Improved synthesis of ZnO nanowalls for gas sensing application: effects of chemical bath deposition time and annealing temperature

Zinc Oxide (ZnO) nanowalls (NWLs) are interesting nanostructures for sensing application. In order to push towards the realization of room-temperature operating sensors, a detailed investigation of the synthesis effect on the electrical and optical properties is needed. This work focuses on the low-cost synthesis of ZnO NWLs by means of chemical bath deposition (growth time of 5, 60, and 120 min) followed by annealing in inert ambient (temperature of 100, 200, and 300 degrees C). The as-grown NWLs show a typical intertwined network of vertical sheets whose features (thickness and height) stabilize after 60 min growth. During thermal annealing, NWLs are converted into ZnO. The electric transport across the ZnO NWL network radically changes after annealing. A higher resistivity was observed for longer deposition times and for higher annealing temperatures, at which the photoluminescence spectra resemble those obtained for ZnO material. A longer deposition time allows for a better transformation to ZnO during the annealing, thanks to the presence of ZnO seeds just after the growth. These findings can have a significant role in promoting the realization of room-temperature operating sensors based on ZnO NWLs.