Available online at www.sciencedirect.com

www.elsevier.com/locate/elecom

Electrochemistry Communications 10 (2008) 506–509

Novel procedure for the template synthesis of metal nanostructures

Rosalinda Inguanta *, Salvatore Piazza, Carmelo Sunseri

Dipartimento di Ingegneria Chimica dei Processi e dei Materiali, Universita di Palermo, Viale delle Scienze, 90128 Palermo, Italy

Received 14 December 2007; received in revised form 14 January 2008; accepted 15 January 2008Available online 31 January 2008

Abstract

In this work we describe a novel method for the fabrication of a regular and uniform array of Cu nanowires into anodic aluminamembranes. It is based on galvanic contact between the metal sputtered film covering the bottom of template and a less noble metal.The growth rate was estimated as function of the immersion time. Nanowires with aspect ratio from 12 to 286 were obtained by adjustingthe deposition time. Copper nanowires were found to be polycrystalline with an average crystalline size of about 40 nm. This procedurecan be applied for the preparation of a wide range of metallic nanostructures and it can be easily scaled up for industrial processing.� 2008 Elsevier B.V. All rights reserved.

Keywords: Displacement deposition; Template synthesis; Nanostructures; Metallic nanowires; Alumina membranes

1. Introduction

Nanostructured materials have attracted much interestbecause of their unique properties. In fact, due to theirstructure features and size effects, they show physical prop-erties that are different from bulk materials [1]. Manymethods have been developed for the fabrication of nano-wires. Among these methods, template synthesis is consid-ered as quite useful, because it can be used for thepreparation of different types of nanostructures [2,3]. Thehigh order degree of the porous structure of anodic alu-mina membrane (AAM) (consisting in a close-packed arrayof columnar hexagonal cells, each containing a centralcylindrical pore normal to the surface), makes it an idealtemplate for the fabrication of nanostructured materials,suitable for applications in optoelectronics, sensors, mag-netic memories and electronic circuits [4–6]. Besides, theinterest in AAM is due to the ease of production. In fact,the morphological parameters (membrane thickness, porediameter and density) can be easily controlled by adjustingthe anodizing conditions (voltage and time, anodizing bathcomposition and temperature) [7,8]. For these reasons, a

1388-2481/$ - see front matter � 2008 Elsevier B.V. All rights reserved.

doi:10.1016/j.elecom.2008.01.019

* Corresponding author. Tel.: +39 0916567232; fax: +39 0916567280.E-mail address: r.inguanta@dicpm.unipa.it (R. Inguanta).

variety of nanostructures (metals, alloys, semiconductors,oxides and polymers) with different morphologies (tubules,fibers or wires, rods) have been fabricated by utilizingAAM as template in sol–gel process [9], microwave plasmachemical vapor deposition [10], electrodeposition and elec-troless deposition [8,11,12].

Many previous works focused on the fabrication ofcopper nanowires [13–15], because of the potential appli-cations in the microelectronics industry and, in particu-lar, for interconnection in electronic circuits. We haverecently demonstrated a novel route for fabricating metalnanowires [16], we investigate here for growth of crystal-line copper nanowires. The procedure is based on metaldisplacement reaction [17] (cementation of copper ions)leading to the growth of copper nanowires into the poresof commercially available alumina membranes. The gal-vanic displacement reaction for the synthesis of core/sheet nanostructured materials have been investigated inthe literature [18–20]. The key steps of the proposedapproach is the use of nanostructured materials as tem-plate and suitable salt precursor solution. On the con-trary, in this work we investigated the direct growth ofcopper nanowires due to the immersion of coupled differ-ent metals into an electrolytic solution containing copperions.

R. Inguanta et al. / Electrochemistry Communications 10 (2008) 506–509 507

2. Experimental

Commercial anodic alumina membranes (Anodisc 47made by Whatman) having an average pore diameter ofabout 210 nm, a nominal thickness of 60 lm, porosityof about 28% and surface pore population of the orderof 1013 pores/m2, were used as templates.

In this work, for the cementation of Cu nanowires, wehave used an aluminum foil as active metal having a thick-ness of 1.5 mm and a purity of 99.99%. In order to depositcopper inside the channels of AAM, prior to cementation athin conductive layer of Au was sputtered on one side ofthe AAM using a conventional sputter coater to make thissurface electrically conductive. A portion of AAM wasmounted onto the aluminum support (active metal) bymeans of a conductive paste and delimited by an insulatingfilm. Since the displacement deposition process needs anelectrolytic contact of the active metal surface, a small areaof the aluminum support (of the order of 0.25 cm2) wasexposed to the deposition solution. A scheme of thearrangement used for the fabrication of copper nanowiresis reported in Fig. 1. This arrangement was placed horizon-tally in a beaker and covered with 25 ml of a 0.2 M coppersulphate and 0.1 M boric acid solution having pH 3. Theexperiments were conducted at room temperature. The sur-face area of the AAM exposed to the deposition solutionwas of the order of 1 cm2. A fresh solution was used foreach experiment. Experiments were carried out for differenttimes of deposition (from 7 h to 7 days).

The crystallographic structure of the nanowire arrayswas investigated by X-ray diffraction analysis (XRD).XRD patterns were obtained using a Philips generator(mod. PW 1130) and a PW goniometry (mod. 1050); all dif-fractograms were obtained in the 2h range from 10� to 100�with a step of 0.02� and a measuring time of 0.5 s for step,using the Cu Ka radiation (k = 1.54 A). Nanowiresmorphology was investigated using a scanning electronmicroscope (SEM, Philips, ESEM XL 30) equipped with

Fig. 1. Scheme of the arrangement used for the fabrication of coppernanowires into the AAM template.

an X-ray energy dispersive spectrometer. SEM analysiswas carried out before and after dissolution of AAM in1 M NaOH aqueous solution at room temperature. Priorto SEM examination, sample surface was sputter coatedwith a thin layer of gold in order to form a conducting filmthat avoids electrostatic charging under the electron beam.

3. Results and discussion

Because the standard reduction potential of the Cu2+/Cu redox pair (0.34 V vs the normal hydrogen electrode(NHE)) is higher than that of the Al3+/Al redox pair(�1.70 V vs. NHE), copper ions are reduced to copperwhen an aqueous solution containing these ions is in con-tact with aluminum

3Cu2þ þ 2Al! 3Cuþ 2Al3þ ð1ÞThe Au layer covering the back-side of the AAM served

as initial substrate for copper deposition. In fact, this Aulayer was in direct contact with the solution permeatingthe membrane pores so that the deposition process takesplace preferentially into the pores rather than on the activemetal surface. Without this conductive layer, copper couldnot be deposited into the AAM channels, since the metaldisplacement deposition could occur only at aluminum sur-face. The conductive paste ensures the electrical contactbetween aluminum layer and Au film necessary for the elec-tron transport from the anodic area (aluminum surface) tothe cathodic one (pore bottom).

Fig. 2 shows the XRD pattern of copper nanowires intoAAM obtained after 7 h of immersion into the depositionbath. Cu nanowires were polycrystalline, as revealed bythe presence of five diffraction peaks at 2h angles of43.23�, 50.43�, 74.13�, 89.93� and 95.14�, which correspondto Cu(111), Cu(200), Cu(2 20), Cu(311) and Cu(222),respectively. All peaks can be attributed to the crystallinecubic form of metallic copper, in good agreement with

Fig. 2. X-ray diffraction pattern of as-deposited copper nanowires in theAAM template. The presence of Au peaks is due to the film sputtered onback membrane surface.

508 R. Inguanta et al. / Electrochemistry Communications 10 (2008) 506–509

the literature values [21], while the broad band at lower 2hvalues (from 25� to 30�) is characteristic of the amorphousalumina. The strong and sharp peaks indicate that coppernanowires have high crystalline order degree. Using theScherrer’s equation [22], a crystalline grain size of about

Fig. 3. Cross-sectional views of copper nanowire arrays obtained afterdifferent times of deposition: (a) 7 h; (b) 3 days; (c) 7 days. Inset of Fig. 3cshows the regular cylindrical shape of self-standing Cu nanowires.

39.4 nm was evaluated for the main peak. Similar patternswere observed for samples with different deposition times.Little differences are only in the intensity of copper peaksthat increases with increasing the deposition time.

The length of nanowires at different cementation timeswas estimated by SEM analysis of the cross-section ofAAM template. Fig. 3 shows SEM pictures of coppernanowires deposited into AAM with different times of dis-placement deposition. In all pictures the formation of anarray of perfectly aligned copper nanowires can beobserved. Nanowire arrays are straight, dense and contin-uous with a uniform diameter throughout their entirelength. The nanowire length increases with the immersiontime. Fig. 3a shows the SEM image of Cu nanowires after7 h of deposition. All nanowires started to grow from thebottom of the pores up to a length of about 2.5 lm. After3 days of deposition, the copper nanowires were about44 lm long, as shown in Fig. 3b. A longer deposition time(1 week) produce a continuous Cu film onto the AAM sur-face with a thickness of about 150 lm. For this sample,after the removal of the Au layer (with a 1200 grade abra-sive paper) and the complete dissolution of the AAM tem-plate (in 1 M NaOH solution for 3 h), self-standing Cunanowires, vertically oriented to the substrate surface, have

Fig. 4. (a) Top view of nanowire array of Fig. 3c after removal of Au layerwith an abrasive paper and after total dissolution of AAM template in1 M NaOH for 3 h and (b) EDS spectrum of copper nanowires.

R. Inguanta et al. / Electrochemistry Communications 10 (2008) 506–509 509

been obtained, as shown Fig. 3c. In the inset of Fig. 3c, itcan be observed also that nanowires are uniform in size,parallel, and with a regular cylindrical shape. In all sam-ples, the height of wires was uniform along the AAMcross-section. This finding indicates that the nanowiresgrow at the same rate in each pore during the displacementdeposition.

The diameter of Cu nanowires was approximately210 nm, equal to the pore diameter of the alumina mem-brane. The aspect ratio increases from 12 to 286 withincreasing the immersion time in the cementation solution(from 7 h to 1 week, respectively).

Fig. 4a shows a top view SEM image of the coppernanowire array of Fig. 3c after complete dissolution ofAAM template. It can be observed that all channels of tem-plate were filled up; therefore, the population of nanowireswas of the order of 1013 nanowires/m2, equal to the porepopulation of the AAM template.

Energy dispersive X-ray spectroscopy (EDS), employedfor evaluating the chemical composition of the as-preparednanowires, revealed that they consist of pure copper metal.Fig. 4b shows a typical EDS spectrum recorded on thenanowire arrays after the dissolution of AAM template.

4. Conclusions

In conclusion, we have fabricated regular and uniformarrays of Cu nanowires into AAM using a displacementdeposition process. The length of nanowires can be easilycontrolled by adjusting the immersion time in the electro-lytic solution. SEM pictures showed that high aspect rationanowires (ranging from 12 to 286) were obtained. XRDanalysis revealed the formation of polycrystalline coppernanowires. This work showed also that this technique offabrication is very easy to control and cheap, because thecost of the sources and equipments necessary for the pro-cess is very low. A further advantage of this approach, incomparison with other technologies, is the possibility tofabricate metal nanowires by using a very large templatearea. Taking into account that copper is one of the mostimportant metals in modern electronic technology, thisprocess can be useful for industrial manufacture of coppernanowires. Besides, since this procedure is only dependenton the difference between redox potentials of the two cou-pled metals, many other metallic nanowires could be pre-pared. Further investigation on the influence of different

parameters (like temperature, pH, composition of the dis-placement deposition bath, ratio of the AAM area to thearea of active metal exposed to the solution) on the rateof growth and nanowires morphology is in progress.

Acknowledgements

This work was supported financially by Universita diPalermo – APQ Ricerca della Regione Siciliana deliberaCIPE No. 17/2003, ‘‘Laboratorio dell’ innovazione nel set-tore dei beni culturali: Sperimentazione di Nanotecnologiee Nanomateriali.”

References

[1] R. Kelsall, M. Geoghegan, Nanoscale Science and Technology,Wiley, Chichester, 2006.

[2] C.R. Martin, Chem. Mater. 8 (1996) 1739.[3] C.R. Martin, Acc. Chem. Mater. 28 (1995) 61.[4] S. Shingubara, J. Nanopart. Res. 5 (2003) 17.[5] Y. Piao, H. Lim, J.Y. Chang, W. Lee, H. Kim, Electrochim. Acta 50

(2005) 2997.[6] H. Hillebrenner, F. Buyukserin, J.D. Stewart, C.R. Martin, J.

Nanosci. Nanotechnol. 7 (2007) 2211.[7] H. Masuda, K. Fukuda, Science 268 (1995) 1466.[8] R. Inguanta, M. Butera, C. Sunseri, S. Piazza, Appl. Surf. Sci. 253

(2007) 5447.[9] G. Zhang, J. Chen, J. Electrochem. Soc. 152 (2005) A2069.

[10] T.-M. Chen, F.-M. Pan, J.-I. Hung, L. Chang, S.-C. Wu, C.-F. Chen,J. Electrochem. Soc. 154 (2007) D215.

[11] R. Inguanta, S. Piazza, C. Sunseri, Nanotechnology 18 (2007) 485605.[12] R. Inguanta, C. Sunseri, S. Piazza, Electrochem. Solid-State Lett. 10

(2007) K63.[13] C. Fang, E. Foca, S. Xu, J. Carstensen, H. Foll, J. Electrochem. Soc.

154 (2007) D45.[14] Y. Shimotsuma, T. Yuasa, H. Homma, M. Sakakura, A. Nakao, K.

Miura, K. Hirao, M. Kawasaky, J. Qiu, P.G. Kazansky, Chem.Mater. 19 (2007) 1206.

[15] M. Motoyama, Y. Fakunaka, T. Sakka, Y. Ogata, S. Kikuchi, J.Electroanal. Chem. 584 (2005) 84.

[16] R. Inguanta, S. Piazza, C. Sunseri, It. Pat., VI-2007-A000275, 2007.[17] M. Paunovic, M. Schlesinger, Fundamentals of Electrochemical

Deposition, Wiley, Pennington, 2006 (Chapter 9).[18] X. Wen, S. Yang, Nano Lett. 2 (2002) 451.[19] Y. Sun, B.T. Mayers, Y. Xia, Nano Lett. 2 (2002) 481.[20] F. Xiao, B. Yoo, H.L. Kyu, N.V. Myung, J. Am. Chem. Soc. 129

(2007) 10068.[21] Joint Committee Powder Diffraction Standards, Power Diffraction

file, International Centre for Diffraction Data, Card No. 4-836,Pennsylvania, USA, 2001.

[22] A.R. West, Solid State Chemistry and its Applications, Wiley,Chichester, 1985 (Chapter 3).