Available via license: CC BY 4.0
Content may be subject to copyright.
Received: 18 July 2022
|
Revised: 25 November 2022
|
Accepted: 25 November 2022
DOI: 10.1002/cey2.340
RESEARCH ARTICLE
A study of highly activated hydrogen evolution reaction
performance in acidic media by 2D heterostructure
of N and S doped graphene on MoO
x
Kubra Aydin
1
|Seongwon Woo
2
|Vinit Kaluram Kanade
1
|Seulgi Choi
3
|
Chisung Ahn
4
|Byungkwon Lim
2
|Taesung Kim
1,3
1
SKKU Advanced Institute of Nanotechnology (SAINT), Department of Nano Science and Technology, Sungkyunkwan University, Suwon‐si,
Gyeonggi‐do, Republic of Korea
2
School of Advanced Material Science and Engineering, Department of Nano Science and Technology, Sungkyunkwan University, Suwon,
Gyeonggi‐do, Republic of Korea
3
School of Mechanical Engineering, Sungkyunkwan University, Suwon‐si, Gyeonggi‐do, Republic of Korea
4
Heat & Surface Technology R&D Department, Korea Institute of Industrial Technology, Siheung‐si, Gyeonggi‐do, Republic of Korea
Correspondence
Chisung Ahn, Heat & Surface Technology
R&D Department, Korea Institute of
Industrial Technology, 113‐58
Seohaean‐ro, Siheung‐si, Gyeonggi‐do
15014, Republic of Korea.
Email: cahn@kitech.re.kr
Byungkwon Lim, School of Advanced
Material Science and Engineering,
Department of Nano Science and
Technology, Sungkyunkwan University,
Suwon, Gyeonggi‐do 16419, Korea.
Email: blim@skku.edu
Taesung Kim, SKKU Advanced Institute of
Nanotechnology (SAINT), Department of
Nano Science and Technology,
Sungkyunkwan University,
2066 Seobu‐ro, Jangan‐gu, Suwon‐si,
Gyeonggi‐do 16419, Republic of Korea.
Email: tkim@skku.edu
Funding information
Korea Institute of Industrial Technology,
Grant/Award Number: KITECH
EO‐22‐0005; National Research
Foundation of Korea,
Grant/Award Numbers:
2022R1A3B1078163, 2022R1A4A1031182,
2022R1A2C2005701
Abstract
Herein, a layer of molybdenum oxide (MoO
x
), a transition metal oxide (TMO),
which has outstanding catalytic properties in combination with a carbon‐
based thin film, is modified to improve the hydrogen production performance
and protect the MoO
x
in acidic media. A thin film of graphene is transferred
onto the MoO
x
layer, after which the graphene structure is doped with N and
S atoms at room temperature using a plasma doping method to modify the
electronic structure and intrinsic properties of the material. The oxygen
functional groups in graphene increase the interfacial interactions and
electrical contacts between graphene and MoO
x
. The appearance of surface
defects such as oxygen vacancies can result in vacancies in MoO
x
. This
improves the electrical conductivity and electrochemically accessible surface
area. Increasing the number of defects in graphene by adding dopants can
significantly affect the chemical reaction at the interfaces and improve the
electrochemical performance. These defects in graphene play a crucial role in
the adsorption of H
+
ions on the graphene surface and their transport to the
MoO
x
layer underneath. This enables MoO
x
to participate in the reaction with
the doped graphene. N‐and S‐doped graphene (NSGr) on MoO
x
is active in
acidic media and performs well in terms of hydrogen production. The initial
overpotential value of 359 mV for the current density of −10 mA/cm
2
is
lowered to 228 mV after activation.
Carbon Energy. 2023;e340. wileyonlinelibrary.com/journal/cey2
|
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https://doi.org/10.1002/cey2.340
This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided
the original work is properly cited.
© 2023 The Authors. Carbon Energy published by Wenzhou University and John Wiley & Sons Australia, Ltd.
Kubra Aydin and Seongwon Woo contributed equally to this study.
KEYWORDS
heteroatom‐doped graphene, hydrogen evolution reactions, metal‐free catalysts, transition
metal oxides, van der Waals (vdWs) heterostructures
1|INTRODUCTION
The energy produced from coal, whose demand is ever‐
increasing, is the main contributor to global warming and is
predicted to account for 38% of global CO
2
emissions
between 2020 and 2030.
1,2
This environmental problem has
led to numerous studies investigating new sources of
renewable and sustainable energy to address this global
issue.
3
Among them, hydrogen (H
2
) energy, generated by
electrochemical reactions, is the most promising candidate
as an energy carrier and fuel source
4,5
owing to its
theoretically high mass‐energy density (120 MJ kg
−1
)and
zero emissions of greenhouse gas (such as carbon dioxide
[CO
2
], methane [CH
4
], nitrous oxide [N
2
O], and ozone
[O
3
]).
6–8
To date, platinum (Pt) has been considered the
most effective catalyst for the hydrogen evolution reaction
(HER) in electrochemical electrolytes; however, its high cost
and low stability in an electrochemical environment limit its
practical application.
9
These disadvantages have led to the
development of new alternative catalysts for the HER, such
as graphene doped with single or multiple atoms,
10,11
metal
phosphides,
12
transition metal dichalcogenides (TMDCs),
13
transition metal oxides (TMO),
14
and their heterostructures.
A heterostructure has many advantages for application
as a catalyst in which it enables electronic structures to be
tuned and crystal structures to be altered; moreover, it offers
highly synergistic effects.
15
Among them, heterostructure
materials based on two‐dimensional (2D) structures are
excellent candidates owing to their large active surface area
compared with their 3D counterparts. In particular,
graphene is a promising material with high electrical
conductivity and many reactive sites on the surface;
moreover, it easily forms heterostructures with other types
of materials such as metals and their derivatives, for
example, chalcogenides, phosphides, and nitrides. From
this perspective, many studies have focused on the use of
graphene‐based heterostructures as catalysts for electroche-
mical water splitting. However, the practical application of
these heterostructures is limited because of their low
chemical stability and highly activated Fermi level resulting
from their highly unstable intrinsic bonding energy
state.
16–18
To enhance their chemical activity and stability,
methods that entail doping with heteroatoms such as N, S,
B, and/or F are widely used.
19,20
Owing to their electro-
negativity, these heteroatom dopants increase the electrical
activity for the adsorption of water molecules, which is
critical for the catalytic activity and stability of the modified
energy state.
21,22
Research has also focused on 2D TMOs,
owing to their common catalytic activity and semi-
conductive properties. TMOs are among the critical
members of the family of functional materials because of
their advantages such as low cost and natural abundance.
23
A feature that distinguishes MoO
x
from other inexpensive
TMOs (such as FeO
x
,CoO
x
,andNiO
x
)isthatMo‐based
materials are considered nonmetallic materials with respect
to the HER.
24
Therefore, MoO
x
is a suitable material for the
design of nonmetallic catalysts. However, the lack of
adsorption sites for hydrogen and the corrosion of MoO
x
in acidic solutions render it unfavorable for the HER in
acidic environments.
25–27
Thus, most studies have focused
on alkaline environments.
28
In addition, MoO
x
materials are
widely used as core materials with a surrounding active
shell, which prevents MoO
x
from corrosion in acidic
environments and provides a good HER response.
29,30
The
conductivity of 2D graphene (shell‐like) supported on a
layer of MoO
x
(core‐like) material as vertically stacked
heterostructures may enhance the HER response. This is
because additional holes are created in graphene owing to
the injection of electrons from graphene to MoO
x
.
31
The
MoO
x
core enables fast electron transport in the graphene/
MoO
x
heterostructure because of its good catalytic propert-
ies for the HER.
14,32,33
The introduction of N and S as
dopants to the upper layer of material serves to increase the
level of defects in the graphene structure. These defects act
as active sites and promote proton (H
+
) penetration into the
graphene surface.
34,35
Once protons penetrate the graphene
surface, the HER can also take place on the material that
forms the lower layer, MoO
x
, in this study. This synergetic
effect of N‐and S‐doped graphene (NSGr)@MoO
x
enables
the HER activity of both materials to be enhanced.
36
Despiteallthesedevelopments,heteroatomdoping
requires the development of methodical approaches that
require high temperatures, pressures, and harmful chemical
materials in the reactive procedure. Single/co‐doping of
graphene materials is mostly carried out via chemical vapor
deposition (CVD)
37
and chemical processes.
38,39
However,
these approaches involve high processing temperatures,
complex procedural steps, and long processing times. With
this in mind, plasma‐enhanced CVD (PECVD) has been
investigated to achieve a high doping/deposition level at low
temperatures; this method
40
is widely preferred to avoid the
use of potentially dangerous precursors.
41
Compared with
the thermal‐CVDmethod,thepresenceofaccelerated
energetic electrons, excited molecules, atoms, free radicals,
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|
AYDIN ET AL.
photons, and other active species in the plasma offers
significant advantages such as a relatively low substrate
temperature and short processing time.
40
Herein, we propose doping the graphene structure
on MoO
x
with S and N atoms (NSGr@MoO
x
)usinga
plasma doping process at room temperature (RT). The
NSGr@MoO
x
heterostructure delivers significantly im-
proved HER performance through the activated graphene
surface in combination with the highly conductive MoO
x
material for fast electron transfer. The plasma reaction‐aided
RT doping process is shown to be an effective approach for
doping without drastically changing the properties of in‐
plane graphene owing to the effect of heating. The results of
this study confirmed the increasing intrinsic activity of the
two layers through van der Waals (vdW) stacking and
the enhancement of the number of active sites by the
heterostructure consisting of doped graphene on MoO
x
.We
consider the proposed structure to constitute an important
mechanism that enables MoO
x
to participate in the HER in
an acidic environment.
2|EXPERIMENTAL SECTION
2.1 |Graphene synthesis sequence
by Thermal‐CVD process
Copper foil (25‐µm‐thick, 99.99% purity; Alfa Aesar) was
used on which to grow the graphene layer using thermal
CVD. The copper foil was pre‐cleaned with isopropyl alcohol
(IPA), deionized water (DIW), acetone, and DIW by
sonication for 5 min for each liquid. After pre‐cleaning, the
copper foil was immersed in a 10% HF solution to remove
unwanted particles from the surface; this was followed by a
drying process using N
2
gas and insertion in a quartz tube.
Ar:H
2
gas was introduced into the quartz tube at 200:30 sccm
to heat the chamber to 1050°C, and the Cu foil was annealed
at 1050°C for 2 h to remove residual material from the foil
surface. The growth process was completed under an
Ar:CH
4
:H
2
100:5:100 sccm atmosphere for 2 h (Table S1).
The final cooling step was performed under the same
atmospheric gas conditions as the growth step.
2.2 |MoO
x
synthesis sequence via
plasma oxidation process
The synthesis conditions for MoO
x
via plasma processing
are listed in Table S2. First, Mo (3 mm Dia × 3 mm Th
pellet; 99.95%) was deposited on a glassy carbon
electrode (GCE) via e‐beam evaporation at a deposition
rate of 0.1 Å s
−1
. Then, oxidation of the Mo seed layer
was completed in a 13.56 MHz inductively coupled
plasma (ICP) chamber. The residues and unwanted
particles on the Mo seed layer were removed by
introducing 20 sccm of Ar into the chamber for 10 min.
Subsequently, 20 sccm of H
2
plasma was introduced into
the chamber for 15 min (200 mTorr of chamber pressure,
200 W of power) to remove the native oxide from the
metal surface, and the chamber was heated to 150°C
under Ar cleaning conditions. An oxidation step was
carried out with 10 sccm of O
2
plasma at 50 mTorr of
pressure and 550 W of power for 15 min at 150°C.
2.3 |Method for transferring graphene
onto the MoO
x
The thermally CVD‐processed graphene layer was
transferred to MoO
x
using a PMMA‐based transfer
method. Poly(methyl methacrylate) PMMA (Microchem)
was coated onto the graphene layer via spin coating. The
PMMA/graphene/copper structure was cured by heating
at 90°C for 10 min after fully covering it with PMMA.
The PMMA‐coated material was then kept in a vacuum
chamber for a few hours to allow strong bonds to form
with the graphene interface. The copper foil was etched
using a copper etchant and the remaining PMMA/
graphene layer was washed with DIW. Finally, the layer
was transferred to a plasma‐processed synthesized MoO
x
layer and the PMMA was melted in acetone. As a result,
we obtained the final graphene/MoO
x
/GCE structure.
2.4 |N and S atom doping via
the plasma doping process
All doping processes were performed in a 13.56 MHz ICP
chamber at plasma temperature, under conditions similar to
those listed in Table S3.Thisisanew,cost‐effective, facile,
and rapid doping process. The graphene structure was
doped with S atoms in a 10:10 sccm Ar:H
2
Splasmamixed
atmosphere at a pressure of 50 mTorr and a power of 550 W
for 5 min. Next, N atom doping was carried out at 50 mTorr
of pressure and 100 W of power for 10 min under 5 sccm of
N
2
plasma, which was chosen as the N dopant because of its
low cost, abundance, nontoxicity, noncorrosive benefits, and
eco‐friendly production of N‐doped graphene compared
with NH
3
gas.
42
2.5 |Characterization of materials
Using Raman spectroscopy (Alpha300 M+; WITec
GmbH, at a wavelength of 532 nm), the vibrational
phonon modes of the molecules were analyzed.
AYDIN ET AL.
|
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The surface of the material was imaged using an optical
microscope (OM, U‐MSSP4; Olympus) to confirm
homogenous and continuous material growth. The phase
imaging and film thickness of the graphene nanosheet on
the SiO
2
substrate were measured using atomic force
microscopy (AFM, NX‐100; Park System). Chemical
analysis of the MoO
x
, NSGr material, and NSGr@MoO
x
heterostructure was carried out using X‐ray photo-
electron spectroscopy (XPS, XPS/AES/UPS; Thermo). A
contact angle (CA) system (FEMTOBIOMED; Smart-
drop) was used to characterize the surface wettability of
each electrocatalyst using DIW. The NSGr@MoO
x
heterostructure was etched using a focused ion beam
(FIB, JIB‐4601F; JEOL Ltd.) for the TEM cross‐section
analysis.
2.6 |Electrochemical measurement
Electrochemical measurements were performed using a
glassy carbon rotating disk electrode (RDE; Pine Research
Instrumentation) connected to a CHI600D electrochemical
analyzer (CH Instruments). An Ag/AgCl electrode (satu-
ratedKCl)andagraphiterod(AlfaAesar)wereusedasthe
reference and counter electrodes, respectively. The electro-
lyte was a 0.5 M sulfuric acid solution diluted to 95%
(Samchun Chemical) using DIW. NSGr@MoO
x
,NSGr,
MoO
x
, and graphene electrodes were used as the working
electrodes loaded onto the glassy carbon RDE with a
geometric area of 0.196 cm
2
.TheHERperformancesofthe
catalysts were evaluated at a rotating speed of 1600 rpm in
the RDE to diffuse the evolved hydrogen. NSGr@MoO
x
was
activated via cyclic voltammetry (CV) in the range from 0 to
−0.5 V versus RHE at a scan rate of 50 mV/s for 1000 cycles.
Chronopotentiometric curve measurements were performed
using a WBCS‐3000 (Xeno Co.) instrument in 0.5 M H
2
SO
4
solution at a current density of −10 mA/cm
2
.Electroche-
mical impedance spectroscopy (EIS) measurements were
performed by applying a −0.2 V alternating current (AC)
excitation signal over the frequency range of 100 kHz to
0.1 Hz. All potential values were converted to the reversible
hydrogen electrode (RHE) scale using the equation E
RHE
=
E
Ag/AgCl
+E
0
Ag/AgCl
+ 0.059 × pH, where E
Ag/AgCl
is the
measured potential using the AgCl reference electrode,
and E
0
Ag/AgCl
is the standard potential of the AgCl electrode
at 25°C (0.197 V, saturated KCl) unless otherwise stated.
3|RESULTS AND DISCUSSION
Scheme 1shows the synthesis of the NSGr@MoO
x
heterostructure via PECVD. Details of the optimized
synthesis conditions for the deposition of the graphene
layer, oxidation of the Mo seed layer, and co‐doping
process, are provided in Tables S1,S2, and S3, respec-
tively. The steps of the synthesis process are described in
detail in the experimental section.
Raman spectroscopy was used to analyze the
NSGr@MoO
x
structure to confirm that it had been
successfully fabricated. The spectrum in Figure 1A
confirms the formation of the multilayered graphene
film in accordance with the I
2D
/I
G
peak intensity on the
MoO
x
/SiO
2
/Si substrate.
43,44
Conversely, the peaks at 349
and 467 cm
−1
of Mo‐S vibration modes were not observed
in the spectrum.
45
XPS analysis was performed to
examine the chemical content of the material and the
results are shown in Figure 1B–Ffor Mo 3d, O 1s, C 1s, N
1s, and S 2p, respectively, when the NSGr@MoO
x
was
deposited on the GCE before the HER measurement. The
Mo
4+
doublet peak is centered at 230.9 and 228.5 eV, as
shown in Figure 1B. The intensity of the Mo
5+
peak of
the NSGr@MoO
x
heterostructure was higher compared
SCHEME 1 Schematic of synthesis steps to build the NSGr@MoO
x
2D thin film heterostructure on a GCE via a 13.56 MHz ICP plasma
process
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|
AYDIN ET AL.
with that of the unprocessed MoO
x
material, and it could
be identified by comparing Figure 1B to Figure S1A.In
addition, the increased Mo
5+
peak intensity suggests that
the HER activity of the material could be expected to
increase because this peak indicates improved conduc-
tivity and electrochemical kinetics of the material.
46
The
Mo 3d spectrum revealed the presence of multiple
oxidation states, Mo
6+
,Mo
5+
, and Mo
4+
, after placing
on the GCE.
47
Thus, the enhanced multiple oxidation
levels of the MoO
x
material are likely to facilitate faster
electron transfer and increase the HER rate because of its
enhanced electrical properties. The intensity of the peak
representing species related to the oxygen defects shown
in Figure 1C is higher than that of the oxygen defect‐
related peak for the MoO
x
material on the O 1s spectrum
(Figure S1B). According to the literature, the O
2−
ions in
the oxygen defect regions have higher mobility than
lattice oxygen and can play a crucial role in the catalyst
activity with enhanced material conductivity.
48
The C 1s
spectrum indicates that N and S atoms formed bonds
with C atoms to form functional groups, as shown in
Figure 1D. The N 1s spectrum of the NSGr@MoO
x
heterostructure (Figure 1E) showed an increased peak
intensity of pyrrolic‐and pyridinic‐N atoms compared
FIGURE 1 (A) Raman spectroscopy analysis of the NSGr@MoO
x
heterostructure on a SiO
2
/Si substrate. XPS analysis of NSGr@MoO
x
heterostructure on the GCE before HER for the (B) Mo 3d, (C) O 1s, (D) C 1s, (E) N 1s, and (F) S 2p core peak spectra.
AYDIN ET AL.
|
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with the NSGr thin film shown in Figure S5A.
Since these two N structures are relatively more defective
structure than graphitic N, they will be an effective
pathway for adsorbing H
+
ions. In addition to these,
the absence of a Mo 3p
3/2
peak around 394 eV in the N 1s
spectrum indicates that N atoms do not penetrate the
MoO
x
layer.
49
The absence of this peak indicates that
only the graphene structure is doped. Further, the
addition of nitrogen to the graphene structure is shown
in detail in Scheme S1. As shown in Figure 1F,we
confirmed the S‐H binding energy at 161.5 eV from the S
2p core peak spectra.
50
The peaks at 163.9 eV (C–S–C),
165.0 eV (C═S), 169.1 eV (C–SO
2
–C), and 170.0 eV
(C–SO
3
–C) confirm that S atoms were successfully
incorporated into the carbon structure.
22,51
An analysis was conducted to observe the character-
istics of the graphene and MoO
x
2D thin films before
stacking them to build the NSGr@MoO
x
heterostructure.
The characterization results of the raw graphene layer
obtained using Raman spectroscopy, OM, and AFM are
shown in Figure S2. Additionally, the results of the
Raman spectroscopy and XPS survey scan analysis are
detailed in Figure S3, and the results of the high‐
resolution transmission electron microscopy (HRTEM)
analysis of the MoO
x
material are shown in Figure S4.
Further, the co‐doped graphene was also analyzed using
Raman spectroscopy and XPS, and the results are shown
in Figure S5. Detailed explanations are provided in the
Supporting Information.
The surface wettability of an electrode is one of the
fundamental surface properties that can affect its HER
performance because of the behavior of hydrogen
bubbles on the electrode surface.
52
Removing the
hydrogen bubbles, which cover the surface of a
hydrophobic electrode, is challenging.
53
The bubbles
would prevent contact between the electrode and
electrolyte and reduce the electrocatalytic activity.
Therefore, CA measurements were performed to verify
whether the surface of the material would affect the HER
performance. The CA results are shown in Figure S6 and
explained in the Supporting Information.
The HRTEM images provide information about the
surface and interior structures of the NSGr@MoO
x
heterostructure. Figure 2A shows that the C atoms are
hexagonally bonded, which indicates the graphene
structure of NSGr@MoO
x
. The interlayer distance of
NSGr@MoO
x
was 0.35 nm, which is consistent with the
typical distance between graphene layers (Figure 2B).
54
To further investigate the elemental composition and
distribution of the NSGr@MoO
x
heterostructure, energy
dispersive spectroscopy (EDS) analysis was performed, as
shown in Figure 2C. The atomic percentage (at%) of each
element in the structure is represented by the inset in
Figure 2C. The SEM image (Figure 1D) shows the surface
morphology of the NSGr@MoO
x
. In addition, EDS
mapping of the C, N, and S atoms show that they were
uniformly distributed on the surface of NSGr@MoO
x
throughout the area (Figure 1D–G). The content of doped
N and S atoms was confirmed to be 6% for N and S,
which represents a high doping level for graphene
structures. This is evident from Figure 2H for Mo and
Figure 2I for O, which means that MoO
x
was not only
distributed in the structure together with C, N, and S but
also fully covered with the N‐and S‐doped graphene.
Overall, MoO
x
is covered with N‐and S‐doped graphene,
but certain areas of the MoO
x
surface are uncovered,
which could improve the HER activity to serve the active
sites from heteroatom‐doped graphene and MoO
x
,
respectively.
22
The electrochemical activity of NSGr@MoO
x
was
evaluated using a three‐electrode cell with 0.5 M H
2
SO
4
as the electrolyte. Additional measurements were con-
ducted to compare its activity toward the HER with that
of graphene, NSGr, and MoO
x
as raw catalysts; moreover,
the linear sweep voltammetry (LSV) curves of these raw
catalysts were compared with that of NSGr@MoO
x
,as
shown in Figure 3A. The initial overpotential (η)of
NSGr@MoO
x
at the current density of −10 mA cm
−2
was
359 mV, which was 70 mV lower than that of NSGr. As is
clearly shown by the LSV curve, the graphene and MoO
x
materials did not show sufficient HER activity under the
same conditions, despite the high voltage of −0.5 V that
was applied. This was attributed to the rapid oxidation of
MoO
x
in the acidic electrolyte medium.
55
In addition, the
difference in the activation of the catalysts implies that
superficial NSGr constitutes the main source of active
sites. Tafel slopes were calculated using the equation
η= blog(j)+a, where η,j, and bare the overpotential,
current density, and Tafel slope, respectively, to deter-
mine the relationship between the electrochemical
reaction rate of cathodic HER and the potential applied
to the catalysts. The calculated values are plotted in
Figure 3B. The reaction kinetics of the NSGr@MoO
x
heterostructure improved substantially, as indicated by
the relatively low value of the Tafel slope for
NSGr@MoO
x
(106.8 mV/dec) compared with NSGr
(140.2 mV/dec), graphene (185.5 mV/dec), and MoO
x
(186.9 mV/dec). These results are in agreement with
the determined highest kinetic HER activity. The
electrochemical and electrocatalytic properties of the
materials were subsequently evaluated using EIS at an
overpotential of η=−0.2 V versus RHE. The Nyquist plot
(Figure 3C) is based on the equivalent electrical circuit
model shown in the inset. The initial solution resistance
(R
s
) and charge transfer resistance (R
ct
) of each catalyst
obtained by EIS are summarized in Figure 3D.
6of13
|
AYDIN ET AL.
The summarized data show lower R
ct
values for
NSGr@MoO
x
compared with those for MoO
x
, NSGr,
and graphene, which explains the higher HER catalytic
performance of NSGr@MoO
x
. This is possibly owing to
the synergetic roles of co‐doped atoms at the interface
between the upper and lower materials, which together
have more active sites than pristine or single‐atom doped
graphene, resulting in significantly enhanced catalytic
performance.
50,56
Owing to the large amount of pyrrolic
N that formed in the NSGr@MoO
x
heterostructure, the
HER activity significantly increased. This is because of
the more defective structure of pyrrolic N compared with
those of graphitic and pyridinic N, owing to its suitable
electronegativity and configuration after doping.
57
In
other words, pyrrolic N more actively promotes HER
performance than graphitic N atoms, because pyrrolic
N‐C binding has a lower Gibbs free energy (ΔG
(H)
) than
N atoms in other binding arrangements.
58
Therefore,
hydrogen ions were more likely to be absorbed by the
catalyst surface to produce hydrogen.
59
As mentioned
earlier, the HER activity is improved because of the high
electrical conductivity of the TMO core material with
HER active shell materials, such as NSGr.
As shown in Figure 4A, the LSV curve reveals
the initial and post‐activation HER activity of the
NSGr@MoO
x
heterostructure in 0.5 M H
2
SO
4
. The
activation process was conducted using a 1000‐cycle CV
method in a potential range from −0.5 to 0 V versus the
RHE scale. After activation, the overpotential at a current
density of −10 mA cm
−2
sharply decreased after 1000
cycles from 359 to 228 mV. The corresponding Tafel
slopes of NSGr@MoO
x
before and after activation
decreased from 106.8 to 99.8 mV/s after 1000 cycles of
CV, as shown in Figure 4B. These results show that the
electrochemical kinetics of NSGr@MoO
x
toward the
HER increased after CV cycling. The Nyquist plots
FIGURE 2 (A,B) HRTEM top view image of the NSGr@MoO
x
heterostructure. (C) EDS results of the NSGr@MoO
x
. The inset shows the
at% composition of each element in the heterostructure. (D) SEM image of the NSGr@MoO
x
. Corresponding EDS elemental mapping image
of the (E) C, (F) N, (G) S, (H) Mo, and (I) O.
AYDIN ET AL.
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obtained after activation (Figure 4C) reveal a sharp
decrease in the R
ct
value from 988.91 to 80.77 Ωafter
1000 cycles, which represents the most significant reason
for the decrease in the overpotential value after activa-
tion. We conducted a more in‐depth investigation of the
optimization conditions by performing additional CV
tests for 5000 CV cycles in the potential range from −0.5
to 0 V and 1000 cycles in the potential range from −1to
0 V (Figure S7). After each experiment, the overpotential
at the current density of −10 mA cm
−2
of the
NSGr@MoO
x
was higher than the result from 1000 CV
cycles in the potential range of −0.5 to 0 V, which means
that the NSGr@MoO
x
undergoes degradation in terms of
cyclic potentiometry analysis. This result shows that the
optimized activation condition was 1000 CV cycles in the
potential range from −0.5 to 0 V.
Figure 5A compares the Raman spectra of the
NSGr@MoO
x
heterostructure before and after CV
measurements. The I
D
/I
G
ratio was found to be 1.2
before the HER and 1.4 after CV cycling, indicating that
the defects in the graphene material increased after CV
analysis. The removal of oxygen functional groups from
the graphene structure reduces the graphene to increase
the I
D
/I
G
ratio.
60,61
Conversely, increasing the I
D′
/I
G
peak
ratio from 0.6 before HER to 0.7 after CV cycling
represents an increase in the defect concentration with
doped atoms. Therefore, we can say that H
+
ions were
adsorbed by the graphene surface and caused the D′peak
to increase during the HER analysis. To further study the
increased HER activity after activation, the chemical
structure of NSGr@MoO
x
was analyzed using XPS. The
deconvoluted O 1s spectra showed that the C–O and
C–O–H bonds decreased after activation as shown in
Figure 5B. At the same time, the C 1s spectrum in
Figure 5C shows a decrease in C–O and C═O bonding,
indicating the reduction reaction of NSGr@MoO
x
during
FIGURE 3 (A) Cathodic polarization curves
of bare graphene, NSGr, MoO
x
, and the
NSGr@MoO
x
heterostructure in 0.5 M H
2
SO
4
at
a scan rate of 1 mV s
−1
, (B) corresponding Tafel
slope, (C) EIS, and (D) summary of initial
solution resistance (R
s
) and charge transfer
resistance (R
ct
) of each catalyst.
FIGURE 4 (A) Polarization curves of the NSGr@MoO
x
heterostructure before and after activation, (B) corresponding Tafel plot, and
(C) EIS results of NSGr@MoO
x
heterostructure before and after stability test
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|
AYDIN ET AL.
activation. The N 1s spectrum (Figure 5D) showed a
simultaneous decrease in chemisorbed N and oxidized
N–O bonding; in contrast, graphitic N increased drasti-
cally. These results reveal an increase in the electrical
conductivity of NSGr@MoO
x
during the activation
process, which is consistent with the result of the
decreased R
ct
value measured from the EIS analysis.
The decrease in the C═O and C−O peaks after the HER
measurement indicate the reduction of graphene. Con-
siderable deoxygenation of graphene through chemical
and thermal reduction is important to ensure high
conductivity. Simultaneously, the decrease in the O
content on the N 1s spectrum confirms that oxygen
was removed during the HER analysis. Conversely,
increasing the graphitic‐N in the structure enhances
the main active sites because graphitic‐N has the lowest
barrier for electron transfer.
62
This decrease in the
barrier to electron transfer with an increase in
graphitic‐N was also confirmed by the decrease in the
R
ct
value in the EIS data after activation.
A schematic of the surface of the material where the
HER can take place was drawn on the basis of the Raman
spectra. Scheme 2A shows undoped graphene on the
MoO
x
/GCE structure. The absence of defects in the
pristine graphene structure prevents the adsorption of
H
+
ions. Therefore, the nonadsorbed H
+
ions only couple
with electrons on the graphene surface to produce H
2
.In
this case, the MoO
x
material has no role other than
transmitting the electron it receives from the GCE to
graphene. Scheme 2B illustrates the heterostructure used
in this study, where graphene co‐doped with N and S is
shown on top of the MoO
x
. As a result of these N and S
atoms, defects occurred on the graphene surface. Because
of these defects, H
+
ions were adsorbed from the
graphene surface, as confirmed by the higher I
D′
/I
G
ratio
(Raman spectroscopy analysis), and transferred to the
MoO
x
material. Simultaneously, H
2
production occurs on
the graphene surface, while the adsorbed H
+
ions couple
with electrons on MoO
x
and contribute to H
2
production.
Thus, the synergetic effect of the NSGr@MoO
x
hetero-
structure on H
2
production resulted in higher HER
performance than that of NSGr and MoO
x
.
Long‐term stability is another significant requirement
for practical application. The chronopotentiometric curve
of NSGr@MoO
x
was recorded at a constant current
density of −10 mA/cm
2
to estimate the stability of
NSGr@MoO
x
, as shown in Figure 6A. Without iR
compensation, the overpotential of NSGr@MoO
x
was
288 mV for 24 h, during which time the overpotential did
not change. In addition, the overpotential change after
the stability test was only 3 mV, as shown in Figure 6B,
which proves that NSGr@MoO
x
was stable during long‐
term cycling operation at a constant current density.
These results suggest that the heterostructure can serve
not only to easily realize an HER catalyst but also for
various electrochemical applications. The current densi-
ties, overpotential values, and stability test times for the
NSGr@MoO
x
material with other graphene and Mo/
oxide‐based catalyst materials in the literature are
compared in Table S4.
FIGURE 5 (A) Raman spectra of the
NSGr@MoO
x
heterostructure before the HER
and after CV analysis. XPS analysis of
NSGr@MoO
x
: (B) O 1s, (C) C 1s, and (D) N 1s
core peak spectra.
AYDIN ET AL.
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4|CONCLUSION
We demonstrated the synthesis of the NSGr@MoO
x
heterostructure using a plasma‐enhanced CVD pro-
cess. Owing to the plasma effect, the graphene
structure could be doped with N and S at RT. The
NSGr@MoO
x
electrode consisted of a 2D layered
structure of N‐and S‐doped graphene on the surface
of MoO
x
. The outstanding HER activity of the
electrode could be derived from the improved stability
resulting from the graphene‐based surface protection
layer and electrochemically active surface area of the
heterostructure because of the multiple active sites in
the graphene/MoO
x
structure. The NSGr@MoO
x
het-
erostructure initially had a low overpotential of
359mVatacurrentdensityof−10 mA/cm
2
and
showed negligible minimal change in its overpotential
value for 24 h, based on the chronopotentiometry
curve. Thus, we demonstrated that the proposed
catalyst has excellent long‐term stability. Hence, our
approach provides a versatile avenue for the fabrica-
tion of synergetic heterostructured electrodes for HER
and other catalytic applications.
ACKNOWLEDGMENTS
This research was supported by Basic Science Research
Program through the National Research Foundation of
Korea (NRF) funded by the Ministry of Education
(2022R1A3B1078163 and 2022R1A4A1031182) and
was conducted with the support of the Korea Institute
SCHEME 2 Schematic of HER mechanism
according to H
+
ion adsorption on the material
surface. (A) Undoped graphene and (B) N and S
co‐doped graphene on MoO
x
/GCE.
FIGURE 6 (A) Chronopotentiometry curve
of NSGr@MoO
x
at constant a current density of
−10 mA/cm
2
; (B) polarization curves of
NSGr@MoO
x
initially activated and after
stability test.
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AYDIN ET AL.
of Industrial Technology as “Development of intelli-
gent root technology with add‐on modules (KITECH
EO‐22‐0005)”and the NRF grant funded by the
Korean government (MSIT) (2022R1A2C2005701).
CONFLICT OF INTEREST
The authors declare no conflict of interest.
ORCID
Taesung Kim http://orcid.org/0000-0001-6280-7668
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SUPPORTING INFORMATION
Additional supporting information can be found online
in the Supporting Information section at the end of this
article.
How to cite this article: Aydin K, Woo S, Kanade
VK, et al. A study of highly activated hydrogen
evolution reaction performance in acidic media by
2D heterostructure of N and S doped graphene on
MoO
x
.Carbon Energy. 2023;e340.
doi:10.1002/cey2.340
AYDIN ET AL.
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