Electrospun metallic nanowires: Synthesis, characterization, and applications Abdullah 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
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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
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.
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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