氧化鈀奈米片薄膜對CO氣體感測特性研究

Abstract

本研究以氧化鈀奈米片薄膜對一氧化碳氣體進行感測性質研究。我們於室溫下以活性濺鍍法於SiO2/Si基材上沉積氧化鈀奈米片薄膜，經 SEM、XRD、XPS與電性量測分析來探討薄膜濺鍍時間與退火溫度對於其表面形貌、結晶性、化學組成以及導電率之影響，並於真空環境下於不同溫度及濃度條件進行氣體感測的實驗。實驗結果顯示氧化鈀奈米片於薄膜濺鍍時間十五分鐘後具有較完整之片狀結構分布，在200-400 oC退火後，由非晶變成結晶性佳之結構，其導電性也隨之上升。氧化鈀奈米片薄膜對CO氣體感測研究結果顯示，PdO薄膜在CO氣體分子吸附後，電阻值明顯上升，由於PdO為p-type半導體，此現象應該和CO吸附分子傳輸負電荷至PdO薄膜內，因而降低主載子電洞密度有關。PdO薄膜在操作溫度為200 oC時出現最佳的響應值，當通入10 torr一氧化碳響應時間甚至可低於10 sec。此現象可歸咎於奈米片厚度僅10-20nm，表面因為氣體分子吸附伴隨之電荷轉移會在PdO薄片內形成大範圍之空間電荷，同時因為具有高比表面積，薄膜可吸附大量的CO氣體分子，因而提升CO的感測靈敏度。此外，X光光電子儀(x-ray electron spectroscopy，XPS)分析顯示，於200 oC感測CO環境下，PdO薄膜表面被還原成Pd，當溫度大於200 oC其響應值逐漸下降，其可能原因和氧化鈀薄膜表面組成中金屬鈀比例逐漸提高，而導致薄膜整體導電性變佳有關。本研究結果初步顯示，此PdO奈米片薄膜對CO氣體感測具有相當的應用潛力。

This study prepared PdO nanoflake thin films for the carbon monoxide sensor application. The PdO nanoflake thin films was reactive-sputter deposited on the SiO2/Si substrate at the room temperature, followed by thermal anneal at temperatures >200oC. The PdO thin film was characterized by scanning electron microscopy (SEM), x-ray diffraction (XRD) and x-ray photoelectron spectroscopy (XPS). The PdO thin film is composed of interconnected bowed flakes, and well-crystallized after the thermal anneal. The change in the electrical conductivity of the PdO thin film upon CO adsorption was conducted under a vacuum condition. We have investigated the dependence of the CO sensing sensitivity of the PdO thin films on the operating temperature and the CO gas pressure. The CO sensing study shows that the electrical resistivity of the PdO thin film increases upon CO adsorption, indicating that the depletion region is formed in the nanoflake because PdO is a p-type semiconductor. This observation suggests that negative charges transfer from CO adsorbates to the PdO nanoflake, resulting in the reduction in the majority carrier density. The PdO thin films has the best CO sensing performance at 200oC with good reproducibility. Because of the nanometer-scaled thickness of the nanoflake, the PdO thin film has a short CO sensing response time (less than 10 s at 10 torr). When CO adsorption occurs on the nanoflake, a large space charge region can be created due to charge transfer between CO adsorbates and the PdO thin film. The nanoflake thin film can adsorb a large amount of CO molecules as a result of high surface to volume ratio, thereby enhance the electrical response of the thin film. XPS analysis shows that the PdO surface is reduced to metallic Pd at 200oC. When the CO sensing temperature is larger than 200oC, the sensing sensitivity gradually declines; this may be due to the formation of the metal palladium surface layer, leading to a better conductivity of the thin film. The preliminary result of the study demonstrates that the PdO nanoflake thin film is a potential for the CO sensing application.