Photocatalytic performance of visible active boron nitride supported ZnFe2O4 (ZnFe2O4/BN) nanocomposites for the removal of aqueous organic pollutants

Article abstract of DOI:10.1039/d0nj01272f

Visible active novel ZnFe₂O₄/BN heterostructured nanocomposites with different wt% of BN nanosheets were prepared using a solvothermal method. Their structural, morphological and optical properties were studied by X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), Energy-Dispersive X-ray spectroscopy (EDX), Fourier-Transform Infrared Spectroscopy (FT-IR), UV-Visible spectrophotometry, Photoluminescence (PL) and X-ray Photoelectron Spectroscopy (XPS) techniques. The photocatalytic performance of ZnFe₂O₄/BN hybrid nanocomposites was evaluated towards oxidative degradation of organic pollutants such as Congo red (CR) and tetracycline (TC) under visible light irradiation. The porous BN nanosheets act as a catalytic support and enhance the photoactivity of ZnFe₂O₄ in the nanocomposite. The results also indicated that the presence of BN nanosheets lowers the recombination rate and shows significantly improved separation of the photo-induced electron-hole pairs in ZnFe₂O₄/BN when compared to bare ZnFe₂O₄. Among the different ZnFe₂O₄/BN nanocomposites, 9.3 wt% of BN loaded ZnFe₂O₄ hybrid was found to show the most efficient photocatalytic activity towards the degradation of Congo Red and tetracycline under visible light irradiation. The apparent rate constant values of ZnFe₂O₄/9.3% BN were found to be three and five times higher than that of bare ZnFe₂O₄ in the degradation of CR and TC respectively.

Full text of DOI:10.1039/d0nj01272f

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Photocatalytic performance of visible active boron nitride supported ZnFe2O4
(ZnFe2O4/BN) nanocomposites for the removal of aqueous organic pollutants
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Elangovan Erusappan a, Sivakumar Thiripuranthagan a*, Mani Durai b,
Sakthivel Kumaravel a and Thanigaivel Vembuli a
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aCatalysis Laboratory, Department of Applied Science and Technology, A.C. Tech, Anna
University, Chennai, Tamilnadu, India
bCentre for Nanoscience and Technology, A.C. Tech, Anna University, Chennai, Tamilnadu, India
*Corresponding author Email: tssivakumar@yahoo.com
Graphical Abstract

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Highlights
✓ ZnFe2O4/BN nanocomposites were synthesized by solvothermal method.
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✓ The HR-TEM and FE-SEM/EDX images revealed uniform dispersion of ZnFe2O4
nanospheres on the surface of BN nanosheet.
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✓ Photoluminescence studies showed lower electron-hole pair recombination.
✓ ZnFe2O4/9.3% BN nanocomposite catalyst showed outstanding photocatalytic activity
compared to bare and nanocomposite catalysts.
✓ COD and TOC studies confirm the formation of non-toxic fragments.
Abstract
Visible active novel ZnFe2O4/BN heterostructured nanocomposites with different wt % of
BN nanosheets were prepared by solvothermal method. Their structural, morphological and optical
properties were studied by X-ray diffraction (XRD), Scanning Electron Microscopy (SEM),
Energy-Dispersive X-ray spectroscopy (EDX), Fourier-Transform Infrared (FT-IR), UV-Visible
spectrophotometer, Photo Luminescence (PL) and X-ray Photoelectron Spectroscopy (XPS). The
photocatalytic performance of ZnFe2O4/BN hybrid nanocomposite was evaluated towards
oxidative degradation of organic pollutants such as Congo Red (CR) and Tetracycline (TC) under
visible light irradiation. The porous BN nanosheets act as catalytic support and enhance the
photoactivity of ZnFe2O4 in the nanocomposite. The results also indicated that the presence of BN
nanosheets lowers the recombination rate and shows significantly improved separation of the
photo-induced electron-hole pairs in ZnFe2O4/BN when compared to bare ZnFe2O4. Among the
different ZnFe2O4/BN nanocomposites, 9.3 wt % of BN loaded ZnFe2O4 hybrid was found to show
the most efficient photocatalytic activity towards the degradation of Congo red and Tetracycline
under visible light irradiation. The apparent rate constant values of ZnFe2O4/9.3% BN were found
to be three and five times higher than bare ZnFe2O4 in the degradation of CR and TC respectively.
Keywords: ZnFe2O4/BN, nanocomposites, photocatalysts, Congo Red (CR) and Tetracycline
(TC)

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1. Introduction
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Over the past few years research has been focused on photodegradation of organic
pollutants present in wastewater using Advanced Oxidation Processes (AOPs) involving
semiconductor photocatalysts such as TiO2, ZnO, Fe2O3, etc.,1-10 These semiconductors have
several advantageous characteristics which include low cost, abundant and non-toxicity. However,
some of their inherent properties limit their widespread practical applications.11-15 Hence it is
imperative to address the limitations of semiconductor based photocatalyst. While the wide
bandgap semiconductors require light of intense energy (which is economically not attractive), the
narrow band gap semiconductors show quick recombination of e-/h+. When both these limitations
are addressed, there comes an issue of separation of catalyst and the cost. Hence the new
heterostructured catalyst should meet the requirements of visible activity, low or zero
recombination of electrons, ease of recovery and should be cheap. Designing & developing low
cost, visible light-driven catalyst is the need of the hour. ZnFe2O4 with its spinel structure is a
narrow band gap semiconductor (1.9 eV). Owing to the attractive features of ZnFe2O4 such as low
cost, high photochemical stability, optimal band gap to be driven by visible radiation, unique
electrical & optical properties and more importantly its magnetically separable nature, it has been
extensively used as photocatalyst & water-splitting catalyst.16- 22 Its magnetically separable nature
makes it as a recyclable, cost-effective catalyst. In spite of all the above, ZnFe2O4 still does not
satisfy the requirements for practical applications due to its limited adsorption property, poor
dispersion in aqueous solution & fast recombination.23-28 In order to address the limitations of
ZnFe2O4, it is proposed to develop a 2D heterostructured ZnFe2O4 hybrid with graphene-like boron
nitride (BN).29-33
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Boron nitride is another fascinating 2D material like graphene as thin layer of BN exhibits
many wonderful thermal, electrical, magnetic & metallic properties. BN also called as white
graphene and non-carbon graphene is a wide band gap material (5.2-5.5ev) 34,35 with high surface
area (3300 m2/g-1 ) 36 and high thermal conductivity (600W m−1 K−1) and hence has great potential
for various applications in the areas of adsorption,37,38 catalysis39,40 and energy storage devices.41,42
As BN is a very good electron transporter-like graphene, its presence along with a photocatalyst
will aid electron transport to the surface whereby the transient species are generated for the
degradation of organic pollutants. Hence BN has been coupled with several photocatalysts
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BiOBr/BN,43 Fe2O3/BN, 44 AgBr/BN,45 TiO2/BNNT,46 BN/AgPO4, ZnO/BN,48 TiO2/h-BN,49 g-

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C3N4/h-BN,50 etc either to improve the visible activity or to prevent recombination or for both.
However, to the best of our knowledge, only little and scarce attempts have been made with the
heterostructured ZnFe2O4 hybrids.
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Herein, we report the construction of heterostructure comprising of a spinel
semiconductor oxide (ZnFe2O4) and BN support (ZnFe2O4/x% BN where x: 6.4, 9.3 and 12.1%).
The synthesized nanocomposites were characterized by various physicochemical & instrumental
techniques and finally subjected towards the photodegradation of congo red & tetracycline under
visible irradiation. ZnFe2O4/BN heterostructures show enhanced absorption in the visible region
when compared to the bare spinel oxide nanospheres. The kinetics studies reveal that the
degradation followed pseudo-first-order kinetics and based on the results a possible mechanism
was also proposed.
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2. Experimental
2.1 Materials
Boric acid (98 % purity), urea (≥99 % purity), NH4HCO3 (≥99.7 % purity), zinc acetate
dihydrate (≥ 99 % purity) were obtained from Sisco Research Laboratory Chemicals, India. Ferric
nitrate monohydrate (99.99 % purity) and ethylene glycol were purchased from Sigma Aldrich,
India. Congo Red (98 % purity) dye and Tetracycline were purchased from Colourise Industries,
India.
2.2 Synthesis of Boron nitride (BN)
BN was synthesized by using boric acid and urea as reported elsewhere.51 In a typical
synthesis, boric acid and urea in the ratio of 1:24 were taken dissolved in water and heated at 65
oC overnight. The dried white solid obtained was then heated at 900 oC for 6 hr in N2 atmosphere
in the tubular furnace to get white BN powder.
2.3 Synthesis of ZnFe2O4/BN
The synthesized BN (50 mg) was dispersed in 30ml ethanol and sonicated for 30 min (A)
then, 3 mmol of Zn (CH3COO)2.2H2O and 6 mmol Fe (NO3)3.9H2O were dissolved in 120ml of
ethylene glycol (B). The two solutions A and B were mixed together and 90 mmol NH4HCO3 was
added to the reaction mixture at room temperature. The whole reaction mixture was stirred
vigorously for 30 min. The solution was then transferred into Teflon-lined stainless-steel autoclave
and subjected to solvothermal treatment at 180 oC for 24 hr. The obtained precipitate was filtered,

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washed with ethanol followed by deionized water for several times and dried at 60 oC. Further, the
powder was finally calcined at 450 oC for 10 hr to obtain ZnFe2O4/BN 6.4 Wt % nanocomposite.
In the same way, ZnFe2O4/BN 9.3 Wt % and ZnFe2O4/BN 12.1 Wt % nanocomposites were also
prepared by taking 75 and 100 mg of BN respectively. The bare ZnFe2O4 was also synthesized by
the above procedure but without adding BN.
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2.4 Characterization
Powder X-Ray diffraction (XRD) patterns of the solid products were analyzed using the
Power X-ray diffractometer (Rigaku, Miniflex) with graphite-monochromatized Cu Kα radiation
at 30KV and 15mA. UV-vis diffuse reflectance spectra of the obtained samples were analyzed on
a UV-vis spectrophotometer (JASCO V-750, Jasco, Japan) with BaSO4 as reference. Nitrogen
adsorption-desorption isotherms were obtained at -196 °C using a (BEL SORP-mini, Beljan). Prior
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to the adsorption measurement, the sample (ca.0.1g) was evacuated at 150 C for 12 hr. FT-IR
spectra were recorded using Thermo Scientific NICOLET 4700 with resolution 4 cm-1 at room
temperature after pelletizing the sample with KBr salt. Photoluminescence spectra were recorded
by using Micro-PL analyzer using 320 nm excitation wavelength (MORIBA LabRam HR). The
specific surface area, pore size distribution and pore volume distribution were obtained from N2
adsorption-desorption isotherms of the samples using a volumetric sorption analyzer
(Micromeritics ASAP 2020, Norcross, GA, USA). The morphology was observed using a Hitachi
S-8000 Scanning Electron Microscope (FE-SEM) coupled with energy-dispersive X-ray (EDX)
analyzer. Transmission Electron Microscopy (TEM) images were taken using Hitachi H9500 TEM
at an acceleration voltage of 300 kV.
2.5 Evaluation of Photocatalytic activity
The photocatalytic activities of the synthesized ZnFe2O4/BN nanocomposite catalysts were
evaluated towards the degradation of CR dye and TC antibiotic under visible light irradiation. The
initial concentration of 10 ppm of dye, antibiotic was taken for the present study. A 500 W tungsten
lamp was used as the visible light source which was located inside a quartz cell with a circulating
water bath. 50 mg of catalyst was dispersed in 100 ml of dye/antibiotic and kept in the dark for 30
min to reach adsorption-desorption equilibrium between catalyst and pollutants. Then the
suspension was exposed under visible light irradiation. During the reaction, 3 ml of the solution
was withdrawn at definite time intervals, centrifuged and analyzed by UV-vis spectrophotometer

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(Hitachi U 2000). The % degradation of CR dye and TC antibiotic was determined by using the
following equation:52
D %= C0-Ct/C0 × 100
(1)
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C0= initial concentration and Ct=concentration at time T
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In order to check whether the decolorization was followed by degradation or not, Chemical
Oxygen Demand (COD) and total organic carbon (TOC) analysis were also made by using
Shimadzu TOC-L analyzer. The degradation percentage of dyes was calculated following the
procedure as report by as and other 53,54
3. Results and Discussion
3.1 XRD analysis
The XRD patterns of bare BN, ZnFe2O4, and ZnFe2O4/BN nanocomposites were recorded and
shown in Fig 1. The characteristic diffraction peaks at 2θ = 18.25°, 30.06°, 35.15°, 42.9°, 53.2°,
56.8°, 62.3°, 70.50°, 73.5°, 78.7°, 86.4°, 89.13° correspond to (111), (220), (311), (400), (422), (511),
(440), (620), (533), (444), (642) and (731) planes of ZnFe2O4, respectively (JCPDS card no.22-
1012).55,56
Figure 1
XRD patterns of (A) bare BN, (B) bare ZnFe2O4 and ZnFe2O4/x% BN
composite catalysts (x = 6.4, 9.3 and 12.1%)
The wide-angle diffraction peaks at 2θ value 26.2° and 42.3° are assigned to (002) and (100)
planes of hexagonal BN (JCPDS 85-1068).57 However, there was no typical pattern of BN

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observed in the composites which may be due to the low BN content in the samples.58 The absence
of any other peak confirms the high purity of the composite. The average crystalline size of
prepared materials was determined by using the following formula
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D=K/β cos
(2)
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Where D is the average crystalline size, K is constant (0.89), -is the wavelength of the Cu kα
radiation (λ=0.15418 nm), β-is full width at half maximum and -is the diffraction angle. The
average grain sizes of the ZnFe2O4/x% BN (x= 6.4, 9.3 and 12.1%) composites are given in Table
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Table 1
Physicochemical characteristics of the synthesized catalysts.
Catalysts
Crystalline sizea
Band gapb
(eV)
Surface areac
Pore sizec
(nm)
Pore volumec
(cm3/g)
0.25
(nm)
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(m2/g)
ZnFe2O4
ZnFe2O4/6.4% BN
ZnFe2O4/9.3% BN
ZnFe2O4/12.1% BN
a By XRD
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31.04
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35.18
0.30
b By UV DRS
c By BET
3.2 FT-IR Spectra
FT-IR analysis was carried out to find out the surface functional groups and chemical bonding
present in the catalysts. Figure 2 shows the FT-IR spectra of bare and composite catalysts. The
bands at 3424 cm-1 and 1620 cm-1 are associated with O-H stretching and O-H-O bending
vibrations of the surface adsorbed water molecules.59 The characteristic stretching vibration band
observed between 400- 500 cm-1 in bare ZnFe2O4 is attributed to the Fe-O stretching vibration.60
The FTIR spectrum of BN shows two main characteristic absorption bands at 1378 cm-1 and 805
cm-1 corresponding to the B-N stretching and B-N-B out of plane bending vibrations
respectively.61,62

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Figure 2
FT-IR spectra of bare BN, ZnFe2O4 and ZnFe2O4/x% BN composite catalysts
(x = 6.4, 9.3 and 12.1%)
3.3 BET Surface area
The N2 adsorption-desorption isotherms were constructed and their corresponding Barrett-
Joyner-Halenda (BJH) pore size distribution curves were plotted for bare ZnFe2O4 and ZnFe2O4/
BN (6.4%, 9.3% and 12.1%) nanocomposites to get an insight into their porous nature (Fig.3a).
Type IV N2 adsorption-desorption isotherms with hysteresis loop obtained clearly indicates the
porous nature of ZnFe2O4/BN (6.4%, 9.3% and 12.1%) nanocomposites and the considerable
hysteresis loop at high relative pressure (P/P0) range of 0.60–1.00 confirms that the BN and porous
ZnFe2O4 nanospheres coexist in the synthesized samples.63

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Figure 3
(a) N2 adsorption-desorption isotherms and (b) pore size distribution (PSD)
plots of bare ZnFe2O4 and ZnFe2O4/x% BN composite catalysts (x = 6.4, 9.3
and 12.1%)
The Brunauer-Emmett-Teller (BET) specific surface area values of the catalysts were
calculated to be 33, 32, 34 and 35 m2 g-1 for bare ZnFe2O4 and ZnFe2O4/ BN (6.4%, 9.3% and
12.1%) nanocomposites catalyst respectively. The bare BN also showed a very low surface area
of 52 m2/g (Measured by Single point method) due to the high calcinations temperature (900 Deg)
employed in the synthesis. Their corresponding BJH pore size distributions were found in the range
of 8-28 nm (Table 1) (Fig. 3b). This wider pore size distributions can be attributed to the irregular
shapes and sizes of the pores derived from the accumulation of nanospheres.
3.4 XPS analysis
To investigate the valence and chemical state of the elements present in ZnFe2O4/9.3% BN
catalyst XPS analysis was performed. Fig 4a. demonstrates the survey spectra of the ZnFe2O4/9.3%
BN nanocomposite. Fig.4 (b-f) shows high-resolution core-level spectra of Zn, Fe, O, B, and N
present in the composite. The high resolution spectra of Zn and Fe [Fig.4 (b & c)] show
characteristic Zn 2p 3/2, Zn 2p1/2 and Fe 2p3/2, Fe 2p1/2 peaks of Zn2+ and Fe 3+ ions with the binding
energies of 1022 eV, 1045 eV (for Zn2+) and 710.6 eV, 724 eV (for Fe3+).64,65 The additional peaks
observed around 720.5 eV and 731.5 eV corresponding to Fe 2p3/2 and Fe 2p1/2 s are satellite
peaks.66 The peak at 530.1 eV is attributed to O 1s (Fig 4 (d)). The peaks of B 1s and N 1s appeared
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at the binding energies of 198.8 eV and 398.8 eV
as shown in Fig. 4 (e & f) respectively.

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These observations are in agreement with the previous reports and hence this study confirmed that
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BN has been successfully attached with ZnFe2O4.
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Figure 4
XPS (a) Full survey spectrum and (b-f) core level spectra of ZnFe2O4/9.3% BN
catalyst

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3.5 Morphological study
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FE-SEM was used to know the surface morphology and to determine the particle size and
shape of bare ZnFe2O4, BN and ZnFe2O4/BN composites (Fig. 5). In the formation of ZnFe2O4
nanospheres, the replacement of Fe3+ ion with Zn2+ at the tetrahedral site takes place as Zn2+ always
prefers the tetrahedral sites. The ion exchange by Kirkendall effect was also reported between Zn2+
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ion and Fe2+ ion Fe3O4. Herein the synthesized bare ZnFe2O4 hollow spheres show the crystal
diameter of 170 nm ( Fig 5c) whereas the same ZnFeO4 in the composites have the particle size
less than 80 nm (Fig 5f)
Figure 5
FE-SEM images of (a-c) ZnFe2O4 (d) BN (e & f) ZnFe2O4/9.3% BN composite
catalysts
.
The EDAX mapping further confirms the intact nature of the composites and the presence of
all the elements such as B, N, O, Zn, & Fe. (Fig. 6).

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Figure 6
EDAX mapping of ZnFe2O4/9.3% BN composite catalyst
The TEM and HRTEM images were also recorded to get detailed morphology of
ZnFe2O4/9.3% BN composite catalyst and they are shown in Fig. (7 a-c). It further confirms the
nano hollow spherical morphology of ZnFe2O4 with average particle diameter in the range between
10 to 20 nm dispersed in BN 2D sheet. However few particles with diameter around 60 to 70 nm
were also seen in the TEM image.

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Figure 7
(a-c) HR-TEM (d) SAED pattern of the most active ZnFe2O4/9.3% BN
composite catalyst
3.6 DRS UV-vis spectra
Fig. 8a shows the UV-Vis diffuse reflectance spectra of bare ZnFe2O4 and ZnFe2O4/BN (x = 6.4,
9.3 and 12.1%) nanocomposites. These absorption spectra were used to investigate the effect of
BN modification on the electronic structure of ZnFe2O4. All the catalysts exhibit the absorption
edges in the visible region. BN modified ZnFe2O4 nanocomposites show a steep edge in the visible
region can be attributed to intrinsic band transition.71,72 All are visible active as they absorb > 700
nm.73 Incorporation of BN in ZnFe2O4 has marginal effect on the bang gap value. The presence of
BN prevents recombination as it is a good electron transporter.

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Figure 8
(A) UV-vis Diffuse reflectance spectra and (B) Band gap calculations of bare
ZnFe2O4, and ZnFe2O4/x% BN composite catalyst (x=6.4, 9.3 and 12.1%)
Fig. 8b shows the band gap energy of prepared materials calculated by plotting (αһν)2 Vs
photon energy (TAUC plot).74,75 The calculated band gap values of ZnFe2O4, ZnFe2O4/6.4% BN,
ZnFe2O4/9.3% BN and ZnFe2O4/12.1% BN were found to be 1.77 eV, 1.68 eV, 1.75 eV and 1.70
eV respectively.
3.7 PL spectra
The rate of recombination of electrons and holes in a semiconductor can be studied by the PL
emission spectra. Generally, the lower PL intensity indicates the lower recombination of the photo-
generated carriers and higher photocatalytic activity.76 Fig. 9 shows the PL spectra of bare ZnFe2O4
and ZnFe2O4/BN composite catalysts. The bare ZnFe2O4 shows a high-intensity emission peak
around 360 nm and the intensity of this emission peak decreased significantly for composites and
among the composites ZnFe2O4/9.3% BN showed the peak with the lowest intensity indicating
lowest recombination of photo-generated carriers. The result clearly indicated that the introduction
of boron nitride improves the separation efficiency of photo-generated electron-hole pairs and its
presence is expected to enhance the photocatalytic activity.

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Figure 9
Photoluminescence spectra of bare ZnFe2O4 and ZnFe2O4/x% BN composite
catalysts (x = 6.4, 9.3 and 12.1%)
3.8 Electrochemical impedance spectra
The charge transfer resistance and recombination properties of the prepared composites were
evaluated by using electrochemical impedance spectroscopy (EIS). The EIS spectra were recorded
in the range of 1-100 MHz at an amplitude of 10 mV. Fig 10 shows the Nyquist plots of ZnFe2O4,
ZnFe2O4/6.4% BN, ZnFe2O4/9.3% BN and ZnFe2O4/12.1% BN. In general, lower the radius of the
arc, lower is the electron transfer resistance.77,78 Among all the prepared catalysts, ZnFe2O4/9.3%
BN composite catalysts showed the arc with a smaller radius than bare ZnFe2O4 and ZnFe2O4 BN
loaded catalysts. This reveals that the electron transfer resistance is the lowest in ZnFe2O4/9.3%
BN composite catalyst. Thus, EIS studies indicate efficient charge separation and better electron
transport character due to the incorporation of 9.3% BN over ZnFe2O4 catalysts.

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Figure 10
Impedance spectra of bare ZnFe2O4 and ZnFe2O4/x % BN composite catalysts
(x = 6.4, 9.3 and 12.1%).
3.9 TG and DTA
Thermal stability of the most active catalyst ZnFe2O4/9.3% BN was determined by TG and
DTA measurements in nitrogen atmosphere from ambient temperature to 1000 °C at a heating rate
of 10 °C min and the thermograms are shown in Fig.11. The result clearly indicates that the
composite catalysts are highly stable and about 3% wt loss was observed in the entire temperature
range due to moisture and residual organic species that may be present.79 The first weight loss that
occurs below 110 °C is due to loss the of physically adsorbed water on the surface of the catalyst
and it is about. 1.5%. Another 1% second mass loss that occurs between 110°C and 520 °C may
be due to the combustion of residual organic group if any present.

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Figure 11
TG-DTA thermograms of ZnFe2O4/9.3% BN composite catalyst
4. Photocatalytic activity
The photocatalytic activities of the as-prepared bare ZnFe2O4 and ZnFe2O4/BN
nanocomposites were evaluated towards the degradation of Congo red (CR) and Tetracycline (TC)
under visible light irradiation. Before illumination, the adsorption experiments were first
conducted in the dark for 30 min and after the adsorption-desorption equilibrium the catalysts were
evaluated for their photocatalytic activities towards the decolorisation of CR and TC and the results
are shown in Fig 12.

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Figure 12
Photodegradation of (a) CR and (b) TC by the as-synthesized catalysts
Fig. 12 clearly indicates that the photolysis experiment (without the catalyst) resulted in
insignificant degradation of CR and TC. Bare ZnFe2O4 catalyst showed better degradation than
obtained in the photolysis experiment but showed less degradation when compared to composite
catalysts. The catalytic activities of the composite catalysts were found to be significantly high due
to the presence of 2D material (BN sheet). Among the composite catalysts, ZnFe2O4/9.3% BN
showed the highest photocatalytic activity in the degradation of Congo red (92%) and Tetracycline
(80%) after irradiation for 240 and 180 min respectively.
The rates of photocatalytic reaction in the degradation of dye and antibiotic catalyzed by
both bare and composite catalyst were calculated by modified Langmuir-Hinshelhood
expression:11
kapp. t = -ln (Ct/C0)
(3)
Where, kapp -apparent reaction rate constant, C0 - initial concentration of organic dye, Ct –
concentration of dye at the reaction time. The kinetic plots are shown in Fig.13.

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Figure 13
Kinetic plots in the photodegradation of (a) CR and (b) TC
by the as-synthesized catalysts
The straight lines in the figure indicate that the degradation followed pseudo-first-order reaction.
From the slopes, the rate constant (min−1) and t1/2 values were determined for each catalyst and
given in Table 2. Among them, the rate constant values of ZnFe2O4/9.3% BN were found to be
0.00982, 0.01019 min−1 in the photodegradation of dye (CR) and antibiotics (TC) respectively.
The highest rate constant of ZnFe2O4/9.3% BN may be attributed to the significant synergistic
effect between ZnFe2O4 and Boron Nitride sheets.
Table 2
Kinetic parameters and R2 values obtained in the degradation of CR and TC.
Catalysts
Congo Red
Tetracycline
Kapp (min-1) t1/2 (min)
R2
Kapp (min-1) t1/2 (min)
R2
Blank
0.00042
0.00304
0.00692
0.00982
1650
227.96
100.14
70.57
0.99064
0.99695
0.95107
0.96695
0.00035
0.00282
0.00469
0.01019
1980
245.13
147.76
68.02
0.97394
0.98764
0.93696
0.98327
ZnFe2O4
ZnFe2O4/6.4% BN
ZnFe2O4/9.3% BN
ZnFe2O4/12.1% BN
0.00840
82.51
0.94555
0.00767
90.35
0.96966

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The UV-Visible absorption evolution spectra of CR and TC obtained over ZnFe2O4/9.3%
BN composite catalyst at different time intervals are shown in Fig. 14. It shows that the absorbance
of Congo red and Tetracycline decreased gradually at their irrespective λ max values 497 & 363
nm respectively with increase in the reaction time over ZnFe2O4/9.3% BN composite under visible
light irradiation. Almost complete degradation of dye and significant degradation in the case of
antibiotics were observed.
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Figure 14
Absorption evolution spectra of (a) CR and (b) TC over ZnFe2O4/9.3% BN
catalyst under visible light irradiation at different intervals of time.
To analyze the % photo mineralization in the photocatalytic degradation process, the total
organic carbon (TOC) and Chemical Oxygen Demand (COD) analysis were carried out for the
samples catalyzed by the active composite catalyst i.e ZnFe2O4/9.3 % BN under optimized reaction
conditions. The samples were collected at different time intervals such as 30,120 and 240 min for
CR and 30,120 and 180 min for TC were analyzed for their TOC content and COD made and the
corresponding percentage mineralization values are given in Table.3. The TOC and COD values
were found to be lower than the % decolorisation. The result suggested that the dye and antibiotic
molecules are not completely degraded into minerals such as CO2, H2O during the photocatalytic
reaction and the product mixture may contain petite amounts of various organic salts such as
formates, acetates,etc.,80,81

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Table 3 % Degradation and % Mineralization of Congo red and Tetracycline over
ZnFe2O4/9.3 % BN catalyst at different time intervals.
Congo red
Tetracycline
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Reaction time (min)
% degradation by visible
spectrophotometry
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240
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% mineralization by TOC
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% mineralization by COD
The economic feasibility and photostability of the catalyst were analyzed by recyclability
studies. After the first run, ZnFe2O4/9.3% BN composite catalyst was filtered, washed with de-
ionized water and ethanol and dried for 60 oC and reused for another four successive photocatalytic
results. The activities obtained after each run are shown in Fig 15. It can be observed that there
was almost no activity loss even after four repeated runs.
Figure 15
Photo-stability of ZnFe2O4/9.3% BN composite catalyst in the degradation of
(a) CR and (b) TC.

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The spent ZnFe2O4/9.3% BN photocatalyst were further characterized by XRD and
FESEM. There was no observable change in the morphology of the FESEM image as shown (Fig.
16). This further confirms the high stability and durability of the photocatalyst.
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Figure 16
(a) XRD patterns and (b) HR-SEM image of used ZnFe2O4/9.3% BN
composite catalyst
4.1 Trapping experiments
In order to determine the main active species responsible for the degradation of CR dye the
radical trapping experiments were conducted over ZnFe2O4/ BN composite catalyst. Three
different quenchers viz., tert-butanol, ammonium oxalate (AO) and p-benzoquinone were used as
+
–
the hydroxyl radical (•OH), hole (h ) and superoxide radical anion (O2 ) scavengers,
respectively.82 Fig.17 shows the degradation of CR dye over ZnFe2O4/9.3% BN composite with
different scavengers.
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Figure 17
Effect of quenchers in the photocatalytic degradation of CR over
ZnFe2O4/9.3% BN composite catalyst under visible light irradiation
•
+
–
•
The results indicate that all the above active transient species (OH , h , O2 ) are
responsible for the oxidation of CR dye but to different extents. Among them, p-benzoquinone
–
•
reduced the activity from 80% to 66% which clearly proves that superoxide radical anion (O2 ) is
the most active oxidizing species in the CR decolorisation. The addition of ammonium oxalate
also reduces the degradation percentage from 80 to 55 of CR which indicates that the hole (h+) is
the second dominant oxidizing species in this photocatalytic degradation process. The addition of
tert-butanol to the reaction system has very less influence on CR degradation which indicates that
hydroxyl radicals play a minor role in this photocatalytic system.
4.2 Photocatalytic mechanism
On the basis of the above experimental results, a possible visible light photocatalytic
mechanism of CR degradation by ZnFe2O4/BN composites is proposed (Fig. (14)). Upon the
irradiation of ZnFe2O4/BN composite, photo-induced electrons and holes are generated. The
photoinduced electrons are then excited from the valence band (VB) to the conduction band (CB)

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of ZnFe2O4 whereby the electron are then transferred to the surface of BN sheets,83 leaving the
holes to the VB of ZnFe2O4. The high surface area of BN coupled with its excellent transportation
property made it easier to transfer the photoinduced electrons to the adsorbed oxygen and produce
8
9
–
•
superoxide radical anion (O2 ), which oxidizes the harmful organics. As observed in the above
experiment, hydroxyl radicals do not play a major role in this photocatalytic system because the
VB of ZnFe2O4 is negative than the standard redox potential of E0 = (–OH/•OH) (1.99 eV vs NHE),
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suggesting that the generated holes in ZnFe2O4 cannot directly oxidize H2O to the hydroxyl
radicals (•OH).
Figure 18
A possible degradation mechanism of ZnFe2O4/BN nanocomposite in the
photocatalytic reaction.
5 Conclusion
Bare ZnFe2O4 nanospheres and ZnFe2O4/BN heterostructured nanocomposites with
different BN nanosheets amounts were synthesized successfully by solvothermal method. The
successful incorporation of BN with ZnFe2O4 was confirmed by XRD, FTIR as well XPS and

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EDAX mapping. Photoluminescence spectra confirm that the presence of nanosheets decreases
the recombination of e-/h+ pairs due to its excellent transport property and hence effectively
increases photocatalytic activity under the visible light when compared to bare ZnFe2O4
nanospheres. The enhanced photocatalytic activity is due to higher charge separation efficiency of
photogenerated electron-hole pair by the introduction of graphene-like BN nanosheets.
ZnFe2O4/BN heterostructured nanocomposites could serve as better photocatalyst to remove the
organic pollutants present in the wastewater and other ecosystems.
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Conflicts of interest
There are no conflicts to declare.
Acknowledgment
The third author is grateful for CSIR through SRF (Award letter No. 09/468/0516/2018
EMR-I, 13/04/2018) and the last author thank DST-TPD (Ref. No. DST/TDT/TDP-06/2017) for
financial support.
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