Photocatalytic Polymerization Of Tio <sub> 2 </ Sub> Photoelectric Conversion Film And Interface Structure Of The Nature Of The Preparation Of Polypyrrole / Nano

Photoelectric devices based on dye-sensitized nanoscale semiconductors have attracted much attention. It is necessary to search for new sensitizers as alternatives to the original ruthenium bipyridyl dyes because they are too expensive and hard to synthesize. Potential candidates for the sensitizers areπ-conjugated polymers since their photoelectric properties are suitable for various photoelectric applications including photovoltaic cells and light emitting diodes. Our interest in polypyrrole (PPy), one of the conjugated polymers, is due to its high thermal stability and strong absorption in the range from visible light to near infrared.The key technology for developing PPy-sensitized solar cell is how to prepare highly efficient PPy/TiO₂ nanocomposite film photo-anode. So far, two methods have been usually used in this filed, that is electrodeposition and oxidization polymerization. However, the as-prepared polymer simply deposits on the film without adequate binding. In electrodeposition, a positive voltage, which allows an electrochemical polymerization of pyrrole, is applied to the flat electrode at the backcontact of the TiO₂ film. Since the nanoparticles are electrical insulators at the applied potential, the polymerization only occurs in the film's mesopores where pyrrole can diffuse to reach the electrode below the TiO₂ film. As a result, parts of the TiO₂ nanoparticles are usually uncovered. In this case, the photosensitize effects are not optimized due to the incomplete coverage. In order to improve the photoelectric properties further, the ultimate way is to search for a brand-new method to fabricate the film. We believe that the photocatalytic polymerization initiated by nano-TiO₂ film provides a novel and easy-to-perform approach. So in this thesis, we try to synthesize PPy-TiO₂ nanocomposites films by photopolymerization. We studied the photoresponsive properties and interfacial structure of PPy-TiO₂ films. In order to illuminate the common rules of photopolymerization mechanism, we at last studied the interfacial structure of PMMA-TiO₂ nanocomposites. The main results are outlined as follows: 1. PPy photosensitized photoanode was successfully fabricated through a novel polymerization of pyrrole initiated by TiO₂ nanoparticles under UV irradiation. During preparation, conjugated PPy continuously grows on the surface of the TiO₂ film, resulting in fully covered film with the stably bound PPy layer.2. The results from XPS and Raman analysis consistently indicate that the as-prepared PPy layer is chemisorbed on TiO₂ film. The presence of -N⁺ reveals that the PPy is oxidized, and n electrons of the pyrrole rings have been donated to the empty 3d orbit of Ti⁴⁺ to form an acceptor-donor complex. This chemisorption interaction is as well named coordination.3. PPy layer is an efficient photo-sensitizer in the composite electrode.Moreover, this electrode has enhanced photocurrent density (144μA/cm²) ascompared to the electrodeposited electrodes (50-100μA/cm²). Meanwhilethe separation of electron-hole pairs is more efficient in the as-prepared composite film.4. According to the energy levels of PPy and TiO₂, the photoelectric conversion mechanism is summarized as follows:The LUMO level of as-prepared PPy is higher than that of electrodeposited PPy, providing a stronger driving force for the electron injection into the TiO₂.5. After studying the interfacial structure of PMMA-TiO₂ model composites, a bidentate complex involves both carboxyl and carbonyl groups of PMMA is approved. That is the -OH group interact with PMMA by hydrogen bonding. Meanwhileπelectrons of carbonyl oxygen and the sole electron pairs of carboxyl oxygen coordinate with the empty 3d orbits of Ti⁴⁺. The coordination interaction is consistent with the result of PPy-TiO₂ photoanode.