To facilitate understanding of the performance improvement, we conducted Raman spectroscopy of Cr2O3/rGO on the carbon fiber paper before and after OER. It was found that the characteristic peak positions remained unchanged after the exfoliation process. Compared to Figure , an additional peak (1006 cm -1 ) of Cr2O3 was more clearly observed as it was not covered by the peak of SiO2 substrate (Figure ). However, after OER test at 10 mA cm -2 for over 4 h (Figure ), main characteristic peak of Cr2O3 at around 543 cm -1 significantly decreased, while two new peaks representing Cr 6+ at 842 cm -1 and 888 cm -1 have appeared. This indicates that the Cr located between GO interlayer was further oxidized to Cr 6+ . This is consistent with observations that Cr based OER catalyst show high activity upon conversion to Cr 6+ in the interplay with graphene. The fact that the occurrence of Cr 6+ increases the OER activity is supported by the observation of increasing activity with increasing OER cycle repetitions (Figure ). And this rapid activation was not observed in bulk Cr2O3. further studies may investigate whether angstrom-confined sub-unit cell Cr2O3 have a lower oxidation energy barrier between Cr 3+ and Cr 6+ . Additionally, the interplay of Cr 6+ and graphene in increased OER activity needs further analysis. To extend our synthesis approach to other non-vdW TMO, we electrochemically synthesized 2D iron oxide, α-Fe2O3, within the rGO films (Figure ). Similar to Cr2O3, α-Fe2O3 grows on the basal plane of rGO. The 2D form is further confirmed by the occurrence of a Raman peak previously observed in 2D α-Fe2O3. XRD spectra measuring the dry interlayer space of Fe2O3/rGO (Figure ) confirms that the thickness of the grown material was limited to ~5. Future studies may also explore different transition metal or rare-earth metal ions as well as combinations of them. Interlayer space variations may result in different crystal configurations which could further optimize the catalytic performance. Furthermore, detailed studies of the electronic structure may allow deeper understanding of the catalytic process in this type of material .