Core–shell ZnO@ZIF-8-derived nitrogen-doped carbon nanotubes for the effective removal of methyl orange

The nitrogen-doped carbon nanotubes (NC) are synthesized via pyrolysis of zeolitic imidazolate framework-8 (ZIF-8) coated on ZnO nanorods, followed by acid etching. Morphological studies reveal that the NC shows a porous, tube-like structure with a diameter of 194 nm. The ZIF-8 template on the ZnO nanorods forms thin, shell-like layers surrounding the core ZnO. X-ray diffraction (XRD) analysis reveals the amorphous structure of NC, while FTIR/XPS spectra affirm nitrogen doping, Zn-N interaction in ZnO@ZIF-8, and the N-C environment in NC. The BET surface areas for ZnO@ZIF-8, ZnO@NC, and NC are 724.81 ± 12.77, 638.92 ± 1.29, and 787.11 ± 4.26 m² g⁻¹, respectively. ZnO@NC and NC successfully removed MO at concentrations of 10 and 20 mg L⁻¹, achieving equilibrium adsorption capacities of 50.23 and 100.50 mg g⁻¹, respectively. The optimum conditions for ZnO@NC and NC were a pH of 6, a contact time of 60 min, and an adsorbent dosage of 0.2 g L⁻¹, respectively. The high correlation coefficient R² (0.9983) from the Langmuir isotherm indicated monolayer adsorption on NC with the maximum adsorption capacity (q_max = 126.58 mg g⁻¹). NC followed pseudo-second-order kinetics with a high R² (0.999), substantiating the chemical interaction between the NC surface and MO. ZnO@NC showed chemical adsorption, while NC underwent physical and chemical adsorption, as identified through the isotherm (D-R) and kinetic (Elovich) models. The results show that NC proves a large BET surface area, a tubular structure, heterogeneous nitrogen-doped carbon, and abundant mesoporous channels that beneficially facilitate effective MO adsorption.