(Invited) Development of in situ Electrochemical Small-Angle Neutron Scattering (eSANS) for Simultaneous Structure and Redox Characterization of Nanoparticles

Abstract

The demand for engineered nanomaterials in expanding research sectors, such as energy and healthcare, require materials with widely disparate physicochemical properties. The oxidation and reduction (redox) properties of nanomaterials are crucial for catalysis, matching semiconductor band gaps for photovoltaics, as well as toxicology, since they may cause oxidative stress by generating reactive-oxygen species. Optical spectroscopy analysis of molecular and nanomaterials undergoing redox reactions quantifies the electron transfer and energetics of the reaction via potential control and are the most common analytical techniques employed for this purpose. However, with conjugation to organic ligands and smaller, well-defined particle sizes, engineered nanomaterial redox characterization would benefit by techniques that provide nanometer sensitivity to nanoparticle core and ligand structure. We developed an in situ electrochemical small-angle neutron scattering (eSANS) method to measure simultaneously the redox properties and size, shape, and interactions of solution-dispersed nanomaterials [1]. By combining multi-step potentials and chronocoulometry readout with SANS, the structure and redox properties of engineered nanomaterials are followed, simultaneously in one experiment. Specifically, ZnO nanoparticles were examined as dilute dispersions in pH buffered deuterium oxide solutions under negative electrode potentials. The ZnO disk-shaped nanoparticles undergo an irreversible size transformation upon reduction at the vitreous carbon electrode. The decrease in average nanoparticle size near a current maximum shows the reduction reaction from ZnO to Zn occurs. The eSANS method provides nanometer scale sensitivity to the nanoparticle size and shape changes due to an electrochemical reaction that is crucial to understand in energy, healthcare, and other applications. These experiments utilized the EQ-SANS beamline at the Spallation Neutron Source, Oak Ridge National Laboratory that uniquely provides the ability to follow the redox kinetics under quasi-stationary potential step-scans. The methodology, contrast variation, and scattering properties of the micro and nanoporous carbon electrodes will also be described. [1] V.M. Prabhu and V. Reipa, J. Phys. Chem. Lett. 3, 646 (2012)