标题: | 氧化钯修饰对二氧化锡于一氧化碳低温感测行为之影响 Effects of PdO Decoration on CO Sensing Behavior of SnO2 at Low Temperatures |
作者: | 李光中 Lee, Kuang-Chung 潘扶民 Pan, Fu-Ming 材料科学与工程学系所 |
关键字: | 氧化钯修饰;二氧化锡薄膜;一氧化碳感测器;氧化钯还原;金属钯;PdO decoration;SnO2 thin film;CO gas sensor;PdO reduction;metal Pd |
公开日期: | 2015 |
摘要: | 钯金属(Pd)常用以修饰二氧化锡(SnO2)来改善对气体的感测效果,然而在高温下,钯会被氧化为氧化钯(PdO),并增进二氧化锡对气体的感测效果,所以在本研究中以反应性溅镀法沉积二氧化锡薄膜并以同样的方法以氧化钯修饰(decoration)二氧化锡薄膜。 二氧化锡的感测行为与氧离子吸附态、温度有关,随感测温度升高表面吸附氧离子型态会由氧分子离子(O2-)转变为过氧离子(O22-),加快了一氧化碳的氧化反应,且在200oC以上二氧化锡的表面晶格氧离子也能参与一氧化碳的氧化反应,增加表面电子浓度的变化量,提升对一氧化碳感测的响应值。 经氧化钯修饰后,因二氧化锡与氧化钯之间电子转移作用生成的PN接面,让薄膜在室温下的电阻值提升了约100倍,同时提升对一氧化碳的感测效果,而感测表现随着温度变化。在100oC以下以下,表面化学态在感测过程并无明显变化,响应值的大小和一氧化碳在氧化钯表面的吸附能力、数量有关 ,因一氧化碳分子吸附于氧化钯表面对PN接面的影响,增进了对CO的感测效果。 在150oC以上,气体感测除了气体分子间的反应外,也包含了氧化钯与一氧化碳之间的氧化还原反应,同时强烈的影响对一氧化碳的感测行为;由于氧化钯能加快一氧化碳与二氧化锡表面晶格氧离子和吸附氧离子的反应速率,故在通入一氧化碳时,量测的电流值能迅速提升,但后续由于钯引致的氧分子裂解吸附作用会使电流值逐渐下降。 除此之外,经氧化钯修饰后,在150、200oC的回复时间也大幅缩减,原因可能与钯金属引致的spill over作用,让氧离子更容易吸附于二氧化锡表面上并修补在感测过程生成的氧空缺。我们藉由在250oC下感测不同浓度的一氧化碳(CO)及X光光电子能谱仪的表面分析实验,推论出氧化钯修饰对一氧化碳感测效果的影响及气体感测机制。 Pd is widely used as a sensitizer to improve the gas sensing performance of SnO2. Because of the high operation temperature, oxidized Pd is usually believed to be an active phase to improve the sensing performance. Therefore, understanding the sensitization mechanism of PdO is important to the development of Pd-sensitized SnO2 sensors of better sensing performance. In this thesis, we studied the CO gas sensing behavior of SnO2 thin films decorated with PdO nanoparticles at temperatures below 250oC. Both the SnO2 thin film and the PdO nanoparticles were prepared by reactive sputter deposition. After the PdO decoration, the conductivity of the SnO2 thin film decreases by 2 order of magnitude at room temperature because of the formation of the PN junction between the SnO2 and PdO, in which a depletion layer is developed in the SnO2 thin film. The PdO decoration greatly increases the sensing response of the SnO2 thin film toward CO. The sensor signal of the PdO-decorated sensor reaches about three times that of the bare SnO2 sensor at 150oC and above. The sensing behavior of the sensor at 100oC and below can be understood simply by the oxygen ionosorption model. PdO nanoparticles play a role modifying charge density distribution in the depletion zone in SnO2 by means of CO adsorption on the nanoparticles. At temperatures above 100oC, PdO reduction in the CO gas mixture greatly influences the sensing behavior of the sensor. Upon the CO exposure at 150oC and above, the sensor demonstrates a rapid and strong sensing response, followed by a gradual conductance decay. The strong response is due to improved reduction kinetics of superoxide ions (O2) by incoming CO molecules. The succeeding conductance decay is ascribed to dissociative oxygen adsorption on Pd nanoislands, which are produced via PdO reduction by CO at 150oC. The Pd nanoislands can be reoxidized when extensive dissociative oxygen adsorption occurs on Pd nanoparticles; the Pd reoxidation reduces induced negative charges in the SnO2 support and thus increases the conductance of the sensor. An equilibrium between the PdO reduction and Pd reoxidation will be eventually established during the sensing test, leading to a steady sensing current. The PdO decoration can also significantly reduce the recovery time of the SnO2 sensor. The faster recovery time of the PdO-decorated sensor is ascribed to the spillover effect. After the cutoff of the CO sources, oxygen adatoms may diffuse from residual Pd nanoislands to the SnO2 support, and quickly repair oxygen vacancies formed under the CO sensing condition. We have proposed the reaction mechanism for reducing gas sensing of the PdO-decorated SnO2 thin film in the temperature range between 50 and 250oC. |
URI: | http://140.113.39.130/cdrfb3/record/nctu/#GT070251507 http://hdl.handle.net/11536/126531 |
显示于类别: | Thesis |