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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.