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Electrochemical and SECM Investigation of MoS2/GO and MoS2/rGO
Nanocomposite Materials for HER Electrocatalysis
Sriram Kumar,†,‡ Prasanta Kumar Sahoo,§ and Ashis Kumar Satpati*,†,‡
†
Analytical Chemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India
Homi Bhabha National Institute, Anushaktinagar, Mumbai 400094, India
§
Centre for Nano Science and Nano Technology, Siksha ‘O’ Anusandhan University, Bhubaneswar 751030, Odisha, India
‡
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S Supporting Information
*
ABSTRACT: Development of advanced materials for electrocatalytic and
photocatalytic water splitting is the key in utilization of renewable energy. In
the present work, we have synthesized MoS2 nanoparticles embedded over the
graphene oxide (GO) and reduced graphene oxide (rGO) layer for superior
catalytic activity in the hydrogen evolution process (HER). The nanocomposite materials are characterized using different spectroscopic and
microscopic measurements. A Tafel slope of ∼40 mV/decade suggested the
Volmer−Heyrovsky mechanism for the HER process with MoS2/GO
composite as the catalyst, which indicated that electrochemical desorption of
hydrogen is the rate-limiting step. The small Tafel slope indicates a promising
electrocatalyst for HER in practical application. MoS2/GO composite material
has shown superior catalytic behavior compared to that of MoS2/rGO
composite material. The HER catalytic activity of the catalysts is explored using
scanning electrochemical microscopy (SECM) using the feedback and redox
competition mode in SECM. The activation energy for HER activity was calculated, and the values are in the range of 17−6 kJ/
mol. The lower value of activation energy suggested faster HER kinetics.
2H phases. The catalytic activity of 2H phase is mostly through
the edges of the catalytic system, and this has been supported
using experimental and theoretical studies,6 and the basal plane
was found to have no significant catalytic activity. Therefore, to
improve the catalytic activity using 2H-MoS2, it is essential to
have a higher percentage of active edge sites and there has been
some report about the catalytic HER reaction using 2H-MoS2
phase.7 The activity of the MoS2 has also been investigated to
improve the catalytic activity by making nanophase-based
materials. Electrical conductivity of the material has been
improved by incorporating Co, Ni, or Fe into nanoscaled
MoS2.8 Incorporation of Au,9 activated carbon,10 carbon
paper,11 or graphite12 has also been reported to have improved
catalytic activity.
There has been tremendous improvement in the electrical
conductivity of 1T-MoS2 compared to 2H-MoS2, and this has
been reflected in the improvement of the HER activity.13 Even
the basal plane of the 1T-MoS2 is quite electrochemically active
for the HER catalytic activity.13 Therefore, if 1T phase of MoS2
is formed, there should not be any limitation of its use only
through the edge planes. Conductivity of graphene is well
understood and accepted for the fabrication of nanophase
composite materials of high conductivity.14 The growth of the
1. INTRODUCTION
The limitation of petroleum fuels and global environmental
pollution encourage the researchers to think about an ideal,
clean, and efficient alternative source to traditional sources of
energy. Hydrogen serves as one of the important alternatives
for replacing petroleum fuels for the future. Traditional ways of
production of hydrogen involve release of the greenhouse gas
CO2 and a high temperature reaction, and the production of
hydrogen through such processes are being phased out.1
Generation of hydrogen through electrochemical and photoelectrochemical splitting of water is being considered as the
favored route for the generation of hydrogen. Such processes
require a good electrochemical catalyst which should be cost
effective, environment friendly, efficient, and useful in
prolonged generation of hydrogen from the splitting of water.
As an electrocatalyst, the most important aspect is to decrease
the overpotential of splitting water.2 In acidic solution, Pt group
metals are most effective catalysts for the generation of
hydrogen but due to the high cost of the Pt group metal
elements, large-scale application using these elements is not
feasible.3 Because of the earth’s abundant nature, different
transition metal alloys, carbides, polymeric carbon nitrides, and
transition metal chalcogenides have been investigated for the
hydrogen evolution process (HER) catalysis.4 Molybdenum disulfide has been preferred as a catalyst for HER due to the low
cost and high chemical stability.5 The HER catalytic activity of
MoS2 has been discussed comparing the activity of its 1T and
© 2017 American Chemical Society
Received: May 26, 2017
Accepted: October 2, 2017
Published: November 2, 2017
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Figure 1. XRD pattern of (A) MoS2/GO and (B) MoS2/rGO. Raman spectra of different vibrational modes of (C) MoS2/GO and MoS2/rGO.
Scanning electron microscopy (SEM) images and transmission electron microscopy (TEM) and selected-area electron diffraction (SAED) patterns
of (D−F) pristine MoS2 and (G−I) MoS2/rGO. TEM and SAED patterns of MoS2/GO are shown in the Supporting Information (Figure S2).
catalyst nanophases all over the graphene substrate has further
improved the charge-transport property of the catalysts and
hence enhanced the HER activity.4a
In a previous report, good charge-transfer property between
the adjacent layers of MoS2 and graphene has been reported,15
which has been the key point in making MoS2 graphene
composites as HER catalysts. Therefore, 1T-MoS2, with a
highly conducting basal plane, when forming composite with
graphene, would be the favorable combination for effective
transformation into an HER catalyst. Under the present
investigation, 1T phase of MoS2 has been synthesized through
the hydrothermal route and the improvement in the catalytic
activity of the 1T phase when forming a composite with
graphene oxide (GO) and reduced graphene oxide (rGO) has
been investigated. Because the entanglement of MoS2 between
the GO and rGO substrate would have some differences, it
would be interesting to observe the possible differences in their
catalytic activities. The scanning probe electrochemical
technique and scanning electrochemical microscopy (SECM)
experiments have been employed to map the catalyst substrate
using the redox completion mode, and this has reflected the
difference in the catalytic behavior between the two composite
materials. Probe approach plots at different applied potentials
and their transformation from the positive to the negative feed
due to the introduction of the redox competition mode
between the tip and the substrate has been investigated.
2. RESULTS AND DISCUSSION
2.1. Characterization of the Materials. MoS2/GO and
MoS2/rGO samples were characterized by X-ray diffraction
(XRD) and the diffraction patterns are shown in Figure 1A,B,
respectively. Three diffraction peaks 2θ = 12, 43, and 57.5°
correspond to (002), (006), and (110) planes of MoS2,
respectively [powder diffraction file (PDF no. 37-1492)], which
is due to the metallic 1T structure of MoS2.16 The intensity
corresponding to the (002) plane of MoS2/GO is much lower
than that of MoS2/rGO, which suggests that the lower stacking
height is along Z-axis with more exposure of its active sites. The
1T phase of MoS2 with a trigonal crystal structure and
octahedral orientation is shown to have significant difference in
the electronic property compared with the hexagonal 2H
phase.17
Raman spectroscopy was used for further characterization of
phases of MoS2. The three peaks at 283, 365, and 414 cm−1
correspond to the hexagonal vibration modes E1g, 1E2g, and A1g
of MoS2, respectively. The in-plane 1E2g and out-of-plane A1g
vibrational mode resulted from the opposite vibration of the S
atom with respect to the Mo atom, and out-of-plane vibration
of only S atom to Mo atom18 suggests the formation of pure
MoS 2 phase. The two bands at 1357 and 1577 cm −1
corresponding to the D and G band are clear evidence of
graphene sheets in the nanocomposite. As shown in Figure 1C,
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Figure 2. AFM images of MoS2/GO (A) before the electrochemical test using chronopotentiometry and (D) after the test using
chronopotentiometry at 10 mA/cm2 current density for 4 h. EDS images of (B, C) pristine MoS2 and (E, F) MoS2/rGO.
Supporting Information showed nanostructural characteristics
similar to those of the MoS2/rGO.
The SEM images with EDS results of both the materials are
shown in Figure 2. The presence of Mo and S along with C and
O is observed. The surface morphology of the catalyst-modified
substrate was examined using atomic force microscopy (AFM)
measurements using the MoS2/GO-modified substrate. The
morphology has shown a regular granular pattern of the catalyst
embedded all over the GO substrate, and the morphology of
MoS2/rGO is shown in Figure S3 of the Supporting
Information. The average particle size (diameter) for MoS2/
GO was obtained as 60 nm (the corresponding histograms are
shown in Figure S3 in the Supporting Information).
2.2. Electrocatalytic HER Activity. The electrochemical
HER activity of the catalyst was investigated in 0.5 M H2SO4
solution by depositing catalyst ink on the glassy carbon
electrode (GCE) using the three-electrode system, as discussed
in the Experimental Procedures. In the polarization curve, the
potential is corrected with iR drop and with the potential with
respect to the reversible hydrogen electrode (RHE). The linear
sweep voltammetry (LSV) and cyclic voltammetry (CV) plots
for both the catalysts along with the commercially available Pt/
C catalyst are shown in Figure 3. The LSV plots of both of the
composite materials have shown a sharp increase in the catalytic
hydrogen evolution current after the onset potential. The onset
potential for the MoS2/GO is lower by ∼0.13 V than that for
MoS2/rGO; however, when compared with the Pt/C catalyst,
the ID/IG ratios of graphene oxide and reduced graphene oxide
composite were calculated as 0.92 and 1.07, respectively, which
confirmed that GO is reduced to rGO.19 No significant change
in the vibrational mode was observed with the MoS2 phase
during the reduction of GO to rGO. The surface area was
measured using the Brunauer−Emmett−Teller method, and
the values for MoS2/GO and MoS2/rGO were obtained as 65
and 79 m2/g, respectively.
The morphology and lattice parameters were characterized
by SEM and TEM images. Figure 1D,G shows the SEM
micrograph of the pristine MoS2 and the MoS2/rGO composite
materials. The granular nature of the composite for the MoS2
and sheet-type nature of composite for the MoS2/rGO
composite were observed. TEM images of pristine MoS2 and
MoS2/rGO are shown in Figure 1E,H, respectively, and the
TEM images of MoS2/GO are shown in Figure S2 of the
Supporting Information; both the composite materials have
shown a layer type of structure. The selected-area electron
diffraction (SAED) of MoS2 has shown the (002) plane and the
(110) plane of MoS2. The random interconnection between the
GO or rGO and MoS2 network and random stacking of the
(002) plane is vulnerable in MoS2 for the decrease in the
catalytic activity.20 The clear observation of the (002) plane
from the SEM and TEM measurements rule out the random
stacking of the MoS2 network. The TEM and SAED of the
MoS2/GO nanocomposite materials shown in Figure S2 of the
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Figure 3. (A) Polarization curve of various catalysts. The magnified image at lower current density is shown as the inset. (B) Polarization curve of
catalysts with and without the hydrodynamic effect. The magnified image at lower current density is shown as the inset. (C) Effects of the scan rate
on current density. (D) Effect of hydrodynamic conditions on the catalytic activity.
both the materials have shown good electrocatalytic properties
and the onset potential is not so inferior to that of the
commercially available Pt/C catalyst materials. As seen from
Figure 3A, the catalytic activity of the only GO and only rGOmodified electrode has shown no reduction current due to the
reduction of proton. Therefore the GO and rGO played only a
synergistic role in catalyzing the hydrogen evolution process
along with MoS2. The onset position of the LSV plots was
zoomed and shown as the inset of Figure 3A,B; the reduction
current in both the composite materials started increasing at 0.2
V and separated from the base line current of the GO and rGO
composite-modified substrates. A peak shape was generated at
−0.027 V, just before the onset potential for the hydrogen
evolution process. This reduction peak is the partial reduction
of MoS2 at the Mo4+ center; later, it was oxidized back to its
original oxidation state.21
The LSV plots were further recorded under hydrodynamic
conditions, and corresponding results in comparison with the
data under static condition are shown in Figure 3B. At 1500
rpm, the onset potential was improved in both the composite
materials compared to that in the static condition. However, the
catalytic current at higher applied potential, beyond −0.15 V,
remained the same at static and hydrodynamic conditions.
Interestingly, the peak current for the peak observed just before
the onset potential was increased under hydrodynamic
conditions. LSVs were also compared at two different rotation
speeds, as shown in Figure 3C, the onset potential of the
hydrogen evolution process was improved in both the materials
when the rotation speed was increased from 500 to 3000 rpm.
The improvement in the catalytic activity at the low-current
region due to the hydrodynamic conditions is due to the
enhanced mass transfer attained by the hydrodynamic mass
flow.
From the LSV measurements, as discussed in the previous
section, the MoS2/GO has shown better catalytic activity for
the hydrogen evolution process in terms of the better onset
potential and the high catalytic current density. To delineate
the better catalytic activity of the MoS2/GO composite, the
electrochemical surface area of the catalyst surface was
measured from the double-layer capacitance (Cdl) measurements. CV experiments were carried out at different scan rates,
as shown in Figure S5 in the Supporting Information, and the
current sampled at three potentials 0.1, 0.15, and 0.2 V, where
no significant redox process observed, was plotted with respect
to the applied scan rates. The results are shown in Figure 3C;
from the slope of the linear plot, the double-layer capacitance
was determined and tabulated in Table 1. The “Cdl” value was
calculated from the slope of the current density and scan rate
plot.22
The Cdl values of MoS2/GO and MoS2/rGO are obtained as
0.34 and 0.72 mF, respectively. The effect of hydrodynamics on
the Cdl value shows that the Cdl value increases for MoS2/rGO
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significant effect of rotation on catalytic activity was observed at
the higher overpotential range but at lower overpotential the
catalytic current is modified significantly. At 2 mA/cm2 current
density, the applied potential was improved by 13.1 mV in the
case of MoS2/GO and improved by 7.4 mV in the case of
MoS2/rGO when the rotation speed was increased from 500 to
3000 rpm. Detailed kinetic information about the contribution
from the mass flow and the charge-transfer kinetics was
investigated by introducing the Koutechy−Levich analysis24
using the equations as follows
Table 1. Electrochemical Parameters as Obtained from the
Cyclic Voltammetry Measurements at Different Scan Rates
name of catalysts
MoS2/GO
MoS2/GO-1500 rpm
MoS2/rGO
MoS2/rGO-1500 rpm
potentials
(V)
0.20
0.15
0.10
0.20
0.15
0.10
0.20
0.15
0.10
0.20
0.15
0.10
Cdl (F)
2.53
2.55
2.27
1.85
1.85
1.72
5.3
5.31
5.76
5.06
5.58
6.81
×
×
×
×
×
×
×
×
×
×
×
×
Rf = Cdl/60 μF cm−2
10
10−4
10−4
10−4
10−4
10−4
10−4
10−4
10−4
10−4
10−4
10−4
−4
4.21
4.25
3.78
3.08
3.08
2.87
8.83
8.85
9.6
8.43
9.3
11.35
1
1
1
=
+
i
ik
il
1
1
1
=
+
i
ik
Bω1/2
where ik = nFAkC0 and B = 0.2nFC0D2/3ν−1/6 and, i, ik, and il are
the measured current, kinetic current, and limiting current,
respectively. ω is the rotation rate of the electrode in rpm. k is
electron-transfer rate constant; n is the number of electron
transfers; F = 96 500 C/mol; A is the surface area of RDE in
cm2; C0 is the concentration of H+ in bulk solution in mol/cm3;
D is the diffusion coefficient of proton in 0.5 M H2SO4, and its
value corresponds to 9.3 × 10−5 cm2 s−1; and ν is the kinematic
viscosity of 0.5 M H2SO4, and its value is 0.01 cm2 s−1.25
Currents were sampled at different applied potentials of the
LSV plot (Figure 4) during the Koutechy−Levich analysis, and
the corresponding kinetic parameters for both the composite
materials are shown in Table 2. At a lower applied potential
below the onset potential, the number of electrons transferred
was obtained as close to one, at a higher applied potential (at
−0.21 V) the number of electrons transferred increased
significantly. Such an unreasonably high value of the number
of electrons transferred for the HER process is accounted for by
the enhanced mass flow which resulted in the increased surface
concentration of the H+ compared to its bulk concentration.
The electron-transfer rate constant of the HER process in
MoS2/rGO catalyst was marginally higher than that in the
MoS2/GO catalytic system (Table 3).
from 0.72 to 0.87 mF; however, the Cdl value decreases for
MoS2/GO from 0.34 to 0.27 mF. The roughness factor was
calculated from the double-layer capacitance using the
following equation
R f = Cdl /60
(2)
(1)
where, the value “60” represents the specific capacitance of a
smooth surface in μF cm−2.23 As seen from Table 1, the Rf
value for MoS2/rGO is higher than that of MoS2/GO, so it is
HER activity should have been higher than that of MoS2/GO;
however, the polarization curve shows that MoS2/GO has
better catalytic activity. This contradicts the above observation.
Therefore, the observation of higher catalytic activity in the case
of MoS2/GO compared to that in the case of MoS2/rGO
indicates that the higher catalytic current in the case of MoS2/
GO is not related to the surface area of the materials and has
something to do with the inherent characteristics of the HER
process over the catalyst’s substrate.
The improvement of the onset potential due to the
introduction of the hydrodynamic conditions has been
mentioned with the discussion of the results in Figure 3.
Hydrodynamic conditions were further discussed by determining the kinetic parameters. As shown in Figure 3D, no
Figure 4. Koutechy−Levich plot of (A) MoS2/GO and (B) MoS2/rGO.
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Table 2. Analysis Results from the Hydrodynamic Voltammetric Measurements Using the Koutechy−Levich Analysisa
a
name of catalyst
potential (V)
slope
intercept
1/intercept, ik in mA/cm2
n
MoS2/GO
MoS2/GO
MoS2/GO
MoS2/rGO
MoS2/rGO
MoS2/rGO
0.1
−0.1
−0.21
0.1
−0.1
−0.21
−4.413
−6.66
−0.181
−5.72
−10.585
−2.431
−3.5726
−0.552
−0.0969
−2.0819
−0.70643
−0.4076
−0.2799
−1.811
−10.3199
−0.48
−1.4155
−2.45339
1.15
0.76
27.91
0.88
0.47
2.08
k, cm/s
5.06
49.45
7.66
11.26
61.41
24.44
×
×
×
×
×
×
10−3
10−3
10−3
10−3
10−3
10−3
The final values of n and k are rounded off to two decimal places.
results for the MoS2/GO materials are shown in Figure 5A. The
reference electrode was corrected for temperature using the
following equation.26
Table 3. Activation Energies Obtained on Two Catalytic
Systems at Different Applied Potentials
ln i = ln io − Eaapp/RT
overpotential, V
0.10
0.15
0.21
Ea
app
of MoS2/GO, kJ/mol
17
14
6
E° (V)Ag/AgCl = 0.23695 − 4.8564 × 10−4 t
Eaapp of MoS2/rGO, kJ/mol
20
19
10
− 3.4205 × 10−6 t 2
(3)
LSVs at different temperatures varying from 5 to 70 °C were
recorded. The Ag/AgCl reference electrode was calibrated for
different temperatures using the above equation. Figure 5A
shows the temperature-dependent LSV for the MoS2/GO
catalyst, and the results indicated the improvement of the onset
potential for the HER process with rise in temperature.
Temperature-dependent electrochemical measurements were
carried out in a custom-made cell, in which water can be filled
in the outer jacket for temperature control; corresponding
Figure 5. (A) Temperature-dependent polarization curve for MoS2/GO. Arrhenius plot is shown as inset; (B) Tafel plot of the two-catalyst system
in comparison with that of the standard Pt/C catalyst; (C) stability test using chronopotentiometry; and (D) stability test using chronoamperometry
of MoS2/GO catalyst.
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followed by slow electrochemical desorption step 7, the Tafel
slope will be ∼40 mV/decade and in that case, the HER
mechanism would be the Volmer−Heyrovsky mechanism.
When the electrochemical discharging step is the rate-limiting
step, the Tafel plot will be ∼120 mV/decade and the
mechanism will be through the Volmer step as the ratedetermining step.28,29
The observed Tafel slope of ∼40.6 mV/decade in the present
case, as seen from Figure 5B for the MoS2/GO hybrid catalyst,
suggests that the electrochemical desorption step would be the
rate-limiting step of the present system. In the case of the
MoS2/rGO hybrid catalyst, the Tafel slope was observed as
71.8 mV/decade. The mechanism of the hydrogen evolution
process over the MoS2 catalytic system is such that the
discharge step predominates over the Mo center and S and
graphene centers are responsible for the adsorption and further
recombination process for liberating hydrogen gas out of the
catalytic system. Mo centers are similar in both the catalytic
systems, and the major difference would arise due to the
difference in the GO and rGO in the catalysts. The significantly
higher Tafel slope in the case of the MoS2/rGO hybrid catalyst
system is indicative of mixed mechanism operating across the
catalyst substrate. A part of the catalyst substrate having Mo
centers would have a fast discharge process, whereas the rest of
the surfaces result in the slow discharge kinetics making the
overall Tafel slope high.7c,30 In the case of the MoS2/GO
hybrid catalyst system, there are plenty of exchangeable H+ ions
all over the matrix; because of such exchange of H+ ions
between the solution and the −COOH and −OH groups
present over the catalytic substrate, the discharge step would be
quite fast.31 The electrochemical reduction of H+ has been
consolidated and presented in Scheme 1.
Corresponding results for the MoS2/rGO composite materials
are shown in Figure S6 of the Supporting Information. To
evaluate and extract the activity of the electrocatalysts, the
apparent activation energy for hydrogen evolution (Eaapp) is
estimated using the following equation.27
∂ ln i
( T1 )
=−
∂
Eaapp
R
(4)
app
where Ea is evaluated at different applied potentials, i is the
current density at a given applied potential, T is the absolute
temperature, and R is the universal gas constant. After
recording the LSVs at different temperatures, the observed
currents were sampled at three different applied potentials and
ln i versus 1/T plots at three different applied potentials are
shown in the inset of Figure 5A. It was observed that the slopes
of the plots were decreased with the application of more
cathodic potential, indicating potential-dependent apparent
activation energy for the overall process. The apparent
activation energy for MoS2/GO composite material is relatively
lower than that of the MoS2/rGO at all applied potentials.
Because the activation energy is calculated from the change in
the overall current with the change in temperature of the
process (mass transfer and charge transfer), the effect of the
change in temperature will be reflected in the activation energy
calculation. Therefore, the relatively low activation energy in
MoS2/GO material might be due to the enhanced mass transfer
compared to that of MoS2/rGO.
Tafel treatment was applied to the LSV plot, and the linear
portion of the Tafel plot in Figure 5B was fitted using the Tafel
equation, η = a + b log j, where j is the current density and b is
the Tafel slope. The Tafel slopes for the corresponding catalyst
materials MoS2/GO and MoS2/rGO are obtained as ≈ 40.6
and ≈ 71.8 mV/decade, respectively. The overpotential is iRscorrected and on the scale of the RHE value, as shown in eq 9.
The Tafel slope is used to elucidate the mechanisms involved in
the HER process. There are three possible reaction steps in
acidic aqueous medium for the HER process.28 First, the
discharge step (Volmer reaction)
H3O+ + e− → Hads + H 2O
Scheme 1. Proposed Mechanism of the HER Process over
the MoS2/GO Substrate
(5)
where the Tafel slope, b = 2.3RT/α, F ≈ 120 mV/decade, R is
the universal gas constant, T is the absolute temperature, α =
0.5 is the symmetry coefficient, and F is the Faraday constant.
The second step is the combination step (Tafel reaction)
Hads + Hads → H 2
(6)
b = 2.3RT /2F ≈ 30mV/decade
The third step is the electrochemical desorption step
(Heyrovsky reaction)
Hads + H3O+ + e− → H 2 + H 2O
(7)
b = 2.3RT /(1 + α)F ≈ 40 mV/decade
The Tafel slope is an inherent property of the catalyst that is
determined by the rate-limiting step of the HER process.
Generally, the fast-discharging step 5 is followed by either the
combination step 6 or electrochemical desorption step 7. If the
fast-discharging step 5 is followed by the rate-limiting
combination step 6, the Tafel slope will be ∼30 mV/decade;
in this case, the overall mechanism of the process would be the
Volmer−Tafel mechanism. If the fast-discharging step 5 is
Furthermore, the stability of the catalyst was tested by the
chronopotentiometry and chronoamperometry method. Chronopotentiometry experiments were carried out at the current
density of 10 mA/cm2 for 4 h. As seen from the results in
Figure 5C, the MoS2/GO catalyst is quite stable at the
experimental time period and the applied potential remained
below −0.25 V for the chosen current density. Chronoamperometric experiments were carried out at an applied potential of
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Figure 6. (A) Nyquist plot of MoS2/GO and MoS2/rGO. Inset: zoomed portion of the Nyquist plot at the high-frequency region and the
corresponding Bode plot at the entire frequency range. (B) The equivalent circuit used for fitting the impedance results.
overall process, as discussed in the Tafel analysis, the resistance
R1 corresponds to the Volmer step and the resistance at the
low-frequency region R2 corresponds to the desorptive chargetransfer process, the Heyrovsky step. A smaller value of R1 in
both the materials suggests a better charge-transfer possibility.
The semicircular loop at the low-frequency region corresponds
to the desorptive charge-transfer process, and this process has a
dominant role in the overall HER kinetics. A considerably low
value of this desorptive resistance in the case of MoS2/GO
composite material compared to that in the case of MoS2/rGO
material has indicated that the difference between the HER
activity between these two materials is due to the difference in
this adsorption resistance.34 The Bode phase plot is shown as
the inset of Figure 6A, and it has been observed that at 0.215 V
of overpotential, the phase angle maxima value is 37° for GO
composite, which is smaller than that of the rGO composite at
44°. The lower phase angle maxima suggests an improved
Faradaic process in the case of GO composite materials
compared to that in the case of rGO composite materials.35
Similar to the CV measurements, the roughness factor (Rf) was
determined from the capacitance value obtained from the
impedance measurements and the values are obtained as 4.33
and 11.5 for MoS2/GO and MoS2/rGO composite materials,
respectively.
Additionally, the phase angle maxima for the relaxation
process associated with surface intermediates falls in the range
of 10−100 Hz. The relaxation frequencies for MoS2/GO and
MoS2/rGO composites are 100 and 20.89 Hz, respectively.
This relaxation process is due to the nonhomogeneous charge
transfer by the surface species. Above the onset potential of
hydrogen evolution, the relaxation due to nonhomogeneous
charge distribution dominates, with a minor contribution from
the double-layer capacitance and charge-transfer components,
as shown in Figure 6A. However, the phase angle maxima for
the GO composite being at a higher frequency than that of the
rGO composite suggests that there is significant masking due to
the double-layer capacitance on the HER activity in the case of
the GO composite. This double-layer masking effect should
limit the performance of MoS2/GO composite but the
performance of MoS2/GO is better than that of rGO, as
shown in Figure 6A. This anomalous property can be explained
through the functionalized GO with hydroxyl and carboxylic
acid groups that might increase the double-layer masking and at
the same time increase the mass flow of protons through the
exchange mechanism from the acidic solution, as shown in
Scheme 1.
−0.20 V, and it was observed that after an initial drop in
current, it remained stable for the experimental time period of 4
h. Even though the 1T phase of MoS2 is said to be in the
metastable phase, composites of 1T-MoS2 have been stable
even after long-term testing.13a GO and rGO might have an
important role in stabilizing the 1T-MoS2 phase on prolonged
HER catalysis.
The catalyst-modified electrode was examined using AFM
measurements after the stability test, that is, the chronoamperometric experiments, for 4 h. As seen from Figure 2D, the
general morphology of the materials remained similar to that of
what there was before the electrochemical test. From the
histogram and AFM micrograph (Figures S3 and S4 of the
Supporting Information), the average particle size was
decreased from 60 to 40 nm after the electrochemical test.
The layered structure of composite materials well appears after
the electrochemical test. Because of the energetic changes
during the electrochemical test, the materials might have
relaxed, which has resulted in the minor modification in the
morphology of the composite materials. A similar observation
was reported previously on the Mo oxide materials, where in
prolonged electrochemical cycles, the size of the Mo oxide
nanoparticles was decreased; such decrease in the size of the
nanoparticles and the morphological change were described
due to the redox activities at the catalytic center in prolonged
electrochemical cycles.32
2.3. Electrochemical Impedance Measurements. The
charge-transfer efficiency of the electrocatalyst was investigated
by electrochemical impedance spectroscopy using the CH
Instrument by applying an alternating current voltage of 10 mV
amplitude in a frequency range of 100 000−0.1 Hz.
Corresponding results in the form of the Nyquist plot and
the circuit used for fitting the Nyquist plot are shown in Figure
6. The zoomed portion of the high-frequency region of the
Nyquist plot is shown as the inset of Figure 6. The Nyquist plot
was characterized with two semicircular regions, one at a high
frequency and the other at the low frequency. The Nyquist
plots were fitted with the equivalent circuit model, as shown in
Figure 6. Here, Rs represents solution resistance, R1 is the
charge-transfer resistance, and R2 is the resistance incorporated
to account for the second semicircle.30a,33 The total resistance
for the Faradaic process of HER is the sum of R1 and R2. From
Figure 6A, the R1 value for MoS2/GO is 6 Ω, which is lower
than that of MoS2/rGO, 8 Ω. The value of R2 in the case of
MoS2/GO (32 Ω) is significantly lower than that in the case of
MoS2/rGO (120 Ω). Corresponding to the mechanism of the
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Figure 7. Probe approach curve (PAC) for the MoS2/GO catalyst at different substrate potentials (A) 0.225 V, (B) 0.125 V, (C) 0.024 V, (D)
−0.075 V, (E) −0.08 V, (F) −0.125 V, and (G) −0.175 V, keeping a constant probe potential at −0.075 V.
the diffusion coefficient, and a is the radius of the UME disk. In
bulk solution, H+ ions got reduced at the UME tip and
produced a steady-state current limited by hemispherical
diffusion. As the tip approached the substrate, the hydrogen
atom formed after reduction of the H+ ions at the tip oxidized
at the substrate, when the potential applied at the substrate is
more positive than the tip potential and a positive feedback
response was observed;36b although the catalyst substrate was
not meant for the oxidation of hydrogen to proton, at an
applied positive potential it should oxidize hydrogen to proton.
Feedback responses at different substrate potentials are shown
in Figure 7, where the normalized current, the tip current
during approach (iT), is divided by the steady-state tip current
when the tip was in the bulk solution (iTinf), which is plotted
with respect to the normalized distance L (d/a), where d is the
tip-to-substrate distance and “a” is the radius of the tip
electrode. Positive feedback response was obtained when the
substrate potential was more positive or equal to the tip
potential, and negative feedback responses were obtained when
substrate potentials were more negative than the tip potential.
This is due to the enhanced mass flow of H+ to the tip at a
relatively positive applied potential at the substrate. The
negative feedback response was due to the redox competition
2.4. Scanning Electrochemical Microscopy Measurements. Scanning electrochemical microscopy (SECM) was
employed to characterize the charge-transport processes and to
obtain the local electrochemical activity of the substrates. A Pt
ultramicro electrode (UME) of diameter 10 μm was used as the
working electrode (probe or tip electrode), Pt wire was used as
the counter electrode, the saturated Ag/AgCl electrode as the
reference electrode, and the glassy carbon electrode (GCE)
modified by the catalyst was used as the substrate electrode.
The approach of probe to the surface of the catalyst was
performed by the probe approach curve (PAC) technique, in
which a constant potential of −0.075 V versus RHE was applied
to the probe and different potentials in the range from 0.225 to
−0.175 V were applied to the substrate. Electrochemical signals
were measured by measuring the current at the UME tip as a
function of the precise tip position over the substrate at the
approach distance, and the SECM imaging was obtained. The
steady-state probe current is given by36
Id = 4nFCDa
(8)
where Id is the diffusion-limited current, n is the number of
electrons transferred at the electrode tip, F is Faraday’s
constant, C is the concentration of H+ ions in solution, D is
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Figure 8. Scanning electrochemical microscopy (SECM) images of MoS2/GO catalyst at different substrate potentials (A) 0.225 V, (B) 0.125 V, (C)
0.025 V, (D) −0.075 V, (E) −0.125 V, and (F) −0.175 V.
between the tip and the substrate. When the potential at the
substrate was same or more negative, reduction of H+ ions
became prominent at the substrate; hence, negative feedback
responses were obtained. Therefore, at the same applied
potential to the tip and the substrate, the substrate could
impose a negative feedback response to the tip due to the
introduction of the redox competition mode into the system.
This is essentially due to the higher surface area of the substrate
and also the good catalytic activity of the substrate. The
feedback responses were fitted with standard models, as seen
from Figure 7, where most of the positive feedback responses
could be fitted reasonably well; however, the negative feedback
responses could not be fitted due to the redox completion
mode operating between the tip and the substrate.
After the probe approached the substrate, the catalystmodified substrate was scanned for the electrochemical imaging
of the substrate by SECM using the steady-state current
response from the tip.36a The SECM scanning was carried out
in the constant height mode, where the distance between the
probe and the substrate was kept constant at the approach
distance of 1.1 μm and the probe was scanned in the X−Y
plane. The SECM scans for the MoS2/GO materials are shown
in Figure 8. It was observed that at the relatively positive
substrate potential, the overall probe current was high all over
the substrate; there were some high-current regions spread
across the whole substrate. With a shift in the applied potential
toward more negative direction, the spread of the high-current
regions were decreased and only a few high-current inlands
were observed at the substrate potential of 0.025 V. At −0.075
V, the transition potential between the positive and the negative
feedback response, the entire substrate was covered with low
current response. At further negative applied potential to the
substrate, the probe recorded very low negative currents across
the substrate with some inlands of positive current, as seen in
Figure 8E. The positive current response from the probe was
further increased at even more negative applied potential to the
substrate (cf. Figure 8F). The probe approach plots and the
SECM scans for the MoS2/rGO materials are shown in Figure
S7 of the Supporting Information. A nearly similar observation
as that for the MoS2/GO hybrid material was observed in the
probe approach plot and SECM scan at different potentials.
When the corresponding figures of Figure 8E,F were compared
with Figure S7E,F, it was observed that the positive current
response from the tip was higher in the case of MoS2/GO than
in the case of MoS2/rGO. Thus, the more prominent positive
current response at the substrate potentials of −0.125 and
−0.175 V in the case of MoS2/GO hybrid materials compared
to that in the case of the MoS2/rGO hybrid material, indicates a
better catalytic hydrogen evolution process over the MoS2/GO.
SECM was used previously for the investigation of HER
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GO composite-modified substrate compared to that in the
MoS2/rGO composite-modified substrate.
processes and it was reported that the strained S vacancy of
MoS2 has much higher HER activity than an unstrained one.37
The present result on SECM revealed that both the composite
materials have imposed the oxidation reaction at the Pt tip at an
applied substrate potential of −0.075 V; however, the oxidation
process at the tip due to the redox competition process is
induced predominantly by the MoS2/GO composite materials
than the MoS2/rGO composite materials. Both the materials
have shown good catalytic activity for the HER process, with
relatively higher activity for MoS2/GO composite material,
which has been revealed from the LSV measurements and
supported by the impedance and hydrodynamic measurements.
Previous investigation on SECM with surface interrogation has
revealed the Mo−H bond formation during the HER catalysis
process using MoS2 catalyst.38 This Mo−H bond formation
might be facilitated due to the presence of adjacent
exchangeable protons in MoS2/GO composite materials.
The difference in the work function between graphene and
MoS2 has been favorable for the electron to flow from MoS2
toward the graphene sheet; good coupling between the MoS2
and the graphene sheet would always make this flow of electron
fast for an efficient HER process.15
The present observation of the enhanced HER process in the
case MoS2/GO composite materials is explained from the
direction of the electron flow from MoS2 to the graphene
sheets. The electron would transfer from the electrode to MoS2,
which will further be transferred to the graphene sheet, from
which the electron will be transferred to H+ ions, and the
charge-transfer reaction will take place for the HER catalytic
reaction. Because there are exchangeable protons already
present in the graphene sheet of GO, the overall chargetransfer process would always be favored in the case of MoS2/
GO composite materials compared with MoS2/rGO composite
materials.39
4. EXPERIMENTAL PROCEDURES
4.1. Preparation of MoS2 Nanoparticle. MoS2 was
synthesized by the hydrothermal method. Ammonium
molybdate ((NH4)6Mo7O24·4H2O, 0.44 gm) was dissolved in
5 mL of deionized water and then hydrazine hydrate (N2H4·
H2O, 86%, 4 mL) as the reducing agent was added dropwise
under stirring condition. The reaction mixture was stirred for
0.5 h, and then sodium sulfide (Na2S, 1.32 g) dissolved in 5 mL
of deionized water was added into it; then, the mixture was left
for 10 min for incubation; 5 mL of 2 M HCl was added
dropwise to that mixture. Then, the reaction mixture was left
for 10 min again for incubation. Thereafter, the reaction
mixture was transferred into a 50 mL Teflon-lined stainless
steel autoclave and heated at 180 °C for 24 h. After 24 h, the
autoclave was allowed to cool at room temperature and then
black product was washed with distilled water several times and
then with ethanol. The product, as obtained, was dried at 60 °C
for 12 h in the vacuum oven. The mole ratio of the reactants
was kept at Mo/N2H4/Na2S of 1:357:48 during the synthesis
process.
4.2. Synthesis of Graphene Oxide. Graphene oxide was
synthesized using the modified Hummer’s method.40 Concentrated H2SO4 (300 mL) and H3PO4 (40 mL) were taken in
a round-bottomed (RB) flask of 1 L capacity. The RB flask was
kept in an ice bath at 0−5 °C. The whole setup was kept in a
magnetic stirrer; then, graphite (2 g) powder was added into
the RB flask slowly and the mixture was kept for stirring for 2 h.
Then, KMnO4 (12 g) was added slowly to this mixture under
stirring condition; this reaction mixture was stirred for 3 days at
room temperature. Thereafter, the reaction mixture was kept in
an ice bath; H2O2 (20 mL) was added slowly to terminate the
reaction, followed by washing with HCl (10%) and then
distilled water several times to achieve the neutral pH. The
graphene oxide thus obtained was dried in a vacuum oven and
used for further experiments. Graphene oxide was reduced by
treating GO suspended aqueous solution using hydrazine
monohydrate by stirring for 1 h, and then the solution mixture
was kept under hydrothermal conditions at 180 °C for 12 h.
The rGO thus obtained was washed and dried for further use.
4.3. Procedure of Electrochemical Studies and
Instrumentation. MoS2 and GO or rGO were mixed in
mortar (in 1:4 ratios). Then, 1 mg of the sample mixture was
suspended in ethanol and water solution (in a 1:1 ratio) and
kept in stirring conditions for 24 h. The suspension was
sonicated for 1 h, and then 100 μL of nafion (5 wt %) was
added and again sonicated for 30 min. After sonication, a
homogeneous mixture was formed and then 5 μL of catalyst ink
was drop-casted onto the glassy carbon (GC) electrode, which
then dried under an IR lamp. Electrochemical measurement for
the hydrogen evolution catalysis process was carried out using
the CH Instrument model 920D. Electrochemical studies were
performed in 0.5 M H2SO4 solution using a typical threeelectrode setup using the catalyst-modified electrode as the
working electrode, Pt wire as the counter electrode and Ag/
AgCl electrode as the reference electrode. Linear sweep
voltammetry (LSV) and cyclic voltammetry (CV) were
performed to evaluate HER performance. The glassy carbon
electrode was polished to a mirror finish using alumina powder
of 0.05 μm size and ultrasonically cleaned in distilled water for
10 min, followed by drying in an IR lamp. The electrolyte
3. CONCLUSIONS
The composites of GO ad rGO with the MoS2 have been
synthesized. Material characterization has revealed the 1T
phase of MoS2. Both the composite materials have shown very
good catalytic activity for HER process and their catalytic
activities are not so inferior to the commercially available Pt/C
catalyst. Electrochemical investigations with Tafel analysis have
indicated the Volmer−Heyrovsky mechanism for the HER
process in the MoS2/GO catalytic system. LSV experiments
were carried out in the hydrodynamic mode and the results are
shown to have a marginally higher electron-transfer rate
constant for MoS2/rGO composite materials, where the current
density and the onset potential for the HER process was
comparatively favorable in the case of MoS2/GO. Such anomaly
in the observation has indicated the enhanced mass transfer
process for MoS2/GO; such enhanced mass transfer has been
ascertained from the exchange of proton at the functional group
over the GO matrix and the bulk acidic solution. Electrochemical impedance measurements have shown to have high
desorptive charge-transfer resistance for MoS2/rGO composite
material responsible for the comparatively low HER. SECM
experiments were carried out using the catalyst-modified
substrate, and the probe approach plot has shown the
transformation of the composite-modified electrode as the
substrate from oxidation of proton to the efficient HER catalyst
with the modulation of the applied potential. The SECM
substrate scan has shown the enhanced oxidation current from
the tip electrode at a cathodic applied potential to the MoS2/
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solution was purged with N2 gas for 30 min prior to the
electrochemical measurements to remove dissolved oxygen.
Before measurements, the samples were cycled at a scan rate of
10 mV s−1 50 times to refresh the catalytic surface. All of the
electrochemical measurements are reported against the
potential versus reversible hydrogen electrode (RHE) as the
reference electrode. The Ag/AgCl reference electrode was
calibrated in a three-electrode system using a cleaned Pt
electrode as the working electrode and Pt wire as the counter
electrode. H2SO4 solution (0.5 M) was used as electrolyte
which is purged using high purity H2 gas before and during
measurements. LSVs were recorded at the scan rate of 1 mV
s−1, and the potential where the current was zero is taken as the
reference potential of the hydrogen electrode and was found to
be −0.2246 V from Figure S8. All of the electrochemical
potentials applied and measured using the Ag/AgCl reference
electrode were converted to RHE using the following equation;
in addition to conversion of the potential with respect to RHE,
potentials were also corrected for the iRs drop.
Ecorrected = EAg/AgCl + 0.2246 V − iR s
plot for calibration of the Ag/AgCl reference electrode
with the RHE (PDF)
■
*E-mail asatpati@barc.gov.in.
ORCID
Ashis Kumar Satpati: 0000-0002-2732-8706
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
Funding
This project is fully funded by our institute, Bhabha Atomic
Research Centre, Government of India.
Notes
The authors declare no competing financial interest.
(9)
ACKNOWLEDGMENTS
We thank Dr. P.D. Naik, Associate Director Chemistry Group,
Bhabha Atomic Research Centre, for his encouragements and
support during the course of the present work.
■
where, EAg/AgCl is the Ag/AgCl electrode potential, 0.2246 V is
the corrected potential, Rs is the resistance of solution, iRs is the
potential drop due to solution resistance that is measured using
electrochemical impedance measurements reported in the later
section of this article.
All of the measurements were iRs-compensated in the present
study, and the values of Rs were found by conducting
impedance measurement in 0.5 M H2SO4 and were in the
range of 6−7 Ω for both MoS2/GO and MoS2/rGO composite
materials. The temperature during electrochemical measurements was 25 ± 1°C. The overpotential “η” for hydrogen
evolution was calculated using the following equation
η = E RHE − 0 V − 0.0591 pH − iR s
ABBREVIATIONS
CV, cyclic voltammetry; LSV, linear sweep voltammetry; SEM,
scanning electron microscope; TEM, transmission electron
microscope; SAED, selected-area electron diffraction; HER,
hydrogen evolution process; SECM, scanning electrochemical
microscopy; RDE, rotating disk electrode; GC, glassy carbon;
GO, graphene oxide; rGO, reduced graphene oxide; MoS2,
molybdenum sulfide; RHE, reversible hydrogen electrode;
CPE, constant phase element
■
(10)
Atomic absorption spectrometry (AAS) was used for the
chemical quantification of the catalyst; the Mo content was
determined using the AAS instrument model Contra AA-300
from Analyticjena, Germany. The Mo content in both the
samples were quantified at 20% (wt %). The presence of GO
and rGO in the composite was determined by measuring the
total carbon content in the sample using a carbon sulfur
analyzer from Eltra. The GO and the rGO contents of the
sample were kept the same, and the percentage composition of
GO and rGO in the catalysts sample were determined at ∼60%.
■
AUTHOR INFORMATION
Corresponding Author
■
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ASSOCIATED CONTENT
S Supporting Information
*
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acsomega.7b00678.
X-ray diffraction of pristine MoS2; HRTEM images and
the SAED pattern of MoS 2 /GO nanocomposite
materials; histogram of AFM of MoS2/GO composite
materials before and after the chronopotentiometry test;
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