One-pot solvothermal coating of carbon spheres with ZnS nanocrystallites and their use in the photodegradation of dyes

Article abstract of DOI:10.1002/ejic.201301288

In the present investigation, an easy approach for the synthesis of nanostructures composed of carbon spheres (CSs) and ZnS was demonstrated. CSs were prepared by aerosol-assisted chemical vapor deposition of toluene at 1050 °C and were characterized by different techniques, including X-ray diffraction, Raman spectroscopy, scanning electron microscopy, transmission electron microscopy, and thermogravimetric analysis. Further, their composite with ZnS nanoparticles (NPs) was synthesized by using solvothermal decomposition of a single-source precursor in the presence of ethylene glycol. The competence of the resultant composite material was tested in the photodegradation of eriochrome black-T and methylene blue, and the results were compared with those obtained for the bare ZnS NPs. The substantial increase in the photodegradation efficiency of the composite is attributed to the large surface area provided by the CSs to the ZnS NPs for dye reduction. Copyright

Full text of DOI:10.1002/ejic.201301288

FULL PAPER
DOI:10.1002/ejic.201301288
One-Pot Solvothermal Coating of Carbon Spheres with
ZnS Nanocrystallites and Their Use in the
Photodegradation of Dyes
[a]
[a]
Kanchan M. Samant, Jagruti S. Suroshe, and
Shivram S. Garje*[
a]
Keywords: Carbon / Composite materials / Nanoparticles / Thin layers / Chemical vapor deposition / Solvothermal
synthesis
In the present investigation, an easy approach for the synthe-
sis of nanostructures composed of carbon spheres (CSs) and
ZnS was demonstrated. CSs were prepared by aerosol-as-
sisted chemical vapor deposition of toluene at 1050 °C and
were characterized by different techniques, including X-ray
diffraction, Raman spectroscopy, scanning electron micro-
sition of a single-source precursor in the presence of ethylene
glycol. The competence of the resultant composite material
was tested in the photodegradation of eriochrome black-T
and methylene blue, and the results were compared with
those obtained for the bare ZnS NPs. The substantial in-
crease in the photodegradation efficiency of the composite is
scopy, transmission electron microscopy, and thermogravi- attributed to the large surface area provided by the CSs to
metric analysis. Further, their composite with ZnS nanopar-
ticles (NPs) was synthesized by using solvothermal decompo-
the ZnS NPs for dye reduction.
Introduction
reported a very simple method to prepare well-ordered hol-
low spheres of mesoporous N-doped carbon by using aceto-
Carbon is a versatile element that exhibits various forms
of structures including carbon nanotubes (CNTs), carbon
fibers, graphene, carbon spheres (CSs), and fullerenes. Ow-
ing to the unusual chemical, optical, electrical, and mag-
netic properties of these materials, they find applications in
many areas such as catalysis, emission devices, electrochem-
istry, and gas storage. Among these, CSs are considered to
[11]
nitrile as a source of carbon.
Other methods involve
chlorination of ferrocene at high temperature, self-assembly
of CNTs, and surfactant-assisted chemical vapor deposition
[12–17]
(CVD).
Recently, the inclination towards synthesizing hybrid
nanomaterials by surface modification to produce tailored
[18–20]
nanostructure materials has effectively increased.
Tre-
[
1]
be promising candidates as catalyst supports, biomateri-
mendous work comprising the immobilization of nanopar-
als,[ adsorbents, and anodes in secondary lithium-ion
2]
[3]
[21–24]
ticles (NPs) of either metal or semiconductor
on CSs
[
4]
batteries. They possess structural uniformity, high ther-
mal stability, large packing density, high chemical inertness,
is in progress because of the prospective use of these materi-
als in drug delivery, catalysis, energy storage, and separation
technologies. Additionally, such hybrid structures success-
fully modify the properties of the CSs such as crystallinity,
electrical conductivity, adsorption properties, and chemical
[
5]
tunable surface properties, and excellent conductivity.
Various methods have been reported for the synthesis of
CSs. Among the synthetic routes, the sacrificial template
method is widely used, as it helps to tune the pore size as
well as the surface morphology of the CSs. Silica spheres
and aluminosilicate spheres are used as templates along
[5,24–26]
reactivity.
Given that ZnS NPs are wide band gap semiconductors
with a band gap energy of 3.7 eV, they find applications in
many fields such as optical coatings, solid-state solar cell
windows, electrooptic modulators, photoconductors, field-
effect transistors, sensors, transductors, and photocataly-
with a carbon precursor that gets carbonized in an inert gas
to produce hollow CSs.[
6–9]
Sun et al. developed the hydro-
thermal synthesis of CSs by using sodium dodecyl sulfate
SDS) and glucose as the starting materials.[ Xia et al.
10]
(
[
27,28]
sis.
Similar to other wide band gap semiconductors,
the appreciable role of ZnS NPs in the degradation of dyes
and other organic pollutants fascinated us to choose it as
a coating material on the surface of CSs. As photocatalytic
processes are highly susceptible to surface structures and
properties, it is quite interesting to study the role of CSs
associated with ZnS NPs in photocatalysis. Though there
[
a] Department of Chemistry, University of Mumbai,
Vidyanagari, Santacruz (East), Mumbai 400098, India
E-mail: ssgarje@yahoo.com
[29]
ssgarje@chem.mu.ac.in
http://www.mu.ac.in/science/chemistry/profile.html
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201301288
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are various methods reported for the synthesis of ZnS, very (101), reported for hexagonal graphitic carbon (JCPDS No.
little work has been performed on the synthesis of hybrid 75-1621). The broad nature of the peak indicates the pres-
[
23]
structures of CSs and ZnS.
ence of amorphous carbon in the sample.
In the present work, we synthesized the hybrid material
in two steps. In the first step, we prepared CSs by aerosol-
assisted chemical vapor deposition (AACVD) of toluene at
1050 °C in the presence of nitrogen. Previously, the use of
benzene/ferrocene-based aerosols was reported for the pyro-
[
30]
lytic synthesis of aligned CNTs in the presence of Ar.
However, we did not employ any catalyst or template in the
preparation of these nearly monodispersed spheres. In the
second step, successive deposition of ZnS NPs on the sur-
face of the CSs was achieved by solvothermal decomposi-
tion of a single-source precursor (SSP), that is, ZnCl2-
(
cinnamtscz) (cinnamtscz = cinnamaldehyde thiosemicarb-
2
Figure 2. (a) XRD pattern and (b) Raman spectrum of pristine CSs
prepared by AACVD of toluene.
azone). The SSP molecule contains both metallic as well as
nonmetallic parts, which are required for the synthesis of
semiconducting NPs. Additionally, the use of a SSP mini-
mizes toxicity and avoids contamination that occurs upon peaks at about 1582 and 1354 cm and a single broad 2D
the separate introduction of two or more precursors. The band centered at 2830 cm are observed. The peak at
In the Raman spectrum, two significant bands having
–1
–1
–1
solvothermal synthesis allows precise control over size, 1582 cm , referred to as the G band, is associated with the
shape, and particle distribution and avoids the use of ex- E2g mode of hexagonal graphite and is related to the vi-
2
pensive chemicals. To the best of our knowledge, this is the bration of the sp -hybridized carbon atoms in the graphite
first time that the use of a SSP has been reported for the layer. A distinct band at 1354 cm–1 (D band) is associated
preparation of a CSs–ZnS composite material by using the with the vibration of carbon atoms in the disordered graph-
solvothermal method. We tested this material further in the ite.[31] Its intensity increases with the number of graphitic
photodegradation of methylene blue (MB) and eriochrome layers present. The relative intensity ratio (I /I ) signifies
D
G
black-T (EBT) to emphasize the role of the CSs in the pho- the degree of disorder in the graphitic structures. A higher
tocatalytic behavior of the ZnS NPs.
ratio of I /I indicates a higher degree of disorder in the
D G
synthesized material. This ratio was found to be 0.76, which
implies a relatively low proportion of defects in pristine
CSs.
[
32]
Results and Discussion
A second feature in the Raman spectrum is a broad 2D
Crystalline CSs
–1
band at ca. 2830 cm . This broadening could be due to the
combination of the D and G bands, induced because of
Crystalline, monodispersed CSs were synthesized by
AACVD of toluene at 1050 °C. The scanning electron mi-
croscopy (SEM) images recorded for the pristine product
are shown in Figure 1 (a,b). Nearly isolated, uniformly
spread spherical structures with an outer diameter of ca.
[
33]
disorder.
Raman analysis suggests the presence of gra-
phitic carbon in the product obtained by AACVD and is
consistent with the result obtained from XRD. All these
data reveal the formation of CSs in the sample obtained
from the AACVD of toluene.
To ensure the purity and thermal stability of the CSs,
thermogravimetric analysis (TGA) was performed under an
700 nm are observed. Energy-dispersive X-ray spectrometry
(EDS) revealed the presence of 100% carbon (Figure S1,
Supporting Information).
inert atmosphere of N (Figure 3, a) as well as in air (Fig-
2
ure 3, b). For TGA performed under an inert atmosphere,
about 17% weight loss was observed, which was attributed
to the loss of volatile solvent molecules, the oxidation of
different functional groups associated with the CSs, which
might be incorporated during the synthesis, and trace
amounts of amorphous carbon present. In addition to this,
the average particle size, surface area, and relative fractions
of the edge sites in the spheres are known to contribute to
Figure 1. SEM images of pristine CSs prepared by AACVD of tol-
uene: (a) low-magnification image and (b) high-magnification im-
age.
[34]
the oxidation behavior of graphitic carbon materials.
These factors can further contribute to the decomposition
behavior observed in the present case. No significant weight
The XRD pattern and Raman spectrum of the CSs are loss was observed up to 1000 °C, which suggests that the
shown in Figure 2. A strong diffraction peak at 25.5° and CSs are thermally stable. This could be assigned to the
a weaker peak at 43.3° in the XRD pattern of the CSs were highly ordered graphitic structures. For TGA performed in
assigned to characteristic diffractions, that is, (002) and air, as a result of the availability of sufficient oxygen, the
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CSs were completely oxidized to give gaseous products and
there was almost 100% weight loss up to 1000 °C. Similar
observations were reported earlier.[
16,17]
2
Figure 3. TGA profile of pristine CSs (a) under a N atmosphere
–1
and (b) in air; heating rate: 10 °Cmin .
CSs–ZnS Composite and ZnS NPs
The CSs–ZnS composite was prepared by heating the
CSs with ZnCl (cinnamtscz) in ethylene glycol at reflux.
2
2
ZnCl (cinnamtscz) acts as a SSP to give ZnS NPs. SSPs
2
2
have many advantages over multiple source precursors. The
main advantages of SSPs are their air stability, low toxicity, Figure 4. XRD patterns of the (a) ZnS NPs and (b) CSs–ZnS com-
stoichiometric control of the synthesized materials, and low posite showing hexagonal phase of ZnS (JCPDS: 79-2204).
decomposition temperature. Similarly, the solvothermal
method is beneficial owing to its simplicity and cleanliness.
The experimental parameters that need to be controlled in
a solvothermal route are very simple. This includes concen-
tration of precursor, nature of solvent, and experimental the (001), (002), and (003) crystal planes of the hexagonal
time. phase of ZnS. A peak at 25.5° corresponding to the charac-
The utility of ZnCl (cinnamtscz) as a SSP for obtaining teristic (002) reflection of graphitic plane (JCPDS No. 75-
The peaks at 2θ values of 45, 55, and 70° correspond to
2
2
ZnS can be tested from TGA studies. TGA of this precursor 1621) present in the CSs is merged with the (002) plane of
under an inert atmosphere and in air gave ZnS and ZnO, ZnS. No other impurity or additional peaks are seen. Thus,
respectively (Figure S2, Supporting Information). The there is exclusive formation of the CSs–ZnS composite.
weight loss (82.36%) observed in the TGA curve under an
inert atmosphere matches that expected (82.18%) for ZnS composite, and the ZnS NPs are shown in Figure S4 (Sup-
Figure S2a, Supporting Information). Whereas in air, the porting Information). The absorption band at 279 nm ob-
The UV/Vis spectra recorded for the CSs, the CS–ZnS
(
experimental weight loss (91.85%) tentatively matches the served for pristine CSs remains steady in the CSs–ZnS com-
theoretical weight loss (85.12%) expected in the formation posite. This band could be assigned to the absorption of an
[
36]
of ZnO (Figure S2b, Supporting Information).
aromatic π system in the graphitic structure.
The ZnS
The formation of the CSs–ZnS composite can be ex- NPs show absorption at 339 nm (Figure S4c, Supporting
plained in two ways. First, the capping solvent molecules Information), which is blueshifted to 329 nm in the CSs–
on the surface of ZnS can have interatomic bonding with ZnS composite (Figure S4b, Supporting Information). This
the surface of the CSs, which results in the formation of the could be assigned to a strong interaction between the CSs
composite. Second, there may be direct deposition of ZnS and ZnS, which might involve electron transfer from ZnS to
NPs over the surface of the CSs. Such types of interactions CSs. The corresponding photoluminescence (PL) emission
are feasible as a result of the partial functionalization of the (excitation wavelength 320 nm) is shown in Figure 5 (a,b)
CSs during the synthesis. This is evident from the IR spec- for ZnS and ZnS in the CSs–ZnS composite. The PL spec-
trum of the pristine CSs (Figure S3, Supporting Infor- trum shows a weak shoulder at 432 nm and a strong emis-
mation). Similar kinds of interactions were proposed by Hu sion at 460 nm in the case of pure ZnS NPs. The former
et al. They proposed that the functional groups present on could be assigned to interstitial sulfur vacancies and lattice
the surface of the CSs can bind metal cations through coor- defects and the latter ZnS emission could result from Zn
dination or electrostatic interaction.[
35]
vacancies.
[37]
For the CSs–ZnS composite, the respective
The XRD patterns of the ZnS NPs and the CSs–ZnS peaks are blueshifted to 406 and 426 nm. A drastic blueshift
composite are shown in Figure 4 (a,b). The position of the in the PL emission again suggests strong interaction be-
diffraction peaks for CSs–ZnS and ZnS are in good agree- tween the ZnS NPs and the CSs; this is established through
ment with the previous data and match well with the an electron-transfer process from the ZnS NPs to the CSs
[
38]
JCPDS file No. 79-2204.
as previously observed for CNTs.
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reflections for CSs and planes for ZnS-coated CSs. Selective
deposition of the ZnS NPs on the carbon spheres was ob-
served and there is no unspecific precipitation in the solu-
tion. This may be attributed to interactions between the
CSs and the ZnS NPs, as mentioned earlier. Moreover, the
CSs provide surface for the growth of ZnS NPs. This is
clearly seen in the TEM image of the composite.
Figure 5. PL spectra of (a) ZnS NPs and (b) the CSs–ZnS compos-
ite.
Comparison of the Raman spectrum of pristine CSs with
that of the CSs–ZnS composite shows that the characteris-
tic bands of CSs are weakened as a result of ZnS coating
Figure 7. TEM images of (a) CSs and (b) the CSs–ZnS composite
prepared by using the solvothermal method. Inset in (a) shows iso-
lated CSs and the inset in (b) shows CSs coated with ZnS NPs.
(Figure S5, Supporting Information).
The SEM images of the ZnS NPs and CSs–ZnS compos-
ite synthesized by using the solvothermal method are shown
in Figure 6 (a,b). The ZnS NPs formed are uniform and
have a spherical morphology. They seem to be agglomer-
ated at certain positions. Close inspection of Figure 6 (b)
reveals a coating of ZnS NPs all over the surface of the
CSs. Additionally, at certain points accretion of the CSs is
seen, which is absent in the bare CSs (refer to Figure 1).
This could be due to accretion of the CSs in ethylene glycol
prior to its coating with the ZnS NPs. Such a type of ac-
cretion was previously reported by Kang et al.[39] upon tre-
ating CSs with acetone. According to that report, the
spheres are made up of graphitic flakes and are believed to
exhibit many dangling bonds at their surfaces. As a result,
they become chemically active, which leads to accretion.
Photocatalytic Degradation Study
If a semiconductor material is exposed to the radiation of
energy higher than its band gap energy, the photocatalytic
process is initiated. In the case of ZnS NPs, the process can
be represented as follows:
Electrons are scavenged by molecular oxygen to produce
two intermediates, an oxygen radical and hydrogen perox-
ide. These immediately react amongst themselves to pro-
duce hydroxy radicals. These radicals are good oxidizing
agents and are therefore responsible for the degradation of
[
35]
most of the dye molecules.
The photodegradation efficiency of the CSs–ZnS com-
posite was studied by recording the UV/Vis spectra for MB
and EBT in the presence and absence of the catalyst. The
results were compared with those obtained for ZnS NPs.
Figure 8 (a,b) shows the photodegradation behavior of MB
and EBT under UV irradiation performed in the presence
of bare ZnS NPs and the CSs–ZnS composite. For compari-
son, the spectrum was also recorded for a solution without
Figure 6. SEM images of (a) ZnS NPs and (b) the CSs–ZnS com-
posite prepared by using the solvothermal method.
Figure 7 (a) reveals the TEM image recorded on pristine
CSs. It shows nearly homogeneous, spherical, and trans-
parent CSs with an average diameter of ca. 700 nm. The
inset shows a single, isolated sphere overlapped with the
other particles. Transparency is clearly visible in this over-
lapping. Figure 7 (b) shows ZnS NPs coated on CSs. The
spheres look like faceted particles. The inset shows a single
CS coated with ZnS NPs. The selected-area electron diffrac-
tion (SAED) pattern (Figure S6, Supporting Information)
Figure 8. A comparative study of photodegradation of (a) MB and
(
b) EBT in the presence of (i) no catalyst (blank), (ii) ZnS NPs, and
recorded on these samples clearly indicates (002) graphitic (iii) the CSs–ZnS composite.
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any added catalyst. MB showed a greater decrease in the preheated furnace. After the stipulated time, the humidifier was
turned off to discontinue atomization, and the furnace was cooled
optical density in the presence of the CSs–ZnS composite
in the presence of nitrogen. Then, the quartz tube was taken out,
than the sample with ZnS. Similarly, a decrease in optical
and the grayish product deposited on the wall of the tube was
density of the sample with ZnS was found to be greater
scratched and collected in a sample vial. The amount of precursor
relative to the optical density of the blank.
consumed was estimated by measuring the volume of the precursor
solution before and after the experiment.
For EBT, an absorption band at 550 nm is observed, but
the intensity of this band decreased substantially after UV
irradiation in the presence of ZnS and CSs–ZnS. It seems
that under identical experimental conditions, the rate of
photodegradation is appreciably higher for the CSs–ZnS
composite than for the bare ZnS NPs. This enhanced pho-
tocatalytic activity could be attributed to the presence of
Synthesis of the CSs–ZnS Composite and ZnS NPs by Using a SSP
by the Solvothermal Method: The single-source precursor,
ZnCl
by the method reported by our group.
thiosemicarbazone dissolved in THF was added to a stoichiometric
quantity of ZnCl in THF, and the reaction mixture was stirred at
2 2
(cinnamtscz) , used in the present investigation was prepared
[41]
Thus, cinnamaldehyde
2
CSs in the composite. This can be further explained by room temperature for about 36 h. Then, the solvent was removed
[
40]
using the study previously performed by Sun et al.
Hu et al.
and under vacuum, and the resulting yellow solid was repeatedly
[
35]
According to them, the surface of the CSs is washed with cyclohexane to remove any impurities present. The
product was dried under vacuum (m.p. 103 °C). The reaction
scheme is shown in Equation (1).
hydrophilic and carries some functional groups on their
surface; these groups have the capability to coordinate with
metals. Moreover, there are some nanosized pores that are
distributed on the surface of the spheres. This porosity not
only provides the surface area but also promotes the pene-
tration of some reactive species on the surface. Owing to
this, a greater number of ZnS NPs is deposited on the sur-
face of the CSs, which in turn facilitates the separation of
the electrons and holes and avoids their recombination.
This results in a higher concentration of the dye molecule
near the ZnS NPs associated with the CSs, which ultimately
leads to the effective degradation of the dye molecule.
For the synthesis of the CSs–ZnS composite and ZnS NPs, we
slightly modified the procedure that was reported previously by our
group for the synthesis of ZnS NPs.[ In a typical experiment, a
41]
250 mL, two-necked, round-bottomed flask was charged with CSs
(
60 mg). Ethylene glycol (20 mL) was then added, and the flask was
sonicated for 30 min. Then, this flask was kept in a rotamantle,
and the grayish black colored suspension obtained after sonication
was stirred at reflux in the presence of nitrogen. In another separate
2
50 mL round-bottomed flask, ZnCl
2
(cinnamtscz)
2
precursor
Conclusions
(
100 mg) was dissolved in ethylene glycol (20 mL). This solution
A simple solvothermal method with the use of a SSP was was then injected by syringe into the above refluxing solution of
used for the synthesis of a CSs–ZnS composite. To the best CSs in ethylene glycol. The reaction was stirred for 1 h. After this
2
of our knowledge, this is the first report on the synthesis of time, the reaction mixture was cooled under an atmosphere of N .
The suspension obtained was centrifuged at 9000 rpm, washed
thrice with dry methanol, and finally vacuum dried. The black-
colored solid powder obtained was characterized by UV/Vis spec-
troscopy, PL spectroscopy, and Raman spectroscopy, in addition
to XRD, SEM, and TEM techniques.
such a hybrid material by using a combination of AACVD
and the solvothermal method and its successful use in the
photodegradation of MB and EBT. The significant increase
in photocatalytic efficiency was attributed to the large sur-
face area and the porosity provided by the CSs for dye re-
duction.
To synthesize the ZnS NPs, a 250 mL, two-necked, round-bot-
tomed flask was charged with ethylene glycol (20 mL) and heated
to reflux under an atmosphere of N
necked round-bottomed flask, ZnCl
2
. In another 250 mL, single-
(cinnamtscz) (100 mg) was
2
2
dissolved in ethylene glycol (20 mL). This solution was then in-
jected by syringe into the round-bottomed flask containing re-
fluxing ethylene glycol. After 1 h, the reaction mixture was cooled
Experimental Section
Synthesis of CSs by Using AACVD: The setup used for the synthe-
sis of the monodispersed CSs consisted of a single-zone furnace
with an alumina tube (60 cmϫ5 cm) and a thermocouple pro-
grammed with a temperature controller. A fused silica tube with a
precleaned surface (70 cmϫ3 cm) and having a standard B24 joint
was placed inside the single zone. A 250 mL round-bottomed flask
containing toluene (40 mL) was attached to this tube with the help
of a connector. The other end of the quartz tube was connected to
a gas trap containing methanol to dissolve the byproducts formed
during the reaction. The whole assembly was flushed with nitrogen
gas, and nitrogen flow was continued throughout the experiment
2
under an atmosphere of N . The precipitate obtained was centri-
fuged at 5000 rpm, washed thrice with dry methanol, and finally
vacuum dried. The white-colored solid powder obtained was char-
acterized further with UV/Vis spectroscopy and PL spectroscopy,
in addition to XRD, SEM, and TEM techniques.
Material Characterization: Powder XRD studies of the materials
were performed with an XRD-7000, Shimadzu X-ray dif-
–
1
fractometer by using Cu-K
α
radiation at a scan rate of 0.5°min .
Raman spectra of the CSs and CSs-ZnS composite were recorded
3
+
by using the 514 nm line from a diode pumped Nd :YAG laser
(RENISHAW in via Raman Microscope, Laser 514 nm, Lens
100ϫ). FTIR spectra were recorded with a Perkin–Elmer FTIR
spectrophotometer. Analysis was performed by mixing the dried
–1
at a rate of ca. 190 mLmin . The temperature of the furnace was
raised to 1050 °C. Then, the mist of toluene generated by keeping
the above flask on an ultrasonic humidifier was passed into this
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samples with KBr followed by pressing it into a tablet. TGA of the
CSs was performed with a Perkin–Elmer instrument, Pyris Dia-
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with a FEI Quanta – 200 scanning electron microscope at an acce-
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attached to an SEM. TEM and SAED were performed with a
TECNAI-G2 20 ultrawin FEI Netherland microscope at an acce-
lerating voltage of 200 kV. Samples for the TEM images were pre-
pared by dispersing the dried powder in methanol and placing sev-
eral drops of the suspension on holey carbon films supported by
copper grids. The PL spectra were recorded with a Perkin–Elmer
LS 55 fluorescence spectrometer, whereas absorption spectra were
recorded by using a UV-2450 PC Shimadzu UV/Vis spectropho-
tometer at room temperature. For both measurements, a weighed
dried powder sample was dispersed in methanol and used to record
the spectra.
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MB and EBT: Photocatalytic activity studies of both the CSs–ZnS
composite and ZnS NPs were performed. The photodegradation
of MB and EBT in the presence of these materials was studied.
Visible light from a 500 W Xe lamp with a 420 nm cutoff filter was
used. MB and EBT of analytical reagent grade were used without
any further purification. Initially, a stock solution (δ = 200 ppm) of
each dye was prepared by dissolving its specific quantity in double-
distilled water. Further concentrations were prepared by diluting
the solution from the respective stock.
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