A Review of Zinc Oxide (ZnO) ‎Nanostructure Based Humidity Sensor

Numerous metal oxide nanomaterials, such as titanium dioxide (TiO2</sub>), tin oxide (SnO2</sub>), and zinc oxide (ZnO), are highly suitable for the fabrication of effective humidity sensors. Comparatively, ZnO is considered as metal oxide with the highest potential due to its unique properties, such as the enormous excitation binding energy of 60 meV, a direct wide bandgap (3.37eV), and the ability to be synthesized and grown at low temperatures. To further enhance the sensing performance of ZnO structure for humidity sensing, parameters such as the morphology and crystallinity of the ZnO structure can be optimized through control of synthesis conditions, such as precursor concentration, reaction time, temperature, and pH. Although various fabrication and characterization of ZnO nanostructures composited with other metal oxides have been published, there are insufficient investigations that highlight the performance of humidity sensors using ZnO alone. Therefore, this study provides a comprehensive analysis of the application of ZnO nanostructure in developing humidity sensors. The discussion in this review includes a summary of the recent development of humidity sensors and the parameters used to measure their sensing performances such as doping and compounding method. The study also highlighted the unique features of ZnO and the numerous methods used to synthesis ZnO, including sol-gel immersion, two-step solution, hydrothermal synthesis, and spin-coating process. In short, the intriguing development of ZnO-based humidity sensors would offer an alternative option to employ effective humidity-sensing devices in thin-film solar cells and ultraviolet (UV)-based applications.</em>