Synergy of Intercalation and Stacking Fault Formation to Increase the Optical Properties of N-CQDs/ZnO NCs via Nonmetal–Metal Interface and the Investigation of Its Photocatalytic Activities for Degradation of Rhodamine B

Structural defects in carbon quantum dots (CQDs), zinc oxide nanoparticles (ZnO NPs), and N-CQDs/ZnO nanocomposites (N-CQDs/ZnO NCs) for photocatalytic application are complex and challenging to investigate. In this work, we reported the mechanism of formation of defects and its effect on the properties of the ZnO layer. The existence of the defect structure of the materials was confirmed by X-ray diffraction (XRD). The N-CQDs exhibited a heterogeneous multilayered structure with a weak degree of graphitization (D.G% = 34.87). The crystal defect of the ZnO NPs and N-CQDs/ZnO NCs is dominated by higher dislocation density (31.76 × 10^-4 and 31.96 × 10^-4, respectively) than wurtzite ZnO (5.94 × 10^-4). Atomic details of mismatch dislocation at the interface of ZnO NPs were imaged by high-resolution electron microscopy (HRTEM) with Burger vector of the type 1/3[101̅0] in the hexagonal close-packed (HCP) lattice, which confirmed the existence of the basal stacking fault (BSF). The design of a hybrid system of N-CQDs/ZnO NCs involves the process for generating the intercalation construction by an interfacial interface between ZnO NPs (the host) and N-CQDs (the intercalant). Both the stacking fault formation and intercalation are possible mechanisms to generate oxygen vacancy (V_O) or zinc vacancies (V_Zn) whose energy level is <1 eV below conduction band (CB) and above valence band (VB), respectively, which significantly affects the optical properties of N-CQDs/ZnO NCs as a direct Z-scheme photocatalyst that allows photoinduced electrons and holes (e^-/h⁺) pairs to take part in photocatalytic reactions. Under UV-B light irradiation, the N-CQDs/ZnO NCs show a photocatalytic efficacy of 75.82% in the degradation of Rhodamine B (RhB) 15 ppm with pH = 7.2 for 60 min at room temperature. The electrical energy per order (E_EO) and total operating cost (TOC) analysis confirm that the N-CQDs/ZnO NCs are economical photocatalysts, and worth investigating further.