Green synthesis optimization of graphene quantum dots by Doehlert design for dye photodegradation application

Abstract

In this work, graphene quantum dots (GQDs) were synthesized by hydrothermal carbonization (HTC) of brewer’s spent grains. Doehlert matrix design was used to evaluate the effects of temperature and residence time on the GQDs’ properties, including their photocatalytic activity, totaling 9 experiments. Methyl orange (MO) was used as a model molecule for the photodegradation assays. The GQDs presented pH values smaller than 3.13, high electric conductivity (1.0–3.0 mS cm−1) and positive Zeta Potential values. The GQDs sample, when excited by UV light at 365 nm, emitted a blue color, confirming their self-doping with nitrogen. Hexagonal rings in zigzag edge structures were observed using Transmission Electron Microscopy. The GQDs presented quantum yields ranging from 0.3 % to 4.9 %. The GQDs produced at 150 °C for 14 h presented the best photocatalytic performance. The photodegradation of the MO occurred by a first-order mechanism, which comprises two stages. The stage 1 is attributed to the adsorption of MO by the GQDs and stage 2 is attributed to the degradation of the MO by eBC− and/or O2- radicals. Finally, it can be concluded that GQDs with photocatalytic activity were produced by green synthesis, collaborating for a circular and sustainable dynamic in industrial processes.