Facile and one-step synthesis of a free-standing 3D MoS2-rGO/Mo binder-free electrode for efficient hydrogen evolution reaction

Article abstract of DOI:10.1039/c7ta05440h

A free-standing 3D MoS₂/Mo electrode was prepared via a facile, one-step hydrothermal method wherein the Mo foil itself acted as the Mo source and thio-urea as the S source. When directly used as a hydrogen evolution reaction (HER) electrode, the as-prepared MoS₂/Mo exhibited a low onset potential of 120 mV, requiring an overpotential of only 334 mV to reach a current density of 100 mA cm^-2 while maintaining its catalytic activity for over 20 h. The activation energy of the reaction was determined to be ～120 meV by temperature-dependent HER studies. In addition, we have also shown that MoS₂ structures can be repetitively grown on Mo foil for subsequent HER application by etching MoS₂ with H₂O₂. The HER performance was enhanced further by incorporating reduced graphene oxide (rGO) into the 3D MoS₂/Mo electrode, which facilitated better electron transfer through rGO. The onset potential of the MoS₂-rGO/Mo was 100 mV (which is 20 mV lower than that of the MoS₂/Mo electrode) and it required 291 mV to achieve a current density of 100 mA cm^-2. The catalysts were also found to be highly durable.

Full text of DOI:10.1039/c7ta05440h

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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS2 nanocages as a lithium ion battery anode material
rsc.li/materials-a

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Journal of Materials Chemistry A
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DOI: 10.1039/C7TA05440H
Facile and one-step synthesis of free-standing 3D MoS2-rGO/Mo binder-free
electrode for efficient hydrogen evolution reaction
Barun Kumar Barman, Debanjan Das and Karuna Kar Nanda*
Free-standing 3D MoS2/Mo and MoS2-rGO/Mo electrode are prepared via a facile, one-step
hydrothermal method wherein the Mo foil itself acts as Mo source and thio-urea as the S source. The as-
prepared MoS2/Mo when directly used as a hydrogen evolution reaction (HER) electrode, exhibits a low
onset potential of 120 mV, requiring an overpotential of only 334 mV to reach a current density of 100
mA/cm2 while maintaining its catalytic activity for over 20 h. The HER performance could be enhanced
further by incorporating reduced graphene oxide (rGO) into 3D MoS2/Mo electrode that facilitates better
electron transfer through rGO. The onset potential of MoS2-rGO/Mo is 100 mV (which is 20 mV lower
compared to MoS2/Mo electrode) and requires 291 mV to achieve a current density of 100 mA/cm2 with
long durability.
0
-25
-50
20 % Pt/C
MoS /Mo
2
-75
-100
-125
-150
-175
MoS2-rGO/Mo
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
Potential (V vs. RHE)

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Received 00th January 20xx,
Facile and one-step synthesis of free-standing 3D MoS2-rGO/Mo
binder-free electrode for efficient hydrogen evolution reaction
Barun Kumar Barman#, Debanjan Das# and Karuna Kar Nanda*
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Free-standing 3D MoS2/Mo electrode is prepared via a facile, one-step hydrothermal method wherein the Mo foil itself
acts as Mo source and thio-urea as the S source. The as- prepared MoS2/Mo when directly used as a hydrogen evolution
reaction (HER) electrode, exhibits a low onset potential of 120 mV, requiring an overpotential of only 334 mV to reach a
current density of 100 mA/cm2 while maintaining its catalytic activity for over 20 h. The activation energy of the reaction
was determined to be ~120 meV by temperature-dependent HER studies. In addition, we have also shown that MoS2
structures can be repetitively grown on the Mo foil for subsequent HER application by etching the MoS2 with H2O2. The
HER performance could be enhanced further by incorporating reduced graphene oxide (rGO) into 3D MoS2/Mo electrode
that fecilitates better electron transfer through rGO. The onset potential of MoS2-rGO/Mo is 100 mV (which is 20 mV lower
compared to MoS2/Mo electrode) and requires 291 mV to achieve a current density of 100 mA/cm2. The catalysts are also
found to be highly durable.
MoS2 has perhaps received the greatest attention as a low-
cost alternative to Pt-based electrocatalysts8-23 since the
hydrogen binding energy of MoS2 is close to that of Pt.
Introduction
The main fossil fuel which forms the predominant class of
energy source in the present world is not only a major
contributor to global warming but also faces a threat of
depletion in near future. Therefore, there is an urgency to find
clean and renewable energy alternatives.1 Hydrogen, an
abundant and renewable energy source generating only water
vapours upon combustion is a promising alternative to fossil
fuels.2 Electrochemical water splitting which produces
hydrogen and oxygen in an electrolyzer is an attractive route
to generate high-purity hydrogen on a large scale in a
However, the performance of MoS2-based materials in
comparison to Pt is still far from satisfactory, primarily because
of two major factors. First, it has been found that the HER
activity of MoS2 is due to the edge sites rather than the basal
planes. Second, the intrinsically low conductivity of MoS2
greatly hinders electron transfer process which adversely
affects its efficiency towards HER.24,25 Various strategies have
been adapted to expose more of the active sites and increase
the conductivity of MoS2 either intrinsically or with the aid of
highly conducting substrates such as carbon cloth, graphene or
carbon nanotubes.26-33
sustainable manner.3 Water-splitting, however, is
a
thermodynamically uphill reaction and requires a catalyst for
both the anodic oxygen-evolution reaction (OER) and cathodic
hydrogen evolution reaction (HER) to take place in a practical
manner.
Pt and its alloys4-7 currently form the state-of-art HER
electrocatalysts, however, the acute scarcity and prohibitive
costs severely impedes their large scale application. Therefore,
it is of utmost importance to develop non-precious and earth
abundant materials based electrocatalysts. In recent times,
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is 20 mV lower compared to MoS2/Mo) and it requires 172 and
291 mV over-potential to achieve a current density of 10 and
100 mA/cm2. The catalysts are found to be highly durable.
Moreover, the rate constant of a reaction increases with
temperature (Arrhenius equation), though supplying heat to
enhance HER kinetics would mean additional energy
expenditure. It is worth mentioning at this point that summer
temperatures in various parts of the world, viz. parts of
Arizona and southern California, southern Australia, sub-
Saharan desert, middle-east Asia and western India
consistently record temperatures beyond 45oC (113 0F),
wherein, it may be possible to exploit the heat to achieve
enhanced HER activity thus avoiding any external heat supply.
A proof-of-concept application has been demonstrated by
heating the electrolyte at various temperatures to monitor the
change in the reaction kinetics which was indeed found to
increase with temperature.
Fig. 1 Schematic presentation of synthesis of MoS2/Mo hybrid
nanostructures by recycling method the Mo foil.
Furthermore, prior to their electrochemical tests, the
catalysts are immobilized on a current collector using a
polymer binder, for example, Nafion© which are found to
increase the series resistance34 besides blocking the active
sites, thereby reducing the overall catalytic activity. Therefore,
fabrication of a self-supported binder-free catalyst would go a
long way in solving the above issues as shown by the recent
developments.35-38 Kong et al,35 and Tan et al,36 synthesized
free-standing 3D MoS2 electrodes by chemical vapour
deposition (CVD) which were then directly applied for HER.
However, their catalytic activity was found to be rather
unsatisfactory. Tour’s group developed nanoporous MoS2 film
on Mo foil as a flexible electrode for HER and supercapacitors
devices.37 Though the as-fabricated electrode is mechanically
robust and delivered a good HER catalytic activity, the
synthesis protocol involved anodizing the Mo foil followed by
subsequent sulfurization carried out in a CVD furnace which
made the process instrument-intensive and technically difficult
to emulate. A facile strategy to develop free-standing MoS2
Experimental Section
Molybdenum foil (0.127mm, 99.99% trace metal basis) was
purchased from Sigma Aldrich. Thiourea and 30% H2O2 were
acquired from SDFCL. Millipore water obtained from a Milli-Q
system was used throughout the experiment. The
molybdenum foil was cut into 5 cm x 1cm pieces. The as-
obtained pieces were cleaned with 2M HCl, followed by
ultrsonication in 3:1 v/v H2O: Acetone solution for 20 minutes.
The foil was then washed with copious amount of water and
air-dried. 1.5 g thiourea was dissolved in 30mL water by
stirring for 10 minutes, the solution was then placed in a
Teflon-lined autoclave of 50 mL capacity in which the Mo foil
was placed in an inclined position. The autoclave was then
electrodes is thus called for.
Herein, a simple, one-step hydrothermal method was
employed directly to grow free-standing MoS2/Mo electrode
consisting of MoS2 nanostructures on Mo foil (Fig. 1) which
does not require any external toxic chemical agent or any
sophisticated instrumentation. In this procedure, Mo foil not
only acted as a mechanical support for the growth of MoS2
sheets but also the source of Mo in presence of thio-urea
(NH2CSNH2). The MoS2 grew on the Mo substrate directly due
to the reaction between the electronegative sulphide ions (S2-)
and electropositive Mo metal. The as-prepared electrode was
directly employed as a catalyst which exhibited a superb
activity towards HER in acidic media with excellent stability.
Two factors mainly contributed to the outstanding HER
activity. First, the densely arranged MoS2 nanosheets exposed
a large number of active sites to the electrolyte. Second, direct
and efficient electron transfer from the MoS2 nanosheets to
the Mo foil greatly enhanced overall conductivity. These
favorable attributes resulted in making our catalyst among the
best of the MoS2-based HER electrocatalysts reported so far
with low onset potential of 120 mV, requiring overpotential of
only 188 and 334 mV to reach a current density of 10 and 100
mA/cm2 while maintaining its catalytic activity for at least 20 h.
Most importantly, we have devised a novel strategy to recover
the molybdenum foil used in the process via facile H2O2-
assisted etching of the MoS2 nanostructures. The as-obtained
Mo foil could be used again to regrow the MoS2
nanostructures using the exact same procedure as a fresh Mo
foil. Interestingly, the HER catalytic activity of the regrown
MoS2/Mo catalysts remains indistinguishable to that of a
freshly grown one. The HER performance could be enhanced
further by incorporating reduced graphene oxide (rGO) into 3D
MoS2/Mo electrode with an onset potential of 100 mV (which
0
0
heated to 180 C at a ramping of 6 C/min and maintained for
18 h to obtain MoS2/Mo. The autoclave was then allowed to
cool naturally. The foil was removed and washed with copious
amount of water and ethanol and air-dried under ambient
condition. The MoS2-rGO/Mo was obtained by adding GO into
the reaction mixture. During the reaction, GO gets reduced
hydrothermally to form rGO on the Mo substrate and MoS2
grows on the rGO layer to form 3D MoS2-rGO/Mo electrode.
Physical and electrochemical characterizations
The Raman measurement was performed on a Raman system
(WITec) with confocal microscopy at room temperature using
a Nd:YAG laser (532 nm) as an excitation source. Field emission
Scanning electron microscope (FE-SEM) images and Energy-
dispersive X-ray spectroscopy (EDS) spectra was acquired on a
FE-SEM, FEI-INSPECTF50 instrument by FEI technology.
Transmission electron microscope (TEM), high resolution TEM
(HRTEM) images and selected area electron diffraction (SAED)
patterns were obtained with
a TEM, JEOL- JEM-2100F
operated a 200kV accelerating voltage. The TEM sample was
prepared by carefully scratching off a portion from the Mo foil
which was then dispersed in ethanol by ultrasonication. 2 ꢀL of
the solution was drop-casted on carbon coated copper grid
(300 mesh), and then dried for TEM analysis. X-ray
2 | J. Name., 2012, 00, 1-3
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photoelectron spectroscopy (XPS) was performed for the
elemental analysis which were carried out on an ESCALAB 250
(Thermo Electron) with a monochromatic Al Kα (1486.6 eV)
source. The surface atomic concentrations were determined
from photoelectron peaks areas using the atomic sensitivity
factors reported by Scofield. The binding energy was calibrated
by placing the principal C1s peak at 284.6 eV.
Fig. 2 (a and b) SEM images of as synthesized MoS2/Mo
structure. Inset of (a) shows the flexible nature of MoS2/Mo. (c
and d) TEM and HRETM image of MoS2 nanostructures. (e)
Structural model of MoS2 nanostructure. The Mo and S atoms
are coloured cyan and yellow, respectively
All the electrochemical analyses were performed using an
electrochemical workstation (CHI760E CH Instrument, USA)
with a typical three electrode system. Pt coil was used as the
counter electrode, Ag/AgCl (3 M KCl) as the reference
electrode and the MoS2/Mo foil was directly used the working
electrode. The electrolyte used was 1 M H2SO4 saturated with
N2 gas flow for 30 minutes to remove the dissolved oxygen.
During the measurement, flow of N2 was maintained on the
headspace of electrolyte to minimize the disturbance due to
gas purging. The Linear Sweep Voltammetry (LSV)
measurements were at a scan rate of 5 mV/s. For testing the
stability of the catalyst Chronoamperometry (CA) was
performed at an overpotential of (η) of 210 mV. All the
measurements were done at room temperature. All the
potential values were measured against Ag/AgCl (3 M KCl)
electrode which was then converted to RHE according to the
following equation: ERHE (V) = EAg/AgCl + 0.235 V (at 250C in the
acidic solution having pH=0).
Fig. 2 (a and b) show the high and low magnification
scanning electron microscopy (SEM) images of three-
dimensional (3D) MoS2/Mo synthesized via hydrothermal
method. Fig. 2(a) shows a uniform growth of the MoS2
nanostructures which are tightly bound to the Mo substrate.
Inset photograph of Fig. 2(a) and low magnification SEM image
of the same (ESI-1) ensures the flexible, 3D nature of electrode
with a large-areal conformal MoS2 coverage. The elemental
mapping of the as-synthesized 3D MoS2/Mo where blue and
cyan color correspond to Mo Lα1 and S Κα1, respectively (ESI-
), reveal that both Mo and S are uniformly present indicating
2
that MoS2 nanostructures are uniformly coated on the Mo foil.
Fig. 2(c) displays the transmission electron microscopy (TEM)
image of MoS2 nanostructures consisting of curled, randomly
arranged, crumpled sheets. Furthermore, the high-resolution
TEM (HRTEM) image of a nanosheet is shown in Fig. 2(d). An
interplanar spacing of 0.71 can be clearly observed in the
nanosheet, which is consistent with the expanded d-spacing of
the (002) planes of hexagonal MoS2 (Fig.2 (e)). It also shows a
lattice spacing of 0.275 nm which is consistent with lattice
plane (100) of Mo atom in the crystalline plane of MoS2 along
with amorphous region marked by the red circle in (ESI-3) and
Results and Discussion
(b)
(a)
(c)
(e)
comparable lattice spacing with the commercial MoS2 (ESI-
).39,40 Overall, both SEM and TEM images clearly reveal the
4
formation of MoS2 on Mo substrate.
(a)
(b)
1
A
g
(d)
1
E
2g
0.71 nm
250
300
350
400
450
-1
500
Raman Shift (cm
)
(c)
3d
(d)
S 2p
5/2
2p
3/2
2p
1/2
Mo 3d
3d
3/2
S 2s
226
236
234
232
230
228
224
168
166
164
162
160
158
Fig. 3 (a and b) Raman and EDS spectra oBfiMndiongSE/nMergoy.(e(Vc) and d)
Binding Energy (eV)
2
HRXPS spectra of Mo 3d and S 2p
The formation of MoS2 was characterized further by
Raman spectroscopy. Fig. 3(a) clearly shows two prominent
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peak centred around 373 and 402 cm-1 corresponding to the 1/T. (f and g) 1st, 2500th and 5000th polarization curves with
E1 and A1 peak frequency, respectively.42 Energy dispersive scan rate 50 mV/s and chronoamporometry up-to 20 h at 198
2g
g
spectrum (EDS) as shown in Fig. 3(b) clearly reveals the mV overpotential.
presence of Mo and S. X-ray photoelectron spectroscopy (XPS)
To demonstrate the electrochemical performance of the
3D MoS2/Mo for the HER, the linear sweep voltammetry (LSV)
was used to investigate the chemical states of Mo and S in the
MoS2/Mo 3-D hybrid structures. Fig. 3(c and d) displays the
high resolution XPS (HRXPS) spectra of Mo-3d and S-2p and polarization studies were performed in 1M H2SO4 solution and
the survey spectra is also reveals the presence of Mo and S
ESI-5). In the spectrum of Mo 3d (Fig. 3(c)), the peaks around
compared with 20 % Pt/C as shown in Fig.4 (a). It may be
noted that 3D MoS2/Mo exhibits excellent cathodic HER
current density, while the bare Mo foil as well as commercial
(
228.6 and 231.7 eV can be attributed to Mo 3d5/2 and Mo 3d3/2
binding energies, respectively. In the spectrum of S 2p (Fig.
3(d)), the peaks at about 161.5 and 162.45 eV are due to S
MoS2
electrochemical reaction, the extra energy needed to
overcome the reaction which is known as over potential (
(Sigma-Aldrich) exhibit very poor HER activity. In the
2p3/2 and
S 2p1/2 binding energies, respectively. The
η
)
stoichiometric ratio of Mo:S estimated from the respective
integrated peak area of XPS spectra is 1.8 suggesting the
structure is close to MoS2 while a slightly Mo rich character in
the MoS2 structures may be attributed to the Mo foil.7
and the onset potential stems from the extent of the barrier in
energy conversion process. Lower the onset potential/over
potential, higher is the catalytic activity and lower is the
energy consumption in the catalytic process. The onset
potential of MoS2/Mo is 120 mV. Additionally, the current
density of 10, 50 and 100 mA/cm2 is reached at over potential
at 188, 266 and 334 mV, respectively suggesting the 3D
MoS2/Mo electrode as prominent HER catalyst. The HER
activity of our catalysts were compared with the recently
developed state-of-art molybdenum chalcogenide based HER
electrocatalysts and was found to be superior or at least
comparable (Table-S1).
Commercial
MoS2
0
-25
0.24
0.21
0.18
0.15
0.12
0.09
0.06
(a)
(b)
Mo foil
-50
MoS2/Mo
-75
MoS2/Mo foil
64 mV/dec
Pt/C
-100
-125
-150
Pt/C
32 mV/dec
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
2
Potential (V vs. RHE)
Log J(mA/cm )
Three possible steps have been suggested for HER in acidic
medium.41-43
0.19
0.18
0.17
0.16
0
-25
(c)
(d)
The first one is the Volmer step,
H3O++ e- = Had + H2O (H adsorbs onto the catalyst)
-50
(1)
(2)
rt (25 deg)
40 deg
52 deg
65 deg
78 deg
-75
b= 2.3RT/αF = 120 mV (α = 0.5)
-100
-125
-150
Followed by either the Heyrovsky reaction;
Had + H3O+ + e-= H2 + H2O, b =40 mV
or a Tafel reaction,
-0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0
290
300
310
320
330
340
Potential (V vs. RHE)
Temperature (K)
Had + Had= H2, b = 30 mV
(3)
4.6
Tafel slope was calculated using the Tafel equation (
η
= bLogJ +
0
-25
(e)
(f)
a, where
η = overpotential, J = current density, b = Tafel slope)
4.4
4.2
4.0
-50
and presented in Fig. 4 (b). The Tafel slope of the 3D MoS2/Mo
is found to be 64 mV/dec which indicates that the rate
determining step of the HER follows Volmer mechanism.36 Fig.
-75
-100
-125
-150
st
1
cycle
th
2500 cycle
th
4
(c) presents the temperature-dependent HER activity of 3D
5000 cycle
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
MoS2/Mo. With the increase of solution temperature (298 to
338 K), the HER activity gradually enhanced. The reduced
0.00280 0.00294 0.00308 0.00322 0.00336
Potential (V vs. RHE)
1/T (K)
-8
overpotential (
is found to improve with temperature and is presented in Fig.
(d). The Arrhenius plot between ln J at -0.3 V and 1/T as
shown in Fig. 4(e) yields an activation energy of ~120 meV. Fig.
(f) displays stability towards HER performances of the 3D
η
) to generate a current density of 10 mA/cm2
(g)
-9
4
-10
-11
-12
4
MoS2/Mo. It reveals the catalyst is stable upto 5000th cycles.
Fig.4 (f) shows the chronoamperometric plot indicating the
stability upto 20 h at an overpotential of 188 mV (that
generate a current density of ~10 mA/cm2). In fact, the current
density increases with operando time. The spikes are due to
the release of H2 bubbles from hybrid electrode.
2
4
6
8
10 12 14 16 18 20
Time (h)
Fig. 4 (a) HER activity of 3D MoS2/Mo in 1 M H2SO4 solution
compared with 20 % Pt/C at a scan rate of 5 mV/s. (b) Tafel
plot. (c and d) Temperature-dependent HER activity and the
temperature-dependency of
η
(for 10 mA/cm2). (e) ln (J) vs
4 | J. Name., 2012, 00, 1-3
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(b)
(a)
1
0
(b)
A
2g
(a)
g
1
E
-25
-50
MoS /Mo (Initial)
2
-75
After H2O2 treatment
MoS /Mo
2
-100
-125
-150
After H2O2
tretment
MoS @Mo (Regrown)
2
200
300
400
500
600
)
700 (c)
-0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0
(d)
C-1s
-1
Raman Shift (cm
Potential (V vs. RHE)
O-1s
0
(d)
Mo-3d
(c)
Mo-3p
-25
s-2p
1st
2nd
3rd
-50
-75
-100
-125
-150
100
200
300
400
500
600
Binding Energy(eV)
0.3
0
Liberated H2 bubble
(e)
(f)
-0.4
-0.3
-0.2
-0.1
0.0
-25
-50
Potential (V vs. RHE)
20 % Pt/C
0.2
0.1
0.0
MoS /Mo
2
-75
MoS2-rGO/Mo
-100
-125
-150
-175
Fig. 5 (a and b) HER and Raman spectra of 3-D MoS2/Mo hybrid
electrode before and after treatment with the H2O2. (c) HER
polarization curves of three consecutive regrowth and recycle
of Mo foil with a scan rate of 50 mV/s. (d) Photograph of
reaction container of H2 evolution from 3-D electrode surface
when a bias is applied.
c
e
d
/
V
m
1
3
:
C
/
t
P
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
1.2
2
Potential (V vs. RHE)
Log (J(mA/cm ))
-9.00
-9.25
-9.50
-9.75
-10.00
25
0
(h)
(g)
-25
The recyclability of the MoS2/Mo electrode was
monitored by SEM and Raman spectroscopy while the activity
was evaluated by LSV polarization experiments. The SEM
images of the virgin and recovered Mo foils are shown in ESI-5
which reveal different surface structures. When MoS2/Mo
-50
st
1
-75
MoS2-rGO/Mo
th
th
2500
5000
-100
-125
-150
-175
0
2
4
6
8
10
12
-0.6
-0.4
-0.2
0.0
hybrid was treated with 30
% H2O2, spontaneous self-
Time (h)
Potential (V vs. RHE)
accelerated dissolution of MoS2 nanostructures was observed
which can be explained by following equation; 44
30
21
(j)
(i)
14
7
o
o
M
/
O
MoS2 + 9 H2O2 = MoO22+ + 2SO42- + 2H+ + 8H2O.
G
r
-
20
10
0
S
o
M
0
The fact that the MoS2 nanostructures were completely
dissolved following the H2O2 treatment was established by the
absence of the signature Raman peaks corresponding to MoS2
as well as the loss of HER activity as shown in Fig. 5 (a and b).
However, the HER activity of MoS2 grown on virgin or
recovered Mo foil are almost indistinguishable as evident from
M
/
S
o
M
-7
-14
-21
Mo foil
10 mV/s
50 mV/s
100 mV/s
25 mV/s
75 mV/s
150 mV/s
25
50
75
100 125 150
0.04
0.08
0.12
0.16
0.20
Scan rate (mV/s)
Potential (V vs. Ag/AgCl)
Fig. 6 (a and b) TEM images of MoS2@rGO/Mo hybrids. (c)
HRTEM images of layer structure of MoS2 on rGO. (d) XPS
survey spectra of MoS2@rGO/Mo hybrid. (e and f) display the
HER performances and Tafel slope of hybrid nanostructures,
respectively. (g and h) display the LSV stability curves up-to
5000th cycle and chronoamporometry up-to 12 h (at 172 mV
overpotential). (I and j) CV curves of 3-D MoS2-rGO/Mo
electrode in the 1 M H2SO4 solution with different scan rate
Figure 5(c). Approximately, ~ 6.5 ꢀm of the initial 100 ꢀm thick
foil was consumed during each cycle (ESI-6). Thus a single Mo
foil could be reused a number of times (~ 7 times) for the
growth of MoS2. Figure 5(d) displays the optical photograph of
hydrogen evolution when a bias is applied. Large H2 bubbles
are clearly seen to have accumulated on the electrode surface.
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to access the electrochemical active surface area (ECSA), the
electrochemical double layers capacitance (Cdl), which is
directly proportional to ECSA, has been determined by cyclic
voltammetry (CV) studies in non-redox regime. It can be seen
that Cdl of MoS2-rGO/Mo (0.090 F/cm2) is higher than that for
MoS2/Mo (0.072 F/cm2) and bare Mo foil (0.003 F/cm2) in
acidic medium (Fig. 7 (i and j). This result indicates that the
electrochemically active sites were enhanced after the
incorporation of rGO. Electron impedance spectroscopy (EIS-
12) study reveals a smaller charge transfer resistance (Rct) for
and comparison of difference in current density variation (ꢁJ=
Ja-Jc) at 0.1 V plotted against scan rate to a linear regression
enables the estimation of Cdl of different electrode.
In an attempt to improve the HER activity further and to
prove the versatility of the synthesis methodology, the 3D
MoS2-rGO/Mo hybrid has been synthesized by adding
graphene oxide (GO) in the reaction medium. GO gets reduced
to reduced graphene oxide (rGO) with simultaneously growth
of MoS2 resulting in the formation of MoS2-rGO/Mo hybrid.
Fig.6 (a and b) display the low and high magnification TEM
images which clearly show the crumpled structures with MoS2
layer gown on rGO (schematic in inset of Fig. 6(b)). The HRTEM
images at the centre and side of the nanosheet are shown in
Fig.6 (c), respectively. An interplanar spacing of 0.71 nm can
be clearly observed in the few layer MoS2 nanosheets
corresponding to the expanded d-spacing of the (002) planes
of hexagonal MoS2 on rGO substrate. TEM images of different
places also provide similar morphology (ESI-7). The HRTEM
images of the MoS2 consist with the d-spacing 0.275 nm
corresponding to the lattice plane of the (100) of Mo atom in
the crystalline plane of MoS2 (ESI-8). The XPS survey spectrum
MoS2-rGO@Mo (4
ꢂ) as compared to MoS2@Mo (5.3 ꢂ)
pointing to a faster electron transfer. Overall, rGO not only
provides additional active sites but also simplifies the charge
transfer from the catalyst to the electrode.45, 46
(b)
(a)
T
(c)
(d)
(Fig. 6 (d)) clearly reveals the presence of Mo-3d, S-2p and C-
1s. HRXPS spectra of Mo-3d and S-2p also clearly reveal the
formation of MoS2 (ESI-9). The de-convoluted C-1s spectra of
rGO in MoS2-rGO/Mo and GO further confirm that GO gets
reduced under hydrothermal condition via removal of oxygen
containing functional group from graphene sheet (ESI-10). This
graphene sheet provides the conducting pathway to transfer
the electron from catalyst surface to electrode surface and
enhances the catalytic sites towards HER performance of the
MoS2-rGO/Mo. The formation of the MoS2-rGO hybrid
nanostructures is also characterized by Raman spectroscopy.
The Raman spectrum clearly reveals the characteristic peaks of
MoS2 and rGO (ESI-11). It exhibits two dominant Raman peaks
at 1351 cm-1 (D band) and 1589 cm-1 (G band) corresponding
to graphene in the hybrid structures along with the strong
50 nm
(f)
O-1s
(e)
O-1s
Mo-3d
C-1s
Mo-2p
C-1s
Mo-3d
Mo-3p
S-2p
S-3p
100
200
300
400
500
600
100
200
300
400
500
600
Binding Energy (eV)
Raman peaks at 373 cm-1 (E1
)
and 401 cm-1 (A1 )
Binding Energy (eV)
2g
g
corresponding to the MoS2. Fig.
6
(e) displays the HER
Fig.7 (a-d) TEM and HRTEM images of MoS2 and MoS2-rGO
hybrid nanostructures after the 12 h of HER performance. (e
and f) survey XPS spectra of MoS2/Mo and MoS2-rGO/Mo
hybrid nanostructures after the 12 h of HER performance in
acidic medium.
performances of the MoS2-rGO/Mo compared with the
MoS2/Mo and precious bench mark Pt/C catalyst. With the
addition of rGO, the HER performances enhances and is
believed to be due to improved conductivity. The onset
potential is 100 mV of the MoS2-rGO/Mo which is 20 mV lower
as compared to MoS2/Mo. It requires only 172 and 292 mV to
achieve 10 and 100 mA/cm2 which lower potential compared
with the MoS2/Mo and the Tafel slope is 52 mV/dec (inset Fig.
6(e)). The hybrid catalyst is very stable towards HER
performances and only 6% loss of current density is recorded
after 5000th consecutive cycle (Fig.6 (g)). Chronoamperometric
performance displayed in Fig. 6(h) reveals that the MoS2-
rGO/Mo electrode is stable for up-to 12 h without any
apparent change in the current density (10 mA/cm2). In order
In order to shed some light on the higher stability of our
catalysts, we have characterized them after the
electrochemical studies. Fig.7 (a-d) display the TEM and
HRTEM images of the MoS2/Mo and MoS2-rGO/Mo hybrids
after the HER studies in acidic medium, while Fig.7 (e and f)
display the XPS survey spectra. Overall, the structural and
compositional studies ensure no change in the XPS peak
positions or interlayer spacing and are the main reason for
high stability towards HER performances.
6 | J. Name., 2012, 00, 1-3
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ARTICLE
12. M. A. Lukowski, A. S. Daniel, F. Meng, A. Forticaux, L. Li, S. Jin, J. Am.
Chem. Soc., 2013, 135, 10274.
Conclusions
In conclusion, a 3D MoS2/Mo electrode is developed by
hydrothermal method. The as-synthesized electrode exhibits
highly efficient HER activity in acidic medium due to large
number of exposed edge site by the virtue of the vertically-
aligned nanosheets coupled with fast electron transfer to the
Mo foil. The temperature-dependent HER activities of the 3D
MoS2/Mo reveal the activation energy to be around 120 meV.
Most interestingly, we have also demonstrated the recycling of
the Mo foil and regrowth of MoS2/Mo and their repetitive use
without compromising the HER activity. Further enhancement
of the HER performance of hybrid nanostructures has been
realized by incorporating rGO into MoS2/Mo which also prove
the versatility of the synthesis method. The TEM and XPS
analysis of both the hybrid structures regain after the long-
time HER performance indicate that the both the binder-free
electrode towards the HER activity are very steady.
13. X. Huang, Z. Y. Zeng and H. Zhang, Chem. Soc. Rev., 2013, 42, 1934-1946.
14. J. Wang, W. Cui, Q. Liu, Z. Xing, A. M. Asiri, and X. Sun, Adv. Mater., 2016,
28, 215-230.
15. Z. C. Xing, Q. Liu, A. M. Asiri and X. Sun, Adv. Mater., 2014, 26, 5702-
5707.
16. E. J. Popczun, C. G. Read, C. W. Roske, N. S. Lewis and R. E. Schaak,
Angew. Chem., Int. Ed., 2014, 53, 5427-5430.
17. M. A. Lukowski, A. S. Daniel, C. R. English, F. Meng, A. Forticaux, R. J.
Hamers and S. Jin, Energy Environ. Sci., 2014, 7, 2608-2613.
18. A. I. Carim, F. H. Saadi, M. P. Soriaga and N. S. Lewis, J.
Mater. Chem. A, 2014, 2, 13835-13839.
19. P. Jiang, Q. Liu, C. J. Ge, W. Cui, Z. H. Pu, A. M. Asiri and
X. Sun, J. Mater. Chem. A, 2014, 2, 14634-14640.
20. J. Q. Tian, Q. Liu, A. M. Asiri and X. P. Sun, J. Am. Chem.
Soc., 2014, 136, 7587-7590.
Acknowledgements
The authors acknowledge Department of Science and
Technology (DST) and Council of Scientific and Industrial
Research (CSIR) India for the financial support. The authors
also acknowledge Chemical Science Division in IISc for
providing access to FETEM facility.
21. Q. Liu, J. Q. Tian, W. Cui, P. Jiang, N. Y. Cheng, A. M. Asiri
and X. P. Sun, Angew. Chem., Int. Ed., 2014, 53, 6710–6714.
22. P. Jiang, Q. Liu, Y. H. Liang, J. Q. Tian, A. M. Asiri and
X. Sun, Angew. Chem. Int. Ed., 2014, 53, 12855–12859
23. W. F. Chen, S. Iyer, S. Iyer, K. Sasaki, C. H. Wang, Y. M. Zhu,
J. T. Muckerman and E. Fujita, Energy Environ. Sci., 2013, 6,
1818-1826.
References
1. A. J. Bard, M. A. Fox, Accounts of Chemical Research 1995, 28, 141-145.
24. J. Kibsgaard, Z. Chen, B. N. Reinecke, T. F. Jaramillo, Nat Mater, 2012, 11,
963.
2. J. A. Turner, Science 2004, 305, 972-974.
25. M. Sun, A. E. Nelson, J. Adjaye, Journal of Catalysis 2005, 233, 411.
3. M. Carmo, D. L. Fritz, J. Mergel, D. Stolten, Int. J. Hydrogen Energy 2013,
38, 4901-4934.
26. J. D. Benck, Z. Chen, L. Y. Kuritzky, A. J. Forman, T. F. Jaramillo, ACS Catal.
2012, 2, 1916.
4. J. Greeley, T. F. Jaramillo, J. Bonde, I. Chorkendorff, J. K. Norskov, Nat
Mater 2006, 5, 909-913.
27. X. Zhang, Y. Zhang, B. B. Yu, X. L. Yin, W. J. Jiang, Y. Jiang, J. S. Hu, L. J.
Wan, J. Mater. Chem. A, 2015, 3, 19277.
5. S. Bai, C. Wang, M. Deng, M. Gong, Y. Bai, J. Jiang, Y. Xiong, Angew. Chem.
Int. Ed, 2014, 53, 12120-12124.
28. Y. Yan, B. Xia, N. Li, Z. Xu, A. Fisher, X. Wang, J. Mater. Chem. A 2015, 3,
131.
6. J. R. McKone, E. L. Warren, M. J. Bierman, S. W. Boettcher, B. S.
Brunschwig, N. S. Lewis and H. B. Gray, Energy Environ. Sci., 2011, 4,
3573-3583.
29. Y. G. Li, H. L. Wang, L. M. Xie, Y. Y. Liang, G. S. Hong and
H. Dai, J. Am. Chem. Soc., 2011, 133, 7296-7299.
7. C. Lia and Y. Yamauchi, Phys. Chem. Chem. Phys., 2013, 15, 3490-3496
30. M. A. Lukowski, A. S. Daniel, F. Meng, A. Forticaux, L. S. Li
and S. Jin, J. Am. Chem. Soc., 2013, 135, 10274-10277.
8. B. Hinnemann, P. G. Moses, J. Bonde, K. P. Jørgensen, K. J. H. Nielsen, S.
Horch, I. Chorkendorff, J. K. Nørskov, J. K. J. Am. Chem. Soc., 2005, 127,
5308.
31. Y. Li, H. Wang, L. Xie, Y. Liang, G. Hong, H. Dai, J. Am. Chem. Soc., 2011,
133, 7296-7299.
9. M. Zeng and Y. Li, J. Mater. Chem. A, 2015, 3, 14942-14962.
32. D. J. Li, U. N. Maiti, J. Lim, D. S. Choi, W. J. Lee, Y. Oh, G. Y. Lee, S. O. Kim,
Nano Letters 2014, 14, 1228-1233.
10. E. J. Popczun, J. R. McKone, C. G. Read, A. J. Biacchi, A. M. Wiltrout, N. S.
Lewis and R. E. Schaak, J. Am. Chem. Soc., 2013, 135, 9267-9270.
33. Y. Luo, J. Jiang, W. Zhou, H. Yang, J. Luo, X. Qi, H. Zhang, D. Y. W. Yu, C.
M. Li, T. Yu, J. Mater. Chem., 2012, 22, 8634-8640.
11. D. Voiry, M. Salehi, R. Silva, T. Fujita, M. Chen, T. Asefa, V. B. Shenoy, G.
Eda, M. Chhowalla, Nano Letters 2013, 13, 6222.
34. D. Kong, H. Wang, J. J. Cha, M. Pasta, K. J. Koski, J. Yao, Y. Cui, Nano
Letters 2013, 13, 1341-1347.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 7
Please do not adjust margins

Please do not adjust margins
Journal of Materials Chemistry A
Page 9 of 9
View Article Online
DOI: 10.1039/C7TA05440H
ARTICLE
Journal Name
35. Y. Tan, P. Liu, L. Chen, W. Cong, Y. Ito, J. Han, X. Guo, Z. Tang, T. Fujita, A.
Hirata, M. W. Chen, Adv. Mater., 2014, 26, 8023-8028.
36. Y. Yang, H. Fei, G. Ruan, C. Xiang, J. M. Tour, Adv. Mater., 2014, 26, 8163-
8168.
37. S. T. Finn, J. E. Macdonald, Adv. Energy. Mater., 2014, 4, 1400495.
38. Z. Zeng, Z. Yin, X. Huang, H. Li, Q. He, G. Lu, F. Boey, H. Zhang, Angew.
Chem. Int. Ed, 2011, 50, 11093-11097.
39. S. Zhang, B. V. R. Chowdari, Z. Wen, J. Jin and J. Yang, ACS Nano, 2015, 9,
12464-12472.
40. C. Lee, H. Yan, L. E. Brus, T. F. Heinz, J. Hone, S. Ryu, ACS Nano 2010, 4,
2695-2700.
41. S. Deng, Y. Zhong, Y. Zeng, Y. Wang, Z. Yao, F. Yang, S. Lin, X. Wang, X. Lu,
X. Xia, J. Tu, Adv. Mater. 2017, 29, 1700748.
42. J. Durst, A. Siebel, C. Simon, F. Hasche, J. Herranz, H. A. Gasteiger, Energy
Environ. Sci 2014, 7, 2255-2260.
43. X. Zhao, H. Zhua, X. Yang, Nanoscale, 2014, 6, 10680-10685.
44. V. Ruiz, E. Meux, M. Schneider, V. Georgeaud, Ind. Eng. Chem. Res.,
2011, 50, 5307-5315.
45. H. Jin, J. Wang, D. Su, Z. Wei, Z. Pang and Y Wang, J. Am. Chem.
Soc., 2015, 137, 2688-2694
.
46. H. Li, K. Yu, C. Li, Z. Tang, B. Guo, X. Lei, H. Fu and Z. Zhu, Sci. Rep. 2015,
, 18730.
5
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