Graphene oxide (GO) is a less ordered two-dimensional material and is actively studied in energy, electronic devices, separation of water, materials engineering, and medical technologies.  In 1859, Brodie first described the synthesis of GO, and Hummers is credited with popularizing research into GO in his research in 1958.  However, GO was quite difficult for fundamental understand due to the instability and chemical complexity, leading to a nonproductive research situation for a long time.  In 2010, the Nobel prize in Physics was given for the fundamental research on graphene.  Then, the explosive research on GO of the two-dimensional structure similarity to graphene has started from both aspects of the fundamental science and industrial applications with advanced methodologies.  GO is a precursor material for graphene-like materials which ca simple reduction of GO can massively obtain.

 The world market of powder GO in 2022 is 125 million USD and it will become 200 million USD by 2025.¹ GO is mainly used as a filler of the composite materials to improve the mechanical, electrical, and thermal performance. Recently, the UK government has invested US$7 million in research of graphene materials for biomedical applications (Graphene BioMed). Elucidation of the unstable nature of GO is an urgent issue for promotion of the fundamental science and industrial applications.

We reported a superhigh surface area-reduced GO of 2200 m² g^-1 and a flexible-natured nanoporosity of reduced GO.^2,3 The experience of GO studies showed us that GO was delicate and of an ever-changing nature.

GO must have an authentic GO for the common understanding. When we wrapped vnanocrystals very carefullywith GO to develop a new type of rapid membranes, we convinced the necessity of elucidation of the ever-changing nature of GO on observation of color changes of GO dispersion from yellow brown to dark blackish color. The color change of GO dispersion indicates the structural change associated with the p-electron structure of graphene. Therefore, the change in physicochemical properties should be accompanied by the color change. We measured the optical absorption spectra of GO dispersion with ripening time at different temperatures to grasp the color changes over a very long time. Ripening experiment at 348 K for 2 days provides information on the color change for about half a year at 298 K. That is, we can anticipate the state of GO after half a year at ambient temperature with the ripening at 348 K. We have introduced a concept of reduced time for overall understanding of color changes at 298 to 348 K, demonstrating the presence of three GO states: Intrinsic, metastable, and transient states⁴. The transient state of GO is different from reduced GO. To confirm the presence of the three GO states, we determined physical properties like magnetic properties other than optical absorption upon ripening. We obtained satisfactory results on physical properties inherent to three GO states.

Intrinsic GO is recommended to be used as the authentic GO for any research on metastable and transient GOs to establish GO science.  As the three GOs have different chemical components and their physical properties, we cannot get a common understanding of GO.  In our experience, we found that intrinsic GO was the best for wrapping nanocrystals for high-performance membranes⁵ due to its high flexibility.  Thus, we use the intrinsic GO having the p-p* transition peak at 230.5 ± 0.5 nm for highly reproducible preparation of excellent membranes.  However, the lifetime of the intrinsic GO is very short; it is only about 5 days at ambient temperature.  We sought the stabilization method of the intrinsic GO.

The intrinsic state is stabilized by storing GO dispersions in the frozen state below 255 K. The addition of an oxidant, ammonium peroxydisulfate, is also effective for suppression of the transformation into the metastable state; the oxidant is easy to remove by washing with water before usage.

 In conclusion, we must start the research on GO with the declaration of the GO state. If not, we cannot get a common understanding of GO even with quite active studies.

References

1. sdki.inc https://prtimes.jp/main/html/rd/p/000002524.000072515.html
2. Wang, S. Kaneko, K. et al. Activation routes for high surface area graphene monoliths from graphene oxide colloids. Carbon 76, 220-231 (2014).
3. Wang, S. Kaneko, K. et al. Distorted Graphene Sheet Structure-Derived Latent Nanoporosity. Langmuir 32, 5617-5622 (2016).
4. Otsuka, H. Kaneko, K. et al. Transient chemical and structural changes in graphene oxide during ripening. Nat. Commun. 15, 1709 (2024). https://doi.org/10.1038/s41467-024-46083-4
5. Otsuka, H. Kaneko, K. et al. Graphene oxide-zeolite colloidal mixture to wrapping formation: Creation of graphene-zeolite interlayer channel. J. Colloid Inter. Sci., in preparation.