Solvent-less synthesis of zinc oxide nanostructures from Zn(salen) as precursor and their optical properties
Graphical abstract
Highlights
► Precursor Zn(salen) was synthesized by a solid–solid reaction, and then ZnO nanoparticles were obtaioned by thermal treatment of Zn(salen) at 500 °C for 5 h. ► Room temperature photo-luminescence spectrum of ZnO nanostructures are dominated by green emission attributed to oxygen vacancy related donor–acceptor transition.
Introduction
ZnO is an important functional oxide with direct wide band gap (3.37 eV) and large exciton binding energy (60 meV), exhibiting interesting properties including near-UV emission (Lao, Wen, & Ren, 2002), transparent conductivity (Choi, Lichtenegger, Stucky, & McFarland, 2002), and piezoelectricity (Gao & Wang, 2003). Nanosized ZnO has great potentiality for use in preparing solar cells (Wang et al., 2001), gas sensors (Lin, Tzeng, Hsiau, & Tsai, 1998), chemical absorbents (Rosso, Galletti, Bizzi, Saracco, & Specchia, 2003), varistors (Singhal, Chhabra, Kang, & Shah, 1997), electrical and optical devices (Feldmann, 2003), electrostatic dissipative coating (Kitano & Shiojiri, 1997), catalysts for liquid phase hydrogenation (Hamminga, Mul, & Moulijn, 2004), and catalysts for photo-catalytic degradation (Curridal, Comparelli, Cozzli, Mascolo, & Agostiano, 2003) instead of titania nanoparticles (Fotou & Pratsinis, 1996). In the past decade, various physical or chemical synthetic approaches have been developed to produce ZnO, including vapor phase oxidation (Hu, Li, Wong, Lee, & Lee, 2002), thermal vapor transport and condensation (TVTC) (Lao et al., 2003, Lao et al., 2002), chemical vapor deposition (CVD) (Wu & Liu, 2002), precipitation (Paul et al., 2003, Pesika et al., 2002, Radovanovic et al., 2002, Seelig et al., 2003), sol–gel (Hoyer & Weller, 1995), microemulsion (Guo, Ji, Xu, Simon, & Wu, 2002), hydrothermal (Cheng & Samulski, 2004), solvothermal (Salavati-Niasari, Davar, & Khansari, 2011), sonochemical (Qian, Jiang, & Hansen, 2003), and solid state (Salavati-Niasari, Khansari, & Davar, 2009) methods.
Solid-state chemical reaction provides a relatively simple and powerful method for controlling the size, shape and dimension of nanoparticles. ZnO nanoparticles and nanorods were synthesized by solid-state reaction. Salavati-Niasari, Khansari, et al. (2009), Salavati-Niasari, Ghanbari, and Davar (2009), Sun, Liu, Zhang, and Jia, (2006) reported synthesis of ZnO nanorods by solid state reaction of zinc acetate dihydrate, sodium hydroxide and cetyltrimethylammonium bromide (CTAB) at room temperature. Zhu and Zhou (2008) synthesized ZnO nanoparticles by a one step solid-state reaction.
Solvent-free syntheses have recently attracted considerable attention in the field of green chemistry, and many solvent-less syntheses for organic compounds have been reported. From the viewpoint of green chemistry, we succeeded in preparing high-purity metal complexes without any chemical isolation procedure by solvent-free reactions, and here we report the synthesis of Zn(salen) via simple solvent-less reactions using zinc acetate and N,N′-bis(salicylaldehyde)ethylenediamine (H2salen), and then Zn(salen) was used for preparing ZnO nanoparticles via a thermal treatment process under mild conditions, which has the advantages of short time, high yield, low cost and low power consumption. Morphology of the nanostructured ZnO produced was studied by transmission electron microscopy, and the optical properties, by room temperature photoluminescence and infrared spectroscopy, followed by discussion of the observed luminescence.
Section snippets
Materials
All the chemical reagents used in experiments were of analytical grade and used as received without further purification. Zinc(II) acetate dihydrate, salicylaldehyde, 1,2-ethylenediamine and ethanol were purchased from Merck Co. and used as received.
Characterization
Fourier transform infrared (FT-IR) spectra were recorded on Shimadzu Varian 4300 spectrophotometer in KBr pellets. Room temperature PL was studied on an F-4500 fluorescence (Perkin Elmer) spectrophotometer. Thermo-gravimetric analysis (TGA) was
1H NMR analysis
1H NMR analysis shown in Fig. 1 confirms the formation of the inorganic complexes. The peak observed for H2(salen) at 13.23 ppm was not seen for Zn(salen). Fig. 1(a) shows the 1H NMR (CDCl3) of H2(salen): δ 13.23 (br s, 2H, OH).
XRD analysis of products
Fig. 2(a) displays a typical XRD pattern of the precursor prepared in a molar ratio of 1:1 at 60 °C. All the reflection peaks in this pattern could be readily indexed to crystalline Zn(salen). No obvious peaks of impurities were seen in this pattern. The XRD patterns shown
Conclusion
Nanoparticles ZnO powders were synthesized by means of a novel and simple method. First, precursor Zn(salen) was successfully synthesized by a mechano-chemical solid–solid method, and then ZnO nanoparticles were obtaioned by thermal treatment of Zn(salen) at 500 °C for 5 h. The appearance of excitonic emission in the PL spectra of the ZnO nanoparticles indicates their highly crystalline quality, while the presence of broad visible emission dominated by green emission band indicates the presence
Acknowledgements
The authors are grateful to the councils of Iran National Science Foundation and University of Kashan for their unending effort in providing financial support in this work.
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