2:45 PM - 3:15 PM
[MIS17-05] Surface-Selective Vibrational Micro-Spectroscopy Unveils Physicochemical States and the History of Crystals
★Invited Papers
Keywords:vibrational spectroscopy, titanium dioxide, silica, carbonaceous chondrite, surface, water
Vibrational spectroscopies, such as infrared (IR) absorption and Raman scattering ones, provide detailed information on the structures, orientations, and environments of the molecules and crystals that we focus on. The merits of these spectroscopies are that they do not require vacuum system and are easy to combine with optical microscopes. These properties provide us an advantage for investigating buried water / material interfaces as well as inhomogeneous material surfaces including water molecules at optional points. Here I introduce some examples of our researches on the surface analyses of crystals utilizing vibrational spectroscopies with combining surface-selective techniques and optical microscopes.
The first example is detecting surface hydroxyl (OH) groups on titanium dioxide (TiO2) on single crystals. By combining polarization-modulation technique to external reflection IR spectroscopy (PM-ER-IR), we realized surface-selective measurements of OH groups buried under adsorbed water layers in atmospheric condition. We discriminated the surface OH groups from adsorbed water molecules by real-time cancellation of the signals derived from isotopically oriented water molecules. We discuss their binding site on TiO2 surfaces and the origin of the super-wettability emerged under UV light irradiation.1)
The second example is probing the nanoscopic wetting phenomena of water molecules adsorbed on silica (SiO2) surfaces by combining heterodyne detection technique with surface-selective vibrational sum frequency generation spectroscopy (HD-VSFG).2) The heterodyne detection allows us to extract “phase” information of the SFG signals and to obtain “true” absorption spectra with the molecular orientation information. With changing relative humidity (RH%) of the environments and comparing the spectra with those obtained by ATR-IR technique, we clarified inhomogeneous growth of the wetting site on the SiO2 surfaces, especially under 50 RH %. 2) We have also applied IR spectroscopic techniques to in-situ & real-time monitoring for the crystallization and the melting of the ice nanocrystals confined in reversed micelles, and the melt structures of triacylglycerol self-assemblies. 3,4)
The final one is polymorphism discrimination of microcrystals embedded on the surfaces of carbonaceous chondrite meteorites utilizing Raman microscopy. Carbon materials and water molecules in phyllosilicate such as serpentinite are abundantly embedded in carbonaceous chondrites. They are believed to be one of the important origins of carbon atoms and water molecules to the early Earth. By probing the physical and/or chemical states of carbon materials and discriminating the polymorphism of the minerals that bring water molecules, we discussed the origin and the history of various types of carbonaceous chondrites. However, abundant amorphous carbons on them generally emit strong fluorescence, hindering us from discussing slight changes of Raman shift. Now we are considering another type of Raman microscopy utilizing near-infrared (NIR) excitation and/or non-linear Raman scattering techniques to avoid strong backgrounds of the fluorescence and enhance the ability to discriminate slight changes in crystal structures that should carry important information on the origin of the organic carbons and water molecules at the initial stage of the Earth and the solar system.
Acknowledgements
I would like to express my sincere thanks to the research collaborators: Dr. Shu-hei Urashima, Dr. Toshinori Morisaku, Mr. Taku Uchida, and Mr. Aruto Kashima (Tokyo University of Science), Dr. Akira Yamaguchi and Dr. Naoya Imae (National Institute of Polar Research), and Dr. Yuki Araki (Ritsumeikan University).
References
1) K. Takahashi & H. Yui, J. Phys. Chem. C, 113, 20322 (2011).
2) S. Urashima, T. Uchida, & H. Yui, Phys. Chem. Chem. Phys., 22, 27031 (2020).
3) A. Suzuki & H. Yui, Langmuir, 30, 7274 (2014).
4) H. Yui, Y. Isozaki, & T. Morisaku, Anal. Sci., 33, 75 (2017).
The first example is detecting surface hydroxyl (OH) groups on titanium dioxide (TiO2) on single crystals. By combining polarization-modulation technique to external reflection IR spectroscopy (PM-ER-IR), we realized surface-selective measurements of OH groups buried under adsorbed water layers in atmospheric condition. We discriminated the surface OH groups from adsorbed water molecules by real-time cancellation of the signals derived from isotopically oriented water molecules. We discuss their binding site on TiO2 surfaces and the origin of the super-wettability emerged under UV light irradiation.1)
The second example is probing the nanoscopic wetting phenomena of water molecules adsorbed on silica (SiO2) surfaces by combining heterodyne detection technique with surface-selective vibrational sum frequency generation spectroscopy (HD-VSFG).2) The heterodyne detection allows us to extract “phase” information of the SFG signals and to obtain “true” absorption spectra with the molecular orientation information. With changing relative humidity (RH%) of the environments and comparing the spectra with those obtained by ATR-IR technique, we clarified inhomogeneous growth of the wetting site on the SiO2 surfaces, especially under 50 RH %. 2) We have also applied IR spectroscopic techniques to in-situ & real-time monitoring for the crystallization and the melting of the ice nanocrystals confined in reversed micelles, and the melt structures of triacylglycerol self-assemblies. 3,4)
The final one is polymorphism discrimination of microcrystals embedded on the surfaces of carbonaceous chondrite meteorites utilizing Raman microscopy. Carbon materials and water molecules in phyllosilicate such as serpentinite are abundantly embedded in carbonaceous chondrites. They are believed to be one of the important origins of carbon atoms and water molecules to the early Earth. By probing the physical and/or chemical states of carbon materials and discriminating the polymorphism of the minerals that bring water molecules, we discussed the origin and the history of various types of carbonaceous chondrites. However, abundant amorphous carbons on them generally emit strong fluorescence, hindering us from discussing slight changes of Raman shift. Now we are considering another type of Raman microscopy utilizing near-infrared (NIR) excitation and/or non-linear Raman scattering techniques to avoid strong backgrounds of the fluorescence and enhance the ability to discriminate slight changes in crystal structures that should carry important information on the origin of the organic carbons and water molecules at the initial stage of the Earth and the solar system.
Acknowledgements
I would like to express my sincere thanks to the research collaborators: Dr. Shu-hei Urashima, Dr. Toshinori Morisaku, Mr. Taku Uchida, and Mr. Aruto Kashima (Tokyo University of Science), Dr. Akira Yamaguchi and Dr. Naoya Imae (National Institute of Polar Research), and Dr. Yuki Araki (Ritsumeikan University).
References
1) K. Takahashi & H. Yui, J. Phys. Chem. C, 113, 20322 (2011).
2) S. Urashima, T. Uchida, & H. Yui, Phys. Chem. Chem. Phys., 22, 27031 (2020).
3) A. Suzuki & H. Yui, Langmuir, 30, 7274 (2014).
4) H. Yui, Y. Isozaki, & T. Morisaku, Anal. Sci., 33, 75 (2017).