## 2 Results and Discussion  
### Synthesis and Characterization of the Co@g-C\({}_{3}\)N\({}_{4}\)/G Heterostructure Composites  
g-C\({}_{3}\)N\({}_{4}\) was fabricated using a simple and low-cost strategy according to previous work. A schematic illustration of the synthesis process of Co@g-C\({}_{3}\)N\({}_{4}\)/G is shown in Scheme 1, that is, CoCl\({}_{2}\)\(\cdot\)6H\({}_{2}\)O was dispersed into the GO suspensions, accompanied by the addition of as-prepared g-C\({}_{3}\)N\({}_{4}\) by vigorous sonication to obtain a homogeneous precursor solution. As a result, the cobalt cations could be dispersed uniformly, enabled by the dual-anchoring effect of g-C\({}_{3}\)N\({}_{4}\) and GO. This phenomenon can be attributed to the following two major aspects: (1) It has been authenticated that 2D g-C\({}_{3}\)N\({}_{4}\) nanosheets can be as an ideal platform to trap transition metal/metal ions firmly on account of its abundant polygonal cavities and high-density N atoms, as well as electronegativity [41; 42], and (2) the rich oxygen-containing functional groups (-OH, -COOH, etc.) grafted on the surface of graphene oxide could also in situ anchor metal cations under an aqueous solution state via the electrostatic force or coordination [43], namely implying that GO nanosheets can behave as a surfactant and thus uniformly disperse the anchored Co\({}^{2+}\) ions effectively [44]. As expected, highly dispersed ultrafine-Co-species-nanodot-interspersed g-C\({}_{3}\)N\({}_{4}\) nanosheets and graphene heterostructure nanocomposites were obtained eventually after calcination treatment. The crystal structures of as-prepared samples were investigated using XRD, as shown in Figure 1a. A characteristic peak was observed at 26.5\({}^{\circ}\) in both Co@G and G samples, and this peak was assigned to the C (002) diffraction peak [45]. Besides, the Co@G sample had an extra peak at 42.4\({}^{\circ}\), corresponding to the (200) crystal facet of CoO (JCPDS No. 75-0533) [46]. This was further confirmed from the magnified curve in the appointed region (Figure S2a). Moreover, it was easily found that the peak intensity (26.5\({}^{\circ}\)) of CoO@G was much stronger than that of the G sample, which is mostly attributed to the Co-species introduced to improve the structure orderliness of carbon at a relatively high temperature to some extent. In contrast, there were no characteristic peaks at 26.5\({}^{\circ}\) in both Co@g-C\({}_{3}\)N\({}_{4}\)/G and g-C\({}_{3}\)N\({}_{4}\)/G samples. This phenomenon may be attributed to the fact that some of the g-C\({}_{3}\)N\({}_{4}\) in the g-C\({}_{3}\)N\({}_{4}\)/G and Co@g-C\({}_{3}\)N\({}_{4}\)/G precursor under thermal treatment was partially decomposed [37; 38], and the decomposed product could destroy the structure of carbon and result in an amorphous structure. Furthermore, the Co atoms could bind the N atoms to construct Co-N\({}_{x}\) bonds [47; 48], implanting the Co@g-C\({}_{3}\)N\({}_{4}\)/G material. Thereby, these mutual effects enhance the disorder degree of obtained materials. In addition, there were no obvious diffraction peaks of Co-species in the Co@g-C\({}_{3}\)N\({}_{4}\)/G sample compared to CoO/G. The differences may be ascribed to the highly dispersed ultrafine Co-species in the final product, and thus, it is hard to detect a characteristic signal, evidencing that the addition of g-C\({}_{3}\)N\({}_{4}\) nanosheets could effectively alleviate the aggregation of Co\({}^{2+}\) cations under an aqueous solution state. In addition, the annealing temperature as an influencing factor in the formation of the structure and composition of the obtained Co@g-C\({}_{3}\)N\({}_{4}\)/G products was further investigated. As displayed in Figure 1b, three samples almost exhibited similar XRD patterns at the corresponding temperature. However, an extra diffraction peak appeared at -43.7\({}^{\circ}\) after calcination at 600 \({}^{\circ}\)C (red line in Figure 1b), corresponding to the (220) crystal plane of Co [46].