KOH-Assisted Purification of Graphene Oxide: Effect on Sulfur Residues, Dispersion Behavior and Thermal Reducibility

Graphene oxide (GO) is a key precursor of functionalized graphene materials and reduced graphene oxide (rGO), but its wider use is limited by difficulties in obtaining materials with reproducible composition, controlled oxidation degree and low post-reaction impurity content. This study compares two purification routes for GO obtained by a modified Hummers method. After synthesis, the post-reaction mixture was divided into two parts: one was purified conventionally in acidic HCl medium (GO-HCl), whereas the other was neutralized with KOH, washed with water and finally treated with HCl until approximately neutral pH was reached in order to remove soluble Mn2+ and K+ compounds (GO-KOH). KOH-assisted purification reduced the number of washing cycles from 18 to 5. WAXS/XRD, SEM/EDS, XPS, FTIR, Raman spectroscopy, DSC and TGA showed that GO-KOH sedimented faster, formed a powder after drying, reached a low residual sulfur level after a much shorter purification route (0.27 vs. 0.55 at.% by EDS), and had a higher C/O ratio (2.01 vs. 1.57), a lower Raman ID/IG ratio (1.62 vs. 1.88), and a lower oxygen-group removal enthalpy (521 vs. 1080 J g−1). XPS analysis also showed that the HCl-purified sample was strongly affected by non-uniform charging, whereas the KOH-treated sample did not show significant charging during acquisition. Sequential pH change is therefore an effective tool for shortening GO purification and controlling its dispersion behavior and thermal reducibility.