Surface decorated coral-like magnetic BiFeO3 with Au nanoparticles for effective sunlight photodegradation of 2,4-D and E. coli inactivation

Article abstract of DOI:10.1016/j.molliq.2021.115372

In this report, gold nanoparticle-decorated on the coral-like magnetic BiFeO₃ (Au-BiFeO₃) composite has been successfully fabricated by facile two-steps hydrothermal technique. Incorporation of Au nanoparticles on the BiFeO₃

Full text of DOI:10.1016/j.molliq.2021.115372

Journal of Molecular Liquids 326 (2021) 115372
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Surface decorated coral-like magnetic BiFeO3 with Au nanoparticles for
effective sunlight photodegradation of 2,4-D and E. coli inactivation
a,b
Sze-Mun Lam a,b,c, , Zeeshan Haider Jaffaric,d, Jin-Chung Sin a,b,e, Honghu Zenga,b, , Hua Lin
,
⁎
⁎
Haixiang Li a,b, Abdul Rahman Mohamed f, Ding-Quan Ngd
a
College of Environmental Science and Engineering, Guilin University of Technology, Guilin 541004, China
Guangxi Key Laboratory of Theory and Technology for Environmental Pollution Control, 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
b
c
d
Department of Environmental Engineering and Management, Chaoyang University of Technology, No. 168, Jifeng E. Rd, Wufeng District, Taichung 413310, Taiwan
Department of Petrochemical Engineering, Faculty of Engineering and Green Technology, Universiti Tunku Abdul Rahman, 31900 Kampar, Perak, Malaysia
School of Chemical Engineering, Universiti Sains Malaysia, Engineering Campus, 14300 Nibong Tebal, Pulau Pinang, Malaysia
e
f
a r t i c l e i n f o
a b s t r a c t
Article history:
In this report, gold nanoparticle-decorated on the coral-like magnetic BiFeO3 (Au-BiFeO3) composite has been
successfully fabricated by facile two-steps hydrothermal technique. Incorporation of Au nanoparticles on the
BiFeO3 enabled effective harvesting sunlight for 2,4-dichlorophenoxyacetic acid (2,4-D) photodegradation and
Escherichia coli (E. coli) inactivation on the heterojunction structures. The Au-BiFeO3 composite exhibited amelio-
rated photoactivities toward pollutant photodegradation and bacterial disinfection as compared to that of bare
BiFeO3. The findings of these studies implied that decorating of Au on the coral-like BiFeO3 can greatly delay
the recombination process of the charge carriers produced by photon absorption and thus enhanced the
photoactivity of the BiFeO3. The possible photocatalytic mechanism was also postulated and supported by
photoluminescence and electronic investigations. The radical trapping and electron spin resonance outcomes
supported the role of the oxidative active species and its interaction for the degradation of 2,4-D. The intermedi-
ates responsible for the photodegradation have also been detected. Additionally, the prepared composite can be
easily recovered through an external magnet and demonstrated good reusability within minimal release of de-
posited Au after six runs.
Received 28 November 2020
Received in revised form 7 January 2021
Accepted 9 January 2021
Available online 13 January 2021
Keywords:
BiFeO3
Au
Magnetic
Composite
Photocatalysis
Antibacterial
© 2021 Elsevier B.V. All rights reserved.
1. Introduction
their continuous wastewater treatment operations. Ergo, it was prime
importance to develop magnetic catalysts, which enabled the integra-
The ever-increasing green environment and renewable energy de-
mands have enthused considerable effort to explore new and sustain-
able technologies. Photocatalysis technology on semiconductor
catalysts using abundant solar energy have aroused tremendous atten-
tion in the recent years owing to its tantalizing potential in the environ-
mental and energy applications [1,2]. A plethora of catalysts, including
titanium dioxide (TiO2), zinc oxide (ZnO), tin dioxide (SnO2), cerium di-
oxide (CeO2) and cadmium sulfide (CdS) have been invented by the rea-
sons of their particular optical, electronic and catalytic traits [3,4].
Nonetheless, the bare catalyst materials agonized from ultralow
photoactivities and the limited photo-responding range greatly ham-
pered their photoactivities in the visible region. Moreover, the difficulty
in isolating the solid catalysts from the treated solution also rendered
tion of photocatalytic process with magnetic separation.
Among several magnetic-based materials, bismuth ferrite (BiFeO3)
is a unique and versatile because of its widespread of applications, in-
cluding sensor, photovoltaic device, supercapacitor, electrocatalysis
and photocatalysis [5]. BiFeO3 is a multiferroic material with narrow
band gap energy ~2.3 eV, demonstrating sufficient photocatalytic visible
light response, high chemical stability and good magnetic separation
ability [5,6]. It was well known that the fabrication technique and mor-
phology were very crucial to the photoactivities of catalysts. So far,
many BiFeO3 morphological structures have been revealed, such as
wire, plate, spherical-like, sheet, cube and bowl array [7–10]. The mor-
phological alteration can enhance light harvesting and provided larger
interface contacting area with electrolyte for the photocatalysis
[11,12]. However, the high surface-charge recombination rate of the
pure BiFeO3 still restricted the utilization of charge carriers and thus
compromising its photoactivity.
⁎
Corresponding author at: College of Environmental Science and Engineering, Guilin
University of Technology, Guilin 541004, China.
E-mail addresses: lamsm@utar.edu.my (S.-M. Lam), zenghonghu@glut.edu.cn
(H. Zeng).
On the other hand, some researchers have highlighted that adequate
noble metal (Ag, Pd, Pt and so on) co-catalysts decoration on the catalyst
https://doi.org/10.1016/j.molliq.2021.115372
0167-7322/© 2021 Elsevier B.V. All rights reserved.

S.-M. Lam, Z.H. Jaffari, J.-C. Sin et al.
Journal of Molecular Liquids 326 (2021) 115372
material surface were an effective strategy to enhance the photocata-
lytic performance. It has been revealed that noble metal cocatalysts
can serve as electron reservoirs to suppress the recombination of charge
carrier of the adjacent catalysts as well as provided more active catalytic
sites [12]. Moreover, such “electron reservoirs” approach is able to
strengthen the electron density of noble metals and raised the intrinsic
photoactivity of catalysts, which can make them act as better candidates
for visible light driven applications [13]. Among gold (Au) has always
been one of the broadly explored materials in variety areas of research
owing to its excellent optical and electrical properties. The Au nanopar-
ticle demonstrated good visible light absorption because of the
interband transition (5d band to 6sp band) [14].
As the wastewater from agricultural and industrial activities not
only comprised the waste pollutants, but also several microorgan-
isms which can cause environmental issues and human health, re-
searchers have always been keen to explore multifunctional
materials that can be exploited for more than one application. In
this report, the gold-decorated on the coral-like magnetic BiFeO3
(Au-BiFeO3) composites have been fabricated by simple two-
steps hydrothermal method. The as-fabricated composite with
coral-like structure displayed outstanding ability for sunlight
photodegradation of 2,4-dichlorophenoxyacetic acid (2,4-D). 2,4-
D as a typical herbicide compound which was widely used in agri-
cultural production and posing a significant adverse effect [4]. The
incorporation of Au nanoparticle on the coral-like BiFeO3 also dem-
onstrated a good antibacterial activity toward Escherichia coli
(E. coli). The active radicals during photodegradation process
were also assessed by radical trapping test and the electron spin
resonance (ESR) technique and thus the photocatalytic mechanism
was postulated.
2.3. Material characterization
The surface morphology of bare BiFeO3 and Au-BiFeO3 samples were
captured using a field-emission scanning electron microscopy (FESEM,
Quanta FEG 450), transmission electron microcopy (TEM, Tecnai
G220), high-resolution transmission electron microscopy (HRTEM,
JEM-200 CX). Crystalline structure was measured using an X-ray diffrac-
tion (XRD, Philips PW 1820) diffractometer. The elemental composition
of the samples was assessed using an energy dispersive X-ray (EDX)
instrument coupled with a FESEM. The X-ray photoelectron spectra
(XPS) was examined using a Thermo Escalab 250Xi spectrometer. The
N2 physisorption isotherm was recorded using a Brunauer Emmett
Teller analyzer (BET, ASAP 2020 M). The diffuse reflectance spectra
of bare BiFeO3 and Au-BiFeO3 samples were taken using a Perkin
Elmer L35 spectrophotometer. The photoluminescence (PL) spectra of
as-prepared materials were recorded using a Perkin Elmer S55 spectro-
photometer. The magnetic hysteresis (M-H) loops of the samples were
determined using a MicroSense 10 Mark ΙΙ analyzer. The zeta potential
of the Au-BiFeO3 composite was recorded using a Malvern Zetasizer
Nano-Z.
2.4. Photoelectrochemical measurements
Photoelectrochemical analyses on the samples were investigated
using an electrochemical workstation (Gamry Interface 1000) in a
three-electrode system containing 0.5 M sodium sulfate as an electro-
lyte solution. Samples were coated on fluorinated tin oxide (FTO)
glass, which acted as a working electrode. Ag/AgCl and platinum wire
were applied as a reference electrode and a counter electrode, respec-
tively. Transient photocurrent response was investigated with 30 s off/
on cycles with a bias potential of 0.4 V vs Ag/AgCl. The Nyquist plot
was performed in the frequency from 0.1 to 104 Hz and linear sweep
voltammetry (LSV) was executed at 50 mV/s scanning rate.
2. Experimental section
2.1. Reagents
2.5. 2,4-D photodegradation test
Iron nitrate (Fe(NO3)3·9H2O, 98%), methanol (98%), tetrachlorogold
(III)-saurehydrat (HAuCl4, 99%) and nitric acid (HNO3, 65%) were ob-
tained from ChemSoln. Bismuth nitrate (Bi (NO3)3·5H2O, 98%) and
2,4-dichlorophenoxyacetic acid (2,4-D, 97%) was acquired from Merck
and Sigma-Aldrich, respectively. Potassium hydroxide (KOH, 85%) and
urea (99.5%) were purchased from R & M Chemicals. All reagents were
utilized as received and deionized water (Milipore System, 18.2 Ω)
was utilized as solvent.
The photoactivity of the composite was investigated in a designated
photoreactor setup comprising light source and two mounted fans in an
acrylic black enclosure. Typically, 1 g/L of the catalyst was placed in a
100 mL of 20 mg/L 2,4-D solution. The test was first conducted under
dark condition for 30 min to achieve adsorption-desorption equilibrium
between the solids and solution under magnetic stirring. The solution
was then placed under sunlight with the measured light intensity of
865.7 W/m2. Throughout the test, the air with a flow rate of 2 mL/min
was constantly bubbled in the solution. At different time intervals, the
sample solution was determined using a high performance liquid chro-
matography (HPLC, Perkin Elmer Series 200) equipped with a C18 col-
umn (150 mm × 4.6 mm × 5 μm). The wavelength of UV detector was
fixed at 280 nm and a solvent mixture of acetonitrile, acetic acid and
water in the ratio of 69:1:30 (v/v) with a flow rate of 1 mL/min. The
chemical oxygen demand (COD) of samples was tested using a UV–vis
Hach DR6000 spectrophotometer. The Cl− ions left in the solution was
monitored using a Metrohm 792 Basic Ion Chromatography (IC)
coupled with a Metrosep C4 column. In order to ascertain the active spe-
cies that influence the 2,4-D degradation, 1 mM of capturing agents, in-
cluding isopropanol, benzoquinone, potassium iodide and catalase were
2.2. Synthesis of Au-BiFeO3 composites
In a typical synthesis of bare BiFeO3, 0.006 mol of Bi(NO3)3.5H2O,
0.006 mol of Fe(NO3)3.9H2O, 0.1 mol of urea and 2 mL of HNO3 were dis-
solved in deionzed water (20 mL) under stirring until completely dis-
solved. After that, the mixed solution was added in the KOH (10 M).
The mixture was then shifted into a Teflon-lined autoclave reactor and
heated at 125 °C for 15 h. After cooling down to room temperature,
the obtained product was centrifuged, washed with ethanol and deion-
ized water and dried in an oven at 80 °C.
The Au-BiFeO3 composites were prepared using a hydrothermal
technique in the presence of HAuCl4. In this experiment, a fixed amount
of HAuCl4 in methanol (40 mL) was sonicated in ultrasonication bath
(40 kHz/100 W) for 20 min. Subsequently, 1 g of BiFeO3 was inserted
in the solution and further sonicated for 30 min. After that, the solution
was moved into a Teflon-lined autoclave reactor and heated at 100 °C
for 4 h. The autoclave was then allowed to cool down naturally. The or-
ange precipitation of Au-BiFeO3 was collected by filtration, washed and
oven dried at 80 °C for overnight. A schematic diagram of Au-BiFeO3
composites synthesis using a two-steps hydrothermal technique is re-
vealed in Scheme 1.
adopted to scavenge hydroxyl (•OH) radical, superanion oxide (•O2−
)
radical, hole (h+) and hydrogen peroxide (H2O2), respectively. Addi-
tionally, electron spin resonance (ESR, Bruker EMX plus) with 5,5-di-
methyl-1-pyridine N-oxide (DMPO) as a quencher agent was used to
further confirm the •OH radical.
Recycling test of the Au-BiFeO3 composite was performed over six
cyclic runs. The Au-BiFeO3 composite in each run was centrifuged,
cleaned with deionized water and dried at 120 °C for 4 h. Afterwards,
the dried sample was again inserted in fresh 2,4-D solution for the
next run. The dissolved Fe3+ and Auo in the treated solution were
2

S.-M. Lam, Z.H. Jaffari, J.-C. Sin et al.
Journal of Molecular Liquids 326 (2021) 115372
Scheme 1. Schematic diagram of the synthesis of Au-BiFeO3 composites using a two-steps hydrothermal process.
analyzed via an inductively coupled plasma mass spectroscopy (ICP-MS,
Perkin Elmer AA-6650). All tests were at least duplicated and the aver-
age values were presented with standard deviation error bars.
3. Results and discussion
3.1. Physicochemical characteristics of Au-BiFeO3
The morphology and the detailed structures of the as-fabricated
samples were analyzed by FESEM, TEM and HRTEM as presented in
Fig. 1. It can be seen in Fig. 1a that the bare BiFeO3 powders demon-
strated coral-like morphologies which were interconnected as net-
works with particle sizes of approximately 110–140 nm. Apparently,
composite in Fig. 1b–e showed similar morphological features as those
observed in the bare BiFeO3. Nonetheless, the difference between
these samples was that a small amount of Au nanoparticles was depos-
ited on the BiFeO3 surfaces. The distribution of Au nanoparticles on
coral-like BiFeO3 surfaces observed from TEM image (Fig. 1f) further
confirmed by the realization of Au-BiFeO3 heterostructures formation.
The HRTEM image in Fig. 1g displayed the lattice fringes with 0.27 nm
2.6. Antibacterial assay
Minimum inhibitory concentration (MIC) was defined as the mini-
mum quantity of solid samples which capable to completely inhibit
the visible growth of bacteria. The MIC of Escherichia coli (E. coli) bacte-
ria (ATCC 25922) in the presence of bare BiFeO3 and Au-BiFeO3 samples
was assessed in the current study [15]. All the glassware was sterilized
by autoclaving at 120 °C for 20 min, and the antibacterial activity was
carried out under sterile conditions. The Au-BiFeO3 composite was seri-
ally diluted to achieve a 96-well plate. These wells were then inoculated
with bacterial concentration of 5 × 105 CFU/mL and the plates were in-
cubated at 37 °C for 24 h before examining the bacterial concentrations.
Fig. 1. FESEM images of (a) bare BiFeO3, Au-BiFeO3 decorated at different Au loadings of (b) 0.25 wt%, (c) 0.5 wt%, (d) 1 wt%, (e) 2 wt%, (f) TEM image, (g) HRTEM image and (f) SAED
pattern of 1 wt% Au-BiFeO3.
3

S.-M. Lam, Z.H. Jaffari, J.-C. Sin et al.
Journal of Molecular Liquids 326 (2021) 115372
and 0.24 nm can be assigned to d spacings of (110) of rhombohedral pe-
rovskite BiFeO3 and (011) planes of Au, respectively [16,17]. Many dif-
fraction rings in the SAED pattern in Fig. 1h also revealed the
polycrystalline nature of Au-BiFeO3 composite.
Additionally, it was well nothing that all signals of Bi 4p, Fe 2p and O
1 s in the composite were slightly shifted to higher energy level as com-
pared to that of bare BiFeO3. These small shift in the binding energy
demonstrated some degree of electron transfer between the Au and
BiFeO3 and intense interaction within the composites [24].
The crystalline structures and elemental composition of the as-
fabricated catalysts have been studied by XRD and EDX, respectively.
As shown in Fig. 2a, bare BiFeO3 displayed well-defined peaks occurring
at 2θ = 22.6o, 32.2o, 32.4o, 39.9o, 45.9o, 51.8o, 52.4o, 56.6o, 58.2o, 67.2o,
67.8o, 71.1o, 71.9o 76.5o and 76.9o, which can be respectively indexed
to the (012), (104), (110), (202), (024), (116), (122), (018), (214),
(208), (220), (131), (036), (128) and (134) diffraction planes of rhom-
bohedral phase with space group R3c (JCPDS NO. 86–1518) [8,9]. After
incorporation of Au into BiFeO3, three small peaks at 38.4o (111),
44.3o (200) and 64.5o (220) corresponded to the face-centered cubic
Au (JCPDS NO. 65–2870) appeared distinctly, deducing the effective
combination of Au with BiFeO3 [18]. No other new peaks can be seen
in all the obtained samples, suggesting the high purity of the Au-
BiFeO3 composites. Additionally, EDX in Fig. 2b presents also gold, bis-
muth, iron and oxygen as the major peaks. The EDX elemental mapping
is shown in Fig. 2c–f.
To examine the surface area and pore size distribution, N2
adsorption-desorption isotherms of the bare BiFeO3 and composite
were carried out. As depicted in Fig. 4a, both the samples displayed typ-
ical type IV adsorption isotherms associated with the mesoporous struc-
tures. The BET surface area of Au-BiFeO3 was 19.6 m2/g higher than that
of 11.2 m2/g of bare BiFeO3. The total pore volume and pore size distri-
bution for both samples was determined from the desorption isotherms
from BJH plot are shown in Fig. 4b. The total pore volume and mean pore
size diameter of bare BiFeO3 was estimated to be 0.0028 cm3/g.nm and
21.6 nm, respectively. After decoration with Au nanoparticles, the total
pore volume increased to 0.0043 cm3/g.nm and mean pore size diame-
ter decreased to 17.1 nm. It should be noted that the high surface area of
Au-BiFeO3 afforded more active sites for pollutant molecules and ex-
pected to increase the photocatalytic oxidation efficiency.
The chemical states of the samples have also been evaluated by XPS.
The high resolution XPS spectra of Bi 4f, Fe 2p, O 1 s, and Au 4f for bare
BiFeO3 and Au-BiFeO3 are exhibited in Fig. 3. In the high resolution XPS
spectrum of Au 4f peaks (Fig. 3a), the doublet peaks for Au 4f7/2 and Au
4f5/2 were positioned at 83.3 and 87.3 eV, respectively, indicating that
the existence of Au cocatalyst present in the composite [19,20]. Two
XPS peaks appeared at 158.4 and 163.7 eV associated with Bi 4f7/2 and
Bi 4f5/2, respectively, validating that the chemical state of Bi was triva-
lent oxidation state (Fig. 3b) [21]. The typical high resolution Fe 2p spec-
trum in Fig. 3c illustrated doublet peaks at 710.8 and 724.2 eV
corresponding to the Fe 2p3/2 and Fe 2p1/2, respectively and a satellite
peak observed at 718.8 eV can be viewed as the Fe element in the
form of Fe3+ [22]. The O 1 s spectrum in Fig. 3d can be resolved into
two peaks located at 529.6 and 530.9 eV, corresponding to the
metal‑oxygen bond and chemisorbed oxygen, respectively [23].
3.2. Optical and electrochemical properties
The optical absorption property of Au-BiFeO3 composite was studied
using UV–vis DRS analysis (Fig. 5a). Compared with bare BiFeO3, the Au-
BiFeO3 composites displayed enhanced visible light absorbance around
the 500–800 nm. Moreover, this absorption characteristic improved sig-
nificantly with Au loadings in the heterojunction composites. Fig. 5b de-
picts the Kubelka-Munk (K-M) plots according to the obtained UV–vis
DRS data. The optical band gap (Eg) of bare BiFeO3 and Au-BiFeO3 com-
posite prepared at 1.0 wt% were measured to be 2.60 and 2.35 eV, re-
spectively. The declined Eg value suggested that the Au-BiFeO3 can
afford strong ability to harvest the full solar spectrum for the photo-
oxidation process. The conduction band (CB) and valance band (VB) po-
tential of BiFeO3 can be determined as follows [25,26].
Fig. 2. XRD patterns of (a) bare BiFeO3, and Au-BiFeO3 composites prepared at different Au loadings, (b) EDX spectrum of Au-BiFeO3 and (c-f) EDX elemental mapping validating Au-
BiFeO3.
4

S.-M. Lam, Z.H. Jaffari, J.-C. Sin et al.
Journal of Molecular Liquids 326 (2021) 115372
Fig. 3. High resolution XPS spectra of Au-BiFeO3 composite (a) Au 4f, (b) Bi 4f, (c) Fe 2p and (d) O 1 s.
qffiffiffiffiffiffiffiffiffiffiffiffi
EVB ¼ X–Ee þ 0:5Eg
ECB ¼ EVB–Eg
ð1Þ
ð2Þ
ðmþnþlÞ
XðAmBnClÞ ¼
XmA XnBXlC
ð3Þ
As for BiFeO3, the absolute electronegativities of Bi, Fe, and O were
4.69 eV, 4.06 eV and 7.54 eV, respectively. Hence, from the aforemen-
tioned formula, the computed X(BiFeO3) was 6.06 eV. This value was
fitted in the Eqs. (1) and (2), the calculated EVB and ECB of BiFeO3 was
2.86 and 0.26 eV, respectively.
where, ECB is the conduction band potential, EVB is the valance band
potential, Ee is the free electron energy on hydrogen scale (4.5 eV), band
gap energy (Eg) of BiFeO3 (eV) and X is the Pearson absolute electroneg-
ativity. This X value can be determined as show in Eq. (3) [27,28].
Fig. 4. (a) N2 adsorption-desorption isotherm and (b) variations in total pore volume and pore size diameter of the bare BiFeO3 and Au-BiFeO3.
5

S.-M. Lam, Z.H. Jaffari, J.-C. Sin et al.
Journal of Molecular Liquids 326 (2021) 115372
Fig. 5. (a) UV–Vis DRS spectra, (b) corresponding K-M plots of bare BiFeO3 and Au-BiFeO3 composites, (c) PL spectra, (d) transient photocurrent responses, (e) EIS Nyquist plots and (f) LSV
curves of bare BiFeO3 and Au-BiFeO3 composites with various Au loadings.
The PL analysis is a method used to assess the separation and trans-
fer dynamics of photogenerated electrons and holes. Fig. 5c presents the
PL spectra of Au-BiFeO3 composite materials together with the bare
BiFeO3. The intense PL intensity of bare BiFeO3 indicated the rapid
recombination of excitons. In contrast, PL spectrum of 1.0 wt% Au-
BiFeO3 composite was entirely quenched because of its low recombina-
tion rate of the charge carriers, which was a crucial factor in the photo-
catalytic reactions. In order to further examine the recombination rate
of excitons, electrochemical studies, including transient photocurrent
response, EIS Nyquist and LSV measurements of bare BiFeO3 and Au-
BiFeO3 composite was performed. The transient photocurrent response
was applied to investigate the mobility of photogenerated charge carrier
under light irradiation. Fig. 5d depicts the transient photocurrent
responses with light on and off cycles using the bare BiFeO3 and Au-
BiFeO3 composites. A substantial enhancement in the transient photo-
current response of 1 wt% Au-BiFeO3 composite inferred that greater
photogenerated electron-hole separation efficiency and higher mobility
in comparison with the bare BiFeO3 [29,30]. The EIS Nyquist plot was
executed to measure the interfacial charge transfer resistance in the
prepared samples. Fig. 5e illustrates that 1 wt% Au-BiFeO3 composite
demonstrated the lowest arc radius, signifying the excellent charge sep-
aration and lowest charge transfer resistance at the interface among the
sample. Additionally, the LSV analysis was also conducted to further
confirm the photoelectrochemical performance of the catalyst mate-
rials. As displayed in Fig. 5f, the highest current density and lowest
overpotential of 1 wt% Au-BiFeO3 composite obviously indicated that
it can more appropriately improve the separation and transfer of charge
carriers than those of the remaining catalysts. These ameliorated optical
absorption and improved electrochemical features of the Au-BiFeO3
composite materials deduced that they can be adopted as an effective
sunlight-activated catalyst.
can be seen that the 2,4-D concentration hardly changed under both the
dark and photolysis conditions. After the introduction of bare BiFeO3,
about 50% 2,4-D was degraded within 120 min irradiation. For the Au-
BiFeO3 composites, 1.0 wt% Au-BiFeO3 composite exhibited the opti-
mum photoactivity with the degradation efficiency as high as 98.5%.
This finding conveyed the critical role of Au cocatalyst in enhancing
the photoactivity of coral-like BiFeO3. When comparing the different
Au loadings in the composites, the photoactivities of the Au-BiFeO3
composites were first rose up and then restrained with the increasing
loadings of Au. The reasons could be attributed to the Au nanoparticles
at the Au-BiFeO3 interface can prevent the photoexcited electrons to re-
turn the BiFeO3 surface and prolonged the charge carrier recombination
time [31]. On the contrary, the declination photoactivity could be con-
sidered to the photons scattering effect by Au and the blocked the inci-
dent light absorption by the BiFeO3 [32,33]. Thus, the loading amount of
Au was kept a desirable balance between the BiFeO3 and Au cocatalyst.
In order to quantitatively check the catalyst activity, the
photodegradation kinetics of 2,4-D was also studied. Considering the
2,4-D low concentration (20 mg/L) used in this study, the pseudo-
first-order kinetic model was chosen. The kinetic curves of 2,4-D
photodegradation under sunlight irradiation are illustrated in Fig. 6b.
Besides, the histogram kinetic constant in the inset of Fig. 6b manifested
that the 1.0 wt% Au-BiFeO3 sample (0.0283 min−1) was higher those of
bare BiFeO3 and other Au-BiFeO3 composites.
3.3.1. Influence of solution pH
Photodegradation rate was intensely influenced by the solution pH
as the pH variation can affect 2,4-D molecules adsorption on the Au-
BiFeO3 catalyst surface. To scrutinize the influence of solution pH on
the photocatalytic efficiency, four different pH values at 3, 5, 7 and 9
was prepared and other experiment conditions at initial 2,4-D concen-
tration of 20 mg/L and catalyst loading of 1 g/L were fixed and the find-
ings are presented in Fig. 7a. As can be seen, the pH variations were
effective in the 2,4-D photodegradation efficiency wherein with the so-
lution pH reduction the photodegradation efficiency increased and pH 2
was the highest photodegradation efficiency. Semiconductor metal
3.3. Sunlight photodegradation of 2,4-D using Au-BiFeO3 composites
Fig. 6a shows the photoactivities of bare BiFeO3 and Au-BiFeO3 com-
posites with various Au loadings for the sunlight degradation of 2,4-D. It
6

S.-M. Lam, Z.H. Jaffari, J.-C. Sin et al.
Journal of Molecular Liquids 326 (2021) 115372
Fig. 6. (a) Photodegradation of 2,4-D and (b) pseudo-first-order reaction kinetics curves over Au-BiFeO3 composites under sunlight irradiation. Inset of (b) the histogram of kinetic rate
constants for 2,4-D photodegradation.
oxide typically displayed an amphoteric behavior, which it can affect the
surface zero-point charge (zpc) characteristic of the catalyst when the
reaction occurred on the semiconductor surface. The zpc of Au-BiFeO3
composite was extrapolated to be pH 2 (as recorded in Fig. 7b).
Hence, the surface of Au-BiFeO3 catalyst was positively charged at
pH < pHzpc, while negatively charged at pH > pHzpc. Additionally,
the electric charge property of pollutant was also observed to serve as
a pivotal role on the photodegradation process. The 2,4-D has a pKa of
2.98. It was preferably presented in the molecular form rather than
ionic in the alkaline condition [4]. At low pH, a strong adsorption of
the 2,4-D on the Au-BiFeO3 particles was found due to the electrostatic
reactions of the negatively charged catalyst with the pollutant. While in
the alkaline condition, the 2,4-D molecules were in ionic form, their ad-
sorption were expected to be affected by the negative surface charge of
Au-BiFeO3. Therefore, owing to Coulombic repulsion, the 2,4-D were
hardly adsorbed.
level and thereafter, the photodegradation rate happened at a lower
rate. At a low catalyst loading, sunlight transfer at a higher rate to the so-
lution. This phenomenon caused the 2,4-D photodegradation increased
since the increment of the catalyst loadings promoted the contact and
active sites between the catalysts and pollutants. However, when the
catalyst was overloaded, the increased solution turbidity and limited
light penetration into the suspension resulted in the decrease in the
photoactivity. Therefore, 1 g/L was selected as the optimal loading.
3.4. Intermediate identification and possible photodegradation pathway
The photodegradation of 2,4-D over 1.0 wt% Au-BiFeO3 composite
was studied by evaluating the mixture solution at different treatment
periods through HPLC technique. Four aromatic intermediates, namely
2,4-dichlorophenol (P1) (Retention time (RT) = 2.75 min), 4-
chlorophenol (P2) (RT = 2.32 min), chlorobenzoquinone (P3) (RT =
2.10 min), and phenol (P4) (RT) = 1.92 min) were identified (Fig. 8a)
after compared with the standard chemicals. These intermediates
were started to display in the middle of the reaction and progressively
disappeared when the irradiation time prolonged. At the end of the
test, none of the intermediate was found in the HPLC profiles. The de-
gree of mineralization of 2,4-D was monitored by COD analysis, while
the ion chromatography (IC) assay was employed to analyze the con-
centration of free chloride (Cl−) ion in the solution (Fig. 8b). The
3.3.2. Influence of catalyst loading
Catalyst loading was one of the crucial factors than can govern the
photodegradation rate of 2,4-D. Fig. 7b illustrates the influence of cata-
lyst loading on the 2,4-D photodegradation using Au-BiFeO3 examined
at the range of 0.5–2 g/L at the initial 2,4-D concentration of 20 mg/L
and optimal pH 3. As can be seen, with addition of the catalyst loadings
to the initial photodegradation process, it can increase up to a specific
Fig. 7. (a) Influence of pH on the photodegradation of 2,4-D by Au-BiFeO3, (b) zeta potential graph and (c) influence of catalyst loading on the photodegradation of 2,4-D by Au-BiFeO3.
7

S.-M. Lam, Z.H. Jaffari, J.-C. Sin et al.
Journal of Molecular Liquids 326 (2021) 115372
Fig. 8. (a) HPLC profiles for 2,4-D solution treated by the Au-BiFeO3 composite at different irradiation times and (b) COD removal and chloride ion concentration of 2,4-D over Au-BiFeO3
composite.
obtained data revealed that the treatment time needed for the mineral-
ization process was relatively higher than the photodegradation of
mother compound (120 min for 20 mg/L 2,4-D). This phenomenon ex-
plicated that 2,4-D produced numerous by-products prior to the com-
pletion of mineralization. Meanwhile, the concentration of free Cl− ion
formed during the photodegradation of 2,4-D was gradually increased
along the treatment time and 5.4 mg/L Cl− ion concentration was de-
tected after the 120 min irradiation.
Based on the outcomes of the HPLC and corresponding chemical
components produced upon the oxidation reaction (Fig. 8a and b), a
plausible photodegradation pathway of 2,4-D over the Au-BiFeO3 cata-
lyst surface is postulated in Scheme 2. The photodegradation of 2,4-D
was initiated by the rupturing of its side chain via the breakage of
C\\O bond to yield 2,4-dichlorophenol (2,4- DCP) (P1). These 2,4-DCP
molecules can be dechlorinated to form 4-chlorophenol (4-CP) (P2) or
oxidized to produce chlorohydroquinone or chlorobenzoquinone
(CBQ) (P3). Both the 4-CP and CBQ molecules can then be converted
to the phenol (P4) and BQ or hydroquinone, respectively via the
dichlorination process. Thereafter, low molecular weight aliphatic car-
boxylic acids were produced from the cleavage of benzene rings of BQ
or hydroquinone over the catalyst surface. Ultimately, these carboxylic
acids were mineralized to carbon dioxide, water and Cl− ions as final
products.
3.5. Enhancement behind 2,4-D photodegradation mechanism
3.5.1. Detection of ROS
The radical oxidative species (ROS) in the photodegradation process
was assessed via various capturing agents. As can be observed in Fig. 9a,
the photodegradation efficiency of 2,4-D did not change significantly
when BQ was introduced, suggesting that •O−2 radical contributed to a
minor extent. Nonetheless, after adding IPA, KI and catalase, the
photoactivity was dramatically decreased 2,4-D photodegradation to
27.6%, 34.9% and 29.2%, respectively, implying that •OH, h+ and H2O2
were the dominated ROS. This hypothesis can be further validated by
electron spin resonance (ESR) analysis. Fig. 9b indicates that a strong
typical signal of the DMPO-•OH was witnessed in the presence of Au-
BiFeO3 composite after 5 min light irradiation and the signal intensity
improved when extended the irradiation time. Nevertheless, no signal
can be detected in dark condition. This data was corroborated with
the outcomes of trapping experiments. These data confirmed that the
Scheme 2. Schematic representation of 2,4-D degradation pathway upon the photocatalytic reaction.
8

S.-M. Lam, Z.H. Jaffari, J.-C. Sin et al.
Journal of Molecular Liquids 326 (2021) 115372
Fig. 9. (a) Radical trapping assessment of ROS during the 2,4-D photodegradation over Au-BiFeO3 composite and (b) DMPO spin-trapping ESR spectra of Au-BiFeO3 composite for DMPO-
•OH.
Au-BiFeO3 composite can boost the yield of •OH radicals and conse-
quently participated in the 2,4-D photodegradation.
than that of BiFeO3, the SB formed at the interface led to an upward
band bending. The height of SB (ɸSB) can be determined using the fol-
lowing formula [37]:
3.5.2. Reasonable photodegradation mechanism
ɸSB ¼ ɸm–χs
ð4Þ
According to the findings of aforementioned analyses, a reasonable
2,4-D photodegradation mechanism of Au-BiFeO3 composite was put
forward as depicted in Scheme 3. Under exposure of sunlight irradia-
tion, the electron (e−) of BiFeO3 were activated to its CB and subse-
quently left the primordial hole (h+) in the VB. For the bare BiFeO3,
the photogenerated charge carrier recombination rate was fast and
only limited numbers of them could reach the catalyst surface, which
resulted in low photoactivity. In contrast, decoration of Au on the
coral-like BiFeO3 surface can induce the energy band bending at the in-
terface and a Schottky barrier (SB), which enabled the e− capture at the
heterojunction. This free e− can transport between BiFeO3 and Au be-
cause of the differences in their work functions. The Au had a higher
work function (ɸm) of 5.1 eV [34,35] than that of the BiFeO3 (ɸs) of
4.7 eV [36], which can facilitate the e− migration from the CB of
BiFeO3 to the Au cocatalyst. Moreover, owing to the higher ɸm of Au
where ɸSB is the height of Schottky barrier, and χs (3.3 eV) is the elec-
tron affinity of BiFeO3 []. The obtained ɸSB value was 1.8 eV and the en-
ergy band bending can decelerate the e− transfer from BiFeO3 to Au at
equilibrium (Scheme 3b). Moreover, more photogenerated h+ can be
accumulated at the interface and this condition improved the oxidizing
ability and the generation of •OH radicals. Consequently, the Au cocata-
lyst can act as an effective cocatalyst in extending the lifetime of
photogenerated charge carrier and hence boosted the photocatalytic
performance of composite. For the 2,4-D photodegradation, on one
hand, the photoexcited e− in the Au-BiFeO3 could be captured by O2
molecules and H+ ions to form H2O2 because of the standard redox po-
tential (Eo(O2/H2O2) = +0.695 V vs. NHE) lower than CB edge potential
(+0.29 eV) [38,39]. Meanwhile, the generated •OH by oxidizing OH− or
H2O was due to the Eo(•OH/OH−)(+1.99 eV vs. NHE) and Eo(•OH/H2O)
Scheme 3. Schematic illustration of energy band mechanism of Au-BiFeO3 composites in contact before and after equilibrium under sunlight exposure.
9

S.-M. Lam, Z.H. Jaffari, J.-C. Sin et al.
Journal of Molecular Liquids 326 (2021) 115372
(+2.68 eV vs NHE) lower than the VB edge potential (+2.79 eV) [40].
The obtained H2O2, h+, •OH radicals were then vigorously attack on
the 2,4-D molecules and eventually converted into the mineralized
products.
3.6. Magnetic property and cyclic degradation
Based on the earlier reports [5,6], BiFeO3 is ferromagnetic at room
temperature and the Au-BiFeO3 composite might also displayed the fer-
romagnetic feature. Fig. 10 depicts the magnetic hysteresis (M-H) loops
of the bare BiFeO3 and Au-BiFeO3 composite recorded at room temper-
ature. The M-H loops of bare BiFeO3 and Au-BiFeO3 presented small hys-
teresis loops, which demonstrated that the samples have ferromagnetic
features with the saturation magnetization (Ms) value of 4.80 emu/g
and 3.95 emu/g, respectively. The low Ms value of Au-BiFeO3 was due
to the presence of non-magnetic Au nanoparticles decorated on the
coral-like surfaces. Nevertheless, the Au-BiFeO3 composite still pos-
sessed a higher Ms value than those of other reported magnetic-based
composites [41–43]. The magnetic characteristic of the Au-BiFeO3 com-
posite can have practical applications. The magnetic separability of Au-
BiFeO3 was evaluated by applying a magnet bar closed to a flask-bearing
the solid materials. The Au-BiFeO3 was attracted to the magnet side
within 15 min (inset of Fig. 10a).
Fig. 11. Inactivation efficiency of the Au-BiFeO3 composite against E. coli at different times.
around 89.6% after sixth cyclic run. In this study, the Fe3+ and Auo
ions leakage throughout the cyclic degradation tests were also moni-
tored using ICP-MS technique. No leakage of Fe3+ was found in the
tests, while low concentrations of Auo ranged from 0.035 to
The stability and reproducibility of the catalyst were studied over six
cyclic runs. As presented in Fig. 10b, the photocatalytic performance of
bare BiFeO3 was significantly decreased from 50.0% to 32.6%, while the
Au-BiFeO3 composite was maintained well with slight declination
Fig. 10. (a) Magnetization curves of bare BiFeO3 and Au-BiFeO3 composite. (b) photodegradation of 2,4-D and Au leakage using bare BiFeO3 and Au-BiFeO3 composite over six cyclic runs,
(c) XRD patterns of the fresh and used Au-BiFeO3 composites and (d) transient photocurrent response of used Au-BiFeO3 composite compared with the fresh one. Inset of (a) the image of
magnetic separation for 15 min.
10

S.-M. Lam, Z.H. Jaffari, J.-C. Sin et al.
Journal of Molecular Liquids 326 (2021) 115372
Table 1
Comparison of photocatalytic and antibacterial activities of various Au-decorated composite catalysts.
Composite
catalyst
Magnetic Light source
Pollutant and
concentration
Removal efficiency
Microbe
Antibacterial
efficiency
Ref.
Au-PdO/GO
×
100 W Hg lamp,
λ = 254 nm
Solar lamp
Tetrodotoxin
95% within 60 min under
0.15 mmol/L H2O2
~88% within 60 min
E. coli, S. aureus,
E. coli
99.5%
[49]
[50]
Au@α-Fe2O3
√
Rhodamine B,
10 mg/L
Methylene blue
Ciprofloxacin,
20 mg/L
OD600: 0.4490
⁎
Au@Fe2O3
Au-TiO2
√
×
100 W bulb
300 W Xe lamp,
λ = 420 nm
Solar light,
1250 × 100 lx
2 × 8 W black UV
lamps
94% within 50 min
84.1% within 240 min
E. coli, B. subtilis
E. coli
ZOI : 20, 18 mm
[51]
[52]
88.2% within
720 min
⁎
Ag-Au/ZnO
Au-ZnO
×
×
√
Acid red,
100% within 90 min
25% within 160 min
98.5% within 120 min
B. subtilis, E. coli, P. aeruginosa, S.
aureus, S. pyogenes
E. coli, S. aures, E. ashbyii
ZOI : 11, 12, 9, 12, [53]
15 mm
5 × 10−4
M
⁎
Methylene blue,
2 × 10−5
2,4-D, 20 mg/L
ZOI : 4.3, 7.0,
[54]
M
9.75 mm
56.0% within
300 min
Au-BiFeO3
Sunlight,
E. coli
Current
study
865.7 W/m2
⁎
ZOI = Zone of inhibition.
0.024 mg/L was identified. However, these Au concentrations were still
lower than the reported literatures [44–46]. Both fresh Au-BiFeO3 and
the composite after sixth cyclic run were assessed by XRD and transient
photocurrent response. As indicated in Fig. 10c, no obvious change in
the XRD diffraction peaks was observed after the cyclic degradation, im-
plying that the phase composition and structure of the sample has not
diminished. Besides, the transient photocurrent response on the used
Au-BiFeO3 composite was also monitored as illustrated in Fig. 10d. It
can be observed that both the photocurrent density plots were highly
similar over the six cyclic runs. This work indicated that the Au-BiFeO3
composite had a good stability as well as highly feasible for continuous
wastewater treatment.
techniques to check on the morphological, structural, optical, electronic
and magnetic traits of the catalyst. Compared to bare BFeO3, the Au-
BiFeO3 composite exhibited
a pronounced sunlight-responsive
photoactivities for the 2,4-D degradation and E. coli inactivation. The Au
nanoparticles dispersed on the coral-like BifeO3 structure can afford
more photocatalytic active sites, facilitated the h+ accumulation at the
interface and further enhanced oxidizing ability as well as the formation
of •OH radicals. In-depth PL and electronic investigations also concluded
the remarkable photocatalytic performance. The possible photocatalytic
mechanism was elucidated and supported with the data according to
the radical trapping and ESR experiments. Additionally, intermediates
identification results evidently indicated the 2,4-D decomposition effect.
The Au-BiFeO3 can be easily recovered via an external magnet and dem-
onstrated great reusability with a minimal leakage of Au after six runs.
This work offers the viable and tantalizing prospective of utilizing the
noble metal-based magnetic catalyst in the multifunctional photocata-
lytic areas.
3.7. Bactericidal effect
Fig. 11 shows the bactericidal effect of the Au-BiFeO3 along with bare
BiFeO3 and control (absent of composite). Au-BiFeO3 has a better bacte-
ricidal effect than that of bare BiFeO3. On the contrary, no significant in-
activation of E. coli cell was happened on the control experiment. It can
be observed that the Au-BiFeO3 possessed a strong bactericidal effect
which should stem from the effective charge carrier separation and
transportation processes in the Au-BiFeO3 during the sunlight exposure.
The interaction between the Au-BiFeO3 heterostructure and bacteria
gave rose to the membrane integrity leakages, like cytoplasm, potas-
sium ion, DNA and ribosome molecules owing to the ROS attack on
the bacterial species surface, which led to the enhancement of bacterial
inactivation [47,48]. In the actual wastewater treatment, microorgan-
ism contamination can greatly restrain the photocatalysis efficiency.
Such a high antibacterial capability of the Au-BiFeO3 composite can
avoid it from being polluted by the microorganisms, which were highly
conducive for the viability of this catalyst in the actual wastewater puri-
fication application.
Declaration of Competing Interest
None.
Acknowledgments
This research was supported by Ministry of Higher Education
(MoHE) through Fundamental Research Grant Scheme (FRGS/1/2019/
TK02/UTAR/02/4), Universiti Tunku Abdul Rahman (UTARRF/2020-C1/
S04 and UTARRF/2020-C2/L02), Research Funds of The Guangxi Key
Laboratory of Theory and Technology for Environmental Pollution Con-
trol, China (1801K012 and 1801K013), ASEAN Young Talented Scientist
Program of Guangxi and special funding for Guangxi “Bagui Scholar”
construction project.
The photocatalytic capacity comparison with the present Au-BiFeO3
and different kinds of Au-based composites cited from references are pro-
vided in Table 1. It was difficult to quantitively compare the photoactivities
with previous reported systems because of the various experimental con-
ditions for photocatalytic process. For instance, the operational parameters
of catalyst loading, degraded contaminants, light source, duration of treat-
ment and etc. greatly affected the photodegradation. Nonetheless, it can
still be observed that the magnetic-based catalyst prepared in this work
had excellent photocatalytic and antibacterial activities.
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