機能性金/二氧化錫核/衛星異質組裝結構之氣體感測應用

Abstract

二氧化錫奈米粒子(SnO2 NPs)及其組裝結構已被廣泛地設計合成，藉由對象氣體在粒子表面吸附(adsorption)或脫附(desorption)時所產生之劇烈電阻變化，可作為氣體感測之用。本研究主要討論機能性金/二氧化錫核/衛星異質組裝結構(Au-SnO2 CSHA)之化學法製備及以光學為基礎的氣體感測應用。透過異質結構參數之調控，來達到以光學訊號感測CO濃度之目的。結構製備主要分為兩個步驟：首先，利用直接還原法合成Au NPs (~13 nm)，其次透過熱分解合成SnO2 NPs (~2 nm)，並在Au NPs表面自組裝形成異質結構。由高解析穿透式顯微鏡(High resolution transmission electron microscopy, HRTEM)發現，組裝層之SnO2 NPs形貌及尺寸可利用溶液pH值及前驅物濃度來精確控制。合成之Au-SnO2 CSHA經過反覆地離心清洗，而後沈積於經氧電漿親水處理之石英基板上，進行紫外光-可見光吸收光譜分析及CO之光學感測實驗。 從大氣及室溫下之吸收光譜得知，隨著SnO2 NPs組裝層厚度的增加(從5 nm至9 nm)，Au-SnO2 CSHA的吸收波峰從520 nm最高紅位移至540 nm，此時Au-SnO2 CSHA之膠體溶液顏色由紅變紫。利用Mie理論搭配簡化之Au-SnO2核殼模型，進行吸收光譜之模擬計算發現，當緻密SnO2殼層厚度為7 nm，Au核球直徑為13 nm時，其波峰位於556 nm，相較於合成所得SnO2組裝層厚度7 nm CSHA波峰位置大幅紅位移。為了進一步趨近此理論與實驗厚度上之差異，故進一步利用等效介質理論(EMT)，假設SnO2組裝層為SnO2NPs及水所建構之非緻密殼層(SnO2體積佔有率為0.5時)，即可獲得與實驗值相同之波峰位置540 nm。當SnO2在殼層體積佔有率越低，波峰發生藍移(Blue shift)。此高依存性之關係，可以作為SnO2組裝層厚度及體積佔有率定量之用。 在Au-SnO2 CSHA的光學CO感測方面，將具7 nm SnO2組裝層之Au-SnO2 CSHA置於不同濃度的CO環境中(5 ppm~10000 ppm)，在不同溫度下進行臨場吸收光譜檢測時發現，隨著CO濃度的增加，全測試波長範圍(200 nm~1100 nm)之吸收度均微幅增加，不同吸收波段對感測的響應略有不同，其中最大的吸收變化量約出現在544 nm。利用此波長進行感測特性之數值分析發現，此CSHA在1000 ppm以下有較佳之感測靈敏度，最高吸收度變化率(Absorbance change ratio, ACR)為0.01885。值得注目的是，本研究成功地將傳統感測器所需操作溫度(>300 oC)降低至室溫。

SnO2 nanoparticles (NPs) and their assemblies have been widely designed, synthesized and applied for gas sensing by screening the drastic changes of electrical resistance while absorbing or desorbing targeting gases. In this work, we mainly focus on the chemical preparation and optic-based gas-sensing application of functional Au-SnO2 core-satellite heteroassemblies (Au-SnO2 CSHA). The fabrication of such heteroassemblies involving two steps, i.e. the fabrication of Au cores (~13 nm) by direct reduction method and the following formation and assembling of SnO2 NPs (~2 nm) on the suspended Au cores, is especially designed and precisely optimized to approach purposes of both CO sensing and optical observation. From high resolution transmission electron microscopy (HRTEM) images, it is found that the assembled thickness as well as the morphology of the tiny SnO2 NPs can be precisely controlled by varying pH value and concentration of Sn precursors. The prepared Au-SnO2 CSHA were repeatedly washed with de-ionized water and then deposited onto plasma-treated glass substrates for a series of UV-visible absorption and CO sensing characterizations at various CO concentrations and substrate temperatures. From the UV-visible spectra measured at atmosphere at room temperature, as the thickness of the SnO2 NPs assemblies increased from 5 nm up to 9 nm, the absorption peak of the prepared Au-SnO2 CSHA greatly red shifted from 520 nm to 540 nm, where the solution color changed from red to purple. Based on classical Mie theory and the assumption of a simplified Au-SnO2 core-shell spherical model, the calculated spectrum peak of a 13 nm Au core coated with a dense 7 nm SnO2 shell located at 556 nm which is greatly red-shifted compared with the experimental one with a porous 7 nm SnO2 shell at 540 nm. In order to approach the real structural parameters for obtaining a peak at 540 nm, we further introduced the effective medium theory (EMT) for a theoretical estimation of the Au-SnO2 CSHA with a porous 7 nm SnO2 and found that with the assumption of a mixture of 50% (volume ratio) SnO2 NPs and 50% water, the calculated peak is coincident with the experimental one. The Au-SnO2 CSHA (7 nm SnO2) was applied for the in-situ investigation of CO sensing properties under controlled CO concentration from 5 ppm to 10000 ppm at various temperatures from room temperature to 300 oC. It was found that with the increase of CO concentration, the absorption intensity accordingly decreased over the full measured range from 200 nm to 1100 nm and a maximum change was found at around 544 nm which is the characteristic absorption of Au NPs. By analyzing the CO concentration dependent variation of the Au absorption intensity, CSHA exhibit a significant CO sensing sensitivity below 1000 ppm where an excellent absorbance change ratio (ACR) as high as 0.01885 can be approached at room temperature.