TiO2 composited with carbon nanofibers or nitrogen-doped carbon nanotubes synthesized using coal fly ash as a catalyst: bisphenol-A photodegradation efficiency evaluation

Abstract

Coal fly ash (CFA) was used as a catalyst for the synthesis of nitrogen-doped carbon nanotubes (NCNTs) and carbon nanofibers (CNFs) by chemical vapor deposition. Carbon nanomaterials (CNMs) were successfully purified by sequential treatment in 5% HF and then in a dil. HNO₃/H₂SO₄ mixture, as was shown by SEM, TGA, and XRD. The purified NCNTs and CNFs were composited with TiO₂ nanoparticles at varying loadings (i.e. 1, 5 and 20% CNF/NCNT loadings) by a surfactant wrapping sol–gel/hydrothermal method and used for the photodegradation of bisphenol-A (BPA) in water with light being sourced from a solar simulator. It was shown by TEM that the CNMs were completely coated with TiO₂ nanoparticles and interactions between the CNMs and TiO₂ were demonstrated using PXRD and laser Raman spectroscopy. Photoluminescence measurements showed that compositing TiO₂ with CNMs, especially NCNTs significantly reduced its emission intensity suggesting a reduced electron/hole recombination rate. Unbound TiO₂ was used to optimize the photodegradation experimental conditions, i.e. solution pH, the mass of the photocatalyst, the initial concentration of BPA and solution temperature. The photocatalytic efficiency of the various TIO₂ and CNFs/NCNTs was assessed using the optimized conditions where it was observed that the composites containing 1 and 5% loadings of CNMs outperformed TiO₂. The photocatalytic efficiency of the NCNT based composites was higher than that of the CNF based composites. This work shows that CFA, a toxic material, could be used to synthesize materials useful for cleaning water.