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铁电材料是指在一定温度范围内具有自发极化, 且极化方向能被外加电场改变的材料, 而水是一种普遍存在的极性溶剂. 由于极性作用, 铁电材料与水及水溶液的界面存在着复杂的相互作用. 理解这些物理过程以及机制对于理论研究和实际应用都具有重要意义. 本工作利用同步辐射衍射技术研究了(001)方向极化BaTiO3单晶的表面结构, 并且研究了不同pH值液体对表面结构的影响. 结果表明, BaTiO3单晶含有一个电子密度较小的表面层, 并且由于极性的作用, BaTiO3单晶表面吸附了2.6 nm的水层. 表面滴加纯水后, BaTiO3的表面层结构没有明显的改变. 低温原位掠入射X射线衍射实验表明表面存在冰, 进一步验证表面吸附水层的存在. pH = 1的盐酸溶液也对BaTiO3表面结构没有显著影响, 可能是由于酸性溶液能稳定原有的极化方向. 但pH = 13的NaOH溶液可以使表面层变厚, 可能由于碱性溶液可以使表面极化减弱, 从而改变表面退极化场以及表面层厚度.Ferroelectric material is a kind of material with spontaneous polarization, and water is a common polar solvent. Due to polarity, there are complex interactions at the interface between ferroelectric materials and water/aqueous solutions. Understanding these physical processes and mechanisms is of great significance for both theoretical research and practical applications. Herein, the surface structure of (001) orientated BaTiO3 with (001) direction polarization single crystal is studied by synchrotron radiation diffraction technology, and the effects of liquids with different pH values on surface structure of BaTiO3 single crystal also also investigated. The results show that BaTiO3 single crystal contains a surface layer with a low electron density, and due to the effect of polarity, a 2.6 nm-thick water layer is adsorbed on the surface of BaTiO3 single crystal. After adding deionized water on the surface, there is no significant change in the surface layer structure of BaTiO3. Low temperature in-situ grazing incidence X-ray diffraction experiments indicate the presence of ice on the surface, further confirming the existence of adsorbed water layers on the surface. A hydrochloric acid solution with pH = 1 has no significant effect on the surface structure of BaTiO3, either, which is possibly due to the ability of acidic solutions to stabilize the original polarization direction. However, an NaOH solution with a pH = 13 can thicken the surface layer, which possibly results from the weakening of surface polarization caused by alkaline solutions, thereby changing the surface depolarization field and surface layer thickness.
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Keywords:
- ferroelectric polarization /
- pH value /
- surface structure
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图 2 BTO单晶的PFM和SEM图(标尺: 400 nm)
Fig. 2. PFM and SEM images of BTO single crystal (Scale bar: 400 nm).
图 3 BTO单晶的XRR图及拟合结果, 黑色方块为实验结果, 红线为拟合结果, 插图为电子密度随深度变化曲线 (a)未做任何处理; (b)表面滴加pH = 1的盐酸; (c)滴加去离子水; (d)滴加pH = 13的NaOH溶液
Fig. 3. XRR patterns and fitting results of BTO single crystal, black square represents experimental data, red curve represents fitting results. Inserts: electron density profile: (a) Without any treatment; (b) hydrochloric acid (pH = 1) on the surface; (c) deionized water on the surface; (d) NaOH solution (pH = 13) on the surface.
图 4 BTO单晶面内晶格常数随入射角的变化
Fig. 4. In-plane lattice constants of BTO single crystal vary with incident angles.
图 5 BTO单晶应变随深度变化图 (a)未做任何处理; (b)表面滴加pH = 1的盐酸; (c)滴加去离子水; (d)滴加pH = 13的NaOH溶液
Fig. 5. Strain variation with depth of BTO single crystal: (a) Without any treatment; (b) hydrochloric acid (pH = 1) on the surface; (c) deionized water on the surface; (d) NaOH solution (pH = 13) on the surface.
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