One-step flame synthesis of hydrophobic silica nanoparticles

Hydrophobic silica (SiO2) nanoparticles are synthesized directly from the combustion of hexamethyldisiloxane (HMDSO) in a co-flow diffusion flame burner, employing diluted Ar flow in a branch to further control the flow rate of the precursors. In this study, it is found that the diluted Ar flow could change the surface wetting properties of the silica products synthesized by flame approach. The morphology of the silica products is observed using SEM, and the particle size is estimated through BET measurement. The results show that the change of diluted Ar flow rate has great influence on the size of the silica particle. With the increase of the flow rate, the particle size decreases dramatically and reach a minimum value of 7 nm at the flow rate of 7 L/min. All the contact angles of the samples prepared at the diluted Ar flow rate of 7 L/min are larger than 120°, suggesting that the surface of sample is hydrophobic. Fourier transformed infrared spectroscopy (FT-IR), 29Si CP/MAS NMR and 13C CP/MAS NMR spectroscopy are used to characterize the surface chemistry and grafted group information of silica nanoparticles. It could be concluded that the surface of the silica nanoparticles synthesized at Ar flow rate of 7 L/min are modified by methyl groups, which leads to the surface hydrophobicity of silica nanoparticles.