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CHAPTER 5
SYNTHESIS OF MESOPOROUS CADMIUM SULFIDE
NANOPARTICLES AND THEIR STRUCTURAL, OPTICAL
AND PHOTOCATALYTIC PROPERTIES
5.1 INTRODUCTION
Tuning of the optical, electrical, magnetic and mechanical properties of
semiconductor nanostructures by varying their size, structure and morphology has
been extensively explored (Marandi et al 2006, Chen et al 2006 and Park et al
2005). Since, the properties and applications of synthesised materials depend upon
the structures, many attempts have been made to prepare nanomaterials with
different morphologies and structures. Among the semiconductor nanomaterials
CdS, ZnS, CdSe, ZnSe etc are proved to be versatile materials because of their
applications in optoelectronic devices due to large variation in the band gap as a
function of particle size. This class of materials has interesting electronic, optical,
and magnetic properties. The methods of building macroscopic solids consisting of
two or three-dimensional periodic structures of nanoparticles and the properties of
such nanoparticle assemblies attract more attention (Murray et al 2000 and
Shipway et al 2000).
Cadmium sulfide (CdS) exhibits many remarkable characteristics
including good thermal, mechanical and size-dependent optical properties, which
has potential application in lasers, light-emitting diodes and optical devices (Jie et
al 2006, Karunakaran and Senthilvelan 2005). CdS is photochemically active and
can photosensitize biochemical oxidation–reduction reactions. CdS nanoparticles
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could be excited by visible light to produce photogenerated electrons and holes.
Hence, it is significant to obtain novel nano-/microstructures for improving the
properties of CdS in practical applications. In earlier reports it was stated that the
cubic phase cadmium sulfide is more effective than hexagonal structure as a
photocatalyst (Walter and Joachim 1997, Albert and Allen 1984). In particular,
CdS nanocrystals emit fluorescence in the spectral range from UV to red (Min et al
2004). At the same time, the luminescence of all colors in nanocrystals can be
excited by only one source.
Mesoporous materials constitute an important host for semiconductor
NPs due to well controlled particle size and distribution of NPs. Wang et al (2002)
incorporated CdS inside the SBA-15 channels by ion exchange mechanism. Xu et
al (2002) discussed about the formation of CdS within the modified MCM-41 and
SBA-15 channels. Liu et al (2003) synthesised nanowire arrays of CdS with
MWD-SBA-15.
In this study incorporation of huge quantity of CdS nanoparticles inside
the meso channels of silica (SBA-15) with different pore diameters has been
achieved by easy and inexpensive method. After removal of silica, highly ordered
two dimensional arrangements of mesoporous cadmium sulfide, with high surface
area compared to the previous methods were synthesised. Their structural, optical
and textural properties were analysed. The photocatalytic activity of the
synthesised CdS nanostructure was investigated by the degradation of Methylene
violet-2B dye using visible light irradiation.
5.2 EXPERIMENTAL DETAILS
5.2.1 Materials
Tetraethyl orthosilicate (TEOS) and triblock copolymer poly
(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol)
(Pluronic P123, M.W = 5800, EO20PO70EO20) were obtained from Aldrich.
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2M Cd(ac)2.2H
2O + SBA-15-x
in ethanolAddition of 2M Thiourea Stirring for 6 h
Solvent evaporation
Filtering & washing with ethanol to remove
SBA-15 template
Washed with 2M NaOH @
60C – 3 hrs
Mesoporous cadmium
sulfide (M-CdS-x)
Thermal treatment:
250C/12 hrs
x = 100C, 130C,
150C
2M Cd(ac)2.2H
2O + SBA-15-x
in ethanolAddition of 2M Thiourea Stirring for 6 h
Solvent evaporation
Filtering & washing with ethanol to remove
SBA-15 template
Washed with 2M NaOH @
60C – 3 hrs
Mesoporous cadmium
sulfide (M-CdS-x)
Thermal treatment:
250C/12 hrs
x = 100C, 130C,
150C
Cadmium acetate dihydrate (Cd(CH3COO)2.2H2O) and thiourea (CH4N2S)
were obtained from Wako pure chemicals, Japan and Nacalai tasque
respectively and used without further purification.
5.2.2 Synthesis of Mesoporous Silica SBA-15
SBA-15 was synthesized using the amphiphilic triblock copolymer
P123 (Vinu et al 2003). A typical synthesis was performed as follows: 4 g of
P123 was dispersed in 30 g of DI water (resistivity 10-18
m) with stirring for
3-4 h and 120 ml of 2 M HCl aqueous solution was added and stirred for
2 h at 40 C. Thereafter, 9 g of TEOS was added slowly with continuous
stirring for 24 h at 40 C to form homogeneous solution. The resulting gel
was finally aged at 100 C, 130 C and 150 C for 48 h. After filtering and
washing, the obtained solids were calcined in O2 atmosphere at 540 C to
decompose the triblock copolymer. The synthesised SBA-15 aged at 100 C,
130 C and 150 C was referred as SBA-15-100, SBA-15-130 and SBA-15-
150 respectively.
5.2.3 Synthesis of Mesoporous Cadmium Sulfide
2 M of Cd(Ac)2.2H2O was dispersed in 5 ml of ethanol, with that
500 mg of SBA-15 (100, 130, 150) was added under vigorous stirring for 2 h
with the addition of 2 M of thiourea (Tu) to the mixed solution. The mixture
was kept stirred until the solvent get evaporated completely. The reaction
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mixture was transferred to a petri dish and kept for calcination at 250C for
12 h in the N2 atmosphere. During the heating process, the thiourea gets
decomposed to release sulfur ions and reacts with the cadmium ions which
were inside the channels of SBA-15 to form CdS. Instead of twice/thrice
filling with low concentration of Cd and S sources as per the previous reports,
this method with high molar concentration completely fills the meso channels
of silica SBA-15 template. Silica-CdS composites were soaked with 2M
NaOH in PP bottle with stirring at 60C and filtered, washed with ethanol for
several times and the obtained silica free CdS were kept dried. Cadmium
sulfide synthesised with mesoporous silica SBA-15-100, SBA-15-130 and
SBA-15-150 was referred as M-CdS-100, M-CdS-130 and M-CdS-150
respectively which was represented schematically in the above flow chart.
The synthesised mesoscopic cadmium sulfide has been
characterized by various physic-chemical techniques such as powder X-ray
diffraction (XRD), nitrogen adsorption and desorption, high resolution field
emission scanning electron microscopy (HR-FESEM), energy dispersive X-
ray spectroscopy (EDX), and elemental mapping. The low and high angle
powder XRD patterns were recorded on a Rigaku X-ray diffractometer with
CuKα ( = 0.154 nm) radiation. The diffractograms were recorded in the 2range of 0.6–10 with a 2 step size of 0.01 and a step time of 10 s and for
2 range of 20-70 with step size of 1/min. To measure the textural
parameters such as surface area, pore volume and pore diameter, nitrogen
adsorption and desorption isotherms were performed using Quantachrome
Autosorb analyzer. The Hitachi S-4800 HR-FESEM with an acceleration
voltage between 10 and 15 kV were used to evaluate the morphology and
elemental analysis of the synthesised mesoporous cadmium sulfide.
High-resolution transmission electron microscopy (HRTEM) studies of the
synthesised mesoporous CdS were carried out on JEOL-2100EX2. UV-Vis
diffuse reflectance spectra were recorded on a Lambda 750 Perkin Elmer
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spectrophotometer. Photoluminescence study was carried out using a
Princeton Instruments monochromator and Hamamatsu R955 PMT
spectrophotometer. The excitation source was Kimmon HeCd laser (325 nm)
with an excitation power of ~5 mW focused onto approximately 1 mm
diameter. Optical phonon modes of mesoporous CdS was analysed using
Horiba Jobin-Yvon T64000, photon design micro Raman spectrophotometer
using Ar ion laser with the excitation of 514.5 nm.
5.2.4 Preparation of CdS Catalysts and Methylene Violet Solution
for Photodegration
Aqueous solution of methylene violet-2B (MV) (10-4
M, 50 ml)
with suspended mesoporous CdS catalyst (M-CdS-100) (0.1g/l) as stirred in a
100 ml beaker. The out-side walls of the beaker were covered with aluminum
foil to reflect back astray radiations. Before irradiation, the suspensions were
magnetically stirred in dark condition for 30 min. to ensure the establishment
of an adsorption/ desorption equilibrium of the dye on the surface of the CdS
particles. Samples (3 ml) were taken from the reactor at regular intervals of
illumination and centrifuged to remove the photo-catalyst before analysis by a
UV-Vis spectrophotometer at 582 nm corresponding to the maximum
absorption wavelength (max) of MV.
5.3 RESULTS AND DISCUSSION
5.3.1 X-ray Diffraction Analysis
The small angle X-ray diffraction (SAX) pattern of silica-free CdS
nanostructures shown in Figure 5.1 (a) reveals the formation of two
dimensional hexagonally packed arrangements with ordered pore structure.
After the complete removal of SBA-15, impregnated CdS particles into the
pore channels shows a similar SAX diffraction pattern with peaks indexed as
(100), (110) reflections of the hexagonal space group p6mm, which appears at
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the same positions as those of the parent SBA-15, implying an excellent
stability of the mesostructured framework during the hard-templating process.
The wide angle XRD pattern obtained for silica free mesoporous
CdS with different pore diameters are shown in Figure 5.1 (b). Diffraction
peaks at 26.67, 44.27, 52.19 can be indexed as (111), (220), (311) planes,
which are identified for cubic zinc blende phase of CdS (10-454). As a result
of extensive peak broadening, the XRD pattern reveals the reduced dimension
of crystallites for the synthesised mesoporous CdS (M-CdS). In the case of
spherical crystallites, the particle sizes were calculated using the Scherrer
equation from the intense broad (111) peak. The corresponding crystallite
sizes calculated are 2.89, 2.95 and 3.02 nm, respectively for M-CdS-100,
M-CdS-130 and M-CdS-150. This clearly indicates that the similar size (~3
nm) of CdS particles are filled in the mesochannels of SBA-15 synthesised
with different pore diameters.
Figure 5.1 (a) Small angle X-ray diffraction of template free
mesoporous CdS nanostructure. Inset shows the exposed
spectrum for M-CdS-100
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Figure 5.1 (b) Wide angle X-ray diffraction of template free mesoporous
CdS nanostructure.
5.3.2 Morphological Analysis
The morphology of mesoporous cadmium sulfide (M-CdS) with
different pore diameters was studied using a high-resolution field emission
scanning electron microscope (HR-FESEM). Figure 5.2 (a-i) shows the
FESEM images of the M-CdS-100, M-CdS-130 and M-CdS-150 with
different magnifications. The appearance of silica free M-CdS nanoparticles is
almost similar to that of the mesoporous SBA-15 silica template. All the
samples exhibit rod like morphology with nanoparticles that are uniform in
size and shape and aggregated as bundles. The observed similarity in the
surface morphology of the samples confirmed that the morphology of the
synthesised mesoporous CdS is retained in all the samples after complete
removal of parent silica template.
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(a) (b) (c)
(d) (e) (f)
g) (h) (i)
Figure 5.2 (a-i) FESEM images of template free mesoporous CdS
(M-CdS), synthesised using silica SBA-15 with different
pore diameters
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5.3.3 HRTEM and Compositional Analysis
HRTEM was also used to observe the structural order and the
arrangement of nanoparticles of the synthesised mesoporous CdS materials
with different pore diameters. The typical high resolution TEM images shown
in Figure 5.3 (a-f), reveal the template free mesoporous cadmium sulfide for
M-CdS-100, M-CdS-130 and M-CdS-150 respectively, with different
magnifications. The well aligned mesoscopic arrangements of channels for
the synthesised materials can directly be observed by HRTEM, which has the
same morphology of the parent silica SBA-15. It also reveals that the
synthesised CdS was completely filled inside the channels of SBA-15 and has
no significant effect on the original mesopore structure after the removal of
silica template. Inset in Figure 5.3 (a, c and e) shows the selected area
diffraction pattern (SAED) for M-CdS-100, M-CdS-130 and M-CdS-150,
confirming the crystallinity of the synthesised CdS nanostructures.
Composition of the synthesised products analysed by energy dispersive X-ray
spectroscopy (EDX) and the elemental mapping for M-CdS-100 is displayed
in Figure 5.4 (a) and (b). It reveals the presence of Cd and S peaks, and the
purity of the synthesised CdS nanoparticles. There is no peak present for
silica which confirms the synthesised product was template free.
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(a) (b)
(c) (d)
(e) (f)
Figure 5.3 HRTEM images of the silica free mesoporous cadmium
sulfide nanostructures. (a, b) M-CdS-100, (c, d) M-CdS-130
and (e, f) M-CdS-150. Inset in (a, c and e) shows the SAED
pattern respectively.
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Figure 5.4 (a) EDX spectrum of mesoporous cadmium sulfide
nanoparticles
Figure 5.4 (b) Elemental mapping of mesoporous cadmium sulfide
nanoparticles
5.3.4 BET Surface Area Analysis
The textural properties of mesoporous CdS (M-CdS) were
investigated by the nitrogen adsorption-desorption isotherm and Barrett-
Joyner-Halenda (BJH) methods to determine the specific surface area, pore
volume and pore size distribution. Figure 5.5 (a) and (b) shows the N2
CdS
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adsorption-desorption isotherm with the evident hysteresis phenomenon and
the pore-size distribution of the mesoporous cadmium sulfide, M-CdS-100,
M-CdS-130 and M-CdS-150 respectively. The isotherm can be ascertained as
type IV, which is typically the characteristic of mesoporous materials. In
addition, the sharpness of the capillary condensation step and the shape of the
hysteresis loop in the nitrogen adsorption isotherm are similar to those of the
pure SBA-15, further confirming that the order of the mesopore structure is
retained. The observed textural parameters such as specific surface area, pore
volume and pore diameter for all samples are compiled in Table 5.1. In this
method we obtained the high specific surface area for the mesoporous
cadmium sulfide (M-CdS-X) compared to the earlier reports. The pore
diameter of the M-CdS materials increases with increasing the pore diameter
of the silica templates used. Among the synthesised mesoporous cadmium
sulfide prepared using SBA-15-X as a template, M-CdS-100 exhibits high
specific surface area (150 m2/g) with pore diameter (3.47 nm) compared to the
earlier reports (Liu et al 2003 and Gao et al 2003).
Table 5.1 Textural parameters of Mesoporous Cadmium Sulfide
Nanostructures
Sample Name
Specific
surface area
(m2/g)
Pore diameter
(dp, BJH) (nm)
Specific pore volume
(Vp) (cc/g)
M-CdS-100 150.5 3.47 0.327
M-CdS-130 118.7 4.82 0.323
M-CdS-150 82.3 6.43 0.297
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Figure 5.5 (a) Nitrogen adsorption/desorption isotherm of mesoporous
CdS with different pore diameters
Figure 5.5 (b) Barrett-Joyner-Halenda (BJH) pore-size distribution
plot of mesoporous CdS with different pore diameters
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5.3.5 UV-Vis Absorbance Studies
It is well-known that II–IV semiconductor nanoparticles, especially
for CdS particles, exhibit a huge change in their optical absorption when their
size reduces to a few nanometers. In the case of CdS particles of a few
nanometers in diameter, the effect is very sensitive to the small changes in the
particle size. From the reflectance-UV absorption spectra and in Figure 5.6
(a), it can be seen that the excitonic absorption peaks are well defined with
maxima at about 464 nm, 467 nm and 473 nm respectively for M-CdS-100,
M-CdS-130 and M-CdS-150. The direct bandgap values of the samples have
been obtained from (αhν)2 vs hν plot as shown in the Figure 5.6 (b). This
optical bandgap was calculated using the Tauc relation which is given by the
formula (Tauc 1974) in Equation 5.1.
(αhν) = A(hν – Eg)n
(5.1)
Where α is the absorption coefficient, hυ is photon energy, Eg is the
optical bandgap of the material, A is a constant and n = 1/2 for direct band
gap material. When (αhυ)2 is plotted as a function of (hυ), the linear portion of
the curve is extrapolated to (αhυ)2= 0, the bandgap value of the M-CdS-100,
M-CdS-130 and M-CdS-150 was found to be 2.68 eV, 2.64 eV and 2.61 eV
respectively. A comparison with the value of bulk CdS (512 nm) (Murray et
al 1993) shows that the band edge is blue shifted, indicating the quantum size
effect of the CdS nanostructures. This increase in the band gap arises shows
the quantum confinement effect.
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Figure 5.6 (a) DRS UV-Vis absorbance spectra of mesoporous CdS
nanoparticles
Figure 5.6 (b) Tauc plot of mesoporous CdS nanoparticles
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5.3.6 Photoluminescence Studies
The photoluminescence (PL) spectra of the synthesized mesoporous
cadmium sulfide (M-CdS) nanostructures with different pore diameters are
shown in Figure 5.7. The room temperature PL spectra were recorded with an
excitation wavelength of 325 nm at a fixed power of ~5 mW. The distinct
emission peaks were observed at about ~685 nm for M-CdS-100 and
M-CdS-130 and a relatively broad emission around 580 nm for M-CdS-150
with a lesser intensity compared to M-CdS-100, M-CdS-130. These infrared
bands are associated with structural defects that result from trap states arising
from the excess of sulfur or core defects on the nanoparticle surfaces (Xia et
al 2008). It was also reported that this longer wavelength emission originated
from transitions of electrons trapped at surface states to the valence band of
CdS nanoparticles due to surface defects (Zhou et al 2007, Blandin et al 2000
and Cao et al 2005). It may also be due to the combination of the band edge
emission with trap states emission, resulting in a very broad yellow-green
emission. Such strong luminescence is quite suitable to mark polymer
components for photoluminescence sensing. A significant change in the peak
position for M-CdS-150 was observed due to the samples prepared using
SBA-15 synthesised with different temperatures. The PL intensity decreased
because of a luminescence quenching probably due to cluster aggregation or
due to the removal of sulfur anion vacancies (Herron et al 1990).
If the reaction time increases, the formation of nanoparticles stops
and the particles start growing via the Ostwald ripening mechanism, i.e. larger
particles grown on account of the dissolution of the smaller ones, this result in
an increment on particle size and a decrement in the total number of particles
formed. The increment in particle size produce a red shift in the maximum of
the emission band and the decrement in the total number of particles produce
a decrement in intensity. It was reported that, 355 nm excitation in PL spectra
results with significant peak broadening at longer wavelength. This is
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probably due to the excitation of CdS nanoparticles with different sizes,
where the larger particles emit more to the red end of the spectrum (Castanon
et al 2010).
The nanoconnectors as observed in the porous CdS, are important
in forming ordered assemblies appear to affect the optical properties of the
CdS. Generally, the surface state emission appears around 530–650 nm in the
literature. It is observed that in the emission spectra, PL intensity for all the
samples shows stronger, which may arise from the excess of sulfur or core
defects. It was reported that the synthesis of CdS using SBA-15 as a template
produces nanowires of high spectroscopic quality possessing narrow excitonic
emission with almost insignificant surface/defect long wavelength emission
(Thiruvengadathan and Regev 2005). It was speculated that the existence of
defects such as sulfur vacancies in these hierarchical architectures after the
formation of the reaction solution results in the strong yellowish green
emission (Xiong et al 2007).
Figure 5.7 Photoluminescence spectra of the template free mesoporous
CdS nanostructures with 325 nm excitation wavelength
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5.3.7 Raman Studies
Raman spectroscopy has been chosen as an experimental tool since
it allows for probing of active optical phonon modes, as well as confined
electronic structure of quantum dots (Rolo et al 1998). Figure 5.8 shows the
room-temperature micro-Raman spectra of a mesoporous cadmium sulfide
with different pore diameters. One can clearly see the two strong
characteristic A1 mode, longitudinal optical (LO) phonon peaks corresponding
to CdS at about 300 cm-1
(1LO) and its first overtone band occurred at 600
cm-1
(2LO), together with a weak third harmonic peak at 900 cm-1
. All kinds
of CdS nanostructures exhibited similar Raman spectra, showing mainly the
two typical LO modes. No marked change in the peak position as a function
of particle size (2-5 nm), shape (rods, spheres) or excitation wavelength was
observed. The intensity changes were caused by the great strength of
exciton–phonon coupling, due to phonon confinement in the transverse
directions and the transfer of elementary excitation particles
(carriers, excitons, and phonons) (Pan et al 2005 and Shiang et al 1993).
Figure 5.8 Micro-Raman spectra of the template free mesoporous CdS
nanostructures, excited with 514.5 nm
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5.3.8 Photocatalytic Activity
CdS is one of the most well-known visible light-driven
semiconductors due to its narrow band gap. It can absorb most of the visible
light in the solar spectrum and has great potential application in solar cells
and photocatalysis. In order to investigate photocatalytic activity of the
synthesised high surface area mesoporous CdS, photodegradation of
methylene violet-2B (MV) dye was studied. Figure 5.9 (a) shows the
absorption spectra of aqueous MV dye solution (10-4
M, 50 ml) in the
presence of 5 mg of mesoporous CdS (M-CdS-100) under visible light for
various irradiation time. According to the UV-Vis absorption spectra the
spectral changes of MV in aqueous M-CdS-100 dispersions (Figure 5.9 (b)),
shows that the maximum absorbance is close to zero after 100 min in the
visible light irradiation. The complete degradation of MV dye under visible
light irradiation was achieved using the high surface area CdS nanostructures.
At the end of decolorisation, no absorption bands could be detected
in both UV and visible regions, implying that a complete oxidation of MV
occurred in the presence of mesoporous CdS nanostructures under visible
light. During the course of degradation, the color of the solutions became less
intense and finally transparent. Photocatalytic activity of semiconductors is
mainly determined by crystal structure, surface area, particle size, band-gap
energy and morphology (Testino et al 2007). Small-sized nanoparticles with
high surface area are effective substrates for absorption of UV or visible lights
(Wang et al 2008). The particles with small surface area and the crystal
defects cannot give very fast interfacial charge carrier transfer and it can
largely reduce the bulk electron–hole recombination during the migration of
electrons and holes to the surface of CdS. Hence the results indicate that the
synthesised silica free mesoporous CdS nanostructures with high surface area
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can shorten the bulk-to-surface migrating distance of electrons and holes and
react with the adsorbed reactants, which greatly improve the visible light
responsive for photocatalytic activity of CdS. This superior photodegradation
property of the CdS nanostructures is attributed to their higher surface area,
smaller crystal sizes and the crystallinity in the obtained samples.
Specifically, a large surface can furnish more active adsorption/desorption
sites for photocatalytic reaction, a smaller crystal size can lead to powerful
redox ability due to the quantum-size effect.
Figure 5.9 (a) Absorption spectrum of methylene violet-2B solution
(10-4
M, 50 ml) in the presence of 5 mg of M-CdS-100 under
exposure to visible light
MV-dye degradation with M-CdS-100
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Figure 5.9 (b) Photodegradation of MV under visible light using
mesoporous CdS nanostructure catalysts
5.4 CONCLUSION
Highly ordered mesoporous CdS nanostructures with different pore
diameters have been synthesised using mesoporous silica (SBA-15) as a hard
template. Small angle X-ray diffraction study indicates the presence of
mesoporosity and it was also confirmed through HRTEM analysis. The
mesoscopic CdS with cubic zinc blende phase and its crystalline nature was
revealed by wide angle XRD and SAED pattern respectively. The crystallite
size of the synthesised CdS nanoparticles was calculated to be ~3 nm using
Scherrer formula. Upon removal of template the materials possess good
textural properties with high specific surface area of 150, 118 and 82 m2/g
with tunable pore diameter of 3.4, 4.8 and 6.4 nm for M-CdS-100,
M-CdS-130 and M-CdS-150 respectively. The absorption edges of all the
samples are significantly blue shifted to ~2.6 eV compared to bulk CdS,
confirming the quantum confinement effect. Room temperature
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photoluminescence excited at 325 nm shows a strong yellowish green
emission, probably related to the recombination of electrons and holes at
surface traps due to the presence of sulfur vacancies. Raman spectra confirm
the presence of fundamental and overtone bands corresponding to the
cadmium sulfide. Photocatalytic activity for the mesoporous CdS analysed
with the degradation of methylene violet (MV) demonstrates that the rate of
dye-degradation increases with time under visible light irradiation. It clearly
shows the synthesised mesoporous CdS has excellent photocatalytic activity,
possibly originating due to the ordered arrangement of nanoparticles with
large specific surface area, smaller crystallite size and good crystallinity.