氧化鈀奈米片薄膜對CO與CH4低溫氣體感測特性之研究

Abstract

本研究以氧化鈀奈米片薄膜對一氧化碳與甲烷氣體進行低溫氣體感測性質之探討。我們利用反應性濺鍍製程於二氧化矽基板上沉積氧化鈀奈米片狀薄膜，使用分析儀器SEM、XRD、ESCA與電性量測系統來鑑定在不同濺鍍溫度與退火溫度成長條件下之氧化鈀薄膜的表面形貌、結晶性、化學組成及導電率，並在真空環境於不同溫度與濃度條件下進行CO與CH4氣體感測實驗。實驗結果顯示氧化鈀奈米片薄膜隨沉積溫度上升而晶粒變大，結晶性變好，經過400℃退火後，結晶性更佳，導電性大幅提升。根據氧化鈀奈米片狀薄膜對CO與CH4氣體感測結果顯示，氧化鈀薄膜感測CO與CH4時電阻值升高，此與CO與CH4氣體分子吸附於氧化鈀奈米薄片表面傳輸負電荷至薄膜內，又氧化鈀薄膜為p型半導體，使其主要載子電洞濃度減少有關。於室溫條件下進行氣體感測，CO與CH4分子無法脫附氧化鈀奈米薄片表面；於100℃進行感測，氣體分子可於氧化鈀表面脫附，即是在100℃低溫條件下氧化鈀已具感測之回復性。本研究之氧化鈀奈米片薄膜對於CO與CH4之感測響應時間相當短，響應時間可低達5秒，此歸因於奈米薄片厚度僅15 - 20 nm，氣體分子吸附後電荷轉移會在薄片內形成大範圍空間電荷區，同時因氧化鈀奈米片薄膜具有較高表面積-體積比，薄膜在CO與CH4吸附時可提供較多感測面積，因此在感測上有較靈敏之表現。根據本研究的分析顯示，氧化鈀奈米片薄膜對CO與CH4在低溫氣體感測具有良好的應用潛力。

This study prepared PdO nanoflake thin films for carbon monoxide and methane sensor applications at low temperatures under a vacuum condition. PdO nanoflake thin films were reactive-sputter deposited on the SiO2 substrate at room temperature and 200℃, followed by thermal annealing at 400℃. The morphology, crystallinity and composition of the PdO nanoflake thin films are identified by SEM, XRD and XPS. The crystallization and the grain size of the PdO nanoflake thin films were dependent on the deposition temperature. After thermal annealing at 400℃, the PdO thin film displays a better crystallinity and an improved electrical conductivity. We have investigated the sensing behavior of the PdO thin film to CO and CH4 gases under a vacuum condition at different sensing temperatures and gas pressures. The electrical resistivity of the PdO thin film increases upon CO and CH4 adsorption, suggesting that negative charges transfer from CO and CH4 adsorbates to PdO, which is a p-type semiconductor, resulting in the reduction in the majority carrier density. CO and CH4 molecules can hardly desorb from the surface of the PdO thin film at room temperature, but can easily desorb at 100℃. As a result, the PdO thin film exhibits full recovery at 100℃. The PdO thin film has a quick response to CO and CH4 with a response time less than 5 seconds. The quick response is ascribed to nanometer-scaled thickness of the flake structure. Because the PdO nanoflake thin film has a high surface to volume ratio, it can provide a large amount of adsorption sites for CO and CH4 adsorbate molecules. Moreover, the space charge region induced by CO or CH4 adsorption occupies most of the volume of the nanoflake, resulting in the enhancement in the sensing response of the PdO thin film to the reducing gases. According to this study, the PdO nanoflake thin film has a great potential for the use as the gas sensor material for CO and CH4 at low temperatures.