Nano-Fe3O4@SiO2–TiCl3 as a novel nano-magnetic catalyst for the synthesis of 4H-pyrimido[2,1-b]benzothiazoles

Article abstract of DOI:10.1007/s11164-018-3498-6

Abstract: Fe₃O₄@SiO₂–TiCl₃ NPs, a novel core shell catalyst, was synthesized via preparing Fe₃O₄@SiO₂ as a magnetic support followed by treatment with titanium tetrachloride (TiC

Full text of DOI:10.1007/s11164-018-3498-6

Res Chem Intermed
https://doi.org/10.1007/s11164-018-3498-6
Nano-Fe3O4@SiO2–TiCl3 as a novel nano-magnetic
catalyst for the synthesis of 4H-pyrimido[2,1-
b]benzothiazoles
Seyede Azita Fazeli-Attar1 Bi Bi Fatemeh Mirjalili1
•
Received: 18 March 2018 / Accepted: 31 May 2018
Ó Springer Nature B.V. 2018
Abstract Fe3O4@SiO2–TiCl3 NPs, a novel core shell catalyst, was synthesized via
preparing Fe3O4@SiO2 as a magnetic support followed by treatment with titanium
tetrachloride (TiCl4). The structure, morphology and magnetic properties of this catalyst
were recognized by different techniques including Fourier transform infrared spec-
troscopy, field emission scanning electron microscopy, X-ray diffraction, vibrating
sample magnetometry as well as thermo-gravimetric analysis. To consider the catalytic
activity of this magnetic catalyst, it was used in a multicomponent reaction between
2-aminobenzothiazole, aldehydes and ethyl acetoacetate to synthesize 4H-pyrimido[2,1-
b]benzothiazole derivatives. Our results indicated that the compounds were synthesized
with high yield and purity in a short reaction time. Furthermore, the obtained results
indicated that this catalyst can be reused for at least three times without no significant
change in the efficiency.
Graphical Abstract
Electronic supplementary material The online version of this article (doi:https://doi.org/10.1007/
s11164-018-3498-6) contains supplementary material, which is available to authorized users.
&
Bi Bi Fatemeh Mirjalili
fmirjalili@yazd.ac.ir
1
Department of Chemistry, College of Science, Yazd University, P.O. Box 89195-741, Yazd,
Islamic Republic of Iran
123

S. A. Fazeli-Attar, B. B. F. Mirjalili
Keywords Nano-Fe3O4@SiO2–TiCl3 Á 4H-pyrimido[2,1-b]benzothiazole Á Solid
acid catalyst Á Solvent free condition Á Multicomponent reaction
Introduction
Among heterocyclic compounds, fused heterocycles showed a substantial role in
pharmaceutical industry [1]. Benzothiazole derivatives were categorized as the
fused heterocycles. They have been known to have significant applications in
development of medicinal chemistry [2, 3]. To synthesis the pharmaceutical
compounds, it is necessary to use a convenient and straightforward method without
extra purification. The 4H-pyrimido[2,1-b]benzothiazole derivatives are compounds
with the potential ability in the preparation of drugs. In recent years, these
compounds were known to be functioning as anti-tumor [4], anti-inflammatory [5],
anti-bacterial and anti-fungal [6] materials. The synthesis of 4H-pyrimido[2,1-
b]benzothiazole is a three-component condensation reaction between aldehyde, b-
ketoester and 2-aminobenzothiazole. The catalysts including Fe3O4@nano-cellu-
lose-TiCl [7], nano-TiCl2/cellulose [8], PdCl2 [9], C–Ti O2–SO3–SbCl2 [10],
chitosan [11], N-sulfonic acid modified poly(styrene–maleic anhydride) SMI-SO3H
[12], FeF3 [13], tetrabutylammonium hydrogen sulfate (TBAHS) [14], hydrotalcite
[15], AlCl3 [16] and 1,1,3,3-N,N,N0,N0-tetramethylguanidinium trifluoroacetate
(TMGT) [17] have recently been applied to synthesis of these compounds. To
reduce disadvantages of some of these catalysts, a new and powerful catalyst was
prepared in this study. Our results showed that this catalyst was also stable and
friendly to the environment.
In recent years, the heterogeneous catalysts in organic reactions have been
extremely interesting. They showed the advantages including simple product
separation and purification steps as well as simplicity of their storage and handling.
Researchers have tried to improve these catalysts through decrease of their size,
increase of their surface areas and activity. Nanoparticles (NPs) were known as the
intermediate of homogeneous and heterogeneous catalysts that can be efficiently
dispersed in the reaction medium and then, improved the reaction [18]. However,
these catalysts had a small size, and; therefore, their separation was difficult.
Therefore, magnetic nanoparticles (MNPs) have been selected as the catalyst
support [19–22]. These nanoparticles can be easily separated from reaction medium
by an external magnet without using extra chemicals [23].
To control the oxidation and aggregation of these magnetic nanoparticles, the
surfaces of Fe3O4 nanoparticles were coated [24]. In recent years, different coatings
for Fe3O4 nanoparticles have been applied. They included surfactants [25],
biopolymers [7, 22, 26], silica [19, 21] and carbon [27–29]. The studies indicated
that a silica coating showed the efficient properties including nontoxicity, excellent
biocompatibility, stability and ability to be grafted with different modifiers. These
characteristics have caused the (Fe3O4@SiO2) core–shell structure to have more
attention and applied as a catalyst [21, 23, 24]. In the present study, Fe3O4@SiO2–
TiCl3 NPs was used as a powerful and ecofriendly catalyst. Its effect was assessed in
123

Nano-Fe3O4@SiO2–TiCl3 as a novel nano-magnetic catalyst…
the reaction of ethyl acetoacetate, aldehydes and 2-amino benzothiazole for the
synthesis of 4H-pyrimido[2,1-b]benzothiazole.
Experimental
General
All compounds were purchased from Merck, Aldrich and Fluka chemical companies
and used without any additional purification. FT-IR spectra were run on a Bruker,
Equinox 55 spectrometer. The Bruker (DRX-400 Avance) NMR was used to record
1
the H-NMR spectrum. Melting points were determined by a Buchi melting point
B-540 B.V.CHI apparatus and were uncorrected. The X-ray diffraction (XRD)
pattern was obtained by a Philips Xpert MPD diffractometer equipped with a Cu Ka
anode (k = 1.54 A) in the 2h range from 10° to 80°. Field emission scanning
electron microscopy (FESEM) photographs were obtained on a Mira 3-XMU.
Transmission electron microscopy (TEM) photographs were recorded by a Leo
912AB OMEGA instrument. The XRF analysis was done with a Bruker, S4
Explorer instrument. The VSM measurements were performed by using a Vibrating
Sample Magnetometer (Meghnatis Daghigh Kavir Co. Kashan Kavir, Iran).
Preparation of Fe3O4 NPs
A mixture of FeCl2Á4H2O (1 g, 5 mmol) and FeCl3Á6H2O (2.7 g, 10 mmol) was
dissolved in deionized water (25 mL) and heated in a water bath until the
temperature was 80 °C. Then, 17 mL of NH3 (30%) was added drop wise and
stirred by using a mechanical stirrer. After stirring the mixture for 30 min, the black
magnetic nanoparticles were aggregated by an external magnet, washed with
distilled water three times and dried at 80 °C for 2 h.
Preparation of Fe3O4@SiO2
At first, the mixture of 1.4 g of Fe3O4 NPs in 10 mL of ethanol and 3 mL of NH3
(35%) (mixture A), were sonicated in an ultrasonic bath for 30 min. Then, the
solution of 0.85 mL of Si(OEt)4 in 10 mL of ethanol was added drop wise to the
mixture A and stirred by using a mechanical stirrer for 1 h at room temperature.
Finally, the obtained mixture was filtrated and washed with ethanol.
Preparation of Fe3O4@SiO2–TiCl3 NPs
To obtain the mixture of Fe3O4@SiO2 (1.7 g) in dichloromethane (10 mL), TiCl4
(1.7 mL) was added drop wise and stirred in a well-ventilated system for 1 h at
room temperature. After removing HCl, the mixture was filtered and washed with
dichloromethane to remove unreacted TiCl4 and dried at room temperature to obtain
Fe3O4@SiO2–TiCl3 NPs.
123

S. A. Fazeli-Attar, B. B. F. Mirjalili
General procedure for the synthesis of 4H-pyrimido[2,1-b]benzothiazole
derivatives
In a 25 ML round bottom flask, aldehyde (1 mmol), ethyl acetoacetate (1 mmol),
2-aminobenzothiazole (1 mmol) and Fe3O4@SiO2–TiCl3 NPs (0.04 g) were
charged. The obtained mixture was heated to 100 °C in an oil bath and stirred by
a mechanical stirring motor with a speed of 50 rpm. The progress of the reaction
was monitored by TLC. After completing the reaction, the reaction mixture was
dissolved in ethanol. The catalyst was separated by using an external magnet and
then water was added to the solution. A pure solid appeared in high yield.
Results and discussion
To synthesize Fe3O4@SiO2–TiCl3 NPs as a new catalyst, Fe3O4@SiO2 were
prepared by using the pre-prepared nano-Fe3O4 and Si(OEt)4. Then, TiCl4 was
added to Fe3O4@SiO2. This process led to generation of this acidic heterogeneous
catalyst (scheme 1).
Field emission scanning electron microscopy (FESEM) and transmission electron
microscopy (TEM) were used to provide more accurate information about the
particle size of the catalyst. As the results obtained from TEM and FESEM showed,
the dimensions of Fe3O4@SiO2–TiCl3 NPs were almost 20 nm (Fig. 1).
To study the structure of Fe3O4@SiO2–TiCl3 NPs, the FT-IR spectrum of
Fe3O4@SiO2–TiCl3 NPs (d) was measured and compared with the FT-IR spectra of
nano-Fe3O4(a), Fe3O4@SiO2 (b) and TiCl4 (c) (Fig. 2). The peaks observed at 463
and 554 cm-1 are attributed to Fe/O in Fe3O4 nanoparticles [30]. After coating the
surface of Fe3O4 with SiO2, the appearance of two signals at 1088 and 806 cm-1
exhibited asymmetric and symmetric stretching vibration of a Si–O–Si bond,
respectively. The stretching and bending vibrations of the O–H bond were visible at
3388 and 1617 cm-1, respectively. It is notable that the Fe/O–Si band was appeared
Scheme 1 Preparation of Fe3O4@SiO2–TiCl3 NPs
123

Nano-Fe3O4@SiO2–TiCl3 as a novel nano-magnetic catalyst…
Fig. 1 a FESEM and b TEM image of Fe3O4@SiO2–TiCl3 NPs
Fig. 2 FT-IR spectra of (a) nano-Fe3O4, (b) Fe3O4@SiO2, (c) TiCl4 and (d) Fe3O4@SiO2–TiCl3 NPs
at approximately 584 cm-1 which overlapped with the Fe/O signal. In the spectrum
of the catalyst, the Si–O–Ti peak at 940–960 cm-1 indicated titanium successfully
bonded to the surface of SiO2.
Figure 3 shows the X-ray diffraction (XRD) pattern of the Fe3O4@SiO2–TiCl3
NPs. The values of 2h and FWHM are recorded in Table 1. The peaks at 2h of
31.07°, 36.51°, 42.00°, 53.80°, 56.82° and 63.03° were consistent with FWHM of
2.5190, 0.2362, 0.2362, 0.2362, 1.8893 and 2.3040, respectively. These
123

S. A. Fazeli-Attar, B. B. F. Mirjalili
Fig. 3 The XRD pattern of Fe3O4@SiO2–TiCl3
Table 1 Fe3O4@SiO2–TiCl3
No.
Pos. (°2h)
FWHM (°2h)
NPs reflexes in XRD
diffractogram
1
15.7764
18.3811
31.0748
32.1442
36.5149
42.0020
49.4550
52.7252
53.8054
56.8205
63.0364
0.3149
0.2362
2.5190
0.3149
0.2362
0.2362
0.2362
0.2362
0.2362
1.8893
2.3040
2
3
4
5
6
7
8
9
10
11
observations confirmed the crystalline cubic spinel structure of Fe3O4 nanoparticles
[31]. The broad peak observed at 2h of 20°–30° range described amorphous SiO2
[32]. Presumably, three other peaks in 2h of 36.51°, 42.00° and 53.8° revealed that
Ti was bonded to SiO2.
The X-ray fluorescence (XRF) analysis was used to measure the chemical
composition of catalyst (Table 2). The comparison between Kilo Counts Per
Seconds (KCPS) values of Ti and Cl in XRF catalyst and KCPS values of the same
elements in pure TiO2 and NaCl showed that the amount of Ti and Cl 4.2 g
Table 2 The XRF analysis of catalyst and pure samples of NaCl and TiO2
Elemental component
Fe3O4@SiO2–TiCl3
NaCl
TiO2
KCPS
wt%
KCPS
wt%
62
KCPS
wt%
60
Cl
82.9
162.7
41.9
11.1
15.5
18.6
56
516.5
TiO2
SiO2
Fe2O3
2318.4
819.5
123

Nano-Fe3O4@SiO2–TiCl3 as a novel nano-magnetic catalyst…
(0.088 mol) and 10 g (0.28 mol), respectively. These results indicated that Ti:Cl
ratio is approximately 1:3 in Fe3O4@SiO2–TiCl3.
The components of the Fe3O4@SiO2–TiCl3 NPs were analyzed using energy-
dispersive X-ray spectroscopy (EDX) analysis (Fig. 4). The peaks of Fe, O, Si, Cl
and Ti were observed and the compositions of Fe3O4@SiO2–TiCl3 NPs were
reported to be 17.09, 31.06, 3.39, 30.41 and 18.06% for Fe, O, Si, Cl and Ti,
respectively.
The thermal gravimetric analysis (TG–DTA) pattern of Fe3O4@SiO2–TiCl3 NPs
was also performed through heating from 100 to 800 °C (Fig. 5). According to the
curves, the 4% weight loss in the temperature 80–100 °C can be attributed to the
water desorption (exothermic). Two subsequent weight losses of the catalyst
corresponded to 10 and 2% in the temperature 100–400 °C and 400–800 °C,
respectively (endothermic). These observations showed that the catalyst was
stable till 100 °C. The char yield of catalyst at 800 °C is 86% of original sample
weight.
Brunauer–Emmett–Teller (BET) theory was applied to measure the specific
surface area of the catalyst. The single point surface area was 2.73 m2 g-1 at P/
P0 = 0.984, while the measured mean pore diameter and the total pore volume were
9.5287 nm and 6.5119 9 10-3 cm3 g-1, respectively. The N2 adsorption isotherm
of catalyst is shown in Fig. 6.
The magnetic property of Fe3O4@SiO2–TiCl3 NPs was evaluated by a vibrating
sample magnetometer (VSM) at room temperature (Fig. 7). The figure shows that
the catalyst (Fe3O4@SiO2–TiCl3 NPs) was superparamagnetic. The results indicated
that the coercivity value was zero, and there was no hysteresis loop and remanence.
The saturation magnetization (Ms) values of Fe3O4 and Fe3O4@SiO2–TiCl3 NPs
were 46.67 and 6.67 emu g-1, respectively. Although the magnetization of Fe3O4
decreased after coating, the catalyst can be easily separated from the solution with
an external magnet.
After discovering the catalyst’s characteristics, the reaction of ethylacetoacetate,
benzaldehyde and 2-aminobenzothiazole was chosen as the model reaction to
Fig. 4 The EDX spectra of Fe3O4@SiO2–TiCl3 NPs
123

S. A. Fazeli-Attar, B. B. F. Mirjalili
Fig. 5 The thermal gravimetric
analysis pattern of
Fe3O4@SiO2–TiCl3 NPs
Fig. 6 a BET (Brunauer–Emmett–Teller), b adsorption/desorption isotherm and c BJH (Barrett–Joyner–
Halenda) plots of Fe3O4@SiO2–TiCl3 NPs
Fig. 7 Magnetization loops of
(a) Fe3O4@SiO2–TiCl3 NPs and
(b) Fe3O4
evaluate its catalytic activity and the reaction conditions. The results are presented
in Table 3. After evaluating of the reaction medium and selecting a solvent free
condition as the best medium, the effect of different temperatures and amounts of
catalysts were checked. Then, the amount 0.04 g of the catalyst at 100 °C was
selected.
The reusability is one of the most important qualities of a catalyst. Therefore,
after using the catalyst in the model reaction, it was separated by an external
magnet, washed by chloroform and then, dried. The results obtained from reusing
123

Nano-Fe3O4@SiO2–TiCl3 as a novel nano-magnetic catalyst…
Table 3 The reaction of 2-aminobenzothiazole, benzaldehyde and ethyl acetoacetate in the presence of
catalyst under various conditionsa
Entry
Catalystb (g)
Solvent/condition
Time (min)/yield (%)c
1
0.04
0.04
0.04
0.04
0.04
0.04
0.04
0.04
0.04
0.02
0.03
0.05
H2O/reflux
C2H5OH/reflux
Acetonitrile/reflux
–/70 °C
–/80 °C
–/90 °C
–/100 °C
–/110 °C
–/120 °C
–/100 °C
180/–
180/–
180/50
150/71
150/90
75/70
45/90
30/70
40/73
45/61
45/68
45/54
2
3
4
5
6
7
8
9
10
11
12
–/100 °C
–/100 °C
aThe molar ratio of 2-aminobenzothiazole:benzaldehyde:ethyl acetoacetate is 1:1:1
bFe3O4@SiO2–TiCl3 NPs
cIsolated yield
Fig. 8 The reusability of the
catalyst
the catalyst showed no significant loss of its catalytic activity after four times
(Fig. 8).
To establish the catalytic activity, other 4H-pyrimido[2,1-b]benzothiazole
derivatives were synthesized by using different aromatic aldehydes under
optimized conditions. According to Table 4, aldehydes carrying electron-
withdrawing groups showed more rectivity than aldehydes with electron-donating
groups.
123

S. A. Fazeli-Attar, B. B. F. Mirjalili
Table 4 The synthesis of 4H-pyrimido[2,1-b]benzothiazole derivativesa
Entry
R
Time (min)
Yield (%)b
M.P. (ref.)
1
H
45
60
90
175–178 [14]
153–156 [17]
140–142
2
4-NO2
4-Cl
88
3
120
150
35
83.6
84
4
4-Br
107–109 [17]
218–222
5
4-OH
79
6
2-NO2
2-Cl
45
89
160–164
7
120
40
90
130–132 [14]
218–220 [7]
259–261 [8]
112–116
8
3-NO2
3-OH
83.6
87.7
82
9
90
10
11
12
2,4-(Cl)2
2,4-(OMe)2
3,4-(OH)2
120
120
25
83
164–166
83
227–229 [7]
aThe molar ratio of 2-aminobenzothiazole:aldehyde:ethyl acetoacetate is 1:1:1
bIsolated yield
A plausible mechanism for the reaction of ethylacetoacetate, aldehyde and
2-aminobenzothiazole was outlined in scheme 2. At first, the carbonyl groups in
aldehyde and b-ketoester have been bonded to Ti in catalyst and activated for
further condensation. Condensation of activated aldehyde and b-ketoester was done
by the Knoevenagel reaction and produced compound (I). Then, 2-aminobenzoth-
iazole reacted with compound (I) through a Michael addition to make an iminium
ion. After proton transferring and cyclization, 4H-pyrimido[2,1-b]benzothiazole
compounds were formed.
Conclusions
In summary, we have described the preparation and characterization of Fe3O4@-
SiO2–TiCl3 NPs as a novel, powerful and ecofriendly heterogeneous catalyst. This
catalyst showed excellent catalytic activity in a multicomponent reaction for the
synthesis of 4H-pyrimido[2,1-b]benzothiazoles. Some significant advantages of this
protocol were high yield, short reaction time, easy work-up, facile separation and
reusability of the catalyst.
123

Nano-Fe3O4@SiO2–TiCl3 as a novel nano-magnetic catalyst…
Scheme 2 A proposed mechanism for preparation of 4H-pyrimido[2,1-b]benzothiazole derivatives
Acknowledgements The Research Council of Yazd University is gratefully acknowledged for the
financial support for this work.
References
1. Y.D. Gong, T. Lee, J. Comb. Chem. 12, 393 (2010)
2. S. Seth, Antiinflamm. Antiallergy Agents Med. Chem. 14, 98 (2015)
3. P. Chander Sharma, K. Kumar Bansal, A. Deep, M. Pathak, Curr. Top. Med. Chem. 17, 208 (2017)
4. M.T. Gabr, N.S. El-Gohary, E.R. El-Bendary, M.M. El-Kerdawy, Eur. J. Med. Chem. 85, 576 (2014)
5. S. Nalawade, V. Deshmukh, S. Chaudhari, J. Pharm. Res. 7, 433 (2013)
6. M.S. Chaitanya, G. Nagendrappa, V.P. Vaidya, J. Chem. Pharm. Res. 2, 206 (2010)
7. S. Azad, B.F. Mirjalili, RSC Adv. 6, 96928 (2016)
8. S. Azad, B.F. Mirjalili, Res. Chem. Intermed. 43, 1723 (2016)
9. M. N. Bhoi, M. A. Borad, E. A. Pithawala, H.D. Patel, Arab. J. Chem. (2016) (in press)
10. M. Kour, S. Paul, J.H. Clark, V.K. Gupta, R. Kant, J. Mol. Catal. A: Chem. 411, 299 (2016)
11. P.K. Sahu, P.K. Sahu, S.K. Gupta, D.D. Agarwal, Ind. Eng. Chem. Res. 53, 2085 (2014)
12. M.M. Heravi, E. Hashemi, Y.S. Beheshtiha, K. Kamjou, M. Toolabi, N. Hosseintash, J. Mol. Catal.
A: Chem. 392, 173 (2014)
13. A.B. Atar, Y.S. Jeong, Y.T. Jeong, Tetrahedron 70, 5207 (2014)
14. L. Nagarapu, H.K. Gaikwad, J.D. Palem, R. Venkatesh, R. Bantu, B. Sridhar, Synth. Commun. 43, 93
(2013)
15. P.K. Sahu, P.K. Sahu, R. Jain, R. Yadav, D.D. Agarwal, Catal. Sci. Technol. 2, 2465 (2012)
16. P.K. Sahu, P.K. Sahu, J. Lal, D. Thavaselvam, D.D. Agarwal, Med. Chem. Res. 21, 3826 (2012)
17. A. Shaabani, A. Rahmati, S. Naderi, Bioorg. Med. Chem. Lett. 15, 5553 (2005)
123

S. A. Fazeli-Attar, B. B. F. Mirjalili
18. F.P. Ma, P.H. Li, B.L. Li, L.P. Mo, N. Liu, H.J. Kang, Y.N. Liu, Z.H. Zhang, Appl. Catal. A 457, 34
(2013)
19. V. Polshettiwar, R. Luque, A. Fihri, H. Zhu, M. Bouhrara, J.M. Basset, Chem. Rev. 111, 3036 (2011)
20. R. Hudson, V. Chazelle, M. Bateman, R. Roy, C.J. Li, A. Moores, A.C.S. Sustain, Chem. Eng. 3, 814
(2015)
21. R.B. Nasir Baig, R.S. Varma, Ind. Eng. Chem. Res. 53, 18625 (2014)
22. J. Safari, L. Javadian, RSC Adv. 4, 48973 (2014)
23. D. Wang, D. Astruc, Chem. Rev. 114, 6949 (2014)
24. T.K.H. Ta, M.T. Trinh, N.V. Long, T.T.M. Nguyen, T.L.T. Nguyen, T.L. Thuoc, B.T. Phan, D. Mott,
S. Maenosono, H. Tran-Van, Colloids Surf. A: Physicochem. Eng. Asp. 504, 376 (2016)
25. K. Khoshnevisan, M. Barkhi, D. Zare, D. Davoodi, M. Tabatabaei, Synth. React. Inorg. Met.-Org.
Chem. 42, 644 (2012)
26. V.A.J. Silva, P.L. Andrade, M.P.C. Silva, L.D.L.S. Valladares, J.A. Aguiar, J. Magn. Magn. Mater.
343, 138 (2013)
27. S. Zhang, H. Niu, Z. Hu, Y. Cai, Y. Shi, J. Chromatogr. A 1217, 4757 (2010)
28. B.Y. Chen, J.H. Qiu, H.X. Feng, Int. J. Miner. Metall. Mater. 23, 234 (2016)
29. M. Liu, H. Jin, E. Uchaker, Z. Xie, Y. Wang, G. Cao, S. Hou, J. Li, Nanotechnology 28, 155603
(2017)
30. A.M. Jubb, H.C. Allen, A.C.S. Appl, Mater. Interfaces. 2, 2804 (2010)
31. A. Alizadeh, M.M. Khodaei, M. Beygzadeh, D. Kordestani, M. Feyzi, Bull. Korean Chem. Soc. 33,
2546 (2012)
32. M.S. Moorthy, Y. Oh, S. Bharathiraja, P. Manivasagan, T. Rajarathinam, B. Jang, T.T.V. Phan, H.
Jang, J. Oh, RSC Adv. 6, 110444 (2016)
123