九州大学大学院総合理工学府報告第 42 巻 第 1 号 27-34 頁 令和 2 年 9 月

Engineering Sciences Reports, Kyushu University
Vol. 42, No. 1, pp. 27-34, SEP, 2020

Syntheses of Cu2O and Cu Nanoparticles
by Using a Polyol Method
Masaharu TSUJI*1,*2† Yuki YOSHIDA*3 Mika MATSUNAGA*1 Keiko UTO*1
Masashi HATTORI*1 Jun-Ichiro HAYASHI*1,*2 and Takeshi TSUJI*4
E-mail of corresponding author: tsuji@cm.kyushu-u.ac.jp†(Received August 24, 2020, accepted August 28, 2020)
Copper-containing nanoparticles were synthesized by using a polyol method. Ethylene glycol(EG), which is a typical polyol, was used as a reductant and a solvent. Cu(OAc)2·H2O andpolyvinylpyrrolidone (PVP) were employed as a reagent and a protecting agent, respectively. The
product nanoparticles obtained at each reaction stage were characterized by using transmission
electron microscopic (TEM), TEM-energy dispersed X-ray spectroscopic (EDS), scanning electron
microscopic (SEM), X-ray diffraction (XRD), and ultraviolet (UV)-visible (Vis) extinction spectroscopic
measurements. Results showed that Cu2O particles with a mixture of various morphologies were
initially formed at 175 oC. They consisted of aggregates of small nanoparticles in most cases. After
further heating at 175 oC for 15 min, Cu2O particles were reduced to Cu nanoparticles rapidly. After
heating at 175 oC for 60 min, monodispersed spherical Cu nanoparticles with an average diameter ofabout 200 nm were finally obtained. UV-Vis extinction spectra suggested that Cu2O particle solution
was stable more than 10 days, whereas complete precipitation occurred for Cu nanoparticle solution
after 3 days due to oxidation.
Key words: Cu2O particles, Cu nanoparticles, Polyol method, Ethylene glycol, Two-step reduction,

Thermal decomposition, Stability, Oxidation, Plasmon band

1.

Introduction

Mono-metallic nanoparticles and their oxides
are of great interest because they can haveunique physical, chemical, and opticalproperties that are different from those of theatoms or of their bulk counterparts. The large
surface area to volume ratio can contribute to
some
of
the
uniqueproperties
ofnanoparticles.1-6) Among them, Cu and Cu2O
are industrially important materials that are
widely used in fields such as magnetic storage
media,
solar
energytransformation,
electronics, sensors, catalysis, batteries, andimaging.7-13) Cu and Cu2O have beensynthesized by sonochemical methods,14)
microwave
irradiation,15)
photochemical*1*2

Institute for Materials Chemistry and Engineering
Department of Applied Science for Electronics andMaterials*3 Department of Automotive Science, Graduate
Student*4 Department of Materials Science, Shimane
University

methods,16) hydrothermal and solvothermal
methods,17,18) electrochemical methods,19-21)
sol-gel methods,22,23) solid-state reactions,24)
and chemical reduction and decompositionroute.25-28)
Polyol method is a typical chemical
reduction method for the preparation of
Cu-containing nanoparticles in liquid phase. It
has been used to prepare Cu and their oxides
since a pioneering study of Fiévet et al. on
Cu2O.5,29-34) In the polyol process, the liquidorganic compound, a polyol, including 1,2-diols
and glycol ether, acts both as a reducing agent
and a solvent of the precursor metallic-salt.
Possible reduction processes using polyol havebeen reported in a review article by Rodrigues
et al.4) As well as its role of reductant, thepolyol medium offers several other advantages(i) their high boiling points allow syntheses atrelatively high temperature, resulting in highdegree of crystallinity products, (ii) the
reducing medium prevents as-prepared metal
particles from oxidation, (iii) its ability to28Syntheses of Cu2O and Cu nanoparticles by polyol method

coordinate metal precursors and particle
surfaces, which minimizes aggregation andcoalescence, and (iv) the high viscosity of themedium favors a diffusion-controlled growth ofparticles, leading to well-controlled shapes,
sizes, and compositions.
We have succeeded in syntheses of suchCu-containing nanoparticles as Au@Cu,
Ag@Cu,
Cu@Ag,AgCu,
AuAgCu,phase-separated AuAgCu/Cu, Au@Ag@Cu, and
Au@Pd@Cu
particlesusing
polyolmethods.35-44) We could obtained new
information on growth mechanisms of
Cu-containing
biand
tri-metallic
nanoparticles and effects of crystal structure
and composition for antioxidative properties of
Cu component in various Cu-containing
nanoparticles.
In this study, we prepared Cu2O and Cu
nanoparticles using a polyol method. Crystal
structures were characterized by TEM,
TEM-EDS, SEM, XRD, and UV-Vis extinction
spectral data. Their formation mechanisms in
ethylene glycol (EG) and stability in EG
solution after preparation are discussed.

2.

Experiment

2.1

Materials and experimental procedures

For use in this study, Cu(CH3COO)2·H2O =Cu(OAc)2·H2O (>99.0%) and EG (>99.5%) werepurchased from Kishida Chemical Industry
Ltd. and polyvinylpyrrolidone (PVP: Mw = 55k) was obtained from Sigma-Aldrich Corp. A
high purity Ar gas (>99.999%) was obtained
from Japan Fine Products. All reagents wereused without further purification.
30 mg of Cu(OAc)2·H2O in 17 mL EG and
0.56 g of PVP in 17 mL EG (MW: 55 000 interms of monomer units) were mixed in a 100
mL three necked flask and Ar was bubbled atroom temperature for >20 min to remove
oxygen dissolved in the solution. The final
concentrations of Cu(OAc)2·H2O and PVP in
EG were 6 and 200 mM, respectively.
The reagent solutions were soaked in an oilbath preheated at 186 oC under bubbling Ar.
The temperature profile of solution is shown in
Fig. 1. The solution temperature increases
rapidly from a room temperature to about 170
oC after heating for about 5 min, slowly
increased from 170 to 175 oC in the 5–10 min,and it was kept at 175 oC for 60 min. The
reagent solution was sampled at various
reaction times to examine shape, size,
composition changes of intermediate andproduct particles. The sample solution was

rapidly cooled in a water bath after a desired
reaction time finished.
2.2

Characterization of nanoparticles

Shape, size, and composition of productparticles were examined by using TEM and
TEM-EDS (200 kV, JEM-2100F; JEOL) andSEM (SU8000; Hitachi High-Tech Science
Corp.). Samples for TEM, TEM-EDS, and SEM
observations were obtained by centrifuging thecolloidal solution three times at 15,000 rpm for
30 min. The XRD patterns of the samples were
measured using Cu K radiation operating at
50 kV and 300 mA (RINT-TTR III, Rigaku
Corp.). Extinction spectra of the productsolutions were measured in the 3001200 nm
region using a spectrometer (UV-3600;
Shimadzu Corp.).

3.

Results and Discussion

3.1
Formationof
Cu-containing
nanoparticles from Cu(OAc)2·H2O in EG in thepresence of PVP
Figure 2 shows changes in color of reagent
solution after heating. When Cu(OAc)2·H2O
was injected to the hot EG solution containing
PVP, color of solution was changed from blue(125 oC) to green (160 oC), olive-green (175 oC
and 175 oC 7.5 min), and red copper (175 oC
3060 min). Figure 3 shows UV-Vis extinction
spectraofproducts
obtainedfrom
Cu(OAc)2·H2O/PVP/EG system after heatingfrom room temperature to 175 oC and keeping
at 175 oC for 7.560 min. Just after heating, aFig. 1. Temperature profile of reagent
solution after soaking reagent solution to an
oil bath at 186 oC. Spikes observed in curves
originate from sampling reagent solution byusing a pipette.

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九州大学大学院総合理工学報告第 42 巻第1号

29

Fig. 2. Changes in color of reagent solution after heating.

Fig. 3. UV-Vis extinction spectra of products
obtained from Cu(OAc)2·H2O/PVP in EG
after heating from room temperature to 175
oC and keeping at 175 oC for 7.560 min.
weak broad band is observed in the 4501200
nm range with a peak at about 750 nm. At
125160 oC, similar broad bands are observed,
although peak intensity becomes slightly
strong at 160 oC. After heating at 175 oC and at
175 oC 7.5 min, strong long tail bands withweak shoulder peaks at about 450 nm appear
in the 3501250 nm region. The appearance ofthe 450 nm peak suggests that main product is
Cu2O in this reaction time range.32) After
heating at 175 oC 15 min, 30 min, and 1 h, longtail bands change into broad bands with a peakat about 600 nm. It is known that Cu
nanoparticles give a surface plasmon
resonance (SPR) band with a peak at ≈600
nm.38,44) Thus, the spectral changes indicatethat major products changes from Cu2O to Cu
during heating at 175 oC for 15 min. Similar
two-step formation of Cu nanoparticles via
decomposition of Cu2O in polyol solvents have

Fig. 4. TEM images of products obtained at
160 oC and 175 oC 0–60 min.

30

Syntheses of Cu2O and Cu nanoparticles by polyol method

Fig. 5. TEM and SEM images of various shapes of products at 175 oC.
been reported, although the reaction
temperatures, where Cu2O → Cu reduction
occurs, are higher than that observed in thiswork in most cases.31,32,34) This will be
discussed in more detail in Sec. 3.3.
Figures 4a-4g show TEM images of products
obtained at 160 oC and 175 oC 0–60 min. At
160 oC (Fig. 4a), a mixture of small particles
with an average diameter of about 100 nm anda cubic-type large particles with an average
diameter of about 300 nm is obtained.
At 175 oC and 175 oC 7.5 min (Figs. 4b and4c), mixtures of various shapes of aggregatesare formed. To examine crystal structures ofthese aggregates in more detail, expanded
TEM and SEM images at 175 oC are measured(Fig. 5A-5D). Four kinds of particles are found.
They are spherical aggregates with an average
diameter of about 1 m (type A), cubic particles
with an average diameter of 200 nm (type B),
larger cubic-shaped aggregates with anaverage diameter of about 1 m (C), andhollow spherical aggregates with an average
diameter of about 1 m (type D). All of theseparticles are expected to be Cu2O on the basis
of UV-Vis spectral data (Fig. 3).
To obtain information on composition of
products at 175 oC, TEM-EDS measurements
were carried out. Figures 6a-6f display TEM,
TEM-EDS, and line analysis data, where
hollow particles and cubic particles areobserved. TEM-EDS and their line analysis

data indicate that both particles are composedof uniformly distributed Cu and O components,
and Cu : O atomic ratio was about 2 : 1. These
results suggest that major products at 175 oC
are Cu2O, which is consistent with theprediction from the UV-Vis spectral data (Fig.
3).
At 175 oC 15, 30, and 50 min (Figs. 4d-4f),
hollow particles disappear and mixtures ofspherical and cubic-type particles with anaverage diameter of about 200 nm are formed.
At 175 oC 1 h (Fig. 4g), spherical particles withan average diameter of about 200 nm areformed. On the basis of UV-Vis extinction
spectra (Fig. 3), main component of theseparticles is expected to be Cu.
To obtain definite information on the
crystalline structure of products, XRD patterns
of products were measured. Figures 7(a) and7(b) show XRD patterns of product powders
obtained at 175 oC and 175 oC 1 h, respectively.
At 175 oC, major peaks are attributed to cubic
Cu2O and weak peaks arising from cubic Cu
are observed. The Cu2O and Cu ratio wasestimated to be 96 : 4. At 175 oC 1 h, only Cu
peaks are found, indicating that Cu2O iscompletely converted to Cu after heating Cu2O
particles in EG at 175 oC for 1 h. Results
obtained from XRD analyses were consistent
with drastic spectral changes from broad Cu2O
peak to more sharp Cu SPR peak observed in
the UV-Vis extinction spectra (Fig. 3).

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九州大学大学院総合理工学報告第 42 巻第1号

31

Fig. 6. TEM and TEM-EDS data of products obtained at 175 oC. Line analysis data (e) are
obtained along a blue box shown in (d).
3.2

Fig. 7. XRD patterns of products obtainedat (a) 175 oC and (b) 175 oC 1 h.

Stability of Cu2O and Cu nanoparticles

Stability of Cu2O and Cu nanoparticles after
preparation was examined by monitoring timechanges in UV-Vis spectra. Figure 8a shows
UV-Vis spectra of Cu2O obtained at 175 oC and
12 days after preparation. Although Cu2O peakat 484 nm decreases by 3% after 12 days, nosignificant reduction of peak intensity isobserved. This implies that stability of Cu2O
particles is relatively high in EG.
Oxidation of Cu nanoparticles gives rise toprogressive damping of their characteristic
SPR band because of the growth of the oxideouter shell.45,46) Consequently, the extent ofoxidation of Cu nanoparticles can be monitoredby observing the intensity changes in the Cu

32

Syntheses of Cu2O and Cu nanoparticles by polyol method

band at ≈600 nm. Figure 8b shows SPR bandsof Cu nanoparticles at 0–3 days after
preparation. The SPR band of Cu particles at≈600 nm decreases rapidly with increasing in
the reaction time and becomes zero after 3
days because of their oxidation. These results
suggest that dispersion of Cu nanoparticles ismuch more unstable than that of Cu2O
particles in EG, so that precipitation is formed
within a few days.

green crystalline compound was precipitated.
Single-crystal X-ray diffraction data suggested
that the precipitation was the coordination
polymer Cu3(OAc)2(C2H4O2)2. According totheir analysis, this crystal belongs to thetriclinic space group P1 and has a 2D network
structure generated by bridging bidentate
acetate and deprotonated EG ligands. In their
DTA-TG
measurementsofdry
Cu3(OAc)2(C2H4O2)2 in air showed that it isstable up to 200 oC and decomposes in an
exothermic reaction to Cu2O.
In this study, we obtained a green solution
at 160 oC (Fig. 2), where no precipitation was
obtained. Precipitation was not found in ourstudy, even reaction temperature increased
from 160 oC to 175 oC and it was kept in thistemperature for 1 h. In our work, we did not
hold the temperature constant before elevatingthereaction
temperature
from
room
temperature to 175 oC. Rapid increase in thereaction temperature around 120 oC in thiswork probably prevents the reagent solution
from the precipitation. Although Teichert etal.34) reported that Cu3(OAc)2(C2H4O2)2 isstable in air up to 200 oC, we found here theformationof
Cu2O
particlesfrom
Cu(OAc)2·H2O/PVP/EG system after heating at
a lower temperature of 175 oC in EG. It isthereforereasonabletoassumethat
Cu3(OAc)2(C2H4O2)2 intermediate is not formed
or rapidly decomposed into smaller solubleintermediates in our condition.
We found here that Cu2O particles with a
Cu+ component were formed, when the reagent
solution was heated to 175 oC at a relatively
fast heating rate of about 30 oC/min. The
following reduction potentials in aqueous
solutions can be used for the reduction of Cu2+
to Cu0 via Cu+, because their relative
magnitude is similar in EG and water.47,48)

Fig. 8. Time evolution of UV-Vis extinction
spectra of (a) Cu2O and (b) Cu nanoparticles
in EG/PVP solutions after preparation.

Cu2+ + 2e- → Cu0
Cu2+ + e- → Cu+
Cu+ + e- → Cu0

3.3
FormationmechanismofCu
nanoparticles via reduction of Cu2O particles

It is expected that direct Cu2+ → Cu0 reduction(1) does not occur, but the reduction of Cu2+ions into Cu0 proceeds through two stages in
EG solution:5,31,34)

Teichert et al.34) studied the formation
mechanism of Cu nanoparticles via Cu2O in
Cu(OAc)2/EG system on the basis of XRD and
DTA-TG analyses of intermediate and final
products. They reported that Cu(OAc)2
completely dissolved in EG at ca. 80 oC giving abright turquoise solution. When the reagent
solution was further heated to 120 oC, a darkー

E ° = +0.342 V
E ° = +0.153 V
E ° = +0.521 V

Cu(OAc)2-EG → Cu2O → Cu0
Cu2+ → Cu+ → Cu0

(1)
(2)
(3)

(4a)
(4b)

At first, Cu2+ ions, which are stable in EG atambient temperature, are formed fromCu(OAc)2·H2O reagent and then they are令和 2 年度

九州大学大学院総合理工学報告

reduced to Cu+ through process (2). In
principle, since the standard potential
associated with the Cu+ → Cu0 reduction (3) ismore positive than that of the Cu2+ → Cu+
reduction (2), the Cu+ species produced throughreaction (2) are unstable and should be readily
reduced to Cu0. However, the Cu+ ions
immediately form sparingly soluble Cu2O, a
compound that is more difficult to reduce aspredicted by the more negative redox potential
associated with Eq. (5)Cu2O + H2O + 2e− → 2Cu0 + 2OH−E ° = −0.36 V.

Actually, the SEM, TEM, and XRD data
indicate that these Cu2O particles are notfurther reduced, and therefore, they areformed by the aggregation of primarynano-sized particles at 175 oC and 175 oC 7.5
min. Similar aggregation processes have been
observed in other cases.49-51) Cu2O particles in
EG solution is stable more than 10 days.
After further heating Cu2O/PVP/EG solution
at 175 oC for more than 15 min, Cu+ component
in Cu2O particles is rapidly reduced to Cu0.
Then,monodispersed
spherical
Cu
nanoparticles with an average diameter ofabout 200 nm are finally formed. When leaving
these particles in a bottle, precipitation occurs
nearly completely after three days due tooxidation.
In the last section, we compared the Cu2O
→ Cu conversion temperature observed in thisstudy with those reported previously. Orel etal.31) have first observed conversion of uniform
colloidal Cu2O spheres to copper in polyols (EGand tetraethylene glycol (TEG)) by using
Cu(OAc)2 as a reagent. They reported that theconversion from Cu2O to Cu occurred at 170 oC
in EG, though no detailed experimental
conditions (e.g. reaction time) and XRD data
were shown. They gave more detailed
experimental data, when TEG was used. Pure
Cu nanoparticles were prepared at 240 oC in
TEG, which was much higher than 170 oC in
EG. Zhao et al.32) obtained spherical Cu
particles with a similar size obtained in thisstudy
bymicrowave
heatingof
Cu(OAc)2·H2O/EG solution at 197 oC. Since noprotecting agent was added in theirexperiment, precipitate was obtained. Teichert
et al.34) prepared Cu contained products fromCu(OAc)2·H2O/EG system. After the formationof Cu3(OAc)2(C2H4O2)2 intermediate at 120 oC,
solid alkoxide Cu(C2H4O2) was formed at

第1号

33

180–200 oC. Cu nanoparticles were obtained
under reflux conditions of Cu(C2H4O2) at ahigher temperature of 210 oC. As describedabove, there are several reports that Cu
nanoparticles could be prepared throughdecomposition of Cu2O in polyol solvents, oursimple condition, heating at 175 oC for 15 minin EG, provides pure Cu nanoparticles at a
relatively low temperature in a short heating
time.

4.
(5)

第 42 巻Summary and Conclusion

A simple polyol reduction method wasapplied to the syntheses of Cu2O and Cu
nanoparticles. Cu(OAc)2·H2O was reduced in
EG in the presence of PVP as a protecting
agent. When the reagent solution was heating
to 175 oC at a heating rate of about 30 oC/min,Cu2O particles with a mixture of various
morphologies were prepared with a yield of
96%. The remaining 4% was Cu particles.
After heating at 175 oC for 15–60 min, Cu2O
particles were completely reduced to Cu
nanoparticles. UV-Vis spectral changes
suggested that the Cu2O → Cu conversion
occurs within a few minutes. Although theformationofCu
nanoparticles
via
decomposition of Cu2O particles in polyols has
been reported,31,32,34) higher temperatures
above about 200 oC were required forconversion in most cases. Thus, this work
provides a new simple lower temperaturesynthesis method of monodispersed Cu
nanoparticles in EG.
Acknowledgments
This work was supported by JSPS
KAKENHI Grant Numbers JP25286003,
JP25550056.
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