exhaustive

experimentation to develop a relationship between precursor

chemistry and the final product. The available literature does

not address these issues and therefore this is an important

direction for research.

TABLE I. Summary of the literature related to electrospinning of metallic NWs.

Solution composition

Reference Metal Precursor salt

Polymer

matrix Solvent

Diameter of

NW (nm) Comments

Graeser et al.32 Fe, Co Cobalt dinitrate, iron

trinitrate

PVB Water-isopropyl

alcohol

200 NWs showed higher coercivity with high

anisotropy

Bognitzki et al.33 Cu Cu nitrate PVB Water and isopropanol 270 NWs showed high electrical conductivity, how-

ever, much lower than bulk Cu wires which was

attributed to irregular surface of NW

Wu et al.34 Fe, Co, Ni Fe, Co, Ni nitrate PVA Water 40–54 NWs showed lower magnetization but almost

double coercivity as compared with bulk wires

due to their anisotropic and single domain nature

Kim et al.38,39 Pt Hexachloroplatinic

acid hydrate

PVP Water and ethanol 30 Nanowires showed higher catalytic activity than

nanoparticles

Barakat et al.35 Ni Nickel(II) acetate tet-

rahydrate

PVA Water 200 NWs showed lower magnetization but higher

coercivity

Wu et al.36 Cu Cu acetate PVA Water 100 NWs showed high photo-current and high flex-

ibility due to very high aspect ratio

Kim et al.40 Pt Hexachloroplatinic

acid hydrate

PVP Water and ethanol 100–150 Nanowires showed higher catalytic activity than

nanoparticles

Hansen et al.37 Cu, Ni, Fe, Co Cu, Ni, Fe, Co acetate PVA Water-acetic acid 100–200 NWs showed high coercivity which was found to

decrease with increasing temperature and de-

creasing crystal size

171301-5 Khalil et al. J. Appl. Phys. 114, 171301 (2013)

Whereas the type of chemicals used in the precursor

defines the chemical composition of final product; the rela-

tive proportion of chemicals governs the morphology of final

product. Using higher proportion of polymer will lead to

increased solution viscosity which will ultimately cause the

formation of much thinner fibers, whereas lower polymer

concentration will decrease the solution viscosity and more

beaded fibers will be produced46 leading to more defective

and discontinuous metal NWs. Although the viscosity is also

enhanced by the addition of salt, the contribution of polymer

is much more pronounced. Hansen et al.37 used very high

salt loading (far above the maximum solubility limit) and

very high metal salt to polymer mass ratio for the precursor.

They proposed that such approach could be advantageous for

obtaining much longer and continuous NWs. However, in

such case, since the significant proportion of salt particles is

suspended instead of dissolved, the inhomogeneity in solu-

tion will increase the instabilities and also the final micro-

structure of the metal NWs will become very defective due

to nonuniform distribution of metal atoms. This was evident

from the TEM micrographs shown by Hansen et al., where

the NW structure was extremely defective and the various

crystalline domains were poorly connected. Thus, an ideal

precursor would be one which is saturated with salt without

suspended particles and having such polymer concentration

which can impart good electrospinability to the precursor. It

is worth mentioning that polymer less electrospinning has

been demonstrated by Choi et al.82 for obtaining silica nano-

fibers. They showed that the viscosity of a sol-gel comprising

a salt and solvents only can be brought to an electrospunable

level through its aging. This approach eliminates the poly-

mer burning step and is thus expected to improve the mor-

phology of metal NWs. However, there would be another

chemical step needed to convert the sol-gel fibers into the

pure metal NWs. This polymer less electrospinning for

obtaining metal NWs could be an interesting direction of

research.

B. Jet instabilities

As the jet is ejected from the cone, it continues to thin

down as it moves towards the collector. However, as the jet

narrows down, the radial charge repulsion may become dom-

inant due to the variation in the charge density.83 This results

in a chaotic motion of the jet about the jet axis resulting in

the instability which is usually referred to as “bending” or

“whipping” instability. Thanks to this instability as the major

fiber extension and reduction in the diameter occurs in this

instability region. The radial charge repulsion may also

cause the splitting of primary jet into much thinner jets (a

phenomenon called as “splaying”) resulting in the formation

of much thinner fibers.84 However, the other two types of

axisymmetric instabilities, i.e., the classical “Rayleigh”

instability and the “electric field induced” instability have

also been observed during the electrospinning process. These

three types of instabilities are shown in Fig. 4. Shin et al.85

have shown that the relative contribution of each of the three

instabilities depends upon the fluid properties and the operat-

ing parameters. The electric field induced axisymmetric

instability increases with decreasing charge density of the

solution which causes the modulation in surface charge den-

sity of the jet. As a result, undesirable periodic “beads” are

formed along the fiber axis causing defects and discontinu-

ities in fiber structure. The Rayleigh axisymmetric instabil-

ity, governed by the surface tension of the liquid, is the most

general type of instability encountered by the falling stream

of fluids even in the absence of the electric field. This insta-

bility results in the splitting of liquid stream into tiny drop-

lets and thus could play major role in bead formation. In

electrospinning, the surface tension of the fluid is usually

low enough to avoid this Rayleigh instability. However,

the electric field induced instability is strongly the function

of solution charge density. In case of metallic NWs, this

instability is expected to be less due to an increase in solu-

tion charge density. Since the precursor has significant pro-

portion of the salt, the ions will increase the charge density

and hence less axisymmetric instability is expected to be

observed. This increased charge density is also expected to

cause greater whipping instability and hence more ultrafine

fibers are expected.

C. Length

Electrospinning is capable of producing extremely long

fibers having length of several kilometers. However, such

length scales have been reported in case of polymers only

due to high homogeneity of the precursor having only single

polymeric phase. In case of metallic NWs, such length

scales are not expected due to the presence of different

components in the precursor which will cause discontinuity

and instabilities during the electrospinning process.

Moreover, the polymer burning step is also expected to

cause some microstructural defects and length discontinu-

ities in the metallic NWs. This aspect has not been explic-

itly highlighted in the available literature on electrospun

metallic NWs. However, the obtained lengths so far for the

reported metallic NWs were found to be ranging from 100

lm (Ref. 34) to couple of centimeters.36 The desired length

of the NW depends upon the target application, and means

to precisely control the NW length during electrospinning is

equally important.

FIG. 4. Different types of jet instabilities during electrospinning. (a)

Bending or whipping. Reprinted with permission from Yarin et al., J. Appl.

Phys. 89, 3018 (2001). Copyright 2001 American Institute of Physics. (b)

Rayleigh. Raleigh instability image. Reproduced by permission from Y. J.

Kim, Ph.D. thesis, University of Washington, 2011. Copyright University of

Washington Libraries 2009–2013. (c) Axisymmetric. Reproduced by per-

mission from C. P. Carroll and Y. L. Joo, J. Non-Newtonian Fluid Mech.

153, 130 (2008). Copyright 2008 by Elsevier BV.

171301-6 Khalil et al. J. Appl. Phys. 114, 171301 (2013)

D. Alignment

Not only from the application aspect but also for conven-

ient and reliable characterization, it is desirable to have the

electrospun NWs in a highly oriented form. Unfortunately,

because of the instabilities, the NWs are collected in random

orientations in the form of meshes and nonwoven mats. To

overcome this difficulty, several approaches have been used.

Some authors have proposed a collector in the form of rotat-

ing drum88 and rotating wheel.89 These methods somehow

turned out to be better for improving fiber alignment over the

larger length scale. However, the alignment was still not up

to the level so that these NWs could be studied and applied in

an isolated manner. A much efficient way was proposed by

Li et al.90 They used a split electrode with a gap of few milli-

meters to centimeters. Such configuration results in the col-

lection of uniaxially aligned fibers, however, over a smaller

length scales (few mm to a cm). This makes the handling and

characterization of nanofibers very easy. Moreover, the

aligned arrays can be easily and accurately transferred to

various substrates for fabricating different devices such as

electrodes. Similarly, these aligned metallic NWs can be

sandwiched between two electron beams deposited metallic

nanolayers to form nano laminates. However, in case of me-

tallic NWs, it is important to ensure that the lateral welding

of these aligned NWs does not occur during the polymer

burning and reduction steps. Fig. 5 shows the different collec-

tor geometries which have been discussed.

E. Reduction

The composite electrospun fibers comprising metallic

salt and polymeric phase, obtained after evaporation of vola-

tile liquid, are heated under high temperature to selectively

remove the polymeric phase. Most of the studies33,34,36 have

shown that the metallic NWs end up in the form of their

oxide after the polymer burning step. As a result, these

oxides need to be reduced through a suitable way to be con-

verted into metallic form. The method used so far involves

reducing the oxide NWs in a hydrogen atmosphere for suita-

ble duration under elevated temperatures. However, some

studies have shown that metallic phase NWs are obtained

directly after the polymer burning step.35,37 It is interesting

to note that these studies used the metal acetate salt, whereas

those required reduction as a necessary step used metal

nitrate salts. Wu et al.36 also used copper acetate salt but

they reported reduction as a necessary step for converting

copper oxide NWs to pure copper NWs. However, the reduc-

tion time and temperature was much lower than other

studies.

Although hydrogen is the most common and active gas

for the reduction of metal oxide nanostructures91 and thin

films,92 there are several other possibilities besides conven-

tional reduction in hydrogen atmosphere which does not

only consumes more time and energy but also unsafe from

environmental perspective. Using suitable reducing agents,

such as “hydrazine,” the reduction of metal oxides can be

done much quickly and in a much cheaper way. It has been

shown that hydrazine acts as an effective reducing agent for

converting metal oxides to pure metals.93,94 Similarly, other

reducing agents such as alcohols, carboxylic acid, and alde-

hydes have been reported as effective reducing agents for

metal oxide films.95 In addition to these purely chemical

techniques, electrochemical methods96,97 have also been pro-

posed to be very effective for converting metal oxides to

pure metals. We believe that these reduction techniques can

also be applied for reducing metallic oxide NWs. Since these

techniques are much faster and less energy consuming, they

can be more practical when it comes to commercial produc-

tion of metallic NWs.

F. Control of microstructure

The microstructure of electrospun metallic NWs has

been found quite defective as revealed from TEM images.

The defects and irregularities across the electrospun NWs

are produced primarily due to two reasons. First, as the jet

undergoes instability, the composite fiber is stretched in ran-

dom directions due to simultaneous action of various force

components. This causes the inherent microscopic defects

inside the nanofiber and consequently in the final metallic

NW. Second, and more importantly, it is the selective

FIG. 5. Different types of collector geo-

metries: (a) Rotating drum. Reproduced

by permission from Kenawy et al., J.

Controlled Release 81, 57 (2002).

Copyright 2002 by Elsevier BV. (b)

Rotating wheel. Reprinted with permis-

sion from J. Kameoka and H. G.

Craighead, Appl. Phys. Lett. 83, 371

(2003). Copyright 2003 American

Institute of Physics. (c) Split electrode.

Reproduced by permission from Nano

Lett. 3, 1167 (2003). Copyright 2003 by

American Chemical Society.

171301-7 Khalil et al. J. Appl. Phys. 114, 171301 (2013)

removal of polymer which results in unevenness across the

NW, especially at the NW exterior. No matter how uniform

the solution is, there will always be a slight heterogeneity in

polymer-metal distribution at the microscopic level. Fig. 6

compares the morphology of solution processed and electro-

spun metallic NWs. Whereas, solution processed NWs are

very homogenous and uniform throughout their length, the

electrospun NWs are inhomogeneous, defective and nonuni-

form in diameter across their length. This nonuniform and

defective morphology is one of the current main drawbacks

of electrospun metallic NWs. By subjecting these metal

NWs to controlled annealing treatments, it is expected to get

improvement in microstructures and hence various physical

properties of metallic NWs.

As far as the diameter of Electrospun metal NW is con-

cerned, there is significant amount of variance in the reported

literature. The minimum average diameter was reported as

40 to 50 nm by Wu et al.,34 whereas diameters as large as

270 nm were reported by Bognitzki et al.33 The value of 100

to 200 nm was observed by others.35–37,98 The large variance

is obvious due to the different solution compositions and

electrospinning parameters used in these studies. It is inter-

esting to note, this key benefit of electrospinning, that by

varying the solution properties and electrospinning

parameters, metal NWs of desired diameter can be obtained

which may be suitable for particular application. The diame-

ters reported by Wu et al.,34 i.e., 40 to 50 nm are comparable

with those reported by the authors who used more sophisti-

cated techniques such as soft solution processing18 and nano-

imprint lithography.14 This shows that electrospinning has

equal potential for producing ultra-fine metallic NWs.

IV. PROPERTIES AND APPLICATIONS

A. Compositional and morphological analysis

The compositional and morphological characteristics of

electrospun metal NWs have been determined using the con-

ventional approaches such as SEM, TEM, XRD, and energy

dispersive spectroscopy (EDS). However, it is important to

realize the limitations of some conventional techniques while

using them for evaluating nanostructures. For example, the

SEM can give rough estimate about the true diameter of the

NW due to limitation posed by maximum resolution, and

very little information can be obtained about the morphology

and crystalline structure even for very well prepared sam-

ples. The use of TEM is thus inevitable if one needs reliable

information about NW diameter and structure. Even the

images of TEM have to be carefully analyzed due to arti-

facts produced as the wire diameter approaches 10 nm or

below.99 Atomic Force Microscope (AFM) has also turned

out to be useful in estimating the NW diameter. The profile

generated by AFM tip while scanning the surface in contact

mode can give good indication about the average diameter

of NWs.36

The compositional analysis of electrospun metal NWs

has been carried out using XRD and EDS.35,36 The most

important aspect of interest is to determine whether the

obtained NWs after the polymer removal step are pure metal

or metal oxide. It can be understood that due to limitations

posed by resolution of these techniques, there could be sig-

nificant level of uncertainty in the results. For example, if

one is using SEM based EDS for point analysis over the

length of NW, the results could be unreliable due to inaccur-

acy in pin-pointing the desired location and consequently

the error caused by substrate elements. This error can be

minimized by depositing a very dense mesh of NWs on the

substrate which can be easily done by simply increasing the

electrospinning duration. Then the area analysis rather than

point encompassing larger amount of NWs and operating at

lower accelerating voltage will maximize the probability that

the collected signals represent the NWs. Using lower accel-

erating voltage will limit the electron penetration to NWs,

and the substrate effects will be minimized.

Similarly, XRD will be more informative about phase

determination in NWs if the substrate used for collecting

NWs is amorphous in nature, e.g., glass, so that the XRD sig-

nals are not overlapped by the crystalline substrate. Taking

care of these aspects, both EDS and XRD35,36 were found to

be useful for compositional analysis of electrospun metal

NWs. A modified form of XRD which is called as “grazing

incidence XRD (GIXRD)” (also called “glancing incidence

XRD”) is a more practical choice if one wants to carry out

the phase analysis of NWs lying randomly over the substrate.

This technique, which was originally developed to analyze

very thin films deposited over a substrate,100 relies on the

principle of total external reflection.101 Since the refractive

index for most of the materials is less than 1 in case of

X-Rays, one can achieve the total external reflection by keep-

ing the incident angle to a very low value. This limits the

X-ray penetration in the specimen to only fraction of nano-

meters rather than few microns and hence the contribution of

FIG. 6. Morphology of solution processed Ag [Reproduced by permission

from Sun et al., Nano Lett. 2, 165 (2002). Copyright 2002 by American

Chemical Society] and electrospun Fe. Reproduced by permission from Wu

et al., Chem. Mater. 19, 3506 (2007). Copyright 2007 by American

Chemical Society and Reproduced by permission from Hansen et al., Small

8, 1510 (2012). Copyright 2012 by Wiley-VCH Verlag GmbH & Co KGaA.

And metallic NWs as observed through TEM.

171301-8 Khalil et al. J. Appl. Phys. 114, 171301 (2013)

substrate becomes negligible in the diffracted signals. Further

optimization of slit configuration before the detector allows

only the parallel beam of X-rays to reach the detector and

hence highly tuned signals can be obtained representing the

nanomaterial only lying over a substrate.102 Several studies

have shown the effectiveness and importance of GIXRD in

analyzing different NWs such as ZnO (Ref. 103) and Pd

(Ref. 104) lying over different substrates.

The distribution of metal ions (or salt molecules) across

the polymer molecules is an important aspect which does not

only affect the electrospinning process itself but also the final

morphology of the metal NW. In this regard, TEM could be

a good tool in analyzing the morphology of electrospun com-

posite nanofibers comprising both the metallic and polymeric

phase. For example, Fig. 7(a) shows the TEM image of the

electrospun titania/PVP (poly-vinyl pyrrolidone) composite

nanofiber and Fig. 7(b) shows the TEM image of the same

nanofiber after calcination.105 It can be seen that no discrete

domains are visible in the composite nanofibers which reflect

the high degree of homogeneity in the salt/polymer distribu-

tion. As a consequence, the calcined nanofibers, which are

pure titania, have very fine morphology and very high uni-

formity in diameter throughout the length. Similarly, Fig. 8

shows the TEM micrograph of electrospun PEO (poly-ethyl-

ene oxide) nanofibers containing 28 wt. % magnetite nano-

particles.106 The dark regions in this image represent the

domains where the magnetite concentration is high and vice

versa. However, such TEM imaging of electrospun compos-

ite nanofibers has not been presented in any of the studies

related to metallic NWs. Hansen et al.37 showed the TEM

images of electrospun metal salt/PVA composite nanofibers

calcined at 400 �C. As an example, the images for Cu and Fe

are shown in Figs. 9(a) and 9(b). The authors argued that

these fibers are pure metal NWs, however, their morphology

suggest that the unevenly distributed darker spots may be the

residues left after polymer burning because the temperature

required for complete degradation of PVA including all the

bi-products is around 500 �C (Ref. 107), whereas the calcina-

tion temperature selected by Hansen et al. was only 400 �C.

When the same composite fibers were calcined at 800 �C, the

morphology of obtained NWs was found to be highly defec-

tive as depicted by TEM images shown in Figs. 9(c) and

9(d). This may be attributed to uneven polymer/metal distri-

bution due to very high loading of salt content, as discussed

previously. One possible reason for the lack of TEM analysis

of composite nanofibers could be the irradiation sensitivity

of polymers. The polymer at the nanoscale will quickly de-

grade upon exposure to high energy electron beam in

TEM.108 A much better technique, therefore, to analyze the

polymer/metal distribution could be AFM operating in

dynamic mode.109 The phase contrast images obtained in

amplitude modulation AFM can give comprehensive infor-

mation about the different elements present in the medium at

the nanoscale without affecting the compositional or mor-

phological features of the medium. An example of such

approach was given by Aviles et al.67 where they identified

different domains within an electrospun PZT microfiber

through phase contrast images using scanning probe micro-

scope as shown in Fig. 10. Similarly, other composite elec-

trospun fibers can also be characterized in terms of

compositional homogeneity following same approach. Since

the nanofiber might be too thin for informative phase con-

trast images, an alternative approach could be the dynamic

AFM analysis of the precursor prepared for electrospinning.

The precursor may be applied over a glass substrate and after

drying, the phase images of the precursor can be obtained

FIG. 7. TEM images of electrospun PVP/titania nanofibers. (a) Before calci-

nation and (b) after calcination. Reproduced by permission from D. Li and

Y. Xia, Nano Lett. 3, 555 (2003). Copyright 2003 by American Chemical

Society.

FIG. 8. TEM micrograph of electrospun PEO nanofibers containing 28 wt.

% magnetite nanoparticles. Reproduced by permission from Wang et al.,Polymer 45, 5505 (2004). Copyright 2004 by Elsevier Ltd.

FIG. 9. TEM image of electrospun (a) Cu and (b) Fe NW after calcination at

400 �C. (c) and (d) are the images of wires shown in (a) and (b), respec-

tively, but after calcination at 800 �C. Scale bar is 200 nm. Reproduced by

permission from Hansen et al., Small 8, 1510 (2012). Copyright 2012 by

Wiley-VCH Verlag GmbH & Co. KGaA

171301-9 Khalil et al. J. Appl. Phys. 114, 171301 (2013)

via AFM giving information about compositional homogene-

ity which is vital for obtaining uniform and defect free elec-

trospun metallic NWs. However, it is important to realize

that AFM is more of a surface characterization technique

and limited information can be obtained about the interior of

the structure. Therefore, AFM could be useful as long as the

different phases are uniformly distributed across the nano-

fiber. If the salt/metal ions are mostly concentrated towards

the center of the nanofiber and are enclosed in a polymeric

shell, little information can be obtained about

the distribution quality. In such case, TEM will be more

effective.

B. Conductivity

Metals are well known for their high electrical and ther-

mal conductivity and the reason behind this behavior is well

understood. The electrical conductivity for metals is higher

because of high density of free electrons. As far as thermal

conductivity is concerned, it is a function of both free elec-

tron density and the contribution from lattice vibrations (also

called “phonons”). Since the metals have highly ordered

lattice structure, the contribution of phonons is also signifi-

cant and as consequence, metals display very high thermal

conductivity. However, at the nanoscale, the metals were

found to exhibit lower thermal and electrical conductivity.

The lower electrical conductivity of NWs has been explained

in terms of quantum dissipations110 and electron scattering

from the wire boundary. These effects become more pro-

nounced as the wire diameter approaches the mean free elec-

tron path of the bulk material. Also, the electron scattering

from the uneven wire boundaries, especially in case of elec-

trospun metal NWs,33 has been proposed as another reason

for this behavior. It should be noted that the role of atomic

defects is negligible when we talk about the electrical con-

ductivity of bulk metals. However, when the diameter of the

wire is in the nanoscale, few defects can play important role

in electron scattering as these defects could be significant

proportion of the total NW diameter.

Even with lower electrical conductivities, the future of

low cost MEMS and NEMS seems to be largely driven by

these metallic NWs. The expensive techniques, such as

lithography which are used to pattern the micron and nano

sized conducting paths, hinder the mass production and com-

mercial usage of MEMS and NEMS. With much cheaper

techniques like electrospinning, it will be possible to produce

the metallic NWs in mass quantity and make their wide-

spread usage in various MEMS and NEMS. However, there

are two main hindrances in using electrospun metallic NWs

in these miniaturized systems. The first aspect is the con-

trolled deposition of the NWs which is very difficult in con-

ventional electrospinning process. However, Sun et al.111

have demonstrated that it is possible to deposit the nanofibers

in a fairly controlled fashion using “near-field electro-

spinning” technique. This approach involves using very

small nozzle-collector distances (few millimeters) and appli-

cation of very low applied voltages to minimize the inherent

instabilities in the electrospinning process. The second diffi-

culty is the reliable measurement of their electrical proper-

ties before they can be applied. The isolation of single NW

from the electrospun nonwoven and then measuring its

conductivity is a very complicated process and requires

some custom designed setup. For instance, Bognitzki et al.33

utilized low-energy electron point source (LEEPS) micro-

scope for measuring electrical conductivity of single Cu NW

as shown in Fig. 11. They measured the conductivity to be

8500 S/cm. However, in the other studies conducted by Wu

and Hansen et al., a conventional two probe37 and four

probe36 method was employed to measure the electrical

characteristics of electrospun Cu NW sheet and due to uncer-

tainty in NW length, only the sheet resistance or conductivity

was reported which is a representative of combined effect

produced by various NWs in random directions with fused

junctions. At the same time, due to poor crystalline structure,

the reported values may not be the ideal ones for a NW. We

believe that a controlled and prolonged heat treatment of

electrospun metallic NWs can lead to their better crystalline

structure and hence enhanced conductivity.

Thermal conductivity of the NWs has also been found to

be less as compared to bulk materials due to similar reasons.

At nanoscale, the effects of “mean free path” and “mean free

time” available for phonons to transfer thermal energy

become significant as opposed to macroscopic scale where

these effects get averaged out and play negligible role. It has

been shown for silicon that despite fine crystalline structure,

the NWs have nearly half of the thermal conductivity as

compared to bulk silicon.112 The possible reasons proposed

FIG. 10. SPM phase contrast image of electrospun PZT microfibers.

Reproduced by permission from Santiago-Aviles et al., Appl. Phys. A:

Mater. Sci. Process. 78, 1043 (2004). Copyright 2004 by Springer-Verlag

FIG. 11. LEEPS setup for measuring electrical conductivity of a single NW.

(a) Schematic representation and (b) image taken during conductance mea-

surement. Reproduced by permission from Bognitzki et al., Adv. Mater. 18,

2384 (2006). Copyright 2006 by John Wiley and Sons

171301-10 Khalil et al. J. Appl. Phys. 114, 171301 (2013)

for this were the increased phonon scattering at the bounda-

ries and modification in phonon spectrum at the nanoscale.

The thermal conductivity was, however, found to increase

with increased temperature. Zhou et al.113 have obtained

similar results for the thermal conductivity of indium arse-

nide NWs and they explained their results in the similar

context. Using molecular dynamic simulations, Kosevich

and Savin114 have demonstrated how the phonon scattering

becomes dominant in case of NWs with rough surfaces and

edges.

Although their efficiency is less, the metallic NWs have

tremendous potential to act as heat conductors in future

MEMS and NEMS. The lower thermal conductivity of

NWs does not affect their potential application as nano heat

transfer elements because of the huge surface area offered

by them which is another key property for efficient heat

transfer. There has been an increasing interest in developing

micro and nano heat exchangers for enhancing heat transfer

in miniaturized electronic devices.115 The size reduction

of heat exchangers offered by metallic NWs will overcome

the disadvantage of their lower heat transfer efficiency.

Moreover, further refinement in their microstructure and

reduction in surface defects can further improve their effi-

ciency to the level where they can be applied in advanced

engineering systems with more reliability.

C. Magnetic properties

A very interesting property of electrospun metal NWs

has been found to be their extremely high coercivity as com-

pared to their bulk counterparts. This is the key property

required for high density data storage and therefore these

NWs are expected to have important applications in mass

data storage devices,32,34,35,37 Fig. 12. The high coercivity of

NWs has been explained in terms of their ID-single domain

nature. Although there are several other challenges regarding

the integration of these randomly oriented NWs in data

storage devices, the low cost of electrospinning and progress

in micro/nanofabrication technologies can lead to their

successful use in future mass data storage devices. The satu-

ration magnetization was, however, found to be very low for

these electrospun metal NWs as compared to their bulk

counterparts which was attributed to the formation of mag-

netically dead oxide layer on the NW surface due to their

high surface area.35

D. Optical properties

Due to higher and consistent specular and diffusive

transmittance as well as very high aspect ratios, the metallic

NWs are expected to serve as transparent flexible electrodes

in solar cells and future electronic devices.16,36 Fig. 13

depicts the performance of Cu NW networks as compared

to conventional indium tin oxide (ITO) films. It can be seen

that the transmittance of NWs is not only higher but also

consistent with wavelength as compared to ITO films.

Higher aspect ratio of the electrospun metallic NWs is one of

the key characteristic which imparts them matchless flexibil-

ity which cannot be achieved even with extremely thin sput-

tered metallic coatings. Even if the length of the NW is only

1 cm, the diameter of the order of 100 nm results in the

aspect ratio of the orders of 100 000 which is extremely

high. This allows the NWs to easily release the built-up

strain during repeated stretching without any cracking. Thus,

another key benefit of electrospinning is obtaining very long

metal NWs as compared to other conventional techniques

which yield NWs which are only few micrometers long.

E. Mechanical properties

Another important aspect is the mechanical properties of

these electrospun metallic NWs which have not been

addressed in any of the published studies so far. Of course, it

is again due to the difficulties associated with the mechanical

characterization at the nanoscale. However, an intelligent

use of AFM can give us some idea about the nanoscale

mechanical integrity of the metallic NWs. In this way, we

will be able to analyze their potential as mechanical rein-

forcements. Although it is possible to cut the tensile speci-

mens from the electrospun nonwovens, the tensile properties

will be the representative of the net effect produced by

FIG. 12. High coercivity of electrospun Ni NWs. Reproduced with permis-

sion from Barakat et al., J. Phys. Chem. C 113, 531 (2009). Copyright 2009

by American Chemical Society.

FIG. 13. Higher and more consistent specular transmittance of Cu NWs as

compared to ITO films. Reproduced by permission from Wu et al., Nano

Lett. 10, 4242 (2010). Copyright 2010 by American Chemical Society

171301-11 Khalil et al. J. Appl. Phys. 114, 171301 (2013)

various randomly oriented wires and very little information

can be obtained for the individual NW. Even if the nonwo-

ven comprises highly aligned wires, the tensile tests will be

useful if the aligned nonwoven is to be used in certain appli-

cation. Again, there will be a high uncertainty in the proper-

ties of single NW. Although it is almost impossible to carry

out the tensile test on a single NW, yet some nanoscale char-

acterization devices such as AFM can be very effective for

mechanical characterization of individual NW.116 Bellan

et al.117 have demonstrated the use of AFM for measuring

the elastic modulus of poly-ethylene and silica nanofibers.

The testing configuration is similar to the 3-point bending

test in which a flat AFM tip is pressed against a NW sus-

pending across rigid supports. The NW can be fixed from

two ends using FIB which will weld the NW end with the

support trench. This is schematically represented in Fig. 14.

Given the dimensions of NWs and the force-deflection

curves obtained from AFM, the elastic modulus can be esti-

mated with a high degree of accuracy using the relationship

for 3-point bending test. The observed value of elastic modu-

lus was found to be significantly higher than the bulk materi-

als due to high degree of molecular orientation. Using a

similar 3 point bending configuration in AFM, Lee et al.118

have measured the elastic modulus of titania nanofibers.

However, the measured value was found to be significantly

lower than the bulk titania and this was attributed to high

degree of uncertainty in crystalline orientation and effects

of diffusional creep and shear deformations. Gu et al.119

have also reported extremely high elastic modulus for poly-

acrylonitrile (PAN) nanofibers as compared to bulk PAN.

They also used AFM for this purpose but in a different con-

figuration. The one end of the fiber was directly attached to

the AFM tip and it was bent against a rigid support using the

other end of the fiber as an anchoring point. These studies

demonstrate that AFM can be a very useful tool for having a

good estimate for the elastic modulus of metallic NWs.

Breaking the NW in the 3-point bending configuration can

give us an estimate about its strength. The mechanical prop-

erties estimated in this way can give a good understanding

about the feasibility of these NWs in flexible applications

such as transparent flexible electrodes.

An alternative way of employing AFM for determining

mechanical properties of NWs is to directly press the NW

through AFM tip which is laterally lying on the substrate

in a horizontal position. This approach, demonstrated in

Fig. 15, has been followed by Ko et al.120 for estimating the

modulus of carbon nanotubes (CNTs) based composite nano-

fibers. This method was actually used by Kracke et al.121 for

determining the nanoelasticity of thin gold films. An impor-

tant requirement for implementing this approach is that the

fiber diameter should be significantly larger than the tip con-

tact radius. The deflection, Dz, for a given applied load can

be translated to the elastic modulus of the NW.

Recently, in-situ characterization techniques122–127 have

turned out to be very useful for mechanical testing of NWs.

However, due to lack of standardization and several com-

plexities, there is large variance in the reported values, and

the elastic properties for these NWs were found to be very

different from their bulk counterparts. Several metallic NWs

have been tested using in-situ electron microscopy methods.

Taking an example of Au NWs, it has been demonstrated

that the deformation mechanisms in the nanostructures are

very different from their bulk counterparts.128 Whereas the

dislocation emission and multiplication is the dominating

deformation mechanism in bulk materials, the partial dislo-

cations emitted from the surface governs the deformation

mechanism in NWs. Similar observations have been made

elsewhere in case of Au NWs.124 In another example,

extremely high strength for individual Ni NWs was observed

through in-situ tensile testing.129 Similarly, Zhang et al.126

have carried out in-situ tensile testing of Co NWs inside

SEM, however, they observed very low modulus for these

NWs as compared to bulk Co. They attributed this behavior

to the stiffness of the soldering portions, specimen misalign-

ment, microstructure of the NWs and the experimental mea-

surement uncertainty. Size dependent tensile properties for

these NWs have also been an interesting topic. For example,

Asthana et al.127 have demonstrated the compressive testing

of titania nanofibers via AFM tip operating inside TEM.

They observed strong size dependence for modulus. Within

FIG. 14. Schematic illustration of 3-point bending test of NW via AFM tip. FIG. 15. Lateral pressing of nanofiber lying on a rigid substrate.

171301-12 Khalil et al. J. Appl. Phys. 114, 171301 (2013)

the diameter range of 40 nm to 110 nm, the modulus was

found to decrease with increasing diameter after which it

became stable. This interesting observation was explained in

terms of combined effects of surface relaxation and long

range interactions present in the ionic crystals, which led to

much stiffer surfaces than the bulk. They proposed that

for wires having larger diameter, the surface-to-volume

ratio decreases and hence the surface stiffness effect also

decreases. Fig. 16 shows some examples of in-situ mechani-

cal testing inside electron microscopes.

It is important to realize that the characterization techni-

ques for NWs, in fact, for all classes of nanomaterials, are at

the stage of development and researchers are evaluating dif-

ferent techniques to come up with a reliable testing methods.

Due to lack of standardization and high uncertainties, differ-

ent and contrasting results are observed. Current trend shows

more inclination towards AFM as compared to in-situtensile/compression testing for mechanical characterization

of NWs. This is most probably due to the fact that most of

the AFM systems are capable of such experimentation,

whereas not all SEM and TEM systems are equipped with

specialized in-situ testing holders. Such holders are neither

economical nor standardized in terms of required sensitivity

and accuracy. It is worth mentioning that the in-situ tensile/

compressive testing of NWs could be more reliable and

practical because it treats the NW as a continuum. On the

contrary, the AFM measurements for mechanical properties

are localized in nature and do not provide good level of

confidence about the macroscopic response of NWs.

Nevertheless, AFM and in-situ electron microscopy manifest

themselves as strong and practical techniques when it comes

to the mechanical testing of these one-dimensional nano-

structure. In future, we can expect some non-destructive

testing (NDT) techniques for mechanical testing of these

nanostructures. An interested reader may go through a com-

prehensive review article by Rohlig et al.130 where different

techniques for evaluating elastic properties of NWs have

been covered.

The electrospun metallic NWs have great potential to

act as mechanical reinforcements. This is mainly due to two

reasons. First, since the reinforcement effect is governed by

the aspect ratio, even very small length nanofibers of few

millimeters can be very effective due to extremely high

aspect ratio. Second, because of very high surface area, these

NWs will have very good adhesion with the matrix and the

strengthening will be at the molecular level due to the size

range of NWs. Because of better strengthening effect, lower

contents of the reinforcement are required as compared to

those when macroscopic fibers are used. Limited attempts

have been made to study the influence of polymeric nanofib-

ers as reinforcements because of the difficulties associated

with nanofiber orientation and dispersion inside the matrix.

Bergshoef et al.131 have reported 35 times and 4 times

improvement in the stiffness and the strength, respectively,

for epoxy when reinforced with polyamide nanofibers.

Similarly, Kim et al.132 have shown around 35% improve-

ment in the modulus of epoxy when reinforced with poly-

benzimidazole nanofiber mats. Based on these results,

metallic NWs are expected to have strong reinforcing effect

for different metallic and polymeric matrices. However, as

stated earlier defect free microstructure of the NWs is very

important to take their mechanical advantage.

F. Sensing characteristics

Metal NWs produced through electrochemical growth

have shown promising sensing characteristics, especially the

sensing of various gases. The higher sensitivity of NWs is

due to their very high surface energy which allows them

to reversibly react with different gases in the surrounding

environment. For example, sub-micron Pd NWs have shown

excellent hydrogen sensing properties due to reversible

hydride formation,2 Fig. 17. Similarly, the electrochemically

deposited Pd NWs133 have also shown excellent hydrogen

sensitivity as depicted in Fig. 18. The temporary change

from pure metal to metal hydride due to hydrogen exposure

causes the change in electrical properties and consequently

the change in current flowing characteristics with time which

can be measured and amplified to produce a detectable sig-

nal. To improve the reliability and functionality of these sen-

sors, the arrays of NWs can also be employed as sensors as

shown elsewhere.134 At the same time, this higher reactivity

could turn out be disadvantageous because of oxidation of

these NWs. However, Wu et al.36 have shown that the oxida-

tion tendency of Cu NWs is not too high and it becomes of a

FIG. 16. In-situ mechanical testing of NWs inside SEM (left) [Reproduced

by permission from Zhang et al., Nanotechnology 20, 365706 (2009).

Copyright 2009 by IOP Publishing] and TEM (right) [Reproduced by per-

mission from Asthana et al., Nanotechnology 22, 265712 (2011). Copyright

2011 by IOP Publishing.]

FIG. 17. Hydrogen sensing characteristics of sub-micron Pd wire.

Reproduced by permission from Yun et al., Nano Lett. 4, 419 (2004).

Copyright 2004 by American Chemical Society

171301-13 Khalil et al. J. Appl. Phys. 114, 171301 (2013)

lesser concern if the NWs are to be used in the applications

where they are embedded underneath the other material. In

addition to gas sensors, the Mo NWs containing the beads of

other metallic nanoparticles, such as gold, have also been

proposed as very effective bio-sensors.135 Depending upon

the surface chemistry of nanoparticle, the specific nano/bio

species present in the environment may interact with it caus-

ing an overall change in the current carrying capacity of the

NW. This concept is depicted in Fig. 19. The high surface

energy and hence chemical reactivity/affinity is therefore

one of the key characters of metal NW which makes them

strong candidate for future sensors and related micro/nano

devices. Moreover, the variety of electrospun metal NWs

with different diameters and morphology will further lead to

cost reduction and mass production of such sensors.

Following the same approach for obtaining aligned electro-

spun metal NWs across two electrodes,90 the arrays of NW

acting as sensors can be obtained. The challenge however

remains that how these electrospun NWs can be integrated

with other micro/nano devices so that low cost and reliable

sensors may be developed.

G. Catalysts

Due to very high surface area, electrospun metal NWs

act as strong and active catalytic elements as shown in some

studies. Kim et al.38,39 have shown that electrospun Pt NWs

have much higher catalytic performance as compared to the

nanoparticles, as shown in Fig. 20. They explained this

behavior in terms of better electron transport in NWs due to

less collision across interfaces. Such NWs could thus serve

as efficient electrode materials for low temperature fuel cells.

Kim et al.40 have also shown that the rough surface and the

defective structure of electrospun Pt NWs can be advanta-

geous for better catalysis. The rough surfaces in these NWs

serve as active sites for effective electron transport and hence

enhance the catalytic activity. Based on these observations,

metal NWs seem to have important future applications as

electrodes in low cost fuel cells as well as in several chemi-

cal and electrochemical catalytic applications.

V. OUTLOOK

Despite several challenges, electrospinning manifests

itself as the most economical method for mass production of

metallic NWs. Moreover, providing excellent control over

final NW structure and properties is one of the key benefits

of this process over other methods. However, there is a

strong need to develop three clear connections in this regard.

First, the connection between the precursor properties and

the structure of NWs. Second, the connection between

the electrospinning parameters and the structure of NWs.

Finally, the connection between the structure and the differ-

ent properties of the NWs. Once these connections are well

developed and understood, the process can be commercial-

ized for mass production of metallic NWs for desired appli-

cations. At the same time, focus is needed to optimize the

electrospinning process for obtaining highest quality NWs

and to carefully understand their nanoscale response from

various perspectives. Also, the means to integrate the elec-

trospun metallic NWs with other materials and devices need

FIG. 18. Hydrogen sensing characteristics of a Pd NW. Reproduced by

permission from Walter et al., Microelectron. Eng. 61–62, 555 (2002).

Copyright 2002 by Elsevier BV.

FIG. 19. Schematic diagram of a beaded NW-based sensor. Depending on

the chosen metal and/or the presence of recognition elements, the interaction

of a specific molecule with the surface of the particle induces a conductivity

change. Reproduced by permission from Walter et al., Surf. Interface Anal.

34, 409 (2002). Copyright 2002 by John Wiley & Sons Ltd.

FIG. 20. Maximum power density for a given current in case of Pt NWs and

the Pt nanoparticles Reproduced by permission from Kim et al., Electrochem.

Commun. 11, 446 (2009). Copyright 2009 by Elsevier, Inc.

171301-14 Khalil et al. J. Appl. Phys. 114, 171301 (2013)

to be explored. This will pave the way towards the successful

and large scale usage of these electrospun metallic NWs in

future engineering applications. Advancements and innova-

tions in electrospun metal NWs can be foreseen at this stage.

For example, by using a combination of different metallic

salts, bimetallic or alloyed NWs can be produced. An exam-

ple of this approach has been demonstrated by Kim et al.136

for obtaining electrospun PtRh and PtRu NWs. In future,

we can expect electrospun metallic core shell NWs (nano-

pipes) and porous metallic NWs (nanofoams) via similar

approaches which have already been used for producing

core shell and porous polymeric nanofibers. Such metallic

NWs with customized morphology will open new doors of

research in the area of nanomaterials.

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