Aerosol Synthesis and Characterization of Heterogeneous Bimetallic Nanoparticles

As the proverbial noose tightens around humanity’s resource spending, research is focused on utilizing materials to their fullest potential. Nanotechnology is the ultimate way to economize by splitting objects into smaller parts and dispersing the material properties over greater surface to volume ratios. Emerging quantum effects at the nanoscale present unique tweaking opportunities in applications. This thesis deals with creating and characterizing heterogeneous nanoparticles, including Janus and core-shell nanoparticles: segregated structures where the different parts having different properties allow for multiple functionalities within individual units. Heterogeneous nanoparticles that have already attracted interest in fields ranging from catalysis to biotechnology are typically made by chemical methods. Here, aerosol technology has been used to realize these nanostructures, as such physical synthesis hold advantages in improved purity of
the product, and reduced waste from the process.

The two main approaches that have been developed in this work to create bimetallic heterogenous nanoparticles, surface segregation and condensational growth, both use spark ablation as the material source. From the optical emission in the electrical discharges, we use machine learning to determine the composition of bimetallic AuAg nanoparticles. Thermally induced surface segregation in CuAg agglomerates forming Janus and core-shell nanoparticles have been studied on- and off-line with aerosol metrology and electron microscopy. Compared to analogue works where the particles sit on a substrate, the aerosol phase is ideal to study surface segregation of “free” nanostructures. A more general route toward arbitrary metal-metal core-shell combinations is explored with condensational growth by thermal evaporation and photolysis. To understand the condensation inside a custom thermal evaporator designed in this work, a novel approach to measure the residence time distribution of aerosol nanoparticles is presented. Condensational growth of aerosol nanoparticles by
photolysis of metal-organic precursors is a new route that can be carried out at room temperature. The process therefore allows for formation of core-shell particles of miscible materials and avoids thermophoretic losses of particles experienced in conventional thermal evaporation.

Combining on-line compositional monitoring with the unique, precursor-less
pathways to create heterogeneous nanoparticles that aerosol technology enables, this thesis is a step toward more sustainable synthesis of tailored bimetallic nanostructures with applications in, for instance, catalysis, sensors, and electronics.