Coating of Silica Nanolayers on Carbon Nanofibers via the Precursor Accumulation Method.

Surface modification of nanocarbons, for example, by coating with oxide nanolayers, is a research topic of significant interest because of the drastic changes in the physicochemical properties of the modified nanocarbons. One simple method of creating these oxide nanolayer coatings on nanocarbons is the precursor accumulation (PA) technique, which entails the following: (1) a precursor solution is dropped onto nanocarbon powder; (2) the solvent is dried, leaving accumulated precursor on the nanocarbon surface; and (3) hydrolysis or decomposition of the precursor in air leads to the formation of oxide nanolayers on the nanocarbons. In this study, tetraethoxysilane (TEOS) was used as a precursor for coating silica nanolayers onto carbon nanofibers (CNFs). TEOS is so stable that it hardly undergoes hydrolysis on the surface of pristine CNFs. By treating CNFs with H2SO4/HNO3, acidic functional groups were introduced onto the CNF surfaces. Silica nanolayers successfully synthesized on these acid-treated CNFs via PA coating because the acidic functional groups catalyzed the hydrolysis of TEOS accumulated on the CNF surfaces. Scanning transmission electron microscopy (STEM) indicated the thickness of silica layer is approximately several nanometers. Pore size distribution analysis for the silica nanolayer suggested the presence of nano-pore with 3~5 nm. The TEOS molecules could have accessed the functional groups through the nano-pore; therefore, the number of silica nanolayers formed increased with the number of PA coatings. Finally, we compared the PA coating with conventional sol-gel and atomic layer deposition (ALD) techniques.