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.
Recently, Professor Kaihong Luo, Adjunct Professor from Shanghai Institute for Advanced Study, Zhejiang University and his collaborators deployed the reactive force-field molecular dynamics (ReaxFF MD) to gain atomic insights into the initial stage of carbon coating on the titania nanoparticle.
Eighteen typical hydrocarbon species in flames, including chain aliphatic species with 1–4 C atoms and aromatic species with 1–10 aromatic rings were selected as carbon source for coating and their coating performance on a 3 nm titania nanoparticle was simulated for 2 ns under different temperatures ranging from 400 K to 2500 K.
Fig. 1. Illustration of the initial simulation configuration for carbon coating on a 3 nm titania nanoparticle (a); Composition of surrounding CmHn species for different simulation cases. Light blue region denotes short-chain aliphatic species, while light yellow region denotes polycyclic aromatic hydrocarbon (PAH) species. The black dot indicates the species is a radical (b); Molecular structure for C4H2, C4H3, C4H4, C4H5, C4H6 and PAH species (c)
The authors managed to quantitatively describe the carbon coating outcome as well as compare the relative contribution of physical attraction and chemical bond formation to carbon coating.
Fig. 2. Illustration of the approach to differentiating between physically coated carbon atoms and chemically coated carbon atoms. The snapshot of 600 C4 molecules/radicals (Case 4 in Fig. 1(b)) coating on the 3 nm titania nanoparticle (Ti420O728) at 2 ns, T=800K is taken as the example
They 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.
Fig. 3. (a) Illustration of the chemical bonds formed after 600 C4 molecules/radicals coating on a 3 nm titania nanoparticle (Ti420O728) at 800 K (120 C4H2 + 120 C4H3 + 120 C4H4 + 120 C4H5 + 120 C4H6, Case 4 in Fig. 1(b)) for 2 ns; (b), (c) and (d) are snapshots obtained by zooming in local regions of the particle surface to demonstrate the newly formed bonds during carbon coating. (b) shows two C-Ti bonds formed between one C4H2 molecule and two Ti atoms on the particle surface; (c) shows one C-Ti bond and one C-O bond formed between one C4H2 molecule and the particle surface; (d) shows dangling C-Ti bonds or C-O bonds formed between hydrocarbon species and the particle surface
The optimal temperature range for carbon coating is T≤1200K, because C-Ti/O bonds are unstable at higher temperatures. At T≥1500K, 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.
Fig. 4. The total number of carbon atoms in carbonaceous clusters (nC,cluster)after different hydrocarbon species coating on a 3 nm titania nanoparticle (Ti420O728) at various temperatures for 2ns. The number of C atoms in carbonaceous clusters (nC,cluster in each cell is obtained by averaging the simulation results during 1950–2000ps) under corresponding simulation conditions. For easy comparison, the cells are colored in blue–white–red scale based on the value of nC,cluster–The darker the red, the larger the nC,cluster; While the darker the blue, the smaller the nC,cluster; White color represents nC,cluster in between
In this work, the authors compared the roles of chemisorption and physical adsorption during C coating, and concluded that optimal temperature range for carbon coating was T≤1200 K. They further proposed a carbon coating mechanism for both aliphatic and PAH species and demonstrated that the number of C–C triple bonds in an aliphatic species affected its coating capability.
The work was published as ‘Atomic insights into mechanisms of carbon coating on titania nanoparticle during flame synthesis’ in Carbon, and could be accessed at https://www.sciencedirect.com/science/article/pii/S0008622322007266#d1e875