Journal of Science: Advanced Materials and Devices 3 (2018) 331e341
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Journal of Science: Advanced Materials and Devices
journal homepage: www.elsevier.com/locate/jsamd
Original Article
Facile synthesis of MnWO4/BiOI nanocomposites and their efficient
photocatalytic and photoelectrochemical activities under the visiblelight irradiation
V. Ramasamy Raja a, b, A. Karthika a, A. Suganthi a, c, *, M. Rajarajan d, **
a
P.G & Research Department of Chemistry, Thiagarajar College, Madurai 625 009, Tamil Nadu, India
K.P. National College of Arts and Science, Batlagundu 624 202, Tamil Nadu, India
Mother Teresa Women's University, Kodaikanal 624 102, Tamil Nadu, India
d
Madurai Kamaraj University, Madurai 625 021, Tamil Nadu, India
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 8 April 2018
Received in revised form
17 June 2018
Accepted 8 July 2018
Available online 24 August 2018
The novel MnWO4/BiOI nanocomposite materials were successfully synthesized by a precipitation
deposition method. The as-prepared photocatalyst was characterized by XRD, SEM, EDAX, UV-DRS, FT-IR,
TEM and BET techniques. The as-prepared MnWO4/BiOI nanocomposites were further utilized to study
the degradation of the Celestin blue aqueous solution under visible-light irradiation. Absorption range
and band gap energy, which are responsible for the observed photocatalyst behavior, were investigated
by the DRS spectroscopy. The photocatalytic test suggested that MnWO4/BiOI nanocomposites possess a
higher activity for the degradation of these pollutants than the pure BiOI and MnWO4 under the visiblelight irradiation. Among the as-prepared nanocomposites, the one of 3% MnWO4/BiOI displays the best
photocatalytic activity of the degradation. Factors, such as the effect of the catalyst dosage, the solution
pH and the initial dye concentration affecting the photocatalytic activity were investigated. The investigations of the adsorption kinetics and isotherm demonstrate that the adsorption process follows the
pseudo-first-order kinetic model and the Langmuir adsorption isotherm, respectively.
© 2018 Publishing services by Elsevier B.V. on behalf of Vietnam National University, Hanoi. This is an
open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
Keywords:
MnWO4-BiOI
Nanocomposite
Visible light
Celestin blue
Photocatalytic activity
1. Introduction
With the rapid development of economy and society, water
contamination has been an increasing environmental problem for
humans [1,2]. To solve this trouble, abundance of efforts have been
devoted to develop efficient methods for decomposing environmental pollutants, such as filtration, solvent extraction, chemical
oxidation, adsorption, floatation and photocatalytic degradation.
Photocatalytic approaches have been devoted to extensive studies
on the environmental purification [3e7].
Transition metal tungstates belong to an important family of
inorganic materials which have potential application in versatile
* Corresponding author. Mother Teresa Women's University, Kodaikanal 624 102,
Tamil Nadu, India. Fax: þ91 4542 241121.
** Corresponding author. Fax: þ91 4542 241121.
E-mail addresses: suganthiphd09@gmail.com (A. Suganthi), rajarajan_1962@
yahoo.com (M. Rajarajan).
Peer review under responsibility of Vietnam National University, Hanoi.
fields and have been studied for many years [8]. MnWO4 is a
narrow bandgap semiconductor (Eg ~ 2.8 eV), which is expected
to be a novel excellent photocatalyst [9]. MnWO4 has nailed its
extensive research interests due to its novel electrochromic,
multiferroic, ionic properties and its significance in gas sensor,
electrochemical and catalytic etc. applications [10,11]. However,
the issues regarding the control of shape and size are considered
as the negative effect on their applications. As reported in literatures, semiconductor photocatalysts, such as MnWO4/Polyaniline [12], MnWO4/Reduced Graphene oxides [13], exhibit
efficient photocatalytic degradation. In addition to that MnWO4
when added to BiOI the degradation property seems to be
enhancing.
Amongst the materials, BiOI exhibits as the most attractive
semiconductor the most promising visible-light e harvesting
photocatalytic properties, because of its narrow band gap and
efficient sunlight-harvesting nature [14e16]. BiOI has been recognized as one of the most promising photocatalytic materials also
because of its strong oxidation ability and high stability, etc. [17].
Owing to its open and layered crystalline and electronic structure,
https://doi.org/10.1016/j.jsamd.2018.07.003
2468-2179/© 2018 Publishing services by Elsevier B.V. on behalf of Vietnam National University, Hanoi. This is an open access article under the CC BY license (http://
creativecommons.org/licenses/by/4.0/).
332
V. Ramasamy Raja et al. / Journal of Science: Advanced Materials and Devices 3 (2018) 331e341
the BiOI shows outstanding photocatalytic performance [18,19]. It is
known that materials with an excellent photoabsorption in solar
spectrum have been proved to be promising photocatalytic materials for the treatment of wastewater [20]. Till now, numerous BiOI
based heterostructured composite catalysts have been reported,
including BiOI/Zn2SnO4 [21], BiOBr/BiOI [22], BiOI/ZnGe2O4 [23],
BiOI/BiCOOH [24], p-BiOI/n-SnS2 [25], BiVO4/BiOI [26], BiOI/BiPO4
[27], Bi2O2CO3/BiOI [28], BiOI/BiOBr [29], and C3N4/BiOI [30]. All
these heterostructured composite catalysts show enhanced visible
light activity as compared to pure BiOI. The reason for their better
photocatalytic ability is the formation of the interface junction
which facilitates the effective separation and transfer of
electronehole pairs.
In the present work, MnWO4/BiOI samples with different molar
ratios were synthesized by the precipitation deposition method. The
photocatalytic activity of MnWO4/BiOI under visible-light was
evaluated by the use of CB dye as the pollutant. The applied synthesis
methods have demonstrated a good reproducibility, and proved the
possibility to obtain the uniform small particle sizes and the production of high purity products. As-prepared samples were studied
by different techniques. The obtained MnWO4/BiOI nanocomposites
exhibit significantly enhanced visible photocatalytic and photoelectrochemical (PEC) performances in comparison with the bare,
MnWO4 and BiOI individually modified electrodes. The visible light
separation appeared in the merged hetrostructure presents the
attributed enhanced photocatalytic and PEG activities. This
approach draws a potential application in synthesizing a broad
range of hybrid nanostructures with prominent applications in
photocatalysis and other relevant areas.
2. Experimental
2.1. Materials
All chemicals and reagents such as manganese nitrate trihydrate
(Mn(NO3)2$3H2O), glycine, sodium tungstates (Na2WO4$2H2O),
bismuth nitrate pentahydrate (Bi(NO3)3$5H2O), ethylene glycol
(EG) and potassium iodide (KI) were purchased from Merck
Chemical India. All reagents were of analytical grade and used
without further purification. Deionized (DI) water was used
throughout the experiments.
2.2. Synthesis of MnWO4 nanoparticles
MnWO4 nanoparticles were prepared by the precipitation deposition method. In a typical procedure, an aqueous solution of
Na2WO4$2H2O (1 mmol) in the presence of different capping agents,
such as valine, glycine and asparagine, was mixed with
Mn(NO3)2$3H2O (1 mmol) aqueous solution and the solution was
heated up to 80 C for 10 min. The brown precipitate was then
centrifuged, washed out with distilled DI water and methanol for three
times. Finally, the final product obtained was dried at 80 C for 24 h.
2.3. Synthesis of MnWO4-BiOI nanocomposites
The MnWO4-BiOI nanocomposite was synthesized in two
different BiOI concentrations (2% MnWO4-BiOI and 3% MnWO4-BiOI)
according to the procedure previously reported in [31]. Initially Bismuth nitrate (Bi (NO3)3$5H2O) and potassium iodide (KI) were dissolved in ethylene glycol. This solution was added dropwise into the
previously sonicated MnWO4 in double distilled water. The mixture
was stirred at room temperature for 4 h, after that the product was
washed with ethanol, double distilled water and dried at 80 C for
24 h. BiOI was also synthesized by the same procedure without the
addition of MnWO4.
2.4. Characterization
The optical properties were investigated using a UVevis-DRS.
The spectra were recorded in air at room temperature in the wave
length range of 200e800 nm using the Shimadzu UV-2450 Spectrophotometer. The crystal structure of the as-prepared photocatalysts was identified by the X-ray diffraction method using CuKa radiation (l ¼ 1.5418 Å) with a scanning angle (2q) of 10 e85 at
a scan speed of 4 ( )/min and a voltage of 40 kV and current of
300 mA. The surface morphology of the sample was characterized
with a JSM 6701F-6701 scanning electron microscope (SEM)
operated at 25 kV attached with an energy dispersive electron
microscopy (EDX) device. Transmission electron microscopy (TEM)
images and selected area electron diffraction (SAED) patterns were
taken on a JEOL JEM-2100 electron microscope with an accelerating voltage of 200 kV. The BET (Brunauer‒Emmett‒Teller) surface area was derived from the N2 adsorption e desorption
isotherm by the Barrer-Joyner-Halenda (BJH) technique. The FT-IR
spectrum of the as-prepared catalysts was recorded using the FT-IR
spectrometer JASCO-4200. Photodegradation experiments were
performed in a HEBER immersion type photoreactor (HIPRMP125).
2.5. Photodegradation experiments
The photocatalytic behavior of the as-synthesized samples were
evaluated by degrading the Celestin blue under the visible light
radiation. A cylindrical quartz photoreactor was used to conduct
the experiments, whereas a 300W Xenon arc lamp was used as a
source of visible light. The lamp was surrounded by a circulating
water jacket for cooling. Air flowing was kept bubbling continuously into the aliquot by an air pump in order to provide a constant
source of dissolved oxygen. 0.60 g/L of the 3% MnWO4-BiOI was
dispersed in a 300 ml CB solution (6 mM). Then, the pH of the solution was adjusted using 0.1 M H2SO4 (or) 0.1 M NaOH, and the
required amount of the photocatalyst was added into the vessel.
The CB solution was stirred for 30 min in darkness before the visible
light irradiation, to ensure the adsorptionedesorption equilibrium
at regular time intervals. 5 ml aliquot was taken and centrifuged,
filtered and the filtrate was analyzed in a UVeVis spectrometer to
estimate the residual CB concentration, using the following
defenition:
Photodegradation (%) ¼ C0 C/C0 100,
(1)
where, C0 is the concentration of CB before the irradiation (t ¼ 0),
and C is the concentration of CB after a certain irradiation time.
2.6. Photoelectrochemical measurements
The EIS was investigated with an electrochemical analyzer
(CHI660B, Chen Hua Instruments, Shanghai, China) in a standard
three-electrode system, which used a platinum wire as the counter
electrode, a saturated calomel electrode (SCE) as the reference one
and an ITO slice as the working electrode, respectively. Typically,
5 mg sample powder was dispersed ultrasonically in 1 mL ethylene
glycol, and 20 mL of the resulting colloidal dispersion was drop-cast
onto a piece of the ITO slice with a fixed area of 0.5 cm2 and then
dried at 80 C. A 500 W Xenon arc lamp was utilized as the light
source. The electrochemical impedance spectra (EIS) were
measured in the frequency range from 0.01 Hz to 100 kHz at 0.24 V,
and the amplitude of the applied sine wave potential in each case
was 5 mV. An 0.1 M Na2SO4 aqueous solution was used as the
impedance liquid.
V. Ramasamy Raja et al. / Journal of Science: Advanced Materials and Devices 3 (2018) 331e341
3. Results and discussion
3.1. Optical absorbance analysis
Two important factors, namely the optical absorption and the
energy band feature of a semiconductor was employed for the
determination of its photocatalytic activity. Fig. 1 shows the DRS
spectra of the as-prepared samples. For the MnWO4 nanoparticles,
strong absorption peaks at 400 nm and for the BiOI nanoparticles,
an absorption edge at around 570 nm are observed, respectively.
For the MnWO4-BiOI composites, the absorption within the visible
light range is apparently increased and a red shift appears when
compared with pure BiOI. The enhanced light absorption may lead
to the formation of more electronehole pairs. The above result
indicates that dispersing MnWO4 on the BiOI surface leads to the
enhanced absorption in the visible light range, electronehole pairs
are formed due to the enhanced light absorption and this would be
the promising mechanism for the photocatalysis application
[32,33]. Tauc relation was used to measure the optical band gap of
the nanocomposites. The UVevis absorption curves were observed
and the absorption edges were calculated using the following
formula:
a,h,n¼ A (h,n ‒ Eg)n/2
Fig. 1. UVeVis-DRS of MnWO4, BiOI, 2% MnWO4-BiOI, 3% MnWO4-BiOI.
333
(2)
where a, h, n, A, Eg are the absorption coefficient, the Planck's
constant, the incident light frequency, a proportionality constant,
and the band gap energy, respectively. The obtained values of the
band gap energy for MnWO4, BiOI, 2% MnWO4-BiOI and 3%
MnWO4-BiOI are found as 2.65 eV, 1.80 eV, 2.15 eV and 2.05 eV,
respectively and they are displayed in Fig. 2. Hence, 3% MnWO4BiOI absorbs more visible light than the pure materials MnWO4,
BiOI.
Fig. 2. Tauc Plots of (a) MnWO4 (b) BiOI (c) 2% MnWO4-BiOI (d) 3% MnWO4-BiOI.
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Fig. 3. FT-IR spectrum of MnWO4, BiOI, 2% MnWO4-BiOI, 3% MnWO4-BiOI.
3.2. FT-IR spectrum
The infrared spectrum of the MnWO4-BiOI nanocomposite is
shown in Fig. 3. The main characteristic peaks of MnWO4 and BiOI
are in accordance with the reported respective results. The absorption peak appears at 1084 cm1 corresponds to the existence of
MnWO4. The peak at 515 cm1 is characteristic for the absorption of
the MneO bond. The vibration modes of MnWO4 are observed at
455, 592, 735, 792, and 885 cm1, while the feature at 1587 cm1 is
due to the deformation vibration of physisorbed or chemisorbed
water molecules [34]. The peak at 489 cm1 corresponds to the
v(BieO) spectrum of the pure BiOI [35]. The above IR characteristic
results have revealed the formation and present of the MnWO4BiOI nanocomposite that is also confirmed by the XRD results as
discussed below.
MnWO4, BiOI, 2% MnWO4-BiOI and 3% MnWO4-BiOI were determined as 20.15 nm, 26.45 nm, 23.09 nm and 26.54 nm, respectively,
on the basis of Debye-Scherrer equation [36].
3.4. Morphological studies
The detailed morphology and microstructure of the pure
MnWO4, BiOI, and the 3% MnWO4-BiOI composite were investigated by SEM and EDAX. As seen in Fig. 5 (a), (b) and (c), the
morphology of the pure MnWO4 shows a sphere-shaped configuration in which the core part is not clear. For BiOI, the picture shows
3.3. X-ray diffraction
XRD technique was used to study the phase composition of
synthesized samples of the pure MnWO4, the pure BiOI and the
MnWO4-BiOI photocatalysts and results are shown in Fig. 4. The
observed XRD peaks of the MnWO4 correspond to the cubic phase,
matching well with standard pattern of the JCPDS card (file no:
#14-0688). All the diffraction peaks of BiOI are in accordance with
the orthorhombic phase, which is in agreement with the database
pattern of the JCPDS card (file no: # 43-0543) as well. As it is seen
for the MnWO4-BiOI heterojunctions, the corresponding characteristic diffraction peaks of MnWO4 and BiOI coexist in the XRD
patterns of the composite. No other peaks are detected in the
heterostructure spectra, indicating that no impurity species are
formed between BiOI and MnWO4. This suggests that BiOI is well
dispersed on the MnWO4 surface. The average crystal sizes of
Fig. 4. XRD Pattern of MnWO4, BiOI, 2% MnWO4-BiOI, 3% MnWO4-BiOI.
V. Ramasamy Raja et al. / Journal of Science: Advanced Materials and Devices 3 (2018) 331e341
the porous texture of the material aggregates. The 3% MnWO4-BiOI
composite appears to be of a small spherical structure and the
average particle size is below 28 nm. The EDAX data shown in Fig. 5
(d) also confirm the presence of the constituting elements, such as
Mn, W, Bi, I and O in 3% MnWO4, BiOI composites.
The TEM measurement was performed in order to get more
detailed information about the crystalline structure of the as-prepared photocatalysts. In Fig. 5(e), that the TEM image taken on the
3% MnWO4-BiOI nanocomposite sample shows the surface
morphology of this sample. From this, the standard grain diameter
of the 3% MnWO4-BiOI composite varies between 23e47 nm. The
removal of intercalated and hydroxyl water adsorbed in the structure, release pore spaces and consequently create the configuration
of mesoporous structures on the surface. Fig. 5 (f) shows the SAED
(selected area electron diffraction) pattern taken on the 3%
MnWO4-BiOI nanocomposite. The 3% MnWO4-BiOI composite is
the composition of the cubic phase of MnWO4 and the
335
orthorhombic phase of BiOI which have been well identified with
the XRD patterns. The SAED patterns of the 3% MnWO4-BiOI
nanocomposite confirm that the product is well of polycrystralline
nature [37].
3.5. BET surface area analysis
Generally, photocatalytic reaction takes place on the surface of
the photocatalysts, and ultimately, it draws effects on its photocatalytic activity. The specific surface area and the pore structure of
the as-prepared MnWO4, BiOI and 3% MnWO4-BiOI nanocomposites were measured at 195.675 C and the nitrogen
adsorptionedesorption isotherm was taken in order to explain the
dissimilarity of the following photocatalytic presentation and results are depicted in Fig. 6. The three isotherms are identified according to the IUPAC classification as type IV by the mesoporous
nature, indicated by the presence of the pores of 2e50 nm [38]. The
Fig. 5. SEM images of (a) MnWO4; (b) BiOI; (c) 3% MnWO4-BiOI; (d) EDAX spectrum of the 3% MnWO4-BiOI composite; (e) TEM image of 3% MnWO4-BiOI; and (f) SAED pattern of
3% MnWO4-BiOI.
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Fig. 6. N2 adsorptionedesorption isotherms and pore size distribution plots for samples of (a) MnWO4; (b) BiOI; and (c) 3% MnWO4-BiOI.
hysteresis loops can be classified as type H3. The hysteresis loop
appears for the relative pressure P/P0 between 0.6 and 1. Table 1
summarizes the BET surface area, the pore volume, and the
average pore size of the as-synthesized samples as calculated on
the basis of the recorded isotherms. Both MnWO4, BiOI and the 3%
MnWO4-BiOI nanocomposite possess all larger surface areas, and
thus, they can provide more surface active sites for the adsorption
of reactant molecules which is beneficial for achieving higher
photocatalytic activity [39].
3.6. Electrochemical analysis
Nyquist plots from electrochemical impedance spectra (EIS)
are significant in elucidating the migration and transfer
Table 1
Parameters obtained from N2 adsorptionedesorption measurements.
Sample
Specific surface
area (m2/g)
Average pore
diameter (nm)
Pore volume
(cm3/g)
MnWO4
BiOI
3% MnWO4-BiOI
45.36
34.75
75.45
29.32
20.77
46.50
0.2986
0.9161
1.5699
processes of photogenerated electronehole pairs in semiconductors [40]. The prepared BiOI and 3% MnWO4/BiOI samples
were used as electrode materials to measure their EIS and result
is shown in Fig. 7 (a). The semicircle in the high frequency region is ascribed to the charge-transfer resistance (Rct), and it
confirms the charge transfer through the electrode/electrolyte
interface. The smaller radius of 3% MnWO4/BiOI semicircle
implies a higher efficiency of charge transfer in this composite
than in the pure BiOI. Hence, the coupling of MnWO4 with
BiOI is considered beneficial in electron transport, and in the
enhancement of the separation efficiency of the photogenerated
charge carriers [41].
The photo-induced holes or the formed hydroxyl radicals are the
two main species responsible for the oxidization of the organic
pollutants. Disodium Ethylene Diamine Tetra Acetate (EDTA-2Na)
was used as the hole scavenger and tert-butanol was used as an
electron acceptor in investigating the main species involved in CB
photodegradation over the 3% MnWO4/BiOI composite. As is shown
in Fig. 7 (b), the addition of EDTA-2Na greatly inhibited the
degradation of CB, while the addition of tert-butanol could only
slightly diminish the degradation rate of CB. Therefore, it confirmed
that both the holes and hydroxyl radicals play significant role in the
CB photodegradation while the holes are the main reactive species.
V. Ramasamy Raja et al. / Journal of Science: Advanced Materials and Devices 3 (2018) 331e341
337
Fig. 8. Time dependent UVeVis spectral changes of CB (6 mM) in the presence of the 3%
MnWO4-BiOI composite (0.60 g/L).
activity towards the degradation of CB as compared to the bare
MnWO4 and BiOI.
3.7.2. Possible photocatalytic activity enhancement mechanism
Based on the results mentioned above, a possible mechanism
for the photocatalytic degradation of organic compounds by
MnWO4/BiOI nanocomposite is proposed as displayed in Fig. 9.
Further, the conduction band (CB) bottom and the valence band
(VB) top of MnWO4 and BiOI can be calculated by the following
equations:
Fig. 7. Electrochemical impedance spectroscopy of BiOI (a) and 3% MnWO4-BiOI
nanocomposite (b). Comparison of the photocatalytic activities of 3% MnWO4-BiOI
nanocomposite for the degradation of CB under visible light irradiation with and
without adding EDTA-2Na and t-butanol.
3.7. Photocatalytic activity
3.7.1. Photodegradation of CB
The photodegradation was monitored by examining the variations of the maximal absorption with respect to the irradiation
time in the UVeVis spectra. The photocatalytic activity of the 3%
MnWO4-BiOI nanocomposite was evaluated from the degradation
of 6 mM of CB with an initial pH of 9. The changes in the UVeVis
spectra of CB as a function of irradiation time using 3% MnWO4BiOI (0.60 g/L) are presented in Fig. 8. It is important to notify that
the contact time prior to the irradiation was 30 min under the dark
condition indicating the absorption of dyes on the active sites of
the synthesized photocatalysts. With the increase of the irradiation time, the absorption peak at lmax ¼ 645 nm decreases gradually and disappears after 180 min of irradiation. The
photocatalytic degradation of CB in an aqueous solution in the
presence of the 3% MnWO4-BiOI composite results in 90% degradation efficiency. The degradation efficiency of the photocatalysts
follows the order as: 3% MnWO4-BiOI > 2% MnWO4BiOI > BiOI > MnWO4 according to the results presented. The 3%
MnWO4-BiOI heterojunction exhibits the best photocatalytic
EVB ¼ X e Ee þ 0.5 Eg
(3)
ECB ¼ EVB e Eg
(4)
where X is the electronegativity of the semiconductor, Ee is the
energy of the free electrons on the hydrogen scale (4.5 eV) and Eg
is the band gap energy of the semiconductor. As is it known, the
estimated CB and VB of MnWO4 material are 2.07 and 4.72 eV, and
of BiOI 0.8 and 2.6 eV, respectively. As illustrated in Fig. 9, under the
visible-light irradiation, both the g-MnWO4 and BiOI are easily
excited and engender the corresponding photogenerated
electronehole pairs. Once the electrons in the VB of MnWO4 and
BiOI are excited to the CB of MnWO4 under visible-light irradiation,
the photoinduced electrons on the CB of MnWO4 are transferred to
the CB of BiOI, whereas the photogenerated holes on the VB of BiOI
to the VB of MnWO4 [42e44] respectively. These processes cause
the gathering of electrons on the CB of BiOI and holes on the VB of
the MnWO4 surface. As an outcome, the separation of electrons and
holes proceeds and completes causing the decrease of the recombination process leading to a highly efficient photocatalytic activity
of the MnWO4/BiOI nano composites. The mechanism of the
degradation reaction can be summarized as the following:
MnWO4/BiOI þ hn (Vis) / MnWO4/BiOI (hþ þ e)
(5)
MnWO4/BiOI (e) þ O2 / MnWO4/BiOI þ O
2
(6)
2O
2 þ 2H2O / 2OH þ H2O2 þ O2
(7)
H2O2 þ MnWO4/BiOI (e) / OH þ OH þ MnWO4/BiOI
(8)
þ
CB þ O
2 / OH/h / intermediates / degradation products
(9)
338
V. Ramasamy Raja et al. / Journal of Science: Advanced Materials and Devices 3 (2018) 331e341
Fig. 9. Schematic diagram of the electron transfer in the 3% MnWO4-BiOI under visible-light irradiation.
The CB electrons (e) are accumulated on the surface of
MnWO4/BiOI. Some of them are then scavenged by the oxygen on
the surface of the photocatalyst to form the superoxide radicals
(O
2 ), which can react with H2O to form H2O2. Some of them can be
scavenged then by the H2O2 to form the hydroxyl radicals (OH).
These are three active species being involved in the photocatalytic
process.
3.8. Optimization of the reaction parameters
3.8.1. Effect of pH
Generally, the pH of the solution is an important parameter in
the photocatalytic process. The effect of pH on the photodegradation of the CB was studied in the pH range of 4e9 with an
initial of the CB concentration of 6 mM, and the catalyst dosage of
0.60 g/L. The results are displayed in Fig. 10(a). The photodegradation of the CB increases from pH 4e8 and then decreases
as the pH increases to 9. The alkaline or acidic conditions were
adjusted by adding the appropriate amount of NaOH or H2SO4 to
the solution. This is due to the change in the electrostatic
attraction or repulsion between the pollutant molecules and the
catalyst. The CB is a cationic dye and therefore, the electrostatic
attraction between the dye molecules and the catalyst is greatly
improved at pH ¼ 9 [45].
3.8.2. Effect of the catalyst dosage
In order to optimize the photocatalyst dosage on the degradation of the CB, experiments were carried out with varying the
3% MnWO4-BiOI dosage from 0.30 g/L to 0.70 g/L, at constant the
CB concentration of 6 mM and pH ¼ 9. The results are shown in
Fig. 10(b). The photodegradation of the CB is negligible in the
absence of the catalyst. As the concentration of photocatalyst
increases from 0.30 g/L to 0.60 g/L the degradation of the CB also
increases. In contrast, it causes the decrease in the photodegradation due to the presence of more active sites and also
due to the increase in the total active surface area of the
photoreaction [46].
3.8.3. Effect of the concentration
The effect of the initial dye concentration on its photodegradation was investigated from 2 mM to 8 mM, at pH ¼ 9 and at
the 3% MnWO4-BiOI dosage of 0.60 g/L. The results are shown in
Fig. 10(c). With the initial concentration increasing the CB photodegradation decreases. At high concentrations, interruption by dye
molecules of photons takes place- before reaching the catalyst
surface, as a result of which the formation of reactive oxygen species on the photocatalytic surface is reduced. The 3% MnWO4-BiOI
concentration is the same for all initial concentrations of CB and
therefore, the formation of OH remains constant [47].
V. Ramasamy Raja et al. / Journal of Science: Advanced Materials and Devices 3 (2018) 331e341
339
Fig. 10. (a) Effect of pH on the photodegradation of CB; (b) Effect of catalyst dosage on the photodegradation of the CB; (c) Effect of the concentration on the photodegradation of the
CB; and (d) Kinetic regime on the photocatalytic degradation of the CB.
3.8.4. Kinetics of the CB photodegradation
The kinetics of the CB under the visible light over all the photocatalysts was investigated by applying the following lpseudo-first
order equation:
ln (C0/C) ¼ k,t
(10)
where, k is the rate constant of pseudo-first order. The plot of ln
(C0/C) versus irradiation time (where C0 is the initial concentration
of the dyes and C is the dye concentration in the reaction time) was
found to be linear as shown in Fig. 10(d). This put forward that the
photodegradation reactions follow the kinetics of the pseudo-first
order. The linear regression method was used to calculate the
rate of the reaction constant (k) and the rate law for the photocatalytic degradation of the CB. The apparent rate constants for
MnWO4, BiOI, 2% MnWO4-BiOI, 3% MnWO4-BiOI were determined
as 0.0115 102 S1, 0.0078 102 S1, 0.235 101 S1,
0.326 101 S1, respectively. The observed results show that the
3% MnWO4-BiOI, nanocomposite has a higher rate constant than
the other modified and their bare catalysts [48].
3.8.5. COD
The COD was used as a measure of the oxygen equivalent to the
organic content in a sample that was susceptible to oxidation to
carbon-dioxide and water by a strong oxidant. Ideal conditions
were maintained for conducting the photocatalytic experiments.
Table 2
COD removal (mg/L) of CB during the photodegradation using the 3%
MnWO4-BiOI under visible light irradiation.
Time (min)
COD removal efficiency (%)
Celestin blue
0
30
60
90
120
150
180
0
15.68
28.37
43.67
60.45
75.34
90.00
Test samples were collected at the time intervals of every 30 min.
The COD before and after the irradiation under visible light was
estimated and results are shown in Table 2. After the photodegradation at 180 min under the optimum conditions the solutions have a significant decrease in COD up to 90.00%. As an
outcome smaller species were obtained by the degradation of
organic matters (especially inorganic compounds) and hence the
COD required decreases.
4. Conclusion
In conclusion, a novel MnWO4/BiOI composite with visible light
photocatalytic activity was successfully synthesized via a
340
V. Ramasamy Raja et al. / Journal of Science: Advanced Materials and Devices 3 (2018) 331e341
precipitation deposition method and characterized by various
techniques. XRD results indicate that the as-prepared catalysts
composed of the cubic phase MnWO4 and the orthorhombic phase
BiOI. The 3% MnWO4-BiOI nanocomposite was efficient for the
degradation of the CB under visible light irradiation due to their
unique morphology and optical property. Evaluation and optimization of the effect of pH, catalyst dosage throughout the COD experiments indicated that under the studied conditions the
parameters as pH ¼ 9, 0.60 mg/L MnWO4-BiOI and 180 min irradiation time favor the COD efficiency. Under the above condition
approximately 90% of degradation was achieved within 180 min of
irradiation. Correlation of photodegradation kinetics was considered with the model of the pseudo-first-order. A plausible mechanism for the photodegradation was proposed. To conclude this
research work has proved that the synthesized nanocomposites can
be utilized as a favorable photocatalyst for the degradation of
organic dyes in the aqueous environment.
[18]
[19]
[20]
[21]
[22]
[23]
Acknowledgements
[24]
The authors thank the Management of Thiagarajar College for
providing necessary laboratory facilities to carry out this work.
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