Topochemical fluorination and defluorination as a method to develop novel photocatalysts with tailored optical properties

光催化分解水产生的氢气可以提供一种替代不可再生燃料的方法，以增加可持续能源的使用。人们对开发能够利用太阳光进行光催化反应的宽禁带氧化物半导体越来越感兴趣，例如从水或碳氢化合物中生产氢气。为了实现这一目标，有必要开发具有良好光催化性能和高稳定性的新材料用于光催化过程。在这项拟议的工作中，我们旨在开发基于InDATE的Ruddlesden-Popper(RP)型材料，并研究其光催化应用。本项目的主要目标是系统地研究LnAEInO4(Ln=稀土元素，Y；AE=钙，锶，钡)系列化合物，除了传统的固相法外，还通过溶胶-凝胶法和水热法等湿化学法合成，以获得具有高比表面积和合适形貌的相纯晶体化合物。在获得相纯的RP型氧化物后，我们的目标是利用母体氧化物的可逆氟化和脱氟作为一种方法来改变材料的带隙能，从而改变材料的带隙能量，并研究RP型氧化物的氟化化学基础。为了得到氧化物的氟化和脱氟相，我们将主要采用Chimie Douce拓扑化学反应路线，在保持In~(3+)氧化态的情况下，通过聚偏二氟乙烯(PVDF，(CH2CF2)n)与氧化物起始物质反应生成LnAEInO4-xF2x(0≤x≤2)。然后将尝试使用诸如NaH、CaH2或n-丁基锂(n-BuLi)的还原剂选择性地对这些材料进行脱氟。这些反应在足够低的温度下进行，以保持初始粉末的形态。将根据所选择的反应条件，详细研究氟化和还原脱氟过程中发生的结构变化。得到的氧化物、氧氟化物和还原脱氟的氧氟化物将被研究用于太阳能收集。在这里，重点将集中在确定它们光催化分解水产生氢气的潜力，并研究光催化性能随组成的变化，以创建对潜在的结构-性质-关系的详细了解。此外，我们将以电化学方法为目标来研究体和薄膜氧氟化物的脱氟行为。我们将更详细地研究还原产物的光学性质引起的变化。到目前为止，这种以阴离子为中心的化学及其改变/定制光学性质的可能性还没有得到系统的研究。

H2 generated by photocatalytic water splitting can provide an alternative to non-renewable fuels to increase sustainable energy use. There is a growing interest in the exploitation of wide gap oxide semiconductors with the ability to use sunlight to bring about photocatalytic reactions such as the production of hydrogen from water or hydrocarbons. To achieve this, it is necessary to develop new materials with desirable photocatalytic properties with high stability for the photocatalytic process. In this proposed work, we aim to develop indate-based Ruddlesden-Popper (RP) type materials and study their photocatalytic application. The principal objective of this project is to systematically study the series LnAEInO4 (Ln = lanthanides, Y; AE = Ca, Sr, Ba) synthesized via wet-chemical routes such as sol-gel and hydrothermal methods in addition to a conventional solid-state method in order to achieve phase pure crystalline compounds with high surface area and suitable morphologies. After achieving the phase pure RP-type oxide, we aim to employ reversible fluorination and defluorination of the parent oxide as a method to alter the bandgap energy of the materials upon insertion/extraction of fluoride ions into/from the host oxide crystal framework and to study the underlying science of the fluorination chemistry of the RP-type indates. In order to obtain fluorinated and defluorinated phases of oxides, we will mainly use chimie douce topochemical reaction routes, by reacting polyvinylidene difluoride (PVDF, (CH2CF2)n) with oxide starting materials to form LnAEInO4-xF2x (0 ≤ x ≤ 2) under maintenance of the In3+ oxidation state. These materials will then be attempted to be defluorinated selectively using reductants such as NaH, CaH2, or n-butyllithium (n-BuLi). These reactions are performed at temperatures sufficiently low to maintain the initial powder morphology. A detailed study of the structural changes occurring on fluorination and reductive defluorination will be performed in dependence of the reaction conditions chosen. The obtained oxides, oxyfluorides, and reductively defluorinated oxyfluorides will be investigated to be used for the solar energy harvesting. Here, the focus will be set on determining their potential for photocatalytic water splitting to generate hydrogen, and on studying composition dependent changes of the photocatalytic properties to create a detailed understanding of the underlying structure-property-relationships. Further, we will target the electrochemical method to study the defluorination behaviour of bulk and thin film oxyfluorides. Induced changes in optical properties for the reduced products will be studied in more detail. So far, this anion-centered chemistry and its potential for altered/tailored optical properties has not been investigated systematically.