化學浴沉積法製備孔洞性四氧化三鈷奈米片薄膜應用於一氧化碳氣體感測之特性研究

Abstract

四氧化三鈷為p型氧化物半導體，在各種功能性材料被廣泛應用，文獻中指出四氧化三鈷在低溫下有良好的一氧化碳催化效果，而被認為是一個合適的低溫一氧化碳氣體感測材料。在本研究中，我們利用0.15 M四水合醋酸鈷、10 wt%尿素為溶質以及去離子水為溶劑，以化學浴沉積法的方式在SiO2/Si基板上成長孔洞性Co3O4奈米片薄膜且研究其對一氧化碳氣體感測特性。孔洞性Co3O4奈米片薄膜擁有巨大表面積可供一氧化碳吸附，且孔洞性結構有助於氣體分子在Co3O4奈米顆粒間擴散。我們使用X光繞射分析儀( XRD )、掃描式電子顯微鏡( SEM )及穿透式電子顯微鏡( TEM )分析Co3O4薄膜微結構及形貌、化學分析電子能譜儀 (ESCA )分析試片表面的化學成分。首先，我們使用不同沉積時間經400oC大氣煅燒之Co3O4薄膜、在大氣氛圍下對一氧化碳進行感測實驗，感測響應值隨著沉積時間增加而上升，因為厚度增加，使薄膜能與一氧化碳反應的表面積變大，響應值提升；其次，使用沉積12小時經300oC-600oC大氣煅燒之Co3O4薄膜、在大氣氛圍下對一氧化碳進行感測，感測響應值隨著鍛燒溫度增加而下降，因為煅燒溫度增加使Co3O4奈米顆粒成長，薄膜比表面積下降，響應值降低。以沉積12小時、400oC煅燒之Co3O4薄膜調控工作溫度在大氣氛圍下進行一氧化碳氣體感測，感測響應值隨著一氧化碳濃度上升而提高，且在工作溫度150oC、一氧化碳濃度250 ppm有良好感測效果，有高響應值與最快響應速率，並有良好再使用性及高CO/H2選擇比。

Tricobalt tetraoxide is a p-type semiconductor oxide with many applications as a functional material. The catalytic ability of Co3O4 toward CO oxidation has been widely reported in the literature, and it is considered as a potential oxide semiconductor for CO gas sensing applications. In this study, we synthesized porous Co3O4 nanoflake thin films on the SiO2/Si substrate by chemical bath deposition using cobalt(II) acetate tetrahydrate and urea as the precursors and deionized water as the solvent, and studied their gas sensing response toward CO. The porous Co3O4 nanoflake thin films has a large surface area and the porous structure can effectively facilitate gas diffusion. The microstructure and morphology of the porous Co3O4 nanoflake thin films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and the surface chemical compositions was analyzed by electron spectroscopy for chemical analysis technique (ESCA). The CO sensing behavior of the Co3O4 thin films was studied under the atmosphere condition. We first performed the gas sensing experiment as a function of the deposition time of the Co3O4 thin film. We found that the sensing response increased with the deposition time, and ascribed the result to the increase in the surface area for CO adsorption. When the gas sensing experiment was conducted as a function of the calcination temperature of the Co3O4 thin film, we found that the sensing response decreased with increasing the calcination temperature as a result of the decrease in the surface to volume ratio of the thin film. For the Co3O4 thin film deposited for 12 hr and later treated by 400oC calcination, the gas sensing behavior of the sensor toward 250 ppm CO at 150oC exhibited a high sensing response, good reproducibility and a high CO/H2 selectivity.