Z-scheme MoS2/Bi2O3 heterojunctions: Enhanced photocatalytic degradation performance and mechanistic insight

Article abstract of DOI:10.1039/c9nj02521a

A new Z-scheme MoS₂/Bi₂O₃ heterojunction photocatalyst was successfully prepared using a facile and practical hydrothermal method. The Z-scheme MoS₂/Bi₂O₃ heterojunction exhibits a superior degradation efficiency compared to pure MoS₂ and Bi₂O₃ for removing industrial and domestic pollutants under visible light, such as 2-mercaptobenzothiazole, tetracycline and rhodamine B. The enhancement in the photocatalytic performance mainly stems from the interfacial interaction between the MoS₂ nanosheets and Bi₂O₃ rods, which can increase the specific surface area and accelerate the separation of the photo-induced electron-hole pairs. To explain this interfacial interaction between MoS₂ and Bi₂O₃ in the Z-scheme MoS₂/Bi₂O₃ heterojunction, several characterization techniques were applied. In addition, the photocatalytic mechanism for degrading pollutants was demonstrated using trapping experiments and electron spin resonance. This work could provide a promising approach for the formation of other Z-scheme transition metal compound heterojunctions with outstanding photocatalytic activity.

Full text of DOI:10.1039/c9nj02521a

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Z-scheme MoS /Bi O heterojunctions: enhanced
2
2 3
photocatalytic degradation performance and
Cite this:DOI: 10.1039/c9nj02521a
mechanistic insight†
a
b
c
d
a
Rong Ji, Changchang Ma, * Wei Ma, Yang Liu, Zhi Zhu
and
a
Yongsheng Yan
*
A new Z-scheme MoS
2
/Bi
2
O
3
heterojunction photocatalyst was successfully prepared using a facile and
/Bi heterojunction exhibits a superior degradation
for removing industrial and domestic pollutants under visible
light, such as 2-mercaptobenzothiazole, tetracycline and rhodamine B. The enhancement in the photocatalytic
performance mainly stems from the interfacial interaction between the MoS nanosheets and Bi
rods, which can increase the specific surface area and accelerate the separation of the photo-induced
electron–hole pairs. To explain this interfacial interaction between MoS and Bi in the Z-scheme
MoS /Bi heterojunction, several characterization techniques were applied. In addition, the photocata-
practical hydrothermal method. The Z-scheme MoS
2
2 3
O
efficiency compared to pure MoS and Bi
2
2 3
O
2
2 3
O
Received 16th May 2019,
Accepted 19th June 2019
2
2 3
O
2
2 3
O
lytic mechanism for degrading pollutants was demonstrated using trapping experiments and electron
spin resonance. This work could provide a promising approach for the formation of other Z-scheme
transition metal compound heterojunctions with outstanding photocatalytic activity.
DOI: 10.1039/c9nj02521a
rsc.li/njc
Recently, photocatalysis has become more and more attrac-
tive because it has been described as a potential method to deal
1
. Introduction
6
–8
Mercaptans, one of several significant chemical products, have with industrial and domestic pollutants.
Thus, a range of
found comprehensive applications in various fields during the single photocatalysts can be applied to remove organic pollu-
1
–3
rapid development of the chemical industry.
mercaptans, 2-mercaptobenzothiazole (MBT), an important mental problems.
mercaptan, has been widely used in chemical manufacturing, semiconductor, stands out among the other metal semicon-
Among these tants in order to solve the increasing and devastating environ-
9
,10
Bismuth trioxide (Bi O ), as a metal oxide
2
3
1
1,12
for use in medicines, antidotes, rubber vulcanization, and so ductor materials.
Bi O has been used in a lot of fields,
2 3
forth. Although MBT has many purposes, there is no doubt that such as for use in sensors, supercapacitor electrodes, and solid
1
4–16
MBT has a certain virulence, which irritates the skin and oxide fuel cells.
It has a low cost and a suitable band gap of
mucous membrane resulting in skin ulcers. Aside from the 2.5–2.8 eV. In addition, unlike other popular semiconductor
abovementioned industrial pollutants, there are some domestic photocatalysts, such as TiO and SrTiO , Bi can be excited by
Nevertheless, rapid recombination of charge
B (RhB). These can cause damage to the functioning organs of carriers in pure Bi O frequently leads to its poor photocatalytic
4
13
2
3
2 3
O
1
7–19
pollutants in water, such as tetracycline (TC) and rhodamine visible light.
2
3
the body in high concentrations. Therefore, there is an urgent activity. Thus, a bare Bi O semiconductor photocatalyst cannot
2
3
1
3
need to reduce and remove MBT, TC and RhB from water to meet the demands of daily life. In order to overcome this
5
protect the environment and human health from harm.
drawback, numerous researchers have reported various means
to modify Bi , including semiconductor compositing, carbon
introduction and metal/nonmetal ion doping.
2 3
O
2
0–22
However,
a
the above described approaches have several shortcomings
such as a high cost, low efficiency and a lack of stability. It is
well known that the fabrication of Z-scheme heterojunctions
with a suitable band structure has been researched widely and
can accelerate charge separation effectively and give the rest of
Institute of the Green Chemistry and Chemical Technology, School of Chemistry
and Chemical Engineering, Jiangsu University, Zhenjiang 212013, P. R. China.
E-mail: yys@mail.ujs.edu.cn
b
Research Center of Fluid Machinery Engineering and Technology, Institute of the
Green Chemistry and Chemical Technology, Jiangsu University, Zhenjiang 212013,
P. R. China
2
3–26
c
the electrons and holes a higher oxidation–reduction ability.
Jiangsu United Chemical Co., Ltd, Zhenjiang 212013, P. R. China
d
Until now, a lot of Bi
O
2 3
heterojunction photocatalysts have been
School of Physics, Jilin Normal University, Siping 136000, P. R. China
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/c9nj02521a studied for enhancing the photocatalytic performance, such as
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Bi
and heterojunction structures, building a Z-scheme heterojunc- Bi(NO ) Á5H O. Finally, the Z-scheme MoS /Bi O heterojunc-
3
/g-C
–ZnO.
3
N
4
/Bi
2
O
3
, BiOCl/Bi
2
O
3
, RGO–Cu
2
O/Bi
2
O
3
, and Cs
2
O– 180 1C for 24 h. Subsequently, the Bi
2
O
3
solid rods were
2
7–30
2
O
3
By combining the abovementioned Z-scheme attained using a typical synthesis procedure using 4.8 g of
3
3
2
2
2 3
tion photocatalyst seems to be an excellent method with great tion photocatalyst was prepared via a hydrothermal process.
potential to handle the disadvantages of Bi O . Thus, a semi-
conductor needs to be found which is well matched with Bi O
2 3
X-ray diffractometry (XRD), scanning electron microscopy
(SEM), transmission electron microscopy (TEM), X-ray photo-
2
3
to form a Z-scheme heterojunction to further increase the electron spectroscopy (XPS) and other measurements were used
photocatalytic performance. to characterize the samples. Photocatalytic tests, trapping experi-
Among numerous semiconductors, molybdenum disulphide ments and photoelectrochemical measurements were all carried
MoS ) as a flake transition metal sulfide has been commonly out to detect the photocatalytic performance of the Z-scheme
researched for the enhancement of the photocatalytic MoS /Bi O heterojunction. The details of the experimental
(
2
2
2 3
3
1–34
efficiency.
It acts as a two dimensional (2D) sheet structure, process, characterization and measurements are described in
which is comparable, and even better in some cases, to other the ESI.†
multidimensional metal materials in the photocatalytic
3
5–38
field.
2
Very recently, MoS as a metal dichalcogenide, has
received a large amount of attention for the degradation of 3. Results and discussion
organic pollutants owing to its large surface area, high electrical
3.1. XRD analysis
3
9–42
conductivity, and stable chemical properties.
A variety of
The crystalline phase of the different samples was determined
using XRD analysis. As shown in Fig. 1a, the as-synthesized
studies have reported the ability of MoS2 heterojunctions for
the degradation of organic pollutants, such as MoS /g-C N ,
2
3 4
4
3–45 Bi
2
O
3
solid rods and MoS
diffraction peaks, which suggest a good crystalline phase for
6,47 these samples. The sharp diffraction peaks of Bi and MoS
JCPDS No. 41-1449;
2
nanosheet samples present strong
MoS /Bi WO and MoS /Mn0.2Cd0.8S/MnS heterojunctions.
2
2
6
2
2
Moreover, the unique 2D layered crystalline structures of MoS can
4
2
O
3
2
ensure the separation and transportation of the charge carrier.
In addition, the conduction band (CB)/valence band (VB) of MoS
is located at À0.38 eV/1.52 eV, which conforms well with Bi
0.16 eV/2.98 eV) to create a Z-scheme heterojunction. Therefore, it
2 3
correspond with the standard spectra (Bi O
2
MoS JCPDS No. 37-1492) respectively, and no other impurity
2
2 3
O
diffraction signals were found. In addition, for the Z-scheme
(
MoS
similar to that of pure MoS
Z-scheme MoS /Bi heterojunction was successfully con-
structed. It should be noted that the diffraction peaks are
consistent with the (002) lattice plane of MoS and the (012)
lattice plane of Bi in the MoS /Bi heterojunction and both
/Bi O
2 2 3
heterojunction all of the diffraction peaks were
is hoped that preparation of a Z-scheme heterojunction by
recombination of MoS2 with Bi O will promote the charge
2
and Bi , which means that the
2 3
O
2
3
2
2 3
O
carrier separation for the photocatalytic removal of various
pollutants under visible light. Finally, it must be emphasized
that there are no reports published to date detailing research on
2
O
2 3
2
2 3
O
the Z-scheme MoS
2 2 3
/Bi O heterojunction and its application in
become weaker compared with those of the single MoS and
2
the photocatalysis field.
Bi O respectively. This indicates that the intense interaction
2
3
In light of the above described considerations, in this paper
a Z-scheme MoS /Bi O heterojunction was achieved by a facile
2 2 3 2 2 3
between MoS and Bi O appears in the Z-scheme MoS /Bi O
2
2 3
heterojunction, and this is beneficial to the transport and
separation of charge carriers between them, and therefore the
and practical hydrothermal method. In order to evaluate the
photocatalytic activity of the Z-scheme MoS /Bi O heterojunc-
2
2 3
1
4
photocatalytic activity is enhanced. On the one hand, it is the
same as the (012) lattice plane of Bi , and the characteristic
peaks of (101) and (110) lattice plane of MoS are too weak in the
composite. This is mainly because the content of MoS is not
tion, a series of MBT, TC and RhB degradation experiments
were carried out under visible light. These results all proved
2 3
O
2
that the Z-scheme MoS
strengthens the visible-light photocatalytic efficiency compared
with pure MoS and Bi , but also possessed outstanding
2 2 3
/Bi O heterojunctions not only distinctly
2
2
2 3
O
stability and reusability. As expected, the enhanced photocataly-
tic performance for the degradation of various organic pollutants
is on account of the strong interfacial interaction between MoS2
and Bi O and the fast transportation of the electron hole pairs
2 3
in the interface, which also extends the lifetime of the photo-
generated charge carriers.
2. Experimental
The Z-scheme MoS /Bi O heterojunction photocatalyst was
2
2 3
Fig. 1 (a) XRD patterns for the pure MoS
MoS /Bi heterojunction; (b) EDX spectrum of 15% MoS
junction and the mass and atomic ratios of the 15% MoS
2 2
)
and heating at junction (inset).
2 2 3
nanosheets, Bi O rods and 15%
successfully prepared using a facile and practical hydrothermal
method. The MoS nanosheets were prepared by combining
.21 g Na MoO O and 1.56 g CS(NH
Á2H
2
2
O
3
2
/Bi
/Bi
2
O
3
hetero-
hetero-
2
O
2
2
3
1
2
4
2
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significant in the composite. Therefore, there is no characteristic performed and is shown in Fig. S2 (ESI†). From this image the
peak for MoS observed in the Z-scheme MoS /Bi heterojunc- lattice fringes of the MoS nanosheets in the MoS /Bi
2
2
O
2 3
2
2
2 3
O
tion except for the peak for the (002) lattice plane. Moreover, an heterojunction can be clearly observed and the lattice spacing
elemental composition of 15% for the MoS /Bi O heterojunction was measured to be 0.2604 nm, which corresponds with a
2
2 3
was detected from the EDX. As shown in Fig. 1b, the elements typical (100) plane for MoS . Meanwhile, the (120) plane of
2
Mo, S, Bi and O were observed in the Z-scheme MoS
heterojunction, confirming that the MoS nanosheets were MoS
successfully combined with the Bi rods. The aforementioned the HRTEM analysis further prove that the composite structure
results further demonstrate that the MoS nanosheets were is a Z-scheme MoS /Bi heterojunction.
deposited on the surface of Bi
/Bi O
2 2 3
Bi
2
O
3
with a lattice spacing of 0.32 nm was also found in the
2
2
/Bi heterojunction nanocomposite. Thus, the results of
2 3
O
2 3
O
2
2
2 3
O
2 3
O .
3.3. XPS analysis
3
.2. Microstructural analysis
The morphology and microstructure of MoS
MoS /Bi heterojunction were characterized using SEM and
2 3
TEM analysis. From Fig. 2a, it can be observed that bare Bi O
The surface chemical constituent of the Z-scheme MoS /Bi O
2 2 3
heterojunctions and the chemical interactions between MoS2
and Bi O were further examined using XPS. The XPS spectrum
2
2 3
, Bi O and the
2
3
2
2 3
O
of the Z-scheme MoS /Bi O heterojunction includes the ele-
2
2 3
ments Mo, S, Bi and O, and is shown with the XRD, EDX and
elemental mapping results in Fig. 3a. The XPS spectrum of
Mo 3d can be observed from Fig. 3b, and it reveals two strong
peaks at 229.1 and 232.5 eV belonging to Mo 3d5/2 and Mo 3d3/2
respectively, which obviously indicates that the valence state
of the molybdenum element is 4+ in the MoS /Bi O
2 3
possesses high uniform micron-sized rods with a diameter of
approximately 0.5 mm, and that pure MoS2 presents small
irregular nanosheets with a tendency to aggregate, as shown
in Fig. 2b. Fig. 2c and d show typical SEM and TEM images of
the coupling photocatalytic 15% MoS
mentioned images, it can be seen that the MoS
have been successfully absorbed onto the surface of the Bi
2
/Bi
2
O
3
. From the above-
2
2
nanosheets
4
8
heterojunction. As shown in Fig. 3e, in the high resolution
XPS spectrum of Bi 4f, two peaks appearing at 158.6 and
2 3
O
micron-sized rods during the hydrothermal process and the
formation of MoS has no obvious effect on the Bi O . The SEM
1
64.2 eV are evident, corresponding to Bi 4f7/2 and Bi 4f5/2 in
2
2 3
4
9,50
the sample respectively.
Meanwhile, the peaks at 161.8 and
image shown in Fig. 2c demonstrates that the small irregular
1
62.9 eV can be ascribed to S 2p3/2 and S 2p1/2 because of the
spin orbit separation of the S element, which also certifies the
MoS nanosheets are uniformly distributed on the surface of
2
Bi
describes the tight connection between the MoS
and Bi micron-sized rods, which allows the interparticle
photo-induced transfer of the carriers. Furthermore, elemental
2 3
mapping has also been performed on the Z-scheme MoS /Bi O
2
O
3
. Meanwhile, the TEM image shown in Fig. 2d also
2
À
51,52
emergence of S in the final photocatalyst.
The O 1s peaks
2
nanosheets
in Fig. 3d can be fitted to the two peaks at 529.9 and 531.3 eV, they
belong to the surface hydroxyl groups (Bi–O–H) and Bi–O bond in
2 3
O
53
the sample respectively. Fig. 3g displays the XPS pattern of S 2p,
there are two distinct peaks at 162.4 and 163.8 eV which are
matched with S 2p3/2 and S 2p1/2. Finally, it is worth noting that
the XPS spectrum of Mo 3d and Bi 4f is evidently shifted to a
higher binding energy (Fig. 3c and f) comparing with the pure
2
heterojunction, which also confirms the presence of all ele-
ments (Mo, S, Bi and O) in the heterojunction (Fig. 2e). This
result is in accordance with the EDX analysis, and reveals that
MoS
2 2 3
has been successfully combined with Bi O . In addition,
high-resolution TEM (HRTEM) of the heterojunction was also
Fig. 2 SEM images of pure: (a) Bi
c) 15% MoS /Bi heterojunction; (d) TEM images of the 15% MoS
heterojunction; and (e) elemental mapping of the 15% MoS /Bi hetero-
2
O
3
rods; (b) single MoS
2
nanosheets;
(
2
2
O
3
/Bi O
2 2 3
2
2 3
O
junction showing the presence of the constituent elements (Mo, S, Bi
and O).
Fig. 3 XPS spectra of the 15% MoS
spectrum; (b) and (c) Mo 3d; (d) O 1s; (e) and (f) Bi 4f; and (g) S 2p.
2 2 3
/Bi O heterojunction: (a) survey
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abovementioned results correspond with previously published
reports. Thus, by combining the XPS spectral data and previous
reports, it can be determined that the band gap of MoS is 1.9 eV,
2
meanwhile, the conductance bands of MoS and Bi O are À0.38
2
2 3
1
3,56
and 0.16 eV respectively.
Based on the above described
2 2 3
findings, it can be confirmed that MoS and Bi O can form a
perfect Z-scheme heterojunction.
3.5. Photocatalytic tests
The photocatalytic ability of Z-scheme MoS /Bi O heterojunc-
2
2 3
tion photocatalysts depend on the content of MoS
different MoS contents in MoS /Bi heterojunction photo-
catalysts must be studied for degrading MBT under the visible
light. As shown in Fig. 5a, it was noted that the bare MoS and
Bi display a poor photocatalytic performance, and only
3.5% and 37.0% of MBT is removed respectively. Moreover,
the photocatalytic degradation efficiency of MBT was 57.6%,
2
, therefore
2
2
2 3
O
2
2 3
O
2
Fig. 4 (a) UV-vis DRS of different samples; (b) the plots of (ahn) versus
hn) of Bi ; (c) XPS valence band spectra of MoS ; and (d) Bi
1
(
2
O
3
2
2 3
O .
8
1.2%, 89.6%, 71.2% and 66.2% for the 5% MoS /Bi O , 10%
2 2 3
2 2 3 2 2 3 2 2 3 2
MoS /Bi O , 15% MoS /Bi O , 20% MoS /Bi O and 25% MoS /
Bi O heterojunction after irradiation for 120 min. As expected,
2 3
MoS
action between MoS
prove the existence of a strong interaction between MoS
Bi O in the Z-scheme MoS /Bi O heterojunction, which could
promote the migration of an electron–hole pair, and also
increase the photocatalytic performance and stability.
2 2 3
and Bi O , this may be due to the heterojunction inter-
5
4
2
and Bi
2
O
3
.
The above described results
and
the Z-scheme MoS
a better photocatalytic performance compared to simple MoS
and Bi , which can also obviously enhance the photocatalytic
performance. More interestingly, when the content of MoS is
2 2 3
/Bi O heterojunction photocatalysts exhibit
2
2
2
3
2
2 3
2 3
O
2
15 wt% (0.02 g), the highest degradation rate achieved is 89.6%.
However, when the content of MoS grows to 25%, the degrada-
3.4. UV-vis adsorption of the as-prepared photocatalysts
2
tion rate is only 66.2%. This is mainly due to the higher content
of MoS on the Bi O rods which may restrain the absorption of
2 2 3
visible light and the light transmission capacity, after that, it
will cause a negative influence on the photocatalytic process.
Using UV-vis diffusion reflectance spectra is a common method
to analyze the light absorption and energy band features of a
photocatalyst. Therefore, the UV-vis absorption spectra of the
different samples are shown in Fig. 4. As shown in Fig. 4a, the
On the contrary, with a lower content of MoS
2
, there is not
absorption edge of Bi
bits a photo-response to visible light. In comparison, the pure
MoS nanosheets present a broad absorption spectra because
2 3
O is approaching 440 nm, which exhi-
enough MoS nanosheets inserted into the Bi O rods to create
2
2 3
the Z-scheme heterojunction, which will reduce the degradation
rate and refraction effect of the visible light. Thus, the reduced
photons migrating onto the surface of the photocatalysts may
2
of their black color, which indicates that they can absorb all of
the photons from visible light and this is similar to previously
1
3
reported results. Furthermore, after being added to MoS , a
2
striking full spectral absorption can be observed in the absorp-
tion spectrum of the MoS
due to the transition of MoS
2
/Bi
2
O
3
composite samples, which is
and Bi . Meanwhile, there are
2
2 3
O
other factors affecting the photocatalytic efficiency achieved,
such as the optical range, containment of the charge transfer,
specific surface area and the suppression of the recombination
of electron–hole pairs of the Z-scheme MoS /Bi O heterojunc-
2
2 3
tion. Moreover, in order to determine the relative band gap and
the position of Bi , the band gap energy (E ) of the sample
was counted using the following formula based on the differ-
2
O
3
g
5
5
ential reflectance spectroscopy (DRS) results: ahn = A(hn À
n/2
E
(
g
)
g 2 3
. From Fig. 4b, the E of Bi O counted using a plot of
2
ahn) versus (hn) was found to be 2.82 eV.
Furthermore, in order to ensure the VB and CB of the edge
potential of the semiconductors, samples were analyzed using
XPS and previous reports. From Fig. 4c and d, it can be seen that
Fig. 5 (a) Photocatalytic degradation of MBT in the presence of the
as-prepared samples under visible light irradiation; (b) absorption spectra
the VB of MoS
2 2 3
and Bi O were 1.52 and 2.98 eV, as detected
of MBT over the 15% MoS
order reaction kinetics; and (d) values of reaction rate constants over the
2 3
the band gap of Bi O
was found to be 2.82 eV. Interestingly, the as-prepared samples.
2 2 3
/Bi O heterojunction; (c) the pseudo-first-
using XPS. By combining this with the result of the UV-vis,
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cause a relatively low removal efficiency. From Fig. 5b, it can be
observed that the absorbance peak intensity of the MBT solution
over the 15% MoS /Bi O composite gradually decreased during
2
2 3
the degradation process. Furthermore, the reaction kinetics of
MBT degradation over the Z-scheme heterojunction photo-
catalysts were investigated using the model of ln(C
which the slope k is the apparent reaction rate constant (Fig. 5c
and d). Evidently, the Z-scheme MoS /Bi heterojunctions
and Bi , and
0
/C) = kt, in
2
2 3
O
show a higher k value compared with pure MoS
2
2 3
O
À1
the 15% MoS
2
/Bi
2
O
3
(0.01936 min ) was about 15.6 and
À1
5
.1 times as high as that of MoS (0.00124 min ) and Bi O
2
2
3
À1
(0.00382 min ) respectively. To further confirm the adsorption
of the catalysts, adsorption tests for MBT were also carried out
and the results are presented in Fig. S1 (ESI†). The above
described results all demonstrate that the 15% Z-scheme
2 2 3
MoS /Bi O heterojunction distinctly promotes the separation
Fig. 7 (a) Photocatalytic reduction curves for TC over the as-prepared
efficiency of the charge carriers and mineralization of the MBT
molecules can be achieved.
2 2 3
different samples; (b) absorption spectra of TC over the 15% MoS /Bi O
composite; (c) photocatalytic degradation curves; and (d) absorbance
variations of RhB solution over the as-prepared different samples under
visible light.
It is important for the photocatalyst to be reusable and
stable to allow its practical application. Thus, to evaluate the
reusability and stability of the Z-scheme MoS /Bi O hetero-
2
2 3
2 2 3 2 2 3
junction, the 15% MoS /Bi O heterojunction was assessed via heterojunction. As expected, the 15% Z-scheme MoS /Bi O
the degradation of MBT under visible light irradiation. It can be heterojunction exhibits the highest efficiency and a reduction
seen in Fig. 6a that there is no distinct inactivation in the of 79.3% is achieved, which is a factor of 4.65 and 1.94 times
photocatalyst, which indicates that the 15% Z-scheme MoS
2
/
2 2 3
higher than that of simple MoS (18.6%) and Bi O (40.8%)
Bi heterojunction possesses a high stability. Furthermore, after 120 min respectively. Meanwhile, as can be seen from
2 3
O
in order to carefully detect the reusability and stability of the Fig. 7b, the characteristic absorption peak intensity of TC
photocatalysts, XRD characterization was performed before and increasingly declines and there is no shift of the maximum
after the degradation process, the results are shown in Fig. 6b. absorption wavelength position at 357 nm. Furthermore, the
As expected, the catalyst does not show any obvious change in reaction kinetics of TC over different samples are shown in
the intensity after the degradation process, this means that the Fig. S3a and b (ESI†). The rate constant for the 15% MoS
5% Z-scheme MoS /Bi heterojunction possesses an out- heterojunction is higher than that observed for pure MoS
standing photocatalytic recyclability and stability in practical Bi
applications. Bi
2
/Bi
and
. Thus, these results all prove that the Z-scheme MoS
heterojunctions have the ability to accelerate the separa-
2 3
O
1
2
2
O
3
2
2
2
O
O
3
3
2
/
Moreover, it is well known that TC, as a universal antibiotic tion efficiency of the charge carriers and improve the photo-
drug, has greatly harmed the environment and food sources catalytic performance compared to pure MoS and Bi O .
2
2 3
owing to its excessive use in human and animals. Therefore, we
also evaluated the photocatalytic performance of the as pre- pollutant to determine the universal effect of the Z-scheme MoS
pared samples via the degradation of TC under visible light. Bi heterojunctions. As shown in Fig. 7c, 90% of RhB can be
Fig. 7a shows the visible-light photocatalytic degradation curves removed by the 15% MoS /Bi heterojunction under visible
of TC over pure MoS , Bi rods and the 15% MoS /Bi light within 120 min and the degradation rate is higher than that
In addition, RhB was chosen as a representative dye and model
2
/
2 3
O
2
2 3
O
2
2
O
3
2
2 3
O
2 2 3
Fig. 6 (a) Five cycle degradation kinetic curves of MBT; (b) XRD patterns of 15% MoS /Bi O heterojunction after five reaction cycles.
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of pure MoS
2
and Bi
2
O
3
. The inset in Fig. 7c shows the color photocatalytic activity of the 15% Z-scheme MoS
2
/Bi
2 3
O hetero-
changes occurring every 20 min, it can be observed that the junction is much higher than that of pure Bi O . This result
2 3
color of the solutions changes gradually from pink to colorless. also indicates that the surface area of the photocatalysts is an
Moreover, Fig. 7d reveals the absorbance variation of the RhB important factor that affects the photocatalytic activity.
solutions over the 15% Z-scheme MoS /Bi O heterojunction. It
2
2 3
3
.6. Photocurrents and electrochemical impedance spectral
is noted that the characteristic absorption peak intensity of RhB
553 nm) distinctly decreases with each time interval. As with
analyses
(
the degradation of TC, the reaction kinetics of RhB over In order to further understand the transfer behavior of the
different samples is shown in Fig. S3c and d (ESI†). As expected, charge carriers in the heterojunctions, photocurrent experiments
2 2 3
the rate constant for the 15% MoS /Bi O heterojunction is the were carried out. The photocurrent responses of the pure MoS2,
highest among all of the samples. Finally, the above described Bi O and 15% Z-scheme MoS /Bi O heterojunction are shown
2
3
2
2 3
degradation experiments demonstrate that the Z-scheme hetero- in Fig. 9a. Evidently, the 15% MoS /Bi O heterojunction gives
2
2 3
junctions can greatly improve the photocatalytic performance for the best photocurrent response, which is 6.47 and 2.21 times
the degradation of industrial and domestic pollutants under that of pristine MoS and Bi O . This result confirms that the
2
2 3
visible light.
Z-scheme MoS /Bi O heterojunction has a lower recombination
2 2 3
2
Fig. 8 shows the N adsorption–desorption isotherms for the rate for the charge carriers, which means the photocatalytic
prepared samples. It is well known that the surface area performance was greatly enhanced. Moreover, after several
significantly affects the photocatalytic activity of photocatalysts, cycles, the steady and reproducible photocurrent response sug-
and it was found that the adsorption–desorption isotherms gests that the 15% Z-scheme MoS /Bi O heterojunction pos-
2
2 3
exhibited a classical type IV with a hysteresis loop. From Fig. 8a, sesses a good photophysical stability. In order to further research
the Brunauer–Emmett–Teller (BET) surface area (SBET) of pure the interfacial charge transfer abilities of the photocatalysts, EIS
2
À1
Bi
Bi
2
2
O
O
3
3
was found to be 8.5 m g , clearly revealing that pure tests were performed. As shown in Fig. 9b, the radius of the 15%
has a low surface area. Compared with the pristine Bi
MoS /Bi O heterojunction was the smallest compared to that of
/Bi hetero- pure MoS and Bi O , and it exhibits a faster interfacial charge
2 3
O ,
2
2 3
the SBET of the as-prepared 15% Z-scheme MoS
junction photocatalyst was found to be 18.5 m g , and this transfer rate in the Z-scheme MoS /Bi O heterojunctions. Thus,
2
2
2 3
O
2
2 3
À1
2
2 3
was larger than that found for the simple Bi
2 3
O (Fig. 8b). This the photocurrent and EIS results indicate that the 15% Z-scheme
obviously indicates that the adsorption activity of 15% MoS2/ MoS /Bi O heterojunction can efficiently control the recombina-
2
2 3
Bi O and Bi O conform well to the surface area. Therefore, the tion of the electron–hole pairs and the Z-scheme heterojunction
2
3
2 3
Fig. 8
2
N adsorption–desorption isotherms of the as-prepared samples.
Fig. 9 (a) Transient photocurrent response; and (b) electrochemical impedance spectroscopy of the as-prepared samples.
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2 2 3
Fig. 10 m/z of degrading MBT over the 15% MoS /Bi O heterojunction:
(
a) degradation during 30 min; (b) degradation in 1 h; (c) degradation in 2 h;
and (d) possible intermediate products of degradation of MBT over the 15%
MoS /Bi heterojunction.
Fig. 11 (a and b) Photocatalytic degradation ratios of MBT using different
2
2 3
O
radical scavengers over the 15% MoS
2 2 3
/Bi O heterojunction under visible
light irradiation. DMPO spin-trapping ESR spectra of MBT solutions after
visible light irradiation: (c) 15% MoS
MoS /Bi –H O/DMPO.
2
/Bi
2
O
3
–CH
3
OH/DMPO; and (d) 15%
2
2
O
3
2
structure can remarkably accelerate the charge transfer via intimate
contact between the MoS and Bi , which also increases the
photocatalytic performance of the MoS /Bi heterojunctions.
2
2 3
O
2
O
2 3
ꢀ
scavengers being added. This implies that OH is not the main
radical involved in the degradation of MBT. In order to further
confirm the emergence of the abovementioned active species,
the ESR technique was used to explain the appearance of the
3.7. Intermediates and mineralization ability tests
The intermediates during the degradation of MBT were detected
using high performance liquid chromatography mass spectro-
metry (HPLC-MS) experiments. From Fig. 10a–c, it can be seen
that the peak at m/z = 167 of MBT reduces during the degrada-
tion reaction and had almost disappeared after two hours. All of
the results indicate that MBT was degraded into small mole-
ꢀ
À
ꢀ
57,58
O2 and OH radicals within the photocatalytic processes.
ꢀ
À
Fig. 11c exhibits the DMPO– O characteristic peaks of
2
MoS , Bi O and the 15% Z-scheme MoS /Bi O heterojunction
2
2
3
2
2 3
under visible light irradiation. It is worth noting that there are
ꢀ
À
no evident characteristic signals of DMPO– O
2
in Bi
radicals. In contrast
, the 15% MoS /Bi heterojunc-
tion shows the strongest intensity, which indicates more O
2 3
O , which
cules, such as CO
2
2
and H O. Taking into account the deep
ꢀ
À
suggests that pure Bi
2
O
3
cannot form O
2
analysis of the photodegradation process, the possible inter-
mediate products were tested by the change in the measured
mass, which is shown in Fig. 10d. As can be observed in Fig. 10d,
the MBT A (m/z = 167) is fragmented into B (m/z = 187) by the
addition reaction. Then C (m/z = 169) is formed by losing the
to the simple MoS and Bi
2
2
O
3
2
2 3
O
ꢀ
À
2
can be generated in the photocatalytic degradation reaction.
Similarly, it was also observed that pristine Bi O and 15% MoS /
2
3
2
ꢀ
Bi O heterojunction can create DMPO– OH. There are four
2
3
–OH group by the elimination reaction. In the proceeding
ꢀ
characteristic peaks of DMPO– OH (1: 2 : 2 : 1 quartet pattern)
found in the ESR spectra (Fig. 11d). Obviously, the intensity of all
signals for the bare Bi O are largely weaker than those of the
2 3
reaction, D (m/z = 152) is fragmented by losing –NH . After that,
2
D is further resolved to E (m/z = 110) by an addition reaction.
Next, F and G are created by wiping out –SH and by an addition
reaction. In the end, these small intermediate products may be
15% MoS /Bi O
2 2 3
heterojunction, which means that the 15% Z-
ꢀ
scheme MoS /Bi O
2 2 3
heterojunction can create more OH active
2 2
further degraded into H O and CO .
species compared to the single Bi O . In summary, these results
2
3
demonstrate that all three active species play a significant role in
the removal reaction, and the degree of their effect on the MBT
3
.8. Trapping experiments and ESR
To gain a better understanding of the process mechanism of
the Z-scheme MoS /Bi heterojunction, radical trapping tests
were carried out by adding the different trapping agents of
ꢀ
À
+
ꢀ
degradation process is in the order O
2
4 h 4 OH.
2
2 3
O
+
ꢀ
3.9. Possible photocatalytic reaction mechanism
TEOA, IPA and Vc to catch holes (h ), hydroxyl radicals ( OH)
and the superoxide radical ( O
ꢀ
À
2
) respectively. As shown in In order to further comprehend the transfer method of the
Fig. 11a and b, when Vc and TEOA were added to the MBT photo-induced charges, a possible photocatalytic mechanism
solution, the photodegradation rate greatly reduced to 16.3% of the Z-scheme MoS /Bi O heterojunction was proposed.
2
2 3
and 25.7% when compared with the addition of no radical Hypothetically, as shown in Fig. 12, when the MoS /Bi O
2 3
2
ꢀ
À
+
scavengers, indicating that the O2 and h are major active heterojunction is excited by visible light, the electrons move
species in the process of removing MBT. Upon the addition of from the CB of the MoS nanosheets to the CB of the Bi rods.
IPA into the MBT solution, the degradation rate was 50.8%, Meanwhile, the holes will transfer from the VB of the Bi to
which decreased by almost 39% compared with no radical the VB of the MoS . However, in theory, the photo-induced
2
2 3
O
2 3
O
2
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efficiency within 120 min. In addition, the increase in the photo-
catalytic efficiency could be attributed to the combination of
Bi O and MoS2 extending the light absorption and the for-
2
3
mation of Z-scheme MoS /Bi O heterojunction by the tight
2
2 3
face-to-face connection with a good energy band match between
the MoS and Bi . The interfacial interaction between MoS
and Bi can accelerate the separation of the photo-induced
2
2
O
3
2
2 3
O
carriers and amplify the specific surface area. Finally, this work
provides a novel design and preparation method for use with
other high performance Z-scheme heterojunctions for degrading
various industrial and domestic pollutants in water.
Fig. 12 Schematic illustration of the band structure diagram and photo-
induced carrier transfer for the Z-scheme MoS
the degradation of various organic pollutants under visible-light irradiation.
2 2 3
/Bi O heterojunction for
Conflicts of interest
There are no conflicts to declare.
2 3
electrons in the CB of Bi O (0.16 eV vs. NHE) cannot react with
ꢀ
À
ꢀ
2À
O
2
to produce O
in the VB of MoS
2
(O
2
/ O , À0.046 eV vs. NHE) and the holes
À À
ꢀ
ꢀ
Acknowledgements
2
cannot oxidize OH to yield OH (OH / OH,
5
9,60
2
.38 eV vs. NHE),
the above described hypothesis goes
This work was supported by the National Natural Science
Foundation of China (No. 21676115), and the Natural Science
Foundation of Jiangsu Province (No. BK2018088).
against the results of the electron spin resonance (ESR) tech-
nology and active species trapping experiments. Therefore, the
Z-scheme electron–hole pairs transfer methods are illustrated
in Fig. 12. On account of the fact that the Z-scheme MoS /Bi O
2
2 3
heterojunction can be irradiated using visible light to generate
photo-induced electron–hole pairs, at the same time, the elec-
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