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Nida Qutub, Ph.D.Thesis 2013, A.M.U., India

CHAPTER 3 CADMIUM SULPHIDE

NANOPARTICLES

Nida Qutub, Ph.D. Thesis 2013, A.M.U., India�� 60

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CHAPTER 3

CADMIUM SULPHIDE NANOPARTICLES …………………………………………………………………………………………

3.1. INTRODUCTION:

Cadmium sulphide (CdS) is a II-VI semiconductor which is insoluble in water, but

soluble in dilute mineral acids. It exhibits intrinsic n-type of conductivity caused by

sulphur vacancies due to excess cadmium atoms1. CdS in bulk has band gap energy of

2.42eV at 300K with absorption maxima at 515nm2,3. It can attain three types of

crystal structures namely wurtzite, zinc blend and high pressure rock-salt phase

(Figure 3.1). Among these, wurtzite is the most stable of the three phases and can be

easily synthesized. Wurtzite phase have been observed in both the bulk and

nanocrystalline CdS while cubic and rock-salt phases are observed only in

nanocrystalline CdS4,5. The wurtzite form comprises of hexagonal close packing (hcp)

in which the stacking sequence of the atoms is ABABAB…, while, the zincblende

and rock salt structure have the stacking sequence of the atoms as ABCABCA…, i.e.,

called cubic close packing (ccp). In hexagonal wurtzite and cubic zinc blend, each

atom is coordinated to four other atoms in tetrahedral fashion such that each atom has

four neighboring atoms of the opposite type1, whereas in rock-salt each atom is

coordinated to six other atoms in octahedral fashion such that each atom has six

neighboring atoms of the opposite kind.

Figure 3.1: A representative diagram for the unit cell for crystal structure of CdS,

showing (a) wurtzite (hcp), (b) zinc blend (ccp) and (c) rock salt (ccp) phases.

Chapter 3

61 ��Nida Qutub, Ph.D. Thesis 2013, A.M.U., India�

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The nanoparticles of CdS show unique physical, chemical and structural properties

from the bulk. The melting point, electronic absorption spectra, band gap energy

crystal structure, and other properties of cadmium sulphide nanoparticles (CdS-NP)

are affected by size6-8. Thus, CdS on the whole is an attractive system for practicing

synthetic chemistry for nanocrystals and for understanding the chemistry, growth

history of nanomaterials and also for various technical applications9,10. Colloidal

dispersions of CdS semiconductor nanoparticles can display spectacular color changes

of their fluorescence depending on the size of the particle7. The CdS nanoparticles

shows quantum size effect, due to which the size of the cadmium sulphide particles is

directly related to the absorption wavelength9,11. The structure of the nanocrystalline

CdS can play an important role in determining the electronic properties. It can

crystallize in different structures upon size reduction, depending upon the reaction

conditions4.

Due to high stability, excellent physical, chemical and structural properties,

availability, ease of preparation and handling, CdS nanomaterials can be exploited in

various fields of life. Owing to quantum size effects and surface effects, CdS

nanoparticles can display novel optical, electronic, magnetic, chemical and structural

properties that might find many important technological applications. In addition to

size/volume ratio, the distribution of atoms over the surface is found to be a key

component of CdS semiconductor electrodes12. CdS-NPs are also used as pigment in

paints and in engineered plastic due to their good thermal stability1,13. CdS have large

band gap energy of 2.42eV at room temperature that enables its nanoparticles to be

remarkable in optoelectronics, photonics, photovoltaics and photocatalysis. Due to

photon-induced conduction, 1D CdS nanoparticles can be used in optoelectronics for

making photocells, light emitting diode (LED)14, lasers15, field-effect transistors

(FETs)16 and address decoders17. In photonics, due to its photoconducting and

electrical properties can be used in sensors, photodetectors, optical filters, and all

optical switches4,18-21. It exhibits high photosensitivity and its band gap appears in the

visible spectrum22, enabling it to be useful for many commercial and potential

applications in photovoltaics, as hetero-junction solar cells and thin film solar

cells4,21,23,24. In photocatalysis25, owing to its photochemical and catalytic properties,

CdS nanoparticles can be used for water splitting (hydrogen production)26,27 as well as

for water and air purification13,28. CdS NP can be used for the diagnosis and treatment

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of cancer due to its high optical and fluorescence properties. Diagnosis or imaging of

cancer cells can be done by accumulating CdS nanoparticles inside cancer cells,

which then can be easily visualized by irradiated with ultra violet radiation. For

treatment of cancer, photo activation of fluorescent CdS nanoparticles (photodynamic

cancer therapy) accumulated within cancer cell with radio sensitizing agents could

induce cell death29,30. In ophthalmology the CdS nanoparticles can be used for the

purposes of visualization as well as for drug delivery to the tissues of the eye,

including retina and cornea31.

Due to wide range of applications in different fields of life, CdS nanomaterials have

been synthesized extensively. Various techniques have been applied to fabricate CdS

in the form of thin films or powder, such as RF-magnetron sputtering technique32,

physical evaporation33, thermal evaporation34, hydrothermal synthesis35,36, electron

beam vacuum evaporation technique37, electrodeposition38, physical vapor deposition

(PVD)39, pulsed laser deposition40, laser ablation method41, spin-coating technique14,

solvothermal method42, template synthesis43, chemical bath deposition (CBD)44,

chemical precipitation method45, chemical vapor deposition (CVD)46, simulating

biomineralization technique13, biological synthesis using bacteria, fungi, yeast etc30.

Out of these synthetic methods, Chemical precipitation method is considered to be the

most appropriate due to its ease and simplicity. The chemical method usually required

simple lab equipments, ambient environmental conditions and the experiment usually

complete within hours, whereas other methods often required sophisticated

equipments, extreme environmental conditions (temperature, pressure etc.) and large

time interval. The particle sizes and stability are controlled either by restricting the

reaction space within matrices viz., zeolites, glasses, silica, polymers, reverse

micelles, vesicles and LB films32,42,47, or by using stabilizers and capping agents, like

thiols, phosphates, phosphine oxides, mercaptoacetic acid, long chain alkyl xanthates,

thiourea, and thioglycerol10,27. Another factor affecting the particle size of the CdS

nanoparticle is the solvent. Solvents are known to affect the kinetics and equilibria of

synthesis reactions, the spectroscopic properties of solutes and even the facets present

on crystals47-50.

Synthetic textile dyes and other industrial dyestuffs are one of the largest groups of

water pollutants in the world because of their displeasing, noxious, mutagenic,

consistent nature13,51-53. The photocatalytic degradation is one of the most efficient

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and economical method for non-destructive physical water treatment processes as

most of the conventional physicochemical and biological treatment methods being

inadequate for their effective removal51,54-58. In the last decade, photocatalytic

degradation using semiconductors have been shown to be effective for degradation of

pollutants in water. Several semiconductors such as ZnO, ZnS, TiO2 and Fe2O3 have

been used for heterogeneous photocatalytic degradation of organic wastes in

water25,58,59. CdS is a significant visible-light-sensitive semiconductor, which makes it

possible to utilize solar energy efficiently. CdS have been extensively used for

photocatalytic splitting of water for hydrogen production60 and for photodegradation

of organic or inorganic pollutants in air and water61 under visible light (VL).

Specifically, these materials have a relatively narrow band gap with the conduction

band edge sufficiently more negative than the reduction potential of protons, and thus

can efficiently absorb visible light62,63. However, the reported quantum efficiency of

the photocatalytic reactions by CdS is fairly poor due to the fast recombination of

photo-generated charge carriers. Various attempts to improve the efficiency of the

photocatalytic activity of CdS include changing the surface structure of CdS

nanoparticles by controlling morphology (size and structure)64, depositing CdS to

Nafion membranes or polymers to get homogeneously distributed quantum sized CdS

nanoparticles13,51,65, doping of transition metal ions into CdS28, and coupling of two

semiconductors66-68. Recently, CNTs (Carbon Nano tubes) decorated with CdS

nanoparticles and nanowires have been reported69,70.

In the present work, CdS nanoparticles were synthesized by six different

combinations of chemical precursors using Hydrogen Sulphide (H2S), Sodium

Sulphide (Na2S) and Ammonium Sulphide ((NH4)2S) as source of (Sulphide) S2- ions.

CdS nanoparticles were grown by simple chemical precipitation reactions in aqueous

medium at room temperature. The effect of stabilizers on the stability and size of CdS

nanoparticles was studied. The effect of different S2- ion sources ((NH4)2S, H2S and

Na2S) on the size of nanoparticles, respective band gaps and crystalline structure were

studied. Finally, the series of synthesized nanoparticles were exploited for the

degradation of Acid Blue-29 (AB-29), under visible light. The photocatalytic

efficiency of the synthesized nanoparticles, using different reactant combinations,

were compared and optimized.

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3.2. MATERIALS AND METHODS:

3.2.1. Synthesis:

CdS nanoparticles were grown by simple chemical precipitation reactions in aqueous

medium at room temperature and pressure. All chemicals were of analytical grade and

used as received without further purification. Six different reaction combinations were

set to synthesize CdS nanoparticles. A stock solution of Cadmium Nitrate (Cd(NO3)2)

(0.085M) was prepared and six different syntheses reactions were performed using

(NH4)2S, H2S and Na2S as S2- ion sources in presence and absence of stabilizing

agents. The reactions are summarized as below:

Reaction 1 (R1): Synthesis of CdS nanoparticles using Ammonium sulphide:

100mL aqueous solution of Cd(NO3)2 (0.085M) was added drop wise to 100 mL

aqueous solution of (NH4)2S (0.1M) with vigorous stirring. Stirring was continued for

5 hours. The dark yellow precipitates of CdS nanoparticles were obtained. This

reaction was similar to the reaction performed by P.P. Favero et al. 200612.

Reaction 2 (R2): Synthesis of CdS nanoparticles using Ammonium sulphide and

1-thioglycerol:

100mL aqueous solution of Cd(NO3)2 (0.085M) solution was stirred vigorously for 10

minutes. 1.6mL of 1-thio glycerol (98%) (0.18M) was added drop wise into the

solution with continuous stirring for 30 minutes. 20mL of (NH4)2S (20%) (0.5M) was

added to the solution under ambient conditions and the stirring was continued for

additional 5 hours, which yielded a dark yellow solution.

Reaction 3 (R3): Synthesis of CdS nanoparticles using Hydrogen Sulphide:

100mL Cd(NO3)2 (0.085M) solution was kept in H2S atmosphere for 1 minute with

vigorous stirring for additional 5 hours upon which the solution turned transparent to

yellow.

Reaction 4 (R4): Synthesis of CdS nanoparticles using Hydrogen Sulphide and

Methanol:

50mL Methanol (CH3OH) (24.44M) was added to a 100mL Cd(NO3)2 (0.085M) drop

wise with continuous stirring. The reaction was then carried out in H2S atmosphere

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for 1 minute with vigorous stirring continued for additional 5 hours upon which the

solution turned transparent to light yellow.

Reaction 5 (R5): Synthesis of CdS nanoparticles using Sodium Sulphide:

100mL aqueous solution of Na2S (0.1M) was added drop wise to a 100mL Cd(NO3)2

(0.085M) with continuous stirring for additional 5 hours. As the formation of

nanoparticles started the reaction system gradually changed from transparent to light

yellow. This reaction is similar to the reaction reported by V. Singh et al. 200971.

Reaction 6 (R6): Synthesis of CdS nanoparticles using Sodium Sulphide, Sodium

Hydroxide and Methanol:

100mL aqueous solution of Sodium Hydroxide (NaOH) (0.1M) and 50mL Methanol

(MeOH) (24.44M) was added slowly to 100mL aqueous solution of Cd(NO3)2

(0.085M) with continuous stirring continued for 1/2 hour. To this a 100mL aqueous

solution of Na2S (0.1M) was added drop-wise with vigorous stirring continued for

additional 5 hours, and a green-yellow solution was obtained. This reaction is similar

to the reaction reported earlier72. The precipitates thus obtained from the above

reactions were washed 3-4 times with water and acetone (used as non solvent) and

were air dried.

3.2.2. Characterization:

The functional and elemental analyses were carried out by Fourier Transform Infrared

Spectroscopy (FTIR) Spectroscopy and Energy Dispersive X-ray Spectroscopy

(EDS). The structural and morphological properties were studied by X-Ray

Diffraction (XRD) Spectroscopy, Scanning Electron Microscopy (SEM) and

Transmission Electron Microscopy (TEM). Thermal properties were determined by

Thermal Gravimetric Analysis (TGA), Differential Thermogravimetry (DTG) and

Differential Thermal Analysis (DTA) while the Optical properties were determined by

employing UV-Visible Spectroscopy.

3.2.3. Photocatalytic Experiment:

The photocatalytic activity of the CdS nanoparticles was studied by studying the

decolorization of a derivative Acid Blue-29 (AB-29) in presence of visible light.

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The photocatalytic experiments were performed in an immersion well photoreactor

made of Pyrex glass consisting of inner and outer jacket and equipped with a

magnetic bar, a water circulating jacket and an opening for molecular oxygen.

Irradiation was carried out using 500W halogen liner lamp (9500Lumens). The

catalyst dosage was optimized by irradiating the aqueous solution of the dye with

different strengths of CdS catalyst. 180mL of the dye solution of desired

concentration (0.06mM) containing the appropriate quantity of the catalyst (1gL-1)

was magnetically stirred in dark, in presence of atmospheric oxygen for at least 20

minutes to attain adsorption–desorption equilibrium between dye and catalyst surface.

A 5mL blank sample (0 minute) was taken out prior starting the irradiation. Other

samples (5mL) were collected at regular intervals during the irradiation and analyzed

after centrifugation. The suspensions were continuously purged with molecular

oxygen throughout each experiment and a constant temperature (20±0.3°C) was

maintained using refrigerated circulating liquid bath. The decolorization of AB-29

was monitored by the change in absorption spectroscopy using UV-vis. spectroscopic

analysis technique (Shimadzu UV-Vis 1601). The concentration of dye was calculated

by standard calibration curve obtained from the absorbance of the dye at different

known concentrations. For the purpose of practical implementation, it is essential to

evaluate the stability and reuse of the catalyst. The photocatalytic performances of the

nanomaterials were studied for five consecutive cycles using the same portion of

catalyst nanomaterials and a fresh solution of dye sample every time under similar

conditions.

3.3. RESULTS AND DISCUSSION:

The CdS nanoparticles obtained showed color variation from dark yellow to green-

yellow. This change in color from higher wavelength to shorter wavelength (blue

shift) might be due to the decrease in particle size7. Figure 3.2 shows the synthesized

CdS nanoparticles obtained as such in suspension form, and Figure 3.3 shows the

CdS NPs obtained after washing and drying. The effect of different S2- ions source

and the presence or absence of stabilizers are summarized in Table 3.1.

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Figure 3.2: The synthesized CdS nanoparticles (R1, R2, R3, R4, R5 and R6)

obtained as such in suspension form.

Figure 3.3: The synthesized CdS nanoparticles (R1, R2, R3, R4, R5 and R6)

obtained after washing and drying.

Table 3.1: The effect of different sulphide ion sources on synthesized CdS

nanoparticles (R1, R2, R3, R4, R5 and R6).

S.no. S2-

ion source Stabilizing agent Agglomeration Average Particle size

R1 (NH4)2S Absent Present 10.0nm

R2 (NH4)2S Present Absent 9.0nm

R3 H2S Absent Present 6.5nm

R4 H2S Present Absent 6.0nm

R5 Na2S Absent Present 5.0nm

R6 Na2S Present Absent 4.5nm

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3.3.1. ELEMENTAL ANALYSES:

The elemental analyses and determination of composition of synthesized CdS NPs

were carried out by Fourier Transform Infrared Spectroscopy (FTIR) Spectroscopy

and Energy Dispersive X-ray Spectroscopy (EDS).

3.3.1.1. Fourier Transform Infrared (FTIR) Spectroscopy:

FTIR is used to study the purity and composition of the synthesized products. It is

used to determine the functional groups and types of bonds present in the system. The

dried CdS nanoparticles mixed with KBr were characterized with FTIR. The FTIR

spectra could be explained by various peaks (Figure 3.4) obtained by the sample.

Table 3.2 contains the explanation of the peaks obtained by all the synthesized CdS

nanoparticles73,74. The absorption peak in the range of 3600-3100cm-1 could be

attributed to the –OH group of water adsorbed by the samples. The weak absorption

band at 1635cm-1 was assigned to CO2 adsorbed on the surface of the particles. In

fact, adsorption of water and CO2 are common for all powdered samples exposed to

atmosphere and are even more pronounced in case of nanosized particles with high

surface area9. Small peak near 400-470cm-1 indicated the formation of CdS

nanoparticles as this region was assigned to metal-sulphur (M-S) bond73-75. The peak

at 405cm-1 corresponded to the characteristic peak of CdS76-78.

Table 3.2: Interpretation of the peaks obtained by the FTIR spectra of the synthesized

CdS nanoparticles (R1, R2, R3, R4, R5 and R6).

Peak Region Intensity Significance

A 400-410 Small and weak Cd-S bond (CdS nanoparticles)

B 570-620 Small and weak S-S bond (crystal S-S bond)

C 820-850 Sharp S-S-S bending or C-H stretching

D 1060-1120 Sharp C-O or S-O (acetone or sulphate)

E 1380-1420 Sharp or Broad C-H bending of CH3 (Acetone)

F 2340-2360 Small and weak S-H bond (Free H2S)

G 3140-3470 Broad Intermolecular H-bonds (Lattice water)

69 ��Nida Qutub, Ph.D. Thesis 2

Figure 3.4: The FTIR spectra of

R5 and R6).

3.3.1.2. Energy Dispersive X-ray

Figure 3.5 reveals the EDS spec

and R6), the presence of Cd and

no other elemental impurity. T

55.5:44.5 for R1, 54.0:46.0 for R

R5 and 56.5:43.5 for R6. Other

and silicate, were due to sputter

not considered in elemental analy

Chapter 3

s 2013, A.M.U., India�

of the synthesized CdS nanoparticles (R1, R2, R3

ay Spectroscopy (EDS):

ectra of the synthesized CdS NP (R1, R2, R3, R

nd S peaks confirmed the formation of pure CdS

The average atomic percentage ratio of Cd:S

R2, 52.5:47.5 for R3, 55.0:45.0 for R4, 51.5:48

er peaks in this figure corresponded to carbon, ox

er coating of glass substrate on the EDS stage and

alysis of Cd and S.

R3, R4,

R4, R5

S with

:S were

48.5 for

oxygen

nd were

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Figure 3.5: The EDS spectra of synthesized CdS nanoparticles (R1, R2, R3, R4, R5

and R6).

3.3.2. STRUCTURAL ANALYSES:

The structural and morphological properties were determined by diffraction studies

(using X-Ray Diffraction (XRD) Spectroscopy) and microscopic studies using

Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy

(TEM).

3.3.2.1. Diffraction Studies:

The Diffraction Studies were carried out using X-Ray Diffraction (XRD)

Spectrometer.

X-Ray Diffraction (XRD) Spectrometer:

The XRD data revealed the formation of hexagonal-wurtzite type and cubic-zinc

blend type structured CdS NP (R1, R2, R3, R4, R5 and R6). The XRD pattern

displayed in Figure 3.6 showed that the crystal structure changed from hexagonal to

cubic with the decrease in particle size. The XRD pattern for CdS NP R1 (Figure 3.6)

can be consistently indexed on the basis of the hexagonal, W-type structure1,45,71 in

which the six prominent lines correspond to the reflections at 2�=25.182˚ (100),

26.816˚ (002), 28.465˚ (101), 37.372˚ (102), 47.206˚ (103) and 51.534˚ (112). The

weak 43.938˚ (110) peak was also observed. The peaks at 2��37˚ and 47˚ are

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characteristic for hexagonal W-type structure33,42,71. The estimated crystallite size for

R1 was ~9nm based on the FWHM of the (101) peak.

Similarly the XRD spectrum from R2 exhibited peaks at 2�=24.563˚ (100), 26.359˚

(002), 28.668˚ (101), 37.077˚ (102), 43.879˚ (110) and 51.063˚ (112) corresponding to

hexagonal, W-type structure. The estimated X-ray size for R2 (based on the FWHM

of the (101) peak) was ~7nm.

The XRD spectrum from R3 and R4 apparently exhibited only three broad peaks,

centered at 2��27˚, 43˚ and 51˚. The main broad peak at 27˚ on close observation was

found to be an overlap of multiple peaks, comprising of shoulders on both the sides at

2�� 24˚ and 28˚, respectively, resulting from the overlap of (100), (002) and (101)

peaks of hexagonal W-type structure. The increase in overlap in R3 and R4 was

clearly a result of line broadening due to the smaller particle size in these samples as

compared to R1 and R2. However, the three most prominent peaks for cubic CdS

with Z-type structure also occur at 27˚ (111), 43˚ (220) and 51˚ (311). Thus, the

presence of cubic CdS could not be ruled out based on XRD data. Therefore, it can be

concluded that R3 and R4 exhibited pronounced features of both phases and had a

distorted structure resulting due to the partial contents of both the phases6,79. The

estimation of the mean crystallite size was not possible for R3 and R4 due to the peak

overlap as well as the possible presence of a mixture of cubic and hexagonal phases.

The XRD pattern for R5, also exhibited three broad peaks centered at 2��27˚, 43˚ and

51˚ (Figure 3.6), but there were two main differences between the diffraction patterns

of R5 and R3/R4. In R5, the width of the diffraction peak at 27˚ was significantly

smaller, and it was much more symmetric. These observations indicated that the 27˚

peak in R5 is a single peak and not an overlap of multiple peaks. Thus, the XRD

peaks for R5 can be identified as 2�=26.864˚ (111), 43.281˚ (220), 51.403˚ (311)

peaks for cubic Z-type structure9,27,45,51. The mean crystallite size of R5, calculated

from the FWHM of the peak 26.864˚ (111) was ~5nm.

The nature of XRD pattern for R6 was similar to R5, but the widths of all the three

peaks were significantly larger. The mean particles size for R6 was ~4nm. Table 3.3

lists the crystal structures and mean particle size derived from the X-ray diffraction

data, for the different samples of CdS nanoparticles.

Figure 3.6: XRD spectra

R6).

Table 3.3: The crystalline

CdS

Nanoparticles

R1

R2

R3 Hex

R4 Hex

R5

R6

3.3.2.2. Microscopic Stu

Microscopic Studies we

Transmission Electron M

Chapter 3

Nida Qutub, Ph.D. Thesis 2013, A.M.U., Ind

tra of synthesized CdS nanoparticles (R1, R2, R3

ine phase and average crystallite size obtained by

Crystalline

Phase

FWHM (in

degree)

Crys

Hexagonal 0.9037

Hexagonal 1.12

exagonal +cubic -

exagonal +cubic -

Cubic 1.74

Cubic 2.0145

tudies:

ere done using Scanning Electron Microscope

Microscope (TEM).

India�� 72

3, R4, R5 and

y XRD data.

ystallite size

(nm)

9.068

7.320

-

-

4.693

4.053

pe (SEM) and

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Scanning Electron Microscopy (SEM):

The SEM micrographs showed amorphous mass with very fine particle structure. A

decrease in porosity was observed indicating reduction of particle size. Definite

particle shape was not visible due to more of a fine amorphous powder. Figure 3.7

shows the SEM images of the synthesized CdS NP at 5000 times magnification (5kx).

Figure 3.7: SEM images of the CdS nanoparticles (R1, R2, R3, R4, R5 and R6) at

5000x magnification.

Transmission Electron Microscopy (TEM):

Figure 3.8, displays the TEM images of the CdS-NP showing spherical quantum dots

with particle size less than 10nm. TEM images revealed that smaller sized NP were

formed when H2S (R3 and R4) was used as a source of sulphide ions instead of

(NH4)2S (R1 and R2), while Na2S (R5 and R6) gave the smallest sized NP. The

reason could be that, (NH4)2S is less active source of S2- ions than H2S and liberates

S2- ions less readily, and H2S liberates S2- ions even with lesser ease than Na2S47. This

means that the rate of precipitation has direct effect on the particle size. Hence, in

case of Na2S, the particle sizes were smallest of all. Also, the presence of stabilizers

showed a reduction in the aggregation and coagulation of NP (R2, R4 and R6). The

particle sizes obtained from the TEM images are listed in Table 3.4.

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Figure 3.8: TEM images of synthesized CdS nanoparticles (R1, R2, R3, R4, R5 and

R6).

Table 3.4: Particle size of synthesized CdS nanoparticles (R1, R2, R3, R4, R5 and

R6) obtained by TEM.

Sample R1 R2 R3 R4 R5 R6

Particle Size 10.0nm 9.0nm 6.0nm 6.5nm 5.0nm 4.5nm

3.3.3. THERMAL ANALYSES:

The thermal studies were done by using Thermal Gravimetric Analysis (TGA),

Differential Thermogravimetry (DTG) and Differential Thermal Analysis (DTA). The

TGA, DTA and DTG curves revealed high thermal stability of the synthesized

nanoparticles (R1, R2, R3, R4, R5 and R6), with high melting point and absence of

any impurity or intermediate complex. The synthesized CdS nanoparticles were found

to be thermally stable upto temperature as high as 700°C. Thereafter a gradual weight

loss was observed.

Thermal Gravimetric Analysis (TGA):

Figure 3.9 shows the TGA thermograph of synthesized CdS nanoparticles. CdS

nanoparticles showed good thermal stability upto 700°C and thus, can be used as

pigment in paints and in engineered plastic.


Figure 3.9: The TGA spectra of

and R6) showing thermal stabilit

Differential Thermogravimetry (

Figure 3.10 shows the DTG cur

with time against the temperature

compounds were found to be ab

700°C to 900°C due to phase cha

Figure 3.10: The DTG curve for

R5 and R6).

Differential Thermal Analysis (D

The DTA curve (Figure 3.11) sh

to thermal decomposition.

Chapter 3


of synthesized CdS nanoparticles (R1, R2, R3, R

lity upto 700°C temperature.

y (DTG):

urve, obtained by plotting the rate of change of w

ure and the result revealed that the melting points

above 1000°C with a slight weight loss in the r

hange.

for the synthesized CdS nanoparticles (R1, R2, R3

(DTA):

showed endothermic arrest leading to weight los

R4, R5

f weight

ts of the

e region

R3, R4,

loss due

Figure 3.11: The DTA cu

R5 and R6) showing endo

3.3.4. OPTICAL ANALY

Optical properties were d

UV-Visible (UV-Vis.) Spe

The UV-vis. spectroscop

range 200 to 800nm of s

515nm while in prepared

wavelengths smaller than

This blue shift was in go

to lower wavelength (blu

the synthesized CdS nan

(Figure 3.12). Figure 3

nanoparticles in the ran

nanoparticles R2 showed

bulk CdS (515nm), this

stabilizer preventing furt

(465nm and 445nm respe

reason that H2S is a mo

absorption edge (445nm)

methanol which acted as

blue shift (445nm) than R

Chapter 3


curve for the synthesized CdS nanoparticles (R1

dothermic peaks.

YSIS:

determined by employing UV-Visible Spectrosco

pectroscopy:

opy gave the absorption spectra of the nanoma

f solar spectrum. The absorption edge in bulk Cd

red CdS nanoparticles the absorption edges were

an the bulk which indicated a blue shift in abs

good agreement with the results reported before4,

lue shift) in absorption spectra than the bulk was

anoparticles as decrease in the particle sizes w

3.13, displays the absorption spectra of synt

range 400nm to 600nm of the visible spectr

ed greater blue shift (470nm) than R1 (475nm) as

is was due to the presence of thio-glycerol whic

urther crystal growth. R3 and R4 showed great

spectively) in the absorption edge than R1 and R

ore active source of S2- ion than (NH4)2S. R4

m) than R3 (465nm). This might be due to the

as a stabilizing agent. Similarly, R5 (425nm) sh

R4, as Na2S is more active source of S2- ion tha

India�� 76

1, R2, R3, R4,

scopy.

aterials in the

CdS is found at

ere observed at

bsorption edge. ,11. Also, shift

as observed in

were observed

nthesized CdS

ctra. The CdS

as compared to

hich acted as a

eater blue shift

R2 due to the

R4 had smaller

the presence of

showed greater

han H2S47. And

Chapter 3


�

finally, R6 (415nm) showed maximum blue shift due to the presence of stabilizing

agent (NaOH and MeOH) and Na2S as a source of S2- ions.

Figure 3.12: UV-visible absorption spectrum of synthesized CdS nanoparticles (R1,

R2, R3, R4, R5 and R6) showing blue shift in absorption edge.

Figure 3.13: UV-visible absorption spectrum of synthesized CdS nanoparticles (R1,

R2, R3, R4, R5 and R6) showing absorption edge.

Based on Tauc relation1,80, Figure 3.14, shows the plot of (�h�)2 versus h�, whose

intercept on energy axis gave the band gap energy (Eg) of the nanoparticles. The plot

showed a shift towards higher band gap. Figure 3.15 displays the obtained band gap

energy of the nanoparticles which are listed in Table 3.5.

Figure 3.14: The band g

towards higher band gap e

Figure 3.15: The band ga

R4, R5 and R6).

The particle radius (R) o

Brus Equation81-83 keepin

for CdS11,71, and Eg=(as o

Brus equation and band g

observed that the values

Chapter 3


d gap energy curve based on Tauc relation show

p energy of CdS NPs (R1, R2, R3, R4, R5 and R

gap energy of the synthesized CdS nanoparticles

of the synthesized CdS nanoparticles were cal

ping Eg0=2.42eV (bulk CdS), me=1.73x10-19, mh

observed, Table 3.5). The particle sizes (2R) cal

d gap energy are listed in Table 3.5. Form this ta

es of the band gap of synthesized CdS nanop

India�� 78

owing the shift

R6).

es (R1, R2, R3,

calculated from

h=7.29 x10-19,

calculated using

table it can be

oparticles were

Chapter 3


�

higher than the bulk band gap of CdS (2.42eV)2,3 and the band gap energy became

larger with the decrease in particle size. Thus, it can be concluded that the synthesized

CdS nanoparticles followed the quantum confinement effect84,85.

Table 3.5: The band gap energy and particle size (2R) of synthesized CdS

nanoparticles (R1, R2, R3, R4, R5 and R6) obtained by absorption spectra.

CdS nanoparticles R1 R2 R3 R4 R5 R6

Absorption wavelength (nm) 475 470 465 445 425 415

Band gap (eV) 2.61 2.64 2.70 2.8 2.93 3.0

Particle size (nm) 10.18 8.06 7.04 5.16 4.26 3.96

3.3.5. PHOTOCATALYTIC PROPERTIES:

The photocatalytic activity of the synthesized CdS nanoparticles (R1, R2, R3, R4, R5

and R6) was studied by photo-degradation experiment of a dye derivative AB-29 in

presence of visible light. The blank experiments were also separately carried out by

irradiating the aqueous solution of the dye derivative in absence of the photocatalyst

and in presence of the photocatalyst under dark condition. Analysis of the samples in

both cases did not show any appreciable loss of the dye (Figure 3.16). In order to

attain the superior photocatalytic activities of CdS nanoparticles, the activity of all the

six samples were compared. The effect of catalyst, synthesized by six different

modes, on the removal of dye was studied and the results are presented in Figure

3.16.

Figure 3.16 shows the relative change in the concentration (C/C0) of AB-29 in the

presence and absence of different photocatalysts (R1, R2, R3, R4, R5 and R6) as a

function of time. The kinetic results revealed that R6 had the highest activity and

almost completely decolorized the solution in a period of only 90 minutes. The

percentage decolorization of the dye followed the order; R1 (54%) < R2 (66%) <R3

(69%) <R4 (72%) <R5 (76%) <R6 (79%). R6 had smallest size so highest

surface/volume ratio and largest band gap, thus, highest photocatalytic activity. On

the other hand, in the absence of photocatalyst, no observable decrease in the dye

concentration could be seen.

Figure 3.16: Change in

absence of synthesized Cd

The highest photocatalyt

factors. The mechanism

nanoparticles can be expl

the absorption of a photo

nanoparticles, producing

leaving behind a positiv

aqueous solutions on sem

holes generated by pho

positive holes, the photog

holes in order to improv

methods is to increase t

decrease the recombinatio

Many researchers have

follow Langmuir-Hinshel

as:

��

��

��

��

Where k is the reaction ra

reactant (mM-1), t is the

Chapter 3


in concentration of AB-29 with time in the p

CdS nanoparticles (R1, R2, R3, R4, R5 and R6).

lytic activity of sample R6 could be the resul

m behind the enhancement of photocatalytic act

plained as follows. The photocatalysis over CdS i

ton with energy equal or comparable to the band

g photoexcited electrons (e-) in VB which migra

tive vacancy known as hole (h+). Photocatalytic

semiconductor particles is effected by electrons

hotoexcitation. Since the electrons tend to rec

togenerated electrons should be separated effectiv

rove the photocatalytic efficiency. One of the m

the band gap of the semiconductor catalyst w

tion of electron and hole86.

e reported that photocatalytic decolourization o

helwood kinetic model87-89 which can be genera

rate constant (mMmin-1), K is the adsorption coef

he reaction time and C is the dye concentration

India�� 80

e presence and

.

ult of multiple

activity in CdS

S is initiated by

and gap of CdS

grate to the CB

tic reaction, in

ns and positive

ecombine with

tively from the

most effective

which thereby

of most dyes

rally expressed

[3.1]

oefficient of the

n (mM). If the


concentration C is very small,

Equation 3.1 could be simplified

��

��

However, the degradation curve

exponential decay curve suggest

the rate constant was calcula

concentration as a function of irra

On the basis of the following equ

��

��

Where kapp is the apparent pseu

time (min), C0 is the initial conce

at time t (mM).

For our experimental conditions

pseudo first-order reaction as al

time as shown in Figure 3.17.

Figure 3.17: Change in conce

absence of synthesized CdS nano

Chapter 3


l, KC will be negligible with respect to unity so

ied to an apparent pseudo-first-order kinetics13,88.

[3.2]

ve (Figure 3.16) could be fitted reasonably well

esting pseudo first order kinetics. For each experi

lated from the plot of natural logarithm of

irradiation time13,52,88.

quation:

[3.3]

eudo-first-order rate constant (min-1), t is the rea

ncentration of dye (mM) and C is the dye concent

ns, data (Figure 3.16) were in good agreement

also depicted by plotting ln (C0/C) versus irrad

centration of AB-29 with time in the presence

noparticles (R1, R2, R3, R4, R5 and R6).

so that

ll by an

eriment,

of dye

reaction

ntration

ent with

adiation

nce and

Chapter 3


�

The slope of the plot of ln (C0/C) vs time gave the rate constant (kapp, apparent

pseudo-first-order rate constant) of the catalytic reaction, which was used to calculate

the decolorization rate. The correlation constant (R2) for the fitted lines was calculated

to be about 0.99 for all the experiments.

The decolorization rate of the dye was calculated using the formula given below87:

�d�C�

dt� k�C�n [3.4]

k=rate constant (molL-1min-1), C=concentration of the dye (mM), n=order of reaction.

The decolorization rate Figure 3.18 for the decomposition of AB-29 in the presence

of different photocatalysts (R1, R2, R3, R4, R5 and R6) revealed that the

decolorization of AB-29 proceeded faster as the sizes of the CdS nanoparticles

decreased. The decolorization rate followed the order R1 (3.7 10-4) < R2 (4.4 10-4) <

R3 (4.5 10-4) < R4 (4.8 10-4) < R5 (5.0 10-4) < R6 (5.2 10-4molL-1min-1).

Figure 3.18: The decolorization rate of AB-29 in the presence of different

synthesized CdS nanoparticles (R1, R2, R3, R4, R5 and R6).

An increase in decolorization rate was observed on the decrease in particle size, which

can be attributed to the increase in the catalyst surface area90. Also, due to decrease in

particle size, band gap energy increases, which diminished the recombination of

charge carriers. It is generally accepted that a larger band gap corresponds to more

powerful redox ability91,92. Hence, R6 possessed largest surface area, increased band

Chapter 3


�

gap and high redox capability with small photocorrosion which was responsible for its

highest photocatalytic activity.

The mechanism involving photogenerated electrons and holes can be given as; after

excitation of CdS the electrons in the VB jumped into the nearby CB, leaving behind

a hole in VB (Equation 3.5). Consequently, the as generated photoexcited electrons

and holes acted as redox centers and induced reduction and oxidation reactions

respectively on the catalyst surface. The electrons were scavenged by molecular

oxygen (O2) to yield the superoxide radical anion O2•− (Equation 3.6) and hydrogen

peroxide H2O2 (Equation 3.7) in oxygen-equilibrated media. These new formed

intermediates then interacted to produce hydroxyl radical •OH (Equation 3.8). It is

well known that the •OH radical is a powerful oxidizing agent capable of degrading

most pollutants (Equation 3.9)10 thus would lead to the degradation of dye into the

final products. However, the photo-generated holes in CdS nanocrystals cannot

oxidize hydroxyl groups to hydroxyl radicals due to its valence band potential. This

could result in the photocorrosion of CdS, forming cadmium cations10,93. Figure 3.19

gives the schematic representation of the mechanism involved in photocatalysis by

CdS NPs.

Figure 3.19: Schematic representation of the mechanism involved in photocatalysis

by CdS NPs (R1, R2, R3, R4, R5 and R6).

Chapter 3


�

The reactions of the mechanism involved in the photoreaction by CdS NPs can be

given as:

CdSh�� CdS��h� �e� [3.5]

e �O2 �� O2� [3.6]

e� �O2 �2H�� H2O2 [3.7]

H2O2 �O2� �� OH OH O2 [3.8]

�OH dye� � degradation�products [3.9]

Even though, the photo-generated hole formed in the VB of CdS cannot oxidize

hydroxyl groups (OH¯ ) and water (H2O) molecule to produce hydroxyl radicals, they

possibly oxidized dye molecules to reactive intermediates, and further to final

products to some extent (Equation 3.10)94.

CdS e h�� Dye � CdS�e� Dye�� CdS�e� degradation�products[3.10]

For the purpose of practical implementation, it is essential to evaluate the stability and

reuse of the catalyst. Figure 3.20 shows the repetitive photodegradation of AB-29

during five consecutive cycles with the same 1gL−1 catalyst at 0.06mM dye

concentration. After each cycle, the nanocomposite catalyst was washed with double

distilled water and a fresh solution of AB-29 was added before each photocatalytic

run. The relative decolorization using CdS nanocatalysts (R1, R2, R3, R4, R5 and

R6) for the 5 cycling reuse after 90 minutes of reaction time are given in the Table

3.6.

The results showed that the catalytic activity of CdS nanocatalysts (R1, R2, R3, R4,

R5 and R6) decreased after first cycles. Among them R1 showed relatively maximum

stability as compared to other CdS nanocatalysts this might be due to the reason that

hexagonal CdS are more stable than cubic CdS. The decrease in the stability of CdS

during photocatalytic degradation reactions might be the result of photocorrosion of

CdS, forming cadmium cations as already mentioned before. CdS leaching out is a

serious concern. Firstly, because catalytic activity is lowered as the amount of

photocatalyst decreased. Secondly, the Cd2+ ions are hazardous to health93. Therefore,

the use of naked CdS as photocatalyst for water purification is questionable. An

Chapter 3


�

approach is to be pursued to make CdS photocatalyst stable and applicable for water

purification.

Figure 3.20: The relative decolorization using synthesized CdS nanomaterials (R1,

R2, R3, R4, R5 and R6) for the 5 cycling reuse after 90 minutes of reaction time

under visible light irradiation.

Table 3.6: The relative decolorization using CdS nanocatalysts (R1, R2, R3, R4, R5

and R6) for consecutive 5 cycling reuse after 90 minutes of reaction time.

Cycle R1 R2 R3 R4 R5 R6

I 53.8% 66.0% 68.6% 72.1% 76.5% 78.9%

II 51.0% 58.8% 62.2% 67.8% 72.9% 76.2%

III 49.7% 55.9% 60.1% 64.2% 68.4% 71.5%

IV 46.7% 52.1% 56.2% 60.8% 64.2% 67.0%

V 45.1% 49.8% 52.3% 56.1% 61.0% 63.6%

Chapter 3


�

3.4. CONCLUSION:

Cadmium sulphide nanoparticles were synthesized via chemical precipitation method,

using different sulphide ion sources ((NH4)2S, H2S, Na2S) and in presence and

absence of stabilizing agents. The S2- ion source affected the nanoparticles size, the

more active source lead to smaller sized CdS-NPs. The presence of stabilizing agent

prevented the agglomeration of the nanoparticles. The CdS-NPs showed good thermal

stability and fine elemental purity. They exhibited quantum size effect and showed an

increase in band gap with the decrease in particle size. All the synthesized NPs

showed considerable blue shift in absorption edge with respect to the bulk CdS.

Crystalline nature of the CdS-NPs was confirmed by the presence of hexagonal and

cubic type phases. The average particles sizes obtained by TEM, XRD, UV-visible

spectroscopy were found to be in good agreement with each other and were in the

range 4-10nm. The synthesized cadmium sulphide nanoparticles were exploited for

the successful photodegradation of an azo dye, Acid Blue-29, in aqueous medium.

The rate of degradation was found to increase with the decrease in particle sizes. But,

the stability of the CdS NPs decreased with time after consecutive degradation cycles.

This might be due to the photocorrosion of CdS. Thus, some effort has to be taken in

order to utilize the useful photocatalytic properties of CdS, by making it less

photocorrosive and more stable.

Chapter 3


�

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