氧化鈀奈米片薄膜對還原性氣體之低溫感測特性研究

Abstract

本研究以活性濺鍍法沉積氧化鈀(PdO)奈米片薄膜，探討PdO對一氧化碳(CO)、氫氣(H2)等其它還原性氣體的低溫感測響應行為。此奈米片結構比表面積大，呈現巨大的開放空間，可快速且大量的吸附氣體分子；奈米片的厚度極薄，當氣體分子於奈米片上進行吸脫附作用時，會形成較大比例的空間電荷區，因而提升感測靈敏度。 氣體感測實驗顯示，於不同感測溫度區間，PdO薄膜對於CO感測具有不同之電性變化與感測機制。感測溫度低於、等於100 oC時，薄膜導電率變化適用於氧離子吸附理論；感測溫度為150 oC時，因CO分子可還原PdO生成金屬Pd，而此Pd相又會被O2分子再度氧化，氧化還原交互作用使得薄膜電性變化有震盪現象；當感測溫度高於、等於200 oC時，除了氧離子吸附理論，氧空缺理論也開始適用於解釋薄膜對CO氣體的感測現象。 PdO薄膜於不同感測溫度區間，同樣對H2呈現不同的感測行為與感測機制。當感測溫度小於、等於100 oC時，薄膜導電率之變化主要與H2在PdO表面形成氫氧基吸附物(hydroxyl adspecies)有關；當感測溫度為150 oC時，PdO與H2的還原反應促進金屬Pd的成長，同時幫助氧的裂解吸附與後續的Pd氧化反應，因而氧化還原反應相互競爭使薄膜有複雜的感測響應行為；當感測溫度高於、等於200 oC時，薄膜的感測響應值隨著H2濃度增加而上升，其感測行為與Mars–van Krevelen機制相符。 我們於PdO奈米片薄膜上沉積鉑(Pt)奈米粒子，並於感測溫度250 oC以下，探討此Pt修飾之PdO薄膜對CO氣體的感測響應行為。根據X射線光電子能譜(XPS)與熱程控脫附實驗(TPD)，Pt奈米粒子會改變PdO的電子結構，進一步促進PdO的還原反應。因而在100至200 oC的溫度區間內，Pt-PdO薄膜的CO感測行為主要是受到PdO的氧化還原反應所影響。感測溫度低於、等於100 oC時，Pt-PdO薄膜的感測特性與PdO薄膜相似，因於此低溫條件下PdO的還原反應無法進行；而當感測溫度為100 oC時，Pt-PdO薄膜有明顯的回復電流延遲現象，其與Pt表面CO吸附物被氧氣置換有關；Pt-PdO薄膜在150 oC的CO感測行為與原始PdO薄膜有很大的差異，因此時PdO的還原反應與Pd的再氧化反應有劇烈的動態競爭；而當感測溫度高於、等於200 oC時，薄膜的感測行為可利用氧空缺理論來解釋。

In this thesis, we studied the gas sensing behavior of PdO thin films toward reducing gases, such as CO and H2 in the temperature range between 25 and 250 oC. The thin film was deposited on the SiO2 substrate by reactive sputter deposition, and had a flake-like nanostructured morphology, which had a large open surface area providing tremendous surface sites for gas adsorption. Moreover, the nanoflakes were so thin (< 20 nm) that the space charge region induced by gas adsorption occupied a large volume ratio of the thin film, resulting in a sensitive gas sensing response. In addition to the gas sensing of the bare PdO thin film, we also studied the effect of Pt decoration on the gas sensing response of the nanoflake thin film. The PdO thin film exhibits a complex gas sensing response behavior to CO and H2 over the studied temperature range. At temperatures at and below 100 oC, the CO gas sensing behavior of the thin film can be simply described by the oxygen ionosorption model. However, the PdO thin film can be reduced by CO at 150 oC forming Pd nanoislands, which are then reoxidized by oxygen in the gas mixture of CO/dry air. The alternating oxidation-reduction reaction causes oscillatory electrical response during the CO sensing process. At temperatures at and above 200 oC, besides the ionosorption model, the oxygen vacancy model is used to account for the CO sensing response of the PdO thin film. The PdO thin film demonstrates a sensing behavior toward H2 very different from toward CO as a result of its stronger reducing power. At temperatures at and below 100 oC, the H2 sensing response of the sensor is primarily governed by hydroxyl adspecies produced as a result of dissociative hydrogen adsorption on the PdO surface. At 150 oC, PdO reoxidation kinetically competes with PdO reduction, leading to a very complicated sensing characteristic. Vigorous PdO reduction by H2 drives the continuous growth of Pd nanoislands, facilitating dissociative oxygen adsorption and thus the subsequent Pd reoxidation in the gas mixture of H2/dry air. At temperatures at and above 200 oC, the PdO thin film exhibits a sensor signal monotonically increasing with the H2 concentration and the H2 sensing behavior is consistent with the Mars-van Krevelen redox mechanism. When the PdO nanoflake thin film is decorated with Pt nanoparticles, the Pt-PdO sensor demonstrates an enhanced CO sensing sensitivity. According to X-ray photoelectron spectroscopy and temperature programmed desorption, Pt nanoparticles modify the electronic structure of the PdO thin film, thus improving the kinetics of PdO reduction by CO. The CO sensing response of the Pt-PdO sensor is greatly influenced by PdO reduction and reoxidation in the temperature range between 100 and 200 oC. The CO sensing characteristics of the Pt-PdO sensor at 100 oC and below are similar to that of the bare PdO sensor because PdO reduction is negligible in the temperature range. However, the Pt-PdO sensor shows a distinct delay in the recovery profile at 100 oC as a result of the sluggish replacement of CO adspecies by oxygen on Pt nanoparticles. At 150 oC, the Pt-PdO sensor has a sensing behavior dramatically different from the bare PdO sensor because the Pt decoration significantly enhances the kinetics of PdO reduction and thus the growth of Pd nanoislands, on which dissociative oxygen adsorption takes place. The CO sensing behavior of the Pt-PdO sensor at temperatures above 200 oC can be understood on the basis of the oxygen vacancy model.