Synthesis and characterization of Ce3+-doped flowerlike ZnS nanorods
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- Shanmugam, N., Cholan, S., Viruthagiri, G. et al. Appl Nanosci (2014) 4: 359. doi:10.1007/s13204-013-0217-x
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Cerium-doped zinc sulfide nanorods with flower-shaped morphology have been successfully synthesized in air atmosphere through simple chemical precipitation method. The incorporation of Ce3+ was confirmed by X-ray diffraction and energy-dispersive spectrum. The doping of Ce3+ ions has significant influence on the optical properties of the synthesized rods. UV–Vis absorption spectroscopy measurements have shown that the absorption peak of doped ZnS was red shifted compared to undoped ZnS. Photoluminescence measurements reveal that luminescence intensity of Ce3+ was enhanced considerably by the energy transfer from ZnS. The existence of functional groups was identified using Fourier transform infrared spectrometer. Field emission scanning electron microscope results show a uniform growth pattern of the nanorods with flowerlike structure. High resolution transmission electron microscopy results showed that the doped ZnS nanocrystals are composed of uniform nanorods with average diameter of 22 nm and length of 140 nm. The thermal stability of the nanorods was confirmed using thermo gravimetric and differential thermal analysis.
KeywordsZnS nanorodsFlowerlike structureFESEMHRTEMEDS
In the modern scientific era, optical properties of doped semiconductor nanoparticles have shown great impact as their electronic structure and electro magnetic fields are drastically modified due to quantum confinement effects. Tailoring the color output of nanomaterials has been playing an important role for their applications as light emitting displays, field emitters (Fang et al. 2007; Bando et al. 2007), lasers, sensors (Fang et al. 2009) and optoelectronic devices to multiplexed biological labeling (Torimoto et al. 2007; Yu et al. 2010). The luminescence properties of ZnS nanoparticles have been tuned by doping with varies transition metals and rare-earth metals (Manzoor et al. 2004; Hu and Zhang 2006; Kim 2009; Ageeth and Meijerink 2001). Since ZnS has wide band gap of 3.68 eV at room temperature, its nanocrystals are the suitable host materials for the doping elements such as RE and transition metal ions which are optically and magnetically active. Rare-earth ions are categorized by group of elements known as the lanthanides and are most stable in their triply ionized form. Furthermore, ZnS is one of the eco friendly materials, the RE-doped ZnS nanocrystals can be used in producing efficient phosphor materials with a gamut of colors (Bhargava 1996). ZnS host is able to produce red, blue, and green luminescence due to various RE ion dopants. So far, different synthesis routes have been suggested for the preparation of RE-doped ZnS nanorods under low temperature and in particular, the effect of dopant on the structures and optical properties are limited (Wang and Fan 2006). In this paper, we have attempted with a simple chemical precipitation method to prepare the Ce3+-doped flower-shaped ZnS nanorods. From this method, cerium-doped nanorods of ZnS with flowerlike morphology have been obtained.
Zinc acetate dihydrate [Zn(CH3COO)2·2H2O], thiourea [NH2CSNH2], cerium(III)chloride hepta hydrate[(CeCl3)·7H2O] were purchased from Merck. All chemicals were used as received since they were of analytical regent grade with 99 % purity. The glass wares used in this experimental work were acid washed. Ultrapure water was used for all dilution and sample preparation.
Synthesis of Ce3+-doped ZnS nanorods and undoped ZnS nanocrystals
For the synthesis of Ce3+-doped ZnS nanorods, 3 g (0.35 M) of zinc acetate [Zn(CH3COO)2·2H2O] in 40 ml of deionized water–ethanol matrix (equal volume) and an appropriate amount of cerium in 10 ml aqueous were mixed drop by drop. Then, 3 g (1 M) of thiourea [NH2CSNH2], in 40 ml of deionized water–ethanol matrix was added drop by drop to the above mixture. The entire mixture was stirred magnetically at 80 °C until the homogeneous solution was obtained. Finally, the product was dried in a hot air oven at 120 °C for 2 h. A similar method of preparation, without the addition of cerium, was used to synthesize undoped ZnS nanocrystals.
The X-ray diffraction (XRD) patterns of the powdered samples were recorded using X′ PERT PRO diffractometer with Cu-Kα radiation (λ = 1.5406 Å). The crystallite size was estimated using the Scherrer equation. The optical absorption spectra of all the samples in deionized water were recorded using LAMDA 25 PERKIN ELMER spectrometer. Fluorescence measurements were performed on a VARIAN spectrophotometer. The FT-IR spectra were recorded from a SHIMADZU-8400 spectrometer using KBr pellet technique. The morphology of the product was observed by a HITACHI S-4700 field emission scanning electron microscope (FESEM). Energy-dispersive spectrum (EDS) measurement was carried out with the EDS arrangement enclosed with HITACHI S-4700 FESEM. High-resolution transmission electron microscopy (HRTEM) analysis was performed using JEOL 3010 HRTEM to study the morphology and size of the nanocrystals. Thermo gravimetric analysis (TGA) and differential thermal analysis (DTA) studies have been performed using Perkin Elmer Diamond TGA/DTA instrument at a heating rate of 20 °C/min in air.
Result and discussion
Optical absorption studies
From the figure, it is observed that ZnS nanocrystals composed of uniform nanorods with average diameter of 22 nm and length of 140 nm were obtained. This is in good agreement with the size obtained from FESEM and XRD analyses. The structure of the grown rods was revealed by the selected area electron diffraction (SAED) pattern (Fig. 7d). The SAED pattern shows three rings corresponding to (111), (220), and (311) planes, respectively, which is in agreement with the XRD patterns.
To confirm the ZnO formation, the nano ZnS was annealed to 1,000 °C and then XRD was taken. The XRD pattern revealed the presence of hexagonal wurtzite ZnO with a very weak peak related to cubic ZnS (Figure not shown). In addition, above 1,100 °C, there is a sudden downtrend in DTA curve with significant weight loss. This may be due to release of residual sulfur ions from the sample.
We have synthesized Ce3+-doped ZnS nanorods with flowerlike structure for the first-time using simple chemical precipitation method. The substitution of parts of lattice Zn of ZnS by Ce3+ ions is confirmed by XRD and EDS techniques. The doping of Ce3+ ions has tuned the band gap and photo luminescent properties of ZnS nanorods. The absorption edge of Ce3+-doped ZnS nanorods was shifted to lower energy side relative to that of undoped ZnS. Undoped ZnS exhibits an emission maximum at 498 nm, whereas on doping two peaks, one at 356 nm and another at 604 nm, were obtained as a result of 5d → 4f transition in Ce3+ ions. Morphological features indicated that the Ce3+-doped ZnS nanocrystals were composed of mono dispersed nanorods with flowerlike structure. The doping of Ce3+ enhanced the UV-emission efficiency of nanocrystalline ZnS. The thermograms have confirmed the stability of the synthesized products.
The authors wish to thank Dr. S. Barathan, Professor and Head, Department of Physics, for providing necessary facilities to carry out this work. We also like to thank Dr. S. T. Balasubramaniyam, Director CAS in Marine Biology, Dr. S. Vijayalakeshmi, Scientist, COMAPS project and Dr. N. Krishnakumar, Assistant Professor, Department of Physics, Annamalai University, for their stimulating discussions during the period of this work.
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