A high performance MoO3&at;MoS2 porous nanorods for adsorption and photodegradation of dye

Article abstract of DOI:10.1016/j.jssc.2020.121652

A porous MoO₃&at;MoS₂ core-shell nanorod was synthesized by hydrothermal method using MoO₃ nanorod as the precursor. The investigation indicated that at high RhB concentration, MoO₃&at;MoS₂ nanorod exhibits excellent adsorption ability (Q_max ?= ?326.8 ?mg/g), while at the low concentration, it exhibits high-performance photocatalytic degradation ability. The adsorption of RhB on MoO₃&at;MoS₂ nanorod fitted well with the pseudo-second-order kinetic model and the adsorption process was mainly controlled by intraparticle diffusion process. The initial fast adsorption process of RhB may localized on both homogeneous (monolayer) and heterogeneous (multilayer) active sites, and then localized on the heterogeneous active sites for the multilayer adsorption matching with the Freundlich isotherm model. The light absorption of MoO₃&at;MoS₂ nanorod in ultraviolet and visible regions increased significantly due to forming core-shell structure. The results of trapping experiments and EPR analysis showed that in MoO₃&at;MoS₂ system h⁺ and ·OH play critical roles in the photodegradation of RhB. Due to forming Z-scheme mechanism, the reducibility of electron in the CB of MoS₂ increases, while the oxidability of hole in the VB of MoO₃ also enhances. Therefore, MoO₃&at;MoS₂ nanorods display excellent photocatalytic activity under simulated sunlight irradiation, implying promising application in wastewater treatment.

Full text of DOI:10.1016/j.jssc.2020.121652

Journal of Solid State Chemistry 291 (2020) 121652
Contents lists available at ScienceDirect
Journal of Solid State Chemistry
journal homepage: www.elsevier.com/locate/jssc
A high performance MoO @MoS porous nanorods for adsorption and
3
2
photodegradation of dye
Jialiang Chen , Ya Liao , Xia Wan *, Shaolong Tie a , Binglin Zhang , Sheng Lan ,
Xingsen Gao
a
a
a
,
,
**
b
c
d
a
School of Chemistry, South China Normal University, Guangzhou, 510006, PR China
School of Information Technology in Education, South China Normal University, Guangzhou, 510006, PR China
Guangdong Provincial Key Laboratory of Nanophotonic Functional Materials and Devices, And School of Information and Optoelectronic Science and Engineering, South
b
c
China Normal University, Guangzhou, 510006, PR China
d
Guangdong Provincial Key Laboratory of Quantum Engineering and Quantum Materials, and Institute for Advanced Materials, South China Academy of Advanced
Optoelectronics, South China Normal University, Guangzhou, 510006, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords:
A porous MoO @MoS core-shell nanorod was synthesized by hydrothermal method using MoO3 nanorod as the
3
2
MoS
MoO
Adsorption
Kinetic study
Photocatalysis
2
precursor. The investigation indicated that at high RhB concentration, MoO
adsorption ability (Qmax ¼ 326.8 mg/g), while at the low concentration, it exhibits high-performance photo-
catalytic degradation ability. The adsorption of RhB on MoO @MoS nanorod fitted well with the pseudo-second-
order kinetic model and the adsorption process was mainly controlled by intraparticle diffusion process. The
initial fast adsorption process of RhB may localized on both homogeneous (monolayer) and heterogeneous
3 2
@MoS nanorod exhibits excellent
3
@MoS
2
3
2
(multilayer) active sites, and then localized on the heterogeneous active sites for the multilayer adsorption
matching with the Freundlich isotherm model. The light absorption of MoO @MoS nanorod in ultraviolet and
3
2
visible regions increased significantly due to forming core-shell structure. The results of trapping experiments and
þ
EPR analysis showed that in MoO
3
@MoS
2
system h and ⋅OH play critical roles in the photodegradation of RhB.
Due to forming Z-scheme mechanism, the reducibility of electron in the CB of MoS
ability of hole in the VB of MoO also enhances. Therefore, MoO @MoS nanorods display excellent photo-
catalytic activity under simulated sunlight irradiation, implying promising application in wastewater treatment.
2
increases, while the oxid-
3
3
2
1
. Introduction
photocatalysis or adsorption. For example, MoS
black N–TiO -x@MoS core-shell nanoparticles [8], Ag
composite material [9], and C /MoS /Bi24 31Cl10 (CN/MS/BOC)
2
/RGO hybrids [7], 3D
2
2
3
PO /MoS
4
2
With growing population and demand, a large amount of organic
3
N
4
2
O
toxic substances are discharged into the environment without any
treatment or substandard treatment. Human beings face huge challenges
in protecting and repairing the environment. Many methods and tech-
nologies have been developed to solve the problem, such as photo-
catalysis [1], membrane separation [2], electrochemical catalysis [3],
and adsorption [4]. In them, adsorption and photocatalysis are consid-
ered to be effective and economical methods.
composite material [10] were used to photocatalytic degrade organic
dyes, such as methylene blue (MB), methyl orange (MO), and Rhodamine
B (RhB). The results indicated that the enhanced photocatalytic activity is
mainly ascribed to more effective separation of photogenerated charge
carriers and stronger oxidation and reduction ability through a Z-scheme
system. In addition, MoS
decorated by MoS [12], and hollow MoS
were used to adsorb organic dyes.
2
sponges [11], hollow Fe /Fe
3
O
4
1
ꢀ
x
S nanocube
2
2
(h-MoS ) microspheres [13]
2
2
Molybdenum disulfide (MoS ) has a S–Mo–S layered structure bound
by covalent bonds and is a two-dimensional material similar to graphite
structure. It is used as lubricant, catalyst, sensor and battery negative
There are a few reports about the materials which exhibit both
adsorption and photocatalytic ability. For example, Li et al. [14] reported
2
material [5,6]. MoS -based materials can be used to remove dyes by
2
a molecularly imprinted carbon nanosheets supported TiO which
*
*
Corresponding author.
* Corresponding author.
E-mail addresses: 651930823@qq.com (J. Chen), wanxia@scnu.edu.cn (X. Wan), tiesl@scnu.edu.cn (S. Tie).
https://doi.org/10.1016/j.jssc.2020.121652
Received 26 June 2020; Received in revised form 26 July 2020; Accepted 6 August 2020
Available online 14 August 2020
0022-4596/© 2020 Elsevier Inc. All rights reserved.

J. Chen et al.
Journal of Solid State Chemistry 291 (2020) 121652
Table 1
50 mL 0.4 mol/L HCl for 5 min, then 0.456 g thiourea was added and
magnetically stirred for 1 h to obtain a mixed solution. The mixed solu-
tion was transferred to a 100 mL Teflon-lined stainless steel autoclave
Kinetic parameters of RhB adsorption based on the pseudo-second-order kinetic
model at different initial concentration.
e(exp) (mg⋅g-1
)
Pseudo-second-order kinetic model
ꢁ
Initial conc
Q
reactor and reacted at 220 C for 2 h. After being cooled to room tem-
(mg⋅g-1
(g⋅mg-1⋅min-1
)
R2
perature, the product was washed three times with deionized water and
ethanol respectively, dried under vacuum at 60 C for 24 h, then
Q
e
)
k
2
ꢁ
2
1
2
5
0 mg/L
19.98
93.40
184.1
285.8
20.01
93.90
184.8
289.0
0.3145
0.9999
0.9996
0.9999
0.9999
3 2
MoO @MoS nanorods were obtained.
00 mg/L
50 mg/L
00 mg/L
0.03979
0.01673
0.006921
2.2. Adsorption experiment
0
.1 g MoO
3
@MoS
2
nanorods were added into 100 mL 20–500 ppm
exhibited remarkable synergy of adsorption-photocatalysis and high
selectivity in both aspects. Yu et al. [15] synthetized a hybrid TiO /-
SiO /g-C composite with superstructure which exhibits enhanced
synergistic effect of adsorption and in-situ photodegradation ability for
removing berberine hydrochloride (BH) in seawater solution. Yang et al.
ꢁ
RhB solution, magnetically stirred at 25 C for 60 min under dark con-
dition. At a regular interval, about 2 mL suspension was withdrawn and
filtered with 450 nm membrane filtration. The filtrate was detected with
a UV spectrophotometer (Shimadzu UV-2700, Japan) at a wavelength of
54 nm (maximum absorption wavelength of RhB). The adsorption ca-
pacity Q (mg/g) of RhB is calculated using the following formula:
2
2
3 4
N
5
[
2
16] synthesized a mesoporous C/MoS composite which exhibits su-
t
perhigh adsorption capacity for methyl orange (~450 mg/g) and excel-
lent photocatalytic antimicrobial capacity. Some literatures have been
ðC
o
ꢀ C
t
ÞV
Q
t
¼
reported about the properties of MoO
in adsorption and photocatalysis. For example, Zhang et al. [17] syn-
thetized a novel MoO @MoS /TiO photocatalyst for photocatalytic
hydrogen production. Song et al. [18] synthetized a MoS –MoO3-x hybrid
cocatalyst deposited on AgIn nano-octahedrons for enhanced H
production photoactivity. Zhu et al. [19] synthetized a h-MoO /1T-MoS
3 2 3 2
/MoS or MoO @MoS materials
m
3
2
2
where V is the solution volume (L); C0 and C are the concentration of
RhB (mg/L) at initial and remaining at time t respectively; m is the mass
of the sample (g). When the concentration of RhB reaches the equilibrium
concentration C , the adsorption capacity of RhB reaches equilibrium, Q
t
2
S
5 8
2
2
3
e
e
heterostructures for enhanced photoelectrocatalytic performance. Nau-
jokaitis et al. [20] synthetized a 1T/2H MoS /MoO hybrid assembled
with glycine as efficient and stable electrocatalyst for water splitting. As
(mg/g) of RhB is calculated as:
2
3
ðC
o
ꢀ C
e
ÞV
Qe
¼
m
far as we know, there have been no reports that porous MoO
materials have both adsorption and photocatalytic properties.
3
@MoS
2
The adsorption efficiency of the sample is calculated using the
following formula:
MoO
monoclinic and hexagonal. According to the literature results [21], MoO
can be converted into MoS
with high specific surface area and adsorption capacity, MoO
as the substrate and in situ react with thiourea to generate MoS
layered structure. Therefore, we successfully synthesized porous
3
usually has three different crystal structures: orthorhombic,
3
C
o
ꢀ C
t
2
by sulfuration. In order to obtain material
was used
with
Adsorption efficiency ð%Þ ¼
ꢂ 100%
C
o
3
2
2.3. Photocatalytic experiment
MoO
MoO
3
@MoS
@MoS
2
core-shell nanorods by hydrothermal method. The porous
nanorods exhibit excellent adsorption capacity for RhB
3
2
The photocatalytic activity of the sample was estimated by degrading
RhB under simulated sunlight irradiation (Xe lamp, 300 W). In a typical
experiment, 10 mg sample was added to 50 mL RhB solution (10 ppm).
After being magnetically stirred for 30 min in the dark to ensure the
adsorption-desorption equilibrium, the suspension was irradiated for
120 min by simulated sunlight. At a regular interval, about 2 mL sus-
pension was withdrawn and centrifuged at a high speed to remove the
particles. The concentration of RhB solution was measured on UV–Vis
spectrophotometer.
(
326.83 mg/g). The adsorption equilibrium and kinetics were studied in
detail. Meanwhile, the photocatalytic performance of porous MoO @-
MoS sample at low concentration of RhB was investigated and the
photocatalytic mechanism was explicated. The results show that porous
MoO @MoS core-shell nanorods exhibit excellent adsorption and pho-
3
2
3
2
tocatalytic properties, implying promising application in wastewater
treatment.
2. Experiments
2.4. Characterization
2
.1. Synthesis
The crystal phase structure of sample was examined by X-ray
2
3
.1.1. Synthesis of MoO nanorod
diffraction (XRD, AXS D8-Advance, Bruker, Germany). The morphology
of sample was characterized by field emission scanning electron micro-
scope (SEM, ZEISS ULTRA 55, Carl Zeiss NTS GmbH, Germany) and
transmission electron microscope (TEM, JEM-2100HR, Electronics Co.
In our experiments all raw materials were analytical reagents and
were not purified before use. In a typical experiment, 20 mL of saturated
sodium molybdate dihydrate (Na MoO ⋅2H O) solution was slowly
added into 70 mL 4 mol/L hydrochloric acid (HCl) solution, and
magnetically stirred for 1 h to obtain a mixed solution. The mixed solu-
tion was transferred to a 200 mL Teflon-lined stainless steel autoclave
reactor and reacted at 220 C for 24 h. After being cooled to room
temperature, the product was washed three times with deionized water
and ethanol respectively, dried under vacuum at 60 C for 24 h, then
MoO nanorods were obtained.
3
2
4
2
3 2
Ltd, Japan). The surface functional groups of MoO @MoS nanorods
before and after adsorption were determined with Fourier transform
infrared spectroscopy (FTIR, PerkinElmer, Germany). X-ray photoelec-
tron spectroscopy (XPS) was conducted on a Thermo Scientific™ K-Alpha
ꢁ
™
(
spectrometer equipped with a monochromatic Al K
1486.6 eV) operating at 100 W. The specific surface area and pore size of
the sample were characterized by nitrogen adsorption apparatus (ASAP
020, Micromeritics, USA). The zeta potential of sample was measured
α X-ray source
ꢁ
2
2
.1.2. Synthesis of porous MoO
Based on the method of reference [22], we changed L-cysteine (sul-
furation agent) to thiourea to synthesize core-shell MoO @MoS mate-
rials. 0.288 g as-prepared MoO nanorod were ultrasonically dispersed in
3 2
@MoS nanorod
using a zeta potential analyser (Zetasizer Nano ZS90, Malvern, England).
UV–Vis diffuse reflection spectroscopy (DRS) was performed on UV–Vis
spectrophotometer (UV-2700, Shimadzu, Japan). EPR spectra were
recorded at room temperature by using a Bruker A-300-EPR (Bruker,
3
2
3
2

J. Chen et al.
Journal of Solid State Chemistry 291 (2020) 121652
Germany). The electrochemical measurements were carried out using an
electrochemical workstation (CHI660C, Chenhua Instrument Co., China)
and 0.5 M Na SO solution was used as the electrolyte. The electro-
2 4
better confirm the core-shell structure, HAADF-STEM and corresponding
EDX mapping of Mo, O and S elements were conducted and the results
were shown in Fig. 2(f and g). It is clear that the signals of Mo, O and S
elements distribute uniformly across the entire nanorod. The result of
chemical measurements were performed using the three-electrode cell.
2
The as-prepared sample (1 cm effective area) was used as a working
XRD show that there are MoS
was synthesized by sulfuration reaction on the surface of MoO
2
and MoO
3
in the composite sample which
precur-
electrode, a Pt foil was used as a counter electrode, and a saturated
calomel electrode (SCE) was used as a reference electrode.
3
sor. Therefore, the core-shell structure MoO
The Raman spectra of MoO and MoO
revealed that the signals of MoS
to the low content and the nanoscale size of MoS
3
@MoS has been formed.
2
3
3
@MoS samples in Fig. 3a
2
3. Results and discussion
2
is much weaker than these of MoO
3
due
2
. The Raman peaks
-
1
-1
3
.1. Properties of materials
between 400 and 200 cm and less than 200 cm corresponded to MoO
octahedral bending vibrations and lattice modes coming from MoO
6
3
1
-1
The XRD patterns of MoO
3
and MoO
3
@MoS
2
nanorods were shown in
crystal [26]. The E 2g (376 cm ) mode of MoS
2
was masked by the B1g
-
1
-1
Fig. 1. The diffraction peaks of MoO
3
nanorod located at 2θ: 12.8(020),
(378 cm ) mode of MoO
3
, only the A1g mode at 401 cm (in the enlarged
2
3.3(110), 25.7(040), 27(021), and 39 (060) were assigned to a-MoO
3
graph) which correspond to the vibrational mode within the S–Mo–S
layer in MoS crystal was observed [27]. In addition, the Raman peaks of
MoO in MoO
the single MoO
MoO in MoO @MoS
material of MoO and MoS
(
PDF#05-5058) [23]. For MoO
3
@MoS
2
sample, except for the diffraction
2
-
1
peaks of MoO
3
, the peaks located at 2θ: 14.1(002), 36.0(101) and 39.7
(PDF#75-1508), implying that MoS was
successfully loaded on the MoO nanorods. Since MoS has a unique
3
3
@MoS
, implying the change of the chemical environment of
sample. These results proved that a composite
2
sample caused a red shift of 10 cm compared to
(
103) were assigned to MoS
2
2
3
3
2
3
3
2
layered structure and nanoscale size, its XRD diffraction peaks are
broadened and their intensities are weak [21]. Therefore, the diffraction
3
2
with single layer and multilayers structure
2
was obtained. The UV–Vis DRS spectra and the plots of (
MoO and MoO @MoS
played that MoO sample shows light absorption only in the ultraviolet
region, and its E was estimated to be about 3.30 eV [28]. MoO @MoS
sample exhibits strong light absorption in both UV and visible regions.
According to the plot of (ahv)2 vs hv of MoO
@MoS sample, there are
αhν
)
vs. h
ν
of
peak at 14.1 (2θ) attributed to MoS
The SEM and TEM images of samples were shown in Fig. 2a–e. The
SEM result of Fig. 2a indicated that MoO show a rod shape which has a
diameter of 200 nm and a length of 1–2
Fig. 2b, c shows a porous nanorod shape, implying that MoO
2
is broadened.
3
3
2
samples are shown in Fig. 3b. The result dis-
3
3
g
3
2
μ
m. MoO
3
@MoS
2
sample in
nanorods
3
3
2
were consumed by sulfuration reaction. The HRTEM images of
MoO @MoS nanorods in Fig. 2d showed that MoS possesses multilayer
three linear regions. The intercepts obtained from the extension line of
the straight lines on the x axis were estimated to be about 3.50, 2.03 and
3
2
2
structure. The thickness of the layer is about 0.256 nm and the layer
spacing is about 0.65 nm which correspond to the (002) crystal plane of
1.32 eV, respectively. 3.50 eV is attributed to the E
MoO @MoS , 2.03 eV should be attributed to the E
layered structure, and 1.32 eV may be attributed to a new deep trap level
coming from vacancy oxygen in MoO . These results indicated that
porous MoO @MoS core-shell composite exhibits excellent light ab-
g
of MoO
3
in
3
2
g
of MoS with
2
MoS
nm of MoS
sites for physical or chemical reaction [25]. The d spacing of 0.345 nm is
assigned to the (040) interplane spacing of MoO , at the same position,
the MoS with layered structure is observed. This indicates that MoO
and MoS nanoparticles are attached to each other. The result of Fig. 2e
local enlarged view) shows that the lattice constant as a ¼ b ¼ 0.32 nm,
consistent with the single layer of MoS [24]. So, the monolayer and
multilayer structures exist simultaneously in the prepared sample. To
2
. This layer spacing is slightly larger than the standard value 0.615
2
[24]. The lattice expansion may lead to an increase of active
3
3
2
3
sorption ability in ultraviolet and visible regions.
2
3
Fig. 4a displayed the high-resolution XPS spectra of Mo element in
2
MoO
3
@MoS
2
nanorod. The peaks at 228.6 eV and 231.8 eV were
. The peaks at 229.5
(
attributed to the 3d3/2 and 3d5/2 of Mo (4þ) in MoS
2
2
eV and 232.7 eV were attributed to the 3d3/2 and 3d5/2 of Mo (5þ),
which may come from the S–Mo charge transfer or a partial reduction of
the 6þ molybdenum oxide into 5þ and 4þ states [29 21]. The peaks at
2
32.4 eV and 235.6 eV were attributed to 3d3/2 and 3d5/2 of Mo (6þ) in
MoO
3
[30]. In addition, the peak at 226.2 eV corresponded to the 2s of S
2-) [29]. Molybdenum disulfide (MoS ) typically exists in two phases of
T and 2H which correspond to the octahedral metallic phase and the
(
1
2
trigonal prismatic semiconducting phase, respectively [31]. Normally, 1T
phase MoS has been demonstrated to superior electrocatalytic property.
2
In the high-resolution XPS spectra of S 2p in Fig. 4b, the peaks at 161.6 eV
and 163.3 eV were attributed to the 2p3/2 and 2p1/2 of S (2-) in 1T phase
MoS
1/2 of S (2-) in 2H phase MoS
with the results coming from XRD and HRTEM.
2
. The peaks at 162.8 eV and 164.4 eV were assigned to the 2p3/2 and
2p
2
[32]. These XPS results are consistent
3.2. Adsorption performances
The N
Barrett-Joyner-Halenda (BJH) pore-size distribution of MoO
nanorods were shown in Fig. 5a. According to IUPAC classification, the
adsorption-desorption isotherms of MoO @MoS sample were identified
as the type III. The BET surface area of MoO @MoS sample was esti-
2
adsorption-desorption isotherms and the corresponding
3
@MoS
2
3
2
3
2
2
-1
mated at about 43.5 m g . The average pore size is about 15.8 nm and
the pore size distributes from 5 to 40 nm, implying relatively uniform
distribution.
The adsorption of RhB on the sample can be proved by Fourier
transform infrared spectroscopy (FTIR). The IR spectra of MoO
3 2
@MoS
sample before and after adsorption of RhB were shown in Fig. 5b. It can
ꢀ1
Fig. 1. XRD patterns of MoO
3
@MoS
2
and MoO
3
nanorods.
3 2
be seen that for MoO @MoS sample, the peak at 1592 cm corresponds
3

J. Chen et al.
Journal of Solid State Chemistry 291 (2020) 121652
Fig. 2. (a) SEM image of MoO
3
sample; (b, c) SEM images of MoO
3
@MoS
2
sample; (d, e) HRTEM images of MoO
3
@MoS
2
sample; (f) EDX spectrum and element
@MoS sample. (For
composition of MoO
3
@MoS
2
sample from red area of the image (g1); (g) HAADF-STEM image (g1) and Mo (g2), O (g3), S (g4) mapping of MoO
3
2
interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
4

J. Chen et al.
Journal of Solid State Chemistry 291 (2020) 121652
3 3 2
Fig. 3. Raman (a) and DRS (b) spectra of MoO and MoO @MoS samples.
3 2
Fig. 4. XPS spectra of Mo 3d (a) and S 2p (b) in MoO @MoS sample.
2 3 2 3 2
Fig. 5. (a) N adsorption-desorption isotherm for MoO @MoS nanorods (inset: the pore size distribution); (b) FTIR spectra of MoO @MoS nanorods before and after
adsorption of RhB.
ꢀ
1
to in-ring C–C stretching vibration in aromatic ring, while the 1340 cm
seen that for both low and high concentrations of RhB, the adsorption
reached saturation almost in 10 min, indicating that MoO @MoS
nanorod shows a very fast adsorption velocity. The adsorption amount of
RhB increased from 19.98 mg/g to 285.8 mg/g with the concentration
from 20 ppm to 500 ppm, meanwhile, the removal rate of RhB decreased
from 99% to 57.3%. The maximum adsorption capacity (Qeq) of porous
ꢀ1
and 1412 cm peaks attribute to C–aryl bond and C–N stretching vi-
bration respectively, which can be assigned to RhB [33]. Before the
adsorption, the peak at 554 cm- was assigned to the stretching vibration
3
2
1
mode of Mo–O coming from MoO
3
[34]. After the adsorption the peak
shifted to 564 cm , which implies the strong adsorption of RhB onto
MoO @MoS nanorods.
The adsorption ability of porous MoO
investigated in detail. Fig. 6a displayed the adsorption capacities (Q
RhB in MoO @MoS system at different initial concentrations. It can be
-
1
3
2
MoO
ported value 213.84 mg RhB/g [11]. The zeta potential analysis (the
figure is not shown) indicated that the surface of MoO @MoS nanorods
is negatively charged (-36.7 mV), indicating that the surface easily
3 2
@MoS nanorod reached 326.83 mg/g, which higher than the re-
3
@MoS
2
nannorods for RhB was
) of
t
3
2
3
2
5

J. Chen et al.
Journal of Solid State Chemistry 291 (2020) 121652
Fig. 6. (a) The adsorption capacities (Q
t
) of MoO
3
@MoS
2
nanorods at different concentration; (b) Plots of the pseudo-second-order kinetic model at different initial
concentration of dye; (c) Intraparticle diffusion kinetic plot for the adsorption of RhB onto MoO
3 2
@MoS nanorods; (d,e) Langmuir (d) and Freundlich (e) isotherm
models for the adsorption of RhB; (f) Adsorption efficiency of RhB onto MoO @MoS nanorods in 8 cycles.
3
2
adsorb the positively charged RhB dye group by electrostatic attraction.
of RhB are greater than 0.999, and the calculated equilibrium adsorption
capacities Q agreed well with the experimental values Qe(exp). These
results indicated that the adsorption of RhB on MoO @MoS nanorods
follows the pseudo-second-order kinetic model. In addition, the rate
constant (k ) decreased with increasing initial concentration of RhB.
So, porous MoO
RhB.
3
@MoS
2
nanorod shows a high adsorption capacity for
e
3
2
If an adsorption kinetics on the material follows the second-order
kinetic model [11], the formula is as follows:
2
The research has proved that the adsorption process on a porous
adsorbent is multi-step process. In general, there are three consecutive
steps: (i) mass transfer (film diffusion) across the external boundary layer
film of liquid surrounding the outside of the particle; (ii) adsorption at a
site on the surface (internal or external), this step is extremely fast
because it depends on physical or chemical forces; (iii) diffusion of the
adsorbate molecules to an adsorption site either by a pore diffusion
t
Q
1
1
¼ k
þ ꢂ t
Q
e
(1)
2
t
2
Q
e
-
1
where Q
and at time t (min), respectively, and k
stant of the second order adsorption. If the second-order kinetics is
applicable, the plot of t/Q vs t should be straight. According to the results
of Fig. 6b, the pseudo-second-order kinetic model provided a perfect
matching with the kinetic results. According to the results of Table 1, the
e t
and Q are the adsorbed amounts of dye (mg⋅g ) at equilibrium
-1
-1
2
(g⋅mg ⋅min ) is the rate con-
t
(
intraparticle diffusion) process or by a solid surface diffusion mecha-
nism [35]. The intraparticle diffusion model can be described as follows
36]:
[
2
correlation coefficients (R ) for both the low and the high concentrations
6

J. Chen et al.
Journal of Solid State Chemistry 291 (2020) 121652
1
=2
3.3. Photocatalytic performances
Q
t
¼ Kt ⋅ t þ C
(2)
ꢀ
where K
(
t
(mg⋅g 1⋅min1/2) is the intraparticle diffusion rate and C
To confirm whether MoO
ability for low concentration dye, RhB (10 mg/L) is chosen as the
degradation target. As shown in Fig. 7a, MoO @MoS nanorods exhibit
higher degradation efficiency than MoO nanorods. The plots of ln (C
) vs reaction time t were shown in Fig. 7b. It can be seen that the plots
show good linear relationship, indicating that the photocatalytic degra-
dation reactions of RhB in MoO and MoO @MoS systems conform to
the pseudo-first-order kinetic model. The rate constant k (0.01796 min )
3 2
@MoS nanorods have photocatalytic
ꢀ
1
1/2
t
mg⋅g ) is a constant. According to this model, if the plot of Q vs t is
linear and passes through the origin, the intraparticle diffusion is the sole
rate-limiting step. If the plot is multi-linear, two or more steps control the
3
2
3
0
/
1
/2
adsorption process [37]. The plot of Q
t
vs t
was shown in Fig. 6c and
C
t
the result indicated that there are two linear phases corresponding to two
diffusion stages. The first line stage does not pass through the origin,
indicating that intraparticle diffusion was not the only rate-controlling
step, but the stage corresponds to the diffusion of RhB molecules to the
pores. In the second linear stage, the diffusion remains fairly constant in
which RhB is bound onto the actives sites [38]. Thus, the diffusion of RhB
3
3
2
-1
of MoO
3
@MoS
2
nanorod is 6 times that of MoO
3
nanorod (0.00282 min-
1
), indicating that the formation of core-shell structure can significantly
improve the photocatalytic ability.
into the pores of MoO
3
@MoS
2
nanorods, as well as adsorption on the
In order to explore the photocatalytic mechanism, trapping agents
have been added into the degradation system to study the effects on
degradation rates to confirm what active particles control the photo-
available surface is probably responsible for the adsorption process.
The adsorption isotherm in a solid-liquid equilibrium system can be
fitted by Langmuir (eq (3)) [39] and/or Freundlich (eq (4)) [40] formulas
respectively.
2
catalytic reaction. Na EDTA, benzoquinone (BQ) [41], isopropanol
(i-PrOH) and potassium bromate (KBrO
3
) [42] are used to capture hole
þ
ꢀ
(
h ), superoxide anion radical (⋅O
2
), hydroxy radical (⋅OH) and electron
C
Q
e
1
C
e
-
(
e ), respectively. If the degradation reaction is significantly weakened,
¼ Q þ Q
(3)
(4)
e
m
b
m
the captured active particle plays an important role in the photocatalytic
process. The capture experiment results were shown in Fig. 7c and the
1
þ
results indicated that h and ⋅OH play major roles in the degradation of
ln Q
e
¼ ln k
f
þ n ln C
e
RhB. Moreover, the addition of BQ resulted in a moderate decrease in the
ꢀ
degradation rate, implying an indispensable role of ⋅O
2
in the degrada-
where C
mg/g) is the adsorption capacity of RhB, and b (L/mg) is the Langmuir
isotherm parameter related to the adsorption affinity of the binding site.
e m
(mg/L) is the concentration of dye solution at equilibrium. Q
tion. The addition of potassium bromate slightly increased the degrada-
(
-
tion rate, implying the neglected contribution of e to the degradation.
þ
Therefore, in MoO
degradation of RhB, meanwhile, ⋅O
3
@MoS
2
system h and ⋅OH play critical roles in the
-
1
1
e f
Q (mg⋅g ) is the amount of dye adsorbed at equilibrium. k (L/g) and n
ꢀ
2
play an indispensable role in the
are the Freundlich isotherm constant related to adsorption capacity and
adsorption strength. The Langmuir isothermal model (eq (3)) assumes
that the adsorption process is a monolayer adsorption on a homogeneous
adsorbent surface that the adsorption occurs at specific homogeneous
sites within the adsorbent. The Freundlich isothermal model (eq (4))
represents a particular adsorption system when the adsorption takes
place on a heterogeneous surface with interaction between the adsorbed
ions.
degradation.
ꢀ
In order to further verify the generation of ⋅OH and ⋅O
2
, EPR spec-
troscopy using 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) as a spin trap
was performed and the result of MoO @MoS nanorod was shown in
Fig. 8a and b. It can be seen that in DMPO-H O system (Fig. 8a), under
3
2
2
dark condition weak signals were detected, and under visible light irra-
diation the strong signals attributed to ⋅OH [43] were detected. Similarly,
ꢀ
in DMPO-MeOH system (Fig. 8b), the strong signals attributed to ⋅O
2
Langmuir and Freundlich isotherm models were used to fit the
adsorption results of RhB onto MoO
[
44] were detected. Hence, under simulated sunlight irradiation porous
3
@MoS
2
nanorods and the plots of
ꢀ
MoO
3
@MoS
2
nanorod can efficiently generate ⋅OH and ⋅O
2
radicals to
C
e
/Q
e
vs C
e
and lnQ
e
vs lnC
e
were displayed in Fig. 6d and e, respectively.
oxidize and reduce pollutant. This result is consistent with the results
coming from capture experiments.
2
For Langmuir isotherm model, the R value extracted by linear fitting of
C /Q vs C is 0.9831 when the C is below 50 mg/L, and the calculated
e e e e
maximum adsorption capacity is 425.5 mg/g which significantly higher
than that of the experimental value (326.83 mg/g). Actually, Langmuir
isotherm model is not matched with the whole adsorption process, that
To investigate the charge separation efficiency, EIS was measured at
open-circuit potential and the EIS Nyquist plots of MoO
MoS samples were presented in Fig. 8c. Obviously, the Nyquist plot
shows a semicircle at high frequency region which characterizes the
charge transfer process. MoO @MoS nanorods show smaller semi-circle
than MoO nanorods, implying lower charge transfer resistance and
3 3
and MoO @-
2
3 2
is, the adsorption of RhB on porous MoO @MoS nanorods is not local-
ized on a monolayer and all the adsorption sites on the adsorbent are not
3
2
2
3
homogeneous [11]. When C
linear fitting of ln Q vs ln C
model is matched very well. This indicates that the adsorption process of
RhB onto porous MoO @MoS nanorods is plausibly controlled by
Freundlich isotherm model under the Ce exceeding 80 mg/L. Therefore,
the initial fast adsorption process of RhB onto porous MoO @MoS
e
exceeds 80 mg/L, the R value extracted by
higher charge transfer rate [45].
Mott-Schottky (M-S) analysis is commonly used to determine both
dopant density and flat band potential at solid-liquid contact [46]. The
e
e
is 0.9983, that is, the Freundlich isotherm
3
2
M–S plots of MoO
The slopes of M-S plots for MoO
3
and MoO
3
@MoS
2
nanorods were shown in Fig. 8d.
@MoS samples are positive,
3
and MoO
3
2
3
2
implying that they are n-type semiconductors. For n-type semi-
conductors, their conduction band potentials are very close to the
flat-band potentials [22]. It can be seen that the conduction band of
nanorods may localized on both homogeneous (monolayer) and hetero-
geneous (multilayer) active sites, and then mainly localized on the het-
erogeneous active sites for the multilayer adsorption, which matches
with the Freundlich isotherm model.
θ
ꢀ
MoO
3
nanorods locates at -0.2 V, less negative than E (O
2
/⋅O
2
) which
locates at -0.33 V vs. NHE, so MoO
3
nanorods do not have the ability to
Desorption is the process of removing adsorbed substance from the
ꢀ
generate ⋅O
2
. For MoS
2
sample, its valence band position (1.42 V) is
adsorbent. The RhB dye adsorbed on the MoO
removed by tetrahydrofuran solution with low-polarity. Fig. 6f showed
the adsorption efficiency of RhB onto MoO @MoS nanorods in 8 cycles
and the result indicated that the removal rate of RhB still reached 92.4%
after eight consecutive cycles, indicating the excellent recycling ability
3
@MoS
2
nanorods was
θ
-
lower than E (⋅OH/OH ) which locates at 2.38 V vs. NHE, therefore it
does not have the ability to generate ⋅OH [47]. However, the EPR results
3
2
ꢀ
(
Fig. 8a, b) showed that MoO
3
@MoS
2
can generate ⋅OH and ⋅O
2
radicals,
implying that MoO
3
@MoS
2
is a Z-scheme photocatalyst [48]. The con-
duction band of MoS
more negative than E (O
2
in MoO
3
@MoS
2
nanorod locates at -0.4 V, which is
for porous MoO
3
@MoS
2
nanorods.
θ
ꢀ
2
/⋅O
2
), so the photogenerated electron in the CB
7

J. Chen et al.
Journal of Solid State Chemistry 291 (2020) 121652
Fig. 7. (a) Photocatalytic degradation kinetics of RhB in MoO
3
and MoO
3
@MoS
2
systems; (b) Plots of ln (C
0
/C
t
) vs degradation time; (c) Effect of additives on the
3 2
degradation of RhB in MoO @MoS system.
ꢀ
of MoS
2
can combine with O
2
adsorbed on the surface to generate ⋅O
in MoO
nanorod locates at 3.30 V, which is more positive than
2
H O þ h → ⋅OH þ Hþ
þ
(8)
(9)
radical. According to the results of Fig. 3b, the VB of MoO
3
3
@-
2
MoS
2
-
þ
θ
-
-OH þ h → ⋅OH
E (⋅OH/OH ), so the photogenerated hole in the VB of MoO
3
can combine
-
with the adsorbed OH or H
2
O on the surface to generate ⋅OH radical.
þ
RhB þ ⋅OH → (RhB) ox → Inorganic small molecules þ CO
2
þ H
2
O
(10)
(11)
Based on the above results, a tentative mechanism for the degradation
of dye in the Z-scheme MoO @MoS system was proposed. As shown in
Fig. 9, when MoS @MoO sample are irradiated by simulated sunlight,
the electrons in the VBs of both MoS and MoO can be activated and
RhB þ hþ → (RhB) ox → Inorganic small molecules þ CO
þ
þ H
O
3
2
2
2
2
3
Due to forming the core-shell structure, the light absorption of
MoO @MoS sample in ultraviolet and visible regions increased signif-
icantly. Under the action of Z-scheme mechanism, the separation effi-
2
3
3
2
jump to the CBs of MoS
2
and MoO
3
(eq (5)). The photogenerated elec-
ꢀ
trons in the CB of MoS
2
react with O
2
adsorbed on the surface to form ⋅O
2
ꢀ
ciency of electrons and holes is greatly increased. The reducibility of
(
eq (6)). ⋅O
2
radical can reduce RhB directly to be decomposed into
and H O (eq (7)). According to the Z-
scheme mechanism, the photogenerated electrons in the CB of MoO
recombine with the holes in the VB of MoS , resulting in the remain of
separated holes in the VB of MoO . This interface e -h recombination
between MoS and MoO causes the accumulation of the electrons in the
CB of MoS and the holes in the VB of MoO . The photogenerated holes in
the VB of MoO can directly oxidize RhB to be decomposed into inorganic
small molecules, CO
electron in the CB of MoS
VB of MoO also enhances. As the result, MoO
excellent photocatalytic activity under simulated sunlight irradiation.
2
increases, while the oxidability of hole in the
inorganic small molecules, CO
2
2
3
3
@MoS nanorods display
2
3
2
-
þ
3
4
. Conclusions
2
3
2
3
In this paper, the porous MoO
3
@MoS
2
core-shell nanorods were
nanorod
as the precursor. The absorption and photocatalytic performances were
investigated in detail. Because the surface of MoO @MoS nanorods is
negatively charged, it interacts with RhB group with positively charged
through electrostatic interaction. In addition, MoO @MoS nanorods
possess a large number of mesoporous, and MoS possesses the layered
structure. These factors lead MoO @MoS nanorods to exhibit superhigh
adsorption ability (326.83 mg/g) for RhB. The adsorption kinetics of RhB
on MoO @MoS nanorod fitted with a pseudo-second-order kinetic
3
successfully synthesized by hydrothermal method using MoO
3
2
and H
2
O (eq (11)). In the other way, photo-
-
generated holes in the VB of MoO
3
can react with H
2
O or OH groups
3
2
chemically adsorbed on the surface to form ⋅OH (eq(8) and (9)). ⋅OH
radicals oxidize RhB to be decomposed into inorganic small molecules,
3
2
2 2
CO and H O (eq (10)). The photocatalytic equation is described as
2
follows:
3
2
-
þ
MoS @MoO
2
3
þ hv → MoS
2
@MoO
3
(e , h )
(5)
(6)
(7)
3
2
-
ꢀ
O
2
þ e → ⋅O
2
model and the adsorption process is mainly controlled by the intra-
particle diffusion process. The initial fast adsorption process of RhB may
ꢀ
ꢀ
RhB þ ⋅O
2
→ (RhB) red → Inorganic small molecules þ CO
2
þ H
2
O
8

J. Chen et al.
Journal of Solid State Chemistry 291 (2020) 121652
Fig. 8. (a,b) DMPO spin-trapping EPR spectra of MoO
3
@MoS
2
nanorods (a:aqueous dispersion, b:methanol dispersion); (c) EIS Nyquist plots of the samples electrodes;
sample exhibites photocatalytic degradation ability. Due to forming core-
(d) Mott-Schottky plots of the sample electrodes.
3 2
shell structure, the light absorption of MoO @MoS sample in ultraviolet
and visible regions increased significantly. By means of Z-scheme
mechanism, the separation efficiency of electrons and holes is greatly
increased. The reducibility of the electron in the CB of MoS
while the oxidability of the hole in the VB of MoO also enhances. As the
result, MoO @MoS nanorods display excellent photocatalytic activity
2
increases,
3
3
2
under simulated sunlight irradiation.
Credit author statement
Jiaoliang Chen: Validation, Data handling, Writing original draft; Ya
Liao: Experiment, Data curation; Xia Wan: Supervision, Writing-
Reviewing and Editing; Shaolong Tie: Conceptualization, Supervision,
Project administration, Financial support; Binglin Zhang: Investigation,
Writing original draft; Sheng Lan: Supervision, Financial support; Xing-
sen Gao: Supervision, Financial support.
Declaration of competing interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Fig. 9. Schematic diagram showing direct Z-scheme photocatalytic mechanism
of MoO @MoS nanorod for pollutant degradation.
3
2
Acknowledgments
localized on both homogeneous and heterogeneous active sites, and then
localized on the heterogeneous active sites for the multilayer adsorption
matching with the Freundlich isotherm model. Our study showed that at
The authors acknowledge the financial support from the National Key
Research and Development Program of China (No. 2016YFA0201002). S.
L. Tie, and X. S. Gao acknowledge the financial support from the National
Natural Science Foundation of China (Grant No. 11674110 and
the high concentration of RhB, MoO
3
@MoS
2
sample exhibites the su-
@MoS
perhigh adsorption ability, while at the low concentration, MoO
3
2
9

J. Chen et al.
Journal of Solid State Chemistry 291 (2020) 121652
1
1874020), the Natural Science Foundation of Guangdong Province,
China (Grant No. 2016A030308010, 2016A030308019 and
018A030313854).
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