Biogenic Hierarchical MIL-125/TiO2@SiO2 Derived from Rice Husk and Enhanced Photocatalytic Properties for Dye Degradation

Article abstract of DOI:10.1111/php.12873

The hierarchical porous heterostructured MIL-125/TiO₂@SiO₂ composites are successfully obtained by biogenic-templated method using rice husk both as biomorphic. The products are characterized by X-ray diffraction, N₂-adsorption-desorption analysis, scanning electron microscopy and Fourier transform infrared spectroscopy. The results reveal that MIL-125/TiO₂@SiO₂ possesses a hierarchical porous structure in scale from micrometer to nanometer with high Brunauer–Emmett–Teller (BET) surface area (385.7–902.7 m² g^?1). The photocatalytic mechanism was discussed in detail. Meanwhile, the activity of MIL-125/TiO₂@SiO₂ for the photocatalytic degradation of rhodamine B in aqueous medium is significantly higher than that of P25. The current work provides some concept and methods in the development of MOF-based photocatalysts.

Full text of DOI:10.1111/php.12873

Article type
: Research Article
2 2
Biogenic Hierarchical MIL-125/TiO @SiO Derived from Rice Husk and Enhanced
Photocatalytic Properties for Dye Degradation
1
1,2,*
1
1
1
1
Huang Chaohui , Chen Hui , Zhao Lei , He Xuan , Li Weixin , Fang Wei , Wang
2
Guanghui
1
The State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science &
Technology, Wuhan 430081, P.R. China
2
HuBei Province Key Laboratory of Coal Conversion and New Carbon Materials, College
of Chemical Engineering and Technology, Wuhan University of Science & Technology,
Wuhan 430081, P.R. China
*Corresponding author’s email: chenhui86@wust.edu.cn (Chen Hui)
Abstract
2 2
The hierarchical porous heterostructured MIL-125/TiO @SiO composites are
successfully obtained by biogenic-templated method using rice husk both as biomorphic. The
products are characterized by X-ray diffraction, N -adsorption-desorption analysis, scanning
2
electron microscopy and Fourier transform infrared spectroscopy. The results reveal that
This article has been accepted for publication and undergone full peer review but has not
been through the copyediting, typesetting, pagination and proofreading process, which may
lead to differences between this version and the Version of Record. Please cite this article as
doi: 10.1111/php.12873
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MIL-125/TiO
2
@SiO
2
possesses a hierarchical porous structure in scale from micrometer to
2
nanometer with high Brunauer-Emmett-Teller (BET) surface area (385.7 - 902.7 m /g). The
photocatalytic mechanism was discussed in detail. Meanwhile, the activity of
2 2
MIL-125/TiO @SiO for the photocatalytic degradation of rhodamine B in aqueous medium
is significantly higher than that of P25. The current work provides some concept and methods
in the development of MOF-based photocatalysts.
Introduction
TiO
solar cells (1). Researchers are trying to improve its light-harvesting and photocatalysis
efficiency. One approach is to fabricate TiO in various nanostructures such as nanoparticles,
nanotubes or nanoporous frameworks. In recent years, hierarchically structured porous TiO
2
is one of the most promising semiconductors used in the field of photocatalytic and
2
2
composites have shown promising advantages over traditional photocatalyst. Meanwhile,
abundant efforts (2-4) have demonstrated that the porous structure ranging from microns to
nanometer of natural plants (especially leaves or coconut shell) strongly favors light
harvesting including the propagation, multiple scattering and absorbance of light. Another is
2
to improve light response range and quantum productivity properties of TiO by constructing
heterostructures (5), depositing metal (6), doping ions (7), or sensitizing by dyes (8).
Especially, heterostructures demonstrate a great potential to promote photocatalytic
performance because excited electron–hole pairs can be efficiently separated and charge
carriers can easily transfer across the interface of the heterostructure to restrain
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recombinations (9). Zhang et al. reported that TiO
heterostructures (TiO /g-C /G) prepared by a simple self-assembly photochemical
reduction method could catalyze hydrogen evolution under visible light (10). Also, 3D black
N-TiO -x@MoS core-shell nanostructures synthesized by a mild and effective strategy
2 3 4
nanowire/g-C N nanosheet/graphene
2
3 4
N
2
2
combined with a typical hydrothermal reaction and an in situ solid-state chemical reduction
◦
method followed by 350 C calcination under an argon atmosphere, which exhibited good
photocatalytic activity towards hydrogen evolution (11).
At present, metal-organic frameworks (MOFs) have received extensive attentions due to
their potential applications in sensors, catalysis, separation, gas storage, electrochemical and
photochemical devices (12-21). Series of MOFs with good photocatalytic performance were
2 2
developed, such as MOF-5, MIL-125(Ti), NH -MIL-125(Ti), UiO-66, NH -UiO-66 (22-27).
Comparing with the traditional catalyst, the structures and the functions of MOFs can be
effectively adjusted by changing organic ligands and metal elements. Besides, high specific
surface area and porous structure of MOFs can provide a large number of catalytic active
sites, optical electronic transport channels and chances for ions doping (28,29). In fact,
combined MOFs and inorganic semiconductors provide an effective method and avenues for
integrated optimization of semiconductor performance. More recently, photocatalytic activity
of MOFs and its composite with nanomaterials have been explored under UV radiation, such
3 4
as Au@MOF-5, Fe O @MOF-5, ZnO@ZIF-8 and Pt@ZIF-8 (30-34). The semiconductors or
nanoparticles, coupled with MOFs, may enhance the efficiency of photocatalytic process by
separating charges and extending the edge of absorption. However, thermal and chemical
stability and recoverability should be further researched.
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Our approach for artificial catalyst synthesis is to construct hierarchically porous
structures with combining semiconducting particles and MOFs to comprehend efficient
light-harvesting and photocatalytic degradation of dye. Herein, we demonstrate a facile
method to fabricate uniform MIL-125/TiO
rice husk template hydrothermal method. Of all the biogenic materials, rice husk (RH)
contains 70-75 mass % organics and 15-20 mass % amorphous silica (SiO ). Due to its
biological hierarchical porous SiO , was widely used as a construction material for concrete
35), as adsorbent (36,37) or as catalyst support (38), which fulfils the physical characteristics
and chemical composition of mineral admixtures (39-40). The hierarchical porous SiO
obtained from the decomposition of rice husk are both the temple and porous catalytic carrier,
which are favorable for light harvesting and replicated on the MIL-125/TiO @SiO . The
results show that MIL-125/TiO @SiO composites performed sensitization effect and
2 2
@SiO hierarchically porous composites based on
2
2
(
2
2
2
2
2
extended the wavelength range of absorbed light by material. The synergy effect of TiO and
2
MOFs component can restrain photoinduced electronic-hole in composite, thus improving the
photoelectric conversion efficiency.
MATERIALS AND METHODS
Materials. All chemicals were analytical grade and used without further purification.
2
Hydrochloric acid (HCl), Zinc chloride (ZnCl ), N,N-Dimethylformamide(DMF), absolute
methanol, tetrabutyl titanate, titanium acid isopropyl ester, terephthalic acid, isopropanol and
glacial acetic acid were purchased from the Sinopharm Group Co. LTD.. Distilled water was
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applied for all the synthesis and treatment processes. The rice husk (RH) was from Jingmen
of Hubei Province in China.
2
Preparation of biogenic hierarchical MIL-125/TiO . 10 g rice husk were stirred with 1 wt %
hydrochloric acid (100 mL), at 100 ºC for 1 h (denoted by HRH). After being rinsed with
º
distilled water to pH 7.0 and dried in an oven at 100 C for 24 h, the treated rice husk was
2
added to 10 wt % zinc chloride (ZnCl ) solution at a mass ratio of 1/10 under vacuum at room
temperature for 24 h. The resulting solution was dried at 120 °C for 72 h. As ZnCl is
2
introduced, the rice husk turned from yellow to dark. The dark powders, denoted by ZRH,
were washed with distilled water for 3 times. 4 g pretreated ZRH were added into 14.41 g
titania sols and impregnated for 24 h, after which the mixtures were dried in an oven at 100
ºC for 24 h. The dried powders were heat treated at 450 ºC for 3 h with a heating speed of 5
°C/min in a flow of nitrogen (1.0 L/min) in a tubular electric furnace to obtain the
morph-TiO composites. Subsequently, 54 ml of N, N-dimethyl formamide (DMF) was
2
added and stirred continuously for 30 min and mixed thoroughly with 6 ml of anhydrous
methanol , 3 g terephthalic acid and titanium acid isopropyl ester (1.56 ml) (23). Three parts
of the above solution were prepared by adding 0.5 g, 1.0 g and 1.5 g morph-TiO
2
, and
sonicated for 20 min respectively.
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The prepared solutions were then transferred to a Teflon-lined stainless steel auto-clave
100 mL) and the synthesis was carried out on oven at 150 ºC for 24 h. The resultant
(
suspension was filtered, washed with DMF and C H OH, respectively. The powder was
2
5
collected by filtering and drying under vacuum at 100 ºC for 12 h. The samples obtained by
adding 0.5 g, 1.0 g and 1.5 g of morph-TiO are denoted as S1, S2 and S3, respectively.
2
Besides, the whole process for the preparation of the catalyst was presented in Scheme 1.
Characterization. In order to determine the crystalline phases of the final powder samples,
X-ray diffraction measurements (XRD, X’Pert Pro, Philips, Netherlands) were carried out
using a conventional Bragg-Brentano diffractometer with Ni-filtered Cu Kα radiation. The
diffraction angle was scanned from 7 to 90° 2θ, at a rate of 10 °/min. The morphology of the
samples was examined by a field emission scanning electron microscope (FESEM, Quanta
400, FEI Company, USA) equipped with an energy dispersive X-ray spectroscope (EDS,
Noran 623M-3SUT, Thermo Electron Corporation, Japan) and transmission electron
microscopy (TEM, JEM-2100UHRJEOL, Japan). The specific surface area and the pore
diameter were determined by nitrogen adsorption at 77.15 K using a JWGB BK112T
Micromeritics instrument and by Brunauer-Emmett-Teller (BET) method, using the software
of Micromeritics. Nitrogen was used as cold bath (77.15 K). The Barrett-Joyner-Hanlenda
(
BJH) method was used to calculate the mesoporous distribution. The tested sample mass was
.2-5 g and heating at 537.15 K for 3 h at degassing conditions under vacuum pump. Fourier
0
Transform Infra-Red (FTIR) spectra were taken with a Spectrum One FTIR
spectrophotometer (Perkin-Elmer, America) at room temperature. The UV-vis diffuse
reflectance spectroscopy (DRS) measurements were performed using a Shimadzu UV-2600
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spectrophotometer equipped with an integrating sphere attachment. The analysis range was
from 220 nm to 800 nm, and BaSO was used as a reflectance standard.
4
Photocatalytic reactions. The photocatalytic degradation of Rhodamine B (RhB) was carried
out in a cylindrical glass reaction tube with air atmosphere upon UV-light irradiation at room
-3
temperature. Before the photocatalytic reaction, every 25 mL RhB (10 mol/L) add 0.025 g
of photocatalyst, stirred at room temperature and kept in dark for 3 h. The reaction was
performed under the irradiation of a 500 W mercury lamp. Degradation degree was measured
by UV-Vis spectrophotometer every 20 minutes. Commercial P25 (nano-titanium dioxide) is
mixed crystalline phase with anatase and rutile weight ratio of about 80/20, which is currently
the most widely used commercial photocatalyst. Therefore, we compare the photocatalytic
degradation efficiency of prepared samples with that of P25 under the same conditions.
2
.5 Electrochemical measurements
Electrochemical impedance spectroscopy (EIS) tests were carried out with AC
impedance-potential model in a standard three-electrode system. A silver chloride electrode
was as a reference electrode, a Pt foil as the counter electrode, and samples as the working
electrodes. The progress was performed under dark condition at -0.3 V potential over the
frequency range between 50 KHz and 100 mHz, with an AC voltage magnitude of 5 mV in
0.2 M Na
2 4
SO solution. For the preparation of composite electrodes, 0.04 g of the sample and
0.01 g of carbon black were uniformly dispersed in anhydrous methanol, and a dropwise
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addition of 1.5 % poly (3,4-ethylenedioxythiophene) -poly (styrenesulfonate) was used as a
binder to obtain a mixed slurry. The slurry was then mixed with foamed nickel and dried in
vacuo at 70 °C.
RESULTS AND DISCUSSION
2 2
Characterization of MIL-125/TiO @SiO
SEM technique was used to comprehensively investigate the morphology and
composition of the prepared MIL-125/TiO @SiO . Fig. 1 presents the SEM images of
2
2
MIL-125, S1, S2 and S3. Generally, the morphological feature in biomorphic hierarchical
structure still retains (Fig. 1). It can be seen that the MIL-125 is in cake shape with smooth
surfaces and diameter ranging at 300 - 550 nm (Fig. 1(a)). The cake-shaped MIL-125 was
observed in S1 and S2 with a diameter in size of 1-3 μm (Fig. 1(b, c)). The morphology of
2 2
MIL-125 changed into decahedral structure with increasing the quantity of TiO @SiO in
Fig. 1(d).
XRD patterns of MIL-125(Ti), S1, S2 and S3 are respectively shown in Fig. 2. The
diffraction patterns of composites were nearly the same as that of MIL-125(Ti), suggesting
that the crystalline characters of MIL-125(Ti) were preserved in the composites and all the
prepared products contained MIL-125(Ti) phase. Besides, peaks appeared for S1, S2 and S3
2
because of the presence of SiO phase in rice husk.
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Fig. 3 shows fourier transform infrared spectra (FT-IR) of MIL-125(Ti), S1, S2 and S3
-1
over the range of 1800 - 400 cm . It can be seen diffraction patterns of S1, S2 and S3 are
nearly the same as that of MIL-125(Ti) which presents characteristic bands of carboxylate at
-1
1
380 and 1600 cm (23). Characteristic absorption of benzene rings is located at 1013 and
−1
-1
7
40 cm (41). At 1730 cm , a weak band corresponding to the TiO surface adsorption was
2
−1
H
2
C
2
O
4
asymmetric stretching vibration. Bands between 400 and 800 cm are attributed to
Ti-O-Ti stretching vibrations in TiO
2
(42).
The nitrogen adsorption-desorption isotherms, pore size distribution of the samples and
corresponding parameters are shown in Fig. 4 and Table 1. The BET surface area are
undertaken at multi-point BET method (3-5 points) with the relative pressure range of
0
.05-0.3 for correlation coefficient higher than 0.999. Total pore volume of samples is
calculated by BJH adsorption cumulative surface area of pores between 17.000 Å and
000.000 Å diameters. Average pore diameter selected by BJH adsorption average pore size.
3
As shown in Fig. 4, the as-obtained samples are of type IV isotherm according to the IUPAC
classification (43). Compared with pure MIL-125(Ti), the BET surface area and total pore
volume of S1, S2 and S3 are decreased. The BET surface area of bare MIL-125 (Ti) is 1242
2
−1
2
−1
m g , while that of S1, S2 and S3 are respectively 902.9, 694.1 and 385.7 m g due to
presence of rice husk SiO . The average pore diameter of MIL-125 is 1.968 nm suggesting
2
that abundant micropore exists in MIL-125(Ti). The average pore diameter of S1, S2 and S3
are 2.151, 2.301 and 3.621 nm, indicating the presence of mesoporous in materials.
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To further investigate surface chemical composition and chemical state of elements
within the S2, X-ray photoelectron spectroscopic (XPS) measurement was carried out. The
survey spectra of all elements and the high resolution XPS spectra of C1s, O1s, Ti2p and
Si2p are shown in Fig. 5. As shown in Fig. 5(a), it indicates that the composite includes O,
Ti, C and Si elements with atomic content about 34.17 %, 6.04 %, 56.89 % and 0.87 %,
respectively. For C1s spectra (Fig. 5(b)), four distinct peaks at 284.5, 284.6, 284.83 and
2
88.49 eV are observed. The first peak at 284.5 eV is ascribed to C-C single bond to which
the benzene ring is attached to the -COOH. The second one (at 284.6 eV) is attributed to the
C-H in MIL-125. The peak at 284.83 eV is the C-O in carboxyl. The last one is assigned to
C=O bond. The O 1s spectra of S2 shown in Fig. 5(c) splits into three peaks. The peak at
5
2
29.84 eV and 531.6 eV are assigned to the Ti-O in TiO and MIL-125, resepectivly. The
peak at 530.8 eV is assigned to the Si-O in SiO . The Ti2p spectra of S2 shown in Fig. 5(d)
2
splits into three peaks. The peak at 458.6 eV is ascribed to Ti-O in TiO . Peaks at 463.95 eV
2
and 464.85 eV are assigned to the Ti-O in MIL-125.
Photocatalytic Activity
2 2
The light-harvesting properties of MIL-125/TiO @SiO were characterized by
UV-Vis spectroscopy. Fig. 6 displays the UV-Vis absorbance of MIL-125, S1, S2 and
S3. The absorption edges of S1, S2 and S3 are at around 360 nm corresponding to the
2
absorption of MIL-125 in UV range. The intercepts of tangents of the plots of (Ahv)
versus photon energy for hollow MIL-125, S1, S2 and S3 are shown in the inset of Fig.
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6
(insert). The estimated band gaps were 3.7, 3.5, 3.3 and 3.3 eV for MIL-125, S1, S2
and S3, respectively.
The photocatalytic performances of MIL-125, S1, S2, S3 and P25 powders were
characterized by degradation of rhodamine B (RhB) under UV light. Fig. 7 exhibits UV-Vis
spectra of RhB aqueous solutions after photocatalytic degradation for various times under UV
light and the graph of RhB residual rate. It is clear that the intensity of characteristic RhB
peak at 551 nm decreases with the increase in the photodegradation time, and the location of
the characteristic peak of RhB is not significantly changed. Fig. 7(a-d) compares the
photodegradation behavior of the samples synthesized by various processes, and Fig. 7(f)
shows their corresponding photodegradation rates. In the Fig. 7(f), Ct/Co is the absorbance
ratio of RhB aqueous solution at each time during the photodegradation process to the
absorbance of original RhB aqueous solution. It observes that S1 shows the best
photocatalystic property that it can absorb the RhB about 17.25 % after kept in dark for 3 h.
Photodegradation of the RhB was about 55 % in 2 h under UV irradiation. The adsorption
and photocatalytic performance of all samples is better than that of P25. In addition, as the
2 2
amount of TiO @SiO increases, the corresponding reduction in specific surface area also
leads to a decrease in photodegradation efficiency. Fig. 8 shows the regression curve of the
relationship between RhB concentration and absorbance. The relationship between the
concentration of RhB aqueous solution and the absorbance was obtained by measuring the
absorbance at a wavelength of 551 nm with a series of standard concentrations of RhB
-3
aqueous solution. The concentrations of RhB were 0.43, 0.45, 0.5, 0.6 and 0.72 (10 M),
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respectively, when the photocatalytic degradation of RhB by MIL-125, S1, S2, S3 and P25
was calculated by the regression curve.
Photocatalytic mechanism
The recombination of electron-hole pairs combines releasing energy, which can be
detected by PL emission. A low PL intensity indicates a low recombination of electron-hole
pairs, resulting in higher photocatalytic activity (44). The PL spectra of samples were
presented in Fig. 9, and the upper right of Fig. 9 is the excitation spectrum. According to the
excitation spectrum, monochromatic light having a wavelength of 268 nm was selected as the
excitation light. MIL-125 has a strong PL emission peak at around 385 nm. It is attributed to
2
the terephthalic acid (H BDC) ligand (45). Incomparison with MIL-125, the peaks intensities
of S1, S2 and S3 are much lower. It shows that the recombination inhibition of electrons and
holes in S1, S2 and S3 are more efficient than that of the pure MIL-125. Especially, the
lowest emission intensity of S1 at 385 nm suggests the lowest recombination rate of
photoinduced electron-hole pairs.
Electrochemical impedance spectroscopy (EIS) measurements were carried out to study
the interface charge transport behavior in the hybrid system. Fig. 10 shows the EIS responses
of MIL-125, S1, S2, and S3. The radius of arc on the EIS Nyquist plot reflects the reaction
rate occurring at the electrode surface. The decrease of the arc radius indicates a fast
interfacial charge transfer. (46) Clearly, a smaller arc radius was observed for the S2 and S3
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as compared to those of MIL-125 and S1. However, there is no significant change in S1
compared to MIL-125. This may be due to the fact that the C left in the heat treatment of the
rice husk in a nitrogen atmosphere improves the conductivity, so that the charge transfer
2
resistance decreases with the increase of morph-TiO .
The schematic illustration of MIL-125/TiO @SiO photocatalytic degradation of
2
2
rhodamine B under UV Light is shown in Fig. 11. The ground state electrons located on
valence band in TiO and MIL-125 is excited to the conduction band under UV irradiation.
Then the excited electrons in TiO migrate to conduction band of MIL-125 and holes in
2
2
2
MIL-125 migrate to valence band of TiO . The excited electrons on conduction band of
-
MIL-125 restored O
2
to •O
2
, and the holes oxidizes RhB to CO
2
and H O.
2
Conclusions
2 2
In summary, heterostructured MIL-125/TiO @SiO composite was successfully
constructed via a mild chemical process and hydrothermal method. During this process,
TiO @SiO was firstly synthesized by a mild chemical process, followed by the
2
2
2 2
hydrothermal method synthesis of MIL-125 on the surface of TiO @SiO . After combined
with TiO @SiO , the morphology of MIL-125 in composite changed from circular to
2
2
octahedral and the specific surface area of composites decreased. It was found that S1 is an
efficient UV-light response photocatalyst for the photoreduction of RhB as well as MIL-125.
On the basis of TEM, SEM, XPS, UV-Vis, EIS and PL analyses, a possible mechanism was
proposed for the photocatalytic reaction process. In summary, this work demonstrated that
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2 2
MIL-125/TiO @SiO could be accurately prepared, and dis-played an excellent
photocatalytic performance.
Acknowledgements
This work was financially supported by the National Natural Science Foundation of
China (61604110), Key Technology R&D Program of Hubei Province (2015BCA253), China
Postdoctoral Science Foundation (2015M572210, 2016M602376), Natural Science
Foundation of Hubei Provincial China (2016CFB297), Department of Education Science
Research Program of Hubei Province (Q20161110), and Open Foundation of Key Laboratory
of Green Chemical Process (Wuhan Institute of Technology) Ministry of Education
(NRGCT201503), Training Programs of Innovation and Entrepreneurship for Undergraduates
of Province (201510488022).
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Table 1. From the corresponding Fig. 4 in the pore structure parameters of the sample.
Parameters
2
-1
3
-1
S
BET/m •g
V
t
/cm •g
D
p
/nm
Cc
Samples
MIL-125
1241.9
902.9
694.1
385.7
0.715
0.486
0.342
0.349
1.968
2.151
2.301
3.621
0.99982
0.99987
0.99952
0.99971
S1
S2
S3
Note: SBET: BET surface area; V : total pore volume; D : average pore diameter; Cc:
t
p
correlation coefficient.
FIGURE CAPTIONS
2 2
Scheme 1. The illustration for the preparation of MIL-125/TiO @SiO photocatalyst.
Figure 1. SEM images of MIL-125 and MIL-125/TiO @SiO : (a) MIL-125, (b) S1, (c) S2
2
2
and (d) S3.
2 2
Figure 2. XRD patterns of (a) MIL-125(Ti) and (b-d) MIL-125/TiO @SiO : (b) S1, (c) S2,
and (d) S3.
Figure 3. FT-IR spectra of (a) MIL-125(Ti), (b) S1, (c) S2, and (d) S3. Wavenumber range
-1
1
800 - 400 cm .
2
Figure 4. (A) N adsorption/desorption isotherms of (a) MIL-125(Ti), (b) S1, (c) S2, and (d)
S3. (B) Pore size distribution.
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Figure 5. XPS survey spectrum and the deconvoluted XPS of the MIL-125/TiO
2 2
@SiO . (a)
Typical wide survey. High resolution XPS spectra of (b) C1s, (c) O1s and (d) Ti2p.
Figure 6. UV-Vis spectra of MIL-125(Ti), S1, S2 and S3. The top right corner is to calculate
bandwidth of the graph.
Figure 7. UV-Vis spectra of RhB aqueous solutions after photocatalytic degradation for
various times under UV light. (a) P25, (b) MIL-125, (c) S1, (d) S2, (e) S3 and (f) RhB
residual rate changes over time.
Figure 8. The regression curve of the relationship between RhB concentration and
absorbance.
Figure 9. Photoluminescence spectra (PL) of the samples.
Figure 10. EIS Nyquist plots of the samples under dark condition.
2 2
Figure 11. Schematic illustration of photocatalytic RhB reduction over MIL-125/TiO @SiO
under UV-light irradiation.
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This article is protected by copyright. All rights reserved.

This article is protected by copyright. All rights reserved.

This article is protected by copyright. All rights reserved.

This article is protected by copyright. All rights reserved.

This article is protected by copyright. All rights reserved.

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