Construction And Photoelectric Properties Of Graphene Quantum Dot Composites

Graphene quantum dots（GQDs）have received widespread attention for their continuously adjustable band gap width,structural stability,resistance to strong acid and alkali corrosion,low cost,and green environmental protection,and have shown great potential and application value in the fields of electronics,catalysis,photonics,and cross-disciplines.However,GQDs have quantum size effect and domain-limiting effect,and their photogenerated electrons and holes are easily compounded with each other,which limits its application in photoelectric conversion.Researchers have compounded GQDs with other materials to enhance their photoelectric conversion capability,but research on such GQDs composites are still limited and their photoelectric properties are not ideal,and it is necessary to construct new composite systems.In this paper,we synthesized light-emitting GQDs with stable performance,then introduced manganese dioxide（MnO₂）and gold nanoclusters（Au NCs）to successfully construct two nanocomposites,GQDs/MnO₂and GQDs/Au NCs,by different methods,and then investigated their photoelectric properties by steady-state and transient spectroscopic techniques and electrochemical measurements,and found that the GQDs/MnO₂composites can effectively detect glutathione（GSH）.The specific contents and results are as follows:（1）GQDs were synthesized by pyrolysis using tartaric acid and urea as reactants.Under the observation of transmission electron microscope,atomic force microscope,Fourier transform infrared spectrometer and X-ray photoelectron diffraction,the prepared GQDs were uniform in size with an average particle size of4.87 nm and lattice spacing of 0.24 nm,corresponding to the graphene（1120）crystal plane.The optical properties of GQDs were investigated by steady-state fluorescence spectroscopy,and their emission peak positions were excitation wavelength dependent,with a characteristic emission maximum at 460 nm when excited at the optimal excitation wavelength of 380 nm.（2）Using GQDs as the donor for electron transfer and MnO₂ as the acceptor for electron transfer,GQDs/MnO₂ composites satisfying the type-Ⅱ energy band structure were prepared using the electrostatic interactions between the surface groups of the two nanomaterials,and their applications in photoelectric conversion and GSH detection were investigated.Fluorescence quenching,lifetime shortening and transient absorption data of GQDs/MnO₂ composites date all confirm that electrons can be efficiently transferred from photoexcited GQDs to MnO₂ with an electron transfer efficiency of about 14%driven by type-Ⅱ energy band alignment.In the photocurrent test,the photocurrent of GQDs/MnO₂ composites was 4.54 times higher than that of GQDs alone and 2.36 times higher than that of MnO₂alone.In addition,the fluorescence quenched by MnO₂ on GQDs can be recovered to some extent by GSH,showing the fluorescence response behavior between GQDs/MnO₂ composites and GSH.The electron transfer in GQDs/MnO₂ composites inhibits the direct recombination of excitons and improves the photoelectric properties of GQDs.（3）Using GQDs as the acceptor for electron transfer and Au NCs as the donor for electron transfer,GQDs/Au NCs composites satisfying the type-Ⅱ energy band structure were constructed using non-covalent interactions between the surface groups of the two nanomaterials,and the photoelectric properties and electron transfer kinetics of the composites were investigated.The introduction of Au NCs compensated for the lack of light absorption of GQDs in the visible region,and the GQDs/Au NCs composites exhibited significant fluorescence quenching compared to the strong fluorescence of individual Au NCs.The time-resolved fluorescence and transient absorption kinetics clearly showed that the photogenerated electrons of Au NCs could be efficiently transferred to the electron acceptor GQDs with an electron transfer efficiency of about 75%.The electron transfer efficiency obtained from the dynamic analysis was in good agreement with that of the steady-state fluorescence quenching.In the photocurrent test,the photocurrent of GQDs/Au NCs was increased by 4.80 and 2.69 times,respectively,in comparison to GQDs and Au NCs alone.The GQDs/Au NCs composites obviously exhibited stronger light trapping ability and photoelectric properties.In conclusion,we successfully prepared GQDs and then synthesized two composites,GQDs/MnO₂ and GQDs/Au NCs,based on them,and investigated their photoelectric properties.The successful construction of both composites enhanced the photoelectric properties of GQDs and provided a promising strategy for the development of photoelectric composites based on GQDs.