Optical and mechanical study of size-controlled Cu particles synthesized by electrodeposition

Abstract

Optical and mechanical properties of size-controlled Cu particles (1.5 µm, 500 nm and 50 nm) fabricated by one-step electrodeposition were studied. First, surface morphology and composition were characterized by SEM and EDS, with crystal structure by TEM, SAED and XRD. Antioxidant ability of 50nm was verified by TGA. In the simple and novel synthesis process, Cu particles of 1.5 µm with polyhedron morphology were firstly synthesized. The increase of current density and addition of potassium ferrocyanide trihydrate played key roles in the grain refinement to 500 nm and 50 nm, respectively. Then, particular focus was given to the improvement of optical and mechanical properties with size reduction, by SERS, UV-Vis and nanoindentation. These properties were gradually enhanced with the decrease of particle size, and Cu particles of 50 nm show the best performance.

Full text

Research Article Vol. 4, No. 12 / 15 Dec 2021 / OSA Continuum 2988
Optical and mechanical study of size-controlled
Cu particles synthesized by electrodeposition
BAO SHU O YAN G,1YUAN AI,2A ND XIAOWEI LI U1,*
1Institute of Technological Sciences, Wuhan University, Wuhan 430072, China
2School of Mechanical Science & Engineering, Huazhong University of Science and Technology, Wuhan
430074, China
*2019206520004@whu.edu.cn
Abstract:
Optical and mechanical properties of size-controlled Cu particles (1.5
µ
m, 500 nm
and 50 nm) fabricated by one-step electrodeposition were studied. First, surface morphology
and composition were characterized by SEM and EDS, with crystal structure by TEM, SAED
and XRD. Antioxidant ability of 50nm was verified by TGA. In the simple and novel synthesis
process, Cu particles of 1.5
µ
m with polyhedron morphology were firstly synthesized. The
increase of current density and addition of potassium ferrocyanide trihydrate played key roles in
the grain refinement to 500 nm and 50 nm, respectively. Then, particular focus was given to the
improvement of optical and mechanical properties with size reduction, by SERS, UV-Vis and
nanoindentation. These properties were gradually enhanced with the decrease of particle size,
and Cu particles of 50 nm show the best performance.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
In recent years, Cu nanoparticles have become attractive materials and been found widespread
application for electrical, optical, catalytic, mechanical applications [1–5]. Their unusual
properties like optical and mechanical performance often differ significantly from the bulk
materials, depending on the size, morphology, dispersibility and uniformity. Plentiful studies
have been carried out for the synthesis of Cu particles with improved properties, one of the key
aspect is controlling of smaller particle size [6–9].
In addition to other routes such as physical vapour deposition, chemical vapour deposition,
reverse micelle and chemical reduction, electrodeposition is by far accepted as the most popular
methods due to its low-costs, convenience, low toxic, highly production rates, easy to industrialize
and mass-produce [10–13]. More importantly, it is a feasible technique to produce Cu particles
with a range of sizes, from micron to nanometer dimensions for different requirements for
size-effected performance [7,14,15]. Through varying the electrolysis conditions (voltage,
current density, pH, temperature, composition and concentration of electrolyte), it is possible to
control the size of products by 1
∼
2 orders of magnitude. Nevertheless, the main drawback of
electrodeposition, at present, is typically leading deposits to a wide size distribution, especially
in nano-size, most of the reported synthesis produce particles of large polydispersity (
≥
20%)
[16–18]. However, a narrow size distribution is a key parameter to obtain reproducible and
controllable optical, chemical and physical properties of products. Therefore, how to realize
delicate control over particle size is one of the hottest research areas [19]. Indeed, another
drawback is the enhanced tendency to oxidize under ambient conditions as size is reduced, bring
challenge of stabilization during particle refinement [20,21]. The study on the synthesis of Cu
particles of controlled size is thus less developed than Ag or Au, many groups have worked on
the stabilization of Cu particles against oxidation but sometimes to the detriment of the size and
shape control [22,23].
As is known, SERS phenomenon can be easily observed from the surface of Au, Ag and
Cu [24,25]. The signal is influenced by several significant parameters, such as size or shape
#434492 https://doi.org/10.1364/OSAC.434492
Journal © 2021 Received 18 Jun 2021; revised 26 Oct 2021; accepted 31 Oct 2021; published 21 Nov 2021

Research Article Vol. 4, No. 12 / 15 Dec 2021 / OSA Continuum 2989
of substrate [26,27]. It is important to develop smaller size in order to maximize their SERS
signals. Researchers have less research on Cu materials because their chemical properties are
unstable and SERS signal is weak compared to Au and Ag [26,27]. However, due to the low
cost and abundant resources, they still have great application prospects especially in exploring
the size-dependent SERS enhancement [28,29]. On the other hand, the wavelength of UV-Vis
absorption spectroscopy or Light extinction spectrometry also have strong relationship with the
size of particles [13,30,31]. Inappropriate size or aggregation would seriously affect the LSPR
mode and greatly weaken the intensity of peaks. It can be identified that only particles with
good dispersion and uniform size can create the intense LSPR peak, which plays a key role in
several technological fields such as chemical and biological sensing, ultrasensitive biosensing and
nanophotonics [30,32,33]. Although some studies have been performed on influence of different
size or shape of metal particles on them, there has been very little research simultaneously
on synthesizing Cu particles for a definitive understanding. In many reports, the mechanical
properties of particles also have a strong correlation with size, usually the smaller the better,
based on a uniform particle size distribution [34–36]. Therefore, it will be very interesting
to conduct size-controlled synthesis of Cu particles by electrodeposition while retaining high
stability and see how their properties being increased by decreasing particle size [13].
In view of aforementioned, the goal of this work is to synthesize Cu particles with controlled
size, uniform distribution, less liable to oxidation, by one-step electrodeposition method and study
their size-effected optical and mechanical properties. Different electrodeposition conditions were
investigated in order to deposit stable Cu particles of 1.5
µ
m, 500 nm and 50 nm. These particles
were characterized by SEM, EDS, TEM, SADE and XRD for surface morphology, composition
and crystal structure analysis. Antioxidant ability was verified by TGA. Spectroscopic techniques
(SERS and UV-Vis) and nanoindentation were used for a quantitative examination of the
relationship between different size, showing potential applications in control of optical and
mechanical performance.
2. Experimental
2.1. Reagents and materials
Potassium ferrocyanide trihydrate (99%) was obtained from J&K Scientific Ltd.. R6G (
≥
99.5%),
CuSO
4·
5H
2
O, anhydrous ethanol, and hydrochloric acid were purchased from Sinopharm
Chemical Reagent Co. Ltd., China. All reagents were used as received without further
purification. Fresh solutions were prepared before experiments to avoid their deterioration with
time.
2.2. Electrodeposition of Cu particles
The synthesis process took place in a one-compartment, two-electrode square container (5 cm
×
5 cm
×
5cm). Highly purified Cu discs (99.99 wt %), with a diameter of 10 mm and 3 mm
thick, were used as cathodic substrates. Commercially pure steel discs with the same size was
positioned at a distance of 5 mm from the upper surface of the substrates as anode. The same
size of cathode and anode could ensure a steady electric distribution, furtherly a more uniform
particle size. Two kinds of electrolytes were used to complete the preparation of Cu particles.
Electrolyte solution A (for 1.5
µ
m and 500 nm) was prepared with CuSO
4•
5 H
2
O (100 g/L)
in deionized water. The 1.5
µ
m particles were obtained at a constant potential of 5 V, current
density of 0.1A/cm
2
after 10 min in solution A. Tunning the current density to 1A/cm
2
but
other conditions remain unchanged, the 500 nm particles were generated. Electrolyte solution B
was composed of solution A and potassium ferrocyanide (30 mg/L). The 50 nm particles were
observed when the reaction time was reduced to 5min in Electrolyte solution B.

Research Article Vol. 4, No. 12 / 15 Dec 2021 / OSA Continuum 2990
Prior to electrodeposition, both sides of discs were mechanically polished by silicon carbide
sheets (from #400 to #7000) to obtain a mirror finish, and then ultrasonically cleaned in
hydrochloric acid and deionized water to remove dust or greasy medium that accumulated on the
surface. After electrodeposition, the substrate covered with particles was washed with deionized
water and anhydrous ethanol successively for cleaning, then wiped with lint free paper, dried in
the air for SEM characterization. For other characterizations, Cu particles were peeled from the
substrates and dispersed in ethanol as suspension. For better reproducibility and comparability,
all the samples were prepared in short time prior to the characterization and detection.
2.3. Morphology and composition characterizations
Scanning electron microscopy (SEM) was used for characterizing the morphology, size, and
distribution of Cu particles. Simultaneously, Energy dispersive X-ray spectroscopy (EDS)
patterns for elemental composition. The images and patterns were obtained by MIRA 3 LMH
(TESCAN Brno, s.r.o) microscope operating at 5 kV and 20kV. Samples were prepared by
mounting the deposited substrates directly on the holder with carbon tape.
Transmission electron microscopy (TEM) and selected-area electron diffraction (SAED) were
used for characterizing the structure. All images were obtained by JEOL 2010 plus transmission
electron microscope operating at an acceleration voltage of 200 kV, and recorded by a Gatan 1k
×
1k slow scan CCD camera. Samples were prepared by casting a drop of the suspension further
grind with a mortar on a 200 mesh carbon-coated Cu grid before drying them in vacuum. The
suspension, composed of milled Cu particles and anhydrous ethanol, was sonicated for 5 min in
deionized water.
For X-ray diffraction (XRD) measurements, XRD patterns were recorded using a Bruker
D8 X-ray diffractometer (Cu K
α
source) with a 2
θ
range of 20
°
to 80
°
. After centrifugation, Cu
particles obtained from the precipitate of the as-prepared suspension were coated on a glass slide
to form films prior to the vacuum dry.
Thermal analysis (TGA) was carried out on Mettler-Toledo TGA2 / DSC3 by heating 10 mg of
Cu particles to 100
°
C for 10 min in a flow of air (25 mL min
−1
) to remove moisture, followed by
increasing the temperature to 600 °C at a rate of 10 °C min−1.
2.4. Optical and mechanical properties characterizations
Rhodamine 6G (R6G) molecules were used for the study of Cu particles in Surface-enhanced
Raman scattering (SERS). SERS spectra were recorded using a confocal Raman spectrometer
(Renishaw inVia) coupled to Zeiss microscope with a 50
×
objective in backscattering geometry.
The 532 nm laser was used as the excitation source. The backscattered signals from 11 mg
pure R6G and 1 mg Cu particles (1.5
µ
m, 500 nm, 50 nm, respectively) mixed with 10 mg
R6G were collected in the range of 300 cm
−1
-1800 cm
−1
. UV-visible absorption spectra
(UV-vis) were carried out with a UV-Vis spectrophotometer (UV-2550, Shimadzu), at room
temperature, between 400 nm and 800 nm, using 1-mm-pathlength quartz cuvettes. The tested Cu
particles were separated from the substrates, dispersed in anhydrous ethanol, and then subjected
to ultrasonic treatment for 30 minutes. Nanoindentation was conducted on TI 950 Tribolndenter
with 30
µ
N of loading force and 30 s of loading time. Samples were prepared by mounting the
deposited substrates directly on the holder with crystal glue.
3. Results and discussions
3.1. Size and morphology analysis
The synthetic process was in Cu-based electrolyte containing CuSO
4·
5H
2
O (100 g/L) at an
applied potential of 18 V and current density of 0.1A/cm
2
for 10 min, the related processing
conditions are given in the Experimental Section. Figure 1(a) and (b) show a uniform distribution

Research Article Vol. 4, No. 12 / 15 Dec 2021 / OSA Continuum 2991
and octahedral Cu crystals with regular shape and an average size of
∼
1.5
µ
m. All the polyhedral
particles have smooth surfaces, clear edges and corners. Particle diameters was reduced to
∼
500
nm with the current density increased to 0.5A/cm
2
, as shown in Fig. 1(c) and (d). There was
broader size distribution of particles compared to that under the current density of 0.1A/cm
2
,
but the regular octahedral shape of Cu crystal was maintained. This analysis suggests that there
are some effects from current density on particle size dispersion. Furtherly, when potassium
ferrocyanide was added into the basic electrolyte solution as a grain refiner, an obvious grain
refinement (
∼
50 nm) was observed (see Fig. 1(e) and (f)), which was consistent with the grain
refinement rule of potassium ferrocyanide reported by Wenbo Lou et al [37]. As evident in the
images, the size distribution restored to a narrow level (
<
10%), but it was difficult to find regular
structure of polyhedral crystals. It can be explained by the fact that small-sized crystals have not
grown into polyhedrons with regular shapes. At the same time, EDS patterns in Fig. 1(a), (c) and
(e) furtherly demonstrate the clear crystal structure of the synthesized Cu particles.
139 The synthetic process was in Cu-based electrolyte containing CuSO4·5H2O (100 g/L) at an
140 applied potential of 18 V and current density of 0.1A/cm2 for 10 min, the related processing
141 conditions are given in the Experimental Section. Fig. 1(a) and (b) show a uniform distribution
142 and octahedral Cu crystals with regular shape and an average size of ~1.5 μm. All the
143 polyhedral particles have smooth surfaces, clear edges and corners. Particle diameters was
144 reduced to ~500 nm with the current density increased to 0.5A/cm2, as shown in Fig. 1(c) and
145 (d). There was broader size distribution of particles compared to that under the current density
146 of 0.1A/cm2, but the regular octahedral shape of Cu crystal was maintained. This analysis
147 suggests that there are some effects from current density on particle size dispersion. Furtherly,
148 when potassium ferrocyanide was added into the basic electrolyte solution as a grain refiner,
149 an obvious grain refinement (~50 nm) was observed (see Fig.1(e) and (f)), which was consistent
150 with the grain refinement rule of potassium ferrocyanide reported by Wenbo Lou et al [37]. As
151 evident in the images, the size distribution restored to a narrow level (<10%), but it was difficult
152 to find regular structure of polyhedral crystals. It can be explained by the fact that small-sized
153 crystals have not grown into polyhedrons with regular shapes. At the same time, EDS patterns
154 in Fig. 1 (a), (c) and (e) furtherly demonstrate the clear crystal structure of the synthesized Cu
155 particles.
156
157
158 Fig. 1. SEM images of Cu particles with different sizes: 1.5 μm (a-b), 500 nm (c-d), and 50 nm (e-f),
159 the illustrations in the upper right corner are the EDS patterns.
160
161 3.2 Structure and stability analysis
162 Cu particles were grinded and deposited on ultra-thin carbon film for TEM characterizations to
163 study crystalline structure. Fig. 2(a), (c) and (e) show the clear morphology of pieces of milled
164 Cu particles (1.5 μm, 500 nm and 50 nm). The typical HRTEM image of the particles are shown
165 in Fig. 2(b), (d) and (f), as noted, the fringe spacing of 2.088 Å, 1.808 Å and 1.2780 Å can be
166 indexed to the lattice plane (111), (200) and (220) of face-centered cubic (FCC) Cu (JCPDS
167 No. 65-9743), respectively. The inset SAED patterns can be indexed to plane of FCC structure
168 of FCC Cu (JCPDS No. 65-9743). At the same time, some dislocations and other defects can
169 also be seen in the figures, which is common phenomena in the electrodeposition process.
170
Fig. 1.
SEM images of Cu particles with different sizes: 1.5
µ
m (a-b), 500 nm (c-d), and 50
nm (e-f), the illustrations in the upper right corner are the EDS patterns.
3.2. Structure and stability analysis
Cu particles were grinded and deposited on ultra-thin carbon film for TEM characterizations to
study crystalline structure. Figure 2(a), (c) and (e) show the clear morphology of pieces of milled
Cu particles (1.5
µ
m, 500 nm and 50 nm). The typical HRTEM image of the particles are shown
in Fig. 2(b), (d) and (f), as noted, the fringe spacing of 2.088 Å, 1.808 Å and 1.2780 Å can be
indexed to the lattice plane (111), (200) and (220) of face-centered cubic (FCC) Cu (JCPDS No.
65-9743), respectively. The inset SAED patterns can be indexed to plane of FCC structure of
FCC Cu (JCPDS No. 65-9743). At the same time, some dislocations and other defects can also
be seen in the figures, which is common phenomena in the electrodeposition process.
Cu particles of three particular sizes were further characterized by XRD (Fig. 3(a)), with
scanning range from 20
°
to 80
°
. There were three characteristic peaks at 43.2
°
, 50.3
°
and 74.1
°
marked with indices of (111), (200), and (220) crystal planes of FCC Cu (JCPDS No. 65-9743).
As previously stated, these results are well consistent with those reported previously for metallic
Cu by other methods [38–40]. The sharp and strong peaks indicate high crystallinity in the
synthesized Cu particles. The intensity ratios of the (2 0 0) to (1 1 1) diffraction peaks suggest
that all these three kinds of particles are dominated by (111) crystal plane orientation and have

Research Article Vol. 4, No. 12 / 15 Dec 2021 / OSA Continuum 2992
171
172 Fig. 2. TEM characterizations of Cu particles with different sizes: 1.5 μm (a-b), 500 nm (c-d) and 50 nm (e-f), insets
173 of panel b, d and f show the corresponding selected area electron diffraction (SAED) patterns obtained from
174 corresponding area.
175
176 Cu particles of three particular sizes were further characterized by XRD (Fig. 3(a)), with
177 scanning range from 20° to 80°. There were three characteristic peaks at 43.2°, 50.3° and 74.1°
178 marked with indices of (111), (200), and (220) crystal planes of FCC Cu (JCPDS No. 65-9743).
179 As previously stated, these results are well consistent with those reported previously for
180 metallic Cu by other methods [38-40]. The sharp and strong peaks indicate high crystallinity in
181 the synthesized Cu particles. The intensity ratios of the (2 0 0) to (1 1 1) diffraction peaks
182 suggest that all these three kinds of particles are dominated by (111) crystal plane orientation
183 and have higher hardness and corrosion resistance. Furthermore, no other impurities such as
184 Cu2O, CuO or Cu(OH)2 were detected, confirming the high purity of Cu particles obtained by
185 this synthetic method.
186
187
188 Fig. 3. X-ray diffractograms of Cu particles with different sizes: 1.5 μm, 500 nm and 50 nm (a)
189 and TGA analysis of 50nm (b).
190
Fig. 2.
TEM characterizations of Cu particles with different sizes: 1.5
µ
m (a-b), 500 nm
(c-d) and 50 nm (e-f), insets of panel b, d and f show the corresponding selected area electron
diffraction (SAED) patterns obtained from corresponding area.
higher hardness and corrosion resistance. Furthermore, no other impurities such as Cu
2
O, CuO
or Cu(OH)
2
were detected, confirming the high purity of Cu particles obtained by this synthetic
method.
171
172 Fig. 2. TEM characterizations of Cu particles with different sizes: 1.5 μm (a-b), 500 nm (c-d) and 50 nm (e-f), insets
173 of panel b, d and f show the corresponding selected area electron diffraction (SAED) patterns obtained from
174 corresponding area.
175
176 Cu particles of three particular sizes were further characterized by XRD (Fig. 3(a)), with
177 scanning range from 20° to 80°. There were three characteristic peaks at 43.2°, 50.3° and 74.1°
178 marked with indices of (111), (200), and (220) crystal planes of FCC Cu (JCPDS No. 65-9743).
179 As previously stated, these results are well consistent with those reported previously for
180 metallic Cu by other methods [38-40]. The sharp and strong peaks indicate high crystallinity in
181 the synthesized Cu particles. The intensity ratios of the (2 0 0) to (1 1 1) diffraction peaks
182 suggest that all these three kinds of particles are dominated by (111) crystal plane orientation
183 and have higher hardness and corrosion resistance. Furthermore, no other impurities such as
184 Cu2O, CuO or Cu(OH)2 were detected, confirming the high purity of Cu particles obtained by
185 this synthetic method.
186
187
188 Fig. 3. X-ray diffractograms of Cu particles with different sizes: 1.5 μm, 500 nm and 50 nm (a)
189 and TGA analysis of 50nm (b).
190
Fig. 3.
X-ray diffractograms of Cu particles with different sizes: 1.5
µ
m, 500 nm and 50
nm (a) and TGA analysis of 50nm (b).
The stability of 50nm Cu particles aganist oxidation was also examined through thermal
analysis which was performed under a flow of air up to 600
°
C. The analysis result shows that Cu
particles are stable even if reduced to a range of tens of nanometers in air up to 200
°
C, above
which there is a stepwise weight gain (Fig. 3(b)) and appeared similar to the work of M. Ibrahim
Dar et al [8]. The total weight gain observed is 23%, which results from the formation of CuO
and is in accordance with the theoretical value of 25% [38].

Research Article Vol. 4, No. 12 / 15 Dec 2021 / OSA Continuum 2993
3.3. SERS study
Typically, SERS can be easily observed with coinage metals such as Au, Ag and Cu. These
materials exhibit localized surface plasmon resonance (LSPR) bands in the visible region due
to excitation of the conduction electrons after irradiation with light. Since Cu particles in this
work provide many sharp edges and corners, which can confine EM fields and generate strong
near-fields concentrated at vertices, it is expected to have visible enhancement in SERS. At the
same time, R6G has been widely applied in SERS research owing to its mature vibration property
[41–46], and was selected as probe molecules to prove SERS performance.
Figure 4presents the Raman spectra of pure R6G powder and the same weight of R6G mixing
with different Cu particles (1.5
µ
m, 500 nm and 50 nm, respectively) under 532 nm close to
the plasmonic peak. We selected some typical fingerprint vibrational bands of R6G that are
significantly obvious and consistent with those reported in other literatures. The characteristic
peaks of 613 cm
−1
, 775 cm
−1
and 1183 cm
−1
belonged to C-C-C ring in-plane bending, C-H
out-of-plane bending and C-H in-plane bending, respectively. In addition, 1310 cm
−1
, 1361 cm
−1
,
1505 cm
−1
, 1573 cm
−1
and 1648 cm
−1
were associated with the stretching modes of fragrant C-C
in-plane [47]. These peaks described above are illustrated by the dashed lines, all the signals
except 1310 above of different samples have been enhanced, but there are also some obvious
differences in enhancement effect of different particle size. The sample mixed with 50 nm has the
highest degree of enhancement, and 1.5
µ
m has the lowest enhancement effect. By quantitatively
comparing the intensities at 613 cm
−1
, the Raman enhancement ratio is about 1.5, 2, 4 between
pure R6G and mixing with 1.5
µ
m, 500 nm and 50 nm, respectively, and agreeing with 1-2 orders
proposed by Zhong-Qun Tian [33]. To varying degrees, the enhancement effect of other peaks
is increased by a factor of about 2
∼
4 due to the higher surface-to-volume ratio as particle size
reduced. Interestingly, as the size reduced from 1.5
µ
m to 500 nm, a clear peak of 1310 appears,
and when further to 50 nm, a clear peak of 1410 also appears. This new signal is enhanced when
particle size was reduced to 50 nm and no other change of the position of any peaks is observed.
This implied that 50 nm is a more suitable size for Cu particles for SERS chemical applications.
In general, the enhancement is determined by the wavelength of the exciting light, composition,
temperature, surface morphology and so on [47,48]. Given that the only difference among these
samples is particle size (and as indicated earlier), we conclude that particle size plays a key role
in the improvement of Raman signal. The larger SERS signal at smaller sizes demonstrates
the importance of controlling particle size of Cu in optical applications. And it is important to
remark that these spectrograms possess the same features than those reported [48,49].
3.4. UV-Vis study
Optical absorption of Cu particles dispersed in anhydrous ethanol was investigated via UV-vis
spectroscopy at room temperature under ambient conditions. In doing so, Fig. 5shows the fitted
peaks of Cu particles at different particle sizes. The 1.5
µ
m, 500 nm and 50 nm Cu particles have
well-defined LSPR extinction bands and maximum absorbance near 650 nm, 620 nm, and 600 nm,
respectively. These characteristic absorption peaks confirm the presence of Cu particles and have
red-shift and higher intensity as the particle size decreases, just as the same phenomenon already
discussed by Chenhuinan Wei et al [38]. The 1.5
µ
m and 50 nm have highest and lowest peak
intensity, respectively. At the same time, the 500 nm has a much broader absorption peak when
well dispersed in solution, which indicates a broad size distribution of Cu particles without any
aggregation and keeps consistent with the broad size distribution analyzed from Fig. 1(c) and (d).
In addition, since Cu particles were monodisperse, the coupling effect is not considered. The peak
width is sensitive to the size distribution of Cu particles, corresponding quite well to that reported
by Taekyung Yu in Au nanoparticles [44]. Furthermore, there exists no observable absorption
tail at 800 nm, indicating the absence of the CuO phase in the samples. Thus, size-controlled
synthesis of Cu particles with narrow size distribution would be possible with this approach.

Research Article Vol. 4, No. 12 / 15 Dec 2021 / OSA Continuum 2994
191 The stability of 50nm Cu particles aganist oxidation was also examined through thermal
192 analysis which was performed under a flow of air up to 600 °C. The analysis result shows that
193 Cu particles are stable even if reduced to a range of tens of nanometers in air up to 200 °C,
194 above which there is a stepwise weight gain (Fig. 3 (b)) and appeared similar to the work of M.
195 Ibrahim Dar et al [8]. The total weight gain observed is 23 %, which results from the formation
196 of CuO and is in accordance with the theoretical value of 25 % [38].
197 3.3 SERS study
198 Typically, SERS can be easily observed with coinage metals such as Au, Ag and Cu. These
199 materials exhibit localized surface plasmon resonance (LSPR) bands in the visible region due
200 to excitation of the conduction electrons after irradiation with light. Since Cu particles in this
201 work provide many sharp edges and corners, which can confine EM fields and generate strong
202 near-fields concentrated at vertices, it is expected to have visible enhancement in SERS. At the
203 same time, R6G has been widely applied in SERS research owing to its mature vibration
204 property [41-46], and was selected as probe molecules to prove SERS performance.
205
206
207 Fig. 4. Raman spectra of pure R6G powder and R6G mixed with dispersed Cu particles with different sizes:
208 1.5 μm, 500 nm and 50 nm.
209
210 Fig. 4 presents the Raman spectra of pure R6G powder and the same weight of R6G mixing
211 with different Cu particles (1.5 μm, 500 nm and 50 nm, respectively) under 532 nm close to the
212 plasmonic peak. We selected some typical fingerprint vibrational bands of R6G that are
213 significantly obvious and consistent with those reported in other literatures. The characteristic
214 peaks of 613 cm−1, 775 cm−1 and 1183 cm−1 belonged to C-C-C ring in-plane bending, C-H out-
215 of-plane bending and C-H in-plane bending, respectively. In addition, 1310 cm−1, 1361 cm−1,
216 1505 cm−1, 1573 cm−1 and 1648 cm−1 were associated with the stretching modes of fragrant C-
217 C in-plane [47]. These peaks described above are illustrated by the dashed lines, all the signals
218 except 1310 above of different samples have been enhanced, but there are also some obvious
219 differences in enhancement effect of different particle size. The sample mixed with 50 nm has
220 the highest degree of enhancement, and 1.5 μm has the lowest enhancement effect. By
221 quantitatively comparing the intensities at 613 cm-1, the Raman enhancement ratio is about 1.5,
222 2, 4 between pure R6G and mixing with 1.5 μm, 500 nm and 50 nm, respectively, and agreeing
Fig. 4.
Raman spectra of pure R6G powder and R6G mixed with dispersed Cu particles
with different sizes: 1.5 µm, 500 nm and 50nm.
223 with 1-2 orders proposed by Zhong-Qun Tian [33]. To varying degrees, the enhancement effect
224 of other peaks is increased by a factor of about 2~4 due to the higher surface-to-volume ratio
225 as particle size reduced. Interestingly, as the size reduced from 1.5 μm to 500 nm, a clear peak
226 of 1310 appears, and when further to 50 nm, a clear peak of 1410 also appears. This new signal
227 is enhanced when particle size was reduced to 50 nm and no other change of the position of
228 any peaks is observed. This implied that 50 nm is a more suitable size for Cu particles for SERS
229 chemical applications. In general, the enhancement is determined by the wavelength of the
230 exciting light, composition, temperature, surface morphology and so on [47,48]. Given that the
231 only difference among these samples is particle size (and as indicated earlier), we conclude that
232 particle size plays a key role in the improvement of Raman signal. The larger SERS signal at
233 smaller sizes demonstrates the importance of controlling particle size of Cu in optical
234 applications. And it is important to remark that these spectrograms possess the same features
235 than those reported [48,49].
236 3.4 UV-Vis study
237 Optical absorption of Cu particles dispersed in anhydrous ethanol was investigated via UV-vis
238 spectroscopy at room temperature under ambient conditions. In doing so, Fig. 5 shows the fitted
239 peaks of Cu particles at different particle sizes. The 1.5 μm, 500 nm and 50 nm Cu particles
240 have well-defined LSPR extinction bands and maximum absorbance near 650 nm, 620 nm, and
241 600 nm, respectively. These characteristic absorption peaks confirm the presence of Cu
242 particles and have red-shift and higher intensity as the particle size decreases, just as the same
243 phenomenon already discussed by Chenhuinan Wei et al [38]. The 1.5 μm and 50 nm have
244 highest and lowest peak intensity, respectively. At the same time, the 500 nm has a much
245 broader absorption peak when well dispersed in solution, which indicates a broad size
246 distribution of Cu particles without any aggregation and keeps consistent with the broad size
247 distribution analyzed from Fig. 1(c) and (d). In addition, since Cu particles were monodisperse,
248 the coupling effect is not considered. The peak width is sensitive to the size distribution of Cu
249 particles, corresponding quite well to that reported by Taekyung Yu in Au nanoparticles [44].
250 Furthermore, there exists no observable absorption tail at 800 nm, indicating the absence of the
251 CuO phase in the samples. Thus, size-controlled synthesis of Cu particles with narrow size
252 distribution would be possible with this approach.
253
254
255 Fig. 5. UV-visible spectra of Cu particles with different sizes: 1.5 μm, 500 nm and 50 nm.
Fig. 5.
UV-visible spectra of Cu particles with different sizes: 1.5
µ
m, 500 nm and 50 nm.

Research Article Vol. 4, No. 12 / 15 Dec 2021 / OSA Continuum 2995
3.5. Mechanical study
Mechanical properties of Cu particles have the same size-effect law as optical performance. As
shown in Fig. 6, Cu particles of different sizes were subjected to the same loading force (30
µ
N), and unloaded after maintaining a consistent loading time of 30 s. Finally, the 50 nm leave
the shallowest indenter pit of less than 2 nm, while 6 nm and 12 nm for 500 nm and 1.5
µ
m,
respectively. Results indicate that 50 nm has the highest hardness while the 1.5 µm the lowest.
256
257 3.5 Mechanical study
258 Mechanical properties of Cu particles have the same size-effect law as optical performance. As
259 shown in Fig. 6, Cu particles of different sizes were subjected to the same loading force (30
260 μN), and unloaded after maintaining a consistent loading time of 30 s. Finally, the 50 nm leave
261 the shallowest indenter pit of less than 2 nm, while 6 nm and 12 nm for 500 nm and 1.5 μm,
262 respectively. Results indicate that 50 nm has the highest hardness while the 1.5 μm the lowest.
263
264
265 Fig.6. Nanoindentation curves of Cu particles with different sizes: 1.5 μm, 500 nm and 50 nm.
266
267 4. Conclusion
268 This paper presented a simple and efficient electrochemical formation of well-antioxidant Cu
269 particles with an average size of 1.5 μm, 500 nm and 50 nm, by altering the current density and
270 adding additives. The synthesized Cu particles were characterized by SEM, EDS, TEM, SEAD,
271 XRD and TGA for size, morphology, structure and stability analysis, which conformed that
272 particles had uniform size distribution and good crystallinity, especially the 50 nm. More
273 importantly, a similar relationship between optical and mechanical properties on the size effect
274 was discovered by SERS, UV-Vis and nanoindentation. It is concluded that size has a visible
275 impact on the peak of the Raman spectrum and the UV absorption spectrum of Cu particles,
276 and the 50 nm has the best performance. Meanwhile, the position of UV-Vis peak also shifts to
277 left in small degree with decreasing size and narrower size distribution brings sharper peaks.
278 This work opens the door to the size-controlled synthesis of Cu particles from 1.5 μm to 50 nm,
279 not only improved optical and mechanical properties by size refinement but also good
280 understanding of size effect.
281 Funding
282 National Natural Science Foundation of China (Grant Nos. 51801138 and 51727901).
283 Acknowledgements
284 The authors acknowledge financial support from the National Natural Science Foundation of China (Grant Nos.
285 51801138 and 51727901).
286 Data Availability
Fig. 6.
Nanoindentation curves of Cu particles with different sizes: 1.5
µ
m, 500 nm and
50 nm.
4. Conclusion
This paper presented a simple and efficient electrochemical formation of well-antioxidant Cu
particles with an average size of 1.5
µ
m, 500 nm and 50 nm, by altering the current density
and adding additives. The synthesized Cu particles were characterized by SEM, EDS, TEM,
SEAD, XRD and TGA for size, morphology, structure and stability analysis, which conformed
that particles had uniform size distribution and good crystallinity, especially the 50 nm. More
importantly, a similar relationship between optical and mechanical properties on the size effect
was discovered by SERS, UV-Vis and nanoindentation. It is concluded that size has a visible
impact on the peak of the Raman spectrum and the UV absorption spectrum of Cu particles, and
the 50 nm has the best performance. Meanwhile, the position of UV-Vis peak also shifts to left in
small degree with decreasing size and narrower size distribution brings sharper peaks. This work
opens the door to the size-controlled synthesis of Cu particles from 1.5
µ
m to 50 nm, not only
improved optical and mechanical properties by size refinement but also good understanding of
size effect.
Funding. National Natural Science Foundation of China (51727901, 51801138).
Acknowledgements.
The authors acknowledge financial support from the National Natural Science Foundation of
China (Grant Nos. 51801138 and 51727901).
Disclosures. The authors declare no conflicts of interest.
Data Availability.
Data underlying the results presented in this paper are not publicly available at this time but may
be obtained from the authors upon reasonable request.
Supplemental document. See Supplement 1 for supporting content.
References

Research Article Vol. 4, No. 12 / 15 Dec 2021 / OSA Continuum 2996
1.
H. Wang, Z. H. Zhang, H. M. Zhang, Z. Y. Hu, S. L. Li, and X. W. Cheng, “Novel synthesizing and characterization
of copper matrix composites reinforced with carbon nanotubes,” Mater. Sci. Eng., A 696, 80–89 (2017).
2.
Y. P. Deng, H. Q. Ling, X. Feng, T. Hang, and M. Li, “Electrodeposition and characterization of copper nanocone
structures,” CrystEngComm 17(4), 868–876 (2015).
3.
P. Wang, D. Zhang, and R. Qiu, “Extreme wettability due to dendritic copper nanostructure via electrodeposition,”
Appl. Surf. Sci. 257(20), 8438–8442 (2011).
4.
D. V. Ravi Kumar, I. Kim, Z. Zhong, K. Kim, D. Lee, and J. Moon, “Cu(ii)–alkyl amine complex mediated
hydrothermal synthesis of Cu nanowires: exploring the dual role of alkyl amines,” Phys. Chem. Chem. Phys.
16
(40),
22107–22115 (2014).
5.
E. J. Broadhead, A. Monroe, and K. M. Tibbetts, “Deposition of Cubic Copper Nanoparticles on Silicon Laser-Induced
Periodic Surface Structures via Reactive Laser Ablation in Liquid,” Langmuir 37(12), 3740–3750 (2021).
6.
M. C. Daniel and D. Astruc, “Gold Nanoparticles: Assembly, Supramolecular Chemistry, Quantum-Size-Related
Properties, and Applications toward Biology, Catalysis, and Nanotechnology,” Chem. Rev.
104
(1), 293–346 (2004).
7.
Dr. Guannan Yang, Xian Zeng, Pengyu Wang, Chao Li, Guangdong Xu, Zhen Li, Prof. Jiye Luo, Prof. Yu Zhang, and
Prof. Chengqiang Cui, “Size Refinement of Copper Nanoparticles: A Perspective from Electrochemical Nucleation
and Growth Mechanism,” ChemElectroChem 8(5), 819–828 (2021).
8.
F. Parveen, B. Sannakki, M. V. Mandke, and H. M. Pathan, “Copper nanoparticles: Synthesis methods and its light
harvesting performance,” Sol. Energy Mater. Sol. Cells 144, 371–382 (2016).
9.
M. I. Dar, S. Sampath, and S. A. Shivashankar, “Microwave-assisted, surfactant-free synthesis of air-stable copper
nanostructures and their SERS study,” J. Mater. Chem. 22(42), 22418–22423 (2012).
10.
G. Oskam, P. M. Vereecken, and P. C. Searson, “Electrochemical deposition of copper on n-Si/TiN,” J. Electrochem.
Soc. 146(4), 1436–1441 (1999).
11.
C. C. Hu and C. M. Wu, “Effect of deposition modes on the microstructure of copper deposits from an acidic sulphate
bath,” Surf. Coat. Technol. 176(1), 75–83 (2003).
12.
D. Grujicic and B. Pesic, “Electrodeposition of copper: The nucleation mechanisms,” Electrochim. Acta
47
(18),
2901–2912 (2002).
13.
SEF Kleijn, SCS Lai, and MTM Koper, “Electrochemistry of Nanoparticles,” Angew. Chem. Int. Ed.
53
(14),
3558–3586 (2014).
14.
F. Nasirpouri, S. J. Bending, L. M. Peter, and H. Fangohr, “Electrodeposition and magnetic properties of three-
dimensional bulk and shell nickel mesostructures,” Thin Solid Films 519(23), 8320–8325 (2011).
15.
KI Popov, SS Djokić, ND Nikolić, and VD Jović, “Electrochemically Produced Metal Powders BT - Morphology of
Electrochemically and Chemically Deposited Metals,” Springer International Publishing, Cham, 205–232 (2016).
16.
M. BenAissa, B. Tremblay, A. Andrieux-Ledier, E. Maisonhaute, N. Raouafi, and A. Courty, “Copper nanoparticles
of well-controlled size and shape: a new advance in synthesis and self-organization,” Nanoscale
7
(7), 3189–3195
(2015).
17.
R. M. Penner, “Mesoscopic Metal Particles and Wires by Electrodeposition,” J. Phys. Chem. B
106
(13), 3339–3353
(2002).
18.
H. Liu, F. Favier, K. Ng, M. P. Zach, and R. M. Penner, “Size-selective electrodeposition of meso-scale metal
particles: a general method,” Electrochimica Acta 47(5), 671–677 (2001).
19.
A. Courty, “Silver Nanocrystals: Self-Organization and Collective Properties,” J. Phys. Chem. C
114
(9), 3719–3731
(2010).
20. M. D. Susman, Y. Feldman, A. Vaskevich, and I. Rubinstein, “Chemical Deposition and Stabilization of Plasmonic
Copper Nanoparticle Films on Transparent Substrates,” Chem. Mater. 24(13), 2501–2508 (2012).
21.
D. S. Jung, H. Y. Koo, S. E. Wang, S. B. Park, and Y. C. Kang, “Ultrasonic spray pyrolysis for air-stable copper
particles and their conductive films,” Acta Materialia 206, 116569 (2021).
22.
A. Sarkar, T. Mukherjee, and S. Kapoor, “PVP-Stabilized Copper Nanoparticles: A Reusable Catalyst for “Click”
Reaction between Terminal Alkynes and Azides in Nonaqueous Solvents,” J. Phys. Chem. C
112
(9), 3334–3340
(2008).
23.
L. Yang, P. Li, H. Liu, X. Tang, and J. Liu, “A dynamic surface enhanced Raman spectroscopy method for
ultra-sensitive detection: from the wet state to the dry state,” Chem. Soc. Rev. 44(10), 2837–2848 (2015).
24.
G. Xiao, B. Ying, Q. Chao, and C. Jiang, “Individual nanostructured materials: fabrication and surface-enhanced
Raman scattering,” Chem. Commun. 48(56), 7003–7018 (2012).
25.
M. Guo, M. Liu, W. Zhao, Y. Xia, W. Huang, and Z. Li, “Rapid fabrication of SERS substrate and superhydrophobic
surface with different micro/nano-structures by electrochemical shaping of smooth Cu surface,” Appl. Surf. Sci.
353
,
1277–1284 (2015).
26.
F. Bao, J.-F. Li, B. Ren, R.-A. Gu, and Z.-Q. Tian, “Synthesis and characterization of Au@Co and Au@Ni core-shell
nanoparticles and their applications in surface-enhanced Raman Spectroscopy,” J. Phys. Chem. C
112
(2), 345–350
(2008).
27.
F. Hubenthal, D. Blázquez Sánchez, N. Borg, H. Schmidt, H.-D. Kronfeldt, and F. Träger, “Tailor-made metal
nanoparticles as SERS substrates,” Appl. Phys. B 95(2), 351–359 (2009).
28.
C. L. Haynes, A. D. McFarland, and R. P. Van Duyne, “Surface-Enhanced Raman Spectroscopy,” Anal. Chem.
77(17), 338 A–346 A (2005).

Research Article Vol. 4, No. 12 / 15 Dec 2021 / OSA Continuum 2997
29.
J. Cejkova, V. Prokopec, S. Brazdova, A. Kokaislova, P. Matejka, and F. Stepanek, “Characterization of copper
SERS-active substrates prepared by electrochemical deposition,” Appl. Surf. Sci. 255(18), 7864–7870 (2009).
30.
A. V. Markin, N. E. Markina, J. Popp, and D. Cialla-May, “Copper nanostructures for chemical analysis using
surface-enhanced Raman spectroscopy,” TrAC, Trends Anal. Chem. 108, 247–259 (2018).
31.
P. Tuersun, C. Zhu, X. Han, K. Ren, and Y. Yin, “Light extinction spectrometry for determining the size distribution
and concentration of polydisperse gold nanospheres,” Optik 204, 163676 (2020).
32.
K. Chul-Hyun, C. Sang-Ho, S. C. Kim, M. Song, J. Lee, W. S. Shin, M. Sang-Jin, J. H. Bahng, N. A. Kotov, and S.-H.
Ji, “Silver nanowire embedded in P3HT:PCBM for high-efficiency hybrid photovoltaic device application,” ACS
Nano 5(4), 3319–3325 (2011).
33.
Z. Q. Tian, B. Ren, and D. Y. Wu, “Surface-Enhanced Raman Scattering: From Noble to Transition Metals and from
Rough Surfaces to Ordered Nanostructures,” J. Phys. Chem. B 106(37), 9463–9483 (2002).
34.
L. Wenbo, C. Weiquan, L. Ping, S. Junling, and Z. Shili, “Additives-assisted electrodeposition of fine spherical
copper powder from sulfuric acid solution,” Powder Technol. 326(15), 84–88 (2018).
35.
M. Demirel and B Aksakal, “Effects of powder particle size on mechanostructure and cell viability of the synthesised
bioceramic bone grafts,” J. Ceramic Process. Res. 19(1), 5–10 (2018).
36.
H. Z. Wang, L. Gao, and J. K. Guo, “The influence of SiC particle size on microstructure and mechanical properties
of Al2O3-SiC nanocomposites,” J. Adv. Mater. 32(1), 60–64 (2000).
37.
J. Xinghua, C. Yuansheng, S. Cuixia, X. Chunping, and S. Zhaorui, “Effect of Copper Particle Size on Properties of
W-40Cu Alloy Parts Prepared by Thixo-die-forging in Pseudo-semi-solid State,” Special Casting & Non-ferrous
Alloys. 33(2), 101–103 (2013).
38.
C. H. N. Wei and Q. M. Liu, “Shape-, size-, and density-tunable synthesis and optical properties of copper
nanoparticles,” CrystEngComm 19(24), 3254–3262 (2017).
39.
H.-C. Wu, C.-S. Chen, C.-M. Yang, M.-C. Tsai, and J.-F. Lee, “Decomposition of Large Cu Crystals into Ultrasmall
Particles Using Chemical Vapor Deposition and Their Application in Selective Propylene Oxidation,” ACS Appl.
Mater. Interfaces 10(44), 38547–38557 (2018).
40.
Y. Gao, W. Li, H. Zhang, J. Jiu, H. Dawei, and K. Suganuma, ““Size-Controllable Synthesis of Bimodal Cu Particles
by Polyol Method and Their Application in Die Bonding for Power Devices,” IEEE Trans. Compon., Packag.
Manufact. Technol. 8(12), 2190–2197 (2018).
41.
K. M. Mayer and J. H. Hafner, “Localized Surface Plasmon Resonance Sensors,” Chem. Rev.
111
(6), 3828–3857
(2011).
42.
S. Kadkhodazadeh, J. R. De Lasson, M. Beleggia, H. Kneipp, J. B. Wagner, and K. Kneipp, “Scaling of the Surface
Plasmon Resonance in Gold and Silver Dimers Probed by EELS,” J. Phys. Chem. C 118(10), 5478–5485 (2014).
43.
C. Chibwe and M. Tadie, ““An Experimental Review of the Physicochemical Properties of Copper Electrowinning
Electrolytes,” Mining, Metallurgy & Exploration 38(2), 1225–1237 (2021).
44.
H. S. S. Sharma, E. Carmichael, and D. Mccall, “Fabrication of SERS substrate for the detection of rhodamine 6G,
glyphosate, melamine and salicylic acid,” Vib. Spectrosc. 83, 159–169 (2016).
45.
Z. Hao, M. Mansuer, Y. Guo, Z. Zhu, and X. Wang, “Ag-nanoparticles on UF-microsphere as an ultrasensitive SERS
substrate with unique features for rhodamine 6G detection,” Talanta 146, 533–539 (2016).
46.
J. Reguera, J. Langer, D. J. D. Aberasturi, and L. M. Liz-Marzán, “Anisotropic metal nanoparticles for surface
enhanced Raman scattering,” Chem. Soc. Rev. 46(13), 3866–3885 (2017).
47.
Q. Shao, R. Que, M. Shao, L. Cheng, and S. Lee, “Copper Nanoparticles Grafted on a Silicon Wafer and Their
Excellent Surface-Enhanced Raman Scattering,” Adv. Funct. Mater. 22(10), 2067–2070 (2012).
48.
D. Xu, D. Jing, Z. Song, and C. Jian, “Fractal theory study and SERS effect of centimeter level of copper nanobranch
detectors by solid-state ionics method,” Sens. Actuators, A 271, 18–23 (2018).
49.
T. Yu, R. Kim, H. Park, J. Yi, and W. S. Kim, “Mechanistic study of synthesis of gold nanoparticles using
multi-functional polymer,” Chem. Phys. Lett. 592(30), 265–271 (2014).