Effects of Precursor Concentration on the Transparency of Hydrothermally Grown Zinc Oxide

The electronic gas sensor is an essential aspect of operational safety in the industry as well as in personal settings. Efforts in developing electronic gas sensors have been focused on improving the device’s sensitivity and selectivity. Nanostructured oxide semiconductors, including Zinc Oxide (ZnO), have been considered to be one of the most promising materials for gas detection. The strong surface interaction with the target gas combined with a high surface area per unit mass is among its merits. Hydrothermal growth using Zinc acetate dihydrate (ZAD) was suggested to be an economically viable approach to produce ZnO with minimum impurity. However, such reports are very limited. This work aims to grow and characterize nanostructured ZnO suitable for electronic gas sensor. Hydrothermal growths were performed by employing ZAD and Hexamethylenetetramine (HMTA) precursors for 3 hours at 90°C on sapphire (001) substrate. The effects of total solution concentration and precursor concentration ratio on the ZnO transparency were systematically investigated. UV-vis spectrophotometry results indicated that transmittance in the visible wavelength region (transparency) slightly decreased with an increased total concentration from 0.05 M to 0.1 M. This was attributed to the increase of supersaturation which drives nucleation and crystal growth as Scanning Electron Microscopy (SEM) images showed an increased density of ZnO microrod. At a total concentration of 0.1 M, ZAD: HMTA concentration ratios were set to 1:3, 1:1, and 3:1 and a transparency maxima of 87% occurred at a 1:1 precursor concentration ratio. Images obtained by SEM showed that ZnO microrods were grown towards $[001]$ on sapphire with a hexagonal cross-section, consistent to the ZnO crystal structure.