Synthesis and characterization of Au@Zn core@shell aerosol nanoparticles generated by spark ablation and on-line PVD

An interesting subset of nanoparticles is core-shell nanoparticles: encapsulating a core particle of one material with a shell of another material can be utilized to combine and even extend the properties of the respective material. Strategies to synthesize core-shell nanoparticles in the aerosol phase remain relatively unexplored, despite the benefit of the continuous, ambient pressure nature of the process. Arguably, the most straightforward way to accomplish the core-shell morphology is to condense the shell material onto pre- formed core particles via physical vapor deposition (PVD). Previous works have utilized a second tube furnace in the aerosol circuit to evaporate the shell material and condense it onto the core particles (Karlsson, et al. 2004, Harra, et al. 2015). However, heating the entire aerosol may lead to unintended alloying of core and shell materials (Karlsson, et al. 2004). In this work we revisit the thermal evaporation approach using a coating chamber in which the evaporating material is only locally heated. As a test system, we coat Au nanoparticles generated by spark ablation with Zn due to the high evaporation rates achievable even at low heating temperatures.
The coating setup, shown schematically in Fig. 1, uses a tandem DMA setup to size select the aerosol prior to, and after condensational growth in the growth
chamber. The first DMA and tube furnace allows us to introduce a monodisperse, spherical Au aerosol into the growth chamber, after which the Zn growth is readily measured by scanning mobility diameter shift using the second DMA and an electrometer at different heater temperatures (Fig. 2). Further, the second DMA enables size selection of the core-shell particles corresponding to a desired shell thickness. The inset in Fig. 2 demonstrates a clear condensational growth up to heater temperatures of 400 °C, after which growth decreases, presumably due to homogenous nucleation of Zn. We will further discuss the characterization of the aerosol using electron microscopy and elemental characterization, as well as process limitations and opportunities.