Synthesis of BI2O3/SiO2 core-shell nanoparticles by an atmospheric CVS/CVD process and their modification by hydrogen or electron-beam induced reduction

Bismuth oxide thin films in the nanometer range (1-11 nm) were deposited onto silica substrate particles of 60-120 nm by a continuous, integrated chemical vapor synthesis (CVS)/chemical vapor deposition (CVD) process at atmospheric pressure. The film thickness, measured by transmission electron microscopy (TEM), was adjusted by the partial pressure of the precursor bismuth triphenyl and the reactor temperature, but must be kept below certain limits to avoid homogeneous decomposition. Thermal stability of the core-shell structures was tested in air for 5 h and no change in structure was observed up to 400 °C, higher temperatures leading to break up of the film and sintering to bigger crystallites. In-situ electron beam induced reduction in the TEM or reduction in hydrogen at atmospheric pressure at 200-250 °C was used for restructuring of the core-shell particles and led to finely dispersed bismuth nanoparticles of 4-11 nm on the silica substrate particles. The size of the bismuth nanoparticles is a function of the initial film thickness. High-resolution transmission electron microscopy and electron energy loss spectroscopy were used to prove the metallic character of the bismuth nanoparticles and surface diffusion and coalescence could be observed as growth mechanism during in-situ TEM observations. The finely dispersed bismuth nanoparticles on the silica substrate could be reoxidized in atmospheric air at 400 °C without a significant change in the particle size distribution.