Green synthesis of reduced graphene oxide nanosheets decorated with zinc-centered metal-organic film for epoxy-ester composite coating reinforcement: DFT-D modeling and experimental explorations

Abstract

Hydrazine is a potent traditional reducing agent of graphene oxide with high deoxygenation capability. However, it has been categorized as a toxic/poisonous substance with serious environmental issues. Due to the hazardous/toxic nature of this substance, numerous researches have been carried out to find sustainable/nontoxic reductants. For the first time, the high quality reduced graphene oxide (rGO) particles were obtained through reduction and covalent-functionalization of GO at the same time with the natural Esfand seed (ES) extract (ESE) that includes some green/harmless reactive nitrogen-rich molecules, e.g., Harmol (C12H10N2O) and Harmalol (C12H12N2O). The rGO-ESE sheets were then modified by metallic zinc ions, and rGO-ESE-Zn(II) sheets were obtained. X-ray diffraction (XRD) analysis revealed the significant increment of GO sheets d-spacing after interaction with ESE-Zn(II). The FT-IR results indicated the disappearance of carbonyl (CO) absorption peak, and significant reduction of COC bond intensity, all affirming the successful GO reduction in the presence of ESE molecules. The C/O ratio calculated based on the EDS data are 0.18 and 0.33 for the rGO-ESE and GO sheets, respectively, indicating the increase in O content after GO reduction proving the ESE adsorption on the GO sheets. Tensile test results depicted the maximum increase of tensile strength (78%), Young's modulus (102%), and fracture energy (83%) of the coating in the presence of rGO-ESE-Zn(II) sheets. FE-SEM analysis revealed the effective toughening mechanisms of the rGO-ESE-Zn(II) sheets in the polymer matrix. Besides, the TGA results evidenced the significant improvement of the epoxy ester coating thermal stability (about 62%) in the presence of rGO-ESE-Zn sheets.