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Nano Research
https://doi.org/10.1007/s12274-017-1804-z
One-step synthesis of novel snowflake-like Si-O/Si-C
nanostructures on 3D graphene/Cu foam by chemical
vapor deposition
Jing Ning1,2,§, Dong Wang1,2,§ (), Jincheng Zhang1,2 (), Xin Feng1,2, Ruixia Zhong1,2, Jiabo Chen1,2,
Jianguo Dong1,2, Lixin Guo3, and Yue Hao1,2
1
The State Key Discipline Laboratory of Wide Band Gap Semiconductor Technology, Xidian University, Xi’an 710071, China
Shaanxi Joint Key Laboratory of Graphene, Xidian University, Xi’an 710071, China
3
School of Physics and Optoelectronic Engineering, Xidian University, Xi’an 710071, China
§
Jing Ning and Dong Wang contributed equally to this work.
2
Received: 10 June 2017
ABSTRACT
Revised: 13 August 2017
The recent development of synthesis processes for three-dimensional (3D)
graphene-based structures has tended to focus on continuous improvement of
porous nanostructures, doping modification during thin-film fabrication, and
mechanisms for building 3D architectures. Here, we synthesized novel snowflakelike Si-O/Si-C nanostructures on 3D graphene/Cu foam by one-step low-pressure
chemical vapor deposition (CVD). Through systematic micromorphological
characterization, it was determined that the formation mechanism of the
nanostructures involved the melting of the Cu foam surface and the subsequent
condensation of the resulting vapor, 3D growth of graphene through catalysis
in the presence of Cu, and finally, nucleation of the Si-O/Si-C nanostructure
in the carbon-rich atmosphere. Thus, by tuning the growth temperature and
duration, it should be possible to control the nucleation and evolution of such
snowflake-like nanostructures with precision. Electrochemical measurements
indicated that the snowflake-like nanostructures showed excellent performance
as a material for energy storage. The highest specific capacitance of the Si-O/Si-C
nanostructures was ~963.2 mF/cm2 at a scan rate of 1 mV/s. Further, even after
20,000 sequential cycles, the electrode retained 94.4% of its capacitance.
Accepted: 15 August 2017
© Tsinghua University Press
and Springer-Verlag GmbH
Germany 2017
KEYWORDS
snowflake-like,
nanostructures,
graphene,
chemical vapor
deposition (CVD)
1
Introduction
Owing to the high specific surface areas, large pore
volumes, strong mechanical strength, and fast mass
and electron transport, three-dimensional (3D) graphenebased nanostructures can be of advantage in a wide
range of applications in the fields of energy storage,
photonic devices, and so on [1, 2]. 3D graphene frames
Address correspondence to Dong Wang, [email protected]; Jincheng Zhang, [email protected]
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Nano Res.
with various morphologies, structures, and properties
can solve problems with commercialized electrode
materials (graphite and silicon, among others) in energy
storage, such as lithium-ion batteries (LIBs) and supercapacitors, which are unable to meet the requirements
for wearable and high-power sources [3–8]. The most
efficient way of solving this problem would be to
develop alternative anode materials with novel structures such that they exhibit high specific capacities [9–15].
However, most carbon materials have a disordered
structure and exhibit poorly defined surface chemistry;
this makes understanding and controlling their surface
reactions a challenge [16]. Moreover, because of strong
p–p stacking and van der Waals interactions between
the interfaces, graphene sheets show a dramatic
decrease of the surface area. Silicon is another promising electrode material because of its superior
theoretical capacity, which is as high as 4,200 mAh/g.
Unfortunately, Si has poor conductivity and large
volume expansion, which usually lead to poor cycling
and rate performance and thus dramatically limit its
practical applicability in LIBs [17, 18]. To solve these
problems, the engineering of novel nano-architectures
and combining Si with other highly conductive components have been shown to be the most favorable
strategies for buffering the large volume changes
induced in Si, as well as for enhancing the electrical
conductivity of Si anodes [19–28]. Ruoff et al. fabricated
a 3D Si/graphene/ultrathin graphite foam electrode by
drop casting a composite consisting of graphene-coated
Si nanoparticles on ultrathin graphite foam [18]. The
gravimetric capacity per unit mass of the entire electrode
was 983 mAh/g after the first cycle, and it remained
as high as 370 mAh/g after 100 cycles for a Si loading
density of 1.5 mg/cm2. Dong et al. synthesized disordered,
hexagonal-platelet-like carbon-coated [email protected] nanocapsules (NCs) using a fast and simple direct current
(DC) arc-discharge plasma method [29]. They found
that the interfacial evolution from carbon to SiO2
shells endowed the [email protected] NCs with enhanced
photocatalytic activity owing to the hydrophilic and
transparent nature of the SiO2 shells, as well as because
of the photosensitivity of the SiC nanocrystals. Yu
et al. reported the fabrication of a rigid 3D structure
composed of SiC [email protected] sheets, which
was prepared using a high-frequency heating process
[30]. The nanostructures showed a strong synergistic
effect, which resulted in improved thermal conductivity
as compared to that in the case of a simple mixture
containing the SiC nanowires and graphene sheets as
fillers in the same proportion. Shi and Wang reported
the fabrication of a hollow core–shell-structured SiO2/
carbon composite anode that was synthesized by the
aerosol spraying of a mixture of polyvinyl alcohol
(PVA) and SiO2 nanoparticles, then coating polyacrylonitrile (PAN) onto the SiO2/PVA surface, and finally
performing an annealing treatment at 800 °C [31].
Compared to the [email protected] composite, the SiO2/porous
[email protected] composite exhibited a higher specific charge
capacity of 669.8 mAh/g at a current density of
100 mA/g after 100 charge/discharge cycles and showed
a capacity retention rate of 98.6%. Thus, combining
3D graphene, Si, SiO2, and SiC or forming appropriate
composite nanoarchitectures for use as electrode
materials is a feasible strategy for realizing improved
energy storage performance.
In this work, we attempted to synthesize novel
snowflake-like Si-O/Si-C nanostructures on 3D graphene/
Cu foam using a simple and effective method by just
one-step CVD. Based on a series of characterization
measurements, including high-resolution field-emission
scanning electron microscopy (HR-FESEM), transmission
electron microscopy (TEM), Raman spectroscopy, X-ray
photoelectron spectroscopy (XPS), and electrochemical
measurements, we discuss the formation mechanism
of the nanostructures, with the aim of developing a
reliable and controllable technique for fabricating such
nanostructures. Furthermore, we evaluate the potential
of the nanostructures for use in energy storage
applications.
2
2.1
Experimental
Fabrication of nanostructures
The snowflake-like nanostructures were grown on
commercial Cu foam with a thickness of 3 mm (homemade, purity of 99.8%) using a laboratory-made lowpressure chemical vapor deposition (CVD) system at
1,030 °C using ultrahigh-purity methane and hydrogen
(>99.99%) as the precursors. First, the surface of the
Cu foam was chemically etched using a (NH4)2(SO4)2
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solution (0.1 mol/L) for 5 min to remove the oxides,
impurities, and organic residues present. After being
flushed with deionized water, the chemically etched
Cu foam was rinsed with analytical-grade acetone,
ethanol, and deionized water (18.2 M) for 5 min
each. Next, the foam was dried with ultrahigh-purity
nitrogen, and the Cu foam/Si (111) stack was placed
into the quartz furnace tube of the CVD system at a
base vacuum of ~0.1 Pa. Ultrahigh-purity hydrogen
was introduced into the furnace tube at a gas flow
rate of 10 sccm and a partial pressure of 21.5 Pa. Once
the temperature reached 1,030 °C, ultrahigh-purity
methane was introduced into the furnace tube (flow
rate of 50 sccm) as the carbon source for 4 h (total
pressure of 101.6 Pa). The tube was then cooled slowly
at a cooling rate of ~5 °C/min.
2.2 Characterization of nanostructures
The micromorphology of the snowflake-like nanostructures formed on the 3D graphene/Cu foam was
investigated by HR-FESEM, (EOL, JSM-7200F) and
HR-TEM (JEOL, JEM-2100F). The elemental distribution
of the nanostructures was investigated using energydispersive X-ray spectroscopy (EDS) (Oxford Instruments, X-MaxN 100TLE). The crystalline quality of the
nanostructures was evaluated using high-resolution
X-ray diffraction (XRD) analysis (Bruker, D8 Discovery)
in the Bragg (reflection) geometry; pure Cu Kα1
radiation (wavelength, λ, of 1.54056 Å) was used. The
Raman spectrum of the nanostructures was determined
with a Raman spectroscopy system (Horiba JY, LabRam
HR 800) using an Ar+ laser (wavelength of 414 nm)
as the excitation source. The surfaces of the nanostructures were analyzed by XPS (ESCALAB, 250Xi).
Finally, the energy storage performance of the nanostructures was characterized using an electrochemical
workstation (Princeton, PARSTA MC-CHS08A).
3
Results and discussion
To ensure greater purity and controllability, the novel
snowflake-like Si-C/Si-O nanostructures were synthesized
on the 3D graphene/Cu foam using a low-pressure
CVD system. Figure 1 presents a schematic diagram
of the formation mechanism of the snowflake-like
Si-C/Si-O nanostructures.
In the beginning, the forming gases, namely, CH4
and H2, were introduced into the furnace tube. The
pretreated 3D Cu foam and Si substrate were placed
at the appropriate position in the tube (center). As the
temperature was increased to more than 900 °C, CH4
started to decompose owing to the catalytic effect of
the Cu foam, thus acting as the carbon source for the
3D graphene frame and the Si-C nanostructures. At
almost the same time, the surface of the Si substrate
began to melt partially and vaporize, thus acting as
the Si source. With continued heating, the surface of
the Cu foam melted and vaporized, with the vapor
subsequently condensing into droplets because of the
surface tension [32]. However, the furnace tube also
Figure 1 Schematic diagram of formation and evolution of snowflake-like nanostructures.
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contained other gases and vapors at this time, including
H2, CH4, C (vapor), Cu (vapor), and Si (vapor). In
addition, it is likely that a small amount of H2O was
also present within the furnace. Finally, the inner surface
of the quartz furnace tube acted as an O source.
As a result, snowflake-like nanostructures formed
under the above-described conditions. To begin with,
most of the carbon present was transformed into
graphene along the surface of the 3D Cu foam. This
prevented the further melting of the Cu foam surface.
On the other hand, atoms of C, Si, and O dissolved into
the surface of the Cu foam during the condensation of
the Cu vapor. This resulted in the formation of Si-C
or Si-O nanostructures, which eventually nucleated at
locations with defects or dust particles. A large number
of nanostructures formed during the condensation
process. However, there was not enough time for these
structures to coalesce into a ball of Cu. Instead, Cu
droplets with long tails formed in different directions.
This is probably the reason why the formed nanostructures were palm-like.
This hypothesis was confirmed through a subsequent
investigation. As shown in Fig. 1, SEM images of the
nanostructures provided the clearest evidence. It can
be seen that nanostructures of various shapes formed
owing to the melting of the Cu foam surface and the
condensation of the resulting vapor into droplets.
Further, it appears that the starting and ending points
of the melting and condensation processes were not
the same and that the processes occurred randomly.
It can also be seen that, in every region, Cu droplets
formed from all directions. Based on these images, it
can be concluded that these droplets eventually formed
the snowflake-like nanostructures. In the rest of the
paper, we focus on describing the formation mechanism
of the snowflake-like nanostructures based on the
results of HR-FESEM, TEM, XRD analysis, Raman
spectroscopy, and XPS.
The growth temperature was one of the main
factors determining the synthesis of the Si-C/Si-O
nanostructures. As can be seen from the HR-FESEM
images in Fig. 2, the temperature had a strong effect
on the growth of the nanostructures. At a low temperature (830 °C), neither the nanostructures nor the
graphene could be seen (Fig. 2(a)), and only a large
number of irregular dust particles (average size of
~300 nm) were present on the surface of the Cu foam;
these became the nucleation centers for the nanostructures [33]. When the temperature was increased
to 880 °C, spherical Si-O nanostructures formed at the
locations where the dust particles or defects were
present, as shown in Fig. 2(b). Several smaller nanostructures (~10 nm) were also present along the grain
boundaries. We believe that these nanostructures may
have been the nuclei of the Si-C nanostructures. When
the growth temperature was increased further to 930 °C,
an even greater number of incomplete snowflake-like
nanostructures were observed, as shown in Fig. 2(c).
Further, a few graphene pieces with an average size
of ~500 nm were observed among the nanostructures.
When the temperature was increased beyond 980 °C,
shown in Figs. 2(d)–2(f), the snowflake-like nanostructures spread all over the surface of the 3D Cu foam.
Surprisingly, the nanostructures were not single-layered
but consisted of stacks of multiple layers, with the
stacked layers spreading over the entire surface of
the 3D Cu foam. In contrast to the case for planar Cu
foil and other similar metal substrates, the graphene
domains grew along the surface of the 3D Cu foam,
even between the snowflake-like nanostructures.
Figure 2 HR-FESEM images of snowflake-like Si-C/Si-O
nanostructures formed on 3D graphene/Cu foam. The images were
obtained at different temperatures. Scale bar is 200 nm.
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The growth time was another key factor that affected
the fabrication of the Si-C/Si-O nanostructures. Figure 3
shows HR-FESEM images of the Si-C/Si-O nanostructures for different growth times; the white dashed
lines indicate the graphene domains. In the beginning,
most of the nanostructures formed were incomplete;
further, the number density of the nanostructures was
low (see Fig. 3(a)). With an increase in the growth
time from 10 to 150 min, the number density of the
snowflake-like nanostructures increased from ~107
to 2.7 × 108 mm–2, whereas the average size deceased
from ~370 to ~100 nm.
These results indicate that there was competition
between the average size and number density. From
SEM images, we found that the Si-O/Si-C nanostructures
were densely covered along with the whole surface
of the 3D Cu frame. At the beginning, the nuclei density
of the Si-O/Si-C nanostructure was low enough to
nucleate larger-scale nanostructures. The average size
of nanostructures seemed to increase. With increasing
nuclei density, the space for nanostructure nucleation
was restricted and distances between nanostructures
became more and more narrow. Thus, the average size
and nuclei density seemed to be competitive, and
thus the average size of the nanostructures decreased.
Thus, by tuning the growth time, it should be possible
to effectively control the size and number density of
the snowflake-like nanostructures. This should be true
for the 3D graphene domains as well.
Figure 4 shows HR-SEM images of the snowflake-like
Si-O/Si-C nanostructures formed on the 3D graphene/Cu
foam substrate. Figure 4(a) shows the inner structure
of the 3D graphene/Cu foam. The lower-magnification
images (Figs. 4(a) and 4(b)) indicate the presence of
Figure 3 (a)–(e) SEM images of snowflake-like nanostructures for growth periods of 10–150 min. (f) Time-dependent variation in
average size and number density of nanostructures.
Figure 4 SEM images of snowflake-like nanostructures grown on 3D graphene/Cu foam.
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many branch-like gullies all over the surface of the
3D graphene/Cu frame. Figure 4(c) shows a highermagnification image, confirming the presence of these
gullies, which are covered by the snowflake-like
nanostructures and are probably the surface pits of
the 3D Cu frame.
We believe that these dented gullies are formed by
the chemical etching process that was performed before
the CVD growth process. Figures 4(d) and 4(e) show
images of different spots on the surface of the 3D
graphene/Cu frame. As can be seen from Fig. 4(d),
the surface of the 3D graphene/Cu foam was not fully
covered by the snowflake-like nanostructures. If one
focuses on the region that is not covered by the
nanostructures (Fig. 4(e)), one can see a wrinkled or
layered graphene film. Further, one can also see two
hexagonal nanostructures under the graphene film.
We believe that these are of nanoscale single-crystal
3C-SiC [34]. This is direct evidence to support of the
assumption made above. In addition, spherical Si-O
nanostructures of different sizes can be observed
in both Figs. 4(d) and 4(e). Figure 4(f) shows a highresolution cross-sectional image of the snowflake-like
nanostructures. It can be seen that the nanostructures
have a height of ~120 nm and are ~700 nm in size.
Figure 5 shows TEM and EDS images of the
snowflake-like nanostructures formed on the 3D
graphene foam.
The 3D Cu frame was removed before the TEM
imaging. As observed in the SEM images, the low-
Figure 5 (a) TEM image of snowflake-like nanostructures on
3D graphene; (b) HR-TEM image. Insets are filtered inverse FFT
images of areas enclosed within red dashed squares in HR-TEM
image, whereas the left-top inset shows corresponding FFT diffraction pattern; and (c)–(e) EDS scans of snowflake-like nanostructures.
magnification TEM images also confirm that the
snowflake-like nanostructures were unordered and
consisted of stacked layers, with the average size
being ~450 nm (Fig. 5(a)). Without the supporting 3D
Cu frame, the 3D graphene foam acts as a film (lightcolored area) beneath the snowflake-like nanostructures.
Figure 5(b) shows an HR-TEM image of the region
enclosed within the yellow dashed line in Fig. 5(a). It
can be seen that most of the nanostructures, that is,
both the spherical and the snowflake-like nanostructures,
are amorphous. However, areas with a two-component
phase consisting of 3C-SiC and graphene are also
present. The left-top inset shows a magnified image of
the finger-like area of the snowflake-like nanostructure in question and the corresponding fast Fourier
transform (FFT) diffraction pattern. The lattice fringe
spacing of 0.25 nm is in keeping with the crystallographic
(111) plane of 3C-SiC [35]. The left-bottom inset shows
a magnified image of the interval between the two
nanostructures. The lattice fringe spacing of 0.32 nm
matches that of graphene [36, 37].
The EDS maps of elemental C and Si of the
snowflake-like nanostructures are shown in Figs. 5(c)–
5(e). From the scanning tunneling electron microscopy
(STEM) image shown in Fig. 5(c), it can be seen
that three structures are present in the imaged area:
graphene, the snowflake-like nanostructures, and the
Al2O3 substrate. The map of elemental C (Fig. 5(d))
indicates that elemental C is distributed within the
entire region of the snowflake-like nanostructures, with
the shape of the map matching those of the regions
covered with the nanostructures and graphene. Further,
Fig. 5(e) shows that only the snowflake-like nanostructures contain elemental Si, confirming that one
of the primary constituent elements of the nanostructures
was Si. In addition, these results match those of the
HR-FESEM and TEM imaging.
Figure 6(a) shows the XRD pattern of the snowflakelike nanostructures formed on the 3D graphene/Cu
foam.
The high-resolution XRD measurements were performed in the Bragg (reflection) geometry using the
pure Cu Kα1 line with a wavelength, λ, of 1.54056 Å.
The narrow high-intensity peak at 2θ of 28.4 is
related to the (111) crystal plane of the Si substrate
[38]. The diffraction peaks at 35.6, 60.1, and 71.8
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Figure 6 (a) XRD patterns and (b) Raman and (c)–(f) XPS spectra of snowflake-like nanostructures.
are attributable to the (111), (220), and (311) crystal
planes, respectively, of 3C-SiC [39]. All the XRD peaks
could be indexed using the JCPDS database (card
No. 29-1129). The diffraction peak at 2θ of 44.5 is
related to the (100) plane of few-layer graphene (JCPDS
No. 01-0646) [37]. As mentioned earlier, most of the
snowflake-like nanostructures were amorphous. Thus,
no XRD peaks related to the Si-O nanostructures
were observed.
Figure 6(b) shows a typical Raman spectrum of the
snowflake-like nanostructures. The highest-intensity
peak at 521 cm−1 is the standard Raman peak of the
Si substrate. This peak is usually used to calibrate
the Raman system. The peak located at 231 cm−1 can
be assigned to the folded transverse acoustic mode of
3C-SiC [40]. The strong and wide peak at 300 cm−1
can be attributed to CuO [41]. We believe that this
peak is related to the oxidation of the 3D Cu foam.
The weak peak at 439 cm−1 can be attributed to glassy
Si [42], and it indicates that the evaporation of
the Si substrate occurred throughout the process for
synthesizing the snowflake-like Si-O/Si-C nanostructures, with the substrate continuing to act as the
Si source. On the right of the Si peak is a sharp peak
at 620 cm−1. This peak can be assigned to the folded
longitudinal acoustic mode of 3C-SiC [40]. These
peaks are indicative of the amorphous nature of the
snowflake-like nanostructures. Next, two peaks can
be observed at 826 and 945 cm−1, respectively. These
peaks are related to the Si–O molecular ring or chain
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that resulted in the formation of the snowflake-like
nanostructures [43]. The Raman shifts of these two
peaks are related to the size of the Si–O molecular
ring/chain. The peak at 976 cm−1 can be assigned to
the transverse optical phonon mode of 3C-SiC [40].
The broadening and shifting of the SiC signals is
attributable to a decrease in the crystal size and an
increase in the density of the stacking faults, respectively. The peaks at 1,349, 1,583, and 2,696 cm−1 are the
typical peaks related to the D, G, and 2D modes of
graphene, respectively [44]. Based on the ratio of the
intensities of the 2D and G bands, we could confirm
that the graphene consisted of more than one layer.
In Fig. 6(b), the 2D mode of graphene was slightly
shifted (16 cm−1) as compared to that of the monolayer
graphene. In general, the 2D band, which is related to
in-plane strain, is a second-order double-resonance
band near the K point [45]. It represents the as-grown
graphene with little compressive stress. Moreover, the
graphene structures collapsed once the 3D Cu frame
was removed. The 2D peak broadened because of layers
of stacking graphene. Further, all the above-mentioned
results were in accordance with those of the SEM,
TEM, and XRD measurements.
XPS measurements were performed to investigate
the chemical surface states of the Si-C/Si-O-based
nanostructures. Figures 6(c)–6(f) present the XPS
O 1s, C 1s, and Si 2p spectra of the snowflake-like
nanostructures. The survey scan spectrum (shown
in Fig. 6(c)) exhibits XPS Cu 2p3, O 1s, C 1s, and Si 2p
peaks. As shown in Fig. 6(d), the O 1s peak, with a
binding energy of 532.5–532.9 eV, can primarily be
attributed to SiOx/Si or SiO2 species [46]. Although we
did not use any source containing elemental O in the
low-pressure CVD system, we believe that the quartz
furnace tube may have released isolated oxygen at
high temperatures (greater than 1,000 °C in this work).
Further, the negligibly small amount of water vapor
that leaked in from the environment may also have
acted as an oxygen source. This is a supposition
for the source of the O element. As we know, the
desorption vacuum of water vapor is approximately
0.01 Pa, whereas the base vacuum of the low-pressure
CVD system in this work was only ~0.1 Pa and the
system did not have any purification accessory. It may
be that some amount of water vapor was induced
into the furnace tube. At high temperatures (over
1,000 °C), the water vapor can decompose and be used
as an O element source. On the other hand, under a
high temperature and H2 atmosphere, the inner surface
of the quartz furnace tube may have partly decomposed
and become a source of O element. The Si 2p peak at
130.63 eV (Fig. 6(e)) can be assigned to the main Si–O
bond or to minor Si–C bonds (Si 2p3/2) [46]. Because
the number density of the Si-C nanostructures was not
high enough, no C 1s peak attributable to the Si–C
bond can be seen in Fig. 6(f), which shows two peaks
at different binding energies: The one at 284.54 eV
can be attributed to the C=C sp2 hybridized C species
of the graphene bond [47], whereas the one at
286.08 eV can be assigned to the –[C6H5]– hexatomic
ring species [46].
When the Si-O/Si-C nanostructures started to
form the snowflake-like morphology, the ordered 3D
structures allowed significant access of the electrolyte
to the integrated nanosheets, which may have decreased
the interface contact resistance between the parallel
directions of the electrodes and the electrolyte. The
electrochemical supercapacitive performance of the
snowflake-like Si-C/Si-O nanostructures formed on
the 3D graphene/Cu foam was evaluated through cyclic
voltammetry and galvanostatic charging/discharging
measurements, which were performed using half-cells.
A three-electrode cell setup was used, with the
Si-C/Si-O nanostructures on the graphene/Cu foam
as the working electrode, Ag/AgCl as the reference
electrode, and Pt as the counter electrode and a KOH
solution (3 M) as the electrolyte. Figure 7(a) shows
the CV curves for scanning rates of 1, 5, 10, 25, 50, and
100 mV/s (0–0.8 V versus Ag/AgCl).
The intensity of the cathode peak as well as that
of the anode peak increased as the scan rate was
increased from 10 to 100 mV/s. No obvious change
in the shape of the CV curves or in the potential
separation between the peaks was observed, indicating
that the nanostructures exhibited excellent charge/
discharge reversibility. During the cathodic process, a
cathodic peak was observed at approximately 0.5 V;
it corresponded to the oxidation of the Cu foam,
which resulted in the release of Cu2+ and Cu+ ions
into the KOH solution under the constant current.
Further, based on previously reported experimental
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Figure 7 Electrochemical performance of snowflake-like Si-C/Si-O nanostructures formed on 3D graphene/Cu foam. (a) CV curves at
scan rates of 1–100 mV/s; (b) galvanostatic charge/discharge curves at different current densities; (c) Nyquist plots; and (d) capacitive
retention rate as function of cycle number at current density of 20 mA/cm2.
results, the other redox peaks could be attributed to
the transition between the SiOx and SiOx+12− species as
follows [48]
SiOx + 2OH–  SiOx+12− + H2O
The specific electrochemical capacitance (C) of an
active material can be calculated from C   IdV / ( vmV ) ,
where I is the electric current, v is the potential scan
rate, m is the mass of the electrode material, and V is
the potential window. The highest specific capacitance
of the Si-C/Si-O nanostructures on the graphene/Cu
foam was ~963.2 mF/cm2, which was observed at a scan
rate of 1 mV/s as determined based on the total weight
of the porous graphene. The specific capacities at
different discharging rates were determined from
galvanostatic discharge measurements, as shown in
Fig. 7(b). The values were calculated using the expression C = (IΔt)/(mΔV), where I, Δt, m, and ΔV are the
discharge current, time, mass of the electrode material,
and potential change, respectively. In this case, the
highest specific capacitance was 953.2 mF/cm2 at a
current density of 5.5 mA/cm2, as determined based
on the total weight of the porous graphene; this value
is close to the value obtained from the CV measurements. The specific capacitance decreased with
increasing current density until reaching a value of
100.3 mF/cm2 at a current density of 17.5 mA/cm2.
Normally, the CV curves of SiC electrodes at different
sweep rates present a rectangular shape, indicating a
predominant double-layer storage mechanism [49].
Thus, we could not find the obvious peak corresponded
to SiC from the CV curves in Fig. 7(a). Moreover, the
number density of the Si-C nanostructures was not
large enough; no C 1s peak attributable to the Si–C
bond can be seen in Fig. 6(f). From these reasons, the
SiC parts contributed to the overall electrochemical
performance from two aspects: the SiC-assisted formation of the snowflake-like nanostructures, which
can increase specific surface area of the electrodes.
The Nyquist plots of the Si-C/Si-O nanostructures
on the graphene/Cu foam contained an arc and a spike
in the high- (10 kHz) and low-frequency (10 mHz)
regions, respectively, as shown in Fig. 7(c). Based
on the diameter of the semicircle in the very-highfrequency region, it was surmised that the fabricated
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Research
10
Nano Res.
Si-C/Si-O graphene electrode had a low charge-transfer
resistance. The cycling stability of the Si-C/Si-O
graphene electrode was tested through galvanostatic
charge/discharge cycling at a current density of
20 mA/cm2 (see Fig. 7(d)). After 10,000 charge/discharge
cycles, the electrode showed a capacitance retention
rate of 96%. Further, even after 20,000 sequential cycles,
the electrode retained 94.4% of its capacitance.
4
Conclusions
In this work, we synthesized novel snowflake-like
Si-O/Si-C nanostructures on 3D graphene/Cu foam
using a low-pressure CVD system in a one-step process.
The formation mechanism of the nanostructures,
investigated by HR-FESEM, TEM, EDS, Raman
measurements, and XRD analysis, indicated that the
nanostructures were formed by melting of the Cu
foam surface and subsequent condensation of the
resulting vapor, growth of 3D graphene through
catalysis by Cu, and nucleation of the Si-O/Si-C
nanostructures in the carbon-rich atmosphere. By
tuning the growth temperature and duration, it should
be possible to control the nucleation and evolution of
such snowflake-like nanostructures with precision. In
particular, the results of electrochemical measurements
indicated that the snowflake-like nanostructures
showed excellent performance and would be highly
suited as a material for energy storage. The highest
specific capacitance of the Si-C/Si-O nanostructures
formed on the graphene/Cu foam was ~963.2 mF/cm2
at a scan rate of 1 mV/s based on the total weight
of the porous graphene. Further, even after 20,000
sequential cycles, the Si-C/Si-O graphene electrode
retained 94.4% of its capacitance.
Acknowledgements
The work was supported by the National Natural
Science Foundation of China (Nos. 61604115 and
61334002), the Natural Science Basic Research Plan in
Shaanxi Province of China (No. 2016ZDJC-09), the
Key Research and Development program in Shaanxi
Province (No. 2017ZDCXL-GY-11-03), the China
Postdoctoral Science Foundation (No. 2015M580814),
the Postdoctoral Science Research Plan in Shaanxi
Province of China and the Fundamental Research
Funds for the Central Universities (Nos. XJS15066 and
JB161103).
Electronic Supplementary Material: Supplementary
material (HR-FESEM images of snowflake-like Si-C/
Si-O nanostructures under different hydrogen flow
rates) is available in the online version of this article
at https://doi.org/10.1007/s12274-017-1804-z.
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