Insight into the ultrasonication of graphene oxide with strong changes in its properties and performance for adsorption applications

A sonochemical approach is widely used as a routine protocol for graphene oxide (GO) preparation prior to adsorption applications, but there is a limited understanding of its effects on the GO sorption ability. Herein, using different types of sorbates, which are cation, cationic and anionic dye, the changes in sorption capacity along with the characteristics of sonicated GO, are shown. The ultrasound treatment unexpectedly removes oxygen functional groups from GO with a concomitant decrease in GO adsorption capacity, rather than an increase in its adsorption as expected from the use of ultrasound in dispersing or exfoliating GO. The C/O ratio increases from 2.1 in pristine GO up to 2.5 in sonicated GO. The loss of oxygen functional groups is also accompanied by an increase in disorder and defects on GO sheets, along with the weakening of p-p interaction. The pathways of GO's structural and functional groups changed by ultrasound are proposed, based on the generation of OH center dot radicals from ultrasonic cavitation. These radicals can react with the facile oxygenated groups of GO, such as those of epoxide, hydroxyl, and carboxyl, leading to the partial removal of these groups and the breakage of GO sheets. GO's adsorption capacities for cation and different dyes can decrease up to 75% after sonication, which is in agreement with the above changes in GO's characteristics. The effects are readily observed even in mild sonication such as using an ultrasonic bath, signifying their importance in many applications employing ultrasonication to disperse GO.