Abstract
A new and simple method was applied for the synthesis of ZnO nanoparticles with an average size of 20 nm. In this microwave-assisted combustion method, glycine as a fuel and zinc nitrate as precursor were used. The final product was obtained very fast with high yield and purity. The synthesized nanoscale ZnO was characterized by X-ray Diffraction (XRD), Energy Dispersive X-ray spectroscopy (EDX), and Fourier transform infrared spectroscopy (FT-IR). The size and morphology of the ZnO nanoparticles have been determined by field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) techniques. This is a simple and fast method for the preparation of ZnO nanoparticles with no need for expensive materials or complicated treatments.
1. Introduction
Zinc oxide (ZnO), which is an n-type direct band gap group II–VI semiconductor material, has received a great deal of attention for its use in various fields [1]. Due to its high optoelectronic efficiencies relative to the indirect band gap group IV crystals, its wide band gap (3.37 eV), high exciton binding energy (E60 meV), and high dielectric constant, ZnO is considered as an important material for variety of applications in the visible and near ultraviolet regions [2, 3]. Nanometer-size zinc oxide has many new exciting properties and wide technological applications such as photocatalysis, chemical remediation, photoinitiation of polymerization reactions, quantum dot devices, solar energy conversion, biochemical sensors, chemical electrode, cosmetics, and pigments [4–13]. Furthermore, ZnO nanoparticles also have good biocompatibility to human cells [14] and their antibacterial and antifungal activities have been already demonstrated [15, 16].
ZnO has exhibited various kinds of nanostructures including nanoneedles, nanobelts, nanoflowers, nanorods, nanobows, nanonails, nanoparticles, and nanowires [17]. Various methods have been used for the synthesis of these zinc oxide nanostructures, including hydrothermal, direct precipitation, thermal decomposition, chemical vapor deposition, sol-gel, spray pyrolysis, emulsion precipitation, and further more routes [18–26]. Despite the numerous reported synthetic methods for ZnO nanoparticles, most of them require high temperature, expensive substrates, tedious procedures, sophisticated equipments, and rigorous experimental conditions. Hence, it is necessary to find out a simple and low-cost method for the synthesis of ZnO nanostructures to tackle the problems. In view of this, we have designed a new facile combustion technique for fabrication of uniform ZnO nanoparticles using microwave heating and glycine as fuel. This is a simple, inexpensive, environmentally benign, template-free, and fast method and therefore can be considered as superior route to most of the reported ones.
2. Experimental
ZnO nanoparticles were prepared by mixing of Zn (NO3)2·6H2O and glycine in the molar ratio of 1 : 2, respectively. These two starting materials were thoroughly mixed and grounded and the obtained white liquid after grinding was transferred into a porcelain crucible. The mixture was then heated in a microwave oven for 2 minutes with 80% power. After evaporation, the crystallization water the mix ignited and released a great deal of foams yielding a fluffy grey white product in the container. Finally, the obtained fine solid was washed with deionized water several times and then with ethanol to remove unreacted materials. The solid ZnO nanoparticles were dried in an oven at 100°C for 3 hours.
3. Results and Discussion
X-ray diffraction (XRD) patterns of the sample were taken with a PW1840 Philips X-ray diffractometer at room temperature using Cu Kα radiation wavelength of λ = 1.542 . The peak position and intensity were obtained between 10 and 80° with a velocity of 0.02° per second. Figure 1 shows the powder X-ray diffraction pattern of the as-synthesized ZnO. The diffraction lines are consistent with the values reported in the database of ZnO (JCPDS card no. 0-3-0888) providing clear evidence for the formation of hexagonal wurtzite-type structure with space group of P63mc for the synthesized ZnO. The as-fabricated ZnO nanoparticles were of pure sample and no diffraction peaks from any other impurities were detected. All the diffraction peaks are rather sharp which indicates that the ZnO sample has high degree of crystallinity. Using amino acid glycine as fuel generates a great deal of heat during the combustion process and the produced heat is sufficient for the crystallization of nanoparticles Therefore, there is no need for further heat treatment which is usually required for the synthesis of this oxide. From the XRD data, the crystallite size () of the as-prepared ZnO particles was calculated to be 21 nm, using the Debye-Scherrer equation, In this equation, is the crystallite size (nm) of the phase under investigation, is the Scherer constant (0.9), λ is the wavelength of X-ray of Cu Kα = 0.1542 nm, β is the full width at half maximum (FWHM) of plane (101), and θ is the Bragg’s angle.
The morphology and size of the ZnO particles were examined by transmission electron microscopy (TEM) using a Philips CM10-HT 100 KV microscope. The FESEM images were obtained by Hitachi Japan S4160 field emission scanning electron microscope. The FESEM images of ZnO nanoparticles are shown in Figure 2. These images reveal the presence of voids and pores in the surface of ZnO sample. These pores are attributed to the release of a large amount of gases during the combustion process. The FESEM images show that the particles are somewhat aggregated which is due to the enormous heat generated during the combustion reaction.
Figure 3 shows TEM images of the prepared ZnO nanoparticles and as it is seen, the ZnO particles are nearly spherical in shape with a little aggregation. The ZnO nanoparticles prepared by utilizing this combustion method are rather well separated with an average size of approximately 20–25 nm which is in a very good agreement with the grain size calculated by the Debye-Scherrer formula.
The purity of the as-prepared nanoscale ZnO particles was further confirmed by energy-dispersive X-ray spectroscopy (EDS). The EDS spectrum of the ZnO nanoparticles is displayed in Figure 4. This spectrum indicates the presence of only Zn and O elements in 1 : 1 ratio in the analyzed ZnO sample.
The FT-IR spectrum of the as-fabricated zinc oxide nanoparticles is presented in Figure 5.
A strong peak at ν = 436 cm−1 is attributed to the stretching vibrations of Zn–O bonds [27]. The peak observed at about ν = 3450 cm−1 indicates the presence of –OH residue, probably due to the atmospheric moisture. This finding is in close agreement with the previously reported study [28]. According to the obtained data, it can be stated that we have successfully synthesized ZnO nanoparticles using a simple, inexpensive, and rapid microwave-assisted combustion procedure. The glycine which is used as a fuel might first form a complex precursor with zinc metal cation [29]. This formed glycine complex is then easily decomposed by microwave heating to produce ZnO nanocrystals in high yield and purity. The release of N2, NO2, and CO2 gases from the oxidation of nitrate ion by glycine is the cause of spongy ZnO product. Zinc oxide is one of the most extensively studied materials with potential applications in many different fields of technology and finding new facile method for the synthesis of this oxide is of a great importance. Compared to the other used combustion procedures, where the product contains some impurity [29], the as-prepared ZnO sample in our method is highly pure.
4. Conclusion
In summary, ZnO nanoparticles with mean size of 20 nm have been prepared by an easy and rapid microwave-assisted combustion method using glycine as an organic fuel and oxidizing agent. The as-prepared final ZnO nanoparticles were characterized by XRD, FESEM, TEM, EDS, and FT-IR techniques. This method provides a pure and high yield of ZnO sample with high crystallinity and well-dispersed nanoparticles. Furthermore, there was no need for complicated treatments or expensive precursors in this facile preparative method which makes it rather superior to other reported methods.
Acknowledgment
The authors are grateful for the financial support provided by the Research Council of Shahid Chamran University, Ahvaz, Iran.