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低温基板上に成長させた結晶氷のプロトン秩序と物性に迫る和周波発生振動分光
杉本, 敏樹
Permalink : http://hdl.handle.net/2115/77766
JaLCDOI : 10.14943/lowtemsci.78.61
KEYWORDS : プロトン秩序;界面水分子;低温氷;強誘電氷;和周波発生振動分光;Proton ordering;Interfacial water;Low-temperature ice;Ferroelectric ice;Sum-frequency generation spectroscopy
Abstract
固体表面に吸着した水分子凝集系においては，水分子の並進構造のみならず配向構造（プロトン配
置）によって化学的特性や物性が大きく変化する．したがって，プロトン配置は水分子凝集系の重要
な構造情報であるが，これをプローブすることは従来の表面科学的実験手法では困難であった．我々
は，プロトン配置に敏感になり得るヘテロダイン検出和周波発生振動分光法を低温Pt（111）基板上の
吸着水や氷の研究に世界に先駆けて応用することで，それらの系のプロトン配置を決定することに成
功した．さらに，氷と基板の界面において形成される異方的なプロトン秩序によって，バルク氷では
発現しない新奇な強誘電物性が氷膜全体に創発されることを見出した．
Interfacial water is ubiquitous in nature and plays crucial roles in a variety of disciplines. In such a symmetrybreaking system, not only adsorption geometry but also anisotropic molecular orientation (i. e., H-up/H-down configuration) is a key structural parameter that determines the unique physicochemical properties of interfacial water systems. However, orientation of water molecules in interfacial hydrogen bond networks has been extremely difficult to investigate with traditional experimental techniques, such as electron diffraction, grazing X-ray scattering, and even scanning probe microscopy because hydrogen has only a single electron and thus respond extremely weakly to the probes used in these techniques. Therefore, the determination of molecular orientation of interfacial water has been an experimental challenge. We have used recently developed heterodyne-detected sum frequency generation (SFG) spectroscopy in our pioneering investigation of the orientation and structure of water molecules on the metal surfaces of the Pt(111). Then, we succeeded in directly demonstrating that the adsorbed first layer water molecules prefer an H-down configuration. The H-down ordering in the first layer was significantly pinned by the Pt (111) substrate and was subsequently propagated to the overlayer during the growth of a multilayer ice film. Temperature dependence of the SFG spectra revealed that such an exotic proton ordering is thermodynamically stable and has an extremely high critical temperature of ～173 K, which is more than twice as large as that of ferroelectric bulk ice XI (Tc = ～72 K). It was demonstrated that anisotropy at the heterointerface is key for stimulating novel exotic proton ordering in many-body proton systems such as ice.
Interfacial water is ubiquitous in nature and plays crucial roles in a variety of disciplines. In such a symmetrybreaking system, not only adsorption geometry but also anisotropic molecular orientation (i. e., H-up/H-down configuration) is a key structural parameter that determines the unique physicochemical properties of interfacial water systems. However, orientation of water molecules in interfacial hydrogen bond networks has been extremely difficult to investigate with traditional experimental techniques, such as electron diffraction, grazing X-ray scattering, and even scanning probe microscopy because hydrogen has only a single electron and thus respond extremely weakly to the probes used in these techniques. Therefore, the determination of molecular orientation of interfacial water has been an experimental challenge. We have used recently developed heterodyne-detected sum frequency generation (SFG) spectroscopy in our pioneering investigation of the orientation and structure of water molecules on the metal surfaces of the Pt(111). Then, we succeeded in directly demonstrating that the adsorbed first layer water molecules prefer an H-down configuration. The H-down ordering in the first layer was significantly pinned by the Pt (111) substrate and was subsequently propagated to the overlayer during the growth of a multilayer ice film. Temperature dependence of the SFG spectra revealed that such an exotic proton ordering is thermodynamically stable and has an extremely high critical temperature of ～173 K, which is more than twice as large as that of ferroelectric bulk ice XI (Tc = ～72 K). It was demonstrated that anisotropy at the heterointerface is key for stimulating novel exotic proton ordering in many-body proton systems such as ice.
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