17:15 〜 18:30
[MIS17-P07] 溶液セルを用いた透過型電子顕微鏡による気泡の核生成のその場観察
キーワード：核生成、溶液TEM、透過型電子顕微鏡、水、放射性分解
Recently, the development of solution cells sandwiched between graphene or silicon plates has made it possible to overcome the high vacuum environment unless the spatial limitation of a TEM sample chamber. As a result, the "in-situ" observation of phenomena in solution using TEM is being actively conducted [e.g., Nielsen et al. Science 345 (2014) 1158]. We have been working on this method since its early days and have succeeded in directly observing nucleation processes. For instance, we found multistep nucleation in the crystallization process of lysozyme protein crystal, which crystallized via dense liquid [Yamazaki, Kimura et al. PNAS 114 (2017) 2154]. Since dehydration is a largest barrier for crystallization in an aqueous solution, it seems that lysozyme molecules agglomerate together with their hydrated layers at first and then gradually dehydrate to be a crystal. In addition, the crystallinity gradually becomes higher due to mobile defects and occasionally undergoes a phase transition to a more stable crystal [Yamazaki, Van Driessche, Kimura Soft Matter, 16 (2020) 1955]. In these in-situ TEM observations, the effects of the electron beam are largest concern and must be considered to discuss an observed process.
When observing an aqueous solution by TEM, water radiolysis due to electron beam irradiation is inevitable. Electrons generate protons, ions, and radicals in the aqueous solution. Then, solution environment varies and, therefore, it makes difficult to discuss crystal growth and conduct experiments that simulate environments such as living organisms. Furthermore, bubbles may be generated depending on the conditions and, in case, the experimental run cannot be continuing anymore. Therefore, estimation of the solution environment, such as temperature, pH, and concentrations of ion, hydrogen, and oxygen, has been attempted in response to electron beam intensity by solving many equations of possible chemical reactions [Schneider et al, J. Phys. Chem. C. 118 (2014) 22373; Ambrožič et al. Chem. Sci. 10 (2019) 8735]. Generally, electron beam irradiates only on an area of the window on a liquid cell, and there are abundant aqueous solutions around it that are not directly irradiated by the electron beam. Even if we predict the relationship between the amount of generated hydrogen and oxygen and their equilibrium concentrations, we cannot expect a bubble formation, because bubbles are generated through a nucleation after supersaturation. Therefore, observed conditions such as the magnification and the brightness (electron dose rate) of the field of view are adjusted based on experience. To overcome these difficulties, in this study, we conducted an experiment to observe the moments of nucleation of bubbles in situ while accurately measuring the electron dose rate using a Faraday cup. We will show the detail experimental results and imprecations on the bubble formation for future in-situ observation.
When observing an aqueous solution by TEM, water radiolysis due to electron beam irradiation is inevitable. Electrons generate protons, ions, and radicals in the aqueous solution. Then, solution environment varies and, therefore, it makes difficult to discuss crystal growth and conduct experiments that simulate environments such as living organisms. Furthermore, bubbles may be generated depending on the conditions and, in case, the experimental run cannot be continuing anymore. Therefore, estimation of the solution environment, such as temperature, pH, and concentrations of ion, hydrogen, and oxygen, has been attempted in response to electron beam intensity by solving many equations of possible chemical reactions [Schneider et al, J. Phys. Chem. C. 118 (2014) 22373; Ambrožič et al. Chem. Sci. 10 (2019) 8735]. Generally, electron beam irradiates only on an area of the window on a liquid cell, and there are abundant aqueous solutions around it that are not directly irradiated by the electron beam. Even if we predict the relationship between the amount of generated hydrogen and oxygen and their equilibrium concentrations, we cannot expect a bubble formation, because bubbles are generated through a nucleation after supersaturation. Therefore, observed conditions such as the magnification and the brightness (electron dose rate) of the field of view are adjusted based on experience. To overcome these difficulties, in this study, we conducted an experiment to observe the moments of nucleation of bubbles in situ while accurately measuring the electron dose rate using a Faraday cup. We will show the detail experimental results and imprecations on the bubble formation for future in-situ observation.