Structural, morphological, optical and mechanical studies of annealed ZnO nano particles
Introduction
ZnO material is a promising material in the family of metal oxides. It has wide band gap 3.37eV, a large binding energy of 60 meV and high electron mobility of ~2000cm2/(V.s) at 80K with good transparency. Due to these properties ZnO material is a potential candidate in the fields of optoelectronic applications such as solar cells, wave guides, laser diodes, gas sensors, spintronics and in bio imaging [[1], [2], [3], [4], [5]]. It is a single semiconductor material which manifests itself as 1-D nano rods, nano plates and nano needles [[6], [7], [8], [9]]. Different methods have been used for the synthesis of ZnO nano powders namely organo metallic precursor method, micro emulsion synthesis, sol-gel process, PVD (Physical Vapour Deposition), Precipitation, chemical vapour deposition [10], chemical bath deposition, Solvothermal and hydro thermal methods [11,12]. In the present study, ZnO nano powders have been prepared from the chemical route (Sol-Gel method) in view of the following 1. Low processing temperature, 2. Short annealing times, 3. High purity of materials and 4. Good control of the size and shape of the particles at atomic level [13,14]. The influence of annealing temperatures on the synthesized samples play a vital role and it is reported in the literature that ZnO nanostructures prepared at room temperature have good stability at high temperatures too [15,16]. ZnO nano powders annealed at high temperatures bare good photo luminescence with enhanced crystallinity [17]. ZnO nanoparticles photoluminescence behaviour would get deteriorated upon annealing as per the literature survey in the perspective of its applications in the fields of optoelectronics and in bio imaging [18]. Further, high temperature annealing provides scope for Zn atom to accommodate itself in the presence of impurity [19,20]. Many device fabrication processes involve high temperature annealing (dopant activation, ohmic contact formation, implantation repairs), and hence the understanding the effect of temperature on the properties of the ZnO nano structures is crucial for the realization of ZnO based Nano devices [21,22]. It has been found that annealing can improve the field-emission (FE) characteristics of ZnO nano structures [23]. The stability at higher temperatures is a prerequisite for the application of ZnO nano structures as oxygen gas sensor, which usually operates at high temperatures [24].
The optical properties of ZnO nanostructures have been extensively studied in the past two decades. However, the mechanical properties of ZnO nanoparticles have not been investigated in spite of its potential applications in mechanical engineering. Li-Yu Lin et al. have studied the effect of annealing temperature on the tribological behaviour of ZnO films prepared by Sol-Gel method. They have reported that the wear resistance of the ZnO films improved when the annealing temperature was increased above 550 °C [25]. They have also noticed that films annealed at high temperatures had better mechanical and wear resistant properties [26]. Huan-Pu Chang et al. studied the influence of oxygen vacancies on the frictional properties of nano crystalline zinc oxide thin films in ambient conditions. The study of the influence of the defects on mechanical properties such as friction and wear resistance plays prominent role in Nano Electro Mechanical Systems (NEMS) [27,28]. Hence, understanding the role of various types of defects generated during the annealing process of ZnO nanoparticles on wear resistance and friction have to be addressed in detail for its use in NEMS.
In the present work, ZnO nanoparticles were prepared by sol-gel method by using PVA as chelating agent. These ZnO nanoparticles are annealed at various temperatures from 500 °C to 1000oCwith a step size of 100 °C to change the defect concentration in the material. The mechanical properties such as wear resistance and friction coefficient of these samples are measured. It is found that the wear resistance increased with increase in temperature and friction coefficient was also found to vary with temperature. The contribution of defects formed during the annealing process to wear resistance and friction are discussed in detail.
Section snippets
Materials and methods
The ZnO nanoparticles were obtained from Sol – Gel method. The materials used in the preparation of ZnO nanoparticles were obtained from HI-Media with 99% purity, and are utilized without any further purification. The deionised water was utilized as a solvent for the preparation of ZnO nanoparticles. The preparation procedure is schematically represented in Fig. 1. For the preparation of ZnO nanoparticles, analytical grade Zn(NO3)26H2O (Zinc Nitrate Hexa hydrate) was mixed with little amount of
Results and discussion
X-ray diffraction patterns of the ZnO nanoparticles are shown in Fig. 2(a). The prominent reflection planes (100) (002) (101) (102) (110) (103) (112) (201) (004) (202) (104) observed from the XRD confirmed the hexagonal wurtzite structure of ZnO. The intensity of these reflection peaks decreased as the temperature increased from 500 °C to 800 °C and there is slight enhancement in the intensity beyond 800 °C as shown in Fig. 2(b). Debye Scherrer formula has been used to estimate the size of the
Conclusion
Nano particles of ZnO have been synthesized from Sol-Gel method, a chemical route using PVA as chelating agent. The nanoparticles were annealed at different temperatures (500°C-100°C–1000 °C). XRD studies have unravelled the formation of hexagonal wurtzite phase in ZnO. TEM results on ZnO nanoparticles annealed at 900 °C, yielded an average particle size of 21 nm, the particles are observed to possess hexagonal geometry. A slight variation in lattice parameters and cell volume has been observed
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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