Thermal analysis of the improved Hummers’ synthesis of graphene oxide

The improved Hummers’ synthesis of graphene oxide (GO) from graphite is investigated to monitor how the functional groups form during the synthesis steps. To achieve these, samples are taken after every preparation step and analyzed with TG–DTA/MS, FTIR, XRD and SEM–EDX techniques. It was found that the main characteristic mass loss step of GO was around 200 °C, where at first the carboxyl and lactone groups were released, and the evolution of sulfonyl groups followed them right away in a partially overlapping step. It became clear that in the as-prepared acidic GO sample the presence of H2SO4 originating from the reaction solution was still dominant. The functional groups were formed only after washing the as-prepared GO with HCl. The consecutive washing step with distilled water did not alter the functional groups or the thermal properties significantly; however, it made the GO structure more ordered. The reduction of the GO structure back to reduced GO resulted in the loss of the functional groups, and a graphitic material was obtained back.