Iron oxide nanoparticle dispersions - solvation structures & interfacial phenomena using X-rays and neutrons

Thomä, Sabrina Louisa Jeanette; Zobel, Mirijam (Thesis advisor); Förster, Stephan (Thesis advisor)

Aachen : RWTH Aachen University (2023)
Dissertation / PhD Thesis

Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2023

Abstract

Iron oxide nanoparticles (IONPs) in dispersion are already widely applied in many different areas like in biomedicine or for waste water treatment. In these dispersions the nanoparticles interact with the dispersion medium and an interfacial region with different structure from bulk exists. In case of water, this region with enhanced ordering is called hydration shell and for other dispersion media solvation shell. An understanding of the order in this interfacial region is crucial, since it affects the whole panoply of solvent-nanoparticle interactions. Therefore, this doctoral thesis has been placed within the theme of the investigation of IONP-solvent interfaces. As starting point of this thesis, a high-energy X-ray pair distribution function (PDF) analysis study on aqueous IONP dispersions conducted during my master thesis and complemented with thermogravimetric analysis (TGA) during this thesis, has been published. In this study a double-difference PDF (dd-PDF) signal from the IONP dispersion minus bulk water minus the IONP powder was received. This signal, from the 5 g/L IONP dispersions freshly re-dispersed after purification, with three sharp peaks within 3 Å and oscillations at higher distances reaching out to 15 Å, has been interpreted as hydration shell in accordance with literature. The astonishing high height of the first peak, explained by hydrogen bonding at the surface, left open questions about the hydrogen bonding network at the surface and individual contributions to it. These questions have been tackled throughout this thesis with further techniques. The developed dd-PDF approach served as perfect platform to investigate IONPs in further solvents. It contributed to a study in which truncated iron oxide cubes were self-assembled to mesocrystals. Depending on the solvent polarity the resulting morphology and symmetry of the built superlattice was influenced. Distinctly different solvation shells for cylcohexane and tetrahdyrofurane (THF) were retrieved with the dd-PDF method. In agreement with analytical ultracentrifugation (AUC) analysis it has been claimed, that the more extended solvation shell in case of cyclohexane is able to modify the shape of the cubes towards spheres, which influences the superlattice morphology. Solvation shells, thus, can be a pivotal driving force during self-assembly. An optimization of the synthesis strategy used in the first study, to obtain aqueous IONP dispersions with concentrations up to 100 g/L was a pre-requisite for following neutron PDF and X-ray absorption spectroscopy (XAS) studies. Further, in this work with combined TGA and elemental analysis the surface composition of the IONPs was characterized, and the co-existence of surface water, residual solvent from synthesis and ligand molecules was proven and quantified. The highly concentrated IONP dispersions in (heavy) water were investigated with neutron PDF at two different facilities. Likewise as for the X-ray data the dd-PDF approach was successfully applied. Even though no pronounced hydration shell signal could be retrieved, hints towards an adsorbed water layer have been found. The modelling of nuclear and magnetic PDF of the IONP powder data revealed, that citrate as capping agent contributes to the PDF up to 20 Å. Already in nominally dry powders surface hydroxyls seem to modify the surface structure. Since no conclusive new insight into the adsorbed water layer has been gained with neutron PDF, a XAS study was conducted to quantify the surface hydroxyl in aqueous IONP dispersion. Surprisingly, no surface hydroxyl has been detected on the investigated IONPs dispersions. Therefore, the high-energy X-ray PDF study of aqueous IONP dispersions was repeated with the improved synthesis strategy and no dd-PDF signal has been observed anymore. The lack of hydration shells could be finally resolved. By reconsideration of the surface coverage with latest literature, it has been found, that at least citrate-capped IONPs are fully covered and therefore also no pronounced hydration shell is expected. Further, the dd-PDF signal observed for samples of the first study, was traced back to ethanol (EtOH) impurities from the purification. The arrangement of these EtOH molecules in dilute aqueous solution was investigated and interestingly structural motifs differing from bulk EtOH with EtOH molecules being closer together, have been elucidated. Thus, it is shown, that in the brilliant synchrotron X-ray beam even insight into the structural details of signatures of ethanol clusters in water next to 5g/L IONPs can be gained. Furthermore, this minor portion of EtOH has significant impact on beam-induced redox chemistry of the IONPs in the brilliant X-ray beam of European Synchrotron Radiation Facility- Extremely Brilliant Source (ESRF-EBS). Upon extended exposure of one minute the Bragg peaks of the IONPs in this dilute EtOH-H2O mixture were observed to shift towards lower Q-values. The shift was elucidated to originate from the reduction of non-stoichiometric, mostly Fe3+ containing IONPs, to magnetite upon exposure with the X-ray beam. In presence of EtOH, the oxidizing radicals from the radiolysis of water are scavenged and a reducing atmosphere is created. By comparison to data before the upgrade this could be clearly correlated to the tremendously increased flux and corresponding higher radiation dose after the EBS upgrade. Concluding, with this thesis it has been shown, that nanoparticle-solvent interfaces are complex and specific systems, which need rigorous investigations. The results of this thesis add to the growing pool of fundamental knowledge regarding nanoparticulate interfaces. Further, neutron PDF has been shown as promising tool to investigate nanoparticle-water interfaces.

Institutions

• Division of Earth Sciences and Geography [530000]
• Chair and Institute of Crystallography [542110]