| Summary: | 博士 === 國立臺灣大學 === 化學工程學研究所 === 93 === 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.
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