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Colloids and Surfaces A: Physicochem. Eng. Aspects 466 (2015) 189–196

Contents lists available at ScienceDirect

Colloids and Surfaces A: Physicochemical andEngineering Aspects

journa l h om epage: www.elsev ier .com/ locate /co lsur fa

reparation of stable sub 10 nm copper nanopowders redispersible inolar and non-polar solvents

artha Pratim Chowdhury, Aabid Hussain Shaik, Jayanta Chakraborty ∗

epartment of Chemical Engineering, IIT Kharagpur, 721302, India

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 Cunanopowder.Cu nanopowder was redispersed inwater, DMF, DMSO, chloroform.Concentrated Cu colloid has beensuccessfully phase transferred.Toluene soluble Cu nanopowdershave been successfully produced.

g r a p h i c a l a b s t r a c t

r t i c l e i n f o

rticle history:eceived 14 June 2014eceived in revised form5 September 2014ccepted 16 October 2014vailable online 23 October 2014

eywords:anopowder

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 wetchemical 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 dispersesin water, DMF, DMSO and choloroform and produces a stable colloid. To increase the copper contentof the powder, washing of the colloid using (i) high speed centrifugation and (ii) ethanol addition andcentrifugation at lower speed have been successfully conducted. The high speed centrifugation increasesthe size of the colloid to 50 nm after re-suspension while the size remains sub-10 nm when washing is

opper nanoparticlesolymer stabilized copper nanoparticleshase transferentrifugation

conducted using ethanol. Particles were also phase transferred successfully from the aqueous polymericsolution 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 loadedcolloid efficiently. Stable organic dispersible nanopowders have been produced from the organosol. Theparticle size is preserved after phase transfer and redispersion in organic solvent.

© 2014 Elsevier B.V. All rights reserved.

. Introduction

Nanoparticles have become a popular product in the marketor 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),

abid.iitkgp@iitkgp.ac.in (A.H. Shaik), jayanta@che.iitkgp.ernet.in (J. Chakraborty).

ttp://dx.doi.org/10.1016/j.colsurfa.2014.10.031927-7757/© 2014 Elsevier B.V. All rights reserved.

optoelectronics [3], absorbents [4] etc. Copper nanoparticles are ofspecial interest because of its favourable properties and low cost. Ithas applications in various fields such as ink jet printing technology[5], catalysis [6], medicine [7], antimicrobial agents [8], etc. Whilea lot of other nanoparticles are available as nanopowder, copper

nanoparticles are mostly obtained as colloidal solution. Literatureon production of copper nanopowder is very sparse. The aim ofthis article is to demonstrate production of stable, re-dispersiblesub-10 nm copper nano-powders.

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Nanopowders have been produced for a variety of materi-ls using a number of techniques. Majority of the materials arexide since they already have market for bulk applications. Forxample, TiO2 nanopowder finds applications in solar cells ands produced in larger amount. Other oxide nanopowders such asnO, CdO and NiO are also produced in large amount. Mainly com-ustion [9], high energy ball milling [10] and sol–gel process [11]re used for production of such oxide nanopowders. RF plasma12], laser ablation [13] and solution precipitation methods [14]re also employed for production of oxide nanopowders. Unlikexide materials, production of metal nanopowders is not so com-on. Probably because many metals form oxide when in contactith air. Nevertheless, some metal nanopowder like gold [15],

ilver [16], copper [17,18] and aluminium [19] have been pre-ared.

In most cases nanopowders contain aggregates with primaryarticles in the range of 30–100 nm [12]. In some cases, however,ell separated particles have been produced [16,20]. Even for such

ases, mostly physical methods have been followed. One particularase that needs special mention here is the particles synthesizedsing Brust’s method [15]. Here small 2–4 nm thiol coated verytable gold or silver nano-powder can be prepared using chemi-al method. Gautam et al. [21] also produced silver nanopowdersing polyol process.

Although a large number of synthesis protocols exist forreparation of copper colloid containing a range of particle size,reparation of copper nanopowder is rarely reported. Song et al.18] prepared organic solvent compatible thiol coated copperanopowder of ∼60 nm size using Brust’s method [15]. Nekoueit al. [17] reported copper nanopowder using electrochemicalethod. In the later work, particles mostly exist in the form

f agglomerates. Sub-10 nm well separate, re-dispersible copperanopowder has not been reported so far.

In this paper, we demonstrate for the first time preparation ofolymer (PVP–PEG) stabilized copper nanopowders that can beedispersed in various solvents. The relative role of the polymers inhe stability and the threshold concentration required has also beenxplored. Washing of the hydrosol has also been studied using var-ous methods and the re-dispersibility and size of the redispersedol has been investigated. The excess polymer has been recycled toroduce fresh copper hydrosol.

Another important aspect explored in this work is the phaseransfer of polymer stabilized copper hydrosol. Although phaseransfer of copper nanoparticles have been studied recently [22,23],hase transfer of polymer stabilized particles has not been studied.uch phase transfer is challenging because of higher particle andolymer loading of the aqueous system. In this work we demon-trate phase transfer of polymer stabilized concentrated hydrosolo organic phase and also produce organic phase compatible copperanopowder of sub 10 nm size.

. Experimental

.1. Materials required

Copper chloride dihydrate (CuCl2·2H2O), hydrazine hydrate80%) (N2H4·H2O), polyethylene glycol (MW-6000), ammonia solu-ion (25% pure), dimethyl sulfoxide (DMSO) and toluene wereurchased from Merck Chemicals, India. PolyvinylpyrrolidoneK30, MW-40000) was bought from SRL Chemicals, India. Mer-aptosuccnic acid (MSA) was purchased from LOBA Chemicals,

ndia. Tetraoctylammoniumbromide (TOAB) was purchased fromigma–Aldrich, USA. 1-Dodecanethiol (DDT) was obtained fromD Fine Chemicals, India. Ethanol (AR 99.9%) was purchased fromiangsu Huaxi International, China. All the chemicals were used as

sicochem. Eng. Aspects 466 (2015) 189–196

received. Nitrogen/argon gas with less than 50 ppm impurity wasused for degassing the solvents.

2.2. Deoxygenation of solvents

Solvents used in the synthesis were deoxygenated by purgingwith 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 protocolgiven by Tian et al. [24]. In a typical synthesis, 4.8 g PEG and 3.6 g PVPwere added to 40 ml of double distilled water (deoxygenation wasnot required) under vigorous stirring until complete dissolution.Then 0.136 g CuCl2·2H2O was added to the solution. This solutionshowed a pale blue colour. Next, ammonia solution was added tothe above solution drop wise under vigorous stirring until the pHof the solution reached 11. The colour of the solution changed toinkish blue after this step. Then the solution was heated to 50 ◦Cin a water bath and maintained at that temperature for 2 h undercontinuous stirring. The colour of the solution turned muddy browngradually. Then 20 ml of 0.3 M hydrazine hydrate was added forreduction. The solution turned colourless immediately and thenbecame yellowish orange. Later the temperature was changed to60 ◦C after hydrazine hydrate addition. The stirring was continuedfor another 20 min during which the solution turned light reddishand finally became deep wine red colour indicating the formationof copper nanoparticles.

2.4. Washing and redispersion of copper nanoparticles

The colloid with surfactant can be dried by evaporating thewater either by mild heating or inert gas bubbling. To wash thesurfactant, the colloids were centrifuged at a speed of 11,500 rpmfor 30 min. After centrifugation, a clear supernatant was obtainedthat was removed by decanting. Fresh degassed water was added tothe precipitate deposited at the bottom of the centrifuge tube andredispersed using ultrasonication. This step was repeated twice toobtain a shining copper coloured solid. This solid readily re-dispersein deoxygenated water to form hydrosol.

Washing was also conducted using ethanol. Equal volumes ofcolloid and ethanol were mixed and the mixture was allowed tostand overnight. Then it was centrifuged at a speed of 7000 rpmfor 30 min. This also produced shining copper coloured solid whichreadily re-disperse in to deoxygenated water.

2.5. Recycling of polymers

The supernatant obtained from centrifugation step containsexcess of ammonium hydroxide and hydrazine hydrate. The liq-uid was first aerated to decompose excess hydrazine hydrate. Thepresence of hydrazine hydrate was judged by its reducing actionon copper salt. After sufficient aeration, the solution is poured ona petri dish and kept in hot air oven for a day at 80 ◦C until theevaporation of solvent. A uniform coating of polymer was formedin the petri dish. This polymer was considered to be a mixture ofPVP and PEG at the proportion they were fed to the synthesis mix-ture. This polymer was used in synthesis along with a make-up offresh polymer to account for the mass loss in reaction-purificationsteps.

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

A: Physicochem. Eng. Aspects 466 (2015) 189–196 191

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ently shaken for about 2 min. Then 1 ml 50 mM tetraoctylammo-ium bromide (TOAB) in degassed toluene was added to the copperydrosol and shaken vigorously for another 2 min. The toluenehase becomes reddish indicating the transfer of copper nanoparti-les to toluene. The aqueous phase retained a faint colour indicatinghat the phase transfer is not 100% efficient.

To produce organic dispersible nano-powder, the solvent wasvaporated using inert gas bubbling. Then the resulting waxy mix-ure was stored in the vacuum dessicator until the smell of tolueneas absent. This remains a dark brownish waxy solid. It immedi-

tely re-dispersed into toluene to give a brownish red colloid.

.7. Characterization

The surface plasmon resonance of copper nanoparticles washaracterized using UV-Visible spectrophotometer (Shimadzu UV-800). The size and morphology of copper sols were characterizedy transmission electron microscope (Tecnai G2 20S Twin). The sizeistribution of the sols is characterized by dynamic light scatteringModel: Malvern Zetasizer Nano ZS). The oxidation behaviour ofopper nanopowder was characterized by using X-ray diffractionechnique (Model: X-ray Diffractometer PW-17291710).

. Results and discussion

Stable copper nanoparticles can be prepared in aqueous phasey charge stabilization or steric stabilization or a combination ofoth. However, for production of nano-powder, concentrated cop-er sol in aqueous phase with very good stability is required andolymeric stabilization is the only feasible option in such a case. Inhis work, copper nanoparticles have been prepared by modifica-ion of standard protocols as detailed in the experimental section.sually, for stable concentrated copper sol, a surfactant (e.g. CTAB)nd/or a polymer (PVP) are used as protective ligands [24,25]. Usef two protective ligands seems to be beneficial for forming smalltable particles [24]. We also used similar strategy, but a pair ofolymers, PVP–PEG has been used instead of polymer–surfactantair to obtain small stable particles at a copper salt concentrationf 0.02 M. The concentrations of polymers were maintained at theame level as reported by other investigators [26–28].

In the following sections we shall discuss various properties ofhis colloid and the powder formed from this colloid. In this workstability’ refers to the tendency of particles to resist aggregation,xidation resistance refers to resistance of particles against oxida-ion of surface and re-dispersibility refers to the ability of particleso form a colloid after formation of dry powder. Size may or mayot be preserved during this process.

Fig. 1 shows the UV–vis spectra of PVP–PEG stabilized copperanoparticles. These particles show a prominent UV–vis peak at571 nm immediately after synthesis which signifies the presencef copper nanoparticles. The particles are very stable and remainuspended even after two months. These sols have also been char-cterized using dynamic light scattering (DLS) and transmissionlectron microscopy (TEM). DLS show a particle size of 14 nm butEM shows sub 10 nm particles (Fig. 2(A)), possibly due to the pres-nce of polymeric chains on particles. It can be noted that if theample is drop casted on the carbon coated copper grid and dried,he polymer layer becomes very thick and forms a visually opaqueayer. The contrast becomes very low (Fig. S1) for such samples ando increase the contrast we ‘wicked’ the sample soon after dropasting using a filter paper. This wicking step increased the contrast

o an acceptable level as can be seen in Fig. 2(A). The two monthsld sample show a slight shift in plasmon peak to 574 nm and thearticle size becomes 50 nm as seen in TEM micrographs (Fig. S3

n Supporting information). Since production of nanopowder is the

Fig. 1. UV–vis spectra of PVP–PEG stabilized Cu nanoparticles immediately aftersynthesis and after 2 months.

main aim of, aggregation over such long time scale is not a matterof concern here.

3.1. Production of aqueous dispersible nanopowder

The amount of polymers used in this synthesis protocols is veryhigh. 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 formany other protocols as summarized in Table 1. In our case, suchhigh proportion of polymer offers very good stability and small size:these colloids can be dried and re-dispersed without affecting theparticle size (results not shown). However, the powder is mostlythe polymers. If nano-powder for commercial use is needed, theamount of polymer per gram of metallic copper must be reduced.One of the ways to reduce the amount of polymer in the sol is bywashing. Washing can be conducted using two different methods:(i) high speed centrifugation and (ii) using precipitating agent alongwith centrifugation at a lower speed and re-dispersed the washedcolloid to a variety of solvents.

First, washing without any precipitating agent was tried. Thecolloid was washed twice using centrifugation at 11,500 rpm for30 min and the precipitate was separated from the suspension.After each wash a clear, colourless supernatant was obtained. Thesupernatant was stored for recycling of the polymer. The sedimentcan be dried using a jet of nitrogen gas to form the powder whichcan readily be re-suspended in fresh degassed water by ultrasoni-cation. Hence, purified, re-suspendable copper nano-powder couldbe produced. The particles are completely re-dispersible in watereven after two washes. More number of washes were unnecessaryas >90% of polymer was removed by the two washes. The particlessettling at the bottom of the centrifuge tube show a shining metal-lic copper colour (Fig. 3(A)) after the second wash. However, theparticle size in washed colloid increased to ∼50 nm after the firstwash 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 ethanolfollowed 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

192 P.P. Chowdhury et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 466 (2015) 189–196

F wice wt ation

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ig. 2. TEM images of (A) unwashed PVP–PEG stabilized copper nanoparticles, (B) twice washed PVP–PEG stabilized copper nanoparticles using ethanol via centrifugia centrifugation.

arger particles in the previous case was instigated by high speedentrifugation and not due to removal of polymers.

As expected, the stability and oxidation resistance of the washedydrosol is poorer than the original colloid for both types of wash-

ng procedures. The original sol contains excess hydrazine hydratehich produces N2 gas and also contains a hyper excess of poly-ers which make the sol very stable and resistant to oxidation. Theashed sol has less of these two components and hence less sta-

le. However, the oxidation of the washed sol can be minimized ifhey are dried with nitrogen immediately after production and inn airtight bottle. It may be noted that the original unwashed solan be stored in ordinary vials which are not perfectly air tight.

The washed and re-dispersed sols form flock after a day buthe flock structure can be broken by ultrasonication. The ultraso-icated sol shows the same character as the original washed sol as

able 1he data on polymer stabilized copper nanoparticles. The third column shows the range oorresponding range of particle size. As the mass ratio increases, the particle size decreas

References Stabilizing agent Mass ratio of stabil

Qing-ming et al. [30] PVP 47.214

Wu et al. [28] PVP 47.74

Dang et al. [31] PVP 62.95–188.85

Lai et al. [27] PVP 21–173.22

Dang et al. [26] PEG 566.39–849.59

This work PVP+PEG 1.65–165.68

ashed PVP–PEG stabilized copper nanoparticles: washed using centrifugation, (C)and (D) recycled polymer stabilized Cu nanoparticles: twice washed using ethanol

measured by UV and DLS. However, in general it seems that there-dispersed sol is not suitable for a prolonged storage but ratherfor immediate use. On the other hand, the powder obtained (asshown in Fig. 3) can be stored in a sealed bottle for a prolongedperiod (more than a month) without any degradation in quality asjudged by UV–vis peak and DLS size. It has been observed that thecopper nano-powder is not prone to oxidation except in a moistenvironment.

Copper nanopowder obtained after washing and drying isreadily 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 thesupernatant during washing step. The amount of polymer in the

f mass ratio of polymeric stabilizing agent to copper and the last column shows thees.

izing agent and copper Size of Cu nanoparticles (nm)

126–375 (depending on other parameters)3.4

72–6150–80

28–4200–2

P.P. Chowdhury et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 466 (2015) 189–196 193

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10 nm. It is also clear that a major portion of polymers used duringsynthesis are not adsorbed on the surface of nanoparticles. Hence,unadsorbed polymers provide good stability and dispersibility by

Copper nanoparticles before exposure to air

Copper nanoparticles after exposure to air

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Cu (200)

Cu (220)Cu2O (111)

ig. 3. (A) Precipitated copper nanoparticles by centrifugation. (B) Redispersionanoparticles. (D) Optical microscopic image of copper nanopowder.

upernatant can be quantified by evaporating the water slowly.he excess hydrazine and ammonia also evaporates during thisrolonged storage at relatively high temperature (70 ◦C). Resultsrom such experiments show that approximately 90% of the poly-

ers can be recovered. Such polymers can be recycled for synthesisalong with a make-up amount of fresh polymer) of particles andound to produce particles similar to those produced by fresh poly-

er as shown in Fig. 2(D). Hence, the recycled polymers can be usedor synthesis.

.2. Oxidation resistance of copper nanopowder

To understand the oxidation resistance of the nano-powder pro-uced, we performed XRD study of the dry nanopowder stored in aealed vessel as well as those exposed to air overnight. These resultsre shown in Fig. 4. It can be seen that the nanopowder stored in

sealed vessel did not show any oxide peak. On the other hand,anopowder exposed to air shows a peak which correspond to 1 1 1

ace of Cu2O [32,33,34]. It may be noted that even in air exposedowder, only a small portion get oxidized.

.3. Quality of particles produced by lesser amount of polymers

It is not clear why such high proportion of polymer is neededuring the synthesis. To test quantitative effects, we reduce bothhe polymers concentrations by a factor of 10 (by keeping the ratioxed). In this case, stable colloid is formed with some amountf precipitated mass. However, upon examination under TEM, weound large aggregates of size ∼200 nm (not shown) are presentn the colloid. Further reduction of overall polymer concentration

eads to unstable colloid. This data suggest that such high concen-ration of polymer is indeed needed.

If we keep PEG concentration at the initial higher level andeduce PVP concentration by a factor of 10, we obtain a stable sol

ecipitated copper nanoparticles by ultrasonication. (C) Fully redispersed copper

with very little precipitation. While the precipitate is of micronsize, the dispersed particles are sub 10 nm as shown in Fig. 5. Ifthe PVP concentration is maintained at the initial high value andPEG concentration is reduced by a factor of 10, around 20 nm par-ticles (DLS) are formed. The stability of various PVP–PEG stabilizedcopper nano colloids are summarized in Table 2. It is clear fromthe table that the two polymers have different role in producinga stable colloid of small size. While PEG is vital for stability (row5 of Table 2), PVP is required to reduce the particle size to sub

806040202-theta

Fig. 4. X-ray diffraction of copper nanopowder stored in sealed environment (solidline) and exposed to air (dashed line).

194 P.P. Chowdhury et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 466 (2015) 189–196

Table 2The 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 Months1 Original concentration Original concentration 1 month 1 month2 Original concentration Original concentration Days 1 week0 Reduced 10 times Original concentration Months 1 month

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he mechanism of ‘depletion stabilization’ [35,36]. “Depletion sta-ilization” is a lesser known route for colloidal stability and occurst a much higher concentration than required for depletion floccu-ation. Typical, polymer concentration for depletion flocculation is.05–0.2% whereas the typical concentration used for polymer sta-ilized copper nanoparticles is ∼14%. Depletion stabilization occurs

ue to the depletion of concentration of free polymers between theurfaces of the approaching particles. This justifies the use of highoncentration of polymers in the synthesis process.

ig. 5. TEM image of copper nanoparticles produced by 10 times reduction of PVPoncentration.

ig. 6. TEM images of (A) unwashed PVP–PEG stabilized copper nanoparticles phase tranopper organosol by inert gas.

centration Months Weeks times Months Days0 times Unstable Unstable

3.4. Production of organic dispersible nano-powder

Next we try to phase transfer the copper hydrosol to organicphase. There are a variety of phase transfer methods to transfernanoparticles from aqueous to organic phase [37]. The simplestone is the replacement of the surface ligand through a biphasicmixture. Simple shaking the biphasic mixture make a coating ofhydrophobic surfactant on the particle at the interface and facili-tate the phase transfer. We tried such methods for the unwashedand washed colloids but we could not phase transfer the particlesusing this method. It was observed that most of the particles aggre-gated at the interface which should be due to partial coating by thioland retention of charge on particle surface.

Next we tried ethanol mediated phase transfer. In this case, thehydrophobic ligand (dodecanethiol) is added to the hydrosol withethanol so that it creates a single phase. This results in better possi-bility of thiol coating on the particles [38]. Even this method failedto phase transfer the polymer stabilized sol. Here also the parti-cles aggregated at the interface and stick to walls of the vial. Thesame result was obtained even with other hydrophobic ligands likeoctadecanethiol (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 alayer on the particles. Then the organic phase (toluene) contain-ing TOAB is used for phase transfer by a mechanism known as ‘ionpair formation through electrostatic interaction’. It may be notedthat the particles could be transferred without any washing. TEMimages of the nanoparticles transferred to toluene are shown inFig. 6(A). It can be seen that the particles retain their size (<10 nm)

during phase transfer. However a few large particles have also beenobserved in the phase transferred particles (not shown).

Organic dispersible nanopowder can be prepared by drying theorganosol using an inert gas. It may be noted that if the organosol

sferred to toluene. (B) Redispersed copper nanoparticles in toluene after drying the

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s heated for drying, it becomes slightly unstable and the particleize increases from 10 nm to 50 nm in such cases. The ‘powder’ inhis case is a waxy mass. This powder can be re-dispersed into freshegassed toluene by gently shaking for a minute. The TEM of redis-ersed colloid is shown in Fig. 6(B). It can be observed from Fig. 6(B)hat the particles remain sub 10 nm.

. Conclusions

Stable copper hydrosol of small particle size (2–7 nm) has beenrepared at high concentration (0.013 M) by modifying a reportedrotocol. The colloids are stable for months showing negligiblehange in UV–vis spectra when stored in a capped vial whichignify strong resistance of these particles towards oxidation. Itas been shown that for preparation of such concentrated hydro-olloid, hyper excess of polymer (polymer to Cu ratio ∼ 100:1) iseeded. Lesser amount of polymers produces an unstable colloid or

arger particles. Depletion stabilization has been speculated as pos-ible reason for requirement of hyper excess polymer. The powderbtained by drying the colloid can be readily re-dispersed into vari-ty of solvents including water, DMF, DMSO, chloroform. However,he copper content of the powder is very less (1%).

Nanopowder containing much higher proportion of copper haseen obtained by washing the nano-colloid using precipitatinggent and/or centrifugation. Centrifugation alone produces parti-les of larger size (∼50 nm). A few nano-rods of length about 200 nmnd aspect ratio 7 are also observed. Ethanol precipitated powders,owever, preserved the original size. The washed nanopowdersould be redispersed into water and other polar solvents easily.he polymer obtained in the supernatant could be recycled for pro-uction of nano-colloid. Such colloids do not show any significantifference from those synthesized by fresh polymers.

The particles produced could be transferred to toluene using phase transfer protocol previously used for nanorods. Thishase transfer protocol uses mercaptosuccinic acid (MSA) andetraoctylammonium bromide (TOAB) for phase transfer. Otherell practised phase transfer protocols were unsuccessful for

his case. The phase transferred particles could be dried and re-ispersed readily in toluene. The particle size remains preserveduring drying and redispersion.

cknowledgements

We thank and acknowledge SRIC (ISIRD Grant), IIT Kharagpuror financial assistance and Prof. S. De for allowing us to performLS measurements. We also thank Mr. Sudipto Mondol, Centralesearch Facility for his help in characterizing the copper nanopar-icles using TEM.

ppendix A. Supplementary data

Supplementary data associated with this article can beound, in the online version, at http://dx.doi.org/10.1016/j.colsurfa.014.10.031.

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