Synthesis of 2D MoO3−x/N-doped-carbon nanocomposites via in situ carbonization of layered (NH4)Mo3O9–(NH4)2Mo4O13-organic hybrid nanomaterials for exceptionally efficient adsorption and separation of organic dyes

Abstract

MoO_3−x/N-doped-carbon (MoO_3−x/N-C) nanocomposites with nanobelt structures were prepared by the in situ carbonization of the (NH₄)Mo₃O₉–(NH₄)₂Mo₄O₁₃/oleylamine–oleic acid hybrids, and were designed using a two-step tailored nanoemulsion method followed by a solvothermal process. The MoO_3−x/N-C nanocomposites possess high stability in a wide pH range of 1–11. In addition to the formation mechanism, detailed studies on adsorption properties, including adsorption isotherms, kinetics, mechanism, and reusability of the MoO_3−x/N-C nanocomposites were systematically studied. The effects of various parameters on the adsorption characteristics, such as adsorbate concentration, contact time, pH, and temperature, were also evaluated. The detailed investigation of the kinetics and adsorption mechanism of MB from an aqueous solution showed that the adsorption process follows a pseudo-second-order kinetics and intraparticle diffusion model. The adsorption isotherm study demonstrated that the Langmuir isotherm model could illustrate the experimental data with a maximum adsorption capacity value of ∼1360 mg g⁻¹, which turns out to be the highest among the previously reported adsorbents based on MoO₃ nanomaterials, and is attributed to multiple adsorption mechanisms including electrostatic, π–π stacking, and H-bonding interactions between the MB dye and the MoO_3−x/N-C nanocomposites. Thermodynamic analysis suggested that MB adsorption onto the MoO_3−x/N-C nanocomposites was spontaneous and endothermic. Additionally, the adsorption and desorption cycles were examined for 100 ppm aqueous MB solution, where the removal efficiency by the MoO_3−x/N-C nanocomposites remained at ∼99% even after four regeneration cycles. Furthermore, the MoO_3−x/N-C nanocomposites could selectively adsorb MB from a binary solution mixture containing two times higher concentrations of an anionic dye, methyl orange (MO), and the separation efficiency obtained was ∼99% at neutral pH. More significantly, the MoO_3−x/N-C nanocomposites could successfully and simultaneously adsorb several cationic dyes, including MB, malachite green (MG), crystal violet (CV), safranin O (SO), and separate an anionic dye, MO, from quaternary and pentanary dye mixture solutions with a separation efficiency of ∼75% and 62%, respectively, at neutral pH. To the best of our knowledge, the effective separation of a single component by molybdenum oxide-based nanomaterials via a simple batch separation technique from a four/five-component mixed interfering analyte, close to a real matrix system, in an aqueous medium at neutral pH has not been reported so far. Therefore, the MoO_3−x-based nanocomposite could be a new promising material for application in dye wastewater treatment.