Atomic insights into mechanisms of carbon coating on titania nanoparticle during flame synthesis

Carbon-metal oxide (CMO) nanocomposites have seen increasing research due to their extraordinary properties for energy storage materials and photocatalysts. Flame aerosol synthesis provides a promising route to producing CMO nanocomposites. Various CMO nanocomposites have been successfully synthesized through flame aerosol techniques in laboratories. However, a detailed understanding of the formation and growth mechanisms of such materials is lacking. Therefore, in this study, the reactive force-field molecular dynamics (ReaxFF MD) was deployed to gain atomic insights into the initial stage of carbon coating on the titania nanoparticle. We performed a large number of simulations of carbon coating with 18 typical hydrocarbon species in flames including aliphatics of C1-C4 species and polycyclic aromatic hydrocarbons (PAHs) at temperatures ranging from 400 K to 2500 K. We found that the titania nanoparticle can not only serve as a nucleus for physical adsorption of the surrounding hydrocarbons, but also can form C-Ti/O bonds with them, and abstract H atoms from the surrounding hydrocarbons. The optimal temperature range for carbon coating is T <= 1200 K, because C-Ti/O bonds are unstable at higher temperatures. At T >= 1500 K, hydrocarbons tend to gather to form larger carbonaceous species instead of coating onto the particle surface, as the formation of C-C bonds is promoted at high temperatures. Small aliphatics are favored to be chemically coated on the particle, while PAH molecules tend to be physically absorbed on the nanoparticle surface due to their stable electronic structure and large size. Coating tendencies of aliphatics are closely related to the number of C-C triple bonds.