Fabrication of La2O3/Bi2O3/silver orthophosphate Heterojunction Catalyst for the Visible Light Mediated Remediation of Refractory Pollutants

Article abstract of DOI:10.1016/j.materresbull.2021.111299

The development of silver orthophosphate based ternary composite catalyst for the augmented visible light assisted photocatalytic abatement of toxic refractory pollutants was accepted. It was confounded that the synthesized catalyst effectively degrade toxic organic dyes including methylene blue (MB), methyl orange (MO), rhodamine B (RhB) and acid red 18 (AR 18) with complete decolourisation and above 90 % mineralization. The hazardous pesticides such as 2, 4-dichlorophenoxyacetic acid (2,4-D), highly toxic insecticide acephate and the pharmaceutical antibiotic tetracycline were succesfully degraded. Here, it is the first time reporting La₂O₃/Bi₂O₃ doped silver orthophosphate ternary catalyst for the removal of toxic organic pollutants in a short time with excellent mineralization. The better reproducibility and accountable stability of the composite catalyst paved the way for making it a promising catalyst for future applications.

Full text of DOI:10.1016/j.materresbull.2021.111299

Materials Research Bulletin 140 (2021) 111299
Contents lists available at ScienceDirect
Materials Research Bulletin
journal homepage: www.elsevier.com/locate/matresbu
Fabrication of La2O3/Bi2O3/silver orthophosphate Heterojunction Catalyst
for the Visible Light Mediated Remediation of Refractory Pollutants
Thomas Abraham, Ragam N. Priyanka, Subi Joseph, Anu Rose Chacko, Beena Mathew*
School of Chemical Sciences, Mahatma Gandhi University, Kottayam-686 560, Kerala, India
A R T I C L E I N F O
A B S T R A C T
Keywords:
The development of silver orthophosphate based ternary composite catalyst for the augmented visible light
assisted photocatalytic abatement of toxic refractory pollutants was accepted. It was confounded that the syn-
thesized catalyst effectively degrade toxic organic dyes including methylene blue (MB), methyl orange (MO),
rhodamine B (RhB) and acid red 18 (AR 18) with complete decolourisation and above 90 % mineralization. The
hazardous pesticides such as 2, 4-dichlorophenoxyacetic acid (2,4-D), highly toxic insecticide acephate and the
pharmaceutical antibiotic tetracycline were succesfully degraded. Here, it is the first time reporting La2O3/Bi2O3
doped silver orthophosphate ternary catalyst for the removal of toxic organic pollutants in a short time with
excellent mineralization. The better reproducibility and accountable stability of the composite catalyst paved the
way for making it a promising catalyst for future applications.
Silver orthophosphate
Ternary composite
Photodegradation
Organic pollutants
1. Introduction
develop a new catalyst for reducing the recombination tendency and
improving the catalytic efficiency. Several silver based photocatalyst
The excess use of hazardous pesticide and toxic organic dyes are a
severe threat to human health and the environment. Most of the
commonly used dyes such as methylene blue, methyl orange, acid red 18
and rhodamine B (RhB) contaminate aqueous solutions and causes
heavy water pollution. The use of organochlorine (OC) compounds such
as 2,4-dichlorophenoxyacetic acid (2,4-D) [1], organophosphorus (OP)
compounds such as acephate [2] and the antibiotic tetracycline [3] will
accumulate in the water sources which causes harm to the bioorganisms.
2,4-D is a highly toxic water-soluble herbicide that is a hormonally
active agents and hence referred to as endocrine-disrupting agents
causing chromosomal mutations. Acephate is an insecticide commonly
used in fruits and vegetables that can harm the environment due to the
accumulation of its residues [4]. Tetracycline is a low-cost antibiotic in
which its uncontrolled use may harm the living organisms [5]. Hence it
is inevitable to develop a suitable catalyst that can control all these toxic
dyes and pesticides in less time with more efficiency. Even though
various degradation techniques are available, photodegradation is
green, cost-effective and low energy consumption. The development of
semiconductor-based photocatalyst heterojunction shows excellent
degradation efficiency with accountable mineralization. The main
drawback of these catalyst heterojunctions is their rapid recombination
tendency. So the main challenge for these degradation techniques was to
such as AgBr [6], Ag2CO3 [7], Ag3PO4 [8], Ag2CrO4 [9], etc shows better
photocatalytic activity. Silver orthophosphate (Ag3PO4) having a nar-
row bandgap which shows high photocatalytic activity in the
silver-based family. The noticeable peculiarity of Ag3PO4 was its very
low valence band level which promotes the generation of more active
photogenerated holes and electrons [10–12]. However, the main
drawback of Ag3PO4 was its poor photostability and a high tendency for
photoreduction of silver ions (Ag+) from Ag3PO4 into metallic silver.
These challenges can be reduced by proper doping of Ag3PO4 with
highly active photocatalyst composite [13]. Recently reported Ag3PO4
based binary composite such as Ag3PO4/WO3 [14], Ag3PO4/NaTaO3
[15], Ag3PO4/NiFe2O4 [16], Co3(PO4)2/Ag3PO4 [17], etc. and ternary
composite including Ag3PO4/ZnO/Fe3O4 [18], BiVO4/RGO/Ag3PO4
[19], gC3N4/MoS2/Ag3PO4 [20], etc. shows good degradation property.
Lanthanum based oxides are the nontoxic catalyst which possesses
excellent stability and enhanced catalytic efficiency. Lanthanum oxide
(La2O3) is one of the active rare-earth metal oxides for the effective
degradation of dyes and pesticides. Several reported La2O3 based com-
posite such as La2O3@TiO2 [21], ZrO2@La2O3 [22], ZnO@La2O3 [23],
etc shows pronounced catalytic activity. The role of La2O3 in the com-
posite is the proper transfer and rapid separation of photogenerated
electrons and holes. The coupling of La2O3 with a bismuth-based
* Corresponding author.
E-mail address: beenamathew@mgu.ac.in (B. Mathew).
https://doi.org/10.1016/j.materresbull.2021.111299
Received 30 November 2020; Received in revised form 16 February 2021; Accepted 5 March 2021
Available online 8 March 2021
0025-5408/© 2021 Elsevier Ltd. All rights reserved.

T. Abraham et al.
Materials Research Bulletin 140 (2021) 111299
catalyst shows excellent photocatalytic activity [24–26]. Hence it was
planned to develop a binary composite with La2O3 and bismuth-based
catalyst. We aimed to develop an efficient photocatalyst for the
removal of toxic dyes and pesticides in a short time. In this paper,
La2O3/Bi2O3 binary catalyst doped silver orthophosphate ternary com-
posite was developed for the successful treatment of refractory pollut-
ants with excellent mineralization and pronounced catalytic efficiency.
3. Results and Discussion
3.1. Structure and morphology
The bonding interactions of the synthesized composite was devel-
oped by Fourier transform infrared (FT-IR) spectroscopy. Fig. 1a shows
the sharp peak at 644 and 578 cmꢀ 1 are due to the formation of La2O3
nanoparticle. The peak at 505.57 cmꢀ 1 corresponds to the vibrations of
Bi-O bond of Bi2O3 nanoparticle. Also the peaks at 860 and 660 cmꢀ 1
show the interatomic vibration modes of Bi-O bond. The sharp peak at
542.36 and 942.67 cmꢀ 1 determines the vibration modes of phosphate
molecules. The structural determination of the composite was done with
the X-ray diffraction (XRD) technique. The planes (011), (012), (003),
(111), (112) and (005) represent the angle 28.54, 39.43, 44.67, 54.56
and 77.46◦, respectively of hexagonal La2O3 with the primitive crystal
lattice (JCPDS No: 83-1344). Bi2O3 formation is manifested with the
diffraction planes of (120), (-211), (130), (131) and (231) at the angle of
27.43, 32.54, 37.21, 41.45 and 45.67◦, respectively of monoclinic Bi2O3
system with primitive crystal lattice (JCPDS No: 41-1449). The forma-
tion of La2O3/Bi2O3 binary composite was confirmed by the peaks at
32.12, 36.34, 43.54, 47.52 and 62.18◦ for the planes of (-211), (130),
(003), (111) and (241), respectively. The structural confirmation of 10
wt. % La2O3/Bi2O3/Ag3PO4 was mainly assigned with the presence of a
phosphate group in the composite. The sharp peaks at 21.32, 30.32,
33.45, 36.67, 48.31, 52.75 and 57.35◦ for the planes of (110), (200),
(210), (310), (222), (320) and (321), respectively show the planes of
Ag3PO4. The peaks at 62.34 and 77.43◦ determine the peak of Bi2O3 and
La2O3, respectively. All these diffraction studies confirm the formation
of the ternary nanocomposite (JCPDS No: 06-0505) (Fig. 1b). The
diffused reflectance spectra (DRS) of the various modified composite
shows the wavelength shifting to redshift in the visible region shows the
improvement in the photocatalytic activity of the composite (Fig. 1c).
The calculation of the bandgap reveals the reason for the high activity of
the composite. The bandgap of the binary composite can be found using
the Tauc plot. The ternary composite shows less binding energy (1.81
eV) on comparing with the binary composite (2.62 eV). The measure-
ment of the bandgap of the synthesized composite can be done with the
Kubelka- Munk equation,
2. Experimental
2.1. Materials
Lanthanum nitrate (La(NO3)3.6H2O, Bi(NO3)3.5H2O, sodium
hydrogen phosphate (Na2HPO4), silver nitrate (AgNO3) and tetracycline
(C22H24N2O8) of analytical grade were supplied from Merck, India and
are used as such. Distilled water was used throughout the experiment.
2.2. Synthesis of La2O3/Bi2O3 composite
La(NO3)3.6H2O (0.01 g) was dissolved in 15 mL deionized water and
Bi(NO3)3.5H2O (0.015 g) was dissolved in 15 mL ethylene glycol. The
two solutions are mixed and kept for 1 h continue stirring. Then the
solution is allowed for 10 min microwave irradiation and the resultant
dark yellow solution is calcined for 3 h at 600 ◦C in a muffle furnace and
the obtained powder was used for further characterizations.
2.3. Synthesis of La2O3/Bi2O3/Ag3PO4 composite
Add La2O3/Bi2O3 (0.01 g) composite into 15 mL deionized water and
sonicated for 10 min. The solution was allowed for continuous stirring
and add an equimolar amount of Na2HPO4 into the above solution and
after 10 min add AgNO3 solution into it with vigorous stirring. Ag3PO4
was formed in the binary composite results in the formation of La2O3/
Bi2O3/Ag3PO4.
2.4. Characterization techniques
∝hϑ = A(hϑ ꢀ Eg)1/n
(1)
Fourier transform infrared (FT-IR) spectroscopy was done for un-
derstanding the bonding interaction between the composites using
Perkin Elmer 400 spectrometer. The structural characterization of the
composite was determined using X-ray diffraction (XRD) analysis using
Bruker AXS D8 Advance X-ray diffractometer of wavelength (λ = 1.5406
Where ∝ represents absorption coefficient, hϑ is the incident photon
energy, A is a constant and n depends on the type of optical transition.
Here the combination of the composite results n = 2, describes indirect
transition [27]. The optical band gap calculated using the Tauc plot is
used to find the valence band and conduction band potentials of the
particle which results in the mechanism of photodegradation. The
following equations are used to found the valence band and conduction
band potential,
A◦) with Cu K
α radiation. The surface morphology of the catalyst com-
posite was examined using field emission scanning electron microscope
(FESEM) with VEGA 3 TESCAN. The inner structure of the composite
was done with a high-resolution transmission electron microscope
(HRTEM) with a specification of the JOEL-JEM-2100 microscope. The
quantification of elemental states was analyzed with X-ray photoelec-
tron spectroscopy (XPS) using a VG Multi-Lab 2000 system containing
EVB
=
χ
–Ee + 0.5 Eg
(2)
(3)
ECB = EVB – Eg
Mg Kα radiation at 20 kV. The diffused reflectance spectroscopy (DRS) is
analyzed with Shimadzu-UV-3600 Plus of Japan for measuring the band
gap using the Tauc plot. The surface area of the composite catalyst was
determined using the Brunauer-Emmet-Teller (BET) of Thermo Fischer
Scientific. The chromatographic separation of the pesticides and anti-
biotics was done using high-performance liquid chromatography
(HPLC) using SPD-20A, Schimadzu Corporation, Japan. The organic and
mobile phase was acetonitrile and 0.02 M ammonium acetate with 0.1 %
3.2. Structure and morphology
The activity of the catalyst depends on its surface morphology.
Hence, field emission scanning electron microscopy (FESEM) analysis is
carried out. Fig. 2(a & b) shows the non-uniform irregular shaped La2O3
and Bi2O3 nanoparticles, respectively. The fabrication of the binary
composite La2O3/Bi2O3 morphology was depicted in Fig. 2c. The
development of spherical shaped silver orthophosphate was successfully
depicted in Fig. 2d. The binary composite La2O3/Bi2O3 was anchored to
the Ag3PO4 nanoparticle was successfully observed here. To confirm the
elemental composition and the arrangement of various elements in the
formic acid respectively with an injection volume of 5 μL with a flow
rate of 1 mL/min. The total organic carbon (TOC) analysis was done
with TOVCPH total organic carbon analyser, Shimadzu Corporation,
Japan. The path of the photodegradation was examined using ultra-
performance liquid chromatography coupled to quadrupole time-of-
flight mass spectrometry (Waters Xevo G2 Q TOF) in positive and
negative electrospray ionization.
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Materials Research Bulletin 140 (2021) 111299
Fig. 1. (a) FT-IR, (b) XRD, (c) DRS and (d) Tauc plot of La2O3 (LaO), Bi2O3(BiO), 47 wt % La2O3/Bi2O3 (LaO/BiO) and 10 wt % La2O3/Bi2O3/Ag3PO4.
Fig. 2. FESEM of (a) La2O3, (b) Bi2O3, (c) 47 wt % La2O3/Bi2O3 and (d) 10 wt % La2O3/Bi2O3/Ag3PO4.
3

T. Abraham et al.
Materials Research Bulletin 140 (2021) 111299
whole composite, elemental mapping was done. Fig. 3a represents the
elemental mapping of all the elements present in the catalyst composite.
Each element are represented by different colours and the mapping
image of each of the elements are separately shown (Fig. 3(b–f)).
The internal structural aspects of the synthesized catalyst composite
were depicted with transmission electron microscope (TEM) analysis,
which was manifested in 100 nm scale. As analysed by FESEM, the inner
nonuniform particles of La2O3 and Bi2O3 are revealed in Fig. 4a & b. The
blending of the irregular La2O3 and Bi2O3 was shown in Fig. 4c. The
doping of La2O3/Bi2O3 on the pristine spherical shaped Ag3PO4 was
confirmed in Fig. 4d. The adjunction of La2O3/Bi2O3 binary composite
on the spherical shaped Ag3PO4 accounts for its enhanced photocatalytic
activity. The high resolution TEM shows the fringe width of the com-
posite and was found that the fringe width of the bulk Ag3PO4 in the
composite was 0.135 nm. The bright spots in the selected area electron
diffraction (SAED) confirm the crystalline nature of the developed
ternary composite (Fig. 4e & f).
available surface area and the pore diameter. The surface area of the
composite was analyzed with Brunauer-Emmet-Teller (BET) and the
pore diameter with the Barret-Joyner-Halenda (BJH) plot. The surface
area possessed by silver orthophosphate is only 0.68 m2/g and the pore
diameter was 5.26 nm. On coupling with more surface area possessed
composite (La2O3/Bi2O3), the surface area of La2O3/Bi2O3/Ag3PO4 be-
comes 12 times higher than the pristine Ag3PO4 nanoparticle (Fig. S1).
Table 1 shows the surface area and pore diameter possessed by various
modified composites.
The chemical states of the elemental composition in the composites
are illustrated with X-ray photoelectron spectroscopy (XPS). Fig. 5a
represents the La 3d spectrum of La2O3, La2O3/Bi2O3 and La2O3/Bi2O3/
Ag3PO4 composites. All the spectra reveal the characteristic satellite
splitting of La in the range of 834.58 and 851.31 eV, which is the
splitting due to La 3d 5/2 and La 3d 3/2, respectively. These peaks show
the presence of La3+ state of the element in the composite. Also, the
energy gap between the two satellite peak is at about 16.73 eV, which
reveals the formation of La2O3 phase. The figure reveals the slight
shifting of the two satellite peaks in La2O3/Bi2O3 and La2O3/Bi2O3/
Ag3PO4 which manifested the formation of binary and ternary com-
posites. Fig. 5b shows the elemental state of bismuth in the fabricated
3.3. Surface area and elemental quantification of the composite
The activity of the catalyst composite was determined with their
Fig. 3. Elemental mapping of all the elements in 10 wt % La2O3/Bi2O3/Ag3PO4.
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T. Abraham et al.
Materials Research Bulletin 140 (2021) 111299
Fig. 4. TEM image of (a) La2O3, (b) Bi2O3, (c) 47 wt % La2O3/Bi2O3, and (d) 10 wt % La2O3/Bi2O3/Ag3PO4, HRTEM (e) and (f) SAED pattern of 10 wt % La2O3/
Bi2O3/Ag3PO4.
range of 532.05 and 532.53 eV reveal the formation of O-La in La2O3.
Table 1
Also, the prominent peak at 528.91 and 531.07 eV attributed to O-Bi
BET surface area and BJH pore diameter of various modified composites.
bond formation. The minor shifting observed for these two peaks in
Sample
Surface Area (m2/g)
Pore Diameter (nm)
La2O3/Bi2O3 and La2O3/Bi2O3/Ag3PO4 confounded the composite for-
mation (Fig. 5c). The sharp peaks at 366.81 and 372.76 eV of Ag cor-
responds to 3d 5/2 and 3/2 supports the formation of Ag3PO4. The
formation of phosphate molecule was examined with the prominent
peak at 131.42 (2p 3/2) and 132.37 (2p 1/2) eV corresponds to the
formation of phosphate molecule. All the results are in well agreement
with binary and ternary composite formation (Fig. 5d & e).
La2O3
12.35
15.51
13.17
0.68
14.21
18.24
15.15
5.26
Bi2O3
47 wt % La2O3/Bi2O3
Ag3PO4
10 wt % La2O3/Bi2O3/Ag3PO4
8.602
3.24
composites. The two prominent peaks at 159.02 and 164.37 eV reveals
the formation of Bi-O bond, which occurs due to the splitting of Bi 4f 2/7
and Bi 4f 2/5, respectively. The energy separation between the two
symmetric peaks are at about 5.4 eV which is attributed to the formation
of Bi3+ oxidation state of Bi2O3. The slight shift observed for the Bi 4f
symmetric peaks for La2O3/Bi2O3 and La2O3/Bi2O3/Ag3PO4 shows the
formation of composite materials. Moreover, the O 1s spectra at a peak
3.4. Photocatalytic degradation
The photocatalytic activity of the synthesized catalyst was analyzed
with various toxic acidic and basic dyes such as methylene blue (MB),
methyl orange (MO), rhodamine B (RhB) and acid red 18 (AR 18). The
low bandgap energy possessed by the catalyst composite shift to longer
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Materials Research Bulletin 140 (2021) 111299
Fig. 5. XPS of (a) La 3d of La2O3, La2O3/Bi2O3 and La2O3/Bi2O3/Ag3PO4, (b) Bi 4f of Bi2O3, La2O3/Bi2O3 and La2O3/Bi2O3/Ag3PO4, (c) O 1s of La2O3, Bi2O3, La2O3/
Bi2O3 and La2O3/Bi2O3/Ag3PO4, (d) Ag 3d, and (e) P 2p of La2O3/Bi2O3/Ag3PO4.
wavelength which results in the absorption of solar radiation in the
visible region. Moreover, the synthesized binary composite can able to
form an Z-scheme pathway in the presence of visible light which will be
easier for the generation of photogenerated electrons and hole pairs and
are responsible for the abatement of refractory pollutants.The optimi-
zation of the photocatalytic abatement was done with MB and observed
an accountable degradation efficiency. Fig. 6a shows the UV–vis. spectra
of the photocatalytic degradation of MB with and without the catalyst. It
was shown that the presence of catalyst shows an enhanced degradation
of 97.42 % degradation efficiency within 5 min. Fig. 6b shows the
degradation efficiency in the C/Co vs. time graph. The % of degradation
is calculated with the following equation,
dyes are in better agreement with the pseudo-first-order kinetics during
the process of photodegradation (Fig. S2, S3 & S4). These high efficiency
and very low time interval make the catalyst a promising one for future
industrial applications.
Even though photodegradation of these toxic dyes occurs with
complete decolourisation, the extend of mineralization of the degraded
dyes has to be analyzed. For that, total organic carbon (TOC) analysis
was done for all the dyes before and after degradation. Fig. 7 shows the
TOC analysis data which reveals the TOC removal percentage of MB,
MO, AR 18 and RhB as 91.36, 95.18, 94.55 and 93.44 %, respectively.
The data confirms the complete decolourisation and mineralization of
these dyes with the developed composite catalyst within a short time
[28]. Table 2 represents the previously reported catalyst for the photo-
degradation of various dyes. The data helps to compare the efficiency of
the synthesized catalyst with other early reported catalysts which re-
veals the excellent efficiency and accountable time for the complete
decolourisation and mineralization of the toxic organic dyes.
% of degradation = [(Co – Ct) × 100]/Co
(4)
Here C0 represents the analyte initial concentration and Ct represents the
concentration at time t. The photodegradation efficiency of the modified
composite is compared with the bar diagram and confirmed the very
high degradation efficiency for 10 wt % La2O3/Bi2O3/Ag3PO4 ternary
composite (Fig. 6c). The kinetics of the photocatalytic study was
analyzed by plotting –ln C/C0 vs time graph. Fig. 6d shows the kinetic
study and reveals that the experiment follows pseudo-first-order kinetics
with a high rate constant of the order k =0.6111 minꢀ 1. To extend the
application of the developed catalyst composite, a photodegradation
study was also done for MO, RhB and AR 18. It was observed that the
complete decolorisation and mineralization occur for MO at a wave-
length of 463 nm within 5 min with 97.33 % degradation efficiency. At
the same time, the degradation study for RhB and AR 18 realized
accountable degradation efficiency with excellent decolorisation and
mineralization. The RhB degraded within 2 min at the wavelength of
553.5 nm with 96.26 % efficiency whereas AR 18 degraded within 10
min at the wavelength of 536 nm with an efficiency of 95.79 %. All these
The activity of the synthesized catalyst can be verified with various
pesticides and antibiotics. The organochlorine pesticide 2,4-dichloro-
phenoxyacetic acid (2,4-D), organophosphorus insecticide acephate
and antibiotic tetracycline were used for this. All the pesticides and
antibiotics were treated with the synthesized catalyst and undergo
visible light-mediated photodegradation. The determination of the
degradation tendency of the catalyst was observed with the high-
performance liquid chromatography (HPLC). The standard of 2,4-D,
acephate and tetracycline were optimized in the retention time of
8.10, 17.06 and 2.88 min, respectively. The effect of the photo-
degradation study was done by comparing the intensity of the standard
solution with the degraded solution after several hours. The intensity of
the peak of 2,4-D, acephate and tetracycline were almost decreased after
180, 60 and 120 min, respectively. These suggest the accountable
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T. Abraham et al.
Materials Research Bulletin 140 (2021) 111299
Fig. 6. (a) UV–vis. spectra of MB, (b) C/C0 vs time graph, (c) bar diagram for comparison of degradation efficiency, and (d) kinetic study of the composite.
Table 2
Comparison with previously reported works.
No
Catalyst
Irradiation
Light
Pollutants
Degraded
Time
Reference
(min)
1
2
3
4
5
Fe3O4/ZnO/
Ag3PO4
Visible Light
Visible Light
Visible Light
RhB
100
[29]
[30]
[31]
[32]
[33]
PANI/Ag3PO4/
NiFe2O4
RhB
MO
MO
40
25
80
Ag3PO4/
nanocellulose
Ag3PO4/PAN
200 W
MB
60
21
mercury lamp
Visible Light
Ag3PO4/
RhB
bentonite
6
7
Ag3PO4/HHSSa
La2O3/Bi2O3/
Ag3PO4
Visible Light
Visible Light
MO
12
5
[34]
MB
This work
MO
5
RhB
AR 18
2
10
a
Fig. 7. TOC removal (%) of MB, MO, AR 18 and RhB with 10 wt % La2O3/
HHSS – hierarchial hollow silica spheres.
Bi2O3/Ag3PO4.
mineralization, the nature of the degraded product should be
mentioned. Thus high-resolution mass spectra (HRMS) are analyzed for
comparing the standard and degraded product of 2,4-D, acephate and
tetracycline. The HRMS results confirm that all the degraded products
obtained were less toxic than the standard solution. The 2,4-D (m/z =
219.96) get converted in to 2,4-dichloro anisole (m/z = 175.97) and
acephate (m/z = 183) degraded to form a small compound (m/z =
111.04) and the heavier tetracycline antibiotic (m/z = 445) degraded in
to bicyclic compound with an m/z of 210 (Figs. S5 &S6). The degrada-
tion route of 2,4-D, acephate and tetracycline are described below
(Fig. 10).
activity of the developed photocatalyst (Fig. 8)
The mineralization of the composite catalyst with these pesticides
and antibiotics can be determined with the total organic carbon (TOC)
analysis. Fig. 9 shows the TOC removal % of 2,4-D, acephate and
tetracycline in different periods. About 65.98 % TOC removal occurs for
2,4-D after 180 min whereas acephate shows a TOC removal of 89.56 %.
Tetracycline shows an effective TOC removal of about 92.93 %. These
results manifested the enhanced mineralization of toxic organic pesti-
cides and antibiotics.
Even though these toxic pesticides and antibiotics show better
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Materials Research Bulletin 140 (2021) 111299
Fig. 8. HPLC and C/C0 of (a & b) 2,4-D, (c &d) acephate, and (e &f) tetracycline, respectively.
recyclability which was analysed by doing photodegradation studies of
about four different cycles. It was observed that an enhanced photo-
degradation efficiency occurs in all the different cycle runs suggesting
the excellent recyclability of the catalyst. To use a catalyst for industrial
purposes, the stability of the catalyst needs to be evaluated. The struc-
tural properties of the composite were identified by comparing XRD, FT-
IR and XPS survey spectrum of the fresh and used samples. The results
reveal that there occurs only a slight variation of the peaks which is
attributed to the high stability of the developed composite (Fig. 11a–d).
The photodegradation mechanism of the catalyst composite needs to
be explained. Several experiments are analyzed for the enhanced ac-
tivity of the catalyst. Fig. 12a shows the fluorescence spectra of the
various modified composite. The intensity of the fluorescence spectra of
the optimized ternary catalyst composite decreases as compared with
the unmodified system. This reveals the less recombination tendency
possessed by the developed catalyst which allows the rapid movement of
electron-hole pairs in the composite. To account for the active species of
degradation steps, a radical scavenger experiment was carried out. The
experiment was done with ethylenediamminetetraacetic acid (EDTA)
for holes, benzoquinone (BQ) for superoxide radical and isopropyl
alcohol for hydroxyl radical. Fig. 12b reveals the order of active species
for photodegradation was h+ > O2ꢀ .> OHꢀ .. Thus holes (h+) were
Fig. 9. TOC analysis of 2,4-D, acephate, and tetracycline 10 wt % La2O3/
Bi2O3/Ag3PO4.
The essential criteria needed for an effective catalyst was its
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Materials Research Bulletin 140 (2021) 111299
Fig. 10. Degradation route of 2,4-D, acephate, and tetracycline with 10 wt % La2O3/Bi2O3/Ag3PO4.
Fig. 11. (a) Recycling runs of the composite for the degradation of MB, (b) XRD, (c) FT-IR, and (d) XPS survey spectra of fresh and used catalyst.
considered as the main active species for photodegradation [35]. The
effect of photocurrent was analyzed using an amperometric experiment
done in 0.1 M Na2SO4 solution. The experiment was done with a proper
switch on and off method with the various modified composite. It was
found that the modified ternary composite shows an enhanced current
which is twice as possessed by the bare GCE suggesting the better cat-
alytic activity of the composite (Fig. 12c). The electrochemical activity
of the synthesized composite was analyzed with the potentio electro-
chemical impedance spectroscopy. The impedance analysis was done in
the solution of 0.1 M KCl and 5 mM ferric ferrocyanide solution with a
9

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Materials Research Bulletin 140 (2021) 111299
Fig. 12. (a) Fluorescence spectra, (b) scavenger study, (c) amperometric experiment and (d) Nyquist plot of the various modified composite.
frequency ranges from 5 Hz to 25 MHz. The radius of the obtained
Nyquist plot describes the resistance possessed by the composite.
Fig. 12d shows a very low radius of the developed composite than the
other unmodified systems reveals the low resistance shown by the
composite. This low resistance reveals the high transfer of electrons
throughout the composite which corroborates the excellent electro-
chemical activity of the composite [36].
pollutants result in the successful degradation of these refractory pol-
lutants (Scheme 1). It was also confirmed with the scavenger study as the
active species for photodegradation were holes (h+) and super oxide
ꢀ
•
radicals ( O2 ). The low fluorescence intensity for the modified com-
posite reveals the less recombination property of the modified composite
suggesting the accumulation of more holes in the valence band of
Ag3PO4. On irradiation of visible radiation, La2O3, Bi2O3 and Ag3PO4 get
excited and surrounded with photoactive holes and electrons (Eqs. 5, 6
& 7). The combination of Ag3PO4 with electron and Bi2O3 with holes get
interconverted into Ag3PO4 with holes and Bi2O3 with electrons. The
formation of superoxide radicals occurs from electrons surrounded by
3.5. A plausible mechanism of photodegradation
All these experiments opened the platform to explain the photo-
catalytic mechanism of the developed catalyst. On irradiation of visible
light, photogenerated holes and electrons are formed. The conduction
band potential (ECB) of Bi2O3 was about +0.11 V, which is not suitable
•
for the production of super oxide radical (ꢀ O2). The E◦ of O2/ꢀ O2 is -0.33
•
V vs NHE which tells the theoretical difficulty for the formation of ꢀ O2
An interesting fact is that super oxide radicals can be developed in the
gaseous phase reaction system when adequate amount of adsorbed O2 is
present. Since high intensified solar radiation comprised of small per-
centages of ultraviolet radiations. Moreover, the configuration of La2+
consist of a d electron which results in its intrinsic instability. Thus, La2+
release the trapped electrons to the adsorbed O2 molecule and thus
•
forming ꢀ O2 [37,38]. There occurs rapid transfer of electrons from the
valence band of Bi2O3 to its conduction band results in the accumulation
of a large number of holes. The photogenerated holes from the valence
band of Bi2O3 get easily transferred to the valence band of Ag3PO4 since
the potential difference was only 0.40 V. The highly active electrons in
the conduction band of Ag3PO4 get easily move to the conduction band
of Bi2O3 which results in the generation of active holes in the valence
band of Ag3PO4. These holes get combine with the toxic organic
Scheme 1. Plausible mechanism of photodegradation of La2O3/Bi2O3/Ag3PO4.
10

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Materials Research Bulletin 140 (2021) 111299
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La2O3 or Bi2O3 particle (equations 8 & 9) [18].
La2O3 + hʋ → La2O3 (h+) + La2O3 (eꢀ )
Bi2O3 + hʋ → Bi2O3 (h+) + Bi2O3 (eꢀ )
Ag3PO4 + hʋ → Ag3PO4 (h+) + Ag3PO4 (eꢀ )
Ag3PO4 (eꢀ ) + Bi2O3 (h+) → Ag3PO4 (h+) + Bi2O3 (eꢀ )
Bi2O3 (eꢀ ) → O2-. + La2O3(eꢀ ) or Bi2O3
(5)
(6)
(7)
(8)
(9)
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for the efficient treatment of refractory pollutants was established. The
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Organochlorine herbicide 2,4-D, organophosphorus insecticide ace-
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CRediT authorship contribution statement
Thomas Abraham: Conceptualization, Methodology, Resources,
Writing - original draft. Ragam N. Priyanka: Data curation, Visuali-
zation. Subi Joseph: Data curation, Visualization. Anu Rose Chacko:
Data curation, Visualization. Beena Mathew: Conceptualization, Su-
pervision, Project administration, Writing - review & editing.
¨
[18] N. Güy, K. Atacan, E. Karaca, M. Ozacar, Role of Ag3PO4 and Fe3O4 on the
photocatalytic performance of magnetic Ag3PO4/ZnO/Fe3O4 nanocomposite under
visible light irradiation, Sol. Energy 166 (2018) 308–316.
[19] Y. Li, X. Xiao, Z. Ye, Fabrication of BiVO4 RGO/Ag3PO4 ternary composite
photocatalysts with enhanced photocatalytic performance, Appl. Surf. Sci. 467
(2019) 902–911.
Declaration of Competing Interest
[20] L. Tian, X. Yang, X. Cui, Q. Liu, H. Tang, Fabrication of dual direct Z-scheme g-
C3N4/MoS2/Ag3PO4 photocatalyst and its oxygen evolution performance, Appl.
Surf. Sci. 463 (2019) 9–17.
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
[21] M. Uzunova, M. Kostadinov, J. Georgieva, C. Dushkin, D. Todorovsky,
N. Philippidis, I. Poulios, S. Sotiropoulos, Photoelectrochemical characterisation
and photocatalytic activity of composite La2O3-TiO2 coatings on stainless steel,
Appl. Catal. B Environ. 73 (2007) 23–33.
Acknowledgements
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heterogeneous ZrO2 supported metaloxide catalysts for brown grease esterification
and sulfur removal, Bioresour. Technol. 102 (2011) 2380–2386.
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La2O3 composite modified by potassium for phenol degradation, Mater. Lett. 113
(2013) 190–194.
The current work was supported by DST-PURSE PII (SR. 417 & SR.
416 dated 27-2-2017) and Mahatma Gandhi University, Kottayam,
Kerala, India.
[24] X. Zou, C. Yuan, Y. Dong, H. Ge, J. Ke, Y. Cui, Lanthanum orthovanadate/bismuth
oxybromide heterojunction for enhanced photocatalytic air purification and
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Appendix A. Supplementary data
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.materresbull.20
21.111299.
[25] Y. Li, G. Chen, H. Zhang, Z. Li, Electronic structure and photocatalytic water
splitting of lanthanum-doped Bi2AlNbO7, Mater. Res. Bull. 44 (2009) 741–746.
[26] W. Meng, R. Hu, J. Yang, Y. Du, J. Li, H. Wang, Influence of lanthanum-doping on
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