3D porous framework composed of exfoliated ultrathin nanosheets is a hot topic in the field of energy storage and conversion. A facile method to prepare 3D mesoporous C3N4 with few-layered nanosheets interconnected in large quantity via H2SO4 intercalation and subsequent thermal treatment was described herein. The obtained thermally-exfoliated C3N4 (TE-C3N4) was thoroughly characterized by scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR) spectroscopy, atomic force microscopy (AFM), UV-Vis diffuse reflectance spectroscopy (DRS) and Brunauer-Emmett-Teller (BET) measurements. The detailed analysis indicated that TE-C3N4 possessed enlarged inter-layer space, enhanced UV-light adsorption and high specific surface area with 3D interconnected structure composed of ultrathin 2D nanosheets. Compared to bulk C3N4, TE-C3N4 showed an enhanced photocatalytic activity for photodegradation of Rhodamine B under UV-light irradiation and exhibited no significant loss of photocatalytic activity after 11 recycled runs. The pseudo-first reaction rate constant for TE-C3N4 was about four times higher than that for pure bulk-C3N4. The better photocatalytic performance could be attributed to more active catalytic sites, prolonged photo-excited carrier lifetime and shorted pathway of the carriers to the reaction sites.

The heterogeneous Cu-based solid catalysts have attracted enormous attention of researchers in different potential applications. In this work, a graphitic carbon nitride/copper-doped carbon dots (g-C3N4/Cu-CDs) nanocomposite with both intrinsic peroxidase- and oxidase-like (POD- and OXD-like) activities was successfully prepared. Due to the synergistic catalytic enhancement and electron transmission provided by g-C3N4, both POD- and OXD-like activities of g-C3N4/Cu-CDs were significantly improved compared with those of g-C3N4 and Cu-CDs. Moreover, upon the addition of H2O2, g-C3N4/Cu-CDs could catalyze the oxidation of colorless o-phenylenediamine (OPD) to form a yellow fluorescent product 2,3-diaminophenazine (DAP) with yellow fluorescence. Interestingly, the OPD + H2O2 + g-C3N4/Cu-CDs system could be inhibited by phenolic compounds, which could efficiently decrease the DAP fluorescence. Based on this, a method for the quantitative detection of total phenolic substances was established. Meanwhile, the use of OXD-like activity of nanocomposites was extended for the degradation of phenols (e.g., 2-CP), which showed a good degradation efficiency. Based on the result that the conversion of Cu+/Cu2+/Cu0 plays pivotal roles in promoting the generation of radicals (i.e., •OH and •O2 –), a possible catalytic mechanism of g-C3N4/Cu-CDs was deduced. These findings showed that the proposed g-C3N4/Cu-CDs exhibit great potential to become a green catalyst for the degradation of phenolic pollutants in the environment.

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