The role of nitrogen plasma surface treatment and TiO2 seeding onto carbon fibers on the microwave-assisted growth of radially aligned TiO2 nanowires

Abstract Radially aligned titanium dioxide (TiO2) nanowires were synthetized on nitrogen plasma-treated carbon fibers (CF) using a two-step microwave-assisted solvothermal method. The TiO2 synthesis consisted in seeding TiO2 nuclei on CF followed by TiO2 nanowire growth. To assess the role of the nitrogen plasma surface treatment and TiO2 seeding, CF and CF-TiO2 hybrids were analyzed by Raman spectroscopy, X-ray diffraction, scanning electron and atomic force microscopies, after each step of the TiO2 synthesis. In addition, X-ray photoelectron spectroscopy was used to analyze the CF chemical composition upon the plasma treatment and TiO2 seeding. Single fiber tensile tests were conducted on CF and CF-TiO2 hybrids to track the mechanical changes induced by the surface treatment and TiO2 synthesis. It was found that both, the nitrogen plasma surface treatment and TiO2 nuclei seeding were essential to homogeneously synthesize radially aligned TiO2 nanowires onto CF. No significant changes in the CF surface roughness after plasma treatment and TiO2 seeding were found; however, upon TiO2 nanowire growth, the surface roughness increased by about twelve times with respect to the pristine ones. The plasma surface treatment generates oxygen- and nitrogen-containing functional groups that promote TiO2 nucleation onto CF. In addition, the CF internal heating due to microwave irradiation triggers a preferential nucleation of TiO2 on the fibers rather than in the reaction medium. The tensile strength of CF remained without significative changes after the surface treatment and TiO2 nanowire growth. The CF-TiO2 hybrid materials developed in this work could be exploited in the development of multifunctional structural composites.