preparation, characterization and physicalproperties of cds nanoparticles with

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676Journal of Applied Sciences Research, 8(2): 676-685, 2012ISSN 1819-544XThis is a refereed journal and all articles are professionally screened and reviewed

ORIGINALARTICLES

Corresponding Author: R. Seoudi, Department of Spectroscopy, Physics Division, National Research Center, Giza, 12622,EgyptTel.: +20 23308157; fax: +20 23370931. E-mail: [email protected]

Preparation, Characterization and Physical Properties of CdS Nanoparticles withDifferent Sizes

1El- Bially A.B., 2,3Seoudi R., 2Eisa W., 2Shabaka A.A., 1Soliman S.I., 1Abd El-Hamid R.K. and4Ramadan 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.4Basic Science Center, Misr University for Science and Technology, 6

thOctober 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 resultsindicated 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 noeffect of the particle sizes on the position of this band. Dependence of the blue shift and optical band gap on thequantum size effect was confirmed by UV-Visible spectroscopy. The dielectric properties are studied in thefrequency 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 propertiesthat 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 theirpossible application in solar cell, photo detector, laser, high density magnetic information storage and manyothers in semiconductor industries. Semiconducting optoelectronic materials play functional role in variety of

applications due to their extraordinary optical, electrical, magnetic and piezoelectric properties. Modifications ofthe optical, electrical, magnetic and physical properties of semiconductor materials strictly depend upon thesizes, structures and morphologies (Tai et al., 2010: and Hu et al., 1998). Due to these changes in propertieswith the crystallites size, researchers interest turn towards the synthesis of semiconductor particle in the fewnanometer range with dimensions comparable to the Bohr radius. The semiconductor nanoparticles within thedimension of Bohr radius exhibit strong size dependent properties. Such particles may lead to quantum dotlasers, 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 aneasy to handle conditions of precursor, solvent and temperature etc. Cadmium sulphide (CdS) is a brilliant IIVIsemiconductor material with a direct band gap of 2.42 eV at room temperature with many outstanding physicaland chemical properties, which has promising applications in multiple technical fields including photochemicalcatalysis, gas sensor, detectors for laser and infrared, solar cells, nonlinear optical materials, variousluminescence devices, optoelectronic devices and so on (Erra et al., 2007: Lakowicz et al., 2002: Ushakov etal., 2006: and Venkatram et al., 2005). Cadmium sulphide (CdS) has excellent visible light detecting propertiesamong 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 etal., 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 thepreparation 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 sodiumsulfide 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 theoptical properties with the particle was size discussed from FTIR and UV-Visible spectra. The dependence ofthe dielectric on the sizes will be obtained.


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2. Experiment:

2.1 Synthesis of CdS Nanoparticles:

All chemicals were of analytical grade and used as received without further purification. CdS nanoparticleswere prepared by the chemical precipitation method at room temperature. In this method aqueous solution of thereactants 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 solutionturned to yellow color immediately due to the formation of CdS. The stirring was continued for some specifictime in order to facilitate complete nanoparticle precipitation. The precipitate was then separated bycentrifugation and washed with deionized water and ethanol repeatedly to get rid of unreacted species and byproduct. The sample was dried at 35 oC for 6h and the free standing powder was collected and preserved in anairtight container. The same procedure was used to synthesize different nanoparticle size of CdS by varying theratios of CdCl2 to Na2S in the mixture.

2.2 Instruments:

The shape, morphologies and the particle size were studied using JEOL JEM 2010 transmission electronmicroscope operated at 200KV accelerating voltage. The structure of the prepared samples were determinedfrom X-ray Diffractometer Philips (PW 13900) equipped with CuK as radiation source ( = 1.54A). Thevibrational 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 therange of 1000- 200nm using Jasco V- 570 UV/VIS/NIR spectrometer. The dielectric constant (/) was carriedout using an RLC bridge (HIOKI model 3530) Japan. The accuracy of measurement for both parameters wasless than 3%. Dielectric constant was calculated using the relation; /(f, T) = C(f, T)d/oA, (area A and distanced 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 sizedistribution. Direct imaging provides a fast automated image analysis solution. The transmission electronmicrograph images of CdS nanoparticles and its particle size histogram for the micrograph were shown inFigures (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 theparticles are to a large extent well-separated from one another and appears to be uniformly distributedthroughout the field of the micrograph. From the particle size histogram, it can be observed that the grain sizedecreases 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 andweakness of the mean diffraction peak at 2=26.5o was seen and it is indicated that a small dimensions of CdSnanoparticles 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):

cos

KD

Where D is the particle diameter, K is a constant equal 0.9, is the X- ray wavelength and is thediffraction angle.


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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.


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The calculated values of crystalline particle size (D) were listed in Table (1). It can be seen that, the CdCl 2

to Na2S volume ratios in our results are the main factors that controlling the particle size. Also it can be noticedthat 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 8.40.7 26.57 3.35 6.2

0.8 26.61 3.35 5.41 26.69 3.34 4.8

1.3 26.72 3.33 4.32 26.77 3.33 4.14 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 metalchalcogenide clusters. In addition, low frequency vibrational spectroscopy used in the characterization of thenanocomposite structures and monitoring changes in bonding accompanying structural changes during growthof nanoclusters from molecular precursors (Loveret al., 1997). Far-IR absorption spectra of CdS nanoparticleswith different particle sizes are presented in Figure (3). It can be seen that the CdS nanoparticles had a broadabsorption 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 CdSnanoparticles prepared at different ratios of Cd to S ions, it can be noticed that, there is remarkable change in thepeak 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- 400cm-1). The absorption peak in the range from 3600 to 3200 cm-1 corresponding to the OH group of wateradsorbed by the samples. The week absorption band at 1635 cm-1 was attributed to CO2 adsorbed on the surfaceof the particles. In fact, adsorption of water and CO2 are common for all powder samples exposed to atmosphereand 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 0.6to 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 effectsand as well as approve the formation of smaller particles (Murray et al., 1993: and Wang et al., 2003). In asemiconductor, the increase in the band gap between the valence and conduction band results from the decreasein the particle size. Consequently, the excitation of electron from valence band to conduction band requireshigher energy, which results in the blue shift or light absorption in higher energy region or lower wavelengthregion .The prepared samples exhibit an interesting example of color variation with crystallite size. The colorchanged from red orange to yellow and then to faint yellow as the volume ratio of CdCl2:Na2S changed from 0.6to 4. These results confirmed that series of CdS nanoparticles were successfully prepared. Tuning theconcentration of reactants was done to vary the growth rate at a particular instant of time. The absorption peakof 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) relativeto the bulk material whereas that of the ratio 0.6 show the smallest shift (60 nm). This indicates that the particlesize 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 (winteret 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 opticalband gap of the nanoparticle, and n = 1/2 for direct band gap and n = 2 for indirect band gap. The absorptioncoefficient (), at the corresponding wavelengths, was calculated from Beer-Lambert's relation (Sahay et al.,2007):


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l

A303.2

where lis the path length andA is the absorbance. CdS had a direct band gap calculated from Figure (6) andlisted in Table (2). Form this table it can be observed that the values of the band gap of CdS nanoparticle arehigher 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.


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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.


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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 2.50.7 450 2.570.8 445 2.61 438 2.64

1.3 430 2.702 427 2.754 422 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) atdifferent temperature of CdS nanoparticles with different particle sizes is shown in Figure (7). The dielectricconstant / was calculated using the following equation:


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low frequencies, the permanent dipoles align themselves along the field and contribute fully to the totalpolarization of the dielectric. At higher frequencies, the variation in the field is too rapid for the dipoles to alignthemselves, so their contribution to the polarization and, hence, to the dielectric permittivity can becomenegligible. Therefore, the dielectric permittivity decreases with increasing frequency. The values of dielectricconstant observed for CdS nanostructures are higher than the bulk. This is attributed to the large space chargepolarization 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. Thesamples 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 ofCdCl2 to Na2S ions. The calculated band gaps of CdS are higher than that of the bulk.

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preparation, characterization and physicalproperties of cds nanoparticles with

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