氧化鈀修飾對二氧化錫於一氧化碳低溫感測行為之影響

Abstract

鈀金屬(Pd)常用以修飾二氧化錫(SnO2)來改善對氣體的感測效果，然而在高溫下，鈀會被氧化為氧化鈀(PdO)，並增進二氧化錫對氣體的感測效果，所以在本研究中以反應性濺鍍法沉積二氧化錫薄膜並以同樣的方法以氧化鈀修飾(decoration)二氧化錫薄膜。 二氧化錫的感測行為與氧離子吸附態、溫度有關，隨感測溫度升高表面吸附氧離子型態會由氧分子離子(O2-)轉變為過氧離子(O22-)，加快了一氧化碳的氧化反應，且在200oC以上二氧化錫的表面晶格氧離子也能參與一氧化碳的氧化反應，增加表面電子濃度的變化量，提升對一氧化碳感測的響應值。 經氧化鈀修飾後，因二氧化錫與氧化鈀之間電子轉移作用生成的PN接面，讓薄膜在室溫下的電阻值提升了約100倍，同時提升對一氧化碳的感測效果，而感測表現隨著溫度變化。在100oC以下以下，表面化學態在感測過程並無明顯變化，響應值的大小和一氧化碳在氧化鈀表面的吸附能力、數量有關 ，因一氧化碳分子吸附於氧化鈀表面對PN接面的影響，增進了對CO的感測效果。 在150oC以上，氣體感測除了氣體分子間的反應外，也包含了氧化鈀與一氧化碳之間的氧化還原反應，同時強烈的影響對一氧化碳的感測行為；由於氧化鈀能加快一氧化碳與二氧化錫表面晶格氧離子和吸附氧離子的反應速率，故在通入一氧化碳時，量測的電流值能迅速提升，但後續由於鈀引致的氧分子裂解吸附作用會使電流值逐漸下降。 除此之外，經氧化鈀修飾後，在150、200oC的回復時間也大幅縮減，原因可能與鈀金屬引致的spill over作用，讓氧離子更容易吸附於二氧化錫表面上並修補在感測過程生成的氧空缺。我們藉由在250oC下感測不同濃度的一氧化碳(CO)及X光光電子能譜儀的表面分析實驗，推論出氧化鈀修飾對一氧化碳感測效果的影響及氣體感測機制。

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.