Surface Conductometric Gas Sensor Based On ZnO Micro/Nanostructures-Controllable Synthesis And Gas Sensing Properties

As continuing to promote the process of industrialization in the global production, it really has improved people's living standard, while at the same time, it also damaged the survival environment in varying degrees. Among them, the emissions and leaks of industrial waste gas, car exhaust and many other gases made our living environment flooded with poisonous and harmful gases, threatening human health and production safety. Therefore, it is particularly necessary to real-time monitor the poisonous and harmful gases in environment. Born in 1960 s, semiconducting metal oxide gas sensor has attracted much attention by scholars and industry due to its high sensitivity, easy to prepare, low cost, and dominanted in the market. Based on this, its sensing performance has been gradually improved. However, recent years, as the harmful gas in environment has becomed diversified and complicated, as well as the improvement of people self-protection awareness, it is necessary to develop gas sensors with high performance. In this thesis, ZnO nano-sensitive material was selected as a carrier. Through controlling the morphology and structure of ZnO nanomaterials, as well as investigating the synergistic cooperation between ZnO and other semiconducting metal oxides, we attempted to construct the relationship between material properties, sensor performance and sending mechanism, sought the effective method to improve sensing performance, and deeply understood its internal sensing mechanism, in order to specifically carry out material design for gas sensing. The main contents and results are as follows:?1? Due to its high surface area, radial dimension compare to Debye length, and one-dimensional carrier transport properties, one dimensional nanostructure has broad application prospects in the field of gas sensor. Based on this, we selected ZnO nanowire arrays as the starting point for research. High-quality ZnO nanowire arrays with controllable degrees over size, orientation, uniformity and periodicity were successfully fabricated with focused ion beam and low-temperature hydrothermal method. The study took advantage of the etching and ion Implantation of FIB, resulting in surface amorphization, which induced the nucleation and growth of ZnO crystal in the growth hole. Research shows that the depth, size and period of the patterned holes have decisive impacts on the morphology of resulting arrays. Particularly, the physical constraints caused by deep hole make growth space insufficient, and the ZnO nanowires in one individual hole merge together. That is the key to the growth of high-quality ZnO nanowires array.?2? Through precisely controlling the concentration of the precursor and the related growth conditions, sphere, cauliflower and sisal-like hierarchical ZnO microstructures were synthesized using low-temperature hydrothermal method. For the sphere-like ZnO microstructure, it presents three unique structures radially from the center to the outside, based on this, a nucleation-growth mechanism was proposed based on the concentration of the precursor. The three microstructures were developed as gas sensors to detect H2 S. Results shows that the responses of the three sensors are higher than 3.8 to as low as 50 ppb H2 S, and can detect the gas concentration range from 50 ppb to 5000 ppb. In addition, the responses of the three sensors are quite different, and the sphere-like ZnO microstructure exhibits the highest sensor response. Through deeply investigating the model of the sensing layer, we hold that reducing the effective contact area between the particles helps to improve transducer function of sensing layer, thereby enhancing the overall performance of the device.?3? Since a single material possesses specific property limitations, to improve the sensing performance of a sensor still needs to use collaborative composite materials. As an attempt, we prepared ZnO-Cu O core-shell nanowire and ZnO/Cu O heterojunction nanowire using hydrothermal method, electrodeposition and thermal oxidization, and measured the sensing properties to the oxidizing gas NO₂. Results show that, compared with the pristine ZnO nanowire, the sensor response of ZnO-Cu O core-shell nanowire is relatively small, exhibiting the behavior of p-type semiconductor. This indicates that it is the Cu O shell dominant the sensor response. While the sensor response of ZnO/Cu O heterojunction nanowire is remarkable improved, and can detect NO₂ as low as 250 ppb. Further analysis found that, the directional transfer of the carrier in the pn heterojunction and the pn potential are the key factors to improve the sensing performance of ZnO/Cu O heterojunction nanowire.?4? Based on the above study, we further investigated the structure, morphology and material combination. The ZnO nanoparticle-loaded electrospun Sn O2 nanotube n–n heterostructures were successfully synthesized by electrospinning combined with facile thermal decomposition. The sensing properties of the pristine Sn O2 nanotubes and ZnO/Sn O2 heterostructures were investigated toward the representative oxidizing?NO₂? and reducing?H2, CO? gases. Results show that the response of ZnO/Sn O2 heterostructures is 30.84 for 5 ppm NO₂, six times higher than that of Sn O2 nanotubes?5.09?. While for 50 ppm H2, the response of ZnO/Sn O2 heterostructures?1.44? becomes nearly one-fifth of the SnO2 nanotubes?6.89?. Results indicate that ZnO/Sn O2 heterostructures exhibit selectively enhanced and diminished sensing performances respectively for oxidizing and reducing gases, exhibiting high selectivity. Since the local heterojunction changes the contact potential barrier in the Sn O2 nanotubes, ZnO/Sn O2 heterostructures becomes sensitive to the oxidizing gas, showing enhanced selectivity.In the thesis, through exploring the effective approaches to improve the sensing properties of ZnO nanomaterials and its composite structures, we initially constructed the relationship between the material properties, gas sensing mechanism and sensor performance in term of the typical structures, which can provide a certain reference for the targeted design of gas sensing materials.