Solid-state synthesis of ultrathin MoS2 as a cocatalyst on mesoporous g-C3N4 for excellent enhancement of visible light photoactivity

Article abstract of DOI:10.1016/j.jallcom.2020.155401

An ultrathin MoS₂ nanosheet was synthesized using a simple solid-state method with sulfur powder as the sulfur source and SiO₂ as a template. The confinement effect of SiO₂ spheres inhibited the growth of MoS₂ along the z-axis to produce a MoS₂ nanosheet composed of a few layers (2–9 layers). The SiO₂ template method was simultaneously used to prepare mesoporous g-C₃N₄, which exhibited a higher specific area and faster charge transfer than bulk g-C₃N₄ and thus higher photocatalytic activity for RhB degradation. The ultrathin MoS₂ nanosheets loaded onto mesoporous g-C₃N₄ provide active sites to obviously promote photocatalytic RhB degradation. The unique ultrathin structure of the MoS₂ nanosheet decreases the distance of electron transfer in MoS₂, inhibits the recombination probability of carriers, and strongly adsorbs RhB, thus facilitating the photocatalytic reaction for RhB degradation. Control experiments confirmed that ?O₂^? is the most important active species, and CO₂ is the dominant degraded product during the photocatalytic reaction. This study provides guidance for the design of photocatalysts and an in-depth understanding of the reaction mechanism of pollutant photodegradation.

Full text of DOI:10.1016/j.jallcom.2020.155401

Journal of Alloys and Compounds 836 (2020) 155401
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: http://www.elsevier.com/locate/jalcom
Solid-state synthesis of ultrathin MoS as a cocatalyst on mesoporous
2
g-C N for excellent enhancement of visible light photoactivity
3
4
a
,
*
a
a
a
a
b
Wei Chen , Mei Liu , Shaojie Wei , Xiying Li , Liqun Mao , Wenfeng Shangguan
a
Henan Engineering Research Center of Resource & Energy Recovery from Waste, Henan University, Kaifeng, 475004, PR China
Research Center for Combustion and Environmental Technology, Shanghai Jiao Tong University, Shanghai, 200240, PR China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 6 January 2020
Received in revised form
An ultrathin MoS2 nanosheet was synthesized using a simple solid-state method with sulfur powder as
the sulfur source and SiO as a template. The confinement effect of SiO spheres inhibited the growth of
MoS along the z-axis to produce a MoS nanosheet composed of a few layers (2e9 layers). The SiO
template method was simultaneously used to prepare mesoporous g-C , which exhibited a higher
specific area and faster charge transfer than bulk g-C and thus higher photocatalytic activity for RhB
degradation. The ultrathin MoS nanosheets loaded onto mesoporous g-C provide active sites to
obviously promote photocatalytic RhB degradation. The unique ultrathin structure of the MoS nano-
sheet decreases the distance of electron transfer in MoS , inhibits the recombination probability of
2
2
2
2
2
31 March 2020
3 4
N
Accepted 26 April 2020
Available online 6 May 2020
3 4
N
2
3 4
N
2
Keywords:
Photocatalytic degradation of RhB
2
carriers, and strongly adsorbs RhB, thus facilitating the photocatalytic reaction for RhB degradation.
Ultrathin MoS
Porous g-C
2
ꢀ
N
4
Control experiments confirmed that ∙O
2
is the most important active species, and CO
2
is the dominant
3
Charge separation
Degradation product
degraded product during the photocatalytic reaction. This study provides guidance for the design of
photocatalysts and an in-depth understanding of the reaction mechanism of pollutant photodegradation.
©
2020 Elsevier B.V. All rights reserved.
1
. Introduction
used in photocatalytic pollutant removal and solar energy conver-
sion [17,18] due to its suitable energy structure, low cost and high
chemical stability, since it was first discovered in 2009 by Wang
Photocatalytic technology has attracted increasing attention due
to its potential applications in alleviating environmental deterio-
ration and energy crisis [1e4]. Upon irradiation with light, semi-
conductor photocatalysts produce electrons and holes that reduce/
oxidize organic/inorganic pollutants to innocuous substances,
providing a promising approach for purifying environmental water
or gas [5e7]. Three factors generally affect the photocatalytic ac-
tivity: (1) the band gap and energy band structure of the photo-
catalyst, (2) the ability to separate photogenerated electrons and
holes, and (3) the numbers of active sites on the photocatalyst
3 4
et al. [19]. However, bulk g-C N has been shown to have a low
photocatalytic activity because its small specific surface area does
not provide sufficient sites for a photocatalytic reaction, and the
long distance for carrier transfer from the bulk to the surface results
in a low carrier separation efficiency [20,21]. Recently, pore for-
mation has been used an effective method to increase the specific
surface area of g-C
Additionally, a thin g-C
3
N
4
through the confinement effect of templates.
wall forms in the gap between the
3 4
N
template particles, which decreases the charge migration distance
and promotes charge separation [22e24].
surface. Although numerous photocatalytic materials, such as TiO
8e10], BiOCl [11e13] and CdS [14e16], have been prepared and
2
[
As the photocatalytic reaction occurs on the surface of photo-
catalysts, the creation of surface active sites is indispensable for
obtaining high photocatalytic activity. In general, active sites are
provided by loading photocatalysts with noble metal cocatalysts,
such as Pt, Pd and Au, possessing high work functions and low
redox overpotentials [10,25,26]. However, the scarcity of noble
metals hinders their larger scale applications. Most recently, noble
used in photocatalytic reactions to date, most of these photo-
catalysts have excessively wide band gaps (UV response only),
inefficient charge separation and an insufficient number of active
sites. Thus, highly active semiconductor photocatalysts are urgently
needed.
3 4
Graphitic carbon nitride (denoted as g-C N ), has been widely
2
metal-free MoS has been widely used as a substitute for noble
metals in catalysis, microelectronics and supercapacitors due to its
unique morphology and good electronic and optical properties
*
Corresponding author.
E-mail address: chanwee@henu.edu.cn (W. Chen).
https://doi.org/10.1016/j.jallcom.2020.155401
925-8388/© 2020 Elsevier B.V. All rights reserved.
0

2
W. Chen et al. / Journal of Alloys and Compounds 836 (2020) 155401
[
27e29]. Similar to graphene, MoS
structure that is formed by the growth of three stacked atom layers
SeMoeS) along the z-axis [30,31]. Therefore, bulk MoS is exfoli-
2
possesses a typical layered
template (Scheme 1). A 1.0 g mass of SiO
added to 20 mL of an aqueous solution of ammonium molybdate
2
(particle size: 15 nm) was
(
2
containing 0.3 g of (NH
4
)
6
Mo
7
O
24$4H
2
O, and the suspension was
ꢁ
ated by destroying the interlayer weak van der Waals forces. With
more edge sites and a shorter transfer distance of carriers, ultrathin
MoS
catalytic activity than bulk MoS
exfoliation (LPE) [29] and chemical vapor deposition (CVD) [36,37]
are effective methods for fabricating ultrathin MoS nanosheets.
evaporated to dryness in a water bath at 60 C with constant stir-
ring. The obtained white powder was mixed with 0.3 g of S powder,
and then transferred to a sealed steel container to vulcanize at
2
nanosheets have exhibited considerably higher photo-
ꢁ
ꢁ
ꢀ1
2
[32e35]. Currently, liquid phase
400 C (at a ramping rate of 20 C ,min ) for 2 h in a tubular
furnace under N flow to form MoS /SiO . Subsequently, the MoS
SiO was immersed in 30 mL of a 10% HF aqueous solution for 6 h
and then washed several times with water to remove SiO . The
obtained product was washed with CS to remove unreacted S.
2
2
2
2
/
2
2
Unfortunately, the low yield and stringent experimental re-
quirements limit the large-scale practical application of these
2
2
methods [38]. Most recently, mesoporous MoS
a template method with CS as a sulfur source. With more exposed
edge sites, mesoporous MoS showed excellent performance for an
2
was prepared using
Finally, the powder was washed with ethanol and distilled water
ꢁ
2
several times and dried at 60 C to obtain the ultrathin MoS
nanosheet.
2
2
electrocatalytic hydrogen evolution reaction [39]. However,
considering the strong flammability and explosive properties of
Synthesis of porous g-C
was prepared using a thermal polycondensation method with
melamine as the raw material and SiO spheres as the template
(Scheme 1). A1.0 g mass of SiO (Macklin reagent, 15 nm) and 2.0 g
3 4 3 4
N : The porous g-C N photocatalyst
CS
However, sulfur powder might also serve as a sulfur source for
MoS , which is frequently used in CVD. Thus, the use of sulfur
powder as the sulfur source in combination with hard templates to
restrain the growth of MoS along the z-axis via the confinement
effect is simple method for producing MoS nanosheets
composed of a few layers.
Here, a simple solid-state synthesis method was proposed to
prepare a MoS nanosheet composed of a few layers using SiO as
the template and sulfur powder as the sulfur source. An ultrathin
MoS nanosheet was obtained due to the confinement effect of SiO
spheres, which hindered the growth of MoS along the z-axis. The
SiO template method was simultaneously used to synthesize
mesoporous g-C with a high specific area and rapid charge
transfer, thus promoting photocatalytic activity for RhB degrada-
tion. More importantly, after the ultrathin MoS nanosheet was
loaded on mesoporous g-C as a cocatalyst, the unique structural
features and physical properties of MoS clearly resulted in an in-
crease in the photocatalytic activity of g-C for RhB degradation.
In this study, the effect of the ultrathin MoS nanosheet on
increasing photocatalytic activity of g-C was investigated in
2
, the synthesis process at high temperature involves risk.
2
2
2
(Acros, 99.9%) of melamine were mixed by ball milling. The mixture
ꢁ
was placed in a crucible with a cover and heated at 550 C (at a
ꢁ
ꢀ1
2
ramping rate of 5 C min ) for 4 h to obtain g-C
g-C /SiO was immersed in 32 mL of a 10% HF aqueous solution
for 6 h to remove the SiO
3 4 2
N /SiO . Then, the
a
2
3
N
4
2
2
template. After several washes with
ꢁ
ethanol and water and drying at 60 C, yellow porous g-C
3
N
4
was
2
2
obtained (0.9e1.0 g, yield: 45e50%). Finally, 0.9 g of porous g-C
3
N
4
ꢁ
was placed on an Al
2
O
3
plate and heated at 500 C at a ramping rate
ꢁ
ꢀ1
2
2
of 5 C min for 2 h in air to obtain light yellow exfoliated porous
g-C with a yield of approximately 40%.
Loading the ultrathin MoS nanosheet on porous g-C
mass of the ultrathin MoS nanosheet were dispersed in 30 mL of
ethanol and ultrasonicated for 10 min. The desired volume of the
suspension solution containing the desired amount of MoS (4.5
mL/0.75 mg, 9 mL/1.5 mg, and 18 mL/3 mg) was extracted and
diluted to 30 mL. Then, 0.15 g of porous g-C was added to the
2
3 4
N
2
2
3 4
N : 5 mg
3
N
4
2
2
2
3 4
N
2
3 4
N
3
N
4
suspension while stirring. After 5 h, the product was collected by
ꢁ
2
filtration and dried at 50 C for 3 h to obtain MoS
2 3 4
/g-C N at a mass
3 4
N
ratio of 0.5e2%.
detail. Additionally, the main degraded products of RhB were
detected, and a reaction mechanism for photocatalytic RhB degra-
dation was proposed based on control experiments.
2.2. Characterization
The crystal structure of the samples was characterized using an
X-ray powder diffractometer (Bruker-AXS, Germany) with Cu K
radiation ( ＝0.15406 nm). The morphology and microstructure of
the samples were characterized using a Carl Zeiss GeminiSEM300
field emission scanning electron microscope at 5 kV and a JEM-
2100F transmission electron microscope at 200 kV. The UVe-vis
absorption spectrum was measured using a TU-1901 UVe-vis
2
. Experimental section
.1. Synthesis of materials
Synthesis of the ultrathin MoS
a
l
2
2
nanosheet: The nanosheet was
as the
synthesized using a simple solid-state method with SiO
2
2 3 4
Scheme 1. Schematic of the synthesis of ultrathin MoS /porous g-C N .

W. Chen et al. / Journal of Alloys and Compounds 836 (2020) 155401
3
spectrophotometer (Purkinje, China). The specific surface area and
pore size were determined using the Brunauer-Emmett-Teller (BET)
method with N adsorption at 77 K (AutosorbeiQ-MP-C, Quan-
tachrome Instruments, USA). The surface electronic state was
analyzed using X-ray photoelectron spectroscopy (XPS) with a
Thermo Fisher Scientific K-Alpha spectrometer equipped with a
the position of the diffraction peak of porous g-C
change, indicating that the g-C
pore creation process. However, the peak intensities of porous g-
are clearly weaker than those of bulk g-C . This result is
potentially attributed to the formation of mesoporous structures in
the g-C , which has a lower degree of crystal order. The MoS
3
N
4
does not
3 4
N structure is preserved in the
2
C
3
N
4
3 4
N
3
N
4
2
ꢁ
ꢁ
ꢁ
ꢁ
monochromatic Al K
energy values were calibrated using the standard value of adven-
a
source (h6 ¼ 1253.6 eV). All the binding
spectrum exhibits reflections at 14.4 , 32.7 , 39.6 , and 58.4 , which
are indexed to the (002), (100), (103) and (110) lattice planes,
titious carbon (C1s ¼ 284.6 eV) as a reference. Photoluminescence
respectively, of hexagonal MoS
nanosheet was synthesized using the simple solid-state method
with S powder as the S source. For MoS /porous g-C samples,
obvious refection peaks of MoS are not observed, which is likely
attributed to the low loading amounts and good dispersity of MoS
on the surface of g-C [41]. Fig. 2b shows the DRS spectra of bulk
g-C , porous g-C and 1.0% MoS /porous g-C . The absorp-
tion edge of bulk g-C is 450 nm, corresponding to a band gap of
, the absorption edge of porous g-
displays a blueshift of ca. 5 nm, which is probably due to the
2 2
(JPCDS 65e1951). Thus, the MoS
(
PL) spectra and time-resolved photoluminescence (TRPL) decay
spectra were recorded using an Edinburgh FLS980 fluorescence
spectrometer at an excitation wavelength of 325 nm.
2
3 4
N
2
2
2.3. Photoelectrochemical measurements
3 4
N
3
N
4
3
N
4
2
3 4
N
The photocurrent response and a Nyquist plot were obtained
3
N
4
using a CHI760D instrument with a three-electrode system: 0.5 M
Na SO aqueous solution as the electrolyte, platinum wire as the
3 4
2.75 eV. Compared to bulk g-C N
2
4
3 4
C N
counter electrode, and Ag/AgCl as the reference electrode. The
working electrodes were prepared as follows: 5 mg of photocatalyst
were added to 1.0 mL of a 0.1 wt% Nafion aqueous solution and
ultrasonicated for 12 h. Then,100 mL of the slurry was injected onto
quantum effect caused by the ultrathin g-C nanosheet during
3
N
4
pore formation. After loading MoS onto g-C N , a new absorption
2
3 4
band appears at a wavelength longer than 450 nm, which is
attributed to the absorption of MoS . The result confirms the suc-
cessful loading of the MoS cocatalyst onto porous g-C
The SEM image of porous g-C in Fig. 2a shows a highly
porous architecture composed of thin walls, which is attributed to
the confinement effect of the SiO template and the subsequent
thermal oxidation exfoliation. Fig. 2b and c shows SEM images of
ultrathin MoS nanosheets prepared using the simple solid-state
method with S powder as the S source and SiO nanoparticles as
templates. Most of the MoS nanosheets are thinner than 5 nm,
indicating that the confinement effect of the SiO2 template effec-
tively restrains the growth of MoS along the z-axis. However, in
Fig. 2a and b, the MoS morphology clearly differs from g-C
although both these materials were prepared using the same SiO
template. This result is obtained because porous g-C
thesized by the thermal polycondensation method, which typical
produces a regular core-shell structure of g-C @ SiO , resulting in
a regular porous structure after the removal of the SiO template.
However, MoS was prepared by loading the Mo salt on SiO fol-
lowed by sulfuration, which does not facilitate the formation of the
regular core-shell structure of MoS @ SiO , while inhibiting the
MoS nanosheets thinness. Additionally, the strong 2D orientation
enables MoS form an ultrathin nanosheet structure. The TEM
images show 2e9 layers of MoS . From the selected area electron
diffraction image (Fig. 2f), the diffraction rings are indexed to the
diffractions of the (100), (103), and (110) planes of hexagonal MoS
consistent with the XRD results. The N sorption isotherms of both
the porous g-C and the ultrathin MoS nanosheet are repre-
2
ITO glass (1 cm ꢂ 1 cm). The obtained electrodes were dried at
2
3 4
N .
ꢁ
6
0 C for 3 h. A 300 W Xenon lamp (780 nm >
l
> 420 nm) served as
3 4
N
the light source. Before conducting the electrochemical test, Ar was
introduced into the electrolyte to remove the dissolved O
2
.
2
2.4. Measurement of the photocatalytic activity
2
2
Photocatalytic activities for RhB degradation were tested in a
00 mL glass reactor, and a 300 W Xenon lamp with a 420 nm UV-
2
3
cutoff filter was used as a light source. For each reaction, 50 mg of
2
ꢀ1
catalysts were added to 150 mL of the 10 mg L
7
3
RhB (TOC:
.0 mg∙L ) aqueous solution. The suspension was stirred for
0 min in the dark to establish an adsorption-desorption equilib-
2
3
N
4
,
ꢀ1
2
3 4
N was syn-
rium. In the photocatalytic process, the reaction system was
ꢁ
maintained at 20 ± 2 C using a circulating water bath. At 15-min
3
N
4
2
intervals, the RHB concentration was detected with a TU-1901
spectrophotometer based on the RhB absorbance peak near
2
2
2
5
54 nm. Various scavengers were added to the RhB solution,
including 1.0 mmol of isopropanol (IPA), EDTA-2Na, K and p-
benzoquinone (BQ) for quenching hydroxyl radicals (∙OH), holes
2
S
2
O
8
2
2
2
þ
ꢀ
ꢀ
2
), respectively, to
(
h ), electrons (e ) and superoxide radicals (∙O
detect the active species during the photocatalytic reaction. Control
experiments were performed with and without O in a continuous-
flow system using a 250 mL top-irradiation-type reactor with a
2
2
2
2
,
ꢀ
1
quartz window. 50 mg of catalysts and 150 mL of the 10 mg L RhB
solution were added to the aforementioned reactor. Then, a 20%
2
3
N
4
2
O
2
þ 80% N
2
gas mixture or Ar was bubbled into the reactor at a rate
sentative type-IV curves with a H3-type hysteresis loop, indicating
a typical mesoporous structure (Fig. 3). Based on a BJH model, the
of 5 mL/min with constant stirring using a mass flowmeter. The
reactor temperature was maintained at 20 ± 2 C. The aforemen-
ꢁ
pore sizes of the porous g-C
are 31 nm and 9.6 nm, respectively. The pores in the porous g-C
are evidently larger than the SiO particles, which is likely attrib-
uted to the thermal oxidation of the porous g-C surface,
3 4 2
N and the ultrathin MoS nanosheet
tioned 300 W Xenon lamp (
source. Before light irradiation, the reaction was purged with 20%
þ 80% N gas or Ar for 1 h to remove the CO dissolved in the
aqueous solution. The potential gaseous products, such as CO , CO
or H2, were detected using an online GC with an FID detector (MS-
3X) and a TCD detector (TDX-01).
l
> 420 nm) was used as the light
3 4
N
2
O
2
2
2
3 4
N
2
resulting in the formation of large pores [42]. Notably, benefiting
from the porous structure, the specific surface area of porous g-
2
ꢀ1
1
C
C
3
N
N
4
is 153.3 m ∙g , a value that is 12.7 times higher than bulk g-
. A large specific surface area promotes pollutant adsorption
3
4
3
. Results and discussion
and provides more active sites for redox reactions, thereby
increasing the photocatalytic activity. Additionally, the specific
2
ꢀ1
Fig. 1 shows the XRD patterns and DRS spectra of g-C
3
N
4
, MoS
2
surface area of the MoS
which is likely ascribed to its ultrathin structure.
The 1.0% MoS /porous g-C sample was investigated using
TEM to examine the morphology of MoS loaded on porous g-C
and the distribution of MoS on the surface of porous g-C . As
2
nanosheet is as high as 154.4 m ∙g ,
ꢁ
2 3 4
and MoS /g-C N . As shown in Fig. 1a, the diffraction peaks at 13.1
ꢁ
and 27.4 , corresponding to typical (100) inter-plane packing and
002) inter-facial stacking [40], respectively, are observed in both
bulk g-C and porous g-C samples. Compared to bulk g-C
2
3 4
N
(
2
3 4
N
3
N
4
3
N
4
3
N
4
,
2
3 4
N

4
W. Chen et al. / Journal of Alloys and Compounds 836 (2020) 155401
3 4 3 4 2 2 3 4 3 4 3 4 2 3 4
Fig. 1. The XRD patterns of bulk g-C N , porous g-C N , MoS and MoS /porous g-C N (a) and DRS spectra of bulk g-C N , porous g-C N and 1.0% MoS /porous g-C N .
Fig. 2. SEM images of porous g-C
3
N
4
(a) and the ultrathin MoS
2 2
nanosheet (b and c); TEM images of the ultrathin MoS nanosheet (d and e) and selected area electron diffraction
(
SAED) image of the ultrathin MoS
2
nanosheet (f).
2 3 4 3 4 2
Fig. 3. N sorption isotherms (a) and pore size distribution (b) of bulk g-C N , porous g-C N and ultrathin MoS nanosheets.
shown in Fig. 4a, porous g-C
nanosheet structure with MoS
Fig. 4b and c) clearly show lattice spaces of 0.63 nm and 0.233 nm,
which are consistent with the (002) and (103) planes of hexagonal
MoS [43,44], respectively. This result is consistent with the XRD
and selected area electron diffraction results. The elemental maps
show the presence of C, N, Mo and S elements in the MoS /porous
g-C sample. Additionally, Fig. 4d shows the relatively even
3
N
4
exhibits a typical 2D mesoporous
on the surface. The HRTEM images
2 3 4
distribution of the MoS cocatalyst on the surface of porous g-C N .
The elemental binding energies were measured using X-ray
photoelectron spectroscopy (XPS) to investigate the chemical states
2
(
2 3 4
of MoS -loaded porous g-C N and the results are shown in Fig. 5.
2
As shown in Fig. 5a, the N, C, O, Mo and S elements are indexed from
the XPS survey spectrum. In the N1s HRXPS spectrum, the main N1s
peak is deconvoluted into three components at 398.6 eV, 399.3 eV
2
3
N
4
3
and 400.9 eV, corresponding to CeN]C, N-(C) , and CeNeH,

W. Chen et al. / Journal of Alloys and Compounds 836 (2020) 155401
5
2 3 4
Fig. 4. (HR)TEM images and elemental mappings of 1.0% MoS /porous g-C N .
2 3 4
Fig. 5. X-ray photoelectron spectroscopy (XPS) spectra of 1.0% MoS /porous g-C N : survey (a), N1s (b), Mo3d (c) and S2p (d).
respectively, which is consistent with the results of g-C
3
N
4
of S2- in MoS
loading of the ultrathin MoS
The photocatalytic activities of bulk g-C
MoS /porous g-C were evaluated by measuring RhB degrada-
tion rates and are shown in Fig. 6a. As shown in Fig. 6a, the porous
g-C photocatalyst clearly exhibits a higher RhB decomposition
rate than bulk g-C . More importantly, the photocatalytic activity
of porous g-C was noticeably increased after the ultrathin MoS
2
[40,43]. The XPS results confirmed the successful
nanosheet onto porous g-C
, porous g-C
composed of a few layers [45]. The Mo 3d spectrum displays two
peaks at 228.7 eV and 231.9 eV, which are attributed to 3d5/2 and
2
3 4
N .
N and
3 4
3
N
4
3
d
3/2 of the Mo4 species [40]. The absence of a peak at 235.7 eV
þ
2
3 4
N
6
þ
corresponding to 3d3/2 of Mo suggests that the raw material
NH Mo 24 vulcanized completely to MoS [43]. In the S 2p
HRXPS spectrum, the two major peaks with binding energies of
61.7 eV and 162.9 eV are assigned to 2p3/2 and 3d1/2 respectively,
(
4
)
6
7
O
2
3 4
N
3 4
N
1
3
N
4
2

6
W. Chen et al. / Journal of Alloys and Compounds 836 (2020) 155401
Fig. 6. Photocatalytic degradation rates for RhB degradation on bulk g-C
3
N
4
, porous g-C
3
N
4
, MoS
2
-loaded porous g-C
3
N
4
(a) and ultrathin MoS
l > 420 nm).
2
nanosheets (b). Reaction conditions:
ꢀ
1
ꢀ1
50 mg of photocatalyst, 150 mL of the 10 mg L RhB or 20 mg mg L RhB aqueous solution and a 300 W Xe lamp (
nanosheet was loaded on its surface. In particular, the 1.0% MoS
porous g-C sample shows the highest photocatalytic activity, as
0% of RhB was decomposed within 15 min. Notably, in Fig, 6b, the
ultrathin MoS nanosheet shows a strong adsorption capacity for
RhB but very low photocatalytic activity for RhB degradation. Thus,
the increase in photocatalytic activity originates from the syner-
2
/
2 3 4 2
mediated by loading MoS onto porous g-C N shows that MoS
3
N
4
further promoted the separation of photogenerated carriers. Elec-
trochemical impedance spectroscopy (EIS) was also performed on
the samples, and the results are shown in Fig. 7b. In a Nyquist plot, a
small arc radius indicates a small charge transfer resistance across
the electrode/electrolyte interface. As shown in Fig. 7b, among the
8
2
gistic effect of the ultrathin MoS
2
nanosheet and porous g-C
3
N
4
.
2 3 4
three samples, the 1.0% MoS /porous g-C N sample possesses the
During a photocatalytic reaction, the ability to separate photo-
induced electrons and holes determines the photocatalytic activity.
The photoelectrochemical performance and photoluminescence
smallest arc radius, indicating that this sample has the lowest
resistance and the fastest interfacial charge carrier transfer [48].
Fig. 7c shows the PL intensity of bulk g-C
1.0% MoS /porous g-C . Generally, a higher PL intensity corre-
sponds to high charge recombination. As shown in Fig. 7c, bulk g-
exhibits a strong PL peak, indicating rapid charge recombi-
nation [49,50]. The PL intensity of the porous g-C sample is
clearly weaker than that of bulk g-C . In addition, the PL intensity
decreases further after its surface is loaded with
, suggesting that charge recombination was signifi-
loading. The result is consistent with the
3 4 3 4
N , porous g-C N and
(
PL) of bulk g-C
3
N
4
, porous g-C
3
N
4
and 1.0% MoS
2
/porous g-C
3
N
4
2
3 4
N
were examined to explain the difference in the photocatalytic ac-
tivity of the samples. Fig. 7a shows the photocurrent responses (i-t)
3 4
C N
of bulk g-C
photocurrent density of porous g-C
indicating more effective charge separation [46,47]. This result is
obtained because the thin nanosheet structure of porous g-C
3
N
4
, porous g-C
3
N
4
and 1.0% MoS
2
/porous g-C
3
N
4
. The
3 4
N
3
N
4
is higher than bulk g-C
3
N
4
,
3 4
N
of porous g-C
1.0 wt % MoS
3
N
4
3 4
N
2
decreases the distance of transfer of the carriers from the bulk to
the surface, thus decreasing the probability of the recombination of
carriers. Additionally, the increase in the photocurrent density
cantly inhibited by MoS
2
photocurrent performance and EIS properties. Time-resolved
photoluminescence (TRPL) spectroscopy was employed to analyze
Fig. 7. Transient photocurrent response (a), EIS Nyquist plots (b), PL spectra (c) and time-resolved transient PL decay spectra (d) for bulk g-C
porous g-C
3 4 3 4 2
N , porous g-C N and 1.0% MoS /
3 4
N .

W. Chen et al. / Journal of Alloys and Compounds 836 (2020) 155401
7
Table 1
PL lifetimes from time-resolved photoluminescence spectra for porous g-C
.0% MoS /porous g-C
migrate from the interface of porous g-C
exterior MoS
tating the degradation reaction. However, as shown in Fig. 6a, the
photocatalytic activity decreases when the amount of MoS loaded
exceeds 1.0%, and the degradation ratio on 2.0% MoS /porous g-
for RhB was lower than 1.0% MoS /porous g-C . This finding
is attributed to the “shielding effect”, as excess MoS on the surface
of g-C will block the light adsorption of porous g-C [55].
N
and MoS
to the
3
4
2
3 4
N and
2
surface, inhibiting carrier recombination and facili-
1
2
3 4
N .
sample
Porous g-C
t
1
(ns)
a
1
(%)
t
2
(ns)
a
2
(%)
t
ave (ns)
2
3
N
4
2.3
1.5
54.7
55.9
11.8
8.6
45.3
44.1
9.9
7.3
2
1
.0% MoS /porous g-C
2
N
3 4
C
3
N
3 4
N
4
2
2
3
N
4
3 4
N
the effect of MoS
decay spectra of porous g-C
shown in Fig. 7d. Based on the curves, the average calculated life-
times of porous g-C N and 1.0% MoS /porous g-C are 9.9 ns and
.3 ns, respectively (Table 1). The shorter PL lifetime of 1.0% MoS
porous g-C is likely attributed to rapid electron transfer from
porous g-C to the ultrathin MoS nanosheet [51e53]. The con-
duction band (CB) potentials in g-C and MoS
are ca. ꢀ0.89 V
and þ0.23 V (RHE vs pH ¼ 0), respectively [54]. As the CB position of
MoS is obviously lower than that of g-C , the photogenerated
electrons in the g-C CB tend to migrate to the MoS CB, while the
, resulting in the spatial separation of
2
on increasing charge separation, and the TRPL
Scavengers were introduced in the photocatalytic reaction sys-
tem to investigate the photocatalytic mechanism and reaction
3
N
4
and MoS loaded porous g-C are
2
3 4
N
pathway. Here, isopropanol (IPA), EDTA-2Na, K
2 2 8
S O and p-ben-
3 4
N
zoquinone (BQ) were used to quench hydroxyl radicals (∙OH), holes
3
2
þ
ꢀ
ꢀ
2
7
2
/
(h ), electrons (e ) and superoxide radicals (∙O ), respectively
3
N
4
[56e58]. As depicted in Fig. 8a, in the reaction with IPA (∙OH
ꢀ
3
N
4
2
scavenger) and K
2
S
2
O
8
(e scavenger), an obvious effect on the
N
4
2
photocatalytic degradation efficiency of 1.0% MoS
not observed, implying that ∙OH and e are not the dominant
reactive species. In contrast, the in addition of BQ (∙O
considerably decreases RhB degradation, indicating that ∙O
2
/porous g-C
3
N
4
is
3
ꢀ
ꢀ
scavenger)
2
3
N
4
2
ꢀ
N
4
2
2
is the
3
holes remain on g-C
carriers.
3
N
4
key reactive species. Moreover, the degradation rate of RhB de-
þ
creases moderately after the introduction of EDTA-2Na (Hh
þ
Based on the analysis described above, the difference in the
photocatalytic performances of photocatalysts in this study for RhB
scavenger), suggesting that h also plays an important role in RhB
degradation. Ar and 20% O
system in control experiments to examine the degraded products
and further clarify that ∙O is the reactive species, and the quan-
2
þ80% N
2
were bubbled into the reaction
degradation are explained below. Although bulk g-C
3
N has a suit-
2
able energy band structure, its photocatalytic activity for RhB
degradation is low due to its weak charge separation ability and
insufficient numbers of active sites. With a high specific surface
area and a thin nanosheet structure derived from the confinement
tities of the degraded products are shown in Fig. 8b. As shown in
Fig. 8b, under the 20% O
products are CO and a small quantity of CO. Only 4.5
generated over 90 min, corresponding to 1.8% of the generated CO
(24.4 mol). Additionally, less CO evolution is detected by GC than
is actually generated because of the high solubility of CO in water,
which may also explain why CO is detected after 54 min when RhB
has been completely degraded. Based on these results, CO is the
dominant product in the RhB degradation reaction. Surprisingly,
2
þ80% N
2
atmosphere, the degraded
2
mmol of CO are
effect of SiO
more active sites, thus resulting in higher photocatalytic activity.
When MoS is loaded on porous g-C , RhB tends to clump on the
MoS surface due to ability of MoS to strongly absorb RhB, which is
then degraded by electrons from porous g-C . In particular, the
ultrathin structure of MoS decreases the distance electrons
2
, porous g-C
3
N
4
exhibits better charge separation and
2
m
2
2
3
N
4
2
2
2
2
3
N
4
2
2
Fig. 8. Photocatalytic degradation rates of RhB in the presence of different radical scavengers over 1.0% MoS
photocatalytic RhB degradation under different atmospheres (b), schematic of charge separation and photocatalytic RhB degradation on ultrathin MoS
and recycling experiments using 1.0% MoS /porous g-C for RHB photodegradation (d).
2
/porous g-C
3
N
4
(a), gaseous reaction products generated during
2
-loaded porous g-C (c),
3 4
N
2
3 4
N

8
W. Chen et al. / Journal of Alloys and Compounds 836 (2020) 155401
under the Ar atmosphere (without O
obviously lower than under the 20% O
2
), the CO
þ80% N
is a crucial species for RhB degradation and that
2
evolution rate is
Acknowledgments
2
2
atmosphere,
indicating that O
2
This study was supported by grants from the National Natural
Science Foundation of China (51602091) and the Natural Science
Foundation of Henan Province (182300410205).
ꢀ
∙O
2
is the key reactive species in the photocatalytic system.
Therefore, based on the experimental results and characterization
analysis, a schematic of the mechanism of MoS loaded on porous
g-C is proposed and shown in Fig. 8c. Upon irradiation with
visible light, the electrons in the VB of g-C are stimulated and
shift into the CB of g-C , thus forming photogenerated electron-
hole pairs. Subsequently, the electrons rapidly migrate to MoS , and
the holes remain on g-C , which inhibits the recombination of
electrons and holes. The electrons reduce O
reduce the RhB adsorbed on MoS to form CO
neously, the holes on g-C also oxidize RhB on g-C
O, thereby degrading RhB. Notably, no H was detected, indi-
2
3 4
N
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3 4
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