Ultrasound-assisted method to improve the structure of CeO2?polyprrole core-shell nanosphere and its photocatalytic reduction of hazardous Cr6+

Article abstract of DOI:10.1016/j.ultsonch.2019.104738

In this work, the CeO₂?polypyrrole (CeO₂?PPy) core-shell nanosphere has been synthesized via an ultra-sonication method using bath type (WUC-D22H, Daihan Scientific, Korea) and they are utilized for the photo-reduction of hazardous Cr⁶⁺ to benign Cr³⁺. The ultrasonic frequency and power were 20 kHz and 100 W, respectively. The PPy shielded CeO₂ in aqueous solution could prevent the dissolution of CeO₂ and to improve the photocatalytic ability of CeO₂. X-ray diffraction was used to confirm the crystalline structure of as prepared CeO₂?PPy core-shell and FT-IR was used to identify the functional groups. The uniform sized core of PPy and shell of CeO₂ were observed by transition electron microscopy. The ultrasonic assisted synthesized CeO₂?PPy core-shell exhibits a narrow bandgap (UV-DRS) and good reduction efficiency with higher reusability and stability compared to pure CeO₂, PPy and mechanical mixing of CeO₂?PPy. Moreover, the synergistic effect of CeO₂ and PPy core-shell structure facilitate a higher electron transfer rate and prolong lifetime of photogenerated electron-hole pairs which can achieve good reduction rate of 98.6% within 30 min. In particular, the pH, catalyst, and Cr⁶⁺ concentration effects were optimized in photocatalytic reduction reactions. Meanwhile, this photocatalysis with fast and effective electron transfer mechanism for the Cr⁶⁺ reduction was elucidated. This method opens a new window for simple fabrication of conducting polymers-based metal oxide nanocomposite towards wastewater remediation and beyond.

Full text of DOI:10.1016/j.ultsonch.2019.104738

Ultrasonics-Sonochemistry59(2019)104738
Contents lists available at ScienceDirect
Ultrasonics - Sonochemistry
journal homepage: www.elsevier.com/locate/ultson
Ultrasound-assisted method to improve the structure of CeO2@polyprrole
core-shell nanosphere and its photocatalytic reduction of hazardous Cr6+
Vellaichamy Balakumar, Hyungjoo Kim, Ramalingam Manivannan, Hyorim Kim, Ji Won Ryu,
Gisu Heo, Young-A. Son⁎
Department of Advanced Organic Materials Engineering, Chungnam National University, 220 Gung-dong, Yuseong-gu, Daejeon 305-764, South Korea
A R T I C L E I N F O
A B S T R A C T
Keywords:
In this work, the CeO2@polypyrrole (CeO2@PPy) core-shell nanosphere has been synthesized via an ultra-so-
nication method using bath type (WUC-D22H, Daihan Scientific, Korea) and they are utilized for the photo-
reduction of hazardous Cr6+ to benign Cr3+. The ultrasonic frequency and power were 20 kHz and 100 W,
respectively. The PPy shielded CeO2 in aqueous solution could prevent the dissolution of CeO2 and to improve
the photocatalytic ability of CeO2. X-ray diffraction was used to confirm the crystalline structure of as prepared
CeO2@PPy core-shell and FT-IR was used to identify the functional groups. The uniform sized core of PPy and
shell of CeO2 were observed by transition electron microscopy. The ultrasonic assisted synthesized CeO2@PPy
core-shell exhibits a narrow bandgap (UV-DRS) and good reduction efficiency with higher reusability and sta-
bility compared to pure CeO2, PPy and mechanical mixing of CeO2@PPy. Moreover, the synergistic effect of
CeO2 and PPy core-shell structure facilitate a higher electron transfer rate and prolong lifetime of photo-
generated electron-hole pairs which can achieve good reduction rate of 98.6% within 30 min. In particular, the
pH, catalyst, and Cr6+ concentration effects were optimized in photocatalytic reduction reactions. Meanwhile,
this photocatalysis with fast and effective electron transfer mechanism for the Cr6+ reduction was elucidated.
This method opens a new window for simple fabrication of conducting polymers-based metal oxide nano-
composite towards wastewater remediation and beyond.
CeO2@polyprrole
Core-shell nanosphere
Photocatalytic degradation
Hexavalent chromium
Reusability and stability
1. Introduction
efficient catalysts to remove Cr6+ still have been recognized as one of
the tasks in the photocatalysis community. Nanoceria have been used
In general, heavy metals are easily entered in aquatics environment
which causing serious contamination and a toxic effect to human and
animals [1]. Among the heavy metals, chromium (Cr) is customarily
used in cement industries, leather tanning, paint formulation, etc [2].
Its high valent Cr6+ has been categorized as a group I human carci-
nogen by the IARC (International Agency for the Research on Cancer)
and the WHO (World Health Organization) has assigned 0.05 mg/L as a
limit of Cr6+ [3]. Therefore, the fast and efficient removal of Cr6+ in
aquatic systems is still needed for environmental safety and analysis. Up
to now, Cr6+ removal has been employed by various methods including
adsorption [4], electrochemical precipitation [5], reverse osmosis [6],
ion exchange [7], solvent extraction [8], catalytic reduction [1,3] and
photocatalytic degradation [9–11]. Among them, photocatalytic
method has been investigated as one of the efficient conversion of toxic
to benign form of organic and inorganic pollutants. In past decades,
many studies have been explored on the photocatalytic removal of
environmental pollutant, Cr6+ [12–15]. But, the cost effective and
for research in many fields including catalysis, electronic, medicine and
optics. Besides, the 3.2 eV band gap of cerium oxide (CeO2) have some
limitations of its application in photocatalysis which is deficient light
absorption, separation and fast recombination of electron-hole pairs
[16,17]. To conquer the above problems, many researchers have been
developed responses such as CeO2 coupled with metals (Ag [18], Au
[18], Pt [19], Pd [16], Fe [20] and Cu [21]), metal oxides (TiO2 [9],
Bi2WO6 [10], SnS2 [11]), carbon based materials (graphene [22] and g-
C3N4 [23]) and heteroatom (N [24]) to form nanocomposites. Inter-
estingly, CeO2 based heterogeneous catalyst attracted attention by
showing robust and stable photocatalytic reduction of Cr6+ [25].
Recently, conducting polymers supported heterogeneous photo-
catalysts have been attracted worldwide attention due to their excellent
energy production and conversion, environmental remediation and
beyond. Among polymers, polypyrrole (PPy) has a good utilization ef-
ficiency of visible-light for photocatalysts due to π-conjugated electron
system of PPy [26]. In addition, the high electron donors and
⁎ Corresponding author.
E-mail address: yason@cnu.ac.kr (Y.-A. Son).
https://doi.org/10.1016/j.ultsonch.2019.104738
Received 13 April 2019; Received in revised form 12 August 2019; Accepted 18 August 2019
Availableonline19August2019
1350-4177/©2019ElsevierB.V.Allrightsreserved.

V. Balakumar, et al.
Ultrasonics-Sonochemistry59(2019)104738
Scheme 1. Schematic representation of the CeO2@PPy core-shell nanosphere synthesized through ultrasonic method.
Fig. 1. XRD patterns of PPy (curve a), CeO2 (curve b), CeO2@PPy core-shell
(curve c) synthesized via ultrasonic method and CeO2@PPy (curve d) synthe-
sized via mechanical mixing.
Fig. 2. FT-IR spectra of PPy (curve a), CeO2 (curve b) and CeO2@PPy core-shell
nanosphere (curve c).
have synthesized α-Fe2O3/PPy nanophotocatalyst to achieve a re-
markable activity toward methylene blue degradation. L. Wang et al.
[29] have studied near infrared light driven degradation of different
organic pollutants by using PPy/W18O49 nanorods. J. Xu et al. [30]
have reported an enhanced specific surface area of PPy-BiOI composite
resulting in good adsorption and generation of more active sites to
degrade organic pollutants. F. Duan et al. [31] used visible light active
transporters properties can be achieved with the enhancement of the
absorption capacity, charge separation and electron-hole pairs. Up to
now, many researchers have used to PPy as active and reusable support
for preparation of photocatalysts with good results. For example, L. K.
Krehula et al. [27] have prepared PPy/TiO2 nanocomposite to enhance
the visible light active photocatalytic activity. F. A. Harraz et al. [28]
2

V. Balakumar, et al.
Ultrasonics-Sonochemistry59(2019)104738
Fig. 3. (A–D) Different magnification SEM images of CeO2@PPy core-shell nanosphere synthesized via ultrasonic method and (E&F) Different magnification SEM
images of CeO2@PPy synthesized via mechanical mixing.
PPY/Bi2WO6 nanocomposite for photoreduction of rhodamine-B and
methylene blue in aqueous solution.
good visible light active photocatalytic detoxification activity and sta-
bility than pure CeO2 and PPy. Finally, the efficient charge separation
mechanism and stability of the nanocomposite has been demonstrated
and investigated in detail. The proposed photocatalytic mechanisms are
likely to a new window for the fabrication of conducting polymers-
based metal oxide nanocomposite for wastewater remediation and be-
yond.
Compare to conventional methods, a rapid, economic and en-
vironmental friendly ultrasound-assisted method is used to develop
nanocomposites with controlled size, morphology, phase purity and
uniform distribution of particles on supportive surfaces [12,17,32,33].
A positive effect of ultrasound has enhanced the photocatalytic per-
formance because of uniform distribution of particles by enhancing the
surface-active sites for degradation process. Recently, modification of
Bi2Sn2O7-C3N4/Y nanophotocatalyst with various ratios of zeolite have
shown good photocatalytic activity towards tetracycline [34]. Sono-
precipitation dispersion of ZnO nanoparticles over graphene oxide na-
nocomposite enhanced the photocatalytic reactivity and stability in-
fluenced by irradiation time and power [35]. Sonocatalytic degradation
of a textile dye using CeO2-biochar nanocomposite are some examples
of ultrasound irradiation used to improve the photocatalyst perfor-
mance [17].
2. Experimental sections
2.1. Materials
In this work, all of the reagents were purchased from analytical
grade and used directly without further purification. Pyrrole, cerium
(III) nitrate hexahydrate (Ce(NO3)3·6H2O, 99%) and urea were pur-
chased from Sigma–Aldrich, South Korea.
Herein we report on a novel and stable of CeO2@polypyrrole core-
shell nanosphere photocatalyst has been synthesized via ultrasonic
method. The synthesized core-shell photocatalyst demonstrated that
The CeO2 core is not dissolved in aqueous solution because shell of PPy
could prevent the dissolution. Therefore, the CeO2@PPy enhancing the
structural stability. The core-shell of CeO2@PPy nanospere exhibits a
2.2. Synthesis of CeO2 nanosphere
0.5 g of Ce(NO3)3·6H2O were dispersed in 10 ml deionized water
and 12 g of urea were dissolved in 40 ml of water and then the dissolved
urea was transferred in to the above solution slowly. The resultant so-
lution was then transferred to an autoclave vessel, heated to 260 °C, and
3

V. Balakumar, et al.
Ultrasonics-Sonochemistry59(2019)104738
Fig. 4. (A) TEM image of CeO2, (B&C) different magnification TEM images of CeO2@PPy core-shell nanosphere and (D) SAED pattern of CeO2@PPy core-shell
nanosphere.
maintained at this temperature for 24 h. The obtained samples were
collected and washed with water and ethanol in twice. Then, it was
dried at air oven and subjected to calcination for 2 h at 500 °C to get the
final product.
microscope (TEM, FEI TECNAI T20 G2) operating at 100 kV. Finally,
HEBER immersion type photoreactor (HIPR-MP125) were used to per-
form photocatalytic experiments.
2.5. Photocatalytic measurements
2.3. Synthesis of CeO2@PPy core-shell nanosphere
The photo-reduction ability of CeO2@PPy were evaluated towards
Cr6+ under visible light irradiation (λ > 450 nm) at 30 °C and the
average light intensity is found to be 29.36 W/m−2. In a standard ex-
periment, 100 ml aqueous solution of Cr6+ (0.1 mM) which was pre-
pared from K2Cr2O7 and 50 mg of catalysts was added in to the above
aqueous Cr6+ solution. Then, the suspension was stirred under dark
conditions for 30 min to ensure the adsorption-desorption equilibrium.
During the irradiation, the solution (5 ml) were taken at every 5 min
and centrifuged at 1000 rpm and measured UV–visible absorbance
spectroscopy. After completing the reduction process, the catalyst was
separated and it was used to carry out the next catalytic reduction
process of fresh substrates. The reduction percentage was calculated
using (C0 − Ct/C0 × 100) where, Ct and C0 are the instantaneous con-
centration of Cr6+ at time t and initial concentration of Cr6+ respec-
tively.
0.2 g of synthesized CeO2 and 0.5 ml of pyrrole were dispersed in
25 ml deionized water and 1.5 g FeCl3 (1:1.25 pyrrole: FeCl3 ratio) was
dispersed into 25 ml of 1 M methanesulphonic acid. The resulting
second solution was dropwise transferred in to the aqueous solution
pyrrole and CeO2. After that the solution was ultra-sonicated for 6 h
(irradiation power 100 W). Then it was filtered, washed, and dried at
110 °C for 12 h. The same reaction was also carried out using simple
mechanical mixing and compare the results. Pure polypyrrole was
prepared by the same procedure (ultrasonication) without adding CeO2
nanosphere. The preparation process of PPy, CeO2 and core-shell na-
nosphere of CeO2@PPy are shown in Scheme 1.
2.4. Instrumentations
The UV–DRS spectra were used to record the absorption value of
samples on a Shimadzu Solid Spec-2700 instrument using BaSO4 as the
reference. Infrared spectra experiments were collected on ALPHA-P
spectrometer. The XRD measurements were performed on a Cu Kα ra-
diation (λ = 1.54056 Å) using (PHILIPS, X'Pert-MPD System, Max P/N:
3 kW/40 kV, 45 Ma, Netherlands). For SEM analysis were carried at
LYRA3 XMU. High-resolution surface structure and selected area elec-
tron diffraction (SAED) were obtained with a transmission electron
3. Result and discussions
3.1. Structural characterization
XRD was analyzed to determine the crystalline structure of PPy,
CeO2 and CeO2@PPy core-shell nanocomposite. XRD pattern shows a
broad peak appears at around 24.21° which is corresponding to the
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V. Balakumar, et al.
Ultrasonics-Sonochemistry59(2019)104738
Fig. 5. (A) UV–visible diffuse absorption spectra of the PPy (curve a), CeO2
(curve b) and CeO2@PPy core-shell nanosphere (curve c) and (B) Tauc’s plots of
PPy (blue line), CeO2 (black line) and CeO2@PPy core-shell nanosphere (red
line).
Fig. 6. (A) Photocatalytic reduction of Cr6+ to Cr3+ in the presence of CeO2@
PPy core-shell nanosphere and (B) Photocatalytic Cr6+ reduction curves of C/
C0 vs time.
amorphous nature of pure PPy as shown in Fig. 1 (curve a). Pure CeO2
was observed at cubic fluorite structure of diffraction peaks indexed
(JCPDS no. 34-0394). This phase diffraction peaks observed at 28.66°,
33.09°, 47.52°, 56.40°, 59.05°, 69.45°, 76.78°, 79.20° and 88.57° (Fig. 1
(curve b)) correspond to (1 1 1), (2 0 0), (2 2 0), (3 1 1), (2 2 2), (4 0 0),
(3 3 1), (4 2 0), and (4 2 2) planes for CeO2, respectively [36]. The
average crystallite size of CeO2 was calculated, resulting in about
197.2 nm from (1 1 1) peak of XRD using Debye-Scherrer equation [37].
Compared with bare CeO2, the XRD patterns of CeO2@PPy core shell
(curve c) nanosphere (ultrasonic synthesis) did not affect the reflections
of CeO2 nanoparticles indicating that crystal structure of CeO2 core was
not affected by PPy shell. Moreover, PPy diffraction peaks are invisible
in the CeO2@PPy, which is due to the low concentration and crystal-
linity of PPy. Compared to this method, mechanical mixing method
synthesized CeO2@PPy exhibits a new peak at 24° which indicates the
PPy. Herein we concluded that mechanical mixing method synthesized
CeO2@PPy was not formed in core-shell, which is confirmed by SEM
images.
(curve a) pure PPy has display bands at 3405, 1711, 1548, 1461 and
1177 cm−1. The peak observed at 3405 cm−1 is attributed to eNH
stretching vibration of PPy while the peaks at 1711 and 1548 cm−1 are
represent to CeC ring stretching and C]C backbone stretching, re-
spectively [26]. The PPy, CeH in-plane and CeN stretching vibrations
are observed at 1461 and 1177 cm−1 [29]. For pure CeO2, (Fig. 2
(curve b)) peaks at 522, 1027, 1647 and 3402 cm−1 with all absorption
peaks appeared in proper position which is good agreement with earlier
reported results [38]. It can be seen that the FT-IR spectra of CeO2@PPy
(Fig. 2 (curve b)) photocatalysts, the intensity of both PPy and CeO2
peaks decreases because PPy was coated on the shell of CeO2.
Fig. 3A–D different magnification SEM images of CeO2@PPy has
core-shell nanosphere showing that average particles size was 200 nm.
As shown in Fig. 3E &F, the mechanical mixing method to synthesize
CeO2@PPy was not formed in core-shell. TEM study were used to find
out the size and shape of the as-prepared CeO2 and core-shell structure
of CeO2@PPy. As shown in Fig. 4A, the uniform sized nanospheres and
the average diameter was found to be 200 nm of CeO2 particles. Fur-
ther, the core-shell structure CeO2@PPy has been given in Fig. 4B&C. It
Fig. 2 shows the FT-IR spectra of PPy, CeO2 and core-shell structure
of CeO2@PPy nanocomposite photocatalysts. As can be seen in Fig. 2
5

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Ultrasonics-Sonochemistry59(2019)104738
Table 1
Photocatalytic activity of different CeO2 based catalysts for the photo-reduction of Cr6+
.
Materials
Catalyst weight (mg)
Cr Concentration
Time (min)
Efficiency (%)
Catalyst stability cycles
Ref.
CeO2-TiO2
50
2 mM
180
60
99.6
99.6
–
3
–
5
4
–
6
–
5
9
CeO2@Bi2WO6
CeO2/SnS2
20
8 mg/L
10
300
50
50 mg/L
0.1 mM
5.0 mg/L
20 mg/L
100 mg/mL
0.1 mM
80
11
Pd/CeO2/g-C3N4
CeO2@Pt@TiO2
CeO2-QDs/BiOX
Y-CeO2/PCN
CeO2@PPy
40
–
25
30
150
60
99.0
97.0
98.1
98.6
40
50
41
120
10
240
45
42
This work
can be seen clearly that the outside of the as-prepared CeO2 particles
surface was uniformly coated with PPy shell showing distinctly dif-
ferent from the CeO2 core. The SAED lattice fringes of CeO2@PPy core-
shell nanosphere exhibited a spacing of 0.27 nm (Fig. 4D), which is in
good agreement with the XRD results.
In acidic pH the catalyst surface can create positively charges and Cr6+
gets negative charged species (HCrO4− and Cr2O72−), leading to strong
adsorption of Cr6+ on the catalyst surface, consequently increase to
photocatalytic reduction rate [1,14]. On the other hand, in higher pH,
the catalyst surface became negatively charged, which repels Cr6+
species and the conversion products of Cr3+ can be easily precipitated
in basic conditions [43]. According to this, the amount of Cr6+ ab-
sorption and rate of reduction efficiency was achieved at lower pH.
In order to find an optimum dosage of the CeO2@PPy core-shell, the
pH and Cr6+ concentration was kept constant and the amount of cat-
alyst varying from 0.5, 1.0 and 1.5 mg were tested. In addition, the
degradation percentage was calculated as shown in Fig. 7B. According
to the results, it can be seen that 1.0 mg of the catalyst loaded solution
has been achieved higher photocatalytic detoxification. Moreover,
lower or higher amount of catalyst loaded solutions had poor activity
because of fewer active sites and solar light inhibition in solution, re-
spectively [44,45]. These are the reasons to reduce the degradation
percentage.
Diffuse reflectance spectroscopy is used to measure the optical ab-
sorption property of the PPy, CeO2 and core-shell structure of CeO2@
PPy nanosphere as hown in Fig. 5A. The pure PPy showed the char-
acteristic absorption edge at about 500 nm as shown in Fig. 4a (curve
a). The pure CeO2 light absorption peak was observed at 310 nm
(Fig. 5A (curve b)) and the corresponding to a band gap (Eg) of 3.01 eV
[25,38] due to electron transition from oxygen to cerium [39]. When
PPy shell was coated on the surface of CeO2 core (Fig. 5A (curve C)),
optical absorption was enhanced to the range of the visible region
compared to pure CeO2, resulting from the PPy. The Tauc’s equation
was used to determine the band gap energy of PPy, CeO2 and CeO2@
PPy core-shell nanosphere and the graph plotted (αhν)2 against the
photon energy (hν). The band gap values were found to be 1.95 (PPy),
3.01 (CeO2) and 2.46 Ev (CeO2@PPy) in Fig. 5B.
Fig. 7C shows the effect of Cr6+ initial concentration on detox-
ification using CeO2@PPy core-shell under visible light irradiation. The
good degradation percentage (98.2%) were achieved in the initial
concentration of Cr6+. It was clearly observed that the enhancement of
Cr6+ concentration reduce the rate of conversion since the higher Cr6+
prevents light on the catalyst with the reduction of generation for
electrons and hole-pairs [45,46]. Furthermore, the minimum con-
centration of Cr6+ completely adsorbed on to the catalyst surface and
generation of active species increases leading to the enhancement of the
conversion percentage.
3.2. Photocatalytic detoxification of Cr6+
In Fig. 6A, the photocatalytic activities of PPy, CeO2 and CeO2@PPy
were described, showing the reduction of hexavalent chromium to tri-
valent chromium under the visible light. The Cr6+ photolytic de-
gradation was negligible in the absence of the catalyst. The Cr6+ de-
gradation in the presence of pure PPy and CeO2 were investigated and
the results showed slightly degradation (48 and 69%, respectively) as
shown in Fig. 6B. In contrast, nearly 100% of Cr6+ was degraded with
(30 min) irradiation in the presence of CeO2@PPy photocatalyst under
the same conditions. As shown in Fig. 6A, the absorption peak intensity
of Cr6+ ions were drastically decreased and finally disappear as much
as irradiation time increased [25]. Complete reduction of toxic Cr6+ to
Cr3+ was clearly observed within 30 min. Also the yellow color of Cr6+
was changed to colorless, indicating the reduction of hazard Cr6+ to
benign Cr3+. These results indicated that core-shell CeO2@PPy photo-
catalyst is obviously superior to pure CeO2 and PPy as well as me-
chanical mixing of CeO2@PPy. The Cr6+ reduction rate constant k of
photocatalysts was evaluated by ln (C0/Ct) against t, showing that the
value for PPy, CeO2 and CeO2@PPy core-shell are found to be 0.0012,
0.0097 and 0.0639 min−1, respectively. The CeO2@PPy core-shell is
compared with other reported CeO2 based photocatalysts for the re-
duction of Cr6+ in Table 1. [40,10,41,9,11,25,42].
3.4. Possible charge separation and degradation mechanism
Based on the experimental analysis above, a charge separation and
Cr6+ degradation mechanism of CeO2@PPy nanospheres has been
proposed in Fig. 8. It was well-known that both PPy and CeO2 can
generate electrons and holes under visible light irradiation. The gen-
erated electrons migrated from HOMO of PPy and VB of CeO2 to LOMO
of PPy and CB of CeO2 respectively. Further, the LUMO of PPy electrons
can be directly injected into the CB of CeO2. However, the LUMO po-
tential of PPy is more negative than CB of CeO2. Finally, the gathered
electrons in CB of CeO2 can react with surface adsorbed oxygen mole-
cules to generate super oxide radical anions [38,47,48]. Simulta-
neously, the excited holes produced by CeO2 are injected into the
HOMO of PPy due to the difference in valence band edge potentials and
then it was reacted with water molecules to generate hydroxide radicals
[49]. These generated super oxide radical anions and hydroxide radi-
cals are easily degrading the adsorbed contaminants. Obviously, the
upgrading of photocatalytic activity induced by the synergistic effect of
CeO2 and PPy had efficient charge separation and reduce the rate of
recombination.
3.3. Optimization of various parameters
In various pH (pH 2, 5, 7 and 9), the Cr6+ degradation efficiency
was studied by fixing other parameters (catalyst weight, Cr6+ con-
centration) constant as shown in Fig. 7A. This figure can clearly show
that the maximum detoxification of Cr6+ was observed in acid solution.
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V. Balakumar, et al.
Ultrasonics-Sonochemistry59(2019)104738
Fig. 8. CeO2@PPy core-shell nanosphere fast and efficient electron transfer
mechanism for the reduction of toxic Cr6+
.
Fig. 9. The repeated photocatalytic reduction experiments using CeO2@PPy
core-shell nanosphere photocatalyst.
noticeable changes. Therefore, ultrasound-assisted synthesis is a useful
method for improvement of degradation efficiency and catalyst stabi-
lity.
4. Conclusions
In this paper, CeO2@PPy core-shell structure of photocatalyst has
been synthesized via ultrasound-assisted method and the resulting
samples were compared to mechanical mixing preparation. As observed
that ultrasound irradiation improves the uniform distribution of parti-
cles and formation of core of CeO2 and PPy shell. The ultrasonic assisted
synthesis of CeO2@PPy exhibits a narrow bandgap and good reduction
efficiency compared to pure CeO2 and PPy. The synergistic CeO2 and
PPy core-shell structure facilitate a higher electron transfer rate and
prolong lifetime of photogenerated electron-hole pairs which could
achieve good reduction rate of 98.6% within 30 min. Obviously, the
ultrasound irradiation has a positive effect on photocatalysts synthesis
and reduction of hazardous Cr6+ to benign Cr3+ in aqueous solutions.
This method opens a new window for simple fabrication of conducting
polymers-based metal oxide nanocomposite towards wastewater re-
mediation and beyond.
Fig. 7. (A) Effect of pH (B) catalyst dotage and (C) Cr6+ concentrations.
3.5. Stability and reusability
To identify the stability and reusability of the CeO2@PPy core-shell
were evaluated in Fig. 9. The reduction of Cr6+ has been achieved
above 97% even after five catalytic cycles. The slight loss of efficiency
can be attributed to unavoidable loss of catalyst mass or blocking of
surface of the catalyst by degraded product. Further, XRD and SEM
analysis (Fig. 10A&B) of after five recyclable catalyst was observed no
7

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Ultrasonics-Sonochemistry59(2019)104738
using faujasite zeolite, Procedia Eng. 33 (2012) 52–57.
[8] M.A. Elbagermi, A.I. Alajtal, H.G.M. Edwards, Spectrophotometric determination of
chromium (VI) in nitric acid by means of solvent extraction with molten mixtures of
naphthalene and biphenyl, APCBEE Procedia 5 (2013) 378–382.
[9] H. Jiang, M. Li, J. Liu, X. Li, L. Tian, P. Chen, Alkali-free synthesis of a novel het-
erostructured CeO2-TiO2 nanocomposite with high performance to reduce Cr(VI)
under visible light, Ceram. Int. 44 (2018) 2709–2717.
[10] Z. Lv, H. Zhou, H. Liu, B. Liu, M. Liang, H. Guo, Controlled assemble of oxygen
vacant CeO2@Bi2WO6 hollow magnetic microcapsule heterostructures for visible-
light photocatalytic activity, Chem. Eng. J. 330 (2017) 1297–1305.
[11] C. Hou, Y. Zhang, J. Li, A. Zhu, In-situ hydrothermal synthesis of CeO2/SnS2 het-
erojunction for use as a new efficient visible-light-driven photocatalyst, Mater. Lett.
213 (2018) 154–157.
[12] P. Wang, W. Ji, M. Li, G. Zhang, J. Wang, Bi25VO40 microcube with step surface for
visible light photocatalytic reduction of Cr(VI): enhanced activity and ultrasound
assisted regeneration, Ultrason. Sonochem. 38 (2017) 289–297.
[13] S.L. Prabavathi, P.S. Kumar, K. Saravanakumar, V. Muthuraj, S. Karuthapandian, A
novel sulphur decorated 1-D MoO3 nanorods: facile synthesis and high performance
for photocatalytic reduction of hexavalent chromium, J. Photochem. Photobiol. A
356 (2018) 642–651.
[14] L. Zhang, C.-G. Niu, C. Liang, X.-J. Wen, D.-W. Huang, H. Guo, X-F. Zhao, G-
M. Zeng, One-step in situ synthesis of CdS/SnO2 heterostructure with excellent
photocatalytic performance for Cr(VI) reduction and tetracycline degradation,
Chem. Eng. J. 352 (2018) 863–875.
[15] M. Abinaya, K. Saravanakumar, E. Jeyabharathi, V. Muthuraj, Synthesis and
characterization of 1D-MoO3 nanorods using Abutilon indicum extract for the pho-
toreduction of hexavalent chromium, J. Inorg. Organomet. Polym. Mater. 29 (2019)
101–110.
[16] D. Channei, A. Nakaruk, P. Jannoey, S. Phanichphant, Preparation and character-
ization of Pd modified CeO2 nanoparticles for photocatalytic degradation of dye,
Solid State Sci. 87 (2019) 9–14.
[17] A. Khataee, P. Gholami, D. Kalderis, E. Pachatouridou, M. Konsolakis, Preparation
of novel CeO2-biochar nanocomposite for sonocatalytic degradation of a textile dye,
Ultrason. Sonochem. 41 (2018) 503–513.
[18] M. Mittal, A. Gupta, O.P. Pandey, Role of oxygen vacancies in Ag/Au doped CeO2
nanoparticles for fast photocatalysis, Sol. Energy 165 (2018) 206–216.
[19] N. Zhang, X. Fu, Y.-J. Xu, A facile and green approach to synthesize Pt@CeO2 na-
nocomposite with tunable core-shell and yolk-shell structure and its application as a
visible light photocatalyst, J. Mater. Chem. 21 (2011) 8152–8158.
[20] D. Channei, B. Inceesungvorn, N. Wetchakun, S. Ukritnukun, A. Nattestad, J. Chen,
S. Phanichphant, Photocatalytic degradation of methyl orange by CeO2 and
Fe–doped CeO2 films under visible light irradiation, Sci. Rep. 4 (2014) 5757.
[21] K.S. Ranjith, C.-L. Dong, Y.-R. Lu, Y.-C. Huang, C.-L. Chen, P. Saravanan, K. Asokan,
R.T. Rajendra Kumar, Evolution of visible photocatalytic properties of Cu-doped
CeO2 nanoparticles: role of Cu2+-mediated oxygen vacancies and the mixed-va-
lence states of Ce ions, ACS Sustainable, Chem. Eng. 67 (2018) 8536–8546.
[22] A. Murali, Y.P. Lan, P.K. Sarswat, M.L. Free, Synthesis of CeO2/reduced graphene
oxide nanocomposite for electrochemical determination of ascorbic acid and do-
pamine and for photocatalytic applications, Mater. Today Chem. 12 (2019)
222–232.
Fig. 10. XRD patterns of pre- and post- CeO2@PPy core-shell nanosphere and
(B) SEM image of CeO2@PPy core-shell nanosphere photocatalyst.
[23] X. She, H. Xu, H. Wang, J. Xia, Y. Song, J. Yan, Y. Xu, Q. Zhang, D. Du, H. Li,
Controllable synthesis of CeO2/g-C3N4 composites and their applications in the
environment, Dalton Trans. 44 (2015) 7021–7031.
Acknowledgements
[24] C. Wu, Solvothermal synthesis of N-doped CeO2 microspheres with visible light-
driven photocatalytic activity, Mater. Lett. 139 (2015) 382–384.
This study was supported by the Basic Science Research Program
through the National Research Foundation of Korea (NRF) funded by
the Ministry of Science, ICT and Future Planning (Grant No. NRF-
2017R1E1A1A01074266). This work was supported by the Industrial
Fundamental Technology Development Program (10076350) funded by
the Ministry of Trade, Industry and Energy (MOTIE) of Korea.
[25] K. Saravanakumar, R. Karthik, S.-M. Chen, J. Vinoth Kumar, K. Prakash,
V. Muthuraj, Construction of novel Pd/CeO2/g-C3N4 nanocomposites as efficient
visible-light photocatalysts for hexavalent chromium detoxification, J. Colloid
Interface Sci. 504 (2017) 514–526.
[26] B. Vellaichamy, P. Periakaruppan, R. Arumugam, K. Sellamuthu, B. Nagulan, A
novel photocatalytically active mesoporous metal-free PPy grafted MWCNT nano-
composite, J. Colloid Interface Sci. 514 (2018) 376–385.
[27] L.K. Krehula, J. Stjepanović, M. Perlog, S. Krehula, V. Gilja, J. Travas-Sejdic,
Z. Hrnjak-Murgić, Conducting polymer polypyrrole and titanium dioxide nano-
composites for photocatalysis of RR45 dye under visible light, Polym. Bull. 76
(2019) 1697–1715.
References
[1] B. Vellaichamy, P. Periakaruppan, B. Nagulan, Reduction of Cr6+ from wastewater
using a novel in situ-synthesized PANI/MnO2/TiO2 nanocomposite: renewable,
selective, stable, and synergistic catalysis, ACS Sustain. Chem. Eng. 5 (2017)
9313–9324.
[28] Farid A. Harraz, Adel A. Ismail, S.A. Al-Sayari, A. Al-Hajry, Novel α-Fe2O3/poly-
pyrrole nanocomposite with enhanced photocatalytic performance, J. Photochem.
299 (2015) 18–24.
[29] L. Wang, S. Jiao, C. Li, Y. Qu, B. Wang, Promoting infrared light driven photo-
catalytic activity of W18O49 nanorods by coupling polypyrrole, Res. Chem.
Intermed. 44 (2018) 5455–5466.
[2] A. Dandapat, D. Jana, G. De, Pd nanoparticles supported mesoporous γ-Al2O3 film
as a reusable catalyst for reduction of toxic CrVI to CrIII in aqueous solution, Appl.
Catal. A 396 (2011) 34–39.
[30] J. Xu, Y. Hu, C. Zeng, Y. Zhang, H. Huang, Polypyrrole decorated BiOI nanosheets:
efficient photocatalytic activity for treating diverse contaminants and the critical
role of bifunctional polypyrrole, J. Colloid Interface Sci. 505 (2017) 719–727.
[31] F. Duan, Q. Zhang, D. Shi, M. Chen, Enhanced visible light photocatalytic activity of
Bi2WO6 via modification with polypyrrole, Appl. Surf. Sci. 268 (2013) 129–135.
[32] M. Stucchi, G. Cerrato, C.L. Bianchi, Ultrasound to improve both synthesis and
pollutants degradation based on metal nanoparticles supported on TiO2, Ultrason.
Sonochem. 51 (2019) 462–468.
[3] B. Vellaichamy, P. Periakaruppan, A facile, one-pot and eco-friendly synthesis of
gold/silver nanobimetallics smartened rGO for enhanced catalytic reduction of
hexavalent chromium, RSC Adv. 6 (2016) 57380–57388.
[4] J. Sun, Z. Zhang, J. Ji, M. Dou, F. Wang, Removal of Cr6+ from wastewater via
adsorption with high-specific-surface-area nitrogen-doped hierarchical porous
carbon derived from silkworm cocoon, Appl. Surf. Sci. 405 (2017) 372–379.
[5] N. Kongsricharoern, C. Polprasert, Electrochemical precipitation of chromium
(Cr6+) from an electroplating wastewater, Water Sci. Tech. 31 (1995) 109–117.
[6] K. Chon, S.J. Kim, J. Moon, J. Cho, Combined coagulation-disk filtration process as
a pretreatment of ultrafiltration and reverse osmosis membrane for wastewater
reclamation: an autopsy study of a pilot plant, Water Res. 46 (2012) 1803–1816.
[7] H. Mekatel, S. Amokrane, A. Benturki, D. Nibou, Treatment of polluted pqueous
solutions by Ni2+, Pb2+, Zn2+, Cr+6, Cd+2 and Co+2 ions by ion exchange process
[33] S. Mallakpour, S. Shamsaddinimotlagh, Ultrasonic-promoted rapid preparation of
PVC/TiO2-BSA nanocomposites: characterization and photocatalytic degradation of
methylene blue, Ultrason. Sonochem. 41 (2018) 361–374.
[34] S. Heidari, M. Haghighi, M. Shabani, Ultrasound assisted dispersion of Bi2Sn2O7-
C3N4 nanophotocatalyst over various amount of zeolite Y for enhanced solar-light
photocatalytic degradation of tetracycline in aqueous solution, Ultrason. Sonochem.
8

V. Balakumar, et al.
Ultrasonics-Sonochemistry59(2019)104738
43 (2018) 61–72.
41 (2000) 1104–1107.
[35] M. Zarrabi, M. Haghighi, R. Alizadeh, Sonoprecipitation dispersion of ZnO nano-
particles over graphene oxide used in photocatalytic degradation of methylene blue
in aqueous solution: influence of irradiation time and power, Ultrason. Sonochem.
48 (2018) 370–382.
[43] Q. Wu, J. Zhao, G. Qin, C. Wang, X. Tong, S. Xue, Photocatalytic reduction of Cr(VI)
with TiO2 film under visible light, Appl. Catal. B: Environ. 142–143 (2013)
142–148.
[44] Microscale hierarchical three-dimensional flowerlike TiO2/PANI composite:
synthesis, characterization, and its remarkable photocatalytic activity on organic
dyes under UV-light and sunlight irradiation, J. Phys. Chem. C 118 (2014)
18343–18355.
[36] R. Magudieshwaran, J. Ishii, K.C.N. Raja, C. Terashima, R. Venkatachalam,
A. Fujishima, S. Pitchaimuthu, Green and chemical synthesized CeO2 nanoparticles
for photocatalytic indoor air pollutant degradation, Mater. Lett. 239 (2019) 40–44.
[37] B. Vellaichamy, P. Periakaruppan, Ag nanoshell catalyzed dedying of industrial
effluents, RSC Adv. 6 (2016) 31653–31660.
[45] B. Samai, S.C. Bhattacharya, Conducting polymer supported cerium oxide nano-
particle: enhanced photocatalytic activity for waste water treatment, Mater. Chem.
Phys. 220 (2018) 171–181.
[38] K. Saravanakumar, M.M. Ramjan, P. Suresh, V. Muthuraj, Fabrication of highly
efficient visible light driven Ag/CeO2 photocatalyst for degradation of organic
pollutants, J. Alloys Compd. 664 (2016) 149–160.
[46] R. Djellabi, M.F. Ghorab, Photoreduction of toxic chromium using TiO2-im-
mobilized under natural sunlight: effects of some hole scavengers and process
parameters, Desalin. Water Treat. 55 (2015) 1900–1907.
[39] S. Li, J. Cai, X. Wu, B. Liu, Q. Chen, Y. Li, F. Zheng, TiO2@Pt@CeO2 nanocomposite
as a bifunctional catalyst for enhancing photo-reduction of Cr (VI) and photo-oxi-
dation of benzyl alcohol, J. Hazard. Mater. 346 (2018) 52–61.
[47] L. Wu, S. Fang, L. Ge, C. Han, P. Qiu, Y. Xin, Facile synthesis of Ag@CeO2 core–shell
plasmonic photocatalysts with enhanced visible-light photocatalytic performance,
J. Hazard. Mater. 300 (2015) 93–103.
[40] J. Yang, Y. Liang, K. Li, G. Yang, S. Yin, One-step low-temperature synthesis of 0D
CeO2 quantum dots/2D BiOX (X = Cl, Br) nanoplates heterojunctions for highly
boosting photo-oxidation and reduction ability, Appl. Catal. B: Environ. 250 (2019)
17–30.
[48] N. Wetchakun, S. Chaiwichain, B. Inceesungvorn, K. Pingmuang, S. Phanichphant,
Andrew I. Minett, J. Chen, BiVO4/CeO2 nanocomposites with high visible-light-
induced photocatalytic activity, ACS Appl. Mater. Interfaces 4 (2012) 3718–3723.
[49] B. Vellaichamy, P. Periakaruppan, Synergistic combination of a novel metal-free
mesoporous band-gap-modified carbon nitride grafted polyaniline nanocomposite
for decontamination of refractory pollutant, Ind. Eng. Chem. Res. 57 (2018)
6684–6695.
[41] H. Yang, B. Xu, S. Yuan, Q. Zhang, M. Zhang, T. Ohno, Synthesis of Y-doped CeO2/
PCN nanocomposited photocatalyst with promoted photoredox performance, Appl.
Catal. B: Environ. 243 (2019) 513–521.
[42] S. Tsunekawa, R. Sahara, Y. Kawazoe, Y. Kawazoe, Origin of the blue shift in ul-
traviolet absorption spectra of nanocrystalline CeO2-x particles, Mater. Trans. JIM
9