氧化鈀奈米片薄膜在低於250oC對丙酮氣體之感測機制研究

Abstract

丙酮(acetone)是揮發性有機物中相當重要的一種，常見於室內化學場所，而糖尿病患者呼出的丙酮氣體濃度較正常人略高，因此，丙酮也是人體呼氣檢測疾病的生物指標之一，藉由精確地檢測人體呼出的丙酮氣體濃度，可以診斷受試者是否患有糖尿病。 P-型氧化物半導體材料氧化鈀(PdO)對於還原性氣體有很好的感測效果。在本研究中，我們便以氧化鈀(PdO)來研究丙酮氣體在低於250oC的環境下的感測機制。發現在100oC以下時丙酮在氧化鈀薄膜上的反應性很差，但當溫度升到100oC和更高溫時，可以在電性量測的結果中發現非常獨特的電性反應，包括一開始的電流值快速下降、隨後的電流值緩緩上升、以及最後電流值緩慢的降低等三個主要部分。從這些帶有特徵的電性譜圖中，我們推論是丙酮分子在氧化鈀上的表面反應造成電性變化。所以，藉由X光光電子能譜儀(XPS)分析表面元素組成和化學態，我們得知丙酮在100oC以上會在氧化鈀表面形成帶有烷氧基(C-O)、羰基(C=O)、羧基(COO)的中間產物，也從漫反射傅立葉轉換紅外光譜儀(DRIFTS)和質譜儀(MS)的分析結果中觀察到有水(H2O)、二氧化碳(CO2)生成。這些吸附物和氣態產物的生成來自丙酮氣體在PdO上的表面反應，而這些表面反應的反應速率和丙酮的濃度以及感測溫度有關，並可能是特徵電性訊號的來源。因此，我們比較這些產物分析的結果和不同丙酮濃度、溫度條件下的特徵電性訊號來解釋丙酮在氧化鈀上的反應機制。

Acetone is a ubiquitous volatile organic compound (VOC) existing in indoor and outdoor environments and in human body. It is also an important breath marker for many diseases. Breath of diabetic patients generally contains a very high concentration of acetone compared with that of healthy people. Correct qualitative and quantitative detection of acetone exhaled from diabetic patients is useful for early diagnosis and design of effective therapy for the patients. PdO is a p-type oxide semiconductor with intriguing sensing properties toward reducing gases. In this work, we studied gas sensing reactions of PdO exposed to acetone at temperatures below 250oC. The PdO sensor responses poorly to acetone below 100oC. However, the PdO thin film shows distinct response features when exposed to acetone of low concentration at 100oC and above; it demonstrates three electrical response regions at 100oC and above, including a prompt conductivity drop upon acetone exposure, followed by a moderate increase and a subsequent slow decay. From the characteristic response feature, several surface processes are likely to take place on the PdO surface during the acetone sensing test. According to x-ray photoelectron spectroscopy (XPS), alkoxy, carbonyl and carboxyl adspecies are formed on the PdO surface after acetone exposure at 100 oC and above. H2O and CO2 are detected by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and mass spectroscopy (MS). The detection of the adsorbed and gaseous species is a result of concurrent occurrence of various surface reactions during the acetone exposure. The reaction rate of these surface processes depends on the acetone concentration and the sensing temperature, resulting in the characteristic sensing response profile for the PdO sensor. We have proposed surface reaction mechanisms to describe the characteristic sensing response of PdO to acetone.