Synthesis of ZnO Nanoparticles using environmentally friendly Zinc-Air System
Zinc-air batteries have high specific energy, environmental compatibility and use low-cost materials. They have long been considered to be attractive as potential power sources for electronic applications. In the operation of these batteries, zinc oxide (ZnO) is formed as a byproduct. The present study investigates the form of the ZnO produced and suggests the potential of the method to synthesize ZnO nanostructures. Current methods of synthesizing ZnO nanostructures are expensive and complex, while requiring good vacuum and high temperatures. They are corrosive and evolve high toxic gasses. We present an on-going research that investigates the feasibility of producing ZnO nanostructures using an electrochemical, zinc-air cell system that is also a voltage generator. The electrolyte used in the study is readily available lye or sodium hydroxide (NaOH). The measured parameters are electrolyte concentration, zinc plate size, open-circuit cell voltage and discharge time into a calibrated load. The experimental has two aspects. The first aspect is the measurement of the output cell voltage versus electrolyte concentration and cell voltage output at constant ohmic load. The second aspect is the surface characterization of the zinc electrode substrate using SEM, EDS, UVVis and XRD techniques to investigate the formation of ZnO as a function of electrolyte concentration. Conclusions are then drawn by correlating the electrical performance of the cell in the first part versus the surface products formed in the second part. The potential application of the study is therefore twofold; firstly, we suggest the study as an alternative to large scale manufacture of ZnO and secondly, we suggest a way to optimize the power output of the cell as a function of the surface products formed. A layer of well-aligned zinc oxide (ZnO) nano-needles was synthesized on a zinc plate at room temperature using an environmentally friendly zinc-air cell system (ZACs). The zinc plate was the anode, and the air cathode was composed of steel wire. A porous voidpaper separated the electrodes and in the presence of a low concentration NaOH electrolyte also formed the medium of transferring electrons from the anode to the cathode. In this study, the open-circuit voltage, Voc, were monitored as a function of the electrolyte concentration. The electrolyte concentrations were varied from 0.4M to 2M. The measured values of Voc were approximately 1.2V for all the five different concentrations used. The effect of concentration on orientation and lengthwise growth of the synthesized ZnO nano-needles were determined through scanning-electron microscopy (SEM) and X-ray diffraction (XRD). It was found that the growth and orientation of the resulting ZnO nano-needles is highly dependent on the electrolyte concentration. The SEM images show good length properties of nano-needles with average sizes between 780 nm and 2200 nm. In addition, using XRD, UVVIS spectrometry (UV-vis) and Field Emission Scanning electron microscopy (FE-SEM) techniques the effects of varying the annealing temperature from 400◦C to 600◦C on the structure, morphology and optical properties of the synthesized ZnO nano-needles were also investigated. XRD measurements indicated that the synthesized ZnO nano-needles exhibit the hexagonal wurtzite structure with no impurities. In general, with the annealing the particle size increased and the nano-needles became more orientated with average height between 538.1 nm and 1195 nm. Ultraviolet-Visible (UV-Vis) spectroscopy showed a slight decrease in absorbance and the absorption edge shifted slightly to lower energy. The apparent increase in the band gap energy was from 3.29 to 3.30 eV over the temperature range, although it cannot be reliably attributed to adverse effects such as high-temperature defect formation since it is within the measurement uncertainty 2%. The nano-needles exhibit strong absorption peaks, in the wavelength range of 360nm to 380nm. The peaks appear to decrease with annealing temperature with increased crystallization strength. The absence of impurities after annealing was confirmed using Energy Dispersive Spectroscopy (EDS). Overall the ZAC method appears to be a feasible alternative method to produce ZnO nanostructures.