Effect of Long-Range and Local Order of Exfoliated and Proton-Beam-irradiated WSe2 Nanosheets for Sodium Ion Battery Application

Article abstract of DOI:10.1002/bkcs.11456

WSe₂ nanosheets were synthesized by mechanical exfoliation via sonication of bulk WSe₂ in organic solvent. The exfoliated WSe₂ nanosheets were separated into two different particle size ranges, and were then irradiated by

Full text of DOI:10.1002/bkcs.11456

Article
DOI: 10.1002/bkcs.11456
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Y.-G. Chun et al.
KOREAN CHEMICAL SOCIETY
Effect of Long-Range and Local Order of Exfoliated and Proton-Beam-
irradiated WSe2 Nanosheets for Sodium Ion Battery Application
*
Yu-Gyeong Chun, Won-Jae Lee, Minseop Lee, and Seung-Min Paek
Department of Chemistry, Kyungpook National University, Daegu 41566, Republic of Korea.
*E-mail: smpaek@knu.ac.kr
Received February 19, 2018, Accepted March 21, 2018
WSe2 nanosheets were synthesized by mechanical exfoliation via sonication of bulk WSe2 in organic sol-
vent. The exfoliated WSe2 nanosheets were separated into two different particle size ranges, and were then
irradiated by a proton beam for modification of their local structure, along with the original bulk sample.
According to X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) analyses of the samples,
the mechanical exfoliation reaction mainly affected the long-range order of the WSe2 samples, whereas
proton beam irradiation influenced the local structure of the two-dimensional WSe2. To examine the struc-
tural dependence of exfoliated and proton-beam irradiated WSe2 in sodium ion battery (NIB) applications,
charge/discharge experiments were carried out, which showed that proton beam irradiation could effec-
tively enhance both the discharge capacity and cycling performance of the WSe2 samples. This synthetic
procedure via proton beam irradiation is expected to be used for the development of new anode materials
with increased energy density in NIBs.
Keywords: Layered WSe2, Sodium ion battery, Proton beam irradiation, X-ray absorption spectroscopy
Introduction
are easily accessible to the intersheet region of transition
metal dichalcogenides owing to the large interlayer spacing,
even though transition metal dichalcogenides have a similar
lamellar structure (space group P63/mmc) to graphite. We
chose tungsten diselenide (WSe2) as a representative exam-
ple for anodes in NIBs, because WSe2 has quite a large
interlayer spacing of 0.65 nm between adjacent selenium
ions.8 Even though a reaction mechanism for the charge/
discharge of anodes in NIBs is quite complex and still
under debate, it is known that a transition metal dichalco-
genide, such as WSe2, follows conversion and alloying
reactions for sodiation/desodiation.9 However, WSe2 has an
inherent limitation of rapid volume expansion and capacity
fading in the process of charge/discharge of sodium ions,
which are larger than lithium ions.10 In order to overcome
this limitation, we have attempted to examine the effect of
morphological changes of WSe2 in NIBs, because the size
of the primary particle could have a significant influence on
the charge/discharge process. To date, numerous studies
have been devoted to the development of anode materials
in NIBs; however, systematic studies to examine the effect
of the lateral size of lamellar materials are quite scarce.
WSe2 can be a good candidate for reducing its particle size,
because it can be exfoliated into single sheets and separated
into different fractions by centrifugation depending on the
lateral size, as shown in the schematic illustration
(Figure 1). A key strategy in this study was to synthesize
macroporous WSe2 of different lateral sizes and thicknesses
without any deterioration of the crystal and electronic struc-
ture of 2H-WSe2. The effect of a structural disorder in the
Lithium ion batteries (LIBs), as rechargeable batteries, have
attracted much research interest from scientists as well as
industry, and are soon becoming a mainstream technology
for mobile and automotive applications owing to their
promising properties such as high energy density and good
durability.1,2 Although there is currently a stable supply of
lithium, there might be a lithium shortage in the near future
because of uneven distribution and insufficient supply.3,4
As the demand for sustainable energy storage devices
increases, sodium ion batteries (NIBs) could be one of the
alternatives for secondary batteries, because sodium is
abundant, low in cost, and similar to lithium in terms of its
physicochemical properties. Most cathode materials for
LIBs, such as layered transition metal oxides, can be used
for NIBs, since lithium intercalation chemistry is applicable
in a similar way to a cathode in NIBs.5 However, graphite,
which is commonly used as an anode in LIBs, cannot inter-
calate sodium ions in electrochemical cells, because a
sodium ion is 55% larger than that of lithium.6 In this
regard, there is urgent need to identify new anode materials
for NIBs with high energy density.
Transition metal-containing anodes for NIBs could be
one of the reliable alternatives, because they can accommo-
date very large amounts of sodium ions via a conversion
reaction.7 In particular, transition metal dichalcogenides
(MX2, where M = Mo, W, Nb and X = S, Se, Te) tend to
have a layered structure due to weak van der Waals interac-
tions between adjacent sheets. Therefore, sodium cations
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KOREAN CHEMICAL SOCIETY
H+
exposed to proton beam bombardment at room temperature
for 1 h, in which the proton beam energy was 7 MeV.
Hereafter, three kinds of samples (bulk WSe2, WSe2-1.5,
and WSe2-15) after proton-beam irradiation (PI) were
abbreviated as WSe2-PI, WSe2-1.5-PI, and WSe2-15-PI,
respectively. Total dose for proton beam irradiation was
adjusted to the 3.2 × 1011 particle/cm2. Proton beam irradi-
ation using TR102 was conducted with pelletized samples
in ambient atmosphere.
Sample Characterization. The crystal structures of prod-
ucts were examined by X-ray diffractometer (XRD, Bruker
D2 phaser, Billerica, MA, USA) using Cu Kα radiation
(λ = 1.5418 Å). The X-ray absorption spectroscopy (XAS)
including the X-ray absorption near edge structure
(XANES) and the extended X-ray absorption fine structure
(EXAFS) was conducted by using the 8C beamline of
Pohang Accelerator Lab (PAL) in Pohang, Korea. The
XAS analysis at W L3-edge and Se K-edge was carried out
by the standard procedure as described previously.13–15 The
morphological evolution of products was probed by using
scanning electron microscope (SEM, Hitachi SU-8220,
Tokyo, Japan) and transmission electron microscope (TEM,
Hitachi HT-7700, Tokyo, Japan).
Charge/Discharge Experiments. To evaluate sodium stor-
age property of samples, electrochemical charge/discharge
experiments were carried out with battery test equipment
(Maccor K4300, Tulsa, Oklahoma, USA). About 70% active
material, 25% conductive carbon (Super P), and 5% poly-
acrylamide were mixed in N-methyl-2-pyrrolidone (NMP).
The obtained slurry was casted onto copper foil by doctor
blade, followed by drying under vacuum at 100ꢀC overnight,
and introduced into an Ar-filled glovebox without any expo-
sure to air. Standard CR2032-type coin cell was constructed
with the active electrode and sodium foil counter electrode,
in which 1 M NaClO4 in propylene carbonate was used as
an electrolyte. A constant current with a current density of
50 mAh/g was applied for charge/discharge experiment, in
which a voltage window was between 0.01 and 2.0 V.
H+
H+
H+
H+
Tungsten
diselenide
(WSe2)
WSe2-1.5
WSe2-1.5-PI
Solvent
for exfoliation
Proton-beam
irradiation
Mechanical
exfoliation
Centrifugation
at 1.5 k rpm
Sonication
H+
H+
H+
H+
H+
Centrifugation
of supernatant
Sediment
WSe2-15
WSe2-15-PI
at 15 k rpm
Proton-beam
irradiation
Figure 1. Schematic illustration of the sample synthesis using
exfoliation, subsequent self-restacking, and proton-beam
irradiation.
short-ranged region was also examined by the introduction
of local structural variations using ion beam irradiation.
High energy proton beams from a cyclotron and/or linear
accelerator could provide enough energy to induce defect
sites and/or local disorder in electrode materials for NIBs.11
The discharge capacity and cyclic performance of the sam-
ples upon sodium ion storage were compared in order to
understand the effects of changes in the particle size and
local structural variation.
Experimental
Sample Preparation and Proton Beam Irradiation. The
mechanical exfoliation was conducted by sonication of the
bulk WSe2 (Aldrich, St. Louis, Missouri, USA) in an
appropriate solvent as previously described.12 N-cyclo-
hexyl-2-pyrrolidone (CHP), suitable for dispersing WSe2,
was used as solvent for mechanical exfoliation. WSe2 pow-
der was put into CHP at the concentration of 40 mg/mL,
followed by ultrasonic treatment using a horn probe sonica-
tion tip for 1 h at the power of 750 W (SONICS Vibra-Cell
VCX 750, Newtown, CT, USA). In order to separately col-
lect the exfoliated WSe2 nanosheets according to the parti-
cle size, the colloidal dispersion of WSe2 was centrifuged
at the rpm of 1.5 k for 30 min. After the sediment was
washed with acetone, the first sample (WSe2-1.5) was
obtained, which had smaller lateral size than the bulk
WSe2. By further centrifuging the supernatant at the rpm of
15 k for 30 min, the second sample (WSe2-15) with the
smallest lateral size was collected.
Proton beam irradiation was used to induce local struc-
tural disorder of samples. Each sample was irradiated with
proton beam by using the MC-50 cyclotron at the Korea
Institute of Radiological and Medical Sciences (KIRAMS)
and/or proton beam accelerator (TR102) in Korea Multi-
purpose Accelerator Complex (KOMAC). Three kinds of
samples such as bulk WSe2, WSe2-1.5, and WSe2-15, were
dispersed in isopropyl alcohol at the concentration of
15 mg/mL, respectively. And then, each dispersion was
Results and Discussion
Figure 2(a) shows XRD patterns of the exfoliated and self-
reassembled WSe2 derivatives after mechanical exfoliation
in CHP solvent, along with the bulk WSe2 for comparison.
WSe2 with lamellar structure was exfoliated in CHP, fol-
lowing which exfoliated WSe2 with different particle sizes
was collected by controlling the speed of centrifugation.
The XRD pattern of bulk WSe2 can be indexed to a space
group of P63/mmc with a hexagonal unit cell.16 The cell
parameters of a = 3.282 Å and c = 12.961 Å after least-
squares refinement are in good agreement with those in a
previous report.17 The basal spacing of bulk WSe2 was
6.51 Å, calculated from the (002) peak appearing at
2θ = 13.6ꢀ. After exfoliation and subsequent self-restacking
with centrifugation at 1.5 k rpm, the intensity of the (002)
peak of WSe2-1.5 was slightly decreased, indicating a
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Proton-beam-irradiated WSe2
KOREAN CHEMICAL SOCIETY
decreases. These phenomena are due to structural disorder
in the in-plane direction, caused by weakened van der
Waals interactions between the adjacent layers and distur-
bance of the atomic arrangement in WSe2 layers. Such a
result is due to destructive interference from diffracted X-
ray waves, caused by long-range disorder of WSe2 after
exfoliation and a restacking reaction.
Figure 2(b) shows XRD patterns for the samples after
ion beam irradiation. For all the proton-beam-irradiated
samples, the overall intensities of XRD peaks decrease
when compared to those before proton beam irradiation. It
is worthwhile to note that the (002) peaks in WSe2-1.5-PI
and WSe2-15-PI appearing at 13.6ꢀ are divided into two
peaks appearing at 13.4ꢀ and 13.6ꢀ, suggesting that WSe2-
1.5-PI and WSe2-15-PI contain different repeating distances
after proton beam bombardment. The (002) peak in WSe2–
PI shows a much broader shape on the lower angle side.
These changes in the intensities and shapes of the (002)
XRD peaks after proton beam irradiation are ascribed to
structural disorder of the lattices along the crystallographic
c-axis, because an energetic proton beam could provide the
high energy necessary for atomic displacement.
Direct evidence for structural changes arising from the
exfoliation and proton beam treatment was obtained from
SEM and TEM analysis (Figure 3). The left-side and right-
side figures are before and after proton beam irradiation,
respectively. Bulk WSe2 was composed of the largest parti-
cles, having widths ranging from 10 to 50 μm, whereas the
lateral size of WSe2–1.5 was below 20 μm. The insets in
Figure 3(a), (c), and (e) show the edge-side of each sample,
in which stacking between adjacent layers can be observed.
The thicknesses of the samples gradually reduce as the lat-
eral particle size decreases. The WSe2-15 sample consisted
of very small flakes, in which the widths of the primary
particles were below 1 μm. Moreover, in the WSe2-15 sam-
ple, very small and thin plates were stacked with irregular
patterns, showing that these plates are composed of self-
restacked layers after a full degree of exfoliation. As shown
in Figure 3(g), TEM images for the self-restacked WSe2–
15 indicated very thin nanosheets with pale contrast, show-
ing the fully exfoliated nature of WSe2-15. The thin nature
of WSe2-15 is also observed after proton beam irradiation
(Figure 3(h)), in which the basal spacing between adjacent
layers can be observed as seen in the inset. The overall
morphology for all the proton-beam-irradiated samples such
as WSe2-PI, WSe2-1.5-PI, and WSe2-15-PI (Figure 3(b),
(d), (f ), and (h)) is quite similar to those before ion beam
treatment. However, the particle sizes of proton-beam irra-
diated samples were slightly smaller than those of the sam-
ples before proton-beam irradiation, implying that proton
beam bombardment affects both the local structure and
macromolecular characteristics.
(a)
8000
6000
4000
2000
WSe2-15
12.5 13.0 13.5 14.0 14.5
2 Theta (degree)
0
8000
6000
4000
2000
WSe2-1.5
12.5 13.0 13.5 14.0 14.5
2 Theta (degree)
0
8000
6000
4000
2000
0
bulk WSe2
12.5 13.0 13.5 14.0 14.5
2 Theta (degree)
10
20
30
40
50
60
70
2 Theta (degree)
(b)
8000
WSe2-15-PI
WSe2-1.5-PI
WSe2-PI
6000
4000
2000
12.5 13.0 13.5 14.0 14.5
2 Theta (degree)
0
8000
6000
4000
2000
12.5 13.0 13.5 14.0 14.5
2 Theta (degree)
0
8000
6000
4000
2000
0
12.5 13.0 13.5 14.0 14.5
2 Theta (degree)
10
20
30
40
50
60
70
2 Theta (degree)
Figure 2. XRD patterns of (a) the bulk WSe2 and self-restacked
WSe2 derivatives and (b) samples irradiated by proton beam
(inset: Enlarged XRD patterns in the range of 12.5–14.5ꢀ).
smaller lateral size of WSe2-1.5 compared to that of bulk
WSe2. The (002) peak intensity of WSe2–15 is approxi-
mately 37% of that of bulk WSe2, whereas the full width at
half maximum (FWHM) of WSe2-15 is much larger than
that of bulk WSe2. Such an XRD result highlights the smal-
ler particle size of WSe2-15 due to disorder in the long-
range region caused by exfoliation. From this result, it is
expected that the thickness of samples also decreases as the
centrifugation speed increases. The average particle sizes of
the samples were estimated by the Debye–Scherrer equa-
tion.18 The particle sizes of bulk WSe2, WSe2-1.5, and
WSe2-15, were determined to be approximately 190, 110,
and 70 nm, respectively. Although the Debye–Scherrer
equation is generally used for evaluating the particle size of
globular particles, it is quite certain that the separation of
exfoliated WSe2 was well-performed with respect to their
particle sizes. When we compared the intensity ratios (IR)
of the (100) peak to those of the (002) peak, the IR values
were 0.17, 0.16, and 0.04 for bulk WSe2, WSe2-1.5, and
WSe2-15, respectively. This result confirms that in-plane
ordering of the samples also decreases as the particle size
of the samples decreases. Moreover, when the XRD peaks
at above 30ꢀ are compared, we can observe decreased XRD
intensity and broadened FWHM as the particle size
To investigate local structural and electronic features of
samples upon the exfoliation and the proton beam irradia-
tion, we performed X-ray absorption spectroscopy (XAS)
analysis such as the extended X-ray absorption fine
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Proton-beam-irradiated WSe2
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WSe2-15 was much larger than that of the bulk WSe2.
WSe2-1.5 and WSe2-15 consisting of smaller particles have
much larger amount of exposed atoms on the surface, lead-
ing to easier oxidation of atoms than those in bulk state.
Meanwhile, overall shapes of XANES spectra after proton
beam irradiation is quite analogous to those before proton
beam treatment, suggesting that electronic structures of
samples were not deteriorated by proton beam.
(a)
(b)
(d)
(f)
5.0 µm
5.0 µm
50.0 µm
50.0 µm
(c)
(e)
(g)
Figure 4(c) and (d) display radial distribution functions
(RDFs) of k3-weighted EXAFS signals (non-phase-shift-
corrected), which were obtained from the Fourier
5.0 µm
5.0 µm
50.0 µm
50.0 µm
5.0 µm
5.0 µm
10.0 µm
10.0 µm
(h)
100
100 nm
Figure 3. SEM and TEM images of samples before and after ion
beam irradiation: The left side figures ((a), (c) and (e)) represent
SEM images for bulk WSe2, WSe2-1.5, and WSe2-15 samples,
respectively, while the right side figures ((b), (d), and (f )) are
SEM images of WSe2-PI, WSe2-1.5-PI, WSe2-15-PI samples after
ion beam irradiation. (g) and (h) represent TEM images for WSe2-
15 and WSe2-15-PI after ion beam irradiation.
structure (EXAFS) and X-ray absorption near edge struc-
ture (XANES) as shown in Figure 4. In XANES spectra at
W L3-edge (Figure 4(a)), white lines are mainly ascribed to
the electronic transitions from 2p3/2 to vacant 5d orbitals.19
The threshold energy of main edges of samples at W L3-
edge was shifted to the higher energy position after proton
beam irradiation, revealing that tungsten atoms in WSe2
samples were slightly oxidized by proton beam. This result
implies that the proton beam treatment induced local struc-
tural stresses for WSe2 samples, which is in good agree-
ment of XRD results showing the increased basal spacing
after proton beam irradiation. Se K-edge XANES spectra
(Figure 4(b)) also showed shifted main edges to the higher
energy. It was found that the ion beam comprising H+ cat-
ions caused oxidation of not only tungsten but also sele-
nium atoms in the WSe2 frameworks. The extent of
oxidation could be estimated by the edge position, which
displays that the oxidative tendency of in WSe2-1.5 and
Figure 4. XANES spectra of samples at (a) W L3-edge and (b) Se
K-edge. Radial distribution function (RDF) of Fourier-transformed
(FT) EXAFS signal for samples at (c) W L3-edge and (d) Se
K-edge.
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Proton-beam-irradiated WSe2
KOREAN CHEMICAL SOCIETY
Transformation. In the RDFs at W L3-edge (Figure 4(c)),
the first FT peak appearing around 2.3 Å is assigned to the
first neighboring Se atoms covalently bonded to W atoms,
while the second peak near 3.1 Å, relatively smaller peak
than the first one, is due to the W atoms in the second
neighboring shells from the absorbing W atoms.20 Since
(a)
(b)
(d)
120
2.0
1.5
1.0
0.5
0.0
500
100
bulk-WSe -PI
2
400
300
200
100
0
80
60
40
20
0
30, 20, 10, 2, 1 cycle
bulk WSe -PI
2
bulk WSe
2
anisotropic interaction between Se atoms is rather stronger
0
10
20
30
0
100
200
300
400
Capacity (mAh/g)
21,22
Cycle number
than that between the other elements in WSe2,
the FT
(e)
(f)
2.0
1.5
1.0
0.5
0.0
120
100
80
60
40
20
0
amplitude of the second shell had much smaller than that of
the first shell. In addition, Figure 4(d) shows the local fine
structural features at Se K-edge. Here, two kinds of strong
peaks are shown, in which the first FT peak at 2.4 Å is
covalently bonded Se atoms and the second FT peak at
2.9 Å is assigned to the nearest W atoms. According to
RDF spectra, overall local structures of samples were pre-
served even after the proton beam treatment, however, the
intensities of the first peak owing to the chemical bonds
between W and Se were slightly decreased after proton
beam irradiation. This reduced amplitude of the first shell is
attributed to the disturbance and/or variation in bond length
of (W Se), as a result of local disorder introduced by pro-
ton beam. From the above XAS analysis, it is certain that
ion beam could control the local structures of WSe2 sam-
ples, leading to the displacement and/or structural disorder
of W and Se atoms induced by energetic proton beam.
To evaluate the electrochemical performance of the pre-
pared samples before and after proton beam irradiation, half
cells of the samples were assembled for NIB application,
and their galvanostatic charge/discharge experiments were
performed at a current density of 30 mA/g and in the cutoff
voltage range of 0.01–2.0 V (Figure 5). The sodiation and
desodiation mechanism in NIBs is still under debate; how-
ever, the first discharge curve is ascribed to the sodium
intercalation reaction and/or conversion reaction as shown
in Figure 5 (a-c). The reversible discharge capacity of the
bulk WSe2 was 270 mAh/g at the second discharge as seen
in Figure 5(d). This discharge capacity of the bulk WSe2
slowly faded to 120 mAh/g after the 30th cycle, which was
only approximately 44% retention of the reversible
discharge capacity. Such a large capacity fading upon
successive charge/discharge is due to the formation of a
solid-electrolyte interphase (SEI), incomplete conversion
reaction, and electrolyte reduction during cycling.23–25
After proton beam irradiation, WSe2-PI had a reversible
discharge capacity of 270 mAh/g, similar to that of bulk
WSe2 itself. However, the discharge capacity of WSe2-PI
after 30 cycles was 170 mAh/g, or 63% of the reversible
capacity. Such an enhancement in cyclability implies that
proton beam irradiation could be used as an effective
method to relieve capacity fading. As seen in the SEM and
XAS results, a high energy proton beam can modify both
the morphology and local structure of WSe2 and its deriva-
tives. Since proton-beam-irradiated samples have much
higher flexibility than unirradiated samples, local disorder
of samples after proton beam bombardment facilitates facile
and reversible sodiation and desodiation, leading to an
500
400
300
200
100
0
WSe -1.5-PI
2
30, 20, 10, 2, 1 cycle
WSe -1.5-PI
2
WSe -1.5
2
0
100
200
300
400
500
0
10
20
30
Capacity (mAh/g)
Cycle number
(c)
120
100
80
60
40
20
0
2.0
500
400
300
200
100
0
WSe -15-PI
2
1.5
1.0
0.5
0.0
30, 20, 10, 2, 1 cycle
WSe2-15-PI
WSe2-15
0
100
200
300
400
500
0
10
20
30
Capacity (mAh/g)
Cycle number
Figure 5. Charge/discharge curves of (a) bulk WSe2-PI,
(b) WSe2-1.5-PI, and (c) WSe2-15-PI after proton beam irradia-
tion. The cycling performance of (d) bulk WSe2, (e) WSe2-1.5,
and (f ) WSe2-15 before and after proton beam irradiation (filled
triangle: discharge capacity, empty triangle: charge capacity,
square: Coulombic efficiency).
enhanced cyclability. Similar increased discharge capacity
was also found in WSe2–1.5 after proton beam irradiation,
as shown in Figure 5(e). WSe2-15, with the smallest parti-
cle size among the samples, exhibited the largest first dis-
charge capacity because of additional sodiation via SEI
formation. However, the charge/discharge experiment for
WSe2-15 revealed that it had a very small discharge capac-
ity of 40 mAh/g at the 30th cycle, due to a low Coulombic
efficiency, as seen in Figure 5(f). After the proton beam
treatment, the discharge capacity of WSe2-15-PI was
160 mAh/g, which is four times larger than that before pro-
ton beam irradiation. It is highly plausible that local struc-
tural defects developed via proton beam treatment can be
used as buffer spaces during volume changes upon succes-
sive sodiation/desodiation, leading to increased cyclability.
Among the three WSe2 samples, WSe2-15 showed the
highest increase in discharge capacity after proton beam
treatment. Such a result is possibly due to the smallest par-
ticle size, largest surface area, and the most disordered
structures of WSe2-15. The large flexibility of WSe2-15
could facilitate the efficient development of buffer spaces
via ion beam treatment, leading to the enhanced electro-
chemical properties. In case of the bulk WSe2-PI irradiated
by proton beam accelerator (not shown), discharge capacity
was nearly similar to that before irradiation, even though
the same total dose was used for the proton beam irradia-
tion. This suggests that the pulsed beam from accelerator in
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KOREAN CHEMICAL SOCIETY
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Conclusion
It was demonstrated that proton beam irradiation can be
used for the development of new anode materials with
increased cyclability for NIB applications. According to
XRD and XAS results, the long-range order of crystals can
be controlled by mechanical exfoliation of WSe2 whereas
the local structure of WSe2 is modifiable by proton beam
irradiation. Electrochemical charge/discharge experiments
of the samples clearly revealed that the discharge capacity
and cyclability of the samples were enhanced after proton
beam treatment, emphasizing that the present synthetic
route using proton beam treatment could be extended to the
fabrication of new nanostructured NIB electrodes with
enhanced electrochemical properties.
Acknowledgments. We gratefully acknowledge financial
support from the National Research Foundation of Korea
(NRF: 2016M2B2A4912195, 2017M2B2A4049463, and
2015R1D1A3A01016279). This work was also partly sup-
ported by Dongil Culture and Scholarship Foundation of
Korea.
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www.bkcs.wiley-vch.de
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