Advanced Characterization Techniques for Evaluating Porosity, Nanopore Tortuosity, and Electrical Connectivity at the Single-Nanoparticle Level

Abstract

We demonstrate quantification of porosity, nanopore tortuosity, and electrical connectivity at the single-nanoparticle (NP) level for NPs synthesized by nanodroplet-mediated electrodeposition. Focused ion beam nanoslice tomography was used to slice NPs with ca. 10 nm slice resolution followed by imaging using scanning electron microscopy (SEM), allowing measurement of these parameters on NPs not amenable to transmission electron microscopy. Slices were reconstructed in three dimensions and revealed pores with an average size of 3 ± 2 nm and relative nanopore tortuosity of 46.8 ± 24.5. We also demonstrate a new technique to evaluate electrical connectivity at the single-NP level by taking advantage of material-selective electrodeposition. The rate of Cu electrodeposition differs significantly on Pt compared to carbon, implying Cu can be selectively electrodeposited onto Pt NPs adsorbed onto a carbon support. Following the Cu electrodeposition step, NP connectivity was determined by the presence of Cu on Pt, as studied by energy-dispersive X-ray spectroscopy and SEM. We demonstrate that NPs synthesized by electrodeposition have >97% connectivity with underlying highly oriented pyrolytic graphite (HOPG) or amorphous carbon electrodes. The same method was employed to study connectivity of citrate-capped Pt NPs (diameter of 70 nm) on HOPG and amorphous graphite adsorbed by drop-casting. Surprisingly, <80% of these NPs had connectivity on HOPG and <40% had connectivity on amorphous carbon. These techniques will find applications in nanomaterials characterization, particularly in the fields of electrocatalysis and energy storage and conversion.