PAPER www.rsc.org/materials | Journal of Materials Chemistry

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Formation of air-stable copper–silver core–shell nanoparticles for inkjetprinting†

Michael Grouchko, Alexander Kamyshny and Shlomo Magdassi*

Received 27th November 2008, Accepted 4th February 2009

First published as an Advance Article on the web 17th March 2009

DOI: 10.1039/b821327e

Copper nanoparticles can be utilized as a low-cost replacement for silver and gold nanoparticles which

are currently used in inkjet printing of conductive patterns. However, the main obstacle for using

copper nanoparticles is their spontaneous oxidation at ambient conditions. Here we describe the

synthesis of nonoxidizable copper nanoparticles by coating them with a silver shell, and inkjet printing

of these particles. The formation of these core–shell nanoparticles is driven by a transmetalation

reaction on the surface of copper nanoparticles, where the copper atoms present on the particles’

surface are used as the reducing agent for the silver. This process results in formation of solely

copper–silver core–shell nanoparticles, with no individual silver particles. It was found that coating 40

nm copper nanoparticles with a 2 nm layer of silver prevents oxidation of the copper core and preserves

its metallic characteristic. Characterization of these nanoparticles by HR-TEM, SEM, EDS, XRD,

spectrophotometry and XPS confirm the core–shell structure and their stability to oxidation. Inkjet

printing of concentrated aqueous dispersions of these copper–silver nanoparticles was done on various

substrates, and it was found that conductive and decorative patterns with metallic appearance, stable to

oxidation (up to 150 �C) are formed.

1. Introduction

The unique properties of metallic nanoparticles (NPs) pave the

way to new applications and possibilities of making new

products such as electronic, optical, and magnetic devices,1–4

nanoelectromechanical systems,5 conductive coatings and

pigments for inkjet inks,6 energy conversion and photothermal

devices,7 catalysts,8 biosensors, biolabels and drug delivery

systems.9 The most widely studied are nanoparticles of noble

metals such as Au, Ag and Pt.10,11 The main difficulty in utili-

zation of non-noble metals such as Co, Ni, Fe and Cu arises from

their tendency toward oxidation at ambient conditions, partic-

ularly as their size gets smaller.12,13

The use of silver NPs in conductive inks and its direct imaging

by inkjet printing technology14 has been known for years.

However, the very high cost of silver limits the wide industrial

applications as conductive, and obviously decorative metallic

inks. Since copper is much cheaper than silver but possesses

a very high conductivity (only 6% less than that of Ag) and

mirror-like appearance, Cu NPs can be considered as a replace-

ment for silver NPs. The main challenge in production and

utilization of Cu NPs is prevention of their spontaneous oxida-

tion in air. A well-known approach to overcome the oxidation

problem is based on the protection of NPs with a nonoxidizable

shell formed by ligands,15 polymers16 or silica.17,18 The main

disadvantage of this approach is the formation of a nonmetallic

coating. It was reported that successive reduction of a thin

Casali Institute for Applied Chemistry, Institute of Chemistry, The HebrewUniversity of Jerusalem, Jerusalem, 91904, Israel. E-mail: magdassi@cc.huji.ac.il

† Electronic supplementary information (ESI) available: XPS spectrum.See DOI: 10.1039/b821327e

This journal is ª The Royal Society of Chemistry 2009

nonoxidizable metal layer over the preformed core2,19–21 can

protect it from oxidation, while leaving most of the core proper-

ties unchanged. Such a process may lead to undesirable formation

of individual particles of the second metal, in addition to forma-

tion of a shell around the preformed NPs.22 To overcome this

drawback, a selective reduction method, which should take place

only at the surface of the core particle, is required. We present here

the formation of copper–silver core–shell (CucoreAgshell) NPs

with a tunable silver shell by the transmetalation method,23–25

i.e. reduction of silver ions on the surface of preformed Cu NPs,

while the copper is the reducer. By this method, the obtained

CucoreAgshell NPs have the optical and electric properties of both

metals, while the oxidation of the copper core is prevented.

Evaluation of the CucoreAgshell NPs stability and the use of

aqueous dispersions of these nanoparticles as conductive and

decorative inks for printing metallic patterns, which are stable to

oxidation on various substrates, are presented as well.

2. Result and discussion

2.1 Synthesis and characterization

To achieve the core–shell structure, we used a two-step process as

schematically presented in Fig. 1. At the first step aqueous

dispersion of Cu NPs is prepared by reduction of Cu(NO3)2 with

an excess of hydrazine hydrate in the presence of polyacrylic acid

sodium salt as a polymeric stabilizer, as we reported previously.26

The large excess of hydrazine prevents oxidation of the Cu NPs

in the aqueous dispersion, but only if it is kept in closed vials.

Exposure of such dispersion to air leads to immediate oxidation27

with a color change from red to bluish green.

At the second step a silver salt is added, and by trans-

metalation reaction the reduction of silver ions by the copper

J. Mater. Chem., 2009, 19, 3057–3062 | 3057

Fig. 1 Schematic illustration of a single Cu NP synthesis and the formation of a silver shell by the transmetalation reaction. The surface copper atoms

serve as reducing agents for the silver ions.

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metal takes place directly on the surface of CuNPs (Ag to Cu

atomic ratio of 0.12), thus leading to a shell of silver on the

copper core.

A problem which arises at the second step (due to the hydr-

azine excess) is the reduction of silver ions to silver NPs in the

solution, and therefore the result is a mixture of Ag and Cu NPs,

with some CucoreAgshell NPs. The difference in standard redox

potentials of silver (Ag+ + e�/ Ag, E0¼ 0.799 V) and hydrazine

(N2H4 + 4OH�/ N2 + 4H2O + 4e�, E0¼�1.16 V) is about 1.96

V; that is, much higher than the corresponding difference of

redox potential of silver and copper (0.46 V). In order to prevent

the reduction of silver ions in solution to form free silver NPs, the

hydrazine concentration must be reduced to zero. This was

achieved by the addition of acetaldehyde (C2H4O + N2H4 /

C2H6N2 + H2O) prior to the addition of silver nitrate.

The obtained dispersions of CucoreAgshell NPs are character-

ized by an orange–reddish color which, in contrast to the

uncoated Cu NPs, remains unchanged in an open vial. After

drying the dispersion and exposure to air, the color of the powder

also does not change.

As seen from the redox reaction stoichiometry (Fig. 1), each

copper atom should reduce two silver ions, thus the obtained

core–shell nanoparticles should have a similar, slightly larger,

particle size compared to the original Cu NPs. As follows from

high-resolution transmission electron microscope (HR-TEM)

images and dynamic light scattering (DLS) measurements

(Fig. 2), the size distribution of the obtained CucoreAgshell NPs is

indeed similar to that of the Cu NPs without coating. According

to DLS, the average size of CucoreAgshell NPs and Cu NPs is

about 34 and 32 nm, respectively.

Characterization of core–shell structures by TEM is reported

in many researches.21,28 However, in the case of thin shells (<2

nm), the differences between the core and the shell cannot be

revealed by TEM only (Fig. 2b, inset). XRD pattern, absorption

spectra and EDS data in scanning transmission electron

microscopy (STEM) mode are presented in Fig. 3. The results

confirm a core–shell structure as follows: (a) the XRD pattern of

the powder (Fig. 3a) indicates the presence of both copper and

silver, with fcc crystal structures (not their alloy); (b) the surface

plasmon spectra (Fig. 3b) of the uncoated Cu NPs (solid line) is

characterized by the 570 nm copper plasmon peak only; after the

addition of increasing amounts of silver nitrate (at Ag to Cu

atomic ratio of 0.02 to 1.0, dotted lines to dashed line), a silver

shell formation is accompanied by the appearance and growth of

the 410 nm silver plasmon peak; (c) EDS analysis in STEM mode

by scanning an individual CucoreAgshell NP (at Ag to Cu atomic

ratio of 0.12) along its diameter (Fig. 3c), indicates the presence of

3058 | J. Mater. Chem., 2009, 19, 3057–3062

the two metals in a single particle: a typical silver profile shows

a slightly higher intensity at the edges than at the center, while the

same scan for copper shows a complementary profile with higher

intensity at the center (Fig. 3d). Calculation of the expected shell

thickness for a 40 nm CucoreAgshell NP, based on the reagents

concentrations (assuming the complete reduction of the Cu and

Ag) gives a thickness of 1 to 2 nm, which are equivalent to 4 to 8

atomic layers of metallic silver. However, evaluation of the shell

thickness of such NPs (analysis of 10 NPs) according to the

differences in EDS profiles of silver and copper (Fig. 3d) leads to

a thickness of 2 to 5 nm (the difference between the two evalua-

tions may be explained by the non-uniform coverage of the silver

shell as appeared from the EDS profile of silver, Fig. 3d, yellow).

The XPS pattern (see ESI) of these NPs is characterized by the

peaks characteristic of both metals, and does not show any

indications for the presence of copper oxides. From these results,

it can be concluded that the Cu NPs are indeed coated by a thin

layer of silver. Since this layer should provide protection of

the copper against oxidation, the following experiment was

performed in order to verify this assumption. The changes in

XPS spectrum of a dried powder of the CucoreAgshell NPs were

monitored, while gradually etching the surface layer (with an

ionized argon beam) followed by exposure to oxygen. The XPS

peaks of 2p Cu are shown in Fig. 3e. The solid line spectrum is

characteristic of unoxidized copper, in spite of exposure of the

NPs to air. After etching, a drastic increase in the intensity of

copper bands is observed due to removal of the silver shell

(Fig. 3e, dashed line). After this stage, the sample was exposed to

air: as shown in Fig. 3e (dotted line), the intensity of metallic

copper peaks decreases, and new peaks of CuO appear.29

These results clearly indicate that the Cu NPs are indeed

protected from oxidation in both aqueous dispersion and in

powder form (at ambient conditions).

2.2 Inkjet printing

As stated earlier, Cu NPs are very attractive for obtaining

conductive and decorative patterns due to the high conductivity,

metallic appearance and lower price compared to silver. Since the

oxidation was prevented, these particles were further evaluated in

inkjet printing.

The inkjet ink formulation contained aqueous (25 wt% metal)

dispersion of CucoreAgshell (at Ag to Cu atomic ratio of 0.12) NPs

and a wetting agent, as described in the Experimental section.

The ink, which has a viscosity of 1.9 cP and surface tension of

23.9 mN m�1 was printed with a single nozzle MicroFab printer

and also with a Lexmark office printer. A drop of ink jetted from

This journal is ª The Royal Society of Chemistry 2009

Fig. 2 TEM and HR-TEM images of (a) the uncoated Cu NPs, (b) with

CucoreAgshell NPs, and (c) their particle size distribution according to

DLS (dotted line—Cu NPs, solid line—CucoreAgshell NPs).

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a 60 mm print-head orifice of the MicroFab printer is shown in

Fig. 4a. An optical image of a printed 300 mm wide line is shown

in Fig. 4b. HR-SEM cross-section image of this line (Fig. 4c)

reveals that the printed line (400 nm thickness) is composed of

closely packed nanoparticles. An example of an RFID antenna

printed by a Lexmark office printer on an inkjet paper is

presented in Fig. 4d.

This journal is ª The Royal Society of Chemistry 2009

2.2.1 Decorative inkjet printing. In order to evaluate the

applicability of the CucoreAgshell dispersion as decorative metallic

ink for inkjet printing, a 5 wt% dispersion was printed on glass

slides and on inkjet photo paper. An example of a pattern with

metallic brightness and mirror-like reflection is presented in

Fig. 5. The gloss values measured with the use of a glossy meter

were found to be 111 � 6 on glass and on inkjet photo paper,

quite close to the glossiness of a mirror, 124 � 4.

2.2.2 Conductive inkjet printing. Printed patterns composed

of metallic NPs are usually sintered at elevated temperatures in

order to decrease their resistivity. It was found that heating these

patterns in air to 150 �C did not cause any change in color.

Heating above that temperature resulted in color changes that, as

revealed by XRD analysis, are the result of CuO and Cu2O

formation. The copper oxide formation upon heating, based on

XRD analysis, is shown in Fig. 6a. At temperatures up to 150 �C

no oxide was detected, which can be explained by the protective

action of a silver shell. However, at 200 �C, about 30 wt% of the

copper was oxidized and up to 54 wt% at 400 �C. Further

analysis of the XRD results (by Shearer equation) reveals that at

these conditions the copper oxidation is accompanied by growth

of silver crystallites. Up to 150 �C, the size of the silver crystallites

does not change, but starts to increase at higher temperatures.

The resistivity of these patterns after heating to this tempera-

ture range, was found to be greater than 3.0 � 104 mUcm. Such

high resistivity (more than five orders greater than bulk copper)

is probably due to the oxide formation. In order to examine this

assumption, the printed pattern’s resistivity was measured after

heating under N2. It was found that conductive patterns could be

obtained and the resistivity decreased with the increasing

temperature (Fig. 6b). The resistivity after heating at 300 �C was

11 mUcm, which is only 7 times greater than that of bulk copper

(the resistivity measured after heating at 225 �C and lower

temperatures was above the upper threshold of the ohmmeter).

XRD analysis of the printed patterns which were heated under

N2 (at 300 �C) showed an increase in the size of the Ag crystallites

(similar to that presented in Fig. 6a), but copper oxides were not

detected. The absence of copper oxides indicates that the high

resistivity of the pattern below 225 �C is not due to copper

oxidation, but rather the inherent property of the Cu NPs pattern

(for example, the presence of the insulating organic stabilizer

between the NPs).

The heating process was followed by observing the printed

patterns by HR-SEM (Fig. 7).

As can be seen, at room temperature and after heating to

200 �C (under N2) the particles have irregular morphology with

numerous corners. However, after heating to 250 �C, the particle

surface becomes smoother, and at 275 �C sintering of the parti-

cles while forming continuous interconnections occurs. The sin-

tering process is accompanied by the appearance of new, smaller

NPs, which grow upon further heating to 300 �C, and according

to EDS analysis are Ag NPs. Such growth of Ag NPs correlates

well with the silver crystals size increase as revealed from the

XRD analyses. As illustrated in Fig. 8, during heating the silver

shell transforms into small particles, which are attached to the

copper core surface; at 250 �C, the Cu NPs are no longer coated

by a silver shell. Therefore, if exposed to oxygen at this elevated

temperature they can undergo oxidation, as was indeed found

J. Mater. Chem., 2009, 19, 3057–3062 | 3059

Fig. 3 (a) XRD pattern of a dried dispersion of CucoreAgshell NPs at room temperature with characteristic d-spacings of copper (2.089, 1.809 and 1.279 �A)

and silver (2.355, 1.444 and 1.231 �A). (b) Absorption spectra of diluted dispersions of CucoreAgshell NPs at increasing silver to copper atomic ratio (0, solid

line, 0.02 to 1.0, dotted lines to dashed line). (c) STEM image of a 40 nm particle. (d) Copper and silver elemental profile along the particle diameter according

to EDS analysis. (e) XPS analysis of a dried CucoreAgshell NPs powder (solid line), after Ar etching (dashed line) and after exposure to air (dotted line).

Fig. 4 Printing of a 25 wt% dispersion of CucoreAgshell NPs. A drop jetted

from a 60 mm print head orifice of MicroFab (a), a 300 mm wide pattern

obtained from the jetting along a line on a glass slide (b) and a HR-SEM

image of the pattern cross-section (c). An antenna printed using the same

dispersion with the use of a Lexmark office printer on an ink-jet paper (d).

Fig. 5 Decorative printing of a 5 wt% dispersion of CucoreAgshell NPs

with the use of a Lexmark office printer on an inkjet photo paper.

3060 | J. Mater. Chem., 2009, 19, 3057–3062

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while heating in air. However, it was found that after cooling to

room temperature, the patterns are stable to oxidation, and no

oxides were found (according to repeated XRD analysis) even

after several months.

2.3 Conclusions

We have developed a simple and effective transmetalation

approach for the synthesis of 10–50 nm CucoreAgshell NPs with

tunable shell thicknesses. These nanoparticles are characterized

by optical and conductivity properties of both metals, and by the

chemical stability of the silver shell. These particles can be

printed by an office inkjet printer and yield a conductive and

decorative patterns. Heating these particles to 200 �C causes the

destruction of the silver shell, thus causing exposure of the

This journal is ª The Royal Society of Chemistry 2009

Fig. 6 (a) The silver crystallite size (open circles) and the copper oxides

(CuO and Cu2O) percentage in heated patterns (filled circles) as a func-

tion of temperature. (b) The resistivity calculated for printed patterns (on

glass) heated under N2 as a function of temperature.

Fig. 8 Schematic illustration of the growing silver crystallites forming

the shell.

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copper core to the atmosphere, followed by oxidation. Further

research will focus on preventing the destruction of the shell

upon heating. According to our knowledge, there is no

commercial decorative metallic inkjet ink in the market, due to

the high cost of silver and gold, and the spontaneous oxidation of

Fig. 7 HR-SEM images of the printed pattern

This journal is ª The Royal Society of Chemistry 2009

cheaper metals. We believe that such CucoreAgshell inks can serve

as low-price metallic inks.

3. Experimental

3.1 Synthesis

Cu NPs were synthesized by the reduction of copper nitrate with

an excess of hydrazine hydrate in the presence of polyacrylic

acid sodium salt as described previously.26 For synthesis of

CucoreAgshell NPs, 100 ml of the obtained dispersion of Cu NPs

was diluted with 1000 ml of triple distilled water followed by

dropwise addition of 1.0 ml acetaldehyde. After 5 min of stirring,

0 to 29.4 ml of 1 wt% silver nitrate was added while stirring.

3.2 Characterization methods

TEM, HR-TEM and STEM images were obtained with a Tecnai

F20 G2 transmission electron microscope operating at 200 kV for

samples prepared on a carbon-coated nickel grid. HR-SEM

characterization was performed on Sirion (FEI) scanning elec-

tron microscope. X-Ray diffractograms (XRD) were obtained

for the dried samples using the D8 Advance (Bruker AXS)

diffractometer. The UV-vis absorption spectra were recorded

with the use of Cary 100 Bio (Varian) spectrophotometer. DLS

particle size analyses were performed using Zetasizer nano-SZ

(Malvern Instruments). XPS spectra were obtained with Axis

Ultra (Kratos Analytical) X-ray photoelectron spectrometer.

s heated under N2 at various temperatures.

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The gloss measurements were taken in the use of micro-TRI-

gloss m (Gardner) glossy meter at the position of 85�.

3.3 Ink formulation

For preparation of conductive inks, the dispersion of

CucoreAgshell NPs was washed and concentrated by a precipita-

tion–centrifugation–decantation process. More specifically,

as-synthesized CucoreAgshell NPs dispersion was acidified to pH

2.9 by a dropwise addition of 6.5% nitric acid while stirring. Then

the dispersion was centrifuged for two minutes at 800 g and

decanted. The obtained precipitate was redispersed by adjusting

the pH to 9 with 2-amino-2-methyl-1-propanol. This precipita-

tion–centrifugation–decantation process was repeated three

times. The concentration of metal in the obtained formulation

was adjusted to 25 wt% by the addition of a precise amount of

triple distilled water. BYK 348 at a concentration of 0.1 wt% was

added as a wetting agent. The obtained formulation was treated

in an ultrasonic bath (Branson 1510) for 10 min. Surface tension

and viscosity of the ink formulation were measured with the use

of FTA 125 (First Ten Angstroms) tensiometer and Brookfield

(LVDV-II+) viscometer, respectively. For the decorative ink

formulation, the same procedure was carried out, except that the

metal concentration was adjusted to 5 wt %.

Inkjet printing. The obtained ink was printed on glass slides

using a JetDrive III controller (MicroFab) with a 60 mm orifice

print head with the following pulse shape: Rise: 5 ms, Dwell: 20

ms, Fall: 5ms, Echo: 50 ms, Final rise: 5 ms, and voltage between

100 and �100 V at a frequency of 60 Hz. The same ink was also

printed with a commercial Lexmark Z615 office printer on Plus

Glossy (Canon) inkjet photo paper.

The resistivity of printed patterns was calculated by measuring

the printed line resistance by a four-probe milliohm meter

(EXTECH) and measuring the thickness profile by HR-SEM.

Acknowledgements

This project was supported by the European Community Sixth

Framework Program through a STREP grant to the SELECT-

NANO Consortium, Contract No.516922.03/25/2005.

3062 | J. Mater. Chem., 2009, 19, 3057–3062

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