Powder Technology 250 (2013) 91–96

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Copper nanopowder synthesis by electrolysis method in nitrate andsulfate solutions

Rasoul Khayyam Nekouei, Fereshteh Rashchi ⁎, Arsalan RavanbakhshMetallurgy and Materials Engineering, School of Metallurgy and Materials Engineering, University College of Engineering, University of Tehran, Tehran, Iran

⁎ Corresponding author at: Metallurgy andMaterials EnandMaterials Engineering, University College of Engineeri11155/4563, Tehran, Iran. Tel.: +98 21 88012999; fax: +

E-mail addresses: r.nekouei@ut.ac.ir (R.K. Nekouei), raa.ravanbakhsh@ut.ac.ir (A. Ravanbakhsh).

0032-5910/$ – see front matter © 2013 Elsevier B.V. All rihttp://dx.doi.org/10.1016/j.powtec.2013.10.012

a b s t r a c t

a r t i c l e i n f o

Article history:Received 23 February 2013Received in revised form 9 September 2013Accepted 11 October 2013Available online 21 October 2013

Keywords:Copper nanopowderElectrolysisNitrate solutionDirect currentPulse current

Chemical electrodeposition (electrolysis) is one of the methods which can be used for production of high-puritycopper powder. In this work, synthesis of nano-sized copper powder particles by means of electrolysis has beeninvestigated. Parameters such as electrolysis solution, type of current (direct and pulse), and nitric acidconcentration have been studied. The media of study were nitrate and sulfate. Particle size of powder obtainedby the direct current in the nitrate solution was even smaller than those of the pulse current state in the sulfate.Thus, the nitrate solution was chosen for investigation of the nitric acid concentration (25–45g/L). The obtainedpowders were characterized by X-ray diffraction (XRD), scanning electronmicroscopy (SEM), energy dispersivespectroscopy (EDS), and laser particle size analyzing (LPSA) methods. The powders produced in the sulfatesolution (in pulse state along with the polyvinylpyrrolidone (PVP) additive) were less than 200 nm in size;however, those produced in the nitrate solution (in direct state without additive)were less than 100nm. Currentefficiency of the nitrate solution (about 30%) was significantly less than that of the sulfate solution (about 70%).The powders produced in the nitrate solution were of high purity, similar to the sulfate solution.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

In recent years, synthesis of nanopowders (less than 100 nm) hasdrawn attention. Due to its significant physical and chemical properties,the copper nanopowder is extensively used in oil lubrication, electricconductive pastes, medicine, biologic filters, and for other purposes[1,2].

Fine-grain copper powders are produced by differentmethods such asatomization, copper oxide reduction, chemical and electrochemicaldeposition. Among these methods, the powder produced by electro-chemical method results in the highest rate of purity, suitable greenstrength, and low oxygen content [2].

Many researchers [1–11] have investigated the production of copperpowder from sulfate acidic solutions by galvanostatic or potentiostaticmethods. Most of them have studied morphology of the powderproduced at overvoltages (over-potentials) higher and lower than thelimiting current density. Based on the obtained results, many para-meters affect the morphology and physical and chemical properties ofthe produced powder. Among these parameters are overvoltage, rateof released hydrogen, current density, copper ion concentration, acidconcentration, temperature, distance and materials of electrodes, andelectrolyte drift velocity. With regard to these parameters, the obtained

gineering, School of Metallurgyng, University of Tehran, Po Box98 21 88006076.shchi@ut.ac.ir (F. Rashchi),

ghts reserved.

powder morphology can be grain, dendrite, and cauliflower in shapes[3–6]. To produce copper powder, the parameters should be selectedin a way that the deposited particles do not stick on electrodes andcan be easily separated from cathode. On the other hand, the cathodeefficiency should be in economically cost-effective.

One of the variables in electrochemical method is the mediumemployed. The most common medium employed for copper nano-powder synthesis is sulfate [1–11]. The most important advantage ofsulfate solution is their very high current efficiency [2]. In recent years,regarding these sulfate solution limits, the use of additives [8,9],increasing overvoltage, and change in type of current from direct topulse have been used for the reduction of powder particle size.According to various researches, presumably one of problems withsulfate solution is the unfeasibility to obtain nano-sized particles inthis solution without adding different kinds of additives [10]. Althoughthe additives lead to morphology control and finer size of the particles,their overuse makes the powder contaminated. Of these additives usedin copper electrolysis, one can refer to polyvinylpyrrolidone (PVP) [8]and sodium dodecyl sulfate (SDS) [9].

On the other hand nitrate solutions are usually used for recoveringcopper from scraps and copper tailings. In this state, the copper scrapand tailings are dissolved in nitrate solution and are then recoveredby solvent extraction or electrolysis method [12–15]. The mostimportant problem with these methods is the presence of impuritiesaccompanying the copper. However, if the initial solution is pure, theproduction of high purity nanopowder is possible.

Naseri et al. [15] studied the production of copper nanopowder innitratemediumof the solution obtained from recycled electronic scraps.

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Due to the high acidity of the solution after recovering, electrolysis ofcopper in thenitrate solutionwasdifficult. Therefore, they first removedthe nitric acid by tributyl phosphate (TBP) organic solvent up to acertain concentration by the use of solvent extraction method.Moreover, Choi et al. [12] worked on the production of 99.99% purecopper by nitrate residue solution through electrolysis.

In this research, the sulfate solution (alongwith an optimum amountof the additive) was first studied by using either direct or pulse current.In continuation, nitrate solution was studied and optimization of theacid concentration and type of the current in this medium wereperformed. The properties studied were powder purity, morphology,and particle size.

2. Materials and research method

2.1. Materials

The cathodic copper (99.9% pure) was selected as the cathode andthe anode. Copper sulfate pentahydrate (CuSO4·5H2O), sulfuric acid(H2SO4, 96%, nitric acid (HNO3, 35%)), copper nitrate pentahydrate(Cu(NO3)2·5H2O), polyvinylpyrrolidone (PVP = (C6H6NO)n), acetone,and ethanol (Germany Merck Company) were used as the rawmaterials. For acids, the reported concentrations (e.g. in Table 1) referto the acid quantity and not to the diluted reagent.

2.2. Synthesis

In this research the copper powder was produced by a rectifier(model SL20RPC-IPC) with the precision of two decimal digits on thevoltage and current readings. Two pieces of the cathodic copper sheet(25× 40×4mm3) were used as the two electrodes. After washing theelectrodes with deionized (DI) water they were polished with sandpaper (of SiC material) up to 1200 mesh size. In continuation, theywere treated in acetone and ultrasonic bath (BANDELIN sonorex,30 W) for 10 min to remove grease. After rinsing with DI water anddrying in air, they were immediately inserted in 10% sulfuric acid bath,then inserted in the ultrasonic bath for 5min in order to remove surfaceoxidized layer. Then, the electrodes were rinsed again and theelectrodes were inserted into the electrolyte solution.

Themean cathodic current density in all the testswas 0.2A/cm2 [10].For the current density less than this value, the adhesion of the copperpowders on the cathode increases and in the current densities morethan this value, the current efficiency drops and hydrogen productiongreatly increases [2]. The mean cathodic current density was selectedto be two times greater than the mean anodic current density [16,17].For this reason, one of the anode surfaces was covered with nail polishafter degreasing.

The electrolyte solution in the sulfate bath was a mixture of water,sulfuric acid, copper sulfate, and PVP in a 500mL glass beaker. Also, thenitrate bath was a mixture of water, nitric acid, and copper nitrate. Theperformed tests, test conditions, electrolyte compositions, resultedparticle size and current efficiency are shown in Table 1. The materialsweremixed by amagnetic stirrer (model IKA RH Basic 2) and electrolytetemperature was raised to 50 °C and controlled within ±3 °C. Theturning speed of the stirrer was constant. Time duration for the

Table 1The performed tests, their specifications, the bath composition and results.

Test num. Type of test Acid con. (g/L( Cu2+ con. (g/L) Mean overvo

1 Sulfate-direct current 140 (sulfuric acid) 2 1.552 Sulfate-pulse current 140 (sulfuric acid) 2 8.963 Nitrate-direct current 1 45 (nitric acid) 2 2.274 Nitrate-direct current 2 25 (nitric acid) 2 3.055 Nitrate-pulse current 25 (nitric acid) 2 10.20

deposition of powder on the cathode for all the tests was constant andselected to be 10min.

With regard to display of the rectifier and its precision, the meanvoltage of the system in each test was recorded and registered. Thepowder was removed by a clean spatula and then washed with DIwater for several times and in order to prevent oxidation it was pouredinto acetone [16,17]. Since the copper powder obtained from thismethod is generally in the form of agglomerated particles; hence, itwas dispersed by the magnetic stirrer for 20 min and then in theultrasonic bath for 40min. To ensure the accuracy of the results, eachtest was performed two times and the mean value of the particle sizewas reported.

The mechanism and reactions of copper powder electrolysis aredescribed in details in references [7,10].

2.3. Characterization

In order to identify and ensure purity of the cathode copper,quantometric analysis (spark emission spectrometry) was done. It wasfound that the purity of the copper sheet was higher than 99.9%.Efficiency of the current was obtained by weighing the copper powderand comparing with the theoretical weight [1] according to Eq. (1).

Percentageof currentefficiency

¼ practical weight of deposit=theoretical weight of depositð Þ� 100:

ð1Þ

To identify the purity and existing phases in the copper powder, X-ray diffraction analysis (XRD, X'pert Philips) and energy dispersivespectroscopy (EDS) were used. To study the powder morphology andparticle size, scanning electron microscopy (SEM, CamScan MV2300)and a laser particle size analyzer (LPSA, Cilas 1061) were employed.For the test number of 1 and 2 in addition to SEM images, the LPSAwas used for further investigation [7,10] and the powder was dispersedfairly after ultrasonication. On the other hand, due to finer size ofparticles for tests 3 and 4 and their tendency towards agglomerationonly SEM images were used for calculation of particle's diameterwhich gave us a wider range of values.

3. Results and discussion

Among additives used in copper electrolysis, and with regard tofunctional mechanism, PVP has the most effective role on the reductionof particle size [8], acting like a strong inhibitor. Therefore, this additiveis used as a particle size reducer in sulfate solution. Since its optimumvalue in our previous study was 2 g/L [10], the same value was alsoselected as the constant value. Another parameter which is effective inthe reduction of particle size is type of current. With regard to theresearch done by the writers of the article concerning optimization ofpulse current, a 100Hz frequency with a duty cycle of 20% was selectedas the optimal state. Other parameters of the process such as the acidconcentration, copper ion concentration, temperature, and mean ofthe current density were optimized by the writers in previous study[7] and were employed in this research (Table 1).

ltage Other conditions Particle size (nm) Current efficiency (%)

2 g/L PVP ~500 702 g/L PVP, 100 Hz, duty cycle 20% 100–120 60– 50–100 30– 50–100 30100Hz, duty cycle 20% – 20

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A comparisonwasmade betweendirect and pulse currents in sulfatesolution; sulfate and nitrate, nitrate solution with different concen-trations of nitric acid, and nitrate solution with direct and pulsecurrents.

3.1. Effect of type of current on sulfate solution

According to Haas et al. [8], application of pulse current (with thepresence of ultrasonic field) leads to the reduction of grain size

Fig. 1. SEM image of copper powder produced in sulfate solution by (a) direct current(experiment 1); (b) pulse current (experiment 2).

(crystallite size). What should be taken into consideration is that theapplication of pulse current does not always result in the reduction ofparticle size. The most significant advantage of pulse current is thereduction of thickness of the double layer which provides conditionfor an increase in the limiting current density [18]. The double layeron the cathode surface is thicker in direct current than in pulsingcurrent. During the off time of the current in pulse state, the doublelayer discharges partly and becomes thinner.

The first point is that by the use of the pulse current, the limitingcurrent density can be increased. Consequently, higher overvoltages

Fig. 2. SEM image of copper powders produced in nitrate solution by direct currentand (a) 45 g/L of nitric acid concentration (experiment 3); (b) 25 g/L of nitric acidconcentration (experiment 4).

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can be applied. According to Eq. (2) [1], by application of higherovervoltages, the rate of nucleation (υ) is increased.

υ ¼ K1 exp−K2ηj j

� �ð2Þ

where K1 and K2 are constants; and η is the applied overvoltage. Theequation presented for very high overvoltages is not correct, becausein very high current densities, occurrence of auxiliary reactions (suchas hydrogen evolution) leads to severe occurrence of polarizationaround the cathode. Also, since the hydrodynamic conditions adjacentto electrodes change, this equation cannot be used in very high currentdensity in direct current state. The critical overvoltage for this equationdepends on different factors such as type of current and the additives.

The second point concerning the mechanism of pulse currentfunction is the reduction of hydrogen production. For the production ofpowder, evolution of hydrogen is favorable. Hydrogen current efficiencymust be less than 30% of the theoretical current efficiency [2]. In additionto generating turbulence on the surface of double layer, production ofhydrogen reduces adhesiveness of powder to substrate with a positiveeffect for powder production. However, in the pulse state, with a

Fig. 3. (a) XRD analysis and (b) EDS analysis of copper powders produced in nitrate solu

reduction in the evolution of hydrogen, thickness of the double layer isseverely reduced. In addition, the thicker double layer means the morethe powder is refined with less adhesiveness. Because when this layeris more thickened, it leads to a diffusion-controlled produced powderreaction and the rate of copper atom reduction on cathode surfacebecomes higher than the diffusion rate of the atom into the doublelayer and crystalline lattice of the particle surface. As a result, the copperatoms do not have required time for taking on regular crystalline latticeand the number of nucleuses is increased [11]. Over-evolution ofhydrogen increases turbulence in the boundary layer; reduces thethickness of the double layer; and has a negative effect on the reductionof particle size. Ultimately, it can be said that the particle size iscontrolled by the competition between two factors: thickness of thedouble layer and rate of the evoluted hydrogen.

Fig. 1 shows the SEM image of copper powder produced in thesulfate solution using direct and pulse currents, respectively. Eachimage shows agglomerates containing some individual particleswhich can be identified with precise investigation. From these figuresit can be concluded that in the presence of the PVP additive, change ofthe type of the current does not have a considerable effect on theparticle size, even if it is possible that it has an effect on the grain size

tion by direct current and the concentration of 45 g/L of nitric acid (experiment 3).

Fig. 4. SEM image of copper powders produced in nitrate solution by pulse current(experiment 5).

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(crystallite size) of the powder. With regard to the above-mentionedpoints, it can be concluded that the negative effect of the reduction onthickness of the double layer and the positive effect of the increasedapplied overvoltage, cancel out each other and the particle size is notsignificantly affected.

3.2. Effect of type of solution

Figs. 1(a) and 2 show the comparison between sulfate and nitratesolution, respectively. Nitrate and sulfate ions have the role of electroncarriers or the conductivity of the solution. After performing the tests, itwas found that the production of powder in the nitrate solution enjoysmuch less current efficiency (about 30%); however, this value wasabout 70% in the sulfate. With regard to this significant difference incurrent efficiency, it can be concluded that, this may be one reason whynitrate solution attracted a small portion of attention among researchersfor production of powder. Another factor for the disinclination ofresearchers towards nitrate can be high acidity of nitric acid [15].Extraction of copper from nitric acid, even at low acid concentration, isvery energy-consuming, requiring precise temperature control. Addi-tionally, due to the low concentration of the acid, conductivity of thesolution is low and in order to reach similar current density with sulfatestate, higher potential is required. On the other hand, due to theapplication of higher overvoltage, consumption of energy increases;more heat is delivered to the electrolyte; and temperature increasesfaster. Of course, achieving nano-powder with mean particle size about50–100 nm can compensate for this deficiency in current efficiency. Forthe role of nitrate ions in molecular scale there is not a tangiblemechanism in electrolysis field yet.

3.3. Effect of concentration of nitric acid

Fig. 2 shows the SEM images of the produced powders in nitratesolution with different acid concentrations. In this image, whiteparticles represent the copper nanoparticles. According to references[16,17], the more the concentration of the acid, the smaller the particlesize will be. By an increase in concentration of acid, conductivity of theelectrolyte is increased while overvoltage is reduced. On the otherhand, the limiting current density is increased and more overvoltagecan be applied (according to Eq. (2)). Moreover, an excessive increasein concentration of the acid results in a decrease in the electrolyteelectric resistance, reducing the overvoltage [1]. As a result, a decreasein the overvoltage leads to coarser powder particles. Therefore, theconcentration range of the selected nitric acid was in a way that theinitial overvoltage changed (2.27–3.05 V) up to the extent that theenergy consumption was within a reasonable limit and variation inthe solution temperature was easily controllable.

According to Fig. 2, it can be said that the dimensions of theproduced powders are less than 100 nm and by changing the acidconcentration, the size distribution of the produced powders changewithin a small range. These small changes in the size of the particlesmight result from the small range selected for the acid concentration.In fact, it is clear that the nature of the nitrate has a significant effecton the reduction of particle size. The XRD analysis and EDS of test 3are shown in Fig. 3. Based on these two analyses, it can be said thatthe purity of the produced powders is very high. In other words, noimpurity was observed in them. Small peaks were indexed for thegold coating and the carbon which were not related to the synthesisprocess, but to the SEM sample preparation. The outcome of these twoanalyses for the test numbers of 1, 2 [10], and 4 was the same as theoutcome of the test number 3.

3.4. Effect of type of current on nitrate solution

According to the details mentioned in Section 3.1 and by the samereasoning, the pulse current test was used for the nitrate solution. The

electrolyte and the pulse conditions were respectively selectedaccording to test 5 in Table 1 and a frequency of 100Hz with a workingperiod of 20%. After performing the test, the result was completelyincompatible with what was expected. Fig. 4 shows the SEM image ofthe powder produced in test 5 conditions. The current efficiency in thepulse state relative to the direct state decreased once more (about20%) and the powder deposited only on the edges of the cathode.

According to Fig. 4, it can be found that the morphology of thepowder is completely changed into slab and disk forms. The powderagglomeration is greatly increased requiring more studies such aspowder zeta potential, morphology and mechanism governing it.

4. Conclusions

In this research, high purity copper nano-powder with a diameter of50–100 nm was synthesized by direct electrolysis in nitrate solutionwithout any additives. Also, a comparisonwasmade among the powdersproduced using sulfate solution with direct and pulse current; nitrateand sulfate; nitrate with different concentrations of nitric acid; andnitrate with direct and pulse current leading to the following results:

A. The use of pulse current has a positive effect on the reduction ofparticle size in the copper powder synthesis in sulfate solution, butthis effect is not very significant.

B. Sulfate solution along with additives reduces the particle size to lessthan 200 nm. However, in nitrate solution, spherical particles arewell synthesized to dimensions less than 100 nm (50–100 nm).Efficiency of the nitrate solution is very low and it was about 30%.The produced powders are of very high purity.

C. An increase in the nitric acid concentration from 25 to 45g/L leads tosmall changes in the particle size.

D. Change of type of the current from direct to pulse in the nitratesolution leads to severe agglomeration of the powder; generationof disk morphology; and severe reduction in the current efficiency.Pulse current in the nitrate solution (at least under the testedconditions) did not produce nanopowders.

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References

[1] W. He, X.C. Duan, L. Zhu, Characterization of ultrafine copper powder prepared bynovel electrodeposition method, J. Centr. South Univ. Technol. 16 (2009) 708–712.

[2] G. Orhan, G. Hapci, Effect of electrolysis parameters on the morphologies of copperpowder obtained in a rotating cylinder electrode cell, Powder Technol. 201 (2010)57–63.

[3] M.G. Pavlovic, Lj. Pavlovic, V. Maksimovic, N. Nikolic, K. Popov, Characterization andmorphology of copper powder particles as a function of different electrolyticregimes, Int. J. Electrochem. Sci. 5 (2010) 1862–1878.

[4] N. Nikolic, K. Popov, L. Pavlovic, M. Pavlovic, The effect of hydrogen codeposition onthe morphology of copper electrodeposits. I. The concept of effective overpotential,J. Electroanal. Chem. 588 (2006) 88–98.

[5] K. Popov, T. Kostic, N. Nikolic, E. Strojilkovic, M. Pavlovic, A new approach to metalelectrodeposition at a periodically changing rate part I. The reversing overpotentialmethod, J. Electroanal. Chem. 464 (1999) 245–251.

[6] V. Maksimovic, L. Pavlovic, M. Pavlovic, M. Tomic, Characterization of copperpowder particles obtained by electrodeposition as function of different currentdensities, J. Appl. Electrochem. 39 (2009) 2545–2552.

[7] R.K. Nekouei, F. Rashchi, A.A. Amadeh, Using design of experiments in synthesis ofultra-fine copper particles by electrolysis, Powder Technol. 237 (2013) 165–171.

[8] I. Haas, S. Shanmugam, A. Gedanken, Pulsed sonoelectrochemical synthesis ofsize-controlled copper nanoparticles stabilized by poly(N-vinylpyrrolidone), J.Phys. Chem. B 110 (2006) 16947–16952.

[9] M.Y. Wang, Z. Wang, Z.C. Guo, Preparation of electrolytic copper powders with highcurrent efficiency enhanced by super gravity field and its mechanism, Trans.Nonferrous Met. Soc. China 20 (2010) 1154–1160.

[10] R.K. Nekouie, F. Rashchi, N.N. Joda, Effect of organic additives on synthesis of coppernano powders by pulsing electrolysis, Powder Technol. 237 (2013) 554–561.

[11] A. Durisinova, Factors influencing quality of electrolytic copper powder, PowderMetall. 34 (1991) 139–141.

[12] J.Y. Choi, D.S. Kim, Production of ultrahigh purity copper using waste copper nitratesolution, J. Hazard. Mater. B99 (2003) 147–158.

[13] M.S. Lee, J.G. Ahn, J.W. Ahn, Recovery of copper, tin and lead from the spent nitricetching solution of printed circuit board and regeneration of the etching solution,Hydrometallurgy 70 (2003) 23–29.

[14] F.J. Alguacil, A. Cobo, M. Alonso, Copper separation from nitrate/nitric acid mediausing Acorga M5640 extractant part I: solvent extraction study, Chem. Eng. J. 85(2002) 259–263.

[15] N.N. Joda, F. Rashchi, Recovery of ultra fine grained silver and copper from PC boardscraps, Sep. Purif. Technol. 92 (2012) 36–42.

[16] W.B. Eisen, B.L. Ferguson, R.M. German, R. Iacocca, P.W. Lee, D. Madan, H. Sanderow,Y. Trudel, ASM Metals Handbook, 10th ed., Powder Metal Technologies andApplications, vol. 7, ASM International, Ohio, 1998.

[17] In: O.D. Neikov, S.S. Naboychenko, G. Dawson (Eds.), Handbook of Non-ferrousMetal Powders, first ed., Elsevier, United Kingdom, 2009, pp. 331–368.

[18] M. Chandrasekar, M. Pushpavanam, Pulse and pulse reverse plating — conceptual,advantages and applications, Electrochim. Acta 53 (2008) 3313–3322.