Sulfur, nitrogen co-doped graphene quantum dots (S, N-GQDs) with high crystallinity were obtained by a top-down strategy. The as-prepared S, N-GQDs were investigated and the results indicate that S, N-GQDs exhibit a transverse dimension about 20 nm and a topographic height of 1–2 layers graphene. The incorporation of S, N can effectively reduce the layers of GQDs and strip the graphene sheets. Moreover, the S, N-GQDs reveal an absorption band located at 405 nm and exhibit an adjustable fluorescence characteristic in the excitation-visible range. Meanwhile, the S, N-GQDs shows a high specific capacitance of 362.60 F g−1 at a fixed scan rate of 5 mV s −1. This high performance is ascribed to the additional high pseudocapacitance provided by the doped S, N and the doping state acting as a trap state to enhance the charge storage capacity. The high specific capacitance advantages of S, N-GQDs illustrate their potential prospects in the capacitors.

ZnS-graphene quantum dot (GQDs) composites were synthesized by a simple solvothermal method, in which GQDs were prepared by a hydrothermal cutting process. The products were characterized by transmission electron microscopy, atomic force microscopy, X-ray diffraction and ultraviolet-visible absorption spectroscopy. The results show that GQDs were obtained by size tailoring of 1–4 graphene layers and combined with cubic ZnS nanoparticles to form ZnS-GQDs composites. The photocurrent and electrochemical behavior of the products were evaluated by transient photocurrent responses and electrochemical impedance spectra. The photocurrent density of ZnS-GQDs achieves the value of 2.32 × 10⁻⁵ A/cm², which is 2.4-times as high as that of ZnS-graphene. GQDs serve as an electrical conducting material, which decreases the conductive path and accelerates the electron transfer. The charge-transfer resistance of ZnS-GQDs is much lower than that of ZnS-graphene and pure ZnS due to the effective electron separation and transfer ability upon the incorporation of GQDs.