Study on the role of Iodine in synthesizing ZnO nanoparticles and effect of heat treatment to its properties

The number of studies on Zinc oxide (ZnO) have increased tremendously over the last ten years due to its widespread applicability as optoelectronic device, sensor, catalyst, transparent conductive oxide layer (TCO) in solar cells and piezoelectric device. The potential compliance of ZnO in those various field is due to its unique properties which exhibit direct wide band gap (~ 3.37 eV) semiconductor, large exciton binding energy (60 meV), possess good piezoelectric characteristics, chemical stability as well as biocompatibility. At nanometer scale, ZnO nanostructures such as nanorod, nanoneedle, nanobelt and nanoparticles have shown to exhibit different properties from the bulk. Likewise, they demonstrated significance enhancement of ZnO performance due to high surface to volume ratio characteristics. Among the various structures, ZnO nanoparticles offers great extent of applications ranging from chemical sensor, catalyst, anti-bacterial activities, transparent UV protection film, bio-imaging to bio medical application. However, the overall cost of producing these devices is relatively high compared to Si due to high cost of producing ZnO nanoparticles. Realizing of the advance functional device that ZnO nanoparticle is capable of, it is crucial to devise a method to synthesize ZnO nanoparticles for mass production. Up to now, a variety of techniques have been employed for the synthesis of ZnO nanoparticles via physical vapor deposition (PVD) and solution based method. PVD techniques, such as spray pyrolysis, sputter deposition, template assisted growth and chemical vapor deposition requires very complicated equipment, expensive raw materials, high temperature, high pressure and long deposition times which suggest that they are not suitable for large scale production at relatively low cost. In contrast to PVD, solution based techniques such as sol-gel, hydrothermal, sonochemical, microemulsion and precipitation are more feasible to be industrialized as they are low cost and produce high output volume. Comparing both physical and chemical techniques, the latter method is favored in this study. In solution technique, the selection of reactants is crucial as they are involved in the construction and formation of the particles. Most of the reagents employed for synthesizing ZnO nanoparticles are in zinc salt substance which will react with basic solution to provide a medium for nucleation and growth. Zinc salt commonly used are zinc acetate, zinc nitrate, zinc sulphate, and zinc chloride whereas for basic solution; sodium hydroxide, ammonium hydroxide, ammonium carbonate are the reagents that commonly employed. Only a few works that used elementary elements as the reactant in ZnO synthesis have been reported. For example, C. Wang and co-workers have synthesized ZnO nanoparticles using Zn and I
₂
which they have discovered that I
₂
plays some part in the nucleation of ZnO nanoparticles. However, the role of I
₂
in ZnO formation is not thoroughly analyzed and discussed [1]. In this work, we present the study of I
₂
role and improvement in ZnO nanoparticles synthesis by introducing Diethanolamine (DEA) as initiator and capping agent as well as O
₂
diffusion as catalyst during nanoparticles formation. Moreover, we employed post-heat treatment to removes the byproduct. ZnO nanoparticles properties (structural, morphology and optical) produced under different calcination temperatures at 700, 850 and 1150 °C were characterized using X-ray powder diffractometer (XRD), Scanning electron microscope (SEM) and room temperature Photoluminescence (PL) spectroscopy. Based on the SEM results, uniformly spherical particles are observed for all the samples prepared in the presence of I
₂
whereas microsphere with honey comb like structure were observed for samples without the presence of I
₂
. Samples heated at 850°C prepared in the I
₂
existence shows high intensity of blue-violet emission at 411 nm whereas sample without I
₂
exhibit two emission peak centered at 380 nm (UV emission) and 523nm (green emission). Possible reason of the red shifted peak for samples prepared with I
₂
with respect to UV may be attributed to zinc interstitial or zinc vacancy defects created via interaction with I
₂
during nanoparticles formation. Weight % calculated for samples synthesized with I
₂
is ~ 1.795% whereas without I
₂
is ~ 0.9% indicating enhancement in terms of output volume.