PREPARATION OF CUBE-SHAPED CdS NANOPARTICLES BY SONOCHEMICAL METHOD

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1 International Journal of Nanoscience, Vol. 1, Nos. 5 & 6 (2002) 1 5 c World Scientific Publishing Company PREPARATION OF CUBE-SHAPED CdS NANOPARTICLES BY SONOCHEMICAL METHOD HUI WANG, YINONG LU, and JUNJIE ZHU, Laboratory of Mesoscopic Materials Science, Department of Chemistry College of Materials Science and Engineering Nanjing University of Technology, Nanjing, , P. R. China jjzhu@nju.edu.cn Received 26 November 2002 Revised 24 December 2002 Cube-shaped CdS nanoparticles have been successfully prepared by a sonochemical method in an oil-in-water microemulsion. The product was characterized by using techniques including X-ray powder diffraction, high-resolution transmission electron microscopy, energy-dispersive X-ray analysis and UV-visible absorption spectroscopy. This microemulsion system in the presence of high-intensity ultrasound irradiation provides special conditions for the nucleation and growth of the CdS nanoparticles. Keywords: CdS; nanoparticles; sonochemistry; microemulsion; cube-shaped. There has been increasing interest in the preparation and characterization of chalcogenides due to their important physical and chemical properties. 1 Among the chalcogenides, CdS is one of the most attractive semiconducting materials owing to its special properties and wide application in various fields, especially in photoconducting cells. It is known that the chemical and physical properties of nanosized semiconductors depend on their size and morphology. Therefore, control over both size and morphology has become the focus of interest of many synthetic chemists and materials scientists. In the past few years, nanocrystalline CdS with various morphologies including spherical nanoballs, 2 nanorods or nanowires, 3 nanoribbons, 4 hollow spheres, 5 and peanut-like particles 6 have been successfully synthesized via several novel synthetic routes. However, up to now, designing new strategies for the fabrication of cube-shaped CdS nanoparticles still remains a great challenge. Currently, ultrasound irradiation offers a very attractive method for the preparation of nanosized materials and has shown very rapid growth in its application to materials science due to its unique reaction effects and its ability to induce the formation of novel materials with unusual properties. 7 It has been reported that exposing an aqueous solution containing cadmium salts and sodium thiosulfate will lead to the formation of spherical CdS nanoparticles. 8 Sonochemical preparations of spherical assemblies composed of CdS spherical particles in CS 2 - water-ethylenediamine (CWE) system 9 have also been recently reported. Herein, 1

2 2 H. Wang, Y. Lu & J. Zhu we design a new strategy to prepare cube-shaped CdS nanoparticles sonochemically in a microemulsion. To the best of our knowledge, the preparation of cube-shaped CdS nanoparticles has never been reported before. In a typical procedure, 5 ml sulfur-toluene solution (STS) was mixed with 95 ml aqueous solution containing mol cadmium chloride (CdCl 2 ), 0.4 g sodium dodecyl sulfate (SDS) and 5 ml ethylenediamine. STS was prepared by stirring 1.0 g sulfur powder in 50 ml toluene at ca. 313 K till the sulfur powder was completely dissolved. Then the mixtures were exposed to ultrasound irradiation under ambient air for 30 minutes. Ultrasound irradiation was accomplished with a high-intensity ultrasound probe (Xinzhi Co., China; JY92-2D; 0.6 cm-diameter; Ti-horn, 20 khz, 60 W/cm 2 ) immersed directly in the reaction solution. The sonication was conducted without cooling so that a temperature of about 333 K was reached at the end of the reactions. When the reactions finished, a yellow precipitate occurred. The precipitate was separated by centrifugation, washed with distilled water, absolute ethanol and acetone in sequence, and dried in air at room temperature. The final products were characterized by using techniques including X-ray powder diffraction (XPRD), high-resolution transmission electron microscopy (HRTEM), energy-dispersive X-ray analysis (EDAX) and UV-visible absorption spectroscopy. The XPRD analysis was performed by Shimadzu XD-3A X-ray diffractometer with graphite monochromatized Cu Ka radiation ( = nm). The HRTEM images were obtained by employing a JEOL-4000EX high-resolution transmission electron microscope with a 200 kv accelerating voltage. EDAX measurements were performed by employing the PV9100 instrument. Ruili UV-2400 photospectrometer was used to record the UV-Visible absorption spectrum of the as-prepared particles. The morphology and structure of the as-prepared CdS nanoparticles were characterized by HRTEM. The HRTEM images (Figs. 1(A) 1(C)) reveal that the product is composed of cube-shaped nanoparticles with 3 6 nm in size. Though they are agglomerated, cubic morphology and hexagonal lattice fringes can be seen clearly. Figure 1(D) provides further insight into the structure of an individual CdS nanoparticle. It is obvious that this particle crystallizes in a hexagonal structure with cell constant a = b = 0.41 nm, as measured directly in Fig. 1(D). The spaces of lattice planes are close to those obtained from the bulk CdS crystals. The XPRD pattern of the product (Fig. 2) provides further evidence to its crystalline structure. All the diffraction peaks can be indexed to be a pure hexagonal phase for CdS. The intensities and positions of the peaks are in good agreement with the literature values. 10 The average size of the as-prepared CdS nanoparticles is estimated to be 5 nm according to the Debye Scherrer formula. 11 The product is also characterized by EDAX measurements for evaluation of their composition and purity. The EDAX results reveal that the as-prepared CdS has high purity and the Cd:S ratio is calculated to be approximately 1:1. Figure 3 shows the UV-visible absorption spectrum of the as-prepared CdS nanoparticles dispersed homogeneously in ethanol solution (the concentration is

3 Preparation of Cube-Shaped CdS Nanoparticles by Sonochemical Method 3 A B C D Fig. 1. HRTEM images of the as-prepared CdS nanoparticles. 002 Intensity / a.u θ / degree Fig. 2. XPRD pattern of the as-prepared CdS nanoparticles mg/ml). The maximum at about 475 nm is assigned to the optical transition of the first exited state. 3 The absorption edge of CdS is blue-shifted from the absorption edge of the bulk CdS (512 nm), indicating the presence of quantum size effects in this sample. The blue-shift of the absorption peak is attributed to the increase of the band-gap of the nanoparticles, which indicates the size quantum effects.

4 4 H. Wang, Y. Lu & J. Zhu 1.4 Absorbance / a.u nm Wave length / nm Fig. 3. UV-Vis absorption spectrum of the as-prepared CdS nanoparticles. The present synthetic design was motivated by the known simple interaction between sulfur and ethylenediamine, 12 but it provided special conditions for the formation and growth of the sulfide nanoparticles in such a microemulsion system in the presence of high-intensity ultrasound irradiation. Sulfur-toluene solution (STS) was chosen as the sulfur source. When STS was mixed with water, a liquid liquid heterogeneous system was formed. Sulfur powder was dissolved in toluene, the oil phase, and [Cd(en)x] 2+, and excessive ethylenediamine was in the water phase. The cavitation behavior of ultrasound irradiation can lead to extraction, mixed phase reactions and emulsification in such a liquid liquid heterogeneous system. Thus, an o/w microemulsion of toluene-in-water formed under ultrasound irradiation. Toluene droplets became homogenized and sulfur began to react with ethylenediamine at the oil water interface to give H 2 S. Then the released H 2 S combined with [Cd(en)x] 2+ to form CdS nuclei in the water phase. SDS played a critical role in this process. It not only promoted the formation of the microemulsion as the oil droplet stabilizer, but also provided the sites for the reactions by bridging the oil water interface. The details of the formation of cube-shaped CdS nanoparticles in such a microemulsion under ultrasound irradiation is still under study. In summary, a novel sonochemical method for the preparation of cube-shaped CdS nanoparticles in a microemulsion has been successfully established. It may be extended to the preparation of some other chalcogenide nanoparticles and provides a promising way to produce nanoparticles with cubic morphology. Acknowledgments This work is supported by the National Natural Science Foundation of China (Grant Nos and ) and the Jiangsu Advanced Science and Technology Program (BG ).

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