Angstrom‐Confined Electrochemical Synthesis of Sub‐Unit‐Cell Non‐Van Der Waals 2D Metal Oxides

Abstract Bottom‐up electrochemical synthesis of atomically thin materials is desirable yet challenging, especially for non‐van der Waals (non‐vdW) materials. Thicknesses below a few nanometers have not been reported yet, posing the question how thin can non‐vdW materials be electrochemically synthesized. This is important as materials with (sub‐)unit‐cell thickness often show remarkably different properties compared to their bulk form or thin films of several nanometers thickness. Here, a straightforward electrochemical method utilizing the angstrom‐confinement of laminar reduced graphene oxide (rGO) nanochannels is introduced to obtain a centimeter‐scale network of atomically thin (<4.3 Å) 2D‐transition metal oxides (2D‐TMO). The angstrom‐confinement provides a thickness limitation, forcing sub‐unit‐cell growth of 2D‐TMO with oxygen and metal vacancies. It is showcased that Cr ₂ O ₃ , a material without significant catalytic activity for the oxygen evolution reaction (OER) in bulk form, can be activated as a high‐performing catalyst if synthesized in the 2D sub‐unit‐cell form. This method displays the high activity of sub‐unit‐cell form while retaining the stability of bulk form, promising to yield unexplored fundamental science and applications. It is shown that while retaining the advantages of bottom‐up electrochemical synthesis, like simplicity, high yield, and mild conditions, the thickness of TMO can be limited to sub‐unit‐cell dimensions.