Preparation of submicrometer-sized copper and silver crystallites by a facile solvothermal complexation-reduction route

Article abstract of DOI:10.1016/j.jssc.2005.03.009

Pure submicrometer-sized copper and silver crystallites have been directly synthesized via solvothermal treatment of CuCl₂·2H ₂O or AgNO₃ in ethylenediamine (EDA) at 80-180 °C for 15-20 h, and characterized by means of X-ray powder diffraction, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and transmission electron microscopy. It was suggested that the formation of copper and silver crystallites in this solvothermal system be through a typical complexation-reduction process, in which EDA serves not only as a reducing reagent, but also as a complexing solvent.

Full text of DOI:10.1016/j.jssc.2005.03.009

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Journal of Solid State Chemistry 178 (2005) 1609–1613
www.elsevier.com/locate/jssc
Preparation of submicrometer-sized copper and silver crystallites
by a facile solvothermal complexation–reduction route
Ã
Ã
Yong Cai Zhang , Gui Yun Wang, Xiao Ya Hu , Rong Xing
College of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou City 225002, China
Received 20 January 2005; received in revised form 27 February 2005; accepted 2 March 2005
Abstract
Pure submicrometer-sized copper and silver crystallites have been directly synthesized via solvothermal treatment of CuCl ꢀ 2H O
2
2
or AgNO in ethylenediamine (EDA) at 80–180 1C for 15–20 h, and characterized by means of X-ray powder diffraction, Fourier
3
transform infrared spectroscopy, X-ray photoelectron spectroscopy and transmission electron microscopy. It was suggested that the
formation of copper and silver crystallites in this solvothermal system be through a typical complexation–reduction process, in
which EDA serves not only as a reducing reagent, but also as a complexing solvent.
r 2005 Elsevier Inc. All rights reserved.
Keywords: Metals; Crystal growth; Characterization; Solution-phase synthesis; Solvothermal
1
. Introduction
pressure [26–29], hydrothermal and solvothermal
30–32], etc. Nevertheless, some of the above-mentioned
[
Compared with the conventional bulky counterparts,
synthetic techniques either need special precursors and
harsh conditions and cost high in the equipments, or
have difficulties in purification of the final products, or,
even worse, sometimes the as-produced metal powders
may suffer from several unwanted characteristics,
including low yield, poor crystallinity, uncontrollable
size, and surface oxidation, etc.
The solvothermal method provides a simple and
efficient way for the synthesis of high-quality metal
powders in the intermediate thermal (100–250 1C) region
[32–35]. The choice of appropriate organic solvents
played a key role in the solvothermal synthesis, because
the solvent properties such as redox, polarity, com-
plexation and viscosity, vapor tension, etc. strongly
influence the solubility and transport behavior of the
precursors involved in such heterogeneous liquid–solid
reactions [33–35]. Ethylenediamine (EDA) was found to
be an excellent solvent for solvothermal synthesis of
novel nanomaterials [33–36], in which the polarity
and bidentate ligand of the solvent can successfully
control the size and shape of the resultant products.
Moreover, due to its strong complexing capacity and
small-grained copper (Cu) and silver (Ag) crystallites
exhibit notable improvements in mechanical [1,2],
catalytic [3,4], optical [5–7] and other physical and
chemical properties [1,8,9], which have spurred con-
siderable scientific and technical interest recently [1–29].
In particular, high-pure, well-crystalline, submicrom-
eter-sized Cu and Ag powders are highly desirable for
production of conductive paste in electronic industry
[
26]. As a result, a great variety of physical and chemical
methods have been developed for preparing submic-
rometer- or nanometer-sized Cu and Ag crystallites up
to date, such as inert gas condensation [10], vacuum
vapor deposition and vapor–solid reaction growth
[
11,12], ball milling [13], laser, UV and microwave
irradiation [14–16], radiolytic reduction [17], sonochem-
ical [18], templating [19–22], microemulsion [23,24],
electrochemical [25], chemical reduction of various
copper and silver compounds in solutions under normal
Ã
Corresponding author. Fax: +86 514 7975244.
E-mail address: zhangyc@yzu.edu.cn (Y.C. Zhang).
0022-4596/$ - see front matter r 2005 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2005.03.009

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Y.C. Zhang et al. / Journal of Solid State Chemistry 178 (2005) 1609–1613
1610
easy formation of relatively stable N-chelate compound
with various metal ions, EDA has also been used to
activate the metal surface by removing the surface
oxides [37,38]. However, to our knowledge, though the
reductibility of EDA under high-pressure and high-
temperature liquid environments has already been
known for a long time, the utilization of it as a reducing
reagent, as well as a complexing solvent, was seldom
explored in solvothermal synthesis of metal crystallites
images were taken on Philips Tecnai12 microscope
operating with the accelerating voltage of 120 kV.
3. Results and discussion
Fig. 1 shows the XRD patterns of the resultant products
via solvothermal treatment of CuCl ꢀ 2H O in EDA at
2
2
180 1C for 15 and 20 h. The strong and sharpdiffraction
[
33,34]. Herein, we report a facile route to pure
peaks suggested that the resultant products were well
crystalline. All the XRD peaks can be indexed to face-
centered cubic structure Cu, and no obvious XRD peaks
arising from the possible impurity phases such as CuCl,
submicrometer-sized Cu and Ag crystallites via the
low-temperature (80–180 1C) solvothermal reduction of
CuCl ꢀ 2H O and AgNO by EDA, which simulta-
2
2
3
neously acts as a complexing solvent in an autoclave.
The as-prepared Cu and Ag crystallites are characterized
using X-ray powder diffraction (XRD), Fourier trans-
form infrared spectroscopy (FTIR), X-ray photoelec-
tron spectroscopy (XPS), and transmission electron
microscopy (TEM). It is worthwhile to point out that
in the present solvothermal route, no extreme conditions
such as an absolutely non-aqueous, non-oxygen envir-
onment are required. A small amount of water (for
example, the crystal water in the starting material,
CuCl ꢀ 2H O; or even mixed solutions of water and
CuO and Cu O were observed. The lattice constants
2
calculated from the XRD data were a ¼ 3:6165 and
˚
3
.6159 A for the products obtained at 180 1C for 15 and
20 h, respectively, which also agree with the literature
value given by JCPDS file No. 4-836. The yields of both
products were above 62%, with reference to the amount of
CuCl ꢀ 2H O used. However, when the reaction tempera-
2
2
ture was lower than 170 1C (20 h) or the reaction time was
shorter than 10 h (180 1C), no Cu powders were obtained.
This fact indicated that suitable reaction temperature (e.g.,
180 1C) and time (e.g., 15 h) were needed to bring about
the redox reactions between CuCl ꢀ 2H O and EDA
2
2
EDA in the case of synthesizing Ag powders) and
oxygen proves not to be a serious problem.
2
2
under the solvothermal conditions.
+
By contrast, since the oxidation ability of Ag (E
É
+
(
Ag /Ag) ¼ 0.7991 V) is much stronger than that of
2+ É 2+
Cu
between AgNO and EDA under solvothermal condi-
(E (Cu /Cu) ¼ 0.3394 V), the redox reaction
2
. Experimental
3
tions can be initiated at a rather lower temperature (e.g.,
All the reagents used in our experiments are of
analytically pure grade. In a typical synthesis procedure,
8
0 1C). Fig. 2 shows the XRD patterns of the resultant
products via solvothermal treatment of AgNO in EDA
3
0
.005 mol of CuCl ꢀ 2H O or AgNO powders was put
2
2
3
or a mixed (v/v ¼ 1:1) solution of distilled water
and EDA at 80–160 1C for 20 h. All of the resultant
products displayed similar XRD peaks corresponding to
cubic phase of Ag (JCPDS file No. 4-783), and no
into a Teflon-lined stainless-steel autoclave of 50 mL
capacity, to which 40 mL of EDA or a mixture of EDA
and water was added. The autoclave was maintained at
8
0–180 1C for 10–20 h in an electric oven, and then air
cooled to room temperature. The resulting solid was
filtered, washed with distilled water and ethanol, and
purified by ultrasonication in ethanol to remove residue
EDA attached on the surface. During the filtration, the
topsurface of the pr oduct was always covered with
liquids to prevent its exposure to the air. The products
were dried in vacuum and stored in ethanol.
(111)
(200)
(220)
XRD patterns of the obtained products were recorded
(b)
(a)
2
2
on a Japan Mac Science Corp. M03XHF X-ray
diffractometer with CuKa radiation at room tempera-
ture. FTIR spectra in the wavenumber range of
ꢁ
1
4
00–4000 cm
were recorded on a Nicolet AVAR-
AT360 FT-IR Spectrometer at room temperature with
samples in a KBr wafer. XPS measurements were
performed using a PHI-5300/ESCA system with MgKa
radiation as the exciting source, where the binding
energies were calibrated by referencing the C 1s peak
2
0
30
40
50
60
70
80
2
θ (deg.)
Fig. 1. XRD patterns of the resultant products via solvothermal
treatment of CuCl O in EDA at 180 1C for (a) 15 and (b) 20 h.
ꢀ 2H
(
284.6 eV) to reduce the sample charge effect. TEM
2
2

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Y.C. Zhang et al. / Journal of Solid State Chemistry 178 (2005) 1609–1613
1611
FTIR analysis result indicated that the as-obtained
products were free of the solvent. The preparation of
naked metal nanoparticles may offer an opportunity
that different surface functionalities can be readily
introduced for template-assisted or self-assembly-based
fabrication methods for nanoscale devices [39]. Fig. 4(a)
shows the XPS spectrum of Cu 2p of the product derived
from solvothermal treatment of CuCl ꢀ 2H O in EDA at
(111)
(200)
(311)
(220)
(
e)
(d)
2
2
(
c)
1
80 1C for 20 h. Two peaks corresponding to Cu 2p3=2
and Cu 2p1=2 appeared at 932.6 and 952.4 eV,
respectively. The peak position/binding energy, line-
shape and peak-to-peak separation (19.8 eV) of the XPS
spectrum were all indicative of elemental Cu free of
surface oxidation [40]. Fig. 4(b) shows the XPS
spectrum of Ag 3d of the product derived from
(
b)
a)
(
20
30
40
50
60
70
80
2
θ (deg.)
solvothermal treatment of AgNO in EDA at 100 1C
for 20 h. The binding energies for Ag 3d5=2 and Ag
3d3=2 were identified at 368.0 and 374.0 eV, respectively,
which were in better agreement with those of metallic Ag
3
Fig. 2. XRD patterns of the resultant products via solvothermal
treatment of AgNO
in (a) a mixed (v/v ¼ 1:1) solution of distilled
water and EDA at 100 1C for 20 h, or in pure EDA at (b) 80, (c) 100,
d) 140 and (e) 160 1C for 20 h.
3
(
and not with those of Ag O [40]. Thus, it can
2
be concluded on the basis of the above XRD, FTIR,
XPS and TEM analysis results that pure, submi-
crometer-sized Cu and Ag crystallites have been
synthesized through this simple and mild solvothermal
processing.
characteristic XRD peaks of the possible Ag O impurity
2
can be visible, indicating the preparation of phase-pure
Ag powders under the milder solvothermal conditions
than those needed in synthesizing Cu. The yields of the
products (with respect to the amount of AgNO used)
3
In our solvothermal synthesis of Cu and Ag crystal-
lites, the source materials used were merely
CuCl ꢀ 2H O or AgNO and EDA, respectively. Under
were enhanced from about 58% to 90%, when the
reaction temperatures were increased from 80 to 160 1C,
keeping the other synthetic parameters unchanged.
Furthermore, the diffraction peaks of the resultant Ag
powders became obviously sharper as the reaction
temperatures were increased from 80 to 160 1C, suggest-
ing that the products grew larger and better crystallized
at higher reaction temperatures according to the well-
known Debye–Scherrer formula.
2
2
3
the high-pressure and alkalescent solvothermal condi-
tions, EDA mainly takes the role of electron transfer,
displaying moderate intensity reductive characteristic
[33,34,41]. Meanwhile, being a strongly chelating and
polarizing solvent, EDA can greatly enhance solubility,
diffusion and reactivity of the reactant species and
subsequent crystal growth of the products. Especially,
2
+
+
The TEM images of the resultant Cu powders via
solvothermal treatment of CuCl ꢀ 2H O in EDA at 180 1C
when EDA was chelated with Cu
relatively stable complex ions, [Cu(EDA)x]
or Ag to form
2
+
(one of
2
2
for 15 and 20h are shown in Fig. 3(a) and (b), while those
of the resultant Ag powders via solvothermal treatment of
the evidences was that the solution color changed
rapidly from transparent to light then to dark blue,
when CuCl ꢀ 2H O was added into EDA) or [Ag(E-
AgNO in EDA at 80 and 100 1C for 20 h are shown in
3
2
2
+
Fig. 3(c) and (d), respectively. As can be seen from Fig.
3(a) and (b), the Cu powders derived from solvothermal
DA)] , the short distances between copper or silver and
nitrogen atoms in the complex ions were favorable for
2
+
+
electron to transfer from N to Cu or Ag , resulting in
treatment of CuCl ꢀ 2H O in EDA at 180 1C for 15 and
2
2
2
+
+
or [Ag(EDA)] to
2
8
0 h consisted of elliptoid crystallites with the sizes of
0–230 and 190–250 nm, respectively. The TEM images in
the reduction of [Cu(EDA)x]
elemental Cu or Ag. Furthermore, the elevated pressure
autogenerated in the solvothermal system afforded a
greater driving force for the reaction processes to occur
[42,43]. Therefore, it was suggested that in our case, the
formation of Cu and Ag crystallites by solvothermal
treatment of CuCl ꢀ 2H O or AgNO powders in pure
Fig. 3(c) and (d) revealed that the solvothermally
synthesized Ag comprised irregular morphological crystal-
lites with the sizes increasing from 70–320 to 150–580 nm
when the reaction temperature was raised from 80 to
100 1C (20 h). These characterization results of the
2
2
3
products by TEM are also consistent with those of
XRD analysis, indicating that the sizes of the resultant
metal powders strongly depended on their preparation
conditions, such as the reaction temperature and time.
The purity of the as-prepared Cu and Ag crystallites
was further confirmed by FTIR and XPS analysis. The
EDA at low temperatures (80–180 1C) be via a typical
complexation–reduction process:
CuCl2 ꢀ 2H2O þ xEDA
solvothermal
2
!
½CuðEDAÞxꢂ þ ꢁ! Cu;

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Y.C. Zhang et al. / Journal of Solid State Chemistry 178 (2005) 1609–1613
1612
Fig. 3. TEM images of the resultant Cu powders via solvothermal treatment of CuCl
the resultant Ag powders via solvothermal treatment of AgNO
2
ꢀ 2H
2
O in EDA at 180 1C for (a) 15 and (b) 20 h, and those of
3
in EDA at (c) 80 and (d) 100 1C for 20 h.
AgNO þ EDA
and non/less-complexing solvents, the starting materials
remained essentially unchanged, implying that no
3
solvothermal
!
½AgðEDAÞꢂþ ꢁ! Ag:
solvothermal redox reactions between CuCl ꢀ 2H O
2
2
and these solvents took place. So, in the whole
solvothermal reaction processes, EDA played important
roles in the formation of pure Cu and Ag crystallites.
To improve our understanding of the proposed
solvothermal complexation–reduction mechanism, ex-
periments were conducted under similar or even more
rigorous conditions in the case of synthesizing Cu
crystallites, when EDA was replaced by several non/
less-reducibility and non/less-complexing solvents such
as tetrachloride carbon, trichloromethane, benzene,
pyridine, N,N-dimethylformamide, etc. The analysis
results demonstrated that in these non/less-reducibility
4. Conclusions
In summary, a facile solvothermal route has been
adopted to synthesize submicrometer-sized Cu and Ag
crystallites, which made use of the reactions between

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Y.C. Zhang et al. / Journal of Solid State Chemistry 178 (2005) 1609–1613
1613
Foundation (No. BK2002045), and the Yangzhou
University Natural Science Foundation (No.
HK0413155).
Cu 2p3/2
Cu 2p1/2
References
[
[
[
1] Y. Wang, Nature 419 (2002) 912.
2] L. Lu, Y. Shen, X. Chen, Science 304 (2004) 422.
3] J.R. Monnier, J.W. Medlin, M.A. Barteau, J. Catal. 203 (2001)
3
62.
[
[
4] G. Vitulli, M. Bernini, S. Bertozzi, Chem. Mater. 14 (2002) 1183.
5] Y. Takeda, C.G. Lee, Nucl. Instrum. Meth. Phys. Res. B 191
(
2002) 422.
[6] D.H. Jeong, Y.X. Zhang, M. Moskovits, J. Phys. Chem. B 108
2004) 12724.
9
30
940
950
960
970
(
(a)
Binding energy (eV)
[
[
[
7] A. Maali, T. Cardinal, M.T. Delapierre, Physica E 17 (2003) 559.
8] C.T. Campbell, S.C. Parker, D.E. Starr, Science 298 (2002) 811.
9] P.V. Kamat, J. Phys. Chem. B 106 (2002) 7729.
Ag 3d5/2
[
[
10] H. Gleiter, Prog. Mater. Sci. 33 (1989) 223.
11] Z.W. Liu, Y. Bando, Adv. Mater. 15 (2003) 303.
Ag 3d3/2
[12] M.Y. Yen, C.W. Chiu, C.H. Hsia, Adv. Mater. 15 (2003) 235.
[
[
13] M.A. Thein, M. Gupta, Mater. Sci. Tech. 19 (2003) 1473.
14] T.Y. Chen, S.F. Chen, H.S. Sheu, J. Phys. Chem. B 106 (2002)
9
717.
15] S. Kapoor, D.K. Palit, T. Mukherjee, Chem. Phys. Lett. 355
2002) 383.
[
(
[
[
[
[
16] H. Yin, T. Yamamoto, Y. Wada, Mater. Chem. Phys. 83 (2004) 66.
17] A. Henglein, J. Phys. Chem. B 104 (2000) 1206.
18] V.G. Pol, D.N. Srivastava, O. Palchik, Langmuir 18 (2002) 3352.
19] E. Hao, K.L. Kelly, J.T. Hupp, J. Am. Chem. Soc. 124 (2002)
1
5182.
20] G.L. Hwang, K.C. Hwang, Y.T. Shieh, Chem. Mater. 15 (2003)
353.
[
3
64
368
372
376
380
1
(b)
Binding energy (eV)
[
[
[
[
[
21] S. Behrens, J. Wu, W. Habicht, Chem. Mater. 16 (2004) 3085.
22] J. Wang, M. Tian, T.E. Mallouk, J. Phys. Chem. B 108 (2004) 841.
23] M.P. Pileni, J. Tanori, A. Filankembo, Langmuir 14 (1998) 7359.
24] R.P. Bagwe, K.C. Khilar, Langmuir 16 (2000) 905.
Fig. 4. XPS spectrum of Cu 2p of the resultant Cu powders via
solvothermal treatment of CuCl O in EDA at 180 1C for 20 h (a),
ꢀ 2H
and that of Ag 3d of the resultant Ag powders via solvothermal
treatment of AgNO in EDA at 100 1C for 20 h (b).
2
2
25] M.E.T. Molares, V. Buschmann, D. Dobrev, Adv. Mater. 13
2001) 62.
3
(
[
[
[
26] A. Sinha, B.P. Sharma, Mater. Res. Bull. 37 (2002) 407.
27] Y. Sun, B. Mayers, T. Herricks, Nano Lett. 3 (2003) 955.
28] P. Raveendran, J. Fu, S.L. Wallen, J. Am. Chem. Soc. 125 (2003)
CuCl ꢀ 2H O or AgNO and EDA at 80–180 1C for
2
2
3
1
29] Z. Zhang, M. Han, Chem. Phys. Lett. 374 (2003) 91.
3940.
1
5–20 h in an autoclave. The formation of pure Cu and
[
[
[
Ag crystallites in this system was thought to be through
a typical solvothermal complexation–reduction mechan-
ism, in which EDA acted as both the reducing agent and
the complexing solvent. Such a soft solution synthetic
strategy to metal submicrometer-sized crystallites used
only common and cheapchemicals, and required no
stabilizing agents, elaborating ambience control (e.g., no
water and oxygen) and expensive equipment, which will
ensure easier operation and greatly reduce the produc-
tion cost, and thus offer great chance for scale-up
preparation of high quality metal powders.
30] Z. Liu, Y. Yang, J. Liang, J. Phys. Chem. B 107 (2003) 12658.
31] Y.C. Zhang, R. Xing, X.Y. Hu, J. Cryst. Growth 273 (2004) 280.
[32] G. Wei, C. Nan, Y. Deng, Chem. Mater. 15 (2003) 4436.
[
[
[
33] Z. Zhang, D.A. Blom, Z. Gai, J. Am. Chem. Soc. 125 (2003) 7528.
34] Y. Gao, H. Niu, C. Zeng, Chem. Phys. Lett. 367 (2003) 141.
35] W.S. Sheldrick, M. Wachhold, Angew. Chem. Int. Ed. 36 (1997)
2
36] Z.-X. Deng, C. Wang, Y.-D. Li, Inorg. Chem. 41 (2002) 869.
07.
[
[37] E. Ramli, T.B. Rauchfuss, C.L. Stern, J. Am. Chem. Soc. 112
1990) 4043.
(
[
[
[
[
38] G. Henshaw, I.P. Parkin, G.A. Shaw, J. Chem. Soc. Dalton
Trans. (1997) 231.
39] D.D. Evanoff Jr., G. Chumanov, J. Phys. Chem. B 108 (2004)
1
3948.
40] C.D. Wagner, Handbook of X-ray Photoelectron Spectroscopy,
Perkin-Elmer Corporation, Minnesota, 1979, p. 519, 529.
41] Y.C. Zhang, T. Qiao, X.Y. Hu, J. Solid State Chem. 177 (2004)
Acknowledgments
4
093.
This work is supported by the National Natural
Science Foundation of China (No. 20175023 and
[
[
42] P.R. Bonneau, R.F. Jarvis Jr., R.B. Kaner, Nature 349 (1991) 510.
43] C.H. Wallace, S.H. Kim, G.A. Rose, Appl. Phys. Lett. 72 (1998)
596.
2
0375034), the Jiangshu Province Natural Science