Engineering Science and Technology, an International Journal xxx (2016) xxx–xxx

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Engineering Science and Technology,an International Journal

journal homepage: www.elsevier .com/ locate / jestch

Short Communication

Effect of reducing agents in the conversion of Cu2O nanocolloidto Cu nanocolloid

http://dx.doi.org/10.1016/j.jestch.2016.09.0032215-0986/� 2016 Karabuk University. Publishing services by Elsevier B.V.This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

⇑ Corresponding author.E-mail address: andal.che@kcgcollege.com (V. Andal).

Peer review under responsibility of Karabuk University.

Please cite this article in press as: V. Andal, G. Buvaneswari, Effect of reducing agents in the conversion of Cu2O nanocolloid to Cu nanocolloid, ETech., Int. J. (2016), http://dx.doi.org/10.1016/j.jestch.2016.09.003

V. Andal a,⇑, G. Buvaneswari b

aDepartment of Chemistry, KCG College of Technology, Chennai, IndiabMaterials Division, SAS, VIT University, Vellore, India

a r t i c l e i n f o a b s t r a c t

Article history:Received 3 February 2016Revised 23 August 2016Accepted 2 September 2016Available online xxxx

Keywords:Cu nano-colloidAscorbic acidX-ray diffractionElectronic spectroscopy

Current work reports the conversion of copper (I) oxide, nano-colloid to stable copper nanocolloid.Different reduction conditions were attempted to control the stability and size of the Cu nanoparticles.Hydrazine hydrate, ascorbic acid and glucose are found to be good reductants. In our work stable coppernanoparticles are obtained by reducing Cu2O without any special protections like inert gas atmosphereetc. Ascorbic acid, a natural vitamin C not only reduces cuprous oxide but protects the new born copperdue to its antioxidant properties. A red shift is observed when Cu2O nanospheres get converted to Cu. UV,XRD, FTIR and TEM were used to characterize the prepared Cu nanoparticles. The mechanism for thegrowth process of Cu nanomaterials are discussed.� 2016 Karabuk University. Publishing services by Elsevier B.V. This is an open access article under the CC

BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction

There has been a significant interest in the synthesis of metaloxide and metal nanoparticles due to their peculiar propertiesand potential applications in diverse fields [1]. Among severalmetal particles, copper nanoparticles have been our interest dueto their intriguing properties such as catalytic, optical, electronicsand gas sensors [2–5]. Copper nanoparticles are extremely sensi-tive to air, and the formation of oxide layer on the surface isunavoidable. Due to the surface oxidation, the electrical conductiv-ity of copper is expected to vary and hence it cannot be consideredas an alternative to expensive metals in electronics industry.

Several reduction methods have been developed for the synthe-sis of colloidal copper nanoparticles such as microwave reduction,sonochemical reduction, electrochemical reduction, photochemicalreduction, thermal reduction and chemical reduction [6–11].Chemical reduction method is more convenient method of synthe-sizing copper nanoparticles by varying experimental parameterssuch as temperature, concentration, pH etc. The major disadvan-tage associated with the synthesis of Cu nanoparticles by reductionmethod is its stability. The newly formed copper nanoparticlesundergo oxidation under ambient conditions when prepared inaqueous media irrespective of the nature of reducing agents used[12].

Cu2O an important semiconductor has been investigated exten-sively due to its potential applications in solar energy conversion,lithium ion batteries, catalysis, gas sensors etc [13]. Various meth-ods have been practised to synthesize Cu2O such as hydrothermal,microwave, reduction, electrochemical, reverse micelle, microemulsion [13–16].

Yu et al. reported the synthesis of copper colloid in two differ-ent solvents (water and ethylene glycol) by using ascorbic acid asreducing agent. He found that copper colloid synthesized by ethy-lene glycol required less time (1 h) than water (8 h) [17]. SimilarlySarkar et al. [18] prepared PVP protected spherical shaped coppercolloid by hydrazine hydrate reductant in two different solventsDMF (N,N,dimethyl formamide) and FA (Formamide).

Colloidal copper is prepared using NaBH4/LiCl mixture as reduc-ing agent in diglyme solvent in the presence of 3-(6-mercaptohexyl) thiophene. The synthesized colloid is stable forsix months at low temperature [19]. Colloidal cubic nanoparticlesare obtained using sodium borohydride and sodium ascorbate byseed mediated method in aqueous condition [20]. Colloidal coppernanoparticles stabilized by DBS have been synthesized bymicroemulsion method which shows long term stability in thepresence of N2 atmosphere [21]. Kim et al. reported bulk Cu2Oslurry reduction to micron sized Copper powder using hydrazinehydrate [22]. Wang et al. converted Cu2O hollow nanospheres toCu hollow nanospheres using H2 as reducing agent [23]. Yanget al. synthesized copper nano powder by two step reductionmethod using two reducing agents glucose and ascorbic acid[24].

ng. Sci.

2 V. Andal, G. Buvaneswari / Engineering Science and Technology, an International Journal xxx (2016) xxx–xxx

In all the above mentioned methods Cu colloid has been synthe-sized using organic solvents, in the presence of N2 atmosphere andusage of irradiation. Hence there is a need for the synthesis ofstable copper colloid without using inert gases, special techniquesand in shorter duration. The current work reports a simple methodof preparation of stable Cu nano-colloid from Cu2O nano-colloid.The impact of three different reducing agents such as glucose,ascorbic acid and hydrazine hydrate has been investigated.

2. Experimental

2.1. Materials

Copper sulphate, CuSO4�5H2O (98.5%), Ascorbic acid (98%) andD-glucose (99%) were procured from Qualigens, India. Sodiumborohydride (98%), Sodium alginate, Hydrazine Hydrate (99%) fromSd fine chemical Ltd India.

2.2. Preparation of Cu2O Seed nanoparticles

The method reported in our paper [25] was followed for thesynthesis of Cu2O nanocolloid.

2.3. Preparation of copper nanoparticles from Cu2O seeds

In the synthetic procedure, Copper nanoparticles were obtainedvia a wet chemical reduction route by varying the reducing agentconcentration, temperature and time (Table 1). Hydrazine hydrate,Ascorbic acid and Glucose are used to reduce Cu2O nanocolloid toCu nanoparticles. Cu2O nanocolloid (20 ml) was used as the precur-sor for copper nanoparticles.

2.4. Hydrazine hydrate

Hydrazine hydrate (5–20 ll) was added to the Cu2O nanocolloidboth at room temperature (26 �C) and at elevated temperature(70 �C) to obtain stable copper colloid. At room temperature, onvarying the concentration of hydrazine hydrate the time requiredfor the conversion of Cu nanocolloid was 10 min and the obtainedcolloid was not stable. At elevated temperature (70 �C), on varyingthe concentration of hydrazine hydrate a stable maroon colloidwas obtained for 20 within 3 min. The colloid was stable formonths (Table 1).

Table 1Amount of reducing agent in the formation of stable Cu at different temperature and time

Amount of reducing agent Temperature &time

Glucose (g) 100 �C (30 min)0.10.20.25

Ascorbic acid (g) 70 �C/100 �C (20 min)0.010.02 70 �C (20 min)0.03 70 �C (20 min)0.053 70 �C/100 �C (20 min)

Hydrazine hydrate (ll) Room temperature (10 min) & 70 �C (2–3 mi5101520

Please cite this article in press as: V. Andal, G. Buvaneswari, Effect of reducingTech., Int. J. (2016), http://dx.doi.org/10.1016/j.jestch.2016.09.003

2.5. Ascorbic acid

Ascorbic acid (0.01–0.053 g) is dissolved in 10 ml water andadded dropwise to the Cu2O colloid. Then the reaction was inves-tigated at two different temperatures (70 �C &100 �C). The requi-site time for the conversion of Cu2O nanocolloid was 20 minregardless of the amount of ascorbic acid and temperature. For0.02 & 0.03 g of ascorbic acid, the yellow colour of the Cu2O turnedto yellowish brown which indicates the impure phase formation ofCu colloid. Though the maroon colloid was obtained for 0.01 and0.053 g of ascorbic acid. The maroon colloid obtained at 0.053 gwas found to be stable for months (Table 1).

2.6. Glucose

To the Cu2O nanocolloid 0.1 g glucose was added and heated at100 �C for 30 min till a yellow colour colloid is obtained. The syn-thesis was repeated by increasing the concentration of glucose to0.2 and 0.25 g keeping the other parameters constant. A stableCu colloid (Table 1) was obtained for 0.25 g of glucose.

2.7. Characterization

The Cu colloids were analysed by UV–vis spectroscopy (HitachiDouble beam spectrophotometer Model U2800), Powder X-raydiffraction (Cu Ka, PANalytical,), FTIR spectroscopy (Thermo Nico-let – 330 spectrometer) and TEM (JEOL 3010)/

3. Results and discussion

3.1. Structure and morphology of copper nanoparticles

In this work we used three different reducing agents viz, hydra-zine hydrate, ascorbic acid and glucose for the reduction of Cu2O toCu. The Corresponding mechanism is proposed below. From themechanism it is clear that the Cu2O gets reduced to Cu nanoparti-cles with all the reducing agents.

Cu2Oþ C6H12O6 ! 2Cuþ C6H12O7 ð1Þ

2Cu2Oþ N2H4 ! 4Cuþ N2 þ 2H2O ð2Þ

Cu2Oþ C6H8O6 ! Cuþ C6H6O6 þH2O ð3ÞThe reducing power of glucose and ascorbic acid depends both

on the concentration of the reducing agents and the reaction tem-perature. As seen from the Table 1, it is clear that 0.25 g glucose isrequired to reduce Cu2O completely at 100 �C. Similarly, ascorbicacid reduces Cu2O at 70 �C. Among these two mild reducing agents

.

Colour of colloid Phase formed/stability

Yellow CuO and Cu

CuO and CuCu (stable)

Maroon colloid Cu, Cu2O, CuO

Yellowish brown Cu2OYellowish brown Cu2OMaroon colloid stable Cu (stable)

n) Maroon colloid Cu (unstable after one day)

Maroon colloid Cu (unstable after one day)Maroon colloid Cu (unstable after one day)Maroon colloid Cu (stable)

agents in the conversion of Cu2O nanocolloid to Cu nanocolloid, Eng. Sci.

a c b

500 600 700 800

c

b

a

603

678587

Abs

orba

nce

(a.u

)

wavelength (nm)

200 400 600 800

448

Abso

rban

ce(a

.u)

wavelength (nm)

Fig. 1. UV–visible spectra of Cu nanoparticles using a) hydrazine hydrate. b)ascorbic acid c) glucose [inset – UV–vis spectrum of Cu2O colloid, photo of Cucolloids].

d

0)

(111)

V. Andal, G. Buvaneswari / Engineering Science and Technology, an International Journal xxx (2016) xxx–xxx 3

ascorbic acid induces faster reduction (20 min) at low temperature.In addition, the amount of ascorbic acid is also less. On addinghydrazine hydrate to the colloid at room temperature, the colourof the colloid changes to maroon which indicates the significantreduction.

Hydrazine hydrate reduces Cu2O nanocolloid at room tempera-ture. Whereas ascorbic acid and glucose required heating to reduceCu2O the colloid went through a series of colour change from yel-

1200 1100 1000 900 800 700 600 500

a

% T

rans

mitt

ance

wavenumber (cm-1)

c

b

d

Fig. 2. FTIR spectra of Cu2O nano-colloid a) before reduction and after reductionusing b) hydrazine hydrate c) ascorbic acid d) glucose.

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low, light brown to maroon. The UV–vis spectra of the resultingcolloids on reduction of Cu2O by reducing agents, a red shift isobserved compiled in Fig. 1. The absorption bands for Cu nanopar-ticles are in the range of 550–600 nm [26,27]. The absorption max-imum of Cu nanoparticles prepared from hydrazine hydrate andascorbic acid was observed at 587 nm and 600 nm. The broad bandobserved in the case of glucose indicates the formation of Cunanoparticles (<50 nm). As per the quantum size theories thebroadening of the band may be due to the small particle size [28].

FTIR spectrum provides information to confirm the formation ofCu nanoparticles. The spectrum is compared with the IR spectrumof sodium alginate capped Cu2O nano-colloid in Fig. 2. The IR spec-trum of Cu2O colloid recorded before the reduction showed a peakaround 620 cm�1 which is the characteristic of Cu(I)–O vibrationmode. After the reduction, the disappearance of the band confirmsthe formation of Cu nanoparticles. Existence of the features due tothe alginate skeleton established the stabilization of the Cu nanoparticles by alginate groups.

The XRD patterns of the residues (Fig. 3) show the peaks char-acteristic of metallic Cu. All the diffraction peaks can be indexedbased on the standard XRD data of metallic copper (PDF File No.85-1326). Fig. 4 shows TEM images of the Cu nanoparticlesobtained. Cu nanoparticle prepared using hydrazine hydrate ishexagonal shape and the size is around �10–20 nm. Ascorbic acidproduces �20 nm size particles which are spherical in shape andhomogeneously distributed. Irregular shape particles of size inthe range of 50–100 nm are obtained in the presence of glucose.The particle size distribution graph for the ascorbic acid reducedCu nanoparticle is shown in Fig. 4. The average particle size isfound to be 4 nm. The reason for the larger size of Cu nanoparticles

10 20 30 40 50 60 70

a

(111)

(220)

(200)

(111)

(110)

Inte

nsity

(a.u

)

2θ

b

(200)

(200)

(111) c

(20

Fig. 3. X-ray diffraction patterns of a) Cu2O b) Cu (hydrazine hydrate) c) Cu(ascorbic acid) d) Cu (glucose).

agents in the conversion of Cu2O nanocolloid to Cu nanocolloid, Eng. Sci.

a b

c

0 1 2 3 4 5 6 7 8 9 10 11120

10

20

30

40

50

Fre

qu

ency

Diameter (nm)

d

Fig. 4. TEM images of Cu nanoparticles obtained using a) hydrazine hydrate b) ascorbic acid c) glucose d) size histogram.

4 V. Andal, G. Buvaneswari / Engineering Science and Technology, an International Journal xxx (2016) xxx–xxx

using glucose as reducing agent is due to the polar groups of glu-conic acid capped on Cu nanoparticles. More the polarity of the sta-bilizing groups leads to more agglomeration which increases thebulkiness. Ascorbic acid reduced Cu nanoparticles possess smallersize because the dehydroascorbic acid capping agent undergoesintra-intermolecular hydrogen bonding and prevent the agglomer-ation [29].

Though hydrazine is a stronger reducing agent than ascorbicacid and glucose, the impact of addition of reducing agent has animpact on the size and shape of Cu nanoparticles.

3.2. Effect of reducing agents and temperature

Hydrazine hydrate (strong reducing agent), glucose and ascor-bic acid (mild reducing agent) has been explored very much forreducing copper salts to copper nanoparticles. These reducingagents are used to synthesize stable Cu nanoparticles either inthe presence of N2 atmosphere [30] or in non aqueous mediumto prevent oxidation.

By varying reducing agent, temperature and fixing the concen-tration of Cu2O colloid (2 � 10�5 M) the optimum condition for theformation of stable copper colloid is achieved. The results areshown in Table 1. At room temperature, on addition of glucoseand Ascorbic acid to the Cu2O, colour of the colloid diminishesand the absorbance is suppressed. The suppression of the absor-

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bance may be due to the reaction between the capping agent(sodium alginate) and glucose or ascorbic acid.

Glucose and Ascorbic acid requires alkaline medium, inert gasatmosphere and temperature to reduce the copper salts [31,32].In the current work, no base is externally added to create alkalinemedium. Besides pH = 7.5, already available during the synthesis ofCu2O colloid (NaBH4 reduction) is sufficient for the reducing agentto reduce Cu2O. The free –CHO group of glucose reduces the freshlyformed Cu2O to Cu in the absence of alkaline nature, which in turngets oxidised to –COOH group. On reducing Cu2O to Cu, ascorbicacid becomes dehydro ascorbic acid.

The reductive power of glucose and ascorbic acid depends bothon the concentration of reducing agent and the temperature of thereaction. As seen from the Table 1, it is clear that 0.25 g glucose isrequired to reduce Cu2O completely at 100 �C. Similarly ascorbicacid (0.053 g) reduces Cu2O at 70 �C. When the reaction tempera-ture and the amount of reducing agent are less than the abovementioned CuO and Cu2O predominates.

Among the mild reducing agents (glucose and ascorbic acid)ascorbic acid induces fast reduction (20 min) at a lower tempera-ture than glucose. In addition to this the amount required forascorbic acid is less. The copper colloid and powder prepared fromascorbic acid is stable because of the antioxidant property of ascor-bic acid. Similarly in the case of glucose the –COOH group formedstabilizes the newly formed Cu nanoparticles.

agents in the conversion of Cu2O nanocolloid to Cu nanocolloid, Eng. Sci.

Fig. 5. UV–visible spectrum of eight months aged Cu nanocolloid (hydrazinehydrate).

V. Andal, G. Buvaneswari / Engineering Science and Technology, an International Journal xxx (2016) xxx–xxx 5

Hydrazine hydrate is capable of reducing Cu2O both at roomtemperature and elevated temperature (70 �C). The surface plas-mon resonance of Cu colloids formed under both the conditionsshowed similar absorbance at 577 nm. The duration for the com-pletion of reduction is comparatively more at room temperature(10 min) than at 70 �C (2–3 min). This could be due to faster nucle-ation process at higher temperature. Though the conversion isachieved using negligible amount (5 ll) of hydrazine hydrate, thestability of the copper colloid is poor (Table 1). To obtain a stableCu nanocolloid, 20 ll (0.02 M) hydrazine hydrate is required. Basedon the reduction time and stability of the colloid the activity of thereducing agents follows the trend: Hydrazine hydrate > Ascorbicacid > Glucose.

3.3. Stability

The stability of the synthesized metallic copper colloids hasbeen checked by storing in closed containers them under ambientcondition and characterized by UV–vis Spectroscopy. The coppercolloids obtained by using glucose and ascorbic acid are stablefor 3 months and undergoes agglomeration whereas hydrazinehydrate reduced copper colloid is stable for eight months. To fur-ther confirm the stability, the eight months stable colloid was char-acterized by UV and the obtained surface plasmon resonance wassimilar to the fresh colloid (Fig. 5). This could be attributed to N2

atmosphere generated inside the closed container due to hydra-zine. In the case of ascorbic acid, the stability is attributed to theantioxidant property of ascorbic acid and the formation of dehydroascorbic acid. The –COOH group of gluconic acid formed stabilizes

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the newly formed Cu nanoparticles when glucose is used as reduc-ing agent [33].

4. Conclusion

The present investigation demonstrates a simple, easier andnovel method for the preparation of stable copper nanocolloidand powder from Cu2O nanocolloid at room temperature. Amongthe three reducing agents, hydrazine hydrate induces a fasterreduction than ascorbic acid and glucose.

Acknowledgement

The authors thank VIT University for providing all requiredfacilities to carry out the experiments.

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agents in the conversion of Cu2O nanocolloid to Cu nanocolloid, Eng. Sci.