A Reduced Graphene Oxide Quantum Dot-Based Adsorbent for Efficiently Binding with Organic Pollutants

This paper, for the first time, presents a novel, robust, and efficient high-quality reduced graphene oxide quantum dot (GQD)-based adsorbent for addressing the alarming environmental pollution issues nowadays. Such GQDs were fabricated in a facile manner via a combined green chemistry (GC) and temperature-controlled sonication irradiation (TSI) process. The mechanism underlying the fragmentation of the reduced graphene oxide (RGO) sheets prepared by GC with vitamin C (VC) as the only reactant to regulate the microstructure of graphene oxide (GO) was clarified through various characterization techniques such as X-ray diffraction, Raman, Brunauer–Emmett–Teller specific surface area measurements, transmission electron microscopy, scanning electron microscopy, elemental mapping, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, photoluminescence spectroscopy, selected area electron diffraction, and energy dispersive spectroscopy. The GQDs were assumed to be generated via cutting the nanometer-size sp2 domains or clusters out of the RGO sheets along the defect sites decorated with oxygen groups and sp3 carbons. The adsorption of the methylene blue (MB) dye onto the prepared GQDs was fast and obeyed the pseudo-second-order kinetic model, while the Freundlich adsorption isotherm was more suitable to fit the experimental data, as a result of the preferential enrichment of the organic dye molecules onto the edges rather than the homogeneous adsorption over the entire surface of GQDs. This could also be evidenced by the finding that the smaller was the lateral size of the RGO sheets, the stronger were the adsorption interactions with the MB dye. The dangling bonds with nonbonding electrons on the RGO edges were thus believed to play a significant role in the heterogeneous adsorption of the dye molecules through donor–acceptor charge transfer interactions. Impressively, the striking edge effect rendered the GQDs highly adsorptive toward the organic dye, with the maximum adsorption capacity calculated to be 827.5 mg g–1 for MB, outstripping most of the reported adsorbents. In addition, this GQDs-based adsorbent was found to be durable for many runs of repeated usage, and its universality was also demonstrated through efficiently binding with rhodamine B.