反應性濺鍍氧化鈀奈米片薄膜對CO常壓感測之研究

Abstract

本研究以反應性濺鍍法沉積氧化鈀奈米片薄膜(PdO)，探討氧化鈀奈米片薄膜(PdO)對一氧化碳(CO)氣體感測的電性響應。此奈米片薄膜比表面積大，呈現巨大的開放空間，可提供許多氣體分子之吸附位置；薄片的厚度極薄，當氣體分子於氧化鈀奈米片表面有吸脫附之作用時，表面載子空乏層厚度變化所占比例大，進而導電率變化會較為靈敏。 本氣體感測實驗顯示，氧化鈀薄膜(PdO)於不同感測溫度下，對於一氧化碳(CO)感測具有不同之電性變化與感測機制。感測溫度小於100oC時，薄膜電性變化主要來自於一氧化碳(CO)與氧化鈀薄膜(PdO)間的電荷交互作用；感測溫度150oC時，一氧化碳(CO)還原氧化鈀(PdO)生成鈀金屬(Pd)，而造成電性變化有震盪現象；感測溫度高於200oC時，一氧化碳(CO)會與預吸附之氧分子離子及氧化鈀(PdO)之氧原子反應並達動態平衡，使得電流值變化可達飽和。

This study deposited PdO nanoflake thin films on the SiO2/Si substrates by reactive sputter deposition, and investigated CO sensing properties of the PdO thin film. The thin film has a flake-like nanostructure with a large surface area and, therefore, the film can provide a large amount of adsorption sites for CO molecules. Moreover, because of the ultrathin thickness of the nanoflake, the space charge region induced by O2 or CO adsorption will occupy most volume of the nanoflake, resulting in a very sensitive change in the electrical resistance of the film. According to this study, the PdO thin film exhibits different electrical response behavior toward CO adsorption at different temperature ranges. At low temperatures (≤100oC), CO molecules react with (or replace) anionic oxygen molecules (O2-) and thus change electrical properties of the thin film. Because PdO is a p-type semiconductor, the depletion region in the nanoflake, which is formerly formed due to oxygen ionosorption, is widened upon CO adsorption, leading to the increase in the electrical resistivity of the PdO thin film. At 150oC, the CO molecules can react with surface oxygen atoms, thereby reducing PdO. However, in this temperature range, subsurface oxide can be formed on the reduced Pd surface, and the alternative oxidation-reduction reaction causes oscillatory electrical response during the CO sensing process. At higher sensing temperatures ≥200oC, the change in the sensing current of the PdO thin film upon CO exposure reaches a steady level, suggesting that reactions between adsorbed CO, pre-adsorbed oxygen ion and lattice oxygen are in dynamic equilibrium.