N-doped graphene quantum dots modified with CuO (0D)/ZnO (1D) heterojunctions as a new nanocatalyst for the environmentally friendly one-pot synthesis of monospiro derivatives

Abstract

Although graphene quantum dots (GQDs) have attracted great research interest owing to their various applications, the performance of N-doped GQD nanocomposites has rarely been focused on in the catalysis field. A simple one-step chemical strategy for the fabrication of a CuO nanoparticle/ZnO nanorod heterojunction was introduced for the first time in this paper. According to the obtained results, the selection of an appropriate molar ratio, temperature, and pH are the most important factors in the heterojunction preparation procedure. Then, N-doped graphene quantum dots modified with CuO/ZnO nanoarrays were synthesized via a green hydrothermal process. The catalytic activities of the CuO/ZnO heterojunctions, pure N-GQDs, and CuO/ZnO@N-GQDs nanocomposite were compared and, based on the empirical results, it can be concluded that the activities are in the order: CuO/ZnO@N-GQDs > N-GQDs > CuO/ZnO. Therefore, the proposed nanocomposite was used to facilitate the one-pot synthesis of monospiro-2-amino-4H-pyrans in an aqueous system at ambient temperature with excellent yields (89–98%) within a short reaction time. In this multicomponent reaction, three new σ bonds (one C–O and two C–C bonds) were formed in a single operation, which led to the generation of the spiro skeleton. Moreover, the CuO/ZnO@N-GQDs heterogeneous nanocatalyst was also recoverable after seven cycles without significant loss of activity.