DOI: 10.14457/tu.the.2018.918
title: Effects of ultrasonication and annealing atmosphere on the properties of graphene oxides: characterizations and their adsorption uses

description: The characteristics and adsorption abilities of pristine graphene oxide (GO), ultrasonicated GO, and thermally annealed GO were investigated to evaluate the effects of ultrasound and different annealing atmospheres on the properties of pristine GO. The sorption of graphene oxide (GO) and thermally treated graphene oxide (TGO) toward cations and organic dyes was investigated by various factors such as pH, contact time, temperature and adsorption isotherms, exhibiting that these materials can be efficient sorbents for the tested sorbates. Various spectroscopic techniques were employed to characterize the as-prepared materials.Ultrasonic treatment of GO is a general treatment protocol that is normally used to better disperse or exfoliate GO in an aqueous solution prior to sorption process. Using various types of sorbates with different sorption pathways, which are elemental cations (cesium and strontium), cationic dye (methylene blue) and anionic dye (methyl orange), the effects of ultrasound on the structure as well as sorption abilities of GO are found to be significant. Two different types of sonicator, tank-type (ultrasonic bath) and probe-type (probe sonicator), were employed to obtain systematic investigation and showed that the sorption ability can reduce by half after two hours of sonication, by both tank-type and probe-type sonicators. The exfoliation and structural fragmentation of GO by sonochemical approach were observed by Scanning Electron Microscopy (SEM), Dynamic Light Scattering Sizer (DLS), Fourier Transform Infrared (FTIR), X-ray Photoelectron Spectroscopy (XPS), Raman Spectroscopy, and Ultraviolet-visible Spectroscopy (UV-vis). The strong changes in sorption performance of sonicated GO are due to the elimination of various oxygen-functional groups and the fragmentation of GO into smaller sheets, which creates more defects and weakens the π-π interactions between GO and sorbates. The possible pathways for GO’s structural and functional groups change by ultrasonication, leading to decreased adsorption capacities, are proposed based on the generation of bubble cavitation during ultrasonic treatment, creating H· and OH· free radical which lead to the further reactions with GO sheets, as shown in the following graphical abstract. In order to obtain better separation between sorbent and sorbate solution after the sorption process, thermally treated graphene oxide (TGO) have been synthesized as a more hydrophobic material. The radionuclides sorption efficiency of TGO is found to strongly depend on the annealing atmosphere, with TGO annealed in ambient air at 500°C performs six times better than that in nitrogen, in term of sorption capacity. The annealing temperature was also varied to observe the evolution of GO in air and in nitrogen along with temperature. Different transformation paths between GO undergoing thermal reduction in air and in nitrogen are proposed, based on the results from FTIR, XPS, Raman, XRD. Oxygen in ambient air is found to be the main factor causing the differences between TGO obtained from the two atmospheres. There are more defects and oxygen-containing groups such as epoxide and carboxyl formed during thermal annealing of TGO in air, leading to better sorption ability. On the contrary, the annealing in nitrogen gas is mainly the elimination of numerous functional groups and the restoration of sp2 structure. The different properties of TGO annealing in air and in nitrogen at similar temperature, which leads to the changes in sorption capacities toward cation, are displayed in the figure below. The findings from these works can contribute to the understanding of graphene oxide and reduced graphene oxide in a pure system without additional composites and can be useful in order to develop an optimal route to prepare graphene-based materials for different fields of applications.