Photodynamics of Silica Coated CdS Nanoparticles

Transcription

1 Photodynamics of Silica Coated CdS Nanoparticles Kalaiyar Swarnalatha 1, Subramaniam Kamalesu 2, Thangaraj Shankar 3, T. Antony Rajasekar 4 1 Department of Chemistry, Manonmaniam Sundaranar University, Tirunelveli , Tamil Nadu, India Abstract - Silica coated CdS nanoparticles were synthesized using different types of surfactants (CTAB, SDS) as stabilizers. Bare CdS nanoparticles are prepared by reverse micelle based method using AOT surfactants. SiO 2 provides a sufficient contrast to show the core homogeneously surrounded by the silica shell. The coating can be made thicker by the addition of ammonia and TEOS resulting in monodispersed silica spheres with CdS cores placed at these centers. The influence of surfactants on the particle size of the nanoparticles are well characterized by various techniques such as scanning electron microscopy (SEM), X-ray diffraction, UV-Vis and fluorescence spectroscopy. Silica coated CdS nanoparticles show excellent emission and can be used in various therapeutic diagnosis and treatment. The photophysical studies of the silica coated and non coated CdS naoparticles are carried out and the effect of stabilizer on the luminescence properties of the nanoparticles are investigated in detail. Index Terms Silica coated CdS nanoparticles, stabilizer - surfactants, photodynamics I. INTRODUCTION Synthesis of nanoparticles (NPs) has received a great deal of interest because of their potential biomedical applications in imaging, drug targeting and delivery. Modern biomedical imaging technologies have led to significant advances in diagnosis and therapy [1-5]. Several types of nanoparticles have been employed for bioimaging applications, including metals, metal oxides and semiconductor nanocrystals [6, 7]. Water-soluble functional NPs are indispensable for various biomedical applications. The coating helps to convert hydrophobic NPs into hydrophilic water-soluble particles and introduce chemical functionality onto the particle surface so that different chemicals and biomolecules can be covalently attached. Silica coating is one of the most widely used methods for the surface modification of NPs [8]. Silica acts as a robust, inert layer against the degradation of optical properties and imparts water solubility. This coating method can introduce a crosslinked silica shell to protect the core NPs from the external environment [9]. Rogach et al. encapsulated water-soluble CdTe QDs within nm silica spheres using a modified Stober method, which resulted in reduced emission intensity with broadening of the spectrum [10]. Silica-coated QDs exhibit water solubility, colloidal stability, photostability, efficient fluorescence, and low nonspecific adsorption under aqueous biological conditions. Only a few methods have reported silicacoated NPs of <20 nm in size with high water solubility and colloidal stability [11,12]. The incorporation mechanism of hydrophobic QDs into silica spheres is clearly elucidated and polyethyleneglycol (PEG)-modified silica-coated core shell (SiO 2 /CdSe - ZnS) NPs have been synthesized by Koole et al [13]. The synthesized hydrophobic trioctylphosphine oxide (TOPO)-capped CdSe QDs can be coated with silica in reverse microemulsion method, employing Igepal as the nonionic surfactant [14]. Dyes encapsulated within silica shells had an enhanced lifetime and luminescence due to improved chemical stability and photostability [15, 16]. Silica coated CdS QDs would be expected to be less toxic than bare CdS cores, making CdS SiO 2 core-shell QDs more attractive for biological labeling and down conversion in solid-state light source [17]. The main aim of this paper is to convert the hydrophobic NPs into hydrophilic NPs. In this paper we report the synthesis of Silica coated CdS (CdS@SiO 2 ) nanoparticles by microemulsion method using CTAB and SDS as stabilizer. The photophysical properties of the synthesized nanoparticles are studied and the role of the stabilizer in the synthesis of the above NPs were investigated. II. EXPERIMENTAL PROCEDURE Cadmium nitrate, Sodium sulphide, Ammonium hydroxide, Tetraethyl orthosilicate (TEOS), Ethanol, cetyl trimethyl ammonium bromide (CTAB), sodium dodecyl sulphate (SDS), Sodiumbis(2-ethylhexyl) sulphosuccinate (or) aerosol OT (AOT) were purchased from SD- fine chemicals. n-hexane (HPLC grade) was purchased from Merck and it is used without further purification. All the other reagents used are of reagent grade. 2.1 Synthesis of CdS Nanoparticles [18] Cadmium sulphide nanoparticles were prepared by the reverse micelle method. The solution of mol (AOT) in 100 ml n-hexane was prepared. The aqueous solutions of cadmium acetate (0.2mol/L) and sodium sulphide (0.2mol/L) were separately added to the two portions of AOT containing n-hexane solution. The mixed solutions were stirred for two hours. both solutions were mixed together and stirred for another 1 hour for obtaining CdS nanoparticles. However, CdS nanoparticles prepared by this method represent the small CdS seeds. Therefore, a certain amount of Na 2 S containing micelles was further added into the reverse micelle solution containing these seeds. 660

2 After stirring for 3 hours, the equal amount of Cd(NO 3 ) 2 micelle solution was added into the reverse micelle solution. After the seed growth procedure, we obtained the larger CdS nanoparticles. Same procedure was followed using CTAB and SDS instead of AOT surfactant and yellow precipitate of CdS nanoparticles. 2.2 Synthesis of silica coated CdS nanoparticles [19] For the synthesis of CdS-SiO 2 nanoparticles, the ammonia solution (10 L, 25wt %) was added as a catalyst into the obtained CdS solution. After 10 minutes, the 0.96 ml Tetraethyl orthosilicate (TEOS) was introduced under vigorous magnetic stirring. The reacting system was stirred at a room temperature for 24 hours. By hydrolysis and condensation of TEOS, Silica shell was formed on the surface of CdS nanoparticle. The synthesized nanoparticles are characterized by various techniques such as X-ray diffraction using Brucker AXS D8 Advanced powder X-ray diffractometer. Optical absorption spectra of the nanoparticles were recorded using UV Vis Perkin Elmer Lambda 25 spectrophotometer. Emission measurements were done with JASCO F6300 Spectrofluorometer. The photodynamics of the above synthesized nanoparticles is carried out. The size of the CdS@SiO 2 nanoparticles synthesized using CTAB surfactant is found to be larger (~ 4.0 nm) compared to the nanoparticles prepared using SDS (~2.5 nm) surfactant. The energy dispersive X-ray spectroscopy (EDX) spectra taken for CdS@SiO 2 nanoparticles using CTAB and SDS as stabilizer are shown in figure 4, 5. The EDX spectrum of the silica coated CdS nanoparticles shows the additional signal corresponding to C, Si & O elements apart from CdS. This indicates that the samples are free of any contamination. (a) 3.1 Morphology Study III. RESULT AND DISCUSSION 3.1.1SEM Characterization Figure 1 shows the SEM images for the bare CdS nanoparticles. CdS nanoparticles prepared by microemulsion method (reverse-micelle based template) are spherical, uniform and monodispersed [20]. As determined from the images, the average size of the obtained spherical CdS nanoparticles are in the range >1 nm. (b) Figure 2. SEM images of (a) CdS@SiO 2/SDS and (b) Cds@SiO 2/CTAB (A) (B) (C) Figure 1. SEM images of CdS nanoparticles with the magnification of (A) 50 nm, (B) 10 nm and (C) 5 nm. The SEM images of the CdS@SiO 2 nanoparticles prepared using CTAB and SDS surfactants as stabilizer (Figure 2.a,b) showed that the nanoparticles are spherical with an uniform size (about 1 to 5 nm), however, the partly aggregate may result from the hydroxylated dehydrated cross-link between different particles. 661 Figure 3. EDAX pattern of CdS@SiO 2 nanoparticles using SDS surfactant as stabilizer. Element Weight Atomic C N O Si S Cd Total

3 Figure 4. EDAX pattern of 2 nanoparticles using CTAB surfactant as stabilizer. Element Weight Atomic C N O Si S Cd Total Structural Characterizations X-ray Powder Method Figure 5a-c illustrates the XRD pattern of synthesized CdS and CdS@SiO 2 nanoparticles. The XRD peaks are found to be broad which indicates the very fine size of the grain [20]. CdS nanoparticles have broad peaks at 2θ values 26.75, 44.11, 51.76, The broaden peaks are indicating that the sizes of the particles are in nano range. The particle size was determined from the width of XRD peaks using Scherrer s formula D = (0.94λ) / (β cosθ) and is found to be 2.6 nm in diameter. The XRD patterns for CdS@SiO2/CTAB and CdS@SiO2/SDS are shown in (figure 5b&c). The peaks are found to be very broad indicating the sizes of the nanoparticles fall in nano regime. 3.3 Photodynamics of CdS@SiO 2 nanoparticles Absorption spectra The absorption spectrum of CdS@SiO 2 nanoparticles shown in Figure 6 & 7. The particle size of the CdS, CdS@SiO 2 /CTAB and CdS@SiO 2 /SDS analyzed with the help of absorption spectra. The absorption maximum observed at 450, 424, 415 nm for noncoated and coated nanoparticles. The decrease in the absorption values on coating with silica indicated the consequence of quantum confinement and the particle size could also be calculated in virtue of the following expression [22], D = ( *10-8 ) λ 3 + (1.9557*10-4 ) λ 2 (9.2352*10-2 ) λ The particle diameters of the synthesized CdS@SiO 2 nanoparticles are ~ nm and are given in the table 1. The particle size determined using the absorption maximum values are well coincided with the particle size obtained from the SEM images. Table 1. Particle size of the nanoparticles determined using absorption values. Nanoparticles λ max, nm Particle size (D) CdS@SiO 2/ CTAB CdS@SiO 2 /SDS Figure 5. XRD pattern of (a) CdS, (b) CdS@SiO 2/CTAB, (c) CdS@SiO 2/SDS nanoparticles 662 Figure 6. UV-Visible Absorption Spectrum of noncoated CdS nanoparticles.

4 3.3.2 Emission spectra The emission spectra of the synthesized 2 /CTAB and 2 /SDS, excited at their corresponding absorption maximum is given in figure 8 & 9. The emission spectrum of the synthesized CdS@SiO 2 /CTAB exhibits two bands at 545 and 610nm (Figure 8). The band appeared in the range of 545nm provides yellow emission. The yellow emission was attributed to recombination via surface localized states and a transition from interstitial cadmium to valence band. It also exhibits a broad band with a maximum at 610 nm and it corresponds to red emission [24]. Red emission was observed due to the presence of sulphur vacancies. The wavelength shifted to red region on coating with silica. Figure 9. Absorption and emission spectra of CdS@SiO 2 (using SDS surfactants). The emission spectra of the synthesized CdS@SiO 2 /SDS exhibits a sharp band at 490 nm corresponds to green emission (Figure 9). The emission due to electronic transition from the conduction band to an acceptor level due to interstitial sulphur. The small hump was also observed around 580 nm, called as yellow emission. Figure 7. UV-Visible Absorption spectra of coated CdS nanoparticles using surfactant stabilizers. Figure 8. Absorption and emission spectra of CdS@SiO 2 (using CTAB surfactants). 663 Figure 10. Emission spectra of CdS and silica coated CdS Nanoparticles. Figure 10 shows the emission spectra of CdS/CTAB exhibits two bands at 460, 585 nm. The sharp band at 460 nm corresponds to green emission. The green emission band was associated with the emission due to electronic transition from the conduction band to an acceptor level due to interstitial sulphur. The band appeared in the range of 585nm provides yellow emission [23, 24]. The yellow emission was attributed to recombination via surface localized states and a transition from interstitial cadmium to valence band. Effect of surfactants on the size of the NPs In the field of nanoscience, these surfactants are used as stabilizers in the synthesis of nanoparticles. The main and the important role of these stabilizers are to achieve monodispersity.

5 We have used two different surfactants say, CTAB and SDS in the preparation of CdS nanoparticles. Effects of added surfactants (stabilizer) are explored on the photophysical properties and the influence of the surfactants on the surface properties of the nanoparticles. The SEM images of the 2 nanoparticles prepared using CTAB and SDS surfactants as stabilizer are given in figure 1 & 2. From the SEM analysis, we conclude that the size of the CdS@SiO 2 nanoparticles synthesized using CTAB surfactant is found to be larger compared to the nanoparticles prepared using SDS surfactant. From the absorption spectra of CdS@SiO 2 nanoparticles prepared using CTAB and SDS surfactants, we observed and reported that the size of the CdS@SiO 2 nanoparticles synthesized using CTAB surfactant (4.2 nm) is found to be larger compared to the nanoparticles prepared using SDS surfactant (3.9 nm). Larger size particles can be achieved using CTAB surfactants as stabilizer. The emission spectra of the synthesized CdS@SiO 2 /CTAB exhibit a broad band and correspond to red emission. But, the photoluminescence emission spectra of the synthesized CdS@SiO 2 /SDS exhibit a sharp band and correspond to green emission. So CTAB facilitated CdS@SiO 2 is found to be a promising candidate for improving the luminescence performance. This due to the presence of more number (15) of carbon units in the alkyl chain in CTAB compared to SDS (12 carbon units) and also the presence of cationic head group in CTAB. IV. CONCLUSION In summary, Silica coated CdS nanoparticles were synthesized using different types of surfactants (CTAB, SDS) as stabilizers. The broad XRD peaks indicated that the sizes of the nanoparticles fall in nano regime. FESEM image shows the size of the CdS@SiO 2 /CTAB nanoparticles is found to be larger compared to the CdS@SiO 2 /SDS, due to the longer chain length and increase in number of carbon units. Size of the nanoparticles determined using absorption values. The chemical purity of synthesized NPs was confirmed by EDX measurement. The emission spectra exhibit a broad band with red emission at 610nm for CdS@SiO 2 /CTAB. But, the photoluminescence emission spectra exhibit a sharp band with green emission at 490nm for CdS@SiO 2 /SDS. So CTAB facilitated CdS@SiO 2 is found to be a promising candidate for improving the luminescence performance. Acknowledgment Kalaiyar Swarnalatha is grateful to Dr. K. Jeganathan, Center for Nanoscience and Nanotechnology, School of Physics, Bharathidasan University, Tiruchirappali for SEM measurements. The authors are also expressing their sincere thanks to the University for their partial financial support under University Seed Money Project for completing this work. REFERENCES [1] Selvan, S. T.; Tan, T. T. Y.; Yi, D. K.; Jana, N. R., Langmuir, 2010, 26, [2] Medintz, I. L.; Uyeda, H.T.; Godman, E.R.; Mattoussi, H., Nat. Mater, 2005, 4, [3] Weissleder, R., Science, 2006, 312, [4] Rosi, N.L; Mirkin, C.A., Chem. Rev, 2005, 105, [5] Murphy, C. J.; Gole, A. M.; Stone, J. W.; Sisco, P. N.; Alkilani, A. M.; Goldsmith, E. C.; Baxter, S. C., Acc. Chem. Res, 2008, 41, [6] Jun, Y.W.; Lee, J.H.; Chenon, J., Angew. Chem., Int. Ed, 2008, 47, [7] Samia, A. C. S.; Chen, X.; Burda, C., J. Am. Chem. Soc, 2003, 125, [8] Zhelev, Z.; Ohba, H.; Bakalova, R., J. Am. Chem. Soc, 2006, 128, [9] Santara, S.; Yang, H.; Holloway, P. H.; Stanley, J. T.; Mericle, R. A., J. Am. Chem. Soc, 2005, 127, [10] Rogach, A. L.; Nagesha, D.; Ostrander, J. W.; Giersig, M.; Kotov, N. A., Chem. Mater, 2000, 12, [11] Larsen, E. K. U.; Nielsen, T.; Wittenborn, T.; Birkedal, H.; Vorup-Jensen, T.; Jacobsen, M.H.; Ostergaard, L.; Horsman, M.R.; Besenbacher, F.; Howard, K.A.; Kjems, J., Nano, 2009, 3, [12] Schroedter, A.; Weller, W., Angew. Chem. Int. Ed, 2002, 41, [13] Koole, R.; Van Schooneveld, M. M.; Hilhorst, J.; Donega, C.D.; t Hart, D. C.; van Blaaderen, A.; Vanmaekelbergh, D.; Meijerink, A., Chem. Mater, 2008, 20, [14] Selvan, S. T.; Tan, T. T.; Ying, J. Y., Adv. Mater, 2009, 21, [15] Lal, M.; Levy, L.; Kim, K. S.; He, G.S.; Wang, X.; Min, Y.H.; Pakatchi, S.; Prasad, P.N., Chem. Mater, 2000, 12, [16] Chang, S.Y.; Liu, L.; Asher, S.A., J. Am. Chem. Soc, 1994, 116, [17] Qian, L.; Bera, D.; Teng-Kuan Tseng; Holloway, P.H., App. Phys. Lett, 2009, 94, [18] Wei, C.; Zhou. W.; Du, Y.; Xu, J.; Yang, P., Coll. J, 2010, 72 (2), [19] Teng, F.; Tian, Z.J.; Xiong, G.X.; Xu, Z.S., Catalysis Today, 2004, 93-95, 651. [20] Song-yuan Chang; Liu, L.; Asher, S.A., J. Am. Chem. Soc, 1994, 116, [21] Duan, J.; Feng, Y.; Yang, G.; Xu, W.; Li, X.; Liu, Y.; Zhao, J., Ind. Eng. Chem. Res, 2009, 48, [22] Yu, W. W.; Qu, L.; Guo, W.; Peng, X., Chem. Matter, 2003, 15, [23] He, X.; Wang, K.; Cheng, Z., Nanomed. Nanobiotech, 2010, 2, [24] Iwasaki, K.; Torimoto, T.; Shibayama, T.; Takahashi, H.; Ohtani, B., J. Phys. Chem. B, 2004, 108,