Gas Sensing Behavior of Sputter-Deposited Zinc Oxide Thin Film toward Reducing Gases
| Autor: | Lin, Chun-Yu, 林駿宇 |
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| Rok vydání: | 2019 |
| Druh dokumentu: | 學位論文 ; thesis |
| Popis: | 107 ZnO is an n-type metal oxide semiconductor (MOS). In this study, ZnO gas sensor was prepared by reactive sputtering on the silicon oxide substrate in order to study the sensing behavior of ZnO toward reducing gas at the temperatures below 250℃. The study was conducted in two parts. The first part is the study on the gas sensing mechanism of ZnO toward ethanol and acetone at different concentrations and temperatures. The second part is the study on the effect of humidity on the sensing behavior of ZnO toward carbon monoxide and hydrogen; the reason for studying the two simple gas molecules is because of their simple surface processes on the ZnO thin film. The ZnO sensor responses poorly to ethanol and acetone below 100℃. When the temperature is increased to 150℃ and higher, the sense current rises rapidly followed by an inverse. Ethanol and acetone molecules are dissociative adsorbed on the ZnO surface, producing hydrogen adatoms, which can react with preadsorbed oxygen ions, and thereby decreasing the resistance of the ZnO sensor by releasing electrons to the ZnO conduction band. The ZnO surface can also be reduced by the reducing gases, creating undercoordinated Zn lattice atoms and oxygen vacancies, further increasing the sensing current. Migration and nucleation of undercoordinated Zn atoms will form metallic Zn nanoclusters, on which oxygen molecules are dissociatively adsorbed. Positive charges are induced in the ZnO sensor by the electronegative oxygen adatoms, resulting in the characteristic sensing current inverse. ZnO has a similar gas sensing behavior toward ethanol and acetone, but the current inversion temperature and the sensor response are different because the two gases have different dehydrogenation activity leading to a difference in the electrical response due to different surface kinetics. The gas sensing sensitivity of the ZnO sensor decreases significantly with increasing the relative humidity (RH) since water molecules compete with target gas adspecies for surface adsorption sites. For gas sensing at 250℃ with a gas concentration of 15 ppm,the RH that reaches the detection limit is RH 80% and 60% for CO and H2, respectively. CO has a higher detection limit because more CO molecules can be adsorbed per unit surface area than H2. Only one single surface site is needed for CO adsorption. On the other hand, adsorption of H2 requires two adjacent surface sites. Under the same water molecular coverage, more CO molecules can be adsorbed on the ZnO surface so that the detection limit of carbon monoxide under high humidity is higher. When the ZnO sensor is decorated with Pt nanoparticles, the sensing response can be significantly increased; the sensor still have a high sensing response toward the two gases at a RH of 90%; this is particular true for H2 sensing. It is likely that Pt may modify the electronic structure of the ZnO substrate and thus improve the sensitivity. In addition, H2 and water molecules are susceptible to dissociatively adsorb on Pt nanoparticles, creating hydrogen adatoms, which can enhance reduction of ZnO around the Pt nanoparticles via spillover effect. Key Words: ZnO, ethanol, acetone, CO, hydrogen, metal oxide semiconductor, humidity, Pt decoration, spillover |
| Databáze: | Networked Digital Library of Theses & Dissertations |
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