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

• Applicant: Dr. Shama Perween, Ph.D.
• Subject Area: Solid State and Surface Chemistry, Material Synthesis
• Term: since 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 518952364

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.

• DFG Programme: Research Grants