1. Colloids and Surfaces A: Physicochem. Eng. Aspects 466 (2015) 189–196
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Colloids and Surfaces A: Physicochemical and
Engineering Aspects
journal homepage: www.elsevier.com/locate/colsurfa
Preparation of stable sub 10 nm copper nanopowders redispersible in
polar and non-polar solvents
Partha Pratim Chowdhury, Aabid Hussain Shaik, Jayanta Chakraborty∗
Department of Chemical Engineering, IIT Kharagpur, 721302, India
h i g h l i g h t s
• Sub 10 nm PVP–PEG protected cop-
per nanoparticles have been pre-
pared.
• Centrifugation and ethanol precip-
itation employed to prepare Cu
nanopowder.
• Cu nanopowder was redispersed in
water, DMF, DMSO, chloroform.
• Concentrated Cu colloid has been
successfully phase transferred.
• Toluene soluble Cu nanopowders
have been successfully produced.
g r a p h i c a l a b s t r a c t
a r t i c l e i n f o
Article history:
Received 14 June 2014
Received in revised form
25 September 2014
Accepted 16 October 2014
Available online 23 October 2014
Keywords:
Nanopowder
Copper nanoparticles
Polymer stabilized copper nanoparticles
Phase transfer
Centrifugation
a b s t r a c t
In this work we prepare 10 nm copper nanopowders redispersible in various polar and non-polar solvents.
First, concentrated colloid of copper nanoparticles protected by PVP–PEG is synthesized by a simple wet
chemical method using hydrazine hydrate as reducing agent. Extremely stable copper nano-powder con-
taining sub 10 nm particles can be readily prepared by drying this colloid. This powder readily disperses
in water, DMF, DMSO and choloroform and produces a stable colloid. To increase the copper content
of the powder, washing of the colloid using (i) high speed centrifugation and (ii) ethanol addition and
centrifugation at lower speed have been successfully conducted. The high speed centrifugation increases
the size of the colloid to 50 nm after re-suspension while the size remains sub-10 nm when washing is
conducted using ethanol. Particles were also phase transferred successfully from the aqueous polymeric
solution to toluene using a protocol previously used for gold nano-rods. Many other simpler phase trans-
fer techniques have also been tried but such methods failed to transfer particles from such polymer loaded
colloid efficiently. Stable organic dispersible nanopowders have been produced from the organosol. The
particle size is preserved after phase transfer and redispersion in organic solvent.
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
Nanoparticles have become a popular product in the market
for a variety of applications such as catalysis [1], biosensors [2],
∗ Corresponding author. Tel.: +91 3222283950.
E-mail addresses: ppc.sodepore@gmail.com (P.P. Chowdhury),
aabid.iitkgp@iitkgp.ac.in (A.H. Shaik), jayanta@che.iitkgp.ernet.in (J. Chakraborty).
optoelectronics [3], absorbents [4] etc. Copper nanoparticles are of
special interest because of its favourable properties and low cost. It
has applications in various fields such as ink jet printing technology
[5], catalysis [6], medicine [7], antimicrobial agents [8], etc. While
a lot of other nanoparticles are available as nanopowder, copper
nanoparticles are mostly obtained as colloidal solution. Literature
on production of copper nanopowder is very sparse. The aim of
this article is to demonstrate production of stable, re-dispersible
sub-10 nm copper nano-powders.
http://dx.doi.org/10.1016/j.colsurfa.2014.10.031
0927-7757/© 2014 Elsevier B.V. All rights reserved.

2. 190 P.P. Chowdhury et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 466 (2015) 189–196
Nanopowders have been produced for a variety of materi-
als using a number of techniques. Majority of the materials are
oxide since they already have market for bulk applications. For
example, TiO2 nanopowder finds applications in solar cells and
is produced in larger amount. Other oxide nanopowders such as
ZnO, CdO and NiO are also produced in large amount. Mainly com-
bustion [9], high energy ball milling [10] and sol–gel process [11]
are used for production of such oxide nanopowders. RF plasma
[12], laser ablation [13] and solution precipitation methods [14]
are also employed for production of oxide nanopowders. Unlike
oxide materials, production of metal nanopowders is not so com-
mon. Probably because many metals form oxide when in contact
with air. Nevertheless, some metal nanopowder like gold [15],
silver [16], copper [17,18] and aluminium [19] have been pre-
pared.
In most cases nanopowders contain aggregates with primary
particles in the range of 30–100 nm [12]. In some cases, however,
well separated particles have been produced [16,20]. Even for such
cases, mostly physical methods have been followed. One particular
case that needs special mention here is the particles synthesized
using Brust’s method [15]. Here small 2–4 nm thiol coated very
stable gold or silver nano-powder can be prepared using chemi-
cal method. Gautam et al. [21] also produced silver nanopowder
using polyol process.
Although a large number of synthesis protocols exist for
preparation of copper colloid containing a range of particle size,
preparation of copper nanopowder is rarely reported. Song et al.
[18] prepared organic solvent compatible thiol coated copper
nanopowder of ∼60 nm size using Brust’s method [15]. Nekouei
et al. [17] reported copper nanopowder using electrochemical
method. In the later work, particles mostly exist in the form
of agglomerates. Sub-10 nm well separate, re-dispersible copper
nanopowder has not been reported so far.
In this paper, we demonstrate for the first time preparation of
polymer (PVP–PEG) stabilized copper nanopowders that can be
redispersed in various solvents. The relative role of the polymers in
the stability and the threshold concentration required has also been
explored. Washing of the hydrosol has also been studied using var-
ious methods and the re-dispersibility and size of the redispersed
sol has been investigated. The excess polymer has been recycled to
produce fresh copper hydrosol.
Another important aspect explored in this work is the phase
transfer of polymer stabilized copper hydrosol. Although phase
transfer of copper nanoparticles have been studied recently [22,23],
phase transfer of polymer stabilized particles has not been studied.
Such phase transfer is challenging because of higher particle and
polymer loading of the aqueous system. In this work we demon-
strate phase transfer of polymer stabilized concentrated hydrosol
to organic phase and also produce organic phase compatible copper
nanopowder of sub 10 nm size.
2. Experimental
2.1. Materials required
Copper chloride dihydrate (CuCl2·2H2O), hydrazine hydrate
(80%) (N2H4·H2O), polyethylene glycol (MW-6000), ammonia solu-
tion (25% pure), dimethyl sulfoxide (DMSO) and toluene were
purchased from Merck Chemicals, India. Polyvinylpyrrolidone
(K30, MW-40000) was bought from SRL Chemicals, India. Mer-
captosuccnic acid (MSA) was purchased from LOBA Chemicals,
India. Tetraoctylammoniumbromide (TOAB) was purchased from
Sigma–Aldrich, USA. 1-Dodecanethiol (DDT) was obtained from
SD Fine Chemicals, India. Ethanol (AR 99.9%) was purchased from
Jiangsu Huaxi International, China. All the chemicals were used as
received. Nitrogen/argon gas with less than 50 ppm impurity was
used for degassing the solvents.
2.2. Deoxygenation of solvents
Solvents used in the synthesis were deoxygenated by purging
with argon or nitrogen in a conical flask for 30 min.
2.3. Synthesis of copper nanoparticles in aqueous phase
Copper nanoparticles were prepared by modifying a protocol
given by Tian et al. [24]. In a typical synthesis, 4.8 g PEG and 3.6 g PVP
were added to 40 ml of double distilled water (deoxygenation was
not required) under vigorous stirring until complete dissolution.
Then 0.136 g CuCl2·2H2O was added to the solution. This solution
showed a pale blue colour. Next, ammonia solution was added to
the above solution drop wise under vigorous stirring until the pH
of the solution reached 11. The colour of the solution changed to
inkish blue after this step. Then the solution was heated to 50 ◦C
in a water bath and maintained at that temperature for 2 h under
continuous stirring. The colour of the solution turned muddy brown
gradually. Then 20 ml of 0.3 M hydrazine hydrate was added for
reduction. The solution turned colourless immediately and then
became yellowish orange. Later the temperature was changed to
60 ◦C after hydrazine hydrate addition. The stirring was continued
for another 20 min during which the solution turned light reddish
and finally became deep wine red colour indicating the formation
of copper nanoparticles.
2.4. Washing and redispersion of copper nanoparticles
The colloid with surfactant can be dried by evaporating the
water either by mild heating or inert gas bubbling. To wash the
surfactant, the colloids were centrifuged at a speed of 11,500 rpm
for 30 min. After centrifugation, a clear supernatant was obtained
that was removed by decanting. Fresh degassed water was added to
the precipitate deposited at the bottom of the centrifuge tube and
redispersed using ultrasonication. This step was repeated twice to
obtain a shining copper coloured solid. This solid readily re-disperse
in deoxygenated water to form hydrosol.
Washing was also conducted using ethanol. Equal volumes of
colloid and ethanol were mixed and the mixture was allowed to
stand overnight. Then it was centrifuged at a speed of 7000 rpm
for 30 min. This also produced shining copper coloured solid which
readily re-disperse in to deoxygenated water.
2.5. Recycling of polymers
The supernatant obtained from centrifugation step contains
excess of ammonium hydroxide and hydrazine hydrate. The liq-
uid was first aerated to decompose excess hydrazine hydrate. The
presence of hydrazine hydrate was judged by its reducing action
on copper salt. After sufficient aeration, the solution is poured on
a petri dish and kept in hot air oven for a day at 80 ◦C until the
evaporation of solvent. A uniform coating of polymer was formed
in the petri dish. This polymer was considered to be a mixture of
PVP and PEG at the proportion they were fed to the synthesis mix-
ture. This polymer was used in synthesis along with a make-up of
fresh polymer to account for the mass loss in reaction-purification
steps.
2.6. Phase transfer of PVP–PEG stabilized copper nanoparticles to
toluene and formation of organic dispersible nanopowder
2 ml of 26 mM aqueous solution of mercaptosuccinic acid (MSA)
was added to 2 ml Cu hydrosol in a glass vial and the vial was

3. P.P. Chowdhury et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 466 (2015) 189–196 191
gently shaken for about 2 min. Then 1 ml 50 mM tetraoctylammo-
nium bromide (TOAB) in degassed toluene was added to the copper
hydrosol and shaken vigorously for another 2 min. The toluene
phase becomes reddish indicating the transfer of copper nanoparti-
cles to toluene. The aqueous phase retained a faint colour indicating
that the phase transfer is not 100% efficient.
To produce organic dispersible nano-powder, the solvent was
evaporated using inert gas bubbling. Then the resulting waxy mix-
ture was stored in the vacuum dessicator until the smell of toluene
was absent. This remains a dark brownish waxy solid. It immedi-
ately re-dispersed into toluene to give a brownish red colloid.
2.7. Characterization
The surface plasmon resonance of copper nanoparticles was
characterized using UV-Visible spectrophotometer (Shimadzu UV-
1800). The size and morphology of copper sols were characterized
by transmission electron microscope (Tecnai G2 20S Twin). The size
distribution of the sols is characterized by dynamic light scattering
(Model: Malvern Zetasizer Nano ZS). The oxidation behaviour of
copper nanopowder was characterized by using X-ray diffraction
technique (Model: X-ray Diffractometer PW-17291710).
3. Results and discussion
Stable copper nanoparticles can be prepared in aqueous phase
by charge stabilization or steric stabilization or a combination of
both. However, for production of nano-powder, concentrated cop-
per sol in aqueous phase with very good stability is required and
polymeric stabilization is the only feasible option in such a case. In
this work, copper nanoparticles have been prepared by modifica-
tion of standard protocols as detailed in the experimental section.
Usually, for stable concentrated copper sol, a surfactant (e.g. CTAB)
and/or a polymer (PVP) are used as protective ligands [24,25]. Use
of two protective ligands seems to be beneficial for forming small
stable particles [24]. We also used similar strategy, but a pair of
polymers, PVP–PEG has been used instead of polymer–surfactant
pair to obtain small stable particles at a copper salt concentration
of 0.02 M. The concentrations of polymers were maintained at the
same level as reported by other investigators [26–28].
In the following sections we shall discuss various properties of
this colloid and the powder formed from this colloid. In this work
‘stability’ refers to the tendency of particles to resist aggregation,
oxidation resistance refers to resistance of particles against oxida-
tion of surface and re-dispersibility refers to the ability of particles
to form a colloid after formation of dry powder. Size may or may
not be preserved during this process.
Fig. 1 shows the UV–vis spectra of PVP–PEG stabilized copper
nanoparticles. These particles show a prominent UV–vis peak at
∼571 nm immediately after synthesis which signifies the presence
of copper nanoparticles. The particles are very stable and remain
suspended even after two months. These sols have also been char-
acterized using dynamic light scattering (DLS) and transmission
electron microscopy (TEM). DLS show a particle size of 14 nm but
TEM shows sub 10 nm particles (Fig. 2(A)), possibly due to the pres-
ence of polymeric chains on particles. It can be noted that if the
sample is drop casted on the carbon coated copper grid and dried,
the polymer layer becomes very thick and forms a visually opaque
layer. The contrast becomes very low (Fig. S1) for such samples and
to increase the contrast we ‘wicked’ the sample soon after drop
casting using a filter paper. This wicking step increased the contrast
to an acceptable level as can be seen in Fig. 2(A). The two months
old sample show a slight shift in plasmon peak to 574 nm and the
particle size becomes 50 nm as seen in TEM micrographs (Fig. S3
in Supporting information). Since production of nanopowder is the
Fig. 1. UV–vis spectra of PVP–PEG stabilized Cu nanoparticles immediately after
synthesis and after 2 months.
main aim of, aggregation over such long time scale is not a matter
of concern here.
3.1. Production of aqueous dispersible nanopowder
The amount of polymers used in this synthesis protocols is very
high. Other synthesis protocols also use such a large excess of poly-
mer or surfactant to synthesize copper sol at high concentration.
The protocol given by Tian et al. [24] uses a mass ratio of stabiliz-
ing agent to copper as 93. Similar trends have been observed for
many other protocols as summarized in Table 1. In our case, such
high proportion of polymer offers very good stability and small size:
these colloids can be dried and re-dispersed without affecting the
particle size (results not shown). However, the powder is mostly
the polymers. If nano-powder for commercial use is needed, the
amount of polymer per gram of metallic copper must be reduced.
One of the ways to reduce the amount of polymer in the sol is by
washing. Washing can be conducted using two different methods:
(i) high speed centrifugation and (ii) using precipitating agent along
with centrifugation at a lower speed and re-dispersed the washed
colloid to a variety of solvents.
First, washing without any precipitating agent was tried. The
colloid was washed twice using centrifugation at 11,500 rpm for
30 min and the precipitate was separated from the suspension.
After each wash a clear, colourless supernatant was obtained. The
supernatant was stored for recycling of the polymer. The sediment
can be dried using a jet of nitrogen gas to form the powder which
can readily be re-suspended in fresh degassed water by ultrasoni-
cation. Hence, purified, re-suspendable copper nano-powder could
be produced. The particles are completely re-dispersible in water
even after two washes. More number of washes were unnecessary
as >90% of polymer was removed by the two washes. The particles
settling at the bottom of the centrifuge tube show a shining metal-
lic copper colour (Fig. 3(A)) after the second wash. However, the
particle size in washed colloid increased to ∼50 nm after the first
wash and remains the same after second wash as shown in Fig. 2(B).
Some nanorods with large aspect ratio were also observed (Fig. S2)
in the washed samples.
On the other hand, if the particles are precipitated using ethanol
followed by centrifugation at lower speed (∼8000 rpm), the parti-
cle size remains unaltered after re-dispersion as shown in Fig. 2(C).
Very high centrifugal field is known to induce aggregation in par-
ticles [29] and hence it may be concluded that the formation of

4. 192 P.P. Chowdhury et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 466 (2015) 189–196
Fig. 2. TEM images of (A) unwashed PVP–PEG stabilized copper nanoparticles, (B) twice washed PVP–PEG stabilized copper nanoparticles: washed using centrifugation, (C)
twice washed PVP–PEG stabilized copper nanoparticles using ethanol via centrifugation and (D) recycled polymer stabilized Cu nanoparticles: twice washed using ethanol
via centrifugation.
larger particles in the previous case was instigated by high speed
centrifugation and not due to removal of polymers.
As expected, the stability and oxidation resistance of the washed
hydrosol is poorer than the original colloid for both types of wash-
ing procedures. The original sol contains excess hydrazine hydrate
which produces N2 gas and also contains a hyper excess of poly-
mers which make the sol very stable and resistant to oxidation. The
washed sol has less of these two components and hence less sta-
ble. However, the oxidation of the washed sol can be minimized if
they are dried with nitrogen immediately after production and in
an airtight bottle. It may be noted that the original unwashed sol
can be stored in ordinary vials which are not perfectly air tight.
The washed and re-dispersed sols form flock after a day but
the flock structure can be broken by ultrasonication. The ultraso-
nicated sol shows the same character as the original washed sol as
measured by UV and DLS. However, in general it seems that the
re-dispersed sol is not suitable for a prolonged storage but rather
for immediate use. On the other hand, the powder obtained (as
shown in Fig. 3) can be stored in a sealed bottle for a prolonged
period (more than a month) without any degradation in quality as
judged by UV–vis peak and DLS size. It has been observed that the
copper nano-powder is not prone to oxidation except in a moist
environment.
Copper nanopowder obtained after washing and drying is
readily redispersed in a variety of solvents such as chloroform, DMF,
DMSO and ethanol. Copper nanopowders redispersed in these sol-
vents shows the prominent peak of copper nanoparticles as shown
in Fig. S6.
It can be seen that huge portion of polymer is wasted in the
supernatant during washing step. The amount of polymer in the
Table 1
The data on polymer stabilized copper nanoparticles. The third column shows the range of mass ratio of polymeric stabilizing agent to copper and the last column shows the
corresponding range of particle size. As the mass ratio increases, the particle size decreases.
References Stabilizing agent Mass ratio of stabilizing agent and copper Size of Cu nanoparticles (nm)
Qing-ming et al. [30] PVP 47.214 126–375 (depending on other parameters)
Wu et al. [28] PVP 47.74 3.4
Dang et al. [31] PVP 62.95–188.85 72–6
Lai et al. [27] PVP 21–173.22 150–80
Dang et al. [26] PEG 566.39–849.59 28–4
This work PVP+PEG 1.65–165.68 200–2

5. P.P. Chowdhury et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 466 (2015) 189–196 193
Fig. 3. (A) Precipitated copper nanoparticles by centrifugation. (B) Redispersion of precipitated copper nanoparticles by ultrasonication. (C) Fully redispersed copper
nanoparticles. (D) Optical microscopic image of copper nanopowder.
supernatant can be quantified by evaporating the water slowly.
The excess hydrazine and ammonia also evaporates during this
prolonged storage at relatively high temperature (70 ◦C). Results
from such experiments show that approximately 90% of the poly-
mers can be recovered. Such polymers can be recycled for synthesis
(along with a make-up amount of fresh polymer) of particles and
found to produce particles similar to those produced by fresh poly-
mer as shown in Fig. 2(D). Hence, the recycled polymers can be used
for synthesis.
3.2. Oxidation resistance of copper nanopowder
To understand the oxidation resistance of the nano-powder pro-
duced, we performed XRD study of the dry nanopowder stored in a
sealed vessel as well as those exposed to air overnight. These results
are shown in Fig. 4. It can be seen that the nanopowder stored in
a sealed vessel did not show any oxide peak. On the other hand,
nanopowder exposed to air shows a peak which correspond to 1 1 1
face of Cu2O [32,33,34]. It may be noted that even in air exposed
powder, only a small portion get oxidized.
3.3. Quality of particles produced by lesser amount of polymers
It is not clear why such high proportion of polymer is needed
during the synthesis. To test quantitative effects, we reduce both
the polymers concentrations by a factor of 10 (by keeping the ratio
fixed). In this case, stable colloid is formed with some amount
of precipitated mass. However, upon examination under TEM, we
found large aggregates of size ∼200 nm (not shown) are present
in the colloid. Further reduction of overall polymer concentration
leads to unstable colloid. This data suggest that such high concen-
tration of polymer is indeed needed.
If we keep PEG concentration at the initial higher level and
reduce PVP concentration by a factor of 10, we obtain a stable sol
with very little precipitation. While the precipitate is of micron
size, the dispersed particles are sub 10 nm as shown in Fig. 5. If
the PVP concentration is maintained at the initial high value and
PEG concentration is reduced by a factor of 10, around 20 nm par-
ticles (DLS) are formed. The stability of various PVP–PEG stabilized
copper nano colloids are summarized in Table 2. It is clear from
the table that the two polymers have different role in producing
a stable colloid of small size. While PEG is vital for stability (row
5 of Table 2), PVP is required to reduce the particle size to sub
10 nm. It is also clear that a major portion of polymers used during
synthesis are not adsorbed on the surface of nanoparticles. Hence,
unadsorbed polymers provide good stability and dispersibility by
80604020
Copper nanoparticles before exposure to air
Copper nanoparticles after exposure to air
Intensity(a.u)
2-theta
Cu (111)
Cu (200)
Cu (220)Cu2O (111)
Fig. 4. X-ray diffraction of copper nanopowder stored in sealed environment (solid
line) and exposed to air (dashed line).

6. 194 P.P. Chowdhury et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 466 (2015) 189–196
Table 2
The stability of copper nanoparticles in aqueous phase with respect to polymer concentrations.
Number of washings PVP concentration used in synthesis PEG concentration used in synthesis Oxidation resistance Flocculation time
0 Original concentration Original concentration Months Months
1 Original concentration Original concentration 1 month 1 month
2 Original concentration Original concentration Days 1 week
0 Reduced 10 times Original concentration Months 1 month
0 Reduced 100 times Original concentration Months Weeks
0 Reduced 10 times Reduced 10 times Months Days
0 Reduced 100 times Reduced 100 times Unstable Unstable
the mechanism of ‘depletion stabilization’ [35,36]. “Depletion sta-
bilization” is a lesser known route for colloidal stability and occurs
at a much higher concentration than required for depletion floccu-
lation. Typical, polymer concentration for depletion flocculation is
0.05–0.2% whereas the typical concentration used for polymer sta-
bilized copper nanoparticles is ∼14%. Depletion stabilization occurs
due to the depletion of concentration of free polymers between the
surfaces of the approaching particles. This justifies the use of high
concentration of polymers in the synthesis process.
Fig. 5. TEM image of copper nanoparticles produced by 10 times reduction of PVP
concentration.
3.4. Production of organic dispersible nano-powder
Next we try to phase transfer the copper hydrosol to organic
phase. There are a variety of phase transfer methods to transfer
nanoparticles from aqueous to organic phase [37]. The simplest
one is the replacement of the surface ligand through a biphasic
mixture. Simple shaking the biphasic mixture make a coating of
hydrophobic surfactant on the particle at the interface and facili-
tate the phase transfer. We tried such methods for the unwashed
and washed colloids but we could not phase transfer the particles
using this method. It was observed that most of the particles aggre-
gated at the interface which should be due to partial coating by thiol
and retention of charge on particle surface.
Next we tried ethanol mediated phase transfer. In this case, the
hydrophobic ligand (dodecanethiol) is added to the hydrosol with
ethanol so that it creates a single phase. This results in better possi-
bility of thiol coating on the particles [38]. Even this method failed
to phase transfer the polymer stabilized sol. Here also the parti-
cles aggregated at the interface and stick to walls of the vial. The
same result was obtained even with other hydrophobic ligands like
octadecanethiol (ODT) [39] and dodecylamine (DDA)[38].
Next we explore another protocol previously used for trans-
ferring gold nanoparticles and nanorods [40,41]. First, a thio-acid,
marcaptosuccinic acid, was added to the hydrosol which coats a
layer on the particles. Then the organic phase (toluene) contain-
ing TOAB is used for phase transfer by a mechanism known as ‘ion
pair formation through electrostatic interaction’. It may be noted
that the particles could be transferred without any washing. TEM
images of the nanoparticles transferred to toluene are shown in
Fig. 6(A). It can be seen that the particles retain their size (<10 nm)
during phase transfer. However a few large particles have also been
observed in the phase transferred particles (not shown).
Organic dispersible nanopowder can be prepared by drying the
organosol using an inert gas. It may be noted that if the organosol
Fig. 6. TEM images of (A) unwashed PVP–PEG stabilized copper nanoparticles phase transferred to toluene. (B) Redispersed copper nanoparticles in toluene after drying the
copper organosol by inert gas.

7. P.P. Chowdhury et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 466 (2015) 189–196 195
is heated for drying, it becomes slightly unstable and the particle
size increases from 10 nm to 50 nm in such cases. The ‘powder’ in
this case is a waxy mass. This powder can be re-dispersed into fresh
degassed toluene by gently shaking for a minute. The TEM of redis-
persed colloid is shown in Fig. 6(B). It can be observed from Fig. 6(B)
that the particles remain sub 10 nm.
4. Conclusions
Stable copper hydrosol of small particle size (2–7 nm) has been
prepared at high concentration (0.013 M) by modifying a reported
protocol. The colloids are stable for months showing negligible
change in UV–vis spectra when stored in a capped vial which
signify strong resistance of these particles towards oxidation. It
has been shown that for preparation of such concentrated hydro-
colloid, hyper excess of polymer (polymer to Cu ratio ∼ 100:1) is
needed. Lesser amount of polymers produces an unstable colloid or
larger particles. Depletion stabilization has been speculated as pos-
sible reason for requirement of hyper excess polymer. The powder
obtained by drying the colloid can be readily re-dispersed into vari-
ety of solvents including water, DMF, DMSO, chloroform. However,
the copper content of the powder is very less (1%).
Nanopowder containing much higher proportion of copper has
been obtained by washing the nano-colloid using precipitating
agent and/or centrifugation. Centrifugation alone produces parti-
cles of larger size (∼50 nm). A few nano-rods of length about 200 nm
and aspect ratio 7 are also observed. Ethanol precipitated powders,
however, preserved the original size. The washed nanopowders
could be redispersed into water and other polar solvents easily.
The polymer obtained in the supernatant could be recycled for pro-
duction of nano-colloid. Such colloids do not show any significant
difference from those synthesized by fresh polymers.
The particles produced could be transferred to toluene using
a phase transfer protocol previously used for nanorods. This
phase transfer protocol uses mercaptosuccinic acid (MSA) and
tetraoctylammonium bromide (TOAB) for phase transfer. Other
well practised phase transfer protocols were unsuccessful for
this case. The phase transferred particles could be dried and re-
dispersed readily in toluene. The particle size remains preserved
during drying and redispersion.
Acknowledgements
We thank and acknowledge SRIC (ISIRD Grant), IIT Kharagpur
for financial assistance and Prof. S. De for allowing us to perform
DLS measurements. We also thank Mr. Sudipto Mondol, Central
Research Facility for his help in characterizing the copper nanopar-
ticles using TEM.
Appendix A. Supplementary data
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/j.colsurfa.
2014.10.031.
References
[1] D.T. Thompson, Using gold nanoparticles for catalysis, Nano Today 2 (2007)
40–43.
[2] K. Saha, S.S. Agasti, C. Kim, X. Li, V.M. Rotello, Gold nanoparticles in chemical
and biological sensing, Chem. Rev. 112 (2012) 2739–2779.
[3] K. Tanabe, Optical radiation efficiencies of metal nanoparticles for optoelec-
tronic applications, Mater. Lett. 61 (2007) 4573–4575.
[4] L. Feng, M. Cao, X. Ma, Y. Zhu, C. Hu, Superparamagnetic high-surface-area
Fe3O4 nanoparticles as adsorbents for arsenic removal, J. Hazard. Mater.
217–218 (2012) 439–446.
[5] Y. Lee, J.-R. Choi, K.J. Lee, N.E. Stott, D. Kim, Large-scale synthesis of copper
nanoparticles by chemically controlled reduction for applications of inkjet-
printed electronics, Nanotechnology 19 (2008) 415604.
[6] Z. Huang, F. Cui, H. Kang, J. Chen, X. Zhang, C. Xia, Highly dispersed silica-
supported copper nanoparticles prepared by precipitation-gel method: a
simple but efficient and stable catalyst for glycerol hydrogenolysis, Chem.
Mater. 20 (2008) 5090–5099.
[7] J. Bhattacharya, U. Choudhuri, O. Siwach, P. Sen, A.K. Dasgupta, Interaction
of hemoglobin and copper nanoparticles: implications in hemoglobinopathy,
Nanomedicine 2 (2006) 191–199.
[8] A. Esteban-Cubillo, C. Pecharromán, E. Aguilar, J. Santarén, J.S. Moya, Antibac-
terial activity of copper monodispersed nanoparticles into sepiolite, J. Mater.
Sci. 41 (2006) 5208–5212.
[9] M.S. Nguyen, V.T. Ho, N.T.T. Pham, The synthesis of BaMgAl10O17:Eu2+
nanopowder by a combustion method and its luminescent properties, Adv.
Nat. Sci.: Nanosci. Nanotechnol. 2 (2011) 045005, 4 pp.
[10] S. Zouari, M. Ellouze, A. Nasri, W. Cherif, E.K. Hlil, F. Elhalouani, Morphol-
ogy, structural, magnetic, and magnetocaloric properties of Pr0.7Ca0.3MnO3
nanopowder prepared by mechanical ball milling method, J. Supercond. Novel
Magn. 27 (2013) 555–563.
[11] S. Hu, F. Li, Z. Fan, Enhanced visible light activity and stability of TiO2 nanopow-
der by co-doped with Mo and N, Bull. Korean Chem. Soc. 33 (2012) 1269–1274.
[12] K.-S. So, H. Lee, T.-H. Kim, S. Choi, D.-W. Park, Synthesis of silicon nanopowder
from silane gas by RF thermal plasma, Phys. Status Solidi A 211 (2014) 310–315.
[13] A.P. Safronov, O.M. Samatov, A.I. Medvedev, I.V. Beketov, A.M. Murzakaev, Syn-
thesis of strontium hexaferrite nanopowder by the laser evaporation method,
Nanotechnol. Russ. 7 (2012) 486–491.
[14] H. Ashassi-Sorkhabi, D. Seifzadeh, M. Raghibi-Boroujeni, Analysis of electro-
chemical noise data in both time and frequency domains to evaluate the effect
of ZnO nanopowder addition on the corrosion protection performance of epoxy
coatings, Arab. J. Chem. (2012), http://dx.doi.org/10.1016/j.arabjc.2012.02.018.
[15] M. Brust, M. Walker, D. Bethell, D.J. Schiffrin, R. Whyman, Synthesis of thiol-
derivatised gold nanoparticles in a two-phase liquid–liquid system, J. Chem.
Soc., Chem. Commun. 80 (1994) 801–802.
[16] J.-W. Song, D.-J. Lee, F. Yilmaz, S.-J. Hong, Effect of variation in voltage on the
synthesis of Ag nanopowder by pulsed wire evaporation, J. Nanomater. 2012
(2012) 1–5.
[17] R.K. Nekouei, F. Rashchi, A. Ravanbakhsh, Copper nanopowder synthesis by
electrolysis method in nitrate and sulfate solutions, Powder Technol. 250
(2013) 91–96.
[18] X. Song, S. Sun, W. Zhang, Z. Yin, A method for the synthesis of spherical copper
nanoparticles in the organic phase, J. Colloid Interface Sci. 273 (2004) 463–469.
[19] A. Sossi, E. Duranti, C. Paravan, L.T. DeLuca, A.B. Vorozhtsov, A.A. Gromov, Y.I.
Pautova, M.I. Lerner, N.G. Rodkevich, Non-isothermal oxidation of aluminum
nanopowder coated by hydrocarbons and fluorohydrocarbons, Appl. Surf. Sci.
271 (2013) 337–343.
[20] Y.S. Kim, L.T. Linh, E.S. Park, S. Chin, G.-N. Bae, J. Jurng, Antibacterial performance
of TiO2 ultrafine nanopowder synthesized by a chemical vapor condensation
method: effect of synthesis temperature and precursor vapor concentration,
Powder Technol. 215–216 (2012) 195–199.
[21] A. Gautam, G.P. Singh, S. Ram, A simple polyol synthesis of silver metal
nanopowder of uniform particles, Synth. Met. 157 (2007) 5–10.
[22] M.K. Niranjan, J. Chakraborty, Synthesis of oxidation resistant copper nanopar-
ticles in aqueous phase and efficient phase transfer of particles using
alkanethiol, Colloids Surf. A: Physicochem. Eng. Aspects 407 (2012) 58–63.
[23] A.H. Shaik, J. Chakraborty, Use of repeated phase transfer for preparation of thiol
coated copper organosols at higher particle loading, Colloids Surf. A: Physic-
ochem. Eng. Aspects 454 (2014) 46–56.
[24] K. Tian, C. Liu, H. Yang, X. Ren, In situ synthesis of copper nanoparti-
cles/polystyrene composite, Colloids Surf. A: Physicochem. Eng. Aspects 397
(2012) 12–15.
[25] L.Q. Pham, J.H. Sohn, C.W. Kim, J.H. Park, H.S. Kang, B.C. Lee, et al., Copper
nanoparticles incorporated with conducting polymer: effects of copper con-
centration and surfactants on the stability and conductivity, J. Colloid Interface
Sci. 365 (2012) 103–109.
[26] T.M.D. Dang, T.T.T. Le, E. Fribourg-Blanc, M.C. Dang, Synthesis and optical prop-
erties of copper nanoparticles prepared by a chemical reduction method, Adv.
Nat. Sci.: Nanosci. Nanotechnol. 2 (2011) 015009, 6 pp.
[27] D. Lai, T. Liu, G. Jiang, W. Chen, Synthesis of highly stable dispersions of cop-
per nanoparticles using sodium hypophosphite, J. Appl. Polym. Sci. 128 (2012)
1443–1449.
[28] C. Wu, B.P. Mosher, T. Zeng, One-step green route to narrowly dispersed copper
nanocrystals, J. Nanopart. Res. 8 (2006) 965–969.
[29] B.-S. Yin, J.-Q. Hu, S.-Y. Ding, A. Wang, J.R. Anema, Y.-F. Huang, et al., Identifying
mass transfer influences on Au nanoparticles growth process by centrifugation,
Chem. Commun. 48 (2012) 7353–7355.
[30] Q. Liu, T. Yasunami, K. Kuruda, M. Okido, Preparation of Cu nanoparticles with
ascorbic acid by aqueous solution reduction method, Trans. Nonferrous Met.
Soc. China 22 (2012) 2198–2203.
[31] T.M.D. Dang, T.T. Tuyet Le, E. Fribourg-Blanc, M. Chien Dang, The influence of
solvents and surfactants on the preparation of copper nanoparticles by a chem-
ical reduction method, Adv. Nat. Sci.: Nanosci. Nanotechnol. 2 (2011) 025004,
7 pp.
[32] M. Grouchko, A. Kamyshny, S. Magdassi, Formation of air-stable copper–silver
core–shell nanoparticles for inkjet printing, J. Mater. Chem. 19 (2009)
3057–3062.

8. 196 P.P. Chowdhury et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 466 (2015) 189–196
[33] C.-H. Chen, T. Yamaguchi, K. Sugawara, K. Koga, Role of stress in the self-limiting
oxidation of copper nanoparticles, J. Phys. Chem. B 109 (2005) 20669–20672.
[34] M. Mansour, L. Favergeon, M. Pijolat, Kinetic modeling of low tempera-
ture oxidation of copper nanoparticles by O2, Thermochim. Acta 570 (2013)
41–50.
[35] R.I. Feigin, D.H. Napper, Depletion stabilization and depletion flocculation, J.
Colloid Interface Sci. 75 (1980) 525–541.
[36] R.J. Hunter, Foundations of Colloid Science, second ed., Oxford University Press,
New York, 2001.
[37] A. Kumar, H. Joshi, R. Pasricha, A.B. Mandale, M. Sastry, Phase transfer of silver
nanoparticles from aqueous to organic solutions using fatty amine molecules,
J. Colloid Interface Sci. 264 (2003) 396–401.
[38] J. Yang, J.Y. Lee, H.-P. Too, A general phase transfer protocol for synthesiz-
ing alkylamine-stabilized nanoparticles of noble metals, Anal. Chim. Acta 588
(2007) 34–41.
[39] N. Lala, S.P. Lalbegi, S.D. Adyanthaya, M. Sastry, Phase transfer of aqueous gold
colloidal particles capped with inclusion complexes of cyclodextrin and alka-
nethiol molecules into chloroform, Langmuir 17 (2001) 3766–3768.
[40] J. Yang, J.-C. Wu, Y.-C. Wu, J.-K. Wang, C.-C. Chen, Organic solvent dependence
of plasma resonance of gold nanorods: a simple relationship, Chem. Phys. Lett.
416 (2005) 215–219.
[41] H. Yao, O. Momozawa, T. Hamatani, K. Kimura, Phase transfer of gold nanopar-
ticles across a water/oil interface by stoichiometric ion pair formation, Bull.
Chem. Soc. Jpn. 73 (2000) 2675–2678.