Structural-morphological and adsorption properties of hollow balls of oxidized graphene obtained by auto-combustion of saccharose

Abstract

In this work, the atomic structure and morphology of the carbon material obtained by self-combustion of the composite mixture of saccharose (52 wt., %), potassium nitrate (44 wt., %), and sulfur (4 wt., %) were investigated. The proposed mechanisms for sample formation are as follows. Hollow balls of oxidized graphene are created due to the action of pyrolysis gases (vapor H₂O, CO₂). Further carbonization of the bubble shells leads to the formation of hollow carbon particles with a diameter of (10−100) nm and (300–3500) nm. Individual carbon atoms in graphene are oxidized to groups C-OH, -СООН, and –ОСООН during auto combustion. Another oxidation pathway of carbon atoms is carried out with the participation of KNO₃. The N^5 + cations of this compound are converted into N₂ molecules by electrons captured from carbon atoms. Carbon atoms, oxidized in this way, transform into the =C²⁺ cations and attach O^2- anions. The obtained material was investigated using XRD, FTIR, XPS, Raman, and TG/DTG/DTA analysis. Also, the surface area and porous size distribution were measured using low-temperature N₂ adsorption/desorption isotherm. The oxidized state of carbon atoms in the graphene structure performs its new valuable properties, such as adsorption properties. The oxidized graphene can bind halogen anions in an aqueous medium. The kinetics of adsorption of iodine anions from the KI solution, equilibrium adsorption, and dependence of adsorption on pH were experimentally studied. The Elovic and diffusion kinetic models, Lagergren's models based on pseudo-first and pseudo-second-order equations, and the Langmuir-Hinshelwood kinetic model were applied to the obtained results. Equilibrium adsorption was analyzed using the theories of Langmuir, Freundlich, and Henry. The supposed mechanism is related to forming adsorption centers =C²⁺O^2-. The number of adsorption centers was quantified for the first time. It was found that the surface of oxidized graphene with an area of 10 nm² contains ∼ 24 groups = C²⁺O^2-, which can exchange O^2- anions, for example, for halogen anions. Hollow balls of carbon adsorbent can adsorb 618 mg∙g⁻¹ iodine anions from the aqueous solutions, which is a unique result since most carbon adsorbents and MOFs adsorb only molecular iodine.