Evolution of copper nanowires through coalescing of copper nanoparticles induced by aliphatic amines and their electrical conductivities in polyester films

Mingxia Tian,Aili Wang,Hengbo Yin

Faculty of Chemistry and Chemical Engineering,Jiangsu University,Zhenjiang 212013,China

Keywords:Nanomaterials Nanostructure Electronic materials Copper nanowires Electrical conductivity Polyester film

ABSTRACT Copper nanowires were synthesized by the wet chemical reduction method using copper sulfate as the copper precursor,aliphatic amines (methylamine,ethanediamine,1,2-propanediamine) as the inducing reagents,and hydrazine hydrate as the reductant through the aging and reduction processes.The high-resolution transmission electron microscopy (HRTEM) images reveal that the copper nanowires were synthesized by coalescing extremely small-sized copper nanoparticles with the particle sizes of 1–6 nm in copper complex micelles.A longer aging time period favored the coalescing of the copper nanoparticles to form thinner copper nanowires in the following reduction process.The coalescing extent of copper nanoparticles in copper nanowires was highly enhanced by ethanediamine and 1,2-propanediamine as compared with that by methylamine.The copper nanowire-filled polyester films had higher electrical conductivity than the copper nanoparticle-filled ones.

1.Introduction

Researchers have made significant efforts on the synthesis of one-dimensional electrical conduction nanomaterials,such as carbon nanotube [1],silicon [2–5],SnO2[6],Cu [7,8],AuPd [9],FePd[10],Ag [11],and Si/Ag [12] nanowires because these one dimensional conducting materials have potential applications in the sensing [3,6,8],catalysis [9,10],electrical and thermal conduction[4,7,11],adsorption/desorption [5],electrochemistry [12–14],tribology [15],piezoresistivity [16],and antimicrobial [17] fields.

Among the metallic nanowires,the synthesis and application of copper nanowires have attracted great attention of researchers because metallic copper not only has a high thermal and electrical conductivity and but also has a high abundance and a low price[18–25].The synthesis of copper nanowires could be classified into the electrochemical,organic chemical vapor deposition,hydrothermal,solvothermal,and wet chemical reduction methods.The progresses in the above-mentioned methods were briefly summarized as follows.

By the electrochemical method,the copper nanowires could be simply synthesized by electrochemical growth in porous polycarbonate and porous alumina cathodes using CuSO4as the copper precursor and H2SO4,H2SO4/H3BO3,and (NH4)2SO4/diethylenetriamine as the electrolytes,respectively [26–30].The copper nanowires in the hollow arrays orientated along a direction perpendicular to the (1 1 1) crystal plane [29].In this method,the removal of porous templates with the use of solvent is a tedious work to some extent.

Copper nanowires were synthesized by the chemical vapor deposition method with the use of SBA-15 as the template,copper(II) acetylacetonate as the copper precursor,and H2as the reductant [31].On silicon substrate at 200–300 °C,the free-standing copper nanowires were synthesized by the chemical vapor deposition method with the use of Cu(etac)[P(OEt)3]2as the precursor[32,33].It was suggested that the copper nanowires grew up on their five (1 1 1) planes by the chemical vapor deposition of Cu(etac)[P(OEt)3]2[33].

Copper nanowires could be hydrothermally synthesized at 120–180°C using CuCl2[18,24,34,35]as the copper precursor and alkylamines (hexadecylamine,octadecyl amine,and oleyl amine)[18,24,34–36] as the inducing (reducing) agents with [18,35] or without[24,34]reductant.The copper nanowires were synthesized through the following steps.Cu2+cations and alkyl amine molecules firstly formed complexes,the slow reduction of the complexes allowed the formation of twinned copper seeds,which grew up to form nanowires by consuming newly generated copper seeds [18].

Copper nanowires were synthesized by the solvothermal method at 80 °C in ethanol solution with the use of CuCl2as the copper precursor,polyvinylpyrrolidone as the structure-directing agent,and hydrazine as the reductant [37].The researchers suggested that the facets(1 0 0) of the multiply-twinned Cu nanoparticles were covered by polyvinylpyrrolidone molecules and the reduced copper atoms were preferentially deposited onto (1 1 1)facets,leading to the anisotropic growth of copper nanowires.

Similar to the solvothermal method,the copper nanowires were also synthesized in an aqueous solution by the wet chemical reduction method using copper salts (Cu(NO3)2and CuCl2) as the copper precursors,hydrazine hydrate and glucose as the reductants,and aliphatic amines (ethanediamine and hexadecylamine)as the inducing reagents in an NaOH aqueous solution or deionized water at 30–100 °C [21,38–42].It was suggested that the copper nanowires grew upviathe reducing Cu2+complexes on the copper nanoparticles [38,39,43] along the (1 1 0) planes of copper nanoparticles [41,43].

Until now,the growth mechanism of copper nanowires has been generally ascribed to the anisotropic growth mechanism of copper nanoparticles.However,copper nanoparticles are produced with an abundant amount,accompanied by the formation of copper nanowires.Recently,it was reported that copper nanowires with rough and smooth surfaces were synthesized with and without stirring,respectively [44].The growth mechanism of copper nanowires is still unclear and worthy of investigation.

In our present work,the copper nanowires were synthesized by the wet chemical reduction method with the use of CuSO4as the copper precursor,methylamine,ethanediamine,and 1,2-propanediamine as the inducing reagents,and hydrazine hydrate as the reductant in an alkaline aqueous solution.The evolution mechanism of copper nanowires was investigated by TEM and HRTEM techniques.We found that the copper nanowires grew up by coalescing the copper nanoparticles entrapped in organic micelles under our present experimental conditions.It was different from the anisotropic growth mechanism of copper nanowires grown on copper nanoparticle seeds.The addition of the assynthesized copper nanowires in polyester films obviously increased their electrical conductivities as compared with the addition of the controlled copper nanoparticles.

2.Experimental

2.1.Materials

Pentahydrate copper sulfate (CuSO4?5H2O),sodium hydroxide(NaOH),hydrazine hydrate (85% (mass),N2H4?H2O),methylamine aqueous solution (25% (mass),CH3NH2),ethanediamine (NH2CH2-CH2NH2),1,2-propanediamine (NH2CH2CH(NH2)CH3),polyvinylpyrrolidone (MW=50,000),and anhydrous ethanol (CH3CH2OH)were guaranteed reagents and purchased from Shanghai Chem.Reagent Co.Ltd.Polyester paint was purchased from Jinhai Fuqian Science and Technology Co.Ltd.

2.2.Synthesis of copper nanowire

A copper sulfate (0.15 mol?L-1,10 ml) aqueous solution and a sodium hydroxide aqueous solution (4.5 mol?L-1,10 ml) were added into a 100 ml flask.When the copper sulfate and sodium hydroxide solutions were mixed together at 30 °C,light bluecolored flocculent precipitation occurred.And then,22 ml of methylamine aqueous solution,4 ml of ethanediamine,or 5 ml of 1,2-propanediamine were added under stirring at 15 r?min-1and 30 °C for different aging time periods of 0.5–8 h.The mole ratio of amine group in the aliphatic amine to Cu2+was set at 80:1.After the addition of aliphatic amine,the light blue-colored flocculent precipitate disappeared and the solution color became dark blue.After aging at 30 °C for a given time period,the reaction solution was heated to 70 °C,and then a hydrazine hydrate aqueous solution (2 ml of hydrazine hydrate 85% (mass) diluted to 50 ml) was added by a peristaltic pump in 3.5 h.Red metallic copper precipitate was formed during the reduction process.The precipitate was centrifuged,washed with anhydrous ethanol for 7 times,and stored in anhydrous ethanol.The synthesis procedures are illustrated in Fig.1.

The controlled copper nanoparticle sample was synthesized by the following procedures.0.125 g of polyvinylpyrrolidone was dissolved in 100 ml of copper sulfate (0.1 mol?L-1) aqueous solution and 100 ml of sodium hydroxide (0.25 mol?L-1) aqueous solution,respectively.The sodium hydroxide aqueous solution was added into the copper sulfate aqueous solution at 30 °C under stirring at 300 r?min-1.Then,500 ml of hydrazine hydrate aqueous solution(0.25 mol?L-1)was added into the resultant copper hydroxide suspension.After the addition of hydrazine hydrate,the reduction reaction was conducted out for 2 h.The resultant copper nanoparticles were obtained by the centrifugation at 8000 r?min-1and washed with anhydrous ethanol for at least 5 times.The asprepared copper nanoparticle sample was stored in anhydrous ethanol.

2.3.Characterization

The as-synthesized metallic copper samples were measured by the powder X-ray diffractometer (D8 super speed Bruke-AEX,Cu Kα radiation at λ=0.154056 nm)to determine their crystal phases.The morphologies and the crystal structures of the as-synthesized copper nanowires were observed on a high-resolution transmission electron microscope (HRTEM,JEM-2100).

The copper nanowire-or copper nanoparticle-filled polyester paint films were prepared by mixing a given amount of copper sample in the paint at room temperature (25 °C).The mixed paint was coated on smooth glass surface by a doctor blading method and dried in air atmosphere at room temperature for 6 h.The thicknesses of the films were approximately 1 mm.The sheet resistances of the copper nanowire-or copper nanoparticle-containing polyester films were measured on a four-point probe resistivity measurement instrument (RTS-8).

Fig.1.Synthesis procedures of copper nanowires.

3.Results and Discussion

3.1.XRD patterns of copper samples

The powder X-ray diffraction patterns of the copper samples showed that their XRD peaks appeared at 2θ values of 43.2°,50.3°,and 74.1°,respectively (Fig.2).These peaks were ascribed to those of the (1 1 1),(2 0 0) and (2 2 0) crystal planes of the face-centered cubic metallic Cu (JCPDS 01-1241).No XRD peaks of copper oxide and hydroxide were observed in the samples.The results showed that the as-synthesized copper samples were of pure metallic copper phase.

3.2.Morphology and crystal structure of copper nanowire

TEM and HRTEM were used to analyze the evolution process of the copper nanowires(Figs.3–5).When methylamine was used as the inducing reagent,the copper nanowires composed of copper nanoparticles with the particle sizes of 2–6 nm formed in methylamine micelles(Fig.3).The average diameters of the copper nanowires were 99,35,34,and 33 nm when the aging time periods were 0.5,2,4,and 8 h,respectively.Interestingly,it was found that prolonging the aging time period caused the formation of thin copper nanowires.The average lengths of the copper nanowires were 0.73,1.47,1.48,and 2.20 μm.It has been reported that Cu2+cations can chemically interact with amine groups of organic amines to form complexes [45–47] and amine-terminated micelles can be used for the direct synthesis of nano ZnO[48].In our present work,methylamine could react with Cu2+cation to form dissoluble complex because Cu(OH)2precipitate disappeared when methylamine was added in the aqueous Cu(OH)2suspension.The Cu2+containing methylamine complexes could form micelles since the reduced copper nanoparticles were clearly observed in the methylamine micelles according to the TEM images.The Cu2+containing complex micelles shrank along the radial direction,extended,and grew up along the length with the prolonging of the aging time period.When the Cu2+cations in these complex micelles were reduced with hydrazine,the copper nanowires with different diameters and lengths formed.

Fig.2.X-ray diffraction patterns of the copper samples.The copper samples were synthesized by stirring the CuSO4,NaOH,and aliphatic amine aqueous solution at 30 °C and 15 r?min-1 for different aging time periods of 0.5–8 h and then reducing with hydrazine hydrate at 70 °C for 3.5 h.Inducing reagents:(a–d) methylamine,(e–h) 1,2-propanediamine,(i-l) ethanediamine;aging time periods:0.5,2,4,and 8 h.

The lattice fringes of the copper nanoparticles present in the copper nanowires were approximately 0.21 nm,which were ascribed to the lattice spacing of the (1 1 1) plane of the facecentered cubic(fcc)metallic copper.The results showed that these copper nanoparticles were of pure metallic phase.Interestingly,the HRTEM images show that the coalescing extent of the copper nanoparticles present in the copper nanowires increased upon prolonging the aging time period.Especially when the aging time period was 8 h,the copper nanowire obviously formed by tightly coalescing copper nanoparticles (Fig.3d).The adjacent primary copper nanoparticles could coalesce to form larger sized ones because their interfaces were active.On the other hand,the reduced copper atoms could join the primary copper nanoparticles together by depositing at their interfaces.Therefore,it was reasonable to suggest that the copper nanowires were synthesized by coalescing copper nanoparticles in methylamine micelles.

On the other hand,the copper nanoparticles also formed apart from the copper nanowires.The particle sizes of the copper nanoparticles were approximately 40,86,100,and 96 nm when the aging times were 0.5,2,4,and 8 h.The ratio of the copper nanoparticle number to the copper nanowire number was less than 1:1.The formation of the copper nanoparticles aside the copper nanowires could be explained as that the Cu2+cations could not be completely encapsulated in the Cu2+containing complex micelles.The reduction of the Cu2+cations aside the Cu2+containing complex micelles led to the formation of individual copper nanoparticles.

When 1,2-propylenediamine was used as the inducing reagent,the as-synthesized copper nanowires composed of copper nanoparticles with the particle sizes ranging from 1 to 4 nm formed in 1,2-propylenediamine micelles (Fig.4).The average diameters of the copper nanowires synthesized by aging for 0.5,2,4,and 8 h were 81,68,57,and 50 nm,respectively.The average lengths of the copper nanowires were 1.42,1.72,1.94,and 2.71 μm.With the prolonging of the aging time,the shrinking and extending of 1,2-propylenediamine micelles led to the formation of thinner and longer copper nanowires.

The lattice fringes of the copper nanoparticles present in the copper nanowires were around 0.21 and 0.18 nm,which could be ascribed to the lattice spacings of the (1 1 1) and (2 0 0) planes of metallic copper.The HRTEM images show that when the aging time period was 0.5 h,the as-synthesized copper nanowires were composed of the individual copper nanoparticles.Upon prolonging the aging time to 2–8 h,HRTEM images clearly show that the copper nanowires formed by closely coalescing individual copper nanoparticles.

The particle sizes of the copper nanoparticles apart from the copper nanowires were approximately 100 nm.The ratio of the copper nanoparticle number to the copper nanowire number was less than 1:1.

When ethanediamine was used as the inducing reagent,the assynthesized copper nanowires were composed of copper nanoparticles with the particle sizes of 1–4 nm (Fig.5).The average diameters of the copper nanowires were 68,54,58,and 30 nm when the aging time periods were 0.5,2,4,and 8 h,respectively.The average lengths of the copper nanowires were 1.25,0.97,1.33,and 0.89 μm.With the prolonging of the aging time,the shrinking of ethanediamine micelles led to the formation of thinner copper nanowires.When ethanediamine was used as the inducing reagent,the resultant copper nanowires were shorter than those synthesized with the use of methylamine and 1,2-propylenediamine as the inducing reagents.

The lattice fringes of the copper nanoparticles present in the copper nanowires were approximately 0.21 nm,which could be ascribed to the lattice spacing of the (1 1 1) plane of metallic copper.Interestingly,the HRTEM images show that even the aging time period was 0.5 h,the small-sized nanoparticles could tightly coalesce in the copper nanowires.The results revealed that ethanediamine exhibited a higher activity for the coalescing of copper nanoparticles to form copper nanowires.The activities of the aliphatic amines for coalescing copper nanoparticles in the copper nanowires were in an order of ethanediamine>1,2-propylenediamine>methylamine.

Fig.3.TEM and HRTEM images of the copper nanowires synthesized using methylamine as the inducing reagent.The aging time periods were(a)0.5,(b)2,(c)4,and(d)8 h,respectively.

3.3.Evolution mechanism of copper nanowire induced by aliphatic amine

When the copper sulfate aqueous solution was mixed with a NaOH aqueous solution,the light blue colored Cu(OH)2flocculent precipitate formed.When aliphatic amine aqueous solution was added into the aforementioned suspension,the amine molecules combined with the Cu(OH)2to form dissoluble Cu2+complexes[39,43].The Cu2+complexes could form micelles in water because the nonpolar methyl group and carbon chains are present in the amine inducing reagents.With prolonging the aging time,the Cu2+-aliphatic amine micelles shrank in radial direction to form thinner micelles.When the strong reductant hydrazine hydrate was added,the Cu2+complexes were firstly reduced to smallsized copper nanoparticles and then these copper nanoparticles coalesced together to form copper nanowires.At the same time,the Cu2+complexes apart from the micelles could be reduced to form individual copper nanoparticles.The evolution mechanisms of the copper nanowires and nanoparticles are illustrated in Fig.6.

Fig.4.TEM and HRTEM images of the copper nanowires synthesized using 1,2-propanediamine as the inducing reagent.The aging time periods were(a)0.5,(b)2,(c)4,and(d) 8 h,respectively.

3.4.Crystal structure and morphology of controlled copper nanoparticle sample

The X-ray diffraction pattern of the controlled copper nanoparticle sample showed that three diffraction peaks appeared at (2θ)42.9°,50.1°,and 73.8° (Fig.7),which could be ascribed to those of metallic Cu (JCPDS 01-1241).The TEM image shows that the sample was composed of copper nanoparticles with the average particle size of 84 nm.

3.5.Electrical conductivities of copper nanowire-and copper nanoparticle-filled polyester films

The electrical conductivities of the copper nanowire-or copper nanoparticle-filled polyester films were expressed as their sheet resistances.When the controlled copper nanoparticles were added in the polyester paint films,the sheet resistances decreased from 1980.2 to 248.6 kΩ upon increasing the copper mass contents from 15%to 40%(Fig.8).When the copper nanowires synthesized using methylamine and ethanediamine as the inducing reagents were used as the additives in the polyester films,their sheet resistances decreased from 1251.3 to 134.3 and from 1016.3 to 0.626 kΩ,respectively,upon increasing the copper nanowire mass contents from 15% to 40%.

Fig.5.TEM and HRTEM images of the copper nanowires synthesized using ethanediamine as the inducing reagent.The aging time periods were(a)0.5,(b)2,(c)4,and(d)8 h,respectively.

The results indicated that the addition of copper nanowires obviously increased the electrical conductivity of the polyester film as compared with the addition of copper nanoparticles.Furthermore,when the copper nanowires synthesized using ethanediamine as the inducing agent were used as the additives in the polyester films,the polyester films exhibited lower sheet resistances than those films containing the copper nanowires synthesized using methylamine as the inducing reagent.It could be explained as that ethanediamine could result in the formation of copper nanowires by more tightly coalescing copper nanoparticles than methylamine,giving a lower sheet resistance.On the other hand,the thinner copper nanowires synthesized with the use of ethanediamine as the inducing reagent could construct more networks in the polyester film than the thicker copper nanowires and copper nanoparticles,which more efficiently increases the conductivity of the composite film.

4.Conclusions

Copper nanowires could be synthesized using aliphatic amine as the inducing reagent and hydrazine hydrate as the reductant in an alkaline aqueous solution.The evolution of the copper nanowires was suggestedviathe coalescing of the extremely smallsized copper nanoparticles in copper complex micelles.The extremely small-sized copper nanoparticles formed by the reduction of copper complexes in copper complex micelles.

Fig.6.Evolution mechanisms of copper nanowires and nanoparticles induced by aliphatic amines,methylamine,ethanediamine,and 1,2-propanediamine.

Fig.7.XRD pattern and TEM image of the copper nanoparticle sample synthesized using polyvinylpyrrolidone as the capping reagent.(a)XRD pattern;(b)TEM image.

The addition of copper nanowires more effectively increased the electrical conductivity of the polyester composite film than the copper nanoparticles.The diamines favored the formation of copper nanowires by tightly coalescing copper nanoparticles,which endowed the polyester composite film with a higher electrical conductivity.

Fig.8.The sheet resistances of the copper nanoparticle-and copper nanowire-filled polyester films.(a) Copper nanoparticles,(b) copper nanowires synthesized using methylamine as the inducing reagent and aging for 8 h.(c) Copper nanowires synthesized using ethanediamine as the inducing reagent and aging for 8 h.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was financially supported by the fund from the Jiangsu Science and Technology Department,China(FZ20180919).We also sincerely thank Dr.Kangmin Chen for his helpful support on TEM and HRTEM analyses.