▎ 摘　　要

This work adopts one- and two-step modified Hummers' method to chemically exfoliate graphene nanosheets (GNs) from graphite flake powders and carbon nanotubes (CNTs). The protocol for making the GN products involves one- and two-step exfoliation using KMnO4 oxidation agent. Experimental results reveal that the two-step exfoliation induce high efficiency of chemical oxidation, especially for the synthesis of GNs from CNT precursor. The thermal reduction enables the removal of surface functionalities, thus narrowing the band gap of GN-based electrodes, e.g., from similar to 2.12 eV to similar to 1.60 eV. The two series of graphene samples, prepared from graphite flake and CNTs, are chemically cut into nanoscale and serve as graphene quantum dots (GQDs). The photoluminescence (PL) excitation behavior is strongly affected by oxidation level and band gap of GQD products. After the thermal reduction, the PL peak shifts to lower wavelength region (i.e., 427-451 nm), offering a blue-shift behavior. The blue-shift behavior primarily originated from surface functionalization, particle size and shape of GQDs. Without any thermal reduction, the GQD suspensions exhibit an improved PL intensity, attributed to fast carrier-carrier scattering dominates over the electron-phonon scattering onto the basal plane of graphene sheets due to their high oxidation level and large band gap. As a result, this study offers an efficient way to tune the band gap of GQDs, favoring the development of optoelectronic and biological applications using graphene-based materials in the future.