Novel Magnetic Retrievable Visible-Light-Driven Ternary Fe3O4?NiFe2O4/Phosphorus-Doped g-C3N4 Nanocomposite Photocatalyst with Significantly Enhanced Activity through a Double-Z-Scheme System

Article abstract of DOI:10.1021/acs.inorgchem.9b02996

Nickel ferrite (NiFe₂O₄) and magnetite (Fe₃O₄) are established earth-abundant materials and get tremendous attention because of magnetic and high photocatalytic activity. First we fabricated novel Fe₃O₄?20 wt % NiFe₂O₄/phosphorus-doped g-C₃N₄ (M?NFOPCN) using a convenient simple coprecipitation method followed by calcination at 400 °C. Then M?NFOPCN composites were prepared by the in situ growth of Fe₃O₄ nanorods and cubes on the surfaces of a porous agglomerated NFOPCN nanostructure, varying the weight percentage of Fe₃O₄. A series of characterizations like X-ray diffraction, UV-vis diffuse-reflectance spectroscopy, photoluminescence, Fourier transform infrared, thermogravimetric analysis-differential thermal analysis, vibrating-sample magnetometry, scanning electron microscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy techniques confirm that changing weight percentage of M can constructively control the textural characteristics, internal strain, size of the crystals, and other aspects meant for photocatalytic activity. When M was coupled with NFOPCN, magnetic loss was lowered and also an appreciable saturation magnetization (M_s) was obtained. 40 wt % M?NFOPCN showed admirable photostability and was capable of evolving 924 μmol h^-1 H₂ when irradiated under visible light. The percentage of degradation for ciprofloxacin (CIP) by this ternary nanocomposite was almost 2-fold greater than those of the pure M and NFOPCN photocatalysts. A plausible photocatalytic mechanism for the degradation of CIP antibiotic was established. Hence, this study presents a reusable, low-cost, noble-metal-free, environmentally friendly, fast, and highly efficient 40 wt % M?NFOPCN photocatalyst, achieving 90% degradation of CIP antibiotic under visible light. The double-Z scheme triggers charge separation and migration, enhances visible-light harvesting, and helps in internal electric-field creation, thus headed toward dramatic augmentation of the photocatalytic activity.

Full text of DOI:10.1021/acs.inorgchem.9b02996

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Article
Novel Magnetic Retrievable Visible-Light-Driven Ternary
Fe3O4@NiFe2O4/Phosphorus-Doped g‑C3N4 Nanocomposite
Photocatalyst with Significantly Enhanced Activity through a
Double-Z-Scheme System
Priti Mishra, Arjun Behera, Debasmita Kandi, Satyajit Ratha, and Kulamani Parida*
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ABSTRACT: Nickel ferrite (NiFe2O4) and magnetite (Fe3O4) are
established earth-abundant materials and get tremendous attention
because of magnetic and high photocatalytic activity. First we
fabricated novel Fe3O4@20 wt % NiFe2O4/phosphorus-doped g-
C3N4 (M@NFOPCN) using a convenient simple coprecipitation
method followed by calcination at 400 °C. Then M@NFOPCN
composites were prepared by the in situ growth of Fe3O4 nanorods
and cubes on the surfaces of a porous agglomerated NFOPCN
nanostructure, varying the weight percentage of Fe3O4. A series of
characterizations like X-ray diffraction, UV−vis diffuse-reflectance
spectroscopy, photoluminescence, Fourier transform infrared,
thermogravimetric analysis−differential thermal analysis, vibrat-
ing-sample magnetometry, scanning electron microscopy, trans-
mission electron microscopy, and X-ray photoelectron spectroscopy techniques confirm that changing weight percentage of M can
constructively control the textural characteristics, internal strain, size of the crystals, and other aspects meant for photocatalytic
activity. When M was coupled with NFOPCN, magnetic loss was lowered and also an appreciable saturation magnetization (Ms) was
obtained. 40 wt % M@NFOPCN showed admirable photostability and was capable of evolving 924 μmol h−1 H2 when irradiated
under visible light. The percentage of degradation for ciprofloxacin (CIP) by this ternary nanocomposite was almost 2-fold greater
than those of the pure M and NFOPCN photocatalysts. A plausible photocatalytic mechanism for the degradation of CIP antibiotic
was established. Hence, this study presents a reusable, low-cost, noble-metal-free, environmentally friendly, fast, and highly efficient
40 wt % M@NFOPCN photocatalyst, achieving 90% degradation of CIP antibiotic under visible light. The double-Z scheme triggers
charge separation and migration, enhances visible-light harvesting, and helps in internal electric-field creation, thus headed toward
dramatic augmentation of the photocatalytic activity.
and average grain size obtained by excitation under large flux
density, are highly appreciated by researchers because of their
range of applications like water treatment, magnetic devices,
catalysis, lithium-ion batteries, microwave-absorbing materials,
gas sensors, and medical care.5−10 Among them, mixed spinel
1. INTRODUCTION
With a growing population and recent developments, this new
era has headed toward a level of crisis in energy shortage and
an increase in environmental pollution. Auspiciously, utilizing
solar energy, heterogeneous photocatalysis is a potential
platform for these crises.1 It helps in the conversion of
environmental contaminates to nontoxic products as well as in
the harvest of renewable energy through water splitting.2
Therefore, to extend the best way of utilizing renewable solar
light, researchers from all over the globe have stimulated their
skills and experiences through productive photocatalysis by
using low-cost or waste materials.3 Furthermore, recent
developments of composites acquire stronger oxidizing and
reducing abilities through a Z-scheme-based system.4 In recent
times, soft magnetic ferrites like MFe2O4 (M = Fe, Ni, Cu, Co,
Zn), because of their crystalline, anisotropic magnetic nature,
oxides NixFe −xO4 (0 < x < 1) have been investigated for their
3
photocatalysis, admirable enduring stability, ease of recovery
by magnetic separation, and ability to check secondary
pollution for treated water.11−14 Hence, because of their
Received: October 11, 2019
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Table 1. Comparison Study of a Magnetic Photocatalyst
photocatalytic degradation
measure
0.029 min−1
0.0235 min−1
material (magnetic component in bold)
ZnO/AgBr/Fe3O4/Ag3VO4
NiAl-layered double hydroxide/Fe3O4/reduced graphene
oxide
model pollutant
source of light
ref
rhodamine B
ciprofloxacin
50 W light-emitting diode
500 W xenon lamp (>420 nm
20
21
filter)
Fe3O4/TiO2
Bi2MoO6/ZnFe2O4
CoFe2O4/polyaniline
reactive brilliant red 3 300 W xenon lamp
0.03−0.035 min−1
0.0034 min−1
22
23
24
rhodamine B
150 W xenon lamp
methyl orange
10 W light-emitting diode
85% degradation in 2 h
unmatched supremacy in separation and recycling, magnetic
nanoparticles (NPs) have been extensively used as a platform
for the construction of a range of efficient photocatalysts.15,16
In particular, NiFe2O4 (NFO), because of its high Curie
temperature, environmental benignity and stability, high
electrical resistivity, and low price and the large abundance
of nickel, has became the most important magnetic material.
Within this inverse spinel, the Ni2+ and Fe3+ ions at octahedral
sites and balanced Fe3+ ions at tetrahedral sites having
antiparallel spins give rise to ferrimagnetism. The Fe2+ ion
present in the system and hopping of electrons from Fe2+ to
Fe3+ are accountable for the conductivity as well as the n-type
behavior, whereas Ni3+ is responsible for p-type activities, and
the shifting of holes from Ni3+ to Ni2+ gives rise to
conductivity.6 Also, as reported by Ding and co-workers, the
use of minor or noble metals should be given the least priority
for the assembly of proper sustainable systems, and recently
nickel NPs have been used in place of platinum NPs for
photocatalytic H2 evolution.16 However, again the fast reunion
of photoelectron−hole pairs in NFO has made it a poor
photocatalyst. Hence, to enhance the activity, composite
formation with NFO is advisible. As reported by our group,
after NFO was combined with phosphorus-doped g-C3N4 (P-
CN), there was a tremendous enhancement in the photo-
catalytic activity. This variation endowed carbon nitride
materials with superior optical absorption, outstanding H2
generation under visible light by water splitting, and improved
dye adsorption and degradation. These were all possibly due to
the increased surface area, better charge-transfer rate, and
narrowing of the band gap by phosphorus doping.7 We
deliberately designed a novel magnetic NFO-based composite
photocatalyst paired with P-CN and coupled with Fe3O4.
Meticulously, Fe3O4 is suitable for the best recycling
properties, excellent chemical stability, wide frequency ranges,
large saturation magnetization, electromagnetic wave absorp-
tion in the high frequency range, high complex permeability,
and eddy current loss as well as low alternating-current (ac)
electrical conductivity.17−19
Some recently published works related to this are given in
Table 1.20−24
From our previous work, we have already proven that
NFOPCN not only is a promising photocatalyst but also has
many more appreciable properties. However, to check further
the secondary pollution for treated water, we tried to furnish a
more environmentally benign photocatalyst with enhanced
catalytic properties by reaping the benefits of the previous
work.10,25 A series of Fe3O4-deposited NFOPCN nano-
composites were synthesized by simple in situ coprecipitation
methods by varying the concentration of Fe3O4 (30−50%).
These were all investigated mainly for photocurrent, photo-
catalytic antibiotic degradation, i.e., ciprofloxacin (CIP), and
H2 evolution and uses as supercapacitors under visible-light
irradiation. The 40 wt % Fe3O4 (M)-deposited NFOPCN
sample exhibits optimum photocatalytic activity for CIP
degradation and H2 production and also shows satisfactory
photostability after four photocatalytic cycles. Also, the
coupling of M with a NFOPCN lattice gives rise to new
dopant energy levels suitable under visible-light irradiation, for
the enhancement of interfacial charge transport of both holes
(h+) and electrons (e−) for better photoredox reaction of
H2O.26,27
2. EXPERIMENTAL SECTION
2.1. Chemicals Used in the Reactions. Fe(NO3)3·9H2O
(99.9%), FeCl3·9H2O (99.5%), FeCl2·4H2O (98%), Ni(NO3)2·
6H2O (99.9%), melamine (99.9%), citric acid (C6H8O7; 99.9%),
and NaOH (97%) were all of analytical grade (Sigma-Aldrich) and
were used with no further purification. Required aqueous solutions of
all chemicals used throughout the experiment were prepared from
deionized water (DW), which was obtained by means of a double-
distillation unit.10,27,28
2.2. Synthesis Procedure. 2.2.1. Fabrication of NFO and P-CN
Nanocomposites (NFOPCN). As reported earlier, the synthesis of
pure NFO and P-CN was furnished through sol−gel and chemical
routes, respectively, followed by calcination. The composite
NFOPCN (20 wt % NFO@P-CN) was also obtained by a calcination
route.10
2.2.2. Preparation of M (Fe3O4). Taking FeCl3 and FeCl2 as
precursors, the magnetite particles M (Fe3O4) were synthesized by a
coprecipitation method from their aqueous solutions at strongly basic
pH (pH = 12). The molar ratio maintained between the precursors
was Fe2+:Fe3+ = 1:2 ([Fe3+] = 0.5 M and [Fe2+] = 0.25 M). At 30 °C,
a 2 M NaOH solution was added dropwise to maintain the pH under
vigorous stirring in the presence of N2 gas. Complete chemical
precipitation was achieved after stirring for 5 h at 70 °C. Finally, the
product was collected after cooling, magnetically separating, and
washing thoroughly with DW followed by acetone. The obtained
blackish M was dried in an oven at 60−70 °C.27,28
2.2.3. Preparation of x wt % Fe3O4 @NiFe2O4/P-g-C3N4 (x wt %
M@NFOPCN). The x wt % (x = 30−50) M@NFOPCN nanomaterials
were synthesized by an in situ coprecipitation method. In a distinctive
procedure, different weight percentages (30−50 wt %) of M with a
fixed weight of NFOPCN were dispersed in 250 mL of DW/ethanol
(2:1) and ultrasonicated for 5 h at ambient temperature. A dark-
In this paper, we have synthesized Fe3O4@20 wt %
NiFe2O4/phosphorus-doped g-C3N4 (M@NFOPCN), a Z-
scheme-based heterostructure-agglomerated porous rod like
composites through an in situ calcined route that is pertinent
to a range of nanomaterials. The obtained results shed new
light on the interface and composition-related photocatalytic
properties like H2 evolution and antibiotic degradation.19 The
prominent appreciable activity of Fe3O4 as well as ferrites
against environmental pollution has already been established
by many research. Without much effort, we combined the
above two and modified the result for our purposes. It was
observed that the ternary M@NFOPCN nanocomposite
showed significantly enhanced activity compared to Fe3O4
and 20 wt % NFOPCN photocatalysts, and it could be
effortlessly recovered from the polluted water using a magnet.
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Scheme 1. Synthesis of M@NFOPCN by the Coprecipitation of Sol Gel via an In Situ Method
brown suspension was produced in almost all cases followed by
centrifugation. Further it was washed twice with DW and ethanol and
finally obtained through magnetic separation. The produced dark-
brown material was dried in an oven at 60 °C for 24 h. The
composites were finally ground properly and then calcined at 400 °C
for further analysis.29,30
2.3. Formation Mechanism. The in situ growth method has
been extensively used to synthesize a range of nanocomposites. A
similar approach was chosen for the synthesis of M@NFOPCN
hybrid nanomaterials, as given inScheme 1. Ultrasonic dispersion
helped in the deposition of nickel and iron ions on the surface of P-
CN via chemical adsorption. The use of basic NaOH during
precipitation helped to control the growth of iron ions by conversion
to Fe3O4 NPs. Thus, through ethanol sonication, the uniform
distribution and deposition of Fe3O4 NPs were effectively performed
on the surfaces of the P-CN sheets. Again the double-Z-scheme-type
heterojunction led by the as-prepared Fe3O4 NPs at the interface of
NFOPCN and M (Fe3O4) in the synthesized nanocomposite system
reduces the surface energy. The agglomeration of M (Fe3O4) NPs is
also avoided because of in situ growth.10,31
2.4. Methods of Characterization. A Rigaku Miniflex Advance
powder X-ray diffractometer (set at 30 kV and 15 m A) using Cu Kα
radiation (λ = 1.54056 Å) at a scan rate of 2° min−1 with 2θ degree
angles ranging from 10° to 70° and a step size of 0.01° was used for
analysis of the crystal structure of the nanocomposites. The IR spectra
were analyzed at a resolution of 4 cm−1 by a JASCO FT/IR-4600
Fourier transform infrared (FTIR) spectrophotometer using KBr as
the reference diluent. Within a frequency range of 400−4000 cm−1,
the vibrational modes and chemical compositions of M@NFOPCN
were analyzed. The composition and surface framework of the as-
synthesized composite, M@NFOPCN, and neat materials were
measured by scanning electron microscopy (SEM; Hitachi S-
3400N). Transmission electron microscopy (TEM) images at
different scales were examined by a JEOL-2010 200 kV instrument.
This helps to study the morphology interface, interaction between M
and NFOPCN, and microstructure of the prepared catalysts. The
magnetic study was carried out by using vibrating-sample magneto-
metry (VSM; Microsense EZ9). The room-temperature photo-
luminescence (PL) spectra of M, NFOPCN, and 30−50 wt % M@
NFOPCN were investigated utilizing a fluorescence spectrophotom-
eter (JASCO FP-8300) equipped with a xenon (Xe) lamp with an
excitation wavelength of 330 nm. UV−vis diffuse-reflectance spec-
troscopy (DRS) spectra were found using a UV−vis spectropho-
tometer (JASCO 750), and transformation from reflection to
absorbance for the Kubelka−Munk method was chosen by using
BaSO4 as the reference. A monochromatic X-ray source (Mg Kα X-
ray) was used in order to accomplish X-ray photoelectron
spectroscopy (XPS) by a VG Microtech Multilab ESCA 3000 surface
analyzer.
electrode system was used for photocurrent studies and measure-
ments. The electrolyte used for the study was an aqueous solution of
Na2SO4 (0.1 M) at pH = 6.7. Again, for the reference, working, and
counter electrodes, Ag/AgCl, the prepared electrode, and platinum
wire were used, respectively. For the source of light, a 300 W xenon
lamp through an UV cut-off filter (λ = 400 nm) was used. A powdered
photocatalyst (0.02 g) with iodine (0.02 g) in acetone (30 mL) was
dispersed and then used for the preparation of electrodes by
electrophoretic deposition. The two parallel fluorine-doped tin oxides
(FTOs) were dipped in the solution at a distance of 10−15 mm at 60
V bias underneath potentiostat control for 3 min. Then the FTOs
having a coated area of almost 1 cm2 were dried. A frequency range
from 0.1 Hz to 100 kHz and an ac voltage of 10 mV amplitude were
used to determine EIS. Under dark conditions at 1500 Hz, the MS
plots of the nanocomposites were well illustrated. The polarization
curves (LSV) were furnished by a potential sweep from −0.5 to +1.5
V.
2.6. Photocatalytic Experiment. 2.6.1. Measurement of
Photocatalytically Furnished H2. A gas distribution, an evacuation
system, and a Pyrex quartz reactor enclosed in a 100 mL sealed round-
bottomed flask were used for the H2 evolution reactions. A 150 W
xenon arc lamp (λ > 420 nm) of medium pressure as the light source
was taken to activate the reaction using a 1 M NaNO2 solution as a
UV cut-off filter. The distance from the photoreactor was almost 20
cm. The measured density of normal light was found to be 120 mW
cm−2. A total of 20 mL of an aqueous solution containing the target
powdered catalyst of 20 mg and a solution of 10% methanol was
taken. N2 gas was purged for 20 min for the total removal of CO2 and
O2 dissolved in H2O prior to irradiation. The H2 gas produced was
collected by downward displacement of H2O.
2.6.2. Experiment for CIP Reduction. The photocatalytic
degradation efficiency of the synthesized nanocomposites was
established by decomposing CIP under solar-light irradiation.
Aqueous solutions of CIP and photocatalyst were taken in 100 mL
stoppered conical flasks and agitated in the dark using a magnetic
stirrer for 30 min. Then they were irradiated under solar light for 1 h
from 11:00 am to 12:00 noon. The average luminosity throughout the
course of the experiments was found to be 102000 lx. Then, the
suspension was centrifuged, followed by filtration to reobtain the
catalyst aliquots. The concentration of the remaining CIP was
analyzed using a UV−vis spectrophotometer. Further, in addition to
that, the initial concentration of the CIP solution, time duration of
exposure to visible light, and effects due to the weight percent
variation in the composition of photocatalysts on the probable
photocatalysis of M@NFOPCN were investigated.
3. STUDY OF THE MORPHOLOGY, CRYSTAL
STRUCTURE, AND COMPOSITION OF AN
AS-SYNTHESIZED PHOTOCATALYST
2.5. Photoelectrochemical (PEC) Measurements. Cyclic
voltammetry (CV), Mott−Schottky (MS) plots, linear-sweep
voltammetry (LSV), and electrochemical impedance spectroscopy
(EIS) were carried out under the essential conditions to analyze the
PEC response of the synthesized nanocomposites. An Autolab
electrochemical analyzer (Ivium potentiostat) with a standard three-
3.1. Crystal Phase and Phase Purity by PXRD Studies.
As obtained from PXRD analysis, the phase, crystalline nature,
and interlayer stacking of synthesized samples are displayed in
Figure 1, which reveals the XRD patterns of neat M (Fe3O4), a
NFOPCN composite, and the hybrid nanocomposites x wt %
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Figure 2. FTIR spectra of M, NFOPCN, and 30−50 wt % M@
NFOPCN composites.
Figure 1. XRD patterns of M, NFOPCN, and 30−50% M@
NFOPCN composites.
O−H groups, respectively.10,33 This finding specifies the
existence of crystal H2O in the compound.9 The distinctive
bands in the area ranging from 1240 to 1640 cm−1 are assigned
to either the bridging C−NH−C or trigonal C−N(−C)−C
units (full condensation).34 The P−N stretching vibration in
NFOPCN is well identified by a feeble peak present at 950
cm−1.10 Again, the sharp peak located at 810 cm−1 can be
related to the breathing vibration of the typical s-triazine
ring.35 The weak typical low-frequency peak at 560 cm−1 is
ascribed to Fe−O stretching in the tetrahedron and the band at
608 cm−1 to metal−oxygen deformation in the octahedron.
The possibility of formation of other phases for M could also
be eliminated because the existence of a Fe−O vibrational
mode at 560 cm−1 is only ascribed to the M magnetite phase,
whereas the band at 470 cm−1 typical for γ-Fe2O3 is absent.36,37
Further, in the case of pure M, the peak at 586 cm−1 is ascribed
to the symmetric stretching vibration of the Fe−O band in the
tetrahedral FeO6 groups of spinel-type compounds.34 It is also
observed that all of the main distinctive peaks of NFOPCN
and M are well maintained in the M@NFOPCN composite
spectra, further confirming the evidence of NFO and M on the
surface of P-CN.
M@NFOPCN (x = 30, 40, 45, and 50 wt %), which are used
to clarify the structural parameters. The well crystallization is
confirmed from the strong and sharp diffraction peaks. In
NFOPCN, the peaks at 18.4°, 30.3°, 35.7°, 37.3°, 43.4°, 53.8°,
57.4°, and 62.9° are ascribed to the (111), (220), (311),
(222), (400), (422), (511), and (440) planes of NFO, as
shown in Figure 1 (JCPDS 10-0325). Also, the other two
distinct diffraction peaks of lower intensity than g-C3N4, at
27.41 and 13.11 indexed as the (002) and (100) planes, were
obtained for neat P-CN, as reported in our previous paper.10
The obtained diffraction peaks of neat M are in good harmony
with JCPDS 65-3107 for pure cubic M.27 The peaks at 30.1°,
35.5°, 43.4°, 57.4°, and 62.9° are attributed to the (220),
(311), (400), (511), and (440) planes of M. After hybrid-
ization with NFOPCN, it is observed that the crystal phase of
M does not change, but there is a slight dislocation toward
higher angles in comparison to pure M, signifying a strong
interaction within NFOPCN and M.27,28 Although there are
two ferrites, there are no double reflections in the PXRD
pattern due to broadening of the diffraction lines.29 It is also
well observed that, with a gradual increase in the wt % of M
from 30% to 50%, an increase in the intensities of the ferrite
peaks and a decrease in the intensity of the P-CN peaks along
with the appearance of a peak at 33.5°, which is attributed to a
tendency of varying phases of M to support Fe2+ → Fe3+
oxidation-state shuttling.30,31 Further, as was already reported,
the increasing magnetic interaction within the magnetic
materials also supports that above.28 The above XRD pattern
of the as-synthesized M@NFOPCN photocatalyst given in
Figure 1 matches well with the magnetite (Fe3O4; JCPDS 65-
3107) and maghemite (γ-Fe2O3; JCPDS 39-1346) structures.
Because of the similarity in the XRD patterns of maghemite
and magnetite, XPS measurements are taken into account to
avoid unambiguity.15,32
3.3. SEM. The influence of [Ni2+], [Fe2+], and [Fe3+] on the
morphology, size, elemental distribution/composition, and
structure of the sintered composites is revealed by electron
microscopy techniques [SEM, TEM, high-resolution TEM
(HRTEM), and selected-area electron diffraction (SAED)].9
The SEM images of the nanocomposite in Figure 3a−c clearly
reveal the presence of pores and voids in the sample. This may
be due to the discharge of a huge amount of gases during the
combustion process adopted for the synthesis. In a spongy
structure, fine multigrain crystallites are found to undergo
agglomerations.38 Pure NFO having granular unstructured
morphology and flakelike, globular agglomerates of P-CN were
already reported in our previous paper.10 Figure 3b shows
cubical agglomerated M with some rodlike morphology.
Further, as shown in Figure 3a−c, there are numerous smooth
rodlike nanocrystals of M, allowing a large amount of
mesoporous agglomerated nanocrystals on the surface of
those M rods. These nanomaterials linked to each other and
shaped the successive conductive network at [Ni2+]. These
thin nanorods and cubes of M are quite capable of increasing
the photoredox reactions during photocatalysis.39 This type of
3.2. FTIR. To illustrate the surface chemistry and different
bonds of the as-synthesized 30−50 wt % M@NFOPCN
composite, FTIR spectroscopy was used, as given in Figure 2.
The broad peaks at around 3100−3500 cm−1 are indicative of
surface-adsorbed H2O molecules and uncondensed N−H
stretching vibrations of the P-CN spectrum. The two enlarged
broadening bands almost centered around 3367 and 1638
cm−1 belong to the stretching and bending vibrations of the
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Figure 3. (a) SEM image of NFOPCN and rod and cubic agglomerated porous structures of (b) M (Fe3O4) and (c) 40 wt % M@NFOPCN hybrid
nanomaterial.
Figure 4. (a and d) TEM images of M (Fe3O4) and NFO agglomerated over the P-CN sheets in 40 wt % M@NFOPCN. (b and e) HRTEM
images presenting the lattice fringes of the planes of a 40 wt % M@NFOPCN nanocomposite and neat M. (c and f) SAED images of 40 wt %
MNFOPCN and M.
increased surface area by rod and cubic morphology as well as
agglomerated porous NFOPCN facilitates the enhancement of
electron channelizing, enhancing the photocatalytic activity.40
The specific surface area of the M@NFOPCN NPs estimated
by the Brunauer−Emmett−Teller equation is 147.143 m2 g−1,
which is much higher than that of NFOPCN, i.e., 37.316 m2
g−1.37
3.4. TEM. The crystalline orientation and texture of the two
neat samples NFOPCN and M and composite M@NFOPCN
are confirmed by TEM, as shown in Figure 4a,b,d,e.
Agglomerated porous structures composed of numerous
pseudocubic crystals or small cubes signify magnetite, and its
rodlike structure having lengths from 50 to 200 nm is
observed. The arrangement of M NPs on a NFOPCN seed
most likely leads to diverse structural parameters.29 This
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Figure 5. (a) EDS layered spectrum and (b) elemental EDS spectrum of M@NFOPCN.
Figure 6. (a) TGA for thermal stability. (b) VSM study for magnetic behavior of neat M and NFOPCN and M@NFOPCN.
noteworthy rodlike and pseudocubic crystal or small cube
formation of a typical magnetite also goes well with the XRD
patterns of as-prepared magnetite, which were obtained by
annealing the precursor for M at 400 °C for 2 h. However, as
soon as the composites of M and NFOPCN are obtained,
agglomerated porous structures of numerous petite nanocryst-
als on the surface of nanorods appeared.41 This signifies the
addition of Ni2+ ions to the system. The measured lattice
spacing in Figure 4b are 0.251 and 0.148 nm, which assigned
well with the (311) and (440) planes having corresponding d
values (d = 0.2514 and 0.148 nm) of NFO (JCPDS 10-0325),
respectively. Similarly, in Figure 4d, the lattice fringe spacings
of 0.252 and 0.292 nm ensure the (311) and (220) planes of
M.12,13 The SAED patterns of the photocatalyst shown in
Figure 4c,f depict distinct rings and spots. It sheds light on the
polycrystalline nature and structure of the composite in the
synthesized materials. Agglomerated spherical NPs with a wide
range of size distributions (5−15 nm) are observed. During the
photocatalytic activity, the nanorod morphology of M is quite
promising and facilitates the photoredox reactions by delaying
the recombination and helping in the electron channelizing.
The lattice plane present in the agglomerates is well identified
by SAED, confirming the presence of the spinel phase. The
existence of elements like P, C, N, O, Fe, and Ni in the M@
NFOPCN nanocomposite is confirmed by elemental color
mapping obtained by EDS analysis, as given in Figure 5a,b.
The obtained EDS spectra of M and NFOPCN, as shown in
Figure 5b, distinctly show the well-defined spatial arrangement
of all elements C, N, O, Ni, Fe, and P, confirming formation of
the M, NFOPCN, and M@NFOPCN composite. The weight
percentages of the elements are displayed in the inset of Figure
5b. However, the analogous ferrites cannot be differentiated by
this. Hence, XPS spectra are needed for further confirmation.17
3.5. Thermogravimetric Analysis (TGA) and VSM.
TGA and differential thermal analysis have been carried out to
investigate the thermal stability and actual content of M on
M@NFOPCN nanocomposites from 25 to 1000 °C in a N2
atmosphere, and the results are shown in Figure 6a. As was
already reported, an overall weight loss of the ferrite sample is
almost 7.6%.42 Becaise of the burning of P-CN, the
decomposition of P-CN occurs almost up to 800 °C. From
earlier reports due to thermal decomposition, CN has two
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weight loss regions from 100 to 400 °C and from 550 to 800
°C. Some weight loss within 100−400 °C may also be
attributed to the adsorption of surface-bound H2O.15 The
criteria of exothermic measures were taken into account for the
related heat-flow effects. The endothermic affair from 25 to
400 °C could be ascribed for the physical adsorption and
removal of crystallized H2O, from both the interlayer space and
outer surface.34 Again, as was already reported by Deng et al.,
the decomposition of hydroxides and carbonates that occurs
between 400 and 800 °C as a second event is also
endothermic, as shown in Figure 6a.42 From TGA, the weight
loss obtained in the M@NFOPCN composites is around
21.034%, which is quite low in comparison to other reported
ferrite g-CN composite samples.43 This confirms that the
coupling of M with NFOPCN increases the thermal stability.
The presence of NFO and P-CN may be responsible for the
relatively low decomposition temperature in comparison to
pure CN.34 Hence, it is concluded that the stability of M is
retained.
VSM was used to measure the magnetic measurements of as-
prepared M@NFOPCN with a peak field of 15 kOe. The
characteristic behavior of prepared soft magnetic material
(samples V1 = M@NFOPCN and V2 = NFOPCN) are shown
by the hysteresis loops in Figure 6b. An increase in the
saturation magnetization (Ms) value is observed for V1 = M@
NFOPCN rather than V2 = NFOPCN in Figure 6b. The Ms
value of M@NFOPCN is higher than that of NFOPCN. As
shown in Figure 6b, without any remanence and coercivity,
M@NFOPCN possesses superparamagnetic character. The
magnetic effect of M is well observed from Scheme 2. NFO is
3.6. XPS. An XPS study has been carried out to give further
confirmation about the surface chemical composition as well as
oxidation states. The high-resolution spectra of Ni 2p, Fe 2p, C
1s, N 1s, O 1s, and P 2p for the M@NFOPCN photocatalyst
and Fe 2p and O 1s for the M photocatalyst are represented in
Figure 7a−f. As given in Figure 7a, the P 2p binding energy
peak at ca. 133.7 eV confirms the presence of P incorporated in
the CN structure through P−N coordination. In Figure 7b, the
peak at 285.5 eV is attributed to C−O interaction.33 The peaks
at 284.5 and 287.7 eV are for neat graphitic carbon (C−C) and
the distinctive peak for N−CN bonding in P-CN,
respectively. As shown in Figure 7c, the deconvoluted XPS
spectra for N 1s have three peaks. The two at 398.1 and 399.4
eV confirm the sp2-hybridized nitrogen in pyridine like C
N−C and tertiary pyrrolic N−(C)3 groups.44 Another at
401.06 eV reveals the typical graphitic C−N−C peak.10 A blue
shift in the binding energy of about 0.4 eV is observed for the
O 1s peak for the M@NFOPCN nanohybrid, confirming a
strong coupling within M and NFOPCN. The peaks at 529.8
and 531.2 eV are ascribed to the chemisorbed oxygen of
surface-adsorbed hydroxyl (OH−) or H2O-type oxygen
vacancies and oxygen species, respectively, as depicted in
Figure 7d, whereas the peak at 532.7 eV may be assigned for
interstitial oxygen.45 In M@NFOPCN, without much conflict
with the previously reported literature, two main peaks of Ni
2p3/2 and Ni 2p1/2 are noticed at 856.3 and 875.3 eV as
binding energies of Ni2+, as shown in Figure 7e. This core level
is absent in the iron oxide XPS spectra, thus confirming it as
nickel XPS spectra. After hybridization of the NFOPCN
nanocomposites with M via in situ coprecipitation, followed by
calcination growth, there is an upshift of ca. 2.7 eV for Ni 2p3/2
and 3.5 eV for Ni 2p1/2 at the binding energy of the peaks in
M@NFOPCN compared with those for Ni in NFOPCN.10
Two shakeup peaks of Ni at around 860.5 and 891.2 eV as
satellite peaks are observed at the high binding-energy regions
of the Ni 2p1/2 and Ni 2p3/2 edges. This blue shift of the
binding energies for the Ni 2p peaks accounts for the firm
coupling between NFOPCN and M in the nanohybrid, maybe
via the Ni−O−Fe bands during the in situ calcination
treatment.18 NFO and M show almost similar XRD patterns
but different valences of the iron ion. This is cleared by their
XPS spectra. The detailed chemical state of the iron ions was
confirmed through deconvolution. M as a mixture of the Fe3+
and Fe2+ chemical states can be expressed as FeO/Fe2O3.26 In
Figure 7f of the high-resolution XPS spectrum of Fe 2p, two
major peaks of Fe 2p1/2 and Fe 2p3/2 situated at 711.2 and
725.4 eV are found.10,36,43 The Fe 2p peak well-fitted with the
Fe3+ and Fe2+ peaks confirms the presence of M in the
oxidation layer. Again the upshifts of ca. 2.0 eV for Fe 2p3/2
and 4.3 eV for Fe 2p1/2 for the binding energy of the peaks in
M@NFOPCN further confirm the strong coupling within M
and NFOPCN.
Scheme 2. Schematic Representation of the Interactions
within Two Magnetic Materials in Their Magnetic Field
4. OPTICAL BEHAVIOR AND ELECTRONIC BAND
STRUCTURE
an inverse spinel-type soft magnetic material having a crystal
structure of (Fe3+)A[Ni2+,Fe3+]BO42−. Among the three main
types A−O−A, A−O−B, and B−O−B, the superexchange
interaction in A−O−B among the A and B sites is the major
one. Because of less decomposition of the M and NFO phases,
there is an increase in [Ni2+], resulting in the improved
superexchange interactions in A−O−B, thus favoring the
increase of Ms.37,38
4.1. UV−Vis DRS. Parts a−d of Figure 8 depict the UV−vis
DRS spectra of 30−50 wt % M@NFOPCN, NFO, pure P-CN,
neat M, and hybrid nanocomposite samples in the range of
200−800 nm. The photon receptive behavior of the M@
NFOPCN nanocomposite is expressed through the optical
band gap and absorbance range. As reported in our previous
paper, a sharp absorption edge at 450 nm for P-CN
corresponding to a band gap of 2.7 eV and a wide range
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Figure 7. XPS spectra of 40 wt % M@NFOPCN: (a) P 2p; (b) C 1s; (c) N 1s; (d) O 1s; (e) Ni 2p; (f) Fe 2p.
Figure 8. (a) UV−vis spectra of all neat compounds and composites. (b−d) UV−vis DRS spectra of pure NFO, M, and pure P-CN.
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within 200−720 nm for NFO analogous to a band gap of 1.7
eV were already established.10 In Figure 8a, a bathochromic
shift toward the near-IR range was apparently observed with an
increase of the M weight percentage in M@NFOPCN, which
may be ascribed to the interface interaction between
NFOPCN and M in comparison to NFOPCN. With increased
M weight percentage, the absorption intensity of the as-
prepared different weight percentage samples also notably
strengthened with more light-harvesting capacity.17
and 30−50 wt % M@NFOPCN with an excitation wavelength
of 330 nm and a Gaussian-like peak at ca. 435 nm.47 The PL
emission spectra give details about the recombination,
migration, and shifting of photogenerated charge carriers.
From Figure 9, it can be observed that the peak intensity of the
40 wt % M@NFOPCN photocatalyst is quite low in
comparison to those of M, NFOPCN, and other prepared
photocatalysts. This indicates an improved charge-separation
efficiency in M@NFOPCN. As given in our previous work, the
foremost emission peak for the pure P-CN sample is centered
at about 440 nm and is responsible for the band−band PL
trend, with the light energy almost equal to the band-gap
energy of P-CN.10 As was already established, phosphorus
doping enhances photogenerated electron−hole pair separa-
tion, which was initiated by an n → π* electronic transition in
P-CN obtained from the lone pair of electrons available on the
nitrogen atom.10 Also, a weak peak for NFO was obtained
because of defects of NFO from excitation PL as well as from
the surface oxygen vacancies. Combining NFO with P-CN
brings a noticeable drop off in the intensity of the PL
spectra.10,45 However, the UV−vis absorption statistics of M
and the 30−50 wt % M@NFOPCN nanocomposite do not
show any peak as given above in the Gaussian wavelength
range. Hence, structure-related defects are more accountable
than band-edge transitions. Because of its narrow-band-gap
value (1.6 eV), M might show high PL intensities, leading to
fast recombination. The d-orbital electrons responsible for the
narrow band gap of M are accountable for the high electrical
conductivity. Further, in a restricted volume, quantum
confinement of the charge carriers of M due to size-related
quantization may be responsible for the strong emission peaks.
This effect may be ascribed to the electron-channelizing
capacity of P-CN; it traps the photoexcited electrons of M on
its π skeleton and checks the e−/h+ recombination rate. As a
result, the electron−hole pair recombination rate is suppressed,
and the charge separation increases in the M@NFOPCN
nanocomposite.10 Therefore, M@NFOPCN nanocomposites
are likely to achieve first-rate photocatalytic activity toward the
degradation of antibiotics. Further, EIS measurement justifies
these PL data.
4.3. Photogenerated Electron−Hole Transport from
EIS Analysis. Among the prepared photocatalysts, 40 wt %
M@NFOPCN has the best efficiency. This was further
confirmed by EIS measurement. Distinctive Nyquist diagrams
representing the reaction rate at the surface of the electrode,
charge separations obtained by incident photons, charge
transfer, and resistance given by the photocatalysts are given
in Figure 10. According to its principle, the arc radius of the
Nyquist plot varies inversely with the charge-separation
efficiency of the working electrode; i.e., the smaller the
semicircle radius of the photocatalyst, the less the interface
charge-transfer resistance and thus the more active the
separation of photogenerated e−/h+ pairs and vice versa.48
Figure 10 shows the Nyquist plots for neat M, NFOPCN, and
M@NFOPCN composites at zero biasing potential. Generally,
the conductivity of the catalyst is expressed by the semicircular
part in the high-frequency region and the Warburg resistance
by the straight line in the low-frequency region.49 The arc
radius for M, NFOPCN, and synthesized M@NFOPCN are
found to be 63, 120, and 49 Ω, respectively. Delocalization of
the lone electron pairs in P-CN, electron hopping in M from
Fe2+ to Fe3+, and hole shifting from Ni3+ to Ni2+ in NFO are
the main sources of conductivity.10 Hence, M@NFOPCN,
Further, the Kubelka−Munk equation was used to estimate
the band gap value.
αhv = k(hv − Eg)n/2
(1)
where α = absorption coefficient, n = type electronic transition,
Eg = optical band-gap energy, ν = frequency of light, and A =
proportionality constant, i.e., n = 1/2 for a direct type transition
and n = 2 for an indirect type transition, and in our case, the
materials suffer direct transitions, respectively.
In the case of M, the strong absorption with an energy of 1.6
eV covering at ∼690 nm is due to the transition of an electron
from the valence band (VB) O 2p to the crystal field (eg) on
the tetrahedral site. Another absorption around ∼565 nm
assigned to the recombination of electrons from eg → t2g on
the octahedral site and the transition of t2g → eg and O 2p →
t2g in the tetrahedral site. The oxygen vacancies as well as
electron traps on the tetrahedral site are responsible for near-
IR absorption at around ∼840 nm. Similarly, in NFO, the
transition of Ni 2+−O−Fe3+ to Ni+−O−Fe4+ is accountable for
a strong absorption up to around 720 nm. The n−p* transition
in the heptazine/triazine ring system of P-CN, due to the lone
pair of electrons on the nitrogen atoms, is liable for a
absorption band edge at 450 nm.10 Hence, under the same
visible-light irradiation, the superior light absorption by the
prepared nanocomposites helps in the production of more
electron−hole pairs, which consequently results in better
photocatalytic activity. However, when the amount of M is
increased in M@NFOPCN, M may act as a reunion center for
photogenerated exciton pairs, thus reducing the photocatalytic
activity of the nanocomposites.46
4.2. PL Spectral Analysis for the Confirmation of
Photogenerated Electron−Hole Separation. Figure 9
represents the characterized PL spectra for pure M, NFOPCN,
Figure 9. PL spectra of neat M (Fe3O4), NFOPCN, and 30−50 wt %
M@NFOPCN at excitation wavelength λex = 330 nm.
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character, while negative slope is p-type character. The flat-
band potentials are +1.54, −1.47, and −0.6 V for NFO, P-CN,
and M versus Ag/AgCl electrode, respectively, or +2.11, −0.87,
and −0.0012 V versus normal hydrogen electrode (NHE). The
Efb value of p-type semiconducting materials is close to the VB
potential and that of n-type materials is close to the conduction
band (CB) potential. Again, to be in harmony, considering the
carrier concentration and effective mass of the electron (me),
the conduction band minimum (CBM) is 0.1 V lower than
Efb.50 The Nernst equation as given below is used for the
conversion.
Efb(NHE)=Efb(pH
= 0, vs Ag/AgCl) + E(Ag/AgCl) + 0.059pH
(2)
Figure 10. Nyquist plots for the NFOPCN, M, and M@NFOPCN
As explained and reported earlier, because of the shifting of
holes from Ni3+ to Ni2+, NFO accounts for p-type behavior.
The actual oxidation and reduction potentials in the nano-
composites are determined from the band-edge potentials (VB
and CB) of the samples calculated from the extent of the band
gap (Eg = VB − CB). The VB positions as obtained from the
Tauc/Davis−Mott model derived from the UV−vis DRS
spectra are found to be 2.11, 1.8, and 1.68 eV for NFO, P-CN,
and M, respectively, and the CB positions are +0.4, −0.8,
−0.0012 eV for NFO, P-CN, and M, respectively. Because of
the CB and VB positions, we put on extra impending into the
mechanism during this MS plot.10
nanocomposites.
with the smallest arc indicating the utmost electron−hole pair
separation and with the enhanced straight line part for superior
conductivity, proved to be a better photocatalyst. P-CN π
conjugation is the most important reason behind this result
because it channelizes the photoexcited electrons through the
framework and thus decreases the reunion process.
4.4. Electrochemical Study for MS, LSV, and PC
Measurements. The flat band potentials (Efb) of NFO, P-
CN, and M were investigated by the MS plots (Figure 11a−c).
The positive slope of the MS plot is representative of n-type
Figure 11. MS plots of (a) NFO, (b) P-CN, and (c) M.
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4.4.1. LSV. The charge-separation efficiencies of M,
NFOPCN, and M@NFOPCN were predicted by their
photocurrent densities. Under light irradiation, it was clearly
found out that M produces an anodic photocurrent of nearly
1.37 mA cm−2, thus an n-type semiconductor. Furthermore, it
was also shown that NFOPCN produced both anodic
(+531.38 μA cm−2) and cathodic (−127.43 μA cm−2)
photocurrents at +1.5 and −0.5 V, respectively.10 However, a
noteworthy increase was observed toward the anodic direction
with respect to the applied bias, confirming an n-type
semiconductor, whereas NFO is proven to be a p-type
semiconductor under light conditions showing a cathodic
current density of −96.64 μA cm−2 and finally ending in a
saturated value.10 In contrast, under the forward and reverse
applied bias potentials, a 40 wt % M@NFOPCN n−n−p-type
nanocomposite generates different photocurrents. Further,
with positive bias potential, there is a sharp increase in the
photocurrent and nearly 1.87 mA cm−2 is achieved at +1.5 V,
whereas a negligible reverse photocurrent is experienced at
−0.5 V, as shown in Figure 12. Both the oxidation and
4.4.2. Supercapacitive (SC) and Charge−Discharge (CD)
Study by CV Analysis. SC nature of M@NFOPCN has been
evaluated by means of CV. Standard Ag/AgCl, M@NFOPCN
composite, and platinum electrodes were used as reference,
working, and counter electrodes, respectively. The scan rate
was calibrated at a scan rate of 10 mV s−1 by taking 3 M KOH
as the standard electrolyte. Transition-metal ions Ni2+/3+ and
Fe3+/2+ present in M@NFOPCN undergo redox reactions and
are thus responsible for oxidation reduction peaks of CV, as
shown in Figure 13a. The scan rate was varied as 10, 20, 40,
and 100 mV s−1 while keeping the potential window constant.
During the electrochemical process at high scan rates, only
surface species of the electrode can participate, whereas at low
scan rates, time allows for the diffusion of ions into the
electrode material, thus utilizing active sites of the electrode.51
With increasing scan rate, there is no significant change in the
shape of the CV curves, indicating a fast surface kinetic
response from the electrode material with respect to the
rapidly varying potential step and firm synergetic interaction
within the electrode surface and electrolyte. Thus, the
pseudocapacitive behavior of the M@NFOPCN electrode is
the output of those redox peaks. Again, this confirms its large
active surface area for energy storing. The typical CD plots of
the electrodes at 0−0.5 V Ag/AgCl are shown in Figure 13b.
The shape of the discharging curves implies Faradaic
interaction, thus leading to SC properties.
5. INVESTIGATION OF THE PHOTOCATALYTIC
PERFORMANCE OF M@NFOPCN
The photocatalytic efficiencies of M@NFOPCN were
evaluated by CIP degradation as well as H2 evolution under
visible-light irradiation.
5.1. Photocatalytic Reduction of CIP. In order to
identify the photocatalytic efficiency of M@NFOPCN, CIP, a
major antibiotic of ecosystem, was considered as a specimen to
be degraded under solar-light irradiation. The photocatalytic
performance was investigated by taking into account the
degradation of a 20 mg L−1 aqueous CIP solution exposed to
sunlight. The same experimental conditions were followed for
a comparison study of the prepared ternary and parent
catalysts. The type of reaction was a photocatalysis process
rather than photolysis, which was also confirmed by perform-
ing degradation without the photocatalyst. It was observed that
not more than 1% of the initial concentration of CIP was
Figure 12. Photocurrent densities of M, NFOPCN, and M@
NFOPCN nanocomposite.
reduction responses of M@NFOPCN are due to the anodic
and cathodic photocurrents. Hence, the combined synergetic
effect of M, NFOPCN, and M@NFOPCN resulted in a higher
photocurrent response.
Figure 13. (a) CV analysis and (b) CD curves of M@NFOPCN.
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Figure 14. (a) Kinetic study of CIP degradation by M@NFOPCN in every successive time interval. (b) Kinetics of CIP decolorization in the first
cycle. (c) Assessment of the degradation percentage of CIP by NFO, P-CN, and 30−50 wt % M@NFOPCN nanocomposites. (d) Reusability plot
of CIP degradation over M@NFOPCN for four consecutive degradation experiments.
Figure 15. (a) XRD of the used and unused M@NFOPCN. (b) SEM image of the used M@NFOPCN.
decreased. Initially the adsorption−desorption of CIP on the
catalysts was carried out in the dark by stirring for 30 min at
pH = 6. The accomplishment of adsorption−desorption
equilibrium was confirmed from the invariance in the C/C0
value, as shown in Figure 14a, and the results of the kinetic
study performed, as shown in Figure 14b, are given under 5.2.
The residual concentration of CIP was measured after 1 h of
exposure to sunlight at an outdoor temperature (35 5 °C).52
Degradations of around 43% and 48% for neat NFOPCN and
M are observed, whereas, surprisingly, an excellent 90%
degradation was achieved by the 40 wt % M@NFOPCN
nanocomposite, as shown in Figure 14c. CIP degradations by
30, 45, and 50 wt % M@NFOPCN nanocomposites were 52%,
73%, and 62%, respectively. With increasing weight percentage
of M, the number of M@NFOPCN nanocomposites and
adsorption sites available on the M@NFOPCN surface also
improved, resulting in an augmentation in the degradation
efficiency. However, a further increase in the concentration of
M in M@NFOPCN (from 40 to 50 wt %) decreases the
degradation. This may be due to light scattering, agglomeration
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•
Figure 16. (a) Percentage degradation of CIP over M@NFOPCN in the presence of scavenging agents. (b) Confirmation of OH generation
•
through a TA test. (c) O2− generation by a NBT test in M@NFOPCN.
of nanoparticles, and a screening effect, which hinders solar
light from reaching active sites and thus decreases the
photocatalytic degradation efficiency from 90% to 62%.10,52
5.2. Kinetics of CIP Reduction. A kinetic study of the
solar photodegradation of CIP solutions by M@NFOPCN was
carried out in detail. Initially, to accomplish adsorption−
desorption stability, 20 mg of M@NFOPCN was dissolved in
20 mL of the antibiotic CIP solution and set aside in the dark
under constant stirring for 30 min. Then, at the outset, a CIP
solution with the best obtained photocatalyst, as given above,
was taken and subjected to solar irradiation for 1 h, giving a
time interval of 15 min. The remaining solution was collected
at 15 min time intervals and studied to determine the reaction
kinetics. A 20 mg L−1 CIP solution was degraded by 20 mg L−1
M@NFOPCN in 60 min under solar-light irradiation.
Centrifugation, followed by filtration, was used to extract
degraded CIP and then analyzed using a UV spectropho-
tometer (Figure 14c).
solution at reaction time t, initial concentration of the solution,
and reaction time, respectively, as shown in Figure 14a,b.53,10
The rate of degradation was found by the following equation:
photodegradation rate = (C0 − C/C0) × 100. In summary,
from all of the above results, the rate constant in the first-order
kinetics of 40 wt % M@NFOPCN is too high compared to
those of neat NFOPCN (0.0490 min−1) and M (0.0210
min−1), proving that a better composite has been formed.
5.3. Reusability and Stability Study. The reusability and
stability of 40 wt % M@NFOPCN after degradation of CIP
were also tested for industrial-scale applications. The obtained
nanocomposite was washed carefully using ethanol and H2O,
followed by air oven drying prior to the subsequent
degradation experiment. As shown in Figure 14d, although
there is a little reduction in the photocatalytic performance, the
catalyst has proven to be stable for up to four cycles. As shown
in Figure 15a, there is a decrease in the intensity in XRD, and
in Figure 15b, a SEM image of the used sample again confirms
the stability.
From the results obtained, it was concluded that M@
NFOPCN efficiently irradiated 90% CIP under the above
given experimental conditions. The rate constant of CIP
degradation was found to be 0.0990 min−1, and the data
appeared to be fitted to a pseudo-first-order reaction, Ct/C0 =
e−kt, where Ct, C0, and t stand for the concentration of the
5.4. Effect of Scavengers for the Proposed Mecha-
nism of CIP Degradation. During the photocatalytic activity,
various photogenerated active species covering hydroxyl
radicals (•OH), superoxide radicals (•O2 ), holes (h+), and
−
electrons (e−) were used in the photocatalytic reactions
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Scheme 3. Schematic Presentation of Degradation of CIP and Evolution of H2 over M@NFOPCN
Scheme 4. Structure of CIP and Its Oxidation and
Reduction
Table 2. Amount of H2 Evolution by the Prepared Magnetic
Photocatalyst
neat and
composites
H2 evolution
light source used
(μmol h−1
)
ref
M@NFO
xenon arc lamp (l >
420 nm)
80
prepared
photocatalyst
NFOPCN
xenon arc lamp (l >
904
450
520
774
924
prepared
420 nm)
photocatalyst
30 wt % M@
NFOPCN
45 wt % M@
NFOPCN
50 wt % M@
NFOPCN
40 wt % M@
NFOPCN
xenon arc lamp (l >
420 nm)
xenon arc lamp (l >
420 nm)
xenon arc lamp (l >
420 nm)
xenon arc lamp (l >
420 nm)
prepared
photocatalyst
prepared
photocatalyst
prepared
photocatalyst
prepared
photocatalyst
headed for the degradation of pollutants. Trapping experi-
ments were performed to identify their effects. During this
study, isopropyl alcohol (IPA), 1,4-benzoquinone (p-BQ),
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Figure 17. (a) Rate of H2 evolution shown by various prepared catalysts. (b) Run graph showing the photostability toward H2 evolution.
ethylenediaminetetraacetic acid (EDTA), and dimethyl sulf-
photocatalyst using a UV spectrophotometer. As shown in
Figure 16c, the concentration of NBT appeared to be
•
•
−
oxide (DMSO) were used as scavengers to arrest OH, O2 ,
h+, and e−, respectively.10 Without any scavenger, the
degradation efficiency of M@NFOPCN for 10 ppm of CIP
was 90% in 60 min and increased to almost 98% for DMSO as
the scavenger. Conspicuously, 5 mM IPA and p-BQ decreased
the degradation efficiency to almost 35% and 55%,
respectively. By using EDTA as the scavenger, the degradation
was found to be almost the same as that of the neat
photocatalyst.52 Hence, it was observed that, after the addition
of IPA and PBQ, around 55% and 35% of the CIP removed
was quenched, indicating the significant contributions of the
•OH and •O2− radicals for CIP degradation. The consequence
of the percentage of degradation after using scavenging species
is displayed in Figure 16a. The types of reactive oxygen species
produced in the M@NFOPCN system are identified by the
scavengers.53 As p-BQ and IPA were added, there were
significant inhibitory effects on CIP degradation. From Figure
16a, it is clearly observed that •OH is one of the major
oxidative species for oxidizing CIP, obtained by the reaction
between •O2− and H2O. Hence, it is concluded that the major
active species should be •OH rather than •O2− and h+, which is
in good agreement with the obtained results.10
−
decreased, confirming the presence of •O2 during the
degradation process.
5.7. Proposed Z-Scheme Mechanism in M@NFOPCN.
O the basis of the obtained band energies and band-edge
positions from MS and Tauc plots as well as foregoing
analyses, three types of mechanisms for charge transfer can be
predicted [lane-1(a,b) and lane-2], as given in Scheme 3. M,
NFO, and P-CN of M@NFOPCN are excited under solar
light, and this thus allows their photoinduced electrons and
holes to accumulate on their CB and VB and permit them to
be channelized in the proper path, respectively. As was already
established in our previously published literature, there exists a
Z scheme at the interface of NFO and P-CN in 20 wt %
NFOPCN, proving it as one of the best photocatalysts. Further
taking into account the interfaces obtained by HRTEM of the
neat materials and composites, three types of charge transfer
can be considered within M, NFO, and P-CN, as shown in
Scheme 3. Through the lane-1a mechanism (i.e., type II), if M
would have attached to NFO, then charge transfer at the M−
NFO interface would have taken place by channelizing the
electrons from the CBM of M to that of NFO and holes in the
opposite direction. Therefore, there would be a drastic
•
5.5. Test for OH Radical Confirmation [Terephthalic
•
reduction in the formation of OH radicals due to the VB of
Acid (TA) Test]. TA was used as a probe reagent to confirm
•
M (1.68) being less positive than 1.99. However, this is against
the OH radicals as the major active species. Because TA is
•
the scavenger test, and the generation of OH was already
insoluble in a neutral or acidic medium, 5 mM TA was
dispersed in an equimolar amount of NaOH, followed by the
required quantity of catalyst, kept under solar light for 30 min,
and then subjected to PL measurement in order to confirm the
formation of 2-hydroxyterephthalic acid (TAOH). There was
no reaction between TA with other radicals and molecules
confirmed by the TA test. Hence, it can be confirmed here that
this type of lane-1 (a) charge transfer at the M−NFO interface
does not exist in M@NFOPCN.
Further, within M and P-CN, because the CBM of M
(−0.0012) is lower than that of P-CN (−0.84), electrons
migrate to M. Also, the VBM of M is less positive than that of
P-CN; hence, holes gathered at M, which makes it a
recombination center, as presented in lane-1b. This goes
against the experimental observations of the PL study.
Moreover, with this type of charge transfer, the feasibility of
•O2− and •OH formation at the M−P-CN interface is obsolete
because the CBM of M is more positive than the O2/•O2−
potential (−0.33 eV vs NHE) and the VBMs of M and P-CN
are less positive than OH/•OH (+1.99 eV vs NHE). However,
−
•
(•O2 , OH, and H2O2) in the solution, and it voluntarily
•
interacts with OH. The emission peak at λ = 424 nm proves
the formation of TAOH, as shown in Figure 16b. The higher
•
the intensity of the PL peak, the more the OH radicals are
formed. The obtained •OH radicals in H2O could be related to
the fluorescence intensity of TAOH:53 •OH + TA = TAOH.
−
5.6. Confirmatory Test for •O2 Radicals by a
Nitroblue Tetrazolium (NBT) Test. The formation of
−
•O2 radicals is confirmed by a NBT test as follows. A total
•
through NBT and TA tests, adequate triggering of OH and
of 0.01 g of the prepared catalyst M@NFOPCN was dispersed
in 10 mL of a prepared 5 × 10−5 M solution of NBT, and the
resulting solution was irradiated for 30 min under solar light.
Then the solution was analyzed after separation of the
−
•O2 radicals in the 40 wt % M@NFOPCN nanocomposite
confirms them as the major active species. Thus, the VBM of
•
NFO is confirmed as taking the lead on OH generation.
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Additionally, the CBM of NFO is more positive than the
wt % M on NFOPCN, the rate of H2 evolution increases to
924 μmol h−1 under visible light compared to NFOPCN (904
μmol h−1) and M (240 μmol h−1). The efficiency of the
catalyst is monitored based on the fact that the potential of the
CB should be more negative than the reduction potential of
H2O/H2 (0 V vs NHE, pH = 0) for H2 evolution.1,26,55 Out of
the working catalysts, P-CN and M are suitable for producing
H2 because of their CBM positions (−0.84 and −0.0012 eV).
Hence, it is clear from the results that all three components in
this ternary system are well participants for H2 evolution.
Again, the two binary Z-scheme entities during formation of
the CICZ system properly channelize and enhance the
separation of e− and h+. On the basis of our previous literature,
although NFO is inactive and P-CN is less active toward H2
evolution (234 mmol h−1) under visible-light irradiation,
NFOPCN is quite competent for producing H2 (904 μmol
h−1). H2 evolution by the as-prepared 30−50 wt % M@
NFOPCN nanocomposites, neat and M@NFO, is summarized
in Table 2. From the study, if the M interface would have
attached to NFO, then it would have resulted in negligible H2
production. However, because the result was different, M must
have interracially composited at P-CN side, confirming the
CICZ. Again from the rate of H2 evolution, the accumulation
of enough e− at the CB of P-CN is confirmed, thus further
giving a green signal to the CICZ. Hence, finally the electrons
from the VBMs of M and NFO through Z schemes
accumulated at the CBM of P-CN, enhancing H2 evolution.
O2/•O2− potential; hence, it is not capable of producing •O2 .
−
From the above results, the proposed lane-1a and -1b pathways
are not feasible for M@NFOPCN to be a good photocatalyst.
However, 40 wt % M@NFOPCN could degrade 90% of 20
ppm of CIP in 60 min. So, ultimately, it can be inferred that a
closed interface-coupled Z-scheme (CICZ) type of charge-
transfer mechanism should exist here. Thus, it further confirms
the existence of one Z scheme within NFO (VB, +2.1 eV; CB,
+0.4 eV), P-CN (VB, +1.8 eV; CB, −0.8 eV) and another Z
scheme within M and P-CN, resulting in an inverted CICZ
system.50 Thus, this type of system is beneficial with oxidant
(PCN), reductant (NFO), and magnetically recyclable
component M. At this point, both ferrites, NFO and M, are
good donors and P-CN is a good acceptor, working all
together under a wide range (UV to near-IR).54 Hence, the
introduction of M enables the dual Z-scheme charge-transfer
path to be accomplished through better separation efficiency
and stronger redox capability of photogenerated holes and
electrons. The oxidation and reduction of CIP are shown in
Scheme 4.
CIP through Its Oxidation and Reduction. The oxidation
and reduction of CIP are shown in Scheme 4.
Step 1: Photoexcitation of an e−/h+ pair.
Fe3O @NFOPCN + hν
4
→ Fe3O @NFOPCN(e−cb + h+νb)
(3)
(4)
(5)
(6)
(7)
4
Fe3O + hν → Fe3O (e− + h+)
4
4
M@NFOPCN + 2h+ + 2e−CBM(P‐CN) + h+VBM(M)
+ h+VBM(NFO)
P‐CN + hν → P‐CN(e− + h+)
NFO + hν → NFO(e− + h+)
NFO@P‐CN(e− + h+) → NFO(h+) + P‐CN(e−)
Step 2: Formation of a OH• radical
→ 2e−CBM(P‐CN) + 2H+ + H2( ↑ )
(13)
The plausible conversion efficiency of photocatalytic H2
evolution (924 μmol h−1) was calculated by means of the
following equation:
Fe3O @NFOPCN(h+νb) + H2O → H + •OH
NFO(h+) + H2O → H+ + •OH
(8)
(9)
4
apparent conversion efficiency
= (stored chemical energy/incident light energy) × 100
(14)
Step 3: Conversion of the adsorbed oxygen into the
superoxide radical
−
Fe3O @NFOPCN(e−cb) + O2 → •O2
stored chemical energy = combustion heat of H2
(ΔHc in kJ mol−1) × number of moles of H2
(10)
(11)
4
P‐CN(e−) + O2 → P‐CN + •O2
−
produced after the completion of reaction
•
−
•
(15)
Step 4: Neutralization of O2 to HO2 by protonation
−
•
•O2 + H+ → HO2
where
(12)
Step 5: Degradation of CIP by radicals
1
2
ΔHc = 285.8 kJ/mol for H2O → H2 + O2
•
•
−
•OH, HO2 , O2 + CIP → CO2 + H2O + other simpler
(16)
molecules
5.8. Proposed Photocatalytic H2 Evolution Mecha-
nism. An inferential schematic mechanism proposed for the
water splitting and photocatalytic H2 evolution of the as-
synthesized 30−50 wt % M@NFOPCN, M, and pristine
NFOPCN is shown in Scheme 3. All of the synthesized
composites are tested for H2 evolution under the effect of
visible-light irradiation (λ ≥ 400 nm). Under the same ambient
setting, a reference testing was performed only with pure
methanol before the proposed experiment was performed.
However, no evolution of H2 gas was found. After loading 40
The stored chemical energy was 0.073 × 10−6 mol s−1 × 285.8
kJ mol−1 = 0.000073 kJ = 0.073 W. Again, the incident light
energy was measured to be 70 mW cm−2 × 3.14 × (1.5)2 =
0.4947 W.
Hence, the apparent conversion efficiency of photocatalytic
H2 was 14%.
The extent of H2 furnished by the photocatalyst with
stability and time was evaluated through a reusability test. The
stability was found to be maintained for up to four runs, as
shown in Figure 17a,b.
P
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6. CONCLUSION
pubs.acs.org/IC
Article
different contaminants under visible light. J. Photochem. Photobiol., A
2019, 374, 161−172.
In summary, the investigation reports on the in situ fabrication
of a double-Z-scheme-based novel magnetically retrievable
Fe3O4@NiFe2O4/P-g-C3N4 nanocomposite, followed by com-
bustion and calcination. The formation of a double-Z-scheme-
based nanocomposite at the interface of Fe3O4, P-g-C3N4, and
NiFe2O4 was recognized by XPS, HRTEM, and scavenger test
analysis. Among the fabricated composites, the 40 wt % M@
NFOPCN nanocomposite showed superior activity under
optimized conditions. The remarkable photocatalytic activity
(90% CIP degradation) of M@NFOPCN compared to M and
NFOPCN can be assigned to functionalized e− and h+ charge
separation, which was justified by EIS and PL analysis. The
anodic photocurrent at +1.5 V around 1.87 mA cm−2 and SC
nature shown by M@NFOPCN further support it as a better
photocatalyst. A good amount of H2(energy) is confirmed with
14% conversion efficiency. Above all, the incorporation of M
significantly enhances the recyclability and stability of the
photocatalyst. The Ms value of 28 emu g−1 of M@NFOPCN
establishes it with superparamagnetic nature. Of course, with
higher weight percentage of M coupling, the photocatalytic
effectiveness is reduced due to agglomeration of M and thus
decreasing active sites. However, an effective construction like
M@NFOPCN can accomplish an excellent photocatalytic
performance with a recyclable, wide-bandwidth photocatalyst
under sunlight.
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AUTHOR INFORMATION
■
Corresponding Author
Kulamani Parida − School of Basic Sciences, Indian Institute of
Technology, Bhubaneswar 752050, Odisha, India;
orcid.org/0000-0001-7807-5561;
(12) Srivastava, M.; Ojha, A. K.; Chaubey, S.; Singh, J. In-situ
synthesis of magnetic (NiFe2O4/CuO/FeO) nanocomposites. J. Solid
State Chem. 2010, 183, 2669−2674.
Email: kulamaniparida@soa.ac.in
(13) Mahalingam, S.; Ahn, Y. H. Improved visible light photo-
catalytic activity of rGO−Fe3O4−NiO hybrid nanocomposites
synthesized by in situ facile method for industrial wastewater
treatment applications. New J. Chem. 2018, 42, 4372−4383.
(14) Shekofteh-Gohari, M.; Habibi-Yangjeh, A.; Abitorabi, M.;
Rouhi, A. Magnetically separable nanocomposites based on ZnO and
their applications in photocatalytic processes: a review. Crit. Rev.
Environ. Sci. Technol. 2018, 48, 806−857.
(15) Kumar, S.; Kumar, B.; T, S.; Baruah, A.; Shanker, V. Synthesis
of magnetically separable and recyclable g-C3N4−Fe3O4 hybrid
nanocomposites with enhanced photocatalytic performance under
visible-light irradiation. J. Phys. Chem. C 2013, 117, 26135−26143.
(16) Chen, L.; Huang, H.; Zheng, Y.; Sun, W.; Zhao, Y.; Francis, P.
S.; Wang, X. Noble-metal-free Ni3N/g-C3N4 photocatalysts with
enhanced hydrogen production under visible light irradiation. Dalton
Transactions. 2018, 47, 12188−12196.
Authors
Priti Mishra − School of Basic Sciences, Indian Institute of
Technology, Bhubaneswar 752050, Odisha, India
Arjun Behera − School of Basic Sciences, Indian Institute of
Technology, Bhubaneswar 752050, Odisha, India
Debasmita Kandi − School of Basic Sciences, Indian Institute of
Technology, Bhubaneswar 752050, Odisha, India
Satyajit Ratha − School of Basic Sciences, Indian Institute of
Technology, Bhubaneswar 752050, Odisha, India;
orcid.org/0000-0002-8708-2349
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.9b02996
(17) Mansingh, S.; Padhi, D. K.; Parida, K. Bio-surfactant assisted
solvothermal synthesis of Magnetic retrievable Fe3O4@ rGO
nanocomposite for photocatalytic reduction of 2-nitrophenol and
degradation of TCH under visible light illumination. Appl. Surf. Sci.
2019, 466, 679−690.
(18) Li, P.; Ma, R.; Zhou, Y.; Chen, Y.; Liu, Q.; Peng, G.; Liang, Z.;
Wang, J. Spinel nickel ferrite nanoparticles strongly cross-linked with
multiwalled carbon nanotubes as a bi-efficient electro catalyst for
oxygen reduction and oxygen evolution. RSC Adv. 2015, 5, 73834−
73841.
(19) Asadzadeh-Khaneghah, S.; Habibi-Yangjeh, A.; Seifzadeh, D.
Graphitic carbon nitride nanosheets coupled with carbon dots and
BiOI nanoparticles: boosting visible-light-driven photocatalytic
activity. J. Taiwan Inst. Chem. Eng. 2018, 87, 98−111.
(20) Shekofteh-Gohari, M.; Habibi-Yangjeh, A. Novel magnetically
separable ZnO/AgBr/Fe3 O 4/Ag 3VO4 nanocomposites with tandem
Notes
The authors declare no competing financial interest.
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