Diffusion of water, diethylene gylcol and citrate molecules on iron oxide nanoparticle surfaces via quasielastic neutron scattering

• Applicant: Professorin Dr. Mirijam Zobel
• Subject Area: Physical Chemistry of Solids and Surfaces, Material Characterisation
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 457807997

Iron oxide nanoparticles (NP) and related mixed metal ferrites are of increasing interest in e.g. heterogeneous catalysis or biomedicine, and solid-liquid interfaces are omnipresent in many applications. Yet the experimental characterization of these interfaces is challenging, since the contribution of the volume signal often dominates the interfacial signal. Although the influence of size or strain effects of the solid state structure of NPs onto their properties is being investigated since long, phenomena on the liquid side are rarely addressed. For instance, X-ray absorption and Raman spectroscopy have previously investigated hydrogen bond networks around NPs, and via pair distribution function analysis of X-ray scattering data we achieved insight into the structure of restruc-tured hydration shells around 7–15 nm sized iron oxide NPs.Since restructuring comes along with modified dynamic properties, this project shall complement our structural studies with dynamical insight. Relaxation times of rotational and translational diffusion of water molecules around ions in solution differ from the corresponding times in bulk water. Hence, modified dynamical properties of water molecules are also to be expected at NP surfaces. With quasielastic neutron scattering (QENS), we can measure dynamical properties of single molecules in spatial and time domain on time scales of 10-13 to 10-7 s, corresponding to typical relaxation times of diffusion processes. This project has the aim to establish QENS for the characterization of the dynamics of water and ligand molecules on surfaces of iron oxide NPs. Along a known synthesis route, 7 nm sized iron oxide NPs will be produced, and their structure and stability over a larger range of temperatures and relative humidities will be extensively characterized, among others with XRD/PDF, CHN, TGA, TEM, SQUID. Water sorption isotherms as a function of relative humidity will allow us to adjust nominally dry powders, as well as different numbers of water layers on the NPs. Powders of ‘naked’, diethylene glycol- and citrate-functionalized iron oxide NPs with defined water content will be measured at backscattering and time-of-flight spectrometers at neutron sources, in order to characterize the impact of relative humidity onto diffusion modes. From the QENS data, we will be able to determine the different types of diffusion behaviour of water and ligands molecules on the NPs, e.g. translational, rotational or more complex diffusional modes. Temperature-dependent measurements will allow us to access the activation energies of the motions. This project will enable a deeper understanding of the dynamics of water and ligand molecules around metal oxide NPs, relevant for nanomaterials, mineralogy or catalysis. In particular, the aspect of the diffusion dynamics of ligand molecules at NP surfaces is scarcely experimentally addressed so far, but of great importance towards the synthesis and long-term stability of NP dispersions.

• DFG Programme: Research Grants