Carbon nitride is an organic semiconductor, which can be prepared from easily accessible and inexpensive precursors. In addition to an appropriate bandgap to generate electron-hole pairs under visible light (about 2.7 eV), CN provides nitrogen-rich cavities to accommodate and stabilize singleatom catalysts. We synthetized and characterized the CN support and the Co-CN photocatalyst according to previously reported procedures (for details see the Supporting Information). In a first step, bulk CN was prepared by heating dicyanamide as source of carbon and nitrogen. After exfoliation through thermal treatment, the obtained CN sheets were dispersed in water with cobalt(II) chloride as metal precursor. This was followed by a reduction treatment and washing steps to obtain the Co-CN material, which was characterized through a combination of techniques. The X-ray powder diffraction (XRD) pattern of Co-CN shows two distinct diffraction peaks at 2𝜃 equal to 12.8° and 27.5° corresponding to the (100) and (002) planes, respectively, of the graphite-like carbon nitride system (Figure , Supplementary Information) that match well with those reported in literature. The weak diffraction peak at 12.8° indicates an intralayer structural packing motif, specifically trigonal N linkage of tri-s-triazine units, while the intense sharp peak at 27.5° is due to the interplanar stacking of aromatic structures. The XRD diffraction pattern of Co-CN showed no additional peaks other than the characteristic ones of the metal-free CN sheets, indicating that the structure of CN is well preserved after incorporation of the metal (Figure , Supplementary Information), and that no Co-based species such as chlorides, nitrides or oxides as crystalline metal clusters or particles are present thereby suggesting the presence of atomic dispersion of Co atoms on the CN support (vide infra).