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

通过光催化水分解产生的H2可以提供不可再生燃料的替代品，以增加可持续能源的使用。人们对开发具有利用太阳光进行光催化反应（例如从水或烃类产生氢）的能力的宽禁带氧化物半导体的兴趣越来越大。为了实现这一目标，有必要开发具有所需光催化性能的新材料，并具有光催化过程的高稳定性。在这项工作中，我们的目标是开发基于茚三酮的Ruddlesden-Popper（RP）型材料，并研究其光催化应用。本项目的主要目的是系统地研究系列LnAEInO 4（Ln =镧系元素，Y; AE = Ca，Sr，Ba）合成通过湿化学路线，如溶胶-凝胶和水热法除了传统的固态方法，以实现相纯晶体化合物具有高比表面积和合适的形态。在获得纯相RP型氧化物后，我们的目标是采用可逆的氧化物和脱氟的母体氧化物作为一种方法，以改变材料的带隙能量时，插入/提取的氟离子到/从主机氧化物晶体框架和研究的RP型indates的氧化物化学的基础科学。为了获得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，通过聚偏氟乙烯（PVDF，（CH 2CF 2）n）与氧化物起始原料反应，在保持In 3+氧化态的情况下生成LnAEInO 4-xF 2x（0 ≤ x ≤ 2）。然后将尝试使用还原剂如NaH、CaH 2或正丁基锂（n-BuLi）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形态。将根据所选择的反应条件对在脱乙酰基和还原脱乙酰基上发生的结构变化进行详细研究。所得氧化物、氟氧化物和还原脱氟氧化物将被研究用于太阳能收集。在这里，重点将放在确定它们的光催化水分解产生氢气的潜力，并研究光催化性能的组成依赖性变化，以创建一个详细的了解潜在的结构-性能-关系。此外，我们将针对电化学方法来研究散装和薄膜氟氧化物的脱氟行为。将更详细地研究还原产物的光学性质的诱导变化。到目前为止，这种以阴离子为中心的化学反应及其改变/定制光学性质的潜力尚未得到系统的研究。

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.