Hydrothermal Growth Temperature Dependence of Nanostructured Nickel Oxide Transparency

Oxide semiconductors are promising materials for various electronic devices ranging from the gas sensor to photodetector. Nickel oxide (NiO) is particularly interesting to be one of the few p-type oxide semiconductors. In NiO, p-type semiconductivity arises from the hole generation due to the existence of Nickel vacancy. NiO is visibly transparent owing to a wide bandgap of $\sim3.6$ eV. The capability to synthesize the p-type oxide semiconductor is detrimental in realizing important future applications such as transparent solar cells and transparent devices for wearable or skin electronics. However, very few reported on a cost-efficient synthesis of NiO or correlated the growth parameter to the optical transparency. This work presents a successful synthesis of nanostructured NiO by hydrothermal growth employing nickel (II) nitrate hexahydrate and hexamethylenetetramine as the precursors. Growth time was set to 3 hours and the transparent c-sapphire was used as the substrate. The effects of growth parameters, i.e. growth temperature and precursor ratio on the transparency were thoroughly evaluated. The correlation between the growth parameters to transparency was established and it was found that the transparency decreases with increasing temperature. Increased rate of reaction probably occurred at higher temperatures promoting more material deposition which led to reduced transparency. The growth mode and sample morphology were significantly affected by the precursor ratio. Scanning Electron Microscopy (SEM) images revealed that the deposited NiO morphology was in the form of a flaky nanostructure. The morphology changed into microrod structure with the use of a low Ni precursor concentration.