Deposition of an organic–inorganic hybrid material onto carbon fibers via the introduction of furfuryl alcohol into the atomic layer deposition process of titania and subsequent pyrolysis

Exposing bundles of carbon and quartz fibers as well as silicon wafers to sequential pulses of gaseous TiCl4/furfuryl alcohol (FFA) and H2O/TiCl4/furfuryl alcohol, respectively gave rise to conformal organic/inorganic coatings on all of these substrates. The deposition during the FFA pulse is self-limiting and the process shows a linear growth per cycle of (0.65 ± 0.05) nm/cycle in the absence of water pulses and (1.68 ± 0.04) nm/cycle in the presence of water pulses. A possible reaction mechanism comprises classic atomic layer deposition steps as well as a polymerization route of furfuryl alcohol, which leads to the release of water during the furfuryl alcohol pulse and further comprises hydrolytic ring opening of furan rings and/or hydrolytic cleavage of Ti–Cl bonds. Since an additional subsequent water pulse significantly enhances the growth, it seems that without additional water pulse these reactions are limited by the amount of water that is formed or desorbed. Oxidizing the coated carbon fibers at 700 °C gave rise to thin-walled titania microtubes with wrinkled tube walls. Titania/carbon-hybrid microtubes were obtained by coating quartz fibers with TiO2/FFA, pyrolysis at 600 °C under nitrogen atmosphere, and subsequent etching with hydrogen fluoride. Energy dispersive x-ray spectroscopy and x-ray photoelectron spectroscopy (of coated silicon wafers) confirm that the coating consists predominantly of titanium, oxygen, carbon, and comprises residual chlorine. The stoichiometry is TiO2C3.5Cl0.1 before and TiO1.8C1.4 after the pyrolysis. According to XPS and thermogravimetric analysis, the molar ratio of titania to furfuryl alcohol is approximately 1.