Fabrication of Cu nano particles by direct electrochemical

reduction from CuO nano particles

Won-Kyu Han a, Jae-Woong Choi a, Gil-Ho Hwang a, Seok-Jun Hong a,Jai-Sung Lee b, Sung-Goon Kang a,*

a Division of Materials Science and Engineering, Hanyang University, Seoul 133-791, Republic of Koreab Department of Metallurgy and Materials Science, Hanyang University, Ansan 426-791, Republic of Korea

Received 4 March 2005; received in revised form 18 April 2005; accepted 21 April 2005

Available online 15 June 2005

www.elsevier.com/locate/apsusc

Applied Surface Science 252 (2006) 2832–2838

Abstract

In this report, Cu nano particles have been prepared by direct electrochemical reduction from CuO nano particles and the

reduction mechanism was investigated. To investigate the reduction mechanism, CuO has been deposited on the AISI 430 by

magnetron sputtering in various Ar/O2 ratio and the cyclic voltammetry (CV) was performed in 0.5 M NaCl solution at 300 K.

This result indicated that the oxygen from the CuO was ionized at �0.874 V (versus SCE) and reduced to Cu. To fabricate

Cu nano particles, we employed CuO nano particles, which were prepared by a conventional mechanical milling, with a dc

rectifier and the specific electrochemical cell. The structure of the films and nano particles were analyzed by XRD, SEM/EDS

and XPS.

# 2005 Elsevier B.V. All rights reserved.

Keywords: Cu nano particles; Direct electrochemical reduction; Cyclic voltammetry; XPS

1. Introduction

Metallic nano particles have received significant

attention by researchers due to their unique properties

such as color, conductivity, melting temperature,

magnetism, specific heat and light absorption in

comparison with bulk metal [1–5].

* Corresponding author. Tel.: +82 2 2220 0404;

fax: +82 2 2296 4560.

E-mail addresses: altosjw74@hanmail.net (J.-W. Choi),

sgkang@hanyang.ac.kr (S.-G. Kang).

0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved

doi:10.1016/j.apsusc.2005.04.049

Especially, gold and silver nano particles are the

most widely researched material and these nano

particles exhibit very useful properties in catalysis and

biosensing [3–5]. Since 1990s, copper nano particles

have attracted much attention of researchers due to

their applications in catalysis [6]. In other words,

copper nano particles show much higher specific

catalytic efficiency than bulky copper due to their

enormous surface area [7].

Many techniques such as radiation methods [8],

micro emulsion techniques [9–11], super critical tech-

niques [12,13], thermal reduction [14], sonochemical

.

W.-K. Han et al. / Applied Surface Science 252 (2006) 2832–2838 2833

reduction [14,15], laser ablation [16], metal vapor

synthesis [17], vacuum vapor deposition [18] and

chemical reduction [19,20] have been developed to

synthesize copper nano particles. Also, it was reported

that the new process of electrochemical method for the

direct reduction of TiO2 at 800 8C under argon [21] but

this process led to the agglomeration of particles

(sintering) because of its high processing temperature.

Therefore, it is thought that this process is not suitable

to produce the nano particles.

In this study, we have reported that the reduction

mechanism of CuO thin film to Cu thin film.

Furthermore, we also fabricated Cu nano particles

by using a direct electrochemical reduction from CuO

nano particles without agglomeration of particles.

2. Experimental procedure

CuO thin films were prepared by magnetron

sputtering on the AISI 430 in various Ar/O2 ratio.

The AISI 430 substrates were cleaned with acetone and

ethanol before setting in the deposition chamber. The

distance between the copper target and the substrate

was 6 cm. Prior to the copper oxide deposition, theAISI

Fig. 1. Schematic diagram of electrochemical cell for direct elec-

trochemical reduction of CuO nano particles.

430 substrate was in situ etched by ion sputtering. The

argon flow rate was fixed constantly at 25 sccm and the

oxygen flow rate was varied from 0 to 12 sccm. Thus,

the base pressure was 5 � 10�6 Torr and the working

pressure was between 1.1 � 10�2 and 1.6 � 10�2 Torr.

To measure the reduction potential of CuO to Cu, the

cyclic voltammetry was conducted by using a conven-

tional three electrodes cell with 0.5 M NaCl solution at

300 K. And the CuO thin film was potentio-statically

reduced at the potential obtained from the cyclic

voltammetry.

To fabricate Cu nano particles, we used CuO nano

particles, which were prepared by a conventional

mechanical milling, with a dc rectifier and the specific

electrochemical cell. The experimental apparatus was

represented in Fig. 1. The glass container, filled with

CuO nano particles of about 100 nm in diameter, was

Fig. 2. XRD curves of copper oxide deposited by magnetron

sputtering with a function to the oxygen flow rate (constant argon

flow rate, 25 sccm): (a) 3 sccm; (b) 4 sccm; (c) 8 sccm

W.-K. Han et al. / Applied Surface Science 252 (2006) 2832–28382834

immersed into 0.5 M NaCl solution and Pt cathode

was dipped into the nano particles. And a voltage of

16 V, considering iR drop, was applied between the

cathode and anode for 1 h. The characteristics of the

films and nano particles were analyzed with XRD,

SEM/EDS and XPS.

3. Results and discussion

3.1. Preparation of CuO thin film

The phases of the thin films, prepared by

magnetron sputtering, were strongly dependent on

the oxygen flow rate as can be seen in Fig. 2. As soon

Fig. 3. Cyclic voltammograms (CV) of bulky copper (

as the oxygen was introduced into the chamber, Cu2O

was obtained. When the oxygen flow rate was 4 sccm,

single Cu2O phase was detected. Because the range of

the oxygen flow rate for single Cu2O is very narrow.

Indeed, as the oxygen flow rate went up to 5 sccm, the

diffraction peaks of Cu3O4 was detected except those

of Cu2O. On the other hand, when the oxygen flow rate

was more than 8 sccm, single CuO phase was formed

and we can obtain single CuO deposited thin films.

3.2. Electrochemical reduction of CuO thin film

Fig. 3(a and b) shows the cyclic voltammograms

(CV) of bulky Cu and CuO thin film, respectively, in

0.5 MNaCl solution at 300 K. As can be seen in Fig. 3,

a) and CuO thin film deposited on AISI 430 (b).

W.-K. Han et al. / Applied Surface Science 252 (2006) 2832–2838 2835

Fig. 4. Cross-sectional SEM images and EDS of CuO thin film (a) and Cu thin film potentio-statically reduced at �0.874 V (vs. SCE) (b).

the current waves 50 and 60 were observed in only CV

curve of CuO thin film. Therefore, it was thought that

the current wave 50 and its anodic counter part wave 60

originated from the existence of oxide scale. To

ascertain the direct reduction of CuO, CuO thin film

was reduced potentio-statically for 1 h at the potential

of wave 50, �0.874 V (versus SCE). Fig. 4(a and b)

shows the cross-sectional SEM/EDS of CuO thin film

and electrochemically reduced Cu thin film, respec-

tively. As shown in Fig. 4(a), the thin film was about

4 mm in thickness and composed of copper and

oxygen. However, the thin film reduced potentio-

statically was composed of only copper in Fig. 4(b).

X-ray photoelectron spectroscopy (XPS) was

employed to ascertain the reduction of CuO thin film

and the results were represented in Figs. 5 and 6. As

can be seen in Fig. 5(a), the XPS spectra of CuO thin

film shows a Cu 2p3/2 peak at 933.6 eV (corresponding

to CuO), a shake-up satellite peak at about 9 eV higher

than the Cu 2p3/2 peak and some peaks above 950 eV

arising from spin–orbit coupling. Fig. 5(b) shows the

spectra of Cu thin film electrochemically reduced after

ion sputtering for 20 s to remove the native oxide. As

shown in Fig. 5(b), the peak of about 932.5 eV

W.-K. Han et al. / Applied Surface Science 252 (2006) 2832–28382836

Fig. 5. XPS spectra of CuO thin film deposited on AISI 430 (a) and

Cu thin film potentio-statically reduced at �0.874 V (vs. SCE) (b).

Fig. 7. XRD curves of CuO nano particles (a) and Cu nano particles

potentio-statically reduced (b).

corresponds to Cu was detected. Also, the disappear-

ing of strong satellite peaks for Cu 2p indicates the

reduction of CuO. Fig. 6 shows the spectra of O 1s of

CuO thin film and electrochemically reduced Cu thin

film. As shown in Fig. 6(a), the spectra of O 1s of CuO

is asymmetric. This result indicates that at least two

oxygen species exist in the nearby region. The peak at

about 530 eV corresponds to O–Cu bond due to CuO,

whereas the peak at about 531 eV corresponds to O–H

bond due to chemisorbed oxygen on the surface. As

can be seen in Fig. 6(b), only one peak, corresponds to

O–H bond, is observed after electrochemical reduc-

tion of CuO thin film. These results indicate that the

CuO thin film was transformed to the Cu thin film by

direct electrochemical reduction.

Fig. 6. XPS spectra of CuO thin film deposited on AISI 430 (a) and

Cu thin film potentio-statically reduced at �0.874 V (vs. SCE) (b).

3.3. Fabrication of copper nano particles

As mentioned previous section, CuO nano particles

were reduced electrochemically for 1 h with the

specific electrochemical cell represented in Fig. 1.

Fig. 7 shows the XRD curves of CuO nano particles

and Cu nano particles obtained by electrochemical

reduction. As shown in Fig. 7, the XRD curve of Cu

nano particles is composed of Cu and Cu2O. It was

thought that the Cu2O phase in the XRD curve of Cu

nano particles may be formed during handling the Cu

nano particles under air. Fig. 8 shows the FE-SEM

morphological images of CuO nano particles and Cu

nano particles. As can be seen in Fig. 8, the spherical

shape and the size of CuO nano particles were

maintained and the agglomeration of Cu nano

particles was not observed after electrochemical

reduction.

W.-K. Han et al. / Applied Surface Science 252 (2006) 2832–2838 2837

Fig. 8. FE-SEM images of CuO nano particles (a) and Cu nano particles potentio-statically reduced (b).

4. Conclusions

It was ascertained that the CuO thin film was

electrochemically reduced to Cu at the potential of

�0.874 V (versus SCE) by using a cyclic voltam-

metry. And the CuO thin film was reduced potentio-

statically at the reduction potential. Also, XRD, SEM/

EDS and XPS indicated that the reduction of CuO

thin film to Cu thin film. Consequently, CuO nano

particles, prepared by a conventional mechanical

milling, were directly reduced to Cu nano particles

without agglomeration.

Acknowledgement

This work was supported by the research fund of

Hanyang University (2004).

W.-K. Han et al. / Applied Surface Science 252 (2006) 2832–28382838

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