One-Pot-Architectured Au-Nanodot-Promoted MoS2/ZnIn2S4: A Novel p-n Heterojunction Photocatalyst for Enhanced Hydrogen Production and Phenol Degradation

Article abstract of DOI:10.1021/acs.inorgchem.9b01105

Developing effective and simple one-pot synthetic strategies regarding the formation of heterojunction photocatalytic semiconductors remains an intense challenge in research pursuits. Further scheming of the p-n heterojunction has sustained renewed interest in catalysis, photocatalysis, energy storage, and conversion because they easily accelerate the bulk charge separation efficiency. Thus we have successfully designed a Au-MoS₂/ZnIn₂S₄ heterojunction photocatalyst for the first time by adopting a simple one-pot hydrothermal technique, followed by a deposition-precipitation method. By adjusting the mole ratio of Mo with that of Zn and In precursors, we have fabricated a MoS₂/ZnIn₂S₄ p-n heterojunction photocatalyst, and the established p-n heterojunction between MoS₂ and ZnIn₂S₄ is demonstrated by various physicochemical and morphological characterizations. An interfacial junction is created between MoS₂ and ZnIn₂S₄ at the depletion region via an in situ formation mechanism, leading to the enhancement of the charge separation through the p-n heterojunction and thus improving the photocatalytic activity. Moreover, the photocatalytic activity is projected to further improve by the incorporation of Au nanodots on the surface of MoS₂/ZnIn₂S₄ photocatalysts. The increase in activity is due to the generation and participation of a large number of direct-electron-transfer-induced hot electrons in the photochemical reaction. From the experimental results, Au-MoS₂/ZnIn₂S₄ heterojunction photocatalysts with only 1% MoS₂ and 1% Au loading content displayed a 561.25 μmol/h H₂ evolution rate and 84% degradation of phenol, which are nearly 15 and 6 times higher than those neat ZnIn₂S₄. In addition Au-MoS₂/ZnIn₂S₄ photocatalysts exhibit a photocurrent density of ～2.56 mAcm^-2, which is nearly 2.4 times higher than that of the MoS₂/ZnIn₂S₄ heterojunction photocatalyst. This exertion represents the synergetic enhancement of photocatalytic activity through the p-n heterojunction as well as the hot-electron participation by the metal nanocatalyst, which is an inspiration for developing efficient photocatalysts.

Full text of DOI:10.1021/acs.inorgchem.9b01105

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
One-Pot-Architectured Au-Nanodot-Promoted MoS2/ZnIn2S4: A
Novel p−n Heterojunction Photocatalyst for Enhanced Hydrogen
Production and Phenol Degradation
Gayatri Swain, Sabiha Sultana, and Kulamani Parida*
Centre for Nano Science and Nanotechnology, Siksha O Anusandhan (Deemed To be University), Bhubaneswar 751030, Odisha,
India
S
* Supporting Information
ABSTRACT: Developing effective and simple one-pot synthetic strategies regarding the formation of heterojunction
photocatalytic semiconductors remains an intense challenge in research pursuits. Further scheming of the p−n heterojunction
has sustained renewed interest in catalysis, photocatalysis, energy storage, and conversion because they easily accelerate the bulk
charge separation efficiency. Thus we have successfully designed a Au-MoS2/ZnIn2S4 heterojunction photocatalyst for the first
time by adopting a simple one-pot hydrothermal technique, followed by a deposition−precipitation method. By adjusting the
mole ratio of Mo with that of Zn and In precursors, we have fabricated a MoS2/ZnIn2S4 p−n heterojunction photocatalyst, and
the established p−n heterojunction between MoS2 and ZnIn2S4 is demonstrated by various physicochemical and morphological
characterizations. An interfacial junction is created between MoS2 and ZnIn2S4 at the depletion region via an in situ formation
mechanism, leading to the enhancement of the charge separation through the p−n heterojunction and thus improving the
photocatalytic activity. Moreover, the photocatalytic activity is projected to further improve by the incorporation of Au
nanodots on the surface of MoS2/ZnIn2S4 photocatalysts. The increase in activity is due to the generation and participation of a
large number of direct-electron-transfer-induced hot electrons in the photochemical reaction. From the experimental results,
Au-MoS2/ZnIn2S4 heterojunction photocatalysts with only 1% MoS2 and 1% Au loading content displayed a 561.25 μmol/h H2
evolution rate and 84% degradation of phenol, which are nearly 15 and 6 times higher than those neat ZnIn2S4. In addition Au-
MoS2/ZnIn2S4 photocatalysts exhibit a photocurrent density of ∼2.56 mAcm−2, which is nearly 2.4 times higher than that of the
MoS2/ZnIn2S4 heterojunction photocatalyst. This exertion represents the synergetic enhancement of photocatalytic activity
through the p−n heterojunction as well as the hot-electron participation by the metal nanocatalyst, which is an inspiration for
developing efficient photocatalysts.
increasing attention in photocatalysis due to their suitable band
gap, variety of catalytic functions, easy availability, and low
cost.7,8 Among them, ZnIn2S4 (ZIS), having a well-defined
proper band gap and band-edge potential and well within the
visible-light region, has been a suitable candidate for the fields
of photocatalytic hydrogen evolution and organic pollutant
degradation.9,10 The structural unit of hexagonal ZIS is made
up of a packet consisting of S−Zn−S−In−S−In−S stacking
layers by van der Waals forces of attraction, which provide a
INTRODUCTION
■
The current research agenda of the scientific community has
focused on the flourishing development of inorganic graphene
analogues, which have attained the utmost attention due to
their large specific surface areas and abundant surface sites.1−3
Among various graphenes and their analogues, transition-metal
chalcogenides like MoS2 and ternary metal chalcogenides like
CaIn2S4, ZnIn2S4, and CdIn2S4 have garnered great research
interest and have been proven to be the most effective toward
photocatalytic applications.4−6 Compared with the conven-
tional wide-band-gap semiconductor, important ternary metal
chalcogenides, II−III2−VI4 semiconductors, have attracted
Received: April 16, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.9b01105
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
vacancy layer between two sulfur layers and are helpful for
better photocatalysis.6,11 However, the unmodified ZIS
photocatalyst suffers from low migration ability as well as
low charge separation efficiency, which limits its application
toward greater photocatalysis.12,13 Many previous studies have
revealed that a number of efforts have been devoted to
improve the photocatalytic efficiency of pristine hexagonal ZIS
by a variety of techniques such as noble-metal loading,
coupling to metal oxides as well as metal sulfides and metal
phosphides, and so on.7,14−16 However, making a p−n
heterojunction with MoS2 is considered to have potential for
becoming a low-cost material toward photocatalytic applica-
tions.
fascinating cost-effective way to avoid the above drawbacks of
each individual component and realize a synergistic interaction
toward improving the efficient photoexcited charge generation
and separation, p−n heterojunction structures have been
demonstrated. In this regard, we have developed a one-pot
synthetic route to the MoS2/ZIS photocatalyst.
Throughout the literature survey regarding MoS2 and ZIS, it
is convincing that MoS2 is a p-type and ZIS is an n-type
semiconductor.26,6 The main reason for the rational design and
synthesis of the p−n heterojunction system by taking MoS2
and ZIS is their ideal band gaps and matched band-edge
potentials, which perfectly fulfill the thermodynamic achieve-
ment that is required for the construction of p−n
heterojunction material. Owing to the big difference in the
work function between a p-type and an n-type semiconductor,
in a p−n heterojunction system, a strong internal electrostatic
field is developed across the junction interface which drives the
transfer of photoinduced electron hole charge carriers.31
However, some studies have revealed that the photocatalytic
performance of p−n heterojunction systems is somehow
limited by some drawbacks; that is, some energy is lost during
the transfer of electrons to the conduction band (CB) and
holes to the valence band (VB) at more positive and negative
potentials, respectively, which has a small negative impact on
the separation of photoexciton pairs.32 To alleviate the above
difficulties, noble metal such as Au nanodots, a catalytic
component, can be introduced into the p−n heterojunction
photocatalyst to improve the photocatalytic activity of the p−n
heterojunction system. Nevertheless the highly capacitive and
spontaneous Au nanodots make the transition-metal chalco-
genide heterojunction photocatalyst extra special by regulating
its carrier concentration as well as the transport barrier.
Furthermore, the emergence of high photocatalytic activity is
attributed to the participation of highly efficient direct-
electron-transfer (DET) hot electrons generated by Au
nanodots, which can also extend the light absorption spectrum
of the photocatalyst. Again, noble metals behave likes electron
reservoirs and can favor the acceleration and migration of
photoinduced electrons to the CB of heterojunction photo-
catalyst. Until now, few studies have been reported with
respect to ternary heterojunction photocatalysts toward
photocatalytic applications such as Pd-NiFe2O4/2D MoS2,32
Ag/Ag2S/BiVO4,33 BiOCl/PANI/Pd,34 and C3N4/Pd/
Cu2O.35
Inspired by the above-mentioned advantages, herein we have
demonstrated the rational designing of Au-nanodot-modified
MoS2/ZIS heterojunction photocatalysts by a one-step hydro-
thermal treatment. In the procedure, first, 0D MoS2 nano-
particles are in situ grown on the 2D ZIS nanosheets; next, Au
nanodots, which are in the range of nanometer size, are
uniformly immobilized on both MoS2 nanoparticles as well as
ZIS sheets. To the best of our knowledge, no reported work
has been established on the Au-MoS2/ZIS heterojunction
photocatalyst. With our approach, the designed ternary
nanocomposite photocatalyst has greatly overcome the draw-
backs of each individual component with a high charge
separation efficiency and shows an excellent photocatalytic
activity toward H2 evolution and phenol degradation.
Furthermore, compared with the traditional hybrids of
heterostructures, the current heterojunction photocatalyst
provides a large specific surface area with a greater number
of active sites for the photocatalytic performance. Lastly, the
The development of the research interest in the photo-
catalysis field has diverted the attention toward MoS2, which is
a rising star in the graphene analogue family.17,18 Besides
photocatalysis, MoS2 also has applications in the areas of
electrocatalysis, biosensors, pollutant degradation, drug deliv-
ery, and so on.2,19−21 The structural units of the semi-
conducting phase of MoS2 arrange themselves in a way that the
transition-metal atom exhibits six-fold coordinates, which are
hexagonally packed and covalently bonded in between the two
trigonal chalcogen atomic layers in the S−Mo−S fashion,
forming a sandwich layer.22,23 It is of particular interest that
MoS2 is a narrow-band-gap semiconductor exhibiting indirect
to direct band-gap (1.29 to 1.9 eV) characteristics; however, its
photocatalytic activity is very unsatisfactory. Previously
reported studies have revealed that MoS2 alone has poor
conductivity toward the photocatalytic field due to the low
conductivity power through the layers of MoS2 and also due to
the low charge-carrier density.23−25 But whenever it is coupled
to other semiconductors, it can improve the charge separation
efficiency as well as enhance the light absorption capacity of
the primary photocatalyst to a greater extent. A few works have
been proposed (of which some are given here) that are focused
on the development of a MoS2-based heterojunction photo-
catalyst by taking MoS2 as a p-type semiconductor. Zhang et al.
demonstrated the MoS2/CdS p−n heterojunction system and
showed the application toward photocatalytic hydrogen
evolution.26 Zhao et al. designed the n-BiVO4@p-MoS2
core−shell structure, which exhibited the photocatalytic
performance toward Cr6+ reduction and CV oxidation.27 The
novel p-MoS2/n-CeO2 and p-MoS2/n-CaIn2S4 p−n hetero-
junction photocatalysts have been previously constructed by
our group, and both of the heterojunctions exhibited high
visible-light activity toward H2 evolution.28,23 Reviewing the
literature regarding MoS2 and ZIS, it is deduced that a few
composites have been designed by combining materials. Of
such works, some examples, such as Liu et al., developed MoS2
anchored on the ZIS microsphere through a two-steps
hydrothermal method by using dimethylformamide (DMF)
as the dispersing agent.13 Again, ZIS hybrid with MoS2 was
designed by Huang et al., followed by a two-step hydrothermal
method.29 In addition, Zhang et al. demonstrated the MoS2
quantum dot (QD) with S-vacancy ZIS composites via a two-
step hydrothermal technique, followed by the lithiation process
and DMF as the solvent.30 However, it is well known that the
synthesis of the above photocatalyst proceeding through two
or more complicated synthetic processes is relatively time
consuming, thus limiting the industrialized utilization of the
hybrids. Furthermore, most of the synthetic process requires a
mixed-phase solvent, a toxic organic solvent, and surfactants
that are environmentally harmful as well as cost-effective. As a
B
DOI: 10.1021/acs.inorgchem.9b01105
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
detailed possible photocatalytic mechanistic pathway of this
Au-MoS2/ZIS is proposed.
cysteine through Au−S bonding. (iii) Third, the Au atoms
were easily reduced and deposited on the surface of the MoS2/
ZIS heterojunction photocatalyst by the subsequent addition
of L-ascorbic acid, which acted as a reductive agent. To know
the crystal structure and the morphology of the as-synthesized
composites, X-ray diffraction analysis (XRD), X-ray photo-
electron spectroscopy (XPS), and high-resolution transmission
electron microscopy (HRTEM) analyses were carried out and
are described below.
EXPERIMENTAL SECTION
■
Reagents and Materials. Zinc acetate [Zn(CH3COO)2·2H2O],
sodium molybdate [Na2MoO4·2H2O], and thioacetamide [C2H5NS]
were purchased from Merck India. Indium nitrate [In(NO3)3·6H2O]
was purchased from Sigma-Aldrich. Distilled water was used as the
solvent throughout the experiment. All reagents were analytical grade
and used without further purification.
X-ray Diffraction. XRD spectra of neat ZIS, 1%MoS2/ZIS,
and 1%Au−1%MoS2/ZIS samples are demonstrated in Figure
1 to illustrate the crystallographic structure and phase purity.
One-Pot Synthesis of MoS2/ZIS Heterojunction Photo-
catalyst. The given heterojunction photocatalyst was prepared by
following a one-step in situ hydrothermal technique. In the present
procedure, 60 mL of solvent (distilled water) was taken in a 100 mL
beaker containing 0.6 mmol of [Zn(CH3COO)2·2H2O], 1.2 mmol of
[In(NO3)3·6H2O], and 0.004 mmol of [Na2MoO4·2H2O] and stirred
for approximately a few minutes until all of the materials were
completely dissolved. Then, 3.02 mmol of [C2H5NS] was mixed into
the above suspension and again stirred for another ∼2 h at room
temperature to get a homogeneous solution. Then, the whole solution
was transferred to a stainless-steel autoclave and heated to 200 °C for
24 h. After being cooled naturally to ambient temperature, the
obtained dark-yellow precipitate was washed through centrifugation
several times with distilled water and ethanol and dried in a vacuum
desiccator for the whole night. The controlled loading of MoS2 (0.5,
1, 3, and 5%) was adjusted by the appropriate loading content of
Na2MoO4·2H2O, following the same synthesis procedure to prepare
all percentages of the photocatalysts. The obtained photocatalysts
were denoted as 0.5%MoS2/ZIS, 1%MoS2/ZIS, 3%MoS2/ZIS, and
5%MoS2/ZIS. For comparison, free MoS2 and ZIS were synthesized
in the absence of Zn and In precursors and Mo precursor,
respectively.
Figure 1. XRD spectra of neat ZIS, 1%MoS2/ZIS, and 1%Au−1%
MoS2/ZIS photocatalysts
Synthetic Protocol for Au-Modified MoS2/ZIS Heterojunc-
tion Photocatalyst. As-fabricated 1%MoS2/ZIS photocatalyst was
modified by 0D gold nanodots via L-cysteine as a linker. Typically, 25
mg of the aforementioned MoS2/ZIS dried samples was suspended in
10 mL of distilled water and sonicated for 30 min to get a complete
dispersion of the sample. Then, 25 mL of 1 mM L-cysteine aqueous
solution was added to the above solution, and the whole solution was
subjected to sonication for another 30 min. Then, the sonicated
solution was transferred to a beaker under vigorous stirring. 25 mL of
0.3 mM chloroauric acid (HAuCl4) was loaded in a dropwise manner
and kept under dark conditions to facilitate the complete adsorption
of the gold sample on the composite sample. With vigorous stirring
for ∼30 min, a solution containing 30 mL of 5 mM L-ascorbic acid
was quickly added to the solution, and the reaction was left to rapidly
stir for ∼3 h. Finally, the products were collected by separation
through centrifugation, repeatedly washed with distilled water to
eliminate impurities, and dried in a vacuum oven at 40 °C for 36 h. A
gray product was obtained, denoted as 1%Au−1%MoS2/ZIS.36
In general, a sharp, well-defined diffraction peak indicates the
good crystallinity of the photocatalysts. Regarding the
hexagonal phase, according to JCPDS no. 01-072-0773, ZIS
pertaining to the diffraction peaks at scattering angles (2θ) of
21.8, 27.6, 32.1, 34.2, 36.1, 39.7, 45.7, 47.5, 52.0, 56.2, 67.3,
70.8, and 75.4° indexed to the (006), (102), (105), (106),
(0010), (108), (1010), (112), (1012), (203), (2010), (1017),
and (211) planes, respectively.3 The XRD patterns concluded
that there is no other impurity-related diffraction peak
regarding binary sulfides, untreated reactant, and metal oxide,
explaining the high purity of the photocatalyst. As shown in
Figure S1a, MoS2 has XRD peaks located at angles of 32.67,
44.0, 49.29, 58.21, and 72.80°, corresponding to the (100),
(006), (105), (110), and (203) planes, respectively. The main
two peaks at 32.67 and 58.21° suggest the existence of a
hexagonal phase of the MoS2 (JCPDS file no. 037-1492) with
lattice constants a = b = 0.311 and c = 1.211 and space group
P63/mmc.37 The weak peak intensity of MoS2 suggests the
synthesis of the few-layered hexagonal MoS2. Moreover, the
1%MoS2/ZIS heterojunction photocatalyst exhibited a similar
diffraction pattern as neat ZIS, and there are no observed
diffraction spectra for MoS2, suggesting the good dispersion
and excellent coupling interaction property of MoS2 in the
composite photocatalyst. Furthermore, it can be found that
going from a lower content to a higher content of MoS2 in the
MoS2/ZIS heterojunction photocatalyst, the diffraction peak
intensity decreases may be due to the in situ synthesis method.
The XRD pattern of MoS2/ZIS photocatalyst with various
loading amounts of MoS2 (0.5, 3, 5%) was also analyzed and is
shown in Figure S1b along with 1%MoS2/ZIS. Upon the
introduction of the Au nanodots, the diffraction peak position
of the MoS2/ZIS photocatalyst remained almost unchanged,
with a notably huge decrease in peak intensity. Moreover, in
RESULTS AND DISCUSSION
■
The MoS2/ZIS heterojunction photocatalyst was prepared by a
simple one-pot hydrothermal method. This method enhances
the intimate coupling interaction between the MoS2 and ZIS
and also modifies the textural structure of the composite
photocatalyst. To further enhance the activity, Au nanodots
were incorporated into the as-obtained photocatalyst through a
simple deposition precipitation method, and the overall
procedure proceeded through three reaction mechanisms. (i)
First, the linker L-cysteine acted as a binder and bound to the
MoS2/ZIS composite surface through its carboxylate group
and facilitated the growth of Au nanodots on the surface of the
MoS2/ZIS heterojunction composite. (ii) Second, during the
addition of the gold salt precursor (HAuCl4) onto the surface
of the L-cysteine-functionalized MoS2/ZIS, the chloroauric
ions (AuCl4)− were easily attracted to and captured by the L-
C
DOI: 10.1021/acs.inorgchem.9b01105
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
the XRD spectra of the 1%Au−1%MoS2/ZIS heterojunction
photocatalyst, no characteristic diffraction peaks for Au were
detected, ascribed to low content and uniform distribution,38
but with Au loading, the rapid decrease in peak intensity may
be due to the heavy atom effect of Au nanodots.39 The
formation of Au-modified MoS2/ZIS heterojunction photo-
catalysts has also been supported by XPS and energy-dispersive
X-ray analysis (EDAX). Through the XRD analysis, according
to the Scherrer equation, we can also calculate the average
crystallite size of the neat ZIS, x%MoS2/ZIS composite
photocatalyst, and 1%Au−1%MoS2/ZIS by broadening the
006 plane.
Morphological Analysis. The morphological character-
istics as well as the interposition relationship of the as-
synthesized photocatalyst were elucidated by field-emission
scanning electron microscopy (FESEM) and HRTEM
techniques. Figure 2a−c shows the FESEM images of the 1%
Au−1%MoS2/ZIS heterojunction photocatalyst, in which a
number of ZIS nanoplates are randomly arranged with what
look likes a flake-type morphology with MoS2 nanoparticles
deposited on it, but Au nanodots are not clearly distinguished
due to the smaller particle size of Au. From the HRTEM
images of 1%MoS2/ZIS in Figure 2d, the nanoplate-like
morphology of ZIS is clearly observed, whereas some black
dense structures, that is, nanoparticles of MoS2, are randomly
deposited on ZIS. The fringe patterns corresponding to the d-
spacing values of 0.273 nm ascribed to the (100) plane for
Kλ
β cos θ
Lh,k,l
=
40
MoS2 and 0.41 nm ascribed to the (006) plane for ZIS3 are
From the equation, the crystallite size (Lh,k,l) is perpendicular
to the reflected crystallite plane. Here K is the shape factor of
the crystallite size, and its value is 0.9 for all base peaks. β is the
half width of the diffraction intensity peaks, λ is the X-ray
wavelength, and θ is the Bragg angle in degrees. The crystallite
size of the various photocatalysts decreases after the loading
content of MoS2 is changed in comparison with neat ZIS. The
crystal-phase determined values of different photocatalysts are
shown in Table 1.
observed, as shown in the inset of Figure 2d, confirming the
existence of both of the photocatalysts in 1%MoS2/ZIS
composite photocatalyst. A clear interface between ZIS and
MoS2 is visible, which further confirms the close interaction in
the heterojunction photocatalyst, but the image clearly shows
that MoS2 nanoparticles are distributed in some of the region
of the ZIS nanoplates due to the low content of MoS2. In the
1%Au−1%MoS2/ZIS HRTEM image (Figure 2e,f), similar
type of morphology is obtained, and, in addition to it, some
dot-shaped particles are also observed, which may be due to
the Au nanodots. A further HRTEM image is taken on that
portion, and it is seen that some nanodot-shaped Au particles
with an average size of ∼1.5 nm represented in the yellow
dotted circles are decorated over the ZIS nanoplates. HRTEM
images revealed two types of well-resolved lattice fringes with
interplanar distances of 0.273 and 0.41 nm indexed to the
(100) plane of MoS2 and the (006) plane of ZIS, respectively.
However, no lattice fringe patterns were observed for Au
nanodots, which may be ascribed to the complete dispersion of
Table 1. Average Crystallite Size of Neat ZIS in Comparison
with x wt % MoS2/ZIS and 1%Au−1%MoS2/ZIS
name of the photocatalysts
crystallite size (nm)
neat ZIS
1.84
1.59
1.74
1.68
1.64
1.45
0.5%MoS2/ZIS
1%MoS2/ZIS
3%MoS2/ZIS
5%MoS2/ZIS
1%Au−1%MoS2/ZIS
Figure 2. (a−c) FESEM images of 1%Au−1%MoS2/ZIS. (d) HRTEM image of 1%MoS2/ZIS and fringe pattern of 1%MoS2/ZIS (inset figure).
(e,f) HRTEM image and fringe pattern of 1%Au−1%MoS2/ZIS heterojunction photocatalysts.
D
DOI: 10.1021/acs.inorgchem.9b01105
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 3. XPS comparison spectra of 1%Au−1%MoS2/ZIS heterojunction photocatalysts with those of neat ZIS. (a) Survey scan, (b) Zn 2p, (c) In
3d, (d) S 2p, (e) Mo 3d, and (f) Au 4f.
Au nanodots in the heterojunction matrix. The various
Debye−Scherrer ring patterns for both 1%MoS2/ZIS and 1%
Au−1%MoS2/ZIS in Figure S2a,b correspond to the hkl plane
as (006), (102), (106), (112), and (203), demonstrating the d
-spacing values, which have good agreement with the XRD.
The accurate coexistence of various constituent elements in 1%
MoS2/ZIS and 1%Au−1%MoS2/ZIS heterojunction photo-
catalysts is confirmed from the EDAX spectroscopy mapping
image, which is established in Figure S2c,d. Furthermore,
energy-dispersive spectroscopy (EDS) elemental mapping
spectra of 1%Au−1%MoS2/ZIS heterojunction photocatalysts
from Figure S3a−f have suggested the uniform distribution of
each element (Zn, In, Mo, S, and Au) in the composite
photocatalysts.
X-ray Photoelectron Spectroscopy. The information
regarding the elemental composition and bonding config-
uration of the neat ZIS and 1%Au−1%MoS2/ZIS was explored
by the help of XPS. The survey spectrum of neat ZIS and 1%
Au−1%MoS2/ZIS photocatalyst is demonstrated in Figure 3a,
which shows the predominant signals for Zn, In, and S and Zn,
In, S, Mo, and Au, along with the presence of C and O,
respectively. No additional peak in the survey spectra of both
samples suggests the purity of the sample that fully matches the
EDAX spectrum. The surface adventitious C 1s peak (BE =
284.8 eV) was taken as the reference component in the
instrument to correct the binding energy of other existing
elements in the photocatalyst, which can be attributed to the
use of carbon tape during the preparation of the sample. The O
1s peak in the survey spectrum was assigned to the
atmospheric oxygen absorbed by the sample during analysis.
The electron-transfer mechanism in support of the formation
of the p−n heterojunction in the 1%Au−1%MoS2/ZIS
photocatalyst system can be explained by the XPS comparison
study of the composite photocatalyst with that of the neat
counterpart.
As shown in Figure 3b, for pure ZIS, the peak centered at
binding energies of 1022.09 and 1045.16 eV for Zn 2p3/2 and
2p1/2, respectively, suggesting the +2 oxidation state of the Zn,
whereas the binding energies in Figure 3c, at 445.20 and
452.77 eV assigned to 3d5/2 and 3d3/2, confirmed the +3
oxidation state of indium. The core-level spectra of S in pure
ZIS from Figure 3d have fitted into a doublet corresponding to
the binding energies 161.96 (2p3/2) and 163.11 eV (2p1/2),
exhibiting the −2 oxidation state of sulfur (S2−) in the ZIS
sample.16 After making a heterojunction with MoS2, the
binding energies of Zn 2p3/2, Zn 2p1/2, In 3d5/2, In 3d3/2, S
2p1/2, and S 2p3/2 shift to 1022.0, 1045.07, 444.63, 452.19,
162.6, and 161.4 eV, respectively. The previously described
negative shifting in the peak positions of Zn, In, and S in the
composite photocatalyst specifies that the electron transfer
takes place from a more electron-rich system, p-type MoS2 and
Au nanodots, to an n-type ZIS with a poor electron system. As
a result, the electron density of the ZIS in the composite
photocatalyst increases, resulting in lower binding energy.16,23
In the Figure 3e, the deconvolution of Mo 3d peaks gives a
weak peak at the binding energy 229.8 eV, assigned to 3d3/2
,
suggesting the dominant existence of Mo4+ in MoS2, and this is
due to the low loading content of MoS2 in the MoS2/ZIS
composite. In addition to this, another peak at the binding
energy 226.8 eV is determined due to the existence of the S 2s
peak.41
The high-resolution XPS spectrum of Au from Figure 3f has
deconvoluted into three peaks. The two binding energy
positions in the XPS spectra of 1%Au−1%MoS2/ZIS are
E
DOI: 10.1021/acs.inorgchem.9b01105
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
observed at 83.9 (Au 4f7/2) and 87.7 eV (Au 4f5/2), with a peak
splitting of ∼3.8 eV. The difference in the two binding energies
unambiguously suggests that Au is present in the nanodot form
and confirms that Au nanodots are loaded on the surface of
MoS2/ZIS heterojunction photocatalyst. Besides, the small and
linear portion of (αhν)1/n versus hν. The value of n manifests
the kind of optical transition of a semiconductor photocatalyst
that is produced in the Tauc’s expression graph. Throughout
the observation, the optical transitions of ZIS, MoS2, 1%MoS2/
ZIS, and 1%Au−1%MoS2/ZIS photocatalysts are directly
allowed, and the obtained values of band-gap energies are
2.59, 1.70, 2.45, and 2.33 eV, respectively. This decrease in
band-gap value suggests that the visible-light-capturing capacity
of the composite photocatalyst in a broad range facilitates the
production of a greater number of charge carriers and hence
causes impressive photocatalytic activity.
broad peak observed at the binding energy 91.4 eV
42
corresponds to Zn 3p1/2
.
UV−Visible Spectroscopy. To determine the photo-
absorptive behavior as well as the cause of the photocatalytic
effect, UV−vis diffuse reflectance spectroscopy (DRS) spectra
of all deliberate photocatalysts were evaluated, and the
comparison study is shown in Figure 4. Moreover, the shape
By using the following expressions, we can also estimate the
band-edge potential of the semiconductor photocatalysts
EVB = χ − Ee + 0.5Eg
ECB = EVB − Eg
Here ECB, EVB, and Eg are the corresponding energy levels of
the CB and VB and the band-gap energy of the semiconductor,
respectively. χ represents the absolute electronegativity value of
the semiconductor (4.85 for ZIS and 5.32 for MoS2), and Ee is
the energy of free electrons with reference to the hydrogen
scale, whose value is ∼4.5 eV. Now, the calculated values of VB
and CB for ZIS are 1.98 and −0.61 eV, and those for MoS2 are
1.67 and −0.03 eV, respectively. The above type of band-edge
position suggests the straddling type of band alignment in the
heterojunction photocatalyst.
Photoluminescence. Furthermore, the comparison study
of the photoluminescence (PL) spectra of the aforementioned
samples with different weight percentages of MoS2 is shown in
Figure 5 to investigate the surface processes involving the
Figure 4. UV−vis DRS spectra of neat ZIS, x wt % MoS2/ZIS, and
1% Au−1%MoS2/ZIS along with color variation image of different
photocatalysts.
of the absorption band of bare ZIS with steep edges ends in the
visible-light region around 560 nm, indicating that the visible-
light absorption was due to the band-gap transition and not
due to the transition from impurity levels to the CB, as
observed for some ion-doped photocatalysts.6 In addition,
MoS2 has greater absorption ability, ranging from the UV−
visible to the IR region, which is in accordance with the black
color of the material. The incorporation of MoS2 in the ZIS
hybrid can crucially affect the spectral behavior of MoS2/ZIS
heterostructures, thus enhancing the photocatalytic activity.
Furthermore, with the loading content of MoS2, all absorption
peaks have red-shifted due to the photosensitizing effect of
MoS2, which is supported by the color of the photocatalysts,
which changed from light yellow to dark yellow. Despite this,
with the introduction of Au nanodots, 1%Au−1%MoS2/ZIS
showed a higher wavelength absorbance, which may be
attributed to the presence of a large number of hot electrons.
Again, the UV−vis absorption spectra of Au-MoS2/ZIS do not
exhibit any surface plasmon resonance (SPR) band around the
high-wavelength region, and the reason is that with decreasing
particle size, there is a damping in the SPR band.43
Figure 5. Photoluminescence spectra of neat ZIS, x wt % MoS2/ZIS,
and 1%Au−1%MoS2/ZIS.
charge separation, migration, and transfer efficiency as well as
the lifetime of the photoinduced excitons. The PL spectra
result from the emission of the photon that is acquired by the
recombination of photoexciton pairs when the photocatalyst is
irradiated. All samples were excited with the excitation
wavelength of 400 nm at room temperature. As displayed in
Figure 5, all samples exhibited three emission peaks at
approximately 469, 492, and 537 nm. The main two emission
peaks at around 469 and 492 nm were found to be caused by
the band-edge free exciton, and at 537 nm, all photocatalysts
showed a wide and weak emission peak mainly associated with
the band-gap recombination of photoexcitons.44,45 Remark-
ably, the peak intensities of PL spectra have become much
weaker with the increase in the loading percentage of MoS2;
From the UV−vis DRS plot, we have also estimated the
band gap as well as the band-edge potential of all photo-
catalysts (Figure S4a,b) by fitting the absorbance data with the
Schuster−Kubelka−Munk function-transformed reflectance
spectra and Tauc’s plot via the following equation
(αhν)1/n = A(hν − Eg)
Here α is the absorption coefficient, h is the Planck constant, ν
is the frequency of the incident light, A is the arbitrary
constant, and Eg is the energy of band gap of the
semiconductor that can be obtained by extrapolating the
F
DOI: 10.1021/acs.inorgchem.9b01105
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 6. (a) EIS spectra and (b) linear sweep voltammetry (LSV) spectra of neat ZIS, 1%MoS2/ZIS, and 1%Au−1%MoS2/ZIS heterojunction
photocatalysts in dark (D) and light (L). MS plot of (c) ZIS, 1%MoS2/ZIS, and 1%Au−1%MoS2/ZIS and (d) MoS2.
notably, the intensity of the PL peak further seems to be much
more weakened than those of pristine ZIS and x%MoS2/ZIS
when Au nanodots are incorporated onto the surface of the 1%
MoS2/ZIS heterojunction photocatalyst. This observation
regarding the PL spectra has ensured that the recombination
rate of photoexcited electrons and holes of ZIS is suppressed
after modification with MoS2 and Au nanodots, and this
phenomenon increases the lifetime of each charge carrier at the
respective band-edge position, which is beneficial for the
excellent photocatalytic reaction. To further elucidate the
charge-transfer dynamics of photogenerated charge carriers,
time-resolved photoluminescence (TRPL) analysis is provided
in Figure S5. By fitting the TRPL decay spectra to the
following equation, the average emission lifetimes of ZIS and
1%Au−1%MoS2/ZIS were calculated to be 1.31 and 2.11 ns,
respectively. The 1%Au−1%MoS2/ZIS heterojunction photo-
catalyst has a longer PL lifetime than the neat ZIS, suggesting
that the rate of recombination of photoinduced charge carriers
is slow in the 1%Au−1%MoS2/ZIS photocatalyst, and this
causes a shorter transport distance in the bulk phase.30
interface. The semicircle arc for 1%MoS2/ZIS is much smaller
than that of neat ZIS. Thus the presence of a very low amount
of MoS2 in the composite greatly suppresses the charge
transfer-resistance and, in other words, boosts the photo-
conductivity.47 Here the p−n junction efficiently separated
photoexcitons, ultimately minimizing the chance of recombi-
nation losses occurring in ZIS. Moreover, Au-loaded MoS2/
ZIS shows a much lower Rct value as compared with its binary
counterpart, and it was concluded that Au further delays the
charge recombination process and hence significantly improves
the photoconductivity.7
Linear Sweep Voltammetry Analysis. Upon irradiation
with energy larger than the band gap, excitons were generated
and efficiently separated at the p−n junction. Then, the
electrons and holes started drifting toward the opposite
electrodes in the presence of an external electric field, resulting
in the generation of a photocurrent. Under the application of a
forward bias, charge carriers were injected across the junction,
leading to the enhanced charge-carrier transportation rate.
Thus, with the application of an external bias and light
irradiation, a photocurrent was produced.48 The photocurrent
density value of various photocatalysts is shown in Figure 6b.
For neat ZIS, the photocurrent density was calculated to be
0.025 mA·cm−2, whereas for 1%MoS2/ZIS, it was enhanced up
to 1.07 mA·cm−2. Furthermore, the decoration of the Au
nanodot exceptionally accelerated the exciton generation and
separation, thus giving a a high amount photocurrent density,
that is, 2.56 mA·cm−2. To verify the effective charge-transfer
ability as well as the photocurrent response over ZIS and 1%
Au−1%MoS2/ZIS, a transient photocurrent test (I−t curve)
was introduced at 0.2 V (EAg/AgCl), as shown in Figure S6. For
this test, four light “on” cycles under visible-light illumination
(300 W xenon lamp with 420 nm cutoff filter) and four light
2
B1τ12 + B2τ2
τ =
B1τ1 + B2τ2
Electrochemical Impedance Spectroscopy Analysis.
The electrochemical impedance spectra (EIS) of the photo-
catalysts is presented in Figure 6a to show the detail of the
charge transportation resistance of the photoanode under both
dark and light irradiation. In the presence of light, the
semicircle arc of all photoanodes is greatly diminished in
comparison with that in the dark, which confirms that the
exciton transfer is efficiently carried out under light
irradiation.46 In general, the smaller semicircle implies the
lower charge-transfer resistance at the electrode−electrolyte
G
DOI: 10.1021/acs.inorgchem.9b01105
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 7. (a) Hydrogen evolution rate of neat ZIS, 1%MoS2ZIS, and 1%Au−1%MoS2/ZIS. (b) Reusability test of H2 production for 1%Au−1%
MoS2/ZIS heterojunction photocatalysts.
“off” cycles were conducted at every 30 s time interval for both
ZIS and 1%Au−1%MoS2/ZIS. As can be seen in the graph,
under same experimental condition, the single ZIS exhibited a
relatively lower transient photocurrent density (2.53 μA·cm−2)
compared with that of 1%Au−1%MoS2/ZIS (4.68 μA·cm−2).
This type of results confirms that both MoS2 and Au accelerate
the charge separation as well as the charge-transfer efficiency
through the p−n heterojunction mechanism, thus improving
the rate of the photocatalytic reaction.
Mott−Schottky Analysis. To estimate the flat band
potential and carrier density at the electrode−electrolyte
interface, the Mott−Schottky measurement was performed.
The measurement can also provide qualitative insights into the
formation of p−n junctions from the shift in the flat band
potential.49 From Figure 6c, the Mott−Schottky plot of ZIS
represented a positive slope because it is an n-type semi-
conductor; similarly, MoS2 in Figure 6d showed a negative
slope of p-type characteristic behavior.23 An inverted shape
plot was then observed for MoS2/ZIS and Au-MoS2/ZIS,
which confirms the formation of a p−n junction between ZIS
and MoS2.50 In comparison with neat ZIS, the composite
junction materials exhibited shifts of flat band potential, which
indicates that a rearrangement occurred in making the p−n
junction, where the Fermi level of the p-type MoS2 shifted
upward, whereas the n-type ZIS shifted downward. From the
Mott−Schottky plot, we can also estimate the carrier density of
the materials, which indirectly gives a perfect idea about the
enhanced photoactivity.7 The p−n junction materials exhibited
a lower slope and hence a higher photocarrier density.
photocatalytic H2 evolution due to its low charge-carrier
density as well as the presence of the basal plane, which takes
part in the reaction. In comparison with the neat counterpart,
the MoS2/ZIS composite photocatalyst showed a better result
toward photocatalytic H2 generation; however, the rate of H2
evolution increased after loading only a small amount of Au
solution under visible-light illumination by facilitating the
electron channelizations, constructing a p−n heterojunction
pathway. The rate of H2 production of as-synthesized
photocatalysts with absorbance characteristics is illustrated in
Figure 7a. Pure ZIS exhibits a H2 production rate of 38.21
μmol/h. The trend of H2 evolution of various MoS2-loading
ZIS composite photocatalysts has the sequence of 5% MoS2/
ZIS < 0.5% MoS2/ZIS < 3% MoS2/ZIS < 1% MoS2/ZIS
(200.1, 238.6, 285.44, and 379.1 μmol/h, respectively) and is
shown in Figure S7. From the above data, the photocatalytic
efficiency first increased, then decreased with the loading
content of MoS2 because the excess amount of MoS2 shielded
the light-absorbing capacity of ZIS, thus weakening the light
intensity that falls on the surface of the ZIS and hence blocking
the active site. The rate of H2 production increased up to
561.25 μmol/h after loading of Au nanodots to the 1%MoS2/
ZIS photocatalyst, which is nearly 15 times higher than the rate
obtained by neat ZIS. The apparent conversion efficiencies of
H2 generation by 1% MoS2/ZIS and 1%Au−1% MoS2/ZIS
heterojunction photocatalysts were found to be 6.07 and 9.1%,
respectively.23 Table S1 delivers the photocatalytic H2
evolution activity of some recent studies of ZIS-based
composite photocatalysts.
Photocatalytic Activity of As-Synthesized Au-Modi-
fied MoS2/ZIS Heterojunction Photocatalyst. Hydrogen
Production under Visible-Light Irradiation. The photo-
catalytic activities via H2 production of neat ZIS, x wt %
MoS2/ZIS, and Au-MoS2/ZIS heterojunction photocatalyst
were examined under visible-light irradiation (λ ≥ 400 nm).
The photocatalytic performance was performed by taking 0.02
g of photocatalysts in aqueous mixture containing 0.25 M
Na2SO3 and 0.35 M Na2S solution with a continuous reaction
time of ∼1 h. The detection of no H2 gas from the blank
experiment (with no photocatalyst as well as no light)
suggested that the hydrogen evolution experiment is a
photocatalytic reduction reaction over the prepared photo-
catalyst. The photocatalytic activity of neat ZIS is very low,
which may be attributed to the high rate of recombination of
photoexcited charge carriers as well as the lack of reactive
active sites. Interestingly pure MoS2 is highly inactive toward
Aside from the highest photocatalytic activity, the photo-
catalytic durability of the photocatalyst was also investigated
throughout the activity and is shown in Figure 7b. For the
account of this reusability test, all samples were tested under
the same reaction conditions for repeated cycles. The 1 wt %
Au−1%MoS2/ZIS heterojunction photocatalyst cycled con-
tinuously with a time interval of 1 h for four consecutive cycles.
After four cycles, the photocatalyst exhibited slight deactivation
upon the recycling process. In addition to this, we also
performed the XPS analysis of 1 wt %Au−1%MoS2/ZIS
photocatalysts after four cycles of photocatalytic experiments,
and we have provided the figures in Figure S8. After the
photocatalytic reaction, there is a minute positive shifting in
the binding energy, with no additional product observed,
confirming that the photocatalyst still remained somewhat
active for further activity tests. However, in the case of Au XPS
spectra, the decrease in the peak intensity (Au 4f7/2) was
H
DOI: 10.1021/acs.inorgchem.9b01105
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 8. (a) % of phenol degradation efficiency of various photocatalysts, (b) photocatalytic phenol degradation concentration, (c) first-order
kinetic study of all photocatalysts, and (d) reusability study of 1%Au−1%MoS2/ZIS heterojunction photocatalyst.
mainly due to the dislodging/leaching of some Au nanodots
because of the loose attachment to the surface of MoS2/ZIS.51
Activity Test for Phenol Degradation under Solar Light
Illumination. To further verify the versatile photocatalytic
activity of the optimized photocatalyst, a 1 wt %Au−1%MoS2/
ZIS, degradation study of phenol as the model colorless
organic contaminant was examined under direct natural solar
light illumination and compared with the neat ZIS and x%
MoS2/ZIS photocatalyst. A negligible amount of phenol
oxidation was noticed in the blank test, that is, without a
photocatalyst or sunlight, indicating the good response of the
photocatalyst and the good stability of the organic pollutant
phenol toward visible light. The investigation of the phenol
degradation test was illustrated by considering different
parameters such as the loading content of photocatalyst,
different pH effects, and the kinetics of degradation efficiency.
Effect of Loading Concentration of Photocatalyst. To
determine the effect of optimum photocatalyst loading on the
photodegradation of phenol, five different concentrations of
photocatalyst, that is, 5, 10, 15, and 20 mg, at a fixed phenol
concentration were examined under solar light irradiation. The
effect of concentration loading is illustrated in Figure S9a, and
from the figure, it is observed that for 10 mg of photocatalyst,
the maximum degradation of phenol is achieved. With the
loading content of photocatalyst, the degradation efficiency
increased to 10 mg of photocatalyst, and after it gradually
decreased due to the agglomeration of the catalyst that blocks
the active sites of the photocatalyst.
the efficiency of phenol degradation was found to be the
highest under solar light irradiation. The sequence of efficiency
at different pH values follows the order pH 6 > pH 4 > pH 2 >
pH 8.
Kinetics Study of Phenol Degradation. The kinetics
study of all photocatalysts was analyzed by taking 0.01 g of
photocatalyst with an optimum pH 6 at different time intervals.
No detectable phenol degradation was achieved under dark
conditions, but a reasonable change was made under light
illumination. The phenol degradation rates of MoS2, ZIS, 1%
MoS2/ZIS, and 1%Au−1%MoS2/ZIS photocatalysts after 2 h
of experiment are shown in Figure 8a. For the kinetics study,
the plot for phenol degradation was drawn between
concentrations versus time and is explained in the Figure 8b.
From this observation, we concluded that neat ZIS (14%) and
neat MoS2 (2%) predicted a very negligible amount of
oxidation product of phenol arising due to the low charge
separation efficiency and the low migration of photoexcited
charge carriers. However, the enhancement of the high
photocatalytic degradation of phenol was achieved after the
formation of the heterojunction between MoS2 and ZIS, and
the sequential order for the degradation of phenol over x%
MoS2/ZIS was 5% MoS2/ZIS (28%) < 0.5% MoS2/ZIS (42%)
< 3% MoS2/ZIS (54%) < 1% MoS2/ZIS (69%). However, the
photocatalytic phenol oxidation rate further increased after the
incorporation of Au nanodots in the MoS2/ZIS heterojunction
photocatalyst, which arose due to the generation of hot
electrons by the Au nanodots, and the rate of degradation
reached up to 84%. The percentage of degradation efficiency of
the phenol was monitored in time intervals of 30 min for ∼2 h,
and after analysis with UV−vis DRS, a linear plot with a
negative slope (concentration vs time) was obtained, which
represents the first-order reaction kinetics (Figure 8c). Table
Effect of pH on Phenol Degradation. In addition, pH is
another component that affects the degradation efficiency of
phenol. The effect of pH was examined and is shown in Figure
S9b by carrying out the degradation process with different pH
values ranging from basic to acidic (pH 2, 4, 6, and 8). From
the experiment at slightly acidic pH, that is, at optimum pH 6,
I
DOI: 10.1021/acs.inorgchem.9b01105
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Scheme 1. Band-Gap and Band-Edge Alignment of p-MoS2 and n-ZIS with Respect to Vacuum Level (a) before Contact and
(b) after Contact, (c) Possible Charge-Transfer Mechanism in 1%MoS2/ZIS, and (d) 1%Au−1%MoS2/ZIS Heterojunction
Photocatalysts
S2 illustrates the rate constant value of all photocatalysts along
with regression coefficient.
demonstrated by the low peak intensity that was obtained by
the MeOH scavenger spectra.52,53
The photostability of the 1%Au−1%MoS2/ZIS hetero-
junction photocatalyst as further evaluated by the phenol
oxidation process by operating the photoreaction for 8 h at a
time interval of 2 h for four consecutive cycles (Figure 8d). No
apparent deactivation of cycles suggested the advantageous
photostability of the photocatalyst, which is again supported by
the XRD and H2 evolution data observed in the cyclic
experiment.
The photodegradation efficiency of phenol by some ZIS-
based composites and MoS2-based composite photocatalysts
under visible-light irradiation is illustrated in Table S3.
Protocol for the Electron-Transfer Mechanism in 1%
MoS2/ZIS and 1%Au−1%MoS2/ZIS Heterojunction Pho-
tocatalyst. The overall photocatalytic mechanism of the novel
heterojunction photocatalyst arose from the p−n hetero-
junction pathway, followed by the generation of hot electrons.
According to consensus, it is very much well known that the
efficient separation of the photogenerated electrons and holes,
which are the two cornerstones of the photocatalyst, eventually
supports the enhanced photocatalytic activity. On the basis of
the result described from PL and EIS, it has been deduced that
the rate of recombination of photogenerated electron and
holes is delayed when a heterojunction architecture is framed
Scavenger Test for Phenol Oxidation. The mechanism
for the degradation of phenol is governed by the scavenger
experiment under solar light illumination, which determines
the types as well as the roles of the active species that
participate in the reaction mechanism. In this regard, a series of
radical quenching experiments of 1%Au−1%MoS2/ZIS hetero-
junction photocatalyst was examined by taking various
scavengers such as IPA, p-BQ, and MeOH along with the
photocatalyst under the same analysis procedure. In detail, 1
mM IPA (•OH radical trapping), 1 mM p-BQ (O2 radical
•−
trapping), and MeOH (h+ trapping) were added to the
photocatalytic solution containing 10 mL of phenol solution
and 10 mg of photocatalyst under direct solar light illumination
for ∼2 h. The rate of photodegradation of phenol with
different scavengers is shown in Figure S9d, and the results
reveal the order of degradation of phenol to be IPA < p-BQ <
•
MeOH, respectively. The above results suggest that OH and
•−
O2 are the two main reasonable active species for the
photooxidation of phenol; it is alternatively illustrated that e−
is the primary participating active species that helps to produce
•OH and O2•− radicals. In addition, h+ has a minimal effect on
the rate of the oxidation process of phenol, which is
J
DOI: 10.1021/acs.inorgchem.9b01105
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
between MoS2 and ZIS and the incorporation of Au nanodots,
which significantly ameliorates the controlled photocatalytic
performance toward H2 evolution as well as the phenol
degradation under visible-light illumination. To gain deeper
insight into the extent of band bending in a p−n
heterojunction system and the electron-transfer mechanism
between Au, MoS2, and ZIS in the 1%Au−1%MoS2/ZIS
heterojunction photocatalyst, band-gap and band-edge poten-
tials, Fermi level alignment, and work functions are the crucial
parameters that are determined from both the UV−vis DRS
graph as well as Mott−Schottky analysis. In accordance with
the Mott−Schottky results, ZIS is an n-type semiconductor
with a positive slope, and MoS2 is a p-type semiconductor with
a negative slope. Derived from the UV−vis plot (Tauc graph),
the band gaps of neat MoS2 and neat ZIS are 1.70 and 2.59 eV,
respectively. Again, the band-edge potentials obtained from the
UV−vis plot (Mulliken electronegativity function) are 1.67
(VB) and −0.03 eV (CB) for MoS2 and 1.98 (VB) and −0.61
eV (CB) for neat ZIS. According to the Mott−Schottky graph,
the VB and CB positions obtained for MoS2 are 1.65 and
−0.05 eV, and those for ZIS are 1.98 and −0.61 eV,
respectively; those values are nearly equal to those obtained
in UV−vis results. The Fermi levels were found to be 1.55 and
−0.51 eV for MoS2 and ZIS, respectively. In the case of an n-
type semiconductor, the Fermi level lies just below (0.1 eV)
the CB and above (0.1 eV) the VB in the case of the p-type
semiconductor.54 In general, the Fermi level determines the
direction of the flow of charge carriers from one semi-
conductor to another. By using the following expression, we
can convert the above values of CB and VB from the normal
hydrogen electrode (NHE) to the vacuum level.55
ZIS because there is a greater chance to quicken the
recombination, thus retarding the rate of photocatalytic
activity. Hence the proper band-edge alignment is necessary
to assist in a meaningful photocatalytic reaction. In this regard,
the p−n heterojunction architecture is constructed through the
one-pot synthesis technique between the p-type MoS2 and the
n-type ZIS, which is essential to modulate the mechanism of
the photocatalytic reaction by creating a charge separation
chamber at the interface. An in-depth understanding of this
fact is elaborately described in the current session. When MoS2
nanoparticles are assembled to come close to ZIS before
contact (Scheme 1a), the transfer of electrons takes place from
n-ZIS to p-MoS2, which gives rise to a negative charge segment
near the junction site of p-MoS2; simultaneously, holes move
from p-MoS2 to n-ZIS, creating a positive section in the n-ZIS
adjacent to the junction. This process of electron hole
migration from one semiconductor to another will continue
until the Fermi level of both individual photocatalysts gets
equilibrated. This condition is known as the thermal
equilibrium state, and in this process, an internal electric
field is created at the interface that is directed from n-ZIS to p-
MoS2 and from p-MoS2 to n-ZIS at the same time, which
concurrently builds up to stop the charge diffusion from n-ZIS
to p-MoS2. This results in band bending by shifting of the
Fermi level with the corresponding VB and CB edges in such a
way that there is no change in the work function of each neat
counterpart.31,55 In consequence, this band bending leads to
bending of the vacuum level with the bending of the reduction
potential of H2 (4.5 eV, pH 0) as well as the potential that is
required for the formation of the superoxide (4.45 eV, pH 0)
in the same manner.55 However, the band edges of p-type and
n-type semiconductors bend downward and upward, respec-
tively. As a result, a built-in electric field followed by a
depletion region is created at the junction interface of both
MoS2 and ZIS, which enhances the charge separation
efficiency. The built-in potential with respect to the vacuum
level is determined by the qVbi = ϕp − ϕn expression, where ϕp
and ϕn represent the work functions for p- and n-type
semiconductors.55,57 In this case, the built-in potential value is
assumed to be nearly 2.0 eV. At the depletion layer, because of
the carrier concentration gradient, +ve charges are accumu-
lated at the surface of ZIS interface sites and −ve charges are
accumulate at the MoS2 interface surface sites for which across
the transition region there is a difference in the equilibrium
potential. However, on the basis of this contact potential, when
the MoS2/ZIS heterojunction photocatalyst is illuminated
under a visible-light source, an excitation treatment of
electrons from VB to CB occurs, leaving holes at the VB
edge. Consequently the electrons that are generated at the CB
of MoS2 could easily diffuse to the CB of ZIS, and holes that
are generated at VB could easily be transferred from ZIS to the
VB edge of MoS2, which significantly reduces the rate of
recombination as well as accelerates the efficient separation of
photogenerated excitons (Scheme 1b). This charge separation
technique efficiently utilizes the visible light, thus improving
the photocatalytic activity. Electron density further increases at
the CB edge of ZIS, which leads to the reduction of H2O to
produce hydrogen, simultaneously helping to generate a
superoxide ion.
ENHE = −4.5 − EVac
Here ENHE and EVac are the energy potentials on the NHE scale
and at the vacuum level, respectively. EVac is the energy level
that lies just above the semiconductor solid, where electrons
are present with zero kinetic energy with respect to the surface
of the sample (i.e., EVac = 0 eV). With reference to the vacuum
level, the CB and VB edges of neat ZIS are estimated to be
−3.89 and −6.48 eV, and those of MoS2 are −4.45 and −6.15
eV, respectively. Hence, in the present case, the Fermi energy
level is calculated to be approximately −3.99 for ZIS and
−6.05 eV for MoS2 nanoparticles, respectively, versus the
vacuum level. In addition, the work function is another
parameter that is calculated by using the mathematical
expression given below after knowing the position of Fermi
level reference with regard to the vacuum level.55
work function (ϕ) = EVac − Ef
Hence the calculated work functions obtained for MoS2 and
ZIS in 1%MoS2/ZIS are 6.05 and 3.99 eV, respectively.
Scheme 1a shows the band gap, band edge, Fermi level
position, and work function values of both MoS2 and ZIS with
respect to the vacuum level before contact, and it has been
illustrated that neat MoS2 and ZIS have a type-I band structure
(straddling gap type) in the 1%MoS2/ZIS heterojunction
photocatalyst.56 The above nested type of band structure
between MoS2 and ZIS is unable to boost the charge
separation efficiency.31 According to the general charge-
transfer mechanism, because the CB of MoS2 is lower than
that of ZIS, electrons can flow easily from the CB of ZIS to the
CB of MoS2, but holes cannot flow from the VB of MoS2 to
that of ZIS due to the higher positive potential of the VB of
In detail, at the CB surface of the ZIS sheet, a large number
of H2O molecules are adsorbed and get dissociated to H+ and
OH−; then, H+ ions are reduced to H2 (0 eV at NHE) by the
help of electrons present at the CB of ZIS. Similar to electrons,
K
DOI: 10.1021/acs.inorgchem.9b01105
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
holes also have a vital role in the enhancement of
photocatalytic activity. In the case of the H2 evolution
mechanism, the holes diffused at the VB of MoS2 are
consumed by scavengers, that is, the Na2S/Na2SO3 solution,
and give rise to S2O3 and S2 (Scheme 1c). In addition,
aqueous Na2S/Na2SO3 has a great part in the H2 evolution
reaction because it continuously supplies the sulfide ions to the
reaction solution, thus stabilizing the photocatalyst material as
well as preventing corrosion of the photocatalyst. Equations
1−7 represent the H2 evolution pathway
some energy on behalf of the transfer of the photogenerated
charge carrier from one semiconductor to another in the case
of the p−n heterojunction system, the photocatalytic activity
by this system is somehow limited. Throughout the
experimental study, it has been formulated that the photo-
catalytic activity for H2 evolution and phenol degradation is
further increased after hybridization of the MoS2/ZIS
heterojunction photocatalyst with Au nanodots. Going through
the literature survey, it has been found that under ambient
conditions, the work function of the Au lies in between 4.4 and
4.7 eV.58 When Au nanodots are introduced to the 1%MoS2/
ZIS, Au nanodots are deposited on the surface of ZIS
nanosheets in a greater amount than that deposited on MoS2
nanoparticles. Scheme S1 represents the Fermi level position of
Au nanodots in 1%MoS2/ZIS with respect to the vacuum level,
and in Scheme 1d, the electron-transfer mechanism from the
Au nanodot surface to the CB of ZIS is shown with respect to
their respective Fermi levels. Initially, novel metals are rich in
free electrons near their Fermi energy level. Whenever the 1%
Au−1%MoS2/ZIS heterojunction photocatalyst is subjected to
visible-light illumination, more hot electrons are generated, and
these free hot electrons are jumped into a higher energy
excited state. This movement of hot electrons results in an
upward shift of the Fermi level of Au nanodots closer to the
CB of ZIS so that the electrons can easily move from Au
nanodots to ZIS. Now, the electrons that reside at the higher
excited state of Au nanodots are rapidly transferred to the CB
of ZIS and take part in the photocatalytic reaction, resulting in
an enhancement in the photocatalytic activity as well as the
photocurrent density. This photocatalytic achievement occurs
at both the heterojunction photocatalyst surface and the Au
nanodot surface. From the Mott−Schottky analysis of the 1%
Au−1%MoS2/ZIS photocatalyst, it is elucidated that there has
been no change in the nature of the plot, which is explained by
the fact that the p−n heterojunction behavior of the MoS2/ZIS
remains unaltered. The introduction of Au nanodots can easily
trap the incident light due to the presence of a scattering center
(photon-to electron conversion) and thus extend the region of
the visible-light absorption range of the photocatalyst. The
above results proved that the Au nanodots are effective centers
that accelerate the H2 evolution rate and the degradation
efficiency of phenol 1.5- and 1.2-fold, respectively, compared
with the 1%MoS2/ZIS heterojunction photocatalysts. This
novel idea of the electron-transfer pathway provides excellent
evidence of the positive effect of the current heterojunction
photocatalyst on the photocatalytic activity, such as the
hydrogen production as well as the phenol degradation.
2−
2−
−
MoS2/ZIS + hν → MoS2(hVB+) + ZIS(eCB
)
(1)
−
+
Au + hν → Au(eCB + hVB
)
(2)
(3)
(4)
Au(eCB−) + MoS2/ZIS → Au + MoS2/ZIS(eCB
2H2O + 2eCB → H2 + 2OH−
)
−
−
2−
2hVB+ + 2S2− → S2
(5)
(6)
(7)
S2 + SO3 → S2O3 + S2−
SO3 + S2− + 2hVB+ → S2O3
2−
2−
2−
2−
2−
Overall:
2−
2−
2−
H2O + SO3 + S2− → H+ + S2O3 + S2
Again, the dissolved oxygen that is present in the aqueous
phenolic solution captures and reacts with the photogenerated
electrons at the CB of ZIS, producing a superoxide radical at
−0.046 eV (at NHE), which further reacts with hydrogen ions
to generate a hydrogen peroxide molecule. Again, by the attack
of photogenerated electrons, the above initiated H2O2
molecules are converted into strong oxidizing-agent hydroxyl
radicals (OH•), which have a great contribution toward the
degradation of phenol by giving (Scheme 1c). The overall
mechanism is a chain reaction in which H2O2 is the significant
intermediate species. On the contrary, photogenerated holes
directly take part in the degradation process of phenol, forming
CO2 and H2O. Equations 8−16 represent the photo-
degradation of the phenol mechanism pathway.
−
MoS2/ZIS + hν → MoS2(hVB+) + ZIS(eCB
)
(8)
(9)
−
+
Au + hν → Au(eCB + hVB
)
−
Au(eCB−) + MoS2/ZIS → Au + MoS2/ZIS(eCB
)
(10)
(11)
(12)
(13)
(14)
(15)
(16)
−
•
−
eCB + O2 → O2
•
−
e− + O2 + H2O → H2O• + OH
CONCLUSIONS
■
Ternary heterojunction photocatalyst Au-MoS2/ZIS with
different molar ratios of Mo has been successfully designed
for first time via a one-pot hydrothermal technique, followed
by a deposition−precipitation method. Each individual
component present in the composite photocatalysts contrib-
utes to the enhancement of the unprecedented photocatalytic
activity toward H2 production and the photodegradation of
phenol under visible light. The intimate junction constructed
at the interface of both MoS2 and ZIS boosts the photo-
catalytic activity compared with that of the neat counterpart,
thus suppressing the recombination of photoexcited charge
carriers. The p−n heterojunction also accelerates the migration
of photoexcitons from one semiconductor to another,
enlarging the visible-light absorption capacity of the systems.
e− + H2O• + H+ → H2O2
e− + H2O2 → OH• + OH−
Au(hVB+) + MoS2(hVB+) + OH− → Au + OH•
OH• + phenol → degraded products
From the above study, it is concluded that the one-pot-
synthesized 1%MoS2/ZIS heterojunction photocatalyst
showed an enhanced photocatalytic activity in which the H2
evolution rate is enhanced ten-fold and the degradation
efficiency is enhanced five-fold compared with the neat ZIS.
However, some experience revealed that due to the loss of
L
DOI: 10.1021/acs.inorgchem.9b01105
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
(6) Zhang, Z.; Huang, L.; Zhang, J.; Wang, F.; Xie, Y.; Shang, X.; Gu,
Y.; Zhao, H.; Wang, X. In Situ Constructing Interfacial Contact
MoS2/ZnIn2S4 Heterostructure for Enhancing Solar Photocatalytic
Hydrogen Evolution. Appl. Catal., B 2018, 233, 112−119.
(7) Liu, Q.; Lu, H.; Shi, S.; Wu, F.; Guo, J.; Deng, K.; Li, L. 2D
ZnIn2S4 Nanosheet/1D TiO2 Nanorod Heterostructure Arrays for
Improved Photoelectrochemical Water Splitting. ACS Appl. Mater.
Interfaces 2014, 6, 17200−172072.
(8) Yuan, L.; Yang, M.-Q.; Xu, Y.-J. A Low-Temperature and One-
step Method for Fabricating ZnIn2S4−GR Nanocomposites with
Enhanced Visible Light Photoactivity. J. Mater. Chem. A 2014, 2,
14401−14412.
(9) Liu, H.; Jin, Z.; Xu, Z.; Zhang, Z.; Ao, D. Fabrication of
ZnIn2S4−g-C3N4 Sheet-on-Sheet Nanocomposites for Efficient
Visible-Light Photocatalytic H2-Evolution and Degradation of organic
pollutants. RSC Adv. 2015, 5, 97951−97961.
(10) Li, S.; Dai, D.; Ge, L.; Gao, Y.; Han, C.; Xiao, N. Synthesis of
layer-like Ni(OH)2 Decorated ZnIn2S4 Sub-Microspheres with
Enhanced Visible-Light Photocatalytic Hydrogen Production Activity.
Dalton Trans. 2017, 46, 10620−10629.
(11) Chen, Y.; Hu, S.; Liu, W.; Chen, X.; Wu, L.; Wang, X.; Liu, P.;
Li, Z. Controlled Syntheses of Cubic and Hexagonal ZnIn2S4
Nanostructures with Different Visible-Light Photocatalytic Perform-
ance. Dalton Trans. 2011, 40, 2607−2613.
(12) Kale, S. B.; Kalubarme, R. S.; Mahadadalkar, M. A.; Jadhav, H.
S.; Bhirud, A. P.; Ambekar, J. D.; Park, C.-J.; Kale, B. B. Hierarchical
3D ZnIn2S4/Graphene Nano-Heterostructures: In-Situ Fabrication
with Dual Mimics in Solar Hydrogen Production and Anode for
Lithium Ion Battery. Phys. Chem. Chem. Phys. 2015, 17, 31850−
31861.
(13) Liu, C.; Chai, B.; Wang, C.; Yan, J.; Ren, Z. Solvothermal
Fabrication of MoS2 Anchored on ZnIn2S4 Microspheres with
Boosted Photocatalytic Hydrogen Evolution Activity. Int. J. Hydrogen
Energy 2018, 43, 6977−6986.
Furthermore, the photocatalytic activity of 1%MoS2/ZIS
heterojunction photocatalysts is improved by the interference
of Au nanodots at the surface of MoS2/ZIS. Because of the
generation of greater numbers of DET hot electrons in Au
nanodots, more energetic electrons are provided to the surface
of photocatalysts for better photocatalytic reactions. This p−n
heterojunction-based work demonstrates a potential inspira-
tion for developing a number of suitable hetrojunction
semiconductor photocatalysts for the improvement of photo-
catalytic activity.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.9b01105.
Details of photocatalytic experimental setup; character-
ization of the photocatalysts; XRD pattern of neat MoS2
and x wt % MoS2/ZIS; SAED and EDAX analysis of
neat ZIS and 1%Au−1%MoS2/ZIS; EDS spectra of 1%
Au−1%MoS2/ZIS; band-gap potential of neat ZIS and
neat MoS2; transient fluorescence spectrum and
transient photocurrent tests of neat ZIS and 1%Au−
1%MoS2/ZIS; hydrogen evolution study of x wt %
MoS2/ZIS; XPS spectra of used 1%Au−1%MoS2/ZIS;
study of phenol degradation with various loading
contents of photocatalysts at different pH values, x wt
% MoS2/ZIS, and absorbances of scavenger test; and
comparison study of H2 evolution and phenol
degradation data (PDF)
(14) Li, Y.; Zhang, K.; Peng, S.; Lu, G.; Li, S. J. Photocatalytic
Hydrogen Generation in the Presence of Ethanolamines over Pt/
ZnIn2S4 under Visible Light Irradiation. J. Mol. Catal. A: Chem. 2012,
363, 354−361.
AUTHOR INFORMATION
Corresponding Author
*E-mail: kulamaniparida@soauniversity.com.
■
(15) Wei, L.; Chen, Y.; Zhao, J.; Li, Z. Preparation of NiS/ZnIn2S4
as a Superior Photocatalyst for Hydrogen Evolution under Visible
Light Irradiation. Beilstein J. Nanotechnol. 2013, 4, 949−955.
(16) Li, X.-L.; Wang, X.-J.; Zhu, J.-Y.; Li, Y.-P.; Zhao, J.; Li, F.-T.
Fabrication of Two-Dimensional Ni2P/ZnIn2S4 Heterostructures for
Enhanced Photocatalytic Hydrogen Evolution. Chem. Eng. J. 2018,
353, 15−24.
(17) Voiry, D.; Mohite, A.; Chhowalla, M. Phase Engineering of
Transition Metal Dichalcogenides. Chem. Soc. Rev. 2015, 44, 2702−
2712.
ORCID
Kulamani Parida: 0000-0001-7807-5561
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We thank SOA management for their cooperation and
encouragement.
■
(18) Kumar, N. A.; Dar, M. A.; Gul, R.; Baek, J.-B. Graphene and
Molybdenum Disulfide Hybrids: Synthesis and Applications. Mater.
Today 2015, 18, 286−298.
(19) Chai, B.; Yan, J.; Wang, C.; Ren, Z.; Zhu, Y. Remarkably
Enhanced Photocatalytic Hydrogen Evolution over MoS2 Nanosheets
Loaded on Uniform CdS Nanospheres. Appl. Surf. Sci. 2017, 391,
376−383.
(20) Huang, X.; Zeng, Z.; Zhang, H. Metal Dichalcogenide
Nanosheets: Preparation, Properties and Applications. Chem. Soc.
Rev. 2013, 42, 1934−1946.
(21) Liu, T.; Shi, S.; Liang, C.; Shen, S.; Cheng, L.; Wang, C.; Song,
X.; Goel, S.; Barnhart, T. E.; Cai, W.; Liu, Z. Iron Oxide Decorated
MoS2 Nanosheets with Double PEGylation for Chelator-Free Radio
Labeling and Multimodal Imaging Guided Photothermal Therapy.
ACS Nano 2015, 9, 950−960.
(22) Huang, Y.; Guo, J.; Kang, Y.; Ai, Y.; Li, C. M. Two Dimensional
Atomically Thin MoS2 Nanosheets and Their Sensing Applications.
Nanoscale 2015, 7, 19358−19376.
(23) Swain, G.; Sultana, S.; Moma, J.; Parida, K. Fabrication of
Hierarchical Two-Dimensional MoS2 Nanoflowers Decorated upon
Cubic CaIn2S4 Microflowers: Facile Approach To Construct Novel
REFERENCES
■
(1) Jana, M. K.; Rao, C. N. R. Two-Dimensional Inorganic
Analogues of Graphene: Transition Metal Dichalcogenides. Philos.
Trans. R. Soc., A 2016, 374, 20150318.
(2) Rao, C. N. R.; Gopalakrishnan, K.; Maitra, U. A Comparative
Study of the Potential Applications of Graphene, MoS2 and other 2D
Materials in Energy Devices, Sensors and Related Areas. ACS Appl.
Mater. Interfaces 2015, 7, 7809−7832.
(3) Chaudhari, N. S.; Bhirud, A. P.; Sonawane, R. S.; Nikam, L. K.;
Warule, S. S.; Rane, V. H.; Kale, B. B. Ecofriendly Hydrogen
Production from Abundant Hydrogen Sulfide Using Solar Light-
Driven Hierarchical Nanostructured ZnIn2S4 Photocatalyst. Green
Chem. 2011, 13, 2500−2506.
(4) Yang, J.; Shin, H. S. Recent Advances in Layered Transition
Metal Dichalcogenides for Hydrogen Evolution Reaction. J. Mater.
Chem. A 2014, 2, 5979−5985.
(5) Jo, W. K.; Sivakumar Natarajan, T. Facile Synthesis of Novel
Redox-Mediator-free Direct Z-Scheme CaIn2S4 Marigold-Flower-like/
TiO2 Photocatalysts with Superior Photocatalytic Efficiency. ACS
Appl. Mater. Interfaces 2015, 7, 17138−17154.
M
DOI: 10.1021/acs.inorgchem.9b01105
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Metal-Free p−n Heterojunction Semiconductors with Superior
Charge Separation Efficiency. Inorg. Chem. 2018, 57, 10059−10071.
(24) Zong, X.; Yan, H.; Wu, G.; Ma, G.; Wen, F.; Wang, L.; Li, C.
Enhancement of Photocatalytic H2 Evolution on CdS by Loading
MoS2 as Cocatalyst under Visible Light Irradiation. J. Am. Chem. Soc.
2008, 130, 7176−7177.
(25) Meng, F.; Li, J.; Cushing, S. K.; Zhi, M.; Wu, N. Solar
Hydrogen Generation by Nanoscale p-n Junction of p-type
Molybdenum Disulfide/n-type Nitrogen-Doped Reduced Graphene
Oxide. J. Am. Chem. Soc. 2013, 135, 10286−10289.
(26) Zhang, J.; Zhu, Z.; Feng, X. Construction of Two-Dimensional
MoS2/CdS p−n Nanohybrids for Highly Efficient Photocatalytic
Hydrogen Evolution. Chem. - Eur. J. 2014, 20, 10632−10635.
(27) Zhao, W.; Liu, Y.; Wei, Z.; Yang, S.; He, H.; Sun, C. Fabrication
of a Novel p-n Heterojunction Photocatalyst n-BiVO4@p-MoS2 with
Core-shell Structure and its Excellent Visible-Light Photocatalytic
Reduction and Oxidation Activities. Appl. Catal., B 2016, 185, 242−
252.
(28) Swain, G.; Sultana, S.; Naik, B.; Parida, K. Coupling of
Crumpled-Type Novel MoS2 with CeO2 Nanoparticles: A Noble-
Metal-Free p−n Heterojunction Composite for Visible Light
Photocatalytic H2 Production. ACS Omega 2017, 2, 3745−3753.
(29) Huang, T.; Chen, W.; Liu, T.-Y.; Hao, Q.-L.; Liu, X.-H. ZnIn2S4
hybrid with MoS2: A Non-noble Metal Photocatalyst with Efficient
Photocatalytic Activity for Hydrogen Evolution. Powder Technol.
2017, 315, 157−162.
(40) Tang, R.; Yin, R.; Zhou, S.; Ge, T.; Yuan, Z.; Zhang, L.; Yin, L.
Layered MoS2 Coupled MOFs-Derived Dual-Phase TiO2 for
Enhanced Photoelectrochemical Performance. J. Mater. Chem. A
2017, 5, 4962−4971.
(41) Ma, S.; Xie, J.; Wen, J.; He, K.; Li, X.; Liu, W.; Zhang, X.
Constructing 2D Layered Hybrid CdS Nanosheets/MoS2 Hetero-
junctions for Enhanced Visible-Light Photocatalytic H2 Generation.
Appl. Surf. Sci. 2017, 391, 580−591.
(42) Yu, C.; Yu, Y.; Xu, T.; Wang, X.; Ahmad, M.; Sun, H.
Hierarchical Nanoflowers Assembled with Au Nanoparticles Deco-
rated ZnO Nanosheets toward Enhanced Photocatalytic Properties.
Mater. Lett. 2017, 190, 185−187.
(43) Amendola, V.; Pilot, R.; Frasconi, M.; Marago, O. M.; Iati, M.
A. Surface Plasmon Resonance in Gold Nanoparticles: A Review. J.
Phys.: Condens. Matter 2017, 29, 203002.
(44) Yang, G.; Chen, D.; Ding, H.; Feng, J.; Zhang, J. Z.; Zhu, Y.;
Hamid, S.; Bahnemann, D. W. Well-Designed 3D ZnIn2S4 Nano-
sheets/TiO2 Nanobelts as Direct Z-Scheme Photocatalysts for CO2
Photoreduction into Renewable Hydrocarbon Fuel with High
Efficiency. Appl. Catal., B 2017, 219, 611−618.
(45) Yuan, Y.-J.; Chen, D.; Zhong, J.; Yang, L.-X.; Wang, J.; Liu, M.-
J.; Tu, W.-G.; Yu, Z.-T.; Zou, Z.-G. Interface Engineering of Noble-
Metal-Free 2D-2D MoS2/Cu-ZnIn2S4 Photocatalyst for Enhanced
Photocatalytic H2 Production. J. Mater. Chem. A 2017, 5, 15771−
15779.
́
(46) Zhang, H.; Tian, W.; Li, Y.; Sun, H.; Tade, M. O.; Wang, S.
(30) Zhang, S.; Liu, X.; Liu, C.; Luo, S.; Wang, L.; Cai, T.; Zeng, Y.;
Yuan, J.; Dong, W.; Pei, Y.; Liu, Y. MoS2 Quantum Dots Growth
Induced by S Vacancy in ZnIn2S4 Monolayer: Atomic-Level
Heterostructure for Photocatalytic Hydrogen Production. ACS Nano
2018, 12, 751−758.
(31) Reddy, K. H.; Parida, K. M.; Satapathy, P. K. CuO/PbTiO3: A
New-Fangled p−n Junction Designed for The Efficient Absorption of
Visible Light with Augmented Interfacial Charge Transfer, Photo-
electrochemical and Photocatalytic Activities. J. Mater. Chem. A 2017,
5, 20359.
(32) Fu, W.; Xu, X.; Wang, W.; Ye, M.; Shen, J. In-situ growth of
NiFe2O4/2D MoS2 p-n heterojunction immobilizing palladium
nanoparticles for enhanced visible-light photocatalytic activities.
ACS Sustainable Chem. Eng. 2018, 6, 8935−8944.
(33) Wei, Z.; Benlin, D.; Fengxia, Z.; Xinyue, T.; Jiming, X.; Lili, Z.;
Shiyin, L.; Leung, D. Y. C.; Sun, C. A Novel 3D Plasmonic p-n
Heterojunction Photocatalyst: Ag Nanoparticles on Flower-like p-
Ag2S/n-BiVO4 and its Excellent Photocatalytic Reduction and
Oxidation Activities. Appl. Catal., B 2018, 229, 171−180.
(34) Tian, Y.; Li, W.; Zhao, C.; Wang, Y.; Zhang, B.; Zhang, Q.
Fabrication of Hollow Mesoporous SiO2-BiOCl@PANI@Pd Photo-
catalysts to Improve the Photocatalytic Performance under Visible
Light. Appl. Catal., B 2017, 213, 136−146.
Heterostructured WO3@CoWO4 Bilayer Nanosheets for Enhanced
Visible-Light Photo, Electro and Photoelectro-Chemical Oxidation of
Water. J. Mater. Chem. A 2018, 6, 6265−6272.
(47) He, K.; Xie, J.; Liu, Z.-Q.; Li, N.; Chen, X.; Hu, J.; Li, X. Multi-
Functional Ni3C Cocatalyst/g-C3N4 NanoHeterojunctions for Robust
Photocatalytic H2 Evolution under Visible Light. J. Mater. Chem. A
2018, 6, 13110−13122.
(48) Mukherjee, S.; Maiti, R.; Katiyar, A. K.; Das, S.; Ray, S. K.
Novel Colloidal MoS2 Quantum Dot Heterojunctions on Silicon
Platforms for Multifunctional Optoelectronic. Sci. Rep. 2016, 6,
29016.
(49) Liu, Y.; He, H.; Li, J.; Li, W.; Yang, Y.; Li, Y.; Chen, Q.
Preparation and Enhanced Photoelectrochemical Performance of p-n
Heterojunction CuFe2O4/WO3 Nanocomposite Film. RSC Adv.
2015, 5, 99378−99384.
(50) Pareek, A.; Kim, H. G.; Paik, P.; Borse, P. H. Ultrathin MoS2−
MoO3 Nanosheets Functionalized CdS Photoanodes for Effective
Charge Transfer in Photoelectrochemical (PEC) Cells. J. Mater.
Chem. A 2017, 5, 1541−1547.
(51) Gao, M.; Peh, C. K. N.; Pan, Y.; Xu, Q. H.; Ho, G. W. Fine
Structural Tuning of Whereabout and Clustering of Metal−Metal
Oxide Heterostructure for Optimal Photocatalytic Enhancement and
Stability. Nanoscale 2014, 6, 12655−12664.
(35) Yin, W.; Bai, L.; Zhu, Y.; Zhong, S.; Zhao, L.-H.; Li, Z.; Bai, S.
Embedding Metal in the Interface of p-n Heterojunction with Stack
Design for Superior Z-Scheme Photocatalytic Hydrogen Evolution.
ACS Appl. Mater. Interfaces 2016, 8, 23133−23142.
(36) Yin, Z.; Chen, B.; Bosman, M.; Cao, X.; Chen, J.; Zheng, B.;
Zhang, H. Au Nanoparticle-Modifi ed MoS2 Nanosheet-Based
Photoelectrochemical Cells for Water Splitting. Small 2014, 10,
3537−3543.
(37) Wang, D.; Zhang, X.; Shen, Y.; Wu, Z. Ni-doped MoS2
Nanoparticles as Highly Active Hydrogen Evolution Electrocatalysts.
RSC Adv. 2016, 6, 16656−16661.
(38) Zhang, N.; Xie, S.; Weng, B.; Xu, Y.-J. Vertically Aligned ZnO-
Au@CdS Core-Shell Nanorod Arrays as an All-Solid-State Vectorial
Z-Scheme System for Photocatalytic Application. J. Mater. Chem. A
2016, 4, 18804−18814.
(39) Wang, Y.; Shen, Y.; Xie, A.; Li, S.; Wang, X.; Cai, Y. A Simple
Method To Construct Bifunctional Fe3O4/Au Hybrid Nanostructures
and Tune Their Optical Properties in the Near-Infrared Region. J.
Phys. Chem. C 2010, 114, 4297−4301.
(52) Chai, B.; Yan, J.; Wang, C.; Ren, Z.; Zhu, Y. Enhanced Visible
Light Photocatalytic Degradation of Rhodamine B over Phosphorus
Doped Graphitic Carbon Nitride. Appl. Surf. Sci. 2017, 391, 376−383.
(53) Chai, B.; Liu, C.; Yan, J.; Ren, Z.; Wang, Z. J. In-situ Synthesis
of WO3 Nanoplates Anchored on g-C3N4 Z-Scheme Photocatalysts
for Significantly Enhanced Photocatalytic Activity. Appl. Surf. Sci.
2018, 448, 1−8.
(54) Guo, F.; Shi, W.; Wang, H.; Han, M.; Li, H.; Huang, H.; Liu,
Y.; Kang, Z. Facile Fabrication of a CoO/g-C3N4 p−n Heterojunction
with Enhanced Photocatalytic Activity and Stability for Tetracycline
Degradation under Visible Light. Catal. Sci. Technol. 2017, 7, 3325−
3331.
(55) Nashim, A.; Parida, K. M. n-La2Ti2O7/p-LaCrO3: A Novel
Heterojunction Based Composite Photocatalyst with Enhanced
Photoactivity Towards Hydrogen Production. J. Mater. Chem. A
2014, 2, 18405−18412.
(56) Hu, W.; Yang, J. Two-Dimensional Van der Waals
Heterojunctions for Functional Materials and Devices. J. Mater.
Chem. C 2017, 5, 12289−12297.
N
DOI: 10.1021/acs.inorgchem.9b01105
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
(57) Annathurai, S.; Chidambaram, S.; Baskaran, B.; Prasanna
Venkatesan, G. K. D. Green Synthesis and Electrical Properties of p-
CuO/n-ZnO Heterojunction Diodes. J. Inorg. Organomet. Polym.
Mater. 2019, 29, 535.
(58) Kahn, A. Fermi level, work function and vacuum level. Mater.
Horiz. 2016, 3, 7−10.
O
DOI: 10.1021/acs.inorgchem.9b01105
Inorg. Chem. XXXX, XXX, XXX−XXX