Binding modes of Fe(III) with graphene oxide in aqueous solutions. Competition with Sr2+, Cs+, Na+ ions and Fe(III) chelators

Reduced graphene oxide -MnO2 nanocomposite for CO2 capture from flue gases at elevated temperatures

The newly prepared reduced graphene oxide-MnO₂ (rGO-MnO₂) nanocomposite has exhibited highly selective CO₂ adsorption from gaseous mixtures at elevated temperatures. The Mn²⁺ basic sites are scattered over the rGO-MnO₂ nanocomposite which produce an effective BET surface area of 710 m² g^-1 for selective CO₂ capture. The selective adsorption of CO₂ (5.87 mmol g^-1) over N₂ (0.36 mmol g^-1) and CH₄ (0.41 mmol g^-1) at 298 K/1 bar was achieved by the nanocomposite. The heat of adsorption followed a unique correlation with the quantity of CO₂ adsorbed and fits well to the Fowler-Guggenheim equation. The mechanism of CO₂ adsorption on the nanocomposite was complemented with molecular modelling and simulations. The rGO-MnO₂ have shown better CO₂ adsorption capacity of 28.5 mmol g^-1 at 323 K/20 bar as compared to zeolite derivatives, MOFs, and carbons as reported in the literature. The formation of inert frameworks with 3-6 nm porous structure in the nanocomposite thermally stabilizes to capture CO₂ repeatedly. The nanocomposite with adsorption capacity of 3.69 mmol g^-1 at 373 K/1 bar is quite close to real-life conditions for flue gas treatment.

Principles governing control of aggregation and dispersion of aqueous graphene oxide

Controlling the structure of graphene oxide (GO) phases and their smaller analogues, graphene (oxide) quantum dots (GOQDs), is vitally important for any of their widespread intended applications: highly ordered arrangements of nanoparticles for thin-film or membrane applications of GO, dispersed nanoparticles for composite materials and three-dimensional porous arrangements for hydrogels. In aqueous environments, it is not only the chemical composition of the GO flakes that determines their morphologies; external factors such as pH and the coexisting cations also influence the structures formed. By using accurate models of GO that capture the heterogeneity of surface oxidation and very large-scale coarse-grained molecular dynamics that can simulate the behaviour of GO at realistic sizes of GOQDs, the driving forces that lead to the various morphologies in aqueous solution are resolved. We find the morphologies are determined by a complex interplay between electrostatic, [Formula: see text]-[Formula: see text] and hydrogen bonding interactions. Assembled morphologies can be controlled by changing the degree of oxidation and the pH. In acidic aqueous solution, the GO flakes vary from fully aggregated over graphitic domains to partial aggregation via hydrogen bonding between hydroxylated domains, leading to the formation of planar extended flakes at high oxidation ratios and stacks at low oxidation ratios. At high pH, where the edge carboxylic acid groups are deprotonated, electrostatic repulsion leads to more dispersion, but a variety of aggregation behaviour is surprisingly still observed: over graphitic regions, via hydrogen bonding and "face-edge" interactions. Calcium ions cause additional aggregation, with a greater number of "face-face" and "edge-edge" aggregation mechanisms, leading to irregular aggregated structures. "Face-face" aggregation mechanisms are enhanced by the GO flakes possessing distinct domains of hydroxylated and graphitic regions, with [Formula: see text]-[Formula: see text] and hydrogen bonding interactions prevalent between these regions on aggregated flakes respectively. These findings furnish explanations for the aggregation characteristics of GO and GOQDs, and provide computational methods to design directed synthesis routes for self-assembled and associated applications.

Carboxyl groups do not play the major role in binding metal cations by graphene oxide

In this study, we investigate the chemical interactions of Mn²⁺ ions with graphene oxides, prepared by Hummers' (HGO) and Brodie's (BGO) methods in aqueous solutions by means of NMR relaxation. Carboxyl groups, which are always present in HGO in significant quantities, are often considered as the main binding sites for metal ions. Here we demonstrate that metal ions are bound efficiently by BGO, containing a negligibly small quantity of carboxyl groups. The difference in the shape of the relaxation curves is due mostly to the difference in the solubility and exfoliation degree of the two GO samples in aqueous media. HGO binds Mn²⁺ in the broad pH range, including highly acidic solutions, while BGO binds only at pH > 6, since it is not dispersible in water at lower pH values. The ability of BGO to chemically bind Mn²⁺ despite lacking sulfate and carboxyl groups, coupled with our earlier published findings, strongly suggests that carboxyl groups do not play the main role in binding metal ions by GO, as is commonly believed. We propose that metal ions initiate a significant transformation in the GO structure to attain the most efficient coordination of metal ions. This reorganization might involve the metal cation induced C-C bond cleavage with the formation of enols at the newly formed edges.