Flame synthesis of carbon metal-oxide nanocomposites in a counterflow burner

Core-shell carbon metal-oxide nanocomposites are synthesized in a well-defined counterflow burner. Benefitting from the two-nozzle configuration, the nanocomposite morphology can be controlled by adjusting the precursor loading rate and fuel mass fraction independently. In-situ laser diagnostic method and transmission electron microscopy (TEM) technique are used to investigate the formation mechanism of nanocomposites. In the presence of a 532 nm laser pulse, laser-induced incandescence, Ti atomic emissions, and C2 swan spectra that appear simultaneously can indicate the particle volume fraction, titanium component, and carbon component, respectively. By increasing the precursor loading rates, both the measured laser diagnostic signals and the TEM images demonstrate that the titania component rises proportionally with the precursor concentration while the carbonaceous layer thickness remains unchanged. As further revealed by the axial profiles of the three signals, the formation and growth of the nanocomposites is composed of two steps. The first step is the reaction, nucleation, collision, and coagulation of metal oxides in the oxidizer side of the flame sheet. The second step is the formation of carbonaceous layers surrounding the metal oxides, which can be regarded as the heterogeneous nucleation of the carbonaceous species in the fuel-rich zone. By adjusting the precursor concentration and the fuel mass fraction, we can achieve independent control of the first and the second steps, respectively, thereby actively tailoring the core size of metal oxides and the carbonaceous layer thickness.