Post-deposition heat treatment effect on pH sensing behavior of chemical bath deposited nanostructured zinc oxide

Abstract

The post-deposition heat treatment process was studied to improve zinc oxide (ZnO) ‘s pH sensing performance as the sensing electrode (SE) of extended-gate field effect transistor (EGFET) pH sensor. The low-temperature chemical bath deposited ZnO nanostructured thin film was heated from 200 to 400 °C before being applied as the SE. The heat treatment enhanced the agglomeration of ZnO nanoparticles, and the formation of nanonails was seen at the highest temperature of 400 °C. It was proven by XRD results that the crystallinity of ZnO samples were improved when the ZnO SE was heated at 300 °C and formed different growth orientations at (100), (002) and (101) before turning to (101) growth orientation at 400 °C heat treatment. The heat treatment process also increased the ZnO particle size from 15.47 nm to 260 nm at 300 °C. Further increasing the heat treatment up to 400 °C decreased the particle size to 44.30 nm. The ZnO morphology was correlated with the pH sensor performance. The clustered structure of the untreated ZnO sample exhibited a good pH sensitivity of 52.4 mV/pH with a linearity of 0.93 and a hysteresis value of 46.71 mV. On the other hand, almost similar pH sensitivity was obtained from the sample treated at 300 °C having poly-structure morphology, which was 52.9 mV/pH, but with a significantly improved linearity of 0.99, and hysteresis values of 41.99 mV respectively. The 300 °C heat treatment also improved the stability of ZnO SE performance. The difference in the performance of these two samples can also be related to the crystalline quality of the film.