化學修飾電極應用於感測及光電元件

Applications of Chemically Modified Electrodes on Sensing and Electro-Optical Devices

廖鎔榆(Jung-Yu Liao)

指導教授 ： 何國川

國立臺灣大學/工學院/化學工程學研究所/博士(2004年)

https://doi.org/10.6342/NTU.2004.10074

摘要

本論文主要目的是要探討化學修飾電極在氣體感測以及光電元件上的應用。
在氣體感測方面，由真空蒸鍍而成的TTF-TCNQ導電薄膜在室溫下用來感測氧氣及二氧化氮。在這個以TTF-TCNQ錯合物為主的氣體感測系統中，發現其不可逆之感測行為。根據文獻的報導及我們自己的分析實驗，這種不可逆的行為主要導因於薄膜與氣體分子間的反應。完全不可逆的行為發生在TTF-TCNQ薄膜接觸到二氧化氮氣體的時候。而當接觸氧氣時，卻只有部分不可逆的現象發生。而這種不可逆程度的差別主要是因為二氧化氮與氧氣不同的脫附及反應的競爭比例。根據上述競爭的概念我們提出了一個理論，並且推導一個通式足以表達三種可能發生在氣體感測系統的行為（完全可逆、完全不可逆及部分不可逆的情形）。當反應及吸附的範圍局限於薄膜表面時，氧氣的感測數據符合部分不可逆的理論。
另一方面，探討了TTF-TCNQ薄膜感測二氧化氮氣體的感測行為，包括了電導隨時間的變化以及感測靈敏度等。完全不可逆的感測特性可以由擴散控制的『失澤膜』理論來解釋。當滿足了特別的感測條件，如極短的接觸時間或低感測濃度範圍時，可以由電導變化率對二氧化氮氣體濃度的作圖當中，得到線性的關係，即為8.74×10-3 μS/sec．ppm的靈敏度。在室溫下，電導變化率對二氧化氮氣體濃度的線性關係一直維持到約30 ppm的感測上限。並且，比較文獻值與由真空蒸鍍薄膜所估算的導電率發現薄膜內分子的排列應是隨機而較無秩序的。
在光電的應用方面，探討的是染料敏化光電池以及光電致色變元件。我們研究以染料敏化硫化鋅/氧化鋅複合薄膜所組成的光電池。硫化鋅薄膜以熱真空蒸鍍及電化學沉積在FTO導電玻璃上；然後在其上塗佈一層氧化鋅奈米顆粒並燒結使其形成硫化鋅/氧化鋅複合層。可見光用來激發吸附在氧化鋅表面的釕染料，而硫化鋅層相信是用來提供跨越氧化鋅介面障礙的另外的電子傳遞通道。這種具有另外的電子傳遞通道的複合層提供了比單層的氧化鋅或硫化鋅較高的效率。此外，一種具電洞傳輸能力的p型半導體薄膜PEDOT也被嘗試取代具有再生I-離子的白金催化薄膜。儘管p型半導體的性質提高了開環電位(Voc)，但是I-離子在PEDOT表面的再生速率卻比在白金上差，導致電池的效率下降。
同時，我們展示了含有光電轉換及電致色變物質的光電致色變元件。由於PEDOT具有280 cm2/C的高著色效率足以降低光電流的需求，所以選擇為本系統的電致色變材料。細微的二氧化鈦顆粒以及吸附其上的釕染料分子，在照光時提供光電流還原PEDOT薄膜而成為著色狀態。紫外光及可見光分別是二氧化鈦顆粒以及釕染料分子的激發光源。甲醇選作為不可逆的二氧化鈦-紫外光系統的電洞消耗劑，而I-/I3-氧化還原對則用於可逆的釕染料-可見光光電變色元件系統。由於二氧化鈦傳導帶的電位不夠負，元件在波長630 nm的穿透度變化僅約20 %。當一個不可逆且光電轉換及電致色變物質複合的光電致色變元件受光初始照射時，由於氧化態的p型PEDOT薄膜也同時被光誘導造成開環電位(Voc)的提昇。如果說系統或元件可以得到一個穩定的開環電位值，那麼對於此光電致色變元件而言，這個值將可以被視為一個如電化學理論中所述的『階梯電位』來分析。

關鍵字

二氧化鈦 ； 著色效率 ； 硫化鋅 ； 開環電位 ； TTF-TCNQ ； 染料敏化光電池 ； 氧化鋅 ； 光電致色變元件 ； 二氧化氮氣體感測器 ； PEDOT

並列摘要

The main purpose of this dissertation is to discuss the applications on gas sensing and electro-optical devices of chemically modified electrodes.
In the gas sensing aspect, a vacuum deposited and conductive TTF-TCNQ thin film is used to detect O2 and NO2 gases at room temperature. The irreversible sensing behavior is investigated for TTF-TCNQ complex based gas sensing system. Reactions with gas molecules, which have been reported and supported by our own analytical data, are the main reason that causes the irreversibility. The totally irreversible behavior was found when TTF-TCNQ thin film contacts NO2 gas. As for O2 gas, however, only partially irreversible phenomenon is monitored. The different levels of irreversibility are caused by the different competition ratios of desorption rate and reaction rate for NO2 and O2 gases with TTF-TCNQ sensing thin film. The theory based on the competition concepts was proposed and a general expression for three possible behaviors (totally reversible, totally irreversible and partially reversible cases) in gas sensing system is obtained. The O2 sensing data match the partially irreversible theory when reaction and adsorption ranges are limited at the surface.
On the other hand, the gas sensing properties for the NO2 sensing using TTF-TCNQ thin film, including conductance transient and sensitivity, are discussed. The irreversible sensing characteristics can be explained by the tarnishing film theory which is mainly governed by diffusion. When the special sensing condition of short contact time or lower concentration range is satisfied, a linear relationship is obtained by plotting the rate of conductance change vs. NO2 concentration, from which a sensitivity value of 8.74×10-3 μS/sec．ppm is obtained. The rate of conductance change measured at room temperature is linear with respect to the NO2 gas concentration up to 30 ppm. Moreover, by comparing with the literature, the conductivity calculated for the vacuum evaporated TTF-TCNQ thin films suggests that the molecular structure of the complex is randomly oriented.
In the electro-optical application, the dye-sensitized solar cells (DSSCs) and photoelectrochromic devices (PECDs) are discussed. A photovoltaic cell containing a dye-sensitized ZnS/ZnO composite thin film was studied. ZnS was thermally evaporated or electrodeposited onto conducting fluorine-doped tin oxide (FTO) glass; then a nano-particulate ZnO layer was pasted and sintered to form a ZnS/ZnO composite layer. A visible light source was utilized to excite the Ru-dye which was adsorbed onto the surface of the ZnO. The ZnS layer is believed to provide an alternative pathway for electrons to move across ZnO barriers. This alternative pathway with the composite layer structure provides higher power efficiency than does a single layer of ZnO or ZnS. In addition, a hole-injecting, p-type poly(3,4-ethylenedioxythiophene) (PEDOT) thin film was also introduced to substitute for the Pt catalytic layer which helps with the rejuvenation of I- ions. Although the p-type semiconductor behavior increased the open circuit voltage (Voc), the power efficiency decreased because the I- rejuvenation rate was much slower on PEDOT than on Pt.
Meanwhile, a photoelectrochromic device, consisting of a photovoltaic species and an electrochromic material, is presented. PEDOT (poly(3,4-ethylenedioxythiophen)) was chosen to be the electrochromic material because of its high coloration efficiency of 280 cm2/C at 630 nm, a value high enough to lower the required photoinduced charge in applications. The TiO2 fine particles and Ru-dye molecules adsorbed on the particles are used to provide the photocurrent that reduces the PEDOT thin film to a darkened state. UV and visible light are the activation sources for TiO2 particles and Ru-dye molecules, respectively. Methanol was chosen as the hole scavenger for an irreversible TiO2-UV system, and the I-/I3- redox couple was chosen for a reversible Ru-dye visible PECD system. Owing to the insufficient negative potential of the conduction band for TiO2, the transmittance change of PECDs at 630 nm was only about 20 %. This photoinduced, oxidized p-type PEDOT is responsible for the rising open circuit voltage (Voc) when an irreversible, composite PECD is firstly illuminated. If a constant value of the Voc is obtained, it can be regarded and analyzed as a “potential step” in electrochemical modeling for a composite type PECD system.

並列關鍵字

Photoelectrochromic device ； Open circuit voltage ； Dye-sensitised solar cell ； ZnO ； NO2 gas sensor ； Coloration efficiency ； Poly(3,4-ethylenedioxythiophene) (PEDOT) ； TTF-TCNQ ； TiO2 ； ZnS

參考文獻

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