Effect of various factors on the dispersity of copper nanopowders produced by reduction of copper salts with glycerol

Article abstract of DOI:10.1134/S1070427209060093

Effect of the nature of a copper salt and initiators on the dispersity of copper powders was studied. The dispersity was determined by electron microscopy, X-ray phase analysis, and method of small-angle X-ray scattering, as well as from the specific surf

Full text of DOI:10.1134/S1070427209060093

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2009, Vol. 82, No. 6, pp. 981–985. © Pleiades Publishing, Ltd., 2009.
Original Russian Text © I.I. Obraztsova, G.Yu. Simenyuk, N.K. Eremenko, 2009, published in Zhurnal Prikladnoi Khimii, 2009, Vol. 82, No. 6, pp. 926–930.
PHYSICOCHEMICAL STUDIES
OF SYSTEMS AND PROCESSES
Effect of Various Factors on the Dispersity
of Copper Nanopowders Produced by Reduction
of Copper Salts with Glycerol
I. I. Obraztsova, G. Yu. Simenyuk, and N. K. Eremenko
Kemerovo Branch of the Institute of Solid-State Chemistry and Mechanochemistry, Siberian Branch,
Russian Academy of Sciences, Kemerovo, Russia
Received June 18, 2008
Abstract—Effect of the nature of a copper salt and initiators on the dispersity of copper powders was studied.
The dispersity was determined by electron microscopy, X-ray phase analysis, and method of small-angle X-ray
scattering, as well as from the specific surface area found by the BET technique.
DOI: 10.1134/S1070427209060093
Of considerable recent interest in view of the
intensive development and wide introduction of com-
puter technologies and various electronic equipment in
all fields of science, technology, and industries is the
replacement of noble metals with composites based on
ultradispersed copper nanopowders, because the latter
are nearly as good in electrical and heat conductivity,
but are substantially less expensive and more ac-
cessible. Therefore, development of techniques for
production of nanosize copper particles and materials
based on these powders is a topical task of indubitable
importance.
copper powders can yield composites with increased
stability (epoxy composites, >10 years; novolac com-
–6
posites, >6 years) and electrical conductivity (10 –
–
7
10 Ω m) [4]. The influence exerted by the nature of a
reducing agent on the dispersity, stability, and
electrical conductivity of copper powders has been
analyzed [5].
In this study, the influence exerted by the nature of
a copper(II) salt and initiators on the dispersity of
copper powders produced by reduction of copper salts
with glycerol was examined. New dispersity data were
compared with data on the electrical conductivity of
novolac composites based on these powders, reported
in [4, 6].
Electronic engineering for the most part uses
powders produced by explosion of conductors, con-
densation techniques, and electrolysis, which requires
specialized equipment and involves gross energy
expenditure. The most economically efficient are
chemical techniques for production of copper powders
by reduction of copper salts. This enables wide
variation of the size and shape of powder particles and
of the impurity content and, consequently, allows
purposeful control over their properties.
EXPERIMENTAL
Copper nanopowders (NPs) produced by reduction
of copper salts with glycerol were used in the study.
As starting copper compounds served salts of both
inorganic (sulfate, basic carbonate) and organic
(formate, acetate) acids. As initiators of the process of
copper sulfate reduction with glycerol were used
various organic acids-reducing agents (ascorbic,
acetylsalicylic, and citric acids). The procedures used
were described in [4].
Previously, methods for production of ultra-
dispersed copper powders by reduction of various cop-
per(II) salts with glycerol, ascorbic acid, and sodium
hypophosphite have been developed [1–3]. It has been
shown that chemical modification of ultradispersed
Copper nanopowders were studied using high-
resolution transmission electron microscopy (HRTEM,
9
81

9
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OBRAZTSOVA et al.
Table 1. Physicochemical properties of copper nanopowders produced by reduction of various copper salts with glycerol
2
–1
Sample no.
Copper(II) salt
Cu(HCOO)
Cu(CH COO)
CuCO ·Cu(OH)
CuSO ·5H
τ
decomp, min
T
decomp, °C
D
Cu, nm
S
sp, m g
1
2
3
4
2
30
40
135–140
145–155
155–160
180–185
29±1
38±2
44±2
51±3
2.90±0.14
2.00±0.12
1.40±0.07
0.50±0.03
3
2 2
·H O
3
2
14
4
2
O
180
PEM-125), X-ray phase analysis (XPA, DRON-3.0,
The aim of this study was to determine the specific
surface area and average crystallite sizes of the copper
nanopowders obtained. The results are listed in Table 1.
CuK radiation, λ = 0.15418 nm), and small-angle X-
α
Cu
ray scattering method (SAXS, KRM-1). The specific
surface area of the powders was determined by the
BET method from the argon adsorption. The average
particle size DCu was calculated by the Warren method
with allowance for the instrumental width of the
diffractometer, found by analyzing a well-annealed
homogeneous copper powder with particle sizes of 1–
It can be seen that, in reduction of readily
decomposing salts, it is possible to raise severalfold
the specific surface area of powders, compared with
copper sulfate, and to make substantially smaller the
constituent crystallites. As the decomposition tempera-
ture of a complex decreases, the specific surface area
grows and the average crystallite size becomes smaller
1
0 μm [7].
The particle size distribution was determined from
small-angle X-ray scattering curves [8].
(
according to XPA data). X-ray diffraction patterns of
copper NPs produced by reduction of various copper
salts with glycerol are shown in Fig. 1.
It is known [9] that, in the general case, the
chemical reduction of metal salts in solutions occurs
with acceleration and, as a rule, includes three
characteristic transformation regions: induction period
and stages of acceleration and decay of the trans-
formation.
It was found that, in the case of copper(II) sulfate
reduction with glycerol, the duration of the induction
period is several hours. Further, the reaction rate
noticeably increases because, presumably, the metallic
clusters being formed catalyze this process, in
agreement with the concepts accepted in the literature.
It can be assumed that introduction of reducing
agents, such as various organic acids, into the reaction
mixture makes shorter the induction period of the
reduction process and raises the dispersity of the
powders obtained.
It has been shown previously that organic acids-
reducing agents (L-ascorbic, oxalic, citric, formic, or
acetylsalicylic), added in small amounts, initiate the
reduction of copper sulfate by glycerol. Introduction of
these acids enables the reaction to proceed at lower
temperatures, makes the process substantially faster
(
with acetyl-salicylic acid, the process duration
becomes 7 times shorter), leads to an increase in the
dispersity of copper powders, and raises the electrical
conductivity and stability of their composites with
novolac resin, which is confirmed by HRTEM and
XPA data (Figs. 2, 3).
Fig. 1. X-ray diffraction patterns of copper nanopowders
produced by reduction of various copper(II) salts with
glycerol. (2θ) Bragg angle; the same for Fig. 3. (1–4) Sample
numbers in Table 1.
It can be seen in Fig. 2 that, compared with copper
NPs produced by copper sulfate reduction with
glycerol in the absence of initiators (1), the NPs
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EFFECT OF VARIOUS FACTORS ON THE DISPERSITY
983
Fig. 2. Electron micrographs of copper NPs produced by reduction of copper(II) sulfate with glycerol in the presence of initiators,
organic acids. (1–4) Sample numbers in Table 2; the same for Figs. 3, 4.
obtained with initiators (L-ascorbic, acetylsalicylic,
and citric acids) have a higher dispersity. The highest
dispersity is observed in the cases of citric (4) and
acetylsalicylic (3) acids. The minimum particle sizes in
Fig. 2 are 10–40 nm. As a rule, aggregates of the
particles are not larger than 100–200 nm. With L-
ascorbic acid (2), the amount of aggregates with sizes
of 100–200 nm is somewhat larger than that with citric
acid. The powders obtained without initiators are more
aggregated (1) and contain aggregates up to 1–2 μm in
size, composed of finer particles.
no. 4). Apparently, the powders obtained with ascorbic
and citric acids as initiators contain almost no copper
oxide impurities. If, however, the role of initiator is
played by acetylsalicylic acid decomposing below the
reaction temperature (Tdecomp = 140°C), copper(I) oxide
is present in the powders.
The peak broadening in the X-ray diffraction
patterns was used to calculate the average crystallite
size by the Warren method. The data obtained are
listed in Table 2.
Figure 3 shows X-ray diffraction patterns of copper
powders produced by copper(II) sulfate reduction with
glycerol without an initiator (sample no. 1) and in the
presence of initiators: L-ascorbic (sample no. 2),
acetylsalicylic (sample no. 3), and citric acids (sample
Apparently, the introduction of organic acids-
initiators results in the decrease of crystallite size and
the growth of specific surface area, compared with a
sample obtained without an initiator. The smallest
crystallite size and largest specific surface area are
Table 2. Physicochemical properties of copper nanopowders produced by reduction of copper(II) sulfate with glycerol in the
presence of initiators
2
–1
Sample no.
c
Indicator (acid)
–
T
decomp, °C
τ
decomp, min
D
Cu, nm
S
sp, m g
1
2
3
4
1:10.0
180–185
140–145
155–160
170–175
180
51±3
49±2
44±2
31±1
0.50±0.03
0.70±0.03
1.80±0.10
2.70±0.13
1:7.5:0.020
1:10.0:0.050
1:7.5:0.060
L-Ascorbic
Acetylsalicylic
Citric
40
25
100
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9
84
OBRAZTSOVA et al.
the presence of L-ascorbic acid as initiator, and at 60–
0 nm for acetylsalicylic acid, with the second peaks
7
beginning at 40–50 nm in the first two cases. With
acetylsalicylic acid, the second peak is very broad
(
from 10 to 200 nm) and has an almost constant
intensity in the range from 50 to 90 nm. The only
exception is the powder produced with citric acid as
initiator. In this case, only a single peak is clearly seen,
with the maximum at 25–30 nm.
Separately are the first peaks presented of the
distribution functions (Fig. 4b), calculated using a
special procedure [10]. It can be seen that for all the
powders the maximum of the peak is at 5–6 nm. For
powders produced in the presence of ascorbic and
citric acids, the maximum is shifted to the left. The
peak has the highest intensity for the copper powder
produced by reduction in the presence of citric acid as
initiator.
Fig. 3. X-ray diffraction patterns of copper nanopowders
produced by reduction of copper sulfate with glycerol.
X-ray data (XPA, SAXS) are in good agreement
with results obtained using electron microscopy.
Apparently, the highest dispersity is observed with
citric acid used as initiator.
observed for powders produced by reduction with
glycerol, with citric acid as initiator. In addition, these
powders contain no admixtures of copper(I) oxide.
Comparison of the dispersity data obtained in this
study with previously reported data on the electrical
conductivity and stability of the composites [4, 6]
suggests that introduction of organic acids-reducing
agents not only intensifies the reduction process and
raises the NP dispersity, but also enables synthesis of
composites with prolonged stability (>6 years) and
high electrical conductivity even with a novolac resin.
The highest stability is observed for the composites
produced with an organic acid that is the most stable
against decomposition (at the temperature of the
reaction of copper sulfate reduction by glycerol), citric
Figure 4 shows weight particle size distributions
calculated from small-angle scattering curves.
Apparently, a bimodal particle size distribution is
observed for almost all powders, with the first peak at
5
–10 nm, and the second, at 60–100 nm. It can be seen
that, with initiators used, the peak maxima are shifted
to smaller particle sizes. For example, the maximum of
the main (second) peak lies at 100–110 nm for the
copper powder produced without initiators (Fig. 4,
sample no. 1), at 80–85 nm for the powder produced in
(
a)
(b)
Fig. 4. Mass particle size distribution functions in the ranges (a) 0–200 and (b) 0–20 nm for copper nanopowders produced by
reduction of copper(II) sulfate with glycerol in the presence of organic acids. (d) Particle diameter and (I) intensity of the
distribution peak.
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EFFECT OF VARIOUS FACTORS ON THE DISPERSITY
985
acid, as initiator. This is presumably accounted for by
stabilization (liganding) of copper particles in the
course of the reaction with this acid.
(4) The highest dispersity was observed in reduc-
tion of copper formate with glycerol and in reduction of
copper sulfate in the presence of citric acid as initiator.
This circumstance makes it possible to synthesize
copper nanopowders with high dispersity, stability, and
electrical conductivity not only from the expensive
copper(II) formate and acetate, but also from a less
expensive and more accessible salt, copper(II) sulfate
pentahydrate. Moreover, the copper nanopowder
produced from this compound in the presence of citric
acid is almost as good in dispersity as the powder
based on copper formate, but, in contrast to the latter,
contains no admixtures of copper(I) oxide and enables
fabrication of composites with conductivity stable for
more than six years (~4 10–6 Ω m).
REFERENCE
1
. Obraztsova, I.I., Efimov, O.A., Eremenko, N.K., and
Simenyuk, G.Yu., Neorg. Mater., 1995, vol. 31, no. 4,
p. 798.
2
3
4
5
6
7
. Obraztsova, I.I., Simenyuk, G.Yu., and Eremenko, N.K.,
Zh. Prikl. Khim., 2004, vol. 77, no. 3, p. 386.
. Obraztsova, I.I., Simenyuk, G.Yu., and Eremenko, N.K.,
Zh. Prikl. Khim., 2006, vol. 79, no. 5, p. 717.
. Obraztsova, I.I., Simenyuk, G.Yu., and Eremenko, N.K.,
Zh. Prikl. Khim., 2002, vol. 75, no. 11, p. 1772.
. Obraztsova, I.I., Simenyuk, G.Yu., and Eremenko, N.K.,
Zh. Prikl. Khim., 2006, vol. 79, no. 10, p. 1626.
CONCLUSIONS
(
1) It was shown that, in reduction of readily
. Obraztsova, I.I., Simenyuk, G.Yu., and Eremenko, N.K.,
Neorg. Mater., 1999, vol. 35, no. 8, pp. 937.
decomposing copper salts or copper(II) sulfate in the
presence of initiators, the specific surface area of the
resulting powders can be made larger and the
crystallite size can be diminished, with the electrical
conductivity and stability of the powders simulta-
neously raised.
. Gusev, A.I., Nano: materialy, struktury, tekhnologii
(
Nano: Materials, Structures, Technology), Moscow:
Fizmatgiz, 2005.
8
. Fizika tverdogo tela: Laboratornyi praktikum “Metody
polucheniya tverdykh tel
i
issledovaniya ikh
(
2) It was found by means of transmission electron
struktury” (Solid-State Physics: Laboratory Manual
“Methods for Production of Solids and for Analysis of
Their Structure”), book 1, Khokhlov, A.F., Ed.,
Moscow: Vysshaya shkola, 2001.
microscopy that introduction of initiators leads to a
decrease in the size of copper particles. Copper
particles with a minimum size of 10–50 nm were
shown to be present.
9
. Pomogailo, A.D., Rozenberg, A.S., and Uflyand, I.E.,
Nanochastitsy metallov
Nanoparticles in Polymers), Moscow: Khimiya, 2000.
v
polimerakh (Metal
(
3) The method of small-angle X-ray scattering was
used to demonstrate that introduction of initiators
strongly affects the particle size distribution.
10. Dodonov, V.G., Z. Kristallogr., 1991, no. 4, p. 102.
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