A facile reflux method for in situ fabrication of a non-cytotoxic Bi2S3/β-Bi2O3/ZnIn2S4ternary photocatalyst: A novel dual Z-scheme system with enhanced multifunctional photocatalytic activity

Article abstract of DOI:10.1039/d0ta06129h

In this study, we have developed a novel non-cytotoxic Bi2S3/β-Bi2O3/ZnIn2S4 ternary nanocomposite using a mild in situ reflux method for multimodal applications in the fields of waste water remediation, H2 energy production and microbial disinfection. Initially, the metastable β-Bi2O3 phase with nanoplate morphology was synthesized by thermal decomposition of Bi2O2CO3. The co-assembly of Bi2S3 and ZnIn2S4 with β-Bi2O3 is achieved by a tailor made in situ reflux route using thioacetamide as the sulfur source. Comprehensive characterization of the ternary composite revealed close microscopic contact between the semiconductors, fast electron channelization, enhanced charge carrier separation and a prolonged life time (11.25 ns) of the excited state. The ternary composite exhibits excellent visible light assisted photocatalytic activity for aqueous phase tetracycline (TCN) degradation and Cr(vi) reduction achieving 96.3% (Kapp = 0.0868 min-1) and 93.7% (Kapp = 0.2236 min-1) removal in a short reaction span of 40 min and 12 min, respectively. The ternary composite also afforded a H2 generation rate of 2243 μmol h-1 g-1 with an apparent conversion efficiency of 14.3%. A bacterial inactivation study conducted using Enterobacter cloacae suggested good visible light assisted bacterial degradation activity of the coupled semiconductor system. The MTT assay study confirms the non-cytotoxic nature of the photocatalyst as well as the treated TCN and Cr(vi) solutions. A synergistic dual Z-scheme electron migration mechanism is deduced from radical trapping experiments and the ESR study which accounts for the excellent photocatalytic activity of the ternary composite material. This journal is

Full text of DOI:10.1039/d0ta06129h

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A facile reflux method for in situ fabrication of
a non-cytotoxic Bi2S3/b-Bi2O3/ZnIn2S4 ternary
photocatalyst: a novel dual Z-scheme system with
enhanced multifunctional photocatalytic activity†
Cite this: DOI: 10.1039/d0ta06129h
Dibyananda Majhi,a Krishnendu Das,a Ranjit Bariki,a Subash Padhan,b Abtar Mishra,c
a
Rohan Dhiman,c Priyanka Dash,c Bismita Nayakc and B. G. Mishra
*
In this study, we have developed a novel non-cytotoxic Bi2S3/b-Bi2O3/ZnIn2S4 ternary nanocomposite
using a mild in situ reflux method for multimodal applications in the fields of waste water remediation,
H2 energy production and microbial disinfection. Initially, the metastable b-Bi2O3 phase with nanoplate
morphology was synthesized by thermal decomposition of Bi2O2CO3. The co-assembly of Bi2S3 and
ZnIn2S4 with b-Bi2O3 is achieved by a tailor made in situ reflux route using thioacetamide as the sulfur
source. Comprehensive characterization of the ternary composite revealed close microscopic contact
between the semiconductors, fast electron channelization, enhanced charge carrier separation and
a prolonged life time (11.25 ns) of the excited state. The ternary composite exhibits excellent visible light
assisted photocatalytic activity for aqueous phase tetracycline (TCN) degradation and Cr(VI) reduction
achieving 96.3% (Kapp ¼ 0.0868 minꢀ1) and 93.7% (Kapp ¼ 0.2236 minꢀ1) removal in a short reaction span
of 40 min and 12 min, respectively. The ternary composite also afforded a H2 generation rate of 2243
mmol hꢀ1 gꢀ1 with an apparent conversion efficiency of 14.3%. A bacterial inactivation study conducted
using Enterobacter cloacae suggested good visible light assisted bacterial degradation activity of the
coupled semiconductor system. The MTT assay study confirms the non-cytotoxic nature of the
photocatalyst as well as the treated TCN and Cr(VI) solutions. A synergistic dual Z-scheme electron
migration mechanism is deduced from radical trapping experiments and the ESR study which accounts
for the excellent photocatalytic activity of the ternary composite material.
Received 21st June 2020
Accepted 27th September 2020
DOI: 10.1039/d0ta06129h
rsc.li/materials-a
efforts lead to fabrication of semiconductor heterostructures, in
particular type-II heterostructures with signicant enhance-
ment of the photocatalytic efficiency.10,11 Formation of type-II
1. Introduction
In recent years, semiconductor photocatalysis has emerged as
one of the most promising, sustainable and cost-effective
technologies to mitigate the pressing global issues of environ-
mental pollution and energy crisis. Photocatalytic methods are
widely used in diverse areas like waste water decontamination,
reduction of heavy metal ions, biomass conversion, water
splitting reaction, CO2 reduction, organic transformations,
nitrogen xation and air purication.1–5 With an urge to develop
innovative and efficient photocatalytic materials, extensive
research efforts have been put forward to prepare new photo-
catalytic systems with enhanced solar light absorption and
improved charge carrier separation properties.6–9 Many of these
heterostructures furnishes accelerated charge carrier separa-
tion, however, the photogenerated eꢀ and h+ move in a cyclic
manner in these photosystems which weakens the redox
ability.10 Mimicking the natural photosynthesis mechanism in
the formation of direct Z-scheme heterostructure systems is an
efficient methodology to achieve higher photocatalytic activity
and improved solar light harvesting. Unambiguously, the band
alignment in the Z-scheme system is similar to that of the type-
II system but charge carrier migration occurs in a different way
due to the energy proximity of donor and acceptor bands.
Unlike the type-II system, in this assembly the eꢀ migrates from
the conduction band (CB) of one semiconductor component to
the h+ rich valence band (VB) of the other semiconductor which
is facilitated by the electrostatic attraction. Z-scheme hetero-
structure materials are characterized by their optimal redox
ability and higher charge carrier separation properties.10,11
Moreover, the construction of ternary double Z-scheme heter-
ostructure photocatalysts produces a higher number of redox
aDepartment of Chemistry, National Institute of Technology, Rourkela-769008,
Odisha, India. E-mail: brajam@nitrkl.ac.in
bDepartment of Applied Chemistry, IIT Dhanbad, Jharkhand-826004, India
cDepartment of Life Science, National Institute of Technology, Rourkela-769008,
Odisha, India
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d0ta06129h
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centers which leads to their better photocatalytic activity than situ fabrication of a novel, non-toxic Bi2S3/b-Bi2O3/ZnIn2S4 dual
their binary counterparts. Some of the recently studied ternary Z-scheme photocatalyst system. The photocatalytic activity of
dual Z-scheme photocatalysts include rGO/InVO4/Fe2O3, WO3/g- the ternary semiconductor materials is evaluated for tetracy-
C3N4/Bi2O3, g-C3N4/Ag2CO3/GO, ZnMoO4/BiFeWO6/rGO, Bi2S3/ cline (TCN) degradation, Cr(VI) reduction, bacterial inactivation
BiVO4/MgIn2S4, CdS/rGO/WO3, PANI/BiOBr/ZnFe2O4 and PANI/ and H2 energy production. The anchoring of Bi2S3 and ZnIn2S4
Ag3PO4/CNO which show improved performance for CO2 materials over the Bi2O3 component is substantiated using
reduction, degradation of refractory organic pollutants and H2 XRD, FESEM, TEM, elemental mapping and XPS techniques.
evolution reaction.12–19 In recent past, we have developed a one The accelerated channelization of electrons through a dual Z-
pot hydrothermal method for the synthesis of a CdS/BiOBr/ scheme mechanism in the heterostructure system is probed
Bi2O2CO3 ternary double Z-scheme photocatalyst which shows using photoelectrochemical study, radical trapping experiment
excellent photocatalytic activity for aqueous phase degradation and ESR study.
of atrazine herbicide.20 In continuation of our interest to
develop novel ternary photosystems, in this work, we have
developed a mild reux method for the synthesis of a Bi2S3/b-
2. Experimental
Chemicals and reagents
Bi2O3/ZnIn2S4 ternary composite with multimodal photo-
catalytic applications.
The chemicals and reagents used in this study were procured
from Merck Specialities Pvt. Ltd. All the chemicals were of
analytical grade and were used without further purication. The
deionized water prepared in our laboratory was used for all the
experiments in this study. The Mueller–Hinton Agar (MHA) and
Mueller–Hinton Broth (MHB) that were required for culture of
the organisms in solid and liquid media, respectively, were
procured from Himedia Pvt. Ltd. India.
Bi-based oxides emerged as effective alternative base mate-
rials to traditional photocatalysts due to their non-toxic nature,
special electronic structure and suitable band gap for visible
light absorption.21 Additionally, the metastable b-Bi2O3 shows
good photocatalytic properties due to its narrow band gap
energy (ꢁ2.4 eV) and suitable valence band maximum (VBM) for
oxidation of H2O/OHꢀ to cOH radicals. Hence, consciously
combining metal suldes having high ‘ꢀve’ conduction band
minima (CBM) with the b-Bi2O3 material can lead to direct Z-
scheme binary/ternary photocatalytic systems with accelerated
charge carrier separation and improved redox ability. In addi-
Sample preparation
Synthesis of Bi2S3, ZnIn2S4 and b-Bi2O3. The detailed
tion, such types of combinations will lead to the development of methods of the synthesis of pure Bi2S3 (BS), ZnIn2S4 (ZIS) and b-
highly efficient photocatalytic systems for multifunctional Bi2O3 (BO) are presented in the ESI le.† The pictorial repre-
applications such as pollutant degradation and H2 evolution sentation of the facile synthesis of ZIS under reux conditions is
reaction. Many mono-metal suldes including NiS, MoS2 and depicted in Fig. S1.†
Bi2S3 have been coupled with Bi2O3 to form direct Z-scheme
In situ synthesis of Bi2S3/b-Bi2O3/ZnIn2S4 (BZxBO) and Bi2S3/
photocatalytic systems for energy and environmental applica- b-Bi2O3 (BS/BO) heterostructure materials. The ternary Bi2S3/b-
tions.21–23 A recent study shows that mixed metal suldes Bi2O3/ZnIn2S4 (BZxBO) materials were synthesized by a one pot
possess excellent photo-stability, high electron conductivity and method using a facile reux route. Initially, 1 g of b-Bi2O3 was
superior redox properties in comparison to their mono metal dispersed in 60 ml of water by ultrasonication to which 2 mmol
sulde counterparts.24 Among multicomponent suldes, of thioacetamide was added and reuxed for 1 h. To this
ZnIn2S4 is a promising eco-friendly photocatalyst belonging to aqueous suspension, required amounts of Zn(NO3)2$6H2O,
the AB2X4 semiconductor family and has been studied exten- InCl3 and thioacetamide (TAA) in a 1 : 2 : 4 molar ratio were
sively for hydrogen evolution and other photocatalytic reduc- added and further reuxed for another 3 h. The resulting solid
tion reactions owing to its high ‘ꢀve’ CBM.24 To the best of our was separated by centrifugation and washed repeatedly with
knowledge, there are no studies on Bi2O3 and ZnIn2S4 based water and ethanol to obtain the BZxBO ternary composite. The
binary and ternary heterostructure materials for photocatalytic ZnIn2S4 content in the composite is varied in the range of 5–
applications.
30 wt%. The obtained ternary heterostructure materials are
Concisely, facile synthesis of binary and ternary Z-scheme labeled as BZ5BO, BZ10BO, BZ20BO and BZ30BO, respectively.
heterostructure materials by a one pot method is a practical The binary Bi2S3/b-Bi2O3(BS/BO) material was also prepared by
approach and provides a nest path to acquire better micro- the same procedure by reuxing a mixture of 1 g of b-Bi2O3 with
scopic contacts among different phases in the heterostructure 2 mmol of thioacetamide in 60 ml water for up to 3 h. The BS/BO
system which leads to improved photocatalytic activity. Addi- materials are also prepared using sodium thiosulfate and
tionally, preparation of non-toxic photocatalysts with multi- thiourea as sulfur sources and are labeled as BS/BO-ST and BS/
functional activity is a sustainable and realistic maneuver to BO-TU, respectively. The pictorial representation of the
mitigate environmental pollution and energy crisis. The devel- synthesis of BS/BO and BZxBO composites is depicted in
opment of a simplied, less time consuming and mild one pot Scheme 1. For a comparative study, the ZnIn2S4/b-Bi2O3 (ZIS/
method for the fabrication of ternary photocatalysts with BO) and Bi2S3/ZnIn2S4 (BS/ZIS) composite materials were also
potential for multifunctional applications is rarely reported in prepared by a self-assembly approach. The detailed method for
the literature. Hence, taking into consideration all these the synthesis of these binary composites is presented in the ESI
aspects, herein we have developed a simple reux route for in le.†
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were carried out according to a previously reported procedure
using a human leukemia monocyte cell line (THP1).20 The THP1
cells were treated with different volumes of parent 20 ppm TCN
and 10 ppm Cr(VI) solutions as well as photocatalytically treated
samples collected at different time intervals. The TCN solutions
collected aer 20 min (solution A) and 40 min (solution B) of
photocatalytic treatment were evaluated for the cytotoxicity
study. The Cr(VI) solution at the end of 12 min of photocatalytic
treatment was chosen for the cytotoxicity study. The cytotoxic
effect of the BZ20BO photocatalyst material was also tested in
the concentration range of 200–500 mg Lꢀ1. Before the experi-
ment the photocatalyst was well dispersed in distilled water by
ultrasonication for 3 h. The cell viability (%) was calculated by
using the formula presented in the equation.1
OD562
of test sample
of control
nm
% cell viability ¼
ꢂ 100
(1)
OD562
nm
3. Results and discussion
The phase and crystallinity of the as-prepared photocatalysts
are assessed using the powder X-ray diffraction technique. The
XRD patterns were recorded in the 2q range of 20–60 degrees at
a scan rate of 2ꢃ minꢀ1. Fig. 1I presents the XRD patterns of pure
Bi2S3, Bi2O3 and ZnIn2S4 materials. All the materials exhibit well
dened and sharp XRD peaks which suggest their highly crys-
talline nature. The diffraction peaks of the BS material are well
indexed to the orthorhombic Bi2S3 phase (JCPDS no. 75-1306,
Scheme 1 Pictorial representation of the in situ synthesis of binary
Bi2S3/Bi2O3 and ternary Bi2S3/b-Bi2O3/ZnIn2S4 materials by a reflux
route.
˚
space group-Pbnm) with unit cell constants a ¼ 11.11 A, b ¼
˚
˚
11.25 A and c ¼ 3.97 A. The XRD peaks obtained for the BO
material can be readily assigned to the tetragonal Bi2O3 phase
ꢀ
(JCPDS no. 27-0050, space group-P421c) with unit cell constants
Characterization techniques
˚
˚
a ¼ b ¼ 7.74 A and c ¼ 5.63 A. The diffraction pattern of ZIS is
well matched to the hexagonal crystalline structure of ZnIn2S4
(JCPDS no. 72-0773, space group-P63mc) with cell constants a ¼
The as-synthesized photocatalysts were characterized using
various analytical techniques including XRD, UV-Vis, FTIR,
Raman, eld emission scanning electron microscopy (FESEM),
transmission electron microscopy (TEM), photoluminescence
(PL), time resolved uorescence (TR-PL), linear sweep voltam-
metry (LSV), transient photocurrent measurements, impedance
spectroscopy and XPS. The detailed methods of characteriza-
tion of the as-prepared samples are presented in the ESI le.†
˚
˚
b ¼ 3.85 A and c ¼ 24.68 A. The observed XRD patterns of the
pure semiconductors are consistent with the earlier
reports.21,25,26 The XRD pattern of the binary BS/BO material
prepared using thioacetamide as the sulfur source indicates the
co-existence of both orthorhombic Bi2S3 and tetragonal b-Bi2O3
phases (Fig. 1II(a)). Fig. S2† shows the XRD patterns of BS/BO-
ST and BS/BO-TU binary heterojunctions prepared using
sodium thiosulfate and thiourea as sulfur sources, respectively.
Photocatalytic activity test
The photocatalytic activity of the ternary heterostructure mate-
rial was evaluated for tetracycline (TCN) degradation, chro-
mium(VI) reduction, bacterial inactivation and H2 production
using visible light irradiation (Xe lamp, 250 W tted with
a 420 nm cut off lter). The experimental procedures for the
above-mentioned photocatalytic studies are appended in the
ESI le.†
Cytotoxicity test
The MTT assay technique was adopted to explore the cytotoxic
effect of the photocatalytically treated tetracycline solution and
Fig. 1 PXRD patterns of (I) (a) Bi2S3, (b) Bi2O3, (c) ZnIn2S4 and (II) (a) BS/
Cr(VI) solution and the BZ20BO photocatalyst. The experiments BO, (b) BZ5BO, (c) BZ10BO and (d) BZ20BO samples.
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The BS/BO-ST material exhibits well crystalline diffraction TEM techniques, respectively. The FESEM images of pristine BS,
peaks due to the Bi2O3 phase, however, the XRD peaks due to BO, ZIS, BS/BO and BZ20BO materials are presented in Fig. 2.
the Bi2S3 phase are weak, less intense and broad in nature The pure BS material contains agglomerated nanoparticles with
(Fig. S2†). For the BS/BO-TU material, no diffraction peaks due size ranging between 15 and 25 nm (Fig. 2a) whereas the BO
to the crystalline Bi2S3 phase could be discerned. This material material exhibits nanoplate morphology with undened edges
contains diffraction features of the tetragonal Bi2O3 phase only. (Fig. 2b). The planar dimensions of BO nanoplates are
The XRD study suggests that the sulfur source is crucial for measured in the range of 100 to 250 nm (Fig. 2b). The ZIS
generation of the binary heterostructure. In the present study, material exhibits unique ower like microspheres having an
thioacetamide shows the highest rate of S2ꢀ ion release in average diameter of 7 mm composed of ultrathin ZIS nanopetals
aqueous solution which accounts for its intense sulfurization (Fig. 2c). The uniform distribution of Zn, In and S elements
activity leading to the formation of the crystalline Bi2S3 phase in throughout the microscopic specimen is conrmed from the
the binary heterostructure.27 In the case of the BZ5BO and elemental mapping study (Fig. S4(a–d)†). The FESEM image of
BZ10BO ternary systems, no assignable XRD peaks corre- the BS/BO binary composite displays the presence of Bi2S3
sponding to either ZnIn2S4 or Bi2S3 are observed due to the low nanoparticles anchored over Bi2O3 nanoplates (Fig. 2d). The
content of these phases in the ternary composite (Fig. 1II(b and elemental mapping study reveals good dispersion of both Bi2S3
c)). In the case of the BZ20BO material both Bi2S3 and Bi2O3 and Bi2O3 phases in the BS/BO binary heterostructure material
phases are noticed, but still there is absence of diffraction peaks (Fig. S4(e–g)†). The BZxBO materials exhibit both plate like and
for the ZnIn2S4 phase which is due to the dominant high sheet like morphology together with uniformly distributed Bi2S3
intensity peaks of Bi2S3 and Bi2O3 phases (Fig. 1II(d)). However nanoparticles (Fig. 2e and S5(a and b)†). The elemental
for BZ30BO, the XRD peak corresponding to the (110) plane of mapping images of the BZ20BO material presented in Fig. S5(c–
ZnIn2S4 is observed along with diffraction patterns of both h)† display even distribution of Bi, O, Zn, In and S elements
Bi2O3 and Bi2S3 phases (Fig. S3†).
throughout the selected micron range.
The surface morphology and microstructural properties of
Additionally, the FESEM image and elemental mapping study
the synthesized materials are further studied using FESEM and of the BZ30BO material reveal the presence of isolated ZIS sheets
Fig. 2 FESEM images of (a) BS, (b) BO, (c) ZIS, (d) BS/BO and (e) BZ20BO and (f–i) TEM images of the BZ20BO ternary composite material.
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(Fig. S6†). This observation suggests lower microscopic contacts range of 528–532 eV for the composite materials, which could be
between the BO and ZIS phases in the BZ30BO material. Fig. 2f–i deconvoluted into two Gaussian peaks with maxima at 530.1–
present the TEM images of the BZ20BO material. It is obvious 530.5 eV and 531.5–531.8 eV (Fig. S7†). These spectral features
from Fig. 2f that the BZ20BO material is composed of numerous originate from the lattice oxygen of Bi2O3 and the adsorbed water
nanoplates and nanosheets in contact with each other. Fig. 2g molecules, respectively.21 The broad peak with binding energy
displays uniform distribution of small Bi2S3 nanoparticles values between 230.5 and 230.8 eV for the composite materials
(particle size between 15 and 20 nm) over Bi2O3 nanoplates. The can be assigned to the photoelectron emission from the S 2s
HRTEM image shown in Fig. 2i displays lattice fringes with d- energy state. The structural Zn2+ ions from ZnIn2S4 in BZ20BO
spacing values of 0.27, 0.29 and 0.32 nm corresponding to the and ZIS/BO exhibit a spectral doublet in the spectral range of
(301) plane of Bi2S3, (002) plane of b-Bi2O3 and (102) plane of 1020–1045 eV due to photoelectron emission from Zn 2p3/2 and
ZnIn2S4, respectively. The HRTEM study reveals the crystalline Zn 2p1/2 energy states (Fig. S7I and III†). Similarly, the spectral
nature of all the phases and also supports the observations ob- doublet in the binding energy range of 440–445 eV is assigned to
tained from XRD analysis of the BZ20BO material. The close the 3d5/2 and 3d3/2 energy levels of In3+ ions from ZnIn2S4 (Fig. S7I
microscopic contacts among the three semiconductor phases and III†).30 In the present study, for the ternary composite the
result in the formation of a ternary heterostructure.
spectral features of Zn(II) and In(III) ions in ZnIn2S4 have shied
The surface chemical composition and oxidation state of towards the lower energy side whereas the Bi(III) peak has shied
constituent elements in the ternary BZ20BO as well as binary BS/ towards the higher energy side in comparison to the binary
BO and ZIS/BO materials are studied from XPS analysis (Fig. S7†). composites.29,30 This observation suggests effective channeliza-
The XPS survey spectrum in Fig. S7I(a)† validates the presence of tion of electrons in the ternary composite due to intimate inter-
Bi, O, S, Zn and In elements on the surface of the ternary facial contact and heterojunction formation between the
composite material together with adventitious carbon. The high constituent phases. The microstructural properties of the pho-
resolution spectrum for Bi3+ exhibits a doublet at binding energy tocatalysts were further studied using Raman spectroscopy
values 158.9 eV and 164.2 eV corresponding to Bi 4f7/2 and Bi 4f5/2 techniques. The Raman data are presented in Fig. S8I and the
states with a spin–orbit splitting of 5.3 eV (Fig. S7I(b)†).28,29 For results are discussed in the ESI le.†
the binary composites, the Bi 4f peaks appear red shied by
The UV-Vis diffuse reectance spectroscopy technique is
ꢁ0.3–0.4 eV in comparison to the ternary composite which can be employed to investigate the optical absorption feature and band
ascribed to the change in the chemical environment due to het- gap energy of the heterostructure materials (Fig. 3I and S8II†). The
erojunction formation. The broad peak due to O 1s appears in the absorption edge of both Bi2O3 and ZnIn2S4 has commenced near
Fig. 3 (I) UV-Vis-DRS spectra of (a) BO, (b) ZIS, (c) BS, (d) BS/BO, (e) BZ5BO, (f) BZ10BO, (g) BZ20BO, and Mott–Schottky plots for (II) BO, (III) ZIS
and (IV) BS materials.
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550 nm whereas pristine Bi2S3 shows absorption from the near where ND ¼ concentration of donor impurities, q ¼ elementary
infrared (NIR) region. Aer heterostructure formation, signicant electronic charge, and 30 and 3r correspond to vacuum permit-
red shiing is observed for BS/BO and BZxBO samples as tivity and relative permittivity, respectively.
compared to neat BO and ZnIn2S4 materials (Fig. 3I). All the
The at band potentials (V) vs. Ag/AgCl obtained from the
binary and ternary heterostructure materials show complete MS plots are 1.14 eV, ꢀ0.13 eV and ꢀ1.05 eV for BS, BO and ZIS
absorption in the visible light region which extends up to NIR. materials, respectively. Aerward the V w.r.t NHE is calculated
This observation suggests that our developed ternary hetero- using the following expression in eqn (3).
structure material can be used as an excellent photocatalyst under
Vfb(NHE,pH¼7) ¼ Vfb(Ag/AgCl) ꢀ 0.059
visible light irradiation as well as near infrared irradiation. The
(7 ꢀ pH of electrolyte) + 0.198
(3)
band gap of as prepared pure samples is obtained from Tauc's
plot. The neat BO, ZIS and BS materials exhibited band gap energy
values of 2.48, 2.20 and 1.40 eV, respectively (Fig. S8II†).
Mott–Schottky (MS) plots are used to determine the type of
semiconductivity and band edge potential of the bare semi-
conductor components. The pristine BO and ZIS show a positive
slope in the MS plot which suggests n-type semiconductivity of
these materials (Fig. 3II and III). However, the BS material
exhibits a negative slope revealing its p-type nature (Fig. 3IV).
Subsequently, the at band potential (V) is obtained by
extrapolating the linear portion of the MS plot to abscissa. The
slope ((1/C2)/V) in the MS plot is expressed by the following
equation (eqn (2)).31
The conduction band (CB) of an n-type semiconductor is
generally observed to be ꢁ0–0.2 eV more negative than the V of
the material.31,32 The calculated CB positions of pure BO, BS and
ZIS are ꢀ0.04 eV, ꢀ0.09 eV and ꢀ1.01 eV, respectively.
The elevated separation and movement of charge carriers in
a photocatalyst is a desirable characteristic for enhanced pho-
tocatalytic activity of a semiconductor material. The trans-
portation and separation properties of photo-assisted charge
carriers in the photocatalysts are studied through the linear
sweep voltammetry (LSV) technique. A high anodic photocur-
rent of 0.42 mA cmꢀ2 is observed for the BZ20BO ternary pho-
tocatalyst whereas 0.18 mA cmꢀ2 and 0.08 mA cmꢀ2
photocurrent is observed for BS/BO and BO materials (at 1.0 V
(1/C2)/V ¼ 2/NDq303r
(2)
Fig. 4 (I) Photocurrent density, (II) electrochemical impedance spectroscopy (EIS) Nyquist plots and (III) PL spectra of (a) BS, (b) BO, (c) ZIS, (d) BS/
ZIS, (e) BS/BO, (f) ZIS/BO and (g) BZ20BO materials, and (IV) time-resolved fluorescence decay for the BZ20BO material.
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bias potential vs. Ag/AgCl) (Fig. S9†). This observation suggests temperature solid state uorescence measurements are carried
easy channelization of electrons and accelerated separation of out at an excitation wavelength of 320 nm for all samples
excitons in the ternary heterostructure material. To substantiate (Fig. 4III). A signicant reduction of peak intensity at 465 nm is
this charge-transfer prociency, the transient photocurrent observed for the BZ20BO material in comparison to pure BO
response (I–t) is recorded for the as-prepared samples. The I–t and binary composites which indicated a lower recombination
plot presented in Fig. 4I reveals high photocurrent density for rate of eꢀ and h+ species.
the BZ20BO sample as compared to the BS/BO binary composite
The photophysical behavior of the charge carriers is also
and pure semiconductors (BO, BS, and ZIS) which suggests studied by nano-scale level time-resolved uorescence
improved charge carrier separation and migration in the ternary measurements carried out at room temperature. The uores-
composite material. The photocurrent response of the BZ20BO cence decay proles for BZ20BO, BO and binary composites are
ternary composite is also compared with those of BS/ZIS and presented in Fig. 4IV and S10.† The average decay time (savg) of
ZIS/BO binary composites to access the role of each semi- the charge carriers is calculated using the general formula
conductor component. The I–t response of the ternary BZ20BO presented in eqn (4).33
,
material is signicantly higher than those of the binary
n
n
X
X
savg
¼
aisi2
aisi
(4)
composites suggesting that the cooperative action of the three
components is essential to achieve improved charge migration
properties. The results obtained from photocurrent measure-
ments are also in good agreement with the impedance study
(Fig. 4II). The lower arc radius in the EIS Nyquist plot for the
ternary BZ20BO material in comparison to pure semi-
conductors and binary composites suggests lower charge
transfer resistance which allows efficient separation of eꢀ and
h+ in the ternary material. Further for deep investigation of the
charge carrier recombination in the photocatalysts, the room
i¼1
i¼1
where si ¼ individual life time and ai ¼ corresponding ampli-
tude value. The ternary BZ20BO material exhibited a prolonged
life time decay value of 11.25 ns as compared to pristine BO
(6.45 ns) and binary BS/BO (8.80 ns) & ZIS/BO (9.35 ns), mate-
rials. The prolonged life time value for BZ20BO species suggests
lower recombination of charge carriers providing more free eꢀ
and h+ for effective utilization in the photocatalytic process.
Fig. 5 (I) Photocatalytic degradation of TCN, (II) pseudo first order kinetic plots for BS, BO, ZIS, and BZxBO ternary composite materials, (III) effect
of initial TCN concentration on the photocatalytic activity of the BZ20BO material and (IV) comparison of percentage TOC removal during TCN
degradation.
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9.4 and 2.3 times greater than that of BO and BS/BO materials,
respectively. The photocatalytic experiments are optimized by
varying the TCN concentration and catalyst weight (Fig. 5III and
S11†). The optimization study revealed that 300 mg Lꢀ1 of the
BZ20BO photocatalyst can be used for the degradation of
20 ppm TCN solution with >95% PDE.
Further, in-depth investigation of the degradation process of
TCN over the BZ20BO ternary photocatalyst was carried out by
TOC analysis, LC-ESI-MS and the three-dimensional uores-
cence excitation emission (3D-EEMS) study. A total organic
carbon (TOC) removal efficiency of 22.4, 70.1 and 91.2% is
observed aer 40 min of reaction time for BO, binary BS/BO and
BZ20BO ternary composites, respectively (Fig. 5IV).
The high value of TOC removal for the BZ20BO photocatalyst
suggests effective mineralization of TCN over the ternary pho-
tocatalyst. The degradation process of TCN is further accessed
by the 3D-EEMS study in an excitation (lex)/emission wave-
length (lem) range of (270–450)/(300–550) nm, respectively. The
3D-EEMS spectra of the parent TCN solution together with
treated solutions at different reaction times are presented in
Fig. S12.† A predominant peak is observed in the excitation/
emission wavelength range of (305–330)/(430–450) nm for
samples collected aer 10 and 20 min of photocatalytic reac-
tion. This peak is ascribed to the humic acid-like substances
formed during TCN degradation.34,35 However, samples
collected aer 0 min and 40 min of reaction do not show any
such peak. In order to further investigate the degradation
pathway of TCN over the BZ20BO photocatalyst surface, the LC-
ESI-MS study is performed. Four aromatic compounds are
detected as intermediates based on which a plausible pathway
for photocatalytic degradation of TCN is proposed (Fig. S13†).
The degradation of TCN occurs in a series of steps involving
demethylation, dehydroxylation, ring cleavage and deamination
to form aromatic intermediates which are further mineralized
over the BZ20BO surface to innocuous products (ESI le†).36,37
Evaluation of photocatalytic activity
Enhanced photocatalytic degradation of TCN. The photo-
catalytic activity of the BZ20BO ternary composite material is
studied for tetracycline (TCN) degradation. Initially, the degra-
dation reaction of TCN is carried out under visible light illu-
mination in the presence as well as in the absence of the
BZ20BO photocatalyst to check whether the decomposition
process occurs in a photolytic or photocatalytic manner. A
negligible degradation of TCN is observed aer 2 h of reaction
time in the absence of a photocatalyst under illumination
conditions which suggests that TCN is resistant to photolysis.
As presented in Fig. 5I, neat BS, BO, and ZIS exhibit 20.8%,
30.4% and 40.9% degradation of TCN, respectively. The pho-
tocatalytic degradation efficiency (PDE) is substantially
enhanced aer heterojunction formation. For binary hetero-
junction samples, PDE in the range of 64–85% is achieved in
40 min of reaction time (Fig. S11†). In contrast, all the BZxBO
ternary heterostructure materials display >85% PDE in the same
time span. The best result is obtained for the BZ20BO material
achieving 96.3% degradation of TCN in 40 min. However, the
BZ30BO material exhibits a lower PDE (90.5%) than the BZ20BO
material (Fig. S11†) due to poor interfacial charge transfer
caused by local agglomeration of Bi2O3 and ZnIn2S4 phases. The
photocatalytic process can be tted well to the pseudo-rst
order kinetic model with a correlation coefficient (R2) value of
>98% using the following expression in eqn (5).
Ao
ln
¼ Kappt
(5)
A
where Kapp represents the pseudo-rst order rate constant
(minꢀ1), and A and Ao denote TCN concentrations at time t and
0 min, respectively. The Kapp values for degradation of TCN over
different photocatalysts were obtained form ꢀln A/Ao vs. t plots
(Fig. 5II).
The kinetic data and PDE of different photocatalyst samples
towards TCN degradation are shown in Table 1. The ternary
BZ20BO heterostructure material displays a higher Kapp value of
8.68 ꢂ 10ꢀ2 minꢀ1 for TCN degradation which is approximately
Photocatalytic reduction of Cr(VI)
The photocatalytic reduction of carcinogenic Cr(VI) to innoxious
Cr(III) species is studied using BZxBO ternary composites in the
presence of tartaric acid (TA) as a hole scavenger (Fig. 6I). No
photoreduction of Cr(VI) is noticed in the absence of the pho-
tocatalyst (blank) under darkness as well as illumination
conditions suggesting that the reduction process occurs in
a photocatalytic manner (Fig. S14†). The change of Cr(VI)
absorbance in the presence of the BZ20BO photocatalyst and
4 mM TA at different time intervals is shown in Fig. S14I.† The
pristine BO material showed 36.6% photoreduction efficiency
(PRE) within 12 min of light illumination. However, aer het-
erostructure formation the PRE is substantially enhanced. A
PRE value of 64.4, 78.1 and 93.7% is observed for BZ5BO,
BZ10BO and BZ20BO materials, respectively (Fig. 6I).
Table 1 Kinetic data and photodegradation efficiency (PDE) for TCN
degradation over BZxBO ternary composite materials
Catalyst
Kapp (minꢀ1
)
R2
Standard errora
(%) PDEb
BS
BO
ZIS
BS/ZIS
BS/BO
ZIS/BO
BZ5BO
BZ10BO
BZ20BO
0.0046
0.0092
0.0132
0.0233
0.0375
0.0432
0.0513
0.0628
0.0868
0.9852
0.9868
0.9369
0.9788
0.9569
0.9932
0.9920
0.9937
0.9831
0.0002
0.0004
0.0018
0.0015
0.0019
0.0015
0.0020
0.0022
0.0050
20.8
30.4
40.9
64.4
77.5
84.7
86.6
91.5
96.3
a
The photoreduction process is found to follow the pseudo-rst
(I) Standard error values are for the slope in the pseudo rst order
kinetic equation. (II) PDE denotes the photocatalytic degradation order kinetic model (R2 > 97). The Kapp values are calculated from
efficiency. (III) Experimental conditions: 30 mg catalyst, visible light
b
the slope of ꢀln C/Co vs. t plots presented in Fig. 6II. The BZ20BO
material exhibits a Kapp value which is 7 times higher than that of
the pure BO and 3 times higher than those of the BS/BO binary
(250 W Xe lamp), 100 ml of 20 mg Lꢀ1 TCN solution, 40 min reaction
time.
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Fig. 6 (I) Photocatalytic reduction of Cr(VI), (II) pseudo first order kinetic plots for BS, BO, ZIS, and BZxBO ternary composite materials, (III) H2
evolution reaction for BO, ZIS, BS/BO, ZIS/BO, and BZ20BO photocatalysts in aqueous suspension and (IV) recyclability study for the BZ20BO
ternary heterostructure photocatalyst towards H2 evolution.
heterostructure materials (Table 2). The effect of TA on the PRE of adsorption of Cr(VI). The photoreduction efficiency of the BZ20BO
BZ20BO is further examined by varying the concentration of TA material is also compared with those of BS/BO, ZIS/BO and ZIS/
in the photocatalytic experiments (Fig. S14II†). An optimized PRE BS binary composite materials in Fig. S15.† The optimal
of 93.7% is observed by using 4 mM TA for the BZ20BO ternary BZ20BO material exhibits signicantly higher photoreduction
photocatalyst. A further increase in TA concentration decreases efficiency in comparison to the binary composites. The photo-
the Cr(VI) reduction which is due to the predominant occupation catalytic method developed in this work offers distinct advan-
of the photocatalyst surface by TA molecules which hinders the tages in terms of lower reaction time and usage of less energy to
effectively perform Cr(VI) reduction.
Table 2 Kinetic data and photoreduction efficiency (PRE) for Cr(VI)
reduction over different photocatalysts
Photocatalytic activity for H2 evolution
The photocatalytic activity of the BZ20BO material is further
evaluated for visible light induced H2 energy production by
water splitting reaction. The H2 production reaction is carried
out using 10 vol% methanol as the sacricial agent to capture
the holes generated during the course of reaction. Fig. 6III
shows the H2 generation in the presence of BO, ZIS, BS/BO, ZIS/
BO and BZ20BO materials under visible light irradiation. Pure
BO and ZIS show a very low photocatalytic H2 evolution rate of
329 and 783 mmol gꢀ1 hꢀ1, respectively. The binary BS/BO and
ZIS/BO materials could able to generate 1034 mmol gꢀ1 hꢀ1 and
1330 mmol gꢀ1 hꢀ1 which are higher than those of the pure
semiconductors. The BZ20BO ternary heterostructure material
Catalyst
Kapp (minꢀ1
)
R2
Standard errora
(%) PREb
BS
BO
ZIS
BS/ZIS
BS/BO
ZIS/BO
BZ5BO
BZ10BO
BZ20BO
0.0239
0.0308
0.0454
0.0559
0.0682
0.0748
0.0807
0.1205
0.2236
0.9867
0.9800
0.9973
0.9879
0.9879
0.9829
0.9836
0.9778
0.9740
0.0011
0.0018
0.0009
0.0009
0.0030
0.0040
0.0042
0.0073
0.0148
24.0
36.6
40.8
46.3
56.8
59.9
64.4
78.1
93.7
a
(I) Standard error values are for the slope in the pseudo rst order
b
kinetic equation. (II) PRE denotes the photocatalytic reduction
efficiency. (III) Experimental conditions: 30 mg catalyst, visible light
(250 W Xe lamp), 100 ml 10 mg Lꢀ1 Cr(VI) solution, 12 min reaction time.
exhibits the highest rate of H2 production (2243 mmol gꢀ1 hꢀ1
)
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with an apparent conversion efficiency of 14.3% (calculation obtained from Mott–Schottky plots using the following gener-
presented in the ESI le†). The H2 evolution rate of the BZ20BO alized formula.
ternary photocatalyst is nearly 6.8 and 2.9 times more than that
Eg ¼ EVB ꢀ ECB
(6)
of pure BO and ZIS, respectively. The recyclability of the BZ20BO
photocatalyst is studied up to three consecutive cycles for the
photocatalytic H2 production reaction (Fig. 6IV). Aer the third
photocatalytic cycle a negligible decrease of 2% H2 production
is noticed under visible light irradiation.
where Eg, EVB and ECB represent the band gap energy, valence
band and conduction band edge potential of a semiconductor,
respectively. The band gap energy and band edge potentials vs.
NHE of bare BS, BO, and ZIS are schematically presented in
Fig. S16I.† All three components in the ternary composite
possess favorable band gap energies to produce eꢀ–h+ pairs
under visible light illumination. Based on the band alignments
of the three constituent semiconductors, two types of eꢀ–h+
migration pathways can be conceived for the ternary BZ20BO
photocatalyst i.e. traditional type-II and direct Z-scheme
mechanisms (Fig. 8). Among the two probable pathways the
double direct Z-scheme electron transfer mechanism is vali-
dated on the basis of active species trapping experiments and
the ESR study (detailed experimental procedure is presented in
the ESI le†). The PDE of the BZ20BO photocatalyst for TCN
degradation in the presence of different radical scavengers is
presented in Fig. S16II.† It is clear from the experiment that in
Antibacterial properties
The antibacterial properties of the BZ20BO material are evaluated
for Enterobacter cloacae using the literature reported standard
protocol.38 The colony forming unit (CFU) assay was carried out to
comprehend the contributions of BZ20BO heterostructure mate-
rials during the photocatalytic inactivation process.38,39 The
growth of bacterial colonies under different conditions on the
Mueller–Hinton Agar (MHA) plate is portrayed in Fig. 7.
The synthesized BZ20BO NPs showed convincing antibacte-
rial behavior due to its photocatalytic nature. When exposed to
visible light irradiation, the BZ20BO material generates reactive
oxygen species (ROS) i.e. hydroxyl and superoxide radicals. The
radicals upon interaction with the bacterial cells perform cell
disruption which leads to the death of the organisms resulting
in their decreased growth on plates.38–40 The digital images of
the plates under different experimental conditions shown in
Fig. 7 conrm the excellent antibacterial properties of the
BZ20BO ternary photocatalyst which causes ample cell deaths
aer 60 min of visible light irradiation.
In this study, the photocatalytic application of the in situ
synthesized BZ20BO ternary photocatalyst has been successfully
demonstrated for TCN mineralization, Cr(VI) reduction, H2
generation and bacterial inactivation. The enhanced photo-
catalytic activity of the ternary photocatalyst can be ascribed to
accelerated charge carrier migration and separation. The in situ
fabrication of the ternary photocatalyst helps in achieving close
microscopic contacts among the three constituent phases. The
affluent channelization of photo-assisted charge carriers
through the grain boundaries of Bi2S3, Bi2O3 and ZnIn2S4 pha-
ses leads to enhanced photoactivity of the BZ20BO material.
ꢀ
the presence of cOH and cO2 scavengers, the PDE of the
BZ20BO material is severely suppressed whereas a moderate
decrease in PDE is observed in the presence of the h+ scavenger.
Transient photocurrent measurements of BZ20BO in the pres-
ence of 20 ppm TCN solution also exhibited an increase in
photocurrent density due to utilization of the photogenerated
holes in oxidation of TCN through direct and indirect routes
resulting in good separation of charge carriers.41
ꢀ
The scavenger study reveals that cOH and cO2 are major
active species responsible for TCN degradation. The in situ
formation of these reactive oxygen species (ROS) in the aqueous
suspension of the BZ20BO photocatalyst is also conrmed from
the terephthalic acid photoluminescence (TA-PL) probing
technique and nitroblue tetrazolium chloride (NBT) test.20,21
The uorescence intensities of aqueous suspensions of BO, BS/
BO, ZIS/BO and ternary BZ20BO photocatalysts in the presence
of terephthalic acid (TA) are presented in Fig. 9. Signicantly
higher uorescence intensity is noticed in the case of the
BZ20BO photocatalyst than the pure BO and binary composite
samples (Fig. 9II). This observation suggested the availability of
a higher quantity of free VB h+ species for the transformation of
Electron-transfer mechanism in BZxBO ternary composite
materials
OHꢀ to cOH (EOH /cOH ¼ 1.99 eV). The produced cOH radicals
ꢀ
The VB and CB positions of BS, BO and ZIS materials were
calculated from band gap values and at band potentials (V)
react with TA molecules to form uorescent 2-
Fig. 7 Digital images of the CFU of Enterobacter cloacae (a) light control, (b) dark control experiments and (c and d) treated with 300 mg Lꢀ1 of
BZ20BO catalysts after 40 and 60 min visible light irradiation.
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Fig. 8 Plausible photocatalytic mechanism for the BZxBO ternary composite material.
hydroxyterephthalic acid (HTA) which is determined by the O2cꢀ radicals due to the sufficient potential of CB electrons to
uorescence study. The O2cꢀ radical formation over the pho- reduce dioxygen species to O2cꢀ radicals (EO2/O2c ¼ ꢀ0.33 eV).
42
ꢀ
tocatalyst surface is further conrmed from the NBT test
Although pristine BS and BO materials do not perform
(Fig. 9III and S17†). A negligible amount of O2cꢀ radicals is dioxygen reduction, the binary BS/BO material is capable of
formed using pure BO as the photocatalyst. However, pure ZIS, producing a noticeable amount of O2cꢀ radicals. This is due to
binary ZIS/BO and ternary BZ20BO photocatalysts can generate the change in band realignment of both BO and BS
Fig. 9 (I) PL spectra of the aqueous suspension of the BZ20BO photocatalyst containing 5 ꢂ 10ꢀ4 M terephthalic acid (excitation wavelength:
320 nm), (II) comparison of PL intensity for (a) BO, (b) BS/BO, (c) ZIS/BO and (c) BZ20BO materials, (III) UV-Vis spectra of 5 ꢂ 10ꢀ5 molar nitro blue
tetrazolium solution containing 300 mg Lꢀ1 of BZ20BO photocatalyst and (IV) ESR spectra of the DMPO spin adduct formed in BZ20BO
suspension in (a) methanol and (b) water under dark and illumination conditions (10 min).
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eꢀ + O2 / O2cꢀ
(8)
(9)
semiconductors due to the formation of the p–n heterojunction.
The highest NBT transformation rate observed for the BZ20BO
material can be ascribed to the enhanced charge carrier sepa-
ration in the ternary composite (Fig. S17V†). To get solid
evidence of O2cꢀ and cOH radical formation in the BZ20BO
photocatalyst suspension, the electron spin resonance (ESR)
study is performed using DMPO as the radical trapping agent.
The ESR spectra of the spin adduct formed in methanol as well
as the aqueous suspension of the BZ20BO material under dark
and illumination conditions (10 min) are presented in Fig. 9IV.
Under dark conditions, no ESR signal could be detected in
either solution. However, under illumination conditions the
characteristic ESR peaks due to DMPO–cOH and DMPO–O2cꢀ
adducts are clearly noticed.43 The ESR study further conrms
the formation of O2cꢀ and cOH radicals in the BZ20BO photo-
catalyst suspension.
The radical trapping study supports the occurrence of a dual
Z-scheme electron transfer mechanism while ruling out tradi-
tional type-II electron transfer in the BZ20BO ternary
composite. The type-II electron transfer mechanism cannot
explain adequately the formation of the ROS species observed
experimentally (Fig. 8I). The radical trapping experiments also
support the occurrence of the Z-scheme electron transfer
process individually in BS/BO and ZIS/BO binary composites as
both binary composites are capable of generating O2cꢀ and cOH
radicals in aqueous suspension. Thus, the ternary composite
which is obtained by integration of the two binary photosystems
is expected to exhibit dual Z-scheme electron migration. The
occurrence of the dual Z-scheme mechanism can be further
inferred from the XPS study where a shi of the Bi 4f peak to
higher energy is noted for the ternary composite in comparison
to the binary materials. The density of states (DOS) calculation
study of b-Bi2O3 suggested that the conduction band is mainly
Bi in character.44 The efficient channelization of electrons from
the CB of b-Bi2O3 can account for the observed shi in BE to the
higher energy side in the XPS study. The dual Z-scheme electron
transfer mechanism also provides a satisfactory explanation for
photocatalytic water splitting as well as Cr(VI) reduction. During
the photocatalytic reaction, the reduction of water and Cr(VI)
species can take place in the CB of ZIS and BS to generate
molecular H2 and Cr(III) ions, respectively. Moreover, the
superoxide ions can also effectively reduce Cr(VI) species
through an indirect two step mechanism, where electrons from
the CB are consumed by dissolved O2 to form O2cꢀ radicals
which further facilitate the reduction of Cr(VI) to Cr(III).45,46 The
photogenerated holes in the VB of BO are quickly consumed by
MeOH and TA, which are good h+ scavenging agents. Thus, the
photogenerated charge carriers are well separated with a pro-
longed life time in the composite system which is responsible
for the improved performance towards photocatalytic hydrogen
generation and Cr(VI) reduction.
O2cꢀ + H2O / cHO2 + OHꢀ
cHO2 + H2O / H2O2 + cOH
H2O2 + O2cꢀ / cOH + OHꢀ + O2
cOH + TCN / oxidation products
h+ + H2O / cOH + H+
(10)
(11)
(12)
(13)
(14)
h+ + TCN / oxidation products
(b) Cr(VI) reduction
h+ + TA / Rc
(15)
(16)
(17)
cOH + TA /CO2 + H2O
Cr(VI) + 3eꢀ / Cr(III)
(c) H2 production
1
H2O þ 2hþ/ O2ðgÞ þ 2Hþ
(18)
(19)
2
2H+ + 2eꢀ / H2(g)
(d) Bacterial inactivation
Cytotoxicity study
Study of the cytotoxic effect of the catalysts as well as the water
samples aer photocatalytic treatment is an important aspect
for practical application. In this regard, the cytotoxic effects of
TCN and Cr(VI) solutions as well as the BZ20BO photocatalyst
are studied by the MTT assay technique. Initial optimization
studies for the cytotoxicity experiments a carried out by taking
2–10 mL of parent and treated solutions (Fig. S18†). The cell
viability (%) of 10 mL parent tetracycline and treated aqueous
solutions (i.e. solution A and solution B obtained aer 20 and
40 min of photocatalytic treatment) is presented in Fig. 10I.
Solution A (87% cell viability) and solution B (94% cell viability)
are found to be less toxic than the parent TCN solution (70% cell
viability). Similarly almost all cells are viable in the photo-
catalytically treated solution of Cr(VI) which suggests that the
hazardous Cr(VI) is reduced to less toxic Cr(III) (Fig. 10II). The
photocatalyst is also found to be non-cytotoxic between the
concentrations of 200 and 500 mg Lꢀ1 (Fig. 10III).
The radical mechanisms for photocatalytic degradation of
TCN, reduction of Cr(VI), H2 production and bacterial inactiva-
tion are presented in eqn (7)–(19):
(a) TCN degradation
BZ20BO + hn / eꢀ + h+
(7)
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Fig. 10 Cytotoxicity study of the (I) 20 ppm tetracycline solution before and after photocatalytic treatment (solution A and solution B refer to 20
and 40 min photocatalytic treatment respectively), (II) 10 ppm Cr(VI) solution before and after photocatalytic treatment and (III) different
concentrations of the catalyst in aqueous solution.
BZ20BO photocatalyst is probed under continuous illumination
of the Xe lamp (250 W) for 2 h (Fig. S19V†). The BZ20BO
material shows excellent photostability with nominal loss of
photocurrent under prolonged exposure.
Catalyst recyclability and stability study
The stability and reusability of a catalyst are important aspects
in the eld of catalysis. In this study, the recyclability test for the
BZ20BO photocatalyst is performed for ve catalytic runs. Aer
successive catalytic runs, catalyst particles are separated by
centrifugation and washed with ethanol which is then subjected
to vacuum drying at 80 ꢃC for 12 h. The BZ20BO material is
found to be recyclable up to the 5th catalytic run with negligible
loss in photocatalytic efficiency for TCN degradation and
a minimal loss of 3% activity for Cr(VI) reduction (Fig. S19I†).
The structure and morphological changes of the regenerated
photocatalyst are accessed using XRD and FESEM studies
(Fig. S19†). No noticeable changes in the morphology and
crystallographic phase of the BZ20BO photocatalyst could be
observed aer the catalytic cycles (Fig. S19II†). This observation
conrms the excellent reusability and stability of the BZ20BO
photocatalyst. The elemental mapping study of the BZ20BO
photocatalyst aer the 5th catalytic cycle is presented in
Fig. S19III.† The occurrence of adsorbed Cr species is inferred
from the elemental composition study. It has been noted in
earlier studies that Cr(VI) upon photoreduction to Cr(III) can
remain as an adsorbed impurity on the photocatalyst surface.47
To further conrm the nature and oxidation state of the
adsorbed Cr species on the photocatalyst surface, an XPS study
is performed. The high resolution XPS spectra of Cr exhibited
a doublet at 577.8 and 587.1 eV which correspond to the Cr(III)
species (Fig. S19IV†). The XPS study further conrms the pho-
tocatalytic reduction of Cr(VI) to Cr(III) species on the BZ20BO
photocatalyst surface. The long term photo-stability of the
4. Conclusion
A facile reux method was developed in this work for in situ
synthesis of a novel ternary Bi2S3/b-Bi2O3/ZnIn2S4 dual Z-
scheme heterostructure photocatalyst. The heterostructure
material contained Bi2S3 nanoparticles (15–25 nm) nely
dispersed over a continuous matrix of interleaved Bi2O3 nano-
plates and ZnIn2S4 nanosheets. The in situ synthesis method
facilitated high interfacial contact among the constituent pha-
ses which accounted for the improved photoelectrochemical
properties of the ternary composite. The photocatalytic efficacy
of the ternary heterostructure was successfully demonstrated
for multidimensional application in waste water remediation,
H2 production and bacterial disinfection. The ternary photo-
catalyst exhibited dramatic improvement in photocatalytic
performance towards visible light assisted TCN degradation
and Cr(VI) reduction achieving 96.3% photodegradation effi-
ciency and 93.7% photoreduction efficiency, respectively,
within a short span of time. During TCN degradation the
ꢀ
involvement of cOH and cO2 radicals as reactive species was
validated from radical trapping experiments and ESR. The non-
cytotoxic nature of the photocatalyst as well as the treated TCN
and Cr(VI) solution was veried by the MTT assay study. The
ternary heterostructure material also exhibited promising
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potential for photocatalytic H2 generation (2243 mmol gꢀ1 hꢀ1 15 P. J. Mafa, B. Ntsendwana, B. B. Mamba and A. T. Kuvarega,
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21 D. Majhi, P. K. Samal, K. Das, S. K. Gouda, Y. P. Bhoi and
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22 R. Ji, C. Ma, W. Ma, Y. Liu, Z. Zhu and Y. Yan, New J. Chem.,
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Conflicts of interest
There are no conicts of interest to declare.
23 B. Shao, X. Liu, Z. Liu, G. Zeng, Q. Liang, C. Liang, Y. Chemg,
W. Zhang, Y. Liu and S. Gong, Chem. Eng. J., 2019, 368, 730–
745.
Acknowledgements
24 X. Y. Yu and X. W. Lou, Adv. Energy Mater., 2017, 1701592.
The research was supported by the Council of Scientic & 25 A. Helal, F. A. Harraz, A. A. Ismail, T. M. Sami and
Industrial Research (CSIR), India. We would like to thank Mr I. A. Ibrahim, Appl. Catal., B, 2017, 213, 18–27.
Malaya Kumar Debata (NIT, Rourkela, India) for his immense 26 H. Wang, H. Ye, B. Zhang, F. Zhao and B. Zeng, J. Phys. Chem.
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27 X. Liu, W. Wang, Y. Liu, B. Huang, Y. Dai, X. Qin and
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28 G. Zhu, Y. Liu, M. Hojamberdiev, J. Han, J. Rodriguez,
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