Investigating the effects of various synthesis routes on morphological, optical, photoelectrochemical and photocatalytic properties of single-phase perovskite BiFeO3

Article abstract of DOI:10.1016/j.jpcs.2021.110342

Herein, various BiFeO₃ morphologies, including sheet-like, coral-like and rod-like structures, were synthesized via co-precipitation (CP), hydrothermal (HT), and sol-gel (SG) synthesis routes, respectively. The as-synthesized samples were characterized by physicochemical techniques to investigate their crystal structure, optical and photoelectrochemical properties. The SG-BiFeO₃ sample exhibited remarkable direct sunlight photocatalytic degradation of phenol (98.95%), superior to those of the HT-BiFeO₃ (77.4%) and CP-BiFeO₃ (66.9%) in 120 min. The SG-BiFeO₃ sample was the most effective among all due to the lower energy band gap value and highest separation of photogenerated charge carriers, which was validated by the UV–vis absorption, photoluminescence (PL) and photoelectrochemical measurements. The recycling and ferric (Fe³⁺) ion leakage test suggested that the SG-BiFeO₃ sample was highly stable for up to six consecutive runs. The radical scavenger studies implied that the photogenerated hole (h⁺), hydrogen peroxide (H₂O₂) and hydroxyl radicles (?OH) were the dominant reactive species. Finally, based on these, a possible photocatalytic mechanism for phenol degradation over SG-BiFeO₃ sample was also postulated.

Full text of DOI:10.1016/j.jpcs.2021.110342

Journal of Physics and Chemistry of Solids 160 (2022) 110342
Contents lists available at ScienceDirect
Journal of Physics and Chemistry of Solids
journal homepage: www.elsevier.com/locate/jpcs
Investigating the effects of various synthesis routes on morphological,
optical, photoelectrochemical and photocatalytic properties of single-phase
perovskite BiFeO3
a
a
b
,
c
d,e
Shui-Wen Chang Chien , Ding-Quan Ng , Dileep Kumar , Sze-Mun Lam
Zeeshan Haider Jaffari
,
a,*
a
Department of Environmental Engineering and Management, Chaoyang University of Technology, No. 168, Jifeng E. Rd, Wufeng District, Taichung, 413310, Taiwan
Division of Computational Physics, Institute for Computational Science, Ton Duc Thang University, Ho Chi Minh City, Viet Nam
Faculty of Applied Sciences, Ton Duc Thang University, Ho Chi Minh City, Viet Nam
b
c
d
e
College of Environmental Science and Engineering, Guilin University of Technology, Guilin, 541004, China
Department of Environmental Engineering, Faculty of Engineering and Green Technology, Universiti Tunku Abdul Rahman, 31900, Kampar, Perak, Malaysia
A R T I C L E I N F O
A B S T R A C T
Keywords:
BiFeO
Herein, various BiFeO3 morphologies, including sheet-like, coral-like and rod-like structures, were synthesized
via co-precipitation (CP), hydrothermal (HT), and sol-gel (SG) synthesis routes, respectively. The as-synthesized
samples were characterized by physicochemical techniques to investigate their crystal structure, optical and
3
Chemical synthesis
Magnetic
photoelectrochemical properties. The SG-BiFeO
degradation of phenol (98.95%), superior to those of the HT-BiFeO
The SG-BiFeO sample was the most effective among all due to the lower energy band gap value and highest
separation of photogenerated charge carriers, which was validated by the UV–vis absorption, photoluminescence
3
sample exhibited remarkable direct sunlight photocatalytic
Photoelectrochemical
Photocatalysis
3
(77.4%) and CP-BiFeO (66.9%) in 120 min.
3
3
(
PL) and photoelectrochemical measurements. The recycling and ferric (Fe3 ) ion leakage test suggested that the
+
SG-BiFeO
3
sample was highly stable for up to six consecutive runs. The radical scavenger studies implied that the
+
photogenerated hole (h ), hydrogen peroxide (H
2
O
2
) and hydroxyl radicles (•OH) were the dominant reactive
species. Finally, based on these, a possible photocatalytic mechanism for phenol degradation over SG-BiFeO
sample was also postulated.
3
1
. Introduction
photocatalytic semiconductor materials with smaller energy band gaps
and higher charge carrier separation efficiency.
Semiconductor photocatalysis, as a non-hazardous and environment-
3
Perovskite bismuth ferrite (BiFeO ) is one of the rare single-phase
friendly technique, has been widely applied for eliminating environ-
mental contamination as well as the energy crisis [1,2]. To date,
multiferroic semiconductors which is visible light-responsive due to a
small energy bandgap (2.1 eV) [7,11]. Besides, it is non-toxic and has
outstanding chemical stability [12]. The low decomposition tempera-
2
numerous semiconductors such as TiO , ZnO and CuO have been
investigated for the photodegradation of organic contaminants [1,3].
Nonetheless, the practical applications of the photocatalysts as
mentioned above are still restricted due to wide bandgap energies and
rapid recombination of photogenerated charge carriers [4,5]. To over-
come these shortcomings, various attempts, including noble metal
doping [6,7], ion substation [8,9] and heterojunction formation [5,10],
have been made. All these investigations have achieved substantial
progress, yet some intrinsic problems are not entirely resolved. Hence,
from the practical application point of view, it is necessary to develop
3
ture of bismuth salts makes it difficult to synthesize BiFeO material
without any impurities using conventional solid-state synthesis process
at higher sintering temperatures [13]. Chemical techniques including
sole-gel [14], hydrothermal [15], rapid liquid phase sintering [16] and
co-precipitation [17] methods have been applied to synthesize BiFeO
3
materials with the required morphologies. The photocatalytic applica-
tions of these studies suggested that the control over particle size and
shape had an important role in the performance due to the influence of
bandgap energy and the number of surface reaction sites [18,19].
*
Corresponding author.
E-mail address: jaffarizh@hotmail.com (Z.H. Jaffari).
https://doi.org/10.1016/j.jpcs.2021.110342
Received 15 June 2021; Received in revised form 5 August 2021; Accepted 15 August 2021
Available online 16 August 2021
0
022-3697/© 2021 Elsevier Ltd. All rights reserved.

S.-W. Chang Chien et al.
Journal of Physics and Chemistry of Solids 160 (2022) 110342
Fig. 1. Schematic illustration of the BiFeO
3
synthesis using three different synthesis routes (a) hydrothermal route, (c) co-precipitation technique, and (c) sol-
gel synthesis.
Recently, various BiFO
wires [21], mesh, mat-shaped [22], nanotubes [23] and bowl-arrays
24], have been successfully synthesized using different methods.
However, a comparison among the various synthesis routes of BiFeO
and their effects on photocatalytic performance is not well documented
in the literature. Hence, it is essential to synthesize BiFeO materials
3
morphologies, including spindle-like [20],
stirring. The mixture was then shifted into a 100 mL Teflon-lined
◦
autoclave reactor for the hydrothermal reaction at 125 C for 15 h. At
[
the end of the reaction, the HT-BiFeO
3
particles were collected, washed
◦
3
with water/ethanol multiple times, and dried in an oven at 70 C for 4 h.
In the co-precipitation (CP) method, equal molar Bi(NO
3
)
3
⋅5H
2
O and
3
Fe(NO O were dissolved in 2 M HNO solution at room temper-
3
)
3
⋅9H
2
3
using different techniques to investigate their influence on morphology,
optical, photoelectrochemical and photocatalytic properties.
ature. Both nitrate and ammonia solution were then added dropwise in
the above mixture to maintain a solution pH of 10. The colloidal pre-
cipitate was settled through centrifugation and washed with ultrapure
water until a solution pH of 7.0 was achieved. Afterward, the obtained
In the current study, three synthesis techniques were applied to
3
fabricate BiFeO materials with various morphologies and the average
◦
particle sizes ranging from nanometer to micrometer. The physico-
chemical, optical and electrochemical properties of as-synthesized ma-
terials were determined using numerous techniques. Phenol was applied
as a model organic pollutant to investigate the photocatalytic activities
of the as-synthesized materials. Finally, the recycling potential and
particles were dried in an oven at 60 C for 6 h. Finally, the dried par-
◦
ticles were calcinated at 550 C for 1 h to obtain the CP-BiFeO
3
sample.
O and 0.005
O were dissolved into a 0.2 M aqueous HNO so-
In the sol-gel (SG) synthesis, 0.005 mol of Bi(NO
mol of Fe(NO
3
)
3
⋅5H
2
3
)
3
⋅9H
2
3
lution. The above solution was then inserted into a 0.0025 M solution of
3
possible photocatalytic mechanism over BiFeO material were studied
polyvinyl alcohol (PVA) under strong magnetic stirring. The mixture
◦
through electrochemical measurements and hydroxyl radical and reac-
tive species detection during the photodegradation process.
was then heated at 60 C until a gel was formed. The gel was then cal-
◦
cinated in a muffle furnace at 550 C for 2 h to obtain the final SG-
3
BiFeO powder.
2
. Experimental section
2
.2. Characterizations
2
.1. Synthesis of BiFeO
3
The crystalline structure of the synthesized products was investi-
gated using X-ray diffraction (XRD, Rigaku D/Max 2550) at 40 kV, with
Cu K as a source of radiation. The surface morphology of the samples
was captured using a field emission scanning electron microscope
FESEM, JSM-6701 F). The elemental analysis was performed using
All the chemicals were of analytical grade obtained from Sigma
Aldrich and applied directly without any additional processing. Fig. 1
displays the three different routes to synthesize single-phase perovskite
α
BiFeO
thermal route, the equal molar ratio of ferric nitrate (Fe(NO
mmol, 98%) and bismuth nitrate (Bi(NO O, 5 mmol, 98%) were
⋅5H
dissolved in a 20 mL aqueous solution of nitric acid (HNO , 2 mL, 65%)
3
samples with various shapes and particle sizes. In the hydro-
(
3
)
3
⋅9H O, 5
2
energy dispersive X-ray (EDX) coupled with FESEM. The surface func-
tional groups on the synthesized samples were recorded in the range
between 400 and 4000/cm using Perkin Elmer Lambda 35. The ab-
sorption spectra of the samples were taken in the range of 200–800 nm
using a PerkinElmer Lambda 750 S. The photoluminescence (PL) spectra
were conducted at room temperature with 325 nm as excitation
3
)
3
2
3
and urea (0.1 mol, 99.5%) under magnetic stirring. After 30 min of
magnetic stirring, the whole mixture was inserted dropwise into a 10 M
solution of potassium hydroxide (KOH, 85%) under strong magnetic
2

S.-W. Chang Chien et al.
Journal of Physics and Chemistry of Solids 160 (2022) 110342
wavelength in a PerkinElmer S55. The magnetic properties were studied
using the magnetic hysteresis (M ꢀ H, MicroSense 10 Mark ІІ) loop.
2
.3. Photoelectrochemical measurements
Photoelectrochemical measurements were performed in a three-
2 4
electrode cell using 0.5 M sodium sulfate (Na SO , 99%) as an electro-
lyte. The fluorinated tin oxide (FTO) glass coated with the BiFeO
3
samples was applied as working electrodes. Silver chloride (Ag/AgCl)
and platinum (Pt) wire acted as the reference and counter electrodes,
respectively. The transient photocurrent response (TPR) spectra,
Nyquist plot and linear sweep voltammetry (LSV) were investigated in a
Gamry electrochemical workstation. Precisely, the photocurrent curve
was investigated by controlling on-off light for 20 s with a bias voltage of
0
.4V. The Nyquist plot was obtained at a frequency ranged between 0.1
Hz and 10 KHz. Finally, the LSV was executed at a scan rate of 50 mV/s.
2
.4. Evaluation of photocatalytic test
3
Fig. 2. XRD patterns of BiFeO samples synthesized using different routes.
For a typical reaction, 100 mg of the synthesized photocatalyst was
dispersed in a 100 mL of 5 mg/L phenol aqueous solution to produce the
reaction suspension. After the suspension was magnetically stirred for
3. Results and discussion
3
0 min under dark conditions, it was exposed to direct sunlight irradi-
3.1. Physiochemical characterizations
2
ation with a light intensity of 875.4 W/m . As the photodegradation
reaction proceeds, a certain volume of solution was taken from the re-
Fig. 2 depicts the XRD diffraction patterns obtained for the BiFeO
3
action mixture, filtered using a 0.22
μ
m syringe filter to eliminate the
samples, namely HT-BiFeO
using three different routes. All samples matched the rhombohedral
perovskite structure of BiFeO
(JCPDS No. 20–0169) [25]. The strong
and sharp peaks suggested that the as-synthesized BiFeO materials
3 3 3
, CP-BiFeO and SG-BiFeO , synthesized
photocatalyst particles. The filtered solution was then analyzed using a
UV–vis spectrophotometer (PerkinElmer Lambda 900 spectrometer) at
3
2
70 nm wavelength. The chemical oxygen demand (COD) was investi-
3
◦
gated using COD vials incubated in a COD digester for 2 h at 150 C. The
phenol COD can be stoichiometrically calculated using the following
phenol degradation reaction.
were highly crystalline. Additionally, no foreign characteristic peak
for impurity was detected in the synthesized samples, indicating a
high product purity.
The morphology and elemental composition of the as-synthesized
C
6
H
5
OH + 6O
2
→6CO
2
+ 3H
2
O
(1)
3
BiFeO samples were investigated by FESEM and EDX analyses. The
different fabrication routes induced dissimilar morphologies and parti-
cle sizes. The FESEM images showed that HT-BiFeO3 (Fig. 3(a)) pos-
sesses a coral-like morphology with an average particle size around 130
7 × 32
94.11
(
COD)Phenol = [C]Phenol
×
(2)
nm, the CP-BiFeO
average width and diameter of 10 nm and 150 nm, respectively, while
SG-BiFeO (Fig. 3(c)) had a rod-like structure with an average diameter
of 350 nm and a length ranged between 1.5 and 2.5 m. The elemental
samples were composed
3
(Fig. 3(b)) had a sheet-like morphology with an
where C is the concentration of phenol at a certain time. The COD of
intermediated can be calculated using the following equation;
3
(
COD)Interm ediates = (COD)Total
̶
(COD)Phenol
(3)
μ
analysis (Fig. 3(d–f)) showed that all the BiFeO
3
Additionally, the recycling potential of the SG-BiFeO
3
material was
of Bi, Fe and O elements. The weak C peak in the spectra can be asso-
ciated with the supporting carbon tape. The elemental distributions in
studied by executing six recycling reactions with 90 min for each run. At
the end of each run, the photocatalytic material was magnetically
separated from the solution, washed with distilled water and finally
BiFeO
3
samples using EDX mapping (Fig. 3(g–i)) showed that Bi, Fe and
◦
3+
O elements were uniformly distributed throughout the samples, which
could be helpful for the effective charge carrier separation/transfer.
dried at 80 C in an oven for a successive run. The dissolved Fe ions
concentration in the final solution was also investigated using inductive
coupled plasma mass spectroscopy (ICP-MS).
3
The surface functional groups of BiFeO samples were investigated
with the FTIR technique. As displayed in Fig. 4, all the BiFeO
3
samples
ꢀ 1
ꢀ 1
exhibited sharp absorption peaks around 450 cm
and 554 cm
,
2
.5. Active species assessments
which could be originated from the Fe–O–Fe bending and Fe–O
stretching mode of the FeO
6
octahedral perovskite, respectively
To examine the participation of reactive species in the photocatalytic
─1
[
26–28]. The weak peak around 1620 cm could be attributed to the
+
reaction, triethanolamine (TEOA, 1 mM, h scavenger), benzoquinone
–
OH bending mode of physisorbed water on the surface of BiFeO
3
, while
ꢀ
(
(
BQ, 1 mM, quencher for superoxide radicals (•O
2
)), isopropyl alcohol
, 1 mM)
─1
the strong and broad absorption peak at 3410 cm could be assigned to
IPA, 1 mM, quencher for •OH) or catalyze (quencher for H
2 2
O
the O–H stretching mode of –OH group on the surface of the catalysts
was introduced into the phenol solution.
[
3
29,30]. Primarily, it can be noticed that the SG-BiFeO sample
The amount of •OH radicals generated on the SG-BiFeO
3
sample was
exhibited stronger intensities of O–H bending and stretching mode of
OH groups than that of the remaining BiFeO samples (insert of Fig. 4).
also measured using TAPL analysis [21]. Typically, 0.2 mM of tereph-
thalic acid (TA, 99%) was ultrasonically dissolved in 100 mL of 0.025
mM sodium hydroxide (NaOH) solution. 0.1 g of prepared sample was
added in the above solution and further sonicated for 1 h under dark
conditions and then exposed to direct sunlight irradiation. The reacted
solution was magnetically separated, and the TAPL spectra were recor-
ded at 325 nm excitation wavelength using a spectrophotometer (Per-
kinElmer LS-55).
–
3
The presence of –OH group in large quantity ensures their effective
photodegradation performance. The –OH group served as a key
quencher for the photogenerated charge carriers that led to the gener-
ation of active •OH radical, which is highly conducive for organic pol-
lutants degradation [4].
3

S.-W. Chang Chien et al.
Journal of Physics and Chemistry of Solids 160 (2022) 110342
Fig. 3. FESEM images of the (a) HT-BiFeO
3
, (b) CP-BiFeO
3
and (c) SG-BiFeO
3
samples; EDX analysis (d) CP-BiFeO
3
, (e) HT-BiFeO
3
and (f) SG-BiFeO
3
samples; EDX
(4)
3 3 3
mapping (g) CP-BiFeO , (h) HT-BiFeO and (i) SG-BiFeO samples.
2
(
α
hv) = A(hv-E
g
)
where
stant, and E
CP-BiFeO , HT-BiFeO
and 2.05 eV, respectively (Fig. 5(b)). The decline in the bandgap en-
ergies indicated that the SG-BiFeO sample has more light absorption
ability to facilitate the excitation of an electron from the valance band
VB) to the conduction band (CB), which might be advantageous for the
α
is the optical absorption, hv is the photon energy, A is a con-
is the bandgap energy. The estimated bandgap energies for
and SG-BiFeO samples were around 2.12, 2.09
g
3
3
3
3
(
photodegradation reaction. The CB and VB potential of BiFeO
3
can be
calculated using the Mulliken electronegativity theory [33,34].
e
E
E
VB = X – E + 0.5E
g
(5)
(6)
CB = EVB – E
g
where ECB, EVB, E
e
, E
g
and X are the conduction band potential, valance
band potential, free electron energy on hydrogen scale (4.5 eV),
bandgap energy of BiFeO (eV) and absolute electronegativity of BiFeO
3
3
,
respectively. The absolute electronegativity value can be estimated
using the following equation [34,35].
3
Fig. 4. FTIR spectra of BiFeO samples synthesized using different routes.
√
̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅
(
m+n+1)
m
n
1
X(AmBnCl) =
X X X
(7)
A
B
C
3
.2. Optical and electrochemical traits
The absolute electronegativity values for Bi, Fe and O elements in
3
BiFeO were 4.69 eV, 4.04 eV and 7.54 eV, respectively [34]. Hence, the
Fig. 5(a) shows the optical absorption properties of the as-
synthesized BiFeO samples via three different synthesis routes. It can
be seen that different synthesis routes can influence the optical prop-
erties of BiFeO . The SG-BiFeO sample exhibited an absorption cut-off
wavelength around 624 nm, which was higher than that of the HT-
BiFeO and CP-BiFeO samples for absorption cut-off wavelength of 618
nm and 610 nm, respectively. The bandgap energies of BiFeO samples
hv) vs. energy bandgap (E
according to the well-known Kubelka-Munk (K-M) theory [31,32].
estimated X(BiFeO3) value from the above equation was 6.06 eV. After
substituting these values in equations (5) and (6), the obtained ECB and
3
E
VB values were around 0.53 eV and 2.59 eV, respectively.
3
3
It is a well-known fact that the catalytic performance of a photo-
catalyst highly depended on the separation and transfer capabilities of
photogenerated charge carriers. Therefore, a deep understanding of the
dynamics of charge carriers on as-synthesized BiFeO3 samples was a
crucial issue. To interpret the insight into the generation of photo-
3
3
3
2
can be estimated from the graph between (
α
g
)
3
generated charge carriers on BiFeO samples, the PL emission of the
4

S.-W. Chang Chien et al.
Journal of Physics and Chemistry of Solids 160 (2022) 110342
2
Fig. 5. UV–vis DRS spectra of various BiFeO
3 g 3
samples and (b) Plots of (αhv) vs. photon energy (E ) of absorbed light for BiFeO samples.
3
Fig. 6. (a) PL spectra, (b) TPR, (c) EIS Nyquist plot and (d) LSV of BiFeO samples fabricated via different techniques.
synthesized samples were recorded. Fig. 6(a) depicts the PL spectra of
BiFeO samples synthesized using three different synthesis techniques. It
can be seen that the PL intensity of the SG-BiFeO sample was lower than
that of the HT-BiFeO and CP-BiFeO sample, suggesting that the charge
recombination efficiency. Besides, electrochemical impedance (EIS) is
also a vital indicator of the photogenerated charge carrier’s separation
and transfer capability on the photocatalyst surface. The smaller the arc
radius in the impedance figure, the inferior the impedance posed by the
material [34,36]. Fig. 6(c) shows the various synthesis routes highly
3
3
3
3
carrier’s separation efficiency could be boosted using the sol-gel syn-
thesis technique and thereby gave an ameliorated photocatalytic activ-
ity [7].
affected the electrochemical impedance of BiFeO
electrochemical impedance happened to the SG-BiFeO
kΩ), which was only 0.79 and 0.69 times that of the HT-BiFeO
kΩ) and CP-BiFeO (5.5 kΩ), respectively, indicating that the SG-BiFeO
3
samples. The smallest
sample (3.81
(4.78
3
The TRP spectra is a common indicator to study the charge carrier
separation efficiency on the photocatalyst surface. A superior photo-
current value means better charge carrier separation/transfer and
3
3
3
sample owned the best charge carrier’s separation efficiency. Moreover,
the LSV spectra were also performed to investigate the photo-
electrochemical properties of synthesized samples further. As shown in
Fig. 6(d), the lowest overpotential and maximum photocurrent density
higher photodegradation performance [36]. As illustrated in Fig. 6(b),
2
the highest photocurrent intensity (2.49
the SG-BiFeO
of the HT-BiFeO
respectively, signifying that the SG-BiFeO
μ
A/cm ) was recorded using
3
sample, which was 1.36 and 1.17 times higher than those
2
2
3
(1.82
μ
A/cm ) and CP-BiFeO
3
(2.12
μ
A/cm ) samples,
3
of SG-BiFeO sample suggested that it can appropriately enhance pho-
3
had the least charge carrier
togenerated charge carrier separation and transfer than the remaining
5

S.-W. Chang Chien et al.
Journal of Physics and Chemistry of Solids 160 (2022) 110342
Fig. 7. UV–vis absorption spectra of phenol degradation with various direct sunlight irradiation time over SG-BiFeO
photolysis, dark absorption and various BiFeO samples under direct sunlight irradiation, (c) corresponding pseudo-first-order plots and rate constant k values over
numerous samples, (d) effect of photocatalyst loading on the phenol degradation over SG-BiFeO , (e) effect of initial substrate concentration on photodegradation of
phenol using SG-BiFeO and (f) COD variations versus time.
3
, (b) photocatalytic degradation of phenol over
3
3
3
two samples. The LSV analysis was in accordance with the UV–vis DRS,
PL, TPR and EIS findings, which further confirm that the SG-BiFeO
photocatalytic degradation was best suited for the pseudo-first-order
3
kinetics, and the rate constant (k) was corresponding to the line slope
sample can show effective charge carrier separation, thus, the highest
photocatalytic performance.
(Fig. 7(c)). As can be seen that the k values of SG-BiFeO
CP-BiFeO samples were 0.03804/min, 0.01433/min and 0.00907/min,
respectively. The SG-BiFeO sample showed the highest k value for
3 3
, HT-BiFeO and
3
3
phenol degradation, which was around 2.65 and 4.19 times higher than
that of the HT-BiFeO and CP-BiFeO samples, respectively. Hence, the
3
.3. Photocatalytic degradation of phenol
3
3
3
SG-BiFeO sample was then chosen for the following experiments.
The photodegradation performance of differently synthesized
BiFeO
3
samples was estimated against the degradation of phenol solu-
3.3.1. Effect of catalyst dosage
tion under direct sunlight irradiation. Before the sunlight irradiation, the
suspension was stirred under dark conditions to attain an adsorption-
desorption equilibrium state. Fig. 7(a) depicts the absorption spectra
of phenol solution obtained at 30 min intervals in the existence of the
The effect of SG-BiFeO3 dosage on the photodegradation reaction
was investigated, and results are presented in Fig. 7(d). When SG-BiFeO3
dosage was increased from 0.25 g/L to 1.5 g/L, the photocatalytic
degradation of phenol was increased from 40.95% to 100%. However,
further enhancement in SG-BiFeO3 dosage to 2.0 g/L caused a
decreasing efficiency to 86.4%. At a lower catalyst dosage, the catalyst
absorbs fewer photons to initiate the photodegradation reaction, thus
lower photocatalytic performance. Upon enhancing the catalyst dosage,
the number of active sites and light absorption centers on the catalyst
surface also increased, thereby improving the photocatalytic perfor-
mance [37]. Besides, the optimized catalyst dosage can also enhance the
generation of photogenerated charge carriers, which led to an
enhancement in the production of reactive radical species. Nonetheless,
when the catalyst dosage was enhanced beyond the optimum amount,
the limited light diffusion into the suspension and enhanced solution
turbidity led to a decline in the photocatalytic degradation performance
[34,38]. Hence, 1.5 g/L catalyst dosage was selected in the following
study.
3
SG-BiFeO sample. The main absorption peak of phenol was situated
around 270 nm, whose intensity dropped rapidly with the rise in light
irradiation time. After 120 min of sunlight irradiation, the peak almost
became a flat line, suggesting a complete degradation of phenol
molecules.
Fig. 7(b) shows the changes in C/C
absorption and photodegradation tests in the existence of CP-BiFeO
HT-BiFeO and SG-BiFeO samples. The comparative studies suggested
o
vs. time for the photolysis, dark
3
,
3
3
that when the phenol solution was irradiated in the absence of BiFeO
samples or when the phenol solution in the presence of SG-BiFeO
3
3
sample was stirred under the dark conditions, an insignificant amount of
phenol was eliminated. In the existence of BiFeO samples, the direct
sunlight irradiation for 120 min resulted in a 66.9% and 77.4% phenol
degradation using CP-BiFeO and HT-BiFeO samples, respectively. In
contrast, the SG-BiFeO particles led to a 98.95% phenol degradation
after a similar duration of sunlight irradiation. The superior photo-
degradation performance of BiFeO particles could be attributed to the
3
3
3
3
3.3.2. Effect of initial concentration
3
To investigate the effect of initial phenol concentration on the SG-
BiFeO3 photocatalytic degradation, the photocatalytic trials were
repeated by altering the initial phenol concentration from 10 mg/L to
smaller energy bandgap and well-defined rod-like morphology, which
could reduce the photogenerated charge carrier recombination and
enhance the number of active sites on the photocatalyst surface. The
6

S.-W. Chang Chien et al.
Journal of Physics and Chemistry of Solids 160 (2022) 110342
Fig. 8. M ꢀ H loop of SG-BiFeO
3
sample, insert of the figure is the magnetic separation of SG-BiFeO
3
, (b) Six consecutive recycling runs in the photocatalytic
3
degradation of phenol under direct sunlight irradiation, (c) XRD patterns of recycled and fresh SG-BiFeO sample and (d) TPR comparison among the fresh and
3
recycled SG-BiFeO samples.
5
0 mg/L (Fig. 7(e)). A decline in photocatalytic degradation from 100%
3.4. Magnetic properties and cyclic photodegradation
to 54.9% was recorded upon increasing the initial phenol concentration
from 10 mg/L to 50 mg/L. The highest and lowest photodegradation
rates were recorded at 10 mg/L and 50 mg/L, respectively. The decline
in photodegradation efficiency at higher phenol concentration could be
credited to the decrease in photocatalytic active sites accessibility to the
phenol molecules, which ultimately led to a decline in reactive species
generation to partook in photodegradation reaction [39,40]. Another
possible reason for declined photodegradation performance could be
due to the higher light absorption by phenol molecules than that of the
Fig. 8(a) illustrates the M ꢀ H loops of the synthesized SG-BiFeO
3
sample. The M ꢀ H loops displayed negligible coercivity and remanence
magnetization in the absence of an external magnetic field, indicating
the superparamagnetic nature of the synthesized sample [40]. Unlike
other magnetic materials, superparamagnetic materials could be
extremely useful in environmental applications due to their reversible
magnetic interaction. When the external applied magnetic field was
3
removed, the magnetic moment vanished, and the SG-BiFeO particles
SG-BiFeO
3
[40].
were redispersed in the aqueous solution (inset of Fig. 8(a)), thus
establishing the prospect of their recycling properties. A clear specific
saturation magnetization of about 4.91 emu/g was recorded, which was
3
.3.3. Chemical oxygen demand (COD) measurements
The intermediate products generated via partial oxidation of organic
higher than that of the reported BiFeO
Fig. 8(b) depicts the photocatalytic stability of the SG-BiFeO
for six consecutive runs against phenol degradation. At the end of each
run, the SG-BiFeO particles were collected using a small external
3
literature [43–45].
pollutants might lead to competitive adsorption on the photocatalyst
surface or become more harmful than the initial one [41]. Hence, it was
crucial to study the complete oxidation through the investigation of COD
contents. It can be seen from Fig. 7(f), the total COD and COD of phenol
decreased gradually as the reaction proceeds. In contrast, the concen-
tration of reaction intermediates increased initially and then decreased
until the photocatalytic reaction.
3
sample
3
magnet, washed, dried and reused in fresh phenol solution degradation.
A slight decline in the photocatalytic performance was recorded. At the
3
end of the sixth run, the SG-BiFeO sample still showed more than 92%
degradation efficiency. In the current study, Fe3 ion leaching during
+
The photodegradation caused decomposition and ring-opening of
phenol molecules resulted in the intermediate product generation,
recycling tests was also verified using the ICP-MS technique. It found
that no Fe3 ion leaching was detected, proving the high stability of the
+
which ultimately mineralized into carbon dioxide (CO
2
) and water
photocatalytic material. These findings were also consistent with other
(
H
2
O) [42]. When the phenol concentration decreased, an excess
BiFeO
To further confirm the stability of photocatalyst, XRD and TPR an-
alyses of the recycled SG-BiFeO were performed. Fig. 8(c) depicts the
XRD patterns of SG-BiFeO before and after six cycles, confirming that
there were no noticeable changes in XRD patterns. Moreover, Fig. 8(d)
shows the TPR comparison among the fresh and used SG-BiFeO
3
-based literature reports [11,15].
amount of reactive species and surface-active sites became available to
facilitate the remaining phenol molecules and possible intermediate
products. Consequently, around 95.05% reduction in total COD was
recorded after 90 min of direct sunlight irradiation.
3
3
3
7

S.-W. Chang Chien et al.
Journal of Physics and Chemistry of Solids 160 (2022) 110342
Fig. 9. Effect of different radical scavengers on phenol degradation using SG-BiFeO
3
sample and (b) TA-PL spectral changes with irradiation time for SG-BiFeO
3
sample in aqueous TA solution.
3
.5. Reasonable photodegradation mechanism
Identifying the prominent reactive species during the photo-
degradation reaction is essential for proposing the photocatalytic
mechanism. Hence, it is imperative to perform a radical scavenging
study. Herein, we applied 1 mM of TEOA, BQ, IPA, and catalyze as
+
ꢀ
scavengers for h , •O
2
, •OH and H
2 2
O , respectively [7,46]. From Fig. 9
(
a), it can be seen that the addition of BQ had a lesser effect on phenol
degradation. In contrast, the addition of TEOA, IPA and catalase led to a
dramatic decrease in phenol degradation to 45.2%, 39.8% and 45.7%,
+
respectively, suggesting that the h , •OH, and H
2 2
O were the dominant
reactive species. The active role of •OH radical was further investigated
by the TAPL analysis. Literature reports suggested that the •OH radicals
can react with the aqueous TA molecules to generate a highly fluores-
cent 2-hydroxyterephthalic acid (TA-OH) compound with a PL peak
situated around 425 nm [22]. As shown in Fig. 9(b), no PL signal was
recorded under dark conditions. Nonetheless, strong PL peaks for
Fig. 10. Schematic diagram of direct sunlight photocatalytic degradation
mechanism of BiFeO sample.
3
3
SG-BiFeO were recorded after 30, 60 and 90 min of light irradiation,
validating the •OH radicals were undoubtedly a key reactive species in
samples. The results suggested that both the TPR intensities were almost
similar over the six runs, proving high stability and an ideal candidacy of
phenol degradation reaction.
3
The above-stated results suggested that the SG-BiFeO sample with a
SG-BiFeO
3
material for continuous wastewater treatment.
rod-like structure seems to have a higher photocatalytic degradation due
+
to the generation of h , H
2
O
2
, and •OH radicals. The schematic charge
transfer process and reactive oxidizing species generation for the SG-
BiFeO reaction are portrayed in Fig. 10. Under direct sunlight
3
Table 1
3
A comparison of the previously reported literature on the organic pollutant degradation using BiFeO with findings in the current study.
Synthesis route
Morphology
Energy bandgap
eV)
Pollutant and concentration
Catalyst loading
(g/L)
Light source
Removal efficiency
Ref.
(
Sol-gel
Spherical
shaped
2.21
̶
Mordant Blue 9, 50 mg/L
Methylene blue, 10 mg/L
Methyl orange, 10 mg/L
1.0
0.5
2.0
1.0
1.0
0.5
1.0
1.0
1.0
Direct sunlight
Visible light
77.5% within 180
min
[48]
[23]
[49]
[50]
[28]
[22]
Template-technique
Hydrothermal
Nanotubes
75% within 180
min
Nanosphere
Nanofibers
Cube-like
2.1
Simulated
sunlight
78% within 90 min
Electrospinning
technique
2.1
Tetracycline hydrochloride,
10 mg/L
300W Xe lamp
74% within 120
min
Hydrothermal
2.1
Rhodamine B, 100 mg/L
300W Xe lamp
Direct sunlight
Direct sunlight
Direct sunlight
Direct sunlight
18.76% within 150
min
Electrospinning
technique
Mesh shaped
Sheet-like
Coral-like
Rod-like
2.39
2.12
2.09
2.06
Methylene blue, 10 mg/L
Phenol, 10 mg/L
84.7% within 240
min
Co-precipitation
66.9% within 120
Current
Study
min
Hydrothermal
Sol-gel
Phenol, 10 mg/L
77.4% within 120
Current
Study
min
Phenol, 10 mg/L
98.95% within 120
min
Current
Study
8

S.-W. Chang Chien et al.
Journal of Physics and Chemistry of Solids 160 (2022) 110342
irradiation, the photogenerated electron on the BiFeO
3
surface would
[7] S.-M. Lam, Z.H. Jaffari, J.-C. Sin, H. Zeng, H. Lin, H. Li, A.R. Mohamed, Insight into
3
the influence of noble metal decorated on BiFeO for 2, 4-dichlorophenol and real
herbicide wastewater treatment under visible light, Colloids Surfaces A
Physicochem. Eng. Asp. 614 (2021) 126138.
migrate from filled VB to CB band and leaving an equal number of holes
in VB. These promoted CB electrons of BiFeO
(+0.53 eV) can react with
the oxygen molecules to generate H molecules (E (O
0.695 eV) [35,47]. Simultaneously, the holes situated in the VB band
3
o
O
2
/H
2 2
O
2
) =
[8] Y. Zhang, Y. Yang, Z. Dong, J. Shen, Q. Song, X. Wang, W. Mao, Y. Pu, X. Li,
2
3
Enhanced photocatalytic activity of Ba doped BiFeO by turning morphologies and
+
band gap, J. Mater. Sci. Mater. Electron. 31 (2020) 15007–15012.
o
of BiFeO
3
(+2.59 eV) would react with the OH‾ (E (•OH/OH‾) = +1.99
[
9] W. Mao, Q. Yao, Y. Fan, Y. Wang, X. Wang, Y. Pu, X. Li, Combined experimental
+
eV) to generate the •OH radicals [46]. Subsequently, these h , H
2
O
2
and
and theoretical investigation on modulation of multiferroic properties in BiFeO
ceramics induced by Dy and transition metals co-doping, J. Alloys Compd. 784
3
•
OH radicals progressively attacked the surface adsorbed phenol mol-
(
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Table 1 was assembled to compare the present work results with earlier
[
3 4
N
reported literature on organic pollutants elimination using BiFeO
photocatalysts.
3
(
[
[
[
3
decontamination, Separ. Purif. Technol. 168 (2016) 134–140.
4
. Conclusion
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Enhancement of the photoelectrochemical water splitting by perovskite BiFeO
3
via
In this report, three BiFeO
3
morphologies, including sheet-like, coral-
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like, and rod-like structures, were successfully synthesized using three
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powders fabricated by
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HT-BiFeO and CP-BiFeO techniques was attributed to the lower energy
3
sample with rod-like struc-
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3
3
[
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loaded BiFeO3 microcomposite with enhanced visible light photocatalytic
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bandgap value and higher separation of photogenerated charge carriers.
The process parameters such as catalyst dosage and initial phenol con-
centration were also found to show their effects on phenol degradation.
2
36 (2020) 116195.
[
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R. Katiyar, J. Zhang, D.J. Sellmyer, U.N. Roy, others, Magnetic and electrical
3
Additionally, the SG-BiFeO sample still sustained higher photocatalytic
activity and no Fe3 ion leaking after six recycling runs. Based on the
+
properties of single-phase multiferroic BiFeO
[17] M.H. Basiri, H. Shokrollahi, G. Isapour, Effects of La content on the magnetic,
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+
radical trapping test, h , H
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2
and •OH radicals were the vital reactive
3
oxidizing species in the phenol photocatalytic degradation reaction. This
study opened up some new avenues for synthesizing distinct magnetic
semiconductor materials, which can be structurally tuned for the
effective degradation of organic pollutants.
1
2 3
O
nanostructures with different morphologies for efficient photocatalytic
degradation of oxytetracycline, Appl. Surf. Sci. 464 (2019) 115–124.
[
19] M. Karmaoui, L. Lajaunie, D.M. Tobaldi, G. Leonardi, C. Benbayer, R. Arenal, J.
A. Labrincha, G. Neri, Modification of anatase using noble-metals (Au, Pt, Ag):
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CRediT authorship contribution statement
[
[
[
Shui-Wen Chang Chien: Formal analysis, Methodology, Resources.
Ding-Quan Ng: Writing-Editing & Review. Dileep Kumar: Writing-
Editing & Review. Sze-Mun Lam: Resources. Zeeshan Haider Jaffari:
Conceptualizations, Formal analysis, Methodology, Software, Writing-
original draft.
21] F. Gao, Y. Yuan, K.F. Wang, X.Y. Chen, F. Chen, J.-M. Liu, Z.F. Ren, Preparation
and photoabsorption characterization of BiFeO3 nanowires, Appl. Phys. Lett. 89
(2006) 102506.
22] S. Bharathkumar, M. Sakar, V.K. R, S. Balakumar, Versatility of electrospinning in
the fabrication of fibrous mat and mesh nanostructures of bismuth ferrite (BiFeO
and their magnetic and photocatalytic activities, Phys. Chem. Chem. Phys. 17
3
)
(
2015) 17745–17754.
Declaration of competing interest
[23] X. Zhang, L. Chen, H. Sun, H. Zhang, W. Si, W. Wang, L. Wang, J. Li, others,
3
Enhanced magnetic and photocatalytic properties of BiFeO nanotubes with
ultrathin wall thickness, Vacuum 184 (2021) 109867.
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.
[
24] X. Zhang, B. Wang, X. Wang, X. Xiao, Z. Dai, W. Wu, J. Zheng, F. Ren, C. Jiang,
Preparation of M@BiFeO
nanocomposites (M= Ag, Au) bowl arrays with
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organic framework on a magnetic BiFeO : a highly efficient separable
3
2
[
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