Investigating the emergence of electronic and atomic structure during the synthesis of transition-metal sulfides

The synthesis of nanomaterials in solution often involves multiple intermediate steps from the precursor to the final product, as outlined in the non-classical nucleation theory. The first step in a one-pot reaction typically begins with the dissolution of the precursors in a solvent, often promoted by the coordination of metal ions with specific ligands. Upon heating, a series of chemical reactions are triggered, leading to the nucleation and growth of the desired nanoparticles. Understanding all steps in the synthesis of nanomaterials is essential for controlling and designing their formation mechanism.Especially transition metal oxide and sulfide nanomaterials are used in a variety of applications. While mechanistic studies are often reported for the case of metal oxides, the key reaction pathways influencing the synthesis of many transition metal sulfides are still under debate. This thesis elucidates the formation of ZnS nanomaterials in solvothermal reactions through in situ X-ray spectroscopy and scattering techniques as a benchmark and shows preliminary characterization of FexSy nanomaterial synthesis.We utilized in situ valence-to-core X-ray emission spectroscopy (vtc-XES) combined with in situ high-energy-resolution fluorescence-detected X-ray absorption spectroscopy (HERFD-XAS) supported by density functional theory (DFT) calculations to resolve the metal-ligand coordination during the synthesis of sphalerite ZnS nanorods (s-ZnS). The study elucidates the dissolution of Zn(Ac)2 in oleylamine (OA), proposing [Zn(OA)4]2+ and [Zn(OA)6]2+ complexes, and detailing the ligand exchange towards oleylthioamide (SOA) upon the addition of elemental sulfur to the reaction mixture at room temperature. Throughout the synthesis of s-ZnS, in situ vtc-XES and HERFD-XAS analyses were employed to calculate an in situ HOMO/LUMO gap, showing the emergence of a band structure during the synthesis of s-ZnS. This enables the size prediction of the intermediate structure, the ZnS nanoparticle in the wurtzite phase (w-ZnS), based on their band gap. Complementary in situ powder X-ray diffraction (PXRD) and small angle X-ray scattering (SAXS) analysis confirm the identity of w-ZnS and s-ZnS and provide additional information about their size and morphology.We also propose and study the synthesis of ZnS nanoparticles using a two-phase water-toluene solvothermal reaction. The method allows the obtaining of colloidally stable monodispersed ZnS nanoparticles in the sphalerite phase. The diffusion of Zn2+ ions at the water-toluene interface was monitored by spatially resolved HERFD-XAS analysis, where the coordination of Zn2+ ions in each phase was resolved using DFT calculations. The dissolution of Zn(Ac)2 in water leads to the formation of a [Zn(H2O)6]2+ complex. The Zn2+ ion diffuses into the toluene phase already at room temperature, forming a [Zn(OA)6]2+ complex. In situ HERFD-XAS during the heating of the reaction solution up to 155°C reveals the formation of ZnS nanoparticles in the sphalerite phase, undergoing a [Zn(RS)4]2+ cluster and ZnS nuclei as intermediate structures. The findings were complemented by in situ PXRD and total X-ray scattering (TS) analysis, which verified the occurrence of these intermediates and assigned domain sizes to them. To validate the versatility of the method, we applied the complementary analysis to understand the formation of Fe3S4 nanomaterials in a solvothermal reaction. Since the reaction of Fe(acac)3 with elemental sulfur and oleylamine dissolved in toluene failed to achieve the required reaction temperature (180°C) to yield Fe3S4 nanomaterials, the solvent was exchanged to benzyl alcohol (BA) and the sulfur source to thioacetamide. In this reaction, no addition of oleylamine was required. BA is well known as a solvent that promotes the formation of ultrasmall and monodisperse nanoparticles.The preliminary analysis of in situ HERFD-XAS and vtc-XES data of the synthesis of Fe3S4 nanomaterials uncovers the formation of an intermediate structure above 50°C, which can be attributed to the coordination of Fe(acac)3 by BA under the formation of a [Fe(acac)2(BA)2] complex, as resolved by HERFD-XAS analysis and supported by DFT calculations. Furthermore, the formation of Fe3S4 nanocrystals undergoes an intermediate formation of FeS nanocrystals, as revealed by combined HERFD-XAS and FEFF analysis.The in situ approach of vtc-XES reveals changes in donor orbitals during the first 50 minutes of the reaction, confirming the emergence of the [Fe(acac)2(BA)2] complex. Through complementary in situ PXRD analyses, we identified the formation of FeS nanosheets and their conversion into Fe3S4 nanocrystals, which are approximately 22 nm in size.This thesis presents an analysis of the coordination chemistry and structural evolution during the synthesis of ZnS and FexSy nanomaterials. It aims to propose reaction mechanisms for transition metal sulfides under solvothermal conditions by combining in situ X-ray spectroscopy and scattering analysis with quantum-mechanical calculations. This complementary approach offers insights into the formation of transition metal sulfide molecular clusters, the emergence of transition metal sulfide nuclei, and the crystallization processes of transition metal sulfide nanoparticles, nanorods, and nanosheets.