# Nanostructured LaFeO3-MoS2 for efficient photodegradation and photocatalytic
hydrogen evolution

Subrata Das, Sagar Dutta, Angkita Mistry Tama and M. A. Basith
<EMAIL_ADDRESS>Nanotechnology Research Laboratory, Department of
Physics, Bangladesh University of Engineering and Technology, Dhaka-1000,
Bangladesh.

DOI: 10.1016/j.mseb.2021.115295

###### Abstract

Fabrication of heterogeneous photocatalysts has received increasing research
interest due to their potential applications for the degradation of organic
pollutants in wastewater and evolution of carbon-free hydrogen fuel via water
splitting. Here, we report the photodegradation and photocatalytic hydrogen
generation abilities of nanostructured LaFeO3-MoS2 photocatalyst synthesized
by facile hydrothermal technique. Prior to conducting photocatalytic
experiments, structural, morphological and optical properties of the
nanocomposite were extensively investigated using X-ray diffraction analysis,
field emission scanning electron microscopy and UV-visible spectroscopy,
respectively. Nanostructured LaFeO3-MoS2 photodegraded $\sim$96% of rhodamine
B dye within only 150 minutes which is considerably higher than that of LaFeO3
and commercial Degussa P25 titania nanoparticles. The LaFeO3-MoS2
nanocomposite also exhibited significantly enhanced photocatalytic efficiency
in the decomposition of a colorless probe pollutant, ciprofloxacin eliminating
the possibility of the dye-sensitization effect. Moreover, LaFeO3-MoS2
demonstrated superior photocatalytic activity towards solar hydrogen evolution
via water splitting. Considering the band structures and contribution of
reactive species, a direct Z-scheme photocatalytic mechanism is proposed to
rationalize the superior photocatalytic behavior of LaFeO3-MoS2 nanocomposite.

##  I Introduction

In recent years, semiconductor photocatalysts have manifested promising
potential for the production of clean hydrogen (H2) fuel via water splitting
and for the removal of various toxic pollutants from wastewater under solar
illumination Appavu ; Guo ; Hu ; Yendrapati ; Mirzaee ; Basith1 ; Wen .
Notably, perovskite metal-oxide lanthanum ferrite (LaFeO3) with relatively
narrow band gap ($\sim$2.34 eV) as compared to titania (TiO2) photocatalyst,
has emerged as a promising photocatalyst because of its visible-light
responsive photocatalytic activity, non-toxicity and excellent structural
stability under the photochemical reaction conditions Ismael ; Peng ; Scafetta
. Moreover, LaFeO3 has been reported as a promising ferroelectric material
possessing spontaneous polarization potential at room temperature Acharya2010
. Although in recent years, significant amount of research is being conducted
for synthesizing nanostructured LaFeO3 Mihai ; Parida ; Tijare  with high
photocatalytic efficiency, it has not been possible yet to employ LaFeO3
nanoparticles for industrial applications due to their high tendency of
agglomeration Peng , considerably low charge separation efficiency and
consequently, fast recombination rate of photoinduced $e^{-}-h^{+}$ pairs
Ismael ; Peng ; Mihai .
Therefore, currently, numerous research endeavors are being undertaken to
alleviate the shortcomings of LaFeO3 nanoparticles which include surface
modification, metals and non-metals doping, engineering heterogeneous
nanocomposites and introducing lattice defects Peng ; Ren ; Wu2015 ; Wu2018 ;
Yang . A number of recent investigations have reported that the photocatalytic
efficiency of LaFeO3 nanoparticles can be substantially increased by forming
their Z-scheme heterojunctions with other semiconductor photocatalysts such as
g-C3N4, SnS2 which have suitable band gap energy and appropriate band edge
positions Wu2018 ; Acharya2017 ; Luo . However, according to previous reports,
the performance of direct Z-scheme photocatalysts depends largely on the
energy band configurations, crystallographic structure, surface states and
interfacial features of their constituents which makes it quite challenging to
design and develop efficient Z-scheme systems Zhang . Therefore, despite
showing promising potential, LaFeO3 based Z-scheme photocatalyst
nanocomposites are yet less explored.
In recent years, molybdenum disulfide (MoS2), a layered transition metal
dichalcogenide, is being effectively employed in photocatalysis as a promising
co-catalyst owing to its unique structure, tunable band gap and intriguing
optical properties Chen ; Han ; Splendiani . Especially, mono or few-layer
MoS2 nanosheets have been reported to introduce significant improvement in the
photocatalytic performance of various pristine metal oxide nanoparticles e.g.
ZnO, YVO4 etc. by forming type-II or Z-scheme heterojunctions with them Chen ;
Qian . These two-dimensional (2D) ultrathin nanosheets also provide enormous
specific surface area and prevent the agglomeration of photocatalyst
nanoparticles owing to their favorable dispersion and synergetic effect
Jaramillo ; Tama . Moreover, MoS2 nanosheets yield a plenty of exposed active
edge sites which can efficiently catalyze the photocatalytic H2 evolution
reaction Chen ; Qin . Hence, considering the compatible band structures and
promising properties, it is highly intriguing to develop a heterojunction
between LaFeO3 nanoparticles and 2D MoS2 nanosheets Acharya2020  and
extensively investigate its photocatalytic performance.
Therefore, in the present investigation, we have synthesized LaFeO3
nanoparticles via a facile sol-gel method Hu ; Ahsan  and incorporated it with
ultrasonically exfoliated few-layer MoS2 nanosheets by adapting a low-
temperature hydrothermal synthesis technique Tama ; Das ; Sabarinathan  to
investigate comprehensively the photodegradation and photocatalytic hydrogen
evolution efficiency of LaFeO3-MoS2 nanocomposite. Interestingly, LaFeO3-MoS2
has outperformed both LaFeO3 and commercial Degussa P25 TiO2 nanoparticles in
the photodecomposition of a textile dye, rhodamine B (RhB) in aqueous solution
as well as solar H2 generation via water splitting. It is noteworthy that
colorful organic dyes e.g. RhB may pose some limitations in evaluating the
performance of photocatalysts because of their dye-sensitization effect
Barbero . Hence, we have further investigated the photocatalytic activities of
LaFeO3-MoS2 nanocomposite towards the photodegradation of a colorless probe
pollutant, ciprofloxacin antibiotic which cannot directly absorb visible light
to cause photosensitization Wen . Notably, the outcome of this experiment
provided additional evidence for the superior photocatalytic performance of
as-synthesized LaFeO3-MoS2 nanocomposite towards photodegradation.

##  II Materials and Methods

###  II.1 Materials

The chemical reagents used were analytical grade lanthanum nitrate hexahydrate
[La(NO3)3.6H2O], ferric nitrate nonahydrate [Fe(NO3)3.9H2O], citric acid
[C6H8O7], ammonium hydroxide [NH4OH] and molybdenum disulfide [MoS2] powder
(manufacturer of all reagents is Sigma-Aldrich, Germany).

###  II.2 Sample preparation

####  II.2.1 Synthesis of LaFeO3 nanoparticles

LaFeO3 nanoparticles were synthesized by a citrate based sol-gel technique Hu
; Ahsan . Initially, La(NO3)3.6H2O, Fe(NO3)3.9H2O and C6H8O7 were taken in the
molar ratio of 1:1:2 and dissolved in deionized (DI) water under magnetic
stirring to form a homogeneous solution. To ensure stability, the pH of this
solution was then adjusted to 11 by adding NH4OH. Afterward, the resulting
mixture was heated at 85 ∘C for 3 hours until it turned into viscous gel. The
viscous gel was further heated at 95 ∘C for 3 more hours to initiate auto
combustion. After combustion, the obtained dry gel was ground for 1 hour and
finally, calcined for 2 hours at air in a programmable furnace (Carbolite, UK)
with a heating and cooling rate of 3 ∘C/min to acquire the desired LaFeO3
nanoparticles by removing volatile substances and carbonaceous materials. We
have calcined LaFeO3 nanoparticles at three different temperatures i.e. 600,
700 and 800 ∘C to optimize the calcination temperature in terms of
crystallinity and uniform surface morphology. Notably, the crystallite size of
LaFeO3 significantly enhanced when calcined at 900 ∘C. Hence, we did not
further increase the calcination temperature beyond 800 ∘C. For brevity,
LaFeO3 nanoparticles calcined at x ∘C will be referred to as LaFeO3(x) from
now on.

####  II.2.2 Synthesis of few-layer MoS2 nanosheets

Ultrathin few-layer MoS2 nanosheets have been prepared directly from their
bulk powder with a high yield of 60% via a facile ultrasound assisted
exfoliation technique, which has been described in details elsewhere Das ;
Sahoo . Initially, a 5 mg/mL solution of MoS2 powder and isopropanol was
prepared. The solution was then ultrasonicated for 90 minutes in a simple
ultrasonic bath (Powersonic 510, maximum power 100 W, sonication frequency 37
kHz or 80 kHz) by applying a continuous ultrasound wave having constant 100 W
power and 80 kHz frequency. After 4 hours of rest, residue bulk powder and
sheets with micron-size thickness were removed from the ultrasonicated
solution via careful decantation. Finally, the collected supernatant was dried
for 5 hours in a universal drying oven (Labtech, Republic of Korea) at 80 ∘C
to obtain MoS2 nanosheets.

####  II.2.3 Synthesis of LaFeO3-MoS2 nanocomposite

Low-temperature hydrothermal technique is considered beneficial for
synthesizing efficient nanostructured photocatalysts because of its ability of
inhibiting grain growth, ensuring small particle size with narrow
distribution, enhancing crystallinity and minimizing impurities due to the low
processing temperature Basith1 ; Shi . Therefore, in this investigation, we
have adopted the facile, low-cost hydrothermal technique to incorporate MoS2
nanosheets with LaFeO3(800) nanoparticles Chen ; Tama . At first, 1 mmol of
LaFeO3(800) and 0.05 mmol of MoS2 were dissolved in 50 mL of DI water (i.e. 5
mole% MoS2 to LaFeO3 content). After stirring rigorously for 2 hours, the
mixture was heated at 150 ∘C for 14 hours in a Teflon-lined autoclave. After
cooling down the obtained suspension to room temperature (RT) naturally,
centrifugation and continuous rinsing was carried out using DI water and
ethanol. Finally, the mixture was dried at 85 ∘C in the oven for 12 hours to
get the desired LaFeO3-MoS2 nanocomposite.

###  II.3 Characterization techniques

The powder X-ray diffraction (XRD) spectra of as-prepared nanomaterials were
obtained by a diffractometer (PANalytical Empyrean, UK) equipped with
Cu-$K_{\alpha}$ radiation (wavelength, $\lambda$ = 1.5418 Å). Further,
FullProf Suite Software was used for extensive analysis of the experimentally
acquired XRD data via Rietveld refinement method Rodriguez-Carvajal . Field
emission scanning electron microscopy (FESEM) imaging along with the energy
dispersive X-ray (EDX) analysis was carried out using a scanning electron
microscope (XL30SFEG; Philips, Netherlands) to investigate the surface
morphology and elemental composition of the samples. A ferroelectric loop
tracer (Marine India, India) coupled with an external amplifier (10 kV) was
used to determine the leakage current density as well as polarization vs.
electric field (P-E) hysteresis loops of the samples Basith1 . The driving
frequency was maintained at 50 Hz during the measurement of P-E loops. The
diffuse reflectance spectra (DRS) of as-prepared materials were recorded at RT
by a UV-visible spectrophotometer (UV-2600, Shimadzu, Japan) using barium
sulfate powder as the reference (wavelength range 200-1100 nm) Yendrapati . A
Spectro Fluorophotometer (RF-6000, Shimadzu, Japan) was used to carry out
photoluminescence (PL) analysis.

###  II.4 Photocatalytic degradation of dye and antibiotic

The photocatalytic efficiency of prepared nanomaterials and commercial TiO2
nanoparticles was evaluated in the photodegradation of RhB dye and
ciprofloxacin antibiotic in aqueous medium under solar irradiation Tama ; Wei
. The details of the photocatalytic experiments are provided in electronic
supplementary information (ESI). In order to evaluate the precision and
experimental validation, all the photocatalytic activity tests were carried
out three times at exactly same experimental conditions. The experimental
values are mean of triplicate measurements and their standard deviations have
been presented in the figures as error bars at each data point.

###  II.5 Photocatalytic hydrogen evolution

The photocatalytic H2 evolution test was performed using a slurry-type
photochemical reactor containing 40 mg of photocatalyst homogeneously
suspended in 60 mL of DI water Basith1 . Initially, the system was purged with
argon gas for 30 minutes to remove the dissolved oxygen and then irradiated by
the 500 W Hg-Xe lamp (Hamamatsu Photonics, Japan). Under illumination, the
evolved H2 gas was collected at 1 hour interval and then, analyzed and
measured in the gas chromatography (GC) device (Shimadzu, Japan) equipped with
a thermal conductivity detector (TCD) and a gas analyzer. The GC programming
was set up with a reverse polarization so that the hydrogen peaks could be
attained in the upward direction. The gas analyzer compared the peak
intensities of extracted gases to the peaks observed for standard hydrogen
gas. The amount of produced hydrogen gas after 4 hours of irradiation was
calculated in $\mu$mol g-1 catalyst from this comparison. For comparison, the
hydrogen evolution test was also performed using commercially available TiO2
nanocatalyst under identical experimental conditions. The error analyses were
also carried out for these experiments.

##  III Results and discussions

###  III.1 Structural characterization

####  III.1.1 Crystal structure

Figure 1: XRD patterns of LaFeO3 nanoparticles calcined at (a) 600 ∘C, (b) 700
∘C and (c) 800 ∘C.

The crystallographic structure and phase of as-synthesized nanomaterials have
been investigated by analyzing their powder XRD spectra via Rietveld
refinement method. Fig. 1(a), (b) and (c) display the Rietveld refined powder
XRD patterns of LaFeO3 nanoparticles calcined at 600, 700 and 800 ∘C,
respectively in the same scale. As shown in the figures, the diffraction peaks
exhibited by all these samples can be indexed at (101), (121), (220), (202),
(240), (242), and (204) planes (JCPDS card file no 37-1493) which conform to
the orthorhombic crystal phase (Pnma space group) of LaFeO3 perovskite
Acharya2017 ; Zhao . No impurity or secondary phases were detected in these
XRD spectra, implying that the phase-purity of LaFeO3 nanoparticles was
unperturbed by the varying calcination temperatures. However, the intensity
and sharpness of the XRD peaks have considerably enhanced with the increase in
calcination temperature which clearly indicates that LaFeO3(800) nanoparticles
possess higher degree of crystallinity as compared to the other two samples.
Our observation agrees well with several previous investigations reporting
that higher activation temperature ensures better crystallization for LaFeO3
nanoparticles Parida ; Wiranwetchayan . The average crystallite sizes of
LaFeO3 nanoparticles calcined at 600, 700 and 800 ∘C are estimated to be about
18, 31 and 45 nm, respectively from the XRD patterns by means of the Scherrer
equation Scherrer .

The XRD spectra of MoS2 nanosheets and LaFeO3-MoS2 nanocomposite are presented
in Fig. 2. As shown in Fig. 2(a), the diffraction peaks demonstrated by MoS2
nanosheets can be readily indexed to single-phase 2H hexagonal structure
(P63/mmc space group) (JCPDS no. 37-1492) Chen ; Liu2014 . The observed small
intensity of the XRD peaks of MoS2 revealed their few-layer sheet-like
structure which is qualitatively in good agreement with our previous
investigation Das . Fig. 2(b) exhibits that all the diffraction peaks of
LaFeO3 nanoparticles along with the most intense peak corresponding to (002)
plane of MoS2 nanosheets are present in the XRD spectrum of LaFeO3-MoS2,
implying the successful formation of the nanocomposite. However, the other
diffraction peaks of MoS2 are not visible in this XRD spectrum because of
their relatively low intensity which can be attributed to the small molar
ratio of MoS2 to LaFeO3 in the nanocomposite. Clearly, the Bragg positions
evince the simultaneous existence of two phases corresponding to LaFeO3 and
MoS2 in the crystal lattice of the nanocomposite. The average crystallite size
of LaFeO3-MoS2 nanocomposite has been calculated to be $\sim$36 nm by the
Scherrer equation which is smaller than that of pristine LaFeO3(800)
nanoparticles ($\sim$45 nm).
Table 1 presents different structural parameters and constituent phases (in
wt%) for as-synthesized nanomaterials. The errors on the last significant
digit of the crystallographic parameters are indicated by the numbers in the
parentheses. Notably, the calculated lattice parameters of LaFeO3 and MoS2 are
well consistent with the values reported in literature Ren ; Liu2014 . The
unit cell volume of LaFeO3 nanoparticles was observed to nominally enhance
with increasing calcination temperature. It is worth noticing that the
calculated percentages of the constituent LaFeO3 and MoS2 phases, 92.92% and
7.08%, respectively, in the nanocomposite are close to their mole percentages
used during synthesis. Moreover, the structural variables corresponding to
these two phases remained reasonably unaltered in the LaFeO3-MoS2
nanocomposite, implying that no deformation occurred in the crystallographic
structure of individual phases during the preparation of the nanocomposite.
For further insight, we have tabulated the calculated atomic coordinates of
the samples along with the reliability (R) factors (Rwp, Rp, RExp and
$\chi^{2}$) of Rietveld refinement in ESI Table S1. The quality of the
refinement is evident from the modest R values, in particular the goodness of
fit parameter ($\chi^{2}$) of as-synthesized nanomaterials.

Figure 2: XRD spectra of (a) few-layer MoS2 nanosheets and (b) LaFeO3-MoS2 nanocomposite.  Table 1: Structural parameters of synthesized nanomaterials. Sample | Constituent | Crystallographic phase (wt%) | Space group | a (Å) | b (Å) | c(Å) |  V (Å3)
---|---|---|---|---|---|---|---
LaFeO3(600) |  LaFeO3 | Orthorhombic (100%) | Pnma | 5.551(0) | 7.856(0) | 5.551(1) | 242.10(0)
LaFeO3(700) |  LaFeO3 | Orthorhombic (100%) | Pnma | 5.557(1) | 7.854(2) | 5.555(1) | 242.47(0)
LaFeO3(800) |  LaFeO3 | Orthorhombic (100%) | Pnma | 5.569(0) | 7.853(0) | 5.555(0) | 242.90(0)
MoS2 |  MoS2 | Hexagonal (100%) | P63/mmc | 3.149(0) | 3.149(0) | 12.250(2) | 105.18(0)
LaFeO3-MoS2 |  LaFeO3 | Orthorhombic (92.92%) | Pnma | 5.569(3) | 7.854(6) | 5.554(4) | 242.90(0)
|  MoS2 | Hexagonal (7.08%) | P63/mmc | 3.160(3) | 3.160(3) | 12.277(2) | 106.14(0)

####  III.1.2 Surface morphology

Figure 3: FESEM images of MoS2 nanosheets : (a) sheet-like structure and (b)
nanoscopic thickness. LaFeO3 nanoparticles calcined at (c) 600 ∘C, (d) 700 ∘C
and (e) 800 ∘C. (f) Particle size distribution histogram of LaFeO3
nanoparticles calcined at 800 ∘C. (g) FESEM image and (h) corresponding
histogram of LaFeO3-MoS2 nanocomposite.

The surface morphology of as-synthesized samples was investigated by FESEM
imaging as shown in Fig. 3. The ultrathin lamellar structure of vertically
grown 2D MoS2 nanosheets is demonstrated in Fig. 3(a). To show the
morphological change of MoS2 due to exfoliation, we have provided two
differently magnified FESEM images of bulk MoS2 powder in ESI Fig. S1. It can
be seen that bulk MoS2 consisted of numerous tightly stacked indistinguishable
MoS2 monolayers which have effectively delaminated to few-layer MoS2
nanosheets after exfoliation. The thickness of individual sheet can be
estimated to be between 8 to 15 nm, suggesting that each nanosheet comprises
$\sim$11-21 MoS2 monolayers (Fig. 3(b)) Splendiani ; Das .
Fig. 3(c), (d) and (e) are the FESEM images of LaFeO3 nanoparticles calcined
at 600, 700 and 800 ∘C, respectively. As evident from the figures, at lower
calcination temperatures (600-700 ∘C), the particles are irregular in shape
with non-homogeneous and porous surface morphology. However, with a further
increment in calcination temperature to 800 ∘C, the surface morphology was
considerably improved and the degree of agglomeration was reduced to a
moderate extent albeit at the cost of slightly increased particle size. Such
reduction in agglomeration might be due to the enhancement of total specific
surface area and the free energy of the system, whereas the increment in
particle size can be imputed to the increased crystal growth rate of the
particles at elevated temperature Ahsan ; Wiranwetchayan ; Pang . The size
distribution histogram in Fig. 3(f) demonstrates that the particle size of
nanostructured LaFeO3(800) lies in the range of $\sim$50-140 nm with an
average of $\sim$87 nm.
Fig. 3(g) and (h) show the FESEM image and corresponding size distribution
histogram of LaFeO3-MoS2 nanocomposite. As can be clearly seen, LaFeO3
nanoparticles are successfully assembled on the surface of the 2D MoS2
nanosheets with almost no agglomeration. The satisfactorily homogeneous and
non-porous surface of the as-prepared nanocomposite reveals that the growth of
LaFeO3 was much more controlled in the presence of MoS2. The improved growth
of nanoparticles can be attributed to the presence of LaFeO3-anchoring
hydroxyl group over the MoS2 surface Das ; Vattikuti . Additionally, we
anticipate that the 2D MoS2 nanosheets have provided sufficient surface sites
for anchoring LaFeO3 to prevent aggregation Han ; Tama . Noticeably, the shape
of the particles has become nearly spherical and reasonably uniform after
incorporating MoS2. Moreover, the particle size distribution of the
nanocomposite was found to be considerably narrower ($\sim$45-70 nm) with
smaller average size ($\sim$59 nm) as compared to LaFeO3 nanoparticles which
is qualitatively consistent with the outcome of our XRD analysis. However, for
both the nanomaterials, the SEM images exhibit slightly larger average
particle size as compared to their crystallite size calculated by Scherrer
equation which is a common phenomenon according to other reported works
Liu2009 . Finally, based on the observed surface morphology, the as-
synthesized LaFeO3-MoS2 nanocomposite can be expected to exhibit better
adsorption and photocatalytic efficiency than LaFeO3 nanoparticles owing to
their higher degree of uniformity in shape and size as well as increased
surface to volume ratio Basith1 ; Peng ; Ren .

####  III.1.3 Elemental composition

The Table S2 inserted in ESI presents the elemental composition of LaFeO3(800)
nanoparticles obtained both via EDX analysis and theoretical calculation
Wiranwetchayan . As can be seen in Table S2 of ESI, the experimentally
obtained mass and atomic percentages of desired elements La, Fe and O are in
excellent agreement with the theoretical values, providing further evidence
for the successful synthesis of LaFeO3 nanoparticles without impurities.
Moreover, the La:Fe atomic ratio in the as-prepared sample has been found to
be 1:1 which is same as the molar ratio of precursors La(NO3)3 and Fe(NO3)3
used in the reaction solution. This indicates that the entire metal contents
of the precursors were successfully carried over to the final sample under the
synthesis conditions. It is worth mentioning that no carbon content was
detected in these nanoparticles which can be ascribed to the complete
decomposition of carbonaceous compounds from the gel by the calcination
process at optimized temperature i.e. 800 ∘C. ESI Fig. S2 demonstrates the EDX
spectra of LaFeO3(800) nanoparticles and LaFeO3-MoS2 nanocomposite. As shown
in the Fig. S2 of ESI, along with the peaks of La, Fe and O atoms, the EDX
spectrum of LaFeO3-MoS2 contains additional peaks of Mo and S at around 2 keV,
confirming the successful formation of the nanocomposite.

###  III.2 Electric characterization

We have determined the ferroelectric polarization (P) as well as the leakage
current density (J) of the as-prepared nanomaterials for varying electric
fields (E) of up to $\pm$ 60 kV/cm. The driving frequency was maintained at 50
Hz while increasing the applied electric field. As demonstrated in ESI Fig.
S3, the leakage current density of the LaFeO3-MoS2 nanocomposite was
considerably smaller as compared to that of the LaFeO3(800) nanoparticles. The
high leakage current of the nanoparticles can be imputed to their surface
porosity as was evident from SEM analysis Van . We anticipate that the leakage
current has been reduced for the nanocomposite due to their non-porous
homogeneous surface morphology. ESI Fig. S4(a) and (b) demonstrate the P-E
hysteresis loops of LaFeO3(800) nanoparticles and LaFeO3-MoS2 nanocomposite,
respectively. As can be observed, the P-E loop of LaFeO3 is more dominated by
leakage current in comparison with the nanocomposite Deng . Freely movable
charges might be considered responsible for their cigar-shaped unsaturated
hysteresis loop Basith1 ; Deng . Interestingly, after incorporating MoS2
nanosheets, the P-E loops tended towards saturation at high electric fields,
indicating the improved ferroelectric behavior of the nanocomposite Deng ;
Scott . The remanent polarization and coercive field of the nanocomposite
enhanced with increasing applied voltage since stronger electric fields
contribute more to the reversal of ferroelectric domains due to higher driving
power Wu2012 . Moreover, it is noteworthy that the breakdown voltage of
LaFeO3-MoS2 (50 kV) was significantly higher than that of LaFeO3(800)
nanoparticles (10 kV). The interaction between the free charges of 2D MoS2 and
bound charges of LaFeO3 can be associated with the superior ferroelectric
behavior of the nanocomposite Jin  which is an important characteristic for
improving photochemical reactivity Khan . The improved ferroelectric behavior
of LaFeO3-MoS2 nanocomposite can be anticipated to facilitate spatially
selective adsorption of dye and $H_{2}O$ molecules on their surface by
creating electric field in the surrounding medium due to surface polarization
Basith1 ; Acharya2010 ; Cui . To be specific, different regions of the surface
may experience different extents of band bending depending on the surface
polarization and thus, promote different carriers to move to spatially
different locations. This phenomenon might result in spatially selective
adsorption at surface and facilitate surface redox reactions. Additionally,
the internal space charge layer in this ferroelectric nanocomposite may
considerably increase the charge separation and migration efficiency by
driving the $e^{-}-h^{+}$ pairs in opposite direction and thus, suppress their
recombination Acharya2010 ; Cui  and improve photocatalytic performance.

###  III.3 Optical characterization

####  III.3.1 Optical band gap

Figure 4: (a) Tauc plots for band gap estimation and (b) steady-state
photoluminescence (PL) spectra (excitation wavelength= 300 nm) of LaFeO3
nanoparticles calcined at 800 ∘C and LaFeO3-MoS2 nanocomposite.

Optical band gap energy of semiconductors is considered as a crucial factor to
assess their photocatalytic activity. Therefore, we have measured the UV-
visible DRS of as-prepared photocatalysts and estimated their band gaps using
Tauc model Scafetta  based on eqn. (1)-

$F(R)\times h\nu=A(h\nu-E_{g})^{n}$ (1)

where F(R) is a parameter calculated from DRS by Kubelka–Munk function which
is proportional to the absorption co-efficient Basith1  and $h\nu$, A, $E_{g}$
and n signify the energy of photons, proportionality, optical band gap and the
electronic transition type, respectively. Notably, Scafetta et al. have
reported direct-forbidden transition in bulk LaFeO3 and hence, we have
considered n=3/2 to obtain the desired Tauc plots Scafetta . Moreover, from
the steady-state photoluminescence (PL) spectroscopy, we observed evidence in
support of the direct band gap of the synthesized nanomaterials which will be
discussed later. As shown in Fig. 4(a), the direct optical band gaps of as-
prepared LaFeO3(800) and LaFeO3-MoS2 have been estimated to be 2.1 and 2.14
eV, respectively from the abscissa intercepts of the tangents to the linear
region of the plots Acharya2017 . The nominal difference between the observed
band gap values can be associated with the synergetic interaction developed
due to incorporation of MoS2 nanosheets Tama . However, for both as-prepared
nanomaterials, the band gaps are smaller than that of bulk LaFeO3 ($\sim$2.34
eV) Scafetta , suggesting their superior potential to exploit broader range of
the solar spectrum and exhibit enhanced visible light-responsive
photocatalytic activities.

####  III.3.2 Photoluminescence (PL) spectra at steady-state

The charge-carrier recombination phenomenon in semiconductor photocatalysts
can be predicted to a certain extent by performing steady-state
photoluminescence (PL) analysis. Generally, an intense PL peak indicates high
radiative recombination rate of photogenerated $e^{-}-h^{+}$ pairs. Fig. 4(b)
presents the steady-state PL spectra of as-prepared LaFeO3(800) and
LaFeO3-MoS2 for an excitation wavelength of 300 nm Wu2018 . From the position
of the PL peaks, the band gaps of LaFeO3 and LaFeO3-MoS2 can be estimated as
2.06 and 2.08 eV, respectively. These values closely match with the band gaps
obtained by UV-visible spectroscopy (Fig. 4(a)), providing evidence for the
direct transition in these nanomaterials. Moreover, it can be clearly observed
that the PL peak intensity of LaFeO3-MoS2 nanocomposite is lower than that of
LaFeO3 nanoparticles which suggests that comparatively smaller number of
photogenerated electrons and holes have radiatively recombined in the
nanocomposite. Such observation implies that incorporation of few-layer MoS2
nanosheets can effectively suppress the radiative recombination of charge-
carriers. Notably, our observation agrees well with a number of previous
studies reporting that MoS2 nanosheets can substantially quench PL of
different metal-oxide photocatalysts by developing heterostructured
nanocomposite Tama ; Qin . However, it should be mentioned that quenched PL
peak does not always correspond to enhanced photocatalytic activity since
information about the non-radiative recombination cannot be extracted from PL
characterization.

###  III.4 Photocatalytic degradation of rhodamine B (RhB) dye

Figure 5: Spectral changes during the decolorization of RhB over (a) LaFeO3
nanoparticles calcined at 800 ∘C and (b) LaFeO3-MoS2 nanocomposite under solar
irradiation. (c) RhB concentration change as a function of irradiation time.
(d) First-order kinetic plots for the photodegradation of RhB. (e)
Recyclability of LaFeO3-MoS2 nanocomposite for 4 successive runs. (f) XRD
patterns of LaFeO3-MoS2 nanocomposite before and after four cycles of
photocatalytic experiments

The photocatalytic efficiency of the synthesized catalysts was evaluated
towards the degradation of a well-known cationic dye, RhB under solar
illumination Basith2 . Initially, we had carried out the experiment at
different pH and found out that the degradation percentage was increased at
acidic reaction solution. Hence, pH= 3 was taken as the optimum pH to study
the photochemical reactions. Further, a blank test was performed to examine
the nature of RhB dye under our specified experimental conditions. Only
$\sim$4% of RhB was found to be decomposed after 150 minutes of irradiation,
suggesting its negligible self-photolysis potential. Additionally, prior to
irradiation, dark adsorption tests were carried out for all the photocatalysts
which demonstrated a nominal amount of RhB ($\sim$4%) degradation after 1 hour
at dark due to chemi-adsorption of the dye molecules on the surface of the
photocatalysts. Therefore, considering the outcomes of these two sets of
experiments, the decomposition of RhB under solar irradiation can be
attributed almost entirely to photocatalysis.

Fig. 5(a) and (b) present the change in the visible absorbance spectra of RhB
photodegraded over LaFeO3(800) nanoparticles and LaFeO3-MoS2 nanocomposite,
respectively. Here, the gradual decrement in the intensity of the
characteristic absorption peak at 553 nm indicates the successful
decolorization of RhB via photochemical reactions. Clearly, among the two
samples, the peak decreased more rapidly and took less time to ultimately
disappear in case of LaFeO3-MoS2 which implies its higher photocatalytic
efficiency.

####  III.4.1 Kinetics of RhB degradation.

Fig. 5(c) demonstrates the degradation profiles of RhB as a function of time
under both dark and light conditions in the presence of as-prepared
LaFeO3(600), LaFeO3(700), LaFeO3(800) and LaFeO3-MoS2 photocatalysts as well
as commercially available TiO2 nanoparticles. The result of blank test has
also been included in the figure. The percentage of degradation has been
calculated using eqn. (2)-

$Photocatalytic\;\;degradation(\%)=\frac{C_{0}-C}{C_{0}}\times 100$ (2)

Here, C0 and C denote the initial and residual concentrations measured at 30
minutes of interval, respectively. It was found that among the three samples
of LaFeO3 nanoparticles, LaFeO3(800) nanoparticles have manifested the best
photocatalytic performance which might be associated with its highest degree
of crystallinity Othman . However, it should be noted that despite possessing
better crystallinity, LaFeO3(700) nanoparticles showed lower photocatalytic
efficiency as compared to LaFeO3(600) nanoparticles which suggests that along
with crystallinity other factors i.e. crystallite size, surface area may also
influence the photocatalytic performance.

As can be seen in Fig. 5(c), LaFeO3(800) nanoparticles degraded $\sim$75% of
RhB dye after 150 minutes of illumination which was reasonably higher than
that of ($\sim$60%) TiO2 nanoparticles. Interestingly, the degradation
percentage of RhB increased significantly to $\sim$96% when LaFeO3-MoS2
nanocomposite was employed as the photocatalyst under the same experimental
conditions. To further assess the kinetics of RhB degradation, the
experimental data were fitted to zero-order (eqn. (3)), first-order (eqn. (4))
and second-order models (eqn. (5)).

$C=-k_{0}t+C_{0}$ (3) $ln(C\textsubscript{0}/C)=k_{1}t$ (4)
$1/C=1/C_{0}+k_{2}t$ (5)

Here, t denotes illumination time and k is the overall photodegradation rate
constant Mishra . The obtained coefficient of determination ($R^{2}$) and
average relative errors revealed that RhB degradation over the photocatalysts
followed the first-order kinetic model. In Fig. 5(d), the slope of the linear
fit plots provide the value of k1 for respective photocatalysts. The rate
constant in the first order kinetics of LaFeO3-MoS2 nanocomposite was found to
be 0.02329 min-1, which was almost 3 and 4.25 times higher than that of the
LaFeO3(800) nanoparticles (k1 = 0.00932 min-1) and TiO2 nanoparticles (k1 =
0.00548 min-1), respectively. Hence, LaFeO3-MoS2 nanocomposite can be
considered as a superior photocatalyst as compared to the other two samples.

Figure 6: (a) UV-visible absorption spectra of RhB before and after being
photodegraded by LaFeO3-MoS2 nanocomposite under solar irradiation. (b) Effect
of different scavengers on the degradation of RhB over LaFeO3-MoS2 under solar
irradiation.

####  III.4.2 Photostability and non-toxicity study

To evaluate the potential for industrial-scale application, we assessed the
photostability of LaFeO3-MoS2 nanocomposite by performing a recyclability test
Basith1 ; Mishra . As shown in Fig. 5(e), the photocatalytic efficiency of the
nanocomposite only slightly reduced after 4 consecutive photodegradation
cycles which can be imputed to the inevitable loss of the materials during the
recycling process. Furthermore, a comparison of the XRD patterns of the
LaFeO3-MoS2 nanocomposite before and after the 4 photocatalytic cycles is
shown in Fig. 5(f). No apparent change was observed in the XRD patterns before
and after the experiments, suggesting that the nanocomposite did not undergo
any structural deformation during the photocatalysis. Finally, we inspected
whether any toxic degradation products were evolved in the reaction mixture.
For this, the UV-visible absorbance spectra (i.e. ranging from 200 nm to 1100
nm) of RhB were examined before and after being photodegraded by LaFeO3-MoS2.
As evident from Fig. 6(a), no additional peaks have emerged after illumination
which implies that the photocatalyst did not introduce any carbonaceous
molecules or benzene radicals in the solution. Therefore, our as-synthesized
LaFeO3-MoS2 nanocomposite can be considered as a promising candidate for the
remediation of industrial dye effluents owing to its high photocatalytic
efficiency, excellent photostability, reusability and non-toxicity.

####  III.4.3 Effect of scavengers

To determine the major reactive species responsible for RhB degradation over
LaFeO3-MoS2 nanocomposite, we have performed trapping experiments using IPA,
acrylamide, EDTA-2Na and K2Cr2O7 as scavengers of hydroxyl ($\cdot OH$)
radicals, superoxide ($\cdot O_{2}^{-}$) radicals, holes ($h^{+}$) and
electrons ($e^{-}$), respectively Acharya2017 ; Tama . As evident from Fig.
6(b), the addition of acrylamide significantly decreased the degradation
efficiency, i.e. from 96% to 50%, suggesting that $\cdot O_{2}^{-}$ radicals
were the dominant active species. Moreover, it was found that the $\cdot OH$
radicals, $h^{+}$ and $e^{-}$ also had considerable effect on the
phototcatalytic process since IPA, EDTA-2Na and K2Cr2O7 had reasonably reduced
the degradation of RhB.

###  III.5 Photodegradation of colorless Ciprofloxacin in aqueous medium

Figure 7: (a) Spectral changes during the photodegradation of ciprofloxacin
over LaFeO3-MoS2 nanocomposite. (b) Degradation percentage of ciprofloxacin
after 4 hours of irradiation.

Fig. 7(a) demonstrates the spectral presentation of ciprofloxacin degradation
in the presence of LaFeO3-MoS2 nanocomposite under solar illumination.
Clearly, the characteristic absorption peaks at 280 and 320 nm gradually
decreased, indicating successful decomposition of ciprofloxacin Tama . As
shown in Fig. 7(b), after 4 hours of irradiation, 80% of ciprofloxacin was
photodegraded by LaFeO3-MoS2 which is reasonably higher as compared to
LaFeO3(800) nanoparticles (60%). The kinetics for the decomposition of
ciprofloxacin was observed to be first order (see ESI Fig. S5). The blank
ciprofloxacin had a very small degradation rate (k1 =0.00197 min-1), implying
its low self-degradation potential. The rate constant for LaFeO3-MoS2
(k1=0.00808 min-1) is found to be almost 2 fold higher than LaFeO3(800)
(k1=0.00445 min-1) which further justifies the superiority of the synthesized
nanocomposite photocatalyst.
Therefore, it is evident that the photocatalytic ciprofloxacin degradation
efficiency of LaFeO3 was significantly increased due to incorporation of few-
layer MoS2 nanosheets. It is worth noting that a previous investigation Sora
has reported 38% and 90% photocatalytic degradation of ciprofloxacin after 5
hours of solar irradiation in the presence of LaFeO3 and LaFeO3+H2O2,
respectively. Notably, ingestion of H2O2 exerts toxic effects on human cells
and results in morbidity via corrosive damage, oxygen gas formation and lipid
peroxidation Watt . Without using such a toxic reagent, we could achieve as
high as 80% degradation of ciprofloxacin employing our as-prepared LaFeO3-MoS2
photocatalyst.

###  III.6 Photocatalytic hydrogen generation

Figure 8: Amount of evolved H2 via water splitting as a function of
irradiation time.

Photocatalytic H2 generation potential of as-synthesized nanomaterials were
evaluated through water splitting under solar illumination Basith2 . Fig. 8
demonstrates the amount of produced H2 gas by LaFeO3(800), LaFeO3-MoS2 and
commercial TiO2 photocatalysts as a function of time for 4 hours of
irradiation. From the figure, the H2 evolution rate (HER) for LaFeO3-MoS2
nanocomposite has been calculated to be $\sim$179 $\mu$mol g-1 h-1, which is
higher than that of both LaFeO3(800) ($\sim$157.5 $\mu$mol g-1 h-1) and TiO2
nanoparticles ($\sim$137 $\mu$mol g-1 h-1). The enhanced H2 evolution
potential can be attributed to the abundant catalytically active edge cites
provided by few-layer MoS2 nanosheets as well as increased interfacial charge
transfer efficiency and high oxidation and reduction potential (will be
discussed later) of the nanocomposite Zhang ; Jaramillo .
For further insight, we have carried out a comparative analysis between the
outcome of our experiments and some related previous investigations which is
presented in Table 2. It is well known that photochemical reactions depend
highly on experimental conditions. Therefore, for rational comparison, we have
also provided details regarding light source and sacrificial reagents used in
the investigations. Noticeably, the H2 evolution rate (HER) of our as-prepared
nanocomposite is reasonably higher than that of different LaFeO3 based
nanocomposites and alkaline-earth doped LaFeO3 nanoparticles Xu ;
Vijayaraghavan . For instance, Xu et al. have reported that maximum HERs of 7
and 121.2 $\mu$mol g-1 h-1 can be achieved by modifying LaFeO3/g-C3N4
heterojunctions with CdS and NiS co-catalysts, respectively Xu . However, the
reported synthesis route involved more precursors and complex steps as
compared to our adopted technique. Additionally, our synthesized LaFeO3-MoS2
has also shown significantly higher HER than a number of similar
heterostructured photocatalysts such as MoS2/TiO2, MoS2/RGO, MoS2/YVO4 etc
Chen ; Kanda ; Min . It is noteworthy that all the referred previous studies
had used sacrificial reagents to facilitate the catalytic reactions, whereas
our as-prepared LaFeO3-MoS2 nanocomposite has demonstrated satisfactory
performance in H2 evolution without using any additional reagents or external
energy input. Therefore, the present investigation might open-up a new avenue
for low-cost solar H2 production by water splitting.

Table 2: Comparison between the hydrogen evolution rates of LaFeO3-MoS2 and
related photocatalysts.

Photocatalyst | Light source | Sacrificial reagent/Co-catalyst | H2 yield | Reference
---|---|---|---|---
|  |  | ($\mu$ mol g-1 h-1) |
2% CdS modified LaFeO${}_{3}/g-C_{3}N_{4}$ | 300 W Xe lamp; $\lambda$>400 nm | 10 vol. % triethanolamine aqueous solution | 7 | Xu
2% NiS modified LaFeO${}_{3}/g-C_{3}N_{4}$ | 300 W Xe lamp; $\lambda$>400 nm | 10 vol. % triethanolamine aqueous solution | 121.2 | Xu
0.3 molar Mg doped LaFeO3 | Medium pressure 150 W Hg visible lamp | 10 vol. % methanol aqueous solution | 178.4 | Vijayaraghavan
2% Au/LaFeO3 | 500 W Xe lamp | Na${}_{2}S/Na_{2}SO_{3}$ aqueous solution | 155 | Huang
0.047 mass% MoS${}_{2}/TiO_{2}$ | 300 W Xe lamp; $\lambda$>300 nm | 5% formic acid aqueous solution | 2.2 | Kanda
1 wt% $MoS_{2}/TiO_{2}$ | 300 W Xe lamp AM 1.5G filter | 20 vol. % methanol aqueous solution | 119.5 | Liu2013
$MoS_{2}/RGO$ | 300 W Xe lamp; $\lambda$ $\geq$ 420 nm | 15 vol. % triethanolamine aqueous solution | 38.3 | Min
2.5% $MoS_{2}/YVO_{4}$ | 300 W Xe lamp | 20 vol. % methanol and 0.002 M $H_{2}PtCl_{6}$ aqueous solution | 134 | Chen
$LaFeO_{3}-MoS_{2}$ | 500 W Hg-Xe lamp | Aqueous solution (no reagent) | 179 | Present work

###  III.7 Photocatalytic mechanism

To elucidate the rationale behind the superior photocatalytic activities of
LaFeO3-MoS2 nanocomposite, we have critically assessed the possible mechanisms
for photodegradation of RhB dye. According to the results of the Tauc plot
(Fig. 4(a)) and our previous investigation Das , the band gap ($E_{g}$) values
of as-synthesized LaFeO3 and few layer MoS2 nanosheets are about 2.1 and 1.21
eV, respectively. Therefore, both LaFeO3 and MoS2 can act as photosensitizers
under visible light irradiation.
The band edge positions of the semiconductor photocatalysts can be determined
using the following empirical formulas Nethercot .

$E_{CB}=\chi-E_{c}-\frac{1}{2}E_{g}$ (6) $E_{VB}=E_{CB}+E_{g}$ (7)

where $\chi$ is the absolute electronegativity, expressed as the geometric
mean of the absolute electronegativity of the constituent atoms, which is
defined as the arithmetic mean of the atomic electron affinity and the first
ionization energy; $E_{c}$ is the energy of free electrons on the hydrogen
scale ($\sim$4.5 eV); $E_{g}$ is the band gap of the semiconductor; $E_{CB}$
and $E_{VB}$ are the conduction and valence band potentials, respectively. The
CB and VB potentials of LaFeO3 are calculated as -0.03 and 2.07 eV whereas
these values for MoS2 nanosheets are found to be -0.14 and 1.07 eV,
respectively Acharya2017 ; Zeng ; Jia . Considering the band gap and band edge
positions, two types of photogenerated charge-carrier transfer mechanism can
be predicted for this photocatalytic system: (i) the traditional type-II
double charge transfer mechanism and (ii) a direct Z-scheme mechanism. As
illustrated in Fig. 9(a), in the case of type-II mechanism, the photoexcited
electrons will flow from the CB of MoS2 to the less negative CB of LaFeO3
whereas, the flow of holes will be from the VB of LaFeO3 to the less positive
VB of MoS2. However, the accumulated electrons in the CB of LaFeO3 are
incompetent to reduce surface-absorbed O2 to yield $\cdot O_{2}^{-}$ radicals
since the CB potential of LaFeO3 (-0.03 eV) is less negative than the redox
potential of O2/$\cdot O_{2}^{-}$ (-0.046 eV vs. NHE) Wu2018 . Moreover, the
VB potential of MoS2 is less positive than the potential at which $H_{2}O$
molecules (or surface $OH^{-}$) are oxidized to $\cdot OH$ (1.99 eV vs. NHE)
Wu2018  which suggests that the holes in the VB of MoS2 cannot produce $\cdot
OH$ radicals in the reaction mixture. However, the trapping experiments have
already confirmed that $\cdot O_{2}^{-}$ played the vital role in the
photodegradation of RhB and $\cdot OH$ was one of the subsidiary reactive
species, which certainly would not be the case if type-II mechanism was
followed. Therefore, it can be inferred that the photocatalytic reaction over
LaFeO3-MoS2 nanocomposite did not occur via the double charge-transfer
mechanism path.

Figure 9: Schematically (a) type-II double charge transfer mechanism and (b)
direct Z-scheme electron transfer mechanism in the photodegradation of RhB
over LaFeO3-MoS2 nanocomposite under solar irradiation

The photocatalytic pathway for RhB degradation can be well explained by a
mediator-free direct Z-scheme mechanism Acharya2017 ; Mishra . As shown in
Fig. 9(b), the photogenerated electrons from the CB of LaFeO3 migrate to the
more positive VB of MoS2 via the heterojunction and recombine with the holes
Acharya2017 . Then, the left behind electrons in the CB of MoS2 successfully
reduce O2 to $\cdot O_{2}^{-}$ radicals since the CB potential of MoS2 is more
negative than the redox potential of this half-reaction. Similarly, the holes
assembled in the VB of LaFeO3 react with $H_{2}O$ molecules and yield $\cdot
OH$ radicals owing to their sufficiently high positive potential to perform
this oxidation. Further, the produced $\cdot O_{2}^{-}$ and $\cdot OH$
radicals along with the photogenerated electrons and holes react with RhB
molecules and cause degradation and thereby, yield non-toxic degradation
products. Rationally, all these statements are also applicable for
ciprofloxacin degradation over LaFeO3-MoS2 nanocomposite. The proposed
Z-scheme mechanism for photodegradation of RhB dye over LaFeO3-MoS2
nanocomposite can be summarized as follows:

$LaFeO_{3}+h\nu\rightarrow LaFeO_{3}(e^{-}+h^{+})$ (8)
$MoS_{2}+h\nu\rightarrow MoS_{2}(e^{-}+h^{+})$ (9)
$LaFeO_{3}-MoS_{2}(e^{-}+h^{+})\rightarrow LaFeO_{3}(h^{+})+MoS_{2}(e^{-})$
(10) $LaFeO_{3}(h^{+})+H_{2}O\rightarrow H^{+}+\cdot OH$ (11)
$MoS_{2}(e^{-})+O_{2}\rightarrow\cdot O_{2}^{-}$ (12) $RhB+(\cdot
O_{2}^{-},\cdot OH,e^{-},h^{+})\rightarrow Degradation\;\;products$ (13)

The results of photocatalytic $H_{2}$ evolution tests further substantiate
that LaFeO3-MoS2 photocatalyst is more likely to follow direct Z-scheme
mechanism, rather than type-II double charge transfer mechanism. Both
mechanisms for solar $H_{2}$ generation are schematically presented in ESI
Fig. S6. According to thermodynamic principles, for successful water
splitting, the photogenerated electrons must have more negative potential than
the reduction potential of $H^{+}$/$H_{2}$ (0 eV vs. NHE) and simultaneously,
the holes must be more positive than the oxidation potential of
$H_{2}O$/$O_{2}$ (1.23 eV vs. NHE) Yendrapati . Therefore, if type-II
mechanism would have existed, LaFeO3-MoS2 photocatalyst could not generate
$H_{2}$ via water splitting (see ESI Fig. S6(a)) since the VB potential of
MoS2 is less positive than the oxidation potential of $H_{2}O$/$O_{2}$.
Contrarily, in the case of direct Z-scheme mechanism, the holes in the VB of
LaFeO3 can produce $O_{2}$ and $H^{+}$ by oxidizing the surface-adsorbed
$H_{2}O$ molecules owing to their adequately high redox potential. Likewise,
the electrons accumulated in the CB of MoS2 will also be able to successfully
reduce $H^{+}$ and evolve $H_{2}$ gas. Hence, it can be inferred that
LaFeO3-MoS2 photocatalyst has produced solar $H_{2}$ via direct Z-scheme
mechanism. Moreover, the unsaturated active S atoms with mono- or two-
coordination on exposed edges of MoS2 nanosheets might have strong bonds with
the $H^{+}$ ions present in the solution and thus, can easily capture them
Chen ; Tama . As a consequence, the reduction of $H^{+}$ to $H_{2}$ would have
been accelerated, resulting in high HER which is consistent with our
experimental findings.
Based on the outcomes and above analysis, the superior photocatalytic
performance of as-synthesized direct Z-scheme LaFeO3-MoS2 photocatalyst can be
attributed to its enhanced charge separation and transportation efficiency at
heterojunction as well as high oxidation and reduction potentials of
photoinduced holes and electrons. In addition to these, the excellent
crystallinity, favorable surface morphology and improved ferroelectric
behavior of the nanocomposite after the incorporation of MoS2 nanosheets might
have played significant role to enhance the photocatalytic efficiency Basith1
.

##  IV Conclusions

We observed that the as-prepared LaFeO3-MoS2 photocatalyst has considerably
higher degradation efficiency of RhB as compared to pristine LaFeO3 and
commercially available titania nanoparticles. Moreover, the superior
performance of the nanocomposite towards the photodecomposition of a colorless
micropollutant has ensured that the degradation is solely attributed to
photocatalytic process rather than dye-sensitization effect. Such enhanced
photocatalytic performance of this nanocomposite can be mainly ascribed to the
formation of a direct Z-scheme heterojunction between its constituents which
facilitated charge separation via interfacial interaction and provided charges
with strong redox ability to drive photocatalysis. Further, the plethora of
exposed active edge sites on the surface of lamellar MoS2 might effectively
promote the H2 evolution reaction, resulting in significant amount of H2
generation via water splitting. It is worth mentioning that the LaFeO3-MoS2
nanocomposite with high photocatalytic efficiency, non-toxicity and excellent
recyclability, can be considered as a promising photocatalyst for numerous
practical applications such as removal of harmful textile and pharmaceutical
pollutants from wastewater, solar water disinfection, CO2 reduction, low-cost
photocatalytic H2 evolution etc. The present investigation might also open up
new avenues for the design and synthesis of novel 2D transition-metal
dichalcogenides based Z-scheme photocatalysts for environmental remediation,
solar energy conversion and related applications.

## Acknowledgements

We acknowledge the Committee for Advanced Studies and Research (CASR),
Bangladesh University of Engineering and Technology for financial assistance.
This work was financially supported partly by the Ministry of Education,
Government of Bangladesh (Grant No. 37.20.0000.004.033.020.2016./PS20191017).

## Appendix A. Supplementary data

Supplementary data to this article can be found online. See
DOI:10.1016/j.mseb.2021.115295

## Data availability

The raw and processed data required to reproduce these findings cannot be
shared at this time due to technical or time limitations.

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