Chemical Engineering Journal 424 (2021) 130393
Contents lists available at ScienceDirect
Chemical Engineering Journal
journal homepage: www.elsevier.com/locate/cej
Recent progress in Tungsten disulphide based Photocatalyst for Hydrogen
Production and Environmental Remediation
M. Sridharan, T. Maiyalagan *
Electrochemical Energy Laboratory, Department of Chemistry, SRM Institute of Science and Technology, Kattankulathur, Tamil Nadu, India
A R T I C L E I N F O
A B S T R A C T
Keywords:
Hydrogen production
Dye degradation
Cr(VI) reduction
Semiconductor-based photocatalysis has dramatically increased interest in the field of photocatalysis, because of
its ability to directly utilize solar energy into fuels and for the degradation of various pollutants. However, the
photocatalytic performance of semiconductor-based photocatalys still lower due to the quick recombination
photogenerated electron–hole pairs and low visible light utilization. Therefore, numerous efforts have been made
to solve these complications. Particularly, cocatalysts supported semiconductor have been extensively applied in
designing and developing highly effective composite photocatalysts for hydrogen photocatalytic application.WS2
has attracted enormous attention in photocatalysis due to its unusual properties like enhancing visible lightharvesting, charge transfer dynamics and surface reactions of a photocatalytic system. In this review, we
begin by describing synthesis route, different morphologies and brief sketch properties of WS2. A brief discussion
of the WS2 supported metal oxide, metal sulphide, carbon based materials, silver based materials and bismuth
based materials photocatalysts is then provided. While various plausible photocatalytic mechanisms of
photogenerated-electrons and holes in WS2 composite should be proposed. The applications of WS2 as cocatalyst
in the Photocatalytic hydrogen production, organic contaminant degradation and Cr(VI) removal. This review
may offer motivation for designing and fabricating novel and efficient WS2 based composite photocatalysts for
highly efficient photocatalytic applications.
1. Introduction
to fuel conversion and dye degradation) is created a considerable
attention for energy conversion and environmental pollution control.
Photocatalytic technology has several key benefits over conventional
catalytic reactions having tedious steps, high pressure and temperature,
using transition metal-based catalyst [14–16]. Generally, photocatalysis
is a process where photocatalysts absorb light energy to generate
photoinduced electrons and holes to drive the oxidation and reduction
reactions [17]. A photocatalytic reaction contains three important steps:
i) Absorption of light energy (UV, Visible and infrared region), ii) pho­
togenerated charge carrier separation, iii) to drive the electron and
proton for reduction and oxidation reactions. The overall photocatalytic
efficiency of a photocatalytic system can be measured by the kinetics of
these three important reaction steps [18,19]. In recent years, most of the
researchers focused to design advanced photocatalytic materials for
high light-harvesting capability, reducing the fast recombination of
photo excited electrons and excellent photocatalytic activity [20,21].
However, the efficiency of energy conversion and dye degradation are
unsatisfactory. Therefore, the development of high light-harvesting,
stable and low-cost photocatlyst fabrication is a great challenge in the
Nowadays, a sustainable energy supply and environmental pollution
control lie at the heart of our modern lifestyles thus have become an
impressive role to our mobility, prosperity and daily comfortability
[1–3]. At present energy consumption is supplied by fossil fuels, while
burning fossil fuels to vast emission of volatile organic compound, SOx,
NOx and CO2 into the air, which causes environmental pollution [4–6].
Among the various energy source, solar energy is widely distributed and
renewable energy sources to alternate fossil fuel [7]. About 3,850,000
exajoules (EJ) of solar power irradiating to the earth by every year,
while 1% of the solar power can be properly utilized, it would meet the
energy requirement of humans at the current energy consumption rate
[8]. The maximum utilization of solar power may lead to increase in the
world’s energy security by the development of sustainability, environ­
mental pollution control, and minimize fossil fuel usage [9]. Various
catalytic technologies such as, adsorption [10,11] and microwave
catalysis [12,13] and photocatalysis have been applied for removal of
organic contaminant. Among these technologies, photocatalysis (solar
* Corresponding author.
E-mail addresses: maiyalagan@gmail.com, maiyalat@srmist.edu.in (T. Maiyalagan).
https://doi.org/10.1016/j.cej.2021.130393
Received 3 February 2021; Received in revised form 13 April 2021; Accepted 14 May 2021
Available online 20 May 2021
1385-8947/© 2021 Elsevier B.V. All rights reserved.
M. Sridharan and T. Maiyalagan
Chemical Engineering Journal 424 (2021) 130393
field of photocatalysis [22].
In 1972, Fujishima and Honda reported the photoelectrocalalytic
splitting of water by using the TiO2 electrode as a photoanode and Pt
cathode under UV light [23]. In 1976 Carey et al. discovered the
degradation of organic pollutants on powdered TiO2 photocatalyst [24].
Some metal oxide semiconductor such as TiO2 [25–28], ZnO [29–33],
ZrO2 [34–37]and CeO2 [38–41] can act as photocatalyts for solar energy
conversion and environmental remediation, this wide band gap metal
oxide materials absorb only UV light. In terms of solar spectrum ac­
counts 5% of UV light, 45% of visible light (400–700 nm) and 50% of
Infrared light (above 700 nm), Therefore, the photocatalytic activity of
metal oxide semiconductor under solar light irradiation is not sufficient
[42–44]. Additionally, metal sulphide (CdS [45–47], and ZnInS4
[48–50]), carbon-based materials [51–55] and metal organic frame­
works [56,57] are widely used to utilize more visible light from the solar
energy in the field of photocalysis, but their photocatalytic efficiencies
are quite limited due to its quick recombination of photo-generated
charge carrier. When cocatalyst loaded on the surface semiconductor
photocatalyst surface significantly accelerate the visible light absorption
and reduce the charge recombination rate and enhance the efficiency of
the photocatalytic system [58,59]. These cocatalysts are classified into
two types (i) noble metal-based cocatalyst, (ii) non-noble-metal based
cocatalyst [60–62]. Noble metal cocatalyst such as Ag [63], Au [64,65],
Pt [66–69] and Ru [70–72] are highly desirable to enhance the photo­
catalytic activity of semiconductor-based photoctalyst. However, the
low abundance and high cost of noble metals significantly hindered their
practical applications. Therefore, the development of earth abundant
and low-cost co-catalyst combined semiconductor photocatalyst along
with high visible light absorpion and high stability is still a big challenge
for its practical application.
During the past few years, many transition metal sulphide and metal
oxide has been used as a noble metal-free co-catalyst for semiconductor
photocatalysis, which includes NiS [73–76], Cu2O [77,78], NiCo2O4
[79–81], and WS2 [82]. Among these, WS2 has attracted much attention
due to its excellent optical absorption properties, good stability, low
cost, and environmental friendliness. However, WS2 exhibit excellent
performance under visible light and infra-red radiation due to their good
photogenerated electron-hole pair transportation capacity, boosting the
lifetime of photogenerated carriers [83,84]. Additionly, modifications of
semiconductor photocatalyst by WS2 is much meaningful strategy to
improve the photocatalytic activities of semiconductor based photo­
catalysis, which can reduce the fast recombination of photogenerated
electrons of semiconductor photocatalyst. From Fig. 1, the number of
articles on WS2 catalyst and WS2 photocatalyst has increased
dramatically over the past 8 years, and more than 150 papers has been
published. With respect to catalysis and photocatalysis in WS2, there is
significant improvement in number of publications since 2018. Notably
WS2 serve as a new research direction for the development of a novel
photocatalytic system for energy conversion and environmental
remediation.
Herein, we begin by describing synthesis route, different morphol­
ogies and brief sketch properties of WS2. A brief discussion of the WS2
supported metal oxide, metal sulphide, carbon based materials, silverbased materials and bismuth-based materials photocatalysts is then
provided. While various plausible photocatalytic mechanisms of
photogenerated-electrons and holes in WS2 composite should be pro­
posed. The applications of WS2 cocatalyst in the photocatalytic
hydrogen production, organic contaminant degradation and Cr(VI)
removal. This review may offer motivation for designing and fabricating
novel and efficient WS2 based composite photocatalysts for highly effi­
cient photocatalytic applications.
2. Synthesis of WS2 based composites
The synthesis method plays an essential role in the fabrication of the
photocatalyst. As well known, the photocatalytic efficiency of different
photocatalyst suffers from their morphology, crystallite size, and shape,
which can be controlled by changing their synthesis parameter [85–87].
Therefore, in this section, we reviewed the important strategies for the
synthesis of WS2 and WS2 based composites. The key to the successful
synthesis of WS2 composites is to control the sulfurization by different
reaction condition for preferential morphologies and uniform growth.
The synthesis methods and properties of WS2 and composites are shown
in Table 1. Till to date, a variety of synthetic methods have been
established, which can be categories into (i) hydrothermal method [88].
(ii) solvothermal method and (iii) calcination method
2.1. Hydrothermal method
The hydrothermal method is a very powerful approach to synthesize
various nanoscale materials. Such a method operates on elevated tem­
peratures and water as a solvent in confined volume to create high
pressure. The product crystal growth is widely depends on the solubility
of the precursor [89,90]. By carefully analysing the previous reports on
the synthesis of WS2 and WS2 based composite material by the hydro­
thermal route, we can understand that the hydrothermal approach could
be used to prepared WS2 nanomaterials with various morphologies,
nanowire, nanoflower and nanosphere. Cao et al., prepared WS2 with
different morphologies via hydrothermal route, by changing the amount
of CTAB. This result showed that many irregular nanosheets are formed
in the reaction time for 4 hrs. The reaction time extended to 24 h, a
number of nanosheets assembled to provide flowerlike morphology.
Additionally, CTAB molecules also help to assemble the sheet like
morphology into 3D flowerlike morphology. Furthermore, the
morphology of the prepared WS2 can be controlled by the primary fac­
tors, reaction temperature, and reaction duration. This results demon­
strated that changing the experimental conditions has a crucial role in
the morphology of WS2 [91]. Hydrothermal growth of WS2 based
composites preparation classified into two types; i) In situ hydrothermal
growth ii) solution based mixing method.
2.1.1. In-situ hydrothermal growth
In-situ the strategy involves the growth of WS2 on other semi­
conductors under hydrothermal condition, which can help to the uni­
form growth of WS2. Wu et al., have revealed that the uniform coating of
layered WS2 (~4 layers) on TiO2 nanosheets by using sodium tungstate
and L- cysteine as a precursor. During the hydrothermal reaction, the W
= O bonds connected with plate-to-plate stacked structured TiO2
nanosheets. The hydrothermal synthesis bare WS2 exist microsphere
structure and it has petal-like features, which can be achieved by WS2
Fig. 1. Number of scientific articles published on WS2 based material over the
past decade ((source: http://www.Sciencedirect.com; Search term: “WS2 cata­
lyst” and “WS2 photocatalyst”).
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M. Sridharan and T. Maiyalagan
Chemical Engineering Journal 424 (2021) 130393
Table 1
Synthesis method and properties of WS2.
S.No
Catalyst
WS2 Synthesis method
Morphology of WS2
1
2
3
4
5
6
7
8
9
10
11
12
13
14
WS2/CdS
WS2/WO3
WS2/g-C3N4
WS2/g-C3N4
WS2/g-C3N4
WS2/In2.77S4
WS2/MoS2
WS2/AgI
WS2
WS2
WS2
WS2
WS2
WS2
Hydrothermal
Solvothermal
Calcination method
Calcination method
Calcination method
Calcination method
Calcination method
Calcination method
Hydrothermal
Solvothermal
Hydrothermal
Hydrothermal
Hydrothermal
Hydrothermal
nanosheet
3D flower
nanoparticles
nanosheet
sandwich
nanosheet
nanoparticles
nanosheet
flower-like nanosphere
flower like nanorod
mesoporous
hexagonal platelets
nanosheet
nanorod
nanosheets are combined together. When the hydrothermal growth of
WS2 on TiO2 nanosheets, the TiO2 surface changed to rough. The loading
amount of WS2 can be controlled by changing the ratio of TiO2 and W
source concentration [92].
Surface area (m2/g)
25
40.64
99.8
197
94.6
7.5
10.8
Band gap (eV)
1.35
1.44
1.66
1.7
1.6
1.35
1.44
1.91
1.99
1.92
References
93
97
98
99
100
102
103
104
91
115
114
113
116
116
2.2. Solvothermal method
The solvothermal method was established from the hydrothermal
method, the solvent usage is one of the main differences between the
two methods. While hydrothermal method using water as a solvent but
the solvothermal method is organic solvents are used to disperse the
reaction precursor, this organic solvent act as a reducing agent and also
inhibit the aggregation of nanoparticle, due to their high viscosity [95].
Xiao et al., prepared the WS2 quantum dots by sonication solvothermal
method using bulk WS2, and then the hydrothermally prepared BiOCl
dispersion was mixed with the WS2 quantum dots in ethanol solvent, the
obtained mixture stirring 24 h at room temperature to form the WS2/
BiOCl composite (Fig. 2(b)). Similarly, WS2 was also synthesized by a
sonicatication solvothermal method using bulk WS2 as a precursor and
DMF is solvent. Then obtained WS2 coupled with BiOCl through the onepot hydrothermal method, during this one-step reaction, the Bi3+ was
hydrolyzed to produce the BiO+ ions. Then, layered structured
[Bi2O2]2+ obtained through BiO+ and Cl- was inserted into the layered
structure. Simultaneously, the negative charged WS2 QDs were inter­
acted by the positive charged BiO+ ions, resulting in a uniform and tight
distribution of WS2 quantum dots on BiOCl [96]. Guiping et al., reported
2.1.2. Solution based mixing method
Almost all the solution mixing methods follows the two-step process.
WS2 nanocrystals are usually prepared by using a hydrothermal
approach and then using other methods to combine WS2 cocatalyst with
other semiconductors to form composites. Gopannagari et al., prepared
the few-layer WS2-CdS composite. In this composite preparation first
step is the hydrothermal synthesis of few-layer WS2 by using thio­
acetamide and tungsten chloride precursors. Here thioacetamide plays
dual role as a reducing agent and sulphur source. The second step pre­
pared WS2 and CdS dispersed in DMF then exfoliated by ultrasonication
the scheme was predicted in Fig. 2 (a) [93]. J. Zhou et al. reported the
fabrication of WS2 sheets- ZnIn2S4 particles. The sheet like WS2 was first
synthesized, and then these nanosheets were dispersed with water by
using ultrasonication. Next, the WS2 sheets were linked to these ZnIn2S4
via simple hydrothermal method. Finally, the loading amounts of WS2
were controlled by changing the WS2:ZnIn2S4 ratio [94].
Fig. 2. Schematic representation of synthesis of WS2 composites (a)WS2/CdS (Reprinted with permission from Ref [93] copyright from Elsevier), (b) WS2/BiOBr
(Reprinted with permission from Ref [96] copyright from RSC), (c) WS2/WO3 (Reprinted with permission from Ref [97] copyright from MDPI), (d) WS2/g-C3N4
(Reprinted with permission from Ref [100] copyright from Elsevier), (e) β-Bi2O3/ WS2 (Reprinted from Ref [111] with permission from the Chinese Chemical Society
(CCS), Peking University (PKU), and the Royal Society of Chemistry).
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M. Sridharan and T. Maiyalagan
Chemical Engineering Journal 424 (2021) 130393
WS2 coupled with WO3 can be obtained by the solvothermal synthesis of
WS2, N-methyl-2-pyrrolidone was used as the solvent to enhance the
vulcanization of tungstic chloride, and followed by partial oxidation
method introduced to produce WS2/WO3 hybrid material, the schematic
representation of WS2/WO3 are shown in Fig. 2(c) [97].
synthesized the WS2@Bi2O3 heterojunction. The n-type ultrathin WS2
nanosheets exfoliated from a mixture of solvent methanol and water
dispersed bulk WS2 through 48 h sonication (Fig. 2(e)). Then ultrathin
WS2 nanosheets loaded on solvothermal prepared β-Bi2O3 hollow mi­
crospheres through a simple stirring method [111].
2.3. Calcination method
3. Properties of WS2
The calcination method has been widely used to prepare WS2 and its
composite photocatalysts. For example, Yidong et al., reported the
mesoporous graphitic carbon nitride–WS2 composite. They used
(NH4)2WS4 as a precursor and impregnation combined calcination
method to produce the WS2/g-C3N4 composite under the flowing of 10%
H2S–90% H2 gas mixture [98]. Huu et al., reported the synthesis of WS2/
g-C3N4 composite by solid state calcination of tungstic acid and thio­
urea. In this case, thiourea acted as a sulphur source for WS2 and pre­
cursor of g-C3N4. The loading amount of WS2 in the composites
controlled by changing the concentration of thiourea [99]. Dongyao et
al., also fabricated sandwich-structured WS2/g-C3N4 composite by onepot calcination process, during this process WO3 and thiourea as starting
material for WS2 and g-C3N4, then uniformly grinded mixture calcinated
under Ar atmosphere, the synthesis scheme of WS2/g-C3N4 are shown in
Fig. 2(d) [100]. Similarly, Zhou et al., reported WS2/g-C3N4 composite
prepared by one step calcination method using sodium tungstate and
thiourea, during this one-step reaction, changing the concentration of
sodium tungstate to altering the WS2 content [101]. Wu Xiang-Feng et
al., reported the fabrication of WS2/In2.77S4. The Wrinkled WS2 nano­
sheets were synthesized by calcination of WO3 and thiourea under N2
atmosphere, and then these nanosheets linked to In2.77S4 through the insitu hydrothermal method [102]. Li et al., synthesized a composite of
dual-petals nanostructured WS2@MoS2, Firstly, ball milling combined
calcination of WO3 and S to produce WS2, and then these WS2 coupled
with MoS2 by hydrothermal method [103]. Finally, Xieng-Feng et al.,
reported WS2/AgI hybrid. They used calcination of WO3 and thiourea to
produce WS2, and then coupled with AgI by in-situ method [104].
Dongmei et al., designed WS2/TiSi2 composites, in this work ball-milled
TiSi2 and (NH4)2WS4 calcinated under Ar atmosphere to produce WS2/
TiSi2 composites by changing the weight ratio of (NH4)2WS4 and TiSi2 to
control the content of WS2 [105].
WS2 exhibit excellent photocatalytic properties and good visible
light absorption capacity, and it is enormously used as a cocatalyst for
photocatalysis, such as photocatalytic dye degradation and photo­
catalytic hydrogen production. Tungsten disulfide (WS2) has layered
structure, which is analogy to graphite-like structure, this layer consist
of unit S–W–S atomic trilayers. Each layer constructed by W atoms
sandwiched between two sulphur atoms. Bulk WS2 has a layered
structure stacked together by van der Waals force. The W-S covalent
interaction within the layer is stronger than van der Waals interaction
between neighbouring unit, therefore plane sliding is allowed [91]. WS2
nanosheets have large surface area, which helps to enhance the binding
or loading of nanoparticles. Bulk WS2 (which contain less than five
monolayer) are reduced to a single monolayers, which undergoes
indirect-to-direct gap transition [107,108]. Its narrow band gap of 1.5
eV is highly encouraging to absorb visible light. WS2 have excellent
visible light absorption ability compared with metal oxide (TiO2 [25],
ZnO [29], CeO2 [41]), metal sulphide (CdS [45], ZnInS4 [48],) carbon
based materials (g-C3N4 [53]). A detailed understanding of the funda­
mental properties of WS2 and WS2 based composite is quite necessary for
the further improvement of related photocatalytic applications [112].
Unique properties of WS2 are described in the following section,
including the morphology, optical properties, and photo corrosion in­
hibition ability.
3.1. Morphology
The morphology of WS2 has a significant role in their electrical and
optical properties, thus determining their photocatalytic efficiency. In
the following section, the morphological feature of WS2 photocatalyst
was briefly summarized. Scrutinizing the specific morphological rela­
tion between the WS2 photocatalysts and its photocatalytic performance
not only plays an essential role in boosting the performance of WS2
photocatalysts but also helps to induce the potential utilization of WS2
based composite photocatalysts [83]. Many researchers have reported
the photocatalytic behavior of WS2 photocatalysts with specific
morphology. Cao et al., synthesized Flower-like WS2 nanosphere (Fig. 3
(a,b)) via hydrothermal method using CTAB as a surfactant. They also
reported that CTAB was considered to be a crucial role in growing the
flower like morphology. The interaction forces of inside the CTAB helps
to made uniformly grown flower like morphology. The obtained flower
like sphere shows excellent light absorption ability and it may have
potential applications as a visible light active catalyst [91]. Same group
prepared nanospheres (Fig. 3 (c,d)), nanorods (Fig. 3(e,f)) and nanobelts
(Fig. 3 (g,h)) by hydrothermal method using CTAB as a surfactant. The
three different morphologies were obtained by changing the concen­
tration of CTAB. The light absorption properties of the prepared nano­
structure were investigated by UV–Visible spectroscopy analysis, this
result shows all WS2 have blue shift compared to bulk WS2, which in­
dicates the presence of strong quantum confinement effect in the WS2
nanostructure [84]. Vattikuti et al., reported group reported the prepa­
ration of hexagonal WS2 platelets (Fig. 3(i,j)) by hydrothermal method.
The reaction time and temperature influence the uniform morphology of
WS2. Through FESEM results, they found that when the reaction tem­
perature is 150 ◦ C, only hexagonal WS2 platelets were obtained. They
have demonstrated the WS2 platelets results in good photocatalytic ac­
tivity, due to their good stability and open reactive sites [113].
Numerous efforts have been devoted to synthesize a 2-dimensional
metal sulphide semiconductor for its various applications. Moreover,
2.4. Sonication assisted method
Recently, the sonication method has been widely used for the
fabrication of novel nanomaterials with unusual properties. The chem­
ical effects of sonication arise from acoustic cavitation. When, the liq­
uids are irradiated through sonication, the compressive and alternating
expansive acoustic waves generate bubbles and the bubbles are oscil­
lated. The oscillated bubbles can effectively store ultrasonic energy, very
short time implosive collapse of bubbles producing concentrated energy,
using this extreme condition to prepare various nanomaterials [106]. Xu
et al., have employed ultrasound-assisted liquid exfoliation method to
produce ultrathin WS2. The bulkWS2 dispersed in NMP and then soni­
cated (150 W output power) to obtain ultrathin WS2 NSs, The growth of
Cu on the surface of the WS2 through the photochemical method [107].
Yajun et al., also reported the preparation of CdS/WS2/CN composite.
They used Bulk WS2 as a starting materials and 1-methyl-2-pyrrolidine
(NMP) as a dispersion medium. The NMP dispersed bulk WS2 soni­
cated for 5 h resulted in the WS2 sheets and then coupled with g-C3N4
and CdS [108]. Yueyao et al., used a similar strategy and water as a
dispersion medium to prepare the WS2 then coupled with ZnS [109].
Furthermore, some sonication methods can use a mixture of solvent as a
dispersion medium for exfoliated WS2 from bulk WS2. For example Atkin
et al., fabricated WS2/carbon dot composite, the 2D WS2 was first syn­
thesized by grinding sonication of bulk WS2 and mixture of water and
ethanol solution as a dispersion medium and then coupled with carbon
dot to obtain WS2/carbon dot composite [110]. Similarly, Li et al et al.,
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M. Sridharan and T. Maiyalagan
Chemical Engineering Journal 424 (2021) 130393
Fig. 3. (a&b) SEM image of nanoflower (Reprinted with permission from Ref [91]copyright from Springer), (c&d) nanosphere, (e&f) nanowire, (g&h) nanobelt
(Reprinted with permission from Ref [84] copyright from IET), (i&j) hexagonal WS2 platelets (Reprinted with permission from Ref [113] copyright from Elsevier), (k)
SEM image of flower like nanorod and (l) TEM image of flower like nanorod (Reprinted with permission from Ref [115] copyright from Elsevier).
most researchers to date reported that the morphology dependence of 2dimensional nanosheets materials for photocatalytic application. The
nanosheet like structured WS2 can be prepared by using various
methods. Vattikuti et al., reported the synthesis of mesoporous WS2
nanosheets by a hydrothermal approach with cetyltrimethylammonium
bromide (CTAB) used as a surfactant. The mesoporous WS2 nanosheets
exhibit high photocatalytic activity, due to their higher surface area
(197 m2g− 1) [114]. A hydrothermal method was reported by YongChuan et al., for hyacinth flower-like WS2 nanorods synthesis in the
presence of L-cysteine as a sulphur source at 200 ◦ C. The hydrothermally
synthesized flower-like WS2 nanorods (Fig. 3(k,l)) showed a length in
the range of 2.00 μm and diameters 0.42 μm, which exhibit good pho­
tocatalytic performance [115]. Waseem et al., prepared WS2 nanorods
and nanosheets by using hydrothermal technique and thiourea as a
sulphur source. In this study, the authors have come up with an inter­
esting result as they have synthesized nanorods and nanosheets both
under the hydrothermal condition and CTAB as a surfactant. The WS2
nanosheets were formed at pH 6.15 and WS2 nanosheets at pH 7.25. WS2
nanosheet has shown better photocatalytic performance than WS2
nanorod, due to their enhanced pore size [116].
enhanced Visible and NIR light absorption has also been comprehen­
sively studied in other works on the WS2 based photocatalysts. For
example, Xiang-Feng and co-workers designed that the fabrication of p-n
heterojunction through the combination of In2.77S4 and WS2, Moreover
WS2 enhance the visible and NIR light absorption of In2.77S4 [102]. In
similar cases, the WS2 quantum dots coupled with Bi2S3 [118], and WS2
nanosheet combined with ZnS. This finding shows a sensitive response
to visible light [109]. Zhang et al., designed WS2/CdS composite, bare
CdS shows the absorption edge at 520 nm (Fig. 4 (a)). But WS2 nano­
sheets combined CdS exhibit remarkable light absorption at a higher
wavelength than 520 nm. Fig. 4 (b) shows the after mixing of WS2 and
CdS to minimize the emission intensity of pure CdS [119].
3.3. Photocorrosion stability
To date, the researcher focused on improves the photocorrosion in­
hibition of semiconductors for photocatalytic application. However, the
photocatalytic system affects from photocorrosion under illumination,
photogenerated electron and hole involve the decomposition of a pho­
tocatalyst, which result in the photocatalytic activity steadily decreased
with increases the irradiation time. Basically, the photocorrosion of
semiconductor photocatalys depends on the arrangement of conduction
band minimum relates to reduction potential or valance band maximum
relates to oxidation potential [120]. When, photocatalyst with a valance
band maximum lower than oxidation potential (O2/H2O) will deterio­
rate from oxidation by unconsumed holes (for example. ZnS). Likewise,
the photocatalyst with conduction band minimum higher than reduction
potential (H+/H2) will affect from reduction by unconsumed electrons
(CdS) [121]. Zhong et al., designed broad spectrum responded WS2/ZnS
photocatalyst for hydrogen production. The lower photocatalytic
hydrogen production of bare ZnS is lower, due to their higher band gap
(3.8 eV), the fast recombination of photogenerated charge carriers, and
the photo corrosion occurred by photogenerated holes. The band gap
value and valence band edge of WS2 is 1.18 eV and 1.76 eV respectively.
Thus, WS2/ZnS composite, CB position of WS2 is lower than that of ZnS.
The VB of ZnS is more positive than WS2. Therefore, WS2/ZnS composite
follow type-I heterojunction mechanism, which means that, the photo­
genrated electron transfer from ZnS CB to photocatalytic hydrogen
production active sites on WS2 surface. The photogenrated holes transfer
from ZnS VB to WS2 VB. Then, the WS2 VB having photogenerated holes
are depleted by sacrificial agent, and protecting the ZnS nanoparticle
corrosion [109].
Zhong et al., introduced the CdS/WS2 heterostructure for H2 pro­
duction. Under illumination, CdS dissolved to Cd2+ and S2-, due to the
photocorrosion. When increase the illumination time with Cd2+ and S2concentration also increased in the solvent. Under irradiation, CdS/WS2
3.2. Optical absorption property
Light absorption is the primary step in the photocatalytic reaction,
measuring the efficiency of a photocatalyst by absorbing light energy to
create efficient electron-holepairs for the catalytic reaction. The ab­
sorption of visible and NIR regions in the solar spectrum is playing an
essential role in photocatalysis. Different types of metal oxide, metal
sulphide, and metal carbide have been used as a photocatalyst for
environmental pollution remediation. Unfortunately, most of the semi­
conductor is absorbing only UV light, and these materials not effective
under visible and NIR region. More recently WS2 is widely used as a
UV–Visible and NIR absorbing materials for solar cell and photocatalytic
applications [117]. To understand the effect of WS2 deposited semi­
conductor on light-harvesting ability, optical band gap, valance band
(VB), and conduction band (CB) edges of the semiconductor. For
example, WS2 nanosheet composited with g-C3N4, the UV–Visible
diffuse reflectance spectroscopy was used to demonstrate that WS2 helps
to enhance the UV–Visible and NIR absorption ability of bare g-C3N4.
Accordingly, UV–vis DRS spectra and Tauc’s plot employed to measure
the optical band gap of semiconductor photocatalyst [99]. Moreover,
the flat band potential is helpful to estimate the electronic band struc­
tures of photocatalyst, which can be measured through the MottSchottky plot. Basically, the plot band potential of P-type semi­
conductor is located to their VB edge and n-type semiconductor plot
band potential value is located to their CB edge. In fact, such an
5
M. Sridharan and T. Maiyalagan
Chemical Engineering Journal 424 (2021) 130393
Fig. 4. (a) UV–Visible spectra, (b) Photoluminescence spectra of CdS/WS2 (Reprinted with permission from Ref [119] copyright from ACS), (c) The amount of Cd2+
and S2- decomposed from CdS, (d) The amount of Cd2+ and S2- decomposed from CdS/WS2, (e) Schematic representation of photo corrosion effect of CdS and
recrystallization of CdS on WS2 nanosheet (Reprinted with permission from Ref [122] copyright from Elsevier).
having large size CdS dissolved to Cd2+ and S2- and then S2- easily
connects with the sulphur vacancies of WS2. According to the ratetheory analysis of surface roughness, the CdS recrystallization rate is
lower. It produces smaller size and uniformly distributed CdS nano­
particles on WS2 surface. After 100 h recycling reaction dissolution and
recrystallization reaches equilibrium and the H2 production rate remain
unchanged. This results in accordance with Inductively Coupled Plasmaatomic emission spectroscopy (ICP-AES) analysis result, the concentra­
tion of W4+, Cd2+ and S2- are 15.2, 51.52 and 12.66 ppm (Fig. 4(c,d)) in
solution respectively, after 90 h recycling reaction. CdS/WS2 having
Cd2+ and S2- concentration is lower than pure CdS concentration in
solution. This results demonstrate that photocorrosion- recrystallization
led to enhance the photocatalytic activity. The recrystallization process
and photocorrosion effect of CdS/WS2 composites are schematically
shown in Fig. 4(e) [122].
4. Basic principles of photocatalytic activity
In semiconductor photocatalysis, Initiate or accelerate the reduction
and oxidation reaction by the photocatalyst under illumination.
Generally the photocatalytic mechanism involves the following steps
(Fig. 5), Firstly generating photoelectrons from the excitation of valance
band to conduction band, holes are generated in the valance band.
Secondly, photogenrated electrons and holes move to the surface.
Thirdly, The CB electrons have +0.5 to 1.5 V chemical reduction po­
tential versus normal hydrogen electrode (NHE) and reveal strong
reduction ability, VB holes having strong oxidative potential. Both
photogenerated electrons and holes can acts as a oxidant and reductant
6
Chemical Engineering Journal 424 (2021) 130393
M. Sridharan and T. Maiyalagan
electron reduce the surface adsorbed oxygen species to .O–2, subse­
quently, active anion radical degraded the pollutants. The crystal defects
or scavengers are served as a recombination centre for electron and
holes. The crystal defects not only acted as active site and also induce the
recombination. Therefore, good crystalinity with controlled defect is
reducing the recombination and combined with other cocatalyst, which
involves the enrichment in separation of photogenerated electron-hole
pairs and electrical conductivity. Based on the photocatalysis working
principle, the recombination of photogenerated electron and the hole is
adverse effect on the efficiency of a semiconductor based photocatalysis
[20,27,123,124]. For good photocatalytic efficiency, the effective sep­
aration of photogenerated charge carrier and reduce the fast recombi­
nation of photogenerated electrons. To develop the photocatalyst
activity and visible light absorption, the access that has mostly applied
to form semiconductor photocatalyst coupled with cocatalyst.
5. WS2 based composites
Coupling WS2 with other semiconductor materials including, metal
oxide, metal sulphide, carbon-based nanomaterials and Ag, Bi-based
materials can affect the photocatalytic activity in comparison with
pure semiconductor photocatalyst. The, construction of WS2 hybrids and
their nanocomposites is a suitable approach for the future development
of WS2-based photocatalysts. This review describes the WS2 based
hybrid nanostructures which were applied to investigate the photo­
catalytic activity. The photocatalytic dye degradation and hydrogen
production efficiency of WS2 based composites are shown in Tables 2
and 3.
Fig. 5. The schematic representation of cocatalyst supported semiconductor
photocatalytic mechanism.
and its react with semiconductor surface desorbed electron donar and
electron acceptor. The excited state electron and holes can recombine
and deplete the input energy is released in the form of light or heat. The
excited electron involve different reactions for phohocatalytic hydrogen
production, Cr(VI) reduction and organic dye degradation. (i) The
excited electrons involve the reduction of H+ to H2 (For photocatalytic
hydrogen producton), (ii) The excited electron undergo the reduction of
Cr(VI) to Cr(III) (For photocatalytic Cr(VI) reduction), (iii) The excited
Table 2
Summary of WS2 Based Composites for Photocatalytic Dye Degradation.
S.
no
Catalyst
Catalyst loading
(mg)
Contaminant
concentration
Light source
Contaminant
Reaction
duration
Degradation
efficiency
References
1
WS2
100
30 ppm
UV and Sunlight
120 mins
71.20%
83
2
Hexagonal WS2
5
10 ppm
300 mins
97%
113
3
4
Mesoporus WS2
Flower like WS2
100
50
100 mL of 4 ppm
50 mL of 20 ppm
Rhodamine-B
Rhodamine-B
30 mins
270 mins
97%
91%
114
115
5
WS2 nanosheet
50
100 mL of 5 ppm
300 W Xe lamp/Visible
light
150 W UV lamp
500 W Xe lamp/Visible
light
100 W Xe lamp
malachite
green
Rhodamine-B
60 mins
99.83%
116
6
WS2 nanodots/
TiO2
WS2/N-doped
TiO2
WS2/Bi2O3
WS2/MoS2
20
100 mL of 20 ppm
300 W Xe lamp
Methylene
blue
Rhodamine-B
120 mins
86.10%
127
50
100 mL of 20 ppm
500 W Tungsten lamp
Congo red
300 mins
94%
128
40
100
40 mL of 10 ppm
100 mL of 40 ppm
500 W/Visible light
500 W/Visible light
60 mins
120 mins
85%
95%
111
103
WS2Q.Dots/
BiOCl
WS2/g-C3N4
20
100 mL of 20 ppm
20 mins
80.10%
96
100
90 mL of 30 ppm
300 W Xe lamp/Visible
light
100 W/Visible light
ofloxacin
Methylene
blue
Rhodamine-B
6h
85.30%
99
50
0.24
150 mL of 10 ppm
1 ppm
30 mins
10 mins
91.2
12%
104
110
14
15
16
WS2/AgI
2D WS/Carbon
dots
Bi2S3/WS2
WS2/MoS2
WS2/BiOCl
20
25
50
100 mL of 5 ppm
50 mL of 10 ppm
100 mL of 10 ppm
90 mins
40 mins
45 mins
88.40%
93%
98.4%
147
150
160
17
18
WS2/BiOCl
WS2/BiOBr
50
40
100 mL of 10 ppm
40 mL of 10 ppm
Methyl orange
Rhodamine-B
Malachite
green
Cr(VI)
Lanasol red 5B
120 mins
40 mins
94.90%
99%
160
164
19
WS2/BiOBr
40
40 mL of 20 ppm
Lanasol red 5B
60 mins
99%
165
20
WS2/In2.77S4
50
150 mL of 50 ppm
Cr(VI)
60 mins
86.60%
102
21
WS2/ZnIn2S4
100
200 mL of 10 ppm
Cr(VI)
120 mins
93.60%
148
7
8
9
10
11
12
13
300 W/Visible light
150 W Xe lamp/Visible
light
300 W Xe lamp
300 W Xe lamp
500 W Xe lamp
500 W Xe lamp
500 W Xe lamp/visible
light
500 W Xe lamp/visible
light
300 W Xe lamp/visible
light
150 W Xe lamp/visible
light
7
methylene
blue
Rhodamine-B
Congo red
M. Sridharan and T. Maiyalagan
Chemical Engineering Journal 424 (2021) 130393
electron-hole pairs [127].
Similarly, nitrogen doped TiO2 nanosheets were synthesized by hy­
drothermal and calcination method using urea as a nitrogen source.
Elangovan et al., reported N-TiO2/WS2 for photocatalytic dye degrada­
tion. Such composite can effectively enhance the visible light response.
The photocatalytic activities for congo red degradation were carried out
under visible light. Compared with N doped TiO2, N doped TiO2/WS2
executed the photocatalytic performance, which could remove 90% of
Congo Red in 300 min. In this case, the composite was introduced WS2,
resulting in utilization of visible light. The photogenerated electrons
could be effectively separated. Accordingly, the possible photocatalytic
mechanisms of N-TiO2/WS2 were proposed as illustrated in Fig. 6 (a)
The photocatalytic durability of N-TiO2/WS2 are show in Fig. 6 (b)
[128].
Table 3
Summary of WS2 Based Composites for Photocatalytic Hydrogen Production.
S.
no
Catalyst
Catalyst
loading
Light source
H2 production
rate µmol⋅h− 1g−
1
WS2/gC3N4
20
599.7
100
3
WS2/gC3N4
100
154
101
4
WS2/Cu
3
6400
107
5
WS2/ZnS
10
308
109
6
WS2/CdS
3
1410
119
7
WS2/CdS
100
373.41
122
8
WS2/CdS
100
420
138
9
WS2/CdS
20 mg
6110
140
10
WS2/CdS
50 mg
1773
141
11
WS2/gC3N4
50 mg
101
152
12
WS2/gC3N4
10 mg
331
153
13
WS2/gC3N4
25 mg
300 W Xe
lamp/visible
light
300 W Xe
lamp/visible
light
Xe lamp/
visible light
300 W Xe
lamp/visible
light
Xe lamp/
visible light
300 W Xe
lamp/visible
light
350 W Xe
lamp/visible
light
Xe lamp/
visible light
300 W Xe
lamp/visible
light
300 W Xe
lamp/visible
ligh
300 W Xe
lamp/visible
ligh
300 W Xe
lamp/visible
ligh
350.75
154
1
References
5.1.2. WS2/ZrO2
Zirconium oxide is an n-type semiconductor photocatalyst with a
bandgap of 5 eV and it exhibits good photocatalytic activity under UV
irradiation and also the photogenerated charge carriers have high redox
behaviour due to their wide-band gap. ZrO2 has three different types of
crystal structure i) monoclinic ii) tetragonal and iii) Cubic. It has good
thermal and chemical stability, and low cost. ZrO2 photocatalytic ac­
tivity is still limited under visible light irradiation. Thus, ZrO2 is coupled
with cocatalyst in order to improve the photocatalytic performance
[129]. Opoku et al., reported reported tuning the electronic properties
and interfacial interactions of WS2 and ZrO2 (0 0 1). The calculated work
functions of ZrO2(0 0 1) 6.02 eV (Fig. 7 (a)) were smaller than ZrO2
(1 0 1) surface (6.31 eV), Similarly WS2 single layer, WS2 double layer
and WS2 triple layer sheets were 5.40, 5.04, and 4.79 eV (Fig. 7 (b-d)).
Monolayer and few–layer WS2 sheets work function was lower than the
(0 0 1) surface, the potential difference between the two phases are very
large. Thus electron transfer from 2D layered WS2 to the ZrO2 (0 0 1)
surface and Fermi level of the two monolayers aligned. ZrO2 electronrich environment would be negatively charged, and WS2 would be
positively charged. The work function of WS2 single layer/ZrO2 (0 0 1),
WS2 double layer/ZrO2 (0 0 1) and WS2 triple-layer/ZrO2 (0 0 1) reduced
due to transfer and separation charge carrier creates on surface dipole
pointing towards the ZrO2. In Fig. 7 (e-g) shows the number of layer in
the WS2 sheets increase with gradually reduced in the work function of
the WS2/ ZrO2 (0 0 1) hetero structures and this signifies the movement
of electron WS2 to ZrO2 (0 0 1) [130]. Vattukutti et al., reported WS2/
ZrO2 hybrid for photocatalytic activity. The WS2/ZrO2 hybrid catalysts
exhibited significantly higher H2 production activities and CV degra­
dation performances than pure WS2 and ZrO2 photocatalyst under
simulated solar light and UV irradiation, which is due to intimate in­
teractions between the ZrO2 NPs (0D), and layered (2D) WS2 nanosheets
and also enhance UV vis light-absorption capacity. The WS2/ZrO2
hybrid H2 production was 7311.44 µmol h− 1 g− 1, which was 8.3 and
5.28 times higher than those for ZrO2 (872.32 µmol h− 1 g− 1) and WS2
(1383.61 µmol h− 1 g− 1) respectively, additionally the hybrid catalyst
exhibited excellent photocatalytic and chemical stabilities [131].
5.1. Metal oxide-WS2 composites
During the past few years, many researchers have reported various
types of heterostructured composites of WS2 with metal oxides showing
improved efficiencies relative to their bare materials. The earthabundant and non-noble metal oxides such as TiO2, ZrO2, WO3 and
Fe3O4 were the most widely used photocatalyst and it has been
employed as a promising candidate for constructing WS2 based
composite.
5.1.1. WS2/TiO2
Several studies on the fabrication of TiO2/WS2 composite have been
reported to induce photocatalytic activities. Jing et al., fabricate nano­
sized WS2 coupled mesoporous TiO2 to enhance the visible light
adsorption of bare TiO2. It was observed that the nanosized WS2 loaded
on TiO2 resulted in fluorescence quenching of the TiO2 emission. An
obvious enhancement of hydrogen production rate 2.13 μmol g− 1h− 1
was also observed [125]. Similarly, Zheng et al., reported TiO2 nano­
sheets integrated with layered WS2 by hydrothermal reaction. The asprepared TiO2/WS2 composites exhibited excellent photocatalytic
Methyl orange degradation activity than TiO2 and WS2, due to the well
visible light absorption and minimized the charge carrier recombination
[126]. Wu et al., reported Layered WS2/TiO2 nanocomposites synthe­
sized via the hydrothermal method, As synthesized novel Layered WS2/
TiO2 nanocomposites exhibited high photocatalytic performance for the
degradation of RhB under visible light irradiation as compared to bare
WS2 and TiO2 nanosheets [92]. Wu et al., synthesized WS2/TiO2 for
photocatalytic activity. The WS2 nanodots were loaded on the inner wall
of TiO2 nanotubes. Under light irradiation for 120 min achieved
degradation rate of 86.1% by 10% WS2 nanodots loaded different TiO2,
which is mainly due to the synergistic effect between WS2 and TiO2, high
specific surface area, the low recombination rate of the photogenerated
5.1.3. WS2/WO3
Tungsten oxide is an important material in photocatalytic applica­
tions. WO3 can absorb visible light irradiation and thus can be widely
used as a visible-light-driven photocatalys. Many research teams pre­
pared Tungsten oxide composite by various methods for photocatalytic
applications. Because Tungsten oxide composite reduces the fast pho­
togenerated charge carrier recombination and enhances the potocata­
lytic activity of WO3. Paola et al., fabricated WS2/WO3 composites
fabricated via oxidation of WS2 and sulfidation of WO3. This WS2/WO3
composite effectively degrades the organic pollutant. The degradation
efficiency of the WS2/WO3 composite was higher than bare WS2 and
WO3 [132].
Huo et al., prepared WS2 nanosheets by thiourea reduction method
using vacuum tube furnace, and then WS2 nanosheets were surface8
M. Sridharan and T. Maiyalagan
Chemical Engineering Journal 424 (2021) 130393
Fig. 6. (a) Proposed photocatalytic mechanism of N-TiO2/WS2, (b) Photocatalytic durability of N-TiO2/WS2 (Reprinted with permission from Ref [128] copyright
from ASP).
Fig. 7. (a) The calculated electrostatic potential of ZrO2 (0 0 1), (b) Monolayer WS2, (c) double-layer WS2, (d) Three layer WS2, (e) WS2-single layer/ZrO2, (f) WS2doublr layer/ZrO2 and (g) WS2-three layer/ZrO2 (Reprinted with permission from Ref [130] copyright from Elsevier).
functionalized with tetragonal WO3 nanoparticles. This type-II struc­
ture, which favours the transfer and separation of photogenerated
electron-hole pairs, resulting good photocatalytic activity [133].
Sumantha et al., fabricated WS2/WO3 via electrodeposition and air
annealing. Under illumination, The photogenerated WS2 CB electrons
transferred to the WO2 conduction band and holes in the valance band of
WO3 transfer to WS2 VB. The photogenerated electron reacts with O2 to
O2− , photogenerated hole reacts with OH− to form OH*. Then reactive
species react with organic contaminant to produce small molecules. The
photodegradation of phenol under different conditions are shown in
Fig. 8 (a) and photocatalytic mechanism are illustrated in Fig. 8 (b)
[134].
5.1.4. WS2/Fe3O4
Fe3O4 is a p-type semiconductor and it is non-toxic and low cost. Bare
Fe3O4 photocatalyst possesses quite smaller photocatalytic activity.
Fe3O4 combines with other semiconductors to form composites, It
favorable for separation of charge carrier and recyclability. The KIT-6/
9
M. Sridharan and T. Maiyalagan
Chemical Engineering Journal 424 (2021) 130393
Fig. 8. (a) Photodegradation of phenol under different conditions, and (b) schematic representation for photocatalytic mechanism of WS2/WO3 (Reprinted with
permission from Ref [134] copyright from Elsevier).
WS2-Fe3O4 nanocomposite was prepared by in situ growth of WS2
nanoparticles on the pores of mesoporous KIT-6 material through the
hydrothermal method. This KIT-6/WS2-Fe3O4 nanocomposite shows
well a photodegradation efficiency of chlorpyrifos under visible light.
The higher photocatalytic activity was achieved to its high specific area
and low recombination rate due to the synergic effect between WS2,
Fe3O4 and KIT-6 [135].
Based on the previous studies, it can be concluded that incorporation
of WS2 to metal oxides mainly take advantage of the enhanced visible
light absorption. Compared to bare metal oxides, WS2 incorporated
metal oxides shows excellent photocatalytic performance due to the
effective transportation and separation of photogenerated electrons.
This WS2 incorporation into metal oxides is highly appreciable for
fabrication of various visible light active photocatalyst.
surface to be favorable to for enhance hydrogen generation and reduce
the photocorrosion effect [122]. Zong et al., prepared WS2 loaded CdS
that showed increased activity under light irradiation. The 1.0 wt% of
WS2 loaded structure demonstrated enhanced visible light harvesting
and hydrogen production. Which is 28 times higher than bare CdS
[138]. Su et al., used hemispherical shell-thin lamellar WS2 porous
structures hybridized CdS and demonstrated enhanced visible light ab­
sorption due to the highsurface area and good electron transport ability,
resulting in improved photo-conversion and a better hydrogen produc­
tion [139]. He et al., synthesized CdS nanorods coupled with WS2
nanosheets by solvothermal/exfoliation method; it showed enhanced
hydrogen production of 61.1 mmol/g/h.
The possible photocatalytic mechanism of CdS nanorods coupled
with WS2 nanosheets in visible light irraditionare shown Fig. 9 (a,b)
[140]. Danyun et al., obtained exfoliated WS2 nanosheeets/CdS com­
posites (Fig. 9 (c)) and compared their hydrogen evolution rate and
stability with those of bare CdS. It was originate that the exfoliation of
WS2 crystal and facilitated visible light absorption due to the strong
interaction of the WS2 and CdS, which eventually led to reduce recom­
bination of the photoexcited charge carriers, WS2/CdS shows higher
hydrogen production rate compared with CdS and WS2 (Fig. 9 (d))
[141]. Zhang et al., reported that a (2D)-2D heterostructured CdS/WS2
photocatalyst liberated 8-fold more H2 production than CdS; this was
mainly because of the efficient separation of photogenerated charge
carrier due to large contact region. the photoexcited elctrons from CdS
CB transferred to CB of WS2 because its CB potential is higher than that
of CdS also the 2D WS2 acted as a good electron collector [119]. Vat­
tukutti et al., prepared CdS anchored porous WS2 hybrid show the ab­
sorption of Visible light. This unique porous WS2 hybrid design
displayed enhanced photocatalytic activity, which was attributed to the
facile electron transfer path from CdS to WS2 [142]. Velpandian et al.,
synthesized CdS@WS2 heterostructure. It was found that the excitance
of WS2 enhanced the visible light absorption and reduce charge
recombination of CdS. This composite shows enhanced Cr(VI) reduction
and RhB degradation compared with bare CdS [143].
5.2. WS2/metal sulphide composites
Very recently, many researchers have designed and reported various
types metal sulphides combined with WS2 showing improved effi­
ciencies compared to their bare materials. The earth-abundant metal
sulphides such as CdS, ZnS, ZnInS4 and MoS2 were the most widely used
photocatalyst and it has been employed as suitable materials for
developing WS2 based composite.
5.2.1. WS2/CdS
Cadmium sulphide as a kind of II-VI transition metal dichalcogenide
has extensively studied photocatalyst in the past few years. It has been
devoted more attention because of its unique properties like high elec­
tronic mobility and excellent charge transport properties. Based on the
advantages, CdS have been widely used as photocatalyst on the removal
of organic contaminants. WS2 incorporating with other semiconductor
material, which can enhance the light absorption capacity of the pho­
tocatalysts and also enhance the photogenerated electrons transfer and
separation [136,137]. In some recent work, a new approach to synthe­
size CdS/WS2 nanocomposites was developed, and the relationship be­
tween the WS2 and the respective visible light photoactivity was studied.
Gopannagari et al., investigated the effect of WS2 on the optical prop­
erties and photocatalytic activity of CdS, The optimized WS2 loaded
photocatalyst shows the rate of H2 production of 185.79 mmol h− 1 g− 1
under simulated solar light irradiation, which is 33 times higher than
that of pristine CdS [93].
Zhong et al.,obtained the best results for a nanocomposite containing
40% WS2 with CdS to show enhanced hydrogen production (373.41
mmol g-1h− 1) compared with WS2 and CdS photocatalysts. Under visible
light irradiation the photocorrosion and recrystallization of CdS on WS2
5.2.2. WS2/ZnS
ZnS is an important II-VI group semiconductor and it has been
extensively researched due to its excellent physicochemical and optical
properties. It exhibits in two forms i) sphalerite ii) wurtzite and their
calculated bandgap 3.72 eV and 3.77 eV respectively. Up to the present
ZnS was applied in many research areas due to high energy conversion
efficiency. Under illumination, ZnS mostly working in the UV region for
photo-generated electron-hole pair separation also decomposed into the
component in the absence of sacrificial electron donor [14,144,145]. In
10
M. Sridharan and T. Maiyalagan
Chemical Engineering Journal 424 (2021) 130393
Fig. 9. (a) Schematic representation of the photcatalytic mechanism of CdS/WS2 in lactic acid solution (Reprinted with permission from Ref [140] copyright from
ACS), (b) schematic illustration of visible light induced photocatalytic mechanism of CdS/WS2 [140], (c) Schematic representation of the 1–2 layered 2H-WS2/CdS
and its photocatalytic hydrogen production mechanism [141] and (d) The photocatalytic hydrogen production rate of CdS/WS2 under visible and simulated sunlight
(Reprinted with permission from Ref [141] copyright from ACS).
order to solve this concern, ZnS was successfully synthesized in com­
posite fabrication features. Zhong et al., reported WS2/ZnS hetero­
structures show good photocatalytic performance. By combining ZnS
with WS2 sheets, the visible light absorption property of ZnS was
enhanced and reduces the quick recombination of photoinduced
electron-hole pairs. The 0.5 wt% WS2 NSs loaded ZnS shows highest H2
evolution rate of 308 μmol h− 1 g− 1 (Fig. 10 (a)). This efficiency is 3 times
higher than pure ZnS photocatalyst and also shows good catalytic
activity under 50 h irradiation (Fig. 10 (b)) [109].
5.2.3. WS2/Bi2S3
Bismuth sulfide is a type of lamellar structure semiconductor and
their bandgap is 1.3–1.6 eV [146]. Nanostructures of Bismuth sulfide
have numerous attentions in photocatalysis due to their visible light
absorption capacity. However, the photocatalytic activity of bare Bi2S3
remains limited because the material shows lower efficiency and
Fig. 10. (a) Rate of photocatalytic H2 evolution on ZnS loading with different amount of WS H2, (b) Cyclic runs for the photocatalytic H2 evolution WS2/ZnS
(Reprinted from Ref [109] with permission from the Centre National de la Recherche Scientifique (CNRS) and the Royal Society of Chemistry).
11
M. Sridharan and T. Maiyalagan
Chemical Engineering Journal 424 (2021) 130393
stability, showing quick recombination of photogenerated charge­
carrier. Vattikuti et al reported Bi2S3 nanorod/2D exfoliated WS2
nanosheet heterojunction for photocatalytic activity. as prepared com­
posites exhibits1.42 and 1.19 times higher photocatalytic activity than
pure and Bi2S3 and WS2. The enhanced performance can be attributed to
1D/2D porous structure, large specific surface area, enhanced charge
separation, and transfer efficiency, reduced photoelectron–hole recom­
bination [147].
and low cost [149]. Li et al., prepared WS2@MoS2 composites by 2step an approach involving ball-milling, annealing, and hydrothermal
method, this composite provided a higher efficiency than pure WS2 and
MoS2. Under illumination, it follows the type II heterojunction electron
transfer mechanism. Photogenerated electron could be injected into the
conduction band of MoS2 from that of WS2 [117]. Similarly, Luo et al.,
coupled WS2 with MoS2 to increase the visible light absorption capacity
of the photocatalyst. After being illuminated, MoS2/WS2 shows the
highest photodegradation rate constant, which is almost five times
higher than pure MoS2 [150].
To sum up, introducing WS2 to metal sulphides (CdS, ZnS, Bi2S3,
ZnInS4 and MoS2) can form composites, which can enhance the sepa­
ration and transfer the photogenerated electrons. In addition, the WS2
incorporation effectively inhibits the photocorrosion effect of metal
sulphides. The composite fabrication of WS2 with metal sulphides may
be a hopeful strategy for potential application of Photocatalytic dye
degradation, Cr(VI) reduction and hydrogen production
5.2.4. WS2/ZnInS4
ZnIn2S4 is an emerging celebrity ternary sulfide semiconductor
photocatalyst recent years, such as non-toxicity, good chemical stability,
and good chemical crystallinity. The photocatalytic performance of
ZnIn2S4 itself is low; therefore, ZnIn2S-4based composite fabrication have
received more attention [67]. Pudkon et al., synthesized hetero­
structures through hydrothermal method the composite of WS2 loaded
on ZnIn2S4 achieved the highest photocatalytic performance Different
concentration of WS2 loaded ZnIn2S4 photocatalytic hydrogen produc­
tion efficiency is shown in Fig. 11 (a) which is higher than bare ZnIn2S4
and WS2.
In this work, ZnIn2S4/WS2 composites shows good photocatalytic
performance under UV–visible irradiation compared with visible irra­
diation (Fig. 11 (b)). moreover photocatalytic Cr(VI) reduction effi­
ciency of WS2, ZnIn2S4, and WS2/ZnIn2S4 composites are 4.2%, 78.4%
and 93.6% respectively [148] Zhou et al., synthesized WS2/ZnIn2S4
composites by hydrothermal method. The obtained WS2/ZnIn2S4 com­
posites resulted in much higher photocatalytic performance for
hydrogen evolution than bare ZnIn2S4, and the photocatalytic perfor­
mance of ZnIn2S4 was significantly affected by the cocatalyst WS2
loading amount. 3% of WS2 cocatalyst loaded ZnIn2S4 composite ach­
ieved the best hydrogen production rate of 199.1 mmol/h/g, which was
higher than conventional Pt/ZnIn2S4 photocatalysts. The enhanced
photocatalytic performance of WS2/ZnIn2S4 composites could be fav­
oured to the efficient photogenerated electron-hole pair’s migration and
separation [94].
5.3. WS2/Carbon based materials
During the past few years, carbon materials have attracted intensive
attention to enhance photocatalytic activity. Carbon materials such as
carbon quantum dots and g-C3N4 enhancing the performance of metalfree semiconductor based photocatalysis.
5.3.1. WS2/g-C3N4
Graphitic carbon nitride (g-C3N4) has much attention as metal-free
polymer n-type semiconductor photocatalysis because of its intrinsic
features, high activity and unique 2D layered structure, non-toxicity and
efficient visible-light absorption. It accommodates only the earthabundant carbon and nitrogen, good chemical and thermal stability
due to the conjugated layer structure containing a strong covalent bond
between carbon and nitrogen atoms. g-C3N4 bandgap value 2.7 eV (4 6 0)
nm, which is helpful to harvest the visible light as well as conduction
band and valance band edge position suitable for both water reduction
and oxidation. Moreover, the photoactivity performance of bare g-C3N4
is low due to its high photogenerated charge carrier’s recombination.
Various methods have been used to overcome these problems such as
doping with metals and non-metals, composites formation with some
other metal oxides and metal sulphides [54,151].
Hou et al., employed WS2 as the co-photocatalyst hybridizing WS2/
mesoporous g-C3N4 composite by impregnation–sulfidation approach.
When controlling the content of WS2 as 0.3 wt%, the composite showed
the greatest photocatalytic hydrogen production. In the photocatalytic
reaction, WS2 obviously encouraged charge separation and transfer,
5.2.5. WS2/MoS2
Molybdenum disulfide (MoS2) is a layered structured transition
metal dichalcogenide. It has enormous attention for various fields of
dye-sensitized solar cells, photocatalytic hydrogen production and
degradation of organic and inorganic contaminants, Because of its
structural similarities to graphene. MoS2 shows high charge carrier
transport and high wear resistance, those physical properties similar to
that of graphene. Moreover, MoS2 has excellent properties over gra­
phene such as good visible light absorption capacity, earth-abundant
Fig. 11. (a) Rate of photocatalytic H2 production on ZnInS4 loading with different amount of WS2, (b) photocatalytic H2 production rate of ZnInS4, ZnInS4/WS2
under UV–visible and visible light irradiation (Reprinted with permission from Ref [148] copyright from RSC).
12
M. Sridharan and T. Maiyalagan
Chemical Engineering Journal 424 (2021) 130393
thus inhibiting the photoelectron-hole pair’s recombination [98]. Lin
et al., used one pot calcination method to prepare sandwich-structured gC3N4/WS2 composite, When 7.49 wt% of W having WS2 was deposited
on g-C3N4, the composite exhibited the excellent photocatalytic activity
(599.7 μmol h-1g− 1), which is around 25 times higher than that of pure
g-C3N4 [100].
Maxwell et al., used a gas–solid reaction to synthesize WS2/g-C3N4
composites. Different ratios of WS2 were growth on g-C3N4 to form the
heterojunction and the composite revealed enhanced photocatalytic H2
production under visible light irradiation. The 0.01 wt% WS2 loaded
composite shows the highest H2-production rate of 101 μmol g− 1h− 1,
which is higher than Pt deposited g-C3N4. The enhanced photocatalytic
H2 production was attributed to the heterojunction formation between
g-C3N4 and WS2 cocatalyst [152]. Jianjian et al., synthesized 1T- WS2
and then coupled with g-C3N4 and used it as a photocatalyst for the H2
production reaction. 1T-WS2 play a dual role in photocatalytic reaction
(i) The better electrical conductivity of 1T-WS2 helps to transfer the
photogenerated electron. (ii) 1T- WS2 can provide more active sites for
hydrogen production reactions on the basal plane (Fig. 12 (a)), more­
over, it helps to reduce electron travel length and recombination of
photogenerated holes. High hydrogen production rate of 331.09 µmolg1 − 1
h was achieved by WS2/g-C3N4, which is 43.3 times higher than bare
g-C3N4 [153]. Zhou et al., prepared WS2/g-C3N4 photocatalysts through
one-pot synthesis. The 0.3 wt% WS2 loaded g-C3N4 achieved the highest
photocatalytic H2 production rate of 154 mmol h-1g− 1. Which is higher
than 0.3 wt% Pt loaded g-C3N4 and 34 times higher than Pure g-C3N4
[101]. Huang et al., reported the colloidal synthesis of 1T/2H-WS2
nanoflakes growth on 2D g-C3N4. This composite shows that the pho­
tocatalytic H2 production 350.75 µmolg-1h− 1, Which is higher than 1T/
2H- WS2 and 2D g-C3N4 as well as other WS2/g-C3N4 composites without
Pt. The photocatalytic hydrogen production mechanism of WS2/g-C3N4
is shown in Fig. 12 (b). This composite could be expressively beneficial
for the separation of the electron-hole pairs, prevent the charge
recombination [154]. Similarly, Tran et al., reported WS2/g-C3N4 com­
posites for photocatalytic degradation of methylene blue. The concen­
tration of WS2 to g-C3N4 affects photocatalytic degradation efficiency of
the composites. 1:7 wt ratio WS2 to g-C3N4 shows the highest photo­
catalytic activity compared with bare g-C3N4 and WS2 [99].
solubility in water, and easy surface modification and surface func­
tionalization [155,156]. Nowadays, numerous effort has been devoted
to CDs coupled with other material and employed for both photo­
catalytic and electrocatalytic applications. Atkin et al., designed 2D
tungsten disulfide nanoflakes hybridized with carbon dots for photo­
catalytic applications. The CDs strongly adhered to the WS2 basal plane
through van der Waals attraction forces. The composite material was
then shown higher photocatalytic Congo Red degradation efficiency.
The assessment of the electronic band structures of CDs, 2D WS2 and
Congo Red was suggested that the improvement is due to induced the
affinity of Congo Red onto the surface of the WS2 flakes rather than a 2D
WS2/CD [110].
In summary, WS2 can be introduced into Carbon based materials to
form WS2/Carbon based composites. This WS2 incorporation into these
composite photocatalyst can induce them with unique properties of WS2
possibly produce new properties, such as extended visible light ab­
sorption, and effective separation of photogenerated electrons, which
boosted the overall photocatalytic activity. These carbon based mate­
rials are low cost and less toxic. Therefore, WS2/Carbon materials based
composite provide invigorating way on the noble metal free and visible
light active photocatalyst fabrication.
5.4. WS2 with Bismuth based materials
Bismuth-based materials have considerable attention in photo­
catalytic activity because of its excellent visible light absorption prop­
erties. Several Bi3+ containing materials have narrow bandgap and
enhanced visible light absorption due to the hybridization of O 2p and Bi
6s2 valence bands [157,158]. Various Bi based materials such as Bi
based oxides and Bi based oxyhalides (BiOCl and BiOBr) are widely used
in photocatalytic activity. However, the photocatalytic activity of bare
Bi based materials is still limited because of the quick recombination of
photogenerated charge carrier.
5.4.1. WS2/BiOCl
BiOCl photocatalysts have received great attention due to its UV and
Solar irradiation absorption ability, because of their indirect bandgap
(3.2 eV) and VB is mainly composed of O 2p states and Cl 3P states. But
bare BiOCl not efficient to visible light photocatalysis. BiOCl photo­
catalyst coupling with co-photocatalysts is one of the effective methods
to enhance the visible light photocatalytic activity [159]. The lower
active oxygen species production hindered the electron-hole pair
5.3.2. WS2/Carbon dots
Carbon dots (CDs) have attracted attention in photocatalysis due to
their fascinating properties including biocompatibility, less toxicity,
Fig. 12. (a) Schematic illustration of charge transfer mechanism of WS2/ g-C3N4 (Reprinted with permission from Ref [153] copyright from Elsevier), (b) Photo­
catalytic H2 production mechanism of 3D composite structured WS2/ g-C3N4 (Reprinted with permission from Ref [154] copyright from Elsevier).
13
M. Sridharan and T. Maiyalagan
Chemical Engineering Journal 424 (2021) 130393
formation and separation in BiOCl photocatalytic activity. Xiao et al.,
reported that WS2 incorporated BiOCl for photocatalytic activity. Under
visible light, the photogenerated electrons could transfer from CB of WS2
to BiOCl due to the suitable band alignment. This promoted charge
carrier separation leading to an enhance RhB degradation efficiency
[96]. Similarly, Ashraf et al., designed 2D/2D BiOCl/WS2 heterojunction
by solution-based sonication method (Fig. 13 (a)). The content of the
WS2 nanosheets in the composite can affect the efficiency of MG
removal. ECB and EVB edges of bare WS2 nanosheets are about 0.165 eV
and 2.15 eV respectively. And for bare BiOCl nanosheets, 0.195 eV and
3.525 eV, respectively. EVB and ECB edge of WS2 higher than EVB and
ECB edge of bare BiOCl respectively. So, under illumination, the
photoexcited WS2 conduction band electrons can easily transfer to BiOCl
conduction band and BiOCl VB having holes easily moves to the WS2
nanosheets (Fig. 13 (b)). An enhanced efficiency degradation of MG is
observed when the WS2 content is increased from 1% to 2%. However,
further raising the concentration of WS2 leads to hinder MG degradation
efficiency. The possible reason for the negative impact of excess amount
of WS2 could be the decrease active sides of the BiOCl [160].
investigated the photocatalytic activity of flower-like WS2/BiOBr het­
erojunction. The formation of heterojunction between BiOBr and WS2
enhance the light absorption capacity. Under Visible light irradiation,
photogenerated electrons and holes are formed on the surface of WS2
and BiOBr. The WS2 CB having photogenerated electrons transferred to
the CB of BiOBr, while photogenerated holes are moved from the VB of
BiOBr to WS2. The photogenerated holes in the VB of WS2 could not
react with H2O, because VB potential of WS2 more negative than the
redox potential of •OH/H2O (+2.38 eV vs. NHE). The WS2 introduction
into BiOBr could enhance photogenerated electron and hole pair
transfer and separation (Fig. 14 (a)). Based on their results,The degra­
dation efficiencies of a various contaminant in the following order:
Lanasol Red 5B (99%), metronidazole (97%), tetracycline (92%),
oxytetracycline (92%), rhodamine B (90%), CIP (83%), methylene blue
(78%), methyl orange (62%), bisphenol (42%) and phenol (40%)
(Fig. 14 (b)).
The photocatalytic degradation of ciprofloxacin was explored under
different conditions, and that result demonstrated that high concentra­
tion of ciprofloxacin, lower pH and concentrations of ions (PO4 3− ,
2
4−
2+
HPO24 , H PO , and Cu ) reduced the photocatalytic degradation ef­
ficiency [165].
5.4.2. WS2/BiOBr
BiOBr is now paid attention in photocatalysis field due to its high
photocorrosion stability, non-toxicity and good chemical stability. The
visible-light photocatalytic activity of BiOBr is still limited in practical
application, due to its lower visible light absorption and fast recombi­
nation of photogenerated electrons [161–163]. The extensive researcher
has shown that the heterojunction formation with other materials,
improved the electron hole pair separation, extend the lifetime of pho­
togenerated electron and enhance the photocatalytic activity. Fu et al.,
reported a WS2/BiOBr for the long-term photocatalytic removal of
organic and inorganic pollutant removal. Based on their results, the
organic and inorganic contaminant degradation feature is highly WS2
concentration dependent. 10 mL of WS2 quantum dots loaded BiOBr
heterostructure shows outstanding photocatalytic performance. To­
wards normally, the catalyst concentration, dye concentration and
presence of secondary effluents or organic contaminants affect the
photocatalytic efficiency of the catalyst. The high CIP concentration and
2+
low catalyst concentration and addition of various ions (PO3and
4 , Cu
Ca2+) reduce the degradation of CIP. The other organic pollutant
degradation efficiency of organic pollutant also studied. The coexistence
of other organic contaminants (LR5B, RhB and TC) also, reduce the
photocatalytic activity of the catalyst. The removal of various pollutant
including Ciprofloxacin (92%), 99% (Lanasol Red 5B), 95% (Rhodamine
B), 96% (tetracycline), 41% (Bisphenol A), and 85% (metronidazole)
was obtained under visible light irradiation [164]. Same group have
5.4.3. WS2/(BiO)2CO3
Most of the Bi-based oxide, catalyst possesses strong visible light
absorption capacity and good photocatalytic activity. However the
practical usage of individual catalyst is still limited because of their fast
recombination of photogenerated charge carriers [166]. (BiO)2CO3 is an
n-type semiconductor, it’s a typical silane phase and belongs to
Aurivillius-related oxide family. In recent years, the number of re­
searchers reported (BiO)2CO3 in the field of photocatalysis due to its
advantages like 2D layered crystal structure, good stability and low
toxicity. But the large bandgap (ca. 3.4 eV) of (BiO)2CO3 limited their
visible light activity. To further enhance the visible light absorption and
charge carrier separation of (BiO)2CO3 by the fabrication of hetero­
junction with other materials [167]. Li et al., reported flower like Zscheme WS2/(BiO)2CO3 composites synthesized by one-pot hydrother­
mal method. Under illumination both WS2 and (BiO)2CO3 excited,
subsequently the photogenerated electrons and holes are stimulated.
Because of both higher VB and CB potentials of WS2 than those of
(BiO)2CO3. Therefore it follows Z-scheme mechanism, CB electrons of
WS2 react with O2 to form .O−2 and the hole in the VB of (BiO)2CO3 react
with H2O to produce OH. The average life time of (BiO)2CO3 and WS2/
(BiO)2CO3 were 3.5 and 3.2. The decrement of life time indicates that
some non-radiative process occurs in WS2/(BiO)2CO3 heterojuction after
the deposition of WS2 (Fig. 15(a)). The active trapping experiments were
Fig. 13. (a) Schematic representation of the synthesis procedure of BiOCl/WS2 composite, (b) Schematic representation of the photocatalytic mechanism of BiOCl/
WS2 (Reprinted with permission from Ref [160] copyright from RSC).
14
M. Sridharan and T. Maiyalagan
Chemical Engineering Journal 424 (2021) 130393
Fig. 14. (a) Schematic diagram of band level with possible photocatalytic mechanism of WS2/BiOBr, (b) Photocatalytic degration efficiency of WS2/BiOBr
(Reprinted with permission from Ref [165] copyright from Elsevier).
Fig. 15. (a) Time-resolved emission decay of (BiO)2CO3 and WS2/(BiO)2CO3, (b) degradation efficiency of WS2/(BiO)2CO3 using different active species (Reprinted
with permission from Ref [168] copyright from Elsevier).
shown in (Fig. 15 (b)). The obtained active species react with organic
pollutant to form smaller molecules. This result demonstrated that
flower-like Z-scheme WS2/(BiO)2CO3 composites show 3.23 times better
Lanasol Red 5B degradation efficiency than bare (BiO)2CO3, moreover
95% ciprofloxacin removal efficiency within 90 min [168].
Fig. 16. (a) Photocatalytic mechanism of WS2/Bi2MoO6, (b) The XRD pattern of WS2/Bi2MoO6 before and after durability test (Reprinted with permission from Ref
[170] copyright from Springer).
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M. Sridharan and T. Maiyalagan
Chemical Engineering Journal 424 (2021) 130393
5.4.4. WS2/Bi2MoO6
Bismuth molybdate (Bi2MoO6) is an n-type semiconductor with layer
layers and perovskite-like
structure which is assembled by Bi2O2+
2
layered [MoO4]2− . It is widely used in photocatalysis because of their
advantages like narrow bandgap (2.4–2.8 eV), high visible-light absor­
bance and high chemical stability [169]. The lower quantum yield and
fast recombination of photogenerated charge carrier still restrict its
photocatalytic activity. Recently, Bi2MoO6 composite fabrication shown
great potential for emerged and advanced in the field of photocatalysis.
Under visible-light irradiation both WS2 and Bi2MoO6 are excited,
the photoexcited Bi2MoO6 conduction band electrons easily transfer to
WS2, because the conduction band potential of Bi2MoO6 (− 0.31 eV) is
more negative than WS2 conduction band potential. CB of Bi2MoO6
having a few electrons reacted with dissolved oxygen to form O2− . The
main active species like holes and O2− react with pollutant to form CO2
and H2O (Fig. 16 a). Enhanced efficiency in RhB degradation is observed
when the concentration of WS2 is increased from 1% to 5%. However,
further increasing concentration of WS2 to 7% leads to a decrease in RhB
degradation. The promising reason for the opposite effect of the excess
amount could be the blocking of active sides of Bi2MoO6, although 5% of
WS2 exhibits remarkably enhanced photocatalytic activity and good
stability. After four cycles, the XRD pattern of WS2/Bi2MoO6 shows
similar XRD pattern to initial XRD pattern (Fig. 16 (b)) [170].
application toward photocatalysis for clean environment and sustain­
able energy production.
5.5. WS2/Ag based materials
Silver-based materials have been widely used in photocatalytic ac­
tivity, due to their superior utilization of visible light. Among the various
silver-based materials, AgI [172–175] and Ag3PO4 [176–179] have
proved the most important materials to utilize the visible light irradia­
tion and good photocatalytic activity. However both AgI and Ag3PO4
suffer from photo corrosion at long time under light irradiation.
Therefore, most of the researcher reported that Ag3PO4 and AgI coupled
with other co-photocatalysts or conductive materials. Researchers take
concern of WS2 as a suitable co-catalyst due to its unique property like
high photocorrsion stability. Wu et al., synthesized AgI/WS2 composites
by a simple in situ growth of AgI on WS2 nanosheet. AgI nanoparticles
were uniformly distributed on the WS2 nanosheets, both component
very close contact with each other. The optimized photocatalytic ac­
tivity of AgI/WS2 composite on RhB was 91.2% on 30 min. The possible
photocatalytic mechanism of the synthesized composites shown in
Fig. 18 (a). Under the illumination, VB electron in WS2 and AgI can be
excited to CB, VB forming the photogenerated holes. CB of AgI photo­
generated electrons moves to CB of WS2. This electron reacts with O2 to
form ⋅O2− . Finally, ⋅O2− can react with RhB to produce small molecule
[104].
WS2 can be also combined with Ag3PO4 for photocatalytic applica­
tions. Hongjian et al. prepared WS2/Ag3PO4 heterostructures by pho­
tocatalytic degradation of RhB. The theoretically calculated valance
bands of Ag3PO4 and WS2 are 2.9 eV and 2.11 eV respectively, similarly
the conduction band of Ag3PO4 and WS2 are 0.45 eV and 0.21 eV.
Because the photogenerated electrons transferred from the conduction
band of WS2 to Ag3PO4 (Fig. 18 (b)) and also hole in the valance band of
Ag3PO4 migrates to WS2. The photogenerated holes react with
contaminant to produce small molecules. Bare Ag3PO4 can completely
degrade the contaminant after 33 min. But Ag3PO4 deposited on WS2
degrade within 9 min, The results show that the existence of WS2 sheets
can expressively enhance the photocatalytic performance of Ag3PO4
[180].
In conclusion, introduction of WS2 into Ag based materials can
promote photocatalytic performance, being attributed to minimize the
fast recombination of photogenerated electron and holes. More impor­
tantly, the WS2 afford high specific area, which helps to enhance the
deposition of nanostructured Ag based materials.
5.4.5. WS2/Bi2O3
Bi2O3 as an efficient visible light active material for photocatalysis
[171]. Li et al., reported the synthesis of WS2@Bi2O3 composite for
photocatalytic application. The synthesized composite optimized by
changing the concentration of WS2 (1% to 12%) to WS2@Bi2O3 com­
posite. The 8% WS2 loaded WS2@Bi2O3 composites shows higher pho­
tocatalytic activity compared with bare WS2 and Bi2O3. The calculated
band gap value of WS2 and β-Bi2O3 are 1.41 eV and 2.36 eV respectively
and the band structure are shown in the Fig. 17(a,b). The noticeable
difference between the bandgap of both WS2 and β-Bi2O3 would be
useful for the harvesting of visible irradiation. p-type and n-type semi­
conducting nature of β-Bi2O3 and WS2 confirmed through the fermi level
position (0.797 eV and 1.023 eV VB of WS2 and β-Bi2O3). The calculated
work functions of β-Bi2O3 and WS2 are shown in Fig. 17(c–g). The CB of
WS2 having photogenerated electrons easily transferred to the conduc­
tion band of β-Bi2O3, because the conduction band edges of WS2 higher
than β-Bi2O3. At the same time, photogenerated hole is transfers from
valance band of β-Bi2O3 to VB of WS2 [111].
In conclusion, the incorporation of WS2 to Bi-based materials to
achieve higher quantum yields and enhanced visible light absorption
capacity. Therefore, these substantial developments and methods for
customization of photocatalyst show potential activity for their real time
Fig. 17. (a&b) The geometries of WS2 and β- Bi2O3, (c&d) The band structure of β- Bi2O3 and WS2 (c,d), the band edge placement of valence band minimum, (e)
fermi level, (f) conduction band minimum β- Bi2O3 and WS2 (Reprinted from Ref [111] with permission from the Chinese Chemical Society (CCS), Peking University
(PKU), and the Royal Society of Chemistry).
16
M. Sridharan and T. Maiyalagan
Chemical Engineering Journal 424 (2021) 130393
Fig. 18. (a) The photocatalytic mechanism of AgI/WS2 (Reprinted with permission from Ref [104] copyright from IET), (b) Schematic representation of the
photcatalytic mechanism of WS2/Ag3PO4 (Reprinted with permission from Ref [180] copyright from RSC).
6. Conclusion
[2]
Photocatalyst are probable to be an upcoming trend, since nano­
structured photocatalysts have shown considerable superior perfor­
mance than their bulk counterparts. Over the past decades different
methods have been used to enhance the photocatalyst optimizing the
photons and electrons prominent to enriched catalytic activity such as
(i) fabrication of both Inorganic and/or organic semiconductors heter­
ojunctions, (ii) band-gap engineering strategies, (iii) usage dye molecule
as a sensitizer, (iv) Usage of the cocatalyst. Among the various ap­
proaches, cocatalyst loading has emerged as an innovative type for
photocatalysis, In fact, WS2 loaded on TiO2, WO3, MoS2, CdS and gC3N4, have been proven to be highly efficient catalysts for photocatalytic
applications. Although noticeable progress has been reached, the studies
in WS2 based photocatalysis are the primary stage and further im­
provements are needed. First, new synthesis methods introduced to
improve WS2 or WS2 based composites. Second, the catalytic perfor­
mance of WS2 is lowered by the quantity of these materials and their
structures. Innovative synthesis methods and computational help are
essential towards the design of site-selective loading of WS2 on another
semiconductor photocatalyst for enhanced performance. Additionally,
more studies are needed to develop the understanding of WS2 based
photocatalytic mechanism. During the photocatalytic reaction, the sta­
bility of the photocatalyst is a major concern in metal sulphide based
photocatalysis, because the stability of the metal sulphide composites
remains outstanding issues. Before forwarding to the large scale syn­
thesis and application of metal sulphide composite, the analysis of any
secondary pollution from its composites is a serious consideration for
photocatalysis.
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
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.
[13]
Acknowledgement
[14]
The authors acknowledge the financial support from the Scheme for
Promotion of Academic and Research Collaboration (SPARC) of the
Ministry of Human Resource Development (MHRD), Government of
India, SPARC Grant No. SPARC/2018-2019/P1122/SL.
[15]
[16]
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