Quasi-Elastic Neutron Scattering of Citrate-Capped Iron Oxide Nanoparticles: Distinguishing between Ligand, Water, and Magnetic Dynamics

Experimental access to the diffusional properties of organic ligand molecules on nanoparticle (NP) surfaces is scarce, although surface functionalization is widespread in synthesis and for the control of functional particle properties. This work focuses on the dynamics of citrate ligands and water molecules on the surface of 6 nm iron oxide NPs (IONPs) equilibrated at a relative humidity of 8% by quasi-elastic neutron scattering. Given the complex quasi-elastic scattering signal including the magnetic nature of the IONPs, we build on fixed window scans to separate multiple dynamic processes, namely, phonons, magnetic relaxations, and hydrogen dynamics. In addition, deuterated samples allowed us to separate the ligand and water dynamics. With a simultaneous fit approach, multiple fixed window scans and energy-resolved spectra are described to determine the activation energies and relaxation times. It is found that surface-bound citrate ligands rotate continuously with E-a = 240 meV and tau(0) = 0.21 ps, while surface water diffuses translationally with E-a = 190 meV and tau(0) = 0.12 ps, significantly slower than bulk water. The separation of the coexisting dynamic processes in this study proves the high potential of quasi-elastic neutron scattering to reach a detailed understanding of interfacial processes in nanostructured materials.