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i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 4 3 ( 2 0 1 8 ) 2 0 3 8 2e2 0 3 9 1
Available online at w
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One-pot synthesis of CdS-MoS2/RGO-E nano-heterostructure with well-defined interfaces forefficient photocatalytic H2 evolution
Xing-Liang Yin*, Lei-Lei Li**, Da-Cheng Li, Jian-Min Dou***
Shandong Provincial Key Laboratory of Chemical Energy Storage and Novel Cell Technology, School of Chemistry and
Chemical Engineering, Liaocheng University, Liaocheng, Shandong 252059, China
a r t i c l e i n f o
Article history:
Received 24 July 2018
Received in revised form
30 August 2018
Accepted 10 September 2018
Available online 11 October 2018
Keywords:
One-pot method
Water splitting
Photocatalysis
Hydrogen evolution
Graphene modification
* Corresponding author.** Corresponding author.*** Corresponding author.
E-mail addresses: [email protected]://doi.org/10.1016/j.ijhydene.2018.09.0470360-3199/© 2018 Hydrogen Energy Publicati
a b s t r a c t
Quality of interfaces is a key factor determining photoexcited charge transfer efficiency,
and in turn photocatalytic performance of heterostructure photocatalysts. In this paper,
we demonstrated CdS-MoS2/RGO-E (RGO-E: reduced graphene oxide modified by ethyl-
enediamine) nanohybrid synthesized by using a facile one-pot solvethermal method in
ethylenediamine, with CdS nanoparticles and MoS2 nanosheets intimately growing on the
surface of RGO. This unique high quality heterostructure facilitates charge separation and
transportation, and thus effectively suppressing charge recombination. As a result, the
CdS-MoS2/RGO-E exhibits a state-of-the-art H2 evolution rate of 36.7 mmol g�1 h�1 and an
apparent quantum yield of 30.5% at 420 nm, which is the advanced performance among all
the same-type photocatalysts (see Table S1), and far exceeding that of bare CdS by higher
than 104 times. This synthesis strategy gives an inspiration for the synthesis of other
compound catalysts, and higher performance photocatalyst may be obtained.
© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction
Carbon-neutral energy is the pursuit for today's society owing
to severe environmental pollution caused by fossil energy
excessive consumption [1e5]. Hydrogen energy is a perfect
candidate with highest energy of 142 MJ kg�1 and only clear
H2O discharge. Photocatalytic hydrogen evolution from water
splitting is an environmental and simple technique to produce
hydrogen in mass degree, and has potential to be applied in
future. Since the early 1970s, when TiO2 was firstly reported
having good photocatalytic activity in water splitting, this
(X.-L. Yin), 88lileilei@163.
ons LLC. Published by Els
research project has attracted widespread interest and ach-
ieved great progress [6e8]. But most reported photocatalysts
can only harvest UV light which makes up only about 3% of
total solar energy. Therefore, much attention should be paid
on the photocatalysts driven by visible light which accounts
for about 45% of total solar energy.
CdS is widely employed as a visible-light-driven photo-
catalyst owing to its suitable conduction band edge for H2
generation, and relative narrow band gap. However, ultrafast
recombination of electron-hole pairs resists its photocatalytic
performance further enhancement. Combing cocatalysts with
CdS to construct heterostructures is an effective approach to
com (L.-L. Li), [email protected] (J.-M. Dou).
evier Ltd. All rights reserved.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 4 3 ( 2 0 1 8 ) 2 0 3 8 2e2 0 3 9 1 20383
enhance the photocatalytic activity [9,10]. Noble metal Pt
working as cocatalyst exhibits high catalytic performance
because it requires low overpotential for Hþ reduction, but its
low reserve and cost of up-scaling severely hindered its
application. Recently, a great number of earth abundant ma-
terials have been developed as alternatives, such as MoS2[11e16], WS2 [17], Ni2P [18,19], CoP [20], carbon dot [21],
graphdiyne [22] and black phosphorus [23,24]. Among them,
MoS2 shows excellent performance and has been intensively
studied [25e30]. Both computational and experimental results
revealed that the edges of MoS2 layers were the active sites for
hydrogen evolution, but their basal planes were catalytic inert
[31,32]. Therefore, increasing the amount of edge sites and
accelerating electron transfer frombasal to edge planeswill be
beneficial for the enhancement of photocatalytic activity. In
previous study, we developed solvothermal approach to syn-
thesize MoS2/CdS heterostructure with amorphous-like MoS2anchored on CdS nanorods, which exposed a great many ac-
tivity sites for H2 evolution, but the weak conductor of MoS2curbed electron transfer and in turn affected H2 generation
activity [33,34]. It is well known that graphene is an ideal
conductor and has been widely applied in photo-catalysis
[35e37], electro-catalysis [38e41] and solar cell [42e44].
MoS2-RGO has been proved to be good dual-cocatalysts
[12,45e47], but the high quality interface between MoS2 and
RGO is still the pursuit. Moreover, the synthesis of CdS/MoS2-
RGO ternary catalysts usually needs multistep, which is time
consumption and energy intensive.
Usually, graphene oxide (GO) was used as precursor for the
synthesis of graphene based photocatalysts [27,35,48e50]. But
its electronegativity resists the absorption of precursor MoO42�
used in this manuscript. It was reported that the graphene
modified with ethylenediamine rendered it easily absorbing
electronegativity precursors and transition metals owing to
the electrostatic attraction and complex effect [51e53].
Inspired by this, herein, we adopted one-pot solvothermal
approach in ethylenediamine solution to synthesize CdS-
MoS2/RGO-E ternary catalyst with CdS nanoparticles and
amorphous-like MoS2 nanosheets in-situ growth on the sur-
face of RGO, which guarantees intimately interfacial contact
between composites, and offers a great many active sites for
H2 generation. This well-defined nano-heterostructure makes
for photo-excited charge separation and transportation, and
thus significantly retarding charge recombination. As a result,
the optimized CdS-MoS2/RGO-E exhibits high performance
with H2 evolution rate of 36.7mmol g�1 h�1, which is 104 times
higher than that of pure CdS, indicating it has potential for
applications.
Table 1 e Theoretical and actual compositions in all theprepared nanohybrid samples.
Samplesa 1 2 3 4 5
MoS2 (theoretical (wt %)) 4 4 4 0 67
MoS2 (actual (wt %)) 3.4 3.6 3.4 0 65
RGO (theoretical (wt %)) 2 2 0 2 33
RGO (actual (wt %)) 1.7 1.6 0 1.4 35
a sample 1e5 represent CdS-MoS2/RGO-E, CdS-MoS2/RGO-W, CdS/
MoS2eE, CdS/RGO-E and MoS2/RGO-E, respectively.
Experimental
Chemicals
Cadmium acetate dihydrate (Cd(CH3COO)2$2H2O, 98%), So-
dium molybdate dihydrate (Na2MoO4$2H2O, 99%), Thiourea
(CN2H4S, 99%), ethylenediamine and graphite (>99.8%) were
purchased from Alfa Aesar chemical co., USA. All agents were
used directlywithout further purification. GOwas synthesized
by amodified Hummers'method [54,55]. Deionizedwater with
a resistivity of 18.2 MU cm, produced by using a Milli-Q
apparatus (Millipore), was used in all the experiments.
Synthesis
Synthesis of CdS-MoS2/RGO-EThe CdS-MoS2/RGO-E composites were synthesized through a
facile one-pot solvothermal method. In a typical preparation,
2 mL GO solution (1.0 mg/mL), 25 mL ethylenediamine, a
varying amount of Na2MoO4$2H2O solution (0.08 M), 0.2 g
Cd(CH3COO)2$2H2O and 0.3 g CN2H4S were mixed together
with strong stirring, and followed by sonication for 10 min.
Then themixture was added into 50 mL Teflon-lined stainless
steel autoclave, and held at 210 �C for 24 h. After naturally
cooled to room temperature, yellow-green powders were
collected by centrifuging, washing with deionized water and
then drying at 80 �C for 12 h.
Synthesis of control samplesAs a control, CdS, MoS2, CdS-MoS2-E, CdS/RGO-E, and MoS2/
RGO-E were synthesized in parallel following the same pro-
cedure except for no addition of (GO þ Na2MoO4$2H2O),
(GO þ Cd(CH3COO)2$2H2O), GO, Na2MoO4$2H2O, or
Cd(CH3COO)2$2H2O, respectively. CdS-MoS2/RGO-W was also
fabricated at the same reaction conditions except for addition
of deionized water instead of ethylenediamine. Note that all
the loadings in percentage in this manuscript represent the
theoretical mass ratios of MoS2 or graphene to CdS given that
the reactants were completely converted into the products.
The actual loading amounts of MoS2 to CdS in all samples
were tested using an inductively coupled plasma-atomic
emission spectrometry (ICP-AES, ICPE-9000 Shimadzu), and
the actual content of graphene to CdS was calculated by using
subtraction method. The results were listed in Table 1.
Evaluation of photocatalytic activities
Photocatalytic H2 evolution was performed in a Pyrex glass
cell which had a flat, round upside-window with an irradia-
tion area of 38 cm2 for external light incidence. A 300WXenon
arc lamp with a 420 nm cut-off filter (CEL-HXF 300, Beijing
China Education Au-light Co., Ltd) was used to simulate the
visible light source. The illumination intensitywas adjusted to
100 mW cm�2. The H2-solar system (Beijing China Education
Au-light Co., Ltd) with a gas chromatogram (GC), equipped
with a thermal conductivity detector (TCD), TDX-01 column
and Ar carrier gas, was used to collect and on-line detect
evolved H2. 0.02 g of photocatalyst was suspended in glass cell
i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 4 3 ( 2 0 1 8 ) 2 0 3 8 2e2 0 3 9 120384
with 72 mL of deionized water containing 8 mL lactic acid as
hole scavenger. The cell was kept at 5 �C by using a circulating
water system. Before irradiation, the reaction system was
pumped to vacuum. The H2 evolution rate was determined by
GC. The apparent quantum yield (Ø) was estimated by the
following equation:
∅% ¼ ne�
np� 100 ¼ 2nH2
np� 100
np ¼ q
hn¼ I� t� S
hn
where Ø is the apparent quantum yield, ne- is the number of
reacted electrons, np is the number of incident photos, nH2 is
the number of evolved H2 molecules, q is the total energy of
incident photos (J), h is the Planck constant (J s�1), n is the
frequency of light (Hz), I is the illumination intensity (W m�2)
determined with a ray virtual radiation actinometer, t is the
irradiation time (s), S is the irradiation area (m2).
Characterization
Transmission electron microscopy (TEM) images were ob-
tained on a JEM 2100F (JEOL, Japan) operated at 200 kV. X-ray
powder diffraction (XRD) was carried out with a Rigaku D/
max-7000 using filtered Cu Ka irradiation. Raman spectrum
was recorded on a Thermo Scientific DXR confocal Raman
Microscope equipped with a 532-nm laser. X-ray photoelec-
tron spectroscopy (XPS) data were recorded with an ESCALab
220 i-XL electron spectrometer from VG Scientific using 300W
Al Ka radiation, in which the binding energies were referenced
to the C1s line at 284.8 eV from adventitious carbon. The
UVevisible absorption spectra were recorded with a
UVevisible spectrophotometer (UV-2550, Shimadzu, Japan).
Fourier transform infrared spectra (FTIR) were obtained on a
FTIR spectrometer (Bruker Tensor 27). The ethylenediamine in
centrifuged solution was tested by using GC with a FID de-
tector equipped with an Rtx-1701 Sil capillary column (Shi-
madzu GC-2014C). Transient photocurrent wasmeasured on a
CHI 760 E electrochemical system (shanghai, china) using Ag-
AgCl as reference and Pt as counter electrodes. The work
electrode was prepared by dispensing sample suspension in
ethanol onto ITO/glass of fixed area (1.96 � 10�5 m2). The
electrolyte is lactic acid solution (1.33 M) which was filled in a
quartz cell with a side window for external light incidence.
Light on and off was controlled by a baffle installed on a
stainless steel black box. Brunauer-Emmett-Teller (BET)
measurements were performed on a Micro-meritics's Tristar
3000. Inductively coupled plasma atomic emission spectrom-
etry (ICP-AES, ICPE-9000 Shimadzu) was used to measure the
Mo and Cd contents.
Results and discussion
The synthesis process for CdS-MoS2/RGO-E is schematically
illustrated in Fig. 1a. GO, CN2H4S, Na2MoO4 and Cd(CH3COO)2were added into ethylenediamine solution in one batch, and
reacted at 210 �C for 24 h to get the end-products (Detailed
experiments see experimental section). The morphology of
GO and as prepared products were detected by transmission
electron microscope (TEM). As shown in Fig. 1b, the raw
material GO is cleanly wrinkled nanosheets, providing large
surface for catalysts deposition. After reaction, TEM images
(Fig. 1c) of the end-products show that nanoparticles with
radius ranging from ca. 15e185 nm firmly grew in-situ on
graphene nanosheets. Detailed statistics (see Fig. S1) display
that nanoparticles with radius less than 60 nm account for
about 71.5%. The further enlarged images of nanoparticals as
shown in Fig. 1d indicate that the lattice fringes spacing is ca.
0.36 nm, which is well corresponding to the (100) planes of
hexagonal CdS [33,34]. Besides, the irregular nanosheets
(delineated by blue dashed line in Fig. 1d) were also observed
growth on graphene or CdS nonoparticles. High-resolution
TEM (HRTEM) images exhibit short-range continuous lattice
fringes on these nanosheets. The lattice spacing of 0.27 and
0.61 nm can be indexed as d-spacing of crystallographic (101)
and (003) planes of rhombohedral MoS2 [33,34,56]. Generally,
the lattice fringes of MoS2 are some amorphous features,
indicating that they are partially crystalline MoS2 nano-
sheets. Those amorphous-like MoS2 can efficiently enhance
H2 evolution performance owing to the exposed edges
providing a great many active sites for HER, which has been
verified in our previous study [33]. Furthermore, the energy
dispersive X-ray spectroscopy (EDS) mapping analysis
(Fig. 1e) of the obtained products shows elemental S and Cd
signal are homogeneously distributed on the surface of
nanoparticles, manifesting that the nanoparticles on gra-
phene are of CdS. However, the signal of C distinctively
presents two sections of strong and weak, which can be
attributed to graphene, and carbon membrane on copper
grid for supporting sample, respectively. The relatively weak
but homogeneous signal of Mo matches well with the sheet-
like structure MoS2 grown on CdS nanoparticles and
graphene.
The further chemical state information of those elements
was detected by X-ray photoelectron spectroscopy (XPS). As
shown in Fig. 2a, the XPS survey spectrum reveals the exis-
tence of C, S, Cd and Mo in the ternary composite. The high-
resolution XPS spectrum in Fig. 2b displays two peaks at
412.0 and 405.1 eV, which are in good consistency with the
characteristic binding energies of Cd2þ 3d3/2 and Cd2þ 3d5/2 in
CdS, respectively [57]. XPS spectrum in Fig. 2c shows a typical
strong doublet at 231.4 and 228.2 eV, which match well with
the binding energies of Mo 3d3/2 and Mo 3d5/2, respectively,
suggesting the dominant existence of Mo4þ species in product
[13]. The peak for S 2p (Fig. 2d) can be well deconvoluted into
two separate peaks at around 162.4 and 161.1 eV, which are
the typical XPS signals of S 2p1/2 and S 2p3/2 in form of S2� [33].
The XPS signal (Fig. 2e) of C1s can be well fitted into four
separate peaks centered at 284.6, 285.7, 286.9 and 288.7 eV,
corresponding to (C-C and C-H), (C-OH and C-N), (C-O-C) and
(O-C]O and N-C]O), respectively. In comparison with C 1s
XPS signal (Fig. S2) of GO, there exists significantly decrease
for the signal of oxygen functionalities, but an additional peak
at 285.7 (C-N), suggesting that the GO thermally treated in
ethylenediamine solution is successfully reduced into RGO
and modified by ethylenediamine. Together with TEM
Fig. 1 e (a) Schematic illustration of the synthesis process for the CdS/MoS2-RGO-E heterostructure. (b) TEM images of pure
graphene oxide. (c) TEM images, (d) HRTEM images and (e) STEM images and EDS elemental mapping of CdS-MoS2/RGO-E
heterostructure.
Fig. 2 e (a) Summary XPS of optimized CdS-MoS2/RGO-E heterostructure. (bee) High resolution spectroscopies of Cd 3d (b),
Mo 3 d (c), S 2P (d) and C 1s (e). (f) XRD patterns of pure CdS and CdS-MoS2/RGO-E heterostructures with graphene mass ratio
fixed at 2 wt% vs. CdS, and MoS2 mass ratio of 4, 10, 20 and 30 wt%, respectively.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 4 3 ( 2 0 1 8 ) 2 0 3 8 2e2 0 3 9 1 20385
i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 4 3 ( 2 0 1 8 ) 2 0 3 8 2e2 0 3 9 120386
observation, these results corroborate that the as-obtained
products are CdS-MoS2/RGO composites.
The phase structure and crystallinity of pure CdS as well as
CdS-MoS2/RGO-E nanohybrids with graphene mass ratio fixed
at 2 wt% vs. CdS, and MoS2 mass ratio from 4 to 30 wt% vs.
(CdS-RGO), were investigated by X-ray powder diffraction
(XRD) (Fig. 2f). All the XRD peaks for the pure CdS correspond
to the hexagonal CdS (JCPDS Card no. 65e3414). However, no
graphene and MoS2 characteristic patterns were detected for
the nanohybrid samples compared with the pristine CdS,
although they can be clearly observed in TEM images as
mentioned above, which can be reasonably attributed to the
low loading for the graphene [35], and the low crystalline of
MoS2 in keeping well with the TEM characterization results
[33,34].
To uncover the role of ethylenediamine for the synthesis of
CdS-MoS2/RGO-E, a control experimentwas carried out, which
kept equal reaction conditions except for adding deionized
water instead of ethylenediamine, and the obtained catalyst is
designed as CdS-MoS2/RGO-W. Compared with CdS-MoS2/
RGO-E, TEM characterization (Fig. S3) of CdS-MoS2/RGO-W
reveals that similarly dimensional CdS nanoparticles grew on
graphene. But it is distinctly different for the MoS2 nao-
nosheetswhich presents flowerlike aggregation. Furthermore,
equal mixture of GO, Na2MoO4 and CH4N2S were added into
ethylenediamine and deionized water to synthesize MoS2/
RGO composites marked as MoS2/RGO-E and MoS2/RGO-W,
respectively. As shown in Fig. 3a, small MoS2 nanosheets with
radius ca. 10 nm tightly and uniformly grew on the surface of
RGO for MoS2/RGO-E. However, the MoS2 in MoS2/RGO-W
(Fig. 3b) exhibits large layered structure random distributing
on RGO. Besides, the different image contrast for theMoS2 and
RGO in Fig. 3b indicates that most parts of MoS2 stretched
outside of RGO, demonstrating the weak contact between
MoS2 and RGO, which will result in its low cocatalytic activity.
Those observations further indicate ethylenediamine can
significantly affect the amorphous of MoS2 and the contact
state between MoS2 and RGO. This influence may stem from
the modification of RGO by ethylenediamine. To prove this
hypothesis, RGO-E and RGO-W were synthesized through
solvothermal treatment of pure GO in ethylenediamine and
water, respectively. It should bementioned here, the obtained
Fig. 3 e (a, b) TEM images of MoS2/RGO-E (a) and MoS2/RGO-W
deionized water.
RGO-E was thoroughly washed to remove the adsorbed eth-
ylenediamine, and the GC analysis of the last centrifuged so-
lution shows that no characteristic peaks of ethylenediamine
appeared. The Fourier-transform infrared (FT-IR) spectra of
GO, RGO-E are shown in Fig. 4a. In comparison, after sol-
vothermal treatment, there is a dramatic decrease intensity
for the peaks of oxygen-containing functional groups at 1727
(COOH stretching vibration peak), 3425, 1399 (eOH deforma-
tion vibration peaks), 1224 (epoxy) and 1060 cm�1 (alkoxy) in
RGO-E in compared with that of GO, but meanwhile, new
peaks at 1564 and 1260 cm�1 appeared in RGO-E, which can be
ascribed to the strong in-plane C-N scissoring absorptions and
C-O stretching vibrations [58]. Raman spectra (Fig. 4b) of GO
and RGO-E show that two peaks appeared at 1343 and
1582 cm�1 corresponding to the D- and G- band of graphene,
where the D band can be attributed to the edges, defects, and
disordered carbon, while the G band is assigned to the vibra-
tion of ordered sp2 C atoms [59e61]. Here, the ID/IG (ratio of
peaks intensity) is used to assess the reduction degree of GO.
The higher of this value represent the higher reduction degree
of GO. Obviously, the ID/IG of RGO-E (1.13) is higher than that of
GO (0.85). The above analysis of FT-IR and Raman spectra
further conform the successful reduction of GO and modifi-
cation of RGO [51]. In addition, the efficient modification of
RGO was further verified by the dispersity of RGO-E and RGO-
W in water, as shown in Fig. 3c. The RGO-E suspension is
stable but the RGO-W can quickly precipitate out of solution
within about 5min. The good stability for the RGO-Emanifests
its good dispersion and hydrophilia owing to the modification
of eNH2 which can effectively retard aggregation and provide
anchored sites for the precursors. Therefore, it can be
reasonably deduced that the eNH2 on RGO easily absorb
MoO42� and provide confined environment for the growth of
MoS2, thus rendering small MoS2 nanosheets intimate growth
on RGO.
The photocatalytic performances of synthesized samples
were assessed under visible light irradiation (l � 420 nm).
Lactic acid, as a green and efficient sacrificial agent, was
employed in photocatalytic HER. Through the oxided reaction
of lactic acid to generate pyruvic acid [29,62], the photo-
generated holes were successfully captured and consumed,
and thus the photocorrosion and charge recombination were
(b). (c) Photographs of RGO-E and RGO-W dispersed in
Fig. 4 e (a) FTIR and (b) Raman spectra of GO and RGO-E.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 4 3 ( 2 0 1 8 ) 2 0 3 8 2e2 0 3 9 1 20387
suppressed in some degree. The effect of mass percentage of
RGO and MoS2 on photocatalytic activity was first investi-
gated, every sample was repeatedly synthesized and tested
for five times. The screening experiment results are shown in
Fig. 5a and b, respectively. It demonstrates that the CdS-
MoS2/RGO-E with RGO and MoS2 mass ratio of 2 and 4 wt%
exhibits the best photocatalytic performance with the
average H2 evolution rate of 36.7 mmol g�1 h�1, correspond-
ing to the apparent quantum efficiency of 30.5% at 420 nm,
which was calculated using the formula listed in experiment
section. Additionally, the quantum efficiency of optimized
CdS-MoS2/RGO-E dependence on light wavelength was
tested. As shown in Fig. S4, with the increment of mono-
chromatic light wavelength, the quantum efficiency reduced.
This tendency is similar with that of light absorption of CdS-
MoS2/RGO-E with light wavelength less than 520 nm as
Fig. 5 e (a, b) Average photocatalytic H2 evolution rate is depen
respectively. (c) Photocatalytic activity comparison of CdS-MoS2
RGO-E and CdS. (d) The stability test of optimized CdS-MoS2/RGO
(e) Transition current response (recorded at the potential of 0 V
absorption and photocatalytic H2 evolution rate (marked by ,
monochromatic light irradiation of optimized CdS-MoS2/RGO-E
under visible-light irradiation (l ≥ 420 nm).
mentioned later in this manuscript, indicating the photo-
catalytic activity was significantly affected by light
wavelength.
This optimized catalyst exhibits highest performance
among the same type catalysts (see Table S1). Considering its
good performance, all the following experiments were carried
out by using this sample, which was marked by CdS-MoS2/
RGO-E for brevity, hereafter. It should be mentioned that the
H2 evolution rate in this manuscript represents the normal-
ized value. The real gram-scale reaction results have been
listed in Table S2. It shows that the H2 evolution rate was not
proportional enhanced in compared with that of the
milligram-scale. This can be reasonably attributed to the
agglomeration of nano-catalyst and the light shielding effect
[29]. Maybe proportionally enlarged reactor will achieve pro-
portional enhancement of H2 evolution rate. Fig. 5c-d shows
dents on the mass ratio of graphene (a) and MoS2 (b),
/RGO-E, CdS-MoS2/RGO-W, CdS/MoS2-E CdS/RGO-E, MoS2/
-E with RGO and MoS2 mass ratio 2 and 4 wt%, respectively.
vs. Ag/AgCl in 1.33 M of lactic acid solution), and (f) UVeVis
and C for CdS-MoS2/RGO-E and CdS, respectively) under
and CdS. All the measures from (a) to (e) were performed
i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 4 3 ( 2 0 1 8 ) 2 0 3 8 2e2 0 3 9 120388
the rate of H2 evolution on CdS-MoS2/RGO-E, CdS-MoS2/RGO-
W, CdS/MoS2-E, CdS/RGO-E, CdS and MoS2/RGO-E, as well as
the long-term stability test for the CdS-MoS2/RGO-E. All the
synthesized processes of those catalysts see details in exper-
imental section. The real composition of all the prepared
samples was analyzed and listed in Table 1, which is matched
well with the theoretical values. No H2 signal was detected for
the MoS2/RGO-E, suggesting that it is not active for photo-
catalytic water splitting. Pristine CdS shows low photo-
catalytic activity with H2 evolution rate of 0.35 mmol g�1 h�1
owing to the ultrafast charge recombination. But the opti-
mized CdS-MoS2/RGO-E can significantly increase the photo-
catalytic activity with H2 evolution of 36.7mmol g�1 h�1 which
is 104 times higher than that of bare CdS, indicating the good
cocatalytic activity of MoS2/RGO-E. The CdS/MoS2-E and CdS/
RGO-E show inferior activity of 18.5 and 1.7 mmol g�1 h�1,
respectively, in compared with that of CdS-MoS2/RGO-E. Be-
sides, the sum H2 evolution rate of CdS/MoS2-E and CdS/RGO-
E is much lower than that of CdS-MoS2/RGO-E (20.2 vs.
36.7 mmol g�1 h�1), manifesting that RGO and MoS2 have
synergistic effect for boosting photocatalytic H2 generation.
But this synergistic effect is significantly affected by the con-
tact station between RGO and MoS2, which is deduced from
the activity comparison of CdS-MoS2/RGO-E and CdS-MoS2/
RGO-W, the H2 evolution rate of the former is 1.5 times higher
than that of the later (36.7 vs. 24.3 mmol g�1 h�1). The BET (see
Fig. S5.) specific surface areas of CdS-MoS2/RGO-E (66.7 m2/g),
and CdS-MoS2/RGO-W (66.3 m2/g) are similar, which indicates
the effect of specific surface for the photocatalytic activity can
be ignored. The CdS-MoS2/RGO-E heterostructure with well-
defined interfaces between MoS2 and RGO as mentioned
from the HRTEM characterization results, will accelerate
photogenerated charge separation and transportation. How-
ever, the MoS2 and RGO in CdS-MoS2/RGO-W have a weak
contact, which deteriorates charge transportation and sepa-
ration, and thus results in the lower H2 generation perfor-
mance in compared with that of CdS-MoS2/RGO-E. The
outstanding catalytic activity of CdS-MoS2/RGO-E is further
verified by the video in supporting information. It can be
observed that no bubbles generate without light irradiation,
but bubbles rapidly generate once the xenon lamp is turned
on. The H2 generation dependence on time for four cycling
runs and 24 h of tests shows that no obvious activity deteri-
oration for CdS-MoS2/RGO-E, indicating its good stability. In
addition, the XRD and XPS characterizations (see Figs. S6 and
7) indicate the samples after 24 h reaction still keep its original
phase and surface chemical state, further suggesting its good
stability.
Fig. 6 e (aec) Tauc plots of CdS (a), MoS
Additionally, the photocatalytic performance dependence
on sacrificial donor concentrationwas studied over CdS-MoS2/
RGO-E. The results (see Fig. S8) show that the H2 evolution rate
enhances along with the concentration increment of lactic
acid before volume ratio up to ca. 25%. However further
increment concentration of lactic acid will result in the dete-
rioration, which can be ascribed to the high formation of in-
termediates at high concentration of lactic acid, being similar
to the previous report [29].
Transition photocurrent technique was employed to
directly investigate charge separation efficiency. As shown in
Fig. 5e, ultrafast photocurrent responses were observed under
chopped light illumination, which directly correlate with the
charge separation efficiency. The photocurrent density of the
CdS-MoS2/RGO-E is ca. 10 times higher than that of pristine
CdS, demonstrating that CdS-MoS2/RGO-E generates more
carriers than that of CdS under light illumination. The reason
can be ascribed to the formation of well-defined interfaces in
CdS-MoS2/RGO-E, which efficiently retards charge recombi-
nation, and thus leading to more electrons involving in H2
generation. Fig. 5f shows theUVeVis absorption spectrum and
wavelength-dependence of photocatalytic H2 evolution rate
under monochromatic light irradiation of pure cdS and CdS-
MoS2/RGO-E. The absorption wavelength for the pristine CdS
is strictly limited to l < 520 nm (denoted by red arrow) which
corresponding to the band gap energy of CdS (2.38 eV). How-
ever, the formation of CdS-MoS2/RGO-E widens light absorp-
tion scope to infrared regions, which is due to the narrow
bandgap of MoS2 [33,34,56]. The source of photo-excited
electrons was investigated through activity comparison of
pristine CdS and CdS-MoS2/RGO-E undermonochromatic light
irradiation with wavelength at 435, 450, 475, 500, 550, 600, 650
and 700 nm, respectively. Before 520 nm, the activity of CdS-
MoS2/RGO-E is superior than that of CdS, but after 520 nm no
H2 signal is detected for both CdS and CdS-MoS2/RGO-E even
there still have light harvest for the CdS-MoS2/RGO-E. This
manifests that the photo-excited electrons stem from CdS
rather than MoS2/RGO-E, the MoS2/RGO-E here only works as
cocatalyst.
The band gap values of pristine CdS and MoS2 were
calculated by using the equation:
ðahnÞn ¼ Aðhn� EgÞwhere a is the absorption coefficient, h is the Planck con-
stant (J s�1), n is the frequency of light (Hz), n is equal to 1/2
and 2 for indirect and direct band gap, respectively, A is a
constant, Eg is the optical band gap energy. The corre-
sponding Tauc plots are shown in Fig. 6, where the intercept
2 (b, n ¼ 1; c, n ¼ 2), respectively.
Fig. 7 e (a,b) Valence-band XPS spectra of CdS (a) and MoS2 (b).
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 4 3 ( 2 0 1 8 ) 2 0 3 8 2e2 0 3 9 1 20389
of the tangents on horizontal axis presents the band gap
energies [56,63]. It can be seen that the band gap for CdS is
2.38 eV (Fig. 6a), whereas the band gap of MoS2 can be
determined as 1.49 (Figs. 6b) and 1.68 eV (Fig. 6c) when n is
chose as 1/2 and 2, respectively. The band-gap value of MoS2
obviously lies between ones of indirect (1.3 eV) and direct
(1.8 eV) band gap [64e66]. Furthermore, the VB-maximum of
CdS and MoS2 was investigated by using XPS, and the results
were presented in Fig. 7. In view of flat band potentials
estimated by Mott-Schottky plots (Fig. S9), the VB-position of
CdS and MoS2 can be ultimately determined at 1.78 (Figs. 7a)
and 1.43 V (Fig. 7b) (vs. NHE), respectively. Combing the band
gap of CdS and MoS2, the CB deposition of CdS and MoS2 can
be determined at �0.60 V and �0.25 ~ �0.06 V (vs. NHE),
respectively.
Based upon the analysis above, the tentative photo-
catalytic mechanism of CdS-MoS2/RGO-E is proposed and
schematically illustrated in Fig. 8. Under.
Light irradiation, CdS nanoparticles generate electron-hole
pairs, and then the electrons directly transport to MoS2 with
more positive conduction-band edges in compared with that
of CdS [7,67] or first migrate to graphene and then to MoS2.
Following that, the electrons accumulated on the surface of
MoS2 involve H2 evolution reaction. Thanks to the formation
of heterostructure with well-defined interface, which inten-
sively retard charge recombination, and results in the high
performance for the CdS-MoS2/RGO-E.
Fig. 8 e Schematic illustration of the photocatalytic H2
evolution mechanism of CdS-MoS2/RGO-E nano-
heterostructure.
Conclusions
In summary, the novel ternary photocatalyst CdS-MoS2/RGO-
E was synthesized by a facile one-pot solvothermal method
and applied in photocatalytic water splitting. Thanks to the
modification of ethylenediamine on graphene, well-defined
interface is formed between MoS2 and graphene, which ac-
celerates photo-irradiated charge transportation from CdS
and significantly suppresses charge recombination, and thus
allows more electrons involving in H2 generation. As a result,
the optimized catalyst of CdS-MoS2/RGO-E shows high pho-
tocatalytic performance with H2 evolution rate of
36.7 mmol g�1 h�1 which is outperforming all reported similar
reaction systems. Besides, this photocatalyst shows high
stability with no obvious activity decrease after 24 h reaction.
Although the real application, especially under aerobic con-
ditions still suffered from photocorrosion and back reaction
that should be addressed in the future study, this novel design
concept of catalysts will give inspiration for developing low-
cost efficient photocatalysts for H2 evolution from water.
Acknowledgements
This work was financially supported by Shandong Province
Natural Science Foundation (Grant No. ZR2017PB002,
ZR2018PB001), National Natural Science Foundation of China
(Grant No. 21801106), Research Fund for the Doctoral Program
of Liaocheng University (Grant No. 318051640, 318051643).
Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.ijhydene.2018.09.047.
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