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

光催化水分解产生的氢气可以替代不可再生燃料，增加可持续能源的使用。人们对宽间隙氧化物半导体的开发越来越感兴趣，这种半导体具有利用阳光进行光催化反应的能力，例如从水或碳氢化合物中产生氢。为了实现这一目标，有必要为光催化过程开发具有理想的光催化性能和高稳定性的新材料。在本研究中，我们的目标是开发基于数据的rudlesden - popper （RP）型材料并研究其光催化应用。本课题的主要目的是在常规固相法的基础上，通过溶胶-凝胶法和水热法等湿化学途径合成LnAEInO4系列（Ln = lanthanides， Y; AE = Ca, Sr, Ba），以获得具有高表面积和合适形貌的相纯晶体化合物。在获得相纯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化作为一种方法，在氟离子进入/从宿主氧化物晶体框架中插入/提取时改变材料的带隙能量，并研究rp型氧化物氟化化学的基础科学。为了获得氧化物的氟化相和脱氟相，我们将主要采用chimie - douce拓扑化学反应路线，在保持In3+氧化态的情况下，将聚偏二氟乙烯（PVDF, (CH2CF2)n）与氧化物原料反应生成LnAEInO4-xF2x（0≤x≤2）。然后将尝试使用还原剂（如NaH、CaH2或正丁基锂（n-BuLi））选择性地对这些材料进行去氟化。这些反应在足够低的温度下进行，以保持初始粉末形态。根据所选择的反应条件，对氟化和还原性脱氟过程中发生的结构变化进行详细的研究。将研究所得的氧化物、氟氧化物和还原性去氟化氟氧化物用于太阳能收集。在这里，重点将放在确定它们光催化水分解产生氢的潜力上，并研究光催化性能的组成依赖变化，以详细了解潜在的结构-性能关系。此外，我们将针对电化学方法研究大块和薄膜氟氧化物的脱氟行为。将更详细地研究还原产物光学性质的诱导变化。到目前为止，这种阴离子中心化学及其改变/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.