The functional groups and physical properties of graphene oxide (GO) are found to be sensitive to and can be controlled by the water content in the reactions when GO samples are prepared at different concentrations of sulfuric acid using a modified Hummers method. GO prepared with 93% sulfuric acid (H2SO4) showed fewer structural defects, less π–π conjugation, and larger interlayer spacing than GO prepared with 99% H2SO4. The intensity ratio of the D-band to the G-band of the Raman spectrum is 0.89 ± 0.01 and 1.02 ± 0.01, corresponding to average interlayer spacing of 0.91 nm and 0.86 nm, respectively. The yield and carbon to oxygen ratio of the GO sheets prepared from different concentrations of H2SO4 are nearly identical. More importantly, compared with GO synthesized with 99% H2SO4, GO prepared with 93% H2SO4 contains more carbon–oxygen single bonds, such as epoxy groups and hydroxyl groups, but fewer carbonyl groups.
