Nanoscience and Nanotechnology: An International Journal. Original Article

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1 Available online at Nanoscience and Nanotechnology: An International Journal Universal Research Publications. All rights reserved Abstract Original Article Synthesis and study of optical properties of CdS nanoparticles using effective surfactants * Ramamurthy. N, Rajesh Kumar. M and Murugadoss. G Department of Physics, Annamalai University, Annamalai nagar , Tamilnadu, India. Corresponding author: drnramamurthy@gmail.com Received 20 October 2011; accepted 04 November 2011 High quality and monodispersed CdS nanoparticles were synthesized through chemical method using Polyvinylpyrrolidone (PVP) and Sodium polyphosphate (SPP). The synthesized nanoparticles have been characterized by X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), Ultraviolet Visible (UV Vis) and Photoluminescence (PL) spectroscopy. The absorption peak of the surfactants capped particles were significantly blue shifted. An enhanced PL emission was observed by the PVP and SPP capped particles. The nanocomposites would have potential application as photocatalyst, novel luminescence and photoelectron transfer devices Universal Research Publications. All rights reserved Key words: CdS, surfactants, surface defect, quantum confinement, PL 1. Introduction Nanostructured II VI semiconductors have been studied very intensively in recent times due to their industrial implementation in nanoelectronic devices. The synthesis of semiconductor nanoparticles has attracted much interest due to their size-dependant properties and great potential in several applications such as nonlinear optics, photoelectrochemical cells, heterogeneous photocatalysis, optical switching and single electron transistors [1 8]. Surfactants, amphiphilic block copolymers (ABCPs) and dendrimers have been used for the preparation of semiconductor nanoparticles [1 21]. Among the synthetic processes of semiconductor nanoparticles, synthesis of semiconductor nanoparticles using surfactants is prominent. That is because polymer matrixes provide for processability, solubility and control of nanoparticle growth and morphology besides acting as a stabilizer. In addition, unlike the case where surfactants and dendrimers create only a semiconductor nanocolloid solution, surfactants can form a nano-colloid solution as well as nanoparticles dispersed in polymeric film. Since inorganic nanoparticles have very unique properties owing to the quantum size effects and the large number of unsaturated atoms, polymeric films containing inorganic nanoparticles such as semiconductor nanoparticles, magnetic nanoparticles and metal oxide nanoparticles can exhibit novel catalytic, magnetic, and optical properties [11 17]. However, surfactants are very expensive materials and can be obtained only through an extremely complicated synthetic process, so the application of polymer films containing semiconductor nanoparticles is highly limited. This has led us to develop a new amphiphilic material and two-step process for the synthesis of nanocomposite films. In this work, CdS nanoparticles were synthesized using two different surfactants such as Polyvinylpyrrolidone (PVP) and Sodium Hexametaphosphate (SHMP) in aqueous medium at air atmosphere. The particles were characterized by different experimental techniques and studied by their optical properties. This work is mainly focused on synthesis of monodispersed CdS nanoparticles for sensors, solar and laser applications. 2. Experimental 2.1. Materials Cadmium acetate dehydrate [Cd (CH 3 COO) 2. 2H 2 O], Sodium Sulfide (Na 2 S), Polyvinylpyrrolidone (PVP, MW-40,000) and Sodium Hexametaphosphate (SHMP, MW ) were analytical grade reagents and were used as 12

2 Fig. 1 X-ray diffraction spectra of uncapped, PVP and SPP capped CdS nanoparticles. received. Double-distilled water, ethanol and acetone were used. 2.2 Synthesis of CdS nanoparticles The CdS nanoparticles were synthesized by chemical precipitation method using water-soluble surfactants such as PVP and SHMP. The synthesis reaction was carried in the aqueous medium at air atmosphere. Solutions of 0.5M Cd (C 2 H 3 O 2 ) 2, 0.5M of Na 2 S and 0.5g PVP were prepared in 50 ml de-ionized water separately. First, the PVP solution was added with Cd (C 2 H 3 O 2 ) 2 solutions by stirring continuously. Next, 50 ml of Na 2 S solution was added to the above solution. The yellow color precipitate was collected from the solution by adding a known volume of Acetone. Immediate flocculation of nanoparticles occurred. To remove the last traces of adhered impurities, the particles were washed several times using de-ionized water and Acetone. The washed particles were dried at 80 C in air for 2h. For the synthesis of SHMP -capped CdS nanoparticles, SHMP was used instead of PVP with same precursor composition. The uncapped CdS nanoparticle was also synthesized using the same precursor materials without adding surfactant Characterization The X-ray diffraction (XRD) patterns of the powdered samples were recorded using X pert PRO diffractometer with a CuKα radiation (λ= Ǻ). The crystallite size was estimated using the Scherrer equation (0.9λ)/ (β cosθ) at the full width at half maximum of the XRD peaks. The size and morphology of the nanoparticles were determined using TEM (PHILIPS-CM200; kV). The Fig. 2a-c SEM micrographs of uncapped, PVP and SPP capped CdS nanoparticles. 13

3 Fig. 4 UV-Visible spectra of uncapped, PVP and SPP capped CdS nanoparticles. Fig. 3a and b TEM micrographs and corresponding SAED pattern of the SPP capped CdS nanoparticles. optical transmission/absorption spectra of the sample in deionized water were recorded using UV-1650PC SHIMADZU Spectrometer. Fluorescence measurements were performed on a RF-5301PC Spectrophotometer. 3. Results and discussion 3.1 Structural and Morphological study XRD pattern of the prepared uncapped, PVP and SHMP -capped CdS samples are shown in Fig. 1. It shows three diffraction peaks at 2θ values corresponding to reflections from (1 1 1), (2 2 0), and (3 1 1) planes of the cubic (Zinc blend) phase [JCPDS ]. The broadening of the diffraction peaks in Fig. 1 indicates the nanocrystalline nature of the samples. This broadness is increased for the capped particles due to decrease of the particles size. The average crystallite size is calculated from the full width at half maximum (FWHM) of the diffraction peaks using the Debye Scherrer formula [22], D = kλ/βcosθ; where k is a geometric factor (= 0.9), λ is the X-ray wavelength (1.546 Å), β is the FWHM of diffraction peak and θ is the diffraction angle. The grain size of the uncapped, PVP and SHMP 14 Fig. 5 PL spectra of uncapped, PVP and SPP capped CdS nanoparticles. capped CdS particles are calculated from the most intense peak at (1 1 1) plane approximately at 4.7, 4.0 and 3.8 nm. The reduction of particle size indicates the effect of the capping agent for prepared colloidal CdS nanoparticles in aqueous medium. All lines could be indexed to the cubic structure of CdS in the entire range of compositions studied. No extra peaks were found, indicating the single phased nature of the samples. It indicated that the surfactants are not disturbed in the CdS lattice. It is only the physical adsorption (Van der Waals force) on the surface of CdS nanoparticles. SEM and TEM micrographs are used to study the morphology and particles size of the samples. Fig. 2a c show SEM micrographs of uncapped, PVP and SPP capped CdS nanoparticles. As shown in the figures, the synthesized particles are in the nanoscale range (100 nm <). Comparing

4 the uncapped CdS with surfactants capped nanoparticles, the uncapped particles are highly aggregated due to the absence of the capping agent in the prepared solution. All nanoparticles are spherical in shape as shown in the Fig. 2a c. Fig. 3a and 3b show TEM micrograph and corresponding selected area electron diffraction pattern of SPP capped CdS nanoparticles. As seen in the figure 3a, the particles are very narrow and spherical shaped. The calculated particle size is 3.8 nm. The small size and monodispersion indicated necessity of the capping agent for synthesis of the nanoparticles. Fig. 3b shows the corresponding SAED pattern of the sample which consists of a central halo with concentric broad rings. It is identified to have cubic structure with diffraction planes corresponding to (1 1 1), (2 2 0) and (3 1 1). No planes corresponding to any impurity phase could be identified in the SAED pattern. The average diameter of the nanoparticles obtained from TEM image is in well agreement with the average grain size obtained from the X-ray diffraction pattern. It is known that semiconductor nanoparticles have unique size-dependent chemical and physical properties [14, 16, 23 26]. As the size of semiconductor particles decreases to the nanoscale, the band gap of the semiconductor increases, causing a blue shift in the UV Vis absorption spectra. The bulk CdS materials exhibit an absorption peak of about 515 nm. If the absorption peak of the obtained sample appeared blue-shifted compared with that of bulk CdS, it could indicate the presence of quantum confinement effect. The UV Vis spectrum of the uncapped, PVP and SPP capped CdS nanoparticles are shown in Fig. 4. As illustrated in Fig. 4, the surfactants capped CdS nanoparticles exhibited an absorption peak of CdS/PVP at 421 nm and CdS/SPP at 412 nm. They are considerably blue-shifted relative to the absorption peak of bulk CdS (515 nm), indicating quantum confinement effect. The absorption peak of the uncapped CdS is centered at 442 nm. Comparing the uncapped CdS nanoparticles, the absorption peaks of the surfactants capped are significantly blue shifted. It indicates reduction of the particles size by the surfactants. The PL spectrum of the uncapped, PVP and SHMP capped CdS a nanoparticle is shown in Fig. 5. The PL spectrum of the CdS nanoparticles was dominated by very strong and narrow emission peak at around 570 nm. The strong emission was attributed to deep trap sites due to the sulfur vacancy. The fluorescence spectrum of surfactants capped CdS nanoparticles showed similar features as that of CdS nanoparticle with slightly larger fluorescence intensity. One can draw the conclusion that the fluorescence of uncapped CdS could be enhanced by the surfactants. The fluorescence enhancement may be attributed to a much higher concentration of radiative recombination centers. Since these centers were likely to be Sulfur vacancies (V S 2+ ), the concentration of V S 2+ must be increased. It is well known that, Sulfur is more soluble in organic solvents of low dielectric constant than in water [27]. Hence, one can say that the excess Sulfur ions might be dissolved from the nanoparticles surface, increasing the number of V S 2+ which in turn removes the local trap sites from the surface (decreases the number of the hole scavengers), thus, significantly increasing the intensity of the emission. By comparing the surfactant capped CdS nanoparticles, the PL intensity of SPP caped particles is dramatically enhanced than that the PVP capped particles. The enhanced PL intensity indicates complete removal of the surface defect. It may be due to presence of the Na ions in the SPP. Due to similar electric charge of the Na ions of SPP between the two CdS interface, it may create a strong electrostatic repulsion, resulting the CdS particles isolated with homogeneity. In addition the surfactants capped particles are significantly blue shifted from the uncapped particles, which may be due to reduction of the particles size. 4. Conclusion Uncapped, PVP and SPP capped CdS nanocomposites were successfully synthesized via simple synthetic route at 80 C, atmospheric pressure. The SEM and TEM data indicated that, nanocomposites were formed with spherical shape. The XRD analysis showed that both uncapped and capped CdS nanoparticle have cubic structure. The synthesized particles sizes are in 3 5 nm range. UV Vis and Photoluminescence spectra indicated that the presence of surfactants could influence the optical properties of the CdS nanoparticles. 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