Role of the Exposed Polar Facets in the Performance of Thermally and UV Activated ZnO Nanostructured Gas Sensors

Abstract

ZnO nanostructures with different morphologies (nanowires, nanodisks, and nanostars) were synthesized hydrothermally. Gas sensing properties of the as-grown nanostructures were investigated under thermal and UV activation. The performance of the ZnO nanodisk gas sensor was found to be superior to that of other nanostructures (Sg ∼ 3700% to 300 ppm ethanol and response time and recovery time of 8 and 13 s). The enhancement in sensitivity is attributed to the surface polarities of the different structures on the nanoscale. Furthermore, the selectivity of the gas sensors can be achieved by controlling the UV intensity used to activate these sensors. The highest sensitivity value for ethanol, isopropanol, acetone, and toluene are recorded at the optimal UV intensity of 1.6, 2.4, 3.2, and 4 mW/cm2, respectively. Finally, the UV activation mechanism for metal oxide gas sensors is compared with the thermal activation process. The UV activation of analytes based on solution processed ZnO structures pave the way for better quality gas sensors.

Ultraviolet Sensors Based on Two-Dimensional Zinc Oxide Structures

“…This band was related to the surface defects; the source of these defects may be attributed to the oxygen vacancies or zinc interstitials [57,60]. Furthermore, the strong dependence of the PL peak locations and intensities of ZnO nanostructures on the size and shape of the nanostructures was noted [35,61]. Several 2D ZnO structures, such as 2D plate [27], nanodisk [35], and nanosheet [59] ZnO structures, show low intensity of the near edge UV emission, which was attributed to the surface states being nonradioactive centres due to their large surface-to-volume ratio [60] and higher intensity in the visible broadband.…”

“…Furthermore, the strong dependence of the PL peak locations and intensities of ZnO nanostructures on the size and shape of the nanostructures was noted [35,61]. Several 2D ZnO structures, such as 2D plate [27], nanodisk [35], and nanosheet [59] ZnO structures, show low intensity of the near edge UV emission, which was attributed to the surface states being nonradioactive centres due to their large surface-to-volume ratio [60] and higher intensity in the visible broadband. This was attributed to the wide surface area of the 2D structures that make the density of surface defects higher compared to the other ZnO structures, such as nanorods and nanowires [57,60].…”

“…This UV emission had been attributed to different origins; it may be attributed to either excitonic or defect-related emission [59]. It was also attributed to the near band-edge emission of ZnO (3.37 eV) [35].…”

“…Several precursors were used to grow the ZnO nanostructure, such as zinc nitrate (Zn(NO 3 ) 2 ) with hexamethyltetramine (C 6 H 12 N 4 -[HMT]) [34]. Alenezi et al [35,36] reported using zinc sulphate (ZnSO 4 ) with HMT to grow 2D ZnO nanostructures. In both cases, the precursors are dissolved in deionised water and the substrate with the seed layer is immersed in the solution.…”

“…Using this method, Tian et al [34] succeeded in growing 2D ZnO with plate (nanoplates) shapes. On the other hand, Alenezi et al [35,36] used ZnSO 4 to synthesise ZnO by employing a hydrothermal process. The ZnO structures were grown in 2D nanodisks.…”