The effects of functionalization on graphene oxide for organic dye adsorption: An experimental-theoretical study using electronic structure calculations and statistical mechanical modeling

Abstract

In this work, an alternative theoretical–experimental approach was combined to investigate the influence of the degree of functionalization of reduced graphene oxides (rGO) on the capacity and adsorption mechanisms of two model organic dyes. The experimental adsorption equilibrium data for methylene blue (MB) and indigo carmine (IC) were adjusted and studied using an efficient Multi-Layer Finite Model (MLFM) based on statistical mechanics. Additionally, density functional calculations (DFTB and DFT) were carried out to study the molecular interaction geometry and adsorption energies. With this approach, a greater amount of information about the adsorption process can be obtained in relation to commonly used methodologies for adsorption at a solid–liquid interface. Herein, kinetic and adsorption equilibrium experiments were conducted on rGOs prepared via thermal reduction at different temperatures (300, 700, and 1000 °C) in order to obtain very different contents of oxygenated functional groups. The highest adsorbed amounts are obtained for rGO1000, reaching 208 and 320 mg g−1 for MB and IC, respectively. The MLFM results revealed that the rGO-dye interaction occurs preferentially via π-stacking, and with the formation of up to two adsorption layers. The elimination of functional groups from the surface of rGOs is also revealed to be favorable, decreasing the adsorption energy and increasing the amount and fraction of molecules adsorbed in parallel orientation.