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Analysis of Carrier Dynamics in Photocatalytic Materials Using Ultrafast Spectroscopy
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| Title: | Analysis of Carrier Dynamics in Photocatalytic Materials Using Ultrafast Spectroscopy |
| Other Titles: | 超高速分光法をもちいた光触媒材料内のキャリアダイナミクスの解析 |
| Authors: | 市原, 文彦 Browse this author |
| Issue Date: | 25-Sep-2020 |
| Publisher: | Hokkaido University |
| Abstract: | Photocatalytic hydrogen evolution and carbon dioxide reduction using photocatalysts are expected as promising technologies for the realization of a sustainable society. However, high efficiency and stability under sunlight is required to fulfill the industrial demand. There are various approaches to improve photocatalytic activity, such as element doping, surface structure modification, implanting defect sites on the surface, using a photosensitizer, and loading of a cocatalyst as an active site. Since the mechanism of activity improvement has not been elucidated, it is not easy to obtain a guideline for developing new materials. Photocatalytic reaction is driven by photo-generated electron hole pairs. Thus, understanding the reaction mechanism and the carrier dynamics right after light irradiation in photocatalytic materials by using ultra-fast spectroscopy is highly desired. In this study, transient absorption spectroscopy was applied to evaluate the effect of dopants in oxide based photocatalyst and the interaction between cocatalysts and inorganic, and organic photocatalytic materials. This dissertation is composed of 6 chapters, including introduction and conclusions.
In Chapter 2, the configuration of the transient absorption spectroscopy used in this study was described, and the conventional fitting method of the transient decay curve and the fitting method adopted in this study were outlined.
Chapter 3 describes the doping effects over the carrier dynamics of La, Cr-doped SrTiO3, which is a visible light responsive photocatalyst. Element doping is a method often used for making a wide gap semiconductor visible light responsive, and is a method of forming an impurity level between bands and promoting light excitation using visible light. Indeed, doping can make photocatalyst visible light responsive and shows photocatalytic activity under visible light. However, the doped photocatalyst has lost the high photocatalytic activity under ultraviolet light, which originally possessed by wide-gap semiconductors. In order to clarify the mechanism of this problem, we selected La and Cr-doped SrTiO3 as model materials, and measured the effects of La and Cr dopants using transient absorption spectroscopy. Based on the results, it can be inferred that La doping would extend the lifetime of photogenerated charge carriers due to the hybridization of La 5d band with conduction band of SrTiO3 which could contribute to secure the excited electrons and thus increased total photocatalytic performance. Interestingly, after Cr doping, the transient feature of SrTiO3 has been changed under UV light irradiation, which displayed less reactivity and lifetime, could account for decreasing the photocatalytic activity. Accordingly, in order to design a superior photocatalyst for effective solar energy harvesting, one should strive for enhancing the electron reactivity as well as retaining the excitation state of SrTiO3.
Chapter 4 describes the carrier dynamics in Pt/SrTiO3. Pt is a typical cocatalyst indispensable for improving photocatalytic hydrogen evolution activity. A cocatalyst represented by a noble metal element has a function of greatly reducing the overpotencial of hydrogen generation and a function as an active site. Therefore novel metal cocatalysts are an indispensable element for designing a highly active photocatalyst. Furthermore, loading cocatalysts are expected to improve charge separation ability. However, theoretical calculations suggests that the broad density of states of Pt over rapping with conduction band and valence band of the wide gap semiconductors, and capturing electrons and holes from conduction band and valence band, respectively. In this study, to examine the loading effect of Pt cocatalysts, the transient absorption spectra were measured over different amount of Pt and Ni loaded SrTiO3 as a comparison. Focusing on the transient signal derived by hole in the SrTiO3, it is found that with the increase Pt cocatalyst loading amount, the decay constant which represent the speed of carrier consumption linearly increase. While in the case of Ni loading, the decay constant was almost constant. This linear relationship between Pt loading amount and decay constant suggests that Pt cocatalyst on SrTiO3 capture both photogenerated electron and hole. Therefore, for the cocatalyst, the localized density of state and an appropriate hydrogen absorption/desorption energy property are highly desired for efficient overall water splitting.
Chapter 5 proposes the charge carriers transfer process of conjugate polymer photocatalysts for carbon dioxide reduction reaction. Monomers and polymers with similar structures were synthesized using Suzuki-Miyaura coupling and Sonogashira coupling, and their photocatalytic activity and carrier dynamics were evaluated. By measuring time-resolved microwave conductivity, it is found that in the polymer synthesized by the Sonogashira coupling reaction, electrons generated by photoexcitation are delocalized due to the alkyl bond between molecules. Whereas in the polymer synthesized by the Suzuki-Miyaura coupling, a single bond is formed between the molecules, the photoexcited electrons can be localized with in benzene part of the conjugate polymer. Also, in ps-ns transient absorption spectroscopy, photoexcited electrons can easily attract strong interaction with cocatalyst through a C-C single bond in the material,thus electron can easily transfer to the cocatalyst. On the other hand, conjugate polymer composed by alkyl bonds, the electron transfer is difficult between conjugate polymer and cocatalyst. Furthermore, the sample prepared using the Suzuki-Miyaura coupling has evaluated by using the time-resolved PL spectrometry. The results shows the greatly shorten of the PL lifetime with the presence of the cocatalyst and carbon dioxide, indicating that electrons generated in the conjugated polymer are consumed in the carbon dioxide reduction reaction. As described above, the charge transfer process during the reaction was proposed by observing the interaction with the cocatalyst due to the bonding state in the conjugated polymer semiconductor. This result suggests that different polymer materials composed of C-C single bond may be useful for carbon dioxide reduction reaction.
Chapter 6 summarizes the above works and give an outlook for the future. As described above, the carrier dynamics in a typical oxide photocatalyst has been discussed for searching further active photocatalysts. In the doping method, by observing charge carriers dynamics, the advantage and disadvantage of doping an element into the semiconductor has been clarified. In the cocatalyst loading, by tracing the trapping process of holes into the Pt cocatalyst, it is found that there is the trade-off relationship between active site of the reaction and recombination center. In addition, the mechanism of the carbon dioxide reduction reaction using a conjugate polymer has evaluated. The electrons transfer process were tracked by various types of ultra-fast spectroscopy to obtain guidelines for material design based on the state of intermolecular bonding. |
| Conffering University: | 北海道大学 |
| Degree Report Number: | 甲第14253号 |
| Degree Level: | 博士 |
| Degree Discipline: | 理学 |
| Examination Committee Members: | (主査) 教授 村越 敬, 教授 佐田 和己, 教授 忠永 清治, 客員教授 葉 金花, 客員教授 白幡 直人 |
| Degree Affiliation: | 総合化学院（総合化学専攻） |
| Type: | theses (doctoral) |
| URI: | http://hdl.handle.net/2115/90497 |
| Appears in Collections: | 課程博士 (Doctorate by way of Advanced Course) > 総合化学院(Graduate School of Chemical Sciences and Engineering)
学位論文 (Theses) > 博士 （理学）
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