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Persistent URL http://purl.org/net/epubs/work/67299502
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Record Id 67299502
Title Complexity in simple things: Defect-control and formation studies of manganese dioxide nanostructures
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Abstract With the climate crisis and the raising need for efficient technologies, it becomes necessary to develop novel materials whose structure and size can be tuned to respond to specific applications. Nanomaterials have attracted interest for the reason that their properties differ highly from their bulk counterpart due to a strong effect of high specific surface area and high surface-to-volume ratio, often coupled with local structure deviating from their bulk crystal counterpart. It becomes crucial to understand, from a fundamental point of view, the formation mechanism of (nano)materials and in return to be able to control their synthesis. Manganese oxide is an ideal study system in that aim. On one hand, manganese ions are stable simultaneously into three oxidation states, yielding a rich electrochemistry, interesting for catalysis and battery applications. On a second hand, manganese oxides show a rich polymorphism, i.e. they can crystallise into various crystal structures, which offers a large panel of properties depending on the sought application. These two characteristics of manganese oxides make this system complex to investigate despite the simple chemical composition. The combination of multiplicity of oxidation states and crystal structures leads to several structural defects that makes any analysis of manganese oxide structures challenging. In this thesis, I investigate the effect of synthesis on defect formation in manganese dioxide along with their characterisation, as well as the formation mechanism of different polymorphs. The development of new equipment for their characterisation as well as data analysis approaches are addressed as well. After a first chapter dedicated to theoretical background on the investigate system and characterisation techniques employed for this thesis, the feasibility of X-ray total scattering experiments using a MultiLayer Mirror as a monochromator at the DanMAX beamline at MAX IV is discussed in a second chapter. In this study, we show that such experiment can be performed with this instrumentation and Pair Distribution Function analysis can be obtained even on systems that don’t show any long-range order. Furthermore, this monochromator provides a higher flux than a regular Double Crystal Monochromator, allowing to yield high quality PDF within few tens of milliseconds on metal oxo-clusters. ix In the third chapter, I present the development of a methodology for the characterisation of intergrowth defects in a specific phase of manganese dioxide, γ-MnO2, using X-ray diffraction and PDF. I generate a large library of supercells comprising different amounts and distribution of intergrowth defects and modelling their corresponding XRD pattern and PDF. It is possible to determine laws to easily quantify defect distribution in a real sample based on the analysis of the PDF and XRD patterns generated. In a second time, this library of supercells is used to perform a “structure-mining”, where the supercells models are refined against experimental XRD and PDF data. The results of this approach are finally used as inputs for a Machine Learning-based algorithm and precise size distribution of intergrowth domains is extracted. Using this methodology, we can thus emphasise the structural reordering of γ-MnO2 nanoparticles throughout synthesis time. In the fourth chapter on this thesis, we investigate the effect of synthesis conditions on the formation of H-related defects in the same γ-MnO2 phase. Hydrogen atoms can insert in two different ways in the structure and depending on the local environment, properties of the materials are impacted differently. We thus investigate how synthesis temperature and time impact the nature and distribution of these defects by performing PDF combined with X-ray and Neutron Spectroscopy and Thermal analysis. Not only we reveal that H- related defects concentration and nature are impacted differently by these two synthesis parameters, but also we discuss the way they organise in the structure. A small section at the end of this chapter is dedicated to the control of defects in layered manganese dioxide by the insertion of alkali-earth and aluminium ions. Finally, in the last chapter of this thesis, we address the formation mechanism of different manganese dioxide polymorphs using in situ PDF and X-ray spectroscopy. While the first technique provides insights into the nature of intermediate species and growth mechanism of the different polymorphs, X-ray spectroscopy allows to retrieve information on the oxidation pathway of manganese ions. We thus reveal that the selectivity of manganese oxide polymorph originates from a selectivity of metastable clusters in solution prior to the formation of any solid-state entity. Furthermore, they undergo different growth mechanisms that are defects-driven. Finally, oxidation of Mn2+ ions in solution follows a two-step mechanism where the actual oxidiser introduced at the beginning of the reaction is responsible for a single electron transfer, while the further oxidation of Mn3+ into Mn4+ is occurring in the solid-state.
Organisation ISIS , ISIS-TOSCA , STFC
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Language English (EN)
Type Details URI(s) Local file(s) Year
Thesis PhD, University of Copenhagen, 2023. https://researchp…-formation-studies-/ 2023