Surface Characterization of Nanoparticles: Insights from In Situ Transmission Electron Microscopy

Nanoparticles are used heavily in several fields such as catalysis, solid oxide fuel cells, biomedical applications, sensing technologies. Due to high surface-to-volume ratio, nanoparticle surfaces have unique properties compared to bulk materials. There are many outstanding questions regarding nanoscale surface characterization under reaction conditions, such as: defect chemistry, surface rearrangements, and charge redistribution under light illumination. To address some of these questions, techniques centered around aberration-corrected high resolution transmission electron microscopy (AC HRTEM) coupled with in situ capabilities were developed and employed to visualize and characterize dynamic surface phenomena.To sense point defects (oxygen vacancies) at the surface of a reducible oxide (ceria), strain mapping was employed on HRTEM phase contrast images. This method provides identification of defect sites at surface and sub surfaces in a ceria nanoparticle via strain field analysis on the cation sublattice. A concept called fluxional strain has also been used to identify the location of stable and unstable defects at surface sites. Density functional theory (DFT) has been used to corroborate the results from the experimental observations. In situ TEM is also employed to study the dynamic behavior of these defects under different redox conditions. In situ TEM was leveraged to gain insights in the dynamic behavior of a metal nanoparticle during gas-solid interactions. Pt supported on ceria was used to study the structural rearrangements and dynamics in the presence of CO gas. High frame rate time-series images (resulting in noisy images) were captured with high spatial resolution to explore the dynamics. Strategies for denoising were developed using unsupervised deep video denoising. Denoised imaged revealed different states of structural motifs at the surface and sub-surfaces of the nanoparticle. In situ TEM coupled with electron holography was also used to characterize surface charge due to electron and photon illumination from rhodium doped strontium oxide nanoparticles. At first, the effect of electron beam dose rate on secondary electron based specimen charging was characterized. Further using in situ light illumination, a 3-fold increase in photoconductivity from the nanoparticles was also identified reducing the overall charging of the nanoparticles.