Optimization of T-ZnO Process for Gas and UV Sensors

In this study, a rapid and scalable technological approach was developed to grow various types of tetrapodal ZnO (T-ZnO) based on a modified flame transport synthesis. The morphological, compositional, structural, and sensing properties of the T-ZnO particles were investigated in detail using scanning electron microscopy (SEM) and transmission electron microscopy (TEM), which allowed observation of the influence of the arm morphology of T-ZnO microparticles on their physical and sensing properties to UV light and volatile organic compounds (VOCs). With increasing synthesis temperature from 920 to 1000 °C, a considerable decrease in tetrapod arm diameter (from 2-8 μm to 50-150 nm) toward a higher aspect ratio was observed. Further, structural analysis revealed monocrystalline c-axis-oriented growth of the tetrapod arms regardless of temperature. However, at higher temperatures of 990 and 1000 °C, a 10-15 nm thin amorphous layer of SiO_x was evidenced by energy-dispersive X-ray spectroscopy (EDS), covering the T-ZnO particles. Both the change in aspect ratio and the formation of the amorphous SiO_x layer affect the sensing properties, e.g., leading to an increased response to UV light. Further design optimizations are critical in order to obtain high-performance UV and volatile organic compound (VOC) sensor structures that work efficiently even at room temperature. The influence of the applied bias voltage and the relative humidity on the performance of the UV photodetectors was examined in detail. Gas sensing measurements demonstrated the possibility of detecting low concentrations of VOC vapors with a detection limit of ∼0.5 ppm at 20 °C. The UV and gas detection mechanisms correlated to the morphology of the samples are tentatively reported. The presented study is of high importance in understanding the role of morphology and aspect ratio of ZnO’s tetrapodal structures and surface modifications on its sensing performance for industrial and biomedical applications.