One-Step Reduction and Functionalization of Graphene Oxide with Phosphorus-Based Compound to Produce Flame-Retardant Epoxy Nanocomposite

9,10-Dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) has been grafted onto the surface of graphene oxide (GO) by reacting epoxy ring groups together with the reduced graphene structure (DOPO-rGO). X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, and UV–vis spectroscopy confirm that DOPO not only covalently bonded to the GO, as a functionalization moiety, but also partly restored the conjugate structure of GO, as a reducing agent. A pelletlike structure of DOPO on rGO sheets was observed by means of transmission electron microscopy (TEM), contributing to good dispersion of rGO in nonpolar toluene. Furthermore, the flame retardancy and thermal stability of DOPO-rGO/epoxy nanocomposites containing various weight fractions of DOPO-rGO were investigated by the limiting oxygen index (LOI) test and thermogravimetric analysis (TGA) in nitrogen. Significant increases in the char yield and LOI were achieved with the addition of 10 wt % DOPO-rGO in epoxy, giving improvements of 81% and 30%, respectively. DOPO-rGO/epoxy nanocomposites with phosphorus and graphene layer structures were found to contribute to excellent flame retardancy compared to that of neat epoxy. Therefore, the synergestic effect of DOPO-rGO is quite useful, and this material can be utilized as a potential flame retardant.