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標題: Modelling and Simulation of Potential-pH Response of Titanium Oxide Electrode
作者: 余介文
Yu, George
關鍵字: Titanium Oxide;氧化鈦;pH;Hydrogen Diffusion;Sensor;pH;氫擴散;傳感器
出版社: 材料工程學研究所
本論文提出兩個動力模式來定量描述氧化物電極之pH感應,以不同的pH緩衝液(pH buffer)監測氧化鈦膜之pH電位感應以確定此動力模式的有效性。氧化鈦膜之氫離子吸附速率常數(adsorption rate constant)、去離速率常數(desorption rate constant)及整體氫離子濃度(bulk concentration)的數值是以穫得之pH電位感應曲線調合而得到。 本研究依據pH電位感應量測來測定氧化鈦膜之氫離子擴散係數,pH電位感應之時漂 (drift) 被認為是水結合層(hydrated layer)之氫離子擴散至氧化膜層所引起的。 此pH電位感應量測來測定氧化鈦膜之氫離子擴散係數的方法的特點是相對地簡易並且可避免傳統滲透方法(conventional permeation method)的複雜性及滲透膜內的氫陷井(hydrogen traps)的影響, 本方法所測得的氧化鈦膜之氫離子擴散係數值比傳統滲透方法所測得值約小兩個冪次(order)。 本研究也致力於氧化鈦膜電極之pH電位感應的機制探討,氧化膜之熱處理顯出可改善pH電位感應的敏感度(pH-response sensitivity)及穩定度(stability)是由於極化電阻Rp (resistance of polarization)及電荷轉移電阻Rct (resistance of charge transfer)之增大,此已為人熟知,當這些電阻被降低時,氧化膜厚度縱使增加也不能改善pH電位感應的敏感度及穩定度。本研究使用電化學交流阻抗頻譜(electrochemical impedance spectroscopy ,EIS)得到氧化鈦之極化電阻Rp及電荷轉移電阻Rct,跟據diffusion-site-binding模式、multiphase模式及實驗結果認定氧化鈦膜電極之pH電位感應的機制有電荷吸附(charge adsorption mechanism)、氧化還原平衡機制(redox equilibrium mechanism) 、氫離子擴散機制(proton diffusion mechanism) 、氧缺乏機制(oxygen deficit mechanism)及腐蝕溶解機制(corrosion dissolution mechanism)。由於表面氧化鈦被修改使得大部份這些機制會暫時性改變有效絕緣之電容(insulator capacitance),因此氧化鈦膜電極之pH電位感應強列地受到氧化組成所影響及鈦電極熱處理過程(thermal history)所關連。本研究發現經由600 oC熱處理10分鐘之氧化鈦-鈦電極有相對最佳的pH電位感應的敏感度及穩定度,若是經由700 oC以上熱處理則顯示嚴重的時漂。

Two kinetic models are presented which quantitatively describe the potential-pH response of titanium oxide electrode. The validity of the kinetic model is examined by monitoring the potential-pH response of the oxide film at different pH buffers. The pH response curves obtained were fitted to the adsorption model to obtain values of the proton adsorption rate constant, desorption rate constant and bulk concentration in the titanium oxide film. In this study, a method based on potential-pH response measurement was used to determine proton diffusion in titanium oxide films. The drift in the potential-pH response is believed to be due to the hydrated layer, which affects hydrogen ion diffusion onto the oxide film. The unique feature of the potential-pH response method is its relatively simple experimental procedure, which eliminates complications arising from surface related effects and / or presence of hydrogen traps in membrane such as those found in the conventional permeation method. Proton diffusion coefficient for titanium oxide films obtained from this method was about two order smaller than the hydrogen diffusion coefficient obtained from the conventional electrochemical permeation technique. This study also attempts to explain the mechanism behind the observed pH response of titanium oxide/titanium electrode. Heat-treated oxide films are known to exhibit improved pH-response sensitivity and stability due to increased Rp (resistance of polarization) and Rct (resistance of charge transfer). When these resistances are reduced, no such improvement is observed even when the oxide film thickness is increased. Using electrochemical impedance spectroscopy (EIS), values of Rp (resistance of polarization) and Rct (resistance of charge transfer) were obtained for the titanium oxide/titanium electrode. Based on the diffusion-site-binding, multiphase model and the experimental results, possible pH-response mechanisms of the titanium oxide thin films were identified such as the charge adsorption mechanism, redox equilibrium mechanism, proton diffusion mechanism, oxygen deficit mechanism and corrosion dissolution mechanism. Most of these mechanisms operate in terms of a temporal change in the effective insulator capacitance resulting from the TiOx surface modification. Hence pH response of the titanium oxide/titanium electrode is strongly influenced by the oxide composition. Correlation of the pH response with thermal history of the titanium electrode was established. The TiOx/Ti electrode annealed in air, at 600 oC for 10 min showed the best long-term stability and highest sensitivity, while the TiOx/Ti electrode annealed above 700 oC showed serious drifts.
Appears in Collections:材料科學與工程學系

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