| Summary: | 碩士 === 國立臺灣大學 === 化學工程學研究所 === 92 === Based on the research results in the past years, photo-catalytic activity of Au/K2La2Ti3O10 for water decomposition under visible light irradiation was better than that of Ni/K2La2Ti3O10. In order to explore the reasons, two experiments were designed. It was found that Au itself can catalyze water decomposition under visible light irradiation. From its UV-Vis spectrum, it is likely that nano-size gold possesses the character of semiconductor. Owing to small band gap, e--h+ pairs can be excited to reduce and oxidize water into hydrogen and oxygen. Even so, the amount of hydrogen evolved was limited. Loaded Au on high energy gaps semiconductors, TiO2 or K2La2Ti3O10, can tremendously improve photo-catalytic activity in water decomposition under visible light irradiation. High energy gap semiconductors, TiO2 or K2La2Ti3O10, prevent the recombination of e--h+ pairs originated in nano-gold.
The catalytic activities of Ag/TiO2 and Ni/TiO2 were compared in this research. It was found that the activity of Ag/TiO2 was higher than that of Ni/TiO2 for photodecomposition of water. In visible light region, there was mainly surface plasmon resonance for Ag and primarily d-d orbital interband transition for Ni. Thereby, it suggests that the enhancement of photo-catalytic activity by surface plasmon resonance may be superior to that by d-d orbital interband transition. The excited conduction band electrons (SPR) and valence band electrons (d-d transition) possessed enough energy to reduce water into hydrogen. Nevertheless, there may be another reason for better activity of Ag/TiO2. It takes excess energy to transfer electrons from TiO2 to Ni due to the Schottky barrier between Ni and TiO2. Ag/TiO2 showed higher photo-catalytic activity as a result of Ohmic contact between Ag and TiO2.
The modeling of absorption spectra of Au, Ag, and Ni loaded on solid mediums were attempted in this research based on Mie Theory. It was found that the SPR and d-d transition band of Au, Ag, and Ni shifted to longer wavelength (red-shift) in higher dielectric constant medium. The shifts are consistent with the experimental results. However, the intrinsic absorption of medium could not be shown from the model. This is because the basic assumption of Mie Theory does not consider the absorption of the medium.
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