Hindawi
Journal of Nanomaterials
Volume 2018, Article ID 1698357, 10 pages
https://doi.org/10.1155/2018/1698357

Research Article
Large-Scale Cu Nanowire Synthesis by PVP-Ethylene Glycol RouteJhon L. Cuya Huaman , Iori Urushizaki, and Balachandran Jeyadevan
Department of Material Science, School of Engineering, The University of Shiga Prefecture, Hikone, Japan
Correspondence should be addressed to Jhon L. Cuya Huaman; cuya.j@oﬃce.usp.ac.jp
Received 14 March 2018; Revised 29 May 2018; Accepted 12 June 2018; Published 8 August 2018
Academic Editor: Elisabetta Comini
Copyright © 2018 Jhon L. Cuya Huaman et al. This is an open access article distributed under the Creative Commons Attribution
License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Cu nanowire (NW) is a promising cost-beneﬁt conducting material that could be considered for the development of transparent
conducting ﬁlms (TCF). However, the development of Cu NW as an alternating material for Ag or Au is not only limited by its
stability in atmospheric conditions in the nanometer range but also due to the nonavailability of a simple synthetic route toproduce them in high yields and in large-scale. Here, a scheme to synthesize Cu NWs by reducing Cu nitrate in a Cl− ionpolyvinylpyrrolidine- (PVP-) ethylene glycol (EG) system is proposed. Cu NWs with average diameter around 60 nm andaverage length of about 40 μm was obtained under optimized experimental conditions. Furthermore, the formation of Cu NW
was elucidated to be through the progression of the following sequential reduction steps: at ﬁrst, Cu ions underwent partial
reduction to form spherical Cu2O. Then, the spherical Cu2O particles were redissolved and reduced to metallic Cu0 atoms that
subsequently formed the Cu seeds. Thereafter, Cu seeds underwent etching to form multiply-twinned particles (MTP). Finally,
these Cu MTP grew unidirectionally to form metallic Cu NWs.

1. Introduction
In recent years, the development of TCF such as touching
screens, thin-ﬁlm solar cells, organic light-emitting diodes,
and sensors using metallic NWs as an alternative conducting
material for indium tin oxide is being researched intensively[1–3]. In fact, the optical and electrical properties of TCF prepared using Ag NWs with diameters ~20 nm and lengths
higher than 20 μm have outperformed the ones preparedusing commercial ITO and exhibited transmittance higher
than 85% and electrical resistivity less than 45 Ω/sq. [2, 3].
However, the material cost and migration caused in moist
conditions are considered detrimental to their use in TCF
and researches on alternative metallic NWs are being considered. Consequently, the research on the development of Cu
NWs are on the rise due to the availability of cheap and abundant resources, high electrical properties, and low migration
problem despite the fact that Cu has showed low stability inmoist and oxidizing atmospheres [4–8]. Consequently, several physical and chemical routes have been developed tosynthesize Cu NWs; however, in most cases, the chemicalroutes are considered due to simplicity and scalability ofthese techniques in contrast to other routes [9–22]. Amongthe developed chemical techniques, aqueous synthetic routeusing hydrazine as the reducing agent is researched intensively [13–18]. However, this technique uses high concentrations of sodium hydroxide (NaOH) and requires stringent
washing technique to remove the alkali that could hamper
the conductivity of Cu NWs due to surface oxidation. On
the other hand, long alkyl amine-mediated synthesis of Cu
NWs has gained importance due to the possibility of dispersing and storing the products in organic solvents that could
avoid rapid oxidation [19–22]. In this method, alkyl amines
function as the reducing, protecting, and growth-controlling
agent. The adsorption of alkyl amine on speciﬁc {100}planes of Cu seeds facilitates the unidirectional growth
the seeds in the [110] direction [20]. Though the organicroute is considered simpler than the aqueous route, theheterogeneous dispersion of NWs has been observed when
the suspension was deposited on substrates; thus, additional processing steps such as ligand exchange are
required to enhance the dispersing ability of NWs in polarand nonpolar solvents. Even though the issues such as
inhibition of oxidation and enhancement in the stability

2of suspension remain, large-scale synthesis is also considered a serious issue that needs to be resolved.
On the other hand, the success of Ag NWs as an alternative material for ITO in TCF could be ascribed to their largescale production using a simple and scalable polyol processthat enables dimensional control of NWs through the control of concentrations of Cl− ion, AgNO3, and PVP in ethylene glycol [23–26]. In addition, the ﬁne tuning of thedimensional properties of NWs such as the length and diameter could also be achieved by introducing chemical additives at various stages of the process [24–28]. However, it
should be noted that the above technique has not been successfully applied to large-scale synthesis of Cu NWs withdimensional properties suitable for conductive electrodes,that is, 20 nm in diameter and 20 μm in length [2, 3]. Oneof the reasons could be the low redox potential of Cu ionscompared to Ag ions, which requires the synthesis at elevated temperatures or the use of weak reducing agents suchas either sodium hypophosphite or sodium borohydridealong with alcohol that enhances the overall reducing abilityof the system [29–31]. However, the products obtained were
mostly Cu nanoparticles (NPs) and not Cu NWs. And also,
unlike in the case of Ag NWs, the formation of intermediatephases such as cupric and cuprous compounds [32–34]
seems to play an important role in determining the reduction rate, which is quite vital and decisive step deﬁning theﬁnal morphology of metallic Cu. Cu NWs have beenobtained in alkylamine-glucose or glycerol systems at relatively high temperatures [4]. Although, alkylamine enhanced
the reducing ability of polyol and controlled the unidirectional growth on {100} planes, this was not adequate to avoid
the generation of NPs in large quantities. On the other hand,the synthesis of Cu NWs by using propylene glycol as reducing agent and PVP as surfactant has also been reported [35].
Here, the use of tetraphenylphosphonium chloride is considered to have played an important role in obtaining nanowires. Though the yield of Cu NWs was very low and theirdiameters were thick (<300 nm), this technique can be
improved by considering stronger reducing agents.
In the case of Ag NWs in EG-PVP system, two closely
related elements, namely, Cl− ions and PVP molecules thatspeciﬁcally adsorb on the {100} planes of Ag NPs, arereported to facilitate the unidirectional growth of Ag seeds[23–26, 28, 36–40]. In addition, PVP molecules are reported
to enhance the reduction of metal salts by forming complexes
through coordination between metallic ions and C=O groups
and also stabilize the products subsequently [36, 37]. However, the speciﬁc adsorption of PVP for the unidirectional
growth of metallic seeds is only realized at speciﬁc Cl− ionconcentrations [28, 36, 40]. At conditions suitable for the
reduction of Ag ions to metallic Ag, Cl− ions control the morphology of Ag products through the etching processes thatgenerate multitwinned Ag seeds. The etching mechanismthat consists of selective removal of twinned structures into
solution by the presence of an oxidative pair such as Cl−/O2
or Fe3+/Fe2+ was initially proposed by Wiley et al. [23]. They
claimed that the shape of the particles can be controlled bycontrolling the degree of etching or in other words by controlling the concentration of Cl ions. For example, in the

Journal of Nanomaterials
absence of Cl− ions, where the etching does not occur, theproduct will be constituted of spherical and polyhedral particles only. However, at higher Cl− ion concentrations, the total
dissolution of nanoparticles occurs. Furthermore, at appropriate concentration of Cl− ions, the formation of MTP isfacilitated and product becomes almost only of NWs.
Here, the synthesis of Cu NWs in high yield using the
EG-PVP system under optimized conditions is reported.
The formation mechanism of Cu NWs based on the analytical results acquired from the solution as well as the solids
obtained during synthesis of Cu NWs using UV-visible spectroscopy (UV-Vis), scanning electron microscopy (SEM),
and X-ray diﬀractometry (XRD) is also proposed. The study
revealed that Cu NWs originated from the dissolution of theintermediate oxide compounds such as Cu2O. The dissolution rate of Cu2O and the subsequent reduction to metallic
Cu were considered to be the critical and rate determining
steps for the formation of Cu NW.

2. Materials and Methods
2.1. Preparation of Cu NWs. Cu(NO3)2·3H2O (10 mM),
polyvinylpyrrolidone (PVPK30, 0 to 75 mM considering
the monomer unit), and cetyltrimethylammonium chloride(CTAC, 0 to 10 mM) were introduced into a 200 ml twonecked ﬂask containing 100 ml of EG. This solution washeated between 172 and 186°C for 4 h under continuous
mechanical stirring and N2 purging (0 to 200 ml/min).
The solid products were collected by centrifuging the suspension at 3000 rpm for 10 min. The recovered sediment
was washed by methanol several times, dispersed in methanol and stored.
2.2. Characterization of Cu NWs. Identiﬁcation of the crystal
phases present in the sediment was analyzed by X-ray diﬀractometry (XRD, Rigaku SmartLab) using Cu Kα in the angle
2θ ranging from 20 to 80 degrees. The optical properties ofCu NWs suspended in methanol were evaluated using ultraviolet visible spectroscopy (UV4900, Rigaku) in the range of
300 to 800 nm using quartz cells. The morphology of the
products was examined by using ﬁeld emission scanningelectron microscopy (FE-SEM, Hitachi 4400) and transmission electron microscopy (TEM, Hitachi-8100). The adsorption of PVP on the Cu NW surface was veriﬁed by Fouriertransformed infrared spectroscopy (FT-IR) in transmittancemode in the range of 500 to 4000 cm−1 (Nicolet iS5, Thermo
Scientiﬁc), and the concentration of PVP attached to the NW
surface was determined by thermogravimetry (TG) under N2
gas ﬂow from room temperature to 500°C by heating at therate of 10°C/min using TG8120-Shimadzu device.

3. Results and Discussion
Taking the experimental conditions to obtain Ag NWs in EGPVP system into consideration, the tuning of Cl− ion concentration was considered vital to control the etching mechanism
and the subsequent formation of NWs. Thus, in the ﬁrst
instance, the concentration of Cl− ions required to obtain
NWs was examined while keeping the concentration ofJournal of Nanomaterials
PVP constant. And then, the inﬂuences of PVP concentration and other parameters such as the reducing atmosphereand temperature were also studied.
3.1. Inﬂuence of Halide Ion Source and Its Concentration on
Cu NW Formation. Figure 1 shows the SEM images of Cubased products obtained in the EG-PVP system using 0.5 to
2.5 mM of CTAC as the source of Cl− ions. At Cl− ion concentration less than 0.5 mM, Cu nitrate was reduced by ethylene glycol to irregular and faceted metallic Cu particles
ranging between 1 and 2 μm along with few rods. When theconcentration of Cl− ions ranged between 0.5 and 1.0 mM,
the Cu particles became larger in size, and rods, plates, andtriangular crystals were also observed (Figure 1(a)). The formation of Cu particles with diﬀerent shapes could be due topresence of twinned Cu crystals that were not etched suﬃciently due to the deﬁciency of Cl− ions. A remarkablechange was observed when the alcoholic solution contained
1.0 mM CTAC; the product was constituted mainly ofthicker rods with an average aspect ratio (length/diameter)of 10 and small quantities of Cu particles with triangularfacets that originated from decahedral or icosahedral crystals(Figure 1(b)).
When Cl− ion concentration reached 1.25 mM, the product was constituted only of Cu NWs (Figure 1(c)). Under
these conditions, the average diameter and length of NWs
were 74.1 nm and 31.5 μm, respectively (aspect ratio of423). However, when the concentration of Cl− ion concentration resulted to be too high, the shape of NW was notretained and the yield decreased progressively (Figure 1(d))
and disappeared totally at 10 mM. On the contrary,micrometer-sized spherical particles were formed at higher
concentration of Cl− ions (Figure 1(d)). Usually in the presence of high concentrations of Cl− ions, the formation of
nanocubes is favored as in the case of Ag NPs due to the preferential adsorption on the {100} planes [23, 24, 28]; however,this was not observed here. The XRD pattern of the sphericalparticles corresponded to the cubic phase of Cu2O. Thissuggests that the reduction of Cu2O to Cu was delayed or
reduction and dissolution occurred repeatedly under highconcentration of CTAC and subsequently neither the formation of Cu NWs nor nanocubes was realized.
The above results suggested that at low Cl− ion concentrations, the reduction rate of Cu was too high for the seeds
to undergo etching to generate preferential planes prior to
growth, resulting in the formation of irregular and faceted
nanostructures. On the other hand, at high Cl− ion concentrations, the etching was too strong and the concentrationof Cu ions in the solution became high and led to the formation of spherical shaped Cu2O particles (Figure 1(e)). Thus,
the optimum Cl concentration was determined to be around1.25 mM, or in other words, the Cl/Cu molar ratio was 0.125.
This value was relatively high compared to the one used by
Whitcomb, which is 0.026 [35]. Although, the low concentration of Cl− ions could be one of the reasons for low yield,
the formation of insoluble or partially soluble intermediate
compound, CuCl could be another. Though the presenceof CuCl was not detected when the concentration of Cl− ions
was low, their presence was conﬁrmed in the structural and3morphological analysis in the products obtained during thesynthesis of Cu NWs under high CTAC concentrations(5 mM) (Figure S1).
Then, to determine the inﬂuence of halide type on theformation of NWs, CTAC was replaced by equal concentrations of sodium ﬂuoride (NaF), sodium chloride (NaCl),
cetyltrimethylammonium bromide (CTAB), and potassium
iodide (KI). As observed in the SEM photographs shown in
Figures 1(f)–1(i), high yield of NWs was obtained in NaCl
and CTAB cases. The above results suggested that the optimal halide concentration is very vital than the type or sourceof halide for the formation of Cu NWs in high yield. On theother hand, the formation of cuprous halides and their solubility product constants (Ksp) [41] could be an importantfactor to consider in the control of the ﬁnal structure andthe complete reduction of Cu salts as shown in Figure 1(j).
Thus, the synthesis of high-yield Cu NWs proceeded in thefollowing order:
CuCl 1 72 × 10−7 > CuBr 6 27 × 10−9 > CuI 1 27 × 10−121Hence, the reduction and formation of Cu NWs isfavored at high solubility of intermediate products in the solvent as in the cases of CuCl and CuBr. But, in the case of low
solubility CuI, the products resulted to be a mixture of CuI,
Cu2O, and Cu phases (Figure S2). On the other hand, inthe case of highly soluble CuF and the instability to form
CuF2 enhances the formation of Cu NPs than Cu NWs.
The formation of NWs is inﬂuenced by the control over
the reduction of Cu ions to form metallic Cu seeds and theirsubsequent etching to form MTPs that facilitate the formation of NWs. The dimensional properties of the NW areinﬂuenced very much by the unidirectional growth facilitatedby the preferential adsorption of the capping agent on the
crystal planes of the NWs. Thus, the role of the capping agent
on the yield as well as the dimensional properties of Cu NWs
will be discussed in the next section.
3.2. Inﬂuence of PVP on the Yield and Dimensional Propertiesof Cu NWs. The molecular weight and concentration of PVP
have been reported to control the yield and dimensions of
NWs [36–39] due to their preferential adsorption on {100}planes of metallic surface compared to {111}. Thus, the eﬀectof PVP concentration on the synthesis of Cu NWs at thereaction temperature of 182°C is investigated, while keeping
Cl−/Cu2+ molar ratio and the amount of N2 purged constant.
Figure 2 shows that decreasing the PVP concentration facilitated the formation of irregular-shaped Cu NPs at theexpense of Cu NWs. At low PVP concentration of 25 mM,
the nucleus grew isotropically due to insuﬃcient concentration of capping agent. In contrast, at higher PVP concentration of 75 mM, the Cu NWs became thicker and shorter.
When the concentration was increased further, only NPs
were formed. This is believed due to the nonpreferential
adsorption of PVP molecules on the crystal faces of Cu
NPs, even on the {111} planes that facilitate the unidirectional growth of NWs as shown in Figure 2(g). The FT-IR
spectra of pure PVP and the Cu NWs are shown in4

Journal of Nanomaterials

10 휇m
(a)

(b)

(Cl−)

Low
Cu0

(c)

(d)

High

Cu0
Fast

Cu2O
Slow

Reduction(e)

10 휇m
(f)

(g)

CuF
Unstable
Cu0

(h)

CuCl
(Ksp: 1.72 ×10−7)

CuBr(Ksp: 6.27 × 10−9)

Cu0

(i)

CuI
(Ksp: 1.27 × 10−12)
Cu2O
CuI

Solubility(j)Figure 1: SEM images of Cu nanostructures obtained by reducing Cu nitrate in EG-PVP system in the presence of (a) 0.5, (b) 1.0, (c) 1.25, and(d) 2.5 mM of CTAC and (e) a schematic illustration showing the eﬀect of Cl− ion concentration in the formation of Cu NWs. SEM images of
halide type in the synthesis of Cu NWs: (f) NaF, (g) NaCl, (h) CTAB, and (i) KI at a concentration of 1.25 mM and (j) a schematic illustrationthat shows the eﬀect of Ksp of Cu(I) halides in the formation of Cu NWs.

Figure 2(e), and the adsorption of PVP on the surface of
NWs was conﬁrmed. The spectrum of the NWs synthesizedusing the above technique shows absorption bands at
3400 cm−1 that could be ascribed to the overtone of C=O
bond. In addition, a band corresponding to O-H groups ataround 3500 cm−1 was also recorded. This could due to thepresence of methanol used during washing. The stretchingvibrations of C-H and C=O were observed at 2920 cm−1
and 1640 cm−1, respectively. Likewise, the absorption bandcorresponding to the stretching mode of C-N was also
observed at around 1300 cm−1. Furthermore, the PVP concentration adsorbed on the metallic surface was evaluatedby thermogravimetry under N2 gas ﬂow. As observed in
Figure 2(f), the weight loss at temperatures below 180°C
was around 8% and it could be ascribed to the removal of solvents such as methanol and adsorbed water. The weight loss
between 180 and 200°C was steep and amounted to almost
15%. Then, the weight loss progressed slowly and an additional 5% decrease was recorded at around 600°C. These
results suggested that PVP is strongly adsorbed on the Cu

surface, and thermal treatments at higher temperatures arenecessary to remove PVP molecules completely.
The optimal concentration of Cl− ions to facilitate theformation of MTPs and the optimal concentration of the
capping agent for their subsequent growth have been studied
in the previous sections. The diameter of the NW is verymuch inﬂuenced by the size of the seed as well as the MTP
formed subsequently. This is believed to depend on the
reduction and the etching rates of the reactions. The etchingrate is believed to depend on the concentration of oxygen in
the reaction system besides the concentration of Cl− ions.
Thus, the inﬂuence of dissolved oxygen amount on the synthesis of Cu NWs is discussed in the next section.
3.3. Eﬀect of the Amount of N2 Gas Flow on the Dimensional
Properties of Cu NWs. Cl− ions are assumed to form an oxidant Cl−/O2 pair with atmospheric oxygen that etches anddissolves the preferential planes of the nucleus to form
MTP. Thus, the control of this mechanism is important tofacilitate the formation rate of multitwinned metallic seeds

Journal of Nanomaterials

5

10 휇m
(a)

(b)

(c)

(d)

)

VP

PVP-Cu NWs

−5

w
Lo

−10

Optimum

Weak adsorptionon (100)

(PVP)

−15
−20

Optimum adsorptionon (100)

h
)

VP

20.3%

−25

(P

Weight variation (%)

PVP

(P

ig
H

Transmittance (a.u.)

0

−30
3500

2500

1500

Wave number (cm−1)

500

0

100 200 300 400 500 600
Temperature (°C)

(e)

(f)

Adsorption on[100] and [111](g)Figure 2: SEM images of Cu products obtained under diﬀerent PVP concentrations: (a) 25, (b) 45, (c) 55, and (d) 75 mM (scale bar: 10 μm).
(e) FT-IR spectra of pure PVP and PVP-capped Cu NWs. (f) Thermal behavior of PVP-capped Cu NWs measured under N2 ﬂow. (g)Schematic illustration describing the eﬀect of concentration on the adsorption of PVP on Cu surface.

that grow unidirectionally in [110] orientation with the assistance of the preferential adsorption of the capping agent in{100} planes. On the other hand, Cu NPs are unstable in air
inclusively during synthesis and it is common that the reduction needs to be carried out under inert atmospheres using
either N2 or Ar gas for the formation of Cu NWs [42, 43].
The reaction atmosphere also inﬂuences the rate of the
reduction. Thus, the change in the N2 ﬂow rate between 0
and 150 ml/min inﬂuenced the morphology of Cu nanostructures as shown in Figure 3. It is clear that EG-PVP system
can reduce Cu even in atmospheric conditions, and Cu
NWs with diameters around 60.6 nm were obtained; however, nanometer-sized Cu oxide particles attached to thesurface of NW were observed. On the other hand, at N2 ﬂow
rate of 150 ml/min, the wires became thicker, ~87.5 nm, andthe formation of nanocrystals with diﬀerent sizes and shapes
was observed at the expense of low yield of NWs. Here, theamount of O2 is reduced by adding N2 gas or other inert
gases to the system, and subsequently, the selective dissolution of speciﬁc crystal planes could be inhibited favoring the
growth of cubic crystals as observed in other metallic systems [44]. Thus, the formation of wires and the inhibitionof oxidation of the synthesized NWs and NPs were realized
under an optimum N2 ﬂow rate of 100 ml/min.
Optimization of the reaction atmosphere facilitated thecontrol over the etching rate and also the stability of the Cu
nanostructures. The average diameter and length of the
NWs were 67.4 nm and 44 μm, respectively, and the aspectratio was around 650. However, for further reduction in the
diameter, the reduction rate should be enhanced. One ofthe parameters that could enhance the reduction rate is thereaction temperature. Thus, the results obtained by varying
the reaction temperature and their inﬂuence on the dimensional properties of Cu NW are discussed in the next section.
3.4. Eﬀect of Temperature. The synthesis of Cu NPs at low
temperatures is realized only when additives that promote
the reduction such as sodium hypophosphite monohydrate
or sodium borohydride were added to the alcoholic system[29–31]. Increasing the reaction temperature near to theboiling point of ethylene glycol should enhance its reducing
ability and facilitate the reduction of Cu2+ ions within a shortperiod of time. Figure 4 and Figure S3 show the eﬀect ofreaction temperature on the synthesis of Cu NWs, while allother experimental parameters were kept constant. At 174°C,
the formation of spherical Cu2O particles was concluded
from SEM and XRD measurements (Figure S4). At thistemperature, Cu2+ ions were reduced to form Cu2O but not
to Cu metal even after a reaction time of 10 h. Even though
the formation Cu NWs was conﬁrmed at 178°C, the presenceof Cu2O particles was also identiﬁed as shown in Figure 4(b).
This suggested that the reaction temperature was not highenough to reduce the Cu ions completely. However, theproducts obtained at 182°C contained Cu NWs and free ofoxide NPs (Figure 4(c)). At higher temperatures, high yieldof thicker NWs free of NPs was obtained as shown in6

Journal of Nanomaterials

Without N2

(a)

(b)

Etching process
With N2

1 휇m
(c)

(d)

Growth(e)

Figure 3: SEM images of Cu NWs synthesized under diﬀerent N2 ﬂow rates: (a) 0, (b) 50, (c) 100, and (d) 150 ml/min. (e) Schematic
illustration describing the eﬀect of N2 ﬂow rate in the formation of Cu NWs.

Figure 4(d). The kinetic of the reaction is expected to get
accelerated at higher temperatures, and the formation of large
particles is expected. In this case, the nanowires can grow inaxial and radial directions easily. Thus, it was found that thediameter of Cu NWs obtained at diﬀerent temperatures isinﬂuenced by the reaction temperature and NWs withaverage diameters ~58 nm and ~103 nm were obtained at
178°C and 186°C, respectively. Figure 4(e) shows the eﬀect oftemperature in the reduction and formation of nanowires.
Our results demonstrated that Cu NWs can be synthesized in high yield in a Cl− ion-EG-PVP system by controllingthe experimental parameter such as type and concentrationof chloride source, concentration of PVP, the amount of N2
gas ﬂow, and the reaction temperature. However, the physical characteristics of Cu NWs, specially the diameter,
obtained under optimal conditions are not suitable to be usedin the development of TCF. Thus, understanding the formation mechanism of Cu NWs is considered vital for further
optimization of the process to develop of Cu NWs with highaspect ratio. In the next section, we attempted to elucidate theformation mechanism of NWs through structural and spectral analyses of the reaction solution and solids using XRD,UV-Vis, and SEM.
3.5. Formation Mechanism of Cu NWs. Figure 5 shows the
XRD patterns of samples collected at diﬀerent reaction times
using the following experimental conditions: 10 mM Cu
nitrate, 0.125 mM of CTAC, 50 mM of PVP, 182°C, andunder a N2 gas ﬂow rate of 100 ml/min. During heating, attemperatures below 150°C, neither color variation of thealcoholic solution nor precipitation occurred. However, attemperatures between 150 and 160°C, the color of the solution turned from light blue to orange with vigorous evolutionof NO2 [45]. The XRD analysis of precipitated powders taken
after completion of the evolution of gases showed a uniquephase that matched well with cubic structured Cu2O, and thisphase remained stable even at higher temperatures as shownin Figure 5. It is clear that at this temperature, the reducing

potential of ethylene glycol was only suﬃcient to reduce
Cu2+ ions to Cu+, but not to Cu0 due to the presence of chloride which delayed the reduction as observed above. The Cu
oxide phase remained for nearly 100 min at 182°C, and then,the orange colored solution turned progressively to red
caused by the transformation of Cu2O to Cu as conﬁrmed
by XRD. The reduction of Cu2O to Cu0 was totally completed
in 10 minutes, and no further change was observed until theend of reaction. It is necessary to emphasize that intermediate phases such as hydroxide, chloride, or oxide (CuO) were
not observed during the reduction of Cu nitrate to Cu metal.
Cu2O and Cu NPs possess localized surface plasmon resonance at around 400 to 700 nm [32], which depends on theparticle size and morphology. Thus, the progression of thereaction Cu2+ → Cu+ → Cu0 during heating at 182°C at different reaction times was analyzed using UV-Vis spectroscopy (Figure 5). At 50°C and at 150°C, the UV-Vis spectra
revealed a broad absorption peak at around 800 nm that
could be ascribed to Cu2+ ions coordinated with ethylene glycol and disappeared totally when Cu2O was formed at higher
temperatures. When the temperature reached 182°C, a broad
absorption band centered at around 440 nm was observed.
This could be assigned to the presence of Cu2O particles,
and this was not present in the spectra measured at temperatures lower than 182°C (inset Figure 5). After 60 min, at
182°C, the UV-Vis spectrum exhibited a shift in the peak corresponding to Cu2O from 440 nm to 470 nm, which isbelieved due to particle growth. After 100 min, at 182°C, thecolor changed from orange to red and a new absorption peak
appeared at 570 nm. This could be as a consequence of Cu2O
reduction to metallic Cu seeds [32]. The reduction reaction
from Cu2O to Cu was relatively faster and the Cu2O absorption peak disappeared totally in 10 minutes leaving the peakcorresponding to Cu NWs at 575 nm [21]. No change wasobserved after 120 min, and absorption peak corresponding
to Cu remained even at the end of the reaction.
The FE-SEM images of these samples (Figure 6) showedsphere-shaped Cu2O particles with a broad size distribution

Journal of Nanomaterials

7

wT
Lo

°C

Cu2O

Optimum T °C
(a)

(b)

Cu2O

Hi
gh

Cu0

T

°C

1 휇m
(c)

Cu0

(d)

(e)

Figure 4: SEM images of Cu nanostructures obtained by reducing Cu nitrate in EG-PVP under diﬀerent reaction temperatures: (a) 174, (b)178 (c) 182, and (d) 186°C. (e) Schematic illustration of the eﬀect of temperature on the formation of Cu NWs.

Absorbance (a.u.)

Cu

240 min

Absorbance (a. u.)

Intensity (a.u.)

180°C: 240 min
180°C: 120 min
180°C: 110 min
180°C: 100 min

50°C
150°C

120 min400

110 min

500

600

700

800

Wavelength (nm)

100 min180°C: 60 min
180°C: 0 min

60 minCu2O
Cu2O (JCPDS
number 05-0667)
CuO (JCPDS
number 05-0661)
20

30

40

Cu (JCPDS number
04-0836)

0 min

Cu2O
50
60
2휃 (degrees)

70

80

300

400

Cu
500

600

700

Wavelength (nm)Figure 5: XRD patterns and UV-Vis spectra of Cu products collected during heating at 182°C. Inset of UV-Vis spectra corresponds to the
UV-Vis spectra of solution at 50 and 150°C.

and an average size of 500 nm in the ﬁrst few minutes of reaction at 182°C. After 60 min of reaction, the spherical Cu2O
particles began to dissolve as shown in the inset. However,the dissolved particles precipitated again as Cu2O and noother oxide phase was detected in the XRD pattern. As
reaction progressed, the dissolution proceeded, and after
100 min, the dissolved oxide was reduced to Cu metal in alcoholic solution. Then, due to the presence of Cl− ions and PVP,
the metallic Cu seeds grew in a preferential direction, whereas
rest of the Cu ions in the solution precipitated into small
Cu2O cubes as observed in the inset. However, it is not clear
whether these cubes originated during cooling the Cu2+ ionsthat remained in the solution or they existed prior to cooling.
After 120 min of reaction, product constituted mainly of Cu
NWs; however, the presence of Cu2O on the NW surfaces still
remained. At the end of reaction, Cu NWs were the unique
nanostructure. The diameter and length were ~72 nm and39 microns (aspect ratio of ~540), respectively. The TEM
analysis and the selected area electron diﬀraction (SAED) of
one section of Cu NWs are shown in Figures 6(g) and 6(h).
The microphotographs showed that the NW had an average
diameter of 71 nm and grew in the [110] direction [20]. Likewise, the sample exhibited diﬀraction spots corresponding to
Cu2O phase. A careful observation of the microphotograph8

Journal of Nanomaterials

0 min

60 min(a)

110 min

100 min

(b)

(c)

(d)

240 min

120 min

111

111

002
(110)
71 nm

002
- 111

111

1 휇m
(e)

(f)

(g)

(h)

Figure 6: SEM images of Cu nanostructures obtained during heating at diﬀerent reaction times: (a) 0, (b) 60, (c) 100, (d) 110, (e) 120, and (f)240 min. Insets in (b) and (c) correspond to the dissolution of the synthesized spherical Cu2O particles and the formed Cu2O cubes,
respectively. (g) TEM image of Cu NWs showing the direction of growth. (h) SAED pattern of the Cu NWs.

Dissolution
seedformation

Partialreduction

Cu2+

Cu2O

1D growth

Cu seeds

Cu nanowires

Figure 7: Formation scheme proposed for the synthesis of Cu NWs in Cu(NO3)2-EG-PVP system.

revealed the presence of a thin oxide layer on the wire surface
indicating that PVP did not oﬀer a good protection to theoxidation as already observed in the synthesis of Cu NPs byusing 1-heptanol as reducing agent [43].
With the above results, the following scheme can beproposed for the formation of Cu NWs. At the beginning,
Cu nitrate and ethylene glycol react to give Cu2O particles
through the decomposition of copper nitrate. The evolution of NO2 gas is detected. At higher temperatures, theCu2O particles undergo dissolution and subsequently Cu+
ions get reduced to form Cu0. The dissolution and subsequent reduction of Cu+ ions to Cu0 is the rate determining a step that strikes a balance between nucleation of
metallic particle and their subsequent etching to form
the MTP seed. Then, the unidirectional growth of the
MTPs is facilitated by the preferential absorption of the
capping agent, PVP on the {100} crystal planes of Cu
NWs. A graphical scheme for the formation of Cu NWs
is shown in Figure 7.

mechanism was proposed and summarized as follows: (1)partially reduction of Cu ions to Cu2O, (2) redissolution of
Cu2O and reduction to Cu0, (3) formation of Cu seeds, and(4) unidirectional growth of Cu0. The concentration of Cl−
ions and the dissolved oxygen was found vital to control
the etching rate of copper seeds. Furthermore, the reaction
temperature was important to obtain copper seeds in large
quantities, which get etched subsequently to form and control the number of MTPs. Finally, the PVP concentration
was a decisive factor that facilitates the unidirectional growthof the MTPs through speciﬁc adsorption on the {100} planes
to inhibit the lateral growth of the NWs. The NWs obtained
under the optimum conditions derived in this study resulted
in the synthesis of Cu NWs that were more 40 microns inlength and 60 nm in diameter or aspect ratio higher than
667 that could become a potential material for the fabricationof conducting ﬁlms.

Data Availability
4. Conclusions
The synthesis of Cu nanowires (NWs) in high yield has beendeveloped using Cu(NO3)2-EG-PVP system. A formation

The data used to support the ﬁndings of this study areincluded within the article and as well as in the supplementary materials.

Journal of Nanomaterials

9

Conflicts of Interest

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The authors declare that there is no conﬂict of interest
regarding the publication of this paper.

Acknowledgments
This study was supported by Grant-in Aid for ScientiﬁcResearch (B) 16H03843 and (C) 16K04891 from the Ministryof Education, Science, Culture and Sport of Japan.

Supplementary Materials
Figures S1–S4: comprehensive analysis of the formation of
Cu2O and CuCl particles and the eﬀect of halide sources
and reaction temperature. (Supplementary Materials)

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