自上世纪以来，随着围绕“固液界面”研究的生物芯片、微流控制系统、化学催化、燃料电池、能源转换与存储等领域的快速发展，研究和控制固液界面的结构、界面分子/离子吸附、电荷转移过程及伴随着的能量转换过程引起了人们的广泛关注。其中表面带电的固体可以在固液界面形成电场，对能量存储、化学催化、细胞增殖等都具有重要的促进作用，从而逐渐成为研究的热点。然而，由于通常固体表面的带电性很难实现人为的控制，因此调控、设计固液界面的结构一直以来都是一个巨大的挑战。铁电材料作为一种具有本征的非易失的、可控的极化和高密度的表面电荷，因此可以作为一个很好的调控固液界面结构的材料。本文的研究重点和核心集中在铁电氧化物薄膜与水溶液的界面结构、电荷转移及伴随着的能量转换过程的研究及调控。本文的主要研究成果总结如下： 1. 我们通过调控铁电表面与水溶液中H+/OH-离子浓度来控制铁电氧化物表面结构，从而实现了完全只利用水溶液的原位可逆铁电极化翻转。通过铁电翻转电滞回线、X射线光电子能谱测试发现水溶液处理后BiFeO3（BFO）表面会有新的化学键的生成，即金属-氧-氢键（metal-O-H, M-O-H），形成新的表面态。利用扫描透射电子显微镜和第一性原理计算我们在原子尺度上对BFO表面与水溶液的界面结构进行探测和分析，并对水溶液翻转铁电极化的机理进行了深入的探索和理解。当水溶液与BFO表面接触时，由于静电库伦作用溶液中的H+、OH-离子会分别与极化向上的BFO表面的O原子及极化向下的BFO表面的Fe原子成键,并导致Fe原子相对于铁氧八面体产生向上或向下的位移，从而翻转铁电极化。这种固液界面间的离子成键过程还会导致的表面电荷的增加，使表面的化学势能减小，静电能增加，进而使体相的铁电极化翻转，实现化学能与静电能的相互转化。利用水溶液翻转铁电极化我们还初步实现了利用铁电薄膜对表面带电的丝蛋白纳米纤维的原位“释控”过程。同时，结合光刻技术，我们还可以实现绿色、低成本的铁电极化的纳米尺度的“活字印刷”。这为后续铁电极化在芯片实验室、药物释控、纳米尺寸的混合、催化、信息存储等工业应用提供了可能。 2. 基于上述对铁电与液体界面结构的认识，发现水溶液大面积印刷极化的方法适用于更多铁电氧化物（BaTiO3、PbTiO3等）。进一步以BFO为模型体系，我们通过调控其极化方向实现了选择性的BFO表面光催化分解水产生氢气或氧气的控制。铁电极化向上的BFO表面由于受到内建电场的作用使光激发产生的电子迁移至表面，与水溶液中的氢离子结合，产生氢气；对于极化向下的BFO表面上光生空穴则会与水分子发生氧化反应，产生氧气。另外，我们发现通过在BFO表面形成新的表面态后（与OH-键合），BFO薄膜表面既可以实现产生氢气和氧气的全解水过程，而且产氢、产氧的效率得到明显提高。并且通过人为在BFO表面形成氢离子键合时，发现新的表面态同样会提高产生氢气的效率。利用扫描隧道显微镜探测样品表面的电子结构，发现新的表面的形成会使表面的导带和价带上移，增加与水分子氧化、还原的电势差，从而促进表面的电子、空穴与水溶液的离子进行迁移和交换，降低了固液界面的电子迁移电阻，这与原位的交流阻抗测试结果很好的符合。同时，结合大面积纳米铁电极化的“印刷”技术，通过构造“棋盘状”微纳畴结构（极化向上与向下的铁电畴间隔排列），进一步促进了光生载流子的分离，从而提升光催化效率。而且随着纳米铁电畴结构的尺寸的降低，光催化效率会显著提高，当畴结构的大小达到与BFO的扩散长度相当时，BFO表面的水分解效率将会进一步提升。这为我们实现高效的光催化反应提供了新的思路，为铁电材料在光催化、能源领域的应用提供了新的平台。

Over the past century, exploration and artificial control of molecular/ion absorption, charge transfer and associated structures across the solid–liquid interface is of particular interest for diverse communities including life science (biochip), micro-flow system, chemical catalyzing, efficient energy transition and storage, which has attracted widespread attention. Solid materials with charged surface can emit electric field across the interface of solid and liquid which will promote the efficiency of energy conversion, chemical catalyse and cell proliferation etc. However, due to the hard control of the charge of the surface, it is still challenging to design and control of the interfacial structure of the solid and liquid. Benefiting from the non-volatile/reversible electric polarization and high charge density at surfaces, ferroelectrics could be a distinguished model system to artificially construct and switch the solid–liquid interfacial states in liquid environment, which has been proposed to play critical roles on. In this paper, we focus on the study and control of the the interface structure of BFO ferroelectric oxide film and aqueous solution, charge transfer and associated energy conversion process. The main research results of this paper are summarized as follows: 1. Reversibly polarization switching can be achieved via controlling the consentration of H+/OH- and the energy conversion process at the interface of ferroelectrics and aqueous solution. According to the piezoresponse (electrostatic) force microscopy, quasi-static piezoresponse hysteresis loops and X-ray photoelectron spectroscopy, scanning transmission electron microscopy, we discover that the reversible polarization switching is ascribed to the sufficient formation of nchemical bonds (metal-O-H) at the surface of BiFeO3 (BFO) thin film. We discover that H+/OH- bond with O and Fe ions of the upward and downward polarized BFO surface induced a strong atomic displacement of Fe atom with respect to oxygen octahedral in its bulk, which leads to the polarization switching and has been evidenced by spectroscopic/microscopic measurements and understood by first-principles calculations. That significant formation of chemical bonds also induce the accumulation of negative (positive) charge on the BFO surface which increase the electrostatic energy and decrease the chemical energy of the BFO and lead to the bulk polarization reversal. Dynamic control of the charged silk protein nanofibers in aqueous environment can also be achieved assited by the aqueous solution induced polarization switching process. Furthermore, this water-induced ferroelectric switching allows us to construct large-scale type-printing of polarization using green energy and opens up new opportunities for lab-on a chip, drug delivery, sensing, high-efficient nanoscale catalysis, and data storage. 2. Based on the understanding of the interface of the ferroelectrics and liquid, we can achieve selective hydrogen or oxydren generation by tunning the ferroelectric polarization direction. Due to the internal electric field electrons will migrate at the BFO surface with upward polarization which induced hydrogen generation, and vice versa. Moreover, a high efficient water splitting process performed with the hydroxylation and hydrogenation of the BiFeO3 surface. The conduction and valence band of BiFeO3 surface bending away from the fermi level which enhanced the charge transfer from BiFeO3 to water after hydroxyl and hydrion modification as performed by scanning tunneling spectroscopy measurement and analyzed by firsrt principle calculation. Futhermore, a higher efficient of water splitting can be achieved with the assistance of the manufacture of periodical upward and downward polarization domain patterns, which can greatly enhance the separation of photogenerated carriers (especially when the domain size is compare to the diffusion length of BFO). That surface modification and nano domain enhanced carrier speration provide a new platform for ferroelectric based high efficient photocatalyst and energy conversion process.