-
676 Journal of Applied Sciences Research, 8(2): 676-685, 2012
ISSN 1819-544X This is a refereed journal and all articles are
professionally screened and reviewed
ORIGINAL ARTICLES
Corresponding Author: R. Seoudi, Department of Spectroscopy,
Physics Division, National Research Center, Giza, 12622, Egypt
Tel.: +20 23308157; fax: +20 23370931. E-mail:
[email protected]
Preparation, Characterization and Physical Properties of CdS
Nanoparticles with Different Sizes 1El- Bially A.B., 2,3Seoudi R.,
2Eisa W., 2Shabaka A.A., 1Soliman S.I., 1Abd El-Hamid R.K. and
4Ramadan R.A. 1Department of Spectroscopy, Faculty of Girls for
Art, Science and Education, Ain Shams University. 2Department of
Spectroscopy, Physics Division, National Research Center, Giza,
12622, Egypt. 3Department of Physics, College of Applied Science,
Umm Al-Qura University, Makkah, Saudi Arabia. 4 Basic Science
Center, Misr University for Science and Technology, 6th October
city. Egypt. ABSTRACT
Cadmium sulfide nanoparticles were synthesized with different
sizes by chemical precipitation method. Transmission electron
microscopy (TEM) and X-ray diffraction pattern (XRD) used to study
the morphologies, distribution, and crystallinty of the CdS
nanoparticles and to calculate the values of their sizes. The
results indicated that the CdS were formed with cubic structure and
the particle size decreases with increasing the Cd+2 ions. The Cd-S
stretching vibration band appeared in the far infrared region at
about 250 cm-1 and there is no effect of the particle sizes on the
position of this band. Dependence of the blue shift and optical
band gap on the quantum size effect was confirmed by UV-Visible
spectroscopy. The dielectric properties are studied in the
frequency range (2.5 KHz-5MHz) at different temperatures. Key
words: cadmium sulfide nanoparticles; TEM; XRD; UV-visible;
dielectric properties. 1-Introduction The semiconductor
nanoparticles exhibit structural, optical, luminescence and photo
conducting properties that are very different from their bulk
properties (Alivisatos, 1996: Peng et al., 2000: Bawendi et al.,
1990: Trindade et al., 2001: Bawendi et al., 1992: and Colvin et
al., 1994). It is very attractive because of their possible
application in solar cell, photo detector, laser, high density
magnetic information storage and many others in semiconductor
industries. Semiconducting optoelectronic materials play functional
role in variety of applications due to their extraordinary optical,
electrical, magnetic and piezoelectric properties. Modifications of
the optical, electrical, magnetic and physical properties of
semiconductor materials strictly depend upon the sizes, structures
and morphologies (Tai et al., 2010: and Hu et al., 1998). Due to
these changes in properties with the crystallites size, researchers
interest turn towards the synthesis of semiconductor particle in
the few nanometer range with dimensions comparable to the Bohr
radius. The semiconductor nanoparticles within the dimension of
Bohr radius exhibit strong size dependent properties. Such
particles may lead to quantum dot lasers, single electron
transistors and also have biological applications (Yin et al.,
1998: and Chan and Nie, 1998). It is important to synthesize
nanoparticle at the desired size within a narrow size distribution
and in an easy to handle conditions of precursor, solvent and
temperature etc. Cadmium sulphide (CdS) is a brilliant IIVI
semiconductor material with a direct band gap of 2.42 eV at room
temperature with many outstanding physical and chemical properties,
which has promising applications in multiple technical fields
including photochemical catalysis, gas sensor, detectors for laser
and infrared, solar cells, nonlinear optical materials, various
luminescence devices, optoelectronic devices and so on (Erra et
al., 2007: Lakowicz et al., 2002: Ushakov et al., 2006: and
Venkatram et al., 2005). Cadmium sulphide (CdS) has excellent
visible light detecting properties among the others semiconductors
(Ghasemi et al., 2009). In the last decades, many techniques have
been reported on synthesis of CdS nanoparticles (Chatterjee and
Patra, 2001: Ghows and Entezari, 2010: and Li et al., 2000). The
possibility of finding new experimental methodologies that can
yield very low cost and low size- and shape-dispersion
nanoparticles at a low cost. In last few years, researchers have
been devoted to the preparation of high-quality CdS nanoparticles
and the investigation of their various properties. In this study,
CdS nanoparticles were prepared using different ratios of cadmium
chloride and sodium sulfide precursor to control the particle sizes
by simple chemical route. The particle size distribution,
morphologies and crystallinty, studies using TEM and XRD. The
vibrational structure and the change of the optical properties with
the particle was size discussed from FTIR and UV-Visible spectra.
The dependence of the dielectric on the sizes will be obtained.
-
677 J. Appl. Sci. Res., 8(2): 676-685, 2012
2. Experiment: 2.1 Synthesis of CdS Nanoparticles: All chemicals
were of analytical grade and used as received without further
purification. CdS nanoparticles were prepared by the chemical
precipitation method at room temperature. In this method aqueous
solution of the reactants was prepared. 0.01 M CdCl2 and 0.01M Na2S
uses as the reactant materials. The reaction mixture was prepared
by adding 4 mL CdCl2 (0.01M) and 4 mL Na2S (0.01M) into 40 mL
deionized water. The solution turned to yellow color immediately
due to the formation of CdS. The stirring was continued for some
specific time in order to facilitate complete nanoparticle
precipitation. The precipitate was then separated by centrifugation
and washed with deionized water and ethanol repeatedly to get rid
of unreacted species and by product. The sample was dried at 35 oC
for 6h and the free standing powder was collected and preserved in
an airtight container. The same procedure was used to synthesize
different nanoparticle size of CdS by varying the ratios of CdCl2
to Na2S in the mixture. 2.2 Instruments: The shape, morphologies
and the particle size were studied using JEOL JEM 2010 transmission
electron microscope operated at 200KV accelerating voltage. The
structure of the prepared samples were determined from X-ray
Diffractometer Philips (PW 13900) equipped with CuK as radiation
source ( = 1.54A). The vibrational spectra of the investigated
samples were carried out by using FTIR spectrophotometer (Jasco,
Model 6100, Japan) in the absorbance mode at a resolution of 4
cm-1. The UV- Visible spectra were measured in the range of 1000-
200nm using Jasco V- 570 UV/VIS/NIR spectrometer. The dielectric
constant (/) was carried out using an RLC bridge (HIOKI model 3530)
Japan. The accuracy of measurement for both parameters was less
than 3%. Dielectric constant was calculated using the relation;
/(f, T) = C(f, T)d/oA, (area A and distance d of the plan parallel
electrode system; capacitance C; and o permittivity of the vacuum =
8.85 x 10-12 F/m). 3-Results and discussion 3.1. Transmission
Electron Microscope (TEM) of CdS Nanoparticles:
Transmission electron microscope approach would provide and
obtain morphology, shape and particle size distribution. Direct
imaging provides a fast automated image analysis solution. The
transmission electron micrograph images of CdS nanoparticles and
its particle size histogram for the micrograph were shown in
Figures (1: a, a/-g, g/). From the images it can be clearly seen
that a number of well-dispersed nanoparticles with a fairly even
size distribution. From the images it can be suggested that CdS
nanoparticles have an external spherical shape and the particles
are to a large extent well-separated from one another and appears
to be uniformly distributed throughout the field of the micrograph.
From the particle size histogram, it can be observed that the grain
size decreases from 14 to 5 nm as the CdCl2:NaS2 volume ratio
changes from 0.6 to 4. 3.2 X- ray diffraction pattern data: Figure
(2) shows the X- ray diffraction (XRD) pattern of CdS nanoparticles
with different particle sizes. Three remarkable peaks were observed
at 2=26.5, 43.4 and 51.7. These peaks corresponds to the (111),
(220), and (311) planes of the cubic CdS, respectively according to
(JCPD No.10-454). The broadness and weakness of the mean
diffraction peak at 2=26.5o was seen and it is indicated that a
small dimensions of CdS nanoparticles was formed. The reduction in
particle size was confirmed by increasing the ratios of Cd to S
ions. The particle size was calculated from Scherrer equation
(Georgekutty et al., 2008):
cosKD
Where D is the particle diameter, K is a constant equal 0.9, is
the X- ray wavelength and is the diffraction angle.
-
678 J. Appl. Sci. Res., 8(2): 676-685, 2012
Fig. 1: (a, a/ - g, g/): TEM micrographs and histogram of the
particle size distribution of CdS nanoparticles prepared with
different volume ratios of CdCl2 to Na2S, (a, b, c, d, e, f, g) was
(0.6, 0.7, 0.8, 1, 1.3, 2, 4).
Fig. 2: X-ray diffraction pattern of CdS nanoparticles prepared
different volume ratios of CdCl2 to Na2S.
-
679 J. Appl. Sci. Res., 8(2): 676-685, 2012
The calculated values of crystalline particle size (D) were
listed in Table (1). It can be seen that, the CdCl2 to Na2S volume
ratios in our results are the main factors that controlling the
particle size. Also it can be noticed that the average nanocrystal
diameter is significantly decreased with increasing the ratios of
Cd:S ions. Table 1: The d-spacing and the crystallite size
calculated from XRD analysis of pure CdS nanoparticles.
Volume ratios of CdCl2 :Na2S 2o d () Size (nm) 0.6 26.52 3.36
8.4 0.7 26.57 3.35 6.2 0.8 26.61 3.35 5.4 1 26.69 3.34 4.8
1.3 26.72 3.33 4.3 2 26.77 3.33 4.1 4 26.81 3.32 3.6
3.3 FTIR Spectroscopic: Far-infrared spectroscopy (400-150 cm-1)
is a valuable technique for the characterization of metal
chalcogenide clusters. In addition, low frequency vibrational
spectroscopy used in the characterization of the nanocomposite
structures and monitoring changes in bonding accompanying
structural changes during growth of nanoclusters from molecular
precursors (Lover et al., 1997). Far-IR absorption spectra of CdS
nanoparticles with different particle sizes are presented in Figure
(3). It can be seen that the CdS nanoparticles had a broad
absorption band in the wavenumber range from 300 to 200 cm1and this
is in agreement with Nyquist and Kagel (Nyqusit and Kagel, 1971).
This absorption band can be assigned to the stretching vibration of
CdS. The appearance of this band indicated the formation of CdS
nanoparticle. By comparing the IR spectra of CdS nanoparticles
prepared at different ratios of Cd to S ions, it can be noticed
that, there is remarkable change in the peak position. This
indicates that CdS stretching vibration is unaffected with
decreasing particle size. Figure (4) shows the mid infrared
absorption spectra of CdS nanoparticles in the spectral range
(4000- 400 cm-1). The absorption peak in the range from 3600 to
3200 cm-1 corresponding to the OH group of water adsorbed by the
samples. The week absorption band at 1635 cm-1 was attributed to
CO2 adsorbed on the surface of the particles. In fact, adsorption
of water and CO2 are common for all powder samples exposed to
atmosphere and are even more pronounced for nanosized particles
with high surface area. 3.4 UV-Visible absorption spectroscopic
data: The absorption spectra of the CdS nanoparticles prepared at
different volume ratio of CdCl2:Na2S from 0.6 to 4 are shown in
Figure (5). The spectra of all samples exhibit absorption peak in
the range of (400- 480 nm). This peak was assigned to the optical
transition of the first excitonic state and shifted gradually to
the lower wavelength (blue shift) as the ratio of Cd to S ions
increased. This shift may be due to the quantum size effects and as
well as approve the formation of smaller particles (Murray et al.,
1993: and Wang et al., 2003). In a semiconductor, the increase in
the band gap between the valence and conduction band results from
the decrease in the particle size. Consequently, the excitation of
electron from valence band to conduction band requires higher
energy, which results in the blue shift or light absorption in
higher energy region or lower wavelength region .The prepared
samples exhibit an interesting example of color variation with
crystallite size. The color changed from red orange to yellow and
then to faint yellow as the volume ratio of CdCl2:Na2S changed from
0.6 to 4. These results confirmed that series of CdS nanoparticles
were successfully prepared. Tuning the concentration of reactants
was done to vary the growth rate at a particular instant of time.
The absorption peak of CdS bulk was appeared at 515 nm (Hongmei et
al., 2007). It is evident that the CdS synthesized from the volume
ratio 4 shows the largest blue shift (90 nm) relative to the bulk
material whereas that of the ratio 0.6 show the smallest shift (60
nm). This indicates that the particle size decreases with
increasing the Cd:S volume ratio. These results are consistent with
that in the literature (Herron et al., 1990: and Babu et al.,
2007). The optical band gap has been calculated from absorption
coefficient data as a function of wavelength by using Tauc Relation
(winter et al., 2005: and Ethayaraja et al., 2007): nnpEhBh where;
is the absorption coefficient, h is the photon energy, B is the
band tailing parameter, Enp is the optical band gap of the
nanoparticle, and n = 1/2 for direct band gap and n = 2 for
indirect band gap. The absorption coefficient (), at the
corresponding wavelengths, was calculated from Beer-Lambert's
relation (Sahay et al., 2007):
-
680 J. Appl. Sci. Res., 8(2): 676-685, 2012
lA303.2
where l is the path length and A is the absorbance. CdS had a
direct band gap calculated from Figure (6) and listed in Table (2).
Form this table it can be observed that the values of the band gap
of CdS nanoparticle are higher than the band gap of bulk was (2.42
eV) (Brus, 1984). This is due to the strong quantum confinement.
The band gap energies gradually increases from 2.5 eV (Cd:S=0.6) to
2.8 eV (Cd:S=4).
Fig. 3: Far infrared absorption spectra of CdS nanoparticles
prepared by different ratios of Cd to S ions.
-
681 J. Appl. Sci. Res., 8(2): 676-685, 2012
Fig. 4: Mid-infrared absorption spectra of CdS nanoparticles
with different particles sizes.
Fig. 5: UV-visible spectra of CdS anoparticles prepared by
different ratio of Cd to S ions.
-
682 J. Appl. Sci. Res., 8(2): 676-685, 2012
Fig. 6: Graph of (hv)2 vs hv of CdS nanoparticles prepared by
different ratios of Cd to S ions.
Table 2: The optical parameters of CdS nanoparticles with
different particles sizes.
Volume ra tios of CdCl2 : Na2S max (nm) Energy gap Enp (eV) 0.6
454 2.5 0.7 450 2.57 0.8 445 2.6 1 438 2.64
1.3 430 2.70 2 427 2.75 4 422 2.81
3.5 Dielectric Properties: The change of the real part of
dielectric constant (/) with frequencies in the range (2.5 KHz -5
MHz) at different temperature of CdS nanoparticles with different
particle sizes is shown in Figure (7). The dielectric constant /
was calculated using the following equation:
-
683 J. Appl. Sci. Res., 8(2): 676-685, 2012
)()()(
20
/
mAmdFC
where, C is the capacity in Farad, d is the film thickness in m,
A is the thin film area in m2 and 0 is the relative permittivity of
vacuum = (8.85 10-12 Fm-1).
Fig. 7: Variation of dielectric constant (/) of CdS
nanoparticles prepared by different ratios of Cd to S ions as
function of frequency at different temperature. The nature of
dielectric permittivity related to free dipoles oscillating in an
alternating field may be described in the following way. It can be
found that, at very low frequencies dipoles follow the field and
the real part nearly constant; / s (value of the dielectric
constant at quasi-static field and frequency much less than the
reciprocal of the relaxation time
-
684 J. Appl. Sci. Res., 8(2): 676-685, 2012
low frequencies, the permanent dipoles align themselves along
the field and contribute fully to the total polarization of the
dielectric. At higher frequencies, the variation in the field is
too rapid for the dipoles to align themselves, so their
contribution to the polarization and, hence, to the dielectric
permittivity can become negligible. Therefore, the dielectric
permittivity decreases with increasing frequency. The values of
dielectric constant observed for CdS nanostructures are higher than
the bulk. This is attributed to the large space charge polarization
taking place at the interfaces of nanostructured materials (Anoop
et al., 2011). 4. Conclusion: Controlling of the nanoparticle size
of CdS was done by the volume ratios for cadmium to sulfur ions.
The samples were synthesized in a cubic structure form and the
particle size decreases with increasing the Cd+2 ions. The effect
of the particle size on the UV-VIS spectra exhibit blue shift with
the change of the volume ratio of CdCl2 to Na2S ions. The
calculated band gaps of CdS are higher than that of the bulk.
References Alivisatos, A.P., 1996. Semiconductor clusters,
nanocrystals, and quantum dots. Science, 271: 933. Anoop Chandran,
Soosen M. Samuel, Jiji Koshy and K.C. George, 2011. Dielectric
relaxation behavior of CdS
nanoparticles and nanowires. J. Mater Sci., 46: 4646-4653. Babu,
K.S., C. Vijayan and P. Haridoss, 2007. Properties of size-tuned
PBS nanocrystalets stabilized in a
polymer template. Mater. Res. Bull., 42: 1251-1261. Bawendi,
M.G., P.J. Caroll, W.L. Wilson and L.E. Brus, 1992. Luminescence
properties of CdSe quantum
crystallites: Resonance between interior and surface localized
states. J. Chem. Phys., 96: 1335. Bawendi, M.G., M.L. Steigerwald
and L.E. Brus, 1990. The quantum mechanics of larger
semiconductor
clusters (quantum dots). Annu. Rev. Phys. Chem., 41: 477. Brus,
L.E., 1984. Electronelectron and elec tron-hole interactions in
small semiconductor crystallites: The size
dependence of the lowest excited electronic state. J. Chem.
Phys., 80: 4403. Chan, W.C.W. and S.M. Nie, 1998. Quantum dot
bioconjugates for ultrasensitive nonisotopic detection.
Science, 281: 2016. Chatterjee, M. and A. Patra, 2001. Cadmium
sulfide aggregates through reverse micelles. J. Am. Ceram.
Soc.,
84: 1439. Colvin, V.L., M.C. Schlamp and A.P. Alivisatos, 1994.
Light emitting diodes made from cadmium selenide
nanocrystals and a semiconducting polymer. Nature, 370: 354.
Erra, S., C. Shivakumar, H. Zhao, K. Barri, D.L. Morel and C.S.
Frekides, 2007. An effective method of Cu
incorporation in CdTe solar cells for improved stability. Thin
Solid Films, 515: 5833. Ethayaraja, M., C. Ravikumar, D.
Muthukumaran, K. Dutta and R. Bandyopadhyaya, 2007. CdS-ZnS
Core-
Shell Nanoparticle Formation: Experiment, Mechanism, and
Simulation. J. Phys. Chem., (C); 111: 3246. Georgekutty, R., M.K.
Seery and S.C. Pillai, 2008. A highly efficient Ag-ZnO
photocatalyst: synthesis,
properties, and mechanism. J. Phys. Chem. (B); 112: 13563.
Ghasemi, Y., P. Peymani and S. Afifi, 2009. Quantum dot: magic
nanoparticle for imaging, detection and
targeting. Acta. Biomed., 80: 156. Ghows, N. and M.H. Entezari,
2010. A novel method for the synthesis of CdS nanoparticles without
surfactant.
Ultrason. Sonochem., 18: 269. Herron, N., Y. Wang and H. Eckert,
1990. Synthesis and Characterization of surface- cap, Size
Quantized of
CdS Clusters. Chemical Control of Cluster Size. J. Am. Chem.
Soc., 112: 1322. Hongmei Wang, Zhe Chen, Pengfei Fang and Shaojie
Wang, 2007. Synthesis, characterization and optical
properties of hybridized CdS-PVA nanocomposites. Materials
Chemistry and Physics, 106: 443-446. Hu, K., M. Brust and A. Bard,
1998. Characterization and surface charge measurement of
self-assembled CdS
nanoparticle films. J. Chem. Mater, 10: 1160. Lakowicz, J.R., I.
Gryczynski, G. Piszczek and C.J. Murphy, 2002. Emission spectral
properties of cadmium
sulfide nanoparticles with multiphoton excitation. J. Phys.
Chem. (B); 106: 5365. Li, Y., F. Huang, Q. Zhang and Z. Gu, 2000.
Solvothermal synthesis of nanocrystalline cadmium sulfide. J.
Mater. Sci., 35: 5933. Lover, T., G.A. Bowmaker, John M. Seakins
and R.P. Cooney, 1997. Vibrational Spectroscopic study of
thiophenolate- capped nanoclusters of CdS and of Cadmuim
thiophenolate compolexes. Chem. Mater, 9: 967-975.
Murray, C.B., D.J. Norris and M.G. Bawendi, 1993. Synthesis and
characterization of nearly monodisperse CdE (E=S,Se,Te)
semiconductor nanocrystallites. J. Am. Chem. Soc., 115: 8706.
Nyquist, R.A. and R.O. Kagel, 1971. Infrared Spectra of
Inorganic Compounds, Academic Press, New York, 253.
-
685 J. Appl. Sci. Res., 8(2): 676-685, 2012
Peng, X.G., L. Manna, W.D. Yang, J. Wickham, E. Scher, A.
Kadavanich and A.P. Alivisatos, 2000. Shape control of CdSe
nanocrystals. Nature, 404: 59.
Sahay, P.P., R.K. Nath and S. Tewari, 2007. Optical properties
of thermally evaporated CdS thin films. Cryst. Res. Technol., 42:
275.
Tai, G., J. Zhou and W. Guo, 2010. Inorganic salt-induced phase
control and optical characterization of cadmium sulfide
nanoparticles. Nanotechnology, 21: 175601.
Trindade, T., P. OBrien and N.L. Pickett, 2001. Nanocrystalline
semiconductors: synthesis, properties and perspectives. Chem.
Mater, 13: 3843.
Tripathi, R., A. Kumar and T.P. Sinha, 2009. Dielectric
properties of CdS nanoparticles synthesized by soft chemical route.
Pramana C-journal of physics, 72: 969.
Ushakov, N.M., G. Yurkov, Yu, K.V. Zapsis, D.A. Baranov, N.A.
Kataeva, I.D. Kosobudski and S.P. Gubin, 2006. Optical properties
of cadmium sulfide nanoparticles on the surface of
polytetrafluoroethylene nanogranules. Opt. Spectrosc, 100: 414.
Venkatram, N., D.N. Rao and M.A. Akundi, 2005. Nonlinear
absorption, scattering and optical limiting studies of CdS
nanoparticles. Opt. Express, 13: 867.
Wang, W., I. Germanenko and M.S. El-Shall, 2003.
Room-temperature synthesis and characterization of nanocrystalline
CdS, ZnS, and CdxZn1-xS. Chem. Mater, 14: 3028.
Winter, J.O., N. Gomez, S. Gatzert, C.E. Schmidt and B.A.
Korgel, 2005. Variation of cadmium sulfide nanoparticle size and
photoluminescence intensity with altered aqueous synthesis
conditions. Colloids and Surfaces A: Physicochem. Eng. Aspects,
254: 147.
Yin, Y., X. Ling, X. Ge, C. Xia and Z. Zhang, 1998. Synthesis of
cadmium sulfide nanoparticles in situ using -radiation. Chem.
Commun., 16: 1641.