Synthesis of ZnO Nanoparticles using environmentally friendly Zinc-Air System
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Date
2015-01
Authors
Malevu, T. D.
Journal Title
Journal ISSN
Volume Title
Publisher
University of the Free State
Abstract
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.
Description
Keywords
Dissertation (M.Sc. (Physics))--University of the Free State, 2015, Zinc oxide -- Synthesis, Nanoparticles, Nanotechnology, Storage batteries, Electric batteries