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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: xingliangyin@126.comhttps://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), dougroup@163.com (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.

r e f e r e n c e s

[1] Rivero MJ, Iglesias O, Ribao P, Ortiz I. Kinetic performance ofTiO2/Pt/reduced graphene oxide composites in thephotocatalytic hydrogen production. Int J Hydrogen Energy2018. https://doi.org/10.1016/j.ijhydene.2018.02.115.

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 120390

[2] Wan J, Pu C, Wang R, Liu E, Du X, Bai X, et al. A faciledissolution strategy facilitated by H2SO4 to fabricate a 2Dmetal-free g-C3N4/rGO heterojunction for efficientphotocatalytic H2 production. Int J Hydrogen Energy2018;43:7007e19.

[3] Zhang T, Shao X, Zhang D, Pu X, Tang Y, Yin J, et al. Synthesisof direct Z-scheme g-C3N4/Ag2VO2PO4 photocatalysts withenhanced visible light photocatalytic activity. Separ PurifTechnol 2018;195:332e8.

[4] Liang S, Zhang T, Zhang D, Pu X, Shao X, Li W, et al. One-potcombustion synthesis and efficient broad spectrumphotoactivity of Bi/BiOBr:Yb,Er/C photocatalyst. J Am CeramSoc 2018;101:3424e36.

[5] Gao M, Zhang D, Pu X, Li H, Li W, Shao X, et al. Combustionsynthesis of Fe-doped BiOCl with high visible-lightphotocatalytic activities. Separ Purif Technol 2016;162:114e9.

[6] Low J, Cao S, Yu J, Wageh S. Two-dimensional layeredcomposite photocatalysts. Chem. Commun.2014;50:10768e77.

[7] Chang K, Hai X, Ye J. Transition metal disulfides as noble-metal-alternative co-catalysts for solar hydrogen production.Adv. Energy Mater. 2016;6:1502555.

[8] Song J, Zhao H, Sun R, Li X, Sun D. An efficient hydrogenevolution catalyst composed of palladium phosphoroussulphide (PdP~0.33 S~1.67) and twin nanocrystal Zn0.5Cd0.5Ssolid solution with both homo- and hetero-junctions. EnergyEnviron Sci 2017;10:225e35.

[9] Wang J, Wang Z, Qu P, Xu Q, Zheng J, Jia S, et al. A 2D/1D TiO2

nanosheet/CdS nanorods heterostructure with enhancedphotocatalytic water splitting performance for H2 evolution.Int J Hydrogen Energy 2018;43:7388e96.

[10] Du J, Wang H, Yang M, Zhang F, Wu H, Cheng X, et al. Highlyefficient hydrogen evolution catalysis based on MoS2/CdS/TiO2 porous composites. Int J Hydrogen Energy2018;43:9307e15.

[11] Zong X, Yan H, Wu G, Ma G, Wen F, Wang L, et al.Enhancement of photocatalytic H2 evolution on CdS byloading MoS2 as cocatalyst under visible light irradiation. JAm Chem Soc 2008;130:7176e7.

[12] Chang K, Mei Z, Wang T, Kang Q, Ouyang S, Ye J. MoS2/graphene cocatalyst for efficient photocatalytic H2 evolutionunder visible light irradiation. ACS Nano 2014;8:7078e87.

[13] Hou Y, Laursen Anders B, Zhang J, Zhang G, Zhu Y, Wang X,et al. Layered nanojunctions for hydrogen-evolutioncatalysis. Angew Chem Int Ed 2013;52:3621e5.

[14] Liu C, Chai B, Wang C, Yan J, Ren Z. Solvothermal fabricationof MoS2 anchored on ZnIn2S4 microspheres with boostedphotocatalytic hydrogen evolution activity. Int J HydrogenEnergy 2018;43:6977e86.

[15] Yu J, Chen Z, Chen Q, Wang Y, Lin H, Hu X, et al. Giantenhancement of photocatalytic H2 production over KNbO3

photocatalyst obtained via carbon doping and MoS2

decoration. Int J Hydrogen Energy 2018;43:4347e54.[16] Li S, Pu T, Wang J, Fang X, Liu Y, Kang S, et al. Efficient

visible-light-driven hydrogen evolution over ternary MoS2/PtTiO2 photocatalysts with low overpotential. Int J HydrogenEnergy 2018;43:16534e42.

[17] Chen J, Wu X, Yin L, Li B, Hong X, Fan Z, et al. One-potsynthesis of CdS nanocrystals hybridized with single-layertransition-metal dichalcogenide nanosheets for efficientphotocatalytic hydrogen evolution. Angew Chem Int Ed2014;54:1210e4.

[18] Sun Z, Zheng H, Li J, Du P. Extraordinarily efficientphotocatalytic hydrogen evolution in water usingsemiconductor nanorods integrated with crystalline Ni2Pcocatalysts. Energy Environ Sci 2015;8:2668e76.

[19] Indra A, Acharjya A, Menezes Prashanth W, Merschjann C,Hollmann D, Schwarze M, et al. Boosting visible-light-driven

photocatalytic hydrogen evolution with an integrated nickelphosphideecarbon nitride system. Angew Chem Int Ed2017;129:1675e9.

[20] Cao S, Chen Y, Wang C, Lv X, Fu W. Spectacularphotocatalytic hydrogen evolution using metal-phosphide/CdS hybrid catalysts under sunlight irradiation. Chem.Commun. 2015;51:8708e11.

[21] Liu J, Liu Y, Liu N, Han Y, Zhang X, Huang H, et al. Metal-freeefficient photocatalyst for stable visible water splitting via atwo-electron pathway. Science 2015;347:970.

[22] Han Y, Lu X, Tang S, Yin X, Wei Z, Lu T. Metal-free 2D/2Dheterojunction of graphitic carbon nitride/graphdiyne forimproving the hole mobility of graphitic carbon nitride. Adv.Energy Mater. 2018;0:1702992.

[23] Ran J, Wang X, Zhu B, Qiao S. Strongly interactive 0D/2Dhetero-structure of a ZnxCd1-xS nano-particle decoratedphosphorene nano-sheet for enhanced visible-lightphotocatalytic H2 production. Chem. Commun.2017;53:9882e5.

[24] Zhu M, Zhai C, Fujitsuka M, Majima T. Noble metal-free near-infrared-driven photocatalyst for hydrogen production basedon 2D hybrid of black phosphorus/WS2. Appl Catal B2018;221:645e51.

[25] Wei L, Chen Y, Lin Y, Wu H, Yuan R, Li Z. MoS2 as non-noble-metal co-catalyst for photocatalytic hydrogen evolution overhexagonal ZnIn2S4 under visible light irradiations. Appl CatalB 2014;144:521e7.

[26] Wang J, Guan Z, Huang J, Li Q, Yang J. Enhancedphotocatalytic mechanism for the hybrid g-C3N4/MoS2nanocomposite. J Mater Chem A 2014;2:7960e6.

[27] Xiang Q, Yu J, Jaroniec M. Synergetic effect of MoS2 andgraphene as cocatalysts for enhanced photocatalytic H2

production activity of TiO2 nanoparticles. J Am Chem Soc2012;134:6575e8.

[28] He H, Lin J, Fu W, Wang X, Wang H, Zeng Q, et al. MoS2/TiO2

edge-on heterostructure for efficient photocatalytichydrogen evolution. Adv. Energy Mater. 2016;6:1600464.

[29] Kumar DP, Song MI, Hong S, Kim EH, Gopannagari M,Reddy DA, et al. Optimization of active sites of MoS2

nanosheets using nonmetal doping and exfoliation into fewlayers on CdS nanorods for enhanced photocatalytichydrogen production. ACS Sustainable Chem Eng2017;5:7651e8.

[30] Hong S, Kumar DP, Kim EH, Park H, Gopannagari M,Reddy DA, et al. Earth abundant transition metal-doped few-layered MoS2 nanosheets on CdS nanorods for ultra-efficientphotocatalytic hydrogen production. J Mater Chem A2017;5:20851e9.

[31] Hinnemann B, Moses PG, Bonde J, Jørgensen KP, Nielsen JH,Horch S, et al. Biomimetic hydrogen evolution: MoS2nanoparticles as catalyst for hydrogen evolution. J Am ChemSoc 2005;127:5308e9.

[32] Jaramillo TF, Jørgensen KP, Bonde J, Nielsen JH, Horch S,Chorkendorff I. Identification of active edge sites forelectrochemical H2 evolution from MoS2 nanocatalysts.Science 2007;317:100.

[33] Yin X, Li L, JiangW, Zhang Y, Zhang X, Wan L, et al. MoS2/CdSnanosheets-on-nanorod heterostructure for highly efficientphotocatalytic H2 generation under visible light irradiation.ACS Appl Mater Interfaces 2016;8:15258e66.

[34] Yin X, He G, Sun B, Jiang W, Xue D, Xia A, et al. Rationaldesign and electron transfer kinetics of MoS2/CdS nanodots-on-nanorods for efficient visible-light-driven hydrogengeneration. Nanomater Energy 2016;28:319e29.

[35] Li Q, Guo B, Yu J, Ran J, Zhang B, Yan H, et al. Highly efficientvisible-light-driven photocatalytic hydrogen production ofCdS-cluster-decorated graphene nanosheets. J Am Chem Soc2011;133:10878e84.

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 20391

[36] Liu J, Pu X, Zhang D, Seo HJ, Du K, Cai P. Combustionsynthesis of CdS/reduced graphene oxide composites andtheir photocatalytic properties. Mater Res Bull 2014;57:29e34.

[37] Zhang D, Gao Y, Pu X, Li W, Su C, Cai P, et al. One-Stepcombustion synthesis of magnetic Fe2O3/TiO2/graphenehybrids and their visible-light-driven photodegradation ofmethylene blue. Sci Adv Mater 2014;6:2632e9.

[38] Qu K, Zheng Y, Zhang X, Davey K, Dai S, Qiao S. Promotion ofelectrocatalytic hydrogen evolution reaction on nitrogen-doped carbon nanosheets with secondary heteroatoms. ACSNano 2017;11:7293e300.

[39] Wei X, Li Y, Xu W, Zhang K, Yin J, Shi S, et al. From two-dimensional graphene oxide to three-dimensionalhoneycomb-like Ni3S2@graphene oxide composite: insightinto structure and electrocatalytic properties. Roy. Soc. OpenSci. 2017;4.

[40] Ren C, Li H, Li R, Xu S, Wei D, Kang W, et al. Electrocatalyticstudy of a 1,10-phenanthroline-cobalt(ii) metal complexcatalyst supported on reduced graphene oxide towardsoxygen reduction reaction. RSC Adv 2016;6:33302e7.

[41] Yue Q, Zhang K, Chen X, Wang L, Zhao J, Liu J, et al.Generation of OH radicals in oxygen reduction reaction at Pt-Co nanoparticles supported on graphene in alkalinesolutions. Chem. Commun. 2010;46:3369e71.

[42] Zhou H, Yin J, Nie Z, Yang Z, Li D, Wang J, et al. Earth-abundant and nano-micro composite catalysts ofFe3O4@reduced graphene oxide for green and economicalmesoscopic photovoltaic devices with high efficiencies up to9%. J Mater Chem A 2016;4:67e73.

[43] Yin J, Zhou H, Liu Z, Nie Z, Li Y, Qi X, et al. Indium- andplatinum-free counter electrode for green mesoscopicphotovoltaics through graphene electrode and graphenecomposite catalysts: interfacial compatibility. ACS ApplMater Interfaces 2016;8:5314e9.

[44] Yin J, Wang J, Li H, Ma H, Li W, Shao X. Unique ZnS nanobunsdecorated with reduced graphene oxide as an efficient andlow-cost counter electrode for dye-sensitized solar cells. J.Energy Chem. 2014;23:559e63.

[45] Yu X, Du R, Li B, Zhang Y, Liu H, Qu J, et al. Biomolecule-assisted self-assembly of CdS/MoS2/graphene hollowspheres as high-efficiency photocatalysts for hydrogenevolution without noble metals. Appl Catal B2016;182:504e12.

[46] Ben Ali M, Jo W-K, Elhouichet H, Boukherroub R. Reducedgraphene oxide as an efficient support for CdS-MoS2

heterostructures for enhanced photocatalytic H2 evolution.Int J Hydrogen Energy 2017;42:16449e58.

[47] Kumar DP, Hong S, Reddy DA, Kim TK. Ultrathin MoS2 layersanchored exfoliated reduced graphene oxide nanosheethybrid as a highly efficient cocatalyst for CdS nanorodstowards enhanced photocatalytic hydrogen production. ApplCatal B 2017;212:7e14.

[48] Ma B, Wang X, Lin K, Li J, Liu Y, Zhan H, et al. A novelultraefficient non-noble metal composite cocatalyst Mo2N/Mo2C/graphene for enhanced photocatalytic H2 evolution.Int J Hydrogen Energy 2017;42:18977e84.

[49] Zhang D, Ding Q, Pu X, Su C, Shao X, Ding G, et al. One-stepcombustion synthesis of NiFe2O4-reduced graphene oxidehybrid materials for photodegradation of methylene blue.Funct. Mater. Lett. 2013;07:1350065.

[50] Zhang D, Pu X, Ding G, Shao X, Gao Y, Liu J, et al. Two-phasehydrothermal synthesis of TiO2-graphene hybrids withimproved photocatalytic activity. J Alloy Comp2013;572:199e204.

[51] Kim NH, Kuila T, Lee JH. Simultaneous reduction,functionalization and stitching of graphene oxide withethylenediamine for composites application. J Mater Chem A2013;1:1349e58.

[52] Zawisza B, Baranik A, Malicka E, Talik E, Sitko R.Preconcentration of Fe(III), Co(II), Ni(II), Cu(II), Zn(II) and Pb(II)with ethylenediamine-modified graphene oxide. Microchim.Acta 2016;183:231e40.

[53] Wang S, Wang J, Zhang W, Ji J, Li Y, Zhang G, et al.Ethylenediamine modified graphene and its chemicallyresponsive supramolecular hydrogels. Ind Eng Chem Res2014;53:13205e9.

[54] Hummers WS, Offeman RE. Preparation of graphitic oxide. JAm Chem Soc 1958;80. 1339-1339.

[55] Kovtyukhova NI, Ollivier PJ, Martin BR, Mallouk TE,Chizhik SA, Buzaneva EV, et al. Layer-by-layer assembly ofultrathin composite films from micron-sized graphite oxidesheets and polycations. Chem Mater 1999;11:771e8.

[56] Ma S, Xie J, Wen J, He K, Li X, Liu W, et al. Constructing 2Dlayered hybrid CdS nanosheets/MoS2 heterojunctions forenhanced visible-light photocatalytic H2 generation. ApplSurf Sci 2017;391:580e91.

[57] Yuan J, Wen J, Zhong Y, Li X, Fang Y, Zhang S, et al. Enhancedphotocatalytic H2 evolution over noble-metal-free NiScocatalyst modified CdS nanorods/g-C3N4 heterojunctions. JMater Chem A 2015;3:18244e55.

[58] Bose S, Kuila T, Uddin ME, Kim NH, Lau AKT, Lee JH, et al. In-situ synthesis and characterization of electrically conductivepolypyrrole/graphene nanocomposites. Polymer2010;51:5921e8.

[59] Eckmann A, Felten A, Verzhbitskiy I, Davey R, Casiraghi C.Raman study on defective graphene: effect of the excitationenergy, type, and amount of defects. Phys Rev B 2013;88,035426.

[60] Bosch-Navarro C, Coronado E, Marti-Gastaldo C, Sanchez-Royo JF, Gomez MG. Influence of the pH on the synthesis ofreduced graphene oxide under hydrothermal conditions.Nanoscale 2012;4:3977e82.

[61] Huang T, Luo Y, Chen W, Yao J, Liu X. Self-assembled MoS2-GO framework as an efficient cocatalyst of CuInZnS forvisible-light driven hydrogen evolution. ACS SustainableChem Eng 2018;6:4671e9.

[62] Harada H, Sakata T, Ueda T. Effect of semiconductor onphotocatalytic decomposition of lactic acid. J Am Chem Soc1985;107:1773e4.

[63] Wen J, Xie J, Yang Z, Shen R, Li H, Luo X, et al. Fabricating therobust g-C3N4 nanosheets/carbons/NiS multipleheterojunctions for enhanced photocatalytic H2 generation:an insight into the trifunctional roles of nanocarbons. ACSSustainable Chem Eng 2017;5:2224e36.

[64] Radisavljevic B, Radenovic A, Brivio J, Giacometti V, Kis A.Single-layer MoS2 transistors. Nat Nanotechnol 2011;6:147.

[65] Chang K, Li M, Wang T, Ouyang S, Li P, Liu L, et al. Drasticlayer-number-dependent activity enhancement inphotocatalytic H2 evolution over nMoS2/CdS (n�1) undervisible light. Adv. Energy Mater. 2015;5:1402279.

[66] Conley HJ, Wang B, Ziegler JI, Haglund RF, Pantelides ST,Bolotin KI. Bandgap engineering of strained monolayer andbilayer MoS2. Nano Lett 2013;13:3626e30.

[67] Iqbal S, Pan Z, Zhou K. Enhanced photocatalytic hydrogenevolution from in situ formation of few-layered MoS2/CdSnanosheet-based van der waals heterostructures. Nanoscale2017;9:6638e42.