Synthesis and characterization of hollow nanoparticles in RF dusty plasma

Hollow particles have high surface-to-mass ratio, low density, high optical contrast, well-defined geometrical properties and have the capability to host or encapsulate another phase in their interior. These properties lead to a variety of applications and potential uses in catalysis, in photonic and imaging applications or as hosts for nanoencapsulation and controlled release of guest molecules. Here we report the formation of hollow nanoparticles by low-pressure processing. The presence of hydrocarbons in a glow discharge leads to the formation cross-linked solids, often referred to as plasma polymers. We show that plasma polymerization can under certain conditions lead to the formation of submicron spherical particles with a hollow center. These particles are formed in the plasma, become trapped in the electrode sheath and eventually deposit on the electrodes when the plasma is extinguished. The samples are analyzed under TEM, by FTIR and by image analysis. Sixteen organic vapors, both aliphatic and aromatic, were investigated of which 14 were found to result in reproducible formation of submicron spheres with a hollow center. Particle sizes vary from 70 nm to almost a micron while the size of the core varies from 20 nm to 150 nm. We hypothesize that the formation of solid particles is preceded by the formation of a liquid-like droplet which solidifies under prolonged exposure of its surface to the plasma. This hypothesis predicts a linear relationship between core and outside diameter which is verified experimentally for all 14 precursors that produce hollow particles.