Abstract
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 minutes) followed by annealing in inert ambient (temperature of 100, 200, and 300 °C). The as-grown NWLs show a typical intertwined network of vertical sheets whose features (thickness and height) stabilize after 60 minutes 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.
Highlights
Zinc Oxide (ZnO) nanostructures, thanks to biocompatibility, non-toxicity, low cost, earth abundance, and chemical and thermal stability, have attracted a large industrial and academic interest for applications in gas sensing [1,2,3,4,5]
Thermal annealing chemical bath deposition are investigated as a function of the growth time and annealing can induce bothInthe nucleation the growth
Particular, weand assume that theofinitial successive seeds already exist, the thermal annealing can directly induce the growth of grains, evolution of the nanostructures affecting the electrical and optical properties
Summary
Zinc Oxide (ZnO) nanostructures, thanks to biocompatibility, non-toxicity, low cost, earth abundance, and chemical and thermal stability, have attracted a large industrial and academic interest for applications in gas sensing [1,2,3,4,5]. Chemosensors 2019, 7, 18 electron-hole pairs in ZnO, and the photo-induced carriers can interact with adsorbed gaseous species [7,9] This can allow for the realization of gas sensors UV-activated at room temperature using. Among the ZnO nanostructures, ZnO nanowalls (NWLs), i.e., 2D layers of few atomic planes grown perpendicularly to the substrate, are characterized by a huge surface-to-volume ratio and extremely thin wall thickness These nanostructures, thanks to their large specific area and numerous active sites for gas adsorption [8], can further improve the interaction with the gas target in order to obtain extremely efficient gas sensors [8,15]. This strategy can be very promising because it does not require alteration of the nanostructures by means of doping or surface functionalization, which can compromise the versatility of the sensor [7]
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