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: rsmawed@yahoo.com

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.

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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 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.

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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 2.5 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

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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

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